Energy & Fuels 1998, 12, 1055-1060
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Adsorption Separation of Low Concentrations of CO2 and NO2 by Synthetic Zeolites Zheng-Ming Wang,* Tsuyoshi Arai, and Mikio Kumagai Chemical Research Division, Kashiwa Lab, Institute of Research and Innovation, Takada 1201, Kashiwa-shi, Chiba 277, Japan Received May 5, 1998. Revised Manuscript Received August 13, 1998
The CO2 equilibrium adsorption and breakthrough activity of CO2-NO2 mixtures on A- and faujasite-type zeolites and Na- and proton-type (HM) mordenites were studied to determine the appropriate zeolite for separation of low-concentration mixtures of CO2 and NO2. Adsorption of CO2 correlated with the polarity of the zeolites as a result of dominant adsorption via interaction of the quadrupole moment and polar surface sites. Competitive adsorption of NO2 and CO2 gives nearly zero breakthrough times for CO2 on 5A- and faujasite-type zeolites and HM. Adsorption separation of CO2 and NO2 by zeolites depends on the NO2 adsorptivity, which is determined not only by surface polar sites but also by pore geometry and dimension. 13X had the greatest NO2 adsorptivity, but the thermal instability in NO2 adsorptive sites is the main obstacle for its use as a substrate for the separation of CO2 and NO2. Sufficient protonation of NaM leads to zero CO2 adsorption and still keeps a great NO2 adsorption. Because of the thermal stability and low NO2 desorption temperature, HM is the most promising adsorbent for separation of CO2 and NO2 using the thermal-swing adsorption method.
Introduction Dissolution of spent nuclear fuel produces large amounts of off-gas containing high concentrations of NOx and low concentrations of 14CO2. After several treatments, such as water scrubbing and adsorption by zeolites,1-3 low concentrations of NOx and 14CO2 remain in the off gas. The environmental impact of 14CO2, which has a half-life of 5730 years, is considered to be small. It is possible, however, that in the future removal of 14CO2 to a level as low as reasonably achievable (ALARA) will be desirable. Because the polarity of NOxsprimarily NO2 and NOs is greater than that of CO2, it is rather difficult to adsorb only CO2 from the mixture with the adsorption method, one of the dry methods. Thus, it is necessary to separate CO2 and NOx first and then adsorb CO2 in a second process. Zeolites are widely used as molecular sieves for gas separation4,5 because of the large variations in pore dimension and geometry. On the other hand, zeolites have many polar sites, such as surface cationic sites. Thus, zeolites can also be used as an adsorbent for the separation of gases with different polarities.4 The chemisorption effect of zeolites can be especially effective for the separation of gases present in low concentrations. The physical sieving and chemisorption effects of zeolites have been studied extensively.6-15 While ad(1) Henrich, E.; Hufner, R. Proceedings of the 16th DOE Nuclear Air Cleaning Conference, 1980; p 507. (2) Ringel, H. Proceedings of the 17th DOE Nuclear Air Cleaning Conference, 1983; p 664. (3) Fumoto, H.; Sato, S.; Ito, W.; Tamura, T.; Yoshiki, N.; Kobayashi, Y. Nucl. Technol. 1986, 75, 96. (4) Breck, D. W. Zeolites Molecular Sieves-Structure, Chemistry, and Use; John Wiley & Sons: New York, 1974. (5) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: New York, 1978.
