Comparison of SiO2-supported molybdena, tungsta and rhenia catalysts for olefin metathesis

• Published in: Catalysis science & technology Vol. 14; no. 16; pp. 4716 - 4724
• Main Authors: Zhang, Bin, Wachs, Israel E
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 12.08.2024
• Subjects: Catalysts; Chemical synthesis; Dehydration; High temperature; Metal oxides; Metathesis; Propylene; Silicon dioxide; Surface stability; Thermal stability; Titration
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/d4cy00730a

The characteristics of supported MOx/SiO2 catalysts (M = Re, Mo, W) for olefin metathesis were compared side-by-side to understand the differences among these catalyst systems. The series of SiO2-supported metal oxide (ReOx, MoOx, WOx) catalysts, with maximum achievable dispersion of surface MOx sites, was synthesized by incipient-wetness impregnation of the corresponding aqueous precursors, molecularly characterized (in situ Raman and DRIFTS), chemically probed with transient studies (C3H6-TPSR-MS, C3H6-TPSR-IR, and C2H4/2-C4H8 titration with C2H4-TPSR-MS) and steady state activity for C3H6 metathesis. Under dehydrated conditions, the initial SiO2-supported surface metal oxide sites are fully dispersed as isolated sites on SiO2 and present as isolated tri-oxo (O=)3ReO sites, isolated di-oxo (O=)2MoO2 sites and a mixture of isolated di-oxo (O=)2WO2 and mono-oxo (O=)WO4 sites (dioxo ≫ mono-oxo) anchored at the isolated Si–OH surface hydroxyls of SiO2 support. High temperature propylene pre-treatments were required to activate the SiO2-supported MOx catalysts, but also resulted in MOx volatilization (ReOx > MoOx ≫ WOx). The number of activated surface MOx sites and TOF values varied with the specific MOx site, activation temperatures and reaction temperatures, which are related to the stability of the surface M=O oxo and M=carbene bonds. The higher thermal stability, activity and TOF of the surface WOx sites on SiO2 at elevated temperatures relative to the supported ReOx/SiO2 and MoOx/SiO2 catalysts account for adoption of the supported WOx/SiO2 catalyst as the industrial olefin metathesis catalyst among these supported MOx/SiO2 catalysts.