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THE PREPARATION OF SMALL METAL PARTICLES ON ZEOLITES AND OTHER SUPPORTS (MICROWAVES, MOESSBAUER, FISCHER-TROPSCH, FERROMAGNETIC RESONANCE)

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8512151
M cM ahon, K erry Charles
THE PREPARATION OF SMALL METAL PARTICLES ON ZEOLITES AND
OTHER SUPPORTS
Ph.D.
The University of Connecticut
University
Microfilms
International
1985
300 N. Zeeb Road, Ann Arbor, Ml 48106
Copyright 1985
by
McMahon, Kerry Charles
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THE PREPARATION OF
SMALL METAL PARTICLES
ON ZEOLITES AND OTHER SUPPORTS
Kerry Charles McMahon,
Ph.D.
The University of Connecticut, 1985.
Small metal particles
selective
catalysts for
in the cages of
the
zeolites can be
production of
hydrocarbons
from carbon monoxide and hydrogen.
Our research involves
several
preparation of
different methods
for the
particles of metal in zeolites.
small
Iron and cobalt were the
metals investigated.
Success
microwave
has
been
induced
achieved
argon plasma
carbonyls and organometallics to
metal particles.
their
chemical,
Fischer-Tropsch
through
which
the
use
decomposes
of
a
metal
form small ferromagnetic
Characterization of these samples as to
physical
synthesis
and
catalytic properties
has been
done.
for
Details
preparations and reactions are presented.
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of
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THE PREPARATION OF
SMALL METAL PARTICLES
ON ZEOLITES AND OTHER SUPPORTS
Kerry Charles McMahon
B.S., Geneva College,
1979
A Dissertation
Submitted in Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
at
The University of Connecticut
1985
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APPROVAL PAGE
Doctor of Philosophy
THE PREPARATION OF
SMALL METAL PARTICLES
ON ZEOLITES AND OTHER SUPPORTS
Presented by
Kerry Charles McMahon, B.S.
Major Adviser_
Steven L. Suib
Associate Adviser
John Tanaka
Associate Adviser
Robert G. Michel
The University of Connecticut
1985
- ii -
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Copyright by
Kerry Charles McMahon
1985
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To Rete and Babum,
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ACKNO W LEDGEM ENTS
First of
into
all,
I ’d like to
chemistry and
graduate school.
I'd like to
thank the Lord who
helped me
led me
through undergraduate
and
He was always there when I needed Him.
thank Steve for guiding me
into a chemist.
and molding me
I appreciate all the extra effort he put
into directing the research group.
A special thanks
goes to Dr.
Tanaka
whose late night
advice helped me get my dissertation done.
I'd also like
to thank Dr. Michel for helping me with my dissertation.
I want
to say
thanks to
the entire
Suib group
from
Ovid, Dimitri and Dan to Katy, Ann, Rich, Art, Jim, Norma,
Zong-Chao,
Janet and the
unforgetable Mr.
Willis.
You
made the four and a half years bearable.
I want to thank Lennox Iton
for exposing me to zeolite
synthesis and making Argonne a fun experience.
Finally,
I'd like to thank
my wife,
Minnie,
support, love and patience she has given me.
for the
I did it all
for her.
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Funding for this and other
term
of my
dissertation was
work carried out during the
provided
by the
following
agencies:
Atlantic Richfield Foundation of the Research Corporation,
Petroleum Research Fund of the American Chemical Society,
National Science Foundation under Grant CHE 82 ^ 1 7 ,
Department of Energy, Basic Energy Sciences,
and
The University
of
Connecticut Research
Foundation,
Dissertation Fellowship.
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TABLE
OF
CONTENTS
APPROVAL P A G E .......................................... ii
ACKNOWLEDGEMENTS
.......................................
iv
Chapter
page
I.
INTRODUCTION
.....................................
1
O v e r v i e w ................................................ 1
B a c k g r o u n d Of T h e W o r k ............................. 2
...................
2
Transition Metal Compounds
S u p p o r t e d M e t a l C a t a l y s t s .....................
2
H e t e r o g e n e o u s C a t a l y s i s ........................
3
Spectroscopic Studies of Catalysts . . . .
5
Relevant Literature
................................
6
Zeolites
...........................................
6
Zeolite Synthesis
...........................
6
Zeolite Structure
...........................
7
E x c h a n g e a b l e C a t i o n s ........................
13
New Z e o l i t e s ........................... 15
P r e c u r s o r s Of S u p p o r t e d M e t a l s . . . . . .
M e t a l L o a d e d Z e o l i t e s ......................... 26
I r o n L o a d e d Z e o l i t e s ....................... 26
C o b a l t L o a d e d Z e o l i t e s .....................
M e t a l A t o m V a p o r i z a t i o n ..................27
Bimetallic Zeolites
........................
I r o n An d C o b a l t On O t h e r S u p p o r t s ......... 29
M e t h o d s o f A c t i v a t i o n Of S u p p o r t e d M e t a l s .
T h e r m a l A c t i v a t i o n ...........................
Chemical Activation
........................
P h o t o c h e m i c a l A c t i v a t i o n ...................
M i c r o w a v e A c t i v a t i o n ........................
F i s c h e r - T r o p s c h S y n t h e s i s .....................
S h a p e S e l e c t i v i t y ......................... 38
C a t a l y s t A c i d i t y ............................ 38
I n s t r u m e n t a l T e c h n i q u e s W h i c h AreU s e f u l . . .
Methods For Pro bing E lect ro n ic P r o pe rt ie s
.
M o s s b a u e r S p e c t r o s c o p y . ...................
Ferromagnetic Resonance
...................
Infrared Spectroscopy
.....................
Methods For Probing StructuralProperties
.
Electron Microscopy
........................
X-ray Powder Diffraction . . . . . . . .
Me th ods For Probing Chemical Properties . .
C-as C h r o m a t o g r a p h y ......................... 49
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18
27
28
30
30
33
33
34
35
39
39
39
45
48
48
48
49
49
Gravimetric Techniques .................
U9
R a t i o n a l e .......................................50
Catalyst Synthesis
........................ 50
Catalytic Reactions ........................ 51
Spectroscopic Characterization
........... 51
F o c u s ........................................... 52
II.
EXPERIMENTAL METHODS
............................
53
Introduction ................................... 53
Mossbauer Spectroscopy ........................ 53
Electron Paramagnetic Resonance/Ferromagnetic
R e s o n a n c e ................................ 56
.......................... 56
Electron Microscopy
X-ray Powder Diffraction ...................... 57
Infrared Spectroscopy
........................ 57
Gas C h r o m a t o g r a p h y .............................. 57
Thermal Analysis Techniques
................. 60
Microwave Generator
.......................... 60
R e a g e n t s ......................................... 60
Sample Preparation M e t h o d s ..................... 62
Iron Carbonyl Zeolites
.................... 62
Hydrogen Reductions ........................ 65
Sodium Vapor Reductions .................... 66
Bimetallic Zeolite Preparations ........... 69
New Aluminoferrisilicate Zeolite
Preparations
........................ 69
Microwave Discharge Preparations.. ......... 73
Microwave Generation of Color Centers . . . 78
Inert Atmosphere Dry Box Procedures . . . . 79
Fischer-Tropsch Reactions ................. 79
III.
R E S U L T S ............................................. 82
Investigations of Samples By Fischer-Tropsch
R e a c t i o n s ................................ 82
Catalysts Prepared By Microwave Reduction . 82
Cobalt Catalysts ........................ 82
Iron C a t a l y s t s ............................88
Catalysts Prepared By Hydrogen Reduction
103
Iron Carbonyl Catalysts
.............
103
Bimetallic Zeolite Catalysts .........
109
Spectroscopic Characterization of Catalysts
117
Catalysts Prepared By Microwave Reduction
117
Catalysts Prepared By Hydrogen Reduction
128
Iron Carbonyl Catalysts
.............
128
Bimetallic Zeolites
.................
1^1
Aluminoferrisilicate Zeolites . . . .
1^8
Catalysts Prepared By Sodium Vapor
R e d u c t i o n ............................. 159
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IV.
D I S C U S S I O N ..................................... 164
Fischer-Tropsch Results
...................
164
Catalysts Prepared By Microwave Reduction
164
Cobalt Catalysts ......................
164
Iron C a t a l y s t s ...........................166
Catalysts Prepared By Hydrogen Reduction
169
Iron Carbonyl Catalysts
.............
169
Bimetallic Catalysts .................
171
Spectroscopic Charaterization Of Catalysts . 173
Catalysts Prepared By Microwave Reduction 173
Catalysts Prepared By Hydrogen Reduction
177
Iron Carbonyl Zeolites ...............
177
Bimetallic Zeolites
.................
180
Aluminoferrisilicate Zeolites
. . . .
183
Catalysts Prepared By Sodium Reduction
. 184
Comparison of Sample Reduction Methods . . .
185
Completeness of Reduction ...............
185
Length of Reduction T i m e .................. 187
Comparison of Particle S i z e .................. 188
Mechanisms of Reduction of Metal Complexes
in Z e o l i t e s ...........................191
Comparison to Zeolite Catalysts Prepared by
O t h e r s ................................... 193
V.
C O N C L U S I O N S ........................................ 196
Information Gained ..........................
196
Future Experiments ..........................
197
Catalytic Studies ........................
197
Characterization Studies
...............
197
New P r e p a r a t i o n s ...........................198
R E F E R E N C E S .............................................. 201
VITA
213
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L IS T
OF T A B L E S
Table
P age
1.
Description of Z e o l i t e s ............................ 11
2.
Zeolite Properties .................................
3.
Mossbauer Data For Iron C o m p o u n d s ................. 41
4.
Electronic Configurations of Iron and Cobalt . . .
47
5.
Weight Percent Values
59
6.
Vaporization Temperatures of Na and K
7.
Synthesis Conditions ...............................
71
8.
Reactant Concentrations
72
9.
Catalytic Properties of Iron and Cobalt Catalysts
10.
Catalytic Properties of Bimetallic Samples . . .
11.
Values of g A p p a r e n t ............................... 127
12.
Iron Carbonyl Samples, Unreduced ...............
129
13.
Iron Carbonyl Samples, Reduced ..................
131
14.
Bimetallic Samples, Unreduced
..................
14?
15.
Bimetallic Samples, Reduced
....................
144
16.
Aluminoferrisilicate Zeolites, Mossbauer Results
150
17.
Mb'ssbauer of Reduced Aluminoferrisilicates . . .
152
18.
Ion-Exchanged Zeolites, Mossbauer Results
...
160
19.
Mossbauer Results After Sodium Reduction . . . .
162
20.
Comparison of Reduction Methods
190
21.
Comparison of Catalysts With Literature
12
............................
............. 68
..........................
99
116
................
. . . .
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195
L IS T
OF
F IG U R E S
Figure
page
1.
Structure of Zeolite X ............................... 9
2.
Sites in Z e o l i t e s ................................... 14
3.
Zeolite Framework Replacement
4.
Structure of Fe(CO)c .................................19
D
5.
Structure
of Fe2 (C0 ) g .............................. 20
6.
Structure
of Co2 (C0 ) g .............................. 22
7.
Structure
of F e r r o c e n e .............................. 24
8.
Structure
of Nitroprusside Anion ..................
25
9.
Mossbauer Experiment ...............................
43
10.
In-situ Mossbauer C e l l .............................. 55
11.
Inverted U T u b e ..................................... 64
12.
Sodium Vapor Reduction Apparatus ..................
13*
Sample T u b e ......................................... 74
14.
Microwave L i n e ....................................... 76
15.
Reaction L i n e ....................................... 81
16.
Percent Conversion Cobalt Catalysts
17.
Co Catalysts Methane Production
..................
85
18.
CoZSM-5 Product Distribution ......................
86
19.
CoX Product D i s t r i b u t i o n ............................ 87
20.
Percent Conversion of Iron on A
....................
.............
17
67
84
.................... 89
21 . Percent Conversion of Iron onZ S M - 5 ................ 91
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22.
Catalytic Properties of Iron onZ S M - 5 .............. 92
23.
Percent Conversion of Iron on X
24.
Catalytic Properties of Iron on X
25.
Percent Conversion of FeX S i n t e r e d ................. 97
26.
Percent Conversion of Iron on Y
27.
Catalytic Properties of Iron on Y
28.
Catalysis of Fe(C0)5Y
29.
Methane and Ethane Production of Fe(C0)^Y
30.
ftp Desorption of F e ( C 0 ) ^ Y ......................... 108
31.
Catalytic Properties of C o Y ................
111
32.
CoY Activity Versus Temperature
112
33*
Catalytic Properties of R u Y .................... 11 if
....................94
.................. 95
................... 101
................. 102
..............................104
. . .
................
106
3*1. RuY Reaction and Helium P u r g e ..................... 115
35.
FeY FMR S p e c t r a .................................... 118
3 6 . FeY g Value Versus T e m p e r a t u r e ..................... 120
37.
FeY FMR Linewidth Versus Temperature ............
121
38.
FMR of C o X ........................................... 123
39.
CoX g Value Versus T e m p e r a t u r e .................. 12-4
40.
CoX Linewidth Versus T e m p e r a t u r e ...................125
41.
Mossbauer Spectrum of Fe(C0)^Y... ................
42.
TEM of Fe(CO)cY
0
43.
Mossbauer of Fe(C0),-Y C a r b i d e ..................... 139
44.
Mossbauer Spectrum of C o Y ......................... 146
45.
DTA of A l u m i n o f e r r i s i l i c a t e ....................... 154
46.
DTA of A l u m i n o f e r r i s i l i c a t e ....................... 155
47.
FMR of A l u m i n o f e r r i s i l i c a t e ....................... 157
48.
FMR of A l u m i n o f e r r i s i l i c a t e ....................... 158
132
.................................... 136
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Chapter I
INTRODUCTION
1.1
OVERVIEW
The research
the
study of
described in
transition
this dissertation
metals
supported on
involves
zeolites.
Several different methods were used to prepare and charac­
terize small metal particles (less
the pores of zeolites.
than 11 Angstroms)
in
Samples prepared by various meth­
ods were studied as catalysts in the Fischer-Tropsch reac­
tion.
The goals of this research were to synthesize novel
supported metal
systems,
Tropsch catalysts,
and to
to
prepare selective
to study various activation procedures
investigate the chemical,
properties
of small
Fischer-
metal
physical
particles.
and catalytic
Throughout
this
work, several spectroscopic experiments were used to char­
acterize these
metal supported zeolite
catalysts before,
during and after catalytic reactions.
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2
1.2
BACKGROUND OF THE WORK
1.2.1
Transition Metal Compounds
Transition metals,
which include elements
like iron,
cobalt, ruthenium, etc., have very interesting properties.
Some of these properties of transition metal compounds are
various coordination geometries, variable oxidation states
and the ability to form brightly colored compounds.
sition metals are also important as catalysts.
Tran­
Catalysts
are materials that speed up reactions but are not signifi­
cantly consumed in the reaction.
1.2.2
Supported Metal Catalysts
Transition metals
when in a
often act as
supported metal form.
system that results when
a
catalysts particularly
A supported
metal is a
transition metal is dispersed
on an inert material (support) such as Si02 or Al^O^.
El­
ements can be used as supports, the most common being car­
bon.
Oxides, such as alumina (A1?0^), silica (SiO? )
titania (Ti02 )
are also commonly used.
and
Zeolites are an­
other oxide material that can be used as a support.
Dif­
ferent supports have different loading capacities, differ­
ent surface
areas and different
degrees of acid
or base
character.
Because these metals are
of the metal is small and
dispersed,
the particle size
the surface area is large since
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3
surface area is inversely related
to particle size.
One
way to obtain a highly dispersed metal is to prepare a mo­
lecular cluster
of metal
cluster compound.
atoms which
is called
a metal
Metal clusters are compounds containing
two or more atoms of the
same or different metals usually
with metal-metal bonds and ligands attached.
The
relationship between
clusters has been suggested
supported
ported metals have
those of metal clusters.
characteristics of metals such
also have similar characteristics
clusters which have
only a few metal
magnetic properties
than bulk
supported metals
1 -ii
exist between the prop­
Sup­
to metal
atoms and different
materials.
may illuminate the
Sup­
as the
absence of attached groups and zero oxidation state.
ported metals
metal
by Muetterties and others
There are some relationships that
erties of bulk metals and
metals and
The
study of
relationship between
metals and metal clusters and improve theories about bond­
ing and catalytic reactions.
1.2.3
Heterogeneous Catalysis
Present theories
catalysts,
lysts.
suggest that there
homogeneous catalysts
Homogeneous catalysts are
are two
types of
and heterogeneous cata­
used when the catalyst
and reactants are in the same phase such as liquids react­
ing with a catalyst dissolved in solutions.
Heterogeneous
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u
catalysts incorporate different phases such as a solid ca­
talyst and gaseous reactants.
tion deals with heterogeneous
catalysis,
the initial
The work in this disserta­
systems.
In heterogeneous
step is often the
adsorption (or
physical binding) of the reactants onto the active site or
metal.
The reaction takes place
on the surface and then
the products desorb from the catalyst.
detach from
a surface.
To adsorb
To desorb means to
means to attach
to a
surface.
The products that are formed
lyst are
self.
governed by the
and desorb from the cata­
properties of the
The activity of the catalyst
catalyst speeds
up a reaction.
lectivity which is
is the rate that the
The
how much total product is formed.
catalyst it­
activity determines
The catalyst has a se­
the ability of a catalyst
to form the
desired product rather than several side products.
Transition metals
can speed up
reactions such
as the
Fischer-Tropsch reaction which is the production of hydro­
carbons from hydrogen and carbon
monoxide.
We have pre­
pared supported metal catalysts that are active and selec­
tive
Fischer-Tropsch
catalysts.
heterogeneous catalytic reactions
tions which involves the
bons from olefins
Other
important
are hydrogenation reac­
formation of saturated hydrocar­
and the oxidation of
ammonia by oxygen
to form nitric oxide".
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5
The properties
factors that are
the active
caused by the shape
sites.
sition metal
also
of catalysts are affected
In many
The oxidation
cases active sites
activity and
are tran­
Electronic factors
selectivity of
states of transition
sponsible for the
tion.
and distribution of
atoms on the catalyst.
affect the
by geometric
catalysts.
metals are
electronic effect in a
often re­
catalytic reac­
Many studies of supported metals appear in the lit­
erature.
Several of
these studies will be
discussed in
sections 1 .3*3 and 1.3 .^.
1.2.4
Spectroscopic Studies of Catalysts
Heterogeneous catalytic reaction
cult to understand.
One of the
mechanisms are diffi­
main features of our re­
search is the characterization of catalysts before, during
and after
a catalytic
reaction.
Several
spectroscopic
methods can be used to probe the geometric, electronic and
catalytic properties of these
systems.
structural studies were done with
diffraction
and
scanning
Our geometric or
the use of X-ray powder
electron
microscopy
methods.
Electronic properties were probed with Mossbauer spectros­
copy
and electron
resonance)
flow
paramagnetic resonance
techniques.
reactors and
Catalytic reactions were done in
the products
chromatography and mass
(ferromagnetic
were
analyzed with
spectrographic methods.
gas
Further
background information can be found in section 1.3*
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6
1.3
RELEVANT LITERATURE
1.3.1
Zeolites
One type of
zeolite.
con,
oxide that can be
used as a support
is a
Zeolites are aluminosilicates composed of sili­
aluminum and oxygen atoms.
They were first discov­
ered in 1756 by Baron Cronstedt, a mineralogist of Swedish
birth.^
Since that
time,
many uses have
been found for
7
zeolites from replacement for phosphates in detergents to
7
abrasives in toothpaste . Several potential uses of zeolQ
ites have also been suggested .
stedt discovered existed in
The zeolites that Cron­
nature.
Naturally occurring
zeolites are given names such
as faujasite and chabazite.
Other zeolites are man made.
Man-made zeolites are given
names such as
A,
X and ZSM-5.
Some synthetic zeolites
like Zeolon, marketed by the Norton Company,
analogs such as mordenite.
unique.
have natural
Other synthetic zeolites are
The study of man-made zeolites involves the area
of zeolite synthesis.
