Reduction of Bi 2O 3MoO 3 catalyst during the ammoxidation of propylene in the absence of gaseous oxygen

Abstract

The reduction of a P 2O 5Bi 2O 3z.sbnd;MoO 3 SiO 2 catalyst (a catalyst for the acrylonitrile process), consisting of Bi 2O 3z.sbnd;MoO 3 + BiPO 4 + amorphous SiO 2, during the ammoxidation of propylene to acrylonitrile in the absence of gaseous oxygen was studied quantitatively by X-ray diffraction techniques and by catalytic activity measurements. Amorphous SiO 2 and BiPO 4 are not affected in synthesis. The overall reduction of catalytically active Bi 2O 3 · 3MoO 3 to metallic Bi and MoO 2 proceeds according to the following two consecutive reactions, where Bi2O 2 · MoO 3 (koechlinite) is an intermediate: Bi 2 O 3·3 Mo O → ( C 3 H 6, NH 3) Bi 2 O 3· Mo O 3+2 Mo O 2+ O 2 Bi 2 O 3·3 Mo O → ( C 3 H 6, NH 3)2 Bi+ Mo O 2+2 O 2 The catalytic activity decreases rapidly with the increasing reduction of the catalyst. The phase changes of the catalyst and the product formation in synthesis correspond quantitatively to the depletion of lattice oxygen from the catalyst. In the absence of gaseous oxygen, the optimum activity for acrylonitrile of silica supported Bi 2O 3z.sbnd;MoO 3 catalysts is at a Bi:Mo atomic ratio of about 1. For the catalytic ammoxidation of propylene to acrylonitrile on Bi 2O 3z.sbnd;MoO 3 based catalysts, a steady state mechanism involving simultaneous reduction and reoxidation of the catalyst surface with gaseous oxygen is suggested.