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Low temperature microwave driven C1 reactions: The catalytic partial oxidation of methanol to formaldehyde and the gasification of coal

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THE FLORIDA STATE UNIVERSITY
COLLEGE OF ARTS AND SCIENCES
LOW TEMPERATURE MICROWAVE DRIVEN C1 REACTIONS: THE
CATALYTIC PARTIAL OXIDATION OF METHANOL TO
FORMALDEHYDE AND THE GASIFICATION OF COAL
By
MARK CROSSWHITE
A dissertation submitted to the
Department of Chemistry and Biochemistry
in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
Degree Awarded:
Summer Semester, 2012
Mark Crosswhite
All Rights Reserved
UMI Number: 3539542
All rights reserved
INFORMATION TO ALL USERS
The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMI 3539542
Published by ProQuest LLC (2012). Copyright in the Dissertation held by the Author.
Microform Edition © ProQuest LLC.
All rights reserved. This work is protected against
unauthorized copying under Title 17, United States Code
ProQuest LLC.
789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, MI 48106 - 1346
UMI Number: 3539542
All rights reserved
INFORMATION TO ALL USERS
The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMI 3539542
Published by ProQuest LLC (2012). Copyright in the Dissertation held by the Author.
Microform Edition © ProQuest LLC.
All rights reserved. This work is protected against
unauthorized copying under Title 17, United States Code
ProQuest LLC.
789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, MI 48106 - 1346
The members of the committee approve the dissertation of Mark Ray
Crosswhite defended on June 19, 2012
Albert Stiegman
Professor Directing Dissertation
Jeff Chanton
University Representative
John Dorsey
Committee Member
Alan G. Marshall
Committee Member
The Graduate School has verified and approved the above-named
committee members.
i
I dedicate this to Evelyn Crosswhite
Thank you for the personal sacrifice that you made for my education.
ii
ACKNOWLEDGEMENTS
I acknowledge Patty Crosswhite for her support in my life.
I acknowledge Al Stiegman for his investment in my professional
development.
I acknowledge my father and mother for teaching me not to give up on a
goal.
I acknowledge Kyle Serniak and Taylor Southworth for providing me
with excellent assistance in all aspects of my research.
iii
TABLE OF CONTENTS
ABSTRACT ...........................................................................................................................................................VIII
CHAPTER 1: INTRODUCTION TO MICROWAVE HEATING AND HETEROGENEOUS CATALYSIS ................................. 1
Microwaves .....................................................................................................................................................2
Microwave Heating of Liquids .........................................................................................................................2
Microwave Heating of Solids ...........................................................................................................................4
Impact of Selective Heating on Reaction Rates ...............................................................................................5
Heterogeneous Catalysis .................................................................................................................................7
Model for Selective Heating Applied to Heterogeneous Catalysis.................................................................11
Magnetic Spinel Oxide Nanoparticles as Microwave Selective Oxidation Catalysts .....................................13
Microwave Reactor Common to All Experiments ..........................................................................................20
Microwave Driven Heterogeneous Catalysis: Role in Green chemistry .........................................................22
CHAPTER 2: RAPID LOW-TEMPERATURE MICROWAVE SYSTHESIS OF FORMALDEHYDE ....................................... 23
Background ...................................................................................................................................................23
Experimental .................................................................................................................................................24
INITIAL STUDIES ............................................................................................................................................. 24
CATALYST PREPARATION.................................................................................................................................. 25
CATALYST CHARACTERIZATION .......................................................................................................................... 25
QUANTITATICE ANALYSIS OF FORMALDEHYDE ....................................................................................................... 27
HEATING ANALYSIS......................................................................................................................................... 28
OXIDATION OF METHANOL .............................................................................................................................. 29
MECHANISM OF METHANOL OXIDATION OVER AN OXIDE CATALYST .......................................................................... 32
CONSUMPTION OF METHANOL AND OXYGEN ....................................................................................................... 33
NMR ANALYSIS ............................................................................................................................................. 34
CHAPTER 3: LOW TEMPERATURE STEAM-CARBON GASIFICATION ....................................................................... 40
Background ...................................................................................................................................................40
Materials and Instruments ............................................................................................................................45
MICROWAVE REACTOR ................................................................................................................................... 45
INITIAL CARBON SOURCE (ACTIVATED CARBON) ................................................................................................... 47
GAS COLLECTOR ............................................................................................................................................ 48
iv
GAS CHROMATOGRAPHS................................................................................................................................. 49
Experimental .................................................................................................................................................51
Results ...........................................................................................................................................................52
CHAPTER 4: GENERAL CONCLUSION..................................................................................................................... 70
REFERENCES......................................................................................................................................................... 71
BIOGRAPHICAL SKETCH........................................................................................................................................ 82
v
LIST OF FIGURES
FIGURE 1 ELECTROMAGNETIC SPECTRUM AND INTERACTIONS WITH MATTER ........................................................ 3
FIGURE 2 SIMPLIFIED DEPICTION OF MECHANISM OF MICROWAVE HEATING OF SOLID DIELECTRIC MATERIALS ..... 5
FIGURE 3 TRADITIONAL (LEFT) THERMAL HEATING WHERE HEAT IS CONVECTIVELY TRANSFERRED THROUGH
GLASSWARE AND SOLVENT BEFORE HEATING THE REACTANTS VERSUS (RIGHT) INSTANTANEOUS
MICROWAVE HEATING OF ENTIRE SYSTEM WITH HETEROGENEOUS CATALYST .............................................. 6
FIGURE 4 REPRESENTATION OF REACTANT ADSORBING TO HOT CATALYST SURFACE, REACTING AND BEING
EJECTED INTO A COOL MEDIUM ................................................................................................................... 10
FIGURE 5 SOLUTIONS TO DIFFERENTIAL EQUATIONS FROM ENERGY BALANCE MODEL DEPICTED IN FIGURE 4 ..... 12
FIGURE 6 BALL AND STICK REPRESENTATION OF UNIT CELL OF NORMAL SPINEL OXIDE AB2O4 MATERIALS
2+
3+
SHOWING A , TETRAHEDRAL (PINK) AND B , OCTAHEDRAL (GREEN) SITES. ............................................. 14
FIGURE 7 A) XRD DATA OF COBALT CHROMATE NANO-PARTICLES SPINEL MADE BY PRECIPITATION OF COBALT
NITRATE AND CHROMIUM NITRATE, B) STANDARD SILICON USED FOR CALIBRATION AND C)STANDARD OF
COBALT CHROMIUM OXIDE.......................................................................................................................... 16
FIGURE 8 TUNNELING ELECTRON MICROSCOPE IMAGE OF COBLAT CHROMATE CATALYST SHOWING NANOSCALE
PARTICLES. .................................................................................................................................................. 17
FIGURE 9 COMMERCIALLY AVAILABLE MICROWAVE REACTOR FROM THE PRESENT STUDIES ............................... 21
FIGURE 10 CALIBRATION CURVE GENERATED FOR DETERMINATION OF FORMALDEHYDE .................................... 27
FIGURE 11 HANTZSCH REACTION USED TO REACT FORMALDEHYDE AND GENERATE PRODUCT WITH ABSORBANCE
MAXIMUM AT 415 NM ................................................................................................................................. 28
FIGURE 12 THERMAL IMAGES (UPPER PANELS) OF CHROMATE SPINELS A) FE, B) CU AND C) CO WITH THE
CORRESPONDING (LOWER) HEATING CURVES IN NON-ABSORBING SOLVENT ............................................... 29
FIGURE 13 CONVERSION OF METHANOL TO FORMALDEHYDE FOR MICROWAVE DRIVEN AND TRADITIONAL
(THERMAL) REACTION AND HEATING RATE SHOWN FOR EACH SPINEL......................................................... 32
FIGURE 14 CONSUMPTION OF OXYGEN AND METHANOL IN THE OXIDATION TO FORM FORMALDEHYDE ............... 33
FIGURE 15 PROTON NMR SPECTRA SHOWING FORMALIN PEAK GROWING AND METHANOL BEING DEPLETED AS
WATER REMAINS CONSTANT ....................................................................................................................... 34
FIGURE 16 CONVERSION PERCENTAGE SHOWN FOR EACH SPINEL AS A FUNCTION OF A) OXYGEN PRESSURE AND B)
METHANOL CONCENTRATION ...................................................................................................................... 37
FIGURE 17 THERMAL IMAGES OF MICROWAVE CAVITY, SAMPLE AND QUARTZ CELL (UPPER LEFT PANEL),
ACTIVATED CARBON AT 100 AND 200 W (UPPER CENTER AND RIGHT, RESPECTIVELY). ON THE RIGHT OF THE
UPPER PANELS IS THE TEMPERATURE SCALE THAT IS CUSTOMIZED FOR EACH THERMAL IMAGE. THE LOWER
PANEL SHOWS THE TEMPERATURE VERSUS TIME PLOTS FOR A SERIES OF RUNS FROM 10 – 200 W .............. 43
FIGURE 18 SHOWS THERMAL IMAGES OF ORGANIC MATTER AT VARIOUS STAGES IN COALIFICATION PROCESS. IN
THE UPPER LEFT, CENTER LEFT, LOWER LEFT, UPPER RIGHT CENTER RIGHT AND LOWER RIGHT ARE SAMPLES
DOE 5, LIGNITE, SS, GRAPHITE, PEAT, AND DOE 3, RESPECTIVELY........................................................... 44
vi
FIGURE 19 CUT AWAY VIEW OF THE MICROWAVE REACTION CHAMBER AND THE HOME MADE QUARTZ VESSEL .. 46
FIGURE 20 IMAGE OF FISHER BRAND ACTIVATED CARBON WITH 1 MM SCALE TAKEN WITH OPTICAL MICROSCOPE.
