Density Gradation of Open Metal Sites in the Mesospace of Porous

Jul 26, 2017 - The prevalence of the condensed phase, interpenetration, and fragility of mesoporous coordination polymers (meso-PCPs) featuring dense ...
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Density Gradation of Open Metal Sites in the Mesospace of Porous Coordination Polymers Jingui Duan,*,†,‡ Masakazu Higuchi,‡ Jiajia Zheng,‡,⊥ Shin-ichiro Noro,§ I-Ya Chang,∥ Kim Hyeon-Deuk,∥ Simon Mathew,‡,¶ Shinpei Kusaka,‡ Easan Sivaniah,‡ Ryotaro Matsuda,‡ Shigeyoshi Sakaki,⊥ and Susumu Kitagawa*,‡,# †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Centre for Advanced Materials, Nanjing Tech University, Nanjing 210009, China ‡ Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan § Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan ∥ Department of Chemistry, Kyoto University, Kyoto, 606-8502, Japan ⊥ Fukui Institute for Fundamental Chemistry, Kyoto University, Nishi-hiraki cho, Takano, Sakyo-ku, Kyoto 606-8103, Japan # Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: The prevalence of the condensed phase, interpenetration, and fragility of mesoporous coordination polymers (meso-PCPs) featuring dense open metal sites (OMSs) place strict limitations on their preparation, as revealed by experimental and theoretical reticular chemistry investigations. Herein, we propose a rational design of stabilized high-porosity meso-PCPs, employing a low-symmetry ligand in combination with the shortest linker, formic acid. The resulting dimeric clusters (PCP-31 and PCP-32) exhibit high surface areas, ultrahigh porosities, and high OMS densities (3.76 and 3.29 mmol g−1, respectively), enabling highly selective and effective separation of C2H2 from C2H2/CO2 mixtures at 298 K, as verified by binding energy (BE) and electrostatic potentials (ESP) calculations.

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INTRODUCTION Mesoscale materials continue to provide low-cost and efficient solutions for energy and environmental sustainability.1a,b However, despite the progress in constructing mesopores in silica, zeolites, and activated carbons, the synthesis of regular and periodic mesoporous materials exhibiting both high surface area and high porosity is still challenging,2a,b since these materials possess certain limitations due to their inherent properties such as low crystallinity, poorly tunable macromolecular design, and uncontrolled pore arrangement.3a,b Among porous materials, porous coordination polymers (PCPs) or metal−organic frameworks (MOFs), featuring metal ions bridged by organic ligands, exhibit high crystallinity and facile tunability due to an unlimited number of organic/ inorganic hybrid combinations. It is the sheer tunability of these systems that makes them amenable toward the construction of new mesoporous PCPs (meso-PCPs).4a−e Nevertheless, several prerequisites need to be considered for designing and assembling meso-PCPs with predetermined pore properties.5 A general approach to increasing PCP pore size exploits a linear extension of organic linkers in a given network topology. However, the connectivity of ligands must match the © 2017 American Chemical Society

coordination geometries of corresponding metal clusters within the mesoporous network, as highlighted by reticular chemistry investigations.6a−c The increasing molecular size and complexity of ligands induce interpenetration or catenation of coordination networks and limit their synthetic accessibilty (i.e., solubility, reactivity, and byproduct).7 Thus, although a number of meso-PCP architectures have been reported, the hierarchical suprastructure suffers from the well-known disadvantages of decreased surface area, small pore volume, and greater degree of molecular disorder.8 Inspired by the idea of topological induction and ligand extension, we succeeded in rationally designing meso-PCPs employing a small, low-symmetry ligand.9a,b The insertion of an additional aromatic ring into fully symmetric ligands such as trimesic acid (H3BTC) achieves a partial extension of ligand length while preserving its connectivity mode. In contrast to the case of high-symmetry frameworks (tbo-based HKUST-1), the nonuniform distances between the coordinating groups of such ligands can benefit the formation of twisted or semiregular Received: June 12, 2017 Published: July 26, 2017 11576

