Composites of Metal-Organic Frameworks Preparation and Application in Adsorption

Materials Today Volume 17, Number 3 April 2014 

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Composites of met etal al–o –org rgan anic ic frameworks: Preparation and application in adsorption Imteaz Ahmed and Sung Hwa Jhung* Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 702-701, Republic of Korea

Metal–organic frameworks (MOFs) are one of the most discussed materials of the last decade. Their extraordinary porosity and functionality from metals and organic linkers make them one of the most promising materials promising materials for a for a vast array of applications. of applications. The The easy tunability of their of their pore pore size and shape from the micro- to meso-scale, by changing the changing the connectivity of the inorganic moiety and the nature of the organic linkers, makes these materials special. Moreover, by combining with combining with other suitable materials, the properties of MOFs of MOFs can be improved further for further for enhanced enhanced functionality/stability, ease of preparation of preparation and selectivity of operation. of operation. In this review, various methods and paths for the for the preparation of composites of composites are discussed, especially for those which have been successfully applied to gas and liquid phase adsorptions. In the second part of this of this paper, several applications in adsorptive processes are discussed. Introduction Remarkable progress on porous materials has been achieved because of developments in mesoporous materials [1,2] and metal–organic frameworks (MOFs) [3–5] [3–5],, which are one of the most important and rapidly growing groups of porous materials. MOF MOF mate materi rial alss are composed of metal ions (or clusters) and coordinating linkers which impart high porosity to the MOF structures. The particular interest in MOF materials is due to the easy tunability of their of their pore size and shape from a microporous to a mesoporous scale, by changing the connectivity of the inorganic moiety and the nature of the organic linkers. Recently, MOF materials have been used in many applications including gas adsorption/storage, separation, catalysis, adsorption of organic molecules, drug delivery, luminescence, electrode materials, carriers for nanomaterials, magnetism, polymerization, imaging, membranes and so on [3–11] [3–11].. Thei Theirr potential increases every year due to the easy modification of MOFs, which make them a prominent group of materials in a vast number of published reports. As mentioned, MOFs show very promising physical and chemical properties for various applications; moreover, their properties ies can can be further improved by several means. Some of these are grafting active groups [12] [12],, chan changi ging ng organic linkers [13] [13],, Corresponding ng author. author. Jhun *Correspondi Jhung, g, S.H. S.H. ([email protected] [email protected]))

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impregnating suitable active materials [14] [14],, postsynt postsynthet hetic ic ligand and ion exchange [15] [15],, making making composites with suitable materials [16–20],, and so on. A composite is a multi-component material [16–20] with multiple phases which has at least one continuous phase [21] [21].. Recently, MOF composites have been receiving tremendous interest due to their various applications, including adsorption. This is a relatively new concept and several reports have been published recently with successful syntheses and promising applications of MOF composites. By composing MOFs with suitable materials, the synthesis kinetics [22] [22],, morp morpho holo logy gy [15,23] [15,23],, physi physico coch chemi emical cal properties [24,25] [24,25],, stability [26–28] and potential applications [24,25] can be largely improved. Adsorption receives more attention every year for its superiority to othe otherr techniques in the removal of hazardous materials, purification of fuels or water and storage of gases such as hydrogen and methane. Because adsorption can be carried out at low temperatures, it is less energy intensive and hence of comparatively low cost. cost. It also has the advantages of a of a wide range of applications, of applications, simplicity of design, easy operation, and low harmful secondary products. MOF-type materials are very well known for their applicability in adsorption processes due to their high surface area with adequate pore openings [29,30] [29,30].. Addition Additionally ally,, MOFs contain special and specific chemical functionalities which are useful to selectively adsorb some species, and these functionalities are one of the great properties of MOFs. Recently MOF composite

13691369-702 7021/0 1/06/$ 6/$ - see front front matter matter  2014 2014 Elsev Elsevier ier Ltd. Ltd. All rights rights reserv reserved. ed. http://dx.doi.org/1 http://dx.doi.org/10.1016/j.mattod. 0.1016/j.mattod.2014.03.002 2014.03.002


