CYCU-3: an Al(III)-based MOF for SO₂ capture and detection

Abstract

The Al(III)-based MOF CYCU-3 exhibits a relevant SO₂ adsorption performance with a total uptake of 11.03 mmol g⁻¹ at 1 bar and 298 K. CYCU-3 displays high chemical stability towards dry and wet SO₂ exposure. DRIFTS experiments and computational calculations demonstrated that hydrogen bonding between SO₂ molecules and bridging Al(III)–OH groups are the preferential adsorption sites. In addition, photoluminescence experiments demonstrated the relevance of CYCU-3 for application in SO₂ detection with good selectivity for SO₂ over CO₂ and H₂O. The change in fluorescence performance demonstrates a clear turn-on effect after SO₂ interaction. Finally, the suppression of ligand–metal energy transfer along with the enhancement of ligand-centered π* → π electronic transition was proposed as a plausible fluorescence mechanism.

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Introduction

Air pollution is a major concern throughout the world due to the presence of elevated concentrations of greenhouse and toxic gases in the atmosphere.¹ The US Environmental Protection Agency (EPA) has identified the principal polluting chemical species: carbon dioxide, nitrous oxides, fluorinated gases, carbon monoxide (CO), ground-level ozone, sulfur dioxide (SO₂), lead, ammonia (NH₃), particulate matter (PM), volatile organic compounds (VOCs), and hydrogen sulfide (H₂S).² Together, these gases/vapors contribute to a range of contemporary issues, including the greenhouse effect and environmental degradation.³ Air pollution sources are generally divided into anthropogenic emissions (human activities) and natural sources.⁴ SO₂ is one of the most harmful and less investigated pollutants in both cases. For example, recent Popocatépetl volcanic activity in Mexico led to a high release of SO₂.⁵ In this scenario, SO₂ can cause severe damage to human health and direct environmental hazards.⁶ Furthermore, SO₂ can produce sulfurous acid (H₂SO₃) in the presence of water, promoting acid rain and causing significant disturbance to different ecosystems.⁷ Also, SO₂ can cause severe respiratory problems even at low concentrations.⁸ Therefore, reducing the concentrations of SO₂ in the environment is necessary.

Different strategies have been developed to tackle anthropogenic emissions, and flue-gas desulfurization (FGD) technology is one of the most commonly employed technologies for SO₂ removal.⁹ However, the FGD process generates a large amount of waste and leads to corrosion in pipelines.¹⁰ Therefore, the design and implementation of effective absorbent materials are fundamental to implementing new alternatives for SO₂ removal. Metal–organic frameworks (MOFs) have emerged as promising candidates for the efficient adsorption of different gases.^11–13

MOFs are crystalline hybrid materials formed by organic linkers and metal ions.¹⁴ Selected MOFs have shown high chemical and thermal stability, displaying outstanding surface areas, chemical mutability and tunable pore sizes.¹⁵ Based on their remarkable properties, MOFs have been widely applied in different research fields, particularly catalysis,¹⁶ adsorption of pollutants,¹⁷ detection,¹⁸ and drug delivery.¹⁹ In the case of SO₂ removal, MOFs that have been successfully implemented feature high chemical stability toward SO₂ and ideally participate in SO₂ physisorption processes,²⁰ since, in some cases, metal–oxygen bonds can dissociate after SO₂ adsorption. For example, although MOF-177 holds the SO₂ uptake record (25.7 mmol g⁻¹ at 298 K and 1 bar), after SO₂ adsorption, the crystal structure collapses.²¹ Therefore, designing chemically stable metal–organic frameworks is fundamental to achieving effective SO₂ sequestration. Recently, Mg₂(dobpdc) and MIL-101(Cr)-4F(1%) were reported to exhibit outstanding SO₂ uptake (19.5 and 18.4 mmol g⁻¹ at 298 K and up to 1 bar, respectively) and stability, even under humid conditions.^22,23 The coordination environment of the metal centers (building units) allowed them to interact with SO₂ molecules without breaking the fundamental coordination bonds.²⁴ Additionally, some Al(III)-based MOFs have been effective as SO₂ adsorbents, showing high chemical stability, low toxicity and sustainability.²⁵

