(544ck) Ice-Templating Fabrication of Hierarchical TS-1 Monoliths with Steam-Assisted Crystallization for Enhanced Benzene Hydroxylation

TS-1 zeolite exhibits superior catalytic performances for oxidation reactions using H₂O₂ as the oxidant. However, the intracrystalline diffusion in pelleted TS-1 catalysts inhibits the access of reacting molecules to active sites within zeolitic pores and the removal of product molecules, keeping both reaction rate and selectivity low. Many efforts have been made to relieve this diffusion limitation. One strategy is to construct hierarchically porous zeolites with auxiliary porosity using either bottom-up or top-down method. Hierarchical zeolites constructed by the bottom-up method are favourable to overcome the dilemma of low-level conversion and selectivity, but the requirement for expensive mesostructured templates with the complex procedure of fabrication is a drawback. The top-down method usually suffers from titanium leaching together with a reduction of effective areas. Since the introduction of second mesopores usually sacrifices some active sites, the conversion of benzene and the selectivity of phenol could not be enhanced at the same time when hierarchical zeolites synthesised by the top-down method are employed.

The ice-templating of porous structures by the solidification of water is an eco-friendly way to build hierarchically-structured bulks, and the engineering of pore morphologies and their connections could be achieved. The wealth of pore morphologies and the directional macropores together with easily accessible meso- and micro-pores would make these ice-templated materials promising candidates as solid catalysts.

Therefore, we come up with a novel method to synthesise TS-1 monolithic matrices with a hierarchical pore structure by using the ice-templating of small TS-1 grains followed with the steam-assisted crystallization of the formed monolithic bulk. The catalytic performance of the synthesised TS-1 monolithic matrix in benzene hydroxylation will be checked using a loop flow reactor. Since the existence of macropore, the pressure drop would be intensively decreased. A plenty of intercrystalline mesopores would reduce the diffusion resistance of reacting species, leading to a remarkable improvement in both reaction rate and selectivity for benzene hydroxylation.

Firstly, the hierarchical TS-1 monoliths were fabricated by ice-templating nano-sized TS-1 grains followed by steam-assisted crystallization. Based on structure characterization using SEM, TEM, XRD, BET, FT-IR as well as ²⁹Si NMR etc., the well-connected macropore structure was formed within TS-1 monoliths after the ice-templating. In the process of steam-assisted crystallization, silica sol and synthetic gel could be transformed into zeolitic crystals. Additionally, a great amount of intracrystalline mesopores appeared after steam-assisted crystallization. The hierarchical monolith had a total pore volume of 0.40 cm³/g and the specific surface area of 432 m²/g. In the following catalytic evaluation, the monolith exhibited low pressure drop and excellent mass diffusion properties. It was demonstrated that fabricated monoliths had excellent catalytic performances compared with parent TS-1 grains. The reaction yield could be up to 24.6% for benzene hydroxylation catalyzed by using hierarchical TS-1 monoliths, accounting for a 20% increase over parent TS-1 grains.

This work was supported by the National Natural Science Foundation of China (21136008).