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materials have been used to improve functionality and porosity and, hence, improve the performance of adsorbents for their selectivity or adsorption capacity. These materials have been applied to both gas/vapor phase and liquid phase adsorptions. Gas phase adsorptions are carried out mainly to remove toxic gases. There are a large number of reports showing high adsorption capacity for the adsorption of gases over MOFs. Recently, MOF composites have also been used for gas phase adsorption to improve the capacity and selectivity of adsorbents [30]. A large number of studies have shown promising gas storage capabilities with MOF composites which are also based on the adsorptive phenomenon [31–33]. By imparting special functionality, selective adsorption of specific gases can be achieved. Therefore, it is very important to improve the selectivity of MOF materials by combining them with suitable materials for removal, storage and separation of gaseous materials. Several toxic gases such as H2S, NH3 and NO2 could be adsorbed and hence separated by, for example, graphite oxide/MOF composites [24,25,34,35]. MOF composites have also been used for gas storage. A few reports have been published for hydrogen storage using carbon nanotube (CNT)/MOF composites, and in some cases, metals were also loaded to these composites to improve the storage capacity [16,36]. Liquid phase adsorptions have also been widely discussed and applied to the various fields of MOF composites. Presently, fuel processing is an important area of research, and every year MOFs are being used to purify fuels or to remove harmful components from fuels. In most cases, organic compounds are removed or separated through MOFs using their selective interaction behavior. One of the most discussed adsorption processes is the adsorptive removal of sulfur or nitrogen containing compounds from fossil fuels as shown by a few recent review articles [37,38]. Besides these, water purification using MOFs is also gaining interest every year. Understanding their true potential, researchers are turning their interests to the adsorptive nature of MOF materials, and a vast number of publications related to MOFs have been published in the last decade. The opportunity to study pure MOFs has become narrower due to the broadening field, and hence, researchers now are experimenting with modified MOF materials with plenty of room for studies on MOF composites. In this review, we will discuss various methods and aspects for the synthesis of MOF composites. The improvements in MOFs made by combining with different materials will also be discussed. Applications of MOF composites in adsorption will be explained through the enhancement of porosity (surface area and pore volume) or improved interaction between adsorbents and adsorbates (by acid–base interaction or p-complexation, and so on).

Discussion Preparation of MOF composites Several methods and approaches have been applied for the synthesis of MOF composites. In general, there are two basic types of MOF composites according to the formation of the composite [39]. In MOF composites, MOFs can be in both the discontinuous phase and the continuous phase. However, the first type of composites (discontinuous MOFs) is not commonly used for application in adsorption and are prepared mainly for purposes such as composites that require different sizes/shapes and that demand easy

