J. Phys. Chem. C 2010, 114, 22025–22031
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Potassium Chabazite: A Potential Nanocontainer for Gas Encapsulation Jin Shang,† Gang Li,† Ranjeet Singh,† Penny Xiao,† Jefferson Z. Liu,‡ and Paul A. Webley*,† Department of Chemical Engineering, and Department of Mechanical and Aerospace Engineering, Monash UniVersity, Clayton, Victoria 3800, Australia ReceiVed: August 8, 2010; ReVised Manuscript ReceiVed: October 20, 2010
Gas storage in a safe and economical way is an important aspect of modern life. This study reports on the behavior of the zeolite potassium chabazite (Si/Al ) 2.2) as a nanocontainer to store N2 and CH4 by a “temperature” controlled nanovalve. We reveal the effect of extra-framework cation type and density on the adsorption characteristics by measuring a series of isotherms. Temperature programmed desorption (TPD) experiments are conducted to identify the working temperature for gas storage. The results show a N2 storage capacity of 3.02 mmol/g if loaded above 266 K and 5 MPa and held below 223 K and at atmospheric pressure. For CH4 storage, a more mild working temperature (i.e., above 279 K) is observed due to its larger molecular size than that of N2. Chabazite is a promising gas storage container because its features can be tailored to encapsulate a series of gases of different size by tuning its extra-framework cation type and density. 1. Introduction Gas separation and storage is a widely studied field due to increasing demand for pure gases such as nitrogen, oxygen, argon, and more specifically rare gases. Energy security and environmental concerns have recently directed researchers toward alternatives to conventional fossil fuels (gasoline and diesel) such as hydrogen and methane.1,2 The general objective for storage of gases is to ensure relatively high density, high purity, and safety. Several methods have been proposed for this purpose such as high-pressure cryogenic tanks, chemisorption (metal hydrides, hemicarcerand system, etc.), physisorption by microporous solids (carbon nanotubes, carbon molecular sieve (CMS), metal organic frameworks, etc.), and encapsulation of gases. The process of gas encapsulation3-6 involves forcing a gas molecule at elevated temperatures and pressures into pores of a zeolite, which are normally not accessible under ambient conditions because of limited size of the pore windows. At elevated temperatures (hundreds of degrees Celsius normally), the enhanced thermal vibrations of the ring-window-oxygen atoms and, if applicable, the oscillation and migration of the extra-framework cations usually acting as sentinels, combined with the increased kinetic energy of guest gas molecules, lead to the entrance of molecules into the cavities of the host zeolites. Upon quenching the system to a certain lower temperature, the gas molecules are trapped inside the micropores and cannot escape at reduced pressure. Encapsulation offers a means of gas storage and of controlled release of trapped gases, either by reheating the zeolite or by treating with small polar molecules.7-10 As compared to gas adsorption, encapsulation presents certain advantages because the stored gas can be held in a nonpressurized container and thus would be easier for transport, safer in the event of container rupture, as well as lighter in terms of container weight. The utilization of zeolites as a storage medium for small gas molecules has been extensively examined. Early studies were conducted on some simple zeolites including tridymite, cristo* Corresponding author. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Mechanical and Aerospace Engineering.
