Composites ''lithium halides in silica gel pores

0 downloads 0 Views 227KB Size Report
tion and sorption properties due to (i) dispersion of the salt ..... Pore structure parameters of initial silica gel and composites LiCl(21 wt.%)/SiO2 and LiBr(29 wt.

Available online at www.sciencedirect.com

Microporous and Mesoporous Materials 112 (2008) 254–261 www.elsevier.com/locate/micromeso

Composites ‘‘lithium halides in silica gel pores’’: Methanol sorption equilibrium Larisa G. Gordeeva

b

a,*

, Angelo Freni b, Tamara A. Krieger a, Giovanni Restuccia b, Yuri I. Aristov a

a Boreskov Institute of Catalysis, Pr. Ak. Lavrentieva 5, 630090 Novosibirsk, Russia Istituto di Tecnologie Avanzate per l’Energia ‘‘Nicola Giordano’’, S. Lucia Sopra Contesse 5, 98126 Messina, Italy

Received 29 June 2007; received in revised form 27 September 2007; accepted 27 September 2007 Available online 1 October 2007

Abstract This paper presents the methanol sorption equilibrium and phase composition of two composite sorbents ‘‘LiCl and LiBr confined to mesopores of silica gel’’. Chemical phase analysis of the samples was performed by the differential dissolution method. Phase transformation of the composites during methanol sorption was characterized in situ by X-ray diffraction analysis. The isobars of methanol sorption on the composites were measured in the temperature range T = 293–423 K and for methanol pressure PMeOH = 21–300 mbar using a thermo-gravimetric technique. It was found that the two composites showed an outstanding sorption capacity wmax = 0.8 g/g and an energy storage ability Emax = 1.0 kJ/g, which could be attractive for development of efficient adsorptive cooling/heating and energy storage systems driven by a low temperature heat source.  2007 Elsevier Inc. All rights reserved. Keywords: Silica gel; Lithium halides; Composite materials; Methanol; Adsorption

1. Introduction Silica based nanocomposites have received significant attention because they exhibit unusual, sometimes unique magnetic, electrical, adsorption and catalytic properties, which could have promising applications in technology [1–4]. Dispersion of the components to a nano-sized scale is an important tool resulting in the appearance of new properties as a consequence of a large fraction of surface atoms [4–6]. Encapsulation of an active substance inside a porous matrix is a useful route for the synthesis of nanocomposites [4,7–9], in order to avoid their natural tendency to form aggregates. Incorporation of inorganic salts (halides, sulphates, nitrates of alkaline and alkaline-earth metals) into common adsorbents (alumina, silica gel, porous carbons) resulted in the formation of hybrid materials *

Corresponding author. Tel.: +7 383 3269454; fax: +7 383 3309573. E-mail address: [email protected] (L.G. Gordeeva).

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.09.040

‘‘salt inside porous matrix’’ with enhanced affinity to water [8–11], ammonia [12] and methanol vapour [13–15]. Due to their physical structure the materials take an intermediate position between solid adsorbents and pure salts and can be manufactured in such a way to demonstrate the best features of both systems. These composites are considered promising for various applications like adsorptive heating/cooling [16–19], heat storage [20], gas drying or removal of methanol vapour or ammonia from gaseous mixtures, etc. The confinement of a salt inside the matrix’s mesopores alters significantly its chemical, phase composition and sorption properties due to (i) dispersion of the salt to nano-sized particles [8,10], or size effect [4–6,21]; (ii) surface interaction between the matrix and the salt particles during preparation of the composites [11], so-called guest–host interaction [4,22]. Methanol adsorbents are widely used in various applications, e.g. in adsorption cooling technology [23–25], in methanol separation from gaseous mixtures [26–28], for

