MetalâOrganic Frameworks as a Potential Platform for Selective

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Metal−Organic Frameworks as a Potential Platform for Selective Treatment of Gaseous Sulfur Compounds Kowsalya Vellingiri,† Akash Deep,*,‡ and Ki-Hyun Kim*,† †

Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 04763, Korea Central Scientific Instruments Organisation (CSIR-CSIO), Sector 30 C, Chandigarh 160030, India

‡

ABSTRACT: The release of various anthropogenic pollutants such as gaseous sulfur compounds into the environment has been accelerated as globalization has promoted the production of high-quality products at lower prices. Because of strict enforcement of mitigation technologies, advanced materials have been developed to efficiently remove gaseous sulfur compounds released from various source processes. Metal−organic frameworks (MOFs) are promising materials to treat sulfur compounds via adsorption, catalysis, or separation. Nonetheless, the practical applicability of MOFs is limited by a number of factors including loss of structural integrity after use, limited reusability of spent MOFs, and low stability toward omnipresent molecules (e.g., H2O). Here, we provide a comprehensive assessment of MOF technology for the effective control of gaseous sulfur compounds. This review will thus help expand the fields of real-world application for MOFs with a roadmap for this highly challenging area of research. KEYWORDS: sulfur, MOF, postsynthesis, adsorption, separation, catalysis

1. INTRODUCTION The energy crisis along with the implementation of stringent environmental regulations has facilitated a transition of fuel consumption patterns from fossil fuels to natural gas.1 Purification technologies for natural gas, diesel, and petroleum products to reduce the release of potentially harmful gaseous compounds into the atmosphere are therefore facing strong demand. Common hazardous compounds such as sulfurcontaining compounds (e.g., H2S, SOx, and organothiols), nitrogen-containing compounds (e.g., NOx, NH3, and hydrogen cyanide) , and many others can lead to the degradation of outdoor air quality; note that these compounds are generally the byproducts of fuel combustion.2 Among the pollutants listed above, sulfur compounds are perhaps the most harmful due to their potential reactivity with other small molecules (e.g., water); thus, it is important to develop efficient technologies for capture/degradation or any other effective abatement of these compounds.1,3,4 Many technological approaches have been developed and employed to remove gaseous sulfur such as hydrodesulfurization (HDS),5,6 catalytic oxidation,7 biological processes,8−10 and wet (or dry) scrubbing processes.11 These options, however, have several technical and economical disadvantages (e.g., high temperature and pressure requirements, high processing costs, and problems associated with the disposal of harmful byproducts). Chemical and physical adsorption processes are alternative treatment options.12 Porous adsorbent materials such as zeolite, activated carbon, and metal oxides are well-known for their superior performance in the removal of a wide variety of sulfur compounds under both harsh and moderate conditions.13−17 In addition to these conventional © 2016 American Chemical Society

porous materials, porous coordination polymers (PCPs) such as metal−organic frameworks (MOFs) have recently attracted attention because of their high sorption capacities, structural flexibility, and thermal/chemical stabilities.12,18,19 The easy tunability of MOF ligands regarding target specificity holds great promise for the effective sorptive removal of sulfurous compounds. Here, we provide a comprehensive review of the factors and issues associated with the use of various MOFs for the treatment of gaseous sulfur compounds. We discuss adsorption mechanisms based on physical (e.g., surface area and pore volume) and chemical (e.g., the collapse of the framework and color change) properties before and after the capture of sulfur compounds. The postsynthetic modification is an effective way to reduce structural damage and to improve the stability of MOFs. We discuss the effectiveness of structural modification applicable to MOFs by citing relevant examples and describe how such process can enhance their uptake capacity. Separation and catalysis of sulfur gases by MOFs to mitigate and control these pollutants is also described. Technical barriers currently hindering the commercial application of MOFs are also evaluated with respect to access cost, biodegradability, toxicity, and other factors. Finally, the future potential of MOFs is assessed, and various options are proposed to expand their applications toward sorption, separation, and catalysis. Received: August 21, 2016 Accepted: October 11, 2016 Published: October 11, 2016 29835

DOI: 10.1021/acsami.6b10482 ACS Appl. Mater. Interfaces 2016, 8, 29835−29857

Review

ACS Applied Materials & Interfaces

Figure 1. Flowchart of commercially available desulfurization techniques.

2. MOTIVATION FOR MITIGATION TECHNOLOGIES Emission sources and atmospheric dispersal are two common and important means by which harmful substances enter the atmosphere. Effective control on the disposal of hazardous pollutants at the source may help protect the environment. For instance, fluid catalytic cracking (FCC) naphtha is one of the main contributors to sulfurous emissions worldwide; it is blended with 200−7000 ppm by weight (ppmw) of various sulfur compounds.3 The major sulfur components (60−70 wt %) in the FCC naphtha solution are H2S, thiols, disulfides, thiophene, and alkyl derivatives.7,20 As control measures, the European directive (2009/30/CE) and the United States Environmental Protection Agency (EPA) have imposed a limit of 10 ppmw and 30−80 ppmw FCC naphtha for gasoline products, respectively.3 To date, source control efforts have reduced the emission of sulfurous compounds by 30−40%. Various techniques that have been used include liquid-phase chemical scrubbing,11,21−23 hydrodesulfurization,3,24−28 biological processes (e.g., biofilters, bioscrubbers, and biotrickling filters),8−10,29−32 catalytic oxidation,7,33 and adsorption.12,13,15,34−37 Nonetheless, it is still important to reduce their concentration to parts per billion (ppb) or parts per trillion levels (ppt).19 In addition, most of the above technologies are based on chemical adsorption, which invariably requires higher energy and chemical processing penalties than physical adsorption.3,12,26,28,38 Thus, it becomes important to develop new technologies that employ

anticorrosive and moisture-stable materials in order to ensure improved energy efficiency, low-cost processing, and possible reusability. 2.1. Gaseous Sulfur Compounds and Their Characteristics. H2S is a colorless, toxic, and flammable gas that needs to be removed from most systems. It is a poisonous substance for most metal catalysts18 and because of its acidic nature can induce corrosion of transition metals in fuel cells applications.39 H2S can explode if it encounters water during the burning process.40 However, after complete combustion, it can be converted and released as SO2 into the atmosphere.41−43 Mercaptans are a class of highly undesirable sulfur compounds as they cause foul odors, metal corrosion, and SOx emissions.44,45 Related materials (e.g., dimethyl sulfide, tetrahydrothiophene, methanethiol, and ethanethiol) are generally used for odorizing purposes; for example, such odorous compound can be used to alert a leakage of gas from pipelines.46,47 However, the introduction of small amount of methanethiol into pipelines can corrode turbines48 and degrade the quality of gasoline products by forming gum through a series of combustion reactions.44 SOx is mainly produced during combustion in gasoline and diesel-powered engines. 49 Generally, SO 2 occurs as a combustion product of H2S; it then remains as sulfate particles (e.g., SO3) through reaction with exhaust moisture (H2O).50 In the sulfur cycle, SO2 is a main intermediate component that is oxidized further in the atmosphere to cause smog and acid rain.51 29836


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Figure 2. Schematic diagram of mitigation technologies used for sulfur gas removal. Panel (a) reproduced with permission from ref 66. Copyright 1999 Taylor & Francis. Panels (b)−(d) reproduced with permission from ref 38. Copyright 2006 Canadian Society of Agricultural Engineering. Panel (e) reproduced with permission from ref 33. Copyright 2015 American Chemical Society. Panel (f) reproduced with permission from ref 46. Copyright 2010 American Chemical Society.

isomerization usually do not contain sulfur because of the high boiling points of sulfur-containing compounds in fuels58 and the preferable hydration of feedstock.59 Over the past few decades, HDS has been the dominant technology for sulfur removal because of its high efficiency, low cost, and yield of useful products.6 Nonetheless, this technology has several potential drawbacks such as efficiency reduction under the presence of trace quantities of nitrogen-containing compounds in the gas mixture.5,60−64 This chemical interference can induce hydrodenitrogenation (HDN) with the potential of reducing the octane number of gasoline because of the saturation of the resulting olefin.65 2.2.2. Membrane Technology. Membrane technology has diverse advantages over other remediation techniques. It requires low capital investment with small space for operation; it is also easy to install and operate with low maintenance requirements.66,67 As such, its performance is also usually superior to that of HDS. In membrane technology, a gas mixture is fed into the membrane at high pressure to induce permeation of gas. Slowly moving gas molecules cannot permeate or be enriched at the retentate side (Figure 2a).66,67 Thus, a separation is accompanied by partial pressure differences of gases on the feed and permeates sides. For instance, rubbery polymeric membranes have been demonstrated to have good permeability toward CO2, H2S, and water vapor (faster gas (or permeate))

Thiophene and its aromatic derivatives are among the major objectionable sulfur compounds present in petroleum fractions.52 They are highly reactive and can get converted into smaller molecules such as H2S, SO2, and mercaptans during exposure. Because of their bulky nature, the thiophenic derivatives require high amounts of energy to remove them from gasoline products.53 2.2. Types of Mitigation Technologies for S Gases. As the S components show strong reactivity toward the storage system, desulfurization becomes an important step in gasoline and natural gas systems to mitigate the associated problems of pollution hazards. Commercially used desulfurization techniques are summarized in Figure 1. Detailed explanations are provided in the following sections. 2.2.1. Hydrodesulfurization (HDS). HDS is a process in which various types of sulfur compounds with diverse structures and molecular weights react with hydrogen. The process occurs at different reaction rates depending on the reactants. Concomitantly, the process can also hydrogenate other nonsulfur-containing molecules or fragments. In some cases, nonsulfur containing compounds can be cracked by fluidized catalytic cracking (FCC) units to be reformed or isomerized.6 The catalyst usually comprises a metallic sulfide supported on alumina or silica.54−57 HDS is generally carried out in the temperature range of 500−700 K at an H2 pressure of 150−3000 psig.6,24,25 Materials produced by reforming and 29837


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Figure 3. Commonly used MOFs for the sorption, separation, and catalysis of sulfur compounds (figures obtained from CuBTC,3 MIL frameworks,1 ZIF-8,89 Prussian blue,98 F-MOF-2,94 Hofmann clathrates,51 and NOTT-202a120).

