4-Pyrazolecarboxylic Acid-based MOF-5 Analogs Framework with High Adsorption and Separation of Light Hydrocarbons①

FU Hong-Ru YAN Li-Bin XIE Tao

?

4-Pyrazolecarboxylic Acid-based MOF-5 Analogs Framework with High Adsorption and Separation of Light Hydrocarbons①

FU Hong-Ru②YAN Li-Bin XIE Tao

(471934)

One porous framework [Zn4(4-O)(4-4-pca)3]·2(DEF)·2(H2O) (1, 4-H2Pca = 4-pyrazolecarboxylic acid, DEF = N,N-diethylformamide) with MOF-5 type topology has been synthesized solvothermally. Significantly, this compound exhibits high capacity of C2 hydrocarbons. C2H2capacity could compare with the highest value of the reported MOFs, far exceeding that of MOF-5, as well as the high selectivity adsorption of C2over C1.

4-pyrazolecarboxylic acid, MOF-5-type, light hydrocarbons, adsorption and separation;

1 INTRODUCTION

Light hydrocarbons are great important raw materials for chemical industry, energy security and global climate. The methane as the principal com- ponent of natural gas is the priori alternative fuel. C2 hydrocarbons (C2) play a crucial role in the Indus- trial process. However, it’s necessary to develop more efficient separation approach to purify paraffin, adsorption-based storage method in porous materials, which is considered to be very promising[1], andmaterials that can preferentially capture C2H2and CO2over CH4are very desirable.

Compared with traditional porous materials such as zeolites and activated carbon, metal-organic frameworks (MOFs) as a class of crystal materials with ultra-high pore volume and surface area[2, 3]and outstanding tunability[4-8]have shown great advan- tages for gas storage and separation[9, 10]. Recently, MOFs have been regarded as potential materials for the adsorption and purification of light hydrocarbons owing to the tunable microenvironment of the pore and surface. Chen and coworkers have done a lot of pioneering studies on metal-organic frameworks for the purification and storage of C1, C2 and C3 light hydrocarbons[11, 12]. Li group revealed the light hydrocarbon-induced structure transformation of flexible MOFs[13, 14]. In addition, some parts of JLU and FIR series were reported by Liu and Zhang groups, respectively[15-18].

Efficient gas adsorption and separation of MOFs depend on high surface areas, narrow pore width distributions and chemical functionality[19]. Ultramic- roporous materials (pore width < 0.7 nm) display remarkableperformance as separating agents, and feature the advantageous properties in close relationship with the intrinsic characteristics ofthe gas molecules (such as size, shape and chemical/e- lectronicproperties), which could dramatically enhance the adsorption of gas species[20], thus todesign and construct porous materials with the pore size near the ultramicropore is an efficient approach to improve the capacity of small gas molecules. Selecting the rigid linker with smaller scale is the straightest and simplest method to synthesize ultramicroporous MOFs.

Here, our synthetic strategy focuses on the small linker 4-pyrazolecarboxylic acid-based metal-organic frameworks. The compound [Zn4(4-O)(4-4- pca)3]·2(DEF)·2(H2O) was synthesized solvother- mally. This compoundexhibits high capacity of C2as well as high selectivity of C2over CH4.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All commercially available solvents and chemicals were of analytical grade. Elemental analyses for carbon, hydrogen, and nitrogen atoms were performed on a Vario EL III elemental analyzer (Elementar, Germany). The crystal was determined on a Bruker SMART APEX II CCD diffractometer (Madison, WI, USA) equipped with a graphite-mo- nochromatized Moradiation (= 0.71073 ?). Gas adsorption was measured using an ASAP 2020 instrument (Micromeritics, USA). Powder X-ray diffraction(PXRD) pattern was recorded on a RigakuD/Max-2500 diffractometer at 40 kV and 30 mA with aCu-target tube and a graphite-mono-chromator. Thermalgravimetric analysis (TGA) was conducted on aNETZSCH STA 449F3 instrument in flowing N2ata heating rate of 5 ℃/min.

