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Electrochemical catalysis of styrene epoxidation with films of manganese dioxide nanoparticles, and, Synthesis of mixed metal oxides using ultrasonic nozzle spray and microwaves

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Electrochemical Catalysis o f Styrene Epoxidation with Films o f MnC>2 Nanoparticles,
and Synthesis o f Mixed Metal Oxides Using Ultrasonic Nozzle Spray and Microwaves
Laura Espinal, Ph.D.
University of Connecticut, 2005
Films of polyions and octahedral layered manganese oxide (OL-1) nanoparticles on
carbon electrodes made by layer-by-layer alternate electrostatic adsorption were active
for electrochemical catalysis of styrene epoxidation in solution in the presence of
hydrogen peroxide and oxygen. The highest catalytic turnover was obtained by using
applied voltage -0.6 V vs. SCE, O2 , and 100 mM H2 O2 . 180 isotope labeling experiments
suggested oxygen incorporation from three different sources:
molecular oxygen,
hydrogen peroxide and/or lattice oxygen from OL-1 depending on the potential applied
and the oxygen and hydrogen peroxide concentrations. Oxygen and hydrogen peroxide
activate the OL-1 catalyst for the epoxidation. The pathway for styrene epoxidation in the
highest yields required oxygen, hydrogen peroxide and a reducing voltage, and may
involve an activated oxygen species in the OL-1 matrix.
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Laura Espinal - University of Connecticut, 2005
Multicomponent metal oxide (MMO) crystallites were prepared by spraying a reactant
solution into a receiving solution or air under microwave radiation at atmospheric
pressure.
The injection of nitric acid solution through an ultrasonic nozzle into a
receiving solution of metal precursor and the use of microwave radiation were combined
to form a novel preparation technique called the nozzle-spray/microwave (NMW)
method. The inclusion of an additional step, the in situ mixing of precursor solutions
prior to their injection through the ultrasonic nozzle spray, led to another procedure called
the in sfru/nozzle-spray/microwave (INM) method.
For comparison, MMO materials
with the same metal constituents as those prepared by our novel techniques were
prepared by conventional hydrothermal (CH) methods.
Fresh materials prepared by
NMW, INM and CH methods were heat treated to study the effect of calcination. All
materials were characterized before and after calcination using XRD, SEM, Bet, and ICP.
The NMW method produces particles with rod-like morphologies different from those
obtained using CH methods. The INM method produces an amorphous material that
crystallizes
after calcination into
morphologies.
small
(-200
nm) particles
with interesting
Notably, calcination of materials prepared by both NMW and INM
reduces particle size and increases surface area. The present work paves the way to use
NMW and INM to prepare MMOs with unique morphologies.
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Electrochemical Catalysis o f Styrene Epoxidation with Films o f MnC>2 Nanoparticles,
and Synthesis o f Mixed Metal Oxides Using Ultrasonic Nozzle Spray and Microwaves
Laura Espinal
B.S., Simon Bolivar University, 1999
A Dissertation
Submitted in Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
at the
University of Connecticut
2005
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UMI Number: 3205569
Copyright 2005 by
Espinal, Laura
All rights reserved.
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Copyright by
Laura Espinal
2005
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APPROVAL PAGE
Doctor of Philosophy Dissertation
Electrochemical Catalysis of Styrene Epoxidation with Films of Mn02 Nanoparticles,
and Synthesis of Mixed Metal Oxides Using Ultrasonic Nozzle Spray and Microwaves
Presented by
Laura Espinal, B.S.
Major Advisor
Steven L. Suib
Associate Advisor
James F. Rusling
Associate Advisor
Amy R. Flowed
University of Connecticut
2005
11
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Dedicated with love to
Hugo Espinal, Trina Thielen de Espinal,
Anais Espinal, Diana Espinal,
and Chris Forrey
iii
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ACKNOWLEDGEMENTS
I would like to thank my major advisor, Dr. Steven L. Suib for his support throughout
my graduate studies, Dr. James F. Rusling for his guidance and advice on my research,
and Dr. Amy Howell for considerate advice.
I would also like to acknowledge my colleagues from both Dr. Rusling’s group and Dr
Suib’s group: Abhay Vaze, Liping Zhou, Carmelita Estavillo, Bernard Munge, Maricar
Tarun, Jing Yang, Jackie Yu, Katana Ngala, Jun Nable, Vincent Crisostomo, Ruma Gosh,
Maggie Gulbinska, Weina Li and Amos Mugwero for their cooperation, assistance and
friendship in the past years. Especially thanks to my very good friend Kinga Malinger
who was always very willing to help and be a friend at all times.
Thanks to my friends Morty and Isabel, who treated me like a daughter since I started
my Ph.D. Thank also to my friends Erik, Jimbo, Ian and Tom who were always making
sure we also had fun in graduate school.
Finally, I would like to thank my little sister Anais Espinal and my boyfriend Chris
Forrey for making me laugh all these years at all times and for always trying to make me
a better person.
iv
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TABLE OF CONTENTS
CHAPTER
I.
ELECTROCHEMICAL
CATALYSIS
OF
STYRENE
EPOXIDATION WITH FILMS OF M N 02 NANOPARTICLES
1-1. INTRODUCTION ...............................................................................................
1
1-2. EXPERIMENTAL SECTION
3
Chemicals
...........................................................................................................
Preparation of films
3
...........................................................................................
4
..................................................................................................
5
............................................................................................................
6
Instrumentation
1-3. RESULTS
.........................................................................
Film assembly
.....................................................................................................
6
Cyclic Voltammetry
...........................................................................................
8
Styrene epoxidation
............................................................................................
13
180-labeling studies
............................................................................................
19
.....................................................................................................
21
.................................................................................................
27
.........................................................................................................
28
....................................................................................................
35
1-4. DISCUSSION
1-5. CONCLUSIONS
1-6. APPENDIX
I-7. REFERENCES
CHAPTER II. SYNTHESIS OF MIXED METAL OXIDES USING ULTRASONIC
NOZZLE SPRAY AND MICROWAVES
II-1. INTRODUCTION
...........................................................................................
v
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39
II-2. EXPERIMENTAL SECTION
.......................................................................
43
Chemicals
...........................................................................................................
43
Synthesis
............................................................................................................
43
Characterization
.................................................................................................
48
..........................................................................................................
48
XRD studies
........................................................................................................
49
SEM studies
........................................................................................................
56
II-3. RESULTS
BET surface area measurements
.......................................................................
65
..............................................................................................
67
....................................................................................................
67
...............................................................................................
70
...................................................................................................
70
Elemental analysis
II-4. DISCUSSION
II-5. CONCLUSIONS
II-6. REFERENCES
vi
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LIST OF TABLES
Table 1-1. Oxidation of styrene by OL-1 film at 4° C a for 1 hr..................................
15
Table 1-2. Oxidation of styrene by OL-1 film a at 4°C b using electrolysis in the
presence of 1 8 0 2 and/or H2 1 8 0 2 ......................................................................................
20
Table 1-3. Source o f the oxygen in styrene oxidation by OL-1 film a at 4°C b using
electrolysis in the presence of O2 and/or H2 O2 .............................................................
Table II-1.
26
Metal precursors and synthetic method used to prepare the fresh
materials...........................................................................................................................
46
Table II-2. BET surface area measurements and elemental analysis.........................
66
vii
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LIST OF FIGURES
Figure 1-1. QCM frequency shifts for films assembled from OL-1 nanoparticles
(0.01 M) and PDDA (2 mg/mL) on a bed of PDDA/PSS/PDDA silver resonators.
7
Figure 1-2. Cyclic voltammograms at 0.05 V-s'1 in pH 5.5 buffer for OL-1 film at
different conditions. OL-1 film:
PG-(PSS/PDDA)3/OL-l/(PDDA/OL-l)9, 10
layers of OL-1 in total were assembled onto basal plane pyrolytic graphite
electrodes with geometric area 0.16 cm2, (a) no O2 / no H2 O2 , (b) O2 / no H2 O2
(reduction of oxygen), (c) no 0 2 / 10 mM H20 2, (d) 0 2 /1 0 mM H20 2, (e) no 0 2 /
100 mM H20 2 and (f) 0 2 / 100 mM H20 2.................................................................
9
Figure 1-3. Cyclic voltammograms at 0.05 V-s'1 in pH 5.5 buffer. Polyion film:
PG-(PSS/PDDA)3,
OL-1
film:
PG-(PSS/PDDA)3/OL-l/(PDDA/OL-l)9.
Multilayer films were assembled onto basal plane pyrolytic graphite electrodes
with geometric area 0.16 cm2, (a) polyion film: O2 , (b) polyion film: O 2 /10 mM
H2 O2 , (c) OL-1 film: O2 and (d) OL-1 film: O2 / 10 mM H2 O2 . (High sensitivity
inset on right.)
............................................................................................................
Figure 1-4. Cyclic voltammograms at 0.05 V-s'1 in pH 5.5 buffer for OL-1 film at
different conditions. OL-1 film:
PG-(PSS/PDDA)3/OL-l/(PDDA/OL-l)9, 10
layers of OL-1 in total were assembled onto basal plane pyrolytic graphite
viii
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10
electrodes with geometric area 0.16 cm2, (a) O2 /IOO mM H 2 O2 and (b) O2 /IOO
mM H 2 O2 / saturate styrene...........................................................................................
12
Figure I-Al. Mass spectra of a standard SI 6 0 (styrene oxide with oxygen 16).
28
Figure I-A2. Mass spectra of the styrene oxide obtained from bulk electrolysis
using Mn02 film. Experimental conditions: T = 4°C (controlled with a circulating
water bath), time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl),
constant applied potential o f -0.6 V, 35 mL/min 160 2, no H20 2 was added...............
29
Figure I-A3. Mass spectra of the styrene oxide obtained from bulk electrolysis
using M n02 film. Experimental conditions: T = 4°C (controlled with a circulating
water bath), time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl),
constant applied potential o f-0.6 V, 35 mL/min 160 2, 100 mM H2160 2....................
30
Figure I-A4. Mass spectra of the styrene oxide obtained from bulk electrolysis
using M n02 film. Experimental conditions: T = 4°C (controlled with a circulating
water bath), time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl),
constant applied potential of -0.6 V, 3 mL/min 18C>2 , no H2 O2 was added.................
Figure I-A5. Mass spectra of the styrene oxide obtained from bulk electrolysis
using M n02 film. Experimental conditions: T = 4°C (controlled with a circulating
water bath), time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl),
ix
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31
constant applied potential o f-0.6 V, 35 mL/min 1602, 14 mM H2 18C>2 .
32
Figure I-A6. Mass spectra of the styrene oxide obtained from bulk electrolysis
using MnC>2 film. Experimental conditions: T = 4°C (controlled with a circulating
water bath), time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl),
constant applied potential o f -0.6 V, 3 mL/min
18C>2 ,
100 mM H2 16C> 2 ...........