L.G. Gordeeva et al. / Microporous and Mesoporous Materials 112 (2008) 254–261

shifting conversion in methanol synthesis to favour product yield by means of methanol separation [29], etc. In spite of great interest in methanol adsorbents, most of the studies on methanol adsorption deal with common porous materials like hydrophobic zeolites and porous carbons. Their sorption capacity is rather low and usually does not exceed 0.15–0.4 g/g. Very few studies are available on innovative materials with advanced methanol sorption properties like active carbon fiber [30], chemically modified zeolites or carbons [27,31,32], and hybrid materials, metal doped alumina [14], pillared clays [32] or ‘‘salt inside porous matrix’’ [13,15]. So far, among the composites ‘‘salt inside porous matrix’’ methanol sorption on calcium chloride confined to the pores of various silica gels has been studied [13]. Two types of methanol sorption equilibrium were found: the formation of solid crystalline solvate CaCl2 Æ 2CH3OH at low methanol uptake and the formation of CaCl2–methanol solution inside the pores at high uptake. The encapsulation of CaCl2 into silica mesopores affects essentially the sorption equilibrium of the salt and its crystalline solvate CaCl2 Æ 2CH3OH. In contrast, the sorption properties of confined CaCl2–CH3OH solution remain the same as for the bulk solution. High methanol sorption ability (0.7 g/g) and energy storage capacity (0.8 kJ/g) were found. A number of other composites based on different salts (LiCl, LiBr, NiBr2, MgCl2, CaBr2) presented methanol sorption ability as high as 0.4–0.8 g/g and could be of great interest for practical applications, in particular for sorption heating/cooling [15]. The aim of this paper is a comprehensive study of the phase composition and methanol sorption equilibrium for two proposed composites based on LiCl and LiBr encapsulated into pores of silica gel, which are considered to be the most promising for adsorption cooling. 2. Experimental 2.1. Materials and synthesis The commercial silica gel Grace Davison 8926.02 (average pore diameter dav = 14 nm, specific surface area Ssp = 361 m2/g, pore volume Vp = 1.3 cm3/g) was used as a host matrix. Lithium chloride and lithium bromide were purchased from Aldrich and used as delivered. The silica gel was dried at 473 K for 2 h, impregnated with an appropriate amount of aqueous LiCl or LiBr solution to complete filling of the pores and subsequently dried again at 423 K until the sample weight became constant. Composites with various salt content LiCl(31 wt.%)/SiO2, LiCl(21 wt.%)/SiO2, LiBr(29 wt.%)/SiO2 and LiBr(24 wt.%)/SiO2 were prepared. 2.2. Characterization The isobars of methanol sorption on the composites were measured in the temperature range T = 293–423 K and for methanol pressure PMeOH = 21–300 mbar using a CAHN C2000 thermal balance. Before measurements were

255

taken, a 15–30 mg mass sample was heated up to 433 K under continuous evacuation (the residual pressure 0.01 mbar) until reaching the dry weight m0 to make sure that all water was completely eliminated from the sample. Then, the sample was exposed to methanol vapour with a fixed pressure and cooled to a fixed temperature to start the equilibrium tests. The methanol vapour pressure was a saturated pressure of liquid methanol in an evaporator, whose temperature was set by a thermostat with an accuracy of ± 0.1 K. During the experiments the temperature ranged from 293 to 423 K and methanol pressure was from 30 to 300 mbar. The amount of methanol sorbed at equilibrium m(PMeOH, T) was measured as the final increase in the sample weight at fixed T and PMeOH. The methanol sorption was characterized by the methanol content w = m(PMeOH, T)/m0, or by the equilibrium number of methanol sorbed molecules related to one molecule of the salt nðP MeOH ; T Þ ¼ ½ðmðP MeOH ; T Þ=lMeOH Þ=ðm0  C s =ls Þ; where lMeOH and ls are the molecular weights for the methanol and the salt, respectively, m(PMeOH, T) is the equilibrium amount of methanol sorbed at fixed PMeOH and T, and Cs is the salt content in the sorbent. Methanol sorption isosters were measured by a sorption isosteric method [33]. About 50 g of the sample of known dry weight was placed inside a vacuum chamber and pumped to a residual pressure of 0.01–0.05 mbar at T = 393 K and the chamber was connected to an evaporator filled with a known amount of liquid methanol, so that the sample was saturated with methanol vapour up to a fixed uptake. Then the chamber was disconnected from the evaporator and the sample was kept at a closed volume for 24–48 h in order to reach a uniform methanol distribution inside the sample. Finally, the sample was heated to a fixed temperature and the methanol equilibrium pressure over the sample was measured. During the experiment the temperature range was adjusted so that the pressure ranged from 20 to 400 mbar. During the heating phase a change in methanol uptake due to its partial desorption into the ‘‘dead’’ volume over the sample was estimated to be Dn  (3–6)·103 mole CH3OH/mole of the salt or Dw  103 g/g. The chemical phase analysis of the samples was performed using the differential dissolution (DD) method [11,34]. A solvent flux was passed through the sample placed into a flow reactor. The solvent composition was changed from distilled water to HCl (0.1 M) and finally to HF (0.2 M). The solution from the reactor outlet was injected into the multi-element detector-analyzer ICP AES that recorded the chemical composition of the solution as a function of time. Both stoichiometry of the salt composition and the contribution of individual phases were extracted from these curves using a special computer program [35]. Nitrogen adsorption isotherms were measured by a Micromeritics ASAP 2400 instrument at 77 K to obtain

