Advancing MagnesiumâOrganic Porous Materials through New

Article pubs.acs.org/crystal

Advancing Magnesium−Organic Porous Materials through New Magnesium Cluster Chemistry Quan-Guo Zhai,† Xianhui Bu,*,‡ Xiang Zhao,† Chengyu Mao,† Fei Bu,† Xitong Chen,† and Pingyun Feng*,† †

Department of Chemistry, University of California, Riverside, California 92521, United States Department of Chemistry and Biochemistry, California State University, Long Beach, California 90840, United States

‡

S Supporting Information *

ABSTRACT: While synthesis of materials with novel topologies is important, it can be even more impactful to create new modular structural units capable of serving as a versatile pool of connectors for rational design of targeted solids. Here we report five magnesium−organic frameworks (denoted as CPM-201 to -205) exhibiting step-by-step evolution of trimeric clusters from unprecedented core-less [Mg3(COO)6], to OH-centered [Mg3(μ3-OH)(COO)6]−, and eventually to two variations of pentamers [Mg5(μ3-OH)2(COO)8] formed from fusion of trimers. New clusters are joined by dicarboxylates into trigonalbipyramidal, cuboidal, and octahedral cages embedded in the labyrinth of hexagonal and cubic channels. These materials exhibit high CO2 uptake and CO2/CH4 selectivity.

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many highly porous crystalline frameworks such as MIL-88,34 MIL-101,35 and PCN-19.36 However, prior to our work,37 the magnesium form of such trimers was unknown, which led to our perception that such Mg-trimers were difficult to synthesize. We reasoned that the difficulty is apparently due to magnesium’s lack of the trivalent state, because such trimers are often found in all M3+ or mixed M2+/M3+ compositions. To overcome the inherent limitation (i.e., inaccessible Mg3+) in Mg-trimer creation, our initial synthesis employed an approach which involved identifying ligands with functional groups and their geometric distribution favoring metal trimers. From literature examples based on 3d metals, we were successful in finding a triangular pyridine−carboxylate ligand, which indeed helped lead to a porous Mg-MOF (CPF-3) built from [Mg3(μ3-OH) (COO)6]− trimer.37 Such approach, while successfully demonstrating the feasibility of synthesizing Mgtrimers, is not generally usable because there are few such ligands. In this study, we turned our focus to solvent-induced effects. For metal ions such as Mg2+ with relatively large solvation enthalpy, solvents are known to play dominant roles in crystallization. However, prior to this study, there was no clue as to which solvent system could meet our goal of creating new magnesium clusters. It first became clear to us that the use of common amide solvents such as N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMA) led to Mg-MOFs

INTRODUCTION High crystallinity, diverse compositions, and topologies, together with tunable functionalities, have made metal−organic frameworks (MOFs) promising candidates for many applications.1−18 Currently, MOFs are predominantly derived from 3d or 4f metals, even though light main-group elements (e.g., Li, Be, B, Mg, or Al) are receiving increasing interest. Among these light elements, Mg appears to offer the greatest flexibility, in part because the Mg−O bond consists of a good mix of covalency and ionicity. For example, the greater ionicity of the Li−O bond limits the hydrothermal stability of Li-MOFs while greater covalency of the Al−O bond limits reversibility of Al−O bond formation, making Al-MOFs hard to crystallize, especially in large-enough size for single-crystal study. So far, Mg-MOFs are still relatively few,19−24 even though Mg-MOF-74 (also called CPO-27) is well-known, as a result of its huge CO2 uptake capacity.25 It is observed that while Mg tends to exhibit structural similarity to 3d elements in some MOFs such as MOF-74 with chains, symmetrical Mg-clusterbased secondary building units (SBUs) such as trigonal prismatic trimers or paddle-wheel dimers have rarely been observed in Mg-MOFs although such trimers or dimers are frequent occurrences with transition metals. Given the impressive performance of Mg-MOF-74, the accessibility to Mg-cluster-based SBUs is highly desirable and could provide a versatile pool of connectors for the rational design of targeted MOFs.26−32 Among different forms of clusters, [M3(μ3-O/μ3-OH) (COO)6] (M = Cr, Fe, Co, and Ni, etc.), a well-known type of trinuclear clusters,33 have been used in the construction of © XXXX American Chemical Society

