CO2 Adsorption in the RHO Family of Embedded Isoreticular Zeolites

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

CO Adsorption in the RHO Family of Embedded Isoreticular Zeolites Jung Gi Min, Kingsley Christian Kemp, Hwajun Lee, and Suk Bong Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09996 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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The Journal of Physical Chemistry

1

CO2 Adsorption in the RHO Family of Embedded Isoreticular Zeolites

Jung Gi Min, K. Christian Kemp, Hwajun Lee, and Suk Bong Hong*

Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 37673, Korea

ABSTRACT

More effective carbon capture techniques and materials are still required to

combat ever-increasing anthropogenic CO2 emissions. Here we report the CO2 adsorption isotherms, at 298 – 373 K and 0 – 1.0 bar, on various alkali cation forms of the five members of the RHO family of embedded isoreticular zeolites (i.e., Rho, PST-29, ECR-18, ZSM-25, and PST-20). The adsorption behavior of CO2 on this new family was found to differ significantly according not only to the zeolite framework flexibility but also to the type of extraframework cation. Among the zeolites studied here, the adsorption isotherms on the Cs+tetraethylammonium cation form of ZSM-25 and PST-20 are characterized as one-step, with working capacities of 1.3-1.4 mmol g-1 at 298 – 343 K which may have a clear advantage for the industrial temperature swing adsorption process. These two zeolite adsorbents also have large CO2 selectivities (ca. 100 and 50, respectively) over N2 and CH4, as well as good longterm stability.

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2 INTRODUCTION

Carbon dioxide capture and storage (CCS) is the most effective way to reduce the concentration of atmospheric carbon dioxide (CO2), a major contributor to global warming and ocean acidification.1,2 Physical adsorption has been considered a promising technique for industrial CCS applications due to its low cost, easy manipulation, and broad applicability over a wide range of temperatures and pressures.3-5 Zeolites and metal-organic frameworks (MOFs) are the two most widely studied ordered porous materials for this purpose because of their structural diversity, high porosity, and adjustable adsorption properties.6-8 However, the unparalleled physicochemical stability of the former materials makes them more valuable. While the CO2 adsorption efficiency of coal-fired power plants is highly dependent on the energy intensive amine solvent regeneration step,9 on the other hand, the industrial regeneration process for solid-state adsorbents is usually performed via pressure swing adsorption (PSA), vacuum swing adsorption (VSA), and temperature swing adsorption (TSA), or a combination of these processes.10 The VSA process can maximize the CO2 working capacity of commercially applied zeolites, because their isotherms are usually characterized as Langmuir type I.11 However, it suffers from a demanding energy cost owing to difficulties with applying vacuum to large volumes of gas stream.12 Therefore, many researchers have focused on how to enhance the CO2 working efficiency of zeolites. We were the first to recognize the existence of a family of embedded isoreticular zeolites (EIZs) with the same body-centered cubic symmetry but increasing structural complexities, denoted the RHO family, and to predict and synthesize its many members. 13-16 The structures of the RHO family are shown in Figure 1: the 26-hedral ([4126886]) lta cages are located at the centre and vertex of the cube, and a pair of 10-hedral ([4882]) double 8-ring (d8r) and 18-hedral ([41286]) pau cages are consecutively added along the cube edges (between two lta cages) with


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3 increasing generation. The embedded cages, i.e., 14-hedral ([466286]) t-plg, 8-hedral ([4583]) toto, 10-hedral ([4684]) t-gsm, and 12-hedral ([4785]) t-phi cages, are neatly filled inside the cubic scaffold by sharing 8-ring windows. It is worth noting that the mixed Na+-tetraethylammonium (TEA+) cation form of zeolites ZSM-25 (framework type MWF) and PST-20, the fourth (RHOG4) and fifth (RHO-G5) generations of the RHO family, shows excellent CO2/CH4 selectivity (ca. 20) even at high pressure (25 bar), as well as relatively large CO2 uptakes.17 As a continuation of our studies on the adsorption properties of the RHO family of EIZs, we have prepared a series of alkali cation-exchanged forms of Rho (RHO; RHO-G1), PST-29 (RHO-G2), ECR-18 (PAU; RHO-G3), ZSM-25 (MWF; RHO-G4), and PST-20 (RHO-G5), and investigated their CO2 adsorption characteristics. The RHO-G2 structure was first proposed when Gordon and co-workers reported the structure of paulingite, the natural analog of ECR18, in 1966.18 Very recently, we have synthesized this structure using N,N´-dimethyl-1,4diazabicyclo[2.2.2]octane (DMD2+) cations as an OSDA, together with Na+ and K+, and named it as PST-29.19 Here we show that the CO2 adsorption behaviour of RHO-family members differs notably according to a combination of their framework flexibility and type of extraframework alkali cations. More interestingly, the adsorption isotherms of the Cs+-TEA+ form of ZSM-25 and PST-20 were found to show an unusual one-step isotherm with CO2 working capacities of 1.3 – 1.4 mmol g-1 at 298 – 343 K, making them attractive as industrial TSA adsorbents.

