Microporous LeadâOrganic Framework for Selective CO2 Adsorption

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Microporous Lead−Organic Framework for Selective CO2 Adsorption and Heterogeneous Catalysis Miroslav Almásǐ ,† Vladimír Zeleňaḱ ,*,† Róbert Gyepes,‡,§ Sandrine Bourrelly,⊥ Maksym V. Opanasenko,‡ Philip L. Llewellyn,⊥ and Jiří Č ejka‡ Department of Inorganic Chemistry, Faculty of Science, P. J. Šafárik University, Moyzesova 11, SK-041 54 Košice, Slovak Republic Department of Synthesis and Catalysis, J. Heyrovský Institute of Physical Chemistry of the ASCR, v.v.i., Dolejškova 2155/3, CZ-182 23 Prague 8, Czech Republic § Department of Education, University of J. Selye, Bratislavská cesta 3322, SK-945 01 Komárno, Slovak Republic ⊥ Aix-Marseille University, CNRS, MADIREL, F-133 97 Marseille Cedex 20, France † ‡

S Supporting Information *

ABSTRACT: A novel microporous metal−organic framework, {[Pb4(μ8-MTB)2(H2O)4]·5DMF·H2O}n (1; MTB = methanetetrabenzoate and DMF = N,N′-dimethylformamide), was successfully synthesized by a solvothermal reaction and structurally characterized by single-crystal X-ray diffraction. The framework exhibits a unique tetranuclear [Pb4(μ3-COO)(μ2-COO)6(COO)(H2O)4] secondary building unit (SBU). The combination of the SBU with the tetrahedral symmetry of MTB results in a three-dimensional network structure, with one-dimensional jarlike cavities having sizes of about 14.98 × 7.88 and 14.98 × 13.17 Å2 and propagating along the c axis. Due to the presence of four coordinately unsaturated sites per one metal cluster, an activated form of compound 1 (i.e., desolvated form denoted as 1′) was tested in gas adsorption and catalytic experiments. The studies of gas sorption revealed that 1′ exhibits a surface area (Brunauer−Emmett−Teller) of 980 m2·g−1. This value is the highest reported for any compound from the MTB group. Interactions of carbon dioxide (CO2) molecules with the framework, confirmed by density functional theory calculations, resulted in high CO2 uptake and significant selectivity of CO2 adsorption with respect to methane (CH4) and dinitrogen (N2) when measured from atmospheric pressure to 21 bar. The high selectivity of CO2 over N2 is mostly important for capturing CO2 from the atmosphere in attempts to decrease the greenhouse effect. Moreover, compound 1′ was tested as a heterogeneous catalyst in Knoevenagel condensation of active methylene compounds with aldehydes. Excellent catalytic conversion and selectivity in the condensation of benzaldehyde and cyclohexanecarbaldehyde with malononitrile was observed, which suggests that accessible lead(II) sites play an important role in the heterogeneous catalytic process.

I. INTRODUCTION Metal−organic frameworks (MOFs) have attracted tremendous interest in the recent years because of their unique structural features and perspective applications in gas storage and separation,1 catalysis,2 sensing,3 drug delivery,4 and nanoparticle preparation.5 This is because of their diversity and modularity, which offers the opportunity to systematically finetune and control their properties and performance. Usually MOFs are constructed from commercially available linear or bent poly(carboxylic acid)s, which act as linkers between metal ions or their clusters to give final three-dimensional (3D) porous structures.6 However, the demand to better control the final framework architecture leads to the design and synthesis of new linkers with specifically tailored dimensionality.7 Among them, the ligands with tetrahedral symmetry have received considerable interest recently because they are by nature 3D and extended linkers. Especially, carboxylic acids like © XXXX American Chemical Society

methanetetrabenzoic acid (H4MTB) and its germanium- or silicon-centered8 derivatives or adamantanetetracarboxylic9 and adamantanetetrabenzoic10 acids and others were to date used in the MOF design. Porous MOFs were shown to be effective sorbents of different gases, notably carbon dioxide (CO2), of which removal from the atmosphere or a flue gas produced in industry (especially in coal-powered stations) is becoming a serious environmental problem, causing global warming. The recent studies demonstrate that MOFs are promising CO2 adsorbents with the potential to replace existing materials for CO2 capture at low concentrations, ambient humidity, and moderate temperatures.11 To enhance the CO2 adsorption capacity and selectivity of MOFs, different approaches have Received: September 27, 2017

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DOI: 10.1021/acs.inorgchem.7b02491 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry been reported. The first is based on the building of a porous framework with a suitable pore size for CO2 adsorption either by use of a linker of appropriate dimensionality or by design of the structure with adjusted interpenetration.12 Another way to increase the CO2 adsorption capacity is to enhance the binding affinity between the framework and adsorbed CO2. This can be done, for example, by the generation of unsaturated coordination sites on metal ions, generated by the removal of coordinated terminal ligands from central atoms or by ion exchange, forming charged frameworks. CO2 adsorption can be increased also by the incorporation of polar functional groups as accessible strong Lewis basic sites (−OH, −NH2, −SH, etc.), which decorate organic linkers, or by postsynthetic modification of the final framework, which provides sites for interaction with CO2, thereby improving the affinity of MOFs toward CO2.13 The MOFs are also very interesting for applications in catalysis and liquid-phase reactions because MOFs combine the advantages of heterogeneous (recycling, stability, ease of separation, etc.) and homogeneous (selectivity, high efficiency, controllability under mild reaction conditions, etc.) catalysts. The possibility of tailored synthesis and organization of metal nodes and organic linkers in MOFs (playing the role of active catalytic sites), and their functionalization on the nanoscale level, opens the potential to build up MOFs particularly modified for catalysis.14 To date, MOFs have been used as solid catalysts in a variety of organic transformations such as Knoevenagel15 and Friedlander condensations,16 Pechmann,17 Henry,18 and Prins19 reactions, Huisgen cycloaddition,20 Beckmann rearrangement,21 and Friedel−Crafts alkylation/ acylation.22 In this study, we were particularly interested in the Knoevenagel reaction. The Knoevenagel reaction of active methylene compounds with aldehydes has been widely used in the synthesis of fine chemicals as well as heterocyclic compounds of biological importance. The Knoevenagel reaction is conventionally catalyzed by ionic liquids,23 surfactants,24 alkali metals,25 organic amines,26 and polyoxometalates27 under homogeneous conditions. Recently, different solid catalysts have been used to study this reaction such as zeolites,28 surface-modified mesoporous silica,29 covalent− organic frameworks (COFs),30 zeolitic imidazolate frameworks (ZIFs),31 and MOFs: MIL-100,32 MIL-101,33 IRMOF-3,34 UiO-67,35 MOF-76,36 and others. In the present study, a new MOF compound of the MTB4− series with the composition {[Pb4(μ8-MTB)2(H2O)4]·5DMF· H2O}n (1) was successfully prepared and characterized. Singlecrystal X-ray diffraction (XRD) showed a 3D polymeric framework with a one-dimensional jarlike cavity and active metal nodes. An activation study of the compound was monitored by a combination of IR measurements, obtained after heating of the compound to different temperatures, and thermogravimetric analysis (TGA) coupled with simultaneous evolved gas analysis (EGA) measurements (TGA−EGA) and powder X-ray diffraction (PXRD). After activation, 1 exhibits a stable porous framework with a specific Brunauer−Emmett− Teller (BET) surface area of 980 m2·g−1 and high CO2 adsorption capacity and high selectivity compared to those of methane (CH4) and dinitrogen (N2) at 303 K and pressure up to 21 bar. The catalytic properties of 1 were tested in the Knoevenagel condensation of cyclohexanecarbaldehyde and benzaldehyde with active methylene compounds (methyl cyanoacetate, ethyl acetoacetate, and malononitrile). The results confirmed that the compound shows high catalytic

