A Selenophene-Incorporated Metal–Organic Framework for Enhanced CO2 Uptake and Adsorption Selectivity

A new metal–organic coordination polymer [Zn2(sedc)2(dabco)] (1se; sedc2− = selepophene-2,5-dicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane) was synthesized and characterized by single-crystal X-ray diffraction analysis. This MOF is based on {Zn2(OOCR)4N2} paddle wheels and is isoreticular to the family of [Zn2(bdc)2(dabco)] derivatives (1b; bdc2− = 1,4-benzenedicarboxylate) with pcu topology. The gas adsorption measurements revealed that 1se shows a 15% higher CO2 volumetric uptake at 273 K and 28% higher CO2 uptake at 298 K (both at 1 bar) compared to the prototypic framework 1b. Methane and nitrogen adsorption at 273 K was also investigated, and IAST calculations demonstrated a pronounced increase in CO2/CH4 and CO2/N2 selectivity for 1se, compared with 1b. For example, the selectivity factor for the equimolar CO2/CH4 gas mixture at 1 bar = 15.1 for 1se and 11.9 for 1b. The obtained results show a remarkable effect of the presence of selenium atom on the carbon dioxide affinity in the isoreticular metal–organic frameworks with very similar geometry and porosity.


Introduction
Carbon dioxide is a corrosive gas and a major atmospheric pollutant, causing a greenhouse effect. Selective capture and sequestration of carbon dioxide is one of the challenging problems in the modern chemical industry. Typically, a chemical sorption of CO 2 by amines is highly effective; however, the corrosive nature of the chemosorbents and substantial energy penalties during their regeneration demand new viable alternatives [1][2][3][4]. Porous coordination polymers or metal-organic frameworks (MOFs) are recently emerged materials with the greatest potential for adsorption of gases. Remarkable porosity, sufficient stability, as well as vast versatility of the structural and functional design make this class of porous materials among the most perspective adsorbents for selective CO 2 capture from natural gas or industrial flue gas [5][6][7][8][9][10]. Modular design is a widely used principle to tailor structural and/or functional properties within a certain type of isostructural MOF family, such as IRMOF-1 [11][12][13], MIL-53 [14][15][16], or UiO-66 [17][18][19], which are typically based on the linear 1,4-benzenedicarboxylate (bdc 2− ) linker. Other than a bent shape, the striking feature of ditopic dicarboxylate linkers based on heterocyclic core (Scheme 1) is the polarity of the core and the polarizability of a heteroatom, enabling additional induced dipole interactions with a substrate. Recently, a number of research groups have shown that the incorporation of 2,5-furanedicarboxylate (fdc 2− ) or 2,5-thiophenedicarboxylate (tdc 2− ) into porous MOF structures imbue such materials with quite unique features such as selective ion adsorption, luminescence sensing [20][21][22], dielectric bistability [23], and gas/vapor uptake [24,25].

Structure Description
The coordination polymer 1se was synthesized by the solvothermal reaction of Zn(NO 3 ) 2 ·6H 2 O, H 2 sedc, and dabco in DMF at 100 • C for 24 h. According to single-crystal X-ray diffraction data, 1se crystallizes in the tetragonal space group P-42 1 c. The asymmetric unit contains two crystallographically independent Zn(II) atoms with the same coordination environment of four O atoms of four bridging carboxylate groups and one N atom of dabco bridge. Zn-O distances lie in the range 2.015(2)-2.062(2) Å. Zn-N distances are 2.040(2) and 2.047(2) Å. Zn(1) and Zn(2) atoms form binuclear 'paddlewheel' blocks {Zn 2 (OOCR) 4 } interconnected by four sedc 2− anions into square-grid network. The angular shape of the sedc 2− ligands results in a considerable distortion of the binuclear blocks ( Figure 1a) and corrugation of the square-grid layers (Figure 1b). These layers are bound by dabco bridges to form a three-dimensional porous framework with a primitive cubic topology (pcu)-the same as the other MOFs of the isoreticular family [Zn 2 (bdc) 2 (dabco)]. The coordination network in 1se contains a three-dimensional system of intersecting channels, with the widest channels running along the c axis, across the windows of the square-grid layers (see Figure 1b). The apertures of these channels are 5 × 8 Å. Two different types of smaller channels across the main channels of the tetragonal structure have smaller the apertures of ca. 2.5 × 5 Å or 3.5 × 4 Å, depending on the particular arrangement of arks of the sedc 2− ligands ( Figure 1c). Importantly, all selenium atoms of the heterocycles are immediately accessible for interactions with potential substrate molecules, allowing a clear verification of the hypothesis of this work. We should also point out that the crystal structure 1se is similar to the known compound [Zn 2 (tdc) 2 (dabco)] (1t), based on 2,5-thiophenedicarboxylate [31] although the distortions of the binuclear blocks {Zn 2 (OOCR) 4 } and the square-grid layers in 1t are even more pronounced as a result of stronger bending of tdc 2− ligand [34], compared with that of sedc 2− .

