M. Clayton Wheeler Associate Professor   • B.S. University of Texas at Austin, 1992 • M.S. University of Texas at Austin, 1996 • Ph.D. University of Texas at Austin, 1997   Phone: 207-581-2280 Fax: 207-581-2323 Email: cwheeler@umche.maine.edu

Research Interests

Biofuels • Catalysis • Chemical Sensors

Current Research

• Biofuels and Catalysis
  • Thermal conversion technologies for converting lignocellulosics to hydrocarbon fuels and chemicals.
  • Thermal deoxygenation of biomass-derived organic acids
  • Hydrodeoxygenation of pyrolysis oils
  • Hydrogenation of mixed ketones to mixed alcohols
  • Esterification of mixed organic acids
  • Hydrogenolysis of mixed esters
  • Microcalorimetric arrays and catalyst screening
• Chemical Sensors
  • Selective sensor materials and sensing algorithms
  • Integrating sensor platforms into microchemical systems using micromachining techniques
  • Characterization of sensing mechanisms for selectivity enhancement

Biofuels and Catalysis

There is a pressing need for the development of renewable fuels and energy derived from biomass, wind, geothermal heat and solar radiation in order to meet future economic and environmental requirements. Among these renewable resources, biomass is considered to be the only sustainable and carbon-neutral source for the production of liquid fuels. The US has the potential to sustainably produce biomass which can replace more than one-third of the 2004 U.S. petroleum consumption. As an alternative feedstock for oil, biomass is also inexpensive at $12 to $24 per barrel of oil equivalent. Compared to agricultural biomass, forest lignocellulosic biomass has particular advantages because it does not need fertile soil, has a 3-4 times higher bulk density, and one order of magnitude lower ash content.

Some strategies which are being pursued to produce liquid fuels from lignocellulosic biomass include

• Hydrolysis of cellulose and xylose to monomeric sugars followed by
  • Thermal catalysis to convert sugars to hydrocarbons
  • Fermentation of sugars to produce alcohols
  • Mixed Culture Robust (MCR) fermentation of sugars to produce organic acids
• Pyrolysis of wood, or its components such as lignin
• Conversion of cellulose to levulinic acid and thermal deoxygenation of the acid to produce Levulene, a hydrocarbon fuel mixture

One focus of my group is upgrading of organic acids that have been produced by MCR fermentation of pulp mill extracts and algae processing residues. During fermentation, buffers such as calcium carbonate are used to maintain appropriate pH. After drying, the resulting organic acid salts, e.g. calcium acetate and calcium butyrate can be converted to ketones by thermal deoxygenation. We then hydrogenate the mixed ketones to alcohols which can be dehydrated and oligomerized to hydrocarbon fuels. The reaction kinetics are studied using both batch and trickle bed reactor systems.

The major challenge in making hydrocarbon fuels from lignocellulosic biomass is removing the oxygen. If wood is heated to 500°C for a few seconds, it will pyrolyze and form an oil which has large quantities of alcohol, carboxylic acid, and methoxyl functionalities. These oxygenated compounds contribute to high acidity and make the oils unstable. They also significantly reduce the energy density of the oil. We are developing and testing new classes of catalysts for selectively reducing the oxygenated species.

My group has also been working on a new process for converting cellulose-derived levulinic acid to diesel, gasoline and jet fuels. Our process uses no catalysts and no hydrogen to thermally deoxygenate the levulinic acid and produce a crude oil. The product is a mixture of straight-chain and aromatic hydrocarbons which phase separate from water and have neutral pH. Major efforts in this area include understanding the mechanisms and kinetics of the reactions, developing continuous processes based on the bench-scale batch experiment results, and optimizing the integration with the levulinic acid production process. This process is very promising because it is particularly tolerant of impurities in the feed and can use many different feedstocks such as municipal solid wastes, recycled paper, forest residues, and pulp mill wastes.

Chemical Sensors

Applications for chemical sensors are diverse and include areas such as combustion products analysis, detection of chemical warfare agents, air quality monitoring, and medical diagnostics. Semiconducting metal-oxides such as SnO₂, TiO₂, and WO₃ are widely used as gas sensing materials because the electrical conductivities of these materials increase by orders of magnitude when they are exposed to reducing gases such as carbon monoxide or hydrogen. However, commercial oxide sensors are not selective (i.e. they cannot differentiate between different analytes), and their relatively large masses result in high power requirements and long thermal stabilization times. Recent developments in microfabrication have provided opportunities to reduce the power requirement of sensor devices by utilizing micromachining techniques to thermally isolate sensor platforms from their underlying substrates. In addition to lower power requirements, such microsensor devices fabricated on silicon wafers have dramatically shorter thermal equilibration times than their traditional counterparts. Thus, it is possible to cycle microsensor temperatures on millisecond timescales, and we can take advantage of chemical kinetic effects to enhance sensor selectivity.

