Novel 3D Nitrogen-Rich Metal Organic Framework for Highly Efficient

Jun 13, 2018 - MOF-Zn-1 was prepared by solvothermal crystallization method. ... Typically, 34.5 mmol propylene oxide (PO), MOF-Zn-1 catalyst 0.1 g an...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Novel 3D Nitrogen-Rich Metal Organic Framework for Highly Efficient CO2 Adsorption and Catalytic Conversion to Cyclic Carbonates under Ambient Temperature Jianwen Lan,†,⊥ Mengshuai Liu,‡,⊥ Xingyuan Lu,§ Xiao Zhang,*,† and Jianmin Sun*,†,∥

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†

State Key Laboratory of Urban Water Resource and Environment, MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150080, China ‡ College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China § School of Science, Northeast Forestry University, Harbin 150040, China ∥ Shenzhen Key Laboratory of Organic Pollution Prevention and Control, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China S Supporting Information *

ABSTRACT: A novel Lewis acid−base bifunctional Zn(II)-based MOF-Zn-1 [Zn2L2MA·2DMF] (MA = melamine, H2L = 2,5thiophenedicarboxylic acid), with abundant micropores and free -NH2 groups was facilely assembled by incorporating zinc(II) ion with nitrogen-rich melamine and 2,5-thiophenedicarboxylic acid ligands. The constructed MOF-Zn-1 presented an excellent affinity toward CO2 molecules due to the Lewis-base property together with abundant micropores. The Zn active sites could be used for epoxide activation. The acid−base synergistic effects facilitated CO2 conversion into cyclic carbonates under ambient temperature using the porous MOF-Zn-1 as a heterogeneous catalyst. Moreover, the MOF-Zn-1 exhibited satisfied stability and versatility, and it was easy to recycle with no obvious decrease of catalytic activity. Then the feasible synergistic mechanism of MOF-Zn-1/Bu4NBr catalysts for CO2 conversion was proposed. KEYWORDS: carbon dioxide, cyclic carbonate, Lewis acid−base, Zn-based MOF, heterogeneous catalysis

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INTRODUCTION

Scheme 1. Synthesis of Cyclic Carbonates from CO2 and epoxides

Nowadays, human are using more and more fossil fuels to meet energy demands, which has released massive amounts of carbon dioxide into atmosphere, and it has resulted in global surface temperature increase and subsequent climate changes.1 Therefore, it is extremely urgent to realize the CO2 emissions reduction.2 Also, CO2 is a cheap, abundant and nontoxic C1 feed stock, the promising and attractive strategy is to utilize and convert CO2 into high-value chemicals.3 Catalytic conversion of CO2 and epoxides into cyclic carbonates is an alternative and potential study in this field (Scheme 1). The cyclic carbonates as reaction medium and chemicals have important applications, including green nonprotonic solvent, precursors for polymer, intermediates of medicines and pesticides, electrolytes of lithium-ion battery and so on.4−7 However, the industrial synthesis of cyclic carbonate usually needs high reaction temperature and pressure, due to the thermodynamic stability of CO2 molecule.8 In order to overcome this issue, various catalysts have been reported in the past decades. The homogeneous catalysts such as ionic liquids (ILs),9 organic and metal complexes10 exhibited © XXXX American Chemical Society

outstanding activities for catalytic conversion of CO2 under moderate conditions, while the difficulties of catalyst separation, purification and recycling make them fail to satisfy the demands of sustainable and economical industrial applications. Meanwhile, more attentions are focused on the development of heterogeneous catalysts like functional zeolites,11 porous ionic polymers,12 graphite oxide.13 And especially, MOFs with high Received: March 7, 2018 Revised: May 24, 2018