256

L.G. Gordeeva et al. / Microporous and Mesoporous Materials 112 (2008) 254–261

specific area Ssp (BET), pore volume Vp (BJH method, desorption branch) and average pore diameter dav (BET). Phase transformation of the composites during methanol sorption was characterized in situ by an X-ray diffraction (XRD) using a Siemens D-500 diffractometer with Cu Ka radiation and a graphite monochromator on the diffraction beam. The high temperature in situ experiments were conducted using an X-ray reactor chamber installed at the diffractometer [36]. The sample was placed in a camera-reactor placed at the diffractometer and heated up to T = 373 K in a helium environment and the dry sample’s diffraction pattern was recorded. Then the sample was exposed to helium saturated with methanol vapour and cooled to a fixed temperature to start the sorption. The sample was kept at a constant temperature for 1–2 h and the diffraction pattern was recorded again. The temperature of the sample ranged from 323 to 303 K with a step of 3–5 K. The diffraction patterns were recorded by 0.05 step scanning at the 2h angle range from 25 to 65.

3. Results and discussion 3.1. Chemical composition The pattern of DD analysis is presented in Fig. 1 for LiBr(29 wt.%)/SiO2 composite. Monitoring the lithium and silicon concentrations in the outlet solution vs. the fraction of dissolved lithium provides information about various forms of the salt in the composite. Three different forms of Li+ cations were found. The quantity of Li+ dissolved in water was 97%. This form is probably LiBr stabilized in the silica gel pores. The remaining 3% of Li+, dissolved in hydrochloric acid, was probably associated with the Li+ cations adsorbed on the silica surface during impregnation of the silica gel with an aqueous salt solution according to the following reactions [11]:

And finally, a negligible amount of Li+ cations, dissolved together with the silica gel in hydrofluoric acid, was probably an impurity contained in the original silica gel. Thus, the salt confined into the silica gel pores consisted predominantly of the stoichiometric salt and a minor amount of Li+ which formed surface complexes with silica gel „Si– O  Li+. The DD analysis of the other samples also revealed the two mentioned forms of Li+ cations. 3.2. Phase composition The XRD patterns of composites LiCl(31 wt.%)/SiO2 and LiBr(29 wt.%)/SiO2, recorded under helium flow at T = 373 K indicates that a crystalline phase of LiCl (JCPDS No. 04-0664) and LiBr (not presented) with a cubic structure formed inside the silica pores. The size of coherently scattering domains was 20–30 nm, which was close to the pore diameter. Bulk crystals of LiCl and LiBr were not detected in the composites. Phase transformation in composite LiCl(31 wt.%)/SiO2 during methanol sorption was monitored by an XRD in situ (Fig. 2). The XRD patterns of the composite recorded under methanol vapour at T > 313 K were similar to those recorded for the dry sample. The broadening of the spacing parameter from 0.5149 to 0.5171 nm at T = 318 and 373 K, respectively attributed to a thermal expansion of the sample, was detected. In contrast, the sample saturated with methanol at T < 318 K appeared to become an X-ray amorphous which could indicate the formation of LiCl–methanol solution inside the pores. Surprisingly, intermediate phases of crystalline solvates LiCl Æ CH3OH or LiCl Æ 3CH3OH formed in the bulk system LiCl–CH3OH [37–39], were not detected during methanol sorption on the composite.

BSi  O þ Liþ  BSi  O    Liþ ; BSi  OH þ Liþ  BSi  O    Li þ Hþ :

Fig. 1. Pattern of DD analysis for LiBr(29 wt.%)/SiO2. Concentrations of lithium (solid line) and silicon (dashed line) in the outlet solution as a function of the dissolved lithium fraction.

Fig. 2. XRD in situ pattern of LiCl(31 wt.%)/SiO2 composite measured in a helium flow at T = 373 K (1), as well as in the helium flow saturated with methanol at T = 318 (2) and 303 K (3).

