Towards the use of metal-organic frameworks for water reuse: A review of the recent advances in the field of organic pollutants removal and degradation and the next steps in the field

Elton M. Dias1 and Camille Petit1

1Department of Chemical Engineering, Imperial College London, London, United Kingdom

KEYWORDS: Metal-organic frameworks, water purification and reuse, H2 production, photocatalysis, adsorption

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ABSTRACT

Water reuse is becoming increasingly important as more and more areas in the world are facing water stress issues. Treatment of wastewater to attain the purity required for various usages from culture irrigation to drinking water is therefore key. Several water treatment options are already in place and while we will continue to use them, parallel efforts are required to: (i) address the removal of the most persistent chemicals in water and (ii) provide solutions for local communities. Recently, several studies on the use of metal-organic frameworks (MOFs) for the adsorption and photocatalytic degradation of organics in water have been reported. This enthusiasm originates from the large porosity and chemical tunability of MOFs – beneficial for adsorption – as well as their catalytic nature – beneficial for degradation. The present reviews proposes a comprehensive and critical analysis of the most recent studies on the use of MOFs for organics adsorption and photocatalytic degradation. The potential to use MOFs to catalyze the production of H2 from organic molecules, like water contaminants, is also addressed. Overall, the discussion is organised based on the type of organic pollutants targeted and encompasses those released in industrial, domestic and agricultural wastewater streams.

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1. INTRODUCTION

Access to clean freshwater represents a key challenge for the upcoming decades. For instance, predictions show that in 2025, more than one billion people living in arid regions will face absolute water scarcity (1). While harvesting groundwater or freshwater from seawater is a clear way to address these issues, it might not be a straightforward or possible short-term option for populations with limited resources (2) and/or those living in remote areas. Therefore, beyond the purification of water, the concept of water reuse, and specifically potable water reuse, is becoming increasingly important. On the other hand, it still faces a number of challenges such as treatment costs, potential chronic toxicity and public acceptance (3-5). From a technical viewpoint, the installation of distribution systems at centralized facilities has yet to be done, and it is clear that different technologies can been applied depending on local conditions and requirements making the ‘one-size-fits-all’ approach unlikely. In addition, the intended use of the water and the related water quality standards must be clearly defined (6). The conventional methods to treat water include adsorption, coagulation, sedimentation, filtration, chemical and membrane technologies.

Although these processes are commonly used, they do not completely eliminate the waste, involve high operating costs and can generate toxic secondary pollutants (7). Industrial wastewaters contain a complex range of organic pollutants including dyes, detergents, oils, and other organic compounds (8), some of them being carcinogenic and exhibiting other side effects (9). Domestic wastewater is also not considered innocuous and often contains trace levels of pharmaceuticals and personal care products (PPCPs) (10) among others (11). Those organic pollutants are not always fully degraded by conventional methods since many of those organic substances are toxic and/or resistant to biological treatment. An option to treat these organic pollutants is through the use of adsorbents such as activated carbon, polymeric resins, clay minerals, biomaterials, zeolites and industrial solid wastes (12-14). Another

3 option to remove organics from water is via their degradation through advanced oxidation processes (AOP). These methods usually involve the in-situ generation of highly reactive radicals that partially oxidize organic molecules into less toxic ones or fully degrade them to form CO2 and water. The latter pathways is called full mineralization and is described in

Equation (1) (15, 16).

C푥H푦O푧 + (푥 + 0.25푦 − 0.5푧)O2 → 푥CO2 + 0.5푦H2O (1)

Generally, AOPs include photochemical degradation processes (e.g. UV/O3), photocatalysis

(e.g. TiO2/UV) and chemical oxidation processes (e.g. O3) (17). TiO2 is certainly the most studied photocatalyst used to degrade organics in water. While TiO2 suspensions tend to be more efficient than supported TiO2, the cost and energy for separating and recycling TiO2 particles makes the use of supported TiO2 a more attractive approach. A number of studies have been reported on this topic and commonly involve the use of polymeric films, ceramics, glass fibers/beads or carbon as the support (18).

Organic pollutants may also be seen as a source of energy, in the form of chemical energy stored in bonds. That energy can be harvested by the conversion of organic compounds into other compounds that can more easily be used for energy generation, such as H2. Indeed, if an

AOP allows for the partial mineralization of organic pollutants, H2 can be obtained from the organics degradation, as shown in Equation (2) (19, 20).

C푥H푦O푧 + (2푥 − 푧)O2 → 푥CO2 + (2푥 + 0.5푦 − 푧)H2 (2)

This approach not only brings the benefits of degrading the pollutants but also adds another dimension of energy generation to the treatment process. To allow for the possibility of H2 evolution, it is required that energy levels of the photogenerated electrons are capable of reducing hydrogen molecules (21). Having an excess amount of sacrificial electron donors

4 such as organic acids, alcohols, phenolic compounds, dyes, etc., increases the hydrogen production (20).

Metal-organic frameworks (MOFs) are virtually infinite crystalline structures composed of metal clusters bound via coordination to organic ligands (22, 23). These structures are highly porous and flexible owing to their structural and chemical tunability. MOFs have been used as adsorbents for separations in gas-phase and liquid-phase (24). Some compounds that

MOFs are able to adsorb include organics (25), H2 (26), CO2 (27-29), chemical warfare agents (30) to name a few. More recently, a number of studies on MOFs have focused on their catalytic and particularly photocatalytic properties. In fact, it has been shown that: (i) the optic properties of MOFs can be tuned to improve light-harvesting, and (ii) MOFs can act as hosts for other more photosensitive molecules (31, 32). As coordination compounds, MOFs are potentially unstable in water (e.g. MOF-5) depending on the nature of the bond between the metallic sites and the organic ligands as well as its structure. However, as a greater understanding is reached on the water stability of MOFs, a new breed of water-stable MOFs is being developed. These materials can withstand immersion in water for extended periods of time, in large pH ranges (0 to 12) (33). Overall, MOFs can appear as promising candidates for the treatment of waste water streams owing to their high porosities and specific adsorbate/adsorbent interactions. Furthermore, their catalytic properties could be used for the degradation of the toxic compounds and the production of H2 from organics. The recent enthusiasm around the use of MOFs for water purification is highlighted in Figure 1, which indicate the number of papers on the topic published in the last few years.

This review paper seeks to provide a critical analysis of the latest - i.e. past couple of years - efforts/strategies reported on the use of MOFs for the adsorption of organic pollutants in water, the photocatalytic degradation of these chemicals and the catalytic generation of H2 from organics and water. It must be noted that the latter aspect is usually not studied as being

5 part of a water purification process. It is however included here as it represents a potential way to clean water while producing energy. An overview of the three topics outlined above and covered in this paper is presented in Figure 2.

2. DISCUSSION

2.1. MOFs for organics adsorption from water

The use of MOFs for water contaminant adsorption has received an increased interest in the past five years, likely due to: (i) a greater awareness around the presence (and its consequence) of emerging contaminants and (ii) the realization that MOFs’ unique textural and chemical properties can not only be useful to gas separations but also to liquid separations. Recently, two reviews reported the use of MOFs for separation in the liquid phase, of which water contaminants removal was a sub-part (25, 34). Particularly, Van de

Voorde, Bueken (34) review paper focused on the use of MOFs for adsorption in liquid phase for either organic or aqueous phase. In the latter case, the discussion revolved around aqueous solutions containing low concentrations of organics and the separation of bio-based chemicals. More recently, Hasan and Jhung (25) published a review highlighting the potential interaction mechanisms between MOFs and hazardous organics in water. The selective adsorption mechanisms reported were classified into: electrostatic, hydrogen bonding, acid- base, π-π interactions/stacking, pore/size-selective adsorption and hydrophobic interactions

(Figure 3). The effect of the framework metal on the binding energy with water and the pore size/accessibility of the organic molecules was also explored. Another earlier review from

Khan, Hasan (35) presented a compilation of MOFs used for the adsorptive removal of hazardous materials from liquid and gaseous streams. The authors organized the review according to the pollutants targeted including: sulphur-containing compounds (SCCs) and nitrogen-containing compounds (NCCs) in fuels, organic contaminants and heavy metals in

6 waste water, and toxic gases. The main adsorption mechanisms were highlighted and they corroborated the trends indicated by the review of Hasan and Jhung (25).

The section below highlights the results of various studies on the adsorption of organic contaminants from water using MOFs. These studies have been grouped, based on the targeted contaminants, speacifically: (i) “common” contaminants whose adverse effects have been known for some time and (ii) emerging contaminants. All these contaminants can originate from industrial (e.g. dyes, phthalates, phenols, nitrobenzene), municipal (e.g. pharmaceutical and personal care products) or agricultural (e.g. veterinary drugs, herbicides, pesticides) waste streams. Overall, the discussion below provides an account of the most recent findings on organics adsorption from aqueous solutions. It must be pointed out though that a few pioneering studies are still reported here for some areas in which no significant progress has been made in the past couple of years (i.e. pharmaceuticals removal from water).

The results from the various studies are summarised in Table 1 and are discussed below.

Removal of “common” contaminants: Dyes are among the most common industrial pollutants and water coloration can be regarded as an effective and immediate indication of water pollution (36). Haque, Lo (37) investigated the adsorption of anionic (methyl orange,

MO) and cationic (methylene blue, MB) dyes using an amine functionalised MOF, NH2-

MIL-101(Al). This MOF exhibited a much higher adsorption capacity for MB than MO and performed better than MIL-101(Al). The adsorption mechanism on NH2-MIL-101(Al) was attributed to a reactive adsorption resulting in the MOF structure collapse. The reaction was confirmed by the high values of the thermodynamic properties, loss of aluminium metal concentration in the MOF and changes in the X-ray diffraction patterns. Attempts to recycle the NH2-MIL-101(Al) used to adsorb MB were unfruitful. On the other hand, the

7 recyclabillity of NH2-MIL-101(Al) used to adsorb MO was possible. This could be due to the lower binding affinity for MO in comparison to MB. Yi, Li (38) studied the adsorptive removal of dyes MB, MO and Rhodamine B (RhB) using MOFs based on tri-cadmium-ion secondary building units and the hexa[4-(carboxy-phenyl)oxamethyl]-3-oxapentane acid

(H6L) ligand. The MOFs exhibited varied topologies owing to the different N-donor bridges.

