K+ Exchanged Zeolite ZK-4 as a Highly Selective Sorbent for CO2

Article pubs.acs.org/Langmuir

K+ Exchanged Zeolite ZK‑4 as a Highly Selective Sorbent for CO2 Ocean Cheung, Zoltán Bacsik, Panagiotis Krokidas, Amber Mace, Aatto Laaksonen, and Niklas Hedin* Department of Materials and Environmental Chemistry, Berzelii Center EXSELENT on Porous Materials, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden S Supporting Information *

ABSTRACT: Adsorbents with high capacity and selectivity for adsorption of CO2 are currently being investigated for applications in adsorption-driven separation of CO2 from flue gas. An adsorbent with a particularly high CO2-over-N2 selectivity and high capacity was tested here. Zeolite ZK-4 (Si:Al ∼ 1.3:1), which had the same structure as zeolite A (LTA), showed a high CO2 capacity of 4.85 mmol/g (273 K, 101 kPa) in its Na+ form. When approximately 26 at. % of the extraframework cations were exchanged for K+ (NaK-ZK-4), the material still adsorbed a large amount of CO2 (4.35 mmol/g, 273 K, 101 kPa), but the N2 uptake became negligible (4.5 mmol/g (at 273 K, 100 kPa).7 Shao et al.8 and Siriwardane et al.9 showed that zeolites NaX and NaY had higher capacity for adsorption of CO2 than the other zeolites studied. The reported uptake of CO2 on zeolites NaX and NaY was 5.2 mmol/g. (393 K, 2.0 MPa) and 4.8 mmol/g (303 K, 80 kPa), respectively. Bae et al. compared a range of zeolite adsorbents and showed that zeolite NaX performed well in terms of selectivity (with an ideal adsorption solution theory (IAST) separation factor of 310) and high CO2 uptake at 15 kPa (3.4 mmol/g, 313 K, 15 kPa).10 Other groups, including us, have studied zeolite A as a CO2 adsorbent.11−15 Zeolite A is inexpensive and offers a high CO2 adsorption capacity (around 4 mmol/g, 273 K, 101 kPa) in most forms. There are 4-, 6-, and 8-membered ring (MR) windows in zeolite A, constructed by the stacking of sodalite cages. The framework of zeolite A bares a negative charge. These charges are balanced by exchangeable cations. In the case of zeolite A in its sodium form (NaA), the cations are located: 8 close to the 6MR (site I), 3 in the 8MR (site II), 1 close to the 4MR (site III), per α cage.16 Because of the strong interaction of CO2 with the extraframework cations, the observable CO2 capacity of zeolite A at low pressures of CO2 is high. Palomino et al. synthesized a range of high silica LTA zeolites and found that the CO2 adsorption capacity could be as

INTRODUCTION In recent years, considerable efforts have been put into research targeting an efficient new technology for carbon capture and storage (CCS). One potential approach is to implement adsorption driven CO2 separation processes at point sources, such as fossil-fuel burning power plants, cement factories, and steel mills. Swing adsorption processes for CO2 separation are still under development, and a suitable adsorbent is required for their efficient operation. Adsorbents with high heat of CO2 adsorption appear to be more suitable for high pressure gas mixtures where the partial pressure of CO2 is low. This was suggested by Wilmer et al.1 Other characteristics for a good adsorbent include rapid adsorption kinetics and high CO2 selectivity. Many different porous compounds have been investigated as CO2 adsorbents. Among those are zeolites, (alumino- and silicoalumino-) phosphates, activated carbons, amine modified silica, and metal−organic frameworks. These solid adsorbents have been reviewed recently.2−5 The microporous and crystalline nature of zeolites gives them several potential advantages as CO2 adsorbents. They typically exhibit relatively high heat of adsorption for CO2. They have well-defined porous channels (or cages) with defined pore window apertures. In some cases, the effective size of the window can be tuned. Several zeolites can also be produced at a low cost with sustainable methods. Shang et al.6 studied ion-exchanged zeolites with the chabazite structure as adsorbents for CO2. They observed that the © 2014 American Chemical Society

