A microporous yttrium metal–organic framework of an unusual nia topology for high adsorption selectivity of C₂H₂ and CO₂ over CH₄ at room temperature

Abstract

A new microporous yttrium metal–organic framework Y-H₃TDPAT [H₆TDPAT = 2,4,6-tris(3,5-dicarboxylphenyl-amino)-1,3,5-triazine] with Lewis basic sites on the pore surface was solvothermally synthesized and structurally determined as an unusual (6,6)-connected nia topology. The small pores and the Lewis basic sites within the desolvated Y-H₃TDPAT have improved the affinity for C₂H₂ and CO₂ which has been revealed in their large isosteric heats of adsorption (38.2 and 30.9 kJ mol⁻¹). In addition, highly selective separation of C₂H₂/CH₄ and CO₂/CH₄ at room temperature makes Y-H₃TDPAT possess a potential application in natural gas purification.

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Introduction

Nowadays, the energy crisis has caused worldwide concern and some global issues need to be resolved with urgency. What is worrying the world greatly now is the fossil fuel shortage in the near future, including coal, oil and natural gas. Improving the energy efficiency of fossil fuel would provide a roadmap to utilize resources rationally. Among them, natural gas purification has drawn more attention from materials scientists and chemists in recent years.^1–3 Methane (CH₄), as the main ingredient of natural gas and biogas, is a clean fuel for residential use and a useful C₁ feedstock for the production of various chemicals in petrochemical industries, such as chloromethane and formaldehyde.^4,5 However, impure CH₄ as a starting material may complicate the separation processes of products for petrochemical and chemical applications. The major impurities in naturally occurring CH₄ are C₂ hydrocarbons, typically C₂H₂. Furthermore, CO₂ can also be found in natural gas in large quantities. Some impurities may inhibit the conversion of CH₄. For example, CO₂ poisons the Li/MgO catalyst that converts CH₄ into ethane and ethylene.⁶ Materials that can preferentially capture C₂H₂ and CO₂ over CH₄ are very desirable. The commonly used cryogenic distillation technology for the separation of C₂H₂ and CO₂ from CH₄ is very energy consuming. Therefore, it is urgent to develop new effective adsorbents for reduction of the cost to produce high-purity natural gas.

Metal–organic frameworks (MOFs) are a unique class of crystalline porous materials that have attracted growing interest on account of their extensive application scope in gas storage,^7–9 separation,^10–12 catalysis^13–15 and optical devices.^16–18 Owing to their high porosities, adjustable chemistry and tunable pores/shapes, a number of crucial and required gas separations, such as CO₂/N₂ separation,^19,20 O₂/N₂ separation,^21,22 light hydrocarbon separation,^23,24etc., have been achieved by utilizing novel MOFs as absorbents. Among the diverse gas separations, separation of C₂H₂ and CO₂ over CH₄ is practical and needs to be further developed.

The establishment of the permanent porosity is absolutely vital to explore the gas separation performance of Ln-MOF materials. Lanthanide ions have higher coordination numbers than transition-metal ions, which is beneficial to stabilize the frameworks.^25–27 To date, compared with the huge amounts of transition-metal MOFs, there are obviously less porous Ln-MOFs that have been exploited for the application of gas storage and separation.^28,29 In addition, the incorporation of specific sites, such as Lewis basic sites (LBSs) onto the walls of porous MOFs can dramatically improve their gas storage and separation capacities.^30–32 Although the present research of porous MOFs for the separation of C₂H₂/CH₄ and CO₂/CH₄ has seen some progress, examples of Ln-MOFs with rich LBSs for high adsorption selectivity of these gas molecules at room temperature are relatively scarce.

Recently, we have prepared a dendritic six-carboxylate ligand, 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine (H₆TDPAT).³³ Self-assembly of the H₃TDPAT³⁻ ligand with Y³⁺ ions under solvothermal conditions afforded a 3D nia-type Ln-MOF called Y-H₃TDPAT. The resultant microporous Y-H₃TDPAT features rich LBSs, which endow it with interesting performances in the separation of C₂H₂/CH₄ and CO₂/CH₄ at room temperature.

