Sr-, Zn- and Cd-exchanged zeolitic materials as water vapor adsorbents for thermal energy storage applications

Applied Thermal Engineering 106 (2016) 1217–1224

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Research Paper

Sr-, Zn- and Cd-exchanged zeolitic materials as water vapor adsorbents for thermal energy storage applications P. Aprea 1, B. de Gennaro 1, N. Gargiulo 1, A. Peluso, B. Liguori, F. Iucolano, D. Caputo ⇑ ACLabs – Laboratori di Chimica Applicata, Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Naples, Italy

h i g h l i g h t s
We prepared Sr-, Zn- and Cd-exchanged zeolitic materials as water vapor adsorbents.
Water vapor adsorption isotherms at different temperatures were collected.
Water vapor adsorption isotherms were modeled by means of the Dubinin equation.
The isosteric heat of water vapor adsorption was successfully estimated.
The specific heat storage density of the adsorbents was successfully calculated.

a r t i c l e

i n f o

Article history: Received 28 April 2016 Revised 7 June 2016 Accepted 10 June 2016 Available online 11 June 2016 Keywords: Zeolite Ion exchange Water vapor Adsorption Thermal energy storage

a b s t r a c t This paper reports the characterization of Sr-, Zn- and Cd-exchanged zeolitic materials as water vapor adsorbents, in order to evaluate the influence of the extraframework species on their adsorption properties. Both synthetic and natural substrates are taken into account. Water vapor adsorption isotherms on each ion-exchanged sample have been obtained at 298, 318, 338, and 358 K and have then been modeled using the Dubinin-Astakhov equation. Focusing on the possible implementation of such adsorbents in thermodynamic cycles, an estimation of their specific heat storage densities has been expressed. Results revealed that adsorbents of natural origin are not suitable for a valid employment in thermodynamic cycles, while FAU-type zeolite X samples exchanged with Sr2+ or divalent transition metal ions (i.e., Zn2+ or Cd2+) show a significant potential as heat storage media. The same trend of the specific heat storage density with the cationic content of the adsorbent can be identified for both series of synthetic and naturally originating materials (i.e., Zn > Sr > Cd > Na), confirming how ion exchange allows effective tuning of zeolitic substrates when employed in thermodynamic cycles based on the reversible adsorption of water vapor. Ó 2016 Published by Elsevier Ltd.

1. Introduction Adsorption refrigeration is a technology based on the storage of thermal energy in materials capable to release it in a reversible way, by means of an adsorption/desorption process. This technology appears very promising in terms of sustainable development and environment protection, because, differently from traditional refrigeration systems, it can be implemented using adsorbents and adsorbates with low or even null environmental impact

Abbreviations: 13X, synthetic FAU-type zeolite; CLINO, clinoptilolite-bearing tuff; OF, original cationic form; CE, cation-exchanged form. ⇑ Corresponding author. E-mail address: [email protected] (D. Caputo). 1 These authors equally contributed to this work. http://dx.doi.org/10.1016/j.applthermaleng.2016.06.066 1359-4311/Ó 2016 Published by Elsevier Ltd.

(i.e., zero Ozone Depletion Potential). Moreover, the energy required by a refrigerator based on adsorption/desorption cycles can be easily obtained from waste heat (e.g., coming from combustion engines or thermal plants) or solar radiation (i.e., zero Global Warming Potential) [1–10]. Due to these attractive premises, this technology is receiving much interest since about twenty years: many studies have been carried out about the more appropriate adsorbent/adsorbate couples [11,12], and different adsorption-based refrigeration cycles and applications based on solar or waste energy utilization have been considered [1]. The most suitable adsorbent/adsorbate couples for thermal energy storage should have the advantage to be completely nonflammable, thermally stable, and inexpensive. From this point of view, water vapor can be considered the de facto standard

