An Emerging Family of Hybrid Nanomaterials: MetalâOrganic

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An Emerging Family of Hybrid Nanomaterials: MOF/Aerogel Composites Zeynep Inonu, Seda Keskin, and Can Erkey ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01428 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Nano Materials

An Emerging Family of Hybrid Nanomaterials: MOF/Aerogel Composites Zeynep Inonu1, Seda Keskin1,2, Can Erkey1,2 1Department

2Koç

of Chemical and Biological Engineering, Koç University, Rumelifeneri Yolu, 34450 Sariyer, Istanbul, Turkey

University Tüpraş Energy Center (KUTEM), Koç University, Rumelifeneri Yolu, 34450 Sariyer, Istanbul, Turkey

Submitted to ACS Applied Nano Materials Corresponding Author: [email protected]

Abstract Metal organic frameworks (MOFs) are crystalline nanoporous coordination polymers made of metal ions and organic linkers. Aerogels are highly nanoporous amorphous polymers which can be organic, inorganic or hybrid. Both of these unique materials have been extensively investigated in many laboratories around the world for a wide range of applications ranging from separations to catalysis resulting in thousands of published articles in a wide variety of journals. Composites of MOFs and aerogels (MOFACs) are a new class of nanostructured materials attracting increasing attention due to their favorable properties. The combination of micro/mesoporosities of MOFs with meso-/macroporosities of aerogels makes MOFACs hierarchically multimodal porous materials. MOFACs with their high surface areas, combined morphological, mechanical, physicochemical and functional properties of both MOFs and aerogels have demonstrated outstanding performances in various applications. Herein, we provide an overview of the techniques used to synthesize MOFACs in various shapes such as monoliths or particles based on incorporating MOFs into the porous network of aerogels along with literature examples. The synthesis of aerogel supported metal/metal oxides using MOFACs as precursors are also described. Use of these composites for several applications such as adsorption, separation, catalysis, energy conversion and storage devices such as batteries and supercapacitors are

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reviewed. Future prospects in synthesis techniques and applications are provided to address opportunities and challenges in the field of MOFACs. Keywords: MOF, aerogel, composite, MOFAC, adsorption, catalysis, energy 1. Introduction: Porous materials are increasingly contributing to the developments in many diverse fields ranging from electronics to drug delivery. According to IUPAC (International Union of Pure and Applied Chemistry), materials with pore sizes smaller than 2 nm are classified as microporous, materials with pore sizes between 2 to 50 nm are classified as mesoporous and materials with pore sizes larger than 50 nm are classified as macroporous. Materials that contain multimodal interconnected pores are called hierarchically porous structures.1 Structural hierarchy may be bimodal such as micro-micro, micro-meso or micro-macroporous or they can be multimodal like micro-meso-macroporous. These materials which mimic natural systems have enhanced properties and/or they perform multiple functions. For example, combined micro-/mesoporosities lead to high surface areas, high active site accessibilities, short diffusion paths and low mass transport limitations that make them perfect candidates for applications in tissue engineering and drug delivery,1 catalysis,2 energy conversion and storage,3 adsorption and separation.4 One such strategy for obtaining hierarchical multimodalities is combining two materials which have two different nanoporous structures. Two remarkable classes of nanostructured materials are aerogels and metal organic frameworks (MOFs). Aerogels were first synthesized in 1932 by Kistler by replacing the liquid in a gel without collapsing its porous network. These synthetic materials which are prepared by special drying techniques have extremely low densities which can be as low as 0.16 mg/cm3 and have extremely high porosities which can be as high as 99,9%. Because of their high porosity, 2 ACS Paragon Plus Environment

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aerogels are sometimes termed as “solid air”. This high porosity results from the internal solid nanostructure of aerogels consisting of cross-linked polymeric nanoparticles with a large fraction of air filled pores. The size of these pores which can be tuned are in the mesopore range and the pore size distribution can be very sharp. Kistler synthesized many different types of inorganic and organic aerogels such as silica, alumina, gelatin and egg albumin.5 Since then, aerogels have been used in some niche and low volume applications such as components of Cherenkov detectors and space shuttle tiles which utilized silica aerogels due to their superior thermal insulation performances. With the growing importance of saving energy, research efforts were concentrated on developing products which contained aerogels with superior thermal insulation properties than the conventional products in the market. Blankets made from silica aerogels with thermal conductivities as low as 15 mW/mK were developed by Aspen Aerogel and a plant was constructed to produce them on the large scale which started operation in 2007. Cabot Corporation developed silica aerogel granules under the trade name of Nanogel® with excellent insulation, water-repelling, and light transmittance properties for use in glazing systems for energy-efficient daylighting. This was followed by the development of organic polyurethane aerogels by BASF which is currently marketed with the brand name Slentite. Carbon aerogels are another class of aerogels which are obtained by pyrolysis of the organic aerogels at elevated temperatures under an inert atmosphere. Recently developed graphene aerogels which belong to the class of carbon aerogels have very high electrical conductivities and therefore they are promising candidates as electrodes for electrochemical power sources, for energy storage, catalysis and sensors. Organic aerogels can also be polysaccharide based such as alginate, cellulose or starch. Polysaccharide aerogels are very promising in food and cosmetics industries and drug storage and delivery applications since they are biocompatible and biodegradable. Another advantage of aerogels is that their pore properties can be tuned by 3 ACS Paragon Plus Environment


changing reactant concentrations and synthesis conditions. However, aerogels have several drawbacks such as poor mechanical properties due to their very high porosities and a rather wide pore size distribution with almost no pores in the micropore region. Metal organic frameworks (MOFs), also referred as porous coordination polymers, are selfassembled crystal structures which are composed of metal nodes and organic linkers. Although MOFs were discovered back in 1965, they have attracted great interest with the studies of by Omar Yaghi and coworkers since 1999.6 MOFs are crystalline with well-defined pores and they are obtained using “reticular synthesis” in which the predetermined ordered structures are produced by choosing the appropriate molecular building blocks.7 The opportunity of creating crystal structures with desired pore sizes and functionality makes MOFs very attractive. With their ordered structures, high pore volumes and large surface areas, MOFs have attracted great interest for a large variety of chemical and biological applications including gas storage and separation,8 catalysis,9 sensing,10 drug storage and delivery,11 energy storage and conversion.12 Storage of H2 and CH4, selective CO2 separation from natural gas and flue gas are the most widely investigated applications for MOFs.6,13 BASF commercialized a small variety of MOFs and small start-up companies have been producing tailor-made MOFs for a wide variety of applications. In spite of their fascinating properties and industrial potential, MOFs are generally obtained in powder form which may be prohibitive in several industrial applications. Such powders cause mass transfer limitations, high pressure drops and entrainment in packed beds and are difficult to handle due to dust formation.14 In order to avoid this problem, significant efforts are being exhausted to either shape MOFs into free-standing monoliths as metal organic aerogels


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(MOAs),15 films,16 membranes17 or as composites with porous materials such as polyHIPEs,18,19 foams,20 or sponges21 which usually end up with micro-/macroporous bimodalities. Incorporating microporous MOFs into mesoporous aerogel matrices to make MOF/aerogel composites (MOFACs) is one strategy to achieve highly desirable products where hierarchical micro-/mesoporosity can be easily controlled or tuned. Both MOFs and aerogels belong to the class of nanostructured materials, have high surface areas, high pore volumes, high porosities and low densities and therefore they are suitable for similar applications. Some general properties of MOFs, aerogels and their composites are listed in Table 1. The multimodal hierarchical structures of MOFACs offer great potential for catalysis, selective sorption and separation, and power applications since mesopores and macropores intensify the diffusion and mass transfer whereas micropores create good interaction between host and guest molecules.22 Table 1. General properties of MOFs, aerogels and MOFACs. Bulk MOFs

Aerogels

MOFACs

Pore size Structure

Microporous Crystalline

Mesoporous and microporous Amorphous with or without crystalline domains

Shape

Powder

Mesoporous Amorphous Powder, monolith, bead, particle, fiber

Powder, monolith, bead, particle, fiber

Consequently, MOFACs have been under extensive investigation in the last decade. These materials are intended to offer advantage of both nanostructured materials with enhanced chemical or physical properties to improve performance in various applications. The rapid increase in the number of publications in this field can be attributed to the combination of superior properties of both materials, controllable micro-/mesoporosity ratio, ease of preparation, control over morphology, single step MOF activation and aerogel formation. Moreover, it has been demonstrated in a number of studies that incorporation of MOFs enhances the mechanical properties of aerogels.23,24 5 ACS Paragon Plus Environment


In this article, synthesis and applications of MOFACs are reviewed. Synthesis methods are presented in detail in Section 2 together with examples from the literature which demonstrate the enhanced structural and morphological properties of these novel materials. The formation of supported metals or metal oxides by using MOFACs as precursors is described in Section 3. The use of MOFACs for several chemical applications such as catalysis, sorption and separation and power source applications such as batteries, fuel cells and supercapacitors are given in Section 4. New application areas for MOFACs together with the needs for further development of these materials are presented in Section 5. 2. Synthesis Techniques of MOFACs: A MOF/aerogel composite is synthesized by incorporation of MOF into the aerogel matrix where MOF can generally be considered as the dispersed phase and the aerogel matrix is the continuous phase. MOFACs can be shaped into various structures that are obtainable for aerogels such as monoliths, beads, particles and fibers. MOFACs can be prepared using two different methods: direct mixing of pre-synthesized MOF with gel precursors followed by gelation and drying and in situ MOF synthesis inside the pores of the hydrogel or aerogel. These two methods are described in detail in Sections 2.2 and 2.4 together with examples of these materials reported in the literature in Sections 2.3 and 2.5. Since aerogel synthesis and drying techniques are important aspects of both of these methods, they are described in detail in Section 2.1. 2.1. Aerogel Synthesis and Drying Techniques: In aerogel synthesis, first gels which are composed of a liquid phase within a three dimensional solid network are synthesized using various gelation techniques such as sol-gel method, cross-linking of polymers or freeze-thaw induced gelation. If the liquid phase is water, the gel is called a hydrogel. If the liquid phase is an organic solvent, the gel is called an 6 ACS Paragon Plus Environment

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organogel. Gels can be synthesized in various forms such as monoliths, beads, particles or fibers using different techniques. The modification of these techniques to synthesize MOFACs is explained in detail under section 2.2. The prepared gels are then dried to remove the liquid inside the gel network. When gels are dried under ambient conditions, as the solvent evaporates from the pores of the gel, due to presence of a vapor-liquid interface, huge capillary stresses cause the porous network to collapse.25 The resultant materials are called xerogels. Xerogels usually have high densities, low porosities, cracks and very large volume shrinkage. However, with special drying techniques such as supercritical drying or freeze drying, the liquid inside the gel is removed and replaced by air while preserving the original gel structure to obtain aerogels. With both techniques, the liquid-vapor interface and capillary pressures are avoided so that gels are dried to aerogels while preserving their high pore volume. In supercritical drying, the solvent that is present inside the pores of the gel is extracted by a fluid that is above its critical temperature and pressure, which is called a supercritical fluid. Supercritical CO2 (scCO2) is widely used for this purpose as it is non-toxic and has mild critical conditions. To use this technique, the solvent inside the pores has to be miscible or partially miscible in scCO2. If the solvent is not scCO2 miscible like water, then it is exchanged by another solvent like ethanol that is miscible in scCO2. Supercritical drying time is an important consideration for industrial scale application and varies directly with the thickness of the gel. For small particles, supercritical drying of the gels takes about minutes. Commercial supercritical driers with various capacities are available from manufacturers such as Natex and Uhde which can also be used to produce MOFACs. Ambient temperature subcritical drying with high pressure liquid CO2 is another technique that is widely used. However, after drying subcritically, during depressurization, a vapor-liquid interface cannot be avoided therefore the pores may collapse 7 ACS Paragon Plus Environment


depending on the surface tension of CO2 at high pressures, the size of the pores and the strength of the solid matrix. In freeze drying, the solvent inside the pores is removed by lowering the temperature below freezing point and pressure below sublimation pressure so that the solvent sublimes inside the pores. Aerogels that are obtained by this method are sometimes called cryogels. Freezing of the solvent molecules may sometimes induce crystal growth inside the pores which results in the formation of macropores, especially if the solvent is water since water expands during freezing. Solvent exchange may also be carried out to prevent this effect. Freeze drying is also an established technology used on an industrial scale. 2.2. Direct Mixing Method: In the direct mixing method, the procedure for synthesis of a particular aerogel as described in the preceding section is modified by the addition of MOF particles either to the liquid precursor solution or to the sol before gelation as shown in Figure 1. Gelation of the matrix entraps the dispersed MOF in the gel and this step is followed by supercritical drying or freeze drying to obtain hierarchically porous hybrid MOFACs.


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Figure 1. MOFAC preparation by MOF incorporation into gel matrices using direct mixing method.

Three common methods used for the synthesis of MOFACs as monoliths, small particles, beads and fibers are shown in Figure 2. Monolithic composites are formed by first mixing the gel precursor in an appropriate solvent. Followed by the introduction of MOF powder and homogenization, gelation is induced by the addition of a gelation trigger and the matrix is allowed to gel in a mold which will give the overall shape of the composite. The choice of the material of the mold is as important as the preparation conditions in order to obtain a crack-free MOFAC monolith. The mold should not stick to the gel surface allowing easy release, and also should not interact with the sol before and after the gelation. Teflon and polypropylene molds are better compared to glass molds. Moreover, the surface of the mold should be very smooth since molds with high surface roughness leads to aerogels with increased surface irregularities.26 Sometimes, gels are aged to increase their mechanical strength. The last step is the removal of the solvent from the pores of the gel.



(a)

(b)

(c)

Figure 2. Scheme of (a) monolithic MOFAC synthesis in molds, (b) emulsion gelation method to make micron sized particles, (c) dripping method to make spherical beads or fibers.