balite, heulandite, stilbite, sodalite, and cancrinite with noble gases such as He, Ne, Ar, and Kr as target guests.7,11,12 After Barrer, nearly all the encapsulation work focused on type A zeolites as gas containers with different types of exchangeable cations due to its regular structure. Fraenkel and Shabtai13 conducted hydrogen encapsulation experiments on Cs-A molecular sieves and reported that at 573 K the loading was about 14 cm3/g(STP) at 8.96 MPa and 65 cm3/g(STP) at 91.69 MPa. Weitkemp et al.14 demonstrated that zeolite containing the maximum number of sodalite cages exhibits the highest hydrogen storage capacity at 573 K and 10 MPa. Gesser et al.4 investigated the encapsulation of methane in Linde 3A molecular sieves and observed that the methane loading at 633 K and 414 MPa was 6.9 wt % for 2 h. However, most of the previous studies are merely of theoretical significance with little practical application due to the high operating pressures and temperatures. Moreover, the reported materials for encapsulation were mainly limited to type A zeolites. Therefore, the exploration of new encapsulation materials with viable working conditions is of great interest. It was observed in an earlier study from our group that the K+ form of chabazite (CHA) exhibited pore blockage phenomenon for gases such as N2 and Ar15 at 77 and 87 K, respectively. In this study, we investigated the feasibility of encapsulation of nitrogen and methane in CHA zeolite structures under mild operating conditions, which may open a new pathway for gas storage and separation at ambient temperature conditions. 2. Experimental Section 2.1. Synthesis of Chabazite. Potassium chabazite was synthesized from zeolite Y (CBV400) following the reported procedure16 with gel composition 0.17Na2O:2.0K2O:Al2O3: 5.18SiO2:224H2O. A typical procedure involved addition of 25 g of zeolite Y powder in 198.2 mL of deionized water and 9.5 M KOH (26.8 mL) in a polypropylene bottle. The mixture was shaken for about 30 s and placed in an oven for 15 d at 368 K. The product obtained was filtered, washed with deionized water, and dried in an oven at 373 K. The as-synthesized chabazite was ion exchanged to the potassium form as follows: 200 mL of 1 M KCl was added to
10.1021/jp107456w 2010 American Chemical Society Published on Web 11/30/2010
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5 g of as-synthesized chabazite (solution-to-zeolite ratio of 40), and the mixture was refluxed at 343 K under stirring for 24 h. The supernatant solution was decanted, and the solid was washed 2-3 times with deionized water. The above procedure was repeated twice to obtain fully exchanged potassium chabazite (labeled as KCHA1). After the final wash, the product was placed in a porcelain dish and dried in an oven at 353 K. The calcium (CaCHA) and sodium forms of chabazite (NaCHA) were prepared from as-synthesized chabazite by five consecutive ion exchanges of as-synthesized chabazite with 1 M CaCl2 and 1 M NaCl (solution-to-zeolite ratio ) 30) aqueous solution, respectively. The mixture was stirred and heated at 353 K for 12 h under reflux conditions. The supernatant solution was decanted, and fresh solution was added each time. Finally, the CaCHA and NaCHA were filtered off and washed 2-3 times with deionized water and dried at 353 K overnight (labeled as CaCHA and NaCHA). High silica (Si/Al ratio ) 17 and 50) chabazite was synthesized for comparison purposes by following the reported procedure.17 Typically, a gel was prepared from 5.1 g of Ludox AS-30 (Sigma-Aldrich), 2.0 g of N,N,N-trimethyl-1-adamantammonium iodide, 12.1 mL of water, 0.4 g (or 0.0884 g in the case of Si/Al ratio of 50) of Al2(SO4)3 · 18H2O, and 1.09 g of potassium hydroxide, poured into the Teflon cup of a Parr 4545 reactor, which was closed, sealed, and heated at 423 K for 6 d, under autogenous pressure while being rotated at 30 rpm. After being cooled, the contents were filtered, and the solids were washed five times with water followed by methanol and acetone. The resulting zeolite was dried in air at 353 K. The samples were labeled as KCHA2 (Si/Al ratio ) 17) and KCHA3 (Si/Al ratio ) 50). 2.2. Characterization. All the samples (as-synthesized and ion-exchanged) were characterized by powder X-ray diffraction (Philips SEI diffractometer) using CuKR (λ ) 1.540598 Å) radiation for phase purity and identification. Elemental analysis for inorganic ions as well as Si/Al ratios was determined by inductively coupled plasma-mass spectroscopy (ICP-MS) as well as by energy dispersive X-ray detector (EDX). Crystal size, shape, and morphology were examined by scanning electron microscopy (SEM) (JEOL JSE-6300). 