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Direct Evidence: Enhanced C2H6 and C2H4 Adsorption and Separation Performances by Introducing Open Nitrogen-Donor Sites in a MOF Xiu-Yuan Li,†,§ Zhen-Jing Li,†,§ Yong-Zhi Li,† Lei Hou,*,† Zhonghua Zhu,‡ and Yao-Yu Wang*,† †

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Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China ‡ School of Chemical Engineering, The University of Queensland, Brisbane 4072, Australia S Supporting Information *

ABSTRACT: To comparably analyze the influence of a porous environment on the gas adsorption in MOFs, based on an imidazoledecorated MOF, {[Zn(imtp)]·DMA·1.5H2O}n (1-im, H2imtp = 2(imidazol-1-yl) terephthalic acid), an analogue MOF, {[Zn(tztp)]· DMA}n (1-tz, H2tztp = 2-(1H-1,2,4-triazol-1-yl) terephthalic acid) has been synthesized by replacing imidazole with triazole motifs. The two MOFs are isostructural frameworks containing 1D channels; however, they possess different porous wall environments. The open nitrogendecorated channels in 1-tz lead to significantly enhanced C2H6 (76.5 cm3 g−1) and C2H4 (73.1 cm3 g−1) uptakes at 298 K and 1 atm, which are 5 times of the adsorption amounts of C2H6 and C2H4 in 1-im that is the absence of exposed N atoms in the channels. Furthermore, the activated 1-tz also reveals higher adsorption selectivities for C2H6 and C2H4 over CH4. The different sorption properties were further uncovered by theoretical simulations.



INTRODUCTION CH4 as a widespread use of clean energy mainly comes from biogas and natural gas, however, which is usually mixed with hydrocarbons.1−4 Isolation and purification of CH4 not only promote the quality of gas but also provide a basic starting material for industrial process of producing ethane, liquefied petroleum gas, and so on.5−8 In this context, the existing highly energy-wasting conventional separation methods have caused serious pollution to the environment, so adsorption by utilizing porous adsorbents based on a molecular sieve effect has been developed as an environmentally friendly and economic way.9 Traditional adsorbents including the zeolites and activated carbons have limits in structural design of functionalization and regulation to interact with specific gas molecules.10 Alternatively, exploiting new types of adsorption and separation materials is being pursued. By virtue of large surface areas,11−14 modulatable porous diameters15−17 and decoratable porous surfaces,18−23 metal− organic frameworks (MOFs), as a newly emerged class of crystalline porous materials, have attracted extensive attention in the adsorption and separation domain. 24−27 Some impressive strategies were adopted to increase the uptake of gas molecules in MOFs,28,29 such as inserting polar functional groups to porous walls,30−34 generating open metal sites by removing coordinated solvent molecules,35−37 and diminishing porous sizes by interpenetration as well.38−40 Usually, © XXXX American Chemical Society

increasing uptake along with decreasing adsorption selectivity; thus it is still very challenging to attain large gas uptake and high adsorption selectivity simultaneously. Generally, because of larger quadruple moment and polarizability of C2H6 and C2H4 relative to CH4, forming open metal sites or embedding polar group sites has witnessed an effective approach to improve the adsorption amount of a MOF for C2H6 and C2H4 as well as selective capture for them over CH4.41−45 The free −NH2 groups and exposed N atoms can interact with C2H6 and C2H4 molecules to enhance the loading.46 However, there is no direct experiment and theoretical calculation to systematically compare or explore the effect of pore environment on C2H6 and C2H4 uptakes and separations over CH4 in MOFs with the same structures. For this goal, the judicious design of organic linkers is crucial. We have recently utilized an imidazole-decorated terephthalic acid ligand to build a porous MOF {[Zn(imtp)]·DMA·1.5H2O}n (1-im, H2imtp = 2-(imidazol-1-yl) terephthalic acid) containing 1D rhombic channels.47 To precisely regulate the porous wall, we expanded our work by replacing the imidazole group with triazole and constructed a framework {[Zn(tztp)]·DMA}n (1-tz, H2tztp = 2-(1H-1,2,4-triazol-1-yl) terephthalic acid) (Scheme 1). 1-tz is isostructural to 1-im; however, the channel Received: August 3, 2018

