Porous metal–organic frameworks as emerging sorbents for clean air

Xue Han, Sihai Yang* and Martin Schröder*

School of Chemistry, University of Manchester, Manchester, M13 9PL (UK)

E-mail: Sihai.Yang@manchester.ac.uk; M.Schroder@manchester.ac.uk

Abstract

Sulphur dioxide (SO2) and nitrogen oxides (NOx, primarily NO2) generated by anthropogenic activities are air pollutants that cause serious environmental problems and pose significant health threats. Although established methods for emission desulfurisation and denitrogenation already exist, more efficient and flexible technologies are still required. In this Review, we highlight state-of-the-art examples in which metal–organic frameworks (MOFs), an emerging class of porous sorbents, have been applied to the adsorptive removal of SO2 and NO2. MOFs can simultaneously exhibit superior adsorption capacities and exceptional selectivities for SO2 and NO2 in the presence of other flue and exhaust gases, while maintaining their structural integrity. The highly crystalline nature and rich chemical functionality of MOFs have enabled the elucidation of host–guest interactions at a molecular level to afford insights and new knowledge that will inspire and inform the design of new generations of adsorbents.

Anthropogenic emissions of SO2 and NOx from the combustion of fossil fuels are pollutants that cause serious environmental problems and pose significant health threats.1,2 These gases are the most toxic components in smog, which comprises of a mixture of particulates, hydrocarbons, and oxides of nitrogen and sulphur. Smog is a serious environmental problem in many highly populated cities and countries, and it is especially harmful for people with heart and lung conditions. Cumulative exposure to smog has also been reported to increase the likelihood of premature death from respiratory diseases.3 Smog can further react in the atmosphere to form tropospheric ozone and acid rain, which is known to have many adverse ecological effects, such as acidifying aquatic environment and damaging forests.4 These effects combined with other environmental pressures can lead to greater impacts on our interconnected ecosystem.5 Over the past few decades, awareness of the harmful effects of air pollution has been raised significantly, resulting in air quality being more closely monitored, pollution emissions being more strictly regulated, and environmental legislation being more frequently updated. All of these efforts have sparked the development of new and various emission control technologies, but mitigation of air pollution is intrinsically a complex issue.6 New technologies for desulfurisation and denitrogenationare urgently required, and here we focus on some very recent advances in the use of porous metal organic frameworks (MOFs) for the capture and removal of the air pollutants SO2 and NO2. Although MOFs have been reported previously for hydrogen storage7 and carbon dioxide capture,8 their applicability to the capture of SO2 and NO2 is less well explored, especially for NO2. However, even within the limited examples available, some exciting results signpost the huge potential of MOFs as a platform technology in this area. In this review, we analyse and discuss the potential of MOFs compared to existing technologies for abatement of SO2 and NO2, and illustrate their unique design and synthetic flexbility which affords huge opportunities for the discovery of new generations of advanced functional materials.

[H1] SO2 abatement

The largest source of SO2 in the atmosphere comes from the burning of fossil fuels by power plants and other industrial facilities. Other sources of anthropogenic SO2 emissions include industrial processes such as extracting metal from ore, shipping and other vehicles and heavy equipment that burn fuel with a high sulfur content.1 Regulations to limit SO2 emissions in stationary power generation, on-road and marine transport have become more stringent over time.9 For example, for the marine transport sector, which used to be one of the least regulated emission sources, the maximum sulfur content of the marine fuel used by ships operating in the sulfur emission control areas (SECAs) — the Baltic Sea, the English Channel and the North Sea — was restricted from 3.5% to 1.0% in July 2010 and further reduced to 0.1% in January 2015.10 Compliance with these regulations therefore demand the development of efficient emission reduction technologies. Flue gas desulfurisation (FGD) processes can be classified as "once-through" or "regenerable", depending on the treatment of the sorbent material after adsorption.11 In once-through FGD processes, SO2 is permanently bound to the sorbent, such as lime or limestone, and the resultant materials must be disposed of as a waste or used as a by-product such as gypsum. Alternatively, regenerable materials can be used to capture SO2, which is then released and processed further to obtain a saleable end product such as elemental sulfur or sulfuric acid. The regenerated sorbent material is then re-used for further absorption of SO2. Both wet and dry processes have been developed in each category. The selection and application of any FGD methodology result from careful consideration of both technical and economic aspects to assure high efficiency of SO2 capture, to minimize the use of energy and the production of waste, to lower the overall cost and to consider adoption of the same approach for different plants. Comment by Graziano, Gabriella: What was the previous value? Comment by Graziano, Gabriella: Of what kind of reaction gypsum is the by-product of? Please clarify.

Amongst the various FGD technologies, the wet once-through method based upon lime and limestone scrubbing remains the most popular due to its inherent simplicity, availability of limestone and high removal efficiency.12 In this case, an alkaline aqueous slurry of lime (Ca(OH)2) or limestone (CaCO3) is sprayed into the top of a flue-gas scrubber vessel, where the SO2 containing flue gas is enters and reacts with finely ground lime or limestone particles to form sulfite or sulfate in the form of solid waste (Figure 1a). Although this FGD process is based upon relatively simple principles, it requires complex equipment and operation because the incomplete oxidation of the calcium sulfite can cause scaling problems. Furthermore, this method generates a large amount of solid waste that contains CaSO3, CaSO4 and CaCO3, which are normally disposed of in landfill. For example, a typical limestone-based wet scrubber that processes flue gas at 680,000 m3/h with SO2 content of 289 kg/h produces 3729 kg/h solid waste and 32,666 t/year.13