Micron to millimeter sized particles can be prepared with emulsion gelation method where the sol-MOF suspension can be dispersed in an oil phase to make water in oil (w/o) emulsion.27 A surfactant may be added to achieve a fine dispersion of sol in oil phase, stabilize the emulsion and decrease the particle size. Ratio of the dispersed to continuous phases, concentration and type of surfactant, geometrical parameters such as mixer shape and diameter, physical parameters like density and viscosity, process parameters such as stirring rate are some of the important parameters that determine the particle size distribution and particle shapes.28 From these parameters, stirring rate is the easiest parameter to control and increasing stirring rate results in smaller sized microspheres. Viscosity of the phases and stirring rate are important parameters for obtaining particles with a narrow size distribution. 10 ACS Paragon Plus Environment

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Spherical beads and fibers can be synthesized by addition of sol-MOF suspension into a regeneration bath that induces cross-linking. If the solution is added dropwise, spherical beads are produced.29 The regeneration bath is stirred gently but continuously to avoid agglomerations. Spheres with approximately 5 mm diameter can be produced with this method. The size of the drops depends greatly on the diameter of the nozzle. This technique is used on the industrial scale for production of various types of hydrogel beads. If the solution is added as a continuous stream, fibers can be drawn.30 Based on this method, it is possible to synthesize periodic aerogel microlattices via 3D printing technique.31 2.3. Literature Examples of MOFACs Synthesized by the Direct Mixing Method: The direct mixing method was first used by Ulker et al.22 to synthesize a monolithic MOFAC by incorporating a microporous MOF, Cu-BTC (copper benzene-1,3,5-tricarboxylic acid, HKUST-1) into silica aerogel. Composites with different MOF concentrations were synthesized with a modified two-step sol-gel method which is composed of dispersing Cu-BTC inside a TEOS (tetraethylorthosilicate) and ethanol solution followed by hydrolysis and condensation reactions leading to gelation. The solutions were poured into a mold just before gelation. After the aging step and solvent exchange, the composite gels were supercritically dried to form Cu-BTC/silica aerogel composites which were comprised of microporous Cu-BTC domains embedded in mesoporous silica aerogel network. The gelation time of the composites was observed to increase with increasing MOF content. As FTIR (Fourier-transform infrared spectroscopy) spectra showed no significant change in the chemical structure of silica aerogel, the delay of gelation time was attributed to the change in sol concentration with their adsorption on MOF particle surfaces and formation of the network around the MOF particles retarding the network formation in solution. Although Cu-BTC is prone to decomposition when in contact with 11 ACS Paragon Plus Environment


water, the XRD (X-ray diffraction) spectra revealed that crystal structure of Cu-BTC remained intact as the composite contained the characteristic peaks of pure crystalline Cu-BTC. BET (Brunauer–Emmett–Teller) results indicated an increase in specific surface area and micropore volume with increasing MOF content but with a slightly lower ratio which is probably due to the small fraction of micropore blockage. Clogging of MOF micropores by monomers and small oligomers of the gel precursors is undesirable. In order to prevent micropore blocking, pre-polymerization of the gel precursors was suggested.32,33 Since larger oligomer chains are less prone to diffuse into the pores of MOF and block its pores, by pre-polymerizing the precursors, MOF pores can thus be protected. CuBTC/silica aerogel monoliths were synthesized by Nuzhdin et al.33 by adding Cu-BTC as a suspension in isopropanol into pre-synthesized sol just before gelation to preserve the MOF structure and to prevent blockage. It was reported that micropore volume and ratio of micropore surface area to BET surface area were directly proportional to Cu-BTC weight percent meaning that Cu-BTC micropores were not blocked at all. Another strategy to prevent pore blocking and achieve a more homogeneous distribution of MOF particles inside the gel network was suggested by Wickenheisser et al.32 According to their study, addition of a small amount of extra water formed an aqueous layer around the MOF and prevented pore blockage. These water added composites resulted with higher BET surface areas than estimated values for their MOF/xerogel composites. Direct mixing method may also result in clogging of aerogel pores. For example, Ramasubbu et al.34 prepared a Cu-BTC/titania aerogel composite by dispersing Cu-BTC into titania sol and experienced a decrease in BET surface area with increasing Cu-BTC content. SAXD (small angle X-ray scattering) results revealed the structural integrity of Cu-BTC in titania 12 ACS Paragon Plus Environment

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aerogel network even after exposure to aqueous medium and ammonia vapors during the synthesis. The reduction of surface area was attributed to the clusters of Cu-BTC blocking the TiO2 aerogel pores. Apart from inorganic aerogels, composites of MOFs with biodegradable organic aerogels were also prepared. Zhu et al.24 incorporated several MOFs into cellulose aerogels. The composites were synthesized by mixing and entrapping MOF in cellulose nanocrystal (CNC) to form a stable colloidal suspension which was then cross-linked to carboxymethyl cellulose (CMC) that formed a gel. Followed by freeze drying, MOF/cellulose aerogel composites were obtained with various MOFs such as ZIF-8, UiO-66 and MIL-100(Fe). The resultant composites were uniform, flexible and mechanically robust. The hierarchical multimodal pore structure consisted of micropores of MOFs, mesopores between the cross-linked CNC and CMC which make up the walls of the structure and macropores that were formed due to growth of ice crystals during freeze drying. It was observed that the MOF particles were dispersed within the walls as shown in Figure 3 with no agglomerate formation.

Figure 3. Schematic representation of MOF/cellulose aerogels (up) and Scanning Electron Microscopy (SEM) images of aerogels (down) with 50 wt% UiO-66 at different magnifications



(a) 100 µm, (b) 2 µm, (c) 500 nm. Reprinted with permission from reference 24. Copyright 2016 John Wiley and Sons. Generally, it is desirable to achieve a fine dispersion of MOF particles in the gel matrix. Just before the formation of a gel, MOF particles should be well dispersed in the solution without any concentration gradients. This can be accomplished by constant stirring of the solution. Although some MOFs are easily dispersed in gel medium, agglomerates of MOFs are obtained especially in highly viscous solutions and at high MOF loadings. This leads to inhomogeneous distribution of MOF particles inside the aerogel matrix. These agglomerations of MOF clusters can be a reason of reduction of interconnected continuous porous network structure thus result in collapse of the structure. For example, ZIF-8/konjac glucomannan (KGM) aerogel composites were synthesized via sol-gel method.35 The effect of MOF loading on the composite morphology was studied. At low MOF concentrations (7:5 KGM/ZIF-8 ratio), a good dispersion and uniform porous structure was observed as shown in Figure 4 (a) and (e). With increasing MOF concentration (such as 7:7.5 KGM/ZIF-8 ratio), agglomeration of excess ZIF-8 particles lead to asymmetrical MOF distribution, a non-uniform pore distribution (Figure 4 (c) and (g)) and eventually collapse of the porous network (Figure 4 (d) and (h)).


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Figure 4. SEM images of pure KGM (a and e), composite with 7:5 KGM/ZIF-8 ratio (b and f), composite with 7:7.5 KGM/ZIF-8 ratio (c and g), and composite with 7:10 KGM/ZIF-8 ratio (d and h) at different magnifications. Reprinted with permission from reference

35.

Copyright 2018 Elsevier B.V. MOF/graphene aerogel composites were studied by several groups due to the fact that both MOFs and graphene aerogels are promising materials for similar applications such as energy storage and adsorption and their synergistic effect may lead to the development of superior materials. Graphene is an attractive material due to its high surface area, mechanical, thermal and chemical stability. Graphene aerogel (GA) which is a 3D network of 2D graphene sheets is especially promising to support MOFs. Graphene aerogels are generally synthesized by first forming an aqueous solution of graphene oxide (GO) prepared using methods such as Hummer’s method.36 MOF particles or suspensions can be easily mixed with GO solution at this step. After gelation, the hydrogel is then dried either by supercritical drying or freeze drying to obtain MOF/graphene aerogel composites. GO can be reduced either before or after the drying step. Zhang et al.37 synthesized MIL-101(Cr)/graphene aerogel (GA) composites. They mixed MIL-101 into a graphene oxide (GO) dispersion, freeze dried the resulting gel and reduced GO under hydrazine vapor. As shown in Figure 5, MIL-101 particles were immobilized on GO sheets which was attributed to the interaction between positively charged coordinatively unsaturated metal sites of MIL-101 and negatively charged GO sheets. This immobilization prevented the agglomeration of MIL-101 particles and improved their dispersion on graphene sheets.



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Figure 5. Schematic representation of MIL-101/graphene aerogel synthesis (top). SEM images of (a) graphene aerogel and (b-c) MIL-101/graphene hybrid aerogel (bottom). Reprinted with permission from reference 37. Copyright 2016 Royal Society of Chemistry.

Mao et al.38 synthesized ZIF-8/reduced graphene oxide (rGO) aerogel with a one-step selfassembly method where metal sites of MOFs were used synergistically for chemical reduction and cross-linking. ZIF-8 was first dispersed in methanol and then added to an aqueous solution containing GO nanosheets and hydrazine hydrate. GO sheets acted as binding sites for ZIF-8 and Zn2+ ion of ZIF-8 acted as a coordinator for removal of hydrophilic groups such as carboxyl, hydroxyl and epoxy groups of graphene oxide (GO), therefore accelerated the gelation process of GO nanosheets by metal-oxygen covalent bonding. Followed by freeze drying, ZIF-8/reduced graphene

aerogel

(rGA)

was

obtained

with

enhanced

mechanical

properties

and

superhydrophobic nature with high water repellency and oil adsorption capacity. SEM images 16 ACS Paragon Plus Environment

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revealed that ZIF-8 crystals were uniformly distributed on rGO walls containing micro/macropores. FTIR analysis showed that ZIF-8 was deposited on the rGO aerogel and the covalent bond of Zn–O between ZIF-8 and rGA was observed in both the FTIR and Raman spectra. Similarly, Xu et al.39 mixed a wide variety of MOFs in graphene oxide (GO) solution where GO sheets and MOFs spontaneously self-assembled into 3D structures to form hydrogels. Followed by freeze drying, MOFACs were obtained which included Ni-MOF, ZIF-8, MOF-5, Sn-MOF, Co-MOF and bimetallic Fe-MOF/Ni-MOF. Li et al.40 also synthesized ZrMOF(MMU)/graphene aerogel in the form of a disk for use in solid phase extraction (SPE). The SEM images showed that the honeycomb-like structure of pure graphene aerogel was conserved but the smooth GA walls became rougher when MMU was wrapped around the GA. However, XRD pattern revealed that the composite lost several characteristic peaks of MMU which was attributed to the change in the MMU crystal structure after being incorporated into GO matrix. Taking advantage of amphiphilic nature of MOFs and GO, Zhang et al.41 synthesized ZrMOF/GO composites by forming a pickering emulsion in which Zr-MOF (Zr-BDC-NO2) and GO solids interacted with hydrogen bonding and accumulated at the water-oil interface (Figure 6). When the emulsion of oil droplets dispersed in water phase was freeze dried to remove the oil and water, a macroporous solid composite was obtained. The droplets of the emulsion were used as the template of composite pores and the pore sizes of the composites were consistent with the emulsion’s droplet sizes. The effect of MOF concentration on pore size was investigated. It was found that with increasing MOF content, average droplet size of the emulsion first decreased and then increased. The decrease in droplet size was attributed to the increase in emulsifier amount. Subsequent increase in droplet size was described by the strengthening of the interactions



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between Zr-BDC-NO2 and GO at higher concentrations which resulted in reduction of amphiphilicities of both emulsifiers.

Figure 6. Pickering emulsion stabilization mechanism of Zr-MOF (a) and formation of ZrBDC-NO2/GO composite (b). Reprinted with permission from reference

41.

Copyright 2017

Royal Society of Chemistry.

Other than monoliths, micron sized particles can be prepared with emulsion gelation method as explained previously. Shalygin et al.42 synthesized Cu-BTC/silica aerogel particles by coupling sol-gel method and emulsion gelation method. By dispersing Cu-BTC powder inside silica sol and dispersing this suspension in an oil phase which consisted of pentadecane, a water in oil emulsion was formed under continuous stirring. The droplets were composed of the dispersed phase therefore the gelation of the particles took place inside the emulsion. Followed by washing to remove pentadecane, the composite particles were dried via supercritical drying. Two different pellet sizes (0.1-0.5 mm and 1-2.5 mm) were formed by changing the stirring rate of the emulsion (which were 380 and 250 rpm, respectively). With direct mixing method spherical composite aerogel beads with approximately 5 mm diameter can also be synthesized by the dripping method. Fe-BTC/alginate aerogels have recently been synthesized by our group. First, Fe-BTC was dispersed in a sodium alginate solution. The prepared suspension was added dropwise into a CaCl2 solution. Spherical gel beads with 4 mm diameter were formed in the solution. The beads were then placed in an ethanol bath for solvent 18 ACS Paragon Plus Environment

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exchange and supercritically dried to obtain spherical MOFAC beads. Micron sized particles were also produced using a modified version of emulsion gelation method explained in the study by Alnaief et al.43 A suspension of Fe-BTC, sodium alginate and CaCO3 was dispersed in paraffin oil which contained a surfactant under constant stirring. The formed w/o emulsion was then subject to a small portion of paraffin oil which contained acetic acid. With the addition of acetic acid, the droplets gelled forming micron sized particles trapping Fe-BTC inside the alginate matrix. After removing the oil phase by centrifugation and washing, the gel particles were subjected to solvent exchange with ethanol and dried supercritically. These MOFACs can be seen in Figure 7.

Figure 7. Photographs of Fe-BTC/alginate aerogel composites produced via (a) dripping method and (b) emulsion gelation method.

