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I. Effects of microwave heating in syntheses of OMS-1 and OMS-2 manganese oxides. II. Coating of sheet and structured materials via microwave heating. III. Microwave-assisted desulfurization of NSR (NOx storage reduction) catalyst

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Effects of Microwave Heating in Syntheses of OMS-1 and
OMS-2 Manganese Oxides.
Coating of Sheet and Structured Materials via Microwave
Heating.
Microwave-assisted Desulfurization of NSR
(NOx Storage Reduction) Catalyst.
Kinga A. Malinger, Ph.D.
University o f Connecticut, 2006
Application o f microwave heating in chemical processes has attracted considerable
attention in recent years. There are many advantages o f using microwave instead o f
conventional heating based on literature reports. Interaction o f a material with microwave
radiation causes so-called volumetric heating and therefore homogeneous nucleation,
shorter induction times, and enhancement o f reaction rates can be achieved. This
automatically leads to energy savings.
This work presents a study o f microwave heating effects on properties o f OMS-type
manganese oxides in comparison with the conventionally prepared materials. Manganese
oxides have a high value o f dielectric constant (10,000) that is an important factor
determining interaction with microwave radiation. Microwave application during the
syntheses o f OMS-1 and OMS-2 type manganese oxides resulted in different chemical,
physical, and catalytic properties o f these materials when compared to the conventionally
prepared ones.
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Kinga A. Malinger — University of Connecticut, 2006
In addition, application o f a high frequency microwave radiation (5.5 GHz) in the
synthesis o f OMS-2 manganese oxide produced a catalyst with a better activity for the
oxidation o f 2-thiophenemethanol as compared to the low frequency OMS-2 (2.45 GHz).
The second part o f this work comprises effects o f microwave radiation in producing
inorganic coatings in comparison with the conventional methods. Due to the specific
mechanism o f microwave interaction with matter, uniform coatings o f inorganic oxides
like alumina, ceria, zirconia, and silica were produced on three-dimensional fibrous
substrates. Substantially uniform coatings along the fibers within the substrates were
obtained. Removal o f a liquid media during the coating process with the conventional
heating resulted in the non-uniform coatings.
A new approach for regeneration o f NOx Storage Reduction (NSR) catalyst via
microwave heating was also investigated. Desulfurization o f the poisoned NSR catalyst
was achieved at temperatures as low as 200 °C in a microwave field and in reducing
atmospheres. Presence o f water in the system lowered the sulfur desorption temperature
to 150 °C. Normally, temperatures above 600 °C are required to promote sulfur
desorption using conventional heating in reducing atmospheres. The application o f
microwave radiation presents a promising method to achieve regeneration o f the NSR
catalyst.
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I. Effects of Microwave Heating in Syntheses of OMS-1 and
OMS-2 Manganese Oxides.
II. Coating of Sheet and Structured Materials via Microwave
Heating.
III. Microwave-assisted Desulfurization of NSR (NOx Storage
Reduction) Catalyst.
Kinga A. Malinger
M. S., A. Mickiewicz University Poznan,
Chemistry Department, Poland, 2001
A Dissertation
Submitted in Partial Fulfillment o f the
Requirements for the Degree o f
Doctor o f Philosophy
At The University o f Connecticut
Storrs, CT 06269
2006
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UMI N um ber: 3205754
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APPROVAL PAGE
Doctor o f Philosophy Dissertation
I. Effects of Microwave Heating in Syntheses of OMS-1
and OMS-2 Manganese Oxides.
II. Coating
of Sheet
and
Structured
Materials
via
Microwave Heating.
III. Microwave-assisted
Desulfurization
of NSR (NOx
Storage Reduction) Catalyst.
Presented by
Kinga A. Malinger, M. S.
Major Advisor
Associate Advisor
A. Dimock
Associate Advisor
L'U r
F. S. Galasso
Associate Advisor
Tanaka
The University of Connecticut
2006
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DEDICATION
This work is dedicated to my family. To my parents; Anna and W ojciech Malinger.
Thank you for being my first teachers, showing me how to find inspiration and
motivation, and thank you for support to achieve my goals. I also thank my wonderful
brother Tomasz Malinger for encouragement.
vi
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ACKNOWLEDGEMENTS
I am deeply grateful to my research advisor, Professor Steven L. Suib, for the support
and trust he has given me during m y years o f study at the University o f Connecticut. I
would like to express my sincere gratitude to Dr. Francis S. Galasso for his constant
guidance and kindness. I am also thankful to my associate advisors, Dr. John Tanaka and
Dr. Arthur Dimock for their valuable advices and support that I needed for preparing this
dissertation.
I wish to thank Dr. D. Scola for help with the variable microwave frequency
syntheses, Dr. P. Fanson for guidance with the NSR desulfurization project, Dr. L.
Murrell for assistance regarding the coating project, Dr. E. Neth for help with the
microwave equipment, and Dr. A. Vaze for valuable suggestions. I really appreciate
collaboration and help that I received from my groupmates:
Dr. Laura Espinal, Dr.
Young-Chan Son, Dr. Vinit Makwana, Dr. Javier Garces, Xiongfei Shen, Yunshuang
Ding, Anais Espinal, and Edward K. Nyutu.
While at the University I have met many people who have become my friends.
Although, there are many to mention here, I would like to especially acknowledge Laura
Espinal and Anais Espinal. They were not only great co-workers in Prof. Suib’s
laboratory, but also became my dearest friends and sisters. They made my journey
through the graduate program much more beautiful. I will always treasure their
friendship.
I thank the NSF NIRT program and TOYOTA Motor Corporation for the financial
support I received during my Ph.D. program.
vii
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Table of Contents
Overview.
15
Background and Novelty o f These Studies.
17
PART I. Effects o f Microwave Heating in the Syntheses o f OMS-1 and OMS-2
Manganese Oxides.
21
Introduction.
21
Experimental.
26
Characterization.
28
1. X-Ray Diffraction.
28
2. Average Oxidation State (AOS) Determination.
28
3. Morphology.
28
4. Thermal Analysis.
29
5. Temperature Programmed Decomposition o f OMS-2.
29
6. Specific Surface Area and Porosity Measurements.
30
7. Acidity and Basicity Measurements o f OM S-1.
30
8. Raman Spectroscopy.
31
9. Catalytic Activity o f OM S-1: Oxidation o f Indene.
31
10. Catalytic Activity o f OM S-1: Oxidation o f Benzyl Alcohol.
31
11. Catalytic Activity o f OMS-2: Oxidation o f 2-thiophenemethanol.
32
12. GC/MS.
32
Results.
33
viii
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1. Synthesis and XRD Results.
33
2.
34
Average Oxidation State Determination.
3. Morphology.
34
4.
52
Thermogravimetric Analyses.
5. Temperature-Programmed Decomposition.
52
6. Surface Area and Pore Size Distribution.
53
7. Acidic and Basic Properties o f OMS-1.
54
8. Raman Spectroscopy.
55
9. Oxidation o f Indene.
60
10. Oxidation o f Benzyl Alcohol.
60
11. Oxidation o f 2-thiophenemethanol.
60
Discussion.
62
1. Effects o f Microwave Heating on Properties o f OM S-1.
62
2. Microwave Frequency Effects on the Synthesis o f Cryptomelane-type
Manganese Oxide and Catalytic Activity o f Cryptomelane Precursor.
63
Conclusions.
66
1. Effects o f Microwave Heating on Properties o f OM S-1.
66
2. Microwave Frequency Effects on the Synthesis o f Cryptomelane-type
Manganese Oxide and Catalytic Activity o f Cryptomelane Precursor.
66
PART II. Coating o f Sheet and Structured Materials via Microwave Heating.
69
Introduction.
69
Experimental.
70
1. Coating Process.
70
ix
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Characterization.
71
Results.
72
Discussion.
82
Conclusions.
83
Introduction.
85
Experimental.
87
1.
Preparation o f the NSR catalyst.
87
2.
Microwave Apparatus.
87
3.
Desufurization Experiments.
87
4.
Catalyst Characterization.
88
Results.
89
1.
Microwave-assisted Desulfurization o f NSR Catalyst.
89
2.
Water Effect on the MW-assisted Desulfurization o f TSCatalysts.
89
3.
Chemical Composition Analyses o f the M W-heated NSR Catalysts.
90
Discussion.
100
Conclusions.
102
x
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List of Tables
Table 1.1. Average Oxidation State (AOS), Average Crystallite Size (ACS), BET
Surface Area, BJH Desorption Average Pore Diameter, and BJH
Desorption Cumulative Pore Volume Data for the OMS-1 Materials.
43
Table 1. 2. N 2 physisorption and Mn average oxidation state (AOS) determination
results for the OMS-2 samples.
44
Table 1.3. Conversion and selectivity in the oxidation o f 2-thiophenemethanol
using the manganese oxide samples as the catalysts.
45
Table 3 .1 . Calculated percentage o f sulfate species removed after treatment in
hydrogen gas, based in FTIR resu ltsa.
97
Table 3. 2. IC data obtained for treated and untreated poisoned NSR catalyst.
98
Table 3 .3 . XPS data o f treated and untreated poisoned NSR catalyst.
99
xi
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List of Figures
Figure 1.1. OMS-1 (a) and OMS-2 (b) manganese oxides.
Figure 1. 2. Model 10-1300 Microwave oven (a) and Model LT 502Xb Variable
Frequency Microwave Furnace (b).
Figure 1.3. Synthesis procedure o f OMS-1: (a) layered Na-OL-1, (b) layered MgOL-1, (c) tunnel-structured Mg-OMS-1.
Figure 1. 4. Synthesis procedure o f OMS-2.
Figure 1.5. X-ray diffraction patterns o f layered precursors OL-1 and: (a) Na-OL1, (b) Mg-OL-1, (c) conventionally prepared OMS-1, (d) microwave
prepared OMS-1.
Figure 1. 6. X-Ray Diffraction o f OMS-2 made: (a) conventionally, (b) at 2.45
GHz, (c) at 5.5 GHz, (d) at variable frequency. XRD pattern for the
precursor (e).
Figure 1. 7. SEM micrographs o f (a) precursor OL-1, (b) conventional OMS-1,
(c), (d) OMS-1 synthesized in the presence o f a microwave field.
Figure 1. 8. TEM bright-field image o f a todorokite cubic-shaped particle (a),
high resolution magnified image o f that particle (b), and its EDX (c).
The inset o f (a) is the [060] zone axis.
Figure 1. 9. FESEM images o f OMS-2 made: (a) conventionally, (b) at 2.45 GHz,
(c) at 5.5 GHz, (d) at variable frequency, (e) depicts the precursor
morphology.
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Figure 1.10. TEM images o f the OMS-2 precursor (a) low magnification, (b) high
magnification.
Figure 1. 11. TEM images o f the low microwave frequency (2.45 GHz) OMS-2
(a) low magnification (the insert shows electron diffraction pattern
for the single fiber), (b) high magnification.
Figure 1. 12. TEM images o f the high frequency (5.5 GHz) OMS-2: (a) and (b)
low magnification (the insert shows the electron diffraction pattern),
(c) high magnification.
Figure 1. 13. Oxygen evolution during Temperature Programmed Decomposition
of: (a) conventional OMS-2, (b) 2.45 GHz OMS-2, (c) 5.5 GHz
OMS-2, (d) variable frequency OMS-2, (e) precursor.
Figure 1. 14. NH3 temperature programmed desorption: (a) from conventional,
and (b) from microwave made OMS-1.
Figure 1. 15. C 0 2 temperature programmed desorption: (a) from conventional
OMS-1, (b) from microwave made OMS-1.
Figure 1.16. Raman spectra o f OMS-2 (a) and the precursor: (b), (c), and (d).
