(183ab) A Commercially-Viable One-Step Synthesis Method to Prepare MWW Zeolite Nanosheets
Zeolites are shape-selective microporous crystals that have been widely used as heterogeneous catalysts in petroleum and chemical industries. However, pore dimensions (4 â 7 Ã ) of typical zeolites often encounter diffusion limitations, especially in applications that involve bulky molecules. MCM-22 (MWW type) zeolite exhibits properties that improve mass transport by providing accessible acid sites on or near its external surface. Conventional layered MCM-22 precursors lead to the direct condensation of MWW layers during calcination, sacrificing significant accessibility to surface acid sites. In our studies, we have demonstrated a novel approach to obtain disordered MWW-type material with high external surface acid sites via a commercially-viable single step protocol.
Disordered MCM-22 was obtained in a one-pot synthesis using a cationic modifier to disrupt the natural alignment of MWW layers. A series of catalysts were prepared with varying Si/Al and a range of layer disorder to assess the degree of impact of external surface area on catalytic performance. Scanning electron micrographs reveal stark differences in the morphology and thickness of ordered and disordered zeolites. Cryo transmission electron microscopy quantified the thickness dependence of the MWW crystallites as a function of cationic surfactant concentration; the control sample (no surfactant addition) were on average a factor of 6-7 times thicker than MWW synthesized in the presence of maximum cationic surfactant, which were mono- or bilayered unit cell. The packing structure of MWW materials and their interlayer distances can be identified from powder XRD patterns in the range 2Î¸ = 6 â 10Â°. The degree of disorder was quantified by the disorder index, which is defined as the ratio between external and total surface areas (varying from 0.3 to 0.7 for ordered and disordered materials, respectively). The impact of modifiers on Al incorporation into disordered MWW materials was explored by solid state 27Al NMR measurements and by varying the synthesis gel Si/Al ratio. We observe notable differences in the locations of Al at tetrahedral sites in the frameworks of ordered and disordered MWW samples. This study provides a new approach for the one-pot synthesis of disordered MWW with high surface area and tunable Al density and siting. The ability to selectively tailor the physicochemical properties of zeolites allows for the development of structure-performance relationships of microporous catalysts.
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 Roth, W. J.; Nachtigall, P.; Morris, R. E.; Äejka, J.; Chem. Rev. 114 (2014) 4807-4837