Post-metalation of porous aromatic frameworks for highly efficient carbon capture from CO₂ + N₂ and CH₄ + N₂ mixtures

Abstract

The development of microporous materials for carbon capture, especially for carbon dioxide and methane, is a rapidly growing field based on the increasing demand for clean energy and pressing environmental concerns of global warming effected by greenhouse gases. To achieve this goal of developing carbon selective porous materials, a new porous aromatic framework featuring carboxyl-decorated pores, PAF-26-COOH, has been synthesized successfully. The modification of PAF-26 materials with representative light metals is exemplified by Li, Na, K and Mg via a post-metalation approach. The obtained PAF-26 products exhibit moderate surface area and controllable pore size at the atomic level. Gas sorption of CO₂, CH₄ and N₂ is carried out on as-prepared PAF-26 samples at mild temperatures (273 K and 298 K). It is found that the PAF-26 materials show high adsorption capacity for CO₂ and CH₄ and low ability toward N₂. Particularly, as-synthesized PAF-26 compounds exhibit remarkably high isosteric heats of adsorption toward CO₂ and CH₄, indicating high affinity for CO₂ and CH₄ gases. The gas selectivity for CO₂–N₂ and CH₄–N₂ mixtures is predicted by the IAST model. High selectivity of 80 for CO₂ over N₂ is obtained for PAF-26-COOMg. In addition, high selectivity values of CH₄ over N₂ are observed. The high performance including high storage capacity and selectivity makes PAF-26 materials promising for carbon capture or sequestration.

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Introduction

One of the most pressing environment concerns of our age is the escalating level of atmospheric greenhouse gases (mainly CO₂), which is deemed as a significant contribution to global warming.¹ Flue gas emissions of power plants are responsible for roughly 30% of total CO₂ emissions. Nitrogen is a main component (>70%) of flue gas whereas the major impurity is CO₂ (10–15%); thus it is required to separate CO₂ from N₂.² Another energy-related separation involves the purification of CH₄ from natural gas, since natural gas is mainly composed of valuable methane, typically 80–95%, with N₂ and CO₂ impurities of 5–10% depending on different gas reservoirs. In addition, landfill gas and coal-bed gas are rapidly growing as complements for natural gas; however, they often contain unacceptable levels of N₂ contaminants. The separation of methane from nitrogen is essential for upgradation of natural gas and purification of landfill and coal-bed gases to improve their purity in order to meet specific criteria. Thus, carbon capture and separation (CCS) is therefore highly demanded. Consequently, sorption-based techniques or processes (usually involving solid adsorbents) have been innovated for CCS, which plays a leading role among versatile approaches.³ Therefore, intensive efforts have been made to investigate the use of solid adsorbents for carbon capture and storage. In the library of solid adsorbents, porous materials, such as zeolites,⁴ carbons,⁵ mesoporous silica-supported amines,⁶ and metal organic frameworks,⁷ have exhibited good performances for practical CCS implementation. Very recently, porous organic frameworks (POFs) have also been proposed as new porous adsorbents for carbon capture and storage.⁸ POFs composed of light elements via robust covalent bonds are emerging as a new family of porous architectures and are attracting increasing attention thanks to their high stability, tunable building blocks, controlled pore connectivity, and featuring opportunities for further functionality.⁹ To achieve the goal of high-performance gas capture and storage with enhanced capacity and selectivity using POF materials, rational modification of POF skeletons with variable functionalities is deemed to be an appropriate approach.¹⁰ Zhou's group has reported that sulfonate-grafted and polyamine-tethered porous polymer networks exhibit exceptionally high binding affinity and selectivity for CO₂.^10a,11g Generally, gas storage in POFs is greatly related to the pore size and the polarity of the pore surface. Controlling the pore properties such as pore size and polarity in POFs is possible by changing the functional group in the organic monomer. Tailoring electrostatic interaction on the pore surface at the atomic level such as introducing different metal ions in the porous skeleton may be a possible way to fine-control the pore property. The incorporation of metal ions can introduce point charges into the host material framework. By changing the atomic numbers of the incorporated metals, we can tune the electrostatic charge-quadrupole and charge-induced dipole interactions with gas molecules on the pore surface.¹¹ In this study, we report the design and synthesis of a carboxyl-functionalized porous aromatic framework (PAF) material with permanent porosity, PAF-26-COOH, and a series of light metalized derivatives, referred to as PAF-26-COOM (M = Li, Na, K, Mg) (Scheme 1). The resulting PAF-26-COOM materials are examined for CH₄, CO₂ and N₂ sorption. Further, the effects of pore size, surface area and isosteric heat of adsorption on a group of PAF-26-COOM materials will be investigated in detail. Besides, the adsorption capacity and selectivity of these metalized PAF materials for CO₂ and CH₄ in CO₂–N₂ and CH₄–N₂ binary mixtures are estimated.

