Benchmarking nitrous oxide adsorption and activation in metal–organic frameworks bearing coordinatively unsaturated metal centers

Abstract

Anthropogenic emissions of N₂O, the third most abundant greenhouse gas after CO₂ and CH₄, are contributing to global climate change. Although metal–organic frameworks (MOFs) have been widely studied as adsorbents for CO₂ and CH₄, less effort has focused on the use of MOFs to remove N₂O from emission streams or from air. Further, N₂O activation would enable its use as an inexpensive oxidant for fine chemical synthesis. Herein, we identify features that contribute to strong binding and high uptake of N₂O at coordinatively unsaturated metal sites in the M₂Cl₂(btdd) (M = Mn, Co, Ni, Cu; btdd²⁻ = bis(1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) and M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn; dobdc⁴⁻ = 2,5-dioxido-1,4-benzenedicarboxylate) series of MOFs. Combined experimental and computational studies suggest that N₂O adsorption at open-metal-sites is primarily based on electrostatic interactions, rather than π-backbonding, causing MOFs with more Lewis acidic metal centers to be superior N₂O adsorbents. As a result, Mg₂(dobdc) demonstrates strong binding and record-setting N₂O uptake (8.75 mmol g⁻¹ at 1 bar and 298 K). Using density functional theory (DFT) to characterize reactive intermediates and transition states, we demonstrate that N₂O activation to form a M(IV)–oxo species and N₂ is thermodynamically favorable in Mn₂(dobdc) and Fe₂(dobdc) but appears to be kinetically limited in Mn₂(dobdc). Our work lays a foundation for understanding N₂O adsorption and activation in MOFs, paving the way for the design of promising next-generation materials for N₂O capture and utilization.

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Introduction

N₂O is the third most prevalent anthropogenic greenhouse gas after CO₂ and CH₄, accounting for 6% of the effective radiative forcing from 1960 to 2019.¹ Although CO₂ and CH₄ are present in higher concentrations in the atmosphere, the global warming potential of N₂O (265) is far greater (1 and 28 for CO₂ and CH₄, respectively) and its atmospheric lifetime (116 year) is far longer (1 and 12 year for CO₂ and CH₄, respectively).² Over the last four decades, global anthropogenic emissions of N₂O have increased by 30%.³ Up to 87% of this increase derives from agricultural practices such as nitrogen additions to soils. By nature, the sources of these emissions (i.e., farmlands) are diffuse, in contrast to point sources of N₂O emissions such as adipic and nitric acid manufacturing.⁴ Diffuse emissions are currently largely uncontrolled, except for preventative measures such as the use of more efficient fertilizers.^5,6 At point sources, N₂O can be catalytically destroyed, but regulations requiring this practice have not been globally adopted.⁷ Thus, the majority of human-caused N₂O emissions are currently unabated, leading to a current estimated rate of increase in atmospheric N₂O concentration of 2% per decade.³ In addition to its global warming potential, N₂O was found to be the dominant ozone-depleting substance emitted in the 21st century,⁴ underscoring the urgency of curtailing N₂O emissions. The environmental effects of anthropogenic N₂O emissions and its long atmospheric lifetime necessitate the development of new materials for N₂O capture.

Selectively capturing gases from diffuse sources such as air is a unique challenge that requires specially designed sorbents. Metal–organic frameworks (MOFs) are an emerging class of materials that have drawn significant interest for their potential applications in greenhouse gas capture,⁸ separations,⁹ catalysis,¹⁰ and beyond.¹¹ They are crystalline, highly porous materials formed by connecting metal secondary building units (SBU) with multitopic organic linkers. MOFs have been designed to selectively bind CO₂ and CH₄ based on electrostatic interactions,^12,13 chemical reactivity,^14–16 hydrogen bonding,¹⁷ and more. In contrast, only a small number of largely unrelated MOFs have been studied for N₂O capture to date.^18–25 The presence of Lewis acidic open-metal-sites has been shown to enhance N₂O binding;¹⁹ however, a general lack of structure–property trends informing the design of new materials hinders improvements in N₂O capture in porous materials.

Herein, we present a structure–activity study of N₂O adsorption in MOFs, with the purpose of identifying features that lead to strong binding and high uptake. Through a combined experimental and computational analysis, we clarify the effects of the ligand field and metal identity on N₂O adsorption at Lewis acidic open-metal-sites in the M₂Cl₂(btdd) (M = Mn, Co, Ni, Cu; btdd²⁻ = bis(1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin)^26–28 and M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn; dobdc⁴⁻ = 2,5-dioxido-1,4-benzenedicarboxylate) series of MOFs.^12,29,30 From this analysis, we identify the M₂(dobdc) series, particularly Mg₂(dobdc) and Ni₂(dobdc), as promising adsorbents that display strong binding and record-setting N₂O adsorption capacities.

