Electronic specialty gases (ESGs), especially fluorinated gases (F-gases), play pivotal roles in the advancement of semiconductor and microelectronics industries [
1–
7]. Hexafluoroethane (C
2F
6) as an important perfluorinated electronic specialty is highly preferred in the advanced electronics industry for its excellent etching performance, high chemical stability, low global warming potential (GWP), and low production cost [
8–
10]. In industry, C
2F
6 is predominantly manufactured by fluorinating 1,1,1,2-tetrafluoroethane (CF
3CH
2F) and pentafluoroethane (CF
3CHF
2), so that small amounts of CF
3CH
2F or CF
3CHF
2 would invariably remain in the crude product [
11]. Given the extremely similar boiling points (CF
3CH
2F, 246.7 K; CF
3CHF
2, 224.6 K; C
2F
6, 195.1 K) and electronic-grade purity requirement, it is difficult and energy-consuming to obtain high purity C
2F
6 industrially using cryogenic distillation methods [
12–
15].
Adsorptive separation process has been increasingly regarded as the most promising gas purification technology due to its high efficiency and low energy consumption [
16–
18]. With the flourishing of adsorbents in the past decade, numerous tricky separations alkanes/alkenes [
19–
22], alkynes/alkenes [
23–
26], aromatics/cyclic aliphatics [
27–
30] and so on have been achieved. However, in contrast to the above gas mixtures, the carbon atoms in C
2F
6, CF
3CH
2F and CF
3CHF
2 are all sp
3 hybridized, with fluorine atoms being the main component of molecules [
31–
34]. This means that the physico-chemical properties of the three gases are much closer. Presently, no clear separation mechanism has been developed for the separation of C
2F
6 from CF
3CH
2F/CF
3CHF
2/C
2F
6 mixture. Compared to C
2F
6, both CF
3CH
2F and CF
3CHF
2 have hydrogen atoms in their molecules. Owing to the electron-withdrawing effect of the F atom, the C—H bonds in CF
3CH
2F and CF
3CHF
2 are highly polarized and the H atoms are relatively active [
35–
37]. As a result, the H atoms in CF
3CH
2F and CF
3CHF
2 are good hydrogen-bond donors. As such, constructing adsorbents with high degree of hydrogen-bond acceptor sites should allow for selective adsorption of CF
3CH
2F and CF
3CHF
2 [
38–
41]. Firmly, in the previous reports, J. Milner, Motkuri and Su
et al. have proven that halogen (X), O or N atoms in the frameworks can form X/O/N···H hydrogen bonds with the hydrogen atoms of hydrofluorocarbons to enhance their adsorption [
42–
44]. Thus, if we functionalize multiple hydrogen bond acceptors in the framework, the selectivities for CF
3CH
2F and CF
3CHF
2 can be similarly improved.
As we know, an eligible adsorbent should not only have good gas selectivity, but also a high adsorption capacity, so as to achieve efficient separation performance [
45–
48]. However, adsorbents with narrow channels often possess high gas selectivity but often show low adsorption capacity because of the strict pore size limitations or immobilization of functional sites will drastically reduce their porosity. It is widely known that cage-like materials have high gas uptakes due to their extremely large cavities which can accommodate many gas molecules [
49–
53]. Consequently, if we can construct a porous material with both cage-like cavities and multiple hydrogen-bond acceptors, high CF
3CH
2F and CF
3CHF
2 adsorption capacity and high CF
3CH
2F/C
2F
6, CF
3CHF
2/C
2F
6 selectivity could be achieved simultaneously (
Scheme 1).
