Mechanistic Insights into Heterogeneous Catalytic Conversion of Light Oxygenates and Olefins from Biomass Derivatives

The decreasing availability of fossil fuel resources has generated growing interests in chemical transformations of small oxygenates and olefin molecules derived from biomass sources into larger hydrocarbon fuel and chemical precursors. Aldol condensation of light oxygenates and alkene oligomerization of light olefins which proceed via C-C bond formation paths show potential for production of valuable chemical and fuel precursors. However, oxygenate coupling by C-O bond formation such as in the reaction of methanol to dimethoxy methane (DMM) can occur together with C-C formation processes and lead to coke formation and catalyst deactivation. First principle density functional theory (DFT) calculations were carried out to probe the C-C as well as C-O coupling pathways over metal oxide as well as zeolite catalysts. More specifically we examined the paths for aldol condensation of aldehydes and ketones over ZrO2 and ZnO and alkene oligomerization and oxygenate coupling in zeolites. These three systems show similarities in the formation and stability of charged intermediates, stabilization of transition states, and intermolecular and catalytic steric effects on the activity for these reactions.
Aldol condensation of C2-C4 aldehydes and ketones over model ZrO2 and ZnO surfaces show that the rates are controlled by the kinetics for enolate formation, with m-ZrO2 being the most active surface for aldol condensation. The rates of reaction in these systems were controlled by the stabilization of an adsorbed enolate that forms on the oxide where smaller aldehydes show higher interaction energies and thus faster rates of reaction.
Alkene oligomerization of C2-C4 olefins which proceed via similar C-C bond formation steps in TON, MFI, and MOR zeolites show similar results, where the relative differences in stabilization of the reacting carbenium ion by the zeolite as well as differences in alkene adsorption largely determined the overall activity for alkene oligomerization in various zeolite structures. TON was predicted to be the most active catalyst as the narrow 1-D channel maximizes the number of potential secondary interactions with the carbenium ion at the transition state.
Oxygenate coupling reactions in zeolites lead to larger oxygenate species that can go on to form coke and deactivate the zeolite catalyst. The methanol to hydrocarbons process, for example, is inhibited by the formation of dimethoxymethane (DMM) formation. This reaction of methanol to DMM was calculated to be controlled by the low stability of the methyl cation that forms. The large size of the CHA cage as well as the exothermic nature of the proposed mechanism suggests that the CHA zeolite can accommodate further coupling reactions of much larger molecules that could ultimately block entrance into the open zeolite cages.