The first part of this study demonstrates the possibility to form a secondary system of parallel pores of uniform size in zeolite crystals. A high energy 238U ion beam was employed to form latent tracks in zeolite crystals, which were etched consequently with diluted HF solution to produce mesopores of uniform size. This model material is used to bring better understanding to the effect of a secondary pore system in the catalytic performance of hierarchical zeolites obtained by chemical etching.The main part of the study is devoted to the treatment in fluoride media with the ultimate goal to obtain hierarchical zeolite crystals with characteristics similar to those obtained by nuclear track imprinting. Buffering HF solutions with NH4F shifts the equilibrium to the most reactive species (HF, FHF-), which are not Al/Si selective. Thus, after a proper adjustment of HF and NH4F concentrations, non-selective chemical etching of the ZSM-5 framework occurs. Consequently the small intergrowth domains in zeolite are disintegrated by fluoride treatment. In addition, large pores penetrating deep in the crystals are created. The resulting materials combine a secondary porosity with retention of the native compositional and structural characteristics of the parent zeolite. The surface area, porosity properties, aluminum content, number and distribution of acid sites of NH4F-HF-treated zeolites are similar to the parent ZSM-5. As a result, the diffusion of molecules in and out is enhanced and the performance of zeolite in catalytic conversion of m-xylene substantially improved.The preparation of hierarchical zeolites with a chemical composition similar to their parent is an important step in the rational design of zeolite catalysts. It provides also a key to unravel the respective contributions of improved diffusion and controlled acidity on the overall catalytic performance.

Mesoporous zeolites by fluoride route

248th National Meeting (2014)

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