Nanoporous Materials with Enhanced Hydrophilicity and High Water Capacity

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1 Universiti Sains Malaysia From the SelectedWorks of Eng-Poh Ng September 1, 28 Nanoporous Materials with Enhanced Hydrophilicity and High Water Capacity Eng-Poh Ng, University of Caen, France Svetlana Mintova, University of Caen, France Available at:

2 Available online at Microporous and Mesoporous Materials 114 (28) 1 26 Review Nanoporous materials with enhanced hydrophilicity and high water sorption capacity Eng-Poh Ng, Svetlana Mintova * Laboratoire de Matériaux à Porosité Contrôlée, UMR-716 CNRS, ENSCMu, Université de Haute Alsace, 3 rue Alfred Werner, 6893 Mulhouse, France Received 24 July 27; received in revised form 7 December 27; accepted 18 December 27 Available online 28 December 27 Abstract The main types of nanoporous adsorbents for water are identified and described with emphasis on the mechanism of adsorption, modification, improvement of the water sorption capacity, possible regeneration, and stabilization. Among the three main groups of water sorbents, i.e. inorganic materials (zeolites, clays, silica), carbon based materials and organic polymers, the first one is described in details. The significance of their porosity, chemical and structural features relative to the water adsorptive properties of each inorganic type materials is reviewed. Important features for silicates, zeolites, aluminophosphates, mesoporous materials and clays are described, which define the interactions between the water and the porous structures. The significant improvements of the water sorption capacity of nanoporous materials under modification of their surface, particle size, morphology, and chemical specificity are explored for each type of inorganic porous compounds. The hydrophilic nanoporous materials are of significant importance for construction of regularly operating sorption equipments, reversible physi- or chemisorption reactors for generation of heat and cold, purification of lubricants, and sensing of water with different concentrations. Ó 27 Elsevier Inc. All rights reserved. Keywords: Water sorption; Nanoporous sorbents; Water capacity; Zeolites, Clays; lica Contents 1. Introduction Definition of hydrophilic materials Measurements of water: characterization techniques Inorganic nanoporous materials as water sorbents lica gels Modification and properties of silica gels lica aerogels Modification and properties of silica aerogels Microporous molecular sieves Zeolites Aluminophosphates and titanosilicates Mesoporous materials Clays and metal oxide pillared interlayer clays (PILCs) * Corresponding author. address: svetlana.mintova@cup.uni-muenchen.de (S. Mintova) /$ - see front matter Ó 27 Elsevier Inc. All rights reserved. doi:1.116/j.micromeso

3 2 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Evaluation of nanoporous sorbents and potential applications Summary and outlook Acknowledgment References Introduction The demands for water and protection of the entire ecosystems is an important issues credited to the entire world s population. n one side, the development of new materials with advanced properties such as high specific surface area, defined degree of hydrophobicity/hydrophilicity, open pore systems, thermal and hydrothermal stability, given monomodal particles is an ongoing task for many environmentally friendly chemical processes. n the other side, the demand for screening of traces of water and controlling the humidity in the air is the other goal. Water sorption behavior in nanoporous materials plays an important role in manufacturing and designing of advanced materials and devices thereafter. The most studied adsorbents for water are microporous materials (zeolites), activated carbon, and silica gels. In real situations, the nanoporous materials exist in a hydrated form and the water in the pores governs their properties. The water sorption behavior of a sorbent depends on many factors such as the structure and the chemical composition of the nanoporous material (e.g. /Al ratio for aluminosilicates), the presence of charged species, type of framework structure, and hydration level. Besides, reversible water adsorption on nanoporous materials constitutes the basis of several technological processes, where different parameters are under precise control. Examples for required reversible water adsorption can be found in the fields of gas separation and purification, gas sensing, atmosphere pollution control, gas storage, etc. In many cases, humidity control by adsorption meets much lower dew points and is less energy demanding compare to compression and condensation methods. Traditionally, highly hygroscopic salts such as LiBr [1 5], LiCl [6,7], KBr, CaCl 2 [1,8] and MgCl 2 [9] are used for humidity control via adsorption of water. However, a crystallization process tends to happen when the salts are used at high water concentrations [5,1,11]. In addition, the high solubility of these salts in water at high humidity limits their application at certain conditions [1,7]. The search for alternative adsorbent other than salts in water sorption to avoid the risk of salt crystallization, led to the evaluation of alkaline hydroxides (NaH KH CsH) as one possible material [5,1,11]. Nonetheless, a partial or total dissolution of these solids occurs if they are exposed at higher than the relative humidity atmosphere, thus resulting in leaking and corrosion problems especially at high temperatures [2]. The demand for controlling the humidity and development of more efficient sorbent technology enhances considerably the interest in new nanoporous materials. There are numbers of commercially available water sorbents used for specific sorption processes. The development of environmentally friendly technologies for sorption of water at different conditions coupled with the preparation of advanced materials with improved sorption properties, recycling possibilities and long-term use are enduring tasks. In general, the efficiency of hygroscopic materials in adsorbing water depends on two factors, i.e. the amount and the type of sorbents in use. Up to date, several water sorbents have been discovered, and they are classified in three main categories: first: inorganic materials (zeolites, clays and silica), second: carbon based adsorbents (activated carbons, graphite, carbon molecular sieves, and pre-shaped carbon fibres and nanotubes), and third: organic polymers. Numerous studies related to the adsorption phenomenon of water on polymers and carbon based materials have been carried out, however they will not be described in this paper. The focus of this review is on the current development of inorganic porous sorbents, and the mechanism of water sorption. Modification methods for enhancement of the sorbent hydrophilicity and water sorption capacity will be discussed for each individual category of sorbents. Additionally, the main techniques for water sorption analysis will be presented, and finally, an outlook and perspectives in the development of advanced nanoporous materials as hydrophilic adsorbents will be reviewed. 2. Definition of hydrophilic materials According to the definition, a hydrophilic material is a substance, which has high affinity to water (water-loving) [12]. In reality, the description of hydrophilicity is under continuous debate since it is general and ambiguous when is applied to complex systems. Hydrophilic solids are described as polar substances such as zeolites and silica gels, but in some cases molecular sieves with neutral framework (aluminophosphates) are ascribed as hydrophilic compounds as well [13]. Apart from that, the mesoporous silicas (e.g. MCM-41, SBA-15, FSM-16, etc.) and some of the carbon based materials, which are classified as hydrophobic supplies show even higher water sorption capacity than zeolites and silica gels [14]. It is also found that some of the above materials have high affinity to both water and organics. Conversely, the hydrophilicity and water sorption capacity of porous sorbents are often discussed independently. A hydrophilic material is not necessary to have a high water sorption capacity, whereby the water sorption capacity is mainly determined from the pore volume of the materials. In fact, the hydrophilicity of sorbents is defined based on its selectivity to polar sorbate (water) compare to other sorbates at particular relative pressure.

4 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) According to lson et al. [15], the hydrophobicity is defined as low affinity of a sorbent to water. The slope of the water sorption isotherm at zero loading was proposed to be used as an indicator for hydrophobicity classification. Using this definition, the sorbent which shows a steep slope in water sorption isotherm is defined as hydrophilic sorbent. However, this definition is often misleading since different types of isotherm might have a similar slope at the origin. Therefore, a clearer definition of hydrophilicity/hydrophobicity is given by Anderson and Klinowski [16] by introducing the term hydrophobicity index (HI). The value for HI is calculated based on the loss of water at 15 C over weight loss at 4 C determined by thermogravimetric analysis: Hydrophobicity index; HI ¼ Weight loss up to 15 C Weight loss up to 4 C when HI = 1, this is representative for a very hydrophobic sample, and when HI =, this is characteristic for a very hydrophilic sample. This model, however, does not always accurately characterize the high silica zeolites [17,18]. Also, while the temperatures of 15 C and 4 C seem to be somewhat arbitrary, the consistent use of those limits should provide an indication for water loosely adsorbed compared to water adsorbed more tenaciously. Later, Weitkamp et al. [18] quantified the hydrophobicity of the materials on the basis of a competitive sorption of a hydrocarbon/water mixture. The HI is given by Hydrophobicity index; HI ¼ X Hydrocarbon X Water where X is the amount of probe molecules sorbed. ne of the advantages of this definition is that it provides a relative measure of the hydrophobicity of the sorbents when a single organic molecule is consistently in use. However, the sorption in some systems may depend on the type of probe molecules introduced into the pores. Thus, the results obtained may not reflect the equilibrium sorption capacities. Another definition of hydrophobicity was proposed by Giaya et al. [17]. The HI is calculated according to the formula: Hydrophobicity index; HI ¼ V t V >15 C V t where V t is the total pore volume of the sorbent, and V >15 C is the volume of water desorbed at temperature above 15 C. According to this definition, sorbents having HI = 1 are classified as a very hydrophobic material, whereas HI = is a characteristic of a very hydrophilic ones. This model provides a broader and more universal comparison between materials such as zeolites, activated carbons, silica gels, etc. Unfortunately, this model is not applicable to sorbents such as AlPs that release water at temperature lower than 15 C. The hydrophilicity of any sorbent is quantitatively and qualitatively classified according to IUPAC based on the type of the sorption isotherms. Basically, the water sorption isotherms are divided in seven types (Fig. 1). The isotherm type I represents a materials with high water sorption capacity and very fast reaction saturation at low partial pressure (P/P ), followed by consistent adsorption over a wide range of P/P due to the water saturation in the pores. These sorbents are classified as very hydrophilic due to high affinity to water even at low P/P. n the other hand, hydrophilic materials exhibit sorption isotherms type II or IV, whereby a considerably high sorption capacity of water at low P/P and moderate P/P is measured. Additionally, the rare type VI step-like isotherm is also characteristic of hydrophilic sorbent having stepwise sorption. In contrast, sorption isotherms types III and V describe hydrophobic or low hydrophilic materials with low sorption at low P/P and sometimes moderate sorption at the middle P/P, and suddenly high water sorption at P/P in close proximity to 1. For highly hydrophobic sorbents, Hydrophilic materials V V V V I P/P o II P/P o IV P/P o VI P/P o Hydrophobic materials V V V III P/P o V P/P o Vll P/P o Fig. 1. Adsorption isotherms classified according to IUPAC: type I: very hydrophilic material, type II: hydrophilic material, type III: hydrophobic/ low hydrophilic material with weak sorbent water interactions, type IV: hydrophilic material, type V: hydrophobic/low hydrophilic material with weak sorbent water interactions, type VI: hydrophilic material with multiple sorbent water interactions and stepwise sorption, type VII: very hydrophobic material.