2.3. Isotherm Measurements. Prior to isotherm measurements, the samples were dehydrated by heating stepwise to 623 K under nitrogen as purge gas on Micromeritics Smartprep degasser and held at this temperature for about 18 h. Furthermore, the samples were thoroughly dehydrated and degassed on a Micromeritics ASAP2010 accelerated surface area and porosity analyzer at 623 K under high vacuo for 18 h prior to measurement. Adsorption and desorption isotherms for N2 and CH4 were measured in the temperature range 77-353 K and at pressures up to 120 kPa. High-pressure adsorption isotherm measurements were performed on Rubotherm magnetic suspension balance at 278 K and pressures up to 4 MPa. 2.4. Encapsulation Measurements. Encapsulation work was performed on a custom built volumetric temperature programmed desorption unit (TPD) (see Figure S2 in the Supporting Information). The TPD system consists of two parts: a manifold (upper part) and a sample cell (lower part), which are connected by a valve. A pressure transducer (MKS PDR2000) and a thermocouple (K-type) are attached to the top of the manifold to measure pressure and temperature, respectively. A gas inlet line and a vacuum line are connected to the manifold as well. In-situ sample activation can be carried out by using a heating jacket coupled with temperature controller (Eurotherm model 2132i) attached to the sample cell. The sample cell temperature
Shang et al. can be increased or decreased to the desired temperature by using an appropriate bath (water bath or liquid nitrogen bath). A typical encapsulation experiment involved introduction of 1.5 g of pretreated sample to the sample cell followed by in situ evacuation at 623 K for 4 h. After the sample was cooled to ambient temperature, the vacuum pump was turned off, and the valve between the manifold and the sample cell was closed. The manifold was dosed to 100 kPa with the analysis gas. With the valve between the manifold and the sample cell turned on, gas was introduced while the sample was being cooled from 333 to 77 K. When equilibrium was reached, the system was evacuated to 0 kPa and held under vacuum to ensure steady entrapment. Finally, the desorption process was initiated by removing the liquid nitrogen (LN2) bath so that the temperature of the sample increased naturally by convection to ambient temperature by absorbing heat from environment, and then raised to 373 K with the help of a heating jacket. The temperature and pressure change during desorption process was continuously recorded. An integrated encapsulation experiment was demonstrated on a high pressure reactor (Parr model 4561 mini reactor) (shown in Figure 1 and Figure S3 in the Supporting Information). The typical procedure involved filling a small glass bottle (15 mL) with approximately 10 g of chabazite, and placing this bottle into the reactor before sealing the reactor. The adsorbent was activated in situ by heating the reactor to 623 K in a vacuum. A high pressure gas (5 MPa) was then introduced into the reactor while the sample was cooled from 323 to 195 K, and the pressure was maintained for 30 min. After that, the pressure was released to ambient by opening the gas release valve while the temperature was maintained at 195 K. The glass bottle was then removed from the pressure reactor and capped with an empty plastic bag. The bottle was placed in a warm water bath (323 K) for accelerated heat transfer. The decapsulated gas was discharged and collected in a plastic bag. The amount of gas stored was determined by the water displacement method. 3. Results and Discussion 3.1. Adsorbent Characterization. The crystallinity and thermal stability of the chabazite structure were confirmed by powder X-ray diffraction (XRD). The XRD patterns (see Figure S1 in the Supporting Information) of chabazite samples prepared in this study were in excellent agreement with reported ones,18 indicating that the products are essentially pure synthetic chabazite. Furthermore, XRD patterns of as-synthesized and ion exchanged chabazite samples were similar, indicating that no significant changes in crystal structure occurred during the ion exchange process. The scanning electron micrograph of KCHA1 sample (Figure 2) shows hexagonal crystals with no amorphous phase. The results of elemental analysis, surface area, and micropore volume are summarized in Table 1. 3.2. Effect of Cation Type. Nitrogen adsorption isotherms of NaCHA, KCHA1, and CaCHA at 77 and 273 K are shown in Figure 3a and b, respectively. Figure 3a clearly shows that the adsorption amount of N2 on KCHA1 at 77 K is much lower than that on the Na+ and Ca2+ form chabazite, which is reflected in terms of low N2 Langmuir surface area of 20 m2/g for KCHA1 (Table 1) in comparison with approximately 599 and 649 m2/g for Na+ and Ca2+ types, respectively. In contrast, at 273 K, the adsorption isotherm of N2 on KCHA1 (Figure 4b) exhibits reasonable loadings with normal Langmurian shape in line with other chabazite samples. The discrepancy is in agreement with Ridha’s work15 in which potassium chabazite was compared to sodium chabazite and lithium chabazite, and
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Figure 1. Schematic representation of integrated encapsulation-decapsulation experiment. (a) Adsorption at 50 bar and 323 K; (b) the adsorbed gas is “fixed” at 50 bar (1 bar ) 0.1 MPa) and 195 K for 30 min; (c) system pressure is released to ambient pressure (1 bar) at 195 K; and (d) decapsulation by raising the temperature to 323 K, and the desorbed gas is collected by a plastic bag.
