Oxidative dehydrogenation of isobutane to isobutene III: Reaction mechanism over CePO 4 catalyst

Abstract

Among rare earth phosphates, only CePO 4 and LaPO 4 exhibit very high activity for oxidative dehydrogenation (ODH) of isobutane. This phenomenon was studied in comparison with the corresponding oxides. Isobutene was formed over NdPO 4, which was used as a representative of inactive phosphate catalysts, by simple dehydrogenation with 30–32% selectivity, C 3H 6 and CO 2 were formed with a selectivity of 25–33% and 20–30%, respectively. Isobutene was obtained at a higher selectivity of 79–86% over CePO 4 and LaPO 4. No H 2 formation suggests that the essential reaction was the ODH of isobutane. In the oxidation over metal oxides, isobutene selectivity was relatively high: 60–70% over CeO 2; however, the essential reaction was considered to be a simple dehydrogenation. Isobutene was formed at 40–47% selectivity, but simple dehydrogenation of isobutane would proceed over Nd 2O 3 and La 2O 3. La 2O 3, Nd 2O 3, and NdPO 4 have only weak acidic sites. CeO 2 has weak acidic sites and those with intermediate strength; LaPO 4 and CePO 4 have relatively strong acidic sites in addition to the weak acidic sites and those of intermediate strength. Oxidative dehydrogenation activity was correlated with strong acidic sites. ODH of isobutane occurred in the reaction between lattice oxygen of CePO 4 and isobutane. Introduction of NH 3 into the reaction system reduced both ODH of isobutane and CO 2 formation; and stopping the NH 3 supply led to a resumption of the activity, which suggests the participation of acidic sites. CePO 4 catalyst contained 2% excess Ce to P; in the prepared CePO 4 sample, Ce 4+ was detected by XPS. We conclude that acidic sites and redox between Ce 4+ and Ce 3+ play important roles in the ODH of isobutane.