. Various regenerable methods for FGD have been developed, with the most widely used being the wet Wellman–Lord process,14 in which SO2 in the flue gas is captured by exploiting the effective equilibrium between sodium sulphite and sodium bisulfite (SO2 + Na2SO3 + H2O ⇔ 2NaHSO3; 2NaHSO3 + O2 ⇒ 2NaSO4). Most of the sodium bisulfite produced after absorption can be converted back to sodium sulfite upon heating. This enables highly efficient removal of SO2 and is flexible in terms of product selection; however, it requires rather sophisticated and expensive equipment. Other wet methods have been developed to overcome these drawbacks, including the Linde–Solinox process15 (Ca(OH)2 + SO2 ⇒ CaSO3 + H2O), the magnesium oxide method16 (Mg(OH)2 + SO2 ⇔ MgSO3 + H2O), and an aqueous ammonia process17 (SO2 + 2NH4OH ⇒ (NH4)2SO3 + H2O；2(NH4)2SO3 + O2 ⇒ 2(NH4)2SO4). Most of the wet methods involve high rates of adsorption and high retention efficiency of the sorbent. However, a major drawback is that a lot of energy is required to cool flue gases prior to adsorption and to re-heat the sorbent for gas release. The need for wet sludge handling also increases the capital and operational costs.

FGD based on regenerable dry methods can greatly reduce energy and water consumption because they do not require gas reheating. Furthermore, the dry nature of the products eliminates corrosion hazards and leads to simpler equipment design and lower capital cost.18 The spray-drying process was initially proposed in the 1970s19 and involves the reaction of SO2 in the hot flue stream with the fine sorbent spray generated by rotary atomizers (Na2CO3 +1/2O2 + SO2 ⇒ Na2SO4 + Na2SO3). This approach is currently the second most used FGD method after the wet limestone one. Various dry methods based on the catalytic oxidation of SO2 have also been developed using metal oxide20 or activated carbon.21,22 The effectiveness of these methods is determined by their ability to simultaneously act as SO2 adsorbents and catalysts for SO2 oxidation to SO3 in the presence of O2, or H2SO4 in the presence of H2O. For example, for anthracite (KUA1) activated by heating to 1173 K, SO2 uptake at 313 K increases from 0.53 mmol g-1 for pure SO2 gas, to 3.12 mmol g-1 for a 1:1 SO2:O2 gas mixture, an increase of 490%.23 The absorption capacity of SO2 by activated carbon can be greatly affected by sample origin, porosity of the sorbent, surface functionality and method of pre-treatment. However, even in the presence of O2, the SO2 uptake by most activated carbon materials is < 5 mmol g-1, which is less than half of the uptake for the best performing MOFs (see below).24-26 Due to the amorphous nature of activated carbon and the lack of functional groups, the study of the catalytic oxidation of SO2 at the carbon surface in the presence of O2 and H2O is often limited to temperature programmed desorption (TPD).27 A sequence of reactions have been proposed in which SO2 and O2 react with the active site of the carbon sorbent to form C–O species, which then react with SO2 to form SO3 and regenerate the active site.28 The carbon loss during regeneration due to the evolution of CO2 and CO, and the combustion of activated carbon remain significant problems for this process. Thus, highly stable MOFs that exhibit superior adsorption capacities (11.01 mmol g-1 at 298 K and 1 bar),25 higher selectivity and full reversibility are valuable alternatives for SO2 adsorption. Given the many combinations of metal ions and organic linkers that are available, MOFs offer a rich structural diversity and tuneable pore environments, thus enabling the synthesis of ad hoc materials for different applications, which is difficult or impossible to achieve with other types of porous materials. Comment by Graziano, Gabriella: Does it mean that the O2 helps SO2 uptake? Comment by Sihai Yang: yes

[H2] MOFs for SO2 removal

Unlike the extensive investigations in the fields of hydrogen storage and carbon capture, it was not until 2008 that several benchmark MOFs (MOF-5, IRMOF-3, MOF-74, MOF-177, MOF-199 and IRMOF-62) were examined for adsorptive removal of SO2.29 These materials feature a wide range of Brunauer–Emmett–Teller (BET) surface areas (632–3875 m2 g-1) and have pores with a large variety of functionalities (for example, amines, aromatics, alkynes, coordinatively unsaturated metal sites and framework catenation) and sizes (pore volume ranging from 0.39 to 1.59 cm3 g-1)29 . The dynamic adsorption capacity of MOFs, which is defined as the quantity of gas adsorbed before the gas concentration in the effluent stream reaches breakthrough (typically 500 ppm; Figure 1b, Table 1), was evaluated29 and compared with the commonly used granular activated carbon (Calgon BPL carbon).22 Only MOF-74(Zn), composed of [Zn2O2(CO2)2]∞ chains bridged by benzene-2,5-dihydroxy-1,4-dicarboxylate linkers, exhibits a significant increase in the dynamic adsorption capacity for SO2 (3.03 mmol g-1) over BPL carbon (0.515 mmol g-1). HKUST-1 is a MOF that features Cu(II) ions bridged by benzene-1,3,5-tricarboxylate linkers to give [Cu2(O2CR)4] paddlewheel fragments. This MOF exhibits comparable SO2 sorption capacity (0.499 mmol g-1) to BPL carbon, whereas the other MOFs tested in this study showed negligible retention of SO2. Interestingly, an irreversible colour change was observed for MOF-74(Zn) upon adsorption of SO2, which was assigned to interaction of SO2 with the five-coordinate Zn(II) centre species and/or with the potentially reactive hydroxy-groups.29

