Selective adsorption behaviour of carbon dioxide in OH-functionalized metal–organic framework materials†

Jinjie Qian *^ab, Jinni Shen ^b, Qipeng Li ^b, Yue Hu *^a and Shaoming Huang *^a
aCollege of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, PR China. E-mail: jinjieqian@wzu.edu.cn; yuehu@wzu.edu.cn; smhuang@wzu.edu.cn; Tel: +86 577 88373064
^bState Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China

First published on 17th August 2017

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The efficient capture and conversion of carbon dioxide (CO₂) is of great importance in realizing a carbon-neutral energy cycle and low-carbon society.¹ Over a long period of time, humans have become heavily reliant on fossil fuels in agriculture and industry, which is widely believed to present the major source for CO₂ emissions, thus leading to the rise of temperatures across the planet, from which a new kind of increasingly serious threat: global warming, may eventually have to be faced.² Although several developments of techniques for the capture of CO₂ have been developed, for example, effective and efficient CO₂ adsorption in alkylamine solutions, there are still a series of issues related to cost performance, storage, transportation, and safety with this.³ Therefore, the strong driving force has long been to use alternative high performance strategies to efficiently remove CO₂.

Currently, microporous metal–organic frameworks (MOFs), a subclass of crystalline hybrid polymers constructed from metallic cations/clusters and organic ligands, are emerging as one of the most promising candidates for CO₂ capture because of their tunable structure, diverse topology, different chemical compositions of the micropores as well as the extra large specific surface area.⁴ Within the field of small molecule adsorption, a large amount of research has been focused on the mutual interaction mechanism between the MOF framework and the entrapped small molecule guests, especially for non-polar linear CO₂ molecule, thus discovering new functional materials with better performance. Recently, the group of Ibarra has demonstrated that alcohol confinement within a previously reported water-stable InOF-1,⁵ built from octahedral indium(III) hydroxide-biphenyltetracarboxylic acid [In₂(OH)₂(BPTC)] chain extended by BPTC ligands, can significantly promote CO₂ capture capacity with an approximately 3.6-fold increase of performance (from 3.8 to 13.7 mmol g⁻¹) at only 1.0 bar and 30 °C.⁶ In this case, more suitable narrow pore sizes which can effectively accommodate the small guest molecules and thus provide a strong overlapping potential have also been confirmed to enhance the CO₂ capacity as well as the sorption affinity.⁶ In this respect, it is also necessary to predict and visualize the CO₂ positions in the hydroxyl (OH)-decorated pore environment within the MOF materials using theoretical calculations and experimental tests.

It has been learned that high capacity storage in MOF materials can be achieved by the introduction of open metal sites (OMS),^7a–c where there exists a strong CO₂ binding strength which forms the Mⁿ⁺⋯C(O)₂ bond because of the direct interaction between CO₂ and the coordinately unsaturated metal centers, causing irreversible physisorption and permanent loss of OMS activity. Fig. 1 shows that traditional alkylamine solutions, alkylamine-appended as well as polar amine-/sulfonate-/ketone-appended MOF materials^7d–h can be extensively utilized for the storage and separation of CO₂, but few examples are available showing that OH-functionalized MOF materials can effectively remove CO₂ because of the limited structural stability of the crystal compounds. In this paper, the optimization of CO₂ molecule sites in the InOF-1 framework using density functional theory (DFT) calculations, and a CO₂ adsorption behaviour comparison with an aluminium-/gallium-(Al/Ga)-based isostructural framework (AlOF-1 and GaOF-1, respectively), is reported, which confirms that all the OH-appended materials possess high CO₂ capacity and selectivity towards nitrogen (N₂) at 273 K, 283 K, and 295 K. Furthermore, it was found that AlOF-1 outperforms in CO₂ uptake because of the large binding energy used to structurally form AlOF-1·4CO₂. More importantly, a high selectivity of CO₂/N₂ at 273–295 K and 1.0 bar, and a large heat of CO₂ adsorption can be achieved at zero coverage (20.37 kJ mol⁻¹for AlOF-1, 18.31 kJ mol⁻¹ for GaOF-1 and 11.98 kJ mol⁻¹ for InOF-1).

