Th-MOF showing six-fold imide-sealed pockets for middle-size-separation of propane from nature gas

doi:10.21203/rs.3.rs-888730/v1

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Th-MOF showing six-fold imide-sealed pockets for middle-size-separation of propane from nature gas

https://doi.org/10.21203/rs.3.rs-888730/v1

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Separation of propane from nature gas is of great importance to industry. However, in light of size-based separation, there still lacks effective method to directly separate propane from nature gas, due to the comparable physical properties for these light alkanes (C1-C4) and the middle size of propane. In this work, we found that a new Th-MOF could be an ideal solution for this issue. The Th-MOF takes UiO-66-type structure, but with the pocket sealed by six-fold imide groups; this not only precisely reduces the size of pocket to exactly match propane, but also enhances the host-guest interactions through multiple supramolecular interactions. As a result, highly selective adsorption of propane over methane, ethane, and butane was observed, implying unique middle-size separation. The actual separation was confirmed by breakthrough experiments, and it is found that both relatively smaller molecules (methane and ethane) and relatively bigger molecules (butane) break through the Th-MOF column within 10 min/g, whereas propane with middle size can maintain very long retention time up to 80 min/g, strongly suggesting middle-size separation and its superior application in direct separation of propane from nature gas. The separation mechanism, as unveiled by both theoretical calculation and comparative experiments, is due to the six-fold imide-sealed pockets that could effectively distinguish propane from other light alkanes through both size effect and host-guest interactions.

Inorganic Chemistry

propane

natural gas

separation mechanism

Nature gas (NG) widely exists on the earth, with abundant reserves, such as oilfield gas, gas field gas, mud volcanic gas, coalbed methane and biogenic gas, and so on. It contains both light hydrocarbon and nonhydrocarbon gases. The major component of light hydrocarbon consists of methane (85%), ethane (9%), propane (3%), and butane (1%)^1-5. Accordingly, propane produced from natural gas is important raw materials for modern industry^8-10.For example, China's propane consumption in 2019 is 16.57 million tons¹¹.

The dynamic diameter of methane, ethane, propane, and butane is 3.8 Å, 4.0 Å, 4.2 Å, 4.3 Å, respectively, implying difficult separation for this mixture, due to comparable size^12-15.But they show different molecule size such as 3.76 Å×3.83 Å×3.99 Å for methane, 4.08 Å×4.29 Å×4.72 Å for ethane, 4.02 Å×4.79 Å×6.20 Å for propane, and 4.02 Å×4.61 Å×7.38 Å for butane. This means that we can separate this mixture through precise design on the pore structure and size. Due to the outstanding importance of propane in the industry, thereby, there demands urgent concern about direct generation of propane from nature gas (mainly quaternary gas mixture of methane, ethane, propane, and butane)^16-20.However, to enforce this target, there still lacks available MOFs or effective molecular design strategy to match exactly the size of propane. On the other hand, in light of size-based separation that is often designed and employed for separation in the field of MOFs, whatever the dynamic diameter and the molecule size of propane is located at the middle among these mixtures, implying a demand of middle-size separation^21-24.However, this is seriously restricted for common molecular design and most available MOFs. To the best of our knowledge, there is still no report about direct separation of propane from nature gas until this work.

The size and configuration of pore in MOFs presents a highly important factor for separation^25-29.MOFs could selective adsorption of gas molecules with smaller size than the pore MOFs, but will exclude these gas molecules with bigger size than the pore MOFs^30,31.And co-adsorption would occur when all the components in the gas mixture with smaller size than the pore MOFs; this often leads to weak or no separation^32-34. Thereby, separation of molecule with middle size among the gas mixture remains a challenging task^35,36. Flexible framework with gate opening could be a good resolution.For example, just the target molecule with middle size can result in the gate opening, and other molecules with relatively smaller size or bigger size could not result in the gate opening, consequently leading to selective adsorption of middle-size molecule. This unique phenomenon has been realized by Zhang et al for separation of styrene from ethylbenzene/styrene/ toluene/benzene mixture³⁷.However, this type separation is highly dependent on the magnitude of host-guest interactions, thus, restricting its application for other mixtures.

Anchoring functionalized units on the pore wall of MOFs to enhance the host-guest interactions is also an effective method for separation^38-40.This can theoretically selectively adsorb target molecule from the mixture under precise design on the functionalized units. However, this method could be also invalid to perform middle-size separation for these molecules with similar structure and physical properties.

In this work, we found that the combination of size control and anchoring functionalized units is an effective solution to give middle-size separation for propane from nature gas (see Scheme 1). The used MOF show the pore size bigger than methane, ethane, and propane, but less than butane, and the pore wall is decorated by three-fold imide groups that acts as strong H-acceptor for enhance host-guest interactions. Selective adsorption of propane over methane and ethane is due to the relatively stronger host-guest interactions between propane and MOFs, while selective adsorption of propane over butane is due to the big size of butane (relative to the pore of MOF).