Figure 2. 1. Conventionally dried: (a) 1/16” fiber paper coated with 20% silica,
(b) 1/16” fiber paper coated with 30% silica, (c) 1/8” fiber paper
coated with 20% silica, (d) 1/8” fiber paper coated with 30% silica.
Figure 2. 2. Microwave-dried: (a) 1/16” fiber paper coated with 20% silica, (b)
1/16” fiber paper coated with 30% silica, (c) 1/8” fiber paper coated
with 20% silica, (d) 1/8” fiber paper coated with 30% silica.
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Figure 2. 3. HRSEM images o f 20% Silica-coated and Microwave-dried fibers
from the quartz fiber paper.
Figure 2. 4. HRSEM images o f the quartz fiber paper coated with alumina sols o f
different concentrations (10 %, 20%, and 30%) and MW dried.
Figure 2. 5. 10% Ceria coated quartz fiber paper.
Figure 2. 6. 10% Zirconia coated quartz fiber paper.
Figure 2. 7. (a) MW-dried 10% Ceria-coated fiber paper, (b) conv. dried 10%
Ceria-coated fiber paper.
Figure 2. 8. (a) MW-dried 10% Zirconia-coated fiber paper, (b) conv. dried 10%
Zirconia-coated fiber paper.
Figure 3. 1. Thermal desorption o f sulfur from the sulfur-poisoned NSR catalyst
placed in a microwave field and exposed to 1% H2/He between 150
and 200°C.
Figure 3. 2. Thermal desorption o f sulfur from the sulfur-poisoned NSR catalyst
placed in a microwave field and exposed to 10% H2/He between 150
and 200°C.
Figure 3. 3. Thermal desorption o f sulfur from the sulfur-poisoned NSR catalyst
placed in a microwave field and exposed to 1% H2/He and a
controlled input o f water at 150°C.
Figure 3. 4. FTIR spectra o f sulfur poisoned NSR catalysts and catalysts treated
with microwave heating at 200 °C under varying reducing
atmospheres.
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Overview.
This research contains three major parts. The first part focuses on effects o f
microwave energy in synthesis o f OMS-type manganese oxides (Octahedral Molecular
Sieves) particularly OMS-1 (todorokite) and OMS-2 (cryptomelane). The second part
involves applications o f microwave heating in the coating o f sheet and structured fibrous
materials. The third part involves the use o f microwave radiation in the desulfurization o f
NSR (NOx Storage Reduction) automotive catalysts.
In the first part a hydrothermal method o f synthesis was used to prepare OMS-1 and
OMS-2 manganese oxides. Microwave and conventional heating was utilized to assist in
the preparation o f the materials from their precursors.
Both, microwave and
conventionally prepared manganese oxides were thoroughly characterized and their
properties were compared. Furthermore, different microwave frequencies and a variable
frequency microwave program were used to synthesize OMS-2 type manganese oxides.
Finally, microwave and conventionally synthesized OMS-1 and OMS-2 were used as
catalysts in various oxidation reactions, and their catalytic performance was evaluated.
Advantages o f microwave heating during the synthesis have been described.
In the second part microwave heating was employed in the process o f coating o f
fibrous sheets and 3-dimensional structures with various oxides. The oxide coatings were
developed from their sol precursors. Conventional heating was also used in the
preparation o f the same oxide coatings on fibrous sheets and 3-dimensional structures for
comparison. The materials were characterized with optical and electron microscopy, and
the advantages o f microwave heating in the coating formation processes were evaluated.
15
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In the third part o f the research, desulfurization o f an automotive NSR catalyst was
performed in the presence o f microwave radiation. The experiments were performed in
the presence o f various reducing atmospheres and in the presence o f steam. Mass
spectrometry was used to analyze species desorbed from the NSR catalyst. The amount o f
desorbed
sulfur compounds was
estimated with FTIR,
TEM,
XPS,
and Ion
Chromatography.
16
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Background and Novelty of These Studies.
Applications o f microwave radiation in chemistry have been extensively studied in
12
3
•
recent decades. M any chemical reactions and processes can be accelerated ’ ’ and in
some cases selectivity can be improved 4’ 5’ 6 due to the presence o f a microwave field.
Coupling with the electric field o f microwave radiation depends on the material type,
namely on the dielectric constant. The dielectric constant characterizes the ability o f the
material to be polarized by the electric field 1. Heating takes place due to interaction o f
microwaves with either dipolar molecules or ionic species
o
. W ater is a very good
medium for microwave-assisted synthesis due to its high dielectric constant. Energy
created by solvent absorption leads to efficient heating o f the reaction mixture 9’ I0.
There is a significant difference between a variable frequency microwave (VFM) and
single frequency microwave (MW) processes. During a variable frequency microwave
operation a selected bandwidth is being swept around a central frequency in a specified
time, which prevents microwave energy from remaining focused at any given location for
more than a fraction o f a second. Therefore variable microwave frequency processes
result in time averaged heating n ’ 12. In the presence o f standing waves o f electric fields
during constant frequency microwave operation, arcing may occur from a charge build­
up in conductive materials. Arcing problems and localized heating are eliminated using a
VFM technique 13. Application o f different microwave frequencies and a variable
microwave frequency operation during synthesis processes could affect the properties o f
the materials.
17
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I.
Effects of Microwave Heating in the Syntheses of OMS-1 and OMS-2
Manganese Oxides.
Todorokite (OMS-1) and Cryptomelane (OMS-2) are both microporous tunnelstructured manganese oxides composed o f M n06 octahedra. OMS-1 has a tunnel size o f
6.9 x 6.9 A and OMS-2 has a is 4.6 x 4.6 A tunnel size due to their 3 x 3 and 2 x 2
arrangements o f the M n06 building blocks correspondingly. Mixed valency o f OMS-type
manganese makes them good semiconductors and oxidation catalysts. The average
manganese oxidation state has been reported from 3.4 to 3.9 due to the presence o f a
mixture o f Mn4+, Mn3+’ and Mn2+ ions u ’ 15,16 OMS-type manganese oxides are easy and
inexpensive to prepare. Various synthetic routes have been explored, such as re flu x I7,18,
thermal or hydrothermal treatment o f bimessites 19’20, and a sol-gel ro u te 21’ 22.
Applications o f microwave radiation can be beneficial in processing o f materials with
high dielectric constants because absorption o f microwave radiation by a material
depends on its dielectric constant and its dielectric loss factor. M anganese oxides have
been reported to have a very high dielectric constant o f -10,000
. Therefore one can
expect good coupling with microwave radiation 24,25. As an effect o f that, microwaveassisted preparation o f manganese oxides may cause some interesting effects in
properties as opposed to conventional heating synthesis. In addition, syntheses at
different microwave frequencies may also give differences in the properties o f
microwave and conventionally prepared cryptomelane.
18
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II.
Coating of Sheet and Structured Materials via Microwave Heating.
Coatings play an important role in the manufacture and performance o f many articles.
Coatings can modify the surfaces o f a substrate, for example, diminish wear, inhibit
corrosion, and change the physical properties o f a substrate
0f\ on
’
A number o f
techniques have been developed to produce coatings 28>29’30’ 31.
One o f the popular methods to prepare coatings on a substrate includes dipping a
substrate in a liquid mixture containing the coating material and the subsequent removal
o f the liquid
"XO
’ . The major disadvantage o f that technique is the non-uniformity o f the
produced coatings. This problem is more serious when the substrate is textured and/or
porous. The challenge is to make coatings, in which the thickness at the comers or edges
o f a three-dimensional substrate is substantially the same as the thickness at other
positions o f the substrate. The non-uniformity in the coating arises primarily during the
removal o f the liquid, in which the substrate has been dipped. Applications o f microwave
radiation as a source o f heating during the removal o f the liquid could produce uniform
coatings due to the nature o f microwave heating.
In this study the effects o f microwave heating in producing inorganic coatings was
investigated. Inorganic oxide sols (e.g. silica, alumina, zirconia, and ceria) were
deposited on a quartz fibrous network by dipping the substrate in an inorganic oxide sol
and the subsequent removal o f the liquid by microwave heating. Conventional heating
was used to make the same inorganic coatings for comparison with the microwaveprepared ones.
19
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III. Microwave-assisted Desulfurization of NSR (NOx Storage Reduction) Catalyst.
One o f the major worldwide themes to protect the global environment is a growing
demand for cleaner exhaust gases and better fuel economy. Lean-bum engines provide an
effective technology to improve fuel economy. Still, lean-bum engines produce oxygenrich exhaust gases, in which the removal o f NOx is particularly difficult using
conventional 3-way catalysts 34. To overcome this problem, Toyota researchers
developed the NOx storage-reduction (NSR) catalysts 35. The NSR catalyst consists o f a
storage element, typically barium oxide and a reduction-oxidation component, typically
Pt. As a result, NOx species are stored as nitrates during the lean engine operation, and
during rich or stoichiometric operation, the nitrates can be reduced to nitro g en 36.
Unfortunately, the activity o f NOx storage-reduction (NSR) catalysts is greatly
reduced by sulfur poisoning due to the SO 2 present in the exhaust stream. Normally,
desorption o f sulfur species from poisoned NSR catalysts occurs at temperatures above
600 °C using reducing atmospheres and conventional ways o f heating.
In this work, microwave heating has been applied to enhance desulfurization o f
poisoned NSR catalysts. The desulfurization experiments were carried out by treating the
poisoned NSR catalyst with a reducing gas in presence o f a MW field. Desorption o f H 2 S
was observed at temperatures as low as at 200 °C. Desorption at even lower temperatures
(150°C) were observed when water was introduced to the system. In the presence o f
water, sulfur species desorbed as both H 2 S and SO 2 . An overall reduction o f sulfur
species o f about 60% was obtained. The use o f MW heating proves to be an efficient way
to achieve regeneration o f poisoned NSR catalysts.
20
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PART I. Effects of Microwave Heating in the Syntheses
of OMS-1 and OMS-2 Manganese Oxides.
Introduction.
Microwave heating is fundamentally different from conventional heating. In
microwave processes heat is generated internally within the material instead o f
£
originating from external sources . There are two principal mechanisms o f microwave
interaction with matter. One is that polar molecules tend to align themselves and oscillate
in step with the oscillating electrical field o f the microwaves. Collisions and friction
between the moving molecules result in heating. The more polar a molecule, the more
effectively the molecule will couple with the microwave field. On the other hand, ions are
charged species that can couple with the oscillating electrical field o f the microwaves.
The effectiveness or rate o f microwave heating o f an ionic solution are functions o f the
concentration o f ions in solution. Therefore, rapid heating in a microwave field is caused
by efficient absorption o f the radiation by a material that is placed in the field.
There are several important parameters related to microwave heating, which are
described in the equations below:
e
"
tan8 = s" /s '
(1)
tan 8 ~ 1/x
(2)
- dielectric loss
e' - dielectric constant
x - penetration depth
21
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The energy dissipation factor (loss tangent - tan 8) basically describes the material’s
ability to convert electromagnetic energy into heat under microwaves at given frequency
and temperature. The loss tangent is a phase difference between electric field and
polarization o f the material. The higher the dissipation factor the better the transformation
o f electromagnetic energy into heat. The energy dissipation factor is also a measure o f the
penetration depth (x) o f microwave radiation into a material and it is inversely
proportional to x. Microwaves can penetrate only a certain distance into a bulk material.
The equation suggests that there is an advantage o f working at lower frequencies for
larger samples. Penetration depth has been tabulated for only a few materials and at
certain temperature ranges.
e' is the dielectric constant and measures the ability o f a molecule to be polarized by
an electromagnetic field. For water for example s' is relatively high (-80) at low
frequencies (2.45 GHz), but drops to zero at 30 GHz. e" is a dielectric loss factor and its
related to the efficiency o f a medium to convert microwave energy into heat.
The
dielectric loss shows a parabolic profile reaching maximum at 20 GHz.