5 4 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 Amount of water sorbed Strongly hydrophilic (a) (b) (c) Strongly hydrophobic (d) Weight loss / wt% Water capacity = Amount of water desorbed Mass of dry adsorbent 25 g 75 g Temperature / o C Fig. 3. A thermogram of a sorbent illustrating the water loss under continuous heating. = =.33 g/g P/P o Fig. 2. Water sorption isotherms of four nanoporous solids with different degrees of hydrophilicity. isotherm type VII is attributed, which gives a low sorption of water throughout the entire P/P range, and the finite water sorption capacity less than the available pore volume typifies the sorption characteristic of this material. Materials with the same pore volume but different degrees of hydrophilicity, i.e. containing different concentrations of hydrophilic sites are shown in Fig. 2. The adsorption curves display: (a) type I isotherm which is revealing that the solid adsorbs high amount of water at very low P/P until sorption equilibrium is reached, (b) type I isotherm as well but the equilibrium is reached at higher P/P, and (c) type V sorption isotherm. By comparing the curves (a) and (b), it can be concluded that the sample (a) is more hydrophilic than sample (b) since the slope of the curve is steeper, which means adsorbing more water and faster than for (b) especially at low P/P. Further, sample (c) displays type V isotherm, demonstrating that the sample does not or adsorb small amount of water at low P/P until the sorption capacity increases suddenly, i.e. high water sorption resulting from water clusters formation, followed by creation of water monolayer covering the surface at medium P/P. Therefore, this sample can be classified as a hydrophobic or weakly hydrophilic material. n the other hand, sample (d) demonstrates type VII isotherm via adsorbing only traces of water even at high P/P, and hence is judged as a strongly hydrophobic material. In general, there are no ideal hydrophobic solids because their surfaces contain some hydrophilic sorption sites arising from defects, accidental impurities or purposely added components [19 21]. Nevertheless, Gao and McCarthy recently reported on a perfectly hydrophobic fluoro-organic materials in which the ideal hydrophobicity is derived from the fully substitution of fluorine atoms in the oligomers [22]. Furthermore, there are also sorbents, which are sensitive to both polar (e.g. water) and non-polar compounds (e.g. benzene and cyclohexane) [23]. These materials are not selective in mixtures containing multi-compounds with different polarity. In this case, the sorbents are having lyophilic property instead of purely hydrophilic or hydrophobic. The latest observation has to be taken into account for the selection of sorbents especially for separation processes with multi-component systems. 3. Measurements of water: characterization techniques Information on the amount, type and structural characteristics of water sorbed in different solids is essential for deriving the sorptive properties of a desired sorbent com- Table 1 Methods for measuring of water content Method Principle of measurement ven-drying Heating of sample and gravimetric measurement of weight loss Thermogravimetry (TG) Measurement of weight loss under controlled temperature change Sorptometry desiccators method Measurement of mass change under variation of (partial) water vapour pressure Standard contact porometry Measurement of mass change under contact with reference sample at defined humidity Infrared (IR) spectroscopy Measurement of H absorption band in the IR spectra (164, 343 or 52 cm 1 ) Nuclear magnetic resonance (NMR) spectroscopy Measurement of the intensity of hydroxyl and molecular water signals in 1 H NMR spectra at low temperature (chemical shift = 4 6 ppm) Calorimetric method Measurement of heat capacity Karl Fischer titration Titration using Karl Fischer reagent Moisture indicator Measurement of color changes (qualitative test)

6 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Absorbance / a.u Wavenumber / cm -1 Fig. 4. Infrared spectra of a hydrophilic sorbent loaded with different amount of water: H stretching bands at 343 and 3618 cm 1, and bending absorption bands at 164 cm 1. From bottom to top:.4,.9,.12,.14,.16,.18,.2,.26 and.31 g/g, respectively. pound. This information is of high importance as allows further development of preferred sorbents for diverse processes. In the past few decades, several qualitative and quantitative techniques were developed and employed for measuring of water content (Table 1). The oven drying and thermogravimetry (TG) are the most popular methods for determination of water content in the sorbents (Fig. 3) [24,25], whereby the water capacity is determined by dividing the weight loss due to water desorption with the weight of a dry sorbent. Additionally, the standard contact porometry and sorptometry are applied to determine the water capacity based on the mass change of the sorbents under constant water vapour pressure/humidity [26]. Additionally, the standard contact porometry and sorptometry are applied to determine the water sorption capacity based on the mass change of the sorbents under constant water vapor pressure/humidity [26]. Spectroscopic techniques especially infrared (IR) have been proven to be effective for quantitative measurements of water [24,26 28]. ne of the advantages of using IR spectroscopy is that the types of water molecules sorbed can be identified by measuring the frequency of the respective IR peaks, and the amount of water sorbed is measured based on the intensity of absorption band (Fig. 4). Apart from that, water determination by NMR spectroscopy can be carried out by measuring the intensity of signals assigned to H groups and molecular water from static 1 H NMR spectra at low temperature (see Table 1) [27]. Calorimetric approach is applied for the water determination, where the adsorption of water vapor is an exothermic process. Therefore, the surface temperature of the sample is increased during the adsorption process due to the release of heat of adsorption. Based on this principle, calorimetric approach is applied for the water determination. By measuring the temperature or energy change, the amount of water sorbed is calculated from the Clapeyron equation [25,29,3]. Another technique for water determination is based on the Karl Fischer (KF) titration, which is considered as an important and standard method for evaluating the amount of water [27,31,32]. It is simple, fast, and reproducible, and no standards are necessary for the measurements. The volumetric and coulometric KF titration is based on the following two-step reaction: Step 1: S 2 þ R H þ B R S 3 þ HBþ Step 2: R S 3 þ H 2 þ I 2 þ 2B! R S 4 þ 2HBþ þ 2I B Base; R Alkyl In the first step, an alkylsulfate anion is formed through sulfonation reaction. In the second step, the oxidation of alkylsulfate anion by iodine which is requiring of water is observed, whereby the water content is stoichiometrically calculated based on the consumption of the iodine. KF titration requires a direct contact between the water and the reagent, therefore, dissolution of the sorbents in liquids or heating of sorbents to release and direct the water vapor to titration solution is necessary prior to the measurements. Qualitative measurements of water are also performed using moisture indicator through observing color changes. The most common moisture indicators are copper(ii) sulfate, cobalt(ii) chloride and potassium lead iodide in which change of colors upon adsorption of water is observed [33,34]. However, the moisture indicator is a spot local analysis and is not providing reliable information on the quantity of water sorbed. 4. Inorganic nanoporous materials as water sorbents The syntheses of inorganic sorbents gain a lot of attention during the last few decades due to the needs of materials with high sorption capacity and high selectivity toward water at different concentrations. The most common

