Prof. Carsten Sievers, Georgia Tech

Hydrothermal Stability and Surface Chemistry of Heterogeneous Catalysts for Biomass Conversion

Abstract: Aqueous phase processes are expected to play a key role in the production of renewable chemicals and fuels from biomass. Facile separation makes heterogeneous catalysts an attractive option for achieving high efficiency in these processes. Therefore, it is not surprising that a number of recent publications described heterogeneously catalyzed processes in water including aqueous phase reforming and dehydration of sugars. However, little is known about the stability of the catalysts under reaction conditions (liquid water at 150-265 °C) and the mechanisms involved in catalytic conversion of biomass.

γ-Alumina is a very popular support for metal particles. However, in hot liquid water γ-alumina is converted into a crystalline boehmite phase. This transformation results in a drastic decrease of the surface area and the concentration of Lewis acids sites. Moreover, sintering of supported metal particles is observed once a certain fraction of the support has been converted. However, the conversion of γ-alumina to boehmite is significantly retarded when supported Pt or Ni particles are present on the support because the metal particles block basic surface hydroxyl group. These groups also serve as initiation sites for the hydration of γ-alumina to boehmite. To eliminate the remaining surface hydroxyl groups Pt/γ-Al2O3 catalysts can be protected by silylation. Boehmite formation is not observed for silylated samples, and a significantly increased hydrogen production is observed when these catalysts are used for aqueous phase reforming of glycerol. Increased stability is also observed for amorphous silica-alumina supports that are synthesized under well controlled conditions. However, the stability and transformations of these materials strongly depend on the synthesis methods used.

The surface chemistry of biomass derived oxygenates can be studied by ATR-IR spectroscopy. Even at room temperature, Pt/Al2O3 readily activates biomass-derived oxygenates, such as glycerol. As a result, the Pt particles are covered with adsorbed carbon monoxide. In addition, a small amount of chemisorbed molecules are observed on the alumina support. In the presence of molecular oxygen, carbon monoxide is oxidized to carbon dioxide, which readily desorbs. At the same time the formation of aldehydes and carboxylic acids is observed. The present results will help designing catalysts with specific active sites and preventing deactivation by formation of carbonaceous deposits.

Bio: Carsten Sievers obtained his Diplom degree and PhD in Technical Chemistry at the Technical University of Munich, Germany. Under the guidance of Prof. Johannes A. Lercher he worked on heterogeneous catalysts for various processes in petroleum refining including hydrogenation of aromatics in Diesel fuel, alkylation, alkane activation, and catalytic cracking. Additional research projects included novel catalytic system, such as supported ionic liquids. In 2007, he moved to Atlanta to work with Profs. Christopher W. Jones and Pradeep K. Agrawal at the Georgia Institute of Technology as a postdoctoral fellow. His primary focus was the development of catalytic processes for biomass depolymerization and synthesis of biofuels. He joined the faculty at Georgia Institute of Technology in 2009. His research group is developing catalytic processes for sustainable energy production. Specific foci are on the stability of solid catalysts in aqueous environment, surface chemistry of oxygenates, heterogeneous catalysis in aqueous phase, applied spectroscopy, physicochemical characterization of solid materials, and reactor design.

For more information contact Prof. L. Andrew Lyon (404-894-4090).