contamination of the resulting product with trace amounts of microorganisms (Figure 2c). 2.2.3.3. Biotrickling Filters. The working principle of biotrickling filters is similar to that of biofilters except that an aqueous phase nutritive solution is trickled continuously over the packed bed (Figure 2d).9 The efficiency of biological removal appears to be high (e.g., sulfur removal up to 4 ppmv).8 However, there are some potential drawbacks, including (1) environmental risks due to the release of bacteria, (2) deposition of accumulated sulfate particles at the bottom side of the reactor, (3) requirement of an air supply for the growth of microorganisms, and (4) high chemical costs.29,32 These factors remain as major obstacles to broaden the application of biotrickling filters to remove sulfur from gases. 2.2.4. Catalytic Oxidation and Wet Flue Gas Desulfurization (FGD). In this process, a solution of an alkaline chemical reagent (e.g., aqueous caustic solutions and solid bases) is reacted with gaseous sulfur compounds to produce disulfides (Figure 2e).7,33 The process can operate at both moderate (below 150 °C) and high temperatures (700 to 1200 °C).33 2.2.5. Dry Sorbent Injection (DSI). This process is similar to the FGD system except that lime or limestone is first mixed with water and then supplied to the drier. This process is efficient for low-sulfur coal systems because SOx impurities are adsorbed efficiently under dry conditions using carbonaceous materials (e.g., activated carbon or coke).33 In recent years, DSI technology has become one of the preferred treatment options because of its small environmental footprint (i.e., limited production of waste), low water consumption (40% less than the wet FGD process), and low capital costs.33 2.2.6. Adsorption. Along with many technologies discussed above, adsorption processes are widely used because of their relatively low cost, simplicity, and feasibility under ambient

over N2 and CH4 (slower gas (or retentate)) in natural gas reforming processes.68,69 As such, the membrane method is a technically superior option to treat gases. However, this area is still in development and requires greater selectivity at low flux before it can be commercialized. Further, prolonged use of polymeric membranes (PDMS) for the separation of CO2 from CO, H2S, and water reduces the permeability of the membranes toward CO2. This phenomenon is due to competitive sorption of these gases to the polymeric matrix.69 Therefore, the stability of membranes under harsh conditions needs to be improved before real world application of membranes to remove sulfurous gases. 2.2.3. Biological Processes. 2.2.3.1. Bioscrubbers. Bioscrubbing is a process in which the gaseous stream containing various levels of contaminants is scrubbed using chemical reagents (e.g., an alkaline medium) as a first step and the resultant liquid phase is then treated with sulfur-specific microorganisms to selectively remove sulfur impurities from the gaseous stream.38 In this process, sulfur gas is removed from the gaseous stream by absorption into the alkaline medium (e.g., sodium carbonate/sodium bicarbonate/ferric sulfate solution). The resulting liquid is then treated in a bioreactor to enrich the elemental sulfur.8 A compost filter is used to treat the trace sulfur gases released from the bioreactor (Figure 2b).38 Careful prevention of the accumulation of sulfate in the bleed stream maintains proper growth of the microorganisms.38 2.2.3.2. Biofilters. In this technology, the gaseous stream is continuously fed into the biofilter, while a nutrient solution is added discontinuously. Sulfur is removed in the form of sulfide or elemental sulfur through reaction between sulfur-containing compounds (e.g., H2S) and microorganisms (e.g., Thiobacillus novellus, Pseudomonas putida CH11, and Arthrobactor oxidans CH8).10,29−31 A main drawback of this system is the 29838


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ACS Applied Materials & Interfaces conditions.12 In principle, a solid sorbent can adsorb the desired material based on the physical structure of the adsorbent and the analyte. A wide spectrum of adsorbents including zeolite,15,16,70,71 activated carbon,35,36,72 and metal oxides,34,73 has been used for different industrial applications over the past decade. To improve their removal capacities for sulfur, the adsorbents have been impregnated with transition metal ions.13,14,74 The common analytical processes for adsorption of sulfur are described in Figure 2f. Recently, a new class of porous materials, the metal-organic frameworks (MOFs), has drawn attention as next-generation adsorbents for different industrial processes. The MOFs are characterized by structural flexibility, tunability, an extra-large surface area, and thermal/chemical stability. Two types of adsorption processes are commonly practiced using these sorbent materials: (1) pressure swing adsorption (PSA) and (2) temperature swing adsorption (TSA). Bulk separation of gases (e.g., a mixture of CO, CO2, H2S, CH4, and N2) can be carried out using PSA. However, to remove concentrated gases (e.g., sulfur-containing compounds), TSA is recommended.17

3.1.2. MIL Frameworks. The family of MIL frameworks includes both rigid (e.g., MIL-47 (V)) and flexible (e.g., MIL-53 (Al, Cr, and Fe)) structures with the general molecular formula of M(X){O2C−C6H4−CO2} (X = O for M = VIV/OH for M = AlIII, CrIII, FeIII).65,82−85 Their three-dimensional frameworks are built with corner-sharing chains of MIVO6 or MIIIO4(OH)2 octahedra connected through terephthalate moieties.86,87 As another class of rigid MIL frameworks, chromium-solids with general formulas such as MIL-100:Cr 3 F(H 2 O) 2 O{C6H3(CO2)3}2 and MIL-101: Cr3F(H2O)2O{O2C−C6H4− CO2}3 are also important to consider. These MOFs are made of trimers of chromium octahedra linked with trimesate and terephthalate, respectively.87,88 3.1.3. Zn-Based MOFs. ZIF-8 is also a suitable framework for the adsorption of sulfur compounds because of the open uncoordinated metal sites. In general, the external surface of ZIF-8 is dominated by the variable coordination environment of the zinc nucleus. This Zn nucleus is known to undergo crystal transaction upon exposure to various thermal treatments to produce different Zn-free coordinated sites. In general, crystal cleaving occurs during the thermal treatment either during synthesis or pretreatment. Adsorption of sulfur compounds depends on the availability of the Zn-free coordinated sites. The necessary activation procedure can be controlled by different types of thermal pretreatments.89 For instance, ZnII is Lewis acid sites, whereas the N-extremities of imidazolate ligands are Lewis and Bronsted bases.89 Because of the low uniformity of the ZIF-8 molecules, a stereochemicalmodified and hierarchically synthesized ZIF-8 has also been preferably applied for sulfur sorption.65 In one study, the modified materials exhibited enhanced stability along with an elevated external surface area relative to the pristine ZIF-8.65 A disadvantage of ZIF-8 is that it has unstable low-coordinated Zn sites on the crystal defects.89,90 During the desorption process, these sites can be detached from the ZIF-8 framework and gradually form aggregates.91,92 Consequently, the availability of Zn sites limits the sorption capacity at elevated temperatures. Another new class of Zn-based MOF utilized for the sorption of sulfur gases was fluorous MOF (FMOF-2). It has a flexible V-shaped organic building block connected to two zinc atoms. As such, this 2-fold interpenetrated framework also occupies coordinated DMF and ethanol molecules.93,94 The zinc clusters and V-shaped building blocks are self-assembled to form a porous framework upon removal of solvated molecules (such as DMF and ethanol). After the solvent evacuation, the resulting octahedron is bound to six organic building blocks. Repetition of the same unit results in a three-dimensional framework with interpenetrating strands.93,94 Thus, such strands of the FMOF2 can be actively involved in the sorption of acidic sulfur compounds. 3.1.4. Prussian Blue Analogs and Hoffmann Clathrates. Because of the easy collapse of the well-studied MOFs (such as MIL, ZIF-8, and Cu-BTC),3,48 a new class of PCPs was recently introduced including Prussian blue analogues, Hofmann clathrates, and NOTT MOFs.93−98 The Prussian blue analogues have the chemical formula M3II[MIII(CN)6]2·nH2O (M = a transition metal) and are constructed by octahedral MIII(CN)63− complexes with simple cubic lattice bridging M2+ ions. The mentioned metal cyanide complexes (MIII(CN)63−) are basically filled with water molecules in their cubic lattices. Therefore, high temperature pretreatment is used to eliminate these water molecules, leading to the formation of a defect-free, three-dimensional cubic lattice of alternating M2+ and M3+ ions

3. METAL−OGANIC FRAMEWORKS FOR THE REMOVAL OF S GASES The adsorption process on nanoporous materials occurs via specific interactions between the adsorbent framework and guest analytes.75 In this respect, the interplay of the adsorbent material with analytes plays a vital role in achieving its maximum capture capacity as well as the reusability. To date, the structural damages to the adsorbent framework materials have been reported to affect their sorptive performance; for example, highly corrosive sulfur compounds can damage the framework. In the following subsections, we present an overview of various MOFs available for the treatment of sulfur compounds along with their structural characteristics and basic adsorption mechanisms. 3.1. Structural Characteristics of MOFs Commonly Used for Sorptive Removal of Sulfur Gases. The structural composition of the framework is a key parameter to determine its potential applications in the field of sorption, catalysis, and separation. For instance, some of the MOFs possess only removal characteristics,76 whereas other MOFs can be used for molecular separation.75 Also, under certain circumstances, some MOFs can play a dual role of both separation and sorption48 or separation and catalysis.77 Indeed, it is important to study the structural composition and characteristics of the functional molecules in the framework before exploring its application possibilities. Figure 3 summarizes the common MOFs used for the capture of gaseous sulfur compounds. 3.1.1. Cu-Based MOFs. HKUST-1/Cu-BTC/Basolite-300 has been a well-studied MOF in the literature.78 The copper atom in the framework is coordinated by four oxygens of the tricarboxylate (BTC) linkers and water molecules. According to single-crystal data, it forms face-centered cubic crystals that contain large square shaped pores (9.9 Å).18 The water molecules present in the first coordination sphere of the copper ions can be removed by a dehydration process, making a framework with vacant Cu2+ species.79 The coordinatively unsaturated Cu2+ cations constitute a surface ion pair where both cationic and anionic pairs are raised from carboxylate linkers and play a role in the adsorption process.80 Therefore, HKUST-1 was utilized for the sorption of H2S, DMS, and tbutyl mercaptan.48,77,81 29839


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Figure 4. Selectivity profiles of the amount of IL per unit cell of the IL-Cu-TDPAT toward H2S/CH4 separation.114 (a) Snapsots of extracted GCMC simulations for different IL loading conditions of Cu-TDPAT and (b) their corresponding values of selectivity for H2S/CH4.

