An amino-coordination metal–organic framework for highly selective C₂H₂/CH₄ and C₂H₂/C₂H₄ separations through the appropriate control of window sizes

Abstract

The efficient separation of C₂H₂ versus C₂H₄ and CH₄ to obtain high-purity C₂H₂ and C₂H₄ is of significance for making full, economic use of these raw chemicals. Herein, an amino-coordination microporous metal–organic framework ZJU-198, ZnL·DMF (ZJU = Zhejiang University, L = (2E,2E′)-3,3′-(5-amino-1,3-phenylene)diacrylic acid, DMF = N,N′-dimethylformamide), has been demonstrated as a valuable adsorbent for C₂H₂/C₂H₄ and C₂H₂/CH₄ separations. The activated ZJU-198a exhibits moderate C₂H₂ uptakes (99.4 cm³ cm⁻³ for 273 K and 98.4 cm³ cm⁻³ for 298 K under 1.0 bar) and moderately high C₂H₂/C₂H₄ selectivity (5.8 to 7.7 at 273 K and 4.8 to 7.2 at 298 K). Specifically, the C₂H₂/CH₄ selectivity of ZJU-198a reaches up to 497.9 and 391.1 at 273 K and 298 K, respectively. To the best of our knowledge, the C₂H₂/CH₄ selectivity coefficients of ZJU-198a at both 298 K and 273 K are the highest values among the reported metal–organic frameworks, meaning that there is bright potential for ZJU-198a in hydrocarbon storage and separation.

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Introduction

C₂ hydrocarbons are generally recognized as important raw chemicals to the petrochemical industries.¹ The efficient separation of C₂H₂/C₂H₄ can make full, economic use of these raw chemicals. Ethylene is mainly produced from steam cracking of an alkane. However, the presence of spin-off acetylene has an impeditive effect on further polymerization of ethylene. That is why the C₂H₂ purity of the C₂H₂/C₂H₄ mixture must be guaranteed below 40 ppm.^2–4 What’s more, acetylene that is chiefly yielded by the cracking of petroleum generally contains the adverse by-product methane (CH₄).⁵ Therefore, the efficient separation of C₂H₂ over C₂H₄ and CH₄ to obtain high-purity C₂H₄ and C₂H₂ is of economic significance.

Porous materials might offer alternative approaches over energy-consumptive commercial materials. Extensive efforts have been engaged in hydrocarbon adsorption and separation. During the past decades, microporous metal–organic frameworks (MOFs) have attracted substantial attention in gas storage and separation.^6–15 Compared with other porous adsorbents, such as activated carbons and molecular sieves, MOFs have greater potential both at a research and application level by virtue of their attractive advantages.^16–35 On the one hand, the high porosity of MOFs allows them to take up large amounts of gas molecules; on the other hand, the easily-adjustable pore/window sizes and functionalization within MOFs offer significant improvements in gas separation selectivity.^36–40

In terms of gas separation, the pores within MOFs can be judiciously modified through the immobilization of specific sites like Lewis basic sites (LBSs) and functional groups including amino groups to enhance the interaction between the framework and preferred gas molecules.^41–43 However, the aforementioned strategies bring a costly regeneration bill. This worrisome issue can be addressed through the accurate control of the pore/window sizes. Herein, our previously reported amino-coordination microporous metal–organic framework ZJU-198, which was validated as one of the best performing metal–organic frameworks for CO₂/N₂ separation,⁴⁴ has been chosen with the appropriate window sizes and synthesized successfully. The activated ZJU-198a has been further demonstrated as a valuable adsorbent for C₂H₂/C₂H₄ and C₂H₂/CH₄ separations and exhibited moderate C₂H₂ storages (99.4 cm³ cm⁻³ at 273 K and 98.4 cm³ cm⁻³ at 298 K under 1.0 bar) and moderately high C₂H₂/C₂H₄ selectivity (5.8 to 7.7 at 273 K and 4.8 to 7.2 at 298 K). It’s worth noting that ZJU-198a has been further demonstrated for C₂H₂/CH₄ separation with very high selectivity of 63.1 to 497.9 and 69.0 to 391.1 at 273 K and 298 K, respectively. To the best of our knowledge, ZJU-198a exhibits the highest performance for the separation of C₂H₂/CH₄ both at 298 K and 273 K among the reported metal–organic frameworks, meaning that there is bright potential for ZJU-198a in hydrocarbon storage and separations.

