A multifunctional Zr-MOF for the rapid removal of Cr2O72−, efficient gas adsorption/separation, and catalytic performance

Xiurong Zhanga, Xia Wanga, Weidong Fan*a, Yutong Wanga, Xiaokang Wanga, Kai Zhanga and Daofeng Sun*ab
aCollege of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, People's Republic of China. E-mail: weidongfan@163.com; dfsun@upc.edu.cn
bSchool of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, People's Republic of China

Received 3rd October 2019 , Accepted 25th November 2019

First published on 25th November 2019


To achieve the selective adsorption of ions and light hydrocarbons by changing the pore structure, Zr-based metal–organic framework [Zr63-O)43-OH)4(OH)4(H2O)4(TB)2]·2DMF·2H2O (UPC-50) was synthesized based on 3,3′,5,5′-tetra(4-carboxyphenyl)bimesityl (H4TB) with a steric hindrance effect under hydrothermal conditions. UPC-50 features a three-dimensional frame with two-dimensional channels and high hydrothermal and chemical stability. The Zr–OH bond in the Zr6 cluster endows UPC-50 with excellent removal capacity for trace Cr2O72− in water (0.089 ppm) and efficient adsorption rate (<1 min). Moreover, through the single-component gas sorption and selectivity calculated by the IAST method, the channel of 10.6 × 16.4 Å gives UPC-50 a high adsorption capacity for C3H6 (252.67 cm3 g−1), and also shows effective separation of C3H6/CH4. The unsaturated sites on the Zr6 cluster also impart excellent catalytic performance to UPC-50 in the catalytic cycloaddition reactions of CO2 with epoxides. Therefore, UPC-50 is a multi-functional material with a rapid removal capacity for Cr2O72−, efficient gas adsorption/separation, and catalytic performance.


Introduction

With the deepening of industrialization, water pollution has become a problem that plagues the development of human society and the construction of ecological civilization. The industrial wastewater discharged during factory production has become a major cause of water pollution.1–4 A large amount of industrial wastewater has caused serious pollution of natural water bodies. Heavy metal ions (such as Cr(III)/Cr(VI), Pb(II), Hg(II), etc.) in wastewater can’t be degraded naturally, and eventually are enriched into the human body through the biological chain, causing more chronic toxicity, reproductive toxicity, genotoxic diseases and potential carcinogenic hazards.5–7 Therefore, the adsorption of heavy metal ions is a widely studied problem; in particular, hexavalent chromium (Cr2O72−) is mutagenic, carcinogenic and teratogenic to the human body, and is classified as a Class A carcinogen by the US Environmental Protection Agency (EPA). According to the standards recommended by the US EPA, the concentration of hexavalent chromium ions in drinking water should be strictly controlled below 100 ppb.8,9 Therefore, finding an effective and safe heavy metal ion adsorbent has become an urgent problem. A variety of technologies have been developed to detect and eliminate heavy metal ions in wastewater, including adsorption, ion exchange, chemical treatment, biological treatment, fluorescence detection, and membrane filtration.10–14 Adsorption is considered to be one of the most effective methods to eliminate the pollutants in water. Various highly efficient adsorbent materials have been reported, such as activated carbon, graphene, siliceous materials, zeolites, etc.15–19

Metal–organic frameworks (MOFs), as a crystalline porous material, have a periodic network structure formed by the self-assembly of metal ions and organic ligands. Compared with traditional zeolite porous materials, due to their structural diversity and customizability, MOFs have important applications in energy storage, separation, fluorescence detection and catalysis.20–23 In recent years, Zr-based MOFs have been extensively studied due to their excellent water stability and unique Zr6 cluster structure, which can well adsorb Cr2O72− in water. For example, Liu et al.24 reported that the (4,8)-c sqc topology Zr-MOF (JLU-MOF60) with water stability has excellent adsorption capacity (149 mg g−1) and adsorption rate (38.0 mg g−1 min−1) for Cr2O72− in water due to the existence of the Zr–OH bond in the Zr6 cluster. In 2015, Zheng et al.25 designed a cationic Zr-MOF ZJU-101 through the post-synthetic modification of MOF-867. ZJU-101 has an adsorption capacity of 245 mg g−1 for Cr2O72− and a high adsorption rate, which reaches the adsorption equilibrium within 10 minutes. Therefore, the development of stable MOF materials plays an important role in the adsorption of heavy metal ions in industrial wastewater.

