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Determination of Composition Range for “Molecular Trapdoor” Effect in Chabazite Zeolite Jin Shang,†,‡,∥ Gang Li,†,‡,⊥,∥ Ranjeet Singh,†,‡ Penny Xiao,†,‡ Jefferson Z. Liu,*,§ and Paul A. Webley*,†,‡ †

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Melbourne, Australia Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia § Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia ‡

S Supporting Information *

ABSTRACT: Highly selective separation of small molecules, such as CO2, N2, and CH4, is difficult to achieve if all of the molecules can access the internal surface so that the selectivity depends only on differences in interaction of these molecules with the surface. Recently, we reported on a “molecular trapdoor” mechanism (Shang, J.; et al. J. Am. Chem. Soc. 2012, 134, 19246−19253), which provides a record high selectivity through a guest-induced cation deviation process where the adsorbent exclusively admits “strong” molecules (e.g., CO2 and CO) but excludes “weak” ones (e.g., N2 and CH4). In this study, we have investigated the range of zeolite compositions (varying Si/Al and cation type) for which a trapdoor effect is present and summarize this composition range with a simple “rule of thumb”. Cation density and cation type are the controlling factors in achieving the molecular trapdoor effect on chabazites. Specifically, the “rule” requires every pore aperture connecting the supercages to accommodate one door-keeping cation of an appropriate type. This “rule” will help guide the synthesis of “trapdoor” chabazite adsorbents for the deployment of carbon capture as well as help the development of molecular trapdoor adsorbents/membranes for other small-pore zeolites, such as RHO, LTA, and other porous materials.

1. INTRODUCTION Separation of carbon dioxide from various gas streams, such as methane-rich natural gas and nitrogen-rich flue gas, has gained interest worldwide. Carbon dioxide is a major impurity in methane-rich gas streams, such as natural gas, landfill gas, and biogas, where it not only reduces the energy density of the gas stream but also causes pipeline and equipment corrosion/ blockage in the presence of water.1−3 Carbon capture and storage (CCS) also requires the removal of CO2 from flue gas streams as the first step in the CCS chain.4 Accordingly, several technologies have been developed to separate carbon dioxide, including ammonium/dual-alkali absorption,5 cryogenic distillation,6 membrane separation,7 and adsorption.8 Among these, adsorption-based technologiespressure swing adsorption (PSA)9 and vacuum swing adsorption (VSA)2,8are promising owing to their simple operation, low operating and capital costs, and favorable energy efficiency.10,11 An adsorption process for CO2 capture requires an adsorbent with high carbon dioxide working capacity and selectivity. Porous materials, including activated carbons, metal−organic frameworks (MOFs), and zeolites, serve as primary candidates.12 If all molecules of the mixture have access to the internal surface of the porous adsorbents, then adsorption selectivity is based on differences in intermolecular interactions between these molecules and the surface. Carbon dioxide is © XXXX American Chemical Society

usually preferentially adsorbed over CH4 or N2 since it has stronger intermolecular interactions with the surface. Two molecular parameters that are important for physisorption of CO2, N2, and CH4 are the quadrupole moment and polarizability. CO2 has a relatively high quadrupole moment and polarizability (−1.4 × 10−39 cm2 and 29 × 10−25 cm−3, respectively) compared with nitrogen (−4.7 × 10−40 cm2 and 17.4 × 10−25 cm−3, respectively) and methane (0 cm2 and 26 × 10−25 cm−3, respectively). Although CO2 is preferentially adsorbed, these adsorbents still suffer from intrinsic disadvantages: the adsorption selectivity is limited and the selectivity often declines with increasing total pressure due to continuous uptake of the weakly adsorbing component at higher pressures, in contrast to early saturation of the strongly adsorbing component at much lower pressures. This drop in selectivity with increase in pressure is a serious drawback in applying adsorption to high-pressure separation, such as that occurring in natural gas streams. An alternative separation mechanism relies on a steric mechanism (or molecular sieving) for separation. In this case, large component molecules, such as nitrogen and/or methane, Received: February 11, 2013 Revised: May 14, 2013

