Anaerobic oxidation of isobutane: I. Catalysis by Cu–V complex oxides

Abstract The anaerobic oxidation of isobutane over complex oxide catalysts of the type Cu n V 2 O x ( n  = 0, 1, 2, 3, and 5) was studied. V 2 O 5 , CuV 2 O 6 , and Cu 2 V 2 O 7 showed high oxidizing activity, and Cu 3 V 2 O 8 and Cu 5 V 2 O 10 were less active. Over the active catalysts, CO 2 was selectively formed only at the beginning of the reaction, and the rate of CO 2 formation decreased with reaction time. Isobutene formation became dominant 15–30 min after the start of the reaction. The rates of CO 2 and isobutene formation were lower over the less active catalysts, Cu 3 V 2 O 8 and Cu 5 V 2 O 10 . The catalytic activity at 623 K decreased in the order V 2 O 5  > CuV 2 O 6 , Cu 2 V 2 O 7  ≫ Cu 3 V 2 O 8 , Cu 5 V 2 O 10 . Because CO 2 formed at the beginning of the reaction, this system should be applicable for the selective formation of isobutene at appropriate reduction degrees. We concluded that CuV 2 O 6 and Cu 2 V 2 O 7 should be applicable in a thin-layer reactor, in which isobutane and oxygen are separately supplied to opposite sides of the layer; in such a reactor, the catalyst surface on the isobutane side could be kept at a certain degree of reduction. Using the rate constants for isobutane consumption over Cu 2 V 2 O 7 , we calculated the apparent activation energy for anaerobic oxidation of isobutane to be 87.6 kJ/mol, which agrees well with the reported value for a metal oxide catalyst. XPS analysis indicated that not V 5+ but Cu 2+ was reduced during the reduction of Cu 2 V 2 O 7 by H 2 at 473 K. Our results indicate that selective oxidation should be achievable over complex oxides that contain highly redox-active metal ions such as Cu 2+ if the reaction is carried out in a thin-layer reactor.