In addition to ion adsorption, permanent porosity also provides excellent performance in MOF materials for gas adsorption and catalysis. Low carbon hydrocarbons have always been an important petrochemical feedstock. However, they are accompanied by the production of by-product gases during production and purification. In order to obtain high-purity products, the industry often requires a complicated separation process.26 The traditional gas separation technologies are cryogenic distillation and solvent extraction, both of which are energy intensive.27,28 As an emerging adsorption/separation material, MOFs have adjustable pores and functional groups that can separate low-carbon hydrocarbons through molecular sieve effects or host–guest interactions, thereby reducing energy consumption and pollution.29–32 In addition, the active metal sites in the pores also make MOFs a good catalyst for the reaction, such as the carbon dioxide cycloaddition reaction.33–35 Although MOFs have many application potentials, most materials are unstable under high temperatures, acids and bases, which limits their industrial applications, so the stability of MOFs is also worth exploring.36–39

To achieve selective adsorption of ions and light hydrocarbons by changing the pore structure, we introduce a methyl group into tetracarboxylic acids and synthesized 3,3′,5,5′-tetra(4-carboxyphenyl)bimesityl (H4TB) to increase the steric hindrance effect. In this work, we constructed a microporous Zr-MOF [Zr63-O)43-OH)4(OH)4(H2O)4(TB)2]·2DMF·2H2O (UPC-50) using the ligand H4TB and ZrCl4. The secondary building unit (SBU) in the UPC-50 is a classic Zr6 cluster ([Zr6O4(OH)8(H2O)4(CO2)8]). The SBU linked with the ligand and the steric hindrance effect of the methyl group cause UPC-50 to form 2-D channels of 10.6 Å × 16.4 Å, which provides the possibility for selective adsorption of C3/C2/C1. Meanwhile the Zr–OH bond in the Zr6 cluster is conducive to the adsorption of Cr2O72− in water, and the Zr(IV) site provides powerful help for the catalytic carbon dioxide cycloaddition reaction.

Results and discussion

Crystal structures of UPC-50

UPC-50 was synthesized from the ligand H4TB with zirconium tetrachloride under solvothermal reaction conditions. The phase purity for subsequent characterization was confirmed by powder X-ray diffraction (PXRD).

UPC-50 {[Zr63-O)43-OH)4(OH)4(H2O)4(TB)2]·2DMF·2H2O} single crystal X-ray diffraction indicates that UPC-50 crystallizes in the tetragonal space group P4/mnc. Compared with the ligand H4DCBA (Fig. 1a) which does not have asteric hindrance effect in UPC-102-Zr,40 the methyl group at the 2,2′,6,6′-position in H4TB causes 80° twist between the central benzene rings in the process of crystal formation, which leads to the transformation of the crystal structure. The asymmetric unit of UPC-50 contains two crystallographically independent Zr(IV) atoms and a 1/3 deprotonated TB ligand, a μ3-O atom, an OH, and a H2O molecule. Each Zr(IV) chelates with two carboxylic acids from different ligands to form a classical Zr6 cluster.41–44 During the crystal growth process, the benzene ring of the ligand is distorted due to the steric hindrance of the methyl group on the ligand (Fig. 1b). The SBU and the ligand are interconnected to form a three-dimensional network structure which has two-dimensional rhombic channels with a size of 10.6 Å × 16.4 Å along the a and b axes (Fig. 1c). From a topological point of view, the SBUs and ligands are simplified to different nodes (Fig. S1d and e, ESI). Topology analysis shows that the structure of UPC-50 can be simplified to a 2-nodal (4,8-c)-connected network flu topology with the point symbol of {412·612·84}{46}2 (Fig. S1f, ESI), which is similar to LB3-Zr68-flu that was published during the process of writing this paper.45 When comparing UPC-50 with LB3-Zr68-flu, although the ligands used were the same, the degrees of torsion between the benzene rings in the ligands were different, while the central benzene rings of the LB3-Zr68-flu ligand were twisted 73° (Fig. S2, ESI), making it crystallize in the Fmmm space group. Using the SQUEEZE program in the PLATON software, the solvent availability of UPC-50 was calculated to be 71.51% (8557.5 Å of 11966.8 Å unit cell volume) and the density was 0.600 g cm−3.


image file: c9qm00612e-f1.tif
Fig. 1 (a) Structure of the ligand (H4DCBA) in UPC-102-Zr. (b) Structure of the ligand in UPC-50. (c) Structure of UPC-50 in the a,b-axis direction. (d–f) Simplified structure of SBU, ligand, and UPC-50. (The atoms Zr, C, O and N are represented by turquoise, gray, red and blue, respectively, and H atoms are omitted for clarity.)

