Synthesis of Metal Organic Frameworks (MOF) and Covalent Organic Frameworks (COF)

Tyagi, Adish; Kolay, Siddhartha

doi:10.1007/978-981-16-1807-9_16

Adish Tyagi^25,26 &
Siddhartha Kolay^25,26

Part of the book series: Indian Institute of Metals Series ((IIMS))

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1 Citations

Abstract

Since the very beginning, nature has demonstrated its ability to create complex systems with advance functions from atomic-level assembly. Gaining inspiration from nature, an enormous progress has been achieved in constructing crystalline porous material frameworks like metal organic framework (MOF) and covalent organic framework (COF) with predetermined topologies, large surface areas, tunable pore sizes and functionalities. They provide many key features required in industrial applications, like high surface area, uniform nanoporosity, interconnected pore/channel system, accessible pore volume, high adsorption capacity and shape/size selectivity. These features make them an ideal material for gas storage and separation (such as H₂, CH₄, CO₂). In fact, having superior crystallinity, porosity and stability relative to other porous materials, MOFs and COFs affirm their candidature for a wide range of applications. This chapter gives a brief background of porous materials and their classification, followed by a discussion on MOFs, COFs and their properties. The subsequent section includes design and synthesis strategies followed by a detailed discussion related to their applications in various frontline areas.

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Designing, Synthesis, and Applications of Covalent Organic Frameworks (COFs) for Diverse Organic Reactions

Synthesis and catalytic applications of metal–organic frameworks: a review on recent literature

Article Open access 05 December 2018

Metal-Organic Frameworks

Keywords

Acronyms used in the chapter

		Organic linker
MOF	Metal organic framework
COF	Covalent organic framework
ZSM-5	Na_nAl_nSi_96-nO₁₉₂.16H₂O (0 < n < 7)
SBU	Structural building unit
DCC	Dynamic covalent chemistry
POP	Porous polymeric framework
MOF-5	[Zn₄O(C₈O₄H₄)₃]	1,4-benzenedicarboxylate
MOF-74	Framework of M²⁺ with 2,5-dihydroxybenzene-1,4-dicarboxylate	2,5-dihydroxybenzene-1,4-dicarboxylate
MOF-177	[Zn₄O(C₂₇H₁₅O₆)₃]	1, 3, 5-tris(4-carboxyphenyl)benzene or benzene tri benzoic acid
HKUST-1	Cu₃O₁₂C₁₈H₆	1,3,5-benzene tricarboxylic acid
ZIF 8	ZnN₄C₈H₁₀	2-methylimidazole
COF-5	C₉H₄BO₂
IRMOF	Metal Organic Framework. with 1,4-benzodicarboxylic acid
IRMOF-3	[Zn₄O(C₈H₅NO₄)₃]	2-aminobenzene-1, 4 dicarboxylic acid
IRMOF-11	[Zn₄O(C₁₈H₁₂O₄)₃]	4, 5, 9, 10-tetrahydropyrene-2, 7- dicarboxylic acid
MIL	Materials Institute Lavoisier
IRMOF-20	[Zn₄O(C₈H₂O₄S₂)₃]	Thieno[3, 2-b]thiophene-2, 5-dicarboxylic acid
MIL-53(Fe)	3D-[M(μ₄-bdc)(μ-OH)]
Mn-BTC	Mn-MOF with benzene-1,3,5-tricarboxylic acid
DAAQ-TFP	COF from 2,6-diaminoanthraquinone (DAAQ) and 1,3,5-triformylphloroglucinol (TFP)	2,6-diaminoanthraquinone and 1,3,5-triformylphloroglucinol
MIL-100(Cr)	[M ₃O(H ₂O) ₂F _0.8(OH) _0.2){C ₆H ₃(CO ₂)₃} 2 H₂O ()]
MIL-101(Cr)	3D-[Cr₃(O)(bdc)₃(F)-( H₂O)₂]
MCM-41	Mobil Composition of Matter No. 41
COF-320	Synthesized from tetra-(4-anilyl)methane and 4,4′-biphenyldialdehyde	tetra-(4-anilyl)methane and 4,4′-biphenyldialdehyde

16.1 Introduction

Meeting ever-growing energy needs for the over growing population demands an enormous amount of sustainable energy source and their storage medium [1]. Moreover, the power generation through burning of fossil fuels causes significant damage to the environment due to the release of a large amount of CO₂ and SO₂ gases in the burning process. This has led to extensive work not only in searching an alternative non-emissive renewable energy sources like solar, wind and wave or other energy production sources like nuclear power, hydrogen power and electrochemical but also focussing on the storage of the energy for their effective use [2,3,4,5,6]. Porous materials have emerged as the material of choice in the area of energy storage since they already find relevance in the field of gas storage for clean energy production like (H₂, CH_4, etc.), water purification, photocatalysis and heterogeneous catalysis [7, 8]. Porous materials are defined as open framework solids. They are also called molecularly engineered materials since their structures can be tuned at molecular level making them adaptable for a wide variety of applications. They are of great technological significance because of their high surface area, tunable pore size and ability to interact with reactants (atoms, ions and molecules) at their surface and bulk. Researchers have designed novel porous materials by gaining insight from nature. Honeycombs with hexagonal pores, hollow bamboo, alveoli in lungs, bones, etc., are few examples of porous structures found in nature. Some common examples of porous materials designed by researchers include activated carbon, mesoporous silica, zeolites, metal organic frameworks, covalent organic frameworks [9,10,11].

