applied sciences Review Efficient Adsorbent-Desiccant Based on Aluminium Oxide Eugene P. Meshcheryakov 1, Sergey I. Reshetnikov 2, Mariya P. Sandu 3, Alexey S. Knyazev 4 and Irina A. Kurzina 1,* 1 Faculty of Chemistry, National Research Tomsk State University, 634050 Tomsk, Russia; [email protected] 2 Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, 630090 Novosibirsk, Russia; [email protected] 3 Department of Chemistry, Siberian State Medical University, 634050 Tomsk, Russia; [email protected] 4 LLC “Engineering Chemical Technological Center”, 634050 Tomsk, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-913-882-1028 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. Citation: Meshcheryakov, E.P.; Keywords: aluminium oxide; adsorbent; water vapour adsorption; material modification with ions; Reshetnikov, S.I.; Sandu, M.P.; adsorption activity Knyazev, A.S.; Kurzina, I.A. Efficient Adsorbent-Desiccant Based on Aluminium Oxide. Appl. Sci. 2021, 11, 2457. https://doi.org/10.3390/ 1. Introduction app11062457 Adsorbents-desiccants of the air are successfully applied in different fields of industry, particularly in mechanical engineering and for the elimination of excessive moisture in Academic Editor: Mohamed ◦ M. Chehimi amenity premises. As a rule, they are also used at a dewpoint for compressed air below 0 C. The adsorption method is applied in processes in which a high degree of air dehydration Received: 12 February 2021 is required (pharmaceutics, chemical and petrochemical plants, textile factories, food and Accepted: 5 March 2021 electronic industry, etc.) [1–3]. Published: 10 March 2021 The following requirements are imposed on industrial adsorbents-desiccants [4,5]: - Interaction processes between adsorbents and water vapours must be fast. Adsor- Publisher’s Note: MDPI stays neutral bents must have high absorption capacity, which will allow the gas to pass through with regard to jurisdictional claims in adsorbers at a high rate and use compact adsorption plants for dehydration. published maps and institutional affil- - Adsorbents must have gigh stability after multiple regenerations. iations. - Adsorbent grains must have high mechanical compression, (crushing) and abrasion strength. - Adsorbents must be inexpensive and easily regenerated. - Adsorbents must not react chemically during adsorption and regeneration. Copyright: © 2021 by the authors. To meet the abovementioned requirements for the efficiency of the adsorbent, it must Licensee MDPI, Basel, Switzerland. have the following characteristics: This article is an open access article distributed under the terms and - Large internal pore volume conditions of the Creative Commons - Large value of specific surface Attribution (CC BY) license (https:// - Controlled pore-size distribution, preferably in the micropore range creativecommons.org/licenses/by/ - Controlled properties of the surface, owing to selected functional groups 4.0/). Appl. Sci. 2021, 11, 2457. https://doi.org/10.3390/app11062457 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 23 - Large internal pore volume - Large value of specific surface - Controlled pore-size distribution, preferably in the micropore range Appl. Sci. 2021, 11, 2457 - Controlled properties of the surface, owing to selected functional2 ofgroups 23 - Weak interactions between an adsorbate and an adsorbent (in general, physical ad- sorption) - WeakMolec interactionsular sieves between (zeolites), an adsorbate silica andgels, an adsorbentand activated (in general, aluminium physical ad-oxide are usually usedsorption) as adsorbents-desiccants. These adsorbents have their own advantages and disad- vantagesMolecular [5–13 sieves], as (zeolites), shown in silica Figure gels, 1 and. activated aluminium oxide are usually used as adsorbents-desiccants. These adsorbents have their own advantages and disadvan- tages [5–13], as shown in Figure1. —high value, —low value Figure 1. Characteristics of the basic adsorbents-desiccants. Figure 1. Characteristics of the basic adsorbents-desiccants. Zeolites have sufficiently large adsorption capacity by water, but they are costly and difficultZeolites to regenerate. have Prolonged sufficiently time large and temperatures adsorption above capacity 340 ◦C by are water, required but for theirthey are costly and regeneration.difficult to Aregenerate. distinctive featureProlonged of zeolites time is and a high temperature adsorption rates above of water 340 vapour °C are required for (steep rise of isotherms) in the area of low concentrations of water vapour. The amount their regeneration. A distinctive feature of zeolites is a high adsorption rate of water va- of the absorbed water reaches a maximum value with a relative humidity equal to 20% andpour practically (steep remainsrise of constantisotherms) with ain subsequent the area increaseof low in concentrations the air humidity [14of]. water The vapour. The secondamount distinctive of the featureabsorbed of adsorption water reaches of water a vapour maximum on zeolites value is a with weak a dependence relative humidity equal ◦ ofto its 20% adsorption and practically capacity on remains the temperature. constant At aw temperatureith a subsequent of 100 C increase and a pressure in the air humidity ÷ of[14]. 10 mm The of second mercury, distinctive the adsorption feature capacity of ofadsorption zeolites reaches of water 15 16 vapour g/100 gon of thezeolites is a weak adsorbent. Even at 200 ◦C, it is still significant at—3.6 g/100 g of the adsorbent [14]. dependenceThe absorption of its capacity adsorption of silica capacity gel depends on onthe the temperature. temperature of At humid a temperature air and of 100 °C partialand a pressurepressure of vapour:of 10 mm When of themercury, temperature the adsorption increases and capacity the partial of pressure zeolites of reaches 15 ÷ 16 vapourg/100 g decreases, of the adsorbent. this capacity E decays.ven at It200 is expedient°C , it is still to use significant silica gel,having at—3.6 a lowg/100 g of the ad- ◦ regenerationsorbent [14]. temperature 150–200 C (see in Table1[ 5,14]), in desiccants of compressed air with hot regeneration at an adsorption temperature not higher than 35 ◦C and content of water vapourThe absorption in the dehumidified capacity air of not silica lower gel than depends 0.02 vol.%. on The the main temperature disadvantage of humid air and ofpartial silica gels pressure is low water of vapour: resistance W [14hen], which the temperature leads to their destruction increases when and in the contact partial pressure of withvapour water. decreases, this capacity decays. It is expedient to use silica gel, having a low re- generationThe advantages temperature of active 15 aluminium0–200 °C oxide (see (AO),in Table providing 1 [5,14]), its wide in desiccants application forof compressed air gaswith dehydration, hot regeneration are high adsorption at an adsorption capacity as temperature compared to zeolitesnot higher with highthan relative 35 °C and content of humidity; thermal stability; a possibility of multiple regeneration, which determines itswater good vapour indicators in under the dehumidified operation conditions; air not water lower resistance, than 0.02 especially vol.%. resistance The main to disadvantage condensedof silica gels moisture; is low and water relative resistance ease of production, [14], which as well leads as availability to their of destruction raw materials. 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 Appl. Sci. 2021, 11, 2457 3 of 23 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 (γ-, η-, χ-, δ-, θ-, κ-, α-, ρ-) and depend on the structure and properties of their precursors—aluminium hydroxides. Aluminium hydroxides also exist in the form of various modifications that differ in the chemical composition and crystal structure. Therefore, the thermal transformations of each aluminium hydroxide have their own peculiarities [16–22]. Depending on the preparation method, adsorbents can have different crystal structures and a wide range of pores [23,24]. Among the known series of aluminium oxide modifications, gamma (γ), eta