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adsorbate. As regards adsorbents, nanoporous materials belonging to the class of zeolites were often taken into account. Indeed, with respect to other thermochemical energy storage materials such as salt hydrates, zeolites show higher volumetric heat storage densities [13], while, compared to ‘‘salt inside porous matrix” composites, they do not suffer of a significantly worrying pitfall such as the leakage of the salt solution formed during sorption [14]. Moreover, zeolites adsorption properties can be usually tuned by means of modification techniques based, for instance, on cation exchange procedures. These easy and affordable processes involve the replacement of the extra framework cations hosted in the zeolite cavities and are, in general, regulated by several parameters, as Si/Al ratio, cation exchange capacity (CEC), and cation selectivity, which are in turn function of the thermodynamic of the exchange reaction Ka and DG0 [15]. There are many studies reported in the literature about the adsorption of water vapor on as synthesized or alkali/alkaline earth metal ion-exchanged synthetic zeolites [16–24]: however, very few data related to water vapor adsorption on synthetic zeolites exchanged with strontium is available. Moreover, very less attention has been paid to water vapor adsorption on divalent transition metal ion-exchanged synthetic zeolites [25] and, more generally, on ion-exchanged natural zeolites [26,27]. For these reasons, this paper reports some preliminary results concerning the characterization of Sr-, Zn-, and Cd-exchanged zeolitic materials as water vapor adsorbents, in order to evaluate the influence of the extraframework species on their adsorption properties. Both synthetic and natural substrates are taken into account. Water vapor adsorption isotherms on each ion-exchanged sample have been obtained at 298, 318, 338, and 358 K and have then been modeled using the Dubinin-Astakhov equation. Focusing on the possible implementation of such adsorbents in thermodynamic cycles, an estimation of their specific heat storage densities has been expressed. Specific heat storage densities for each adsorbent have been calculated starting from the isosteric heat of adsorption (i.e., the ratio of the infinitesimal change in the adsorbate enthalpy to the infinitesimal change in the amount adsorbed) and typical working temperatures for adsorption-based thermodynamic cycles.

Table 1 Chemical composition of 13X- and CLINO-based materials used in this work. Amount (oxide %)

SiO2 Al2O3 Fe2O3 K2O Na2O CaO MnO MgO ZnO CdO SrO

OF-13X

OF-CLINO

48.57 32.41 – – 19.02 – – – – – –

78.03 13.53 0.08 3.83 0.28 3.31 0.03 0.91 – – –

of cationic content. The cation exchange has been performed by contacting 3 g of sample with 100 ml of a 1 M solution containing the nitrate salt of the ingoing cation at 353 K under continuous stirring for 72 h. The values of the Cation Exchange Capacity (CEC) considered in this work are 1.84 meqg1 for OF-CLINO (previously estimated by de Gennaro et al. [30]) and 4.67 meqg1 for OF-13X (calculated on the basis of the chemical formula). After stirring, liquid and solid were separated by filtration. After the cation exchange process, each sample has been cooled down to room temperature, washed repeatedly, and dried overnight at 378 K. In order to estimate the cation exchange extent, the amount of exchangeable cations has been determined with the same digestion and analysis technique exposed above. Table 2 reports the percentage of such cations in all the investigated samples. In order to verify the structural integrity of the selected materials, ruling out a possible framework collapse after the exchange process, powder diffraction patterns of each sample were determined by X-ray powder diffraction (XRPD) (PANalytical X’Pert Pro automated diffractometer equipped with a XCelerator PIXCEL 1D detector) in a 2H range from 5 to 80°, with the following operating conditions: CuKa radiation, 40 kV, 40 mA, step size 0.0131° 2H, counting time 18.87 s per step. Results confirmed the good crystallinity of each sample, and were processed by means of the GSAS package [31] in order to evaluate the unit cell parameters of the zeolitic phase (reported in Table 3).

2. Experimental 2.2. Water vapor adsorption isotherms 2.1. Sample preparation and characterization Results here reported have been obtained on a synthetic FAU-type zeolite (13X, Carlo Erba reagents) and a HEU-type zeolite, a clinoptilolite-bearing tuff coming from the Eskisßehir region (Turkey), a calc-alkaline, lacustrine, Middle-upper Miocene volcanoclastic deposit (Emirler tuff) [28,29] (hereafter CLINO, supplied by Italiana Zeoliti). The mineralogical composition of the sample, reported elsewhere [30], shows that clinoptilolite (79 wt%), is the only zeolitic phase, with minor amount of opal-CT (15 wt%), feldspar (5 wt%), and quartz (1 wt%). The chemical composition of both the zeolites in their original form was determined submitting to digestion, under microwaveinduced heating (Perkin-Elmer Multiwave 3000 oven), a weighted amount of sample in a mixed HCl, HNO3, and HF solution. After addition of H3BO3 to attain fluoride complexation, the resulting solution was analyzed by ICP atomic emission spectrophotometry, using a Perkin-Elmer Optima 2100 DV ICP-OES apparatus. Table 1 reports the results of such chemical analyses. 13X zeolite has been used in its original form (OF) and after modification by cation exchange (CE) using Sr2+, Zn2+, and Cd2+ as ingoing cations. CLINO has been used in the same CE forms and also in a Na-CE form produced to better compare with 13X-OF in terms

