Pore Space Partitioning of Metal–Organic Framework for C2Hx

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Pore Space Partitioning of Metal−Organic Framework for C2Hx Separation from Methane Jia Li,* Sheng Chen, Lianyan Jiang, Dapeng Wu, and Yanshuo Li* School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China

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S Supporting Information *

coordination between secondary building units (SBUs) and different organic linkers. Herein we report a Zn2(O2CR)4 paddlewheel-unit-based framework, NbU-5 (NbU denotes Ningbo University), namely, [Zn2(ptptc)(bpy)]·2DMF·2.5H2O (H4ptptc = pterphenyl-3,3″,5,5″-tetracarboxylic acid; bpy = 4,4-bipyridine), using a new kind of PSP strategy: Zn2(O2CR)4 paddlewheel units connect tetracarboxylic organic linkers to build a primary framework, and 4,4-bipyridine acting as a pore-partitioning agent is successfully inserted into the cage of the primary framework. Each large cage has been fragmented into six small cages and a 1-D channel caused by the partitioning of 4,4bipyridine ligand, which is very useful to enhance the host− guest interaction (Figure 1).

ABSTRACT: To develop efficient adsorbent materials for C2Hx separation from methane, herein, we design a new 3-D framework (NbU-5) using the pore space partition strategy: Zn2(O2CR)4 paddlewheel units connect tetra-carboxylic linkers to build a primary framework, and 4,4-bipyridine, which acts as a pore-partitioning agent, has been successfully inserted into the cage of the primary framework. Remarkably, NbU-5 exhibits excellent C2H2/ CH4 and C2H6/CH4 selectivity, which is proved by breakthrough experiments.

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as separation is a large investment and an energyconsuming process.1,2 An important example is C2Hx separation from methane, which involves the separation of ethane from natural gas, the recovery of ethane from MTO (methanol to olefin) effluent, as well the demethanizer unit of a typical thermal cracking system. The current state-of-the-art demethanation separation processes involve the cryogenic separation of C2Hx and methane at temperatures as low as −90 °C, a very expensive process due to both the capital cost and the operating costs. Nonthermally driven separation technology using porous materials has received widespread attention with the considerations of cost-efficiency and environmental compatibility.3−6 Compared with traditional porous materials such as molecular sieves, activated carbon, and so on, a new family of porous adsorbent materials, metal−organic frameworks (MOFs), have become powerful competitors due to their attractive characteristics of high surface area, tunable porosity, and controllable structure.7−9 The trade-off between the adsorption capacity and the selectivity of porous materials is a major barrier for efficient gas separation. Recently, a number of strategies have been developed to improve the performance of MOFs for gas storage and separation, including the creation of open metal sites and the implantation of functional organic groups (e.g., −NH2, −F, and −OH) into small pore spaces.10−12 However, these strategies are not workable in MOFs with large pores. Pore space partition (PSP) strategy provides one solution to this problem,13−15 which was proposed by Bu’s group and refers to inserting molecular-scale walls into the original large cage or channel to form smaller segments to enhance the host−guest interactions of the primary frameworks. Although several reports have confirmed that PSP is an effective approach for gas storage, implementing this strategy is still a great challenge, partly because the competition interactions between organic linkers may lead to a less controllable © XXXX American Chemical Society

Figure 1. Illustration of pore space partition through 4,4-bipyridine acting as a pore-partitioning agent.

The single-crystal X-ray diffraction study reveals that NbU-5 crystallizes in a trigonal R3̅m space group. Each Zn2+ ion is coordinated by four oxygen atoms and one nitrogen atom, forming a pentagonal shape. Two Zn2+ ions are linked by four carboxylate oxygen atoms, generating the classic paddlewheel [Zn 2 (COO) 4] SBUs, with one nitrogen atom at the paddlewheel axis. If the pore-partitioning agent is omitted, then each SBU in the primary framework is bridged by four terphenyl carboxylate ligands, and each ligand is linked by four paddlewheel clusters (Figure 2a), thus forming NbO-type topologies with a Schläfli symbol of (6482), which is isoreticular to the structure of NOTT-101.16 In NOTT-101, the metals in the paddlewheel units are usually terminated by pendant solvent molecules along the paddlewheel axis, which could be easily removed, leaving potential open metal sites (OMSs). Herein, for NbU-5, by inserting the bpy ligands, the pendant molecules are superseded by pyridine nitrogen atoms, which means the loss of OMSs and occupation by bpy ligands. Received: February 25, 2019

