Vibrational and Thermodynamic Study of the Adsorption of Carbon

07071 Palma de Mallorca, Spain. Received October 15, 1999. In Final Form: March 3, 2000. Microcalorimetry and vibrational spectroscopy have jointly be...
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Vibrational and Thermodynamic Study of the Adsorption of Carbon Dioxide on the Zeolite Na-ZSM-5 B. Bonelli,† B. Onida,‡ B. Fubini,† C. Otero Area´n,§ and E. Garrone*,‡ Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita` di Torino, via P. Giuria 7, I-10125 Torino, Italy, Dipartimento di Ingegneria Chimica e Scienza dei Materiali, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10126 Torino, Italy, and Departamento de Quimica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain Received October 15, 1999. In Final Form: March 3, 2000 Microcalorimetry and vibrational spectroscopy have jointly been used to study the interaction at room temperature between CO2 and Na-ZSM-5. This interaction consists of a reversible, two-step adsorption on sites approximately all equal and noninteracting, which constitute an ideal ensemble in the thermodynamic sense. From the volumetric and calorimetric adsorption isotherms, standard changes in enthalpy and Gibbs free energy due to adsorption were calculated for both the 1:1 and 2:1 adducts formed by CO2 and Na+ ions, and from these data, the standard changes in entropy were determined. All vibrational modes of adsorbed CO2 have been measured, including the usually elusive intermolecular ones, thus allowing the entropy of the adsorbed phase to be calculated by means of statistical mechanics. The vibrational modes of the carbon dioxide molecule are only slightly perturbed from the corresponding values for the gas phase. The most prominent ν3 mode is at 2356 cm-1 in the 1:1 adduct and at 2352 cm-1 in the 2:1 adduct, as compared to 2349 cm-1 in the gas phase. The ν1 and ν2 modes are at 1382 and 656 cm-1 for both adducts (1388 and 667 cm-1, respectively, in the gas phase). Comparison between volumetric and optical adsorption isotherms has allowed the estimate of the related molar adsorption coefficients for all vibrational modes of the two adducts, with the exception of the corresponding value for the ν3 mode in the 2:1 adduct, because the related band is exceedingly intense.

1. Introduction A major application of zeolite molecular sieves involves their usage as selective adsorbents in gas purification processes. Removal of carbon dioxide from various gas mixtures is needed in such industrial processes as natural gas production, helium extraction, hydrogen production, and air separation plants, to name only a few examples. Alkaline zeolites are the adsorbents of choice for carbon dioxide removal,1 and improvements in their efficiency would benefit from a better understanding of the interaction taking place between the CO2 molecule and the zeolite. From a different point of view, small molecules can be used as IR spectroscopic probes for determining local details of the structure of zeolites and related materials.2-5 For this purpose, carbon monoxide is usually chosen,2,6-10 but other probe molecules such us dinitrogen,8,11-13 dihydrogen,14-17 and several hydrocarbons5,18,19 have also †

Universita` di Torino. Politecnico di Torino. § Universidad de las Islas Baleares. ‡

(1) Towsend, R. P. Properties and Applications of Zeolites; The Chemical Society: London, 1980. (2) Zecchina, A.; Otero Area´n, C. Chem. Soc. Rev. 1996, 25, 187. (3) Lavalley, J. C. Catal. Today 1996, 27, 377. (4) Lercher, J. A.; Gru¨ndling, C.; Eder-Mirth, G. Catal. Today 1996, 27, 353. (5) Kno¨zinger, H.; Huber, S. J. Chem. Soc., Faraday Trans. 1998, 94, 2047. (6) Kustov, L. M.; Kazansky, V. B.; Beran, S.; Kubelkova, L.; Jiru, P. J. Phys. Chem. 1987, 91, 5247. (7) Zecchina, A.; Bordiga, S.; Spoto, G.; Scarano, D.; Petrini, G.; Leofanti, G.; Padovan, M.; Otero Area´n, C. J. Chem. Soc., Faraday Trans. 1992, 88, 2959. (8) Onida, B.; Gabelica, Z.; Lourenc¸ o, J. P.; Ribeiro, M. F.; Garrone, E. J. Phys. Chem. 1997, 101, 9244. (9) Otero Area´n, C.; Tsyganenko, A. A.; Escalona Platero, E.; Garrone, E.; Zecchina, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 3161. (10) Gruver, V.; Fripiat, J. J. J. Phys. Chem. 1994, 98, 8549.

