Ethylene (C
2H
4) is an essential feedstock for the chemical industry, and its downstream chemicals account for the majority of petrochemical products. Presently, three main processes are used to prepare C
2H
4: Naphtha cracking, oxidation and dehydrogenation (ODH) of ethane, and coal-to-ethylene. Nevertheless, in response to increasingly severe environmental issues and cost-cutting, researchers have developed more production processes with lower energy consumption, fewer pollutant emissions, and higher ethylene recovery rates. The oxidative coupling of methane (OCM) for ethylene, which involves a simple and cost-effective process, is a highly promising technique for ethylene production [
1]. In the OCM process, the C–H bond is activated by O
2 to generate radicals, which then undergo dehydrogenative coupling to form C
2H
4, while the extensive thermals released in the process may cause over oxidation of C
2H
4 to produce C
2H
2, CO
2, and other byproducts. For the sake of downstream products made from C
2H
4, removing C
2H
4 from other byproducts is critically important. Considering the cost and environmental impact, porous materials show great potential in separating C
2H
4 from light hydrocarbon mixtures. Since their slight difference in quadrupole moments (C
2H
2: 7.2 × 10
−26 esu cm
2, CO
2: 4.3 × 10
−26 esu cm
2, C
2H
4: 1.5 × 10
−26 esu cm
2) and kinetic sizes (C
2H
2: 3.3 Å, CO
2: 3.3 Å, C
2H
4: 4.2 Å) (Table S1 in Supporting information), one-step purification of C
2H
4 from C
2H
2/CO
2/C
2H
4 ternary mixtures is extremely challenging. Therefore, an effective strategy to address this challenge is to design and synthesize a polar structure, wherein a negative electrostatic potential environment within the pores facilitates the enhancement of adsorption differentiation among gas molecules with distinct polarities. Metal–organic frameworks (MOFs) are a class of porous and inorganic-organic hybrid materials. Currently, the pore size designability and modifiability of MOFs have reached the range of sub-angstrom, thus providing great potential in separating molecules with subtle differences, such as C
2H
2/CO
2 [
2–
4], C
2H
6/C
2H
4 [
5,
6], and xylene isomers separation [
7–
9]. However, most of the reported MOFs have a poor selectivity for both C
2H
2/C
2H
4 and CO
2/C
2H
4 because of their similar physical properties, so it is still difficult to realize one-step purification C
2H
4 from C
2H
2/CO
2/C
2H
4 ternary mixtures [
10–
13]. In the view of pore structure, reducing the pore size of MOFs is one way to improve selectivity, proper apertures allow the framework to selectively capture small gases while excluding larger gases. Therefore, the design and synthesis of new and polar ultra-microporous MOFs is mandatory considering the kinetic sizes and quadrupole moments of C
2H
2, CO
2, and C
2H
4.
Zeolite-like metal–organic frameworks (ZMOFs) are a unique branch of MOFs, which exceed zeolites and other MOFs in gas storage and separation application [
14–
19]. Compared to traditional MOFs, the advantages of ZMOFs primarily lie in their cage-like cavities and ultra-microporous window sizes, which are beneficial for enhancing adsorption capacity and selective adsorption performance. Additionally, the anionic or cationic nature of some ZMOFs can be exploited to tailor the frameworks’ affinity for certain gases, improving separation performance. Compared to other ZMOFs topologies, the sodalite (SOD) net is the most common occurrence, owing to its simple tiling of edge-transitive nets (1121). Specifically, the SOD cage is composed of six four-membered rings (4MR) and eight six-membered rings (6MR), while the sodalite network is a 3D structure formed by the interconnected SOD cages through their coplanar assembly. Through the unremitting efforts of researchers, how to construct the SOD topological ZMOFs has made substantial strides. For example, Eddaoudi's group used the Zr
6 cluster as a tetrahedral building unit (TBU) and judiciously selected the bent ditopic linker with steric hindrance to construct the Zr-sod-ZMOFs [
20]. Subsequently, they utilized the centring structure-directing agents (cSDA) with appropriate geometry to prepare 21 types of ZMOFs with SOD topology, during which Fe-sod-ZMOFs-320 possessed the largest pore volume (3.21 cm
3/g) of sodalite compounds and exhibited the outstanding gas storage performance [
21]. ZIF-8 with SOD topology can be regarded as a potential ideal material for gas separation due to its ultra-microporous pore size (3.4 Å) and high porosity (0.66 cm
3/g). However, ZIF-8 shows poor separation ability in slightly different gases (
e.g., C
2H
2, CO
2, and C
2H
4) owing to its nonpolar skeleton. To solve this problem, a feasible strategy is to construct an interpenetrated SOD structure that can fine-tune the pore size under sub-angstrom and increase the number of active sites per unit volume. However, rare examples of interpenetrated ZMOFs have been witnessed on account of the trait of zeolite type [
22,
23], and theoretically, the SOD net is not self-dual and forbidden self-interpenetration. Therefore, it remains a great challenge for the synthesis of interpenetrated SOD-based ZMOFs.
