Microporosity

Microporosity refers to pores with widths below 2 nanometers (20 angstroms), a category defined by IUPAC as the smallest class of porosity. Materials that exhibit microporosity include activated carbons, zeolites, metal-organic frameworks (MOFs), porous silica, and certain clays and polymers. Microporosity is distinct from mesoporosity (2–50 nm) and macroporosity (greater than 50 nm). The presence of micropores often yields very high surface areas and strong adsorptive interactions, especially for small molecules.

In practical terms, microporosity dominates adsorption at low relative pressure and governs the selectivity and capacity for gas and vapor uptake. Because micropores are narrow, diffusion can be restricted, influencing kinetics, accessibility, and catalytic performance. The energetics of adsorption in micropores are typically stronger per surface area than in larger pores, which can enhance separation efficiency for certain pairs of molecules and improve storage or catalytic activity.

Characterization of microporosity relies on gas adsorption measurements. Nitrogen adsorption at 77 K is commonly used to estimate surface area with the BET method, but micropores can lead to underestimation or overestimation of total surface area if interpreted without care. Techniques such as CO2 adsorption at 0–40 °C, t-plot analysis, and density functional theory (DFT)–based pore size distributions are employed to quantify micropore volume and size distribution. The resulting data inform applications in adsorption, separation, catalysis, and materials design.

Applications of microporous materials span activated carbons, zeolites, MOFs, and porous silica, where microporosity underpins high adsorption capacity, selectivity, and catalytic performance.