Received: December 18, 2013 Published: July 29, 2014 9682

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high as 6.1 mmol/g at high pressure (at 500 kPa, 303 K, for LTA with Si:Al = 2:1).17 An additional advantage of zeolite A is its narrow 8MR windows. Also, certain compositions of zeolite A have a cation placed in the windows. By cation exchange, it is possible to effectively tune the pores to selectivity allow a particular type of molecules to diffuse through and adsorb. The possibility to moderate the effective size of the 8MR pore windows on zeolite A was first established by Breck et al. around 60 years ago.18 We recently explored this property further by tuning zeolite NaA (4A) by partial K+ cation exchange and studied various aspects of the CO2 adsorption. With K+ in the 8MR windows, the effective size of the pore window is smaller than if the window would be populated with Na+. Zeolite NaKA has effective pore window apertures between 0.3 and 0.4 nm depending on the content of Na+ and K+. Some compositions of zeolite NaKA are extremely selective toward CO2 (over N2) when a large fraction of the 8MR pore windows are populated with K+.13 Furthermore, these selective zeolites still offer reasonably fast adsorption kinetics.11 Note that both in zeolite NaA and zeolite KA, the corresponding cations in the 8MR pore windows will have to move to allow transport of N2 or CO2. Akhtar et al.19 evaluated different types of zeolites using the IAST and found that zeolite NaKA could potentially be the best zeolites for the removal of CO2 from flue gas. Hasan et al.20 combined equation driven process calculations with materials science and presented 10 zeolite types that have the potential to separate CO2 from flue gas at a coal-fired power plant. They established that the cost of capturing one ton of CO2 using these adsorbents was less than 30 USD/ton. Of these 10 zeolite types, five exhibited pore limiting diameters similar to the zeolite NaKA and zeolite NaK-ZK-4. The highly hydrophilic property of zeolite A appears to be not very problematic, as Hasan et al.20 estimated that the dehydration cost was around 10 USD/ton. Zeolite ZK-4 is a more siliceous version of zeolite A and was first synthesized by Kerr in 1961.21 The two compounds share the same framework structure (LTA). While the Si:Al ratio of zeolite A is 1:1, for zeolite ZK-4, this ratio is higher than 1. The higher Si content means that the number of Si−O−Si linkage increases, and the net framework charge becomes less negative. Hence, the number of charge balancing cations would decrease. The structural aspects of zeolite ZK-4 have already been investigated.22,23 Kodaira et al.24 examined the interaction between zeolite ZK-4 and the structural directing agent (SDA) usually used for the synthesis of zeolite ZK-4−tetramethylammonium hydroxide (TMAOH). In addition, Zverev et al. and Vasil’eva et al. studied CO2 and NH3 adsorption of various ionexchanged versions of zeolite ZK-4 and calculated the heat of adsorption of CO2 and NH3 on those zeolites. The heat of CO2 adsorption at nonzero loading on zeolite ZK-4 (Na+ form) was around 45 kJ/mol.25,26 On zeolite ZK-4 there are still cations sites available and filled with exchangeable cations, such as Na+ or K+. These cations sites are located close to the 4MR and 6MR or in the 8 MR. On the Na+ form of zeolite ZK-4, the sites in the 8 MR pore windows are unoccupied. On the other hand, Ikeda et al.27 showed that K+ do occupied cations sites in the 8MR windows. We will study the tunable effective pore window size on zeolite ZK-4 by cation exchange. Our aim is to investigate if the high CO2 selectivity observed on zeolite NaKA can also be observed on zeolite ZK-4, whist keeping the high CO2 capacity. Introducing K+ on to zeolite ZK-4 would mean that 8MR windows would become occupied by K+ ions. Our aim is to investigate how K+ occupancy at the 8MR windows of zeolite ZK-4 controls the separation of CO2 from N2.

