Gamma-alumina: An Active Support to Obtain Immobilized Electron Poor Zr Complexes

Abstract

By grafting well-defined metallic complexes on a solid support, surface organometallic chemistry provides a bridge between homogeneous and heterogeneous catalysis. This innovative approach has already proven its efficiency for several reactions: polymerization, olefin and alkane metathesis… However several fundamental questions remain open on the chemical role of the oxide surface, on the grafting mechanism and on the reactivity of the grafted complex. Is a single well-defined surface/metal/ligand complex formed? Is the solid surface just an inert support, or does it play an active chemical role? We combine on the one hand reactivity and spectroscopic experiments with on the other hand total energy calculations and spectra simulations (NMR, IR) in order to propose insights for these open questions, using the example of [Zr(CH 2 tBu) 4 ] complex grafted on partially dehydroxylated gamma-alumina. A molecular level approach to the grafting mechanism is proposed. Alumina plays a dual role, both as support and Lewis acid co-catalyst. The hydroxyl groups permit the covalent grafting through Al-O-Zr linkages, while the neighboring Al Lewis sites allow the formation of an ion pair, by transfer of an alkyl ligand from the Zr toward the Al. The formed cationic Zr center shows an enhanced electrophilic character. The mechanism of the formation of the Zr hydride supported on alumina, upon treatment under H 2 gas is described, with the formation of a mixture of mono-hydride (Zr + -H; Al --CH 2 tBu) and bis-hydride (Zr + -H; Al --H). This Zr hydride supported on alumina is active for butane hydrogenolysis. A mechanism with reasonable energy barriers is proposed, the rate limiting step being the C-C scission.

Surface structural-chemical characterization of a single-site d 0 heterogeneous arene hydrogenation catalyst having 100% active sites

“…However, kinetic poisoning experiments in which the catalytic sites are titrated in situ with H 2 O or t BuCH 2 OH indicate that ≤5% of D-type sites are catalytically significant, likely reflecting, among other factors, the established heterogeneity of alumina surfaces (5,6,9), hence rendering active site structural and chemical descriptions necessarily imprecise. In contrast to these results, chemisorption of such organozirconium precursors on SiO 2 , Al 2 O 3 , and SiO 2 -Al 2 O 3 surfaces having appreciable coverage by weakly acidic OH groups predominantly yields covalently bound, poorly electrophilic Etype species via Zr-CH 3 protonolysis with CH 4 evolution (5,6,10,11). Although the E-type sites may be characterized in some detail by high-resolution solid-state NMR and extended X-ray adsorption fine structure spectroscopy (EXAFS), they display minimal catalytic turnover in the absence of added, complicating activators [e.g., methylalumoxane or B(C 6 F 5 ) 3 ], and the fraction of catalytically significant sites is unknown (12,13).…”

Isolated Surface Hydrides: Formation, Structure, and Reactivity

“…In addition to ν Zr–H = 1622 cm –1 , there is also ν Al–H = 1919 and 1415 cm –1 . These two stretches have been assigned to hydrides on four-coordinate and three-coordinate Al, respectively …”

Unravelling the Metastable Nature of the Single Site Tungsten Hydride Metathesis Catalyst Supported on γ-Alumina from First Principles

“…These grafted hydrides are all neutral. Indeed, it has been impossible to find hydrides with charge separation, as it was the case for zirconium hydrides . Dihydrides H1 – H4 are grafted to the oxide support through two covalent Al–O–W bridges, named as bigrafted here.…”

“…Indeed, it has been impossible to find hydrides with charge separation, as it was the case for zirconium hydrides. 26 Dihydrides H1−H4 are grafted to the oxide support through two covalent Al−O−W bridges, named as bigrafted here. Two OH groups have been transformed in this bigrafting and 4 protons out of 6 remain on the surface in H 2 O or OH groups.…”