# 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
The analysis of acidic and basic properties of the surface of desiccants, performed by the authors of [80], by means of IR spectroscopy using CO and CDCl\({}_{3}\) probe molecules, respectively, showed that the surface modification was accompanied by a decrease in Lewis and an almost complete disappearance of Bronsted acidity, appearance of super strong ones and significant increase in the concentration of strong basic sites, associated apparently with bridged oxygen atoms near alkaline ions. According to the authors, the formation of super strong sites can take place when a proton at the end of the OH group is replaced by a sodium or potassium ion with the formation of a bridged oxygen atom between Na\({}^{+}\) and Al\({}^{3+}\) ions. In this work, a linear correlation has been found between the concentration of strong basic sites on the surface and the dynamic capacity of desiccants, which allowed for the conclusion that the dynamic capacity is related to the strengthening of the main properties of the samples. The best sorption and strength characteristics have been demonstrated by a sample, which was obtained by the impregnation with potassium hydroxide with a potassium content of 3 mass.%. It has been assumed that during modification from carbonate solutions, the carbonates deposited on the surface block both strong and weak basic sites, which leads to a decrease in the dynamic capacity of water. The authors of [81] studied the properties of aluminium oxide granules modified with Li, K and Na cations by the impregnation from an excess of an alkali solution of pseudoboehmite-based aluminium oxide granules. In addition, for the modified adsorbents obtained by the water capacity impregnation, these samples have demonstrated a decrease in the specific surface and an increase in the average pore size against the background of an increase in the specific adsorption capacity of the samples, with an increase in the content of alkali metal cations (Figure 5). Figure 5: Dependence of the capacity of the adsorbent surface unit on the mole content of the modifier. Figure 6 shows the dependence of the equilibrium adsorption capacity of the modified materials (a) on the specific surface (S\({}_{\rm{sp}}\)). Figure 6 shows that the lithium modification has almost no effect on the adsorption capacity. At the same time, the dependence of the capacity of the samples modified with potassium and sodium on the specific surface is extreme in the range of changes in the specific surface of S\({}_{\rm{sp}}\) = 125-290 m\({}^{2}\)/g. The maximum value of the equilibrium adsorption capacity has been observed in the range of S\({}_{\rm{sp}}\) = 240 \(\pm\) 10 m\({}^{2}\)/g. This value of the specific surface corresponded to the modification of the initial material in the amount of 1.2 wt.% of sodium or 2.6 wt.% of potassium. Besides, the introduction of the controlled number of alkali metal cations into AO is possible at the stage of CTA product hydration during the synthesis of an adsorbent-desiccant, consisting of the mixture of low-temperature modifications of aluminium oxide of 60-70% (\(\gamma\) + \(\eta\))-Al\({}_{2}\)O\({}_{3}\) + 30-40% \(\chi\)-Al\({}_{2}\)O\({}_{3}\)[77]. It was shown that changes in the preparation conditions (alkaline modulus and the type of alkali at the hydration stage, the value of the acid modulus at the peptization stage) practically did not influence the static capacity of the desiccants, while an increase in the alkali content at the hydration stage or the use of NaOH instead of KOH increased the dynamic capacity of the desiccant granules. In this way, alkaline cations can be introduced at different stages of desiccant preparation: By the impregnation of finished granules, using alkalis at the stage of preparation of plastic pastes, and in the case of desiccants based on n \(\eta\)-Al\({}_{2}\)O\({}_{3}\), using solutions of alkalis with different concentrations at the hydration stage of the thermal activation product [82]. In the first case, the number of preparation stages increases.