# Highly Selective Separation of C2H2/CO2 and C2H2/C2H4 in an N-Rich Cage-Based Microporous Metal-Organic Framework

Authors: Lingzhi Yang; Wenpeng Xie; Qiuju Fu; Liting Yan; Shuo Zhang; Huimin Jiang; Liangjun Li; Xin Gu; Dandan Liu; Pengcheng Dai; Qingbin Zheng; Xuebo Zhao
Journal: Adsorption Science & Technology
 (2023)

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## Abstract

The separation of acetylene (C2H2) from carbon dioxide (CO2) and the purification of ethylene (C2H4) from C2H2 are quite essential processes for the chemical industry. However, these processes are challenging due to their similar physical properties, including molecule sizes and boiling points. Herein, we report an N-rich cage-based microporous metal-organic framework (MOF), [Cd5(Tz)9](NO3) (termed as Cd-TZ, TZ stands for tetrazole), and its highly efficient separation of C2H2/CO2 and C2H2/C2H4. Single-component gas adsorption isotherms reveal that Cd-TZ exhibits high C2H2 adsorption capacity (3.10 mmol g-1 at 298 K and 1 bar). The N-rich cages in Cd-TZ can trap C2H2 with a higher isosteric heat of adsorption (40.8 kJ mol-1) than CO2 and C2H4 owing to the robust host-guest interactions between the noncoordinated N atoms and C2H2, which has been verified by molecular modeling studies. Cd-TZ shows a high IAST selectivity for C2H2/CO2 (8.3) and C2H2/C2H4 (13.3). The breakthrough simulations confirm the potential for separating C2H2/CO2 and the purification of C2H4 from C2H2.


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## Body

## 1. Introduction

C2H2 and C2H4 are two of the most important chemical raw feedstocks for producing various commercial chemicals [1]. C2H2 is mainly obtained from coal or coal-derived coke with direct or indirect processes [2]. Thus, CO2 is usually contained in the crude acetylene as an unavoidable impurity generally removed in an aqueous sodium hydroxide wash. C2H2 can also be purified by dissolving in organic solvents, such as N-methylpyrrolidone and dimethylformamide. C2H4 is typically produced by the pyrolysis of ethane gas or light naphtha, with a small amount of C2H2 generated. C2H2 in mixtures must be removed, reducing its content to below 5 ppm [3, 4] because of the effects of C2H2 in poisoning catalysis during the polymerization process of ethylene. Solvent extraction and partial hydrogenation are the main commercial techniques to separate trace C2H2 from C2H4. However, both the purification methods of C2H2 and C2H4 used in the chemical industry are with poor separation selectivity and are energy-intensive [5]. Alternatively, selective adsorption and separation processes based on porous sorbents are energy-efficient and sustainable.In recent decades, MOFs [6–12] have been studied widely in separations and purifications for hydrocarbons [13–24]. However, the molecular sizes (3.3×3.3×5.7Å3 for C2H2, 3.2×3.3×5.4Å3 for CO2, and 3.3×4.2×4.8Å3 for C2H4) and kinetic diameter (3.3 Å for C2H2, 3.3 Å for CO2, and 4.2 Å for C2H4) of acetylene, carbon dioxide, and ethylene are very close [7, 25], making the efficient separation of C2H2/CO2 and C2H2/C2H4 a challenge. Until now, MOFs [7, 26–31] with highly selective separation performance for both C2H2/CO2 and C2H2/C2H4 are still rare. Currently, the reported MOF materials with record selectivity for C2H2/CO2 and C2H2/C2H4 is an anion-pillared flexible MOF, UTSA-300a [26]. Because of the strong C–H···F hydrogen-bonding interaction between UTSA-300a and C2H2, C2H2 can easily open the pore and be adsorbed into it while the other gases are blocked out of the pore. Lee and co-workers [28] reported a cationic MOF, JCM-1, which possesses imidazolium and nitrate groups in the pore channel, exhibiting a higher selectivity and adsorption affinity for C2H2 than for CO2 or C2H4. A broad strategy in MOFs to enhance the selectivity of C2H2/CO2 and C2H2/C2H4 is to increase the polar groups or adsorption sites, such as fluorine/nitrogen/oxygen-containing functional groups, to strengthen the interaction with C2H2 over CO2 or C2H4.Herein, we report a microporous MOF [Cd5(Tz)9](NO3) [32] with N-rich cavities for the preferential adsorption of C2H2 and the efficient separation for C2H2/CO2 and C2H2/C2H4. At ambient conditions, Cd-TZ shows a higher adsorption capacity and isosteric heat of adsorption for C2H2 than for CO2 and C2H4. Thus, Cd-TZ exhibits elevated C2H2/CO2 and C2H2/C2H4 selectivity. The modeling studies reveal that the trapped C2H2 molecule is in the center of the cavity and strongly interacts with the surrounding N atoms. The simulated breakthrough curves demonstrate the effective separation of binary mixtures of C2H2/CO2 and C2H2/C2H4.


