Evaluating solvothermal and mechanochemical routes towards the metal–organic framework Mg₂(m-dobdc)

Abstract

Metal–organic frameworks bearing coordinatively unsaturated Mg(II) sites are promising materials for gas storage, chemical separations, and drug delivery due to their low molecular weights and lack of toxicity. However, there remains a limited number of such MOFs reported in the literature. Herein, we investigate the gas sorption properties of the understudied framework Mg₂(m-dobdc) (dobdc⁴⁻ = 4,6-dioxido-1,3-benzenedicarboxylate) synthesized under both solvothermal and mechanochemical conditions. Both materials are found to be permanently porous, as confirmed by 77 K N₂ adsorption measurements. In particular, Mg₂(m-dobdc) synthesized under mechanochemical conditions using exogenous organic base displays one of the highest capacities reported to date (6.14 mmol g⁻¹) for CO₂ capture in a porous solid under simulated coal flue gas conditions (150 mbar, 40 °C). As such, mechanochemically synthesized Mg₂(m-dobdc) represents a promising new framework for applications requiring high gas adsorption capacities in a porous solid.

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Introduction

Metal–organic frameworks (MOFs) are porous, crystalline extended solids that consist of inorganic metal nodes or secondary building units (SBUs) bridged by polytopic organic linkers.¹ Their uniquely modular structures coupled with high internal surface areas have enabled numerous applications in drug delivery, catalysis, chemical separations, and gas storage.^2–5 Frameworks containing coordinatively unsaturated metal centers, also known as open-metal site MOFs, have been extensively studied due to their ability to strongly interact with guest molecules.⁶ In particular, the canonical M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd; dobdc⁴⁻ = 2,5-dioxidobenzene-1,4-dicarboxylate), MOF-74, or CPO-27 family of frameworks features hexagonal one-dimensional channels decorated with a high density of exposed M(II) centers (Fig. 1, left).⁷ Among the reported isostructural metal variants, the Mg analogue, Mg₂(dobdc), is particularly promising due to its low cost, lack of toxicity, and high gravimetric and volumetric adsorption capacities for a range of adsorbates.^8–12 As such, the identification of new porous frameworks bearing accessible Mg(II) sites is highly desirable.

Fig. 1 Structures of M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn; dobdc⁴⁻ = 2,5-dioxidobenzene-1,4-dicarboxylate, left) and M₂(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc⁴⁻ = 4,6-dioxidobenzene-1,3-dicarboxylate, right). Green, gray, white, and red spheres correspond to magnesium, carbon, hydrogen, and oxygen, respectively.

A closely related family of frameworks, M₂(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc⁴⁻ = 4,6-dioxido-1,3-benzenedicarboxylate), have been reported to possess an even higher density of exposed cationic sites than MOF-74 materials due to slight differences in the ligand field around the metal center.¹³ However, Mg₂(m-dobdc) prepared under solvothermal conditions was initially reported to be non-porous due to difficulties associated with removing coordinating solvents such as N,N-dimethylformamide (DMF) or methanol (MeOH) from the framework pores.¹³ Although Mg₂(m-dobdc) was later reported to be porous when prepared under mechanochemical conditions,¹⁴ much about its intrinsic gas sorption properties, such as the accessibility of the Mg(II) sites to guest molecules,⁸ remains relatively unknown.

Herein, we systematically investigate the synthesis and gas sorption properties of fully desolvated Mg₂(m-dobdc). Careful washing and activation of Mg₂(m-dobdc) prepared under traditional solvothermal (ST) conditions, termed Mg₂(m-dobdc)-ST, enables access to a material with a similar 77 K N₂ Brunauer–Emmett–Teller (BET) surface area as closely related Mg₂(dobdc). In addition, we report an improved mechanochemical (MC) method to reliably prepare Mg₂(m-dobdc)-MC using exogenous organic base.¹⁴ Beyond the inherent advantages in scalability and waste minimization offered by mechanochemical syntheses,^15–17 Mg₂(m-dobdc)-MC exhibits higher gas adsorption capacities than Mg₂(m-dobdc)-ST. Indeed, at 150 mbar of CO₂ and 40 °C, conditions relevant to CO₂ capture from coal flue gas,¹⁸ Mg₂(m-dobdc)-MC exhibits an higher CO₂ uptake (6.14 mmol g⁻¹) than even Mg₂(dobdc) (5.28 mmol g⁻¹).^12,19 This represents one of the highest capacities reported to date for CO₂ capture in a porous solid under simulated coal flue gas conditions.²⁰ Overall, our findings suggest that Mg₂(m-dobdc) prepared under mechanochemical conditions represents a promising and scalable alternative to ubiquitous Mg₂(dobdc) for applications in chemical separations, gas storage, and beyond.