Figure 2. SEM images at (a) low and (b) high magnification of the KCHA1 crystals.
TABLE 1: Results of Elemental Analysis and Pore and Surface Area Properties for Different Types of CHA sample name Si/Al
unit cell formula
KCHA1 2.2 KCHA2 17 KCHA3 50.4 NaCHA 2.2 CaCHA 2.3
K11.2[Al11.2Si24.8O72] K2[Al2Si34O72] K0.7[A0.7Si35.3O72] Na10.2K1[Al11.2Si24.8O72] Na0.1K1.2Ca4.9[Al11Si25O72]
Sa Smb Sec Vmd (m2/g) (m2/g) (m2/g) (cm3/g) 20 954 646 599 649
2 727 477 443 479
18 54 33 23 32
0.0008 0.32 0.22 0.20 0.20
a
c
Langmuir surface area by N2. b t-plot micropore area by N2. t-plot external surface area by N2. d t-plot micropore volume.
the author attributed the unexpected low loading of N2 on potassium chabazite at 77 K to “pore-blockage”. The surface areas given by t-plot method are in agreement with those by the Langmuir method. Besides, the t-plot micropore volume of KCHA1 is 0.0008 cm3/g, which is negligible in comparison with those of NaCHA as well as CaCHA.
Interestingly, KCHA1 shows “normal” N2 adsorption capacity at ambient temperatures (273 K) as compared to exceptionally low capacity at lower temperatures (77 K). As all the prepared chabazites have similar crystal structure, the difference in the isotherms can be attributed to the cation size, cation site, and cation density. CHA consists of D6R units arranged in layers that are linked together by tilted four-member rings. The pore structure comprises six eight-member rings of 0.38 × 0.38 nm opening into large ellipsoidal cavities of 0.67 × 1.0 nm (Figure 4). The adsorption properties of chabazite are, to a large extent, attributed to the interactions between guest gas molecules and the extra-framework cations. From previous studies,20-22 four general cation positions in dehydrated chabazite are known to exist: one at the center of the D6R prism (SI), one at the triad axis of the D6R prism but displaced to the supercavity (SII), one in the supercavity at the four-ring window of the (SIII), and one at the sites in the eight-ring window of the channel
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Figure 3. Effect of cation type on the loading of N2 at (a) 77 K and (b) 273 K for KCHA1 (red 9), NaCHA (green 2), and CaCHA (blue b).
Figure 4. Schematic representation of chabazite structure and cation positions.19
(SIII′), as shown in Figure 4. Different types of cations have different preference to sites. Na+ has three favorable sites following the sequence in terms of preference, SIII′ > SII > SI; K+ resides primarily at SIII′ and SIII,23,24 and Ca2+ cations preferentially coordinate SI sites and SII sites.20,25 The size of cations adopted in this study follows the sequence: K+ (0.133 nm) > Ca2+ (0.099 nm) > Na+ (0.097 nm). K+ cation with its large size (r ) 0.133 nm) may render the 8-ring-window inaccessible to N2 due to steric hindrance, combined with its decreasing mobility at low temperature and causes pore blockage. The unit cell (Figure 4) of KCHA1 consists of 11 K+ cations, 5 of which occupy SIII sites and 6 SIII′ sites, which function as guards to the supercavity. In addition, the low temperature may render the oxygen ring of the supercavity less flexibility, which may prevent the N2 molecules diffusing through the window. Breck26 observed that a variation in vibration amplitude of 0.01-0.02 nm in the temperature range of 80-300 K is reasonable in the case of zeolite A structure, and an expansion or dilation of 0.03 nm in the aperture diameter could result from thermal vibration effect alone. Saig et al.27 later tried to verify the structural dilation of zeolite A using molecular dynamics and demonstrated that lattice vibration can yield large fluctuation of zeolite apertures, thus enhancing permeability and observed deviations as high as 7% in the case of sodalite at 503 K. Therefore, we believe that higher temperature facilitates the diffusion of N2 molecules through the pore window due to elevated cation mobility and structure dilation. On the other hand, no pore blockage for N2 was detected at 77 K for either NaCHA or CaCHA. This can be attributed to
Figure 5. Effect of K+ cation density on the loading of N2 at 77K. Red 9 denotes KCHA1 (Si/Al ) 2.2); purple 2 denotes KCHA3 (Si/ Al ) 50.4); and blue b denotes KCHA2 (Si/Al ) 17).