.................................................................................................................................................................... 47
FIGURE 21 SCHEMATIC OF HOME BUILT VOLUMETRIC GAS COLLECTOR ............................................................... 48
FIGURE 22 GAS CHROMATOGRAM SHOWING HE, H2,O2, N2, CO, AND IN THE ZOOMED IN REGION FROM 5.5 TO 12.5
MINUTES CH4 AND CO2 RETENTION TIMES IN MINUTES. .............................................................................. 50
FIGURE 23 REPORTED POSSIBLE REACTIONS A) HOMOGENEOUS WATER GAS SHIFT, B) HETEROGENEOUS WGS, C)
BOUDOUARD REACTION, D) HYDROGENATIVE GASIFICATION AND E) METHANATION WITH OCCUR WITH
CARBON, WATER, CARBON MONOXIDE, CARBON DIOXIDE, AND HYDROGEN. ............................................... 53
FIGURE 24 ACTIVATED CARBON RUN AT 65 WATTS SHOWING PERCENT COMPOSITION OF PRODUCT GASES FROM A)
STEAM CARBON REACTION HYDROGEN, CARBON MONOXIDE AND B) CARBON DIOXIDE FROM HOMOGENEOUS
WATER GAS SHIFT ....................................................................................................................................... 54
FIGURE 25 SHOWS PERCENT HYDROGEN, CARBON MONOXIDE AND METHANE PRODUCED VS. VOLUME OF GAS
PRODUCED AT 200 W FOR A HIGH ANTHRACITE COAL SAMPLE TERMED DOE 5 .......................................... 55
FIGURE 26 GC ANALYSIS OF HEAD SPACE FROM COAL REACTION ........................................................................ 58
FIGURE 27 SHOWS MMOLE HYDROGEN, CARBON MONOXIDE AND METHANE PRODUCED VS. VOLUME OF GAS
PRODUCED AT 200 W FOR A HIGH ANTHRACITE COAL SAMPLE TERMED DOE 5 .......................................... 59
FIGURE 28 PERCENT HYDROGEN, CARBON MONOXIDE AND METHANE PRODUCED VS. VOLUME OF GAS PRODUCED
AT 200 W FOR A BITUMINOUS COAL SAMPLE TERMED STOCKTON SEAM .................................................... 60
FIGURE 29 MMOLE HYDROGEN, CARBON MONOXIDE AND METHANE PRODUCED VS. VOLUME OF GAS PRODUCED AT
200 W FOR A BITUMINOUS COAL SAMPLE TERMED STOCKTON SEAM ......................................................... 62
FIGURE 30 SHOW PERCENT HYDROGEN, CARBON MONOXIDE AND METHANE PRODUCED VS. VOLUME OF GAS
PRODUCED AT 200 W FOR A SUB BITUMINOUS COAL SAMPLE TERMED DOE 3 ............................................ 63
FIGURE 31 MMOLE HYDROGEN, CARBON MONOXIDE AND METHANE PRODUCED VS. VOLUME OF GAS PRODUCED AT
200 W FOR A SUB BITUMINOUS COAL SAMPLE TERMED DOE 3 ................................................................... 64
FIGURE 32 PERCENT HYDROGEN AND CARBON MONOXIDE VERSUS VOLUME OF GAS PRODUCED AT 200 W FOR A
SYNTHETIC GRAPHITE SAMPLE .................................................................................................................... 65
FIGURE 33 MOLES PRODUCED VS. VOLUME OF GAS PRODUCED FOR VARIOUS WATTS .......................................... 67
vii
ABSTRACT
Microwave radiation is known to heat materials in a unique way that
differs significantly from conventional heating. Specifically, materials that
strongly absorb microwaves will be heated selectively and, under certain
circumstances, can attain temperatures higher than that of the surrounding
medium. In the particular case of heterogeneously catalyzed reactions, where
the solid catalyst is itself microwave-absorbing, the use of microwave heating
can produce dramatic enhancements of reaction rates and changes in selectivity
that differ dramatically from those produced by convective thermal heating.
These microwave-specific effects arise, in part, from the fact that microwaves
selectively heat the catalyst to temperatures much higher than the surroundings,
which can rapidly activate substrates that strike the surface and allow the
products to be ejected into the cooler medium. In addition to selective heating,
other microwave-specific effects that can have a strong impact on microwave
driven catalysis have now been well demonstrated, most notably the selective
interfacial heating of molecules adsorbed at the surface. The latter effect can
give rise to activation of substrates at the active site that can accelerate
chemical transformations at the surface. These factors suggest that great
enhancements in turnover number and product selectivity might be realized by
developing microwave driven catalytic reactions.
In the present studies, two microwave driven chemical reaction systems
are investigated. The first is the heterogeneously catalyzed oxidation of
methanol to formaldehyde over a series of microwave absorbing solid catalyst.
This reaction, which is run industrially at high temperatures as a gas-solid
reaction, was found to occur under mild conditions under microwave radiation
viii
when catalyzed by a series of magnetic spinel chromite oxides that have strong
microwave absorption cross sections.
The second reaction is the production of synthesis gas through the
selective heating of carbon and water (carbon-steam reaction) by microwaves.
This is a primary reaction in coal gasification, it has been identified that this
reaction is driven thermally at high temperatures whose reactants are strongly
microwave absorbing. In this study our objective was to heat the reagents using
microwave irradiation in order to drive the reaction at a lower temperature than
traditional heating methods. It has been show that the carbon can be heated
directly to temperatures at which it reacts with steam (also generated using the
microwaves) to produce synthesis gas (CO + H2). This work demonstrates that
microwave specific heating contributes to increased reaction rates in
heterogeneous chemical systems.
ix
CHAPTER 1: INTRODUCTION TO MICROWAVE HEATING AND
HETEROGENEOUS CATALYSIS
Microwave chemistry has its origin in 1946 in a secret research project
called “speedy weenie” Spencer was working in radar research when he began
testing a magnetron. He noted that a chocolate bar in his pocket was heated and
melted though nothing else on his person was heated. He conducted further
experiments and showed that corn kernels placed near a working magnetron
would be popped into popcorn. This started him down the path of studying the
“food effect” of microwaves1.
The food effect is physically based on the fact that the constituents of the
food (primarily water) heat efficiently in a microwave field while other
materials heat less or not at all. This fundamental observation, made in 1946, of
selective heating forms the basis for most recent studies on microwave driven
chemical reactions2. Specifically, the unique manner in which microwaves heat
materials can allow for lower observed reaction temperatures, lower energy
consumption, and higher reaction rates compared to traditional thermal
heating3. For heterogeneously catalyzed chemical reactions the catalysts can, in
principle, be designed or selected so that it will have a high microwave
absorbance. For other, non catalyzed chemical reactions their reactivity will be
governed by their ability to absorb microwaves. Some materials differ by
orders of magnitude in their ability to absorb energy and to convert that energy
to heat4. Systems containing two or more materials with a significantly
different absorptivity can be heated in such a way as to heat one component
selectively. If the portion of the system with the greatest ability to absorb
microwaves is the portion that requires the most energy then less energy will be
required for the reaction. Because of these advantages interest in the area of
1
microwave chemistry has increased in recent years. In 1986 there were only 3
publications on microwave driven chemical reactions and reached 4000 in
2011 (that represents a higher percent growth that Apple and Microsoft shares
in those same years).
Microwaves
Microwaves are electromagnetic radiations with wavelengths ranging
from one meter to one millimeter which in terms of frequencies lie between
0.3 GHz and 300 GHz. In commercially available microwaves used for heating
purposes the frequency is set at 2.54 GHz. Microwave power devices are
required to operate within one of the industrial, scientific and medical (ISM)
radio bands to prevent interference with broadcast and communications bands.
As such, commercial microwave ovens utilize a frequency in the S-band. This
restriction is used to preserve other wavelengths for communication5.
Microwave heating occurs through two fundamentally different dielectric
responses: dipolar and charge carrier. The former explains heating of molecules
in solution and the latter solids materials6.
Microwave Heating of Liquids
2
Figure 1 Electromagnetic spectrum and interactions with matter
For molecular dipolar heating (Figure 1), microwave energy is neither
sufficient to break bonds nor to cause molecular vibrations, but it will cause
molecular rotation. Molecular rotation occurs when the dipole of the molecule
attempts to align itself with the oscillating electromagnetic field. For molecules
in the gas phase which are free to rotate, this gives rise to quantized absorption
of the radiation that is observed as rotational spectra. For molecules in the
condensed phase, the rotation is hindered and the molecules cannot keep in
3
phase with the oscillations of the electromagnetic field (EM) field. This results
in frictional loss processes that manifest themselves as heat 7.
Microwave Heating of Solids
The various loss mechanisms by which solid materials can absorb
microwaves can be found in a number of reviews8-10. Briefly, the loss process
by which electromagnetic radiation transfers energy and, in turn, selectively
heats the substrate is typically treated using the complex form of the
permittivity, , and the permeability, , where the imaginary part ( " and "
represent the loss responsible for heating, and the real part ( ’ and
’ )
represents the anomalous dispersion term at the resonant frequency. The
magnitude of the loss is typically given by the loss tangent, which gets large
when the imaginary part is much greater than the real part10, 11.
ε = ε' + iε"
a)
μ = μ' + iμ''
b)
tanγ = ε"/ε'
c)
tanμ = μ"/μ'
d)
Dielectric heating, through the creation of charge-separation, is the
primary mode of heating for metal oxides, Figure 2. In the case of magnetic
materials, strong resonant interactions are possible through the interactions of
4
the magnetic field component ( ) of the electromagnetic wave with the
magnetic moment of the oxide (there are several different mechanisms
depending on the frequency of the radiation)12, 13. To assess microwave loss in a
material, the real and imaginary parts of the susceptibility and permeability can
be measured directly using established methods. This has been done for a
number of magnetic spinels and closely related materials14-19. The studies show
these materials to be strong microwave absorbers because of the complex
permittivity and permeability6.
--
-e--e
--e ---e -e-- e
--e -e-- e
--e
e-- e
-e-- e
e-- e
e-e-- -- e-- -e
e
e-- -- e-- -- e-- -e -- e -- e
e -- e -e -- e
e
--e--e
--e ---e -e-- e
--e -e-- e
--e
e-- e
--e e
e-- e
Figure 2 Simplified depiction of mechanism of microwave heating of solid dielectric
materials
Impact of Selective Heating on Reaction Rates
5
Figure 3 Traditional (left) thermal heating where heat is convectively transferred
through glassware and solvent before heating the reactants versus (right) instantaneous
microwave heating of entire system with heterogeneous catalyst
Figure 3 compares schematically the process that occur during
conventional convective and microwave heating20. In traditional thermal
heating the heat passes convectively from the heat source though the reaction
vessel, heating both the medium and the catalyst. Depending on how it is
executed, convective heating can potentially result in temperature gradients and
uneven heating of the reaction mixture. For a heterogeneously catalyzed
6
system, the temperature of the catalyst is the same as the temperature of the
bulk solution. This means that sufficient energy must be spent to heat the
solvent and reaction vessel as well as the reactants.
Microwaves heat volumetrically, from the inside out, with the absorbing
components of the system heating directly and, convectively heating the nonabsorbing regions. This allows a reaction to reach the desired temperature
quickly. For heterogeneously catalyzed systems utilizing spinel materials, the
microwaves couple to the metal oxide heating it instantly from the inside. This
yields a higher effective temperature at the surface, which will increase the
reaction at the active sites. In addition to this, the reactive intermediate can,
potentially, couple with the microwave and affect the formation of the product4.