2. 2 Synthesis of the title complex

[Zn4(4-O)(4-4-pca)3]·2(DEF)·2(H2O) (1): A mixture of 4-H2Pca (28.0 mg, 0.25 mmol) and Zn(NO3)2·6H2O (90.0 mg, 0.3 mmol) was dissolved in DEF (4 mL), CH3CN (1 mL) and HNO3(5 drops, 0.1 M in H-2O) in a screw capped vial. The vial was capped and placed in an oven at 100 ℃ for 36 h. Colorless block crystals (0.5 mg, 37%) were obtained. Elemental analysis calcd. (%) for 1 (Zn4O11C22H32N8): C, 31.47; H, 3.81; N, 13.35. Found: C, 29.57; H, 3.83; N, 13.01.

2. 3 Crystal structure determination

Single-crystal X-ray diffraction analysis of the complex was carried out on a Bruker SMART APEX II CCD diffractometer (Madison, WI, USA) equip- ped with a graphite-monochromatized Moradia- tion (= 0.71073 ?) by using the-scan mode at room temperature. The structure was solved by direct methods with SHELXS-2014[21]. The hydrogen atoms were assigned with common isotropic displa- cement factors and included in the final refinement by use of geometrical restrains. A full-matrix least-squares refinement on2was carried out using SHELXL-2014. The disorder guests were removed with SQUEEZE instructionby PLATON soft[22]. The selected bond lengths and bond angles are listed in Table 1.

Complex 1 crystallizes in cubic with space group-3,= 20.1349(4),= 20.1349(4),= 20.1349(4) ?,= 8163.0(3) ?3,= 2,D= 0.9792g/m3,(000) = 2320 and= 2.882mm-1. A total of 1797 reflections were obtained and 433 unique (int= 0.0228) were collected in the range of 3.7880≤≤67.40o by anscan mode, of which 6488 reflections with> 2() were used in the succeeding refinement. The final= 0.0403,= 0.1603 (= 1/[2(F2) + (0.1068)2+ 33.0000], where= (F2+ 2F2)/3), (Δ)max= 0.6263, (Δ)min= –0.8043 e/?3, (Δ/)max= 0.0002 and= 1.0754.

Table 1. Selected Bond Distances (nm) and Bond Angles (o) for Compound 1

Symmetry transformations used to generate the equivalent atoms: #1: 1/2–,,1/2–;#2:,1/2–,1/2–; #3: 1/2–,1/2–,

3 RESULTS AND DISCUSSION

3. 1 Structural description of 1

Compound 1 was synthesized according to the previous reports[23]. It crystallizes in a 3structure with cubic-3space group. As shown in Fig. 1a, the coordination model of nitrogen atom is identical to that of oxygen atom, so that the asymmetry unit consists of two similar building units of Zn4O and Zn-4N, respectively. Thus, each Zn4O cluster forms an octahedral Zn4O(CO2)6secondary building unit (SBU) through edge-bridging coordination of six carboxylate groups, and each Zn-4N cluster forms an octahedral Zn4O(pz)3SBU through edge-bridging coordination of six pyrazolates. The SBUs connect each other to form the non-interpenetrated frame- work, formulated as [Zn4(4-O)(4-4-pca)3]. Totally, the structure of 1 is analogous to the MOF-5 system, with the cross-section windows of ～7.5×7.5 ?2and a void volume of 63.5 % (5291.4 ?3/ 8092.8 ?3, accessible free volume/unit cell volume calculated by PLATON analyses[22]) after removing solvent guests and framework hydrogen, indicating that the size of window is nearly close to the ultramicropore. Comparably, the total unit cell volume of MOF-5 reaches up to 17349.4 ?3with a 76.8% void volume and the 11.5×11.5 ?2open window, which is attributed to the much longer-phthalic acid[24].

Fig. 1. View of the crystal structure of 1. (a) Coordination environment of 1. (b) 3D structure in the direction ofaxis

3. 2 PXRD and thermal properties

In order to check the phase purity of 1, the X-ray powder diffraction (XRPD) pattern was checked at room temperature. As shown in Fig. 2, the peak positions of the simulated and experimental XRPD patterns are in agreement with each other, demon- strating the good phase purity of 1. Also, the XRPD peaks of the activated sample match well with the simulated shape, indicating the sample still maintains the crystalline state.

