Preferential CO2 adsorption and theoretical simulation of a Cu(II)-based metal-organic framework with open-metal sites and basic groups

Accepted Manuscript Preferential CO2 adsorption and theoretical simulation of a Cu(II)-based metal-organic framework with open-metal sites and basic groups

Cong-Li Gao, Ju-Yin Nie PII: DOI: Reference:

S1387-7003(19)30015-2 https://doi.org/10.1016/j.inoche.2019.02.029 INOCHE 7287

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

5 January 2019 18 February 2019 18 February 2019

Please cite this article as: C.-L. Gao and J.-Y. Nie, Preferential CO2 adsorption and theoretical simulation of a Cu(II)-based metal-organic framework with open-metal sites and basic groups, Inorganic Chemistry Communications, https://doi.org/10.1016/ j.inoche.2019.02.029

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ACCEPTED MANUSCRIPT Preferential CO2 adsorption and theoretical simulation of a Cu(II)-based metal-organic framework with open-metal sites and basic groups

Cong-Li Gao1* and Ju-Yin Nie2 Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light

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Industry, Zhengzhou 450002, People's Republic of China Henan Experimental High School, Zhengzhou 450002, People's Republic of China

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* Corresponding author E-mail address: [email protected]

Inorg. Chem. Commun

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ACCEPTED MANUSCRIPT Abstract. By utilizing the  1,3,5-benzenetricarboxylic acid (H3btc) and the N-rich melamine (ma) as the co-ligands, a new microporous cluster-based metal-organic framework [Cu3(btc)2(ma)(H2O)2](DMA)4 (1) containing open metal sites and uncoordinated nitrogen atoms on the internal surface was solvothermally synthesized. The single crystal X-ray study reveals that compound 1 is built up from the {Cu3(ma)(COO)6(H2O)2} cluster-based secondary building unit (SBU) and incorporates two types of polyhedral cages in the framework, which

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results in 1D nanosized channels along the c axis. More importantly, the activated compound 1 exhibits not only high uptake capacity for CO2 molecules at room temperature but also a significant selective adsorption of CO2

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over CH4, which may be ascribed to the proper-sized pores with high density of open metal sites as well as the amine and triazine decorated pore surroundings. Meanwhile, the Grand Canonical Monte Carlo (GCMC) simula-

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tions on CO2 adsorption of compound 1 demonstrate that the both of the open metal sites and melamine backbone

Keywords:

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play key roles in the CO2 binding.

Metal-organic framework; melamine ligand; {Cu3} cluster; Selective CO2 sorption, GCMC simu-

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lation.

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As the major component of natural gas, methane is not only a prevalent and inexpensive fuel for industry and residential use but also a useful C1 feedstock in chemical and petrochemical industries for the production of vari-

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ous C1 and C2 chemicals, such as acetylene and chloromethanes [1]. Furthermore, methane has also been used in the dominions of natural gas heating and fuel gas cars [2]. For chemical and petrochemical applications, impuri-

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ties in methane as a starting material may complicate the product isolation processes. Moreover, certain impurities may inhibit the conversion of methane and lower its efficiency. For example, carbon dioxide poisons the Li/MgO catalyst that converts methane into ethane and ethylene [3]. Natural gas contains 80–95% of methane with various amounts of carbon dioxide presented, which requires further purification prior to various chemical processes. Carbon dioxide sequestration is efficiently achieved by amine scrubbers (chemisorption), whose regeneration, however, is energetically costly [4]. The need for the development of lower cost CO2 capture technology has led to the proposed application of porous solids to that end. 2

ACCEPTED MANUSCRIPT In the past few decades, the field of metal–organic frameworks (MOFs) has underwent a rapid development not only due to their beautiful architectures but also because of their importantly potential applications in gas adsorption and separation, catalysis, fluorescent sensing and so on [5-10]. The adjustable nano-spaces and functionalities combined with accessible structural modification make MOFs especially suitable for the application in the selec-

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tive CO2 sorption [11]. According to the reported results, the selective CO2 sorption performance in MOFs does

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not rely on the high surface areas and large pore aperture but is largely dependent on the framework-CO2 interac-

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tions [12]. To date, various feasibility strategies, such as the introduction of a high density of open metal sites, charged skeleton of MOFs, and decoration with polar substituent groups have been explored to enforce their in-

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teractions and thus enhance the adsorption selectivity of MOFs toward CO2. The high selectivity arises from the

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enhanced framework-CO2 interaction and cooperative CO2-CO2 interactions [13]. To boost the framework-CO2 interaction, MOFs with small pore size or interpenetrated networks are usually welcomed. However, the increased

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CO2 sorption selectivity usually accompanies with declined CO2 sorption capacity [12]. Recent literatures have

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revealed that the MOFs with 1,3,5-triazine decorated backbone could exhibit a balance between large storage capacity and high selectivity for CO2 [14, 15]. To make a further modification of the framework-CO2 affinity, in this

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study, we choose the amino triazine ligand melamine given its high density of Lewis basic sites (LBSs). Interest-

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ingly, the resulting MOF displays excellent adsorption selectivity for CO2 over methane. Meanwhile, the Grand Canonical Monte Carlo (GCMC) simulations on CO2 adsorption of compound 1 demonstrate that both of the open metal sites and melamine backbone play key roles in the CO2 binding.