Water vapor adsorption isotherms at 298, 318, 338, and 358 K have been obtained using a gravimetric technique based on a McBain-type balance [32], whose schematic is depicted in Fig. 1. At ambient temperature, balance resolution is 0.121 mg, the average accuracy in the 10–100 mg range is 0.102 mg, and the average experimental error is 0.124 mg. Such parameters are also consistent with those calculated at different temperatures. Temperature control during adsorption runs is provided (with an uncertainty of 0.1 K) by a circulating thermostatted fluid system. Each sample has been previously degassed under vacuum (108 kPa) at 423 K (heating rate: 10 Kmin1) for 3 h. This temperature, sufficient to obtain the evacuation of every sample without producing any structural change or degradation of the zeolitic framework, has been chosen because representative of the actual desorption temperature in an adsorption-based thermodynamic cycle. A preliminary evaluation of the adsorption kinetic has been performed, checking, for every isotherm, the weight change of the sample every 10 min. When no weight change was observed between two consecutive measures, the final one was performed after one more hour to ensure that equilibrium conditions were reached. Furthermore, in order to verify the reversibility of the adsorption process, the samples were weighed after every

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P. Aprea et al. / Applied Thermal Engineering 106 (2016) 1217–1224 Table 2 Cation content of 13X- and CLINO-based samples expressed as molar %.

K Na Ca Mn Mg Zn Cd Sr

OF-13X

Sr-CE-13X

Zn-CE-13X

Cd-CE-13X

OF-CLINO

Na-CE-CLINO

Sr-CE-CLINO

Zn-CE-CLINO

Cd-CE-CLINO

– 100.00 – – – – – –

– 4.08 – – – – – 95.92

– 60.66 – – – 39.34 – –

– 6.21 – – – – 93.79 –

47.17 5.24 34.24 0.25 13.10 – – –

34.99 65.01 – – – – – –

20.10 7.95 – – – – – 71.95

48.81 – 9.26 – – 41.93 – –

36.04 – – – – – 63.96 –

Table 3 Unit cell parameters of 13X- and CLINO-based materials used in this work.

a b

Sample

a (Å)

b (Å)

c (Å)

b (°)

OF-13Xa Sr-CE-13Xa Zn-CE-13Xa Cd-CE-13Xa OF-CLINOb Na-CE-CLINOb Sr-CE-CLINOb Zn-CE-CLINOb Cd-CE-CLINOb

24.97 25.08 24.90 24.78 17.72 17.74 17.68 17.72 17.68

18.00 18.00 17.96 18.02 18.01

7.45 7.46 7.42 7.44 7.41

116.27 116.33 116.35 116.26 116.17

Crystal system of 13X: cubic (a = b = c, a = b = c = 90°). Crystal system of CLINO: monoclinic (a – b – c, b – 90°, a = c = 90°).

re-degassing step that followed the single adsorption run, showing no significant weight change with respect to the initial degas operation and thus suggesting an essentially reversible phenomenon.