Characterization and Activation
To evaluate the thermal stability of 1se, thermogravimetric analysis (TGA, Figure S1) was performed. The first observed 30% weight loss step occurs between 80 • C and 200 • C, referring to the evaporation of four guest DMF molecules per formula unit (calculated: 30%). The metal-organic framework 1se itself is stable up to ca. 280 • C, after which the irreversible framework decomposition apparently takes place. Such thermal stability is comparable to other reported MOFs containing sedc 2− anions (T decomp = 250 ÷ 320 • C) [32,33]. Such substantial difference between the temperature of the evaporation of guests and the temperature of the framework decomposition makes it possible direct activation of 1se by heating in a vacuum, obtaining the guest-free activated framework [Zn 2 (sedc) 2 (dabco)] (1se ). The infrared spectra of both 1se and 1se ( Figure S2) contain the characteristic bands of Csp 2 -H valence vibrations (3073 cm −1 ), Csp 3 -H valence vibrations (2965 cm −1 and 2935 cm −1 ), antisymmetric (1590 cm −1 ) and symmetric (1360 cm −1 ) carboxylate group vibrations. Infrared spectrum of 1se also contains a characteristic band of CO amide stretchings (1666 cm −1 ), which is absent on the spectrum of 1se , confirming a complete desolvation of metal-organic framework 1se during the activation process. The PXRD data ( Figure 2) suggest the phase purity of the synthesized compound 1se. The powder diffraction pattern of 1se is generally very similar to that of 1se although there is a noticeable shift of some of the reflexes to lower angles. For example, the (2 0 0) reflection at 2θ = 8.57 • (1se) is shifted to 2θ = 8.36 • (1se ) indicating a slight extension of the metal-organic framework upon its activation. Similar guest-assisted breathing of the framework was observed earlier for 1b and 1t [31,35]. The unit cell parameters for 1se were refined according to the powder data using the Powdercell program [36] and provided in Table A2. While the crystallographic parameter c is almost intact, the parameters a and b in 1se are longer by ca. 2.6% than in 1se, likely due to certain straightening of the sedc 2− dicarboxylate bridges. Overall, the unit cell volume is expanded by 5.2% during the activation of the compound.

