Stoichiometry of Reaction between MoO3and OH Groups of Alumina

Abstract

A series of oxidic MoO3/Al2O3catalysts with different Mo loading was prepared by reaction of alumina (172 m2/g) with aqueous slurry of MoO3and by conventional impregnation with ammonium heptamolybdate. Surface concentrations of OH groups on the samples have been determined as a function of pretreatment temperature (160–500°C) and loading (up to 19.3% MoO3) by titration with dimethylzinc reagent. It was found that consumption of OH groups of alumina by MoO3proceeds during catalyst calcination and is completed already at 315°C. Further thermal treatment led only to dehydroxylation of alumina support. The gradual consumption of OH groups by MoO3can be monitored up to saturation loading 14.4% MoO3. It was described by a linear decrease of the number of OH groups up to about 7.8% MoO3, followed by a nonlinear dependence between 7.8–14.4% MoO3. An expression describing the change of OH/Mo stoichiometry (OH consumed per Mo atom) with MoO3loading was derived. The OH/Mo value decreased from about 2 for the lowest MoO3loadings to about 0.8, close to 14.4% MoO3. Above this loading, the content of OH groups decreased to 20–25% of the original value and remained constant irrespective of the excess of MoO3added. As a consequence, the OH/Mo stoichiometry approached zero. The residual hydrogen is ascribed mainly to the nonreactive hydroxyls of alumina and probably to a small extent to the Mo–OH groups originating from MoO3.