Tuning the Number of Active Sites and Turnover Frequencies by Surface Modification of Supported ReO4/(SiO2–Al2O3) Catalysts for Olefin Metathesis
- Lehigh Univ., Bethlehem, PA (United States)
- Stony Brook Univ., NY (United States)
- Stony Brook Univ., NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
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=)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 (Ns). 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. Furthermore, the overall propylene metathesis activity was not related to the local surface ReO4 molecular structure or strength of the Brønsted acid site since the same rhenia structures appeared to be present on all of the active catalysts and the strength of the Brønsted acid sites were comparable for all of the active catalysts, respectively.
- Research Organization:
- Lehigh Univ., Bethlehem, PA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- FG02-93ER14350; SC0012335
- OSTI ID:
- 1775287
- Journal Information:
- ACS Catalysis, Vol. 11, Issue 4; ISSN 2155-5435
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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