Polarization and activation of alkanes on zeolite LaX. La3+ exchanged faujasite type zeolites are of great interest for catalytic applications, such as cracking and alkylation. When alkanes were adsorbed on zeolite LaX (faujasite with a low Si/Al ratio), strong polarization of the sorbates was observed [1]. This polarization was particularly strong for alkanes, which are branched in the 2 position. It was shown by 13C CP NMR spectroscopy that a partial positive charge is induced on secondary and tertiary carbon atoms, whereas a partial negative charge was found on tertiary protons. The polarization is strong enough to readily activate octane isomers with multiple branchings at ambient temperature [2]. The activation occurs via hydride abstraction forming a surface carboxy group/carbenium ion. Once reactive species are present on the surface, typical carbenium ion reactions (cracking, alkylation, isomerization, hydride transfer) are observed.
Isobutane/2-butene alkylation over lanthanum exchanged faujasite type zeolites. Premature deactivation is the key problem for the industrial implementation of solid acid catalysts for industrial isobutane/2-butene alkylation. Deactivation was investigated by performing the alkylation reaction for different times on stream [3]. After reaction, the catalysts were characterized by various physicochemical techniques. It was shown that the deactivation is caused by a combination of site poisoning and pore mouth plugging. Moreover, LaX (Si/Al = 1.1) and LaY (Si/Al = 2.4) were compared as catalysts for isobutane/2-butene alkylation [4]. The catalyst lifetime and alkylate yield were superior for LaX. The differences were assigned to the higher concentration of strong Brønsted acid sites in LaX. This is in contrast to the commonly accepted trend that the strength of Brønsted acid sites in zeolites increases with increasing Si/Al ratio. The reasons for this trend are the presence of residual sodium in LaY and a stronger distortion of the zeolite framework by polyvalent lanthanum cations in LaX.
Tailoring catalytic activity and selectivity in supported ionic liquids. Immobilizing metal complexes in thin films of ionic liquids on silica supports is a novel approach for combining the selectivity of heterogeneous catalysts with the possibility of facile separation of heterogeneous systems. The interaction of the immobilized metal complexes with the supported ionic liquids was investigated by 1H MAS NMR spectroscopy. It was shown that tight solvent cages are formed around the complexes [5]. The selectivity in these cages can be very different from reactions in classic solvents. The enatiomeric excess for the hydrogenation of acetophenone to (S)-1-phenylethanol exceeded the ee found in methanol by far [6]. Depending on the reaction conditions high selectivities were found for the Markownikoff and anti-Markownikoff product for the hydroamination of styrene with aniline [7].
I recently started my work as a postdoctoral fellow at the Georgia Institute of Technology under the supervision of Prof. Christopher W. Jones and Prof. Pradeep K. Agrawal. My research is focused on catalytic pathways for the production of fuels and hydrogen from woody biomass.
Production of fuels and hydrogen from lignocellulosic biomass. Utilization of renewable renewable feedstocks is a research field of rapidly growing interest. Although a few industrial processes (e.g. ethanol from corn) have been implemented, it is clear that economic viability and sustainability of these technologies require further research. Several innovative concepts have been demonstrated in the last years but many catalytic studies have only been performed with model compounds. Our research is focused on the development of an integrated route to fuels and hydrogen from woody biomass. The first step in the chemical processing of biomass feedstock is depolymerization of the lignin and holocellulose fractions of the wood by hydrolysis. The dissolved fractions are analyzed by HPLC allowing the quantification of primary hydrolysis and degradation products. Analysis of the residues by 13C CP MAS NMR provides information on the concentrations of lignin and carbohydrates as well as on chemical modifications, which occur during the depolymerization process. With the information on dissolution and degradation yields, process conditions can be optimized to provide a higher selectivity to sugars or other intermediates, which will be used as feedstock for catalytic upgrading. Moreover, novel reaction media are investigated for reforming of biomass and model compounds. First experiments indicate that reforming can be achieved under surprisingly mild conditions.
Processing of biological feedstocks will be a key part in my future research. In particular, I am planning to focus on the interaction of key compounds in biomass reforming with heterogeneous catalysts. These fundamental studies will be complemented by work on specific application for the productions of chemicals from biomass. Additionally, I am interested in enantioselective heterogeneous catalysis.
References
[1] C. Sievers, A. Onda, R. Olindo, and J.A. Lercher, J. Phys. Chem. C 111 (2007) 5454-5464.
[2] C. Sievers, A. Onda, A. Guzman, K.S. Otillinger, R. Olindo, and J.A. Lercher, J. Phys. Chem. C 111 (2007) 210-218.
[3] C. Sievers, I. Zuazo, A. Guzman, R. Olindo, H. Syska, and J.A. Lercher, J. Catal. 246 (2007) 315-324.
[4] C. Sievers, J.S. Liebert, M.M. Stratmann, R. Olindo, and J.A. Lercher, Appl. Catal. A-Gen. submitted for publication.
[5] C. Sievers, O. Jimenez, T.E. Müller, S. Steuernagel, and J.A. Lercher, J. Am. Chem. Soc. 128 (2006) 13990-13991.
[6] K.L. Fow, S. Jaenicke, T.E. Müller, and C. Sievers, J. Mol. Catal. A-Chem. (2007) accepted for publication.
[7] O. Jimenez, T.E. Müller, C. Sievers, A. Spirkl, and J.A. Lercher, Chem. Commun. (2006) 2974-2976.