A Doubly Interpenetrated Co(II) Framework: Synthesis,Crystal Structure and Selective Adsorption of CO2①

WEI Fng-Fng LI Zi-Yin CHEN Ling-Ji LIN Qun-Jie YE Ying-Xing LIU Li-Zhen ZHANG Zhng-Jing, XIANG Sheng-Chng, ②

a (Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China)b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

A microporous and doubly interpenetrated MOF (FJU-29) was synthesized which possesses paddle-wheel {Co2(COO)4} clusters bridged by pyrazolate H2NDI and H2BDC.The desolvated FJU-29a shows the BET surface area of 560.44 m2×g-1is accompanied with relatively high CO2/N2(18/85) adsorption selectivity of 75.5 at 296 K and 100 kPa based on IAST, implying that FJU-29a is a potential material for the CO2capture from flue gas.

1 INTRODUCTION

Metal-organic frameworks (MOFs) are porous crystalline materials[1,2], which are built from organic and inorganic building blocks and assembled from molecular building units in modular fashion, and therefore can achieve a high degree of well-defined functionality[3,4].This makes them become a fast expanding research area because of their intriguing structural diversity and potential applications as functional materials in gas adsorption and separation[3a,3d], magnetism[5], drug delivery[6],catalysis[7], and chemical sensing[3d,8].The judicious selection of molecular building units is crucial to control the pore surfaces by changing the sizes which can also possibly be taken as a kinetic barrier of adsorption phenomena of particular gas molecules[9].Carbon dioxide (CO2) emitted through human activities and the combustion of fossil fuels is considered as the primary and pressing environmental concern[10].For the purpose of mitigating CO2emissions, CO2capture from a postcombustion flue gas becomes an important and practical approach[11].The commercial technology is based on alkanolamine solvents which are not cost effectively[12].To date, many solid porous adsorbent materials have been used to alternate liquid phase absorption process such as MOFs[3a,4d,11,13,14],porous carbons[15], and zeolites[16].

In this work, we chose bi-pyrazolate 2,7-bis(3,5-dimethyl)dipyrazol-1,4,5,8-naphthalene-tetracarboxydiimide (H2NDI) and terephthalic acid (H2BDC)as dual-ligands which coordinate with the paddlewheel {Co2(COO)4} clusters to form a 3D porous doubly interpenetrated MOF (FJU-29) for the selective adsorption of CO2.X-ray single-crystal and powder diffractions, thermal gravimetric analysis and infrared spectra were used to characterize the as-synthesized FJU-29.The porous property endows FJU-29 with high BET surface area of 560.44 m2×g-1and pore volume of 0.216 cm3×g-1.Gas sorption analysis of the desolvated FJU-29a shows relatively larger uptake of CO2(79.8 and 54.1 cm3×g?1at 273 and 296 K, respectively) under 1 bar pressure with good selectivity over N2.Ideal adsorbed solution theory (IAST) calculation indicates that the selectivity values of CO2/N2(15:85) are 103.5 at 273 K and 75.5 at 296 K, which suggest that FJU-29a is a potential contender for the separation of flue gas.

2 EXPERIMENTAL

2.1 Materials and methods

1,4,5,8-Naphthalene-tetracarboxylic dianhydride(NDA) and terephthalic acid (H2BDC) were purchased from J&K Chemical Co., Ltd..Co(NO3)2·6H2O, N,N-dimethylformamide (DMF),methanol and other reagents were purchased from Shanghai Chemical Reagent Co.All materials were of reagent grade and used without further purification.H2NDI was synthesized similar to published procedures[17].Powder X-ray diffraction (PXRD)was carried out with PANalytical X’Pert3powder diffractometer equipped with a Cu-sealed tube (λ =1.541874 ?) at 40 kV and 40 mA over the 2θ range of 5～30o.The Fourier transform infrared (KBr pellets) spectra were recorded in the range of 400～4000 cm-1on a Thermo Nicolet 5700 FT-IR instruments.Thermal analysis was carried out on a METTLER TGA/SDTA 851 thermal analyzer from 30 to 800 ℃ at a heating rate of 10 ℃ min-1under nitrogen atmosphere.

