Oxidative dehydrogenation of isobutane to isobutene III

Among rare earth phosphates, only CePO4 and LaPO4 exhibit very high activity for oxidative dehydrogenation (ODH) of isobutane. This phenomenon was studied in comparison with the corresponding oxides. Isobutene was formed over NdPO4, which was used as a representative of inactive phosphate catalysts, by simple dehydrogenation with 30-32% selectivity, C3H6 and CO2 were formed with a selectivity of 25-33% and 20-30%, respectively. Isobutene was obtained at a higher selectivity of 79-86% over CePO4 and LaPO4. No H2 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 CeO2; 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 Nd2O3 and La2O3. La2O3, Nd2O3, and NdPO4 have only weak acidic sites. CeO2 has weak acidic sites and those with intermediate strength; LaPO4 and CePO4 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 CePO4 and isobutane. Introduction of NH3 into the reaction system reduced both ODH of isobutane and CO2 formation; and stopping the NH3 supply led to a resumption of the activity, which suggests the participation of acidic sites. CePO4 catalyst contained 2% excess Ce to P; in the prepared CePO4 sample, Ce4+ was detected by XPS. We conclude that acidic sites and redox between Ce4+ and Ce3+ play important roles in the ODH of isobutane.