the size of the cations. In the case of NaCHA, in each unit cell 1 K+ cation is supposed to occupy 1 SIII′ site and 10 Na+ cations are believed to occupy SIII′, SI, SII, and SIII sites. Because the Na+ cation is smaller in size (0.097 nm) as compared to K+ cation (0.133 nm), the difference is large enough, and hence Na+ fails to prevent N2 molecules diffusing through the 8-ringwindows into the supercavity. In other words, steric hindrance is absent or greatly reduced in the case of NaCHA. Similarly, in the case of CaCHA, the size of Ca2+ (0.099 nm) is also smaller than K+, making access of N2 to the supercage possible. Furthermore, the cation density for divalent Ca2+ cations is onehalf that of K+ cations, further reducing the blockage effect. 3.3. Effect of Cation Density. N2 isotherms of KCHA samples with different Si/Al ratios at 77 K are shown in Figure 5. It is noteworthy that both KCHA2 (Si/Al ) 17) and KCHA3 (Si/Al ) 50.4) show “normal” high surface area, while KCHA1 (Si/Al ) 2.2) shows exceptionally low surface area although these three materials have the same structure and extraframework cations. The negligible micropore volume of KCHA1 as compared to KCHA2 and KCHA3 endorses the huge difference in surface area. The only difference is the K+ cation density arising from the difference in Si/Al ratios. Hence, one may conclude that there exists a threshold for the density of K+ (6 K+ per unit cell corresponding to Si/Al ) 5), above which
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Figure 6. Experimental and model data of isobars of (a) N2 and (b) CH4 on KCHA1 at different temperatures.
every 8-ring-window accommodates at least one K+ so that all the windows are effectively impermeable to N2 molecules. However, below this threshold, some of the 8-ring windows become accessible to N2. Therefore, the cations could virtually enhance the adsorption amount due to increased adsorption sites rather than lower the capacity. In the case of KCHA1, the effective density of blocking K+ cations (11 K+ per unit cell) is believed to exceed the threshold resulting in extremely low adsorption capacity of N2, which can be attributed to adsorption by intercrystal mesopores and external surface sites.28 On the other hand, in the case of KCHA2 and KCHA3, the density of cations (2 K+ and 0.7 K+ per unit cell, respectively) is clearly below the threshold, so the higher surface areas were observed as compared to that for low Si/Al ratio potassium chabazite. Adsorption capacity for KCHA2 was found to be higher as compared to KCHA3, which may be attributed to more accessible adsorption sites in the former material created by higher density of K+ per unit cell. 3.4. Temperature Dependence of N2 Adsorption Characteristic. To study the role of temperature on the pore blockage effect and to find the critical temperature beyond which pore blockage is absent, we measured a series of isotherms for N2 on KCHA1 in the temperature range from 195 to 343 K. Figure 6a shows three isobars as a function of temperature. Each isobar is arch shaped, and in the temperature range from 266 to 343 K, at a given pressure, the amount of N2 adsorbed decreases gradually with increase in temperature as expected. However, there is a considerable decline in N2 loading at constant pressure in the temperature range 266-195 K. This is contradictory to conventional physisorption theory. We propose that the unusual shape of the isobars reflects an activated adsorption mechanism wherein the potassium cations residing at the plane of 8-oxygenring apertures of the supercavity act as sentinels forming a barrier for the diffusion of guest N2 molecules into the cavity. Provided that the process is conducted at a sufficiently high temperature, the gas molecules acquire sufficient thermal energy, the blocking cations possess enhanced vibration energy, as well as the framework of chabazite acquires more vibration potential, all of which give rise to successful permeation. The right-hand side of the isobars (beyond the temperature for maximum loading) therefore represents true equilibrium data, while that on the left-hand side is nonequilibrium, encapsulation data. Do29 pointed out that diffusion of N2, O2, and Ar into carbon molecular sieve (CMS) follows an activated diffusion process,
