Phase cooperation between tin and antimony oxides in selective oxidation of isobutene to methacrolein I. Mechanical mixtures of SnO 2 and α-Sb 2O 4

Abstract

Results are reported concerning the cooperation between SnO 2 and α-Sb 2O 4 particles in the selective oxidation of isobutene to methacrolein. The catalysts were prepared by mechanically mixing the corresponding powders. A conspicuous catalytic synergy was observed when methacrolein production and selectivity to methacrolein formation were considered. The catalysts, both fresh and used, were characterized by XRD, 119Sn Mössbauer spectroscopy, XPS, analytical electron microscopy (AEM), and ESR in order to investigate the origin of the synergy observed. The joint use of these techniques yielded no indication that a new phase (or solid solution) formed or that mutual surface contamination during either the preparation of the mixture or the catalytic test took place. Within the sensitivity limits of the techniques used, the mechanical mixtures can be considered as composed of two pure separate phases in good contact. The origin of the observed synergy and the other experimental observations is explained in a satisfactory manner by the existence of a “remote control” mechanism, i.e., that α-Sb 2O 4 produces a mobile oxygen species, namely spillover oxygen, which, by flowing onto the surface of SnO 2, creates on the surface of the latter new selective sites and/or regenerates those which have become deactivated. Spillover oxygen produced by α-Sb 2O 4 seems to control the selective catalytic sites on SnO 2 by inhibiting their transformation to reduced, nonselective sites. Spillover oxygen also inhibits the formation of carbonaceous deposits.