Another relevant property that has recently started to be successfully explored in MOFs is the detection of SO₂.²⁶ Specifically, select MOF materials can change their light-emission properties in response to SO₂ molecules, the concentration of which can thus be evaluated quantitatively or qualitatively.²⁷ Particularly, the fluorescence phenomenon is a suitable technique for this purpose, either by a turn-on or turn-off mechanism, although the turn-on effect has been reported to be the most accurate.²⁸ In other words, a MOF material can exhibit a response after a stimulus (interaction with a specific molecule), and the resulting change in luminescence can be measured. In addition, this behaviour is observed in MOFs in association with different analytes. For example, Zn₂(TCPE) displays a shift in its florescence spectra (related to a turn-on effect) for NH₃.²⁹ With regard to SO₂, a Tb(III)-based MOF showed a characteristic change in the fluorescence of Tb³⁺ after interaction with SO₂.³⁰ Also, MOF-5-NH₂ exhibited a luminescence turn-on phenomenon when exposed to SO₂.³¹ Therefore, implementing certain MOF materials for SO₂ adsorption and detection is a promising avenue in the field of environmental remediation and workplace hazard reduction.

Herein, we report an Al(III)-based MOF named CYCU-3 [Al(OH)(SDC)] (H₂SDC = 4,4′-stilbenedicarboxylic acid) (Fig. 1), which displays octahedral Al(III) centres with hydroxo bridging groups (μ₂-OH), showing approximately 3 nm wide hexagonal pores.³² CYCU-3 shows promise for SO₂ adsorption (even under humid conditions) and detection due to outstanding chemical stability.

Fig. 1 Structure of CYCU-3 (a) along the channel c-axis and (b) metal cluster and linker arrangement along the b-axis. Atom labels: purple: AlO₄(μ-OH)₂ octahedra, grey: carbon, and red: oxygen.

Experimental

Chemicals

Aluminium chloride anhydrous (AlCl₃, 99.99%), 4,4′-stilbenedicarboxylic acid (H₂SDC, 98%), acetic acid glacial (CH₃COOH, 99%), N,N-dimethylformamide (DMF, 99.8%), and acetone ((CH₃)₂CO, 99%) were supplied by Sigma Aldrich. Sulphur dioxide (SO₂, 99.99%), nitrogen (N₂, 99.99%), carbon monoxide (CO, 99.99%), and helium (He, 99.99%) were supplied by INFRA. All reagents and solvents were used as received from commercial suppliers without further purification.

Synthesis of CYCU-3

CYCU-3 was synthesized according to a previously reported methodology.³² Thus, the H₂SDC organic linker (0.1073 g, 0.40 mmol) and AlCl₃ (0.0533 g, 0.4 mmol) were mixed in DMF (10 mL). After that, CH₃COOH was added (0.114 ml, 2.0 mmol) under stirring, and the solution was placed in a Teflon liner inside an autoclave and heated at 180 °C for 72 h. Finally, the resulting powder was washed with DMF (three times) and dried at 100 °C overnight.

Instruments

Detailed information on the instrumental techniques is available in section S1.†

Adsorption experiments and modelling

For SO₂ adsorption experiments, the adsorption–desorption isotherm was recorded at 298 K and up to 1 bar using a dynamic gravimetric gas/vapor sorption analyzer, DVS Vacuum (Surface Measurement Systems Ltd). DRIFTS experiments used an environmentally controlled PIKE DRIFTS cell with ZnSe windows coupled to a Thermo Scientific Nicolet iS50 spectrometer with an MCT/A detector. Absorbance spectra were obtained by collecting 64 scans at a 4 cm⁻¹ resolution. 0.020 g of the sample was pre-treated in situ under an N₂ flow at 453 K for 6 h. After this treatment, the sample was cooled down to room temperature, and then a flow of carbon monoxide (CO: 30 mL min⁻¹; 5% CO diluted in He) was passed through the sample. The spectra of the solid were recorded every 10 min. Moreover, computational modelling was employed using the Gaussian 09 code to perform partial optimization of the systems under study at the B3LYP/6-31 + g(2d,p) level of theory.³³

SO₂ detection measurements

The detection measurements using CYCU-3 were carried out using an excitation wavelength of 346 nm, with a 340-10 nm band-pass filter on the lamp side and a 395 nm long-pass filter on the detector side to remove any remaining light from the excitation source. The measurements were performed with a step size of 1 nm and a dwell time of 0.3 s. The excitation bandwidth was set at 0.50 nm, and the emission bandwidth for the detector was set at 1.00 nm. Half-life spectra were recorded by exciting the sample with a picosecond pulsed light emitting diode EPLED-340 with an excitation wavelength of 335.6 nm, a pulse width of 930.8 ns and a bandwidth of 12.4 nm, at an emission wavelength of 500 nm.