activity in condensation reactions and a high selectivity to desired products, without a significant loss of catalytic activity even after repetition cycles.

II. EXPERIMENTAL SECTION Materials. The chemicals needed in the synthesis of the studied complex and reagents for catalytic reactions were purchased from Sigma-Aldrich or Acros Organics, respectively, and used without purification. Methanetetracarboxylic acid [tetraphenylmethane4,4′,4″,4‴-tetracarboxylic acid (H4MTB)] was prepared according to the literature procedure.37 The aldehydes cyclohexanecarbaldehyde (97%) and benzaldehyde (≥99%) and the active methylene compounds methyl cyanoacetate (99%), ethyl acetoacetate (≥99%), and malononitrile (≥99%) were received as commercial samples. In addition, n-dodecane (≥99%) was used as an internal standard and pxylene (≥99%) as the solvent in catalytic experiments. Compound {[Ni4(μ6-MTB)2(μ2-H2O)4(H2O)4]·10DMF·11H2O}n, used in the adsorption experiments, was prepared by the synthetic procedure described in ref 38. Synthesis of {[Pb4(μ8-MTB)2(H2O)4]·5DMF·H2O}n (1). The mixture of Pb(NO3)2 (10 mg, 0.03 mmol) and H4MTB (20 mg, 0.04 mmol) in 3 mL of N,N′-dimethylformamide (DMF), 3 mL of water (H2O), and 3 mL of ethanol (EtOH) were placed in a 20 mL glass Ace tube. The mixture was slowly heated to 353 K (at a heating rate of 1 K·min−1) and kept at 353 K for 24 h. Then the mixture was cooled to room temperature at a cooling rate of 0.1 K·min−1. The colorless crystals of 1 were filtered off and washed several times with a mixture of DMF/H2O/EtOH in the ratio 1/1/1 (v/v/v). Elem anal. Calcd for Pb4C73H77N5O26 (2269.21 g·mol−1): C, 38.64; H, 3.42; N, 3.09. Found: C, 38.77; H, 3.51; N, 3.12. FT-IR (KBr technique, cm−1): ν(OH) 3440(m, br); ν(CH)ar 3063(vw), 3036 (vw); ν(CH3)asym 2928(w); ν(CH3)s 2975(w); ν(CO) 1659(s); ν(CC)ar 1601(s), 1435(m); ν(COO−) as 1588(s), 1532(s); ν(COO−) s 1389(s); δ(CCH)ar 1187(w), 1138(w), 1101(w); γ(CCH)ar 844(m); δ(COO−) 777(m); γ(CO) 662(w) (s = strong, m = medium, w = weak, br = broad, asym = asymmetric, sym = symmetric, and ar = aromatic). Characterization. Elemental analysis was performed with a Vario MICRO CHNOS elemental analyzer from Elementar Analysensysteme GmbH. IR spectra were recorded with an Avatar FT-IR 6700 spectrometer in the range 4000−400 cm−1. The sample was prepared in the form of KBr pellets with a complex/KBr mass ratio of 1/100. Before IR measurements, KBr was dried at 973 K for 3 h in an oven and cooled in a desiccator. The room temperature spectrum was recorded after the preparation of a pellet of complex 1. The same pellet was then heated at 373, 473, 573, and 673 K for 15 min in an oven, and after this time, the IR spectrum of the sample was measured again. The thermal behavior of compound 1 was studied by TGA with simultaneous differential thermal analysis (DTA) and EGA in the region of 300−1200 K (TGA/DTG−DTA−EGA). The sample with a weight of approximately 30 mg was placed in an aluminum oxide (Al2O3) crucible and heated under a dynamic atmosphere of argon (50 cm3·min−1) at a heating rate of 9 K·min−1 using a Netzsch 409-PC STA apparatus. The residues evolved during thermal decomposition were monitored by an Aëolos QMS mass spectrometer coupled with the STA apparatus in the region of 1−300 amu. The crystallinities of the samples (as-synthesized, activated, and after stability tests in H2O at different pH values and after catalytic runs) were determined by PXRD measurements on a Bruker AXS D8 ADVANCE diffractometer in the Bragg−Brentano geometry using Cu Kα (λ = 1.54056 Å) radiation with a graphite monochromator and a positron-sensitive NaI dynamic scintillation detector (Vantec-1) in the 2θ range 4−40°. Structure Determination and Refinement. Compound 1 crystallizes as colorless prismlike crystals. Diffraction data were collected on a Nonius Kappa CCD diffractometer equipped with an APEX II detector (Mo Kα radiation, λ = 0.71073 Å) at 150(2) K. Data were processed by the diffractometer software.39 The phase problem was solved by direct methods (SHELXS 2013) and the structure refined by full-matrix least squares on F2 (SHELXL 2013).40 Diffraction maxima observed in the cavities could not be refined B


