An indium-based microporous metal–organic framework with unique three-way rod-shaped secondary building units for efficient methane and hydrogen storage

Abstract

A novel microporous indium-based MOF material with unique 3-way rod-shaped secondary building units (SBUs), UTSA-22, was reported and exhibited high methane (CH₄) and hydrogen (H₂) storage. At 298 K and 65 bar, the total CH₄ volumetric uptake of UTSA-22 is 174 cm³(STP) cm⁻³. Moreover, UTSA-22 shows a high CH₄ working capacity of 146 cm³(STP) cm⁻³ in a pressure range of 5–65 bar at 298 K. In addition, UTSA-22 shows a high H₂ gravimetric storage capacity (1.2 wt%) at 298 K and 100 bar.

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As a primary greenhouse gas, carbon dioxide has been released globally to reach a record high level as fossil fuel demand is growing tremendously. Methane (CH₄), a primary component in natural gas, is considered a potential alternative to liquid fossil fuels since it is clean, abundant, and renewable on earth.¹ However, low energy densities have limited its practical applications. In this regard, to utilize methane as a transportation fuel, a suitable adsorbent with high CH₄ storage capacity at low pressures will be required (when methane is used as a transportation fuel stored by an adsorbent).² Therefore, significant interest has been shown in adsorbed natural gas systems to overcome these problems, including filling the tank with porous materials for storing high-density methane at moderate pressures. According to the guidelines of the department of energy (DOE) in the U.S.,³ ambitious targets for volumetric and gravimetric capacities for CH₄ storage are up to 350 cm³(STP) cm⁻³ and 0.5 g/g, respectively, at room temperature (R.T.) for the next generation of clean energy automobiles when considering the ignored loss of the packing adsorbent. Thus, comprehensive research efforts are being devoted to developing novel adsorbent materials with high CH₄ storage capacity to achieve these challenging storage goals.^4,5

Owing to their high Brunauer–Emmett–Teller (BET) surface areas and tunable pore functions, metal–organic frameworks (MOFs) are emerging as a new generation of crystalline materials that are able to outperform conventionally used activated carbon, zeolites, and silica gels in a multitude of different physico-chemical aspects.^6–15 Large numbers of MOFs have been demonstrated to be promising for CH₄ storage, considering both volumetric and gravimetric CH₄ uptake and storage.^16–24 It is worth noting that the CH₄ volumetric working capacity (also known as deliverable capacity) is considered to be a much more important parameter to assess the performance of these absorptive materials for practical applications due to the limitations of gas tanks in vehicles, which reflects the actual driving range using natural gas.^25–27 At present, to achieve a high working capacity, it is necessary to maximize the amount of methane stored at high pressure and minimize methane storage at low pressure (around 5 bar).^28–30 Several strategies have been proven to improve the CH₄ working capacity, such as optimizing pore structure, tuning the framework's flexibility, and incorporating strong binding sites.^31–35 However, it is still challenging to optimize the pore structure with an appropriate CH₄ binding affinity for balancing the trade-off of methane adsorption between low and high pressures to therefore obtain a superior volumetric working capacity.^36–39

Herein, a three-dimensional microporous metal–organic framework [In₅(TTETA)_11/3(OH)₄(H₂O)·30H₂O·19DMF] (UTSA-22, H₃TTETA = 4,4′,4′′-((2,4,6-trimethylbenzene-1,3,5-triyl)tris(ethyne-2,1-diyl)) tribenzoic acid), with unique 3-way rod-shaped SBUs, was synthesized under solvothermal conditions. It was found that the activated UTSA-22 shows a high methane uptake of 174 cm³(STP) cm⁻³ at 298 K and 65 bar, which is higher than those of DUT-4 (164 cm³(STP) cm⁻³),³ Fe-NDC (160 cm³(STP) cm⁻³)⁴⁰ and VNU-22 (155 cm³(STP) cm⁻³).⁴⁰ Moreover, this uptake value is comparable to some of the top performing materials when considering the significantly low surface area of UTSA-22 (2173 m² g⁻¹), such as MOF-205 (183 cm³(STP) cm⁻³),⁴¹ FJI-H23 (179 cm³(STP) cm⁻³),⁴² and BUT-22 (182 cm³(STP) cm⁻³).¹⁶ Additionally, the H₂ storage capacity of UTSA-22 can reach 1.2 wt% (8.45 g L⁻¹) at 100 bar and 298 K.