been used. However, because of the small interaction energy involved, most of these probe molecules yield good results only when measurements are carried out at low temperature. Indeed IR spectroscopy at liquid nitrogen temperature has frequently been the modality of choice, although some room temperature work has also been reported.18,20,21 Carbon dioxide, by virtue of its relatively high polarizability and quadrupole energy,22 is readily adsorbed on zeolites at room temperature, thus allowing IR spectra to be taken without the need for low-temperature facilities. However, to facilitate the use of CO2 as a standard probe molecule for IR studies of zeolites, more knowledge has to be gained on the details of the corresponding IR spectra. Note that while carbon monoxide (11) Geobaldo, F.; Lamberti, C.; Ricchiardi, G.; Bordiga, S.; Zecchina, A.; Turnes Palomino, G.; Otero Area´n, C. J. Phys. Chem. 1995, 99, 11167. (12) Neyman, K. M.; Strodel, P.; Ruzankin, S. Ph.; Schlensog, N.; Kno¨zinger, H.; Ro¨sch, N. Catal. Lett. 1995, 31, 273. (13) Otero Area´n, C.; Turnes Palomino, G.; Geobaldo, F.; Zecchina, A. J. Phys. Chem. 1996, 100, 6678. (14) Kustov, L. M.; Kazansky, V. B. J. Chem. Soc., Faraday Trans. 1991, 87, 2675. (15) Bordiga, S.; Garrone, E.; Lamberti, C.; Zecchina, A.; Otero Area´n, C.; Kazansky, V. B.; Kustov, L. M. J. Chem. Soc., Faraday Trans. 1994, 90, 3367. (16) Jiang, M.; Karge, H. G. J. Chem. Soc., Faraday Trans. 1996, 92, 2641. (17) Makarova, M. A.; Zholobenko, V. L.; Al-Ghefaili, K. M.; Thomson, N. E.; Dewing, J.; Dwyer, J. J. Chem. Soc., Faraday Trans. 1994, 90, 1047. (18) Khodakov, A. Yu.; Kustov, L. M.; Kazansky, V. B.; Williams, C. J. Chem. Soc., Faraday Trans. 1993, 89, 1393. (19) Kustov, L. M. Top. Catal. 1997, 4, 131. (20) Bolis, V.; Fubini, B.; Garrone, E.; Giamello, E.; Morterra, C. Stud. Surf. Sci. Catal. 1989, 48, 159. (21) Garrone, E.; Fubini, B.; Bonelli, B.; Onida, B.; Otero Area´n, C. Phys. Chem. Chem. Phys. 1999, 1, 513. (22) Barrer, R. M.; Gibbons, R. M. Trans. Faraday Soc. 1965, 61, 948.

10.1021/la991363j CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000

Adsorption of CO2 on the Zeolite Na-ZSM-5

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has been extensively studied in this context, comparatively few studies dealt with carbon dioxide, and they refer mainly to zeolites belonging to the structure types A 23 and X 3,24 where cation concentration is very high. We report on a combined microcalorimetric and FTIR spectroscopic study of CO2 adsorption, at room temperature, on the Na-ZSM-5 zeolite. ZSM-5 belongs to the structure type MFI25 and is a high-silica zeolite having a low concentration of extraframework cations. This fact should facilitate thermodynamic and spectroscopic studies, since interaction among molecules adsorbed on nearby cations should be minimal. The combined use of both experimental techniques is a relevant feature of our work, which enables one to derive understanding of both structural details and thermodynamics of the adsorbateadsorbent interaction. 2. Experimental Section The Na-ZSM-5 zeolite was prepared by template-assisted hydrothermal synthesis, using appropriate amounts of sodium silicate, aluminum nitrate, and tetrapropylammonium bromide solutions and following standard procedures.26 The nominal Si: Al ratio was 14:1. The Na:Al ratio was close to 1. Powder X-ray diffraction showed good crystallinity and the absence of any diffraction lines not assignable to the MFI-type structure.26,27 For IR studies, a thin self-supported wafer (surface density about 7 mg cm-2) was prepared and outgassed in a dynamic vacuum (residual pressure < 10-4 Torr) for 2 h at 723 K inside an IR cell which allowed in situ gas dosage. The same outgassing procedure was used before calorimetric measurements. Infrared spectra were obtained, at room temperature, by using a Perkin-Elmer 1720 FTIR spectrometer operated at 2 cm-1 resolution. Microcalorimetric measurements were carried out by means of a Setaram Calvet-type instrument, operated at 303 K. The microcalorimeter was equipped with a volumetric attachment of conventional design for gas dosage and measurement of the corresponding adsorption isotherm.

Figure 1. Room temperature IR spectra of CO2 adsorbed on Na-ZSM-5 outgassed at 723 K. Asymmetric stretch (ν3) region, equilibrium pressure up to 0.26 Torr. Insets: right-hand side, magnification of the 13CO2 asymmetric stretch region; left-hand side, magnification of the spectral features at very low coverages.

3. Results 3.1. Infrared Spectra. CO2 belongs to the D∞h symmetry group and has four fundamental modes: the symmetric stretching ν1, the doubly degenerate bending vibration ν2, and the asymmetric stretching vibration ν3. For symmetry reasons, the ν2 and ν3 modes are infrared active, whereas ν1 is only Raman active. In the free molecule,28 these modes appear at 1388.3 (ν1), 667.3 (ν2), and 2349.3 (ν3) cm-1. For CO2 adsorbed on the faujasitetype Na-X zeolite, several IR adsorption bands were observed in the range 1420-1720 cm-1, which were assigned to carbonate species3,24 formed by CO2 bicoordinated to an extraframework Na+ (through one oxygen end of the CO2 molecule) and to an oxygen atom of the zeolite framework (through the carbon atom). Our IR spectra of CO2 adsorbed on Na-ZSM-5 did not show any absorption bands in the wavenumber region corresponding to carbonate species. Figures 1 and 2 show the spectral region corresponding to the ν3 mode of CO2 adsorbed on Na-ZSM-5; Figure 1 depicts the IR spectra for small CO2 adsorbed amounts (0.0035 up to 0.26 Torr, equilibrium pressure), and the (23) Fo¨rster, H.; Schuman, M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1149. (24) Jacobs, P. A.; Van Cauwelaert, F. M.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1 1973, 69, 2130. (25) Meier, W. M.; Olson, D. H. Atlas of Zeolites Structure Types; Butterworth-Heinemann: London, 1992. (26) Szostak, R. Molecular Sieves; van Nostrand Reinhold: New York, 1989. (27) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J. Phys. Chem. 1981, 85, 2238. (28) Herzberg, G. Molecular Spectra and Molecular Structure; van Nostrand Reinhold: New York, 1950.