Experimental section

Materials and measurements

All solvents and reagents used in this study are commercially available and were used as received without further purification. Fourier-transform (FT-IR) spectra were acquired on a Bruker IFS-66 V/S FT-IR spectrometer. Thermogravimetric measurement was carried on a pre-weighed sample in a N₂ stream using a Netzsch STA449C apparatus with a heating rate of 10 °C min⁻¹. Elemental analyses for C, H, and N were performed on a vario MICRO CHN Analyzer. Powder X-ray diffraction (PXRD) data were obtained using a Bruker D8 ADVANCE diffractometer (Cu-Kα, 1.5418 Å).

Gas sorption measurements

A Micromeritics ASAP 2020 instrument was used to measure gas adsorption characteristics. The guest solvent molecules in the pores of the freshly prepared sample of Y-H₃TDPAT were exchanged with acetone molecules. Then the sample was activated at 393 K for 10 h with high vacuum (the outgas rate was <5 μm Hg min⁻¹). The adsorption experiment was maintained at 273 K with an ice-water bath and 77 K with liquid nitrogen, respectively. As the circulation constant temperature water bath was set up, the temperature for the adsorption isotherms was constant at 298 K.

The virial equation is used to analyze the isotherm data:³⁴

ln(n/p) = A₀ + A₁n + A₂n² +⋯ (1)

where n is the amount adsorbed, p is the pressure, and A₀, A₁, A₂, etc., are virial coefficients. The constant K_H is defined as exp(A₀), and the Henry's law selectivity can then be obtained based on the constant K_H.

Prediction of the gas adsorption selectivity by IAST

Ideal adsorption solution theory (IAST) was employed for prediction of the binary mixture adsorption from the experimental single-component isotherms.^35,36 In order to carry out the integrations required by IAST, the pure-gas isotherms should be fitted by a suitable model. Practices show that several methods are available to process the curve fitting. We found that the dual-site Langmuir–Freundlich equation was successful in fitting for this set of data. As can be seen in Fig. S19–S21 and Table S3 (ESI†), the results from the model match the isotherms very well (R² > 0.99999).

Here, N is the adsorbed amount per unit weight of adsorbing substance (mmol g⁻¹), N^max₁ and N^max₂ are the saturation adsorption capacities on sites 1 and 2 (mmol g⁻¹), P is the pressure of the bulk gas in a state of equilibrium with the adsorbed phase (kPa), b₁ and b₂ represent the coefficients of adsorption affinity on sites 1 and 2 (1 kPa⁻¹), and n₁ and n₂ are the deviations from an ideal uniform surface. The model parameters were then utilized for prediction of a multi-component adsorption process with IAST.

The selectivity S_A/B in a bi-component system of A and B is equal to (x_A/y_A)/(x_B/y_B), where x_i and y_i represent the mole fractions of component i (i = A or B) in the bulk and adsorbed phases, respectively.

GCMC simulation

The density distribution of C₂H_2H molecules was calculated through Grand Canonical Monte Carlo (GCMC) simulation at 1 bar and 298 K. The periodic boundary conditions and Universal force field (UFF) were used.³⁷ The VDW interactions between C₂H₂ molecules and the MOF were calculated using Lennard-Jones potentials. Electrostatic interaction was evaluated through the Ewald summation method. Charge equilibration uses the specified method to redistribute the overall charge on the C₂H₂ molecules and atoms of the framework.³⁸ All the GCMC simulations were carried out using the Sorption module in Material Studio software.³⁹

