ZSM-22 Synthesized Using Structure-Directing Agents of Different Alkyl Chain Lengths for Controlled n‑Hexadecane Hydroisomerizations

ZSM-22 zeolites were synthesized using different structure-directing agents (SDAs) with varying alkyl chain lengths, including n-alkyldiamines (DAB, DAH, DAO, and DAD) and lauryl amine (LA). The characteristics of the resulting samples (denoted as “Z-SDA”) revealed that optimal elongation of SDA alkyl chains brought about a decrease in crystallization time while retaining ZSM-22 crystal phase stability. The morphology and acidity of the samples varied as the alkyl chain length of the SDAs increased. The average crystal length ranged from 13 μm to 250 nm, and the total acid content showed a downward trend, while each sample possessed nonlinear acid site distributions. Additionally, ZSM-22-supported Pt catalysts were obtained and probed using the n-hexadecane hydroisomerization process. It was found that the activity and i-hexadecane selectivity gradually increased with the alkyl chain length of the SDAs (C₄–C₁₀). Notably, Pt/Z-DAD achieved >70% yield under specific reaction conditions, which could be attributed to the abundant mesopore network and high proportion of weak Brønsted acid site distributions.