Beyond reducing its environmental impact, N₂O capture is incentivized by the opportunity to utilize it as a cheap, abundant, and potent oxidant in organic synthesis.^31–33 As a kinetically inert molecule, transition metal catalysts and/or high temperatures and pressures are required to facilitate N₂O activation. Thus, MOFs and zeolites containing Fe(II) sites have been explored as catalysts for N₂O utilization.^34–39 In particular, Fe₂(dobdc) has been demonstrated to catalyze C–H oxidation of hydrocarbons using N₂O as an oxidant.^37,40–42 Bearing strong similarities to enzymatic iron-based catalysts,^43–45 this occurs through a 2 e⁻ transfer from high spin Fe(II) to the oxygen of N₂O, cleaving the N–O bond and forming a high spin Fe(IV)–oxo intermediate. This process is calculated to be followed by σ-attack and H atom abstraction by the Fe(IV)–oxo and radical rebound to generate the oxidized product.⁴⁰ Herein, DFT calculations support that N₂O activation is also thermodynamically favorable in Mn₂(dobdc), and we map out an approximate reaction coordinate for this process using the climbing-image nudged elastic band method (CI-NEB). While combined analysis of computation and experiment suggests that this reaction is kinetically limited in Mn₂(dobdc), our work motivates further study of N₂O activation in related Mn-based MOFs. Overall, our findings stimulate the development of new framework materials for selective N₂O capture and conversion to mitigate its environmentally destructive impact.

Results and discussion

N₂O adsorption analysis

N₂O is a weakly σ-donating and π-accepting ligand, which has limited the number of well-characterized transition metal adducts of N₂O to only a small handful, typically bound as η¹-N or η²-N,N coordinated N₂O through π-backbonding interactions.^46–52 Although N₂O has a relatively weak dipole moment (0.161 D),⁵³ sorbents could compensate by incorporating Lewis acidic sites to strengthen electrostatic interactions between N₂O and the sorbent material. MOFs containing highly Lewis acidic coordinatively unsaturated metal centers are effective adsorbents for a wide variety of gases through strong electrostatic interactions.⁵⁴ To identify robust structure–property trends regarding N₂O binding at open-metal-sites, we targeted isostructural frameworks accommodating a range of metal ions in similar ligand fields. These criteria should effectively yield insights into the interplay of the metal identity and ligand field on the binding strength of N₂O at open-metal-sites in MOFs.

Two groups of MOFs that fulfil these requirements are the M₂Cl₂(btdd) and M₂(dobdc) series. These MOFs are composed of hexagonal-pored structures formed by connecting one-dimensional, rod-like metal SBUs with ditopic organic linkers (Fig. 1). Porous channels extend through the materials and, after activation, become lined with a high density of coordinatively unsaturated metal sites confined in square pyramidal geometries. While the overall structures of the M₂Cl₂(btdd) and M₂(dobdc) series are similar, their SBUs lead to distinctive ligation of the metal sites. In M₂Cl₂(btdd), metal ions are coordinated to three individual triazolate groups through one nitrogen atom per triazolate. Each triazolate coordinates to three distinct metal ions, and two chlorides (μ₂) bridge neighboring metals, forming M–Cl chains. For this study, the isostructural Mn, Co, Ni, and Cu MOFs of this series were synthesized according to reported procedures (Fig. 1a and Sections S3–S6, ESI†).^26,28,55 In the M₂(dobdc) series, the secondary building units are composed of metal-salicylate chains, forming an oxygen-based ligand field. The Mg, Mn, Fe, Co, Ni, Cu, and Zn variants of M₂(dobdc) were synthesized in accordance with the literature (Fig. 1b and Sections S7–S13, ESI†).^{12,30,56–59} All MOFs analyzed in this study were confirmed to be highly crystalline and match the expected structures via powder X-ray diffraction (PXRD). The porosity of all MOFs was confirmed using 77 K N₂ adsorption/desorption measurements. The calculated surface areas are similar to those reported in the literature in every case.