An important consideration in synthesis of MOFACs is the compatibility of MOFs with the solvents used in aerogel synthesis. Since most aerogels are synthesized in aqueous solutions, MOFs should be stable in water during the synthesis stage. It is well known that the oxygen in water is a nucleophile whereas the metal coordination centers in MOFs are electrophilic. If this metal center of MOF is not sufficiently inert, water can coordinate with the metal cluster and distort or destroy the MOF’s crystal lattice structure. Calculation of the free energy change of this 19 ACS Paragon Plus Environment


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hydrolysis reaction may enable one to predict if such a distortion is thermodynamically favorable for a particular MOF. Kinetic factors may also play a role and the activation energy for this reaction may be high due to the steric factors or for hydrophobic MOFs. The degree of stability of a wide variety of MOFs in aqueous solutions are tabulated in an excellent study by Burtch et al.44 which can be used as a guideline before starting to develop MOFACs for a particular application. Solvent exchange is another step in aerogel synthesis where MOFs are subjected to solvents such as acetone or ethanol that can be removed by supercritical drying. Studies in the literature on stability of MOFs in such solvents are not extensive.

Another important

consideration in direct mixing method is the interference of MOFs with the polymerization chemistry for gel formation. The procedures that are developed for synthesis of pure gels/aerogels may not work in the presence of MOF in solution or may lead to a different pore structure than pure gels. This may be due to the effects of adsorption of sol-gel or cross-linking catalysts and monomers on the surface of MOFs which change catalyst and reactant concentrations as well as pH. It is also well known that MOFs can function as catalysts and can influence the delicate reaction network for the formation of gels. Studies are required on how the addition of MOF to precursor solution for gel synthesis affect the gelation behavior. As there are many different MOFs and aerogels that can be combined, selecting a synthetic procedure for a MOFAC that will preserve the properties of pure MOF and pure aerogel can be difficult. Each composite should be approached with great attention where its functionality and stability should be checked with various characterization techniques such as XRD and N2 sorption analysis. 2.4. In Situ MOF Synthesis: The other technique to prepare MOFACs is synthesizing the MOF in situ inside the pores of an aerogel or a gel which is then dried. This method can also be termed as ship-in-a-bottle approach where the MOF is the ship and bottle is the pore of a gel or an aerogel. 20 ACS Paragon Plus Environment

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MOFs can be synthesized at ambient conditions or at high temperatures or pressures by various techniques such as conventional heating, microwave heating, electrochemistry, mechanochemistry and ultrasonic methods primarily by reactions of metal precursors and organic linkers.45 Then the synthesized MOFs are purified and activated which generally is associated with removal of the solvent from the pores of the MOF. For a detailed description of synthesis techniques of MOFs, excellent reviews are present.13,45–48 Using the mentioned techniques, MOFs can be grown either inside the pores of a gel or an aerogel by sequentially adding the metal precursor and the organic linker as depicted in Figure 8. For synthesis in gels, MOF precursors are incorporated to the gel thus MOFs are grown in situ inside the pores of a gel followed by drying to obtain MOFACs. Whereas for synthesis in aerogels, plain gels are synthesized and dried supercritically or via freeze drying. Then the as-synthesized aerogels are immersed in MOF precursor solutions to synthesize MOF inside the pores of the aerogel, preferably followed by another drying to obtain MOFACs.

Figure 8. Preparation of MOFACs in situ on hydrogels and aerogels.



Either using hydrogel or aerogel as substrate, when the necessary conditions are reached for the start of the reaction between metal precursor and organic linker, MOF crystals begin forming, nucleating, growing and precipitating both inside and outside the porous matrix. Next is the purification step which is the removal of unreacted precursors and unbound MOF particles. If MOFs are grown without binding, they are susceptible to be washed out during purification steps. Therefore, binding of MOFs to the gel matrix is highly desirable in this technique. MOFs can be attached to internal surface of gel matrices via interaction between the functional groups of gels and metal ions of MOFs. This way, with the addition of organic linker, already attached metal ions start reacting with the organic linker and nucleating on the attached site. Several groups took advantage of substrates that already contain surface functional groups like graphene oxide whereas some others functionalized their substrates prior to MOF growth. Another strategy is entrapping metal ions of MOF or MOF particles themselves in gel matrix to prevent losses during purification. Even using the metal sites of aerogel for MOF growth is possible. The last step is the removal of solvent from the pores of the MOF and the gel. This is an important step as it facilitates high surface areas. For general MOF synthesis, supercritical activation is considered as a comparably gentler method that retains the potential porosity as conventional methods may result in collapse of the porous network.49 Fortunately, during in situ MOF synthesis, aerogel formation and MOF activation can be accomplished in a single step either by supercritical drying or freeze drying methods. 2.5. Literature Examples of MOFACs Synthesized by the In Situ Method: Wisser et al.50 formed Cu-BTC crystals in hollow fibers of chitin by taking advantage of high affinity of biopolymer to the metal ions. Chitinous networks that were extracted from a marine sponge were immersed in a solution containing Cu metal precursor Cu(NO3)2 and then 22 ACS Paragon Plus Environment

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organic linker BTC (benzene-1,3,5-tricarboxylic acid) was added. As the functional groups of proteins interacted with metal ions, chitin-copper interactions resulted in MOF nucleation mainly inside the hollow fibers of chitin creating a hierarchical porous structure as seen in Figure 9. After MOF formation, unbound MOFs were washed and then the matrix was dried supercritically. The MOF loading reached up to 55% with surface areas up to 800 m2/g and pore volumes of 3.6 cm3/g.

Figure 9. (a,b) Microscopic images and (c,d) SEM images of chitin fibers filled with CuBTC. Reprinted with permission from reference 50. Copyright 2015 John Wiley and Sons.

Bo et al.51 synthesized ZIF-8/cellulose aerogels by first dispersing zinc precursor Zn(NO3)2·6H2O into cellulose solution. Then by chemically cross-linking the polymer with N,N'methylenebisacrylamide (MBA), a cellulose hydrogel was formed where Zn2+ ions were trapped inside. Followed by freeze drying, Zn2+/cellulose aerogels were obtained. Then this aerogel was immersed into organic linker 2-methylimidazole solution to start MOF growth as depicted in 23 ACS Paragon Plus Environment


Figure 10. The composite was washed and freeze dried to obtain ZIF-8/cellulose aerogel. PXRD revealed that the crystallinity of ZIF-8 was preserved in the composite and FTIR exhibited the characteristic peaks of both ZIF-8 and cellulose aerogel. ZIF-8 contents were determined to be 10-30 wt% by TGA. As the MOF content increased, density increased while the total pore volume and porosity decreased even though the composite with 30 wt% ZIF-8 had 95.3% porosity.

Figure 10. Schematic procedure of ZIF-8 growth on cellulose aerogel. Reprinted with permission from reference 51. Copyright 2018 Elsevier.

Another in situ ZIF-8 loading on cellulose was performed by Zhu et al.52 where they synthesized MOF crystals on template of TEMPO (2,2,6,6-Tetramethylpiperidyl-1-oxyl)oxidized cellulose nanofibrils (CNFs). By TEMPO-oxidation, carboxylate groups formed on the surface of CNFs which interacted ionically with multivalent metal ions such as Zn2+, Cu2+ or Co2+. When an appropriate organic ligand precursor is also present in the medium, MOF crystals start to nucleate on fibrous CNF networks so that ZIF-8 crystals adhere to cellulose via Hbonding and ionic interactions forming hierarchically porous structures as depicted in SEM images in Figure 11. MOF loadings and size of MOF crystals were controlled with precursor concentrations and temperature. 24 ACS Paragon Plus Environment

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Figure 11. SEM images of Zn2+ cross-linked cellulose nanofibrils aerogel (a), ZIF-8/cellulose aerogel composites with different ZIF-8 contents: 51 wt % (b), 75 wt % (c), and 81 wt % (d). Reprinted with permission from reference 52. Copyright 2018 American Chemical Society.

Moreover, Ren et al.53 in situ synthesized ZIF-9 and ZIF-12 on pre-synthesized cellulose aerogels by immersing them in MOF precursor solutions. Same precursors but different solvents were used for ZIF-9 and ZIF-12. The composites were then washed and freeze dried again to obtain ZIF/cellulose aerogel composites. Although no change in porous structure of cellulose aerogel was observed, the shape and XRD spectra of MOFs grown inside the pores of cellulose aerogels were observed to be different than their pure counterparts and this was attributed to the influence of aerogel surface on the crystal growth. Composites in the form of microspheres and fibers with channels having honeycomb internal structures were also synthesized by the in situ method.54 ZIF-8/chitosan microspheres with such a structure were prepared by adding a mixture of zinc metal ion precursor Zn(NO3)2 and chitosan to a solution of NaOH with organic linker 2-methylimidazole (Hmim). Gelation of chitosan and growth of ZIF-8 crystals occurred instantly and simultaneously. This way, spheres having a diameter of 2 mm and fibers having diameter about 1 mm were synthesized with the dripping technique. ZIF-8 particles were well dispersed and embedded inside the chitosan matrix and their loadings ranged from 30 wt% to 83 wt% which was mainly influenced by the 25 ACS Paragon Plus Environment


concentration of Zn(NO3)2 precursor. 200 to 500 nm sized ZIF-8 particles were obtained by changing the amount of Zn(NO3)2 precursor concentration without changing the molar ratio of Zn2+ to Hmim. Additionally, by changing the metal ion and organic ligand, ZIF-7/chitosan and ZIF-67/chitosan microspheres were also synthesized. An important substrate to grow MOFs is graphene aerogel. Plentiful functional groups of graphene oxide can well adsorb metal ions and promote heterogeneous nucleation thereby allow formation of uniformly dispersed MOF particles on graphene sheets. There are two strategies for growing MOFs in situ on graphene, either in aqueous graphene oxide suspension or on graphene aerogel. Liu et al.55 synthesized two kinds of Fe-MOF/GA composites. They first synthesized spindle-like MIL-88-Fe particles by adding metal and ligand precursors and growing them in situ on presynthesized graphene aerogel. They also synthesized by first adding Fe3+ ions to graphene oxide solution to make Fe3+-doped graphene aerogels which were then subjected to a solution of the ligand. This way, oriented rod-like MOFs were grown directionally and controllably on (002) lattice plane of graphene surface. The interaction between graphene and MOF was stronger for the latter case. Moreover, MOFs were distributed more evenly due to the homogeneous distribution of Fe3+ ions on the graphene aerogel surface and the composite had a higher specific surface area. Qu et al.56 synthesized Cu-BTC on the surface of Ru/3D graphene aerogel surface by step-by-step self-assembly method. They first synthesized 3D graphene aerogel with metallic ruthenium particles (Ru/GA) and then modified the surface functional groups of Ru/GA in order to form strong chemical bonds with copper ions. With the addition of copper precursor and the organic ligand several times in succession, they were able to synthesize approximately 30 wt% Cu-BTC on Ru/GA surface as determined by TGA. 26 ACS Paragon Plus Environment

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Growing MOFs on graphene oxide sheets in aqueous solution is also possible. For example, Yang et al.57 synthesized ZIF-67 on the residual hydrophilic sites of rGO. The hydrogel was then freeze dried yielding ZIF-67/rGO aerogel that was cross-linked honeycombs containing micro/macropores. Method of in situ MOF synthesis was recently extended by two-step reduction and layer-by-layer assembly to synthesize hierarchically micro-/meso-/macroporous ZIF-8/graphene aerogel.23 Firstly, a graphene oxide solution was partially reduced, frozen and thawed, reduced again to yield graphene hydrogel with large pores which was then used as the template. By immersion of hydrogel in both precursor solutions successively, ZIF-8 was synthesized in situ layer-by-layer on both external and internal surface of graphene. As large pores of graphene minimized the capillary forces and the mechanical strength of graphene increased with incorporation of ZIF-8 crystals, composites were able to resist ambient pressure drying without collapse of the porous structure. MOF loadings were strongly dependent on the oxygen containing functional groups of rGO and they were quantified from the density increase and carbon/nitrogen ratio. The ZIF-8 loadings reached up to 88 wt% with 10 immersion cycles and surface area increased up to 1099 m2/g at the end of the 8th cycle. Domán et al.58 synthesized Cu-BTC/carbon aerogel composites with the aim of increasing the thermal and mechanical stability of Cu-BTC. For in situ synthesis, carbon aerogel was soaked in metal and ligand precursor solutions successively before the solvothermal process. In order to grow Cu-BTC crystals on the surface of carbon aerogel, the surface was made acidic by the addition of acidic surface groups. However, no chemical interaction between carbon aerogel surface and Cu atoms of Cu-BTC was observed, indicating the composite resembled a physical mixture but the mesopores and macropores of the aerogel was filled with the synthesized MOF.



Another MOF/carbon aerogel composite was prepared by Zhao et al.59 They grew MOFs with mixed metal ions on the surface of carbon aerogel to make composites via a modified hydrothermal reaction. By placing carbon aerogel inside an autoclave along with the MOF precursors, a bifunctional MOF(Fe/Co)/carbon aerogel (CA) was prepared with a good chemical stability. Different Fe/Co ratios were studied, yet only a ratio of 2:1 was found to lead to a successful growth on the surface of carbon aerogel to make MOF(2Fe/Co)/CA composite. A different approach is growing MOF in situ on other materials which are then incorporated into aerogel as in the recent work of Zhang et al.60 They synthesized ZIF-8 in situ on carbon nitride (C3N4) nanosheets and then added them to agar aerogel as seen in Figure 12 to increase its flexibility and efficiency for waste water treatment. The maximum dye adsorption capacity was obtained with ZIF-8/C3N4 aerogel which was attributed to the fine dispersion of ZIF-8 on the nanosheets that was achieved by in situ synthesis with an additional benefit of reusability.