33
Figure I-A7. Mass spectra of the styrene oxide obtained from bulk electrolysis
using MnC>2 film. Experimental conditions: T = 4°C (controlled with a circulating
water bath), time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl),
constant applied potential o f -0.6 V, 3 mL/min
18C>2 ,
14 mM H2I802.............
34
Figure II-1. Methods involving nozzle-spray and microwaves: (a) NMW, and (b)
INM..................................................................................................................................
42
Figure II-2. XRD patterns of: (a) Fresh-MMO-A and (b) MMO-A.........................
51
Figure II-3. XRD patterns of: (a) Fresh-MMO-B and (b) MMO-B...........................
52
Figure II-4. XRD patterns of: (a) Fresh-MMO-C and (b) MMO-C...........................
54
Figure II-5. XRD patterns of: (a) Fresh-MMO-D and (b) MMO-D..........................
55
x
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Figure II-6.
SEM images of: (a) Fresh-MMO-A and (b) MMO-A.......................
57
Figure II-7.
SEM images of: (a) Fresh-MMO-A' and (b) MMO-A' ....................
59
Figure II-8.
SEM images of: (a) Fresh-MMO-B and (b) MMO-B.......................
61
Figure II-9.
SEM images of: (a) Fresh-MMO-C and (b) MMO-C.......................
62
Figure 11-10. SEM images of: (a) Fresh-MMO-D, and (b) MMO-D.......................
64
xi
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CHAPTER I. ELECTROCHEMICAL CATALYSIS OF STYRENE
EPOXIDATION WITH FILMS OF M N 02 NANOPARTICLES
1-1. INTRODUCTION
Epoxides are important intermediates for the manufacture of a range of modern
commercial products such as epoxy resins,1 textiles,2 surface coatings,1 and biological
chemicals.3 Currently epoxides are prepared from alkenes on an industrial scale using
oxygen, peroxides and peracids. Ethylene oxide is industrially obtained by gas phase
oxidation of ethylene using a supported silver catalyst in oxygen environments whereas
propylene oxide is produced by metal catalysed liquid phase oxidation of propylene using
peroxides.4,5 Many polyoxometalate salts6,7 and oxometallacycles8 have been used for
epoxidation as well as several homogeneous coordination complexes, e.g. porphyrins,9
salens,10 1,4,7-triazacyclononane 11,12 derived catalysts, and iron tetradentate ligand
complexes.13
Catalytic systems based on different metals such as cobalt,14
molybdenum,15"17 vanadium,17 tungsten,18 manganese19,20 rhenium,21 and titanium22 have
also been reported for the epoxidation of a wide range of alkenes.
The existing
epoxidation processes, especially in the case of propylene oxide, are not environmentally
friendly or give rise to significant amounts o f byproducts.4 Thus, new epoxidation
procedures that are safer or more highly selective towards the epoxides would be
industrially attractive.
1
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Manganese oxides have great potential as selective heterogeneous catalysts,
adsorbents, and battery materials.23 They have been used for a wide range of industrial
catalytic applications, e.g. ozone decomposition,24 photocatalytic oxidation of organic
pollutants,
nitric oxide reduction, '
O C O/C
cyclohexane, ’
ethylbenzene,
'X 'l
selective oxidations of carbon monoxide,
ethanol
1 0
q q A{~\
and 2-propanol, ’
decomposition of
hydrogen peroxide,41 hydrogenolysis of cyclopropane and oligomerization of methane.42
Porous manganese oxides are either amorphous or else crystallize as tunnel-structured
materials (octahedral molecular sieves, OMS) or layered materials (octahedral layered
phases, OL).23,43 Correlations between the porosity of these materials and catalytic
activity and selectivity has been found42’44 Recently, proteins have been combined with
octahedral layered (OL-1) manganese oxide nanoparticles to make electroactive films and
macroscopic helixes with catalytic activity.45
Layer-by-layer electrostatic assembly is a method of ultra-thin film growth based on
the alternate adsorption of oppositely charged polyions.46 The key feature of this method
is excessive surface adsorption at every stage of the polycation/polyanion assembly that
leads to recharging of the outermost surface at every step of film formation. The layerby-layer technique can include polyions, DNA, proteins,47 viruses,48 and charged
nanoparticles49 This technique has been suitable for the construction of electronic,
electro-optic, and charge storage devices, sensors, biocompatible films and bioreactors.50
Enzyme/polyions films grown layer-by-layer have been used to catalyze organohalide
reductions as well as epoxidation of styrene derivatives.51,52 Lvov et al. constructed films
of OL-1 nanoparticles with polycations and the protein myoglobin (M b)45a Films made
with nanoparticles involve three-dimensional film architecture with growth perpendicular
2
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to the solid support resulting in a porous structure. Because Mb was electrochemically
and catalytically active in the protein/nanoparticle assemblies, these films could be used
in electrochemical bioreactors.
Preliminary results showed that protein/OL-1
nanoparticle films catalyzed reduction of oxygen to form hydrogen peroxide, which in
turn reacts with the film to form catalytic species that are capable of transferring oxygen
to styrene.
Surprisingly, both OL-1 nanoparticles and myoglobin are active for
electrochemical catalysis of styrene epoxidation and may act by independent pathways.45
In the presence of -100 mM hydrogen peroxide and oxygen, OL-1 nanoparticles in
films (without proteins) are excellent epoxidation catalysts. Synthetic, electrochemical
and lsO labelling results were used to propose a pathway for styrene epoxidation by OL-1
manganese oxide nanoparticles.
1-2. EXPERIMENTAL SECTION
Chemicals
A
colloidal
solution
of
manganese
oxide
(OL-1)
containing
0.01
M
tetramethylammonium (TMA) permanganate was prepared according to published
procedures.23,43,48
Sodium
poly(styrenesulfonate)
(PSS,
MW
70
000),
poly(dimethyldiallylammonium chloride) (PDDA), catalase and superoxide dismutase
were from Aldrich. Hydrogen peroxide (30%) was from J.T. Baker. Water was purified
by Hydro Nanopure system to specific resistance > 15 MQ cm'2. 1802 (99%, lsO) and
H21802 were from Icon Isotopes.
3
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Preparation of films
Alternate adsorption onto solid substrates from aqueous solutions of PDDA (2 mg/mL,
pH 12), PSS (3 mg/mL) and OL-1 nanoparticles (0.01 M, diameter ~ 40 nm) was used as
described previously.45 Immersion time used for the adsorption was 15 min and surfaces
were rinsed with water between adsorption steps to remove weakly adsorbed molecules.
For QCM experiments, silver-coated quartz crystals were pretreated to generate a
silver oxide layer (negatively charged). This pretreatment consisted in sonicating the
silver coated quartz crystals in a mix of KOH/Ethanol/Water (1:60:39) for 30 s and then
rinsing in water for 10 s. A positively charged polyelectrolyte layer such as PDDA was
then adsorbed. Three layers (PDDA/PSS/PDDA) were typically adsorbed as a precursor
film followed by alternate layers of OL-1 and PDDA
For cyclic voltammetry studies, multilayer films of PDDA and OL-1 were assembled
onto basal plane pyrolytic graphite electrodes with geometric area 0.16 cm2 roughened by
abrading on coarse emery paper (3M Crystal Bay).
Three PSS/PDDA layers were
adsorbed before the first layer of OL-1 was deposited to form a smooth bed for OL-1
adsorption. After the first layer of OL-1 was adsorbed, nine PDDA/OL-1 layers were
adsorbed to form a total of 10 layers of OL-1.
For catalytic oxidation of styrene, OL-1 films were prepared using a similar procedure
in which three PSS/PDDA layers were adsorbed on both sides of 1.5 x 6 cm carbon cloth
prior to OL-1 layer. Only one layer of OL-1 was used for the oxidation experiments to
minimize mass transport limitations.
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Instrumentation
Quartz Crystal Microbalance
A quartz crystal microbalance (QCM, USI System, Japan) was used to monitor the
layer-by-layer growth.
Frequency shifts were measured after carefully washing and
drying the quartz crystal after each step of adsorption. The Sauerbrey equation gives a
relationship between the frequency shift (AF) and the mass change in the absence of
viscolelasticity differences.46 The film mass per unit area M/A (g/cm2) on a resonator of
A = 0.16 ± 0.01 cm2, is given by Eq 1, taking into account characteristics of the 9 MHz
quartz resonators. The nominal thickness of a dry film may be estimated taking into
account the film density53 by Eq 2 which relates frequency shift (AF) and film thickness
(d) of adsorbed film on both sides of the electrode. X-ray reflectivity measurements and
scanning electron microscopy of cross-section images were used to confirm these
calculations. 45a’46
M
AF (Hz)
— (g/cm ) = ------- -— —T
A
-1.83 x 108
(1)
d (nm)--(0 .0 1 6 +0.002) AF (Hz)
(2)
Electrochemistry
Voltammetry was done as described previously453 at 22 ± 2°C.
The cell for
electrolysis employed a saturated calomel reference electrode (SCE), a carbon rod
counter electrode, and a carbon cloth (National Electrical Carbon Corp., 1.5 x 6 cm)
working electrode. The counter electrode was separated from the reaction solution by a
5
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saturated KC1 agar bridge. Electrochemical oxidation of styrene was carried out in 10mL Tris buffer pH 7.4 saturated with styrene (~ 10 mM), usually at -0.60 V vs. SCE.
The cell was saturated with oxygen and temperature was controlled at 4 °C to minimize
loss of reactants and products under gas input conditions. Samples were extracted after 1
h of reaction with hexane and analyzed by GC by a previously described method.51
1-3. RESULTS
Film assembly
Film growth was monitored by frequency changes measured with a quartz crystal
microbalance (QCM). The first three steps represent a polyion bed of PDDA/PSS/PDDA
on the quartz crystal. Subsequent steps represent (OL-l/PDDA)n layers (Figure 1-1).
The data for the (OL-l/PDDA)n films suggest a reproducible film growth process. AF
was 1500 Hz for OL-1 adsorption steps, with a smaller step of 650 Hz for each PDDA
layer. The growth of OL-1/PDDA films proceeded with a regular increase in mass in
which each layer of OL-1 added 8.3 pg/cm2 and PDDA added 3.6 pg/cm2, based on Eq 1.
The corresponding nominal average thicknesses (from Eq 2) for the OL-1 and PDDA
layers were 24±3 nm and 10±1 nm, respectively.
These results demonstrate the
reproducibility of layer and film formation, and are similar to those reported
previously.45a
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Adsorption Cycle
Figure 1-1. QCM frequency shifts for films assembled from OL-1 nanoparticles (0.01 M)
and PDDA (2 mg/mL) on a bed of PDDA/PSS/PDDA silver resonators. Silver-coated
quartz crystals were pretreated, as explained in experimental section, to generate a
negatively charged layer on top of which PDDA was adsorbed.