In addition, the chemical composition o f the material, as well as the physical size and
shape, will affect how it behaves in a microwave field. Microwaves obey laws o f optics
and can be transmitted, reflected, or absorbed. There are materials that reflect
microwaves. These are conductors like metals. The temperature increase in these kinds o f
materials is only marginal. Therefore, microwave devices are internally shielded with a
metal cage to avoid any leakage o f microwaves.
On the other hand, transmitters are
transparent to the MW radiation. Teflon, quartz, pure aluminum oxide (corundum),
22
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plastics, and special glass types are microwave transmitters. Absorbers on the contrary
show strong absorption o f microwaves that consequently leads to the rapid heating.
The use o f microwave radiation in chemical processes has been widely studied during
recent years. A large number o f publications focuses on this topic 4’ 5’ 6’ 7’ 8’ 9’ 10. Many
reaction types have been tested in a microwave field.
Some researchers think that the acceleration o f MW-assisted reactions is due to the
different mode o f transferring heat to the reagents and solvent4’37 Others think that there
might occur some specific activating (non-thermal) effects on the reagent molecules 38,39.
Because microwave radiation provides so-called volumetric heating, homogeneous
nucleation can be achieved. In addition, shorter induction times and enhancement o f
reaction rates in a microwave field are observed as opposed to conventional heating,
which leads to energy savings. This is very advantageous from an economical point o f
view. Literature also reports better crystallinity o f products7’ 40
Microwave heating depends on the MW absorption o f the material in the MW field.
Thus, localized high temperatures: so-called hot spots and hot surfaces can occur within
the material that may lead to a selective formation o f specific morphologies. Reflections
and refractions on local boundaries can cause hot spots, however this topic is still very
IQ
controversial
.
A variable frequency microwave (VFM) and a single frequency microwave (MW)
heating are very different in terms o f the energy distribution. A selected bandwidth is
being swept around a central frequency in a specified time during a variable frequency
microwave operation. Therefore, microwave energy does not remain focused at any given
location for more than a fraction o f a second. Variable frequency microwave processes
23
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result in time averaged heating n ’ 12 During a constant frequency microwave operation,
arcing may occur from a charge build-up in conductive materials in the presence o f
standing waves o f electric fields. Arcing problems and localized heating are eliminated
using a VFM technique13. Conde and co-workers studied effects o f microwave frequency
in the oligomerization o f methane to higher hydrocarbons. They observed changes in
product distribution with changing MW frequency due to differences in transverse
magnetic modes 41. Application o f different microwave frequencies and a variable
microwave frequency operation during syntheses could affect the properties o f the OMStype materials.
Manganese oxide octahedral molecular sieves (OMS) are one-dimensional tunnelstructured materials. They are formed by edge-shared and comer-shared MnC>6 octahedra
42,43. OMS materials are composed o f mixed-valent manganese species (Mn2+, M n3+, and
Mn4+). Their microporous structures have large open tunnels, and high surface areas 44,45,
46. Cryptomelane
and
todorokite-type
manganese
oxide
(OMS-2
and
OMS-1
respectively) materials are types o f important OMS materials. They have been
extensively used in catalysis, separation processes, chemical sensors, and batteries.
Syntheses, characterization, and applications o f OMS-2 and OMS-1 materials have been
investigated extensively for the past 2 decades 47’48,49’ 50’ 51,52,53. Vileno and coworkers
reported better crystallization kinetics for both, todorokite and bimessite, in the presence
o f microwaves. They also found differences in the surface properties between microwave
and conventionally synthesized todorokite, which led to differences in their catalytic
activity in the oxidative dehydrogenation o f ethylbenzene59.
24
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Todorokite, so-called OMS-1 (Figure 1. 1 (a)) is a multivalent manganese oxide that
can be found naturally in terrestrial manganese ore deposits and deep-sea manganese
nodules 54,55’ 56. Todorokite consists o f 3x3 arrangements o f edge-shared MnC>6 octahedra
with the pore size o f 6.9 A. This material is useful as a shape selective catalyst due to its
tunnel structure. In fact, synthetic todorokites are reported to be catalytically active in the
oxidation o f CO 51, H 2 O 2 decom position58, and in the production o f styrene by oxidative
dehydrogenation o f ethylbenzene59.
Synthesis o f todorokite in general involves three major steps: 1) synthesis o f a layered
precursor, namely bimessite (OL-1), 2) ion-exchange o f the precursor and 3)
hydrothermal aging 59. The use o f microwaves instead o f conventional heating in the
hydrothermal treatment step could significantly affect the physical and chemical
properties o f the microwave synthesized manganese oxides.
Cryptomelane-type manganese oxide (OMS-2) has a 2x2 tunnel structure with a pore
size o f 4.6 A (Figure 1. 1 (b)). Among other applications mentioned earlier OMS-2 has
been reported as an easily regenerable catalyst for the acid-catalyzed, selective, and
environmentally friendly oxidation o f alcohols using air as an oxidant 60. Various
synthetic routes have been explored, such as reflux50; thermal, or hydrothermal treatment
o f bimessites 47,61; and a sol-gel ro u te52,53. The reflux method involving the oxidation o f
Mn2+ by KMn 0 4 is the most commonly used route to prepare bulk OMS-2 materials.
25
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Experimental.
1. Microwave equipment.
A model 10-1300 Microwave oven (MMT, Knoxville, TN) equipped with a Micristar
heat control system and a Model LT 502Xb Variable Frequency Microwave Furnace
were used for MW-assisted syntheses. Figure 1. 2 shows the MW equipment. Both MW
furnaces had shielded thermocouples that remained in an intimate contact with the
samples during syntheses. The heating temperatures were controlled and monitored with
the thermocouples. Teflon autoclaves (CEM) were used for the syntheses in a microwave
field. The thermocouples were inserted into the Teflon vessels through tight openings in
the screwed cups.
2. Synthesis.
A hydrothermal treatment was applied to prepare the OMS-1 and OMS-2 materials.
Conventional and microwave prepared OMS-1 were synthesized according to the
following procedure (Figure 1. 3). First, a layered Na-bimessite (Na-OL-1) was made by
the oxidation o f Mn 2+ with potassium permanganate in a sodium hydroxide solution
(Figure 1. 3 a).
The resulting Na-OL-1 was aged hydrothermally as a slurry in a
magnesium chloride (MgCL) solution at 160 °C.
In this process, ion-exchange (to
produce Mg-OL-1 (Figure 1. 3 b)) and hydrothermal aging (to produce Mg-OMS-1
(Figure 1.3 c)) were performed in one step. The obtained OMS-1 was washed with large
amount o f hot (~100°C) distilled de-ionized water. Teflon autoclaves transparent to
microwave radiation were used for microwave processing. M etal autoclaves with Teflon
liners were used for the synthesis in a convection oven. Synthesis o f todorokite in a
26
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microwave oven took 8 hours and the synthesis o f todorokite in a convection oven took 2
days.
In the case o f OMS-2 first, manganese acetate water solution was added drop-wise to
a solution o f potassium permanganate with vigorous stirring at room temperature to form
the precursor material. The obtained brownish precipitate was kept stirred for 24 hours,
followed by a thorough washing with distilled de-ionized water (DDW). Then, the
precursor o f OMS-2 was treated hydrothermally in a convection oven and microwave
ovens at 100 °C (Figure 1. 4). Approximately the same volumes were used for the
conventional and microwave syntheses (70 ml). The synthesis with conventional heating
took up to 24 hours and the microwave-assisted syntheses took 4 hours. Approximately
0.5 g o f each OMS-2 material was obtained. The syntheses were performed at 2.45 GHz,
5.5 GHz, and at variable frequency programs. During the variable frequency program, the
microwave frequency was swept in the 3-5.5 GHz range with a sweeping time o f 1
second. The microwaves were generated at approximately 100 Watts during the
syntheses. The obtained materials were washed and calcined at 120 °C.
27
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Characterization.
1. X-Ray Diffraction.
A Braker D5005 diffractometer with Cu K a X-ray radiation and with a 1.5418 A
wavelength was used to collect the X-ray powder diffraction patterns o f the OMS-1
samples. A beam voltage o f 40 kV and a 40 mA beam current were used. The X-ray
powder diffraction patterns o f the OMS-2 samples were collected using a Scintag 2000
PDS instrument with Cu K a X-ray radiation with a 1.5418 A wavelength. In the case o f
OMS-2 the beam voltage o f 45 kV and a 40 mA beam current were used. The JCPDS
database was employed to identify the phases.
2. Average Oxidation State (AOS) Determination.
The average oxidation state o f manganese in all o f the samples was measured by a
two-step potentiometric titration. In the first step the total manganese content in a sample
is determined. The second step involves the measurement o f the amount o f available
oxygen in the sample. A complete titration procedure is reported in the literature
.
3. Morphology.
Morphologies o f microwave and conventionally synthesized OMS-1 samples were
examined with an Amray 1645 Scanning Electron Microscope. Samples were uniformly
spread on a carbon tape attached to an aluminum sample holder for the analyses. An
Amray model PV 9800 EDX was used for the energy dispersive X-ray EDX analyses.
28
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Transmission Electron Microscopy (TEM) images o f the OMS-1 materials were
obtained with a JEOL 2010 FasTEM at an accelerating voltage o f 200 kV.
OMS-1
samples were dispersed in ethanol and the suspensions were deposited on a Quantafoil
holey carbon coated copper grid (Rl.2/1.3).
Field emission scanning electron microscopy (FESEM) on a Zeiss DSM 982 Gemini
FESEM with a Schottky Emitter at an accelerating voltage o f 2 kV and a beam current o f
about 1 pA was used to study the morphology o f OMS-2 materials. The samples were
prepared for analyses by dispersing them in distilled deionized water (DDW). Droplets o f
the suspensions were placed on a gold-coated silicon wafer and dried in air.
In addition, Transmission Electron Microscopy (TEM) images o f the OMS-2 were
obtained with a JEOL 2010 FasTEM at an accelerating voltage o f 200 kV.
Powder
samples were dispersed in 2 -propanol and the obtained suspensions were deposited on a
Quantafoil holey carbon coated copper grid (Rl.2/1.3).
4. Thermal Analysis.
Thermogravimetric analyses (TGA) were carried out with a TA instrument Model
2950 under a N 2 atmosphere. Temperature was increased from 30 °C to 700 °C at a rate
o f 10 °C/min during the experiments.
5. Temperature Programmed Decomposition of OMS-2.
Temperature-programmed decomposition experiments o f OMS-2 materials were
completed using mass spectrometry (TPD-M S). About 30 mg o f each sample was loaded
into a quartz tube, and placed in a tubular furnace controlled with an Omega temperature
29
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controller. The samples were purged with a He gas (Matheson) for 4 h at room
temperature prior to the TPD experiments. The samples were then heated to 700 °C at a
rate o f 15 °C/min in He. The exhaust gases were monitored with an MKS-UTI PPT
quadrupole residual gas analyzer.
6. Specific Surface Area and Porosity Measurements.
A Micrometries ASAP 2010 accelerated surface area system was utilized for the
nitrogen sorption measurements.
The adsorption and desorption experiments were
performed at 77 K after initial pretreatments o f the samples by degassing them at 130 °C
for 12 hours. The specific surface area o f the samples was determined by the BrunauerEmmett-Teller (BET) method. The micropore size distribution was reported using the
Horvath-Kawazoe (HK) model. The mesopore/macropore size distribution was calculated
by the Barrett-Joyner-Halenda (BJH) method using the desorption data.
7. Acidity and Basicity Measurements of OMS-1.
Temperature Programmed Desorption (TPD) o f NH 3 and CO 2 experiments revealed
acidic and basic properties o f OMS-1. The experiments were done with 20 mg o f each
material. The samples were placed in quartz tubes and secured with quartz wool plugs.