Solvothermal reactions of H₃TTETA with In(NO₃)₃·6H₂O and nitric acid yielded single crystals of UTSA-22. Single-crystal X-ray diffraction analysis revealed that UTSA-22 crystallizes in the trigonal system, space group R3̄c. Three independent In³⁺ atoms, 11/6 TTETA³⁻ ligands, and one μ₂-OH⁻ group were observed in the asymmetric unit of UTSA-22. Both In³⁺ atoms are coordinated with four carboxylate O atoms coming from four different TTETA³⁻ ligands in the equatorial positions and two μ₂-OH⁻ groups in the apical positions. The lengths of In–O and In–OH bonds are in the ranges of 2.050(4)–2.209(6) and 2.026(6)–2.094(6) Å, respectively (Table S2†). For the TTETA³⁻ ligand, two carboxyl groups coordinate with two adjacent In³⁺ atoms in a bi-monodentate coordination, and the remaining carboxyl group coordinates with the In³⁺ atom in a monodentate mode (Fig. 1). The uncoordinated carboxylate O atom (O12) can form hydrogen bonding interactions with a μ₂-OH⁻ group (O2, the distance is 2.6 Å, Fig. S1†). The connection of In³⁺ atoms with carboxylate and μ₂-OH⁻ groups in the order of “In1–In2–In3–In2–In1” results in infinite rod-shaped secondary building units (SBUs). The SBUs are bridged by TTETA³⁻ ligands making a three-dimensional structure with one type of double-wall disordered octahedral cage (Fig. S2†). The diameter of the octahedral cage is about 18 Å. In addition, two types of pore walls with thicknesses of 3.6 and 7.7 Å, respectively, are observed in UTSA-22 (Fig. S3†). Interestingly, the 1D chains in UTSA-22 are arranged in a three-way model, which differs from the commonly observed one-way or two-way models (Fig. 1).^43–46 Similar SBUs have been reported in a recently published work.⁴⁷ It should be pointed out that, while preparing this manuscript, the single crystal structure of UTSA-22 was reported by Li and co-workers, and this MOF was used for the detection of selective antibiotics in water.⁴⁸ The total potential solvent accessible void volume of the framework is 65% of the whole structure as estimated by PLATON.⁴⁹

Fig. 1 (a) Infinite 1D rod-shaped SBUs, (b) structural formula of the ligand, H₃TTETA, (c) the packing of the SBUs, and (d) a three-dimensional framework structure of UTSA-22 viewed along the crystallographic c-axis (color code: In, turquoise; C, black; and O, red; an octahedral geometry constituted by In and O; and hydrogen atoms are omitted for clarity).

The phase purity of UTSA-22 was examined by powder X-ray diffraction (PXRD) measurements (Fig. S5†). The PXRD peaks of the as-synthesized sample match with those of simulated ones obtained from the single-crystal data, proving the high phase purity of UTSA-22. The crystal structure remains intact after activation. In the thermogravimetric analysis (TGA) curve, two steps of weight loss were clearly observed: one is in the temperature range of 21–69 °C with a weight loss of 12%, and the other one is in the temperature range of 69–137 °C with a weight loss of 30% (Fig. S6†). By considering that in the synthesis of UTSA-22, only water, DMF, and HNO₃ (10 μL, 16 M) are used, and the trace HNO₃ will decompose to NO₂ at high temperature, it is thus believed that the two steps of weight loss belong to that of water and DMF in the pores of UTSA-22, respectively. The calculated amount of water and DMF molecules in the pores of UTSA-22 are 30 and 19, respectively, which is reasonable considering the large cell parameters (a = b = 51.504(3) Å, c = 50.096(4) Å; α = β = 90°, γ = 120°; and v = 115 083(17) ų) of the host framework. The framework of UTSA-22 is stable up to ∼400 °C, followed by its decomposition (Fig. S6†). Besides, in the FT-IR spectra, a slight red shift of the characteristic peaks belonging to the carbonyl group in UTSA-22 was observed compared with those of the H₃TTETA ligand, demonstrating the coordination between the carboxylate groups and metals (Fig. S7†). At 77 K, UTSA-22 was examined by N₂ adsorption to achieve permanent porosity (Fig. 2a). The saturated N₂ uptake of UTSA-22 is 581 cm³(STP) g⁻¹, corresponding to 2173 m² g⁻¹ as the BET surface area (Fig. S8 and S9†). Therefore, the experimental total pore volume of UTSA-22 is 0.90 cm³ g⁻¹, which is close to the theoretical value of 0.93 cm³ g⁻¹ by PLATON calculation.

Fig. 2 (a) N₂ adsorption/desorption isotherms for UTSA-22 at 77K; (b) CH₄ isotherms at 273 and 298 K for UTSA-22 up to 80 bar. Solid symbols: adsorption and open symbols: desorption. (c) Volumetric and (d) gravimetric CH₄ uptake (at 5, 35, 65, and 80 bar, respectively)/working capacities (in the pressure ranges of 35–5, 65–5, and 80–5 bar, respectively) of UTSA-22 at 298 K. (e) Q_st of CH₄ for UTSA-22 obtained using the virial method; and (f) comparison of the volumetric CH₄ working capacities (5–65 bar) of UTSA-22 with some benchmark MOF materials.