Received: September 7, 2015 Revised: December 25, 2015

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DOI: 10.1021/acs.cgd.5b01297 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1. Evolution of [Mg3] Trimeric Structural Building Blocks: (a) Linear Trimer, (b) Cyclic Core-less Trimer, (c) OHCentered Trimer, (d) Unsymmetric Pentamer, and (e) Symmetric Pentamer

Table 1. Summary of Crystal Data and Refinement Resultsa name

formula

space group

a (Å)

b (Å)

c (Å)

β (deg)

R(F)

CPM-201 CPM-202 CPM-203 CPM-204 CPM-205

[Mg3(BDC)3(DEA)2(EtOH)]·solvent [Mg3(TDC)3(DEA)3]·solvent [(CH3)2NH2][Mg3(OH) (FDC)3(H2O)3]·solvent [Mg5(OH)2(OBB)4(H2O)6]·solvent [Mg5(OH)2(NDC)4(DMA)3(H2O)]·solvent

P3c1 C2 R32 Pnma C2/c

22.8104(2) 22.803(3) 14.1381(9) 30.3968(8) 53.598(9)

22.8104(2) 13.4571(13) 14.1381(9) 26.0799(7) 16.409(3)

16.8099(2) 15.5637(14) 21.558(4) 13.1050(3) 21.672(4)

90 93.761(7) 90 90 113.85(3)

0.1143 0.0675 0.0869 0.0927 0.1097

a H2BDC = terephthalic acid; H2TDC = 2,5-thiophenedicarboxylic acid; H2FDC = 2,5-furandicarboxylic acid; H2OBB = 4,4′-oxybis(benzoic acid); H2NDC = naphthalene-2,6-dicarboxylic acid; DEA = N,N-diethylacetamide; DMA = N,N-dimethylacetamide; EtOH = ethanol.

with other configurations (e.g., linear trinuclear clusters, Scheme 1a). N,N-Diethylacetamide (DEA), a much less studied amide solvent, was found to be critical in creating unprecedented cyclic hollow trimetric [Mg3(COO)6] in CPM-201 and -202 reported here. Furthermore, we explored a new solvent system by combining amide solvents with recently reported urothermal synthesis approach. Among many uro-amide combinations studied in this work, a particular combination, DMPU-DMA (DMPU = 1,3-dimethyl-3,4,5,6tetrahydro-2(1H)-pyrimidinone) was found to be instrumental in producing an -OH-centered trimer as well as two variations of pentamers resulting from the fusion of two trimers. We further examined effects of various chemical additives and found that the addition of a trace amount of 1,1,1,5,5,5hexafluoro-2,4-pentanedione (HFP) contributes to the growth of large and pure single crystals of CPM-203, -204, and -205. Herein, we report five MgOFs (denoted as CPM-201 to -205, CPM = crystalline porous materials) that demonstrate step-by-step evolution of trimeric SBUs from unprecedented hollow trimetric [Mg3(COO)6] (for CPM-201 and -202), to μ3-OH-bridged trimetric [Mg3(μ3-OH) (COO)6]− (for CPM203), and eventually to pentanuclear [Mg5(μ3-OH)2(COO)8] formed from the fusion of two trimers (for CPM-204 and -205; Scheme 1 and Table 1). This is the first observation on the evolution of various Mg clusters during the formation of MgMOFs. These novel SBUs are connected by dicarboxylates to generate trigonal-bipyramidal cages (CPM-201, -202, and

-204), cuboidal cages (CPM-203), and octahedral cages (CPM-205), which are stacked into 3D frameworks with hexagonal, primitive-cubic, and body-centered-cubic channels.