7 6 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 compounds are silica and alumina based materials such as silica gels, aerogels, zeolites, zeotypes (aluminophosphates and titanosilicates), mesoporous materials and clays lica gels lica gel with the general formula 2 xh 2 isthe mostly studied among the silica-based materials for the purpose of water sorption. Essentially, the silica gel is synthesized by two methods. The first approach is based on polymerization of silicic acid, (H 4 ), and the second on aggregation of colloidal silica particles [35]. licic acid is prepared via hydrolysis of silicon alkoxides, which is unstable and spontaneously polymerize and form siloxane ( ) network. While polymerization is taking place, some of the silanol ( H) groups are not reacting and leave some uncondensed silanol ( H) contributing to the hydrophilicity of silica gels. The synthesis route is briefly described as follow: Step 1: Formation of silicic acid from the hydrolysis of silicon alkoxide and water. R R R R H + 4 H 2 H + 4 RH H H Step 2: Formation of siloxane network from condensation, hydrolysis and polymerization processes. Fig. 5. A surface silanol group interacting with three water molecules via hydrogen bonding (dash line) Modification and properties of silica gels lica gels are widely studied as hydrophilic compounds due to the high affinity to water vapor, large water sorption capacity at low humidity, cheap and easy regeneration (15 C) [36,37]. Basically, the selectivity of silica gel towards polar compounds (e.g. water) is based on its pore structure and the distribution of silanol ( H) groups [36]. The amount of silanol groups in the silica gels is a key parameter that determines the amount of sorbed water and degree of surface modification. Water molecules are attracted and sorbed on the surface of silica via formation of hydrogen bonding with the hydroxyl of the silanol groups ( H). Principally, one silanol group H H H H + H H H H H H H H + 2H 2 H H H H + R R R R H H R R + 2RH The second method, which is a coagulation of silica particles due to van der Waals forces is resulting in cations bridging. Commercially, the silica hydrosol is produced through polymerization of silicon source (e.g. sodium silicate) with a mineral acid (e.g. sulphuric or hydrochloric acids). When the polymerization is completed, white jelly is formed named silica gel, and then is subjected to purification and activation. Thus the silica gels have different surface areas, pore volumes, and particle sizes, which are depending on the synthesis conditions, silica concentration, temperature, ph and activation steps [35]. is able to adsorb three water molecules through a hydrogen atom, and the lone pairs of electrons of oxygen from the silanol group (Fig. 5). Three common types of silanol groups are observed on the surface of silicates, namely H-bonded, isolated, and germinal types; all the three silanol groups are formed during the preparation of silica gel and presented in Fig. 6. The subsequent drying process directs the formation of xerogel through polymerization and condensation of silicic acid, (H) 4 with some silanol groups on the surface. Additionally, silanol groups are created through hydrolysis when the siloxane ( ) chains are exposed to water at ambient conditions. Although the pure silica gels are the most common sorbents for water, additional modification improves the sorp-

8 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Fig. 6. Three types of surface silanol groups accessible on silica materials. tion capacities substantially. The modifications of silica gels can be performed by several approaches, where the effect of grain size, sorption temperature and pressures on the water sorption is followed [38,39]. The sorption of water at low temperature enhanced efficiently the water uptake. It is due to the low mobility and inability of water molecules to desorb from the silanol active sites, thus leading to lower water desorption and high water uptake. Also high water sorption capacity of the silica gel can be achieved through reducing the grain size, whereby the small size of the substances leads to the enhancement of the surface area and pore volume. Several attempts are made to reduce the pore size of silica gels since it plays a significant role in water sorption processes. A small pore diameter of the silica gel is beneficial for improving sorptivity through increment of hydrophilicity and water sorption capacity [36,4]. It is known that sorbents with smaller pore diameter has a higher active sites per surface area than those having larger pore diameter. Thus, the water molecules tend to be attracted and sorbed faster on the active sites, and reach equilibrium more rapidly. In respect to this, Ito et al. [36] reported on the preparation of silica gel with smaller pores through decreasing the solubility of silica during the polymerization process. The principle is that the micropore formation is strongly dependent on the size of the primary particles, which grow during the polymerization and drying of the mono-silicate. The shrinkage of the three-dimensional network formed after polymerization and drying of mono-silicates is achieved, and thus the silica gel will have smaller micropores. Another strategy for reducing the pore diameter of silica gel is based on direct introduction of aluminum ions during the synthesis process [36]. Aluminum ions act as a growth inhibitor of primary particles and reduce the pore diameters of silica gels. Thus the silica gel after modification has higher hydrophilicity, increased sorptivity even at low P/P and more stable sorption characteristics. The relation between the pore size and sorption capacity of materials has been studied comprehensively over the past few years [4,41]. Dawoud et al. [4] explored the role of pore size of silica gels by studying sequentially the sorption capacity of commercial samples towards water vapor. It was proved that the water loading of microporous silica is twice more than in mesoporous, which is explained by an increment of pore volume when the pore size is being reduced (see Fig. 7). This phenomenon is supported by the work of Chua et al. [41] demonstrating that the pore volume and the surface area have a key role in achieving a high water adsorption uptake in the nanoporous materials. It is well known, that the hygroscopic salts have greater water sorption capacity than silica gel and they can be used as water sorbents. However, their performance declines when equilibrium is achieved after the formation of solid crystalline hydrates. Based, several modification strategies for silica gels are developed by combining the advantages of hygroscopic salts with those of the basic silica gels. These strategies resulted in development of hybrid or Water Loading (g/1 g) Time (s) Microporous silica Mesoporous silica Fig. 7. A comparison between the sorption kinetics of water vapor on microporous (dashed) and mesoporous (line) silica at sorption temperature of 35 C and starting pressure of 4 mbar.

9 8 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 Table 2 Water sorption capacity of silica and alumina based materials Adsorbent Preparation Pore diameter, nm BET, m 2 /g H 2 capacity, g/g a Reference lica aerogel 1% 2 with C 2 supercritical drying 1 2 and [55] Alumina aerogel 1% Al 2 3 with C 2 supercritical drying ns [55] Mixed 2 Al 2 3 7% 2 3% Al 2 3 with C 2 supercritical drying 1 15 and [55] LiBr/ 2 aerogel Sol gel method (28.6% LiBr) [49] LiBr/densified 2 aerogel Dry treatment and sol gel method (28.6% LiBr) [49] CaCl 2 / 2 aerogel Sol gel method (29% CaCl 2 ) [48] CaCl 2 / 2 xerogel Sol gel method (28.6% CaCl 2 ) [48] lica gel type A Commercial, particle size of mm [41] lica gel type RD Commercial, particle size of mm [41] CaCl 2 /silica gel Impregnation with saturated CaCl 2 (33.7 wt.%) ns 35.8 [42] CaCl 2 /silica gel Impregnation with CaCl 2 (24 wt.%) [5] ns = no specified. a Measurements were performed at 25 C and P/P = 1.. composite materials by impregnation [42 47] or sol gel [48,49] methods followed by doping with hygroscopic salts (chlorides, bromides, sulfates and nitrates of alkali and alkali-earth metals). The LiBr and CaCl 2 silica gel composites were found to have higher water sorption capacity than both the pure hygroscopic salts and silica gel [43,46,5]. By varying the amount of the salt inside the pores, the water sorption capacity was improved too. Subsequent work by Levitskij et al. [42] demonstrated that the silica gel impregnated with saturated CaCl 2 6H 2 successfully increased the water sorption capacity up to 8 wt.%. Moreover, the concentration of anions (bromide, chloride) plays a role in the water uptake of silica composites [49]. Among the modified silica gels, the composites with high bromide content have superior water sorption capacity (.8 g/g at high P/P ), which is significantly higher than the conventional silica gels (see Table 2). The development of hydrophilic sorbents by introduction of transition metal salts in porous hosts has been conducted. The advantage of transition metal salts in comparison with alkali and alkali-earth metal salts is the ability to change colors upon moisture, and this property is used for qualitative estimation of water sorption capacity of the materials. The transition metal salts can be dispersed in silica gel by (a) mixing a salt with the silica gel, (b) impregnating the silica gel with salt solutions, filtration, washing and drying, (c) mixing of water glass, salt and sulfuric acid, cooking under pressure, filtration and washing, or (d) impregnating the silica hydrogel with salt solutions, decantation and drying. In the latest cases, the dispersed transition metal salt on the surface of the silica gels function as humidity indicator, whereby the colour is changed upon moisture due to hydration of the transition metals [33] lica aerogels lica aerogel is a hydrophilic and extremely porous amorphous silica materials consisting of aggregated 2 networks (Fig. 8). The aerogels are commonly used as superthermal insulators, catalyst supports, dielectric materials and sorbents for organics or water vapors [51,52]. The synthesis of silica aerogel involves two major steps: (1) preparation of an alcogel via sol gel process, and (2) Fig. 8. Structure of silica aerogel consisting of nanoporous 2 network.

10 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) supercritical drying for removing of solvents from the final product. For the preparation of silica aerogels, the silicon source (tetraethylorthosilicate or sodium silicate solution) is hydrolyzed with water under acidic conditions prior conversion into alcogel by solvent exchange. The supercritical drying is performed at high temperature and pressure for improving the texture properties of the silica aerogels. During the supercritical drying, the alcohol (usually ethanol) is removed resulting in the formation of extremely porous inorganic matrix with high surface area and highly hydrophilic surface with a large amount of silanol groups [53] Modification and properties of silica aerogels milarly to the silica gel, the hydrophilicity of the aerogel is attributed to the presence of surface silanol groups by which the water molecules are attracted and sorbed. Therefore, the degree of hydrophilicity of silica aerogel is also depended on the concentration of surface silanol groups. However, the pure siliceous aerogels face difficulties in the applications as water sorbent. The major obstacle is the collapse of the structure during exposure of the aerogels to moisture even at ambient conditions. During the water sorption, the cleavages of siloxane ( ) bridge of silica aerogels through hydrolysis by moisture from the air tend to occur and diminish the water sorption performance [54]. This problem can be overcome by surface modification of the silica aerogels resulted in the high hydrostabilization. With respect to this, Bialon et al. prepared CaCl 2 / silica aerogel hybrid porous materials via sol gel route involving two-step hydrolysis and condensation of tetraethoxysilane [48]. The chemically treated aerogels have improved water sorption performance, and they were in use for 5 cycles without deterioration. Later, LiBr/silica aerogels using the same method were prepared, but they exhibit a lower water sorption capacity and poorer adsorption performance during several sorption/desorption cycles [49]. Thus, this composite is less attractive than the CaCl 2 / silica aerogel composite. The mixing of alumina with silica aerogels is also a promising approach for improving their hydrostability. The synthesis of alumina and mixed silica alumina aerogels carried out using sol gel and supercritical C 2 drying methods has been reported by Knez and Novak [55]. Both modified aerogels demonstrate increased surface hydrophilicity and have excellent water sorption capacity (Table 2). Moreover, the mixed silica alumina aerogels were hydrostable and resistant to moisture even after 1 runs. Thus, by introduction of hetero-atom such as Al, a significant stabilization of the 2 networks in the structure and enhancement of the hydrostability of the aerogels are achieved Microporous molecular sieves Zeolites are a class of crystalline aluminosilicate materials built of 4 and Al 4 tetrahedra; via connecting the 4 and Al 4 tetrahedra in networks the formation of different zeolites structures is achievable. Up to dates, numerous types of synthetic zeolites with variable framework type structures with one-, two- and three-dimensional pore systems have been synthesized. As examples, several zeolites based on sodalite (SD) cage as secondary building units are shown in Fig. 9. Fig. 9. Framework structures of zeolites based on sodalite cage as a secondary building block.

