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Micropore Formation of [Zn(Oxac)(Taz)]•(H2O) via CO Adsorption Moondra Zubir, Atom Hamasaki, Taku Iiyama, Akira Ohta, Hiroshi Ohki, and Sumio Ozeki Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03456 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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Micropore Formation of [Zn2(Oxac)(Taz)2]•(H2O)2.5 via CO2 Adsorption Moondra Zubir,1,2 Atom Hamasaki,1 Taku Iiyama,1 Akira Ohta,1 Hiroshi Ohki,1 and Sumio Ozeki1,∗

1

Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto,

Nagano 390-8621, Japan. 2

Department of Chemistry, Faculty of Mathematics and Natural Science, State University of

Medan, North Sumatera, Indonesia.

ABSTRACT As-synthesized [Zn2(Oxac)(Taz)2]•(H2O)2.5, referred to ZOTW2.5, was prepared from aqueous methanol solutions of Zn5(CO3)2(OH)6 and two kinds of ligands of 1,2,4-triazole (Taz) and oxalic acid (Oxac) at 453 K for 12 hours. The crystal structure was determined by the Rietveld method. The as-synthesized ZOTW2.5 was pretreated at 383 K and 1 mPa for tpt h, ZOTWx(tpth). ZOTWx(≥3h) showed Type I adsorption isotherm for N2 at 77 K having the saturation amount (Vs) of 180 mg/g, but that pretreated shortly showed only 1/10 in Vs. CO2 was adsorbed at 303 K in sigmoid on non-porous ZOTWx(≤2h) and in Langmuir-type on

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ZOTWx(≥3h) to reach the adsorption amount of 120 mg/g at 700 Torr. N2 adsorption on ZOTWx(≤2h)deCO2, degassed after CO2 adsorption on ZOTWx(≤2h), was promoted by 5 times from 180 mg/g on ZOTWx(tpth) and ZOTWx(≥3h)deCO2 up to ca. 1000 mg/g. The interaction of CO2 and H2O molecules in micropores may lead to a new route for micropore formation.

INTRODUCTION Porous materials are widely used as adsorbents, catalytic supports, electric capacitor substrates, etc. The adsorptivity of adsorbents depends on physical properties such as pore size and shape, hydrophobicity and charge density of pore wall, along with chemical properties such as surface functional groups.

Porous coordination polymers (PCPs)1-4 and metal organic

frameworks (MOFs)5-8 are constructed from metal ions and organic linker ligands to give twoand three-dimensional frameworks.

PCPs are attractive porous materials, because they are

crystals which lead to homogeneous definite pores and their structures and functions are easily controlled by changing organic ligands and metal ions. Variously designed PCPs were prepared from physical and chemical points of view.8 For example, gas storage capacity in PCPs was promoted by introducing open metal sites,7 increasing surface area,9 and modifying pores using functionalized organic linker.6,10,11

Using a ligand containing an amino group such as

aminotriazole (ATaz), no micropores appeared, but CO2 can be adsorbed because of interaction between amino groups and CO2.6 A unique adsorption mechanism, the gate phenomenon,2,12 was found that the latent pores of PCPs were opened above a critical pressure.12 A flexible PCP, e.g., ELM-11, adsorbed CO2 in the gate mechanism, which was brought about after dehydration due to heating the pre-ELM-11 having no gas adsorptivity. Such gate adsorption was observed in CO2 adsorption on {Zn2(C2O4)(C2N4H3)2}(H2O)0.5 (C2N4H4 = 3-amino-1,2,4,-triazole),13 referred