Figure 2. Room temperature IR spectra of CO2 adsorbed on Na-ZSM-5 outgassed at 723 K. Asymmetric stretch (ν3) region, equilibrium pressures in the range 0.26-110 Torr. Inset: magnification of the 13CO2 asymmetric stretch region.

spectra corresponding to higher CO2 adsorbed amounts (up to 110 Torr) are shown in Figure 2. All of these spectra are dominated by a main band at 2356 cm-1, which slightly shifts toward lower frequency when the CO2 equilibrium pressure is high (Figure 2). This band can be readily assigned to the ν3 mode of carbon dioxide forming Na+‚‚‚ OdCdO adducts with extraframework Na+ ions of NaZSM-5. It has a counterpart at 2290 cm-1 which corresponds to similar adducts involving 13CO2 (natural abundance of about 1%), magnified in the right-hand side inset in Figure 2. Minor features of the spectra in Figures 1 and 2 are the small bands (or shoulders) at 2344, 2365, 2371, and 2416 cm-1 (left-hand side inset in Figure 1).

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Figure 3. Room temperature IR spectra of CO2 adsorbed on Na-ZSM-5 outgassed at 723 K in the symmetric stretch (ν1) region: equilibrium pressures as in Figure 2.

Figure 4. Room temperature IR spectra of CO2 adsorbed on Na-ZSM-5 outgassed at 723 K in the bending mode region (ν2): equilibrium pressures as in Figure 2.

These minor features will be discussed later. Upon outgassing at room temperature, all IR adsorption bands of adsorbed CO2 were found to disappear. This fact demonstrated the reversible character of CO2 adsorption on Na-ZSM-5. Figure 3 reports the IR spectra in the region of the IRforbidden (for the free molecule) ν1 mode, which is observed at 1382 cm-1. Note that the intensity of this band is relatively low and it was hardly observable for small CO2 doses; the spectra in Figure 3 correspond to those shown in Figure 2 for the ν3 mode. Figure 4 depicts the wavenumber region corresponding to the ν2 mode for the same spectra shown in Figures 2 and 3; the peak of the ν2 mode is observed at 656 cm-1. We note that in both Figures 3 and 4 there are no appreciable shifts in the

Bonelli et al.

Figure 5. Calorimetric and volumetric data concerning the room temperature adsorption of CO2 on Na-ZSM-5 outgassed at 723 K: (a) volumetric isotherm (adsorbed amount vs equilibrium pressure); (b) calorimetric isotherm (evolved heat vs equilibrium pressure).

bands location with increasing equilibrium pressure, in contrast with the data in Figure 2. 3.2. Volumetric Measurements and Microcalorimetry. Figure 5a reports the volumetric isotherm (adsorbed amount versus pressure) for the adsorption of CO2 at 303 K on Na-ZSM-5. Figure 5b reports the corresponding calorimetric isotherm (evolved integral heat versus equilibrium pressure). The reversible nature of the adsorption was checked by carrying out a second adsorption run after desorption. From these data, the plot in Figure 6a (differential heat as a function of equilibrium pressure) was drawn. The first two doses show values of differential heat (62 and 53 kJ mol-1) which appear to be too high (see later). Discarding these values, the differential heat of adsorption is seen to decrease smoothly with coverage: an initial value of about 48 kJ mol-1 can be obtained by extrapolation to vanishing coverage, whereas at higher CO2 coverage the differential heat reaches a value of 32 kJ mol-1. The curve in Figure 6a (interpolating the data) has been obtained not by any fit to the experimental points but through an overall thermodynamic procedure described below. Figure 6b shows the population of the different sites (see below) as a function of CO2 equilibrium pressure. 4. Discussion Several bonding modes can, in principle, be proposed for coordination of CO2 to metal atoms in metal complexes: side-on, C-coordination, chelating bent, end-on bent, and end-on linear.29 All of these bonding modes are possible candidates for the interaction of carbon dioxide with extraframework cations in zeolites. In addition, CO2 can also act as a bidentate ligand (through both oxygen ends) bridging two adjacent cation centers or forming carbonate species. Actually, only the end-on linear and the carbonate bonding modes were found in previous IR (29) Pacchioni, G. Surf. Sci. 1993, 281, 207.