Synthesis of Y-H₃TDPAT

Ultrasonic dissolution with Y(NO₃)₃·6H₂O (19 mg, 0.05 mmol) and H₆TDPAT (15 mg, 0.025 mmol) was performed in 3 mL of DMF and 2 mL of H₂O. Then 0.10 mL of nitric acid was added, and the mixture was placed in a teflon-lined stainless steel vessel and heated at 120 °C for 48 h, then cooled to room temperature over 24 h. Block colorless crystals were obtained to give Y-H₃TDPAT. Anal. calcd for C₂₇H₁₅N₆O₁₂Y (%): C, 46.00; H, 2.13; N, 11.93. Found: C, 45.64; H, 2.20; N, 12.09. IR (cm⁻¹): 3645(w), 3282(m), 3074(m), 2961(m), 2772(m), 2467(m), 1573(s), 1499(s), 1372(s), 1106(m), 1025(m), 846(m), 783(s), 725(m), 609(m), 499(w), 427(m) (Fig. S3, ESI†).

X-ray single-crystal diffraction

Data collection for Y-H₃TDPAT was carried out on a Rigaku RAXIS-RAPID-based diffractometer with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The SADBAS program was applied to the data for the corrections of incident and diffracted ray absorption effects.⁴⁰ The SHELXTL program was used to solve the crystal structure by a direct method and refine the data by the full matrix least-squares against F² values.⁴¹ The non-hydrogen atoms were located from Fourier maps successfully and refined through anisotropic thermal parameters. On account of some disordered solvent molecules in Y-H₃TDPAT, their diffraction contributions were removed by utilizing the PLATON/SQUEEZE.⁴² Crystal data of Y-H₃TDPAT are listed in Table S2 (ESI†).

Results and discussion

Crystals of Y-H₃TDPAT suitable for X-ray analysis were obtained by carefully adjusting the pH, solvent and temperature using a solvothermal method. Single crystal X-ray crystallographic analysis reveals that Y-H₃TDPAT crystallizes in a non-centrosymmetric trigonal P31c space group. The asymmetric unit contains one Y³⁺ ion and one-sixth of the ligand H₃TDPAT³⁻ (Fig. S1, ESI†). In every H₃TDPAT³⁻ ligand, the dihedral angles of the central triazine ring with three terminal benzene rings are all 54.66°. The larger torsion dihedral angles in the ligand are probably due to the semi-rigid connection effect. As shown in Fig. 1a, each Y³⁺ ion is nine-coordinated adopting a distorted monocapped square antiprismatic molecular geometry and among them there are nine oxygen atoms from the six carboxylate groups of H₃TDPAT³⁻ ligands. The H₃TDPAT³⁻ ligand acts as a μ₆-bridge to link six Y³⁺ ions to construct a 3D structure via three bidentate chelating (μ₁-η₁:η₁) carboxyl groups and three monodentate carboxyl groups (Fig. 1b). Their coordination bond lengths are 2.2595(19), 2.3559(19) and 2.754(2) Å, respectively.

Fig. 1 Single crystal structure of Y-H₃TDPAT indicating (a) the coordination environment of Y³⁺ ions in Y-H₃TDPAT, (b) the coordination mode of H₃TDPAT³⁻, and (c) the 3D framework of Y-H₃TDPAT along the b axis.

There are open channels along the b direction in Y-H₃TDPAT, with a diameter of about 7 Å (excluding van der Waals radii) (Fig. 1c and Fig. S2, ESI†). The void space accounts for approximately 51.4% of the whole crystal volume (1154.7 ų out of the 2246.4 ų per unit cell volume) by PLATON analysis.⁴² To achieve a better insight into the structure of Y-H₃TDPAT, topological analysis was carried out.⁴³ Each simplified framework in Y-H₃TDPAT can be topologically represented as a binodal (6,6)-connected nia topology by reducing Y³⁺ ions and H₃TDPAT³⁻ ligands as two different 6-connected nodes (Fig. 2a–c). Yaghi et al. reported that a nia-type porous MOF can be generated by connecting an octahedral Zn₄O cluster as an inorganic building block (IBB) with a trigonal prismatic six-node ligand.⁴⁴ Conversely, Zhu et al. reported that a porous nia-MOF can also be formed by connecting a trigonal prismatic In₃O cluster IBB with an octahedral six-node ligand.⁴⁵ However, nia-type MOFs with a lanthanide metal center and a six-node ligand are uncommon. One should explicitly recognize that the hexatopic linker can be simplified as four 3-coordinated (3-c) branch points as there are a few distinct types of octahedral node in basic nia nets (Fig. 2d).⁴⁶ In this case, the derived net is jjt shown in Fig. 2e.