2.2 Synthesis of {Co2(H2NDI)(BDC)·2H2O·MeOH·0.5DMF}n (FJU-29)

A mixture of Co(NO3)2·6H2O (0.0291 g, 0.1 mmol), H2NDI (0.0227 g, 0.05 mmol), and H2BDC(0.0166 g, 0.1 mmol) was dissolved in DMF/MeOH(4 mL/1 mL) in a screw-capped vial.The vial was heated at 80 ℃ for 1 day under autogenous pressure.Deep purple bulk crystals were obtained after filtration, washed with DMF, and dried in air.Yield 46% (based on H2NDI).Elemental analysis calcd.(%) fo{Co2(H2NDI)(BDC)·2H2O·MeOH·0.5DMF}n(%) Calcd.: C, 50.74; N, 9.05; H, 3.73.Found: C,49.95; N, 9.32; H, 3.61.IR (KBr pellet, cm-1): 3734(w), 3364 (s), 2945 (w), 1721 (m), 1677 (s), 1579 (s),1385 (s), 1243 (s), 1199 (m), 1058 (w), 980 (m), 834(m), 746 (s), 545 (m).

2.3 Crystal structure determination and refinement

Data collection and structural analysis of crystal FJU-29 were collected on an Agilent Technologies Super Nova Single Crystal Diffractometer equipped with graphite monochromatic CuKα radiation (λ =1.54184 ?).The crystal was kept at 150 K during data collection.Using Olex2[18], the structure was solved with the Superflip structure solution program using Charge Flipping and refined with the ShelXL[19]refinement package using full-matrix least-squares minimization.All non-hydrogen atoms were refined with anisotropic displacement parameters.The hydrogen atoms on the ligands were placed in idealized positions and refined using a riding model.We employed PLATON[20]/SQUEEZE[21]to calculate the diffraction contribution of the solvent molecules and thereby produce a set of solvent-free diffraction intensities.All non-hydrogen atoms were refined anisotropically.The hydrogen atoms were placed at the calculation positions.R = 0.0585 and wR = 0.1544 for 4789 observed reflections (I >2s(I)), and R = 0.0726 and wR = 0.1627 for all data.Selected bond, angle parameters and hydrogen bond parameters are listed in Tables 1 and 2, respectively.

Table 1.Selected Bond Lengths (?) and Bond Angles (°) for FJU-29

Table 2.Hydrogen Bond Lengths (?) and Bond Angles (°) in FJU–29

2.4 Gas sorption measurements

After the bulk of the solvent was decanted, the freshly prepared sample of FJU-29 (~0.15 g) was soaked in ~10 mL MeOH for one hour, and then the solvent was decanted.Following the procedure of MeOH soaking and decanting for ten times, the solvent-exchange samples were activated by vacuum at 60 ℃ for 24 hours till the pressure of 5 μmHg to obtain FJU-29a.N2and CO2adsorption isotherms were measured on Micromeritics ASAP 2020 HD88 surface are analyzer for the FJU-29a.The sorption measurement was maintained at 77 K with liquid nitrogen and at 273 K with an ice-water bath (slush),respectively.

2.5 Virial equation analysis

The virial equation can be written[22]as follows:

where n is the amount adsorbed (mol·g-1) at pressure p (Pa).At a low surface coverage, the A2and higher terms can be neglected and the equation becomes

A linear graph of ln(n/p) versus n was obtained at low surface coverage and this is consistent with neglecting the higher terms in Eq.(2).A0is related to the adsorbate-adsorbent interactions, whereas A1describes the adsorbate-adsorbate interactions.

Enthalpies of Adsorption.Zero surface coverage.The isosteric enthalpies of adsorption at zero surface coverage (Qst,n=0) are a fundamental measure of adsorbate-adsorbent interactions and these values were calculated from the A0values obtained by extrapolation of the virial graph to zero surface coverage.

van't Hoff isochore.The isosteric enthalpies of adsorption as a function of surface coverage were calculated from the isotherms using the van't Hoff isochore, which is given by the equation

A graph of lnP versus 1/T at a constant amount adsorbed (n) allows the isosteric enthalpy and entropy of adsorption to be determined.The pressure values for a specific amount adsorbed were calculated from the adsorption isotherms by: (1) assuming a linear relationship between the adjacent isotherm points starting from the first isotherm point; and (2)using the virial equation at low surface coverage.