Figure 12. Schematic procedure of hybrid agar aerogels containing ZIF-8/C3N4 heterostructures. Reprinted with permission from ref 60. Copyright 2017 American Chemical Society. A very interesting study was conducted by Reboul et al.61 who synthesized MOF on a metal oxide aerogel using its metal as a nucleation site. By applying a pseudomorphic replacement process which involves dissolution of the metastable parent product and reprecipitation to a new stable phase, a coordination replication was achieved when organic ligands were introduced into a pre-shaped metal oxide. So the metal oxide was not only the metal ion source but also the 28 ACS Paragon Plus Environment

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“architecture-directing agent”. For this, first alumina aerogels (meso and macroporous) were synthesized and an organic linker H2ndc(2,6-Naphthalenedicarboxylic acid) was added. Followed by microwave treatment, aluminium naphthalene dicarboxylate frameworks were grown using the aerogel as nucleation site, creating MOF crystals into two and three dimensional mesoscopic higher-order structures. As depicted in Figure 13, FESEM analysis showed the dissolution of the metal oxide phase providing metal ions for MOF synthesis and the formation of MOF crystals on alumina pattern without being connected to each other. The same method was applied with two more dicarboxylic acids as well.

Figure 13. The growth of MOF on alumina monitored with respect to time by FESEM. Images of the alumina pattern (a) and after replication for 1 s (b), 4s (c), 6s (d), 10 s (e), 20s (f), 40s (g) and 60s (h). All scale bars are 1 µm. Reprinted with permission from reference

61.

Copyright 2012 Springer Nature. Although a more uniform distribution of MOF crystals inside the composite is achieved with this technique when compared to direct mixing method, MOF growth is not easily 29 ACS Paragon Plus Environment


controlled. Concentration of precursors and reaction environment such as temperature are generally adjusted to control MOF growth inside the pores. As the amount of MOF synthesized cannot be predetermined in this technique, methods such as TGA (thermogravimetric analysis) and weighing are used to estimate the MOF amount in composite. Another important aspect is the ability of gel/aerogel matrix to maintain its structural integrity against the synthesis conditions and solvents used for MOF preparation. For example, solvothermal methods will not be applicable to synthesize composites of MOFs with most organic gels as they will degrade at high temperatures. Furthermore, the loss of mesopore volume due to the growth of MOF particles inside the mesopore volume is another consideration. If such a decrease cannot be tolerated for a particular application, then direct mixing method may be a better choice. The advantages and disadvantages of both methods are given in Table 2 as a brief summary. Table 2. Advantages and disadvantages of the synthesis techniques of MOFACs. Direct mixing method

Advantages  No loss of mesopore volume  MOFs entrapped into gel matrix  Controlled MOF loading

   

In situ MOF synthesis

 Uniform MOF distribution  No MOF agglomerations  No MOF pore blockage

   

Disadvantages MOF may not be stable during aerogel synthesis Sol may clog MOF micropores MOF may effect gelation behavior (interference of MOFs with the polymerization chemistry) Non-uniform MOF distribution and agglomeration possibility Aerogel may not be stable during MOF synthesis Loss of mesopore volume MOF must attach the matrix to prevent loss Uncontrolled MOF loading

Some representative data from the studies in the literature on pore properties of MOFACs are given in Table 3. The increase in surface areas with increasing MOF content is evident which


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is indicative of preservation of the microporous nature of MOFs in the composite. More details on these properties can be found in the corresponding references given in Table 3. Table 3. Comparison of pore and structural properties of some pure MOFs and their MOFACs.

Reference

Ulker et al.

Wisser et al.

22

50

Jiang et al. 23

Zhang et al. 37

Bo et al. 51

Material

MOF wt%

Micropore area (m2/g)

Pure silica aerogel Cu-BTC/Silica Aerogel Cu-BTC/Silica Aerogel Cu-BTC/Silica Aerogel Pure Cu-BTC Pure chitin network Cu-BTC/Chitin Aerogel Pure Cu-BTC

0 4.2 16.3 30.5 100 0 53 100

0 19 59 211 1119

Graphene Aerogel (GA)

0

ZIF-8/GA 2 cycles ZIF-8/GA 4 cycles ZIF-8/GA 8 cycles ZIF-8 pure pure GA MIL-101/GA 0,5 MIL-101/GA 0,51 MIL-101/GA 0,52 ZIF-8/Cellulose Aerogel ZIF-8/Cellulose Aerogel ZIF-8/Cellulose Aerogel ZIF-8/Cellulose Aerogel

71.8 73.5 86.7 100 0 33 50 70 10 15 20 30

BET surface area (m2/g) 926 1025 1036 1138 1352 18±9 730±70 1370±20

Pore Volume (cm3/g)

Micropore volume (cm3/g)

Density (g/cm3)

0 0.006 0.028 0.11 0.591

0.13 0.142 0.154 0.184 0.35

404

0.52

0.003

663 799 1099 1567 272 469.8 1126.6 1560.1

0.4 0.53 0.67 0.52

0.95

0.15±0.08 3.6±1 0.75±0.06

32.62 25.37 21.54 19.33

0.047 0.053 0.067 0.076

3. MOFACs as Precursors of Supported Metals or Metal Oxides: Metals and metal oxides on porous carbon supports are widely investigated materials especially for electrochemical energy storage and conversion applications. When MOFs are subjected to thermal treatment, carbon materials decorated with metal and metal oxides can be 31 ACS Paragon Plus Environment


obtained due to their inorganic-organic nature.62 Further metal leaching leads to purely carbonaceous materials with very high surface areas. Similarly, graphene aerogels and carbon aerogels obtained via pyrolysis of organic aerogels are used directly or doped with metals to achieve hierarchically porous materials.63 Accordingly, MOFACs can be used as sacrificial templates and precursors of various supported metals or metal oxides. Several MOF/aerogel composites were thermally treated via pyrolysis or calcination to obtain carbon supported metals or metal oxides. In some studies, metals were further leached to obtain hierarchically porous carbon materials with high surface areas and narrow pore size distributions. Xu et al.39 synthesized Fe-MOF/GO and Ni-MOF/GO composites with direct mixing method and then converted them into Fe2O3/rGO and NiO/Ni/rGO composite aerogels with a two-step annealing process which comprised first annealing under N2 protection at 450 °C for 1 h, then another thermal treatment in air at 380°C for 1 h. The composites exhibited highly porous 3D structures with interconnected GO sheets which were uniformly coated with nanoparticles. Zhu et al.64 also used direct mixing method and first synthesized the MOF, Fe-bpdc (iron2,2’-bipyridine-3,3’-dicarboxylic acid). Then together with g-C3N4, the MOF was encapsulated into a resorcinol-formaldehyde resin during the precursor polymerization. The formed aerogel was then ball milled and carbonized for 4 h under N2 atmosphere at different temperatures, 700°C, 800°C, 900°C and 1000°C to obtain Fe-bpdc-C3N4/carbon aerogel composites. During calcination, carbon aerogel protected some of the Fe-bpdc which remained unchanged but the rest of the Fe-bpdc which were not totally encapsulated within the carbon aerogel was converted to smaller Fe3C particles encapsulated in a carbon shell. Sui et al.65 synthesized ZIF-67 particles in situ inside the pores of N-doped graphene aerogel (NGA) by first adsorbing Co2+ on graphene sheets followed by the addition of ligand. By 32 ACS Paragon Plus Environment

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calcination in air, ZIF-67 was transformed into Co3O4 forming hierarchically porous Co3O4/ nitrogen-doped graphene network (NGN) composite as shown on Figure 14. They also physically mixed ZIF-67 with NGA obtain ZIF-67/NGA composite which was further calcined to obtain Co3O4/NGN composite.

Figure 14. Schematic procedure of in situ growth of ZIF-67 on the pores of nitrogen-doped graphene aerogel followed by calcination to form Co3O4/NGN composite (top), SEM images of Co3O4/NGN composite (bottom) at different magnifications (a) 5 µm, (b) 1 µm. Reprinted with permission from reference 65. Copyright 2017 American Chemical Society.

Wan et al.66 synthesized graphene aerogel monoliths from GO aqueous solution followed by freeze drying. Then, ZIF-8 and ZIF-67 were grown uniformly on those macroporous graphene aerogels by adding metal precursor and ligand solutions successively. SEM images displayed that the interconnected macroporous structure was preserved after growth of ZIF crystals. The composites were then calcined and etched to remove the metal ions forming hierarchically meso-



/macroporous N-doped carbons supported on 3D graphene to be used as electrode materials for all-solid-state supercapacitors. Xia et al.67 prepared a nitrogen-rich porous carbon−graphene composite (C/NG-A) by first in situ growing Co-MOF on N-doped graphene hydrogel. Followed by supercritical drying, the material was then thermally activated to form Co core-Carbon shell structures. By treatment in air at 100 °C for 24 h, Co cores were oxidized to form hollow CoOx nanoparticles with a uniform size of 35 nm on N-doped graphene aerogel. The composite had a high BET surface area of 1359 m2/g and large pore volume 7.2 cm3/g. The thermally activated composite was also treated with HCl to remove Co. This yielded carbon/N-doped graphene aerogel composite with a lower BET surface area of 814 m2/g, which was explained as the shrinkage of carbon shells caused by the removal of Co. The same group also synthesized Ni-MOF-74/rGO by consecutive addition of ligand and metal precursor in aqueous suspension of graphene oxide followed by freeze drying.68 The composite was then mixed with sublimed sulfur and exposed to 5% H2S–95% Ar gas flow in a tube furnace which led to a hierarchically micro-/mesoporous R-NiS/rGO where rod-like NiS were intertwined with graphene sheets. Jiang et al.69 synthesized Prussian blue (PB, Fe4[Fe(CN)6]3) by adding excessive metal ions to a graphene oxide aqueous solution which contained ligand. This way, PB was ensured to be deposited on graphene oxide surface. After the reduction of graphene oxide, PB particles on the external surface of 3D graphene were removed and entrapped PB nanoparticles were grown into larger and more stable particles by Ostwald ripening. The hydrogel was then freeze dried and PB/3D graphene aerogel (3DG) composite was obtained. Followed by annealing at 250 °C for 2 h in air, PB nanoparticles decomposed into Fe2O3 forming Fe2O3/3DG aerogel. By calcining the composite with Se powder, a carbon shell was obtained forming Fe7Se8@C/3DG aerogel that had 34 ACS Paragon Plus Environment

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a Fe7Se8-enriched inner core and carbon-enriched outer shell encapsulated within a threedimensional graphene network.70 In another study by the same group, PB/3DG aerogel was calcined with sulfur powder and under argon atmosphere and a core–shell FeS@carbon encapsulated within 3DG aerogel was obtained.71 4. Applications: The properties of MOFACs are governed by contributions from their solid network and pore structure. The solid network generally consists of MOF particles on the interconnected particles/fibers which make up the solid backbone of the MOFAC. Interactions of guest molecules and the solid network depends on the chemical nature of the guest molecule and the physicochemical and morphological properties of the solid network. Therefore, different MOFACs interact differently with different molecules. This interaction dictates the thermodynamic limit for applications such as adsorption and separation. The pore structure is equally important in many applications since it governs how fast the guest molecules are transported in the porous structure. Hierarchical porosities enable fast mass transfer rates due to meso-/macro porosities while micropores induce selective transport and also promote strong interactions of adsorbate and adsorbent. In this section, the use of MOFACs which were synthesized by the techniques described in the previous section are presented for various applications. A list of all MOFACs in literature with their synthesis techniques, shapes and applications are summarized in Table 4. Table 4. Summary of MOFACs with synthesis methods, shapes and applications. MOFAC synthesized

Technique

Shape

Al(OH)(ndc)/Alumina Aerogel

In Situ

Monolith

Co-MOF/NGA derived CoOx/NGA

In Situ

Monolith

Cu-BTC/Carbon Aerogel

In situ

Monolith


Application Sorption and Separation Energy Storage and Conversion Adsorption

Ref. 61

67 58


Cu-BTC/Chitin Aerogel

In Situ

Fiber

Cu-BTC/Ru/Graphene Aerogel

In Situ

Monolith

Cu-BTC/Silica Aerogel

Direct Mixing

Monolith


Direct Mixing


Direct Mixing

Particles via emulsion gelation Monolith

Cu-BTC/Titania Aerogel

Direct Mixing

Monolith

Fe-bpdc/C3N4/Carbon aerogel

Direct Mixing

Powder (ball milled)

Fe-MOF/GO and Ni-MOF/GO derived Fe2O3/rGO aerogel and NiO/Ni/ rGO aerogel, ZIF-8/GO, MOF-5/GO, Sn-MOF/GO, CoMOF/GO, Fe-MOF/Ni-MOF/GO

Direct Mixing

Monolith

MIL-88B-NH2/NGA derived Fe3O4/NGA

In situ

Monolith

MIL-88-Fe/Graphene Aerogel

In situ

Monolith

Direct Mixing

Monolith

In situ

Monolith

MIL-101(Cr)/Graphene Aerogel

Direct Mixing

Monolith

Ni-MOF-74/rGO Aerogel derived R-NiS/rGO

In situ

Monolith

Prussian Blue/ 3DGA derived FeS@C/3DG

In situ

Monolith

Prussian Blue/3DGA derived Fe2O3/3DGA

In situ

Monolith

In situ

Monolith

Direct Mixing

Monolith

ZIF-67/ rGO aerogel

In situ

Monolith

ZIF-67/NGA derived Co3O4/N-doped Graphene Network(NGN) , ZIF-8/NGA, UiO66/NGA

In situ

Monolith

ZIF-8/C3N4/Agar Aerogel

In situ and direct mixing

Monolith

ZIF-8/Cellulose Aerogel

In situ

Monolith

ZIF-8/Cellulose Aerogel

In situ

Monolith

MIL-88-Fe/GO Aerogel derived Fe2O3/rGO Aerogel MOF(2Fe/Co)/Carbon Aerogel

Prussian Blue/3DGA derived Fe7Se8@Carbon/3DGA ZIF-67 derived Co/C nanoparticles/Polypyrrole Aerogel


