Abstract
Direct exfoliation of layered zeolites into solutions of monolayers has remained unresolved since the 1990s. Recently, zeolite MCM-56 with the MWW topology (layers denoted mww) has been exfoliated directly in high yield by soft-chemical treatment with tetrabutylammonium hydroxide (TBAOH). This has enabled preparation of zeolite-based hierarchical materials and intimate composites with other active species that are unimaginable via the conventional solid-state routes. The extension to other frameworks, which provides broader benefits, diversified activity, and functionality, is not routine and requires finding suitable synthesis formulations, viz. compositions and conditions, of the layered zeolites themselves. This article reports exfoliation and characterization of layers with ferrierite-related structure, denoted bifer, having rectangular lattice constants like those of the FER and CDO zeolites, and thickness of approximately 2 nm, which is twice that of the so-called fer layer. Several techniques were combined to prove the exfoliation, supported by simulations: AFM; in-plane, in situ, and powder X-ray diffraction; TEM; and SAED. The results confirmed (i) the structure and crystallinity of the layers without unequivocal differentiation between the FER and CDO topologies and (ii) uniform thickness in solution (monodispersity), ruling out significant multilayered particles and other impurities. The bifer layers are zeolitic with Brønsted acid sites, demonstrated catalytic activity in the alkylation of mesitylene with benzyl alcohol, and intralayer pores visible in TEM. The practical benefits are demonstrated by the preparation of unprecedented intimately mixed zeolite composites with the mww, with activity greater than the sum of the components despite high content of inert silica as pillars.
Highlights
We have recently reported a breakthrough in delamination/ exfoliation of layered zeolites by showing direct high yield exfoliation of the zeolite MCM-56 with MWW topology into dispersions of 2.5 nm thick monolayers in a liquid upon treatment with tetrabutylammonium hydroxide solutions (TBAOH).[1]
The solutions of exfoliated layers obtained by the above procedure were diluted 100-fold for Atomic Force Microscopy (AFM) visualization
The analysis conducted in eight different places showed the estimated content of the bifer unilamellar nanosheets to be about 94%, whereas the remaining 6% was associated with multilayer structures (Figure S1)
Summary
We have recently reported a breakthrough in delamination/ exfoliation of layered zeolites by showing direct high yield exfoliation of the zeolite MCM-56 with MWW topology into dispersions of 2.5 nm thick monolayers in a liquid upon treatment with tetrabutylammonium hydroxide solutions (TBAOH).[1] This soft-chemical process[2−4] is one of the most effective delamination procedures that presents many practical benefits, unfeasible with bulk solids, for designing and synthesis of various materials using zeolite monolayers in solutions/liquid dispersion as building blocks.[5−8] Fundamentally, a complete exfoliation represents the ultimate manifestation of the two-dimensional (2D) nature for layered solids.[9] It has not been shown and proven conclusively for 2D zeolites until now.[7,10−14] The tentative explanation for this lack of genuine high yield exfoliation is that suitable synthesis formulations (gel compositions), presumably giving low level of intergrowths, are needed for sufficiently high efficiency and yield of exfoliation, which in practice is very simple to carry out. This showed that solutions of exfoliated zeolite layers can be used to prepare unprecedented materials with mixed, hierarchical, and other advanced structures and compositions
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