Zeolites ZSM-25 and PST-20: Selective Carbon Dioxide Adsorbents at

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Zeolites ZSM-25 and PST-20: Selective Carbon Dioxide Adsorbents at High Pressures Jung Gi Min, Kingsley Christian Kemp, and Suk Bong Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11582 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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The Journal of Physical Chemistry

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Zeolites ZSM-25 and PST-20: Selective Carbon Dioxide Adsorbents at High Pressures

Jung Gi Min, K. Christian Kemp, and Suk Bong Hong* Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 790-784, Korea E-mail: [email protected]

ACS Paragon Plus Environment


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ABSTRACT

Natural gas upgrading requires the separation of its major components, predominantly CH4 and CO2, at the high pressures found at the wellhead. Here, we demonstrate the Na+tetraethylammonium form of ZSM-25 and PST-20, the fourth and fifth generations of the RHO family of zeolites with embedded isoreticular structures, can be efficiently employed to separate CO2 from CH4 over a wide temperature and pressure range (298 - 373 K and 0 - 25 bar). Both zeolites exhibit impressive CO2/CH4 selectivity (> 20) at 298 K and 25 bar, relatively large adsorption capacity (4.0 and 3.6 mmol g-1, respectively) even at 373 K and 25 bar, and long-term durability even in the presence of H2O. These superior separation properties could be linked to the limited framework flexibilities of ZSM-25 and PST-20 mainly arising from the larger ratio of embedded to scaffold cages in the higher generation of the RHO family.


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INTRODUCTION

Selective adsorption of CO2 is an important industrial process that not only allows the conversion of this green-house gas (GHG) into useful products such as fuels and chemicals, but also minimizes its emission effects.1-6 Among a wide variety of CO2 adsorbents investigated thus far, zeolites and related microporous crystalline solids have attracted much interest because of their uniform void spaces of molecular dimensions, high surface areas, and adjustable surface selectivity.7,8 However, few of them have shown comparable CO2 adsorption capacities to those observed for other porous materials such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and amine functionalized polymers.9,10 In terms of physicochemical stability, nevertheless, zeolites have a clear-cut advantage over these state-of-the-art materials. In practical applications, an operation in which adsorption and desorption take place at high and atmospheric pressures, respectively, would be desirable to minimize the energy consumption of CO2 separation, for example, in enhanced oil recovery, natural gas sweetening, and CO2 recovery from landfill gas.11 Little attention has been directed toward the CO2 adsorption behavior of zeolites at pressures higher than 10 bar. Zeolite Rho (framework type RHO) is a small-pore material consisting of a body centered cubic arrangement of 26-hedral ([4126886]) lta cages connected through 10-hedral ([4882]) d8r units.12 Rho has been reported to show a large CO2 uptake and desirable selectivities over other small gases like N2 and CH4.13 However, it has a serious drawback as a CO2 adsorbent, because of the very slow adsorption kinetics for this GHG: at 298 K and atmospheric pressure, about 2 h is needed to achieve equilibrium.19



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We have recently discovered a new series of cage-based, small-pore zeolites with embedded isoreticular structures.19-21 As shown in Figure 1, the scaffold of this zeolite family, designated the RHO family, is built by consecutively adding a pair of 18-hedral ([41286]) pau and d8r cages along unit-cell edges (lta cages) of the RHO structure, thus expanding a by ca. 10 Å. Except its second member which remains hypothetical, the space between the scaffolds is filled by four other cage types, i.e., 14-hedral ([466286]) t-plg, 8-hedral ([4583]) t-oto, 10hedral ([4684]) t-gsm, and 12-hedral ([4785]) t-phi cages. ZSM-25 (MWF) and PST-20 are the fourth and fifth members of the RHO family, and their mixed Na+-tetraethylammonium (TEA+) cation form (denoted NaTEA-ZSM-25 and NaTEA-PST-20) was found to be useful as CO2 adsorbents at low pressures (≤ 1.2 bar).16 Here, we show that among the RHO family of zeolites, NaTEA-ZSM-25 and NaTEA-PST-20 are of potential interest as a selective CO2 adsorbent at pressures as high as 25 bar. Especially, since no detectable decrease in CO2/CH4 selectivity was found even in the presence of H2O, these embedded isoreticular zeolites could find practical applications in natural gas purification.


