A robust aluminum-octacarboxylate framework with scu topology for selective capture of sulfur dioxide

Abstract

The high structural diversity and porosity of metal–organic frameworks (MOFs) promote their applications in selective gas adsorption. The development of robust MOFs that are stable against corrosive SO₂ remains a daunting challenge. Here, we report a highly robust aluminum-based MOF (HIAM-330) built on a 4-connected Al₃(OH)₂(COO)₄ cluster and 8-connected octacarboxylate ligand with a (4,8)-connected scu topology. It exhibits a fully reversible SO₂ uptake of 12.1 mmol g⁻¹ at 298 K and 1 bar. It is capable of selective capture of SO₂ over other gases (CO₂, CH₄, and N₂) with high adsorption selectivities of 60, 330, and 3537 for equimolar mixtures of SO₂/CO₂, SO₂/CH₄, and SO₂/N₂, respectively, at 298 K and 1 bar. Breakthrough measurements verified the capability of HIAM-330 for selective capture of SO₂ (2500 ppm) over CO₂ or N₂. High-resolution synchrotron X-ray powder diffraction of SO₂ loaded HIAM-330 revealed the binding domains of adsorbed SO₂ molecules and host–guest interactions.

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Introduction

By 2050, fossil fuels are predicted to account for 77% of global energy production, with coal emerging as the primary source in the global energy supply matrix.¹ Exhaust gases from pulverized coal combustion consist of 10–12% CO₂ and 500–3000 ppm SO₂;² the latter is a toxic and corrosive gas that will pose significant threats to human health and the environment.^3,4 While current flue gas desulfurization (FGD) technologies can remove most SO₂ from flue gases,^5–7 they generate a tremendous amount of solid waste and suffer from high energy input. Achieving complete removal of trace SO₂ proves challenging, as it necessitates a capture system with low selectivity for both dinitrogen and carbon dioxide coupled with remarkable material stability to withstand the highly corrosive nature of SO₂.^8,9

Selective capture of SO₂ by dry, regenerable adsorbents under ambient conditions has attracted growing interest and is perceived as a promising strategy for eliminating trace SO₂.¹⁰ This new method offers advantages such as lower energy consumption and minimized solid waste compared with the FGD strategy. Additionally, the recovered SO₂ can be used to produce valuable chemicals such as sulfuric acid. While porous materials such as zeolites,⁸ activated carbon,¹¹ and silica¹² have been employed for SO₂ capture, their adsorption capacity is generally low,^13,14 limiting the overall capture efficiency. Metal–organic frameworks (MOFs) have garnered substantial attention for gas adsorption, due to their high porosity, structural diversity, precisely tunable pore dimensions, and highly customizable functionality.^9,15,16 However, to date, only a limited number of MOFs have exhibited reversible SO₂ adsorption and structural stability upon adsorption–desorption cycles.^17–19 This is due to the highly corrosive nature of SO₂, which can cause framework degradation in many MOFs.^1,20–22 Thus, the ability of a MOF to maintain its structural integrity and retain its adsorption capacity after repeated cycles of SO₂ adsorption and desorption is a crucial factor for its practical application in desulfurization processes. Furthermore, highly stable and crystalline MOFs can enable the study of host–guest interactions using in situ X-ray diffraction techniques to understand the underlying mechanism of selective adsorption. To this end, the development of MOFs with reversible SO₂ adsorption and excellent structural stability remains a challenging and important area of research.

Over the past few years, Al-MOFs and Zr-MOFs have demonstrated superior efficacy in the adsorption of SO₂, underpinned by their high stability.^23,24 Their framework robustness arises not only from the strong Al–O and Zr–O bonds but also from the highly connected multinuclear inorganic clusters.²⁵ The ability of these clusters to link with multiple ligands bolsters the stability of the MOFs. Using highly connected organic ligands to synthesize MOFs is an effective strategy for improving framework stability. Thus, the combination of multinuclear inorganic building blocks and multi-topic organic linkers usually leads to MOFs with intriguing connectivity and topology. However, the construction of MOFs from high valence metals (Al³⁺, Zr⁴⁺, Ti⁴⁺, etc.) and organic linkers with high connectivity (>6) is particularly challenging due to the inherent synthetic difficulties. For example, only a handful of Zr-MOFs built on hexacarboxylates and octacarboxylates^26–30 have been reported, while only a single example of Al-MOF incorporating hexacarboxylates exists.³¹ Yet, no examples of Al-MOFs made of an octacarboxylate linker have been reported to date.

