Propane metathesis

C2.7.6.4 PROPANE METATHESIS CATALYSED BY TANTALUM COMPLEXES ANCHORED TO SILICA... 

The product selectivities in propane metathesis can also be explained by using the same model in which [1,3]- and [1,2]-interactions determine the ratio of products. For instance, the butane/pentane ratios are 6.2 and 4.8 for [(= SiO)Ta(= CHfBu)(CH2tBu)2] and [(= SiO)2Ta - H], respectively (Table 5). A similar trend is observed for the isobutane/isopentane ratio, which are 4.1 and 3.0, respectively. The higher selectivity in butanes (the transfer of one carbon via metallacyclobutanes involving [l,3]-interactions) than that of pentanes (the transfer of two carbons via metallacyclobutanes involving [1,2]-interactions) is consistent with this model (Scheme 28). 

Dehydrogenation of methanol, dehydrogenation of propane, metathesis of ethylene and butylene, and cat crackers. (Other crackers in refineries produce olefins too.)... 

In a dynamic reactor for propane metathesis, (=SiO)2Ta-H remains active during several days. The results are comparable to those obtained in batch mode, in terms of turnover number (TON) and selectivity (Figure 3.10). 

The reaction selectivity is better under these conditions at steady state, because an equihbrated ratio is observed between the resulting higher and lower homolog alkanes. In addition, dynamic conditions allow us to vary the contact time to obtain information about primary products and then about the mechanism. It appears that, in the case of propane metathesis in a stationary regime, conversion increases Hnearly with contact time, which shows that the reaction is under dynamic control with no diffusion Hmitation. Under these conditions, decreasing contact time results in an increase of the selectivity for hydrogen and olefin whereas that of alkanes decreases. Similarly, the alkanes/olefins ratio tends to zero as the contact... 

Figure 3.10 Propane metathesis catalyzed by (=SiO)2TaH (3) in a continuous flow reactor (520mg of5.3% Ta/Si02 1 atm, 150°C, 1 mLmin" orVHSV = 38h ) turnover number (TON) (a) and selectivity (b).

Propane Metathesis Comparison between Supported Tantalum and Tungsten Hydrides... 

Various tungsten-hydrido compounds prepared on silica [38], silica-alumina [39] or alumina [40] supports have been tested in propane metathesis under batch conditions to compare their properties with those of the silica or alumina-supported tantalum hydride(s) 3 [41]. 

Reaction between methane and propane requires the right conditions. Firstly, it has a positive free energy of AG° = -1- 2kcalmoT at 150 °C for amethane/propane ratio of 1 but this can be overcome by increasing this ratio, which for a value of 1250 allows 98% propane conversion at 250°C. Secondly, it has to be separated from other reactions catalyzed by tantalum hydride, such as propane hydrogenoly-sis, leading to 1 equiv. of methane and 1 equiv. of ethane, or propane metathesis, leading to 0.5 equiv. of ethane ... 

Table 11.4 Activity and selectivity in propane metathesis by metal-hydrocarbyl and related metal-hydride surface species .

Scheme 17 Proposed mechanism for the propane metathesis (a) formation of linear alkanes and (b) formation of branched alkanes...

Scheme 18 Stability of metallacyclobutane intermediates in propane metathesis reaction...

Synthesis and characterization of several catalyst precursors of [Ta] and [W] on different supports like silica [44], alumina [45], or silica-alumina [70] have been discussed in the previous section. These catalyst precursors were tested mainly in propane metathesis under similar condition in batch reactor at 150°C for 120 h. Catalytic performances revealed that the [Si02-Al203.5oo-W-H] and [AI2O3.500-W-H] have similar and better activity (TON) than the corresponding silica-supported tantalum-based catalyst precursors (Table 2). It was also observed that the product selectivity for tungsten hydrides is narrower than for tantalum hydrides [71]. 

After tremendous success in propane metathesis (lower alkane) reactirMi, catalyst precursor 27 was employed for metathesis of -decane (higher alkane). The n-decane metathesis reactimi carried out at 150°C produced a broad distribution of linear alkanes from methane to C30 (triacontane) with trace amoimt of branched alkanes without any olefinic or cyclic products [74], Interestingly, the formation of lower... 

Figure 2.3 Propane metathesis product distribution of iinear and branched alkanes.

Scheme 2.3 The proposed mechanism of propane metathesis starting from Zr bishydrides.

Scheme 2.6 The postulated mechanism for the production of linear versus branched propane metathesis products. Reproduced from [15]. Copyright 2010 with permission from Accounts Chemical Research.

The alkane metathesis reaction could involve one or two active sites one active site would imply that the formation of the olefin and the carbene occurs within the same coordination sphere of the metal, whereas two active sites would have these two steps each occurring on a distinct metal. To address this issue, three different loadings of the well-defined alkyhdene Mo-1 (2.03, 0.45, and 0.07 wt%) were prepared and used in the propane metathesis reaction [74]. No correlation was found between the site density and the loading of the catalyst. 

Supported Ta-Neopentylidene Complexes A silica-supported Ta-alkylidene [(=SiO) Ta(=CHCMe3)(CH2CMe3)p j] catalyst has been synthesized and well characterized [76, 77]. This Ta-alkylidene catalyst possesses a neopentyl ligand and an alkylidene group, and was found to be active for propane metathesis (TON=33). Since the precursor is not a hydride and a proposed carbene hydride is known to be the intermediate, this implies that Ta-neopentyl and neopentylidene organometaUic species are initially transformed into this key species (Scheme 2.10) [78-83]. Different initiation steps have been suggested. One involves the initial addition of the alkane into the alkylidene moiety to form multiple Ta-alkyl species, which can then decompose via an a-H abstraction to produce a Ta-alkylidene (Scheme 2.10a) [84, 85]. 

Supported W-Neopentylidyne Complexes A supported, Schrock =SiOW(=C Bu) (CH2 Bu)2 complex on silica has also been studied for propane metathesis [89]. Interestingly, when immobilized on silica, W catalysts are highly active for alkene... 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

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