Page 36 of 63

Adsorptive Separation Catalysis Adsorptive Separation

50 56 33

Catalysis

42

Adsorption Photon Absorption Energy Conversion/ Electrocatalyst

22

Energy Storage/ Supercapacitor Energy Conversion/ Electrocatalyst Energy Storage/ Supercapacitor Energy Storage/ Battery Anode Catalysis Adsorptive Separation Energy Storage/ Supercapacitor Energy Storage/ Battery Anode Energy Storage/ Battery Anode Energy Storage/ Battery Anode Electromagnetic Absorption Adsorptive Separation Energy Storage/ Battery Anode Adsorptive Separation Adsorptive Separation Adsorptive Separation

34

64

39

72

55

73 59 37

68

71

69

70

74

75

65

60

51

52

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ZIF-8/Cellulose Aerogel UiO-66/Cellulose Aerogel MIL-100(Fe)/Cellulose Aerogel ZIF-8/Chitosan Aerogel ZIF-7/Chitosan Aerogel ZIF-67/Chitosan Aerogel ZIF-8/Graphene Aerogel ZIF-8/Graphene Aerogel ZIF-67/Graphene aerogel

Direct Mixing

Monolith

Adsorptive Separation

24

In situ

Microspheres, fibers


54

In situ

Monolith

23

In situ

Monolith

Direct Mixing

Monolith

Direct Mixing

Monolith

Direct Mixing

Monolith

Adsorption Energy Storage/ Supercapacitor Adsorptive Separation Adsorptive Separation, Photocatalysis N/A

In situ

Monolith

Catalysis

53

Zn-MOF/Titania Aerogel

Direct Mixing

Monolith

Energy Conversion

77

Zr-BDC-NO2/Graphene Oxide Aerogel

Direct Mixing/Pickering Emulsion

Monolith

N/A

Zr-MOF(MMU)/Graphene Oxide Aerogel

Direct Mixing

Membrane


ZIF-8/Konjac Glucomannan Aerogel ZIF-8/rGO Aerogel, also MOF-5/rGA, MOF199/GA, MIL-88-Fe/rGA, ZnO/rGA, TiO2/ rGA, CuO/rGA ZIF-8/Silica Aerogel ZIF-9/Cellulose Aerogel ZIF-12/Cellulose aerogel

66

35

38

76

41

40

4.1 Sorption and Separation: Storage of gases such as H2 and CH4, separation of gases such as N2 and CO2 are very important fields due to the need to develop new technologies to combat global warming. MOFs are extensively studied for gas separation and sorption applications. Aerogels are also very promising materials in this field.78,79 While very high uptakes and very high selectivities have been obtained with MOFs, aerogels are not only excellent carriers for MOFs but they are also performance enhancers. Consequently, many remarkable and promising results were reported with the synergy of MOFs and aerogels in the areas of gas and vapor uptakes, water purification, dye adsorption, filtration and extraction. For example, significant CO2 uptake of 0.99 mmol/g at 298 K and 1 bar was achieved with ZIF/graphene aerogel composite having a BET surface area of 1099 m2/g.23 CO2 uptake capacity of the composite was more than that of pure MOF (0.70 37 ACS Paragon Plus Environment


mmol/g) and pure graphene aerogel (0.38 mmol/g) alone which can be explained by the synergistic effects of combining MOF and aerogel that provide strong host-guest interaction in pores. Moreover, adsoption kinetics was faster due to fast mass transfer through the mesopores with the additional benefit of mechanical robustness. Domán et al.58 investigated CH4 and water vapor uptakes of Cu-BTC/carbon aerogels. Although Cu-BTC had a remarkably high CH4 uptake of 190 cm3 (STP)/cm3, it was sensitive to water and it became amorphous at high partial pressures.80 For this purpose, Cu-BTC/carbon aerogel composites were prepared to provide protection to Cu-BTC from water vapor. The composites were synthesized either as a physical mixture or Cu-BTC grown on carbon aerogel and compared. Even though the adsorption isotherms were similar, the protective effect of the carbon aerogel was obvious demonstrating the advantage of these composites. In another study, ammonia breakthrough experiments were conducted with Cu-BTC/chitin aerogel composites for removal of toxic industrial gases.50 An ammonia uptake capacity of 39.3 mg/g was reported with Cu-BTC that was grown on hollow chitin fibers with 53 wt% MOF content. Moreover, CuBTC/chitin aerogel composites provided protection from mechanical stress and abrasion with high accessibility through macropores. These hierarchical composites can also be used as packing material for column chromatography. Nuzhdin et al.33 used Cu-BTC/silica aerogel composites as a stationary phase in conventional liquid chromatography for separation of olefins or aromatics from paraffins and cycloparaffins. High efficiency was reported when separating cyclohexene or benzene from cyclohexane. Reboul et al.61 synthesized two types of Al-MOF/alumina aerogel which had micro/mesoporous and micro-/meso-/macroporous structures. Both materials were tested to separate water and ethanol and high water/ethanol separation selectivities (2.56 and 1.84, respectively) were achieved. As compared with MOF powder, micro-/mesoporous composite enhanced mass 38 ACS Paragon Plus Environment

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transfer kinetics, resulting in lower breakthrough times for water and ethanol while preserving separation efficiency. Industrial waste water treatment to remove metal ions and organic dyes are important applications as they are environmentally and ecologically hazardous. Zhu et al.24 tested their MOF/cellulose hybrid aerogels to remove potassium dichromate from aqueous solutions, as Cr(VI) is the most toxic form of chromium. The adsorption kinetics was found to fit to a pseudosecond-order model which implied that molecular transport in cellulose matrix is fast and MOF is the cause of diffusional limitations. The excellent performance of the composites was attributed to the hierarchical structure where macropores facilitated accessibility and capillary effect of mesopores resulted in high water uptake. The flexible composites could be squeezed to remove the contaminated water and they could retain their original shape when put back into the solution. Similarly, Bo et al.51 tested ZIF-8/cellulose aerogels for Cr(VI) removal. Adsorption isotherms could be represented by the Langmuir isotherm and the kinetics of removal was described by a pseudo-second-order kinetic model. The Cr(VI) uptake of 27.9 mg/g for the composite was significantly higher than the uptakes of 7.0 mg/g and 8.3 mg/g for pure CA and for pure ZIF-8, respectively indicating the performance improvement due to synergistic effects in these materials. In another study, Cu(II) removal from water was investigated using soft and compressible ZIF8/chitosan aerogel composites that can float on water surface.54 While the amount of Cu(II) adsorbed on the composite was 7.0 mmol/g, it was 2.3 mmol/g for chitosan aerogels and 7.2 mmol/g for pure ZIF-8. The rate of equilibration was very high especially for fibers. Moreover, in a recent study, the adsorption kinetics of antibiotic ciprofloxacin from water by ZIF-8/konjac glucomannan aerogels was investigated.35 The drug was effectively adsorbed on the composite following pseudo-second-order kinetics with a maximum adsorption capacity of 811.03 mg/g at a



ciprofloxacin concentration of 1500 mg/L which is very promising for solving the problems of antibiotic removal from wastewater. Another industrial wastewater treatment application is removal of hydroquinone which is highly toxic and hazardous to health. Li et al.40 synthesized Zr-MOF/reduced graphene oxide aerogel composites in the form of a disk which was utilized as a solid phase extraction (SPE) disk for the removal of hydroquinone and immobilization of laccase which is an enzyme that oxidizes hydroquinone. The promising results obtained in this study indicate the potential of materials that can be prepared by incorporating additives such as enzymes, drugs, and catalysts into MOFACs. In another study, Zhang et al.37 prepared MIL-101/graphene aerogels as adsorbents for extraction of non-steroidal anti-inflammatory drugs (NSAIDs) and selective enrichment of proteins where they achieved low backpressure and low mass transfer resistance due to the macroporous structure. Dyes are important wastewater effluents due to the fact that they are used extensively in many different applications and dyes that are discharged into water streams are significant pollutants. Several groups studied dye adsorption of MOFACs. For example, ZIF-67 that was grown in situ on reduced graphene showed impressive adsorption capacities up to 1714.2 mg/g for cationic dye crystal violet and 426.3 mg/g for anionic dye methyl orange.57 This composite was reported to have a remarkably high maximum adsorption capacity for crystal violet dye when compared to other adsorbents. The adsorption kinetics was found to fit the pseudo-first-order model for both crystal violet and methyl orange. While the ultrahigh adsorption capacity towards crystal violet is mainly driven by π-π interactions and electrostatic interactions between the composite and dye molecules, high adsorption capabilities towards methyl orange were attributed mainly to the high surface area and electrostatic interactions. Zhu et al.52 investigated ZIF40 ACS Paragon Plus Environment

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8/cellulose nanofibril (CNF) aerogel for adsorption of Rhodamine B and several other dyes. The composite with a 33 wt% ZIF-8 content had a maximum adsorption capacity of 81 mg/g for Rhodamine B. The size of the dye molecules was noted to be larger than the pores of ZIF-8. Therefore, it was suggested that the dye may not be able to enter the pores but the high adsorption efficiency could be due to the strong surface adsorption ability of ZIF-8 crystals. Moreover, the composite selectively adsorbed methyl orange over methyl violet due to the difference in adsorption affinities. 4.2 Catalysis: MOFs are being investigated as heterogeneous catalysts with their metal ions as the active sites and their ligands as structure directing agents to impart high selectivities. MOFs have also been used to heterogenize homogeneous catalysts and to encapsulate molecular catalysts for a wide variety of reactions.9 Aerogels are being developed as supports for a wide variety of catalytically active single and bimetallic nanoparticles such as Pt, Ni, Ru, PtCu and PtPd.81 Therefore, catalysis is an important potential application field for MOFACs as they can be an alternative to aerogel supported nanoparticles. Catalytic degradation of dyes was studied by several groups. Zhang et al.60 tested their ZIF8/C3N4/agar aerogel for dye degradation. While the dye was adsorbed on the surface of ZIF-8, photocatalytic C3N4 sheets regenerated the composite by irradiation of visible light.

The

composite not only removed organic dyes from water but also photocatalytically degraded adsorbed dye Congo Red (CR). Fast adsorption kinetics and a high uptake capacity of 287.35 mg/g were reported and the composites could be regenerated under sunlight. Figure 15 shows the solution contaminated with CR before and after the placement of composite and the picture of the



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composite after being regenerated five times. The composite also showed a maximum adsorption capacity of 154.87 mg/g towards methylene blue (MB).

Figure 15. (a) Photographs of the contaminated CR (50 ppm) aqueous solution before and after adsorption (b) Photograph of the composite with 50.0% ZIF-8/C3N4 with visible light irradiation for 2 h after five regenerations. Reprinted with permission from reference

60.

Copyright 2017 American Chemical Society.

Another study on photocatalytic activity of MOFACs was carried out by Mao et al.38 They investigated ZIF-8/reduced graphene aerogels (rGA) for photocatalytic degradation of MB which followed first-order reaction kinetics. The degradation performance of the composite was remarkably higher than those of pure ZIF-8 and pure graphene aerogel as shown in Figure 16. They also studied the separation of oils and organic solvents from water with ZIF-8/rGA due to its superhydrophobic/oleophilic character. The composites could be regenerated by burning the oil and remained intact as graphene was fire resistant. Moreover, removal of heavy metal ions and benzo pollutants were also studied for water purification application. Metals Pb2+ and Cd2+ were found to have maximum adsorption capacities of 281.5 and 101.1 mg/g, respectively and the composite was also promising for removal of phthalic acid.


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Figure 16. (a) Degradation kinetics of MB by self-degradation, rGA, ZIF-8, and ZIF8/rGA under UV irradiation. (b) Adsorption isotherms for Cd2+ and Pb2+ ions in water on Zn2+/rGA and ZIF-8/rGA, (c) Phthalic acid adsorption capacity of rGA and ZIF-8/rGA with respect to time. Reprinted with permission from reference

38.

Copyright 2017 Royal Society of

Chemistry.

Zhao et al.59 synthesized bimetallic MOF (2Fe/Co)/carbon aerogel and used it as a cathode material in solar photo–electro–Fenton (SPEF) process for wastewater treatment. The carbon aerogel served as a catalyst for H2O2 synthesis via the 2–electron pathway and bimetallic composite had both photocatalytic and electrocatalytic activity with improved generation of •OH radicals to degrade pollutants. The degradation efficiency of two organic pollutants Rhodamine B (RB) and dimethyl phthalate (DMP) was investigated at pH 3 with the initial concentration of 20 mg/L. High catalytic performances were reported such that almost 100% RB was removed in 45 min and 85% DMP was removed in 120 min. Besides photocatalysis, studies on other catalytic uses of MOFACs showed also very promising results. For example, CO oxidation was targeted by Qu et al.56 where Cu-BTC, which has high CO adsorption affinity, was in situ grown on 3D graphene aerogel (GA) containing Ru metal particles. When compared with Ru/GA without Cu-BTC, the catalytic efficiency of the composite was reported to be enhanced almost 48.4% with CO conversion of 19.8% at a weight 43 ACS Paragon Plus Environment


hourly space velocity (WHSV) of 36000 mL/h/g. The enhanced efficiency was reported to be due to the CO adsorption ability of Cu-BTC and the mild pretreatment of the composite at 150 °C for 4 h which tuned the surface chemistry of Ru which in turn resulted in high catalytic activity and durability at room temperature. Shalygin et al.42 used Cu-BTC/silica aerogel composite as a catalyst for isomerization of styrene oxide (SO) to phenyl acetaldehyde. The composite was prepared in pellet form to reduce pressure drop in a continuous packed-bed reactor. The pellets catalyzed the reaction with high selectivity and there were no mass transfer limitations. However, SO conversion was observed to decrease over time due to the formation of carbonaceous deposits on the surface of the composite although the selectivity stayed constant. MOFACs were also used as catalysts for oxidation of environmental pollutants into harmless products. Ren et al.53 synthesized ZIF-9/cellulose aerogel and ZIF-12/cellulose aerogel composites. Their degradation performances were investigated for Rhodamine B (RB), tetracycline hydrochloride (TC) and p-nitrophenol (PNP). Results showed that the composites could effectively degrade 99% of RB in 10 min, 90% of TC in 1 h and 90% of PNP in 1 h at various pH values. They could easily be separated from the solution and reused for at least 3 cycles. Renewable energy related technologies such as metal–air batteries and hydrogen fuel cells, and electrocatalytic or photocatalytic water splitting rely on the electrochemical reactions of oxygen which are either the oxygen reduction reaction (ORR) or oxygen evolution reaction (OER). Pt and Pt-based alloy catalysts are currently considered as the best ORR catalysts. However, the high cost of Pt and the limited supply of such noble metals have severely hindered the widespread commercialization of ORR based devices. To overcome this obstacle, tremendous 44 ACS Paragon Plus Environment

