English

• 0.   Introduction

  Metal-organic frameworks (MOFs) have emerged as a promising porous material for the greatly potential application in gas adsorption and separation, e.g. CO₂ capture, combining with the advantages of large porosity, designable and tunable structure, tailorable and functionalized pore surface and etc^[1-12]. The elaborate selection of special inorganic or organic building units with some useful functionalities and their directed self-assembly through the coordinating bonds may result in the desired functional metal-organic frameworks (FMOFs) with targeted functions^[13-19]. Currently, many investigations in the field of MOFs for CO₂ capture have implied that the polar functional groups including both Lewis acid and base being introduced in the backbone of MOFs are efficient to enhance the affinity of MOFs toward the CO₂ gas molecule adsorbed in the pores^[20-22]. Typically, the amino group introduced in the framework of FMOFs as one Lewis base might play an important role in the improvement of selective CO₂ adsorption property, which has been observed in the CO₂ capture of NH₂-IRMOF-1^[23-24], bio-MOF-11^[25] and etc^[23-29].

  One research interest of our group is the construction of the pillar-layer MOFs based upon T-shaped ligands with both N donor and carboxyl group as the coordination sites and the [Cu₂(COO)₄]-paddlewheel unit, besides, investigate their greenhouse gas (i.e., CO₂ gas) capture performance^[30-33]. Recently, we have published one work, in which the pillar-layer rtl-MOF, SYSU^[34], built from 5-pyridin-4-yl-isophthalic acid and the [Cu₂(COO)₄]-paddlewheel unit, has been selected as a platform, then the N coordination site in its organic ligand was chosen to be shifted to result in the pore size being finely-tuned in the achieved MOF, NJU-Bai7, with the CO₂ capture performance greatly optimized^[30]. To further expand our work, we wonder if combining the amino group into the SYSU platform as one Lewis base with its pore surface functionalized would inherently strengthen the interaction between the framework and the adsorbed CO₂ gas molecule, eventually improving the CO₂ capture property of the NH₂-functionalized rtl-MOF.

  In this work, a new ligand decorated with the amino group (-NH₂), 5-(3-amino-pyridin-4-yl)-isoph-thalic acid (NH₂-H₂L₁), has been designed and synthesized successfully, of which the reaction with the copper salt under the solvothermal reaction condition led to the successful preparation of the NH₂-functionalized pillar-layer rtl-MOF, SNNU-Bai65 (SNNU-Bai for Shaanxi Normal University Bai group). In addition activation, the selective CO₂ adsorption over N₂ and CH₄ of activated SNNU-Bai65 was also investigated.

  1.   Experimental

  1.1   Materials and methods

  All reagents were obtained from commercial vendors and, unless otherwise noted, were used without further purification (CuCl₂·2H₂O, Sinopharm Chemical Reagent, AR 99%; 3-amino-4-iodopyridine, Alfa, 98%; 3, 5-bis(methoxy-carbonyl)phenylboronic acid, Aladdin, 96%). The IR spectra were obtained in the 4 000~400 cm^-1 range on a Tensor27 spectrometer using KBr pellets. ¹H NMR spectra were recorded on a Bruker Ascend 400M spectrometer with tetrame-thylsilane as an internal reference. Thermal gravimetric analyses (TGA) were performed under N₂ atmosphere (100 mL·min^-1) with a heating rate of 5 ℃·min^-1 using a Beijing Henven HTG-1 thermogravimetric analyzer. Powder X-ray diffraction (PXRD) data were collected over the 2θ range of 5°~50° on a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα radiation (λ=0.154 18 nm) at room temperature with a scan speed of 5°·min^-1 at 40 kV and 40 mA.