Motivated by the previous work, the complexes MOF-74(M) (M = Co, Mg, Ni and Zn) (Figure 2a) were screened for their capacity to remove trace SO2 (1000 ppm) from air using fixed-bed breakthrough experiments under both dry and humid conditions.30 Under dry conditions, MOF-74(Mg) shows higher SO2 adsorption (1.60 mmol g-1) than MOF-74(Co) (0.63 mmol g-1), MOF-74(Zn) (0.26 mmol g-1) or MOF-74(Ni) (0.04 mmol g-1), but the presence of water was found to have a detrimental effect on both framework stability and SO2 adsorption for all MOF-74 series. The desorption branch of the breakthrough curves were only reported for MOF-74 (Co, Zn, Ni), with MOF-74(Co) showing the lowest desorption rate suggesting a stronger interaction between adsorbed SO2 to Co than to Ni or Zn. However, desorption data for the best performing MOF-74(Mg) were not reported leaving uncertainty over the mechanism of action. The SO2 capacity of MOF-74(Zn) in this study30 (0.256 mmol/g) is notably lower than that reported previously29 (3.03 mmol/g), and is most likely due to different conditions under which these measurements were performed. For example, in the previous study, the SO2 adsorption capacity was calculated from the dynamic breakthrough experiment that used a 16 mm internal diameter glass tube with MOF packed to 10 mm in height, whereas the later study used significantly less sample in a 4 mm internal diameter tube with a pack height of 4 mm, the gas residence time being only 0.15 s in this latter case. Although the adsorption capacities were reported as per unit weight of samples, the complex effects of mass transfer and adsorption kinetics can directly lead to different adsorption capacities when different size and length of columns are used. In addition, the operating temperature and gas flow rate also varied for these two studies, e.g., the early study was conducted at 25 °C with a gas flow rate of 25 mL/min29whereas the later one run at 20 °C with a flow rate of 20 mL/min,30 thus again affecting the observed adsorption capacity. Comment by Graziano, Gabriella: Was this the case for all MOF-74(M) or just for MOF-74(Mg)? Please clarify.

The organic linker can also influence the SO2 adsorption capacity of MOFs. For example, micro-breakthrough measurements have been performed to test the sorptive properties of UiO-66-ox, a UiO-66 analogue featuring free carboxylic acid groups (Figure 2b), when exposed to a gas stream containing SO2 .31 Interestingly, UiO-66-ox shows an 8-fold enhancement of the adsorption capacity of SO2 in comparison to UiO-66 (from 0.1 mmol g-1 to 0.8 mmol g-1), and this was attributed to chemisorption of SO2 to the free carboxylic acid groups in UiO-66-ox.

The structural stability of MOFs upon adsorption of SO2 is crucial for reversible gas uptake. However, porous MOFs can show structural flexibility as they may convert reversibly between a narrow-pore phase and a large-pore phase under external stimuli (for example, temperature, pressure, guest inclusion and light).32 Thus, structural transitions of MOFs upon adsorption of SO2 can lead to increased observed gas uptakes. For example, FMOF-2, incorporating Zn(II) ions with the fluorinated ligand 2,20-bis(4-carboxyphenyl)hexafluoropropane, exhibits hysteresis in the adsorption–desorption of SO2 consistent with a structural phase change.33 The SO2 uptake at room temperature and 1 bar is 2.19 mmol g-1, and powder X-ray diffraction (PXRD) confirmed the structural stability of FMOF-2 upon SO2 adsorption. Hysteresis has also been observed in two Prussian blue analogues Co3[Co(CN)6]2.nH2O and Zn3[Co(CN)6]2.nH2O that exhibit similar SO2 uptakes, 2.5 mmol g-1 and 1.8 mmol g-1, respectively, at room temperature and 1 bar.34 Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been used to study the mechanism of the competitive adsorption of CO2 and SO2 in Co3[Co(CN)6]2.nH2O, revealing the selective adsorption of SO2.35 MFM-202a, has a doubly interpenetrated structure with one of the networks having only ∼75% occupancy due to the conflicting steric requirements of the ligands leading to a defect structure. Each of the single networks is constructed from mononuclear [In(O2CR)4] nodes bridged by L4− (H4L = biphenyl-3,3’,5,5’-tetra-(phenyl-4-carboxylic acid)) ligands in a 1:1 ratio. Each [In(O2CR)4] node connects to four different deprotonated ligands to give a tetrahedral 4-c center, and vice versa to afford an overall diamondoid network..24 It exhibits an exceptional SO2 uptake of 13.6 mmol g-1 at 268 K and 1 bar. More interestingly, desolvated MFM-202a shows stepwise SO2 adsorption and broadly hysteretic desorption, and an in situ PXRD investigation revealed that MFM-202a (Figure 2d) undergoes an irreversible phase change to MFM-202b (Figure 2e) upon adsorption of SO2. The phase change leads to an increase in pore volume due to the formation of triple π–π interactions between the phenyl rings on the backbone of MFM-202b that lead to an increase in the stability, SO2 uptake and selectivity of the overall framework . Interestingly, other gases such as CO2, CH4, N2, Ar, O2 and H2 do not trigger this phase transition in MFM-202a under the same conditions (Figure 1c). Thus, the SO2-induced framework transition from MFM-202a to MFM-202b leads to both additional uptake and high SO2/CO2 selectivity of 132 at 273K and 84.5 at 293 K.24Comment by Graziano, Gabriella: I suggest you report both MFM-20211a and MFM-202b in figure 2d so that it is possible to appreciate the phase transition.Comment by Xue Han: Figure revised.Comment by Graziano, Gabriella: Are these results still reported in ref 17? If not, please provide a reference for these statements.Comment by Xue Han: These results are reported in ref 17.

Competitive adsorption of other gases can greatly affect SO2 adsorption capacities, and, in reverse, SO2 may compete with adsorption of other substrates such as CO2 and hydrocarbons. Although adsorption selectivity can be readily estimated by analysing single component isotherms, understanding this selectivity at a molecular level is highly challenging. To date, there have been relatively few reports on in situ characterisation of SO2 adsorption processes in MOFs.36 An SO2 uptake of 9.97 mmol g-1 at 298 K and 1.1 bar has been reported for [Ni(bdc)(ted)0.5] (H2bdc = benzene-1,4- dicarboxylic acid; ted = triethylenediamine). In situ infrared spectroscopy was used to study host–guest interactions and revealed that37 SO2 is weakly absorbed in two domains of the pore, as indicated by major peaks at 1326 and 1144 cm-1 . Minor additional species characterized by peaks at 1242 and 1105 cm-1 persist within the framework upon removal of SO2 . Access of CO2 into the pores is blocked by the more strongly bound SO2, the sulphur and oxygen centres of which were suggested to interact with the metal–carboxylate units and the C–H groups of the organic linker, respectively.Comment by Graziano, Gabriella: I’m not sure I fully understand what you mean by in situ adsorption here. Please clarify.