DOI: 10.1021/jacs.7b05702 J. Am. Chem. Soc. 2017, 139, 11576−11583

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Journal of the American Chemical Society

Scheme 1. Schematic Representation of the Route to the Meso-PCPs through the Small and Low Symmetry Ligand or the Smallest Formic Acid

supplementary crystallographic data of PCP-31 and PCP-32, respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Single and Binary Gas Adsorption. In the single gas sorption measurement, ultrahigh-purity grade gases were used throughout the adsorption experiments. All measured sorption isotherms have been repeated twice to confirm the reproducibility within experimental error. Mixed gas adsorptions were carried out using a multicomponent gas adsorption apparatus, Belsorp-VC (MicrotracBEL Corp.). In this apparatus, the total adsorbed amount was calculated by a constant volume method, and the composition ratio of mixed gases was determined using an Agilent 490 Micro GC system equipped with a thermal conductive detector. From these data, we calculated adsorbed amounts and partial pressures for each gas. Computational Methods. All DFT computations were performed with the Becke three-parameter Lee−Yang−Parr (B3LYP) functional in the Gaussian 09 package.10 PCP-31 was divided into two systems, the dense and dilute parts, since PCP-31 includes 2280 atoms. As for the PCP-32, the LanL2DZ11 and 4-31G(d,p) basis sets were applied to Cu and the nonmetallic atoms, respectively. Then, Kohn−Sham orbitals of the optimized PCP structures were refined with a larger basis set, 6-311G(2d,2p). The three-dimensional (3D) ESP was made from the converged Kohn−Sham orbitals by the utility cubegen program of Gaussian 09 package. The 2D ESP contour maps were cut off from the 3D ESP and visualized by the GaussView 5.0 package. Cross sections on each 2D ESP contour map shaded by the blue-colored shapes follow the two definitions: (1) a blue-colored shape must have an isovalue whose absolute value is smaller than 0.02. Note that the isovalue highly depends on which kind of atom (O or Cu or H) is closest to the center of each slide: If it is an O atom, the first-touched isovalue is −0.02. If it is a Cu or C atom, the first-touched isovalue is +0.02. (2) Cross sections must be inside the PCP frame. We carefully fit each 2D ESP map by a blue-colored simple shape satisfying these definitions. The binding energies of C2H2 and CO2 with PCP-31 and PCP-32 were calculated by the B3LYP-D3 functional.12 Because PCP-31 has Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2 paddle wheel units and PCP-32 has Cu2(O2CPh)4 unit, two cluster models, Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2, were employed to investigate gas adsorption at the dilute OMS. To evaluate the binding energy at the dense OMS in PCP-31, two kinds of cluster model (PCP-31-M1 and PCP-31-M2) were employed to consider that the gas molecule is adsorbed at different sites. In PCP-31-M1 and PCP-31-M2, six and

PCPs with increased cage size and offer various coordination modes to realize previously unattainable structures. Given the complexities of periodic cycles within meso-PCPs, the use of small, low-symmetry ligands in combination with metal clusters represents a promising strategy to preserve high surface areas with dense OMSs. Herein, we propose the rational design and preparation of meso-PCPs featuring dense OMS regions, high porosity, and large window apertures, using formic acid as the smallest linker in a compact and low-symmetry ligand (H3L: [1,1′:4′,1″terphenyl]-3,4″,5-tricarboxylic acid) (Scheme 1). The two resulting meso-PCPs (PCP-31 and PCP-32) exhibit radically different topologies, ultralarge pores (23−28 and 18−38 Å, respectively), and dense OMS regions (3.76 and 3.29 mmol g−1, respectively) within high surface area (2858 and 4816 m2 g−1, respectively), allowing unprecedentedly effective roomtemperature separation of C2H2 from C2H2/CO2 mixtures. Further, the asymmetric dense and dilute OMS regions in both PCP-31 and PCP-32 exhibit pressure-dependent acetylene separation efficiency. All of these findings are well rationalized by density functional theory (DFT) calculations, which reveal the role of asymmetric dense and dilute OMSs.

