Adsorption of Nitrogen Dioxide in a Redox-Active Vanadium Metal–Organic Framework Material

Nitrogen dioxide (NO2) is a toxic air pollutant, and efficient abatement technologies are important to mitigate the many associated health and environmental problems. Here, we report the reactive adsorption of NO2 in a redox−active metal–organic framework (MOF), MFM-300(V). Adsorption of NO2 induces the oxidation of V(III) to V(IV) centers in MFM-300(V), and this is accompanied by the reduction of adsorbed NO2 to NO and the release of water via deprotonation of the framework hydroxyl groups, as confirmed by synchrotron X-ray diffraction and various experimental techniques. The efficient packing of {NO2·N2O4}∞ chains in the pores of MFM-300(VIV) results in a high isothermal NO2 uptake of 13.0 mmol g–1 at 298 K and 1.0 bar and is retained for multiple adsorption–desorption cycles. This work will inspire the design of redox-active sorbents that exhibit reductive adsorption of NO2 for the elimination of air pollutants.

I mproving air quality is an important global issue. NO 2 remains a key outdoor air pollutant, with road transport being a major cause, particularly in areas with high population. 1 With increasingly stringent emission standards for vehicles being issued by the World Health Organization (WHO) and other local environmental agencies, 2 refinements and improvements to the existing selective-catalytic-reduction (SCR) systems are required urgently to deliver closer-to-zero emissions of NO 2 . While SCR is based on the reduction of NO x by ammonia or urea at elevated temperatures in the presence of a precious-metal-based catalyst, physio-sorption of gases in porous materials is a well-established method to remove gaseous species. Activated carbons have been long known to remove toxic gases, 3 but these often suffer from low adsorption capacity of NO 2 and/or rapid structural degradation. 4 Metal−organic frameworks (MOFs) show great potential for gas adsorption and storage, and this is attributed to their large surface area and pore functionalities. 5 Systems incorporating a redox active metal center are of particular interest in gas separation 6 and catalysis. 7 However, adsorption of NO 2 in MOFs remains largely unexplored, 8 and reported studies are based upon breakthrough experiments using gas streams containing NO 2 at ppm levels. In most of these cases, the MOF and MOF-containing composites undergo irreversible structural degradation on adsorption of NO 2 . Infrared spectroscopy has been used to characterize the species that are formed upon contacting the surface with highly reactive NO 2 . 9−14 Additionally, MOFs have also been explored as new sensing materials for NO 2 detection with high sensitivity and selectivity. 15 Reversible adsorption isotherms for pure NO 2 have been measured with MOFs in only two cases, namely with MFM-300(Al) 16 and MFM-520(Zn). 17 Both materials are highly structurally robust and remain intact throughout the isothermal measurements.
Here we report the reactive adsorption of NO 2 in a redoxactive MOF, MFM-300(V) [V 2 (OH) 2 (C 16 H 6 O 8 )]. MFM-300(V) is redox active, and both V III and V IV forms, MFM-300(V III ) and MFM-300(V IV ), can be obtained as pure phase materials. 18 Oxidation of turquoise MFM-300(V III ) leads to conversion of the bridging hydroxyl groups to bridging oxo groups in brown MFM-300(V IV ) (Figure 1a,b). The adsorption and binding of CO 2 18 and hydrocarbons 19 exhibit notable differences in these two materials with MFM-300 (V III ) showing higher adsorption uptakes owing to strong hydrogen bonding between substrate molecules and the bridging hydroxyl group.
We introduced NO 2 gas into MFM-300(V III ) and collected a series of high-resolution synchrotron X-ray powder diffraction (SPXRD) data. The first three data sets, noted as rapid scan (RS, RS-1, RS-2; ∼8 min for each scan), were collected immediately after loading of NO 2 ; the final data set, noted as long scan (LS; ∼1.5 h), was collected after adsorption had reached equilibrium, as evidenced by the lack of changes in subsequent SPXRD patterns. Rietveld refinement has allowed the determination of binding domains of adsorbed NO 2 molecules and captured key structural changes of the host framework for these models. In the RS model of MFM-300(V)·(NO 2 ) 1.67 , which represents the kinetically favorable state (Figure 1 c, e), only NO 2 was observed and anchored to the pore interior primarily through the hydrogen bonding interaction with the bridging hydroxyl group of the vanadium chain [O NO 2 ···H hydroxyl = 2.33(2) Å]. The bound NO 2 molecule is additionaly stabilized by 8-fold supramolecular interactions with the aromatic ligand of the framework [O NO 2 ···H aromatic = 3.01(4)−3.59(4) Å; N NO 2 ···C aromatic = 3.37(2)−3.76(2) Å]. Unlike NO 2 @MFM-300(Al), 16  In the RS-2 model MFM-300(V)·(NO 2 ) 1.28 ·(N 2 O 4 ) 1.23 , we observed an increase of the oxidation state of the vanadium centers and dimerization of NO 2 molecules in the pore; the overall structure is similar to that of the equilibriated LS model, which is described now in detail. Upon reaching equilibrium for adsorption of NO 2 in the LS model for MFM-300(V)·(NO 2 ) 1.60 ·(N 2 O 4 ) 1.53 , the coordination environment of the V center changes [V−O = 1.831(1), 1.991(2), 2.056(3)Å (each appears twice); ∠VOV = 136.2(1)°], and BVS calculations confirm the increase of valence of the V centers to 3.7 (Figure 1d). Thus, host−guest charge transfer has occurred via the hydrogen bonds of bridging vanadium hydroxyls, which are deprotonated to bridging oxo groups to accommodate oxidation of the V centers in the LS model. This has been confirmed further by the disappearance of the −OH stretching band at 3627 cm −1 in the FTIR spectrum of the MFM-300(V) on adsorption of NO 2 (Figure 2e). The change in oxidation state of V in MFM-300(V) has also been confirmed by in situ EPR spectroscopy where an intense signal centered at g = 1.955 (9.8743 GHz) with a peak-to-peak line width of ca. 75 G appears on adsorption of NO 2 (Figure 2f), consistent with the formation of d 1 V IV centers. 