Advanced Synthesis of Hierarchically Porous Zeolite Catalysts Utilizing Biomass Templates for the Catalytic Pyrolysis of Stearic Acid into Short-Chain Olefins

In this study, hierarchically porous ZSM-5 catalysts were fabricated by one-pot assembling ZSM-5 particles onto diverse biomass templates (e.g., rice husk, tea seed husk, tung shell, and coconut shell), wherein the biomass template was transformed into bio-SiO₂ or biochar depending on the calcination conditions. The biotemplated ZSM-5 variants, including ZSM-5(RH), ZSM-5(TSH), ZSM-5(TS), and ZSM-5(CS), exhibited significantly improved deoxygenation performance, achieving ∼100.0% deoxygenation efficiency as compared to the untemplated ZSM-5 catalyst (85.3%). Among them, the ZSM-5(TSH) catalyst exhibited the best performance, accompanied by 100% conversion, 99.6% deoxygenation rate, and 82.3% olefin selectivity. Interestingly, the product distribution over biotemplated ZSM-5 was dominant C₄⁼-C₈⁼ (selectivity of ∼100% in total olefins), while long-chain olefins (C₉⁼-C₁₇⁼) was the major product (selectivity of 57.3%) over the untemplated ZSM-5. Moreover, molecular dynamics (MD) simulations revealed that biotemplated ZSM-5 exhibited superior diffusion coefficients of stearic acid (reaction substrate) and anthracene (coke precursor) compared to the untemplated ZSM-5, indicating higher self-diffusion rates and consequently superior activity and stability in the catalytic pyrolysis reactions. Furthermore, in situ DRIFTS results showed stearic acid over ZSM-5(TSH) primarily was converted to the C₁₇H₃₆ intermediate mainly via the decarboxylation route, followed by dehydrogenation pyrolysis and C-C breaking reactions into C₄⁼-C₈⁼ products. Overall, this work developed an effective strategy for manufacturing hierarchically porous zeolite catalysts using biomass-derived bio-SiO₂ or biochar as the platform.