The use of fossil fuel derived fuels and chemicals is problematic for a variety of reasons including depletion of known reserves, adverse effects on the environment, and political complications. Lignocellulosic biomass stands as a promising alternative to potentially replace a significant portion of fossil derived resources, and it can be locally grown in a sustainable fashion. In the foreseeable future, biorefineries will compete with traditional refineries that convert fossil feedstocks using mostly heterogeneously catalyzed processes at elevated temperatures. It therefore seems that in order to realistically process biomass on a large scale, it is highly likely that heterogeneous catalysis will play a significant role.
One challenge associated with the processing of lignocellulosic biomass is that it is comprised of oxygen rich carbohydrates, while coal and oil are carbon and hydrogen rich. Carbohydrates are polar molecules which typically have low vapor pressure which in turn makes them difficult to process via gas phase reactions. Therefore, large scale processing will likely take place in liquid phase, and water is a good solvent candidate due to its availability and the polar nature of carbohydrates. However, the presence of high temperature liquid phase water poses many challenges for catalytic processes especially considering that many common catalysts were developed for gas-phase reactions. Of specific interest is the material stability of these catalysts in high temperature aqueous phase environments.
This dissertation aims to investigate the structural integrity of some common catalytic materials under typical biomass reforming conditions. There are 3 main objectives of this study: 1) identify potentially stable candidates from commonly used materials, 2) understand the mechanism(s) by which these catalysts degrade, 3) design/modify catalysts in an effort to increase their hydrothermal stability.
The first thrust investigates the behavior of zeolites in hot liquid water. Zeolites are aluminosilicate materials with highly ordered porous structures, and are used in many common catalytic upgrading processes (i.e. fluid catalytic cracking). Two framework types, faujasite and ZSM-5, with different Si/Al ratios were subjected to hot water treatment at 200 °C. It was found that the ZSM-5 framework was highly stable over the course of treatment, independent of the Si/Al ratio, which is consistent with its high stability in steaming environments. The stability of faujasite is highly dependent on the Si/Al ratio, where silicon rich materials are less stable. These findings are opposite that of stability trends in steam, and therefore highlight the importance of investigating catalyst degradation in aqueous environments. The integrity of the aluminum rich materials was attributed to the higher number of Si-O-Al bonds which are less susceptible to cleavage than Si-O-Si bonds. It was also found that the addition of lanthanum modifier was effective in increasing hydrothermal stability.
In the second thrust, the hydrothermal stability of γ-Al2 O3 based catalysts in water at 200 °C are thoroughly investigated, as γ-Al2O3 is used as a catalyst support for many aqueous phase reforming reactions. It was found that the alumina support hydrates and undergoes a phase transformation to form crystalline boehmite (AlOOH) with a subsequent loss in surface area and Lewis acid sites. When metal particles are present on the support, the phase change kinetics are slowed. This behavior is attributed to metal particles adsorbed on “basic” surface OH groups, which are presumed to play a key role in the formation of boehmite. It was also observed that as the support crystallized, metal particles underwent sintering. It is therefore concluded that γ-Al 2O3 suffers detrimental changes in aqueous phase reforming environments associated with the material phase change, but capping specific surface hydroxyls presents a viable route to increase the hydrothermal stability.
The third thrust examines how metal precursor affects the stability of a γ-Al2O3 supported catalyst and in turn what effect this has on a model biomass reaction (aqueous phase conversion of cellulose to sorbitol). It was found that the metal precursor used in catalyst synthesis changes the boehmite formation kinetics and also affects alumina support dissolution. It was demonstrated that dissolved aluminum cations are active for cellulose hydrolysis which is the first step in the formation of sorbitol; therefore, catalyst integrity was shown to directly affect activity for a model biomass reforming reaction. This again highlights the importance of hydrothermal stability as it can influence reaction mechanisms.
The fourth and final thrust aims to stabilize a γ-Al2O 3 supported catalyst for aqueous phase reforming of sorbitol to produce hydrogen. The information gained from the previous thrusts was used to modify a Pt/γ-Al2O3 catalyst. A silicon layer was deposited on the catalyst with two different silicon containing precursors. It was found that these silicon treatments are effective in protecting the catalyst from boehmite formation upon exposure to hot liquid water. The increase in stability is attributed to resilient Si-O-Al linkages and an observed decrease in basic surface hydroxyls which were previously found to play a key role in boehmite formation. In addition to increased stability, the silicon treatments result in stabilization against metal particle sintering and an increase in turnover number for hydrogen production. These results illustrate an effective method of increasing the hydrothermal stability of a γ-Al2O 3 supported catalyst for use in aqueous phase reforming reactions. This method may be applicable to other catalyst supports.