The first direct observation of SO2 within the pores of a MOF was in MFM-300(Al) (SO2 uptake of 8.1 mmol g-1 at 273 K and 1 bar) (Figure 2c) using synchrotron PXRD and inelastic neutron scattering (INS), that enabled the identification of two distinct binding sites for SO2 (Figure 1d). 38 SO2(I) resides in a pocket in which it is participates in moderate-to-weak hydrogen bonding with a –OH group [O(δ-)∙∙∙HO–Al], as well as 4-fold supramolecular interactions with the neighbouring phenyl –CH groups. SO2(II) interacts with SO2(I) through intermolecular dipole interactions [OI(δ-)∙∙∙ SII(δ+)∙∙∙OI(δ-)] to form a dimer. The dynamics of host–guest binding were interrogated by a combination of INS measurements and density functional theory (DFT) calculations that revealed the changes on the motions of the hydrogen atoms in the framework due to interactions of SO2 with O-H and C-H moieties.38 Comment by Graziano, Gabriella: Is this just a label? Can I change it to (a)? The same for (II)? Can we change it to all instances to avoid confusion? NOComment by Sihai Yang: It is more common to use Roman numerals in this case.Comment by Graziano, Gabriella: Are these results still reported in ref 30? If not, please provide a reference for these statements.

A comprehensive investigation of SO2 binding has been carried out in the isostructural MFM-300(In).39 This MOF exhibits a high SO2 uptake of 8.28 mmol g-1 at 298 K and 1 bar and, more importantly, predictions based on ideal adsorbed solution theory (IAST) revealed exceptional SO2/CO2 = 60 and SO2/N2 = 5000 selectivities for equimolar mixtures at 298 K and 1 bar. The host–guest interactions were also investigated using a combination of in situ single crystal and powder diffraction, synchrotron micro-IR and INS. As for MFM-300(Al), two independent binding domains for SO2 were found in MFM-300(In). The O=S=O(II)···S=O(I) distances in MFM-300(In) were comparable to those in the crystal structure of solid SO2 (3.10–4.49 Å), confirming the efficient packing of SO2 molecules in MFM-300(In). The competitive binding of SO2/CO2 mixtures were investigated by micro-IR spectroscopy, which showed that SO2 in the gas phase can readily displace CO2 in the pores as a result of the stronger binding withSO2 , thus validating the exceptionally high selectivity of MFM-300(In) towards SO2. Interestingly, MFM-300(In) has recently been coated solvothermally onto a functionalised capacitive interdigitated electrode to form an advanced chemical capacitive sensor for the detection of SO2.40 The interdigitated electrodes (IDEs) were fabricated on a silicon wafer with a 2 μm oxide layer, which was thermally grown for electrical isolation A layer of 10 nm Ti and 300 nm Au was firstly deposited via physical vapour deposition (PVD), followed by photolithography to pattern the electrodes. Thin films of the MFM-300(In) were then grown solvothermally from the reaction solution onto the surface of the pre-functionalized IDE chips This sensor shows exceptional detection sensitivity to SO2 at concentrations down to 75 ppb, with the detection limit around 5 ppb. More importantly, it also exhibits optimal detection selectivity to SO2 in the presence of other small gas molecules, such as CO2, CH4, N2, and NO2. The excellent framework stability of MFM-300(In), its optimal adsorption capacity and selectivity to SO2 pave the way for the development of future MOF-based sensors for a wide range of applications, particularly for portable applications owing to the potential high detection sensitivity of MOFs.

More recently, the dense packing of SO2 clusters was also reported for SIFSIX materials, which are copper coordination networks constructed by inorganic hexafluorosilicate anions, SiF62−, and heterocyclic organic linkers.25 SIFSIX-1-Cu (1 = 4,4′-bipyridine) shows ultrahigh SO2 adsorption capacity of 11.0 mmol g-1 at 298 K and 1 bar due to its unique pore chemistry that enables the absorption of four SO2 molecules per unit cell due to Sδ+··Fδ- and Oδ-··Hδ+ interactions. SIFSIX-2-Cu-i (2 = 4,4′-dipyridylacetylene, i = interpenetrated) exhibits high SO2 capacity at low pressures (4.16 mmol g−1 at 0.01 bar and 2.31 mmol g−1 at 0.002 bar) and SO2/CO2 selectivity of 86–89 confirming its potential in removing trace SO2 from gas mixtures.25 Breakthrough experiments with SO2–N2 and SO2–CO2 mixtures have confirmed the excellent performance of these MOFs that simultaneously afford high adsorption capacity at very low pressures, separation efficiency and materials stability. Although it not currently possible to predict how MOFs might be applied to FGD on an industrial scale, a deep molecular-level understanding of the SO2 adsorption mechanism of MFM-300(M) (M = Al, In) and the SIFSIX materials have not been achieved previously for any sorbent material. Very recently, the zirconium based MFM-601 has been reported to show the highest SO2 uptake capacity at 298 K, 1 bar of 12.3 mmol g-1, and the binding domains for adsorbed SO2 and CO2 molecules in MFM-601 were determined using in situ synchrotron X-ray diffraction experiments.26Comment by Graziano, Gabriella: Is this still reported in Reference 18?Comment by Sihai Yang: yes