With the above consideration in mind, we herein report a cage-like metal–organic framework FJI-W20. On its pore surface, multiple hydrogen-bond acceptors such as F, O and N atoms that can provide tight C—H···F, C—H···O and C—H···N interactions with CF3CH2F and CF3CHF2 molecules are modified. As expected, FJI-W20 exhibits simultaneous preferential and high adsorption for CF3CH2F and CF3CHF2. Adsorption experiments show that it can absorb 132.9 cm3/g of CF3CH2F and 110.1 cm3/g of CF3CHF2 but can only absorb 57.9 cm3/g of C2F6 at 298 K and 1 bar. Due to the large difference in their uptakes, FJI-W20 delivers selectivity as high as 17.6 and 12.3 for respective CF3CH2F/C2F6 (5/95, v/v) and CF3CHF2/C2F6 (5/95, v/v). Breakthrough experiments show that high-purity C2F6 can not only be obtained from the binary CF3CH2F/C2F6 (5/95, v/v) and CF3CHF2/C2F6 (5/95, v/v) mixtures but also can be directly collected from ternary CF3CH2F/CF3CHF2/C2F6 (5/5/90, v/v/v) mixture. After one breakthrough experiment at 298 K and 1 bar, the productivity of C2F6 can reach 20.6 mol/kg. Furthermore, FJI-W20 can maintain its good separation performance at different gas flow rates and temperatures. Computational simulation reveals that multiple electronegative atoms on the pore wall of FJI-W20 can generate substantial hydrogen bonding interactions with CF3CH2F and CF3CHF2, which facilitates the FJI-W20 to achieve an eye-catching CF3CH2F/CF3CHF2/C2F6 separation performance.
FJI-W20 can be afforded as dark orange block-shaped crystals by solvothermal method (Fig. S1 in Supporting information). Single-crystal X-ray experiments reveal that it crystallizes in the
I4
1/amd space group (Table S1 in Supporting information). As depicted in
Fig. 1a, FJI-W20 is constructed by three components,
i.e., BDC—CF
32− anion, BIPYDZ ligand and [Co
8(μ
3-OH)
4(H
2O)
4(SO
4)
2(COO)
8] cluster (Co
8 cluster). In the Co
8 cluster unit of, Co
1 atom is six-coordinated by two carboxylate oxygen atoms from two BDC—CF
32− ligands, two oxygen atoms of two μ
3-OH groups, one oxygen atom from SO
42−, one water oxygen atom and one nitrogen atom from BIPYDZ, leading to octahedral geometry [
54]. Analogously, the coordination mode of Co
2 atom in this Co
8 cluster is also hexa-coordination, which is completed by two carboxylate oxygen atoms from two BDC—CF
32− ligands, two oxygen atoms of μ
3-OH groups and two oxygen atoms of SO
42− anion, respectively. In the structure of FJI-W20, each Co
8 cluster can bind to twelve linker ligands (eight BDC—CF
32− ligands and four BIPYDZ) (
Fig. 1e). This unusual Co
8-based cluster node, in conjunction with two linear ligands BDC—CF
32− and BIPYDZ, jointly constructs a 3D framework with a unique cage-like structure (
Fig. 1b). The solvent accessible volume in fully evacuated FJI-W20 is up to 59.2% [
55]. In FJI-W20, there are two kinds of polyhedral cages, that is, small tetrahedral cage (Cage-A) which is completed by three Co
8 clusters, four BDC—CF
32− ligands and one BIPYDZ ligand and big octahedral cage (Cage-B) which is completed by eight Co
8 clusters, eight BDC—CF
32− ligands and four BIPYDZ ligands (
Figs. 1c and d). Especially, in simultaneous cage-A and cage-B, there are multiple electronegative atoms distributed, such as the F atoms, N atoms and O atoms, which are potential strong hydrogen bond receptors. This indicates that FJI-W20 can provide strong interactions with the gas molecules with the partially positive hydrogen atoms, such as CF
3CH
2F and CF
3CHF
2, thereby increasing the selectivity of CF
3CH
2F/C
2F
6 and CF
3CHF
2/C
2F
6.
