A new microporous metal dicyanamide cluster, Co(hmt)(dca)2 (hmt: hexamethylenetetramine, dca: dicyanamide), with accessible N-donor sites exhibits high CO2/N2 selectivity, 83 at 295 K and 1 bar, for a mixture with a 15:85 CO2 to N2 ratio. Adsorption studies show that the use of hmt and dca moieties as building blocks for solid adsorbents can enhance the CO2:surface interactions due to N atoms available inside the pores, which is confirmed by X-ray single crystal studies. © 2016 Elsevier Inc. All rights reserved

Karadas F.

Karalti O.

Tekin A.

Bilkent University Institutional Repository

lable at ScienceDirectMicroporous and Mesoporous Materials 228 (2016) 153e157Contents lists avaiMicroporous and Mesoporous Materialsjournal homepage: www.elsevier .com/locate/micromesoA metal dicyanamide cluster with high CO2/N2 selectivityAysun Tekin a, Ozan Karalti a, Ferdi Karadas a, b, *a Department of Chemistry, Bilkent University, Ankara, 06800, Turkeyb UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Ankara, 06800, Turkeya r t i c l e i n f oArticle history:Received 4 June 2015Accepted 23 March 2016Available online 25 March 2016Keywords:DicyanamideCO2/N2 selectivityCO2 adsorptionAdsorptionCrystal structure* Corresponding author. Department of Chemistry06800, Turkey.E-mail address: karadas@fen.bilkent.edu.tr (F. Karahttp://dx.doi.org/10.1016/j.micromeso.2016.03.0351387-1811/© 2016 Elsevier Inc. All rights reserved.a b s t r a c tA new microporous metal dicyanamide cluster, Co(hmt)(dca)2 (hmt: hexamethylenetetramine, dca:dicyanamide), with accessible N-donor sites exhibits high CO2/N2 selectivity, 83 at 295 K and 1 bar, for amixture with a 15:85 CO2 to N2 ratio. Adsorption studies show that the use of hmt and dca moieties asbuilding blocks for solid adsorbents can enhance the CO2:surface interactions due to N atoms availableinside the pores, which is confirmed by X-ray single crystal studies.© 2016 Elsevier Inc. All rights reserved.1. IntroductionCO2 capture and sequestration by solid adsorbents has been ofgreat interest in the recent years since CO2 is believed to contributeto the increase in ocean acidity and air toxicity [1e3]. Studies showthat CO2 adsorption is governed by a combination of structuralparameters such as surface area, pore volume, adsorption enthalpy,and functional groups inside the pores, e.g. active metal sites andLewis basic groups [4e8]. Of these, introduction of accessible ni-trogen atoms inside the pores has been found to be one of the mosteffective methods to improve CO2/N2 selectivity given the Lewisacid characteristic of CO2 molecule. The promising results obtainedwith this approach have led research groups to focus on exploringnew coordination compounds with convenient bridging groupsthat contain accessible nitrogen atoms [9e12].Dicyanamide anion (dca), N(CN)2, is a versatile ligandparticularly for incorporating divalent transition metal ions intovarious 3D lattices [13e16]. One of the common strategies inmetal dicyanamide chemistry involves employing a secondbridging ligand, generally a N-donor ligand, to obtain compoundswith interesting magnetic and electronic properties [12,17].These materials hold great potential also as efficient solid ad-sorbents that could selectively adsorb CO2 since almost all of themetal dicyanamide clusters reported in the literature form, Bilkent University, Ankaradas).extended 2D or 3D structures with porous behavior. Further-more, the nature of their cavities, e.g. the surface area and thepolar functional groups inside the pores, can be tuned bychoosing convenient N-donor co-ligands.Herein this study, a new metal dicyanamide cluster was pre-pared, characterized, and investigated for its CO2 and N2 adsorptioncapacities. The well-established synthetic method for the prepa-ration of dicyanamide extended networks that involves the use ofN-donor molecules as co-ligands was applied to obtain a porouscluster with accessible nitrogen donor sites. Hexamethylenetetra-mine (hmt) with four nitrogen