## 2. Materials and Methods

### 2.1. Materials

All reagents and solvents were commercially available and, unless otherwise noted, were used without further purification. Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, AR) was purchased from Aladdin Reagent Co. Ltd. Ethyl tetrazole-5-carboxylate (>95%) was purchased from Bidepharm Co. Ltd.High-purity CO2 (99.999%), C2H4 (99.95%), and C2H2 (99.9%) were purchased from Qingdao Tianyuan Gas Co., Ltd. C2H2 was filtered through activated carbon to remove traces of acetone before use.


### 2.2. Synthesis of Cd-TZ

Cd-TZ was synthesized according to the methods reported in the literature [32]. A mixture of Cd(NO3)2·4H2O and ethyl tetrazole-5-carboxylate was dissolved in 10 mL H2O, then transferred to a 23 mL Teflon-lined autoclave and heated at 433 K for 72 h, followed by cooling to room temperature. Colorless crystals were collected by filtration and washed with H2O and methanol. The activation process of the Cd-TZ sample was conducted under a vacuum at 423 K for 12 h.


### 2.3. Characterizations of Cd-TZ

Thermogravimetric analyses (TGA) were examined using a Netzsch STA 449C instrument under an N2 atmosphere with a heating rate of 5 K min-1. Powder X-ray diffraction (PXRD) data were performed on an X-ray diffractometer (Bruker D8 Adv., Germany) with Cu Kα radiation from 5 to 50° (λ=1.5406) and a step size of 0.0167° in 2θ.


### 2.4. Adsorption Studies for CO2, C2H2, and C2H4

The sample was degassed in a vacuum for 12 h at 423 K to remove guest molecules before adsorption. Equilibrium and kinetic adsorption experiments of CO2, C2H2, and C2H4 were measured using a XEMIS magnetic suspension balance sorption analyzer (Hiden, UK) equipped with a circulating water bath at 273 K and 298 K, respectively.


### 2.5. Calculation of Brunauer-Emmett-Teller (BET) Surface Area and Langmuir Surface Area

Surface areas of Cd-TZ were calculated using the BET equation and Langmuir equation based on the CO2 adsorption isotherm at 273 K.BET surface area:(1)PnP0−P=1Cnm+C−1Cnm·PP0,(2)SBET=nm·A·σm.Langmuir surface area:(3)Pn=Pnm+1bnm,(4)SLangmuir=nm·A·σm,where P is the pressure (bar), P0 is the saturated vapor pressure, n is the adsorption amount under the corresponding pressure, nm is the amount related to the monolayer surface coverage, C (C>0) and b are constants, σm is the cross-sectional area of CO2 (2.18×10−19m2), and SBET and SLangmuir are the calculated BET surface area and Langmuir surface area, respectively.