1.3.1.1
Zeolite Synthesis
There are many
ites
Q-1 2
.
patents concerning the synthesis
Zeolites
claves which are
synthesized in
high pressure reactors usually
of stainless steel.
high temperatures
are typically
of zeoiauto­
made out
Synthesis conditions normally involve
(150°C and
(above one atmosphere).
above)
and
high pressures
These conditions mimic the condi­
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tions in which zeolites are
formed in nature.
occurring zeolites are usually found
oceans.
ammonium
bromide,
The
factors
near volcanoes or in
Synthetic procedures often incorporate the use of
templates which are organic
form.
Naturally
around
which
the zeolite
size of the template
that determines
formed.
molecules such as tetrapropyl
structures
molecule is one
the particular
of the
zeolite that
is
Zeolites are often crystallized from basic solu­
tions and the pH of the
of the synthesis.
nate phases
phase is
solution is a very important part
Variations in
in the zeolite.
pH can introduce alter­
The preparation of
usually desired.
Variations in
procedures can result in the
a pure
the synthetic
preparation of zeolites with
different structures.
1.3.1.2
Zeolite Structure
Zeolites
have
structure.
rahedra of
an
open
three
SiO^ and AlO^ units
joined by the
sharing of
Theories about how these groups can attach
13
to each other have been suggested'-'.
13
framework
They are formed by the vertex linking of tet-
oxygen atoms.
stein
dimensional
proposed
that
unstable and therefore
A1 - 0
for instance, Lowen-
- A1 bonds in
do not exist
- 0 - A1 bonds are much more stable.
A 1C>2 unit has a charge
of minus
a charge of zero.
A1 - 0
In
_
zeolites are
in the framework.
Si
This is because the
one and the SiO^ unit has
- Al,
the minus charges are
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8
too close together and repel
each other making the struc­
ture unstable.
Al,
sion.
In Si - 0 -
there is no such repul­
Si - 0 - Si bonds are also stable because there is
no repulsion.
A specific structure, that of zeolite X, is
shown in Figure 1.
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9
Figure 1:
Structure of Zeolite X
Tetrahedral
Si 0/
u
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10
The three dimensional structure of
the one
for zeolite X,
space which can
contains
such as
a large amount
of void
be filled by molecules of
species like metal carbonyls.
sorption of
a zeolite,
molecules such
have been reported
water or other
Several examples of the ab­
as volatile
carbonyl species
14-22
The specific zeolite structure
can affect the environ-
3+
4+
ment of the A1J and Si
ions in the framework structure.
Solid state magic angle
spinning nuclear magnetic resoO *3—O fc\
nance
has been used to study these affects.
These
studies have
involved the study of
27
A1 and
observe the different A1 and Si environments
environments can
be different because of
29
Si
27-42
.
NMR to
These
various zeolite
compositions.
The composition of zeolites is
tion of
a few common
well known.
zeolite types and
can be found in Table 1.
A tabula­
their structures
A list of some of the properties
of these zeolites is given in Table 2.
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11
TABLE 1
Description of Zeolites
Type
A
Unit Cell Composition
Symmetry
Na12(Al02 )12(Si02 )124.5H2 0
C
Y
Na5 6 (Al02 )56(Si02 )136250H2 0
c
c
M
Na8 (A102 )g(Si02 )4024H20
0
NanAlnSi96_n019216H20
T
X
ZSM-5
44
formulas of A, X, Y and M from Breck
ZSM-5 formula from Kokotailo and coworkers
M - mordenite
C - cubic
0 - orthorhombic
T - tetragonal
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12
TABLE 2
Zeolite Properties
Type
Pores(A)
Stability(-K)— Largest AdsorbateDb
A
it.2
973
ethylene
X
7.5
1045
(CifH9 )3N
Y
7.5
1066
(c1|h9 )3n
M
7.0
1273
ZSM-5
5.5
C6H6
—
liq
a from J a c o b s ^
b from Breck
M - mordenite
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13
Several good
books are available that
7
in -iifi
properties and structure ’ J
of zeolites
great deal
and modified
5 8-55
penments J
are a
The chemical properties
zeolites have
of attention.
X-ray diffraction studies
.
discuss zeolite
also received
Infrared investigations
lie-8?
a
217 48
’ ,
and electron microscopy ex-
few of the areas that
have been ex­
tensively studied.
1.3*1*3
Exchangeable Cations
One feature of zeolites which
is highly related to the
structure and the Si/Al ratio is the presence of exchange­
able cations.
Cations are present
aluminosilicate framework has an
in zeolites since the
overall negative charge.
The general formula for a zeolite is the following^:
M e X / n ( A 1 0 2 > X ( S 1 0 2 > y ‘M H 2 0
where Me can be
a variety
an overall charge
ygen is -2.
of -1.
of cations.The SiO?
of zerosince silicon is +H
and
unit has
each ox­
The AlO^ unit, however, has an overall charge
In oxides, A1 is usually +3 and each oxygen is -2.
This negative charge
is compensated by a
cations can be monovalent
lency (Ca
2+
2+
, Mg ,A1
cation.
These
(Na+ , K+ , etc.) or of higher va-
3+
, etc.).
Some
of the possible ex­
change sites in a zeolite are shown in Figure 2.
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1n
Figure 2:
Sites in Zeolites
hexagonal
rin g
Large
cavity
sodalite
unit
hexagonal
ring
from
reference
56
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15
Interesting
chemistry
results from
these exchangeable cations.
presence
of
These cations are only loose­
ly bound to the zeolite and are,
bile.
the
therefore,
somewhat mo­
These cations can be replaced by other ions such as
rare earth ions to produce samples that luminesce
method commonly
ion-exchange.
ites with
used to replace
these cations
.
One
is called
It routinely involves the stirring of zeol­
aqueous solutions of
Prolonged and
57
the ion to
repeated exchanges
cation that is replaced.
is usually determined,
increase the
amount of
The upper limit of ion-exchange
however,
by the Si/Al ratio since
the number of exchangeable cations
of aluminum ions.
be exchanged.
is equal to the number
Variations in the ion-exchange capacity
of zeolites can be achieved by
the synthesis of new zeol­
ites which can have more or less cation exchange capacity.
1.3-1 .^
New Zeolites
Recently,
new classes of
zeolites have been prepared.
These new zeolites involve substitution of different atoms
into the framework of the
q r R-62
aluminophosphates' *'
or
sophates^,
aluminosilicate.
In this way,
ALPO^'s and
sil icoaluminop'n-
or S A P O ’s, have been formed.
These new zeol­
ites have new
and interesting properties and
have intro­
duced some new
three dimensional structures to
of zeolites.
A l P O ^ ’s
q 158-62
are
the class
electronically neutral
and therefore have no exchangeable cations.
fciO
SAPO’s
con­
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16
tain exchangeable cations.
Iron has also been incorporat­
ed into the zeolite framework forming what has been called
aluminof errisilicates** ^ ^
be used
to incorporate
^ .
Many different methods can
metals into
zeolites,
ion-exchange and framework substitution.
methods will be discussed in
including
Several of these
the next section.
Figure 3
shows how some iron ions can enter the zeolite framework.
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17
Figure 3:
Zeolite Framework Replacement
\
'
Si
o' 0
\
fi
Al
\ /
Si
.0'
Si
/
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18
1.3*2
Precursors Of Supported Metals
Several complexes
metals on supports.
of metals
Carbonyl species
were used in several of
as one of
were employed
the studies.
of iron and cobalt
Ferrocene was used
the starting materials in the
crowave method
method.
to prepare
Ions of iron
reduction by mi­
were ion-exchanged
into zeolites from several starting compounds such as fer­
ric nitrate
and iron
nitroprusside.
The
structures of
several of these starting materials will be discussed.
Several
different
carbonyl
species
were
employed.
Fe(CO)j-f Fe2 (C0)g and Co2 (C0)g were all utilized
as starting materials in our research.
The structures are known for the various iron carbonyls
that were studied.
iron in
bridging
a trigonal bipyramid with
the center and carbonyl
three equatorial
Figure
Fe(CO)^ is
positions.
Fe2 (C0)g
carbonyls and
is a
groups at two
The
linear
three
structure of Fe2 (C0 )g which is
structure is
axial and
shown in
structure with
three
terminal carbonyls.
The
also known as diiron nona-
carbonyl is shown in Figure 5.
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19
Figure 4:
Structure of Fe(CO)^
0
c
/
0
<T>
O u a c ^
F e ^ "
ID
\
0
0
4. 5
from
reference
128
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20
Figure 5:
Structure of Feo (C0).
on
lO
lO
CNJ
OsJ
lO
3.7
from
•
C
O
Fe
refe re nc e
129
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21
Co2 (CO)g is a linear molecule
nal
carbonyls similar
to
with bridging and termi­
Fe^CCO)^.
The structure
Co2 (C0)g is shown in Figure 6 .
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of
Figure 6 :
Structure of Co_(C0 )o
2
8
co
121
<NJ
LO
i
S- 2
from
•
Co
o
C
•
0
reference
187
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?3
Ferrocene is a sandwich compound.
It is called a sand­
wich compound because the iron atom is located between two
cyclopentadienyl rings like a sandwich.
The structure is
shown in Figure 7.
2The structure of
dron.
Fe(CN)cNO
b
is related to
an octahe-
This compound is shown in Figure 8 .
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?.n
Figure 7:
Structure of Ferrocene
Fe
fNJ
CO
kl
from
reference
188
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Figure 8:
Structure of Nitroprusside Anion
0
a
N
V* N
C'
LO
00
LO
-'N
4. 33
from
reference
189
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25
1.3.3
1.3.3.1
Metal Loaded Zeolites
Iron Loaded Zeolites
Morice and R e e s ^
studied ion-exchange of several zeolq
ites including
In many
X and Y with
cases breakdown of
occurred and Fe
2+
solutions of FeJ
+
the structure of
ions oxidized to Fe
“
5+
and Fe
p+
the zeolite
ions.
To avoid this structural breakdown, Collins and Mulay
obtained
iron loaded
Fe(C0)j- and FeCl^
was introduced
zeolite.
in zeolites.
by the
FeCl^ was
Both samples
zeolites
Fe(CO)^ is
deposited from
contained iron
and
the incorporation
spraying of a
Mossbauer spectroscopy,
ceptibility
by
a liquid and
fine mist
an ether
oxide and
electron microscopy.
onto the
solution.
were analyzed
X-ray diffraction,
of
by
magnetic sus­
Other
synthetic
methods have been used to avoid breakdown of the structure
of the zeolite.
f Q - (7A
Delgass and
coworkers
prepared iron
loaded zeol­
ites without structural degradation by the careful adjust­
ment of the solution pH before ion-exchange.
A pH of 3-8
- 4.0, prepared by the addition of of sulfuric acid to the
zeolite-water mixture,
They studied
allowed exchange of the Fe
these samples by Mossbauer
2+
ions.
spectroscopy and
also observed the effect of adsorbates such as ammonia and
oxygen.
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27
Garten and
Fe
P+
coworkers
in mordenite.
71
extended
this work
He studied these samples as catalysts
in the reverse
of the water gas shift reaction
— > H20 +
and the ammoxidation of
CO)
acrylonitrile.
to include
Studies of
Fe
p+
(H2 + C02
propylene to form
ions in ZSM^5
have also
been done.
Petrera and
coworkers
72
have recently done
some Moss­
bauer studies of large coordination complexes of Fe^+ that
were ion-exchanged into ZSM-5.
ious exchange sites
like
cobalt have
They investigated the var­
in this zeolite.
also
been
Other
metals ions
ion-exchanged into
zeolite
ZSM^-57 3 .
1.3.3.2
Cobalt Loaded Zeolites
Cobalt
ions have
Stencel and
with cobalt.
Co
P+
ions in
coworkers
been
73
incorporated into
have
studied zeolites
zeolites
73
.
exchanged
They observed the formation of nonreducible
interior sites sites of
ZSM-5 and reducible
cobalt oxide on exterior sites of ZSM-5.
complexes can also be placed
Cobalt and iron
in zeolites by the technique
of metal atom vaporization.
1.3.3.3
Metal Atom Vaporization
Metal atom vaporization
rate metals into
74-83
has
been used to incorpo­
zeolites and other supports.
Ozin and
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?8
yh
gO
coworkers
have used metal vapors
rather than
metals.
ion-exchange methods,
This
metal under
technique involved
high
of iron and cobalt,
to prepare
supported
the vaporization
of a
vacuum and the subsequent trapping and
solvating ofsmall particles of this metal which were then
dispersed onto a support.
resulting in
The solvent was then removed
the preparation of
the supports.
small metal
Ozin and coworkers
some of their preparations.
Mossbauer spectroscopy and
Tropsch reaction.
7 i i - AO
used
clusters on
zeolites in
These samples were studied by
as catalysts
in the
Butenes were selectively
reaction of carbon monoxide and
Fischer-
produced by
hydrogen over these cata­
lysts .
Guczi
84
has reviewed several
of metals into supports.
methods
Jacobs
of incorporation
85 86
’
has reviewed the in­
corporation of several metals
into zeolites.
zeolites can also be prepared
using similar techniques as
presented by
Guczi
84
and Jacobs
85
by loading
Bimetallic
two metals
into a zeolite.
1 -3-3-^
Bimetallic Zeolites
There has been some recent
work on mixed metal systems
and bimetallic systems, demonstrating the changes in prop­
erties of a metal when it is influenced by another metOy
al
. The term bimetallic in our work refers to the pres­
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29
ence of two transition metal ions which can be in ionic or
metallic states.
the presence
This
is related to our
research since
of another metal
might lower
decrease the
sintering that
temperature or
higher temperature in our samples.
the reduction
occurs at
a
This second metal may
also change the activation energy
of a reaction or influ­
ence the adsorption or desorption
properties of a sample.
One article that was particularly relevant was the work of
Scherzer and Fort
iron and other
zeolite Y.
88
.
They prepared bimetallic samples of
transition metals in the
The procedure
ammonium form of
incorporated the
initial ex­
change of a divalent cation like cobalt or copper followed
by the
subsequent exchange with an
iron anion
ii_
?_
Fe(CN)g
and Fe(CN)^NO
are two of the anion
complexes that
were employed.
formed a complex in the zeolite
form metallic iron.
The cations
species.
and anions
which could be reduced to
Scherzer and Fort
successfully prepared reduced iron
88 89
’
by this method
in zeolites.
We have
prepared some samples using
the method of Scherzer and
QQ Q1
Fort
’ .
Similar systems can be prepared by loading of
metals on other supports.
1 .3•^
Iron And Cobalt On Other Supports
Delgass and
coworkers
92-95
studied
the Mossbauer
and
Fischer-Tropsch properties of iron and iron alloys on oth­
er supports such as silica.
These systems can be compared
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30
to the
iron on
zeolite systems.
Cobalt has
also been
loaded onto silica and other supports.
Meyers and Hall
composition of
96
prepared cobalt on alumina by the de­
Co^(C0)12 .
They
studied
the
Fischer-
Tropsch activity of the resulting sample.
Bartholomew
Co^(C0)12
and
coworkers
and incorporated
well as other supports to
talysts.
also
cobalt nitrate
decomposed
on silica
as
form Fischer-Tropsch active ca­
They prepared catalysts that produced a variety
of hydrocarbons.
fell in
97 98
’
the
A significant percentage of the product
to
range.
treatment of the cobalt
use as a catalyst.
Their work
involved pre­
complex (decomposition)
There are
prior to
many other methods of pre­
treatment or activation of samples.
1.3*5
Methods of Activation Of Supported Metals
Activation procedures are an area of synthesis that has
been thoroughly studied.
There are several types of acti­
vation procedures including thermal, chemical, photochemi­
cal and microwave.
1.3*5.1
Thermal Activation
Heat is a common activation procedure, since many spec­
ies decompose at high temperatures or reduce at high temp-
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31
eratures to form active species.
the work of Huang and Anderson
Huang and Anderson
99
99
One such study involves
.
studied the
Fe^+ on zeolite Y by hydrogen.
would reduce to
ther.
Fe
2+
reduction of Fe
2+
and
They found that Fe^+ ions
but that Fe
2+
would
not reduce fur­
Metallic iron which was the intended product of the
reduction was not produced.
Gao and Rees*'l“,<") also had dif­
ficulty in producing metallic iron.
Gao and Rees10^ studied the
ite A.
son
99
.
reduction of Fe^+ in zeol­
They obtained similar results to Huang and AnderThey could
form ferrous ions but
Other heat treatments
of Fe^+ ions in
no iron metal.
zeolites have been
done.
Kulkarni and Kulkarni101 studied
of Fe^+ in zeolite Y.
the thermal stability
They found that the ferric ions mi­
grated from the supercages to the sodalite cages or hexag­
onal prisms at
500°C.
This migration was
analysis of
differential thermal
diffraction
showed
framework for
lattice
high loadings
determined by
analysis data.
distortion
of ferric
of
ions.
the
X-ray
zeolite
This
was
shown by a decrease in the relative intensities of certain
hkl planes.
lysts.
Many heat activated samples are used as cata-
One example is the work of Petunchi and Hall
1 02
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32
Petunchi and
Hall102 prepared Fe2+
mordenite using
on zeolites
Y and
Q
and coworkers
the procedure of Delgass
and studied the resulting catalytic properties for the ox­
idation of CO with NO, Op and ^ 0 .
tivity and
They compared the ac­
susceptibility to poisoning
of the
two zeol­
ites .
Lo and coworkers1^ ’1®^
heat treated Fe^+ ions
drogen to prepare Fischer-Tropsch
catalysts.
in hy­
These Fe^+
ions were impregnated into ZSM-5 and silicalite.
Silical-
ite is a zeolite that is isostructural with ZSM-5 but the11
oretically has no framework aluminum .
Impregnation in­
volves
the contact
support.
of a
solution
of metal
ion with
a
Usually only enough liquid as necessary to dis­
solve the metal ion precursor
compound is employed.
The
solution is then evaporated off leaving the metal compound
on the support.
This is different than ion-exchange where
only the metal ion is incorporated into the support.
impregnation,
the entire metal
Lo and coworkers
10S 10^4
compound is incorporated.
then studied the catalytic activi­
ty of these samples for Fischer-Tropsch synthesis.
discussion
1.3.6.
of
Fischer-Tropsch
Mossbauer spectra were
Fischer-Tropsch reaction.
the methane to butane
involve heat treatment
derson^,
With
Synthesis,
section
obtained before and after
Most of the products
range.
were in
Most activation procedures
such as the work of
Gao and Rees11^ ,
see
For a
Huang and An­
Kulkarni and Kulkarni1^1 ,
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Pe-
33
tunchi and
Hall
102
other activation
and
Lo and
coworkers
procedures like chemical
103
1
although
activation can
have certain advantages.
1.3.5.2
Chemical Activation
Gunsser and coworkers1
and Lee10^ used a chemical ac­
tivation of iron zeolites.
metal
vapor as
They studied the use of sodium
a reducing
agent.
transfer electrons to the iron
and Na+ in the zeolite.
tion
occurred.
Sodium metal
could
ions to form metallic iron
Unfortunately, incomplete reduc­
Complete
reduction of
iron
has
been
achieved with the use of photochemical activation.
1.3.5.3
Photochemical Activation
Derouane and
coworkers
14
used
photochemical activation step to
bonyl on zeolites
ultraviolet light
as a
decompose iron pentacar-
to form metallic iron.
They reported
the production of highly dispersed, pyrophoric iron parti­
cles.
Ultraviolet light has also been used in photocataihO
lytic studies of Fe(C0)^/zeolites
. Suib jet.al.
stud­
ied the conversion of 1-pentene to cis and trans 2 -pentene
on these
catalysts
10 8
.
Photochemical
the use of heat in pretreatment.
activation avoids
Another activation tech­
nique that does not require heat is microwave activation.