Scheme 1 Schematic representation of the synthesis of PAF-26-COOH and PAF-26-COOM using tetrakis(4-ethynylphenyl)methane and 2,5-dibromobenzoic acid.

Experimental section

1 Chemicals

The chemicals were purchased from Aldrich, Alfa-Aesar, and Aladdin-reagent, and used as received unless it was noted. N,N-Dimethylformamide (DMF) and triethylamine (Et₃N) were dehydrated with CaH₂. Magnesium methoxide solution (7–8%) in methanol was purchased from Alfa-Aesar. Tetrakis(4-ethynylphenyl)methane was prepared according to the previously reported method.^9f 2,5-Dibromo-benzoic acid was purchased from Aladdin-reagent.

2 Synthesis of PAF-26-COOH

PAF-26-COOH was synthesized via palladium-catalyzed Sonogashira–Hagihara cross-coupling reaction. Typically, tetrakis(4-ethynylphenyl)methane (250 mg, 0.6 mmol), 2,5-dibromobenzoic acid (335 mg, 1.2 mmol), tetrakis(triphenylphosphine)palladium (45 mg), and copper iodide (15 mg) were dissolved in a mixture of DMF (15 mL) and Et₃N (15 mL) in a 50 mL two-neck flask. After degassing via three freeze–pump–thaw cycles, the mixture was stirred at 100 °C for 36 h under a N₂ atmosphere. After cooling down to room temperature, the resulting PAF-26-COOH was collected by filtration, followed by consecutive washing with chloroform (10 mL), methanol (10 mL), and water (10 mL) to remove the unreacted monomers and catalysts. As-prepared PAF-26-COOH was further purified via Soxhlet extraction with methanol for 48 h. After drying at 90 °C in vacuum overnight, PAF-26-COOH was obtained in the form of a yellow powder (352 mg, 89% yield).

3 Post-metalation of PAF-26-COOH

3.1 Synthesis of PAF-26-COOLi, PAF-26-COONa and PAF-26-COOK. 10 mg LiOH was added to a mixture of PAF-26-COOH (100 mg) in CH₃OH–H₂O (10 mL/10 mL). The resulting mixture was stirred at room temperature for 1 day. Subsequently, the solid was collected by filtration, washed with water several times to completely remove LiOH residues, and dried to produce PAF-26-COOLi as a yellow powder (95 mg).

PAF-26-COONa and PAF-26-COOK were prepared by the same procedure using NaOH and KOH as alkaline sources, respectively.

3.2 Synthesis of PAF-26-COOMg. Magnesium methoxide (7–8% in methanol solution) is considered as the suitable alkaline source to overcome the poor solubility of Mg(OH)₂ in CH₃OH or water. Typically, 210 μL magnesium methoxide solution was added to a mixture of PAF-26-COOH (100 mg) and anhydrous methanol (15 mL) in a dry 50 mL flask. The resulting mixture was then stirred in a dry atmosphere at room temperature for 12 h. Then, the solid was collected by filtration and washed with anhydrous methanol several times. After drying at 90 °C in vacuum for 10 h, PAF-26-COOMg was obtained as a yellow powder (92 mg).