Synthesis, crystal structure, and characterization of ECUT-Th-10. The MOF, namely ECUT-Th-10, was synthesized by the solvothermal reaction of Th(NO₃)₄, N,N’-bis-(4-benzoic acid)-1,4,5,8-naphthalenediimide (H₂L) and perchloric acid. Light orange octahedral crystal was obtained. The purity of the as-synthesized samples was confirmed by PXRD tests (Supplementary Fig. 1). The synthetic details of the ligand H₂L and MOF was presented in the supporting information. Single-crystal X-ray diffraction analysis shows that ECUT-Th-10 crystallizes in the cubic space group Fd-3m with big unit cell of a=b=c=39.5857(7) Å and large volume of 62032(3) Å³(Supplementary Table 1). The asymmetric unit contains one crystallography-independent Th (IV), which is coordinated by four carboxylate oxygen atoms from four different L^2- ligands, two μ³-OH ions, and two μ³-O^2- ions, forming a square antiprismatic geometry (Fig. 1b). The Th-O bonds are in the normal range, comparable with that observed in the literature. The L^2- ligands afford the bidentate mode to connect to four Th (IV) ions. Due to steric hindrance, the middle imide-containing fragment is rotated and show almost vertical to the two terminal benzene rings, as evidenced by the dihedral angle of 88.97º and 88.53º between them (Fig. 1a).

Through μ³-OH ions and μ³-O^2- ions, six identical Th(IV) ions are integrated to generate a Th₆(μ³-OH)₄(μ³-O^2-)₄ building block, which is very similar with the well-known Zr₆(μ³- OH)₄(μ³-O^2-)₄ building block in UiO-66-type structures. Each Th₆(μ³-OH)₄(μ³-O^2-)₄ core connects to twelve L^2-ligands, and an overall UiO-66-type structure is observed in ECUT-Th-10 (Fig. 1c). Interestingly, as observed in UiO-66, ECUT-Th-10 also affords two different cages of octahedral and tetrahedral cages. However, the octahedral cage is as large as to 2.6 nm, due to the use of long ligand of L^2-(Fig. 1d). The tetrahedral cage in ECUT-Th-10 is divided into two types of pores, and four three-fold imide-sealed pockets are observed at the vertex of tetrahedral cage that gives the aperture of 6.1 Å (Fig. 1e); this is impressively matchable with the size of propane, but bigger than methane and ethane, and smaller than butane, implying its potential for selective adsorption of propane over methane, ethane, and butane, finally leading to middle-size separation. However, UiO-66 just shows one type of pore for tetrahedral cage with size about 8 Å (Fig. 1f). The formation of unique three-fold imide-sealed pockets in ECUT-Th-10 is due to the use of L^2- ligands that show the rotation of imide-containing fragment and results in a vertical configuration for the L^2- ligand between the middle imide-containing fragment and two terminal benzene rings, and finally cause the imide C=O bonds to orient inside the tetrahedral cage. Notably, two-fold interpenetration is observed in ECUT-Th-10, which largely reduces the pore. And the three-fold imide-sealed pockets is then further sealed by additional three imide units from another net, constructing the overall six-fold imide-sealed pockets (Fig. 1g).

To confirm the permanent porosity, we first carried out thermogravimetric analysis (TGA) of ECUT-Th-10, showing that the guest molecules can be removed before 250 ℃ (Supplementary Fig. 2). But this can be decreased through CH₃OH treatment, since the CH₃OH-exchanged samples showed the solvent loss before 100℃. Therefore, the activation of ECUT-Th-10 was prepared at 100℃ under vacuum. To check the porosity, N₂ adsorption-desorption behavior was studied at 77 K. As shown in Fig. 2a, the uptake capacity of N₂ is around 250 cm³/g at 1 bar. The Brunauer−Emmett−Teller (BET) surface area was calculated to be 854 m²/g. The size distribution gives a narrow pore at 0.63 nm and broad pore at 1.3 nm, where the narrow pore belongs to the six-fold imide-sealed pockets and the broad pore belongs to the reduced pore (such as the pristine large octahedral cage) due to interpenetration.

