MECHANISM OF BUTANE ISOMERIZATION OVER INDUSTRIAL ISOMERIZATION CATALYSTS

Double labeled 1,4-C-13 butane, (CH3)-C-13-(CH2)-C-12-(CH2)-C-12-(CH3) -C-13 was used to test four industrial isomerization catalysts based o n chlorinated alumina. Mass-spectrometric analysis of the iso-butane p roduct shows that extensive intermolecular scrambling occurs in the co nversion from n- to iso-butane. Apparently, the reaction mechanism ove r these catalysts involves a bimolecular reaction, leading to an adsor bed C-8(+) intermediate. At low conversion, the fragmentation pattern of n-butane reveals an intramolecular rearrangement of carbon atoms, i .e. place exchange of C atoms in methyl and methylene groups. Both phe nomena had been observed previously for sulfated zirconia (SZ) catalys ts; however, for the present catalysts the distribution of the five is otopic iso-butane molecules does not obey the binomial law. The result s suggest that isomerization of the C-8 intermediate is less extensive than over sulfated zirconia; beta-fission of the 2,4,4-trimethylpenty l ion leads to two fragments with iso-butyl structure with (2+n) and ( 2-n) C-13 atoms, respectively. (C) 1997 Elsevier Science B.V.