3. Theory/calculation The adsorption isotherms have been modeled using the threeparameter Dubinin-Astakhov (DA) isotherm [33]. This is a model often used to deal with microporous materials, such as zeolites, for which a pore filling process controls adsorption phenomena. Unlike other theoretical isotherms, such as the Sips [34–38] or Toth one [39], the DA isotherm does not rely on entirely empirical expressions to describe the dependence of the maximum

adsorption capacity and of the heterogeneity coefficient on temperature. Moreover, the DA model well describes the behavior of adsorbates, such as water vapor, which are characterized by strong molecular lateral interactions. According to the DA equation, the pressure dependence of the adsorbed amount takes the following form:

q¼

W

v mol

¼

W0

v mol

exp½ðA=EÞn  A ¼ RT lnðp=p0 Þ

ð1Þ

In Eq. (1), W is the volume occupied by the adsorbate within the available micropores, vmol is the molar liquid volume of the adsorbate (which is assumed to behave as an ordinary liquid within the micropores). A is the adsorption potential, i.e., the molar energy required for the isentropic compression of the adsorbed species to the saturation pressure, R is the gas constant and p0 is the saturation pressure of the adsorbate at the temperature T. The model parameters are the available micropore volume, W0, which is a textural property of the adsorbent, the characteristic energy E, which is a measure of the adsorption strength between adsorbate and adsorbent, and the heterogeneity coefficient n. One striking characteristic of the DA model is that, using just the three parameters W0, E, n, it is possible to describe not only a single isotherm, but also the dependence of the adsorption process on temperature. In this work, the experimental adsorption data were submitted to non-linear regression using the Origin Fitting Function Builder tool to calculate the optimal values of the parameters that appear in Eq. (1). The data provided by Perry and Green [40] were used to evaluate vmol and p0.

Fig. 1. Schematic of the McBain-type balance used in this work. (A) Balance chamber. (A1) Quartz spring zone. (B) Quartz spring. (B1) Pan. (C) Toroidal oven for sample activation. (C1) Oven programmer. (D) Cylinder of N2 for chamber purge. (E) Thermostatic unit. (E1) Water jacket. (F) Gas inlet. (G) Turbomolecular pump. (G2) Backing/ roughing valve. (H) Rotary pump. (S1, S2) Pressure indicators. (T1, T2) Pressure transducers. (V1) Purge inlet valve. (V2) Purge outlet valve. (V3) Needle valve.

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4. Results and discussion Fig. 2 shows water vapor adsorption isotherms on the different cationic forms of 13X zeolite considered in this work at 298, 318, 338, and 358 K for water vapor pressures ranging from 0 to about 1.5 kPa, together with fits to the DA equation (see below). Fig. 3 shows the corresponding results for CLINO samples. Tables 4 and 5 report tabular data of the experimental points in Figs. 2 and 3, respectively. Examination of these figures reveals that all reported isotherms are IUPAC Type I, as expected for pore filling processes in microporous solids [41]. There is a strong evidence of higher water vapor adsorption capacities in 13X samples with respect to CLINO samples. Indeed, CLINO adsorbents contain phases that are not porous at all, as emerged from the mineralogical analysis. Moreover, the pore network of FAU-type zeolites contains larger cavities than that of HEU-type ones [42]. As 13X and clinoptilolite are zeolitic phases having a low Si/Al ratio, they are both known to show high affinity towards water vapor (i.e., they exploit the most of their total porosity to adsorb water vapor, also at low partial pressures). Consequently, affinities being comparable, the higher pore volume of 13X samples directly implies a higher adsorption capacity with respect to CLINO samples. Anyway, the water vapor maximum adsorption capacities of 13X samples seems sub-par with respect to the well-known capacity of commercial samples of this adsorbent, which is typically higher than 16–17 molkg1. A possible explanation for this inconsistency may be found in the relatively low degas temperature applied before performing adsorption runs. Indeed, common temperatures for full dehydration of zeolites fall in the 473–573 K range but, in this case, a lower degas temperature was chosen on purpose, in order to make it correspond to a typical working desorption temperature in thermodynamic cycles (as better discussed below). Regarding the influence of the extraframework species on the water vapor adsorption performances reported in Figs. 2 and 3, there is a clear evidence of how sodium-rich samples adsorb much less water vapor than samples exchanged with divalent cations. Such trend appears obvious for both 13X and CLINO series of adsorbents. Indeed, the replacement of two monovalent sodium cations by just one divalent cation (due to charge compensation) increases the pore volume, thus providing more place for the water