EXPERIMENTAL SECTION

Adsorbents Preparation. Zeolites Rho, PST-29, ECR-18, ZSM-25, and PST-20 were synthesized according to the procedures in the literature.13,19-21 As-made zeolites were refluxed twice in 1.0 M NH4NO3 solutions (1 g solid/100 mL) at 353 K for 6 h. The resulting NH4+ form



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4 was refluxed twice (or 8 times for Rho) in 1.0 M aqueous nitrate solutions (1 g solid/100 mL) of Li+, Na+, K+, Rb+, and Cs+ cations at 353 K for 6 h and dried at atmospheric temperature overnight. In addition, all as-made zeolites, except as-made PST-20 with poor thermal stability, were calcined at 823 K for 6 h before their conversion into each alkali cation form. General Characterization. Powder X-ray diffraction (XRD) measurements with varying CO2 pressures and phase identifications were carried out on a PANalytical X’pert diffractometer (Cu Kα radiation) with an X’Celerator detector. Prior to their collection, the samples were evacuated at 623 K for 6 h and cooled to the target temperature. Then, CO2 gas was introduced to the samples as a function of pressure. Data was collected with a step size of 0.01o after a CO2 equilibrium time of ca. 30 min. The powder XRD data for the calculation of the unit cell parameters was collected on the 5A beamline at the Pohang Acceleration Laboratory (PAL; Pohang, Korea) using monochromated X-rays (λ = 0.6926 Å ). Prior to the experiments, each zeolite sample was packed into a 0.7 mm quartz capillary. If required, the sample was evacuated under a vacuum of 10-3 Torr at 623 K for 6 h and sealed. Unit cell parameters were calculated using the Fullprof program and/or by the Lebail method using the GSAS suite of programs.22,23 Elemental analysis was performed by the Analytical Laboratory of the Pohang Institute of Metal Industry Advancement. The analysis for Li, Na, K, Rb, Cs, Si, and Al were carried out by a Jarrell-Ash Polyscan 61E inductively coupled plasma (ICP) spectrometer in combination with a Perkin-Elmer 5000 atomic absorption spectrophotometer. The C, H, and N contents of the samples were analyzed by using a Vario EL III elemental organic analyzer. IR measurements of adsorbed CO2 were carried out on a Thermo Nicolet 6700 FT-IR spectrometer using selfsupporting 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 the wafer down to room temperature, a background spectrum was recorded. Then, CO2 gas was introduced into


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5 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. Gas Adsorption and Separation Experiments. The CO2 (99.999%, Linde), CH4 (99.995%, Linde), and N2 (99.999%, Linde) adsorption isotherms were collected using a Setaram PCTPro E&E analyser or a Mirae SI nanoPorosity-XQ analyzer. Prior to the experiments, 0.1 g of zeolite sample was evacuated under a vacuum of 10-3 Torr at 523 K for 3 h. The increment of the reservoir pressure (ΔP) was set as 0.1 bar, and, if required, ΔP was set as 0.004 bar to discriminate pressures at step points. In most cases, the equilibrium conditions were fixed at 98% of the calculated uptake or at the maximum equilibration time of 60 min for each isotherm point. In the case of Na-Rho and K-Rho, however, considerably longer equilibration times up to 360 min were employed to measure CO2 adsorption isotherms. The CO2/N2 and CO2/CH4 selectivities were calculated using the following equation:

𝛼𝐶𝑂2/(𝑁2 𝑜𝑟 𝐶𝐻4) = 𝑛

𝑛𝐶𝑂2 (𝑁2, 𝐶𝐻4)

×

𝑃(𝑁2, 𝐶𝐻4) 𝑃𝐶𝑂2

(1)

where α is selectivity, n uptake of each gas, and P pressure (PCO2 = 0.15 bar and P(N2 and CH4) = 0.85 bar). The breakthrough experiments were performed at 303 K using CO2/CH4/Ar (12:68:20 v/v/v) and CO2/N2/Ar (12:68:20 v/v/v) gas mixtures with a total gas flow rate of 25 cm3 min-1. If required, 3% water vapour was introduced to the gas mixture. In a typical breakthrough experiment, 0.3 g of sample was packed in a vertically placed fixed bed and then dehydrated in flowing Ar (50 cm3 min-1) at 523 K for 6 h. The detailed experimental conditions are the same as those given in our recent study.17 Calculation of Isosteric Heat of CO2 Adsorption. The experimental adsorption isotherms obtained were fitted using the dual-site Langmuir (DSL) equation:



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6

𝑛=

𝑚(𝑏) 𝑏𝑃 1+𝑏𝑝

+

𝑚(𝑑) 𝑑𝑃

(2 )

1+𝑑𝑝

where m(b) and b are the saturation capacity and the affinity parameter on the first set of sites, respectively, m(d) and d the corresponding parameters on the second set of sites, respectively, n uptake, and P pressure. For the stepped adsorption isotherms, in addition, the isotherms were divided into pre- and post-step points, and then fitted using the following two equations, respectively:

𝑛=

𝑛=

𝑚(𝑏) 𝑏𝑃 1+𝑏𝑝

+

𝑚(𝑑) 𝑑𝑃 1+𝑑𝑝

𝑚(𝑏) 𝑏(𝑃+𝑃𝑠𝑡𝑒𝑝 ) 1+𝑏(𝑃+𝑃𝑠𝑡𝑒𝑝 )

+

(if P < Pstep)

𝑚(𝑑) 𝑑(𝑃+𝑃𝑠𝑡𝑒𝑝 ) 1+𝑑(𝑃+𝑃𝑠𝑡𝑒𝑝 )

(3 )

(if P ≥ Pstep)

(4 )

where Pstep is the pressure at the observed step, and the other parameters are the same as those given for Eq. (2). All fitted DSL parameters can be found in Tables S1-S5. The heats of adsorption were calculated by applying the Clausius-Clapeyron equation to the two adsorption isotherms at 298 and 323 K (or 308 K):

𝜕𝑙𝑛𝑃

[𝜕(1/𝑇)]𝑞 =

−∆𝑄

(5 )

𝑅

where ΔQ is the heat of adsorption, P pressure, R the gas constant, and T temperature. Gas Cycling Experiments. All the experiments were carried out using a Setaram PCTPro E&E analyser using high-purity CO2. Prior to the experiments, 0.1 g of sample was activated under the conditions described above. The VSA cycle was repeated 20 times at 343 K from 0


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7 to 1.2 bar. Both adsorption and regeneration times were set at 20 min. The TSA cycle was measured between 298 and 343 K as follows: (1) during the first adsorption cycle, the reservoir pressure and adsorption time were set at 0.5 bar and 30 min, respectively, to reach equilibrium; (2) To desorb CO2, the temperature was first switched from 298 to 343 K with a heating rate of 2.0 K min-1. Then, the regeneration time was set as 30 min; (3) For subsequent adsorption cycles, the temperature was first switched from 343 to 298 K with a cooling rate of 1.2 K min-1. Then, the adsorption time was set as 30 min. Steps 2 and 3 were repeated to complete the necessary cycles. All cycles were performed in a closed system. Desorption curve measurements at different temperatures were carried out in a similar manner.

RESULTS AND DISCUSSION

Powder XRD patterns of the series of RHO-family EIZs with different extraframework compositions show that their as-made form maintains structural integrity during the ionexchange step (Figures S1 and S2). However, although there are no reflections other than those from each zeolite structure, the low-angle (2θ < 15o) X-ray reflections from Rb+- and Cs+exchanged zeolites are noticeably weak in intensity compared to the other alkali cation forms, due to the large absorption cross sections of their alkali cations on the Cu Kα source.24 The anhydrous chemical composition data of various cation forms of the five members of the RHO family are given in Table S6. Although as-made zeolites (i.e., those containing OSDAs), except in the case of zeolite Rho, were used in ion exchange, the degree of alkali cation exchange in each zeolite was found to be normally higher than 70%. However, because of its high hydration energy,25 Li+ gave lower degrees (25 – 64%) of ion exchange. Figure 2 shows the CO2 adsorption isotherms, over the temperature range from 298 to 373 K and 0 to 1 bar, on different alkali cation (Li+, Na+, K+, Rb+, and Cs+) forms of OSDA-



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8 containing DMD-PST-29, TEA-ECR-18, TEA-ZSM-25, and TEA-PST-20. Most of the adsorption isotherms are characterized as Langmuir type I, typical of microporous crystalline solid adsorbents. In addition, their CO2 uptake increases according to the cation present in the order Cs+ < Rb+ < K+ ≤ Li+ < Na+. A quite similar trend was observed for the equilibrium CO2 adsorption capacities of the series of alkali cation-exchanged forms of DMDPST-29 and TEA-ECR-18. Given that the degree of Li+ ion exchange in each member, where the rest of inorganic extraframework cations are NH4+, is significantly smaller than that of the other alkali ions exchanged (Table S6), therefore, the CO2 uptake trend of RHO-family zeolites can be closely related to the order of cationic increase in charge density. Interestingly, NaTEAZSM-25 and NaTEA-PST-20 show an unexpected two-step CO2 adsorption isotherm with large CO2 uptakes (3.5 and 3.2 mmol g-1 at 298 K and 1.0 bar, respectively). As the adsorption temperature increases to 373 K, this step moves to higher pressures, becomes smaller, and finally disappears. Another unexpected finding is that a different type of stepped isotherm is observed on CsTEA-ZSM-25 and CsTEA-PST-20. The isotherms exhibit only one step at low pressures, unlike those on NaTEA-ZSM-25 and NaTEA-PST-20. As shown in Figure 2, however, this step also shifts to higher pressures and disappears with increasing temperatures. To further investigate the CO2 adsorption behavior of the RHO family of EIZs, we also measured the adsorption isotherms on its OSDA-free versions. Here we did not perform measurements on PST-20, because the structural integrity of its fifth generation is compromised on calcination. As shown in Figure 3, Na-Rho, the first generation of this zeolite family, gave the highest CO2 uptake (4.6 mmol g-1) at 298 K and 1.0 bar among the RHO family of zeolites with different framework structures and extraframework cation compositions studied here. While this uptake is in good agreement with the value previously reported,13,26 Na-Rho suffers from extremely slow adsorption kinetics, a serious drawback as an industrial CO2 adsorbent (Figure S3). When the equilibrium conditions were fixed at a maximum of 60 min for each