Results and discussion

We commenced our studies into the synthesis and gas sorption properties of Mg₂(m-dobdc) by optimizing the synthesis of H₄m-dobdc (Fig. 2). In our hands, the previously reported synthesis of H₄m-dobdc from resorcinol via the Kolbe–Schmitt reaction was only modestly reproducible, often yielding the monocarboxylic acid 1 instead of H₄m-dobdc (Fig. 2). Two modifications to the standard preparation were identified to make the synthesis of H₄m-dobdc more reliable (see ESI† section 2 for details). First, starting from commercially available 1 in place of resorcinol improved the reproducibility of the reaction, likely because only a single carboxylation reaction must take place to yield H₄m-dobdc.²¹ Second, the reaction temperature (250 °C) was found to be a critical parameter and best monitored using an internal thermocouple placed directly in the solvent-free reaction mixture. With these modifications in place, we were able to reliably synthesize H₄m-dobdc on >5 g scale in a single batch.

Fig. 2 Synthesis of H₄m-dobdc from 1.

The previously reported small-scale solvothermal synthesis of Mg₂(m-dobdc) employed Mg(NO₃)₂·6H₂O as the Mg precursor in 2 : 1 DMF : MeOH at 120 °C.¹³ In order to identify the optimal solvothermal conditions for preparing Mg₂(m-dobdc), several combinations of amide (DMF or N,N-dimethylacetamide) and alcohol (MeOH, ethanol, H₂O) solvents were evaluated (ESI† Table S1, see ESI† section 3 for details). Other methods previously reported for the preparation of Mg₂(dobdc), such as employing Mg(OAc)₂·4H₂O as a basic Mg precursor, were tested as well.^22–24 Characterization of the produced solids by powder X-ray diffraction (PXRD)²⁵ validated that combining H₄m-dobdc and Mg(NO₃)₂·6H₂O in 1 : 1 DMF : MeOH (0.03 M) at 120 °C for 48 h was optimal to yield highly crystalline Mg₂(m-dobdc)-ST (Fig. 3, ESI† Fig. S5). This synthesis can be readily scaled to produce Mg₂(m-dobdc)-ST on at least 0.5 g scale (Fig. 3, ESI† Fig. S6) and is reproducible as well (ESI† Fig. S17). Soaking the resulting MOF in DMF at 120 °C to remove residual starting materials, then in MeOH at 60 °C to remove DMF, and then in acetone at room temperature to remove MeOH, was sufficient to remove coordinating solvents and soluble impurities from Mg₂(m-dobdc)-ST, as confirmed by acid digestion and analysis of the resulting solution by ¹H NMR (ESI† Fig. S13). Unfortunately, higher reaction concentrations (>0.1 M) led to impure materials (not shown), limiting the scalability of this solvothermal method.²⁶

Fig. 3 PXRD (λ = 1.5406 Å) patterns of Mg₂(m-dobdc)-ST synthesized on small and large scale under solvothermal conditions. The simulated pattern based on the previously reported single-crystal X-ray diffraction structure of the isostructural framework Co₂(m-dobdc) is included for reference.²⁵

