Aerobic epoxidation of olefins over the composite catalysts of Co-ZSM-5(L) with bi-/tridentate Schiff-base ligands

Three tridentate or bidentate Schiff-base ligands inclusive of salicylaldehyde benzoylhydrazone (L-1), vanillic aldehyde benzoylhydrazone (L-2) and 4-methyl benzaldehyde benzoylhydrazone (L-3), have been designed, synthesized and employed to coordinate with Co2+ ions and ion-exchanged Co-ZSM-5 forming several Co-L complexes and Co-ZSM-5(L) composite catalysts. The catalytic epoxidation of several alkenes with dry air has been carried out at 90 degrees C under atmospheric pressure using Co-L complexes and Co-ZSM-5(L) catalysts (using TBHP in small amounts as the initiator). In contrast, the catalysts Co-ZSM-5(L) shows higher catalytic activity than Co-ZSM-5 itself and Co-L complexes. Among three Co-ZSM-5(L) catalysts, Co-ZSM-5(L-1) exhibits the highest activity for the selective epoxidation of alkenes. Recycling studies show the recyclability of Co-ZSM-5(L-1) as a heterogeneous catalyst, which does not lose the catalytic activity after even eight reuses appreciably. Based on the experiments, one possible reaction mechanism is proposed. (C) 2010 Elsevier B.V. All rights reserved.