Miniaturization of chemical sensor technologies through micromachining techniques promises many advantages such as portability, low power operation, rapid thermal cycling, and array-based implementation for improved selectivity. Miniaturization is also accompanied by many challenges that include engineering issues involved in the design of micromachined devices as well as development of platform-compatible sensing materials to achieve chemical sensitivity and selectivity on the microscale. The University of Maine has an established research program dedicated to chemical sensor development in the Laboratory for Surface Science and Technology (LASST), and members of my group collaborate with LASST to develop and characterize sensor systems based on chemically selective sensor materials.

One goal of our research is to take advantage of chemical kinetics mechanisms to enhance the performance of gas sensors. Therefore, we study how temperature and catalytic activity of sensor materials affect the sensitivity of a sensor to particular gas analytes. The differences in chemical activity make it possible to design sensor systems that are selective to particular gas analytes. In this work we are interested in developing chemical sensor systems, as well as using surface science techniques such as temperature programmed desorption and infrared absorption spectroscopy to elucidate the underlying chemical reaction mechanisms.

Another area of current interest is the study of coupling or "cross-talk" between various sensor elements as we work to miniaturize sensor platforms and include multiple sensors in array-based platforms. For instance, experiments using arrays of SnO2-based microsensors have demonstrated that chemical reactions occurring on one sensor can affect the performance of adjacent sensors (see publications below). In addition to chemical crosstalk, we are also interested in addressing issues of thermal and electrical interference in multi-sensor arrays.

Selected Publications

T. J. Schwartz, A. R. P. van Heiningen and M. C. Wheeler, “Energy Densification of Levulinic Acid by Thermal Deoxygenation”, Green Chemistry 12, 1353–1356 (2010).

K. D.Hurley, B. G. Frederick, W. J. DeSisto, A. R. P. van Heiningen, and M. C. Wheeler, Catalytic Reaction Characterization using Micromachined Nanocalorimeters, Appl. Catal. A 390, 84–93 (2010).

T. J. Thibodeau, A. S. Canney AS, W. J. DeSisto, M. C. Wheeler, F. G. Amar, and B. G. Frederick, “Composition of tungsten oxide bronzes active for hydrodeoxygenation,” Appl. Catal. A-General 388, 86-95 (2010).

I. T. Ghampson,C. Newman, L. Kong, E. Pier, K. D. Hurley KD, R. A. Pollock, B. R. Walsh, B. Goundie, J. Wright, M. C. Wheeler, R. W. Meulenberg, W. J. DeSisto, B. G. Frederick, and R. N Austin, “Effects of pore diameter on particle size, phase, and turnover frequency in mesoporous silica supported cobalt Fischer-Tropsch catalysts,” Applied Catalysis A-General 388, 57-67 (2010).

J. Joseph, C. Baker, S. Mukkamala, S. Beis, M. C. Wheeler, W. J. DeSisto, B. Jensen, and B. G. Frederick, Chemical Shifts and Lifetimes for NMR Analysis of Bio-fuels, Energy Fuels 24, 5153–5162 (2010).
W. J. DeSisto, N. Hill, S. Beis, S. Mukkamala, J. Joseph, C. Baker, T-H. Hong, E. Stemmler, M.C. Wheeler, B.G. Frederick, and A.R.P. van Heiningen, “Fast Pyrolysis of Pine Sawdust,” Energy Fuels 24, 2642–2651 (2010).

S. H. Beis, S. Mukkamala, N. Hill, J. Joseph, C. Baker, B. Jensen, E. A. Stemmler, M. C. Wheeler, B. G. Frederick, A. van Heiningen, A. G. Berg, and W. J. DeSisto, "Fast Pyrolysis of Lignins," BioRes. 5(3), 1408-1424 (2010).

M. Bhatia, K. D. Hurley, G. P. van Walsum, and M. C. Wheeler, "Thermal Conversion of Carboxylate Salts (Biomass-to-Mixed Alcohols)" AIChE Annual Meeting Conference Proceedings on CD, New York, NY (2009).

R. Nelson, W. J. DeSisto, B. G. Frederick, A. van Heiningen, M. C. Wheeler, “High Throughput Microcalorimetry for Catalyst Discovery,” AIChE Annual Meeting Conference Proceedings on CD, New York, NY (2008).

Y. Ji , M. C. Wheeler, and A. van Heiningen, "Oxygen Delignification Kinetics: CSTR and Batch Reactor Comparison," AIChE Journal, 53(10) 2681-2687 (2007).

A. G. Shirke, S. Semancik, R. E. Cavicchi, B. G. Frederick, and M. C. Wheeler, “Femtomolar Isothermal Desorption using Microhotplate Sensors” Journal of Vacuum Science Technology A, 25(3) 514-526 (2007).

