A hexagon based Mn(II) rod metal–organic framework – structure, SF₆ gas sorption, magnetism and electrochemistry

Abstract

A manganese(II) metal–organic framework based on the hexatopic hexakis(4-carboxyphenyl)benzene, cpb⁶⁻: [Mn₃(cpb)(dmf)₃], was solvothermally prepared showing a Langmuir area of 438 m² g⁻¹, rapid uptake OF sulfur hexafluoride (SF₆) as well as electrochemical and magnetic properties, while single crystal diffraction reveals an unusual rod-MOF topology.

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Metal–Organic Frameworks, colloquially known as MOFs,¹ are coming of age with advanced² and basic³ textbooks published, and gas storage devices on the market with other applications close to commercialization.⁴ An emerging application is the capture of SF₆, a greenhouse gas some 22 000 times more potent than CO₂, used in industrial settings because of its dielectric properties, its non-toxicity, and thermal stabilty.^5,6

Manganese(II) is an attractive choice for such MOF construction as it is ubiquitous, non-toxic and present in relatively high amounts in most living systems. It is also a less common MOF metal ion, compared to i.e. Zn(II).⁷ In contrast to zinc(II) it is also magnetically and electrochemically active, properties that have recently been investigated.⁸ Higher stability may be induced in such MOFs by having the metal secondary building units (SBUs) forming an infinite rod.^9,10

Here we explore Mn(II) with the hexatopic linker hexakis(4-carboxyphenyl)benzene, cpb⁶⁻, Fig. 1. This hexagon shaped linker,¹¹ has earlier led us to investigate unique topological properties,¹² and recently we implicated the concerted movement of the six carboxyphenyl groups in gate opening CO₂ gas sorption dynamics in a rod-MOF.¹³ We noted that the size of the rhombic channels formed by pairwise stacks of cpb linkers would probably conform to the 7 Å diameter suggested as optimal for SF₆ capture,¹⁴ and therefore targeted this greenhouse gas in the present gas sorption study.

Fig. 1 The linker 1,2,3,4,5,6-hexakis(4-carboxyphenyl) benzene, cpb⁶⁻, and how it is connected to a MOF in [Mn₃(cpb)(dmf)₃], CTH-18.

Solvothermal synthesis in dimethylformamide (dmf) at 120 °C gave the MOF [Mn₃(cpb)(dmf)₃], CTH-18, in good yield. Contrary to some other M(II) cpb MOFs that form 2D kgd-nets with counter ions partly blocking the pores¹² the single crystal structure of CTH-18 shows an infinite rod metal SBU pairwise connecting the six-connected cpb linkers creating two types of channels running parallel with the rods having approximate minimal dimensions 4.3 × 6.0 Å and 3.4 × 4.7 Å when the van der Waals radii have been excluded. Thus, every Mn(II) only connects to four cpb linkers thereby maintaining a neutral framework, see Fig. 2 and Fig. S1 (ESI†).

Fig. 2 X-Ray single crystal structure of [Mn₃(cpb)(dmf)₃], CTH-18 with symmetry independent Mn atoms drawn in different shades of mauve. Gas sorption indicate the severely disordered dmf molecules can be removed by dynamic vacuum, and these have therefore been excluded for clarity. Left: View of the channels using van der Waals radii. Structure and gas sorption indicate apertures of around 5–7 Å. Right: Thermal ellipsoid drawing of the rod metal-SBU showing how the linkers connect pairwise to form the rod.

CTH-18 is stable in organic solvents including ethanol, unstable in 1 M NaOH(aq) and transforms to other crystalline phases in 1 M HCl and deionized water. Thermogravimetric studies in air indicated gradual solvent loss from the framework below 300 °C, and rapid breakdown of the MOF at 425 °C.

Taking the TGA results into account, the material was activated in dynamic vacuum at 250 °C prior to gas sorption analysis. N₂ sorption isotherms recorded at 77 K demonstrated the microporous nature of the material, shown by the steep N₂ adsorption isotherm shape at the very low-pressure regime, Fig. 3 top. However, we noted a very slow N₂ adsorption kinetics on CTH-18, especially at low relative pressures. In fact, repeated N₂ sorption experiments yielded different equilibrium uptakes due to this kinetic effect (and the equilibrium N₂ uptake points at low pressures would take several days to reach). The apparent hysteresis shown by the N₂ desorption isotherm was likely to be an artefact of this slow N₂ adsorption rather than N₂ entrapment, i.e. N₂ adsorption had not reached equilibrium when recording the adsorption isotherm.