Three layers
(PDDA/PSS/PDDA) were adsorbed as a precursor film, followed by alternate layers of
OL-1 and PDDA. Frequency shifts were measured after carefully washing and drying the
quartz crystal after each step of adsorption. (Negative shift is upward).
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Cyclic Voltammetry
OL-1/PDDA films were evaluated by cyclic voltammetry in buffers of 20 mM acetate
(pH 5.5).
Cyclic voltammograms of 10-layer OL-1 films in the absence of oxygen
showed no electroactivity in the 0 to -0.7 V potential range (Figure I-2-a). However, the
films showed a small increase in current in the presence of O 2 at -0.6 V vs. SCE (Figure
1-2-b) due to the reduction of oxygen. These films also showed a large reduction peak in
the presence of H 2 O2 at potentials of -0.35 V vs. SCE (Figures I-2-c, I-2-d, I-2-e and I2-f). Figure 1-3 shows that control polyion films without OL-1 gave no evidence for
reduction of O 2 or H2 O2 . Results indicate that both OL-1 and H2 O2 have to be present for
the reduction reactions to occur, and the data suggest the electrochemical catalytic
reduction of H2 O 2 by OL-1. Similar results were obtained at pH 7.4.
8
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1400
1200
1000
800
600
400
200
0
-200
0
-200
-400
-600
E, mV vs SCE
-800
Figure 1-2. Cyclic voltammograms at 0.05 V s'1 in pH 5.5 buffer for OL-1 film at
different conditions. OL-1 film: PG-(PSS/PDDA)3/OL-l/(PDDA/OL-l)9, 10 layers of
OL-1 in total were assembled onto basal plane pyrolytic graphite electrodes with
geometric area 0.16 cm2, (a) no O 2 / no H 2 O2 , (b) O 2 / no H2 O 2 (reduction of oxygen), (c)
no 0 2 / 10 mM H20 2, (d) 0 2 / 10 mM H20 2, (e) no 0 2 / 100 mM H20 2 and (f) 0 2 / 100
mM H20 2.
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600
100,
500
400
50
■5j_ 300
ys,
200
100
-100
200
0
-200
-400
-600
-800
-100 -150 -200 -250 -300 -350 -400 -450 -500
E, mV vs SCE
E, mV vs SCE
Figure 1-3. Cyclic voltammograms at 0.05 V s"1 in pH 5.5 buffer. Polyion film: PG(PSS/PDDA)3, OL-1 film:
PG-(PSS/PDDA)3/OL-l/(PDDA/OL-l)9. Multilayer films
were assembled onto basal plane pyrolytic graphite electrodes with geometric area 0.16
cm2, (a) polyion film: O2 , (b) polyion film: O 2 /10 mM H2 O2 , (c) OL-1 film: O 2 and (d)
OL-1 film: O2 / 10 mM H2 O2 . (High sensitivity inset on right.)
10
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Figure 1-4 shows cyclic voltammograms of OL-1 films under O 2 and 100 mM H 2 O 2
with and without styrene. The presence of styrene does not seem to affect the behavior of
these films in cyclic voltammetry. Cyclic voltammetry only provides information about
the capability of these films to reduce oxygen and H 2 O2 . Thus, to study the epoxidation
of styrene, electrolysis and product analysis were employed.52
11
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800
600
400
1
200
-200
200
0
-200
-400
-600
-800
E, mV vs SCE
Figure 1-4. Cyclic voltammograms at 0.05 V s'1 in pH 5.5 buffer for OL-1 film at
different conditions. OL-1 film: PG-(PSS/PDDA)3/OL-l/(PDDA/OL-l)9, 10 layers of
OL-1 in total were assembled onto basal plane pyrolytic graphite electrodes with
geometric area 0.16 cm2, (a) O2 /IOO mM H 2 O2 and (b) O 2 /IOO mM H2 O 2 / saturate
styrene.
12
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Styrene epoxidation
Electrolysis experiments were done at potentials where the film reduces oxygen and
hydrogen peroxide. It appears that this process produced activated OL-1, which then
epoxidizes styrene to styrene oxide. Products were measured by gas chromatography.
Chemical reactions with no applied voltage were also studied. Results from incubation of
styrene with OL-1 films, at conditions appropriate for epoxidation, yielded styrene oxide
and benzaldehyde as major products.
Previous work using iron heme proteins as
catalysts reported non-catalytic conversion of styrene to benzaldehyde by hydrogen
peroxide.51
Table 1-1 summarizes results from the epoxidation of styrene under different
conditions. In electrolysis experiments, at a constant applied potential of -0.6 V vs. SCE,
OL-1 in the presence of O 2 produced 0.09 pmoles of styrene oxide (Table 1-1, entry 1),
which indicates that OL-1 is able to oxidize styrene. The applied voltage is on the plateau
of the catalytic H 2 O2 reduction wave in CV. The 6 mM concentration of H2 O2 found at
the end of the reaction showed that the systems reduced O2 to H2 O2 . Other electrolysis
experiments were performed with OL-1 films in the absence of O2 to prevent H 2 O 2
formation (Table 1-1, entry 2).
These experiments gave virtually no styrene oxide
indicating that H 2 O 2 is required for the epoxidation of styrene by OL-1. This result was
confirmed with electrolysis experiments in which the enzyme catalase was added to
rapidly destroy the H 2 O 2 that was formed (Table 1-1, entry 3).51 Results from these
experiments gave no styrene oxide, showing once again that H 2 O2 is necessary for the
epoxidation. Electrolysis in the absence of OL-1 (PDDA film) under oxygen conditions
(Table 1-1, entry 4) produced 1 mM hydrogen peroxide but gave negligible amounts of
13
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styrene oxide supporting the important catalytic role that OL-1 plays in the epoxidation
of styrene. Electrolysis experiments performed without O2 and in the absence of OL-1
resulted in no products.
14
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 1-1. Oxidation of styrene by OL-1 film at 4° C a for 1 hr.
Entry
Th
02b
Film
J-'applied c
[ H 20 2]i d
(V vs SCE)
added,
soe
Be
Turnover
[ H 20 2] f 8
Rate f
found
(h-1)
(mM)
(pmol) (pmol)
(mM)
1
O L-lh
Yes
-0.6
0
0.09
0.06
0.05
6.0
2
OL-1
No
-0.6
0
0
0.01
0
0
3
OL-1
Yes
-0.6
0
0
0.01
0
0
(+ cat.)1
4
PDDA’
Yes
-0.6
0
0.01
0.01
-
1.0
5
OL-1
Yes
-
100
0.11
0.07
0.06
100
6
OL-1
No
-
100
0.07
0.03
0.04
100
a All reactions were in pH 7.4 buffer (50 mM Tris + 50 mM NaCl) saturated with styrene
(~ 10 mM). Time of reaction: 1 hour. Reproducibility was ± 20%. Electrode surface
area used was 9 cm2 (1.5 x 6 cm). b Yes/No: presence/absence of oxygen during the
reaction. c
E appiied:
constant potential applied. d
[ H 20 2]i:
Initial hydrogen peroxide
concentration used. e GC-FID was used for the product analysis. f Turnover Rate ( h 1) =
(moles of product)(moles of catalyst)'1(time)'1. 8
[ H 20 2]f:
Final hydrogen peroxide
concentration determined with an error margin of ± 15% using Quantofix Peroxide 100
test sticks (Macherey-Nagel GmbH & Co., Germany). h OL-1: film with OL-1 as outer
layer deposited onto polyion bed, (PSS/PDDA)3-OL-l. 1 3000 units catalase added. '
PDDA:
polyion fdm with PDDA as outer layer, (PSS/PDDA)3.
k 3000 units of
superoxide dismutase added.
15
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Table 1-1. Continued.
Entry
Film
!h
o2b
J-'applied
c
[ H 20 2]i d
(V vs SCE)
added,
so"
Be
Turnover
[ H 20 2] f 8
Rate f
found
( h 1)
(mM)
(pmol) (pmol)
(mM)
7
PDDA
Yes
-
100
0.01
0.01
-
100
8
OL-1
Yes
-0.6
100
5.20
0.05
3.0
100
9
OL-1
No
-0.6
100
0.39
0.05
0.2
100
10
OL-1
Yes
-0.6
100
5.22
0.05
3.0
100
(+ SOD)k
11
OL-1
Yes
-0.45
0
0
0.01
0
1.6
12
OL-1
No
-0.45
100
0.36
0.01
0.2
100
13
OL-1
Yes
-0.45
100
1.46
0.19
0.9
100
a All reactions were in pH 7.4 buffer (50 mM Tris + 50 mM NaCl) saturated with styrene
(~ 10 mM). Time of reaction: 1 hour. Reproducibility was ± 20%. Electrode surface
area used was 9 cm2 (1.5 x 6 cm). b Yes/No: presence/absence of oxygen during the
reaction. c Eappiied: constant potential applied. d [H202]i: Initial hydrogen peroxide
concentration used. e GC-FID was used for the product analysis. f Turnover Rate ( h 1) =
(moles of product)(moles of catalyst)'1(time)"1. 8 [H20 2]r:
Final hydrogen peroxide
concentration determined with an error margin of ± 15% using Quantofix Peroxide 100
test sticks (Macherey-Nagel GmbH & Co., Germany). h OL-1: film with OL-1 as outer
layer deposited onto polyion bed, (PSS/PDDA)3 -OL-l. 1 3000 units catalase added. '
PDDA:
polyion film with PDDA as outer layer, (PSS/PDDA)3 .
k 3000 units of
superoxide dismutase added.
16
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In electrolysis, oxygen is reduced at the electrode to form H 2 O2 , which in turn may
react chemically/electrochemically with OL-1 to form an active species that transfers
oxygen to the olefinic bond of styrene. Since hydrogen peroxide is essential for styrene
oxide formation, experiments were also done after adding additional hydrogen peroxide
without any potential applied.
In these chemical reactions,
concentration also played an important role.
Reactions
hydrogen peroxide
with added
amounts of 10 mM
and 20 mM H2 O2 produced no styrene oxide. Results from chemical reactions in which
100 mM H20 2 was directly added to the reaction mixture are shown in Table 1-1 (entries
5-7). The amounts of styrene oxide produced in these experiments were 0.11 pmoles of
styrene oxide in the presence of oxygen (Table 1-1, entry 5) and smaller amounts (0.07
pmoles) in the absence of oxygen (Table 1-1, entry 6). These results confirmed that
hydrogen peroxide is an essential component for the activation of OL-1, and can provide
turnover even in the absence of electrolysis.
Chemical reactions without OL-1 (PDDA
film), in the presence of O2 gave no styrene oxide (Table 1-1, entry 7).
Control
experiments with OL-1 film in the absence of both O 2 and H 2 O2 gave no styrene oxide.