The pristine InOF-1 and ethanol (EtOH)-impregnated InOF-1 (InOF-1–EtOH) simultaneously exhibit an outstanding CO₂ capacity under the same conditions.⁶ Therefore, it is of great importance to understand the in-depth mechanism for the binding formation of small molecules and their hosts using direct visualization of the interaction between the CO₂ molecules and the InOF-1 framework. In this paper, the adsorption of CO₂ in one water-stable tetracarboxylate-based framework with an In(III) metal cation, [InOF-1, Fig. 2a and b and S2 (ESI†)] is mainly predicted and explained using DFT calculations. Meanwhile, the electron and energy properties of InOF-1 were calculated using a combination of DFT and plane-wave pseudopotential methods as implemented in the Vienna Ab initio Simulation Package (VASP) code.⁸ Calculations were performed under the Perdew–Burke–Ernzerhof (PBE) approximation⁹ for exchange and correlation with a plane-wave cutoff energy of 380 eV. The optimized unit cell used has 72 and 84 atoms for the bare and four CO₂ molecules loaded materials (InOF-1·4CO₂, where each μ₂-OH group only connects to one CO₂ molecule), respectively. The wave functions were sampled according to the Monkhorst–Pack scheme with a k-point with mesh spacing of ∼0.05 Å⁻¹.

As well as the InOF-1 material, two other isostructural host AlOF-1 and GaOF-1 were also prepared,^7d–f and these possess a hydroxyl-functionalized 4₁ M₂(OH₂)(BPTC) helix chain (M = Al or Ga), whose sizes are ∼6.36 × 9.62 Ų (Fig. 2b). DFT calculations were used to refine the crystal structure for the optimized InOF-1·4CO₂ structures, and this shows that these adsorbed CO₂ molecules interact with the cis-μ₂-OH groups in an end-on mode (Fig. 2c). The O⋯H distance (d_I) between the entrapped CO₂ molecule and the hydroxyl group (H_OH) is 2.330 Å which indicates a relatively moderate to weak hydrogen bond (Table 1). The optimized C–O bond lengths in CO₂ were 1.179 Å (hydrogen bonded end, C–O1) and 1.173 Å (free end, C–O2), and the ∠OCO bond angle was absolutely linear and had a value of 180°. For the coordination environment of each captured CO₂ molecule, the hydrogen bonded O1 atom was also bolstered together by weak cooperative supramolecular interactions between the O1 atom and H atoms from biphenyl C–H groups (H_CH, O⋯H = 3.242 Å and 3.276 Å for d_IIa and d_IIb, respectively, and each occurs twice, Fig. 2d). In this context, the d_IIa and d_IIb distances in the InOF-1·4CO₂ and GaOF-1·4CO₂ structure (d_IIb = 3.202 Å) are slightly larger than that of the AlOF-1·4CO₂ structure (d_IIa = 3.124 Å, d_IIb = 3.197 Å, see Table 1), which obviously indicates more strong interactions between the main MOF framework and the captured CO₂ molecules in the AlOF-1 parent. A top view of the adsorbed CO₂ molecule in the OH-functionalized in-based chain, a total of 5 H atoms (4 H_CH atoms and 1 H_OH atom) attract cooperatively with the O1 charge centres of the captured CO₂ molecules in the tetragonal channel through the combination of the moderate to weak hydrogen bonds and the supramolecular interactions (Fig. 2e). As shown in Fig. 2c–e, the modest hydrogen bond between the O1 atom and the H_OH atom from the M–OH moiety is highlighted in green, and the weak cooperative hydrogen-bond interactions between O1 and H_CH H(dp) from the C–H groups are highlighted in pink for clarity and comparison.

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However, in order to reveal the theory of the adsorption of CO₂ in these three types of isostructures, the binding energy (ΔE), zero point energy correction (ZPEC), thermal energy correction (TEC) and binding enthalpy (ΔH) of AlOF-1·4CO₂, GaOF-1·4CO₂ and InOF-1·4CO₂ were calculated.

The ΔE of CO₂ is evaluated using the following equation:

ΔE = −EMOF·4CO2 + EMOF + 4ECO2	(1)

in which E_MOF·4CO2 is the total energy of the framework and the adsorbed CO₂ molecule, E_MOF and E_CO2 are the energies of the framework and the CO₂ molecule, respectively.

The ZPEC for a system is then calculated as:¹⁰

ZPEC = ZPEMOF·4CO2 − ZPEMOF − 4ZPEBCO2	(2)

where MOF and CO₂ are systems that are considered in isolation and then in combination as MOF·4CO₂ to determine the change in ZPE that results from the systems being placed in the presence of one another.

