Key Roles of Late-Stage Crystallization Time on the Structure and Acid Properties of ZSM-48 for Hydroisomerization

ZSM-48 zeolite, featuring one-dimensional channels with ten-membered rings, has been proven to be a qualified support of Pt for hydroisomerization of long-chain n-alkanes. However, the detailed crystallization behavior of ZSM-48 over time is still not clear, which should have a great influence on its physicochemical properties and even catalytic performance. In this work, a systematic analysis on the crystallization process of ZSM-48 is conducted. Various ZSM-48 samples with different structures and acid properties are synthesized by controlling crystallization time and evaluated in the hydroisomerization of n-hexadecane. Correlation analysis is carried out to reveal the quantitative relationship among late-stage crystallization time, zeolite physicochemical properties, and catalytic performance. For Brønsted acid sites, increased concentration inside the pore channel and decreased concentration at the pore mouth and outer surface are beneficial for increasing activity, while a properly balanced distribution is needed for achieving a high isomerization selectivity. A moderate crystallization period of 44 h generates the optimal ZSM-48 support for efficient hydroisomerization. Compared with traditional catalysts, the isomerization selectivity of Pt/ZSM-48-44h is increased from 87% to more than 93% when substrate conversion is lower than 80%. This work illustrates that the late-stage crystallization time of ZSM-48 plays a key role in tuning its physicochemical property and even catalytic performance.