SYNTHESIS OF ADVANCED CATALYTIC MATERIALS FROM CHAR

Researcher: Jim Kastner, jkastner@uga.edu
Concept: Synthesize Advanced Catalytic Materials Derived from Biomass Chars

In one type of biorefinery process, biomass can be pyrolyzed to generate a bio-oil as a potential liquid fuel and a solid char (Fig. 1). The resultant char can form the basis of tailored adsorbents and carbon supported catalysts, providing additional value added products from the biorefinery. In another process, biomass can be combusted to generate power and heat, but results in an ash that is typically landfilled. However, the resultant materials (ash and char) hold promise as industrial catalysts (Fig. 2). A catalyst is a material that lowers the activation energy of a reaction and/or provides an alternative reaction pathway, yet is not consumed in the reaction. Catalysts typically speed up reactions or lower the temperature at which reactions occur.

Fig. 1: Pyrolysis unit (left) and resultant peanut hull char (far right picture shows a microscopic scale of the char, the bar indicates 2μm)

Carbon supported catalysts have several distinct advantages over alumina or silca supported systems; 1) they are stable under acidic and basic conditions, 2) active material (e.g., metals) can be finely dispersed throughout the carbon structure increasing accessibility of reactants to the catalyst relative to a bulk metal, 3) the amount of metal needed for catalyst development is reduced, 4) a renewable carbon source is used (in the case of ash, a waste material is reused), and 5) metal recovery and potential reuse is enhanced, since the residual carbon can be burned off.

	Fig. 2: SEM analysis of wood fly ash showing presence of microspheres and apparent porous carbon sections. Fig. 3: Cobalt Oxide Carbon Supported Catalyst via ECD. Fig. 4: Yellow powder observed during surface area measurement after treatment of a 500 ppmv H2S stream using wood ash.

Catalysts are used in a wide range of industrial processes such as hydrogenations for polyol sweetener production (e.g., sorbitol and xylitol), hydrocarbon reforming, chemical intermediate formation and liquid fuels (e.g., syngas to diesel), specialty chemicals, NOx reduction in coal fired power plants, and catalytic oxidations of air pollutants (e.g., ammonia from poultry housing). The world market for catalysts is approximately $9 billion dollars with environmental catalysts representing 1/3rd of the total market.

Traditional methods of solid catalyst preparation limit control of key variables in catalyst design and thus limit activity and selectivity. Recent advances in the synthesis of nanocrystalline materials have allowed for generation of advanced catalytic material with better control over key variables such as dispersion, shape, size, crystal composition and structure, and surface area. The primary disadvantages of these techniques are high operating temperatures and pressures, use of organic solvents (harsh, expensive chemicals), potential lack of scalability, and still limited control over crystalline size and dispersion. However, recently working with the nanotechnology group we have demonstrated the use of electrochemical deposition of metals on activated carbon (similar to electroplating) at room temperature (Fig. 3) as a green method of making carbon supported catalysts.

Research Goals
In the near term, our goals are to develop industrial scale catalysts from biomass ash and char, focusing on understanding what phases in the ash and char structure are responsible for catalytic activity, determining how to manipulate char structure to enhance catalytic activity, and trying to understand the mechanism of the reactions occurring on the surface of the char. Intermediate and long term we will focus on engineered manipulation of the char structure and surface to enhance catalytic activity and development of nano-films or nano-particles on the surface of the char to enhance activity and selectivity. Near term our application areas are in catalytic oxidation for air and waste water pollution control (e.g., ammonia oxidation). Intermediate and long-term we will focus on catalysts for synthesis of liquid fuels and chemicals; examples include catalytic transformation of glycerol to synthesis gas, conversion of synthesis gas to mixed alcohols, catalytic cracking and reforming of tar in biomass gasification, and catalytic transformation of bio-oils to liquid fuels.

Recent Results
Our past research has focused on developing control techniques for H₂S and volatile organic sulfur compounds (VOSCs). For example, we have previously demonstrated that wood fly ash could be used as a catalyst to remove H₂S using air only (O₂; Fig. 4) and that methanethiol (a Volatile Organic Sulfur Compound or VOSC) was oxidized in the presence of ozone when using wood fly ash as catalyst at ambient temperatures. We have also worked on the catalytic ozonation of aldehydes as representative VOCs—a typical test bed is shown in Fig. 5.

Very recently, catalytic ozonation of gaseous ammonia was investigated at room temperature using wood fly ash and biomass char as catalysts. Wood fly ash gave the best results (probably due to the presence of certain metals and carbon), removing ammonia (11 ppmv NH₃, 45% conversion) at 23°C at a residence time of 0.34 s, using 5 grams of catalyst or ash at the lowest ozone concentration. However, we believe we can increase catalytic activity by depositing specific metals on the char and have recently demonstrated the capability of forming nano-films of metal oxides on activated carbon using electrochemical deposition method (in collaboration with Nanotechnology at UGA) and demonstrated their activity for removal of air pollutants (Fig. 3).

Fig. 5: Apparatus and Process for Measuring Catalyst Effectiveness. Click figure for larger version.

Relevant Publications
Kastner J.R., Ganagavaram, R, Kolar P., Teja, A., Xu, C. Catalytic Ozonation of Propanal Using Wood Fly Ash and Metal Oxide Nanoparticle Impregnated Carbon. Environ. Sci. Technol. 2008, 42, 556-562.

Kolar, P. 2008. Low Temperature Catalytic Oxidation of VOCs Using Novel Catalysts. Ph.D. Dissertation. Biological Engineering. University of Georgia, Spring.

Kastner J.R., Miller J., Das KC. 2008. Pyrolysis Conditions and Ozone Oxidation Effects on Ammonia Adsorption in Biomass Generated Chars. Journal of Hazardous Materials. [In Review]

Kastner J.R., Miller J., Kolar P., Das KC. 2008. Catalytic Ozonation of Ammonia Using Biomass Char and Wood Fly Ash. Applied Catalysis: Environmental.[In Review]

Praveen Kolar, James R. Kastner, and Joby Miller. 2007. Low Temperature Catalytic Oxidation of Aldehydes Using Wood Fly Ash and Molecular Oxygen, Applied Catalysis B: Environmental (Impact Factor 3.94). 76: 203-217.

Kastner JR, Q Buquoi, R Ganagavaram, K.C. Das. 2005. Catalytic Ozonation of Gaseous Reduced Sulfur Compounds Using Wood Fly Ash. Environ. Sci. Tech. 39(6), 1835-1842.

Kastner, J.R., K.C. Das, J.Q. Buquoi, and N.D. Melear 2003. Low temperature catalytic oxidation of hydrogen sulfide and methanethiol using wood and coal fly ash. Environmental Science and Technology. 37(11): 2568-2574.