  1.2   Synthesis of 5-(3-amino-pyridin-4-yl)-isophthalic acid (NH₂-H₂L₁)

  To a solution of 3-amino-4-iodopyridine (2.2 g, 10 mmol) in 125 mL of toluene was added the mixture of 3, 5-bis(methoxy-carbonyl)phenylboronic acid (3.2 g, 12 mmol) in 30 mL of ethanol, followed by the addition of a solution of Na₂CO₃ (3.5 g, 33 mmol) in 10 mL of water. This solution was degassed using N₂ for 10 min, and then Pd(PPh₃)₄ (0.5 g, 0.43 mmol) was added. The resulting reaction mixture was stirred at 90 ℃ under N₂ overnight. The solvent was then removed by the rotary evaporation, and the residue was dissolved in CH₂Cl₂ and washed with water. Then the organic layer was dried over MgSO₄, filtered, concentrated, and purified by silica gel flash column chromatography with an eluent of acetone/petroleum ether(1:6, V/V). The product was hydrolyzed by refluxing in 2 mol·L^-1 aqueous KOH followed by acidification with 37%(w/w) HCl to afford a pale yellow solid, NH₂-H₂L₁. Yield: 1.2 g (47%). IR (KBr, cm^-1): 1 695, 1 592, 1 542, 1 442, 1 370, 1 218, 1 067, 931, 826, 742, 682, 622, 543. ¹H NMR (DMSO-d₆): δ 13.05 (broad peak, COOH), 8.48 (t, 1H, ArH), 8.21 (d, 2H, ArH), 8.13 (s, 1H, ArH), 7.85 (d, 1H, ArH), 7.05 (d, 1H, ArH), 5.26(s, 2H, NH).

  1.3   Synthesis of [Cu(NH₂-L₁)]·x(Solvent) (SNNU-Bai65)

  A solution of CuCl₂·2H₂O (20.0 mg, 0.117 mmol) in 0.5 mL of methanol was mixed with the NH₂-H₂L₁ (10 mg, 0.039 mmol) in 1.5 mL of DMF. To this was added 0.02 mL of concentrated HCl with stirring. The mixture was sealed in a Pyrex tube and heated to 85 ℃ for 48 h. The green block crystals obtained were filtered and washed with DMF. Yield: 50%. Selected IR (cm^-1): 1 668, 1 587, 1 439, 1 374, 1 255, 1 092, 916, 832, 722, 655, 580.

  1.4   Crystal structure determination

  Single-crystal X-ray diffraction data were measured on a Bruker Apex Ⅲ CCD diffractometer at 243 K using graphite monochromated Cu Kα radiation (λ=0.154 178 nm). Data reduction was made with the Bruker SAINT program. The structures were solved by direct methods and refined with full-matrix least squares technique using the SHELXTL package^[35]. Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. Organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to 1.2U_eq of the attached atom. The unit cell includes a large region of disordered solvent molecules, which could not be modelled as discrete atomic sites. We employed PLATON/SQUEEZE^[36] to calculate the diffraction contribution of solvent molecules and, thereby, to produce a set of solvent-free diffraction intensities; structures were then refined again using the data generated. Crystal data and refinement conditions are shown in Table 1.

  Table 1

  

  Table 1.  Crystal data and structure refinement for SNNU-Bai65

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  Empirical formula	C13H8CuN2O4 (+Solvent)		F(000)	644
  Formula weight	319.76		Crystal size / mm	0.180×0.150×0.130
  Temperature/ K	143(2)		θ range for data collection / (°)	4.085~72.633
  Crystal system	Monoclinic		Index ranges	-14 ≤ h ≤ 14, -16 ≤ k ≤ 16, -17 ≤ l ≤ 17
  Space group	P21/c		Reflection collected	23 399
  a / nm	1.184 73(2)		Independent reflection	4 079 (Rint=0.025 2)
  b / nm	1.336 02(3)		Completeness to θ=20.879° / %	99.8
  c / nm	1.421 30(3)		Refinement method	Full-matrix least-squares on F2
  β / (°)	114.031 0(10)		Data, restraint, parameter	4 062, 0, 190
  Volume / nm3	2.054 68(7)		Goodness-of-fit on F2	1.002
  Z	4		Final R indices [I > 2σ(I)]	R1=0.046 8, wR2=0.318 1
  Dc / (g·cm-3)	1.034		R indices (all data)	R1=0.047 3, wR2=0.321 3
  Absorption coefficient / mm-1	1.598		Largest diff. peak and hole / (e·nm-3)	726 and -1 325

  CCDC: 1971729.

  1.5   Gas sorption measurements

  Low-pressure adsorption isotherms of CO₂ (99.999%, V/V), CH₄ (99.999%, V/V) and N₂ (99.999%, V/V) gas were performed on Quantachrome Autosorb IQ-2 surface area and pore size analyzer. Before analysis, the as-synthesized samples of SNNU-Bai65 were soaked in ethanol for 4 days with ethanol refreshing every 8 hours. Then, the ethanol-exchanged sample was activated under vacuum and elevated temperature following the heating procedure as 50 ℃ for 3 h then 60 ℃ for 2 h and finally 80 ℃ for 9 h to obtain the activated bulk retaining intact as far as possible.