Crystals of [Cu3(btc)2(ma)(H2O)2](DMA)2(H2O)3 (1) could be prepared by reaction of Cu(NO3)2·3H2O, H3btc and ma under solvothermal conditions in good yield. Single-crystal structural analysis reveals that complex 1 crystallizes in the tetragonal space group I4/m and has a three-dimensional (3D) porous neutral framework constructed from trinuclear {Cu3(ma)(COO)6(H2O)2} SBUs. The asymmetric unit of 1 contains one and a half Cu(II)

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ACCEPTED MANUSCRIPT cations, one btc3- ligand, a half ma ligand and one coordinated water molecule (Fig S1). The Cu1 cation on the 2-fold axes is six-coordinated with four carboxyl O atoms, one amido N atom and one coordinated water, forming an octahedral coordination environment; The tetrahedral coordination environment of Cu2 cation is finished by three carboxyl O atoms and one amido N atom. The ma ligand uses its three N donors to “capture ” three different

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Cu atoms to generate a trinuclear {Cu3(ma)(COO)6(H2O)2} cluster (Fig 1a). Furthermore, there are two types of

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polyhedral cages in the framework of 1. The cage-I with the inner hole of 16.3820.32 Å2 is formed by 12

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{Cu3(ma)(COO)6(H2O)3} clusters and 32 btc3- ligands; The cage-II is shaped by 6 {Cu3(ma)(COO)6(H2O)2} clusters and 6 btc3- ligands, which reveals an inner hole of 16.169.72 Å2 (Fig 1b). Each cage-I is surrounded by four

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adjacent cage-II via sharing the four vertexes and two triangular faces. The packing of the two types of cages in

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the three-dimensional direction results in the 3D framework of 1 with two types of 1D channels along the c axis (Fig 1c). The channel A with a pore aperture of 9.72 Å fills with water coordination unsaturated Cu(II) centers,

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and the channel B with a pore aperture of 6.18 Å fills with open O donors, which indicate their usage in the CO2

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binding. PLATON analysis showed that the effective free volume of 1 is 61.2% of the crystal volume (6662 out of 10799 Å3). From a topological point of view, the {Cu3} cluster could be considered as a 6-connected node and the

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btc3- ligand could be judged as a 3-connected node, so the overall 3D network of 1 can be rationalized as a

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(3,6)-connected topology with the point symbol of {4^2.6}2{4^4.6^2.8^9} which has not been reported in the TOPOS database (Fig 1d and Fig S2).

(Insert Fig. 1)

The crystalline phase purity of 1 was confirmed by comparing experimental powder X-ray diffraction (PXRD) patterns with the simulated one from the single-crystal data, and the PXRD measurements show that the framework of 1 can be highly stable in water for more than one day (Fig S3). The thermogravimetric analysis (TGA) curve of 1 shows a continuous weight loss of 34.8% (calcd 34.4%) from 30 C to 270 oC, corresponding to the re4

ACCEPTED MANUSCRIPT lease of four lattice DMA and two coordinated water molecules, followed by a relatively steady plateau up to 370 o

C after which the framework starts to decompose (Fig S4). The activated samples were prepared by exchanging

the solvent in the as-synthesized 1 with CH3OH, followed by evacuation under high vacuum at 100 oC for 10 h to produce the activated 1 (1a). A comparison of PXRD patterns and TGA curves of the as-synthesized and activated

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1 reveals that 1a keeps its framework integrality without any reside solvents in the crystal lattice. Permanent po-

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rosity of 1a was confirmed by N2 adsorption–desorption isotherms at 77 K, which showed a reversible type-I iso-

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therm with the maximum uptake capacity of 400 cm3/g (Fig S5). Based on the N2 adsorption isotherm, the Brunauer–Emmett–Teller (BET) and Langmuir surface areas of 1a are calculated to be 1386 and 1540 m2/g, re-

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spectively. The total pore volume calculated from the N2 isotherms is 0.58 cm3/g, which is similar to the value

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calculated from single-crystal data. Meanwhile, according to the pore size distribution analysis from the N2 adsorption isotherm at 77 K using density functional theory (DFT), the pore size of 1a is in the range of 5.6–9.4 Å),

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which are in good accordance with the channel windows observed in the crystal structure from the [001] direction

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(Fig S5 inset).

