# Efficient Adsorbent-Desiccant Based on Aluminium Oxide
## 4 Industrial Adsorbents-Desiccants and Their Characteristics
### Aluminium Oxide Surface Characteristics  
Active sites and hydroxyls on the AO surface largely determine its physicochemical and adsorption properties. The increased chemical activity and active adsorption sites on the surface appear as a result of the formation of a defective structure during the transition from the amorphous to crystalline state during thermal dehydration [8]. The interaction of water vapour with the AO surface is complex and proceeds under the influence of several different types of forces. Water adsorption by aluminium oxide desiccants is the result of three processes (Figure 4). The first is chemisorption or dissociative adsorption of water molecules on the active sites of the aluminium oxide surface, which leads to the formation of the first layer. The second is physical adsorption, leading to the formation of multilayers due to hydrogen bonds. The third is capillary condensation in pores [8; 14; 56]. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Typical Properties** & **F 200 \({}^{\circ}\)BASF\({}^{\circ}\)**[58] & **AsAsorb 2-5 D \({}^{\circ}\)Axens\({}^{\circ}\)** & **F-1** & **Alasorb 675** & **AO** \\  & & [56] & \({}^{\bullet}\)**Alca\({}^{\circ}\)**[57] & **Salavatski Catalytic Factory** & **Novomichurinski Catalytic Factory** \\ \hline Grain size, mm & 4.7 & 2.0–5.0 & 4.0–8.0 & 7–12 & 2.0–5.0 & 2.8–8.0 \\ \hline Specific surface & & & & & & \\ area, m\({}^{2}\)/g & & & & & & \\ \hline Pore volume, mL/g & 0.50 & 0.44 & 0.44 & 0.40 & 0.60 & 0.54 \\ \hline Statistic capacity & & & & & & \\ with 60\% of relative & 21.0 & 21.5 & 21.0 & - & 21.0 & 11.0 \\ humidity, \% & & & & & & \\ \hline Packed bulk density, & 0.80 & 0.82 & 0.79 & 0.85 & 0.70 & 0.71 \\ \hline \end{tabular}
\end{table} 
Table 2: Characteristics of some industrial adsorbents based on aluminium oxide. The efficiency of adsorbents and their static and dynamic capacity depend on the structural modification of aluminium oxide (\(\eta\)-Al\({}_{2}\)O\({}_{3}\), \(\gamma\)-Al\({}_{2}\)O\({}_{3}\), \(\chi\)-Al\({}_{2}\)O\({}_{3}\)), as well as the preparation conditions that determine the donor-acceptor properties of the oxide surface and their textural characteristics: Specific surface, pore volume and size distribution [46; 57; 59]. In addition, the adsorption properties also depend on the size and shape of the granules [14; 60]. According to modern concepts, the surface of a solid body represents a combination of Lewis and Bronsted sites of the acid and basic type [61]. The surface of aluminium oxide is generally amphoteric. Oxygen atoms are strong basic sites, and aluminium ions act as strong Lewis sites. Four types of Lewis acid sites have been identified, differing in the structure of the second coordination sphere of aluminium [62]. As for the basic Lewis sites, two sites are most clearly shown: Strong and weak sites, associated with the bridging oxygen atoms of Al-O-Al and oxygen atoms of OH groups, respectively. A comparison of the Bronsted acidity and basicity of OH groups conducted by the authors of [62] showed that AO hydroxyls are more basic than acidic. Using spin probes, it has been shown that aluminium oxides have electron-acceptor and electron-donator sites on their surface that are capable of reducing (oxidizing) the molecules adsorbed on them due to the transfer of a single electron (Single Electron Transfer (SET) mechanism) [63; 64; 65]. It is logical to assume that changes in the concentration or strength of the abovementioned sites can influence the physicochemical properties of the surface. The authors of [66] showed that the number of Lewis acid sites (LAS), determined from the FT-IR spectra after pyridine adsorption, decreases in the series of \(\eta\)-Al\({}_{2}\)O\({}_{3}\)\(>\)\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)\(>\) (\(\chi+\gamma\))-Al\({}_{2}\)O\({}_{3}\) and correlates well with the catalytic activity in the reaction of methanol dehydration into dimethyl ether.