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34
1.3*5.4
Microwave Activation
Microwaves
ecules
have been
to form
agents1^ ’11 0 .
used to
are good
mol­
hydrogen atoms
which
This method of
activation avoids the use
of heat and has been
reducing
successful in preparing small parti­
cles of nickel on zeolite
the use of
activate hydrogen
Y.
This work has demonstrated
ferromagnetic resonance to study
the magnetic
properties of the samples and to get an approximate parti­
cle size of the metal.
Other reports of the use of ferro-
magnetic resonance in similar systems can be found
Another report of the use of microwaves
production of
then be used
volved the
a highly energetic
121
involves the
argon plasma
to decompose metal halides.
preparation of electrodeless
.
which can
This work in­
discharge lamps.
The system was a closed unit containing Mnl2 , silica chips
and a
reduced pressure of
done a
lot of work
book which reviews
argon gas.
with plasmas
McTaggart1
and has written
the area of chemical
has
a good
reactions in the
plasma.
Microwaves have
also been
used to
1
2
^ ip
J
i
such as zeolites
(RF)
dehydrate supports
1
2
R
.
Bartley
used radio frequency
waves to modify the catalytic properties of a sample
of iron on SiO^-Al^O^.
rectly by the
The iron particles were heated di­
RF waves and the
by conduction from the iron.
silica-alumina was heated
These catalysts were used in
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35
the hydrogenation
cyclopropane.
activated,
of propylene
and the
As with Bartley’s work
they are often used
1 25
isomerization of
, once samples are
as catalysts in reactions
such as Fischer-Tropsch synthesis.
1.3*6
Fischer-Tropsch Synthesis
Fischer-Tropsch synthesis is one
of the reactions that
1 2 f\
is catalyzed
by iron and cobalt
1 O *7
*
.
This synthesis
involves the production of hydrocarbons from carbon monox­
ide and hydrogen in the following reaction:
nCO
+
(iJn+2)H2
H ( C H 2 )n H
Catalytic studies
of metal containing zeolites
recent past have focused
ity.
+ nH2 0
in the
on efforts to increase selectiv­
Selectivity is the ability of a catalyst to form one
product.
Jacobs has addressed this area directly
109k
.
He
suggests that metal zeolites are important Fischer-Tropsch
catalysts and describes how they will effect the future of
this area
128
.
schemes impose
Deviations from
He states,
"Schulz-Flory polymerization
severe limitations upon
Schulz-Flory kinetics by
tions on dual-component catalysts
this selectivity.
secondary reac­
(classical CO reduction
function mixed with a shape selective acidic zeolite)
prove
range."
the
selectivity
mainly
in
the
gasoline
im­
number
Schulz-Flory polymerization is the mechanism that
is accepted as the pathway
for the formation of hydrocar-
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36
bons in the Fischer-Tropsch reaction.
its the products that can
lie in the
This mechanism lim­
be formed.
Desirable products
gasoline range since this is one
of the reac­
tions studied for the production of synthetic fuels.
1->o
way to change this selectivity, Jacobs suggests
,
use a dual component catalyst,
on a zeolite.
One
is to
such as a metal supported
The metal would provide the active site and
the zeolite would introduce shape selectivity1^3-1^8 .
a discussion on shape selectivity, see section 1 .3 .5 .1 .
i pQ
He also states
,
"The
encagement of these particles
in a stable manner in zeolite
cages seems the most likely
route to prepare selective FT catalysts."
that the metal be _in the zeolite cages.
It is important
The metal parti­
cles must be small enough to fit into these cages (11 Ang­
stroms in
zeolites X and
tempted to prepare
Y).
Several workers
small metal particles on
have at­
the order of
zeolite cages.
Jacobs also
occurs no
was used.
commented
129
matter what the
that
in all
cases sintering
reducing agent or
This results in the
external surface of the zeolite.
which metal
formation of metal on the
This sintering could be
drastically reduced with the use of low reduction temperatures
129
Jacobs proposes
129
in the preparation of
that heat is a very important factor
these samples.
Overheating causes
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37
the metal
and
to sinter and
underheating does
Phillips and
carbonyls
Dumesic
for the
since their
migrate to the
not completely
130
reduce the
have suggested
production
of
the use
small metal
reduction temperature is
for metal salts.
external surface
so much
metal.
of metal
particles
lower than
They suggests that supported metal sys­
tems could eventually approach the atomic dispersion limit
with these samples.
Changes in the catalyst can have some
effect on the mechanism of the reaction.
anism are therefore
Studies of mech­
very important in the
study of cata­
lysts.
Mechanisms of
the Fischer-Tropsch
131-138
suggested 3
0 .
reaction have
Niemantsverdriet and coworkers
gested that
surface carbides
are active
catalysts.
The formation of these
1 33
been
sug­
Fischer-Tropsch
active carbides is an
initial activating step.
Mossbauer
studies
have
been reported
of
iron
car-
1 • 3 Q - 1 2J2
bides
.
These
and bulk carbides.
studies have involved
The method of growth of the hydrocar­
bons has been studied.
Some authors favor a mechanism in­
volving polymerization of surface
prefer the insertion
are factors
CH^ groups while others
of CO into a surface
hydrogenation of the CO.
nometallic aspects
both surface
1 37
Herrmann J
of Fischer-Tropsch
that influence the
chain and then
discussed the orgasynthesis.
mechanism of
There
a reaction.
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33
Some of these
factors are shape selectivity
and catalyst
acidity.
1.3.6.1
Shape Selectivity
Shape selectivity
1ln-i 48
J
is an effect that
as zeolites can have on a reaction.
shape
selectivity due
cages and pores.
lectivity.
to
their
There are
supports such
Zeolites can exhibit
framework structure
of
different types of shape se­
Reactant selectivity
occurs when
reactants
that are small enough enter a pore and react whereas large
reactants
are excluded
144
.
Product selectivity
occurs
when the size of the product is determined by the pore
Metals supported on zeolites
tivity in
144
can introduce product selec­
reactions such as the
Fischer-Tropsch reaction
and gasoline range hydrocarbons could possibly be prepared
using a zeolite with the
correct pore size.
One example
of shape
selectivity is the Mobil
methanol to gasoline
14Q IRQ
synthesis
’
.
ZSM-5 has been shown to
catalyze the
formation of
This is a
gasoline range
hydrocarbons from
possible route for the
methanol.
production of synthetic
fuels.
1.3.6.2
Catalyst Acidity
The acid nature of catalysts
studied in
great detail
151-158
Bronsted and Lewis acid sites.
such as zeolites has been
.
Zeolites contain
both
Bronsted sites are present
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39
in the
form of
Lewis sites
groups.
OH groups
are present
The
in the
aluminum ion in
electron pair acceptor.
acid sites
attached to
form of
A1 and
Si atoms.
A10^ + and
these oxide species
A10+
is an
The strength and number of these
varies over the
different types
making them good cracking catalysts
151
.
of zeolites,
This is important
because cracking reactions form small useable hydrocarbons
from large chain hydrocarbons.
of reactions often involves
to characterize
important to
The
study of these types
many spectroscopic techniques
the catalysts.
The techniques
the research discussed in
that are
this dissertation
will be presented in the next section.
1.4
INSTRUMENTAL TECHNIQUES WHICH ARE USEFUL
FOR STUDYING THESE ZEOLITE SYSTEMS
The study
of catalysts such
can involve many characterization
tronic,
structural and chemical
as metal
loaded zeolites
techniques.
The elec­
properties of a catalyst
can be determined by these techniques.
1.4.1
1.4.1.1
Methods For Probing Electronic Properties
Mossbauer Spectroscopy
Mossbauer spectroscopy is an important technique in the
study of oxidation
many
reports
states of elements.
of
Mossbauer
There
studies
have been
of
zeol-
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40
.. 66-72,74-80,89,99,100,105,106
ites
’ ’
*
.
been thoroughly
studied by Mossbauer
Mossbauer data for Fe(C0)^
Fe(CO),. is
Fe2(C0)g has also been
have
spectroscopy.
The
have been reported^**.
bauer studies of Fe(C0)^ are
tures because
T
.
Iron carbonyls
Moss­
usually done at low tempera­
a liquid
at room
temperature.
studied by MSssbauer spectroscopy,
but since it is a solid, it can be studied at room temperature
154
ety of
.
ligands to
studies
ed
Fe(CO)^ will exchange one CO group for a vari­
154 -160
of
.
form a
FeCCO^L
Fe(C0)1}L species.
type
species
have
Mossbauer
been
report-
The Mossbauer analysis of ferrocene and ferro-
cene derivatives has been done
161
.
Table 3 contains the
Mossbauer data for the iron carbonyls and ferrocene.
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TABLE 3
Mossbauer Data For Iron Compounds
Compound
Fe(C0)R a
Fe? (C07q
FeTC5H5 52
IS (mm/sec)
0.09
0.28
0.07
QS (mm/sec)
2.57
0.54
0.24
a - taken at -195 C
carbonyl values from reference 100
ferrocene value from reference 92
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42
A diagram of the basis of
in Figure 9.
a Mossbauer experiment is shown
A radioactive, gamma ray emitting source is
positioned next to the sample.
Atomic nuclei in the sam­
ple are excited by the absorption
of gamma rays of appro­
priate energies (energies equal to the transition energy).
The gamma
rays that are
emitted by
the source are
of a
fixed energy and vibration of the source can vary this en­
ergy.
In this way, gamma rays of the energy of the tran­
sitions are generated.
All gamma rays other than the ab­
sorbed rays are transmitted to
the detector.
of these transitions is determined
ple such
as oxidation state.
peaks and their position on an
mitted rays can indicate the
in the sample.
The energy
by factors in the sam­
The number
of absorption
energy graph of the trans­
oxidation state of the metal
This information can be obtained from the
Mossbauer spectrum.
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43
Figure 9:
source
Mossbauer Experiment
I
I
naw
J m o tio n
s a m p le
counter
no
a b sorp tion
▲
count
rate
m axim um
ab sorption
♦
0
source
velocity
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44
Several
bits of
information can
Mossbauer spectrum.
value.
obtained from
a
All data is referenced to a standard
This is usually the center
pattern of
be
alpha iron
point of the six line
which is given
mm/sec (velocity of the source).
the value
of zero
One must be careful when
comparing literature data because different references are
often used.
The
shift of a peak or the
center of split
peaks is given the name 'isomer shift'.
tween peaks
is called
isomer shift
tion
the 'quadrupole
can be calculated
splitting'.
from the
The
following equa-
162
IS = cs1 + s2 + s3 +
where
sec.
The distance be­
s4 ) A
are the positions
of the peaks in mm/
This is for a sample with four Mossbauer peaks.
equation can be
changed so that the number of
The
S terms is
equal to the number of peaks
in the spectrum if there are
less than
quadrupole splitting
four peaks.
The
calculated from the following equation
162
can be
:
QS = (S1 + S6 - S2 - S5 )/4
This is also for a sample with four peaks.
er peaks,
the equation is modified.
nals are present
When Mossbauer sig­
for more than one type
the isomer shift and quadrupole
Again for few­
of iron nucleus,
splitting values are cal-
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45
culated separately.
For an indepth discussion
bauer spectroscopy,
’the reader
162 through
Another technique that
168.
tronic information
on a
of Moss­
is directed to references
catalyst is
provides elec­
ferromagnetic reso­
nance.
1.4.1.2
Ferromagnetic Resonance
Ferromagnetic resonance
169
netic properties of samples
mentally,
involves a study of the mag­
versus temperature.
the technique is identical to electron paramag­
netic resonance.
A sample
and the absorption
to calculate a
is irradiated with microwaves
of these waves is plotted
tain range (in Gauss).
The position
"g" value.
For powder
called a g-apparent value since
ticles
Instru-
is fairly
aligned as in a
random.
If
over a cer­
of the peak is used
samples,
this is
the alignment of the par­
the particles
could
crystal then a g-parallel value
perpendicular value could possibly be measured.
values
were anisotropic
in
all crystallographic
be
and a gIf the g
direc­
tions, a g , g and g value could be measured,
x
y
z
The calculation of the g value
lowing equation
170
was done using the fol-
:
g
= hV/BH
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46
where h = Planck's constant, B = the Bohr magneton, V =
the microwave frequency, H = the magnetic field.
romagnetic materials,
the g-apparent value and the linew-
idth change with temperature,
of the
broadness of
The linewidth is a measure
a peak and
maximum to peak minimum.
is calculated
important.
The
from peak
Above a certain temperature, the
g-apparent value becomes constant as
netic species.
For fer­
in a simple paramag­
temperature at which this
occurs is
The exact value of this temperature depends on
the particle size of the ferromagnetic species.
An esti­
mate of the particle size can be obtained from experiments
of this kind
109
Electron paramagnetic resonance (EPR) and ferromagnetic
resonance (FMR)
iron and
sample
are important techniques for the study of
cobalt samples.
are determined
by
The
magnetic properties
the electronic
of a
configuration.
Table 4 lists the electronic configurations for the common
states of iron and cobalt.
Ferric ions are EPR active but
high spin ferrous ions are hard to
orbit coupling.
Co
2+
ions are
see by EPR due to spin
easier to see
than Co
ions especially in the low spin configuration.
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2+
TABLE
4
Electronic Configurations of Iron and Cobalt
substance
electronic
configuration
,6
Fe
CO­
CO3*
Hs*3d 6
4s°3d 5
fa'hi7
Hs‘3d
4s°3d
4s 3d
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48
Electron
paramagnetic resonance
has been
high spin Fe^+ ions i n •zeolites171~ 1
.
used to
study
Another technique
that is useful in probing the electronic properties of ca­
talysts and complexes is infrared spectroscopy.
1.4.1.3
Infrared Spectroscopy
Infrared spectroscopy can be used
to observe the pres­
ence of infrared active groups such as CO, OH, etc.
tions and intensities
tion
about
the
of these signals can
sample
environment
Several reviews have dealt with
ganic systems
175
.
concentration.
infrared studies of inor-
been done on Fe(CO)^
Ferrocene contains
that are infrared active
been done.
give informa­
47 175
’
.
Infrared studies have
Fe2 (C0)g
and
Posi­
175
.
as well as
cyclopentadienyl groups
Studies of this compound has
Infrared spectroscopy
of Co2 (C0)g
has been
1 7CZ
done
.
The infrared
properties of a sample
dependent on its structure.
For this reason,
are often
it is impor­
tant to study the structural properties of a system.
1.4.2
1.4.2.1
Methods For Probing Structural Properties
Electron Microscopy
Electron microscopy
allows an experimenter
the surface morphology (texture,
size of a sample.
structure)
to observe
and particle
X-ray analyzers coupled to an electron
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49
microscope enable
elements.
one to get
a surface
Electron microscopy
surface structural properties.
concentration of
gives information
about
Bulk structural properties
can be obtained from X-ray powder diffraction.
1.4.2.2
X-ray Powder Diffraction
X-ray powder
tures
diffraction can be
because solids
pattern.
give
Changes in this
used to
study struc­
a characteristic
diffraction
diffraction pattern can denote
structure change or structural breakdown.
Metals can also
give diffraction patterns if incorporated
into a solid in
a significant amount.
1.4.3
Methods For Probing Chemical Properties
1.4.3.1
Gas Chromatography
Gas chromatography
q u e ^ ^ ’1^^.
Gaseous
is an important
separation techni-
and volatile liquid samples
analyzed by this method.
can be
Concentrations of gas components
can be obtained for the analysis of a reaction system.
1.4.3-2
Gravimetric Techniques
Thermal gravimetric
analysis and
analysis are two important techniques
materials.
When samples are heated,
differential thermal
in the study of new
the weight often de­
creases due to the loss of some material.
Frequently, wa ­
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50
ter is given off while at
poses.
This
point at
other times,
usually happens at high
which this
weight loss
the sample decom­
temperature.
occurs is
The
significant.
Gravimetric techniques give a great deal of information in
this area.
1.5
RATIONALE
1.5.1
Catalyst Synthesis
Various
synthetic
methods were
small metal particles in zeolites.
to
study the
effect of
Fischer-Tropsch
activity
the
of
employed
prepare
Zeolites were employed
various pore
the
to
sizes on
resulting
the
catalysts.
Shape selectivity of the zeolites on the reaction products
was investigated.
to avoid
Microwave discharge
the introduction of
heat into
methods were used
the preparation.
Heat can sinter particles and force them to migrate to the
external surface of a support.
vide the
sintering.
Microwave discharges pro­
energy for catalyst preparation
without causing
Sodium vapor treatment and hydrogen treatment
were employed at low temperatures to achieve reduction but
avoid sintering.
The effects
of these preparations were
studied with catalytic reactions.
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51
1.5.2
Catalytic Reactions
Fischer-Tropsch reactions were used
lytic properties of
to study the cata­
the iron and cobalt
catalysts.
The
Transient Pulse Method was employed to study surface spec­
ies on the catalyst and
tion times.
A flow reactor line was employed to study the
concentrations of
times.
The
reactions products at short reac­
hydrocarbons produced at
activity of a
catalyst can be
short and long reaction times.
become inactive with time.
long reaction
different at
Initially active sites may
These initial sites may effect
the formation of other sites and effect the catalyst life.
Studies of short and long reaction times can give informa­
tion about the changes that
reaction.
The activity
electronic,
talyst.
a catalyst undergoes during a
of the catalysts depends
on the
structural and chemical properties of the ca­
Various
spectroscopic techniques
were used
to
study these properties.
1.5.3
Spectroscopic Characterization
One novel
feature of
in-situ Mossbauer reactor.
before,
this research is
Mossbauer
the use
of an
analyses were done
during and after Fischer-Tropsch reaction without
any contamination from atmospheric water or oxygen.
bauer spectroscopy
and ferromagnetic resonance
Moss­
were used
to determine the oxidation states of the metals on the ca­
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52
talysts.
Ferromagnetic resonance and electron microscopy
were used to measure the particle size of the metals.
frared spectroscopy was used to
CO or other
In­
analyze for any remaining
ligand groups on the
metals after reduction.
We have tried to use a variety of spectroscopic techniques
to support the results and to
give as much information as
possible about our catalysts.
We recognize the problems
with basing conclusions on information from one technique.
1.6
FOCUS
We have tried to build on
have used
the research of others.
compounds that others
carbonyls,
have used such
organometallic compounds and
are ion-exchanged into zeolite samples.
We
as metal
metal ions which
We have also used
reported techniques like reduction by hydrogen,
sodium va­
por reduction
novelty of
and microwave treatments.
this research,
however,
lies in the use of low reduction
temperatures to minimize sintering,
bonyl species (which decompose at
the use
of microwaves
technique.
The
as a
the use of metal car­
a low temperature)
room temperature
and
activation
Different metal precursors and various reduc­
tion techniques were studied.
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Chapter II
EXPERIMENTAL METHODS
2.1
INTRODUCTION
This chapter is included to explain the instruments and
experimental procedures that were used.
planations of the
More detailed ex­
various techniques can be
references given in each section.
found in the
The importance of each
of these techniques has been given in the introduction.
2.2
MOSSBAUER SPECTROSCOPY
All Mossbauer spectra were recorded in the transmission
mode.
The
instrument incorporated an
Elscint Mossbauer
drive unit, an Elscint function generator model MFG 3A, an
MVT-3 linear
velocity transducer
driving unit.
preformed by
and an
MD-3 transducer
Detection of the transmitted gamma rays was
a Reuter-Stokes Kv-CH4
proportional counter
powered by an Ortec *401 A/456 high voltage power supply and
coupled to
an Ortec 142
PC preamplifier.
Signals were
transmitted from the preamp through an Ortec 571 spectros­
copy amplifier
to a Tracor Northern
analyzer for storage.
NS-701A multichannel
Plots of the data were later gener­
ated on an IBM 360/370 computer.
-
53
-
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The Mossbauer system employed an in-situ treatment cell
similar to that of Delgass
hydrogen,
1 7 fi
.
It allowed treatment with
hydrogen/carbon monoxide mixtures or oxygen and
Mossbauer analysis without any exposure to the atmosphere.
A drawing of this cell is shown in Figure 10.
