STOICHIOMETRY OF REACTION BETWEEN MOO3 AND OH GROUPS OF ALUMINA

A series of oxidic MoO3/Al2O3 catalysts with different Mo loading was prepared by reaction of alumina (172 m(2)/g) with aqueous slurry of Mo O3 and by conventional impregnation with ammonium heptamolybdate. Surf ace concentrations of OH groups on the samples have been determined as a function of pretreatment temperature (160-500 degrees C) and loadin g (up to 19.3% MoO3) by titration with dimethylzinc reagent. It was fo und that consumption of OH groups of alumina by MoO3 proceeds during c atalyst calcination and is completed already at 315 degrees C. Further thermal treatment led only to dehydroxylation of alumina support. The gradual consumption of OH groups by MoO3 can be monitored up to satur ation 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 de pendence between 7.8-14.4% MoO3. An expression describing the change o f OH/Mo stoichiometry (OH consumed per Mo atom) with MoO3 loading was derived. The OH/Mo value decreased from about 2 for the lowest MoO3 lo adings to about 0.8, close to 14.4% MoO3. Above this loading, the cont ent of OH groups decreased to 20-25% of the original value and remaine d constant irrespective of the excess of MoO3 added. As a consequence, the OH/Mo stoichiometry approached zero. The residual hydrogen is asc ribed mainly to the nonreactive hydroxyls of alumina and probably to a small extent to the Mo-OH groups originating from MoO3. (C) 1997 Acad emic Press.