M. C. Wheeler, R. E. Cavicchi, and S. Semancik, “Tin Oxide Microsensor Arrays as Probes for the Oscillatory CO Oxidation Reaction on Supported Platinum,” Journal of Physical Chemistry –C 111, 3328-3332 (2007).

B. J. Meulendyk, M. C. Wheeler, M. Pereira da Cunha, " Generalized and pure shear horizontal SAW sensors on quartz for hydrogen fluoride gas detection," 2007 IEEE Ultrasonics Symposium Proceedings 1-6, 480-483 (2007).

A. Clark, M. Pereira da Cunha, B. Segee, and M. C. Wheeler, "Correlation Of Microhotplate Metal Oxide Sensor Response To Catalytic Fluorocarbon Decomposition Activity," 2007 AIChE Annual Meeting Conference Proceedings on CD, New York, NY, 2007.

S. Winchenbach, M. C. Wheeler, M. Pereira da Cunha, and B. Segee, "Demonstrating A Novel Approach To The Control Of Microhotplate Sensors Utilizing A Distributed Computing Approach And Numeric Modeling," 2007 AIChE Annual Meeting Conference Proceedings on CD, New York, NY, 2007.

B. J. Meulendyk, M. C. Wheeler , B. Segee, and M. Pereira da Cunha, "Sensitivity Of A Surface Acoustic Wave Hydrogen Fluoride Sensor To Quartz Substrate Etching," 2007 AIChE Annual Meeting Conference Proceedings on CD, New York, NY, 2007.

M. Q. Snyder, S. A. Trebukhova, B. Ravdel, M. C. Wheeler, J. DiCarlo, C. P. Tripp, and W.J. DeSisto, “Synthesis and Characterization of Atomic Layer Deposited Titanium Nitride Thin Films on Lithium Titanate Spinel Powder as a Lithium Ion Battery Anode,” Journal of Power Sources 165, 379-385 (2007).

B. J. Meulendyk, M. C. Wheeler, and M. Pereira da Cunha, “Relevance of Considering Power Flow Angle and Sample Geometry to Mitigate Spurious Scattered Signals in Acoustic Wave Reflection Measurements,” Nondestructive Testing and Evaluation 21, 155-169 (2006).

M. C. Wheeler, J. E. Tiffany, R. M. Walton, R. E. Cavicchi, S. Semancik, “Chemical Crosstalk between Heated Gas Microsensor Elements Operating in Close Proximity,” Sensors and Actuators B 77:167-176 (2001).

S. Semacik, R. Cavicchi, M. C. Wheeler, J. E. Tiffany, G. E. Poirier, R.M. Walton, J. S. Suehle, B. Panchapakesan, D. L. DeVoe, “Microhotplate Platforms for Chemical Sensor Research,” Sensors and Actuators B 77:579-591 (2001).

M. C. Wheeler, C. T. Reeves, D. C. Seets, C. B. Mullins, “Experimental Study of CO Oxidation by an Atomic Oxygen Beam on Pt(111), Ir(111), and Ru(001),”  Journal of Chemical Physics 108:3057-3063 (1998).

P. D. Nolan, M. C. Wheeler, J. E. Davis, C. B. Mullins, “Mechanisms of Initial Dissociative Chemisorption of Oxygen on Transition-Metal Surfaces,” Accounts of Chemical Research 31:798-804 (1998).

M. C. Wheeler, D. C. Seets, C. B. Mullins, “Angular Dependence of the Dynamic Displacement of O₂ from Pt(111) by Atomic Oxygen,” Journal of Chemical Physics 107:1672-1675 (1997).

D. C. Seets, M. C. Wheeler, C. B. Mullins, “Trapping-Mediated and Direct Dissociative Chemisorption of Methane on Ir(110): A Comparison of Molecular Beam and Bulb Experiments,” Journal of Chemical Physics 107:3986-3998 (1997)

M. C. Wheeler, D. C. Seets, C. B. Mullins, “Kinetics and Dynamics of the Initial Dissociative Chemisorption of Oxygen on Ru(001),”  Journal of Chemical Physics 105:1572-1583 (1996).

D. C. Seets, M. C. Wheeler, C. B. Mullins, “Kinetics and Dynamics of Nitrogen Adsorption on Ru(001): Evidence for Direct Molecular Chemisorption,” Chemical Physics Letters 257:280-284 (1996).

M. C. Wheeler, D. C. Seets, C. B. Mullins, “Kinetics and Dynamics of the Trapping-Mediated Dissociative Chemisorption of Oxygen on Ru(001),” Journal of Vacuum Science & Technology A 14:1572-1577 (1996).