sorption of NOx on several zeolites has been studied,4,16 a systematic study of CO2 and NOx separation by zeolites has not been reported. To obtain an appropriate adsorbent for separation of CO2 and NO2 in off gas, it is necessary to compare the adsorptivity of CO2 and NO2 on the known zeolites. Experimental results of CO2 and NO2 adsorption separation on zeolites should also be helpful for a fundamental understanding of the interaction of polar molecules with polar adsorbents. In the work reported here, we examined the adsorption and separation of CO2 and NO2 at low concentrations on conventionally synthesized zeolites. The most appropriate zeolite for CO2 and NO2 adsorption separation and the reason for the difference in CO2 and NO2 adsorptivities on zeolites are discussed. Experimental Section Sample Preparation, Characterization, and CO2 Equilibrium Adsorption. A-type zeolites (4A, 5A), faujasite-type zeolites (13X, NaY, and USY), and Na-type mordenite (NaM) were obtained from Tosoh Co. Proton-type mordenite (HM) with a proton ion-exchange ratio of 90% was obtained by (6) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5963. (7) Barrer, R. M.; Peterson, D. L. Proc. R. Soc. 1964, A280, 466. (8) Takahashi, T.; Yusa, A. Trans. Faraday Soc. 1971, 67, 3565. (9) Hyun, S. H.; Danner, P. J. Chem. Eng. Data 1982, 27, 45. (10) Suzuki, M. Adsorption Engineering; Kodansha-Elsevier: Tokyo, 1990. (11) Costa, E.; Calleja, G.; Jimeneze, A.; Pom, J. J. Chem. Eng. Data 1991, 36, 218. (12) Giacobbe, F. W. Gas Sep. Purif. 1991, 5, 16. (13) Kim, J.; Chue, K.; Kim, K.; Cho, S. J. Chem. Eng. Jpn. 1994, 27, 45. (14) Siever, W.; Mersmann, A. Chem. Eng. Technol. 1994, 17, 325. (15) Tantet, J.; Eric, M.; Desai, R. Gas Sep. Purif. 1995, 9, 213. (16) Triebe, R. W.; Tezel, F. H. Gas Sep. Purif. 1995, 9, 223.
10.1021/ef980109g CCC: $15.00 © 1998 American Chemical Society Published on Web 09/19/1998
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Table 1. Dehydration Properties, N2-Pore Volumes, and CO2 Equilibrium Adsorption Amounts of Zeolites zeolites unit cell formula 4A 5A 13X NaY USY NaM HM
Na12Al12Si12O48 Na4Ca4Al12Si12O48 Na86Al86Si136O384 Na56Al56Si136O384 H22.3Al22.3Si136O316.6 Na8Al8Si40O96 H7.2Na0.8Al8Si40O96
DSC peak W0(N2)/ Wl(CO2)/ Ws(CO2)/ temp/K cc g-1 cc-STP g-1 cc-STP g-1 440 469 458 466 476 466 460
0.29 0.37 0.34 0.32 0.18 0.22
80 100 107 89 16 73 36
21.4 11.5 16.2 1.9 0 31.8 0.9
treating NaM with 1 M NH4NO3 at room temperature for 2 days followed by calcination at 823 K for 3 h. Sample names and chemical compositions are listed in Table 1. The crystal structures of all samples were confirmed by X-ray diffraction (Rigaku, Geigerflex), and their dehydration properties were determined by differential scanning calorimetry (Shimadzu, DSC-50). The N2 adsorption isotherm at 77 K and CO2 adsorption isotherms at 333 K were determined with a highresolution volumetric apparatus (Autosorp-1, QuantaChrom). Before adsorption measurement, samples were evacuated below 1 mPa at 773 K for 2 h. Breakthrough Curves of Two-Component Mixtures. Two-component breakthrough curves were determined on an apparatus consisting of a gas mixing unit, an electric furnace capable of temperature control and temperature-programmed desorption (TPD), and a mass spectrometer with a preevacuation unit. Commercial standard NO2 and CO2 were mixed through mass flow controllers to make a mixed stream of 800 ppm NO2 and 950 ppm CO2 in a carrier gas of dried air with a dew point below 203 K. At a total flow rate of 110 cm3/min, the NO2 and CO2 mixture was passed through a 6φ-quartz column filled with 1 cm3 of sample with a particle size of 500850 µm that had been dehydrated in dried air at 723-773 K. The column temperature was maintained at 333 K. After complete breakthrough of both CO2 and NO2, the desorption property was determined by air purge at 333 K for 2 h and successive TPD in dried air with a temperature ramp rate of 10 K/min. Repeat adsorption-desorption experiments were also performed after the TPD procedure.