The PXRD patterns of the as-synthesized FJI-W20 sample are well consistent with the calculated ones from the single crystal, indicating that the prepared FJI-W20 sample has high phase purity (Fig. S2 in Supporting information). Furthermore, FJI-W20 possesses excellent stability in air and common organic solvents, which can be certificated by the well matched PXRD patterns with the fresh sample after imposing FJI-W20 in air for 30 days or various organic solvents for 7 days (Figs. S2 and S4 in Supporting information). What is more, FJI-W20 also demonstrates good thermal stability. The thermogravimetric analysis experiment (TGA) shows that there is no obvious weight loss up to 290 ℃ (Fig. S3 in Supporting information). Variable-temperature PXRD (VT-PXRD) were also carried out and the results show that no significant changes in the PXRD patterns are observed even at 200 ℃ in air atmosphere, implying that the framework can be maintained at such high temperature (Fig. S5 in Supporting information). On the whole, the excellent structural stability endows it with great Given the exceptional stability and large pore of FJI-W20, we next tested its gas adsorption properties. The 77 K N2 was firstly employed to illustrate its porosity. As revealed in Fig. S6 (Supporting information), it can absorb 419.9 cm3/g of N2 at 77 K and 1 bar. The calculated BET surface area is up to 1640.4 m2/g and the pore volume is as high as 0.65 cm3/g, which indicates that FJI-W20 can be used as a promising platform for gas adsorption. Furthermore, owing to the cage-like structure and multiple hydrogen bond acceptors on its pore surface, it can be reasonably inferred that FJI-W20 may exhibits great potential for absorbing gas molecules that possess active hydrogen atoms.
Based on this inference, we immediately afterwards carried out the adsorption experiments for CF
3CH
2F, CF
3CHF
2 and C
2F
6. As anticipated, FJI-W20 showcases simultaneous high uptakes for CF
3CH
2F and CF
3CHF
2 in the wide temperature range. As shown in
Figs. 2a–c, at 273 K and 100 kPa, the adsorption amounts for CF
3CH
2F and CF
3CHF
2 are up to 150.0 and 128.0 cm
3/g, respectively. When elevating the temperature to 298 K, the saturated uptakes of CF
3CH
2F and CF
3CHF
2 are 132.9 and 110.1 cm
3/g. And the adsorption capacity remained almost perfectly consistent in the five consecutive adsorption cycles (Figs. S7–S9 in Supporting information). Notably, even the temperature is increased to 313 K, FJI-W20 can still absorb 124.9 cm
3/g of CF
3CH
2F and 101.5 cm
3/g of CF
3CHF
2. These results demonstrate that the cage based structure of FJI-W20 promotes its gas absorption capacity, which is unmatched by narrow pore materials such as Ni(pba)
2 that we previously reported [
11]. Furthermore, comparing with the high storage capacity of CF
3CH
2F and CF
3CHF
2, FJI-W20 shows relatively low uptake for C
2F
6. The saturated uptakes of C
2F
6 by FJI-W20 at 273, 298 and 313 K are only 87.1, 57.9 and 41.6 cm
3/g, which is much lower than that of CF
3CHF
2 and CF
3CH
2F. More importantly, at low pressure region, the adsorption curves of C
2F
6 increase particularly gently, reflecting that the framework can hardly generate strong interaction with C
2F
6 [
56–
58]. Evidently, the adsorption results reveal that FJI-W20 shows great difference between fluoroethane such as CF
3CH
2F, CF
3CHF
2 and C
2F
6. And the apparently strong affinity for CF
3CH
2F and CF
3CHF
2 relative to C
2F
6 suggests that multiple hydrogen bond receptors in FJI-W20 may favour binding of hydrofluorocarbons.
In order to visually illustrate that FJI-W20 exhibits stronger interactions on CF
3CH
2F, CF
3CHF
2 than C
2F
6, we calculated the adsorption heats (
Qst) for CF
3CH
2F, CF
3CHF
2 and C
2F
6 with the adsorption isotherms at 273 and 298 K. As demonstrated in
Fig. 2d and Figs. S13-S15 (Supporting information), the
Qst values at zero loading for CF
3CH
2F, CF
3CHF
2 are 43.6 and 42.2 kJ/mol, respectively. In contrast, the corresponding
Qst for C
2F
6 is 23.5 kJ/mol, indicating its lower affinity relative to CF
3CH
2F, CF
3CHF
2. This difference may be attributed to the presence of electronegative atoms in the structure of FJI-W20, which enhances the framework-hydrofluorocarbon interactions. Given the remarkable affinity for CF
3CH
2F and CF
3CHF
2, we judge that it has the ability to purify C
2F
6 from CF
3CH
2F/CF
3CHF
2/C
2F
6 mixture. To validate this inference, ideal adsorbed solution theory (IAST) has been employed to determine the IAST selectivity (Figs. S10–S12 in Supporting information). As shown in
Figs. 2e and f, the IAST selectivities for CF
3CH
2F/C
2F
6 (5/95) and CF
3CHF
2/C
2F
6 (5/95) can reach 17.6 and 12.3 at 298 K and 1 bar, which are much higher than other porous materials such as some commercial molecular sieves (12.7/6.3 for β molecular sieve and 13.0/7.1 for TS-1) and the cage-like porous materials without recognized sites (8.6/3.9 for HKUST-1, 8.4/4.8 for UiO-66 and 10.4/4.8 for FJI-H8) (Figs. S16–S37 and Table S2 in Supporting information). And this result sufficiently proves that FJI-W20 can be used as an efficient adsorbent to separate CF
3CH
2F/C
2F
6 and CF
3CHF
2/C
2F
6 and completely indicates that the immobilization of multiple hydrogen-bond acceptor sites in a cage-like metal-organic framework is a verifiable strategy for efficient C
2F
6 purification.