4 Characterizations

FTIR spectra were obtained using an IFS 66V/S Fourier transform infrared spectrometer. Thermogravimetric analysis was implemented using a Netzch Sta 449c thermal analyzer system at a heating rate of 10 °C min⁻¹ under air. Scanning electron microscopy (SEM) imaging was performed on a JEOS JSM 6700. X-ray diffraction (XRD) measurements were carried out on a Riguku D/MAX2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) operating at a voltage of 50 kV and a current of 200 mA. Solid-state ¹³C CP/MAS NMR measurement was carried out on a Bruker Avance III model 400 MHz NMR spectrometer at a MAS rate of 5 kHz. The metal content in metalized porous frameworks was determined by inductively coupled plasma (ICP) spectroscopy (Perkin-Elmer ICP-OES Optima 3300DV). The typical procedure includes: (1) the digestion of samples (∼15 mg) in 3 : 1 HCl–HNO₃ 10 mL solution and heating at 120 °C in a 15 mL autoclave for 12 h; (2) after cooling down to room temperature, the solution was filtered and transferred to a 50 mL volumetric flask with distilled water.

5 Low-pressure gas adsorption measurements

The gas sorption isotherms were measured on a Quantachrome Autosorb iQ2 analyzer. Prior to the measurements, the samples were degassed at 120 °C for 24 h. N₂ sorption isotherms were recorded at 77 K, 273 K, and 298 K; CH₄ and CO₂ sorption measurements were performed at 273 K and 298 K, respectively. Ultra-high-purity grade (99.99%) N₂, CH₄, and CO₂ gases were used for all adsorption measurements. Liquid nitrogen baths were utilized to control the temperature at 77 K. Ice-water and water baths equipped with a temperature sensor were used to control the temperature at 273 K and 298 K.

Results and discussion

Description of PAF-26-COOH and PAF-26-COOM

To build a porous architecture, secondary building blocks with tetrahedral geometry are employed to form 3-D porous organic frameworks via their co-condensation with linear building blocks. Additionally, the functional groups on the building blocks and their connection modes determine the physicochemical properties of organic frameworks. Using this concept, PAF-26-COOH is prepared by the condensation of tetrakis(4-ethynylphenyl)methane as the tetrahedral node, and 2,5-dibromo-benzoic acid as the linear linker via Sonogashira–Hagihara cross-coupling reaction (Scheme 1).¹² Further, PAF-26-COOH bearing free carboxylic groups in the skeleton is post-modified with different light metals, and the corresponding products are designated as PAF-26-COOM. For better illustration of the connectivity and geometry of carbon atoms in the PAF-26-COOH structure, this PAF material was characterized by solid-state ¹³C cross-polarization magic-angle spinning (CP/MAS) NMR and Fourier transform infrared (FTIR) spectroscopy. Fig. 1 shows the ¹³C CP/MAS NMR spectrum of a PAF-26-COOH solid sample. An NMR signal around 168 ppm is observed, which is assigned to the carbon atom in the carboxyl groups. This means that the carboxyl groups remain intact, which makes further modification possible. A well-resolved peak is detected at 64.8 ppm associated with the quaternary carbon atom in tetrakis(4-ethynylphenyl)methane, indicating that tetrakis(4-ethynylphenyl)methane is the main component in the final PAF-26-COOH product.¹³ A distinct NMR shift assigned to the benzene rings at 131.1 ppm occurs, which indicates that the PAF-26-COOH framework is composed of highly conjugated phenyl groups.

Fig. 1 ¹³C CP/MAS NMR spectrum of PAF-26-COOH.

