Article pubs.acs.org/est

Chemical Vapor Deposition on Chabazite (CHA) Zeolite Membranes for Effective Post-Combustion CO2 Capture Eunjoo Kim,† Taehee Lee,† Hyungmin Kim,† Won-Jin Jung,‡ Doug-Young Han,§ Hionsuck Baik,§ Nakwon Choi,∥ and Jungkyu Choi*,†,⊥ †

Department of Chemical & Biological Engineering, College of Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea ‡ Center for Materials Analysis, Research Institute of Advanced Materials, Seoul National University, Gwanangno, Gwanak-gu, Seoul 151-744, Republic of Korea § Korea Basic Science Institute, Seoul Center, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea ∥ Center for BioMicrosystems, Brain Science Institute, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea ⊥ Green School, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea S Supporting Information *

ABSTRACT: Chabazite (CHA) zeolites with a pore size of 0.37 × 0.42 nm2 are expected to separate CO2 (0.33 nm) from larger N2 (0.364 nm) in postcombustion flue gases by recognizing their minute size differences. Furthermore, the hydrophobic siliceous constituent in CHA membranes can allow for maintaining the CO2/N2 separation performance in the presence of H2O in contrast with the CO2 affinity-based membranes. In an attempt to increase the molecular sieving ability, the pore mouth size of all silica CHA (Si-CHA) particles was reduced via the chemical vapor deposition (CVD) of a silica precursor (tetraethyl orthosilicate). Accordingly, an increase of the CVD treatment duration decreased the penetration rate of CO2 into the CVD-treated Si-CHA particles. Furthermore, the CVD process was applied to siliceous CHA membranes in order to improve their CO2/N2 separation performance. Compared to the intact CHA membranes, the CO2/N2 maximum separation factor (max SF) for CVD-treated CHA membranes was increased by ∼2 fold under dry conditions. More desirably, the CO2/N2 max SF was increased by ∼3 fold under wet conditions at ∼50 °C, a representative temperature of the flue gas stream. In fact, the presence of H2O in the feed disfavored the permeation of N2 more than that of CO2 through CVD-modified CHA membranes and thus, contributed to the increased CO2/ N2 separation factor. DDR,12,23 and SSZ-13 (CHA type)24 8-MR membranes have exhibited good CO2 separation performance. Along with other 8-MR zeolites, chabazite (CHA) zeolites, which have a pore size of 0.37 × 0.42 nm2, are promising materials for the separation of CO2 from N2 in flue gas streams from coal-fired power plants.25 The permeation selectivity of CO2 over N2 in CHA zeolite membranes is estimated to be as high as ∼20−30, indicating that CHA zeolites can serve as membrane materials for continuous CO2 separations.15,24,26 However, molecular simulations predict the facile diffusion of N2 over CO2 in CHA zeolites with a CO2/N2 diffusion coefficient ratio (i.e., CO2/N2 diffusion selectivity) being ∼0.5.26

1. INTRODUCTION As an alternative to conventional amine-based absorption processes for CO2 separations, membrane-based CO2 separations have been studied with the overarching goal toward the realization of energy-efficient CO2 separations at the large scale.1−3 In particular, the use of ionic liquid, polymer, and their composites has been widely investigated to develop energyefficient CO2 capture processes via the manufacturing of high performance membranes.1−9 Along with other type membranes, porous zeolite membranes are also promising to separate CO 2 from larger molecules such as N 2 in postcombustion or CH4 in natural gas upgrading through differences in size and adsorption.10−13 In particular, eightmembered ring (8-MR) zeolite membranes are desirable because of the similar pore size to the molecular sizes of CO2, N2, and CH4.14,15 Specifically, the maximum size of 8-MR zeolites is ∼0.43 nm, while the kinetic diameters of CO2, N2, and CH4 are 0.33, 0.364, and 0.38 nm, respectively.16 Indeed, SAPO-34 (CHA type), 17−20 Zeolite-T (ERI/OFF), 21,22 © XXXX American Chemical Society

Received: September 5, 2014 Revised: November 14, 2014 Accepted: November 17, 2014

