(653g) Mesoporous Silica Catalysts Containing Sulfonic Acid And Thiol Groups Organized On The Surface At Varying Distances

Authors: 
Margelefsky, E. L., California Institute of Technology
Zeidan, R. K., California Institute of Technology
Davis, M. E., California Institute of Technology
Dufaud, V., Ecole Normale Superieure de Lyon


There is growing interest in bifunctional heterogeneous catalysts in which multiple organic groups act cooperatively to provide catalytic activity greater than either could achieve alone. In the condensation between phenol and acetone to form Bisphenol A it has been demonstrated that catalysts containing both acid and thiol groups have improved activity and selectivity compared to acid alone, but the effect of acid/thiol distance has never been studied because the catalysts have a random distribution of groups. We have prepared mesoporous silica (SBA-15) materials functionalized with alkylsulfonic acid/thiol pairs at fixed distances via a non-oxidative method. A trialkoxyorganosilane was synthesized containing a 1,3-propanesultone ring. After being grafted onto the surface of SBA-15, these sultone groups can be modified by treatment with a nucleophile such as a hydrosulfide or thiolate anion to generate acid/thiol pairs on the surface. By employing different thiolate anions (such as the monoanion of alkyl and aryl dithiols), both the acid/thiol distance and thiol nucleophilicity can be tuned. The resulting materials were characterized by CP/MAS NMR, acid titration, nitrogen adsorption, and x-ray diffraction. The catalyst with acid and thiol groups separated by three carbon atoms exhibits both high activity and high selectivity in the synthesis of Bisphenol A (per site yield = 78, selectivity = 94%) whereas separating the acid and thiol groups by a greater distance reduces both activity (per site yield = 5 ? 60) and selectivity (65% - 92%). Thus, the distance between the acid and thiol groups is an important factor governing the activity of these bifunctional catalysts. This work represents one of the first examples of the nanoscale organization of two disparate functional groups on the surface of a heterogeneous catalyst.