Results and discussion

Characterization of CYCU-3

The PXRD pattern (Fig. S1†) of the as-synthesized CYCU-3 was in good agreement with the simulated data.³² Well-defined peaks were observed confirming its high crystallinity. FTIR spectra (Fig. S2†) display peaks at 3641, 3026, 1605, and 1435 cm⁻¹, related to the hydroxo group (–OH), aliphatic =CH stretching vibration, carboxyl groups coordinated to the Al(III) centres, and the benzene ring skeleton, respectively.³⁴ TGA analysis (Fig. S3†) showed the thermal stability of CYCU-3. An initial thermal decomposition of 12.1% related to DMF molecules within the pore is observed. Then, it was confirmed that the stability was up to 450 °C.³⁵ The solid-state UV-Vis spectrum of CYCU-3 (Fig. S4†) exhibits a maximum absorption peak at 343 nm. The BET surface area and pore volume were calculated from the nitrogen isotherm (Fig. S5†) at 77 K and they were found to be 2711 m² g⁻¹ and 1.36 cm³ g⁻¹, respectively. These values are in line with those reported previously.³²

SO₂ adsorption–desorption measurements

Prior to adsorption experiments, CYCU-3 was solvent exchanged with acetone for a week and then activated at 180 °C under vacuum for 24 h to ensure the removal of guest molecules from the channels. The adsorption–desorption SO₂ isotherm at 298 K is displayed in Fig. 2. A 1.36 mmol g⁻¹ uptake at 0.1 bar is observed in the low-pressure region. Then, from 0.1 to 0.2 bar, a considerable increase is observed, displaying an overall uptake of 3.24 mmol g⁻¹. Interestingly, from 0.2 to 1 bar, the isotherm exhibits linear uptake of SO₂, achieving a maximum of 11.03 mmol g⁻¹ at 1 bar. In this case, the saturation uptake could not be reached under the experimental parameters (298 K and 1 bar) in CYCU-3 due to its different porosity distribution, displaying micro and mesoporosity with diameters of 16 and 27.4 Å.³² Also, this value can be compared to the SO₂ capture of representative Al(III)-based MOF materials (Table S1†). For example, MIL-160 (7.2 mmol g⁻¹, at 293 K and 1 bar),²¹ DUT-4 (13.6 mmol g⁻¹, at 298 K and 1 bar),³⁶ MOF-303 (7.86 mmol g⁻¹, at 298 K and 1 bar),³⁷ MIL-53 (8.9 mmol g⁻¹, at 298 K and 1 bar),³⁸ MIL-96 (6.5 mmol g⁻¹, at 293 K and 1 bar),³⁹ CAU-23 (8.4 mmol g⁻¹, at 293 K and 1 bar),⁴⁰ and CAU-10 (4.47 mmol g⁻¹, at 298 K and 1 bar)⁴¹ have shown favorable SO₂ capture results due to their high stability.

Fig. 2 SO₂ adsorption–desorption isotherm at 298 K and 1 bar for CYCU-3 (solid circles represent adsorption and open circles show desorption).

Furthermore, the SO₂ isotherm shows the absence of hysteresis due to completely reversible adsorption–desorption behaviour, suggesting a relatively weak SO₂ interaction. The stability of CYCU-3 after the adsorption process was confirmed since no substantial changes were observed in the PXRD pattern (Fig. S6†). The BET surface area remains unchanged after the SO₂ adsorption experiments (Fig. S7†). Also, to further evaluate the structural stability of CYCU-3, an activated sample of CYCU-3 was exposed to humid SO₂ using an in-house designed setup (Fig. S10†). The resulting sample, exposed to humid SO₂, exhibited a PXRD pattern comparable to that of the pristine material (Fig. S6†).

In order to investigate the host–guest interactions between SO₂ and CYCU-3, the heat of adsorption was determined, and DRIFTS experiments were carried out in conjunction with computational calculations. The isosteric heat of adsorption (ΔH) for SO₂ adsorption was calculated at low coverage (Fig. S8 and S9†) using the virial method.⁴² It was estimated that CYCU-3 shows a relatively low heat of adsorption (ΔH = −29.8 kJ mol⁻¹). In this case, the value agrees with the absence of hysteresis in the adsorption isotherm, which is attributed to the saturated Al(III) coordination sphere in CYCU-3, which provides no open metal sites for chemisorption.⁴³ Thus, the low heat of adsorption confirms a physisorption process. The material saturation is not reached under the experimental conditions, characteristic of a mesoporous MOF material. Moreover, this performance is associated with a relatively weak hydrogen bonding interaction between SO₂ and the hydroxo-functional groups,⁴⁴ as it was previously reported for hydroxo-functionalised MOFs. For example, MFM-300(Sc), MFM-300(Al), and DUT-4 displayed negative heat of adsorption values of 36.2,⁴⁵ 30.0,³⁶ and 31.3 kJ mol⁻¹,⁴⁶ respectively. This also suggests that SO₂ directly interacts with CYCU-3 via hydrogen bonding.