Overall, MB was more readily adsorbed by the three MOFs, while RhB was at the end of the spectrum. Even though the interactions between the dyes and MOFs were not explored in detail, the authors inferred about shape and size selectivity towards the differents dye molecules. MB and MO were considered to have small enough sizes while, RhB as the largest dye molecule, could not enter the MOF channels. A study from Zhang, Gao (39) tested the adsorption of MB, MO, RhB and Congo Red (CR) over MOF Zn(bdc)(tib)]·3H2O

(tib = 1, 3, 5-tri (1H-imidazol-1-ly) benzene and H2bdc = 1,4-dicarboxybenzene). The MOF could adsorb CR but not the other dyes. Once again, the adsorption selectivity towards CR over the other dyes was linked to CR molecule size matching that of the pore of the MOF.

Additionally, interactions existed between the coordinated unsaturated sites (CUSs) of the

MOF and the amine groups of CR. The MOF was regenerated using an ethanol wash and the material showed no apparent loss in adsorption capacity for CR over adsorption five cycles.

A study from Jia, Jin (40) demonstrated the use of MIL-100(Fe) for the adsorptive removal of

MB, methyl blue (MyB) and isatin. Isatin, an electrically-neutral dye, could not be adsorbed by MIL-100(Fe). On the other hand, MB (cationic dye) and MyB (anionic dye) were adsorbed by MIL-100(Fe). The interactions adsorbate/adsorbent were considered to be of electrostatic nature with some π-π interactions, which explained the improved adsorption of

MB over the negatively charged MIL-100(Fe) surface compared to the other dyes. The pH and ionic strength did not seem to influence the adsorption. The recyclabilty of MIL-100(Fe) was achived by a HCl/ethanol wash for MB loaded MOF and NaOH/ethanol for the MyB

8 loaded version. The effect of different water sources (ultrapure water, rainwater and river water) on the adsorption capacities was evaluated. The results revealed little influence regarding the water source demonstrating the potential of MIL-100(Fe) for the adsorptive removal of dyes in real conditions. Du, Wang (41) designed a MOF based on bismuth and

1,1’’-(1,4-butanediyl)bis[4,4’-bipyridinium]bis[tetrafluoroborate] (bbpyf), namely

[bbpy][Bi4I16]. The MOF was tested for the adsorptive removal of MB and RhB. The MOF adsorbed both dyes but there was no quantification of the adsorption capacity. Regarding adsorption mechanism, the authors inferred about hydrogen bonding. Another study by

Abbasi, Aali (42) reported the synthesis of a thin-film of MOF NH2-TMU-16(Zn) over a silk fiber’s surface. The MOF@silk material was capable of adsorbing MO and morphine while retaining its thin-film structure. The interactions adsorbates/adsorbent were determined to be due to strong hydrogen bonding. Ethanol washes were to some degree effective in recycling the adsorbent. The adsorption of fluorescein dye was investigated by Tella, Owalude (43) using [Cu(INA)2] (INA = isonicotinate) MOF. A range of tests for different adsorption conditions such as contact time, pH, temperature, dye concentration and adsorbent dosage were performed. The interactions were estimated to be of electrostatic nature. Another natural dye is humic acid. It gives water a dark colour and (44) and can be converted into toxic and carcinogenic by-products upon chlorination (45). Lin and Chang (46) tested the adsorption of humic acid in aqueous solutions using ZIF-8. The study not only showed that humic acid was effectively adsorbed using ZIF-8 but also had superior adsorption capacities when compared to conventional sorbents (e.g. chitin, chitosan, rice husk ash, bentonite, fly ash, palygorskite, graphite and activated carbon). Electrostatic interactions as well as π-π interactions/stacking were the mechanisms that appeared to govern the adsorption of humic acid on ZIF-8, based on pH studies and previous studies (47). Other experiments in this report showed little

9 influence of the ionic strength on the adsorption capacity, but a huge drop in the presence of ionic surfactants even at low concentrations.

Besides the industrial sector, agricultural waste is another source of water contaminants, such as herbicides and pesticides(48, 49). Seo, Khan (50) studied the adsorption of methylchlorophenoxypropionic acid (MCPP) using UiO-66(Zr). MCPP is a general use herbicide and is considered slighty toxic (51). The performance of UiO-66(Zr) was found to be higher and faster than that of an activated carbon (AC). The gap in performance between

UiO-66(Zr) and the AC was reduced at high MCPP concentrations. It was therefore concluded that the real benefit of having a sorbent such as UiO-66(Zr) (over AC) resides in the adsorption at low concentrations. The favourable adsorption of MCPP using UiO-66(Zr) was assigned to the strong electrostatic interactions between sorbent/adsorbate and π-π interactions/stacking with the MOF structure. The contribution of each of these adsorption mechanisms was evaluated over a large pH range, changing from weak electrostatic to strong electrostic interactions and to π-π interactions/stacking as the pH increased. The reusability of

UiO-66(Zr) for the adsorption of MCPP was highlighted over a 3-cycle study. Experiments in the low range of concentrations would be useful to confirm the adsorption capacities of the

MOF for that concentration range. The adsorption of another widely used acidic herbicide,

2,4-dichlorophenoxyacetic acid (2,4-D), using MIL-53(Cr) was reported (52). A similar adsorption mechanism was observed to that of MCPP adsorption (50). Jung, Hasan (52) confirmed that at pH lower than the isoelectric point of the MOF, the adsorbent being positively charged interacted through electrostatic forces with the acidic organic compound.

At higher pH values, adsorption, albeit minimal, occurred via π-π interactions/stacking. In this study, higher adsorption capacities and adsorption rates were measured on MIL-53(Cr) compared to an AC and a zeolite USY. De Smedt, Spanoghe (53) provided an extensive comparison of different solid adsorbents for the removal of highly mobile pesticides such as

10 bentazon, clopyralid and isoproturon. The adsorbents tested included: iron 1,3,5- benzenetricarboxylate, copper benzene-1,3,5-tricarboxylate, MIL-53(Al), DUT-5(Al), (OH)2-

MIL-53(Al), NH2-MIL-53(Al), (OH)2-CAU-1(Al), NH2-CAU-1(Al), MIL-125(Ti) and MOF-

235(Fe), resins, ACs and zeolites. Within the MOFs category, a screening study showed that

CAU-1(Al) and MOF-235(Fe) provided the best adsorption results and were studied in more detail. Overall, it was verified that MOFs adsorbed faster but less compared to the ACs and resins. The decreased adsorption rates of ACs and resins were suggested to be a result of the bigger particle size. The main advantage of most MOFs over the ACs and resins was their good recyclability. The adsorption mechanisms for the MOFs were not studied in depth but some indication of the effect of pore size, hydrophobicity/hydrophilicity, and π–π interactions were made. Glyphosate (GP) and glufosinate (GF) are two organophosphorus harmful pesticides widely used in agriculture (54, 55) and can cause harm to human health (56). In a study by Zhu, Li (57), UiO-67(Zr) was successfully employed to remove GP and GF from aqueous solutions. The adsorption capacities reported were higher than those of other types of adsorbents (e.g. MgAl-layered double hydroxides (LDH), Ni2AlNo3-LDH, etc.). The main interaction mechanism was found to be chemisorption rather than electrostatic adsorption.

Particularly, it was estimated that the Zr-OH groups in the UiO-67(Zr) framework are very reactive (58, 59) and could effectively form complexes with the organophosphorus pesticides.

Removal of emerging organic contaminants: Emerging organic contaminants fall into a group of organic pollutants that include newly developed compounds, compounds newly discovered in the environment and compounds that have only recently been categorised as contaminants

(60). Increasing efforts are therefore dedicated towards finding suitable materials to separate them from water. These contaminants can be released from the industrial, agricultural or municipal sector. An example of a class of emerging contaminants is that of phthalates which

11 are chemicals commonly used as plasticizers. Li, Wu (61) synthesized MIL-53(Al) using different sources of aluminium (i.e. alumina, aluminum oxide, boehmite and aluminum nitrate) and tested them for the removal of dimethyl phtalate (DMP), a known endocrine disruptor (61). The MOFs exhibited different textural properties and performances. However, no direct correlation between adsorption and porosity was observed. The maximum adsorption was obtained for MIL-53(Al) synthesised using aluminum oxide, adsorbing 71% more DMP than that of the least adsorptive material. Further investigation on the effect of the chemical features of the various MOFs on their adsorption capacity could help elucidate the differences observed. Similar conclusions regarding the absence of correlation between textural features and adsorption capacity were reported by Khan, Jung (47). The authors tested ZIF-8, UiO-66(Zr), NH2-UiO-66(Zr) and an AC for the adsorption of phthalic acid

(PA) and diethyl phthalate (DEP). The adsorbent with the highest adsorption capacity for PA was ZIF-8 despite its lower surface area and porosity compared to the other materials. The adsorption of PA and DEP using ZIF-8 was studied in more detail at different pH. It was found that mainly electrostatic interactions were responsible for the binding of PA to the ZIF-

8 surface. In addition, at low pH, the authors pointed to acid-base interactions, which were observed for both virgin and amine functionalized UiO-66(Zr). Overall, the MOF samples adsorbed less DEP than PA. This trend was attributed to the lack of electrostatic interactions between the charged MOFs surfaces and the neutral DEP molecule.

In addition to herbicides and pesticides, agriculture waste streams can also contain veterinary drugs. For instance, arsenical veterinary drugs such as p-arsalinic acid (ASA) and roxarsone

(ROX) are used as feed additives in the poultry and swine industries and have the potential to be converted into highly toxic inorganic arsenic compounds(62, 63). Jung, Jun (63) assessed the ASA adsorption capacities of materials such as an AC, zeolite Y and various MOFs (i.e.

ZIF-8, mesoporous ZIF-8 which had a higher mesopore volume and surface area (64), MIL-

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101(Cr), MIL-53(Cr)). A preliminary adsorption study showed that the mesoporous ZIF-8 and ZIF-8 were the best adsorbents followed by the AC. Again, it was found that the differences in textural properties could not solely explain the variations in adsorption performance. Thus, the authors further explored the adsorption mechanisms in mesoporous

ZIF-8 and ZIF-8. Isoelectric potential measurements and adsorption studies at different pH demonstrated that at pH < 9, the main interaction mechanism was electrostatic interactions between the positively charged MOF surface and the anionic form of the ASA. The fact that mesoporous ZIF-8 had a slightly better performance than ZIF-8 was not fully understood. A study from Jun, Tong (62) explored the adsorption of ASA and ROX over different porous materials including: MIL-53(Cr), MIL-100(Al), MIL-100(Cr), MIL-100(Fe), MIL-101(Cr), an AC, zeolite Y and goethite. Two major observations were that: (i) MIL-100(Fe) was the best adsorbent and (ii) MIL-100(Al) and MIL-100(Cr) exhibited similar adsorption capacities. To understand the influence of the central metal ion in MIL-100 on the adsorption mechanism, MIL-100(Al), MIL-100(Cr) and MIL-100(Fe) were studied in more details. The authors concluded that the variations in adsorption capacity among the different MIL-100 samples were most likely due to the water adsorption energy and the organic molecule adsorption energy. Water was shown to bind more weakly to MIL-100(Fe) than to the other

MIL-100 samples. In addition, ASA and ROX bound more strongly to MIL-100(Fe) when compared to the other MIL-100 samples. Regarding the adsorption mechanisms, it was suggested that the anionic species of ASA and ROX coordinated to CUSs of the MIL-

100(Fe).

