Factors affecting isomer yield for n-heptane hydroisomerization over as-synthesized and dealuminated zeolite catalysts loaded with platinum

Abstract

Pt loaded Y, USY, beta, mordenite, ZSM-12, and ZSM-5 zeolites with different Si/Al ratios and a USY and ZSM-12 sample having enhanced activity were studied for the hydroisomerization of n -heptane. All the possible heptane isomers were obtained over Pt/H-ZSM-12 and the product distribution was closer to that over Y and beta catalysts, but very different from that over Pt/H-ZSM-5. Shape-selective effects in ZSM-12 resulted in a significantly higher isomer yield than the beta and Y-zeolite catalysts. For Y, beta, and ZSM-12 samples having very little extraframework aluminum, changing the Si/Al ratio did not affect the isomer yield. However, USY and ZSM-12 samples that showed enhanced activity were very sensitive to the Pt precursor and Pt loading, which caused huge differences in their activity and isomer selectivity. The Pt dispersion in the enhanced activity catalysts was also much lower. A careful optimization of the precursor, amount of Pt loaded, and pretreatment conditions is essential to achieve good balance between the metal and the acid functions in these catalysts. The optimized enhanced activity catalyst is able to provide isomer yields approaching that obtained over the regular catalysts, which suggests that zeolite acidity is not directly affecting the isomer selectivity. These results indicate that the maximum isomer yield for a Pt/H-zeolite catalyst is obtained when the metal and acidic functions of the catalysts are well balanced. Therefore, it may not be possible to increase the isomer yield beyond this maximum by modifying the acidity characteristics of the catalyst. Mordenite and ZSM-5 catalysts showed low isomer yields, but acid dealumination of the parent mordenite improved the isomer yield, possibly due to creation of mesopores .