Unique Performance and Characterization of a Crystalline SbRe_2O_6 Catalyst for Selective Ammoxidation of Isobutane

摘要

The catalytic performances of a new family of crystalline Re-Sb-O compounds SbRe_O_6, SbOReO_4·2H_2O and Sb_4Re_2O_(13) in selective ammoxidation of isobutane (i-C_4H_(10)) to methacrylonitrile (MAN) have been studied and compared with those of a mechanical mixture of Sb_2O_3 + Re_2O_7, coprecipitated SbRe_2O_x, Sb_2O_3-supported Re_2O_7, bulk Re oxides, and Sb oxides. SbRe_2O_6 efficiently catalyzed the i-C_4H_(10) ammoxidation to MAN at 673 K with the good selectivity of MAN (44.9%) and to the sum of MAN + i-C_4H_8 (84.3%) at a steady-state conversion of 4.4%, while significantly no MAN activity was observed on the other catalysts. No structural change in the bulk and surface of SbRe_2O_6 after the i-C_4H_(10) ammoxidation was observed by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and in situ confocal laser Raman microscopic spectroscopy (LRM). The good performance of SbRe_2O_6 may be ascribed to its specific crystal structure composed of alternate octahedral (Re_2O_6)~(3-) and (SbO)~+ layers. It was found that the presence of NH_3 was prerequisite to the C-H bond breaking of i-C_4H_(10), and the oxidation/dehydrogenation of i-C_4H_(10) never proceeded in the absence of NH_3. The presence of NH_3 was also prerequisite to maintain the crystal structure of SbRe_2O_6 under the reaction conditions. Fourier transformed infrared (FT-IR) spectra showed that NH_x species irreversibly adsorbed on the SbRe_2O_6 catalyst. The i-C_4H_(10) ammoxidation proceeded on SbRe_2O_6 by a redox mechanism, in which the oxidative dehydrogenation of i-C_4H_(10) to i-C_4H_8 was the rate-determining step. Increasing reaction temperature and decreasing GHSV did not give rise to increase in the formation of byproducts CO_2 and acetonitrile. Thus the crystalline SbRe_2O_6 compound may be regarded to be a new promising catalyst for the ammoxidation of light alkanes.