2.6 Prediction of the Gas adsorption selectivity by IAST

The ideal adsorption solution theory (IAST)[23]was used to predict the binary mixture adsorption from the experimental pure gas isotherms.To perform the integrations required by IAST, single-com-ponent isotherms should be fitted by the correct model.In practice, several methods are available; for this set of data we found that the single-site Langmuir-Freundlich equation was successful in fitting the results.

where p is the pressure of the bulk gas in equilibrium with the adsorbed phase (kPa), N is the amount adsorbed per mass of adsorbent (mmol×g-1),Nmaxis the saturation capacities of site 1 (mmol×g-1),b is the affinity coefficients of site 1 (1/kPa) and n represents the deviations from an ideal homogeneous surface.The fitted parameters were then used to predict multi-component adsorption with IAST.The adsorption selectivity based on IAST for mixed CO2/N2are defined by the following equation:

where xiand yiare the mole fractions of component i(i = A, B) in the adsorbed and bulk phases,respectively.

3 RESULTS AND DISCUSSION

3.1 Structure description

FJU-29 was synthesized via solvothermal reaction of Co(NO3)2·6H2O, H2NDI, and H2BDC in DMF/methanol solution at 80 ℃ for one day.The X-ray single-crystal diffraction study reveals that FJU-29 crystallizes in monoclinic space group C2/c.As shown in Fig.1a, there is one crystallographically independent Co2+ion as coordinating with one N atom from the NDI2-ligand and four O atoms from four BDC2-ligands.The framework of FJU-29 contains paddle-wheel binuclear {Co2(COO)4} units(Fig.1b), which are bridged by BDC ligands to form a 2D square grid.The 2D square grids are diagonally pillared by H2NDI ligands, whose nitrogen atoms occupy the axial sites of the {Co2(COO)4} paddlewheels, to form a 3D framework with pcu-topology[24]that can be described as an elongated primitive cubic lattice.The spacious nature (10.87 ×18.34 ?2) of the single network allows another identical network to penetrate it in a normal mode,thus resulting in a doubly interpenetrating array.Meanwhile, the unprotonated N(2) atoms from pyrazole of H2NDI ligands form the hydrogen-bonding interactions (d(N(2)(pyrazole)···O(5)(naphthalimide))= 3.094(6) ?) with O(5) atoms (naphthalimide groups) from H2NDI ligands of neighboring interpenetrating framework which has significant impact on the interpenetration of FJU-29.After elimination of guest solvent molecules, the total accessible volume in FJU-29 is 35.7% by using the PLATON software[20].

Fig.1.Structure of FJU-29.(a) Coordination environments of the Co(II) atoms (Symmetry code: # = 1–x, y, 3/2–z).[Co2(COO)4] unit connects to each other with BDC2- to form 2D layers (b), which are further pillared by H2NDI to produce a 3D open framework viewed along the b axis (c) and the subsequent two crystallographically dependent pcu networks were interpenetrated (red and blue, d & e).H atoms are omitted for clarity

3.2 Powder X-ray diffraction and thermogravimetric analysis

The purity of the as-synthesized FJU-29 was examined by similarities between simulated and experimental PXRD patterns (Fig.2a).The as-synthesized PXRD pattern of FJU-29 is coincident well with the simulated from the single-crystal data,indicating a good purity and homogeneity of the compound.The thermal stability of FJU-29 was examined by the TGA technique in the temperature range of 30～600 ℃ under N2atmosphere.The result reveals the weight loss of 7.3% from 30 to 112 ℃ and 4.6% till 158 ℃, one belonging to the weight of two H2O molecules and one CH3OH molecule (calcd: 6.8%) and the other being a half DMF molecule (calcd: 3.8%) (Fig.2b).Finally,FJU-29 shows a plateau up to 418 ℃ following with sharp weight loss, indicating the collapse of the framework.FJU-29 exhibits high thermal stability which encourages us to further explore the practical applications of this material.