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EXPERIMENTAL SECTION

Materials and Methods. Mg(NO3)2·6H2O, terephthalic acid (H2BDC), 2,5-thiophenedicarboxylic acid (H2TDC), 2,5-furandicarboxylic acid (H2FDC), 4,4′-oxybis(benzoic acid) (H2OBB), naphthalene-2,6-dicarboxylic acid (H2NDC), N,N-dimethylacetamide (DMA), N,N-diethylacetamide (DEA), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), ethanol (EtOH), and 1,1,1,5,5,5hexafluoro-2,4-pentanedione (HFP) were purchased from Aldrich Chemical Co. and used as received without further purification. The FT-IR spectra (KBr pellets) were recorded on a Nicolet Avatar 360 FT-IR spectrometer in the range of 400−4000 cm−1. The powder Xray diffraction patterns (PXRD) were recorded on a Bruker D8 Advance (40 kV, 40 mA) diffractometer (Cu radiation, λ = 1.54056 Å) with a scan speed of 0.5 s/step at room temperature. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer under nitrogen atmosphere (40−1000 °C range) at a heating rate of 5 °C/min. X-ray Structure Determination and Structure Refinement. Single-crystal X-ray analysis was performed on a Bruker Smart APEX II CCD area diffractometer with nitrogen-flow temperature controller using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å), operating in the ω and φ scan modes. The SADABS program was used for absorption correction. The structure was solved by direct methods, and the structure refinements were based on |F|2 with anisotropic displacement using SHELXTL.38 All non-hydrogen atoms in the B



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Figure 1. Trigonal-bipyramidal (a, CPM-201; b, CPM-202; d, CPM-204), cuboidal (c, CPM-203), and octahedral (e, CPM-205) cages embedded in the porous Mg−organic frameworks. [Mg5(OH)2(NDC)4(DMA)3(H2O)]·solvent (CPM-205). In a 20 mL glass vial, 102.6 mg of Mg(NO3)2·6H2O and 43.2 mg of H2NDC were dissolved in a mixture of 4.0 g of DMA and 2.0 g of DMPU. After addition of 28 μL of HFP, the vial was sealed and placed in a 120 °C oven for 5 days. Colorless block crystals were obtained after cooling to room temperature. The yield was about 66% based on Mg. Pure sample was obtained by filtering and washing the raw product with DMA. Gas Adsorption Experiments. Gas sorption isotherms of CPM201−CPM-205 were measured on a Micromeritics ASAP 2020 M surface-area and pore-size analyzer up to 1 atm of gas pressure by the static volumetric method. All as-synthesized samples were immersed in CH2Cl2 for 3 days; during the exchange, the CH2Cl2 was refreshed three times. The resulting CH2Cl2-exchanged samples were then evacuated (10−3 Torr) at 80 °C for 12 h and then at 150 °C (CPM202 and -203) or 220 °C (CPM-201, -204, and -205) for 24 h. All gases used were of 99.99% purity, and the impurity trace water was removed by passing the gases through the molecular sieve column equipped in the gas line. The gas sorption isotherms for N2 and H2 were measured at 77 K. The gas sorption isotherms for CO2, CH4, and C2H2 were measured at 273 or 298 K. The isosteric heat of CO2 adsorption was estimated from the CO2 sorption data measured at 273 and 298 K, by using a virial-type expression.40

