(627g) A Mixed-Solvent Approach to Delaminate Layered Zeolitic Precursors

Two dimensional zeolitic materials built in layered precursors serve as building blocks to synthesize new catalysts, which are equipped with molecular-sized crystalline micropores that enable molecular sieving effects as well as large mesopores that allow fast mass transfer of reactants. Because these zeolitic nanosheets are held together tightly by strong hydrogen bonds, which are mediated by structure-directing agents (SDAs) present in the interlayer region, the first step generally used to obtain zeolitic nanosheets is to weaken strong interactions by intercalation of surfactants or removal of SDAs by acid treatment. Our strategy is to use a mixture of readily available solvents where one type of solvent extracts SDAs from the interlayer while the other interacts with the hydroxyl groups on the surface of zeolitic nanosheets and prevent their excessive condensation after removal of SDAs. The resultant material allows further processing such as facile delamination, swelling under mild conditions, and intercalation of other guest molecules. Here, to demonstrate our approach, we report results from treatment of as-made ITQ-1, a layered precursor of pure-silica MWW-type molecular sieve, with N,N-dimethylformamide (DMF) and ethylene glycol (EG). As-made ITQ-1 was synthesized with dual SDAs of N,N,N-trimethyl-1-adamantammonium (TMAda⁺) and hexamethyleneimine (HMI) according to the literature method [1] and subjected to the treatment of a DMF-EG mixture with various compositions at 423 K. Data from elemental analysis, ¹³C MAS NMR spectroscopy, thermogravimetric analysis, and powder X-ray diffraction indicate that treatment of as-made ITQ-1 with a DMF-EG mixture preferentially removes TMAda⁺ from the interlayer region although the SDA is charged and thus its removal requires another cation to maintain charge neutrality. Subsequent shear mixing of the solvent-treated ITQ-1 in a high-speed mixer yielded a material, which exhibits a 3-fold higher external surface area and 2.5-fold larger mesopore volume than ITQ-1 after calcination as characterized by N₂adsorption experiments. In contrast, shear mixing of as-made ITQ-1 without the solvent treatment barely increased the external surface area, indicating that the solvent treatment renders the layered precursor labile to shear forces.     

[1] Camblor, M. A., Corma, A., Díaz-Cabañas, M. J., Baerlocher, C., Synthesis and structural characterization of MWW type zeolite ITQ-1, the pure silica analog of MCM-22 and SSZ-25. J. Phys. Chem. B 102, 44-51 (1998).