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handling. These include monoliths, beads, fibers, membranes/ films, and so on. Examples of these materials are HKUST-1 monoliths [40], ZIF-8/PVP (polyvinylpyrrolidone) fiber mats [41], composite membranes of MOF-5 and ZIF-8 [42,43], and HKUST-1/PAM (polyacrylamide) beads [27]. The second type of MOF composites is the well-known MOF composites with continuous MOFs for applications such as adsorption, and our discussion will be focused on these materials. For the preparation of MOF composites where MOFs are in a continuous phase, a wide variety of methods have been applied. The methods can be generally categorized into a few major classes. In one of these methods, MOF precursors are mixed with presynthesized composing materials and then the synthesis procedure is carried out. Graphite oxide/MOF (GO/MOF) is one of these types of materials that has been widely reported for various applications [25,35,44,45]. Bandosz et al. published several reports on GO/MOF composites prepared using this procedure and successfully applied them in gas phase adsorptive removal of toxic gases such as NH3, H2S, NO2, and so on [25,35,44,45]. In these composites, the GO layers are somewhat separated and stacked inbetween the planar cage structure of the MOFs (Fig. 1). In some cases, for this type of synthesis, the particles of the composing materials can be trapped in the pores of the MOF and hence can be stabilized or immobilized. This concept is known as ‘bottle around ship’ (BAS) and is shown in Fig. 2(a). In the BAS method, large particles of the composing materials are immobilized inside the cage of the MOF, which is only possible by in situ synthesis of the MOF. The MOF is built from the precursors around the composing materials, and after complete formation of the MOF, the composing materials are trapped inside the pores. Several reports about the encapsulation of polyoxometalates (POM) in MOFs have also been published [22,46,47]. Liu, Su and coworkers also prepared several MOF composites (the preparation method was very similar to the BAS method), named NENU-n, from the in situ synthesis of MOFs (HKUST-1) in the presence of POMs. Fig. 3 shows the structure of two pores of NENU-n, which was unambiguously obtained by single crystal Xray analysis [48]. It was later demonstrated that the NENU-11 (a sodalite-type porous MOF with a POM template as shown in Fig. 3(c)) possessed a good adsorption/decomposition capability for dimethyl methyl phosphonate (from 6 molecules in MOF-5 to 15.5 molecules in NENU-11 per unit formula) [49]. In the other preparation method, the MOF composite is supposed to form from the precursors of the composing materials and the preformed MOF. This method is called sometimes the ‘ship in a bottle’ (SIB) method, as explained by Gascon et al. [19] (Fig. 2(b)). In this case, the composing material forms inside the cages of the MOF and the diffusion of the precursors is usually carried out in the cages by a solvent system. Due to the increased size, the particles stay stable inside of the cages. In most cases, the precursors of the composing materials are added through solutions and diffused into the pores. Then the composite is formed by stabilizing the composing moiety inside the pores (of the MOFs) by means of chemical or thermal methods. One of the best examples of this, shown in Fig. 4, is direct encapsulation of different porphyrines into ZMOFs, and this composite was successfully applied in catalytic oxidation [50]. Although SIB techniques are widely known for zeolite composites [51], their study within the MOF 137

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FIGURE 1

Schematic comparison of the coordination between GO carbon layers and MOF units for different types of MOF networks (MOFs are MOF-5, HKUST-1 and MIL-100). Adapted from Ref. [44].

field has recently started and there are vast opportunities for utilizing this encapsulation method for MOFs. If the solubility of composing materials is not an issue for applications, the composing procedure may be carried out using postsynthesis methods. Simple loading, or impregnation, is one widely applied method of this type. Jhung’s group loaded soluble salts and polyoxometalates (POM) and used them for liquid phase adsorptions [52,53]. One of the best examples of these materials is the loaded metal oxides or insolubilized materials inside the pores, achieved through chemical means (e.g. changing the oxidation state, and so on). This method has been used for the preparation of MOFs composed of metal oxide (e.g. Cu2O) [54]. There have also been reports on the insolubilization of composing materials in which precipitates were formed from the solution by changing the oxidation state of the soluble salts [55,56]. Lu et al. [57] reported a controlled encapsulation strategy that enables surfactant-capped nanostructured objects of various sizes, shapes and compositions to be enshrouded by a zeolitic imidazolate framework (ZIF-8). This strategy worked for the incorporation of several types of functional nanoparticles into the MOF pores. The incorporated nanoparticles are well dispersed and fully confined within the ZIF-8 crystals. They used polyvinylpyrrolidone (PVP) as a surfactant, and the procedure is shown in Fig. 5. Canioni et al. [58] synthesized 138

POM-inserted MIL-100(Fe) with various methods including the in situ synthesis of MIL-100(Fe) and showed the stability of the composite in water. An important type of MOF composite is the metal/MOF composite. Several studies on MOFs loaded with different metal particles are available in the literature. Most of the metals used are dblock transition metals which demonstrate nice functionality in terms of their complex forming capability and acid-base interactions, thanks to their d-block electrons. There have been reports on Au, Ag, Pd, Ni, Ga, Co, Pt, Ru and some other metals being composed with MOFs [12,59–66]. It is also possible to use more than one type of metal atom in composites [67]. The encapsulation method differs widely for different metals and MOF pairs. Ishida et al. used a solid grinding technique to prepare Au-MOF composites by grinding the Au particles along with MIL-53(Al), MOF-5 and HKUST-1 [59,60]. Other techniques used for metal/MOF composites are direct encapsulation [61], impregnation [12,62,63], gas phase infiltration [64,65], coprecipitation [66], and so on. Some composing materials have templating effects for the structures of MOFs. Graphene oxide is one of such example. According to Jahan et al. [23], graphene can be decorated with functional groups on either side of its basal plane, giving rise to a