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Hydrophilic zeolite membranes, including FAU10,27,28 and SAPO-3419,29 membranes, are effective in separating CO2/N2 under dry conditions because of their preferred adsorption of CO2 over N2. However, the high CO2/N2 separation performance is often deteriorated in the presence of H2O,30 especially near the temperatures of the flue gases (∼50−75 °C) from coal-fired power plants. This is attributed to the stronger adsorption of H2O over CO2 in hydrophilic zeolites.31−33 Therefore, hydrophobic zeolite membranes with the appropriate pore size (here, 8-MR pore structure) are desirable for separating CO2 from N2 in the presence of H2O34 by alleviating the adsorption of H2O in the temperature range ∼50−75 °C. Because the size of the H2O molecule (0.265 nm) is close to the size of helium (0.26 nm),16 the smallest molecule, reduction in the adsorption capacity of H2O via controlling the hydrophilicity is a sound approach for minimizing the flux of H2O across membranes, and thus, for maintaining a high CO2/ N2 separation factor at the expense of decreased CO 2 permeation rate. The diffusion coefficient of permanent gases is a strong function of the pore dimension of 8-MR zeolites.14,35 For example, DDR membranes with a pore size of 0.36 × 0.44 nm2 showed a more effective cutoff ability in disfavoring the permeation of N2 (0.364 nm) as compared to SAPO-34 membranes with a pore size of 0.38 nm.23,36 Thus, the pore size of zeolites is often reduced by chemical vapor deposition (CVD) and undesired nonzeolitic (defective) parts were concomitantly decreased, synergistically improving the separation performance of the membranes.37,38 Accordingly, because the pore size of CHA zeolites (0.37 × 0.42 nm2) is slightly larger than the size of N2, modification of the pore mouth of CHA membranes will disfavor the permeation of N 2 considerably more than that of CO2, and thus, increase the CO2/N2 separation performance. Despite the promise of pore size reduction, to the best of our knowledge, an attempt to understand growth on CHA crystals by CVD and demonstrate its viability for CO2 separation through membranes has not yet been carried out. CVD on CHA crystals can be considered as analogous to that of seeded growth for zeolite crystal synthesis,39 which is often critical for directing the growth of a continuous membrane via the epitaxial growth of a seed layer.40,41 Similarly, the fundamental understanding of the effect of CVD process on the properties of CHA particles will be helpful to characterize and eventually control the structure of CHA membranes as desired. For the silica precursor, tetraethyl orthosilicate (TEOS, ∼0.96 nm), which is relatively inexpensive and is a good candidate to reduce the pore mouth size on the outer surface of CHA zeolites, although other silica precursors such as methyldiethoxysilane (MDES, 0.4 × 0.91 nm2) can be used to tune the pore mouth and/or channel size inside CHA zeolites.42,43 In this study, we first measured the adsorption isotherms of CO2 and N2 in all silica CHA (Si-CHA) zeolites at different temperatures. CO2/N2 ideal selectivities evaluated with simulated compositions of CO2 (e.g., ∼13 kPa) and N2 (e.g., ∼77 kPa) in the postcombustion process were estimated to be as high as ∼15−16 at 303 K, supporting the potential of SiCHA zeolites for membrane-based CO2/N2 separations. In addition, we attempted to reduce the effective pore mouth size on the outer surface of Si-CHA zeolites using CVD via the thermal decomposition of TEOS. The amount deposited on the Si-CHA zeolites increased with time and eventually reached a plateau, indicating its saturation capacity. The transient

