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James G. Goodwin, Ph.D. -- Research Activities Removal of Impurities from Biomass Gas
Our research involves the study of heterogeneous catalysis and, especially, the in-situ characterization of surface reaction properties. A major goal of our research is to develop an understanding of the underlying cause(s) of the kinetics of surface-catalyzed reactions. Our research group has especially been focused on the genesis and nature of active sites. Major themes of research that have been followed are: catalyst formulation effects on activity/selectivity, the use of chemical promoters, support-metal interactions, preparation effects, bifunctional catalysis, acid catalysis, CO hydrogenation (methanation, Fischer-Tropsch synthesis, methanol synthesis, higher alcohol synthesis), hydrogenolysis (ethane and cyclopropane), isomerization of alkanes, methane coupling/partial oxidation, MTBE synthesis, catalyst deactivation (attrition, coking, poisoning, sintering, and metal-support compound formation), adsorption (air separation), and chemisorption characterization. Recently, our research group has focused heavily on the catalysts and catalyzed reactions relating to the conversion of biomass to chemicals and fuels especially plant and animal lipids to biodiesel, the removal of ammonia and tars from biomass gasification products, Fe-based catalysts for the Fischer-Tropsch synthesis of syn gas produced from biomass and coal, and alkane isomerization.
Typical reaction studies provide information about the overall kinetics of a catalytic system. However, much catalyst site kinetic information is not separable from the "lumped" kinetic information obtained in these types of studies. Steady-state isotopic transient kinetic analysis (SSITKA) provides perhaps the most powerful technique available for studying the surface kinetics of heterogeneous catalysts and adsorbents under realistic reaction conditions. Our research group has been in the forefront in the last 20 years in developing the capabilities of SSITKA, especially in data handling using moment analysis and mathematical convolution and deconvolution. SSITKA permits the determination of the concentration of the active surface reaction intermediates, the surface residence time of atoms involved in reaction, a measure of the site activity, and the degree of surface site heterogeneity. This technique is routinely used in reaction studies in our laboratories.
Biodiesel is a diesel fuel consisting of mono alkyl esters of long chain fatty acids prepared from renewable lipid feedstocks, such as vegetable oils and animal fats. The synthesis of diesel fuel from biomass (biodiesel) provides an effective way for utilizing more effectively renewable resources, a means to recycle carbon dioxide for a combustion fuel, a way to convert waste vegetable oils and animal fats to a useful product, and produces a fuel that is biodegradable, non-toxic, and has a low emission profile. Fats and oils are primarily made up of triglycerides, esters of glycerol (mono- and diglycerides) and fatty acids (carboxylic acids). Vegetable oils, and even more so animal fats, effectively cannot be used directly as diesel fuels due to their high viscosity, polymerization as a result of the reactivity of C-C double bonds that may be present, incomplete combustion, and coking. Animal fats are much more unsaturated than vegetable oils.
Biodiesel has a number of characteristics that makes it better than petroleum-based diesel fuel: inherent CO2 recycle, higher cetane number, no aromatics, almost no sulfur, and 10 to 11 percent oxygen by weight making it cleaner burning, non-toxic, and biodegradable. In December 2002, the Biomass Research and Development Technical Advisory Committee for the Secretaries of Energy and Agriculture published the Roadmap for Biomass Technologies in the United States (USDA, 2002). In it, a stated, desirable goal was that "transportation fuels from biomass will increase significantly from 0.5 percent of U.S. transportation fuel consumption in 2001 (ca. 20 million gallons) to 20 percent in 2030. The report specifically identified a need for more research to develop chemical catalysts that could advance biotechnology capabilities. Such catalysts are necessary to efficiently and cost effectively convert biomass feedstock into biobased products, including fuels. Although there are currently commercial processes for converting vegetable oils and animal fats to biodiesel, these processes are not as cost-effective or energy efficient as they should be to compete with petroleum-based diesel fuel. Primarily, there is a need to develop continuous processes (rather than batch), to use heterogeneous catalysts (to minimize separation problems and decrease cost/gal.) rather than homogeneous disposable catalysts (NaOH, KOH), and to develop catalysts able to work for long periods of time without losing activity.
Little research has been done to date to understand the fundamentals of solid catalyzed esterification of free fatty acids (FFAs) and transesterification of triglycerides (TGs). This is needed in order to scientifically develop commercial solid acid and base catalysts for use in biodiesel production. While base catalysts are active for transesterification of TGs, only solid acid catalysts are active for both transesterification or TGs and esterification of FFAs. Use of solid acid catalysts would eliminate the need to use liquid acids, would eliminate corrosion problems in process equipment, and possibly could reduce the number of reaction steps required. Research in our laboratories is currently focused on understanding the fundamentals of solid catalyzed transesterification and esterification through the accurate measure of reaction kinetics so that better catalysts can be designed. Reaction studies make use of model compounds as well as animal fats and vegetable oils. Both liquid and gas phase reaction conditions are being investigated. Recent results have shown that reaction on solid Bronsted acids proceed via by a similar mechanism as with sulfuric acid as the catalyst.
Removal of Impurities from Biomass Gas
When biomass is gasified, synthesis gas (CO and H2) is formed that can be used as fuel for turbines to generate electricity, reactants for the formation of chemicals or alternative liquid fuels, or the source of hydrogen for fuel cells. Unfortunately, tars (ca. 10,000 ppm), ammonia (ca. 4000 ppm), and hydrogen sulfide (ca. 100 ppm) are also formed during the gasification process. Prior to use of this synthesis gas, these impurities must be removed. For example ammonia is a precursor for NOx that has to be removed in order to make biomass gas suitable for use in power generation due to stringent emission standards and to remove a potential catalyst poison for numerous possible downstream catalytic processes.