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FIGURE 2

Composite formation by encapsulation with different strategies: (a) ‘bottle around ship’ approach in which the porous host materials are produced around the encapsulated materials; (b) ‘ship in a bottle’ in which guest moieties are prepared in the presence of porous host and; and (c) the composite prepared by any of the two methods. Adapted from Ref. [19].

bifunctional nanoscale building block that can undergo face-toface assembly. They demonstrated that benzoic acid-functionalized graphene (BFG) can act as a structure-directingagent (SDA) in influencing the crystal growth of an MOF. Moreover, BFG can also be imparted into the MOF structure as a linker (Fig. 6). New electrical properties were also imparted into the MOF by intercalation with graphene oxide while the original MOF is insulating. Bajpe et al. showed that a POM/HKUST-1 composite can remarkably improve the kinetics of the synthesis. The interaction

between the two materials even reduced the energy requirement for the synthesis as shown by the fact that room temperature synthesis was possible in a short period of time [22,68,69]. Another study by Bandosz’s group showed not only the enhancement of the properties of virgin MOFs, but also the distinct properties of the composites compared to the parent materials [45]. Interaction between the metal sites of MOFs and the oxygen groups of GO creates new pores in the interfaces between the carbon layers and the MOF units. There is a report where activated carbon (AC) 139

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FIGURE 3

(a) Components of the crystal structures of NENU-n (n = 1–6): (a) a cube of five truncated-octahedral cages sharing square faces; (b) the pore A accommodating the Keggin polyanion, Cu2+ cluster (blue polyhedra), Keggin polyanion (polyhedral in yellow circle), C (gray), and O (red); (c) (1) a Keggin type of POM; (2) three-connected node and hexagonal face (blue) defined by a BTC ligand linked to six adjacent Cu2+ ions; (3) secondary building unit (SBU) and square face (green) defined by four Cu2+ ions; and (4) cube of eight sodalite-like truncated octahedral cages sharing square faces. Reproduced with permission from Refs. [48,49]. Copyright 2009 and 2011 American Chemical Society, respectively.

generated a large proportion of voids in MIL-101 and hence improved the porosity to enhance the hydrogen storage of MIL101 [70]. In this way, the functionality and applicability of MOFs can be improved to a great extent.

we will discuss the applications of MOF composites in different areas through adsorption processes. All the adsorption techniques can be divided into two general categories: (a) gas phase adsorption and (b) liquid phase adsorption.

Applications of MOF composites in adsorption

Gas phase adsorption

The applications of MOF composites not only include all those of individual MOFs, but also include additional fields. Their applicability includes adsorption (both in liquid and gas phases), separation, purification, catalysis, drug delivery, structure modification and so on. The potential applications of MOF composites are summarized in Fig. 7. The mechanisms for the applications of MOF composites may be similar to or different from that of virgin MOFs. Moreover, adsorption is observed in many applications and hence it is important to understand the applicability of MOF composites through adsorption phenomena. In the next part,

All over the world, toxic gases are an alarming environmental concern. One of the best ways to remove these gases is by utilizing an adsorption process. There have been a vast amount of reports on the adsorptive removal of harmful gases. Recently, the use of MOFs has been reported for the removal of gases through their enhanced surface area and distinct interactions [9,29,71–76]. Additionally,gas storage has also been gaining wide attention every year, and MOFs have found their way into gas storage applications due to their extraordinary properties, including a high surface area and pore volume. Recently, MOF composites, similar to the virgin MOFs,

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FIGURE 4

(a) Eight-coordinated InN4 represented as a tetrahedral building unit; (b) [H2TMPyP]4+ porphyrin; and (c) crystal structure of rho-ZMOF (left), hydrogen atoms omitted for clarity, and schematic representation of the [H2TMPyP]4+ porphyrin ring enclosed in rho-ZMOF a-cage (right, drawn to scale). Reproduced with permission from Ref. [50]. Copyright 2008 American Chemical Society.