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EXPERIMENTAL SECTION

Synthesis of PCP-31. For synthesis of [Cu2(L)2(HCOO)2· (H2O)2]·xG (G = solvents) (PCP-31), copper(II) nitrate (25 mg), H3L (10 mg), and HNO3 (80 μL) were mixed with 3 mL of DMF/1,4dioxane/H2O(4:1:1) in a 4 mL glass container and tightly capped in a Teflon vial and heated at 50 °C for 2 days. After cooling to room temperature, the vials were heated at 60 °C for another 2 days and then cooled to room temperature. The resulting green polyhedral crystals were harvested and washed with DMF. Synthesis of PCP-32. For synthesis of [Cu6(L)4 (H2O)6]·xG (G = solvents) (PCP-32), copper(II) nitrate (13 mg), H3L (6 mg), and HNO3 (20 μL) were mixed with 1 mL of DMF/1,4-dioxane/H2O (4:2:0.5) in a 4 mL glass container and tightly capped in a Teflon vial and heated at 65 °C for 2 days. After cooling to room temperature, the resulting high yield green crystals were harvested and washed with DMF. X-ray Study. Crystallographic data and refinement information are summarized in Table S1. CCDC 1490343 and 1490344 contain the 11577

DOI: 10.1021/jacs.7b05702 J. Am. Chem. Soc. 2017, 139, 11576−11583

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Figure 1. Structures and formation of (1) PCP-31 and (2) PCP-32: (a) Initial mesopores with 24 Cu-dimers for PCP-31. (b) Formation of PCP-31 through self-assembly of ligands and formic acid with 2- and 4-connected Cu-dimers. The initial large mesopore, shaded in light blue, became a small mesopore, shaded in deep blue, due to interpenetration. (c) Highly asymmetric dense and dilute OMS regions constituting interpenetrated PCP-31. (d) Dense OMS region of different views in PCP-31. (e) Initial mesopores of different size for PCP-32. (f) Formation of PCP-32 keeping the initial mesopore size. (g) Highly asymmetric dense and dilute OMS regions constituting PCP-32. (h) Dense OMS region of different views in PCP-32. The arrow in panels d and h reveals the region and direction of latent interaction from dense OMS (highlighted in dark blue). Other Cu ions from dense OMS region and dilute OMS region in panels c, d, g, and h are highlighted in turquoise and blue, respectively. two paddle wheel units were considered, respectively. Similarly, two cluster models PCP-32-M1 and PCP-32-M2 consisting of four and three paddle-units, respectively, were employed to investigate gas adsorption at the dense OMS in PCP-32. These model systems are presented in Figure S44. In geometry optimizations of model systems of PCP-31 and PCP32, positions of the outside Cu atoms were fixed to those in the experimental structures (Figure S44). Full optimizations were carried out for Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2. The basis sets were used as follows: the LANL2DZ with the effective core potentials (ECPs)11 for Cu, the 6-31G(d,p) for CO2 and C2H2, the 6-31G(d) for carboxylate group with a set of diffuse functions added to O atoms, and the 6-31G for other atoms. The binding energies (BEs) were calculated with eq 1:

BE = (E PCP + nE L − E PCP·nL)/n

diffraction (Table S1). In this meso-PCP framework, each ligand is connected to three Cu paddle wheel dimer clusters, with two of them coordinating to four discrete ligands (4-foldconnected Cu2(L)4 nodes) and one bonding to two ligands and two formate anions (doubly connected Cu2(L)2(HCOO)2 nodes), with the latter originating from DMF hydrolysis by concentrated HNO3. Among the plethora of previously reported PCPs, this framework is the first example of a doubly connected Cu paddle wheel node, providing access to larger meso pores compared to those observed for 6- (e.g., Cu3(ArCOO)6) and 4-fold-connected clusters. Single-crystal X-ray diffraction revealed that PCP-31 features a framework of distorted truncated cuboctahedrons with large cavities (∼40 Å) packed face-to-face (Figure 1a), with interpenetration by an equivalent framework in different directions along the c-axis reducing the pore size. Nevertheless, the reduced cavity diameter of the two integrated pores equals ∼28 Å, making PCP-31 the first interpenetrated meso-PCP possessing intrinsic mesopores (Figure 1b). Moreover, PCP-31 shows 24 distinct Cu dimer units per pore, corresponding to considerably denser OMSs than those typically observed for supramolecular PCP/ MOF cages (Figure 1c,d), with the calculated latent OMS density (3.76 mmol g−1) being significantly higher than that of a series of important meso-PCPs such as MOF-210 (0 mmol· g−1),15 PCN-777 (1.95 mmol·g−1),16 NU-100 (2.67 mmol· g−1),17 and NOTT-119 (2.48 mmol·g−1)18 (Table S2). It should also be emphasized that PCP-31 exhibits both dense and dilute OMS regions, which results in a fairly asymmetric OMS mesopore space. Figure S6 shows the corresponding unprecedented 3D network featuring doubly and 4-foldconnected nodes and triply coordinated ligands. However, subsuming the doubly connected vertices into the cluster

(1)

where EPCP·nL is the total energy of the PCP model system with n molecules of L (L = CO2 and C2H2), and EPCP and EL are the total energies of PCP model system without gas molecules and one free gas molecule, L, respectively. A positive binding energy means that the corresponding adsorption is exothermic. In evaluating the binding energies, the following basis sets were employed: the (311111/22111/411/11) with Stuttgart−Dresden− Born (SDB) ECPs13a,b for Cu, the 6-311G(2d,2p) for CO2 and C2H2, the 6-311G(d) for other atoms with a set of diffuse functions added to O atoms of carboxylate group. The basis set superposition error (BSSE) was corrected by the counterpoise method.14

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RESULTS AND DISCUSSION A solvothermal reaction of copper(II) nitrate with H3L in DMF/dioxane/H2O containing concentrated HNO3 (80 μL) afforded polyhedron-shaped crystals of [Cu2L(HCOO)· (H2O)2]xGuest (PCP-31) in high yield (Figure S1), which were shown to belong to the R3̅m space group with a = 42.321(5) Å and b = 30.872(6) Å by single-crystal X-ray 11578