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9 adsorption point, in addition, Na-Rho exhibited a step in the temperature range (298 – 373 K) studied. This step is more clearly observed on K-Rho and is characterized by slower adsorption kinetics, whereas Rb-Rho and Cs-Rho exhibit negligible CO2 uptakes at 298 K, probably due to a strong ‘molecular trapdoor’ effect.26 An increase in equilibration time to 360 min at 298 K led to the disappearance of the step in the CO2 adsorption isotherms on both Na-Rho and KRho. However, adsorption kinetic data indicates that equilibrium has not been still reached even after 360 min at each adsorption point. We feel confident in invoking the trapdoor effect in this study as we know from prior structural analyses that while the monovalent cations are located in the 8-ring window, the trap door effect has been observed in Rho and ECR-18.13,19,21,26 It was also found that the cation-containing RHO family zeolites have negligible N2 BET surface areas, indicative of the blockage of their 8-ring windows by cations. An interesting result from Figure 3 is that the adsorption capacity (4.3 mmol g-1) of NaPST-29 at 298 K and 1.0 bar is somewhat smaller than that of Na-Rho, whereas the capacities (3.4 and 3.0 mmol g-1, respectively) of Rb-PST-29 and Cs-PST-29 at the same temperature and pressure are much larger than those of Rb-Rho and Cs-Rho. This indicates a significantly weaker trapdoor effect in the PST-29 structure. Apparently, CO2 molecules can access to the void space of PST-29 not only via d8r windows, but also via single 8-ring (s8r) ones that are located between the t-oto cages and the pau or t-plg cages. Since the alkali cations cannot be more strongly held in ‘thinner’ s8r windows than in d8r cages, it is not difficult to confer that the cations in the former windows would then be more readily relocated than those in the latter ones through the interactions with CO2 molecules and thereby permit other CO2 molecules to pass. The observation of stepped isotherms only in the Na+-TEA+ and Cs+-TEA+ forms of ZSM25 and PST-20 (Figure 2) is interesting in that one would then likewise expect similar isotherms from the K+-TEA+ and Rb+-TEA+ forms of the corresponding zeolites, if the same adsorption



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10 mechanism is operating for all zeolites. It is also remarkable that Na-ECR-18 and Cs-ECR-18 exhibit clear two- and one-step isotherms, respectively (Figure 3), which is not observed on their OSDA-containing form. However, this third generation of the RHO family, when exchanged with K+ or Rb+, shows no stepped isotherms, regardless of the presence of OSDAs in its pores, suggesting that the type of extraframework inorganic cations has a close relationship to stepped isotherms, which will be further discussed below. On the other hand, Na-ZSM-25 shows a typical type I isotherm at 298 K and 0 – 1 bar. Also, no further step can be observed with higher temperatures. However, as shown in Figure 3, a two-step isotherm appears in the high-pressure (< 10 bar) region and a similar tendency is observed with increasing temperature. Cs-ZSM-25 exhibits a sharp one-step isotherm, with the step appearing at a noticeably higher pressure (0.11 and 0.44 bar at 298 and 323 K, respectively) compared to its organic-containing form, which disappears at 348 K. Adsorption steps have frequently been reported for crystalline solid adsorbents including zeolites and MOFs. As far as CO2 adsorption is concerned, however, they have rarely been observed for zeolitic materials, especially for cage-based, small-pore zeolites. Up to now, various mechanisms, which include phase transition (and/or site selective adsorption), breathing effect, freezing effect, and guest-modulated effect, have been invoked for adsorption steps.27-34 However, we were not able to reasonably explain the steps in Figures 2 and 3 based on the known mechanisms. For example, the in situ IR results of adsorbed CO2 on CsTEAZSM-25 and CsTEA-PST-20 at 298 K allowed us to exclude the possibility of phase transition, because their spectra showed no new bands or significant spectral shift with increasing CO2 pressure from 5 to 300 Torr (Figure S4). The same trend was observed for the Na+-exchanged zeolites, i.e., Na-ECR-18, NaTEA-ZSM-25, and NaTEA-PST-20. We can also dismiss the breathing effect because neither the framework of RHO-family members nor their extraframework species can induce unit cell volume shrinkages as high (>