In order to further study the mechanism of FJI-W20 preferentially adsorbing CF
3CH
2F and CF
3CHF
2, theoretical calculations were carried out. As we know, compared with C
2F
6, CF
3CH
2F and CF
3CHF
2 contain not only F atoms, but also partially positive H atoms. The presence of H atoms enables CF
3CH
2F and CF
3CHF
2 to have more interactions with the skeleton. Therefore, the
Qst values of CF
3CH
2F and CF
3CHF
2 in FJI-W20 are higher than C
2F
6. According to
Figs. 3a and b, the calculated results clearly reveal that CF
3CH
2F and CF
3CHF
2 molecules are tightly bound in the corners of the polyhedral hole that possessing abundant F/O/N environment. Specifically shown in
Fig. 3a, CF
3CH
2F possesses eight interactions with FJI-W20, including four C—H···F hydrogen bonds (
DH···F = 2.69, 2.83, 3.27 and 3.30 Å), three C—H···π interactions (
DC···C = 3.58, 3.89 and 3.98 Å) and one C—H···O interactions (
DH···O = 3.60 Å) [
59–
63]. As depicted in
Fig. 3b, each CF
3CHF
2 molecule interacts with the framework through two C—H···F (
DH···F = 2.96 and 3.95 Å), one C—H···N (
DH···N = 3.08 Å), one C—H···O (
DH···O = 3.78 Å) and three C—H···π (
DC···C = 3.66, 3.76 and 4.07 Å) supramolecular interactions. These evidences validate that the electronegative sites on the pores are critical for capturing CF
3CH
2F and CF
3CHF
2 molecules [
64–
67]. As a sharp contrast, there are complete F atoms in C
2F
6 molecule and it can only interact with the framework by C—H···F hydrogen bonds (
DH···F = 2.67, 2.79, 2.82, 3.42 and 3.48 Å) (
Fig. 3c). Correspondingly, the calculated static binding energies of CF
3CH
2F, CF
3CH
2F and C
2F
6 are 52.12, 47.71 and 32.09 kJ/mol, respectively, which is in well agreement with the experimental trend of
Qst under zero coverage. These simulation results demonstrate the rationality of FJI-W20 as a purified C
2F
6 adsorbent.
Based on the above compelling rationales, we next conducted dynamic breakthrough experiments to investigate the practical performance of FJI-W20 for the purification of C
2F
6. Firstly, the binary gas mixture of CF
3CH
2F/C
2F
6 and CF
3CHF
2/C
2F
6 were employed to evaluate the C
2F
6 purification potentiality. As shown in
Figs. 4a and b, FJI-W20 can effectively purify C
2F
6 from CF
3CH
2F/C
2F
6 and CF
3CHF
2/C
2F
6 respectively. In
Fig. 4a, it can be seen that the C
2F
6 effluxes rapidly at 30.9 min whereas the signal of CF
3CH
2F emerges after 391.2 min. High-purity C
2F
6 (≥99.99%) can be collected continuously during 360.3 min with an ultrahigh yield of 35.7 mol/kg. Similarly, FJI-W20 can also achieve efficient CF
3CHF
2/C
2F
6 separation (
Fig. 4b). At 298 K and with the flow rate of 2.2 mL/min, high purity of C
2F
6 can be ceaselessly obtained from 30.9 min to 297.8 min and the corresponding C
2F
6 yield is as high as 25.9 mol/kg. The purification yield of C
2F
6 from CF
3CHF
2/C
2F
6 is slightly lower than from the CF
3CH
2F/C
2F
6 and this is owing to the lower adsorption capacity of FJI-W20 for CF
3CHF
2.