Having optimized the solvothermal synthesis of Mg₂(m-dobdc), we set out to improve the mechanochemical synthesis of this material for comparison. Previously, we reported the mechanochemical synthesis of Mg₂(dobdc) using N,N-diisopropylethylamine (Hünig's base) as both the base required to deprotonate the linker precursor and as the liquid to facilitate liquid-assisted grinding in a planetary ball mill.¹⁴ This method could be generalized to the synthesis of porous Mg₂(m-dobdc) with modest crystallinity. We hypothesized that careful optimization of the Mg precursor (Mg(NO₃)₂·6H₂O, Mg(OAc)₂·4H₂O, or MgO), base (Hünig's base, Et₃N, or 2,6-lutidine), and grinding time (1, 5, or 10 min) would enable the synthesis of Mg₂(m-dobdc)-MC with maximum crystallinity and porosity (see ESI† section 4 for details). Indeed, the combination of Mg(NO₃)₂·6H₂O, Et₃N, and 5–10 min grinding time was optimal to yield crystalline Mg₂(m-dobdc)-MC (Fig. 4, ESI† Fig. S18 and S19). This method could be reproducibly carried out with 10 min of grinding at 600 rpm to produce Mg₂(m-dobdc)-MC on 0.5 g scale in excellent yield (Fig. 3, ESI† Fig. S20 and S31). Notably, this mechanochemical synthesis bypasses the use of toxic DMF,²⁷ representing a green alternative to the solvothermal synthesis of Mg₂(m-dobdc)-ST. No MOF was obtained with 2,6-lutidine, likely because it is not basic enough to fully deprotonate H₄(m-dobdc).

Fig. 4 PXRD (λ = 1.5406 Å) patterns of Mg₂(m-dobdc)-MC synthesized on small and large scale under mechanochemical conditions. The simulated pattern based on the previously reported single-crystal X-ray diffraction structure of the isostructural framework Co₂(m-dobdc) is included for reference.²⁵

With optimized samples of Mg₂(m-dobdc)-ST and Mg₂(m-dobdc)-MC in hand, we compared their crystallite morphologies (Fig. 5) and porosities (Table 1) in order to understand how synthesis procedure affects the physical properties of Mg₂(m-dobdc). Characterization of Mg₂(m-dobdc)-ST by scanning electron microscopy (SEM) revealed that it is composed of crystalline needles >5 μm in length on average (Fig. 5 left, ESI† Fig. S12). A needle-like morphology for Mg₂(m-dobdc)-ST is consistent with that previously reported for single crystals of the isostructural framework Co₂(m-dobdc).^25,28 Similarly, Mg₂(m-dobdc)-MC is comprised of needles <1 μm in length (Fig. 5, right, ESI† Fig. S26). The smaller crystallites for mechanochemically synthesized Mg₂(m-dobdc)-MC likely arise due to rapid deprotonation of H₄m-dobdc by triethylamine during the reaction.^14,29

Fig. 5 SEM images of Mg₂(m-dobdc)-ST (left) and Mg₂(m-dobdc)-MC (right).

Table 1 77 K N₂ BET and Langmuir surface areas of Mg₂(m-dobdc)-ST and Mg₂(m-dobdc)-MC. Literature values reported for related Mg₂(dobdc) are included for comparison

Material	BET surface area (m^2 g^−1)	Langmuir surface area (m^2 g^−1)
Mg2(m-dobdc)-ST	1556 ± 2	1971 ± 3
Mg2(m-dobdc)-MC	1653 ± 2	1964 ± 38
Mg2(dobdc)	1495 (ref. 12)	1905–1957 (ref. 10 and 12)

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Careful activation of Mg₂(m-dobdc)-ST and Mg₂(m-dobdc)-MC under high vacuum (<10 μbar) at 180 °C for at least 24 h was sufficient to fully remove solvent molecules from both frameworks. Their porosities were assessed by collecting 77 K N₂ adsorption isotherms (ESI† Fig. S9 and S23). As expected, Mg₂(m-dobdc)-ST and Mg₂(m-dobdc)-MC are both microporous, with BET and Langmuir surface areas comparable to those reported for related Mg₂(dobdc) (Table 1).^10,12 The BET surface area of Mg₂(m-dobdc)-MC (1653 ± 2 m² g⁻¹) is somewhat higher than that of Mg₂(m-dobdc)-ST (1556 ± 2), reflecting a greater degree of accessible pores and/or the presence of insoluble, amorphous impurities in the latter material. We note that heating Mg₂(m-dobdc) with ramp rates faster than 1 °C min⁻¹ or to temperatures greater than 180 °C consistently led to lower surface areas, likely due to partial pore collapse. Nonetheless, these findings confirm that highly porous and crystalline Mg₂(m-dobdc) can be readily prepared under both solvothermal and mechanochemical conditions.