### 2.6. Calculation of Isosteric Enthalpy of Adsorption

Isosteric enthalpy of adsorption (Qst) was calculated from isotherms measured at 273 K and 298 K for CO2, C2H2, and C2H4. The isotherms were firstly fit to a virial equation:
(5)lnP=lnN+1T∑i=0maiNi+∑i=0nbiNi,where P is the pressure expressed in mbar, N is the amount adsorbed in mmol g-1, T is the temperature in K, ai and bi are virial coefficients, and m and n represent the number of coefficients required to describe the isotherms adequately. To calculate Qst, the fitting parameters from the above equation were used for the following equation:
(6)Qst=−R∑i=0maiNi.


### 2.7. Ideal Adsorbed Solution Theory (IAST) Selectivity Calculation

The gas adsorption isotherms were firstly fitted to a dual-site Langmuir-Freundlich (DSLF) equation:(7)q=q1b1pn11+b1pn1+q2b2pn21+b1pn2,where q (mmol g-1) is the amount of adsorbed gas; p (mbar) is the bulk gas phase pressure; q1 (mmol g-1) and q2 (mmol g-1) are the saturation capacities of sites 1 and 2, respectively; b1 (mbar-n1) and b2 (mbar-n2) are the affinity coefficients of site 1 and site 2, respectively; and n1 and n2 represent the deviations from an ideal homogeneous surface.The adsorption selectivity is defined by(8)Sads=x1/y1x2/y2,where xi and yi are the mole fractions of component i (i=1 and 2) in the adsorbed and bulk phases, respectively.


### 2.8. Density-Functional Theory Calculations

All the geometry optimizations and binding energies were calculated by the periodic density functional theory (DFT) method using the DMol3 module. The host framework and the gas molecule were both regarded as rigid. The structures of the framework were first optimized. Then, guest gas molecules were introduced to the optimized framework, followed by a full structure relaxation. An isolated gas molecule placed in a supercell (with the same cell dimensions as the framework) was also relaxed as a reference to obtain the gas binding energy. The widely used generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional and the double numerical plus polarization (DNP) basis set, the Grimme method for DFT-D correction, and the DFT semicore pseudopots (DSPP) were used. The energy, force, and displacement convergence criteria were set as 1×10−5Ha, 2×10−3Ha, and 5×10−3Å, respectively. The following equations then calculated the static binding energy (at T=0K):
(9)ΔEbinding=EMOF+Egas−EMOF+gas,where ΔEbinding is the static binding energy between the MOF and the gas molecule and EMOF, Egas, and EMOF+gas are the energies of the gas-free MOF, the free gas molecule, and the MOF-gas system, respectively.


### 2.9. Breakthrough Simulation

Breakthrough simulations were performed using the 3P-Sim software. In the simulated separation experiment, the Cd-TZ (1.0 g) was packed into the column with a length of 5 cm and an inner diameter of 0.45 cm. And the simulated operating condition is under 1 bar at 298 K with a continuous gas (C2H2/CO2 (50/50, v/v) or C2H2/C2H4 (50/50, v/v)) flow of 2 mL (STP) min-1.


## 2.1. Materials

All reagents and solvents were commercially available and, unless otherwise noted, were used without further purification. Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, AR) was purchased from Aladdin Reagent Co. Ltd. Ethyl tetrazole-5-carboxylate (>95%) was purchased from Bidepharm Co. Ltd.High-purity CO2 (99.999%), C2H4 (99.95%), and C2H2 (99.9%) were purchased from Qingdao Tianyuan Gas Co., Ltd. C2H2 was filtered through activated carbon to remove traces of acetone before use.


## 2.2. Synthesis of Cd-TZ

Cd-TZ was synthesized according to the methods reported in the literature [32]. A mixture of Cd(NO3)2·4H2O and ethyl tetrazole-5-carboxylate was dissolved in 10 mL H2O, then transferred to a 23 mL Teflon-lined autoclave and heated at 433 K for 72 h, followed by cooling to room temperature. Colorless crystals were collected by filtration and washed with H2O and methanol. The activation process of the Cd-TZ sample was conducted under a vacuum at 423 K for 12 h.