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dx.doi.org/10.1021/jp4015146 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Table 1. Summary of Chabazite Samples of Different Compositions, the N2 Adsorption Capability at 77 K, and the Determination of the Molecular Trapdoor Effect

a

no.

sample namea

Si/Al ratio

unit cell formula

N2 uptake @ 77 K

molecular trapdoor? (Yes/No)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

r1BaNaKCHA r1KLiNaCHA r1KNaCHA r1KCaNaCHA r1CsKCHA r1KMgCHA r1.5KCHA r2SrKCHA r2NaKCHA r2KCHA r2RbKCHA r2CaKNaCHA r2MgNaCHA r2BaNaCHA r2LiKCHA r2CsKCHA r5.5KCHA r6.5KCHA r7KCHA r17KCHA r50KCHA

1.04 1.08 1.13 1.16 1.16 1.19 1.5 2.12 2.2 2.2 2.3 2.3 2.3 2.4 2.5 2.53 5.4 6.5 7 17 50.4

Ba5.9Na3.7K2.2[Al17.6Si18.4O72] K9.2Li5.1Na3[Al17.3Si18.7O72] K16.3Na0.5[Al16.9Si19.1O72] K6.6Ca4.6Na0.9[Al16.7Si19.3O72] Cs14.8K1.9[Al16.7Si19.3O72] K10Mg3.2[Al16.4Si19.6O72] K14.1[Al14.1Si21.9O72] Sr5.3K0.9[Al11.5Si24.5O72] Na10.2K1[Al11.2Si24.8O72] K11.2[Al11.2Si24.8O72] Rb8.7K2.2[Al10.9Si25.1O72] Ca4.9K1.2Na0.1[Al11Si25O72] Mg3.8Na3.3[Al10.9Si25.1O72] Ba5.2Na0.2[Al10.6Si25.4O72] Li9.4K0.9[Al10.3Si25.7O72] Cs9K1.2[Al10.2Si25.8O72] K5.6[Al5.6Si30.4O72] K4.8[Al4.8Si31.2O72] K4.5[Al4.5Si31.5O72] K2[Al2Si34O72] K0.7[Al0.7Si35.3O72]

N N N Y N N N Y Y N N Y Y Y Y N Y Y Y Y Y

Y Y Y N Y Y Y N N Y Y N N N N Y N N N N N

The samples are named by their Si/Al ratios and extraframework cation types.

be met provided that the stoichiometric zeolite chemical composition is within prescribed bounds. In this work, we carefully identify these bounds for “operation” of the molecular trapdoor mechanism by measuring and screening the adsorption equilibria of CO2, N2, and CH4 on chabazites with a wide range of ratio of silicon to aluminum and a series of different cations. In this study we select CO2, N2, and CH4 as illustrative of “strong/weak” molecules to demonstrate the functioning of the molecular trapdoor effect for gas separation. The mechanism is not limited to these three gases and should extend to other molecules, such as CO, NO, N2O, Ar, O2, and H2, as long as an appropriate “strong/weak” molecule combination is adopted. The criterion for “strong” or “weak” depends not only on the polarity (i.e., quadrupole, dipole, and polarizability) of the guest molecule but also on the charge distribution of the gas molecules. The details of the extent to which different molecules are able to induce cation motion is beyond the scope of the current work but was discussed in our earlier study.15 Given that (1) we have provided concrete evidence from both ab initio simulations and molecular level experimental characterizations for the molecular trapdoor model using one typical chabazite sample in the previous study15 and (2) the chabazite samples in this study have a similar composition to the previous one and show similar adsorption properties, we do not provide the atomistic simulations and structural or spectroscopic characterizations for those trapdoor candidates listed in this contribution. In the current study, we use the composition bounds of the zeolite to derive an empirical “rule” that can serve as a working principle for fabricating “molecular trapdoor” chabazite with separation performance comparable to the conventional “molecular sieving” zeolites. We hope it will also inspire the development of other molecular trapdoor zeolites and eventually lead to a database of “molecular trapdoor zeolites” for application in extremely difficult, but commercially important, molecular separations for small molecules.