framework were refined with anisotropic displacement parameters. The large volume fractions of solvents in the lattice pores could not be modeled in terms of atomic sites and were treated using the SQUEEZE routine in the PLATON software package.39 Crystal data as well as details of data collection and refinements were summarized in Supporting Information Tables S1−S3. Synthesis of MOFs. [Mg3(BDC)3(DEA)2(EtOH)]·solvent (CPM201). In a 20 mL glass vial, 51.3 mg of Mg(NO3)2·6H2O and 33.2 mg of H2BDC were dissolved in 2.0 g of DEA and 1.0 g of ethanol. The vial was sealed and placed in a 120 °C oven for 5 days. Large pure block colorless crystals were obtained after cooling to room temperature. The yield was about 85% based on Mg. Pure sample was obtained by filtering and washing the raw product with DEA. [Mg3(TDC)3(DEA)3]·solvent (CPM-202). In a 20 mL glass vial, 51.3 mg of Mg(NO3)2·6H2O and 34.4 mg of H2TDC were dissolved in 3.0 g of DEA. The vial was sealed and placed in a 120 °C oven for 5 days. Pure block colorless crystals were obtained after cooling to room temperature. The yield was about 75% based on Mg. Pure sample was obtained by filtering and washing the raw product with DEA. [(CH3)2NH2][Mg3(OH)(FDC)3(H2O)3]·solvent (CPM-203). In a 20 mL glass vial, 102.6 mg of Mg(NO3)2·6H2O and 31.2 mg of H2FDC were dissolved in a mixture of 4.0 g of DMA and 2.0 g of DMPU. After addition of 28 μL of HFP, the vial was sealed and placed in a 100 °C oven for 5 days. Pure block colorless crystals were obtained after cooling to room temperature. The yield was about 80% based on Mg. Pure sample was obtained by filtering and washing the raw product with DMA. [Mg5(OH)2(OBB)4(H2O)6]·solvent (CPM-204). In a 20 mL glass vial, 102.6 mg of Mg(NO3)2·6H2O and 51.6 mg of H2OBB were dissolved in a mixture of 4.0 g of DMA and 2.0 g of DMPU. After addition of 28 μL of HFP, the vial was sealed and placed in a 120 °C oven for 5 days. Pure block crystals were obtained after cooling to room temperature. The yield was about 72% based on Mg. Pure sample was obtained by filtering and washing the raw product with DMA.

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RESULTS AND DISCUSSION Crystal Structure. The most unusual feature of CPM-201 and -202 is the presence of an unprecedented core-less Mg3 trimer. Three unique Mg centers are bridged by six carboxylate groups to give a noncentrosymmetric triangular [Mg3(COO)6] trimer (Scheme 1b). Each Mg center is of five-coordinated square-pyramidal geometry with four carboxylate oxygen atoms serving as square plane and O from DEA or ethanol solvent occupying the vertex. Mg···Mg distances (about 3.64−3.74 Å) C