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efforts are being devoted to develop noble metal free ORR catalysts such as oxides, oxynitrides, nitrides, metal-free heteroatom-doped carbon materials, and nitrogen-doped carbon supported transition metal catalysts.82 Using MOFACs or their supported metal oxide derivatives as electrocatalysts is a promising area. Zhu et al.64 used carbon shell encapsulated Fe3C particles derived from Fe-bpdcC3N4/carbon aerogel as a catalyst for ORR and the highest ORR activity was obtained using the composite calcined at 800oC with an initial reduction potential of 1.09 V and a half-wave potential reaching 0.96 V, which was better than the commercial Pt/C catalyst. Xia et al.67 also prepared monodisperse cobalt oxide (CoOx) hollow nanoparticles dispersed in N-doped graphene aerogel (NGA) as Pt-free electrocatalyst and tested its performance. They reported that the electrocatalyst had high stability and excellent activity for the ORR demonstrating the highest value of 32.5 mA/cm2 at 0.750 V, which was 9.2 times higher than that of Co/NC catalyst and 3.3 times higher than that of Pt/C catalyst. Similarly, Liang et al.72 prepared Fe3O4/NGA which was derived from MIL-88B-NH2/NGA composite. The resultant Pt-free electrocatalyst had outstanding ORR activity, long-term durability and methanol tolerance when compared with commercial 20% Pt/C catalyst. 4.3 Energy Storage and Conversion: Hierarchically porous materials have attracted tremendous attention as components of devices for energy conversion and storage. High surface areas and interconnected network of multimodalities are especially useful for shortened diffusion paths and fine dispersion of active sites to maximize reaction rates.3 MOFs, MOF derived structures and aerogels are being studied extensively in the fields of energy storage and conversion.62,63,83 Although direct usage of MOFs in electrochemical devices is limited due to their low thermal and chemical stability and their low 45 ACS Paragon Plus Environment


electrical conductivity, metal/carbon complex nanostructures have shown outstanding electrochemical properties.84 Related to that, studies on MOFACs and MOFAC derived supported metals or metal oxides have gained tremendous importance in energy conversion and storage fields. Studies on energy conversion applications of MOFACs have mainly focused on electrocatalysis for fuel cells and light harvesting for dye-sensitized solar cell (DSSC) applications whereas energy storage applications comprise supercapacitors and batteries. Supercapacitors which can be classified as electrostatic double-layer capacitors (EDLCs) and electrochemical pseudocapacitors are capable to store much more energy than conventional capacitors and offer higher power densities than batteries. They are expected to play a central role in meeting the demands of new energy storage devices and systems both for now and in the future with much longer shelf and cycle life compared to the batteries. In EDLCs, the accumulated energy is based only on the electrostatic attraction between ions and the electrode surface which is charged. The charge in EDLCs is physically adsorbed on the double layer without any reaction. Thus, its performance is dependent on the interface between the surface of the electrode and the electrolyte as well as the simple access of charge carriers to this interface. The addition of transition metal oxides along with carbon in the electrodes to improve the energy density of the EDLCs leads to the pseudocapacitance effect through the electrosorption or redox processes. Many literature studies focused on carbon and graphene aerogels as electrode materials due to their advantageous properties such as superior electrical conductivity, high specific surface area and hierarchical meso-/macroporosity that facilitate high rates of ion and electron transfer.85,86 MOFs and metal oxides derived from MOFs were also studied as electrode materials since their micropores are suitable for storage of large amounts of ions.55 Combining 46 ACS Paragon Plus Environment

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meso-/macropores of aerogels that result in high rates of ion transport with micropores of MOFs is very intriguing for high performance electrodes of supercapacitors. Xia et al.67 investigated Co-MOF/NGA derived nitrogen-rich porous carbon−graphene composite (C/NG-A) as a supercapacitor electrode in three electrode configuration. It displayed high capacitance of 421 F/g at a current density of 1 A/g, which was one of the best gravimetric capacities obtained for nitrogen-doped graphene and 305 F/g at 50 A/g which corresponded to a capacitance retention of 72.5%. Besides, when it was used in an all-solid-state symmetric supercapacitor, 99.7% capacitance retention after 20000 cycles was achieved with an energy density of 33.89 Wh/kg at a power density of 500 W/kg and 24.86 Wh/kg at 25000 W/kg. These results were remarkable when compared to other nitrogen-rich carbon-based symmetric all-solid state cells. Qu et al.68 decorated rGA with Ni-MOF-74 derived NiS nanorods. When tested in a supercapacitor, the electrode displayed high specific capacity, superb rate capability, and excellent cycle life, far better than those of other nickel sulfide-based electrodes ever reported. The enhanced electrochemical performance was attributed mainly to the (101) and (110) active site-enriched edges of the R-NiS/rGA hybrid, which exhibited a strong affinity for OH- in 2 M KOH electrolyte. A hybrid supercapacitor was also constructed by coupling this electrode with the capacitive electrode described above as shown in Figure 17. The hybrid device showed very high energy densities and power densities.



Figure 17. Schematic illustration of the two-electrode hybrid device. Reprinted with permission from reference 68. Copyright 2018 Royal Society of Chemistry.

Wan et al.66 synthesized two different carbon/3D graphene aerogel composites derived from in situ grown ZIF-8 and ZIF-67 on graphene aerogels followed by carbonization and etching. Although both performed better than their without graphene aerogel counterparts, the ZIF-67 derived porous carbon/graphene composite showed higher specific capacitance of 53 F/g at a scan rate of 5 mV/s and 24 F/g at a scan rate of 500 mV/s with a 91% capacitance retention and a 92% cycle stability after 1000 cycles when used as electrodes in all-solid-state supercapacitors. In another study, Fe2O3/rGO composite aerogel was synthesized by annealing Fe-MOF/GO aerogels and tested in three electrode configuration by Xu el al.39 The composite electrode had a specific capacitance of 869.2 F/g at 1 A/g and 289.6 F/g at 20 A/g which was much higher than capacitances of rGO and Fe2O3 electrodes. Performance was constant for 5000 cycles at 20 A/g. The composite electrode had an energy density of 79.2 Wh/kg at power density of 405 W/kg and an energy density of 25.8 Wh/kg at power density of 8010 W/kg. In addition to that, a flexible all-solid-state supercapacitor was fabricated using the Fe2O3/rGO aerogel which had a volumetric capacitance of 250 mF/cm3 at 6.4 mA/cm3 with a capacity retention of 96.3% after 5000 cycles at 50.4 mA/cm3 and an energy density of 0.035 mWh/cm3 at a power density of 0.003 W/cm3 and 0.002 mWh/cm3 at 0.072 W/cm3. Liu et al.55 also investigated their oriented rod-like Fe-based MOF/graphene aerogel composite as electrode materials for supercapacitors. The composite electrodes had high capacitive volume, fast charge/discharge rate, and reliable cycling stability with a specific capacitance of 353 F/g at a scan rate of 20 A/g and a retention ratio of 74.4% after 10000 cycles. The composite electrode achieved an energy density of 36 Wh/kg at a power density of 588 W/kg and a power density of 4300 W/kg at an energy density of 48 ACS Paragon Plus Environment

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27 Wh/kg. The performance of these MOFACs can be compared to current energy storage devices using the Ragone plot which gives regions of specific power as a function of specific energy for fuel cells, batteries, capacitors and supercapacitors. As can be seen in Figure 18, the performances of MOFACs are superior.

Figure 18. Ragone plot for various energy storage devices and various MOFACs. Reproduced with permission from reference 87. Copyright 2000 Elsevier Science Ltd.

Other than supercapacitors, MOFACs were investigated for use in anodes or cathodes of batteries. Metal oxides alone suffer from low electrical conductivity and short life due to the change in volume during charge−discharge as they pulverize or aggregate.69 MOFs can be used as sources of porous metal oxide structures which can be incorporated inside graphene aerogels, thus providing a highly conductive matrix with a hierarchical porous structure and high surface area. Therefore, high ion and electron transport rates, capacities and stabilities can be achieved by 49 ACS Paragon Plus Environment


combining MOFs with graphene aerogels.65,69,70 Moreover, core-shell structures are advantageous not only to enhance electrical conductivity but also inhibit volume expansion and contractions during lithiation/delithiation or sodiation/desodiation processes.70,71 Hierarchically porous Co3O4/nitrogen-doped graphene aerogel (NGA) derived from calcination of ZIF-67/NGA composite was studied by Sui et al.65 as anode material of lithium ion batteries (LIBs). The composite exhibited a capacity of 1030 mAh/g at 100 mA/g, rate performance of 681 mAh/g at 1000 mA/g and good cycling stability with 676 mAh/g at 1000 mA/g after 400 cycles. The good electrochemical performance of the composite was attributed to the conductivity of N-doped graphene sheets, fast lithium reaction kinetics and robustness of graphene that minimize volume change and Co3O4 aggregation. Moreover, small Co3O4 particle size reduced the electrolyte diffusion path and hierarchical porosity led to a good contact between the electrolyte and the electrode. They also prepared ZIF-67/nitrogen-doped graphene aerogel composites via physical mixing and calcination but the electrochemical performances of that composite was not as good as the one prepared by in situ growth demonstrating the importance of synthesis method. Jiang et al.69 synthesized Fe2O3/3D graphene aerogel (3DG) composite by calcining PB/3DG aerogel to be used as anode of lithium ion batteries. The flexible composite achieved a high capacity of 1129 mAh/g at 0.2 A/g after 130 cycles, and cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is noted to be the best reported result for Fe2O3-based anode materials. The high performance of the composite was attributed to its high porosity, multidimensional pathways and porous encapsulated structure of Fe2O3 that shorten ion diffusion, ensure an integrated electron and ion transport and lessen pulverization and aggregation of metal oxide during cycling. They also synthesized Fe7Se8@C/3D graphene 50 ACS Paragon Plus Environment

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aerogel that has a Fe7Se8-enriched inner core and carbon-enriched outer shell encapsulated within a three-dimensional graphene network.70 The electrochemical performance of the flexible composite was much better than FeSex@C alone and exhibited a high reversible capacity of 884.1 mAh/g at 0.2 A/g after 120 cycles and 815.2 mAh/g at 1 A/g after 250 cycles. The same group also synthesized a core–shell FeS@C/3D graphene aerogel by calcining PB/3DG aerogel with sulfur powder to be used as flexible anode of sodium ion batteries.71 The composite displayed a specific capacity of 632 mAh/g after 80 cycles at 100 mA/g and rate capacity of 363.3 mAh/g at 1 A/g and 152.5 mAh/g at 6 A/g with a capacity retention of 97.9% after 300 cycles at 1 A/g which is noted to be the best result reported for FeS-based anode materials. Lastly, solar cell applications should be briefly mentioned as solar cells convert light energy to electricity by photovoltaic effect and photon absorption is an important property of MOFACs for photovoltaic and photocatalytic applications. Ramasubbu et al.34 synthesized CuBTC/titania aerogel composites for this purpose. As titania aerogel absorbs photons in UV region and MOF in visible region, their composite is capable of absorbing more photons from both regions and the absorption in the visible region was found to increase with increasing MOF content. The same group published another article recently using Zn-MOF/titania aerogel composite as a photoanode material for DSSC.77 High surface areas led to increased dye adsorption and enhanced short-circuit current density to 6.22 mA/cm2. An overall maximum power conversion efficiency of 2.34% was achieved. But as electron lifetime and power conversion efficiency was comparably lower than P25-based commercial titania DSSCs, it was noted that the MOF content should be decreased to achieve more control of density of electron trapping sites and insulation properties to obtain a better performance. 5. Outlook and Future Research: 51 ACS Paragon Plus Environment


The first MOFAC was synthesized 5 years ago with the aim to create a hierarchically multimodal nanostructured material with superior properties for gas storage. The field is still in its infancy and the number of different types of MOFACs synthesized so far are very few especially in comparison to the nearly infinite number of MOF and aerogel combinations. Despite this, it has been demonstrated that some of the MOFACs have already superior performance than the pure MOF and pure aerogel making up the MOFAC. Such examples surely indicate the potential of these materials and also that the interest in the field is going to grow in the coming years. This is also evident in the increasing number of publications on MOFACs in the scientific literature. MOFACs have been so far synthesized by direct mixing and in situ methods both of which disperse the MOF in the aerogel matrix. The chemistry in both of these techniques is quite complex and influential in determining the physicochemical and morphological properties of MOFACs. New synthesis techniques are expected to emerge in the coming years. For example, recently the 3D printing technique was applied to form a variety of complex aerogel structures.31 The major challenge for this fabrication strategy was to develop printable inks by tailoring the composition and rheology required for reliable flow through fine nozzles and self-supporting shape integrity after deposition. The 3D printed graphene aerogels were lightweight, highly conductive, had large surface areas and exhibited supercompressibility (up to 90% compressive strain). Moreover, the Young’s moduli of the 3D printed graphene aerogels were higher by a factor of 10 than the bulk graphene materials with comparable geometric density. Incorporation of MOFs into these structures either by direct mixing or by in-situ methods seem like a promising way to enhance the properties of such materials. As discussed in the Introduction section, composites of MOFs with a variety of support materials such as foams and polymeric films have been synthesized and investigated for a wide 52 ACS Paragon Plus Environment

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variety of applications. It would be valuable to compare the properties of MOFACs to those composites at identical compositions and experimental testing conditions. Other than MOFs, metal-organic aerogels (MOAs) that are derived from metal organic gels (MOGs) can be incorporated into organic, inorganic or hybrid aerogels.

