Composites ``binary salts in porous matrix'' for

Applied Thermal Engineering 50 (2013) 1633e1638

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Composites “binary salts in porous matrix” for adsorption heat transformation Larisa Gordeeva a, *, Alexandra Grekova b, Tamara Krieger a, Yuri Aristov a a b

Boreskov Institute of Catalysis, Lavrentiev av. 5, Novosibirsk 630090, Russia Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 March 2011 Accepted 25 July 2011 Available online 5 August 2011

A family of Composites “Salt inside Porous Matrix” (CSPM) has been considered as promising for adsorption heat transformation (AHT) due to their high sorption capacity, steep sorption isobars and opportunity to harmonize CSPM properties with boundary conditions of the AHT cycle. In this communication, we extend the harmonizing tools by confinement of one more salt to the matrix pores. Novel CSPMs based on a binary mixture of lithium, calcium, and barium halides inside various mesoporous matrices were synthesized with wide variation of the relative salts content. Their phase composition and sorption equilibrium with water, methanol and ammonia vapour were studied by XRD and TG techniques. It was shown that the formation of a homogeneous solid solution of the salts led to changing the equilibrium temperature (pressure) of the solvation. Thus, the confinement of binary salt systems to the matrix pores can be an effective tool for designing innovative materials with predetermined sorption properties adapted to particular AHT cycles. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Adsorption heat transformation Adsorbent design Composite “salt inside porous matrix” Water Methanol Ammonia

1. Introduction Solid sorption cycles can be energetically advantageous for transformation of heat from various sources, e.g. solar energy, waste heat, etc [1e3]. However, improvement of the performance is obligatory to make AHT competitive with liquid absorption and compression systems. The efficiency of AHT cycle is extremely sensitive to the sorption equilibrium of the working pair “adsorbenteadsorbate” [4e7]. It was shown that the Coefficient Of Performance (COP) of adsorption chillers increases with the rise in the amount of refrigerant, exchanged per cycle Dw, asymptotically approaching to the ratio of latent heat to desorption heat DL/DH [8]. Thus, fitting the adsorption equilibrium to requirements of a particular AHT cycle is one of the encouraging ways to enhance the cycle performance. For this reason many efforts have been directed to selection [7,9,10] or synthesis [7,11] of new adsorbent materials suitable for AHT cycles. The working conditions of AHT cycles depend on the number of factors, among which are a purpose of heat transformation (air conditioning, ice making, heating, deep freezing, etc.), temperature of the external heat source used for the sorbent regeneration and climatic conditions of the area where the cycle is used. The demands of particular cycle to desirable properties of the working pair

* Corresponding author. Tel.: þ7 383 3269454; fax: þ7 383 3304573. E-mail address: [email protected] (L. Gordeeva). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.07.040

“adsorbenteadsorbate” vary greatly in accordance with the working conditions of the cycle [7]. Despite the wide variety of the requirements imposed by different AHT cycles, only few working pairs are traditionally used for AHT, namely, silica gels-water, zeolites-water, activated carbons-methanol and activated carbons-ammonia [1,2,7,12]. This conservative practice reflects two past tendencies which now are being overcome: (a) a poor understanding of what adsorbent is optimal for a particular AHT cycle, and (b) a low level of co-operation between materials scientists and thermal engineers. Both issues have been widely discussed in the IMPRES Symposiums in Kyoto [13] and Singapore [14]. As a matter of fact, the modern materials science offers a huge choice of novel porous solids which may be used for AHT. Original and literature data on several classes of novel materials potentially promising for this important application, namely, metalaluminophosphates (AlPOs, SAPOs, MeAPOs), metal-organic frameworks (MILs, ISEs, etc.), ordered porous solids (MCM, SBA, etc.), and various composites (SWSs, AlPO-Al foil) are reviewed in [1]. Moreover, the present-day level of material science allows a target-oriented design of novel porous materials adapted to particular application [7,15]. Tailoring the specific adsorbent in the frame of this approach is divided into two parts: (i) formulating the demands of particular application to the necessary adsorbent properties, and (ii) synthesis of the adsorbent which precisely or nearly fits these demands [7]. A family of composites “salt in porous matrix” was developed for sorption of water, methanol and ammonia for various applications,