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to ZOATW0.5. Amount of solvent and functional groups inside frameworks of PCPs are very important for formation of latent pores and their flexibility as well as pore volume and adsorptivity. In this study, we report a new route for micropore formation of PCPs via CO2 adsorption in micropores containing water. An as-synthesized [Zn2(Oxac)(Taz)2]•(H2O)2.5, ZOTW2.5 having no pores, was prepared by solvothermal reaction of Zn2+, 1,2,4-triazole (Taz) and oxalic acid (oxalate ion is abbreviated to Oxac).14 Although the as-synthesized ZOTW2.5 pretreated gently had little open micropores and thus adsorbed only small amount of CO2, the CO2-adsorbed PCP brought about large micropore volume after being degassed at around room temperature. EXPERIMENTAL SECTION 0.4g of Zn5(CO3)2(OH)6 (Alfa Aesar, Co. Ltd.), 0.4g of oxalic acid (Wako Pure Chemical Industries,Ltd.; WPCI), and 1.4g of Taz (WPCI) were added to a mixed solvent of 12mL of methanol (WPCI) and 2mL of distilled water. The solution containing white precipitates was transferred to a Teflon cell which was set in an autoclave vessel, and heated at 453 K for 12 h. Cooling the system to room temperature, crystals prepared were filtered with a membrane filter and washed with the solvent, followed by drying it in air at room temperature. In order to obtain pure crystals, the molar ratio of reactants was varied in narrow range. Crystals prepared were characterized by a Rigaku X-ray diffractometer (XRD) Multiflex with Cu-Kα at 40 kV and 20 mA and a JEOL scanning electron microscope (SEM) JSM-7600F. The crystal structure was initially estimated by the program EXPO2014,15 and successively refined by the Rietveld method using RIETAN-FP program.16

Thermogravimetric and differential

thermal analysis (TG/DTA) were carried out using a Rigaku Thermo Plus TG8120. Also,


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amount of solvent included in ZOTs was examined gravimetrically as a function of heating time at 383 K. The elementary analysis of C, H, and N was carried out with a Thermo Electron Flash EA1112 elementary analysis equipment. N2 and CO2 adsorption was measured at 77 and 303 K, respectively, using a custom-made volumetric adsorption system.17 Each sample of ca. 100 mg was pretreated at 383 K and 1 mPa for various times, tpt/h. To confirm the N2 adsorption amounts a gravimetric method was also carried out with a quartz spring (the spring constant of 5.829 mg/mm). Carrying out a series of adsorption experiments, an as-synthesized PCP was pretreated only before first adsorption, and subsequent adsorption experiments were done after degassing at 1 mPa and 298 K for 30 min. The infrared (IR) spectra were measured by a JASCO VIR-200 infrared spectrometer, using a vacuum cell with CaF2 windows connected to a vacuum line. The sample powder of 2 mg was filled in 0.5 mm-thickness, evacuated for 5 min, followed by CO2 gas introduction, and then its IR spectra were measured with a transmission method. RESULTS AND DISCUSSION White crystals of Zn-Oxac-Taz complexes (ZOT) were synthesized in the yield of ca. 70 %. The composition of as-synthesized ZOT was estimated [Zn2(C2O4)(C2N3H2)2]·(H2O)2.5, which is referred to as ZOTW2.5, from the results of TG and elementary analyses (C : H : N = 18.02 : 2.27 : 21.02 %). The XRD pattern of a pure crystal (Figure 1) was obtained from assignment of impurity peaks by the plots of XRD intensities of all peaks as a function of composition of reactants in preparing. The XRD peaks could be indexed in the monoclinic system, P21/a. Figure 1 includes the results of the Rietveld refinement using the XRD pattern (2θ/o = 5 ∼100) without tiny impurity peaks around 13o and 17o, whose fit was fair as the residual intensity


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shows. Figure 2, constructed by Vesta program,18 depicts the crystal structure (without H2O) of the as-synthesized ZOTW2.5, having a five coordinated Zn2+ ion connecting to three Taz molecules and an Oxac ion. The crystal has 3-dimensional coordination network and micropores along the c-axis, whose size is estimated ca. 0.30 x 0.31 x 0.40 nm3. The composition and crystal structure were very similar to ZOATW0.5 prepared by using ATaz instead of Taz, although its pore space may be filled with two solvent (water) molecules instead of two amino groups, as suggested by their compositions.6 Figure 3 shows that an as-synthesized ZOTW2.5 has little amount of N2 adsorbed at 77 K, indicating that the micropores are filled with solvent.

As-synthesized ZOTW2.5 for gas

adsorption was pretreated at 383 K, because TG/DTA curves showed that solvent molecules included were removed between 330 and 383 K. By heating as-synthesized ZOTW2.5 at 383 K, its weight measured using the gravimetric method decreased in sigmoid to reach a saturation value (Figure 4). After short pretreatment the crystals still include some water to be expressed as ZOTWx, where x = 2.5–∆ (∆ is the amount of H2O dehydrated). Thus, the pretreatment shorter than 2h brought about little change in adsorption amount of N2, but sufficient pretreatment increased significantly adsorption amount from low pressure, whose adsorption isotherms are classified as Type I,19 indicating the appearance of micropores due to removal of H2O. The ZOTWx pretreated over 12 h, ZOTWx(12h), showed the saturation adsorption amount of ca. 190 mg/g (0.15 mL/g) from the DR-plot and the diameter of 0.7 nm of pores surrounded by van der Waals surfaces from the t-plot.19 Using the structural data refined by the Rietveld method, the pore volumes of ZOTW2.5 and ZOTWx(12h) were estimated with PLATON20 to be 0.033 and 0.201 cm3 g-1 (the porosity of 6.4 and 31.7 %), respectively. The values correspond to the saturation adsorption amounts of N2, Vs, of 26.8 and 163 mg g-1 and also are consistent with the


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experimental Vs values.