Gas adsorption and separation performances. The above results further motivate us to investigate its application in separation of nature gas. First, the single component isotherms of CH₄, C₂H₆, C₃H₈ and n-C₄H₁₀ were measured at 298 K, respectively (Fig. 2b). The uptake at 298 K and 110 kPa is respectively 2.89 mmol/g (C₃H₈), 2.69 mmol/g (C₄H₁₀), 1.72 mmol/g (C₂H₆), and 0.42 mmol/g (CH₄), suggesting selective adsorption of propane over methane, ethane, and butane. Especially, no adsorption for C₄H₁₀ at low pressure before 13 kPa is observed, implying that butane is excluded from ECUT-Th-10 under this condition, since the size of butane is bigger than the pore size of six-fold imide-sealed pockets. As for methane, ethane, and propane, similar phenomenon is not observed, because their sizes are smaller than the pore size of six-fold imide-sealed pockets. Thereby, the adsorption ratio at low pressure (13 kPa) is as high as to 30 for C₃H₈/C₄H₁₀, 3.75 for C₃H₈/C₂H₆, and 17.1 for C₃H₈/CH₄, strongly indicative of selective adsorption towards propane. The sharp increase after 13 kPa observed in the adsorption isotherm for butane is mainly resulted from gate opening, owning to somewhat flexibility of the MOF framework. Gate opening is often encountered in MOFs and is generally used for separation^41-43.

The adsorption selectivity is also evaluated by ideal adsorption solution theory (IAST), in light of the above single component isotherms at 298 K (Fig. 2c)^44-47.Ultrahigh selectivity up to 60 for C₃H₈/C₄H₁₀ (50:50 v/v), 9.31 for C₃H₈/C₂H₆(50:50 v/v), and 54.5 for C₃H₈/CH₄(50:50 v/v) is respectively observed at the onset of adsorption, also suggesting selective adsorption of propane.

We further investigated the host-guest interaction between ECUT-Th-10 and these gases, which is mainly reflected on the isosteric heat of adsorption (Q_st). Thus, the adsorption of these gases at 273 K was carried out (Supplementary Fig. 3). Due to gate opening in C₄H₁₀, its fitting of the adsorption isotherms at 298 K cannot obtain reasonable value, thus its Q_st value cannot be estimated. The Q_st value for other gases gives the hierarchy of C₃H₈ (33.6 kJ/mol) > C₂H₆(27.3 kJ/mol) > CH₄(26.4 kJ/mol), implying stronger host-guest interactions for propane (Fig. 2d).

To evaluate the actual separation ability of ECUT-Th-10 in nature gas, we further carried out breakthrough experiments at 298 K. Initially, we investigated the separation of the equimolar C₃H₈ and C₂H₆ mixtures (50:50, v/v), as both of them holds closer molecule size and smaller size than the pore of six-fold imide-sealed pockets that means inevitable co-adsorption and consequently weak or no separation. As shown in Fig. 2e, C₂H₆was first eluted though the bed within 32 min/g, whereas C₃H₈ break through after 70 min/g, clearly showing separation of C₃H₈ from C₃H₈/C₂H₆ mixture. To confirm this, we repeated the experiments twice, and repeatable results were observed, also suggesting excellent recyclability for the present MOF adsorbent (Supplementary Fig. 4). Then, equimolar 3-component CH₄/C₂H₆/C₃H₈ mixture is measured, where all the components show smaller size than the pore of six-fold imide-sealed pockets (Fig. 2f). Notably, clearly separation for C₃H₈ is also observed, as evidenced by the sequence of outgoing gas such as 10 min/g for CH₄, 20 min/g for C₂H₆, and 48 min/g for C₃H₈. Furthermore, we also measured the equimolar 3-component C₂H₆/C₃H₈/C₄H₁₀ mixture, where C₄H₁₀ shows bigger size than the pore of six-fold imide-sealed pockets. As expected, C₃H₈ can be also completely separated from this mixture, and the retention time for C₂H₆ and C₄H₁₀ is about 10 min/g and the corresponding value for C₃H₈ is as long as 42 min/g (Fig. 2g). Finally, we carried out the breakthrough experiments with equimolar quaternary CH₄/C₂H₆/C₃H₈/C₄H₁₀ mixture at 298 K and the results are shown in Fig. 2h. It is impressive that middle-size separation of C₃H₈ from this quaternary mixture is observed, and very long retention time up to 83 min/g is observed for C₃H₈, while CH₄, C₂H₆, and C₄H₁₀ just render short retention time within 15 min/g, implying excellent separation of C₃H₈ from this quaternary mixture. The results suggest its superior application for direct separation of propane from nature gas.