In this work, we first proposed a feasible 4 + 4 + 2 strategy to prepare a novel ZMOF (namely
JLU-MOF119) with a unique two-fold interpenetrated SOD topology based on the 4 + 2 strategy for ZIF-8 synthesis (
Scheme 1). Specifically speaking, based on the 4 + 2 strategy, besides the TBU served by the 4-connected metal node and a 2-connected linker (such as imidazole) to generate the 6MR, a coplanar 4-connected linker is properly chosen to replace some 2-connected linker and serves as the 4MR. In such 4 + 4 + 2 strategy, lengthening of the 4-connected ligand is crucial to construct interpenetrated structures. Given that, following our previous work of JLU-MOF52 (GIS) [
24], a 2,5-H
4ABTC ligand with low symmetry was utilized as the 4-c ligand. Such ligand may transform its configuration from tetrahedron (in JLU-MOF52) to parallelogram like 3,5-H
4ABTC in PCN-250 in appropriate conditions [
25]. Meanwhile, to increase the number of polar sites, the imidazole in ZIF-8 is replaced by the TAZ ligand. As a result, the prepared
JLU-MOF119 possesses the SOD cage with abundant uncoordinated O and N sites, and the ultra-microporous pore size (4.1 Å and 3.3 Å).
JLU-MOF119 exhibits one-step purification C
2H
4 from ternary mixtures of C
2H
2/CO
2/C
2H
4, reaching 99.2% purity of C
2H
4, as proved by the breakthrough and recycling experiments.
According to the reported ZMOFs, constructing the SOD ZMOFs involves several strategies: (1) 4 + 2 strategy,
i.e., 4-connected single-metal-ion nodes serve as TBU bridge by ditopic linker [
26–
30], (2) 4 + 4 strategy,
i.e., bridge the TBU with a tetrahedral tetratopic linker [
31], (3) 4 + 6 strategy,
i.e., bridge the TBU with a hexacarboxylic linker [
32], (4) face-directed strategy,
i.e., the linker serves as the face of the cage [
33–
36], (5) polyhedral assembly strategy,
i.e., the single-metal building blocks {In(COO)
4} as the TBU to construct the SOD-type frameworks and the internal trinuclear clusters {In
3O} form the tetrahedral cages (Fig. S1 in Supporting information) [
37,
38]. Here, based on the 4 + 2 strategy, a novel 4 + 4 + 2 strategy was proposed and successfully applied to prepare
JLU-MOF119 with unique 2-fold interpenetrated SOD net. Specifically, a portion of the imidazole ligands is replaced by 4-connected 2,5-H
4ABTC ligands, while the remaining imidazole ligands are substituted by TAZ ligands, thereby forming the 4 + 4 + 2 strategy. The employment of elongate 4MR results in two types of 6MR and distorted framework, eventually leading to the interpenetrated SOD structure.
