Organic–inorganic hybrid catalysts for acid- and base-catalyzed reactions

Abstract

Stable heterogeneous catalysts with adjustable base strength have been prepared by grafting organic amine bases into the the pores of the inorganic mesoporous material, MCM-41, and to a porous styrene–divinylbenzene resin. The activities of these catalysts are compared for the formation of the monoglyceride from lauric acid and glycidol, and the Knoevenagel condensation of heptaldehyde with benzaldehyde to form α-n-amylcinnamaldehyde (jasminaldehyde). Also included in the comparison are catalysts prepared by incorporating K 2O, BaO and K 2O/La 2O 3 into MCM-41. The resin-based catalyst suffers from poor thermal and mechanical stability. The organic–inorganic hybrid material containing the strong hindered amine base, TBD (1,5,7-triazabicyclo[4,4,0]dec-5-ene), performs well for the monoglyceride reaction at 110°C, and the catalyst can be re-used for at least 11 cycles with little loss of activity. However, it deactivates if it is used in the coupling reaction with aldehydes. This is obviously caused by loss of the base at the higher reaction temperature of 170°C, and by poisoning of the strong basic sites with benzoic acid which is formed by oxidation or through the Cannizzaro disproportionation of benzaldehyde. The more weakly basic catalysts based on MCM-41 with K 2O/La 2O 3 can be used at a higher reaction temperature to compensate for their lower intrinsic activity, and their activity can be restored by calcination.