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The shift of the ν3 band from 2356 cm-1 at low CO2 equilibrium pressure down to 2352 cm-1 for the highest doses (Figure 2) can be explained in terms of a stepwise adsorption process where an Na+ initially coordinated to one CO2 molecule accepts a second ligand, following the scheme

Figure 6. (a) Differential heats for room temperature adsorption of CO2 on Na-ZSM-5 outgassed at 723 K; (b) calculated populations of sites bearing nil, one, and two CO2 molecules, respectively (see text).

spectroscopic studies of carbon dioxide adsorbed on zeolites.3,23,24 For CO2 on Na-X, Lavalley3 reported IR adsorption bands at 1365, 1431, 1488, and 1711 cm-1, which were all assigned to carbonate species. Since we found no bands in this frequency range, it should be concluded that carbonate species are not formed in the CO2/Na-ZSM-5 system. The contrast to the CO2/Na-X system can be explained in terms of a lower basicity of framework oxygens in ZSM-5 (as compared to faujasites) due to the much higher Si:Al ratio; this smaller basicity of the zeolite framework would preclude formation of carbonate species by adsorbed carbon dioxide. The main IR absorption is that at 2356 cm-1, assigned to the ν3 mode of CO2. This vibration mode of CO2 appears at 2349.3 cm-1 in the gas phase and is shifted to higher frequency when the molecule interacts (end-on) with cation centers: a value of 2361 cm-1 was reported23 for CO2 adsorbed onto the Na-A zeolite. The IR absorption of the ν3 mode is very intense, and the corresponding band goes out of scale for moderate CO2 equilibrium pressures (0.40.8 Torr). Figure 2, reporting the spectra at medium to high coverages, shows bands truncated at the optical density of 2.0. To follow the spectral changes, recourse has to be made to the 13CO2 isotopic band at 2290 cm-1 (inset in Figure 2). The peak of this band shows a shift with coverage from 2290 to 2286 cm-1, which corresponds to a shift in the main 12CO2 band from 2356 to 2352 cm-1: such a shift is indeed visible as a displacement of the center of mass of the band initially at 2356 cm-1, shown by the horizontal arrow. A component also grows at 2274 cm-1 (inset to Figure 2) corresponding to 2339 cm-1 in the 12 CO2 band. Parallel experiments on silicalite (the purely siliceous substance with MFI-type structure) indicate that this band is due to physisorbed CO2 interacting with the zeolite framework.

Z-Na+ + (CO2)g T Z-Na+(CO2)

(1)

Z-Na+(CO2) + (CO2)g T Z-Na+(CO2)2

(2)

where Z represents the zeolite framework. This hypothesis is in agreement with previous studies30 regarding carbon monoxide adsorption on Na-ZSM-5 and suggesting that more than one CO molecule can be adsorbed on the same cation site. Similarly, Hadjiivanov and Kno¨zinger31,32 found that two CO molecules (or two N2 molecules) can be coordinated to the same Na+ in Na-Y. Further evidence to support formation of bicoordinated species comes from our volumetric adsorption measurements. The two adducts will be referred to hereafter as 1:1 and 2:1 species. From its nominal composition, it is inferred that the Na-ZSM-5 sample used contains, at the most, about 1000 µmol g-1 of Na+ adsorbing sites. The volumetric isotherm in Figure 5a shows that the adsorbed amounts of CO2 reach larger values: note that a plateau has not yet been attained. These facts suggest coordination of more than one molecule per adsorption site. The downward shift of the ν3 band due to coordination of the second CO2 molecule (high equilibrium pressure, Figure 2) amounts to about -4 cm-1. This small value is in agreement with the observation made by Hadjiivanov and Kno¨zinger31,32 that coordination of two CO molecules to the same Na+ gives rise to a single unresolved band at a frequency slightly lower than that corresponding to monocoordinated Na+‚‚‚CO adducts. Note that frequency lowering is due to the fact that when two adsorbed molecules share the same cation site, each of them should undergo a weaker interaction with the Na+. We turn now to the minor bands, at 2371, 2365, 2344, and 2416 cm-1 in Figure 1 (see left-hand side inset for the first three bands). The weak band at 2371 cm-1 is assigned to CO2 interacting with Al3+ present in extraframework (aluminum oxide) species. A similar assignment was reported by Morterra et al.33 as far as the bulk Al2O3 was concerned. To further support this assignment, a wafer of Na-ZSM-5 was outgassed at a temperature of 673 K (instead of 723 K). IR spectra of CO2 adsorbed on this sample (not shown) gave a band at 2371 cm-1 significantly less intense. Thermal activation of zeolites (high-temperature outgassing) is known to be a major factor determining the extent of dealumination,7,13,34,35 with attendant formation of extraframework species which usually escape detection by X-ray diffraction.35 Therefore, the observed behavior of the intensity of the band at 2371 cm-1 supports the assignment made. It should also be pointed out that Al3+ in extraframework species have a higher polarizing power than Na+,13,21,34 thus explaining the larger hypsochromic shift of the ν3 mode of adsorbed (30) Bordiga, S.; Escalona Platero, E.; Otero Area´n, C.; Lamberti, C.; Zecchina, A. J. Catal. 1992, 137, 179. (31) Hadjiivanov, K.; Kno¨zinger, H. Chem. Phys. Lett. 1998, 303, 513. (32) Hadjiivanov, K.; Massiani, P.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 1999, 1, 3831. (33) Morterra, C.; Zecchina, A.; Coluccia, S.; Chiorino, A. J. Chem. Soc., Faraday Trans. 1 1977, 73, 1544. (34) Sayed, M. B.; Kydd, R. A.; Cooney, R. P. J. Catal. 1984, 88, 137. (35) Otero Area´n, C.; Turnes Palomino, G.; Escalona Platero, E.; Pen˜arroya Mentruit, M. J. Chem. Soc., Dalton Trans. 1997, 873.

