Green epoxidation of 1-hexene: O2 utilizing cis-MoO2 Schiff base complex

Abstract

The green epoxidation of 1-hexene using [hydrazine-N-salicylidene-N′-salicyloyl-] cis-dioxomolybdenum(VI) and its zeolite-Y composite as catalyst in DMF in the presence of molecular oxygen as oxidant and in the temperature range 333–363 K is reported. The ‘green’ factor of olefin oxidation catalysis using the Mo(VI) dioxo complex is its ability to use atmospheric oxygen which is non-hazardous and inexpensive and produces no harmful by-products. The effects of the initial concentration of the substrate and Mo complex and the temperature in the range 333–363 K on the rate of reaction are examined. The olefin conversion into different products is monitored using gas chromatography. The formation of 1,2-epoxyhexane, 1-hexanal and 2-hexanone is observed under both homogeneous and heterogeneous catalytic conditions; however, 2-hexanone did not form during the reactions using the zeolite composite. 1-Hexene conversion was found to be nearly complete and the product distribution into 1,2-epoxyhexane, 1-hexanal and 2-hexanone being 1 : 20 : 6 under homogeneous conditions for 6 h; while the final product is 1-hexanal under heterogeneous conditions for 6 h. The rate of 1-hexene oxidation showed first order dependence with respect to the catalyst and substrate concentration and it was an endothermic reaction (activation energy, 10.68 Kcal mol−1). A tentative mechanism for the oxidation of 1-hexene is proposed and the observed product formation was reasoned to involve epoxide isomerisation through participation of adventitious water in the reaction mixture and the resulting carbonium ions.