Article

EXPERIMENTAL SECTION

Synthesis. Zeolite ZK-4 was synthesized using an adaptation of the method used by Kerr.28 A typical synthesis requires a mixture containing 21.5 g of sodium aluminate (Sigma-Aldrich, technical grade), 6.0 g of sodium hydroxide, 300 g of 25% tetramethylammonium hydroxide (TMAOH), and 57 g of colloidal silica (Ludox HS-40) in 300 g of deionized water. Crystallization was carried out at 383 K under reflux for 1−5 days. Details of the synthesis procedures can be found in the Supporting Information. The TMAOH was removed by calcination of the as-synthesized zeolite ZK-4 by heating to 773 K (at a rate of 1 K/min) for 600 min in a slow flow of air. The calcination temperature was chosen according to Ikeda et al.27 Ion exchange of zeolite Na-ZK-4 was carried out by following the similar procedures to Liu et al.13 1 g of calcined zeolite ZK-4 was stirred in 100 cm3 of deionized water with different concentrations of potassium chloride (KCl) for 30 min. The ion exchanged zeolite ZK-4 was separated by vacuum filtration, washed with an excess amount of deionized water, and dried overnight at 373 K. The products were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM); the details are listed in the Supporting Information. Gas Adsorption. CO2 and N2 adsorption isotherms of zeolite ZK-4 were recorded using a Micromeritics Gemini V 2390 surface area analyzer. The zeolite ZK-4 samples were pretreated at 623 K under a flow of N2 gas for a minimum of 6 h. Adsorption isotherms were recorded at 273 K; the temperature of the experiments was controlled using a Dewar flask of an ice/water mixture. A helium void volume measurement was performed with an equilibration time of 10 s (0.01% difference, with 100 s delay). It is our understanding that He entrapment in the α cage of zeolite A occurs at significantly lower temperature than that used for the experiments.29,30 The adsorption isotherms recorded using this setup were cross checked with the same isotherms recorded using a Micromeritics ASAP2020 surface area analyzer (discussed in Supporting Information along with a detailed description of the experiments). The adsorption point equilibration time was set to 10 s with a 100 s delay. Some adsorption isotherms recorded using a longer equilibration time (on both analyzers, with up to 30 s, with 300 s delay equilibration time), and adsorption/desorption isotherms are presented in the Supporting Information (Figures S1−S3). The difference in the CO2 uptake (at 101 kPa, 273 K) due to the different equilibration times was less than 1%. The isosteric heat of CO2 physisorption for zeolite ZK-4 was calculated using adsorption (physisorption onlydetails are in the Supporting Information) isotherms recorded at different temperatures using a Micromeritics ASAP2020 surface area analyzer (273−303 K, 10 K steps). The isosteric heat of CO2 physisorption was calculated by following the Clausius−Clapeyron equation; the detailed procedures were highlighted previously.31 CO2 Adsorption Investigation by Infrared Spectroscopy. The infrared (IR) spectra were recorded on a Varian 670-IR FT-IR spectrometer using a mercury cadmium telluride (MCT) detector cooled with liquid nitrogen. The in situ experiments were performed with a high vacuum stainless steel manifold connected to a stainless steel IR transmission cell. The physisorption rate was measured by in situ IR spectroscopy by following the development of a combination band of adsorbed CO2. A detailed description of this experiment has been highlighted previously11,31 and can be found in the Supporting Information. Molecular Modeling. The adsorption isotherms as well as the heat of adsorption of CO2 and N2 were calculated from Monte Carlo (MC) simulations using the Materials Studio software package using the grand canonical MC (GCMC) simulations. A detailed explanation regarding the reconstruction of the unit cell (Table S1, Supporting Information) and the details of the simulation methods can be found in the Supporting Information. We simulated 20 pressure points on the adsorption isotherm, each point with 5 × 106 MC steps for CO2 and 1 × 106 MC steps for N2. The simulations were carried out using a rigid framework. 9683