Figure 1. XRD patterns of zeolites.
Results and Discussion N2 and CO2 Equilibrium Adsorption on Zeolites. Figure 1 shows the typical XRD patterns of 4A, 13X, and NaM. By comparison with the reported data,17 all zeolites were confirmed to have the appropriate crystal structure. All zeolites had an endotherm at 440-480 K in their DSC curves caused by the desorption of water (Table 1). There is no exotherm, and structural changes did not occur until 773 K. These results are similar to results obtained by the previous differential thermal analysis.4 The N2 adsorption isotherms of all zeolites except 4A are of typical Type I. The micropore volumes (W0) of zeolites were calculated from the linear branch of Dubinin-Radushkevich (DR) plots at a low-pressure range (P/P0 ) 0.05-0.1).18 Values of W0 are listed in Table 1. Breck has summarized the framework density (df) and void fraction (Vs) obtained from the water saturation amount for zeolites.4 Thus, the pore volume, V0, based on water adsorption can be calculated from multiplication of Vs and the reciprocal of df. The values of V0 for A-, X-, and Y-zeolites and Na-type mordenite are 0.37, 0.38, 0.37, and 0.17 cm3/g, respectively. The values of W0 for 13X, NaY, USY, and NaM, in our case, (17) Newsam, J. M.; Treacy, M. M. J. Zeolites 1993, 13, 183. (18) Dubinin, M. M. Chem. Rev. 1960, 60, 235.
Figure 2. DR plots of N2 adsorption isotherms at the lowpressure branch.
are close to those of V0 and the other reported results.4 However, the W0 for 5A is much smaller than the corresponding V0 but is also close to the reported results.4 The N2 adsorption isotherm of 4A could not be obtained because of remarkable diffusion-determining adsorption at low temperature.4 Because N2 and CO2 are similar in their quadrupolarities, the adsorption property of N2 at the low-pressure branch can be used as an indicator of zeolite surface polarity. The DR plots of N2 adsorption at P/P0 < 0.01 on all zeolites except 4A are shown in Figure 2. N2 adsorption on HM, NaY, or USY is quite depressed at P < 0.02 kPa (ln2(P0/P) < 72.5). The order of polarity obtained from N2 adsorption is NaM,13X > 5A > HM > NaY > USY, a sequence consistent with values obtained with adsorption energy measurements.19,20 Because CO2 has a 3.5-fold greater
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Figure 3. CO2 adsorption isotherms at 333K.