  2.   Results and discussion

  2.1   Crystal structure of SNNU-Bai65

  Solvothermal reaction of CuCl₂·2H₂O with NH₂-H₂L₁ in the solution of DMF/MeOH containing HCl afforded a high yield of green block crystals of SNNU-Bai65, which crystallizes in monoclinic space group P2₁/c. SNNU-Bai65 exhibits the same topology as the prototypical rtl-MOF, SYSU and its iso-reticular MOF, such as NJU-Bai7 and NJU-Bai8. In the structure of SNNU-Bai65, the inorganic building block, [Cu₂(COO)₄]-paddlewheel unit, is connected by the isophthalic acid fragment in the organic building block, leading to the formation of 2D layer with sql-lattice. Then, the 2D sql-layer is further pillared by the 3-amino-pyridin-4-yl group in the organic building block, resulting in the formation of 3D rtl-MOF. Being perpendicular to the sql-layer, the 1D pore channel is generated within the 3D framework of the new rtl-MOF, SNNU-Bai65. Compared with SYSU, the polar amino group combined in the framework of SNNU-Bai65 functionalizes the pore surface, simultaneously, partitions the 1D pore channel into two parts with the pore size of 0.44 nm (determined by the van der Waals diameter of the inserted pseudo atom) for each part of SNNU-Bai65 reduced from 0.62 nm in SYSU (Fig. 1).

  Figure 1

  

  Figure 1.  Schematic diagram for the NH2-functionalization of MOFs (up); Design of amino group decorated organic ligand, NH₂-H₂L₁(middle); Amino group functionalization of rtl-MOF with the polar amino group decorating the pore surface and partitioning the pore channel into two parts in SNNU-Bai65 (down, C: gray, N: blue, O: red)

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  To better understand the structure, the [Cu₂(COO)₄]-paddlewheel unit is simplified as 6-connected node and the organic ligand is viewed as a 3-connected node, then SNNU-Bai65 may be described as a (3, 6)-connected rtl-topological network with its point (Schlfli) symbol of {4.6²}₂{4².6¹⁰.8³}. The total potential solvent accessible volumes in desolvated SNNU-Bai65 calculated by the PLATON/SOLV program is ca. 48.9% and its density is 1.034 g·cm^-3.

  2.2   Powder X-ray diffraction and thermal stability

  The phase purity of the bulk sample was confirmed by the powder X-ray diffraction (PXRD) in Fig. 2, in which the peaks are well coincident with those in the simulated curve resulted from the single X-ray crystal data. After the exchange of solvent in the pores of as-synthesized sample for EtOH with it being soaked in EtOH for four days, the framework of EtOH-exchanged sample was still retain well, which can be seen from its PXRD pattern in Fig. 2. Finally, with the solvent molecules removal from the pores of EtOH-exchanged SNNU-Bai65, the activated sample also kept the integrality of its framework, which was confirmed by the coincident peaks in PXRD patterns of the activated and as-synthesized sample.

  Figure 2

  

  Figure 2.  PXRD patterns of simulated, as-synthesized, EtOH-exchanged and activated SNNU-Bai65

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  The thermal stability of SNNU-Bai65 was investigated by the thermogravimetric analysis (TGA). As shown in Fig. 3, the first weight-loss of as-synthesized sample took place around 125 ℃, which is corresponding to the loss of solvent molecules packed in the pores of SNNU-Bai65. After 270 ℃, the framework of SNNU-Bai65 began to collapse. For the sample of EtOH-exchanged SNNU-Bai65, the EtOH molecules exchanged into the framework were lost around 75 ℃. Then, the framework of EtOH-exchanged SNNU-Bai65 collapsed around 270 ℃. Finally, after the removal of exchanged EtOH molecules from the pores of EtOH-exchanged SNNU-Bai65, there was still a small loss of weight around 60 ℃ in the TGA curve, which may be resulted from the water molecules adsorbed in the pores during the measurement process. Then, after a large platform, the framework of activated SNNU-Bai65 also collapsed around 270 ℃, which inherently indicates the enough activation of the sample of activated SNNU-Bai65.