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including adsorption heat transformation and storage. CSPMs are found promising for AHT because of their high sorption capacity and steep sorption isobars [7]. The comprehensive study of CSPMs has revealed a dominant contribution of the confined salt to the composite sorption properties. The salt reacts with vapour that results in the formation of salt solvates. This shows itself as a sharp uptake rise on the sorption isobars. Finally the salt-sorbate solution forms inside the pores and sorption increases gradually with the decrease in temperature. The sorption equilibrium of CSPMs can be intently modified by proper choice of the active salt and the host matrix, matrix porous structure, the salt content and synthesis conditions [16,17]. Embedding two salts affecting each other into the matrix’s pores gives one more “tool” for intentional managing of the CSPMs sorption properties [18]. The formation of the solid solutions (SSs) of LiCl and LiBr inside the pores of silica gel results in shifting the equilibrium temperature (or pressure) of the salt solvation. Due to low mutual solubility of the lithium halides the shift of solvation temperature did not exceed 5e15 K thus restricting the possibility to vary the sorption equilibrium of the composites. The main goals of this work are (a) to extend this approach to other binary salt systems (LiCl þ LiBr, CaCl2 þ CaBr2 and BaCl2 þ BaBr2), host matrices (mesoporous silica gel and macroporous expanded vermiculite) and sorbates (water and ammonia) and to prepare novel CSPMs; (b) to investigate their phase composition and sorption equilibrium with a special emphasis on changing the solvation temperature; and (c) to assess the merit of applying the new sorbents for several adsorption cooling cycles. Appropriate single salt composites have been studied before and demonstrated high sorption capacity to water, methanol and ammonia under typical conditions of AHT cycles [8,19e24]. 2. Experimetal 2.1. Materials and synthesis A commercial silica gel Davisil gr. 646 (the average pore diameter dav ¼ 15 nm (BET), the specific surface area Ssp ¼ 293 m2/g (BET), the pore volume Vp ¼ 1.18 cm3/g) and an expanded vermiculite (dav ¼ 6.5 mm (BET), Ssp ¼ 2.0 m2/g (BET), Vp ¼ 2.7 cm3/g) were used as host matrices. The composites were synthesized by a dry impregnation of the matrices with the aqueous solution of two salts (MeClm þ MeBrm) followed by the thermal drying at 433 K [18]. A number of composites were prepared with different molar ratio of the metal chloride to the metal bromide nMeClm:nMeBrm ¼ 6:1, 3:1, 1:1, 1:3, 1:6, indicated parenthetically as follows: (MeClm þ MeBrm (nMeCl:nMeBr))/matrix. The total molar content of the salts nS ¼ nMeClm þ nMeBrm was equal for each raw of the samples nS ¼ 5.6, 2.0 and 1.4 mmole per gram of the composite for (LiCl þ LiBr)/vermiculite, (CaCl2 þ CaBr2)/SiO2 and (BaCl2 þ BaBr2)/ SiO2, respectively.