uptake of CO2 in intact CHA and CVD-treated CHA zeolites indicated that the effective pore mouth of CHA could be reduced by CVD. However, following the CVD of TEOS, whose size is larger than the pore channel in Si-CHA zeolites, ∼10% of the adsorption sites were inaccessible to CO2 adsorbates. Finally, CVD was carried out on siliceous CHA membranes, manufactured by the secondary growth of CHA seed layers. The corresponding CO2/N2 maximum separation factor of CVD-treated CHA membranes was increased by ∼2 fold under dry conditions (50 kPa CO2/ 50 kPa N2) and more desirably, increased by ∼3 fold under wet conditions (48.5 kPa CO2/ 48.5 kPa N2/ 3 kPa H2O), as compared to the intact CHA membranes. Seemingly, H2O, adsorbed on siliceous CHA membranes, disfavored the transport of larger N2 over that of smaller CO2. The CVD-based post-treatment was a sound approach toward reducing both the effective pore mouth size and the undesired defect on CHA membranes, as reflected by the marked improvement in the CO2/N2 separation performance of the CVD-treated membranes.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Si-CHA Zeolites. Si-CHA zeolites were synthesized using a reported method. 44 The detailed experimental procedure for preparing the synthetic precursor is described elsewhere.45,46 In short, TEOS (98%, SigmaAldrich), N,N,N-trimethyl-1-adamantanammonium hydroxide (TMAdaOH, 25 wt % aqueous solution, SACHEM Inc.), and hydrofluoric acid (HF, 48 wt % aqueous solution, SigmaAldrich) in deionized water were mixed to prepare the synthetic precursor at a final molar composition of 10 SiO2/5 TMAdaOH/5 HF/30 H2O. The prepared solution was transferred to a Teflon liner in an autoclave. The autoclave was placed in a rotating rack in an oven and the reaction was carried out at 160 °C (following the original synthesis protocol44) and 190 °C. After ∼42 h at 160 °C and ∼12 h at 190 °C, the reactions were quenched with tap water. The assynthesized particles were recovered by vacuum filtration and were calcined at ∼600 °C for ∼12 h at a ramp rate of 1 °C· min−1 under 200 mL·min−1 of air flow. For convenience, the particles obtained at 160 and 190 °C are referred to as CHA-L and CHA-H, respectively, where L and H represent low and high synthesis temperatures. Two different synthesis temperatures were employed to see the effect of the synthetic condition on the structural and adsorption properties of the resulting CHA particles. 2.2. CVD of TEOS on CHA Zeolite Crystals. The surface of calcined Si-CHA particles was treated by the CVD of TEOS. Approximately 0.2 g of dried CHA particles were placed in a quartz boat, with both ends open. Then, the boat was placed in a quartz tube (outer diameter of 50 mm and wall thickness of 2 mm) and transferred to a tubular furnace. The sample was preheated to ∼550 °C for 1 h with a ramp rate of 5 °C·min−1 under 150 mL·min−1 of Ar flow, and cooled to ∼500 °C prior to feeding the TEOS vapor for the CVD reaction at the total pressure of 1 atm. Considering the vapor pressure of TEOS at room temperature (∼1.5 Torr),47,48 the net flow rate of TEOS under Ar carrier gas flow was ∼0.3 mL·min−1. After a predetermined time (6, 12, 24, 48, and 96 h), TEOS feeding was stopped and the sample was cooled to room temperature under a flow of Ar. A schematic for the CVD process (Thermal CVD system, Scien Tech, South Korea) is illustrated in Scheme S1 in the Supporting Information (SI). The weight change before and after CVD treatments was recorded in order to B

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Figure 1. (a) SEM image of CHA-L particles (synthesized at 155 °C for 42 h) and (b) SEM and (c) TEM images of CVD-treated CHA-L particles (CHA-L-CVD-96). Some thin flakes in CHA-L particles in (a) are indicated by red arrows. A grown part due to the CVD process is marked in blue ellipse in (b) and corresponding humps in the magnified TEM image are pointed out by blue arrows in (c). In addition, as indicated by the red arrows in (b), it appears that the flakes on the surface of the near cubic Si-CHA zeolite shown in (a) were further grown after the CVD process.

average sizes ± the corresponding standard deviations were estimated to be 4.6 ± 2.5 μm for CHA-L and 5.3 ± 2.6 μm for CHA-H (SI Figure S2). Despite the possible inaccuracy in estimating the particle sizes based on the SEM images, the wide size distributions of CHA-L and -H particles were evident. The XRD patterns (SI Figure S1) confirmed that both CHA-L and -H were primarily composed of CHA zeolites, while little amorphous materials were present (indicated by the black arrows in SI Figure S1). 3.1.1. Adsorption Properties of CHA Crystals. SI Figure S3 shows the adsorption isotherms of CO2 and N2 in CHA-L and -H particles along with the fitted curves. The adsorption isotherms that obey the Langmuir-type adsorption and Henry’s law were used to fit the experimental adsorption isotherms of CO2 and N2, respectively. In general, the corresponding isotherm models appropriately described the adsorption behaviors of CO2 and N2 in both the CHA-L and -H particles. The fitting process resulted in the optimal estimates of the Langmuir adsorption parameters and the saturation capacity for CO2 and Henry’s constants for N2. Detailed information is summarized in SI Table S1. The heats of adsorption were calculated by correlating the Langmuir adsorption constants (for CO2) and Henrys’ law constants (for N2) with the van’t Hoff equation. The resulting heats of CO2 adsorption in CHAL and -H were estimated to be 25 ± 1.9 and 24 ± 6.2 kJ·mol−1, respectively, while those of N2 adsorption in CHA-L and -H were estimated to be and 12 ± 7.1 and 14 ± 5.0 kJ·mol−1, respectively. These heats of adsorption of CO2 in CHA-L and -H were in a good agreement with the experimental and simulated heats of adsorption (22.5 and 23.0−23.6 kJ·mol−1, respectively) reported in the literature.52 In SI Figure S4, the adsorption isotherms of CO2 in Si-CHA52 and SAPO-34 particles53 reported in the literature are plotted along with those of CO2 in CHA-L shown in SI Figure S3a1, showing a good agreement between the adsorption isotherm of CO2 in SiCHA found in the literature52 and in CHA-L in this study. Considering the molar composition of flue gases in postcombustion processes (e.g., ∼13 kPa/ ∼ 77 kPa CO2/ N2),54,55 the CO2/N2 ideal sorption selectivities, expected with CHA-L zeolites, were approximately 16, 12, and 8.5 at 303, 323, and 348 K, respectively, which were comparable to the CHA-H counterparts (15, 12, and 9.0) (see SI eqs S1−S3 for the definition of sorption selectivity in the SI). Considering that the theoretically predicted diffusivities of CO2 and N2 in CHA zeolites were ∼2 × 10−9 and ∼3−4 × 10 −9 m2 ·s−1 , respectively,26,56 the CO2/N2 ideal selectivities, which can be

