Exfoliating layered zeolite MFI into unilamellar nanosheets in solution as precursors for the synthesis of hierarchical nanocomposites and oriented films

Abstract

Zeolites have many uses and are continually developed for catalysis and separations as highly active, selective and thermally stable materials with microporous framework structures of various topologies. Among approximately 250 known frameworks about 20, but possibly all eventually, produced 2D forms with layers of thickness up to a few nanometers. Recently, layered forms of the zeolite MWW and a ferrierite-related one have been exfoliated directly by a soft-chemical treatment into solutions of unilamellar nanosheets. They enable the synthesis of layer-based hierarchical and nanocomposite materials with diverse compositions that are unfeasible with solid 3D and 2D forms. The efficiency and yield of zeolite exfoliation can be inhibited by intergrowth so each topology presents a separate challenge. The MFI zeolite can provide particular benefits as one of the most active and versatile frameworks. It has permeable layers due to perpendicular channels and has been used as a membrane. This article describes exfoliation of the layered MFI produced by the bifunctional template developed by Ryoo et al. It affords unilamellar MFI nanosheets in solution with a uniform hydrated thickness of 3.6 nm confirmed by AFM, X-ray diffraction and TEM. The nanosheets can be reassembled top-down into self-standing b-oriented discs or films, proven by X-ray powder diffraction. They preserve high acid site concentration and porosity. The basal spacing contracts from 3.6 nm to 2.8 nm upon calcination. Oriented MFI films were also formed by the evaporation/calcination of nanosheet solutions and surfactant composites enabling simple fabrication on various substrates for application in catalysis and separations.