(27a) Intensified Production of Renewable Fuels and Chemicals: Rational Design of Heterogeneous Aminated Catalysts for Liquid Phase Aldol Reactions
Cooperative effects between amine groups and silanols on a silica surface, were observed and, subsequently, optimized by controlling their concentrations and spatial arrangement with respect to each other. From considerations about the amineâs ability to form the key enamine intermediate and inhibiting imine species, secondary amines were identified as the most suitable amine type. The amine structure and proton affinity was carefully designed by computationaly probing steric and electronic effects. Elucidating promoting group effects, revealed that H-bond donors, such as alcohols, are able to provide the most pronounced cooperativity. Co-feeding small amounts of water inhibits the formation of site blocking species and, hence, improves the catalyst stability. The effects of water and the promoting groups were also rationalized by means of ab initio calculations. However, prolonged exposure to high water concentrations resulted in hydrolysis of the silica support leading to active site leaching and structural changes. This could be reduced, although not entirely avoided, by increasing the support hydrophobicity via incorporation of organic components in the silica framework. Additionally, incorporation of organics allowed to tune the component enrichment inside the catalyst pores and consequently the performance. Organic supports that do not contain Si-O-Si bonds or other structures susceptible to hydrolysis, were identified as crucial for the materialâs long term stability. Finally, an aminated poly(ethylene glycol) methacrylate resin was designed, which is highly active and stable in an aqueous environment.