The crystallography data reveals that
JLU-MOF119 crystallizes in a trigonal system with
R-3 space group (Table S2 in Supporting information). In the skeleton, independent Zn
2+ and 2,5-H
4ABTC adopt four coordination, and TAZ serves as a ditopic linker. Each Zn
2+ is coordinated with two oxygen atoms from two individual 2,5-H
4ABTC ligands and two nitrogen atoms from different TAZ, respectively, to form the low-symmetry TBU, ZnN
2O
2 (
Fig. 1a and Fig. S2a in Supporting information). Each independent 2,5-H
4ABTC ligand is linked to four Zn
2+ and two TAZ, generating the 4MR (
Fig. 1c and Fig. S2b in Supporting information). Moreover, each carboxyl group of 2,5-H
4ABTC contains an uncoordinated O atom and points into the SOD cage. Each individual TAZ linker is coordinated to two Zn
2+ cations and then engenders the 6MR-1 (Fig. S2c in Supporting information), the adjacent TAZ linkers are interlaced, and the uncoordinated N atoms of TAZ point toward the center of the window. There are two kinds of 6MR in the framework, one is the regular 6MR-1, and the other is formed by two TAZ, four half of 2,5-H
4ABTC and six Zn
2+ (
Figs. 1c and d). In the 6MR-2, the TAZ serves as the shorter edge, and the half of 2,5-H
4ABTC serves as the longer edge, the length varies between TAZ and 2,5-H
4ABTC resulting in the window of 6MR-2 is bigger than that of 6MR-1. From the topological perspective, the 4MR, 6MR-1, and 6MR-2 further extend to a 3D framework, which has the same underlying SOD topology as ZIF-8 (
Fig. 1b), yet distinguishes it from the SOD net. In the
JLU-MOF119 framework, the 4MR with low symmetry and represents parallelogram, and other 4MR of ZMOFs with SOD net are mostly square. The twisted 4MR gives rise to the formation of 6MR with different sizes, which causes structural interpenetration and distortion of the framework (
Fig. 1b). Thus, there are two types of apertures for
JLU-MOF119, one is 6MR-1 forming a hexagonal aperture with size of 4.1 Å × 4.1 Å along the [001] direction and the other has a rectangular channel formed by interpenetrated 6MR-2 with size of 4.8 Å × 3.3 Å (
Figs. 1e–g). It is noted that the pore structure of
JLU-MOF119 is distinct from ZIF-8 due to the interpenetrated framework, in which ZIF-8 only has one kind of hexagonal aperture and a significant smaller cage of SOD (11.6 Å
vs. 18.6 Å), yet the cage of
JLU-MOF119 is reduced to 5.9 Å after the interpenetration of the framework (Fig. S3 in Supporting information).
Notably,
JLU-MOF119 possesses two kinds of ultra-microporous pores as well as better stability in various solvents and thermal stability before 210 ℃ (Figs. S4 and S5 in Supporting information), which endows it with potential adsorption and separation ability in light hydrocarbons mixtures. N
2 could not be adsorbed by
JLU-MOF119 for its ultra-microporous window size, thereby, CO
2 adsorption was used to evaluate the porosity of
JLU-MOF119 at 195 K. As shown in
Fig. 2a, the uptake of
JLU-MOF119 is 125 cm
3/g at 195 K and 100 kPa. The Brunauer-Emmett-Teller (BET) and Langmuir surface areas of
JLU-MOF119 are 387 and 433 m
2/g, respectively. Light hydrocarbons are significant raw materials and industrial gases for the chemical industry, and the ultra-microporous window size of
JLU-MOF119 prompts us to hypothesize that it has the potential for the separation of light hydrocarbons. Therefore,
JLU-MOF119 was performed for single-component adsorption of N
2, CO
2, CH
4, C
2H
6, C
2H
4, C
2H
2, and C
3H
8 (
Figs. 2b–d and Fig. S6 in Supporting information) at 273 and 298 K. Apparently, the gases of N
2, CH
4, C
2H
6, and C
3H
8 with bigger kinetic diameters are hardly adsorbed by
JLU-MOF119 at 273 and 298 K, the slight uptake is primarily attributed to the adsorption on the surface of
JLU-MOF119. Simultaneously, the C
2H
2, CO
2, and C
2H
4 can be adsorbed by
JLU-MOF119, these results are consistent with the ultra-microporous pore size, which means that
JLU-MOF119 exhibits a respectable size sieving effect. Moreover, the uptake of C
2H
4 (17.37 cm
3/g) is lower than that of C
2H
2 (45.61 cm
3/g) and CO
2 (31.54 cm
3/g) at 1.0 bar and 298 K, which uncovers the possibility of separating C
2H
2/C
2H
4 and CO
2/C
2H
4. Simultaneously, compared to ZIF-8, the advantage of the interpenetrated framework is more obvious in gas separation, which exhibits an increase in the uptake of gases with smaller size (
e.g., CO
2 and C
2H
2), while a significant decrease of bigger gas molecules (
e.g., C
2H
6, C
3H
8, and C
3H
6) [
39–
41]. Moreover, the adsorption capacity of
JLU-MOF119 for C
2H
2 remains stable after 12 cyclic tests, indicating that
JLU-MOF119 has excellent stability and C
2H
2 sorption capacity (Fig. S7 in Supporting information).
