Ultradeep Removal of Moisture in Gases to Parts-per-Billion Levels

Jan 27, 2018 - Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engin...
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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Ultradeep Removal of Moisture in Gases to Parts-per-Billion Levels: The Exploration of Adsorbents Lin Zhang,†,# Zheng-Zhong Kang,‡,§,# Shi-Chao Qi,† Xiao-Qin Liu,*,† Zhi-Min Wang,† and Lin-Bing Sun*,† †

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China ‡ Department of Chemistry, Zhejiang University, Zheda Road 38, Hangzhou 310027, China § Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden S Supporting Information *

ABSTRACT: Owing to the rigid standards of manufacturing semiconductor components, decrease of the moisture to parts-per-billion (ppb), at a global level, is always vital but extremely nerve-racking in the production of high-purity gases. Herein, typical adsorbents including oxides (SiO2 and γ-Al2O3), zeolites (4A and NaX), and metal−organic frameworks (MOFs; HKUST-1, UiO-66, and ZIF-8) are investigated with respect to the abilities of ultradeep dewatering from N2. Compared with other adsorbents, NaX performs much better on both dewatering efficiency (DE, from 2750 to 66 ppb) and the adsorption capacity (AC, 1.55 × 104 L N2·g−1). Moreover, it is for the first time experimentally and theoretically proved that the dewatering ability of X zeolite mainly depends on its cation species (Na+, Li+, K+, Cs+, Mg2+, Ca2+, Sr2+, or Ba2+), the forces between the zeolite and H2O, and the number of H2O molecules per cell of the zeolite. CaX thus shows a fascinating DE from 2750 to 33 ppb, a huge AC of 9.08 × 104 L N2·g−1, and an ideal reusability, compared with results of the scarce contributions reported to date.

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INTRODUCTION High-purity gases such as nitrogen, helium, argon, and ammonia always act as important raw materials in the production of chip films, photovoltaic cells, and optical fibers. High-purity gases are also widely used in the semiconductor industry for the manufacture of microcircuit devices, and the purity of the gases directly affects the quality of the products.1−3 It is crucial that the gases are essentially free of impurities. For instance, the residual moisture in semiconductor fabrication ought to be decreased to an extremely low level of 50 ppb to ensure that the quality and performance of the semiconductor chips are not degraded. Therefore, the purity of the gases is often required to reach 99.999% (referred as 5N), 99.9999% (6N), or even 99.99999% (7N).4,5 Compared with other contaminants in the gases, moisture is ubiquitous but special in properties. Because of intermolecular hydrogen bonds, the moisture is extremely difficult to remove. To date, many purification approaches to remove the moisture in gases have been tried, such as adsorption,6−8 rectification,9 and membrane separation.10 However, only a very few reports, as far as we know, claimed ultradeep dewatering to parts-per-billion (ppb) level.11 Adsorption is widely used because of its simple operation and low energy consumption with some solid adsorbents, such as activated carbon, SiO2,12 Al2O3,13−15 zeolites,16 and polymers.17,18 For © XXXX American Chemical Society

instance, 3A zeolite has been used to reduce the moisture content in the ammonia flow to ca. 100 ppb.7 Metal−organic frameworks (MOFs) used for moisture harvesting have been proposed recently, but MOFs applied in ultradeep dewatering to ppb level have never been studied.19−21 It is believed that MOFs are extremely sensitive to the moisture because of their open-metal sites.22,23 Yaghi’s group reported that the MOF-801 can harvest 0.25 L·kg−1 of water with 0.6 kPa vapor pressure (at 20% relative humidity and 25 °C).24 Of course, efforts searching for adsorbents for ultradeep dewatering have been made. For example, a Zr−V−Fe alloy was invented by SAES Co. Ltd. and showed an excellent ability to reduce the moisture content to 20 ppb.25 Unfortunately, both the adsorption capacity (AC) and reusability were fatal defects of the alloy. In the present study, the ultradeep dewatering performance of various adsorbents are comprehensively studied. Three kinds of representative adsorbents, i.e., oxides (SiO2 and γ-Al2O3), zeolites (4A and NaX), and MOFs (HKUST-1, UiO-66, and ZIF-8), are tested. Among these adsorbents, NaX has impressive dewatering efficiency (DE, from 2750 to 66 ppb) and AC (1.55 × 104 L·g−1). Furthermore, ion exchange of NaX Received: November 24, 2017 Revised: January 16, 2018