A class of industrial emerging water contaminants is that of phenolic compounds. Those are considered toxic (endocrine disruptor chemical) and carcinogenic, with adverse effects observed even at low concentrations (65). Bisphenol-A (BPA) is an example of a toxic phenolic compound and is typically used as raw material to produce epoxy resins and

13 polycarbonate plastics (66). Park, Hasan (66) removed BPA from water using MIL-53(Cr) with higher adsorptive performance in terms of maximum capacity and adsorptive rates compared to an AC and ultrastable Y zeolite. The specific interactions that favoured adsorption in MOFs were identified as π–π interactions and hydrogen bonding between the hydroxyl groups of MIL-53(Cr) and those of BPA. Qin, Jia (67) have also successfully removed BPA from an aqueous solution using MIL-101(Cr) and MIL-100(Fe). MIL-101(Cr) was capable of adsorbing BPA faster than the AC and in larger quantity than MIL-100(Fe).

These results were attributed to the favourable textural properties of MIL-101(Cr), i.e. larger pore apertures and higher surface area. In addition, the interactions between MIL-101(Cr) and

BPA were suggested to be π–π interactions and hydrogen bonding. Liu, Yang (65) studied the removal of phenol and p-nitrophenol (PNP) from aqueous solutions using MIL-100(Fe),

MIL-100(Cr), NH2-MIL-101(Al) and an AC. In the case of phenol adsorption, the AC outperformed the MOF samples. The very strong binding energy with water for MIL-100(Fe) and MIL-100(Cr) reported in prior studies was used to explain the poor performance.

Regarding the adsorption of PNP, the performance followed the order: NH2-MIL-101(Al) >

AC > MIL-100(Fe) ~ MIL-100(Cr). The authors suggested that the effect of the metal sites in

MOFs were inadequate to explain the adsorption of phenol and PNP from water. Instead, the improved adsorption of PNP by NH2-MIL-101(Al) was attributed to hydrogen bonding between the MOF amine groups and the PNP nitro groups. Han, Xiao (68) reported the adsorption of phenol from water using MIL-68(Al). An improved adsorption capacity of phenol was observed when using a MOF/carbon nanotubes composite. It was suggested that

π–π interactions and hydrogen bonding were the controlling adsorption mechanisms.

Other aromatic compounds, such as nitrobenzene, have been widely used as intermediates in the preparation of dyes, perfumes, fuels, drugs, explosives, pesticides, and other macromolecules organic chemicals (69). Nitrobenzene (NB) is also considered a likely

14 human carcinogen by the United States Environmental Protection Agency (70). Patil,

Rallapalli (71) studied the adsorption of nitrobenzene from aqueous solutions using MIL-

53(Al) over a broad range of pH (2 to 10). Simulation studies showed the effects of π–π interactions and previous reports (72) suggested interactions through hydrophobic/hydrophilic centres between the nitrobenzene and MOF. A follow-up study by

Xie, Liu (73) reported the adsorption of nitrobenzene using several MOFs: MIL-53(Al), MIL-

53(Fe), MIL-100(Cr), MIL-100(Fe), MIL-68(Al), MIL-68(In), UiO-66(Zr), NH2-UiO-66(Zr),

NO2-UiO-66(Zr), ZIF-7, ZIF-8, CAU-1 and CAU-6. The samples MIL-53(Al), MIL-68(Al) and CAU-1 provided the best results for the adsorption of NB. Adsorption in those cases was linked to hydrogen bonding between the OH groups of the MOFs and the hydrogen atoms of

NB (74). The influence of textural properties (75) was also evaluated. Having a larger metal site, MIL-68(In) was less porous than MIL-68(Al). Overall, the MOFs adsorbed less than an

O-group functionalized AC (76).

Another important group of emerging contaminants is that of pharmaceuticals and personal care products (PPCPs). Regarded as an essential and indispensable element for life, they include a large group of substances such as medicines, cosmetics, or veterinary drugs (77,

78). Unlike in the above, here a few “older” references are included as no new studies have been reported in the past couple of years. Cychosz and Matzger (79) investigated the adsorption of furosemide and sulfasalazine from water using MIL-100(Cr). This MOF was selected after screening the water stability of different MOFs such as MOF-177(Zn), MOF-

5(Zn), HKUST-1(Cu), MOF-505(Cu), UMCM-150(Cu), ZIF-8(Zn) and MIL-100(Cr). The adsorption mechanism was not explored in depth, but the authors did infer about the existence of cooperative binding between the organic molecules within MIL-100(Cr), due to the observed S-shape adsorption curve. The adsorption of naproxen and clofibric acid in aqueous medium with MOFs has been extensively studied by Hasan, et al. (81, 82). In their

15 first study (Hasan, Jeon (81)), they investigated the adsorption of these two pharmaceuticals using MIL-101(Cr), MIL-100(Fe) and an AC. It was shown that MIL-101(Cr) was capable of adsorbing faster and larger quantities of both pharmaceuticals when compared to the AC.

MIL-100(Fe) was also capable of adsorbing more naproxen than AC, but less than the MIL-

101(Cr). The interactions between the drugs and the MOFs were considered to be of electrostatic nature, with MIL-101(Cr) performing better than MIL-100(Fe) due to its higher surface area or pore volume. In a later study, Hasan, Choi (82) compared the adsorption of naproxen and clofibric acid using MIL-101(Cr) and its derivative functionalized with either aminomethanesulfonic acid (AMSA-MIL-101(Cr)) or ethylenediamine (ED-MIL-101(Cr)).

ED-MIL-101(Cr) adsorbed more naproxen and clofibric acid than MIL-101(Cr) and AMSA-

MIL-101(Cr), in that order. No links to the textural properties were observed in these cases, suggesting the presence of other factors controlling the adsorption, like for instance the acid- base interactions between the carboxylic group in the pharmaceuticals and the amino group in

ED-MIL-101(Cr).

Summary: Overall, the studies discussed above, whose results are summarised in Table 1, highlight the promising role of MOFs for organic contaminants adsorption from aqueous solutions. The textural properties of MOFs, and particularly their often large porosity, are frequently regarded as one of the main drivers for using MOFs for that purpose. Nevertheless, these textural parameters do not seem to be the major controlling factors of the adsorption mechanism. In fact, the pore window size is often not compared to that of the pollutant.

Often, the molecular size of the organics to be removed is likely to be similar or larger than that of the MOF pore windows. Hence, moving forward the modifications of the textural properties, e.g. increasing the pore window size might become key. The synthesis of mesoporous MOFs is however still subject to intense research (83). While the textural

16 parameters do not appear to be a governing factor in current studies, the presence of the following specific interactions seem to control the adsorption mechanism: π–π interactions, electrostatic interactions, hydrogen bonding, acid-base interactions and coordination to CUSs.

The types of interactions involved are mainly dependent on the organic pollutant being removed and the structure/chemistry of the chosen MOF. Here, techniques such as Post- synthestic modification (PSM) could prove to be a useful tool to improve the MOF/adsorbate interactions. A review from Han, Li (84) compiled several substitution reactions (metal ion, organic ligand and free guest molecule) in MOFs. The substitution reaction reactions could not only modify the MOFs’ textural properties but also their interactions with the adsorbate.

AC is frequently used as a reference for the adsorption of organic pollutants owing to its availability, low cost, ease of preparation, robustness and high porosity. The recyclability and adsorption kinetics of AC tend to be poorer and slower, respectively compared to those of

MOFs. However, MOFs despite exhibiting faster kinetics do not always reach the adsorption capacities measured in ACs. In addition, it must be noted that particle size distribution for each adsorbent can influence the kinetics and this parameter is not often reported. In general, it is noted that comparison with AC – while necessary since AC is the common benchmark for adsorption – must be considered carefully since: (i) the ACs used differ from one study to the other, (ii) the choice of a particular AC versus another is not frequently, if at all, justified while it would be relevant to use ACs employed in water treatment for the waste stream considered, and (iii) different ACs can exhibit very different surface chemistries and textural parameters and these properties are not often reported in papers.

It is only when MOFs’ stability and reusability are guaranteed that MOFs can be used as effective organic pollutants adsorbents for aqueous solutions. Most investigated MOFs were water stable for the duration of the adsorption experiments. Some even exhibited good stability for a large pH range (e.g. UiO-66(Zr) adsorbed MCPP from pH 2 to 9). Some studies

17 also looked at the effect of the ionic strength, in order to probe the effect of the mineral diversity of waste water streams. The recyclability of the MOFs (usually tested over 3 cycles) has often been investigated and successful regeneration of the materials with a simple wash with ethanol has been highlighted in a few papers.

The typical contaminant concentrations targeted ranged from about 1 to 1,000 ppm. Reasons for selecting a specific concentration are usually not provided, while it would be useful to link these to the targeted waste stream. The range of MOF loadings goes from about 30 to

6,000 mg∙L-1. Overall, the differences in pollutant concentrations and MOF loadings make any direct comparison challenging.

2.2. MOFs for organics degradation

Degradation of organic pollutants from water streams can also be seen as another option for water purification. Degradation brings other advantages compared to adsorption such as the absence of a sorbent regeneration step and the fact that no further treatment is required to eliminate the organics. Wang, Li (85)’s recent review paper presents a comprehensive analysis focused on the use of MOFs for the organic pollutants degradation. The discussion below highlights the latest developments in photocatalysis for organics degradation using

MOFs and some of the strategies employed to achieve better photocatalytic performances.

The degradation of dyes is reviewed first followed by that of other aromatic compounds

(other than dyes). A summary of the conditions and performances of the reviewed materials is provided in Table 2. As for the section on adsorption, the discussion below is organised based on the type of contaminants to be removed.