Stability of metal–organic frameworks

As an emerging material, MOFs have been widely used in various fields, but the stability of MOFs limits their application under harsh industrial conditions (high temperature, acidity and alkalinity, etc.).46–48 Therefore, in addition to excellent performance, the design and synthesis of highly stable MOF materials also play an important role in their performance applications. Thermogravimetric analysis (TGA) and the stability of UPC-50 in acid–base aqueous solution under high temperature were tested. TGA showed that UPC-50 lost the solvent molecules in the range of 40–400 °C (Fig. S3, ESI). In addition, to investigate the thermal stability of UPC-50, the samples were heated under a nitrogen atmosphere for 3 hours. Obviously, it can be known from the powder X-ray diffraction pattern that the thermal stability of UPC-50 can reach 400 °C. In addition to thermal stability, water stability is one of the necessary conditions for MOF materials to be used in industry. The samples were placed in aqueous solutions of HCl and NaOH at different pHs for 24 hours. The powder X-ray diffraction patterns showed that UPC-50 maintained a high chemical stability in aqueous solutions of pH = 1–12 (Fig. 2). This excellent thermal and chemical stability may be attributed to the strong synergistic effect between the Zr6 cluster and the carboxylic acid groups, and the hydrothermal stability also provides the possibility for UPC-50 to be applied in different fields.
image file: c9qm00612e-f2.tif
Fig. 2 Powder X-ray diffraction (PXRD) of UPC-50 with different temperatures and pHs.

Permanent porosity

The porosity of UPC-50 was tested due to the existence of a regular two-dimensional channel. The as-synthesized sample was solvent exchanged with methanol and dichloromethane, and then degassed at 100 °C for 10 h under high vacuum to obtain an activated sample. The adsorption of N2 (77 K) exhibits a type I micropore adsorption isotherm with a maximum adsorption capacity of 601.2 cm3 g−1 at 1 bar and a pore distribution of approximately 12 Å (Fig. 3). The specific surface area of Brunauer–Emmett–Teller (BET) is 1731.6 m2 g−1.
image file: c9qm00612e-f3.tif
Fig. 3 N2 sorption isotherms (77 K). Inset, pore size distribution calculated by the density functional theory (DFT) method.

Adsorption of Cr2O72−

The stability and porosity of the MOF prompted us to study its versatility. Since the Zr–OH bond in the Zr6 cluster has a strong attraction to Cr2O72−,24,49,50 the adsorption capacity of UPC-50 for Cr2O72− in water was tested. As shown in Fig. 4a, the characteristic peak (350 nm) of Cr2O72− decreases rapidly (87%) within 30 s after adding UPC-50, and only slightly changes after 1 minute. The color of the solution changed from yellow to colorless (Fig. S6, ESI), indicating that UPC-50 can adsorb Cr2O72− rapidly and reach saturation, which is faster than NU-1000 (Table S3, ESI). The maximum adsorption capacity of Cr2O72− solution with different concentrations (25–400 ppm) was measured to be 56.8 mg g−1, which was comparable to that of SLUG-21 (60 mg g−1)51 and PCN-134 (57 mg g−1),52 and the Langmuir model fits the adsorption isotherm well (Fig. 4c). In addition, in order to simulate the effect of other ions on the adsorption of Cr2O72− in the real state, the adsorption capacity of Cr2O72− in the decuple molar of the interfering ions (Cl, Br, I, NO3) together with K+ or Na+ cations was measured. As shown in the Fig. 4d, UPC-50 can still maintain a great adsorption capacity of Cr2O72− in the presence of other ions. At the same time, in order to evaluate its adsorption performance at a low concentration, 50 mg of UPC-50 was dispersed into the solution for 10 min, and 99.6% Cr2O72− in the solution is adsorbed, with residual concentration of only 0.089 ppm (Fig. S5, ESI), which met the EPA drinking water standard (less than 100 ppb).9 The recovered UPC-50 was analyzed by PXRD and still maintained good crystallinity (Fig. S7, ESI). At the same time, the release experiment was also conducted via the UPC-50 crystals after adsorption being soaked in an aqueous solution. It is worth noting that about 33% of Cr2O72− was released from UPC-50 after 1 h, and about 63% of Cr2O72− was finally released (Fig. S8, ESI). Therefore, UPC-50 can be considered as an excellent Cr2O72− adsorbent.
image file: c9qm00612e-f4.tif
Fig. 4 (a) UV-vis spectra of Cr2O72− in aqueous solution at different times. (b) The adsorption rate with 0–24 hours. (c) The maximum adsorption amount for Cr2O72− (inset: photographs of the 25 ppm Cr2O72− solution before and after adsorption). (d) Effect of interference ions on Cr2O72− adsorption.