vapor molecules and increasing the adsorbed amount. As particularly regards 13X samples, Sr-CE-13X shows the best overall water vapor adsorption capacity: this partially disagrees with the fact that, among the divalent cations considered in this work, Sr2+ has the largest hindrance. According to the literature, the extent of the Na-Sr exchange is significantly greater than that of Na-Zn and Na-Cd exchanges [43–45], thus compensating, in terms of pore volume, for the larger radius of the ingoing cation. On the contrary, in CLINO samples, the smaller hindrance of Zn2+ seems to be the key factor for obtaining a higher pore volume, i.e., a higher water vapor adsorption capacity. Indeed, clinoptilolite is essentially unselective towards the divalent cations considered in this work, in accordance with the Eisenman-Sherry theory, as previously reported [46,47], thus strongly limiting the substitution of the cations present in the OF-CLINO, which, among other things, already contains significant amounts of divalent species. In order to have a more precise understanding of the phenomena examined, a modeling effort was set out using the DA isotherm. The computed values of the parameters of Eq. (1) are reported in Tables 6 and 7 for 13X and CLINO samples, respectively. The comparison between model and experimental results can be observed in Figs. 2 and 3, in which symbols pertain to experimental data and continuous curves pertain to the best fit DA theoretical isotherms. Inspection of Figs. 2 and 3 distinctly points out a good fit between model curves and experimental data. This is also proved by the values of the coefficient of determination R2 reported in Tables 6 and 7. It can be clearly noted how the values of the parameter W0 (i.e., total micropore volume) follow the trend already seen through examination of the experimental data. Indeed, the lowest value of W0 in both series of synthetic and natural adsorbents belongs to the Na-rich forms. On the contrary, the highest value of W0 among 13X samples is found for Sr-CE-13X, whereas the corresponding value for CLINO samples pertains to Zn-CE-CLINO. As regards the characteristic energy E, its physical meaning has to be discriminated from that, e.g., of the interaction energy in the Langmuir model. Indeed, the Langmuir mechanism is a ‘‘monolayer type” adsorption, and the interaction energy is a measure of the interaction between an adsorbate molecule and surface atoms. In the case of micropore filling, the interaction occurs between the

Fig. 2. Water vapor adsorption isotherms on OF-13X (a), Sr-CE-13X (b), Zn-CE-13X (c), and Cd-CE-13X (d) at 298 (circles), 318 (squares), 338 (diamonds), and 358 K (triangles). Continuous lines: best fitting DA theoretical isotherms.

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Fig. 3. Water vapor adsorption isotherms on Na-CE-CLINO (a), Sr-CE-CLINO (b), Zn-CE-CLINO (c), and Cd-CE-CLINO (d) at 298 (circles), 318 (squares), 338 (diamonds), and 358 K (triangles). Continuous lines: best fitting DA theoretical isotherms.

Table 4 Water vapor adsorption isotherms on OF-13X, Sr-CE-13X, Zn-CE-13X and Cd-CE-13X. Temperature (K)

OF-13X Pressure (kPa)

Adsorbed amount (mol/kg)

Pressure (kPa)

Sr-CE-13X Adsorbed amount (mol/kg)

Pressure (kPa)

Zn-CE-13X Adsorbed amount (mol/kg)

Pressure (kPa)

Adsorbed amount (mol/kg)

