Substrate inhibition in the heterogeneous catalyzed aldol condensation: A mechanistic study of supported organocatalysts

In this study, we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support.

Aminopropyl-functionalized mesoporous silica displays a moderate catalytic activity for the aldol condensation. The reason behind this low activity is a substrate inhibition, as determined by kinetic, infrared and solid-state NMR analyses. Small widening of the pores of the support and structural modifications to the supported amine lead to dramatic improvements in catalytic activity. Cooperative interactions make this heterogeneous catalyst orders of magnitude more active than the corresponding homogeneous catalyst.Figure optionsDownload high-quality image (123 K)Download as PowerPoint slideHighlights
► Aminoalkyl-silicas suffer substrate inhibition in catalysis of aldol condensation.
► An increase of only 0.8 nm in pore width improved conversion more than 20-fold.
► Poisoning by Schiff base was identified by kinetics, NMR, infrared spectroscopies.
► Replacing primary amine by secondary eliminates inhibition.
► Heterogeneous catalyst is orders of magnitude faster than homogeneous.