Degradation of dyes: Rhodamine B (RhB) degradation has been one of the most investigated organic dyes degradation using photoactive MOFs. Zhao, Dong (86) recently developed a

18 novel MOF, [Co3(BPT)2(DMF)(bpp)]·DMF, to degrade aqueous RhB under visible light. The proposed reaction mechanism was very similar to that reported in previous papers (87-89) and consisted of an electron being excited with visible light irradiation from the HOMO

(oxygen and/or nitrogen orbitals) to the LUMO (metal cluster). The OH-, from hydrogen

+ peroxide (H2O2) in solution, would interact with the recently created hole (h ) and generate the hydroxyl radical that degraded RhB. The powder X-ray diffraction (PXRD) pattern of the used MOF showed little difference compared to that of the fresh MOF. The use of MIL-

53(Fe) for the photodegradation of aqueous RhB was also tested (90-92). Ai, Zhang (90) reported the importance of using hydrogen peroxide (H2O2) as an effective activator for the reaction of degradation. The photoreaction mechanism was estimated to be through metal- oxo cluster direct excitation. The MOF could be reused for three cycles without apparent performance loss. In an attempt to promote the separation of the photogenerated charge carriers in MIL-53(Fe), Zhang, Ai (92) synthesised a graphene-MOF composite photocatalyst. Nevertheless, the benefits of using such auxiliary structure remained inconclusive. Another MOF-based composite study from Zhang, Ai (91), explored the use of

MIL-53(Fe) rods decorated with Fe3O4 magnetic nanospheres (MN-MIL-53(Fe)), under visible light. Both pure MIL-53(Fe) and MN-MIL-53(Fe) proved to be more efficient to degrade organic dye than other photocatalysts such as Fe2O3, Fe3O4 nanoparticles and commercial P25. The reaction mechanism was described by charge carriers being transferred to the surface of the photocatalyst to participate in redox reactions. Further analysis with trapping experiments revealed that the photogenerated electron and ˙OH radicals are the main contributors to the photocatalytic process. The MOF reusability was confirmed after three cycles. Wang, Yuan (93) successfully used graphitic carbon nitrite (g-C3N4) to improve the photocatalytic performance of MIL-125(Ti), for the degradation of aqueous RhB. The physical mixture of g-C3N4 and MIL-125(Ti) performed better than the pure MOF. g-C3N4 led

19 to an improved charge carrier separation and acted as a good sensitizer for visible-light response (93). In the proposed reaction mechanism, MIL-125(Ti) was not directly excited by visible light. Instead RhB and g-C3N4 acted as sensitizers. Photogenerated charges on these structures then transfered to the titanium-oxo cluster or the heterojunction structure and promoted the redox reactions. The MOF-based composite retained its photocatalytic performance for five cycles. Wang, Dong (94) used two MOFs (Zn(5-aminoisophthalic acid)•H2O and [Cd(5-aminoisophthalic acid)•H2O]n•2nH2O) as active photocatalysts for RhB degradation under UV irradiation. ZnMOF performed about 38% better than CdMOF. No explanation was provided to justify this trend. In a subsequent study from the same group

+ (Wang, Dong (95)), a MOF/polyaniline composite was used (MOF: [CdL]·[ H2N

(CH3)2](DMF)(H2O)3). Polyaniline is known to extend the photoresponse region and has a high separation efficiency of photogenerated electron–hole pairs (96). As a result, an enhanced performance was measured for the composite. In both studies (94, 95), the reusability of the MOFs was successfully tested over five cycles. However, the reaction mechanisms were not explored in detail. Lu, Chen (97) evaluated the use of four MOFs with varying structures for the photodegradation of RhB. Zn(1,3-BDC)(bmimb), Zn(1,4-

BDC)(bmimb), Cd(1,3-BDC)(bmimb) and Cd(1,4-BDC)(bmimb) (1,X-BDC = 1,X- benzenedicarboxylate; bmimb = 4,4’-bis(4-methyl-1-imidazolyl)biphenyl) were chosen due to their hydrophobic nature originating from the methyl groups on the organic linker (98-

100). As suggested by prior studies, the organic dye interacted with the UV light and injected electrons to the MOFs conduction band. The redox reactions occurred afterwards like for instance the oxiditation of the organic molecule by the photogenerated hole. After five degradation cycles, PXRD analysis indicated no obvious loss in crystallinity but no performance results from those cycles were provided. A study from Sha, Chan (101) tested

UiO-66(Zr)/Ag2CO3 composite and used visible-light for the degradation of RhB. The

20 composite adsorbed about 1.7 times faster than pristine UiO-66(Zr). The results revealed the importance of the balance between the available active sites and the specific surface area of

-● the composite. The reaction mechanism was attributed to the presence of O2 (from dissolved oxygen) and h+. Charaterization techniques have demonstrated the existence of metallic Ag, indicating that some photocorrosion occurred. In another study, Sha and Wu

(102) used composites of MOFs (UiO-66(Zr), NH2-UiO-66(Zr) and NH2-MIL-53(Fe)) and a bismuth oxyhalide (BiOBr). The composites exhibited a superior photocatalytic performance

(up to ~60 times faster for UiO-66(Zr)) compared to the pristine MOF. BiOBr/UiO-66(Zr) remained stable after three degration cycles. In Li, Ren (103) study, Zn2(tipm)(BDC)2,

Co2(tipm)(BDC)2 as well as two Co-doped ZnMOFs were tested for RhB degradation under

UV irradiation. Overall, all materials were able to degrade most of the dye (from 90% to

97%). The Co-doped MOFs performed almost twice as good the original CoMOF however no explanation was provided to explain the results. The reaction mechanism was reported to be a result of direct excitation of the metal cluster with electron and holes pairs participating in redox reactions, as previously reported (104, 105). Analysis of the final solution with liquid chromatography mass spectrometer (LC-MS) revealed no other organic species, suggesting that RhB was fully degraded into CO2 and H2O (106, 107).

The degradation of MO was studied by Pu, Xu (108). Here, a UiO MOF with naphthalene-

1,4-dicarboxilic acid (NA) and anthracene-9,10-dicarboxylic acid (AN) organic ligands was used. The results showed that the photocatalytic performance increased with greater

π-conjugation, promoting red-shift in the adsorption spectra. The reaction was based on the semiconductor theory (109, 110) where generated photoexcited holes have a strong oxidant ability and can oxidize the surface adsorbed organic molecules. Peng, Zhao (111) used

Cu(II), Zn(II) and Cd(II) MOFs (prepared with ligands bis(1,2,4-triazol-4-yl)ethane and benzenetricarboxylate) for the degradation of aqueous MO, under UV irradiation. CuMOF

21 had the best performance followed by ZnMOF and CdMOF. The mechanism was regarded as

“complicated and not very clear” but experiments with OH scavengers revealed the importance of ˙OH radicals in the photocatalytic reaction. A study from Liang, Jing (112) tested the influence of noble metals (Au, Pd and Pt) photodeposited onto MIL-100(Fe) for the degradation of MO. Visible light was enough to trigger photogenearated charges in acidic solutions (pH 4) of the dye. The excited electrons would then transfer to the catalytic metal nanoparticles, thereby limiting charge recombination. All the composites performed better than the pure MOF. The Pt version demonstrated the best performance. This was explained by the favourable average particle size that could enhance light adsorption and provide more efficient charge separation. The composite withstood four regeneration cycles with washes in

HNO3 aqueous solutions, with no apparent losses in the performance.

Wang, Li (113) used [Pb(Tab)2(bpe)]2(PF6)4 and its Ag-doped counterpart to photodegrade a range of azo dyes: MO, Acid Orange 7, Orange I, Orange IV, Orange G, CR, Acid Red 27,

Sunset Yellow, Amido Black 10B, Nigrosin, Acid Chrome Blue K and Eriochrome Black T.

The tests were conducted under UV radiation. The use of metal ion dopants could be used to tune the band gap of the photocatalyst and enhance charge separation, therefore promoting the photocatalytic activity (114). Ag-doped MOF always performed better than the pristine

MOF. The narrowing of the band gap was justified by a shift to a lower energy level of the conduction band, owing to the presence of the 4d orbital of Ag+. The mechanism was described by electrons being photoexcited from the valence band (VB) to the conduction band (CB) with further conversion into highly reactive radicals, that could ultimately degrade the azo dyes. Li, Liu (115) demonstrated the degradation of MB using a MOF based on

Cu4(OH)2 and ligands bis(1,2,4-triazol-4-ylmethyl)benzene. It was speculated that the degradation proceeded as in the Fenton-like process (116-118). Mohaghegh, Kamrani (119) synthesized a composite of Ag2O on a copper terephthalate MOF (Ag2O@Cu(tpa)·(dmf)).

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The composite degraded acid blue 92 dye under visible irradation, unlike the pure MOF and isolated Ag2O nanoparticles. The proposed mechanism included the photogenearation of charge carriers at the Ag2O nanoparticles with the MOF providing a large surface. The MOF unique structure could also serve to regulate the charge separation of e- and h+, promoting the photocatalytic reaction. The Ag2O@Cu(tpa)·(dmf) was recycled five times with distilled water and showed a slight decrease in performance.

Degradation of aromatic compounds (excluding dyes): Nitroaromatic compounds have not only been successfully removed from aqueous streams by adsorptive removal onto MOFs, but researchers have also been able to degrade them using photoactive MOFs. Wu, Qi (120) studied the photodegradation of NB, PNP and 2,4-dinitrophenol (2,4-DNP) using

Ag4(NO3)4(dpppda) (dpppda, 1,4-N,N,N’,N’-tetra(diphenylphosphanylmethyl)benzene diamine). Interestingly, the reaction was shown to fit a pseudo-zero-order kinetics model, suggesting that the reaction took place at the surface of the MOF, similar to another study

(Whang, Hsieh (121)). The photodegradation mechanism was based on the usual photo- promotion of electrons from the VB to the CB (122, 123) with ˙OH radicals being the main reaction mediators. The complete degradation of the pollutants into CO2 and H2O was confirmed by analysis of the liquid phase as well as by the detection in the gas phase. The reusability of this MOF was observed over five cycles.

Another class of aromatic pollutants are chlorophenols. These can be used as fungicides as well as in the synthesis of herbicides and insecticides. Due to their toxicity and high solubility in water, they are ranked as severe environmental pollutants (124). 2-Chlorophenol was successfully photodegraded under visible light with two MOFs: [Zn2(Fe-L)2(μ2-

O)(H2O)2]•4DMF•4H2O and [Cd2(Fe-L)2(μ2-O)(H2O)2]•2DMF•H2O (H4L = 1,2- cyclohexanediamino-N,N’-bis(3-methyl-5-carboxysalicylidene) (Li, Yang (125)). ZnMOF performed better than CdMOF, but no explanation was provided for this trend. Experiments

23 with a ˙OH radical scavenger suggested that the photodegradation process was predominantly controlled by ˙OH radicals (126). Based on 2-CP’s molecular dimensions (127) and the

MOFs channel diameters, the authors also inferred about the possibility of the photocatalytic process ocurring inside the MOF channels. ZnMOF retained its catalytic activity over four cycles.