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CO2. We stress the fact that the band at 2371 cm-1 (Figure 1) is rather weak, which implies that CO2 coordinated to the extraframework species constitutes a negligible part of the total amount adsorbed on the zeolite in FTIR experiments. The high values of the differential heat in the first two doses in calorimetric measurements may be related to the presence of some Al3+, because we have evidence that unwanted processes of dealumination occur more readily in the calorimetric frame. The broad IR absorption band at 2416 cm-1 (Figure 1, main body) is assigned to a combination mode ν3 + νis, where νis denotes the cation-oxygen stretching of the Na+‚‚‚OdCdO adduct, hereafter also called intermolecular stretch. This assignment is supported by quantum chemical calculations to be described elsewhere.36 The frequency of the intermolecular stretching mode in the 1:1 complex results therefore in νis ) 60 cm-1. The same band is observed in Figure 2 to shift somewhat with coverage to lower frequency: a rough estimate of its location is 2395 cm-1. This corresponds to a value for νis in the 2:1 complex of about 45 cm-1. The nature of the minor bands at 2365 and 2344 cm-1 is more debatable. We tentatively assign the band at 2365 to CO2 molecules acting as bidentate ligands, and that at 2344 cm-1 to CO2 linearly coordinated to a Na+ less exposed then those originating the main band at 2356 cm-1. Although there is supporting evidence21,30 (from studies on CO adsorption) that not all of the Na+ in Na-ZSM-5 are situated at exactly equivalent local environments, we stress the fact that assignment of these minor bands should remain tentative. Note, however, that 2344 cm-1 is a wavenumber lower that that of free CO2 (2349.3 cm-1) but higher than CO2 adsorbed on silicalite (2341 cm-1). We now consider the other IR modes of adsorbed carbon dioxide. Spectra in Figure 3 show that the IR-forbidden ν1 mode is seen at 1382 cm-1, i.e., the shift from the gasphase value is ∆ν1 ) -6 cm-1. The band steadily grows with increasing CO2 equilibrium pressure and does not shift: we take this as evidence that both the 1:1 and the 2:1 adducts absorb at the same frequency. Two ill-defined and weak bands are also seen at about 1403 and at 1367 cm-1, assigned to positive and negative combination modes, respectively, of ν1 with a soft intermolecular vibration. Comparison with computed vibrational modes36 allows such intermolecular vibration to be identified as a mode of frustrated rotation (libration) with a frequency νfr, so that the two bands have wavenumbers values ν1 + νfr and ν1 - νfr, respectively. Therefore, the value of νfr results in 18 cm-1. The center of mass of the two combination modes (1385 cm-1) differs slightly from the actual value of the ν1 band (1382 cm-1) because of anharmonicity. When comparison is made with the ν3 + νis mode (discussed above) which generates the band at 2416 cm-1, a band is expected at ∼2295 (i.e. 2356 - 60) cm-1, due to the ν3 - νis mode. This is not readily observed, because the band superimposes with that due to 13CO2. The ν2 mode (Figure 4) falls at 656 cm-1, slightly downward shifted with respect to the gas-phase value (ν2 ) 667.3 cm-1), which is seen as a minor component of the spectra. Also in this case, no shift with coverage is seen. Since the populations of 1:1 and 2:1 complexes vary with pressure, in a fashion described in more detail below, it is inferred that both types of adducts share the same ν2 mode at 656 cm-1. It is remarkable that neither the νfr nor the νis mode is coupled with this vibration, and no related combination mode is seen. (36) Garrone, E.; et al. J. Phys. Chem., submitted for publication.

Bonelli et al. Table 1. Vibrational Modes of CO2 in the Gaseous Phase, Adsorbed on Silicalite and on Na Substituted ZSM-5 (cm-1)a vibration ν2 ν1 ν3 ν3 + νis νis ν1 + νfr νfr ν3 (13CO2)

gaseous CO2

CO2/ silicalite

667.3 1388.3 2349.3

662 2341

2284.5

2274

CO2/Na-ZSM-5 lp

hp

656 (24) 1382 (4.2) 2356 (397) 2416 60 1402 20 2290

656 (15) 1382 (2.4) 2352 ≈2395 ≈45 1402 20 2287

a

Low pressure (lp) is equivalent to 1:1 adducts; high pressure (hp), to 2:1 adducts. Wavenumbers are in cm-1; molar absorption coefficients (in parentheses, when determined) are in kilometers per mole.