estimate the increased amount of SiO2 relative to the CHA sample. The results of the CVD of CHA-L and CHA-H are referred to as CHA-L-CVD-x and CHA-H-CVD-x, respectively, where x indicates the duration of CVD in hour. 2.3. CVD on CHA Membranes. CVD treatment, employed for CHA powder, was also conducted on CHA zeolite membranes using the process shown in SI Scheme S1. Prior to CVD treatment, CHA zeolite membranes were manufactured on α-Al2O3 discs following a secondary growth protocol reported previously.45 In brief, a CHA seed layer was obtained on an α-Al2O3 disc by sonicating a glass reactor that contained the CHA suspension (∼0.05 g in ∼40 mL of dry toluene) and a cover-glass-sandwiched α-Al2O3 disc in a custom-made Teflon comb-shaped holder. Subsequently, the seeded α-Al2O3 disc was calcined at 450 °C for 4 h with a ramp rate of 1 °C·min−1 and was hydrothermally treated inside a stainless steel autoclave at 160 °C for 3 d. For the secondary growth of the CHA seed layer, a molar composition of 100 SiO2/20 TMAdaOH/20 NaOH/5 Al(OH)3/4400 H2O was used in reference to the synthesis of SSZ-13.24 The as-synthesized CHA membranes were calcined at 550 °C with a ramp rate of 0.5 °C·min−1 under an air flow of 200 mL·min−1. The CVD treatment of TEOS on the calcined CHA membranes was carried out at 200 °C and 1 atm with varying durations (∼18−72 h) after preheating at 500 °C for 1 h under the Ar flow of 200 mL·min−1. Considering the vapor pressure of TEOS at room temperature (∼1.5 Torr),47,48 the net flow rate of TEOS under Ar carrier gas flow was ∼0.4 mL·min−1. The CVD-treated CHA membranes were further calcined at 550 °C using a ramp rate of 0.5 °C·min−1 under an air flow of 200 mL·min−1. For convenience, CVD-treated CHA membranes are referred to as membrane CHA-L-CVD-x, where L indicates the low temperature of 160 °C analogous to CHA-L particles, and x represents the CVD time in hour.

3. RESULTS AND DISCUSSION 3.1. CHA Crystals. Figures 1a and S1 in the Supporting Information (SI) shows the SEM images of CHA-L and CHAH particles synthesized at 160 and 190 °C, respectively and the corresponding XRD patterns. The SEM images reveal that near cubic particles, typical in Si-CHA zeolites,35,49−51 were observed in CHA-L and -H particles. Both samples had a wide size distribution of ∼1−10 μm and some plate-like flakes, indicated by red arrows in Figure 1a, were also observed for CHA-L particles as similar to our previous report.45 In addition, CHA-L and -H particle sizes were estimated by measuring the side length of the near cubic particles in the SEM images; the C

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Figure 2. (a) Increased weight percentage of CHA-L and -H particles vs the CVD duration (hour) and (b) adsorbed CO2 amount relative to the SiCHA zeolite weight (CO2 (g)/Si-CHA zeolite (g) and CO2 (mol)/Si-CHA zeolite (kg)) inside CHA-L and CHA-L-CVD-x (x = 6, 12, 24, 48, and 96) particles at a CO2 partial pressure of ∼47 kPa vs uptake time (min). Inset: CO2 adsorption isotherm of CHA-L at 323 K (black) shown in SI Figure S3a1 along with the CO2 saturation capacity of CHA-L estimated from the plateau region in TGA in (b).