Fig. 1 General synthesis conditions of (a) M₂Cl₂(btdd) (M = Mn, Co, Ni, Cu) and (b) M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn) MOFs (R = Et, Me, iPr, H). Structures of (c) M₂Cl₂(btdd) and (d) M₂(dobdc). Purple, light green, blue, grey, and red spheres represent metal, chlorine, nitrogen, carbon, and oxygen atoms, respectively. Hydrogen atoms are omitted for clarity.

After synthesis, the MOFs were each evaluated as N₂O sorbents by measuring N₂O adsorption and desorption isotherms at 25 °C, 35 °C, and 45 °C (Sections S3–S13, ESI†). Adsorption data were fit using dual-site Langmuir–Freundlich models (eqn (S1), ESI†). These fits were subsequently used to calculate enthalpies of N₂O adsorption (−ΔH_ads) in each MOF using the Clausius–Clapeyron equation (eqn (S2), ESI†). The resulting −ΔH_ads values as a function of N₂O uptake in both series of MOFs are summarized in Fig. 2 and Table 1. Critically, PXRD and surface area measurements confirm the stability of every MOF towards N₂O except for Fe₂(dobdc), which has been previously shown to react irreversibly with this gas.^37,40–42 As a result, the adsorption data for this MOF were excluded from the analysis below.

Fig. 2 (a) –ΔH_adsvs. N₂O uptake calculated from N₂O adsorption isotherms in (a) M₂Cl₂(btdd) (M = Ni, Co, Cu, Mn) and (b) M₂(dobdc) (M = Ni, Mg, Co, Mn, Zn, Cu) MOFs.

Table 1 N₂O adsorption enthalpies and maximum uptake values at 298 K in M₂Cl₂(btdd) and M₂(dobdc) MOFs

MOF	−ΔHads N2O (kJ mol^−1)	Maximum uptake at 298 K (mmol g^−1)
Mn2Cl2(btdd)	23.0 ± 1.1	2.37
Co2Cl2(btdd)	23.9 ± 0.8	3.10
Ni2Cl2(btdd)	23.8 ± 1.1	3.49
Cu2Cl2(btdd)	19.2 ± 1.1	1.86
Mg2(dobdc)	39.9 ± 0.5	8.75
Mn2(dobdc)	33.4 ± 0.6	7.77
Co2(dobdc)	37.5 ± 0.8	7.26
Ni2(dobdc)	43.8 ± 0.6	6.44
Cu2(dobdc)	24.3 ± 0.7	2.21
Zn2(dobdc)	30.0 ± 0.5	5.37

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The M₂Cl₂(btdd) series were first evaluated as N₂O sorbents. Despite their high density of open-metal-sites, all MOFs of this series only weakly interact with N₂O (−ΔH_ads < 25 kJ mol⁻¹). The difference in binding strength between the variants is minor: Mn, Co, and Ni analogues exhibit similar binding strengths, followed by Cu (Table 1). MOFs bearing coordinatively unsaturated Cu sites are commonly poor adsorbents due to Jahn–Teller axial distortion at the Cu sites.^12,60 Lacking substantial differences in binding strength between variants, the influence of the metal identity appears to be overshadowed by that of the ligand field in the M₂Cl₂(btdd) series. The Lewis acidities of the metals are likely tempered by the electron-donating coordination environment of triazolate and chloride ligands. These ligands produce electron-rich metal centers that are less Lewis acidic than those found in other materials. Although these MOFs are capable of binding polar gases such as NH₃,²⁶ the absence of a significant dipole moment on N₂O makes the M₂Cl₂(btdd) MOFs ineffective N₂O adsorbents. Lacking suitable binding sites, the M₂Cl₂(btdd) series show very limited uptakes; the quantity of N₂O adsorbed under equilibrium conditions, even at 1000 mbar N₂O and 298 K (Table 1), does not come close to saturating the available open-metal-sites.