Summary: Overall, the focus of the studies discussed above was to: (i) minimize the recombination rate of excited carriers in order to promote degradation and (ii) narrow the band gap in order to allow visible light-triggered degradation and enhanced reaction rates.

Generally, two routes were explored to address these two points: (i) the MOFs were directly modified via functionalization of the ligand or modification of the metallic sites or (ii) MOF- based composites were formed (see Figure 4).

In most cases, the photodegradation pathway was mediated by ˙OH radicals formed after the photoexcitation process. Interestingly, most of the studies did not use hydrogen peroxide during the reaction and roughly a quarter of the studies were conducted using visible light rather than UV light (Table 2). As for the adsorption studies, the MOF loadings as well as the contaminant concentrations varied widely from studies to studies: from 100 to 1,000 mg∙L-1 and from 0.005 to 100 ppm, respectively. Again, this makes direct comparisons challenging.

It is also often not made clear how relevant the contaminants concentrations are to a specific waste water stream. Another interesting aspect is the limited diversity of contaminants targeted (mostly organic dyes) in spite of the wide range of existing pollutants. In addition, as for adsorption studies, only simple waste streams (i.e. no mixture of contaminants) were considered.

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While the degradation of the contaminants is usually investigated using UV-vis spectrophotometer by monitoring the disappearance of a specific chemical, the formation of resulting decomposition products (other than CO2 and H2O) is not always studied/reported.

This may be needed to confirm complete degradation. Reusability is also a core criterion for the use of MOFs in the photodegradation of organic compounds. The latest publications have shown that many of the materials studied were able to withstand up to five full cycles without decrease of photocatalytic activity

2.3. MOFs for hydrogen evolution from organics

The preceding two sections focus solely on water purification. Particularly, the discussion on degradation shows how MOFs can be used as photocatalyst to mineralise organics, i.e. convert them into CO2 and H2O. It is interesting to notice that in parallel to those studies, work has been done to photocatalytically produce H2 from water and organic molecules. It is worth noticing that one could integrate wastewater treatment with H2 production following this scheme (Figure 2). For this reason, the generation of H2 from aqueous streams using

MOFs is included here. It is however noted that this field is largely less explored compared to the two topics covered above. In addition, among the studies investigating MOFs for H2 production, the objective is typically to produce H2 from water using an organic sacrificial agent rather than to clean water (and produce energy). Nevertheless, it is interesting to remark that the fundamental materials challenges remain the same whether we consider the problem from one angle or the other. Hence, the topic has been included in this review as a way to open the perspective on MOF utilization in water treatment.

Nasalevich, van der Veen (31) and Wang and Wang (32) review papers described the role of

MOFs in H2 production. Overall, it is agreed that MOFs can adopt one or more of three

25 functions. Firstly, they can act as the primary photocatalyst. In this case, the radiation interacts directly with the MOFs structure, leading the photoexcited charges generated either on the organic ligand or on the metal cluster. Attempts to improve light harvesting properties of MOFs cover strategies such as organic group functionalization and/or change of the metal ions in the framework (31). In addition, MOFs can serve as co-catalysts (32), where other structures (dyes, semiconductors, etc.) harvest light and transfer excited charges to the MOF, where the catalytic reactions occur. Finally, MOFs can be used as nanoreactors whose function is to host photoactive particles. The first two mechanisms are schematized in Figure

5. A recent review from Wang, Han (128) covers strategies used to improve the photocatalytic H2 production in MOFs, via incorporation of organometallic complexes. This section of the present review paper provides an overview of the recent findings (i.e. last couple of years) on the use of MOFs as photocatalysts for H2 production in aqueous systems where organic compounds can simultaneously be degraded into CO2. The results are discussed below and a summary can be found in Table 3.

Degradation of naturally occurring organic compounds: L-ascorbic acid, also known as vitamin C, can be easily dissolved in water. Yuan, Yin (129) used composites of UiO-66(Zr) and Pt nanoparticles to produce H2. L-ascorbic acid (pH 4.0) was used as a sacrificial electron donor and Erythrosin B (ErB) dye as sensitizer. The dye, adsorbed on the MOF surface via different interactions (130, 131), acted as a light absorbing antenna for visible radiation. Pt-

UiO-66(Zr) showed no H2 evolution, but upon increasing the concentration of the dye, a maximum in H2 production was achieved. This peak of performance was explained by a balance between MOF dye sensitization and free dye light-absorbing molecules (covering- effect). The mechanism was explained by excitation of the dye (131) with further charge transfer to the MOFs CB (Figure 5) where the redox reactions would occur. The protons

26 origin was not discussed neither the fate of the sacrificial agent. Following a similar approach, Zhou, Wang (132) grew CdS nanoparticles on UiO-66(Zr) with further photo- deposition of Pt. The system exhibited an enhanced H2 evolution due to more efficient charge separation provided by the different photocatalyst components. L-ascorbic acid (pH 4.0) was used as sacrificial agent under visible light irradiation. The reported reaction mechanism suggested the generation of photoexcited electrons to the CdS’s CB with further charge transfer to the MOF’s CB, where the reaction of reduction of H2 took place. Wang, Gu (133) studied composites of UiO-66(Zr) and g-C3N4 to promote hydrogen evolution from L- ascorbic acid. g-C3N4 was chosen due to its chemical stability and visible light adsorption capabilities (134-138) together with the ease of anchoring to other materials (139-143). It was found that there was an optimal amount of g-C3N4 beyond which a decrease in the H2 production rate was observed. The reaction mechanism was suggested to be a result of g-

C3N4 having light-excited electrons transferred to the MOFs’ CB, similar to a dye sensitized process. Sasan, Lin (144) synthesised a MOF with zirconium as the metallic species and a zinc-based porphyrin as the ligand. The MOF was functionalised with an iron-based complex and used for H2 production in acidic solution (pH 5) under visible light. The enhanced electron transfer from the sensitizer (i.e. porphyrin) to the catalytic centre (i.e. iron complex) was attributed to the intimate contact between the iron species and the zinc sites (145). The

L-ascorbic acid was degraded but the degradation products were not investigated.

Lactic acid is an organic compound with several roles in biochemical processes. Shen, Luo

(146) used this compound in water as a sacrificial electron donor for H2 evolution. The substrate for such reaction was a composite made of UiO-66(Zr) and quantum-sized CdS mixed with molybdenum disulphide (MoS2). CdS facilitated the separation of photogenerated charge carriers and MoS2 acted as the co-catalyst increasing the photocatalytic activity (147-

149). Using the composite, about twice as much H2 was produced compared to using pure Pt

27 particles. Regarding stability, the MOF composite proved to sustain four cycles of photocatalysis while maintaining its chemical and physical integrity. The reaction proceeded via electron transition under visible light from the VB to the CB of the composite with subsequent charge transfer to MoS2.

Methanol is a known organic alcohol that is highly toxic for human consumption (150).

Zhang, Zhang (151) degraded methanol in acidic (pH 1.8) aqueous solution while producing

H2 using a MOF composite exhibiting a UiO like topology under visible light. The MOF built

2+ from [Ru(bpy)3] -derived dicarboxylate ligands and Zr6(μ3-O)4(μ3-OH)4 secondary units

6− served as a host for the encapsulation of a noble-metal-free polyoxometalate ([P2W18O62] =

POM). A view of the resulting structure is shown in Figure 6. The mechanism was explained

2+ by the direct photo-excitation of the [Ru(bpy)3] -derived dicarboxylate ligand, with charge transfer to the active catalytic centre, the guest POM. A multielectron injection mechanism from the organic ligand to the POM promoted faster H2 production rates. Ethanol widely known in industry and alcoholic beverages can be separated from water through a wide range of processes (152). Saha, Das (153) were able to directly use ethanol for H2 evolution under visible irradiation using MOF-based composite. Specifically, quantum dots of CdS were embedded in a low molecular-weight metallohydrogel, which was then used to synthetize the

MOF-CdS quantum composite single crystals. Pt cocatalyst was then deposited. The suggested mechanism was described as follows: CdS acted as sensitizer while charge transfer took place to the MOF and Pt nanoparticles.. The ethanol acted as sacrificial electron donor donating electrons to the photogenerated holes. The photocatalyst showed no significant decrease after 12 cycles.

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Degradation of synthetic organic compounds: Tertiary amines such as triethanolamine

(TEOA) have applications in the cement production (154), cosmetics and medicine.

Nevertheless, it is associated with allergies (155-157), increase incidence of tumours in mice

(158) and toxicity for aquatic species (159). He, Wang (160) used UiO-66(Zr) decorated with

Pt nanoparticles to produce H2. RhB served as sensitizer and TEOA as sacrificial electron donor. The effect of the sensitizer was more evident when RhB was not fully adsorbed onto the MOF, suggesting that RhB in solution could also sensitize UiO-66(Zr), while an excess of

RhB on MOF surface shielded the materials from light, thereby affecting the performance.

The suggested mechanism involved the direct excitation of the dye by visible radiation, followed by photo-excited electrons being directly transferred to the Pt nanoparticles or indirectly via the MOFs CB. Hydronium ions reduction then occurred at the Pt particles surface. The use of a dye sensitizer (ErB) was also reported by Liu, Wang (161). In this case,

MIL-101(Cr) was covered by photodeposited Ni/NiOx nanoparticles (photocatalyst). The photoreaction occurred under visible-light with TEOA used as sacrificial agent. The reaction mechanism was described by the above reported excitation of dye with charge transfer to the catalyst. A more complex approach was studied by Wen, Mori (162). In their study, the already photoactive Pt/NH2-MIL-101(Cr) was sensitized by RhB under visible light. The reported reaction mechanism consisted of both MOF and RhB being excited and transferring

3+ charges to the catalytic sites (Pt nanoparticles and unsaturated Cr ) responsible for H2 evolution. TEOA in solution was then be oxidized due to the photogenerated holes. Toyao,

Saito (163) studied the use of a Ru complex-incorporated Ti-based MOF (Ti-MOF-Ru(tpy)2) with Pt nanoparticles as co-catalyst. Under visible light irradiation, TEOA was used as sacrificial electron donor. The reaction mechanism was assigned to a ligand-to-metal charge transfer (LMCT). A run of three cycles indicated the material’s good photocorrosion resistance.