## 2.3. Characterizations of Cd-TZ

Thermogravimetric analyses (TGA) were examined using a Netzsch STA 449C instrument under an N2 atmosphere with a heating rate of 5 K min-1. Powder X-ray diffraction (PXRD) data were performed on an X-ray diffractometer (Bruker D8 Adv., Germany) with Cu Kα radiation from 5 to 50° (λ=1.5406) and a step size of 0.0167° in 2θ.


## 2.4. Adsorption Studies for CO2, C2H2, and C2H4

The sample was degassed in a vacuum for 12 h at 423 K to remove guest molecules before adsorption. Equilibrium and kinetic adsorption experiments of CO2, C2H2, and C2H4 were measured using a XEMIS magnetic suspension balance sorption analyzer (Hiden, UK) equipped with a circulating water bath at 273 K and 298 K, respectively.


## 2.5. Calculation of Brunauer-Emmett-Teller (BET) Surface Area and Langmuir Surface Area

Surface areas of Cd-TZ were calculated using the BET equation and Langmuir equation based on the CO2 adsorption isotherm at 273 K.BET surface area:(1)PnP0−P=1Cnm+C−1Cnm·PP0,(2)SBET=nm·A·σm.Langmuir surface area:(3)Pn=Pnm+1bnm,(4)SLangmuir=nm·A·σm,where P is the pressure (bar), P0 is the saturated vapor pressure, n is the adsorption amount under the corresponding pressure, nm is the amount related to the monolayer surface coverage, C (C>0) and b are constants, σm is the cross-sectional area of CO2 (2.18×10−19m2), and SBET and SLangmuir are the calculated BET surface area and Langmuir surface area, respectively.


## 2.6. Calculation of Isosteric Enthalpy of Adsorption

Isosteric enthalpy of adsorption (Qst) was calculated from isotherms measured at 273 K and 298 K for CO2, C2H2, and C2H4. The isotherms were firstly fit to a virial equation:
(5)lnP=lnN+1T∑i=0maiNi+∑i=0nbiNi,where P is the pressure expressed in mbar, N is the amount adsorbed in mmol g-1, T is the temperature in K, ai and bi are virial coefficients, and m and n represent the number of coefficients required to describe the isotherms adequately. To calculate Qst, the fitting parameters from the above equation were used for the following equation:
(6)Qst=−R∑i=0maiNi.


## 2.7. Ideal Adsorbed Solution Theory (IAST) Selectivity Calculation

The gas adsorption isotherms were firstly fitted to a dual-site Langmuir-Freundlich (DSLF) equation:(7)q=q1b1pn11+b1pn1+q2b2pn21+b1pn2,where q (mmol g-1) is the amount of adsorbed gas; p (mbar) is the bulk gas phase pressure; q1 (mmol g-1) and q2 (mmol g-1) are the saturation capacities of sites 1 and 2, respectively; b1 (mbar-n1) and b2 (mbar-n2) are the affinity coefficients of site 1 and site 2, respectively; and n1 and n2 represent the deviations from an ideal homogeneous surface.The adsorption selectivity is defined by(8)Sads=x1/y1x2/y2,where xi and yi are the mole fractions of component i (i=1 and 2) in the adsorbed and bulk phases, respectively.


## 2.8. Density-Functional Theory Calculations

All the geometry optimizations and binding energies were calculated by the periodic density functional theory (DFT) method using the DMol3 module. The host framework and the gas molecule were both regarded as rigid. The structures of the framework were first optimized. Then, guest gas molecules were introduced to the optimized framework, followed by a full structure relaxation. An isolated gas molecule placed in a supercell (with the same cell dimensions as the framework) was also relaxed as a reference to obtain the gas binding energy. The widely used generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional and the double numerical plus polarization (DNP) basis set, the Grimme method for DFT-D correction, and the DFT semicore pseudopots (DSPP) were used. The energy, force, and displacement convergence criteria were set as 1×10−5Ha, 2×10−3Ha, and 5×10−3Å, respectively. The following equations then calculated the static binding energy (at T=0K):
(9)ΔEbinding=EMOF+Egas−EMOF+gas,where ΔEbinding is the static binding energy between the MOF and the gas molecule and EMOF, Egas, and EMOF+gas are the energies of the gas-free MOF, the free gas molecule, and the MOF-gas system, respectively.