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Journal of the American Chemical Society linkage, the generated basic unit of PCP-31 can be assigned to a tetracarboxylate linker joining the 4-fold-connected nodes, implying that the 3-c branching points of L3− yield a (3,4)-c framework with a fog topology. Although a number of frameworks with this topology have been reported, this is the first example of an interpenetrated fog meso-PCP framework. The solvent-accessible volume calculated for evacuated PCP-31 using PLATON equaled 69.9%, reaching 83.9% for noninterpenetrated PCP-31 and thus highlighting the excellent mesoporous nature of this framework. Based on the insights obtained for isolated PCP-31, we utilized traditional topological guidances to find new and more porous H3L-based structures with larger accessible volumes. To achieve this goal, less HNO3 was employed to reduce the amount of in situ produced formic acid, allowing Cu dimers to be completely coordinated by the above ligand to afford a [3,4]-c network with tbo topology. However, the solvothermal reaction of H3L with copper(II) nitrate in DMF/dioxane/H2O containing a single drop of HNO3 yielded polyhedron-shaped crystals of a new meso-PCP ([Cu6(L)4(H2O)6]xG, PCP-32) belonging to the Cmc21 space group, with a = 36.542(7) Å, b = 39.322(8) Å, and c = 43.852(9) Å. Compared to PCP-31, PCP32 exhibited a different coordination geometry, comprising purely 4-fold-connected Cu dimer clusters. Additionally, being derived from a [3,4]-c network, PCP-32 exhibited three types of pores, with two of them being irregular mesopores with sizes of ∼30 and 33 Å and mesowindows with sizes of ∼22 and 24 Å, respectively (Figure 1e,f). In the above structure, one cage was formed by coordination of 16 H3L ligands and 8 isophthalate rings of neighboring H3L ligands with 12 Cu dimer clusters, whereas the other one was constructed by coordination of 8 H3L ligands to 12 Cu dimer clusters (Figure 1g,h). The large number of Cu dimer nodes in PCP-32 preserved its highdensity OMS region (3.29 mmol g−1). In analogy to PCP-31, PCP-32 also contained both dense and dilute OMS regions, which resulted in the formation of a similar asymmetric mesopore space. Comprising 4-fold-connected Cu dimer nodes and triply connected linkers, PCP-32 features a tbo/twisted boracite network (Figure S14) similar to that of HKUST-1 with four crystallographically distinct linkers per asymmetric unit. The solvent-accessible volume of PCP-32 was calculated as ∼83.4%, demonstrating the significant porosity of this material. Moreover, the structures and phase purities of PCP-31 and PCP-32 were confirmed by powder X-ray diffraction (XRD) analysis at the Spring-8 BL02B2 synchrotron beamline and by Le Bail analysis (Figure S18−21).19 The large mesopores and dense OMSs of PCP-31 and PCP32 make them great candidates for realizing ultrahigh Brunauer−Emmett−Teller (BET) surface areas. The architectural stabilities of PCP-31 and PCP-32 were confirmed by synchrotron powder XRD and thermogravimetric analyses of the fully activated phase prepared by exchange with acetone and evacuation under vacuum at elevated temperature. Initially, PCP-31 and PCP-32 were subjected to N2 (77 K) and Ar (87 K) adsorption studies. The obtained results (Figure 2) revealed reversible hysteresis-free type-I isotherms and exceptionally high gas uptakes, with the estimated BET surface areas of PCP31 and PCP-32 equaling ∼2858 and ∼4856 m2 g−1, respectively. Despite having a smaller surface area than that of PCN-68,20 NU-100,17 and MOF-210,15 PCP-31 and PCP32 can be classified to the group of mesopore materials having dense OMS regions (Table S2). The calculated pore size of PCP-31 and PCP-32, ∼23−28 Å and ∼19−35 Å, respectively,

Figure 2. Low-pressure Ar (88 K) and N2 (77 K) adsorption isotherms: (a) PCP-31 and (b) PCP-32.

are in excellent agreement with the mesopore size estimated from the single crystal data, indicating effective and complete activation. Inspired by the preferential interaction between OMSs and unsaturated hydrocarbons, we investigated the challenging separation of C2H2 from C2H2/CO2 mixtures at room temperature. CO2 has been identified as a difficult-to-remove impurity in several acetylene-utilizing industrial processes due to the similarity of molecule size (∼3.3 Å) and boiling points (∼189−195 K) of these compounds.21a−c Single-gas adsorption of C2H2 and CO2 by PCP-31 and PCP-32 at 298 K, showing type-I isotherms with completely reversible desorption, was compared to that by two typical PCPs, HKUST-1 and ZIF-8 (Figures 3, S30, and S32). For an equimolar mixture of C2H2 and CO2 at 298 K, the low-pressure acetylene adsorption selectivities of PCP-31 and PCP-32 (calculated using the ideal adsorbed solution theory (IAST) and fitted experimental isotherm data) equaled ∼43 and 23, respectively, being significantly higher than those of USTA-74a (∼20).22 Although a number of porous materials are capable of C2H2/CO2 separation, for example, PCPs with dense OMSs (such as HKUST-1 (12-5), ZJU-40a (17-11),23 NOTT101a (9-8),24 and PCP-33 (10-6)25) and without OMSs (TIFSIX-2−Cu-I (10-6)26 and ZIF-8 (1.7-1.5)), as well as organic porous materials (HOF-3 (14-21)27), the current lowpressure selectivity achieved by PCP-31 and PCP-32 is the highest, which is ascribed to the existence of a dense OMS region in these materials. In contrast, when the pressure was increased to 100 kPa, the adsorption selectivities of PCP-31 and PCP-32 gradually decreased to 7 and 8, respectively (Figure 3a,b), as further confirmed by the results of binary gas adsorption. Coadsorption experiments were performed by exposing PCP-31 11579

