Propene metathesis

Fig. 2 Quantification in the gas phase of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene during propene metathesis (500 equiv.) catalyzed with 13(1 equiv.) at 25 °C...

Figure 3.27 Quantification 3,3-dimethylbutene and ( )-4,4-dimethyl-2-pentene during propene metathesis catalyzed by i-0-Re( Bu)(=CH Bu)(CH 2Bu) at 25 C.

The surface species is an active catalyst for propene metathesis (780 eq., TOI min = 0-89 molpropene hioImo s" for the catalyst obtained by the benzene impregnation route) and ethyl oleate metathesis (lOOeq. TOI mm =0.11 moloieate hioImo s for the catalyst obtained by the benzene impregnation route) [136]. The high catalytic activity of the surface neopentylidene complex has not been modeled by the silsesquioxane analogue [136]. 

ILtSn or ILtPb causes a spectacular improvement of the catalytic performance of Re207/Al203, raising the rate of propene metathesis by 10-100 and also making possible the metathesis of functionalized alkenes where previously no reaction was observed41-43. When R = Me some methane is produced, suggesting that the reaction [Re](CH3)2 — [Re]=CH2 + CH4 is involved in the production of the initiating species. 

The properties of silica- and/or alumina-supported molybdena catalysts for propene metathesis were studied by Handzlik and coworkers. These materials could be prepared by thermal spreading of Mo02(acac)2 with well-dispersed molybdenum in a wide range of its loading. The selective metathesis activity depends on the substrate and on the surface molybdenum concentration. For example, a higher activity is found for the molybdena-alumina system at high Mo loadings. 

Hydrogenation of 1,3,5-trimethylbenzene Methanol synthesis Ethyl acetate synthesis Methyl acetate synthesis Propene metathesis. 

Fig. 8 Simulations and experimental results gas phase exit composition for propene metathesis in fixed-bed catalytic reactor without pressure swing effect. (From Ref. ". )...

The grafting of Mo(allyl)4 or other allyl compounds (Mb, W) on silica and other oxides has also been investigated, but the mechanism of grafting remains unclear [8]. These catalysts had some activity in olefin metathesis, but several activation steps under H2 and O2 at high temperature were usually needed to obtain the best activity. Iwasawa et al. [8] were able to establish a structure-activity relationship showing that Mo(IV) was the most active (Scheme 4 activity for propene metathesis at 90 °C in ton. min ). These steps, however, dramatically modified the coordination sphere of the metal, depending on the thermal treatment applied. 

After CO photoreduction and thermal treatment the silica-supported cubane-type clusters [(Cp Rh)2Mo309(OMe)4] and [(Cp Rh)4Mo40 6] have significant catalytic activity in propene metathesis. ... 

Supported vanadium oxide catalysts have some activity for the olefin metathesis reaction but are not very selective (Banks 1969). In combination with Rc207 a very active catalyst system is obtained with alumina as the support (Nakamura, R. 1977b Xu, X. 1985b). When V2O5/AI2O3 is preheated at about 575°C and treated with Me4Sn it forms an effective catalyst system (V/Al = 3/97 Sn/V = 0.05-0.08) for propene metathesis at 25°C (Ahn, H-G. 1993). 

A comparison has been made of the activity of Al203-supported Mo catalysts prepared in various ways. The order of activity for propene metathesis at I00°C... 

It is concluded that on a reduced M0O3/AI2O3 surface, e.g. after activation in hydrogen, active sites are formed from both isolated Mo(IV) and Mo(VI) precursors, whereas after thermal treatment in inert gas the activity originates exclusively from active sites derived from Mo(VI) precursors. Mo-carbene formation might take place in a way similar to that for photocatalytic propene metathesis see Section 2.7, eqn. (28). 

An ESR study of a reduced Mo03/MgO catalyst (Mo/Mg = 0.5-0.7) indicates that Mo(V) ions with a coordination number less than six are the active sites for propene metathesis at 150°C with this catalyst (Hasegawa, S. 1992). For the... 

Molybdenum oxide films prepared by sublimation in vacuo on to a glass wall are converted into an active catalyst for propene metathesis at 0.4 kPa and 20°C by treatment of the surface with ethene (or propene) and atomic hydrogen at — 196°C (Kazuta 1987). Active sites can also be obtained by exposing the molybdenum oxide film to CH2 radicals produced by the reaction of CH2I2 with A1 or Mg metals deposited near the oxide film (Kazuta 1988, 1990). 

For propene metathesis, it is best to pretreat the catalyst with H2 or CO at 500°C followed by exposure to oxygen at room temperature. This treatment presumably brings the surface compound into the most favourable oxidation state for the formation of the initial metal carbene. The same result can be achieved by treatment first with propene at 450°C and then with oxygen at 80°C (Nakamura, R. 1982 Xu, Y. 1986). y-Irradiation of Re207/Al203 in a reducing atmosphere gives an increase in reaction rate of at least a factor of 10 (Tarasov 1996). 

Re2(CO)io/Al203 is active for propene metathesis after appropriate heat treatment (Danil5uik 1983, 1988). These treatments include (i) 02-adsorption at 120°C on partially decarbonylated Re2(CO)io/Al203 (ii) 02-adsorption at 25°C on Re2(CO)io/Al203 which has been completely decarbonylated at 500°C and (iii) partial reduction at 200-300°C of Re2(CO)io/Al203 preoxidized at 300°C. The processes of oxygen adsorption in the first two stages may be represented as in eqn. (24). The close similarity in the catalytic properties and in the conditions of the... 

An active and stable supported rhenium catalyst for propene metathesis is formed by heating [Re(C0)30H]4/Si02 under a propene-helium atmosphere at 150°C (Kirlin 1985a). [Re(CO)30H]4 acts as a reservoir of rhenium from which catalytically active mononuclear rhenium complexes are formed (Kirlin 1985b). The reaction of [ReBr(CO)5] with y-Al203 leads to the formation of a supported complex, which is the precursor of a catalyst with low activity for propene metathesis at 125°C (Nicolaides 1986). 

Fig. 5.1 Initial rate of propene metathesis, ro, on a 5.8 wt% Re207/Al203 catalyst in a flow system, as a function of temperature and propene pressure P. The lines represent the rates calculated from eqn. (5) (Kapteijn 1981).

Support Pretreatment temperature of support/°C Catalyst activation temperature/ C Initial rate of propene metathesis"/ % min m ... 

Table 5.3 Activities for propene metathesis of catalysts containing surface molybdenum complexes in different oxidation states (Yermakov 1975/76)...

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