structure with ∼70% of the void pore volume of this framework was formed under high-pressure conditions.106 The voids can be utilized for the exothermic sorption of the sulfur compounds at low-pressure and low- temperature regions. Another weak π−π interaction of flexible MOFs toward sulfur compounds was noticed in vanadium carboxylate frameworks [{V(OH) (bdc)}(H2bdc)x].86 This solvated framework loses the guest acid upon heating in air: at the same time, the V3+ ions are oxidized to V4+ without changing its topology. This chemical form was utilized for the sorption of thiophene through π−π interactions.107 During the PSA process, the absence of any electron density in the channels was useful to attain complete desorption of thiophene at highpressure conditions.75 3.2. Pore Properties of MOFs Used for the Sorption and Separation of Sulfur Compounds. Each unit in the MOF can be considered a potential coordination-adsorption site for the target gas.81 Especially, the pore/void volume is an important parameter toward sorption and separation applications. In general, the inherent stabilities of the MOFs against guest molecules can be assessed by measuring the difference in

connected through cyanide bridges. The products formed by the above process are mostly metal cyanide complexes.98 These analogues are stable while maintaining the framework reversibility (high desorption rate) toward the corrosive sulfur compounds. Hofmann clathrates are an alternative class of well-known PCPs with the formula {Fe(pz)[MII(CN)4]} (pz = pyrazine, MII = Ni, Pd, Pt).95−97 These PCPs represent a singular class of bistable chemo-responsive FeII spin crossover (SCO) materials that can switch between the paramagnetic (yellow-orange) high-spin (HS) state and the diamagnetic (deep-red) low-spin (LS) state through guest-adsorption processes at room temperature.99−104 This spin stabilization has been used for the adsorption of SO2 with high reusability of the framework under atmospheric pressure conditions.51 3.1.5. Flexible MOFs (NOTT and Vanadium Carboxylate Frameworks). In the series of flexible MOFs, NOTT-202a is an adsorbent material with a doubly interpenetrated framework structure. One of its networks has only ∼75% occupancy because of the conflicting steric requirements of the ligands that lead to defects in the structure.105 A desolvated diamond-like 29840


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of the highly polar nature of both H2S and IL. On the other hand, in the case of CH4, the above statement is not applicable because of its nonpolar nature. However, it should be reminded that the proper ratio between the IL and the available number of unit cells in the Cu-TDPAT is an important key factor to offer a better selectivity (Figure 4a). For instance, as the inclusion of IL per unit cell increased from 1 to 15, the separation/sorption affinities of Cu/TDPAT also increased. However, as the ratio of IL per unit cell increased up to 30, the sorption/separation of Cu/TDPAT decreased; this phenomenon should be ascribable to the reduction of free void in the composite (Figure 4b).114 This factor clearly supports the importance of free void spaces for adsorption relative to the presence of charged species. As such, it is clear that the creation of the effective pore spaces is an important variable to induce the dispersive/steric hindrance forces of the MOF, which in turn should lead to increases in the sorption capability. 3.3. Sorptive Removal Mechanisms of Various S Species. In the desulfurization process, the removal of S compounds is generally accounted for by three types of mechanisms: (1) physisorption, (2) chemisorption, and (3) the H-donor and H-acceptor concept. Physisorption is generally accompanied by surface interactions of S compounds. In chemisorption, the presence of end products (e.g., ZnS, CuS, and H2O) can lead to the collapse of the initial framework due to the active interaction between the metal ion and ligand. In the case of the H-donor and H-acceptor mechanism, the weak interactive forces are prevalent between the electron-rich framework and the electron-deficient sulfur atoms. As discussed in the previous section, the composition of target sulfur species (such as reduced (e.g., H2S) or oxidized (e.g., SO2) sulfur species) along with the type of the framework structure is crucial to determine/optimize the mechanism of the removal process. 3.3.1. Chemisorption. As discussed above, the physiochemical characteristics of the target compounds are tightly influenced by the sorptive removal properties of sorbent materials. For instance, IRMOF-3, when challenged with a tertiary mixture of S gases (containing H2S, DMS, and ethanethiol), exhibited the adsorption capacity in order of H2S (16 mg g−1), ethyl mercaptan (5.92 mg g−1), and DMS (5.80 mg g−1).12 The removal of this tertiary mixture thus appeared to proceed in a competitive manner either via chemical adsorption (for H2S) or via the H-donor and Hacceptor mechanism (for DMS and ethyl mercaptan).19 In case of chemical sorption, active interactions among amino groups and zinc ions yield ZnS and H2O.115,116 In contrast, as the Hdonor or acceptor mechanism becomes prevalent, the adsorption capacity tends to be lower because of a weak interaction between the lone pair of N atoms (H-donor) and S atoms (H-acceptor).12 As a mechanism controlling the sorption characteristics of the target species, it is important to simultaneously consider the interaction between the structural type of MOFs (e.g., MIL101/MIL-53/MIL-47) and their central metal ions (e.g., Al, Cr, and Fe for MIL-53). For example, one study compared the adsorption behavior of H2S at room temperature with six MOFs (namely MIL-53(Al), MIL-53(Cr), MIL-53(Fe), MIL47(V), MIL-100(Cr), and MIL-101(Cr)).1 The maximum sorption capacity of H2S (1322 mg g−1) was recorded by the MIL-101 framework. However, this MOF exhibited zero reversibility because of the collapse of the framework after the interaction. In contrast, small pore-sized MOFs (e.g., MIL-

pore characteristics of MOFs before and after the inclusion of certain guest molecules.108,109 For instance, the interaction of the sulfur molecules (e.g., H2S) with MIL framework is accompanied by the −OH groups of the inorganic chains with ligand molecules; it can then lead to a closure of the pores at low-pressure conditions.1 In contrast, at high pressure, the pores can be reopened (due to its breathing property) to lead to either strong interaction between H2S−OH or total filling of the pores with weak host−guest interactions.1 MIL frameworks (e.g., MIL101 and MIL-100) are found to favorably interact with the incoming molecules (e.g., H2S). Hence, their large pore-sized frameworks can collapse after the adsorption of sulfur gases (e.g., H2S) to be unreusable. In contrast, the small pore-sized MOFs (e.g., MIL-53 and MIL-47) were demonstrated to be partially reversible under the TSA process or the PSA process.1 The synthesis of the MOFs with very small pores is yet tedious because of their highly sensitive nature for the pretreatment parameters. Thus, the external modifications of the pores are techniques of demand to achieve viable nondestructibility of the original framework structure. For instance, the modification of some MOFs (e.g., MIL framework, MOF-5, and HKUST-1) was recently carried out using GO material to create new pore spaces in their structure.81 Here, MOF-5 (ZnMGO) and HKUST-1 (CuMGO) loaded with GO showed enhanced experimental porosity and surface area relative to the theoretical values. As such, it is possible to confirm the formation of the new pore spaces at the interface of the carbon layer of GO and MOF surfaces.110−113 One of the main reasons for the coordination of GO on the MOF surfaces is the presence of cubic pore cavities in the MOF-5 and HKUST-1. This cubic pores encouraged the network formation between GO and MOF at the interfaces.112 On the other hand, MIL framework loaded with GO (FeMGO) showed a decrease in porosity and surface area compared to the theoretical value. This contrary result suggests the absence of coordination between GO and MOF material. As the MIL framework interacted with GO, the formation of the MIL-GO network was prevented by the spherical arrangement of the pores in MIL through which the assembly of additional MIL units was blocked on the surfaces.112 Therefore, this force retarded the creation of new pore spaces on the interfaces. Although GO materials are useful to enhance the new pore spaces for the physical interactions, the technical gaps can still rise due to the absence of specific functional groups or charged particles needed for an effective attraction of guest molecules inside the tetrahedral or cubic cages. Hence, another way to activate the pores in the MOFs is to introduce charged molecules inside the unit cells of the framework. In this case, the modifier enters the pores of the framework to stimulate the charged ions on the pore entrances through the coordination interactions. This kind of pore characteristics have been assessed critically by Li et al.114 They have modified CuTDPAT MOF using ionic liquid (IL) ([BMIM]+) for the separation of H2S from the binary mixture of H2S/CH4. According to the computational analysis, the adsorption affinity of composite material (IL/Cu-TDPAT) toward H2S increased after incorporating IL; this phenomenon was explained based on the principle of “like dissolves like”.114 This mechanism explains that the molecule that favors stronger attraction toward IL will be adsorbed more efficiently on the pores of the MOF composites. In this regard, the estimated strength of the H2S dissolvation in the pores of IL/Cu-TDPAT increased because 29841


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ACS Applied Materials & Interfaces Table 1. MOFs Used for the Sorptive Removal of Gaseous Sulfur Compounds order

adsorbent

BET surface area (m2 g-1)

feed conc.