298

0.009 0.013 0.116 0.339 0.627 0.908 1.303

3.444 5.692 7.910 8.800 9.440 9.700 9.950

0.003 0.013 0.135 0.357 0.640 0.917 1.316

3.729 8.318 12.190 12.908 13.481 13.911 14.055

0.011 0.056 0.194 0.361 0.634 0.915 1.313

8.596 11.136 12.113 12.699 13.089 13.675 13.675

0.007 0.030 0.191 0.369 0.642 0.920 1.328

5.663 10.193 11.164 11.326 11.487 11.649 11.649

318

0.014 0.029 0.115 0.341 0.627 0.908 1.305

2.828 4.345 6.590 7.480 7.860 8.370 8.620

0.005 0.026 0.153 0.342 0.633 0.910 1.310

3.997 8.422 11.277 11.705 12.134 12.419 12.562

0.013 0.064 0.209 0.374 0.633 0.913 1.319

4.331 7.875 10.631 11.419 12.009 12.403 12.797

0.009 0.047 0.201 0.364 0.641 0.907 1.369

4.059 7.468 10.066 10.228 10.391 10.553 10.715

338

0.027 0.052 0.153 0.331 0.625 0.916 1.320

1.150 2.800 4.970 6.250 7.010 7.270 7.650

0.007 0.046 0.170 0.345 0.632 0.911 1.308

3.774 6.822 9.580 10.741 11.612 11.902 12.192

0.026 0.089 0.211 0.362 0.633 0.911 1.309

3.364 4.947 6.926 8.905 10.092 10.488 10.883

0.019 0.078 0.209 0.353 0.634 0.913 1.314

2.609 4.728 6.685 7.989 9.130 9.782 10.108

358

0.048 0.079 0.181 0.339 0.608 0.903 1.280

0.640 1.540 3.350 4.250 5.530 6.310 6.950

0.026 0.086 0.220 0.364 0.622 0.904 1.313

2.599 4.476 6.353 7.220 8.230 8.808 9.530

0.030 0.104 0.227 0.363 0.620 0.889 1.297

1.400 2.600 3.800 5.000 6.201 7.601 8.401

0.023 0.081 0.217 0.358 0.615 0.891 1.296

1.666 2.727 4.544 5.453 6.665 7.725 8.786

adsorbent and the volume of adsorbate residing within the micropore, and this interaction is equivalent to the characteristic energy [48]. The latter case does not necessarily imply a strict correlation between, e.g., the cationic content of a zeolitic substrate and the intensity of the adsorbent/adsorbate interaction. Indeed, the results reported in Tables 6 and 7 do not allow identifying a trend in the extent of the characteristic energy with the different forms of ion-exchanged 13X and CLINO. As an example, among all 13X

Cd-CE-13X

samples, the one showing the highest value of E is Sr-CE-13X, i.e., one of the samples with a high content of divalent cations. On the contrary, the highest value of the characteristic energy among CLINO samples is registered for Na-CE-CLINO, which is the sample with the highest content of monovalent cations. As regards the heterogeneity parameter n, because of its empirical nature, it does not point to the source of the heterogeneity, but it can be used as a macroscopic measure of the sharpness of the

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Table 5 Water vapor adsorption isotherms on Na-CE-CLINO, Sr-CE-CLINO, Zn-CE-CLINO and Cd-CE-CLINO. Temperature (K)

Na-CE-CLINO Pressure (kPa)

Adsorbed amount (mol/kg)

Pressure (kPa)

Adsorbed amount (mol/kg)

Pressure (kPa)

Adsorbed amount (mol/kg)

Pressure (kPa)

Adsorbed amount (mol/kg)