The PAF-26-COOH sample is further studied by FT-IR spectroscopy. The IR spectra of as-prepared PAF-26-COOH and their starting monomers are shown in Fig. 2. The disappearance of the peak at 3283 cm⁻¹ associated with ≡C–H in the PAF-26-COOH product (Fig. 2c) provides direct evidence for the completed cross-coupling reaction. The IR bands corresponding to the stretching vibrations of the –COO group (1728 and 3431 cm⁻¹) in the PAF-26-COOH sample are similar to those of its monomer 2,5-dibromo-benzoic acid (Fig. 2a), which means that –COO groups are kept unchanged during the reaction. The band related to the C–Br vibration mode at 1153 cm⁻¹ (Fig. 2a) vanishes into the background, which gives distinct proof for the complete reaction between these two monomers (Fig. 2c). The metalized derivatives PAF-26-COOM were also characterized by IR spectroscopy and their IR spectra are recorded in Fig. 3. Shoulder bands around 1683 cm⁻¹, 1683 cm⁻¹, 1681 cm⁻¹ and 1685 cm⁻¹ for PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg samples grow conjointly with the ones at 1722 cm⁻¹, 1721 cm⁻¹, 1720 cm⁻¹, and 1724 cm⁻¹, respectively. These additional bands are assigned to the –COO groups linked to metal cations, which are not observed in the case of PAF-26-COOH. The metal loading in PAF-26-COOM was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). It is found that the protons in the carboxyl groups of PAF-26-COOH are almost stoichiometrically substituted by metal ions (Table 1), affording the corresponding PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg products. This high replacement of protons (about 85%) confirms that nearly all the carboxyl groups in the PAF-26-COOH framework are available for post-metallization, which is in agreement with the IR result. The isolated PAF-26-COOH and its corresponding derivatives PAF-26-COOM powders were subjected to XRD and SEM characterizations. Based on the XRD study (Fig. S1†), all of them exist as amorphous polymers. Spherical aggregates with small particles in the size of 3–5 μm are observed in SEM pictures (Fig. S2†). The thermal stability of PAF26s is studied by TGA (Fig. S3†). A distinct weight loss in the range of 300–400 °C is observed, which corresponds to the decomposition or collapse of the organic skeleton.

Fig. 2 FT-IR spectra of starting materials 2,5-dibromobenzoic acid (a), tetrakis(4-ethynylphenyl)methane (b) and as-prepared PAF-26-COOH (c).

Fig. 3 FT-IR spectra of (a) PAF-26-COOH, (b) PAF-26-COOK, (c) PAF-26-COONa, (d) PAF-26-COOMg, (e) PAF-26-COOLi.

Table 1 Results from chemical analysis (ICP) and N₂-sorption measurements for PAF-26 samples including metal loadings, BET surface areas, pore volumes and their ratio, and pore sizes

Samples	Theoretical metal loading (mmol g^−1)	Actual metal loading^a (mmol g^−1)	BET SSA^b (m^2 g^−1)	V micro (cm^3 g^−1)	V total (cm^3 g^−1)	V micro/Vtotal	Pore size^d (nm)
a The actual metal loading was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). b BET SSA was calculated in the partial pressure (p/p0) range of 0.01–0.05 which gives the best linearity. c Total pore volume at relative pressure p/p0 = 0.99. d The pore size distribution calculated using the DFT method.
PAF-26-COOH		—	717	0.31	0.36	0.86	0.52
PAF-26-COOLi	3	2.45	591	0.24	0.28	0.86	0.52
PAF-26-COONa	3	2.6	483	0.20	0.23	0.87	0.49
PAF-26-COOK	3	2.4	430	0.19	0.20	0.95	0.48
PAF-26-COOMg	1.5	1.3	572	0.23	0.28	0.82	0.57

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Porosity of PAF-26-COOH and PAF-26-COOM

The porosity and pore structures of the PAF-26 series were investigated by nitrogen sorption measurement at 77 K. As shown in Fig. 4, a sharp increase in gas uptake is observed at low pressure in the nitrogen adsorption–desorption isotherms of PAF-26-COOH and PAF-26-COOM, which confirms the existence of micropores in PAFs. A weak hysteretic shoulder in the desorption isotherms proves that the materials also feature a small degree of mesoporosity. For clear illustration, a series of pore parameters derived from the nitrogen isotherms are listed in Table 1, including Brunauer–Emmett–Teller (BET) apparent surface area, pore size, total pore volume and micropore volume. The surface area of PAF-26-COOM is a bit lower than that of PAF-26-COOH (717 m² g⁻¹ for PAF-26-COOH, 591 m² g⁻¹ for PAF-26-COOLi, 483 m² g⁻¹ for PAF-26-COONa, 430 m² g⁻¹ for PAF-26-COOK and 572 m² g⁻¹ for PAF-26-COOMg, respectively). The replacement of hydrogen atoms with metal ions leads to a decrease of surface area, which is consistent with an increase of atomic weights from Li to K.

