LI Go-Peng LIZhen-Zhen XIE Hong-Fng FU Yun-Long③ WANGYo-Yu

a (Key Laboratory of Magnetic Molecules & Magnetic Information Materials of the Ministry of Education,School of Chemistry & Material Science, Shanxi Normal University, Linfen041004, China)

b (Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education,College of Chemistry and Materials Science, Northwest University, Xi’an710127, China)

c (Department of Clinical Laboratory, The Second Affiliated Hospital of Xi’an Jiaotong University, Xi’an710004, China)

ABSTRACT A multi-site functionalized microporous metal-organic framework (MOF), H[Zn2(BDP)0.5(ATZ)3]·0.5H2O·0.5DMF (1), was synthesized through mixed ligands strategy. The pore surface of complex 1 was modified by uncoordinated carboxylate O atoms, phenyl and pyridyl rings as well as -NH2 groups, which strengthen interactions with C2H6, C2H4 and CO2 molecules and lead to efficiently selective C2H6, C2H4 and CO2 uptake over CH4. The selective adsorption mechanism was discussed deeply based on Grand Canonical Monte Carlo (GCMC)simulations. It is expected that this study will provide a new perspective for the rational design and synthesis of MOFs with efficient gas adsorption and separation performance.

Keywords: multifunctional sites, metal-organic framework, selective gas adsorption, molecular simulations;

1 INTRODUCTION

Methane has been widely utilized as an energy source due to its clean burning characteristics. However, natural gas often contains impurities such as CO2and C2hydrocarbons like C2H4and C2H6[1]. Thus, the separation of methane from C2hydrocarbons and CO2is a very important industrial process.Traditionally, light hydrocarbon separations are performed by cryogenic distillation, which is an energy consuming and a huge-investment process[2-4]. In contrast, physical separation methods using porous materials under ambient conditions have attracted global attention[5,6]. Therefore, developing new physical microporous adsorbents as superior candidates for the separation of methane from CO2and C2hydrocarbons is emergent and vital.

Metal-organic frameworks (MOFs) are a type of porous crystalline materials composed of organic ligands and metal ions/clusters[7-9]. Due to their tunable structures, permanent porosities, and large surface areas, MOFs have received considerable attention in the application selective C2hydrocarbon and CO2uptake[10]. In fact, a great number of MOFs have been constructed for the separation or purification of C2hydrocarbon or CO2over CH4[11,12]. Generally, efficient gas storage/separation on MOFs need highly polar pores with open metal sites and Lewis basic groups (such as -NH2and-OH)[13,14]. Porous MOFs with high polarity can be constructed efficiently using Lewis basic groups functionalized rigid symmetric ligands[15,16], however, combination of different active sites in one MOF is also a significant challenge, in view of crystal engineering.

In this work, we chose two commonly used highly symmetric rigid ligands with Lewis basic groups, 2,6-bis(3,5-dicarboxyphenyl)pyridine (H4BDP) and 5-amino-1H-tetrazole(ATZ) as bridging linkers bridging Zn(II) constructed one uncommon microporous cage-based MOF,H[Zn2(BDP)0.5(ATZ)3]·0.5H2O·0.5DMF (denoted as 1).Complex 1 has channels modified by uncoordinated triazolyl N atoms, carboxylate O atoms, phenyl and pyridyl rings and-NH2groups, and displays highly selective capture for C2H6,C2H4, CO2over CH4.

2 EXPERIMENTAL

2. 1 Materials and methods

All chemicals were commercially available and used without further purification. An infrared (IR) spectrum was obtained through an EQUINOX-55 FT-IR spectrometer together with a KBr pellet from 4000 to 400 cm-1. Elemental analyses for C, H, and N were recorded on a PerkinElmer 2400C Elemental Analyzer. Thermogravimetric analyses(TGA) were carried out in a N2stream using a Netzsch TG209F3 instrument at a heating rate of 10 ℃·min-1. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 ADVANCE with CuKαradiation (λ= 1.5418 ?). All gas adsorption isotherms were measured by ASAP 2020 M adsorption equipment.

2. 2 Synthesis of H[Zn2(BDP)0.5(ATZ)3]·0.5H2O·0.5DMF (1)

A mixture of Zn(NO3)2·6H2O (0.1 mmol), H4BDP (12.22 mg, 0.03 mmol), DMF (5 mL), H2O (1mL) and 0.1 mL HNO3(63%, aq.) was blended in a 10 mL glass bottle. The vial was capped and placed in an oven at 95 ℃ for 72 h and then cooled to room temperature at a rate of 5 ℃·min-1. Colorless flaky crystals of 1 were obtained in 50.4% yield. Anal. Calcd.for C15H15N16O5Zn2: Zn: C, 30.09; H, 2.46; N, 35.53%. Found:C, 30.12; H, 2.43; N, 35.45%. IR (KBr, cm-1): 3402(s),2933(m), 2492(w), 2322(w), 2026(w), 1662(m), 1435(w),1327(m), 1279(m), 1253(m), 1179(s), 1102(s), 1062(m),930(m), 860(m), 783(s), 719(s), 657(s), 529(m).