N H 3 and CO 2 gases were passed over the samples for 30 minutes at room temperature.
Then, the samples was purged with He to remove the physisorbed gases.
The
temperature was increased from 25 °C to 700 °C at 10 °C/min rate. The gaseous products
were monitored with an MKS-UTI PPT quadruple mass spectrometer.
30
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8. Raman Spectroscopy.
Raman spectra were recorded in the range o f 100 - 2000 cm ' 1 with a Renishaw 2000
Raman microscope setup. The equipment includes an optical microscope and a CCD
camera for multi-channel detection. A 514-nm argon ion laser was used to record the
spectra for the conventional OMS-2 and the precursor.
9. Catalytic Activity of OMS-1: Oxidation of Indene.
In a 50 mL round bottomed flask, 1 mmol o f indene was dissolved in 10 mL o f
CH 3 CN (Acetonitrile) and 0.05 g o f either microwave or conventionally made OMS-1
was added to the solution. Then, two equivalents o f TBHP (tert-butyl hydroperoxide)
were added drop-wise using a pipette at room temperature. The mixture was stirred and
refluxed overnight at 70 °C, followed by cooling and filtration.
M gS 0 4 (magnesium
sulfate) was added as a desiccator. The reaction mixture was analyzed and quantified
using GC/MS.
10. Catalytic Activity of OMS-1: Oxidation of Benzyl Alcohol.
For the oxidation o f benzyl alcohol 1 mmol o f benzyl alcohol and 10 mL o f toluene
were placed in a 50 mL round bottomed flask. The catalyst (0.05 g) either conventionally
or microwave synthesized OMS-1 was added to the reaction mixture. The contents were
stirred and refluxed at 100 °C for 4 h, and then filtered. The products o f the experiment
were analyzed and quantified using GC/MS.
31
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11. Catalytic Activity of OMS-2: Oxidation of 2-thiophenemethanol.
The following procedure was used for the oxidation o f 2-thiophenemethanol. An
amount o f 1 mmol o f 2-thiophenemethanol and 10 mL o f toluene were placed in a 50 mL
round bottomed flask. Conventionally or microwave synthesized OMS-2 (50 mg) was
added to that mixture as the catalyst. The OMS-2 precursor was also tested for its
catalytic activity. The reaction mixtures were stirred and refluxed at 110 °C for 4 h in air
followed by filtration.. The products o f the experiments were analyzed and quantified
with GC/MS.
12. GC/MS.
A Hewlett-Packard Model 5890 Series II gas chromatograph coupled to a Hewlett
Packard Model 5971 mass selective detector was utilized to analyze and quantify the
reaction products. An HP-1 (nonpolar cross-linked siloxane) column with dimensions o f
12.5 m x 0.2 mm x 0.33 pm was used.
32
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Results.
1. Synthesis and XRD Results.
Synthesis o f OMS-1 in a microwave field took
that took 48 hours. After only
8
hours and the conventional synthesis
8
hours o f microwave heating fully crystalline OMS-1
material was obtained without any traces o f the precursor. The syntheses times were
estimated based on XRD analyses o f the materials. XRD patterns for conventionally and
microwave prepared OMS-1 materials are shown in Figure 1.5. All the samples have
crystal patterns corresponding to synthetic todorokite according to the JCPDS 38-475 o f
the NaMn 6 0 i 2 x D H 2 O. The patterns o f materials synthesized in the convection oven as
well as materials prepared in the presence o f a microwave field did not reveal any extra
phases. Reflections characteristic o f synthetic todorokite at 9.5 A and 4.7 A d spacing
were observed in all the samples. The diffraction peak at 4.7 A d spacing is the most
intense, which is characteristic o f the synthetic OMS-1. Microwave samples are much
more crystalline than conventionally prepared ones based on the peak intensities.
The average crystallite sizes for the samples were calculated based on the FWHM
(Full W idth at H alf Maximum) o f (002) peak (the most intense peak) using the Sherrer
equation.
The average crystallite size value is larger for conventionally prepared
materials compared to that o f the microwave synthesized samples o f tordorokite. The
calculated values are listed in Table 1.1.
The XRD results o f the microwave and conventionally prepared OMS-2 materials are
presented in Figure 1.
6
. The patterns correspond to the Q-phase o f cryptomelane
(KMngOi6 , JCPDS 29-1020). The OMS-2 precursor appears to be an amorphous
manganese oxide with little crystallinity showing
2
broad peaks. The d=2.389 A peak
33
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matches the strongest intensity for the synthetic cryptomelane. Another broad peak
appears at d=3.937 A. The JCPDS pattern o f the synthetic Q-cryptomelane phase is
shown at the bottom o f the Figure 1. 6 . MW-assisted synthesis o f OMS-2 took 4 hours.
Conventional preparation time o f OMS-2 was 24 hours. The end points o f the syntheses
were determined based on XRD analyses o f the materials, on the presence o f all the
characteristic intensities for OMS-2 in the XRD patterns. None o f the precursor XRD
peaks were observed in the patterns o f OMS-2 materials.
2. Average Oxidation State Determination.
Table 1. 1 shows the average oxidation number for manganese for OMS-1 samples.
The AOS o f M n in the microwave samples is higher compared to conventionally made
materials.
The values were calculated based on the potentiometric titration.
The
difference in the M n AOS might control the catalytic activity o f OMS-1 in oxidation
reactions. Table 1. 2 presents among others the results o f the average oxidation state
determination for manganese for the OMS-2 samples. Conventionally made OMS-2
appears to have the
lowest AOS o f manganese. The material prepared at variable
frequency operation
showed the highest value o f the Mn AOS. The AOS o f Mn is
characteristic o f OMS-2 for all the samples.
3. Morphology.
Figures 1. 7 and 1.
respectively.
SEM
8
show SEM and TEM micrographs o f the OMS-1 samples,
images show morphological inhomogeneity o f the microwave
prepared materials.The microwave-synthesized
samples show cubes
among the
34
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characteristic todorokite plate-like shaped particles. The average size o f a cube is 4 pm.
The cubic morphology was found in all microwave prepared OMS-1 materials.
Conventionally made todorokite showed a typical plate-like morphology very similar to
the morphology o f its precursor, Mg-OL-1. EDX analyses revealed that typical elemental
composition o f the cube surface is -1 0 % Mg and -9 0 % Mn. The surface composition o f
plates was 25-35% Mg and 75-65% Mn. This novel cubic morphology could be a result
o f the microwave heating applied during the syntheses. TEM data are in agreement with
the SEM and XRD results. Figure 1.
8
shows TEM images o f the cubic OMS-1 particles
and a selected area electron diffraction pattern o f the cubic particles. After tilting the
sample to the [060] zone axis, the selected area electron diffraction pattern (when
compared to the JCPDS file) confirmed that the cube was made o f OMS-1 platelets. In
addition, the electron diffraction pattern was simulated using Desktop Microscopist,
which also proved the todorokite structure o f the cubes. TEM results indicate that the
cubes are composed o f uniform OMS-1 fibers stacked and oriented along the same
direction. TEM data showed long fiber strands and flakes o f the conventional OMS-1.
Microwave-assisted synthesis o f OMS-1 could cause the agglomeration o f the single
OMS-1 fibers and flakes.
Figure 1. 9 depicts FESEM micrographs o f the OMS-2 materials. All the OMS-2
samples display fibrous morphologies that are typical o f synthetic OMS-2. No differences
in the morphology were observed with respect to the different types o f hydrothermal
treatment. The length o f the OMS-2 fibers is a few hundred nm and they are less than 100
nm in diameter. The OMS-2 precursor revealed a chunky morphology.
35
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TEM micrographs show the details o f the crystal structure o f the prepared materials.
Figure 1. 10 illustrates the morphology o f the OMS-2 precursor. The sample appears to
be mainly amorphous with parts showing some order. OMS-2 prepared at a low
microwave frequency (2.45 GHz) revealed a typical OMS-2 morphology (Figure 1. 11).
The insert o f the Figure 1. 11 (a) presents an electron diffraction pattern for a single fiber
o f the OMS-2 material prepared with microwaves at 2.45 GHz. The d-spacing values
calculated from the diffraction pattern are in good agreement with the XRD data for Qcryptomelane (KMn80 i6 , JCPDS 29-1020).
Figure 1. 12 shows TEM data for OMS-2 prepared at high frequency (5.5 GHz). This
sample revealed a non-uniform morphology consisting o f cryptomelane-like fibers and
smaller crystallites that are less than 100 nm long (Figure 1. 12 (b)). Based on the TEM
data, the relative ratio o f the larger to smaller crystallites is approximately 1:1. The insert
in Figure 1. 12 (a) shows the electron diffraction pattern for OMS-2 prepared with
microwaves at 5.5 GHz. The ring pattern suggests that the crystallites are randomly
oriented. The d-spacings calculated from the ring pattern are consistent with the JCPDS
file for Q-cryptomelane (KMn80 i6 , JCPDS 29-1020).
36
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Figure 1.1. OMS-1 (a) and OMS-2 (b) manganese oxides.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 1 . 2. Model 10-1300 Microwave oven (a) and Model LT 502Xb Variable
Frequency Microwave Furnace (b).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 1 .3 . Synthesis procedure o f OMS-1: (a) layered Na-OL-1, (b) layered Mg-OL-1,
(c) tunnel-structured Mg-OMS-1.
39
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Mn Acetate +
KMn04
3 : 2 Molar Ratio
Stirring
I Room T em perature
OMS-2 PRECURSOR
H ydrothlrm al Aging
(Conventional Ovenl/IW Oven/VFMW Oven)
OMS-2
Figure 1.4. Synthesis procedure o f OMS-2.
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CM
O
O
o
o
o
o
20
40
60
80
26 [degrees]
Figure 1 .5 . X-ray diffraction patterns o f layered precursors OL-1 and: (a) Na-OL-1, (b)
Mg-OL-1, (c) conventionally prepared OMS-1, (d) microwave prepared OMS-1.
41
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20 [degrees]
Figure 1 . 6 . X-Ray Diffraction o f OMS-2 made: (a) conventionally, (b) at 2.45 GHz, (c)
at 5.5 GHz, (d) at variable frequency. XRD pattern for the precursor (e).
42
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T able 1 .1 . Average Oxidation State (AOS), Average Crystallite Size (ACS), BET
Surface Area, BJH Desorption Average Pore Diameter, and BJH Desorption Cumulative
Pore Volume Data for the OMS-1 Materials.
BJH
Desorption
Cumulative
Pore Volume
[cm3/g]
Sample
AON for Mn
ACS [nm]
BET Surface
Area [m2/g]
BJH
Desorption
Average Pore
Diameter
[nm]
Conventionally
Synthesized
OMS-1
3.69
26
17
25
0.120
Microwave
Synthesized
OMS-1
3.76
20
13
21
0.079
43
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Table 1. 2. N 2 physisorption and Mn average oxidation state (AOS) determination results
for the OMS-2 samples.
Micropore
BET Surface
AOS
Area [m2/g]
±0.2
0.0030
67
3.72
0.489
0.0031
80
3.92
Average Pore
Total Pore
Diameter [A]
Volume [cm3/g]
Conventional OMS-2
151
0.272
2.45 GHz OMS-2
245
Volume
[cm3/g]
5.5 GHz OMS-2
189
0.307
0.0042
73
3.75
Variable Frequency OMS-2
271
0.437
0.0061
65
4.02
OMS-2 Precursor
64
0.468
0.0170
273
3.79
44
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Table 1 .3 . Conversion and selectivity in the oxidation o f 2-thiophenemethanol using the
manganese oxide samples as the catalysts.
