The porous

Did you say porous?

What is a porous solid?

A porous solid is a compound consisting of a solid frame which reveals empty spaces - the pores - arranged regularly or not. The internal surface created by the presence of pores allows communication with the outside by letting the fluids pass through or by allowing other molecules to fix in the pores. It is therefore the presence of this porosity which gives the porous solids some of their unusual properties.

Different porous solids for multiple applications

The size of the pores can vary from less than one nanometer (i.e. 10^-9 meters) to several millimeters. These are called microporous solids if their size is under1 nanometer, mesoporous solids between 1 and 50 nanometers and macroporous solids beyond.

The porous solids also differ by their chemical composition. Zeolites - stones that boil in Greek - are inorganic porous solids. But there are also organic porous solids such as activated carbon. Finally, hybrid solids, whose skeleton is composed of both inorganic and organic parts exclusively connected by strong bonds, have been developed since the 1990s.

The structure of a porous solid can be ordered, i.e. crystallized, or disordered, i.e. amorphous.

The size, volume and organization of the pores combined with the composition and internal surface of these solids make them suitable for a wide range of applications going from petroleum cracking to drug vectorization, soil remediation and heat storage.

Examples of porous materials

Zeolites

In 1756, the Swedish mineralogist Axel-Frédéric Crönstedt observes that certain natural minerals are covered with bubbles after heating, as if they were boiling. Crönstedt then gives the name "zeolites" to this family of compounds.

A century later, in 1862, Henri Sainte-Claire Deville, synthesized the first homologue of the zeolites. But it was only in 1930 that Linus Pauling explained the phenomenon observed by Crönstedt by establishing the crystal structure of zeolites by X-ray diffraction.

Very quickly, industrialists use these solids as molecular sieves, ion exchangers, catalysts, gas absorbers. Zeolites are used, for example, as catalysts in the cracking of petroleum and the refining of heavy oils. Zeolites also allow the capture of calcium in phosphate-free detergents, which today represent the largest tonnage of use of these solids.

Porous materials in civil engineering

Building materials such as concrete or wood, but also soils, are disorganized porous materials. The study of their porosity from microscopic to macroscopic scales allows the development of new building materials, the safe storage of nuclear waste, the feasibility of underground storage of CO₂ and the energy optimization of houses.

The strength properties of concrete and their durability (ie their ability to withstand the to which they are subjected) are governed first by their porosity: the less porous the concrete is, the more resistant it is and sustainable. It was by developing concretes with optimized porosity up to the nanometric scale that it was possible to achieve unprecedented strengths and lifetimes, allowing the construction of exceptional structures such as the Channel Tunnel or Millau Viaduct.

The underground rocks are porous. A better understanding of how fluids can be accommodated in their porosity helps to consider different scenarios for underground storage of CO₂, whether in depleted petroleum tanks, in deep coal veins, or in saline aquifers.

The French Cigéo deep waste repository project was designed to be housed in an argillite. Like all subterranean rocks, this argillite is porous. A better understanding of the porous structure of this rock and the way it is altered by the stresses to which it is subjected makes it possible to better understand how their permeability should evolve over time and thus contributes to the security of storage.

Glass wool is used in buildings as a thermal insulation. But if the glass wool is such a good thermal insulator, it is not because the fiberglass itself is insulating (quite the contrary, it is very good conductor of heat!), But because the wool is very porous and contains a lot of air, which is a good insulator!

MOF

Since the 1990s, the development of novel hybrid porous solids or MOF, for Metal-Organic Framework, has opened new perspectives. Their sizes and their properties are extremely modulable, multiplying the possible uses of these in the environment and health domains.

Reducing CO₂ emissions is a major challenge for our societies. But current methods of CO₂ capture are not very efficient and extremely energy intensive. The use of MOFs for CO₂ capture is a much more efficient and environmentally friendly alternative. In general, the potential for gas storage is enormous. The internal surface of one gram of MOFs can exceed 7,000 m² and thus can absorb very large quantities of gas, which release can then be controlled.

In the field of health, MOFs are extremely promising for medical imaging or vectorization of drugs by encapsulating magnetic agents and / or therapeutic molecules. These non-toxic, biodegradable MOFs are designed to reach only the target area (ill tissue or organ), improving both diagnosis and treatment.

Biomaterials

A biomaterial is a non-living material, used in a medical device and designed to interact with biological systems, that can either participate in the constitution of diagnostic apparatus or of a tissue or organ substitute, or to that of a functional substitution device (or assistance). Some polymers take the form of gels or foams whose porous structure is studied and used to improve the colonization by the cells and to functionalize the biomaterials with molecules allowing a better survival of the cells.