Here, we report the incorporation of an octacarboxylate, 4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1′-biphenyl]-3,5-dicarboxylic acid)) (H₈ettbpdc) into a robust aluminum-based MOF, denoted as HIAM-330. It features a 4,8-c scu topology, built on 4-connected Al₃O₂(COO)₄ clusters and 8-connected ettbpdc⁸⁻ linkers. The structure contains two distinct types of cavities and demonstrates a SO₂ adsorption capacity of 12.1 mmol g⁻¹ at 298 K and 1 bar. The adsorption was fully reversible and the adsorption capacity was retained after multiple SO₂ adsorption/desorption cycles. HIAM-330 exhibits selective capture of SO₂ in the presence of CO₂ and N₂. Molecular insights into the selective adsorption mechanism were achieved through X-ray diffraction analysis on SO₂-loaded HIAM-330, which elucidated the binding domains and host–guest interactions.

Results and discussion

H₈ettbpdc was selected as the organic building block for constructing the Al-octacarboxylate MOF due to its well-defined rigidity, planar geometry and facile synthesis. It was synthesized from 1,1,2,2-tetrakis(4-bromophenyl)ethylene via the Suzuki coupling reaction (Fig. S1–S3†). Subsequently, block-shaped crystals of HIAM-330 were solvothermally synthesized at 150 °C by reacting Al(NO₃)₃·9H₂O and H₈ettbpdc in DMF/formic acid/acetonitrile mixed solvent (Fig. S5†). Single-crystal X-ray diffraction analysis revealed that it crystallized in the tetragonal crystal system with a space group of I4/mmm (Table S1†). The crystal structure of HIAM-330 is built on a trinuclear Al₃O₂(COO)₄(H₂O)₆ cluster. In each cluster, the central Al³⁺ is octahedrally coordinated to four carboxylate oxygen atoms and two bridging oxygen atoms, while the terminal Al³⁺ is octahedrally connected to two carboxylate oxygen atoms, one bridging oxygen atom, and three water molecules (Fig. 1 and S6†). Thus, the trinuclear Al₃ clusters act as 4-connected nodes in the structure of HIAM-330, bridged by ettbpdc⁸⁻ organic struts. It is noteworthy that the Al₃ cluster observed here is distinct from the commonly observed triangular trinuclear Al₃ SBU, where a μ₃-O connects three Al³⁺ to form a 6-connected node.²⁵ The octacarboxylate organic linker ettbpdc⁸⁻ is fully deprotonated and connected to sixteen Al³⁺ centers from eight Al₃ clusters, with all carboxylates coordinated in bidentate mode (Fig. S7†). The overall structure of HIAM-330 features 4,8-c scu topology. It is intriguing that its connectivity is in sharp contrast to that of previously reported scu Zr-MOFs. The latter, as exemplified in PCN-606,³² is constructed on 8-connected Zr₆ nodes and 4-connected organic linkers (usually tetracarboxylates, Fig. 1). Nevertheless, in HIAM-330, the Al₃ clusters serve as the 4-connected nodes and the organic ligands act as the 8-connected units. This also leads to a different pore structure. The commonly observed scu-type Zr-MOFs such as PCN-606 or Zr-abtc possess one-dimensional rhombic channels, derived from the cubic cages of 4,12-c ftw-type connectivity.³³ In contrast, in HIAM-330, there are two distinct types of cages, with large and small cages arrayed in alternating sequences (Fig. 1 and S8†).

Fig. 1 Building blocks and crystal structure of PCN-660 (left) and HIAM-330 (right).

The phase purity of HIAM-330 was verified through powder X-ray diffraction (PXRD) analysis (Fig. 2a). Thermogravimetric analysis (TGA) of as-synthesized HIAM-330 displayed a continuous weight loss, while that of the methanol-exchanged sample showed a plateau from 120–270 °C, indicating the successful exchange of high boiling point solvents by methanol (Fig. S9†). During the subsequent activation of the methanol-exchanged HIAM-330 by heating at 150 °C, its crystal structure was fully retained. Our experimental investigation suggests that HIAM-330 exhibits excellent stability, as evidenced by its fully preserved PXRD patterns after being heated at 150 °C in open air for 1 week, immersed in water at 80 °C for 1 week, or exposed to 90% humidity for 1 week (Fig. 2a). The permanent porosity of HIAM-330 was evaluated by N₂ adsorption measurements at 77 K (Fig. 2b). The N₂ isotherm displays a typical type I profile, yielding a BET surface area of 1624 m² g⁻¹ and a pore volume of 0.65 m³ g⁻¹ (Fig. S10†). The pore size distribution curve, determined using the NLDFT model, was centered at approximately 5.5, 8.5, and 13.0 Å (Fig. S11†), which is consistent with the value estimated from Zeo++ software based on the crystal structure (Fig. S12†).

