Homologation of n-alkanes on metal films. A novel aspect of metal—carbene chemistry

Abstract

A novel highly selective reaction has been discovered in the catalytic conversion of n-alkanes in hydrogen on unsintered tungsten films at 420 K. In addition to hydrocracking, substantial formation of the next higher n-alkane may be achieved (homologation), the yields being very sensitive to hydrogen pressure, paraffin structure and chain length. Thus isoalkanes are at least an order of magnitude less efficient than the corresponding n-alkanes and cyclopentane or neopentane gives no trace of homologation. Multiple initial chain growth, i.e., the successive addition of more than one single carbon entity to the reactant, is also observed to an increasing extent with the higher n-alkanes ( C₅). The reaction is discussed in terms of selective addition of adsorbed methylene, formed by chain rupture, to the unsubstituted end of the double bond in intermediate alk-1-enes. The possible relationship to olefin metathesis and Fischer—Tropsch synthesis is evident in these systems and is briefly considered.

In the higher temperature range 520 K homologation is also extensive on palladium and sintered rhodium films, as well as on tungsten but now the intermediate C₆ and larger chains rapidly cyclize and aromatize. Some activity for homologation was also observed with Mo and Nb and to a lesser extent with Re and Pt.

The evolution of different types of reaction with temperature especially on W also affords a deeper insight into the mechanisms of cyclization and 1,2-bond-shift skeletal rearrangement. These aspects together with some general trends in catalytic behaviour of the transition metals for hydrocarbon reactions are briefly discussed.