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Figure 1. The structures of the RHO family of embedded isoreticular zeolites ranging from Rho (RHO-G1) to PST-20 (RHO-G5) and their seven different building units.



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EXPERIMENTAL SECTION

Adsorbents Preparation. Zeolites employed in this study were synthesized according to the procedures reported in the literature.13-17,19 Further details of zeolite syntheses are described in the supporting information, Table S1. Prior to use as adsorbents, as-made zeolites were calcined at 823 K in air for 12 h to remove the occluded organic structure-directing agents (OSDAs). In the case of as-made ZSM-25 and PST-20, calcination was carried out under flowing NH3 (30 ml min-1) from room temperature to 773 K, with a ramp rate of 1 K min-1, and held at the same temperature for 4 h in order to minimize the structural collapse caused by the removal of OSDAs, i.e., tetraethylammonium ions. The calcined zeolites were then refluxed twice in 1.0 M NaNO3 solutions (1 g solid/100 mL solution) for 6 h to convert them into the Na+ form. If required, as-made materials were directly refluxed twice in 1.0 M NaNO3 solutions for 6 h, without calcination, to prepare their Na+-TEA+ form.

Gas Adsorption and Separation Experiments. The high pressure CO2, N2, and CH4 adsorption isotherms were measured using a Setaram PCTPro E&E analyser. Prior to the experiments, 0.3 g of zeolite sample were evacuated under a vacuum of 10-3 Torr at 473 K for 6 h. The temperature of the samples were subsequently reduced under vacuum until the target temperature was reached. The equilibrium conditions were fixed at 98% of the calculated uptake or at the maximum equilibration time of 60 min for each isotherm point. The adsorption isotherms in the low pressure range were carried out on a Mirae SI nanoPorosityXG analyser using 0.1 g of zeolite adsorbent placed in a sample holder. The same equilibrium and pre-treatment conditions as for the high pressure measurements were employed. Both high- and low-pressure adsorption isotherms were obtained using the corrected volumetric method. The CO2/CH4 and CO2/N2 selectivities were calculated using the respective


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equilibrium molar uptakes of CO2, CH4, and N2 at a given pressure. The breakthrough experiments were performed at 298 K and atmospheric pressure using CO2/CH4 (50:50 v/v) and CO2/N2 (50:50 v/v) gas mixtures with a total gas flow rate of 20 cm3 min-1. When necessary, a CO2/CH4/H2O (50:45:5 v/v/v) gas mixture, instead of a CO2/CH4 (50:50 v/v) one, was used. In a typical breakthrough experiment, the sample was packed in a vertically placed fixed bed micro reactor (0.63 cm inner diameter), giving a sample height of ca 0.7 cm, and then fully dehydrated in flowing He (100 cm3 min-1) at 473 K for 6 h. After cooling to 298 K, a gas mixture was passed through the sample at the target temperature. The humidity of the gas mixtures is controlled by adjusting the water temperature. The intensity of each gas passing through the sample reactor was monitored using an inline Hiden Analytical HPR20 gas analysis system detecting ion peaks at m/z+ = 4 (He+), 15 (CH3+), 28 (N2+), 44 (CO2+), and 18 (H2O+). Prior to passing a gas mixture through the samples, the mass spectrometer was stabilized for 30 min with the gas mixtures using a bypass. The feed lines are kept at an elevated temperature with heating tapes and insulation materials to prevent water vapor condensation. The experimental setup for breakthrough measurements of gas mixtures is given in Figure S1 in the supporting information. The CO2 adsorption kinetic experiments at 1.0, 10, and 25 bar and different temperatures for 3 h, at which time uptake of CO2 had reached equilibrium, were carried out on a Setaram PCTPro E&E analyser. The recycling experiments were carried out on the same analyser. Both adsorption and vacuum cycles were conducted for 10 min at the same temperature, and the procedure was repeated for 50 cycles.

Calculation of Isosteric Heat of CO2 Adsorption. The experimental adsorption data for the single gas adsorption isotherms were fitted using the dual-site Langmuir equation:

=

()

+

()

(1)



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where m(b) and b are the saturation capacity and the affinity parameter on the first set of sites, respectively, and m(d) and d are the analogous parameters on the second set of sites. The heats of adsorption were calculated by applying the Clausius-Clapeyron equation to the two adsorption isotherms at 298 and 308 K:

[( /)] =

∆

(2)

where ∆Q is the heat of adsorption, P pressure, R the gas constant, and T temperature.