Given the microporosity of both Mg₂(m-dobdc) samples and the high capacities of closely related Mg₂(dobdc) for a range of adsorbates,^10,12 we assessed the potential suitability of Mg₂(m-dobdc)-ST and Mg₂(m-dobdc)-MC for gas capture applications using CO₂ scrubbing from coal flue gas as a representative separation. As such, CO₂ adsorption and desorption isotherms at 30 °C, 40 °C, and 50 °C and N₂ adsorption and desorption isotherms at 40 °C were collected for Mg₂(m-dobdc)-ST (Fig. 6a, ESI† Fig. S14 and S15) and Mg₂(m-dobdc)-MC (Fig. 6b, ESI† Fig. S28 and S29). In all cases, gas sorption was found to be completely reversible. Both materials exhibit steep uptake at low CO₂ pressures, indicative of strong interaction of CO₂ with exposed Mg(II) sites.¹² The maximum CO₂ uptakes for both materials at 30 °C and 1 bar of CO₂ are 6.50 mmol g⁻¹ for Mg₂(m-dobdc)-ST and 8.69 mmol g⁻¹ for Mg₂(m-dobdc)-MC; the latter value is similar to that predicted for binding one CO₂ per Mg(II) site in this material (8.24 mmol g⁻¹). The higher CO₂ capacity of Mg₂(m-dobdc)-MC than Mg₂(m-dobdc)-ST is consistent with its higher 77 K N₂ BET surface area and suggests that the mechanochemically synthesized MOF has more accessible Mg(II) sites.

Fig. 6 30 °C, 40 °C, and 50 °C CO₂ and 40 °C N₂ adsorption isotherms in a) Mg₂(m-dobdc)-ST and b) Mg₂(m-dobdc)-MC. The lines correspond to individual fits to the dual-site Langmuir model. A data point was considered equilibrated when <0.01% pressure change occurred over a 30 s interval. c) Enthalpy of adsorption (−ΔH_ads) values for Mg₂(m-dobdc)-ST and Mg₂(m-dobdc)-MC determined using simultaneous fits to dual-site Langmuir models. d) Summary of CO₂ and N₂ uptake values and non-competitive CO₂/N₂ selectivities relevant to CO₂ capture from coal flue gas for Mg₂(m-dobdc)-ST and Mg₂(m-dobdc)-MC. The corresponding values for Mg₂(dobdc) reported in the literature are included for reference.¹⁹

To confirm that CO₂ binding in Mg₂(m-dobdc) materials likely occurs at coordinatively unsaturated Mg(II) sites, the CO₂ adsorption isotherms were fit using dual-site Langmuir models both independently and simultaneously (ESI† Tables S2 and S4). The independent dual-site Langmuir model fits are included in Fig. 6a and b and represent good fits to the experimental data. Using the Clausius–Clapeyron relationship, the differential enthalpies of adsorption (−ΔH_ads) as a function of CO₂ loading were calculated (Fig. 6c, ESI† Fig. S16 and S30). The −ΔH_ads values at low coverage are comparable for Mg₂(m-dobdc)-MC (42 kJ mol⁻¹) and Mg₂(m-dobdc)-ST (37 kJ mol⁻¹) and are similar to those previously reported for CO₂ adsorption in MOFs bearing accessible Mg(II) sites as well (38–43 kJ mol⁻¹).^10,30 The −ΔH_ads plots support that Mg₂(m-dobdc)-MC contains more accessible Mg(II) sites than Mg₂(m-dobdc)-ST, as the strong binding of CO₂ drop offs at higher loadings in this material (∼0.7 CO₂ per Mg vs. ∼0.4 CO₂ per Mg in Mg₂(m-dobdc)-ST).