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octahedral, and the apex trans to the μ3-OH is a terminal H2O. The Mg···Mg separation is 3.4934(33) Å, markedly shorter than that in core-less trimers. CPM-203 crystallizes in the chiral R32 space group. Each [Mg3(μ3-OH)(COO)6]− is connected via bent 2,5-furandicarboxylate to six other trimers. However, instead of trigonal bipyramid in CPM-201 and -202, the orientation of the functional groups in FDA led to cuboidal cages (Figure 1 and Supporting Information Figure S3). The edge of the cube is 10.8751(10) Å. Each cube links adjacent six cages to form the 3D framework of CPM-203 with pcu topology, isostructural to MIL-59 constructed from [V3(μ3-OH)(COO)6] and isophthalate.44 An interesting observation with Mg-MOFs reported here is that, unlike some MOFs such as MOF-5 or MOF-74 which form isoreticular series upon the use of longer ligands, 4,4′oxybis(benzoic acid) (H2OBB) and naphthalene-2,6-dicarboxylic acid (H2NDC) led to two different ways of fusion by trimers to give unprecedented unsymmetric or symmetric [Mg5(μ3-OH)2(COO)8] pentanuclear clusters in CPM-204 and -205 (Scheme 1d,e). CPM-204 crystallizes in Pnma space group with the asymmetric unit containing five independent Mg cations and two OBB anions. All Mg atoms are octahedral but exhibit three types of coordination modes. Two μ3-OH groups bridge adjacent three Mg centers to form two [Mg3(OH)] cores, which are further jointed together through vertex sharing to give a [Mg5(OH)2] (Scheme 1d). Taking carboxylate groups into account, half of the pentamer is [Mg3(μ3-OH) (COO)6] trimer, similar to that in CPM-203. But, one terminal H2O is replaced by OH−, which serves as μ3-group linking the other two Mg atoms. Two bidentate carboxylate groups of [Mg3(μ3OH)(COO)6] are also used to join two trinuclear motifs. The existence of three terminal H2O around the fifth Mg center leaves the second trimer open. The Mg···Mg separations and Mg−O−Mg angles of the pentamer are of 2.9−3.9 Å and 93.6− 148.1°, which deviate from those in the regular separate trimer. Each pentanuclear Mg cluster is surrounded by eight OBB anions but connects only six other [Mg5(OH)2] SBUs. Such a distribution of the organic linkers builds up a much elongated cage (Figures 1d and Supporting Information Figure S5), which further led to the 3D structure of CPM-204 exhibiting acs topology. Different from the unsymmetric pentanuclear SBUs in CPM204, the symmetrical [Mg5(μ3-OH)2(COO)8] motif in CPM205 (Scheme 1e) is built from two symmetrically distributed [Mg3(μ3-OH)(COO)4] units through the corner-sharing mode. Four Mg atoms on the edge of each pentanuclear cluster are of five-coordinated trigonal-bipyramidal geometry with three carboxylate O atoms, one μ3-OH group, and one terminal water or DMA molecule. The central Mg is six-coordinated to four carboxylate O atoms and two μ3-OH groups. It is noted that two unique [Mg5(OH)2] clusters exist in CPM-205, which differ in four terminal molecules as shown in Supporting Information Figure S5. All of the Mg···Mg separations (3.1−3.4 Å) and Mg−O−Mg angles (108.3−123.8°) are in accordance with those of the μ3-OH-bridged trimers. Neighboring Mg centers all are double-bridged by two carboxylate groups, and thus eight NDC anions distribute symmetrically above and below each [Mg5(μ3-OH)2(COO)8] SBU (Supporting Information Figure S5). This orientation of the NDC links builds up an elongated octahedral cage with the axial length of 21.672(4) Å (Figure 1e). The octahedral cages are stacked to

