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Nanoporous materials consist of a regular organic or inorganic bulk phase in which a porous structure is present. Nanoporous materials exhibit pore diameters that are most appropriately quantified using units of . The diameter of pores in nanoporous materials is thus typically 100 or smaller. Nanoporous materials include subsets of mesoporous (with typical pores having sizes between 2 and 50 ) and microporous materials (typical pores with diameters <2nm). Pores may be open or closed, and pore connectivity and void fraction vary considerably, as with other porous materials. Open pores are pores that connect to the surface of the material whereas closed pores are pockets of void space within a bulk material. Open pores are useful for molecular separation techniques, adsorption, and catalysis studies. Closed pores are mainly used in thermal insulators and for structural applications.
Most nanoporous materials can be classified as bulk materials or membranes. Activated carbon and are two examples of bulk nanoporous materials, while can be thought of as nanoporous membranes. A porous medium or a porous material is a material containing pores (voids). The skeletal portion of the material is often called the "matrix" or "frame". The pores are typically filled with a fluid (liquid or gas).
These categories conflict with the classical definition of nanoporous materials, as they have pore diameters between 1 and 100 nm. This range covers all the classifications listed above. However, for the sake of simplicity, scientists choose to use the term nanomaterials and list its associated diameter instead.
Microporous and mesoporous materials are distinguished as separate material classes owing to the distinct applications afforded by the pores sizes in these materials. Confusingly, the term microporous is used to describe materials with smaller pores sizes than materials commonly referred to simply as nanoporous. More correctly, microporous materials are better understood as a subset of nanoporous materials, namely materials that exhibit pore diameters smaller than 2 nm. Having pore diameters with length scales of molecules, such materials enable applications that require molecular selectivity such as filtration and separation membranes. Mesoporous materials, referring generally to materials with average pore diameters in the range 2-50 nm are interesting as catalyst support materials and Adsorption owing to their high surface area to volume ratios.
Sometimes classifying by size becomes difficult as there could be porous materials that have various diameters. For example, microporous materials may have a few pores with 2 to 50 nm diameter due to random grain packing. These specifics must be taken into consideration when categorizing by pore size.
Organic nanoporous materials can be further classified into and Amorphous solid networks. Crystalline networks are materials that have a well-defined pore sizes. The pore sizes are so well defined that simply by changing the monomer, one can obtain different pore sizes. COFs are an example of such crystalline structure. In contrast, amorphous nanoporous materials have a distribution of pore sizes and are usually disordered. An example is PIMs. Both categories have various uses in gas sorption and catalysis reactions.
One example of this application is hydrogen storage. With the onset of climate change, there is an increased interest in zero-emission vehicles, especially in fuel cell electric vehicles. By storing hydrogen at high densities using porous materials, one can increase electric car mileage range.
Another use case for nanoporous materials is as a substrate for gas sensors. For example, measuring the electrical resistivity of a porous metal can yield the exact concentration of an analyte species in gaseous form. Since the resistivity of the substrate is proportional to the surface area of the porous media, using nanoporous materials will yield higher sensitivity in detecting trace gaseous species than their bulk counterparts. This is especially useful as nanoporous materials have a higher effective surface area normalized to the top-view surface area
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