## 2.9. Breakthrough Simulation

Breakthrough simulations were performed using the 3P-Sim software. In the simulated separation experiment, the Cd-TZ (1.0 g) was packed into the column with a length of 5 cm and an inner diameter of 0.45 cm. And the simulated operating condition is under 1 bar at 298 K with a continuous gas (C2H2/CO2 (50/50, v/v) or C2H2/C2H4 (50/50, v/v)) flow of 2 mL (STP) min-1.


## 3. Results and Discussion

As shown in Figures1(a) and 1(b), each Cd (II) atom in the framework is surrounded by different tetrazole ligands and coordinated with six nitrogen atoms to form an octahedral coordination structure, resulting in a cationic three-dimensional framework [Cd5(Tz)9]+ which is balanced by NO3-. Cd (II) atoms interconnect such six second-building units to form one-dimensional straight pore channels (4.6 Å), which have large cavities (5.3 Å) parallel to each other along the c axis (Figures 1(c) and 1(d)). The surface of the cavities contains many uncoordinated tetrazole nitrogen atoms, which is conducive to capturing C2H2 molecules. Cd-TZ was successfully prepared by hydrothermal reaction at 433 K, and the phase purity of the material was identified by PXRD patterns which nicely match the calculated patterns from single crystal data (Figure 2(a)). To investigate the thermal stability of Cd-TZ, the synthesized Cd-TZ samples were subjected to TGA under a nitrogen atmosphere, as shown in Figure 2(b). The TGA curves show that the Cd-TZ sample lost most of the free water molecules in the pore channels as the temperature increased to 98°C. When the temperature exceeded 300°C, the sample lost weight rapidly. This phenomenon indicates that Cd-TZ has good thermal stability under a nitrogen atmosphere and can maintain the structure over a wide temperature range (<300°C). The specific surface area of Cd-TZ was characterized using CO2 adsorption experiments performed at 273 K (Figure S1a). And the BET surface area (Figure S1b) and Langmuir surface area (Figure S2) were calculated to be 388 m2 g-1 and 403 m2 g-1, respectively.Figure 1
The framework of Cd-TZ without hydrogen atoms and solvent molecules viewed along thea axis (a) and b axis (b) (Cd, C, N, and O are represented by light yellow, gray, lavender, and red, respectively). The pore structure of Cd-TZ without hydrogen atoms and solvent molecules viewed along the a axis (c) and b axis (d).
(a)(b)(c)(d)Figure 2
(a) PXRD patterns of as-synthesized Cd-TZ and activated Cd-TZ and (b) thermogravimetry curves of Cd-TZ.
(a)(b)Considering the suitable pore size and the presence of abundant uncoordinated nitrogen atoms in the pore surface of Cd-TZ, we were intrigued to explore its potential to capture C2H2 and, further, to separate C2H2/CO2 and C2H2/C2H4. Therefore, single-component adsorption-desorption isotherms of C2H2, CO2, and C2H4 were collected at 273 K and 298 K (Figures 3(a) and 3(b)). The adsorption capacity of C2H2 is higher than those of CO2 and C2H4 in Cd-TZ under the same conditions. The C2H2 capacity is 3.10 mmol g-1 at 298 K and 1 bar, higher than the capacity of CO2 (2.07 mmol g-1) and C2H4 (1.68 mmol g-1) and outperforming many reported MOFs, such as ZU-62-Ni (3.0 mmol g-1) [33], NKMOF-1-Ni (2.7 mmol g-1) [34], Zn(ad)(int) (2.32 mmol g-1) [35], CPL-1-NH2 (1.84 mmol g-1) [36], and Zn-FBA (1.03 mmol g-1) [37]. Furthermore, the C2H2 