CATALYTIC SYNERGY IN THE OXIDATIVE DEHYDROGENATION OF PROPANE OVER MGVO CATALYSTS

Synergetic effects in the oxidative dehydrogenation of propane have be en studied over magnesium vanadate catalysts containing three differen t Mg/V ratios: 1/2, 2/2, and 3/2, denoted as MgV(1/2), MgV(2/2), and M gV(3/2). Four types of catalysts were analysed: (a) pure magnesium van adate oxides, (b) mechanical mixtures of the pure magnesium vanadate o xides, (c) mechanical mixtures of the magnesium vanadate oxides with a lpha-Sb2O4, and (d) impregnated MgV(2/2) and MgV(3/2) with Sb ions. Sy nergetic effects are observed in MgV(3/2) and in MgV(2/2) oxides when they are in presence of (alpha-Sb2O4. In the mixtures of MgV(3/2) with alpha-Sb2O4, the principal effect is an increase in the selectivity w ith a corresponding decrease in propane conversion, whereas in the mix tures of MgV(2/2) with alpha-Sb2O4, there is a strong increase in prop ane conversion with a moderate increase in propene yield. Concerning t he mixtures of MgV(3/2) and MgV(2/2), synergetic effects in the conver sion, in the yield, and in the selectivity are observed. However, no s ynergetic effects in selectivity or conversion are exhibited by MgV(2/ 2) and MgV(3/2) when they are mixed with MgV(1/2). Highly dispersed (S bxOy), formed on impregnated MgV(2/2) and MgV(3/2), sinters and detach es from MgV(2/2) and MgV(3/2) surfaces. No formation of a new phase or contamination in the presence of alpha-Sb2O4 takes place when MgV(3/2 ) is used. Thus, these catalysts contain two separate phases in contac t. In MgV(2/2) + alpha-Sb2O4 mechanical mixtures a new phase, MgSb2O6, (formed during the test) is also present in small quantities. The mix tures of MgV(3/2) with MgV(2/2) reveal neither formation of a new phas e nor contamination. The synergetic effects in the selectivity exhibit ed by MgV(3/2) mixed with MgV(2/2) and with alpha-Sb2O4 is explained b y a remote control mechanism. MgSb2O6, formed in MgV(2/2)-containing c atalysts, is responsible for the increase in complete oxidation and th e strong decrease in selectivity. (C) 1996 Academic Press, Inc.