To disclose the separation mechanism, we further carried on additionally comparative experiment and DFT (density functional theory) calculation. Due to the high similarity of our MOF with UiO-66 in the structural aspect, in conjunction with the difference in the aspect of tetrahedral cage, UiO-66 is selected to give a comparative research. UiO-66 was synthesized by solvothermal method⁴⁸. The purity of the as-synthesized UiO-66 was confirmed by PXRD tests (Supplementary Fig. 5). The tetrahedral cage in UiO-66 is about 8 Å, which is bigger than the molecular size of methane, ethane, propane, and butane. Accordingly, middle-size separation is not expected for UiO-66. Then, we first tested the single component isotherms of CH₄, C₂H₆, C₃H₈ and n-C₄H₁₀ at 298 K, respectively, giving the adsorption capacity in the hierarchy of n-C₄H₁₀(4.5 mmol/g) > C₃H₈(4.0 mmol/g) > C₂H₆ (2.3 mmol/g) > CH₄(0.5 mmol/g) (Fig. 3a). The results means that the adsorption capacity of these gases increases along with increasing molecular size of gas molecules, completely excluding the selective adsorption of middle-size molecule as observed in ECUT-Th-10. Gate opening in UiO-66 is also not observed for butane, since it affords bigger tetrahedral cage than the molecular size of butane. Moreover, the corresponding C₃H₈/C₄H₁₀, C₃H₈/C₂H₆ and C₃H₈/CH₄ adsorption selectivity was also calculated, giving S=0.4, 2.0 and 32 at the onset of adsorption, respectively (Fig. 3b). This value is fairly less than that observed in our MOF such as S=60, 9.31, and 54.5, respectively, which is mainly due to the reduced size and enhanced host-guest interactions from the six-fold imide-sealed pockets in ECUT-Th-10. The adsorption ratio at 13 kPa in UiO-66 is 0.65 for C₃H₈/C₄H₁₀, 1.93 for C₃H₈/C₂H_6,26.15 for C₃H₈/CH_4,indicative of the hard propane extraction (Fig. 3c). The absence of middle-size separation was further confirmed by breakthrough experiments from the equimolar quaternary CH₄/C₂H₆/C₃H₈/C₄H₁₀ mixture at 298 K, where the breakthrough sequence also obeys the order of molecular size (Fig. 3d). In light of these comparative experiment results, we can reason the mechanism of middle-size separation in ECUT-Th-10 from the unique six-fold imide-sealed pockets that gives matchable size and enhanced host-guest interactions with propane over other gases such as methane, ethane, and butane.

Mechanism of gas adsorption by theoretical calculations. To obtain the structural information after adsorption of gas molecules, we then carried out DFT calculation. It is found that the adsorption of these gas molecules is primarily located at six-fold imide-sealed pockets. And each pocket could accommodate one molecule. The calculated binding energies between these gas molecules and the six-fold imide-sealed pocket are -9.65 kJ/mol (CH₄), -17.38 kJ/mol (C₂H₆), -24.12 kJ/mol (C₃H₈), 11.58 kJ/mol (C₄H₁₀), respectively. This means that the adsorption of CH₄, C₂H₆, and C₃H₈ is thermodynamically spontaneous, owing to their molecular size less than the pore of the six-fold imide-sealed pocket, whereas the positive binding energy indicates that the adsorption of C₄H₁₀ is excluded, due to its molecular size bigger than the pore of the six-fold imide-sealed pocket; this is well consistent with the experimental results. In addition, it is found that C₃H₈ in the six-fold imide-sealed pocket is tightly fixed by multiple hydrogen bonds from imide units with C-H∙∙∙O distance of 3.29 (3) Å-4.02(3) Å (Fig. 4b). By contrast, weak hydrogen bonds are observed for CH₄ with distance of 4.19(3) Å-4.31(3) Å and C₂H₆ with distance of 3.35(3) Å-4.15(3) Å (Supplementary Fig. 6 and Fig. 7). Thereby, supramolecular interactions from the imide units enhancing the host-guest interaction for C₃H₈ is also responsible for the middle-size separation.

In summary, we reported a novel MOF, ECUT-Th-10, which holds the UiO-66-type structure, but is constructed by diimide-based ligands and enables an interpenetrated structure with the formation of unique six-fold imide-sealed pockets. These structural merits are beneficial to separate propane from methane/ethane/propane/butane mixture, demonstrated an extremely rare case of middle-size separation, suggesting its big potential in direct generation of propane from nature mixture. The pocket-like cage exhibited an excellent molecular sieving effect to block larger C₄H₁₀, while imide units within this pocket-like cage provide multiple supramolecular interactions with propane, thus enhancing the host-guest interactions for propane, consequently reducing the co-adsorption for methane, ethane, and propane and performing highly selective adsorption towards propane.

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Acknowledgements

We thanks to Open Fund of Jiangxi Province Key Laboratory of Synthetic Chemistry (JXSC202007), Doctoral Scientific Research Start-up Foundation of East China University of Technology (DHBK2018044), the National Natural Science Foundations of China (21966002, 21871047), the Training Program for Academic and Technical Leaders of Major Disciplines in Jiangxi Province (20194BCJ22010), and the Fuzhou youth leading talent project (2020ED64).

Author contributions

F. Luo and L. Wang conceived the experiments and wrote this paper; W. Jia, Y. Ran and L. Chen did experiments; L. Gong carried out computational simulations; R. Krishna calculated the selectivity and Q_st.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/

Competing financial interests

The authors declare no competing financial interest.

There is NO Competing Interest.

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