# Efficient Adsorbent-Desiccant Based on Aluminium Oxide
## 4 Industrial Adsorbents-Desiccants and Their Characteristics
### Aluminium Oxide Surface Characteristics
There is a close relationship between the structure of the initial hydroxides and oxides Al\({}^{3+}\) obtained from them during heat treatment: Gibbsite \(\rightarrow\) (\(\chi+\gamma\))- Al\({}_{2}\)O\({}_{3}\); bayerite \(\rightarrow\)\(\eta\)-Al\({}_{2}\)O\({}_{3}\); boehmite \(\rightarrow\)\(\gamma\)-Al\({}_{2}\)O\({}_{3}\); pseudobemite \(\rightarrow\) pseudocubic \(\gamma\)-Al\({}_{2}\)O\({}_{3}\). The last two \(\gamma\)-modifications of Al\({}_{2}\)O\({}_{3}\) differ significantly in their structural peculiarities, distribution of Al\({}^{3+}\) ions over tetra-(Td) and octa-(Oh) oxygen positions and, consequently, in their acidic properties. Thus, the choice of the structural modification of Al\({}^{3+}\) hydroxide can regulate not only the specific surface area (S\({}_{\rm Sp}\)) and texture of desiccants, but also their acidity, which can influence the sorption properties of desiccants. The acid-base properties of the surface of aluminium oxides obtained by thermal activation differ from the acid-base properties of homogeneous in their phase composition oxides obtained by precipitation. For example, the authors of [67] established that \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) obtained by thermal activation and subsequent autoclaving is characterised by a higher content and strength of Lewis acid sites and a lower content of basic sites than those of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) obtained by precipitation, approaching \(\eta\)-Al\({}_{2}\)O\({}_{3}\), obtained by precipitation. The authors of [68] found that, for oxides obtained by thermal activation and subsequent  
Figure 4: Stages of water vapour adsorption on aluminium oxide. hydration under mild conditions, \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) is characterised by a larger number and strength of Lewis acid sites than those of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) obtained by precipitation. \(\eta\)-Al\({}_{2}\)O\({}_{3}\), on the contrary, is characterised by a significantly higher surface basicity than that of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\). A relatively simple method which makes it possible to change the specific surface value, pore-size distribution, total pore volume, strength and concentration of active surface sites is the controlled heat treatment of AO [16]. Dehydrogenation and dehydroxylation of the surface, proceeding during thermal and chemical transformations of aluminium oxides, lead to the appearance of coordination-unsaturated oxygen atoms (Lewis base) and aluminium (Lewis acid). On the surface of the oxide, there are bridge and terminal hydroxyl groups that are Bronsted acid sites. During dehydration of two neighbouring OH- groups of this type, a water molecule is formed and released, and the Lewis acid (Al\({}^{3+}\)) and basic (O\({}^{2}\)) sites remain on the surface [61; 66]. Usually, the surface of aluminium oxide manifests strong acidity after calcination in vacuum at temperatures above 470 \({}^{\circ}\)C [69]. At temperatures up to 973 K, the number of Lewis sites increases with the decrease in the OH- group content. It has been found that the acidity of aluminium oxide is mainly determined by aprotonic sites, and Bronsted acidity is weakly expressed. Tendencies to increase the acidity and to decrease the basicity of the aluminium oxide surface with increasing temperature during its thermal activation were also noted by the authors of [16]. The comparison of aluminium oxides of various modifications has shown that the set of OH- groups for them is approximately the same [61], and OH- groups on the surface of aluminium oxide have different acidity. At the same time, the concentration of OH- groups significantly depends on the crystal structure and on the conditions of heat treatment of the initial hydroxides. According to the concepts available in the literature, the sorption characteristics of desiccants based on active AO can directly depend on the acid-base properties of AO and on the nature of its acid sites (acceptor/donor, weak/strong, Lewis/Bronsted) [59; 61].