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have been used for gas phase adsorption/storage because of their improved functionality, stability, selectivity and capacity. Table 1 shows the applications of MOF composites in gas phase adsorption with detailed improvements and plausible mechanisms. There are many applications for gas removal/storage through adsorption. In some studies, graphite oxide combined with different MOFs has been used to improve adsorption capacities, as well as other properties. It was found that a GO/MOF composite demonstrates a strong dispersive force which enhances the retention of small molecules such as ammonia. The results showed that composites of MOF-5 and GO possess a synergetic effect in the adsorption of ammonia, compared to the individual materials, and hence the composite material demonstrates good performance as an ammonia adsorbent [24]. The adsorption capacity was improved from 6 mg/g (pristine MOF) to 82 mg/g over the composite surface. Ammonia adsorption was also improved in a GO/HKUST-1 composite. It was observed that the GO/HKUST-1 composite had a maximum adsorption of NH3 compared with other materials [24]. The improved adsorption arises from the development of new porosity at the interface between the GO and the MOF units. It was also reported that different adsorption sites are introduced upon combining the two materials (MOF and GO) [25]. NO2 and H2S removals have also been reported with GO/ HKUST-1 and GO/MIL-100(Fe), respectively [35,44]. Gas storage through gas phase adsorption also has been widely studied using MOF composites. It was observed that MOF-5 has a good storage capability for H2; however, this material is not stable when exposed to atmospheric moisture. Its stability in moisture

FIGURE 5

Scheme of the controlled encapsulation of nanoparticles in ZIF-8 crystals. Through surface modification with surfactant PVP, nanoparticles of various sizes, shapes and compositions can be encapsulated in a well-dispersed fashion in ZIF-8 crystals, themselves formed by assembling zinc ions with imidazolate ligands. Reproduced with permission from Ref. [57]. Copyright 2012 Nature Publishing Group. 141

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FIGURE 6

Schematic of proposed bonding between (a) MOF and graphene and (b) MOF and BFG. Reproduced with permission from Ref. [23]. Copyright 2010 American Chemical Society.

was improved by combining with multi-walled carbon nanotubes (MWCNTs), and the composite was denoted as MOFMC (MOF multi-walled CNT) [16]. This material has an interpenetrating structure and hierarchical nanopores that improve the storage capability of the composite. The MOFMC is also mesoporous, which further improves the storage capacity (2.02 wt% at 77 K and 1 bar). Another report was published using Pt-loaded MWCNT/MOF-5 with a great improvement in H2 storage (from 1.20 wt% in virgin MOF to 1.89 wt% in the composite) [77]. Yang et al. also published a report on H2 storage using MWCNTs composed with MOF-5 [78]. They reported a 50% improvement

in storage capacity compared to virgin MOF-5. They measured the H2 storage capacity for various conditions, and in every case, they found improved adsorption results compared to the virgin MOF-5. Activated carbon (AC) composed with MOF was also reported for H2 storage [70]. Up to 10.1 wt% of H2 storage has been reported with the AC/MIL-101 material where only 0.63% of AC was incorporated. A theoretical study showed that the hydrogen storage capacity is improved to 6.3 wt% or 42 g L1 at 100 bar and 243 K over Li-decorated IRMOF-10 with Li-coated fullerenes (C60) [79]. POM/MOF composites have also been used in hydrogen storage. The introduction of POMs and lithium ions in MOFs leads to strong hydrogen adsorption [80]. CO2 capture is one of the important applications of MOF composites, and an MWCNT (multi-walled carbon nanotube)/ MIL-101 composite has been used to improve CO2 capture [81]. The MWCNT/MIL-101 has the same morphology as the virgin MIL-101, but a 60% improvement in CO2 capture has been noticed (from 0.84 to 1.35 mmol g1). The improvement of CO2 capture with the composite is attributable to the increased micropore volume of the MIL-101 through the MWCNTs. An MOF composite, polyethyleneimine loaded MIL-101 (PIM/MIL-101), showed improved CO2 capture although the porosity of the MIL-101 was drastically reduced by the PIM [82] due to the strong interaction between the alkylamine groups and CO2.