L.G. Gordeeva et al. / Microporous and Mesoporous Materials 112 (2008) 254–261

3.3. Porous structure

257

the possible silica surface blocking by the salt, the normalized surface area (NSA) was calculated as

The data on porous structure of the started silica gel and the composite sorbents LiCl(21 wt.%)/SiO2 and LiBr(29 wt.%)/SiO2 are presented in Fig. 3 and Table 1. Modification of the silica gel by the salt results in the decrease in total pore volume and surface area. The pore volume of the composites per gram of the starting silica was measured to be Vp = 1.13 and 1.11 cm3/g for LiCl and LiBr based composites, respectively. Taking into account the density of LiCl and LiBr q = 2.068 and 3.464 g/cm3 the volume occupied by the salt in 1 g of silica gel can be calculated as Vs = 0.13 and 0.12 cm3/g. Subsequently, the empty pore volume of the composite per gram of silica can be evaluated as Vp(comp) = Vp(SiO2)– Vs = 1.16 and 1.17 cm3/g for LiCl(21 wt.%)/SiO2 and LiBr(29 wt.%)/SiO2, respectively. The calculated values are very close to those experimentally obtained from nitrogen adsorption data (Table 1). The decrease in the surface area of the starting silica gel is more pronounced. The surface area of the composites per gram of the sample was measured to be Ssp = 209.3 and 203.5 m2/g for LiCl and LiBr based composites, respectively, are much less than Ssp = 361.0 m2/g for the starting silica. In order to quantify

NSA ¼

S sp ðcompÞ ; ð1  yÞ  S sp ðsilicaÞ

where Ssp is the specific surface area of the parent silica gel or salt/silica composite, y is the weight fraction of the salt in the sample. The value obtained are NSA = 0.73 and 0.79 for LiCl and LiBr based composites, respectively that is less than unit. This is probably caused by the fact, that the salt particles inside silica pores can block a part of the narrow pores, which contribute more to the pore area than to the pore volume of the sample [4]. The increase in average pore diameter of the composite (Fig. 3 and Table 1) from dav = 14 nm for the starting silica to dav = 17 and 16 nm for the LiCl and LiBr based composites, respectively, confirms the supposition on the partial blocking of the narrow pores of silica gel by the incorporated salt. Thus, the data on XRD and nitrogen adsorption indicate that the salt most likely is encapsulated in the silica gel pores partially blocking narrow pores. 3.4. Sorption equilibrium

Fig. 3. Pore size distribution of the starting silica gel (solid line), LiCl(21 wt.%)/SiO2 composite (dot line) and LiBr(29 wt.%)/SiO2 composite (dash line).

The isobars of methanol sorption on LiCl(31 wt.%)/ SiO2 and LiBr(29 wt.%)/SiO2 composites are presented in Figs. 4 and 5, respectively. The samples demonstrate a strong affinity to methanol. All isobars for each sample show a regular shape but shift toward higher temperatures with an increase in the methanol vapour pressure. The maximum amount of methanol sorbed wmax = 0.75– 0.80 g/g exhibited by both composites is 2–5 times larger than the methanol sorption capacity for non-modified silica gel (0.15 g/g, Fig. 6) as well as for conventional adsorbents like zeolites and activated carbons (0.2–0.4 g/g) [24,25,30]. This great enhancement of adsorptivities is due to the predominant contribution of methanol sorption by the salt embedded in the silica gel matrix. At a high temperature, composite LiCl(31 wt.%)/SiO2 sorbs a small amount of methanol (w 6 0.05 g/g, Fig. 4), which can be attributed to the methanol adsorption on active centres of the silica gel surface. As the temperature decreases, the salt starts to absorb methanol and the uptake

Table 1 Pore structure parameters of initial silica gel and composites LiCl(21 wt.%)/SiO2 and LiBr(29 wt.%)/SiO2 obtained from nitrogen adsorption isotherms Sample

SiO2 LiCl/SiO2 LiBr/SiO2 a b c d

Vpa (cm3/g)

Sspb (m2/g)

Per gram of sample

Per gram of silica

Per gram of sample

1.29 0.88 0.79

1.29 1.13 1.11

361.0 209.3 203.5

Vp – pore volume. Ssp – BET surface area. NSA – normalized surface area. dav – average pore diameter.

NSAc

davd (nm)

– 0.73 0.79

14 17 16

258

L.G. Gordeeva et al. / Microporous and Mesoporous Materials 112 (2008) 254–261

Fig. 4. Isobars of methanol sorption on (solid symbols) and desorption from LiCl(31 wt.%)/SiO2 composite (open symbols) as well as equilibrium data taken from experimental isosters for LiCl(21 wt.%)/SiO2 composite (crossed symbols) at methanol pressure PMeOH = 47 (1), 114 (2), 186 (3) and 300 (4) mbar.