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11 20%) as those found in MOFs (Table 1).35 Furthermore, the adsorption kinetics of guest molecules during the step cannot be fast because of accompanying structural changes.30 However, as shown in Figure S3, zeolites studied here exhibited no significant adsorption kinetic differences between pre- and post-step points: while the Na+-TEA+ and Cs+-TEA+ forms of ZSM-25 and PST-20 reach equilibrium after only 10 min at each step even at 298 K, which is fast enough to apply to dynamic CO2 separation, Na-ECR-18 shows much slower adsorption kinetics (180 min) at lower pressure (< 0.1 bar), probably due to a relatively larger unit cell volume shrinkage (14.2%) under dehydration (Table 1). We can also rule out the freezing effect33 due to the lack of 8-ring channel intersections in the RHO-family zeolites, where the kinetic diameter of CO2 (3.3 Å ) is not big enough to be caught. Finally, the guest modulated effect, which has only been observed in ITQ-12, can be excluded, as this pure-silica 1-D channel zeolite contains no extraframework cations.34 It is well established that the strength of the trapdoor effect observed in zeolite adsorbents differs significantly according to the size and charge density of extraframework inorganic cations present.36,37 If we only consider this effect, then one would expect the K+-TEA+ and Rb+-TEA+ forms of ZSM-25 and PST-20 to exhibit a step. As shown in Figure 2, however, they do not show any step. We have recently proposed that the framework flexibility of the RHO family is dramatically influenced by the ratio of scaffold to embedded cages.17 To confirm this, we determined the unit cell parameters of the hydrated and dehydrated forms of a series of alkali ion-exchanged RHO-family zeolites using synchrotron powder XRD data. It should be noted here that although the OSDAs (DMD2+ and TEA+, respectively) used in the synthesis of PST29 and the other three higher members (ECR-18, ZSM-25, and PST-20) of the RHO family is different from each other, they are encapsulated within the exact same cages (i.e., t-plg and pau cages) of each zeolite,13,19 thereby making it possible to compare the unit cell volume shrinkage of these four zeolites.



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12 As shown in Table 1 and Figure 4, the unit cell volume shrinkage (i.e., the framework flexibility) of the RHO family of as-made EIZs increases in the order PST-29 < ECR-18 < ZSM-25 ≈ PST-20. In addition, this parameter was found to be closely related to the ratio of scaffold (i.e., lta, pau, d8r, and t-plg) to embedded (i.e., t-oto, t-gsm, and t-phi) cages of 0.54 > 0.31 > 0.20 > 0.14 (Table S7). Here, the t-plg cages have been considered as scaffold cages rather than as embedded ones, unlike in our original study on the RHO family,13 because these diagonally interconnected cages can support the entire framework structure of each zeolite, allowing it more rigidity and thus serving as scaffold cages during its crystallization process.15 The unit cell volume shrinkage data in Table 1 and Figure 4 shows that the Na+ form exhibits the largest framework flexibility, regardless of the presence of OSDAs in zeolite pores, among various cations of the RHO family of IEZs, whereas the flexibility observed for the K+, Rb+, and Cs+ forms are comparatively slightly decreased. Regarding the trapdoor effect, it is well known that the smaller Na+ ion, with larger charge density compared to the higher atomic number alkali cations, is easier to move. We also speculate that the greater degrees (12.5 and 12.0%, respectively) of the framework flexibility of NaTEA-ZSM-25 and NaTEA-PST-20 may cause larger structural changes when the adsorbed CO2 molecules interact with their framework. To check whether this speculation is correct, we carried out in situ powder XRD measurements on NaTEA-ZSM-25, CsTEA-ZSM25, and CsTEA-PST-20 as a function of CO2 gas pressure at 323 K (Figure S5) and found that the main X-ray peak of NaTEA-ZSM-25 at 2θ = 27.39o shows a larger shift (Δ2θ = 0.23o) with increasing CO2 pressure (0.1 to 1.0 bar) when compared with the shifts (0.04 and 0.06o, respectively) of CsTEA-ZSM-25 and CsTEA-PST-20. This indicates that the former zeolite exhibits a larger structural change during CO2 adsorption. Therefore, it is most likely that although the trapdoor effect is important for NaTEA-ZSM-25 and NaTEA-PST-20, the framework flexibility is the larger contributor factor towards the observed step.


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13 In the same vein, the one-step isotherms on CsTEA-ZSM-25 and CsTEA-PST-20 can be explained by the stronger trapdoor effect which is likewise effected by the framework flexibility but to a far lesser extent. The absence of a step in the isotherms on the K+-TEA+ and Rb+-TEA+ forms of ZSM-25 and PST-20, which have similar degrees of framework flexibility to those observed for their Cs+-TEA+ form, lends further support to the hypothesis that framework flexibility is less important for these zeolites. This implies that the trapdoor effect becomes stronger mainly due to an increase in cation size or to a decrease in charge density. Thus, a larger number of CO2 molecules, i.e., higher pressure, may be required to move the strained Cs+ to pass through the 8-ring window. Similarly, for OSDA-containing DMD-PST-29 and TEA-ECR-18, in addition, we attribute the absence of any steps in their isotherms to the relatively lower degrees of framework flexibility (Table 1), which originates from its large ratio of scaffold cages to embedded ones and from the rigidity afforded by the OSDAs occluded in pau and t-plg cages. The stepped CO2 adsorption isotherms on various cation forms of OSDA-free zeolites can be explained in a similar manner (Figure 3). Following the same line of reasoning as we applied for OSDA-containing zeolites, we note that the degree of framework flexibility of OSDA-free zeolites increases in the order PST-29 < Rho < ECR-18 (Table 1). This order is somewhat different from the order of their ratios of scaffold to embedded cages: Rho < ECR-18 < PST29 (Table S7). However, we can make an exception of Rho because this first generation of the RHO family consists of only two structural building units (i.e., lta and d8r cages). Furthermore, the lta cage itself plays a role as both scaffold and embedded cages, making it impossible to compare with the higher generations of the RHO family. With respect to OSDA-free ECR-18, we observed larger degrees of framework flexibility than OSDA-free Rho or PST-29 owing to the absence of TEA, which may in our view be the major contributing factor to the observed two-step isotherms. This suggests that the adsorption mechanisms in Na-ECR-18 and Cs-ECR-