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Simulation of Adsorption by Monte Carlo Simulations. The force field used for the nonbonding interactions for CO2 in the framework was developed by Maurin et al.,32 while that for N2 was taken from the work of Pillai et al.33 The molecules of CO2 and N2 are described with the use of three point charge models, which can reproduce the quadruple moment of these molecules. In the case of N2, the massless interaction site in the middle of the triple bond serves as a charge carrier only. Simulation of Adsorption by Molecular Dynamics Simulations. We simulated the diffusion of both CO2 and N2 gas molecules in one single unit cell of zeolite K-ZK-4 (Figure 1) in order to investigate

atoms. The geometry optimization and frequency calculations were performed with the B3LYP exchange and correlation functional together with the 6-31G (d,p) basis set. We studied a composition of zeolite ZK-4 which had a Si:Al ratio of 1.5:1. The cation occupancies on the 6MR and 8MR sites were varied (between K+ and Na+ at the different sites). Note that K+ and Na+ were located at different locations in the 8MR as shown independently by Ikeda et al.27 and Eddy et al.22 We also omitted the 4MR cation site, as the Na+ occupancy was very low and because K+ simply did not occupy the 4MR site at Si:Al > 1.5:1.27

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RESULTS AND DISCUSSION

Here, we will discuss zeolite Na-ZK-4 and the partially K+ zeolite NaK-ZK-4 as CO2 adsorbents. The synthesized zeolite ZK-4 were cubic crystals with high crystallinity (PXRD pattern and SEM micrographs are shown in the Supporting Information) and a Si:Al ratio of approximately 1.3:1. CO2 Adsorption on Zeolite Na-ZK-4. The high quality of the crystals was in some sense further confirmed by the high CO2 adsorption capacity. Under the adopted experimental conditions, the CO2 uptake (Figure 2a: chemisorption and physisorption; Figure 2b: physisorption only) of the calcined zeolite Na-ZK-4 reached 4.8 mmol/g at 273 K (101 kPa). The N2 uptake (Figure 2a) of zeolite Na-ZK-4 was much lower (0.6 mmol/g) at 273 K (101 kPa). The experimental adsorption isotherms were very well described (R2 > 0.99) by the two sites Langmuir isotherm model through regression analyses for all temperatures. The Clausius−Clapeyron equation was applied to the experimental data to calculate the heat of CO2 physisorption. The experimental heat of CO2 physisorption was ∼42 kJ/mol up to 2.5 mmol/g (∼8 CO2 per cage) loading at 293 K (Figure 3a). The values were very similar to our experimental values for zeolite A11 as well as those that are reported in the literature for zeolites, including zeolite ZK-4.17,25,36 Figure 2b displays the CO2 physisorption isotherms of zeolite Na-ZK-4 at 273, 283, 293, and 303 K together with the GCMC simulated isotherms. At 273 K and 101 kPa, the CO2 uptake on zeolite Na-ZK-4 was around 10% less than the “chemisorption + physisorption” isotherm (Figure 2a). The difference of 10% implies that chemisorbed CO2 accounted for around 10% of the total uptake (chemisorption and physisorption) on zeolite Na-ZK-4. The simulated isotherms showed higher CO2 uptake than that observed experimentally from a loading level of around 2−3 mmol/g onward. The lower experimental maximum adsorption capacity as compared with the simulated could indicate that the samples may have contained amphorous fractions that could not be used for CO2 adsorption.

Figure 1. Unit cells of zeolite K-ZK4 (Si/Al = 1.5). Red, yellow, and magenta balls represent oxygen, silicon, and aluminum atoms, respectively. the hindering effect of the K+ cations on the motion of small gas molecules. The simulations were carried out using a molecular dynamics (MD) method (NVT ensemble) in fully flexible 2 × 2 × 2 supercells (both framework atoms and K cations were allowed to move), where the trajectories of the motion of molecules inside the porous medium were calculated for a period of 5 ns. The time step was 1 fs, the simulation temperature was set to 300 K, and the Nosé− Hoover method (NHL)34 was used to control the temperature. The mean-square displacement (MSD) of the diffusing molecules was extracted by their trajectories, and their self-diffusion was calculated using the Einstein equation (more details can be found in the Supporting Information). Simulation of Adsorption by Quantum Chemical Calculations. Density functional theory (DFT) calculations were performed using the Gaussian09 package.35 A fragment of zeolite ZK-4 (consisting of one 4MR, one 6MR, and one 8MR) with one single molecule of CO2 was placed close to these rings and studied using an isolated cluster approach, where the fragment is terminated by spatially frozen H