A

DOI: 10.1021/acs.inorgchem.8b02199 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Evolution of 1-tz from 1-im by Replacing the Imidazole with Triazole Groups

Table 1. Crystal Data and Structure Refinement for 1-tz 1-tz molecular formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Rint reflns collected/unique μ (mm−1) ρ (g/cm3) reflns collected goodness-of-fit on F2 R1,a wR2b [I > 2σ(I)] R1,a wR2b [all data]

surface of 1-tz is decorated by the uncoordinated N atoms from triazolate groups, which is different to the absence of open N-donor sites in the channels of 1-im. As a result, the activated 1-tz presents not only sharply enhanced C2H4 and C2H6 uptakes and larger adsorption heat but also higher selectivity for C2H4 and C2H6 hydrocarbons over CH4 compared to those in 1-im. Grand canonical Monte Carlo (GCMC) simulations further verify the crucial adsorption roles of open N-donor sites toward C2H6 and C2H4 in 1-tz.



a

C10H5O4N3Zn 296.54 296(2) monoclinic C2/c 15.271(11) 16.147(12) 17.017(16) 115.186(11) 3797(5) 8 0.0408 8863/3310 1.299 1.037 8863 1.105 0.0812, 0.2171 0.0885, 0.2229

R1 = ∑(|Fo| − |Fc|)/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.



RESULTS AND DISCUSSION Crystal Structure. 1-tz with a monoclinic C2/c space group reveals a 3D porous network with dinuclear [Zn2(COO)4] secondary building units (SBUs) and 1D rhombic channels. The asymmetric unit is composed of one Zn2+ ion and one fully deprotonated tztp ligand (Figure 1a). The Zn2+

EXPERIMENTAL SECTION

Materials and General Methods. All reagents were commercially available. Fourier transform infrared spectrum (FTIR) was obtained with a Nicolet FTIR 170 SX spectrophotometer. Elemental analyses for C, H, and N were performed with a PerkinElmer 2400C Elemental analyzer. Thermogravimetric analysis (TGA) was measured with a NETZSCH TG 209 thermal analyzer under a nitrogen atmosphere with a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) pattern was recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Gas sorption was tested with a Micrometrics ASAP 2020 M apparatus. Synthesis of {[Zn(tztp)]·DMA}n (1-tz). A mixture containing H2tztp (11.7 mg, 0.05 mmol), Zn(NO3)2·6H2O (29.7 mg, 0.1 mmol), DMA (3 mL), and H2O (3 mL) was sealed in a vessel (15 mL) and heated at 120 °C for 48 h. After cooling to room temperature at a rate of 10 °C h−1, the colorless block-shaped crystals were gained in 65% yield (9.6 mg, based on H2tztp). Anal. Calcd for C14H14N4O5Zn: C, 43.82; H, 3.68; N, 14.60. Found: C, 43.75; H, 3.65; N, 14.63. IR data (KBr, cm−1): 3403(m), 3116(m), 1606(s), 1531(s), 1465(w), 1367(s), 1282(m), 1215(w), 1147(m), 1113(w), 1047(w), 991(m), 879(w), 833(m), 775(m), 657(m), 555(w), 435(w). Synthesis of {[Zn(imtp)]·DMA·1.5H2O}n (1-im). We previously reported the method was used to prepare 1-im which was gained by reacting H2imtp with Zn(NO3)2·6H2O in DMA and H2O solvents at 120 °C.47 Crystallography. The crystal structure of 1-tz was determined at 296(2) K by a Bruker SMART APEX II CCD diffractometer equipped with a Mo Kα radiation source (λ = 0.71073 Å). The structure was solved by direct methods and refined on F2 by fullmatrix least-squares methods using the Olex2 program.48 The nonhydrogen atoms were refined anisotropically, while the hydrogen atoms were added to their geometrically ideal positions and were refined isotropically. The model of DMA solvent molecules cannot be well refined from the difference Fourier map; thereby the SQUEEZE routine of the PLATON program49 was used in structural refinement. The results of structure refinements and selected bond distances/ angles are listed in Tables 1 and S1, respectively.

