Themes

Chemical porosity: characteristics, types and examples

It is represented by the portion of the volume of these cavities divided by the volume of the totality of the material studied. The magnitude or numerical value resulting from this parameter can be expressed in two ways: a value between 0 and 1 or a percentage (value between 0 and 100%), to describe how much of a material is empty space.

Chemical porosity
Kitchen paper is highly porous

Although multiple uses are attributed to it in different branches of pure, applied, materials sciences , among others, the main functionality of chemical porosity is linked to the ability of a certain material to allow the absorption of fluids; that is, liquids or gases.

Porosity characteristics

Medium with high porosity (left) and with low porosity (right). Black shapes represent solids, blue represents porous spaces

Two substances interact

Porosity is the portion of the volume of a supposed solid that is certainly hollow and is related to the way in which two substances interact, giving it specific characteristics of conductivity, crystalline, mechanical properties and many others.

The speed of reaction depends on the  space of the surface of the solid

In reactions that occur between a gaseous substance and a solid or between a liquid and a solid, the speed of a reaction depends largely on the space on the surface of the solid that is available for the reaction to take place.

Accessibility or penetrability depends on the pores

The accessibility or penetrability that a substance can have on the inner surface of a particle of a given material or compound is also closely related to the dimensions and characteristics of the pores, as well as their number.

Types of chemical porosity

Porosity can be of many types (geological, aerodynamic, chemical, among others), but when dealing with chemistry, two types are described: mass and volumetric, depending on the class of material that is being studied.

By referring to the mass porosity the ability of a substance to absorb water is determined. To do this, the equation shown below is used:

% P m = (m s – m 0 ) / m 0 x 100

In this formula:

m represents the proportion of pores (expressed as a percentage).
s refers to the mass of the fraction after being submerged in water.
 describes the mass of any fraction of the substance before it was submerged.

Volumetric porosity

Similarly, to determine the volumetric porosity of a certain material or the proportion of its cavities, the following mathematical formula is used:

% P v = ρ m / [ρ m + (ρ f / P m )] x 100

In this formula:

v describes the proportion of pores (expressed as a percentage).
ρ m refers to the density of the substance (not immersed).
ρ f  represents the density of water.

Examples of chemical porosity

Example of a porous and non-porous medium

The unique characteristics of some porous materials, such as the number of cavities or the size of their pores, make them an interesting object of study.

Thus, a large number of these extremely useful substances are found in nature, but many more can be synthesized in laboratories.

Investigating the factors that influence the porosity qualities of a reagent allows us to determine the possible applications that it has and to try to obtain new substances that help scientists to continue advancing in the areas of materials science and technology.

One of the main areas in which chemical porosity is studied is in catalysis, as in other areas such as gas adsorption and separation.

Zeolites

Zeolite

Proof of this is the investigation of crystalline and microporous materials, such as zeolites and the structure of organic metals.

In this case, zeolites are used as catalysts in reactions that are carried out by means of acid catalysis, due to their mineral properties as oxide porous and there are different types of zeolites with small, medium and large pores.

An example of the use of zeolites is in the catalytic cracking process, a method used in oil refineries to produce gasoline from a fraction or cut from heavy crude oil.

Organic metal structures involving hybrid materials

Another class of compounds that are being investigated are the structures of organic metals that involve hybrid materials, created from an organic fragment, the binding substance and an inorganic fragment that constitutes the fundamental basis for these substances.

This represents a greater complexity in its structure with respect to that of the zeolites described above, therefore it includes possibilities much greater than those imaginable for zeolites since they can be used for the design of new materials with unique properties.

Despite being a group of materials with little study time, these organic structures of metals have been the product of a large number of syntheses to produce materials with many different structures and properties.

These structures are quite stable thermally and chemically, including one of special interest that is the product of terephthalic acid and zirconium, among other reagents.

UiO-66

This substance, called UiO-66, has a large surface area with adequate porosity and other characteristics that make it an optimal material for studies in the areas of catalysis and adsorption.

Others

Finally, there are countless examples in pharmaceutical applications, soil research, in the oil industry and many others where the porosity of substances is used as a basis to obtain extraordinary materials and use them in favor of the sciences.

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