Seed-Assisted Hierarchical H-ZSM-5: Overcoming Diffusion Barriers for Efficient Recyclable Oleic Acid Isomerization to Commercial Isostearic Acid

Isostearic acid, a bio-based and environmentally friendly chemical, has attracted considerable attention because of its high value as an oleic acid derivative. The synthesis of isostearic acid via oleic acid isomerization over commercial H-ZSM-5 zeolites is limited by low yield and acid value, mainly due to diffusion constraints. To address this issue, a hierarchical-pore H-ZSM-5 zeolite (Meso-5) was synthesized using Silicalite-1(S-1) as a seed crystal to enhance diffusion and catalytic performance. The catalyst was comprehensively characterized by XRD, SEM, BET, TGA, FT-IR, and Py-FTIR. Meso-5 efficiently catalyzed the isomerization of oleic acid, attributed to its abundant Brønsted and Lewis acid sites whose synergistic interaction significantly promoted the formation of branched-chain products. The catalyst delivered a selectivity of 87.3% with a 71.1% yield in the first run, and even after five reuse cycles it maintained 82.1% selectivity with a 55.6% yield. Evidently, the hierarchical pore structure of the catalyst significantly facilitated the diffusion of oleic acid molecules and their isomerized intermediates, thereby enhancing the overall catalytic activity. The resulting isostearic acid was purified via a simple recrystallization procedure to a purity of 83.4% and an acid value of 186.7 mg KOH/g, both meeting the standards of commercial isostearic acid. These findings provide a robust basis for further catalyst optimization and scale-up toward industrial isostearic acid production.