sample of 0.1e0.4 g weight was heated up to 443 K under continuous pumping for 3 h. Then the sample was cooled down to a fixed temperature and put in contact with water, methanol or ammonia vapour that initiated sorption. The temperature of the sample was measured with an accuracy of 0.2 K. The vapour pressure was a saturated pressure of a liquid sorbate in an evaporator, whose temperature was set by a thermostat with an accuracy of 0.1 K. The weight of the sample was recorded continuously during the adsorption until the constant value became settled that was considered as the equilibrium one. The sorption was calculated as w ¼ m(P, T)/m0, or as the equilibrium number of sorbed molecules related to one metal atom N(P, T) ¼ [m(P,T)/Mv]/(m0Ns), where m(P, T) is the weight of vapour adsorbed, m0 is the dry weight of the sorbent, Mv is a molecular weight of the sorbate. Accuracy of the uptake measurements was 0.0001 g/g. 3. Results and discussion 3.1. The system (LiCl þ LiBr)/vermiculite The XRD patterns of composites (LiClþLiBr(1:3))/vermiculite (Fig. 1) exhibit symmetrical peaks, which can be assigned as (111), (200), (220), (311) and (222) reflections of a cubic LiBr (space group Fm-3m) (JCPDS No. 06-0319). These peaks are shifted towards larger angles with respect to those of the reference salt (LiBr). This indicates a decrease of the lattice parameter due to the formation of a homogeneous solid solution of LiCl in LiBr. The XRD patterns of composites (LiClþLiBr(1:1))/vermiculite and (LiClþLiBr(3:1))/ vermiculite are quite different. The reflections (111) and (200) become asymmetrical or transform to two clearly separated reflections. This indicates the formation of the mixture of two solid solutions, one of which is enriched with LiCl (SSCl) and the other is enriched with LiBr (SSBr). Considering the linear dependence between the spacing parameters and the composition of solid solutions, the approximate molar content of chloride nCl and bromide nBr ions in each solid solution are evaluated (Fig. 2). Thus, the SS formation is observed for all the composites (LiCl þ LiBr)/ vermiculite. For those with a dominant content of LiBr, the homogeneous SSBr forms inside the pores. Further increase in the LiCl content results in the transformation of this homogeneous solution to the mixture of two solid solutions SSCl and SSBr. A sharp increase in the sorption is observed on each isobar for the single-salt composite which can be attributed to the reactions [18] LiBr þ CH3OH ¼ LiBr$CH3OH,

(1)

LiCl þ 3CH3OH ¼ LiCl$3CH3OH.

(2)

LiBr has a higher affinity to methanol and sorbs it at higher temperature (348e353 K) than LiCl (308e313 K). Essential

2.2. Characterization Phase composition of the new materials was characterized by an X-ray diffraction (XRD) using a Siemens D-500 diffractometer with Cu Ka radiation and a graphite monochromator on the diffraction beam. High temperature experiments were conducted using an Xray reactor chamber installed at the diffractometer. The sample was placed in the reactor, heated up to T ¼ 383e433 K in a helium environment, and a diffraction pattern of the dry sample was recorded. The diffraction patterns were recorded by 0.05 step scanning at the 2q angle range from 25 to 65 . The isobars (or isotherms) of water, methanol or ammonia sorption on the new composites were measured by a TG method. A

Fig. 1. XRD patterns of composites (LiClþLiBr)/vermiculite. nLiCl/nLiBr ¼ 1:3 (1), 1:1 (2) and 3:1 (3).


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Fig. 2. The parameter of cubic crystalline lattice of SSCl and SSBr for the composites (LiCl þ LiBr)/vermiculite.

distinctions are found between the experimental and curves calculated as a sum of the sorption on the single-salt composites NLiCl(P, T) and NLiBr(P, T) taken with the proper weight coefficients N ¼ (NLiCl(P,T) nLiCl þ NLiBr(P,T) nLiBr)/nS

(3)

for all the composites. The experimental step corresponding to the transition SSBreSSBr$CH3OH is shifted towards lower temperature by some 15 K in the bromine enriched composite (LiClþLiBr(1:3))/ vermiculite (Fig. 3a). That means the SSBr reacts with methanol at lower temperature than LiBr in the vermiculite pores. The reason of this shift is likely to be the formation of SSBr with smaller lattice parameter. The transition SSCleSSCl$CH3OH in (LiCl þ LiBr(1:1))/ vermiculite and (LiCl þ LiBr(3:1))/vermiculite shifts towards higher temperature regarding LiCl (Fig. 3b, c). Contrary to (LiCl þ LiBr(3:1))/ SiO2 [18], the increase in the temperature of SSCl solvation in the composite (LiCl þ LiBr(1:1))/vermiculite is quite large (10e15 and 5 K for vermiculite and silica gel based composites, respectively), probably, due to higher LiCl content in the solid solution. The shift of the solvation steps on the sorption isobars can be used for intent modification of sorption properties of CSPMs. For instance, a small additive of LiCl to the LiBr/SiO2 composite diminishes the temperature necessary for regeneration of the sorbent by some 15 K and consequently allows using a heat source with lower temperature at the desorption stage of AHT cycle.

Fig. 3. Isobars of methanol sorption on the composites (LiCl þ LiBr)/vermiculite with different salts content (aec) as well as curves calculated as a linear combination of the sorption on the single-salt composites LiCl/vermiculite and LiBr/vermiculite. P ¼ 10.7 kPa.