2. 3 X-ray structure determination

A Bruker Smart CCD area-detector was utilized to get the crystal data at 180(2) K using anωrotation scan with width of 0.3° and MoKαradiation (λ= 0.71073 ?). The structure was solved by direct methods and refined by full-matrix least-squares refinements based onF2with the SHELXTL 2014 program[17]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added to their geometrically ideal positions. The contribution to scattering from guest molecules was subtracted by using the SQUEEZE routine in the PLATON program[18]. Crystal data for 1:C27H21N31O8Zn4,Mr= 1169.23, orthorhombic space groupImmm,a =9.3960(13),b= 28.469(3),c= 34.177(4) ?,V=9142.4(19) ?3,Z= 4,ρcalcd= 0.849 g·cm-3,μ= 1.018 mm-1,17826 reflections measured (2.25°≤2θ≤57.66°) and 5076 unique (Rint= 0.0661) which were used in all calculations.The finalR= 0.0811 (I> 2σ(I)),wR= 0.2811 (all data), andGOF= 1.106. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

2. 4 Selectivity prediction via ideal adsorption solution theory (IAST)

The experimental isotherm data for gases A and B were fitted at 298 K using a dual Langmuir-Freundlich (L–F)model:

whereqandpare adsorbed amounts and the pressure of componenti, respectively.

The adsorption selectivities for binary mixtures of C2hydrocarbons and CO2/CH4defined by

were respectively calculated using the IAST of Myers and Prausnitz, wherexiandyiare the mole fractions of componentsiin the adsorbed phase and bulk, respectively.

Table 2. Parameters Obtained from the Dual Langmuir-Freundlich Fitting of the Single-component Adsorption Isotherms at 298 K

2. 5 Calculation of sorption heat (Qst) for gas uptake using virial 2 models

The above virial expression was used to fit the combined isotherm data for complex 1 at 273.15 and 298 K, wherePis the pressure,Nthe adsorbed amount,Tthe temperature,aiandbiare virial coefficients, andmandNare the number of coefficients used to describe the isotherms.Qstis the coverage-dependent enthalpy of adsorption andRis the universal gas constant.

Table 3. Parameters Obtained from the Virial 2 Model Fitting of the Single-component Adsorption Isotherms in 1a at 273.15 and 298 K

3 RESULTS AND DISCUSSION

3. 1 Structure description

Single-crystal X-ray study shows that 1 crystallizes in orthorhombic space groupImmm, and the asymmetric unit consists of two half independent Zn(II) ions, quarter of symmetry disordered BDP4-as well as one and two quarters of ATZ-ligands. Zn(1) and Zn(2) are both tetra coordinated with one carboxylic O atom from the BDP4-ligand and three N atoms from three different ATZ-ligands (Fig.1a). The axisymmetric BDP4-is connected with four Zn(II) via four carboxylic groups (Fig.1b). Next, eight BDP4-, twenty-four ATZ-and twenty-four Zn atoms interlink to form one flat ellipsoid cage with an approximate diameter of 2.3 nm (Fig.1c and 1d), which represents one of the larger sizes reported for analogous discrete metal-organic cages. Each cage in 1 contains an equivalent rectangle window of 7.24 × 12.35 ?2(excluding van der Waals radii of the atoms). The neighboring cages are joined together further to generate a threedimensional (3D) porous MOF (Fig.2a). By removing guest molecules, the framework has a free void of 58.5%, calculated with PLATON program. The uncoordinated triazolyl N atoms,carboxylate O atoms, phenyl and pyridyl rings and -NH2groups stand in the porous surface, suggesting a polar character and potential active site adsorption of guests.Topologically, by regarding Zn center and BDP4-all as 4-connected nodes, the extended framework of 1 can be simplified as a new (4,4)-connected net with the point symbol of (3·63·72)(3·65)2(66)2(Fig.2b).

Fig.1. (a) Coordination environment of Zn2+ ion in 1; (b) Coordination modes of BDP4- ligands 1;(c) Cage structure; (d) Three-dimensional pores of the cage structure in 1

Fig.2. (a) 3D framework of 1 viewed along the c axis; (b) 4,4-connect net in 1

3. 2 PXRD and TGA

Experimental powder X-ray diffraction (PXRD) of complex 1 is in good agreement with the pattern simulated from the single-crystal data, implying the good phase purity (Fig.3a).TGA experiment was performed on crystalline samples from 35 to 800 ℃ at a heating rate of 10 ℃·min-1under a nitrogen atmosphere for complex 1 (Fig.3b). The first weight loss of 7.20% (calcd. 7.22%) in 1 below 158 ℃ corresponds to the removal of all H2O and DMF guest molecules. The main framework is thermally stable up to 218 ℃ and then decomposes at higher temperature.