Adsorption Measurements
The textural characteristics of the evacuated compound 1se were studied by a nitrogen porosimetry at 77 K. The adsorption isotherm ( Figure 3) corresponds to the type I with no hysteresis, typical to microporous adsorbents. The pore volume measured at p/p 0 = 0.95 is V pore = 0.57 cm 3 ·g −1 ; the calculated BET surface area is A BET = 1504 m 2 ·g −1 , respectively (see other details in Table S1). The experimental V pore is very much consistent with the theoretically expected value based on a solvent accessible volume of 1se , calculated using PLATON routine [37] (0.58 cm 3 ·cm −3 or 0.55 cm 3 ·g −1 ). The pore-size distribution, calculated by DFT method from the N 2 adsorption isotherm, gives a value of a pore size near 7 Å, which corresponds to the van der Waals diameter of the large cuboidal cages inside the pcu net. For a reference, the pore volume of the activated MOFs based on thiophenedicarboxylic acid (1t ) or benzenedicarboxylic acid (1b ), reported earlier, are 0.68 cm 3 ·g −1 and 0.75 cm 3 ·g −1 , respectively. The gravimetric porosity directly depends on molecular weight of a compound, which is notably higher for 1se, than for 1t and 1b. The comparison of experimental volumetric porosities for these MOFs results in a more or less comparable values: 0.60 cm 3 ·cm −3 (1se ), 0.64 cm 3 ·cm −3 (1t ), 0.62 cm 3 ·cm −3 (1b ), see also Table S2. Most importantly, the compounds 1b, 1t, and 1se represent a suitable family of porous materials where the influence of different heteroatoms on the gas adsorption properties could be systematically analyzed and assessed since the other parameters are almost identical. Being motivated by such an opportunity, we investigated the adsorption properties of 1se towards industrially important gases (CO 2 and CH 4 ) and compared the obtained data with the other prototypes 1b and 1t . The gravimetric CO 2 adsorption uptakes at 1 bar for 1se are 110 cm 3 ·g −1 (273 K, see Figure S3) and 46 cm 3 ·g −1 (298 K). Such numbers are comparable with the literature data of CO 2 adsorption by 1b (122 cm 3 ·g −1 at 273 K, 46 cm 3 ·g −1 at 298 K). Taking into account the crystallographic densities, the corresponding volumetric uptakes for 1se were calculated to be 116 cm 3 ·cm −3 (273 K) and 48.5 cm 3 ·cm −3 (298 K), which exceeds those for 1b by 15% (101 cm 3 ·cm −3 ) at 273 K (see Figure 4) and by 28% (38 cm 3 ·cm −3 ) at 298 K, respectively, convincingly confirming a positive effect of the polarizable heteroatom (Se) on the absorption properties of porous materials [38][39][40][41]. Similarly, the isosteric heat of CO 2 adsorption at zero coverage Q st (0) for 1se (19.9 kJ·mol −1 ) is greater than for 1b (19.0 kJ·mol −1 ), indicating stronger binding of CO 2 with the porous framework containing the selenophene heterocycle. We must mention here that the CO 2 adsorption by 1t is still the highest among the MOFs discussed here, both in terms of the gravimetric uptakes (153 cm 3 ·g −1 at 273 K, 67.5 cm 3 ·g −1 at 298 K, 1 bar) and volumetric uptakes (143 cm 3 ·cm −3 at 273 K, 63.1 cm 3 ·cm −3 at 298 K, 1 bar). The isosteric heat of CO 2 adsorption by 1t (23.7 kJ·mol −1 ) also suggests that the thiophene moieties have greater impact on the CO 2 adsorption. It is probably the polarity of the aromatic ring that contributes to a stronger binding between the polar CO 2 guest and porous MOF host. Based on the experimental data, the following dependence of the CO 2 uptake on the nature of the dicarboxylate anion was established: tdc 2− > sedc 2− > bdc 2− . This dependence strengthens the earlier claim that the substitution of the common terephthalate linkers to heterocyclic ones should enhance the adsorption properties of the MOF material due to induced dipole interactions. In terms of the gas storage, the thiophene-containing tdc 2− seems to be an optimal choice for such substitution since the incorporation of heavier sedc 2− no longer improves the gas adsorption capacity of the framework.
The CH 4 and N 2 adsorption-desorption isotherms for 1se and 1b were measured up to p = 1 bar at 273 K. The gas adsorption isotherms are shown on the Figure 5. For 1se the gravimetric adsorption volumes at 1 bar are 16.2 cm 3 ·g −1 (CH 4 ) and 6.0 cm 3 ·g −1 (N 2 ) adsorption, respectively. For 1b , the corresponding uptakes are 20.0 cm 3 ·g −1 and 6.6 cm 3 ·g −1 . The gravimetric gas uptakes for 1se are slightly lower than for 1b , mainly due to the higher density of the former. On the contrary, the volumetric assessment indicates higher gas adsorption by 1se , than by 1b , supporting the concept of stronger van der Waals interactions of methane/nitrogen with sedc 2− than with bdc 2− .  The sequestration of CO 2 from N 2 or from CH 4 is a critical technology for a reduction of environmental risks and for protection of natural gas pipelines, respectively. The CO 2 /CH 4 as well as CO 2 /N 2 gas adsorption selectivity factors were calculated by three commonly employed approaches: (i) as the ratio of the adsorbed volumes at 1 bar (S V ), (ii) as the ratio of Henry constants (S K ), and (iii) by the ideal adsorbate solution theory (S IAST ). The calculated selectivity values are summarized in Table 1, and the details of the calculations are provided in Tables S3 and S4. By any criteria used, the incorporation of sedc 2− evidently increases the adsorption selectivities of 1se , compared with the prototypic 1b . The calculated CO 2 /CH 4 selectivity factors for 1se (S V = 6.8, S K = 4.8, S IAST = 5.6) are comparable or even exceed those reported for other MOFs with promising application potential for separation of such small molecules [42][43][44]. The CO 2 /N 2 adsorption selectivity factors for 1se are also quite remarkable S V = 18.6, S K = 12.9, S IAST = 15.1 ( Figure 6) for a porous MOF with no unsaturated metal centers. The results obtained for 1se are superior to the CO 2 /N 2 selectivity factors for both 1b and 1t . Particularly, the IAST CO 2 adoption selectivity for the equimolar CO 2 + N 2 gas mixture for 1se is ca. 25% greater than for 1b or for 1t (see also Figure S4). Such remarkable increase should apparently be attributed to the nature of the heterocyclic moiety since the pore geometry and other structural parameters of the investigated MOFs are, essentially, the same. However, the contribution of sieving effect is also possible, as the channels in 1se situated along two of three directions have the apertures smaller than 5Å [45,46]. In terms of the CO 2 /N 2 selective separation, the incorporation of the selenophene-containing anion provides the best performance in the row: sedc 2− > tdc 2− > bdc 2− . Moreover, a rather low CO 2 adsorption enthalpy for 1se (19.9 kJ·mol −1 ) ensures the facile regeneration of the porous adsorbent in a cyclic CO 2 sequestration process. The unique combination of remarkable CO 2 /N 2 adsorption selectivity, high CO 2 uptake, and one of the lowest CO 2 adsorption enthalpies puts the title MOF 1se among the best porous materials for practical purification of the industrial exhausts.  6. IAST selectivity factors on CO 2 mole fraction in binary gas mixtures: CO 2 /N 2 (a) and CO 2 /CH 4 (b) at 273K and P total = 1 bar.