The isosteric heat of adsorption (
Qst) for C
2H
2, CO
2, and C
2H
4 was calculated by the Virial equation to estimate the affinity between gases and the skeleton. As shown in
Fig. 2e, the
Qst of C
2H
2, CO
2, and C
2H
4 are 22.6, 21.5, and 19.55 kJ/mol respectively at zero-coverage, indicating a stronger affinity of C
2H
2 and CO
2 than C
2H
4. Additionally, all the values of
Qst are lower than the majority of MOFs,
e.g., FJI-H36 (36.1, 29.2, and 29.9 kJ/mol) [
42], SNNU-98-Mn (60.2, 29.2, and 30.8 kJ/mol) [
43], and ZNU-8 (33.1, 26.6, 25.7 kJ/mol) [
4], which implies the low energy consumption for regeneration. The simulation of the adsorption isotherms and
Qst plots for the other gases are shown in Figs. S8–S11, Tables S3 and S4 (Supporting information).
To assess the separation capacity, the adsorption selectivity was calculated by the ideal adsorption solution theory (IAST) of dual-site Langmuir-Freundlich model from single component adsorption isotherms. The selectivity for C
2H
2/C
2H
4 (50/50, 1/99, v/v) is 3.4 and 3.3 (
Fig. 2f), higher than those reported MOFs of Fe-MOF-74 (2.08, 50/50) [
44], BSF-1 (2.3, 50/50) [
45], TJT-100 (1.8, 50/50) [
46], JLU-MOF88 (3, 50/50) (Table S5 in Supporting information) [
47]. The selectivity for CO
2/C
2H
4 (50/50, 1/99, v/v) is 2.03 and 2.05, higher than Zn-atz-oba (1.33, 50/50) [
13], NKMOF-1-Ni (0.8, 50/50) [
48]. Compared to reported MOFs in purification C
2H
4 from ternary mixtures of C
2H
2/CO
2/C
2H
4 (Fig. S12 and Table S6 in Supporting information), such as Zn-atz-oba (C
2H
2/C
2H
4, 1.33, CO
2/C
2H
4, 1.43) [
13], Zn-fa-atz (C
2H
2/C
2H
4, 1.5, CO
2/C
2H
4, 1.4) [
10],
JLU-MOF119 is an effective purification candidate for C
2H
4 from ternary mixtures of C
2H
2/CO
2/C
2H
4 by one step.
As is well known, compared to C
2H
4, C
2H
2, and CO
2 possess higher electron cloud densities. Therefore, a polar cage surface demonstrates a stronger affinity for C
2H
2 and CO
2. To thoroughly verify the practical application potential of
JLU-MOF119 for C
2H
2/CO
2/C
2H
4 separation,
JLU-MOF119 was executed the breakthrough experimental tests. The binary mixtures breakthrough experiments of C
2H
2/C
2H
4 and CO
2/C
2H
4 were performed in equimolar at 2 mL/min and 298 K (
Figs. 3a and b). In
Fig. 3a, the C
2H
4 is first flowed out at 17 min, while the C
2H
2 is eluted at 32 min. The experiment result is consistent with the static adsorption isotherm. Compared to separate C
2H
2/C
2H
4, the mixtures of CO
2 and C
2H
4 have closer quadrupole moments, which means it would be more obstacle to separate CO
2/C
2H
4. As indicated in
Fig. 3b, the C
2H
4 is eluted at 17 min, and CO
2 is retained until eluted at 33 min, confirming
JLU-MOF119’s exceptional CO
2/C
2H
4 separation performance. In the OCM process, C
2H
2, CO
2, and C
2H
4 are coexistent, thus the breakthrough experiment of C
2H
2/CO
2/C
2H
4 was executed at 3 mL/min and 298 K (
Fig. 3c). As shown in
Fig. 3c, the C
2H
4 is first flowed out at 14 min, while C
2H
2 and CO
2 are flowed out at 21 min and 22 min. And the purity of C
2H
4 reaches a high value of 99.2% at the outlet before the C
2H
2 and CO
2 flow out further proving
JLU-MOF119 achieves one-step purification of C
2H
4 from C
2H
2/CO
2/C
2H
4 ternary mixtures. To better meet the practical application, cyclic tests on
JLU-MOF119 were performed to evaluate the regenerative capacity. As shown in
Figs. 3d–f, the capacity of
JLU-MOF119 to separate C
2H
2/C
2H
4, CO
2/C
2H
4 and C
2H
2/CO
2/C
2H
4 remains virtually unchanged after 4 cycles. Moreover, the consistent C
2H
2 adsorption capacity after 12 cycles indicates that
JLU-MOF119 retains its selective C
2H
4 purification performance even after 12 breakthrough experiments (Fig. S7).
