Selective ammoxidation of isobutylene to methacrylonitrile on a new familyof crystalline Re-Sb-O catalysts

The catalytic properties of a new family of crystalline Re-Sb-O compounds S bRe2O6, SbOReO4. 2H(2)O, and Sb4Re2O13 in selective ammoxidation of isobuty lene to methacrylonitrile (MAN) have been studied and compared with those o f a coprecipitated SbRe2Ox catalyst, an Sb2O3-supported Re2O7 catalyst, bul k Re oxides, and bulk Sb oxides. The Re-based catalysts were more or less a ctive for MAN synthesis with selectivities of 47.9-83.6% at 673 K, whereas bulk Sb oxides (Sb2O3 and Sb2O4) showed no activity. The results demonstrat e that Re is prerequisite for the ammoxidation catalysis of Re-Sb-O systems . In catalytic systems the presence of Sb also contributes to the ammoxidat ion catalysis for MAN synthesis. Among these catalysts, SbRe2O6 was most ac tive and selective (83.6%) for MAN formation at 673 K. No structural change in the bulk and surface of SbRe2O6 was observed after i-C4H8 ammoxidation by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning e lectron microscopy, and confocal laser Raman microspectroscopy. The good pe rformance of SbRe2O6 may be ascribed to its specific crystal structure comp osed of alternate octahedral (Re2O6)(3-) and (SbO)(+) layers. Pulse reactio n results suggested that adsorbed NH3 species on the SbRe2O6 catalyst facil itated the adsorption and subsequent activation of isobutylene. Increasing reaction temperature and decreasing GHSV did not give rise to increasing fo rmation of by-products CO2 and acetonitrile, while increasing the i-C4H8 co nversion. Thus the crystalline SbRe2O6 compound may be a new promising cata lyst for ammoxidation of light hydrocarbons. (C) 2001 Academic Press.