Enhanced CO₂ adsorption on Al-MIL-53 by introducing hydroxyl groups into the framework

Abstract

A series of hydroxyl functionalized Al-MIL-53 materials (Al-MIL-53-OH_x) containing varying hydroxyl molar ratios (x = 25%, 50%, 75%, and 100%) were synthesized via a mixed-linker approach, wherein x denotes the molar ratio of 2,5-dihydroxy terephthalic acid:(2,5-dihydroxy terephthalic acid + terephthalic acid). All Al-MIL-53-OHs exhibited an identical structure to that of Al-MIL-53. The thermal stability of Al-MIL-53 decreased after introducing hydroxyl groups. The hydroxyl functionalized Al-MIL-53 containing 25 mol% and 50 mol% of hydroxyl group showed higher surface areas (S_BET = 1270 and 1260 m² g⁻¹ for Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀, respectively) than that of Al-MIL-53 (S_BET = 819 m² g⁻¹). A further increase in the OH groups (75 mol% and 100 mol%) led to dramatical compromise of the framework. The presence of hydroxyl groups affected not only the CO₂ adsorption capability but also the ‘breathing effect’ of MIL-53 resulting from the intraframework interaction. The CO₂ adsorption capacities of Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀ at 1 bar at 25 °C were 8.5 and 8.3 wt%, respectively, which are about 19% higher than that of Al-MIL-53 under the identical conditions. Moreover, pronounced improvement in CO₂ adsorption was observed below 0.2 bar, especially for Al-MIL-53-OH₂₅ (5.5 wt% for Al-MIL-53-OH₂₅ vs. 1.7 wt% for Al-MIL-53). This behavior is due likely to the enhanced isosteric heat of CO₂ adsorption. The hydroxyl group plays a positive role in the CO₂ adsorption performance of Al-MIL-53, which is comparable to amino groups. Al-MIL-53-OHx (x = 75 and 100) displayed lower CO₂ adsorption capacities despite the higher isosteric heat of CO₂ adsorption, which might be due to the blocked pores in the presence of dense hydroxyl groups.

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1. Introduction

Elevated CO₂ emission has become one of the highest environmental abatements worldwide. It is imperative to develop effective ways to migrate the intense saturate. Recently, using porous solids for CO₂ capture have been considered as a promising and effective strategy due to their favorable adsorption capacity, high thermal stability and low cost of regeneration compared to conventional amine solutions.^1–3

Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs) or coordination networks, has demonstrated their enormous potential as an adsorbent for CO₂ storage and separation because of their recorded surface areas, adjustable porosity, moderate CO₂ affinity and suitable adsorption kinetics.^4–7 Some MOFs have shown higher saturated CO₂ capacities at room temperature than those of traditional zeolites.^8–11 For example, MOF-177 (Zn₄O₃(H₃BTB) H₃BTB = 4,4′,4′′-benzene-1,3,5-triyl-tribenzoic acid) with a high surface area of 4508 m² g⁻¹ has an unprecedented CO₂ capacity of 59.6 wt% at 25 °C and 35 bar.⁹ MOF-210 with a BET surface area of 6240 m² g⁻¹ and a pore volume of 3.60 cm³ g⁻¹ shows a new record CO₂ capacity of 70.6 wt% at 50 bar and 25 °C.¹¹ Unfortunately, these MOFs give poor CO₂ adsorption capacities (<11 wt%) at low pressure range (<1 bar) due to the weak interaction between the framework and CO₂ molecule.¹²

