Skeletal isomerization of n-heptane over Pd-H4SiW12O40 supported on SiO2:: comparative study with typical bifunctional catalysts

Skeletal isomerization of n-heptane in the presence of hydrogen has been studied over silica-supported bifunctional catalysts, Pd-H4SiW12 O-40/SiO2 with different loadings of H4SiW12O40. The catalytic performances of these catalysts have been compared with those of typical bifunctional catalysts such as Pd-H-beta zeolite, Pt-SO42-/ZrO2, and Pd-WO3/ZrO2. The Pd catalysts with low loadings of H4SiW12O40, Such as 2wt.% Pd-10, 15 and 20 wt.% H4SiW12O40/SiO2, exhibited very high selectivity toward branched heptanes comparable to that of Pd-H-beta zeolite, and demonstrated higher activity than did Pd-H-beta zeolite in the skeletal isomerization of n-heptane. As a result, the space time yield (STY) from forming branched heptanes on 2wt.% Pd-15 and 20wt.% H4SiW12O40/SiO2 was approximately three times higher than that found for 2wt.% Pd-H-beta zeolite, which is the best catalyst known for the skeletal isomerization of n-alkane in the presence of hydrogen at atmospheric pressure at 453 K. These bifunctional heteropoly catalysts were far superior to Pt-SO42-/ZrO2 and Pd-WO3/ZrO2 catalysts in selectivity and activity. Selectivity toward branched heptanes was sensitive to the loading amount of H4SiW12O40; the selectivity increased with a decrease in H4SiW12O40 loading amount. The acid strength/acid amount decreased while dispersion of Pd on H4SiW12O40/SiO2 increased with a decrease in the H4SiW12O40 loading amount. These results, and the changes in product distribution with contact time, indicated that n-heptane was isomerized via a consecutive reaction pathway over Pd-H4SiW12O40/SiO2. Here n-heptane was dehydrogenated on Pd sites, the resulting n-heptene was isomerized to mono-, di-, and tri-branched heptenes via carbenium ions at acidic sites on H4SiW12O40/SiO2, and the branched heptenes were hydrogenated on Pd sites. Cracking products are considered to be produced from dibranched heptenes. (C) 2004 Elsevier B.V. All rights reserved.