298

0.012 0.098 0.241 0.391 0.652 0.921 1.308

1.460 1.910 2.360 2.470 2.580 2.690 2.800

0.010 0.085 0.235 0.382 0.644 0.918 1.330

2.822 4.076 4.703 5.017 5.330 5.644 5.644

0.011 0.094 0.241 0.387 0.670 0.927 1.317

2.970 4.690 5.315 5.628 5.941 6.253 6.253

0.009 0.085 0.236 0.383 0.657 0.920 1.331

2.624 3.937 4.724 4.986 5.249 5.380 5.380

318

0.020 0.104 0.247 0.385 0.652 0.921 1.310

1.340 1.790 2.020 2.130 2.240 2.350 2.460

0.013 0.102 0.250 0.386 0.658 0.930 1.317

2.510 3.294 3.921 4.078 4.235 4.392 4.549

0.022 0.103 0.246 0.388 0.656 0.930 1.325

2.320 3.248 3.867 4.176 4.331 4.485 4.640

0.011 0.095 0.242 0.391 0.653 0.923 1.314

2.369 3.290 3.685 3.948 4.211 4.342 4.474

338

0.022 0.106 0.245 0.387 0.651 0.922 1.316

1.130 1.469 1.695 1.808 1.921 2.034 2.147

0.024 0.108 0.248 0.381 0.652 0.919 1.336

2.062 2.697 3.173 3.332 3.490 3.649 3.649

0.026 0.108 0.250 0.390 0.651 0.937 1.335

1.413 2.513 3.141 3.455 3.612 3.769 3.926

0.019 0.108 0.245 0.387 0.654 0.927 1.316

1.712 2.635 3.030 3.161 3.293 3.425 3.557

358

0.025 0.108 0.248 0.402 0.649 0.915 1.317

0.797 1.139 1.481 1.595 1.709 1.822 1.936

0.026 0.107 0.248 0.387 0.650 0.930 1.319

1.450 1.813 2.296 2.538 2.658 2.900 3.021

0.032 0.111 0.244 0.384 0.648 0.910 1.321

1.217 1.758 2.164 2.435 2.840 2.976 3.111

0.026 0.111 0.245 0.386 0.646 0.936 1.323

1.616 2.101 2.263 2.424 2.586 2.747 2.909

Sr-CE-CLINO

Zn-CE-CLINO

Cd-CE-CLINO

Table 6 DA parameters for water vapor adsorption on OF- and CE-13X zeolites (standard errors in brackets). Parameter 3

1

W0 (cm g ) E (kJ mol1) n R2

OF-13X

Sr-CE-13X

Zn-CE-13X

Cd-CE-13X

0.167 (0.004) 16.443 (0.256) 3.066 (0.254) 0.973

0.249 (0.007) 19.009 (0.450) 2.483 (0.259) 0.953

0.246 (0.005) 16.149 (0.260) 2.654 (0.198) 0.977

0.209 (0.004) 17.530 (0.254) 2.961 (0.207) 0.978

Table 7 DA parameters for water vapor adsorption on CE-CLINO tuffs (standard errors in brackets). Parameter

Na-CE-CLINO

Sr-CE-CLINO

Zn-CE-CLINO

Cd-CE-CLINO

W0 (cm3 g1) E (kJ mol1) n R2

0.052 (0.001) 20.866 (0.525) 1.386 (0.124) 0.976

0.116 (0.006) 16.829 (0.737) 1.091 (0.115) 0.975

0.125 (0.007) 14.878 (0.708) 1.255 (0.148) 0.966

0.114 (0.006) 16.840 (0.847) 1.060 (0.122) 0.971

micropore size distribution. For solids having narrow micropore size distribution (such as a synthetic zeolite), the DA equation with n equal to about 3 is found to describe the data well [48]. Indeed, Table 6 reports a value of n very near to 3 for both the pristine 13X sample and its Cd-exchanged form. On the contrary, Sr- and Zn-exchanged forms significantly deviate towards lower values of the heterogeneity parameter, suggesting a broadening of the micropore size distribution [48] due to a plausible frameworkdistorting effect of the ingoing cations. As regards CLINO samples, Table 7 shows very low values of n, probably due to the coexistence of other phases together with the zeolitic one, thus making the system more complex both in terms of chemical nature of adsorbing surfaces and of pore size distribution. Focusing on the possible implementation of the different cationic forms of 13X and CLINO in adsorption-based thermodynamic

cycles, an estimation of their potential heat storage capacity has to be acquired. The first step towards this aim is the determination, for each adsorbent, of the isosteric heat of adsorption. According to Do [48], the isosteric heat of adsorption (DH) can be computed from the van’t Hoff equation:


 DH @ ln p ¼  @T q RT 2

ð2Þ

Fig. 4 shows the opposite of the isosteric heat of water vapor adsorption on the different cationic forms of 13X and CLINO considered in this work as a function of the adsorbed amount q. Such plots have been obtained following the method proposed by Zukal et al. [49]. Indeed, adsorption isosteres were calculated from water vapor adsorption isotherms in the temperature range of 298–358 K. The

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Fig. 4. Isosteric heat of water vapor adsorption on different 13X (a) and CLINO (b) samples. Circles: OF-13X and Na-CE-CLINO. Squares: Sr-exchanged forms. Diamonds: Znexchanged forms. Triangles: Cd-exchanged forms. Continuous lines: fitting polynomial curves.