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Inorganic Chemistry

Figure 1. (a) Atomic labeling diagram of the [Pb4(μ3-COO)(μ2-COO)6(COO)(H2O)4] cluster with corresponding coordination polyhedra of lead(II) ions. (b) View of the framework of 1 along the c crystallographic axis, after subtraction of solvent molecules. experiments, N2 of 99.9995% purity was used, which was obtained from Linde Gas. The measurements of the gas adsorption up to 21 bar were realized at 303 K with gases CO2, CH4, and N2 using a homemade highthroughput instrument.47 Gas adsorption was measured via a manometric gas dosing system on six samples in parallel. The amounts of gas adsorbed were calculated by an equation of state using the Reference Fluid Thermodynamic and Transport Properties (REFPROP 8.0) software package of the National Institute of Standards and Technology.48 For the measurements, around 100 mg of the sample was used. The sample was thermally activated in situ under vacuum at 483 K overnight. The gases for measurements were obtained from Air Liquide. The purity of N2 and CH4 was 99.9995% (N55), and the purity of CO2 was 99.995% (N45). To evaluate the regenerability of the sample, two measurements with the same parameters were performed on the sample for each gas (CO2, CH4, and N2). Between the two experiments, the sample was submitted to evacuation at 303 K and under a primary vacuum for 80 min. Using this procedure, from the second gas adsorption measurement, the regenerability/recovery of the sample was checked. The selectivity of CO2 over N2 and CH4 was calculated by ideal adsorbed solution theory (IAST) in the range of pressures 1−10 bar. Adsorption selectivities for equimolar CO2/N2 and CO2/CH4 mixtures were predicted using pure-component isotherm fits, defined by

using a chemically plausible model and have thus been discarded using the SQUEEZE procedure in PLATON.41 Hydrogen atoms were refined isotropically and all other atoms anisotropically. Hydrogen atoms residing on aromatic carbon atoms were included in an ideal position with the C−H bond fixed to 0.95 Å and Uiso(H) assigned to 1.2Ueq of the adjacent carbon atom. The absorption correction was semiempirical on symmetry equivalents. The structure figures were drawn using DIAMOND software.42 Crystal data for 1: crystal size 0.52 × 0.16 × 0.13 mm, monoclinic, space group C2/c, a = 41.582(2) Å, b = 20.2045(10) Å, c = 28.205(3) Å, β = 131.6580(10)°, V = 17 704(2) Å3, Z = 8, Dcalcd = 1.758 g·cm−3, μ = 7.660 mm−1. Of the 74859 total reflections collected (−51 ≤ h ≤ 51, −20 ≤ k ≤ 25, −35 ≤ l ≤ 23), 18252 were unique (Rint = 0.0418) and used to solve the crystal structure. On the basis of these data and 748 refined parameters, final R indices [I > 2σ(I)], R1 = 0.0659, wR2 = 0.2021; R indices (all data), R1 = 0.1018, wR2 = 0.2158, and the goodness-of-fit on F2 is 1.147. Molecular Modeling. Density functional theory (DFT) computations of 1 have been performed on the bose cluster at the J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i,. using Gaussian 09, revision D.01. 43 Computations were conceived as spin-unrestricted DFT using Grimme’s B97D functional including a long-range dispersion correction44 and the LANL2DZ core potential45 employed for all atoms. The structure motif of the MOF was represented by 28 atoms (four Pb2+ ions and eight COO− groups), which were kept fixed during geometry optimization. The coordinates of all other atoms were allowed to vary. The interaction enthalpy between the MOF and its adsorbate molecules was computed assuming their infinite separation and employing the Counterpoise46 approach. Gas Adsorption Measurements and Analysis. The N 2 adsorption isotherm was measured using NOVA 1200e (Quantachrome) and ASAP 2020 (Micromeritics) automatic adsorption analyzers at 77 K. Before the adsorption studies, the sample was activated by evacuation at 483 K for 24 h to remove the guest molecules from the cavities. Compound {[Ni4(μ6-MTB)2(μ2H2O)4(H2O)4]·10DMF·11H2O}n, used in adsorption experiments, was activated by the procedure described in ref 38. The N2 adsorption isotherm for desolvated sample 1 (further denoted as 1′) was collected in a relative pressure range from p/p0 = 0.005 to 1. From the N2 adsorption isotherm, the BET specific surface area (SBET) and pore volume (Vp) of the sample were evaluated. In the adsorption

Si / j =

xi/xj yi /yj

where Si/j is the selectivity toward component i in a binary mixture with j (i stands for CO2 and j for CH4 or N2), and xi and yi denote the equilibrium mole fraction for component i in the adsorbed and gas phases, respectively. The IAST is fully explained in the original paper by Myers and Prausnitz.49 Catalysis. The Knoevenagel condensation of active methylene compounds and aldehydes was realized in a liquid phase under atmospheric pressure at temperature 403 K on a multiexperiment workstation StarFish. Before catalytic experiments, single crystals of the catalyst were gently ground to increase their active sites for catalytic experiments and then activated at 473 K for 90 min in a stream of air (50 cm3·min−1). Typically, 4 mmol of aldehyde, 0.4 g of C


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Inorganic Chemistry dodecane (internal standard), 50 mg of catalyst, and 10 mL of p-xylene (solvent) were added to the 25 mL three-necked vessel, equipped with a condenser and a thermometer. The final suspension was stirred and heated. After temperature 403 K was reached, 6 mmol of the methylene compound was added to the reaction vessel through a syringe. During the catalytic experiments, 0.1 mL of the reaction suspension was sampled after 10, 30, 60, 120, 240, 360, and 1440 min. After the sampling, the mixture was immediately centrifuged for 5 min at 4000 rpm. Then, the reaction products were analyzed by gas chromatography (GC) using an Agilent 6850 chromatograph with a flame ionization detector equipped with a nonpolar HP1 column (diameter 0.25 mm, thickness 0.2 μm, and length 30 m). The products were identified using GC/mass spectrometry (MS) analysis (ThermoFinnigan FOCUS DSQ II single quadrupole gas chromatograph/mass spectrometer). Moreover, the identity of the reaction products was also confirmed by 1H NMR spectroscopy (Varian Unity 400 spectrometer). 1H chemical shifts in CDCl3 were given with tetramethylsilane as the internal standard. The observed spectra corresponded to the published data.50 Recyclability of the catalyst was tested in the reaction of cyclohexanecarbaldehyde with malononitrile in 10 repetition cycles. After each test, the catalyst was separated by centrifugation, stirred in solvents with different polarities (EtOH, acetone, and n-hexane), filtered off, reactivated at 473 K for 90 min, and then reused in the next run. To evaluate a leaching of the active species from 1 during catalytic experiments, a fraction of the reaction mixture was separated and immediately centrifuged. The obtained liquid phase was reinvestigated in Knoevenagel condensation under the same reaction condition.

