Tuning the number of active sites and turnover frequencies by surface modification of supported ReO₄/(SiO₂-Al2O3) catalysts for olefin metathesis

A series of supported ReOx catalysts were synthesized by incipient-wetness impregnation of perrhenic acid onto one component (Al2O3 and SiO2) and surface-modified mixed-oxide supports (SiO2/ Al2O3, Al2O3/SiO2, and ZSM-5 (Si/Al = 15)), characterized with in situ molecular spectroscopy (Raman, DRIFTS, UV-vis, and XAS), and chemically probed (ammonia chemisorption, C2H4/C4H8-titration, C3H6- TPSR, and steady-state propylene self-metathesis). The initial dehydrated surface rhenia species were coordinated to the oxide supports as isolated Re7+O4 sites. For the Al-containing supports, dioxo surface (O=L)2Re-(-O)2 sites appear to be the preferred coordination. The number of activated surface ReOx sites during metathesis is determined by the oxide support ligands (3% ReOx/ZSM-5 > 3% ReOx/5% AlOx/SiO2 > 3% ReOx/5% SiOx/Al2O3 > 3% ReOx/Al2O3 ≈ 3% ReOx/SiO2). The specific activity (TOF) is also controlled by the oxide support ligands (3% ReOx/Al2O3 > 3% ReOx/5% SiOx/Al2O3 ≫ 3% ReOx/ZSM-5 ≈ 3% ReOx/5% AlOx/SiO2 ≫ 3% ReOx/SiO2). The overall propylene metathesis activity (N × TOF), however, is dominated by the number of activated sites (N). Consequently, the enhanced overall activity of surface ReOx supported on SiO2-Al2O3 mixed-oxide supports is related to the greater number of activated surface ReOx sites. The overall propylene metathesis activity was not related to the local surface ReO4 molecular structure or the strength of the Bronsted acid site, since the same rhenia structures appeared to be present on all of the active catalysts and the strengths of the Bronsted acid sites were comparable for all of the active catalysts, respectively.