Decreasing the ligand field strength surrounding the open-metal-sites should produce more Lewis acidic metal centers and thus lead to stronger electrostatic interactions with N₂O. Switching from M₂Cl₂(btdd) to M₂(dobdc) preserves the coordination geometry of the metal centers, but the oxygen-based SBU provides a weaker ligand field overall. Indeed, every member of the M₂(dobdc) series binds N₂O more strongly at low pressures than the M₂Cl₂(btdd) MOFs (Fig. 2b). Moreover, the comparatively electron-deficient coordination environments in this series emphasize the influence of the metal identity on N₂O binding strengths; enthalpies of adsorption vary by as much as 20 kJ mol⁻¹ among M₂(dobdc) variants (Table 1). N₂O binding strengths in these series mirror the empirical Irving–Williams series: Cu < Zn < Mn < Co < Mg < Ni.⁶¹ Like Cu₂Cl₂(btdd), Cu₂(dobdc) likely exhibits weak N₂O adsorption because of axial distortion.¹² In contrast, Mg₂(dobdc) is an especially effective N₂O adsorbent due to the hard nature of its Lewis acidic Mg(II) cations. As the effective charge of the transition metal centers increases from Mn to Ni,⁶² the N₂O binding strengths increase as well. As a result, Ni₂(dobdc) is the strongest N₂O adsorbent assessed in this study (−ΔH_ads = 43.8 ± 0.6 kJ mol⁻¹). Compared to other commonly studied gases (CO₂, O₂, N₂), N₂O is generally more strongly bound by the M₂(dobdc) series, which is likely due to its modest dipole moment. In particular, N₂O binding is approximately 2–6 kJ mol⁻¹ stronger than CO₂ in all cases except for in Mg₂(dobdc), in which CO₂ binds more strongly by approximately 3 kJ mol⁻¹.¹² O₂ and N₂ binding enthalpies are consistently 10–20 kJ lower than those of N₂O as well.⁶³ These findings highlight the discrepancy between the comparatively strong binding of N₂O at metal centers compared to CO₂, O₂, and N₂ and the lack of well-characterized N₂O-bound metal complexes.^46–50

Strong N₂O binding is accompanied by relatively high N₂O capacities in the M₂(dobdc) series of frameworks. In particular, the second-strongest adsorbent of N₂O identified in this work, Mg₂(dobdc), adsorbs 8.75 mmol g⁻¹ at 1000 mbar of N₂O and 298 K, surpassing the previous record-holder, MIL-100 (Cr) (5.78 mmol g⁻¹, MIL = Materials Institute Lavoisier), by a significant margin.²⁰ The Mn, Co, and Ni M₂(dobdc) variants also exceed the previous record for N₂O uptake in a MOF. Overall, this comparison underlines the M₂(dobdc) series, especially Mg₂(dobdc), as promising adsorbents that exhibit high gravimetric N₂O capacities coupled with strong and tuneable binding strengths.

Computational survey of N₂O adsorption.

After establishing the M₂(dobdc) series as effective N₂O adsorbents, we further evaluated N₂O binding in this series using DFT calculations (Section S14, ESI†). For these calculations, trimetallic cluster models were generated to approximately study the one-dimensional chain SBU (Fig. 3a). Similar cluster models have been used to evaluate N₂O reduction in Fe₂(dobdc).^40,41 For each member of the M₂(dobdc) series, excluding Mg₂(dobdc) for redundancy, two types of cluster models were simulated: a trimetallic system and a Mg-diluted system. In the Mg-diluted models, both edge metal ions were replaced with Mg(II) ions. The purpose of including the Mg-diluted systems is to simulate only one open-shell metal center and decouple adsorption energetics from the potential influence of metal–metal coupling on N₂O adsorption. All systems are neutral with all M(II) ions in the high-spin state where applicable (Table S13, ESI†).

Fig. 3 (a) Trimetallic (left) and Mg-diluted (right) cluster models of M₂(dobdc). Purple, green, grey, red, and white spheres represent metal, magnesium, carbon, oxygen, and hydrogen atoms, respectively. DFT-calculated negative adsorption energies (−ΔE_ads) of η¹-N (blue) and η¹-O (red) coordinated N₂O in the (b) Mg-diluted and (c) trimetallic systems.

The energies of adsorption (−ΔE_ads) for both η¹-N and η¹-O coordinated N₂O adducts in the model clusters were first calculated (Fig. 3b–c and Table S11, ESI†). In both the trimetallic and Mg-diluted models, η¹-O coordinated N₂O is slightly more stabilized (2–10 kJ mol⁻¹) than η¹-N coordinated N₂O. This aligns with the approximately 60%/40% η¹-O/η¹-N population split determined from neutron diffraction data collected on N₂O-dosed Fe₂(dobdc).³⁷ The calculated formation energies of N₂O adducts in the Mg-diluted series (Fig. 3b) approximately match the trend observed experimentally: Cu ≈ Mn < Fe ≈ Zn < Co < Ni ≈ Mg. However, some divergence is noted in the trimetallic systems (Fig. 3c): the Mn-based trimetallic system binds N₂O considerably more strongly than the equivalent Mg-diluted model, suggesting multiple metal effects that require consideration of metal–metal coupling (see below). Additionally, η¹-N coordination to Mn₂(dobdc) is slightly more favorable than η¹-O in the trimetallic model.