Gas storage/separation

In addition to ion adsorption, permanent porosity enables MOF materials to show excellent performance in gas adsorption, so the adsorption of UPC-50 in H2 (77 K), CO2 (273 K, 298 K) and CH4 (273 K, 298 K) was tested (Fig. 5a). The results showed that the maximum adsorption capacity of UPC-50 for H2 was 131.1 cm3 g−1 (1.17 wt%), which was comparable to IRMOF-9 (1.17 wt%),53 IRMOF-2 (1.21 wt%)53 and ZIF-8 (1.27 wt%).54 The maximum adsorption capacities of CO2 and CH4 were 57.6 and 15.2 cm3 g−1 at 273 K, respectively. When the temperature increased to 298 K, the maximum adsorption capacities of CO2 and CH4 decreased to 47.8 and 12.4 cm3 g−1, respectively. Due to the difference in the adsorption of CO2 and CH4, the selectivity of gas separation was calculated using ideal solution adsorption theory (IAST) (Fig. 5b). At 273 K, the gas separation selectivities of CO2/CH4 are 5.4 and 5.9 (50[thin space (1/6-em)]:[thin space (1/6-em)]50, 10[thin space (1/6-em)]:[thin space (1/6-em)]90), respectively. At 298 K, the gas separation selectivities of CO2/CH4 are 4.6 and 4.8 (50[thin space (1/6-em)]:[thin space (1/6-em)]50, 10[thin space (1/6-em)]:[thin space (1/6-em)]90), which are comparable to ZIF-79 (5.4),55 SIFSIX-2-Cu (5.3),56 and PCN-88 (5.3).57
image file: c9qm00612e-f5.tif
Fig. 5 (a) H2 (77 K), CO2 (273 K and 298 K), and CH4 (273 K and 298 K) sorption isotherms. (b) The selectivity for CO2 over CH4 at 273 K and 298 K.

