Dimethyl branching of long n-alkanes in the range from decane to tetracosane on Pt/H-ZSM-22 bifunctional catalyst

Single long n-alkanes in the range n-C-10 to n-C-24 were hydroisomerized at 233 degreesC in a fixed-bed down-flow vapor-phase reactor loaded with Pt/H -ZSM-22 zeolite catalyst. The multibranched isomer fractions obtained by sk eletal isomerization. of the different n-alkanes were analyzed in detail wi th GC-MS. The large majority of multibranched isomers were dimethyl branche d. In the preferred isomers, the tertiary carbon atoms carrying the methyl branchings were separated by 3 up to 14 carbon atom positions. Further pecu liarities were the preferential formation of isomers with symmetric methyl positions on the chain. Among this family of "symmetric" isomers, the forma tion of 2,x - 3-dimethyl-Cx-2 isomer was favored in conversions of n-C-x al kanes with x equal to or smaller than 17 and disfavored with longer molecul es. In the family of 3,n-dimethyl-Cx-2 isomers obtained from the longest al kanes studied (n-C-20, n-C-22, n-C-24), the preferred n positions were from 8 to 12 and from 14 to 18. In the 4,n-dimethyl-Cx-2 isomers, the preferred n positions were from 8 to 17. It was also found that the formation of a d imethyl-branched isomer occurs preferentially through methyl branching of a monomethyl-branched isomer having already the more centrally positioned me thyl group rather than the one close to the extremity of the chain. These p eculiar positional selectivities and reaction paths can be explained by key -lock catalysis in pairs of micropore openings on the external surface of t he ZSM-22 zeolite crystals. (C) 2001 Academic Press.