Experimental

Materials and methods

All solvents and reagents except the organic ligand were commercially available and used without further purification. The syntheses of the organic linker (2E,2E′)-3,3′-(5-amino-1,3-phenylene)diacrylic acid, simplified as 5-amino-H₂L, and ZJU-198 are referred to in our previously reported work.^{44 1}H NMR spectra were obtained using a Bruker Advance DMX 500 MHz spectrometer with tetramethylsilane (TMS) as an internal standard. Elemental analyses (EA) for C, H, and N were performed using an EA1112 microelemental analyzer. Infrared spectra (IR) were obtained using a Thermo Fisher Nicolet iS10 spectrometer using KBr pellets. Thermogravimetric analyses (TGA) were performed using a Netzsch TG209F3 with a heating rate of 10 °C min⁻¹ under the protection of a high-purity N₂ atmosphere. All gas sorption isotherms were obtained using a Micromeritics ASAP 2020 surface area analyzer. The activation procedure of ZJU-198 also corresponded to our previously reported work.⁴⁴ The sorption measurements were maintained at 196 K with a mixture of drikold and acetone. Ice-water and water baths were used for the adsorption isotherms of C₂H₂, C₂H₄ and CH₄ at 273 K and 298 K, respectively.

Ideal adsorbed solution theory (IAST)

The selectivity of the preferential adsorption of component 1 over component 2 in a mixture containing 1 and 2 can be formally defined as:
In the above equation, q₁ and q₂ are the absolute component loadings of the adsorbed phase in the mixture. These component loadings are also termed the uptake capacity. We calculated the values of q₁ and q₂ using the Ideal Adsorbed Solution Theory (IAST) of Myers and Prausnitz.

The isosteric heat, Q_st

The isosteric heat of C₂H₂, C₂H₄, and CH₄ adsorption, Q_st, defined as
was determined using the Clausius–Clapeyron equation by fitting the adsorption isotherms taken at 273 K and 298 K to a Langmuir expression.

Results and discussion

The solvothermal reaction of the organic ligand 5-amino-H₂L with Zn(NO₃)₂·6H₂O in a mixture of solvents DMF/H₂O/acetonitrile afforded ZJU-198 as colorless layer-shaped crystals, which has a formula of ZnL·DMF (DMF = N,N-dimethylformamide) determined by single-crystal X-ray diffraction and elemental analysis (EA). The purity of the phase was further indicated by powder X-ray diffraction (PXRD).

Single-crystal X-ray diffraction analysis demonstrates that ZJU-198 crystallizes in the space group of Pbca with a (4, 5)-coordination network. It’s clear to see that each Zn1 atom exhibits 5-coordination connecting with O1A, O3, O13, O18, and N9 from four different organic linkers to construct an asymmetric pentahedron as the secondary building unit (SBU) (Fig. 1b). Furthermore, the four oxygen atoms and the nitrogen atom of the same organic linker coordinate to four different Zn1 atoms (Fig. 1a). The O1A atom and O3 atom from the same bidentate carboxylate group connect two different Zn1 atoms while O13 and O18 exhibit the same coordination condition. The SBUs link the organic ligands further to form a three-dimensional (3D) framework. It’s worth noting that the zigzag coordination tendency makes the unique orderly overlapping channels, but with no interpenetration of the framework (Fig. 1d–f). Only two types of channel exist, which are approximately 3.6 Å × 4.1 Å and 2.1 Å × 5.0 Å as viewed from the a axis (Fig. 1c), and the van der Waals radii of the atoms have been already been taken into account when labelling the size of these channels.

Fig. 1 The X-ray single crystal structure of ZJU-198, indicating that (a) the organic ligand 5-amino-H₂L coordinates four Zn²⁺; (b) each Zn²⁺ exhibits 5-coordination that connects four different organic linkers to construct an asymmetric pentahedron at the secondary building site (SBU); (c) the zigzag channels of about 3.6 Å × 4.1 Å and 2.1 Å × 5.0 Å are viewed from the a axis; (d–f) the coordination tendency of the orderly overlapping framework (C, gray; O, red; Zn, cyan; N, blue; H atoms are omitted for clarity).