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Since the size of an N2 molecule is ca. 0.33Ф x 0.44 nm2, the

frameworks seem to be somewhat flexible. CO2 adsorption for ZOTWx pretreated for various tpt/h was examined at 303K, as Figure 5 shows.

The opened micropores of ZOTWx(≥3h) pretreated over tpt/h = 3 accepted CO2

molecules from low CO2 pressure to approach the saturation adsorption amount of up to 120 mg/g. The non-pretreated ZOTW2.5 having no opened micropores adsorbed hardly any CO2 molecule at low pressure, but gradually adsorbed beyond 300 Torr.

With increasing the

pretreatment time up to tpt/h = 2, CO2 was adsorbed in sigmoid or cooperatively in partially H2Ofilled micropores, and approached to the adsorption amount in opened micropores of ZOTWx(≥3h), 120 mg/g, at high pressure region. The behavior may be a kind of gate opening adsorption at around 200 ~ 400 Torr of CO2, although usually the gate phenomenon appeared stepwise.12,13 ZOATW0.5 showed that the open of gate for CO2 brought about the adsorption amount of CO2 of 1.4 times of the saturation amount of 100 mg/g. It was interpreted that the gate phenomenon arised from the presence of symmetrically positioned Zn-O bonds of the Znoxalate units that facilitate subtle swivelling motion.13

Though there are little accessible

micropores for N2 in the case of tpt/h less than 3h, the CO2 adsorptivity of ZOTWx(≤2h) gradually increased with tpt up to 7 mg/g below 200 Torr and significant increase over the pressure. The CO2 adsorption amount expected from the solubility of CO2 in bulk water, 14.5 mg/cm3 at 298 K,21 is less than 0.4 mg/g. Thus, such high CO2 adsorptivity suggests that CO2 should interact strongly with H2O molecules at high CO2 pressure and be assisted by the interaction between CO2 and pore frameworks as in opened micropores of ZOTWx(≥3h). A CO2 molecule having the size of around 0.31Ф nm (x 0.54 nm) seems to be more accessible than an N2 molecule to narrow micropores at 303 K in higher fugacity region.


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The high CO2 adsorptivity seems to require the formation of a specific species occluding CO2 in micropores comprising water and frameworks. In a preliminary IR measurement of ZOTW2.5 under CO2 gas, a peak of CO2 at around 2276 cm-1 appeared just below the band of CO2 gas (Figure S1). The peak increased with increase in CO2 pressure of 240 to 500 Torr (0.067 MPa), and reversibly disappeared by degassing. When CO2 was included in water as a clathrate hydrate at 3.3 MPa and r.t., the Raman bands of the Fermi diad and anti-symmetric stretching vibration of CO2 gas shifted by 7 and 14 cm-1, respectively, to lower wavenumber.22,23 Since the antisymmetric stretching vibration (2349 cm-1) is IR-active, the 2276 cm-1 peak may be assigned to the vibration of CO2 occluded in a clathrate-like species, comprising CO2, water, and polar framework. However, the formation of such clathrate-like structure must require very high pressure. It is reported that the effective pressure of adsorbed phase in micropores less than 1 nm in pore size is very high even under 0.1 MPa because of deep potential energy wells;24-27 e.g., 2 GPa in micropores of CPL-1 ([Cu2(pzpc)2pyz])24 and 1.9 GPa25 and 90 GPa in carbon nanotubes.26 Thus, the 2276 cm-1 band suggests that certain clathrate-like species may be formed in micropores of ZOTW2.5 because of high pseudo-pressure. The lower shift might be brought about by extremely high pressure in micropores. Such high pressure may induce deformation of frameworks including new species and also the decomposition of the species due to CO2degassing may induce the removal of water as well as CO2, followed by the formation of abundant open micropores. Figure 6 shows N2 adsorption isotherms measured at 77K for ZOTWx(tpth)deCO2 which was degassed at 298 K after CO2 adsorption on ZOTWx(tpth). The saturation amounts (ws/mg g-1) of N2 adsorbed were plotted as a function of pretreatment time (Figure 7).


