New perspectives for the isomerization of light n-alkanes
Isomerization of light n-alkanes into the high-octane number branched alkanes is an important challenge for petroleum refining industry. According to the literature, such a transformation needs two different active centres: one having a hydrogenating/dehydrogenating ability and another, a protonic acid function. From this point of view, heteropolyacids (HPAs) in mixing with one or more transition metal appear promising. Mechanical mixtures of alumina-supported platinum or of mixed palladium (or nickel) -cerium oxides and silica-supported HPAs have been tested for the isomerization of n-hexane. The catalysts were characterized before and after tests. Although pure silica-supported HPAs have an initial activity, they are very easily to deactivate by coke deposition. Presence of Pt, Pd, and Ni gives rise to a better activity and a higher selectivity for di-branched isomers, and is really beneficial for retaining the Keggin structure of silica-supported HPAs. The catalytic performances are strongly depending on the composition of the mechanical mixtures. Roughly, the activity is in the same order as the acid strength. 1H and 31P solid-state NMR studies of trimethyl phosphine adsorbed on heteropolytungstate put in the evidence of the acidic sites. It is noteworthy that the presence of metallic Pt, Pd, or Ni metal after the reduction, instead of the original metallic ions, reinforces the generally admitted mechanism with an activation of the alkane. The activation is difficult to obtain by direct attack by an acid (proton), but it can be achieved by the metallic species to the corresponding alkene before formation of a carbenium intermediate on the acid site. However, the formation of alkenes directly by the HPAs and the participation of metallic species for limiting the alkenes concentration are not excluded.