[H1] NOx abatement

NOx represents a family of seven compounds (NO, NO2, N2O, N2O2, N2O3, N2O4 and N2O5), of which NO2 is the most prevalent form in the atmosphere.41 Unlike SO2 emission, which is mainly generated from stationary power plants, approximately half of the NOx emission comes from automobiles, whereas electric power plants only contribute ~20% to the total NOx emission.42 Other NOx sources include industrial boilers, incinerators, gas turbines and steel mills. NOx abatement can be effected by simply decreasing NO2 production and prevent pollution.41 For example, decreasing combustion peak temperature and the residence time can be achieved by using an excess of fuel and air, such that less NOx is generated . NOx production can be also limited by replacing air with oxygen in the combustion process thus avoiding nitrogen as a reactant, or by use of fuel with ultra-low nitrogen content. However, these types of control used on their own are not sufficient , and reduction of NOx emissions will involve add-on technologies. A wide range of add-on controls based on different operating principles are available for all types of combustion systems and satisfactory destruction or removal efficiency (DRE) can be achieved especially when many of these methods are combined and integrated. Three main principles have been applied for the design of add-on control technologies: chemical reduction of NOx, oxidation of NOx followed by absorption of the oxidized product and direct absorption of NOx using sorbent materials.43 Comment by Xue Han: I mean: pollution prevention methods (lower combustion temperature, replacing air with O2..) on their own are not enough to reduce NO2 to emission to satisfying level. Thus Add-on tech is needed to further capture the emitted NO2.

Among all the methods for NOx removal, selective catalytic reduction (SCR), in which NOx in exhaust gas is reduced to N2 by ammonia, urea or hydrocarbons, is the most effective and established method with DRE greater than 90% (Figure 3a).44 A variety of materials have been explored and applied as catalysts for SCR since the 1970s, and these are often based on TiO2-supported V2O5–WO3 and/or V2O5–MoO3.45 In these formulations, V2O5 is catalytically active for NOx reduction even at low loading(<1% w/w). WO3 or MoO3 are employed in larger amounts (~10 %), and afford catalysts with increased thermal stability that minimize surface area loss of anatase and its transformation to rutile. TiO2-anatase is an excellent support compared to other oxides such as Al2O3 and ZrO2, mainly because it is only partially and reversibly sulphated by SO2 in the exhaust gas. Furthermore the wide dispersion of V2O5 on the TiO2 surface gives rise to isolated vanadyl centres, which play a crucial role in achieving high catalytic activity. Despite comprehensive studies on reaction stoichiometry, kinetics, mechanisms of catalysis and by-product generation of the SCR process, several drawbacks remain, such as high initial cost, finite lifetime of the catalyst and problems associated with ammonia loss without reaction with NOx. In addition to metal oxides, other materials such as noble metals (for example, Pt, Pd and Rh),46 zeolites (for example, Fe-, Cu-doped ZSM-5)47 and activated carbons48 have also been investigated as SCR catalysts. It is also worth noting that zeolites and activated carbon49 can both physisorb NO2 into their porous structure and simultaneously act as catalysts for their chemical transformation into other N-containing species. Comment by Graziano, Gabriella: Please briefly explain what are the implications of the formation of isolated vanadyl centres.Comment by Graziano, Gabriella: It is not clear how activated carbon and zeolites behave differently from MOFs. Please explain.

Despite intensive study over the past few decades, there is still no single technology that is optimal for all combustion systems. It should also be noted that NO2 is an important feedstock for chemical industries as a nitrating agent for organic compounds,50 as an inhibitor in acrylates polymerisation51 and as a sterilisation or bleaching agent.52 Therefore, an ideal NO2 capture system should not only afford high capacity, selectivity and stability, but also, reversibility, to enable the release of the absorbed NO2 for further use. Gas adsorption by MOFs, which is based on supramolecular host–guest interactions, is a promising approach that combines high reversibility and low energy penalties for systems regeneration. However, there are even fewer reports on the use of MOFs for NO2 absorption than for SO2 absorption. This is mainly due to the highly reactive and corrosive nature of NO2 that leads to framework degradation and decomposition.

[H2] MOFs for NO2 adsorption

High structural stability and reusability are the prerequisites for any potential applications of MOFs for NO2 adsorption. Unfortunately, most of the studied MOFs or MOF composites show decomposition to some extent upon NO2 adsorption. For example, HKUST-1 and its graphite oxide composites (GO–HKUST-1) exhibit NO2 adsorption capacity of 2.30 mmol g-1 and 2.43–2.91 mmol g-1, respectively, under dry conditions at room temperature53,54. However, upon contact with NO2 under dry or wet conditions, the PXRD of these materials confirm their partial decomposition accompanied by a 90% reduction in total BET surface area. Differential thermogravimetric and Fourier-transform infrared (FTIR) spectroscopic studies reveal that the adsorbed NO2 preferentially reacts with the Cu centre in the framework to form a bidentate nitrate. This leads to cleavage of Cu–O bond(s) inducing the loss of microporous volume and collapse of the main structure.53 Similarly, two Zr(IV)-based MOFs, UiO-66 and UiO-67 55 (constructed from hexamers of 12-coordinate [ZrO6(OH)2] moieties and benzene-1,4-dicarboxylate or diphenyl-4,4ʹ-dicarboxylate linkers, respectively), which are known for their high stability towards moisture and acid, also showed significant reduction in BET surface areas and loss in crystallinity after NO2 adsorption. This is particularly the case for UiO-67, which loses crystallinity completely upon exposure to NO2 under both dry and wet conditions. Amine modification of these two MOFs was undertaken by incorporation of urea or melamine moieties into the bridging ligand structures.56 Although surface species were formed due to the interaction of NO2 with the amines in the MOFs, NO2 adsorption capacities were not enhanced and a substantial loss of BET surface area was still observed after contacting with NO2. Another analogue of UiO-66 featuring free carboxylic acid groups, UiO-66-ox,31 exhibits a significantly higher adsorption capacity (8.4 mmol g-1) compared to UiO-66 (3.8 mmol g-1) as measured in micro-breakthrough experiments. This is possibly due to the strong interaction between NO2 and the free carboxylic acid group in the framework as supported by FTIR spectroscopic studies. However, the PXRD results confirm that structural degradation occurs upon exposure to NO2. The amine-functionalised UiO-66, UiO-66-NH257 has shown extremely highly NO2 adsorption capacities of 20.3 mmol g-1 and 31.2 mmol g-1 under dry and humid conditions, respectively. These adsorption capacities are significantly higher than the theoretical adsorption capacity based upon the crystal structure of this material and cannot be explained by conventional adsorption theory.58 Various techniques including FTIR, NMR, DRIFTS and X-ray photoelectron spectroscopy (XPS) confirm that once adsorbed, the NO2 reacts with the amine group to form a diazonium ion. NO2, HNO2 and HNO3 can react further, most likely with the bridging oxo, to form nitrate ions. Interestingly, these reactions are localised within the material and PXRD analyses do not show any major framework bond cleavage. UiO-66 and UiO-67 have also been doped with Ce(III) aimed at more favourable NO2 adsorption by introducing more active sites.59 The obtained Ce-UiO-66 and Ce-UiO-67 exhibit moderately increased NO2 uptake compared to the parent UiO-66 and UiO-67; however, both structural decomposition and rearrangement of the MOF host occur on adsorption of NO2 as confirmed by PXRD.Comment by Graziano, Gabriella: Please insert the references for theUiO-66 and UiO-67 too or repeat ref 48, if applies. No need to