Figure 2. (a) CO2 and N2 adsorption isotherms of zeolite Na-ZK-4 at 273 K recorded on Micromeritics Gemini 2390. (b) CO2 physisorption isotherms of zeolite Na-ZK-4 (Si/Al ∼ 1.3) recorded on Micromeritics ASAP2020 at 273 (■), 283 (●), 293 (▲), and 303 K (▼); dotted lines are the grand canonical Monte Carlo simulated isotherms (with Si/Al = 1.5). 9684

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Figure 3. (a) Experimental and (b) simulated heat of CO2 physisorption on zeolite Na-ZK-4 at 298 K. The experimental values were calculated using Clausius−Clapeyron equation. The simulated values are expected to have up to 5% error.

in zeolite Na ZK-4; we presumed that site III was not populated. As the K+ content increased, we calculated the fraction of 8MR windows populated by K+ by linear interpolation from the Si−Al ratio and the K+ content (see Table 1). In line

The experimental heat of adsorption remained fairly constant up to a loading of 2.5 mmol/g (∼8 CO2 per cage), which suggested that the cation sites or high energy sites had a CO2 capacity of around 2.5 mmol/g. The decrease in the heat of adsorption above 2.5 mmol/g meant that CO2 began to adsorb on other less energetically favorable sites. Similar decreases in the heat of physisorption of CO2 at high loading levels were observed for other zeolites.37 We understood that the lowering of the heat of physisorption was the effect of pore filling when CO2−CO2 interaction dominated energetically. The values of the GCMC simulated heat of physisorption of CO2 (Figure 2b) were very similar to the experimental values ∼42−44 kJ/mol, but the calculations could not represent the significantly reduced heat of adsorption of CO2 at loadings >2.5 mol/g. The difference between the simulated adsorption isotherms and the values for heat of physisorption of CO2 both became apparent at around 2−3 mmol/g loading. From the experimental data, this level of loading was when CO2 began to populate low-energy sites. The difference could therefore be due to an overestimation of the CO2−CO2 interaction energy or the underesitmation of the CO2−CO2 repulsion energy by the force field used. Selectivity and Adsorption of Ion Exchange Zeolite NaK-ZK-4. Zeolite ZK-4 in its sodium form (Na-ZK-4) was ion-exchanged to zeolite NaK-ZK-4. The level of K+ exchange in each individual sample was confirmed by EDS and confirmed with ICP-OES (Medac Ltd., UK). The term “K+ at. %” or similar terms refer exclusively to the percentage of the total number of extraframework exchangeable cations in zeolite ZK-4 being K+ (the rest of them are Na+). We obtained zeolite NaK-ZK-4 with approximately 10%, 18%, 26%, 45%, 55%, and 79% (with an error of ±10 ppm from the ICP-OES results) of the cations on ZK-4 being K+ (the rest of them are Na+). There were no changes to the morphologies of the crystals upon ion exchange. The uptakes of CO2 and N2 dropped somewhat with an increasing K+ content, which could be related to diffusional hindrance of some kind. We estimated the fraction of 8MR rings with K+ ions, as we could not determine that by X-ray diffraction. We presumed that zeolite Na-ZK-4 (Si:Al = 1.3:1, which has on an average 10.4 cations sites per cage) had a Na+ occupancy per cage: 8.4 close to the 6MR (site I), 2 in the 8MR (site II), and 0 close to the 4MR (site III). Note that Na+ does not occupy the 8MR window sites when the Si−Al ratio is sufficiently high.38 Kerr et al. also indicated that cation depopulation at the 8MR sites occurs before the 6MR sites as the Si:Al ratio increases.39 On the other hand, at a Si:Al = 1.3:1 Na+ is expected to partly populate the 8MR site II

Table 1. CO2 (at 15 kPa) and N2 (at 85 kPa) Uptake and CO2 Selectivity (s) of Zeolite NaK-ZK-4 with Various K+ Content at 273 K (101 kPa) approx K+ content (at. % of the total cations)

CO2 uptake at 15 kPa (mmol/g) (qCO2)