Because of the low CO₂ partial pressure in flue gases (∼0.15 bar), it is of practical importance to understand both the CO₂ adsorption mechanism and improve the CO₂ uptake in MOF materials in the low pressure region for their applications as novel adsorbents to remove CO₂ from flue gases. Some effective strategies have been employed such as introducing open metal sites and surface modification by functional groups.^13–18 The introduction of open metal sites can usually be realized by the removal of weakly coordinating solvents. For example, Mg-MOF-74 (Mg₂(dobdc), dobdc = 2,5-dihyroxyldicarboxylate) represents the highest low-pressure gravimetric (20.5 wt% at 0.15 bar) and volumetric adsorption capacity for CO₂ despite its relatively low surface area (S_BET = 1495 m² g⁻¹). The excellent CO₂ adsorption on Mg-MOF-74 has been explained by a high isosteric heat of CO₂ adsorption (47 kJ mol⁻¹).^13,14 However, decomposition of the frameworks or coordination geometry transformation might happen along with the removal of coordinating solvents. Modification of the pore surface by functional groups, e.g. –NO₂, –NH₂, –CH₃, may tune the polarity and acidity of the porous environment, resulting in an enhancement of the affinity towards CO₂ in some MOFs.^19–21 Amines has been proposed to be one of the most promising functional groups due to the strong interaction between CO₂ and the Lewis basicity of amine.^22–24 Amine functionalized Cr-MIL-101 showed a CO₂ adsorption capacity of up to 40 wt% at 16 °C at 25 bar.²⁴

Similar to amine, hydroxyl group (–OH) is another functional group wherein the oxygen atom serves as an electron-donor center, obtaining dipolar or quadrupolar interactions between the hydroxyl group and CO₂ molecules. Early computational research has demonstrated that the hydroxyl group may enhance the isosteric heat of CO₂ adsorption.^21,25 This enhancing effect has been reported for Zn(bdc)(ted)_0.5 modified with –OH, which showed a 77% higher CO₂ uptake than the parent structure.²⁶ In another study, Biswas and co-workers²⁷ synthesized hydroxyl modified Al-MIL-53 by employing 2,5-dihydroxyl terephthalic acid as the sole ligands in the initial reaction solution. Compared to unmodified Al-MIL-53, a lower CO₂ adsorption capacity was observed for this hydroxyl modified Al-MIL-53, most likely due to the poor porosity. Therefore, it will be desirable to prepare hydroxyl groups containing materials with excellent porosity.

Herein, we introduced various contents of the hydroxyl groups into the framework of Al-MIL-53 by a mixed-linkers strategy and investigated the CO₂ adsorption performance of Al-MIL-53 containing different hydroxyl contents. The results have demonstrated that low hydroxyl contents played a positive role in the low pressure CO₂ adsorption capacity of Al-MIL-53, but high hydroxyl contents did reduce the CO₂ adsorption capacity of Al-MIL-53 due to the poor porosity.

2. Experimental

2.1 Chemicals

AlCl₃·6H₂O, terephthalic acid (H₂BDC, >97%, Alfa-Aesar), 2,5-dihydroxylteraphthalic acid (H₂BDC–(OH)₂, >97%), and N,N′-dimethyformamide (98%) were used as received.

2.2 Synthesis of Al-MIL-53s modified with varied molar percentage of hydroxyl groups

Hydroxyl modified AL-MIL-53s were synthesized according to the literature.²⁷ In a typical procedure, 0.966 g (4 mmol) of AlCl₃·6H₂O, 0.199 g (1 mmol) of 2,5-dihydroxy terephthalic acid and 0.498 g (3 mmol) of terephthalic acid were dissolved in 10 mL of DMF. The mixture was heated at 125 °C for 8 h. The product was filtered and washed with DMF. The yellow raw product was then dispersed in 400 mL of a solution of methanol/water (50/50, v/v). The mixture was heated at 100 °C for 12 h and followed by filtering, washing and drying. The resultant was labelled Al-MIL-53-OH₂₅.

The sample synthesized in the molar ratios of 2,5-dihydroxy terephthalic acid to terephthalic acid (1 : 1, 3 : 1 and 4 : 0) in the initial reaction solution were labelled Al-MIL-53-OH_x (x = 50, 75 and 100).

