# 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
For desiccants prepared on the basis of bayerite-containing hydroxide, the introduction of alkaline cations at the stage of obtaining the plastic paste has not led to an improvement in sorption characteristics. At the same time, an increase in the average pore diameter, a decrease in the number of acid sites and an increase in the number of super strong basic sites was also observed, but the total concentration of all basic sites decreased during modification [68]. The dynamic capacity of the obtained adsorbents, measured at the gas moisture content at the inlet of the adsorber of 15.6-16.6 g/m\({}^{3}\) (dewpoint: 18.5-19.5 \({}^{\circ}\)C), ranged from 3.9 g/100 cm\({}^{3}\) to 5.9 g/100 cm\({}^{3}\). The static capacity at 60% humidity of the synthesized adsorbents was in the range of 17.5% to 23.1%. The content of the \(\chi\)-phase in aluminium oxide adsorbents significantly increased during acid modification, but practically did not change during alkaline modification, which allowed the authors of the work to associate the increase in the moisture capacity only with the process of the chemical modification with alkalis. A correlation was found between the change in the total concentration of the basic sites during modification of desiccants and the change in their dynamic capacity. The study of the acid-base characteristics, modified with K and Na ions at the stage of obtaining the plastic paste of adsorbents by removing the curves of influence of the aqueous suspension pH of desiccant samples on the time, was carried out by the authors of [77]. The authors showed that a sharp alkalinization of the solution occurred in the first seconds of contact of the sample with distilled water. The location of the kinetic curves above the level of neutrality and the high rate of the pH change indicate the presence of strong basic sites of the aprotic type on the surface of these adsorbents. It has been noted [78] that when metal cations are introduced during peptization, there is a natural decrease in the total number of LAS and an increase in the number of basic ones. The values of the adsorption capacity of the samples correlate well with the concentrations of both types of sites and their total number. The life tests of such adsorbents, based on pseudoboehmite and performed on a pilot plant at an increased pressure of 3 MPa, showed [79] that the sample of the desiccant, modified with potassium (\(\sim\)2 wt.%), was not inferior in terms of dynamic capacity under low load to the sample, which was obtained on the basis of bayerite-containing hydroxide. After conducting nine cyclic tests (adsorption-desorption), the phase composition, content of alkaline impurities, specific surface area and crushing strength of the samples did not change within the margin of error of determination. Another way to increase the concentration of the basic sites on the surface of aluminium oxides is to introduce alkaline ions by impregnation of calcined AO granules [80]. It was shown that the impregnation of the initial aluminium oxide granules (\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) with alkaline solutions of sodium or potassium led to an increase in the dynamic capacity of the modified desiccants, while the impregnation of the initial aluminium oxide granules with carbonate solutions of sodium or potassium resulted in a decrease in the dynamic capacity. The static capacity by water of the obtained samples practically did not change. There was a decrease in the specific surface of the modified desiccants and an increase in the average pore diameter and strength of granules (during modification with carbonates, the strength practically did not change).