Facile synthesis of cooperative acid-base catalysts by clicking cysteine and cysteamine on an ethylene-bridged periodic mesoporous organosilica
A Periodic Mesoporous Organosilica (PMO) containing ethylene bridges was functionalized in order to obtain a series of cooperative acid-base catalysts. A straightforward, single-step procedure was devised to immobilize cysteine and cysteamine on the support material via a photoinitiated thiol-ene click reaction. Likewise, PMO materials capped with hexamethyldisilazane (HMDS) were used to support both compounds. This resulted in different materials, where the amine site was promoted by carboxylic acid groups, surface silanol groups or both. The catalysts were tested in the aldol reaction of 4-nitrobenzaldehyde and acetone. It was found that silanol groups have a stronger promoting effect on the amine than the carboxylic acid group. The highest turnover frequency (TOF) was obtained for an amine functionalized material which only contained silanol promoting sites. The loading of the active sites also has a significant effect on the activity of the catalysts, which was rationalized based on a computational study.