Tuning the Number of Active Sites and Turnover Frequencies by Surface Modification of Supported ReO₄/(SiO₂–Al₂O₃) Catalysts for Olefin Metathesis

Abstract

A series of supported ReO x 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 ReO x sites during metathesis is determined by the oxide support ligands (3% ReO x /ZSM-5 > 3% ReO x /5% AlO x /SiO2 > 3% ReO x /5% SiO x /Al2O3 > 3% ReO x /Al2O3 ≈ 3% ReO x /SiO2). The specific activity (TOF) is also controlled by the oxide support ligands (3% ReO x /Al2O3 > 3% ReO x /5% SiO x /Al2O3 ≫ 3% ReO x /ZSM-5 ≈ 3% ReO x /5% AlO x /SiO2 ≫ 3% ReO x /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 ReO x supported on SiO2–Al2O3 mixed-oxide supports is related to the greater number of activated surface ReO x sites. The overall propylene metathesis activity was not related to the local surface ReO4 molecular structure or the strength of the Brønsted acid site, since the same rhenia structures appeared to be present on all of the active catalysts and the strengths of the Brønsted acid sites were comparable for all of the active catalysts, respectively.

Computational Insights into Active Site Formation during Alkene Metathesis over a MoOx/SiO2 Catalyst: The Role of Surface Silanols

“…Total Gibbs energy for each system is estimated by adding the Gibbs energy correction and the DFT-D3­(BJ) , dispersion correction to the PBE0/def2-TZVPP single point energy. The same computational methodology was recently validated and used to study the reactivity of the Phillips CrO x /SiO 2 catalyst. , If not stated otherwise, the reaction pathways are discussed in terms of relative Gibbs energies at T = 298.15 K because the MoO x /SiO 2 system can be active in alkene metathesis even at room temperature. − In most cases, propene has been selected as the reactant, similar to many experimental investigations. ,,,,,− ,,,,, …”

“…6−9 In most cases, propene has been selected as the reactant, similar to many experimental investigations. 6,7,11,13,15,[17][18][19]22,23,26,37,53 All calculations have been performed with the Gaussian 09 set of programs. 84 For the graphic presentation of the structures, the GaussView 6.0 software 85 has been applied.…”

“…Olefin metathesis proceeds according to the well-known carbene mechanism , and metal alkylidene species must be present in the reaction environment. In the case of the supported metal oxide systems, active metal alkylidene sites are usually formed in situ from the surface metal oxide species upon contact with alkene. ,,,− The structure and oxidation state of the metal oxide precursors are not exactly known. The mechanism of their transformation into the active sites is also not well recognised, although several routes for this initiation stage of alkene metathesis were proposed for various catalytic systems, based on experimental studies (Scheme ).…”

“…The mechanism of their transformation into the active sites is also not well recognised, although several routes for this initiation stage of alkene metathesis were proposed for various catalytic systems, based on experimental studies (Scheme ). ,,,,,,− ,− Unambiguous determination of the initiation mechanism is difficult mainly due to typically very low fraction of the surface metal alkylidene species. ,,,,, …”

“…3,7,8,11,12,17,20−22,24−31 Unambiguous determination of the initiation mechanism is difficult mainly due to typically very low fraction of the surface metal alkylidene species. 3,11,13,17,19,22 For high-valent metal oxide species, predominant in calcinated catalysts, the pseudo-Wittig mechanism involving oxametallacyclobutane intermediates was often postulated (Scheme 1). 11,12,17,21 According to this initiation route, the metal site preserves its valency and the aldehyde byproducts are formed.…”