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To gain further insights, adsorption enthalpies (Qst) were calculated utilizing a virial approach.28 Whereas the corresponding values for CO2 adsorption by PCP-31 and PCP-32 (∼30 and ∼26 kJ mol−1) were moderate, those for C2H2 were as high as ∼53 and ∼36 kJ mol−1, respectively. Compared to a group of important porous PCPs such as HKUST-1 (43 kJ mol−1; see Supporting Information), UTSA-74a (33 kJ·mol−1),22 PCP-33 (27 kJ·mol−1),25 and Zn-MOF-74 (23 kJ·mol−1),29 PCP-31 exhibited the highest Qst for C2H2 adsorption. To elucidate the origin of such strong acetylene adsorption by PCP-31/PCP-32 and explore the contribution of their asymmetric dense and dilute OMS regions, we calculated the corresponding BEs utilizing the dispersion-corrected DFT (see Table S3 for comparison of BEs by DFT and DFT-D3), and found that the calculated BEs of CO2 and C2H2 at dense OMSs (Table 1) Table 1. Binding Energies (BE, kJ·mol−1)a of C2H2 and CO2 Calculated with Various PCP Modelsb models

C2H2

CO2

Cu2(O2CPh)4 Cu2(O2CPh)2(O2CH)2 PCP-31-M1 PCP-31-M2 PCP-32-M1 PCP-32-M2

32.7 36.0 58.9 49.0 37.2 37.5

23.8 23.9 38.8 31.7 29.6 29.3

a

BE per gas molecule; a positive value represents exothermic adsorption. bSee Figure S45 for the adsorption structures of C2H2 and CO2 in Cu2(O2CPh)4, Cu2(O2CPh)2(O2CH)2, PCP-31-M2, and PCP-32-M2.

Figure 3. Gas uptakes (black line) and estimated gas selectivity (green line) of C2H2 and CO2 in (a) PCP-31 and (b) PCP-32 at 298 K.

and PCP-32 to 4/1 C2H2/CO2 (v/v) mixtures at 100 kPa and 298 K to reach an equilibrium state, with the adsorbed gas ratios subsequently determined by gas chromatography (Figure 4). Whereas both PCP-31 (93.7%) and PCP-32 (91.5%)

agree with the above experimental results obtained at low coverage and the larger BEs at dense regions than those at dilute ones for both PCP-31 and PCP-32. The difference in BE between C2H2 and CO2 was larger for PCP-31 than for PCP32, in agreement with their high selectivities at low-pressure (Figure 3), indicating the importance of the unique dense OMSs in PCP-31. Since the BEs of C2H2 and CO2 to single Cu paddle wheel unit such as Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2 are much smaller than those for model 1 in PCP-31 (PCP-31-M1) and PCP-32 (PCP-32M1), the interaction of gas molecules with both single and multiple OMSs must be investigated in the dense OMS regions of these meso-PCPs. For PCP-31, we found that C2H2 can interact with a Cu atom of one paddle wheel unit, and simultaneously the H atom of this C2H2 molecule interacts with the formate O (Ofor) of the neighboring paddle wheel unit (Figure 5a). This interaction is attractive due to the proton-like nature of the acetylenic hydrogen, bringing the H atom of the C2H2 molecule in close proximity to the Ofor. This indicates that the dense OMS region in PCP-31 is crucial for increasing the BE of C2H2. In contrast, the corresponding interaction of CO2 via its O atom (OCO2) takes place in an η1-end-on manner, with the orientation different from that of C2H2 (Figure 5b). In this orientation, the positively charged C atom (1.08e) of CO2 (CCO2) prefers a position close to the formate O atom, increasing the BE. The Ofor of the neighbor paddle wheel unit forms attractive interaction with the CCO2 and electrostatic repulsive one with the OCO2. Because OCO2 is more distant from the Ofor than the CCO2, the CO2 binding energy becomes larger with the model 1 than with Cu 2 (O 2 CPh) 4 and Cu2(O2CPh)2(O2CH)2. This is the reason why BE of model 1 is larger than t hose with Cu 2 (O 2 CPh) 4 and