It is worth noting that the dynamic adsorption capacities reported in the literature have all been obtained from breakthrough or micro-breakthrough experiments with NO2 being diluted in a carrier gas (N2 or He) (Table 2). Such an approach allows us to evaluate and predict the performance of a MOF in real-life and industrial applciations. However, comparisons of adsorption capacities derived from different breakthrough and micro-breakthrough experiments are subject to large uncertainties and variations due to the use of different experimental conditions, variations in temperature, flow rate, carrier gas, NO2 concentrations, amount of material used, particle size, reactor size and configuration. Thus, notable variations in reported NO2 adsorption capacities are often observed for a given MOF, for example, with a difference as large as 22 fold reported for HKUST-1.53,57 Therefore, direct measurements of the NO2 isotherms (without a carrier gas) provides a straighforward route for comparing the absolute adsorption uptakes between different materials.

The first isothermal adsorption of NO2 in a MOF, MFM-300(Al) was reported (Figure 3b) only in 2018.58 Importantly, MFM-300(Al) exhibits exceptionally high adsorption capacity (14.1 mmol g-1), with a selectivity of 18.1 for NO2 over SO2, 248 for NO2 over CO2, and >10,000 for NO2 over N2 for equimolar mixtures at 298 K and 1.0 bar (Figure 3b). MFM-300(Al) has also been used successfully in dynamic breakthrough experiments to separate NO2 in both dry and wet conditions from a range of gas mixtures relevant to large-scale applications (Figure 3d). The unprecedented reversible adsorption of NO2 in the robust MFM-300(Al) has been confirmed by synchrotron PXRD, FTIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and INS coupled with DFT and molecular dynamic calculations. This These experimental and simulation techniques enable the direct visualisation of the binding domains, dynamics of host–guest interactions, reactivity and chemical behaviour of adsorbed NO2 molecules within MFM-300(Al). The structure of MFM-300(Al)·(NO2)2·(N2O4)2 shows NO2 molecules interacting end-on to the HO–Al group through moderate-to-weak hydrogen bonds (ONO2···HO = 2.00 Å) that are supplemented by additional four-fold supramolecular contacts of the O(δ-) centre of NO2 to the aromatic hydrogen atoms H(δ+) (O···HC = 2.62, 2.66, 3.35, 3.40 Å). A strong dipolar interaction between the N· centre of NO2 molecule and the C(δ+) centre of the carboxylate group (N···C = 3.11 Å) was also observed. N2O4 molecules are located in the middle of the pore and interact primarily with the NO2 molecules through a three-fold intermolecular dipole interactions (Ndimer···Omonomer = 3.80-3.91 Å). In addition, N2O4 molecules form intermolecular dipole interactions with adjacent N2O4 molecules (Ndimer···Odimer = 2.95, 3.08 Å), comparable to those measured in solid N2O4 (NI···OII = 3.13 Å) using neutron diffraction at 20 K.60 Thus, five types of cooperative binding of NO2 can be observed within the cavity of MFM-300(Al) with up to nine individual contacts (Figure 3c). Interestingly, in situ FTIR spectroscopy and kinetic synchrotron PXRD experiments both confirmed the capture of both the monomer and dimer of NO2 within the pores of MFM-300(Al). The in situ equilibrium 2NO2 ↔ N2O4 within the pores of MFM-300(Al) is found to be pressure-independent, whereas ex situ this equilibrium is an exemplary pressure-dependent first order process. EPR studies confirm the absence of electron transfer from NO2 to the MOF, directly supporting the observed unusual reversibility of NO2 uptake and the stability of the framework. Indeed, MFM-300(Al) is ultra-stable upon NO2 adsorption with no changes in the PXRD pattern after five cycles of NO2 adsorption/desorption at room temperature thus confirming full retention of the absorption capacity over multiple cycles. The high structural stability and facile regeneration of this material post adsorption offer exciting and significant potential in the adsorptive removal of NO2 using MOFs for a wide variety of applications. Comment by Graziano, Gabriella: Please specify. Do you refer to the MD simulations or to the overall approach that includes experiments and calculations?Comment by Graziano, Gabriella: What d and m stand for here?Comment by Graziano, Gabriella: What d stands for here?