C onversion
Selectivity
[%]
[%]
Conventional OMS-2
19
1 0 0
2.45 GHz OMS-2
30
1 0 0
5.5 GHz OMS-2
50
1 0 0
Variable Frequency OMS-2
40
1 0 0
OMS-2 Precursor
89
1 0 0
Sam ple
45
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F igure 1. 7. SEM micrographs o f (a) precursor OL-1, (b) conventional OMS-1, (c),
(d) OMS-1 synthesized in the presence o f a microwave field.
46
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(C)
ik l
* From the support grid.
D em ent
Atomic %
CK
31.7
OK
34.9
M gK
5.7
Mn K
21.6
Cu K
6.1
Total
1Q0.0
1
I
Energy [KeV]
Figure 1 . 8 . TEM bright-field image o f a todorokite cubic-shaped particle (a), high
resolution magnified image o f that particle (b), and its EDX (c). The inset o f (a) is the
[060] zone axis.
47
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Figure 1 .9 . FESEM images o f OMS-2 made: (a) conventionally, (b) at 2.45 GHz,
(c) at 5.5 GHz, (d) at variable frequency, (e) depicts the precursor morphology.
48
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Figure 1.10. TEM images o f the OMS-2 precursor (a) low magnification, (b)
high magnification.
49
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F igure 1.11. TEM images o f the low microwave frequency (2.45 GHz) OMS-2
(a) low magnification (the insert shows electron diffraction pattern for the single fiber),
(b) high magnification.
50
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Figure 1.12. TEM images o f the high frequency (5.5 GHz) OMS-2: (a) and (b)
low magnification (the insert shows the electron diffraction pattern), (c) high
magnification.
51
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4. T herm ogravim etric Analyses.
TGA results for the microwave and conventionally synthesized OMS-1 materials
match the data reported in the literature51. Both, the microwave and conventional OMS1 reveal similar initial water loss (around 2%) followed by two transitions centered at 325
°C and 575 °C. The weight loss at 325 °C is 9% for the microwave-synthesized sample
and
1 2
% for conventionally made material.
The results o f thermogravimetric analyses for OMS-2 are also typical o f the synthetic
OMS-2 63. All the OMS-2 samples synthesized using either M W or conventional heating
show similar thermal behavior. There are three major weight losses between 25 °C and
750 °C. The first weight loss up to 450 °C is about 3% and corresponds to physisorbed
and chemisorbed water. The second weight loss ~ 6 % that occurred from 450 to 650 °C is
due to an oxygen release from OMS-2 materials, as reported in the literature 63. The third
weight loss is approximately 3% and corresponds to the second lattice oxygen release
from 650 to 750 °C.
5. T em p eratu re-P ro g ram m ed Decomposition.
Figure 1. 13 presents release o f oxygen from the OMS-2 precursor and OMS-2
materials with increasing temperature. The oxygen peak appears as the lattice oxygen is
liberated due to heating in a He atmosphere. The oxygen peak shows when the M n -0
bond is broken and it characterizes the thermal stability limit for a material. Based on the
oxygen release temperature the peaks can be classified as low temperature (LT), medium
temperature (MT), and high temperature (HT) peaks according to Yin et al. 64,65, 66. All
the presented here materials show a major oxygen peak between 500 and 560 °C. The
52
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second smaller peak appears at a temperature that is
1 0 0
degrees higher than that for the
first oxygen peak. The O 2 profile for the OMS-2 precursor is dramatically different from
the ones for the OMS-2 samples. For the precursor oxygen starts evolving at 200 °C.
Thermal treatment o f the OMS-2 precursor also gives a sharp maximum at 500 °C. The
second oxygen peak emerges at 700 °C. Low and variable frequency cryptomelanes start
releasing oxygen upon heating at around 365 °C and show the maxima at 500 °C. On the
other hand the high frequency cryptomelane starts releasing oxygen as low as at 300 °C.
The oxygen MS signal gradually increases and reaches the maximum at 520 °C.
Conventionally prepared OMS-2 starts giving off oxygen at a much higher temperature
when compared to the MW-prepared materials, approximately at 430 °C and shows a
maximum at 565 °C.
6. Surface Area and Pore Size Distribution.
The N 2 isotherm adsorption data for the microwave-made OMS-1 materials are
similar to that obtained for conventionally made materials. The isotherm corresponds to
a type II adsorption isotherm with micropore filling at low p/po’s and capillary
condensation at high p/po’s.
The microwave-made OMS-1 show larger macropore sizes compared to the conventional
ones. The BET surface area o f the microwave OMS-1 was measured to be lower than
that o f conventionally made OMS-1. The values are shown in Table 1.1.
Table 1. 2 presents results o f the total pore volume, micropore volume, average pore
diameter, and Brunauer-Emmett-Teller (BET) surface area o f the OMS-2 materials. All
the samples show similar N 2 adsorption/desorption type II isotherms typical o f
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mesoporous materials. The macropore size distribution calculated by the BJH method
gave a major pore size diameter o f -5 3 0 A for the conventionally made OMS-2. All the
cryptomelane samples show a broad pore size distribution and their BET surface areas
are very similar. The OMS-2 precursor showed a relatively narrow pore size distribution
with an average pore diameter o f 64 A. In addition, the BET surface area o f the precursor
is 273 m 2 /g. This is approximately four times higher than the surface area o f OMS-2. The
OMS-2 precursor shows much higher percentage o f micropore volume than the OMS-2
samples. The precursor micropore volume was estimated to be ~ 3.6%. The average
micropore size for the precursor is 5.7 A based on the HK differential pore volume plot.
High frequency OMS-2 (5.5 GHz) also showed a quite high content o f micropores
(~2%), when compared to the remaining OMS-2 samples (~ 1.3%).
7. Acidic and Basic Properties of OMS-1.
Temperature programmed desorption o f NH 3 and CO 2 was used to study acidity and
basicity o f OMS-1 samples, correspondingly. The N H 3 and CO 2 desorption temperatures
reflect the strength o f acidic and basic sites, respectively. The area o f the desorption peak
reflects the amount o f the particular active sites.
Figures 1. 14 a and b illustrate
temperature programmed desorption o f N H 3 from OMS-1 samples. Microwave prepared
OMS-1 showed a maximum N H 3 desorption peak at 340 °C and conventionally made
OMS-1 shows a maximum N H 3 desorption peak at 280 °C. Microwave synthesized
materials have much stronger acid sites than conventionally made OMS-1 that is based
on the temperature o f the desorption peak.
Figures 1. 15 a and b show temperature
programmed desorption data for CO 2 from conventionally and microwave made OMS-1
54
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samples. The strength o f the basic sites is very similar for both samples, however the
microwave made material appears to have more weak basic sites than conventionally
OMS-1.
8. Raman Spectroscopy.
Raman spectra o f the conventional OMS-2 and the OMS-2 precursor are shown in
Figure 1.16. The OMS-2 shows a weak band at 570 cm
1
and a much stronger one at 645
cm '*. The precursor revealed different spectra for the different areas o f the material. The
sample contains a spectrum that is similar to the one for the OMS-2, and also spectra that
could be associated with other types o f manganese oxides. The precursor showed peaks at
500, 560, 635, and 655 cm
Bands at 500 - 510, 575 - 580, and 630-640 cm ' 1 can be
associated with M n 0 2 . The peak at 650 cm
are characteristic o f M n -0 lattice vibrations
1
f\7
is due to Mn 3 0 4 . Peaks at 570 and 650 cm ' 1
.
55
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100
200
300
400
500
Temperature [°C]
600
700
Figure 1.13. Oxygen evolution during Temperature Programmed Decomposition of: (a)
conventional OMS-2, (b) 2.45 GHz OMS-2, (c) 5.5 GHz OMS-2, (d) variable frequency
OMS-2, (e) precursor.
56
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Relative Partial Pressure [a.u.]
0.04
T
0.016
0.03
3
0.012
(0
CL
—
(0
0.02
0.008
(D
CL
g>
0.01
0.004
•■ a
ss
<D
OC
24
224
424
624
24
Temperature [°C]
224
424
624
Temperature [°C]
Figure 1.14. NH 3 temperature programmed desorption: (a) from conventional, and (b)
from microwave made OMS-1.
57
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Relative Partial Pressure [a.u.]
0.0016
r
0.002
4) 0.0016
0 .0 0 1 2
£ 0.0012
0.0008
0.0008
0.0004
■S 0.0004
24
224
424
624
30
Temperature [°C]
230
430
630
Temperature [°C]
Figure 1 .15. C 0 2 temperature programmed desorption: (a) from conventional OMS-1,
(b) from microwave made OMS-1.
58
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12-
10-
400
500
600
700
800
900
Raman Shift [cm-1]
Figure 1.16. Raman spectra o f OMS-2 (a) and the precursor: (b), (c), and (d).
59
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9. Oxidation of Indene.
Application o f conventionally synthesized OMS-1 as a catalyst in the oxidation o f
indene resulted in two products, 40% indenone and 40% phthalic anhydride with the
overall conversion o f 70%.
Indenone is an intermediate in the synthesis o f phthalic
anhydride. Microwave synthesized todorokite gave 100% selectivity to phthalic
anhydride in the oxidation o f indene.
10. Oxidation of Benzyl Alcohol.
The conventionally prepared OMS-1 did not show any catalytic activity in the
oxidation o f benzyl alcohol. However, OMS-1 synthesized by microwave heating gave
16 % conversion and 100% selectivity to benzaldehyde. Crystallinity, average oxidation
number, and the morphology o f microwave-synthesized OMS-1 could be responsible for
the enhanced activity. Oxidation o f alcohols is known to take place in acidic conditions
42. The microwave prepared materials have much stronger acid sites than conventionally
made OMS-1 based on the NH 3 TPD experiments. The improved catalytic activity o f
MW-OMS-1 in the oxidation o f benzyl alcohol could be associated with the stronger
acidity.
11. Oxidation of 2-thiophenemethanol.
Table 1. 3 presents conversion and selectivity results for the OMS-2 samples used as
catalysts in the oxidation o f 2-thiophenemethanol. All the tested materials showed 100%
selectivity to 2-thiophenecarboxaldehyde. The conventional OMS-2 gave the lowest
conversion
(19%) in the oxidation reaction. Moreover, OMS-2 prepared at the high
60
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microwave frequency (5.5 GHz) exhibits significantly higher conversion than the low
frequency OMS-2 (2.45 GHz). Conversion o f 2-thiophenemethanol increases as the
microwave frequency o f the hydrothermal treatment increases. The OMS-2 synthesized
at variable microwave frequencies (3-5.5 GHz) shows the conversion value between
those for the high (5.5 GHz) and the low (2.45 GHz) frequency materials. The OMS-2
precursor revealed the highest 2-thiophenemethanol conversion o f 89% among all the
tested catalysts.
The precursor was washed after the reactions with acetone and
methanol, calcined and reused for the same oxidation reaction. The conversion after the
recycle was 72%.
61
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Discussion.
1. Effects of Microwave Heating on Properties of OMS-1.
Application o f microwave heating during the aging o f layered precursors makes the
synthesis time o f todorokite-type manganese oxide approximately
6
shorter.
Faster
collapsing o f OL-1 layers in the presence o f microwave heating could in quicker
formation o f the 3x3xoo OMS-1 structures. The acceleration o f synthesis time may result
from the microwave coupling to both, water molecules and as well as manganese oxides
due to their high dielectric constant values.
OMS-1 synthesized via microwave heating appears to be more crystalline than the
corresponding conventional materials and has a smaller crystallite size
(2 0
nm) based on
the XRD results. These factors could affect the catalytic activity. In addition, selective
heating or presence o f “hot spots” could cause formation o f OMS-1 cubes in the
microwave field. Moreover, oscillating microwave fields m ay cause the agglomeration
o f todorokite fibers and platelets into cubic structures. TEM data indicate that the cubes
consist o f OMS-1 fibers that are uniformly arranged in one direction.