T (K)

pressure

adsorption capacity (mg g−1)

303 303 303 303 303 303 303 303 293 303−333 298 293 298 298

2.03 MPa 2.03 MPa 2.0 MPa 2.0 MPa 1.6 MPa 1.6 MPa 1.6 MPa 2.0 MPa 1 atm ambient ambient 1 atm 1 bar 1 bar

498 446 569 1322 446 402 290 498 16.7 16.0−1.97 28 90 83 85

19

293 293 293 293 293 293 293 293 268 293 293 298 298 273

ambient ambient ambient ambient ambient ambient ambient ambient 1 bar 1 bar 1 bar 1 bar 1 bar 1 bar

40.4 103 2.56 16.7 1.92 46.1 1.28 2.56 871 248 302 140 173 519

177

121

1 kPa 10 kPa ambient ambient

300 60 340 234 319 154 362

1 atm 1 atm 1 atm′

300 220 90

48

1 kPa ambient

340 5.8

75

ambient

5.92−0.44

12

ref

[1] Hydrogen Sulfide (H2S)

a

1 2 3 4 5 6 7 8 9 10 11 12 13 14

MIL-47 (V) MIL-53(Cr) MIL-100 MIL-101 MIL-53(Cr) MIL-53(Al) MIL-53(Fe) MIL-47 (V) MOF-5 IRMOF-3 ZIF-8 HKUST-1 FMOF-2 M3[Co(CN)6]2·nH2O (M = Co, Zn)

1400a 1500a

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Co-MOF-74 Mg-MOF-74 Ni-MOF-74 Zn-MOF-74 Co-MOF-74 Mg-MOF-74 Ni-MOF-74 Zn-MOF-74 NOTT-202a Fe(pz)[Pt(CN)4] Fe(pz)[Ni(CN)4] FMOF-2 M3[Co(CN)6]2·nH2O (M = Co, Zn) NOTT-300

835 1206 599 496 835 1206 599 496 2220 -

1 2 3 4 5 6 7

Ni(bpb) Ni(bpb) Zn(bpb) V(O) (bdc) V(O) (bdc) HKUST-1 CPO-27-Ni

1600 1600 2200

1 2 3

Cu-BTC MIL-53 (Al) UiO-66

1 2

V(O) (bdc) IRMOF-3

1

IRMOF-3

812

100 ppmv 102 mg m−3 200 ppm 1000 ppm

1928 909 378 700−712

700−712 1370

[2] Sulfur Dioxide (SO2) 1000 mg m−3 1000 mg m−3 1000 mg m−3 1000 mg m−3 1000 mg m−3 1000 mg m−3 1000 mg m−3 1000 mg m−3 100%

100% [3] Thiophene (C4H4S) 30 30 30

293 293 1840 5% molar ratio 373 1423 5% molar ratio 373 [4] Tertiary-butyl Mercaptan (C4H10S) 1504 60 308 1329 60 308 1002 60 308 [5] Dimethyl Disulfide ((CH3)2S) 293 100 mg m−3 303−333 [6] Ethyl Mercaptan (C2H5SH) 101 mg m−3 303−333

19 1 1 1 1 1 1 124 12 65 81 94 98

177 177 177 177 177 177 177 120 51 51 94 98 139

121 121 75 75 3 3

48 48

12

Langmuir surface area.

47 (498 mg g−1) and MIL-53 (Cr and Al: 446 and 402 mg g−1, respectively)) could be regenerated under the PSA process.1 In case of the small pore MOFs, the interactions were accompanied either by the H donor−acceptor concept or by migration of the adsorbed molecules within the pores. This hypothesis was clearly described for the MIL-53(Cr) and MIL47(V) frameworks by Hamon et al.;19 the MIL-47(V) framework was rigid with H-donor (V = O---H−S−H) characteristics during H2S adsorption.117,118 In contrast, MIL53(Cr) exhibited hydrogen acceptor (Cr−OH---SH2) characteristics, although it underwent a structural transition from LP

(large pore) → NP (narrow pore) to NP → LP when the pressure increased from low to high.119 More noticeably, although the above two frameworks had different adsorption mechanisms, they maintained similar uptake rates of H2S at 303 K and 2.03 MPa: 446 mg g−1 for MIL-53(Cr) and 498 mg g−1 for MIL-47(V).19 The findings of these similar uptake capacities might be ascribable to the similar Langmuir surface area (Table 1) in addition to diverse adsorption mechanisms. Therefore, the type of metal ions present in the framework (e.g., V or Cr) was crucial in determining the interaction pathways of guest species. 29842


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Figure 5. Interaction patterns of thiophene with Cu-BTC and Ni(bpb) under dry and humid (60% RH) conditions.121

3.3.2. π-Complexation. The removal mechanism of oxidized S species (e.g., SO2) can be distinguished from that of the reduced species (e.g., H2S, DMS, and DMDS) mainly because of differences in their physiochemical properties. The favorable mechanism of the former process was preceded by structural influences such as π-complexation, synergetic effects, and breathing properties. In case of π-complexation, the metal ion responsible for the adsorption process offered the migration of electrons from the highest occupied molecular orbitals (HOMO) to the lowest unoccupied molecular orbitals (LUMO). In contrast, the breathing effect of the MOFs was induced by the structural respiration of the framework with variations in the partial pressure conditions. For instance, the uptake of SO2 on NOTT-202a (871 mg g−1: at 268 K and 1.0 bar) was ascribed to a strong π−π interaction between frameworks.120 Interestingly, the stability of this MOF was higher than that of chemisorption MOFs (e.g., MIL-53 (Al or Cr)),76 even after the collapse of the framework. This may be ascribable to the observed irreversible stabilization of the framework phases.120 For instance, upon adsorption of acidic SO2, the NOTT-202a frameworks underwent phase transitions to NOTT-202b. This irreversible phase transition upon adsorption was further confirmed by crystal structure data between NOTT-202a and NOTT-202b along with PXRD results.120 3.3.3. Breathing Property. Fernandez et al.94 investigated the breathing properties of FMOF-2 for the adsorption of H2S and SO2; the sorption capacities were 83 and 140 mg g−1, respectively, at room temperature and 1 bar pressure. In addition, the breathing properties of FMOF-2 were confirmed through a series of adsorption/desorption curves with characteristic hysteresis around 1.2 bar; this phenomenon was verified by the highly similar and stable PXRD patterns

recorded before and after the adsorption of H2S and SO2.94 As such, studies on MOFs with the π−π complexation and breathing properties suggest the possible utilization of used MOFs. 3.3.4. Structural Flexibility. The large potential of Prussian blue analogues and Hofmann clathrate PCPs for the capture of gaseous sulfur has been recognized because of their high stability and reusability even after adsorption of highly corrosive gases.51,98 For instance, the sorption capacities of H2S and SO2 on the Prussian blue analogue M3[Co(CN)6]2·nH2O (M = Co, Zn) were 85.2 and 173 mg g−1, respectively, at room temperature and 1 bar pressure without damaging their chemical structure.98 Further, the sorption capacity of SO2 on Hofmann clathrate PCPs (Fe(pz)[Pt(CN)4]) was measured as 302 mg g −1 at ambient conditions. 51 This value is approximately two times higher than that of the Prussian blue analogues. The reversibility of the Hofmann framework was observed at room temperature through spin stabilization. The adsorption mechanism was based on the interaction of PtII with O and S atoms to create Pt−O or Pt−S bonds (chemical sorption).51 The obtained Pt−S bond slightly stabilized the low spin state of FeII ions by shifting the critical temperatures (Tcdown ≈ 291 K and Tcup = 320 K) of the spin transition by 8− 12 K (verified through DFT calculations). A complete reversibility of the adsorbed SO2 could be attained within 24 h at room temperature conditions.51 Likewise, such characteristics appear to be similar to the pore migration (LP to NP or NP to LP) of the MIL frameworks, as discussed above. Indeed, the flexibility and structural stabilization of Prussian blue analogues and Hofmann clathrates are likely to offer new insight into the chemical and structural properties (e.g., pore and cage sizes or surface areas) needed for the design and construction of reusable sulfur-capturing frameworks. 29843


Review


Table 2. Sorption Capacities of Post-Synthetically Modified MOFs for the Capture of Gaseous Sulfur Compounds (pressure conditions: 1 atm or ambient) order 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

GO MOF-5 MG-G0 MG-G1 MG-G2 MG-G3 MG-G4 GO CuMGO a MOF-GOPSN (dry) MOF-GOPSN (moist) b MOF-GOSA (dry) MOF-GOSA (moist) GO HKUST-1 MGO-1 MGO-2 MGO-3

1 2 3 4 5 6 7 8 9 10 11 12 13

UiO-67(bipy)-CuCl2 UiO-67(bipy)-Cu(NO3)2 UiO-67(bipy)-Cu(NO3)2 UiO-67(bipy)-CuSO4 UiO-67(bipy)-CuSO4 UiO-67(bipy)-Cu(acac)2 UiO-67(bipy)-CoCl2 UiO-67(bipy)-Co(NO3)2 UiO-67(bipy)-CoSO4 UiO-67(bipy)-Co(acac)2 UiO-67(bipy)-NiCl2 UiO-67(bipy)-Ni(NO3)2 UiO-67(bipy)-Ni(acac)2

1 2 3 4 5

Cu-BTC BaCO3/Al2O3/Pt Ba/Cu-BTC(CH3COO−) Ba/Cu-BTC(NO3−) Ba/Cu-BTC(Cl−)

1 2 3 4 5 a

adsorbent

ZIF-8 HZIF-8 HZIF-8 HZIF-8 HZIF-8

c

modifier quantity

BET surface area (m2 g−1)

feed conc. (ppm)

T (K)

[1] Sorptive Treatment of H2S by MOF with Graphene Oxide (GO) as Modifier 100 293 812 100 293 5.25 295 100 293 1.75 598 100 293 3.5 648 100 293 5.25 1062 100 293 7 86 100 293 1000 293 10 wt % 1002 1000 293 10 wt % 1722 1000 273 10 wt % 1722 1000 273 10 wt % 1419 1000 273 10 wt % 1419 1000 273 1000 273 909 1000 273 5 989 1000 273 9 1002 1000 273 18 996 1000 273 [2] Sorptive Treatment of H2S by MOF with a Metal Ion (M+) As Modifier 86.80% 1000 293 86.80% 1000 293 60.20% 1000 293 90.40% 1000 293 32.00% 1000 293 23.40% 1000 293 23.00% 1000 293 24.40% 1000 293 10.80% 1000 293 13.40% 1000 293 29.80% 1000 293 4.60% 1000 293 4.30% 1000 293 [3] Sorptive Treatment of SO2 by MOF with a Metal Ion As Modifier 50 473−773 50 473−773 0.76 g 50 473−773 0.78 g 50 473−773 0.81 g 50 473−773 [4] Sorptive Treatment of CH3SH by MOF with a Surfactant As Modifier 1928 200 298 3g 1128 200 298 6g 1446 200 298 9g 1273 200 298 12 g 1266 200 298

adsorption capacity (mg g−1)

ref

2.3 16.7 31.9 43.4 60.1 130 25.1 10 120 109 125 133 241 10 98 200 125 110

124

760 7.80 3.80 9.00 10.0 23.0 0 10.0 0 4.00 0 0 0

125

0.11−0.7 0.6−1.31 0.43−2.70 0.085−1.34 0.026−2.64

123

28.0 ∼95 125 ∼78 ∼70

65

81

77

81

PSN = 4-Ammonium polystyrenesulfonate. bSA = Sulfonic acid. cH = Sodium dodecyl sulfate.