was overwhelmed by a majority of the near cubic CHA-L particles. Also, 29Si MAS NMR spectra (SI Figure S5b) were unable to designate the presence of SiO2, as the intensity of the NMR peak corresponding to Q3 relative to that of Q4 was almost identical among CHA-L and CHA-L-CVD-x (x = 48 and 96) particles. 3.2.1. CO2 Uptake of CVD-Treated CHA Crystals. The reduction in pore mouth size is beneficial for improving the CO2/N2 separation factor via the enhanced molecular sieving of Si-CHA zeolite membranes, primarily by inactivating the diffusion of N2 into Si-CHA zeolites, at the expense of the decreased permeation rate of CO2. In this study, TGA was adopted to investigate the diffusional rates of CO2 into intact and CVD-treated Si-CHA particles. Figure 2b shows the transient weight changes of CHA-L and CHA-L-CVD-x (x = 6, 12, 24, 48, and 96) at a CO2 partial pressure of ∼47 kPa. The TGA experiment supports the difference in the diffusional rates of CO2, as reflected by the more rapid increase in weight in the intact CHA-L than in the CVD-treated CHA-L. This result also indicates the successful reduction in pore mouth size via the deposition of TEOS, which was not supposed to penetrate the Si-CHA zeolite pores (∼0.4 nm) because of its larger molecular size (∼0.96 nm). However, the TGA result also revealed that a fraction of the adsorption sites in CHA-L-CVD particles were not accessible by CO2. For example, the saturation capacity of CHA-L-CVD-48 was reduced by ∼10% compared to that of CHA-L. To verify this, CO2 adsorption isotherms in CHA-LCVD-48 were further measured at 303, 323, and 348 K (SI Figure S6). The fitted parameters for CHA-L-CVD-48 particles are summarized in SI Table S1, along with those for intact CHA particles. The fitted saturated CO2 capacities of intact CHA and CHA-L-CVD-48 particles were comparable to each other, while the adsorption constants were a little smaller for CHA-L-CVD-48 particles seemingly because of the aforementioned reduction of accessible adsorption sites. Considering the fact that the heats of CO2 adsorption were comparable with ∼23−25 kJ·mol−1, it is clear that the CVD treatment did not affect the intrinsic interaction between CO2 and the CHA framework. 3.3. CVD-Treated CHA Membranes. With the profound understanding of the effect of CVD on the CO2 adsorption properties in Si-CHA particles, CVD treatment was extended to CHA membranes in order to enhance their CO2/N2 separation performance. First, siliceous CHA membranes were manufactured on α-Al2O3 discs via conducting the secondary growth of Si-CHA seed layers.45 SEM and XRD (Figure 3a,c and SI

regarded as maximum values expected from the CHA membranes, were estimated to be ∼8, ∼6, and ∼4−5 at 303, 323, and 348 K, respectively, based on the simplest permeation model (SI eq S2). This approximation indicates the high potential of Si-CHA membranes for efficient CO 2 /N 2 separation in postcombustion processes. 3.2. CVD on CHA Crystals. Considering that the diffusivity of guest molecules in a confined environment is highly sensitive to the pore size of channels in zeolites,14,35 we attempted to reduce the pore mouth size by the CVD of TEOS, whose size is ∼0.96 nm, and thus, limit the access of N2 in the Si-CHA pore structure to increase the CO2/N2 selectivity. Figure 2a shows the weight change of the Si-CHA zeolites due to the CVD of TEOS vs the deposition time at the nominal temperature of 500 °C. The weight change reached an asymptotic value after a sufficiently long deposition time, indicating the saturation by CVD. Though simple, we adopted a linear driving force scenario in an attempt to account for the weight increase as a result of CVD. Figure 2a also shows the fitted curves for CHAL (black) and CHA-H (red). The asymptotic weight increases for CHA-L and CHA-H were estimated to be 23 ± 4% and 16 ± 3%, respectively while the apparent driving force constant was close to 0.03 ± 0.01 h−1. The values next to the optimized values are 95% confidence intervals. Because the saturated weight increase can be linearly associated with the total external surface area, we took the square root of the inverse ratio of the saturated weight increase of CHA-L to that of CHA-H and obtained a value of ∼0.82. This value can be considered the ratio of the particle size of CHA-L to that of CHA-H and was in a good agreement with the ratio value of ∼0.87 obtained from the estimated particle sizes of CHA-L and -H (SI Figure S2). The measured weight increase in Figure 2a indirectly confirmed the presence of SiO2 as a result of the thermal decomposition of TEOS during CVD. We further attempted to confirm the presence of SiO2 by other methods. Figure 1b,c shows the SEM and TEM images of CHA-L-CVD-96. Figure 1b showed that the CVD-treated CHA-L now had the rough surface (marked in blue ellipse) on the near cubic Si-CHA particles, as compared to the smooth surface of the intact CHAL shown in Figure 1a. The rough surface was magnified in the TEM image (Figure 1c), revealing protuberant silica grains apparently due to the CVD growth on the outer surface of CHA-L. In addition, some parts, indicated by red arrows in Figure 1b, were likely grown from the flakes observed in Figure 1a. However, XRD results (SI Figure S5a) did not reveal the presence of SiO2 grown by CVD seemingly because its intensity D

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Figure 3. SEM top-view images of membranes (a) CHA-L and (b) CHA-L-CVD-36 along with (c) the corresponding XRD patterns. The simulated XRD pattern of CHA zeolite is also included for reference in (c) and the asterisk (*) represents the XRD pattern from the α-Al2O3 disc.

