# Efficient Adsorbent-Desiccant Based on Aluminium Oxide
## 4 Industrial Adsorbents-Desiccants and Their Characteristics
### Modification of Adsorbents Based on Aluminium Oxide with the Cations of Alkaline Metals  
An increase in the efficiency of aluminium oxide adsorbents can be achieved by changing the strength and the concentration of acid-base sites on their surface. The acid-base properties of the surface can be changed by modification with inorganic acids (HF, H\({}_{2}\)SO\({}_{4}\), H\({}_{3}\)PO\({}_{4}\), H\({}_{3}\)BO\({}_{3}\)), bases (alkalis, carbonates) or bifunctional additives (salts of various acids) [62]. As the aluminium oxide surface is impregnated with alkali, the total surface basicity increases [16]. At the same time, Lewis and Bronsted acidity increases with a low content of the alkaline cation. However, it then decreases with an increase in the alkali content. Modification with small amounts of alkali metal (Li, Na, K) does not lead to a significant change in the specific surface of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) in the phase composition, and it affects only the near-surface layer [71; 72]. When the content of sodium and potassium cations increases from 0.3 mmol/g to 0.9 mmol/g of Al\({}_{2}\)O\({}_{3}\), the concentration of coordination-unsaturated Al\({}^{3+}\) cations on the surface decreases, and the strength of Lewis acid sites (LAS) decreases significantly. The increase in the strength of the sites is due to an increase in the electron density throughout the entire AO lattice. It has been noted that with an increase in the cation radius, the strength of electron-acceptor sites weakens [73], which supports the assumption of electronic suppression of the acidic properties of AO with the introduction of alkali metals. In some cases, with an increase in the content of modifying components, the formation of new phases, in particular, aluminates of alkali metals, has been noted. It has been found that Lewis acidic properties of lithium aluminates correlate with the presence of aluminium cations in their crystal structure in the tetrahedral coordination. The impregnation of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) with NaOH solutions with an alkali content from 0.1 wt.% to 3.0 wt.%, as noted by the authors of [74]. Impregnation also leads to a decrease in the total and local LAC concentration, and the strongest LAS disappears first. When AO is modified with alkalis, the concentration and the strength of weak basic sites increases. When the sodium content is high, strong sites appear due to the appearance of oxygen atoms, which are bound directly to sodium [75]. The authors of [68,76] studied the influence of acid and alkaline modification on the adsorption characteristics of aluminium oxide desiccants prepared from amorphized gibbsite (products of CTA and THA). The authors established that the introduction of the sulphuric acid solution at the stage of obtaining the plastic paste from pseudoboehmite- and bayerite-containing hydroxides makes it possible to increase the dynamic capacity of desiccants based on \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) to the level of \(\eta\)-Al\({}_{2}\)O\({}_{3}\) both by reducing the average pore diameter and by increasing the surface acidity. These processes practically do not influence the phase composition, texture and adsorption characteristics of adsorbents based on bayerite. When aluminium oxides are modified with sulphate ions on the surface, as shown by the authors of [76], strong LAS are formed, the water adsorption on which is accompanied by the formation of strong Brensted acid sites (BAS), which leads to an increase in the sorption capacity of the desiccants modified with sulphate ions by water. During alkaline modification, by introducing the solution of KOH or NaOH at the stage of obtaining the plastic paste based on pseudoboehmite-containing hydroxides, the static capacity of the adsorbent is not changed. However, the dynamic capacity increases against the background of the increased average pore diameter, reduction in the number and strength of LAS, appearance of new super strong basic sites and increase in the total number of the basic sites of varying force.