The first porous material ever used by the mankind was porous carbon (activated charcoal). It had great significance since ancient times due to its miraculous ability to cleanse and detoxify. Its actual discovery is likely to be long before its first recorded use ~3750 BC for melting and combining metals in ancient Egypt. Around 400 BC, first written record of antibacterial and water purification use of activated charcoal was found. Apart from this, people have also used the decolorizing property of activated charcoal [12]. With the progress of time, new porous materials were discovered and utilized for the benefit of human mankind like porous hydraulic cements, used as an important building material in Roman antiquity [13].

Next big thing in the field of porous materials was the discovery of zeolites minerals (stilbite) by the Swedish mineralogist Axel Fredrik Cronstedt in 1756, which he described as the “boiling stones” that is why the word zeolite originates from the Greek words “zeo (to boil)” and “lithos (stone)” [14]. Zeolites are “tectosilicate” minerals, constructed from interconnected TO₄ tetrahedra (where T stands for Si, Al or B) through corner sharing, to produce three-dimensional frameworks [15]. Nanopores of zeolites have been used for many decades in applications ranging from catalysis to gas separation to ion exchange [16]. Realizing the importance of natural zeolites, researchers put significant efforts to synthesize zeolites in laboratory, with first artificial zeolite synthesized in 1950 by Milton and Breck by reactive gel crystallization method [17]. In 1972, the miraculous ZSM-5, a high-silica zeolite was developed by Argauer and Landolt which found enormous applications in the field of catalysis, [18]. Following this, various other porous silicates and phosphates were discovered and got utilized in various industrial applications. However, the difficulty in pre-designing the structure and pre- and post-functionalization of pore channels, to perform highly specific and cooperative functions limited their sophisticated use and further led to search for new ultra-porous materials [19]. Also, the need to achieve uniform pore size, shape and volume has been realized over the years as it can lead to superior properties. For instance, a distribution of pore sizes would severely limit the ability of the porous solid to separate molecules of differing sizes leading to poor selectivity [20].

Design and synthesis of porous materials with ordered tunable pore size having specific functionality are one of the main challenges for improving their performance in the area of gas storage, energy storage, electronic properties, etc. [21]. With untiring efforts of the scientific community worldwide, the understanding about the structure of ordered porous materials and how to control and adjust them has increased considerably. In the ongoing quest, researchers prepare porous solids which involve the coordination of metal ion centre with the organic linkers. In fact, these inorganic–organic porous materials were known since very long time. Early examples include metal cyanide complexes (Hofmann-type clathrates, Prussian blue-type structures) Werner complexes and open diamond-like framework constructed from copper nitrate complexes. During 1990s, metal–organic porous materials gained renewed interest but their inability to maintain permanent porosity and avoid collapse of frameworks upon guest removal or guest exchange were major drawbacks which limits the application of this class of materials.

These shortcomings were overcome by the pioneering work of Omar M. Yaghi and co-workers [22] employing reticular chemistry which led to the discovery of a new type of inorganic–organic hybrid porous materials with permanent porosity in 1998 and were classified as metal organic frameworks (MOF). The hybrid material consists of Zn²⁺ ions, and 1,4-benzene dicarboxylates [22] were termed as MOF-5. MOFs can be defined as a class of hybrid framework solid materials comprised of organized organic linkers and metal cations. On a fundamental level, MOFs epitomize the beauty of chemical structures obtained by combining organic and inorganic chemistry, two disciplines often regarded as dissimilar. Due to the ultra-high uniform porosity, adjustable pore size, MOFs exhibit improved performance in the areas of catalysis, adsorption and separation [9, 23]. However, their inadequate chemical and thermal stability due to reversible nature of the coordinate bond generated during synthesis were few shortcomings needs to be overcome. The thermo-chemical stability of porous materials can be improved by replacing the weak coordinate bonds with strong covalent bond. However, building crystalline organic framework structure by linking organic building blocks through strong covalent bonds is a challenging task as they often result in the formation of amorphous cross-linked polymer [24].