Among MOFs, Mg₂(dobdc) possesses one of the highest reported CO₂ capacities (5.28 mmol g⁻¹) under conditions relevant to CO₂ capture from coal flue gas (150 mbar, 40 °C) (Fig. 6d).^10,18,19 Coupled with minimal uptake of N₂ at 750 mbar and 40 °C (0.50 mmol g⁻¹),¹⁹ this high CO₂ uptake at low pressures makes Mg₂(dobdc) a promising material for CO₂/N₂ separations. While the CO₂ uptake of Mg₂(m-dobdc)-ST at 150 mbar and 40 °C (3.99 mmol g⁻¹) is less than that reported for Mg₂(dobdc), likely due to its dearth of accessible Mg(II) sites, Mg₂(m-dobdc)-MC exhibits a higher capacity for CO₂ (6.14 mmol g⁻¹) than Mg₂(dobdc) under these conditions (Fig. 6d). Previous studies have suggested that the metal centers of M₂(m-dobdc) MOFs are slightly more Lewis acidic than those of M₂(dobdc) MOFs,¹³ which may account for the enhanced CO₂ uptake at low pressures in Mg₂(m-dobdc)-MC. Consistently, the N₂ capacity of Mg₂(m-dobdc)-MC at 750 mbar and 40 °C (0.89 mmol g⁻¹) is higher than that reported for Mg₂(dobdc) as well. Given the unclear suitability of calculating selectivities in open metal site MOFs using ideal adsorbed solution theory (IAST),³¹ we elected to calculate non-competitive CO₂/N₂ selectivities under conditions relevant to coal flue gas capture for Mg₂(m-dobdc)-ST, Mg₂(m-dobdc)-MC, and Mg₂(dobdc) instead (Fig. 6d). The non-competitive CO₂/N₂ selectivity calculated for Mg₂(dobdc) is the highest (53),¹⁹ followed by Mg₂(m-dobdc)-ST (44), and then Mg₂(m-dobdc)-MC (35). The diminished non-competitive CO₂/N₂ selectivity for Mg₂(m-dobdc)-MC is due to the higher uptake of N₂ in this material. Nonetheless, these findings support that Mg₂(m-dobdc)-MC is competitive with the widely studied MOF Mg₂(dobdc) for this representative separation. Further, the superior gas sorption performance of Mg₂(m-dobdc)-MC over Mg₂(m-dobdc)-ST indicates that mechanochemical methods may be preferable for the scalable synthesis of this framework.

Conclusions

Owing to their low cost and high gravimetric gas storage capacities, MOFs bearing high densities of coordinatively unsaturated Mg(II) centers are highly sought after. We have demonstrated that Mg₂(m-dobdc) synthesized under mechanochemical conditions is a promising new Mg-based MOF due to its strong binding and high capacity for CO₂ at low pressures. Notably, the work presented herein represents a rare example in which a mechanochemically synthesized MOF displays superior gas sorption properties compared to material synthesized under traditional solvothermal conditions.^14–17 Moving forward, mechanochemical methods will prove to be a valuable alternative to solvothermal syntheses for the preparation of high-quality MOFs for applications in gas storage, chemical separations, and drug delivery.

Author contributions

P. J. M. and R. M. M. conceived the project. E. Y. C. and R. M. M. carried out all experiments. The manuscript was written through the contributions of all authors, and all authors approved of the final version.

Conflicts of interest

P. J. M. is listed as an inventor on several patents related to the application of MOFs for gas capture.

Acknowledgements

The development of new methods for the synthesis of Mg-based MOFs was supported by the National Institute of General Medical Sciences of the National Institutes of Health under award number R35GM138165. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The characterization of the CO₂ adsorption properties of Mg₂(m-dobdc) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0021000. This work made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC program (DMR-1719875). Some of the ¹H NMR data in this work were collected on a Bruker INOVA 500 MHz spectrometer that was purchased with support from the NSF (CHE-1531632). We thank Cornell University for providing a Summer Experience Grant to E. Y. C.

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Footnotes

† Electronic supplementary information (ESI) available: All procedures and characterization data. See DOI: https://doi.org/10.1039/d2ce00739h

‡ Contributed equally.

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