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Ethylenediaminetetraacetic acid (EDTA) is a chelating agent with many uses in industry and medicine, and is also known as a cytotoxic and genotoxic species (164). EDTA’s very limited biological degradation is also a major issue in water treatment processes (165). Hou, Li (166) used a UiO-67(Zr)-based material to degrade EDTA and produce H2 from an acidic (pH 5) aqueous solution, under visible light. In the material, the ‘usual’ organic ligand was replaced

2+ by structurally similar metal complexes: [Ru(dcbpy)(bpy)2] (dcbpy = 2,2’-bipyridyl-5,5’- dicarboxylic acid), a molecular photosensitizer, and Pt(dcbpy)Cl2, a proton reduction catalyst

(see Figure 7). The advantage was to incorporate both the sensitizer and the catalyst in the same structure to effectively facilitate the separation of the photogenerated charge carriers.

The suggested mechanism seemed consistent with the initial expectations: generation of photoexcited electrons in the Ru unit with further transport to the Pt unit where H2 production took place.

Summary: Overall, it is found that while MOFs may or may not be photoactive on their own, their advantage as substrates lies in their multifunctional nature, where its photocatalytic properties can enhance (e.g. acting as sensitizer for co-catalysts) or be enhanced by other molecular structures (e.g. acting as a catalytic centre enhanced by a sensitizer). When photocatalytic MOFs are used, most efforts are dedicated towards improving the quantum efficiency of H2 production by expanding the light adsorption to the visible range. In the cases where the MOF acts as a sensitizer, then the doping of MOFs with efficient proton reduction catalysts was explored. It is also worth noting that the focus of most studies was to promote H2 evolution while the fate of the organic sacrificial agents was not discussed. In fact, the degradation products of the sacrificial agents were not investigated. As explained before, these organic compounds could well be contaminants found in wastewater. The

30 source of the protons to form H2 was also not explored in detail. Reusability wise, most of the studies looked at MOFs whose performances were maintained over several cycles

CONCLUSIONS AND OUTLOOKS

This paper is the result of a systematic and critical review that covers studies reported on the use of MOFs for water purification during the past couple of years. The clear trend in the increase in the number of papers published on this topic translates the enthusiasm of the research community for this field. Overall, the reviews shows that MOFs have been successfully applied for both the adsorption and photocatalytic degradation of a wide range of organic contaminants in water. Some insights into their mechanisms of action have been gained owing to the use of a range of characterization techniques. Looking forward, further studies should be conducted on the recyclability of these materials, while also targeting more complex wastewater streams. In addition, while the performance of MOFs is often reported to surpass that of common materials used for adsorption (e.g. activated carbon) or degradation

(e.g. titania), the basis for the comparison is not always made clear. Indeed, (i) the particle size of the “reference” materials may differ from that of MOFs, (ii) those materials are not always fully characterized making any conclusions difficult and (iii) the “reference” materials used do not necessarily represent the current state-of-the-art. In fact, the key strength of

MOFs probably goes beyond surpassing current materials and lies in the fact that MOFs can open up routes for novel applications such as the combined water purification and H2 production discussed at the end of the review.

AUTHOR INFORMATION

Corresponding Author

31

*Camille Petit, Imperial College London, Department of Chemical Engineering. South

Kensington Campus, London SW7 2AZ, UK, [email protected], +44 (0)20 7594

3182.

ACKNOWLEDGEMENTS

The authors acknowledge the support from the Department of Chemical Emgineering at

Imperial College London.

ABBREVIATIONS

2-CP, 2-Chlorophenol; 2,4-D, 2,4-Dichlorophenoxyacetic acid; 2,4-DNP, 2,4-Dinitrophenol;

AN, Anthracene-9,10-dicarboxylic acid; AMSA, Aminomethanesulfonic acid; ASA, p-

Arsalinic acid; BPA, Bisphenol A; CB, Conduction band; CUSs, Coordinated unsaturated sites; CR, Congo red; DEP, Diethyl phthalate; DMF, Dimethylformamide; DMP, Dimethyl phthalate; EDTA, Ethylenediaminetetraacetic acid; ED, Ethylenediamine; ErB, Erythrosin B;

GF, Glufosinate; GP, Glyphosate; g-C3N4, graphitic carbon nitrite; MyB, Methyl blue; MO,

Methyl orange; MCPP, Methylchlorophenoxypropionic acid; MB, Methylene blue; NA,

Naphthalene-1,4-dicarboxilic acid; NB, Nitrobenzene; PA, Phthalic acid; PANI, Polyaniline;

PNP, p-Nitrophenol; PSM, Post-synthetic modifications; PXRD, Powder X-ray diffraction;

RhB, Rhodamine B; ROX, Roxarsone; TEOA, Triethalonamine; VB, Valence band

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41 frameworks for photocatalytic hydrogen evolution from aqueous solution. J Mater Chem A. 2015;3(19):10386-94.

42

TABLES

Table 1. Summary of the performance of MOFs for organic pollutants adsorption at ambient pressure. The table is organised in alphabetical order based on the name of the targeted contaminants. Qm is the Langmuir paramater for the maximum adsorption capacity and b is Langmuir constant.

Langmuir parameters

-1 -1 -1 -1 Adsorbate MOF Qm (mg.g ) b (L.mg ) Conditions, T (°C) Conc. Adsorbate (mg.L ) Conc. MOF (mg.L ) Dominant mechanism Reference

556 0.168 25 Electrostatic interaction, 2,4-Dichlorophenoxyacetic acid MIL-53(Cr) 526 0.079 30 10-150 100 (52) π–π interaction 500 0.009 35

p-Arsalinic acid Mesoporous ZIF-8 791 0.032 25 3-350 100 Electrostatic interaction (63)

p-Arsalinic acid ZIF-8 730 0.073 25 3-350 100 Electrostatic interaction (63)

p-Arsalinic acid MIL-100(Fe) 366 n.a. 25 12-200 100 CUSs (62)

d Bentazon NH2-CAU-1 21 n=1.682 n.a. n.a. n.a. n.a. (53)

Bentazon MOF-235(Fe) 17 d n=4.608 n.a. n.a. n.a. n.a. (53)

Bisphenol A MIL-53(Cr) 421 0.815 25 5-80 100 π–π interaction, hydrogen bonding (66)

Bisphenol A MIL-101(Cr) 253 0.022 30 20-300 200 π–π interaction, hydrogen bonding (67)

Bisphenol A MIL-100(Fe) 56 0.008 30 20-300 200 π–π interaction, hydrogen bonding (67)

Clofibric acid ED-MIL-101(Cr) 347 0.024 25 50-150 100 Acid-base interaction (82)

312 0.029 (81) Clofibric acid MIL-101(Cr) 25 50-150 100 Electrostatic interaction 315 0.023 (82)

Clofibric acid AMSA-MIL-101(Cr) 105 0.018 25 50-150 100 Electrostatic interaction (82)

Clopyralid MOF-235(Fe) 5 d n=1.181 n.a. n.a. n.a. n.a. (53)

d Clopyralid NH2-CAU-1 0.7 n=0.634 n.a. n.a. n.a. n.a. (53)

43

Congo red Zn(bdc)(tib)]·3H2O 60a - 25 35 1000 Pore size, CUSs (39)

Dimethyl phtalate MIL-53(Al)* 206a - 25 25 75 π–π interaction, pore size (61)

Dimethyl phtalate MIL-53(Al)* 192a - 25 25 75 π–π interaction, pore size (61)

Dimethyl phtalate MIL-53(Al)* 190a - 25 25 75 π–π interaction, pore size (61)

Dimethyl phtalate MIL-53(Al)* 120a - 25 25 75 π–π interaction, pore size (61)

Fluorescein [Cu(INA)2] 67 1.364 25 3-18 400 Electrostatic interaction (43)

Furosemide MIL-100(Cr) 12a - 25 7.50 1250 n.a. (79)

540 0.074 30

Glufosinate UiO-67(Zr) 310 0.078 25 5.4-72 50 Chemisorption (57)

205 0.107 100

538 0.465 30

Glyphosate UiO-67(Zr) 462 0.891 25 5.1-68 50 Chemisorption (57)

380 0.814 100

70 0.047 20

Humic acid ZIF-8 95 0.076 40 10-80 500 Electrostatic, π–π interaction (46)

112 0.146 60

d Isoproturon NH2-CAU-1 16 n=1.098 n.a. n.a. n.a. n.a. (53)

Isoproturon MOF-235(Fe) 0.9 d n=0.947 n.a. n.a. n.a. n.a. (53)

0.99d n=2.37 4

1.71d n=2.88 27 Methyl blue MIL-100(Fe) 5-25 2500 Electrostatic interaction, π–π interaction (40) 1.88d n=2.72 50

2.37d n=3.01 60

188 0.019 30

Methyl orange NH2-MIL101(Al) 192 0.019 40 20-200 50 Electrostatic interaction (37)

199 0.019 50

a Methyl orange [Cd6(L)2(bib)2(DMA)4] 102 - 25 10 100-600 n.a. (38)

a Methyl orange [Cd3(L)2(bipy)(DMA)4] 17 - 25 10 100-600 n.a. (38)

44

a Methyl orange [Cd3(L)2(tib)(DMF)2] 0.3 - 25 10 100-600 n.a. (38)

NH2-TMU- a Methyl orange 1.27 - 25 32.7 n.a. Hydrogen bonding (42) 16(Zn)@silk

Methylchlorophenoxypropionic acid UiO-66(Zr) 370 n.a. 25 20-170 100 Electrostatic, π–π interaction (50)

762 0.115 30

Methylene blue NH2-MIL101(Al) 1032 0.335 40 20-200 50 Electrostatic interaction (37)

1409 0.773 50

195 0.031 30

Methylene blue MIL-101(Al) 216 0.032 40 20-200 50 Electrostatic interaction (37)

195 0.032 50

8.47d n=1.11 4

6.37d n=1.00 27 Methylene blue MIL-100(Fe) 20-60 2500 Electrostatic interaction, π–π interaction (40) 3.46d n=0.89 50

2.97d n=0.86 60

a Methylene blue [Cd6(L)2(bib)2(DMA)4] 318 - 25 10 100-600 n.a. (38)

a Methylene blue [Cd3(L)2(bipy)(DMA)4] 105 - 25 10 100-600 n.a. (38)

a Methylene blue [Cd3(L)2(tib)(DMF)2] 30 - 25 10 100-600 n.a. (38)

Methylene blue [bbpy][Bi4I16] n.a. n.a. 25 3.2 3000 Hydrogen bonding (41)

NH2-TMU- Morphine 2.00a - 25 28.5 n.a. Hydrogen bonding (42) 16(Zn)@silk

Naproxen ED-MIL-101(Cr) 154 3.25 25 10-18 100 Acid-base interaction (82)