2.3 Characterization

Powder X-ray diffraction (PXRD) patterns of all samples were recorded on a Rigaku-Ultima diffractometer using a Cu Kα radiation source (λ = 0.15432 nm) in the 2θ range from 5° to 80°. Fourier Transform Infrared (FT-IR) spectra were obtained on a Nicolet Fourier transform infrared spectrometer (NEXUS 670). Thermogravimetric analysis (TGA) was performed on a PerkinElmer Pyris Diamond Thermogravimetric Differential Thermal/Analyzer. The samples were heated from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ under an air flow. The pore textural properties, including BET surface area and pore volume, were recorded on a Micromeritics Tristar 3000 adsorption analyzer at −196 °C. Prior to the adsorption measurements, the samples were degassed in situ under vacuum at 150 °C for 10 h. The dead volume of the sample cell was determined in a separate experiment. The weight of a sample obtained after pretreatment was used in various calculations. The BET surface areas were calculated in the adapted pressure range of P/P₀ = 0.01–0.1. Scanning electron microscopy (SEM) was performed on the Au coated samples by a Hitachi S-4800 at 10 kV. CO₂ adsorption measurements were also performed on Micromeritics Tristar 3000 adsorption analyzer using CO₂ (99.999%) without any balanced gas at 8 or 25 °C. Each sample was pre-treated under high vacuum (10⁻⁶ mbar) at 150 °C overnight prior to the CO₂ adsorption measurements.

3. Results and discussion

3.1 PXRD, FT-IR, TG, and N₂ adsorption of Al-MIL-53-OH_x

Fig. 1 shows the PXRD patterns of all hydroxyl-modified Al-MIL-53-OH_x. Al-MIL-53 was synthesized as a reference under similar synthetic conditions and its PXRD pattern is also shown in Fig. 1. It is clear that all Al-MIL-53-OH_x materials gave similar PXRD patterns to that of Al-MIL-53, which indicates that the hydroxyl-modified Al-MIL-53s have been synthesized successfully and they exhibit the same topology as Al-MIL-53. Interestingly, compared to unmodified Al-MIL-53, all diffraction peaks shifted to lower angles with increasing hydroxyl content, indicating that the introduction of hydroxyl groups results in an increase in the lattice parameters.

Fig. 1 PXRD patterns of hydroxyl modified Al-MIL-53s and Al-MIL-53.

The FT-IR spectra of Al-MIL-53 and the hydroxyl-modified Al-MIL-53s are shown in Fig. 2. In the spectrum of Al-MIL-53, the bands at 1589 cm⁻¹ and 1410 cm⁻¹ are attributed to the carboxylate (COO–) asymmetric and symmetric stretching vibrations of terephthalate (BDC). With increasing 2,5-dihydroxylterephthalate content, a shoulder peak at 1603 cm⁻¹ appears, which might be associated with the perturbance of hydroxyl groups (as observed in the case of Al-MIL-53-OH₁₀₀). In addition, characteristic band centered at 1240 cm⁻¹ was observed due to the vibration of the C–OH (Ar-OH) groups. The intensity of this peak increased with increasing ratio of 2,5-dihydroxy terephthalic acid in the synthetic solution. We have roughly calculated the areas of two vibration regions for all samples, such as, 1200–1270 and 1550–1680 cm⁻¹, which were assigned to the Ar-OH and the COO⁻, respectively. The results in Table S1† clearly indicate that the content of hydroxyl groups in each sample is very close to the theoretical value.

Fig. 2 FTIR spectra of hydroxyl modified Al-MIL-53s and Al-MIL-53.

