The literature of olefin metathesis by heterogeneous supported catalysts, both industrial-type supported metal oxides (ReO) and supported organometallic complexes, is comprehensively reviewed. The focus of this Review is supported metal oxide catalysts, but the well-defined supported organometallic catalyst literature is also covered because such model catalysts have the potential to bridge heterogeneous and homogeneous olefin metathesis catalysis. The recent world shortage of small olefin feedstocks has created renewed interest in olefin metathesis as a route to synthesizing small olefins and is reflected in the recent growth of the patent literature. Despite the extensive application of supported metal oxides in industry for metathesis of small and large olefins, the molecular structures and oxidation states of the catalytic active sites, surface reaction intermediates, and reaction mechanisms of this important catalytic reaction have still not been resolved. The absence of reported in situ and operando spectroscopic studies from the olefin metathesis catalysis literature has hampered progress in this area. It appears from this literature review that the topic of olefin metathesis by heterogeneous supported metal oxide catalysts is still a relatively undeveloped research area and is poised for significant progress in understanding of the fundamental molecular details of these important catalytic systems in the coming years.

Supported ReO x /Al 2 O 3 catalysts were investigated for propylene metathesis as a function of surface rhenia loading and extensively characterized with in situ UV−vis, Raman, IR, XANES/EXAFS, and isotopic 18 O− 16 O exchange studies. The experimental studies were complemented with DFT calculations using realistic models of the alumina surface. The surface ReO x sites were found to be isolated surface dioxo (O) 2 ReO 2 species, which represent the most stable surface rhenia structures on alumina as shown by DFT. Two distinct surface ReO 4 species, however, were found to be present and only slightly differ in their bridging Re−O−Al bond lengths brought about by anchoring at different sites of the Al 2 O 3 support. The deformed surface ReO 4 −I species preferentially anchor at more basic μ 1 Al IV and μ 1 Al VI sites and are difficult to activate for propylene metathesis. The surface ReO 4 −II species are formed at more acidic μ 2 Al VI and μ 3 Al VI sites and are the catalytic active sites for propylene metathesis. The surface ReO 4 −II sites were readily activated by propylene while the deformed surface ReO 4 −I sites were almost not affected by propylene, with only a few sites being activated. The steady-state propylene metathesis reaction rates are much higher for the surface ReO 4 −II sites than the deformed surface ReO 4 −I sites. The formation of the less reactive deformed surface ReO 4 −I species could be blocked by occupation of the μ 1 Al IV sites with sacrificial surface TaO x species that resulted in catalysts exclusively containing the more active surface ReO 4 −II sites on alumina. This is the f irst study to demonstrate that the surface ReO 4 −II sites are the precursors for the catalytic active sites for propylene metathesis by supported ReO 4 /Al 2 O 3 catalysts and to molecularly design olefin metathesis catalysts that exclusively contain isolated surface ReO 4 −II sites.

Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2-2.5 Mo atoms nm(-2) ). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.

Supported WO x /SiO2 catalysts were investigated for propylene metathesis as a function of tungsten oxide loading and temperature. The catalysts were synthesized by incipient-wetness impregnation of an aqueous ammonium metatungstate solution onto the silica support and calcined at elevated temperatures to form the supported tungsten oxide phase. In situ Raman spectroscopy under dehydrated conditions revealed that below 8% WO x /SiO2, only surface WO x sites are present on the silica support: dioxo (O)2WO2 and mono-oxo OWO4. The in situ XANES analysis showed that dioxo surface WO4 sites were the dominant surface WO x sites on SiO2 (>90%). The isolated nature of the surface WO x sites was confirmed with in situ UV–vis spectroscopy. The surface WO x sites are activated by exposure to propylene at elevated temperature that removes oxygen from these sites. The activation process produces a highly active surface WO x site that can perform olefin metathesis at ∼150–250 °C. For 8% WO x /SiO2 and higher tungsten oxide loading, crystalline WO3 nanoparticles (NPs) are also present, and their amount increases with greater tungsten oxide loading. WO3 NPs, however, are not active for propylene metathesis. The acid character of the surface WO x sites (Lewis) and WO3 NPs (Brønsted) is responsible for formation of undesirable reaction products (C4–C6 alkanes and dimerization of C2 = to C4 =). This study represents the first time that molecular level structure–activity/selectivity relationships have been established for propylene metathesis by conventionally impregnated supported WO x /SiO2 catalysts.

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