Fig. 2 (a) Polyhedral representation of nia topology; (b) two types of tiles with a face symbol and natural tiling; (c) the (6,6)-connected nia topology of Y-H₃TDPAT; (d) the hexatopic linker with branch points of the underlying net shown as green and magenta balls (respectively 3-c and 4-c vertices). C, gray; O, red; Y, magenta; (e) the jjt topology of Y-H₃TDPAT.

The combined feature of the small pore and the LBSs on the pore surfaces highlights the potential strong binding and high storage of gas molecules. The N₂ sorption isotherm at 77 K showed that Y-H₃TDPAT displayed type-I sorption behaviour with a BET surface area of 962 m² g⁻¹ (Langmuir surface area, 1105 m² g⁻¹) (Fig. 3 and Fig. S4, ESI†). Comparison of PXRD patterns of the as-synthesized sample, guest-free sample and sample after adsorption was made carefully (Fig. S5, ESI†). Moreover, the stabilities of Y-H₃TDPAT in several common organic solvents (CH₂Cl₂, CHCl₃, acetone, MeOH and EtOH) were also tested (Fig. S22, ESI†). Clearly, the main original skeleton of Y-H₃TDPAT remained intact after these tests. The total pore volume calculated from the maximum amount of N₂ adsorbed is 0.42 cm³ g⁻¹, which is in good agreement with the theoretical pore volume (0.48 cm³ g⁻¹).

Fig. 3 N₂ sorption isotherm at 77 K.

The above structural features of Y-H₃TDPAT encouraged us to evaluate its potential application in gas capture. As shown in Fig. 4a and b, Y-H₃TDPAT can take the amount of C₂H₂ (153.7 cm³ g⁻¹), CO₂ (129.2 cm³ g⁻¹) and CH₄ (25.3 cm³ g⁻¹) at 1 bar and 273 K; and C₂H₂ (100.0 cm³ g⁻¹), CO₂ (70.1 cm³ g⁻¹) and CH₄ (17.5 cm³ g⁻¹) at 1 bar and 298 K. The uptake capacity for C₂H₂ in Y-H₃TDPAT at 298 K is lower than some famous MOFs with high density unsaturated metal sites, such as HKUST-1 (201 cm³ g⁻¹),⁴⁷ CoMOF-74 (197 cm³ g⁻¹)⁴⁸ and UTSA-20 (150 cm³ g⁻¹).⁴⁹ However, this value is higher than the ones with LBSs such as UTSA-5 (59.8 cm³ g⁻¹)⁵⁰ and [Zn₄(dmf)(ur)₂(ndc)₄] (32.5 cm³ g⁻¹).⁵¹ It is also significantly higher than some MOFs with steady large pores and high surface areas such as MOF-5 (26 cm³ g⁻¹) and ZIF-8 (25 cm³ g⁻¹),⁴⁷ which represent the best C₂H₂ adsorption property among the MOFs with small pores and LBSs. This illustrates that the combined feature of LBSs (triazine and imino groups) and small pore size lead to the efficient capture of C₂H₂ molecules in Y-H₃TDPAT. A comparison of C₂H₂ adsorption by some other porous MOFs is given in Table S1 (ESI†).

Fig. 4 C₂H₂ (navy), CO₂ (wine) and CH₄ (dark cyan) sorption isotherms of Y-H₃TDPAT at (a) 273 K and (b) 298 K. Solid symbols: adsorption, open symbols: desorption.