CaCl2 þ 2 H2O ¼ CaCl2$2H2O

(4)

CaCl2$2H2O þ 2 H2O ¼ CaCl2$4H2O

(5)

CaBr2 þ H2O ¼ CaBr2$H2O

(6)

CaBr2$H2O þ H2O ¼ CaBr2$2H2O

(7)

CaBr2$2H2O þ 2 H2O ¼ CaBr2$4H2O

(8)

3.2. The system (CaCl2 þ CaBr2)/SiO2 XRD patterns of all the composites (CaCl2 þ CaBr2)/SiO2 exhibit the symmetrical reflexes (Fig. 4) which can be assigned to orthorhombic modification (space group Pnnm), typical of both CaCl2 [JCPDF 74-0992] and CaBr2 [JCPDF 25-1034]. As the chloride content increases, the reflexes gradually move from those of CaBr2 towards lower angles and progressively transform to the reflexes of CaCl2. This indicates the successive reducing the lattice parameters due to the formation of continuous set of homogeneous solid solutions over the whole range of nCaCl2/nS from 0 to 1 that agrees well with the data for the bulk system CaCl2eCaBr2 [25]. Water sorption on the single-salt composites CaCl2/SiO2 and CaBr2/SiO2 results in the formation of crystalline hydrates according to the following reactions

Fig. 4. XRD patterns of the (CaCl2 þ CaBr2)/SiO2 composites.

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which show themselves as steps on the water sorption isobars (Fig. 5a). As CaBr2 has higher affinity to water than CaCl2 the formation of its hydrates occurs at higher temperature. The water sorption isobars of the composites (CaCl2þCaBr2)/ SiO2 differ significantly from the curves calculated as a linear combination of the sorption on the single-salt composites (Fig. 5b, c). The steps caused by the formation of the hydrates SSCl∙2H2O and SSCl∙4H2O in the CaCl2 rich composite (CaCl2þCaBr2(3:1))/SiO2 are situated at higher temperature by 5e10 K as regards the formation of CaCl2∙2H2O and CaCl2∙4H2O. On the contrary, steps attributed to the formation of hydrates SSBr∙2H2O and SSBr∙4H2O in the composite enriched with CaBr2, namely, (CaCl2þCaBr2(1:3))/SiO2, move towards lower temperature by 15e20 K with respect to those of CaBr2. Isotherms of methanol sorption on the single-salt composites CaCl2/SiO2 and CaBr2/SiO2 have two steps attributed to the formation of crystalline methanolates (Fig. 6) CaCl2 þ 2CH3OH ¼ CaCl2∙2CH3OH CaCl2∙2CH3OH þ 2CH3OH ¼ CaCl2∙4CH3OH

(9) (10)

Fig. 6. Isotherms of methanol sorption on the (CaCl2 þ CaBr2)/SiO2 composites. T ¼ 308 K.

CaBr2 þ 2CH3OH ¼ CaBr2∙2CH3OH

(11)

CaBr2∙2CH3OH þ 2CH3OH ¼ CaBr2∙4CH3OH

(12)

CaBr2 demonstrates higher afinity to methanol vapour. The formation of CaBr2∙2CH3OH occurs at very low pressure PCH3OH ¼ 0.3e0.8 kPa approximate to the pressure of CaCl2∙2CH3OH formation (0.7e1.2 Pa). The methanolate CaBr2∙4CH3OH forms at lower methanol pressure (3.1e4.0 kPa) than CaCl2∙4CH3OH (11.5e13.7 kPa). Isotherms of methanol sorption on the (CaCl2 þ CaBr2)/SiO2 composites are located at intermediate pressure range. As the chloride content increases the step corresponding to the formation of methanolate SS∙4CH3OH gradually moves towards higher pressure due to the formation of continuous homogeneous solid solution of the salts. 3.3. The system (BaCl2 þ BaBr2)/SiO2 The phase composition of both single-salt composites BaCl2/ SiO2 and BaBr2/SiO2 is characterized by a mixture of the two modifications of the salts, namely, orthorhombic and hexagonal. With the increase in bromine content, the reflexes on the XRD patterns of the composites (BaCl2 þ BaBr2)/SiO2 transform gradually from those of BaCl2 to the reflexes of BaBr2 (Fig. 7). That shows the formation of continuous row of homogeneous solid solutions in the whole range of nBaCl2/nS ¼ 0e1. Two steps are observed on isotherms of the ammonia sorption on both the single salt composites (Fig. 8). Firstly, at PNH3 ¼ 0e10 kPa the uptake w rises from 0 to 0.03e0.05 g/g due to the ammonia adsorption on active centres of the silica gel. The second rise of sorption from 0.05 to 0.20e0.23 g/g is attributed to the formation of complexes due to the reactions