discussed for Cu-BTC by Britt et al.122 The adsorption capacity of Ni(bpb) was only one-third that of Zn(pbp), but it was still capable of adsorbing thiophene through a synergic mechanism. The hydrophobic nature of the 1D channels of Ni(bpb) was suspected to enhance probable interactions of thiophene (Sdonor atoms) with metal ions (Ni2+ acceptor center) which, in turn, facilitated the adsorption of thiophene even in the presence of water molecules (Figure 5).121 As seen from the above example, it is clear that such synergic mechanism should help to retain the sorption behavior of MOFs even under highly harsh conditions (with a gradual decrease in sorption capacity). This finding indeed projects a new roadmap for the industrial

3.3.5. Synergetic Effect. The synergetic effect is an important mechanism induced by the interplay between two or more frameworks and the guest molecule. This kind of mechanism was noticed for the adsorption of thiophene using highly porous homoleptic Ni(bpb) and Zn(bpb) (bpb:1,4-(4bispyrazolyl)benzene)) under both dry and humid conditions (60% RH).121 Under dry conditions, the maximum adsorbed amounts of thiophene by Ni(ppb) and Zn(bpb) were 300 and 340 mg g−1, respectively, at atmospheric pressure. In contrast, under humid conditions, there was a complete collapse of the Zn(bpb) framework. The collapse mechanism of Zn(bpb) under humid conditions was in line with the earlier theory 29844


Review


Figure 6. Two-step reaction pathway of H2S on HKUST-1 in the presence of moisture (H2O). Reproduced with permission from ref 127. Copyright 2012 Elsevier.

applications of thiophene mitigation as natural gas streams generally contains a high amount of relative humidity. A summary of MOFs available for the sorptive treatment of diverse gaseous sulfur compounds and their sorption capacities are provided in Table 1. The maximum sorption capacity for H2S, SO2, and thiophene molecules was seen from MIL-101 (1322 mg g−1), NOTT-202a (871 mg g−1), and CPO-Ni-27 (362 mg g−1), respectively. Interactions between these materials and sulfur gases can be classified into one of the following mechanisms: (1) hydrogen bond formation, (2) electrostatic interactions, (3) dispersive forces, (4) the breathing effect, or (5) synergic effects. 3.4. Influence of Cocatalysts on the Sorption Performance of MOFs toward Gaseous Sulfur Compounds. Adsorptive removal of toxic pollutants is highly dependent on the surface integrity (e.g., surface area, pore volume, and pore size), surface heterogeneity, and chemical nature of the free functional groups in the adsorbent. In this respect, much attention has been paid to encapsulate diverse, active species as postsynthetic components to enhance the sorptive performance of MOFs toward gaseous guest molecules (Table 2).77,81,123−125 In this section, we summarize the advantages of the postsynthetic modification of parent MOFs using a variety of cocatalysts (e.g., graphene oxide (GO), metal ions, and surfactant) and their enhanced sorption characteristics. 3.4.1. Graphene Oxide (GO). Graphene oxide (GO) has been widely used to increase the surface area and defects in a framework (e.g., due to surface heterogeneity).81 The sheetlike structure of the GO forms a fine layer on the surface of the MOF to induce dispersive forces (intermolecular forces). The intermolecular interactions of the guest species to the framework moieties are highly favored because of the surface heterogeneity through the GO. In contrast, as quantities of GO increased, the framework was protonated to attain hydrophobicity on the final framework. Therefore, selection of an optimum dosage of a modifier on the MOF is key to achieving integrity of the framework toward target species. For instance, a well-known MOF HKUST-1 was doped with various concentrations of GO. After doping pristine HKUST-1 with 5% GO, its adsorption capacity increased from 98 to 200 mg g−1.81 The enhancement in adsorption capacity was accompanied by the creation of new pore spaces between the added GO and MOF (due to surface heterogeneity), which helped increase the synergetic effect due to dispersive forces with the elevated sorption rate.111 However, a further increase in GO resulted in a reduction of the sorption capacity because of increases in the hydrophobicity (protonation) of the sorbent;126 it was noted that an 18% addition of GO reduced the sorption capacity to ∼110 mg g−1.81 A significant reduction in the capture capacity was also accompanied by a favorable decrease in the interaction between Cu2+ and HS− ions to form CuS (Figure 6).127

To date, both hydrophobicity and instability of the framework under humid conditions are large challenges in the field of MOF research. In this respect, it is believed that codoping of functional moieties (such as N or S) along with cocatalysts would increase the stability of the framework. This kind of strategy was applied for the synthesis of S- and N-doped graphite oxides in Cu-BTC.77 As expected, the doped MOF illustrated an almost 100% increase in sorption capacity with varying surface heterogeneity. For example, N-doped GO (MOF-GOPSN) had a smaller BET surface area than S-doped MOF (MOF-GOSA), but the former possessed much higher structural heterogeneity (with low micropore volume).77 As a result, the sorption capacity of MOF-GOSA (133 mg g−1) toward H2S was slightly higher than that of MOF-GOPSN (125 mg g−1) due to the presence of a higher degree of microporosity in MOF-GOSA than MOF-GOPSN.77 Apparently, the presence of moisture (71% RH) further helped increase sorption by MOF-GOSA (241 mg g−1) as supported by possible reactive dissociation of H2S to H+ and HS− in the pore system.128 Review of the PXRD and FTIR results of H2Sloaded samples indicated the formation of crystalline copper sulfides (CuSO3, CuSO4, Cu2S, and CuS) on the framework surface of MOF-GOSA, indicating a full collapse of the framework (dissociation of Cu-ligand bonds). In contrast, in the case of MOF-GOPSN, the only partial collapse of the framework was observed,77 as the reaction took place between the HS− and N atoms of the amine and imide groups.129 As shown in the above example, the doping of the S atom can promote the uptake of H2S even under harsh conditions (up to 71% RH) through micropore formation. In contrast, N doping increased the stability of the framework with a minor decrease in sorption capacity (by about 6 mg g−1). In coordination chemistry, the templating agent plays a vital role as it provides a space for the combination of metals with their corresponding ligand moieties. When this concept first emerged, it was believed that solvent molecules themselves should act as a template for the growth of MOF frameworks. Nonetheless, as the solvent molecules are entrapped in the pores of the MOFs even after synthesis, some special treatments are needed to acquire favorable surface characteristics such as chemical activation or solvent activation.130 Therefore, careful selection of the templating agent is critical, as it determines the final framework composition. On this concept, a Zn-based MOF (MOF-5) was modified with GO using glucose as a templating agent.124 The framework with 5.25% GO showed a higher uptake capacity (130 mg g−1) than pristine MOF-5 (16.7 mg g−1) when tested at various doses. These results indicated that well-matched doses of GO and glucose had synergistic effects because of an increase in dispersive forces near the Zn2+ ions.124 However, the adsorption capacity is dependent not only on the micro/ mesoscopic and surface characteristics of the material but also 29845


Review


Figure 7. Formation of lamellar ZIF-8 with mesopores and micropores in the presence of sodium dodecyl sulfate (SDS). Reproduced with permission from ref 65. Copyright 2014 Elsevier.

g−1, whereas that of copper(II) nitrate@UiO-67(bipy) was 38 mg g−1. In contrast, metal ions loaded with chloride anions experienced the collapse of framework after adsorbing H2S.125 The results of this comparative analysis thus indicate how anions should be selected as along with the activation medium (solvent or thermal treatment) for the optimum treatment of highly reactive gases (e.g., SO2 and H2S). 3.4.3. Surfactant As a Modifier. The adsorption characteristics of any substance are highly dependent on the types of pores (either mesopores or micropores) on the external surface. Thus, many researchers have focused on creating new active sites on the external surfaces of MOFs by adding templating agents (e.g., surfactants) through postsynthetic encapsulation.65 In general, surfactants are surface reactive agents that can reduce surface tension while increasing surface reactivity. The bifunctional (i.e., hydrophobic and hydrophilic) nature of surfactants helps bind the hydrophilic edges with the framework surface. This type of approach is thus very helpful in increasing the thermal, chemical, and water stability of MOFs. For instance, ZIF-8 usually has a uniform pore cavity and limited sorption capacity due to a small number of lowcoordinated Zn sites on the external surface.1889 The reactivity of ZIF-8, when treated with sodium dodecyl sulfate (SDS: surfactant), was found to increase with high thermal, water, and chemical stability (Figure 7).134,135 Along with its stability, the sorption capacity of ZIF-8 against methanethiol also increased from 28 to 125 mg g−1 under ambient conditions.65 This enhanced sorption was accompanied by the effective interaction of S atoms (in mercaptan) with the available low-coordinated Zn sites.136 As we learned from the studies of Petit et al.127 and Ebrahim et al.,77 the selection of an appropriate quantity (ratio) of the templating agent can exert considerable effects on the enhancement of uptake efficiency for the target species. For example, when increasing SDS proportions (from 3 to 12 g) with an accompanied decrease in the sorption of mercaptan (from 95 to 70 mg g−1), removal was optimal at 6 g of SDS.65 An extensive increase in SDS concentration can affect the proportion of the tailing effects (hydrophobic to hydrophilic); such treatment can in turn disturb the reactivity of the

tightly related to macroscopic aspects of the material (size and shape of the grains).124 Chemical binding of H2S to Zn sites affected the formation of carboxylic acid and ZnS, whereas changes in the chemical environment of the BDC linker also contributed to the collapse of the framework.124,131 3.4.2. Metal Ions As Doping Agents. Sulfur gases are believed to be adsorbed on metal-bearing MOFs (e.g., CuBTC) because of potential interactions between metal ions (e.g., Cu2+) and guest gaseous molecules (e.g., H2S and TBM) to yield sulfates/sulfides as final products. However, MOFs based on open metal sites tend to have poor chemical stability under ambient conditions due to rapid interactions with atmospheric moisture.132,133 Thus, the design of MOFs that are highly stable and that have a high sorption capacity toward sulfur gases is a very significant and relevant field of interest. Dathe et al.123 have reported enhanced sorption of SO2 on [Cu3(btc)2] loaded with different Ba2+ metal salts (i.e., BaCl2, Ba(NO3)2, and Ba(CH3COO)2). Among all the tested metal salts, the BaCl2−[Cu3(btc)2] complex showed improved performance relative to the reference material (Table 2). The MOF showed a high degree of structural integrity at low temperatures. A small amount of Ba salts were retained in the pores of the MOF. The adsorption mechanism was based on concomitant adsorption of SO2 on Cu2+.123 Indeed, at high temperatures, the collapse of the framework was reflected by the formation of Cu-sulfates. The impregnation of additional metal ions into the pristine frameworks was effective at raising the sorption capacity. However, the anionic counterions can also exert considerable influences on the framework collapse. Nickerl et al.125 investigated this behavior intensively based on postsynthetic modifications of UiO-67(bipy) (bipy:2,2′-bipyridine-5,5′-dicarboxylic acid) with different metal cations (Cu2+, Ni2+, and CO2+) at varying anionic compositions (Table 2). The framework loaded with Cu ions had a higher adsorption capacity for H2S than the framework loaded with Co or Ni ions (Table 2). Also, the adsorption capacity did not depend only on a number of metal ions inserted into the framework but also on the counteranion of the copper salt used.125 For instance, copper(II) sulfate@UiO-67(bipy) had an uptake value of 9 mg 29846


Review


Figure 8. Adsorptive separation of CO2/SO2 over NOTT-300. (a) Comparison of gas adsorption isotherms at 273 K and 1.0 bar and (b) comparison of the gas-binding interactions of CO2 and SO2 in amine- and hydroxyl-functionalized NOTT-300. Reproduced with permission from ref 139. Copyright 2012 Nature Publishing Group.