The coverage-dependent isosteric heats of adsorption (Q_st) for C₂H₂, CO₂ and CH₄ were calculated using the virial method, which is a dependable and well-established methodology fitting from the pure component isotherms at 298 K and 273 K, as given in Table 1 and Fig. S7–S12 (ESI†). The isosteric heats of adsorption at zero coverage are 38.2, 30.9 and 13.5 kJ mol⁻¹ for C₂H₂, CO₂ and CH₄, respectively. It is noteworthy that Q_st for C₂H₂ is higher than that for USTA-20 (30.8 kJ mol⁻¹)⁴⁹ and HKUST-1 (30.4 kJ mol⁻¹)⁴⁷ with high density unsaturated metal sites and ZJU-5 (35.8 kJ mol⁻¹)⁵² with both unsaturated metal sites and LBSs. The Q_st for CO₂ is also comparable with those for the MOFs with LBSs.⁵³ Furthermore, the Q_st for C₂H₂ and CO₂ is nearly 25 kJ mol⁻¹ and 17 kJ mol⁻¹ larger than that for CH₄ (13.5 kJ mol⁻¹), respectively. The marked feature of Y-H₃TDPAT is the fairly higher Henry's law selectivity of 77.2 for C₂H₂/CH₄ at 298 K. To the best of our knowledge, the C₂H₂/CH₄ adsorption selectivity of 77.2 is the highest one ever reported among the MOFs containing only LBSs (Table 2 and Fig. S13–S18, ESI†). The highly selective adsorption for C₂H₂/CH₄ and CO₂/CH₄ highlights the potential of Y-H₃TDPAT in practical nature gas purification. GCMC simulation reveals that the adsorbed C₂H₂ molecules are associated with particular adsorption sites. Upon adsorption, C₂H₂ can be accommodated between the two 1,3,5-triazine rings (Fig. 5a). These two face to face π(C₂H₂)⋯π(-triazine) stacking interactions cause relatively strong binding between C₂H₂ and Y-H₃TDPAT. The density distribution of the center-of-mass of C₂H₂ molecules obtained from GCMC simulation illustrates that the C₂H₂ molecules prefer to locate at the electron-dense aromatic rings within the framework (Fig. 5b). Moreover, the 1D channel also allows other C₂H₂ molecules to locate inside, and these adsorbed C₂H₂ molecules are distributed in a wave type arrangement with intermolecular C–H⋯π interactions (Fig. S23, ESI†). The π(C₂H₂)⋯π(-triazine) and intermolecular C–H⋯π interactions do not exist between CH₄ and the framework. The difference in the guest/framework interaction strength is likely the reason why the performance of separation for C₂H₂/CH₄ is outstanding in Y-H₃TDPAT at room temperature.

Table 1 Virial graph analysis data for Y-TDPAT and separation selectivities of acetylene and carbon dioxide over methane

Adsorbate	T [K]	K H [mol g^−1 Pa^−1]	A 0 [ln(mol g^−1 Pa^−1)]	A 1 [g mol^−1]	R ^2	S ij	[kJ mol^−1]
a The Henry's law selectivity for the gas component i over CH4 at the speculated temperature is calculated based on the equation: Sij = KH(i)/KH(CH4).
C2H2	273	2.157 × 10^−6	−13.047 ± 0.036	−791.594 ± 21.697	0.99700	147.5	38.2
	298	6.838 × 10^−7	−14.196 ± 0.044	−885.088 ± 33.351	0.99434	77.2
CO2	273	2.211 × 10^−7	−15.325 ± 0.019	−436.273 ± 18.240	0.99304	15.1	30.9
	298	8.561 × 10^−8	−16.273 ± 0.008	−633.389 ± 21.019	0.99561	9.7
CH4	273	1.462 × 10^−8	−18.041 ± 0.002	−307.759 ± 11.384	0.99455	—	13.5
	298	8.850 × 10^−9	−18.543 ± 0.000	−158.466 ± 1.228	0.99976	—

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Table 2 Comparison of several porous MOFs for their C₂H₂/CH₄ separation at 298 K