Fig. 5. Isobars of water sorption on the composites CaCl2/SiO2 and CaCl2/SiO2 (a) and (CaCl2 þ CaBr2)/SiO2 (b, c) as well as a linear combination of the sorption on the composites CaCl2/SiO2 and CaBr2/SiO2. P ¼ 1.3 kPa.

BaCl2 þ 8NH3 ¼ BaCl2∙8NH3,

(13)

BaBr2 þ 8NH3 ¼ BaBr2∙8NH3.

(14)

Again a bromide BaBr2 demonstrates higher affinity to ammonia and the formation of BaBr2∙8NH3 occurs at lower ammonia pressure (70e160 and 550e650 kPa for BaBr2 and BaCl2, respectively, Fig. 8). Isotherms of ammonia sorption on the composites (BaCl2 þ BaBr2)/SiO2 have a similar shape, and two steps are observed (Fig. 8). As the chlorine content increases, the pressure corresponding to the formation of ammonia complexes of the salts


Fig. 7. XRD patterns of the composites (BaCl2 þ BaBr2)/SiO2.

and their solid solution continuously moves from 70e160 to 550e650 kPa. Thus, the results obtained reveal some common features of sorption equilibrium of the composites based on binary salt systems confined to porous matrices. The SS formation in such binary systems leads to distortion of the crystalline lattice of the salts and to change of the spacing parameter. That results in shifting the equilibrium temperature (pressure) of the complexes formation SS þ NV ¼ SS NV regarding the solvation of pure salt S þ NV ¼ S NV. The dissolution of a metal chloride MeClm in the lattice of a metal bromide MeBrm leads to the reducing the spacing parameter that probably hinders the incorporation of the sorbate molecules in the lattice. Therefore, the transition of SSBr to its solvates occurs at lower temperature (higher pressure) than the MeBrm solvation. Dissolution of the MeBrm in the lattice of the MeClm results in the opposite effect: expanding the crystalline lattice occurs that promotes the incorporation of the sorbate molecules in the lattice and raises the solvation temperature (reduces the pressure). The shift of the temperature of the salt solvation due to dissolution of the additional salt can be used for the intent design of the composite sorbents for particular AHT cycles. 3.4. Tailoring the composites (BaCl2þBaBr2)/SiO2 for air conditioning, ice making and deep freezing cycles

