OXIDATIVE DEHYDROGENATION OF ISO-BUTANE TO ISOBUTENE - I - METAL PHOSPHATE CATALYSTS

Metal pyrophosphates catalyse the oxidative dehydrogenation of iso-but ane to iso-butene at 450-550 degrees C using a feed gas of 75 mol% iso -butane and 5% O-2. Ni2P2O7 is the most selective catalyst with the is o-butene selectivity reaching to a maximum value of 82.2% at 550 degre es C. Ag4P2O7 and Zn2P2O7 are also effective, but the iso-butene selec tivities were slightly lower than that of Ni2P2O7. Pyrophosphates of M g, Cr, Co, Mn, and Sn catalyse the oxidative dehydrogenation, but the iso-butene selectivity was 43.8-65.7% at the temperature where the max imum iso-butene yield is observed. The optimum oxygen concentration fo r iso-butene formation was 5-15 mol%, but the increase in Oz concentra tion did not increase the iso-butene selectivity. No adsorbed oxygen s pecies was found by means of TPD. Tile lattice oxygen of the pyrophosp hates began to react with H-2 at 200-400 degrees C. Reactivity of the lattice oxygen of pyrophosphates can be estimated from the value of De lta H-f(0) for the corresponding oxide. More than 2 desorption peaks w ere 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 i so-butene formation. (C) 1998 Elsevier Science B.V.