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-trimethylpentane 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 hydrothermal procedures and characterized by X ray, scanning electron microscopy, and MAS-NMR. Infrared spectra of adsorbed pyridine showed that these molecular sieves differ in their acidity not by the number of acid sites but by their strength. The bifunctional catalysts Pt–Pd/SAPO-31 and Pt–Pd/ZSM-48 were highly selective for the hydroisomerization of n-octane. Monomethyl isomers were the major products. These narrow tubular molecular sieves favored the formation of terminal methyl isomers 2MeC7 and 2,5DMe-C6. Acid strength has almost no effect on the hydroisomerization selectivity, but high acid strength led to a higher reaction rate. Isomerization of 2-methylheptane into 3MeC7 by 1,2-methyl shift or into n-C8 by PCP intermediate discriminates molecular sieves having channels of different diameters, the smaller the dimensions of the pores the lower the ratio of 3MeC7/n-C8.