# Efficient Adsorbent-Desiccant Based on Aluminium Oxide
## 2 Methods of Obtainment of Aluminium Oxide Used in Industry
### Methods of AO Otainment
#### 2.2.3 Rapid Calcination of Baeyer Hydrate (Gibbsite)
It has been found that during hydration of the CTA product of gibbsite in the temperature range of 75-80 \({}^{\circ}\)C, pseudoboehmite is predominantly formed in a slightly acidic medium (acid modulus for HNO\({}_{3}\) = 0.04), and bayerite is formed in an alkaline medium (solution: KOH, pH: 12-13, alkaline modulus: 0.1) [45; 46]. In this way, depending on the hydration conditions (temperature, electrolyte type, time, etc.) of the CTA product, aluminium hydroxides with different phase compositions can be formed. An important stage in the preparation of aluminium oxides which can dramatically influence its composition and characteristics is the calcination stage. Depending on the type of aluminium hydroxide at the heat treatment stage at temperatures \(<\) 600 \({}^{\circ}\)C, low-temperature aluminium oxides (mainly \(\eta\)-, \(\gamma\)- \(\chi\)-forms) are formed which have different properties. The sequence of technological stages for obtaining an adsorbent-desiccant using the CTA product is shown in Figure 3.  
It has been found that aluminium oxide, obtained from amorphized gibbsite (TCA and CTA products), has a higher adsorption activity with respect to water vapour than that of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) obtained by precipitation from pseudobehmite [45; 46]. The comparison of the adsorption properties of Al\({}_{2}\)O\({}_{3}\) samples obtained by the CTA method from a bayerite-containing product (Figure 3) and containing \(\eta\)-Al\({}_{2}\)O\({}_{3}\) has shown that they have a higher adsorption capacity with respect to water vapour than that of AO obtained from pseudobehmite and containing \(\gamma\)-Al\({}_{2}\)O\({}_{3}\). 2.4 Synthesis of Aluminium Oxide from the Basic Aluminium Salt Al\({}_{2}\)(OH)\({}_{5}\)Cl or from Alcoholates Al(OR)\({}_{3}\)  
Schemes for the synthesis of aluminium oxide from the main aluminium salt Al\({}_{2}\)(OH)\({}_{5}\)Cl or from alcoholates Al(OR)\({}_{3}\), obtained by dissolving aluminium in acid or by the reaction of direct interaction of aluminium with alcohol, respectively, are characterised by the use of a large number of aggressive and expensive reagents. They have a large number of stages and a high cost. In the Honeywell UOP (Universal Oil Products) process, the main aluminium salt, Al\({}_{2}\)(OH)\({}_{5}\)Cl obtained during the interaction of hydrochloric acid with metallic aluminium, is mixed with hexamethylenetetraamine ((CH\({}_{2}\))\({}_{6}\)N\({}_{4}\)). This solution is fed through the filter to the heated column, whereby balls are formed. These balls gelatinise during the decomposition of (CH\({}_{2}\))\({}_{6}\)N\({}_{4}\) according to the equation:  
\[(\text{CH}_{2})_{6}\text{N}_{4}+4\text{H}^{+}+6\text{H}_{2}\text{O}\to 6\text{CH}_{2} \text{O}+4\text{NH}_{4}{}^{+}.\]  
After removing the balls from the bottom of the column and crystallising them, a narrow size distribution of crystallites and, as a consequence, uniform pores are obtained [24]. The process of obtaining aluminium oxide from alcoholates is based on the Ziegler reaction [47], which was originally used for the production of higher linear alcohols. Using aluminium of standard quality, hydrogen and ethylene, a number of reactions can be performed:  
Synthesis of triethylaluminium:  
\[2\text{Al}+\text{H}_{2}+6\text{C}_{2}\text{H}_{4}\to 2\text{Al}(\text{C}_{2} \text{H}_{5})_{3}\]  
Figure 3: Technological stages of obtaining aluminium oxide adsorbents using the centrifugal thermal activation (CTA) method. Chain propagation:  
\[\text{Al}(\text{C}_{2}\text{H}_{5})_{3}+3\text{nC}_{2}\text{H}_{4}\to\text{Al}[( \text{C}_{2}\text{H}_{4})_{n}\text{C}_{2}\text{H}_{5}]_{3}\]  
Oxidation of trialkylaluminium:  
\[2\text{AlR}_{3}+3\text{O}_{2}\to 3\text{Al}(\text{OR})_{3}\]  
Hydrolysis of alcoholic to form alcohol and aluminium oxide:  
\[3\text{Al}(\text{OR})_{3}+3\text{H}_{2}\text{O}\to\text{Al}(\text{OH})_{3}+3 \text{ROH}\]  
In view of the high selectivity of the reaction, the impurities, contained in Al remain as insoluble mud, which is removed by filtration and centrifugation.