Oxidative dehydrogenation of iso-butane to iso-butene I. Metal phosphate catalysts Original Research Article

Metal pyrophosphates catalyse the oxidative dehydrogenation of iso-butane to iso-butene at 450–550°C using a feed gas of 75 mol% iso-butane and 5% O2. Ni2P2O7 is the most selective catalyst with the iso-butene selectivity reaching to a maximum value of 82.2% at 550°C. Ag4P2O7 and Zn2P2O7 are also effective, but the iso-butene selectivities were slightly lower than that of Ni2P2O7. Pyrophosphates of Mg, Cr, Co, Mn, and Sn catalyse the oxidative dehydrogenation, but the iso-butene selectivity was 43.8–65.7% at the temperature where the maximum iso-butene yield is observed. The optimum oxygen concentration for iso-butene formation was 5–15 mol%, but the increase in O2 concentration did not increase the iso-butene selectivity. No adsorbed oxygen species was found by means of TPD. The lattice oxygen of the pyrophosphates began to react with H2 at 200–400°C. Reactivity of the lattice oxygen of pyrophosphates can be estimated from the value of ΔHf0 for the corresponding oxide. More than 2 desorption peaks were observed in the TPD spectra of NH3 adsorbed on the pyrophosphates, and a linear correlation was found between the concentration of acid amount of the catalysts and the specific rate of iso-butene formation. This strongly suggests that the acidic sites play a key role in the iso-butene formation.