Besides the adsorption of H2 and CO2, the adsorption and separation of light hydrocarbons are also of great significance for the industrial application of UPC-50. We tested the adsorption capacity of UPC-50 for C1/C2/C3 light hydrocarbons. UPC-50 has the highest adsorption capacity for C3H6, reaching 213.75 and 252.67 cm3 g−1, at 298 K and 273 K, respectively (Table S4, ESI), and has a good cyclicity (Fig. S9, ESI). UPC-50 has a moderate adsorption capacity for C2H2, C2H4, C2H6 and CH4 with 73.42, 67.76, 90.50, and 12.03 cm3 g−1 at 298 K, and 124.94, 103.71, 143.27, and 15.21 cm3 g−1 at 273 K, respectively. It can be seen from Fig. 6 that the adsorption difference of C3, C2 and C1 is obvious in the low pressure region, and C3 and C2 are saturated at low pressure. In contrast, the interaction between CH4 and the skeleton is relatively weak and the adsorption capacity is poor. In order to understand more about the adsorption capacity of UPC-50 for light hydrocarbons, the isosteric heat of adsorption (Qst) for different gases are calculated according to the Clausius–Clapeyron equation based on the adsorption difference at 298 K and 273 K (Table S4, ESI). The zero coverage Qst values of five gases are 18.72 (C3H6), 11.76 (C2H2), 13.43 (C2H4), 14.61 (C2H6) and 8.23 (CH4) kJ mol−1 (Fig. 6c), respectively. At the same time, the curve of C3H6 is steeper due to the higher affinity of UPC-50 to C3H6 under low pressure. Therefore, we used IAST to calculate the separation selectivity of UPC-50 for C3H6/C2 and C3H6/C1 at different temperatures (Table S5, ESI). Obviously, the separation selectivity of C3H6/CH4 is up to 78.28 (50[thin space (1/6-em)]:[thin space (1/6-em)]50) and 54.10 (10[thin space (1/6-em)]:[thin space (1/6-em)]90) at 1 bar and 273 K, respectively (Fig. 6d). When the temperature rises to 298 K, the separation ratio of C3H6/CH4 is 40.03 (50[thin space (1/6-em)]:[thin space (1/6-em)]50), and 29.39 (10[thin space (1/6-em)]:[thin space (1/6-em)]90) (Fig. 6e), respectively, which is lower than that of MFM-202a,58 but higher than that of UPC-32 (Table S6, ESI),59 and C2/C1 (Fig. 6f). In summary, UPC-50 is expected to be a potential separation and purification material for C3H6.


image file: c9qm00612e-f6.tif
Fig. 6 (a and b) The CH4, C2H6, C2H4, C2H2 and C3H6 adsorption isotherms at 273 K and 298 K of UPC-50. (c) The isosteric adsorption enthalpies Qst of UPC-50. (d and e) The C3/C1, C3/C2 selectivity at 298 K and 273 K for UPC-50, calculated by the IAST method (v/v: 50/50 and 10/90). (f) The C2/C1 selectivity at 298 K and 273 K for UPC-50, calculated by the IAST method (v/v: 50/50 and 10/90).

Catalytic activity in carbon dioxide cycloaddition reactions

Industrial organic reactions are increasingly demanding catalysts. Porous materials have highly ordered structures and pores that can accommodate guest molecules, which is important for the catalytic process. Some porous catalysts (such as zeolites) have good catalytic effects, but their application is limited by the harsh reaction conditions. MOFs have great potential for applications in organic catalytic reactions due to their advantages of easy separation, mild reaction conditions and controllability.60–64 Based on the adsorption of CO2 by UPC-50 and the Zr site in the structure, we studied the catalytic activity of UPC-50 for the cycloaddition reaction of carbon dioxide with epoxides to prepare cyclic carbonates, and the products are characterized by GC-MS.

As can be seen from Table 1, UPC-50 plays a good catalytic role in the cycloaddition reaction rate of CO2 with epoxide. The reaction was carried out at 298 K, 1 bar for 24 hours, and the conversion rate of epoxy propane was 65.97%, which was higher than that of HKUST-1 (49.2%),65 but lower than that of UPC-99 (80.21%).66 To improve the conversion, the reaction conditions were slightly changed. The conversion increased to 75.08% at 323 K and 6 bar for 5 hours, and the conversion rate was 74.83% after one cycle. When epoxy propane was replaced by cyclododecane epoxide, styrene oxide and cyclohexene oxide, the catalytic yields were reduced by 20.34%, 32.16% and 15.39%, respectively. The recovered UPC-50 was analyzed by PXRD and still maintained good crystallinity (Fig. S22, ESI). Based on the previous literature,67 we believe that the catalytic mechanism of UPC-50 is that MOF has a high attraction for CO2 due to its exposed Zr sites. The unsaturated orbital of Zr in activated UPC-50 can accept electrons of epoxide O atoms (Fig. S20, ESI). After the epoxide is combined with Zr, some of the electrons are transferred from the O atom to Zr, resulting in a weak C–O bond of the epoxide. Subsequently, n-Bu4NBr (TBAB) attacks the epoxy C atom to open the epoxy ring. Then, the O atom of CO2 attacks the positively charged carbon of the epoxide, and the O atom of the epoxide attacks the C atom of CO2. Finally, the cyclic carbonate is formed by a ring closure step. UPC-50 and TBAB play a synergistic catalytic role throughout the catalytic reaction.

