Acidity and reactivity of some supported molybdenum oxide catalysts

The acidity of MoO3·γ-Al2O3, MoO3·SiO2, MoO3·SiO2·Al2O3 systems has been determined by measuring the heat of immersion in pyridine; a non-monotonic variation with the MoO3 percentage has been observed in the range from 0 - 25 wt% MoO3. The extremes are: 4, 8*, 15 and 25* wt % MoO3 on γ-A2O3; 3,4*, 7, 13*, 16 and 23* wt % MoO3 on SiO2; 1*, 2, 8* and 25 wt % MoO3 on SiO2·Al2O3, where asterisks denote maxima. For the MoO3·Al2O3 and MoO3·SiO2 systems the activity for double-bond isomerization of 1-butene has been measured in the temperature range of 200-245° and apparent activation energies ΔEa have been derived. As a function of the composition, ΔEa shows a minimum for the 14 wt% MoO3·SiO2, a weak maximum (16 wt%) and a steep minimum (8 wt%) for MoO3·Al2O3 catalysts. The selectivity of the isomerisation reaction has been discussed in terms of surface structure and acidity. It is concluded that the formation of various catalytic species rather than the variation of acidity or the deposition of carbonaceous residues accounts for the selectivity of MoO3·Al2O3 and MoO3·SiO2 catalysts towards isomerization of 1-butene to cis- and trans-2-butene and therefore for the variation of the apparent activation energy with composition.