(741g) Role of Water in Aldol Condensation Catalyzed By MCM-41 Functionalized with Sulfonic Acid Groups
A detailed kinetics analysis indicates that the rate limiting step of MCM-41-SO3H-catalyzed aldol condensation reaction is the bimolecular attack of an enol intermediate (formed via protonation/tautomerization) onto an electrophile represented by a second carbonyl molecule. The reaction kinetics strongly depends on the proximity of the two sites. On a catalyst with high acid density, the distance between two adsorbed molecules at adjacent acid sites is close enough to allow for cooperative catalysis. By contrast, when the density of acid sites is low, the cooperative action of two nearby sites is significantly inhibited as they are located farther apart. Consequently, the kinetics switches from a dual-site (L-H) to a single-site (E-R) bimolecular model. In this case, the addition of small amounts of water significantly enhances the activity.
From detailed kinetics studies, solid-state NMR analysis and theoretical calculations, we conclude that water can act as a bridge between an electrophile next to the activated enol and a remote acid site. This bridging action can effectively transfer the polarization exerted by the nearby acid site to the carbonyl group of the electrophile via a chain of H-bonds. Therefore, it is observed that the kinetics goes back to a dual-site L-H model, in which the carbonyl of the adsorbed ketone is polarized by the acid site on the surface facilitating the attack and the rate-limiting CâC coupling.