Hydroisomerization of n-alkanes at low temperatures

Hydroisomerization is an attractive route to boost the octane level of the unleaded gasoline pool based on the fact that branched (iso) alkanes have significantly higher octane numbers than the corresponding n-alkanes. Alkane isomerization increases the octane number without changing the number of carbon atoms and without producing aromatics, the concentration of which is limited by government regulations. This process can be increasingly important in the near future because the phase-out of all ether additives (as methyl-tert-butyl-ether (MTBE)) is in discussion in the US. MTBE emerged in the late 1980s as the preferred octane enhancing oxygenate additive in reformulated gasoline. Bifunctional catalysts are used for hydroisomerization and hydrocracking. These catalysts contain (i) acidic sites for skeletal isomerization and cracking and (ii) metallic sites for hydrogenation/dehydrogenation. The acidic sites can be provided by oxides such as sulfated zirconia or zeolites, the metallic sites by metals from Group VIII of the Periodic Table. Noble metals such as Pt and Pd provide a powerful hydrogenation function with optimum activity being achieved at relatively low metal loading. The higher the acid strength the lower the reaction temperature can be, which favor the formation of multibranched isomers. A further necessary condition to reach bifunctional conversion with n-alkanes is an efficient mass transport of alkenes from the metal towards the acid sites and vice versa. The diffusivity of a given alkene in the pores of the catalyst should exceed that of the intrinsic reaction rate of the alkene conversion over the acid sites. We propose to use composites, consisting of sulfated zirconia and a zeolite or a mesoporous material, or two zeolites with different pore dimensions impregnated with Pt and/or other transition metals for isomerization of n- hexane to n-octane. With this combination of metal containing catalysts we expect: - the incorporation of catalytic functions which may increase the conversion as well as the isomerization selectivity, and - less resistance to the outward diffusion of products, mainly for bulkier product molecules, therefore increasing the selectivity to multibranched isomers.