Only when as-

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synthesized ZOTW2.5 was shortly pretreated for tpt/h≤2, amounts of N2 adsorbed on ZOTWx(≤2h)deCO2 increased markedly up to 1000 mg/g, ca. 5 times larger than the ws value of N2 prior to CO2 adsorption, which were much larger than values expected from the CO2 adsorption.

The reproducibility was checked by the volumetric and gravimetric methods

(diamond in Figure 7). The size of micropores formed was almost unchanged. Such large adsorption amounts of N2 suggest that CO2 interacting with water in micropores may induce the removal of H2O by the gentle degassing process at 298 K to assist the formation of the micropores. Figure 8 shows the XRD patterns of ZOTW2.5deCO2, ZOTWx(1h)deCO2, and ZOTWx(2h)deCO2 (not shown). They were similar to each other and slightly different from that of as-synthesized ZOTW2.5. The (110) peak in the XRD pattern of ZOTW2.5 shifted by 2θ/o = 0.6 to the lower angle, which corresponds to 0.06 nm-increase in (110) spacing. On the other hand, no ZOTWx(≥3h)deCO2 changed their XRD patterns, suggesting that the structure should be stable. ZOTWx(≤2h)deCO2 samples may be a metastable structure, because the XRD pattern approached to the XRD structure of ZOTWx(≥3h)deCO2 after the long storage of 16 months. Therefore, ZOTWx(≤2h)deCO2 will be a mixture of slightly different structures during the structural relaxation. Thus the crystal structure, especially the structure around oxalic acid, cannot be simulated definitely by the Rietveld method.

The crystal structure for

ZOTWx(1h)deCO2 simulated using the molecular structure of oxalate ion (Table S2 and Figure S2) is similar to that of ZOTW2.5, but the positions and rotational angles of Taz ring and ZnOxac complex are slightly different. The difficulty in our structural analysis may arise from swivelling motion around Zn-O bonds of the Zn-oxalate units, as observed in ZOATW0.5, and such rotational flexibility may bring about the remarkable increment in micropore volume. The


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pore volume of ZOTW2.5deCO2 estimated by PLATON is 0.231 cm3 g-1 (the porosity of 36.5 %) or Vs = 187 mg g-1. The Vs value is much smaller than the experimental Vs values. Though the XRD pattern of ZOTW2.5deCO2 is certainly different from ZOTW2.5, as shown in Figure 8, both analyzed crystal structures were quite similar, except for slight difference in the orientation of triazole. Thus, the high adsorptivity of ZOTW2.5deCO2 cannot be explained by the structure obtained. Therefore, the crystal structure of ZOTW2.5deCO2 during N2 adsorption should be different from that during XRD measurement to return to the initial structure almost reversibly, because the crystal was taken out from the adsorption system when the XRD was measured.

CONCLUSION We found a new micropore formation route via CO2 adsorption on ZOTWx(≤2h) which is partially filled with water in micropores. N2 adsorption on ZOTWx(≤2h)deCO2, degassed after CO2 adsorption on ZOTWx(≤2h), was promoted by 5 times from 180 mg/g on ZOTWx(tpth) and ZOTWx(≥3h)deCO2 up to ca. 1000 mg/g.

The interaction of CO2 and H2O molecules in

micropores may lead to abundant micropore formation.

The crystal structure specifies static

pore size and volume, but adsorption amount depends on flexibility of frameworks. The large micropore volume of ZOTWx(≤2h)deCO2 may arise from faint changes in conformation such as the rotation of triazol rings and Zn-O bonds of oxalate ions in the flameworks.