It is difficult to elucidate which of these factors may have the largest impact on
reaction rates.
Heterogeneous Catalysis
Commercial chemical production, which exceeded 400 billion dollars in
the U.S. in 2007, relies heavily on petroleum, both directly as a source of
chemical feed stocks, and indirectly as an energy source in the production of
commodity and fine chemicals21. At the present time, when dwindling
petroleum resources are increasing the cost of energy and concerns about
environmental pollution are becoming acute, there is a pressing need to reduce
the energy consumption and the waste produced in chemical production. A
direct route to this is through improvements in catalysts and catalytic
processes, which are used in the production of many organic chemicals, and
which will reduce both the energy requirements, and the green house and toxic
7
waste emissions arising from chemical production. This can be accomplished
through the design and development of new catalysts that accomplish chemical
transformations with greater efficiency, at lower temperatures and with much
greater selectivity toward the desired product22.
Fundamental studies of catalytic processes are driven by both energy and
environmental needs. Of catalysts used in organic chemical production,
approximately 40% of them are used in various oxidation processes22. Many of
these processes are large in scale and are often very energy-intensive. Two
representative examples are the partial oxidation of methanol to produce
formaldehyde (world production: 18,000 ktons/yr) that is run over either an
iron-doped molybdate metal-oxide or a metallic silver catalyst at temperatures
from 400–650 °C, depending on the catalyst, and the gasification of carbon in
the presence of heated steam (>700 °C)
23-27
. Notably, these and other equally
important commercial oxidation processes take place either over metal
catalysts or complex multi-component metal oxides, the optimum formulations
and performance of which are often determined empirically. In recent years,
there has been considerable interest and effort in reference to developing more
efficient oxidation catalysts and, where needed, to find catalyst systems which
utilize environmentally benign oxidants26-28.
In the area of organic synthesis, the use of microwaves to drive specific
chemical reactions has been an area of increasing interest due to the rapid
reaction rates that can be realized, which at times exceed those observed from
thermal heating by several orders of magnitude29-31. The use of microwaves to
drive heterogeneous catalytic reaction systems has also been studied, albeit less
thoroughly, with many of those studies simply assessing the effect of
microwave
heating
on
existing
heterogeneously
8
catalyzed
reactions,
irrespective of whether they would be expected to absorb microwaves32. As
will be discussed, there are fundamental reasons why oxidation catalysts,
specifically chosen or fabricated to absorb microwaves, will drive oxidation
reactions with vastly higher conversion rates and product selectivity.
Achieving a fundamental understanding of how heterogeneous catalysts
operate and using this to realize more efficient catalysts and catalytic processes
is essential to the goal of energy efficiency and environmental responsibility in
the chemical industry. This work is a thoughtful, well-directed contribution to
achieving this overall goal, based on the development of selective catalytic
heating using microwave radiation. Intrinsic to this work are experiments
aimed at achieving a fundamental understanding of how selective microwave
heating drives reactions at the surface.
The most obvious advantage that microwave radiation affords in driving
a heterogeneously catalyzed reaction is the ability to selectively heat the
catalyst. Many industrial processes utilizing heterogeneous catalysts are high
temperature processes wherein both components of the reaction (i.e., catalyst
and reactants) are heated to the temperature required for the reaction to occur.
Microwave heating can, under appropriate conditions, selectively heat the
catalyst to the temperature required for substrate activation allowing the
medium to remain at a substantially lower temperature. The potential
advantages are that rapid activation of the substrate occurs at the hot surface of
the catalysts, which also imparts kinetic energy that rapidly ejects the product
from the hot surface into the cooler medium, thereby impeding further
reaction31-33(Figure 4).
9
Figure 4 Representation of reactant adsorbing to hot catalyst surface, reacting and
being ejected into a cool medium
In addition, it has also been reported that if this set of conditions is
optimized (i.e., very hot catalyst and cool surroundings), then product
selectivity can favor the kinetic product over the thermodynamic one. This
latter observation is especially true with gas–solid reactions34, 35. In addition,
10
since the microwaves heat the catalysts internally, instead of by convective
heating from outside, attainment of the desired temperature is almost
immediate. As observed in a number of studies, this can result in a much
cleaner and more selective reaction. For an appropriately absorbing substrate,
microwave heating is extremely energy-efficient; moreover, in a reactor system
designed to optimize the advantages of selective heating, significant energy
savings can be realized10, 36, 37.
Model for Selective Heating Applied to Heterogeneous Catalysis
An understanding of the parameters related to selective heating of a
catalyst can be achieved by considering the heat flow from an actively heated
catalyst in a reactant solution. A somewhat simplified model is depicted in
Figure 4 where the microwaves directly heat (q) the catalyst and the medium
through a linear absorption process, which is expressed by the
catI
and
medI
terms, respectively, where I is the energy flux of the microwave radiation in
Watts/m2 and
is the absorption coefficient. Heat is transferred from the
catalyst to the medium by convection, hc(Tcat -Tmed), where Tcat and Tmed are the
effective temperatures of the catalyst and medium, respectively, and hc is the
convective heat transfer coefficient. In this simple model, we are ignoring
radiative heat transfer from the catalyst to the medium and assuming an
adiabatic container, with no heat transfer to the surroundings35.
The heat flow in this model can be represented by several first order
differential equations and their solutions are shown corresponding to the heat
flow parameters defined in Figure 4 and where C is the heat capacity and m is
11
the mass. As can be inferred from Figure 4 and the heat flow equations, the
selective heating of the catalyst is optimized when the absorptivity of the
microwave radiation by the medium (
med)
and the coefficient of convective
heat transfer (hc) is minimized. Note that, in this discussion, the medium can be
either a liquid or a gas and can consist of the neat substrate or the substrate
diluted in a solvent. In cases where
med
and the hc are large, everything is
effectively heated and there becomes little difference between conventional and
microwave heating
Solving this system of differential equations yields the following solutions
Figure 5 Solutions to differential equations from energy balance model depicted in
Figure 4
12
Magnetic Spinel Oxide Nanoparticles as Microwave Selective
Oxidation Catalysts
In a heterogeneously catalyzed system, the catalyst, which is the solid
state, catalyzes the reaction of substrates that are in the gas or solution phase.
The catalytic activity occurs at active sites on the surface of the catalyst. We
chose to study heterogeneous catalysts because, in principle, they can be
tailored to have a large absorption cross section for microwaves and other
reaction specific properties.
They are also easy to separate from the reaction mixture, usually by
filtration and may be reused or reactivated when recalcined. In many cases
recalcining the metal oxide will reactivate it because it will be reoxidized and
any staining species on the surface will be driven off. This contributes to the
metal oxide spinel cost effectiveness. The opposite is true for most
homogeneous catalysts, an example would be acid or base catalysts, which are
harder to separate and reuse. In addition, they are often toxic.
Our approach to the design and selection of microwave active
heterogeneous catalysis is to identify classes of materials that show some level
of thermal activity toward a specific chemical transformation and, at the same
time, are strongly microwave-absorbing. Among the materials that meet these
requirements are magnetic spinel nanoparticles. Their suitability arises from
the fact that they have significant microwave absorption cross sections arising
from loss processes associated with interactions of the electromagnetic field
with the unpaired electrons in the spinel (the imaginary parts of the permittivity
and permeability) Additionally, spinel metal oxides are thermally stable and
can be made on the nano-scale. Because reactions occur on the surface of the
13
spinel, a spinel with a higher surface area will be more active than the same
spinel with a lower surface area.
Because of their tunability to have a high absorptivity for microwave
irradiation, specific reaction activity, high surface area and resistance to high
temperatures metal oxide nano-particles make an ideal catalyst. They are
prepared directly with water as the solvent. In addition to the preceding
chemical reasons for using spinels they are also affordable and easy to
synthesize.
Figure 6 Ball and stick representation of unit cell of normal spinel oxide AB2O4
materials showing A2+, tetrahedral (pink) and B3+, octahedral (green) sites.
Because spinel nanoparticles exhibit both excellent selective microwave
heating properties and oxidative catalytic activity our interest in them was
peaked in using them as microwave-enhanced catalysts.
Spinels, as a class, afford a large, synthetically accessible range of
compositions that permit systematic variations in the reactive sites at the
14
surface thus facilitating optimization of catalytic activity. Moreover, numerous
examples exist of useful oxidations catalyzed by spinels of differing
composition that may be enhanced with selective heating. Finally, many
spinels tend to be strong microwave absorbers. The mechanism of microwave
absorption in spinels is largely due to the loss factors originating from the
interaction of the electromagnetic radiation with the unpaired electron spins in
a material. Since this is a magnetic interaction, the primary spinels that we
focused on were the magnetic ones. In addition, synthetic protocols exist for
the fabrication of nanoparticles of several important classes of spinels. The
advantage to nanoparticles in this effort is two-fold. For catalysis, the
advantages lie in the high surface areas that they afford and in the numerous
catalytically active surface defect sites, which allow unique opportunities for
the sorbed species to interact with the otherwise relatively inert spinel. From
the standpoint of selective microwave heating, the small size will generally be
smaller than the penetration depth of the radiation, which means the particle
will rapidly heat all the way through.
15
TABLE OF CONTENTS
a
b
c
10
20
30
40
50
60
Two theta (deg)
70
80
Figure 7 a) XRD data of cobalt chromate nano-particles made by precipitation of
cobalt nitrate and chromium nitrate, b) standard silicon used for calibration and c)standard of
cobalt chromium oxide
Figure 7 shows the XRD pattern for cobalt chromate spinel that we made
in our lab and it shows that the metal oxide spinel is not pure, but is biphasic,
whereas other spinels used in this study were more phase pure.
16
TEM images of Spinel Nanoparticle
Figure 8 Tunneling electron microscope image of cobalt chromate catalyst showing
nanoscale particles.
Figure 8 shows a tunneling electron microscope (TEM) image of a single
cobalt chromate spinel nanoparticle. These metal oxides are synthesized by the
co-precipitation method described later. They have large surface areas which
provide more active sites and make it a better catalyst.
Spinels are technologically important and an extremely large class of
solid oxides. Spinel itself is a natural gemstone with the formula MgAl 2O4 that
is a paradigm for the class of oxides having the general formula A2+B3+2O4. As
a whole, these oxides represent one of the most extensive series of related
chemical compounds38. The structure consists of a unit cell composed of eight
sub-cells that are each essentially face-centered cubic arrays of oxygen. Four of
the sub-cells contain tetrahedral sites that are occupied by two A2+ cations,
while the remaining four contain octahedral sites occupied by the B3+ cations.