11 1 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 In addition to the classical applications in ion-exchange, separation and catalysis, zeolites are attractive for novel technologies including highly selective membranes, host guest chemistry, chemical sensors, etc. Up to date, about 17 types of zeolites have been discovered [56], and some of them are found in the nature (about 4 types). Synthetically, the zeolites are prepared by hydrothermal treatment starting from aluminosilicate gels or solutions. The uses of organic molecules as template or structure directing agent changed the crystallization systems considerably, thus leading to the preparation of zeolite-like materials such as pure silicates (MFI, MEL, BEA, etc.), aluminophosphates (AEI, AFI, etc.) and titanosilicates (TS-1, ETS-4, etc.) [57,58]. The zeolites are classified in four groups based on their pore size: ultra-large (more than 14-membered ring), large (12-membered ring), medium (1-membered ring) and small (eight-membered ring) pore molecular sieves. In general, the zeolites with medium and large pores are mostly use in catalysis in order to ensure the easy diffusion of molecules and reaching the catalytic active sites in the zeolite pores. n the other hand, those with high concentration of cation exchange sites and small pores are suitable for sorption processes due to the molecular sieving effect. Furthermore, the materials with precise pore shapes are applied in systems where molecular recognition is needed such as shape-selective catalysis, selective adsorption, separation processes, chemical sensors and nanotechnology Zeolites Zeolites are highly hydrophilic sorbents due to their electrostatic charged framework and the abundance of extra-framework cations. Almost all of the zeolites (especially high Al containing) show type I water sorption isotherm, which indicate the high affinity to water at low partial pressure. The most common zeolites and the water sorption capacity respectively are summarized in Table 3. As can be seen, the water sorption capacity generally is proportional to the size of the pores, whereas large pore aluminosilicate zeolites (in Na + form) such as ZSM-2 (FAU/EM intermediate) with 12-membered ring have the highest capacity for water. As disadvantage of zeolites, the dehydration at high temperatures (>2 C) can be considered, which is due to the strong interaction between the electrostatic charged frameworks with the water molecules. The hydrophilicity of zeolites is depending not only on the framework type and extra-framework cations, but also several factors such as defect sites, surface nature, metals in the framework, and amount of coke deposits have to be taken into account. Aluminium-rich zeolites are usually used as desiccants. It is due to their high concentration of hydrophilic active sites, which can enhance the water sorption capacity and hydrophilicity. Indeed, the extra-framework cations and their distinctive interactions with the water molecules determine the sorption ability of the zeolites. The extra-framework cations, which balance the negative charge of the zeolite framework, are mobile and exchangeable. Some of the cations can Table 3 Water sorption capacity of the most common molecular sieves Molecular sieve Framework structure (IZA code) Ring size Water capacity, g/g a Reference Li-X FAU [69] LiNa-X FAU [127] Na-X FAU [6] Mg-X FAU [7] Na-LSX FAU 12.3 [6] Ca-Y FAU [63] H-Y FAU 12.2 [63] K-Y FAU [63] Li-Y FAU [63] Mg-Y FAU [7] Na-Y FAU [63,7] Rb-Y FAU [63] Na-EMT EMT [179] Ca-L LTL 12.2 [179] K-L LTL [179] ZSM-2 FAU/EMT [179] Na-mega MAZ [179] Mordenite b MR [179] Clinoptilolite b HEU 1.14 [18] licalite-1 MFI 1.5 [91] Na-ZK-5 KFI 8.26 [179] CaNa-A-6 LTA 8.16 [63] Mg-A LTA 8.42 [7] Na-A LTA 8.29 [7] ZSM-34 ERI/FF 8.29 [179] Erionite ERI 8.22 [179] Analcime ANA 8.5 [179] K-P GIS 8.12 [179] Na-P GIS 8.24 [179] Ca-P GIS 8.22 [179] a Measurements were performed at 25 C and P/P = 1.. b Natural zeolites. migrate to other sorption sites during the water sorption process depending on their nature [59]. As the sorption process is in progress, the water molecules interact with these cations and form aqua-complexes [24,6 62]. Several zeolites are used as a model system to study the effect of extra-framework cations on the zeolites hydrophilicity. The surface chemistry of water sorbed on HZSM-5 and alkali-metal containing ZSM-5 was studied by Jentys et al. [24]. It is believed that at low P/P, the Lewis acid sites (presumably octahedrally coordinated aluminum) are more responsible for water sorption, while at higher P/P, strong Brönsted acid sites (bridging hydroxyls) are the most important for the water sorption. Several possible orientations of the water molecules bound to the Brönsted acid sites were proposed (Fig. 1), from which the structures 1 and 2 are the most probable. nce the water molecules are adsorbed on these bridging hydroxyl groups at low P/P, water clusters are formed whereby three water molecules can be chemisorbed on the strong Brönsted acid sites. For the alkali metal exchanged ZSM-5 zeolites, all of them show type I isotherms indicating their high hydrophilicity. However, a low amount of water is measured for those exchanging with large cations, which is probably due to the blocking of the

12 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Fig. 1. Possible sites of water molecules adsorbed on HZSM-5 zeolite. pores, and thus reducing the hydrophilic centers in the MFI framework [63]. This phenomenon was observed especially for zeolites exchanged with K +,Rb + and Cs +, and as a result the water molecules are unable to diffuse into the channels resulting in low water sorption capacity [24,6,62,63]. This statement is supported by the work of Hunger et al. demonstrating that the water sorption capacity of FAU depends on the type of extra-framework cations and degree of ion-exchange [6,61]. The degree of ionexchange is dependent on the temperature and size/type of cations in use as well [63]. It was found that the water sorption capacity decreases dramatically from LiX to CsX due to the decrease in the free volume of the large cavities of zeolites as a result of the increased cation dimension. However, in a later publication it was shown that the FAU zeolite partially exchanged with Na +,K +,Rb + and Cs +, only the Na + is interacting with water molecules [64]. The heavier alkali-metal ions present in the cavity of FAU, reduce the water Na + interactions by blocking the cation positions, and hence reduce the water uptake. The water Na + interactions are occurring in the 12-ring of FAU zeolite through formation of cyclic hexamers of water molecules, which are stabilized by hydrogen bonding to the framework oxygen atoms [65,66]. The behavior of water adsorbed on extra-framework alkali-earth cations (e.g. Mg 2+,Ca 2+ and Sr 2+ ) in zeolites was reported by Ward [67]. The existence of structural hydroxyl ( H Al) groups in the cation-deficient zeolites and dissociation of water molecules to activated MH + and H + species were demonstrated (Fig. 11). Thus the cation-deficient zeolites expose to low amount of water creates Brönsted acid sites and promote the catalytic activity [68]. In contrast, no structural hydroxyl groups are observed for the zeolites containing alkali cations. Jänsen et al. [69] have systematically studied the effect of cations on the water sorption uptake. LTA and FAU zeolites were ion-exchanged with different types of cations (Li +,Ca 2+,Mg 2+,Zn 2+,Co 2+,Al 3+ and Fe 3+ ), and they found that the water uptake is practically dependent and proportional to the cation loadings. It is revealed that zeolites exchanged with smaller cations (e.g. Mg-LTA and Mg-FAU) have higher water sorption capacity. This was explained by the replacement of two Na + ions (1.2 Å) by one smaller Mg 2+ (.72 Å), thus enhancing the water sorption through enlarging the pore volume of the zeolite [7]. Furthermore, an increase of the pore volume and water sorption capacity is achieved by ion-exchange of zeolite with small Li + ions (.74 Å). Meanwhile, the hydrophilicity of zeolitic materials was varied by introduction of hetero-metals (Al, Ti, Zr, etc.) into the framework positions; the latest effect is due to the different electronegativity of the incorporated metals. Hence, these materials display different hydrophilic strength and affinity to water molecules. In addition, the

