Abstract
Zeolites are generally defined as three-dimensional (3D) crystalline microporous aluminosilicates in which silicon (Si4+) and aluminum (Al3+) are coordinated tetrahedrally with oxygen to form large negative lattices and consequent Brønsted acidity. Two-dimensional (2D) zeolite nanosheets with single-unit-cell or near single-unit-cell thickness (~2–3 nm) represent an emerging type of zeolite material. The extremely thin slices of crystals in 2D zeolites produce high external surface areas (up to 50% of total surface area compared to ~2% in micron-sized 3D zeolite) and expose most of their active sites on external surfaces, enabling beneficial effects for the adsorption and reaction performance for processing bulky molecules. This review summarizes the structural properties of 2D layered precursors and 2D zeolite derivatives, as well as the acidity properties of 2D zeolite derivative structures, especially in connection to their 3D conventional zeolite analogues’ structural and compositional properties. The timeline of the synthesis and recognition of 2D zeolites, as well as the structure and composition properties of each 2D zeolite, are discussed initially. The qualitative and quantitative measurements on the acid site type, strength, and accessibility of 2D zeolites are then presented. Future research and development directions to advance understanding of 2D zeolite materials are also discussed.
Highlights
Zeolites are generally defined as three-dimensional (3D), crystalline, microporous aluminosilicates that have demonstrated enormous framework variety and pore connectivity [1]
We acknowledge the variety of 2D layered precursors and the derivative zeolite materials, as well as the scope of physical property alteration performed on these materials over the past few decades ; we shift the focus of this article to a summary of acidity characterization on 2D zeolite derivative materials
Different from the acidity strength results obtained from FTIR spectra of adsorbed pyridine, the 31P MAS NMR spectra of TMPO show that 2D MFI nanosheets have high fractions of weak, medium, and strong acid sites, while the fraction of acid sites with medium-high strength is low compared to 3D commercial MFI zeolites
Summary
Zeolites are generally defined as three-dimensional (3D), crystalline, microporous aluminosilicates that have demonstrated enormous framework variety and pore connectivity [1]. 2D zeolites contain extremely thin nanosheet slices of crystals that produce high external surface areas (up to 50% of total surface area compared to ~2% in micron-sized 3D zeolite) and expose a large amount of their active sites on external surfaces. They enable beneficial effects on adsorption and reaction in processing bulky molecules. We focus on the overview of acidity characterization techniques and document results from sparse and isolated studies in individual research laboratories for comparison purposes This forms a collection of acidity information to direct further research and development in this area. The challenges, strategies to overcome these challenges, and future research and development directions to advance 2D zeolite materials are presented
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