2.382(11)−2.714(10) Å for Pb2, 2.379(10)−2.667(12) Å, for Pb3 and 2.44371)−2.757(8) Å for Pb4 (Table S1). The distances of Pb2−O17 and Pb4−O5 [2.714(10) and 2.757(8) Å] are obviously longer than the others, indicating weak interaction (semicoordination). Anyway, the average bond lengths are comparable to those of other reported lead(II) carboxylate complexes.52 In 1, the discrete Pb4 cluster is surrounded by eight different MTB4− ligands and the secondary building unit can be described as a distorted hexagonal bipyramid (Figure S1). Topological analysis of 1 shows an unusual structure that does not imitate simple natural minerals like previously published compounds containing the MTB4− ligand, prepared via a combination of MTB4− with zinc(II) (PtS37 and DIAMOND 53), copper(II) (PtS 54), nickel(II) (CaF 238 and DIAMOND 55 ), cobalt(II) (CaF 2 56 ), and cadmium(II) (CaF257) as metal centers. Figure 1b depicts complex 1 exhibiting a 3D noninterpenetrated framework that contains open channels propagating along the c crystallographic axis. The channels are formed by repeated jarlike cavities, with a narrow entrance (14.98 × 7.88 Å2) and a wider inside pocket (14.98 × 13.17 Å2). Because the guest solvent molecules could not be determined by XRD data to be due to the high thermal disorder, they were proven by elemental analysis, TGA−EGA, and IR measurements at different temperatures. After removal of the guest molecules, the void space in the total crystal volume, determined by the PLATON program, is 8 913 Å3 (cell volume 17704 Å3) and corresponds to 50.3% of the crystal volume. Framework Stability. The thermal robustness of the framework and solvent removal process was studied by a combination of Fourier transform infrared (FT-IR) spectroscopy measured step by step at selected temperatures (HT-IR; Figure S2), TGA−EGA (Figure 2), and PXRD (Figure 3). TGA−EGA (Figure 2) indicates a mass loss of 19.8% in the temperature range 370−570 K, corresponding to the removal of nine H2O and five DMF molecules (calcd 20.1%) in one step (endothermic effect at 390 K on DTA). The MS spectra of the gaseous products of the first stage of thermal decomposition showed signals with 18 and 73 amu, which were assigned to

III. RESULTS AND DISCUSSION Description of the Crystal Structure. In the crystal structure of 1, two crystalographically independent MTB4− molecules and four independent lead(II) ions exist, which have different coordination numbers and geometries. The coordination polyhedron of each lead ion in the [Pb4(μ3-COO)(μ2COO)6(COO)(H2O)4] cluster is strongly distorted because of the stereochemically active 6s2 lone-pair electrons on the central lead(II) atoms, which has also been reported in other polynuclear clusters bridged by oxygen atoms.51 The Pb1 atom has a distorted tetragonal-bipyramidal geometry formed by six oxygen atoms (O7, O8, O10, O13, O17, and O18) from four different carboxylates. The carboxylate O10, O13, and O17 atoms make a bridge between the Pb1 and Pb2 atoms (see Figure 1a). The Pb2 atom is coordinated by seven oxygen atoms (O1, O2, O9, O10, O13, O14, and O17), forming a polyhedron with a distorted monocapped trigonal-prism shape. The carboxylate group, including the carbonyl C28 atom, is coordinated in a chelate−anti−anti fashion and makes a bridge between the Pb1, Pb2, and Pb3 atoms. The Pb3 atom is also 7connected with a distorted monocapped trigonal-prism geometry formed by five oxygen atoms (O5, O6, O9, O15, and O16) from four carboxylate MTB ligands and two coordinated H2O oxygen atoms (O11 and O12). The environment around the Pb4 atom can be described as eightcoordinated distorted bicapped trigonal prism, and it is coordinated with six oxygen atoms (O1, O2, O3, O4, O5, and O15) from four different MTB4− anions and two coordinated H2O molecules (O19 and O20). The lead(II) ions are connected in the sequence of Pb1− Pb2−Pb3−Pb4 by the carboxylate oxygen atoms of the MTB4− molecules, and the distances of Pb1−Pb2, Pb2−Pb3, Pb3−Pb4, and Pb4−Pb2 within the cluster unit are 4.0205(2), 4.1642(2), 4.1400(2), and 4.1520(2) Å, respectively. The Pb−O bond lengths fall in the range of 2.435(9)−2.661(8) Å for Pb1,

Figure 2. Thermoanalytical (TGA/DTG−DTA−EGA) curves of the thermal decomposition of 1 in an argon atmosphere. D