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14 18 are similar to those observed for NaTEA-ZSM-25 and CsTEA-PST-20, respectively. We also believe that while the steps observed for Na-ZSM-25 and Cs-ZSM-25 can be explained in a similar manner, the lack of any step in the isotherms on a series of OSDA-free PST-29 zeolites (Figure 3) can be attributed to the large ratio (0.54) of scaffold cages to embedded ones and hence to the less flexible nature of the PST-29 structure. Table 1 also lists the initial isosteric heats of CO2 adsorption on various alkali cation forms of the five members of the RHO family calculated using the Clausius-Clapeyron equation applied to the adsorption isotherms between 298 and 323 (or 308) K. We should note that we were unable to differentiate between the components of the isosteric heats, as such the reported values may consist of both framework flexing and/or gas/solid interactions. The isosteric heats of adsorption for the zeolites studied here are within the range 28 – 65 kJ mol-1, which can be considered as physisorption. As shown in Figure S6, a slight increase in the heat of adsorption on OSDA-containing RHO-family zeolites is observed due to strong initial adsorbate-adsorbent interactions. For the OSDA-free-zeolites, the initial heats of adsorption follow a more flat curve, indicating a large degree of adsorbent homogeneity,38 and this homogeneity is further exhibited by the decrease in heat of adsorption as a function of loading. Therefore, it is most likely that CO2 interacts more favorably with the OSDA-free zeolites, as there are more cages within which CO2 molecules can freely move. In general, continuous models are used to fit the experimental isotherms followed by putting the model parameters into the Clausius-Clapeyron relation. However, since we were unable to mathematically model the stepped CO2 adsorption isotherms on Cs-ECR-18, CsTEAZSM-25, and CsTEA-PST-20 using continuous equations over the entire pressure range, we separated each isotherm into pre- and post-steps and modeled these two parts using the DSL equation.39 The fitted and calculated CO2 heats of adsorption for the stepped isotherms can be found in Figure S7. An initial heat of adsorption on CsTEA-PST-20 at the pre-step (0 – 0.2


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15 mmol g-1) was calculated to be 35 kJ mol-1. However, the isosteric heat of adsorption jumped to 45 kJ mol-1 at the post-step point and then decreased with increasing CO2 loading to 2.0 mmol g-1. Similar isosteric heat curves were obtained for Cs-ECR-18 and CsTEA-ZSM-25, and this trend could be attributed to different accessible CO2 environments at the pre- and post-step points. It is interesting to note that the isosteric heats calculated for all stepped isotherms still implies a physisorption process, which provides us further evidence to exclude the phase transition and freezing effects. To evaluate the performance of CsTEA-ZSM-25 and CsTEA-PST-20 as regenerative adsorbents, VSA and TSA processes were employed to calculate the working capacities (Figure 5). Both zeolites gave no significant adsorption loss even after 20 cycles at 343 K and 1.2 bar under the VSA process, suggesting good adsorption stabilities. For coal flue gas, TSA processes are generally performed at temperatures above atmospheric temperature and a CO2 partial pressure of 0.15 bar.40,41 On this basis, the position of a step in CsTEA-ZSM-25 and CsTEAPST-20, as well as their reversible desorption curves, make them suited for TSA process, because working capacity can be maximized with minimal energy input. Indeed, both zeolites were found to show fairly large CO2 working capacities (1.30 and 1.38 mmol g-1, respectively), together with outstanding stability over repeated TSA cycles. Notably, these values are more favorable than the values (0.83 and 1.12 mmol g-1, respectively) of the current commercial NaA and Na-X adsorbents (Figure S8). Additionally, we compare our materials to current state of the art materials in Table S8. In practical applications, on the other hand, the used adsorbent needs to be regenerated at higher temperature and atmospheric pressure under 100% CO2 as a purge gas. To confirm the regeneration ability, we repeatedly measured desorption curves at 1.0 bar with increasing temperature. As shown in Figure 5, CsTEA-PST-20 can desorb about 90% of adsorbed CO2 gas at temperatures above 373 K, whereas Na-A and Na-X desorb only 50% or so. NaTEA-ZSM-25 also shows a large working capacity (1.29 mmol g-1) at 0.15 bar



