Mesopores

Mesopores are pores with diameters between 2 and 50 nanometers, a size range defined by IUPAC to distinguish them from micropores (<2 nm) and macropores (>50 nm). Materials possessing such pores are called mesoporous materials. The mesoporous framework enables high surface area and accessible pore channels that facilitate the diffusion of relatively large molecules, making these materials useful in adsorption, catalysis, and separation processes.

Most well-known mesoporous materials are silica-based, including MCM-41, SBA-15, and MCF. They are typically prepared by templating methods in which a structure-directing agent, often a surfactant, forms organized micelles around which inorganic precursor species condense. In soft-templating routes, the surfactant self-assembles into regular mesophases; acid or basic conditions and temperature control the final pore structure. MCM-41 features a two-dimensional hexagonal array of uniform cylindrical pores, whereas SBA-15 possesses larger pores with a more open structure and thicker walls. Hard templating or nanocasting can produce mesoporous carbons and metal oxides by copying a solid template.

Key properties of mesoporous materials include high surface area (often several hundred square meters per gram), tunable pore size, and narrow pore size distribution. Characterization commonly uses nitrogen adsorption-desorption isotherms to yield BET surface area and BJH pore size, along with transmission electron microscopy and small-angle X-ray scattering to reveal pore ordering.

Applications span catalysis and photocatalysis, gas and liquid adsorption and separation, drug delivery, sensing, and energy storage. In addition to silica, other mesoporous materials such as mesoporous aluminosilicates, transition-metal oxides, and carbon-based systems are studied. Functionalization of pore surfaces allows chemical selectivity or anchoring of active centers.