Many industrial catalysts contain isolated metal sites on the surface of oxide supports. Although such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This Review discusses how surface organometallic chemistry (SOMC) engineers surface sites with well-defined structures and provides insight into the nature of the active sites of industrial catalysts; the Review focuses in particular on olefin production and conversion processes.

The development of more effective alkane dehydrogenation catalysts is a key technological challenge for the production of olefins from shale gas, an abundant source of light hydrocarbons. Surface organometallic chemistry provides an original approach to generate nanometric Ga-Pt bimetallic particles supported on partially dehydroxylated silica containing gallium single-sites, which displays high activity, selectivity, and stability in propane dehydrogenation. This catalyst was prepared via sequential grafting of a platinum precursor onto silica possessing site-isolated gallium sites followed by H reduction. Monitoring generation of the reduced species, GaPt/SiO, via in situ X-ray absorption spectroscopy reveals formation of a Ga Pt (0.5< x < 0.9) alloy with a fraction of gallium remaining as isolated sites. This bimetallic material exhibits catalytic performance that far surpasses each of the individual components and other reported Ga-Pt based catalysts; this is attributed to the highly dispersed Ga Pt alloyed structure on a support with low Brønsted acidity containing gallium single-sites.

Molybdenum-based molecular alkylidyne complexes of the type [MesC≡Mo{OC(CH)(CF)}] (MoF, x = 0; MoF, x = 1; MoF, x = 2; MoF, x = 3; Mes = 2,4,6-trimethylphenyl) and their silica-supported analogues are prepared and characterized at the molecular level, in particular by solid-state NMR, and their alkyne metathesis catalytic activity is evaluated. The C NMR chemical shift of the alkylidyne carbon increases with increasing number of fluorine atoms on the alkoxide ligands for both molecular and supported catalysts but with more shielded values for the supported complexes. The activity of these catalysts increases in the order MoF < MoF < MoF before sharply decreasing for MoF, with a similar effect for the supported systems (MoF ≈ MoF < MoF < MoF). This is consistent with the different kinetic behavior (zeroth order in alkyne for MoF derivatives instead of first order for the others) and the isolation of stable metallacyclobutadiene intermediates of MoF for both molecular and supported species. Detailed solid-state NMR analysis of molecular and silica-supported metal alkylidyne catalysts coupled with DFT/ZORA calculations rationalize the NMR spectroscopic signatures and discernible activity trends at the frontier orbital level: (1) increasing the number of fluorine atoms lowers the energy of the π*(M≡C) orbital, explaining the more deshielded chemical shift values; it also leads to an increased electrophilicity and higher reactivity for catalysts up to MoF, prior to a sharp decrease in reactivity for MoF due to the formation of stable metallacyclobutadiene intermediates; (2) the silica-supported catalysts are less active than their molecular analogues because they are less electrophilic and dynamic, as revealed by their C NMR chemical shift tensors.

Alkene metathesis is a widely and increasingly used reaction in academia and industry because of its efficiency in terms of atom economy and its wide applicability. This reaction is notably responsible for the production of several million tons of propene annually. Such industrial processes rely on inexpensive silica-supported tungsten oxide catalysts, which operate at high temperatures (>350 °C), in contrast with the mild room temperature reaction conditions typically used with the corresponding molecular alkene metathesis homogeneous catalysts. This large difference in the temperature requirements is generally thought to arise from the difficulty in generating active sites (carbenes or metallacyclobutanes) in the classical metal oxide catalysts and prevents broader applicability, notably with functionalized substrates. We report here a low temperature activation process of well-defined metal oxo surface species using organosilicon reductants, which generate a large amount of active species at only 70 °C (0.6 active sites/W). This high activity at low temperature broadens the scope of these catalysts to functionalized substrates. This activation process can also be applied to classical industrial catalysts. We provide evidence for the formation of a metallacyclopentane intermediate and propose how the active species are formed.

Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have...

A well-defined silica-supported molybdenum dioxo species, ([triple bond, length as m-dash]SiO)2Mo(O)2, is prepared by grafting Mo(O)2[OSi(OtBu)3]2 on partially dehydroxylated silica SiO2-700, followed by thermal treatment under high-vaccum and calcination. Activated by an organosilicon agent the resulting material is active for olefin metathesis at 30 °C.

Despite the importance of the heterogeneous tungsten-oxo based olefin metathesis catalyst (WO 3 /SiO 2) in industry, understanding of its initiation mechanism is still very limited. It has been proposed that reduced W(IV)-oxo surface species act as precatalysts. In order to understand the reactivity and initiation mechanism of surface W(IV)-oxo species, we synthesized a well-defined silica-supported W(IV)-oxo species (≡SiO)WO(OtBuF 6)(py) 3 (OtBuF 6 = OC(CH 3)(CF 3) 2 ; py = pyridine) (F6@SiO 2-700) via Surface Organometallic Chemistry (SOMC). F6@SiO 2-700 was shown to be highly active in olefin metathesis upon removal of pyridine ligands through the addition of tris(pentafluorophenyl)borane (B(C 6 F 5) 3) or thermal treatment under high vacuum. The metathesis activity towards olefins with and without allylic C−H groups, namely β-methylstyrene and styrene, respectively, was investigated. In the case of styrene, we demonstrate the role of surface OH groups in initiating metathesis activity. In particular, the presence of strong Brønsted acidic surface OH sites, revealed by 15 N-labeled pyridine, likely arises from the presence of adjacent W sites in the catalyst and assist styrene metathesis, an olefin having no allylic C-H bond. In contrast, initiation of olefins containing allylic C−H group (e.g. β-methylstyrene) is independent of surface OH group concentration and likely involves allylic C−H activation mechanism like the molecular W(IV)-oxo species. This study indicates that initiation mechanisms depend on the olefinic substrates and reveals the synergistic effect of Brønsted acidic surface sites and reduced W(IV) sites in the initiation of olefin metathesis.

CuGax alloy nanoparticles supported on SiO2 can be obtained by surface organometallic chemistry. This catalyst is active and selective for the hydrogenation of CO2 to CH3OH, related to the formation of an optimal interface between Cu and GaIIIOx.