indicate strong repulsions between Mg centers, which result in the distortion of this trinuclear cluster. On the other hand, attractions between Mg and carboxylate O atoms (2.297(5)− 2.497(8) Å) help to ease the repulsions and stabilize the magnesium ring. We propose that a key driving force for the formation of the core-less trimer is the lack of access by Mg to the trivalent state. The core-less Mg3 trimer is unusual and is unknown prior to this work. An interesting core-less lithium tetramer, [Li4(COO)6]2−,41 was recently reported; however, no similar Li3 is known. In fact, only discrete [Pd3(COO)6] exhibits a similar core-less feature.42 Trimers of other ions (e.g., Ni, Co, and so on) are known to consist of OH− or O2− at the core. The extra negative charge from OH− and O2− is balanced by the trivalent state of these ions (or extraframework cations, as in CPM-203). Since magnesium has no access to the trivalent state and, in the case of CPM-201 and -202, there are no extraframework charge-balancing cations, it opts to reduce the negative charge by leaving out OH−. It is interesting to note that the geometrical difference in two ligands in CPM-201 and CPM-202 leads to a compression (in CPM-202) on the cages and the overall framework without changing the topological type. CPM-201 and -202 are topologically identical and crystallize in the noncentrosymmetric space group P3c1 and chiral C2, respectively. Each is built up from corner-sharing core-less Mg trimers connected through linear (BDC) or bent (TDC) links (Figure 1). Six separate ditopic organic links lie symmetrically on two sides of the trimer with each carboxylate bonded in a bidentate fashion to adjacent Mg centers. Such a linkage generates a trigonalbipyramidal cage with [Mg3(COO)6] SBUs as vertexes and benzyl or thienyl groups as edges (Supporting Information Figures S1 and S2). The side lengths of the triangular equatorial plane are approximately 13.2 Å for two Mg-MOFs, but the distances between two vertexes along the axis are 16.8054(3) Å (CPM-201) and 15.5637(13) Å (CPM-202), respectively. Clearly, the linker geometry results in a unidirectional compression along the c-axis for CPM-202. Unlike CPM-201 where the carboxylate functionalities are oriented at 180° to each other, in CPM-202, they are oriented at 152°. The closer proximity of carboxylate groups of TDC in CPM-202 produces the compressed version of CPM-201. Each trigonal bipyramid connects adjacent 12 polyhedra to give a 3D packing in the ABAB stacking sequence. Hexagonal pores exist along the c-axis with the centers of [Mg3(COO)6] as vertexes. The diameter of the primary hexagonal channel in CPM-201 is 6.8 Å, which is reduced to 5.6 Å in CPM-202 due to the bend of TDC. Furthermore, [Mg3(COO)6] SBUs can be considered as six-connected nodes, which are joined by two-connected ditopic links. The overall network of CPM-201 and -202 can be described as the acs topology reported earlier for MOF-235.43 In addition to core-less Mg trimer, OH−-centered trimers are also desirable to broaden Mg-based MOFs. For Mg2+, a chargebalancing mechanism is required since [Mg3(OH)(COO)6]− carries a negative charge. Our experiments show that DEA is unable to fulfill this role, either because it does not decompose to provide H2(C2H5)2N+ or such cations are unsuitable chargebalancing cations due to its bulkiness. With the use of the DMPU-DMA solvent, a novel Mg-MOF (CPM-203) containing μ3-OH-bridged [Mg3(μ3-OH)(COO)6]− (Scheme 1c) was obtained. Compared to the core-less [Mg3(COO)6] ring, the μ3-OH group is located on a 3-fold axis leading to three Mg− O(μ3)−Mg angles all equal to 120°. Each Mg center is D



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give the 3D structure of CPM-205 exhibiting bcu topology with pentanuclear clusters as eight-connected nodes and NDC anions as links (Supporting Information Figure S5). Undoubtedly, such formation of aggregated trimeric units [Mg5(μ3-OH)2(COO)8] in CPM-204 and -205, together with the trapping of hollow [Mg3(COO)6] timers in CPM-201 and -202, and [Mg3(μ3-OH)(COO)6] clusters in CPM-203, reveals fascinating synthetic and structural chemistry of Mg-MOFs. These clusters, while rather diverse, are closely interrelated and show strikingly systematic structural evolution. Gas Adsorption. PLATON calculations show that CPM201 to -205 have 63.8%, 62.6%, 63.1%, 55.6%, and 39.3%, respectively, potential guest-accessible volumes (solvent and charge-balancing cations for negative frameworks). They are stable up to about 500 °C for CPM-201, -204, and -205, and 250 °C for CPM-202 and -203 (Supporting Information Figure S6). To study their porosity, samples were activated by soaking in CH2Cl2 for 3 days. The CH2Cl2-exchanged samples were degassed at 80 °C for 12 h and then at 150 °C (CPM-202 and -203) or 220 °C (CPM-201, -204, and -205) for 24 h. FT-IR (Supporting Information Figure S7) and powder X-ray diffraction (Supporting Information Figure S8) show that desolvated samples retain their crystallinity. Two series of absorption bands in the range of 1300−1700 cm−1 are assigned to the νas(C−O) and νsym(C−O) of carboxylate groups. The strong absorption bands near 800 cm−1 can be assigned to the C−H bonds of the aromatic groups. N2, H2, C2H2, CO2, and CH4 sorption isotherms were measured. CPM-201 exhibits the type I isotherm typical of materials of permanent microporosity, but N2 uptakes at 77 K of other samples are negligible (Figure 2 and Supporting Information Figure S9). The Langmuir and BET surface areas are 168 and 117 m2 g−1 for CPM-201. Of these five Mg-MOFs, only CPM-201 exhibits high adsorption of H2 and C2H2. CPM201 adsorbs H2 gas up to 80.8 cm3 g−1 (0.72 wt %) at 77 K and 1 atm and C2H2 gas up to 38.8 cm3 g−1 at 273 K and 1 atm. Considering the low surface area of CPM-201, its H2 and C2H2 uptakes are remarkable. Notably, these Mg-MOFs exhibit significant CO2 uptake capacity (Figure 2). At 273 K and 1 atm, the CO2 uptakes reach 40.7 cm3 g−1 (CPM-201), 40.9 cm3 g−1 (CPM-202), 53.1 cm3 g−1 (CPM-203), 23.6 cm3 g−1 (CPM-204), and 22.0 cm3 g−1 (CPM-205), respectively. Supporting Information Table S5 gives a comparison of CO2 uptake capacity among reported Mg-MOFs. The open Mg sites from removing the coordinated water or solvent molecules likely contribute to the CO2 uptake capacities. In addition, the interactions between charged MgMOF frameworks (CPM-203) and CO2 are obviously enhanced, leading to the highest CO2 uptake capacity among these CPMs. The isosteric heat of adsorption (Qst) of CO2 was determined by fitting the adsorption data collected at 273 and 298 K to the virial model to further understand the adsorption properties. The Qst values for CO2 of CPM-201− CPM-205 are about 23−30 kJ mol−1 near zero coverage (Figure 2), which are comparable to several NH2-decorated MOFs and larger than some well-known MOFs.45−47 The highest Qst value of CPM-203 among these CPMs once again supports the strong interactions of CO2 with the charged frameworks. To our knowledge, only Mg-MOF-74 has a larger Qst value (47 kJ mol−1) at the zero coverage; however, it decreases to about 30 kJ mol−1 (similar to the value for CPM203) at the high loading.25

