The Catalytic Partial Oxidation of Methanol and The

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2012 Low Temperature Microwave Driven C1 Reactions: The Catalytic Partial Oxidation of Methanol to Formaldehyde and the Gasification of Coal Mark Crosswhite

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LOW TEMPERATURE MICROWAVE DRIVEN C1 REACTIONS: THE

CATALYTIC PARTIAL OXIDATION OF METHANOL TO

FORMALDEHYDE AND THE GASIFICATION OF COAL

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

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 dedicate this to Evelyn Crosswhite

Thank you for the personal sacrifice that you made for my education.

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.

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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

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

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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.

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

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

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 non- absorbing 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 catalyzed reactions,

8 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).

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 cat med cat med catalyst to the medium by convection, hc(T -T ), where T and T 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

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.

TABLE OF CONTENTS

10 20 30 40 50 60 70 80 Two theta (deg)

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.

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 MgAl2O4 that 2+ 3+ is a paradigm for the class of oxides having the general formula A B 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, Cu1- xCoxFe2O4] can be synthesized, thereby dramatically extending the rage of materials accessible42, 43. As heterogeneous catalysts, which are our particular interest, chromates 44-47 (MCr2O4) spinels have been used to catalyze a number of reactions . 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 materials52. This study has been followed in recent years by several others that have furthered the demonstration of the advantages of nanoscale spinels for catalytic processes53- 56. 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, 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ where A is: Mg , Mn , Fe , Co , Ni , Cu , Ni0.5 Mn0.5 and 2+ 2+ 2+ 2+ 2+ 2+ 2+ chromates, ACr2O4, where A is: Mn , Fe , Co , Ni , Cu , Zn All of the spinels were synthesized as nanoscale particles, the size of which was estimated from the X-Ray Diffraction (XRD) peaks using the

Scherrer equation and observed directly using transmission electron microscopy (TEM). Surface areas were determined from Brunauer-Emmet- Teller (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.

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.

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.

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 59-61 for a CrO2 oxidation catalyst using thermal imaging techniques . 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

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

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 BET Average size (nm)

CuCr2O surface32.2 area 6e Size FeCr2O4 52.7 12 CoCr2O 59.8 14

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.

QUANTITATICE ANALYSIS OF FORMALDEHYDE

0.8

0.6 Abs 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.

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 C. Each plot correlates to a different metal oxide catalyst. The vessel and the solvent do not absorb microwave energy and hence ⁰ 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.

50 FeCr2O4

45 CoCr2O4 40 CuCr2O4 35 Temp(oC)

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 25 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 2+ 2+ 2+ were a series of chromites, ACr2O4, with the dications Fe , Co , Cu . 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.

Thermal .30 Microwave % Conversion % Heating RateHeating (C/s) 0 5 10 15 20 25 30 30 25 20 15 10 5 0 0 .05 .1 .15 .2 .25 .15 .1 .2 .05 0

FeCr2O4 CoCr2O4 CuCr2O4 Figure Predicted temperature of catalyst

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.

CONSUMPTION OF METHANOL AND OXYGEN

) 9 11 13 15 17 17 15 13 11 9 Concentration (mol/LConcentration 0 20 40 60 80 Time (min) ) Pressure (atmPressure 1 1.5 2 2.5 3 2.5 2 1.5 1 0 5 10 15 20 Time (min) 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

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.

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.

1 atm O2 3 atm O2 a % Conversion% 0 10 20 30 40 30 20 10 0 FeCr2O4 CoCr2O4 CuCr2O4 FeCr2O4 CoCr2O4 CuCr2O4 b % Conversion% 0 10 20 30 40 40 2030 10 0

Figure Predicted temperature1:3.2of catalyst 1:1 1:.5 MeOH : H2O (V:V)

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 water75. 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.

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 reactions79-84. 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 CuCr2O4 (~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).

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 process85. 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 87, 88 and through the disproportionation of carbon and CO2 . 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) %.

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.

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.

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 microwave- specific enhancement of the reaction.

Materials and Instruments

MICROWAVE REACTOR

• Pseudo-Flow system over water immersed sample.

Reaction vessel

Magnetron

IR Temperature sensor

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

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.

7 6

3,4

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.

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 carbon- steam 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 CO2 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.

a) CO + H2O CO2 + H2

b) C + H2O CO + H2

C + CO2 2CO c)

C + 2H2 CH4 d)

CO + 3H CH + H O e) 2 4 2 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.

4 H2H2 CO 3 CO2CO2 2 Product Product gasses (mmol) 1

0 0 100 200 300 400 500 Volume (mL)

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.

60 PercentH2 H2

PercentCO CO 50 PercentCH4 CH4 40

20 Percent CompositionPercent 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.

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.

5 10 15 20 25 30 35 40 45 50 55 Time (min)

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.

4 HH22 3 CO

CHCH44 2 Productgases(mmol) 1

0 0 100 200 300 400 500 Volume (mL)

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.

PercentH H2 40 2 PercentCO CO PercentCH CH4 30 4

20 PercentComposition 10

0 0 50 100 150 200 Volume (mL)

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.

1.2

1 )

0.8

0.6 H2H2 COCO

0.4 CH4CH4

Product gasses (mmol gasses Product 0.2

0 0 50 100 150 200 Volume (mL)

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.

50 Percent H2H2

PercentCO 40 CO

PercentCH 4 CH4 30

20 Percent CompositionPercent 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.

5 ) H2H2 4 COCO

CH4CH4 3

Product gases (mmol gasesProduct 1

0 0 100 200 300 400 500 Volume (mL)

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

60 Percent H2 H2 Percent CO 55 CO

Percent composition Percent 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.

40 H2 30 CO CH4

Mol Percent Mol 20

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.

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 CO2 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 O2 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.

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.

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BIOGRAPHICAL SKETCH

In the spring of 2002 Mark graduated from Mitchell Community College in Statesville, NC with an AA degree. In the spring of 2005 he completed his bachelor’s degree in Chemistry from at the University of North Carolina at Greensboro. Then he began the master’s degree program at FSU which he finished in the fall of 2009. During his master’s degree he was privileged to have studied with Dr. Alan G. Marshall, the co-inventor ICR-MS. After finishing his master’s degree Mark decided to continue towards a Ph.D. which he completed in the summer of 2012 in analytical chemistry under the direction of Dr. Al Stiegman. While working with Stiegman, an inorganic chemist, he applied his analytical skills to the field of catalysis and learned much about materials science. While at FSU Mark researched at the National High Magnetic Field Laboratory, a world class research facility housing the world’s highest resolution and highest mass accurate mass spectrometers and received instruction from some of the world’s most prominent scientist, including Noble Laureate Sir Harry Kroto, ICR-MS co-inventor Alan Marshall, editor Journal of chromatography A John Dorsey and A. E. Stiegman former faculty at Jet Propulsion Laboratory.