Compounds	Uptake of C2H2^a	Uptake of CH4	Q st (CH4)^b	Q st (C2H2)	S ij	Ref.
a The unit for the gas uptake is cm^3 (STP) per gram. b Q st (kJ mol^−1) is the enthalpy at zero coverage. c S ij is the Henry's law selectivity.
Cu-TDPAT	178	28.3	20.7	42.5	127.1	33
Cu-TDPAH	155	22.1	13.8	23.5	80.9	54
Y-H3TDPAT	100.0	17.5	13.5	38.2	77.2	This work
UTSA-50a	90.6	18.8	18.6	39.4	68	55
UTSA-15a	34	2.5	13.6	39.5	55.6	56
UTSA-36a	57	13	24.4	29.0	13.8	57
Zn5(BTA)6(TDA)2	44	10	26.1	37.3	15.5	58
[Zn4(OH)2(1,2,4-BTC)2]	53	10	14.8	28.2	14.7	59
Cu(BDC-OH)	43	13	18.5	25.7	9.26	60
UTSA-38a	64	—	18.9	24.7	5.6	61

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Fig. 5 (a) The C₂H₂ molecule sits right between the two 1,3,5-triazine rings. Multiple-point interactions of the C₂H₂ with the framework: d[center(C₂H₂)⋯center(-triazine)(left)] = 3.657 Å, d[center(C₂H₂)⋯center(-triazine)(right)] = 3.489 Å; (b) density distribution of the center-of-mass of C₂H₂ molecules in the unit cell of Y-H₃TDPAT at 298 K and 1 bar simulated by GCMC.

Since the binary mixture adsorption isotherms cannot be quickly and conveniently measured, it is essential to use a proper adsorption model, such as ideal adsorbed solution theory (IAST), for prediction of the bi-component adsorption from the experimental single-component isotherms. Fig. 6 shows the IAST calculations of the adsorption selectivity for equimolar C₂H₂/CH₄ and CO₂/CH₄ mixtures at 298 K in Y-H₃TDPAT. The C₂H₂/CH₄ selectivity (∼188) and the CO₂/CH₄ selectivity (∼11) have their maximum values at a relatively low pressure and decrease sharply as the pressure rises. The selectivity reaches a plateau at about 40 kPa and stays fairly stable under higher pressures. The values at 100 kPa are estimated to be ∼32 for C₂H₂/CH₄ and ∼7 for CO₂/CH₄, respectively. Fractionation of these gaseous mixtures is expected to be practically feasible.

Fig. 6 Calculations of the adsorption selectivity, S_A/B, for equimolar C₂H₂/CH₄ and CO₂/CH₄ mixtures at 298 K in Y-H₃TDPAT using IAST.

Conclusions

In summary, we have successfully synthesized a porous Ln-MOF Y-H₃TDPAT with a high density of LBSs on the pore surface. The collaborative enforcement of the small pores and LBSs has enabled Y-H₃TDPAT to take up a moderately high amount of C₂H₂ gas molecules of 100 cm³ g⁻¹ at 298 K and 1 bar, and furthermore to exhibit highly selective C₂H₂/CH₄ and CO₂/CH₄ gas sorption with the Henry's law selectivity of 77.2 and 9.7, respectively. The selective adsorption properties of Y-H₃TDPAT make it a promising candidate for CH₄ purification and recycling. Further applications of this MOF, the combination of a dendritic hexacarboxylate linker and other lanthanide metal ions, and other related works are ongoing.

Acknowledgements

We acknowledge National Supercomputing Center in Shenzhen for providing the computational resources and materials studio (version 6.1, module DMol³). This work was supported by the National Natural Science Foundation of China (No. 21601103, 51372125, 21571112 and 51572136), the Taishan Scholars Program of Shandong Province, the Natural Science Foundation of Shandong Province, China (No. ZR2016BP04 and ZR2016BQ28) and the Scientific and Technical Development Project of Qingdao (No. 17-1-1-78-jch).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1475480. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qm00164a

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