Structure and function relationship in novel solid organic-inorganic hybrid base catalysts

The overall objective of this study was to prepare and optimise solid base catalysts for liquid phase reactions, based on silica support materials functionalised with alkyl amine groups. The study included an investigation of the effect of the pore volume, surface area and the surface chemical characteristics of the silica support on the catalyst performance. The role of surface silanol groups remaining on the silica surface after functionalisation working cooperatively with basic amine groups was also studied.
A series of silica gel supports, together with a commercially-available ordered mesoporous silica, SBA-15, were functionalized with tethered aminopropyl groups. Activities were measured in the nitroaldol reaction between nitromethane and benzaldehyde to afford nitrostyrene, and the Knoevenagel condensation reaction between benzaldehyde and ethyl cyanoacetate to form ethyl-2-cyano-3-phenylacrylate.
The results showed large variations in optimum activity depending on the support material, and its porosity in particular. All support materials exhibited an optimum catalyst loading which depended on support surface area, generally coinciding with complete coverage of the surface with a single layer of catalytic groups, based on a nominal surface area occupied by the three tethering groups linking each propylamine group to the surface.
Further experiments were performed with catalytic groups tethered by one and by two (rather than three) groups, which occupy less space on the support surface. In addition, the possible role of silanol groups was investigated by capping free silanols with non-polar methyl groups using trialkoxymethylsilane. This was found to invariably reduce the activity of the supported amine groups. Whether the role of free silanol groups was through an acid-base cooperative catalytic process or whether it was due to the hydrophilicity they impart to the catalyst surface was investigated by altering the polarity of the reaction solvent/solution. The conclusion was that the role of free silanol groups is to provide mildly acidic groups in a cooperative mechanism rather than simply through control of surface hydrophilicity/hydrophobicity.