In order to

proceed this technique, the micropore formation mechanism and stabilization method of micropores should be furthermore investigated. AUTHOR INFORMATION Corresponding Author


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* Email : [email protected] ACKNOWLEDGMENT This research was supported by JSPS Grant-in-Aid for Scientific Research Number 25288003. REFERENCES (1) Kondo, M.; Yoshitomi, T.; Seki, K.; Matsuzaka, H.; Kitagawa, S. Three-Dimensional Frameworks with Channeling Cavities for Small Molecules: {[M2(4,4′-bpy)3(NO3)4]·xH2O}n (M = Co, Ni, Zn). Angew. Chem. Int. Ed. 1997, 36, 1725-1777. (2) Shimomura, S.; Horike, S.; Matsuda, R.; Kitagawa, S.; Guest-Specific Function of a Flexible Undulating Channel in a 7,7,8,8-Tetracyano-p-quinodimethane Dimer-Based Porous Coordination Polymer. J. Am. Chem. Soc. 2007, 129, 10990-10991. (3) Li, W.; Jia, H. P.; Ju, Z.; Zhang, J. A Novel Chiral Cd(II) Coordination Polymer Based on Achiral Unsymmetrical 3-Amino-1,2,4-triazole with an Unprecedented µ4-Bridging Mode. Crystal Growth & Design. 2006, Vol. 6, No. 9, 2136-2140. (4) Garcia-Ricard, O. J.; Morales, P. M.; Martinez, J. C. S.; Curet-Arana, M. C. J.; Hogan, A.; Hernandez-Maldonado, A. J. Carbon dioxide storage and sustained delivery by Cu2(pzdc)2L [L = dipyridyl-based ligand] pillared-layer porous coordination networks. Microporous and Mesoporous Materials. 2013, 177, 54-58. (5) Millward, A. R.; Yaghi, O. M. Metal-organic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature. J. Am. Chem. Soc. 2005, 127, 1799817999.


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(6) Vaidhyanathan, R.; Iremonger, S. S.; Dawson, K. W.; Shimizu, G. K. H. An aminefunctionalized metal organic framework for preferential CO2 adsorption at low pressures. Chem. Commun. 2009, 5230 - 5232. (7) Lin, L. C.; Kim, J.; Kong, X.; Scott, E.; McDonald, T. M.; Long, J. R.; Reimer, J.A.; Smit, B. Understanding CO2 Dynamics in Metal–Organic Frameworks with Open Metal Sites. Angew. Chem. Int. Ed. 2013, 52, 4410-4413. (8) Li, J. R.; Ma, Y.; McCarthy, M. C.; Scullay, J.; Yu, J.; Jeong, H. K.; Balbuena, P. B.; Zhou, H. C. Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks. Coord. Chem. Rev. 2011, 255, 1791-1823. (9) Zhai, Q. G.; Hu, M. C.; Li, S. N.; Jiang, Y. C. A three-dimensional organic–inorganic hybrid solid constructed from novelMo–O–Zn bimetallic oxide networks linked via 3amino-1,2,4-triazole. Inorganic Chemistry Communications. 2008, 11, 1147-1150. (10) Noro, S.; Kitagawa,S. Akutagawa, T.; Nakamura, T. Coordination polymers constructed from transition metal ions and organic N-containing heterocyclic ligands: Crystal structures and microporous properties. Prog. Polym. Sci. 2009. 34, 240-279. (11) Cai, Y.; Zhang, Y.; Huang, Y.; Marder, S. R.; Walton, K. S. Impact of Alkyl-Functionalized BTC on Properties of Copper-Based Metal−Organic Frameworks. Cryst. Growth Des. 2012, 12, 3709-3713. (12) Onishi, S.; Ohmori, T.; Ohkubo, T.; Noguchi, H.; Hanzawa, L. D. Y.; Kanoh, H.; Kaneko, K. Hydrogen-bond change-associated gas adsorption in inorganic–organic hybrid microporous crystals. Applied Surface Science. 2002, 196, 81-88.


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(13) Banerjee, A.; Nandi, S.; Nasa, P.; Vaidhyanathan, R. Enhancing the carbon capture capacities of a rigid ultra-microporous MOF through gate-opening at low CO2 pressures assisted by swiveling oxalate pillars. Chem. Commun. 2016, 52, 1851-1854. (14) Zubir, M.; Hamasaki, A.; Iiyama, T.; Ohta, A.; Ohki, H.; Ozeki, S. Magnetic Field Control of Micropore Formation in [Zn2(Oxac)(Taz)2]•(H2O)x . Chem. Lett. 2016, 45, 362-364. (15) Altomare, A.; Cuocci, C.; Giacovazzo, C.; Moliterni, A. Rizzi, R.; Corriero, N.; Falcicchio, A. EXPO2013: a kit of tools for phasing crystal structures from powder data. J. Appl. Cryst. 2013, 46, 1231-1235. (16) Izumi,F.; Momma, K. Three-dimensional visualization in powder diffraction, Solid State Phenom. 2007, 130, 15. (17) Sakaguchi, A.; Hamasaki, A.; Sadatou, T.; Nishihara, Y. ; Yamamoto, Sekinuma, Y.; Ozeki, S. Magnetic Orientation of Hexagonal Carbon Layers at High Temperatures. Chem. Lett. 2012, 12, 1576-1578. (18) Izumi, F.; Momma, K. “A three-dimensional visualization system for electronic and structural analysis, 2014. (19) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed. Academic Press, London, 1982. (20)