Fig. 5. Isobars of methanol sorption on (solid symbols), desorption from LiBr(29 wt.%)/SiO2 composite (open symbols) at methanol pressure PMeOH = 30 (1), 43 (2), 100 (3) and 160 (4), 227 (5) and 347 (6) mbar.

reaches w = 0.7 g/g or n  3 mole/mole within a narrow temperature range of 3–5 K. Afterwards, with increasing temperature a gradual rise of methanol sorption is observed (Fig. 4). The sorption and desorption isobars nearly coincide, no clear sorption hysteresis was revealed. The uptake gradual growth with a decrease in temperature at n P 3 mole/mole experimentally observed (Fig. 4) is typical for LiCl–CH3OH solution [38,39]. In the methanol vapour pressure range PMeOH 47–300 mbar the amount adsorbed reaches n = 3 mole/mole at temperature T = 303–343 K. The solubility of LiCl in liquid methanol s = 0.438 g LiCl/g CH3OH at T = 303 K, which corresponds to n = 3.03 mole CH3OH/mole LiCl and increases with temperature [37]. Consequently, the LiCl Æ nCH3OH system is liquid at n > 3. It consists of two phases (LiCl–

Fig. 6. Temperature-invariant curves of methanol sorption (solid symbols) and desorption (open symbols) for LiCl(31 wt.%)/SiO2 composite (1), bulk LiCl (2 and 3), silica gel (4) and curve simulated as a linear superposition of methanol adsorption on bulk LiCl and the silica gel (5).

CH3OH solution and CH3OH vapour) and two components (LiCl and CH3OH), so that according to the Gibb’s phase rule, it is divariant. The sharp rise in methanol adsorptivity, detected at 0 < n < 3 indicates the monovariant behaviour of the system LiCl–CH3OH. This means that the system consists of three phases, probably LiCl, LiCl–CH3OH solution and CH3OH vapour, and for each pressure PMeOH the transition occurs at the fixed temperature T = f(PMeOH). A plateau with n = const, corresponding to the formation of stable crystalline solvates LiCl Æ CH3OH and LiCl Æ 3CH3OH, typical for the bulk system LiCl–CH3OH [40,41], was not detected on the methanol sorption isobars by the composite. One mole of LiCl absorbs 3 moles of methanol and transforms directly into LiCl–CH3OH solution inside pores of silica gel. This agrees with the analysis XRD data, indicating that there is no crystalline solvate formation during methanol sorption by the LiCl(31 wt.%)/SiO2 composite. The main features of methanol sorption equilibrium for LiBr(29 wt.%)/SiO2 composite (Fig. 5) are similar to those measured for the LiCl(31 wt.%)/SiO2 composite. The sharp increase in the methanol adsorptivity from n  0 to 1 mole/ mole is followed by a gradual increase in the range n = 1– 7 mole/mole, which is typical for the LiBr–CH3OH solution [42,43]. The formation of crystalline solvates LiBr Æ nCH3OH (n = 1, 3, 4), known for the bulk system, was not detected [37,43]. Sorption hysteresis was absent. All sorption isobars of LiCl/SiO2 and LiBr/SiO2 composites appear to lie satisfactorily along the same curve when presented as the uptake n vs. the free sorption energy F = RTln(PMeOH/Ps), where Ps is a saturation methanol pressure at temperature T (Fig. 6). Consequently, the methanol sorption process on the composites tends to obey the Polanyi principle of temperature invariance [44].

L.G. Gordeeva et al. / Microporous and Mesoporous Materials 112 (2008) 254–261

Comparison of the sorption data of the composite and bulk LiCl (Fig. 6) shows the following changes in sorption equilibrium: • The shape of the sorption isobars branches for the bulk salt is similar to that of the embedded salt but shifted along the free sorption energy axis by about 1150 J/ mol to higher energies. Thus, the LiCl sorption ability increases due to confinement into the silica gel pores that agrees with the water sorption data on the composites ‘‘salt inside porous matrix’’ [9,10]. • No sorption–desorption hysteresis was found for the interaction between methanol and confined salt. This is contrary to dispersed LiCl, where the sorption and desorption branches for the bulk salt differ greatly demonstrating a strong hysteresis. • Like LiCl(31 wt.%)/SiO2 composite no formation of the LiCl Æ CH3OH solvate was observed during sorption by the bulk salt. • The desorption branch of the bulk salt demonstrates a step-wise shape with a pronounced plateau at F = 4250–5100 J/mol that corresponds to w  0.75 g/g or n  1 mole/mole, indicating the formation of stable LiCl Æ CH3OH solvate. This agrees with the data of [37], which described the formation of LiCl Æ CH3OH during the decomposition of the highest crystalline LiCl Æ 3CH3OH solvate. At F > 5100 J/mol the bulk solvate LiCl Æ CH3OH decomposes giving LiCl. From Fig. 6 it is clear that the methanol sorption by the composite can not be represented as a simple addition of the sorption by the bulk salt and the host material (silica) taken with appropriate weight coefficients. This corroborates an earlier supposition of a synergistic effect of both the confined salt and the silica gel matrix on the sorption of the composite [8]. The reason for this nonadditivity could be the dispersion of LiCl to a nano-sized

