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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 J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05702 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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Journal of the American Chemical Society

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 HyeonDeuk∥, 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, Yoshida, Sakyo-ku, 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. ABSTRACT: The prevalence of the condensed phase, interpenetration, and fragility of mesoporous coordination polymers (mesoPCPs) 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, ultra-high 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) calculation.

INTRODUCTION Mesoscale materials continue to provide low-cost and efficient solutions for energy and environmental sustainability.1a, 1b 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, 2b since these materials possess certain limitations due to their inherent properties such as low crystallinity, poorly tunable macromolecular design, and uncontrolled pore arrangement.3a, 3b 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 towards the construction of new mesoporous PCP (meso-PCP)4a, 4b, 4c, 4d, 4e. Nevertheless, several prerequisites need to be considered for designing and assembling mesoPCPs with pre-determined 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 coordination geometries of corresponding metal clusters within the mesoporous network, as highlighted by reticular chemistry investigations.6a, 6b, 6c The increasing molecular size and complexity of ligands induce interpenetration/catenation of coordination networks and limit their synthetic accessibilty (i.e. solubility, reactivity, and by-product)7. Thus, although a number of meso-PCP architectures have been reported, the hierarchical supra-structure 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, 9b 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 highsymmetry frameworks (tbo-based HKUST-1), the non-uniform distances between the coordinating groups of such ligands can benefit the formation of twisted/semiregular 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 mesoPCPs (PCP-31 and PCP-32) exhibit radically different topologies, ultra-large 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 room-temperature separation of C2H2 from C2H2/CO2 mixtures. Further, the asymmetric dense and dilute OMS regions in both PCP-31 and PCP-32 exhibit pressure

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Scheme 1. Schematic representation of the route to the meso-PCPs through the small and low symmetry ligand and/or the smallest formic acid. -dependent acetylene separation efficiency. All of these findings are well rationalized by ab initio calculations, which reveal the role of asymmetric dense and dilute OMSs. EXPERIMENTAL SECTION Synthesis of PCP-31. Synthesis of [Cu2(L)2(HCOO)2·(H2O)2]·xG (G = solvents), (PCP-31): copper(II) nitrate (25 mg), H3L, 10 mg) and HNO3 (80µL) were mixed with 3 mL of DMF/1,4dioxane/H2O(4:1:1) in a 4 mL glass container and tightly capped with a Teflon vial and heated at 50°C for two days. After cooling to room temperature, the vials were heated at 60°C for another two days and then decrease the temperature to room temperature. The resulting green polyhedral crystals were harvested and washed with DMF. Synthesis of PCP-32. 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 with a Teflon vial and heated at 65°C for two 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 1490343-1490344 contain the 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, ultra-high-purity grade were used throughout the adsorption experiments. All measured sorption isotherms have been repeated twice to confirm the reproducibility within experimental error. Mix 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 density functional theory (DFT) computations were performed with the Becke, three-parameter, Lee-Yang-Parr (B3LYP) functional in the Gaussian 09 package10. 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, wave functions of the optimized PCP structures were refined with larger basis set, 6311G(2d,2p). The three dimensional (3D) ESP was made from the converged wave function 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 bluecolored 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 atoms (O or Cu or H) is most close 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 PCP32 were calculated by the B3LYP-D3 functional12. Because PCP31 has Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2 paddlewheel 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 site. In PCP-31-M1 and PCP-31-M2, six and two paddlewheel units were considered, respectively. Similarly, two cluster models PCP-32-M1 and PCP32-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 PCP-32, 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


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Figure 1. Structures and Formation of (1) PCP-31 and (2) PCP-32: (a) Initial meso-pores 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 meso-pore, shaded by light blue, became a small meso-pore, shaded by deep blue, due to interpenetration. (c) Highly asymmetric dense and dilute OMS regions constituting of interpenetrated PCP-31. (d) Dense OMS region of different views in PCP-31. (e) Initial meso-pores of different size for PCP-32. (f) Formation of PCP-32 keeping the initial meso-pore 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 (d) and (h) reveals the region and direction of latent interaction from dense OMS (highlighted by dark blue). Other Cu ions from dense OMS region and dilute OMS region in (c), (d), (g) and (h) were highlighted by turquoise and blue, respectively. 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 atom, and the 6-31G for other atoms. The binding energies (BEs) were calculated with Eq.(1): BE = (EPCP + nEL – EPCP･nL) / n

(1)

where EPCP･nL is the total energy of PCP model system with n molecules of L (L = CO2 and C2H2), EPCP and EL are the total energies of PCP model system without gas molecule 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 StuttgartDresden-Born (SDB) ECPs13a, 13b for Cu, the 6-311G(2d,2p) for C2H2 and CO2, 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 method14. 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 R-3m space group with a = 42.321(5) Å and b = 30.872(6) Å by single-crystal X-ray diffraction (Table S1). In this meso-PCP framework, each ligand is connected to three Cu paddlewheel dimer clusters, with two of them coordinating to four discrete ligands (four-fold-connected 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 paddlewheel node, providing access to larger meso pores compared to those observed for six- (e.g., Cu3(ArCOO)6) and four-fold-connected clusters. Single-crystal X-ray diffraction revealedthat 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 caxis 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 (Figures 1c and 1d), 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 PCP31 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 four-fold-connected nodes and triply coordinated ligands. However, subsuming the doubly connected vertices into the cluster linkage, the generated basic unit of PCP-31 can be assigned to a tetracarboxylate linker joining the four-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 non-interpenetrated PCP-31 and thus highlighting the excellent mesoporous nature of this framework.