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DOI: 10.1021/acs.jpcc.7b11566 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C zeolite was carried out with a series of cations, i.e., Li+, K+, Cs+, Mg2+, Ca2+, Sr2+, and Ba2+, to investigate the effects of cation species on the ultradeep dewatering performance of X zeolite,26 because there was evidence that the cations in the zeolites provided preferential adsorption sites for polar molecules like H2O.27 Simulations on the basis of molecular dynamics (MD) were performed to gain insight into the behaviors of H2O molecules in the zeolite cells. According to the experimental and computational results, it is found for the first time that the performance of zeolites on ultradeep dewatering largely depends on three intrinsic properties, which can be just satisfied by CaX. Therefore, CaX shows a fascinating DE from 2750 to 33 ppb, a good AC of 9.08 × 104 L·g−1, and an ideal reusability.

mL). The mixture was stirred at ambient temperature. Subsequently, the mixture was heated at 140 °C for 24 h. The obtained solids were filtered and then washed by DMF and methanol successively and exchanged with dichloromethane. Characterization. X-ray diffraction (XRD) patterns of the materials were recorded with a Bruker D8 Advance diffractometer with Cu Kα radiation at 40 kV and 40 mA. The N2 adsorption−desorption isotherms were measured by ASAP 2020 at −196 °C. The Brunauer−Emmett−Teller (BET) surface area was calculated with the relative pressure ranging from 0.04 to 0.20. The total pore volume was derived from the amount adsorbed at a relative pressure of 0.99. Fourier transform infrared (IR) spectra of the samples were recorded on a Nicolet Nexus 470 spectrometer. The chemical compositions of samples were obtained by X-ray fluorescence (XRF) spectrometer (ARL ADVANTXP, Thermo). The thermogravimetric analysis (TGA) was performed using a thermobalance (STA-499C, Netzsch). Ultradeep Dewatering Tests. The ultradeep dewatering device was set up as shown in Figure S1. Micromoisture meter (HALO-LP, Tiger Optics LLC) was used to quantify the moisture content in the gas. Nitrogen served as the test gas. The ultradeep dewatering abilities of the materials were evaluated through their adsorption of moisture in 5N nitrogen. 6N nitrogen was employed as activated and purge gas. Before formal experiments, the samples set in the adsorbed bed activated at an appropriate temperature by steam S2 to eliminate the influence of impurities as much as possible, and then 6N nitrogen (through steam S1) was employed to sweep the pipelines. Simultaneously, the concentration of water was monitored at the outlet until it was stable, which means that the impurities have been eliminated. In formal experiments, the feed gas (5N) was imported into the absorbent column through stream S3 with flow velocity of 1200 mL·min−1 and pressure of 0.3 MPa at ambient temperature. Meanwhile, the moisture in the 5N nitrogen at the outlet of the adsorbents bed was monitored. Three targets indicating the ultradeep dewatering abilities of the materials were investigated, i.e., breakthrough time of adsorption, DE from 5N nitrogen, and AC. To meet the criterion in the electronics industry, 100 ppb was set as the penetration point. When the moisture content of the gas was below 100 ppb, the moment was defined as the breakthrough time. AC was the volume of nitrogen treated with per gram of adsorbent, calculated through the breakthrough time and the flow velocity of gas. Simulations of MD. MD simulations were performed to evaluate the interactions between H2O molecules and zeolites.31,32 With periodic boundary conditions, NVT ensemble, and Charmm36 all-atoms force field, all simulations were performed with GROMACS 5.0.4 and VMD packages.33−36 The zeolite was simulated in FAU type, and simple point charge water molecules were used. Dynamics temperature was maintained at 298 K by applying the Nose−Hoover thermostat coupling to the system. For nonbonded van der Waals interactions, the cutoff switching function started at 1.2 nm and ended at 1.35 nm. Particle mesh Ewald summation was employed for the calculation of long-range electrostatic interactions with a cutoff distance of 1.2 nm to separate the direct and reciprocal spaces.37 Linear constraint solver algorithm was applied to constrain the bond lengths.38 Dynamics simulations were carried out with a time step of 2 fs, and the data were saved every 4 ps.

