Physical Adsorption and Chemisorption

Adsorption depends on the existence of a force field at the surface of a solid, which reduces the potential energy of an adsorbed molecule below that of the ambient fluid phase. It is useful to distinguish two broad classes of adsorption (physical adsorption and chemisorption) depending on the nature of the surface forces.

The forces of physical adsorption consist of the ubiquitous dispersion–repulsion forces (van der Waals forces),which are a fundamental property of all matter, supplemented by various electrostatic contributions (polarization, field–dipole and field gradient quadrupole interactions), which can be important or even dominant for polar adsorbents. The forces involved in chemisorption are much stronger and involve a substantial degree of electron transfer or electron sharing, as in the formation of a chemical bond. As a result, chemisorption is highly specific and the adsorption energies are generally substantially greater than those for physical adsorption.

 

Chemisorption is by its very nature limited to less than monolayer coverage of the surface whereas, in physical adsorption, multilayer adsorption is common. In a microporous solid the ultimate capacity for physical adsorption corresponds to the specific micropore volume, which is generally much larger than the monolayer coverage. The economic viability of an adsorption separation process depends on both the selectivity and the capacity of the adsorbent. Because of their high selectivity and low capacity chemisorption systems are generally viable only for trace impurity removal; bulk separation processes almost always depend on physical adsorption. In catalytic processes both physical adsorption and chemisorption can be important.

The surfaces of adsorbents such as activated carbon and high-silica zeolites are essentially non polar although, in the case of carbon adsorbents, oxidation can impart a degree of surface polarity. With non polar adsorbents van der Waals forces are dominant, and relative affinity is determined largely by the size and polarizability of the sorbate molecules and the dimensions of the pores. The influence of the nature of the surface is then secondary so the affinities (for a given sorbate) of a carbonaceous adsorbent or a high-silica zeolite adsorbent of similar pore size are similar. Since non polar adsorbents have a relatively low affinity for water and a higher affinity for most organics, such materials are often described as hydrophobic.

By contrast, in the aluminum-rich zeolites, there are strong intra crystalline electric fields, so that electrostatic forces of adsorption are very important, particularly for polar or quadru polar sorbate molecules. Such adsorbents are classified as hydrophilic because they adsorb polar molecules such as water very strongly. Control of the Si/Al ratio in a zeolite adsorbent thus provides a useful means of adjusting the selectivity of an adsorbent for a particular separation.