Electrolysis combined with 100 mM H2 O 2 for OL-1 films in oxygen environments,
gave dramatically improved turnover (Table 1-1, entry 8). The amount of styrene oxide
obtained from this experiment was 5.2 pmoles of styrene oxide. In the absence of O 2 ,
0.39 pmoles were found (Table 1-1, entry 9). Thus, both O2 and H 2 O2 play an important
role in the epoxidation process on OL-1 -coated electrodes.
It is possible that oxidant species such as superoxide anions could be formed during
this reaction. However, the addition of superoxide dismutase, an enzyme that destroys
17
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superoxide anion,54 did not affect the yields typically obtained (-5.2 pmoles) under the
conditions used (Table 1- 1, entry 10).
OL-1 films showed a steady state catalytic reduction wave at E 1/2 of -0.35 V vs. SCE
in cyclic voltammetry in the presence of H 2 O 2 (Figure I-2-c), which increased in the
presence of oxygen and at larger H2 O2 concentrations (Figures I-2-d, I-2-e and I-2-f).
This electron transfer process was not observed when electrodes coated only with
polyions were used, which suggests that this electron transfer reaction may be involved in
the epoxidation of styrene since the control polyion electrode without OL-1 does not
produce styrene oxide (Table 1-1, entries 4 and 7). Thus, it was important to check if the
reduction peak observed in CV from Figure 1-2 (at -0.35 V vs. SCE) was due to a
reaction involved in the epoxidation mechanism since films without OL-1 did not show
this peak under the same conditions.
Electrolyses with OL-1 films at -0.45 V vs. SCE in the presence of O2 but with no
H2 O2 added, resulted in minimal products (Table 1-1, entry 11).
In addition, the
concentration of H 2 O 2 measured at the end of the electrolysis at -0.45 V vs. SCE (1.6
mM) was smaller compared to that obtained at -0.6 V vs. SCE (6 mM). The amounts of
styrene oxide obtained were negligible at -0.45 V vs. SCE and 0.09 pmoles at -0.6 V vs.
SCE. Electrolysis experiments were also performed at -0.45 V vs. SCE in the presence of
100 mM H 2 O 2 and in the absence of O 2 produced 0.36 pmoles of styrene oxide (Table I1, entry 12), which is comparable with the styrene oxide amount formed in electrolysis at
-0.6 V vs. SCE at the same conditions. For electrolysis at -0.45 V vs. SCE in the
presence of both H 2 O2 (100 mM) and O 2 (Table 1-1, entry 13), the amount of styrene
oxide obtained was almost thirteen times larger (1.46 pmoles) than that obtained from
18
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chemical reactions (0.11 pmoles). Electrolyses at -0.6 V vs. SCE at the same conditions
gave 50 times more styrene oxide (5.2 pmoles) compared to the amounts obtained in
chemical reactions (0.11 jimoles).
These results indicate that the electron transfer
reaction observed by CV at -0.35 V vs. SCE is part of the epoxidation mechanism, and is
definitively voltage dependent. The data obtained from synthetic experiments at lower
applied potentials also confirm that the epoxidation reaction is dependent on the oxygen
and hydrogen peroxide concentrations.
180-labeling studies
Electrolyses were also done with 18C>2 and H2 18C>2 to understand the role of molecular
oxygen, OL-1 and H 2 O2 and the source of oxygen in the product. Table 1-2 shows the
percentage of S160 and S180 obtained for all these experiments by GC-MS. Results
show that 24% of lsO is incorporated when 1802 is used and no H 2 O 2 added. This 1802
incorporation increases to 66% when labeled H2180 2 is used and 1602 is used.
incorporation reaches almost 100% when both H 2 1 8 0 2 and
180 2
I80
are used, which suggest
that oxygen is not coming from the OL-1 lattice under these particular experimental
conditions (14 mM H21802 and 1802 flow rate of 3 mL/min). On the other hand, there
was no lsO incorporation when 140 mM H21602 was used even though 1802 was bubbled
into the reaction solution at 3 mL/min. Results from these labeling experiments confirm
the important contribution of H 2 O2 in the mechanism of epoxidation by OL-1. Amounts
of styrene oxide produced confirm the reaction rate dependence on the 0 2 and H 2 O 2 .
19
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Table 1-2. Oxidation of styrene by OL-1 film a at 4°C b using electrolysis in the presence
of 1 80 2 and/or H 2 180 2 .
Experimental
[H2 O2 ] d
0 2 flow rate
si6<y
s18o*
Total SO g
conditionsc
(mM)
(mL/min)
(%)
(%)
(pmol)
BE e + 180 2
0
3
76
24
0.06
BE e + H2180 2 + 160 2
14
35
34
66
1.15
BE e + H2180 2 + 180 2
14
3
0
100
0.53
BE e + H2160 2 + 180 2
140
3
100
0
6.45
a OL-1: film with OL-1 as outer layer deposited onto polyion bed, (PSS/PDDA)3 -OL-l.
b Temperature was controlled with a circulating water bath. c All reactions were in pH
7.4 buffer (50 mM Tris + 50 mM NaCl). Time of reaction: 1 hour. All results were
averaged for 3 or more reactions in 10 mL of solution. Reproducibility was ± 20%. d
[H2 O2 ]:
Initial hydrogen peroxide concentration used.
e BE: bulk electrolysis
experiments performed at a constant applied potential of -0.6 V. f GC-FID was used for
the product analysis.
g Total SO:
amount of S160 + SlsO produced.
The average
retention time was 6.54 min for styrene oxide and 4.67 min for benzaldehyde. GC-MS
was used for the detection of labeled and non-labeled styrene oxide. Percentages of
products were obtained from calculating the ratios between m/z = 119 and m/z =121
peaks from the mass spectra which correspond to the ratios between SieO and SlsO
respectively.
20
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1-4. DISCUSSION
QCM results (Figure 1-1) showed that the layer-by-layer method was successful for
making stable films of OL-1 nanoparticles in alternation with PDDA.
Lvov found
similar OL-LPDDA ratio (2.1-2.4) in thickness for these nanoparticles-polyion
assemblies, and obtained a bilayer thickness of 27 nm, 18.4 nm for OL-1 and 8.6 nm for
PDDA layers, which gives a OL-LPDDA ratio of 2.1. In the presdent research, a bilayer
thickness of 34 nm was obtained, 24 nm for OL-1 and 10 nm for PDDA layers, which
gives a OL-LPDDA ratio of 2.4.
CV showed that OL-1 films reduce oxygen to hydrogen peroxide, which catalyzes the
epoxidation of styrene-to-styrene oxide. OL-1 films produced hydrogen peroxide as well
as styrene oxide (Table 1-1).
Oxygen was reduced to hydrogen peroxide, a known
reaction for manganese oxides,55 at a potential of -0.6 V vs. SCE (Figure I-2-b). In the
presence of H 2 O 2 , OL-1 films showed a reduction wave at -0.35 V vs. SCE (Figure 1-2c), which could be due to the reduction of an activated OL-1 species from the chemical
reaction between H2 O2 and OL-1.
Manganese oxides particles undergo similar
reactions.41 Once the activated manganese oxide is reduced electrolytically, oxygen
seems to play a very important role in regenerating active sites. Scheme 1-1 shows a
representative pathway of all the reactions involved in this catalytic process: the electrode
reduces both O2 5 5 and activated OL-1 (O OL-1), H 2 O2 activates OL-1 as well as produces
O 2 from its decomposition on OL-1 41 and O2 forms H2 O2 under electrolytic conditions55
as well as regenerate the active sites56 after activated OL-1 is reduced.
The overall
process results in reduction of H2 O 2 to H 2 O.
21
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From electrolysis in the presence of oxygen (Table 1-1, entry 1), 0.09 pmoles of
styrene oxide were produced, which indicate that the generated hydrogen peroxide (6
mM) assisted OL-1 in the oxidation of styrene at relatively small turnover rate. This
result was confirmed when catalase was added to destroy H 2 O2 in a similar experiment
and gave no products (Table 1-1, entry 3). Results from chemical reactions or direct
addition of H2 O 2 (100 mM) with no potential applied gave 0.11 pmoles of styrene oxide
(Table 1-1, entry 5), which proves that OL-1 can be activated by H2 O 2 alone to produce
styrene oxide.
22
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Formation of H20 2 (1)
Regeneration of active sites (5)
Creation of active sites (2)
Formation of Oz (3)
1.
0 2 + 2H+ + 2e' -> H20 2 □ f l
2.
OL-1 + H20 2
3.
OL-1 + H20 2 -> 1/20 2 + OL-1 + H20
4.
O OL-1 + 1e + ArCH=CH2 -» ArCHOCH2 + *OL-1 ■
5.
2 *OL-1 + 0 2 ^ 2 O OL-1 □
OOL-1 + H20 □
□
Scheme 1-1. Suggested pathway for electrochemical styrene epoxidation by OL-1. *OL1 stands for OL-1 with vacant active sites and O OL-1 for OL-1 with active oxidant sites.
Colors beside steps in the pathway correlate to the items in the colored-coded circles.
23
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Addition of H 2 O2 to electrolysis mixtures in the presence of oxygen greatly improved
the yields (Table 1-1, entry 8), which indicates that epoxidation efficiency depends
critically on the H 2 O2 concentration. Similar trends were found when a potential of -0.45
V vs. SCE was used (Table 1-1, entries 11-13). The yields for styrene oxide increased at
this potential when H 2 O2 was added. However, these increments were lower at -0.45 V
vs. SCE compared to equivalent experiments at -0.6 V vs. SCE, suggesting that the rate
of epoxidation depends strongly on the applied potential as well. Electrolyses performed
in oxygen environments in the presence of different added amounts of H2 O 2 : 0 (zero), 14
and 100 mM H2 O 2 gave 0.09 (Table 1-1, entry 1), 1.15 (Table 1-2) and 5.20 (Table 1-1,
entry 8) pmoles of styrene oxide, respectively. These results confirm that the rate of
epoxidation is H 2 O 2 concentration dependent. Analogously, the O2 concentration or O 2
flow rate also affects the rate of epoxidation. Electrolyses in the presence of 14 mM
H 2 O2 at different 0 2 flow rates: 3 and 35 mL/min gave 0.53 and 1.15 pmoles of styrene
oxide, respectively (Table 1-2). Furthermore, as mentioned previously, the absence of
oxygen always resulted in lower amounts of styrene oxide produced. Thus, the rate of
epoxidation is O2 concentration dependent as well.
In general, results agree with the pathway shown in Scheme 1-1: H 2 O 2 needs to
activate OL-141 (step 2) and then O OL-1 is reduced when electrical potential is applied
(step 4), transferring oxygen to styrene producing a manganese oxide with a vacant active
site represented as *OL-1 which will then be regenerated to O OL-1 by O 2 . Step 4
represents the key oxygen transfer to the olefinic bond. The equation as written reflects
the role of catalytic electrochemical activation and regeneration of the active site on the
24
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
catalyst, although the details of this process are still under investigation. Scheme 1-1 also
shows other roles of O 2 , H 2 O2 and potential that indirectly participate in the reaction: O2
can form H2 O2 electrochemically (step 1); H2 O2 can form O 2 after reacting with OL-1
(step 3).