MOAs are 3D

architectures which are derived from MOF precursors using supercritical or freeze drying.15 MOAs do not possess the overall regular pore structure and crystallinity that MOFs possess. Rather, they can be considered as irregularly interconnected MOF nanoparticles.88 These materials can be very advantageous for cases where MOF micropores are limiting mass transport and bimodal micro-/macroporosity is essential, especially for storage and catalysis.15 Therefore, composites of MOAs with aerogels may have interesting properties for various applications. Composites of aerogels such as aerogel supported metal or metal oxide nanoparticles, blankets from aerogels and glass fibers and aerogels with polymers are attracting increased attention for a wide variety of applications ranging from sound and thermal insulation to catalysis. Actually, aerogel composites in the form of blankets has been the first commercial scale production of aerogel based materials. Incorporating MOFs to such composites can enhance the properties and functionality of such materials considerably. The synthetic strategies that are described throughout this manuscript can be utilized with slight modification to make such materials. The ability to shape aerogels into various forms brings many advantages that is not possible with other materials. It was recently demonstrated that UV light can be guided by total internal reflection in water filled channels fabricated in monolithic aerogels due to the very low refractive index of aerogels.89 These water filled channels were then used as a photoreactor for degradation of dyes such as methylene blue. Subsequently, monolithic TiO2 aerogel composites were 53 ACS Paragon Plus Environment


synthesized and channels in these monoliths were used as photocatalytic reactors for degradation of phenol in aqueous solution. MOFACs can be very attractive cladding materials with excellent photocatalytic properties for further improvement of aerogel based photocatalytic microreactors. Such reactors may also find applications in sensing, detection and quantification of a wide variety of molecules and photodynamic therapy. Biomedical applications of MOFACs are another area which deserves attention with a lot of potential. A wide variety of MOFs were investigated for drug storage and delivery, imaging, and sensing.11 Although many MOFs are not biocompatible due to their toxic metal sites such as chromium, bioMOFs which are comprised of biomolecules and biocompatible metal cations are quite intriguing in bio-based applications. Promising results were also obtained with aerogels in drug delivery, tissue engineering, implantable devices, wound care, biosensors and diagnostics.26,90,91 Especially polysaccharide aerogels are very advantageous due to their biocompatible and biodegradable nature. Consequently, MOFACs may be highly promising materials for biomedical applications. Finally, current literature on MOFACs is almost all experimental. Computational studies that will provide atomic-level information about the structural and functional properties of MOFACs will be very useful to guide the rational design of new composites with desired properties. The very large number of available MOFs and aerogels provide a great opportunity to design and develop a large number of MOFACs with predetermined properties for many target applications. Even if only a single aerogel is considered, there are thousands of different MOFs that can be used in the composite. Therefore, this very large materials space also brings a challenge in selecting the most appropriate MOF-aerogel combinations. Molecular simulations can be used to screen MOFs based on some structural properties to identify the most suitable 54 ACS Paragon Plus Environment

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materials that will be used in design and development of new MOFACs. This type of simulations can be also used to compute interactions between MOFs and aerogels which can be used to quantify the stability of the MOFACs.

6. Acknowledgement: Authors acknowledge the support of Koç University Tüpraş Energy Center (KUTEM). AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Tel: +90 212 338 1866 ORCID Can Erkey: 0000-0001-6539-7748 Seda Keskin: 0000-0001-5968-0336 Zeynep İnönü: 0000-0002-2715-3299 7. References: (1)

Yang, X.-Y.; Chen, L.-H.; Li, Y.; Rooke, J. C.; Ment Sanchez, C.; Su, B.-L. Hierarchically Porous Materials: Synthesis Strategies and Structure Design. Chem. Soc. Rev. 2017, 46, 481–558.

(2)

Parlett, C. M. A.; Wilson, K.; Lee, A. F. Hierarchical Porous Materials: Catalytic Applications. Chem. Soc. Rev. 2013, 42, 3876–3893.

(3)

Li, Y.; Fu, Z.-Y.; Su, B.-L. Hierarchically Structured Porous Materials for Energy Conversion and Storage. Adv. Funct. Mater. 2012, 22, 4634–4667.

(4)

Sun, M.-H.; Huang, S.-Z.; Chen, L.-H.; Li, Y.; Yang, X.-Y.; Yuan, Z.-Y.; Su, B.-L. Applications of Hierarchically Structured Porous Materials from Energy Storage and Conversion, Catalysis, Photocatalysis, Adsorption, Separation, and Sensing to Biomedicine. Chem. Soc. Rev. 2016, 45, 3479–3563.

(5)

Kistler, S. Coherent Expanded Aerogels. J. Phys. Chem. 1932, 36 (1), 52–64.

(6)

Czaja, A. U.; Trukhan, N.; Müller, U. Industrial Applications of Metal–organic Frameworks. Chem. Soc. Rev. 2009, 38, 1284–1293.

(7)

Yaghi, O. M.; O ’keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Reticular Synthesis and the Design of New Materials.

(8)

Ma, S.; Zhou, H.-C. Gas Storage in Porous Metal–organic Frameworks for Clean Energy Applications. Chem. Commun. 2010, 46, 44–53. 55 ACS Paragon Plus Environment


(9)

Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal–organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450–1459.

(10)

Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal-Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105– 1125.

(11)

Keskin, S.; Kızılel, S. Biomedical Applications of Metal Organic Frameworks. Ind. Eng. Chem. Res 2011, 50, 1799–1812.

(12)

Liang, Z.; Qu, C.; Guo, W.; Zou, R.; Xu, Q. Pristine Metal-Organic Frameworks and Their Composites for Energy Storage and Conversion. Adv. Mater. 2018, 30 (1702891).

(13)

Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science (80-. ). 2013, 341 (6149), 1230444.

(14)

Bueken, B.; Velthoven, N. Van; Willhammar, T.; Ee Stassin, T.; Stassen, I.; Keen, D. A.; Baron, G. V; Denayer, J. F. M.; Ameloot, R.; Bals, S.; et al. Gel-Based Morphological Design of Zirconium Metal–organic Frameworks. Chem. Sci. 2017, 8, 3939–3948.

(15)

Lohe, M. R.; Rose, M.; Kaskel, S. Metal–organic Framework (MOF) Aerogels with High Micro-and Macroporosity. Chem. Commun. 2009, 6056–6058.

(16)

Shekhah, O.; Liu, J.; Fischer, R. A.; Wöll, C. MOF Thin Films: Existing and Future Applications. Chem. Soc. Rev. 2011, 40, 1081–1106.

(17)

Gascon, J.; Kapteijn, F. Metal-Organic Framework Membranes-High Potential, Bright Future? Angew. Chemie - Int. Ed. 2010, 49, 1530–1532.

(18)

Wickenheisser, M.; Janiak, C. Hierarchical Embedding of Micro-Mesoporous MIL101(Cr) in Macroporous Poly(2-Hydroxyethyl Methacrylate) High Internal Phase Emulsions with Monolithic Shape for Vapor Adsorption Applications. Microporous Mesoporous Mater. 2015, 204 (C), 242–250.

(19)

Schwab, M. G.; Senkovska, I.; Rose, M.; Koch, M.; Pahnke, J.; Jonschker, G.; Kaskel, S. MOF@PolyHIPEs. Adv. Eng. Mater. 2008, 10 (12), 1151–1155.

(20)

Pinto, L. M.; Dias, S.; Pires, J. Composite MOF Foams: The Example of UiO66/Polyurethane. ACS Appl. Mater. Interfaces 2013, 5, 2360−2363.

(21)

Li, S. M.; Song, Y.; Li, H.; Li, M.; Li, W.; Yang, Q.; Li, Y.; Gu, Z. Three Dimensional MOF–sponge for Fast Dynamic Adsorption. Phys. Chem. Chem. Phys 2017, 19, 5746.

(22)

Ulker, Z.; Erucar, I.; Keskin, S.; Erkey, C. Novel Nanostructured Composites of Silica Aerogels with a Metal Organic Framework. Microporous Mesoporous Mater. 2013, 170, 352–358.

(23)

Jiang, M.; Li, H.; Zhou, L.; Xing, R.; Zhang, J. Hierarchically Porous Graphene/ZIF‑8 Hybrid Aerogel: Preparation, CO2 Uptake Capacity, and Mechanical Property. ACS Appl. Mater. Interfaces 2018, 10, 827−834.

(24)

Zhu, H.; Yang, X.; Cranston, E. D.; Zhu, S. Flexible and Porous Nanocellulose Aerogels with High Loadings of Metal–Organic-Framework Particles for Separations Applications. 56 ACS Paragon Plus Environment

Page 56 of 63

Page 57 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Adv. Mater. 2016, 28, 7652–7657. (25)

Şahin, İ.; Özbakır, Y.; İnönü, Z.; Ulker, Z.; Erkey, C. Kinetics of Supercritical Drying of Gels. Gels 2018, 4, 3.

(26)

Ulker, Z.; Erkey, C. An Emerging Platform for Drug Delivery: Aerogel Based Systems. J. Control. Release 2014, 177, 51–63.

(27)

Poncelet, D.; Lencki, R.; Beaulieu, C.; Halle, J. P.; Neufeid, R. J.; Fournier, A. Production of Alginate Beads by Emulsification/Internal Gelation. I. Methodology; 1992; Vol. 38.

(28)

Alnaief, M.; Smirnova, I. In Situ Production of Spherical Aerogel Microparticles. J. Supercrit. Fluids 2011, 55, 1118–1123.

(29)

Robitzer, M.; David, L.; Rochas, C.; Di Renzo, F.; Quignard, F. Nanostructure of Calcium Alginate Aerogels Obtained from Multistep Solvent Exchange Route. Langmuir 2008, 24 (21), 12547–12552.

(30)

Karadagli, I.; Schulz, B.; Schestakow, M.; Milow, B.; Gries, T.; Ratke, L. Production of Porous Cellulose Aerogel Fibers by an Extrusion Process. J. Supercrit. Fluids 2015, 106, 105–114.

(31)

Zhu, C.; Han, Y.-J.; Duoss, E. B.; Golobic, A. M.; Kuntz, J. D.; Spadaccini, C. M.; Worsley, M. A. Highly Compressible 3D Periodic Graphene Aerogel Microlattices. Nat. Commun. 2015, No. 6:6962.

(32)

Wickenheisser, M.; Herbst, A.; Tannert, R. E.; Milow, B.; Janiak, C. Hierarchical MOFXerogel Monolith Composites from Embedding MIL-100(Fe,Cr) and MIL-101(Cr) in Resorcinol-Formaldehyde Xerogels for Water Adsorption Applications. Microporous Mesoporous Mater. 2015, 215, 143–153.

(33)

Nuzhdin, A. L.; Shalygin, A. S.; Artiukha, E. A.; Chibiryaev, A. M.; Bukhtiyarova, G. A.; Martyanov, O. N. HKUST-1 Silica Aerogel Composites: Novel Materials for the Separation of Saturated and Unsaturated Hydrocarbons by Conventional Liquid Chromatography. RSC Adv. 2016, 6, 62501–62507.

(34)

Ramasubbu, V.; Alwin, S.; Mothi, E. M.; Sahaya Shajan, X. TiO2 Aerogel-Cu-BTC Metal-Organic Framework Composites for Enhanced Photon Absorption. Mater. Lett. 2017, 197, 236–240.

(35)

Yuan, Y.; Yang, D.; Mei, G.; Hong, X.; Wu, J.; Zheng, J.; Pang, J.; Yan, Z. Preparation of Konjac Glucomannan-Based Zeolitic Imidazolate Framework-8 Composite Aerogels with High Adsorptive Capacity of Ciprofloxacin from Water. Colloids Surfaces A 2018, 544, 187–195.

(36)

Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc 1958, 80 (6), 1339–1339.

(37)

Zhang, X.; Liang, Q.; Han, Q.; Wan, W.; Ding, M. Metal–organic Frameworks@graphene Hybrid Aerogels for Solid-Phase Extraction of Non- Steroidal Anti-Inflammatory Drugs and Selective Enrichment of Proteins. Analyst 2016, 141, 4219–4226.

(38)

Mao, J.; Ge, M.; Huang, J.; Lai, Y.; Lin, C.; Zhang, K.; Meng, K.; Tang, Y. Constructing 57 ACS Paragon Plus Environment


Multifunctional MOF@rGO Hydro-/Aerogels by the Self-Assembly Process for Customized Water Remediation. J. Mater. Chem. A 2017, 5, 11873–11881. (39)

Xu, X.; Shi, W.; Li, P.; Ye, S.; Ye, C.; Ye, H.; Lu, T.; Zheng, A.; Zhu, J.; Xu, L.; et al. Facile Fabrication of Three-Dimensional Graphene and Metal− Organic Framework Composites and Their Derivatives for Flexible All-Solid-State Supercapacitors. Chem. Mater. 2017, 29, 6058–6065.

(40)

Li, G.; Pang, S.; Wu, Y.; Ouyang, J. Enhanced Removal of Hydroquinone by Graphene Aerogel-Zr-MOF with Immobilized Laccase. Chem. Eng. Commun. 2018, 205 (5), 698– 705.

(41)

Zhang, F.; Liu, L.; Tan, X.; Sang, X.; Zhang, J.; Liu, C.; Zhang, B.; Han, B.; Yang, G. Pickering Emulsions Stabilized by a Metal–organic Framework (MOF) and Graphene Oxide (GO) for Producing MOF/GO Composites. Soft Matter 2017, 13, 7365–7370.

(42)

Shalygin, A. S.; Nuzhdin, A. L.; Bukhtiyarova, G. A.; Martyanov, O. N. Preparation of HKUST-1@silica Aerogel Composite for Continuous Flow Catalysis. J. Sol-Gel Sci. Technol. 2017, 84, 446–452.