Fig. 2. XRPD patterns for complex 1: the as-synthesized patterns,the activated isorthem andthe simulated based on X-ray single-crystal data

Fig. 3. TGA plots of 1 and the activated sample

TG curve for complex 1 is shown in Fig. 3. The TG curve of 1 shows the weight loss (19.2%) at 30～100℃, corresponding to the removal of a free water molecule and the guest organic molecules (calcd.: 20%). The framework keeps stable in the range of 100～400 ℃, then the framework collapses with rapid weight loss. In addition, TG analyses of activated frameworks were performed, and theplateaus range from 40 to 400 ℃ shows well that the initialguest molecules werealmost completely exchanged by methanol, andmethanol molecules can be completely removed by thethermal/vacuum activation at 60℃. The results of theeffective activation laid the foundation for gas sorption.

3. 3 Adsorption properties

The rigidity of the porous framework 1 as well as the high porosity was assessed by solid-gas adsorption experiments with N2and CO2as probe molecule. The N2adsorption isotherms show type-I behaviors, with BET speci?c surface areas of 1138.8 m2×g-1(Fig. 4). The CO2uptakes for 1 are 110.5 and 64.6 cm3×g-1at 273 and 294 K, respectively.

Fig. 4. N2sorption isotherms of compound 1

The adsorption of light hydrocarbons was carried out at 273 K and room temperature (Fig. 5). Porous material 1 exhibits notable adsorption capacities of C2H6, C2H4and C2H2in the following order: C2H2?C2H6? C2H4? CH4. Particularly, the C2H2capacity of 1 is up to 182.1 and 130.6 cm3×g-1at 273 K and room temperature, respectively. Although the surface area in 1 is lower than that of UMCM-150 (3330 m2×g-1)[25], ZIF-8 (1758 m2×g-1)[26], the amount of absorbed C2H2in 1 is higher than the C2H2capacity of UMCM-150 (129 cm3×g-1at 296 K) and ZIF-8 (25 cm3×g-1at 296 K). Furthermore, this value is comparable with that of NOTT-102 (146 cm3×g-1at 296 K)[27].These results mean that compound 1 shows the top adsorption of light hydrocarbons. It is worthy of mention that the hydrocarbon adsorption capacities of 1 far exceeded that of MOF-5 (C2H2, 26 cm3×g-1at 295 K)[25]. Compared to the pore scale of MOF-5 (11.5 × 11.5 × 11.5 ?3), the dimension of pore is only about 7.5 × 7.5 × 7.5 ?3, which is relatively close to the dynamic diameter of small hydrocarbons. The smaller and confined channels enhance the interactions with small hydrocarbons, and further capture the small gases[28].

To evaluate the gas separation ability of 1, the adsorption selectivity of C2/C1and CO2/CH4(equimolarbinary mixtures) was calculated by the ideal adsorption solution theory (IAST)[29].IASTis a method for predicting the adsorption equilibria for components in a mixture using only single-com- ponent adsorptiondata at the same temperature and on the same adsorbent. As shown in Fig. 5, C2H2/CH4adsorption selectivity of 1 is 13.3 and 16.2 at 297 and 273 K, respectively. The separation valve of C2H2/CH4is higher than that of ZJU-30 (9.58, 298 K)[30]and ZJU-48a(7.5, 296 K)[31]. Moreover, the selectivity value of C2H6/CH4of 1 exceeds 11.5 (Mg-MOF-74)[25]and 12 (NOTT-101)[25]at room temperature. Such high selectivity further confirms that this MOF has great potential in the separation of light hydrocarbons.

Fig.5. CO2, CH4, C2H2, C2H4and C2H6sorption isotherms of compound 1:(a) at 297 K, (b) at 273 K. IAST-predicted adsorption selectivity: (c) at 297 K, (d) at 273 K

4 CONCLUSION

In summary, one porous metal-organic framework was successfully synthesized based on 4-carboxy- pyrazole. As expected, this porous material exhibits high capacity of light hydrocarbons. The C2H2uptake of 1 is up to 180.1 at 273 K and 1 bar. More valuably, 1 shows high-efficient selectivity for C2 over CH4. These results indicate this porous material could act as a promising platform for the fuel gas purification and the separation of light hydrocarbons.

(1) Herm, Z. R.; Wiers, B. M.; Mason, J. A.; van Baten, J. M.; Hudson, M. R.; Zajdel, P.; Brown, C. M.; Masciocchi, N.; Krishna, R.; Long, J. R. Separation of hexane isomers in a metal-organic framework with triangular channels.2013, 340, 960–964.