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55
Figure 10:
In-situ Mossbauer Cell
He
H2
vent
■
0
1
co+h 2
0
-
o
X
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56
2.3
ELECTRON PARAMAGNETIC RESONANCE/FERROMAGNETIC
RESONANCE
Paramagnetic
and
ferromagnetic samples
with a Varian E-3 spectrometer.
at an
X-band frequency
250°C using
of approximately
a temperature
heated or cooled nitrogen.
tubes under
vacuum.
studied
The samples were analyzed
temperature of the samples could
to
were
9.5 GHz.
The
be varied between -160°C
controller and
a flow
of
Samples were sealed in quartz
The instrument was
calibrated with
cobalt(II) in MgO.
2.4
ELECTRON MICROSCOPY
Particle size
were
preformed
analysis and surface
using scanning
electron
transmission electron microscopy.
tron microscope was a Hitachi
supported on copper 300 mesh
carbon.
methanol.
mixture prior to
on the grids while dis­
mixture.
done on an AMR
Studies of surface
Samples were
grids which were coated with
of the
contact with the grid aided
croscopy studies were
and
The transmission elec­
Ultrasonification
duction of a homogeneous
ment.
microscopy
model HU200.
The samples were placed
persed in
morphology studies
methanol
in the pro­
Scanning electron mi­
model 1000A instru­
concentrations were done using
Energy Dispersive X^-ray Analysis with an EDAX 9100/60 sys­
tem.
The accelerating voltage employed was 20 keV.
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57
2.5
X-RAY POWDER DIFFRACTION
Samples were studied for bulk
ray powder
diffraction.
These
DIANO-XRD 8000 diffractometer.
copper K-alpha.
structural changes by Xstudies were
The
done on
a
source radiation was
Diffraction patterns were usually run be­
tween 10 and 80 degrees two theta.
2.6
INFRARED SPECTROSCOPY
Infrared spectra were recorded on
283 spectrometer.
a Perkin Elmer model
Powder samples were dispersed in miner­
al oil and placed between two KBr discs.
2.7
GAS CHROMATOGRAPHY
Reaction products
were analyzed
5880A gas chromatograph.
The carrier
helium at a flow rate of 25 mL/min.
ture was
200°C and
the detector
Thermal conductivity was
umns.
Molecular sieve
Hewlett Packard
gas was zero grade
The injector tempera­
temperature was
employed using two sets
13X columns were used
temperature of 35°C to separate
ide.
on a
210°C.
of col­
at an oven
methane and carbon monox­
Alltech VZ-10 columns were used at 50°C to separate
ethane,
ethylene and higher hydrocarbons.
These separa­
tions were compared to standards of natural gas, ethylene,
propane,
multiplied
propylene
by weight
and butane.
factors to
The product
obtain weight
areas were
percent.
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The values
of the weight percents
Product samples
were introduced by
are given in
syringing out
Table
of
in-line septa.
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59
TABLE 5
Weight Percent Values
Compound
methane
ethane
ethylene
propane
propylene
butane
isobutylene
1-butene
trans-2-butene
cis-2‘-butene
nitrogen
oxygen
carbon monoxide
Weight Factor
0.45
0.59
0.585
0.68
0.652
0.68
0.683
0.697
0.658
0.643
0.67
0.80
0.67
taken from McNair and B o n e l l i ^ ^
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60
2.8
THERMAL ANALYSIS TECHNIQUES
Analysis of the aluminoferrisilicates
of a Perkin-Elmer System 7 A
Model 1700.
involved the use
Differential Thermal Analyzer
Samples were heated from 50°C to 750°C in ar­
gon at a heating rate of 20°C/min.
mL/min and all samples were
The argon flow was 11
referenced to alumina.
Sam­
ples were loaded into alumina crucibles for analysis.
2.9
MICROWAVE GENERATOR
The microwave
generator used in the
discharge experi­
ments was marketed by Raytheon model PGM-10.
microwaves of frequency 2*150125 MHz.
It generates
Microwave power was
measured by a Micro Match standing wave ratio bridge.
The
cavity employed was an Evenson quarter wave, coaxial cavi­
ty
incorporating both
tuning
and coupling
adjustments.
All sample tubes were made of quartz.
2.10
REAGENTS
A variety of zeolites were employed in the experiments.
Zeolites NaA,
NaX,
NaY and
ALFA Co. in powder form.
NH^Y were purchased from the
The lot numbers were as follows:
NaA(4A) - 061 576; NaX(1 3X) - 072182;
NH^Y(SK-41)
- 042578.
NaY(SK-40) - 042578;
NaZSM-5 was prepared according to
the patent of Argauer and Landolt
1 71
example 27 which sug­
gests the combination of colloidal silica,
tetrapropylam-
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61
monium bromide,
an autoclave
sodium aluminate
at 175°C for
and sodium hydroxide in
8 days.
HZSM-5
was prepared
from NaZSM-5 by exchange with dilute acid.
nite was obtained from Strem Chemicals,
Sodium morde-
Inc.
and its lot
number was 10282-52.
Bentonite
and
titanium
dioxide
were
obtained
from
Fischer Scientific with their lot numbers being 730635 and
725845 respectively.
Alpha alumina
and amorphous silica
were purchased from the Illinois Minerals Co.
Iron pentacarbonyl was received from the ALFA Co.
lot number was 091179.
Sodium nitroprusside,
Its
The iron carbonyl was 99.5% pure.
Na2Fe(CN),-N0,
and ferrous sulfate,
FeS0jj*7H20, were purchased from the J.
T.
Co. with lot numbers of 52848 and 41898.
Baker Chemical
Copper nitrate,
Cu(N0^)2*6H2 0, and cobalt nitrate,
CoCNO^)2*6H20,
were manufactured
by the General Chemical
Division of Allied Chemical and Dye Corporation with their
respective lot numbers being W030 and H018.
ZnCNO^^* 6H2 0,
Zinc nitrate,
was purchased from Fischer Scientific with
a lot number of 791947.
Iron nitrate, FeCNO^)^ *9H20, was purchased from
ALFA
products,
lot number
06031.
Aluminum
nitrate,
AlCNO^)2 ’9H20, was from Mallinckrodt Chemical
Works,
lot number 3172.
solution was
a 25% by
The tetrapropylammonium hydroxide
weight solution obtained
from ESA
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62
corporation - Organic Products Division
of 2-1728-0581-11.
was
also
from
1788-0482-06.
with a lot number
The tetrapropylammonium bromide powder
RSA
corporation with
a
lot
tion.
The
It
Si02 . The
from E. K.
1,6
sodium silicate used was
1.40 g/mL and
sodium hydroxide
used for pH
Industries.
It contained
Dupont #9 solu­
had a density of
hexanediamine was
5932.
of
The Ludox used in the aluminoferrisilicate
syntheses was Dupont HS-30 colloidal silica.
30$ SiO^.
number
contained 29$
adjustment was
The lot number was 014873.
from Kodak with a
This amine was a solid
lot number
that melts at
dium chloride used was from J.
T.
The
41°C.
of
The so­
Baker Chemical Company
lot number 23264.
All water used in these experiments was first distilled
and then deionized.
2.11
2.11.1
SAMPLE PREPARATION METHODS
Iron Carbonyl Zeolites
Fe(C0)j- was sublimed at
room temperature onto hydrated
and dehydrated supports on a vacuum line.
sublimed onto
a zeolite support
The Fe(CO),. was
in an inverted
"U” tube
like the one in Figure 11.
In
one side
of the
U tube
placed and immediately frozen
0.13 mL
of Fe(CO)^
were
with liquid nitrogen.
One
gram of zeolite was placed on the other side of the U
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63
tube.
The tube was fitted with a stopcock and placed on a
vacuum line.
The system was
the Fe(C0)j- frozen.
evacuated to 10
-5
torr with
The stopcock was then closed and the
iron carbonyl was allowed to slowly sublime onto the zeol­
ite.
The
sublimation was
complete after
five minutes.
The product had a variety of colors ranging from yellow to
orange brown
depending on the
volume of Fe(CO),.
used in
the preparation.
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6H
Figure 11:
Inverted U Tube
to v a c u u m
1
X —'
zeolite
Fe(CO)
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65
Fe2 (CO)g
Seel
177
Jolly
.
178
was
prepared
according
to
the
report
of
Fe(C^H^)2 was prepared according to
.
It was sublimed three times prior to use.
After preparation,
the Fe(CO)^
sample was
with liquid nitrogen.
All samples
were stored under ni­
trogen or in a nitrogen filled
Fe2 (C0)g was heated
glove box until use.
to 35°C by a
bath to promote sublimation.
kept cool
hot air gun or
The
a water
Fe(C0)_ and ferrocene sub-
limed at room temperature under
vacuum so that no heating
was needed.
2.11.2
The
Hydrogen Reductions
samples were
press using
Typically,
pelletized using
pressures in the range
176
usually taken at this time.
to the reduction temperature,
.
16 mm
of 3000 to
300 mg of sample were used.
placed in the Mossbauer cell
a
diameter
^1000 PSI.
The pellets were
A Mossbauer spectrum was
The sample was then heated up
300 to 500°C,
the procedure, in flowing helium,
100 mL/min.
depending on
The sample
was then exposed to flowing hydrogen, 75 mL/min, for up to
hours.
The sample was then cooled to room temperature
in flowing helium and another
Mossbauer spectrum was tak­
en.
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66
2.11.3
Sodium Vapor Reductions
Several experiments
to reduce iron
was
in zeolites as reported
exchanged into
zeolite with 100
trogen.
were attempted using
zeolites by
-5
mixing one
studied were A,
These samples were filtered,
line to 10
by L e e ^ ^ .
mLs of 1055 solutions of
The zeolites
torr.
sodium vapor
Fe^ +
gram of
the
FeSO^ under ni­
Y,
X and ZSM-5.
washed and dried on a vacuum
The samples were then
placed in the
tube shown in Figure 12 with several chunks of sodium that
had been washed in hexane.
um line and
evacuated.
heated to 350°C.
by turning
The tube was placed on a vacu­
While evacuating,
tube was
The sodium was then added to the zeolite
the dumping tube.
collected on the sides of the
placed in
the
a nitrogen
The sodium
tube.
filled glove
vaporized and
The sample was then
box and
loaded in
a
Mossbauer analysis tube.
Reduction with potassium vapor was
also studied due to
the fact that
potassium vaporizes at a
than sodium.
The
lower temperature
same procedure was followed
sodium reduction experiments.
as in the
Table 6 contains data com­
paring the vaporization temperatures
of sodium and potas­
sium at various temperatures.
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67
Figure 12:
Sodium Vapor Reduction Apparatus
to
f
va cu u m
FeY
furnace
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68
TABLE 6
Vaporization Temperatures of Na and K
Metal
Na
K
Pressure(atm)
1
8^3
10^2.
1128
774
333
jjjzi
231
157
10—
121
62
taken from CRC Handbook"' ^
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69
2.11.4
Bimetallic Zeolite Preparations
Iron was
ion-exchanged into zeolites along
metals to prepare bimetallic samples.
and ZSM-5,
trate,
were stirred with
copper nitrate,
one hour.
Typically,
solution were used.
tered,
The zeolites, NH^Y
105# solutions of cobalt ni­
zinc nitrate and iron sulfate for
one gram of
zeolite and 100 mL of
After exchange,
washed with water and dried.
stirred with 100 mL of
with other
the powder was fil­
The powder was then
105& solutions of sodium nitroprus-
side, ammonium ferrocyanide, potassium ferrocyanide or po­
tassium ferricyanide.
washed with water and
The resultant powder was filtered,
dried on a vacuum line to
1X10
torr.
2.11.5
New Aluminoferrisilicate Zeolite Preparations
The author spent
two and a half months
tional Laboratory working with Dr.
synthesis of
ZSM-5,
with iron substituted
silicalite
at Argonne Na­
Lennox E.
and ZSM-5
into the framework.
Iton on the
type zeolites
The
work fo­
cused on some work reported in the patent literature10’'1'1.
The zeolites that were prepared
powder diffraction,
differential thermal analysis,
tron paramagnetic resonance,
bauer spectroscopy.
were analyzed by X-ray
elec­
electron microscopy and Moss­
The samples and their synthesis con­
ditions are given in Table 7.
The amount of iron in these
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70
samples ranged from 2.62 X 10 ^
moles.
The samples
moles up to 10.395 X 10 ^
with the lower amounts
fairly white in color.
to light brown in color.
The
of iron were
higher iron samples were tan
The reactant concentrations are
given in Table 8.
The samples
were prepared
Tropsch catalysts.
to be
studied as
Fischer-
The work was still going on at Argonne
at the time of this dissertation.
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71
TABLE 7
Synthesis Conditions
mple #
38
39
40
41
42
43
44
45
46
47
48
type
1
2
2
1
1
2
1
2
1
1
1
Si/Al
T emp(— C)
Time
140
115
129
11 8
117
129
1 21
150
200
200
175
150
200
150
200
175
175
150
6.8
4.9
3.9
4.9
6.9
6.7
6.7
6.9
6.9
6.7
6.8
type 1 - reference 71
type 2 - reference 72
time - length of synthesis in days
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72
TABLE 8
Reactant Concentrations
Sample #
38
39
40
41
42
43
44
45
46
47
48
mmols Fe
mmol A1
Template
3.^335
2.5622
2.5589
3.427
10.375
0.2587
6.6174
0.0262
6.6088
10.395
10.379
1.1258
NA
NA
1 .355
1 .233
NA
1 .350
NA
1 .354
1 .232
1 .317
HMDA
TPA-Br
TPA-OH
TPA-OH+Br
HMDA
TPA-OH
TPA-OH+Br
TPA+OH
TPA-OH+Br
TPA-OH+Br
HMDA
HMDA - hexamethylene diamine
NA - none added
TPA-OH - tetrapropylammonium hydroxide
TPA-Br - tetrapropylammonium bromide
TPA-OH+Br - a mixture of TPA-OH and TPA-Br
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73
2.11.6
Microwave Discharge Preparations
The details
The microwave
of this
procedure have
discharge experiments
dehydrated supports (heated
10‘5 torr),
been reported
1
ft n
involved the
use of
to 375°C under vacuum
of 1 X
vacuum lines and a nitrogen filled glove box.
These procedures were followed to exclude water and oxygen
from the samples to avoid oxidation of the metals.
The
metal
complex
and the
placed in opposite sides of
in Figure 13.
box.
dehydrated
support
were
the quartz reactor tube shown
This was done in the nitrogen filled glove
The complex was placed in position B and the support
in position C.
Tubes with stopcocks were connected to ei­
ther end of this tube
stopcocks were
closed,
with CAJON ULTRA-TORR unions.
isolating the sample.
could then be removed from
The
The tube
the glove box without exposing
the sample to air.
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71}
Figure 13:
Sample Tube
quartz
-7
metai
B
support
C
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75
The quartz tube was then connected
shown in Figure
14 and the system
to a vacuum line as
was evacuated.
cocks G and A were closed while D remained open.
al complex was
then allowed to sublime
The sublimation was
white zeolite.
Stop­
The met­
onto the support.
evident with a colored
complex and a
The support particles closest to the com­
plex first became colored followed by the rest of the sup­
port.
The sublimation time depended on the volatility of
the metal complex.
ter one half hour.
A typical sublimation was complete af­
At
long sublimation times,
ports became dark in color and
plex were seen
crystals of the metal com­
in the liquid nitrogen
sublimation times,
less
the sup­
trap.
metal was deposited on
At shorter
the sup­
port.
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76
Figure 14:
Microwave Line
in
cr>
in c
u
o
u
CL
z:
O o
>
V) -5
<
LU
Cd
o
LU
8 = 0 = !
o
o
E
3
in
o
I
o ' lu
Cl
CL
l
LU m
Ll1<
cro
CL
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77
The sublimation was then halted,
a liquid
ordinarily by placing
nitrogen trap at position
evacuated.
B and the
system was
Stopcock A was then opened and inert gas from
a gas cylinder was allowed to pass over the sample.
and helium
ments.
have been successfully
used in
Argon
these experi­
A flow controller was adjusted to achieve a pres­
sure of approximately 0.3 torr of inert gas in the system.
This produced
a- flow of
over the sample.
placed around
approximately 1.3 mL/min
of gas
An air cooled microwave cavity was then
the tube
generator was turned on.
at position
C and
the microwave
An inert gas plasma was then ig­
nited using a Zerostat electric discharge gun.
The plasma was highly colored.
Carbonyl complexes pro­
duced a blue plasma due to the emission of CO.
rocene present
the plasma was
initially a
In all cases after a few minutes,
This is
typical for an
sample sometimes changed
ple)
purple color.
the plasma turned pink.
argon plasma.
Agitation
the color back to
indicating that the penetration
is not very far into the
With fer­
of the
blue (or pur­
of the argon plasma
solid and that decomposition was
not yet complete.
The decomposition
was assumed to
be complete
more blue (or purple) color could be generated.
was then halted and the system was evacuated.
and D were
closed and the sample was placed
when no
The flow
Stopcocks A
in the glove
box until further study.
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78
2.11.7
Microwave Generation of Color Centers
During the microwave discharge
ites sometimes became
experiments,
colored faint pink or
even at low microwave powers.
the zeol­
faint purple
Similar color changes have
been reported before for other types of treatments**^** ” ”*®3.
Several experiments were attempted
color centers
existed.
(in a few minutes)
to prove that these
These colors faded
so that
very quickly
preparation in a reactor tube
and transfer in a nitrogen filled glove box to an EPR tube
failed.
Next, some samples were prepared in^situ.
EPR tubes were loaded with
ated.
The tubes
mtorr of
dehydrated zeolites and evacu­
were then filled with
argon gas and sealed
A pink
approximately 50
off with a
centers generated in these tubes
a couple of days.
Quartz
flame.
Color
remained sometimes up to
color was generated in zeolite
NaY and a purple color was generated in NaX.
obtain EPR spectra of these
Attempts to
samples failed and no further
work was done due to lack of time.
It has been reported
1 81
that
tetrahedral sodium centers,
due to Nag
5+
the pink color comes from
N a ^ +.
The purple color is
centers.
Attempts to generate a color center in KY zeolite (pre­
pared from
repeated exchange
of NaY
with KC1
solution)
also failed.
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79
Treatment of
the Na
zeolites with
microwaves without
argon present resulted in no
color centers.
At high mi­
crowave powers (100 Watts),
decomposition of the zeolite
to A1 and Si occurred.
2.11.8
Inert Atmosphere Dry Box Procedures
Many of the samples that were prepared were stored in a
nitrogen filled
dry box.
This
box was
purified almost
daily by recirculating the nitrogen atmosphere over a trap
system containing an oxygen and
a water trap.
This trap
was regenerated every few months by heating it to 250°C in
hydrogen for several
vacuum pump.
hours followed by evacuation
Dishes of
phosphorus pentoxide
with a
were sta­
tioned in the dry box to react with small amounts of water
vapor in the atmosphere of
the box.
The resultant phos­
phoric acid was removed from the box.
2.11.9
Fischer-Tropsch Reactions
Samples
prepared by
the
carbonyl
procedure and
the
bimetallic procedure have been tested to measure their activity for hydrocarbon production
by Li-Min Tau under the
91
.
This work was done
direction of Dr.
Carroll Bennett
in the Department of Chemical Engineering at the Universi­
ty of
Connecticut.
The reaction temperature
The reaction mixture was 10? C0/H? .
mL/min.
was 285°C.
The flow rate was 30
Typically, 25 mg of catalyst were used.
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80
The samples prepared by
the microwave discharge method
were studied on a different
tory.
reactor system in our labora­
Our reaction line is a flow system using helium as
an inert gas
through
purge.
oxygen traps
The helium and
and water
These were regenerated prior to
hydrogen are passed
traps for
use.
purification.
Regulators in each
of the lines allowed regulation of each of the gases sepa­
rately providing
for reaction.
one COrH^.
the possibility of various
These
gas mixtures
experiments used a ratio
The flow rate was
temperature was 250°C.
of one to
30 mL/min and the reaction
This temperature was chosen since
a change in the FMR signal was observed above 250°C.
This
could be due to sintering of the metal.
