Oxidation of isobutane over complex oxides containing V, Nb, Ta, and Mo under aerobic and anaerobic reaction conditions

Abstract

Isobutane oxidation over single metal oxides of V, Nb, Ta, and Mo and over their complex oxides was studied. The most active was V 2O 5, while medium activity was shown by LaVO 4, BiVO 4, CaV 2O 6, Ta 2O 5, TaV 2O 6, ZnNb 2O 6, Nb 2O 5, MgV 2O 6, YVO 4, Cu 2V 2O 7, MoO 3 and MnNb 2O 5. The less active oxides were Na 2MoO 4 and Ag 2MoO 4. Isobutane was oxidized mainly to isobutene and CO x , but isobutene selectivity was generally lower than 50% below 673 K. Relatively high isobutene selectivity was shown by MgTa 2O 6, TaV 2O 6, and ZnNb 2O 6. Methacroleine was only obtained over MoO 3 and methanol was formed only over V 2O 5 and Cu 2V 2O 7. Acetic acid was formed over certain V-containing oxides. No oxygen-containing products were formed over Nb- and Ta-containing oxides. On a unit surface area basis, Nb 2O 5 and Ta 2O 5 have higher oxidizing activity than V 2O 5 and MoO 3. Isobutane oxidation was studied in the absence of gaseous O 2 over MV 2-type complex oxides. Catalytic activity was determined as follows: Cu V 2 O 6 ≫ Co V 2 O 6 > Mg V 2 O 6 > Zn V 2 O 6 ≫ Zr V 2 O 7 > Ca V 2 O 6 . MgV 2O 6 is the most selective for isobutene formation. Combination to easily redoxable metal ions tends to accelerate the deep oxidation process. There may be a threshold for the initial reaction rate of lattice oxygen per unit surface area for selective isobutene formation. Selective isobutene formation continued until a reduction of MgV 2O 6 by at least 4.5%, had finished and the reduced catalyst was recovered by the calcination at 773 K. Selective formation of isobutene made it possible to combine oxidations of isobutane with MgV 2O 6 and with the oxidations of reduced catalysts.