Across the series, the model systems reveal that η¹-O and η¹-N adducts of N₂O are bent, with bond angles and M–N₂O distances in agreement with those solved from the neutron diffraction data in N₂O–Fe₂(dobdc) (Table S12, ESI†), the only definitively characterized structure of N₂O bound to a metal center within a MOF reported to date.³⁷ In the DFT-calculated structure, η¹-N coordinated N₂O exhibits a typical bond angle of 115–123° (122° in N₂O–Fe₂(dobdc)), whereas η¹-O coordinated N₂O exhibits slightly smaller bond angles ranging from 105–120° (117° in N₂O–Fe₂(dobdc)). Bond lengths of η¹-N and η¹-O coordinated N₂O (2.25–2.57 Å and 2.27–2.54 Å, respectively) vary over only a narrow range. These models reinforce that N₂O binding across the M₂(dobdc) series is primarily based on electrostatic interactions rather than π-backbonding, which would be expected to lead to linear η¹-N (V, Cu, Ru, Rh) or side-on η²-N,N (Co, Ni) interactions with N₂O.^46–52

It should be noted that N₂O adsorption has been modelled previously in a small variety of other open-metal-site MOFs. The Fe(II)- and Cu(II)-based paddlewheel nodes in M₃(btc)₂ (btc³⁻ = benzene-1,3,5-tricarboxylate) MOFs show bent η¹-N and η¹-O coordination modes for N₂O.⁵⁴ Likewise, N₂O adducts have been modelled for the trinuclear carboxylate-bridged, oxygen-centered nodes (M₃(μ₃-O)(RCOO)₆, M = V, Cr, Mn, Fe, Co, Ni) common among MOFs such as MIL-100, MIL-101, and MIL-127.⁶⁴ The calculated binding of η¹-N coordinated N₂O at V centers is linear, but other adducts are bent, with similar bond angles and bond lengths as calculated in the M₂(dobdc) series herein. In the Kuratowski-type SBU of Cu-MFU-4l (MFU = Metal–Organic Framework Ulm-University), DFT calculations support an approximately linear η¹-N coordinated N₂O molecule, indicating possible π-backbonding from the Cu(I) centers.¹⁹ Overall, these findings support that N₂O is predicted to bind in a bent fashion at most metal centers in MOFs.

A notable exception to the trends outlined above is the trimetallic Mn cluster, in which η¹-N coordinated N₂O is nearly linear (172°), suggesting that π-backbonding occurs from the Mn d orbitals into the π* orbital of N₂O, which has previously been invoked in linear η¹-N V, Cu, Ru, and Rh adducts of N₂O to justify the stability of those complexes.^{46,48–50,52} Consistently, the Mn–N bond length (1.95 Å) is considerably shorter than in other models (Fig. S68, ESI†). These characteristics are distinct from the equivalent Mg-diluted cluster, suggesting that metal–metal coupling may affect the binding mode of N₂O in these calculations. We thus evaluated N₂O binding in the open-shell trimetallic systems (Mn through Cu) using broken-symmetry density functional theory (BS-DFT) with two spin flip configurations (i.e., in the central metal or in one edge metal) to quantify the metal–metal coupling and to extract coupling constants (Fig. S69 and Section S14, ESI†). To ensure that the BS-DFT calculations converged to the desired states, the spin density was visually inspected (Fig. S70, ESI†). In particular, the energetic difference in the trimetallic Mn system in comparison to the dilute case indicates especially strong metal–metal coupling (Table S14, ESI†). From this data, we calculated strong magnetic coupling along the SBU chains in the trimetallic Mn system (J = 314.27 cm⁻¹), which becomes even stronger after N₂O binding regardless of the coordination mode (J = 857.47 cm⁻¹ for η¹-N, J = 857.47 cm⁻¹ for η¹-O). Consistently, magnetic susceptibility measurements (Fig. S77, ESI†) and prior first principles studies support that Mn₂(dobdc) exhibits antiferromagnetic coupling below approximately 27 K.^62,65 Adsorbate-induced changes in magnetic coupling are also precedented in the M₂(dobdc) series; for example, in Fe₂(dobdc), the ferromagnetic exchange strength along the SBU chains is attenuated by interaction with weak adsorbates (e.g., CH₄, −ΔH_ads = 20 kJ mol⁻¹), and the coupling becomes antiferromagnetic upon interaction with strong adsorbates (e.g., C₂H₂, −ΔH_ads = 47 kJ mol⁻¹).⁶⁶ Our results point to an additional stabilization of the N₂O adducts of Mn₂(dobdc) related to enhanced metal coupling, possibly leading to greater π-backbonding from Mn to N₂O and favoring linear η¹-N coordination. Above 27 K, however, this effect is no longer expected to significantly contribute to the adsorption interactions as the spins become randomly oriented. Consistently, the experimental binding enthalpies do not show enhanced adsorption in Mn₂(dobdc) relative to other MOFs in the series. Nevertheless, if a suitable Mn-based open-metal-site MOF with a higher Néel temperature were to be identified, this feature could potentially be leveraged to enhance N₂O binding.

