Oxidative dehydrogenation of isobutane on MCM-41 mesoporous molecular sieves

MCM-41 mesoporous molecular sieves containing silicon and vanadium have been prepared by a direct hydrothermal route. Aluminium-containing [Si,Al]-MCM-41 and purely siliceous [Si]-MCM-41 solids were synthesised for comparison purposes. 51 V static and magic-angle-spinning NMR studies of [Si,V] -MCM-41 showed one type of VO4 in the as-prepared samples (site I) and two types of monodispersed and distorted to various extent VO4 tetrahedra, chemically bound to the walls of MCM-41 (sites IVa and IVb), in the calcined and rehydrated samples. NMR parameters of vanadium units are given. The catalytic performance of MCM-41 materials was probed in the oxidative dehydrogenation (ODH) of isobutane. The highest selectivities to isobutene (48-59%) were observed for the [Si,V]-MCM-41 materials with Si/V = 85 and 30, respectively. On the contrary, a sample containing V2O5 bulk-like species exhibited much lower selectivity to isobutene, especially at the isobutane conversion higher than 5%. The presence of isolated vanadium species, as found by 51 V NMR, was responsible for the high olefin selectivity in the ODH of isobutane. (C) 2002 Elsevier Science B.V. All rights reserved.