The reactor tube
was a quarter
and the catalyst
inch stainless steel tube
was supported on
fine stainless steel mesh
reactor tube was
placed in a tube furnace
ered by a
temperature controller and the
monitored on a digital thermometer.
screen.
The
which was pow­
temperature was
Analysis of the reac­
tant and product gases was done by an in line gas sampling
valve and by two in-line
The line
was vented to
gas septa for syringing samples.
the hood.
The
experiments were
done on the reactor line shown in Figure 15.
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81
Figure 15:
Reaction Line
O o — CO
vent
a
T
lr
t * . H-
■©— He
c I
a
W at er
b
Oxygen
c
Fu r n ac
trap
trap
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Chapter III
RESULTS
3.1
INVESTIGATIONS OF SAMPLES BY FISCHER-TROPSCH
REACTIONS
3*1.1
Catalysts Prepared By Microwave Reduction
3.1.1.1
Cobalt Catalysts
Cobalt loaded
by microwaves
zeolites were prepared by
of Co2 (C0)g
on zeolites.
were chosen as supports for the cobalt.
the reduction
Three zeolites
A,
a small pore
zeolite, ZSM-5, a medium pore zeolite, and X, a large pore
zeolite,
were all loaded with
the support pore size on
cobalt.
The influence of
the selectivity of the catalysts
was studied.
The influence of the support pore size was shown by the
hydrocarbon product distributions obtained with the cobalt
catalysts.
Cobalt on zeolite A (CoA) formed only methane
as shown in Figure 16.
Also
in Figure 16 are the curves
for the methane produced by cobalt on ZSM-5 (CoZSM-5)
cobalt on X
(CoX).
ZSM-5 was nearly
100? at first but
40? after 10 minutes.
methane.
The methane production
and
by cobalt on
fell to approximately
CoX produced only a minor amount of
Cobalt on ZSM-5,
which
has a larger pore size
than A, also produced some larger hydrocarbons.
_
0?
_
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Figure
CoZSM-5.
17
contains
the product
The
effect of
the pore
production of methane and ethane.
distribution
size was shown
for
by the
X is a large pore zeol­
ite and cobalt on X produced even larger hydrocarbons than
cobalt on ZSM-5.
Figure 18
contains the
Propylene was
the major
product distribution
product at
Ethylene and ethane were also
These catalysts showed
long reaction
the effect
of
the catalyst pore
A small pore support
small hydrocarbon whereas
formed larger hydrocarbons.
time.
formed in small amounts.
size on the hydrocarbons produced.
formed a
for CoX.
a large
pore support
Further characterization of
these catalysts is given in sections that follow.
The percent
conversion of CO
to hydrocarbons
cobalt catalysts is given in Figure 19.
for the
All of the cata­
lysts were very active initially but then decreased in ac­
tivity.
The catalysts leveled off
minutes but
reaction
did not become
times.
CoA
was
in activity after ten
inactive after long
the
most
CoZSM-5 had intermediate activity and
active
(60 min)
catalyst.
cobalt on X had the
least activity.
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84
Figure 16:
Percent Conversion Cobalt Catalysts
• 70
20
o
15
\CoA
-60
o
10
I
\CoZSM-5
-50
o
5
CoX
TIME (min)
R eproduced w ith perm ission o f the copyright owner. F urther reproduction prohibited w itho ut perm ission.
85
Figure 17:
Co Catalysts Methane Production
1001
50CoZS M
o
CoX
30
TIME
( mi n )
R eproduced w ith perm ission o f the copyright owner. F urther reproduction prohibited w itho ut perm ission.
86
Figure 18:
CoZSM-5 Product Distribution
100
o
CH
o
20
-
TIME (m i n )
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87
Figure 19:
CoX Product Distribution
100
80-o
q. 6
0-
o
20-
CH/ C,Hi n
20
TIME
A0
60
(m in )
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
88
3.1.1.2
Iron Catalysts
Iron carbonyl,
F e 2 (C0)g,
and ferrocene,
were loaded into various zeolites
duction by
ites.
microwaves method to
Fe(C^H^)2
and reduced by the re­
iron metal on
the zeol­
Zeolites A, Y, X and ZSM-5 were chosen to study the
influence of pore
were compared
size on these catalysts.
to the cobalt
catalysts.
The results
Fischer-Tropsch
reactions were carried out on these catalysts.
Ferrocene was loaded onto zeolite
A and the sample was
reduced to iron metal by the reduction by microwaves meth­
od.
Figure 20
contains the percent conversion
for this
catalyst which similar to cobalt on A, produced only meth­
ane.
The activity was high at first but then the catalyst
became inactive after fifteen minutes.
Ferrocene was also
loaded onto zeolite ZSMi5.
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89
Figure 20:
Percent Conversion of Iron on A
CONVERSION
40
30
PERCENT
20
10
I
10
f
20
30
TIME ( m i n )
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90
Figure 21 contains
sample 30 (FeZSM-5)
data for the percent
which was prepared from the reduction
by microwaves of ferrocene.
iron on
A in
that it became
Since ZSM-5 is a medium
duced some
lyst.
This catalyst was similar to
inactive after
pore zeolite,
larger hydrocarbons than
Figure
22
conversion of
shows the
15 minutes.
this catalyst pro­
the iron on
hydrocarbons
A cata­
produced
by
FeZSM-5.
Initially, mostly methane was produced.
minutes,
ethane and an unknown which appeared at a reten­
tion time of 1.3 minutes
were formed.
After 5
After 12 minutes,
as the catalyst became less active, the unknown became the
major product.
Similar product distributions were formed
with iron on zeolites X and Y.
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91
Figure 21:
Percent Conversion of Iron on ZSM-5
2
2
O
o
20
.
i—
2
-
LlJ
O
cr
LU
CL
TIME
(mi n )
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
Figure 22:
Catalytic Properties of Iron on ZSM-5
100
80
o
60
AO
o
CH4
20
unknown
5
TIME
10
15
( min )
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93
Two large
pore zeolites,
X and
Y,
preparation of iron loaded zeolites.
were used
Also, two metal pre­
cursors, Fe2(C0)g and ferrocene were used.
tains the
(FeX).
data for
the percent
in the
Figure 23 con­
conversion of
sample 28
This catalyst was prepared by the microwave reduc­
tion of ferrocene
on zeolite X.
The
products formed by
this catalyst were not any higher in molecular weight than
for the products
from iron on zeolite
shows the product distribution.
was produced but the ethane
minutes.
ZSM^.
Figure 2M
Initially, mostly methane
production dominated after 10
This catalyst also became deactivated after fif­
teen minutes
as with the
other iron catalysts.
knowns were produced by this catalyst,
however,
No un^
unknowns
were produced by iron on Y prepared from Fe2 (C0)g.
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QU
CONVERSION
Figure 23:
Percent Conversion of Iron on X
30
PERCENT
20
10
5
10
15
TI ME ( m i n )
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95
Figure 24:
Catalytic Properties of Iron on X
100
CH
80
-
o
o
10
TIME
15
(mi n )
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
96
Figure 25 contains a plot of
sus time for sample 2H (FeY).
by the microwave
Figure 26
lyst.
the conversion of CO ver­
This catalyst was prepared
decomposition of Fe2 (C0)g on
shows the hydrocarbons
At first,
produced by
the major product was
zeolite Y.
this cata­
methane but the
mixture became 50? ethane as the catalyst became inactive.
Two unknown peaks were observed in the gas chromatographic
analyses at retention times of
1.3 and 1.5 minutes.
The
first unknown eluted at the same retention time as the un­
known in
the iron
on ZSM-5 case.
This catalyst
had a
longer life than the other iron catalysts but became inac­
tive after approximately 30 minutes.
Figure 27 contains the percent
of iron on
zeolite X prepared by the
waves of ferrocene.
way as
conversion for a sample
reduction by micro­
This sample was prepared in the same
the previous iron on
X sample except that
it was
accidentally heated up to 275°C before the Fischer-Tropsch
reaction.
The resulting catalyst
produced only methane.
The percent conversion of this catalyst is very high.
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
Figure 25:
Percent Conversion of FeX Sintered
70
(/)
cr
LxJ
>
Z
50
o
o
Ll I
o
Qd
Ll I
30
CL
10
10
20
30
TIME ( m i n )
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
98
The iron catalysts
cobalt catalysts
all became inactive with
remained active.
catalytic properties of
time but the
A comparison
the iron and cobalt
of the
catalysts is
given in Table 9.
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99
TABLE 9
Catalytic Properties of Iron and Cobalt Catalysts
Sample #
23
28
29
30
35
32
31
Sample
% Conv
FeX
FeX
FeA
FeZSM-5
CoA
CoZSM-5
CoX
113.9
3.4
21 .4
3.6
14.1
0.2
6.7
53.6 c
14.0 c 44.2 c
8.5 c 0.09 c
C1/C2+ (a)
-
♦
9.0
0.2
-
0.3
*
0.54 d
-
a - after 2 minutes reaction
b - after 10 minutes reaction
c - after 1 minute reaction
d - after 30 minutes reaction
% conv - percent conversion of CO to products
C.j /C2+ ~ ratio of methane to higher hydrocarbons
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
100
All of the previous catalysts
duction by
were
microwaves method.
prepared by
the reduction
were prepared by the re­
The following
by
catalysts
hydrogen method
tested as Fischer-Tropsch catalysts.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
and
Percent Conversion of Iron on Y
5.0-
PERCENT
CONVERSION
Figure 26:
1------------
10
TIME
20
30
(m i n )
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 02
Figure 27:
Catalytic Properties of Iron on Y
100
80
-
CH
20 -
T I M E (m i n )
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
103
3.1.2
3.1.2.1
Catalysts Prepared By Hydrogen Reduction
Iron Carbonyl Catalysts
Iron catalysts were prepared by the reduction in hydro­
gen of
Fe(C0)j- on several
zeolites was
zeolites.
employed in this
zeolite Y (FeY)
A wide
variety of
research but only
iron on
was tested as a Fischer-Tropsch catalyst.
Figures 28 through 30 show some
of the results of the re­
actions using this catalyst.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 28:
Catalysis of Fe(C0)^Y
120
2700
- 2600
>_
60- 2500
>
o
- 2400
^ 0 0 0 ^ 9 9.QQQQIQGQQ,Q& Q Q
0
10
20
30
I I ME
X
0
40
50
(sec)
CO
C H4
i co2
0 C2 H 6
0 c 3h 8
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Methane was the major product
in Figure 28.
by the
catalyst was
The percent conversion of CO
10.5.
The
activity of
changed with time, as shown in Figure 29.
tivity decreased for about 50
This ac­
seconds of reaction,
these specific reaction conditions,
FeY
Initially, the
catalyst was very active for methane production.
crease again.
as shown
Lesser amounts of C02 , ethane and other hy­
drocarbons were also formed.
in H2
formed by FeY,
under
and then began to in­
A maximum in activity appeared at about 300
seconds followed by a decrease in activity.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 06
Figure 29:
Methane and Ethane Production of Fe(C0),-Y
o
O
ro
o
JD
O
O
r\l
o
Q>
V)
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X
o
o
rsl
o
I
_D
^4X
O
(
a
o
Lf>
o
o
o
lT5
(6-ujLu/ujn) AilMiov
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
107
Hydrogen was passed
seconds of reaction,
from the catalyst.
over the FeY catalyst,
after 320
to observe the desorption of species
Methane
sorbed as shown in Figure 30.
and several hydrocarbons de­
The methane peak consisted
of a peak, a shoulder and a tail.
Ethane, propane and bu­
tane also desorbed from the catalyst in lesser amounts.
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
1 08
F i g ur e
30:
D e s o r p t i o n of F e ( C 0 ) ^ Y
co
Lf>
O
O
o
o
eo
o
a>
ui
X
cj3"
00
X
CO
UJ
O
1
o
<x>
X
O
O
CNJ
CJ>
<r
X
O
o
<T>
O
<£>
(6-Ujiu/ujn)
oo
A1IAI10V
R eproduced w ith perm ission o f the copyright owper. R udder reproduction prohibited w ith o u t perm ission
o
(
<3
109
3.1.2.2
Bimetallic Zeolite Catalysts
Bimetallic
zeolites were
Scherzer and Fort
88
prepared
by
the method
and were reduced in hydrogen.
tion resulted in metallic iron in some cases.
a mixture of oxidation states was obtained.
also contained
zinc, copper,
troduced as
another transition
a metal ion.
It
Reduc­
Other times
The catalysts
metal element
cobalt or ruthenium.
of
such as
This element was in­
was not known
whether the
second metallic element was reduced to the metal.
The ef­
fect of the second metallic element on the catalytic prop­
erties was studied.
Table 9 contains a general
results of a few of
were studied.
comparison of the catalytic
the bimetallic zeolite catalysts that
Column
2 shows the second
was present initially in the catalyst.
lysts
involved the
incorporation of
metal ion that
All of these cataFe(CN)^N0
p-
synthesis and reduction at 400°C for four hours.
Y was the zeolite used in catalysts 14, 18 and 22.
was used in sample 15.
sion of CO in H2 ,
lyst and the
in
the
Zeolite
ZSM-5
Table 9 lists the percent conver­
the number of active sites on the cata­
formation of bulk iron carbide
for the dif­
ferent catalysts.
Figure 31 shows the activity of
duction of hydrocarbons.
est percent
sample 18 for the pro­
Sample 18 (CoFeY) had the high­
conversion of CO
for the
bimetallic zeolite
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1 10
catalysts that were studied.
lyst changed with time similar
ite catalyst.
The
activity of this cata­
to the iron carbonyl zeol­
The activity of this catalyst also changed
with temperature as shown in Figure 32.
This Figure shows
that activity increased as temperature increased.
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111
Figure 31:
Catalytic Properties of CoY
- 2900
450
- 2800
- 2700
E 300
- 2600
- 2500
0
0
25
50
75
100
M E (sec
125
# CH^
X CO
0 c 2 h 6
♦ C3 H8
9 C02
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
1
Figure 32:
CoY Activity Versus Temperature
900
300°C
>300
271 °C
o
0
100
TIME
150
200
(sec)
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12
113
The activity
of sample 22
time as shown in Figure 33.
ane almost entirely.
also
changed with
This catalyst produced meth­
Figure
periment where the CO +
(RuFeY)
shows the data for an ex­
mixture was switched to helium
after 60 seconds of reaction.
When helium was passed over
the catalyst,
C02 was given off as shown by the growth of
the C02 peak.
Methane was also given off as shown by the
peak in the methane curve at 85 seconds.
A comparison of
the catalytic
zeolite samples
results for the bimetallic
is shown in Table 10.
Characterization of these catalysts
and experiments employed to
ies that
form these products
distinguish the surface spec­
are presented
in following
sections.
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Figure 33:
Catalytic Properties of RuY
3000
240
2900
£160
2800
~120
80o
2700
0
25
7
50
100
TI ME’ (s e c )
9
CH4
0
CO
X
•
c2
125
h6
C3 H 8
c4
H 10
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115
Figure 34 •*
200
RuY Reaction and Helium Purge
He
CO * H
■
150
100
>
o
0
20
40
TOTAL
50
TIME
50
100
(sec)
• - ch4
x - C02
R eproduced w ith perm ission o f the copyright owner. F urther reproduction prohibited w itho ut perm ission.
120
116
TABLE 10
Catalytic Properties of Bimetallic Samples
Sample #
1
14
15
18
22
Cation
C
>
Co*
Ru3
% Conv
10.5
2
2
9
5
# Sites
-1-21
2.4
3.3
2.2
2.5
3.8
Cart
2
2
2
1
1
Y
Y
Y
N
N
% conv - percent conversion of CO to products
C.j /Cp+ - ratio of methane to higher hydrocarbons
# sites - the number of active sites from shape of
activity curve
carbide - presence of iron carbide from M'dssbauer
results
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1 17
3.2
SPECTROSCOPIC CHARACTERIZATION OF CATALYSTS
3.2.1
Catalysts Prepared By Microwave Reduction
The samples prepared by the microwave discharge techni­
que did not show any Mossbauer signals.
tration was
below the detection
instrument used.
The iron concen­
limit of
the particular
For this reason, Mo'ssbauer data for the
microwave discharge samples are not presented.
Mossbauer spectra for
iron were obtained
190
materials with a higher
Recently,
loading of
.
The infrared properties of the iron and cobalt zeolites
were studied.
peak
for
Before the microwave treatment, an infrared
carbonyl was
Fe2(C0)g samples.
observed
ples.
2000 cm 1
for
the
A doublet at 2000 cm 1 was observed in
the COgCCOjg zeolite samples.
no signal
at
was observed at
After microwave treatment,
this frequency for
these sam­
Also after microwave treatment, samples were stud­
ied by ferromagnetic resonance.
tra are shown in Figure 35.
Some representative spec­
Shown are the spectra and g
apparent values for an iron on
-160°C (a)
zeolite Y (FeY)
and room temperature
(b).
sample at
The g values were
different at the two temperatures.
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11 8
Figure 35:
FeY FMR Spectra
o
o
O
CO
o
<Nl
II
cn
HP/„XP
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11 9
A plot
of the g
apparent value versus
shown in Figure 36 for the FeY sample.
of inflection at room temperature.
temperature is
There was a point
A plot of the linew-
idth versus temperature is shown in Figure 37.
The linew-
idth decreased with increasing temperature.
R eproduced with perm ission o f the copyright owner. Further reproduction prohibited w ith o u t perm ission.
1 20
Figure 36:
FeY g Value Versus Temperature
o
IX)
o
co <_>
UJ
cr
o
Z>
i—
<
cr
iu
o
CL
00 2 :
UJ
o
CD
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
1 pi
Figure 37:
FeY FMR Linewidth Versus Temperature
o
(—
S
UJ
o
VI
9'i
O'I
( 9 M ) H 1 Q I M 3 N 11
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
122
Representative
zeolite X at
plot of the
ferromagnetic
spectra
25°C and -160°C are shown in
for
cobalt
on
Figure 38.
A
g^-apparent value versus temperature
same sample is shown in Figure
39.
for this
A plot of the linew-
idth versus temperature is shown in Figure ^0.
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
Figure 38:
FMR of CoX
O
O
to
LO
CNJ
o
X
CM
CO
CM
H P /' . X P
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
Figure 39:
CoX g Value Versus Temperature
o
vo
o
. CO
LU
cr
Z)
■o I—
<
ct
LU
Q_
o 2:
■
00
UJ
o
■ VO
ddD
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
125
Figure 40:
CoX Linewidth Versus Temperature
o
o CJ
oo o
UJ
cr
-- o D
<
cr
LU
o
°-
00 X
'
LU
O
<£>
— 1-------- 1------- H-
07
S'l
O'l
(9>l) H 1 Q I M 3 N H
R eproduced w ith perm ission o f the copyright owner. F urth er reproduction prohibited w itho ut perm ission.
12 6
Table
11 lists
preparations
the value
of iron
of g
and cobalt
All of these samples were reduced
crowaves method.
room temperature.
on alumina,
apparent for
on different
several
supports.
by the reduction by mi­
These g apparent values were measured at
Iron and
cobalt carbonyls were placed
titania and silica
tion by microwaves method.
and treated by the reduc­
Ferromagnetic resonance spec­
tra were obtained containing several peaks.
The g values
were very large compared to the other samples.
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127
TABLE
11
Values of g Apparent
Sample #
Metal
Precursor
26
Fep (C0)q
Fep(C0)q
Fep(CO)q
Fe,(CO)'
27
Fep(CO)q
28
FetC,. H . U
23
24
25
29
30
31
32
33
34
36
37
Fe<c|H|)|
Co 2 (C01 q
COp(C0)o
COq(C0)o
COq(CO)o
Co~(C0)p
Fe^(CO)°
Support
Y
X
ZSM-5
SiOp
TiCu
X 2
A
ZSM-5
X
ZSM-5
SiO?
TiOp
HY
B
g(apparent)
2.08
2.08
2.08
2.22
2.18
2.08
2.02
2.04
2.18
2.19
2.32
2.55
2.17
2.08
B - bentonite
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128
3.2.2
3.2.2.1
Catalysts Prepared By Hydrogen Reduction
Iron Carbonyl Catalysts
Fe(C0)j- was deposited on
several zeolites.
