Amine-Functionalized Ordered Mesoporous Silica Transesterification Catalysts

The synthesis, characterization, and catalytic behavior of several amine-functionalized ordered mesoporous silica (OMS) materials are reported. All samples show catalytic activity for the conversion of glyceryl tributyrate with methanol to methyl esters in high yield (>60%). The reactivity of the amines approximately correlates with the pK values of the free amines. The effect of the amine identity, loading, and substrate pore size is also reported. To determine if leached amine was responsible for the observed reactivity, the hot filtrate method was employed. Those experiments show that upon removal of the OMS material the reaction conversion does not increase, indicating surface-tethered amines are primarily responsible for the observed reactivity. The stability of the most promising catalyst, 3-(anilinopropyl)-MCM-41 was analyzed via recycle studies. Leaching of the amine is observed via a decrease in conversion with multiple recycles and a concomitant change in sample color. Capping the residual silanols groups with hexamethyldisilazane leads to an increase in the catalyst stability with a decrease in activity. The work indicates that these solids are interesting altematives to conventional homogeneous catalysts for this class of chemistry.