donor sites has been chosen as a co-ligand to increase the number of sites with Lewis base characterinside the pores.2. Experimental section2.1. SynthesisThe starting materials Co(NO3)2$6H2O, sodium dicyanamide,and hexamethylenetetramine were purchased from Aldrich andused as received. A methanol solution of hmt and sodium dicya-namide were slowly added to a methanol solution of Co(N-O3)2$6H2O (1 g, 3.43 mmol) to achieve a mixture with 1:1:2(Co:hmt:dca) stoichiometric ratio. The solutionwas stirred for oneday and filtered. The precipitate was washed with copiousamounts of distilled water and methanol. The precipitate wasdried in oven at 60 C to obtain a red powder and the yield is 27%.To obtain fine crystals of the compound a 30 mL methanolA. Tekin et al. / Microporous and Mesoporous Materials 228 (2016) 153e157154solution of Co(NO3)2$6H2O (25 mg, 0.086 mmol) was slowlylayered with a 30 mL methanol solution of hmt and sodiumdicyanamide mixture. Red crystals were obtained after threeweeks. Elemental analysis for the compound, Co(hmt)(dca)2,CoC10H12N10: calculated C 36.26, H 3.65, N 42.29; found C 36.02, H3.33, N 41.92. IR bands (cm1): 2979(w), 2925(w), 2333(m),2247(sh), 2184(vs), 1236(m), 1031(m), 932(m).2.2. InstrumentationSingle-crystal X-ray data for the compound was collected on aRigaku MicroMax 007HF diffractometer equipped with mono-chromatic Mo Ka radiation. In a typical experiment, a crystalselected for study was suspended in polybutene oil and mountedon a holder. The data sets were integrated with the Rigaku Crys-talClear software package. For each compound, the data set wasindexed in an orthorhombic unit cell and systematic extinctionsindicated the space group to be Pnma. Solution and refinement ofthe crystal structures was carried out using the SHELX [18] suite ofprograms and Olex2 [19], a graphical interface. Structure solutionby direct methods resolved positions of all metal atoms and mostof the lighter atoms. The remaining non-hydrogen atoms werelocated by alternating cycles of least-squares refinements anddifference Fourier maps. Hydrogen atoms were placed at calcu-lated positions and refined. The final refinement was performedwith anisotropic thermal parameters for all non-hydrogenatoms. A summary of pertinent information relating to unit cellparameters, data collection, and refinements are provided inTable S3. Selected metaleligand bond distances are provided inTable S4. CrystalMaker program was used to display the crystalstructures.The infrared studies were performed using a Bruker Tensor 27model, Fourier transform infrared (FT-IR) spectra were recorded intransmissionmode. A Digi Tech TMDLATGS detector was used witha resolution of 4.0 cm1 in the 400e4000 cm1 range and theFig. 1. Coordination sphere of Cobalt center and binding modes of bridgispectra were recorded by 64 scans. The thermogravimetric analysisthe sample was performed with TA Instruments TGA Q500 model.Measurements weremade at 10 C/min from 30 to 500 C under N2.CHN elemental analysis was performed with Thermo ScientificFlash 2000 model. Gas adsorption measurements were performedusing a Micromeritics Tristar 3000 surface area and pore sizeanalyzer.3. Results and discussion3.1. SynthesisThe use of a N-donor co-ligand (L) in the synthesis of metaldicyanamide clusters have successfully been employed previouslyto obtain metal dicyanamide clusters with the formula; ML(dca)2[14]. The synthesis general involves the reaction of a divalent metalion with a mixture of dca and desired N-donor ligand in polarsolvents such as water, methanol, or ethanol. A similar syntheticmethod has been applied in this study.Metal dicyanamide clusters, [Cd(hmt)(dca)2] and M(dca)2(-H2O)n$hmtn (M ¼ Co and Mn), incorporating hmt as a co-ligandhave already been reported [20,21]. These clusters referencedabove are, however, not convenient materials for gas adsorptionstudies since the main drawback of cadmium cluster is its highmolecular weight and that of cobalt cluster is the water moleculesbound to metal ions in the structure, which could lead to thecollapse of the network under vacuum. Therefore, a new metaldicyanamide with hmt ligand has been targeted in this study bychanging the solvent used in the synthesis.3.2. Structural characterizationThe X-ray structural analysis of the complex clearly indicates a3D coordination network where CoII sites are connected to eachother with hmt and dicyanamide bridging ligands. Each CoII centerng ligands (Hydrogen atoms are omitted for the sake of simplicity).Fig. 2. 