Figure 1. (a) Coordination environment of the Zn2+ ion in 1-tz (Symmetry codes: #1 = 1 − x, −y, 1 − z; #2 = x, −y, z + 1/2; #3 = 1/ 2 − x, 1/2 − y, 1 − z; #4 = x + 1/2, 1/2 − y, z + 1/2.), (b) the dinuclear paddle-wheel [Zn2(COO)4] SBU, (c) a grid layer, (d) exposed N atoms on porous surface, and (e) 3D porous framework.

ion with a tetragonal pyramid coordination geometry is coordinated by four equatorial plane carboxylate O atoms [Zn−O = 2.041(5)−2.090(5) Å, Table S1] from four tztp and one axial triazolate N atom [Zn−N = 2.037(6) Å] from one tztp. Two Zn2+ ions are dimerized by four syn,syn-bridging carboxyl groups of four tztp (Figure S1a) to furnish a paddlewheel [Zn2(COO)4] SBU (Figure 1b). One SBU as a square node is connected by four terephthalate units of four tztp B

DOI: 10.1021/acs.inorgchem.8b02199 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) Gas sorption isotherms of 1a-tz for CO2 at 195 K and N2 at 77 K; inset: porous size distribution of 1a-tz. (b) Gas sorption isotherms of 1a-tz and 1a-im for C2H6, C2H4 at 273 and 298 K. (c) Comparison of adsorbed amounts at 1 atm. (d) C2H6 and C2H4 adsorption heats for 1atz and 1a-im calculated by the virial equation.

ligands to afford a grid layer parallel to the ab crystallographic plane (Figure 1c). The neighboring layers are interlinked by the further coordination between the Zn2+ ions in SBUs and triazolate N atoms of tztp to give rise to a 3D porous framework, which has 1D rhombic channels with the dimensions of about 7.3 × 11.8 Å (which is measured between the diagonal of the rhombic channel, excluding the van der Waals radii of the atoms) as well as the solvent-accessible void of 51.8% (Figures 1d,e and S1b).49 Although the overall framework of 1-tz is isostructural to that of 1-im (Figure S2), however, there exist a large number of uncoordinated triazolate N sites exposed in the channel walls of 1-tz, which obviously differs from the porous environment in 1-im (a void of 55.2%, channel dimensions of about 8.8 × 10.9 Å2). These subtle alterations of porous surfaces could lead to a significant difference in gas storage and separation (Figure 1d). As the dinuclear paddle-wheel [Zn2(COO)4] unit and tztp can be regarded as 6-connected and 3-connected node, respectively, so 1-tz forms a (3,6)connected ant net with a point symbol of (42.6)2(44.62.88.10) (Figure S3). PXRD and TGA. PXRD experiment confirmed the phase purity of 1-tz (Figure S4). TGA shows a weight loss of about 22.7% caused by the loss of guest DMA molecules before 245 °C (calcd. 22.7%); after a thermal stability plateau ending at 330 °C, the framework starts to decompose (Figure S5). Gas Adsorption. The fresh samples of 1-tz were immersed in CH2Cl2 for 72 h, and subsequently heated at 100 °C for 8 h to afford the activated 1a-tz. PXRD and TGA evidenced the complete activation and skeleton intactness of 1a-tz,

respectively (Figures S4 and S5). The porosity of 1a-tz was assessed by N2 sorption experiment carried out at 77 K, which exhibits a reversible type-I sorption isotherm with a saturated loading of 276.8 cm3 g−1 (Figure 2a). The obtained Brunauer− Emmett−Teller (BET) and Langmuir surface areas are 845.9 and 1099.6 m2 g−1, respectively, and the total pore volume of 0.43 cm3 g−1 matches well with the value of 0.44 cm3 g−1 calculated from the crystal structure. The pore size distributions (PSD) based on the density functional theory (DFT) method are about 7.3 and 11.8 Å (Figure 2a), agreeing with the sizes of channels. 1a-tz also displays a type-I sorption isotherm for CO2 at 195 K with an uptake of 244.3 cm3 g−1 at 1 atm (Figure 2a). The same structures but subtle difference in porous wall environments between 1-tz and 1-im provides the desired examples to investigate the effect of porous surface on the gas adsorption and separation performance for light hydrocarbons at ambient temperature. 1-im was desolvated by the same procedure to 1-tz, to form the activated 1a-im (Figures S6 and S7). C2H6 and C2H4 sorption isotherms were measured at 273 and 298 K. The adsorption amounts of 1a-tz reach up to 102.5 and 76.5 cm3 g−1 for C2H6, and 104.0 and 73.1 cm3 g−1 for C2H4 at 273 and 298 K, respectively (Figure 2b). These values are relatively higher than those reported in some MOFs with significant C2H6 and C2H4 uptakes (Tables S2 and S3), such as JLU-Liu5,50 ZJU-31a,51 FJI-C4,52 and PAF-40.53 In sharp contrast with 1a-tz, 1a-im reveals obviously low C2H6 and C2H4 loadings (20.6 and 20.8 cm3 g−1 for C2H6 and C2H4 at 273 K, 15.0 and 15.6 cm3 g−1 for C2H6 and C2H4 at 298 K), which are only 1/5 of the uptakes of 1a-tz (Figure 2b,c), C

