OXIDATIVE DEHYDROGENATION OF ISO-BUTANE TO ISOBUTENE - II - RARE-EARTH PHOSPHATE CATALYSTS

Among various rare earth phosphates, CePO4 and LaPO4 were found to cat alyze the oxidative dehydrogenation of isobutane to iso-butene at 450- 550 degrees C using a feed gas of 75 mol% iso-butane and 5 mol% O-2. A s for the phosphates of Y, Pr, Nd, Gd, Er, and Yb, iso-butene was form ed with a limited selectivity of 20-40% selectivity, and C3H6 formatio n and deep oxidation into CO and CO2 were promoted over the same tempe rature range. Temperature-programmed-desorption spectra of NH3 for CeP O4 and LaPO4 consisted of two large desorption peaks with maxima at 10 0 degrees C and 270 degrees C suggesting that these phosphates have st rong acid sites. But the rest of the phosphates have only weak acid si tes. Rare earth phosphates have no adsorbed oxygen which can be desorb ed up to 550 degrees C. The lattice oxygen of CePO4 and PrPO4 can be r educed at 200 degrees C and that of the rest of the phosphates at abou t 400 degrees C, The amount of oxygen removed up to 550 degrees C in T PR was smaller than the amount corresponding to a monolayer for all ph osphates. Oxygen isotope equilibration experiments over CePO4 and LaPO 4 revealed that only surface lattice oxygen can participate in the equ ilibration at 500 degrees C and 550 degrees C. It can be concluded tha t acidic character is a key factor in the selective oxidative dehydrog enation of iso-butane over the rare earth phosphate catalysts. (C) 199 8 Elsevier Science B.V.