A porous material is a solid containing voids, or pores, within its structure. These spaces may be isolated cavities or interconnected channels and may contain gases or liquids. Porosity occurs in natural substances, including rocks, and in manufactured materials such as foams, ceramics, activated carbon, and crystalline frameworks. Rather than constituting a single chemical class, porous materials are defined by a structural feature that influences fluid accessibility, surface interactions, and mechanical behavior. (pmc.ncbi.nlm.nih.gov)
Pore structure and classification
Porosity is the fraction of a material’s bulk volume occupied by pores, commonly expressed as a percentage. It is distinct from pore size, which describes the dimensions of individual voids. Materials with similar porosities can have different pore shapes, size distributions, and transport properties. Pores may lie between particles or occur within a continuous solid skeleton. (pubs.usgs.gov)
Open pores communicate with the exterior, whereas closed pores are enclosed within the solid. Open pores can form continuous pathways or terminate as dead ends. Connectivity determines whether fluids can move through a specimen, rather than merely enter some of its cavities. A highly porous rock can therefore have low permeability if its voids are poorly connected; this distinction is fundamental to the study of groundwater. (pmc.ncbi.nlm.nih.gov)
In the widely used IUPAC classification, micropores have widths not exceeding about 2 nanometres, mesopores have widths between approximately 2 and 50 nanometres, and macropores exceed approximately 50 nanometres. These categories describe pore dimensions, not the overall size of the specimen. A material can contain several pore-size classes simultaneously. Such hierarchical pore systems combine small cavities with larger access channels, potentially balancing internal surface area and transport efficiency. (pubs.rsc.org)
Principal material families
Porous materials encompass inorganic, organic, metallic, and hybrid solids. Activated carbon contains extensive internal surfaces and usually a distribution of pore sizes. Zeolites are crystalline framework materials with channels and cavities of molecular dimensions. Their composition and pore architecture support applications involving adsorption, ion exchange, and catalysis. Mesoporous silica and other porous oxides provide additional inorganic architectures. (pubs.rsc.org)
Metal–organic frameworks combine metal-containing components with organic linkers, while covalent organic frameworks contain organic building units joined by covalent bonds. Their structures can be designed through reticular chemistry, allowing systematic variation of pore geometry and chemical functionality. Not every framework retains accessible porosity after removal of its synthesis solvent; structural stability and activation conditions remain important. (advanced.onlinelibrary.wiley.com)
Porous polymers include cellular foams, membranes, and interconnected scaffolds. Metallic foams and sponges extend the concept to electrically conducting solids. Their performance depends jointly on composition, macroscopic shape, and pore architecture, rather than on void fraction alone. Aerogels, including silica aerogels, represent another porous family notable for low thermal conductivity. (pmc.ncbi.nlm.nih.gov)
Surface interactions and transport
Small pores can generate substantial specific surface area, meaning accessible surface per unit mass. This supports adsorption, in which molecules accumulate at solid surfaces. Adsorption capacity and selectivity depend on both pore structure and surface chemistry: a large surface area does not by itself guarantee strong or selective uptake of a particular substance. (annualreviews.org)
Narrow pore entrances may admit some molecules while excluding others, producing molecular-sieving behavior. Fluids inside nanopores can also behave differently from bulk fluids because interactions with opposing pore walls overlap and confinement changes adsorption and phase behavior. In larger connected networks, transport depends on pore dimensions, constrictions, and connectivity. Consequently, total pore volume and accessible pore volume are not necessarily equivalent. (annualreviews.org)
Preparation and processing
Manufacturing methods create pores either during solid formation or through subsequent treatment. Gas foaming introduces bubbles into a material. Removable particles, droplets, or other templates define spaces that remain after the template is extracted. Phase separation and freeze-drying provide further routes, especially for polymers and their composites. The selected method influences pore size, morphology, and interconnection. (pmc.ncbi.nlm.nih.gov)
Porous metals can be produced using templates, powder-processing routes, or dealloying, which selectively removes a component from an alloy. 3D printing allows porous solids to be shaped into structured objects with designed channels and external geometries. Printed channels and intrinsic molecular-scale pores occupy different structural levels and can coexist within the same object. (pubs.rsc.org)
Characterization
Gas adsorption measurements are central to nanopore analysis. An adsorption isotherm records uptake as gas pressure changes at constant temperature. The Brunauer–Emmett–Teller method estimates surface area, while suitable adsorption models provide pore-size distributions. Results depend on the probe gas, sample preparation, and model assumptions; inappropriate analysis can produce misleading surface areas or pore distributions. (sol.rutgers.edu)
Mercury intrusion porosimetry complements adsorption when larger pores are present. Imaging methods, including electron microscopy and X-ray computed tomography, reveal aspects of pore morphology and connectivity. Diffraction and scattering characterize framework structure and structural changes. No single technique fully describes every pore scale, so complementary measurements are often necessary. (annualreviews.org)
Applications and design constraints
Porous solids serve as adsorbents, separation media, and supports or active materials for catalysis. They are also investigated for gas storage, electrochemical devices, and carbon capture. Structured porous components can improve access to active surfaces while controlling pressure drop and heat or mass transfer. Silica aerogels are used in thermal-insulation development because of their low thermal conductivity. (pubs.rsc.org)
Useful performance requires balancing porosity with mechanical strength, chemical stability, manufacturability, and regeneration requirements. Increasing void fraction can weaken the solid skeleton, while shaping or processing may alter accessible pores. Application-specific design therefore considers the complete pore network and operating environment, not simply the maximum achievable surface area. (pmc.ncbi.nlm.nih.gov)