Microwave-made OMS-1 was more effective in the catalytic oxidation reactions than
the corresponding conventional OMS-1.
Differences in their properties might be
responsible for the enhanced activity o f MW-OMS-1. Higher oxidation state o f
manganese (3.76) found for MW-OMS-1 and the stronger acidity when compared to the
conventional OMS-1
might play a significant role in improving the catalytic
performance. The amount and strength o f Bronsted sites would have an effect on the
oxidation o f benzyl alcohol. Microwave prepared OMS-1 is also more stable than the
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conventional one according to TGA results. Conventional materials lose more oxygen
when heated, which lowers their oxidation properties.
2. Microwave Frequency Effects on the Synthesis of Cryptomelane-type
Manganese Oxide and Catalytic Activity of Cryptomelane Precursor.
Synthesis times in the presence o f microwave were
6
times shorter than those with
conventional heating. The final points o f the syntheses were determined based on XRD
analyses o f the materials and on the presence o f all the characteristic intensities for OMS2 in the XRD patterns. The precursor peaks were not observed in any o f the patterns o f
the OMS-2. Acceleration o f the syntheses might be due to the strong interaction o f
manganese oxide and water molecules with microwaves.
All o f the OMS-2 samples revealed fibrous morphologies typical o f the synthetic
OMS-2. On the other hand, the OMS-2 precursor consists o f unidentified chunky
particles. From the XRD data one could observe that the precursor’s pattern shows one
major peak that can be associated with the Q-phase o f cryptomelane at d=2.389 A. The
precursor seems to be mainly amorphous, mixed-valent manganese oxide. TEM data
show that the precursor is partially ordered. Lattice fringes appear among the disordered
material (Figure 1. 10). Morphologies o f the OMS-2 synthesized at a high microwave
frequency (5.5 GHz) and the OMS-2 synthesized at a low microwave frequency (2.45
GHz) are different. MW-OMS-2 synthesized at a high frequency (5.5 GHz) consists o f
both small fibers (less then 100 nm in length) and fibers o f a size typical o f OMS-2. The
small crystallites look like building blocks o f the longer fibers. Microwave heating at a
high frequency (5.5 GHz) may lead to the formation o f the smaller particles. The smaller
63
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particle size o f the OMS-2 prepared with microwaves at a high frequency (5.5 GHz)
might be one o f the reasons for the better catalytic activity o f the material. MW-OMS-2
synthesized at a low (2.45 GHz) consists o f fibers that have a size typical o f OMS-2.
The precursor is a mixture o f different manganese oxides based on Raman
spectroscopy and XRD data. Figure 1.16 shows Raman spectra o f the synthetic OMS-2
and the precursor in the range o f 500 - 655 cm _1. Bands in this region can be associated
with mixed-valent Mn in the manganese oxide systems. The precursor gave different
spectra in different areas o f the sample, indicating that the precursor consists o f different
manganese oxide types. One o f the spectra for the precursor is the same as the one for the
synthetic OMS-2. The OMS-2 precursor material gave extraordinary conversion o f 2thiophenemethanol to 2-thiophenecarboxaldehyde (89%) when compared to the OMS-2
catalysts (Table 1. 3). Even after recycling, the precursor preserved its catalytic activity.
The OMS-2 precursor has four times larger BET surface area (273 cm 2 /g) than the OMS2 materials. In addition, the precursor has a significant percentage o f micropores (3.36%)
and its average mesopore size is 64 A. All the other tested materials have significantly
larger pore sizes from 151 to 271 A and the percentage o f micropores is less than 1.3%.
The small size o f the precursor mesopores, the high surface area, and the significant
content o f micropores might enhance its catalytic activity. Small pores might better
accommodate the ~ 4 A molecules o f
2
-thiphenemethanol and therefore provide a
suitable environment for the reaction.
The precursor and the OMS-2 samples show a very different oxygen evolution
profiles. The precursor starts giving o ff oxygen at a temperature as low as 200 °C. All the
OMS-2 materials release oxygen above 300 °C. The low temperature oxygen release
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could potentially supply the oxygen needed for the oxidation reaction. In addition, the
high frequency OMS-2 (5.5 GHz) starts releasing oxygen at around 315 °C that is at a
lower temperature than for the other OMS-2 samples (~ 350 °C). This could be the reason
o f the enhanced conversion using the high frequency OMS-2 when compared to the other
OMS-2 catalysts. Thermogravimetric analyses o f the materials are in good agreement
with the TPD results. There are 3 major weight losses for all o f the OMS-2 samples. The
second weight loss that occurred from 450 to 650 °C reflects the oxygen evolution
observed based on the TPD data.
Microwave heating application during the OMS-2 syntheses resulted in differences o f
the catalytic activity. However, no major differences were observed between OMS-2
synthesized at different frequencies as well as at a variable frequency in terms o f
morphology, surface area, or thermal stability. On the other hand, OMS-2 synthesized
with microwaves at a high frequency (5.5 GHz) showed enhanced conversion in the
oxidation reaction. Similar to the precursor, high frequency material has a substantial
amount o f micropores (2.00%). Hydrothermal synthesis in the presence o f even higher
MW frequencies might result in an OMS-2 catalyst with improved activity in the
oxidation o f 2-thiophenemethanol. Ru-supported materials are reported in the literature as
selective catalysts in the oxidation o f 2-thiophenem ethanol68. OMS-type materials would
be a better option than a precious metal catalyst considering the much smaller
manufacturing expenses.
In the case o f OMS-2 syntheses, effects o f variable frequency heating was not
observed. Variable frequency processing might be beneficial in heating o f larger samples.
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Conclusions.
1. Effects of Microwave Heating on Properties of OMS-1.
Both, conventional and microwave heating were used to prepare todorokite-type
manganese oxides (OMS-1). Application o f microwave heating in the synthesis showed
pronounced effects on chemical, physical, and catalytic properties o f OMS-1.
Microwave-synthesized OMS-1 showed excellent conversion and selectivity to phthalic
anhydride in the oxidation o f indene. Phthalic anhydride is an important intermediate in
the pharmaceutical industry. Furthermore, microwave synthesized OMS-1 was active in
the oxidation o f benzyl alcohol to benzaldehyde, while conventionally made OMS-1 did
not give any conversion in this reaction.
Stronger acidity, higher oxidation state o f
manganese, and better thermal stability o f MW-made OMS-1 seem to be responsible for
its superior activity in the oxidation reactions.
2. Microwave Frequency Effects on the Synthesis of Cryptomelane-type
Manganese Oxide and Catalytic Activity of Cryptomelane Precursor.
Interesting information about the OMS-2 precursor was found while investigating
effects o f microwave frequency on the synthesis o f OMS-2. The precursor showed
spectacular activity in the catalytic oxidation o f
2
-thiophenemethanol (conversion =
89%).
In addition, OMS-2 synthesized with microwaves was also a better catalyst in the
oxidation
of
2-thiophenemethanol
than
the
conventionally
prepared
OMS-2.
Hydrothermal treatment at a higher microwave frequency seems to promote formation o f
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OMS-2 having a smaller crystallite size and larger micropore volume. As a result, more
active catalyst for the oxidation o f 2 -thiophenemethanol can be synthesized.
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Future Work.
Microwave effects on chemical systems are still waiting to be fully investigated and
understood. Regardless, microwave processing shows an alternative way o f producing
these materials and according to the results may provide optimal conditions for the
synthesis o f better catalysts. The suggested work for future could comprise:
1. Study o f the potential use o f the OMS-2 precursor in other catalytic reactions.
2.
Synthesis o f OMS-2 materials at even higher MW frequencies and systematic
studies o f their properties, like the crystallite size, surface area, and micropore
volume.
3. Studies o f microporosity stability o f the precursor.
4. Microwave-assisted
in-situ
experiments
(XRD,
Raman)
and
modeling
calculations.
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PART II. Coating of Sheet and Structured Materials via
Microwave Heating.
Introduction.
Coatings play an important role in the production and performance o f many articles.
They can modify the surfaces o f a substrate, for example, diminish wear, inhibit
corrosion, and change the physical properties o f a substrate 69, 70.
A number o f
techniques have been developed to produce coatings 71,72,73, 74.
One o f the popular methods to prepare coatings on a substrate includes dipping a
substrate in a liquid mixture containing the coating material and the subsequent removal
o f the liquid 75,76. The major disadvantage o f that technique is the non-uniformity o f the
produced coatings. This problem is more serious when the substrate is textured and/or
porous. The challenge is to make coatings, in which the thickness at the comers or edges
o f a three-dimensional substrate is substantially the same as the thickness at other
positions o f the substrate. The non-uniformity in the coating arises primarily during the
removal o f the liquid, in which the substrate has been dipped. Application o f microwave
radiation as a source o f heating during removal o f the liquid could produce uniform
coatings due to the nature o f microwave heating. This study shows an example o f
application o f microwave heating in producing inorganic coatings on a quartz fiber paper
by dipping the substrate in an inorganic oxide sol and the subsequent removal o f the
liquid by microwave heating.
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Experimental.
1. Coating Process.
Craneglas® 500 quartz fiber papers o f different thickness (1/16” and 1/8”) were
coated with silica. The coating was done with a LUDOX colloidal silica water suspension
(Aldrich). Postage-size pieces o f paper were dipped in the colloidal silica. The excess
liquid was removed by contacting the samples with a glass surface. The samples were
dried in a convection oven (120 °C, 1 hour, in air) and in a variable frequency microwave
(VFMW) furnace (120 °C, 30 minutes, in air). The time o f 30 minutes was established to
be sufficient for drying the samples in the microwave furnace. A center frequency o f 4
GHz, sweep time o f 10 seconds, and changing power between 33 and 99 Watts were used
for the VFMW-drying experiments. Variable RF power was set to minimum to assure a
slow temperature increase rate. Pyrex beakers were used for drying in a convection oven,
and Teflon liners transparent to MW radiation were used for drying in the VFMW
furnace. The samples were positioned vertically in the containers. Finally, the samples
were calcined at 600 °C for
6
hours.
Craneglas® 500 quartz fiber papers (1/16” o f thickness) were coated with the
following oxide sols. The coating was done with a LUDOX colloidal silica water
suspension o f various concentrations (Aldrich), A120DW colloidal alumina (Nyacol),
colloidal zirconia (Nyacol), and colloidal ceria (Nyacol). Postage-size pieces (1 x 1”) o f
paper (1/16” paper) were dipped in the colloids. The excess liquid was removed by
contacting the samples with a glass surface and tissue paper. The samples were dried in a
convection oven (120 °C, 1 hour, in air) and in a microwave furnace (120 °C, 30 minutes,
in air. A MW frequency o f 2.45 GHz). 30 minutes time was established to be sufficient to
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dry the samples in the microwave furnace. Pyrex beakers were used for drying in a
convection oven, and Teflon liners transparent to MW radiation were used for drying in
the MW furnace. The samples were positioned vertically in the containers. Finally, the
samples were calcined at 600 °C for
6
hours. Specifically, three types o f experiments
were performed:
1) quartz fiber paper coating with a silica sol o f different concentrations (5%, 10%, and
20 %)
2) sequential coating o f fiber paper with 5% silica sol (MW drying after each dipping
step) to obtain finally 2 0 % silica coating
3) MW-assisted coating o f quartz fiber paper with 10% colloidal alumina, 10% ceria, and
1 0
% zirconia.
Characterization.
Distribution o f the inorganic oxide coatings on a quartz fiber paper was studied with
an optical microscope. Coating o f the separate quartz fibers from the paper and the
thickness o f the inorganic oxide coatings on a quartz fiber paper were studied with field
emission scanning electron microscopy (FESEM) on a Zeiss DSM 982 Gemini FESEM
with a Schottky Emitter at an accelerating voltage o f 2 kV and a beam current o f about 1
juA. The samples were prepared for the analyses by gluing them on a carbon tape
attached to an aluminum holder.