Table 1 summarizes the main IR absorption bands discussed above. For comparison, corresponding values for free carbon dioxide28 and for CO2 adsorbed on silicalite (the pure silica zeolite with MFI structure) are also given. The thermodynamics of the adsorption process will now be discussed. As shown in Figure 6a, the differential heat of adsorption of CO2 on Na+ extrapolates to the value of 48 kJ mol-1 at vanishing coverage, and amounts to 32 kJ mol-1 at a high equilibrium pressure. Dunne et al.37 have recently reported values for the heat of adsorption of CO2 on Na-ZSM-5, ranging between 50 and 30 kJ mol-1 as a function of the adsorbed amount, in excellent agreement with the present data. According to Barrer and Gibbons,22 the enthalpy of adsorption of CO2 on the zeolite Na-X at a low coverage also amounts to 48 kJ mol-1, notwithstanding the formation of carbonate species3,24 which are not formed in the CO2/Na-ZSM-5 system, as shown by our IR spectroscopic results. Coordination of two CO2 molecules to the same cation, as described by eq 2 above, surely affords one reason for the decrease in differential heats of adsorption when the equilibrium pressure is increased. A possible additional cause for the decrease in differential heat of adsorption with increasing CO2 equilibrium pressure could be heterogeneity of Na+ sites; less exposed Na+ would show a smaller interaction energy and would be the last ones to be coordinated by CO2. As already discussed, the weak IR adsorption band at 2344 cm-1 (Figure 1) could be evidence for less exposed Na+. However, the very low intensity of this band (and of that at 2365 cm-1) justifies consideration of the adsorbent system as being essentially homogeneous, at least as far as the CO2 molecule acting as a probe is concerned. As a consequence, the adsorption isotherms may be analyzed under the hypothesis of ideality, as far as the equivalence of sites is concerned, and it is assumed that one or two molecules may enter the coordination sphere of the Na+. Such a model of adsorption has been dealt with by some of us38 and, more recently, by Hadjivanov and Klissurski.39 Defining Nm as the number of cations per gram of adsorbent, K1 and K2 the equilibrium constants for eqs 1 and 2, and θ0, θ1, θ2 the molar fractions of sites, respectively empty, carrying one, and carrying two molecules of CO2, it results (37) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5896. (38) Garrone, E.; Ugliengo, P. J. Chem. Soc., Faraday Trans. 1 1989, 85, 585. (39) Hadjiivanov, K.; Klissurski, D. React. Kinet. Catal. Lett. 1991, 44, 229.

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Table 2. Thermodynamic Quantities Calculated from the Volumetric and Calorimetric Isotherms: K ) Equilibrium Constant; ∆G° ) Standard Change in Gibbs Free Energy; ∆H° ) Standard Change in Entalphy; ∆S° ) Standard Change in Entropya K ∆G° ∆H° ∆S° (Torr-1) (kJ mol-1) (kJ mol-1) (J mol-1 K-1) reaction 1 reaction 2 reactions 1 and 2 av value

0.199 0.015 0.003

+ 4.0 +10.4 +14.4

-49 -21 -70 -35

-96 -181 -277

a Reactions 1 and 2 represent the successive adsorption of one and two molecules; their sum is the formation reaction of the 2:1 adduct. ∆H°(av) is the enthalpy of formation of this latter adduct per CO2 molecule (kJ mol-1).

θ0 ) 1/(1 + K1p + K1K2p2)

(3)

θ1 ) K1p/(1 + K1p + K1K2p2)

(4)

θ2 ) K1K2p2/(1 + K1p + K1K2p2)

(5)

so that the volumetric isotherm is described by

Na ) Nm(K1p + 2K1K2p2)/(1 + K1p + K1K2p2) (6) Data in Figure 5a have been fitted by using eq 6, and the following values have been found: Nm ) 775 µmol; K1 ) 0.199 ( 0.027 Torr-1; K2 ) 0.015 ( 0.0019 Torr-1. The fit was found to be excellent, and the uncertainty on the calculated parameters is small. The Nm value is not far from that expected on the basis of chemical composition. To illustrate the changes in the molar fractions of differently populated sites, Figure 6b reports the variation of θ0, θ1, and θ2 as a function of pressure for the actual values of the equilibrium constants K1 and K2. As expected, the value of θ1 increases in parallel with the decrease of θ0 for low pressures, up to a pressure of 18.2 Torr, at which the population of sites carrying one molecule reaches a maximum. It is readily checked that this pressure equals (K1K2)1/2 and that, at such a pressure, the population of empty sites equals that of doubly occupied sites; i.e., on average, the coverage is 1. At 60 Torr (highest pressure investigated in the present calorimetric measurements) the fraction of empty sites is still 0.04, and the populations of sites carrying one and two molecules are nearly equivalent. The standard Gibbs energy of reactions 1 and 2 are, respectively, ∆G°1 ) -RT ln K1 ) 4.0 kJ mol-1 and ∆G°2 ) -RT ln K2 ) 10.4 kJ mol-1. The positive values of the standard changes in free energy are related to the choice of 1 Torr as the reference for the gas phase, and simply mean that both reactions 1 and 2 are not spontaneous when taking place between their standard states at such a pressure. Taking the value of 1 atm for the reference standard pressure, the standard changes in free energy would have the more familiar negative sign. Table 2 summarizes all the relevant thermodynamic results. The calorimetric isotherm in Figure 5b can be described as

Qint ) Nm[∆H°f(1)K1p + ∆H°f(2)K1K2p2]/ (1 + K1p + K1K2p2) (7) where ∆H°f(1) and ∆H°f(2) are the molar enthalpies of formation of the 1:1 and 1:2 adducts, respectively. While ∆H°1 ) ∆H°f(1), elementary thermodynamics shows that

∆H°f(2) ) ∆H°1 + ∆H°2, because the reaction of formation of the adduct with two CO2 molecules