Fig. 3 Top: N₂ (−195 °C) and CO₂ (−78 °C) adsorption and desorption isotherms of [Mn₃(cpb)(dmf)₃], CTH-18. Adsorption points are shown as solid symbols and desorption points are shown as hollow symbols. The observed hysteresis on the N₂ desorption isotherm was an artefact of the non-equilibrium adsorption isotherm, caused by very slow N₂ adsorption kinetics on CTH-18 at −195 °C Bottom: N₂, CH₄, SF₆ and CO₂ adsorption isotherms of CTH-18 recorded at 20 °C.

The slow diffusion of N₂ is most likely related to the dimensions of the pore channels being very close to the kinetic diameter of N₂ (3.6 Å). We therefore carried out CO₂ sorption experiments at −78 °C as this molecule has a smaller kinetic diameter (3.3 Å) and thus faster diffusion within these narrow pore channels. Indeed, we did not observe slow CO₂ adsorption kinetics on CTH-18 and neither hysteresis upon desorption. The specific BET and Langmuir surface area of CTH-18 estimated using the CO₂ adsorption isotherm were 354 and 438 m² g⁻¹, respectively (values estimated using the N₂ isotherm were 289 and 356 m² g⁻¹).

The pore size distributions (PSD) were estimated using the CO₂ adsorption isotherm by Density Functional Theory (DFT), with the slit pore model. The DFT PSD of CTH-18 are plotted in Fig. S2 (ESI†) and showed two distinct types of pores with estimated diameters of ∼4.3 and ∼5.1 Å. The estimated pore diameters should not be taken as accurate numbers but they were in a comparable range to the crystallographic pore sizes of 4.3 × 6.0 Å and 3.4 × 4.7 Å.

The 20 °C N₂, CH₄, SF₆ and CO₂ adsorption isotherms are shown in Fig. 3. CTH-18 showed selective uptake towards greenhouse gases over N₂. The CO₂ uptake at 1 bar (20 °C) reached over 3.0 mmol g⁻¹ with an isosteric heat of CO₂ sorption of around 30–35 kJ mol⁻¹ at up to 2.5 mmol g⁻¹ loading (Fig. S3, ESI†).

The SF₆ adsorption isotherm showed very sharp increase at low pressures. This sharp increase also indicated that the pore size was close to the kinetic diameter of SF₆ (5.5 Å). As the pore size of CTH-18 was similar to the size of the kinetic diameter of the adsorbate gas (SF₆), enhanced van der Waals interaction between the pore surface and the adsorbate gas was expected.^6,14,15 The enhanced interaction was reflected by the high isosteric heat of SF₆ adsorption (∼40–50 kJ mol⁻¹).

The greenhouse gas selectivity of CTH-18 was estimated using s = (q₁/q₂)/(p₁/p₂). The greenhouse gas selectivities were: for CO₂/N₂ ∼ 16 (15% CO₂, 85% N₂), for SF₆/N₂ ∼ 29 (10% SF₆, 90% N₂) and CH₄/N₂ ∼ 3.9 (50% CH₄, 50% N₂). The SF₆/N₂ selectivity of CTH-18 is thus slightly lower than some highly SF₆ selective MOFs such as Yb-TBAPy or UU-200.^6,14 However, the SF₆ uptake on CTH-18 at 0.1 bar was higher than many other MOFs, including Zn-MOF-74 (∼1.35 mmol g⁻¹ at 298 K)¹⁶ UiO-66-Zr (∼0.75 mmol g⁻¹ at 298 K)¹⁷ or SU-100 (∼1.08mmol g⁻¹ at 298 K).¹⁸

The SF₆ uptake kinetics on CTH-18 is shown in Fig. S3 (ESI†) to be reasonably fast, and over 80% of the equilibrium uptake was reached within 600 s. Moreover, CTH-18 showed high cyclic SF₆ uptake stability (Fig. 4 and Fig. S4, ESI†) with the BET/Langmuir specific area remained unchanged after a number of adsorption/desorption cycles (both with vacuum swing or temperature swing SF₆ adsorption).

Fig. 4 TGA cyclic SF₆ uptake on CTH-18. Adsorption was carried out at 30 °C over 20 minutes, regeneration by heat (desorption) was carried out at 100 °C for 20 minutes.

Water adsorption/desorption isotherms are shown in Fig. S3 (ESI†) indicating that CTH-18 has low water uptake at low relative pressures (p/p₀). Two steps in the water uptake were observed at p/p₀ between 0.1 and 0.3. The shape of the water sorption isotherms are comparable to some MOFs that have been investigated for water harvesting,¹⁹CTH-18 may thus be interesting for water harvesting applications, although this is beyond the scope of the present study.