Table 1 Cycloaddition reactions of CO2 with epoxides catalyzed by UPC-50

image file: c9qm00612e-u1.tif

Entrya Epoxides Products Reaction conditions Yieldb (%)
a Reaction conditions: epoxides (15.0 mmol), UPC-50 (0.01 mmol), TBAB (n-Bu4NBr) (1 mmol).b GC yield. Entries 6–8: these three reactions respectively represent after recycle 1, recycle 2, and recycle 3 of UPC-50.
1 image file: c9qm00612e-u2.tif image file: c9qm00612e-u3.tif RT, 1 bar, 24 hours 65.97
2 image file: c9qm00612e-u4.tif image file: c9qm00612e-u5.tif 50 °C, 6 bar, 5 hours 75.08
3 image file: c9qm00612e-u6.tif image file: c9qm00612e-u7.tif 50 °C, 6 bar, 5 hours 20.34
4 image file: c9qm00612e-u8.tif image file: c9qm00612e-u9.tif 50 °C, 6 bar, 5 hours 32.16
5 image file: c9qm00612e-u10.tif image file: c9qm00612e-u11.tif 50 °C, 6 bar, 5 hours 15.39
6 image file: c9qm00612e-u12.tif image file: c9qm00612e-u13.tif 50 °C, 6 bar, 5 hours 74.83
7 image file: c9qm00612e-u14.tif image file: c9qm00612e-u15.tif 50 °C, 6 bar, 5 hours 73.57
8 image file: c9qm00612e-u16.tif image file: c9qm00612e-u17.tif 50 °C, 6 bar, 5 hours 66.79


Conclusions

In summary, we have developed and characterized a novel microporous Zr-MOF (UPC-50) using a pre-designed functional ligand H4TB. UPC-50 has a three-dimensional network structure with two-dimensional pores with high hydrothermal and chemical stability. Interestingly, due to the affinity of the Zr–OH bond for Cr2O72− in the cluster Zr6, UPC-50 has a great adsorption capacity and adsorption rate for Cr2O72− in water, and a high adsorption capacity for Cr2O72− in the presence of other ions. Moreover, UPC-50 has a better removal capacity for trace Cr2O72− in water, making it an excellent adsorbent for Cr2O72−. In addition, UPC-50 exhibits a large adsorption capacity for C3H6 (252.67 cm3 g−1 at 273 K, 1 bar), and an excellent selectivity for C3H6/CH4 (78.28 at 273 K, 1 bar). Meanwhile, the activated UPC-50 channel has an open Zr site and exhibits excellent catalytic performance in the carbon dioxide cycloaddition reaction. Through the structural design of MOFs, our laboratory is working to develop adsorption and separation materials with high stability and selectivity.

Experimental

Materials and measurement

All the chemical reagents were purchased from chemical vendors and were used without further purification. The 1H NMR spectrum was obtained using a 400 MHz Varian INOVA spectrometer. The PXRD diffractograms were obtained using a Panalytical X-Pert PRO diffractometer with Cu-Kα radiation. Elemental analyses (C, H, N) were performed using a CE instruments EA 1110 elemental analyzer. Infrared spectra (IR) were collected on a Nicolet 330 FTIR Spectrometer within the 4000–400 cm−1 region. Thermo-gravimetric analysis (TGA) measurements were carried out on a Mettler Toledo TGA instrument under a N2 atmosphere with a heating rate of 10 °C min−1 at the range of 40–900 °C. Gas adsorption experiments were carried out on the surface area analyzer ASAP-2020. The UV-vis absorption spectrum is obtained by using a 752 PC UV-vis spectrophotometer.

Synthesis of H4TB ligand

The H4TB ligand was synthesized by our previous report without modification; the detailed synthetic methods are described in the ESI.

Synthesis of UPC-50

ZrCl4 (15.0 mg, 0.0640 mmol), H4TB (10.0 mg, 0.0140 mmol), and benzoic acid (700 mg, 5.74 mmol) were ultrasonically dissolved in DMF (3 ml) solution in a 10 ml vial, and heated to 120 °C in 30 min, kept at 120 °C for 3 days, and then the reaction system was cooled to room temperature slowly at a rate of 0.1 °C min−1. The colorless, block-shaped crystals were obtained, washed with DMF and dried in the air (yield: 72% based on H4TB). The phase purity for subsequent characterization was confirmed by powder X-ray diffraction (PXRD). Elemental analysis calculated (%) for UPC-50: C 50.41; H 0.62; N 1.27. Found: C 50.23; H 0.64; N 1.20. IR (KBr, cm−1): 3410 (m), 3061 (w), 1720 (w), 1605 (s), 1545 (m), 1416 (s), 1174 (m), 1018 (m), 866 (m), 718 (s), 660 (s).