Figure 4. CO2/N2 separation performance of membranes (a1)−(a2) CHA-L, (b1)−(b2) CHA-L-CVD-18, (c1)−(c2) CHA-L-CVD-36, and (d1)− (d2) CHA-L-CVD-72 under dry (left) and wet (right) conditions, respectively as a function of temperature. Error bars represent the standard deviations.

Figure S7a,e) revealed that a continuous, but randomly oriented, CHA membrane (here, denoted as membrane CHA-L) was successfully synthesized by secondary growth. The SEM images in Figure 3b and SI Figure S7b−d indicated that CVD treatments up to 72 h at 200 °C did not provide any noticeable change in the surface morphology as compared to membrane CHA-L (Figure 3a). In addition, XRD (Figure 3c

and SI Figure S7e) confirmed that any silica generated by the CVD process was negligible, as observed for the CVD treatment of CHA particles (SI Figure S5a). Figure 3 and SI Figure S7 support that CVD treatments did not result in any pronounced change in the membrane considering the limitations of SEM and XRD characterizations, which is in a good agreement with the very negligible weight increase after E

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0.6 comparable to the aforementioned expected value of ∼6 under the dry condition. Considering the CO2/N2 SF of ∼2.5 ± 0.3 at 50 °C through the intact CHA-L membrane, the CVD treatment could provide ∼3 fold improvement for the CO2/N2 SF. Considering the modest improvement (from 1.7 to 3.4 CO2/N2 SF) of CVD-treated CHA membranes under dry conditions, the contribution of defects, apparently grain boundary or polycrystalline defects, to the permeation of CO2 and N2 was significant. On the contrary, under wet conditions, H2O, adsorbed on the CHA grains and seemingly more on the defects, would disfavor the permeation rate of slightly bulkier and inert N2 over that of smaller and hydrophilic CO2, thus increasing the CO2/N2 SF. However, further increase of the CVD duration to 72 h rather decreased the CO2/N2 max SF down to ∼2.7 ± 0.4 at ∼100 °C, especially due to the increased N2 permeance. This increase can be attributed to the weak adsorption H2O on the more hydrophobic outer surface and thus, the less inhibition of H2O on the penetration of N2 into the pore structure of CHA zeolites. Although additional, careful characterizations are required, at this moment we speculate that the effect of the adsorbed H2O on the permeation of CO2 and N2 through membrane CHA-L-CVD-72 (Figure 4d2) was decreased due to the more hydrophobic nature (SI Figure S9), as compared to membrane CHA-L-CVD-36 (Figure 4c2). Although the effective pore mouth was further decreased and/or the defects were more repaired by the longer CVD (from 36 to 72 h), the decrease in the adsorption capacity of H2O on CHA grains and defects was more pronounced, thus rendering the permeation rate of N2 increased back and resulting in the deterioration of the CO2/N2 SF. As the high manufacturing cost of supported zeolite membranes has been addressed as an inhibitor for the scaleup,57,58 we would like to further improve the current CO2/N2 SF of 7.5 at 50 °C up to ∼20 and subsequently, increase the CO2 permeance up to ∼3000−5000 GPU (∼1−1.6 × 10−6 mol·m−2·s−1·Pa−1) via the use of high permeable supports (e.g., yttria stabilized zirconia (YSZ) capillary tubes).59 This will contribute to overcoming the high manufacturing cost of supported zeolite membranes, as indicated by the CO2 capture cost trend with respect to the CO2/N2 separation performance.60 3.3.3. Comparison of CO2/N2 Separation Performance. We further compared the CO2/N2 separation performance of membrane CHA-L-CVD-36 (CO2/N2 max SF of ∼8.8) with those of polymeric membranes61 by plotting the CO2/N2 SF vs CO2 permeability (Figure 5). For comparison, the CO2/N2 separation performances of NaY30 and SAPO-3419 membranes under the dry and wet conditions are also included in Figure 5. Detailed information for the separation performance of zeolite and zeolite-like membranes is summarized in SI Table S2. Under dry conditions, the NaY membrane showed a high CO2/ N2 separation performance; however, it was below Robeson’s upper bound.61 Although other FAU membranes exhibited performance superior to Robeson’s upper bound,10,27,28 the NaY membrane reported by Gu et al.30 was employed for comparison in this study, because its separation performance under wet conditions was also reported. Under dry conditions, the CO2/N2 SF was as high as ∼21−32 with a modest CO2 permeability. However, under wet conditions, both CO2 permeability (or permeance) and CO2/N2 SF dramatically decreased at ∼25 and ∼50 °C, apparently due to the hydrophilicity of the NaY zeolites. Nevertheless, at a higher