259

scale inside the silica pores, resulting in quantitative and qualitative altering of its sorption properties (size effect) [8–10]. With decreasing particle size, bulk properties are lost as the fraction of surface atoms becomes large [4– 6,21]. Another reason for the non-additivity could be the surface interaction between LiCl and the silica gel during the composite preparation, or guest-host interaction [6,11]. However, according to the data of DD analysis (see Chapter 2.1) only a minor part of the salt (3%) forms the surface complexes „Si–O  Li+ in the composite, therefore the interaction between the salt and the host matrix is highly unlikely to significantly affect the composite properties. Hence namely the size effect, or dispersion of the salt to nano-sized scale, could be responsible for the alteration of the sorption equilibrium of the system LiCl–CH3OH confined to silica gel pores. The isosteres of methanol sorption on LiCl(21 wt.%)/ SiO2 and LiBr(24 wt.%)/SiO2 composites are straight lines in the Clapeyron–Clausius coordinates (Fig. 7). Isosteric enthalpy DHis and standard entropy DS0 of methanol sorption were evaluated using the Van’t Hoff equation [45] lnP MeOH ¼ DH is =RT  DS0 =R: The data obtained are presented in Table 2. The sorption enthalpy and standard entropy are nearly constant for the whole methanol uptake range for both composites. This indirectly confirms that crystalline methanolates do not form during methanol sorption, since in these complexes methanol molecules would be linked by stronger chemical bonds with the salt [8,10]. The values DHis = 41.7 ± 2.5 and 41.8 ± 2.5 kJ/mol obtained for LiCl(21 wt.%)/SiO2 and LiBr(24 wt.%)/SiO2 respectively, exceed those for metal doped alumina [14] as well as the latent enthalpy of methanol condensation DL = 35.2 kJ/mol. This indicates a strong sorbate–sorbent interaction, which is probably caused by solvation of ions by methanol molecules in the solution and the formation of Li+(CH3OH)n, X

Fig. 7. Isosters of methanol sorption on LiCl(21 wt.%)/SiO2 for n = 0.3 (1), 1.0 (2), 1.3 (3), 2.0 (4), 4.0 (5) and 4.5 (6) mole/mole (a) and on LiBr(24 wt.%)/ SiO2 for n = 0.5 (1), 1.1 (2), 2.0 (3), 2.8 (4), 4.1 (5) and 5.0 (6) mole/mole (b).

260

L.G. Gordeeva et al. / Microporous and Mesoporous Materials 112 (2008) 254–261

Table 2 Isosteric enthalpy and standard entropy of methanol sorption on LiCl(21 wt.%)/SiO2 and LiBr(24 wt.%)/SiO2 composites na (mole/mole)

wb (g/g)

DHis (kJ/mol)

DS0 (J/(mol K))

LiCl(21 wt.%)/SiO2 0.3 0.05 1.0 0.16 1.3 0.21 2.0 0.32 2.7 0.43 4.0 0.63 4.5 0.71

39.3 41.6 43.1 42.4 44.7 41.8 38.7

99.8 108.8 107.2 109.7 101.4 108.0 99.7

LiBr(24 wt.%)/SiO2 0.5 0.04 1.1 0.10 2.0 0.18 2.8 0.25 4.1 0.36 5.0 0.45

40.9 41.7 42.9 43.7 40.7 40.8

85.8 95.5 103.0 110.5 108.8 109.9

salt–methanol solution and methanol vapour) and are monovariant. At methanol uptake n > 3 and 1 mole/mole for LiCl and LiBr based composites, respectively, the salt–methanol solution forms inside silica pores and the systems become divariant. The LiCl sorption equilibrium with methanol vapour changes significantly due to the salt dispersion in the silica gel pores. The methanol sorption enthalpy DHis = 41.7 ± 2.5 and 41.8 ± 2.5 kJ/mol and standard entropy DS0 = 105 ± 10 and 102 ± 10 J/ (mol K) for LiCl(21 wt.%)/SiO2 and LiBr(24 wt.%)/SiO2, respectively were evaluated from the sorption measurements. Due to high sorption capacity wmax = 0.8 g/g and energy storage ability Emax = 1.0 kJ/g of the composites they can be considered promising for adsorptive cooling/ heating and energy storage. Acknowledgments

a

n – methanol sorption, moles of CH3OH per mole of salt. w – methanol sorption, grams of CH3OH per gram of composite sorbent. b