Superoxide anion is ruled out as an oxidant in the epoxidation because the addition of
superoxide dismutase did not influence the amounts of styrene oxide produced (Table I1, entry 10). Other oxidant species such as radicals, peroxy radicals, singlet oxygen or
peroxy anion could possibly be involved. Further experiments using scavengers of these
species would be necessary to elucidate their role, if any. Data from the present work
suggest that at least one of the oxidant species comes from H2 O 2 , but this may also
involve the Mn0 2 nanoparticles.
The precedence of the oxygen transferred to styrene was determined from
electrolysis experiments using 1802 and/or H21802.
Results indicate that the oxygen
transferred to styrene comes from different sources: molecular oxygen, hydrogen
peroxide and/or lattice oxygen from OL-156 depending on the experimental conditions:
oxygen flow rate, hydrogen peroxide concentration and constant potential applied (Table
1-3). When low O2 flow rates are used in the absence of H2 O 2 , the oxygen transferred to
styrene comes from molecular oxygen as well as from the OL-1 lattice.57,58 On the other
hand, when 0 2 is used in the presence of H 2 O2 , there are two tendencies: (1) if high
concentrations of H 2 O2 are used in presence of O 2 , most of the oxygen transferred comes
from H20 2 and (2) if low concentrations of H20 2 are used, the oxygen transferred comes
from both 0 2 and H20 2. Interestingly, oxygen does not come from the OL-1 lattice when
there are both 0 2 and H20 2 present.
25
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Table 1-3. Source of the oxygen in styrene oxidation by OL-1 film a at 4°C b using
electrolysis in the presence of O 2 and/or H2 O2 .
Experimental Conditionsc
O-incorporation (%)
h 2o 2
o2
Lattice-O
3
0
24
76
14
35
66
34
0
140
3
-100
0
0
[H20 2] d
0 2 flow rate
(mM)
(mL/min)
0
a OL-1: film with OL-1 as outer layer deposited onto polyion bed, (PSS/PDDA)3 -OL-l.
b Temperature was controlled with a circulating water bath. c All reactions were in pH
7.4 buffer (50 mM Tris + 50 mM NaCl). Time of reaction: 1 hour. Potential applied: 0.6 V vs. SCE. All results were averaged for 3 or more reactions in 10 mL of solution.
Reproducibility was ± 20%. d [H2 O 2 ]: Initial hydrogen peroxide concentration used. g
Data in this table was taken from data shown in Table 1-2.
26
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1-5. CONCLUSIONS
OL-1 nanoparticle/polyion films on carbon electrodes electrochemically catalyze
styrene epoxidation. The reaction pathway involves H 2 O2 , O2 and a reducing voltage.
Oxygen incorporation to styrene takes place from three possible sources: molecular
oxygen, hydrogen peroxide and/or lattice oxygen from OL-1 depending on the constant
potential applied and on the oxygen and hydrogen peroxide concentrations. A key step
involves the activation of the catalyst by hydrogen peroxide. In this respect, the reaction
has some similarity to hydrogen peroxide-activated oxidations catalyzed by iron heme
enzymes such as peroxidases or cyt P450s.59
27
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1-6. APPENDIX: Mass spectra of products from styrene electrolyses done with 180 2 and
h 218o 2
Abundance
7000
Scan
434
(6.565
m in):
[BSB1 ] LE - 1 4 . 'J
91
6000
500C
4000
3000
1000
61
1000
0
m /z-->
78
49
■
40
50
7 4
■
,
. i .....................
60
70
80
90
1 00
110
120
Figure I-A l. Mass spectra of a standard S160 (styrene oxide with oxygen 16).
28
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Abundance
Scan
421
16.705
14000 :
mini :
[BSR1 ! r.K 1 9 . P
91
12000
10000
8000
; ;
eooo
4000
49
2000
i
C
K d f z - - >
6 b
. ■
40
50
............................
60
70
80
90
■
100
110
120
Figure I-A2. Mass spectra of the styrene oxide obtained from bulk electrolysis using
MnC>2 fdm. Experimental conditions: T = 4°C (controlled with a circulating water bath),
time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl), constant applied
potential of -0.6 V, 35 mL/min 16C>2 , no H 2 O2 was added.
29
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Abundance
3 tt n o
9R4
11
uc,
min; • LE-i-iocn
89
n
0
M /2
•>
30
40
SO
SO
70
30
90
LOO
110
120
Figure I-A3. Mass spectra of the styrene oxide obtained from bulk electrolysis using
MnC>2 film. Experimental conditions: T = 4°C (controlled with a circulating water bath),
time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl), constant applied
potential of -0.6 V, 35 mL/min 16C>2 , 100 mM H216C>2 .
30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Abundance
Scan
439
(6.726
m in):
89
[BSB1]LE-21.D
5000
4500
4000
5 50 0
3000
2500
105
2000
120
o
1500
83
1000
500
65
98
61
68
0
m /z - >
60
70
30
90
100
110
Figure I-A4. Mass spectra of the styrene oxide obtained from bulk electrolysis using
MnC>2 film. Experimental conditions: T = 4°C (controlled with a circulating water bath),
time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl), constant applied
potential of -0.6 V, 3 mL/min l80 2, no H2 O2 was added.
31
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Abundance
Average
of
7.318
to
7.3-12
min
f l OOn
■ LE-H202B
H
i
+. *}
91
0
O
n
M/Z
> 2 0
*3 1
3 0
40
50
50
70
90
90
100
110
120
Figure I-A5. Mass spectra of the styrene oxide obtained from bulk electrolysis using
MnC>2 film. Experimental conditions: T = 4°C (controlled with a circulating water bath),
time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl), constant applied
potential o f -0.6 V, 35 mL/min 16C>2 , 14 mM
32
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Abundance
scan
9S2
(7 323 m i n ) :
L E 1 4 0 2 ML . D
89
6000 :
'>000
1 L 'J
3000 I
i
nn n ■
^
51
"
!
i04
1000
o
M/Z
-
20
30
40
50
60
70
80
90
100
110
120
Figure I-A6. Mass spectra of the styrene oxide obtained from bulk electrolysis using
MnC>2 film. Experimental conditions: T = 4°C (controlled with a circulating water bath),
time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl), constant applied
potential of -0.6 V, 3 mL/min l8C>2 , 100 mM H 2 16C>2 .
33
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Abundance
S c a n 953
(7.327
m i n ) : LE1818.D
91
6000
4000
©
J000
o
63
2000
39
1000
0
M/2
>
; •
30
40
50
60
70
80
90
100 _
110
120
130
Figure I-A7. Mass spectra of the styrene oxide obtained from bulk electrolysis using
MnC>2 fdm. Experimental conditions: T = 4°C (controlled with a circulating water bath),
time of reaction: 1 hour, pH 7.4 buffer (50 mM Tris + 50 mM NaCl), constant applied
potential o f -0.6 V, 3 mL/min l80 2, 14 mM H2I802.
34
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1-7. REFERENCES
(1) Paul, S. In Surface Coatings, John Wiley & Sons Ltd: Chichester, 1996; pp 243-269.
(2) Ito, H.; Sasaki, H.; Tsuji, M.; Kohjiya, S. Textile Res. J. 1999, 6 9 ,473-476.
(3) Nakajima, H.; Takase, S.; Terano, H.; Tanaka, H. J. Antibiot. 1997, 50, 96-99.
(4) Monnier, J. R. Appl. Catal. A 2001, 221, 73-91.
(5) Zuwei, X.; Ning, Z.; Yu, S.; Kunlan, L. Science 2001, 292, 1139-1141.
(6) Grigoropoulou, G.; Clark, J. H., Elings, J. A. Green Chem. 2003, 5, 1-7.
(7) Bosing, M/.; Noh, A.; Loose, I.; Krebs, B. J. Am. Chem. Soc. 1998,120, 7252-7259.
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11946-11954.
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(20) Lane, B. S.; Burgess, K. J. Am. Chem. Soc. 2001,123, 2933-2934.
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705, 5404-5410.
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219-230.
(28) Kijlstra, W. S.; Brands, D. S.; Poels, E. K.; Bliek, A. J. Catal. 1997,171, 208-218.
(29) Yamashita, T.; Vannice, A. J. Catal. 1996,163, 158-168.
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E. K.; Bliek, A. Appl. Catal. B 1996, 7, 337-357.
(31) Smirniotis, P. G.; Pena, D. A.; Uphade, B. S. Angew. Chem., Int. Ed. 2001, 40, 24792482.
(32) Chen, L.; Horiuchi, T.; Mori, T. Appl. Catal. A 2001, 209, 97-105.
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91-105.
(35) Xia, G. G.; W ang, J. Y.; Yin, Y. G.; Suib, S. L. In C atalysis o f O rganic Reactions-,
Herkes, F.E., Ed.; Marcel Dekker: New York, 1998; pp 615-620.
(36) Wang, J. Y.; Xia, G. G.; Yin, Y. G.; Suib, S. L. In Catalysis o f Organic Reactions-,
Herkes, F.E., Ed.; Marcel Dekker: New York, 1998; pp 621-626.
(37) Vileno, E.; Zhou, H.; Zhang, Q.; Suib, S. L.; Corbin, D. R.; Koch, T. A. J. Catal.
1999,187, 285-297.
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(38) Zhou, H.; Wang, J. Y.; Chen, X.; O'Young, C. L.; Suib, S. L. Microporous
Mesoporous Mater. 1998, 21, 315-324.
(39) Chen, X.; Shen, Y. F.; Suib, S. L.; O'Young, C. L. J. Catal. 2001,197, 292-302.
(40) Cao, H.; Suib, S. L. J. Am. Chem. Soc. 1994,116, 5334-5342.
(41) Zhou, H.; Shen, Y. F.; Wang, J. Y.; Chen, X.; O’Young, C. L.; Suib, S. L. J. Catal.
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Brock, S. L.; Marquez, M. Angew. Chem., Int. Ed. 2003,42, 2905-2909.
(44) Giraldo, O.; Brock, S. L.; Willis, W. S.; Marquez, M. Suib, S. L.; Ching, S. J. Am.
Chem. Soc. 2000,122, 9330-9331.
(45) (a) Lvov, Y.; Munge, B.; Giraldo, O.; Ichinose, I.; Suib, S. L., Rusling, J. F.
Langmuir 2000, 16, 8850-8857. (b) Espinal, L.; M.S. Thesis, University of Connecticut,
Storrs, CT, 2001. (c) Gao, Q.; Suib, S. L.: Rusling, J. F. J. Chem. Soc. Chem. Commun.,
2002, 2254-2255.