A

DOI: 10.1021/acs.inorgchem.9b00550 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

exchanged samples were degassed at 80 °C under a dynamic vacuum for 24 h to give fully activated NbU-5. As shown in Figure S2, the stability of the activated NbU-5 has been verified by a powder X-ray diffraction study. NbU-5 exhibits a typical type-I adsorption behavior and N2 uptake of ∼234.5 cm3·g−1 at 1 bar (Figure S5). On the basis of N2 isotherms, the BET surface area and Langmuir surface areas were given to be 671.3 and 1015.8 m2·g−1. The Horvath−Kawazoe method shows a uniform pore distribution of about 4 to 5 Å, which is consistent with the result of structural analysis. Simultaneously, the CO2-adsorptive ability of NbU-5 was also studied (Figure S4). The uptakes of CO2 (1 atm) are 84.4 cm3·g−1 at 273 K and 50.5 cm3·g−1 at 298 K, respectively. It should be noted that these values are slightly lower than those of NOTT-101.19b At zero loading, the enthalpy of CO2 adsorption is 31.9 kJ·mol−1, estimated from the sorption isotherms at 273 and 298 K using the virial equation. Thus the implementation of the PSP strategy, which causes the loss of open metal sites and the division of a large cage in NbU-5, leads to the lower surface area and CO2 uptake. If a PSP strategy is to be successfully implemented to enhance gas adsorption, then the framework must have moderate pore sizes to capture the gas molecules. The uptake of industrially important light hydrocarbons was investigated (Figure S6a,b). As expected, NbU-5 exhibits a higher adsorption capacity for light hydrocarbons, but the values are much lower than those of NOTT-101 (Table1). At zero loading, the values of the adsorption heat for C2H2 (32.1 kJ mol−1), C2H4 (29.1 kJ mol−1), C2H6 (27.3 kJ mol−1), and CH4 (20.6 kJ mol−1) are correlated with the adsorption capacity, showing the trend C2H2 > C2H4 ≈ C2H6 > CH4 (Figure S12). Notably, the adsorption value of C2H2 is remarkable and is much higher than those of the MOFs have nearly the same structures, BET surface areas, and Lewis basic nitrogen sites (Table S2). On the contrary, the C2Hx uptakes are nearly five to eight times greater than those of CH4 at both 273 and 298 K. These results imply that NbU-5 may be a good candidate material for C2Hx separation from methane. To study the role of PSP strategy in gas separation, NOTT101 was prepared according to the corresponding reported references. The pure component sorption isotherms of C2Hx and CH4 at 273 and 298 K were also examined. As shown in Figure S6a,b, the C2Hx uptake of NOTT-101 is about two times greater than that of NbU-5. We determined the C2Hx/ CH4 separation selectivity of both NbU-5 and NOTT-101 with ideal adsorbed solution theory (IAST) calculations (Figure S6c,d). The separation selectivity value of NOTT101 for the mixtures composed of equimolar binary C2H2/ CH4, C2H4/CH4, and C2H6/CH4 are 36.9, 30.0, and 11.1 at 273 K and initial pressure. Under the same condition, the values calculated for NbU-5 reach 52.9, 22.8, and 27.2, respectively. Clearly, the C2H2/CH4 and C2H6/CH4 adsorp-

Figure 2. Four-connected [Zn2(COO)4] paddlewheel unit (a), symmetrically disordered bpy ligands (b), and 3-D framework of NbU-5 (c).

Furthermore, three paddlewheel units connected to three isophthalate groups on each terminus of three H4ptptc ligands form a layer along the ab plane, and a cylindrical channel with a diameter of ∼6.4 Å is formed by connecting two adjacent layers through the bpy ligands. Because the bpy ligands are arranged in two ways (Figure S13), the orientational disorder occurs in the structure of NbU-5 (Figure 2b). As observed from the structural analysis, the strategy of PSP had been successfully implemented for MOFs with the typical paddlewheel SBUs. The accessible voids in NOTT-101 are calculated as ∼70.4% of the total volume and feature intersecting 1-D hexagonal channels with a diameter of ∼7.3 Å passing through the nanocages of ∼13.6 Å diameter.16 In comparison, by the successful implementation of PSP strategy, the nanocages in NOTT-101 were fragmented into seven small cages, which result in two kinds of intersecting 1-D channels with diameters of about 6.4 and 3.8 Å in NbU-5 (Figure 2c) (not taking into account the van der Waals radii). The larger channels (6.4 Å) with hexagonal windows are surrounded by bpy ligands and [Zn2(COO)4] SBUs, whereas small channels (3.8 Å) with approximately triangular windows are encapsulated by six [Zn2(COO)4] SBUs (Figure 2b,c). The PLATON calculation17 shows that the solvent-accessible volume of NbU-5 is 52.0% of the total volume, which is slightly lower than that of NOTT-101. It is well known that the NbO-type NOTT-101 series exhibit exceptionally high C2Hx storage capacities due to their highly open metal sites and adjustable functionalities.18−20 After the successful implementation of PSP strategy onto NOTT-101 to get NbU-5, we explored its potential to achieve high gas selectivity and storage capacity simultaneously toward hydrocarbon separation. The permanent porosity of NbU-5 was checked by N2 gas sorption at 77 K using a 3Flex analyzer (Micromeritics). Prior to gas measurement, the acetonitrile-

Table 1. Summary of Gas Storage and Separation Performance for NbU-5 and NOTT-101.