CO2 IR Spectroscopy. The IR spectra in the 1000-4000 cm-1 region were recorded on a Thermo Nicolet 6700 FT-IR spectrometer using self-supporting zeolite wafers of approximately 30 mg (1.3 cm diameter). Before each measurement, the zeolite wafer was first heated up to 523 K for degassing under dynamic vacuum lower than 10-3 Torr for 6 h inside a home-built IR cell with CaF2 windows. After cooling down the wafer to room temperature, a background spectrum was recorded. Then, CO2 gas was introduced into the IR cell and equilibrated to the target pressure. Finally, the difference IR spectra were measured as a function of CO2 pressure at room temperature.

RESULTS AND DISCUSSION

Figure 2 compares the adsorption isotherms at 298 K and 0-25 bar of CO2, N2, and CH4 on the RHO family, i.e., Na-Rho (Si/Al = 3.8), NaTEA-ECR-18 (PAU; Si/Al = 3.5), NaTEAZSM-25 (Si/Al = 3.4), and NaTEA-PST-20 (Si/Al = 3.1), with those on Na-A (LTA; Si/Al = 1.0) and Na-chabazite (Si/Al = 2.3), the other two well-studied small-pore zeolites for CO2 separation.17,18 All zeolites are phase pure (Figure S2), and their unit cell composition data including TEA+ content are given in Table S2. The high-pressure CO2 adsorption isotherms in Figure 2 are all characterized as Langmuir Type I, like their low-pressure isotherms (Figure


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S3). We also found the same trend in the high-pressure isotherms of the Na+ form of ECR-18, ZSM-25, and PST-20 (Figure S4). It is remarkable that over the pressure range studied here, Na-A and Na-chabazite show non-negligible CH4 and N2 uptakes. This suggests that the energies of CH4 and N2 molecules at higher pressures may be high enough to move the blocking cation from the 8-ring windows into the cages, allowing their passage. On the other hand, considerably smaller CH4 uptakes even at 25 bar are observed for all the RHO family members, which is particularly true for NaTEA-ZSM-25. Therefore, the so-called trapdoor effect17,22 in these zeolites is still effective during CO2 adsorption at high pressures. As shown in Figure 2 and Table 1, however, this is not the case for N2 adsorption, probably due to the higher quadrupole moment of this homonuclear molecule than CH4.22 Table 1 lists the CO2 uptakes and the CO2/CH4 and CO2/N2 selectivities of the zeolites shown in Figure 2, together with those of Na-ECR-18, Na-ZSM-25, and Na-PST-20. Although Na-A and Na-chabazite show larger CO2 uptakes at 25 bar and 298 K than all RHO family members except Na-Rho, their CO2/CH4 selectivities were found to be only 2, making them unattractive as high-pressure CO2 adsorbents. A similar conclusion can be drawn from the low CO2/CH4 selectivities (4-5) of both NaTEA-ECR-18 and Na-ECR-18. The striking results that the CO2/CH4 selectivities of NaTEA-ZSM-25 and NaTEA-PST-20 are considerably higher at 25 bar (> 20) than at atmospheric pressure, in contrast to the trend observed for the other zeolites (Table 1), suggests that these zeolites are potentially useful as selective high-pressure CO2 adsorbents. Thus, they could be largely efficient in practical natural gas separation, given that the CO2 concentration in normal natural gas ranges from 5 to 40%.23 We have recently solved the structure of both ZSM-25 and PST-20 and found that among their seven different types of cages, the TEA+ ions are located only within the pau and t-plg