After sufficient acetone-exchange and suitable activation, the CO₂ sorption at 196 K is further operated to explore the porosity of desolvated ZJU-198a. As depicted in Fig. S5,† the sorption data of CO₂-196 K is consistent with the typical type-I sorption characteristic with a Langmuir surface area and BET of 488.5 m² g⁻¹ and 343.1 m² g⁻¹, respectively. As for the applicable window size and moderate porosity of ZJU-198a, we were encouraged to explore the adsorption performance of C₂H₂, C₂H₄, and CH₄ at low pressures. Herein, the low-pressure adsorption isotherms of C₂H₂, C₂H₄, and CH₄ at 273 K and 298 K are illustrated in Fig. 2. Compared with C₂H₄ and CH₄, it’s exciting that the C₂H₂ adsorption capacity of ZJU-198a zooms up rapidly to reach saturation even at extremely low pressure. Under 0.01 bar, ZJU-198a takes up a great amount of 72.5 cm³ cm⁻³, 37.3 cm³ cm⁻³, and 0.13 cm³ cm⁻³ for C₂H₂, C₂H₄, and CH₄ at 273 K, respectively and 41.4 cm³ cm⁻³, 3.1 cm³ cm⁻³, and 0.03 cm³ cm⁻³ for C₂H₂, C₂H₄, and CH₄ at 298 K, respectively. The saturation capacities of C₂H₂, C₂H₄, and CH₄ are 99.4 cm³ cm⁻³, 92.4 cm³ cm⁻³, and 21.1 cm³ cm⁻³ at 273 K, respectively, and 98.4 cm³ cm⁻³, 89.2 cm³ cm⁻³, and 8.8 cm³ cm⁻³ at 298 K, respectively. It’s worth noting that there is almost no decline in C₂H₂ adsorption with the temperature rise, and the CH₄ molecule has no competitive advantages against the C₂H₂ molecule especially at extremely low pressure.

Fig. 2 The single-component adsorption isotherms for C₂H₂ (violet), C₂H₄ (orange), and CH₄ (blue) of ZJU-198a at 273 K (a) and 298 K (b). The insets exhibit the adsorption data of C₂H₂ and C₂H₄ at extremely low pressure.

The adsorption selectivity for C₂H₂ over C₂H₄ and CH₄ is another prerequisite for assessing an adsorbent material, except for when assessing the mono-component adsorption. As is well-known, the Ideal Adsorbed Solution Theory (IAST) is recognized to estimate the gas adsorption separation. Thus, the results of the IAST calculation for C₂H₂/C₂H₄ (1 : 99, v/v) and C₂H₂/CH₄ (50 : 50, v/v) are expressed in Fig. 3. The selectivity coefficients of C₂H₂/C₂H₄ are 5.8 to 7.7 at 273 K and 4.8 to 7.2 at 298 K, respectively. It’s quite surprising to read that the selectivity of C₂H₂/CH₄ reaches up to 497.9 and 391.1 for 273 K and 298 K, respectively. Table 1 lists the comparison of ZJU-198a with some other promising MOFs for C₂H₂/CH₄ separation.^45–50 We are compelled to admit that the C₂H₂ adsorption capacity of ZJU-198a is moderately limited by the relatively low porosity. However, the C₂H₂/CH₄ selectivity of ZJU-198a drastically outperforms those of other MOFs. To the best of our knowledge, the C₂H₂/CH₄ selectivity coefficients of ZJU-198a at both 273 K and 298 K are the highest values among the as yet reported metal–organic frameworks (MOFs).

Fig. 3 (a) The IAST calculations of C₂H₂/C₂H₄ and C₂H₂/CH₄ adsorption selectivity for ZJU-198a at 273 K; (b) the IAST calculations of C₂H₂/C₂H₄ and C₂H₂/CH₄ adsorption selectivity for ZJU-198a at 298 K.

Fig. 4 The isosteric heat of C₂H₂, C₂H₄, and CH₄ adsorption, Q_st, in ZJU-198a.