are excluded, whereas smaller component molecules, such as carbon dioxide and/or hydrogen, are adsorbed, resulting in high selectivities even at high pressures. This separation mechanism requires that the material has a pore aperture diameter (d) between the molecular diameters of the components in a targeted gas mixture. For the exclusive uptake of CO2 (kinetic diameter σ = 0.33 nm) from a mixture containing N2 (σ = 0.364 nm) and CH4 (σ = 0.38 nm) as remaining components, a molecular sieve with pore aperture diameter of 0.33 < d < 0.364 nm is mandatory. Only a few zeolite molecular sieves satisfy this strict requirement. One typical example is the Engelhard titanosilicate ETS-413 whose pore aperture can be tuned to be the desired size by controlled thermal dehydration. The application of ETS-4 is limited, however, due to its low carbon dioxide capacity and low thermal stability.14 Recently, we have identified a different separation mechanism occurring in some small-pore zeolites. In this “molecular trapdoor” mechanism,15 molecular size is not the dominant parameter. Rather, cations that are present in the pore apertures of these zeolites play a role in controlling the entry of molecule to the supercages. Molecules are admitted to the supercage of the zeolite based on their ability to coordinate with the cation blocking the access, moving the cation temporarily away from the pore aperture. We termed this a “trapdoor” since the cation moves toward the approaching guest molecule, allowing the guest molecule to enter the supercage, before returning to its position in the center of the aperture. Only “strong” molecules, such as CO2 and CO, are able to pay the energy toll to move the “door-keeping” cations (such as Cs+, K+) temporarily and thus gain admittance to the zeolite supercage, whereas “weak” molecules, such as N2 and CH4, are excluded. Therefore, a high selectivity and acceptable capacity of CO2 can be achieved by “molecular trapdoor” zeolites, such as chabazites, even in the high-pressure range. This trapdoor mechanism is only possible, however, if the zeolite has a sufficient number of cations of the appropriate type in the correct locations. This requirement can B



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2. EXPERIMENTAL METHODS 2.1. Sample Synthesis. Chabazite samples with Si/Al = 1.13, 1.5, 2.2, 5.4, 6.5, 7, 17, and 50 were synthesized according to the reported procedures15,16 and subsequently labeled as r1CHA, r1.5CHA, r2CHA, r5.4CHA, r6.5CHA, r7CHA, r17CHA, and r50CHA, respectively. The as-synthesized samples were fully ion-exchanged to various cation types following the published procedure.15 The nearly fully ionexchanged potassium type chabazite with Si/Al = 1.13 (labeled as r1KNaCHA) was partially ion-exchanged to various mixture cation types following a revised procedure: typically, 1 g of r1KNaCHA was added to 100 mL of the respective halide solution (0.08 mol/L LiCl, 0.08 mol/L CaCl2, 0.08 mol/L MgCl2, or 0.1 mol/L BaCl2). The mixture was suspended by stirring for 30 min at 303 K. The sample was then washed 2−3 times with deionized water and dried in an oven at 353 K. 2.2. Sample Characterization. All the samples (assynthesized and ion-exchanged) were characterized by powder X-ray diffraction (Philips SEI diffractometer) using CuKα (λ = 1.540598 Å) radiation for phase purity and identification. Elemental analysis for inorganic ions and Si/Al ratios were determined by inductively coupled plasma-mass spectroscopy (ICP-MS) as well as by energy-dispersive X-ray spectroscopy (EDX). 2.3. Gas Adsorption Measurements. Single component isotherms of CO2, N2, and CH4 were collected at temperatures ranging from 273 to 323 K and pressures up to 120 kPa using a standard volumetric method on a Micromeritics ASAP2010 accelerated surface area and porosity analyzer. The surface areas of the prepared samples were measured by N2 adsorption at 77 K. Prior to each measurement, the samples were thoroughly dehydrated and degassed on a Micromeritics ASAP2010 analyzer by stepwise heating (2 K/min) up to 623 K and held at 623 K under high vacuo for 18 h and then cooled to 295 K, followed by backfill with helium. An equilibrium interval of 20 s was adopted in all the isotherm measurements.