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cages.19 We should note here that the number and chemical shift of the 13C NMR resonances of TEA+ ions in as-made ECR-18 are essentially the same as those observed for as-made ZSM-25 and PST-20 (Figure S5). Since the 13C chemical shift of organic species occluded in zeolites is sensitive to the size of the cavity inside which they end up entrapped during the crystallization process,24 it is not very difficult to reason that NaTEA-ECR-18 also has TEA+ ions only within the pau and t-plg directly connected with pau cages by sharing 8-rings.12 Thus, when they still contain TEA+ ions, their d8r and lta cages cannot adsorb CO2, like the case of pau and t-plg cages. So then, the effective CO2 adsorption sites in NaTEA-ECR-18, NaTEA-ZSM-25, and NaTEA-PST-20 are t-oto, t-gsm, and t-phi cages only. Table 1 shows that the CO2 uptake at 298 K and 25 bar is larger in the order NaTEA-ECR-18 < NaTEAZSM-25 < NaTEA-PST-20. This can be rationalized by considering that the higher generation of this family has a larger portion of t-oto, t-gsm, and t-phi cages per unit cell (Table S3).


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Figure 2. Adsorption isotherms of CO2 (navy), CH4 (green), and N2 (pink) on (a) Na-A, (b) Na-chabazite, (c) Na-Rho, (d) NaTEA-ECR-18, (e) NaTEA-ZSM-25, and (f) NaTEA-PST-20 at 298 K and 0-25 bar. Simultaneous fit of all data with the generalized dual-site Langmuir isotherm (solid line).



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Table 1. CO2 Uptakes at 25 bar, CO2/CH4 and CO2/N2 Selectivities at 25 bar, Isosteric Heats of Adsorption, and Unit Cell Parameters of Cage-Based Small-Pore Zeolites with Different Framework Topologies CO2 uptake (mmol g-1) selectivitiesa unit cell parameter a (Å) -∆Hads ∆V c -1 b adsorbent 298 K 323 K 348 K 373 K CO2/CH4 CO2/N2 (kJ mol ) hydrated dehydrated (%) Na-A 5.1 2 (3) 2 (5) 46 Na-chabazitee 5.7 2 (3) 2 (7) 45 Na-Rho 6.1 5.6 5.3 4.7 8 (23) 11 (31) 44 15.01 14.41 11.5 Na-ECR-18 4.4 4.0 3.3 2.2 4 (20) 3 (11) 34.70 33.16 12.7 NaTEA-ECR-18 4.0 3.7 3.3 2.8 5 (11) 4 (9) 53 35.09 34.54 4.6 Na-ZSM-25 3.9 3.4 3.1 2.4 7 (4) 4 (9) 25 45.03 43.38 10.6 NaTEA-ZSM-25 4.3 4.1 4.0 4.0 45 (22) 4 (10) 65 45.16 44.93 1.5 Na-PST-20 4.4 4.1 3.8 3.2 36 (9) 6 (9) 22 55.05 54.99 0.3 NaTEA-PST-20 4.8 4.3 3.9 3.6 21 (15) 6 (10) 44 55.14 55.14 0.0 a Determined by the comparison of single-component adsorption isotherms at 298 K. The values in the parentheses are the selectivities of each zeolites measure at 1.0 bar and 298 K. bPrior to powder XRD measurements, the zeolite were pretreated under a dynamic vacuum of 0.1 Torr at 523 K for 2 h. dVolume shrinkage defined as (ahyd3-adehyd3)/ahyd3, where a is the unit cell parameter of the cubic RHO family of zeolites. eWhile the N2 sorption experiments reveal that Na-chabazite has a BET surface area of 580 m2 g-1 and a micropore volume of 0.240 cm3 g-1, the textural properties for Na-A and the RHO family of zeolites are not available due to the existence of the trapdoor effect in these small-pore materials.


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To be energy efficient, CO2 adsorbents should possess not only a large adsorption capacity and selectivity, but also fast adsorption kinetics. It has recently been reported that CO2 adsorption on Na-Rho requires ca. 2 h to reach equilibrium at 298 K and atmospheric pressure.19 As shown in Figure 3, by contrast, 10 min is necessary to attain equilibrium at 348 K and 25 bar. The problem for the application of Na-Rho is that its CO2/CH4 selectivity at this temperature and pressure is not so high (8). It is worth noting that while CO2 adsorption on both NaTEA-ZSM-25 and NaTEA-PST-20 at 348 K and 25 bar needs only 5 min to attain equilibrium, their CO2/CH4 selectivities are still higher than 20. Figure 3 also shows that the CO2 uptakes of these two zeolites at 348 K and 10 bar, i.e., under the practical conditions for this high-pressure GHG separation, remain unchanged over 50 adsorption-desorption cycles. Thus, it is clear that NaTEA-PST-20 and NaTEA-ZSM-25 are quite durable CO2 adsorbents at high pressures.