In addition to those octahedral sites, there are twelve additional ones that are
17
not centered in the sub-cell. This brings the number of B3+ to 16 and A2+
tetrahedral sites to 8, or eight AB2O4 formula units per unit cell.
In addition to normal spinels, there are also examples of what is referred
to as inverse spinels. In inverse spinels, the A2+ cation occupies one half of the
octahedral coordination sites, while half of the B3+ cations occupies the other
half of the octahedral coordination sites and all of the tetrahedral sites. Spinels
also tend to show a large amount of cation disorder, and many spinels
containing transition metal ions have inverse or partially inverse structures
attributable to ligand field stabilization effects, which govern the site
preferences of the ions39-41. In addition to the disordered phases, the spinels can
also exhibit large deviations from ideal stoichiometry. All of these variations
can have an effect on the bulk and surface properties. Compositionally, spinels
represent an extremely large range of inorganic oxides. The A2+ ion can vary
over a wide range of ions, usually falling within an ionic radius of 8–11 Å
including, but not exclusively, Mg, Fe, Mn, Zn, and Cu. Typical B3+ ions, with
an ionic radius usually falling between 7–9 Å, are Ti, Cr, Fe, Co, Ga and Al.
Importantly, with the use of co-precipitation synthetic techniques, a ternary
metal system of the general form A1-xAxB2O4 [e.g., Zn1-xCoxFe2O4, Cu1xCoxFe2O4]
can be synthesized, thereby dramatically extending the rage of
materials accessible42, 43.
As heterogeneous catalysts, which are our particular interest, chromates
(MCr2O4) spinels have been used to catalyze a number of reactions 44-47. Among
the chromates, the CuCr2O4 composition is an example of the well known class
of “copper chromite” catalysts that catalyze a number of organic
transformations48. Importantly, many of these examples are either known as
18
strong microwave absorbers or because of their electronic properties, are likely
to be microwave absorbers 14-19, 49.
In recent years, interest has been shown in the magnetic properties of
spinels, particularly the ferrites, of which magnetite, Fe3O4 (a mixed valence
Fe2+/Fe3+ inverse spinel) is a prototypical example50, 51. This has, in part, driven
the development of synthetic protocols for the creation of nanoscale particles
for certain spinel materials with the aim of expanding their magnetic
properties. These will expose sites for chemisorption events that promote
catalytic transformations. The earliest verification of this for spinels, of which
we are aware, are studies by Tsuji et al. on the use of “ultrafine” NiFe2O4
spinels for catalytic production of CH4 from CO2 and H2 (CO2 methanation).
They found that spinel nanoparticles (~16 nm) produced methane yields that
were up to 6 times larger than those produced on bulk materials 52. This study
has been followed in recent years by several others that have furthered the
demonstration of the advantages of nanoscale spinels for catalytic processes 5356
. Finally, in an interesting exploitation of the magnetic properties of ferrite
spinel nanoparticles for the purposes of catalysis, Phan and Jones derivatized
the surface of CoFe2O3 with an organic base catalyst for performing the
Knoevenagel condensation57. The catalysts proved effective and could be
recovered from the reaction mixture using a magnet18.
Two complete series of nano spinels have been synthesized as part of the
preliminary studies. The specific materials we have made are: ferrite, AFe2O4,
where A2+ is: Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Ni0.5
2+
Mn0.5
2+
and
chromates, ACr2O4, where A2+ is: Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+
All of the spinels were synthesized as nanoscale particles, the size of
which was estimated from the X-Ray Diffraction (XRD) peaks using the
19
Scherrer equation and observed directly using transmission electron
microscopy (TEM). Surface areas were determined from Brunauer-EmmetTeller (BET) analysis and found to be between 23 and 38 m2/g.
The compositions listed represent a very systematic array of spinel
oxides from which to base our studies. All of the divalent ions, which span a
range of first row transition and alkali-earth metals, enable us to evaluate the
effect of the trivalent ions with the divalent ion being held constant. Likewise,
within a series, systematic variations across the range of divalent cations can be
determined. Notably, among the materials synthesized is one example of a
mixed metal system, specifically Ni0.5Mn0.5F2O4. From the standpoint of
catalysts for oxidation or reduction reactions, the range of materials represent a
very systematic redox active series. The transition metals can be either an
oxidant or reductant depending on the electron configuration and the reactant.
In addition, many of the ferrites have the inverse spinel structure, meaning that
both the tri- and di-valent ions will occupy both tetrahedral and octahedral
sites, which will be reflected in the reactive site composition. When evaluating
catalysts and correlating surface properties with reactivity this is likely to be a
factor18.
Microwave Reactor Common to All Experiments
The microwave reactor used in these experiments is a commercially
available instrument from CEM, Matthews NC. The CEM Focused
Microwave™ Synthesis System, Discover, is designed to enhance the ability to
perform chemical reactions under controlled conditions on a laboratory scale.
20
The system facilitates either homogeneous or heterogeneous solution phase
chemistry.
Figure 9 Commercially available microwave reactor from the present studies
There is a cooling feature which directs a gas source onto the outside
wall of the reaction vessel. This provides the ability to cool a reaction after
and/or during the application of microwave energy. The cooling feature is
either “on or off”, but the pressure and flow of gas can be held constant.
The Standard Control option provides flexibility in how the user
programs a reaction method and makes greater use of the feedback control data
from the temperature and pressure systems. It applies a specified amount of
power, defined by the user, to reach the control point. It modulates this set
power automatically, based on the sensor feedback data, to ensure the control
point is reached rapidly, but with limited error.
21
Microwave Driven Heterogeneous Catalysis: Role in Green
Chemistry
Microwave driven chemistry is an important part of the larger idea of
green chemistry. Scientists are constantly being asked to consider more
environmentally benign methods to achieve the desired chemical reactions and
attain the needed level of productivity. Using safer solvents and more energy
efficient methods are two fundamental ways in which we can achieve greener
chemistry.
Among the widely accepted principles of green chemistry are two that
apply particularly well to microwave chemistry: energy efficiency and
catalysis. These temperature efficiencies are important when considering
heat/energy lost to the surrounding during reaction. Energy and time saved
during the cooling process can also contribute significantly to the energy
saving.
As will be discussed, intelligently selected reactants and catalysts can be
heated directly by microwaves with high energy efficiencies. This can allow
for an efficient use of microwave energy and can have the least environmental
impact.
Although there have been major advances in recent years in synthetic
chemistry methodology, the use of conductive heating is still the primary way
that chemical transformations are accomplished. It is the opinion of this author
that microwaves will soon become the standard method for heating.
22
CHAPTER 2: RAPID LOW-TEMPERATURE MICROWAVE
SYSTHESIS OF FORMALDEHYDE
Background
The use of microwave radiation instead of conventional thermal heating
can result in a profound increase in reaction rates for certain chemical
reactions. In the particular case of heterogeneously catalyzed reactions, where
the solid catalyst itself is microwave absorbing, the use of microwave heating
can produce dramatic enhancements in the reaction rates and changes in
selectivity13. These microwave specific effects arise, in part, from the fact that
microwaves selectively heat the catalyst to temperatures much higher than the
surroundings, which can rapidly activate substrates that strike the surface and
allow the products to be ejected into the cooler medium58. In an elegant study
by Bogdal et al, the magnitude of this selective heating was directly measured
for a CrO2 oxidation catalyst using thermal imaging techniques59-61. The
catalyst was found to be approximately 200 °C higher than the surrounding
medium. In addition, it has also been reported that if this set of conditions is
optimized (i.e. very hot catalyst and cool surroundings), product selectivity can
favor the kinetic product over the thermodynamic one. The latter observation is
especially true with gas-solid reactions62, 63. In addition to selective heating,
other micro-wave specific effects have been well demonstrated, most notably
the selective interfacial heating of molecules adsorbed at the surface of
heterogeneous materials64.
Because of the potential advantages provided by microwave selective
catalysis we established an effort to develop microwave specific catalyst
23
materials. Our approach was to identify classes of materials that show some
level of thermal activity towards a specific chemical transformation and, at the
same time, are strongly microwave absorbing. Among the materials that meet
these requirements are spinel nanoparticles. Their suitability arises from the
fact that they have significant microwave absorption cross-sections arising
from loss processes associated with interactions of the electromagnetic field
with the unpaired electrons in the spinel (the imaginary parts of the permittivity
and permeability).
The oxidation of methanol to formaldehyde is one of the most
challenging and useful chemical transformations as formaldehyde is an
essential C1 building block that is used in a myriad of products. Industrially, its
synthesis is carried out by the oxidation of methanol, which is heterogeneously
catalyzed either by silver metal or an iron molybdate metal-oxide catalyst65, 66.
Because of its high volatility and reactivity, formaldehyde is generally
produced as an aqueous solution known as formalin24,
67-70
. As part of our
investigations of microwave driven heterogeneous catalysis, we have
developed a rapid, direct, solution-phase synthesis of formalin solutions from
the microwave-specific oxidation of aqueous methanol using nanoscale spinel
oxide catalysts.
Experimental
INITIAL STUDIES
24
Initial microwave reaction studies were carried out using a fixed set of
reaction condition. Specifically, reaction mixtures composed of 6 mL of a 12.3
M aqueous methanol (1:1 v/v MeOH:H2O) solution and 166 mg of the spinel
catalyst were heated to a temperature of 60 °C for 80 minutes. The reactions
took place in a closed Pyrex® cell with constant stirring under 1 atm of O2 with
the temperature monitored internally using a fiber optic thermometer. The
reactions were also carried out under identical conditions (i.e. stirred, closed
container, 1 atm of O2, internal temperature monitoring) but using a regulated
thermal bath to maintain a temperature of 60 °C. The results are shown in
Figure 13.
CATALYST PREPARATION
Spinels were synthesized by dissolving metal nitrate salts of the +2 and
+3 metals separately into nanopure water at 10 % (w/v). The ionic metal
solutions were added in the desired molar ratio to a flask fixed with a
condenser. The mixture was stirred for 12 hours on low heat. Strong ammonia
solution or a solution of NaOH was added drop wise until the pH reached ~8.5.
The solution was vacuum filtered hot yielding the spongy precipitate with a
clear colored to colorless filtrate. The solid product was left to dry overnight at
110 °C and then finely ground by mortar and pestle before being calcined
under ambient pressure at 500 °C for 12 hours.