The CH₄ storage capacity of UTSA-22 was explored from the beginning accordingly. At 273 and 298 K, CH₄ adsorption isotherms were measured from 0 to 80 bar, respectively. As shown in Fig. 2b, at 35 bar and 298 K, the total gravimetric CH₄ uptake of UTSA-22 is 183 cm³(STP) g⁻¹, exceeding the DOE's previous goal (180 cm³(STP) g⁻¹), without regard for packing density loss. At 65 and 80 bar, the total gravimetric CH₄ uptake rates of UTSA-22 are 249 and 268 cm³(STP) g⁻¹ at 298 K, which correspond to 0.179 and 0.192 g/g, respectively, which are much higher than those of some benchmark MOFs such as Ni-MOF-74 (210 (223) cm³(STP) g⁻¹),⁵⁰ VNU-22 (132 (140) cm³(STP) g⁻¹),⁴⁰ and Cu-tbo-MOF-5 (208 (225) cm³(STP) g⁻¹)⁵⁰ under identical conditions (Table S3†). In addition, at 80 bar and 298 K, the volumetric CH₄ uptake is 188 cm³(STP) cm⁻³, comparable to those of VNU-21 (194 cm³(STP) cm⁻³),⁴⁰ BUT-22 (202 cm³(STP) cm⁻³),¹⁶ and MFM-132 (213 cm³(STP) cm⁻³).³⁴

The working capacity is another important factor that needs to be considered while assessing porous materials for practical methane storage. The working capacity is the difference in the total adsorption from 5 to 80 (or 65) bar. As shown in Table S3,† at 298 K, the CH₄ volumetric working capacity (65–5 bar) for UTSA-22 is 146 cm³(STP) cm⁻³, which is comparable or higher than those widely explored MOFs like Ni-MOF-74 (129 cm³(STP) cm⁻³),⁵⁰ FJU-101 (144 cm³(STP) cm⁻³),¹⁷ DUT-4 (124 cm³(STP) cm⁻³),³ and VNU-22 (101 cm³(STP) cm⁻³).⁴⁰ When the temperature is reduced to 273 K, the CH₄ volumetric working capacity (65–5 bar) increases to 157 cm³(STP) cm⁻³, which is higher than some well-known microporous MOFs such as NiMOF-74 (106 cm³(STP) cm⁻³),⁵¹ ZJU-70 (134 cm³(STP) cm⁻³),⁵² MOF-505 (112 cm³(STP) cm⁻³),⁵³ and PCN-14 (153 cm³(STP) cm⁻³).⁵¹ Additionally, the adsorption enthalpy (Q_st) of UTSA-22 is 9.8 kJ mol⁻¹ (Fig. 2e), which is lower than that found for most reported MOFs (Table S3†). Such a low adsorption enthalpy involving host–guest interactions is significantly important when CH₄ gas is released from the gas tank.

The H₂ isotherms of UTSA-22 were recorded up to 100 bar at 273 and 298 K. As shown in Fig. 3, the gravimetric H₂ of UTSA-22 at 298 K and 100 bar is 1.2 wt%, which is higher than the values for most reported MOFs, such as Co₂(BDC)₂(dabco) (0.32 wt%),⁵⁴ Cu₂(BDC)₂(dabco) (0.42 wt%),⁵⁴ JUC-48 (1.1 wt%),⁵⁵ Mg₂(dobdc) (0.8 wt%),⁵⁶ and Cu(peip) (0.46 wt%)⁵⁷ under identical conditions. Besides, UTSA-22 shows a remarkably high H₂ volumetric uptake of 8.45 g L⁻¹ at 298 K and 100 bar, which is higher than some famous MOFs such as NU-1501-Al (8.40 g L⁻¹)⁵⁸ and Mg₂(dobdc) (7.50 g L⁻¹).⁵⁶ The Q_st of H₂ for UTSA-22 is 12.3 kJ mol⁻¹ at zero bar based on the isotherms obtained at 298 and 273 K (Fig. S12†).

Fig. 3 High-pressure hydrogen adsorption isotherms for UTSA-22 at 273 and 298 K.

Conclusions

A novel microporous In-based MOF with unique three-way rod-shaped SBUs, named UTSA-22, has been designed and synthesized for efficient CH₄ and H₂ storage. Owing to its cage-type structure, UTSA-22 has a modest BET surface area of 2173 m² g⁻¹ and is stable up to 400 °C. UTSA-22 shows a high CH₄ gravimetric storage capacity of 268 cm³(STP) g⁻¹ (0.192 g/g) at 80 bar and 298 K. The CH₄ volumetric delivery capacity (65–5 bar) of UTSA-22 is 146 cm³(STP) cm⁻³ at 298 K, comparable to or higher than some benchmark MOF materials. Furthermore, the H₂ gravimetric uptake is 1.2 wt% for UTSA-22 at 298 K and 100 bar. Therefore UTSA-22 can be potentially used in CH₄ and H₂ storage applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Welch Foundation (AX-1730). The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education, in Saudi Arabia for funding this research work through the project number DRI-KSU-572. R.-B. L. wants to thank the NSFC (22101307) for its support. X-ray diffraction studies at the Molecular Foundry and Advanced Light Source was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (DE-AC02-05CH11231). The authors would like to express their sincere appreciation to Dr. Kaijun Lu from the Marine Science Institute at the University of Texas at Austin for assistance with the thermogravimetric analysis.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2204487. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi01956f

‡ These authors contributed equally to this work.

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