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Results.
Craneglas® 500 quartz fiber papers were successfully coated with silica, alumina,
zirconia, and ceria. Microwave-assisted coating processes resulted in very uniform
distribution o f silica, alumina, ceria, and zirconia in the fiber network. In contrast,
conventional heating led to migration o f the oxide solutions to the edges o f the fiber
papers during the drying process and as a consequence non-uniform distribution resulted.
Figures 2. 1 and 2. 2 show optical images o f silica coated conventionally and silica
coated microwave-dried quartz fiber paper (after calcinations), respectively. Figures 2. 1
a, b, c, and d show poor and uneven distributions o f SiC>2 to the edges o f fiber paper
pieces that were dried in a convection oven. In contrast, microwave-dried samples show
(Figure 2. 2) quite uniform distribution o f the oxide and no SiCh accumulation on the
edges o f the paper sheets was observed.
Figures 2. 3 show HRSEM images o f 10% silica coated, MW-dried, and calcined
fiber paper. One can observe a fairly uniform coating along the separate fibers. The
coating fractures are due to mechanical treatment o f the samples that was necessary to
prepare the samples for HRSEM. Conventionally coated quartz fiber paper samples
showed that some regions were not coated at all and some showed accumulation o f silica
between fibers.
Figures 2. 4 show 10%, 20%, and 30% alumina coated, mw-dried and calcined fiber
paper.
The
coating
thickness
on
the
microwave-processed
samples
increased
proportionally to the concentration o f the oxide colloidal solutions.
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Figures 2. 5 and 2.
6
show HRSEM images o f the quartz fibers coated with 10% ceria
and 10% zirconia, respectively. Thickness o f the coatings was -2 0 0 nm when the 10%
sols were used.
Figures 2.
8
and 2. 9 show optical images o f ceria and zirconia-coated, conventionally
or microwave-dried, and calcined fiber papers. Conventionally dried samples show poor
and uneven distributions o f oxides to the edges o f fiber paper pieces. In contrast,
microwave-dried samples show quite uniform distribution o f the oxide. No accumulation
o f oxides on the edges o f the paper sheets was observed.
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Figure 2 .1 . Conventionally dried: (a) 1/16” fiber paper coated with 20% silica, (b) 1/16”
fiber paper coated with 30% silica, (c) 1 / 8 ” fiber paper coated with
2 0
% silica, (d) 1 / 8 ”
fiber paper coated with 30% silica.
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F igure 2 .2 . Microwave-dried: (a) 1/16” fiber paper coated with 20% silica, (b) 1/16”
fiber paper coated with 30% silica, (c) 1/8” fiber paper coated with 20% silica, (d) 1/8”
fiber paper coated with 30% silica.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2 . 3. HRSEM images o f 20% Silica-coated and Microwave-dried fibers from the
quartz fiber paper.
76
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Figure 2.4. HRSEM images o f the quartz fiber paper coated with alumina sols o f
different concentrations (10 %, 20%, and 30%) and M W dried.
77
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Figure 2. 5.10% Ceria coated quartz fiber paper.
78
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Figure 2 . 6 . 1 0 % Zirconia coated quartz fiber paper.
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Figure 2. 7. (a) MW-dried 10% Ceria-coated fiber paper, (b) conv. dried 10% Ceriacoated fiber paper.
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Figure 2 .8 . (a) MW-dried 10% Zirconia-coated fiber paper, (b) conv. dried 10%
Zirconia-coated fiber paper.
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Discussion.
Heating in a microwave occurs due to the interaction o f the material molecules with
electromagnetic field created in the furnace. Consequently, microwave energy can
provide fast and uniform heating. The interaction with microwaves depends on the
dielectric constant o f the material. Used in those experiments inorganic oxides absorb
microwave radiation to a small degree, however water has a relatively high dielectric
constant 11. Therefore, colloidal water suspensions o f the inorganic oxides can be
effectively heated in a microwave field leading to uniform distribution o f the coatings.
Optical microscopy studies o f the inorganic oxide coated materials revealed that the
application o f microwave heating in the drying step allows a uniform distribution o f the
oxide within the quartz fiber paper. No accumulation o f the oxides at the edges o f
microwave-heated samples occurred. FESEM images o f the microwave-heated samples
revealed fairly uniform coatings along the quartz fibers. Uniformity o f the coatings will
affect positively the mechanical properties o f the materials and also will increase the
surface area.
The coating method described here involves contacting a quartz fiber paper with a
mixture comprising a coating composition and a carrier fluid effective to wet the
substrate, and removing the carrier fluid by microwave heating for a time and at a
temperature effective to produce the coating.
In an advantageous feature, the coated
articles produced by the microwave method have uniform coatings in which the thickness
o f the coating at the comers or edges o f the substrate is substantially the same as the
coating thickness at other portions o f the substrate. In conventional heating methods,
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relying exclusively on convection, thermal energy is absorbed on the surface o f an object
to be heated and then is transferred towards the interior o f the object via thermal
conductivity. Because an energy transfer is occurring, the process can be quite slow and
non-uniform.
Microwave heating allows deep penetration by the microwaves, and
therefore energy is absorbed by an object to be heated as a whole and then converted to
heat via dielectric loss mechanisms. Due to the character o f microwave interaction with
polar molecules, the process can be quite rapid.
Furthermore, with microwave
irradiation, the heating is more uniform and less localized, which, with respect to
removing the carrier fluid, results in decreased migration o f the coating composition
during the drying process. This, in turn, results in coatings that may be more uniform and
have fewer or no bare spots.
In addition, as proved by microscopic characterization,
uniform coatings o f desired thickness can be produced on fibrous materials. Moreover,
the overall drying times can be reduced, which leads to manufacturing cost savings.
Other MW-coated articles may be useful in a variety o f applications including ion,
molecule, and gas separation/filtration, ion-exchanging, semiconductors, catalysis, and as
electrodes, among others.
Conclusions.
This study describes a novel procedure to coat sheets or structured materials made
from microfiber papers with colloidal slurries containing oxide materials. The colloidal
particles can be deposited onto the fibers giving a very uniform coating o f the fibers
within the quartz fiber paper. This novel procedure employs microwave drying to achieve
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the uniform distribution o f the coating. The resulting materials (after removal o f the
polymer binder within the fiber paper) possess very strongly attached and substantially
uniform coating o f the fibers.
Future Work.
The fixture experiments could focus on the coating o f larger sheets and structures
using microwave heating and comparison studies with respect to conventional heating.
The microfiber sheets or structures could be coated with various catalyst precursors and
the solution could be removed by microwave drying. Therefore, deposition o f the catalyst
precursor onto the fibers to form catalytic materials with high performance, surface area,
and uniformity for catalytic reactions within the fiber media could be done.
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PART III. Microwave-assisted Desulfurization of NSR
(NOx Storage Reduction) Catalyst.
Introduction.
The NOx storage-reduction (NSR) catalyst has been developed by Toyota researchers
in a worldwide effort to protect the global environm ent35. The NSR catalyst is composed
o f a storage element, typically barium oxide and a reduction-oxidation component,
normally Pt. During the lean operation o f an engine NOx species can be stored as nitrates,
and during the rich or stoichiometric operation, the stored NOx can be reduced to nitrogen
36
Unfortunately, the activity o f NOx storage-reduction (NSR) catalyst is negatively
affected by sulfur poisoning, caused by the SO 2 present in the exhaust stream. The most
difficult problem to solve and the major drawback o f the NSR catalyst is the decrease in
performance caused by deactivation o f the catalyst by sulfur. A competitive sorption o f
sulfur oxides (SOx) resulting from combustion o f sulfur species contained in the fuel
causes poisoning o f the catalyst. SOx species can react with the catalyst’s components
causing the catalyst to become inactive. Sulfur poisoning can occur in a two different
ways 36,78,79. First mechanism is the oxidation o f SO 2 by the precious metal followed by
the reaction with alumina and formation o f the aluminum sulfate. The second one
comprises the reaction o f SOx with the storage component o f the catalyst to form sulfates
(BaSC>4 ) that are more stable than the nitrates. Consequently, when the storage
component contains sulfates, the NSR catalyst becomes inactive. Presently, NSR
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catalysts are used only in countries like Japan, where the concentration o f sulfur in the
fuel is lo w 78.
Desorption o f sulfur species from poisoned NSR catalysts and its regeneration occurs
at temperatures above 600 °C using reducing atmospheres (H 2 ) and conventional heating.
Removal o f sulfur species in a propene or carbon monoxide flow occurs at temperatures
above 650 °C. The equations 3 and 4 describe desulfurization process under H 2 78:
B aS 0 4 (s) + H 2 (g)
B aS 0 4 (s) + 4 H 2 (g)
-» BaO (s) + S 0 2 (g) + H20
BaO (s) + H2S (g) + 3 H20
(3)
(4)
In this research, a novel and alternative method was investigated for the purpose o f
regenerating the NSR catalyst. Microwave (MW) heating was used to promote
desulfurization o f sulfur-poisoned NSR catalysts. The behavior o f the poisoned catalyst
was studied in a MW field, and using different reducing atmospheres and water. The use
o f MW heating was proved to be an efficient way to achieve the regeneration o f poisoned
NSR catalysts.
86
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Experimental.
1. Preparation of the NSR catalyst.
The NSR catalysts poisoned with sulfur were provided by the Toyota Motor
Corporation. The catalysts were prepared using standard wet impregnation with a
nominal composition o f 2 wt% Pt and 20 wt% Ba supported on Y-AI2 O 3 prior to
poisoning with SO 2 . For each experiment, about 30 mg o f the NSR catalyst was placed
between quartz wool plugs in a continuous-flow, quartz, and tubular reactor. The
untreated sulfur-poisoned NSR catalyst and the microwave-treated catalyst are referred in
the text as TS and TS-MW, respectively.
2. Microwave Apparatus.
Sulfur desorption experiments were performed using a MW oven model 10-1300
purchased from Microwave Materials Technology in Knoxville, TN. The temperature o f
the oven was programmed and monitored using a Micristar controller and a K-type
thermocouple, which remained in a close contact with the catalyst during the
experiments. The system was set up for automatic control o f temperature. Microwaves
were generated at 2.45 GHz with a maximum variable power supply o f 1,200 Watts.
3. Desulfurization Experiments.
Around 30 mg o f the TS catalyst was used for the experiments. Sulfur desorption
experiments were carried out at atmospheric pressure in a continuous-flow fixed-bed
tubular quartz reactor with Teflon fittings. The catalysts were purged overnight in He at
87
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room temperature prior to the microwave heating. The experiments were carried out in
the presence o f different gas streams. The reducing gas mixtures were 1% E^/He and
10% E^/He. They were passed at a flow rate o f 40 mL/s. Desorption tests were performed
at temperatures ranging from 150 °C to 200 °C and at a temperature ramp rate o f 10
°C/min. Sulfur desorption analyses were also performed in the presence o f water, under
similar temperature conditions. A peristaltic pump was employed to supply water to the
reactor. Desorbed sulfur compounds were monitored using an on-line MKS-UTI PPT
quadrupole mass spectrometer (MS).
4. Catalyst Characterization.
Ion-Chromatography (IC), X-ray photoelectron spectroscopy (XPS), and Fourier
transform infrared (FTIR) spectroscopy were used to quantify the sulfur species in the TS
catalyst before and after MW treatment. A Dionex IC, model DX-500 was used to
determine the amount o f sulfate present in the catalysts.
FTIR experiments were performed using a Nicolet Magna 750 FT-IR spectrometer in
the 2500-1000 cm ' 1 range using a DTGS detector. The samples were diluted in KBr for
the analyses.