Crystal structure determinations

UPC-50 crystal structure was collected using a Bruker Apex 2 Smart CCD surface detector at 273 K. A graphite monochromator and an (Cu) X-ray source (Cu Kα λ = 1.54184 Å) were used for absorption correction by using Multiscan program SADABS. The crystal structure was analyzed by a direct method with the Sir97 program, and the structure of F2 was refined by a full matrix least square method with the SHELXL program. All non-hydrogen atoms were anisotropically refined. The hydrogen atoms of organic ligands were generated in geometric symmetry. Crystallographic data, thermogravimetric data and elemental analysis were combined to calculate that the solvent molecules in UPC-50 were two DMF molecules and two H2O molecules.

Cr2O72− adsorption in aqueous solution

10 mg UPC-50 samples were added to 10 ml 25 ppm Cr2O72− aqueous solution, and the adsorption kinetics experiment was carried out under the condition of continuous agitation without light. In order to study the maximum adsorption capacity, 10 mg UPC-50 samples were dispersed in Cr2O72− solution with a concentration of 25–400 ppm at 10 ml and stirred continuously for 12 hours without light. Then the liquid was centrifuged, and the supernatant was taken and tested using an ultraviolet-visible spectrophotometer (λ = 300–650 nm). Because the concentration of Cr2O72− has a better linear relationship with its absorbance, the residual concentration of Cr2O72− in the adsorbed solution can be calculated by fitting parameters.24 In addition, 50 mg UPC-50 samples were dispersed in 10 ml 25 ppm Cr2O72− aqueous solution to investigate their adsorption performance at low concentration. At the same time, in order to verify the selectivity of UPC-50 on the adsorption of Cr2O72−, 10 mg UPC-50 samples with decuple molar of the interfering ions (Cl, Br, I, NO3) were added to 10 ml 25 ppm Cr2O72− solution, and then stirred continuously for 10 minutes without light to investigate the effect of interfering ions on the adsorption of Cr2O72−.

Gas sorption measurements

The as-synthesized sample was solvent-exchanged with methanol and dichloromethane three times for 6 hours each time, respectively. The solvent was vacuum-dried and degassed under high vacuum at 100 °C for 10 h to obtain an activated sample. Gas adsorption experiments containing low-pressure H2, N2 gas sorption experiments at 77 K, and CO2, CH4, C2H2, C2H4, C2H6 and C3H6 light hydrocarbon experiments at 273 K and 298 K were carried out using an ASAP-2020 surface area analyzer. A liquid nitrogen bath, ice water bath and normal temperature water bath were used to control the temperature at 77 K, 273 K and 298 K during the test, respectively. The Brunauer–Emmett–Teller (BET) specific surface area and pore distribution were calculated using the N2 adsorption isotherm at 77 K.

Catalyzing Knoevenagel condensation reactions

The catalytic cycloaddition reactions of CO2 and epoxides were implemented under a solvent free environment at a certain temperature, pressure and reaction time. Epoxides (15.0 mmol), UPC-50 (0.01 mmol) and TBAB (n-Bu4NBr) (1 mmol) were put into a 10 ml Schlenk tube. At the end of the reaction, the product is centrifuged and the supernatant is collected for further testing. UPC-50 was centrifuged by DMF and methanol many times for the next cycle catalytic test. The conversion of products was determined by gas chromatography (GC-MS) (DB-1 column, L × I.D. × T.H. 30.0 m × 0.25 mm × 0.25 μm; injector temperature 300 °C). Heating rate: 15 °C min−1. The recovered UPC-50 crystals were characterized by PXRD.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 21875285), Taishan Scholar Foundation (ts201511019) and Key Research and Development Projects of Shandong Province (2019JZZY010331).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Experimental, characterization and additional figures. CCDC 1954445. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qm00612e

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