Micropores

Micropores are pores with diameters less than 2 nanometers. This ultra-small size gives rise to high surface area and strong adsorption potential, making microporous materials particularly effective for trapping and separating small molecules. Microporosity is a key feature of a wide range of solids, including activated carbons, zeolites, and many metal-organic frameworks, as well as certain porous polymers and carbon blacks. In these materials, the pore network is often dominated by sub-2 nm cavities or channels, which significantly influence adsorption equilibria and diffusion behavior.

Porosity is commonly classified by pore size into micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). The presence and distribution of micropores affect selectivity, capacity, and rate of mass transfer, particularly for gases and vapors. Microporosity is exploited in gas separation, carbon capture, and storage applications where small molecules such as CO2, H2, or CH4 are targeted. However, diffusion of larger molecules can be hindered by narrow pores, and accessibility of micropores depends on the surrounding pore architecture.

Characterization of microporosity typically relies on gas adsorption methods. Nitrogen adsorption at 77 K provides overall surface area, but for micropores, techniques such as CO2 adsorption at 273 K, t-plot analysis, and density functional theory (DFT) based pore size distributions are used to quantify micropore volume and dimensions. Parameters of interest include micropore volume (Vmicro) and micropore surface area (Smicro).

Synthesis and control of microporosity are central to material design. Activation of carbon materials (physical or chemical) and templating processes can create extensive microporous networks. Zeolites and many MOFs possess intrinsic microporosity defined by their rigid, well-ordered frameworks, enabling precise control over pore size and chemistry.