Page 16 of 32

16 under TSA conditions, but desorbs only 50% of adsorbed CO2 at 373 K, unlike CsTEA-ZSM25, mainly due to the higher charge density of its alkali cations (i.e., Na+). The adsorption results presented thus far reveal that CsTEA-ZSM-25 and CsTEA-PST-20 are the two most promising CO2 capture adsorbents among the cation-exchanged RHO family of EIZs under TSA conditions. Their CO2, N2, and CH4 adsorption isotherms also show that they are characterized by large CO2/CH4 (102) and CO2/N2 (47) selectivities at 0.15 bar and 298 K (Figure S9). In addition, as shown in Figure S10, CsTEA-ZSM-25 and CsTEA-PST-20 reach equilibrium within 2 min at 1.0 bar and 298 K, which is comparable with commercial NaA and Na-X zeolites. Thus, we conducted breakthrough experiments at 303 K and atmospheric pressure on CsTEA-ZSM-25 and CsTEA-PST-20 using CO2/CH4/Ar (12:68:20 v/v/v) and CO2/N2/Ar (12:68:20 v/v/v) gas mixtures after flushing with Ar (50 cm3 min-1) at 298 K for 30 min (Figure 6). As expected from pure component isotherms, both CsTEA-ZSM-25 and CsTEA-PST-20 can selectively adsorb CO2 under dynamic conditions. However, when 3% water vapor is introduced, the CO2 capacity decreased by ca. 30%. This reduction in capacity, due to a stronger affinity towards water than CO2, is a hurdle that most zeolite adsorbents still need to overcome.

CONCLUSIONS

In summary, a series of various alkali cation-exchanged forms of zeolites Rho, PST-29, ECR18, ZSM-25, and PST-20, with or without OSDA cations present, all of which belong to the RHO family of embedded isoreticular zeolites, have been tested for CO2 adsorption at 1.0 bar over the temperature range from 298 to 373 K. Two different types of stepped adsorption isotherms are observed on their Na+- and Cs+-exchanged forms and are more noticeable with higher generations of this zeolite family and in the absence of OSDAs. The overall


Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17 characterization results strongly suggest that these stepped isotherms are a result of a combination of differences in the degree of the zeolite framework flexibility and differences in the strength of the trapdoor effect: a more flexible framework tends to induce a two-step isotherm, whereas a one-step isotherm is observed when the trap door effect is dominant. More interestingly, CsTEA-ZSM-25 and CsTEA-PST-20 exhibit enhanced CO2 working capacity (1.3 – 1.4 mmol g-1) at 298-343 K and 0.15 bar under TSA conditions when compared with those (0.8 and 1.1 mmol g-1, respectively) of the commercial Na-A and Na-X adsorbents. They also have high CO2/CH4 and CO2/N2 selectivities, together with good long-term durability, making them promising adsorbents for selective CO2 separation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/. Adsorption isotherms, kinetic evolutions, DSL parameters, powder XRD patterns, and difference IR spectra, and isosteric heats of CO2 adsorption on zeolites studied, zeolite chemical compositions, and numbers of different types of cages in each zeolite structure (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jung Gi Min: 0000-0002-4084-1192 K. Christian Kemp: 0000-0002-1478-3515



Page 18 of 32

18 Hwajun Lee: 0000-0003-3791-1943 Suk Bong Hong: 0000-0002-2855-1600 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program 2012R1A3A-2048833) through the National Research Foundation of Korea. We thank Prof. P. A. Wright (St. Andrews) for helpful discussion.


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22 Neaton, J. B.; Smit, B.; Kortright, J. B.; Gagliardi, L.; Bordiga, S.; Reimer, J. A.; Long, J. R. Cooperative Insertion of CO2 in Diamine-Appended Metal-Organic Frameworks. Nature 2015, 519, 303-308. (28) Llewellyn, P. L.; Coulomb, J. P.; Grillet, Y.; Patarin, J.; Lauter, H.; Reichert, H.; Rouquerol,

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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24 Table 1. Unit Cell Parameters and Initial Isosteric Heat of Adsorption of Various Alkali Cation Forms of the RHO Family of Embedded Isoreticular Zeolites adsorbenta Li-RHO Li-PST-29 Li-ECR-18

unit cell parameter a (Å )b hydrated dehydrated 14.9773 14.4043 24.9621 24.2666 34.9611 33.5434

ΔVc (%) -Hads (kJ mol-1)d 11.0 8.8 43.7 (± 0.6) 11.7 27.3 (± 10.7)

Na-RHO Na-PST-29 Na-ECR-18

14.9749 24.9614 34.9344

14.3465 23.9809 33.2007

12.1 11.3 14.2

43.7e 46.8 (± 7.1) -

K-RHO K-PST-29 K-ECR-18

14.9776 24.9356 34.9027

14.5565 24.3100 33.8537

8.2 7.6 8.8

46.0 (± 7.1) 46.3 (± 1.1)

Rb-RHO Rb-PST-29 Rb-ECR-18

14.9599 24.9208 34.9968

14.5466 24.4244 34.0133

8.1 6.0 8.2

45.1 (± 1.2) 45.5 (± 2.2)

Cs-RHO Cs-PST-29 Cs-ECR-18

14.9744 24.9566 34.9379

14.6126 24.5999 34.1429

7.1 4.2 6.7

44.4 (± 1.3) 33.4f

adsorbenta

unit cell parameter a (Å )b hydrated dehydrated

ΔVc (%)

-Hads (kJ mol-1)

