A 2D metal–organic framework composed of a bi-functional ligand with ultra-micropores for post-combustion CO₂ capture

Abstract

A two-dimensional (2D) metal–organic framework (MOF) named NUS-5 was synthesized using a bi-functional ligand 4-pyrazolecarboxylic acid (PyC) containing both carboxylate and pyrazole moieties. NUS-5 is composed of porous grids stacked together into a 2D layered structure exhibiting ultra-micropores. It displays remarkable thermostability and excellent selectivity towards sorption of CO₂ over N₂, making it a good material candidate for post-combustion CO₂ capture.

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Introduction

Metal–organic frameworks (MOFs), hybrid crystalline solids consisting of metal ion (or cluster) based nodes bridged by organic linkers, have attracted great attention recently owing to their unprecedentedly high surface area, enormous structural and chemical diversity, and potential applications in vast areas such as gas storage, separation, catalysis, sensing, etc.^1–4 Because of the chemical versatility and facile synthesis, carboxylate ligands are the most frequently employed organic linkers in the construction of MOFs, leading to various secondary building units (SBUs) such as Zn₄O(CO₂)₆,⁵ paddle-wheel Cu₂(CO₂)₄,⁶ Cr₃O(CO₂)₆,⁷ Zr₆O₄(OH)₄,⁸ etc. Besides carboxylates, N-containing compounds such as imidazolate, pyrazolates, triazolates, tetrazolates, etc. and their derivatives are another important family of ligands capable of linking with metal cations in a tetrahedral configuration, yielding MOFs with zeolite-like framework topologies (such as zeolitic imidazolate frameworks, ZIFs) and high hydrothermal stabilities.^9–11 It is of a natural speculation that bi-functional ligands containing both carboxylic acid and N-containing moieties should offer a wider choice for the tailored synthesis of MOFs with novel structural topologies suitable for specific applications.^12–16 In this work, we report a MOF named NUS-5 (NUS stands for National University of Singapore) which is composed of such a bi-functional ligand 4-pyrazolecarboxylic acid (PyC). NUS-5 has a two-dimensional (2D) layered structure in which the porous grid of each layer is brought together through interlayer interactions into a porous framework. This material exhibits ultra-micropores and selective gas sorption properties with potential applications in gas separation especially post-combustion CO₂ capture.

Experimental

Materials and methods

All reagents were purchased from commercial suppliers and used without purification. Elemental analyses (C, H, and N) were performed on a Vario MICRO series CHNOS Elemental Analyzer. Powder X-ray diffraction patterns were obtained on a Bruker D8 Advance X-ray Powder Diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at a scan rate of 0.02 degree per s. TGA was investigated using SDT TA Instruments 2960 Simultaneous DTA-TGA at a heating rate of 10 °C min⁻¹ under a constant nitrogen flow of 110 mL min⁻¹.

Crystallography

Single crystal X-ray diffraction data of NUS-5 were collected on a SuperNova, Dual, Cu at zero, Atlas diffractometer. The crystals were kept at 100.0(2) K during data collection with a wavelength of 1.54184 Å and K\α. The data were collected with a ω-scan technique and an arbitrary φ-angle. Data reduction was performed with the CrysAlisPro package, and an analytical absorption correction was applied. The structures were solved by direct methods and refined by full-matrix least-squares on F² with anisotropic displacement using the SHELXTL software package. The non-H atoms were treated anisotropically, whereas the aromatic and hydroxy-hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective carbon or oxygen atoms. In these structures, free solvent molecules were highly disordered, and the attempts to locate and refine the solvent peaks were unsuccessful. The diffused electron densities resulting from these residual solvent molecules in NUS-5 were removed from the data sets using the SQUEEZE routine of PLATON and refined further using the data generated. The contents of the solvent region are not represented in the unit cell contents in these crystal data.†

Gas sorption measurements

Low pressure gas sorption isotherms of NUS-5 were measured up to 1 bar using a Micromeritics ASAP 2020 surface area and pore size analyzer. Before the measurements, NUS-5 (∼150 mg) was degassed under a reduced pressure (<10⁻² Pa) at 120 °C for 12 h. UHP grade He, N₂, and CO₂ were used for all the measurements. Oil-free vacuum pumps and oil-free pressure regulators were used to prevent contamination of the samples during the degassing process and isotherm measurement. The temperatures at 77 K, 273 K, and 298 K were maintained with a liquid nitrogen bath, an ice water bath, and at room temperature, respectively. Pore size distribution data were calculated from the N₂ sorption isotherms at 77 K based on non-local density functional theory (NLDFT) model in the Micromeritics ASAP2020 software package (assuming cylinder pore geometry). High pressure gas sorption isotherms of NUS-5 were collected on a HPVA II – High Pressure Volumetric Analyzer.