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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, PCP-32 exhibited a different coordination geometry, comprising purely four-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 (Figures 1e and 1f). 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 (Figures 1g and 1h). The large number of Cu dimer nodes in PCP-32 preserved its high-density 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 four-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 PCP31 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 PCP-32 make them great candidates for realizing ultra-high BrunauerEmmett-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 PCP-31 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 PCP-32 can be classified to the group of meso-pore materials having the dense OMS region (Table S2). The calculated pore size of PCP-31 and PCP-32, ~23 to 28 Å and ~19 to 35 Å, respectively, are in excellent agreement with the meso-pore 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, 21b, 21c 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 (Figure 3, S30 and S32).

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

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.


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For an equimolar mixture of C2H2 and CO2 at 298 K, the lowpressure 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 numb er of porous materials are capable of C2H2/CO2 separation, e.g., PCPs with dense OMSs (such as HKUST-1 (125), ZJU-40a (17-11)23, NOTT-101a (9-8)24 and PCP-33 (10-6)25), and without OMSs (TIFSIX-2-Cu-I (10-6)26, ZIF-8 (1.7-1.5)), as well as organic porous materials (HOF-3 (14-21)27), the current low-pressure 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 (Figures 3a and 3b), as further confirmed by the results of binary gas adsorption. Co-adsorption experiments were performed by exposing PCP-31 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%) outperformed ZIF-8 (84.2%) and HKUST-1 (90.1%) in terms of high-pressure C2H2 separation, the difference between PCP-31 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. 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) 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 PCP-32, 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 paddlewheel 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-32-M1), 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 paddlewheel unit, and simultaneously the H atom of this C2H2 molecule interacts with the formate O ( O for ) 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 O for . 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 ( O CO2 ) takes place in an η1-end-on manner, with the orientation different from that of C2H2 (Figure 5b). In this orientation, the

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 the similar enrichments of C2H2 at the high pressure both of which are higher than the enrichments in ZIF-8 and HKUST-1.

Figure 5. Most stable adsorption structures of C2H2 and CO2 in the dense OMS region. (a) PCP-31-M1 with C2H2, (b) PCP-31M1 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. Table 1. Binding energies (BE, kJ⋅mol-1)a of C2H2 and CO2 calculated with various PCP models.b Models C2H2 CO2 Cu2(O2CPh)4 32.7 23.8 Cu2(O2CPh)2(O2CH)2 36.0 23.9 58.9 38.8 PCP-31-M1 49.0 31.7 PCP-31-M2 37.2 29.6 PCP-32-M1 37.5 29.3 PCP-32-M2 a BE per gas molecule; a positive value represents that the adsorption is exothermic. b See Figure S45 for the adsorption structures of C2H2 and CO2 in Cu2(O2CPh)4, Cu2(O2CPh)2(O2CH)2, PCP31-M2, and PCP-32-M2.



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Figure 6. The 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/molecules, respectively.(See also Figures S47-S50) The area of the ESP cross sections, shaded by the blue-colored shapes whose definition is written 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. positively charged C atom (1.08 e) of CO2 ( C CO 2 ) prefers a posifor tion close to the formate O atom, increasing the BE. The O of the neighbor paddlewheel unit forms attractive interaction with the C CO 2 and electrostatic repulsive one with the O CO2 . Because O CO2 is more distant from the O for than the C CO 2 , the CO2 binding energy becomes larger with the model 1 than with Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2. This is the reason why BE of model 1 is larger than those with Cu2(O2CPh)4 and Cu2(O2CPh)2(O2CH)2. However, the attractive CCO2 -Ofor interaction is somewhat compensated by the repulsive O CO 2 -O for 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 paddlewheel 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 Figures 5c and 5d, gas molecules adsorbed in the dense OMS region of PCP-32 cannot effectively interact with neighboring paddlewheel units, since the OMSs are located at 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 mode 1 in PCP-31 but similar to those in PCP32. 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 PCP31 (93.7%) and PCP-32 (91.5%) achieve similar 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 DFT-optimized 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 (Figures 6a and 6b). Dilute OMS regions (slices 1 and 2 of PCP-31 and slices 5–7 of PCP-32) exhibited large ESP cross-sectional areas (blueshaded), 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 Figures 6c and 6d, where the area of these crosssections fitted by blue-colored simple shapes is plotted as a function of vertical distance from the top of PCP-31 and PCP-32, 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


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at low pressure, they cause molecular clogging at higher pressure and greatly suppress C2H2 adsorption. In fact, CO2 adsorption was less affected by asymmetric cross-sections due to its weaker interaction with OMSs (Figures 3a, 3b, 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/non-equilibrium 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.

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 extention for mesoPCPs. PCP-31 and PCP-32 demonstrate ultra-large 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.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Crystallographic data, TG curves, PXRD, results of density functionaltheory calculations, dual Langmuir−Freundlich isotherm model fitting, isosteric heat of adsorption calculation, IAST calculations of adsorption selectivities, and crystallographic data for PCP-31and PCP-3.

AUTHOR INFORMATION Corresponding Author Email: [email protected]; [email protected].

Present Addresses ¶

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.

ACKNOWLEDGMENT We thank the financial support from National Science Foundation of China (21671102), National Science Foundation of Jiangsu Province (BK20161538), State Key Laboratory of MaterialsOriented 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). We also thank the support from WPI-iCeMS), and the RIKEN SPring-8 Center. We also thank the topology discussion from Prof Michael O’Keeffe.

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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 HyeonDeuk∥, Simon Mathew‡¶, Shinpei Kusaka‡, Easan Sivaniah‡, Ryotaro Matsuda‡, Shigeyoshi Sakaki⊥, and Susumu Kitagawa‡,#*


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Density Gradation of Open Metal Sites in the ... - ACS Publications

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