Figure 2. (a) N2, H2, CO2, C2H2, and CH4 gas sorption isotherms of CPM-203. (b) Comparison on CO2 and CH4 uptake performance of CPM-201−CPM-205 at 273 K. (c) Enthalpy of adsorption (Qst) as a function of CO2 uptakes for five Mg-MOFs.

Compared to the CO2 adsorption, these Mg-MOFs adsorb much less CH4 (Figure 2). The CH4 uptakes for CPM-201− CPM-205 at 273 K and 1 atm are 4.0, 1.7, 3.2, 3.0, and 3.7 cm3 g−1, respectively. The CO2/CH4 uptake ratios are estimated to be ca. 31.0−10.1 (CPM-201), 25.9−24.1 (CPM-202), 19.4− 16.6 (CPM-203), 8.0−7.9 (CPM-204), and 12.6−6.0 (CPM205) between 0.015 and 1 bar, which are higher than the values E



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of many well-known ZIFs and MOFs.48,49 The high uptake capacity for CO2 gas over CH4 in Mg-MOFs may be associated with the quadruple moment of CO2 (−1.34 × 10−39 C m2) which induces efficient interaction with the framework. Overall, other than simple lower mass effects of Mg-MOFs, the unique ionic character of Mg−O bond affords additional uptake of CO2, which also imparts high CO2/CH4 selectivity of CPM201−CPM-205.

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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.cgd.5b01297. Powder X-ray diffraction patterns, TGA curves, FT-IR spectra, gas adsorption isotherms, additional crystal structure figures, crystallographic table, bond lengths and angles, and data on CO2 uptake capacity (PDF) Accession Codes

CCDC 1408963−1408967 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

*E-mail: [email protected] (P.F.). *E-mail: [email protected] (X.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Award No. DE-FG02-13ER46972.

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REFERENCES

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Article

(49) Lin, R.-B.; Chen, D.; Lin, Y.-Y.; Zhang, J.-P.; Chen, X.-M. Inorg. Chem. 2012, 51, 9950−9955.

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