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tailoring the composite which properties fit the demands of particular AHT cycle. Let us demonstrate this for three cooling cycles specified for air conditioning (AC), ice making (IM) and deep freezing (DF). Taking into account the data on ammonia sorption on the composites (BaCl2 þ BaBr2)/SiO2 the pressure corresponding to the formation of the complexes SSBaClnBr(2-n)$8NH3 (w z 0.23 g/g) is plotted vs. the relative content of Br-ions nBaBr2/nS (Fig. 9). This graph allows a precise determination of the solvation pressure for the composite with a fixed ratio nBaBr2/nS. On the other hand, this graph is very convenient for predicting the composition of CSPM which sorbs ammonia at the pre-determined conditions (PNH3 and T). Let us consider the three working cycles specialized for air conditioning, ice making and deep freezing for hot climatic areas. Typical boundary conditions of these cycles are as follows: Tcon ¼ Tads ¼ 313 K, Tev ¼ 280, 270 and 250 K for AC, IM and DF cycles, respectively (Fig. 10). According to the data of Fig. 9, the composite BaBr2/SiO2 sorbs ammonia at PNH3 ¼ 220 kPa at Tads ¼ 312 K that allows estimating Tev ¼ 255 K. This composite (CSPM e DF) can be considered as a proper one for DF cycle (Fig. 10). The rich and weak isosters, which correspond to the ammonia uptakes w ¼ 0.23 and 0.03 g/g, respectively, are schematically presented in Fig. 10. An adsorbent for ice making (Tev ¼ 270 K, Fig. 10) has to sorb ammonia at PNH3 z 380 kPa, hence, its optimal composition should be nBaBr2/nS z 0.6 (Fig. 9). And finally, the composite with BaBr2 content nBaBr2/nS ¼ 0.2 sorbs ammonia at PNH3 ¼ 550 kPa that corresponds to Tev ¼ 280 K (Fig. 9). This adsorbent can be considered as optimal for air conditioning cycle (Fig. 10). Ammonia sorption on each of these sorbents reach wads ¼ 0.23 g/g at the conditions of adsorption stage of the corresponding cycle. Another point of primary importance is the temperature Treg necessary for regeneration of the composite at methanol pressure corresponding to Tcon ¼ 113 K (Fig. 10). For accurate determination of Treg the measurement of the set of adsorption/desorption isobars for each the composite is required. However some reasonable estimation can be done considering that ammonia sorption on the composite obeys the Polanyi principle of temperature invariance [8,26]. According to this principle, at different temperatures Ta and Tb, an equal volume of adsorbed phase V can be achieved at the vapour pressures Pa and Pb, linked in the formula below:

Ta ln

P P0

P ¼ Tb ln P0 b a

(15)

The revealed gradual shift of adsorption isobars (isotherms) due to the formation of solid solution of metal halides can be used for

where P/P0 is the relative pressure of adsorbate.

Fig. 8. Isotherms of ammonia sorption on the silica gel and the composites (BaCl2 þ BaBr2)/SiO2. T ¼ 312 K.

Fig. 9. Ammonia pressure, corresponding to the ammonia uptake w z 0.23 g/g vs. the BaBr2 content in the composites (BaCl2 þ BaBr2)/SiO2 at T ¼ 312 K.

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Acknowledgements The authors thank the Russian Foundation for Basic Researches (projects 09-03-00916a and 10-08-91156) for partial financial support. References

Fig. 10. PeT diagram of the AC, IM and DF cycles as well as the rich (w ¼ 0.23 g/g) and weak (w ¼ 0.05 g/g) isosters of the sorbents CSPM-AC, CSPM-IM and CSPM-DF optimal for these cycles.

Taking into account this principle the following equation can be written

Pev Pcon ¼ Treg ln P0 ðTads Þ P0 Treg

Tads ln

(16)

and regeneration temperature can be estimated as Treg ¼ 378, 399 and 431 K for AC, IM and DF cycles, respectively. Obviously, the rise of affinity of the composites to ammonia vapour due to higher bromine content results in the increase in the regeneration temperature at fixed Pcon. However, even the composite CaBr2/SiO2 specialized for DF cycle can be regenerated at temperature 431 K which is quite available using the solar energy concentrated by parabolic collectors. The amount of ammonia leaved after regeneration at this temperature wreg ¼ 0.05 g/g (Fig. 9) that gives a quite large variation in the uptake per cycle Dw ¼ 0.18 g/g. Thus, the selection of proper composite composition allows an intentional adjustment of the composite properties to particular cooling cycle as well as an exchange of the large ammonia amount at the cycle.

4. Conclusions Phase composition of CSPMs based on various binary salts systems (LiCl þ LiBr, CaCl2 þ CaBr2 and BaCl2 þ BaBr2) confined to the silica and vermiculite pores, and their sorption equilibrium with water, methanol and ammonia are studied. The formation of the salt solid solutions inside the pores results in shifting the equilibrium temperature (or pressure) of the salt solvation reactions. The dissolution of the metal bromides, which demonstrate higher affinity to water, methanol and ammonia than corresponding chlorides, leads to the increase in the equilibrium solvation temperature and appropriate pressure reduction. On the contrary, the additive of chlorides decreases the equilibrium temperature and increases the pressure of the solvation. This temperature tuning is a powerful tool for tailoring CSPMs with predetermined sorption properties which fairly fit requirements of the particular AHT cycle.

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