adsorption isotherms rise rapidly in the low-pressure region at 273 K and 298 K. At 298 K and 0.01 bar, the uptake ratio of C2H2/CO2 and C2H2/C2H4 in Cd-TZ is 1.8 and 3.0, respectively, indicating the stronger interaction between C2H2 and Cd-TZ. The isosteric enthalpies of adsorption for C2H2, CO2, and C2H4 were calculated based on fitting using the virial method (Figure S3-S5) to evaluate the interaction between the framework and the guest molecule. As shown in Figure 3(c), the calculated value of Qst for C2H2, CO2, and C2H4 at zero loading is 40.8 kJ mol-1, 35.6 kJ mol-1, and 31.8 kJ mol-1, respectively. Furthermore, the Qst value of Cd-TZ for C2H2 is higher than those for CO2 and C2H4 over the entire pressure range from 0 to 1 bar. All the results demonstrate that Cd-TZ prefers to adsorb C2H2 and has a stronger affinity for C2H2 than CO2 and C2H4, which prompts us to explore the separation performance of Cd-TZ for the binary gas of C2H2/CO2 and C2H2/C2H4.Figure 3
Single-component adsorption-desorption isotherms of C2H2, CO2, and C2H4 in Cd-TZ at (a) 273 K and (b) 298 K. (c) The isosteric enthalpies of adsorption for C2H2, CO2, and C2H4 in Cd-TZ.
(a)(b)(c)The adsorption selectivity for C2H2/CO2 (50/50, v/v) and C2H2/C2H4 (50/50, 1/99, v/v) was calculated based on the IAST model based on fitting using the DSLF equation (Figure S6-S8). The selectivity of Cd-TZ for C2H2/CO2 (50/50, v/v) is 8.3 at 298 K and 1.0 bar (Figure S9), which is compared to the reported UTSA-74a (9.0) [38], and superior to some of the benchmark MOFs like SIFSIX-21-Ni (7.8) [39], TIFSIX-2-Cu-i (6.5) [40], FJU-90a (4.3) [41], and ZrT-1-tetrazl (2.8) [42] under similar conditions. As shown in Figure S10, the selectivity of Cd-TZ for the binary mixtures of C2H2/C2H4 (50/50, v/v) is up to 20.1 at 273 K and 1 bar, with the temperature increasing to 298 K, and the selectivity is about 13.3, which surpasses most of the reported MOFs, such as cPAF-28 (12.2) [43], MUF-17 (8.73) [30], NbU-1 (5.9) [44], CuTiF6-TPPY (5.47) [45], and Fe-MOF-74 (2.1) [46]. Consider that C2H2 is a minor impurity in the crude C2H4 production by the cracking of ethane. Therefore, the IAST selectivity of C2H2/C2H4 (1/99, v/v) was also calculated in addition to that of C2H2/C2H4 (50/50, v/v). The calculated IAST selectivity value for the binary mixtures of C2H2/C2H4 (1/99, v/v) is 7.3 higher than those of SIFSIX-3-Ni (5.0) [47] and NUM-11a (1.65) [48]. These calculation results further indicate that Cd-TZ is a potential porous material in separating C2H2/CO2 and removing C2H2 from C2H4.The optimal adsorption sites of C2H2, C2H4, and CO2 in the framework of Cd-TZ were elucidated by DFT calculations. The calculated static binding energy of C2H2 in Cd-TZ is 75.1 kJ mol-1, much higher than those of C2H4 with 54.4 kJ mol-1 and CO2 with 35.6 kJ mol-1. The lowest-energy binding configuration of C2H2, C2H4, and CO2 in Cd-TZ is shown in Figure 4, indicating that the adsorbed C2H2 molecule is trapped in the center of the cavity and strongly interacts with the surrounding N atoms, which are highly electronegative. Figure S11 shows that the optimized distance between one H atom of C2H2 and the noncoordinated N atom in the cavity is 2.14 Å, which is smaller than the sum of the van der Waals radius of the H atom (1.20 Å) and N atom (1.55 Å). Moreover, the calculated distances