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Inorganic Chemistry

The stability of the prepared MOF was also tested after treatment of the compound in H2O at different pH values, and the stability of the framework was studied using PXRD and N2 adsorption measurements (Figures S3 and S4). As can be seen from Figures S3 and S4, a 7-day treatment of the sample in H2O at pH 2, 7, and 14 had no effect on the porosity and structural integrity of the sample (Figures S3 and S4). From the obtained results, it could be concluded that the prepared MOF is H2O-stable, which is an important property of the materials for further application in various industrial fields. In summary, the presented results of spectral, thermal, and PXRD studies confirm the stability of the framework after thermal treatment. On the basis of Kitagawa’s classification,58 compound 1 could be classified in the second MOF generation with a stable and robust porous framework, showing permanent porosity after the removal of guest molecules from the pores. Gas Adsorption Properties. The relatively large solventaccessible volume in 1 (50.3% of the cell volume 17704 Å3) made it challenging to investigate the gas adsorption properties of 1 using different gases (N2, CO2, and CH4) and different temperatures and pressures (from vacuum up to 21 bar). The solvents in the cavities of the as-synthesized sample 1 were removed by evacuation under dynamic vacuum and heating at 483 K for 24 h. By such a treatment, the desolvated compound {[Pb4(MTB)2]}n (denoted as 1′ from now on) was obtained. After the activation procedure, a weight loss of 18.9% was observed, and it is in good agreement with the thermogravimetry results. The N2 adsorption measurements at 77 K (Figure S5) revealed a type I isotherm according to IUPAC classification, which is typical for microporous materials. The small difference in the shapes of the ideal (type I isotherm) and measured N2 isotherms for 1′ could be explained by the presence of N2 weaker attractive forces, corresponding to fluid−pore wall attraction, and N2 interaction with a variety of surface active sites (e.g., formed after coordinated H2O elimination from the central ions) and other vacant sites (dislocation and defects in crystals) formed during the activation procedure. At relative pressure p/p0 = 0.95, the N2 uptake capacity reaches 278 cm3·g−1, corresponding to an experimental pore volume of around 0.43 cm3·g−1, and is close to the total microporous volume of 0.67 cm3·g−1 calculated by DFT. The experimental pore volume (Vp) was calculated by eq 1 in Supporting Information. Evaluation of the N2 isotherm using the BET equation gave a specific surface area (SBET) of 980 m2· g−1. It is of note that the observed SBET is the highest compared with other reported MOFs constructed from the methanetetrabenzoate linker such as {[Zn2(μ8-MTB)(H2O)2]·3DMF· 3H2O}n (248 m2·g−1),37 {[Cu2(μ8-MTB)(H2O)2]·6DEF· 2H2O}n (526 m2·g−1),54 {[Zn2(μ4-MTB)(κ4-CYC)2]·2DMF· 7H 2 O} n (644 m 2 ·g − 1 ), 5 3 and {[Ni 4 (μ 6 -MTB) 2 (μ 2 H2O)4(H2O)4]·10DMF·11H2O}n (700 m2·g−1).38 For the complexes {[Co 4 (μ 6 -MTB) 2 (μ 2 -H 2 O) 4 (H 2 O) 4 ]·13DMF· 11H2O}n56 and {[Ni2(μ4-MTB)(κ4-CYC)2]·4DMF·8H2O}n,55 adsorbing a limited amount of N2 [21.6 cm3·g−1 for cobalt(II) and 9.2 cm3·g−1 for nickel(II)], their surface areas, 356 and 141 m2·g−1, respectively, were estimated from the CO2 adsorption isotherms measured at 195 K using the Dubinin−Radushkevich model. The other frequent probe to investigate the sorption properties of MOFs is CO2. The adsorption/desorption isotherm of CO2 measured at 273 K is shown in Figure 4. The maximal storage capacity of CO2 at 1 bar and 273 K for 1′

Figure 3. PXRD patterns of 1 (a) calculated from single-crystal XRD, (b) experimentally measured on an as-synthesized sample, and (c) after 24 h of activation at 483 K.

H2O (18) and DMF (73) molecules. Next, heating of the sample caused thermal decomposition of the polymeric framework accompanied by an increase of the intensity of the EGA signals with 18 amu (H2O), 28 amu (CO), 44 amu (CO2), and 78 amu (benzene) with significant exothermic effects at 633, 708, 788, and 873 K on DTA. The organic part of the framework did not decompose completely upon heating to 1200 K because the decomposition of 1 was studied in an inert gas atmosphere (argon). The carbonized product was still present at 1200 K (residual mass 55.3%). FT-IR spectra of 1 also confirm the gradual release of solvent molecules, H2O and DMF, by decreasing the intensities of their characteristic vibrations in the spectra. Detailed information on the stability and vibrational features are depicted in Figure S2 and reported in the Supporting Information. A comparison of the PXRD pattern of 1 with the simulated pattern derived from the single-crystal XRD data of 1 is shown in Figure 3. A good correspondence between the simulated and experimentally measured patterns was found, which shows the phase purity of the as-synthesized compound. In the case of the desolvated sample 1′, the PXRD pattern is almost identical with the simulated as well as the experimentally measured pattern with only some loss of intensity. This indicates that the activated sample after heating at 483 K for 24 h is highly crystalline and retains its original framework after removal of the lattice solvent molecules without disruption of its structural integrity. E


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Inorganic Chemistry

Figure 4. CO2 adsorption/desorption isotherm measured over 1′ at 273 K up to 1 bar. Figure 5. Repeated high-pressure adsorption measurements of N2, CH4, and CO2 performed at 303 K up to 21 bar (N2 adsorption up to 17 bar) on activated compounds 1 and 2 (activated compounds are marked by single primes).