Figure 9. Selective adsorption of mercaptan from gasoline fuel using hierarchically modified ZIF-8. (a) Inhibition of the hydrocarbon adsorption on ZIF-8 and (b) enhanced selective adsorption of mercaptan on different ZIF-8 species. Reproduced with permission from ref 65. Copyright 2014 Elsevier.

difference in dipole moment) of the analytes toward the chosen framework. For example, the NOTT-300 framework specifically exhibits high selectivity toward SO2 relative to CO2 due to the high reflection in the dipole moment (e.g., SO2 (1.62 D) vs CO2 (0 D)) across both low (∼0.10 bar) and high (50 bar) pressure ranges.139 The calculated pore-filled density ratios of SO2 and CO2 indicate occupation of 69% (0.298 cm3 g−1) and 82% (0.356 cm3 g −1) of the total crystallographic pore volumes (0.433 cm3 g−1).139 According to the selectivity studies carried out on various combinations of gaseous mixtures (e.g., CO2/CH4 (100), CO2/N2 (180), CO2/H2 (105), SO2/CH4 (3620), SO2/N2 (>105), and SO2/H2 (2518)), this MOF exhibited highly selective adsorption toward CO2 and SO2 (Figure 8a). The high selectivity of SO2 was ascribed to the formation of Al−OH---OC(S)O hydrogen bonds supplemented by weaker phenyl C−H---OC(S)O supramolecular contacts by surrounding pores (Figure 8b).139 With that in mind, this phenomenon was contrary to those of MOFs functionalized with amine groups that facilitate the chemisorption mechanism rather than weaker supramolecular forces.106,140,141

framework due to an excessive accumulation of micropores in the interfaces. In other words, as the dispersive force at the interface is minimized, the guest molecules can be repelled by the framework (bottleneck effect), ultimately leading to a reduction in the sorption capacity.137,138

4. MOLECULAR SEPARATIONS Sorption and separation are two main branches of analytical techniques in the fields of industrial chemistry and environmental engineering. Sorption is very useful in storing or capturing highly hazardous components. On the other hand, separation helps purify an individual component from a heterogeneous mixture. In this section, the common methods used for the separation of highly reactive sulfur gases through solid frameworks or membranes are discussed. 4.1. MOFs for Separation. In a technical sense, utilization of MOFs for sorption and separation relies on similar mechanisms. However, in practice, sorption tends to be achieved via strong intermolecular forces such as chemisorption, whereas separation tends to rely on weak interactive forces such as noncovalent interactions.139 In other words, separation relies on the distinguishable physical characteristics (e.g., 29847


Review


Figure 10. Performance comparison of predicted UiO-66(Zr)-(COOH)2-based MMMs for H2S/CH4 separation. (a) Data obtained after the addition of MOF into the polymer and (b) data obtained when considering MOF as filler for polymer membranes. Filled triangular symbols represent data from the literature,68,144,178−181 whereas the empty symbols are MMS data at 0.0 to 0.5 volume fractions. Reproduced with permission from ref 40. Copyright 2015 Elsevier.

between the thiophene and the ligand (bdc) through the formation of weak noncovalent interactions.142 In addition, this weak interaction was helpful in retaining the stability of the framework even after adsorption of CH4 and thiophene. The composition of the mixture of gases is also an important factor in their separation process. For instance, selective adsorption of thiophene over toluene, when tested using HKUST-1 and CPO-27-Ni (using N2 as a purging gas), yielded a sorption capacity of 154 and 87 mg g−1 (and selectivity of 1.8 and 4.3), respectively.3 Interestingly, CPO-27-Ni showed a generally enhanced selectivity over three consecutive cycles (4.3 to 3.0) relative to HKUST-1 (1.8 to 2.1) upon replacement of purging gas with para-xylene. However, the uptake capacity of HKUST-1 (154 to 177 mg g−1) improved while that of CPO27-Ni decreased (87 to 50 mg g−1).3 These results indicated that highly selective separation of thiophene took place in the CPO-27-Ni framework when purged with slightly polar compounds (i.e., p-xylene gas). On the other hand, toluene decreased the sorption capacity because of unfavorable electrostatic interactions and repulsion of the electron-rich framework and the xylene molecules. 4.2. MOF-Based Membranes for Separation. Clear-cut analyses of the adsorptive and dynamic separation properties of membrane materials are prerequisites to determining the separation properties of membranes.40 Also, MOF membranes are analytically and structurally highly complex. Thus, the application of pure MOF-based membranes is limited. To the best of our knowledge, pure MOF-based membranes have scantly been used to separate gaseous sulfur compounds from natural or biogas streams. Here, we summarized the most common MOF-based membranes utilized for the separation of sulfur compounds. The ability of 13 different MOFs was evaluated for the removal of H2S from biogas and natural gas using a combination of experimental measurements and molecular modeling.47 They noticed that the desorption rate at high temperature and the stability of the MOF materials increased with the increases in sulfide concentration when compared to traditional zeolite materials. This phenomenon demonstrated the suitability of the MOF for the desulfurization of gases containing high concentrations of H2S contaminant.47 In another study, Vaesen et al.143 revealed the potential application of the titanium(IV)-based material MIL-125(Ti)-

Competitive or concomitant sorption behavior is another important phenomenon to consider to facilitate the separation through solid microporous material. In this case, a component possessing higher dispersive interaction gains more adsorption sites than the weaker one. To explain this mechanism, separation of TBM from CH4 (or natural gas) is a good example.48 The estimated selectivity of a 60 ppm TBM mixture in 99.9% methane (e.g., selectivity was estimated as a unitless value as follows: STBM,CH4 = (xTBM, adsorbed/xCH4 adsorbed)/ (yTBM, bulk gas/yCH4 bulk gas) was approximately 228 000, 263 000, and 198 000 for Cu-BTC, MIL-53(Al), and UiO-66, respectively.48 These results clearly indicated the competitive adsorption of TBM/CH4. As such, until saturation, both CH4 and TBM were adsorbed almost equally (similar adsorption isotherm curves). The remaining adsorption sites were then preferably populated with TBM relative to CH4 molecules, as the former can preferably interact with the intrinsic micropores of the framework.48 The stereochemical properties (spatial arrangement) of the atoms in the framework also affect the separation of compounds in corrosive feeds. The hierarchically modified ZIF-8 (H-ZIF-8) was utilized for the selective adsorption of methanethiol (200 ppm) from a mixture of toluene (20 wt %), hexane (40 wt %) and cyclohexane (8 wt %). As shown in Figure 9, H-ZIF-8−6 showed an olefin decrement of about 1.0 wt % in a gasoline mixture, while the same material also showed a high sorptive capacity for methanethiol (125 mg g−1); this pattern indicates superior selectivity of hierarchically modified ZIF-8 toward methanethiol.65 A close inspection of results showed that the layered arrangement of the atoms led to the creation of uniform pore cavities; the presence of these cavities then facilitated the selective interaction of suitable molecules by filling the pores through intracrystalline diffusion. In the molecular separation of sulfur or other target gases, it is crucial to consider the formation of noncovalent weak interactions between molecules within the channels of an MOF because this phenomenon can deform or distort the framework during separation processes.75 For instance, the selective removal of thiophene was tested using a mixture of thiophene/CH4/He on V(O)(bdc) at room temperature. Accordingly, a selective separation of thiophene over CH4 molecule was achieved at 0.1 and 10 kPa.75 This highly selective adsorption was ascribed to C−H---π interactions 29848


Review


oxidation of H2S to SO2 can proceed in the presence of quinones (MOF-GOPSN).148 Mass spectrometry analysis confirmed the formation of strong signals of SO2 (m/z = 64) and SO (m/z = 48) in the chromatogram after the addition of H2S due to oxidation.77 Under humid conditions, water acts as a “screening” factor to block probable distortion of well-known Cu2+ centers.77 In fact, this phenomenon has been found to facilitate the catalytic activity of Cu-BTC-GO.