quadrupole moment than N2, it is reasonable to infer that adsorption of CO2 will have the same sequence at lower pressures. The CO2 adsorption isotherms at 333 K, which are also Type I, are shown in Figure 3. CO2 adsorption on 4A can be measured by overcoming the diffusion obstacle at higher temperatures. The equilibrium adsorption amount of CO2 adsorbed at 100 kPa, Wl(CO2)), and at 0.08 kPa, Ws(CO2), which is equivalent to the CO2 partial pressure of 800 ppm in the gas stream at a total pressure of l atm, are listed in Table 1. Ws(CO2) values have the order NaM > 4A > 13X > 5A > NaY, HM, USY, which is almost the same as the order of N2 adsorption at P < 0.02 kPa and the order reported for initial CO2 adsorption energy.5 Therefore, interaction through the quadrupole moment should be the dominant factor for CO2 adsorption at low pressure. On the other hand, Wl(CO2) does not correlate with either Ws(CO2) or pore volume, which indicates that both surface polarity and pore geometry influence CO2 adsorption at higher pressure. Values of Ws(CO2) on NaY, HM, and USY are essentially zero. Separation of CO2 and NO2 on these zeolites depends on the adsorption of NO2, which will be discussed below. Breakthrough Property of NO2-CO2 Mixture. The breakthrough curves of CO2-NO2 mixtures are shown in Figure 4. CO2 passes through all zeolites in front of NO2, which is characteristic of the difference in molecular polarity. All zeolites except NaM actually have zero or near-zero CO2 breakthrough times because of competitive adsorption with NO2. NaM has the greatest CO2 breakthrough time and a replaced passing area at which the concentration level is greater than at the beginning. This indicates that NaM has a relatively strong adsorption of CO2 and that a dramatic replacement of CO2 by NO2 occurs in the column. Because the replaced passing area of CO2 is equivalent to its adsorption area, almost all of the CO2 molecules preadsorbed in the column can be replaced by NO2 molecules after complete passage of both components. On the other hand, the sequence of NO2 breakthrough times on zeolites is 13X > NaM > 5A,NaY,HM > USY,4A. Relative to 5A, the NO2 breakthrough time (19) Tsutsumi, K.; Takahashi, H. J. Phys. Chem. 1970, 74, 2710. (20) Tsutsumi, K.; Takahashi, H. J. Phys. Chem. 1972, 76, 110.
Figure 4. The breakthrough curves of CO2-NO2 mixtures at 333 K. (a) A-type zeolites, (b) faujasite-type zeolites, (c) mordenites (b,9,() NO2; (O,0,)) CO2.
on 4A is greatly reduced, despite the large number of cationic sites. Thus, NO2 adsorption on 4A occurs only near the pore entrance because of pore blockage of preadsorbed molecules. 13X has the largest NO2 breakthrough time of the zeolites, which is probably related both to the large number of cationic active sites and the fact that 13X has the largest pore opening of these zeolites. USY has few NO2 adsorption sites, despite its large pore volume. Although NaM also has a long NO2 breakthrough time and then a large NO2 adsorption, the NO2 adsorption rate is much slower, as shown by a longer mass transfer field. Na-residence, which constructs the strong active center in one-dimensional channels, is most probably responsible for this phenomenon. Even though NaM and HM have the same pore geometry, the NO2 adsorption rate can be greatly accelerated by protonation of NaM, as shown by the much shorter mass transfer field on HM. The dynamic adsorption amounts, Wd, for CO2 and NO2 obtained from the complete adsorption area of breakthrough curves are listed in Table 2. As a result of competitive adsorption with NO2, the Wd values of CO2 are smaller than the respective equilibrium values
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Table 2. Dynamic Adsorption Amount of CO2 and NO2, NO2 Cationic Site Coverage, NO2 Desorption Ratio by Air Purge, and NO2 Maximum Desorption Temperature zeolites 4A 5A 13X NaY USY NaM HM
Wd/cc-STP g-1(mol UC-1) CO2 NO2 4.6(0.3) 1.3(0.1) 2.7(1.6) 0(0) 0(0) 9.6(1.3) 0(0)
8.1(0.6) 19.5(1.5) 67.0(40) 39.9(22) 9.7(4.1) 39.9(5) 27.7(4)
θa (NO2)/%
θde (NO2)/%
Tm/K
5 18 46 41
2 2 18 30 32 22 31
493 519 435 416 417 455 409
71 53
