(5bn) Experimental and Theoretical Studies of Reaction Pathways in Solid Acid Catalysis

I will describe mechanisms for hydrocarbon reactions on zeolites based on microkinetic modeling, computational density functional theory (DFT), and experimental studies. High temperature hydrocarbon-based reaction pathways were investigated in the conversion of light alkanes to aromatics over ZSM-5 based materials (793-823 K). Rate constants and activation energies for elementary steps were obtained based on a microkinetic model comprised of more than 300 reactions that considered surface bound neutral alkoxide species to react via carbenium-ion-like transition states and alkane activation by carbonium-ion-like transition states. Cluster based DFT calculations for olefin protonation mechanisms showed that steric effects involving the interaction of the adsorbed intermediate with the zeolite walls governed stability of the alkoxide species while activation barriers scaled with the protonation site (primary, secondary or tertiary) on the olefin based on the carbocationic nature of the transition state. In a separate study, low temperature hydrocarbon reactions (408-453 K) were probed in the carbonylation of dimethyl ether (DME) to methyl acetate resulting in reactions that selectively form the first carbon-carbon bond and avoid homologation reactions. Steady-state, transient, and isotopic exchange studies were combined with adsorption and desorption studies of probe molecules and infrared spectroscopy in order to identify methyl and acetyl groups as surface intermediates and to propose a DME carbonylation mechanism. Carbonylation is initiated by methylation of acid sites and limited by CO addition to the resulting methyl groups to form acetyl species, which then desorb as methyl acetate in fast methylation reactions without concurrent formation of water and with the re-formation of methyl groups. These steps avoid formation of water and its strong inhibitory effects, present in similar reactions of methanol. In recent work, I am working on low temperature C₁ coupling chemistry to produce highly branched hydrocarbons on solid acid catalysts. For my future research I plan to build upon this experience and explore alternative energy technologies based on biomass and C₁-dervied feedstocks.