N2 uptake at 85 kPa (mmol/g) (qN2)

CO2/N2 selectivity (s) (273 K)

calcd fractions (%) of 8MR windows with K+

0 10 18 26 45 55 79

4.1 3.8 3.4 3.4 3.2 3.1 3.0

0.53 0.40 0.25 0.022 0.0087 0.0030 0.0030

44 53 78 880 >2000 >2000 >2000

0 33 52 92 100 100 100

with the finding of Ikeda,27 we assumed that the K+ first preferred the 8MR windows and became gradually occupied by K+ ions. The K+ occupancy at the 4MR site is close to zero.27 The presence of K+ ions in the 8MR windows introduced an energy barrier for the diffusion of CO2 and N2 through the occupied 8MR windows (K+ effectively hinders sorbate diffusion). When the K+ content increased from 18 to 26 at. %, the number of 8MR windows that were occupied by K+ appeared to have surpassed those needed to reach the percolation threshold for N2 but not for CO2 (see Table 1). The CO2 uptake was not affected to the same extent (the activation barrier for CO2 transport appears to be lower than for N2 transport). Molecular simulations by Mace et al.34,40 showed that the energy barrier for CO2 diffusion through an 8MR window occupied by a K+ ion in zeolite A is less than that of N2. Figure 4a shows that the CO2 uptake was also affected by the increased occupancy of the 8MR windows, but only to a minor extent. As the K+ content was further increased, other sites became increasingly occupied by K+ and the CO2 uptake began to drop (CO2 and N2 adsorption isotherms of zeolite NaK-ZK-4 with K+ content higher than 26 at. % are available in the Supporting Information), possibly due to the presence of the large K+ ion occupying the available space. It is important to note that the presence of K+ did lower the CO2 uptake “actively” and could not be explained by the differential masses of Na+ and K+. (Zeolite Na-ZK-4 had an uptake of 16.0 CO2/cage at 101 kPa and 273 K; zeolite NaK-ZK-4 with 79 at. % K+ had 14.3 CO2/cage under the same conditions.) Although the 9685

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Figure 4. CO2 (a) and N2 (b) adsorption isotherms of various zeolite NaK-ZK-4 at 273 K: 0 (■), 10 (●), 18 (▲), and 26 at. % K+ (▼). These isotherms were recorded using a Micromeritics Gemini 2390; samples were prepared using N2 flow at 623 K. The CO2 adsorption isotherms here show both chemisorption and physisorption contributions.

CO2 uptake did decrease at high K+ content, the uptake of CO2 still seemed much less restricted than that observed on zeolite NaKA with high K+ content.13 On zeolite A (i.e., NaA or NaKA), the overall higher cation density than in zeolite ZK-4 could have induced this difference at a high K+ content; but note that nanosized zeolite NaKA did not display a reduced uptake of CO2 at high loadings as did the regular sized zeolite NaKA.11 The high CO2 uptake, and the minimal N2 uptake, made zeolite NaK-ZK-4 (26 at. % K+) highly selective with a selectivity of over 800. (Note that selectivity of over several hundred, or higher essentially means that N2 is not adsorbed; these numbers should not be interpreted fully quantitatively.) The CO2 and N2 uptake and the CO2 selectivity, as calculated using s = (qCO2/qN2)/(pCO2/pN2), where pCO2= 15 kPa and pN2 = 85 kPa, are shown in Table 1. There was also no decrease in the CO2 uptake between zeolite NaK-ZK-4 with 18 and 26 at. % K+. The increased number of 8MR occupancy apparently had only minor effect on the uptake of CO2 under these experimental conditions. The CO2 uptake rate was affected as indicated by in situ IR (discussed later) due to the increased number of K+ occupied windows, introducing a high energy barrier for diffusion. Rate of CO2 Adsorption on Zeolite NaK-ZK-4. Using in situ IR spectroscopy, we observed fast CO2 physisorption on zeolite NaK ZK-4 with 0-17 at. % of K+ ions. As displayed in Figure 5c, 80% of the capacity to physisorb CO2 (3.62 mmol/g at 273 K) was reached in