Fig. 3 shows TGA curves of Al-MIL-53 and the hydroxyl-modified Al-MIL-53s. Two pronounced weight losses were observed in the TG curves of all samples. All samples showed a weight loss of ∼8 wt% below 100 °C, which was assigned to the release of the solvents (water and methanol). The second weight loss of ∼57.7 wt% was found for unmodified Al-MIL-53, which is in good agreement with the reported value.²⁸ Moreover, the second weight loss increased with increasing hydroxyl content introduced in the Al-MIL-53 framework. The weight losses are 68.6 wt% for Al-MIL-53-OH₂₅, 69.5 wt% for Al-MIL-53-OH₅₀, 70.7 wt% for Al-MIL-53-OH₇₅ and 71.1 wt% for Al-MIL-53-OH₁₀₀. In addition, the decomposition temperature decreased with increasing hydroxyl content introduced in Al-MIL-53. These results indicate that the incorporation of hydroxyl groups reduces the thermal stability of Al-MIL-53, which may be explained by the weakening effect of hydroxyl groups on the coordination strength between Al³⁺ and carboxyl groups. The higher the hydroxyl content is, the poorer the thermal stability is. No weight loss related to the release of DMF was found, revealing that the DMF guests have been removed completely by the heating treatment.

Fig. 3 TGA curves of hydroxyl modified Al-MIL-53s and Al-MIL-53.

The N₂ adsorption isotherms of Al-MIL-53 and the hydroxyl-modified Al-MIL-53s are shown in Fig. 4. It is clear that Al-MIL-53, Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀ display typical type-I isotherms, indicating their microporosity. The amount of N₂ adsorbed by Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀ are comparable and higher than that of Al-MIL-53 below 0.95 bar. However, the first step in the isotherm of Al-MIL-53-OH₇₅ became gentle and the amount of N₂ adsorbed tended to be similar to that of Al-MIL-53 at P/P₀ = 1. Al-MIL-53-OH₁₀₀ showed the lowest amount of N₂ adsorbed. The differentiation in the shape of the isotherms and the amount of N₂ adsorbed is due to a combination of guest-frameworks and the strong intra-framework H-bonding interactions in Al-MIL-53-OH₁₀₀.²⁶ The specific surface areas/pore volumes of Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀ are 1270 m² g⁻¹/0.64 cm³ g⁻¹ and 1260 m² g⁻¹/0.68 cm³ g⁻¹, respectively, which constitutes a modest increase in the surface area of 51% over Al-MIL-53 (819 m² g⁻¹/0.63 cm³ g⁻¹), which may be related to the milder activation process (solvent exchange) for Al-MIL-53-OH₂₅₍₅₀₎ compared to that for Al-MIL-53. Al-MIL-53-OH₁₀₀ showed a very low BET specific surface area of 32 m² g⁻¹ (pore volume: 0.34 cm³ g⁻¹). Owing to an abnormal N₂ adsorption isotherm of Al-MIL-53-OH₇₅, its specific surface areas could not be calculated. The results reveal that the surface area of Al-MIL-53 is improved when relative amount of 2,5-dihydroxylterephthalate does not exceed 50 mol%. The SEM images (Fig. 5) suggest that highly crystalline Al-MIL-53 particles were formed when BDC linkers dominated in the synthetic solution (Fig. 5a and b), whereas amorphous and aggregated particles were formed for Al-MIL-53-OH₁₀₀ (Fig. 5d). The poor morphology of Al-MIL-53-OH₁₀₀ possibly explains why it shows very low surface area.

Fig. 4 N₂ adsorption isotherms of hydroxyl modified Al-MIL-53s and Al-MIL-53.

Fig. 5 SEM images of (a) Al-MIL-53-OH₂₅; (b) Al-MIL-53-OH₅₀; (c) Al-MIL-53-OH₇₅; (d) Al-MIL-53-OH₁₀₀.