CVD treatments. In addition, SI Figure S8 reveals that the thickness of membrane CHA-L was ∼2 μm, which was comparable to the CHA film thickness reported previously.45 3.3.1. CO2/N2 Separation Performance under Dry Conditions. Intact CHA membranes and CVD-treated CHA membranes (i.e., membranes CHA-L-CVD-x, x = 18, 36, and 72) were further tested to see if they could provide molecular sieving ability as means to distinguish CO2 from N2. Despite the apparent continuity observed via SEM (Figure 3a), membrane CHA-L exhibited a maximum (max) CO2/N2 separation factor (SF) of ∼1.7 ± 0.4 at 303 K (Figure 4a1), which was much less than the aforementioned expected value of ∼8 at the same temperature. This strongly suggested the presence of undesired nonzeolitic (i.e., defective) portions on the molecular level. As the CVD duration time was increased from 18 h through 36 to 72 h, the max SFs monotonically increased from ∼2.6 ± 1.0 through ∼2.8 ± 0.2 to ∼3.4 ± 0.8 (Figure 4b1−d1). Notably, CVD treatments led to a modest CO2/N2 max SF of ∼3.4, which was ∼2 times larger than its counterpart (∼1.7) obtained with membrane CHA-L. In fact, taking into account the Knudsen diffusion through bare αAl2O3 discs, the apparent CO2/N2 SF needed to be corrected; specifically, it needed to be divided by the CO2/N2 Knudsen SF (∼0.8) for the intrinsic CO2/N2 SFs due to the CHA membrane. Accordingly, the CO2/N2 max SF only through membrane CHA-L-CVD-72 was likely to be as high as ∼4.3, though this value was still less than the expected value of ∼8. In a previous report, SSZ-13 (CHA type) membranes, prepared by multiple in situ hydrothermal growths (∼4 repetitions), showed ∼11 CO2/N2 max SF at 298 K.24 The lower CO2/N2 SF in this study suggests that the CO2/N2 separation performance of CHA membranes, if appropriately manufactured by secondary growth and post-treated by CVD, could be further improved. 3.3.2. CO 2 /N 2 Separation Performance under Wet Conditions. In addition to the dry conditions, Figure 4a2−d2 shows the CO2/N2 separation performance of membranes CHA-L and CHA-L-CVD-x (x = 18, 36, and 72) under the wet conditions for which saturated water vapor (∼3 kPa) at room temperature was cofed with 50% CO2/ 50% N2. For membranes CHA-L and CHA-L-CVD-18, there was no remarkable change in the CO2/N2 permeation results. However, as the CVD duration was increased to 36 h, unique features were observed, especially at temperatures below 100 °C. Specifically, compared to the dry conditions, the permeances of CO2 and N2 decreased seemingly because of the inhibition of physisorbed water on membrane CHA-LCVD-36. It appeared that the degree of the transport resistance in the presence of water was higher for bulkier N2 than for CO2, thereby increasing the corresponding CO2/N2 max SF at 30 °C from ∼2.8 for membrane CHA-L-CVD-36 under the dry conditions to ∼8.8 under the wet condition. In addition, a monotonic decrease in CO2/N2 SF was observed with temperature mainly due to the increased N2 permeance that was likely associated with the reduced degree of adsorption and thus, less inhibition of H2O. Though Al could be incorporated into the CHA membrane during secondary growth, the siliceous CHA portion (Si to Al ratio of ∼20 from the molar composition in the synthesis solution and ∼7−10 from Energy Dispersive X-ray analysis in SI Figure S8) was the key to reduce the hydrophilicity and accordingly, the adsorbed amount of water. It was noted that the CO2/N2 SF for membrane CHA-LCVD-36 at ∼50 °C, which lies in the temperature range (∼50− 75 °C) of flue gases in postcombustion processes, was ∼7.5 ± F