between C atoms of C2H2 and H atoms of tetrazole are 2.72 Å and 2.74 Å, respectively, shorter than the sum of the van der Waals radius of H and C atoms, 2.90 Å. These results indicate the robust affinity of the framework to the C2H2 molecules. C2H4 has a larger size and planar configuration. Thus, only one end of the =CH2 group is adsorbed in the cavity with side-on orientation and forms a weaker interaction with the surrounding tetrazoles (Figure S12). There are four C=C-H···N dipolar interactions (2.89 Å, 2.92 Å, 3.10 Å, and 3.10 Å) observed between C2H4 and the tetrazoles in the cavity. And the calculated H-C=C···H distances between C2H4 and the tetrazoles in the cavity are 2.70~2.89 Å (2.70 Å, 2.73 Å, 2.87 Å, and 2.89 Å). As shown in Figure S13, CO2 has two weak CCO2···N interactions and three weak OCO2···H interactions with the tetrazoles. The calculated distance of CCO2···N is 3.40 Å and 3.47 Å, while the estimated distance of OCO2···H is 2.85 Å, 3.41 Å, and 3.49 Å, indicating the weak interaction between CO2 and the framework. The simulation calculations confirm that the adsorption affinity of C2H2 is much stronger than C2H4 and CO2, consistent with the experimental results.Figure 4
Comparison of the optimal (a) C2H2, (b) C2H4, and (c) CO2 adsorption sites observed by DFT calculations along the b axis. Cd, C, N, and H in Cd-TZ are represented by dark blue, bright grey, lavender, and white, respectively. C in C2H2 and C2H4 is represented by bright yellow.
(a)(b)(c)The separation performance of Cd-TZ was investigated by breakthrough simulations with the binary gas mixture of C2H2/CO2 (50/50, v/v) and C2H2/C2H4 (50/50, v/v) at 298 K and 1 bar. As shown in Figure 5(a), CO2 gas was first eluted at 31 min from the column, whereas C2H2 was retained in the column until 50 min, demonstrating a stronger binding affinity of the framework to C2H2 molecules. Figure 5(b) shows that Cd-TZ can realize the complete separation of C2H2/C2H4 (50/50, v/v). The C2H4 broke through from the column after 30 minutes, and no C2H2 was eluted out before 53 minutes, indicating that high-purity C2H4 could be obtained with the framework. All the above results demonstrate the great potential of Cd-TZ in the separation of C2H2/CO2 and C2H2/C2H4 in practice.Figure 5
Simulated breakthrough curves for (a) C2H2/CO2 (50/50, v/v) and (b) C2H2/C2H4 (50/50, v/v) at 298 K and 1 bar.
(a)(b)


## 4. Conclusions

In summary, we have investigated the N-rich cage-based microporous metal-organic framework, Cd-TZ, showing a stronger affinity for C2H2 than CO2 and C2H4, and efficiently separating binary mixtures of C2H2/CO2 and C2H2/C2H4. Owing to the abundant uncoordinated nitrogen atoms on the pore surface and the cavities, Cd-TZ exhibits a high capacity for C2H2 (3.10 mmol g-1 at 298 K and 1 bar). The calculated IAST selectivity of Cd-TZ for C2H2/CO2 and C2H2/C2H4 is 8.3 and 13.3, respectively. The breakthrough simulation results well confirm the separation performance of Cd-TZ. In addition, the preferential binding of C2H2 over CO2 and C2H4 is clearly demonstrated by DFT calculations. This study provides an exquisite example of MOF possessing abundant electronegative nitrogen sites on the pore surfaces and the cavities for the challenging separation of C2H2/CO2 and C2H2/C2H4.



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