3.2 CO₂ adsorption property

The low-pressure CO₂ adsorption capacities of Al-MIL-53 and the hydroxyl-modified Al-MIL-53s at 25 °C are shown in Fig. 6. Al-MIL-53 showed a CO₂ adsorption capacity of 7.4 wt%, which is in good agreement with the reported values.²⁸ Compared to unmodified Al-MIL-53, an enhancement of the CO₂ adsorption capacity in the range of 0.15–1 bar was observed when the mole percentage of 2,5-dihydroxyl-terephthalate introduced was below 50%. The CO₂ adsorption of Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀ were 8.8 and 8.5 wt% at 1 bar, respectively. Both values are higher than of Al-MIL-53 by 19% and 15%. With increasing hydroxyl content, the CO₂ adsorption capacities of 4.8 and 4.2 wt% were found on Al-MIL-53-OH₇₅ and Al-MIL-53-OH₁₀₀, respectively. The CO₂ adsorption capacity of Al-MIL-53-OH₁₀₀ is similar to that reported by Stock and coworkers.²⁷ It is noted that the CO₂ adsorption behaviour of Al-MIL-53 in the pressure range of 0.15–0.3 bar was improved significantly, especially for Al-MIL-53-OH₂₅. The CO₂ adsorption capacity of Al-MIL-53-OH₂₅ at 0.5 bar is 5.5 wt%, which constitutes a 223% improvement over Al-MIL-53 (1.7 wt%).

Fig. 6 CO₂ adsorption isotherms of hydroxyl modified Al-MIL-53s and Al-MIL-53 at 25 °C.

In addition to the CO₂ adsorption capacity, the CO₂ adsorption isotherm of Al-MIL-53 at 25 °C was also affect by the introduction of hydroxyl groups. Al-MIL-53-OH₂₅ showed a type-I isotherm. With further increases in the hydroxyl content, a deviation of CO₂ adsorption isotherm of Al-MIL-53-OH_x (x = 50, 75 and 100) from type-I was found. The isotherms showed a distinctive two-step process. The first and second steps occurred at 0.1–0.2 bar and 0.2–0.5 bar. This is tentatively attributed to the ‘breathing behavior’ of the framework wherein the structure alternatives between the NP (narrow pore) and LP (large pore) form.

We performed the CO₂ adsorption cycling measurements on Al-MIL-53-OH₂₅ and the CO₂ adsorption isotherms were shown in Fig. 7. It was clearly observed that the CO₂ adsorption capacity of Al-MIL-53-OH₂₅ was well retained during three cycling measurements, which indicates its good reusability in CO₂ uptake. Moreover, no obvious change in the particle morphology was noticed from the SEM image of the recycled material (Fig. S1†).

Fig. 7 CO₂ adsorption cycling measurements on Al-MIL-53-OH₂₅ at 25 °C.

To understand the adsorption mechanism in more detail, the isosteric heats of CO₂ adsorption for these materials were calculated from the Clausius–Clapeyron equation based on the unfitted isotherms at 25 °C (Fig. 6) and 8 °C (Fig. S2†). Fig. 8 shows the isosteric heat of CO₂ adsorption on all samples as a function of the CO₂ loading. The isosteric heats of CO₂ adsorption on Al-MIL-53 at low CO₂ loadings (<0.2 mmol g⁻¹) was ∼26 kJ mol⁻¹, which is comparable to the reported value (∼23 kJ mol⁻¹).²⁹ Compared to Al-MIL-53, the hydroxyl modified Al-MIL-53s all obtained higher isosteric heat of CO₂ adsorption at low CO₂ loadings. The sequence of the isosteric heat of CO₂ adsorption below 0.1 mmol g⁻¹ is Al-MIL-53-OH₂₅ > Al-MIL-53-OH₅₀ > Al-MIL-53-OH₁₀₀ > Al-MIL-53-OH₇₅. The result reveals that the introduction of hydroxyl groups enhances the isosteric heat of CO₂ on Al-MIL-53, but high hydroxyl contents are not helpful for the low-pressure CO₂ adsorption performance of Al-MIL-53. In addition, the isosteric heat of CO₂ on all Al-MIL-53-OH_x decreased dramatically with the CO₂ loadings and became stable at higher CO₂ loadings, which may be explained by the NP form at the initial CO₂ loadings transforming to the LP form at higher CO₂ loadings.