The CO2 and CH4 adsorption behaviors of compound 1a were further measured to explore the gas storage and

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selective property. As shown in Fig. 2a, the CO2 adsorption capacity of 1a is 192 cm3/g at 273 K and 118 cm3/g at

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298 K and 1 atm. It should be noted that the CO2 adsorption capacity of 1a at 298 K is much higher than many famous MOFs with much higher surface areas such as PCN-61, NOTT-140 and NU-100 under the same conditions [16-18]. The high CO2 adsorption capacity of 1a might be attributed to the polar amine groups, triazine decorated backbone and the high density of open metal sites. The loading amount of CH4 is 31.8 at 273 K and 21.6 cm3/g at 298 K, respectively. As illustrated in Fig. 2a, the uptake values of CO2 are much higher than the corresponding CH4 adsorption amounts at 273 K and 298 K, indicating that 1a might exhibit a selective adsorption of CO2 over CH4. The calculated CO2/CH4 (10:90, V/V) adsorption selectivity based on the ideal absorbed solution

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ACCEPTED MANUSCRIPT theory (IAST) is 33 at 273 K and 10 at 298 K and 1 atm (Fig 2b), which is comparable to the high selectivities of CO2 over CH4 in the some benchmark compounds such as Mg-MOF-74 and Cu(bpy)2(SiF6) [19, 20]. The high CO2 uptake capacity and CO2/CH4 sorption selectivity make complex 1a hold the potential for the practical application for CO2 separation. The isosteric heats (Qst) of 1a for the CO2 and CH4 were calculated using the virial

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method, which is a well-established and reliable methodology fitting from their adsorption isotherms at 273 and

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298 K (273, 288, and 298 K for CO2). For CO2, the isosteric heat at zero coverage is 34 kJ/mol, which is relative

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high among MOFs with both open metals sites and polar N atom sites, reflecting the presence of strong binding sites for the CO2 molecules in the framework of 1a (Fig S9) [21-23]. The isosteric heats further decrease to 24

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kJ/mol with the uptake increased to 3.43 mmol/g, which could be attributed to the occupancy of the favorable

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binding sites by the CO2 molecules. The intermolecular CO2…CO2 interactions may account for the uptrend of Qst at high loading. In comparison, the isosteric heats for CH4 are in the range of 14-12 kJ/mol in the whole loading

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process. The much higher isosteric heats for CO2 than CH4 indicating the framework of 1a is more favorable for

(Insert Fig. 2)

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CO2 than CH4.

To probe the nature of CO2 adsorption at the molecular level, grand canonical Monte Carlo (GCMC) simulation

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was used to evaluate the host–guest interactions between 1a and CO2 at 298 K. At zero coverage, a CO2 molecule is chelated by two Cu(II) ions (Cu-O 2.593 and 2.797 Å) via M···O(δ−)=C(δ+)=O(δ−) (head-on interaction, Fig 3a). At high coverage, the adsorbed CO2 molecules mainly locate around the open metal sites and the N-rich thiazine ligand and the CO2…CO2 interactions can also be observed. As shown in the Fig S10, some of the CO2 molecules arrange themselves into a typical T-shaped configuration where the δ− oxygen of one CO2 interacts with the δ+ carbon of another CO2 molecule, which might account for the uptrend of Qst at high loadings. Furthermore, the slice through the calculated CO2 density distribution was shown in the Fig 3b. Two preferential adsorption 6

ACCEPTED MANUSCRIPT regions are showed in the field. As anticipated, the highest values are located around the unsaturated metal centers and the N-rich ma ligands. (Insert Fig. 3)

In conclusion, we have successfully prepared a new mixed ligand microporous MOF containing both open met-

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al sites and N-donor polar sites. The activated compound 1 exhibits not only high uptake capacity for CO2 mole-

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cules but also a significant selective adsorption of CO2 over CH4 at room temperature. GCMC calculations reveal

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that both of the open metals sites and the N-rich ma ligands within the framework have synergistic effects on CO2

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binding. The selective adsorption properties of 1 make it a promising candidate for methane purification and recycling in the future.

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Acknowledgements

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We are grateful to the support from the National Natural Science Foundation of China (21701150).

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ACCEPTED MANUSCRIPT Captions for schemes and figures Fig. 1

(a) View for the {Cu3(ma)(COO)6(H2O)3} cluster in 1; (b) View for the two different types of cages in 1; (c) View for the 1D channels of 1 along the [001] direction; (d) The simplified (3,6)-connected topological network for 1.

(a) CO2 and CH4 sorption isotherms of 1a at different temperatures; (b) CO2/CH4 selectivity of 1a cal-

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Fig. 2

Fig. 3

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culated using the IAST model based on the dual-site Langmuir-Freundlich model.

(a) GCMC simulation derived primary adsorption site for CO2 at zero coverage and 298 K; (b) The slice

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showing the calculated CO2 density distribution at 298 K and 1 bar.

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Graphical Abstract-Pictogram

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ACCEPTED MANUSCRIPT Graphical Abstract-Synopsis

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A new cluster-based microporous metal-organic framework containing open metal sites and uncoordinated nitro-

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gen atoms on the internal surface was solvothermally synthesized. The activated compound 1 not only exhibits

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high uptake capacity for CO2 molecules at room temperature but also a significant selective adsorption of CO2

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over CH4 at room temperature, which may be ascribed to the proper-sized pores with high density of open metal

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sites as well as the amine and triazine decorated pore surroundings as revealed via the GCMC simulations.

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Research Highlights A new porous mixed-ligand Cu(II)-organic framework has been synthesized.

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It is composed of polyhedral cages and nano-sized 1D channels.

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It exhibits high uptake capacity for CO2 and a significant selective adsorption of CO2 over CH4 at

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room temperature.

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