13 12 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 Al M(H 2 ) 2+ n Al - (n-1)h 2 MH Al H Al Fig. 11. Activation of zeolites via dehydration. concentration of the hetero-metals in the framework affects their hydrophilicity. Recently, lson et al. [15] have investigated the hydrophilic characteristic of HZSM-5 zeolite with various aluminium contents. The results show that the high Al containing HZSM-5 has higher total water uptake and higher water sorption uptake at very low P/P because of the presence of more hydrophilic active sites (see Fig. 12). Conversely, the zeolites with low aluminium content show a decline in the water uptake, and an abrupt uptake in the region B due to the predominant water water interactions leading to water clusters formation by hydrogen bonding in the cavities. At very low partial pressure strong water sorption is demonstrated (region A) due to the hydrophilic centres in the zeolite, i.e., the presence of tetrahedral aluminium and acidic protons. It was shown that the water is sorbed at the Brönsted acid sites of HZSM-5 through formation of hydronium (H 3 + ) ion rather than water hydrogen bonding to the acidic proton [24,71]. Furthermore, they also observed that at low P/P, the Lewis acid sites (presumably octahedrally coordinated aluminium) are more responsible for water sorption, while at higher P/P, strong Brönsted acid sites (bridging hydroxyls) are the most important. The amount and the type of defect sites in zeolites influence on the water adsorption capacity in general. The defect sites are dependent on the method of preparation [72 75], calcination [76], ion-exchange [2], dealumination Water uptake (mg/g) Region 2 A 1 Parameter: 2 :Al 2 3 ratio Region B P/P o ( 1-3 ) Fig. 12. Water adsorption isotherms of HZSM-5 with different /Al ratios at 25 C. [76,77], and on the presence of hetero-elements [21,78]. The defect sites ( H) in the zeolites are classified as follows: (a) defects (siloxy groups) which balance the charge of cations in the zeolite pores, (b) H defects form from bridges via hydrolysis, (c) H groups generated by missing the tetrahedral framework atoms (T vacancies), (d) H groups due to a stacking disorder, and (e) H groups at the external surface [19]. These defect sites interact with the water, and once the number of water molecules in the cavities is increased, water clusters are formed through hydrogen bonding. The effect of the crystal structure and chemical composition on the sorption properties of zeolites X (FAU) and L (LTL) has been studied by Joshi et al. [79]. At identical concentration of extra-framework cations for both NaKX and NaKL materials, different basic character and polarizing ability are observed, thus emphasizing on the effect of zeolite structures on the sorption properties. The high polarization feature of the framework leads to high extent of water framework oxygen and water water interactions. In addition, lack of high dimensionality of the pore system, less open structure, low average framework oxygen charge and less number of accessible non-framework cations are some of the prominent factors which reduce the hydrophilicity of LTL type zeolites compared to FAU. As mentioned before, the hydrophilicity and water sorption capacity of zeolites are affected by the presence of coke deposits. ccasionally, calcination of zeolites containing large amount of organic templates may leave some hydrophobic carbonaceous species or coke as a contaminant in the porous materials, thus leading to pore blocking. The sorbents become less hydrophilic and the pores get narrower after coking, which hinder the diffusion of water molecules [23]. As a result, pore blocking by coke causes a release of water at higher temperature and low water sorption capacity. Through enhancement of the concentration of terminating silanol groups or extra-framework cations in nanosized hydrophilic molecular sieves is expected to have a larger water sorption capacity in comparison to the micron-counterparts [15,8 83]. Fig. 13 illustrates HZSM-5 zeolite ( 2 /Al ) with different crystal sizes upon water sorption. Both materials display type I water sorption isotherm, however, they possess different water loading. For nanosized HZSM-5 crystals (5 nm), the water sorption capacity is much higher than that for the large crystals (1 5 nm) due to the high concentration of terminal

14 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Water uptake (mg/g) Crystal size 5 nm 1-5 nm P/P o ( 1-3 ) Fig. 13. Water sorption isotherms of HZSM-5 ( 2 /Al ) with small and large crystals. silanol groups on their external surface. Furthermore, the two isotherms display different slopes, where the HZSM- 5 nanocrystals have a steeper one (sorb more water at low P/P ). This suggests that the high external surface of the materials play an important role in the enhancement of the hydrophilic properties. The extra-framework cations in the zeolites tend to migrate to the larger channels when they are hydrated while their mobility is increasing [59,84,85]. This phenomenon was explained by a combination of (a) electrostatic interactions between the extra-framework cations and the framework, and (b) steric interactions preventing the cations from occupying the smaller channels. The dipole moment of water molecules confined in low silica Na- LSX zeolites was calculated, and found that it depends on the degree of hydration [66]. Chen has studied the relationship between the amount of water sorbed and the concentration of aluminium in the H-mordenite zeolite framework [86]. He observed a linear stoichiometric relationship in which each hydrophilic site associated with aluminium is coordinated with four molecules of water. It is believed that the specific interaction between the water molecules and the cations (protons) is associated with the tetrahedrally coordinated aluminium. This corresponds to a coordination of four water molecules for each proton associated with tetrahedrally coordinated aluminum in the zeolite. In other words, a cluster of three water molecules around each hydronium ion (H 3 + ) is formed. This finding is well inline with the results obtained by Jentys et al. [24]. Therefore the linear stoichiometric correlation could not be fulfilled in the zeolites having a 2 / A1 2 3 ratio of 1 or lower because of the limited available space within the zeolite pores. In contrast to the hydrophilicity, the hydrophobicity of zeolites is related to the weak interaction of water molecules with the non-polar bonding, and the absence of partially ionic hydrophilic centers associated with the tetrahedrally coordinated aluminum [86]. Pure silica zeolite such as silicalite-1 is a good example to describe such phenomenon. licalite-1 is widely classified as hydrophobic material, and the earliest studies showed readily adsorption of organic molecules over water [57]. The phenomenon of increasing the zeolite hydrophobicity is due to the decrease of alumina content in the framework, and weak interactions of water with non-polar siloxane ( ) framework [15]. However, it is revealed that silicalite-1 sorbs a small amount of water as well [15,87,88]. In fact, the water sorption in silicalite-1 at low P/P is likely associated with adsorption on hydrophilic silanol defect sites [19]. Recently, based on Monte Carlo simulations it is revealed that a small amount of silanol defect sites serve as hydrophilic sites and they promote the adsorption of water vapor via formation of hydrogen-bonded clusters around the defects [89]. Therefore, even at low concentration of silanol groups, the hydrophilicity of the materials is sufficient and the materials have good wetting in water suspensions. However, the mechanism of water binding to the silicalite surface is still not well understood. Experimental and theoretical investigations of the water condensation in hydrophobic nanoporous materials were carried out by Desbiens et al. [9]. The intrusion of water in silicalite-1 at room temperature leads to water condensation inside the pores if high hydraulic pressure is applied, while a spontaneous capillary condensation (drying) is occurred once the pressure is released, thus demonstrating the non-wetting (hydrophobic) character of the silicalite-1 walls. The water repellence (non-wetting) of silicalite-1 surfaces has also been proven by TG and NMR spectroscopy [91,92]. The pore volume occupied by the adsorbed water in silicalite-1 is much smaller than the total volume of the pores. This is explained with the surface of silicalite-1, which is built from only hydrophobic siloxane resisting the water, while trace amount of hydrophilic surface silanol groups are present only in the crystal defects, which serve as water binding sites. Thus, it can be concluded that hydrophilicity of purely silica materials such as silicalite-1 is highly dependant on the number of surface defects. The diffusion of water in silicalite-1 was shown to take place even at room temperature, whereas both extra-crystalline and intracrystalline water is present in the silicalite-1 channels [93,94]. Later, the water silicalite interactions and the water orientations during the diffusion in the channels were studied, demonstrating that the water molecules enter and leave the pores of zeolites by pointing its dipole vector towards the center of the cavities [95] Aluminophosphates and titanosilicates The term zeotype material is occasionally applied to framework compounds, mainly aluminophosphate (AlPs) and titanosilicate (Ti) due to the regular pore systems and networks similar to the zeolites. Up to date, more than 3 types of AlPs materials have been synthesized [56]. Unlike the zeolites, which have extra-framework cations and negatively charge structures, the AlPs type framework is neutral. Thus, there are no ions present in