CATALYST CHARACTERIZATION
25
All of the catalysts were characterized by XRD, and a representative
XRD diffraction pattern for cobalt chromate is shown in Figure 7 The Co and
Fe chromites proved to be phase pure spinels while the Cu chromite proved to
be somewhat biphasic with small amounts of Cr2O3 detected in the XRD.
Control reactions were run with pure Cr2O3 and it was found neither to be
strongly microwave absorbing nor to catalyze methanol oxidation reactions.
The particles produced through precipitation synthesis were nanoscale, with
dimensions estimated from the Scherer equation to be ~ 6 - 14 nm. For the case
of CuCr2O4, the presence and crystalline properties of the nanoscale particles
were confirmed secondarily by transmission electron microscopy (Figure 8).
The surface area and particle size of the catalyst are given in Table 1.
Table 1 Surface area and particle size for the spinel catalysts
Spinel
4
CuCr2O
FeCr2O4
CoCr2O
BET
Average size (nm)
surface32.2 area
e6
(m2/g)52.7
(nm)* 12
59.8
14
Size
4
Surface area is one of the crucial parameters that govern catalytic
activity. Therefore we analyzed the spinels using BET technique and found that
surface area values for these spinels are comparable with those reported in
literature66.
26
QUANTITATICE ANALYSIS OF FORMALDEHYDE
1
0.8
Abs
0.6
0.4
0.2
0
0.00
0.02
0.04
0.06
0.08
0.10
formaldehyde (g/mL)
Figure 10 Calibration curve generated for determination of formaldehyde
Figure 10 shows the calibration curve for formaldehyde which was
obtained by reacting formaldehyde with acetylacetone and ammonium acetate
by the Hantzsh reaction (Figure 11) to form 3:5-diacetyl-1:4dihydrolutidine
(DDL), which absorbs at 415 nm71. Absorbance was plotted vs concentration of
formaldehyde (g/mL) using a PerkinElmer Lambda 900 UV/VIS/NIR
spectrometer with Lambda 900 software.
27
Figure 11 Hantzsch reaction used to react formaldehyde and generate product with
absorbance maximum at 415 nm
The Hantzsch (Figure 11) reaction has been used as a selective method
to quantify formaldehyde in living systems, air and fabrics71, 72.
RESULTS AND DISCUSSION
HEATING ANALYSIS
Figure 12 depicts heating curves, which are plots of temperature versus
time, for the chromates used in this study. On the y-axis is time and on the X
axis is temperature in ⁰C. Each plot correlates to a different metal oxide
catalyst. The vessel and the solvent do not absorb microwave energy and hence
do not heat in a microwave field. All heating of the non-microwave absorbing
solvent is due to the absorption of energy by the metal oxide and the transfer of
that energy to the solvent (Figure 4). This is a qualitative way that we assessed
the ability of a metal oxide to absorb microwave energy. It is clear that not all
metal oxides absorb microwave energy equally efficiently. The microwave
28
heating properties of chromite spinels are shown in Figure 12. It can be seen
that there is variation in heating properties as a function of composition;
however, the best examples reach temperatures of ~55 °C in around two
minutes, which establishes them as strong microwave absorbers73.
60
55
50
FeCr2O4
Temp(oC)
45
CoCr2O4
40
CuCr2O4
35
30
25
Figure 1 Thermal images of a) Fe, b) Co, and c) Cu chromite irradiated at 50 W for 144 sec in a non-absorbing (mesitylene) solvent and d) average temperature
over time of the solutions.
20
0
50
100
150
Time(s)
Figure 12 Thermal images (upper panels) of chromate spinels a) Fe, b) Cu and c) Co
with the corresponding (lower) heating curves in non-absorbing solvent
OXIDATION OF METHANOL
In preliminary studies a range of ferrite and chromite spinels were
screened for alcohol oxidation in general and methanol oxidation in particular.
29
The catalysts which showed significant microwave driven methanol oxidation
were a series of chromites, ACr2O4, with the dications Fe2+, Co2+, Cu2+. These
spinels were made using the co-precipitation method, which generates
nanoscale particles (~6-14 nm) with surface areas ranging from ~30-60 m2/g66.
The relative microwave heating efficiency (Figure 12) was assessed
through thermal imaging of a fixed amount of catalyst in a non-absorbing
solvent (mesitylene). Thermal images of stirred solutions of the three catalysts
after 144 seconds of irradiation at 50 W shows typical characteristics of
volumetric heating (Figure 12). While some thermal in-homogeneities can be
observed, the solutions generally exhibit relatively constant temperatures
throughout with cooler regions relaized at the edge of the container. As can be
seen in Figure 12, all of the spinels heat efficiently, though there are significant
differences between them with the CoCr2O4 showing the most efficient heating
while the FeCr2O4 the least. The heating rates from each catalyst were obtained
from the linear portions of the heating curves.
It is also important to note that the catalytic reactions are run in aqueous
methanol solutions. As such both the catalyst and the solution will be highly
microwave absorbing. Control experiments indicate that no formaldehyde
forms from applying microwaves to aqueous methanol in the absence of a
catalyst, however the presence of an absorbing medium can potentially
attenuate the fraction of the radiation absorbed by the catalyst, though we do
not directly observe this effect in our studies.
The most significant aspect of the reaction is that it is almost uniquely
microwave specific, with only small amount products (≤4% of microwave
conversion) being observed during conventional thermal heating. Notably, the
conversion efficiencies do not correlate with the heating properties of the
30
catalyst. The poorest catayst, CoCr2O4, has the highest heating rate while
CuCr2O4, which heats least efficiently is the best. This suggests that the
observed catalytic activity is dependent on the composition of the catalyst.
Since the +3 cations are the same, this would seem to implicate the +2 cation,
may be the active site. However, the activity may well be one involving both
metal sites acting synergistically. We can further infer from the data that some
of the variation in the reactivity between different catalysts is inherently
microwave specific. This is suggested by the fact that there is no direct
correlation between thermal and microwave activity, which one would expect
if reactivity was governed only by catalyst composition. In particular, while
CuCr2O4 is both the best thermal as well as microwave catalyst, the other two
catalysts show a reverse behavior with FeCr2O4 showing good microwave
conversion and no reactivity while CoCr2O4 shows modest microwave and
thermal reactivity suggesting that the microwave specific interaction play a
role. Taken together, this suggests that the compositional factors may be tied to
specific microwave effects occurring at the interface between the surface and
the substrates64.
Figure 13 shows thermal and microwave driven results from the
oxidation of methanol over the selected materials. One can see that there is a
large significant difference between the effectiveness of the microwave versus
the thermal results.
31
.30
30
.25
.2
.15
.1
Heating Rate (C/s)
.05
0
0
5
% Conversion
25
20
15
10
Thermal
Microwave
FeCr2O4
Figure Predicted temperature of catalyst
CoCr2O4
CuCr2O4
Figure 13 Conversion of methanol to formaldehyde for microwave driven and
traditional (thermal) reaction and heating rate shown for each spinel
MECHANISM OF METHANOL OXIDATION OVER AN OXIDE CATALYST
The net reaction for the oxidation of methanol to formaldehyde occurs as
an oxidation reaction that consumes oxygen and produces water (rxn 1)
CH3OH + 1/2O2  CH2O + H2O
(1)
The reaction is generally thought to take place by a Mars van Krevelen
mechanism where oxidation of methanol occurs through removal of the lattice
32
oxygen of the catalyst. The reduced catalyst is subsequently reoxidized with
O2. The first step in the process will be coordination of the alcohol to vacant
coordination sites on the metal, which for the spinels can be exposed A2+ or B3+
sites. This is followed by C-H bond breaking processes that have been shown
to be the rate-determining step of the reaction over the conventional thermal
catalyst74. For the microwave driven reactions it will be useful to determine
whether they follow the same kinetics as the gas-solid thermal reaction.
Isotopic labelling studies have indicated that the rate limiting step is cleavage
of the methyl C–H bond on methanol75.
Concentration (mol/L)
Pressure (atm)
1 1.5 2 2.5 3 9 11 13 15 17
CONSUMPTION OF METHANOL AND OXYGEN
0
20
40
Time (min)
60
80
0
5
10
Time (min)
15
20
Figure 14 Consumption of oxygen and methanol in the oxidation to form
formaldehyde
33
It was observed that the decrease in the concentration of methanol as
measured by NMR and the decrease of the O2 pressure as measured by the
pressure probe in the microwave reactor shows agreement (Figure 14).
Additionally the increase in formalin concentration measured by NMR and the
colorimetric analysis of the Hantzsch reaction also agrees nicely with the
consumption of MeOH and O2.
NMR ANALYSIS
Figure 15 Proton NMR spectra showing formalin peak growing and methanol being
depleted as water remains constant
34
The evolution of the microwave driven oxidation over the CuCr2O4
catalyst, as a function of microwave irradiation time, was monitored by 1H
NMR spectroscopy (Figure 15). The spectra indicate very selective oxidation
of methanol with no evidence of any products other than formalin (4.8 ppm),
within the detection limits of the NMR76. Notably, this also includes formic
acid, a typical by-product, which would have exhibited a methyl resonance at
6.45 ppm. Using the integrated intensities, calibrated with
known
concentrations of methanol, the decrease in methanol concentration over time
can be measured. The disappearance fits well to an exponential decay
consistent with the reaction being first-order in methanol and yielding a pseudo
first order rate constant of 6.9 10-3 moles/liter sec. This is consistent with a
majority of the rate expressions that have been examined for gas phase
oxidation of methanol over molbydate catalysts75, 77. The turnover number for
the CuCr2O4 catalyst under these conditions, based on the rate of formalin
production and the surface area of the catalysts was determined to be 0.30
moles/sec m2.
The corresponding NMR spectra (Figure 15) show production of 31.8 %
formalin and the consumption of 37.7 % of MeOH. The O2 used and the
methanol decrease determined by NMR show close correlation78. The formalin
quantified by the absorption technique and NMR show close correlation, 31.3
and 31.8 % respectively. Having looked for secondary products and having
found none it has been concluded that some of the formaldehyde (BP = -19 ⁰C)
escapes before forming the formalin complex with water. Therefore we do not
see a “gap” between 32 % formalin being formed and 37 % reagents being
consumed. We observed consumption of enough O2 to account for 36.9 % of
the total needed to oxidize all the MeOH.
35
The effect of applied O2 pressure was investigated across the series of
catalysts. Under the same conditions of reaction temperature, concentrations
and amount of catalyst, the percent conversion of methanol under 1 and 3 atm
of applied O2 was measured. For all catalysts the amount of methanol
converted increased by a factor of 1.49(±0.09) in going from 1 to 3 atm,
suggesting that the order of the O2 dependence of the reaction is less than one,
which is consistent with studies of the gas-solid reaction over molybdates75, 77.
