Oxidative dehydrogenation of saturated hydrocarbons in the presence of polyoxotungstates

Abstract

Polyoxotungstates of the formulas K5FeSi(OH2)W11O39 · 3H2O and Na6MnSi(OH2)W11O39 supported on Al2O3 were studied as catalysts for oxidative transformations of n-heptane, isooctane, cyclohexane, ethylcyclohexane, cyclooctane, decalin, and bicyclohexyl at 150-250°C and a contact time of 2-6 s in an air or helium flow. As a rule, unsaturated hydrocarbons with the same number of carbon atoms were formed. Isooctane underwent partial decomposition. Oxygenated compounds were not found. In the case of monocyclic compounds, dehydrogenation is accompanied to an insignificant extent by ring opening to yield linear dienes. Hydrocarbons with six-membered cycles (with the exception of unsubstituted cyclohexane) are partially dehydrogenated to aromatic compounds. The yield of dehydrogenation products on the iron-containing catalyst decreases in the order: decalin > bicyclohexyl > ethylcyclohexane > cyclohexane > n-heptane > cyclooctane. The use of H2O2 as an oxidant (200°C, 6 s) strongly increases the yield of the dehydrogenation products on Fe polyoxotungstate, especially in the case of bicyclohexyl and decalin.