15 14 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 the materials accounting for the poor ion-exchange properties [96]. It is anticipated that the AlPs molecular sieves will have only weak catalytic and sorption properties due to the presence of a low concentration of surface hydroxyl groups. However, if phosphorous (P) atoms in the framework are replaced by or metallic (Me) atoms, then new properties are generated [97 11]. Most of the AlPs materials are not hydrothermally stable, but several zeotypes such as AlP-5, -11 and -17 have moderate firmness and the structures remain intact after treatment at elevated temperature (below 6 C) [12]. Considering that the hydrophilicity of zeolites originates from the coexistence of cations and framework anion sites, ideal AlPs with a similar structure is expected to have a hydrophobic character. However, AlPs often are described as hydrophilic materials because they sorb water even under low P/P [13,13]. AlPs materials exhibit isotherm type I or V due to the lower hydrophilic strength compared to the electrostatic zeolite, but some of them have very high water sorption capacity [13]. Table 4 summarizes the water sorption capacity of AlPs molecular sieves. Basically, the AlPs are categorized as dense or very small pore (four- and six-membered rings of diameter 3Å), small pore (eight-membered rings of diameter 4Å), medium pore (1-membered rings of diameter 6 Å), large pore (12-membered rings of diameter 7 8 Å), and ultra-large pore (18- and 2-membered rings of diameter > 12.5 Å) materials. As was shown for zeolites, the water filling is dependent on the size of the Table 4 Water sorption capacity of aluminophosphates and titanosilicates classified according to ring size Sorbent Ring size Water capacity, g/g a Reference VPI [179] AlP-36, -37, [12] SAP [179] AlP [181] VAP [179] GeAP-5, MnAP [179] SAP [179] SAP [179] AlP-41, SAP [179,12] AlP-11, MgAP-11, [179,128] SAP-11 VAP [179] SAP [179] GaAP [179] GeAP [179] AlP-18, -34, -35, -42, -43, [12,181] -44, -47 AlP-14, -17, -26, -33, [12,181] SAP [179] SAP [179] AlP-2, -25, [12,181] Na-ETS-4.15 [179] Ca-ETS-4.19 [179] ETS-1.14 [179] H-ETS-1.18 [179] a The water capacity was measured in 1% humidity at 25 C. channels, but for AlPs the water sorption capacity is not only related with the pore size [14]. Fig. 14 shows the water sorption capacity of representative AlPs materials with different ring sizes, i.e. VPI-5 (18R) has the same water sorption capacity (.35 g/g) as AlP-18 (8R), whereas AlP-11 (1R) has larger ring size than AlP- 18, but lower water sorption capacity (.16 g/g). The non-linear relation between the ring size of AlPs and water sorption capacity might be attributed to several factors such as the angle of Al bonding in the framework, the existence of polyhedral Al sites and the dimensions of the framework. In general, different Al angles are able to create different dipole moment and the affinity to water will vary (Fig. 15). These properties are attributed to the atoms in the Al bonding, which control the distance between water being sorbed on the Al sites. The broader Al angles attract water molecules, while the narrower angles prohibit water molecules from being absorbed. In the case of AlP-11, the Al sites have two different angles, the Al sites with narrow Al angle (12 ) prevents adsorption of any water molecule through repulsive interactions between oxygen atom of H 2 and oxygen atoms around the Al sites. In contrast, the water molecules easily sorbed on the Al sites in broader Al angles (15 ) through attractive interactions due to the weak repulsions between the oxygen atoms around the Al and the H 2 molecules. As an example, AlP-18 has all the Al sites with similar angle (135 ), and this angle is broader than those of AlP-11 (12 ) and is sufficient for sorbing water. Apart from that, the three-dimensional framework structure of AlP-18 allows easy diffusion of water molecules into the framework compared to two-dimensional AlP-11 structure. Additional explanations for the high hydrophilicity of AlPs materials are: (a) different concentration of defect sites (Al H and P H) in the structures [96,15], (b) ability of tetrahedral Al framework to form polyhedral (penta- or octahedral Al) [13], and (c) difference in the electronegativity between Al (1.5) and P (2.1) atoms [16]. Although the AlPs are composed of Al P networks, Water Capacity, g/g VPI-5 AlP-5 AlP-11 AlP-18 AlP-2 18 R 12 R 1 R 8 R 6 R Aluminophosphate materials.2 Fig. 14. Water sorption capacity of AlPs materials versus ring size of the framework structures.

16 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Fig. 15. AlP-11 (1R) and AlP-18 (8R) molecular sieves: Al angles in AlP-11 and AlP-18 with repelling and affinity to water. and the molar ratio Al/P is identical, this ratio is slightly varied and as a result, the Al H and/or P H groups appear in order to retain the electrical neutrality of the structure [96,13,17,18]. Unlike zeolites, the interaction of AlPs with water is dominated by hydrogen bonding with a certain number of hydroxyls and non-polar surfaces. Aluminophosphate VPI-5 has attracted considerable attention as a selective sorbent for large organic molecules and/or catalyst due to the extremely large pores [17]. Moreover the hydrophilic character of VPI-5 was confirmed, and stated that the hydrophilicity strongly depends on the local geometry of the Al sites [13]. The Al site with narrow Al angles prevents any water molecules to adsorb by repulsion between the oxygen atom in the H 2 and oxygen around the Al sites (Fig. 15a). In contrast, the water molecules tend to get closer to the Al sites, which have broader Al angles through electrostatic interactions (Fig. 15b). Consequently, the Al sites behave as weak Lewis acid sites and adsorb water, thus forming triple helix chains of water inside the VPI-5 molecular sieves [19]. Further exposure of VPI-5 to water or heat treatment leads to dissolution of phosphates and to conversion into AlP- 8 molecular sieve [11 112]. The sorption properties of another aluminophosphate AlP-5 were studied for mainly nitrogen [112], argon [113], light organic compounds [ ] and water [96,13, ]. Almost all of the sorption isotherms have the usual type I, except for water exhibiting type V due to the chemical interactions between water sorbent and water water through hydrogen bonding. The isotherm type V with steep water uptake starting at a lower P/P =.3 is typical for AlP-5 molecular sieves. The abrupt water uptake is attributed to the water sorption at the moderate hydrophilic sites contributed from the structural defects, i.e. P H or Al H [96]. Besides, a sharp rise in the water uptake as a consequence of capillary condensation in the unique framework of AlP-5 molecular sieve is observed: (a) an uptake below P/P =.25 is a consequence from the low hydrophilic six-membered ring channels, and (b) above P/P =.25 is owing to the capillary condensation in a mild hydrophilic 12-membered ring channels [13]. In addition, several reports demonstrate the unique ability of Al atoms in the AlP framework to attract water molecules. They are either involved in the formation of aquo pentaherally or octahedrally coordinated complexes or remain in tetrahedral coordination form [15,11, ,119]. The possibility of Al atoms to interact directly with the water is depending on both the framework structure and mild electrostatic forces on the surface [114,115]. Recently, the beneficial role of hetero-metals in AlPs on water sorption properties was reported [98]. The effect of Cr,, Mg, Co, V, Zr, Mo, Fe, Cd and Cu into MeAP-5 framework on the water sorption activity is studied. The enhancement of water sorption capacity is also contributed to the structure defects and/or surface H groups created from the metal incorporations [12].

17 16 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 Some AlPs materials were prepared from gels or clear precursor solutions, where the size of the particles is reduced substantially (AEI [83, ], AFI [124,125], and AEL [126]). It was found that the nanosized AlPs possess larger accessible pores due to the extremely high external surfaces. The effect of nanoscale AlP-18 prepared under microwave and conventional heating on the hydrophilicity was reported recently [121]. Both samples showed type V water sorption isotherms with the same hydrophilicity but different water sorption capacity (Fig. 16). The higher water sorption capacity of nanosized AlP-18 was mainly contributed to the high pore volume. In addition, the water desorption of AlP-18 has been studied in comparison with FAU zeolite [121,127]. The results show that micron-sized AlP-18 sorb almost similar amount of water to those of FAU, however, the release of water is at lower temperature, which is explained by the weak interactions of its neutral frameworks to the water molecules [127]. Additionally, nanosized AlP-18 shows greater weight loss than micron-sized crystals revealing a high potential for thermochemical storage at low temperature. The effect of pore size and organic template in AlPs and SAP materials on the water sorption capacity was investigated too [128]. It has been found that the inefficiency of template removal by methanolic acid leads to pore blocking by carbonaceous coke deposits, thus resulting in lower water uptake than those of calcined samples [23]. Additionally, the water sorption capacity of SAP materials is higher than of AlPs owing to the introduction Weight loss / wt% (a) (b) (c) (d) Temperature / o C Fig. 16. TG curves of (a) commercial zeolite X (UP, USA), (b) zeolite Y (Grace, USA), (c) micron-sized AlP-18 and (d) nanosized AlP-18 [121]. of extra-hydrophilic sites ð 4 4 Þ in the SAP framework (see Table 4). For example, SAP-17 has the highest water capacity among the SAPs materials, which can be explained by its eight-membered ring system that has more hydrophilic character and by the existence of extra-hydrophilic sites ð 4 4 Þ. A limited study on the water sorption of titanosilicates (Ti) has been conducted. A high water sorption is measured for amorphous Ti with high Ti 2 concentration due to the presence of high amount of surface hydrolytically unstable titanosiloxane ( Ti) [129]. Titanosiloxane bonds have high tendency to undergo hydrolysis during sorption of water and thus producing H and Ti H, which are responsible for subsequent sorption of water molecules. The water sorption activity and capacity of Ti is much higher than for pure Ti 2 and 2 compounds, which is explained by the presence of octahedrally coordinated Ti in the mixed oxide materials. Titanosilicates (TS-1) and ZSM-5 with different :Ti/ (Al) ratios were synthesized and the sorption of water and organics have been investigated [13]. The polarity of the M (M =, Al or Ti) linkages determines the water sorption capacity of ZSM-5, which is much higher than for TS-1 due to the more polar Al linkages. Furthermore, the water sorption capacity of TS-1 was improved considerably by increasing the amount of the framework titanium. However, the high amount of titanium leads to formation of anatase and decrease the pore volume of the final material. No significant influence on the sorption capacity of TS-1 toward n-hexane, benzene and n-butylamine by changing the titanium loadings was observed. This indicates that the incorporation of titanium does not change the hydrophobic environment ( linkages) in TS-1, but creates more hydrophilic active sites ( Ti linkages) beneficial for water sorption Mesoporous materials Another group of materials with potential applications as water sorbents is represented by mesoporous compounds synthesized by using long-chain surfactant molecules as templates [131]. The common MCM-48, MCM-5, FSM-16, KIT-1, SBA-2, SBA-15 materials have different pore sizes (2 1 nm), large surface areas (>5 m 2 /g), variable dimensions and shapes [14, ]. The mesoporous semi-crystalline compounds are expected to exhibit adsorption characteristics different from ordinary silica gels. The mesoporous materials, regardless of the structure, type and composition have abundant number of silanol groups because of the amorphous surface structure. Therefore, a large amount of water can be sorbed on them followed by capillary condensation. The water sorption capacity of some mesoporous materials is summarized in Table 5. Basically, all the materials show a high water sorption due to their mesoporosity, which is larger than the zeolites. MCM-48 possesses the highest water sorption capacity among the pure silica mesoporous materials due to its bicontinuous cubic pore system, while SBA-1 sorbs the lowest amount of water due to small pore size [14,134]. The MCM-41 is one of the most studied materials due to the structural simplicity (uniform cylindrical/hexagonal pore channels with very narrow pore size distribution), and easy pathway of preparation with negligible pore-blocking effects [14,3, ]. Apart from that, the high thermal, hydrothermal, chemical and mechanical stability are highly conducive for a number of important applications such as adsorption and separation, ion-exchange, catalysis and molecular hosting.