XPS data were collected with a Leybold-Heraeus (LH) Model 10 spectrometer
equipped with a SPECS EA10 MCD hemispherical analyzer. The samples were pressed
in indium foil to minimize charging. Narrow and wide scans o f all elements were
collected for the prominent photoelectron transitions and X-ray excited Auger transitions.
88
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Results.
1. Microwave-assisted Desulfurization of NSR Catalyst.
Heating the TS catalyst in a microwave field and in presence o f 1% H 2 /He gas
resulted in the desorption o f H 2 S at 200 °C (Figure 3. 1). The MS signal for H 2 S shows
that the desorption was not completed after two hours o f conducting the experiment. H 2 S
evolution was accompanied by corresponding H 2 consumption. Reducing atmosphere o f
hydrogen and microwave heating promoted the desorption o f H 2 S from the poisoned
catalyst. A similar experiment was also carried out in 10% H 2 /He. Similarly, the H 2 S
evolution also became significant at 200 °C (Figure 3. 2). In case o f the higher
concentration o f hydrogen in helium ( 1 0 %) the process o f H 2 S desorption seemed to be
completed after 5 hours o f microwave heating.
2. Water Effect on the MW-assisted Desulfurization of TS Catalysts.
Several experiments were carried out in the presence o f hydrogen and water. Water is
known to interact very well with microwaves and has a relatively high dielectric constant
'. The input o f water to the reactor was controlled using a peristaltic pump. The reducing
gas was continuously fed to the reactor. Desorption o f H 2 S occurred at 150°C during the
experiment in the presence o f water. These results show that the presence o f water allows
the catalyst to be heated more efficiently, and consequently lowers the desorption
temperature.
Figure 3. 3 shows the MS signal o f H 2 S and SO 2 when heating the TS catalyst in the
presence o f steam and at 150°C in a microwave field. A significant increase in the H 2 S
89
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relative partial pressure was observed when water was introduced into the reactor. When
the water supply was stopped, the H 2 S MS signal decreased. W hen the water was
supplied for a second time, the H 2 S partial pressure increased again. A similar MS signal
profile was also observed for SO 2 , but the SO 2 relative partial pressure was much lower
as compared to H 2 S. Figure 3. 3 depicts the MS signal for SO 2 that is multiplied by a
factor o f 5.
3. Chemical Composition Analyses of the MW-heated NSR Catalysts.
Fourier transformed infrared experiments (FTIR) spectra o f the TS and TS-MW
catalysts were recorded in the 2500-1000 cm-1 range. Figure 3. 4 shows the FTIR spectra
o f the TS catalyst and the TS catalyst after microwave heating in hydrogen.
There are differences in the intensities for the MW-treated catalyst and the non­
treated TS. Figure 3. 4a shows the 1050 - 1250 cm ' 1 IR absorption region. The bands at
1120 and 1090 cm ' 1 are characteristic o f the S -0 stretching vibrations o f bidentate
sulfates located on the surface o f an alkaline earth metal and correspond to BaSC> 4 present
on the surface o f the catalyst79. The MW treated samples show variations in the intensity
o f the 1185 cm ' 1 band that is characteristic o f bulk barium sulfate species. The band at
1060 cm ' 1 changes its intensity after MW treatment and it corresponds to Al2 (S 0 4 ) 3
species.
TS catalyst normally contains barium in a form o f carbonate and oxide. Figure 3. 4b
shows the IR region that has bands characteristic o f BaCC>3 . The intensity o f the
carbonate band is lower for the MW-treated samples than for the untreated catalyst. The
90
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decrease in the intensity is more pronounced for the TS catalyst heated in the higher
concentration o f hydrogen ( 1 0 %).
The FTIR results provide information about the type o f sulfur species desorbed from
the TS catalyst. Table 3. 1 shows the percentage o f the different sulfur species removed
from the poisoned TS catalysts after MW treatments in hydrogen. The amount o f sulfur
due to aluminum sulfate decomposition is greater than the amount o f sulfur coming from
barium sulfate decomposition. Literature reports that A 12 (S 0 4 ) 3 decomposes at lower
temperatures than BaSCL upon conventional heating
7R
. Table 3. 1 also shows that the
sample MW-heated in 1% H 2 resulted in a greater decomposition o f A 12 (SC>4 ) 3 , compared
to BaSCL, than the MW-heated sample in 10% H2.
Ion Chromatography (IC) and X-ray photoelectron spectroscopy (XPS) were used to
analyze the bulk species and the surface species, respectively. Table 3. 2 shows the IC
results for the TS and MW-TS samples. The amount o f sulfate species in the catalyst was
reduced after MW treatment. The sample treated in 10% H 2 shows the lowest presence o f
SO42'. However, the difference between the samples treated in 1% H 2 and in 10% H 2 is
not significant, even though the time o f the MW heating in 10% H 2 was doubled when
compared to the sample heated in only 1% H2.
The atomic surface concentration o f the elements present in the poisoned NSR
catalysts was analyzed with X-ray photoelectron spectroscopy (XPS). The atomic
concentrations for the S 2p, C Is and O Is o f the untreated poisoned catalyst and treated
TS samples determined by XPS are shown in Table 3. 2. The binding energy (BE) values
obtained for S 2p (169.31-169.42 eV) and C Is (289.98-290.08 eV) are characteristic o f
sulfate species and carbonate species, respectively.
91
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The atomic concentration o f sulfate species in the TS catalyst is reduced after MW
treatment (Table 3. 3). Similarly to the IC results, higher concentration o f H 2 does not
improve the desorption o f H 2 S. The results o f the FTIR, ICP, and XPS confirm a similar
decrease in sulfur species after the MW-assisted processes. In addition, a decrease in
oxygen concentration was detected for the catalysts heated in a MW field due to the
treatment in the reducing atmospheres. The higher the concentration o f H 2 in the reducing
gas, the higher decrease in oxygen on the surface o f the catalysts was observed.
92
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0.025
■S'
0.02
200 °C
0.015
■P
i
0.01
150°C
150 °C
> 0.005
50
100
150
200
Time [min]
Figure 3 .1 . Thermal desorption o f sulfur from the sulfur-poisoned N SR catalyst placed
in a microwave field and exposed to 1% H 2 /He between 150 and 200°C.
93
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0.0002
3
■Si
S 0.00015
150 °C
200 °C
2 0.00005
Time [min]
Figure 3 .2 . Thermal desorption o f sulfur from the sulfur-poisoned N SR catalyst placed
in a microwave field and exposed to 10% F t/H e between 150 and 200°C.
94
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0.0005
“ 0.0004
23
| 0.0003
Q.
■
|
0.0002
A
CL
>
0.0001
0
20
40
60
80
Time [mini
100
Figure 3 .3 . Thermal desorption o f sulfur from the sulfur-poisoned NSR catalyst placed
in a microwave field and exposed to 1% t^ /H e and a controlled input o f water at 150°C.
95
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8
Relative Transmittance
c
— TS
<0
c
2
H
.5
«
Q
C
TS
1050
1100
1150
1200
Wave number (cm-1)
1250
1300
1400
1500
1600
Wavenumber (cm-1)
Figure 3 .4 . FTIR spectra o f sulfur poisoned NSR catalysts and catalysts treated with
microwave heating at 200 °C under varying reducing atmospheres.
96
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T able 3 .1 . Calculated percentage o f sulfate species removed after treatment in hydrogen
gas, based in FTIR resu ltsa.
%
% surface
% bulk
A12 (S 0 4 ) 3
B aS 04
B aS 04
TS-MW 1% H2/ He
85
28
25
TS-MW 10% H2/ He
57
48
47
Sam ple
a The percentages were calculated based on the integrated peak areas from the FTIR
spectra.
97
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Table 3.2. IC data obtained for treated and untreated poisoned NSR catalyst.
Sam ple
SO 4 2‘ w t. %
Tim e o f M W tre atm en t, min
TS
3.7
n.a.
TS-MW 1% H 2 / He
1 .6
150
TS-MW 10% H 2 / He
1.4
300
98
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Table 3 .3 . XPS data of treated and untreated poisoned NSR catalyst.
Time of MW
% Atomic Concentration
Sample
heating lm in ]
s o ? -----------C O ?
o
^
TS
ru i
L74
5?72
86.96
TS-MW 1% H 2
150
0.684
4.77
52.21
TS-MW 10 % H 2
300
0.619
4.68
50.52
n.a.: not applicable
99
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Discussion.
Desorption o f sulfur species present in the TS catalysts was observed at temperatures
ranging from 150 to 200 °C in a MW field and in H2. An effective coupling between
components o f a target material and the oscillating electrical field o f the microwaves
leads to dielectric heating. The catalyst used in this study is a complex material composed
o f barium oxides, sulfates, and carbonates, platinum, and alumina. All o f the components
absorb
differently
MW
radiation.
Microwave
heating
might
lead
to
thermal
inhomogeneities due to the differential MW absorption as compared to conventional
heating. The main components o f the catalyst have low dielectric constant values, and
therefore the interaction with microwaves is not very efficient in this c a s e 77. However, in
the presence o f water the absorption might be enhanced. Highly polar molecules, such as
water, can interact efficiently with the oscillating electric field o f the M W and therefore
generate heat within the catalyst bed. Water showed a positive effect in the desorption o f
sulfur from the TS catalyst. In the presence o f water, desorption o f H2S occurred at 150
°C. On the other hand, H2S desorption in 10% H2/He and in the absence o f steam
occurred at 200 °C. The experiments showed that the presence o f water in this process
allows the NSR catalyst to absorb MW radiation more efficiently, thus lowering the
desorption temperature.
IC and XPS data showed consistent results regarding the total amount o f sulfate
species removed from the NSR catalyst. XPS results give the amount o f sulfate species
removed from the catalyst’s surface and therefore showed higher amounts o f the removed
sulfur species when compared to the amounts determined in the bulk by IC. The FTIR
100
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results showed that Al 2 (S 0 4 ) 3 is preferentially removed from the NSR catalyst when
compared to BaSC>4 .
The concentration o f the reducing gas did not significantly affect the total amount o f
sulfate species removed from the catalyst. Namely, the total amount o f sulfur species
removed did not change drastically when using 1% H 2 /He as compared to 10% H^/He.
On the other hand, the concentration o f the reducing gas seemed to have an effect on the
type o f sulfur species being decomposed. Higher concentration o f hydrogen (10% H 2 /He)
resulted in a higher degree o f BaS 0 4 decomposition. The decomposition o f BaS 0 4 is
beneficial in terms o f regeneration o f the catalyst storage component. Perhaps in the
higher concentration o f the reducing gas, BaSC>4 centers have a better access to hydrogen
and can easily undergo the decomposition.
101
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Conclusions.
This study focuses on a novel method o f the desulfurization o f poisoned NSR
catalyst. In the presence o f MW radiation and hydrogen the desulfurization is achieved at
200 °C. In the presence o f MW radiation, hydrogen, and water, the desulfurization is
achieved at temperatures as low as 150 °C. Water seems to lower the desorption
temperature from 200 °C to 150 °C and also promotes significant evolution o f SO 2 from
the NSR catalyst. A higher concentration o f hydrogen seems to enhance the
decomposition o f BaS 0 4 . Microwave heating appears as a promising and efficient way to
regenerate the NSR catalyst.
102
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Future Work.
Microwave heating constitutes a promising, viable, and efficient way to regenerate
the NSR catalyst. M uch work still needs to be done to understand the mechanism o f the
desulfurization o f the NSR catalyst. Model experiments using BaSC>4 and Al 2 (S 0 4 ) 3 could
be performed to clarify the underlying mechanism o f the process. In addition, using
reducing gases with a tracer gas for the experiments would allow quantification o f the
H 2 S released from the catalyst. Application o f different reducing gases like CO and C 3 H 6
in the MW-assisted desulfurization could be studied. Formation o f COS in presence o f
these gases should be monitored.
103
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