The synthesis of hierarchical ZSM-22 zeolite with only the PHMB template for hydroisomerization of n-hexadecane

Abstract

The ZSM-22 zeolite has become an effective support for the hydroisomerization of n-alkanes due to its one-dimensional pore structure, good hydrothermal stability and simple synthesis process. However, the diffusion resistance of hydrocarbons in the microporous channel still limits the further industrial application of the traditional microporous ZSM-22 zeolites. To overcome the diffusion limitation, the different amounts of polyhexamethylene biguanide (PHMB) was added into the initial gel as the only template instead of 1,6-diaminohexane (DAH) to synthesize a series of hierarchcal ZSM-22 zeolites (Z22-xP), and the corresponding Pd/ZSM-22 catalysts with 0.5 wt% Pd loading were then prepared for n-hexadecane hydroisomerization in this work. The characteristic results show that, the Z22-xP samples exhibit improved mesoporous volume and external surface than those of the traditional microporous Z22-D sample. Additionally, the Z22-xP samples also demonstrates significantly lower Brønsted acid density in comparison with the microporous Z22-D sample because of the proportion decrease of T1-T3 skeleton Al atoms. The catalytic tests results indicate that the employment of PHMB instead of DAH can effectively promote the catalytic performance. The Pd/Z22-0.002P catalyst reveals the highest iso-hexadecane yield of 76.6 % among all catalysts. Therefore, this work provides a more convenient and effective way for the development of potential bifunctional catalysts with promoted catalytic performance.