as those used in this study. We have previously confirmed this behavior spectroscopically and using DFT calculations.15 We define this spontaneous trapdoor opening onset temperature as TS. For a typical trapdoor chabazite, r2CsKCHA (sample no.16), the value of TS falls in the range of 303−333 K.15 The value of TS may vary slightly for trapdoor chabazites of different compositions. Second, if no substantial N2 adsorption is observed far below Ts (for example, at 77 K), but substantial CO2 adsorption is observed below TS (for example, at 273 K), then we have an example of a “trapdoor” effect in which the CO2 molecule has induced cation motion. Conversely, if N2 adsorption is detectable at 77 K (such as typically occurs in large-pore zeolites), then we identify these chabazites as nontrapdoor zeolites. In all of our samples, shown in Table 1, strong adsorption of CO2 at 273 K was observed. In some of the samples, N2 adsorption at 77 K was observed, whereas others showed no N2 adsorption. On the basis of our criteria described above, we assign these latter materials as potential “molecular trapdoor” materials. We add that this criterion is restricted to the current family of zeolites we are examining. In this study, we measured adsorption isotherms on a series of chabazites of different compositions at 273−323 K. Their potential CO2/N2 and CO2/CH4 separation performances were examined accordingly (see the Supporting Information). However, this temperature range of 273−323 K is selected only for convenience of comparison between samples (as adsorbents are usually evaluated in this temperature range in the literature) and it may not be the best working temperature for every trapdoor chabazite. Therefore, some trapdoor chabazites may show noticeable adsorption of N2 and CH4 if their TS overlaps with our experimental temperature. For example, our previous study16 shows that r2KCHA (sample no. 9) exclusively adsorbs CO2 but rejects N2 and CH4 at 253 K; however, admission of N2 and CH4 can be observed at a higher temperature of 273 K (Figure S2, Supporting Information), where the r2KCHA trapdoor opens spontaneously. 3.2. Effect of Cation Density. As the 8MR is the only access to the supercage (which is where the adsorption occurs), each 8MR coordinated by one extraframework cation (doorkeeping cation) constitutes the necessary condition for the molecular trapdoor to take effect. According to eq 1, when Si/ Al = 3, then i = 9. In this context, there will be at most nine cations per unit cell only if all the extraframework cations are univalent. Further, if all the nine univalent cations preferentially coordinate at all the nine 8MR pore apertures on a one-on-one basis, the molecular trapdoor is enabled. In the instance when Si/Al < 3 (Si/Al cannot be smaller than 1 as regulated by Lowenstein’s rule17), there will be more than nine equivalent univalent cations quota per unit cell. As long as nine of them are located at the center of all the nine 8MRs in each unit cell, the molecular trapdoor effect will be retained. The existence of a threshold Si/Al ratio is demonstrated by inspecting N2 adsorption at 77 K on a series of potassiumexchanged chabazites (Figures 1 and 2). Samples with Si/Al < 3 (i.e., Si/Al = 1.17, 1.5, and 2.2) show negligible N2 adsorption, indicating that these samples are potential “trapdoor” candidates. The small amount of adsorption observed for these samples can be attributed to uptake on the external surface and defects of the chabazite nanocrystals. In the instance when Si/Al > 3, there will be absolutely less than nine equivalent univalent cations quota per unit cell, regardless of any combination of cation types. Therefore, gas molecules can readily access the interior of these chabazites

3. RESULTS AND DISCUSSION 3.1. Characterization of Chabazites. The chabazite structure and the high crystallinity of our chabazites were confirmed by powder X-ray diffraction (XRD) (Figure S1, Supporting Information). The unit cell formulas of various chabazite samples were determined by ICP-MS and/or EDX and are shown in Table 1. The unit cell can be expressed with the following formula Min/+n AliSi36 − iO72