Fig. 2 Gas adsorption on HIAM-330. (a) PXRD patterns of HIAM-330 under various conditions. (b) Adsorption–desorption isotherms of N₂ at 77 K by HIAM-330. (c) Adsorption isotherms of SO₂, CO₂, CH₄, and N₂ at 298 K. (d) IAST selectivities of SO₂/CO₂, SO₂/CH₄, and SO₂/N₂ at 298 K in HIAM-330. (e) Q_st curves for SO₂ and CO₂ in HIAM-330. (f) Five adsorption–desorption cycles for SO₂ in HIAM-330 at 298 K and 1 bar.

Single-component adsorption isotherms of SO₂ were collected at 298, 283, and 273 K (Fig. S13†). HIAM-330 exhibits fully reversible adsorption for SO₂ with a high adsorption uptake of 12.1 mmol g⁻¹ at 298 K and 1 bar. This value is higher than those of most of the MOFs studied for SO₂ capture, such as Zr-bptc, UiO-66, UiO-66–NH₂, and Zr-DMTDC,²⁴ but is lower than those of the recently reported MFM-190 series showing higher porosity³⁴ (Fig. S14†). Furthermore, the adsorption uptake increased dramatically at relatively low pressure, indicating its ability to capture trace SO₂. The calculated isosteric heat of adsorption (Q_st) is 38.0 kJ mol⁻¹ at zero coverage. To study the capability of HIAM-330 for selective capture of SO₂ from other light gases that often co-exist in the flue-gas stream, its adsorption towards CO₂, CH₄, and N₂ was also evaluated. HIAM-330 adsorbs 2.60 mmol g⁻¹ of CO₂, 0.68 mmol g⁻¹ of CH₄, and 0.09 mmol g⁻¹ of N₂ at 1 bar and 298 K, substantially lower than that of SO₂ (Fig. 2c). The Q_st of CO₂ calculated from adsorption isotherms at 298, 283, and 273 K is 16 kJ mol⁻¹ at zero coverage, indicating its notably weaker adsorption affinity compared to that of SO₂ (Fig. 2e). The selectivities of HIAM-330 for SO₂/CO₂, SO₂/N₂ and SO₂/CH₄ were calculated via ideal adsorbed solution theory (IAST) at 298 K (Fig. 2d). Due to the negligible adsorption capacity for nitrogen, the calculated SO₂/N₂ selectivity values are unusually high (>1000) and are subject to large uncertainties. HIAM-330 also shows high selectivity values of 60 and 330 for equimolar binary mixtures of SO₂/CO₂ and SO₂/CH₄, respectively at 298 K and a total pressure of 1 bar. The values remain similar when the concentration of SO₂ decreased to 10%, resulting in selectivities of 44 and 239 for SO₂/CO₂ and SO₂/CH₄, respectively. These results suggest the preferential adsorption of SO₂ over other light gases by HIAM-330.

While many MOF materials exhibit high adsorption capacities for SO₂ due to high porosity, only a small number of them show fully reversible adsorption and retain their crystallinity when exposed to highly corrosive SO₂. HIAM-330 demonstrates the ability to maintain its structure after undergoing SO₂ adsorption–desorption tests (Fig. 2a). In consideration of practical applications, cyclic adsorption–desorption experiments were also conducted. In five consecutive adsorption/desorption cycles, no notable loss of SO₂ adsorption capacity (<5%) was observed for HIAM-330 (Fig. 2f), further confirming its high stability to SO₂.

To further verify the ability of HIAM-330 for selective capture of trace SO₂, dynamic breakthrough experiments were conducted using a fixed bed filled with HIAM-330. At 298 K and 1 bar, a mixture of SO₂/N₂ (2500 ppm SO₂, 75% N₂ diluted in He) was passed through the column at a flow rate of 20 mL min⁻¹. The results showed that N₂ eluted at the beginning of the process, while SO₂ was retained in the column for more than three hours (Fig. 3a). Additionally, the calculated SO₂/N₂ separation factor was 745. Similarly, another column breakthrough measurement was performed with a mixture of SO₂/CO₂ (2500 ppm SO₂, 15% CO₂ diluted in He). CO₂ broke through within less than two minutes, while SO₂ was retained for 136 minutes (Fig. 3b). The separation factor of SO₂/CO₂ was calculated to be 65.5. These findings validated the capability of HIAM-330 for selective capture of trace SO₂ from other light gases as a durable adsorbent.

