# 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
When the modifier content is higher than 2-3%, the specific surface area significantly decreases, which is undesirable. In the second case, there is a decrease in the strength of granules as compared to the granules, which are prepared using nitric acid for plasticization, which is consistent with the literature data and may be conditioned by the lower solubility of aluminium hydroxides in alkalis as compared to nitric acid [44]. Negative phenomena, typical of the first two methods, can be avoided, and the content of alkaline cations in \(\eta\)-Al\({}_{2}\)O\({}_{3}\) can be increased by increasing the concentration of alkali at the hydration stage of the thermal activation product. Obtaining strong granules is achievable by optimizing the amount of nitric acid, which is used to prepare the moulded paste [82]. The data, presented by the authors of [82], indicate that the samples, modified with potassium and sodium, have a specific surface of more than 300 m\({}^{2}\)/g. The surface size, pore volume and average pore size of the samples modified with potassium are higher. Meanwhile, they are lower in samples modified by sodium, mainly due to a decrease in the volume of mesopores, since the volume of  
Figure 6: Dependence of the adsorption capacity of the modified materials (a) on the specific surface (S\({}_{\rm{sp}}\)). micropores in the samples is almost the same. An increase in the potassium content at the hydration stage leads to a decrease in the strength of the granules. A similar decrease in the strength of granules was observed earlier when using KOH at the stage of preparation of the moulded paste [68]. The use of NaOH instead of KOH during hydration, as well as an increase in the sodium content, on the contrary, led to an increase in strength. Comparing the characteristics of the adsorbents obtained during modification by the introduction of alkaline cations at the paste preparation stage and the introduction of alkaline cations at the hydration stage of the gibbsite thermal activation product, it can be noted that the previously achieved dynamic capacity and mechanical strength indicators (5.9 g/100 cm\({}^{3}\) and 5.7 MPa) were exceeded [68]. These indicators in the latter case were 7.5 g/100 cm\({}^{3}\) of the desiccant and 8.4 MPa, respectively [82]. The conducted test of the developed modified adsorbent-desiccant in comparison with well-known Russian and foreign analogues [82] has shown that industrial adsorbents, obtained by thermal activation technology, contain mainly \(\chi\)-Al\({}_{2}\)O\({}_{3}\) or \(\gamma\)-Al\({}_{2}\)O\({}_{3}\), while the proposed desiccant contains \(\eta\)-Al\({}_{2}\)O\({}_{3}\). The texture characteristics of the foreign desiccant based on aluminium oxide [82] and the proposed adsorbent have been shown to be quite similar. The higher efficiency of the developed desiccant has been associated by the authors of this work with a higher content of the basic sites, conditioned by the introduction of alkaline cations. It has been shown that the obtained values of the dynamic capacity of desiccants based on AO (industrial and developed) correlate well with the content of sodium cations in them. With an increase in the content of sodium cations, an increase in the dynamic capacity of desiccants was observed, despite the fact that the structural modification of aluminium oxides in these desiccants differs. The study of the dynamic capacity of the developed highly efficient sample of an aluminium oxide adsorbent, as well as the comparison of its adsorption characteristics with those of a commercial adsorbent based on NaX zeolite under similar conditions were presented by the authors of [83]. The grain size of the aluminium oxide adsorbent was characterized by a diameter of 3.55 mm, height of 5.2 mm and NaX dimensions of 4.2 \(\times\) 4.4 mm. In this way, the equivalent diameter of the adsorbents (proportional to the ratio of the grain volume to its outer surface) was approximately the same at d\(\approx\) 4.0 \(\div\) 4.2 mm.