The consumption of O2 was measured by the decrease in pressure. For O2,
5.91 10-3 moles were consumed during the course of the reaction while
1.21 10-2 moles of methanol were converted to formaldehyde yielding a
stoichiometry of ~2:1 MeOH:O2.
36
% Conversion
40
30
20
10
a
0
1 atm O2
3 atm O2
CoCr2O4
CuCr2O4
FeCr2O4
CoCr2O4
CuCr2O4
b
0
% Conversion
20 30 40
10
FeCr2O4
1:3.2
Figure Predicted temperature of catalyst
1:1
MeOH : H2O (V:V)
1:.5
Figure 16 Conversion percentage shown for each spinel as a function of a) oxygen
pressure and b) methanol concentration
Industrially methanol oxidation is carried out as a gas-solid reaction, at
temperatures between 300-440 °C, wherein vaporized methanol and oxygen
mixture reacts over the catalyst. Though there are some disagreements in the
literature, the iron molybdate catalyst used in the industrial reaction is
generally considered to be water sensitive and the desired formalin solutions
are generated at the end of the reaction by addition of water 75. For this reason,
the fact that microwave conversion occurs in high yield in aqueous solution is
somewhat notable. Commercial formalin solutions are typically 37 wt %
37
formaldehyde in water, which are inhibited by methanol in amounts ranging
from 1-15 wt %. Under the reaction conditions described above (160 mg
CuCr2O, 12.3 M MeOH, 80 minutes at 60 °C) we produce 0.0173 moles of
formaldehyde yielding a formalin solution that is only 9.37 wt %
formaldehyde. With the goal of making solutions with higher formalin
concentration directly, we investigated the effect that water had on the
production of formalin. Using the same conditions described above, reactions
were run with methanol to water ratios of 1:3.2, 1:1 and 1:0.5. The percent
conversion obtained from these reactions is shown in Figure 16. In a
coordinating medium such as methanol-water, activation of methanol will be
effected by its competition with water for a vacant coordination site to the
active site. As indicated by the data, both Fe2+ and Co2+ chromites show a
marked increase in conversion efficiency in solutions that are high in methanol.
This is reasonable and attributable to increased coordination of methanol to the
active site. What is not easily reconciled is the fact that CuCr2O3 appears to be
inhibited in solutions that are high in methanol. In fact, it has a very high
conversion efficiency (>40%) in aqueous solutions that are low in methanol.
The origin of this is unclear but it may result from the coordination of more
than one methanol to the active site that, in some way, interferes with the
activation process that involves breaking the C-H bond. Alternatively, it may
also reflect interfacial microwave specific effects that result from changes in
the surface composition as a function of methanol concentration. In such a
scenario the coupling of the microwaves to the interface serves to activate the
methanol at the active site. From a synthetic standpoint, however, this indicates
that, as with traditional thermal catalysis, microwave driven catalysis will vary
depending on the composition of the reaction mixture.
38
The most significant aspect of this study is that it characterizes a
significant oxidation process that occurs almost exclusively through microwave
interactions with a catalyst. We are, of course, not the first to observe
significant microwave effects in hetereogeneously catalysed reactions 79-84.
However, a key aspect of this study that differentiates it, is both the magnitude
of the microwave selectivity, and the fact that the catalysts were developed
specifically for their microwave absorbing characteristics. In particular, the
resulting catalyst systems generate formaldehyde from aqueous methanol under
conventional thermal heating only minimally with the CuCr 2O4 (~4% as
efficient) and not at all with FeCr2O4. Moreover, while it is difficult to make
meaningful comparisons to reactions that are done on a large industrial scale
and whose conditions are highly optimized, the microwave driven oxidation of
methanol takes place with good efficiency under conditions that are mild (60
°C, 1-3 atm. O2).
39
CHAPTER 3: LOW TEMPERATURE STEAM-CARBON
GASIFICATION
Background
The use of microwave radiation as opposed to traditional heating has
only tangentially been applied once to the production of synthesis gas from
carbon, but has never been applied directly to the coal gasification process 85.
Generally, the reaction between superheated steam and carbon to produce
synthesis gas is part of the general category of gasification reactions used to
obtain hydrogen from coal and other carbon rich sources86. Gasification
reactions typically occur at temperatures ≥ 700 °C depending on the carbon
source while industrial processes such as coal gasification are run at much
higher temperatures5. This is to drive the endothermic components of the
primary reactions and to obtain useful reaction rates.
Production of synthesis gas from coal and high temperature steam arise
from a complex equilibria (Figure 23) which produce not only hydrogen and
carbon monoxide, but also methane through the hydrogenative gasification and
methanation and carbon dioxide through the water-gas-shift (WGS) reaction
and through the disproportionation of carbon and CO287, 88. These equilibria
mean that the composition of the gases produced will depend critically on the
temperature and pressure of the reaction.
Because of the industrial importance of this reaction in the production of
hydrogen for direct use as a clean alternative fuel and for the production of
hydrocarbon fuels through the Fischer Trøpse process, the development of less
energy intensive methods for driving this process are desirable. We report here
40
the use of microwaves to convert coal and water to synthesis gas under mild
conditions of temperature and pressure. Microwave radiation selectively heats
the coal to temperatures from 250-450 °C depending on the carbon source,
while at the same time converting the water into steam under ambient pressure.
These conditions result in the facile oxidation of coal and carbon with
evolution of a gas mixture of H2, CO, CO2 and CH4.
The volumetric heating of carbonaceous materials by microwave
radiation at 2.45 GHz generally proceeds efficiently depending on the type of
carbon. Measurements of the permittivity and dielectric relaxation processes in
different kinds of carbon have indicated that heat is produced through space
charge (interfacial) polarization, which is typical for solid dielectric materials.
Qualitatively, this loss mechanism arises from charge carriers (electron-hole
pair), which become trapped at the surface in defect sites and grain boundaries.
The trapping process hinders charge flow thereby dephasing the charge
transport from the oscillating electric field resulting in dielectric loss.
The magnitude of the loss, and hence the degree of heating, varies across
different types of carbon. Activated carbon, such as was used here as proof of
concept, heats very efficiently while even better heating properties are typically
observed for highly graphitic carbon and graphite itself. The latter effect arises
from increased conductivity in those materials.
In the preliminary results with activated carbon run at a constant
microwave power of 65 W (Figure 24) synthesis gas was generated under mild
conditions and our interest of applying this method of heating to samples of
industrially importance, namely coal, for coal gasification. The initial carbon
source used was a high temperature steam activated charcoal, but we soon
41
realized that we could apply our reaction parameters to other samples
(described in the materials section).
As a follow up to the coal studies, one experiment was conducted with
Fisher Brand graphite (Figure 32). This experiment is included because of the
interesting product gases that were produced (described later) although an
extensive study was not performed on graphite samples. It is shown because it
is fundamentally different from the coal and carbon samples, giving three
reactions with three unique types of synthesis gas produced.
The heating of the coal and carbon under microwave irradiation in our
experimental setup was monitored using a thermal imaging camera. Consistent
with volumetric heating of the sample, the surface of the carbon sample is
highest at the center. The temperature declines towards the edges of the
sample, presumably due to heat flow out at the walls of the quartz cell. The
heating rate of a 0.50 g carbon sample as a function of applied microwave
power is shown (Figure 17). As would be expected, the rate of heating
increases steadily as a function of applied microwave power. From the linear
part of the heating curves, prior to the onset of the plateau associated with
thermal equilibrium, the power absorbed by the carbon can be estimated. For
the applied powers of 50 and 200 W, the power absorbed per second is 3.4 and
13.2 W/sec respectively with the absorption varying linearly across the range
of applied powers. The average percent of the applied power absorbed by the
carbon in our experimental configuration was found to be 7.1 (±.5) %.
42
Figure 17 Thermal images of microwave cavity, sample and quartz cell (upper left
panel), activated carbon at 100 and 200 W (upper center and right, respectively). On the
right of the upper panels is a customized temperature scale for each thermal image. The
lower panel shows the temperature versus time plots for a series of runs from 10 – 200 W
Figure 17 shows the heating profiles for activated carbon, measured with
an FLIR E40 thermal camera and Figure 18 shows the thermal images captured
for various coal samples60.
43
DOE 5 200 W
Graphite 150 W
Lignite 200 W
Peat 200 W
SS 200 W
DOE 3 200W
Figure 18 shows thermal images of organic matter at various stages in coalification
process. In the upper left, center left, lower left, upper right center right and lower right are
samples DOE 5, lignite, SS, Graphite, Peat, and DOE 3, respectively.
44
As discussed, the coal gasification and carbon-steam reactions are
extremely high temperature, energy intensive processes, with the reaction
occurring at temperatures typically >700 °C. For specific industrial processes
that make use of this reaction, such as coal gasification, the temperatures are
often >1000 °C with energy provided through the combustion of the coal. Our
hypothesis is that microwave heating of the reaction may result in facile
generation of synthesis gas at much lower energy expenditure. The rationale
for this arises from two properties of microwave heating. One is the
microwaves will selectively heat the carbon to the point where reactivity occurs
without heating the entire system. The second is the possibility of microwavespecific enhancement of the reaction.
Materials and Instruments
MICROWAVE REACTOR
45
• Pseudo-Flow system
over water immersed
sample.
Reaction
vessel
Magnetron
IR Temperature sensor
93
Figure 19 Cut away view of the microwave reaction chamber and the home made
quartz vessel
The CEM microwave chamber (from the instrument shown in Figure 9)
is shown in Figure 1989. The sample was placed in a homemade quartz vessel
and suspended in the microwave field by a microwave attenuator. The vessel
was loaded with water, quartz wool, carbon sample, then followed by
additional quartz wool sequentially. The water and first layer of quartz wool
are separated by a quartz frit. The frit serves to provide a way for the water to
slowly pass from liquid phase to the gas phase and gradually come into contact
with the carbon. It is a control for the introduction of steam to the carbon.
46
INITIAL CARBON SOURCE (ACTIVATED CARBON)
Figure 20 Image of Fisher brand activated carbon with 1 mm scale taken with optical
microscope.
The carbon used for the initial proof of concept was Fisher brand high
temperature steam-activated carbon with an average BET surface area of 900
m^2/g and a mesh size of 50 – 200. The image was captured with a digital
optical microscope camera.
It was soon realized that we could apply our reaction parameters to
carbonized wood and coal samples that were ordered from the Argonne
Premium Coal Sample Program and from the Department of Energy. We have
seen efficient coal gasification of Illinois No. 6, Lewiston-Stockton DOE 3,
and DOE 5. These samples are classified as sub bituminous, bituminous, and
anthracite coal samples. Additionally we applied the above parameters to
graphite and those results are presented here.