DOI: 10.1021/acs.inorgchem.8b02199 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry indicating the exposed N atoms in channels of 1a-tz produce strong interactions with C2 hydrocarbon molecules. Notably, the maximum adsorption capacities of C2H6 and C2H4 in 1a-tz or 1a-im are comparable, but the isotherm of C2H6 is steeper than that of C2H4, illustrating higher affinity of the frameworks toward C2H6, which was caused by the larger polarizability of C2H6 than C2H4. The affinity of the framework toward adsorbates can be reflected by the heat of adsorption (Qst) calculated by the virial model (Figures S8−S11). The Qst in 1a-tz fall within the range of 35.1−32.9 and 33.4−32.4 kJ mol−1 for C2H6 and C2H4, respectively. In contrast, the corresponding Qst of C2H6 (33.0−31.0 kJ mol−1) and C2H4 (21.6−20.2 kJ mol−1) in 1a-im were lower (Figure 2d). To deeply understand the reason for the large difference in the adsorption amounts of C2H6 and C2H4 molecules between 1a-tz and 1a-im, grand canonical Monte Carlo (GCMC) simulations were performed (see the Supporting Information). Analyses revealed three main binding sites existed in 1a-tz for C2H6. C2H6-I and C2H6-III are close to the coordination spheres of Zn(II) centers, in which the methyl group of C2H6 forms multiple C-H···O (H···O = 2.819−2.936 Å) hydrogen bonds with carboxylate O atoms (Figure 3a,b). Expectedly, the

Figure 4. Interactions between C2H4 molecules and 1a-tz (a) and 1aim (b).

The simulations clearly evidence that the exposed uncoordinated N atoms in 1a-tz are direct binding sites toward C2H6 and C2H4 molecules by forming multiple hydrogen bonds. However, this location is occupied by −CH groups of imtp in 1a-im, which is in fact repellent to C2H6 and C2H4 molecules. This subtle difference leads to the significant increases of C2H6 and C2H4 uptakes in 1a-tz relative to 1a-im. Selectivity Behaviors. In contrast to the remarkable C2H6 and C2H4 captures in 1a-tz and 1a-im, very few CH4 were adsorbed, with the adsorption amounts of 15.8 and 6.5 cm3 g−1, respectively, at 298 K and 1 atm (Figure S12). The obvious contrast in adsorption amounts allowed us to investigate the potential applications for small hydrocarbon separation through ideal adsorbed solution theory at 298 K (Figures S13 and S14). For 1a-tz, the C2H6/CH4 selectivity is calculated to be 16.9 for an equimolar C2H6−CH4 mixture at 1 atm, which is comparable to the values reported in LIFM-26 (17)31 and PAF-40-Fe (16.2).53 Meanwhile, this selectivity maintains a high value of 14.1−19.3 with CH4 molar fractions changed from 5% to 95% (Figure 5a). 1a-tz also reveals high C2H4/CH4 selectivity falling in the 8.1−11.5 region with the variable C2H4 molar fractions of 5−95% (Figure 5b). By comparison, although 1a-im also displays moderate C2H6/CH4 and C2H4/CH4 selectivity of 5.1−5.4 and 4.8−4.5 with the variable CH4 concentrations (5−95%), respectively, however, they are only about 1/3 and 1/2 of the values in 1a-tz (Figure 5). Therefore, using exposed N atoms decorated pores is proved to be a successful strategy to enhance adsorbent amount and selectivity for C2H6 and C2H4. Because of significantly high C2H6/CH4 and C2H4/CH4 selectivity in 1a-tz, GCMC simulations were further carried out at 298 K and 1 atm on an equimolar C2H6−CH4 or C2H4− CH4 mixture. As expected, the derived density contours for the C2H6−CH4 or C2H4−CH4 mixture displayed that overwhelming C2H6 or C2H4 molecules were adsorbed in the channels of 1a-tz; however, wherein very few CH4 molecules exist (Figure 6a,b). The simulated results agree well with the calculated values on the basis of measured data.