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the pore mouth could be reduced by the CVD of TEOS; however, a fraction of adsorption sites (∼10%) was not accessible by CO2. Despite the reduced saturation capacity of CO2, the heat of CO2 adsorption in the CVD-treated Si-CHA zeolites was comparable to that in the intact Si-CHA zeolites, indicating the similar interaction between adsorbed CO2 molecules and CHA frameworks. As compared to the conventional approaches based on preferred CO2 adsorption, siliceous CHA membranes with appropriate pore sizes are desirable for effective CO2/N2 separations in the presence of H2O. Although intact CHA membranes did not show a high CO2/N2 separation performance, CVD treatment could improve the CO2/N2 SF by ∼2 fold under the dry conditions and by ∼3 fold under the wet conditions, as compared to the intact CHA membranes. More desirably, in the presence of H2O, the CO2/N2 maximum SF was increased from ∼2.5 for the intact CHA membranes to ∼7.5 for a CVD-treated CHA membrane at ∼50 °C (a flue gas temperature), mainly because the permeance of bulkier N2 was more decreased by the inhibition of the adsorbed H2O. This improvement can be ascribed to the reduction of pore mouth sizes and concomitantly, defects after CVD processes. Currently, the development of an improved synthetic method for intact CHA membranes with high CO2/N2 separation performances is underway in order to apply CVD to those membranes and thus, obtain higher CO2/N2 separation performance.

Figure 5. CO2/N2 SF vs CO2 permeability of membrane CHA-LCVD-36 under both dry (filled) and wet (half-filled) conditions at 30, 50, and 100 °C shown in Figure 4c1−c2 along with those for polymeric membranes.61 For comparison, the CO2/N2 separation performance of NaY30 and SAPO-3419 membranes is included and Robeson’s upper bound is denoted by a blue line. Detailed information is summarized in SI Table S2. As a guide to the eye, the performance change due to the water vapor in the feed is designated by arrows; green and red colors are used to represent the corresponding temperature range.

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temperature (∼100 °C) where the adsorption of H2O is considerably weak, the presence of H2O still led to an inevitable reduction in CO2 permeability with an increased CO2/N2 SF in NaY membranes30 and with an almost constant CO2/N2 SF in SAPO-34 membranes.19 As a guide to the eye, the separation performance according to the environmental change from dry to wet conditions is indicated by the arrows in Figure 5. On the contrary, it was shown that the CO2/N2 SFs for membrane CHA-L-CVD-36 were rather increased under the wet conditions at ∼30 and ∼50 °C at the expense of the modest decrease in CO2 permeability. Thus, the CVD strategy to siliceous CHA membranes can be regarded as a sound approach toward high CO2/N2 separation performance in the presence of H2O, in contrast to conventional approaches19,30 where materials with high CO2 adsorption capacities are mainly used. In fact, a small amount of H2O, even ∼0.1 kPa, would reduce the CO2 adsorption amount almost by 1 order of magnitude at ∼50 °C in NaX (FAU) or Zeolite 5A (LTA).32,33 As compared with the separation performance at 30 and 50 °C, the CO2/N2 SFs at ∼100 °C under the wet condition were comparable to those under the dry condition, presumably because of the less-pronounced adsorption of H2O on CHA zeolites. In summary, the heats of adsorption for CO2 and N2 were estimated to be ∼24−25 and ∼12−14 kJ·mol−1, respectively, confirming the preferred interaction of the Si-CHA zeolite framework with CO2 over N2. The estimated ideal selectivity through Si-CHA membranes for CO2/N2 mixtures was as high as ∼8 at 30 °C, supporting the appropriate choice of Si-CHA zeolites as membrane materials. After conducting the CVD process of TEOS, silica was grown on the outer surface of SiCHA crystals as identified by SEM and TEM. The saturated amount of silica on CHA-L and -H particles indicated that the size of CHA-L relative to that of CHA-H was ∼0.82, which was in good agreement to the value (∼0.87) approximated from SEM images. The CO2 uptake measured by TGA indicated that

ASSOCIATED CONTENT

S Supporting Information *

Detailed characterization methods, schematics for CVD process and permeation systems, the summary for CO2 and N2 adsorptions, and permeation results (Figure 5) are provided, along with the description of sorption, diffusion, and ideal selectivities. In addition, the particle size distribution of CHA particles and the CO2 adsorption isotherms, XRD patterns, and 29 Si NMR spectra of CHA and CVD-treated CHA particles, along with the cross-sectional SEM images of intact and CVDtreated CHA membranes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-2-3290-4854; fax: +82-2-926-6102; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Korea CCS R&D Center (KCRC) (2014M1A8A1049309) and by Basic Science Research Program (2012R1A1A1042450) through National Research Foundation (NRF) of Korea. These two grants were funded by the Korea government (Ministry of Science, ICT & Future Planning). This work was also supported by the Human Resources Development Program (No. 20134010200600) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry, and Energy. SEM and XRD characterizations were conducted at the Korea University Engineering Laboratory Center, and TEM and 29Si NMR characterizations were carried out at Korea Basic Science G

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Institute (KBSI). Personally, J.C. acknowledges the courtesy of Prof. Jinhan Cho with the use of a TGA unit for the CO2 transient measurement.

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