Figure 4. Co-adsorption of C2H2/CO2 mixed gas in ZIF-8, HKUST-1, PCP-31, and PCP-32 at 100 kPa and 298 K. PCP-31 and PCP-32 give similar enrichments of C2H2 at high pressure, both of which are higher than the enrichments in ZIF-8 and HKUST-1.

outperformed ZIF-8 (84.2%) and HKUST-1 (90.1%) in terms of high-pressure C2H2 separation, the difference between PCP31 and PCP-32 was insignificant, being consistent with the similar high-pressure selectivities observed for these compounds. Figure 6 rationalizes the detected pressure dependence of selectivity from the viewpoint of molecular clogging of the asymmetric dense OMS region inside PCP-31 by C2H2. 11580

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Cu2(O2CPh)2(O2CH)2. However, the attractive CCO2−Ofor interaction is somewhat compensated by the repulsive OCO2− Ofor interaction, leading to the smaller enhancement of CO2 BE than C2H2 BE (such compensation is absent in the C2H2 adsorption). As a result, the neighboring paddle wheel unit can enhance both C2H2 and CO2 adsorptions in the dense OMS region, but this enhancement is larger for C2H2 than for CO2, leading to the excellent low-pressure acetylene selectivity of PCP-31. As shown in Figure 5c,d, gas molecules adsorbed in the dense OMS region of PCP-32 cannot effectively interact with neighboring paddle wheel units, since the OMSs are distant well from each other even in the dense region. In such structure, gas molecules can form loose molecular clusters, achieving smaller increase in BE compared to those observed for isolated OMSs, Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2 (Table 1). The BEs of gas molecules with another type of dense OMS (model 2, Figure S45) are smaller than those with model 1 in PCP-31 but similar to those in PCP-32. However, the BEs with model 2 in both PCP-31 and PCP-32 are larger than those with Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2 (Table 1), suggesting the importance of dense OMSs in enhancing gas adsorptions to these two PCPs. Although the low-pressure enhancement of C2H2 adsorption can be well explained by the larger acetylene BE at multiple OMSs, this argument cannot rationalize the qualitatively different gas adsorption behavior observed at higher pressure, when PCP-31 (93.7%) and PCP-32 (91.5%) achieve similar

Figure 5. Most stable adsorption structures of C2H2 and CO2 in the dense OMS region: (a) PCP-31-M1 with C2H2; (b) PCP-31-M1 with CO2; (c) PCP-32-M1 with C2H2; (d) PCP-32-M1 with CO2. C2H2 and CO2 take the η2-side-on and η1-end-on coordination structures, respectively. Distances are given in Å. A C2H2 molecule is attracted from two OMSs in PCP-31, leading to the larger BE listed in Table 1.

Figure 6. ESP contour maps calculated from the 3D wave functions delocalized over the asymmetric dense and dilute OMS regions show positive (green) and negative (red) contour lines, which attract negatively and positively charged atoms or molecules, respectively (see also Figures S47− S50). The area of the ESP cross sections, shaded with the blue-colored shapes the definition of which is provided in Computational Methods, is plotted as a function of distance from the top of each PCP along Z-axis, which vertically crosses through the dilute and dense OMS regions with longest distance. 11581