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DOI: 10.1021/acssuschemeng.8b01055 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) A view of the asymmetric unit and some symmetry-related atoms in MOF-Zn-1. (Symmetry codes: (i) 0.5-x, 0.5-y, -z.), (b) the 2D sheet structure. squares based on F2 was performed were uesed to solve and refine structure.27 FT-IR spectra were within the scope of 4000−450 cm−1 (KBr pellets) by using PerkinElmer Spectrum 100 FT-IR Spectrometer. Powder XRD (PXRD) patterns were measured by using a Bruker D8 Advance X-ray diffractometer with CuKα radiation (40 kV, 40 mA) in the 2θ range of 5−50 o for phase identification. TG-DSC was performed on a NETZSCH STA449F3 simultaneous thermal analyzer under air atmosphere during 25 to 800 °C (10 °C/min). Gas adsorption measurements were performed on a Micromeritics ASAP2020 system, N2 adsorption−desorption isotherms were collected at 77 K, CO2 isotherms were obtained at 273 and 298 K, respectively. ICP-AES measurement was conducted on an ICP-OES Optima 8300 instrument from PerkinElmer. The morphology and particle size of samples were detected by SEM (Hitachi SU8000). Reaction product were determined by GC analysis using Agilent GC7890A. Preparation of MOF-Zn-1 Catalyst. MOF-Zn-1 was prepared by solvothermal crystallization method. Typically, 0.50 mmol Zn(NO3)2· 6H2O (0.15 g), 0.33 mmol melamine (MA, 0.04 g), 0.50 mmol 2,5thiophenedic- arboxylic acid (H2L, 0.08 g) and 10 mL N,Ndimethylformamide (DMF) were mixed uniformly and added in a 25 mL Teflon-lined autoclave. The autoclave was heated to 100 °C with a rate of 5 °C/min and kept at 100 °C for 72 h. After being cooled to 20 °C with a rate of 5 °C/min, colorless prism crystals were obtained. Then crystals were washed with DMF and dried at 60 °C for 24 h. Catalytic Conversion of CO2 into Cyclic Carbonate. All experiments for CO2 conversion were conducted in stainless-steel autoclave (50 mL) and kept suitable stirring. Typically, 34.5 mmol propylene oxide (PO), MOF-Zn-1 catalyst 0.1 g and fixed mass of cocatalyst Bu4NBr were placed in high-pressure autoclave. The air in the autoclave was first replaced by inletting CO2 slowly. Then, the autoclave was heated to 80 °C and CO2 was introduced to target pressure. The autoclave was kept stirring for a given reaction time. After finishing the reaction, stainless-steel autoclave was cooled to room temperature and depressurized slowly. The product was diluted and analyzed on GC. The MOF-Zn-1 was separated by simple centrifugation and washed with ethyl acetate, then dried at 60 °C for 24 h and reused directly for the next run.

surface area, tunable functionalities, adjustable and ordered porous structure, and excellent CO2 adsorption capacity, have been drawn great attentions in CO2 conversion. Recently, typical MOFs such as ZIF-67,14 ZIF-90,15 gea-MOF-1,16 and BIT-10317 have been developed and presented excellent catalytic performance under high temperatures. Even some MOFs such as USTC-253, MIL-53, MIL-101,18 MOF-505,19 UIO-66,18 exhibited moderate catalytic performance under room temperature for over 48 h. To meet the requirement of industrialization for conversion of CO2 into cyclic carbonates,20 more effective MOF catalysts need to be developed for proceeding the reaction under milder conditions, especially at ambient temperature to further reduce the energy consumption and production cost. Pioneering theoretical and experimental results have proved that the synergistic effects of Lewis acid−base sites and the assistance of nucleophile are crucial for CO2 cycloaddition to epoxide,21,22 wherein epoxide substrate and CO2 species are activated by Lewis acid and Lewis base, respectively, then nucleophile helps promote the ring-opening reaction of epoxide.23 Moreover, the existence of accessible hydroxyl group and nitrogen-rich units in the structures of porous MOFs help to further improve the affinity of CO2 molecule to the catalysts.24,25 Taking these into account, the porous MOF-Zn-1 {[Zn2MA(H2L)]} with multifunctional groups of −COOH and −NH2 was designed and facilely assembly prepared, and the catalytic performance was investigated toward the chemical transformation of CO2 to cycle carbonates under different reaction parameters. The scope of substrates as well as catalyst recyclability was examined. According to the experimental results, feasible synergistic mechanism catalyzed by MOF-Zn1/Bu4NBr catalysts was proposed. It should be noted herein that the developed bifunctional MOF-Zn-1 is both an active CO2 adsorbent and catalyst, the activity is comparable or superior to the newly reported MOFs (as shown in Table 3). The MOF-Zn-1 cooperating with Bu4NBr can catalyze the CO2 conversion into cyclic carbonates under room temperature, 1.0 MPa CO2 pressure and solvent-free conditions with excellent product yield and selectivity. Also the heterogeneous MOF-Zn1 can be easily recycled with high structural stability.