(CH3OH)k, Li+(Li+X)s(CH3OH)m and X (Li+X)p (CH3OH)r clusters, where X is Cl or Br [46,47]. The standard entropy of sorption DS0 = 105 and 102 ± 10 J/(mol K) was found for the LiCl(21 wt.%)/ SiO2 and LiBr(24 wt.%)/SiO2, respectively. In order to compare sorption properties for two composites with a different salt content the isosteric method data for LiCl(21 wt.%)/SiO2 was presented as a function of methanol sorbed n vs. temperature at various methanol pressures (Fig. 4). The ‘‘isobars’’ obtained agree quite well with directly measured isobars for LiCl(31 wt.%)/SiO2 composite. Furthermore, the isosteric enthalpy and the standard entropy of methanol sorption evaluated from sorption isobars for LiCl(31 wt.%)/SiO2 DHis(calc) = 43.4 ± 2.5 kJ/ mol and DS0(calc) = 115 ± 10 J/(mol K) are close to those measured for LiCl(21 wt.%)/SiO2. Thus, the main features of sorption equilibrium appear to be similar for the both samples. Taking into account the methanol sorption capacity of the composites wmax = 0.8 g/g and the isosteric enthalpy of methanol sorption DHis = 41.7 kJ/mol, the energy storage capacity can be evaluated Emax = 1.0 kJ/g, which far exceeds those of conventional methanol adsorbents. The results obtained are promising for future research, studying the performance of LiCl/SiO2 and LiBr/SiO2 composites in adsorption heat pumps, cooling and energy storage systems. 4. Conclusions This comprehensive study of methanol sorption equilibrium and phase composition of the composites ‘‘LiCl and LiBr confined to mesopores of silica gel’’ was performed. Two types of sorption equilibrium were observed. At low methanol uptake the systems consist of three phases (salt,

The authors thank Vladislav V. Malakhov and Larisa V. Dovlitova for the DD analysis. This work was partially supported by RFBR (projects N. 05-02-16953 and N. 0508-50223) and CNR-Italy (N. 0030491 11.04.2006). References [1] J.M. Nedejjkovic, Mater. Sci. Forum 352 (2000) 79. [2] A. Tuel, Micropor. Mesopor. Mater. 27 (1999) 151. [3] S.-Y. Jeong, H. Jin, J.-M. Lee, D.-J. Yim, Micropor. Mesopor. Mater. 44–45 (2001) 717. [4] L. Vradman, M.L. Landau, D. Kantorovich, Y. Koltypin, A. Gedanken, Micropor. Mesopor. Mater. 79 (2005) 307. [5] D. Bethell, D.J. Schiffrin, Nature 382 (1996) 581. [6] U. Simon, Adv. Mater. 10 (17) (1998) 1487. [7] H. Wellmann, J. Rathousky, M. Wark, A. Zukal, G. Schulz-Ekloff, Micropor. Mesopor. Mater. 44–45 (2001) 419. [8] Yu.I. Aristov, M.M. Tokarev, G. Cacciola, G. Restuccia, React. Kinet. Catal. Lett. 59 (1996) 325. [9] J. Mrowiec-Bialon, A.B. Jarzebskii, A. Lachowski, J. Malinovski, Yu. I. Aristov, Chem. Mater. 9 (1997) 2486. [10] L.G. Gordeeva, G. Restuccia, A. Freni, Yu.I. Aristov, Fuel Proces. Technol. 79 (2002) 225. [11] L.G. Gordeeva, E.V. Savchenko, I.S. Glaznev, V.V. Malakhov, Yu.I. Aristov, J. Coll. Interf. Sci. 301 (2006) 685. [12] V.E. Sharonov, J.V. Veselovskaya, Yu.I. Aristov, Int. J. Low Carbon Technol. 1 (2006) 191. [13] Yu.I. Aristov, L.G. Gordeeva, Yu.D. Pankratiev, T.M. Plyasova, I.V. Bikova, A. Freni, G. Restuccia, Adsorption 13 (2007) 121. [14] A.K. Khattak, K. Mahmood, M. Afzal, M. Saleem, R. Qadeer, Colloid. Surf. A: Physicochem. Eng. Aspects 236 (2004) 103. [15] L. Gordeeva, A. Freni, G. Restuccia, Yu. Aristov, Ind. Eng. Chem. Res. 46 (2007) 2747. [16] Yu.I. Aristov, G. Resticcia, G. Cacciola, V.N. Parmon, Appl. Therm. Eng. 22 (2002) 191. [17] R. Critoph, Y. Yang, Proc. Inst. Mech. Eng. Part E: J. Mech. Proc. Eng. 219 (2005) 1. [18] X.J. Zhang, K. Sumathy, Y.J. Dai, R.Z. Wang, Int. J. Energy Res. 29 (2005) 37. [19] K. Daou, R.Z. Wang, Z.Z. Xia, Appl. Therm. Eng. 26 (2006) 56. [20] C.Y. Liu, K. Morofuji, K. Tamura, K.-I. Aika, Chem. Lett. 33 (2004) 292. [21] W. Wang, I. Germanenko, M.S. El-Shall, Chem. Mater. 14 (2002) 3028. [22] U. Simon, M.E. Franke, Micropor. Mesopor. Mater. 41 (2000) 1.