18 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Table 5 Water sorption capacity and physico-chemical properties of mesoporous materials Mesoporous materials Structural feature Pore diameter, nm Surface area, m 2 /g Water capacity, g/g a Reference MCM-48 3D highly ordered cubic structure [3] KIT-1 Fully disordered 3D network with worm-like [3] channels and uniform pore size SBA-1 Highly ordered 3D cubic structure with [3] two types of cages SBA-15 2D hexagonal mesopores interconnected with [3] micropores; thick silica wall MCM-41 2D hexagonal array of uniform mesopores [3] AlMCM-41 2D hexagonal array of uniform mesopores (/Al = 6) [23] AlMCM-41 2D hexagonal array of uniform mesopores (/Al = 15) [23] FSM-16 Hexagonal, highly ordered structure made from double 4 layers [134,182] a Measurements were performed at 25 C and P/P = 1.. Purely siliceous mesoporous materials are classified as hydrophobic materials since they sorb more organics than water [134,138,135,139,14]. They show type V water sorption isotherm, indicating relatively weak adsorbent adsorbate interactions between the solid surface and water vapors (Fig. 17). This reveals that the internal surface of mesoporous materials is quite hydrophobic [135,139]. The surface silanol groups of mesoporous materials are three types, namely single/isolated, hydrogen-bonded and geminal H (see Fig. 6) but the amount is lower ( H/nm 2 ) than in the amorphous silica (5. 8. H/nm 2 ) [138]. The water sorption isotherm of MCM-41 materials at low P/P is dominated by hydrophobic interactions, followed by more important water water interactions via hydrogen-bonded water cluster at medium P/P (Fig. 18). The water clusters grow until a certain pressure P/P, and then a capillary condensation is occurred [3,135,141,142]. Water molecules have difficulty to adsorb on the pore surface at the beginning due to surface hydrophobicity (Stage I), and at this stage, water surface interactions are predominantly occurred (usually chemisorption). nce the water molecules are sorbed on the pore surface at higher P/P, water water interactions take place via hydrogen bonding and form water cluster as a single layer (Stage II). Water clusters grow until a certain pressure, and then capillary condensation occurs (Stage III) followed by filling of the pores with water (Stage IV). The nature of silanol species is important in helping to understand the hydrophilic/hydrophobic character of silica mesoporous materials. Recently, Cauvel et al. [143] used microcalorimetry and IR spectroscopy to investigate the nature of water sorbed in pure silica MCM-41 with different concentration of silanol species. They observed the two types of surfaces in siliceous MCM-41, one hydrophobic (isolated silanols weakly or not interact with water and limit the uptake of water) and the other hydrophilic (terminal silanols strongly interact with water, which form H-bonded silanols). The results showed an explicit hydrophilic character of the terminal silanol participating in H-bonding with water molecules at low water pressure, thus forming hydrogen-bonded silanol species. In contrast, isolated silanols barely interact with water molecules even at high water pressure. The surface hydrophilicity of MCM-41 is governed by the heat activation and calcinations as well. A substantial decline in the hydrophilicity Water uptake (g/g) FSM-16 KIT-1 MCM-48 MCM-41 SBA-15 SBA-1 Water uptake (g/g) Stage I Stage II Stage III Stage IV P/P o Fig. 17. Water sorption isotherms of different mesoporous materials at 25 C. 1 P/P o Fig. 18. Sorption and capillary condensation of water in the pores of mesoporous materials at different partial pressure (P/P ).

19 18 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 of the surface of MCM-41 was observed when high temperature heat treatment or calcination (>55 C) was applied, thus resulting in condensation of silanol groups and formation of siloxane ( ) bridges. In contrast to pure siliceous mesoporous materials, Al containing MCM-41 is a more hydrophilic sorbent [139,14], and has high affinity to water because of the presence of a strong electrostatic field created on the framework anion sites. Thus an increase in Al content leads to higher hydrophilic character and higher water sorption capacity [23]. However, further increase of Al in the framework decreases the water uptake, which might be due to the clogging of the pores by water clusters formed around the Al centers. Nevertheless, the sorbed amount of toluene and isopentane is even higher than in pure silica MCM-41, proving that AlMCM-41 has a high lyophilic character [139]. Such phenomenon can be explained by the presence of both the Brönsted and Lewis acid sites on the surface of AlMCM-41 with high tendency to attract and interact with the organic compounds. The hydrophilic characteristics of pure siliceous mesoporous materials change significantly upon the synthesis methods applied. Serrano et al. [139] prepared purely siliceous MCM-41 using sol gel and hydrothermal methods to study the effect of the preparation approach on the hydrophilic/hydrophobic characteristic of mesoporous materials. It was demonstrated that the sol gel MCM-41 with a high structural disorder (worm-like type pore system) and with a low silica condensation in the walls has less hydrophilic character than those prepared via hydrothermal method. This is explained with the generation of a high amount of silanol and siloxane groups in the materials during the preparation. The sol gel modified materials are more selective to hydrophobic compounds (e.g. toluene and isopentane) and therefore their unique surface can be used as a selective adsorbent for removal of volatile organic compounds (VCs) presenting in gases or waste water [135]. The sorption of benzene and water vapors on phenylmodified MCM-41 sorbent was investigated too [144]. The results show that the organic hydrophobic functional group such as phenyl ( C 6 H 5 ) incorporated in MCM-41 significantly decreases the hydrophilicity, thus resulting in a low water uptake. Contrary to that, the benzene sorption was found to be improved, resulting in a type I isotherm, which is indicative for the highly hydrophobic nature of phenyl-modified MCM-41 material. Besides, it has to be mentioned that the MCM-41 and other mesoporous compounds face several problems as water sorbents, because of the high sensitivity to moisture and low hydro(thermal) stability. Treatments in boiling water and/or even in saturated water vapor easily destroy their structures [145,146]. The low stability of mesoporous molecular sieves at high water vapor is due to the cleavage of or Al networks via hydrolysis by water leading to the structural distortion (Fig. 19) [ ]. However, the hydro(thermal)stability of the mesoporous materials can be improved by different approaches and some of them are summarized in Table 6 [138,14, ] Clays and metal oxide pillared interlayer clays (PILCs) milarly to the zeolites, the clays are a class of naturally occurring minerals consisting of silicate and aluminosilicate particles. They are often found as components of soils and sediments and as some large deposits with bentonite (consisting mainly of montmorillonite and beidellite) [152,153]. Basically, the clays have a basic parallel layered structures built up from tetrahedral silicate [ 4 ] and octahedral aluminate [Al 6 ] sheets (Fig. 2), whereby the neg- Before hydrolysis After hydrolysis Al Na H 2 Al H H Na + H H Fig. 19. Schematic diagram of hydrolysis of and Al networks leading to structure distortion of mesoporous materials.