(1)

where M represents the extraframework cation, i is a positive number representing the quantity of aluminum atoms per unit cell (i ≤ 18), and n is an integer representing the valency. Such a unit cell includes one and a half supercavities. Each ellipsoidal supercavity is accessed by six eight-membered rings (8MRs); therefore, our unit cell (one and a half supercavities) contains nine eight-membered rings (8MRs). Table 1 also shows whether N2 adsorption was observed at 77 K, and we use this feature to predict whether the material is a potential trapdoor material based on the following logic. First, we note that, regardless of the molecule characteristics, if the system temperature is sufficiently high, the door-keeping cation will migrate away from the aperture and provide an open eightmembered ring (or “doorway”), allowing for free admission of any gas molecules smaller than the free aperture diameter, such C



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therefore, an overall molecular trapdoor effect may still be present to a limited extent in accordance with the percolation theory.18 3.3. Effect of Cation Types. The types of cations also play an equally important role in enabling the molecular trapdoor effect, apart from the aforementioned minimum requirement of nine cations per unit cell of chabazites. Only those cations that reside at the center of 8MRs (site SIII′19) can serve as the doorkeeping cations. Large univalent cations, such as K+,19 Rb+,20 and Cs+,21 and large divalent cations, such as Ba2+,22 prefer to locate at the center of 8MRs as door-keepers. Figure 3 and

Figure 1. N2 adsorption at 77 K on potassium-exchanged chabazites with different Si/Al ratios. Below Si/Al = 3 (i.e., r1.17, r1.5, and r2.2), very low N2 capacity in the relevant pressure range signifies pore blockage, indicating that the “trapdoor” effect is actuated; above Si/Al = 3 (i.e., from r5.4 up to r50), significant N2 capacity in the relevant pressure range signifies conventional adsorption in the supercage giving a “normal” BET surface area, indicating that the trapdoor effect is absent.

Figure 3. N2 adsorption at 77 K on chabazites with different cation types. Solid symbols denote normal chabazites, and open symbols denote trapdoor chabazites. Note that the cation labeled here is the dominant cation in the case of mixed cation-exchanged chabazites, and the number of each sample is given for indexing from Table 1. The door-keeping cations are highlighted in bold red in the legend. K+-, Rb+-, Cs+-, and Ba2+-exchanged chabazite samples show negligible N2 capacity, whereas chabazites of all the other cation types show a noticeable N2 capacity.

Table 2. BET Surface Area Measured with N2, Demonstrating the Effect of Cation Type on the Molecular Trapdoor Effect for Chabazites with Sufficient Cations Per Unit Cell

Figure 2. BET surface area measured with N2 at 77 K on potassium forms chabazites of various Si/Al ratios, demonstrating the effect of cation density on the molecular trapdoor effect. The negligible surface area for samples with Si/Al < 3 can be attributed to external surface and crystal defects.

a

S (m2/g) sample no.

through cation-free 8MRs, and thus, the trapdoor effect does not apply. For example, the loading of N2 at 77 K is pronounced on chabazite samples with Si/Al > 3 (Si/Al = 5.4, 6.5, 7, 17, and 50, as shown in Figures 1 and 2) and the calculated surface area is comparable to that of other “normal” zeolites, suggesting the absence of the molecular trapdoor effect. Notably, Si/Al = 3 is only a theoretical critical threshold. A chabazite sample with Si/Al in the vicinity of 3 (for example, 3.2), may still show a measurable molecular trapdoor performance due to a practically uneven Si/Al distribution. Although the global Si/Al is 3.2, the local uneven Si/Al distribution dictates that the unit cells with a ratio lower than 3 (which will not allow access) may still be present in the structure. Since the chabazite is a 3D structure, interconnected unit cells with a trapdoor effect will block gas molecules from transferring into the unit cells without a trapdoor effect;

cation

b

Li

Na

K

Rb

Cs

Ca

Ba

653 15

490 9

21 10

16 11

19 16

114 4

66 1

a

The cation listed here is the representative cation in the respective cation-exchanged chabazite. Co-contribution applies if there is any other door-keeping cation. bData of r2LiKCHA is reproduced from our previous work.27

