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.

The deployment of fossil resources such as petroleum, coal and natural gas to produce fuels and chemicals has resulted in excessive carbon dioxide (CO₂) emissions and serious environmental issues. The implementation of CO₂ utilization as a raw material for synthesis of fuels and chemicals has potential to reduce CO₂ emissions and close the carbon cycle. Cyclic carbonates, materials used as building blocks for polymers or green solvents in batteries, can be synthesized from CO₂ and alkylene oxides. Although a variety of catalysts have been synthesized to activate CO₂ and alkylene oxides for transformation into cyclic carbonates, either the catalytic efficiency is low or the catalyst cost is high. In the present work, we report a hierarchical porous metallized poly-melamine-formaldehyde (PMF) polymer catalyst that has a 100-fold monomer cost reduction and six- to several hundred-fold activity enhancement compared to the state-of-the-art catalysts for propylene carbonate synthesis from epoxide and CO₂ under the same reaction conditions. The hierarchically interconnected macro-, meso- and micro-pore structures in PMF that are formed via one-step high internal phase emulsion polymerization facilitate mass transport and accessibility to active sites. The high density of aminal groups and triazine rings in PMF provides multiple sites for CO₂ adsorption and the subsequent reaction with epoxide. The unprecedented low-cost and high-efficiency of metallized PMF as well as the simplicity in its synthesis exhibit great potential in the synthesis of cyclic carbonates for the chemical and plastics industries.