20 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) Table 6 Modifications of mesoporous materials: methods and consequences Method Characteristics Reference Introduction of hetero-atoms (Al or Ti) in the Slight decrease in the surface area of Ti- and Al-MCM-41 under water treatment at [15] framework 25 1 C for 4 h Extensive aging at low temperature (35 14 C) 8 97% of surface area retains after refluxing in water for 24 h [151,183] Salt stabilization Negligible structural changes after heating for 12 h in boiling water [148,149] Restructuring of mesoporous materials The surface area (9%) and pore volume retain after hydrothermal treatment at 9 C [146] Surface silylation High stability in water [15,14] Ion-exchange BET surface area decreases (<1%) under heating at 87 C for 2 h in saturated water [184] vapor Mesoporous matrix with zeolite nanoparticles Structure retains upon steaming at 8 C for 5 h [185] Fig. 2. Idealized layered structure of clays with Al and layers electrically balanced by the equal charge of exchangeable cations. ative charge distributed on the oxygen s of the layer surface is balanced by interlayer cations. The Na + and Ca 2+ are usually the charge balance cations due to the substitution of and Al in the layers by other metals hydrated and located in the interlayer regions loosely bound to the layer surfaces. When guest species are inserted in the interlayer spaces, they are easily replaced via ion-exchange, but the clay structures remain stable [154]. Clays are divided into 1:1 and 2:1 groups according to layer s charge and arrangement of two different sheet structures [155]. For example, montmorillonite is classified as clay 2:1, which means that the layer structures consist of two tetrahedral [ 4 ] layers sandwiching one aluminate [Al 6 ] octahedral layer. Several types of clays widely studied have the 2:1 structure, i.e. montmorillonites, hectorites, micas, vermiculites [ ], while kaolinites and chlorites belong to 1:1 clays [159,16]. As an advantage, the clays are natural and can be directly used in different processes; however, they have a significant amount of impurities. Therefore, clays are prepared in synthetic form by using oxides, glasses and fine powders of minerals and rocks as starting materials gels [161]. The synthetic clays have several advantages such as high purity and the physicochemical properties are adjustable through synthesis modification. Unlike zeolites, the clays have a wide distribution of pore sizes, ranging from micro- (<2 Å) to mesopores (2 5 Å). The porosity of the clays is formed from crevices in the particle surfaces, staggered layer edges, while voids are created by overlapping of stacked layers and interlayer regions [162]. ne of the unique properties of clays is the swelling of water and polar solvents up to a point where no mutual interactions between the clay sheets are remained. After dehydration at 12 C, the clays are restored in the original state; however, dehydration at higher temperatures may cause the irreversible collapse of the structure. In the latest case, the clay platelets are electrostatically bonded by dehydrated cations and hence, lose the adsorption ability. Therefore, further modification is necessary for stabilization and formation of metal oxide pillared interlayer clays (PILCs) with high structural stability. Water sorption process in clays is complicated involving the formation of multi-layers of sorbed water and thus, making the characterization and data interpretation very

21 2 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) 1 26 Amount adsorbed (g/g) M n+ M n+ M n+ P/P o <.1.1 < P/P o <.5 P/P o >.5 1 st 2 nd 3 rd layer P/P o Fig. 21. Water sorption in montmorillonite. complex. In general, the sorption of water on clays is classified as: (i) physisorption on surfaces, (ii) hydration of surface cations, (iii) rehydration of surfaces, and (iv) bulk rehydroxylation (see Fig. 21). The water molecules are sorbed on the external surface of the clay (P/P <.1), and then the sorption of water molecules is accomplished on the interlayer exchangeable cations, until a monolayer is formed (.1 < P/P <.5). Finally, the multi-hydrated layers are formed after the mesopores are completely filled (P/P >.5). Clays have high tendency to swell when are being hydrated: two kinds of swelling are occurring in the clays, namely intracrystalline and osmotic types [163]. Intracrystalline swelling is caused by hydration of the exchangeable cations in the interlayers of clays, while the osmotic swelling is resulted from the large difference between ion concentrations close to the clay surface and in the pores. The essential features of osmotic swelling are not only the large spacing observed between individual clay layers, but also the forces existing between the layers. The forces are osmotic resulting from a balance of electrostatic forces, van der Waals forces and the osmotic pressure exerted by the interlayer cations. The sorption of water in the interlayer spacing is controlled by the size and charge of the interlayer cations, concentration and localization of negative surface charge. If the negative charge is distributed on the octahedral sheets, then the water molecules form weak hydrogen bonding and weakly sorb on the surfaces. In contrast, the formation of strong hydrogen bonding between the water molecules and negative charged oxygen atoms at the surface is favored if the negative charge is distributed on the tetrahedral sheets. Kaolinites and illites are among the most common natural clays found in oil field. It is well known that in oil-saturated sandstones, oil wets preferentially kaolinites, while illites are essentially wetted by water. This behavior discrepancy is difficult to be explained based on crystallographic considerations since no significant structural differences between the kaolinites and illites are existing. Therefore, the kaolinites and illites were compared, whereby asphaltene and water were used to determine the hydrophobicity and hydrophilicity of the samples, respectively [164]. Illites showed more affinity to water due to more hydrophilic character of the surface (4% hydrophilic surface), whereas the kaolinite is exhibiting more hydrophobic characteristic and favorably sorbed asphaltene than water (25% hydrophilic surface). Moreover, cation hydration is shown to be responsible partially for the total hydrophilicity of the clays. Moreover, it is shown that inorganic exchangeable cations enhance their water sorption capacity, while the clays exchanged with organic cations render to be more hydrophobic. The water sorption process on homoionic clays is still a debate due to the uncertainty with the distribution of sorbed water on the external surface, and in the interlamellar space of the montmorillonite [163]. This uncertainty is caused by the different solvation properties of the exchangeable cations and possible changes of the external surface area during an increase of the relative pressure of water vapor. The shape of water sorption isotherm is type VI suggesting complex multi-step mechanism. A change of the external surface area of the aggregated montmorillonite particles during water uptake significantly increased the internal specific area from 43 m 2 /g to 71 m 2 /g. It was concluded that the heat of hydration of exchangeable cations and the surface pressure on the external surface are responsible for the intracrystalline swelling of sodium montmorillonite. The nature of the exchangeable cations in clays plays an important role in the sorption processes, and by increasing the exchanged cationic radius, a large surface area and high total pore volume of clays are reached. Several large inorganic and organic bulky cations such as K +,Cs +,Ca 2+, Cu 2+,Fe 3+ and TMA + are studied [162,165]. All the ionexchanged clays display type II water sorption isotherms except Na-montmorillonite, which shows type III. Furthermore, it is shown that the amount of water sorbed as a function of P/P is increasing gradually. An increment in water uptake demonstrates the enhancement of basal/interlayer spacing as a result of the swelling process. Among the samples, Li-montmorillonite has the highest water sorption capacity followed by Na-, Mg-, Fe-, Ca- and Cu-forms. The phenomenon of creating high surface area and high total pore volume in clays principally is due to the large cations located in the silicate layers, and thus opens a spacing between the layers allowing more molecules (e.g. N 2 and n-pentane) to diffuse and sorb on the clay surface. However, the water sorption in ion-exchanged clays depends mainly on the hydrating power of interlayer cations. In order to explain this phenomenon, Chiou and Rutherford exchanged on montmorillonite with Ca 2+, Na +,K +,Cs + and tetramethylammonium (TMA + ) cations having different hydrating powers [152]. It was found that the clays exchanged with strong solvating cations have

22 E.-P. Ng, S. Mintova / Microporous and Mesoporous Materials 114 (28) the highest interlayer water uptake, i.e. Ca 2+ >Na + > K + >Cs + P TMA +. milar study by Saada et al. [164] confirmed that the exchangeable cations play a significant role on the hydrophilicity of clays, which is based on the different hydration power of the organophilic cations (e.g. TMA +, TEA + and TPA + ) and hydrophilic/inorganic cations (e.g. Na +,K + and Ca 2+ ). The results demonstrated that clays exchanged with inorganic cations are more hydrophilic and showed higher water uptake, which is explained by the high tendency of hydration of the inorganic cations. In contrast, ion-exchanged clays with organophilic TMA + have more hydrophobic properties (originated from alkyl chains of amine), resisting from the cations being hydrated and thus showing lower water uptake. The invention of a new cross-linked clays or metal oxide pillared interlayer clays (PILCs) has attracted worldwide interest in many areas of physical, chemical and engineering sciences. The synthesis of cross-linked clays or metal oxide pillared interlayer clays were firstly reported in the late seventies as a new stable micro and mesoporous material with lower hydrothermal stability than zeolites. Montmorillonite and saponite, which belong to smectite clay mineral group, are the most popular type of clays applied in pillaring processes, whereby the pillared clays can be achieved through cation exchange or intercalation followed by calcination [166]. As a smectite, they tend to swell and thus allow water to enter and expand the clay layers so that ion-exchange is possible. The hydrated cations in the interlayers can be exchanged with larger hydrated metal cations or organic/inorganic complexes giving a virtually constant spacing between the layers (Fig. 22) [161]. The two properties of smectites, i.e. swelling and ion-exchangeability are critical for the successful preparation of PILCs. Detail descriptions of the synthesis of smectites and PILCs are reviewed by Kloprogge [161]. Commonly, the synthesis procedures of PILCs involves: (i) swelling of smectite in water, (ii) exchanging the interlayer cations by partially hydrated polymeric or oligomeric metal cation complexes in the interlamellar region of the clay, and (iii) drying and calcinations of cation exchanged smectite to transform the metal cation complexes into metal oxide pillars [155]. The overall synthesis produces permanent and large openings of the silicate platelets, in which the pillars (metal oxides) are eternally bonded to the clay layers. PILCs using various types of metal cations as pillaring agents (Al 3+,Zr 4+,Ti 4+,Fe 3+,Cr 3+, 4+, etc.) are synthesized either as single or as mixed metal oxides [ ]. A limited study on the sorption of water vapor on PIL- Cs is conducted. Sychev et al. [17] studied the effect of microporosity of Cr-PILC and Ti-PILC materials, and both clays showed type II water sorption isotherms with comparable water sorption capacity. For Ti-PILC, the water sorption sites were of higher energy (hard to desorb water) than those in Cr-PILC, which is attributed to the strong Lewis and Brönsted acid sites distributed over the surface of Ti-PILC. The role of the cations (Na +,Ca 2+,Mg 2+ and Li + )in the alteration of the hydrophilicity of alumina pillared montmorillonite (Al-PILC), resulting in significant improvement of the water sorption especially at low P/P Fig. 22. Cations acting as pillars between the layers and stabilizing the clay structure after calcination.