(46) (a) Lvov, Y. in Lvov, Y.; Mohwald, H., Eds. Protein Architecture: Interfacing
Molecular Assemblies and Immobilization Biotechnology, Marcel Dekker: New York,
2000, pp. 125-167. (b) Lvov, Y. in Nalwa, R. W., Ed., Handbook O f Surfaces And
Interfaces O f Materials, Vol. 3. Nanostructured Materials, Micelles and Colloids,
Academic Press. San Diego, 2001, pp. 170-189. (c) Rusling, J. F. in Lvov, Y.; Mohwald,
H., Eds. Protein Architecture: Interfacing Molecular Assemblies and Immobilization
Biotechnology, Marcel Dekker: New York, 2000, pp. 337-354. (d) Rusling, J. F.; Zhang,
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(48) Lvov, Y.; Haas, H.; Decher, G.; Moehwald, H.; Mikhailov, A.; Mtchedlishvily, B.;
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(50) Hua, F.; Cui, T.; Lvov, Y. Langmuir 2002,18, 6712-6715.
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7372-7377.
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(52) Rusling, J. F.; Zhang, Z. In Handbook o f Surfaces and Interfaces o f Materials, Vol 5,
Biomolecules, Biointerfaces and Applications', Nalwa, H. S., Ed., Academic Press: San
Diego, 2001; pp 33-71.
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(54) Ortiz de Montellano, P. R.; Catalano, C. E. J. Biol. Chem. 1985, 260, 9265-9271.
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1994, 33, 4384-4389.
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6, 1803-1808.
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Biology, Vol. II, CRC Press, Boca Raton, FI, 1991. (b) Schenkman, J.B.; Greim, H., Eds.
Cytochrome P450, Springer-Verlag: Berlin, 1993.
38
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CHAPTER II. SYNTHESIS OF MIXED METAL OXIDES USING ULTRASONIC
NOZZLE SPRAY AND MICROWAVES
II I. INTRODUCTION
Multicomponent mixed oxides have recently gained importance because of their high
catalytic activity for partial oxidation of propane to acrylic acid,1 one of the most
important intermediates for the preparation of fibers, synthetic rubbers, and resins.
Mo-
V-Te-(Nb)-Ox is one of the most common examples of multicomponent mixed oxides for
this application.
These catalysts were developed by Mitsubishi Chemicals and can
achieve around 50% acrylic acid yield.3 Multicomponent metal oxides of this kind have
been prepared by methods that allow the formation of orthorhombic and hexagonal
crystalline structures that are believed to be responsible for their catalytic activity.4,5 One
of the preparation methods consists on freezing the aqueous mixture in a dry-ice acetone
bath, followed by drying under vacuum and calcination.6 Another method involves
reacting aqueous solutions of the metal oxide sources (heptamolybdate, vanadyl sulfate,
tellurium dioxide, and ammonium niobium oxalate) under hydrothermal conditions at
175 °C for 72 hours, and calcining in an inert environment or air.5
Catalytic performance of mixed oxide materials depends not only on the composition
of the oxide elements but also on some specific structures of the resulting complex
oxides.7 The development of new synthetic methods that are able to produce well-defined
materials of nanometer size (especially in a continuous scalable process) remains a
serious challenge. Conventional hydrothermal synthesis approaches provide very limited
39
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control over catalyst properties.
In light of this, the investigation of the preparation
methods for this class of catalysts is very important for the understanding of the effects
on the catalyst structure, and also on the catalytic performance. An alternative approach
is to use a synthetic process that could potentially have control over different catalyst
properties.
According to Lin et al.,7 each and every step of the preparation methods, from mixing,
drying and calcination, can have profound effects on the structures and performance of
MoVTeNb oxide catalysts. The preferred preparation method for an effective MoVTeNb
oxide catalyst includes the intimate mixing of all starting chemicals to form a precursor
solution.
In an effort to address the intimate mixing of all reagents to form a precursor solution
proposed by Lin et a l.7 the concept of spraying a reactant solution into a receiving
solution or air under microwave radiation at atmospheric pressure is explored using two
synthetic methods. One of the methods is called NMW, N stands for nozzle and M for
microwaves (Figure II-1-a). A variation of this method is called INM (in-situ mixing,
ultrasonic Nozzle spray and Microwaves), which involves an additional step of mixing
reactants in-situ prior to the spraying step (Figure II-1-b).
Each of the components of
the NMW and INM processes has previously been used separately to synthesize different
materials.8,9,10 The uniqueness about these methods for making catalysts is the
combination of their components in one process.
Such combination could provide a
better control over compositional, structural, and morphological catalyst characteristics.
The present work demonstrate that both NMW11 and INM11,12,13 are convenient
methods for the preparation of multi-component metal oxide micro-particles whose
40
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particle size, morphology and surface area show a dependence on the preparation used
prior to calcination.
From ongoing studies, the extension of NMW and INM to the synthesis of
nanostructured metals, single metal oxides, polymers, and other materials appears to be
general.
41
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A+B
&
Conventional
procedures of
filtering, washing,
drying and/or
calcining if
required
B7^^
SYRINGE
PUMP
A+B
n
Final Product
Figure II-1. Methods involving nozzle-spray and microwaves: (a) NMW, and (b) INM.
42
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II-2. EXPERIMENTAL SECTION
Chemicals.
Ammonium heptamolybdate tetrahydrate (99%), ammonium metavanadate (99%),
telluric acid (99%), vanadyl sulfate (99%), oxalic acid dehydrate (99%), palladium (II)
nitrate hydrate (42% Pd), tellurium dioxide, and nitric acid were purchased from Aldrich
Chemicals. Niobium ammonium oxalate (20% Nb) was purchased from H. C. Starck.
Synthesis.
Multicomponent Metal Oxide (MMO) materials were obtained by calcining precursor
materials, prepared by methods involving either microwave radiation of aerosols
containing metal precursors or by conventional hydrothermal heating of the metal
precursor solutions in bulk.
The Microwave processing System used is a Wavemat
Model CMPR TM250 (0-1250 W, 2.45 GHz).
Fresh MMO-A was prepared using the NMW method, which is shown schematically
in Figure II-1-a.
Two aqueous solutions were prepared: (1) Solution A - 1 M
ammonium heptamolybdate + 0.3 M ammonium vanadate + 0.23 M telluric acid; (2)
Solution B - 1 M nitric acid. Solution B was injected from the nozzle located at the top
of the reactor to the solution A located at the bottom of the reactor inside the microwave
cavity. To obtain well-controlled droplet size, the ultrasonication nozzle power was set at
5 W, using an injection rate of 6 mL/min. Solution A was irradiated with microwaves at
200 W for 5 min before the addition of Solution B and kept at 200 W until the end of the
43
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addition. The reaction was stopped at the end of the addition. The precipitates formed
were fdtered, washed and dried at room temperature overnight.
Fresh-MMO-B was prepared using the INM method, shown in Figure II-1-b, in
which two solutions are mixed in situ (at the nozzle) using two syringes, a “T” connector
and a syringe pump. The in situ-prepared mixture was injected into the tubular reactor
inside the microwave cavity using the ultrasonic nozzle spray. The precursor solutions
used were:
(1) Solution A -
1 M ammonium heptamolybdate, 0.3 M ammonium
vanadate and 0.23 M telluric acid; (2) Solution B - 0.17 M niobium ammonium oxalate,
0.155 M oxalic acid, 0.24 M nitric acid and 0.01 M palladium(II) nitrate hydrate. The
resulting gel was dried at room temperature overnight.
Fresh-MMO-C was prepared as follows: to a 45 mL Parr Acid Digestion Bomb with
an inner tube made of PTFE, 1.58 g of tellurium dioxide and 30 mL of 1.0 M ammonium
heptamolybdate tetrahydrate in water were added.
The mixture was hydrothermally
treated at 100 °C for 1.5 hrs, followed by the addition of 3.26 g of vanadyl sulfate hydrate
to the bomb at 60 °C under stirring.
The bomb contents were then hydrothermally
treated at 175 °C for 3 days. Black solids formed in the bomb were collected by gravity
filtration, washed with deionized water (50 mL) and dried in a vacuum oven at 25 °C
overnight.
Fresh-MMO-D was prepared as follows: to a 125 mL Parr Acid Digestion Bomb with
an inner tube made of PTFE, 3.15 g of tellurium dioxide and 60 mL of 1.0 M ammonium
heptamolybdate tetrahydrate in water were added.
The mixture was hydrothermally
treated at 100 °C for 1.5 h, followed by the addition of 6.5 g of vanadyl sulfate hydrate
and 30 mL of aqueous solution (0.2 M in Nb) of ammonium niobium oxalate to the bomb
44
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at 60 °C under stirring. The bomb contents were hydrothermally treated at 175 °C for 3
days. Black solids formed in the bomb were collected by gravity filtration, washed with
deionized water (50 mL), dried in a vacuum oven at 25 °C overnight. 15.9 g of solid was
obtained.
A summary of the reactants, preparative compositions and preparation methods used
to make the fresh materials is shown in Table II-1. MMO materials: MMO-A, MMO-B,
MMO-C, and MMO-D were prepared by calcining their respective fresh materials. In a
typical calcination procedure, fresh solids were calcined in two steps as follows:
(1)
under air environment increasing temperature from 25 to 275 °C at 10 °C/min and held at
275 °C for 1 hour and (2) under argon environment increasing temperature from 275 to
600 °C at 2 °C/min and held at 600 °C for 2 hours.
45
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Table II-l. Metal precursors and synthetic method used to prepare the fresh materials.
Preparative Elemental
Sample8
Method
Metal Precursors
Composition
ammonium
Fresh-
heptamolybdate,
M M O -A
ammonium vanadate,
MoVo.3Teo.23
NM W b
MoV0.3Te0.23Nb0.17Pd0.01
IN M C
and telluric acid
ammonium
heptamolybdate,
ammonium vanadate,
FreshM M O -B
telluric acid,
niobium ammonium
oxalate,
and palladium(II)
nitrate hydrate
i
, ,• calcination.
, - • b .r.
a Fresh-MMO: freshly prepared material before
NMW (N: nozzle and MW:
a rn
1
microwaves).
c INM (I: in-situ mixing, N: nozzle, and M: microwaves).
d CH
(conventional hydrothermal method).
46
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Table II-l. Continued.
Preparative Elemental
Sample3
Method
Metal Precursors
Composition
a m m o n iu m
F resh -
MMO-C
h e p ta m o ly b d a te ,
MoVo.4Teo.33
CHd
M o V 0.4T eo.33N b o .2
CHd
v a n a d y l s u lfa te h y d r a te ,
a n d te llu r iu m d io x id e
a m m o n iu m
h e p ta m o ly b d a te ,
F resh -
MMO-D
v a n a d y l s u lfa te h y d r a te ,
t e llu r iu m d io x id e ,
a n d n io b iu m
a m m o n iu m o x a la te
b NMW (N: nozzle and MW:
aa Fresh-MMO: freshly prepared material before calcination.b
T-*
-1 .