L.G. Gordeeva et al. / Microporous and Mesoporous Materials 112 (2008) 254–261 [23] M. Clausse, K.C.A. Alam, F. Meunier, I. Bacardit, Ch. Patterer, in: Proceedings of the International Conference on Heat Powered Cycles 2006, Newcastle upon Tyne, September 11–14, 2006, p. 55. [24] L.W. Wang, R.Z. Wang, J.Y. Wu, K. Wang, S.G. Wang, Energ. Convers. Manage. 45 (2004) 2043. [25] G. Restuccia, A. Freni, F. Russo, S. Vasta, Appl. Therm. Eng. 25 (2005) 1419. [26] I. Dekany, F. Szanto, W. Armin, G. Lagaly, Berich. Bunsen-Ges. Phys. Chem. 90 (1986) 422. [27] J. Goworek, A. Swiatkowski, S. Zietek, Mater. Chem. Phys. 21 (1989) 357. [28] K. Jerabek, Z. Prokop, React. Polym. 18 (1992) 221. [29] M. Kuczynski, W.I. Browne, H.I. Fontein, K.R. Westerterp, Chem. Eng. Sci. 42 (1987) 1887. [30] Y. Hamamoto, K.C.A. Alam, B.B. Saha, S. Koyama, A. Akisawa, T. Kashiwagi, Int. J. Refrig. 29 (2006) 305. [31] O. Chiyoda, M.E. Davis, Micropor. Mesopor. Mater. 38 (2000) 143. [32] J. Pires, A. Carvalho, M.B. De Carvalho, Micropor. Mesopor. Mater. 43 (2001) 277. [33] M. Bulow, D. Shen, S. Jale, Appl. Surf. Sci. 196 (2002) 157. [34] V.V. Malakhov, J. Mol. Catal. 158 (2000) 143.

261

[35] G. Vasilyeva, V.V. Malakhov, L.S. Dovlitova, H. Bach, Mater. Res. Bull. 34 (1999) 81. [36] T.A. Krieger, L.M. Plyasova, T.M. Yurieva, Mater. Sci. Forum 321– 324 (2000) 386. [37] Gmelin Data: 2000–2005 Gesellschaft Deutscher Chemiker licensed to MDL Information Systems GmbH; 1988–1999: Gmelin Institut fuer Anorganische Chemie und Grenzgebiete der Max-PlanckGesellschaft zur Foerderung der Wissenschaften, vol. Li: SVol., pp. 395–440. [38] S.-K.J. Oh, Chem. Eng. Data 42 (1997) 1082. [39] E. Bixon, R. Guerry, D. Tassios, J. Chem. Eng. Data 24 (1979) 9. [40] E. Lloyd, C.B. Brown, D. Glynwyn, R. Bonnel, W.J. Jones, J. Chem. Soc. Part 1 (1928) 658. [41] H. Oosaka, Bull. Chem. Soc. Jpn. 12 (1937) 177. [42] P. Skabichevski, Russ. J. Phys. Chem. XLIII (1969) 2556. [43] W. Raatschen, Thermophysikalische Eigenschaften von Methanol/ Wasser-Lithiumbromidlo¨sungen. Diss. TH Aachen, 1985. [44] M. Polanyi, Trans. Faraday Soc. 28 (1932) 316. [45] P.D. Hopkins, J. Catal. 29 (1973) 112. [46] T. Megyes, T. Radnay, A. Wakisaka, J. Phys. Chem. A 106 (2002) 8059. [47] S. Mochizuki, A. Wakisaka, J. Phys. Chem. A 106 (2002) 5095.

Suggest Documents