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EXPERIMENTAL SECTION Chemicals. Lithium nitrate (LiNO3, purity ≥99.7%) and calcium chloride (CaCl2, ≥ 99.7%) were bought from Guangdong Guanghua Chemical Co., Ltd. Potassium chloride (KCl, ≥99.7%), cesium nitrate (CsNO3, ≥99.7%), magnesium nitrate [Mg(NO3)2, ≥99.7%], strontium nitrate [Sr(NO3)2, ≥99.7%], barium nitrate [Ba(NO3)2, ≥99.7%], and zinc nitrate hexahydrate [Zn(NO3)2·3H2O, ≥99.5%] were supplied by Xilong Chemical Co., Ltd. NaX (the Si/Al ratio is 2.63), 1,3,5benzenetricarboxylate (H3BTC, ≥98%), terephthalic acid (H2BDC, ≥99%), copper nitrate hydrate [Cu(NO3)2·3H2O, ≥ 99.5%], zirconium dichloride (ZrCl4, ≥99.9%), and dimethyl imidazole (C4H6N2, ≥98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. N,N-Dimethylformamide (DMF, ≥99.5%), ethanol (≥99.7%), dichloromethane (≥99.5%), and absolute methanol were obtained from Wuxi City Yasheng Chemical Co., Ltd., China. Allochroic SiO2 and γ-Al2O3 were provided by Lingfeng Chemical Reagent Co., Ltd. 5N and 6N nitrogen were supplied by Nanjing RuiEr Special Gas Co., Ltd. Water used in the experiments was deionized. Preparation of Adsorbents. Spherical particles of NaX zeolite, 4A zeolite, and SiO2 were crushed and then screened into 20−40 mesh particles. The obtained powders were dried at 180 °C. γ-Al2O3 was dried and then pressed into slices at 6 MPa. These slices were granulated to 20−40 mesh particles. The precalcined NaX (4.0 g) was ion-exchanged by 80 mL of chloride solutions (1.0 M) of Li+, K+, or Ca2+ and nitrate solutions of Cs+, Mg2+, Sr2+, or Ba2+ for 24 h under ambient temperature. The ion-exchanged zeolites were calcined at 400 °C for 4 h in air atmosphere and then cooled to ambient temperature under N2 protection. The ion-exchange was repeated three times. The obtained materials are donated as LiX, KX, CsX, MgX, CaX, SrX, and BaX. The MOFs, i.e., HKUST-1, UiO-66, and ZIF-8, were synthesized according to previous reports.28−30 HKUST-1 was prepared as follows: Cu(NO3)2·3H2O (3.6 mmol) and H3BTC (2 mmol) were dissolved into the solvent consisting of ethanol (12 mL), deionized water (12 mL), and DMF (12 mL), followed by heating at 120 °C for 12 h. The obtained solids were filtered, washed with DMF and methanol, and exchanged with dichloromethane in sequence. UiO-66 was synthesized by dissolving ZrCl4 (0.2332 g) and H2BDC (0.161 g) into DMF (50 mL). The mixture was stirred at ambient temperature to homogenize the synthetic system, followed by heating at 120 °C for 48 h. The obtained solids were filtered and washed by DMF repeatedly and then exchanged with dichloromethane. ZIF-8 was synthesized as follows: Zn(NO3)2·6H2O (0.445 g) and 2-methylimidazole (0.245 g) were dissolved into DMF (40 B

DOI: 10.1021/acs.jpcc.7b11566 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Table 1. Characteristics and Ultradeep Dewatering Performances of 4A Zeolite, Nax Zeolite, Γ-Al2O3, SiO2, HKUST-1, UiO-66, and ZIF-8 type

sample

SBET (m2·g−1)

Vp (cm3·g−1)

DE (ppb)a

AC (×103 L N2·g−1)b

AC (H2O wt %)b

oxide

SiO2 γ-Al2O3 4A NaX HKUST-1 UiO-66 ZIF-8

752 179 − 580 1136 883 1295

0.43 0.66 − 0.25 0.53 0.39 0.55

172 44 51 66 306 274 2443

− 8.77 8.67 15.50 − − −

− 0.05 0.05 0.10 − − −

zeolite MOF

a

DE, dewatering efficiency. bAC, adsorption capacity.