Figure 3. Interactions between C2H6 molecules and 1a-tz (a, b) and 1a-im (c).

uncoordinated triazolate N atoms due to exposed on porous walls form direct contacts with both C2H6-I and C2H6-III through double C-H···N (H···N = 2.682−3.262 Å) hydrogen bonds with different fashions. C2H6-II is located between two triazolate motifs through three C-H···N (H···N = 2.726−3.175 Å) hydrogen bonds with two uncoordinated triazolate N and one triazolate N connecting the phenyl ring in tztp (Figure 3b). However, differently, there mainly exist two kinds of C2H6 molecules in 1a-im, which are involved in C-H···O (H···O = 2.643−2.966 Å) and C-H···N (H···N = 2.885 Å) hydrogen bonds with the coordinated imidazolate N and carboxylate O atoms in the framework (Figure 3c). In addition, two different locations of C2H4 molecules were also observed in 1a-tz, in which C2H4-I forms hydrogen bonds with one carboxylate O atom (H···O = 2.925 Å) and three N atoms in one triazolate unit (H···N = 2.694−3.028 Å), while C2H4-II is hydrogen-bonded to two uncoordinated N atoms of two triazolate units (H···N = 2.811−2.969 Å) (Figure 4a). Differently, two C2H4 molecules in 1a-im are attracted by main carboxylate groups and subordinate imidazolate units (Figure 4b).



CONCLUSION In summary, we replaced the imidazole with triazole groups in ligands, which have given rise to two comparable isostructural MOFs 1-tz and 1-im, wherein the porous surface of 1-tz is decorated by the uncoordinated N atoms of triazolate groups; however, the similar open N-donor sites are nonexistent in 1im. Importantly, the subtle porous environment changes, which leads to significantly increased C 2H 6 and C 2 H4 adsorption amounts and selectivity over CH4 in 1-tz compared to 1-im. These differences were further uncovered by GCMC simulations that observed the open N-donor sites in pores directly capture C2H6 and C2H4 molecules in the pores of 1-tz. The contrastive experiment and simulation results in this D

DOI: 10.1021/acs.inorgchem.8b02199 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Compared adsorption selectivity of (a) C2H6/CH4 and (b) C2H4/CH4 in 1a-tz and 1a-im at 298 K with diverse molar mixtures.

Figure 6. Density contours of C2H6 and CH4 (a) and C2H4 and CH4 (b) in 1a-tz simulated from equimolar C2H6−CH4 and C2H4−CH4 mixtures, respectively.

presentation witness an effective and evolvable approach to promote both C2H6 and C2H4 captures in designing MOFs by tuning the porous environment with the accessible N-donor sites.



Zhonghua Zhu: 0000-0003-2144-8093 Yao-Yu Wang: 0000-0002-0800-7093 Author Contributions §

These authors contributed equally to this work.

Notes

ASSOCIATED CONTENT

The authors declare no competing financial interest.



S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02199. Additional structural figures, the detailed calculations on sorption, and bond length/angle tables (PDF)

ACKNOWLEDGMENTS This work is supported by NSFC (21471124, 21871220, and 21531007), NSF of Shannxi Province (15JS113), the Postgraduate Innovation Foundation of Northwest University (YZZ17115), and the Australian Research Council Future Fellowship FT12010072.



Accession Codes

CCDC 1846122 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 [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 Authors

*E-mail: [email protected] (L.H.). *E-mail: [email protected] (Y.-Y.W.). ORCID

Xiu-Yuan Li: 0000-0002-3508-9864 Lei Hou: 0000-0002-2874-9326 E

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DOI: 10.1021/acs.inorgchem.8b02199 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02199 Inorg. Chem. XXXX, XXX, XXX−XXX