.
H -TTk
microwaves).
X
T
. , , , ,
/X T .
c INM (I: in-situ mixing, N: nozzle, and M: microwaves).
d CH
(conventional hydrothermal method).
47
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Characterization.
Structural analysis was performed using powder X-ray diffraction (XRD) methods.
XRD data were collected on a Scintag XDS 2000 diffractometer with Cu K a radiation on
the powder samples.
The morphologies of the prepared materials were studied using a Zeiss DSM 982
Gemini field emission scanning electron microscope (FESEM) with a Schottky emitter.
The sample powder suspension in isopropanol was dispersed on AuPd-coated silicon
chips mounted onto the stainless steel sample holders using silver conductive paint.
The isothermal N2 adsorption/desorption experiments on the powder samples were
conducted on a Micrometries ASAP 2010 accelerated surface area system. Each sample
was degassed at 120 °C for 3 hours. Nitrogen gas was used as an adsorbate at liquid
nitrogen temperature.
Surface areas were calculated by the Brunauer-Emmett-Teller
(BET) method.
The elemental analyses were done at the Environmental Research Institute, Storrs, CT,
using a Perkin-Elmer Model 140 Inductively Coupled Plasma-Atomic Emission
Spectroscopy (ICP-AES) instrument equipped with an autosampler.
II-3. RESULTS
The MMO precursor materials (Fresh-MMO-A, Fresh-MMO-B, Fresh-MMO-C and
Fresh-MMO-D) were each prepared using a unique combination of reactants, elemental
48
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compositions and preparation methods (Table II-l). The preparation of Fresh-MMO-A
and Fresh-MMO-B, for example, involved the use of a nozzle-spray and microwaves. In
contrast, the preparation of Fresh-MMO-C and Fresh-MMO-D involved conventional
hydrothermal conditions.
All freshly prepared materials were subjected to the same
subsequent heat treatment procedure to study the effect of calcination.
Such heat
treatment involved a first step of calcination under air environment increasing
temperature from 25 to 275 °C at 10 °C/min and held at 275 °C for 1 hour followed by a
second step of calcination under argon environment increasing temperature from 275 to
600 °C at 2 °C/min and held at 600 °C for 2 hours. Materials prepared by NMW, INM
and CH were characterized by XRD, SEM, BET, and ICP before and after calcination.
XRD studies.
The identification of crystalline phases was carried out by comparison of our XRD
patterns with standards for various crystalline metal oxides (M0 O 3 H2 O [JCPDS 210569], M0 O 3 [JCPDS 05-0508], V0 .1 3 M 0 0 .8 7 O2 .9 3 5 [JCPDS 48-0766], VM 0 O4 [JCPDS:
18-1454], (V0 .o7 Moo.9 3 )5 0 i4 [JCPDS 31-1437], 3M o02 Nb20 5 [JCPDS 18-0840], T e0 2
[JCPDS 11-0693], and Nb0 .0 9 Mo0 .9 1 O2 .8 0 [JCPDS 27-1310]).
Figure II-2-a shows the XRD pattern of Fresh-MMO-A, which exhibits the
diffraction pattern of M0O3 H2O [JCPDS 21-0569]. A similar diffraction pattern was
obtained for the hexagonal M0O3 as reported by Wagner et al.u Hexagonal
V0.13M00.87O2.935 [JCPDS 48-0766], reported by Guliants et al., also shows a similar
diffraction pattern with the characteristic X-ray reflections at 20 = 9.6°, 16.7°, 19.3°,
49
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25.6°, 29.4° and 45.3
° . 15
Interestingly, hexagonal M 0 O3 structure may exhibit some
activity and selectivity for partial oxidation reactions. 16
The XRD pattern for MMO-A is shown in Figure II-2-b. This diffraction pattern
indicates the presence of (Vo.o7 Moo.9 3 )5 0 i4 [JCPDS 31-1437] and VM 0 O4 [JCPDS 181454] crystal structures. Similar diffraction patterns of Mo-V-(M)-Ox materials were
obtained by Baca et al.1, Katou et al. 17 Lopez-Nieto et al. 18
Fresh-MMO-B shows an amorphous XRD pattern (Figure II-3-a) similar to those
previously reported in Mo-V-Te-Nb-0 systems by Tsuji et al.19 After heat treating this
sample, new crystalline phases appear (Figure II-3-b, MMO-B).
N b 0 .0 9 M o 0 .9 1 O 2 .8 0
The presence of
[JCPDS 27-1310] and/or (Moo^Vo.ovIsOh [JCPDS 31-1437], and
M 0 O3 [JCPDS 05-0508] could be proposed.
Mo-V-Nb-Te-Ox materials exhibiting
similar diffraction patterns were prepared by DeSanto et al?° and Vitry et al.3 These
patterns, with characteristic diffraction angles at 20 = 9.0°, 22.1°, 27.3°, 29.2°, and 35.4°,
were interpreted as revealing the M l orthorhombic phase of Te2 0 M 2 o0 5 6 (M = Mo, V,
Nb).
The presence of these same reflections in the diffraction from Fresh-MMO-B
suggests the presence of orthorhombic M l phase crystallites in MMO-B.
50
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-----0
1
1
1
20
40
60
80
26 [degrees]
Figure II-2. XRD patterns of: (a) Fresh-MMO-A and (b) MMO-A.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
=j
(0
>»
■55
c
<l>
+■>
c
0
20
40
60
80
26 [degrees]
Figure II-3. XRD patterns of: (a) Fresh-MMO-B and (b) MMO-B.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The diffraction pattern of Fresh-MMO-C shown in Figure II-4-a is similar to that of
Te02 [JCPDS 11-0693], The resemblance is probably due to some unreacted precursor
Te0 2 used in the preparation of the fresh material.
The presence of peaks at 20 = 22.1°, 28.2°, 36.2°, 45.2°, and 50.0° in the XRD pattern
of MMO-C (Figure II-4-b) suggests the formation of a TeM 3 0 io (M: Mo, V) bronze
crystal structure as reported by both Lopez-Nieto et a/.18 and Solsona et al? However, the
presence of (Moo^VootIsOh [JCPDS 31-1437], pseudohexagonal MogTe2V024, and
small amounts of TeMo5Oi6 could also be considered.21,20
The XRD pattern for Fresh-MMO-D is shown in Figure II-5-a, of which the
prominent feature is a very broad peak centered at 22°. The breadth of the peak indicates
a largely amorphous material, whereas the fact that there is only a single peak suggests
that order persists in only one dimension. Two recent studies have reported very similar
XRD patterns to that of Fresh-MMO-D. Vitry et a l also interpreted the peak at 22° as
arising from order in a single direction. Dieterle et al. proposed a mixture of M0 5 O 14
and M 0 O3 nano-crystallites, with the small size of the crystallites making them mostly
invisible in the XRD spectrum . 2 2
Heat treatment of Fresh-MMO-D increases crystallinity as seen in the XRD pattern for
MMO-D in Figure II-5-b. The diffraction pattern of MMO-D bears a striking
resemblance to that of TeM3Oio bronze 18 (as was also the case for MMO-C), as well as to
(M0 0 . 9 3 V0 .0 7 ) 5 0 1 4 [JCPDS, 31-1437] and Nb0 .0 9 Mo0 .9 1 O2 .8 0 [JCPDS, 27-1310],
53
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>
*-»
00
c
0)
*-»
c
JUUL
0
10
20
30
O n. I m A ii^ —'*|*~
40
50
60
70
80
26 [degrees]
Figure II-4. XRD patterns of: (a) Fresh-MMO-C and (b) MMO-C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
S
>
4-4
<0
c
4)
4-4
c
0
10
20
30
40
50
60
70
80
26 [degrees]
Figure II-5. XRD patterns of: (a) Fresh-MMO-D and (b) MMO-D.
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SEM studies.
Fresh-MMO-A is composed of long well defined rod-shaped particles with hexagonal
cross sections as shown in Figure II- 6 -a. Wagner et al. reported similar morphologies
for hexagonal M 0 O 3 . 14 The size of the rod-like crystallites of Fresh-MMO-A varies from
0.5 to 2.0 pm in diameter and from 5.0 to 20.0 pm in length. The aspect ratio, however,
appears to remain the same (L/D-10) regardless of particle size. In some particles, the
hexagonal cross section presents smaller rods projecting outward from the surface, as if
the large particle was formed through a process bundling together of thinner rods.
Calcination of the Fresh-MMO-A material produces a material (MMO-A) with
different particle morphology and size distribution (Figure II- 6 -b). Although the MMOA crystallites also have rod-like morphology, their size is clearly reduced during
calcination and the shape of their cross sections change to a ‘pinwheel’ geometry. The
particle diameter of this material varies from 0.05 to 0.5 pm and length from 0.2 - 2.0
pm, with a constant aspect ratio of approximately 4.
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Figure II-6. SEM images of: (a) Fresh-MMO-A and (b) MMO-A.
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As shown in Figure II-7-a, similar morphologies were also obtained when a variation
of Fresh-MMO-A was prepared (Fresh-MMO-A'). Fresh-MMO-A' was prepared using
slightly lower Mo/V and Mo/Te ratios, and 0.7 M
composition used was MoVo.5 Teo.2 8 Ox.
HNO
3
instead of 1 M. The preparative
ICP analysis showed a bulk composition of
M 0 V0 .0 7 , which did not change after calcination. Again, the calcined material (MMOA ') shows rod-like morphology with a ‘pinwheel’ cross-section and smaller particle size
(Figure II-7-b) as compared to the uncalcined material (Fresh-MMO-A'). SEM studies
suggest that the small rod-like particles of the calcined material (MMO-A or A ')
originated from the bigger rod-like particles of the fresh material (Fresh-MMO-A or A ').
Therefore, the particle size of the calcined material might depend on the size of its parent
particle before calcination.
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Figure II-7. SEM images of: (a) Fresh-MMO-A' and (b) MMO-A'
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SEM studies did not reveal the presence of crystallites for the Fresh-MMO-B material
as indicated by the irregular particle morphologies shown in Figure II-8-a.
Crystallization occurs, however, during the calcination treatment. Figure II-8-b shows
the uniform small rod-like crystals of the MMO-B material that aggregate to form sheets.
Vitry et al. report similar cylinder-shaped crystallites for this type of material.3
Orthorhombic MoVNbTeO materials have also been found to have similar structures as
reported by DeSanto et al.20 The rod-like crystals in MMO-B have a ‘pinwheel’ cross
section, similar to that shown for MMO-A and A ' (Figures II-6-b and II-7-b). The
particle size distribution is relatively uniform with diameters between 0.1 - 0.2 pm and
lengths around 0.5 - 1.0 pm.