Figure 1. Breakthrough curves of ultradeep dewatering for (a) SiO2, γ-Al2O3, 4A zeolite, and NaX zeolite and (b) HKUST-1, UiO-66, and ZIF-8. The arrow in panel a shows the fluctuation caused by the replacement of the gas cylinder.

Table 2. Characteristics and Ultradeep Dewatering Performances of LiX, NaX, KX, CsX, MgX, CaX, SrX, and BaX Zeolites

a

sample

degrees of exchange (%)

SBET (m2·g−1)

Vp (cm3·g−1)

DE (ppb)a

AC (×103 L N2·g−1)b

AC (H2O wt %)b

LiX NaX KX CsX MgX CaX SrX BaX

95 − 87 70 72 90 97 93

579 580 550 389 470 588 531 325

0.34 0.35 0.32 0.26 0.29 0.36 0.31 0.23

52 66 46 63 81 33 40 100

12.15 15.50 22.74 11.25 4.59 90.81 19.66 −

0.08 0.10 0.14 0.07 0.03 0.57 0.12 −

DE, dewatering efficiency. bAC, adsorption capacity.

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respectively.42,43 In Figure S3a, the XRD pattern indicates that HKUST-1 is a highly ordered 3D octahedral material.44 In the case of UiO-66 (Figure S3b), the cubic 3D structure can be noted as well.29 The XRD pattern of ZIF-8 (Figure S3c) shows the presence of sodalite topology.45 The experimental XRD results are in good agreement with the simulated ones, thus giving evidence of the successful synthesis of MOFs. Figures S4 and S5 give N2 adsorption−desorption isotherms of different materials, and the corresponding textual parameters are listed in Table 1. As shown in Table 1, SiO2 is an adsorbent of high surface area (752 m2·g−1), while γ-Al2O3 has the lowest surface area (179 m2·g−1) among all the materials. The surface area and pore volume of 4A zeolite is not provided because of its small AC of nitrogen at −196 °C. The size of the nitrogen molecules is too large to go into the micropores of 4A zeolite. NaX zeolite exhibits a surface area of 580 m2·g−1. In Table 1, it can be clearly seen that the surface areas of HKUST-1, UiO-66, and ZIF-8 are much greater than those of other adsorbents. Figure 1 gives the breakthrough curves of samples. The DE of SiO2 is 172 ppb, which indicates that it is not suitable for ultradeep dewatering. It can be seen that the DE of γ-Al2O3 is

RESULTS AND DISCUSSION Preliminary Selection of Adsorbents. The application background of the adsorbents decides the research categories we selected. Both oxides and zeolites have been generally employed in the purification of industrial gases. SiO2 is a most common oxide desiccant, while γ-Al2O3 is widely used as well because its Lewis acidic sites are prone to chemisorb polar molecules. The zeolites with low Si/Al ratios, represented by 4A and NaX, have great affinities toward moisture. In addition, the relatively stable MOFs, represented by HKUST-1, UiO-66, and ZIF-8, are promising candidates of effective adsorbents toward moisture.39,40 Figure S2 displays the XRD patterns of oxides and zeolites. In Figure S2, the XRD pattern of SiO2 shows a single broad diffraction peak centered at 23°, which can be attributed to amorphous silica walls. As for γ-Al2O3, the XRD diffraction peaks located at 38°, 46°, and 67° are well consistent with the JCPDS card.41 The XRD pattern of γ-Al2O3 somewhat broadens, which reveals the crystal of γ-Al2O3 might be unordered. The XRD pattern of 4A zeolite and NaX zeolite show the structure of sodalite topology and faujasite structure, C