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Journal of the American Chemical Society acetylene enrichment efficiencies despite the two times higher selectivity of the former at zero coverage. Figure 6 shows contour maps of ESPs, which are sliced contour lines of the 3D ESPs directly obtained from 3D wave functions of DFToptimized PCP-31 and PCP-32 structures. The above ESPs depend on the molecular orbitals delocalized over the interior mesopore space, featuring continuous positive and negative potentials that attract oppositely charged atoms/molecules. Thus, ESPs represent the actual porous space through which gas molecules diffuse rather than the simple molecular structure based on van der Waals radii (Figure 6a,b). Dilute OMS regions (slices 1 and 2 of PCP-31 and slices 5−7 of PCP-32) exhibited large ESP cross-sectional areas (blue-shaded), indicating that molecules can easily diffuse through this region regardless of pressure. In contrast, dense OMS regions (slices 3 and 4 of PCP-31 and slice 8 of PCP-32) exhibited much smaller cross sections due to intensive OMS aggregation, as demonstrated in Figure 6c,d, where the area of these cross sections fitted by blue-colored simple shapes is plotted as a function of vertical distance from the top of PCP-31 and PCP32, respectively. PCP-31 showed a drastic cross-sectional area decrease in the dense OMS region, with most areas being below 100 Å2. This observation explains the occurrence of pore clogging by C2H2 molecules in PCP-31, especially at higher pressure, since C2H2 exhibits a larger BE and strongly interacts with multiple OMSs, as explained in Figure 5. Interestingly, while both large BE and dense OMSs contribute to efficient molecular adsorption at low pressure, they cause molecular clogging at higher pressure and greatly suppress C2 H2 adsorption. In fact, CO2 adsorption was less affected by asymmetric cross sections due to its weaker interaction with OMSs (Figures 3a,b and 5b) Notably, all molecules adsorbed at high pressure were easily released by decreasing it, and thus no hysteretic adsorption was observed (Figure 3), indicating that clogging by C2H2 does not correspond to kinetic trapping and does not affect the desorption rate. In other words, C2H2 clogging is always observed at a given high pressure regardless of equilibrium/nonequilibrium conditions. As for PCP-32, most of its dense OMS region has areas above 100 Å2, also being much shallower than the dense OMS region of PCP-31, with the respective depths being ∼3 and 12 Å, respectively. Thus, the much less asymmetric ESPs in PCP-32 lead to significantly less pronounced C2H2 clogging in PCP-32, even at high pressure, with PCP-32 showing an overall higher C2H2 uptake than PCP-31 at higher pressure, despite the denser OMSs in the latter.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jingui Duan: 0000-0002-8218-1487 Kim Hyeon-Deuk: 0000-0002-5815-8041 Shinpei Kusaka: 0000-0001-7718-4387 Shigeyoshi Sakaki: 0000-0002-1783-3282 Present Address ¶

Current address: van’t Hoff Institute for Molecular Sciences, Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank National Science Foundation of China (21671102), National Science Foundation of Jiangsu Province (BK20161538), State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201406), ACT-C and ACCEL (JPMJAC1302) project, the PRESTO Program of the Japan Science and Technology Agency (JST), and JSPS KAKENHI Grant-in-Aid for Specially Promoted Research (Grant No 25000007) for financial support. Priority Academic Program Development of Jiangsu Higher Education Institutions. We also thank WPI-iCeMS and the RIKEN SPring-8 Center for support. We also thank Prof Michael O’Keeffe for the topology discussion.

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REFERENCES

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CONCLUSIONS In summary, we have successfully designed and prepared two meso-PCPs combining high surface area and an asymmetric arrangement of dense and dilute open metal sites, utilizing a small low-symmetry ligand and Cu dimers. This result rewrites the traditional guiding principle of using ligand extension for meso-PCPs. PCP-31 and PCP-32 demonstrate ultralarge pores with surface OMS density gradation, which significantly influences guest molecule diffusion and clogging, achieving effective separation of C2H2 from C2H2/CO2 mixtures at room temperature.

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Crystallographic data, TG curves, PXRD, results of density functional theory calculations, dual Langmuir− Freundlich isotherm model fitting, isosteric heat of adsorption calculation, and IAST calculations of adsorption selectivities (PDF) Crystallographic data for PCP-31 (CIF) Crystallographic data for PCP-32 (CIF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05702. 11582

DOI: 10.1021/jacs.7b05702 J. Am. Chem. Soc. 2017, 139, 11576−11583

Article

Journal of the American Chemical Society

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DOI: 10.1021/jacs.7b05702 J. Am. Chem. Soc. 2017, 139, 11576−11583