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RESULTS AND DISCUSSION MOF-Zn-1 Catalyst Characterizations. X-ray Crystallography. Single crystal X-ray diffraction analysis reveals that MOF-Zn-1 crystallizes in the orthorhombic space group Cmcm and displays a 3D framework with channels of dimensions 10.07 Å × 10.11 Å running along the [1 0 0] directions. Figure 1a shows there is one crystallographically unique Zn(II) center, two halves of L2− anion and one-half of MA molecule in the asymmetric unit contains. The Zn(II) center is located in a slightly distorted square pyramid and connected with one N atom from the MA molecule and four O atoms from four

EXPERIMENTAL SECTION

Materials and Characterizations. All chemicals required for this work were commercially available and used as received. Single crystal structure analysis was carried out using an Agilent Technology SuperNova Eos Dual system (Mo−Kα, λ = 0.7107 Å) at 293 K, data reduction and cell refinement were processed using CrysAlisPro.26 The SHELXS-2014 and SHELXL-2014 programs and full-matrix leastB

DOI: 10.1021/acssuschemeng.8b01055 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. (a) The 3D framework, (b) the topology structure of MOF-Zn-1.

Figure 3. (a) PXRD patterns, (b) FT-IR spectra of MOF-Zn-1.

Figure 4. (a) TG-DSC curves of MOF-Zn-1, (b) PXRD patterns of ZnO sample and MOF-Zn-1 remains.

different L2− ligands, in which the four O atoms define a leastsquares plane with average deviation 0.001 Å and the axial position is occupied by N atom with distance 2.473 Å from the least-squares plane. The average distances of Zn−N and Zn−O are 2.05 and 2.02 Å, respectively, which agree with the results in previous reports.28,29 the L2− anions are from H2L ligand and adopts μ4-kO: kO′: kO″: kO‴ bridging modes to coordinate with four Zn(II) centers, that is to say each carboxylate groups of H2L ligand adopts the bis-monodentate mode to coordinate to two Zn(II) centers, therefore, the Zn center and the symmetry-related one (Zn1i) are linked by four L2− ligands into a paddle wheel [Zn2(CO2)4] fragment with Zn−Zn distance of 3.0343(2) Å), and the fragment as subunit connects to four neighboring ones via four L2− ligands to form the 2D sheet

network structure on bc plane with rectangle cavies with dimensions of ≈10.07 Å × 10.11 Å (Figure 1b). The MA ligands are two linkers and utilize two N atoms on the triazine ring to bridge two Zn centers with free amino groups. Thus, MA ligands bridge two adjacent 2D network into a 3D framework with rectangle channels of dimensions 10.07 Å × 10.11 Å running along the [1 0 0] directions (Figure 2a), the free amino groups are inside of the channels. In the view of topological structure, each the paddle wheel [Zn2(CO2)4] fragment can be viewed as a six-connected node and each L2− ligand and MA molecule are linkers, the whole framework can be simplified into a 6-connected topological network with the Schläfli symbol (412·63) (Figure 2b). More details of X-ray crystallography analysis are given in the Supporting Information C

DOI: 10.1021/acssuschemeng.8b01055 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 5. (a) N2 adsorption−desorption isotherm of MOF-Zn-1 and the pore size distribution calculated from desorption isotherm, (b) CO2 adsorption isotherms of MOF-Zn-1.