Fig. 4 N₂ adsorption–desorption isotherms measured at 77 K for PAF-26-COOH, PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg; the adsorption branch is denoted by closed symbols, and desorption branch by open symbols.

Based on the non-local density functional theory (NLDFT), the pore size distribution (PSD) of PAF-26-COOH exhibits a dominant pore diameter of 0.52 nm, while the PSD of PAF-26-COOM exhibits dominant ones of about 0.52, 0.49, 0.48 and 0.57 nm, for PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg, respectively (Table 1 and Fig. S4†). The decreasing trend in the pore size of PAF-26-COOLi, PAF-26-COONa, and PAF-26-COOK samples can be found in comparison with that of the parent sample PAF-26-COOH, which can be explained by the increased ionic radius of Li⁺ (90 pm), Na⁺ (116 pm), and K⁺ (152 pm). The pore size of PAF-26-COOMg is enlarged by exchanging protons with Mg²⁺ ions. This phenomenon can be interpreted with a first cleavage of hydrogen bonds between neighboring carboxylic groups, and further interaction with the Mg²⁺ ion. Additionally, the pores in PAF-26-COOH and PAF-26-COOM are very uniform in the microporous range, which is evidenced by the narrow pore size distribution (Fig. S4†) and high micropore volume ratio (∼90%, Table 1).

Gas adsorption

With the successful preparation of PAF-26 polymers with high porosity, fully available functional groups and a pore size around 0.5 nm, PAF-26-COOH and PAF-26-COOM materials were evaluated for gas adsorption or capture. Typically, the adsorption properties of PAF materials for CO₂, CH₄, and N₂ were studied by gas adsorption measurements, of which CO₂ and CH₄ are two main carbon sources in flue gas, natural gas or landfill gas. Fig. 5 displays CO₂ and CH₄ adsorption isotherms recorded at 273 K. There is a steep uptake at low relative pressures followed by continuous sorption of CO₂ upon increasing the pressure (Fig. 5a). CO₂ uptake for the PAF-26-COOH sample is 52 cm³ g⁻¹ at P = 101 kPa. With the substitution of protons by metal ions, improved sorption ability for CO₂ is attained (Fig. 5a). For a better understanding of the effect of different metal ions on the capture performance, the gas uptakes were normalized by the pore volume (V_total) to assess the sorption capacities of different adsorbents.^5f The uptakes per effective V_totalversus pressure for all PAF-26 materials toward CO₂ are plotted in Fig. 5b. A distinct enhancement is observed in the uptake per effective V_total after metallization with Li⁺, Na⁺, K⁺ and Mg²⁺ if the proton form of the PAF-26-COOH material is taken as a reference (Fig. 5b). PAF-26-COOK can adsorb 572 mg CO₂ per cm³, which is much higher than that of its parent PAF-26-COOH (286 mg cm⁻³). This sheds light on the improved adsorption affinity of PAF-26-COOM toward CO₂. In the same manner, methane adsorption was also carried out. More than 16 cm³ g⁻¹ of methane is adsorbed in PAF-26 samples at 273 K and 101 kPa (Fig. 5c). The dependence of uptakes per effective V_total on pressure for the PAF-26 series towards CH₄ is also calculated and shown in Fig. 5d. It can be found that CH₄ uptake per effective V_total increases from 34 mg cm⁻³ (PAF-26-COOH) to 46 mg cm⁻³ (PAF-26-COOLi, 35% increase), 54 mg cm⁻³ (PAF-26-COOMg, 59% increase), 56 mg cm⁻³ (PAF-26-COONa, 65% increase) and 60 mg cm⁻³ (PAF-26-COOK, 76% increase). Based on the observations above, it can be concluded that introduction of metal active centers would greatly promote gas adsorption capacity (especially for CO₂ and CH₄) and discriminate the adsorption affinity toward specific gases. To probe the influence of water on CO₂ uptake, the pretreated PAF-26-COOMg samples were exposed to wet air (the average relative humidity is about 50%) for 3 days. As indicated in Fig. S8†, CO₂ uptake of the humidified sample represents a 38% drop relative to the outgassed network. This behavior is similar to –OH containing POPs under humid conditions, which also preferentially adsorb water molecules.¹⁴ The cyclic CO₂ adsorption–desorption study is also carried out. As shown in Fig. S8†, the CO₂ uptake of PAF-26-COOMg remains almost constant after exposure to air for more than 4 months, which implies that metalized PAFs are quite stable.

