Abstract
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% O 2. Ni 2P 2O 7 is the most selective catalyst with the iso-butene selectivity reaching to a maximum value of 82.2% at 550°C. Ag 4P 2O 7 and Zn 2P 2O 7 are also effective, but the iso-butene selectivities were slightly lower than that of Ni 2P 2O 7. 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 O 2 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 H 2 at 200–400°C. Reactivity of the lattice oxygen of pyrophosphates can be estimated from the value of ΔH f 0 for the corresponding oxide. More than 2 desorption peaks were observed in the TPD spectra of NH 3 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.
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