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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


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