Comparative evaluation of the catalytic properties of SAPO-31 and ZSM-48 for the hydroisomerization of N-octane: Effect of the acidity

The hydroconversion of n-octane, 2-methylheptane, and 2,2,4-trimethylpentan e over Pt-Pd/SAPO-31 and Pt-Pd/ZSM-48 at 523 K and atmospheric pressure was studied. The acid SAPO-31 and ZSM-48 supports were synthesized by hydrothe rmal procedures and characterized by X ray, scanning electron microscopy, a nd MAS-NMR. Infrared spectra of adsorbed pyridine showed that these molecul ar sieves differ in their acidity not by the number of acid sites but by th eir strength. The bifunctional catalysts Pt-Pd/SAPO-31 and Pt-Pd/ZSM-48 wer e highly selective for the hydroisomerization of n-octane. Monomethyl isome rs were the major products. These narrow tubular molecular sieves favored t he formation of terminal methyl isomers 2MeC(7) and 2,5DMe-C-6. Acid streng th has almost no effect on the hydroisomerization selectivity, but high aci d strength led to a higher reaction rate. Isomerization of 2-methylheptane into 3MeC(7) by 1,2-methyl shift or into n-C-8 by PCP intermediate discrimi nates molecular sieves having channels of different diameters, the smaller the dimensions of the pores the lower the ratio of 3MeC(7)/n-C-8.. (C) 1999 Academic Press.