VOCs adsorption materials are various, such as activated carbon, zeolite and so on. The adsorption and desorption performance of adsorbent materials for VOCs is affected by a variety of factors, the main influencing factors include the structural parameters of the adsorbent, the nature of the molecules of the VOCs gas, and the external conditions, etc., which are explained in detail below:

1、Specific surface area and pore structure

The surface area provides a place for the adsorption process and increases the probability of the adsorbent interacting with VOCs. A large specific surface area implies superior adsorption performance, and the specific surface area of the adsorbent can be increased by opening closed pores or forming new pores. Appropriate acid and alkali treatments can effectively expand the surface area of the material and improve its adsorption capacity, but excessive acid and alkali may also lead to pore destruction or collapse thus reducing the surface area. It was found that the activated carbon with the highest specific surface area did not always show the best adsorption capacity for organic compounds, which proves that the adsorption of VOCs by the material is affected by several factors. Using activated carbon as the adsorbent, the effects of specific surface area and pore structure on adsorption capacity were investigated, and the results showed that the adsorption of VOCs was mainly controlled by pore diffusion, and the smaller the filling density of the adsorbent, the easier it was to penetrate.

The microstructure of carbon adsorbent materials, especially the pore size distribution, determines their adsorption capacity for VOCs, and it was found that the preparation raw materials and conditions affect the specific surface area and pore characteristics. The influence law of different activation temperatures on the physical structure of porous carbon materials, it was found that the effect of activation temperature on the pore size is more significant, with the increase of activation temperature, the pore structure of carbon materials undergoes a process from low to high and back to low.

Generally speaking, micropores are the main site of adsorption, but the increased diffusion resistance in narrow pores also leads to lower adsorption rate; mesopores enhance intra-particle diffusion and shorten the adsorption time. Therefore, the pore size of the adsorbent material determines the size of the VOCs molecules that can be adsorbed, and according to the size exclusion theory, the VOCs molecules can enter the pores of the adsorbent material only when the pore diameter is larger than the molecular diameter of the VOCs. Therefore, optimal adsorption occurs where the pore diameter matches the molecular size of the adsorbent, with micropores favoring the adsorption of small-volume VOCs, and large pores, such as mesopores, being more suitable for the adsorption of large-molecule VOCs. For the same type of VOCs, the larger the diameter of the molecules, the stronger the superposition of the pore walls between the adsorbents; the stronger the adsorption bonding energy, the greater the adsorption capacity of the VOCs, and the larger molecules of the VOCs showed lower adsorption capacity.

The adsorption and desorption behaviors of different target substances such as n-hexane, toluene, and ethyl acetate on adsorbents such as activated carbon, 5A, NaY, 13X, etc., and the adsorption and desorption behaviors of different VOCs molecules on different adsorbents were thermodynamically investigated using chromatography and thermogravimetry, and it was found that the magnitude of the force of physical adsorption was related to the pore size distribution of the adsorbent and the diameter of the molecules, and that when adsorbate molecules were close to the surface of the adsorbent, the Interaction between the solid surface and the molecules occurs, and when the molecules are on two surfaces there is a superposition of potentials (e.g., slit pores), and the potentials of cylindrical or spherical pores will be greater. The pore shape of activated carbon is mainly dominated by slit pores, and molecular sieve has a multi-dimensional pore system with more complex pore shapes.

2、Surface chemical properties

In addition to the morphological structure, the chemical functional groups on the surface of the adsorbent material may also play a role in the adsorption of VOCs, and their types and numbers have a great influence on the adsorption capacity of VOCs, and the chemical modification of the surface of the adsorbent can change the adsorption capacity and selectivity of VOCs. For example, the surface functional groups of carbonaceous adsorbents are related to both the nature of the raw materials and the activation or modification methods such as heating, chemical and electrochemical treatments, and the surface chemical functional groups obtained by different modification methods are different.

The heteroatoms of the surface functional groups determine the surface chemistry of the adsorbent, and the heteroatoms mainly include oxygen, nitrogen, halogen, hydrogen, etc., among which the oxygen and nitrogen groups on porous carbon are considered to be the most important species in the adsorption process. There are three different types of oxygens, acidic, basic and neutral, and the acidic functional groups are -COOH, -OH, -C=O, -CO and -COO-, which are related to the oxidizing phase related to the oxidizing phase, where the carboxyl and hydroxyl groups exhibit strong electron uptake.

In general, liquid-phase oxidation contributes to the formation of carboxylic acids, while gas-phase oxidation promotes the formation of hydroxyl and carbonyl groups. Most of the oxygen groups are a source of surface acidity, which contributes to the adhesion of hydrophilic VOCs to carbon surfaces, and oxidation by acids and ozone is the most efficient way to introduce surface oxygen groups on carbon materials. The presence of oxygen groups may inhibit specific interactions between hydrophobic VOCs and π-electron rich regions on carbonaceous adsorbents. Therefore, hydrophobic VOCs prefer to adsorb on activated carbons without surface oxygen groups. Nitrogen-containing groups are usually introduced by treatment with ammonia, nitric acid and nitrogen-containing compounds, which are mainly alkaline.

The chemical state of the transition metals loaded on the activated carbon also plays an important role in improving the adsorption capacity of VOCs at low temperatures. At present, the metal-modified activated carbon technology is mainly used for the treatment of formaldehyde, toluene and other small relative molecular mass pollutants, and the application of some large relative molecular mass VOCs needs further research.

3、External detection environment

In addition to the impact of adsorbent and adsorbent mass, temperature, humidity and other external conditions on the adsorption performance of VOCs also have a certain impact, the temperature of VOCs adsorption is more pronounced. HJ 644-2013 standard for the ambient air of volatile organic compounds adsorption tube sampling provisions, the adsorption tube sampling temperature of not more than 40 ℃; HJ 734 -2014 standard for fixed source pollution exhaust VOCs using adsorbent material is Tenax GR, Carbopack B, Carbopack C and Carboxen 1000 composite adsorbent material, adsorption sampling temperature of 0 ~ 5 ℃. If you want to realize the adsorption enrichment of some low-boiling VOCs components, the required temperature is even lower, such as C2 components of the sampling enrichment usually need to be -100 ℃ below the low-temperature, only in recent years the development of low-temperature cold traps with -76 ℃, to achieve the enrichment of C2 and C2 above the low-boiling hydrocarbons.

When the actual sampling scenario is a high humidity environment and there is a competition for adsorption between the target adsorbent and water on the surface of the adsorbent, the adsorption of water will have a great impact on the adsorption of VOCs. Therefore, a water removal step is usually added before trapping to reduce the effect of water vapor. In addition, many researchers have improved the hydrophobicity of adsorbent materials by modifying them to make them more suitable for the adsorption of VOCs in high humidity environments.