A Study of the Unreduced Molybdena-Alumina System

• DOI: 10.1006/jcat.1993.1193
• Publication Date: 2002-09-18T12:56:57Z
• Authors: A. Pantoja; J. Sarrin; L. Gonzalez; O. Noguera; M.J.P. Zurita; J.L. Galavis; J. Goldwasser
• Keywords: Induction period; Brønsted–Lowry acid–base theory; 1-Butene

Abstract The isomerization and metathesis of 1-butene was carried out over a series of molybdena-alumina catalysts (1.0-8.2 wt% Mo) prepared by the adsorption equilibrium method. Catalytic conversion measurements (including the effect of preadsorbed water), tracer experiments (1-butene-d0 + 1-butene-d8), pyridine adsorption followed by IR spectroscopy, and ESR studies were used as tools for the characterization of the solids. The pyridine adsorption experiments showed an increase in the number of acid sites as well as in the acid strength with increasing metal loading. The catalytic measurements showed a related increase in the rate of isomerization as the %Mo is increased. The cis/trans ratios decreased with increasing molybdenum loading, suggesting an increase in the acid strength of the remaining alumina hydroxyl groups. The results suggest a Bronsted acid mechanism for the isomerization reaction over catalysts with molybdenum loading ≥ 3.9% Mo. The 1.0% molybdena-alumina catalyst behaved totally different. The results suggest a π-allylic mechanism operative on the unperturbed alumina surface. The rate of formation of ethylene and propylene (products of metathesis) increased dramatically with increasing molybdenum loading. The catalysts evacuated at 500°C were considerably more active for the metathesis transformations than after evacuation at low temperatures. A Mo(V) signal was observed for the 6.0% Mo catalyst after evacuation at 500°C and/or after contacting the catalyst with the olefin. No Mo(V) signal was observed for the 1.0% Mo catalyst. The results suggest that the more easily reducible the catalyst is, the easier it would be to effect olefin metathesis.