In the case of Fresh-MMO-C, SEM studies show the presence of uniform 5.0 - 8.0 pm
long cylinder-like crystals with 0.5 - 1.0 pm pinwheel shaped cross-sections (Figure II9-a). Vitry et al? and Ueda et al.23 previously reported similar morphology for Mo-V(M) oxides. MMO-C crystallites, which are produced from the calcination of FreshMMO-C, also have cylinder-like morphology (Figure II-9-b). However, the pinwheel
shape of the cross section disappears and transforms into a smoother surface with a crosssection of irregular shape. Although the particle diameter appears to be the same after
calcination, the length is slightly longer (-10 pm), and so heat treatment of Fresh-MMOC particles leads to overall larger particle size.
Calcination of these materials also
induces particle agglomeration and interconnection.
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Figure II-8. SEM images of: (a) Fresh-MMO-B and (b) MMO-B
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Figure II-9. SEM images of: (a) Fresh-MMO-C and (b) MMO-C.
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Fresh-MMO-D presents a combination of 1.0 - 2.0 pm long needle-like crystals with a
large amount of amorphous material (Figure II-10-a). Calcination of Fresh-MMO-D
produces no change in the needle-like structures and induces crystallization of the
amorphous region into a mixture of needles, rods and other crystal morphologies (Figure
II-10-b). Most of the particles are short cylinders with irregular cross sections. These
cylinders have a range of sizes, with diameters between 0.2 and 0.5 pm and lengths
between 0.5 and 1.0 pm.
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Figure 11-10. SEM images of: (a) Fresh-MMO-D, and (b) MMO-D.
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BET surface area measurements.
The BET surface areas
(S b e t)
and elemental analysis of the precursor materials
(Fresh-MMO-A, Fresh-MMO-B, Fresh-MMO-C and Fresh-MMO-D) and the calcined
materials (MMO-A, MMO-B, MMO-C and MMO-D) are presented in Table II-2.
The fresh materials prepared by methods involving nozzle-spray and microwaves
(Fresh-MMO-A and Fresh-MMO-B) have lower surface areas than the fresh materials
prepared by conventional hydrothermal methods (Fresh-MMO-C and Fresh-MMO-D).
In detail, Fresh-MMO-A and Fresh-MMO-B materials pose very low surface area (< 1
m2/g), Fresh-MMO-C has slightly higher surface area (~ 6 m2/g), and Fresh-MMO-D has
a much higher surface area (~ 130 m2/g).
Calcination treatment was found to increase the surface area of fresh materials (FreshMMOs) that were prepared by methods involving nozzle-spray and microwaves.
Calcination of Fresh-MMO-A, which has a surface area of 0.55 m2/g (Table II-2, entry #
1) produces a material (MMO-A) which has a surface area of 2.50 m2/g (Table II-2,
entry # 2). Similarly, calcining Fresh-MMO-B (SA ~ 0.45 m2/g) results in a material
(MMO-B) with a SBEt of 3.81 m2/g (Table II-2, entries # 3 and 4).
In contrast, the same calcination treatment decreases the surface area of the fresh
material prepared by conventional hydrothermal methods. For instance, calcination of
Fresh-MMO-C having a surface area of 6.40 m2/g (Table II-2, entry # 5) produces
MMO-C with a surface area equal to 1.7 m2/g (Table II-2, entry # 6). This decrement in
surface area after calcination is more evident in the case of Fresh-MMO-D, in which the
surface area decreases from 130 m2/g to 4.60 m2/g after calcination forming MMO-D
(Table II-2, entries # 7 and 8). .
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Table II-2. BET surface area measurements and elemental analysis.
Entry
Sample
Calcination3
S b e t (m 2/g ) b
Bulk Elemental Composition0
1
Fresh-MMO-A
No
0.55
MoVo.2 9 Teo.i8
2
MMO-A
Yes
2.50
M 0 V0 .O8
3
Fresh-MMO-B
No
0.45
Mo Vo.2 8 Teo. 1sNbo. 1 6Pdo.oi
4
MMO-B
Yes
3.80
Mo Vo.27Teo. 11 Nbo. 1 6Pdo.oi
5
Fresh-MMO-C
No
6.40
MoVo.4 0 Teo.3 2
6
MMO-C
Yes
1.70
MoVo.3 9 Teo.25
7
Fresh-MMO-D
No
130.60
Mo V0 .3 3 T eo.2 8 Nbo.21
8
MMO-D
Yes
4.60
Mo Vo. 3 5 T eo.2 8 Nbo.2 2
a Yes/no: calcined/uncalcined. Calcination treatment involved a first step of calcination
under air environment increasing temperature from 25 to 275 °C at 10 °C/min and held at
275 °C for 1 hour followed by a second step of calcination under argon environment
increasing temperature from 275 to 600 °C at 2 °C/min and held at 600 °C for 2 hours, b
Sbet: surface area calculated by the BET method. Each sample was pre-degassed at 120
°C for 3 h. Nitrogen gas was used as an adsorbate at liquid nitrogen temperature. c Bulk
elemental compositions were determined by 1CP-AES. d N.D.: element not detected.
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Elemental analysis.
The bulk compositions of the fresh (uncalcined) and calcined multicomponent metal
oxides are summarized in Table II-2. The bulk elemental compositions of the fresh
materials (Fresh-MMO-A, Fresh-MMO-B, Fresh-MMO-C, and Fresh-MMO-D) were not
very different from the synthesis compositions shown in Table II-1. In general, Mo, V
and Nb compositions did not change after calcination for all samples. Only in the case of
Fresh-MMO-A, the Mo/V ratio increased. The tellurium (Te) content was lower after
calcination in all the cases except in the case of Fresh-MMO-D.
The elemental
composition of Fresh-MMO-D evidently was not affected at all by calcination.
II-4. DISCUSSION
In preparing materials for catalytic use, the surface area accessible to reagents is of
great importance. It is intriguing therefore that our novel preparation techniques have
produced multicomponent metal oxide precursors that increase in surface area upon
calcination. The opposite effect, of coarsening - where calcination leads to the fusing
together of separate crystallites into larger crystals with a commensurate decrease in
surface area - was observed in our conventionally prepared materials and has previously
been reported for other conventionally prepared Mo-V-Te-(M)-Ox materials. In the case
of Fresh-MMO-A, our XRD results suggest that calcination to MMO-A induces a
crystalline-crystalline phase transition.
Presumably, the NMW preparation method
produced a precursor material that, although crystalline, was trapped in a metastable
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form. Upon further heating, the metastable form was converted to a more stable form.
Such a physical transformation between polymorphic material forms upon heating, with
an associated decrease in particle size, is well known in metamorphic geology, but has
not yet been reported to our knowledge in the preparation of Mo-V-Te-(M)-Ox materials.
It should also be noted that from SEM studies (Figure II-6-a) it appears that a number
of the larger MMO-A precursor particles (Fresh-MMO-A) may be composed of a
bundling of smaller rod-like particles. This raises the question of how the NMW process
alone leads to both the initial formation of the smaller rods and their subsequent bundling
into the larger Fresh-MMO-A particles.
One possible explanation is that the small
particles may form in receiving solution of metal salts alone during the time when the
solution is exposed to microwave radiation, prior to the introduction of nitric acid
solution. The addition of nitric acid, a precipitation agent, would then promote the
aggregation of the small bundles into the Fresh-MMO-A particles.
The calcination
process could somehow be disrupting the larger bundles and the crystalline phase
transition accounting for the shape change of the dispersed smaller particles.
More
experiments are needed to determine whether Fresh-MMO-A particles genuinely arise
from the bundle of sticks motif, whether our proposed ‘stick to bundle’ mechanism
accurately describes the bundling process, and finally whether the decrease in particle
size upon calcination is related to the reverse process, i.e. of bundle disruption.
Fresh-MMO-B, unlike Fresh-MMO-A, is a highly amorphous material. However, the
measured surface area of Fresh-MMO-B is remarkably low for an amorphous material.
When Fresh-MMO-B is crystallized upon calcination, the surface area of the final MMOB material increases by nearly an order of magnitude from 0.45 to 3.80 m2/g. Such an
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increase upon crystallization is highly unusual, as disorder in materials is generally
associated with increased free volume and therefore surface accessibility. The increase in
surface area, while difficult to rationalize, is most likely beneficial in terms of catalysis,
and in fact the surface area values of MMO-B increase to values comparable to materials
produced by other methods.
Mo-Te-V-Nb-Ox materials with orthorhombic crystal structure have been previously
obtained by calcining fresh materials prepared by conventional hydrothermal procedures.
Such conventional hydrothermal procedures take 72 hours.
In the present work, the
preparation of Fresh-MMO-B by INM requires only 20 min. The acceleration of phase
formation could be a result of three possible conditions: good mixing of the reagents in
situ, ultrasonic nozzle spraying, and/or microwave radiation. On one hand, mixing of
reactants during synthesis has been reported to affect the mixed oxide properties.7 On the
other hand, selective heating during microwave radiation could favor appropriate phase
formation.24
Finally, in consideration of the striking differences between Fresh-MMO-A and FreshMMO-B, it is interesting to note that calcination of either species produces particles of
very similar morphology, specifically rod-like morphology with a ‘pinwheel’ crosssection (Figures II-6-b, II-7-b and II-8-b).
This research has simply established new
viable preparation methods for a specific class of multicomponent mixed oxides and
speculated about the underlying physical mechanisms. There remain interesting avenues
to explore regarding our novel preparation techniques, particularly whether the unusual
increase in surface area upon calcination is characteristic of our preparation methods and
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whether the diversity of structures observed in our fresh materials can be exploited for
diverse or exceptional catalytic behavior.
II-5. CONCLUSIONS
The present work demonstrate that both NMW and INM are convenient methods for
the preparation of multi-component metal oxide micro-particles.
Particle size,
morphology and surface area are highly dependent on the preparation method used prior
to calcination.
II-6. REFERENCES
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MoVTeNbO mixed oxide catalysts: study of the phase composition of active and
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(2) Solsona, B.; Lopez-Nieto, J.M.; Oliver, J. M.; Gumbau, J. P. “Selective oxidation of
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catalysts prepared by slurry” Catalysis Today 2004, 91-92, 247-250.
(3) Vitry, D.; Morikawa, Y.; Dubois, J. L.; Ueda, W. “Mo-V-Te-(Nb)-0 mixed metal
oxides prepared by hydrothermal synthesis for catalytic selective oxidations of propane
and propene to acrylic acid” Applied Catalysis A: General 2003, 251, 411-424.
(4) Vitry, D; Morikawa, Y.; Dubois, J.L.; Ueda, W. “Propane Selective Oxidation Over
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(6) Lin, M.; Desai, T. B.; Kaiser, F. W.; Klugherz, P. D. “Reaction pathways in the
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Structural changes of a MoVW mixed oxide catalyst during activation and relation to
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