in which case only the varying thermal energy held by gas molecules plays a major role as cations are absent in CMS. Hence, the activated adsorption process in the case of chabazites is more complex than that in CMS, and the extent of the activated process is a function of temperature. To clarify the temperature dependence of N2 adsorption capacity of KCHA1, a 3D isotherm graph was constructed to show the trend of N2 loading with the change of temperature and pressure. As seen from the N2 isotherms in Figure 7a, we believe that there exists a critical temperature TC dividing the temperature profile into two regions, and only above TC are all the adsorption sites inside the supercavities accessible. By observing a series of N2 isotherms at different temperatures, we can assign TC at around 266 K. It is noteworthy that even below TC the N2 loading on KCHA1 is nonzero although it is relatively low (e.g., 0.02 mmol/g at 195 K), which is believed to represent adsorption on the external surface sites of chabazite28 or a small minority of unblocked cavities resulting from lattice defects. Previous encapsulation work shows that there are two classes of sites in type A zeolite, that is, R-cage and β-cage, which can be accessible to gas molecules at different temperatures.5,27,30,31 In the case of the potassium chabazite/N2 system, although the kinetic diameter of N2 molecule and the effective aperture diameter of D6R unit are comparable (0.364 nm versus 0.26 nm), the limited void volume of D6R unit probably prevents N2 diffusion.32 Therefore, we believe that the effective sites of KCHA1 for N2 are on the external surface and in the supercavities with the former accessible in the entire temperature range, while the latter is only accessible above a certain temperature (TC). The Toth model33 was employed to fit the right-hand side of the isobars and extrapolate to the left-hand side region (shown in Figure 6a), which demonstrates the potential capacity for N2 storage. The Toth model can be written as
q)
qsP
(
1 + pt b
)
1/
(1) t
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Figure 7. 3D isotherms of (a) N2 and (b) CH4 on KCHA1.
TABLE 2: Toth Model Parameters for N2 and CH4 gas N2 CH4
qs (mmol/g) 5.23 2.49
b0 (kPa-1) -5
1.81 × 10 2.57 × 10-6
b ) b0 exp(
Q ) RT
t
Q (J/mol)
0.67 0.88
16 169 22 601
(2)
where q is the adsorbed amount (mmol/g), qs is the monolayer saturated adsorption capacity (mmol/g) when P f ∞ and it is assumed to be independent of temperature, P is the equilibrium pressure (kPa), t is a constant characterizing heterogeneity of the adsorbent, b (kPa-1) is an affinity constant characterizing adsorptive potential, b0 is the pre-exponential factor of b, Q is the heat of adsorption (J/mol), R is the ideal gas constant (J/ (K · mol)), and T is the temperature (K). A fitting procedure was conducted to determine the parameters, which are given in Table 2. An excellent fit was obtained with R2 > 0.999 in both cases. 3.5. Nitrogen Encapsulation. To confirm the temperaturedependent nature of the encapsulation and adsorption and to justify the possibility of employing KCHA1 as nanocontainers for N2 encapsulation, temperature controlled desorption experiments were conducted. As seen in Figure 8, desorption starts at 77 K, proceeding at a steady rate until the temperature reaches 163 K, suggesting desorption of gases from the external surface sites, followed by the thermal expansion of released gas from
Figure 8. Temperature programmed desorption for N2 on KCHA1.
Figure 9. Storage capacity of N2 on KCHA1 at 40 bar.