NH3 decomposition has been studied for over one hundred years as its catalysis is related to NH3 synthesis. Recently, there has been an increased interest in NH3 decomposition as a potential COx-free source of on-site H2 generation for proton exchange membrane fuel cells. The reaction at relatively high temperatures is also important in the cleanup of syngas derived from biomass gasification and coal. Active catalysts include Ru, Fe, Pt, Ni, and W. Carbides and nitrides of V constitute another promising group of catalysts for the decomposition of NH3. Another carbide, WC, offer interesting possibilities as it exhibits catalytic properties similar to those of Pt and other desirable properties such as extreme hardness and thermal stability. Research in our laboratories is underway to investigate ammonia decomposition on WC.
Gasification followed by FTS is currently the most promising method for upgrading low-value coal and biomass to high-value liquid fuels and chemicals. There are sufficient domestic reserves of coal to supply most of US fuel needs for more than one hundred years using FTS. Because biomass is formed by fixation of atmospheric CO2, its use as a fuel feedstock is attractive because this results in virtually no net CO2 emissions. The total biomass produced each year as waste material from agriculture and forest operations could be converted into roughly 40 billion gal/yr of liquid fuels, roughly 25% of the current US gasoline usage.
Bulk iron (Fe) catalysts are the catalysts of choice for converting low H2/CO ratio syngas produced by gasification of biomass or coal to fuels via the Fischer Tropsch synthesis (FTS). These relatively low-cost catalysts have low methane selectivity and high water gas shift activity (which generates H2 in situ). However, development of a bulk Fe FTS catalyst that combines high FT activity, low methane selectivity, high attrition resistance (i.e., ability to withstand physical breakage), and long-term stability (low deactivation rate) is still elusive and presents a widely recognized barrier to the commercial deployment of FTS for coal and biomass conversion. The critical property determining the activity and deactivation of Fe catalysts for FTS appears not to be Fe in the metallic state but the carburized Fe surface.
Current research in our group addresses the issues of the nature, genesis, and maintenance of active Fe sites from a totally different perspective than previous studies. Unlike previous studies of Fe bimetallic catalysts, this work focuses on the ability of second and third metals to form mixed-metal carbides with Fe at reaction or pretreatment conditions. Improvements in activity and deactivation are being sought. Interesting selectivities, especially low methane production, could result as the nature of the active surface carbide and, potentially, the active sites are modified.
The skeletal isomerization of n-alkanes plays an important role in the production of branched, high-octane hydrocarbons for gasoline. Sulfated zirconia (SZ), as well as other modified zirconias, has gained much attention for the isomerization of n-butane because it exhibits high activity and selectivity towards iso-butane even at low temperatures. Initially, this was suggested to be related to the strong acidity of SZ’s, similar to that found for zeolites such as HY, but it has been shown that this is not the case. It is possible that the catalytic ability of SZ for alkane isomerization is related to its capacity to promote redox reactions of hydrocarbons (oxidative dehydrogenation), as some authors have recently suggested.
There is still much controversy about the mechanistic pathway operating for n-butane isomerization on SZ. Several researchers have suggested that the reaction proceeds through a monomolecular mechanism. A monomolecular pathway satisfactorily explains the high selectivity toward iso-butane, especially for short TOS and low conversions.
The other mechanism suggested for n-butane isomerization on SZ is a bimolecular one. The bimolecular pathway is considered to occur via the formation of butene, which subsequently oligomerizes with adsorbed C4+ carbenium ions to produce C8+ oligomeric species. Under this hypothesis, it is assumed that a C8+ species undergoes isomerization and beta-cleavage leading to mainly iso-butane and some disproportionation products. Up until now, however, how the isomerization step of the C8+ oligomer occurs and how it leads mainly to iso-butane have not been clearly explained in the literature. The hypothesized bimolecular mechanism is supported especially by two facts: (1) the observation of disproportionation products such as propane and pentanes and (2) substantial isotopic scrambling for the reaction using 1,4-13C n-butane, with the iso-butane product containing an isotopic distribution from zero to four 13C atoms which can not be explained solely by a monomolecular route. In addition to these two important observations, recently experimental results have shown that the initial formation rate of iso-butane is dramatically enhanced by olefins introduced at low concentrations in the reactant stream. We have recently shown that the activity promoting effect of olefins occurs even when the added olefin is not butene, pointing to a non-specific olefin rate enhancement for iso-butane formation. In addition, excess iso-butane molecules are formed from each olefin molecule added, which suggests that active sites formed by olefin addition last for multiple turnovers. This evidence has led us to conclude that active sites can probably be best described as olefin-modified sites [21, 23]. These observations and the presence of a reaction induction period for the catalyst also support a bimolecular pathway for reaction.
In order to try to account for the seeming contradictory results in the literature, some authors have suggested that n-butane isomerization proceeds by a monomolecular pathway in the early stages of reaction prior to becoming a bimolecular one at long time-on-stream. Others have proposed that a monomolecular isomerization pathway takes place at very low n-butane conversions but becomes a bimolecular route at high conversion yielding disproportionation products. Still other authors have suggested, based on some evidence, that the reaction mechanism is dependent upon reaction temperature.
Recently, based on our research concerning the effect of nonspecific olefin addition on the catalytic activity of SZ for n-butane isomerization and its relationship to the reaction mechanism, we have proposed a comprehensive mechanism exhibiting a duality between monomolecular and bimolecular routes that substantiates all the major facts observed for n-butane isomerization. Thus, the issues of high selectivity, presence of disproportionation products, isotopic scrambling, catalyst deactivation and the effect of nonspecific olefin addition on reaction activity can all be addressed within the context of this mechanistic proposal, which is at the base a bimolecular mechanism but proceeds in a way to externally look partially like a monomolecular one. |
Last Updated:April 18, 2009
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