LiDMD-PST-29 LiTEA-ECR-18 LiTEA-ZSM-25 LiTEA-PST-20

24.9853 35.0256 44.9930 54.8657

24.8064 34.3285 43.7065 53.0130

2.1 5.9 8.3 9.8

NaDMD-PST-29 NaTEA-ECR-18 NaTEA-ZSM-25 NaTEA-PST-20

25.0020 35.0417 45.0532 54.9949

24.7743 33.7819 43.0885 52.6951

2.7 10.4 12.5 12.0

KDMD-PST-29 KTEA-ECR-18 KTEA-ZSM-25 KTEA-PST-20

24.9901 34.9888 45.0846 54.9702

24.7956 34.2458 43.9335 53.6058

2.3 6.2 7.6 7.3

33.8 (± 37.1 (± 48.2 (± 47.9 (±

6.6) 0.4) 4.5) 2.5)

RbDMD-PST-29 RbTEA-ECR-18 RbTEA-ZSM-25 RbTEA-PST-20

25.0074 35.0083 45.1156 54.9839

24.8704 34.4052 44.0137 53.7460

1.6 5.1 7.2 6.6

18.8 (± 40.7 (± 48.1 (± 42.2 (±

9.4) 0.2) 5.8) 1.0)

CsDMD-PST-29 CsTEA-ECR-18 CsTEA-ZSM-25 CsTEA-PST-20

25.0249 35.0153 45.1320 55.0035

24.9218 34.4624 44.0932 53.7701

1.0 4.7 6.7 6.6

29.8 (± 1.2) 40.5 (± 0.2) 27.6f 35.0f

a

31.4 (± 42.8 (± 41.6 (± 43.4 (±

0.7) 0.3) 6.7) 6.3)

42.2 (± 5.0) 46.9 (± 4.0) 65.2e 44.7e

DMD and TEA are N,N´-dimethyl-1,4-diazabicyclo[2.2.2]octane, respectively. b a is the unit cell parameter of RHO family EIZs with cubic symmetry. c The unit cell volume shrinkage defined as (ahyd3-adehyd3)/ahyd3. d The initial heat of adsorption was calculated using the single isotherms measured at 298, 323, and 348 K. Values in parentheses are error range of average heat of adsorption. e The values adopted from ref. 17. f The value was calculated using the single isotherms measured at 298 and 308 K due to inflection step in the isotherms.


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25 FIGURE CAPTIONS Figure 1. Structures of the five members of the RHO family of EIZs and their structural building units: zeolites (a) Rho, (b) PST-29, (c) ECR-18, (d) ZSM-25, and (e) PST20. Adopted from ref 17. Figure 2. CO2 isotherms on zeolites (from left to right) DMD-PST-29, TEA-ECR-18, TEAZSM-25, and TEA-PST-20 in their (a) Li+, (b) Na+, (c) K+, (d) Rb+, and (e) Cs+ forms at 0 - 1.0 bar and 298 (navy), 323 (green), 348 (pink), and 373 K (orange). The solid lines represent simultaneous fits of all data with the generalized dual-site Langmuir isotherm. Adsorption, closed symbols; desorption, open symbols. Figure 3. CO2 isotherms on zeolites (from left to right) Rho, PST-29, ECR-18, and ZSM-25 in their (a) Li+, (b) Na+, (c) K+, (d) Rb+, and (e) Cs+ forms at 0 - 1.0 bar and 298 (navy and light blue), 323 (green), 348 (pink), and 373 K (orange). The solid lines represent simultaneous fits of all data with the generalized dual-site Langmuir isotherm. The isotherms on Na-Rho and K-Rho marked in light blue were measured for a maximum of 360 min at each adsorption point, whereas the equilibrium conditions of all the other isotherms were fixed at a maximum of 60 min at each adsorption point. High-pressure (< 10 bar) CO2 adsorption isotherms on Na-ZSM25 at 298-373 K are also presented. Adsorption, closed symbols; desorption, open symbols. Figure 4. Unit cell volume shrinkage percentage of RHO-family zeolites by dehydration vs type of extraframework cations. Figure 5. CO2 adsorption-desorption behaviours of (top) CsTEA-ZSM-25 and (bottom) CsTEA-PST-20: CO2 adsorption-desorption cycles under (a) VSA conditions at 343 K and 0-1.2 bar and under (b) TSA conditions at 298-343 K and 0.15 bar. (c) Repeated CO2 adsorption-desorption curves at 1.0 bar under different temperatures. Figure 6. CO2/CH4 (top) and CO2/N2 (bottom) breakthrough curves on (a) CsTEA-ZSM-25 and (b) CsTEA-PST-20 using CO2/CH4/Ar (12:68:20 v/v/v) and CO2/N2/Ar (12:68:20 v/v/v) gas mixtures (with and without 3 % H2O present) with a total gas flow rate of 25 cm3 min−1 at 303 K and atmospheric pressure.



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Figure 1


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Figure 2


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Figure 3


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Figure 4



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Figure 5


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Figure 6



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32 TOC Graphic of Min et al., “CO2 Adsorption in the RHO Family of Embedded Isoreticular Zeolites”


CO2 Adsorption in the RHO Family of Embedded Isoreticular Zeolites

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