Skeletal isomerization of n-heptane over Pd-H_4SiW_(12)O_4o supported on SiO_2:comparative study with typical bifunctional catalysts

摘要

Skeletal isomerization of n-heptane in the presence of hydrogen has been studied over silica-supported bifunctional catalysts,Pd-H_4SiW_(12) O_(40)/SiO_2 with different loadings of H_4SiW_(12)O_(40).The catalytic performances of these catalysts have been compared with those of typical bifunctional catalysts such as Pd-H-beta zeolite,Pt-SO_4~(2-_/ZrO_2,and Pd-WO_3/ZrO_2.The Pd catalysts with low loadings of H_4SiW_(12)O_(40),such as 2 wt.% Pd-10,15 and 20 wt.% H_4SiW_(12)O_(40)/SiO_2,exhibited very high selectivity toward branched heptanes comparable to that of Pd-H-beta zeolite,and demonstrated higher activity than did Pd-H-p zeolite in the skeletal isomerization of n-heptane.As a result,the space time yield (STY) from forming branched heptanes on 2 wt.% Pd-15 and 20 wt.% H_4SiW_(12)O_(40)/SiO_2 was approximately three times higher than that found for 2 wt.% 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-SO_4~(2-)/ZrO_2 and Pd-WO_3/ZrO_2 catalysts in selectivity and activity.Selectivity toward branched heptanes was sensitive to the loading amount of H_4SiW_(12)O_(40);the selectivity increased with a decrease in H_4SiW_(12)O_(40) loading amount.The acid strength/acid amount decreased while dispersion of Pd on H4SiWi2O4o/SiO2 increased with a decrease in the H_4SiW_(12)O_(40) 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-H_4SiW_(12)O_(40)/SiO_2.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 H_4SiW_(12)O_(40)/SiO_2,and the branched heptenes were hydrogenated on Pd sites.Cracking products are considered to be produced from dibranched heptenes.