Fig.2.(a) Powder X-ray diffraction (PXRD) patterns for FJU-29.(b) Thermogravimetric analysis (TGA) curves for FJU-29

3.3 Gas adsorption and selectivity

Gas adsorption studies were performed with the activated sample (denoted as FJU-29a) to test its permanent porosity.The as-synthesized FJU-29 was immersed in MeOH for ten times to remove guest molecules, and the phase purity of FJU-29a was checked by using PXRD (Fig.2a).PXRD showed good agreement among the as-synthesized and activated patterns, indicating the retention of porous structure after the solvent exchange and successive elimination of exchanged solvent molecules from the channels.As shown in Fig.3a, N2adsorptiondesorption isotherms at 77 K of FJU-29a display typical type-I sorption behavior with a Brunauer Emmett Teller (Langmuir) surface area of 560.44(600.53) m2×g-1and pore volume of 0.216 cm3×g-1,suggesting microporous nature of the FJU-29 framework.The pore size distribution of FJU-29a was calculated by employing non-local density functional theory (NLDFT), showing that the major pores are located at around 0.5, 0.8 and 1.1 nm (Fig.3a).Single component gas sorption isotherms of CO2and N2were performed at 296 and 273 K and presented in Fig.3b.FJU-29a can absorb 54.1 and 79.8 cm3×g-1of CO2at 296 and 273 K/1 atm, which is higher than the values of most reported MOFs[3a].In sharp contrast to CO2adsorption, FJU-29a only adsorbs limited amounts of N2which are 3.9 cm3g-1at 296 K and 7.5 cm3g-1at 273 K.Such a discrimination of adsorption capacities enables FJU-29a to be a very promising material for the selective separation of CO2/N2mixture.The enthalpy at zero coverage Qst,n=0for CO2adsorption on FJU-29a,using Virial equation analysis, from the isotherms at 273 and 296 K.The comparison of the results from the two methods, the linear extrapolation and the virial equation, shows very good agreement (Fig.3c).The Qstvalue for CO2of FJU-29a was found to be 31.7 kJ mol?1which is comparable to the values for CuBTTri (21 kJ·mol?1)[25], PCN-88 (27 kJ·mol?1)[26],IRMOF-3 (19 kJ mol?1)[27], and NOTT-140 (25 kJ·mol?1)[28], implying that the inner wall of the channels for FJU-29a may be engaged in strong intermolecular interactions with the adsorbed CO2gas molecules.

Fig.3.(a) N2 sorption isotherms of FJU-29a at 77 K.Solid and open symbols represent adsorption and desorption,respectively.(Inset) Pore size distributions of FJU-29a calculated by NLDFT method.(b) CO2 and N2 adsorption isotherms of FJU-29a at 296 and 273 K.(c) Isosteric heats of adsorption (Qst) of CO2 for FJU-29a by virial equation analysis.(d) IAST-predicted selectivity of CO2/N2 (15:85) mixture for FJU-29a

We further studied the potential of FJU-29a for CO2/N2(15:85) selective adsorption which was calculated based on the well-known ideal adsorbed solution theory (IAST) (Fig.3d).It can be seen that FJU-29a exhibits high selectivity towards CO2/N2(15:85) mixture under 100 kPa, and the value is 75.5 at 296 K and 103.5 at 273 K.The adsorption selectivity of FJU-29a for CO2/N2(15:85) is much higher than those of previously reported MOFs:SIFSIX-2-Cu (13.7)[29], PCN-61 (15)[30], ZJNU-44a(15)[31], PCN-88 (18)[26], PMOF-3a (23.4)[32], Cu-BTTri (21)[33], and ZIF-68, (18.7)[34]under similar conditions.The relatively high selectivity further implies that FJU-29a is a promising material for practical flue gas purification.

4 CONCLUSION

In summary, through solvothermal reaction, we have synthesized a novel, porous and doubly interpenetrated MOF (FJU-29) based on dual-ligands of bi-pyrazolate H2NDI and H2BDC.FJU-29 displays high thermal stability up to 418 ℃, and gas sorption measurements of the evacuated FJU-29a shows preferential uptake of CO2at 273 and 296 K under 1 bar over N2.Ideal adsorbed solution theory (IAST)indicated that FJU-29a has high CO2/N2(18/85)adsorption selectivity (75.5) at 296 K and 100 kPa.The relatively high selectivity further implies that FJU-29a is a potential contender for the separation of flue gas.

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