Fig. 5 Gas sorption isotherms of CO₂ (a), CO₂ uptakes per effective V_total (b), CH₄ (c), CH₄ uptakes per effective V_total (d) for PAF-26-COOH, PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg samples at 273 K and 101 kPa.

As we know, the isosteric heat of adsorption (Q_st) is another index to estimate the affinity of an adsorbate to an adsorbent,¹⁵ which is helpful to understand the relationship between apparent sorption capacity and metal ion modified frameworks. Based on the adsorption isotherms at different temperatures (273 K and 298 K, Fig. S5 and S6†), the isosteric heats of adsorption of CO₂ and CH₄ can be determined for each material according to the Clausius–Clapeyron equation (the calculation is detailed in the ESI†). As shown in Fig. 6a, the Q_st values of CO₂ for PAF-26-COOM significantly increased compared with its counterpart PAF-26-COOH. Typically, the CO₂Q_st of PAF-26-COOH at low uptake is 28.1 kJ mol⁻¹ (Fig. 6a and Table 2), while the CO₂Q_st for PAF-26-COONa reaches 35.0 kJ mol⁻¹ and notably high Q_st values for other metalized materials are also obtained (31.8 kJ mol⁻¹, 32.6 kJ mol⁻¹ and 30.0 kJ mol⁻¹ for PAF-26-COOLi, PAF-26-COOK and PAF-26-COOMg, respectively). The present data from adsorption measurements for iso-structural PAF-26 materials with exchanged metal ions show a strong influence of metal sites on isosteric heats of adsorption. More interestingly, there is a decreasing trend in CO₂Q_st values of 35.0, 32.6, 31.8, 30.0 kJ mol⁻¹ with the sequence PAF-26-COONa, PAF-26-COOK, PAF-26-COOLi and PAF-26-COOMg, which coincides with their corresponding basicities originating from compensated alkali or alkaline earth ions (pK_b for PAF-26-COONa, PAF-26-COOK, PAF-26-COOLi, and PAF-26-COOMg are 37.96, 36, 33.95, and 15.08). Because there is less –COOH in the structural unit, the CO₂Q_st of PAF-26-COOH (28.1 kJ mol⁻¹) is lower than that of CMP-1-COOH (33 kJ mol⁻¹).^12c Zhou's group reported a sulfonate-grafted PPN–SO₃H with a CO₂Q_st value of 30.4 kJ mol⁻¹. After lithiation, the CO₂Q_st of PPN-6-SO₃Li reached 35.7 kJ mol⁻¹.^11g In this case, after metalization with Na⁺, the CO₂Q_st of PAF-26-COONa is up to 35 kJ mol⁻¹, which is comparable to PPN-6-SO₃Li and Li doped MOFs.^11h Nevertheless, these Q_st values of PAF-26-COOM are among those of many state-of-the-art excellent porous materials, which can be clearly seen in Table S2.†

Fig. 6 The isosteric heats of adsorption for CO₂ (a) and CH₄ (b) of PAF-26-COOH, PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg as a function of gas uptake.

Table 2 Isosteric heats of adsorption for CO₂ and CH₄, and selectivities of CO₂–N₂ and CH₄–N₂ for PAF-26 materials

Samples	Q st for CO2	Q st for CH4	Selectivity for CO2–N2^a	Selectivity for CO2–CH4^a	Selectivity for CH4–N2^a
a Selectivity was calculated by IAST at 298 K and 110 kPa.
PAF-26-COOH	28.1	14.3	20	4	4.2
PAF-26-COOLi	31.8	16.5	24	5.6	4.4
PAF-26-COONa	35.0	24.0	53	9	5.0
PAF-26-COOK	32.6	23.0	50	8.6	5.8
PAF-26-COOMg	30.0	21.5	73	8.4	6.5