Magnetic measurements on Mn(II) MOFs more often than not reveals antiferromagnetic couplings with a few notable exceptions like [Mn₁₂O₁₂(O₂CR)₁₆(H₂O)₄] (R = Me, Et, etc.) and other so-called single molecule magnets.²⁰ There is no reason to anticipate strong coupling through the cpb linker, and in the chain one would predict antiferromagnetic coupling by the relative close proximity of the d⁵ Mn(II) ions (ca. 4 Å), either directly or through spin polarization of the carboxylate linkers.²¹

This is also what we find for CTH-18, the temperature dependence of the magnetisation shows a Néel-like transition at around T_N ≈ 6 K, Fig. 5. The field-dependent magnetisation curves show the expected linear paramagnetic response in a wide temperature interval, as can be found in Fig. S4 (ESI†).

Fig. 5 Temperature dependence of the magnetization for CTH-18. An applied field of 1 kOe was used. Below the Néel transition at 6 K we can see the magnetic moment decrease as a result of the antiferromagnetic coupling and above 6 K we see paramagnetic behavior.

Calculations based on density functional theory (DFT) reveals an intra-chain antiferromagnetic coupling (5.7 meV Mn⁻¹ lower than the ferromagnetic state) between the Mn moments along the x-axis whereas the magnetic coupling between the chains is negligibly small (tens of μeV). The calculated Mn moments were around 4.5μ_B, establishing the oxidation state of Mn as 2+.

Electrochemistry has also, but less frequently, been investigated for Mn(II) MOFs.^22–24 We note that an easily accessible Mn(IV) state may be beneficial for water oxidation,²⁵ but we found no evidence of this in the cyclic voltammetry of CTH-18 sweeping up to potentials of 1.5 V vs. Ag/AgCl (Fig. S7, ESI†). The sweep rate dependence in a restricted potential region is shown in Fig. 6.

Fig. 6 Cyclic voltammetry at different sweep rates, see legends in phosphate buffer pH = 7. The electrode consisted of carbon paste and [Mn₃(cpb)(dmf)₃], CTH-18.

The redox couple is related to the oxidation of Mn(II) to Mn(III) in the MOF. To simulate the voltammetric response a mechanism separating the electrochemical oxidation and reduction steps was necessary. This is reasonable since the oxidation of Mn(II) to Mn(III) will require that an anion enter the structure.²⁰ The resulting Mn(III)-anion site is reduced on the negative going scan and the anion leaves. Simulated voltammograms are shown in Fig. S6 (ESI†).

The detailed structure of CTH-18 shows no unusual features as far as Mn(II) coordination and cpb conformations are concerned, but in contrast to [La₂(cpb)], CTH-17¹³ the cpb linkers are stacked with a longer spacing (5.7 Å vs. 5.5 Å) and linkers with opposite conformational chirality alter in the stacks, see Fig. S8 (ESI†).

The network topology on the other hand, merits a few words. CTH-18 is an example of a rod-MOF,^26–28 because the metal secondary building unit cannot be reduced to a 0D point forming a dot-MOF, instead it extends in one dimension. However, rod-MOFs pose a descriptive problem as they are commonly analyzed in a different way from dot-MOFs.^26,29,30

The straight rod, STR, approach is the simplest but even that in this case gives an unusual topology. Each cpb connects pairwise to the same points on the rod giving a three nodal 4- and 6-connected net with point symbol {5².6³.7}{4.5².6².7}{4.5⁴.6⁶.7⁴} (Fig. S9, ESI†). The more elaborate points-of-extension method that also considers the metal SBU gives a four-nodal six-connected net. Details of the topology assignment can be found in the ESI.†

In conclusion, CTH-18 shows high SF₆ uptake at 0.1 bar, a pressure practically relevant for SF₆ sorbents, combined with relatively fast kinetics. The SF₆ selectivity compared to N₂ is also higher than for CO₂ and CH₄, and the thermal stability, judged from TGA, is up to 425 °C. It therefore shows some promise for practical applications, shown to be needed by recent reports of industrial safety issues.³¹ ‡

We thank the Swedish Research Council (F. M. A. N, L. Ö.), the Olle Engqvist Foundation (F. M. A. N., L. Ö.), Chalmers areas of Advance Energy, Materials, Health and Nano (F. M. A. N.), the Swedish Research Council for Sustainable Development, #2018-00651 and the Swedish Research Council, #2020-04029 (O. C., M. Å.) and #2016-06959 (P. N., G. S. A.), the Chalmers Materials Analysis Laboratory, Chalmers students Mr A. Jonsson and Mr V. Engdahl for initial synthesis, and Prof. Peter Svedlindh at Uppsala University for discussions.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Crystallographic information files, details of synthesis, characterisation, DFT calculations and topology analysis. CCDC 2226584. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2cc06916d

‡ CTH-18 was prepared by solvothermal methods in dmf, and characterised by SCXRD, PXRD, TGA, SEM, gas sorption, magnetic and electrochemical measurements and elemental analysis. Topologies were analysed with ToposPro³² and SYSTRE³³ and chemical stability tests were performed.

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