−1

is 2.1 mmol·g (9.3 wt %). As can be seen from Figure 4, the adsorption and desorption branches of the isotherm did not close, and the irreversibility is observed even at the lowest equilibrium pressures. Such behavior can arise from a strong interaction between the adsorbate and adsorption sites. A comparison of the adsorption capacity with the literature data has shown that the CO2 adsorption capacities were reported for four compounds containing the MTB ligand, namely, {[Zn2(μ8MTB) (H2O)2]·3DMF·3H2O}n (1.3 mmol·g−1, 5.67 wt %, 1 bar, 273 K),37 {[Co4(μ6-MTB)2(μ2-H2O)4(H2O)4]·13DMF· 11H2O}n (1.6 mmol·g−1, 7.02 wt %, 1 bar, 273 K),56 {[Zn2(μ4MTB)(κ4-CYC)2]·2DMF·7H2O}n (2.4 mmol·g−1, 10.5 wt %, 1 bar, 273 K), 53 and {[Ni4(μ6-MTB)2(μ2-H2O) 4(H2O) 4]· 10DMF·11H2O}n (3.2 mmol·g−1, 12.36 wt %, 1 bar, 273 K).38 As is evident, the adsorption capacity of 1′ is similar to that of the {[Zn2(μ4-MTB)(κ4-CYC)2]·2DMF·7H2O}n complex. Anyway, the adsorption uptake of 1′ is rather high compared to those of other MOFs previously proposed for CO2 separation purposes, such as Nd-RPF9 (0.5 mmol·g−1, 1 bar, 303 K, SBET = 13 m2·g−1),59 Sc2BDC3 (0.7 mmol·g−1, 1 bar, 303 K),60 UiO-66-2COOH (1 mmol·g−1, 1 bar, 303 K),61 and Sc2BDC3-NO2 (1.1 mmol·g−1, 1 bar, 303 K).62 Further, we were interested in exploration of the adsorption potential of 1′ at high pressures for N2, CH4, and CO2 as probe molecules. The repeated adsorption isotherms for 1′ were measured at temperature 303 K up to 21 bar. The excess uptake adsorption isotherms on compound 1′ after three repeated measurements are depicted in Figure 5. The absence of district plateaus in the isotherms, indicated that the maximal capacity was still not achieved and compound 1′ is not saturated by the gases at 21 bar. The amounts adsorbed for CH4 and N2 are relatively low, with saturation capacities of approximately 1.9 mmol·g−1 (21 bar) and 0.8 mmol·g−1 (17 bar), respectively. These adsorption results indicate the absence of specific adsorption sites for these two probes at the MOF surface. On the other side, the activated sample has a better adsorption capacity for CO2 molecules, being ∼4.5 mmol·g−1 at 21 bar. The sample 1′ adsorbs 2−4 times more CO2 than CH4 and N2. The enhanced affinity of the framework of 1′ toward CO2 could be assigned to different quadrupole moments (−Qij) of used gas molecules, 14.3 × 10−40 C·m2 (CO2) > 4.7 × 10−40 C· m2 (N2) > 0 C·m2 (CH4), and their kinetic diameters, which have the opposite trend: CH4 (3.8 Å) > N2 (3.64 Å) > CO2

(3.3 Å). The higher quadrupole moment of CO2 might induce stronger interactions with the framework of 1′, predominantly with coordinately unsaturated lead(II) centers, in comparison to N2 and CH4. Moreover, the observed adsorption trend could also be explained by the different boiling points of the gases: CO2, 195 K; CH4, 112 K; N2, 77 K. Furthermore, all isotherms are completely reversible and repeatable after primary vacuum treatment at 303 K, indicating that a simple regeneration process for this porous solid is possible. This property of the materials is important and desirable for their potential application in practice. For comparison and evaluation of the results obtained for 1, high-pressure adsorption measurements for another compound of the MTB family, namely, {[Ni 4 (μ 6 -MTB) 2 (μ 2 H2O)4(H2O)4]·10DMF·11H2O}n (2), were also performed. 2 was selected because of its similarity to 1. Both compounds are built from a methanetetrabenzoate linker and contain tetranuclear clusters with four unsaturated coordination sites after the desolvation process. Repeated high-pressure isotherms measured on activated compound 2′ are depicted in Figure 5. It is evident that 2′ has a slightly lower adsorption capacity of CO2, 4.1 mmol·g−1 (21 bar), in comparison to 1′. The higher adsorption capacity of 1′ compared to 2′ could be explained by the different surface areas (980 m2·g−1 for 1′ and 700 m2·g−1 for 2′) and the dimensionality of the pore system. Moreover, when we normalized the high-pressure adsorption isotherms from the original parameter (the adsorbed amount of the probe in millimoles per gram of the activated adsorbent) to the adsorbed amount in moles per 1 mol of the support (Figure 6), the adsorption capacity of compound 1′ showed even better adsorption properties because lead(II) is much heavier compared to nickel(II). After recalculation, 1′ adsorbs 8.1 mol·mol−1 (CO2), 3.5 mol·mol−1 (CH4), and 1.5 mol·mol−1 (N2) at maximal measured pressures and 303 K. These values are higher in comparison to those of 2′: 5.0 mol·mol−1 (CO2), 2.6 mol·mol−1 (CH4), and 1.1 mol·mol−1 (N2). On the basis of the presented results, it could be concluded that 1′ shows higher affinity and storage capacity of CO2 compared to 2′. F


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Inorganic Chemistry

molecules were all “pulled” from nearby lead(II) sites. The result coincides with our anticipation of the strong adsorption capacity of lead. Further studies showed that the geometries of the optimized structures for the same types of gas molecules were almost identical. The optimized structures with the highest adsorption energies are shown in Figure 7c,d. The average binding energy of the optimized configuration is −28.01 kJ·mol−1 for CO2 and −12.49 kJ·mol−1 for CH4. The low affinity of CH4 suggests the absence of specific interactions between CH4 and the cluster. The average distance between the oxygen atom of the CO2 molecule and the closest lead(II) atom is 3.081 Å, while the average distance between the hydrogen atom of CH4 and the lead(II) ions is longer, 3.467 Å. The calculated affinities and distances using DFT clearly pointed to a significant interaction of CO2 over CH4 to the lead cluster in 1′. To evaluate the gas adsorption ability of 1′, the IAST was employed for the prediction of the adsorption selectivities from the experimental pure-gas adsorption isotherms fitted for this purpose with a triple-site Langmuir equation.63,45 Figure 8

Figure 6. High-pressure adsorption measurements of N2, CH4, and CO2 at 303 K up to 21 bar on activated (′) compounds 1 and 2 after recalculation of the adsorbed amount per 1 g of activated adsorbent (mmol·g−1) to the adsorbed amount per 1 mol of activated support (mol·mol−1).