Evaluating N₂O activation in the M₂(dobdc) series

N₂O activation to form M(IV)–oxo species and N₂ in MOFs has been principally studied in Fe₂(dobdc) and other Fe(II)-based frameworks.^{37–41,67,68} Unfortunately, Fe(II)-based systems are prohibitively air sensitive for practical applications. Identifying more air-stable materials capable of N₂O activation would be a significant step towards utilizing N₂O as a green oxidant. In order to determine whether this reactivity is unique to Fe₂(dobdc), the same cluster models were used to calculate the favorability of M(IV)–oxo formation in the remainder of the M₂(dobdc) series (Table S15 and Section 14, ESI†). DFT calculations with both the trimetallic and Mg-diluted cluster model series show that, in addition to Fe₂(dobdc), M(IV)–oxo formation is thermodynamically favorable (ΔE_f < 0 kJ mol⁻¹) in Mn₂(dobdc) (Fig. 4a and Section 14, ESI†). Indeed, M(IV)–oxo formation is calculated to be significantly more thermodynamically favorable in Mn₂(dobdc) (−183.1 and −176.1 kJ mol⁻¹ for trimetallic and Mg-diluted systems, respectively) than in Fe₂(dobdc) (−55.5 and −52.7 kJ mol⁻¹ for trimetallic and Mg-diluted systems, respectively). In contrast, M(IV)–oxo formation is endothermic in the Mg, Co, Ni, Cu, and Zn analogues, ruling out these materials as potential catalysts for N₂O activation. This is fairly expected due to the electronic instability of terminal M(IV)–oxo complexes of octahedral symmetry possessing greater than five d electrons.⁶⁹ Precedent for N₂O activation in Mn-based systems is found in manganese oxides, which catalyze the decomposition of N₂O⁷⁰ and the oxidation of 1-butene at high temperatures.⁷¹ Likewise, Mn-substituted polyoxometalates have been shown to activate N₂O and catalyze the epoxidation of alkenes,^72,73 and Mn-substituted zeolites catalyze N₂O decomposition as well.^74,75

Fig. 4 (a) DFT-calculated energies of M–oxo formation (ΔE_f) in the trimetallic (purple) and Mg-diluted (green) models of M₂(dobdc). (b) CI-NEB-calculated approximate reaction coordinate of Mn–oxo formation from η¹-O coordinated N₂O in the Mg-diluted system. Inset: M–O (red) and N–O (blue) bond lengths vs. image number. (c) Structures of the initial (left), transition (center), and final (right) states. Purple, green, blue, grey, red, and white spheres represent manganese, magnesium, nitrogen, carbon, oxygen, and hydrogen atoms, respectively.

To determine the potential viability of N₂O activation by Mn₂(dobdc), an approximate reaction coordinate for this process was modelled using the CI-NEB method (Fig. 4b and Text S1, ESI†). The η¹-O coordinated N₂O adduct of the Mg-diluted Mn₂(dobdc) cluster model was used as the initial state, and the Mn(IV)–oxo-containing Mg-diluted cluster was used as the final state (Section S14, ESI†). During N₂O activation, the Mn(IV)–oxo bond is formed and the O–N bond is broken, forming N₂. From the initial state, the M–O bond length shrinks significantly and is matched by a substantial lengthening of the O–N bond. During this transition, the energy of the system rises sharply early on, after which it falls to roughly the energy of the final state. Likewise, a constant M–O bond length, consistent with Mn(IV)–oxo formation, was observed in the second half of the reaction coordinate, as the rest of the pathway is characterized by O–N bond elongation as unbound N₂ moves away from the cluster. The approximate transition state of the reaction is rather early, with Mn–O and N–O bond lengths of 2.00 Å and 1.39 Å, respectively (Fig. 4c). From the difference in energy between the initial state and this approximate transition state, the kinetic barrier to N₂O activation in this model cluster was calculated to be approximately 113 kJ mol⁻¹. This barrier is comparable to the calculated activation barriers of 167 kJ mol⁻¹ in the Mn-based trinuclear MOF nodes and 109 kJ mol⁻¹ in the Mn-substituted polyoxometalate discussed above.^64,73

