Catalytic oxidation of 1-butene over bismuth molybdate catalysts : III. Reduction of bismuth oxide, molybdenum oxide, bismuth molybdate, and of some nonstoichiometric molybdenum oxides with 1-butene

The reaction of 1-butene with bismuth molybdate, MoO3, and some nonstoichiometric Mo oxides was studied as a function of time and temperature at virtually constant hydrocarbon pressure. The reaction proceeds to four-valent molybdenum and to zero-valent bismuth. It is initially fast on bismuth molybdate, but MoO3 and some of the nonstoichiometric oxides show an induction period. The hydrocarbon products of the reduction are butadiene, CO, and CO2, while also isomerization of 1-butene to cis- and trans-2-butene is observed. The kinetic analysis of the rate as a function of the time was made on the basis of a model proposed by Crank and extended by Haul, Just, and Dümbgen, consisting of a surface reaction followed by oxygen vacancy diffusion in the solid. The model furnishes quantitative values for a surface rate constant and a diffusion constant in the interior. The surface reaction was observed to be fast enough to account for the rate of the oxidative dehydrogenation. Since also the nature of the reduction products is similar to that observed during the catalytic reaction it is concluded that the latter consists of a surface reduction followed by a reoxidation with gaseous oxygen. The diffusion constants observed could be given as log D = log D0 - ED/RTBismuth molybdate log D0 = -3.73 ED = 29.3 kcal mole-1 MoO3 log D0 = -5.62 Ed = 22.1 kcal mole-1 The numerical values for D observed for the MoO3 reduction are equal to those obtainable from the experiments of von Destinon-Forstmann for the reduction of MoO3 by hydrogen.