Table 2 show that chabazites with no less than nine doorkeeping cations (K+, Rb+, and/or Cs+) per unit cell exhibit a negligible BET surface area based on N2 adsorption equilibria at 77 K. Nevertheless, other small univalent cations, including Li+ and Na+, do not favor site SIII′ but choose to stay inside the chabazite supercavity, leading to a nontrapdoor effect.19 This is evident from Figure 3 that, given a similar cation density (∼11 cations per unit cell), Li+- and Na+-exchanged chabazites display considerable N2 loading at 77 K and thus a relatively high surface area and large microporosity. D



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above-mentioned “effective door-keeping cations” (i.e., K+, Rb+, Cs+, and/or Ba2+), regardless of the types of the remaining cations. For example, r1KMgCHA (sample no. 6) has 10 K+ and 3.2 Mg2+ per unit cell, and thus shows the typical trapdoor phenomenon of exclusion of N2 at 77 K (Table 1) with a negligible surface area (32 m2/g). We contend that a cation no smaller than K+ can probably be an eligible door-keeper to enable the molecular trapdoor effect given the fact that large cations prefer the 8MR site to other sites (i.e., 6MR site or 4MR site) due to favorable coordination to more oxygen atoms in the 8MR. Invoking this line of argument, large transitional metals or even lanthanides may work as door-keeping cations. However, the ultimate appraisal of the eligibility of a cation as door-keepers should be provided by concrete experimental techniques, such as powder XRD and solid NMR. 3.4. Summary of the Composition Range for the Molecular Trapdoor Effect. Cation density and cation type are the controlling factors in achieving the molecular trapdoor effect on chabazites. We are able to summarize the boundary conditions in validating the molecular trapdoor chabazites based on the requirement for nine cations per unit cell as follows

We demonstrate the influence of cation type on the trapdoor effect by comparing the adsorption selectivity of CO2 against CH4 on Si/Al ratio = 2 chabazites with different alkali metal cations. As shown by Figure 4, Rb+ and Cs+ type chabazites

1 ≥ Y ≥ 0.25X + 0.25

(2)

and

X≥1

(3)

where Y denotes the fraction of potential door-keeping cations (i.e., K+, Rb+, Cs+, and/or Ba2+) out of the total charges of the silicon−aluminum framework or the number of aluminum atoms per unit cell, and X is the Si/Al ratio. These two controlling factors can be depicted in a schematic diagram, as shown in Figure 5. Zone A represents the strictly theoretical zone in which the molecular trapdoor effect can be achieved since each 8MR is kept by one effective door-keeping cation. For real adsorbents, this zone may extend to encompass zone A′ because of uneven distribution of Si/Al ratio throughout the crystal structure and the partial blockage of these supercages. As a result, the molecular trapdoor effect may be achieved in the zone of A + A′ where high CO2/CH4 selectivity may be obtained given an appropriate working temperature. For example, r1BaNaKCHA (sample no. 1) with an average 8.1 door-keeping cations per unit cell falls in zone A′ in Figure 5, showing a convincing trapdoor effect at 77 K (Tables 1 and 2 and Figure 3). Chabazites falling in zone B impart no molecular trapdoor effect due to unoccupied free open 8MRs. Accordingly, for samples in zone B, the CO2/CH4 selectivity is relatively low. We believe that, for specific separation tasks, high selectivity and appropriate working temperatures can be achieved by optimizing the density and combination of door-keeping cations. For example, the K+ type chabazite with a Si/Al ratio of 1 (sample no. 3) shows an even higher CO2/CH4 selectivity at 10 bar high pressure at 273 K, compared with the Cs+ type chabazite with a Si/Al ratio of 2 (sample no. 16) (Tables S1 and S2, Supporting Information).