Instruments
IR spectra in KBr pellets were recorded in the range 4000−400 cm −1 on a VERTEX 80 spectrometer. Elemental (C, H, N) analysis was made on a varioMICROcube analyzer. Powder X-ray diffraction (PXRD) analysis was performed at room temperature on a Shimadzu XRD-7000 diffractometer (Cu-Kα radiation, λ = 1.54178 Å). Thermogravimetric analysis was carried out on a Netzsch TG 209 F1 Iris instrument. The experiments were carried out under He flow (30 cm 3 ·min −1 ) at a 10 K·min −1 heating rate. Adsorption experiments were performed using Quantachrome Autosorb iQ device.
Low-pressure gas adsorption isotherms at 273 K and 298 K were recorded with a thermostat TERMEX Cryo-VT-12 to adjust temperature with 0.1 K accuracy. The database of the National Institute of Standards and Technology was used as a source of p−V−T relations at experimental pressures and temperatures. Elemental (Zn, Se) ICP-MS analysis was carried out using Agilent 8800. The samples of 1se and 1se were digested in the mixture of HCl 36% water solution and H 2 O 2 30% water solution, then diluted by water prior to ICP-MS.

X-ray Crystallography
Diffraction data for single-crystal 1se were obtained at 130 K on an automated Agilent Xcalibur diffractometer equipped with an area AtlasS2 detector (graphite monochromator, λ(MoKα) = 0.71073 Å, ω-scans with a step of 0.5 • ). Integration, absorption correction, and determination of unit cell parameters were performed using the CrysAlisPro program package [47]. The structures were solved by dual space algorithm (SHELXT [48]) and refined by the full-matrix least squares technique (SHELXL [49]) in the anisotropic approximation (except hydrogen atoms). Positions of hydrogen atoms were calculated geometrically and refined in the riding model. The crystallographic data and details of the structure refinement are summarized in Table A1. CCDC 2026693 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at https://www.ccdc.cam.ac.uk/structures/.

Synthetic Procedures
Synthesis of [Zn 2 (sedc) 2  Synthesis of [Zn 2 (bdc) 2 (dabco)] (1b ) The sample of 1b was activated by keeping in a primary vacuum (10 −9 bar) at 50 • C for 2 h, then at 80 • C for 2 h, and at 100 • C for 12 h with 1 • ·min −1 heating and cooling rates. PXRD data ( Figure S6) confirmed the phase purity of the product.
Supplementary Materials: The following are available online. Figure S1: TG plot of the synthesized 1se; Figure S2: IR spectra of the synthesized 1se and activated 1se'); Figure S3: Gravimetric CO 2 , CH 4 and N 2 sorption isotherms for 1se' (a) and 1b' (b) at 273 K; Figure S4: Dependences of adsorbed CO 2 mole fraction in MOF on CO 2 mole fraction in gas phase for binary gas mixtures: CO 2 /N 2 and CO 2 /CH 4 at 273K and P total = 1 bar; Figure S5: PXRD pattern of the synthesized compound 1b compared to the theoretical one; Figure S6: PXRD pattern of the synthesized compound 1b' compared to the theoretical one for 1b'; Table S1: Textural characteristics of 1se'; Table S2: Calculated virial coefficients for CO 2 isosteric heats of adsorption and Henry constants determination; Table S3: Fit parameters of the isotherms for IAST calculations.

Conflicts of Interest:
The authors declare no conflict of interest.  Table A2. Refined unit cell parameters for 1se .