Shape-selective zeolite catalysis for the tandem reaction of aromatic alcohol dehydration isomerization to internal alkene

Zeolite-catalyzed dehydration of fatty alcohols to alkenes is an effective method for biomass conversion. This study extends the reaction to a tandem reaction process that catalyzes the dehydration isomerization of aromatic alcohols to generate internal alkene under mild conditions. By using three acidic zeolites (HZSM-5, USY, and HBeta) as catalysts, it was found that when using HZSM-5, the conversion rate of 3-phenylpropanol was 100% and the selectivity of 1-propenylbenzene was 99.4% at 150 ℃, the conversion rate and selectivity were significantly lower when using USY and HBeta as catalyst. Grand Canonical Monte Carlo (GCMC) was conducted to understand the product selectivity difference, and the results demonstrate that 3-phenylpropanol adsorbs within the HZSM-5 zeolite channels in a distinctive isolated molecular configuration. This adsorption mode exhibits favorable spatial isolation from neighboring catalytic sites, thereby significantly enhancing the monomolecular dehydration pathway and facilitating selective isomerization to yield 1-propenylbenzene. However, the excessive adsorption of 3-phenylpropanol in the large pores of USY and HBeta will facilitate the formation of ether through bimolecular dehydration. Density functional theory (DFT) calculations showed that the dehydration process of 3-phenylpropanol can be achieved through both concerted and stepwise pathways, respectively. While the selectivity between 1-propenylbenzene and 2-propenylbenzene was affected by the stabilization of HZSM-5 to reaction intermediates, proving shape-selective catalysis. In addition, our calculations found that 3-phenylpropanol can directly generate 1-propenylbenzene through a concerted dehydration isomerization (DHI) pathway in HZSM-5. The reaction kinetics using HZSM-5 catalysts show that the activation energy of the dehydration process was higher than that of the isomerization process, while the initial reaction rate showed the opposite result, which was consistent with the trend of the free energy barriers of the dehydration and isomerization processes in DFT calculations. In addition, the reaction orders of the dehydration and isomerization processes were studied separately, and the elementary reaction steps and rate-determining steps were proposed. The experimental results were consistent with the DFT calculations, and the use of HZSM-5 did not favor multi-molecular dehydration pathways.