[H1] Conclusions and perspectives

Clean air is essential for the development of sustainable and environmental-friendly economy. The current global reliance on fossil fuel is unlikely to be replaced completely by a clean resource over the coming decades; this is not least because of the established infrastructure for energy recovery and utilisation. Powerful drivers therefore exist for the development of new functional materials that can mitigate or reduce emissions of harmful gases arising from combustion processes. As emerging solid sorbents, MOFs show significant advantages and unique properties for selective gas adsorption and separation over state-of-the-art techniques based upon more traditional materials.

Applications of MOFs for capture and sequestration of SO2 and NO2 are still in their infancy, especially for NO2. However, their exceptional design flexibility and structural diversity and the already vast database of MOFs (currently over 60,000 structures in the Cambridge Crystallographic Database) provide an excellent platform to drive future studies in the search of efficient capture systems that afford high capacity, selectivity, stability and reversibility in operation. The measurement of adsorption isotherms of pure gas at ambient conditions (298 K and 1.0 bar) will enable direct comparison of the absolute adsorption capacity between different materials at thermodynamic equilibrium, which is influenced by the fundamental host–guest interactions and physiochemical nature of the host material. Breakthrough experiments are of importance to evaluate performance of the materials under real-life, dynamic conditions. However, unlike the isotherm measurement, which is determined, in principle, only by pressure and temperature, the dynamic breakthrough adsorption is influenced by a variety of experimental conditions, including the quantity, particle size and packing efficiency of the sample, size and geometry of the reactor, concentration and flow rate of the gas stream, in addition to the operating temperature and pressure of the fixed-bed. Moreover, the kinetics of adsorption also play a key role in breakthrough experiments, and its impact on the dynamic adsorption capacity can be difficult to quantify. This makes it difficult to compare selected materials based on the dynamic adsorption capacity..

The recent successes detailed in this Review strengthen further the idea of applying MOFs for SO2 and NO2 removal. But, in order to work with such corrosive and reactive molecules, the porous host must show exceptional stability by retaining its structure, surface area and adsorption capacity under both dry and humid gas contact over many cycles. The materials need to be tested by both static adsorption/desorption cycles and dynamic breakthrough experiments over long durations to validate stability and performance. The proven excellent selectivity between substrates is one of the biggest advantages of using MOF materials over more established and long-standing sorbents. By targeting adsorption of a specific gas of interest, effective and efficient use of pore space and/or of active sites can be established.

Because it is important to consider desorption to evaluate the energy efficiency of the overall process, regeneration of the host material and follow-on conversions of the adsorbed gas are critically important. Multiple moderate-to-weak MOF–gas supramolecular interactions offer great promise for full reversibility and optimisation of the adsorption/desorption process. Ideally, future solid-sorbent-based technologies will involve small changes in pressure or temperature that will alternatively activate adsorption and desorption, allowing facile capture and release of the gas with minimum energy consumption. This, coupled with the production of zero solid waste and reduced environmental impact, gives MOF materials enormous potential as new sorbents for clean air. The potential high production cost of MOFs remains a significant hurdle for their wide applications, but the recent establishment of numerous spin-out and custom synthesis companies reflects a commercial optimism that MOFs can and will be utilised at an industrial scale.

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Figures and Tables

Figure 1. (a) Schematic of wet limestone scrubbing FGD process.12 After removal of fly ash by an electrostatic precipitator, the flue gas is bubbled through the reaction tank, also called a scrubber. The lime or limestone slurry is pumped in and sprayed from the top of the tank, and reacts with the SO2 in the flue gas to form insoluble calcium sulfite (CaSO3), which may be reacted further with oxygen to produce gypsum (CaSO4·2H2O) (b) FGD for selective capture of SO2 by MOFs. (c) Adsorption isotherms of SO2, CO2, CH4, N2, Ar, O2, and H2 in MFM-202a at 273 K at ambient pressure. The adsorption uptake of SO2 is significantly higher than that of all of the other studied gases, which indicates superior selectivity of MFM-202a towards SO2 adsorption24 (d left) View of the crystal structures of MFM-300(Al)·(SO2)2 along the c-axis, showing the alternate packing of adsorb SO2.38 (d right) Detailed view of the binding of SO2 in the functionalised cavity of MFM-300(Al) (Al: green; C: grey; H: white; Oframework: red; S: yellow; OSO2: light green). Comment by Graziano, Gabriella: Please provide a brief explanation of the process.Comment by Graziano, Gabriella: What do we learn from these isotherms. Please expand.

Figure 2. Views of the structures of (a) MOF-74,29,30,61 (b) UiO-66-ox,31 (c) MFM-300,38 (d) MFM-202a and (e) MFM-202b.24

Figure 3. (a) Schematic of selective catalytic reduction (SCR) for NOx abatement. The automobile exhaust gas is filtered to remove particulates before reacting with aqueous urea over the SCR catalyst at high temperature, where NOx is converted to N2. (b) Adsorption isotherms of NO2, SO2, CO2, CH4, CO, N2, Ar, O2, and H2 in MFM-300(Al) at 298 K at ambient pressure, showing that the NO2 uptake is the highest among the studied gases58 (c) View of structure of MFM-300(Al)·(NO2)2·(N2O4)2 along the c-axis showing the packing of NO2 (orange) and N2O4 (dark grey) molecules with the detailed views of the host-guest and guest-guest supramolecular interactions highlighted in dash bonds.58 (d) Dimensionless breakthrough curves of gases through a fixed-bed packed with MFM-300(Al) at 298 K and 1 bar: (top) 0.5% NO2 (5000 ppm) diluted in He/N2 under both dry and wet conditions; (middle) 0.4% NO2 (4000 ppm) and 15% CO2 (v/v) diluted in He; (bottom) 0.16% NO2 (1666 ppm) and 0.34% SO2 (3334 ppm) diluted in He. NO2 can be effectively captured by MFM-300(Al) under both dry and humid conditions, and the clear separation between NO2/CO2 and NO2/SO2 under these dynamic flow conditions has been achieved.58Comment by Graziano, Gabriella: Please provide a brief explanation of the process.Comment by Graziano, Gabriella: What do we learn from these isotherms. Please expand.Comment by Graziano, Gabriella: What do we learn from these curves. Please expand.