of Ws(CO2). On the other hand, the Wd of NO2 depends on both the number of cationic sites and the pore geometry. Even though A-type zeolites have the largest numbers of cationic sites, the smaller pore dimension is the primary obstacle to NO2 adsorption. Replacement of Na+ with Ca2+ in 4A dramatically improves NO2 adsorption, but the Wd of NO2 is still much smaller than Wd values for higher Si/Al faujasite-type zeolites, which have the largest pore openings of the three types studied. The values of Wd for faujasite-type zeolites have the same order as their Si/Al ratio, that is, the number of cationic sites. For NaM, there is no pore size restriction to NO2 adsorption but the number of the cationic sites is less than in 13X, giving rise to a smaller Wd for NO2. To further examine the role of cationic sites in NO2 adsorption, we calculated the percentage of NO2 molecules adsorbed in the total cationic sites as the site coverage of NO2, θa(NO2); values are summarized in Table 2. As already shown above, not all of cationic sites in A-type zeolites may be useful for NO2 adsorption. In addition, Na ions in SI-sites of X- and Y-type zeolites are not accessible to NO2 molecules because the entrance dimension of the hexagonal prism (∼0.22 nm) is much smaller than the molecular diameter of NO2 (0.346 nm).4 Also, the sodium ion situated in the side pocket or in the niche-type cavity of mordenite may not be as easy for direct bonding with NO2.21 Thus, the θa(NO2) can only be considered as a rough indicator of the cationic contribution to NO2 adsorption. From the values in Table 2, it is clear that A-type zeolites have the lowest values of θa(NO2), further indicating that their contribution to NO2 adsorption is most limited by the small pore dimension. 13X and NaY have almost the same value of θa(NO2), which indicates that cationic sites are primarily responsible for NO2 adsorption for faujasite-type zeolites. NaM has the highest θa(NO2) of all zeolites, indicating its cationic sites are the least shielded by the zeolitic framework and, therefore, show the highest efficiency for NO2 adsorption. On the other hand, protonation of NaM gives rise to a lower θa(NO2) compared with the original NaM because of changes in the sites of NO2 adsorption. The solid acidity on HM was considered as the dominant factor for NO2 adsorption.22,23 Desorption of CO2 and NO2 by Air Purge and Temperature-Programmed Heating. Changes in the desorption concentration of CO2 and NO2 with air purge time after complete passage of both components in the adsorption branch are shown in Figure 5. With (21) Tyburce, B.; Kappenstein, C.; Cartraud, P.; Garnier, E. J. Chem. Soc., Faraday Trans. 1991, 87, 2849.
Figure 5. Desorption of CO2 and NO2 by air purge at 333K. (a) A-type zeolites, (b) faujasite-type zeolites, (c) mordenites (b,9,() NO2; (O,0,)) CO2
respect to A-type zeolites that have an effluent of CO2 but no effluent of NO2, other zeolites have almost no effluent of CO2 but a large effluent of NO2. Thus, there is a strong adsorption of NO2 only in A-type zeolites, which have pores with smaller dimensions and a relatively large number of cationic sites. Retention of CO2 in the column of A-type zeolites after complete passage of both components may indicate that one part of CO2 also has a stronger interaction with A-type zeolites. On the other hand, CO2 that is preadsorbed on other zeolites is almost completely replaced by NO2 during the adsorption process. With the exception of USY, NO2 desorption by air purge on these zeolites is a very slow process. The desorption ratios of NO2 up to 2 h of air purge in the total adsorption, θde(NO2), are in the range of 20-32% for these zeolites(Table 2). Changes in the relative desorption intensity of CO2 and NO2 with desorption temperature are shown in Figure 6. Besides a NO2 peak at around 493 K, 4A also has a CO2 desorption peak at around 403 K. CO2 desorption peak may be the result of the formation of surface carbonate species that have a comparable
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Figure 7. Repeating breakthrough curves of CO2 and NO2 mixtures on 13X.