First-principles based calculations with empirical correction for dispersive interactions were performed to investigate the mechanisms that differentiate C
2H
2, CO
2, and C
2H
4 adsorption in
JLU-MOF119 (
Figs. 4a–c) [
49,
50]. In the most plausible adsorption structure (Fig. S13 in Supporting information), C
2H
2 stays within the channel surrounded by Zn(II), TAZ, and dimethyl ammonia cations (DMA) (
Fig. 4a), and the corresponding
Eads is 0.75 eV. C
2H
2 stays around the 6MR-2, it interacts mainly with the uncoordinated O of the 2,5-H
4ABTC at H(C
2H
2)−O(2,5-H
4ABTC) distance of 2.31 and 2.61 Å, respectively, and also interacts electrostatically with the TAZ right below it at C(C
2H
2)−N(DMA) and C(C
2H
2)−C(DMA) distances of 3.46 and 3.53 Å, respectively. C
2H
2 is also distorted slightly upon adsorption, the H–C–C and C–C–H angles are distorted from 180° when freestanding to 176.38° and 179.43°, respectively. These, together with the isosurface plot of charge density difference upon C
2H
2 adsorption suggest that dispersive interactions are formed to stabilize C
2H
2 (
Fig. 4d).
Different from the case of C
2H
2, CO
2 would diffuse into the 6MR-2 opening to interact with the H of adjacent and nearly parallelly arranged TAZ ligands to gain additional stability (
Fig. 4b), and the calculated
Eads is 0.31 eV. In this configuration, positively charged C(CO
2) interacts electrostatically with an H (2,5-H
4ABTC) and a carboxylic-O of uncoordination from adjacent 2,5-H
4ABTC ligands, and two O(CO
2) also interact with the 2 H of different TAZ ligands. The calculated O(2,5-H
4ABTC)−C(CO
2) and H(2,5-H
4ABTC)−C(CO
2) distances are 2.75 and 3.20 Å, respectively. The calculated H(TAZ)−O(CO
2) distances are 2.89 and 3.29 Å, respectively. The O–C–O angle within CO
2 is also distorted from 180° to 177.89°. The isosurface plot of charge density difference upon CO
2 adsorption also suggests the dispersive nature of CO
2 adsorption with cooperative interactions with both negatively and positively charge centres of adjacent ligands that were previously proposed to account for enhanced CO
2 adsorption in microporous amorphous carbon (
Fig. 4e).
The case of C
2H
4 is similar to that of C
2H
2 (
Fig. 4c). Dispersive interactions are formed between C
2H
4 and TAZ ligands. The calculated H(C
2H
4)−N(TAZ) distances are 3.00, 3.05, 3.06, and 3.07 Å, respectively. C
2H
4 is also distorted slightly, the H–C–C angles are distorted from 121.69° when freestanding to 121.87° after adsorption. These together with the charge density difference upon C
2H
4 adsorption suggests that dispersive interactions account for C
2H
4 adsorption (
Fig. 4f). The charge transfer accompanying C
2H
4 adsorption is less significant as compared with that of C
2H
2 and CO
2 (
Figs. 4d and e), so C
2H
4 adsorption would be weaker. Accordingly, the calculated
Eads of C
2H
4 is 0.26 eV and C
2H
4 adsorption is the weakest among the three gases investigated.
In summary, a novel 4 + 4 + 2 strategy is designed to construct the first interpenetrated SOD-based ZMOF, JLU-MOF119. The 4 + 4 + 2 strategy is based on the 4 + 2 strategy of constructing ZIF-8, which is formed by using the 4-c ligand to replace a portion of 2-c ligand. Specifically, the former 4 refers to the TBU of ZnN2O2, which consists of TAZ and 2,5-H4ABTC; the latter 4 refers to the 4-c ligand of 2,5-H4ABTC; and the 2 refers to the 2-c ligand of TAZ. In such framework, the use of elongated and distorted ligand of 2,5-H4ABTC results in the formation of two types of 6MR, and subsequently assemble into a distorted and interpenetrated SOD framework. JLU-MOF119 possesses abundant uncoordinated N and O sites in the SOD cage and the ultra-microporous pore size, which represents a decent separation capacity of C2H2/CO2/C2H4, and the C2H4 purity of 99.2% is obtained by dynamic column breakthrough experiments. The simulations and calculations study gives more insight into the unique structure feature and the distinct affinity between gases of C2H2, CO2, and C2H4 and the framework, accounting for the excellent selectivity of light hydrocarbons. This work pioneers a new and feasible strategy of SOD topology with interpenetrated structure, and explores new MOF materials applied in the field of multi-component light hydrocarbon separation, which also provides an effective pathway for designing structures with specialized functions.
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