Selective oxidation of isobutene to methacrolein on multiphasic molybdate-based catalysts

Synergetic effects in the selective oxidation of isobutene to methacrolein have been studied over catalysts containing several molybdate phases. Two types of catalysts have been analysed: (1) pure bismuth molybdates with Bi/Mo ratios of 2/2 and 2/3 [BiMo(2/2) and BiMo(2/3)] and cobalt molybdate [CoMo(1/1)]; and (2) multiphasic molybdates consisting of three or four molybdates prepared by: (a) coprecipitation of cobalt and bismuth salts and calcination at high temperatures, (b) mechanically mixing in dry conditions the separate molybdates and calcining at high temperatures and (c) dispersing the separate molybdates in n-pentane and calcining at high temperatures. Synergetic effects are observed in multiphasic BiMo(2/2)CoMo and BiMo(2/3)CoMo catalysts. For BiMo(2/2)CoMo catalysts the highest synergetic effects in the conversion, yield and selectivity are observed. For the catalysts before test no new phase is formed after calcination while after test X-ray measurements indicate a practically complete transformation of alpha-Bi2Mo3O12 into gamma-Bi2MoO6 and MoO2 and traces of MoO3 and Bi2xMo1-xO3. Neither contamination between phases is observed. The synergetic effects are explained by the cooperation between separate gamma-Bi2MoO6, MoO3 and (alpha+beta)-CoMoO4 phases in contact. The cooperation between phases is more effective when MoO3 is present in a large proportion.