(2) Sheng, D.; Dan, W. Y.; Luo, G. X.; Deng, M. A new coordination polymer built of 4-(5H-tetrazol)-benzoic acid and 3,5-dimethyl-1H,1,2,4-triazole showing a rarely observed (3,5)-connected lhh topology: synthesis, structure, CO2 adsorption and luminescent property.2016, 2, 264–270.

(3) Wu, X. X.; Fu, H. R.; Han, M. L.; Zhou, Z.; Ma. L. F. Tetraphenylethylene immobilized metal-organic frameworks: highly sensitive fluorescent sensor for the detection of Cr2O72–and nitroaromatic explosives.2017, 17, 6041–6048.

(4) Qiu, S. L.; Xue, M. S.; Zhu, G. Metal-organic framework membranes: from synthesis to separation application.2014,6116–6140.

(5) Gu, C. S.; Hao, X.-M.; Zhang, Z. Y.; Ji, L. L.; Li, Y.; Song, W. D. Syntheses, structures and luminescent properties of the cadmium(II) complex with 3,3?-thiodipropionic acid.2017, 3, 478-484..

(6) Lang, J. P.; Xu Q. F.; Yuan R. X.; Abrahams, B. F. [[WS4Cu4(4,4?-bpy)4][WS4Cu4I4(4,4?-bpy)2]] infinity – an unusual 3D porous coordination polymer formed from the preformed cluster [Et4N]4[WS4Cu4I6].2004, 43, 4741–4745.

(7) Liu, D.; Lang, J. P.; Xu, Q. F.; Yuan, R. X.; Abrahams, B. F. Highly efficient separation of a solid mixture of naphthalene and anthracene by a reusable porous metal-organic framework through a single-crystal-to-single-crystal transformation.2011, 133, 11042–11045.

Wu, H. Y.; Li, H. H.; Zhen, Z. R. Synthesis, crystal structure and characterization of the host-guest type UOF.–

(9) Li, B.; Wen, H. M.; Cui, Y. J.; Zhou, W.; Qian, G. D.; Chen, B. L. Emerging multifunctional metal-organic framework materials.2016, 28, 8819–8860.

(10) Chen, Z.; Weseliński, ?. J.; Adil, K.; Belmabkhout, Y.; Shkurenko, A.; Jiang, H.; Bhatt, P. M.; Guillerm, V.; Dauzon, E.; Xue, D. X.; O’Keeffe, M.; Eddaoudi, M. Applying the power of reticular chemistry to finding the missing alb-MOF platform based on the (6,12)-coordinated edge-transitive net.2017, 139, 3265–3274.

(11) He, Y.; Zhang, Z.; Xiang, S.; Fronczek, F. R.; Krishna, R.; Chen, B. A microporous metal-organic framework for highly selective separation of acetylene, ethylene, and ethane from methane at room temperature.2012, 18, 613–619.

(12) Xiang, S.; Zhou, W.; Zhang, Z.; Green, M. A.; Liu, Y.; Chen, B. Open metal sites within isostructural metal-organic frameworks for differential recognition of acetylene and extraordinarily high acetylene storage capacity at room temperature.2010, 49, 4615?4618.

(13) Nijem, N.; Wu, H. H.; Canepa, P.; Marti, A.; Balkus, Jr. K. J.; Thonhauser, T.; Li, J.; Chabal, Y. J. Tuning the gate opening pressure of metal-organic frameworks (MOFs) for the selective separation of hydrocarbons.2012, 134, 15201–15204.

(14) Li, K. H.; Olson, D. H.; Seidel, J.; Emge, T. J.; Gong, H. W.; Zeng, H. P.; Li, J. Zeolitic imidazolate frameworks for kinetic separation of propane and propene.2009, 131, 10368–10369.

(15) Yao, S.; Sun, X.; Liu, B.; Krishna, R.; Li, G.; Huo, Q.; Liu, Y. Two heterovalent copper-organic frameworks with multiple secondary building units: high performance for gas adsorption and separation and I2sorption and release.2016, 4, 15081–15087.

(16) Liu, B.; Yao, S.; Shi, C.; Li, G.; Huo, Q.; Liu, Y. Significant enhancement of gas uptake capacity and selectivity via the judicious increase of open metal sites and Lewis basic sites within two polyhedron-based metal-organic frameworks.2016, 52, 3223–3226.