Fig.3. PXRD pattern (a) and TGA plot (b) of complex 1

3. 3 Gas adsorption

The coexistence of uncoordinatd triazolyl N and carboxylate O atoms, pyridyl ring phenyl and pyridyl rings as well as -NH2groups on the porous surface implies good selective gas capture performance, and the gas adsorption studies were performed. Before the gas adsorption-desorption experiment, the sample was soaked in CH2Cl2for 72 h and then heated at 393 K under vacuum for 4 h to get the activated sample 1a. The porosity of 1a was confirmed by the N2adsorption experiment performed at 77 K, revealing a reversible microporous type-I isotherm with a saturated loading of 117.57 cm3·g-1under 1 atm (Fig.4a). The corresponding BET was 413.8 m2·g-1(Langmuir surface area is 530.94 m2·g-1) and mean pore width of 11.7 ? based on Horvath-Kawazoe mode. The performance of 1a for the adsorption of small molecule gases (C2H6, C2H4CO2and CH4)was implemented at 273 and 298 K, respectively. As shown in Fig.4b and 4c, the maximum adsorption amounts of C2H4,C2H6, CO2and CH4are 93.24, 76.30, 55.17 and 19.92 cm3·g-1at 273 K and 58.06, 44.68, 35.02 and 12.20 cm3·g-1at 298 K.1a adsorbs much more C2H6than C2H4, CO2and CH4. The C2H6adsorption amount in 1a is higher than that in some reported MOFs, such as UTSA-35 (54.4 cm3·g-1)[19],JLU-Liu6[4](48.8 cm3·g-1) and BUT-70B (53.1 cm3·g-1)[20].The isosteric heats of adsorption (Qst) of CO2, C2H4, C2H6and CH4were calculated based on the adsorption isotherms at 273 and 298 K. TheQstof C2H6, C2H4, CO2and CH4in 1a is in the ranges of 46.9～17.0, 36.6～21.2, 22.5～19.1 and 15.0～15.5 kJ·mol-1, respectively (Fig.4d), reflecting strong affinity of the framework toward C2hydrocarbons and CO2molecules but weak adsorption toward CH4.

Fig.4. (a) N2 sorption isotherm at 77 K (insert: pore width distribution calculated by the Horvath-Kawazoe (HK) method using the slit model);(b) and (c) adsorption isotherms of 1 for C2H6, C2H4 CO2 and CH4 at 273 and 298 K; (d) adsorption heat of 1a for different gases

Because of the significant difference in adsorption amounts between C2H6, C2H4, CO2and CH4, the separation selectivity of 1a for C2H6/CH4, C2H4/CH4and CO2/CH4was calculated at 298 K with the ideal adsorbed solution theory (IAST). 1a reveals a significant C2H6/CH4selectivity for the C2H6–CH4(50%:50%) mixture; the selectivity value of 56.36 under 1 atm is superior to the values reported for NOTT-101[21](11.1)and ZJNU-63[10](10.6) (Fig.5). The C2H4/CH4selectivity of 1a is about 9.80 for equal ratios of the C2H4-CH4mixture,which is comparable with the values reported for[Zn2(NH2-BTB)(2-nim)][22](7) and ZJNU-61[23](7.8).Meanwhile, for a common landfill gas mixture (CO2–CH4=50%:50%), the CO2/CH4selectivity value lies in the range of 5.5～7.0, which is higher than those of most reported MOF-205[24](2.2) and [Cu(INIA)][25](4.3). These results confirm the excellent separation of C2H6, C2H4and CO2over CH4.

Fig.5. IAST sorption selectivity of 1a at 298 K

3. 4 Molecular simulation

To analyze the outstanding adsorption and separation ability of 1a framework, the host-guest binding interactions were studied at 298 K under 1 atm by grand canonical Monte Carlo (GCMC) simulations, and the result suggests two main C2H6molecules existing near the ligands in the cages (Fig.6a).The C2H6–Ivia-CH3groups forms multiple C–H·πinteraction with the benzene and pyridine rings of the BDP4-ligand. The C2H6–Ivia-CH3forms C–H·· O hydrogen bonds contacted with two uncoordinated carboxylic O atoms. The simulations revealed three adsorption sites for C2H4in the framework. C2H4-I contacts the benzene and pyridine rings of the BDP4-ligand through C–H···πinteractions. C2H4-II and-III molecules are located in the proximity of carboxylic O atoms and benzene ringviaCH· O hydrogen bonds and CH··πinteractions (Fig.6b). Three main adsorption sites for CO2molecules were identified in 1a (Figs. 6c and 6d). The C atom of CO2–I is also involved in C··πinteractions with the pyridine rings[26,27]. The CO2-II and -III all located near the ATZ-ligands via strong N–H·· O hydrogen bonds contacted with NH2groups. The simulations revealed the different distributions of C2H6, C2H4and CO2in the pores of 1a,causing distinct types of interaction, which are responsible for different selective adsorption of 1a for C2H6, C2H4and CO2over CH4.

Fig.6. Adsorption sites for C6H6 (a), C2H4 (b) and CO2 (c) in the framework of 1a

4 CONCLUSION

In summary, one multi-site functionalized microporous MOF has been successfully constructed via the solvothermal reaction using mixed ligand strategy. The highly polar pores of 1 were decorated by uncoordinated carboxylate O atoms,phenyl and pyridyl rings as well as -NH2groups, which leads to a significant selective adsorption of C2H6, C2H4and CO2over CH4. GCMC simulations further demonstrated the multiple binding sites for gas molecules in the framework.