Figure 4. Adsorption selectivities of CO2/CH4 on Si/Al ratio = 2 chabazites with different cation types derived from equal molar single component adsorption isotherms, at 273 K and variable pressures (typical conditions for VSA). The sizes of the schematic cations and the chabazite framework (inset) are in proportion. The selectivity is calculated by comparing the equimolar single component adsorption isotherms of CO2 and CH4. r2KCHA is not showing comparable CO2/CH4 selectivity at 273 K as other trapdoor chabazites (r2RbCHA and r2CsCHA) simply because the trapdoor separation of r2KCHA only dominates at temperatures below 253 K.15

(sample nos. 11 and 16) give excellent selectivity as their TS values are well above the working temperature herein (273 K). K+ type chabazite (sample no. 10), although being a trapdoor material, does not show the trapdoor separation effect as with Rb+ or Cs+, because its value of TS is lower than 273 K.16 Na+ and Li+ type chabazites (sample nos. 9 and 15) only show modest separation in the very low pressure range (Henry’s law region) due to conventional equilibrium-based adsorption. Furthermore, small divalent cations, such as Mg2+,23 Ca2+,24 2+ 25 Sr , and Cu2+,26 are also ineffective in imparting the trapdoor effect because they only reside inside the supercavity rather than at the 8MR center. For example, chabazite sample no. 4 with 6.6 K+ and 4.6 Ca2+ per unit cell (total cation number > 9) does not show the trapdoor effect, because Ca2+ cannot serve as a door-keeping cation in a chabazite, and the remaining 6.6 door-keeping cations of K+ per unit cell is substantially lower than the theoretical threshold value of 9. It is usually difficult to obtain 100% cation-exchanged chabazites, which means that there are often cations other than the primary ones in the structure. For chabazites with a given combination of cations, the molecular trapdoor will take effect only if each of the nine 8MRs is allocated to one of the

4. CONCLUSIONS The high selectivity presented by chabazite zeolites results from a molecular trapdoor mechanism. Gas molecules (such as CO2) having sufficient interaction ability to induce the door-keeping cations (such as K+, Rb+, Cs+, and/or Ba2+) to deviate from the center of pore apertures, temporarily and reversibly, can be E



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Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

We acknowledge the University of Melbourne and Monash University. J.S. thanks David Danaci for assisting in adsorption isotherm measurement. Funding for this CO2CRC project is provided by the Australian Government through its CRC program.

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Figure 5. Schematic diagram depicting the composition range and cation type for realizing the molecular trapdoor effect on chabazites in relation to Si/Al ratio (X) and fraction of potential door-keeping cations (i.e., K+, Rb+, Cs+, and/or Ba2+) out of the total charges per unit cell (Y). The gray zone (A′) below the boundary line represents a transitional zone from trapdoor chabazites (zone A) to nontrapdoor chabazites (zone B), where the trapdoor effect may still apply to a lesser extent. Solid symbols are experimental data points labeled with chabazite sample numbers corresponding to those in Table 1. Symbols in light magenta denote chabazites with K+ as primary door-keeping cations and, likewise, red-violet symbols for Rb+ chabazites, blue-violet for Cs+, and green for Ba2+ chabazites. Symbols in black color represent chabazites without any door-keeping cation (i.e., Y = 0). Symbols denoting molecular trapdoor chabazites are marked with a cross.

admitted, whereas those (such as N2 or CH4) that cannot induce this movement are rejected. By evaluating a series of chabazite samples of various silicon/aluminum ratios and various cation types via adsorption measurements, we have summarized a rule to enforce the molecular trapdoor effect in this family of zeolites: tuning the Si/Al ratio and cation type to ensure complete occupation of all “pore aperture doorways” by door-keeping cations. Practically, this rule can guide the fabrication of molecular trapdoor chabazites for application in industrially relevant separations. Theoretically, our work offers working principles for applying the molecular trapdoor mechanism in other similar small-pore zeolites, such as LTA,28 RHO,29 ZK-5,30 etc.

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ASSOCIATED CONTENT

S Supporting Information *

Powder X-ray diffraction patterns of typical chabazite samples and adsorption isotherms data for some chabazite samples. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.A.W.), zhe.liu@ monash.edu (J.Z.L.). Present Address ⊥

Centre for Energy, School of Mechanical & Chemical Engineering, The University of Western Australia, Crawley WA 6009, Australia.

Author Contributions ∥

These authors contributed equally. F



Article

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