Calculations of isosteric heat of adsorption (Q_st)

The CO₂ adsorption isotherms measured at 273 K and 298 K were first fitted to a virial equation (eqn (1)). The fitting parameters were then used to calculate the isosteric heat of adsorption (Q_st) using eqn (2),where P is pressure (mmHg), N is adsorbed quantity (mmol g⁻¹), T is temperature (K), R is gas constant (8.314 J K⁻¹ mol⁻¹), a_i and b_i are virial coefficients, m and n represent the number of coefficients required to adequately describe the isotherms (herein, m = 5, n = 2).

Ideal adsorption solution theory (IAST) selectivity

The CO₂ and N₂ adsorption isotherms were first fitted to a dual-site Langmuir–Freundlich (DSLF) model (eqn (3)),where q is the amount of adsorbed gas (mmol g⁻¹), p is the bulk gas phase pressure (bar), q_sat is the saturation amount (mmol g⁻¹), b is the Langmuir–Freundlich parameter (bar^−α), α is the Langmuir–Freundlich exponent (dimensionless) for two adsorption sites A and B indicating the presence of weak and strong adsorption sites.

IAST starts from the Raoults' Law type of relationship between fluid and adsorbed phase,

P_i = Py_i = P⁰_ix_i (4)

where P_i is partial pressure of component i (bar), P is total pressure (bar), y_i and x_i represent mole fractions of component i in gas and adsorbed phase (dimensionless). P⁰_i is equilibrium vapour pressure (bar).

In IAST, P⁰_i is defined by relating to spreading pressure π,

where π is spreading pressure, S is specific surface area of adsorbent (m² g⁻¹), R is gas constant (8.314 J K⁻¹ mol⁻¹), T is temperature (K), q_i(P_i) is the single component equilibrium obtained from isotherm (mmol g⁻¹).

For a dual-site Langmuir–Freundlich (DSLF) model, we have an analytical expression for the integral,

The isotherm parameters are derived from the previous fitting. For a binary component system the unknowns will be π, P⁰₁, and P⁰₂ which can be obtained by simultaneously solving eqn (5) and (7).

The adsorbed amount for each compound in a mixture is

q^mix_i = x_iq_T (8)

where q_i^mix is the adsorbed amount of component i (mmol g⁻¹), q_T is the total adsorbed amount (mmol g⁻¹).

The adsorption selectivities S_ads were calculated using eqn (10).

In this study, IAST calculations were carried out assuming a CO₂/N₂ binary mixed gas with a molar ratio of 15 : 85 at 298 K and pressures up to 1 bar to mimic the composition and condition of flue gas.

Results and discussion

NUS-5 was obtained as a colourless crystal via a solvothermal reaction between PyC (Fig. 1a) and zinc nitrate in a mixed solvent of dimethylacetamide (DMA), isopropanol, and water (v/v/v: 2/2/1) at 100 °C for 3 days. A single-crystal X-ray diffraction study revealed that NUS-5 crystallized in the monoclinic space group P2₁/n. Zinc adopts a distorted tetrahedral geometry in the SBU of NUS-5 by binding two N from adjacent ligands and two O with one from the third ligand and one from water (Fig. 1b). The ligands are bridged by these SBUs into rhombic grids stacked together through interlayer hydrogen bonding interactions (Fig. 1c) into a corrugated 2D layered structure (Fig. 1d), which is highly porous due to the grid structure of each single layer (Fig. 1e).

Fig. 1 (a) Chemical structure of ligand PyC; (b) SBU of NUS-5; (c) interlayer hydrogen bonding interactions (dash line represents the distance of 1.812 Å from hydrogen atom in terminal water molecule to carbonyl oxygen atom in neighbouring layer); (d) 2D layered structure of NUS-5 viewed along the [010] direction; (e) porous structure of NUS-5 viewed along the [100] direction.