132 2.92 (81) Naproxen MIL-101(Cr) 25 10-18 100 Electrostatic interaction 131 2.93 (82)

Naproxen MIL-100(Fe) 115 1.78 25 10-18 100 Electrostatic interaction (81)

Naproxen AMSA-MIL-101(Cr) 93 1.60 25 10-18 100 Electrostatic interaction (82)

Nitrobenzene MIL-68(Al) 1188 0.0021 30 100-250 100 Hydrogen bonding (73)

Nitrobenzene CAU-1 1172 0.0072 30 100-250 100 Hydrogen bonding (73)

45

Nitrobenzene MIL-53(Al) 625c - 30 10-250 100 Hydrophobic/hydrophilic, π–π interactions (71)

p-Nitrophenol NH2-MIL-101(Al) 192.7 0.064b 30 100-1000b 6667 Hydrogen bonding (65)

Phenol NH2-MIL-101(Al) 759.0 6.376x10-5 b 30 100-1000b 6667 n.a. (65)

13.a - 500 Phenol MIL-68(Al) 30 1000 π–π interaction, hydrogen bonding (68) 117 a - 1000

Phthalic acid ZIF-8 654 n.a. 25 n.a. 100 Acid-base, electrostatic interaction (47)

Phthalic acid NH2-UiO-66(Zr) 224 n.a. 25 n.a. 100 Acid-base, electrostatic interaction (47)

Phthalic acid UiO-66(Zr) 187 n.a. 25 n.a. 100 π–π interaction (47)

a Rhodamine B [Cd6(L)2(bib)2(DMA)4] 67 - 25 10 100-600 n.a. (38)

a Rhodamine B [Cd3(L)2(bipy)(DMA)4] 12 - 25 10 100-600 n.a. (38)

a Rhodamine B [Cd3(L)2(tib)(DMF)2] 4.4 - 25 10 100-600 n.a. (38)

Rhodamine B [bbpy][Bi4I16] n.a. n.a. 25 4.8 3000 Hydrogen bonding (41)

Roxarsone MIL-100(Fe) 387 n.a. 25 25-200 100 CUSs (62)

Sulfasalazine MIL-100(Cr) 6 a - 25 1.40 1250 n.a. (79) a Experimental maximum adsorption capacity; b Assuming a solution density of 1 kg.L-1; c Maximum adsorption capacity calculated with Sips isotherm model: b = 14.87 (g/L)-1/n, 1/n=0.5; d Isotherm parameters based on the Freundlich equations. In this cases b value is replaced with

Freundlich constant n; n.a. Information not available; bbpyf = 1,1’’-(1,4-butanediyl)bis[4,4’-bipyridinium]bis[tetrafluoroborate]; bib = 4,4’- di(1H-imidazol-1-yl)-1,1’-biphenyl; bipy = 2,2’-bipyridine; DMA = dimethylacetamide; DMF = dimethylformamide; H2bdc = 1,4- dicarboxybenzene; H6L = hexa[4-(carboxyphenyl)oxamethyl]-3-oxapentane acid; tib = 1,3,5-tri(1H-imidazol-1-yl)benzene;

46

Table 2. Summary of the performance of MOFs for organics pollutants photocatalytic degradation without hydrogen evolution. The table is organised in alphabetical order based on the name of the targeted contaminants.

Apparent first-order Conditions, T Conc. Organic Conc. MOF Radiation Light normalized Organic molecule MOF Aiders Reference reaction rate constant (h-1) (°C) molecule (mg.L-1) (mg.L-1) range power (W.mL-1)

[Zn2(Fe-L)2(μ2-O) H2O2 @30 %, 1 2-Chlorophenol 73% @ 1.33 h n.a. 40 1000 vis n.a. (125) (H2O)2]•4DMF•4H2O mL

[Cd2(Fe-L)2(μ2-O) H2O2 @30 %, 1 2- Chlorophenol 46% @ 1.33 h n.a. 40 1000 vis n.a. (125) (H2O)2]•2DMF•H2O mL

2,4-

-4 a Dichlorophenoxyacetic Ag4(NO3)4(dpppda) 0.73x10 < 35 92 - 666.67 UV 13.33 (120)

acid

Acid blue 92 Cu(tpa)·(dmf) ~7.8 n.a. 10 - 100 vis n.a. (119)

Amido Black 10B [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.2 h n.a. 100 - 150-250 UV 20 (113)

Amido Black 10B [Pb(Tab)2(bpe)]2(PF6)4 100% @ 0.42 h n.a. 100 - 150-250 UV 20 (113)

Acid Chrome Blue K [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.67 h n.a. 100 - 150-250 UV 20 (113)

Acid Chrome Blue K [Pb(Tab)2(bpe)]2(PF6)4 100% @ 1.0 h n.a. 100 - 150-250 UV 20 (113)

Acid Orange 7 [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.5 h n.a. 100 - 150-250 UV 20 (113)

Acid Orange 7 [Pb(Tab)2(bpe)]2(PF6)4 100% @ 1.5 h n.a. 100 - 150-250 UV 20 (113)

Acid Red 27 [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.83 h n.a. 100 - 150-250 UV 20 (113)

Acid Red 27 [Pb(Tab)2(bpe)]2(PF6)4 100% @ 1.0 h n.a. 100 - 150-250 UV 20 (113)

Congo Red [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.05 h n.a. 100 - 150-250 UV 20 (113)

Congo Red [Pb(Tab)2(bpe)]2(PF6)4 100% @ 0.3 h n.a. 100 - 150-250 UV 20 (113)

Eriochrome Black T [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.25 h n.a. 100 - 150-250 UV 20 (113)

Eriochrome Black T [Pb(Tab)2(bpe)]2(PF6)4 100% @ 0.42 h n.a. 100 - 150-250 UV 20 (113)

47

H2O2 @30 %, 1 Methyl Blue [Cu4(OH)2(btrb)2(1,3,5-btc)2]•4H2O 75% @ 7.5 h n.a. 10 200 UV 0.875 (115) mL

Methyl orange Pt@MIL-100(Fe) 4.55 30 20 H2O2@1 mL/L 125 vis 7.5 (112)

Methyl orange Pd@MIL-100(Fe) 3.30 30 20 H2O2@1 mL/L 125 vis 7.5 (112)

Methyl orange Au@MIL-100(Fe) 2.04 30 20 H2O2@1 mL/L 125 vis 7.5 (112)

Methyl orange MIL-100(Fe) 1.12 30 20 H2O2@1 mL/L 125 vis 7.5 (112)

Methyl orange [Pb(Tab)2(bpe)]2(PF6)4 95% @ 5.0 h n.a. 100 - 150-250 UV 20 (113)

Methyl orange [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 95% @ 0.83 h n.a. 100 - 150-250 UV 20 (113)

[Cu4(OH)2(btre)(1,2,3- H2O2 @30 %, 1 Methyl orange 90% @ 3.0 h n.a. 10 310 UV 0.87 (111) btc)2(H2O)2]•2H2O mL

[Zn4(OH)2(btre)(1,2,3- H2O2 @30 %, 1 Methyl orange 89% @ 4.5 h n.a. 10 310 UV 0.87 (111) btc)2(H2O)4]•4H2O mL

UV (>365 Methyl orange UiO-66(Zr-AN) 65% @ 1.5 h n.a. 20 - 100 0.3 (108) nm)

H2O2 @30 %, 1 Methyl orange [Cd3(btre)(1,2,3-btc)2(H2O)3]•3H2O 21% @ 4.5 h n.a. 10 310 UV 0.87 (111) mL

UV (>365 Methyl orange UiO-66(Zr-NA) 10% @ 1.5 h n.a. 20 - 100 0.3 (108) nm)

Nigrosin [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.33 h n.a. 100 - 150-250 UV 20 (113)

Nigrosin [Pb(Tab)2(bpe)]2(PF6)4 100% @ 0.67 h n.a. 100 - 150-250 UV 20 (113)

-4 a Nitrobenzene Ag4(NO3)4(dpppda) 0.81x10 < 35 61.63 - 666.67 UV 13.33 (120)

-4 a p-Nitrophenol Ag4(NO3)4(dpppda) 0.79x10 < 35 69.56 - 666.67 UV 13.33 (120)

Orange I [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.5 h n.a. 100 - 150-250 UV 20 (113)

Orange I [Pb(Tab)2(bpe)]2(PF6)4 100% @ 2.0 h n.a. 100 - 150-250 UV 20 (113)

Orange IV [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 85% @ 1.0 h n.a. 100 - 150-250 UV 20 (113)

Orange IV [Pb(Tab)2(bpe)]2(PF6)4 80% @ 1.7 h n.a. 100 - 150-250 UV 20 (113)

Orange G [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 95% @ 0.67 h n.a. 100 - 150-250 UV 20 (113)

48

Orange G [Pb(Tab)2(bpe)]2(PF6)4 60% @ 1.0 h n.a. 100 - 150-250 UV 20 (113)

air bubbled @ Rhodamine B BiOBr/UiO-66(Zr) 15.32 n.a. 14.37 500 vis 16.67 (102) 10 mL.min-1

air bubbled @ Rhodamine B BiOBr/NH2-UiO-66(Zr) 12.41 n.a. 14.37 500 vis 16.67 (102) 10 mL.min-1

Rhodamine B MIL-53(Fe) 4.764 n.a. 10 H2O2, 20 mM 400 vis 20 (90)

Rhodamine B GR/MIL-53(Fe) 4.632 n.a. 20 H2O2, 20 mM 400 vis 20 (92)

Rhodamine B g-C3N4/MIL-125(Ti) 3.744 n.a. 50 - 400 vis 3 (93)

Rhodamine B MND-MIL-53(Fe) 3.078 n.a. 10 H2O2, 20 mM 400 vis 20 (91)

air bubbled @ Rhodamine B BiOBr/NH2-MIL-53(Fe) 2.562 n.a. 14.37 500 vis 16.67 (102) 10 mL.min-1

air bubbled @ Rhodamine B Ag2CO3/UiO-66(Zr) 1.44 n.a. 14.37 500 vis 16.67 (101) 10 mL.min-1

H2O2 @30 %, 1 Rhodamine B [Co3(BPT)2(DMF)(bpp)]·DMF 1.15 25 0.024 1000 vis 10 (86) drop/mL

Rhodamine B MIL-53(Fe) 1.098 n.a. 10 - 400 vis 20 (90)

air bubbled @ Rhodamine B UiO-66(Zr) 0.264 n.a. 14.37 500 vis 16.67 (102) 10 mL.min-1