DOI: 10.1021/acs.jpcc.7b11566 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 44 ppb and its AC is 8.77 × 103 L N2·g−1 (Table 1). As for the zeolites, the DE of 4A zeolite (51 ppb) and that of NaX zeolite (66 ppb) are similar to each other. It is interesting that the AC of NaX zeolite (1.55 × 104 L N2·g−1) is almost twice as much as that of 4A zeolite (8.67 × 103 L N2·g−1). As shown in Figure 1b, the DE of HKUST-1 (306 ppb) and that of UiO-66 (274 ppb) are close to each other, and both of them are much better than that of ZIF-8 (2443 ppb). It is obvious that the three MOFs perform badly, compared with other adsorbents. In general, NaX zeolite is the most promising candidate for ultradeep dewatering in view of its DE and AC. Therefore, we paid more attention to the NaX zeolite herein. In consideration of the fluidity of cations in NaX zeolite, a series of ionexchanged NaX zeolites were considered to study the influences of different cations in X zeolites on the ultradeep dewatering performance.46 Structural Characterizations of Ion-Exchanged X Zeolites. As shown in Table 2, the ion exchange degrees of alkali cations nearly accomplished 90%, except Cs+. The sizes of the alkali cations considered in this study increase in the sequence of Li+ (0.068 nm) < Na+ (0.095 nm) < K+ (0.133 nm) < Cs+ (0.169 nm). It is obvious that Cs+ has the largest cation size, and it is consequently more difficult to achieve high ion exchange degree than other cations because of the diffusion problem. Compared with that of alkali metal cations, only half of the amount of alkaline earth metal cations are needed to complete ion exchange because of their divalent charge. This enables the high degree of ion exchange (>90%), except Mg2+.47 Once the ion exchange degree of Mg2+ was up to 50%, further ion exchange would be difficult because the original structure of MgX was destroyed by the new formation of Mg− O bonds.48 Figure 2 shows the XRD patterns of the ion-exchanged X zeolites. In terms of peak intensities, the diffraction patterns of

KX, and SrX maintain the original structure of the X zeolite, while the structures of CsX, MgX, and BaX were degraded to some extent. Figure 3 presents N2 adsorption−desorption isotherms of the ion-exchanged zeolites. The isotherm shapes of ion-exchanged

Figure 3. N2 adsorption−desorption isotherms of LiX, NaX, KX, CsX, MgX, CaX, SrX, and BaX zeolites.

X zeolites are similar to that of NaX. The isotherms of these zeolites are of type I-B, which is consistent with the literature.43 For all samples, the uptake of N2 rises rapidly at low relative pressures, while the isotherms become smooth at relative pressures greater than 0.1. Near the saturated pressure, the adsorption occurs because of the intergranular gaps of zeolites. This is evidence of the presence of microporosities in zeolites. The differences among the zeolites are the ACs at relative pressure below 0.1, which suggest their different surface areas. Some structural parameters calculated from the isotherms are listed in Table 2. The samples LiX and CaX have surface areas of 579 and 588 m2·g−1, respectively, which are similar to that of NaX zeolite (580 m2·g−1). In addition, the surface areas of KX and SrX are 550 and 530 m2·g−1, respectively. Owing to the damage of structures, the surface areas of CsX, MgX, and BaX decrease a lot. For example, the surface area of CsX drops to 389 m2·g−1. This implies that the introduction of different metal cations results in the changes of both surface area and pore volume. These results are well in agreement with that of the XRD patterns. This means that LiX, CaX, KX, and SrX have quite complete structures of X zeolites, while the structures of CsX, MgX, and BaX are partly damaged. Effect of Cations on the Ultradeep Dewatering Performance of X Zeolites. The obtained ion-exchanged zeolites were further applied to ultradeep dewatering in the 5N nitrogen containing 2750 ppb moisture. Figure 4a presents the breakthrough curves of ultradeep dewatering of the zeolites exchanged by alkali metal cations. It is obvious that KX has better DE (46 ppb) and longer breakthrough time (94.73 h) than LiX and CsX. The breakthrough curves of ultradeep dewatering for ion-exchanged zeolites with alkali earth metal cations are shown in Figure 4b. Note that CaX has better DE (33 ppb) and much longer breakthrough time (378.38 h) than the others. CaX performs best among the zeolites. The AC of CaX reaches 9.08 × 104 L N2·g−1, which is much higher than that of the Zr−V−Fe alloy reported previously (1.84 × 103 L N2·g−1).25 These results apparently suggest that the CaX is highly promising for ultradeep dewatering to produce highpurity gases. Reusability of an adsorbent is of great importance in view of practical application. CaX is thus evaluated through six-time recycle of ultradeep dewatering. The zeolites after water

