Needle-like Hierarchical Beta Zeolite Synthesis via Postsynthetic Morphology Control in the Presence of Cetyltrimethylammonium Bromide for Catalytic Dehydration of Sorbitol

Reactant diffusion efficiency plays a critical role in heterogeneous catalytic reactions. In this study, beta zeolite with a unique needle-like morphology was obtained through a designed cascade of postsynthesis treatments. The proposed formation mechanism for the needle-like morphology involves acid treatment to generate isolated silanol groups, followed by alkaline hydrothermal treatment using cetyltrimethylammonium bromide as a growth inhibitor, ultimately yielding the needle-like structure. Additionally, the mesopores created during these treatments─initially around 3.5 nm in size, larger than the conventional 3 nm─diminish with prolonged alkaline treatment and indicate significant morphological evolution beyond typical mesopore formation. These needle-like beta zeolites not only preserved the acidity of the original zeolite but also significantly increased the external surface area and hydrophobicity. Importantly, the intercrystalline spaces formed by the needle-like structure played a crucial role in enhancing catalytic activity, surpassing the impact of intracrystalline mesopores created by conventional methods. The influence of catalyst morphology─particularly the expanded external surface and intercrystalline mesoporosity─on the efficiency of this heterogeneous reaction is thoroughly discussed. Moreover, the turnover frequency for sorbitol conversion of the needle-like beta zeolite was 3-fold higher compared to the parent zeolite. This work advances the post-treatment methods of zeolites and holds significant potential for the utilization of biomass resources.