Initially, this process was developed in order to convert propene into ethylene and butene. Later, due to the increasing request of propene for the synthesis of numerous chemicals polypropylene, acrylonitrile, propene oxide, cumene, and acetone , new processes were developed for the production of propene. This reaction is the reverse of the triolefin process, with quite the same catalyst [ 37 , 38 ]. It produces ca. Another industrial process using olefin metathesis is the synthesis of neohexene from di-isobutene and ethylene Figure Neohexene is mainly used for perfumes where it is a starting material for the obtention of synthetic musks [ 40 ].
The main application of these heterogeneous systems is in petrochemistry and their use in other domains such as organic synthesis, oleochemistry, or polymerization remains very limited, mainly due to the drastic conditions which are required and to their intolerance of functional groups. The first studies were made by Raman and led to the conclusion that the active site was an isolated surface complex of tungsten but of unknown structure [ 43 ]. The first postulated surface species was a pentacoordinated tungsten complex but no experimental justification was given [ 44 ].
In , Basrur et al. By using a combination of Raman and UV-visible spectroscopies in situ , Wachs et al.
It has also been reported that the amount of surface tungsten and the treatment of the catalyst by an inert gas nitrogen, argon, helium [ 45 ] or by hydrogen [ 50 ] have a significant effect on the catalytic activity. Recently, Wachs et al. The results are depicted in Figure The catalytic activity increases with the amount of WO 3 until a value of ca.
At high coverage, the reaction rate is not dependent on the tungsten loading, due to the formation of WO 3 crystallites which are inactive [ 51 ]. There is also an effect of the WO 3 loading on the amount to ethylene and butenes. Wachs et al. These results are in agreement with the UV-visible results.
Olefin Metathesis, Grubbs Reaction
When the tungsten amount is higher than 0. The main conclusion of this study is that tungsten is well dispersed on the silica surface for WO 3 loadings below 8 wt. The catalyst containing 4 wt. The bands characteristics of the mono-oxo and di-oxo species which are the sole species on the solid decrease simultaneously with time and disappear after minutes [ 51 ]. This proves that the two species were activated by propene and led to the formation of carbene species with elimination of oxygen from the coordination sphere of tungsten Figure For catalysts with high loadings 8 wt.
However, Bell et al. The amount of evolved acetone shows that for a catalyst containing 5. This value is similar to what had been reported for MoO 3 supported on silica [ 53 ] and that proposed by Wachs et al. Very recently Stair et al. EXAFS spectra of a pretreated 5.
This decrease was attributed to the transformation of the mono-oxo species into the di-oxo one Figure This increase of the di-oxo concentration could explain the higher activity of this system compared to that obtained after activation under air. Stair et al. Surprisingly, these catalysts can be regenerated by a treatment under nitrogen at high temperature. The main problem is due to the low amount of active sites.
Olefin metathesis: Reaction and Mechanism
As a consequence, it is very difficult to understand the activation mechanism of the catalyst and also its deactivation. The preparation of systems containing a higher amount of active sites could lead to more active and easily regenerated systems and could allow a better characterization and mechanistic study of the initiation and deactivation steps and their rationalization in terms of classical organometallic chemistry. Surface organometallic chemistry SOMC is a choice method for the preparation of silica supported complexes.
Numerous tungsten complexes with a variety of ligands alkyl, carbene, carbyne, oxo, imido, aryloxy, etc. SOMC can be considered as a bridge between homogeneous and heterogeneous catalysis [ 55 , 56 , 57 ]. Its aim is to graft organometallic complexes on oxide surfaces silica, alumina, titania, zirconia, etc. In the case of oxides, the complex can be linked to the support by one or more bonds with surface oxygen atoms. When the support has been previously functionalized, the bonding can be made via other atoms such as P, N, Si, etc.
As it is the case in homogeneous catalysis, these surface organometallic species can be defined by their ligands around the metal. Two types of ligands can be considered, those which will be involved in the catalytic cycle and those which are only spectators such as oxo, alkoxo, amido, or imido groups. The modification of both types of ligands can have a drastic effect on the activity and selectivity of a given catalytic reaction, allowing to establish structure-activity relationships. For example, pretreatment of the support at different temperatures will lead to the synthesis of surface complexes with one, two, or three bonds with the surface.
This new approach has many advantages:. The metal complexes have a limited mobility on the surface, avoiding the bimolecular decomposition reactions which are often observed in homogeneous catalysis [ 58 ]. The catalysts can be characterized easily by use of spectroscopic methods, as all species are identical.
The good knowledge of the structure of the active site allows to propose a reasonable catalytic cycle and to determine how deactivation and regeneration will proceed. A lot of organometallic complexes of groups 4—8 were grafted on a variety of surfaces such as amorphous inorganic oxides [ 55 ], zeolites [ 59 ], or metals [ 60 , 61 ].
This is mainly due to a combination of organometallic synthesis and surface science. The catalytic efficiency of the materials prepared by this way depends on the coordination sphere around the metal, on the number, and the character ionic or covalent of the bonds with the support and on the nature of the oxide support silica, alumina, silica-alumina, etc.
In the case of tungsten SOMC, the choices of the organometallic precursor and of the support are mainly dependent on the expected catalytic reaction and on the intermediates involved in the postulated catalytic cycle. The high oxidation state of tungsten VI allows the possibility of a number of ligands in the coordination sphere leading to both spectators and reactive species in the catalytic cycle.
The reactive species will be hydrides, alkyl, carbenes, and carbynes. During the last few years, many studies were made with such surface complexes in olefin metathesis. We will review here only those containing the oxo ligand as they could be considered as models of the industrial heterogeneous catalysts. There are two principal methodologies which have been developed to achieve well-defined tungsten oxo species on oxide: i grafting of a reactive tungsten carbyne complex followed by transfer of oxygen from the support and ii grafting of an organometallic complex bearing oxo ligand.
In order to avoid this deactivation, these complexes were heterogeneized. Weiss et al. The carbenic ligand was evidenced by its reactivity with acetone via a pseudo-Wittig reaction and indirectly by the catalytic activity in olefin metathesis. This study showed the existence of interactions between protons of residual hydroxyl groups and the alkyl ligands of the supported species.
Species 18 shows a good activity in propene metathesis initial TOF 5. Two mechanisms were proposed explaining the formation of the carbenic ligand. The other possibility is to form directly the carbene by metathesis between the olefin and the carbyne: a metallacyclobutene is formed which decomposes into a carbene-alkenyl tungsten complex Figure Alumina is a complex support as aluminum can be tetra-, penta-, or hexacoordinated and its surface hydroxyl groups can be bound to one, two, or three aluminum atoms.
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