Figure 2. XRD patterns of LiX, NaX, KX, CsX, MgX, CaX, SrX, and BaX zeolites.

LiX and CaX change a little compared with that of NaX zeolite. This means that the Li- and Ca-exchanged zeolites do not change the crystallographic structure of the original zeolite.49 Nevertheless, slight decreases of relative peak intensities of KX and SrX are observed. This means that no major lattice distortions were caused by the ion exchange. Slight variations in the relative intensities of different Bragg reflections could arise because of the different charges and radii of cations. However, differences of the diffraction pattern of CsX, MgX, and BaX can be observed from 10° to 20°.50,51 The absence of a number of XRD reflections indicates that the zeolite structure collapsed partly, similar to the previous report.52 In summary, LiX, CaX, D

DOI: 10.1021/acs.jpcc.7b11566 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Breakthrough curves of ultradeep dewatering for (a) LiX, KX, and CsX and (b) MgX, CaX, SrX, and BaX. The arrows show the fluctuations caused by the replacement of the gas cylinder.

adsorption were calcined at 400 °C for 2 h in N2 atmosphere and then cooled to ambient temperature under N2 protection for regeneration. The results show no discoverable loss of DE (Figure 5a) and AC (Figure 5b) of CaX even after six recycles.

In addition to NaX, a series of X zeolites ion-exchanged with alkali and alkaline earth metal ions were prepared in this report. On the basis of the aforementioned results, it is clear that X zeolites with different cations exhibit quite different ultradeep dewatering performances, and CaX performs much better than others. For insight into the different performances of the zeolites, MD simulations were applied. Three factors that probably affect the performance of the X zeolites were considered, i.e., the forces between H2O molecules and zeolites, number of H2O molecules diffusing in zeolites, and the openframework structures of zeolites. Generally speaking, the stronger forces between adsorbates and adsorbents mean the better performances of adsorbents. The snapshot of the distribution of H2O molecules and the extra-framework cations within CaX are shown in Figure 6.

Figure 5. (a) Dewatering efficiency and (b) adsorption capacity of CaX after six cycles of the regeneration experiment.

TGA curves after dewatering tests were recorded (Figure S6). The results indicate that the moisture adsorbed in the process of ultradeep dewatering can be completely removed as temperature rises from room temperature to ∼400 °C. Ultradeep Dewatering Mechanisms of Adsorbents. Ultradeep dewatering is a key and costly step in the production of high-purity gases. Searching for efficient adsorbents on the basis of insight into the intrinsic mechanisms is an important and practicable approach to reduce the cost of production. According to previous studies, SiO2 adsorbs moisture because of a large number of hydroxyl groups presented on the surface inside the nanoporous structures. The forces between water molecules and hydroxyl groups are so weak that the DE of SiO2 (172 ppb) is bad. As for γ-Al2O3, the DE is up to 44 ppb probably because of the strong forces among Al3+ cations and H2O molecules. However, the AC is limited by its small surface area. Because amounts of organic ligands occupy the internal space of MOFs, widespread nonpolar regions are formed inside the MOFs. The forces between H2O molecules and the MOFs are thus not so strong that the DEs of HKUST-1, UiO-66, and ZIF-8 are unsatisfactory. 4A and NaX zeolites performed very well to capture the moisture because of their great affinities toward water, because the metal cations within zeolites result in a strong electronic field.53 However, the gas-processing capacity of NaX zeolite is almost twice higher than that of 4A zeolite because of the larger pore volume and the special openframework structure of NaX zeolite.