(43)

Alnaief, M.; Alzaitoun, M. A.; García-González, C. A.; Smirnova, I. Preparation of Biodegradable Nanoporous Microspherical Aerogel Based on Alginate. Carbohydr. Polym. 2011, 84 (3), 1011–1018.

(44)

Burtch, N. C.; Jasuja, H.; Walton, K. S. Water Stability and Adsorption in Metal−Organic Frameworks. Chem. Rev. 2014, 114, 10575–10612.

(45)

Stock, N.; Biswas, S. Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites. Chem. Rev 2012, 112, 933–969.

(46)

Férey, G. Hybrid Porous Solids: Past, Present, Future. Chem. Soc. Rev. 2008, 37, 191–214.

(47)

Rubio-Martinez, M.; Avci-Camur, C.; Thornton, A. W.; Imaz, I.; Maspoch, D.; Hill, M. R. New Synthetic Routes towards MOF Production at Scale. Chem. Soc. Rev 2017, 46, 3453.

(48)

Bosch, M.; Yuan, S.; Rutledge, W.; Zhou, H.-C. Stepwise Synthesis of Metal−Organic Frameworks. Acc. Chem. Res. 2017, 50, 857−865.

(49)

Bosch, M.; Zhang, M.; Zhou, H.-C. Increasing the Stability of Metal-Organic Frameworks. Adv. Chem. 2014, 2014, 1–8.

(50)

Wisser, D.; Wisser, F. M.; Raschke, S.; Klein, N.; Leistner, M.; Grothe, J.; Brunner, E.; Kaskel, S. Biological Chitin-MOF Composites with Hierarchical Pore Systems for AirFiltration Applications. Angew. Chemie - Int. Ed. 2015, 54, 12588–12591.

(51)

Bo, S.; Ren, W.; Lei, C.; Xie, Y.; Cai, Y.; Wang, S.; Gao, J.; Ni, Q.; Yao, J. Flexible and Porous Cellulose Aerogels/Zeolitic Imidazolate Framework (ZIF-8) Hybrids for Adsorption Removal of Cr(IV) from Water. J. Solid State Chem. 2018, 262 (December 2017), 135–141.

(52)

Zhu, L.; Zong, L.; Wu, X.; Li, M.; Wang, H.; You, J.; Li, C. Shapeable Fibrous Aerogels of Metal− Organic-Frameworks Templated with Nanocellulose for Rapid and LargeCapacity Adsorption. ACS Nano 2018, 12 (5), 4462–4468. 58 ACS Paragon Plus Environment

Page 58 of 63

Page 59 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(53)

Ren, W.; Gao, J.; Lei, C.; Xie, Y.; Cai, Y.; Ni, Q.; Yao, J. Recyclable Metal-Organic Framework/Cellulose Aerogels for Activating Peroxymonosulfate to Degrade Organic Pollutants. Chem. Eng. J. 2018, 349, 766–774.

(54)

Zhang, Y.; Cai, J.; Zhang, D.; Ke, X.; Zhang, L. Shaping Metal–organic Framework Materials with a Honeycomb Internal Structure. Chem. Commun. 2018, 54, 3775–3778.

(55)

Liu, L.; Yan, Y.; Cai, Z.; Lin, S.; Hu, X. Growth-Oriented Fe-Based MOFs Synergized with Graphene Aerogels for High-Performance Supercapacitors. Adv. Mater. Interfaces 2018, 1701548, 1701548.

(56)

Qu, J.; Chen, D.; Li, N.; Xu, Q.; Li, H.; He, J.; Lu, J. Engineering 3D Ru/Graphene Aerogel Using Metal–Organic Frameworks: Capture and Highly Efficient Catalytic CO Oxidation at Room Temperature. Small 2018, 1800343, 1–9.

(57)

Yang, Q.; Lu, R.; Ren, S.; Chen, C.; Chen, Z.; Yang, X. Three Dimensional Reduced Graphene Oxide / ZIF-67 Aerogel : Effective Removal Cationic and Anionic Dyes from Water. Chem. Eng. J. 2018.

(58)

Domán, A.; Nagy, B.; Nichele, L. P.; Srankó, D.; Madarász, J.; László, K. Pressure Resistance of Copper- Benzene-1,3,5-Tricarboxylate – Carbon Aerogel Composites. Appl. Surf. Sci. 2018, 434, 1300–1310.

(59)

Zhao, H.; Chen, Y.; Peng, Q.; Wang, Q.; Zhao, G. Catalytic Activity of MOF(2Fe/Co)/Carbon Aerogel for Improving H2O2 and ·OH Generation in Solar Photo– electro–Fenton Process. Appl. Catal. B Environ. 2017, 203, 127–137.

(60)

Zhang, W.; Shi, S.; Zhu, W.; Huang, L.; Yang, C.; Li, S.; Liu, X.; Wang, R.; Hu, N.; Suo, Y.; et al. Agar Aerogel Containing Small-Sized Zeolitic Imidazolate Framework Loaded Carbon Nitride: A Solar-Triggered Regenerable Decontaminant for Convenient and Enhanced Water Purification. ACS Sustain. Chem. Eng. 2017, 5, 9347−9354.

(61)

Reboul, J.; Furukawa, S.; Horike, N.; Tsotsalas, M.; Hirai, K.; Uehara, H.; Kondo, M.; Louvain, N.; Sakata, O.; Kitagawa, S. Mesoscopic Architectures of Porous Coordination Polymers Fabricated by Pseudomorphic Replication. Nat. Mater. 2012, 11, 717–723.

(62)

Xia, W.; Mahmood, A.; Zou, R.; Xu, Q. Metal–organic Frameworks and Their Derived Nanostructures for Electrochemical Energy Storage and Conversion. Energy Environ. Sci 2015, 8, 1837.

(63)

Ülker, Z.; Sanli, D.; Erkey, C. Applications of Aerogels and Their Composites in EnergyRelated Technologies. Supercrit. Fluid Technol. Energy Environ. Appl. 2014, 157–180.

(64)

Zhu, H.; Chen, M.; Li, K.; Huang, X.; Wang, F. Composite Electrocatalyst Derived from Hybrid Nitrogen-Containing Metal Organic Frameworks and g-C3N4 Encapsulated In Situ into Porous Carbon Aerogels. ChemElectroChem 2018, 5, 1–10.

(65)

Sui, Z.-Y.; Zhang, P.-Y.; Xu, M.-Y.; Liu, Y.-W.; Wei, Z.-X.; Han, B.-H. Metal−Organic Framework-Derived Metal Oxide Embedded in Nitrogen-Doped Graphene Network for High-Performance Lithium- Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 43171−43178. 59 ACS Paragon Plus Environment


(66)

Wan, L.; Wei, J.; Liang, Y.; Hu, Y.; Chen, X.; Shamsaei, E.; Ou, R.; Zhang, X.; Wang, H. ZIF-Derived Nitrogen-Doped Carbon/3D Graphene Frameworks for All-Solid-State Supercapacitors. RSC Adv. 2016, 6, 76575–76581.

(67)

Xia, W.; Qu, C.; Liang, Z.; Zhao, B.; Dai, S.; Qiu, B.; Jiao, Y.; Zhang, Q.; Huang, X.; Guo, W.; et al. High-Performance Energy Storage and Conversion Materials Derived from a Single Metal−Organic Framework/Graphene Aerogel Composite. Nano Lett. 2017, 17, 2788−2795.

(68)

Qu, C.; Zhang, L.; Meng, W.; Liang, Z.; Zhu, B.; Dang, D.; Dai, S.; Zhao, B.; Tabassum, H.; Gao, S.; et al. MOF-Derived a-NiS Nanorods on Graphene as an Electrode for HighEnergy-Density Supercapacitors. J. Mater. Chem. A 2018, 6, 4003–4012.

(69)

Jiang, T.; Bu, F.; Feng, X.; Shakir, I.; Hao, G.; Xu, Y. Porous Fe2O3 Nanoframeworks Encapsulated within Three-Dimensional Graphene as High-Performance Flexible Anode for Lithium-Ion Battery. ACS Nano 2017, 11, 5140−5147.

(70)

Jiang, T.; Bu, F.; Liu, B.; Hao, G.; Xu, Y. Fe7Se8@C Core–shell Nanoparticles Encapsulated within a Three-Dimensional Graphene Composite as a High-Performance Flexible Anode for Lithium-Ion Batteries. New J. Chem. 2017, 41, 5121–5124.

(71)

Bu, F.; Xiao, P.; Chen, J.; Aly Aboud, M. F.; Shakir, I.; Xu, Y. Rational Design of ThreeDimensional Graphene Encapsulated Core–shell FeS@carbon Nanocomposite as a Flexible High-Performance Anode for Sodium-Ion Batteries. J. Mater. Chem. A 2018, 6 (6414–6421).

(72)

Liang, Z.; Xia, W.; Qu, C.; Qiu, B.; Tabassum, H.; Gao, S.; Zou, R. Edge-Abundant Porous Fe3O4 Nanoparticles Docking in Nitrogen-Rich Graphene Aerogel as Efficient and Durable Electrocatalyst for Oxygen Reduction. ChemElectroChem 2017, 4, 2442–2447.

(73)

Zhang, L.; Liu, W.; Shi, W.; Xu, X.; Mao, J.; Li, P.; Ye, C.; Yin, R.; Ye, S.; Liu, X.; et al. Boosting Lithium Storage Properties of MOF Derivatives Through a Wet-Spinning Assembled Fiber Strategy. Chem. - A Eur. J. 2018, 1–9.

(74)

Sun, X.; Lv, X.; Sui, M.; Weng, X.; Li, X.; Wang, J. Decorating MOF-Derived Nanoporous Co/C in Chain-Like Polypyrrole (PPy) Aerogel: A Lightweight Material with Excellent Electromagnetic Absorption.

(75)

Yang, Q.; Lu, R.; Ren, S. S.; Chen, C.; Chen, Z.; Yang, X. Three Dimensional Reduced Graphene Oxide/ZIF-67 Aerogel: Effective Removal Cationic and Anionic Dyes from Water. Chem. Eng. J. 2018, 348, 202–211.

(76)

Prabhu, A.; Shoaibi, A. Al; Srinivasakannan, C. Preparation and Characterization of Silica Aerogel-ZIF-8 Hybrid Materials. Mater. Lett. 2015, 146, 43–46.

(77)

Alwin, S.; Ramasubbu, V.; Sahaya Shajan, X. TiO 2 Aerogel–metal Organic Framework Nanocomposite: A New Class of Photoanode Material for Dye-Sensitized Solar Cell Applications. Bull. Mater. Sci 2018, 41:27.

(78)

Anas, M.; Gönel, A. G.; Bozbag, S. E.; Erkey, C. Thermodynamics of Adsorption of Carbon Dioxide on Various Aerogels. J. CO2 Util. 2017, 21 (October 2016), 82–88. 60 ACS Paragon Plus Environment

Page 60 of 63

Page 61 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(79)

Anas, M.; Ünsal, S.; Erkey, C. Investigation of Various Aerogels as Adsorbents for Methane Storage. J. Supercrit. Fluids 2017.

(80)

Peng, Y.; Krungleviciute, V.; Eryazici, I.; Hupp, J. T.; Farha, O. K.; Yildirim, T. Methane Storage in Metal−Organic Frameworks: Current Records, Surprise Findings, and Challenges. J. Am. Chem. Soc 2013, 135, 11887–11894.

(81)

Moreno-Castilla, C.; Maldonado-Hó, F. J. Carbon Aerogels for Catalysis Applications: An Overview. Carbon N. Y. 2005, 43, 455–465.

(82)

Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594.

(83)

Cao, X.; Tan, C.; Sindoro, M.; Zhang, H. Hybrid Micro-/Nano-Structures Derived from Metal-Organic Frameworks: Preparation and Applications in Energy Storage and Conversion. 2660 | Chem. Soc. Rev 2017, 46, 2660.

(84)

Guan, B. Y.; Yu, X. Y.; Wu, H. Bin; Lou, X. W. D. Complex Nanostructures from Materials Based on Metal–Organic Frameworks for Electrochemical Energy Storage and Conversion. Adv. Mater. 2017, 29, 1703614.

(85)

Pröbstle, H.; Wiener, M.; Fricke, J. Carbon Aerogels for Electrochemical Double Layer Capacitors. J. Porous Mater. 2003, 10 (4), 213–222.

(86)

Zhang, X.; Sui, Z.; Xu, B.; Yue, S.; Luo, Y.; Zhan, W.; Liu, B. Mechanically Strong and Highly Conductive Graphene Aerogel and Its Use as Electrodes for Electrochemical Power Sources. J. Mater. Chem. 2011, 21, 6494.

(87)

Kötz, R.; Carlen, M. Principles and Applications of Electrochemical Capacitors; 2000; Vol. 45.

(88)

Li, L.; Xiang, S.; Cao, S.; Zhang, J.; Ouyang, G.; Chen, L.; Su, C.-Y. A Synthetic Route to Ultralight Hierarchically Micro/Mesoporous Al(III)-Carboxylate Metal-Organic Aerogels. Nat. Commun. 2013, 4:1774.

(89)

Özbakır, Y.; Jonáš, A.; Kiraz, A.; Erkey, C. Total Internal Reflection-Based Optofluidic Waveguides Fabricated in Aerogels. J Sol-Gel Sci Technol 2017, 84, 522–534.

(90)

Stergar, J.; Maver, U. Review of Aerogel-Based Materials in Biomedical Applications. J. Sol-Gel Sci. Technol. 2016, 77, 738–752.

(91)

Maleki, H.; Durães, L.; García-González, C. A.; Del Gaudio, P.; Portugal, A.; Mahmoudi, M. Synthesis and Biomedical Applications of Aerogels: Possibilities and Challenges. Adv. Colloid Interface Sci. 2016, 236, 1–27.




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