Fig. 3 Separation of SO₂, N₂ and CO₂ by HIAM-330: breakthrough plots for (a) the SO₂/N₂ mixture (2500 ppm SO₂, 75% N₂ in He, and total flow rate: 20 mL min⁻¹) and (b) the SO₂/CO₂ mixture (2500 ppm SO₂, 15% CO₂ in He, and total flow rate: 20 mL min⁻¹) in HIAM-330 at 298 K.

The adsorption domains of SO₂ in HIAM-330 were determined by Rietveld refinements of the high-resolution synchrotron PXRD pattern of SO₂-loaded HIAM-330 (Fig. S22†). The robust framework of HIAM-330 remained highly crystalline, allowing direct visualization of the adsorbed SO₂ molecules in the pores. It was revealed that there were seven SO₂ binding sites in the pores, with one in the small tetrahedral cage (Fig. 4a), two in the large octahedral cage (Fig. 4b), and four in the small pocket surrounded by eight trinuclear Al₃ clusters (Fig. 4c). The total crystallographic uptake of SO₂ was estimated to be 7.35 mmol g⁻¹, corresponding to a crystal structure at a relatively low SO₂ pressure of ∼0.14 bar. It is interesting to observe that most of the adsorbed SO₂ molecules are located nearby the inorganic nodes rather than aggregating in the two main cages. This indicates that the Al₃ clusters are favorable binding sites for SO₂ at low pressure. The SO₂ molecule at site I was stabilized by the organic linker through hydrogen bonds (OSO⋯H–C = 2.44 Å and O₂S⋯H–C = 2.36 Å). At site III, the adsorbed SO₂ was immobilized by the phenyl ring (OSO⋯pi = 2.64 Å, electrostatic interactions) and by one hydrogen bond (OSO⋯H–C = 2.16 Å). In the remaining five sites (sites II and IV–VII), the adsorbed SO₂ molecules were predominantly interacting with the Al₃ cluster (the bridging oxygen or the terminal H₂O) through dipole–dipole interactions with the shortest OSO⋯O and O₂S⋯O distances of 1.97 and 1.91 Å, respectively. The above results offer molecular insights into the guest–host interaction of SO₂ adsorption in HIAM-330 and highlight the important role of the trinuclear Al₃ cluster in SO₂ capture.

Fig. 4 Adsorption domains of SO₂ in HIAM-330 determined by Rietveld refinements of high-resolution in situ synchrotron powder X-ray diffraction. (a) site I, (b) sites II and III, (c) sites IV–VII.

Conclusions

The design and synthesis of highly porous and stable MOFs with multi-topic ligands and high valence metals is important for enriching MOF structure diversity, adsorption, and capture of corrosive gases such as SO₂. We present the construction of an intriguing 4,8-c scu-type Al-MOF using a rigid octacarboxylate linker. The resulting MOF, HIAM-330, exhibits high porosity, excellent stability, high SO₂ uptake (12.08 mmol g⁻¹ at 298 K and 1 bar), and high IAST SO₂/CO₂ selectivity (60, SO₂/CO₂ = 50/50). Mixed-gas column breakthrough experiments further confirmed the effective removal of trace SO₂ by HIAM-330 in the presence of CO₂ and N₂. The excellent chemical and thermal stability of HIAM-330 endows it with good recyclability. Further, Rietveld refinement of high-resolution PXRD patterns of SO₂-loaded HIAM-330 provides valuable information regarding SO₂ adsorption sites and possible guest–host interactions, which is important for understanding the adsorption mechanism. Overall, our findings contribute to rational design strategies for making stable MOFs that hold strong promise for effective capture of target molecules.

Data availability

Data associated to the article are available in the ESI.†

Author contributions

Liang Yu: investigation, methodology, and writing – original draft. Meng He: investigation and methodology. Jinze Yao: investigation and validation. Qibin Xia: conceptualization, supervision, project administration, and writing – review & editing. Sihai Yang: conceptualization, project administration, supervision, and writing – review & editing. Jing Li: conceptualization, supervision, project administration, and writing – review & editing. Hao Wang: conceptualization, supervision, and writing – review & editing.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

This work was supported financially by the Shenzhen Science and Technology Program (No. KCXFZ20211020163818026) and National Natural Science Foundation of China (22178119). Work in the US was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0019902.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2303434 and 2258222. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc01877j

‡ These authors contributed equally.

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