values of ln p corresponding to a series of adsorbed amounts q = 0.1, 1, 2, . . . molkg1 were calculated using the best fitting DA theoretical isotherms. Isosteres obtained in this way were plotted as ln p vs. 1/T. Eventually, DH was calculated from the slope of adsorption isosteres using Eq. (2). Inspection of Fig. 4 allows observing how, for low adsorbed amounts, the isosteric heat of adsorption is significantly higher in CLINO samples than in 13X samples. As particularly regards Fig. 4a, the sodium form of 13X shows the lowest values of (DH), while the Sr-rich form shows the highest ones. Interestingly, the curves concerning the 13X forms exchanged with transition metal ions are almost superimposable. Also among CLINO samples, the Na-rich form has the lowest values of the isosteric heat of adsorption, while the Sr- and Cd-exchanged samples show the highest values with interpolating curves being almost superimposable. Eventually, the curve concerning Zn-CE-CLINO significantly overlaps with the one referring to Na-CE-CLINO at low adsorbed amounts and with the remaining ones at high values of q. As is, the isosteric heat of adsorption provides little information about the potential heat storage capacity of water vapor adsorbents. Usually, the volumetric heat storage density is the most useful parameter for this kind of assessments. Being this study specifically focused on zeolitic materials, i.e., solids characterized by very comparable volumetric mass densities, a parameter that is similar to the volumetric heat storage density and can be easily estimated is the specific heat storage density DHint [50]:

Z

DHint ¼

qmax

DHdq

ð3Þ

qmin

In Eq. (3), qmin and qmax are the water vapor adsorbed amounts at the end of desorption and adsorption phase of the thermodynamic cycle, respectively. In order to assess plausible values of these quantities, working temperatures for a supposed heat storage process must be set. In this work, the same temperatures chosen by Stach et al. [50] have been considered, i.e.: Tads = 333 K, Tdes = 423 K, Tev = Tco = 283 K, where Tads is the temperature of the adsorption phase, Tdes is the temperature of the desorption phase, Tco is the temperature at which desorbed water vapor condenses, and Tev is the temperature at which to-be-adsorbed water molecules evaporate. Starting from these data, the values of the adsorption potential Amax = RTdes ln[p0(Tco)/p0(Tdes)] and Amin = RTads ln[p0(Tev)/p0(Tads)] have been calculated and used to obtain the values of qmin and qmax, respectively, for every adsorbent considered in this work by means of Eq. (1). Eventually, Eq. (3) has been integrated using the polynomial interpolations reported in Fig. 4. The results of this procedure are reported in Table 8: there is clear evidence of how synthetic adsorbents perform much better than the ones of natural origin. Moreover, for both series of 13X- and CLINO-based materials, the

Table 8 Specific heat storage densities of 13X- and CLINO-based zeolitic adsorbents. Adsorbent

DHint (kJkg1 adsorbent)

OF-13X Sr-CE-13X Zn-CE-13X Cd-CE-13X Na-CE-CLINO Sr-CE-CLINO Zn-CE-CLINO Cd-CE-CLINO

420 511 578 500 72 145 175 139

Zn-exchanged forms show the best heat storage capacities, Zn-CE-13X being the sample with the highest value of DHint among all the substrates considered in this work. Very interestingly, the same trend of the specific heat storage density with the cationic content of the adsorbent can be identified for both series of 13X- and CLINO-based materials (i.e., Zn > Sr > Cd > Na), thus confirming how ion exchange allows effective tuning of zeolitic substrates when employed in thermodynamic cycles based on the reversible adsorption of water vapor. 5. Conclusions In this work, an investigation on the water vapor adsorption properties of differently ion-exchanged FAU-type zeolite X and HEU-type clinoptilolite-rich tuff samples has been performed, in order to correlate the content of extra-framework ions with the potential heat storage capacity of the samples. Water vapor adsorption isotherms on each ion-exchanged sample have been obtained at 298, 318, 338, and 358 K and have then been successfully modeled using the Dubinin-Astakhov equation. Best fitting theoretical isotherms where then used to calculate the isosteric heat of adsorption as a basis for the estimation of the specific heat storage density under working temperatures plausible for a supposed heat storage process. Results revealed that adsorbents of natural origin are not suitable for a valid employment in thermodynamic cycles, while FAU-type zeolite X samples exchanged with Sr2+ or divalent transition metal ions (i.e., Zn2+ or Cd2+) show a significant potential as heat storage media. In particular, Zn-exchanged 13X zeolite showed the highest value of the specific heat storage density among all the substrates considered in this work. Very interestingly, the same trend of the specific heat storage density with the cationic content of the adsorbent can be identified for both series of synthetic and naturally originating materials (i.e., Zn > Sr > Cd > Na), thus confirming how ion

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