Acid catalytic function of mesopore walls generated by MFI zeolite desilication in comparison with external surfaces of MFI zeolite nanosheet

• Hierarchical zeolites with similar mesoporosity are synthesized by different methods.
• According to method, concentration and strength of external acid sites are different.
• Zeolite synthesized by structure-directed synthesis excelled in decalin cracking.

Mesopores with zeolite walls were generated through desilication of bulk crystals of MFI zeolite in a NaOH solution in the presence or absence of cetyltrimethylammonium surfactant, as reported in the literature. Catalytic functions of the Brønsted acid sites existing on the mesopore walls were evaluated in comparison with external surfaces of 2.5-nm thick MFI zeolite nanosheets, which were hydrothermally synthesized using a meso-micro dual structure-directing surfactant. Acid sites were characterized with respect to the strength and concentration by 31P NMR signals of adsorbed tributylphosphine oxide. Catalytic functions were evaluated for decalin cracking, and acetal formation of benzaldehyde with pentaerythritol. The result from desilication was quite sensitive to the particle diameters of the initial zeolite samples. In our best case of desilication, the mesopore walls exhibited similar catalytic activity for the acetal formation to the external surfaces of MFI nanosheets. In contrast to the condensation reaction occurring at mild or moderate acid sites, the result for decalin cracking requiring strong acid sites indicated that the desilicated zeolite corresponded to about 60% of the MFI nanosheets.

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