18 This implies that MFM-300(V III ) has undergone redox reaction and the proton of the bridging hydroxyl group is removed upon contacting NO 2 , yielding weakly adsorbed NO and H 2 O that are readily displaced by further molecules of NO 2 . This was confirmed by breakthrough experiments (see below). Interestingly, in contrast to the RS model, 66% of adsorbed NO 2 molecules in the LS model form N 2 O 4 dimers, which are stabilized in the pores of MFM-300(V IV ) by intermolecular dipole and supramolecular interactions with the aromatic ligands (Figure 1f). This results in a different overall packing of NO 2 and N 2 O 4 molecules inside the pores of MFM-300(V IV ) compared to that in NO 2 @MFM-300(Al). 16 In the LS model,   Gravimetric isotherms for uptake of pure NO 2 in MFM-300(V III ) were measured at various temperatures (298−333 K) between 0 and 1.0 bar. All isotherms shown in Figure 2a were measured with the same batch of sample, following the sequence of 298 K, 308 K, 318 K, 328 K, and 333 K. This means that although the sample loaded into the instrument was MFM-300(V III ), after initial contact with NO 2 the followon isotherms are measuring uptake in MFM-300(V IV ), as confirmed by SPXRD results. The NO 2 uptake of MFM-300(V IV ) at 298 K and 1.0 bar is 13.0 mmol g −1 (equivalent to 2.98 NO 2 /V), which is in good agreement with the LS model and similar to MFM-300(Al) (14.1 mmol g −1 or 2.92 NO 2 /Al) under the same conditions. Despite the redox host-to-guest charge transfer, the uptake is fully reversible, and no hysteresis was observed at any temperature. The isothermal uptakes decreased with increasing temperature along the entire pressure range, and the isosteric heats of adsorption (Q st ) increase from 46 kJ mol −1 steadily to 58 kJ mol −1 (Figure 2c), consistent with the presence of strong guest−guest interactions on increased NO 2 loading into MFM-300(V IV ). Compared with NO 2 @MFM-300(Al), 16 a more negative entropy of adsorption (ΔS) at higher NO 2 loading was observed for MFM-300(V IV ), indicating a higher degree of ordering of the N 2 O 4 /NO 2 molecules in the pore, in excellent agreement with the crystallographic study. The same MFM-300(V IV ) sample was then tested across 24 further adsorption−desorption cycles at 298 K, confirming the excellent regeneration of the MOF and no loss of NO 2 uptake (Figure 2b). The crystallinity of MFM-300(V) was retained after all isotherm and cycling measurements (Figure 2d).
Although the initial and the repeated isotherms for NO 2 in MFM-300(V) at 298 K showed equal uptakes at 1.0 bar, when examined more closely, a clear difference of NO 2 uptake can be observed in the low pressure region between these two isotherms. Negligible uptake of NO 2 was recorded in fresh MFM-300(V III ) below 10 mbar, whereas once oxidized to MFM-300(V IV ) a prominent NO 2 uptake of 1.1 mmol g −1 at 10 mbar was observed ( Figure S7). We speculated that when NO 2 is contacted with a fresh sample MFM-300(V III ), the V center is oxidized and NO 2 is then converted to a poorly adsorbing gaseous species, thus leading to the observed low uptake at low pressures. Whereas for the repeated isotherm at 298 K, MFM-300(V IV ) cannot convert further NO 2 and a notable adsorption of 1.1 mmol g −1 was observed at 10 mbar. This hypothesis was validated by dynamic breakthrough experiments, where a 900 ppm of NO 2 balanced flow in He was passed through a glass tubular reactor packed with freshly activated MFM-300(V III ) at 298 K at a total flow rate of 50 mL min −1 . The potential exhaust gaseous products (NO 2 , NO, N 2 O, N 2 , H 2 O, CO 2 ) at the outlet of the fixed-bed were monitored throughout the experiment. After the flow was initiated, a change in the color of the sample from turquoise to brown was clearly observed, starting from the inlet side of the fixed-bed, and this border moved with the gas flow until the entire bed turned brown (Figure 3). This is consistent with the full oxidation of MFM-300(V III ) to MFM-300(V IV ). Significantly, NO is almost immediately detected after the NO 2 flow was initiated. Thus, upon contact with MFM-300(V III ), NO 2 is reduced to NO, which has a much weaker interaction with the framework and is thus readily desorbed and passes rapidly through the fixed bed as confirmed in the dynamic adsorption experiment with NO ( Figure S8). This redox reaction is also accompanied by the production of water, which is gradually displaced by molecules of NO 2 and is observed at the outlet after NO owing to its stronger interaction with the framework. Finally, NO 2 is eluted, while the NO concentration returns to near zero, confirming the completion of the redox reaction at MFM-300(V) and the saturation of the fixed-bed by adsorption of NO 2 . Throughout the experiment, no other nitrogen-containing species was detected. The breakthrough experiment demonstrates the ability of MFM-300(V) for the dynamic adsorption and reduction of NO 2 even at ppm-level concentrations.
In summary, MFM-300(V III ) exhibits redox conversion to MFM-300(V IV ) on adsorption of NO 2 , which is converted to NO. MFM-300(V IV ) shows high capacity and excellent reversibility for NO 2 adsorption suggesting future directions for the development of efficient redox-active sorbent materials to realize adsorptive reduction of NO 2 into nonharmful species.