  Figure 3

  

  Figure 3.  TGA curves of as-synthesized, EtOH-exchanged and activated SNNU-Bai65

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  2.3   Gas adsorption property

  The permanent porosity of activated SNNU-Bai65 was confirmed by the measurement of N₂ adsorption isotherm at 77 K and 100 kPa, as shown in Fig. 4. The N₂ gas adsorption isotherm exhibited a reversible type Ⅰ isotherm without hysteresis on desorption, which indicates that activated SNNU-Bai65 belongs to the microporous material. At 100 kPa and 77 K, activated SNNU-Bai65 could take up the N₂ gas of 262 cm³·g^-1 (STP) with its Brunaer-Emmett-Teller (BET) and Langmuir surface area estimated to be 943 and 1 124 m²·g^-1, respectively, which were a little lower than those of SYSU due to the amino group hanging in the pore^[30]. In addition, the Horvath-Kawazoe (H-K) pore size distribution curve of activated SNNU-Bai65 showed the pore diameter was 0.48 nm, which is consistent with the pore size of 0.44 nm determined from the crystal structure of SNNU-Bai65.

  Figure 4

  

  Figure 4.  N₂ adsorption (closed)-desorption (open) isotherms for activated SNNU-Bai65 at 77 K

  Inset: H-K pore size distribution curve of activated SNNU-Bai65

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  The permanent porosity of activated SNNU-Bai65 prompted us to investigate its CO₂ capture property. At low pressure range of 0~100 kPa, the adsorption isotherms of CO₂, N₂ and CH₄ gas were measured at 273 and 298 K, as shown in Fig. 5a. Activated SNNU-Bai65 could adsorb a large amount of CO₂ with the uptake of 53.4 cm³·g^-1 (~10.5%, w/w) and 113.5 cm³·g^-1 (~22.3%, w/w) at 15 and 100 kPa, respectively, under 273 K, and largely took up CO₂ with the uptake amount high up to 26.3 cm³·g^-1 (~5.2%, w/w) and 79.8 cm³·g^-1 (~15.7%, w/w) at 15 and 100 kPa, respectively, under 298 K. However, it was only able to adsorb N₂ and CH₄ with their amounts of 11.5 cm³·g^-1 (~1.4%, w/w) and 43.7 cm³·g^-1 (~3.1%, w/w), respectively, at 100 kPa and 273 K, and took up N₂ and CH₄ with their uptakes of 4.8 cm³·g^-1 (~0.6%, w/w) and 24.8 cm³·g^-1 (~1.8%, w/w), respectively, at 100 kPa and 298 K. So, it can be indicated that the activated sample of SNNU-Bai65 exhibited a highly selective CO₂ adsorption property.

  Figure 5

  

  Figure 5.  (a) CO₂ adsorption isotherms for activated SNNU-Bai65 at 273 and 298 K; (b) IAST selectivity for CO₂/N₂ (0.15:0.85, V/V) and CO₂/CH₄ (0.5:0.5, V/V) gas mixtures of activated SNNU-Bai65 at 298 K; (c) CO₂ adsorption enthalpy of activated SNNU-Bai65

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  To further investigate the separation ability of activated SNNU-Bai65 for CO₂ over N₂ and CH₄, the selectivities of CO₂/N₂ (0.15:0.85, V/V) and CO₂/CH₄ (0.5:0.5, V/V) gas mixtures were calculated by the ideal adsorbed solution theory (IAST) based upon the pure gas adsorption isotherms(Fig. 5b). At 298 K, the selectivity for CO₂/N₂ (0.15:0.85, V/V) gas mixture was 42.4 at 100 kPa and that for CO₂/CH₄ (0.5:0.5, V/V) gas mixture was 8.1 at 100 kPa. Furthermore, the selectivities of CO₂/N₂ and CO₂/CH₄ based upon the initial slops of gas isotherms were also estimated at 298 K with the values for CO₂/N₂ and CO₂/CH₄ of 29 and 4.5, respectively. Compared with SYSU, both the CO₂ adsorption uptake at 15 or 100 kPa and 298 K and the selectivities for CO₂/N₂ and CO₂/CH₄ at 298 K of activated SNNU-Bai65 are largely improved, although they are still less than those of NJU-Bai7 and NJU-Bai8, which was documented in Table 2.