feature. The hysteresis phenomenon may be caused by hindered diffusion through the narrow pore apertures. The BET surface area and t-plot microporous area for MOF-Zn-1 were 1006.3 m2/g and 924.8 m2/g, respectively, indicating the existence of microporous. The pore size distribution of MOFZn-1 was calculated based on desorption isotherm using nonlocal DFT (density functional theory) and its pore volume was calculated to be 0.40 cm3/g by Horvath−Kawazoe equation. N2 adsorption measurement further confirms the highly porous structure of MOF-Zn-1. High porosity and the existence of abundant nitrogen-rich groups within the framework of MOF-Zn-1 encouraged us to investigate its affinity toward CO2 species. The CO2 adsorption capacity of MOF-Zn-1 was evaluated at 273 and 298 K, respectively. As shown in Figure 5b, MOF-Zn-1 shows excellent CO2 uptake performance with the values of 79.7 cm3/g at 298 K and 108.9 cm3/g at 273 K, which are higher than several reported MOFs. For example, NTU-105,24 USTC-253,18 and [Cu3(cpbda)2(H2O)3](DEF)4,35 showed CO2 uptake values of 94.1, 82.4, and 102 cm3/g at 273 K, with their BET surface areas of 3543, 1800, and 1926 m2/g, respectively. Based on these, the adsorption heat (Qst) was calculated to be 17.8 kJ/ mol,36 showing the relatively weaker interaction between CO2 molecular and Zn-MOF-1 compared to some reported Znbased MOFs such as Zn4O(BDC)(BTB)4/3 (UMCM-1, 12 kJ/ mol), Zn4O(BDC-NH2)3 (IRMOF-3, 19 kJ/mol)37 and [Zn9(OH)2L6](H3O)2(H2O)6 (27 kJ/mol).25 The result demonstrated that the physisorption to CO2 played the main role for the presented Zn-MOF-1, suggesting the ease of desorption and adsorbent regeneration. Coupling of CO2 and Epoxides under Mild Conditions. Previous reports have proved that metal sites in the framework can promote the ring-opening process by forming metal− oxygen adducts.38 The prominent CO2 sorption capability and the exposed Zn sites may give rise to high activity of MOF-Zn-1 for cycloaddition of CO2 to epoxides. Therefore, the catalytic activity was explored for coupling of CO2 and propylene oxide (PO) over MOF-Zn-1 under mild conditions (Table 1). By screening, a little propylene carbonate (PC) was obtained with respective cocatalyst KBr, KI, or NaCl. Moreover, compared with cocatalyst Bu4NI (PC yield, 27%), the cocatalyst Bu4NBr showed superior synergistic effect with MOF-Zn-1, and excellent product yields could be obtained at 80 °C and ambient temperature. As shown in Table 1, MOF-Zn-1/ Bu4NBr catalysts achieved 99% of PC yield under mild reaction conditions (80 °C and 1.0 MPa, 3 h), and even satisfied