Fig. 8 Heat of CO₂ adsorption on hydroxyl modified Al-MIL-53s and Al-MIL-53 as a function of the amount of CO₂ adsorbed.

It has been documented that introducing polar functional groups may improve the low-pressure CO₂ adsorption performance of MOFs.^21,25 The interaction between the localized dipoles of the polar functional groups and the quadrupole moment of CO₂ would induce the dispersion and electrostatic forces to enhance the isosteric heat of CO₂ adsorption. The results suggest that the introduction of hydroxyl group enhances the isosteric heat of CO₂ adsorption, which is consistent with the computational results.²⁵ In addition to the induced dispersion and electrostatic forces, a reduction in the pore size by the introduction of hydroxyl groups might be another reason. The enhanced isosteric heat of CO₂ adsorption did not result in an improvement on the low-pressure CO₂ adsorption capacity of Al-MIl-53 when hydroxyl content exceeded 50 mol%. Unexpectedly, Al-MIL-53-OH₂₅ containing the lowest content of hydroxyl groups obtained the highest isosteric heat of CO₂ adsorption at low CO₂ loadings (59 kJ mol⁻¹ at CO₂ loading of 0.04 mmol g⁻¹) and CO₂ adsorption capacity. We proposed that introducing varying hydroxyl contents in Al-MIl-53 might result in a different porosity and the amount of hydroxyl groups protruded into the pores regardless of the retained structure of Al-MIL-53. When introducing 25 mol% of hydroxyl groups in Al-MIL-53, the pore volume of Al-Mil-53 was retained and almost all hydroxyl groups might be open to the adsorbates within the pores. With increasing hydroxyl groups, the intraframework interaction become stronger. As a result, the adsorption sites (μ-OH) might be blocked by hydroxyl groups and could not interact with CO₂ molecules. The dense hydroxyl groups in Al-MIL-53-OH₇₅ and Al-MIL-53-OH₁₀₀ obtained the pore block, which is corroborated by the N₂ adsorption results. It is noted that the improvement by the hydroxyl group on the CO₂ adsorption capacity of Al-MIL-53 is comparable to that by amine group reported by Biswas and coauthors.²⁷ Based our results, it is crucial that improving CO₂ adsorption capability by modification requires not only a proper functional group but an optimal porous structure where functional groups are open to the adsorbates efficiently.

4. Conclusions

A series of hydroxyl-modified Al-MIL-53s, Al-MIL-53-OH_x (x = 25, 50, 75 and 100), were synthesized via a mixed ligand approach. All hydroxyl-modified Al-MIL-53s showed the same crystal structure as the unmodified Al-MIL-53 parent structure. The thermal stability was reduced once the hydroxyl groups were introduced in the Al-MIL-53 frameworks and became poor with increasing hydroxyl contents. A moderate increase in the specific surface area was obtained when less than 75 mol% 2,5-dihydroxy terephthalate is incorporated as an organic linker. We found that the hydroxyl group enhances the isosteric heat of CO₂ adsorption for Al-MIL-53, obtaining a CO₂ uptake capacity of 5.5 wt% for Al-MIL-53 at 25 °C and 0.2 bar, which constitutes 223% improvement over Al-MIL-53 (1.7 wt%). Owing to the poor porosity, Al-MIL-53-OH₇₅ and Al-MIL-53-OH₁₀₀ displayed lower CO₂ adsorption capacities despite their strong affinity for CO₂ molecules. We conclude that both the adsorption sites and the pore structure must be optimal when modifying a MOF by functional groups for efficient CO₂ capture.

Acknowledgements

This study is financially supported by the Science and Technology Commission of Shanghai Municipality (13ZR1417900) and the National Natural Science Foundation of China (11374204).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09350g

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