Table 1. Summary of Adsorption Capacity of SO2 in MOFs.

MOF

BET surface area

m2/g

Pore Volume

cm3/g

Measurement

methods

Adsorption capacity

(mmol.g-1)

Reference

MOF-5

IRMOF-3

MOF-74

MOF-177

MOF-199

IRMOF-62

2205

1568

632

3875

1264

1814

1.22

1.07

0.39

1.59

0.75

0.99

Breakthrough

298 K

 

<0.016

0.094

3.03

0.016

0.50

<0.016

29

MOF-74(M)

M = Co

M = Mg

M = Ni

M = Zn

 

835

1206

599

496

 

a 

Micro-Breakthrough

298 K

 

Dry Wet

0.63 0.03

1.60 0.72

0.04 0.02

0.26 0.04

30

UiO-66

UiO-66-vac

UiO-66-ox

990

1590

1410

0.426

0.666

0.666

Micro-breakthrough

293 K

0.1

0

0.8

31

FMOF-2

378

a

Isotherm

298K ,1bar

2.19

33

CoCo

ZnCo

712

700

a

Isotherm

298K, 1bar

2.65

2.03

34

NOTT-202(a)

2220

0.953

Isotherm

268 K, 1 bar

13.6

24

Ni(bdc)(ted)0.5

 

Zn(bdc)(ted)0.5 

1783

 

1888

0.74

 

0.84

Isotherm

298K

9.97 @1.13 bar

4.41 @1.01 bar

37

MFM-300(Al)

1370

0.375b

0.43c

Isotherm

273 K, 1 bar

8.10

38

MFM-300(In)

1071

0.419

Isotherm

298 K, 1 bar

8.28

39

MFM-600

2281

a

Isotherm

298 K. 1 bar

5.0

26

MFM-601

3644

a

Isotherm

298 K. 1 bar

12.3

26

SIFSIX-1-Cu

SIFSIX-2-Cu-i

SIFSIX-1-Ni

SIFSIX-1-Zn

a

a

Isotherm

298 K, 1 bar

11.01

6.90

2.74

2.10

25

a. Values were not reported. b. Pore volume was obtained from CO2 adsorption data at 273 K. c. Pore volume was obtained by X-ray structural analysis.

15

Table 2. Summary of Dynamic Adsorption Capacity of NO2 in MOFs.

MOF

Experimental capacity

(mmol g-1)

Theoretical maximum capacity at

294 K

Theoretical maximum capacity at 140 K

BET surface area

(m2 g-1)

Pore volume

(cm3 g-1)

Measurement Method

Sample used to pack fixed-bed (mg)

Condition

Reference

HKUST-1

HKUST-1/GO

2.30 (dry)

1.17 (wet)

2.43-2.91 (dry)

0.83-1.28 (wet)

14.8

16.2-17.8

19.9

21.7-23.8

909

989-1002

0.471

0.515-0.566

Breakthrough

50-120

1000 ppm NO2/Air at 298 K

53

UiO-66

 

UiO-67

1.59 (dry)

0.87 (wet)

1.72 (dry)

2.56 (wet)

14.8

 

22.3

19.9

 

29.8

891

 

1372

0.471

 

0.707

Breakthrough

 

a

 

1000 ppm NO2/Air at 298 K

 

55

U-ZrBDC

 

M-ZrBDC

 

U-ZrBDPC

 

M-ZrBDPC

0.80 (dry)

2.2(wet)

0.06 (dry)

0.22(wet)

1.61 (dry)

3.35(wet)

0.89 (dry)

2.02(wet)

17.3

 

0.126

 

31.0

 

6.05

23.1

 

0.169

 

41.5

 

8.10

1070

 

6

 

2040

 

75

0.549

 

0.004

 

0.984

 

0.192

Breakthrough

 

a

 

1000 ppm NO2/Air at 298 K

 

56

UiO-66

UiO-66-vac

UiO-66-ox

3.8 (dry)

3.9 (dry)

8.4 (dry)

13.4

21.0

21.0

18.0

28.1

28.1

990

1590

1410

0.426

0.666

0.666

Micro-breakthrough

10-15

2138 ppm NO2

at 293 K

31

UiO-66

 

UiO-66-NH2

HKUST-1

 

BPL carbon

8.8 (dry)

13.2 (wet)

20.3 (dry)

31.2 (wet)

6.5 (dry)

26.1 (wet)

8.8 (dry)

15.6 (wet)

13.4-14.8

 

12.6

 

14.8

 

-

18.0-19.9

 

16.9

 

19.9

-

a

 

987

 

a

 

a

a

 

0.40

 

a

 

a

Micro-breakthrough

10-20

500-700 ppm NO2/Air at 293 K

57

Ce-UiO-66

 

Ce-UiO-67

 

2.07 (dry)

1.15(wet)

1.87 (dry)

1.85 (wet)

16.2

 

35.7

21.7

 

47.8

1035

 

2302

0.515

 

1.133

Breakthrough

a

1000 ppm NO2/Air at 298 K

59

MFM-300(Al)

14.0 (dry)

11.8

13.6

15.8

18.1

1370

0.375b

0.43c

Isotherm

 

50-70

pure NO2 at 298K and 1 bar

58

a. Values were not reported. b. Pore volume was obtained from CO2 adsorption data at 273 K. c. Pore volume was obtained by X-ray structural analysis.

Graphical Abstract

SO2 and NO2 are primary causes of air pollution and severe breathing problems worldwide. This Review gives an overview of the recent advances in the use of metal-organic framework materials to capture and remove these toxic gases from air.Comment by Graziano, Gabriella: The original version was very nice, but this TOC paragraph needs to be no longer than 40 words. Please check if my edit is ok.

Capture of gasesComment by Graziano, Gabriella: I really like this image!