On the other hand, a slightly higher affinity of 2′ toward CH4 [2.1 mmol·g−1 (21 bar)] compared to that of 1′ was observed. The N2 adsorption isotherm for 2′ is similar to that measured for 1′, with a maximal capacity of 0.8 mmol·g−1 (17 bar) for 2′. For a better understanding of the enhanced affinity of CO2 over CH4 in 1′, DFT calculations, which are quite well established in the modeling of gas adsorption in MOFs, were performed. For the computational study, only the tetranuclear cluster resolved from the single-crystal X-ray data was chosen as a model (Figure 7a). From the cluster, terminal H2O molecules

Figure 8. IAST selectivities predicted between 0.5 and 15 bar for equimolar binary gas mixtures of CO2/N2 (red circles) and CO2/CH4 (black squares) on 1′ at 303 K.

shows the predicted adsorption selectivities for equimolar CO2/N2 and CO2/CH4 mixtures as a function of the pressure (between 0.5 and 15 bar), which shows that the material is selective toward CO2. The CO2/CH4 adsorption selectivity remains almost constant, around a value of 7, as the pressure increases, despite an increase at low pressure. It can be seen that the predicted CO2/N2 selectivities are not constant as a function of the pressure. The CO2/N2 selectivity at 0.5 bar is approximately 28 and drops to 17 at 15 bar. The high initial selectivity can be explained by the adsorption of CO2 on the open metal sites at low loading, which are the lead(II) clusters acting as Lewis acid sites. When these strongest specific adsorption sites are occupied, the CO2 molecules cannot interact anymore with them and, accordingly, adsorb less strongly on the surface, leading to a weaker selectivity. This effect was also observed in other works.64,65 The higher selectivities of CO2/N2 can be explained by the lower uptake of pure N2 compared to CH4. The same tendencies in the selectivities as a function of the pressure could be expected because CH4 and N2 are both similarly neutral

Figure 7. (a) View of the lead(II) cluster with coordinated H2O molecules (blue ellipsoids), solved from single-crystal X-ray data. (b) Initial model used in the DFT calculation and the final calculated geometries of (c) CO2 and (d) CH4 adsorbing onto an unsaturated [Pb4(μ3-COO)(μ2-COO)6(COO)(H2O)4] cluster using DFT calculations.

were removed and vacant orbitals served as a source of the primary adsorption sites (Figure 7b). To achieve the most stable configuration of the adsorbate−adsorbent structure, geometry optimization on several initial positions of adsorbed molecules was performed. The investigated gas molecules were first placed randomly with different distances and directions from lead(II) metal sites, and subsequently geometry optimization followed. Then we found out that the gas G


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Inorganic Chemistry adsorbates, showing no particularly strong interactions with the coordinately unsaturated metal sites present in the material. However, the CO2/CH4 selectivity is constant in the whole pressure range. This is surprising, but maybe we can explain this result by a higher interaction of CH4 for the adsorbent compared to N2. Indeed, the high polarizability of CH4 (2.59 × 10−3 nm3 > 1.74 × 10−3 nm3 for N2), combined with a possible affinity with the organic part of the material, could balance somehow the strong adsorption of CO2 on the lead(II) CUS, resulting in a moderate constant selectivity. It is of note that this CO2/CH4 predicted selectivity for 1′ is comparable to those of previously studied well-known MOF materials such as MIL-100(Cr) (selectivity 6−8, SBET = 1720 m2·g−1, two unsaturated sites per cluster),64 MIL-53(Al) (selectivity 7−8, SBET = 950 m2·g−1, without unsaturated active sites),66 and HKUST-1 (selectivity 6−9, SBET = 2210 m2·g−1, two unsaturated sites per cluster).67 It is more delicate to compare the selectivities toward CO2 in N2 because, to the best of our knowledge, no experimental CO2/N2 binary isotherms have been reported yet. The review of Sumida et al.68 provides a table of selectivity for selected MOFs, calculated at low pressures (directly from the pure-component isotherms and not predicted by using the IAST). The value obtained at low pressure on 1′ in this study, around 28, is in a comparable order of magnitude with selectivities predicted under the same conditions by Wiersum,65 for HKUST-1 (selectivity ∼20), UiO-66-NH2 (selectivity ∼33), MIL-100(Fe) (selectivity ∼38), and MIL-101(Cr) (selectivity ∼40). Good selectivities may indicate that 1′ has great potential in the adsorptive separation of CO2/N2 and CO2/CH4 gas mixtures. Mainly, the high selectivity of CO2 over N2 is important for the capture of CO2 from the atmosphere to decrease the greenhouse effect on the environment. Knoevenagel Condensation Reactions. Knoevenagel condensation is an important C−C bond coupling reaction in organic chemistry, in which a carbonyl group (aldehyde) reacts with the methylene group activated by two electron-withdrawing groups. This reaction is widely used in the synthesis of pharmaceuticals and fine chemicals.30 The condensation can be acid-catalyzed (Lewis acids) or base-catalyzed, abd it can be homogeneous or heterogeneous.15,23−36 In the present study, Knoevenagel condensation of cyclohexanecarbaldehyde and benzaldehyde with different active methylene compounds was performed. The general scheme of the condensation reaction with reaction conditions and structures of the substrates with corresponding molecular sizes are depicted in Figure 9. The results obtained from the catalytic experiments are listed in Table 1. In the presence of the catalyst, the reactions of benzaldehyde/cyclohexanecarbaldehyde with malononitrile (1/1.5 molar ratio) occurred smoothly and gave 2-benzylidenemalononitrile and 2-(cyclohexylmethylene)malononitrile, respectively, as the only products, with a yield of 100% with complete conversion after 4 h. The catalytic properties of 1′ for the Knoevenagel condensation of aldehydes with methyl cyanoacetate and ethyl acetoacetate were also investigated. We observed a decrease in the activity of 1′ in cyclohexanecarbaldehyde condensation with methyl cyanoacetate, especially with ethyl acetoacetate, in comparison to benzaldehyde. As can be seen from Figure 10 and Table 1, the 100% conversion of methyl cyanoacetate or ethyl acetoacetate with selected aldehydes after 8 h was observed only in the reaction of cyclohexanecarbaldehyde with methyl cyanoacetate.

Figure 9. (a) Scheme of Knoevenagel condensation with experimental conditions. (b) Estimated diameters of the active methylene compounds: malononitrile, methyl cyanoacetate, and ethyl acetoacetate. (c) Aldehydes used in Knoeveneagel condensation: cyclohexanecarbaldehyde and benzaldehyde.