  Table 2

  

  Table 2.  Comparison of selective CO₂ adsorption properties for rtl-MOFs based upon [Cu₂(COO)₄]-paddlewheel unit and bifunctional organic ligand with pyridyl group and isophthalic acid

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  MOFs	SNNU-Bai65	SYSU[30]	NJU-Bai8[30]	NJU-Bai7[30]
  Qst, CO2 / (kJ·mol-1)	29.4	28.2	37.7	40.5
  CO2 uptake (298 K, 15 kPa) / %(w/w)	5.2	3.6	5.4	8.0
  CO2 uptake (298 K, 15 kPa) / %(w/w)	15.7	13.4	11.2	12.8
  S298 K, CO2/N2	29	19.0	58.3	62.8
  S298 K, CO2/CH4	4.5	3.9	15.9	9.4

  To better understand the good CO₂ selective adsorption behavior of activated SNNU-Bai65, its adsorption enthalpy for CO₂ at zero loading was calculated by the virial method based upon the experimental isotherm data with the value of -29.4 kJ·mol^-1, higher than that of SYSU, which indicated a stronger interaction between the framework of activated SNNU-Bai65 and adsorbed CO₂ molecules. All of these may be resulted from the functionalized pore surface and smaller pore diameter partitioned by the introduced amino group.

  3.   Conclusions

  Based upon the amino group decorated organic ligand, NH₂-H₂L₁, the NH₂-functionalized pillar-layer rtl-MOF, SNNU-Bai65, has been successfully synth-esized with the pore surface decorated and the pore channel partitioned. More interestingly, activated SNNU -Bai65 exhibits more highly selective CO₂ adsorption over N₂ and CH₄ than the parent MOF, SYSU.

  
  
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• 

  Figure 1  Schematic diagram for the NH2-functionalization of MOFs (up); Design of amino group decorated organic ligand, NH₂-H₂L₁(middle); Amino group functionalization of rtl-MOF with the polar amino group decorating the pore surface and partitioning the pore channel into two parts in SNNU-Bai65 (down, C: gray, N: blue, O: red)

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  Figure 2  PXRD patterns of simulated, as-synthesized, EtOH-exchanged and activated SNNU-Bai65

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  Figure 3  TGA curves of as-synthesized, EtOH-exchanged and activated SNNU-Bai65

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  Figure 4  N₂ adsorption (closed)-desorption (open) isotherms for activated SNNU-Bai65 at 77 K

  Inset: H-K pore size distribution curve of activated SNNU-Bai65

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  Figure 5  (a) CO₂ adsorption isotherms for activated SNNU-Bai65 at 273 and 298 K; (b) IAST selectivity for CO₂/N₂ (0.15:0.85, V/V) and CO₂/CH₄ (0.5:0.5, V/V) gas mixtures of activated SNNU-Bai65 at 298 K; (c) CO₂ adsorption enthalpy of activated SNNU-Bai65

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  Table 1.  Crystal data and structure refinement for SNNU-Bai65

  Empirical formula	C13H8CuN2O4 (+Solvent)		F(000)	644
  Formula weight	319.76		Crystal size / mm	0.180×0.150×0.130
  Temperature/ K	143(2)		θ range for data collection / (°)	4.085~72.633
  Crystal system	Monoclinic		Index ranges	-14 ≤ h ≤ 14, -16 ≤ k ≤ 16, -17 ≤ l ≤ 17
  Space group	P21/c		Reflection collected	23 399
  a / nm	1.184 73(2)		Independent reflection	4 079 (Rint=0.025 2)
  b / nm	1.336 02(3)		Completeness to θ=20.879° / %	99.8
  c / nm	1.421 30(3)		Refinement method	Full-matrix least-squares on F2
  β / (°)	114.031 0(10)		Data, restraint, parameter	4 062, 0, 190
  Volume / nm3	2.054 68(7)		Goodness-of-fit on F2	1.002
  Z	4		Final R indices [I > 2σ(I)]	R1=0.046 8, wR2=0.318 1
  Dc / (g·cm-3)	1.034		R indices (all data)	R1=0.047 3, wR2=0.321 3
  Absorption coefficient / mm-1	1.598		Largest diff. peak and hole / (e·nm-3)	726 and -1 325

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  Table 2.  Comparison of selective CO₂ adsorption properties for rtl-MOFs based upon [Cu₂(COO)₄]-paddlewheel unit and bifunctional organic ligand with pyridyl group and isophthalic acid

  MOFs	SNNU-Bai65	SYSU[30]	NJU-Bai8[30]	NJU-Bai7[30]
  Qst, CO2 / (kJ·mol-1)	29.4	28.2	37.7	40.5
  CO2 uptake (298 K, 15 kPa) / %(w/w)	5.2	3.6	5.4	8.0
  CO2 uptake (298 K, 15 kPa) / %(w/w)	15.7	13.4	11.2	12.8
  S298 K, CO2/N2	29	19.0	58.3	62.8
  S298 K, CO2/CH4	4.5	3.9	15.9	9.4

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