(SI), including crystal data and structure refinement (SI Table S1), selected bond lengths and bond angles of MOF-Zn-1 (SI Table S2). Powder XRD and FT-IR Characterizations. Figure 3a shows the PXRD patterns of prepared MOF-Zn-1, the results were well matched with the simulated single crystal pattern based on the diffraction data, which confirmed the structural integrity and phase purity of the synthesized MOF-Zn-1sample. FT-IR was conducted to detect and characterize the composition and chemical groups of MOF-Zn-1 (Figure 3b), the double peaks at 3447 and 3401 cm−1 evidenced the existence of primary amine.30 The broad band appeared at 3240 cm−1 and sharp peak at 1657 cm−1 were attributed to O−H and CO stretching vibrations,31 due to the existence of carboxyl groups in L ligand. The peak at 1098 cm−1 was assigned to C−O bending vibration.32 And the bond located at 522 cm−1 was assigned to Zn−O stretching vibration,33 which was consistent with X-ray photoelectron spectroscopy analysis (SI Figure S1). Thermal Analysis. The TG-DSC was conducted to detect the thermal stability and skeleton construction of MOF-Zn-1. As shown in Figure 4a, the DSC curve shows three exothermic peaks which were attributed to the ordered decomposition of skeletal structure of MOF-Zn-1. The DSC result was consistent with the TG analysis as follows. The structure of MOF-Zn-1 was sequentially disintegrated into three steps with the increase of temperature. The initial 19.2% weight loss before 300 °C was assigned to the removal of DMF solvent, this was similar to the theoretical content of 19.6%. While there was no corresponding exothermic peak in the DSC curve, indicating that DMF molecules were sealed in the framework of MOF-Zn-1, and did not form stable coordination bonds with Zn atoms.34 During 300−600 °C, a sharp weight loss of 47.0% was occurred. The weight loss was calculated to the decomposition of two H2tdc organic ligands per formula (cal. 46.2%), suggesting the framework of MOF-Zn-1 was totally collapsed. Subsequently, the other organic ligand MA disintegrated and oxidized in the air, the weight loss was 12.1% (cal. 12.6%). Finally, the remaining mass kept 21.7%, which was due to the formation of ZnO because of the oxidation of Zn atoms in the air (cal. 21.9%). To confirm this, XRD was conducted to further analyze the structure of thermolysis residue (Figure 4b), the main diffraction peaks were matched completely with ZnO. Gas Adsorption Properties. Porosity and surface area of MOF-Zn-1 were confirmed by N2 adsorption analysis at 77 K. As illustrated in Figure 5a, MOF-Zn-1 exhibited type I adsorption isotherm behavior, evidencing its microporous D

DOI: 10.1021/acssuschemeng.8b01055 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6b, the PC yield increased sharply and over 95% PC yield was obtained within 3.0 h. When the time was further extended to 4.0 h, there was no obvious enhancement on the PC yield as most PO has been converted. Figure 6c shows the effect of CO2 pressure on the PC yield. Under a low CO2 pressure of 0.5 MPa, approximate 80% PC yield could be obtained. And it rapidly reached to 95% when CO2 pressure was increased to 1.0 MPa, indicating the positive impact of CO2 concentration at low pressure. However, when reaction pressure was further increased from 1.0 to 4.0 MPa, there appeared a slightly negative fluctuation on the PC yield. On the other hand, higher pressure requires stricter safety management and more production costs. Hence, 1.0 MPa was suitable for the CO2 coupling reaction. As MOF-Zn-1 alone displays no activity, cocatalyst Bu4NBr was introduced to cooperate with MOF-Zn-1 for the coupling of CO2 and epoxides. The influence of cocatalyst dosage on catalytic performance was investigated (Figure 6d). With the increase of Bu4NBr dosage from 25 mg to 100 mg, the PC yield was gradually enhanced. However, no enhancement was observed with further increasing Bu4NBr amount, while the selectivity was almost kept above 99%. Hence, the catalytic system with the combination of 100 mg MOF-Zn-1 and 100 mg cocatalyst Bu4NBr was performed. Recyclability of MOF-Zn-1 Catalyst. Recyclability is a vital advantage for heterogeneous catalyst compared with the homogeneous catalyst. The reusability of MOF-Zn-1 was studied for repeated reactions of the PC synthesis. As shown in Figure 7a, it was observed that MOF-Zn-1 could be well reused, and almost 85% PC yield with above 98% selectivity was still

Table 1. Catalytic Performance of MOF-Zn-1 under Mild Conditionsa cocatalyst (g)

catalyst

Bu4NBr MA MA+Zn(NO3)2•6H2O MOF-Zn-1 MOF-Zn-1 MOF-Zn-1 MOF-Zn-1 MOF-Zn-1 MOF-Zn-1 MOF-Zn-1

a

KI NaCl KBr Bu4NI Bu4NBr Bu4NBr

time (h)

temp. (°C)

yield (%)

3 3 3 3 3 3 3 3 3 24

80 80 80 80 80 80 80 80 80 25

23