NH2 for the concomitant elimination of H2S from biogas and natural gas. They also demonstrated the high selectivity of a chosen framework (∼70) for H2S versus CH4 over all tested pressure ranges (10−20 bar). The selectivity was due to the presence of accessible −OH and −NH2 sites, which facilitated the formation of hydrogen bonds between μ2-OH groups (Hdonor) and the sulfur atom of H2S (H-acceptor).143 The ability of an acid-functionalized UiO-66 mixed matrix membrane (MMM) to separate H2S/CH4 was investigated using Grand Canonical Monte Carlo (GCMC) simulations.40 Separation of H2S/CH4 on UiO-66 (Zr)-(COOH)2 membranes was evaluated for nine different compositions (in terms of H2S levels ranging from 0.1 to 0.9%) at 30 different pressure levels (ranging from 0.00001 to 10 MPa). The MMM with an H2S molar composition of 0.6% had a significantly enhanced permeation rate and selectivity (2.70 × 10−2 m4 N−1 s−1 and 27.8, respectively) relative to carbon nanotubes, another modern membrane material (2.30 × 10−2 m4 N−1 s−1 and 1.38, respectively)144 at 0.1 MPa feed pressure (303 K). The permeability and selectivity of pure MOF membranes and MMMs were additionally verified by performing computations on traditional dimethyl-silicone rubber membranes (Figure 10). The addition of MOF to the polymer matrices (PPOPCOOH/UiO-66(Zr)-(COOH)2)) increased permeability (7.5 × 10−7 to 6.3 × 10−6 m4 N1− s−1) with constant selectivity (10). In contrast, pure polymer membranes showed permeability and selectivity in the ranges of 2.9 × 10−4 to 1.1 × 10−3 m4 N−1 s−1 and 7.70 to 18.2, respectively. These results indicate that addition of the MOF helped achieve a high degree of permeability of the membrane for target gases compared to the traditional polymer.40 Therefore, the polymer/MOF composition can be considered a material of choice for gas separation. Some factors, such as dispersion of particles in the polymer matrices (high permeability) or addition of MOF as filler particles for polymers (low permeability) might still need further optimization for the required permeability scale.

6. CHALLENGES FACING MOF TECHNOLOGY 6.1. Barriers for the Regeneration of MOFs. The most important issue associated with the reusability of MOFs is their structural distortion upon exposure to gaseous substances. Sulfur compounds are by nature highly reactive compounds. This can facilitate the collapse of the MOF framework after the reaction. As described in the preceding sections, CuS or ZnS are the usual indicative end-products of structurally collapsed Cu- and Zn-based MOFs. Only a few reports have explained the mechanism behind the collapsed framework. Therefore, here we discuss the barriers in regenerating two major MOFs, namely HKUST-1 and ZIF-8, using common characterization results. The structural integrity of these MOFs was investigated before and after the exposure to sulfur gases using FTIR, diffused reflectance UV spectroscopy, Raman spectroscopy, and PXRD analysis.18 The complete distortion of the activated (473 K for 1 h) HKUST-1 framework upon exposure to H2S (up to peq= 10 mbar) was confirmed by the IR results, which displayed a complete absence of the asymmetric (2626 cm−1)/symmetric (2614 cm−1) stretching and bending (1182 cm−1) vibrational modes along with complete erosion of the Cu−O vibrational mode (505 cm−1).18 This disappearance of Cu−O and Cu−Cu transitions toward the lower frequency was also confirmed by Raman shifts (e.g., the band at 496 cm−1 shifted to 508 cm−1) along with the appearance of a new peak at 224 cm−1.18,79,80,149 The new product was predicted to be covellite CuS. These results indicate a strong interaction between copper centers and H2S.150 Also, these changes were accompanied by a ligand to metal charge transfer (LMCT) transition and d−d transition of Cu(II) species in distorted octahedral local geometry.18 In the case of ZIF-8, the framework was fragile even at very low H2S dosages (peq = 5 mbar), indicating the formation of a variety of hydrogen-bonded compounds.18 Together, these results confirmed the presence of limited adsorption sites in ZIF-8 relative to HKUST-1. 6.2. Reusability of Spent MOFs. The renewability of adsorbents after sorptive removal is an important factor in minimizing operating costs.65 As discussed above, some of the problems associated with the regeneration of the MOFs might be a less preferable option for MOFs. It is thus desirable to enhance the renewability of pre- and postsynthetically modified MOFs avoiding any structural damage during the release of harmful substances.89,90 In a number of recent studies, the reusability of MOF materials has been achieved for 5−10 cycles, although these materials have not yet been commercialized.12,48 In this section, we summarize some of the reusable MOFs under TSA, PSA, or normal N2 flow conditions. Regeneration of a methanethiol-loaded hierarchically modified ZIF-8 (using the surfactant (SDS)) at 423 K under N2 flow for 2 h at atmospheric pressure resulted in a gentle decrease in its adsorption capacity during the first four regeneration cycles.65 The decrease in adsorption capacity could be related to a decrease in the number of accessible low-coordinated sites.91,92,151 Surprisingly, after the fourth cycle, the aggregation

5. CATALYSIS Porous materials such as zeolite and MOF can exhibit catalytic behavior due to their large, well-defined internal surface areas. Also, the catalytic behavior accompanied by pores/cavities is evidence for heterogeneous reactivity, while catalytic behavior seen from other aspects of the framework (e.g., unsaturated metal sites) is classified as homogeneous catalysis.145 In the present literature, only a very few MOFs have exhibited a catalytic capacity for sulfur compounds; we summarized them here. In general, the adsorption of sulfur compounds is a reactive process because of their highly acidic nature. Hence, some of the frameworks serve as a platform to recover useful end products from the highly hazardous feed. For example, the composites containing Cu-BTC and S-/N-doped GOs have been shown to catalytically convert H2S to SO2.77 A continuous supply of 1000 ppm of H2S in humid conditions (71% RH) to the S-doped MOF-GO (MOF-GOSA) produced about 0.6 ppm of SO2. In contrast, under dry conditions, only 0.2 ppm of SO2 was produced. In the case of an N-doped MOF-GO composite, only 0.1 ppm of SO2 was observed under the same conditions (71% RH), while no catalytic conversion was observed under dry conditions.77 Therefore, the functionalizing material (such as S or N) determines the catalytic efficiency of the MOF-GO. Nitrogen in carbonaceous materials can act as a catalyst to produce superoxide ions.146,147 As such, the catalytic 29849


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ACS Applied Materials & Interfaces Table 3. Comparison of the Market Prices between MOFs and Commercial Materiala order

MOF

trade name

surface area (m2 g−1)

particle size

1

ZIF-8

Basolite Z1200

(A) Metal Organic Frameworks 4.9 μm (D50) 1300−1800b

2

MIL-53(Al)

Basolite A100

31.6 μm (D50)

3

Fe-BTC

Basolite F300

1300−1600

4

magnesium formate

Basosiv M050

400−600

5

Cu-BTC, HKUST-1

BasoliteC 300

6

MOF-177

Basolite Z377

1

zeolite

2

activated charcoal

4−8 mesh

3

activated charcoal

8−20 mesh

15.96 μm (D50)

100−1500

1500−2100

3800−4000 (B) Commercial Adsorbents

bulk density (g cm3) 0.35 0.40 0.160.35 0.30−0.40 0.35

quantity 100 500 100 500 100 500 100 500 100 500 100

cost ($)

g g g g g g g g g g g

1615 6210 1169 6197 1720 6855 993 3967 1820 6648 1446

100 g 500 g 500 g 2.5 kg 500 g 2.5 kg

59 163 43 131 46 126

a

Data for the prices of MOFs (Sigma-Aldrich): http://www.sigmaaldrich.com/materials-science/alternative-energy-materials/metal-organicframeworks.html. Information about (1) zeolite, http://www.sigmaaldrich.com/catalog/product/sigma/96096?lang=ko®ion=KR; and (2) activated carbon, http://www.sigmaaldrich.com/catalog/product/sial/c2764?lang=ko®ion=KR (Sigma-Aldrich). bLangmuir surface area.

adsorption−desorption cycles.48 In contrast, MIL-53(Al) lost adsorption capacity (∼19.9%) and showed a slight decrease in BET surface area (1329 (cycle-1) to 972 m2 g−1 (cycle-5)), whereas Cu-BTC lost its adsorption capacity completely with distortion of the whole framework from 1504 (cycle-1) to 53 mg g−1 (cycle-5). The enhanced stability of MIL-53(Al) and UiO-66 over Cu-BTC can mainly be ascribed to the absence of open metal sites. In other words, the presence of two types of cages in the UiO-66 (namely octahedral and tetrahedral) led to the presence of open pores in the framework surfaces153 during the desorption treatment. Moreover, the absence of open metal sites in the framework endowed the stability of UiO-66 and MIL-53(Al) to be greater than that of HKUST-1 and ZIF-8.48 6.3. Commercial Availability and Material Cost of MOFs. The properties of an adsorbent should essentially include: (1) commercial availability, (2) low cost of production and regeneration, (3) easy disposability, and (4) stability (e.g., with respect to moisture, temperature, and pressure). From an economic viewpoint, the cost of the material is very critical. In this respect, it is important to note that the manufacturing costs of MOF materials are much higher than those of traditional materials such as zeolite and activated carbon. Indeed, commercial use and research with these more traditional materials are still ongoing because of their low production costs.154,155 Table 3 summarizes the market price of some wellknown MOFs produced by a supplier of environmental and process catalysts (e.g., BASF). The market price required to produce 100 g of a commonly used desulfurizing MOF (CuBTC) is about US $1820, while that of the zeolite is only the US $59 (i.e., approximately a 30-fold difference). Also, the price of activated charcoal is lower than that of zeolite (500 g of zeolite costs US $163, whereas 500 g of activated carbon costs US $43). Although MOFs are the noble materials, the high cost of their production (and resulting high market price) is a major factor limiting their practical use or commercial and industrial applications on a large scale.

of low-coordinated Zn sites reached equilibrium with a wellmaintained constant sorption capacity (∼120 mg g−1).65 This observation indicates a good reusability of the used ZIF-8 under mild conditions (e.g., desorption temp: 423 K and desorption pressure: atmospheric pressure). Under these conditions, the uniform pore cavities of the ZIF-8 were retained to continuously allow its adsorption without distortion. Some MOFs have been reported to possess structural reversibility even after exposure to toxic gases. The process of regeneration should be accompanied either by phase transition (conversion of the crystal phase)139 or by the activation of micropores during regeneration (because of high activation temperature and pressure).151 As an example, for a new series of MOFs, called NOTT-300, an exceptionally stable framework structure was recognized even after exposure to corrosive gases like SO2. The consistent structural integrity was confirmed by in situ PXRD measurements after each subsequent addition and removal of SO2 at ambient temperature and pressure (273 K and 1 bar pressure).139 Meanwhile, DFT analysis also indicated the flexibility of the frameworks after the interactions of SO2 molecules with H atoms in the metal bridging units (OSO--HOAl) and the ligand (OSO---HC). Also, the estimated enthalpy values for the adsorption of NOTT-300 (27−30 kJ mol−1) were found to be lower than those for the amine functionalized (40−90 kJ mol−1) frameworks, which may be indicative of the flexible nature of the NOTT-300 MOFs.21,152 In this series, Chavan et al.151 also demonstrated the reversibility of CPO-Ni-27 upon thermal activation at 473 K for 12 h, whereas the uptake capacity of H2S also increased. This observation might reflect the effect of the newly formed active sites on Ni2+ as a result of thermal activation (for desorption). The strong physisorptive interactions were also confirmed by a high enthalpy of adsorption, e.g., 56−58 kJ mol−1 values.151 Investigations on the cyclic regenerability of Cu-BTC, MIL53(Al), and UiO-66 using TSA indicated that UiO-66 (cycle 1 to 5:90 mg g−1) had the highest stability for five consecutive 29850