Similar to ZPEC, TEC is also t calculated as follows:¹⁰

TEC = TEMOF·4CO2(T) − TEMOF(T) − 4TECO2(T)	(3)

where MOF, CO₂ and MOF + CO₂ are as stated previously.

The binding enthalpies at a given temperature are calculated as:¹⁰

−ΔH(T) = H(T)MOF·4CO2 − H(T)MOF − 4H(T)CO2	(4)

where H(T)_MOF, H(T)_CO2, and H(T)_MOF·4CO2 are the enthalpy of the bare MOF without the guest molecule, the enthalpy of the molecule in the gas phase, and the enthalpy of the MOF with the molecule adsorbed, respectively, all at temperature, T. A positive ΔH(T) corresponds to an exothermic adsorption.

The previously described calculated energies are summarized in Table 1. In this table, the larger ΔE and ΔH represent stronger binding. Larger ZPEC and TEC values represent greater correction, that is to say, AlOF-1 (ZPEC = 9.910 kJ mol⁻¹, TEC = 35.827 kJ mol⁻¹) tends to possess greater correction with CO₂ than both GaOF-1 and InOF-1. A stronger binding generates greater confinement and a steeper potential well, and thus yields a larger ZPEC but less thermal motion inside the well.¹⁰ This means that AlOF-1·4CO₂ is easier to form than GaOF-1·4CO₂ or InOF-1·4CO₂. Consequently, agreement is achieved between the previously reported results and this data.

To compare the previously calculated results with experimental CO₂ adsorption tests, InOF-1, AlOF-1 as well as GaOF-1 materials were synthesized according to previous methods reported in the literature for better comparison.^5a,7d,f,11 In order to confirm the permanent porosity, the N₂ isotherms at 77 K were first determined prior to the CO₂ sorption test. The desolvated InOF-1 and GaOF-1 samples showed totally reversible type-I isotherms with the maximum uptake value of 270.9/131.1 m³ g⁻¹ at 1.0 bar and 77 K which corresponds to a Brunauer–Emmett–Teller (BET) and Langmuir surface area of 1065/1093 m² g⁻¹ and 517/570 m² g⁻¹, respectively. In contrast, the activated AlOF-1 powder exhibited barely any N₂ sorption capacity at 77 K, with a maximum value of only 13.4 cm³ g⁻¹ at 1.0 bar and 77 K (Fig. 3a). Despite the microporous window size and the rigid framework for AlOF-1, it was surprising that the low N₂ diffusion into the voids can be observed at 77 K. It was assumed this behaviour might be derived from a strong interaction between the nitrogen molecules and the narrow pore windows by the considerable quadrupole interactions with the electrostatic field gradients near the surface, which subsequently prohibit other molecules from penetrating into the voids,¹² because the AlOF-1 framework possesses only open 1-dimensional channels along the c-axis.

Most importantly, the fascinating internal OH-suspended tetragonal tubes inevitably prompt the further investigation of the practical CO₂ sorption capacity. Single component low-pressure gas sorption isotherms for the three types of desolvated samples toward CO₂ at 273 K, 283 K, and 295 K were collected using a volumetric measurement method and the results obtained are presented in Fig. 4a and Fig. S9–11 (ESI†). Compared to its very low sorption for N₂, the CO₂ isotherms of AlOF-1 at the specific temperatures (273–295 K) show extremely high sorption capacities, with the saturated value of 155.5 cm³ g⁻¹ (6.94 mmol g⁻¹, 305.4 mg g⁻¹) at 273 K and 1.0 bar. Meanwhile, it was also found that the CO₂ adsorption curve quickly reached the value of 42.6 cm³ g⁻¹ (1.90 mmol g⁻¹, 83.7 mg g⁻¹) at 0.15 bar, which is lower than the equivalent partial pressure in flue gas (Fig. 4a). Obviously, this capacity surpasses the InOF-1 adsorption value of 39.1 cm³ g⁻¹ (1.75 mmol g⁻¹, 76.8 mg g⁻¹) at 273 K and 0.15 bar and 140.1 cm³ g⁻¹ (6.25 mmol g⁻¹, 275.2 mg g⁻¹) at 273 K and 1.0 bar. Similar trends can also be found in the uptake of CO₂ at 1.0 bar and 283 K and 295 K, where the AlOF-1 shows a good performance with 118.6 cm³ g⁻¹ and 86.7 cm³ g⁻¹, respectively, whereas the InOF-1 exhibits a lower capability towards CO₂ with 109.5 cm³ g⁻¹ at 283 K and 83.9 cm³ g⁻¹ at 295 K. Furthermore, in terms of CO₂ storage, the Ga-based material shows the lowest capability of 90.3, 71.6 and 56.6 cm³ g⁻¹ [Fig. 4b and S15 (ESI†)]. In this case, it was speculated that there exists a strong interaction between the solvent molecules and the GaOF-1 framework, thus leading to the incomplete desolvation and inadequate utilization of the expected pore volume. However, it should be noted that the CO₂ capacity of AlOF-1 and InOF-1 materials gradually gets closer when the temperature is elevated from 273 K to 295 K (Fig. 4a), which means that the active CO₂ spacing in AlOF-1 is lost faster than that in InOF-1. However, a large amount of CO₂ is still retained under these different temperatures, especially for the Al-based framework, which indicates the excellent retention of the microporous nature before and after the desolvation treatment and gas sorption, as shown in Fig. S6 and S7 (ESI†).