47
GAS COLLECTOR
3
Figure 21 Schematic of home built volumetric gas collector
Figure 21 shows a schematic drawing of the homemade volumetric gas
collector which was used in the carbon-steam and coal gasification reactions.
This apparatus was used to collect and measure the gas produced by the
reactions. As gas was generated it displaced water which was in a reserve. The
48
reserve overflowed into a volumetric graduated cylinder. The amount of water
that overflowed was equal to the amount of gas that had been generated.
A sampling port was designed at the top of the apparatus to allow for
head space sampling. This allowed for samples to be taken and directly injected
into the GC system that was optimized for the product analysis.
GAS CHROMATOGRAPHS
To quantify the products from the carbon and coal reactions with steam a
head space analysis was performed using an HP 58990 Gas chromatograph
with a thermal conductivity detector (TCD). The chromatogram shown in
Figure 22 was generated with this GC. The column used was made by Restek
and is model shin carbon st 80/100 (2 m long at 80 degrees C). This column is
designed to analyze oxygen, nitrogen, methane, carbon monoxide, and carbon
dioxide at room temperature. ShinCarbon ST material, a high surface area
carbon molecular sieve (~1500 m2/g), was chosen because of the high volatility
of the gases that we intended to analyze.
A TCD detector was chosen because it is a bulk property detector. It will
detect any gas that does not have the same thermal conductivity as the
reference gas. This was important because CO2 is not combustible and would
not be able to be detected by flame ionization. Gas chromatograms were
collected and the analyte peaks were integrated using third party software
called Chrom Perect.
To qualitatively show the complexity of the head space gas from the
reactions of coal and steam a PerkinElmer Clarus 400 GC equipped with an
FID was used. The column used was a nonpolar general purpose column with
49
an I.D. of 0.53 mm (15 feet long). The chromatogram shown in Figure 26 was
generated with this GC. The temperature was isothermal at 100 ⁰C. The phase
composition
was
cross-linked/surface
bonded
5%
phenyl,
95%
methylpolysiloxane.
2
6
7
3,4
1
5
Figure 22 Gas chromatogram showing He, H2,O2, N2, CO, and in the zoomed in
region from 5.5 to 12.5 minutes CH4 and CO2 retention times in minutes.
Figure 22 shows a typical gas chromatogram that was generated by
analysis of experimentally produced gas on the HP CG described earlier. The x
50
axis is time in minutes and the y axis is mV. The peaks labeled 1, 2, 3, 4, 5, 6,
7, correspond to the retention times of He, H2, O2, N2, CO CH4 and CO2
respectively. All peaks of interest were resolved and all product gases were
able to be quantified.
Calibration curves were generated for the quantitation of CH4, CO, CO2,
and H2. In the normal fashion various amounts of CH4, CO, CO2 and H2 were
injected with 10 μL of He present in all samples. The peak area corresponding
to each of the gases was divided by the peak area corresponding to He. The
ratio of peak areas was plotted on the y axis and the concentration of the
calibrant gas was plotted on the x axis. The plots were produced. The line of
best fit was later used to calculate the production of product gases.
Experimental
The coal (0.5 g) was spread out across a quartz frit positioned in the
center of the microwave cavity with 3 mL of water in a reservoir just below the
carbon. The microwaves heat the carbon and vaporize the water, which passes
over the carbon. A condenser was placed at the top of the system to return the
water to the system while the synthesis gas is passed out above the condenser
and collected though displacement of liquid (Figure 21). Under these
conditions, the internal pressure remains close to ambient with the partial
pressure of the water vapor surrounding the carbon being relatively constant
through the duration of the run.
51
Results
For all samples the application of microwave radiation to the system
resulted, after a period of induction, in rapid evolution of a gas stream. For all
of the samples there is a distinct induction period before the onset of gas
evolution. This induction period is dependent on a number of factors including
variability in sample placement in the cavity and duration of the Ar purge prior
to reaction. In the latter case, it can be shown that when the system is not
purged with an inert gas and oxygen remains in the pores of the carbon, there is
little if any induction period. We attribute this to the exothermic reaction
between oxygen and carbon, which will occur initially and will act to initiate
the reaction. Power dependence studies of gas evolution revealed a threshold
for gas generation occurred at ~200 watts of applied power, which corresponds
to ~235 °C maximum temperature of the coal sample. At this power, the
induction time is variable.
For all samples the composition of the generated gas was measured as a
function of gas volume collected. As can be seen, for the activated carbon
(Figure 24) sample the constituents are those typically observed for the carbonsteam reaction: H2, CO and CO2. In the early stages of the reaction, the
synthesis gas is displacing the Ar with which the apparatus is purged prior to
application of microwave power. As the argon is displaced and steady-state
conditions are achieved the composition is ~50 mole % H2, with the remainder
being composed of nearly equal amounts of CO and CO2. The production of
CO2 in the steam-carbon reaction can occur through two reactions, the direct
reaction of two water molecules with the carbon surface or through the water
gas shift reaction (Figure 23). The general consensus, from several earlier
52
studies of this reaction at high temperature, is that the primary source of CO 2 is
through the water gas shift reaction.
These preliminary results with activated carbon run at a constant
microwave power of 65 W (Figure 24) generated our interest in using this
method of heating carbon to samples of industrially importance, namely coal
gasification reactions. The initial carbon source used was a high temperature
steam activated charcoal, but soon applied our reaction parameters to other
samples (described in the materials section).
As can be seen for the coal samples (Figure 25, Figure 27, Figure 28,
Figure 29, Figure 30 and
Figure 31) the constituents are different from
observed for the carbon-steam reaction: H2, CO and CH4. The composition
varies in % H2, with the remainder being composed of CO and CH4. The
production of CH4 in the coal gasification reaction can potentially come about
through two reactions, hydrogenative gasification and methanation (Figure 23)
87, 88
.
CO + H2O
CO2 +
H2
a)
C
+ H 2O
CO +
H2
b)
C
+ CO2
2CO
C
+ 2H2
CH4
CO + 3H2
CH4 +
c)
d)
H2O
e)
Figure 23 Reported possible reactions a) homogeneous water gas shift, b)
heterogeneous WGS, c) Boudouard reaction, d) hydrogenative gasification and e)
methanation with occur with carbon, water, carbon monoxide, carbon dioxide, and
hydrogen.
53
8
Product gasses (mmol)
7
6
5
H2
H2
CO
CO
CO2
CO2
4
3
2
1
0
0
100
200
300
Volume (mL)
400
500
Figure 24 Activated carbon run at 65 watts showing percent composition of product
gases from a) steam carbon reaction hydrogen, carbon monoxide and b) carbon dioxide from
homogeneous water gas shift
Figure 24 shows the products from the steam-carbon reaction run at a
fixed power of 65 W. In this experiment (Figure 24) 500 mL of product gas
was collected. These samples produced approximately 7 mmol of hydrogen and
3 mmol of carbon monoxide. These are the results that initially peaked our
interest to apply this methodology to coal samples. One can see that in our
54
product gases there is not a difference between the amount of CO and CO2. The
molar amounts of the two gases overlay.
70
Percent Composition
60
H2
Percent H2
CO
Percent CO
50
CH
Percent4CH4
40
30
20
10
0
0
50
100 150 200 250 300 350 400 450 500
Volume (mL)
Figure 25 Shows percent hydrogen, carbon monoxide and methane produced vs.
volume of gas produced at 200 W for a high anthracite coal sample termed DOE 5
Figure 25 displays the primary product gases from the coal gasification
reaction run with coal sample Department of Energy (DOE) 5 at a fixed power
of 200 W. This sample is commonly known as anthracite, which is
approximately 90 % carbon.
55
In all coal experiments the reactions proceed until the reaction stopped.
There was not a standard volume of 500 mL collected, as was the case in the
activated carbon. This is because the coal samples, keeping the mass of coal
used and the volume of water used constant, did not always generate 500 mL of
product gases.
On the x axis is volume in mL of total gas produced and on the y axis is
the percent composition of H2, CO and CH4. At 200 W fixed power the early
stages of the reaction produce about a 1:4:5 ratio of H2, CO, and CH4
respectively. Soon thereafter, the ratios increased to 60 percent H2 and reached
in CO and CH4 to approximately 25 and 15 %, respectively.
There are some interesting differences in this coal sample when
compared to the activated carbon sample. There is methane in the product
gases of all the coal samples and there was none in the activated carbon
sample. There was carbon dioxide in the activated carbon sample, but there
was none in any of the coal samples. Hydrogen grows to 60 % of the product
gases in the coal samples, but in the activated carbon it reached a maximum of
approximately 50 %.
In the coal sample there is not a significant change in the percent of each
product when comparing the early points with the later points. In the activated
carbon there was as much as a 15 % change in the product gases from the early
stages to the later stages of the reaction. However, in the coal the maximum
percent change is about 10 %.
The percent hydrogen increased from about 50 to 60 % and methane
from about 10 to 15 %, but CO decreased from about 35 to 30 % from early
stages of the reaction to the later stages of the reaction (Figure 25). We propose
that the increase in methane and the decrease in carbon monoxide is due to the
56
methanation reaction where CO and H2 react to form CH4 and H2O. The
hydrogenative gasification reaction would not explain the decrease in the CO
percentage. Although we do recognize that the rich heterogeneity of coal
allows for a large variety of reactions.
We observe that H2 represents 60 % of the total products. Based on the
reactions that we propose (Figure 23) that is not stoichiometrically possible.
We propose that because in coal there are many heterogeneous species, which
could react with CO, H2 and CH4 and form products that we are not interested
in identifying and measuring. These additional products are measured in the
total volume of gas that is being produced, but there is no interest in measuring
them and therefore they skew the plots showing percent of each product.
Additionally there are also other reactions that can produce H2 and
consume CO or CH4 and reduce their percentages as represented in Figure 25.
All of these are possible reason why we see a 60 % H2 composition of the
product gases.
As dictated by the directive of the research goal, the main reaction
processes that we are interested in are only carbon based reaction. As noted by
Gonzalez et al. natural coal contains not only nitrogen and sulfur but also
aromatics, hydrocarbons, tar90. These realities force these data to be slightly
skewed high with respect to H2. With the instrumentation available in our lab
we selectively focused on H2, CO, CO2 and CH4. All the additional products in
industry are scrubbed and discarded. Therefore we did not spend time and
effort to quantify the other products.
