Enhancing Propene Formation in the Metathesis of Ethylene with 2-Butene at Close to Room Temperature over MoOx/SiO2 through Support Promotion with P, Cl, or S

The metathesis of ethylene with 2-butene to propene over catalysts with supported oxides of Mo, Re, or W is attractive both from industrial and fundamental viewpoints. The latter interest is the result of in situ transformation of metal oxides into metal carbenes, which are the active sites. In comparison with previous studies using metal oxide promoters for enhancing catalyst activity, the present study demonstrates that modification of the SiO2 support with P, S, or Cl is an efficient method for facilitating the ability of supported MoOx species to generate molybdenum carbenes at 50 °C and accordingly to produce propene from ethylene and 2-butene. The promoters are ordered in terms of their enhancing effect as follows: P ≫ S ∼ Cl. Modification with phosphorous results in the creation of a special type of hydroxyls, which assists in 2-butene adsorption and therefore facilitates the transformation of MoOx into Mo ═ CH–CH3. Ethylene cannot convert MoOx to any carbenes (particularly to Mo═CH2) at such low reaction temperatures. Mo═CH2 carbenes are, however, formed by ethylene reaction with Mo═CH–CH3. The presence of both of these carbene types is decisive for propene formation through the metathesis of ethylene with 2-butene.