Figure 6. Temperature-programmed desorption of CO2 and NO2 on (a) A-type zeolites, (b) faujasite-type zeolites, and (c) mordenites (b,9,() NO2; (O,0,)) CO2.
intensity with adsorbed NO2 species. Carbonate species can be formed on A-type zeolites, even at room temperature, from the previous research.24 Other zeolites have no signal for CO2 desorption but have several NO2 desorption peaks that depend on the type. Among these zeolites, NaY and HM have the simplest NO2 desorption peak with more than 90% of adsorbed species desorbed at around 523 K. Compared with NaY, 13X has a shoulder peak at higher temperature in addition to a broad peak at around 450 K, which matches the peak for NaY. This shoulder peak is the result of an interaction of NO2 with an additional site on 13X that enhances NO2 adsorption but leads to a higher NO2 desorption temperature. In addition, A-type zeolites, 13X, and NaM have NO2 desorption peaks above 673 K. These NO2 desorption peaks may be related to the formation of nitrate species, which are confirmed by another study using FT-IR.22 The temperatures of the NO2 maximum (22) Wang, Z.-M.; Arai, T.; Kumagai, M. Manuscript in preparation. (23) Ingemar Odenbrand, C. U.; Andersson, L. A. H.; Brabdin, J. G. M.; Javas, S. Catal. Today 1989, 4, 155. (24) Fo¨rster, H.; Schumann, M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1149.
desorption peak (Tm) are listed in Table 2. The order of Tm is HM < NaY < USY < 13X < 4A < 5A. Thus, NaY and HM are the more appropriate adsorbents for use in thermal swing adsorption. Repeat Adsorption-Desorption Properties of 13X. The amount of NO2 adsorbed on 13X is 1.7-fold higher than that found for NaY or HM. While this result suggests that 13X is a good candidate for adsorption separation of low concentrations of CO2 and NO2, low Si/Al ratio zeolites are limited by their heat and H2O resistance. Because of this shortcoming, it was necessary to study the recovery ability of 13X using cycles of adsorption and temperature-programmed desorption. The repeat breakthrough curves of CO2 and NO2 mixtures on 13X for three cycles of adsorption and 773 K desorption are shown in Figure 7. It is clear that the breakthrough time for NO2, which is proportional to the amount of NO2 adsorbed, decreases with increasing cycling number. An equivalent decrease in the relative intensity of the NO2 desorption peak at higher desorption temperature occurs during cycling. Because the crystal structure of 13X is unchanged after repeated adsorption, as confirmed by XRD, several possibilities other than structure destruction can be considered as the reason for the decrease in higher energetic sites. First, the desorption of nitrate species on 13X may be not adequate in the previous TPD run and geometrically influence the NO2 adsorption in the next adsorption run. Second, site migration may possibly occur during desorption of strongly adsorbed NO2 at elevated temperature. Thus, the sites on 13X for NO2 adsorption are not stable and not recovered during adsorption-TPD cycling. While the same instability in NO2 adsorption sites was observed on NaY, a decrease in adsorption of both CO2 and NO2 on NaM and HM is not observed during adsorption-desorption cycling. Therefore, from the standpoint of the thermal stability of NO2 adsorption sites, mordenites are superior to faujasite-type zeolites for the separation of low-concentration mixtures of CO2 and NO2. Conclusion CO2 adsorption at low-concentration levels correlates well with the surface polarity of zeolites. Competitive
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adsorption of CO2 and NO2 leads to a near-zero adsorption of CO2 on 5A, 13X, NaY, USY, and HM. NaM has the largest amount of CO2 adsorption among all of the zeolites. Both pore geometry and cationic sites influence the NO2 adsorption. NO2 adsorption is very restricted by the small pore dimension of A-type zeolites. 13X has the greatest NO2 adsorption because of its pore geometry and the large number of cationic sites. The NO2 desorption temperature on 13X is much greater than those of NaY and HM. The thermal stability of NO2 adsorption sites on mordenites is superior to that on faujasite-type zeolites. HM has a zero CO2 break-
Wang et al.
through time and a high level of NO2 adsorption. Considering separation, NO2 desorption temperature, and thermal stability, HM is the most appropriate adsorbent for separation of CO2 and NO2 using the thermal swing adsorption method. Acknowledgment. This research was supported financially by the Japanese Science and Technology Agency. Z.M.W. thanks Prof. K. Kaneko for helpful discussions. EF980109G