Rhodamine B Zn(5-aminoisophthalic acid)·H2O 0.1865 n.a. 0.0048 - 250 UV 3.75 (94)

Rhodamine B MIL-125(Ti) 0.156 n.a. 50 - 400 vis 3 (94)

[Cd(5-aminoisophthalic acid) Rhodamine B 0.1348 n.a. 0.0048 - 250 UV 3.75 (94) •H2O]n·2nH2O

Rhodamine B [Zn1.54Co0.46(tipm)(1,3-BDC)2] •H2O 90% @ 2.0 h n.a. 140 - 200 UV 8 (103)

Rhodamine B Zn1.952Co0.048(tipm)(1,3-BDC)2 94% @ 2.5 h n.a. 140 - 200 UV 8 (103)

Rhodamine B [Co2(tipm)(1,3-BDC)2] •0.5CH3CN 97% @3.0 h n.a. 140 - 200 UV 8 (103)

Rhodamine B Zn(1,3-BDC)(bmimb) 90% @ 3.0 h n.a. n.a. n.a. n.a. UV n.a. (97)

Rhodamine B [Cd(1,3-BDC)(bmimb)]n 90% @ 3.5 h n.a. n.a. n.a. n.a. UV n.a. (97)

49

Rhodamine B Zn2(tipm)(1,3-BDC)2 95% @ 4.0 h n.a. 140 - 200 UV 8 (103)

Rhodamine B [Zn(1,4-BDC)(bmimb)]n 90% @ 4.0 h n.a. n.a. n.a. n.a. UV n.a. (97)

Rhodamine B [Cd(1,4-BDC)(bmimb)]n 90% @ 5.5 h n.a. n.a. n.a. n.a. UV n.a. (97)

+ PANI/[CdL]•[ H2N Rhodamine B 81.8% @ 8.0 h n.a. 0.0048 - 250 vis 6.25 (95) (CH3)2](DMF)(H2O)3

+ Rhodamine B [CdL]•[ H2N (CH3)2](DMF)(H2O)3 47.9% @ 8.0 h n.a. 0.0048 - 250 vis 6.25 (95)

Sunset Yellow [Pb(Tab)2(bpe)]2(PF6)4•1.64AgNO3 100% @ 0.67 h n.a. 100 - 150-250 UV 20 (113)

Sunset Yellow [Pb(Tab)2(bpe)]2(PF6)4 100% @ 1.0 h n.a. 100 - 150-250 UV 20 (113) a Zero-order reaction parameter, mol.L-1.h-1; n.a. = Information not available; 1,X-BDC = 1,X-benzenedicarboxylate; bmim = 4,4’-bis(4-methyl-

1-imidazolyl)biphenyl; tipm = tetrakis[4-(1-imidazolyl)phenyl]methane; btre = bis(1,2,4-triazol-4-yl)ethane; X,Y,Z-btc = X,Y,Z- benzenetricarboxylate; Tab = 4-(trimethylammonio)benzenethiol; bpe = 1,2-bis(4-pyridyl)ethylene; btrb = 1,4-bis(1,2,4-triazol-4- ylmethyl)benzene; dmf = N,N-dimethylformamide; dpppda = 1,4-N,N,N’,N’-tetra(diphenylphosphanylmethyl)benzene diamine; H4L = 1,2- cyclohexanediamino-N,N’-bis(3-methyl-5-carboxysalicylidene); tpaH = Terephthalic acid

Table 3. Summary of the performance of MOFs for H2 photocatalytic production. The table is organised in alphabetical order based on the name of the organic sacrificial agent used.

Organic Apparent quantum H2 production rate Conc. Organic Conc. MOF Radiation Light normalized MOF Sensitizer Reference -1 -1 -1 -1 -1 molecule efficiency (%) (μmol.h .g MOF) molecule (mg.L ) (mg.L ) range power (W.mL )

L-ascorbic Pt/CdS@UiO-66(Zr) CdS 1.2 @ 420 nm 47000 17612 250 vis 15 (132) acid

50

L-ascorbic UiO-66(Zr)/g-C3N4 g-C3N4 n.a. 1411 17612 500 vis 15 (133) acid

L-ascorbic Pt/UiO-66(Zr) ErB 0.25 @ 420 nm 460 17612 1000 vis 30 (129) acid

L-ascorbic [(í-SCH2)2NC(O)C5H4N]- ZrTCPP n.a. n.a. 3522 n.a. vis n.a. (144) acid [Fe2(CO)6]@ZrPF

EDTA Ti-MOF-Ru(tpy)2 Ru(tpy)2 n.a. ~50 2922 n.a. vis n.a. (163)

2+ [Ru(dcbpy)(bpy)2] - 2+ EDTA [Ru(dcbpy)(bpy)2] /Pt(dcbpy)Cl2 n.a. n.a. 11167 n.a. vis 5 (166) Pt(dcbpy)Cl2@UiO-67(Zr)

EDTA Pt(dcbpy)Cl2@UiO-67(Zr) Pt(dcbpy)Cl2 n.a. n.a. 11167 n.a. vis 5 (166)

500-510 UV-vis Ethanol Pt/CdS@ZAVCl MOF CdS 1.32 @ 420 nm 59 3000 30 (153) 398-418 vis

Lactic acid MoS2/UiO-66(Zr)-CdS UiO-66(Zr)-CdS 23.6 @ 420 nm 32500 122.55 250 vis 3.75 (146)

6- [P2W18O62] @Zr6(μ3-O)4(μ3- Zr6(μ3-O)4(μ3- Methanol 0.01 @ 460 nm 699a 79180 n.a. vis 92 (151) OH)4(L)6](CO2CF3)12, W/Zr = 0.24 OH)4(L)6](CO2CF3)12,

6- [P2W18O62] @Zr6(μ3-O)4(μ3- Zr6(μ3-O)4(μ3- Methanol 0.01 @ 460 nm 193a 79180 n.a. vis 92 (151) OH)4(L)6](CO2CF3)12, W/Zr = 0.48 OH)4(L)6](CO2CF3)12,

Methanol Ti-MOF-Ru(tpy)2 Ru(tpy)2 n.a. ~11.7 79180 n.a. vis n.a. (163)

TEOA Ti-MOF-Ru(tpy)2 Ru(tpy)2 ~0.2 @ 500 nm 181.7 1492 n.a. vis n.a. (163)

TEOA Pt/NH2-MIL-101(Cr) RhB/NH2-MOF n.a. 226 ~583 2000 vis 100 (162)

TEOA Pt/UiO-66(Zr) RhB n.a. 116.1 112 500 vis 3 (160)

TEOA UiO-66(Zr) RhB n.a. 33.9 112 500 vis 3 (160)

TEOA Ni/NiOx@MIL-101(Cr) ErB n.a. n.a. n.a. n.a. vis n.a. (161) a -1 -1 6- In units of μmol.h .g of [P2W18O62] ; n.a.= Information not available; btrb = 1,4-bis(1,2,4-triazol-4-ylmethyl)benzene; dpppda = 1,4-

2+ N,N,N’,N’-tetra(diphenylphosphanylmethyl)benzene diamine; dcbpy = 2,2’-bipyridyl-5;5’-dicarboxylic acid; H2L = [Ru(bpy)3] -derived dicarboxylate ligand; Ru(tpy)2 = bis(4’-(4-carboxyphenyl)-terpyridine)Ru(II) complex; ZAVCl = low-

51 molecular-weight metallohydrogel with NaCl; ZrPF = Zirconium-porphyrin MOF made of Zr6O8(CO2)8(H2O)8 and ZnTCPP, ZnTCCP = tetrakis(4-carboxy-phenyl porphyrin)–zinc complex

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FIGURES

25

Adsorption 20 Degradation

H₂ evolution 15

10 Nr. of publications of Nr.

5

0 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Year

Figure 1. Number of papers published in the past 10 years on the topic of water purification using MOFs. The numbers were determined based on a Scopus search spanning from 2005 to

2015 using keywords: metal-organic frameworks; organics adsorption; organics adsorptive removal, organics photodegradation, organics photocatalysis, photocatalytic hydrogen production/evaluation.

53

Figure 2. Overview of the three topics covered in this paper on water purification, namely:

(i) adsorption of organic molecules from wastewater using MOFs (top); (ii) photocatalytic degradation of organic molecules from wastewater using MOFs (middle) and (iii) photocatalytic H2 production from water and organic molecules using MOFs (bottom).

54

Figure 3. Overview of the various adsorption mechanisms reported for organic molecules adsorption on MOFs. This figure was adapted with permission from Ref. (25), copyright

Elsevier, 2015.

55

Figure 4. Schematic of the possible mechanisms of organic photocatalytic degradation using MOFs. Left: under light, an electron (e-) from the valence band (VB) of the MOF is excited to the conduction band (CB) of the MOF, creating a hole (h+) in the VB. Both the electron and hole can generate radicals that can degrade the organic molecule. Right: under light, an electron (e-) from the VB of the MOF (or a substrate) is excited to the CB of the MOF (or a substrate), creating a hole (h+) in the VB. The excited electron is then transferred to the CB of the substrate

(or MOF), thereby enabling electron-hole separation and preventing charge recombination. Both the electron and hole can generate radicals that can degrade the organic molecule.

56

Figure 5. Schematic of the possible mechanisms of the photocatalytic production of H2 using

MOF. Left: under light, an electron (e-) from the valence band (VB) of the MOF is excited to the conduction band (CB) of the MOF, creating a hole (h+) in the VB. If the potential of the

+ + CB is higher than the redox potential of H /H2, H can be reduced to form H2. Right: under light, an electron (e-) from the HOMO of a dye/sensitizer is promoted to the LUMO. The excited electron is then transferred to the CB of the MOF, thereby enabling electron-hole separation and preventing charge recombination. If the potential of the CB is higher than the

+ + redox potential of H /H2, H can be reduced to form H2.

57

Figure 6. Schematic of a UiO-topology based MOF used for H2 production from methanol.

2+ This MOF contains [Ru(bpy)3] -derived dicarboxylate ligands and Zr6(μ3-O)4(μ3-OH)4

6− secondary units hosting [P2W18O62] ions (purple polyhedral). Zr, cyan; Ru, gold; N, blue; O, red; C, light gray. This figure was reproduced with permission from Ref. (151), copyright

2015, American Chemical Society.

58

Figure 7. Top: UiO-67 MOF using three different ligands (forming RuDCBPY) and Zr.

Bottom: view of the metal centres and ligands (grey: bpdc; yellow: PtDCBPY; purple:

RuDCBPY). This Figure is reprinted with permission, from Ref. (166), copyright 2015,

Royal Society of Chemistry.

59