(17) He, Y. P.; Tan, Y. X.; Zhang, J. Tuning a layer to a pillared-layer metal-organic framework for adsorption and separation of light hydrocarbons.2013, 49, 11323–11325.

(18) He, Y. P.; Tan, Y. X.; Zhang, J. Gas sorption, second-order nonlinear optics and luminescence properties of a multifunctional srs-type MOF built by tris((4-carboxyl)phenylduryl)amine.2015, 54, 6653–6656.

(19) Nandasiria, M. I.; Jambovanea, S. R.; McGrailb, B. P.; Schaefc, H. T.; Nuneb, S. K. Adsorption, separation, and catalytic properties of densified metal-organic frameworks.2016, 311, 38–52.

(20) Adil, K.; Belmabkhout,Y.; Pillai,R. S.; Cadiau,A.; Bhatt,P. M.; Assen,A. H.; Maurin, G.; Eddaoudi, M. Gas/vapour separation using ultra-microporous metal-organic frameworks: insights into the structure/separation relationship.2017, 46,3402–3430.

(21) Spek, A. L. Crystal structure refinement with SHELXL.2015, 71, 3–8.

(22) Spek, A. L. Single-crystal structure validation with the program PLATON.2003, 36, 7?13.

(23) Tu, B.; Pang, Q.; Wu, D.; Song, Y.; Weng, L.; Li, Q. Ordered vacancies and their chemistry in metal-organic frameworks.2014,14465-14471.

(24) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi, S. B.; Choi, E.; Yazaydin, A.?.; Snurr, R. Q.; O’Keeffe, M.; Kim, J.; Yaghi, O. M. Ultrahigh porosity in metal-organic frameworks.2010, 329, 424–428.

(25) He, Y.; Krishna, R.; Chen, B. L. Metal-organic frameworks with potential for energy-efficient adsorptive separation of light hydrocarbons.2012, 5, 9107–9120.

(26) Xiang, S.; Zhou, W.; Gallegos, J. M.; Liu, Y.; Chen, B. L. Exceptionally high acetylene uptake in a microporous metal-organic framework with open metal sites.2009, 131, 12415–12419.

(27) Lin, X.; Telepeni, I.; Blake, A. J.; Dailly, A.; Brown, C. M.; Simmons, J. M.; Zoppi, M.; Walker, G. S.; Thomas, K. M.; Mays, T. J.; Hubberstey, P.; Champness, N. R.; Schr?der, M. High capacity hydrogen adsorption in Cu(II) tetracarboxylate framework materials: the role of pore size, ligand functionalization, and exposed metal sites.2009, 131, 2159–2171.

(28) Das, M. C.; Xu, H.; Xiang, S. C.; Zhang, Z. J.; Arman, H. D.; Qian, G. D.; Chen,B. L. Anew approach to construct a doubly interpenetrated microporous metal-organic framework of primitive cubic net for highly selective sorption of small hydrocarbon molecules.2011, 17, 7817–7822.

(29) Chen, J.; Loo, L. S.; Wang, K. An ideal absorbed solution theory (IAST) study of adsorption equilibria of binary mixtures of methane and ethane on a templated carbon.2011, 56, 1209–1212.

(30) Das, M. C.; Xu, H.; Wang, Z.; Srinivas, G.; Zhou, W.; Yue, Y. F.; Nesterov, V. N.; Qian, G. D.; Chen, B. L. A Zn4O-containing doubly interpenetrated porous metal-organic framework for photocatalytic decomposition of methyl orange.2011, 47, 11715–11717.

(31) He, Y.; Zhang, Z.; Xiang, S.; Fronczek, F. R.; Krishna, R.; Chen, B. L. A robust doubly interpenetrated metal-organic framework constructed from a novel aromatic tricarboxylate for highly selective separation of small hydrocarbons.2012, 48, 6493–6495.

25 September 2017;

12 December 2017 (CCDC949597)

①This work was supported financially by the National Natural Science Foundation of China (No. 21601080), and the Key Scientific Research Projects of Higher Education of He'nan Province (16A150016)

. E-mail: hongrufu2015@163.com

10.14102/j.cnki.0254-5861.2011-1836