Figure 6. Snapshot of H2O molecule distribution and extra-framework cations within CaX, as inferred from MD simulations.

That of other X zeolites is given in Figure S7. The simulation results of the cation distribution in different X zeolites are consistent with that of the experiments in terms of ionexchange degrees. As seen in Figures 7a and S8, the average binding energy consists of binding energy between the water molecules and the framework of the zeolite and that between the H2O molecules and the metal cations. It can be seen that the binding energies between the H2O molecules and the zeolites ion-exchanged with the alkali earth metal ions are higher than that ion-exchanged with alkali metal ions except CsX. The binding energy of CsX is higher than that of MgX and CaX. The number of H2O molecules per cell of the zeolites was also examined. In Figure 7b, the number of H2O molecules in zeolites with alkaline earth metal cations is more than that in E

DOI: 10.1021/acs.jpcc.7b11566 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 7. (a) Average binding energy between H2O molecules and different zeolites and (b) average number of H2O molecules per zeolite cell calculated from MD simulations.

zeolites. CaX can be regarded as a promising adsorbent usable in ultradeep removal of moisture in gases to ppb levels. In addition, it has great advantages in terms of preparation and economic feasibility. In this study, nitrogen was chosen as the test gas, but it is promising to employ CaX zeolite for the ultradeep purification of other gases.

zeolites with alkali metal cations, because the larger number of cations in the latter group excludes more H2O molecules from the zeolite cavities. Cations with large radii occupy bigger spaces, which causes fewer H2O molecules to be adsorbed. Compared with that of other zeolites, CaX cells capture more H2O molecules. In addition, the density of H2O molecules in CaX is obviously greater than that in other zeolites. This implies that the zeolites with alkaline earth metal cations, in particular CaX, have better performance on ultradeep dewatering than that with alkali metal cations. The aforementioned results have demonstrated that the unbroken open-framework structures and the high surface areas of zeolites are the prerequisites for the good performances on ultradeep dewatering. LiX, NaX, KX, CaX, and SrX performed better than CsX, MgX, and BaX. Although MgX and BaX, according to MD simulations, have strong adsorption forces and large AC toward H2O molecules per cell of zeolites, they performed badly in practice, because the frameworks of these two zeolites during ion-exchange have been destroyed, which has been proven by the XRD patterns. In contrast, the good performance of CaX should be largely attributed to the unbroken open-framework structure. On the basis of the aforementioned results and discussion, it is safe to say that the ultradeep dewatering performance of zeolites is attributed to the forces between H2O molecules and zeolites, the AC of zeolites toward H2O, and the openframework structure of zeolites. That CaX performed best on ultradeep dewatering because it won out in the abovementioned three ways.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11566. Device for testing the ability of ultradeep dewatering; XRD patterns and N2 adsorption−desorption isotherms of adsorbents; snapshots of MD simulations; TGA plots of adsorbents (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shi-Chao Qi: 0000-0002-9609-7710 Lin-Bing Sun: 0000-0002-6395-312X Author Contributions #

L.Z. and Z.-Z.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support of this work by the National Natural Science Foundation of China (21676138, 21722606, and 21576137) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions.

CONCLUSIONS Three categories of popular and representative adsorbents, i.e., oxides (SiO2 and γ-Al2O3), zeolites (4A and NaX), and some MOFs (HKUST-1, UiO-66 and ZIF-8), were employed to study their abilities of ultradeep dewatering to ppb level. The experimental results indicate that the performance of NaX is much better than that of other adsorbents. We further investigated the effects of cations exchanged X zeolite on the ultradeep dewatering performances, i.e., with alkali metal and alkaline earth metal cations. It is found that CaX has the most outstanding performance among the cations exchanged X zeolites. The DE from 5N nitrogen and the AC of the CaX even reached 33 ppb and 9.08 × 104 L·g−1, respectively. This is because CaX zeolite, according to both the experimental results and MD simulations, meets three intrinsic factors that decide the ultradeep dewatering abilities of zeolites, i.e., the adsorbate−adsorbents interactions, the volume of per zeolite cell for H2O capture, and the open-framework structure of

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