Use of photoreduction for activation of silica-molybdena catalysts for propylene metathesis: Comparison with thermal reduction

Abstract

Specific activities of silica-molybdena catalysts (1% Mo by weight), prereduced in CO or H 2 either photochemically at 300 K or thermally at 773 K, for the propylene metathesis reaction at room temperature are compared. It is found that preliminary photoassisted reduction of Mo 6+ SiO 2 upon UV irradiation is an effective way to produce active catalysts. Turnover numbers (TON) for photoreduced Mo SiO 2 considerably exceed those for thermally reduced samples (by a factor of 6–10) and are even higher (by a factor of 2–3) than the TON for one of the most active “anchored” organomolybdenum catalysts, Mo(η 3-C 3H 5) 2 SiO 2 . It is shown that propylene metathesis over photoreduced silica-molybdenum occurs via a Langmuir-Hinshelwood mechanism and catalytically active sites involve Mo 4+ ions. Poisoning the catalysts by O 2, N 2O, or NO adsorption is also studied. The possible reasons for higher catalytic activity of photoreduced samples are discussed. A direct comparison is made between the present results and those of M. Anpo I. Tanahashi, and Y. Kubokawa ( J. Chem. Soc. Faraday Trans. 1 78, 2121 (1982)) on the rates of propylene metathesis over Mo 6+ PVG in C 3H 6 atmosphere under UV irradiation.