Dehydration Kinetics over Metal-Modified Zeolites: Impact for Biomass Pyrolysis
David J. Robichaud, Robin Cywar, Lintao Bu, Seonah Kim, Brian Trewyn, Mengze Xu, Matthew Yung, Ryan McDonough, and Mark Nimlos
Catalytic fast pyrolysis over zeolite catalysts is a promising thermochemical technology to convert biomass to transportation fuels or biomaterials. However, several technical issues continue to impede the implementation of this technology including oil yields, oxygen content, and physiochemical properties. Poor understandings of catalytic reaction kinetics (e.g. dehydration, decarbonylation) continue to hinder progress toward addressing these technical issues. Dehydration over acidic zeolites plays an important role in the deoxygenation and coupling reactions during catalytic fast pyrolysis. We have applied a combined experimental/computational approach to investigate two dehydration-related reactions in an effort to generalized our understanding of this important reaction class: dehydration of alcohols to olefins, and aldol coupling/dehydration reactions of small oxygenates to produce furans. Kinetic reaction mechanisms have been investigated over nascent zeolites (MFI, Beta, MOR) to identify descriptors for catalytic reactivity that can be used to design more effective catalysts. Based on these descriptors, modified zeolites were synthesized with transition metals metals. The resulting Brønsted and Lewis acidity upon zeolite modification is assessed and the consequences for the activation barriers are evaluated. Hybrid QM/MM ONIOM model calculations are performed and compared against experimental results for improved reactivity.
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