In previous studies, heating N₂O-dosed Fe₂(dobdc) at only 35 °C was sufficient to partially oxidize the material, and its complete oxidation was accomplished at 60 °C after prolonged heating.³⁷ The activation barrier for Fe(IV)–oxo formation in N₂O-bound Fe₂(dobdc) was calculated to be 94 kJ mol⁻¹ (enthalpy of activation = 82 kJ mol⁻¹).⁴⁰ While we note that the method used to calculate the barrier in Fe₂(dobdc) differs from this work, it is nonetheless a useful comparison. Although the calculated activation barrier for Mn(IV)–oxo formation in Mn₂(dobdc) is notably larger, the conditions that lead to Fe(IV)–oxo formation in Fe₂(dobdc) are mild. As such, we evaluated whether more forceful conditions (i.e., higher temperatures) could enable Mn(IV)–oxo formation in Mn₂(dobdc). To probe this possibility, N₂O adsorption measurements in Mn₂(dobdc) at 180 °C, 250 °C, and 300 °C were collected to identify potential N₂O activation through changes in the adsorption properties of the material (Fig. S72, ESI†). Fully reversible N₂O adsorption was measured at 180 °C; however, measurements at 250 °C and 300 °C consistently yielded anomalous negative adsorption, possibly indicating reactivity with the MOF. For further analysis, a bulk sample of Mn₂(dobdc) was prepared by dosing the MOF with N₂O at 300 °C (Section S15, ESI,†). The BET surface area of Mn₂(dobdc) after heating at 300 °C (1285 ± 3 m² g⁻¹) under vacuum for 24 h is comparable to that of the pristine MOF (1344 ± 3 m² g⁻¹), supporting that this MOF is stable at elevated temperatures. Notably, the BET surface area is significantly attenuated after N₂O dosing at 300 °C (896 ± 2 m² g⁻¹) (Fig. S73, ESI†). The reduction in surface area is accompanied by a color change from orange to brown. PXRD measurements confirm that Mn₂(dobdc) retains its crystallinity after this process, although some peak-broadening was observed, indicative of partial decomposition (Fig. S74, ESI†).

Magnetic susceptibility measurements were used to characterize the product(s) resulting from high-temperature treatment of Mn₂(dobdc) with N₂O. The magnetic moments in Mn₂(dobdc) calculated from susceptibility measurements before (μ_eff = 5.94μ_B) and after (μ_eff = 5.89μ_B) (Fig. S80 and S81, ESI†) N₂O treatment at 300 °C closely match the value expected for Mn(II) with a spin of 5/2 (5.92μ_B), indicating a lack of oxidation at the metal center after N₂O treatment (Fig. 5). Despite this, increased magnetic susceptibility relative to unreacted Mn₂(dobdc) was consistently noted in moment vs. field measurements collected at 5 K after N₂O treatment at 300 °C (Fig. S82, ESI†). This finding may point to reactivity between N₂O and the redox-active linker instead,⁷⁶ as oxidation of the high-spin Mn(II) sites to Mn(IV) should result in a decrease in the magnetic susceptibility.

Fig. 5 Variable-temperature magnetic susceptibility (χ_molT) measurements of Mn₂(dobdc) under an applied field of 1000 Oe before (blue) and after (red) N₂O treatment at 300 °C. The minor increase in χ_molT of the N₂O-treated sample from approximately 2–40 K is attributed to slight O₂ contamination.

To further characterize the reaction of Mn₂(dobdc) with N₂O, we utilized variable-temperature diffuse reflectance Fourier transform (DRIFTS) spectroscopy (Fig. S75 and S76, ESI†). A sample of Mn₂(dobdc) was heated under an atmosphere of N₂O (approx. 1 bar) from 25 °C to 300 °C and held at 300 °C for 15 h. Spectra were collected periodically throughout the duration of the measurement. New Mn–O stretches corresponding to Mn(IV)–oxo (approx. 845 cm⁻¹)⁷⁷ or Mn(III)–OH species (600–700 cm⁻¹)⁷⁸ were not observed (Fig. S76, ESI†). Likewise, stretches corresponding to quinone formation due to linker oxidation were not observed (1657 cm⁻¹).⁷⁶ However, a weak O–H stretching frequency at 3673 cm⁻¹ emerged over time, and the prominent stretch at 1406 cm⁻¹ reduced in intensity after prolonged treatment with N₂O (Fig. S75, ESI†). Overall, while the MOF appears to react with N₂O at high temperatures, the DRIFTS and magnetic data indicate a lack of oxidation at the metal centers. This finding suggests that the high kinetic barrier to Mn(IV)–oxo formation in Mn₂(dobdc) is likely prohibitive, despite its thermodynamic favorability. Elucidating the products of this MOF's reaction with N₂O is the focus of ongoing work.