Mossbauer
spectra of the untreated samples were obtained.
These re­
sults gave information on the oxidation state of the metal
on the catalyst.
these samples.
Table 12 contains the Mossbauer data for
The samples were exposed to air and pelle-
tized before the experiment.
The iron carbonyl zeolite samples
blimation under vacuum.
low in color.
were prepared by su­
The products were initially yel­
Heat was frequently given
samples when exposed to air.
samples turned brown.
dation of the iron.
off from these
When heat was given off, the
This could be an indication of oxi­
Column 1 contains the sample numbers
and column 2 lists the zeolites used in the samples.
value of the
isomer shift (IS)
these samples.
or the center
is given in
column 3 for
The isomer shift is the distance of a peak
of quadrupole split peaks
point or zero point.
Our
from a reference
reference point was the center
point of the six line pattern of metallic iron.
given a value
The
of zero mm/sec.
Column 4 is
the quadrupole splitting (QS) for the sample.
reduced samples,
This was
the value of
For the un­
this was merely the distance between the
peaks in a doublet.
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129
TABLE 12
Iron Carbonyl Samples, Unreduced
Sample #
Zeolite
cm
on.=r
invo
b-
Y
X
A
HM
NaM
NaZSM-5
HZSM-5
IS(mm/sec)
.40
.38
.45
.40
.43
.45
.43
QS(mm/sec)
.80
.95
.90
.90
.85
.90
.95
M - mordenite
R eproduced w ith perm ission o f the copyright owner. Further reproduction prohibited w itho ut perm ission.
1 30
These iron carbonyl loaded zeolites
drogen at
temperatures up
to 500°C.
were heated in hy­
The samples
were
cooled to room temperature and analyzed by Mossbauer spec­
troscopy.
The results of these Mossbauer studies are giv­
en in Table
13-
Column 4 is an
additional column which
contains the hyperfine value that was calculated for these
samples.
This i3 a measure of the magnetic field strength
in a sample.
This can be referenced to a value of 330 kOe
for alpha iron metal
184
.
Figure
41 contains a Mossbauer
spectrum of Fe(CO)^ decomposed on zeolite Y.
This sample
produced a six line pattern centered at zero.
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131
TABLE 13
Iron Carbonyl Samples, Reduced
imple
#
Zeolite
support
IS
(mm/sec)
1
2
3
4
5
6
7
Y
X
A
HM
NaM
NaZSM-5
HZSM*-5
-.09
-.08
-.01
0
J .01
-.01
.03
H
(kOe)
339.0
3^0.6
329.7
335.9
326.6
329.7
335.9
M - mordenite
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1 32
Figure
:
Mossbauer Spectrum of Fe(C0)^Y
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133
o
UJ
CO
>-
o
o
-I
UJ
CM
CM
I
<O
I
CM
CM
CO
co
CO
i N 3 0 a 3 d NI 1 0 3 3 3 3
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1 31*
Infrared spectroscopy
(IR)
was employed to
study the
carbonyl groups on the iron carbonyl zeolite samples.
fore reduction,
1985 and 1960
IR peaks were seen at 2120,
cm 1 .
These peaks correspond
groups in the samples.
also observed.
ites were
2019,
to carbonyl
Characteristic zeolite peaks were
After reduction,
studied by
2052,
Be­
the iron carbonyl zeol­
infrared spectroscopy
to determine
whether any carbonyl was present from the metal precursor.
No carbonyl groups were detected by infrared spectroscopy.
The infrared signal for CO
that was seen before reduction
was absent after reduction.
The electronic
properties of the iron
loaded zeolites
were then
studied by ferromagnetic resonance.
zeolites,
after reduction of the
gen,
um.
The iron
iron carbonyl by hydro­
were loaded into quartz tubes and sealed under vacu­
Ferromagnetic resonance
spectra
temperatures between -160°C to 250°C.
served for the iron loaded
were obtained
at
No peaks were ob­
zeolites after hydrogen reduc­
tion.
The particle
electron
size of
microscopy.
the metals
Samples were
transmission electron microscopy
tron microscopy (SEM).
(TEM)
was then
prepared
studied by
for
both
and scanning elec­
The particular transmission elec­
tron microscope used had a
much higher magnification than
the available scanning electron microscope.
For this rea-
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135
son, more TEM studies were done.
is shown
in Figure 42.
140,000X magnification.
This
A sample TEM micrograph
was a zeolite
This particle
sentative of the other particles
was fairly repre­
studied.
Fe(CO),. on zeolite Y after reduction.
particle at
The sample was
The tiny black par­
ticles on the zeolite are the particles of iron.
erage size
of these
iron particles
was 70
The av­
to 100
stroms.
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Ang­
136
Figure 42:
TEM of Fe(CO).-Y
o
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137
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138
The iron zeolite catalysts
by hydrogen of
iron carbonyl were studied
by Mossbauer spectroscopy.
iron carbide was formed.
spectrum
prepared from the reduction
for iron
on
This was
after reaction
to determine if any
Figure 43 contains the Mossbauer
zeolite Y
after
five minutes
Fischer-Tropsch reaction.
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of
139
Figure 43:
Mossbauer of Fe(C0)^Y Carbide
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140
1N33U33 NI 133333
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1111
3.2.2.2
Bimetallic Zeolites
Table 14 contains
Mossbauer data for the
OO
pared by the method of Scherzer and Fort
.
ites were loaded with two metals,
The reduction of iron was
are reproductions of
samples 15
through 22
these samples
are our
own variations.
on zeolite
All of
NH^Y except
on HZSM-5 and NaZSM-5.
the Mossbauer signal of the
signal before reduction.
not show a Mossbauer signal for
ter reduction.
one of which was iron.
Samples 8 through 14
OO
the work of Scherzer and Fort
and
were prepared
the predominant
These zeol­
studied.
numbers 15 and 16 which were
data are for
samples pre-
for
The
anion which was
Sample
22 did
iron either before or af­
Energy dispersive
X-ray analysis results
indicated that the iron concentration was less than M .
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TABLE 14
Bimetallic Samples, Unreduced
Sample #
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Cation
Fe2
Fe2
Co,
Co,
Ni?
Cu'l
Znp
Cu^
Cu,
f4
Co,
K12
?4
Zn,
Ru
Anion Precursor
K.Fe(CN)fi
(NHj.) j,FeiCN)/K^Fe(CN) /(NHj)11Fe(CN)fi
(NHjKFeCCN)?
Na?Fe(CN) NO
Na^Fe(CN )EnO
Na^Fe(CN)^NO
NapFe(CN)^NO
NapFe(CN)^NO
NapFe(CN)^NO
NapFe(CN)^NO
NapFe(CN)^NO
K~Fe(CN)r
Na^Fe(CN)°N0
IS^
-
0.10
-0-05
0.0
-0.05
-0.05
-
0.20
0.20
-0.15
0.20
0.0
0.20
-
-
-
0.20
0.35
-
0.10
Samples 8-14, 17-22 were on zeolite Y
Samples 15, 16 were on zeolite ZSM-5
a values in mm/sec
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1M3
The bimetallic zeolite samples were reduced in hydrogen
at M00°C for four hours.
were taken
All of
to measure
the samples
M’
o ssbauer peaks,
Mossbauer spectra of the samples
the oxidation
showed the
presence of
except for samples
sample 22 which had no peaks).
Mossbauer spectra which
state of
the iron.
two sets
of
13 and 1M (excluding
Samples 13 and 1M produced
contained only the six
line pat­
tern centered at 0 mm/sec indicating the formation of iron
metal.
All of
the other
bimetallic zeolites
produced
spectra containing the six line pattern and a doublet cen­
tered at
1.2 mm/sec,
which indicates
ferrous ions
present.
The Mossbauer results for the bimetallic samples
after reduction in hydrogen are given in Table 15.
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are
1 ix 4
TABLE 15
Bimetallic Samples, Reduced
Sample #
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Cation
Anion Precursor
Fe ll
Fe *
Co ll
Co 9*
Ni *
Cu t
Zn %
Cu p
Cu %
Fe i
Co
Ni %
Pd
Zn ,
Ru 6
K.FeCCN),
(NHZKFeCCN),
K^Fe(CN)/- 6
(NHjb,.Fe(CN),
(NHZKFeCCN)?
Na^FeC CN),-N0
Na,Fe(CN)^N0
NapFe(CN)^N0
NafFe(CN)XNO
Na,Fe(CN)^NO
Na~Fe(CN)E n O
Na~Fe(CN)EnO
Na^Fe(CN)E n O
Kf Fe(CN)5
Na£Fe(CN)°N0
IS^
H—
-0.08
-0.03
-0.03
-0.09
-0.05
-0.03
0.03
0.0
0.01
-0.01
0.0
-0.04
0.03
-0.04
340.6
337.5
337.5
336.5
331.3
323-5
320.4
311-4
322-0
322-0
33 J.3
244-1
312.7
334.4
Samples 8-14, 17-22 were on zeolite Y
Samples 15, 16 were on zeolite ZSM-5
a in mm/sec
b in kOe
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145
The Mossbauer spectrum of sample
Figure 44.
number 18 is shown in
The major peaks formed a six line pattern for
metallic iron.
A doublet for ferrous ions was also pres­
ent .
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1 46
Figure 44:
Mossbauer Spectrum of CoY
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147
i ---------------1-------------- 1-------------- 1---------------1--------------- r
_ J_________I_________I_________ I_________ I--------------- L
o
—
<m
to
m
l N 3 0 3 3 d NI 1 0 3 3 3 3
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148
Infrared spectroscopic studies of
ites
were performed
iron.
were
to study
Before reduction,
observed.
groups,
These
respectively.
also observed
in these
the bimetallic zeol­
the ligand
IR peaks
peaks
groups on
the
at 2205 and 1950 cm
correspond
to CN
and
NO
Characteristic zeolite peaks were
samples.
After
reduction there
were no signals observed for CN or NO in the samples.
The
characteristic zeolite peaks were still present.
The bimetallic
zeolites were loaded into
quartz tubes
after reduction in hydrogen and sealed under vacuum.
ferromagnetic resonance
spectra of these samples
tained at various temperatures.
The
was ob­
The signals were the same
as in the iron carbonyl zeolite samples.
A straight line
with no peaks was recorded.
The reduced
bimetallic zeolites
electron microscopy to determine the
metals.
These samples had a
were also
studied by
particle size of the
large particle size similar
to the reduction by hydrogen samples.
The iron particles
in these samples were 50 to 80 Angstroms.
3.2.2.3
Aluminoferrisilicate Zeolites
Table 16 contains Mossbauer
silicate zeolites.
The
data for the aluminoferri-
signal for sample 48
is for the
zeolite which was dried overnight at
110°c in an oven.
The sample 49 signal is for the same zeolite after calci-
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1 49
nation at 550°C in air for 6 hours.
The signal for 48 is
a singlet and for 49 a doublet.
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1 50
TABLE 16
Aluminoferrisilicate Zeolites, Mossbauer Results
Sample #
IS(mm/sec)
48
49
Fe(II)Y
Fe(III)Y
0.2
0.3
1.2
0.4
Fe(n)Y from Huang and Anderson
Q S (mm/sec)
0.7
2.4
0.86
79
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151
The aluminoferrisilicate
drogen at 400°C and 1I50°C.
had a Mossbauer
in hy^
Before reduction, the zeolites
doublet with an isomer
mately 0.3 mm/sec.
ions.
zeolites were reduced
shift of approxi­
This signified the presence of ferric
After reduction, the zeolites had a doublet with an
isomer shift of 1.0 mm/sec due to ferrous ions.
bauer results for the
The Moss­
aluminoferrisilicate zeolites after
reduction are given in Table 17.
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152
TABLE 17
Mossbauer of Reduced Aluminoferrisilicates
Sample
£
48R1
48R2
49R3
Fe(II)Y
Fe(III)Y
Reduction
IS
temp.
(mm/sec)
400
450
400
-
1 .0
1.0
1.0
1.2
0.4
QS
(mm/sec)
2.1
2.0
1.9
2.4
0.86
R1
- 48 reduced 400°C
R2
- 48 reduced 450°C
R3
~ 49 reduced 400 C
Fe(n)Y from Huang and Anderson
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153
Differential thermal analysis was employed in the study
of the
aluminoferrisilicate zeolites to observe
of water with heat and
that might occur.
45 and 46.
to study any phase transformations
A few of the runs are shown in Figures
These experiments showed the loss of water as
the zeolites were heated.
ing off of
the loss
Analysis of the substance com­
the zeolite was not done
but the temperatures
suggest that the substance was water.
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Figure *15:
DTA of Aluminoferrisilicate
o
LOt"-
EMPERATURE
(°C)
o
LDLD
O
lo -
o
LO
oxe
iv
opue
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1 55
Figure 46:
DTA of Aluminoferrisilicate
o
LO-
o
o
LO­
UD
LLj <—>
rv- lO-
< O
ce
ld -
2 o
LLi ID CNJ
O
lO-
O
LO
0X0
I V
*
opue
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1 56
The types
of ferric
zeolites was studied with
ions in
the aluminoferrisilicate
ferromagnetic resonance.
ures 47 and 48 show some examples of the results.
Fig­
Signals
were seen for framework ferric ions and non^-f ramework fer­
ric ions.
The framework ion signal
is labeled A and the
non-framework signal is labeled B.
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157
Figure 47:
FMR of Aluminoferrisilicate
HP/..XP
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FMR of Aluminoferrisilicate
H (kG )
Figure 43:
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159
3.2.3
Catalysts Prepared By Sodium Vapor Reduction
Iron was ion^exchanged into several zeolites to be used
in the sodium reduction experiments.
Zeolites A, X and Y
were employed
These
in these experiments.
samples were
studied by Mossbauer spectroscopy to observe the oxidation
states.
The results of these
studies are shown in Table
18.
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160
TABLE
18
Ion-Exchanged Zeolites, Mossbauer Results
Sample #
35
36
37
IS (mm/sec)
.50
.50
.50
Q S (mm/sec)
1.0
0.9
0.9
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151
During the vaporization of the
could be seen
production of a
sodium,
in the reduction tube.
a purple color
This
could be the
color center similar to those
crowave experiments.
The samples were
in the mi­
This color faded quickly.
reduced with sodium vapor
and loaded
into air tight Mossbauer containers in the dry box.
bauer spectra were run to
in the samples.
observe the extent of reduction
Before reduction, these samples showed a
doublet with an isomer shift of 0.5 mm/sec.
tion,
iron on
Y had an isomer shift of
iron on zeolites A and X still
proximately 0.5
mm/sec.
Mossbauer pattern
small doublet
After reduc­
1.3 mm/sec while
had an isomer shift of ap­
Sodium reduction was
ducted on sample 3 (Fe(CO)j. on
six line
Moss­
with an isomer
A).
also con­
This sample showed a
centered at
0 mm/sec
shift value of
plus a
1.2 mm/sec.
Sodium reduction was also done on a bimetallic zeolite ca­
talyst.
Sample 13,
which contained copper and
iron on
zeolite Y, was reduced and showed a doublet with an isomer
shift of 0.4 mm/sec.
The Mossbauer results for these sam­
ples are given in Table 19.
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1 62
TABLE
19
Mossbauer Results After Sodium Reduction
Sample
#
35
36
37
3
13
IS
(mm/sec)
1.3
0.4
0.5
0.0
0.4
QS
(mm/sec)
H
(kOe)
2.6
1 .0
1 .0
1 .0
322.0
F
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163
Potassium vapor
samples.
reductions were
performed on
similar
Some reduction of the iron was detected but the
samples were not reproduced due to lack of time.
tassium vaporized at
a lower temperature than
The po­
the sodium
so that the experiments were run at a lower temperature.
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Chapter IV
DISCUSSION
4.1
FISCHERFTROPSCH RESULTS
4.1.1
Catalysts Prepared By Microwave Reduction
4.1.1.1
Cobalt Catalysts
The cobalt zeolite catalysts that
microwave reduction
of the support
which
of cobalt carbonyl showed
pore size on the
the Fischer^Tropsch
reaction.
contained reduced
small hydrocarbon.
56
The small pore
cobalt formed
The critical molecu-
all have a critical
of 4.2 Angstroms (NaA).
tive for a long time and
n-
diameter of be­
Zeolite A has a pore size
This effect
particles are very small.
Ethane,
Methane could fit inside of A but
was inside the zeolite cage.
vated.
a
The size of the cage would not permit
tween 4.6 and 4.9 Angstroms^.
ethane could not.
zeolite A
only methane,
of methane is 4.00 Angstroms.
propane and n^butane
the effect
hydrocarbons produced in
a larger hydrocarbon to be produced.
lar diameter
were prepared by the
indicated that the cobalt
This implies that the metal
This catalyst also remained ac­
did not appear to become deacti­
Products did not block the pores and cause deacti­
vation .
- 164 -
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1 65
The larger
products.
pore zeolites
produced larger
Cobalt on ZSMi5, which is a larger pore zeolite
than A, formed methane and ethane.
Ethane is a larger hy­
drocarbon than methane and requires
talyst pores.
can
the larger hydrocarbons.
which is a larger pore
takes up more space
formed propy­
in the zeolite.
size of the hydrocarbon product
this to be
The large
formed.
sites were inside of
This indicated that
the zeolite cages.
The metal particles must therefore be very small.
The hydrocarbons
once formed were free to desorb from the catalyst.
sites were on
The hy­
zeolite cages and were lim­
ited in size by the volume of the cage.
active metal
The
was related to the physi­
cal size of the pore in the catalyst.
drocarbons were formed in the
on X,
Propylene is larger than eth­
in zeolite X allowed
the active metal
Cobalt
zeolite than ZSM-5,
lene, ethylene and ethane.
pore size
more space in the caF
ZSM-5 has a pore size of 5.5 Angstroms and
accommodate
ane and
hydrocarbon
the outside of
then the small pore catalyst would
If the
the catalyst,
form the same size hy­
drocarbons as the large pore catalyst.
The activity of the cobalt catalysts changed during re­
action which implied
time.
that the catalyst was
At these temperatures,
al atoms
could move around
increase in particle
size.
changing with
it is possible that the met­
in the catalyst
This was not
and possibly
studied except
for ferromagnetic resonance experiments of the spent cata­
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1 66
lysts.
Cobalt on A and ZSM-5 produced methane and ethane
which are saturated hydrocarbons, whereas cobalt on X pro­
duced propylene and ethylene
carbons.
which are unsaturated hydro­
The metal particles on zeolite
outside of
the zeolite due to
A may be on the
the small pore size
of A.
There might have been another effect influencing the prod­
ucts in the
case of zeolite X
besides shape selectivity.
Zeolite acidity is one possibility.
The physical size of
the hydrocarbon products
was determined by the
of the
degree of saturation
catalyst but the
been determined by some other factor.
pore size
might have
Catalyst acidity or
some reaction condition such as reactant gas concentration
could have an effect.
The exact reason for the formation
of unsaturated hydrocarbons is not known.
*1.1.1.2
Iron Catalysts
The iron zeolite catalysts produced by the reduction by
microwaves of Fe2 (C0)g
fect of the
catalyst pore size on the size
carbons produced.
like cobalt
and ferrocene also showed
of the hydro­
Iron on zeolite A produced only methane
on A.
The
shape of the
graph showed only one peak.
after thirty minutes.
tion of bulk
the ef­
percent conversion
The catalyst became inactive
This is probably due to the forma­
iron carbide as
seen in the
other Fischer-
Tropsch reactions with iron zeolite catalysts.
Verifica­
tion of this theory by Mossbauer spectroscopy could not be
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167
done due to the low
concentration of iron present.
on ZSM-5 produced methane,
Iron
ethane and an unknown species.
This unknown was possibly an oxygenated species since iron
can produce such
products.