As a qualitative analysis of the complex products generated from coal
gasification a GC analysis that was not designed to analyze only H2, CO, CO2
and CH4 is shown in Figure 26.
57
5
10
15
20
25 30 35
Time (min)
40
45
50
55
Figure 26 GC analysis of head space from coal reaction
Figure 26 shows the resulting chromatogram from the experimentally
obtained head space from one of the coal samples. This chromatogram was
collected in order to qualitatively confirm the rich heterogeneity of coal
samples leads to the formation of other products a 50 μL head space aliquot
was analyzed on the PerkinElmer Clarus 400 GC described earlier.
58
7
Product gases (mmol)
6
5
4
H
2
H2
3
CO
CO
CH4
CH
4
2
1
0
0
100
200
300
Volume (mL)
400
500
Figure 27 Shows mmole hydrogen, carbon monoxide and methane produced vs.
volume of gas produced at 200 W for a high anthracite coal sample termed DOE 5
Milimoles of product gases (Figure 27) are plotted versus total volume
of gas collected. In this set of experiments we were able to collect 400 mL of
product gases. These samples produced approximately 6 mmol of hydrogen, 2
mmol of carbon monoxide, and 1 mmol of methane. As will be seen in other
samples, the percentages are approximately the same, but the total number of
moles of gas produced varies greatly. The bituminous coal sample (Stockton
Seam) produced less of all the products of interest.
59
60
Percent Composition
50
H2
Percent H2
40
CO
Percent CO
CH4
Percent CH4
30
20
10
0
0
50
100
Volume (mL)
150
200
Figure 28 Percent hydrogen, carbon monoxide and methane produced vs. volume of
gas produced at 200 W for a bituminous coal sample termed Stockton Seam
Figure 28 shows the primary product gases from the coal gasification
reaction run with bituminous coal sample Stockton Seam at a fixed power of
200 W. This is commonly called “black coal” which is approximately 60-80 %
carbon. This sample has lower carbon content and heats less.
On the x axis is volume in mL of total gas produced and on the y axis is
the percent composition of H2, CO and CH4. At 200 W fixed power the early
stages of the reaction there is no difference between H2 and CO, but CH4 is
produced at approximately a 20 % abundance throughout the reaction. Soon
60
thereafter, the ratio increased to 50 percent H2 and reached in CO while
maintaining 20 % and CH4.
Hydrogen increased to approximately 50 % abundance in both the
activated carbon and coal samples. In the Stockton Seam coal sample there is a
large change in the percent of each product unlike the DOE 5 sample when
comparing the early points with the later points.
Figure 29 shows mmoles of our product gases (hydrogen, carbon
monoxide and methane) plotted versus total volume of gas collected. In this set
of experiments we were able to collect 175 mL of product gases. These
samples produced approximately 1 mmol of hydrogen, 0.4 mmol of carbon
monoxide, and 0.4 mmol of methane. As will be seen in other samples, the
percentages are approximately the same, but the total number of moles of gas
produced varies greatly. The bituminous coal sample SS produced less of all
the products of interest.
61
1.2
Product gasses (mmol)
1
0.8
H2
H2
CO
CO
CH4
CH4
0.6
0.4
0.2
0
0
50
100
Volume (mL)
150
200
Figure 29 mmole hydrogen, carbon monoxide and methane produced vs. volume of
gas produced at 200 W for a bituminous coal sample termed Stockton Seam
Figure 30 shows the primary product gases from the coal gasification
reaction run with sub bituminous coal sample DOE 3 at a fixed power at 200
W. This is commonly called “soft coal” which is approximately 50 % carbon.
On the x axis is volume in mL of total gas produced and on the y axis is
the percent composition of H2, CO and CH4. For this sample we were able to
collect 500 mL of product gases. At 200 W fixed power the early stages of the
reaction there is a difference between H2, CO and CH4 with each gas
representing 35, 20 and 18 % respectively. Soon thereafter, the ratios increased
62
to 50 percent H2 and reached 35 % in CO while maintaining between 12-18 %
in CH4.
70
60
Percent Composition
50
H
Percent
H2 2
CO
40
Percent
CO
CH
Percent4
CH4
30
20
10
0
0
50 100 150 200 250 300 350 400 450 500
Volume (mL)
Figure 30 Show percent hydrogen, carbon monoxide and methane produced vs.
volume of gas produced at 200 W for a sub bituminous coal sample termed DOE 3
Hydrogen increased to approximately 50 % abundance in both the
activated carbon and this coal sample. In DOE 3 coal sample there is a large
change in the percent of each product unlike the DOE 5 sample when
comparing the early points with the later points.
63
6
Product gases (mmol)
5
H2
H2
CO
CO
CH4
CH4
4
3
2
1
0
0
100
200
300
Volume (mL)
400
500
Figure 31 mmole hydrogen, carbon monoxide and methane produced vs. volume of
gas produced at 200 W for a sub bituminous coal sample termed DOE 3
Figure 31 shows mmoles of our product gases (hydrogen, carbon
monoxide and methane) are plotted versus total volume of gas collected. In this
set of experiments we were able to collect 500 mL of product gases. These
samples produced approximately 6 mmol of hydrogen, 2 mmol of carbon
monoxide, and 1 mmol of methane, but when considering the larger volume of
gas produced makes this coal sample very similar to the SS sample. As was
seen in other samples, the percentages are approximately the same, but the total
number of moles varies.
64
65
Percent composition
60
Percent
H2
H2
Percent
CO
CO
55
50
45
40
35
0
100
200
300
400
500
Volume (mL)
Figure 32 Percent hydrogen and carbon monoxide versus volume of gas produced at
200 W for a synthetic graphite sample
Figure 32 shows the primary product gases from the coal gasification
reaction run with synthetic graphite sample at a fixed power of 200 W until 500
mL of product gases were collected. This sample is approximately 100 %
carbon.
On the x axis is volume in mL of total gas produced and on the y axis is
the percent composition of H2 and CO. At 200 W fixed power the early stages
65
of the reaction produce about a 3:2 ratio of H2 and CO respectively. Soon
thereafter, the ratios converged at 50 % each.
There are some interesting differences in this graphite sample when
compared to the activated carbon and coal samples. There is methane in the
product gases in the coal sample and there was none in the graphite samples.
There was carbon dioxide in the activated carbon samples, but there was none
in the graphite. Hydrogen and carbon monoxide converge to 50 % of the
product gases in the graphite samples, but in the activated carbon and coal
samples an ideal synthesis gas was never produced.
In this set of experiment (Figure 32) we were able to collect 500 mL of
product gases. These samples produced approximately 3 mmol of hydrogen
and 3 mmol of carbon monoxide. With the graphite samples there was a 3:2
mole ratio of CO and H2 gases produced early in the reaction, but the products
quickly converged on the ideal 1:1 ratio that would be expected for an ideal
system.
66
60
Mol Percent
50
40
H2
30
CO
CH4
20
10
0
52
Figure 33 Moles produced vs. carbon sample source
Figure 33 shows a plot showing mole percent of products versus coal
type. One can see that the anthracite and sub bituminous samples produced
hydrogen in excess of 50 %, with the sub-bituminous sample producing the
least amount of methane of the coal samples. This has been discussed.
Bituminous coal produced hydrogen at approximately the expected 50 % molar
ratio. Graphite produced ideal synthesis gas by producing a 1:1 ratio of
hydrogen to carbon monoxide.
67
There were several fundamental differences between graphite, activated
carbon and coal. Samples of coal described earlier produced H2, CO and CH4.
The H2 and CO are from the coal gasification reaction. Methane could be
produced directly from carbon and hydrogen or from reaction from carbon
monoxide and hydrogen by either hydrogenative gasification and or
methanation, also called the Fisher Trøpsch reaction, respectively.
Additionally with the coal, there was a very bad odor that was produced
from the reaction of coal. One could distinctly smell the presence of
asphaltenes and a head space analysis on our GC equipped with a flame
ionization detector revealed a very complex head space gas (Figure 26) which
we predicted and attributed to the rich heterogeneity of coal. These addition
products were not characterized. We limited our analysis to H2, CO, CO2, and
CH4. There has been much work done of the analysis of coal, but our primary
interest was to evaluate the enhancement of coal gasification by microwave
irradiation.
The activated carbon samples produced H2, CO and CO2. The H2 and CO
are from the heterogeneous water gas shift reaction. The CO2 could be
produced from either combustion of the carbon or from the water gas shift
reaction involving carbon and water. Combustion is not a likely source of CO 2
in this case because the system was thoroughly flushed with argon and the
amount and ratio of contaminate O2 and N2 observed in the GC remained
relatively constant throughout the experiment. If there were combustion then
the ratio of N2 to O2 would have increased and the amount of contaminate O 2
would have decreased as the reaction progressed.
Graphite samples produced only H2 and CO. This is the ideal sample
because there was only the coal gasification reaction proceeding. We saw
68
neither methane nor carbon dioxide. The ratio of H2 to CO was ideal, at 1:1.
When synthetic graphite was reacted there was no odor, not complex head
space from the secondary GC analysis and a clean reaction was observed.
Two interesting observations are made by noting the thermal images
(Figure 18). The Peat and Lignite samples were not successfully gasified. They
are included in the heating images because they were lower carbon content
samples that were not as far along in the coalification process as the other
samples. These samples did not heat as well as the other samples. Graphite was
the only sample to produce an ideal synthesis gas and it was also the most
efficient heater in a microwave field.
69
CHAPTER 4: GENERAL CONCLUSION
Microwave chemistry had its origin in 1946 and has continued to gain
popularity because of its potential to save energy, time and resources and lower
the impact of chemical synthesis on the environment. Microwave chemistry has
moved far from its humble beginnings with Spenser et al. studying the “food
effect” of microwaves. However, the full implications of the “food effect” are
still not fully understood.
The work presented herein helps to increase the understanding of the
implications of the food effect that was first articulated by Spenser et al. By
understanding the results of the two systems that are presented in this work
(catalytic and non-catalytic) one can better understand how to design a system
for and utilize microwave heating.
In both of the systems (catalytic and non-catalytic) presented in this
dissertation we have shown microwave heating does increase reaction rates and
efficiencies. These enhancements can sometimes be understood by invoking
the efficiency of microwave heating and sometimes are only understood by
invoking a microwave effect.
Microwave chemistry can be tuned particularly well to suit
environmental needs. The work described herein uses water as both a solvent
and reactant, making it not only environmental benign in the efficient use of
energy and time, but also in the elimination (in some reaction systems) of the
need for organic solvents. Chemists are increasingly asked to make strides in
the very important area of green chemistry and one way to do so is to further
utilize microwave heating.
70
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