The effects of the coordination environment and connectivity of Ti on the rate of n-butanal self-condensation over Ti-silica catalysts were investigated. Ti was introduced in two ways, either during the synthesis of mesoporous SBA-15 or via grafting onto amorphous silica with a disordered pore structure. The connectivity of Ti was then characterized by XANES, UV-vis, and Raman spectroscopy. For the lowest Ti loadings, the Ti is found to be predominantly in isolated monomeric species, irrespective of the manner of sample preparation, and as the Ti loading is increased, a progressively larger fraction of Ti is present in oligomeric species and anatase nanoparticles. The turnover frequency for butanal condensation decreased monotonically with increasing Ti loading, and the apparent activation energy increased from 60 kJ mol-1 for monomeric species to 120 kJ mol-1 for oligomeric species. A kinetic H/D isotope effect was observed over isolated titanol and Ti dimer catalysts suggesting that α-H abstraction is the rate-determining step. This conclusion is supported by theoretical analysis of the reaction mechanism. In agreement with experimental results, the calculated activation barrier for alkanal condensation over a Ti dimer is roughly two times greater than that over Ti-OH sites. The cause for this difference was explained by energy decomposition analysis of the enolate formation step which showed that there is a large energetic penalty for the substrate to distort over the Ti-O-Ti dimer than the Ti-OH monomer.

A highly efficient solid-base organocatalyst for the gas-phase aldol self-condensation of n-butanal to 2-ethylhexenal was 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 FTIR measurements demonstrated 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. Blocking the silanol groups on the silica support by silylation or cofeeding water into the reaction stream drastically decreased the reaction rates, demonstrating that weaker acidic silanol groups participate cooperatively with the amine groups to catalyze the condensation reaction. 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 was proposed and then analyzed by DFT calculations. DFT analysis of the reaction pathway suggested 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 agrees reasonably with that measured experimentally. Secondary amines come first: A solid-base organocatalyst achieved by grafting amines onto silica surfaces is applied to the gas-phase aldol self-condensation of n-butanal to 2-ethylhexenal. Silica-supported secondary amine catalysts demonstrate a much higher catalytic activity than the primary amine analogues, owing to the respective formation of enamines as shown by in situ FTIR analysis. The reaction pathway is analyzed by DFT calculations. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.