Z-Na+ + 2(CO2)g T Z-Na+(CO2)2

(8)

is the algebraic sum of reactions 1 and 2. Similarly, ∆G°1 ) ∆G°f(1) and ∆S°1 ) ∆S°f(1), but ∆G°f(2) ) ∆G°1 + ∆G°2, and ∆S°f(2) ) ∆S°1 + ∆S°2. Alternatively, instead of ∆H°f(2), the standard enthalpy of formation of the adduct with two CO2 molecules, the average value ∆H°f(av) ) ∆H°f(2)/2 may be considered, which measures the enthalpy of formation of the 2 molecule adduct per CO2 molecule. Data in Figure 5b have been fitted by means of eq 7, by assuming the values of Nm, K1, and K2 determined from the volumetric isotherm, given above. The only parameters to be determined were therefore ∆H°f(1) and ∆H°f(2). A way to determine these parameters is to linearize eq 7 by rewriting it as

[Qint(1 + K1p + K1K2p2)]/[NmK1p] ) ∆H°f(1) + ∆H°f(2)K2p (9) The quantity on the left-hand side is known, as it implies only experimental values or already determined parameters, and, when plotted against the equilibrium pressure, is expected to yield a straight line. The fit (not shown) was found to be quite satisfactory, and from it the values ∆H°f(1) ) -49 ( 3 kJ mol-1 and ∆H°f(2) ) -70 ( 6 kJ mol-1 were obtained. Note that this last value implies that the enthalpic content corresponding to each CO2 ligand, in the 2:1 adduct, takes the value ∆H°f(av) ) -35 ( 3 kJ mol-1. In terms of absolute values, ∆H°f(av) is smaller than ∆H°f(1) because the second carbon dioxide molecule interacts with an Na+ already coordinated to one CO2 ligand. The standard enthalpy of reaction 2 is only -70 + 49 ) -21 kJ mol-1: this (besides the sign) should also be the limit of the differential heat of adsorption at high coverages, when only reaction 2 takes place. Equations 3-7 can be used to calculate the differential heat of adsorption, dQ/dN, as a function of pressure, which takes the form

dQ/dN ) ∆H°f(1) f(p) + ∆H°f(2) g(p) where

f(p) ) (1 - y)/[1 - y + 2K2p(2 - y)] g(p) ) K2p(2 - y)/[1 - y + 2K2p(2 - y)] y(p) ) (K1p + 2K1K2p2)/(1 + K1p + K1K2p2) (10) It is straightforward to check that the limit of dQ/dN for vanishing pressures is ∆H°f(1), whereas that for very large pressures is ∆H°f(2) - ∆H°f(1). All parameters entering eq 10 are known, and the expected behavior of the differential heat is compared with the experimental results in Figure 6a: the calculated curve represents very satisfactorily the experimental points. Knowledge of ∆H°f(1) from calorimetric measurements, together with the knowledge of ∆G°f(1) from the isotherm, allows the evaluation of ∆S°f(1), the standard entropy of formation of the 1:1 adduct: this results in ∆S°f(1) ) -173 J mol-1 K-1. Taking into account that sø(gas), the standard entropy of CO2 (at 298 K and 1 atm 40), is 213.74 J mol-1 K-1, which corresponds to 269.19 J mol-1 K-1 at 303 K and 1 Torr, the standard entropy of 1:1 adducts sø(1)

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Bonelli et al.

amounts to 96.2 J mol-1 K-1 (11.6 in units of R). Such a value can be compared with that coming from statistical thermodynamics and based on all vibrational modes listed in Table 1 for the 1:1 complex. Being the thermal energy (RT) at 303 K equivalent to 207 cm-1, all vibrations markedly exceeding such a value do not contribute to the entropy of the adsorbate. In practice, only the ν2, νis, and νfr modes contribute (the last one to be counted twice), through the expression

Svib/R ) u(eu - 1) - ln(1 - e-u)

(11)

with u ) hν/kT. Insertion of the corresponding values of νis and νfr, 60 and 18 cm-1, respectively, in eq 11 yields Svib ) 8.8R, in good agreement with the calorimetric estimate of 11.6 R. We would regard a better agreement as being fortuitous, because eq 11 is the expression for the harmonic oscillator, whereas intermolecular modes are known to be strongly anharmonic. Moreover, low-lying vibrational modes of the Na+, which have been neglected, are expected to decrease in frequency upon coordination, thus yielding a positive contribution to the entropy of the adsorbed phase. Slight changes in the location of the cation in zeolites upon adsorption of CO have been documented by EXAFS.41 Following a procedure similar to that used to calculate ∆S°f(1), the value ∆S°f(2) ) -280 J mol-1 K-1 is obtained. The standard entropy of 2:1 adducts per mole of CO2 sø(av) is given by

sø(av) ) [∆S°f(2) - R ln 2 + 2sø(gas)]/2

Figure 7. Integrated optical density of the 2356 cm-1 band as a function of CO2 equilibrium pressure at very low coverages (Henry region). Scheme 1

(12)

