Isobutane metathesis

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). 

Alkylation processes usually combine isobutane with an alkene or with mixed alkene streams (C3-C5 olefins from FCC units). The best octane ratings are attained when isobutane is alkylated with butylenes. Alkylation of higher-molecular-weight hydrocarbons (>C5) is less economic because of increased probability of side reactions. Phillips developed a technology that combines its triolefin process (metathesis of propylene to produce ethylene and 2-butenes) with alkylation since 2-butenes yield better alkylate than propylene.290 Since ethylene cannot be readily used in protic acid-catalyzed alkylations, a process employing AICI3 promoted by water was also developed.291... 

Mixed butenes obtained by ethylene dimerization are used for paraffinic alkylation (isobutane + n-butene —> trimethylpentanes) or to make propene by a subsequent metathesis reaction (ethylene + 2-butene —> 1 propene cf. Section 2.3.3). Higher ethylene oligomers are also used as high-octane-number gasoline components. 

Noteworthy also is the selectivity for higher alkanes. For example the metathesis of propane gives mainly butanes rather than pentanes, and butane rather than isobutane. This is in agreement with stereoelectronic factors that favor the transfer of one carbon on to, preferentially, primary alkyl surface species, leaving tertiary alkyl species unreactive (Scheme 10). The alkane selectivity also depends on the structure of the starting alkane (Figure 1). Overall, this reaction shows the potential of surface organometallic chemistry, and new, unprecedented reactions will be probably discovered in the near future. 

Hydro-metathesis of propene under hydrogen atmosphere, in the presence of TaH/ KCC-1 catalyst, proceeds smoothly under dynamic reaction condition at 150 C for 65 h with 750 TON [83]. In addition to the expected hydrogenation product, propane, ethane, and butane were formed as major products, and methane, isobutane, and isopentanes formed as minor products in case of propene. Similarly in the case of 1-butene, propane and hexanes were formed as major products, and ethane, propene, pentanes, and heptanes were formed as minor products. In case the of butene, the catalyst was found to be stable even after 75 h and cumulative TON up to 1,150 achieved after 75 h of the reaction [83]. The most important issue with this catalyst is the stability and reusability of this Ta-H/KCC-1 catalyst and the high turnover numbers reached compared with the turnover numbers reported for the Ta-H/Si02 catalyst in alkane metathesis reaction. 

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