# Efficient Adsorbent-Desiccant Based on Aluminium Oxide
###### Abstract  
The review describes the main methods of obtaining hydroxides and aluminium oxides (AO) of various structures from gibbsite. The promising techniques of obtaining AO adsorbents are discussed, namely the technique of thermal activation in the mode of pneumatic transport with gibbsite by heated air (TCA Gb) and the technique of thermal activation of gibbsite in centrifugal flash reactors (CTA Gb). The main methods of improving the adsorbent properties of AO, such as the optimisation of texture characteristics and phase composition, as well as the influence of the modification of aluminium oxide adsorbents, obtained using CTA and TCA technologies with cations of alkaline metals, are considered. It is shown that the modification allows a controlled variation of the characteristics of donor and acceptor active sites on the surface of adsorbents and, thus, a substantial increase in their adsorption activity, in particular, with respect to water vapour. aluminium oxide; adsorbent; water vapour adsorption; material modification with ions; adsorption activity +
Footnote †: journal:  
Weak interactions between an adsorbate and an adsorbent (in general, physical adsorption)  
Molecular sieves (zeolites), silica gels, and activated aluminium oxide are usually used as adsorbents-desiccants. These adsorbents have their own advantages and disadvantages [5; 6; 7; 8; 9; 10; 11; 12; 13], as shown in Figure 1.  
Zeolites have sufficiently large adsorption capacity by water, but they are costly and difficult to regenerate. Prolonged time and temperatures above 340 \({}^{\circ}\)C are required for their regeneration. A distinctive feature of zeolites is a high adsorption rate of water vapour (steep rise of isotherms) in the area of low concentrations of water vapour. The amount of the absorbed water reaches a maximum value with a relative humidity equal to 20% and practically remains constant with a subsequent increase in the air humidity [14]. The second distinctive feature of adsorption of water vapour on zeolites is a weak dependence of its adsorption capacity on the temperature. At a temperature of 100 \({}^{\circ}\)C and a pressure of 10 mm of mercury, the adsorption capacity of zeolites reaches 15 \(\div\) 16 g/100 g of the adsorbent. Even at 200 \({}^{\circ}\)C, it is still significant at--3.6 g/100 g of the adsorbent [14]. The absorption capacity of silica gel depends on the temperature of humid air and partial pressure of vapour: When the temperature increases and the partial pressure of vapour decreases, this capacity decays. It is expedient to use silica gel, having a low regeneration temperature 150-200 \({}^{\circ}\)C (see in Table 1[5; 14]), in desiccants of compressed air with hot regeneration at an adsorption temperature not higher than 35 \({}^{\circ}\)C and content of water vapour in the dehumidified air not lower than 0.02 vol.%. The main disadvantage of silica gels is low water resistance [14], which leads to their destruction when in contact with water. The advantages of active aluminium oxide (AO), providing its wide application for gas dehydration, are high adsorption capacity as compared to zeolites with high relative humidity; thermal stability; a possibility of multiple regeneration, which determines its good indicators under operation conditions; water resistance, especially resistance to condensed moisture; and relative ease of production, as well as availability of raw materials. Figure 1: Characteristics of the basic adsorbents-desiccants. These are data based on the fact that granules of aluminium oxide are stronger than the granules of the majority of other desiccants, as they are not grained and abraded in rather harsh operating conditions of the pressure swing adsorption process [15]. The AO properties are determined by the structural peculiarities of its numerous polymorphic modifications (\(\gamma\)-, \(\eta\)-, \(\chi\)-, \(\delta\)-, \(\theta\)-, \(\kappa\)-, \(\alpha\)-, \(\rho\)-) and depend on the structure and properties of their precursors--aluminium hydroxides.