Table 1 The comparison of the adsorption data for selected MOFs

MOFs	Surface area (m^2 g^−1, BET)	C2H2 uptake (at 1.0 bar, RT, cm^3 g^−1)	Selectivity for C2H2/CH4	Qst of C2H2 (kJ mmol^−1)	Ref.
ZJNU-55	450	56.3	64.9	42.4	46
BUT-70A	695	69.5	66.6	25.6	47
UTSA-5a	462	59.8	28.4	30.8	48
Cu-TDPAH	—	177.7	127.1	42.5	45
UTSA-50a	604	90.6	68	39.4	50
ZJU-199a	987	128.0	33.5	38.5	49
ZJU-198a	343	72.9	391.1	26.1	This work

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The moderate gas capacity and excellent gas separation make ZJU-198a a valuable candidate for hydrocarbon adsorption and separation. Furthermore, the regeneration energy cost attributed to the binding energy is another essential consideration. The binding energy between the adsorbed gas molecule and framework is reflected in the isosteric heat of adsorption (Q_st). Fig. 4 illustrates the isosteric heat of C₂H₂, C₂H₄, and CH₄ with ZJU-198a. Unexpectedly, the isosteric heat of C₂H₄ (37.4 kJ mol⁻¹) is higher than that of C₂H₂ (26.1 kJ mol⁻¹), C₂H₄, and CH₄ (16.2 kJ mol⁻¹).

The discrepancy between the C₂H₂ molecule and C₂H₄ molecule is distinctly far less than that between the C₂H₂ molecule and CH₄ molecule. This is a reasonable excuse for the performance of the C₂H₂/C₂H₄ separation of ZJU-198a not looking as outstanding as that of C₂H₂/CH₄. However, there is still a hysteretic circumstance about the adsorption of C₂H₄ versus C₂H₂ especially at the extremely-low pressure referred to in the insets of Fig. 2. Herein, transient breakthrough simulations were carried out to separate the feed gases C₂H₂/C₂H₄ (1 : 99, v/v), the typical industrial ingredient. As is shown in Fig. 5, the C₂H₂/C₂H₄ mixture is clearly separated by ZJU-198a. After a certain time (τ_break, 64.5 minutes), the impurity level of C₂H₂ exceeded the purity level at 40 ppm. During the time 0 ∼ τ_break, the amount of C₂H₂ captured in ZJU-198a is 126.8 mmol L⁻¹, which is still a satisfactory value.

Fig. 5 The breakthrough curves of ZJU-198a for the C₂H₂/C₂H₄ separation (1 : 99; v/v) operated at 1 bar and 298 K. C_A and C₀ are the concentrations of each gas at the inlet and outlet, respectively.

The crystal structure analysis was carried out to demonstrate the excellent C₂H₂/CH₄ and C₂H₂/C₂H₄ separations of ZJU-198a. Note that the empirical kinetic sizes of C₂H₂, C₂H₄, and CH₄ are 3.32 Å × 3.34 Å, 3.28 Å × 4.18 Å, and 3.82 Å × 3.94 Å, respectively.⁵¹ It’s clear to see that there only exists one available window of ZJU-198a of approximately 3.6 Å × 4.1 Å, which is slightly larger than the size of the C₂H₂ molecule, close to that of the C₂H₄ molecule while smaller than that of the CH₄ molecule. Herein, it’s easy to understand that the good-enough window of the framework plays an important role in selecting the preferential C₂H₂ molecule and closing the door on the CH₄ molecule. How does the relatively low porosity of ZJU-198a feature in the moderately high C₂H₂ adsorption? The C₂H₂ molecules were captured in the framework and the diffusion between the adjacent channels was limited by the narrow channels. Therefore, ZJU-198a with orderly overlapping channels exhibits moderately high C₂H₂ storage and excellent C₂H₂/CH₄ and C₂H₂/C₂H₄ separations.

Conclusions

The amino-coordination microporous metal–organic framework ZJU-198 with a good-enough window has been synthesized successfully and further demonstrated as a valuable adsorbent. The activated ZJU-198a exhibits moderate C₂H₂ storage and excellent C₂H₂/CH₄ and C₂H₂/C₂H₄ separations. Especially, the selectivity of C₂H₂/CH₄ is the highest value among the as yet reported metal–organic frameworks (MOFs). Herein, we believe that making the best of the size-sieving effect of the framework to achieve efficient gas separation will be a significant approach for further applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51472217 and 51432001), the Zhejiang Provincial Natural Science Foundation of China (No. LR13E020001 and LZ15E020001), and the Fundamental Research Funds for the Central Universities (No. 2015QNA4009 and 2016FZA4007), and it was partly supported by the Welch Foundation (AX-1730).

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Footnote

† Electronic supplementary information (ESI) available: syntheses; TGA; IR; XRD; breakthrough test. See DOI: 10.1039/c7ra02741a

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