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Parallel Q_st determination of CH₄ for PAF-26 materials was also carried out. The isosteric heats of adsorption for all measured materials are shown in Fig. 6b as a function of CH₄ gas uptake. As clearly seen in Fig. 6b and Table 2, the initial CH₄Q_st values for PAF-26-COOLi, PAF-26-COOMg, PAF-26-COOK and PAF-26-COONa are 16.5, 21.5, 23.0, and 24.0 kJ mol⁻¹, which are remarkably higher than that of PAF-26-COOH (14.3 kJ mol⁻¹). In general, a high isosteric heat of adsorption could lead to high gas storage at low pressures, which has already been exemplified by the adsorption measurements (Fig. 5). Notably, the high Q_st of PAF-26-COOM for CH₄ (16–24 kJ mol⁻¹) guarantees PAF-26 materials to be ranked in the top level of porous materials as can be seen from other outstanding porous materials, such as MIL-53 (18.0–19.0 kJ mol⁻¹),¹⁶ HKUST-1 (16.6–20.7 kJ mol⁻¹),¹⁷ MOF-5 (12.2 kJ mol⁻¹),¹⁸ and BILP-1 to BILP-7 (13–18.4 kJ mol⁻¹).¹⁹ Furthermore, higher Q_st values for PAF-26-COOM provide evidence for the stronger interaction between CH₄ molecules and PAF-26-COOM frameworks in contrast to PAF-26-COOH. Based on the results above, the possible reasons for the strong interaction between PAF-26-COOM materials and CH₄ gas molecules can be explained as follows: on one hand, more charges on metal centers may facilitate polarization of the CH₄ molecule and result in a stronger binding force. On the other hand, pore effect has to be considered. Small pores would result in an overlap of the van der Waals forces of neighboring atoms, which gives rise to a strong interaction.²⁰ From the adsorption observations, one can conclude that PAF-26 materials with permanent small pores and high isosteric heats of adsorption have been synthesized.

Gas selectivity studies

The capture of CO₂ and CH₄ carbon resources is very important in energy conservation. For practical application, the separation of CO₂ from N₂ in flue gas is critical to reduce global greenhouse effect. Upgradation of landfill gas or coal-bed gas is demanded to extract CH₄ from N₂. High adsorption capacities of PAF-26 materials toward CO₂ and CH₄ are demonstrated, while a very small uptake of 3 cm³ g⁻¹ is observed for N₂ at 101 kPa and 298 K (Fig. S7†). Adsorption selectivity is another key parameter in the pressure swing adsorption separation process with porous solid materials toward gases especially for carbon-rich CO₂ and CH₄, which are promising for carbon storage or capture in the future.

In this respect, the selectivity in the capture of CO₂ and CH₄ from CO₂–N₂, CH₄–N₂ gas mixtures is evaluated by the ideal adsorption solution theory (IAST), which is well recognized and employed to predict gas mixture adsorption behaviors in porous materials.²¹ As a result, selectivity studies of theoretical gas mixtures of CO₂–N₂, CH₄–N₂ were conducted using the IAST model utilizing single-component isotherms.

Based on single gas adsorption isotherms using the IAST model, the selectivity of CH₄ over N₂ (1 : 1 in volume) for PAF-26 materials according to the experimental data is calculated and the results are shown in Fig. 7a. The obtained values of the resulting selectivity of CH₄ over N₂ are 4.2, 4.4, 5.5, 5.8 and 6.5 for PAF-26-COOH, PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg, respectively (Table 2). There is no loss in CH₄ selectivity for these four samples in the whole test pressure range (0–110 kPa). As seen in Table 2, the selectivity of CH₄ over N₂ for PAF-26-COOM is higher than that of PAF-26-COOH (4.2), which provides supporting evidence for the enhanced affinity to CH₄ post metalation. The selectivity of all PAF-26 iso-structural materials (PAF-26-COOH and its light metalized derivatives, PAF-26-COOM) for CH₄ over N₂ surpasses the previously reported porous adsorbents, especially for PAF-26-COOMg with an extraordinarily high value of 6.5, ranking PAF-26-COOMg among the best porous adsorbents for separating CH₄ from N₂ (Table S2†).

