(171a) Design of Mesoporous Silica-Supported Alkylamine Catalysts for the Gas-Phase Condensation of Aldehydes and Ketones

Highly efficient solid-base organocatalysts for the gas-phase aldol self-condensation of n-butanal to 2-ethylhexenal have been developed by grafting site-isolated amines on tailored silica surfaces. The catalytic activity depends largely on the nature of amine species, the surface concentration of amine and silanol groups, and the spatial separation between the silanol and amine groups. In situ FT-IR measurements shows that the formation of nucleophilic enamines leads to the enhanced catalytic activity of secondary amine catalysts, whereas the formation of imines (stable up to 473 K) leads to the low activity observed for silica-supported primary amines. This work demonstrates that the spatial separation of the weakly acidic silanols and amines can be tuned by the controlled dehydration of the supporting silica and by varying the linker length of the amine organosilane precursor used to graft the amine to the support surface. A mechanism for aldol condensation is proposed and then analyzed using density functional theory (DFT). DFT analysis of the reaction pathway suggests that the rate-limiting step in aldol condensation is carbon-carbon bond formation, which is consistent with the observed kinetics. The calculated apparent activation barrier that follows from this analysis agrees reasonably with that measured experimentally. Subsequent studies have demonstrated that the co-operative acid-base properties of secondary amines catalysts can be significantly altered by introducing an optimal amount of Al or Ti into the silica matrix. DFT analysis of the reaction pathway suggests that in contrast to the C-C bond forming rate limiting step observed with silica supported amine catalysts, the rate-limiting step is iminium hydrolysis over silica containing Ti-OH or Al-OH groups, which is consistent with the observed kinetics. We have recently demonstrated that ketones, such as 2-butanone and 2-hexanone can be dimerized with high selectivity over silica-supported acid-base catalysts. The lower activity observed for ketone versus aldehyde condensation is explained by DFT calculations.