Elemental analysis of NUS-5 confirmed that the C, H, N, and Zn contents were 32.65, 2.69, 15.74, and 18.96 wt%, respectively, which are quite close to the calculated value of 32.18, 5.06, 14.07, and 21.90 wt% assuming a molecular formula of [Zn·PyC·H₂O]·DMA·H₂O. Activation was done by soaking NUS-5 crystals into anhydrous methanol and evacuating under 100 °C to remove guest and solvent molecules trapped inside the channels. Powder X-ray diffraction (PXRD) patterns of the as-synthesized and the activated NUS-5 were collected (Fig. 2a). We note that some “extra peaks” were presented probably due to the decomposition of the crystal as a result of water instability, which will be discussed later. To understand the nature of “extra peaks”, LeBail refinements were performed (Fig. S1 and S2, ESI†). Details were included in ESI.† Thermogravimetric analysis (TGA) of both the as-synthesized and the activated samples was done under nitrogen atmosphere to evaluate their thermal stability (Fig. 2b). The thermograph of the as-synthesized crystal begins with a 5% weight loss before 100 °C, indicating the removal of free water in the channels. The weight loss between 100 and 400 °C roughly can be attributed to the removal of DMA and the bound water. The framework of NUS-5 does not decompose until the temperature reaches 450 °C, indicating a high thermal stability possibly due to the strong Zn–N bonds.^17,18

Fig. 2 (a) PXRD patterns of NUS-5; (b) TGA curves of NUS-5.

The permanent porosity of the activated NUS-5 was confirmed by N₂ sorption isotherm at 77 K (Fig. 3a). The Brunauer–Emmett–Teller (BET) surface area of NUS-5 was calculated to be 409.8 m² g⁻¹ (611.1 m² g⁻¹ based on Langmuir method), which is comparable to the surface area of other reported 2D MOFs.^19–21 The pore size distribution of NUS-5 calculated through non-local DFT method is centred at 7 and 12 Å agreeing well with the opening of single layer grid from single crystal model. It is interesting to note that the N₂ isotherm exhibits a combined type I/IV shape according to the IUPAC classification.²² Type I isotherm represents the microporous structure (pore size less than 2 nm) of NUS-5 which is confirmed by the pore size distribution. Type IV isotherms are typically associated with mesoporous structures (pore size between 2 and 50 nm) and are featured by a hysteresis between adsorption and desorption branches where the lower closure point that the desorption branch joins the adsorption branch is often located at the lower limit of P/P₀ = 0.42 for N₂ at 77 K regardless of adsorbents.²² However, the presented hysteresis loop of NUS-5 is surprisingly broad compared to a typical type IV isotherm resulting from mesoporous structures. Such a broad hysteresis may come from either structural dynamics^23–25 or interlayer ultra-micropores (pore size less than 5 Å) that can sterically hinder the diffusion of N₂ molecules under cryogenic conditions.^26–28 In order to further verify the former hypothesis, high pressure (up to 100 bar) N₂ and O₂ isotherms of NUS-5 were collected at 298 K. This approach of high pressure gas sorption under supercritical conditions has been adopted previously to study a 2D porous coordination polymer, where a gate opening effect was observed featured by an abrupt increase of gas uptake above a certain pressure threshold attributed to the expansion of inter-layer void.²⁹ The framework dynamics of a 3D MOF was also studied in a similar way.³⁰ In our case, however, the gate opening effect was not observed as both gases exhibit fully reversible Langmuir isotherms (Fig. 3b). Compared to the reported case of 2D MOF where the interlayer force is mainly π–π interactions, NUS-5 has a much stronger interlayer hydrogen bonding interactions that are harder to break with high pressure supercritical gases. Given the obtained information so far, the interlayer ultra-micropores might be the major reason for the wide hysteresis because the imposed steric hindrance can slow down N₂ diffusion under cryogenic conditions, although the possibility of framework dynamics cannot be completely ruled out.

Fig. 3 (a) N₂ sorption isotherms at 77 K with pore size distribution (imbedded); (b) high pressure N₂ sorption isotherms (blue) and O₂ sorption isotherms (red) at 298 K (adsorption, closed; desorption, open).