Conclusions

Herein, we identify features that contribute to strong binding and high uptake of N₂O at Lewis acidic, coordinatively unsaturated metal sites in MOFs. We utilize the M₂Cl₂(btdd) and M₂(dobdc) series as model systems to facilitate comparisons, as their general structures are related but distinguished by the ligation of the open-metal-sites. In the M₂Cl₂(btdd) series, the binding enthalpies of N₂O are generally low (<25 kJ mol⁻¹) and within error, despite variations in the metal identity. In contrast, the salicylate-based SBU in the M₂(dobdc) series provides a weaker, oxygen-based ligand field. N₂O binding strengths in these MOFs mostly mirror the Irving–Williams series, with Ni₂(dobdc) exhibiting the strongest adsorption of N₂O among all tested MOFs (−ΔH_ads = 43.8 ± 0.6). Notably, Mg₂(dobdc) exhibits a record-breaking N₂O gravimetric capacity (8.75 mmol g⁻¹ at 1000 mbar of N₂O and 298 K) among MOFs. While these Lewis acidic sites are effective for N₂O capture, they are not necessarily selective for N₂O among other polar(izable) gases, such as H₂O.⁷⁹ Achieving selective adsorption of N₂O at open-metal-sites will be the focus of future work.

Using the cluster systems to computationally model the oxidation of the M₂(dobdc) series by N₂O, forming M(IV)–oxo species, we find that M–oxo formation is also thermodynamically favorable in Mn₂(dobdc) according to DFT calculations. We thus used the CI-NEB method to map a reaction coordinate for this process and determined an approximate activation barrier of 113 kJ mol⁻¹, which is higher than that calculated for Fe₂(dobdc) (94 kJ mol⁻¹). Although experiments indicate that this kinetic barrier is too high to be overcome in Mn₂(dobdc), these findings suggest that Mn-based MOFs may be promising alternatives to traditionally studied Fe-based materials for N₂O activation.

Overall, this work adds to the growing body of research seeking to utilize N₂O as a green oxidant, in which Mn-based solid-state catalysts remain relatively understudied. The results reported herein will help to drive the identification and development of other effective MOF-based sorbents to mitigate environmentally destructive N₂O emissions.

Author contributions

P. J. M and H. J. K. conceived the project. T. A. P. carried out the synthesis, characterization, and gas sorption measurements of MOF samples under the supervision of P. J. M. P. J. M. and M. E. Z. also synthesized MOF samples. H. J. and A. N. carried out computational studies under the supervision of H. J. K. T. J. A. carried out magnetic measurements. T. A. P. and H. J. prepared the first draft of the manuscript, which was edited and approved by all co-authors.

Conflicts of interest

P. J. M. is listed as a co-inventor on several patents related to metal–organic frameworks.

Acknowledgements

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0021000 (T. A. P., T. J. A., M. E. Z., P. J. M.). Computational modeling was supported by the National Science Foundation under Award Number CBET-1846426 (H. J., A. N., H. J. K.) as well as a National Science Foundation Graduate Research Fellowship under Grant 1122374 (to A. N.). T. J. A. thanks Cornell University for financial support through a recruiting fellowship. We acknowledge the support of a Camille Dreyfus Teacher-Scholar Award to P. J. M. (TC-23-048). This work made use of a Bruker 500 MHz spectrometer, the purchase of which was supported by the National Science Foundation (CHE-1531632). This work made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC program (DMR-1719875). This work used Expanse at the San Diego Supercomputing Center through allocation CHE140073 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation grants 2138259, 2138286, 2138307, 2137603, and 2138296. We thank Prof. Joseph Zadrozny (the Ohio State University) for helpful discussions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, characterization, and gas sorption measurements of all MOFs, additional computational details, and high-temperature N₂O experiments. See DOI: https://doi.org/10.1039/d3tc04492k

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