To corroborate this, in situ DRIFTS experiments were conducted using CO as a probe molecule to corroborate the role of hydroxyl functional groups in the adsorption mechanism (Fig. 3). This spectroscopic technique is a powerful tool to characterise acid/base sites for different porous materials.⁴⁷

Fig. 3 DRIFT spectra of CO adsorbed at different times over activated CYCU-3 at 303 K.

A characteristic red-shift was observed at 3703 cm⁻¹, which can be attributed to the hydroxyl stretching band (Δν(OH) = 4 cm⁻¹) in the presence of CO due to the bridging OH moieties. In this case, the contribution of Brønsted acid sites can form weak hydrogen bonding.⁴⁸ Moreover, a broad new band at 2170 cm⁻¹ is observed due to the CO interaction with the –C=C– site from the arene rings in the ligand. The π-acid character of the CO molecule is suitable since it possesses a π-acceptor character and can interact with the π-density of the ethylene site.⁴⁹ In this context, the bridging Al(III)–OH groups and the –C=C– are the preferential adsorption sites interacting with the CO probe molecule. Even though the chemical properties of CO are different from those of SO₂, this study proved to be useful for gaining insights into the possible adsorption sites, as previously reported.²³

Computational calculations (partially optimised) were performed to gain further insights into the preferential SO₂ adsorption sites. The calculations revealed that SO₂ molecules interacted in five different scenarios within CYCU-3 (Fig. S11†). The interaction of SO₂ through benzene and with the CH site was found to be completely unstable. On the other hand, the other three structures were more stable than the dissociated systems, and in the first scenario, the SO₂ molecule can form a hydrogen bond with one H atom of the –C=C– group from the ligand arene moiety, displaying an energetic value of 4.7 kcal mol⁻¹. The S atom of SO₂ interacts with the hydroxyl O atoms of CYCU-3 with 8.8 kcal mol⁻¹. Finally, the most stable structure was calculated to be 14.4 kcal mol⁻¹, which arises from hydrogen bond formation with the bridging Al(III)–OH groups and one oxygen atom of SO₂. Thus, this corroborates the relatively strong hydrogen bonding, confirming that the bridging OH moieties are the preferential adsorption sites.

SO₂ detection measurements

Up to this point, CYCU-3 has displayed promising potential for application in SO₂ adsorption. Thus, we decided to investigate the feasibility of using this MOF material as a fluorescent SO₂ detector. First, under UV light irradiation at λ_ex = 343 nm (chosen based on solid-state UV-vis spectroscopy experiments, Fig. S4†), the photoluminescence properties of the H₂SDC ligand and CYCU-3 were investigated (Fig. S12†).

The H₂SDC ligand shows strong bands at 465 and 440 nm, related to the presence of conjugation, which leads to ligand-centred π* → π electronic transitions. Moreover, the solid-state emission spectra of CYCU-3 display an intensity decrease and a slight blue shift, showing fluorescence quenching. This behaviour can be associated with the charge transfer between the H₂SDC ligand and Al(III) centres.³⁴

Thus, an activated sample of CYCU-3 was saturated with SO₂ in our in-house designed ex situ adsorption system (Fig. S10†) to measure the photoluminescence properties of SO₂-saturated CYCU-3. Remarkably, the photoluminescence increases considerably (2.22-fold increase) in emission intensity (Fig. 4). Additionally, the quantum yields (QY) of CYCU-3 before and after its exposure to SO₂ were determined, and an increase from 18.65% to 26.81% was observed; such an increment in luminescence QY after SO₂ exposure reinforces the fact that its adsorption on the framework has a positive impact on the light emission efficiency of CYCU-3. Furthermore, this behaviour can be related to the electronic effect that SO₂ generates on CYCU-3. In this case, the hydrogen bonding between SO₂ and bridging Al(III)–OH groups can suppress the energy transfer between the ligand and Al(III) centres, magnifying the π* → π electronic transition.^50,51

Fig. 4 Solid-state emission spectra of activated CYCU-3 (purple line) and after exposure to SO₂ (pink line), CO₂ (pink fuchsia line), and H₂O (blue line). The excitation wavelength was set at 343 nm.

