Silanol assisted aldol condensation on aminated silica: understanding the arrangement of functional groups

Free silanol groups are known to promote the activity of aminated silica. In this work the effect of the silanol-to-amine ratio on the aldol condensation of 4-nitrobenzaldehyde and acetone is investigated in a range from 0 to 2.4. Irrespective of the amine density, identical, moderate turnover frequencies are obtained if the silica exclusively has amines on its surface. The turnover frequency increases with increasing silanol-to-amine ratio until an upper limit is reached at a silanol-to-amine ratio of 1.7. At this upper limit the turnover frequency is a factor5 higher than the turnover frequencies obtained with the monofunctional amine-based catalysts. This increase is ascribed to hydrogen-bridge interactions between the silanols and the carbonyl moiety of the reactants that provoke a more easy interaction between the carbonyl moiety and the amine as required for the aldol condensation. The observation that higher values than one for the silanol-to-amine ratio are required is rationalized by computer simulations. It was found that amine groups were grafted on the silica surface in a clustered manner, originating from positive deviations from ideality in the synthesis mixture, that is, from clustering of the amine precursor in the liquid phase.