2D array of Co(dca)2 fragments connected with hmt groups. (Hydrogen atoms are omitted for the sake of simplicity).A. Tekin et al. / Microporous and Mesoporous Materials 228 (2016) 153e157 155is surrounded by six nitrogen atoms, four of which belongs to nitrilegroup of dicyanamide anion while the remaining two nitrogenatoms belong to hmt groups, resulting in a distorted octahedralgeometry (Fig. 1). The metal ion is located on an inversion center.Dicyanamide ligands that adopt a m1,5-dca coordination mode usetheir terminal nitrogen atoms to bind to metal centers. Eachdicyanamide group is connected to two metal centers forming aCo4dca4 unit with a distorted square geometry (Fig. 2). These unitsform a 2D layer with the formula Co(dca)2. These 2D networks arethen connected to each other with hmt co-ligands. Thus, the 3Dstructure could be best described by 2D arrays of metal-dicyanamide networks that are connected with hmt groups. Itshould also be noted that hmt group uses only two its nitrogenatoms for binding to cobalt ions, leaving two free nitrogen atomsper hmt group (see Fig. 3).Fig. 3. Packing diagram the crystal structure. (Hydrogen atoms aTable S4 reports a selection of coordination bond lengths andangles. The distances and bond angles are in good accordance withthose previously reported for metal dicyanamide and metal hexa-methylenetetramine clusters [17,20].The infrared spectrum of the compound contains three ab-sorptions in the y(C^N) region, 2184, 2247, and 2333 cm1, inaddition to a stretch at 932 cm1, which are attributed to theasymmetric and symmetric cyanide stretches of dicyanamidebridging ligand. The compound also shows two strong bands at1031 and 1236 cm1 and several weak bands in the range2900e3000 cm1, which can be assigned to the CeN and aliphaticy(CH) stretching vibrations of hmt group, respectively. Moreover,elemental analysis performed on powder materials show thatpowder samples used in adsorption studies and single crystalsstudied with X-ray single crystal technique are identical (see Fig. 4).nd solvent molecules are omitted for the sake of simplicity.)Fig. 4. Gas adsorption/desorption isotherms for CO2 and N2 at 273 K and 295 K. Solidsymbols indicate gas adsorption and open symbols indicate gas desorption.Fig. 6. Change in the CO2 over N2 selectivity (for a mixture of 15:85) with pressure.A. Tekin et al. / Microporous and Mesoporous Materials 228 (2016) 153e1571563.3. Gas adsorption studiesThe CO2 and N2 adsorptions of Co(hmt)(dca)2 were measuredup to 1 bar at 273 and 295 K. The isotherms are found to becompletely reversible with maximum CO2 loadings of77.3 cm3 g1 and 44.8 cm3 g1 at 1 bar for the temperatures of273 K and 295 K, respectively. The CO2 isotherms were fitted to adual site Langmuir model (Figs. S3 and S4) and N2 isothermswere fitted to a single site Langmuir model (Figs. S5 and S6) tocalculate the selectivity and heat of adsorption values. A single-site Langmuir type model was not good enough for fitting theCO2 adsorption data, therefore a dual-site model, which indicatestwo available binding sites for the CO2 molecule [22], was used.The parameters obtained from these fittings are reported inTables S1 and S2 in the supporting information. The heats ofadsorption values were calculated using the ClasiuseClapeyronequation (see Eq. (4) in the supporting information). It can beseen from Fig. 5 that the heat of adsorption for the CO2 adsorp-tion increases from about 35 kJ mol1 at low loadings and rea-ches to a plateau of 58 kJ mol1 after 0.9 mmol g1 loading. Thiscould be due to two different available binding sites, N-donorFig. 