Oxidation of lower alkenes by α-oxygen (FeIII–O•−)α on the FeZSM-5 surface: The epoxidation or the allylic oxidation?

Abstract

Reactions of anion-radical α-oxygen (FeIII–O•−)α with propylene and 1-butene on sodium-modified FeZSM-5 zeolites were studied in the temperature range from −60 to 25°C. Products were extracted from the zeolite surface and identified. It was found that main reaction pathway was the epoxides formation. Selectivity for epoxides at −60°C was 59–64%. Other products were formed as a result of secondary transformations of epoxides on the zeolite surface. According to IR spectroscopy, the oxidation of propylene over the entire temperature range and 1-butene at −60°C were not accompanied by the formation of (FeIII–OH)α groups, in distinction to methane oxidation. This testifies that hydrogen abstraction does not occur. In case of 1-butene reaction with α-oxygen at 25°C, hydrogen abstraction occurred but was insignificant, ca 7%. According to DFT calculation ferraoxetane intermediate formation is preferable over hydrogen abstraction. Following decomposition of this intermediate leads to the propylene oxide (PO) formation. The results may be relevant to the low selectivity problem of the silver catalyst in propylene epoxidation and raise doubts about the presently accepted mechanism explaining an adverse effect of allylic hydrogen.