A study of the properties of catalysts for propylene metathesis by means of the temperature programmed reaction method

Abstract

A previous study of propylene metathesis [1] has been extended to active phases formed either by Re 2O 7 plus MoO 3 in various percentages or only by MoO 3, always supported on Al 2O 3. Firstly it has been possible to verify that at 80 C a phase formed by equal weight amounts of MoO 3 and Re 2O 7 gives a higher propylene yield, even if by lowering the selectivity. Temperature programmed desorption and reduction experiments highlighted the result that MoO 3 (less reducible than Re 2O 7) allows the existence of a form of propylene more weakly chemisorbed on the surface, provided with a greater mobility and reactivity. The effect of temperature has been subsequently investigated by using the temperature programmed reaction method; the use of this technique allowed us to point out the variation of the catalytic conversion of propylene with temperature, in accordance with the different content of MoO 3 and Re 2O 7 in the active phase. The kinetic analysis of the results of these experiments allowed us to check that the presence of Re 2O 7 evidences the existence of two temperature ranges, corresponding to different values of the activation energy: an appropriate procedure allowed us to obtain a value of the heat of adsorption of propylene on Re 2O 7 from these results. A correlation has been subsequently carried out between these results and those achieved from the temperature programmed desorption experiments, in order to express hypotheses on the type of the adsorbed propylene involved in the metathesis reaction on the investigated catalysts.