Liquid phase adsorption FIGURE 7

Potential applications of MOF composites. 142

The most prominent liquid phase adsorption using MOFs is the purification of petroleum by removing NCCs (nitrogen containing compounds) and SCCs (sulfur containing compounds) from fuels.

M a t e r i a l s T o d a y 

TABLE 1

Gas adsorption properties of MOF composites. a

MOF

Composing material

Application

Mechanism

Experiment conditions

Result

MOF-5

Multiwalled carbon nanotube (MWCNT)

H2 storage through adsorption

Improved surface area by improving nanopores and improved stability through interaction

77 K temp. and up to 1 bar pressure were used

[16]

MOF-5

Graphite oxide (GO)

Adsorption of NH3

Improved adsorption sites and additional porosity by composing the MOF and GO

HKUST-1

Graphite oxide (GO)

Adsorption of NH3

Improved adsorption sites and additional porosity by composing the MOF and GO

Adsorption improved from 6.2 mmol/g to 8 mmol/g on average

[25]

HKUST-1

Graphite oxide

Adsorption of NH3, H2S and NO2

Synergetic effects between HKUST-1 and GO

NH3 adsorption was doneby diluting with air in a breakthrough analyzing apparatus at ordinary temp. and pressure At a temperature of 25–75 C. Pressure was maintained at 100–120 kPa Flow of the gases diluted by air was used for breakthrough experiments

H2 storage was improved from 1.2 wt% to 1.52 wt% in the composite Adsorption of NH3 was improved from 6 mg/g to 82 mg/g

[45]

MIL-101

Activated carbon (AC)


Reduction of pore size and improvement of pore volume. Additional micropores were formed

77.4 K temp. and up to 600 kPa pressure were used

MOF-5


Secured porosity which acted as H 2 spillover receptor

77 K temp. and up to 1 bar pressure was used

IRMOF-10

Multiwalled carbon nanotube (MWCNT) loaded with Pt Li coated fullerene

H2 storage application

HKUST-1

Polyoxometalate (POM)

H2 adsorption

Electrostatic charge quadrupole and dipole interactions between H2 and Li Interaction between H2 molecule and O atom of POM

243 K temp. and 100 bar pressure were studied 77 and 298 K temp. along with various pressures up to 100 bar

MIL-101

Multiwalled carbon nano-tube (MWCNT)

CO2 storage through adsorption

Increase of micropore volume of the MOF by MWCNT incorporation

298 K temp. and 10 bar pressure

MIL-101

Polyethyleneimine (PEI)

CO2 storage by improving the selectivity

Selectivity of CO2 adsorption improved due to strong interaction between the alkylamine groups of PEI

298 K and 323 K temp. along with 0.15 bar and 1.0 bar were applied

Around 10% and 30% improvement for NH3 and H2S. Little improvement was found for NO2. Nearly 58% improvement of H 2 storage was observed in the composite H2 storage was improved from 1.2 wt% to 1.89 wt% in the composite An improved H2 uptake up to 6.3 wt% Slight improvement in H 2 adsorption. Basically the mechanism was studied. 60% increase (from 0.84 to 1.34 mmol/g) in CO2 uptake was found 140% more amount of CO2 intake was observed

a

8

Ref.

[24]

[77]

[79] [80]

[81]

[82]

Improvements compared to the corresponding virgin MOFs.