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To further investigate the CO2 adsorption behavior of the RHO family, we also measured the adsorption isotherms on Na-Rho, NaTEA-ECR-18, NaTEA-ZSM-25, and NaTEA-PST-20 at four different temperatures. Of particular interest is the fact that NaTEA-ZSM-25 exhibits a fairly smaller decrease in CO2 uptake at 373 K than the other three RHO family members (Table 1 and Figure S6). This is unexpected because the larger member of the RHO family has a higher degree of structural complexity, making its CO2 adsorption pathway more complex, viz., being more difficult to penetrate deep into the interior cages. The isosteric heats of CO2 adsorption for the zeolites studied here were calculated using the equation (2) and are listed in Table 1. As can be found in Figure S4, the isotherm at 298 K of Na-ECR-18


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Figure 3. CO2 adsorption kinetics at 298 K (left) and 348 K (right) on (a) Na-Rho, (b) NaTEA-ZSM-25, and (c) NaTEA-PST-20 at 1.0 (navy), 10 (green), and 25 bar (pink), respectively. Inset, 50 CO2 adsorption-desorption cycles at 348 K and 10 bar. gave an unusual inflection step at a low coverage (P/Po ~ 0.1), making it difficult to calculate an accurate heat of adsorption. Interestingly, NaTEA-ZSM-25 exhibits a heat of adsorption of 65 kJ mol-1 which is highest among the zeolite adsorbents studied here. This appears to be the major influence behind the relatively large CO2 uptake of NaTEA-ZSM-25 even at 373 K. In contrast, the TEA+-free Na-ZSM-25 zeolite exhibits a much lower heat of adsorption (25 kJ mol-1). Table 1 also shows that while the unit cell volumes of the Na+ form of ECR-18, ZSM-25, and PST-20 zeolites change during dehydration at 523 K, a much smaller decrease is



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observed for the higher generation (i.e., PST-20) of the RHO family. A similar trend can be found in their Na+-TEA+ form. But the extent of decrease in unit cell volume upon dehydration is again much smaller (essentially zero) for NaTEA-PST-20, suggesting that the framework flexibility of the RHO family decreases not only with increasing generation, but also with the presence of TEA+ cations much bulkier than Na+. We speculate that this may be due to a combination of the lesser flexibilities of small embedded cages like 10-hedral t-gsm and 8-hedral t-oto cages than those of fairly larger scaffold cages, e.g., 26-hedral lta and 18hedral pau cages, and the larger ratio of embedded to scaffold cages in the higher generation of the RHO family (Table S3).17 If so, highly flexible Rho and ECR-1825,26 cannot be CH4selective at higher pressures, because they should make the CH4 molecules with a kinetic diameter of 3.8 Å, larger than that of CO2 (3.3 Å),27 more easy to diffuse, compared to the considerably more rigid zeolites NaTEA-ZSM-25 and NaTEA-PST-20 (Table 1). On the other hand, one may speculate that the reason NaTEA-ZSM-25 and NaTEA-PST-20 show little change in unit cell parameter upon dehydration is the low vacuum pressure (0.1 Torr) employed in this work. Then, they could still be rather hydrated. Considering that all zeolite adsorbents studied here were treated under the same dehydration conditions, however, it can again be concluded that the former two zeolites may be more rigid in nature than their lower members in the RHO family. To examine the effect of TEA+ on the CO2 adsorption behavior of the three higher generations of the RHO family, we performed in situ IR measurements at different CO2 pressures. As shown in Figure 4, we were able to observe a sharp band appearing at 2346 cm-1, as well as two bands around 3700 and 3600 cm-1, which can be attributed to physisorbed CO2 on NaTEA-ZSM-25. Chemisorbed CO2 species were also observed in the 1800-1200 cm-1 region. While a strong band at 1635 cm-1 is assignable to adsorbed carbonate or bicarbonate