Table 1. Catalytic Results of the Knoevenagel Condensation of Cyclohexanecarbaldehyde and Benzaldehyde with Different Methylene Substrates Performed at 403 Ka aldehyde

methylene compound

run

conversionb

selectivityc

PhCHO

MNT

1st 2nd 3rd 4th 6th 8th 10th

ChCHO

MCA EAA MNT MCA EAA

100 100 100 100 100 100 100 100 68 100 61 57

100 97 98 98 99 97 98 100 87 100 100 55

a

ChCHO = cyclohexanecarbaldehyde, PhCHO = benzaldehyde, MNT = malononitrile, MCA = methyl cyanoacetate, and EAA = ethyl acetoacetate; The yield is calculated by GC using n-dodecane as the internal standard. bConversion after 8 h of reaction. cSelectivity after 8 h of reaction.

In the case of other reactions, the conversions ranged from 57 to 68%, with yields of 100% of the desired products in the reactions with methyl cyanoacetate. On the other hand, selectivities of below 87% (benzaldehyde) and 55% (cyclohexanecarbaldehyde) were achieved in the reactions with ethyl acetoacetate. The observed trend could be explained by the sizes of the molecules: the molecule of cyclohexanecarbaldehyde is bulkier compared to benzaldehyde (Figure 9c), and reactions are limited by the pore/cavity size of the catalyst (14.98 × 7.88 and 14.98 × 13.17 Å2). The decreased reactivity in the order ethyl acetoacetate < methyl cyanoacetate < malononitrile well agrees with the increase in the acid dissociation constant of these active methylene substrates.69 It should be noted that this sequence of substrate reactivities is in good agreement with those in our previous reports.36,38 The conversion of benzaldehyde and malononitrile in Knoevenagel condensation was only slightly dependent on the weight of 1′ (Figure 11a). In the corresponding condensation reaction, 100% conversion of the targeted product after 60 min of reaction was achieved when using 150 mg of 1′, while conversions equal to 94% and 89% were observed in the presence of 100 and 50 mg of 1′, respectively. H


Article

Inorganic Chemistry

Figure 10. Conversion (a) and selectivity (b) curves of the desired products between cyclohexanecarbaldehyde (ChCHO), benzaldehyde (PhCHO), and different methylene compounds: malononitrile (MNT), methyl cyanoacetate (MCA), and ethyl acetoacetate (EAA).

On the other side, the selectivity to 2-benzylidenemalononitrile was almost 100%, independent of the weight of the catalyst. On the other hand, conversion of the respective product, formed in the presence of 1′ (50 mg) in the benzaldehyde condensation with malononitrile, was dependent on the molar ratio of aldehyde/malononitrile used. As can be seen from Figure 11b, when the molar ratio of the reaction substances was 1/1, after 30 min of reaction, the conversion of the desired product was 54%. When the molar reaction of the reactants increased to 1/1.5 and 1/2, the trends of the conversion curves were similar and the corresponding yields at the same reaction time were comparable: 82 and 83%, respectively. To clarify whether the catalysis in the presence of 1′ is heterogeneous or homogeneous, the catalyst was removed by “hot filtration” after 30 min of reaction of malonitrile with benzaldehyde. The filtrate was kept under the same conditions and monitored by GC. As can be seen from Figure S6, after filtration, the conversion of benzaldehyde stops at ∼50% after removal of the catalyst. These results show that the catalysis is heterogeneous and no leaching of the catalytic species from the framework into the liquid phase occurs. The recyclability of 1′ was tested for the reaction of cyclohexanecarbaldehyde with malononitrile. The activity of the catalyst after 10 catalytic runs remains at high levels (conversion 100%; selectivity 97−100%; Figure S7).

Figure 11. (a) Effect of the catalyst weight (50, 100, and 150 mg) in Knoevenagel condensation of benzaldehyde and malonotrile over 1′ performed at 403 K and a molar ratio of 1/1.5 (benzaldehyde/ malononitrile). (b) Effect of the substance molar ratio (aldehyde/ active methylene compound) on conversion of the corresponding product for condensation of benzaldehyde catalyzed by 1′ at the reaction temperature 403 K.

[Pb4(μ3-COO)(μ2-COO)6(COO)(H2O)4] clusters bridged by MTB linkers and exhibits a noninterpenetrated 3D framework with repeated jarlike cavities. The stable desolvated framework with permanent porosity can be prepared by heating the compound to 550 K. The porous nature of the complex was confirmed by various gas adsorption measurements up to 21 bar, showing the highly selective adsorptive separation of CO2 over N2 and CH4. DFT calculations identified directly the functional binding sites located on the lead cluster after removal of terminal H2O molecules. We demonstrated that the open lead(II) coordination sites play an important role in the adsorption process. The results of the catalytic tests clearly confirm the excellent catalytic activity of 1′ for Knoevenagel condensation reactions. Thus, the sustainability and high potential of 1 for application in gas separation and catalysis was demonstrated in multiple experiments, while the origin of such an efficiency was explained using computational approaches. We plan to further investigate compound 1 in Knoevenagel condensation of different bulky aldehydes with active methylene compounds and, in this way, to study more in

IV. CONCLUSIONS We have synthesized and characterized a novel lead metal− organic framework, {[Pb4(μ8-MTB)2(H2O)4]·5DMF·H2O}n (1). Compound 1 is built by asymmetric tetranuclear I


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Inorganic Chemistry

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depth the effect of the pore size of the catalyst on its catalytic performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02491. Selected structural parameters, FT-IR spectra measured at different temperatures, PXRD patterns, results of repeated catalytic experiments, and gas adsorption studies not included in the main text (PDF) Accession Codes

CCDC 1494847 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 data_ [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 Author

*E-mail: [email protected]. ORCID

Vladimír Zeleňaḱ : 0000-0002-6118-1269 Philip L. Llewellyn: 0000-0001-5124-7052 Present Address

Department of Inorganic Chemistry, Faculty of Science, P. J. Šafárik University, Moyzesova 11, SK-041 54 Košice, Slovak Republic. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Scientific Grant Agency of the Slovak Republic (VEGA) under Project 1/0745/17, by the Slovak Research and Development Agency under Contract APVV-15-0520, and by Project VVGS-2016-249 from P. J. Šafárik University. J.Č . acknowledges the Czech Science Foundation for Project P106/12/G015, and R.G. thanks OP VVV “Excellent Research Teams” for Project CZ.02.1.01/0.0/ 0.0/15_003/0000417-CUCAM.

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REFERENCES

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