Fig. 7 IAST-predicted adsorption selectivity of CH₄–N₂ (a) and CO₂–N₂ (b) and CO₂–CH₄ (c) at 298 K and 110 kPa for PAF-26-COOH, PAF-26-COOLi, PAF-26-COONa, PAF-26-COOK and PAF-26-COOMg.

The calculated adsorption selectivity for 15% CO₂ and 85% N₂ mixtures in PAF-26 materials as a function of bulk pressure is presented in Fig. 7b. After metalation, the selectivity of PAF-26-COOM increases significantly in comparison with PAF-26-COOH. At 110 kPa, the selectivity of CO₂ over N₂ in PAF-26-COOH, PAF-26-COOLi, PAF-26-COONa and PAF-26-COOK is estimated to be 20, 24, 50 and 53 in the plateau (Fig. 7b). Exceptionally in the case of PAF-26-COOMg, the selectivity of CO₂ over N₂ reaches 80 under 110 kPa. The excellent CO₂ selectivity in the case of PAF-26-COOMg may be explained by progressive CO₂ adsorption onto the active sites of high-valence magnesium under ambient pressure. The CO₂–N₂ selectivities of PAFs are lower than the N₂-phobic azo-COPs reported by Patel et al., which have the highest CO₂–N₂ selectivity (288) at 323 K.^8f The selectivity of CO₂ over CH₄ (1 : 1 in volume) is also examined as it involves the purification of CH₄ from natural gas. As shown in Fig. 7c, the CO₂–CH₄ selectivity of PAF-26-COOM is also increased in comparison with that of PAF-26-COOH. The selectivity of PAF-26-COONa (9) at 110 kPa is about two times more than that of PAF-26-COOH (4). Combined with adsorption and selectivity studies, one can conclude that the carboxyl-functionalized and post-metalation PAF materials integrate two important characteristics of high sorption capacity and high selectivity. Such PAF materials may be an excellent candidate for carbon capture or storage.

Conclusions

In summary, we have successfully designed and synthesized a carboxyl-functionalized PAF material, PAF-26-COOH. Post-metalation of PAF-26-COOH yields a series of PAF-26-COOM derivatives (M = Li, Na, K, Mg). The structure, morphology and porosity of PAF-26 products are comprehensively investigated by IR, NMR, XRD, SEM and N₂ adsorption measurements. Microporous PAF-26 materials with high surface area, high porosity and tunable pore size are achieved via a post-metalation method. High adsorption capacities toward CO₂ and CH₄ are demonstrated with as-prepared PAF-26 materials, while very low uptake for N₂ is measured with the same samples. Additionally, an enhanced adsorption for CO₂ and CH₄ is observed for PAF-26-COOM in comparison with PAF-26-COOH, which indicates that introduction of light metal ions would improve the adsorption affinity to CO₂ and CH₄ gases. Further, PAF-26 materials exhibit very high isosteric heats of adsorption toward CO₂ and CH₄. Interestingly, the isosteric heats of adsorption for CO₂ follow a sequence of PAF-26-COOMg < PAF-26-COOLi < PAF-26-COOK < PAF-26-COONa, which matches well with their corresponding basicities. The stronger interaction between CH₄ molecules and PAF-26-COOM is verified by their higher isosteric heats of adsorption, which is due to the higher polarity of metal ions than that of protons and the small-pore effect. The selective adsorption toward CO₂ and CH₄ over N₂ is studied and the predicted selectivity is calculated by the IAST model derived from their adsorption isotherms. All measured PAF-26 samples show high selectivity for CO₂ over N₂ with values of 20–80 and significantly high selectivity for CH₄ over N₂ with a highest value of 6.5. The combined merits of high sorption capacity and high selectivity make PAF-26 a potential stepping stone to porous materials for carbon capture and storage.

Acknowledgements

We are grateful for the financial support from the National Basic Research Program of China (973 Program, grant nos 2012CB821700), Major International (Regional) Joint Research Project of NSFC (grant nos 21120102034), NSFC (grant nos 20831002, 21201074) and Australian Research Council Future Fellowship (FT100101059).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures of PAF-26-COOM (IR, XRD, SEM), pore size distribution, calculation of Q_st, basicity and Ideal Adsorption Solution Theory (IAST) calculations. See DOI: 10.1039/c3py00647f

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