The identity of
was not determined but methanol,
ethanol and acetone did
not have retention times equivalent
retention time for
the unknown
to the unknown.
the unknown was 1.3 minutes
The
at a flow
rate of 25 ml/min of helium using Alltech VZ-10 columns at
an oven temperature of 50°C.
Methanol,
ethanol and ace­
tone are not separated by VZ^IO columns and elute with the
air peak at 0.32 minutes under these conditions.
The un­
known’s retention time of 1.3 minutes is between the peaks
for ethylene (1.07 minutes) and propane (1.87 minutes) un­
der these
conditions of flow
If this peak
rate and
is for an oxygenated species,
large molecule that is similar
Iron
on ZSM^5 appeared
it
must be a
to a hydrocarbon since the
unknown was separated from the air
umn.
oven temperature.
peak by the VZ-10 col­
to have two
active sites,
similar to other iron catalysts in this dissertation.
Fischer-Tropsch reaction
about 5 minutes
these specific
showed a
formation.
The
reaction conditions.
probably
formation of iron
firmed since Mossbauer
to the low
conversion maximum
and a second maximum at
quickly became deactivated,
at
12 minutes under
The catalyst
then
due to iron carbide
carbide was
spectra could not be
amount of iron present.
The
It
not con­
obtained due
is possible that
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168
iron carbide
could have
that the iron
formed in
the zeolite
could have migrated to the
pores or
surface to form
iron carbide there.
Two large pore zeolites were
the iron catalysts.
products
formed by
These zeolites were Y
these catalysts
though both catalysts
the same
structure.
and X.
The
were different
even
included iron and were
reduction by
have different
also used as supports for
microwaves method.
Si/Al ratios
The zeolites
and acidities
but the
same
The particle size of the iron was the same for
both catalysts as shown
by ferromagnetic resonance.
pore size of the zeolites was the same,
difference in
the products
was determined
have been bound
Perhaps the
by the
precursor or the acidity of the catalyst.
cursors could
The
so that shape se­
lectivity would form the same size products.
zeolite.
prepared by
metal
The metal pre­
at different sites
in the
Different binding sites can have varying effects
on the metal because of different binding strengths.
Iron
on X was prepared from the reduction by microwaves of fer­
rocene.
This
catalyst formed
larger hydrocarbons
case of cobalt on X.
duction by
methane and
were produced as
ethane.
were formed
No
in the
Iron on Y was prepared from the re­
microwaves of
ethane and two unknowns.
Fe^CCO)^.
It
formed methane,
One of the unknowns was probably
the same unknown as was formed by iron on ZSM-5.
Both of
these catalysts became inactive with time.
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169
4.1.2
Catalysts Prepared By Hydrogen Reduction
4.1.2.1
Iron Carbonyl Catalysts
Fe(CO)j- was placed
hydrogen,
on several zeolites and
reduced in
but only the iron on zeolite Y was studied as a
Fischer-Tropsch catalyst.
All of
the catalysts prepared
by this
method had similar
states.
The samples all contained large particles of iron
metal.
particle sizes
and oxidation
The iron on Y catalysis results were taken as rep­
resentative of samples prepared by this technique.
The iron catalyst prepared by the reduction by hydrogen
of iron carbonyl on zeolite Y had a fairly high conversion
of CO.
28.
The major product was
methane as shown in Figure
The reasons for this high methane production were the
reaction conditions.
These reactions were run
at ?85°C
and a reaction mixture of 10? CO in H2 which represent methanation conditions.
Temperatures above
the formation of methane.
250°C promote
Hydrocarbon chain formation is
inhibited at higher temperatures since molecular motion is
increased and
surface groups are
temperature increases.
bound less
Methane desorbs from the catalyst
before any chain growth can occur.
drogen present
and less carbon
ments than in the reactions
ples.
The reactant
strongly as
There is also more hy­
present in
these experi­
on the microwave reduced sam­
gas mixture is 10? CO
the reactions with the microwave
in
whereas
reduced samples used 50?
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1 70
CO in
.
Lower amounts of
promote methane formation.
CO in the
Iron on
reactant mixture
Y formed other prod­
ucts than methane but in smaller concentrations.
crowave reduced samples
were run at 250°C
mixture of 50? CO in
The mi­
and a reaction
These conditions promoted chain
growth of the hydrocarbons.
Significant amounts
by iron on Y as
of the by-product CO^
indicated by Figure 28.
synthesis ideally involves the
and water.
The oxygen from CO
were formed
Fischer-Tropsch
production of hydrocarbons
is reacted to form water.
This oxygen can also react to form C02 or oxygenated spec­
ies such as methanol.
oxygenated species.
Iron catalysts can produce a lot of
When hydrocarbon chain growth is de­
sired, loss of carbon to a by-product such as CO,, is unde­
sired.
imum.
The amount of CO^ formed is usually kept at a min­
There also appeared to be at least two active sites
on the
catalyst as suggested
curve in Figure
surface of
30.
by the
These sites must be
the zeolite since electron
indicate that the iron particles were
the zeolite cages.
in hydrogen
active initially
One site
show the
microscopy results
too large to fit in
shape selectivity
reduced catalysts did.
and probably was
became active
seconds for
on the outside
For this reason, the catalysts reduced
did not
that the microwave
hydrogen desorption
One
the clean
with time with
these reaction conditions.
site was
iron metal.
a maximum
This
effects
at 300
could have
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171
been the formation of an active carbide species.
Surface
carbides that form early in Fischer-Tropsch reactions have
been shown to be active
gresses,
inactive bulk
catalysts.
carbides
tion of the catalyst.
The
stantly
reduced
changing from
carbides to
time.
form and cause deactiva­
catalysts appeared to be con­
bulk carbides.
Bulk
As the reaction pro­
metal to active
FeY became
iron carbide formation
Mossbauer studies at
surface
deactivated with
was observed
the point where the
in the
catalyst became
inactive.
This is shown in the Mossbauer spectrum in Fig­
ure ^3-
The desorption studies
with three portions.
showed a
methane curve
The three portions (peak,
shoulder
and tail) might be related to the active sites observed in
the reaction product curve.
desorption curve could
same types
of
three portions in the
be due to three
on the catalyst that desorb
be the
These
as methane.
species but
different species
They could also
on sites
with varying
binding strengths.
4.1.2.2
Bimetallic Catalysts
Many samples were prepared which contained iron and an­
other metallic element on a zeolite.
Tropsch catalysis were selected
of reduction.
Samples for Fischer-
according to completeness
The samples containing copper and zinc con­
tained completely reduced iron metal.
chosen with variations in the
Other samples were
second metallic element and
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172
the zeolite.
The other samples contained cobalt, rutheni­
um and zeolites Y or
ZSM-5.
These samples were believed
to be representative of the sample technique.
The bimetallic catalysts showed the
ond metallic element on the
effect of the sec­
percent conversion of CO,
on
the number of active sites on the catalyst and on the for­
mation of bulk iron carbide.
This
is shown in Table 10.
The second metallic element did not, unfortunately,
it the
formation of
large metal
shown in the electron
of iron
are large
particles of
microscopy results.
and therefore must
surface of the zeolite.
These
be on
inhib­
iron,
The particles
the outside
catalysts do not show the
shape selectivity of the microwave reduced catalysts.
catalysts that contained iron and
balt and ruthenium.
zinc or
The rea­
the Fischer-Tropsch activity of co­
The catalysts that contained iron and
copper had low
also produced
The
ruthenium or cobalt had
high activity as catalysts as shown in Table 10.
son for this could be
as
activity.
catalysts with
Cobalt
only one
and ruthenium
active site,
as
shown by the shape of the reaction products curves in Fig­
ures 31 and 33*
to the
There were
formation of another
there were two
no increases in activity due
It is
possible that
active sites that were both
active at the
start of the reaction.
site.
The shape of the reaction product
curve would only indicate more than one site if the activ­
ities had maxima at different times.
This is a possibili­
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1 73
ty that was not investigated.
carbide species
was not detected
taining cobalt and ruthenium.
allic elements also
carbide.
for the
catalysts con­
The presence of these met­
inhibited the formation of
The helium titration
shown in Figure 34 was
bulk iron
of the ruthenium catalyst
conducted at the reaction tempera­
ture and showed the formation
ide.
The formation of an active
of methane and carbon diox­
The carbon dioxide might indicate that there were CO
and 0 groups on the catalyst during reaction.
formation might
indicate that there
groups on the catalyst.
The methane
were also CHx
and H
Under these particular reaction
conditions it appeared that there were more CO groups than
CHx on the catalyst since
more carbon dioxide was formed.
C02 could
formed from the
also have been
surface carbon with 0 groups.
catalysts and
the formation of
combination of
The types of groups on the
bulk iron carbide
inferred by the catalytic results.
can be
A more direct method
of analysis of these types of systems was done using spec­
troscopy .
4.2
SPECTROSCOPIC CHARATERIZATION OF CATALYSTS
4.2.1
Catalysts Prepared By Microwave Reduction
The concentration of the iron was fairly low in the ca­
talysts prepared
by reduction
Mossbauer spectra could be
by microwaves
obtained.
tron microscopy determined the iron to
so that
no
Results from elec­
be less than 1* in
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174
the catalyst.
This was still enough metal to catalyze the
conversion of carbon monoxide
and cobalt
to hydrocarbons.
The iron
carbonyls were completely decomposed
carbonyl peaks were observed
ter microwave treatment.
since no
by infrared spectroscopy af­
The ferromagnetic resonance also
indicated that the decomposition was complete since ferro­
magnetic metal particles were formed.
particles was
smaller
less than
since the
The size of these
20 Angstroms
catalysis results
and probably
even
suggested that
the
metal was inside the zeolite cages.
Ferromagnetic
resonance was
iron and cobalt on supports,
performed
for the
supports in Table 11.
reduction.
The g values
cobalt on
the different
iron and
The
g apparent values measured by
ferromagnetic resonance were the
suggesting that
smallest on the zeolites
the metal particles
zeolites than on the other supports
for iron on X
but g = 2.22 for iron
indicate that
the metal particles
than on zeolite X.
of
such as silica and titania,
that were prepared by microwave
were compared
on samples
were smaller
on the
(for example g = 2.08
on SiO^).
This may
were bigger
on silica
The g apparent values were very close
for the iron prepared from ferrocene and for the iron pre­
pared from Fe^CCO)^.
This indicates that small particles
can be prepared from different metal precursors.
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175
These ferromagnetic species were unstable.
on X
air.
catalyst was pyrophoric
The cobalt
and smoked when
exposed to
The oxidation of these small high surface area par­
ticles appeared to be very exothermic.
The small metal particles generated by the reduction by
microwaves technique
prove that the
the carbonyl compounds without
microwaves might
temperature
sintering the metal.
create local hot
of the
microwaves decompose
spots but
sample during
The
the overall
sample preparation
is
room temperature, as measured by a thermocouple.
The microwave
discharge experiments
had the
number of problems with reproducibility.
ites prepared
by the
reduction by
The cobalt zeol­
microwaves of
carbonyl were fairly easy to reproduce.
prepared by reduction by microwaves
of iron carbonyl were
This could be due
nature of iron metal to
form iron oxide.
oxygen
or
water
oxidation of the metal).
could
causesintering
and low powers could
compose the metal complex)
to
Any trace amount
The
thoroughly dehydrated supports,
microwave power (high powers
sure
tothe reactive
contaminate these samples.
samples were dependent on
composition of supports
cobalt
The iron zeolites
harder to reproduce.
of oxygen or water could
greatest
or de­
fail to de­
and sample handling (any expo­
vapor
Often
results
in
immediate
the contamination of the
sample was not evident until the Fischer-Tropsch reactions
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176
when the catalyst
was found to be
inactive.
Repetition
and careful attention to detail
resulted in increased re­
producibility in these samples.
The microwave discharge
samples often seemed heterogeneous before microwave treat­
ment.
The side of the zeolite closer to the metal complex
and away from the vacuum pump appeared darker.
the metal deposited on this
side,
Initially,
but as time progressed
the rest of the zeolite became saturated with the complex.
A few times after microwave treatment,
the zeolite parti­
cles appeared blacker on the top or on the surface.
did not seem to affect the
This
catalytic activity of the sam­
ples.
The
initial
Fe^CCO)^.
microwave
Carbonyl
volatile enough to
samples involved
the
species were chosen since
sublime and easy enough
use
of
they were
to decompose.
Reduction was not necessary since the iron was in the zero
oxidation state.
flow
system
and
Fe(CO)^ was too
Fe^CCO)^
F e ^ C C O ) ^ was also
of the zeolites.
(it sublimed
was
not
volatile
too large to fit in the
in a
enough.
pores of some
Fe2 (C0)g appeared to be volatile enough
at 35°C)
and
small enough to
The technique was successful in
ticles on
volatile to use
zeolite Y as
shown by
preparing small iron par­
ferromagnetic resonance
but the iron concentration was less than
electron microscopy.
merit study.
Reaction studies
as measured by
were done but the
percent conversion of the CO was fairly low.
Other metal
precursors were investigated.
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1 77
Experiments with
iron samples.
FeCl^ failed
Ferrocene, however,
tion of small
metal particles.
volatile than Fe^CCO)^.
erature.
to prepare
which
the pink of the argon
did yield the produc­
Ferrocene was
much more
It sublimed readily at room temp­
The initial plasma was
the carbonyls)
any reduced
purple (it was blue for
made it harder to
distinguish from
plasma but long decomposition times
resulted in total decomposition.
4.2.2
4.2.2.1
Catalysts Prepared By Hydrogen Reduction
Iron Carbonyl Zeolites
The iron
carbonyl zeolites
spectroscopy
were studied
immediately after
sublimation
carbonyl and before any further treatment.
summarized in Table 12,
on the zeolite.
showed
same as
temperatures.
The signal
ions in zeolites.
These studies,
the condition of the iron
was similar to that
The iron
as shown by the
ferric
The carbonyls were still
infrared results,
somehow changed.
of
probably was oxidized rather
The iron
Fe(C0)^L species where the L
or the zeolite
matrix.The Mossbauer results
types of species have been reported
154-160
but the iron
carbonyl could
have formed an
isomer shifts
iron
reported for iron carbonyl at low
than remaining in the Fe° state.
carbonyl had
of the
The Mossbauer signal for the iron carbo­
nyl was not the
present,
by Mossbauer
.
and quadrupole splittings for
is an
oxygen
for these
The range of
the Fe(C0)^L
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178
species is
similar to the
zeolites.
The use of
careful precaution
prevented the
results for our
iron carbonyl
thoroughly dehydrated supports and
to exclude
formation of
air and
water might
this oxidized
species.
have
The
high temperature that was needed to reduce the iron carbo­
nyl also points to an oxidized
shift and
were not
form of iron.
quadrupole splitting
the same
served in
as those for
for the
The isomer
oxidized species
the unreduced
the Mossbauer spectrum after
iron (ob­
reduction).
The
unreduced iron has an isomer shift and quadruple splitting
very similar to ferrous ions.
temperatures up to
absence of
with X and Y being the
The
isomer shifts
zeolites were close to
greatest.
the iron signal.
0 mm/sec,
This is shown in Table
values for
also different than the rest.
environment in
other smaller pore
kOe.
metallic iron
quadrupole splittings and hyperfine
iron on X and Y were
haps the
indicated that
some unreduced iron.
on all of the
The
pattern and the
Reductions at temperatures below 500°C of­
ten resulted in
13*
The six line
any other peaks
was produced.
for iron
500°C.
The reduction was done at
X and Y
was different
zeolites and that effect
Per­
than the
was shown in
Alpha iron has a hyperfine value of 330
Iron on zeolites A, mordenite and ZSM-5 gave hyper­
fine values close to that of alpha iron.
gave hyperfine values closer to
the value H
is not known.
Iron on X and Y
3^0 kOe.
The experiments
The reason for
were done in
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1 79
the in-situ Mossbauer reactor so
oxygen or water occurred.
ed for
alloy formation
that no contamination by-
High H values have been report-
95
but that is
not the
case here
since iron is the only metal present.
The absence of any carbonyl peaks in the infrared spec­
tra of the reduced iron zeolites indicated that the carbo­
nyl was
completely gone.
Complete
occur during reduction.
not
occur since
the
decarbonylation must
Decomposition of the zeolite did
characteristic
zeolite peaks
were
still present.
The iron carbonyl zeolite samples appeared
at times to be
heterogeneous before reduction (containing
regions of higher and lower metal concentrations).
samples appeared to have more iron
the zeolite,
where contact was
These
carbonyl at the top of
initially made
with the
carbonyl vapor.
Ferromagnetic resonance
indicated that no
ferric ions
were present since ferric ions give a strong electron par­
amagnetic resonance signal due
in the high spin state.
The iron zeolite samples prepared
from the reduction by hydrogen
bulk metallic
This was
iron,
to five unpaired electrons
of iron carbonyl contained
as indicated
also proven by
by the
the electron
straight line.
microscopy results
which indicated a large metal particle size.
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4.2.2.2
Bimetallic Zeolites
The M6'ssbauer signals
for the iron atoms
tallic zeolites before reduction were
ies.
The anion component usually
nal.
Samples 8,
in the bime­
for the anion spec­
gave the stronger sig­
9 and 17 in Table 14 also had Mossbauer
peaks before reduction with an
isomer shift of 1.2 mm/sec
characteristic of
ferrous ions.
The Mossbauer
were similar with
the same anion,
with
due the presence of the second metal.
nal due
to ammonium hexacyanoferrate
fected by the second metal.
p
p^,
with Fe
is not
,
Co
and Ni
seemed to
be unaf­
as the initial second ions.
second metallic
are probably still ions since these
2+
The Mossbauer sig­
It appeared in the same place
still ions or if reduction did occur.
Co
small variations
p^
known whether these
reduce.
results
It
elements are
The zinc and copper
ions are very hard to
The potassium ferricyanide showed a singlet with
and a doublet with Zn
2+
.
The samples containing so­
dium nitroprusside showed similar Mossbauer signals except
when Fe
2+
and Fd
2+
were present.
palladium sample was the most
Tne isomer shift of the
positive value for the sam­
ples studied.
These bimetallic
zeolites reduced to
The samples that reduced the
13 and
14 in Table 15.
peaks for unreduced iron.
varying degrees.
most completely were numbers
These two samples did
Surprisingly,
not show
these two bime-
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tallic zeolites
had very
Tropsch reactions.
low
activity
All of the
in the
Fischer-
other samples showed some
amount of unreduced iron as well as metallic iron by Mossbauer spectroscopy.
The isomer shift of all of the bime­
tallic zeolites was close to
0 mm/sec. The hyperfine val­
ues ranged from 294.1 kOe to 340 kOe.
of H were for the
side.
The lowest
values
samples prepared from sodium nitroprus-
The highest H values
involved combinations of the
iron anion species with iron or cobalt.
Higher values of
H can be an indication of alloy formation.
Alloys of iron
and cobalt in catalysts has been reported.
These samples
showed that the second metal
influenced the degree
of re­
duction of the
electronic properties
of the
iron.
iron and the
It has already been mentioned that the second met­
allic element in the bimetallic
zeolites affected the ac­
tivity and life of a catalyst.
These
samples involved
a
two
Initially, a cation, such as Co
method.
,
was exchanged into the
2like Fe(CH)^N0
was ex­
zeolite.
Secondly,
changed.
When the anion exchanged into the zeolite,
of two things
an anion
2+
fold exchange
must have happened.
If it was
one
a true ex­
change,
then it replaced an anion that was present in the
_
p_
zeolite (N0^ or S0^
from the initial exchange).
If this did not happen then
the cation from the anion (K+
or Na+ ) must have been present somewhere in the zeolite in
order to maintain a neutral charge.
It is
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1 82
not known which process actually occurred.
The important
fact was that the anion did go in and it attached to some­
thing (probably the
transition metal cation)
was not lost in the washing
of anion that
of the catalyst.
exchanged into the zeolites
it produced more
so
that it
The amount
was high since
than 3% iron in the sample
and since it
could be detected by Mossbauer experiments.