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species, which must form due to the presence of adsorbed water,28 for example, a weak band at 1378 cm-1 due to the symmetric O-C-O stretching vibration evidences CO2-Na+ interactions. We should note here that the essentially same IR results were observed for Na-ZSM-25, which is also the cases of ECR-18 and PST-20 (Figure 4). This strongly suggests that the TEA+ effect may not be strong enough to significantly change the isosteric heat of CO2 adsorption on these RHO family zeolites. It is also remarkable that the CO2 uptakes on the Na+ form of ZSM-25 and PST-20 are fairly smaller even at 298 K than those on the Na+TEA+ form of the corresponding zeolites, unlike the case of ECR-18 (Table 1), which can be rationalized by considering that the former two zeolites are structurally less stable than the



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latter one.17 If this is the case, the lower heats of CO2 adsorption observed for these TEA+-free zeolites could then be understood in a similar manner.

Figure 4. Difference IR spectra of adsorbed CO2 on the Na+-TEA+ (left) and Na+ (right) forms of (a) ECR-18, (b) ZSM-25, and (c) PST-20 at different CO2 pressures (5-150 Torr) and 298 K. The adsorption results presented thus far reveal that NaTEA-ZSM-25 and NaTEA-PST-20 are the two most selective high-pressure CO2 adsorbents among the zeolites tested here. This led us to examine their dynamic gas separation properties by breakthrough experiments at 298 K and atmospheric pressure using CO2/CH4 (50:50 v/v) and CO2/CH4/H2O (50:45:5 v/v/v) gas mixtures. As shown in Figure 5, the outlet gas composition passing through NaTEAZSM-25 in the absence of H2O consists of pure CH4 for ca. 120 s before the breakthrough of CO2. This implies that NaTEA-ZSM-25 hardly adsorbs CH4, which corroborates the trapdoor effect mechanism of CO2 adsorption. Figure 5 also shows that the same effect is still working ACS Paragon Plus Environment

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even in the presence of 5% H2O vapor, together with no significant change in CO2 retention time. Finally, we flushed the used NaTEA-ZSM-25 zeolite with He (50 cm3 min-1) at 298 K for 30 min and repeated the breakthrough experiments. In both absence and presence of H2O vapor, there is no detectable decrease in retention time even after seven CO2/CH4 breakthrough cycles, revealing its excellent durability. As shown in Figure 5, quite similar trends were obtained from the breakthrough curve data for NaTEA-PST-20. It thus appears that this RHO family member can also adsorb CO2 selectively under dynamic conditions. We think that such a high stability of NaTEA-ZSM-25 and NaTEA-PST-20 observed during CO2 separation in the presence of water is an indisputable advantage compared to other ordered

Figure 5. Repeated breakthrough curves on (a) NaTEA-ZSM-25 and (b) NaTEA-PST-20 using (left) CO2/CH4 (50:50 v/v) and (right) CO2/CH4/H2O (50:45:5 v/v/v) mixture gas mixtures with a total gas flow rate of 20 cm3 min-1 at 298 K and atmospheric pressure. porous materials like MOFs and COFs.



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CONCLUSIONS

In summary, the RHO family of zeolites with embedded isoreticular structures has been applied at high pressures for CO2 separation. Of the members of this family, NaTEA-ZSM-25 and NaTEA-PST-20 were found to be promising candidates for high-pressure CO2 separation. They not only show higher CO2/CH4 selectivities at 298 K and 25 bar than any of the zeolitic materials tested thus far, but also exhibit relatively high CO2 uptakes (≥ 3.6 mmol g-1) even at 373 K and 25 bar, mainly due to their considerably low framework flexibilities. Moreover, these two zeolites are characterized by fast CO2 adsorption kinetics and long-term stabilities even in the presence of water.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications websi te at DOI: xx.xxxx/ Experimental details, Figures S1-S7, and Table S1-S3.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS

This work was supported by the National Creative Research Initiative Program 2012R1A3A-2048833) through the National Research Foundation of Korea. We thank S. H. Ahn (POSTECH) for assistance with IR measurements and S. Seo and N. H. Ahn (POSTECH) for help in unit cell parameter calculations.

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TOC Graphic of Min et al., “Zeolites ZSM-25 and PST-20: Selective CO2 Adsorbents at High pressures”


Zeolites ZSM-25 and PST-20: Selective Carbon Dioxide Adsorbents at

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