Motivated by the confirmed separation performance of binary mixtures for FJI-W20, we further assessed the C
2F
6 purification capacity from ternary C
2F
6-containing gas mixture of CF
3CH
2F/CF
3CHF
2/C
2F
6. As demonstrated in
Fig. 4c, FJI-W20 can likewise realize high-efficiency separation of C
2F
6 from CF
3CH
2F/CF
3CHF
2/C
2F
6 in the breakthrough experiments, in which C
2F
6 first breaks the packed column, while CF
3CHF
2 and CF
3CH
2F are retained in adsorbent for a period of time. From the breakthrough curve, it can be seen that the sequence of gas outflow at the outlet is C
2F
6, CF
3CHF
2 and CF
3CH
2F, the result is coincident with the gas adsorption amounts. After one three-component separation experiment, about 20.6 mol/kg of high-purity C
2F
6 can be collected. Furthermore, six consecutive breakthrough experiments on FJI-W20 show that the breakthrough times and productivities of C
2F
6 express no obvious loss (
Fig. 4d), indicating that FJI-W20 possesses excellent reusability in C
2F
6 purification process. Moreover, in consideration of the practical industrial situations, the adsorbent should keep good separation performance at different gas flow rates and high temperatures. Thus, in order to examine the separation capacity of FJI-W20 in near-actual conditions, we conducted the breakthrough experiments under different flow rates and high temperatures. As shown in
Figs. 4e and f, FJI-W20 could maintain good C
2F
6 purification ability within the flow rates of 1.7–3.8 mL/min and the C
2F
6 productivities can even remain roughly unchanged, which implies that the separation performance of FJI-W20 could not be influenced by the flow rates. More than that, FJI-W20 as well works well under high temperatures. As depicted in
Fig. 4g, when we gradually elevated the testing temperature, the separation time of C
2F
6 only shows slightly decrease. For instance, the temperature is increased to 318 K, the separation time of C
2F
6 is
ca. 182.1 min and the corresponding C
2F
6 productivity is
ca. 13.8 mol/kg (
Fig. 4h). Remarkably, after exposure to air for 7 days, the separation performance is almost the same as that of fresh samples, which is rarely seen in previous reports (
Fig. 4i). On the whole, above results suggest that FJI-W20 can realize good separation performance under the actual situations.
In summary, herein we report a cage-based framework FJI-W20 whose pore surface is functionalized with multiple electronegative F, O and N atoms as hydrogen-bond acceptors. Cage-like pores and multiple hydrogen-bond sites between the framework and hydrofluoroethanes ensure selectively capture of CF3CH2F and CF3CHF2 with high uptakes. For example, at 298 K and 1 bar, FJI-W20 can absorb 132.9 cm3/g of CF3CH2F and 110.1 cm3/g of CF3CHF2 but only 57.9 cm3/g of C2F6. The selectivity of CF3CH2F/C2F6 (5/95, v/v) and CF3CH2F/C2F6 (5/95, v/v) can reach 17.6 and 12.3. Separation experiments indicate that FJI-W20 can purify C2F6 with high purity from ternary CF3CH2F/CF3CHF2/C2F6 mixtures. After one separation cycle, 20.6 mol/kg of high-purity C2F6 can be directly obtained from CF3CH2F/CF3CHF2/C2F6 (5/5/90, v/v/v) mixture. Additionally, at different gas flow rates and temperatures high-purity C2F6 can also be obtained in one step. This work illustrates that our strategy of immobilizing multiple hydrogen-bond acceptor sites in a cage-like framework is highly successful and effective for designing novel porous materials of purifying of C2F6 from CF3CH2F and CF3CHF2.