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ACS Applied Materials & Interfaces 6.4. Stability of MOFs toward Omnipresent Molecules/Conditions. The influence of omnipresent molecules and conditions (such as moisture and temperature) on the stability of MOFs is inevitable during the adsorption process. The structure and surface area of many MOFs are collapsed or modified when exposed to air.156−158 As such, the decomposition of MOF networks has been linked with the presence of moisture in the air. As water molecules penetrate into the pores, the hydrolysis of carboxylate groups coordinated to metal centers causes the framework to degrade.159 The introduction of water-repellent functional groups within the framework can enhance the hydrophobic properties of MOFs.160 By employing this approach, the stability of MOFs can also be improved to some extent in humid air conditions as hydrophobic pores adsorb less water.161 However, it is extremely difficult to fully exclude the adsorption of water into the framework even with highly hydrophobic pores, especially when the framework is exposed to moisture for a long period. Moreover, even a small quantity of adsorbed water can significantly disrupt the structure of certain MOFs.162 Another option to resolve this problem is the selection of “bioinspired” MOFs, which are based on nontoxic endogenous linkers.163−167 The high basicity values (i.e., higher pKa value) of the related ligands can lead to an improved stability of the framework.168 For instance, highly basic pyrazole (pKa ∼19.8) and imidazole (pKa ∼ 18.6) ligands were found to be stable upon exposure to humid conditions;158,168,169 this was ascribed to the strength of the bond between the metal oxide cluster (Lewis acid) and the bridging linker (Lewis base).158,169 Many of the available MOFs are prone to breakdown in the presence of humid conditions.157,158,161,169−171 As described above, there are some possible solutions to increase the thermal, chemical, and water stability. Research related to bioinspired MOFs or pyrazolate ligands are promising but is still at the laboratory level. All the above issues are critical parameters that are necessary to achieve commercial acceptability of MOFs. Indeed, such efforts should help increase the utilization of MOFs to treat gaseous pollutants (e.g., adsorption, separation, and storage). 6.5. Toxicity and Biodegradability of MOFs. It is important to characterize the toxicity level of any substance to determine the conditions of its safe disposal to prevent environmental damage. Like other chemical substances, MOFs may also contain various toxic molecules (ligands or solvents) within their structures172 and their toxicity levels in the post-application stage also need to be studied. The choice of MOF should be based on its initial toxicity, host−guest interactions, pore size, and hydrophobic/hydrophilic balance.172 MOFs are generally composed of metal ions and ligands that interact via coordination bonds. As such, the interaction of these MOFs with incoming aqueous or gaseousphases can lead to a continuous ligand exchange process between the small molecules (e.g., H2O) within the guest phases and the metal centers of the MOFs.172 Thus, degradation of the MOF matrix will lead to the release of linkers, hazardous gases, and toxic byproducts.173 Increasing applications of MOFs in biomedical applications have further necessitated the study on their toxicity and biodegradability.167,174−176 In light of their application in biomedical processes, some biologically active species such as antigens or antibodies can remain entrapped into the MOF framework, and this requires additional care during the disposal of used MOFs.

In the context of removal/sorption/storage of hazardous sulfur gases with MOFs, there is also concern regarding the disposal of spent MOFs. Therefore, dedicated research is needed to properly assess the toxicity of MOF substances released into the environment after their interactions with the guest molecules. Moreover, it is also important to investigate the (bio) degradability of MOFs in addition to safe disposal methods.

7. CONCLUSIONS AND FUTURE RESEARCH Coordination polymers or MOFs can be used for the adsorption, separation, and catalysis of harmful toxic pollutants, including various sulfur gases. Unique advantages of MOFs include their tunable composition, well-defined crystal geometry, easy functionalization, and accessible metal sites. Hazardous pollutants can be classified into many different categories depending on their chemical characteristics. Because their high acidity and degree of reactivity, volatile sulfur compounds (VSC) are important targets needing their removal from various systems. Traditional materials, such as zeolite and activated carbon, are not capable of capturing these pollutants effectively because of the rigidity of their frameworks. In contrast, MOFs show high efficiency for the adsorptive removal of sulfurous compounds due to their superior physical and chemical properties relative to traditional materials. The increasing popularity of the MOFs is reflected by the growing number of related compounds and their versatile applicability compared to traditional materials. In addition to adsorptive removal, MOFs are suitable options for the separation of sulfur compounds from natural and biogas streams. However, some MOFs exhibit enhanced catalytic capacities, which in turn results in the production of toxic byproducts. Many of pristine MOF forms are often not suitable for the sorptive treatment of the highly reactive and corrosive gases such as H2S and SO2. To overcome this limitation, researchers have suggested structural functionalization of MOFs (e.g., through postsynthetic encapsulation of graphite, metal cations, and surfactants) to achieve better and more selective separation and adsorption of sulfurous compounds. Despite their important benefits, the practical use of MOFs for separation, adsorption, and catalysis is still limited. The most important and critical issues that need to be addressed are (1) reducing the production costs of MOF materials, (2) producing MOFs that can be regenerated or reused, (3) producing MOFs for safe and easy disposal, (4) profiling of toxicity of the MOFs from manufacturing units to disposable units, (5) structurally modifying the MOFs to counter the reactivity from omnipresent molecules (e.g., water vapor), and (6) making the MOFs commercially available for various types of operations. Among these issues, reduction in the production cost and methods to ensure environmental safety from the byproducts are the two most important endeavors that need to be emphasized. The desired developments can potentially be met through functionalization of MOFs with suitable ligand molecules such as “bioinspired” ligands and the use of nontoxic solvents such as water or methanol. In addition, better characterization of MOFs (both before and after adsorption of gaseous sulfur species) will also provide clear information about the adsorption mechanism and the structural integrity of the MOFs after hosting new guest molecules. Simulation of MOF structures before and after real word applications will help avoid the accumulation of various pollutants in the environment. In conclusion, a better knowledge on the 29851


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adaptability of the MOF framework will open up many new avenues for the real-world application of MOFs to treat hazardous sulfurous compounds.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +1-82-2-2220-2325. Fax: +82-2-2220-1945. *E-mail: [email protected]. Tel: +91-172-2657811-452. Fax: +91-172-2657287. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (MEST) (20060093848). A partial financial support from the Council of Scientific and Industrial Research (CSIR-India) through grant no. OMEGA/PSC0202/2.2.5 is also gratefully acknowledged.

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LIST OF ABBREVIATIONS Bpb, 1,4-(4-bispyrazolyl)benzene)) CPO, coordination polymer of Oslo DFT, density functional theory DMS, dimethyl sulfide DSI, dry sorbent injection FCC, fluid catalytic cracking FGD, flue gas desulfurization FMOF, fluorous metal organic frameworks GCMC, Grand Canonical Monte Carlo GO, graphene oxide HDN, hydrodenitrogenation HDS, hydrodesulfurization HKUST, Hong Kong University of Science and Technology LMCT, ligand to metal charge transfer LP, large pore MIL, Material of Institute Lavoisier MMM, mixed matrix membrane MOF, metal organic framework NP, narrow pore PCP, porous coordination polymer ppb, parts per billion PSA, pressure swing adsorption PSN, 4-ammonium polystyrenesulfonate RH, relative humidity SA, sulfonic acid SDS, sodium dodecyl sulfate TBM, tertiary butyl mercaptan TSA, temperature swing adsorption UiO, University of Oslo US EPA, United States Environmental Protection Agency VOC, volatile organic compound VSC, volatile sulfur compound VST, vacancy solution theory ZIF, zeolitic imidazolate framework

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ACS Applied Materials & Interfaces Catalytic Properties of MIL-100 (Fe), an Iron (III) Carboxylate with Large Pores. Chem. Commun. 2007, 2820−2822. (175) Chalati, T.; Horcajada, P.; Gref, R.; Couvreur, P.; Serre, C. Optimisation of the Synthesis of MOF Nanoparticles Made of Flexible Porous Iron Fumarate MIL-88 A. J. Mater. Chem. 2011, 21, 2220− 2227. (176) Soma, C. E.; Dubernet, C.; Barratt, G.; Benita, S.; Couvreur, P. Investigation of the Role of Macrophages on the Cytotoxicity of Doxorubicin and Doxorubicin-loaded Nanoparticles on M5076 Cells In Vitro. J. Controlled Release 2000, 68, 283−289. (177) Glover, T. G.; Peterson, G. W.; Schindler, B. J.; Britt, D.; Yaghi, O. MOF-74 Building Unit has a Direct Impact on Toxic Gas Adsorption. Chem. Eng. Sci. 2011, 66, 163−170. (178) Peterson, E.; Stone, M.; McCaffrey, R.; Cummings, D. Mixedgas Separation Properties of Phosphazene Polymer Membranes. Sep. Sci. Technol. 1993, 28, 423−440. (179) Vaughn, J.; Koros, W. Effect of the Amide Bond Diamine Structure on the CO2, H2S, and CH4 Transport Properties of a Series of Novel 6FDA-based Polyamide−imides for Natural Gas Purification. Macromolecules 2012, 45, 7036−7049. (180) Sridhar, S.; Smitha, B.; Mayor, S.; Prathab, B.; Aminabhavi, T. Gas Permeation Properties of Polyamide Membrane Prepared by Interfacial Polymerization. J. Mater. Sci. 2007, 42, 9392−9401. (181) Robb, W. Thin Silicone Membranes-Their Permeation Properties and Some Applications. Ann. N. Y. Acad. Sci. 1968, 146, 119−137.

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