5. Change in Qst with CO2 loading, calculated using dual-site Langmuir isotherms.atoms available at dca and hmt groups, which is expected since adual-site Langmuir model was needed for the CO2 adsorptionisotherm fitting. The heat of adsorption for the N2 adsorption,however, does not change appreciably with the N2 loading andhas a value of 17.7 kJ mol1. For the binary mixtures, the CO2/N2selectivities were calculated using both the ratio of the initialslopes and the selectivity factor equation (see Eq. (3) in thesupporting information) where loadings were obtained from thepure single component isotherms. A mixture with a 15:85 CO2 toN2 mole ratio was used for these calculations. The change in theselectivity with respect to the pressure is shown in Fig. 6. TheCO2/N2 selectivity is estimated to be 95 and 83 at 273 K and295 K, respectively, for a 15:85 CO2/N2 gas mixture at 1 bar. It isalso observed that CO2/N2 selectivity decreases as the pressure ofgas mixture increases. The selectivity data calculated using theHenry's law ratios (ratio of the initial slopes) from the lowloading region of the single adsorption isotherms are 603 and473 (Fig. S7). The high CO2/N2 selectivity can mainly be attributedto the higher adsorption enthalpy of the CO2 (stronger interac-tion due to a larger quadrupole). Kinetic diameters of CO2 and N2are 3.3 and 3.6 Å, respectively. Two of the pore openings for thecrystal reported are around 3.8 Å, which is closer to N2 kineticdiameter. This could limit the diffusion of N2 into the pores andhence cause a molecular sieving effect that can contribute to thehigh selectivity [23,24]. Furthermore, BET surface area of thecluster, 242 m2/g, is measured by CO2 sorption measurements at273 K since nitrogen sorption studies at 77K failed, which is aunique phenomenon previously observed with some of theclusters studied in this field [4,25].4. ConclusionA new metal dicyanamide cluster, Co(hmt)(dca)2, was success-fully prepared and detailed structural characterization was per-formed by X-ray single crystallography. The structure containsoctahedral CoII sites that are connected to each other with dca andhmt bridging ligands. Each dca group uses its terminal nitrogenatoms while each hmt group uses two of its N-atoms for binding tocobalt center, leaving one and two uncoordinated N atoms in dcaand hmt groups, respectively. High CO2/N2 selectivity and high heatof adsorption value could be attributed to the polarized environ-ment of the pores due to the presence of aforementioned accessibleA. Tekin et al. / Microporous and Mesoporous Materials 228 (2016) 153e157 157N-donor atoms. This study shows that hmt and dca moieties can beused as building blocks for MOFs and this can enhance the CO2/N2selectivity of the solid adsorbents via available N atoms. Given thevast chemistry of metal dicyanamide clusters other metal dicya-namide clusters that have polar functional groups will be investi-gated and introduced to this field.AcknowledgmentThe authors thank to the Science and Technology Council ofTurkey, TUBITAK (Project No: 114Z473) for financial support. A. T.thanks to TUBITAK (2210-C) for scholarship.Appendix A. Supplementary dataSupplementary data related to this article can be found at http://dx.doi.org/10.1016/j.micromeso.2016.03.035.References[1] S. Solomon, G.-K. Plattner, R. Knutti, P. Friedlingstein, Proc. Natl. Acad. Sci.U.S.A. 106 (2009) 1704e1709.[2] N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, C. S.Adjiman, C. K. Williams, N. Shah and P. Fennell, 2010, 3, 1645e1669.[3] R.F. Service, Science 305 (2004) 963.[4] Y.-S. Bae, O.K. 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A metal dicyanamide cluster with high CO2/N2 selectivity

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A metal dicyanamide cluster with high CO2/N2 selectivity

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