Finally, the IAST prediction based upon the experimental CO₂ and N₂ isotherms are clearly presented in Fig. 4b. The adsorption selectivity is defined as S_i/j = (q₁/q₂)/(p₁/p₂), in which q_i is the amount of i adsorbed and p_i is the partial pressure of i in the mixture. At 1.0 bar, the calculated CO₂/N₂ selectivities for AlOF-1 are more variable: 1078.8 and 108.4 at 273 K and 283 K from gas–phase mixtures in a 15:85 molar ratio, whereas at 295 K, it reaches an excessive amount of 58571.6 because of the very low capacity of N₂ at 1.0 bar. Compared to that of the Al-based material, the theoretically calculated CO₂/N₂ selectivity values for GaOF-1 and InOF-1 are more reasonable and trustworthy, where they are 56.7/212.6, 31.1/168.1 and 20.8/219.6 at 273, 283 and 295 K, respectively, [Fig. 4c–e, S20–22 (ESI†)]. More interestingly, the heat of CO₂ adsorption at zero coverage is calculated to be 20.37 kJ mol⁻¹ (AlOF-1), 18.31 kJ mol⁻¹ (GaOF-1), and 11.98 kJ mol⁻¹ (InOF-1) based on the CO₂ isotherms at 283 K and 295 K, which indicates a more reasonable order for this series of M-BPTC structures despite the deficient performance for GaOF-1 in the CO₂ sorption analysis (Fig. 4f). These calculations and observation indicate that all the OH-functionalized materials are promising candidates for practical CO₂ capture and conversion applications.

In summary, with a combination of theoretical calculations and experimental tests, the role of group A(III) metals in three types of OH-appended M-BPTC MOFs, including AlOF-1, GaOF-1 and InOF-1 were thoroughly compared. In this series, it was found that the CO₂ binding to the cis-μ₂-OH groups is in an end-on mode and this is further supported by H-bonding with adjacent biphenyl rings. The experiments and IAST results demonstrate that the largest binding energy and adsorption heat exist in the Al(III)–OH⋯C(O)₂ system which could be empirically predicted by the atomic mass of the cations in the isostructural frameworks. This research provides a useful guideline for the future development of versatile MOFs with functionalized groups.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (51420105002, 21601137, 51602226), and Natural Science Foundation of Zhejiang Province (LQ16B010003), Yunnan Applied Basic Research Projects (2016FD083) and Scientific Research Foundation of the Education Department of Yunnan Province (2016ZZX229).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: For the optimized structures: Re_GaOF-1 (a = 12.1338, b = 12.1339, c = 12.1338, P1), Re_GaOF-1 + 4CO2 (a = 14.9392, b = 14.9392, c = 11.9401, I4122), Re_InOF-1 (unit cell parameters: a = 12.76990, b = 12.77034, c = 12.76990, P1), Re_InOF-1 + 4CO2 (a = 15.7375, b = 15.7375, c = 12.5286, I4122), Re_AlOF-1 (a = 12.13385, b = 12.13386, c = 12.13385, P1), Re_AlOF-1 + 4CO2 (a = 14.9392, b = 14.9392, c = 11.9401, I4122). For more details please see the data in the ESI. See DOI: 10.1039/c7ce01195d

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