Thus, to further test the selectivity of CYCU-3, the fluorescence of CYCU-3 upon exposure to H₂O and CO₂ was investigated (Fig. 4). Interestingly, no significant changes in the shape or intensity of the photoluminescence spectrum were produced by any of these molecules compared to the spectrum of activated CYCU-3. Since the change in fluorescence is only generated by SO₂ exposure, a reproducibility test was conducted. Then, 6 independent experiments were undertaken, and the same SO₂-saturated CYCU-3 sample was re-activated. In all cases, the sample was saturated with SO₂ to obtain the photoluminescence absorption properties during the cycling experiment. The same emission spectrum was consistently observed, with an average 2.22 ± 0.11-fold increase in emission intensity (Fig. S13†).

Once the SO₂ detection capability of CYCU-3 and the reproducibility of the detection of SO₂-saturated CYCU-3 samples were demonstrated, the detection properties of this Al(III)-based material at low SO₂ pressure were investigated. Since low SO₂ pressure could be associated with SO₂ detection at low SO₂ concentrations, exploring any photoluminescence response at only 0.1 bar of SO₂ is fundamental.

Then, an activated sample of CYCU-3 was placed in direct contact with 0.1 bar of SO₂ (in our in-house designed ex situ adsorption system, Fig. S10†) to obtain the corresponding photoluminescence absorption spectrum (Fig. 5). Interestingly, the broad photoluminescence peaks remained centered at λ_max = 450 and 427 nm, with a 1.65-fold increase in emission intensity recorded. Additionally, the reproducibility of this experiment was estimated with 6 independent experiments (re-activation of the sample and re-exposure to 0.1 bar of SO₂), discovering an average 1.65 ± 0.14-fold increase in emission intensity. Importantly, these experiments only evaluate the SO₂ detection by CYCU-3 at 0.1 bar, which cannot be directly interpreted as SO₂ sensing. However, the consistent and reproducible response of the Al(III)-based MOF material towards low-pressure SO₂ is very promising and validates ongoing efforts to develop MOF-based SO₂ sensors.

Fig. 5 Solid-state emission spectra of activated CYCU-3 (purple line) and after exposure to SO₂ at 0.1 bar (pink line). The excitation wavelength was set at 343 nm.

Lastly, time-resolved photoluminescence (TRPL) experiments were conducted using a 340 nm picosecond-pulsed LED as the excitation source (Fig. S14†). Thus, an activated sample and an SO₂-saturated sample were analysed. TRPL decay was measured at an emission wavelength of 450 nm (Table S2†). It was observed that the decay lifetimes slightly decreased after SO₂ interaction with CYCU-3. It is worth mentioning that fluorescence lifetime is independent of fluorescence brightness,⁵² as observed in this work. Thus, the small decay in fluorescence lifetime can be attributed to the cooperative effect of the intramolecular rotational constraint of the framework⁵³ and the emission ligand-centered π* → π electronic transitions.⁵⁴

Conclusions

This study investigated the SO₂ adsorption and detection properties of a chemically stable Al(III)-based MOF named CYCU-3. CYCU-3 exhibits a remarkable SO₂ capture performance (11.03 mmol g⁻¹, at 1 bar and 298 K), displaying high chemical stability upon both dry and wet SO₂ exposure. Moreover, DRIFTS experiments and computational studies corroborated that hydrogen bonding interactions between SO₂ and bridging Al(III)–OH groups form the preferential adsorption sites. Photoluminescence experiments show a substantial change in the fluorescence behaviour of CYCU-3 upon SO₂ exposure, with excellent reproducibility and high selectivity for SO₂ over CO₂ and H₂O. It was proposed that the turn-on effect was generated via the suppression of ligand–metal energy transfer and the enhancement of ligand-centered π* → π electronic transition. Overall, this investigation postulates CYCU-3 as an exciting candidate for SO₂ detection.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. L. O., V. B. L.-C., and C. V. F thank CONACYT for the Ph.D. fellowships (1003953, 1005649, and 1040318). A. M. acknowledges support from DGAPA through Programa de Apoyo para la Superación del Personal Académico de la UNAM (PASPA) and thanks LANCAD-UNAM-DGTIC-141 for computer facilities. Y. A. A.-S. acknowledges the support from the DGAPA-UNAM postdoctoral fellowship grant.

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Footnotes

† Electronic supplementary information (ESI) available: Instrumental techniques, characterization, and experimental data. See DOI: https://doi.org/10.1039/d3dt04073a

‡ These authors contributed equally to this work.

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