Carbon dioxide (CO₂) emission into atmosphere owing to escalating consumption of fossil fuels has become one of the greatest environmental concerns nowadays.^31,32 The urgent need to curb anthropogenic CO₂ emission is promoting the development of efficient carbon capture and sequestration (CCS) technologies.^33–35 Sorption-based post-combustion CO₂ capture is an important approach in CCS due to its low operation cost and footprint, where the key development relies on the finding of efficient adsorbent materials such as MOFs.^36,37 Inspired by the ultra-micropores of NUS-5 which may lead to selective gas sorption based on molecular sieving effect, we collected the N₂ and CO₂ sorption isotherms of NUS-5 under ambient conditions to evaluate its potential applicability in post-combustion CO₂ capture (Fig. 4a and b). The CO₂ uptake of NUS-5 at 298 K and 1 bar is 1.82 mmol g⁻¹, which is even higher than some 3D MOFs with higher surface areas such as MTV-MOF-5-EHI (1.75 mmol g⁻¹, 1176 cm³ g⁻¹),³⁸ IRMOF-11 (1.65 mmol g⁻¹, 2096 cm³ g⁻¹),³⁹ MOF-253 (1.41 mmol g⁻¹, 2160 cm³ g⁻¹),⁴⁰ etc. The CO₂/N₂ selectivity of NUS-5 calculated using the ideal adsorption solution theory (IAST)⁴¹ ranges from 34.8 to 45.9 (Fig. 4c), which is higher than some MOFs and ZIFs such as Ni-MOF-74 (30),⁴² ZIF-78 (30),⁴³ etc. The high CO₂ uptake and CO₂/N₂ selectivity of NUS-5 can be attributed to its ultra-micropores originating from interlayer void which can firmly accommodate CO₂ due to a better molecular size match⁴⁴ as well as the N-rich chemical structure having high affinity towards CO₂. The isosteric heat of adsorption (Q_st) for CO₂ was calculated to quantify the interaction between CO₂ and NUS-5. As can be seen from Fig. 4d, the low-coverage CO₂ Q_st of NUS-5 is −21.2 kJ mol⁻¹, which is comparable to that of other MOFs such as O-coordinated Zn₄(OH)₂(1,2,4-BTC)₂ (−20 kJ mol⁻¹)⁴⁵ and N-coordinated Zn₂(bpy)(TCM) (−20 kJ mol⁻¹).⁴⁶ However, it is surprising to note that the CO₂ Q_st of NUS-5 remains relatively constant over the full CO₂ loading range indicating homogeneous adsorption sites. This observation strongly supports the existence of ultra-micropores in which the interactions with CO₂ are mainly non-directional and can sustain over a large CO₂-uptake range.⁴⁴ The favourable interaction between CO₂ and NUS-5 over a wide CO₂ loading range is in the ideal adsorbate–adsorbent interaction range strong enough for efficient adsorption but also fully reversible, which enables easy regeneration of adsorbent. This important sorption property makes NUS-5 a very promising adsorbent material for use in post-combustion CO₂ capture operations employing pressure swing adsorption technique.⁴⁷

Fig. 4 CO₂ (triangle) and N₂ (circle) sorption isotherms at 273 K (a) and 298 K (b); (c) IAST selectivity of CO₂/N₂ at 298 K with a CO₂/N₂ molar ratio of 15 : 85; (d) isosteric heat of adsorption of CO₂.

To be a good adsorbent for post-combustion CO₂ capture in practice, the compound needs to be stable upon exposure to water. Unfortunately, NUS-5 did not show enough stability in this case. The sharp peaks of PXRD of the crystal disappeared after immersing the compound in water for 24 h. This is likely due to the facile hydrolysis of the Zn–O bonds in the material. Similar poor stability in water has been observed in many Zn-carboxylic based MOFs. New structural design to shield the Zn–O bonds from water attack would significantly enhance the stability of the material. We plan to address this issue in our future work.

Conclusions

In summary, a new 2D MOF named NUS-5 composed of Zn and bi-functional ligand with hybrid N and O coordination mode was synthesized and characterized. NUS-5 contains porous grid layers stacked together through interlayer hydrogen bonding interactions into a framework with interlayer ultra-micropores. It exhibits a high thermal stability up to 450 °C with preferential sorption towards CO₂ that can be used in post-combustion CO₂ capture.

Acknowledgements

H.C. acknowledges the support of a start-up grant from NUS, a Tier 1 grant from Singapore Ministry of Education, a POC grant from National Research Foundation of Singapore, a DSTA grant and the National Natural Science Foundation of China (no. 21233006 and no. 21473164). D.Z. acknowledges the support of National University of Singapore (NUS Start-up Funding R-279-000-369-133, CENGas R-261-508-001-646) and Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410-112, AcRF Tier 2 R-279-000-429-112).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1045563. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra04295j

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