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(21) Hangx, S. J. T.; CATO Work Package WP.4.1. HPT. Laboratory, Department of Earth Sciences Utrecht University. 2005. (22)

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Figure 1.

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The experimental XRD pattern (top) of an as-synthesized ZOTW2.5,

[Zn2(C2O4)(C2N3H2)2]•(H2O)2.5. *: Impurity peaks. Peak positions simulated by the Rietveld method are shown (middle). The residue (bottom) between the experimental and simulated peak intensities are plotted.


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(A)

Zn C N O

(B)

Figure 2.

The crystal structure simulated by the Rietveld method using RIETAN-FP

program. A. Pore structure. B. Local crystal structure.


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300

Amount of N2 adsorbed, w/mg g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

250

tpt /h 12

200

6

150

4

100

50

1 0

0 0

0.2

0.4

0.6

0.8

1

Relative pressure, p/po

Figure 3. Adsorption isotherms for N2 adsorbed on ZOTWx after pretreating ZOTW2.5 for tpt/h at 77K.

Figure 4. Change of x value of ZOTWx with heating time at 383 K and 10 mPa, which was measured by the gravimetric method.


16

Page 17 of 19

2

Amount of CO adsorbed, w/mg g

-1

140

tpt /h

120

12 100

3

80

1

60

2

40 20

0

0 0

100

200

300

400

500

600

700

800

Pressure, p/torr

Figure 5. Adsorption isotherms for CO2 adsorbed on ZOTWx after pretreating ZOTW2.5 for tpt/h at 303K.

-1

1400

2

Amount of N adsorbed, w/mg g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

1200

tpt /h

1000

1

ZOTWx(1h)deCO2

2

ZOTWx(2h)deCO2

0

ZOTW2.5 deCO2

800 600 400

3 12

ZOTWx(12h)deCO2 ZOTWx(12h)deCO2

12 0

ZOTWx(12h) ZOTW2.5

200 0 0

0.2

0.4

0.6

0.8

1

o

Relative pressure, p/p

Figure 6.

N2 adsorption isotherms for ZOTW2.5deCO2 and ZOTWx(tpth)deCO2 (tpt=1~12),

measured at 77K after CO2 adsorption shown in Fig. 2. As references, N2 adsorption isotherms for ZOTW2.5 and ZOTWx(12h) are included.


17

Page 18 of 19

1000

800

600

400 ZOTWx(tpth)deCO2

200

ZOTWx(tpth)

0 0

2

4

6

8

10

12

Pretreatment time, tpt/h

Figure 7.

The saturation amounts of N2 adsorbed on ZOTWx(tpth), circle and blue, and

ZOTWx(tpth)deCO2, triangle and red, were plotted as a function of pretreatment time.

♦: the

saturation amount of N2 determined by the gravimetric method.

tpt /h

Intensity, I / a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Saturation amount of N2 , ws /mg g-1

Langmuir

10

15

12

ZOTWx(12h )deCO2

3

ZOTWx(3h )deCO2

1

ZOTWx(1h )deCO2

0

ZOTW2.5 deCO2

0

ZOTW2.5

20

2θ /

Figure 8.

o

25

30

35

XRD patterns of ZOTW2.5 and ZOTWx(tpth) (tpt=0, 1, 3, 12) degassed after CO2

adsorption.


18

Page 19 of 19

Graphical Abstract

1200 -1

1200

1000

1 CO2

adsorption 800

at 303K 2 Degassed at 298K 3 N2 adsorption

600

400

Opened micropores 200

at 77K Closed micropores with water

1000

800

600

2

at 77K

Amount of N adsorbed, w/mg g

Amount of N2 adsorbed, w/mg g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

400

200

0

0 0

0.2

0.4

0.6

0.8

1



0

0.2

0.4

0.6

0.8

1


19

Micropore Formation of - ACS Publications - American Chemical Society

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