The second term in the right-hand side expression takes into account that a residual entropy stems from the presence of two equivalent CO2 molecules. Equation 12 yields sø(av) ) 126 J mol-1 K-1, (15.2 in R units). The formation of loosely bound adducts (∆H°f(av) ) - 35 kJ mol-1 as compared to ∆H°f(1) ) - 49 kJ mol-1; see above) implies a higher residual entropy: this corresponds to the concept of the compensation effect, which is well-known in adsorption and catalysis. An overall scheme of enthalpy and Gibbs free energy changes with adsorption is given in Scheme 1. A different approach to the quantitative aspects of adsorption is to draw optical isotherms by plotting the intensities of the IR bands of adsorbed CO2 as a function of pressure. Since the 2356 cm-1 band is so intense, when dealing with optical densities less than 2 (the experimental limit), only the Henry region of the isotherm is found, i.e., the portion in which strict proportionality exists between the optical densities at 2356 cm-1 and pressure; the results are shown in Figure 7. From this linear plot, the relationship A(2356) ) 105p was obtained, where A(2356) is the integrated area (cm-1) of the band centered at 2356 cm-1, and p, as before, is the CO2 equilibrium pressure (Torr). From this result it follows that 3K1WSNm ) 105, where S is the geometrical area of the zeolite wafer, W its weight, and 3 is the molar absorption coefficient of the ν3 mode of the CO2 ligand in the 1:1 adduct. The above equation leads to 3 ) 397 km mol-1. Such a value of 3 is to be compared with that corresponding to the free molecule. However, reliable experimental data on the intensity of the ν3 mode at 2349.3 cm-1 for the gas-phase (40) Atkins, W. P. Physical Chemistry; Oxford University Press: Oxford, U.K., 1986. (41) Lamberti, C.; Bordiga, S.; Zecchina, A.; Salvalaggio, M.; Geobaldo, F.; Otero Area´n, C. J. Chem. Soc., Faraday Trans. 1998, 94, 1519.

CO2 are not available, because 3 rather strongly depends on the CO2 pressure. Ab initio quantum mechanics calculations36 give 3 ) 650 km mol-1: coordination of CO2 to Na+ seems to result in a decrease of the specific IR intensity, just as it happens with CO.42 For the other two modes, far less intense than the ν3 mode, it is possible to draw the entire isotherm. It was (42) Ferrari, A. M.; Ugliengo, P.; Garrone, E. J. Chem. Phys. 1996, 105, 4129.

Adsorption of CO2 on the Zeolite Na-ZSM-5

Langmuir, Vol. 16, No. 11, 2000 4983

The same procedure was applied to the ν2 mode, and the values for 2(1) (1:1 adducts) and 2(2) (2:1 adducts) are given in Table 1. For both modes, the molar absorption coefficients for the 1:1 adduct was found to be greater than that corresponding to each molecule in the 2:1 adduct.

Figure 8. Integrated optical density of the 1382 cm-1 band as a function of CO2 equilibrium pressure. Solid curve: linear fit (see text).

observed that the integrated intensities of ν1 and ν2 modes are strictly proportional and that the intensity of the ν2 mode is about 6 times that of the ν1 mode. In Figure 8, the integrated intensity A(1382) of the ν1 mode is reported as a function of the equilibrium pressure. For these data, the expected behavior is similar to that described by eq 7 for the integral heat of adsorption, so that

A(1382) ) Nm[1(1)K1p + 21(2)K1K2p2]/ (1 + K1p + K1K2p2) (13) where 1(1) and 1(2) are the extinction coefficients of CO2 molecules for the ν1 in the 1:1 and 2:1 adducts, respectively. Equation 13 can be linearized as follows:

A(1382)(1 + K1p + K1K2p2)/[NmK1p] ) 1 + 22K2p (14) which yields the values of 1(1) and 1(2) (Table 1), from which the continuous line in Figure 8 has been calculated.

5. Conclusions The joint use of calorimetry and vibrational spectroscopy has allowed a thorough study of the interaction between CO2 and Na-ZSM-5 to be carried out. This interaction consists of a reversible, two-step adsorption on sites approximately all equal and noninteracting, i.e., constituting an ideal ensemble in the thermodynamic sense. This result does not imply that all sites are equivalent from a structural point of view, nor that near ideality is a circumstance to be encountered with any other probe molecule, the evidence provided being valid only for carbon dioxide. The volumetric adsorption isotherm and the corresponding optical and calorimetric isotherms were analyzed accordingly, and the standard changes in enthalpy and Gibbs free energy due to adsorption were calculated for both the 1:1 and 2:1 adducts. From these data, the standard changes in entropy were determined. All vibrational modes of adsorbed CO2 have been measured, including the usually elusive intermolecular ones, thus allowing the entropy of the adsorbed phase to be calculated by means of statistical mechanics. The result was in reasonable agreement with the value determined via classical thermodynamics as far as the 1:1 adduct is concerned and less satisfactory for the 2:1 adduct. The disagreement is probably related, on one hand, to the use of harmonic partition functions and, on the other, to the neglect of the changes in vibrational features of the solid brought about by adsorption. The vibrational modes of the carbon dioxide molecule are only slightly perturbed from the values for the gas phase. The most prominent ν3 mode is at 2356 cm-1 in the 1:1 adduct and at 2352 cm-1 in the 2:1 adduct, as compared to 2349 cm-1 in the gas phase. The other ν1 and ν2 modes were found at 1382 and 656 cm-1, respectively, for both adducts (1388 and 667 cm-1, respectively, in the gas phase). The combined use of the volumetric and optical adsorption isotherms of CO2 on Na-ZSM-5 has enabled IR molar absorption coefficients to be calculated for nearly all vibrational modes of adsorbed CO2, both in the 1:1 and 2:1 adducts with Na+; the only exception being the ν3 mode of the 2:1 adduct which showed an exceedingly high intensity. LA991363J