Before reduction, the anion complex such as Fe(CN)^NO
was probably
on the inside of
the zeolite cage
iron was exchanged into the zeolite.
ion-exchange sites
on the outside
p-
since 3%
There are not enough
of the zeolite
to ex­
change 3% iron into the samples, therefore the anions must
also be exchanging with internal
ions must
be on the inside
sites.
Some of the an­
of the zeolite
before reduc­
tion.
Before reduction, the peaks for CM and NO in the infra­
red spectra of these samples
indicated that the complexes
did exchange into the zeolites.
ter reduction,
The infrared spectra, af­
indicated that the complexes
were decom­
posed, since no CN or NO peaks were observed.
The oxida­
tion state of the initial cation was not known.
The ferromagnetic resonance studies
of ferric ions and the presence
electron microscopy showed
large.
showed the absence
of bulk iron metal.
the metal particle size
The
to be
The metal must therefore be on the outside surface
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183
of the zeolite.
The bimetallic zeolite catalysts did not
show any influence of size selectivity on the hydrocarbons
produced from CO like the
microwave catalysts did.
is further evidence that the metal
This
is on the outside sur­
face of the zeolite.
4.2.2.3
Aluminoferrisilicate Zeolites
These zeolites
450°C.
were reduced in
hydrogen at
400°C and
Before reduction, the Mossbauer signal was that of
ferric ions.
reduction,
The results are
the
shown in Table 16.
signal appeared to
be for
After
ferrous ions.
The results for the reduced samples are in Table 17.
duction to the metal did not occur.
sults were obtained
Similar Mossbauer re­
with the dried and
This suggests that the calcination is
calcined samples.
not a vital step to
the reduction of the iron to the ferrous state.
cination is
a vital step in
from the zeolite.
for the presence
Re­
the removal of
The cal­
the template
The uncalcined samples were not studied
of the template or for
residue from the
template.
The thermal analyses of
ites shows several peaks.
ably due
to the loss of
above are probably
late.
the aluminoferrisilicate zeol­
Low temperature peaks are prob­
water.
The peaks at
due to the decomposition
450°C and
of the temp­
Above 700°C, the zeolite itself might be decompos­
ing .
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184
11.2.3
The
Catalysts Prepared By Sodium Reduction
samples used
in these
experiments were
from the ion-exchange of ferrous
nitrogen.
ions into zeolites under
The Mossbauer experiments on these samples in­
dicated that
ions.
prepared
the ferrous ions
became oxidized
to ferric
Adjustment of the pH was attempted without success.
The ferric zeolites were used
periments.
in the sodium reduction ex­
After reduction, iron on zeolite Y appeared to
be ferrous ions but no
metallic iron was detected.
Iron
on zeolites A and X appeared to be ferric ions as they did
before reduction.
Sample number 3 (iron carbonyl on zeol­
ite A) did reduce to form some metallic iron but some fer­
rous ions were
still present.
and iron bimetallic on zeolite
ions after reduction.
peared to
state.
Y)
reduction of the
never successfully accomplished.
13 (copper
appeared to be ferrous
The failure of the
be in the oxidation
The
Sample number
technique ap­
of the iron to
the ferric
original ferrous
ions was
The potassium reduction
experiments appeared to have potential.
The lower vapori­
zation temperature of the potassium could have caused less
sintering than
were incomplete
did the
and the
sodium reductions.
experiments were
The results
halted due
lack of time.
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to
1 85
4.3
COMPARISON OF SAMPLE REDUCTION METHODS
It
important to
compare reduction
by hydrogen
carbonyl zeolites and bimetallic zeolite samples),
vapor reductions
The various
and reduction by a
(iron
sodium
microwave discharge.
reduction methods varied in
completeness and
quickness of reduction.
4.3.1
Completeness of Reduction
The microwave
discharge method was the
most versatile
and complete method of reduction.
Iron and cobalt carbo­
nyls
ferrocene on
and organometallics
could be reduced to the
successful in
such as
metal.
Hydrogen reductions were
producing metallic iron from
zeolites and in certain bimetallic
produce small metal particles.
15
.
iron carbonyl
zeolites but failed to
The iron carbonyl zeolites
required a much higher temperature
been reported
zeolites
for reduction than has
This could be due to the formation of a
Fe(CO)^/zeolite species.
This species could be harder to
reduce, especially if the iron is in an oxidized state.
The degree of
reduction of the anionic
complex by hy­
drogen to metallic iron in the bimetallic zeolites was in­
fluenced by three factors.
pendent on
the initial
First,
cation.
reduction of iron was achieved with
initial cation.
the reduction was de­
The
highest degree
of
copper or zinc as the
With these cations there was no detecta­
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186
ble unreduced iron.
Different cations interacted differ­
ently with the iron complex.
Perhaps the strength of the
interaction influenced the reduction of the iron.
The
second factor
complex itself.
much more
that influenced
reduction was
the
Sodium nitroprusside, Na2 Fe(CN)^N0,
was
effectively reduced
This complex even reduced at
than the temperature
other complexes.
a lower temperature,
used with the other
effect could be due to the
These ligands
than the
300°C,
samples.
This
ligands present in the sample.
influenced the charge
of the
central iron
atom, the coordination environment of the complex and per­
haps have varying
leaving abilities due to
their binding
strength with the iron.
The third important factor was the support.
Reduction
was easier on NH^Y than on either NaZSM-5 or HZSM-5.
ple 13
differed from samples 15
port.
Sample 13 showed complete reduction whereas 15 and
16 contained some
and 16 only by
Sam­
unreduced iron.
This effect
the sup­
could be
due to the type and strength of sites to which the complex
binds.
Sodium vapor
reductions failed
reduced iron metal.
of Fe
*3+
to produce
completely
This was possibly due to the presence
from the oxidation of Fe
to reduce in zeolites.
2+
.
Several others
Ferric ions are hard
99 100
’
have suggest­
ed that this is a difficult task.
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1 87
4.3.2
Length of Reduction Time
The microwave discharge reduction was one of the quick­
er reduction techniques studied.
Typically, reductions by
this technique could be done in a few minutes.
reductions to the ferrous state
duration.
Complete
The sodium
were also fairly short in
reduction did not occur
reduction.
After the
temperature,
vaporization of the sodium and reduction oc­
curred quickly.
hours.
Samples were heated
reduced in hydrogen
The
reached the
The reduction by hydrogens
long experiments.
higher and
zeolite had
with sodium
samples then had to
room temperature.
faster and could
400°C and
twenty four
be cooled back
down to
The microwave discharge reductions were
be done at room
reductions could have
of the metals.
were fairly
up to
from four to
correct
temperature.
had an effect on
The long
the particle size
The migration of iron to
the surface of
the catalyst could proceed slowly, but since the reduction
time is long,
most of the iron migrates during reduction.
Perhaps reduction
of the
iron species
slow but as the iron migrated
in the
pores was
to the surface of the cata­
lyst, where the hydrogen concentration was greater, reduc­
tion occurred.
tures or the
It is not known whether the high tempera­
long reduction time caused
the sintering of
the iron particles.
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188
H.4
COMPARISON OF PARTICLE SIZE
This was a very important
area in this research.
of the goals of this research
cles that
were small
cages of zeolites.
One
was to produce metal parti­
enough to
fit into
Therefore,
the
the pores
and
particle size of
the
metal is very important in these studies.
The microwave discharge reduction,
was done at room temperature,
particles;
carbonyl zeolites
The
The reduction by hydrogen of iron
prepared particles
These particles were
metal was
zeolite.
prepared the smallest metal
the only metal particles small enough to fit in
the pores of a zeolite.
stroms.
probably because it
probably on
Hydrogen
samples prepared
of 70
to 100
Ang­
larger than zeolite pores.
the external
reduction of
surface of
the bimetallic
metal particles of
50 to
the
zeolite
80 Angstroms.
These particles also were too large to fit in the pores of
zeolites.
The metal
was probably on the
zeolite in this case, too.
particles
Initially,
similar in
size
outside of the
The sodium reductions prepared
to
some iron was in the
all of these preparation methods.
the hydrogen
reductions.
cages of the zeolites in
In all cases except for
the reduction by microwave discharge,
the iron metal was
on the outside surface of the zeolite after the reduction.
At some point, the iron migrated
to the surface.
probably due to the high temperatures employed.
This was
This mi-
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
189
gration could have occurred at
tion.
One possibility
migrated
to
reached the
the
is that the metal
surface before
surface it
were formed.
two points in the prepara­
complex or ion
reduction.
was reduced
After
and large
Another possibility is
particles
that the complex or
ion was reduced
in the zeolite
to the surface
and formed large particles.At high temp­
eratures,
and atoms
ions
Perhaps both of
cage and
are very
it
then it migrated
mobile in
these processes occurred to
zeolites.
some extent.
The microwave discharge experiment was unique in preparing
reduced
metal
cages.
Table
particles
20
this research.
percent reduction
that remained
the
zeolite
compares the reduction methods
used in
It contains lists
of the
metal and
in
of the particle sizes,
the time
needed for
each method.
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190
TABLE 20
Comparison of Reduction Methods
Method
Microwave
Hydrogen
Bimetallic
Sodium
Particle Size
<20
70-100
50-80
large
% Reduction
100
100
100a
0
Particle size in Angstroms
Time in hours
a Totally reduced for some samples.
Others
Time
short
2H
H
80% reduced.
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191
4.4.1
Mechanisms of Reduction of Metal Complexes in
Zeolites
The method of reduction by microwaves involves the pro­
duction of a plasma state
plex.
This
plasma is
since microwaves alone
which decomposes the metal com­
very important
did not reduce the
shown by experiments with no
reduction
complexes,
argon gas present.
microwave powers (100 Watts),
posed and the metal complex
The plasma is
to the
as
At high
the zeolite support decom­
formed large metal particles.
the actual reducing agent in
the method of
reduction by microwaves.
The plasma
is a highly
energetic state
ionized argon species and electrons.
ic state usually emits light.
compose the metal complex
consisting of
This highly energet­
The argon plasma could de­
by several different processes.
The argon plasma could penetrate the zeolite and decompose
the metal complex in the pores.
occurs,
This penetration,
if it
is not too deep since the samples must be agitated
to expose fresh
zeolite to the plasma in
pose all of the metal complex.
of penetration arise
Limitations in the degree
from the fact that
collision
the argon plasma
would
lose its
wall.
The size of the argon ion also limits the degree of
penetration.
energy upon
order to decom­
Miller and coworkers
191
with the
zeolite
have noted that ar­
gon species can be an 80 times bigger the ground state ar­
gon atom.
These large argon ions
would not fit into the
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1 92
pores of a zeolite to decompose the metal complex.
Pene­
tration of the electrons in the plasma into the zeolite is
possible but the
energy of the electron is
to its small mass.
Argon is
probably the species in the
plasma that causes the reduction
the metal
cess.
the zeolite,
since
metal.
the
Reduction of
be a
surface pro­
diffuse to the surface of
the experiments were done
pressures(0.3 torr).
face,
to occur.
complex could alternatively
The metal complex could
much less due
at reduced
Once the complex reaches
argon plasma would
the sur­
reduce the complex
to the
The small metal particles could then diffuse back
into the zeolite pores.
The reduction could also be a vapor state process.
metal complexes are
The
volatile species and could
be vapor­
ized under these reduced atmosphere conditions.
The argon
plasma would decompose the complex in the vapor state pro­
ducing the metal.
The metal particles could then condense
into the pores of the zeolite.
close to the
This vapor state could be
surface of the zeolite.
The zeolite might
promote the vaporization of the complex.
The size of the argon ion in the plasma makes the pene­
tration process questionable.
vapor state
reduction are
production of metal.
The
surface reduction and
both possible
The absence
duction causes the metal particles
routes for
the
of heat during the re­
to remain small.
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For
193
this reason, the metal particles, by whatever process they
are formed, are probably located in the pores and cages of
the zeolites.
Heat could be generated by
of the argon ions with
the microwaves or collision
the zeolite.
Vaporization of the
complex could be promoted by this heat.
This vaporization
may be a necessary step before any postulated mechanism of
argon-metal interaction could take place.
Cooling of the
sample by liquid nitrogen during microwave treatment could
explore this concept.
1}.5
COMPARISON TO ZEOLITE CATALYSTS PREPARED BY OTHERS
Table 21 contains a comparison of our results with some
of
the results
of
Stencel and coworkers
similar
73
studies in
the
literature.
prepared cobalt catalysts from the
impregnation of metal nitrate solutions.
The samples were
reduced at 350°C in hydrogen for 24 hours.
The catalysts
70
prepared by Stencel and coworkers
63$
pentane and
formed 24? methane and
hydrocarbons larger
found that cobalt on ZSM-5
than pentane.
We
formed mostly methane and ethgo
ane.
Ozin
zeolite Y by
lysts
and coworkers
prepared
metal atom vaporization.
formed mostly
butenes whereas
iron and
The
cobalt on
cobalt cata­
the iron
formed methane and butenes as shown in Table 21.
on Y catalyst formed mostly methane and ethane.
catalysts
Our iron
Our cata­
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
lysts formed similar products to
*70
workers
those of Stencel and co-
QQ
and Ozin and coworkers
carbon products are smaller.
ple preparations
except that our hydro­
Perhaps differences in sam­
has influenced the samples.
73
lysts of Stencel and coworkers J
The cata-
could have cobalt on the
outside of the zeolite because of the high reduction temp­
eratures.
Perhaps the particle sizes of the metal in the
catalysts of Ozin
samples.
and coworkers
fin
are different
than our
The effect of the zeolite cage on the hydrocar­
bons produced is more evident in our samples than in these
other cases.
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195
TABLE
Comparison of Catalyst
Metal Support
Coa
Coa
Coa
Co
Coc
Fea
Fec
X
ZSM-5
A
ZSM-5
Y
Y
Y
With Literature
ci
C2
C2T
C3
C31
C4
0
35.1
100
24.4
25
50
19
19.5
43.9
0
2.7
0
50
-
22.0
1 .8
0
0
0
0
2
0
19.3
0
2.7
0
-
58.5
0
0
0.8
5
0
9
0
0
0
5.4
70
0
47
a Our results for microwave reduced samples
b From Stencel and coworkers
c From Ozin and coworkers
Fe is sample III
C1 - methane
C2 - ethane
C 2 ’ - ethylene
C3 - propane
C 3 ’ ~ propylene
C4 - butane and butenes
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Chapter V
CONCLUSIONS
5.1
INFORMATION GAINED
This
research shows
small particles
that it
of iron
is
on various
possible to
supports.
prepare
It
also
shows that these samples are selective Fischer-Tropsch ca­
talysts.
The size of the hydrocarbon produced from CO can
be predetermined
pore size.
by choosing a
Small
zeolite with
the correct
metal particles of cobalt
and iron on
zeolites are ferromagnetic species and give characteristic
ferromagnetic
resonance
spectra.
These
highly reactive and are pyrophoric in air.
cobalt particles can be stabilized
such as
nitrogen or in vacuum
The necessary
lines and inert
conditions for
This technique, however,
are
Small iron and
in an inert atmosphere
for long periods
preparation such
atmosphere glove boxes make
dures not easily transferable
particles
of time.
as vacuum
these proce­
to industrial applications.
is novel and surely will find ap­
plication in other areas besides catalysis.
-
196
-
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197
5.2
FUTURE EXPERIMENTS
5.2.1
Catalytic Studies
Important
through some
values
information
about
these
systems
further catalytic studies.
for iron
and cobalt •on silica
higher than on the zeolites.
The
can
be
g apparent
and alumina
were
A comparison of the catalyt­
ic results of the metals on the zeolites versus silica and
alumina might shed
the supports.
some light on the
differences between
Catalytic studies should show the influence
of zeolite cages on the catalysts.
Another experiment
involves the use of
microwaves in­
stead of heat as an activator in the Fischer-Tropsch reac­
tion.
Rather than heat the catalysts to 250°C in a stain­
less steel reactor at one atmosphere of pressure,
perhaps
the reactions could be run at room temperature on a vacuum
line in a
quartz reactor.
the activation energy.
The microwaves
could provide
The products could be trapped and
analyzed by gas chromatography after the reaction.
5.2.2
Characterization Studies
The unknowns in the catalysis studies of the iron zeol­
ites prepared by
to be identified.
the reduction by microwaves
method need
Gas chromatography with mass spectros­
copy detection should help identify the unknown compounds.
The size of the products as
well as the functional groups
attached will give some information about the reaction.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
198
The homogeneity
couple of ways.
of the samples
If enough iron were present,
«O
Electron Mb'ssbauer
the samples.
could be studied
Spectroscopy
a
in a
Conversion
oc
’
could
be done
on
By varying the energy of the gamma rays that
are analyzed,
the depth of penetration or a depth profil­
ing experiment could be done.
of the iron
In this way, the variation
concentration with sample depth
could be ob­
served.
Also,
surface versus bulk metal concentration could be
studied.
Surface concentrations
electron microscopy or Auger
tration
values could
be
could
be obtained
spectroscopy.
obtained
by
Bulk concen­
by atomic
absorption
spectroscopy.
Mossbauer studies
would be possible
of the
microwave discharge
if more iron could be
deposited on the
zeolite or a stronger source could be used.
of
57
samples
Also, the use
Fe instead of common iron would give a stronger Moss­
bauer signal.
Low temperature studies might also provide
useful informa' 4on.
5.2.3
New Preparations
Several new
preparations are suggested by
of this research.
how samples
The
of iron and
the results
bimetallic zeolite studies showed
either cobalt or
duced catalysts that do not
ruthenium pro^-
carbide and do not apparently
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1 99
deactivate.
lyst,
A cobalt-iron or ruthenium-iron zeolite cata­
perhaps
prepared from carbonyls
of the
metals or
even a mixed metal carbonyl cluster, could form such a ca­
talyst .
The success of the reduction by microwaves of ferrocene
experiment opens
samples.
a wide
range of
dation state.
already in the zero oxi­
Iron ions can be reduced to the metal.
reduction by microwaves of
iron dicyclooctatetrene might give
so that small metal particles
some informa­
iron on the outside of a
This molecule is too large
to fit in a zeolite
could possibly be formed on
the outside surface of the zeolite.
Other carbonyl spec­
ies could be studied such as R u ^ C C O ) ^ or W(C0)g.
tion of cobaltocene
The
a large organometallic complex
tion in the catalytic studies of
zeolite.
new
The starting material does not have to be a car­
bonyl species where the metal is
such as
possibilities for
to cobalt metal would
Reduc­
be interesting
to study the similarities with the cobalt catalysts formed
from cobalt carbonyl.
num would be
Reduction of some complex of plati­
interesting since platinum is
a catalyst in
various reactions.
The failure
of the sodium
that needs work.
reductions is
Experiments with
another area
careful pH control to
2+
stabilize
Fe
might produce
and
careful dehydration
metallic species.
of the
supports
Perhaps reactions
in a
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200
closed
system where
the entire
reactor is
in the
oven
might be successful.
Microwave treatment of
lead to
the aluminoferrisilicates might
further reduction past
non-framework iron
the ferrous
might be reduced to
state.
The
metallic species.
Also microwave treatment of the bimetallic zeolite samples
that reduced completely might form metallic iron.
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213
VITA
The author was born in Pittsburgh, Pennsylvania on Feb.
3> 1957 to Mr. and Mrs.
his elementary
Howard T.
school and
McMahon.
high school
He received
education in
Chartiers Valley school district in Bridgeville,
vania.
He
graduated from
Chartiers Valley
the
Pennsyl­
Senior High
School in June 1975.
His undergraduate degree was
received from Geneva Col­
lege in Beaver Falls, Pennsylvania.
lor of
Science in Chemistry
1979.
Presently,
He received a Bache­
with Research Honors
in May
he is attending the University of Con­
necticut and will receive his
Doctor of Philosophy in In­
organic Chemistry in March 1985.
The author
is a member of
American Chemical Society.
Phi Lambda Upsilon
He
and the
has accepted the position
of staff chemist at Union Carbide Corporation, Linde Divi­
sion, in Tonawanda, New York beginning in March 1985.
R eproduced with perm ission o f the copyright owner. F urther reproduction prohibited w itho ut perm ission.
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