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upon loading, the Cu2+ sites readily turn to Cu+ sites by reacting with the V3+ of MIL-47 and hence a p-complexation site is automatically created through the loading. By contrast, other analogous MOFs [85] having a similar structure, such as MIL-53(Al) and MIL-53(Cr), do not show a reduction ability to form Cu+ because of the lack of reducibility of Al3+ and Cr3+, respectively [56]. In another study the same group reported on BT adsorption using Cu2O/MIL-100(Fe), presenting improved performance [54]. Very recently, they showed that ionic liquid/MOFs are effective in removing SCCs compared to pristine MOFs [86]. Ahmed et al. showed the improved adsorption of quinoline (QUI), one of the typical NCCs, over POM-loaded MIL-101 [53]. It was found that a 1% loading of POM could improve the adsorption of QUI up to 20%. They showed that the adsorption of a neutral NCC, indole, could not be improved by the loading of POM, although basic QUI adsorption was nicely improved through an acid-base interaction. Very recently, it was also reported that GO/MIL-101 was very effective in removing NCCs partly because of the improved porosity of the adsorbent [87]. MOF composites have also been used for water purification, similar to that performed with virgin MOFs [88–91] GO/HKUST-

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upon loading, the Cu2+ sites readily turn to Cu+ sites by reacting with the V3+ of MIL-47 and hence a p-complexation site is automatically created through the loading. By contrast, other analogous MOFs [85] having a similar structure, such as MIL-53(Al) and MIL-53(Cr), do not show a reduction ability to form Cu+ because of the lack of reducibility of Al3+ and Cr3+, respectively [56]. In another study the same group reported on BT adsorption using Cu2O/MIL-100(Fe), presenting improved performance [54]. Very recently, they showed that ionic liquid/MOFs are effective in removing SCCs compared to pristine MOFs [86]. Ahmed et al. showed the improved adsorption of quinoline (QUI), one of the typical NCCs, over POM-loaded MIL-101 [53]. It was found that a 1% loading of POM could improve the adsorption of QUI up to 20%. They showed that the adsorption of a neutral NCC, indole, could not be improved by the loading of POM, although basic QUI adsorption was nicely improved through an acid-base interaction. Very recently, it was also reported that GO/MIL-101 was very effective in removing NCCs partly because of the improved porosity of the adsorbent [87]. MOF composites have also been used for water purification, similar to that performed with virgin MOFs [88–91]. GO/HKUST1 composites were successfully applied in the removal of methylene blue (MB) with improved adsorptive performance. The study showed a maximum adsorption capacity of 183 mg/g for MB in water [92]. It was found that the efficiency of the adsorption was high for a low concentration of MB in water for both virgin HKUST-1 and GO/HKUST-1 composites with fast kinetics. However, in the case of a high MB concentration, the efficiency of HKUST-1 decreased below 50% of the original value, while GO/HKUST-1 still removed the MB up to 80% within 30 min, which indicates the much better adsorption ability of the composite compared to the virgin MOF alone. Another report shows the effective removal of several toxic chemicals (NH3, H2S, and so on) from effluent water with Zr(OH)4/HKUST-1 composites [93]. Besides these applications, there are other adsorptive applications of MOF composites, such as separation with magnetic properties [94], drug delivery [95], static phase material for liquid chromatography [96] and so on. In particular, drug delivery is currently a widely studied topic in the field of MOFs [97–99]. In this case, the concern is more probably desorption rather than adsorption. In many cases, the steady, slow and targeted release of a drug in the body is essential for efficient therapeutics. MOF composites have promising and prospective applications in this field. Several studies on drug delivery have recently been reported using MOF composites [95]. Ke et al. synthesized HKUST-1 with incorporated Fe3O4 nanorods which could be potentially used for drug delivery (Fig. 9(a)) [95]. This kind of material provides an efficient platform for a new strategy of delivering an imaging contrast agent and an anticancer drug by postsynthetic modifications of a highly porous nanoscale metal–organic framework (NMOF). Another report on drug delivery was published by Taylor-Pashow et al. [100]: they demonstrated the synthesis of an iron–carboxylate NMOF with a MIL-101 structure, and post-modification was performed by loading an organic fluorophore and an anticancer drug via covalent modifications of the as-synthesized nanoparticles for targeted drug delivery, magnetic resonance imaging and magnetic separation (Fig. 9(b)) [99]. 

