Selective Ammoxidation of Isobutylene to Methacrylonitrile on a New Family of Crystalline Re–Sb–O Catalysts

Abstract

The catalytic properties of a new family of crystalline Re–Sb–O compounds SbRe2O6, SbOReO4·2H2O, and Sb4Re2O13 in selective ammoxidation of isobutylene to methacrylonitrile (MAN) have been studied and compared with those of a coprecipitated SbRe2Ox catalyst, an Sb2O3-supported Re2O7 catalyst, bulk Re oxides, and bulk Sb oxides. The Re-based catalysts were more or less active 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 demonstrate that Re is prerequisite for the ammoxidation catalysis of Re–Sb–O systems. In catalytic systems the presence of Sb also contributes to the ammoxidation catalysis for MAN synthesis. Among these catalysts, SbRe2O6 was most active 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 electron microscopy, and confocal laser Raman microspectroscopy. The good performance of SbRe2O6 may be ascribed to its specific crystal structure composed of alternate octahedral (Re2O6)3− and (SbO)+ layers. Pulse reaction results suggested that adsorbed NH3 species on the SbRe2O6 catalyst facilitated the adsorption and subsequent activation of isobutylene. Increasing reaction temperature and decreasing GHSV did not give rise to increasing formation of by-products CO2 and acetonitrile, while increasing the i-C4H8 conversion. Thus the crystalline SbRe2O6 compound may be a new promising catalyst for ammoxidation of light hydrocarbons.