Investigation of n-heptane hydroisomerization over alkali-acid-treated hierarchical Pt/ZSM-22 zeolites

Abstract

The design of various hierarchical structures and morphologies of ZSM-22 zeolite catalysts is attempted through a post-treatment using different concentrations of NaOH and subsequently treating with HCl solution including fluosilicic acid (H₂SiF₆) and then loading Pt on their surfaces. The effects of samples on textural characters, acidity, as well as n-heptane catalytic hydroisomerization evaluation were examined. The formation process of the hierarchical structures was further investigated. Test and evaluation results indicated that samples treated with 0–0.5 M NaOH and subsequently treated with 6.0 M HCl and 0.1 M H₂SiF₆ were much more controllable and reasonable due to the better structure-activity relationship of the catalyst by changing crystal morphologies and introducing hierarchical structures. Indeed, PZH-0.3 exhibited 81.1% of the highest n-heptane conversion and 76.4% of total i-heptane yield in the n-heptane catalytic hydroisomerization process at 260 °C, PZH-0.5 displayed 13.6% of the maximum multi-branched i-heptane yield, which was above 104.1% in comparison with PZ-0 at 320 °C. Moreover, PZH-0.3 presented the highest n-butane yield, which possessed a proportion of 65.9% in the butane component of cracking products at 340 °C and improved an average of 16% and 22 times when utilizing PZ-0 and Pt/ZSM-5 catalysts. Additionally, the mechanism of the reaction process is briefly depicted according to the distribution of the reaction products. The regulation of zeolite both on crystal morphologies and pore hierarchy has obvious advantages in the hydroisomerization process, which can improve the composition of multi-branched isomers with high octane number effectively as well as supply raw materials for the chemical industry.