FIGURE 8

(a) Proposed mechanism for the adsorption of benzothiophene (BT) over Cu+ site; and (b) adsorption isotherms for BT over MIL-47 and CuCl2/MIL-47. Adapted from Ref. [55].

These materials have to be removed before commercial use due to environmental problems and catalyst poisoning. As in gas-phase removal, MOF composites are also being widely applied to the removal of contaminates from fossil fuels. MOFs, not in the form of composites, have a large and accessible porosity inside their cage; therefore, this empty space can be utilized by suitable entities for adsorptions [37,38]. Adsorptive removal of obnoxious materials can be further improved by combining MOFs with suitable materials. Khan et al. showed improved adsorption of benzothiophene (BT) over a polyoxometallate (POM)-loaded porous HKUST-1 [52] to understand the effect of POM on the adsorption/removal of BT. The maximum adsorption capacity was improved up to 26% by the loading although the porosity of the composite decreased. Therefore, it was concluded that the chemical interaction, through an acid-base effect (between the acidic POM and slightly basic BT), improved the adsorption. As reported, several metal ions such as Cu+, Ag+, Pd2+ and Pt2+ show p-complexation behavior and hence can improve the adsorptive removal of SCCs. [83,84]. Yang and coworkers have developed adsorbents based on the p-complex for desulfurization [83]. As shown in Fig. 8, Khan et al. [55] demonstrated a remarkable adsorption of BT over CuCl2-loaded MIL-47 through p-complexation. Cu2+ sites are not useful as p-complexating sites; however, 144


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FIGURE 9

(a) Illustration of the synthesis and action of Fe3O4/HKUST-1 nanocomposites adapted from Ref. [95]; and (b) postsynthetic modifications of iron-carboxylate nanoscale MOFs for imaging and drug delivery. Reproduced with permission from Ref. [100]. Copyright 2008 American Chemical Society.

Summary and conclusion For over a decade, individual MOFs have been serving the scientific community with their remarkable properties and wide applications. Additionally, composite materials are currently opening the door for new opportunities and prospects by extending the applicability of MOFs. There are many different ways to prepare an MOF composite, such as encapsulation, impregnation, infiltration, solid grinding, coprecipitation, and so on. Different types of preparation method can produce different properties in the MOF composites and hence can improve the applicability of the composite materials. One of the major improvements in MOF composites is the enhanced porosity and the subsequently improved adsorption capability. Other composites show special functionality and improved practical applications. Some composites show structural modifications and enhanced kinetics in the synthesis of MOFs and have imparted new properties which improve the versatility of the materials in different aspects. The most utilized application of MOFs is adsorption, which can be carried out for different purposes. These include the removalof harmful materials fromliquid/gasphases, gas storage, separation, purification, catalysis, drug delivery and so on. In every case, the applicability and performance of an MOF can be improved by forming a composite with suitable MOFs, mainly because of the

imparted functionality and enhanced porosity. Specially, in such cases where selective adsorption or separation is necessary, MOF composites can show spectacular effects. MOF composites can be applied in every field where MOFs can be applied, and additional new possibilities can also be found in applications where individual MOFs cannot be used. With suitable functional materials, limitless composites are bound only by one’s imagination, and can be prepared for new applications. In the future, there will be many more requirements on the specific capabilities of such materials, and these can be fulfilled by MOF composites. By combining MOFs with suitable materials, the functionality, porosity, ease of synthesis, and thermal/magnetic/electric properties can be improved to meet specific requirements. Therefore, it can be concluded that though the exploration of these composite materials has only just begun, the prospects are vast and will continue to increase.

Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (grant number: 2013R1A2A2A01007176). Authors express their sincere thanks to Dr. N.A. Khan and Dr. Z. Hasan for their helpful discussion. 145

Materials Today Volume 17, Number 3 April 2014

RESEARCH

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