C2Hx uptake at 273 K (cm3/g)

IAST calculated selectivity

C2Hx captured from 0 min to the breakthrough time (cm3/g)

breakthrough time (min)

MOFs

BET (m2/g)

C2H2

C2H4

C2H6

CH4

C2H2/CH4

C2H4/CH4

C2H6/CH4

C2H2

C2H4

C2H6

C2H2

C2H4

C2H6

NbU-5 NOTT-101

671 2930

137 280

105 220

104 240

22 30

52.9 36.9

22.8 30.0

27.2 11.1

50 48

28 34

49.5 36

74.9 73.5

41.4 52.0

73.2 55.1

B

DOI: 10.1021/acs.inorgchem.9b00550 Inorg. Chem. XXXX, XXX, XXX−XXX

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the values of NbU-5 reach 5.1, 2.6, and 5.1, respectively. Apparently, the C2H2/CH4 and C2H6/CH4 adsorption selectivities were enhanced 25 and 55% in the actual separation process after pore space partitioning. In summary, a novel highly symmetric MOF, NbU-5, has been successfully constructed by inserting bpy ligands into a paddlewheel-unit-based framework, which has effectively explored the applied scope of PSP strategy. Thanks to the derived advantages of PSP strategy, NbU-5 shows remarkable C2Hx/CH4 selectivity without the existence of any open metal sites or functional organic groups. Not only are theoretical calculations obtained using the IAST method, but also the efficiency of the separation of C2Hx/CH4 mixtures is demonstrated by breakthrough experiments. Our work here demonstrates that the use of PSP strategy to create suitable pore sizes in MOF materials will offer new ways to address gas separation issues.

tion selectivities were significantly enhanced after pore space partitioning. We further examined the fully activated NbU-5 and NOTT101 in actual adsorption processes for C2Hx/CH4 mixtures through experimental breakthrough studies. An equal molar volume of mixed gas passes through a stainless-steel column at a flow rate of 1.0 mL/min, and highly efficient separations for C2Hx/CH4 mixtures are indeed realized. At 273 K, the breakthrough time of C2H2 takes place at approximately 48 and 50 min for NOTT-101 and NbU-5, which represent about 73.5 and 74.9 cm3 of C2H2 being retained per gram by NbU-5 and NOTT-101 under these dynamic conditions. The excellent separation performance of C2H6/CH4 further indicates that this PSP strategy has played a miraculous role: The breakthrough times of C2H6 for NbU-5 exceed those for the studied NOTT-101 (Figure 3e,f). The amount of C2H6

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00550. Experimental details and measurements and additional figures (PDF) Accession Codes

CCDC 1851494 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*J.L.: E-mail: [email protected]. *Y.L.: E-mail: [email protected]. ORCID

Jia Li: 0000-0002-8392-1125 Yanshuo Li: 0000-0002-7722-7962 Figure 3. Experimental column breakthrough curves for the separation of C2H2/CH4 (a,b), C2H4/CH4 (c,d), and C2H6/CH4 (e,f) mixtures by NbU-5 and NOTT-101.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 21701091, 21761132009, and 21622607), the Open Project of the State Key Laboratory of Physical Chemistry of the Solid Surface (Xiamen University) (201707), and the K. C. Wong Magna Fund in Ningbo University.

captured in NOTT-101 and NbU-5 during the breakthrough process is 55.1 and 73.2 cm3 per gram of adsorbent. We summarized the gas adsorption and separation properties of the two MOFs (Table 1), which indicates that the PSP strategy has a significant ability to enhance gas separation. For the breakthrough curve test of the two materials, there are two points worth noting: First, the C2Hx captured by the two materials from 0 min to the breakthrough time is much smaller than that under equilibrium conditions, especially the dynamic adsorption of NOTT-101. Taking C2H6 as an example, the amounts captured by NOTT-101 and NbU-5 are only 22.9 and 70.0% of their equilibrium adsorption amounts. This implies that the mechanisms of dynamic adsorption and static equilibrium adsorption are different. In addition, the molar ratios of the actual captured molecules of NOTT-101 for C2H2/CH4, C2H4/CH4, and C2H6/CH4 are 4.1, 3.6, and 3.3 from 0 min to the breakthrough time, whereas

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DOI: 10.1021/acs.inorgchem.9b00550 Inorg. Chem. XXXX, XXX, XXX−XXX