163 to 266 K. After that, the rate of pressure change with temperature curve becomes steep until 343 K, indicating desorption of gas from the sites in supercavities. Above 343 K the pressure change rate slows, which may be due to isosteric thermal expansion of all the gas released from both classes of sites. The desorption amount from supercavities is found to be much higher than that from external surface sites, indicating that the sites in supercavities play a major role in the adsorption of N2. On the basis of the experiment above, one can conclude that KCHA1 works as a nanocontainer for N2 storage by temperature regulated “nano-valves”: when KCHA1 is exposed to N2 at a relatively high pressure and above the critical temperature TC (266 K), permeation occurs because the pore window or “valve” is open. When the system is then quenched to below TC with the high pressure maintained, the “valve” closes, trapping the nitrogen molecules. If the surrounding pressure is then released to ambient pressure and the temperature maintained below TC, the gas cannot escape. To measure the potential storage capacity and proper working pressure for storage, N2 adsorption at high pressure was measured, and the resulting isotherm at 278 K is shown in Figure 9. The result shows that at pressure up to 4 MPa, the N2 capacity can be as high as 3.36 mmol/g (9.4 wt %), and it seems that higher capacity could be obtained at elevated pressure. By fitting the isotherm data using the Toth model33 at 313, 323, 333, and 343 K, the maximum capacity of
Potassium Chabazite KCHA1 for N2 was calculated to be 5.23 mmol/g, which is in agreement with the above high pressure experimental data. Results from the encapsulation-decapsulation experiment on the Parr reactor (see Figure 1) give a storage capacity as high as 3.02 mmol/g at a working temperature of 323 K and a working pressure of 5 MPa. As compared to the reported encapsulation work,14,26,34-36 our work is of great practical significance as it can be carried out under mild conditions (i.e., subambient temperature and lower pressure) with high capacities. 3.6. Methane Encapsulation. Considering CH4, having a size similar to N2 (0.38 vs 0.364 nm), isobars were measured to determine the possibility of CH4 storage by KCHA1. As seen in Figure 6b, within the temperature range from 279 to 353 K, at a given pressure the amount adsorbed decreases gradually with increase in temperature, whereas below 279 K the loading increases with temperature. Similar to N2, the right-hand side of the isobars of CH4 was fitted by the Toth model, and the parameters are given in Table 2. The three-dimensional isotherm of CH4 (Figure 7b) has shape similar to that of N2, indicating that pore-blockage phenomenon also happens to CH4. As compared to the situation in N2, we find that the critical temperature TC for CH4 on KCHA1 is around 279 K, which is higher than that of N2 (266 K). In addition, the higher critical temperature for CH4 (279 K) as compared to N2 (266 K) is consistent with their kinetic diameter, that is, 0.38 and 0.364 nm, respectively. A high pressure isotherm measured at 279 K shows that CH4 loading can reach 1.97 mmol/g at 1 MPa. CH4 storage may therefore be possible by selecting the proper temperature and pressure windows. 4. Conclusions Potassium chabazite with Si/Al ratio 2.2 has been studied in detail for N2 and CH4 storage by the encapsulation mechanism. Two classes of adsorption sites for gas accommodation have been shown, the external surface sites and the sites in the supercavities, and the latter works as the primary storage reservoir, which is controlled by a temperature-dependent pore blocking feature. In the case of N2, the theoretical working condition was to dose N2 above 266 K and at 5 MPa and store it at below 266 K and under ambient pressure. A N2 storage capacity of 3.02 mmol/g was successfully achieved. In the case of CH4, a higher critical temperature (279 K) was required to dose CH4 into the chabazite micropores due to the larger size of CH4 molecules as compared to N2. Hence, we can conclude that chabazite materials can be tailored to encapsulate gases with different molecular sizes by carefully tuning the cation type and density in the chabazite unit cells. A peak in the isobar is an indication that the material could be potentially employed for gas storage if proper working conditions are applied. Further work on the encapsulation capability of different gases on a variety of cation exchanged chabazites is currently being undertaken. Acknowledgment. We thank the Australian Research Council (ARC) for supporting this work. Supporting Information Available: X-ray diffraction patterns, schematic representation of custom built volumetric
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