O. M. Yaghi and co-workers [25] were able to solve this long standing problem by synthesizing the first crystalline covalent organic framework (COF) in 2005 using basic principles of dynamic covalent chemistry (DCC). COFs have received considerable interest in recent times, due to their ability to merge the advantages of both porous materials and polymers. COFs exhibit well-defined porosity, easy processability so much so that some of the COFs can be dissolved in solvent and then processed using solution based techniques without destroying porosity. Lastly, the availability of various synthetic routes for COFs enables to fine tune multiple functionalities into the porous frameworks or at the porous surface [26,27,28,29]. Before proceeding further, it will be essential to classify the porous materials on the basis of pore size and depending upon the building blocks.

16.1.1 Classification of Porous Materials

16.1.1.1 Depending upon the Pore Size

Porous solids are classified by IUPAC into three categories on the basis of their pore sizes: (a) Microporous materials—pore diameter in the range of 2 nm and below, e.g. MOFs, (b) Mesoporous materials—pore diameter in the range of 2–50 nm; e.g. Mesoporous silica and alumina and (c) Macroporous materials having pore diameter more than 50 nm, e.g. metal foams (Fig. 16.1) [20].

16.1.1.2 Depending upon the Building Block Framework

Porous materials can also be classified as (i) purely inorganic, (ii) inorganic–organic hybrid and (iii) purely organic on the basis of the constituted framework material type (Fig. 16.2).

(a)
Inorganic porous frameworks

This class of materials includes zeolites, aluminophosphates, metallosilicates (titanosilicates, germanosilicates) and metal phosphates. Zeolites are crystalline aluminosilicates which consist of interconnected TO₄ tetrahedra (T = Si, Al) via corner sharing with a general formula M_x/n[(AlO₂)_x(SiO₂)_y]·wH₂O (M = Na, Li, K, Ca and Mg), where m is the valency of the metal ion which balances the negative charges on aluminosilicates. The presence of these cations in the frameworks leads to exchange characteristics in zeolites. Generally, zeolites are bronsted acids and their framework contains pore having diameter in the range from 3 to 15 Å [30a]. Variety of materials having different chemical functionality can be realized by substituting Si and Al which are substituted with different elements like P or Fe. However, from applications point of view, high-silica materials are most important due to their exceptionally high thermal and hydrothermal stability under process conditions [30b, c].

Another important material of this class is aluminophosphates (AlPOs) obtained by the replacement of the silicon atoms in zeolites with aluminium and phosphorus. Aluminophosphates are usually formed as neutral, due to the presence of octahedral aluminium sites in the framework. Due to low production costs, high chemical and thermal stability, materials of this class were widely employed as industrial adsorbents and catalyst [20, 31].
(b)
Inorganic–organic hybrid porous frameworks

Lack of functionalization and control over the structural integrity of inorganic materials (activated carbons and zeolites) limits their ability to carry out specialized functions. The development of inorganic–organic hybrid porous frameworks leads to the discovery of novel unprecedented structures which allows the tailoring the functionality of both pores and surface by judicious choice of inorganic and organic parts. The common example of this class includes metal organic frameworks (MOFs). Developed by the three-dimensional crystalline assembly of metal containing units (secondary building blocks (SBUs)) and organic ligands, MOFs possess flexible structure of well-defined pore sizes, surfaces areas [32]. The metallic units are present at the node separated by organic spacers. The metal ions employed in the construction of MOFs include metals from alkaline earth metals, transition metals, p-block elements, rare earth elements (RE = Ln, Y, Sc), actinides and even mix metals [33,34,35]. Since MOFs structure and in turn its property depends on the choice of metal ions and organic spacer, a variety of organic ligands of different shape and size have been utilized in the synthesis of MOFs. The commonly used organic ligands are carboxylates, Schiff bases, imidazolate, phosphates, pyrazine and bipyridine.

Construction of MOFs follows the basic rules of reticular synthesis where there is direct correlation between the structure of reactants (building block), the structure of frameworks and porosity. In principle, reticular synthesis can be considered as a process where judiciously designed primary building blocks are stitched together to form secondary building blocks (SBUs), held together by strong bonding interactions leading to a predetermined ordered structure [36] as shown in Fig. 16.3.
Fig. 16.3
Schematic representation of construction of MOF through primary building blocks
Full size image

The amazing thing about primary building blocks used in the reticular synthesis to generate MOFs is that they maintain their structural integrity throughout the construction process unlike traditional solid-state method where reactants do not maintain their structure during synthesis leading to poor or no correlation between reactants and final frameworks.

The topology of the structures constructed by assembling the SBUs through reticular chemistry can be described by a net, assigned by a three-letter symbol such as pts, rht and soc. The details of nets can be obtained from Reticular Chemistry Structure Database (RCSR) [37] or a computer program TOPOS [38] while new nets can be identified by a mathematical program called SYSTRE [39].

It is important to note that reticular synthesis is different from retro-synthesis as well as from supra-molecular assembly. Unlike retro-synthesis, structural integrity of building block used in reticular synthesis remains intact throughout the process and in contrast to weak interactions responsible for the supra-molecular assemblies, building blocks in reticular synthesis are linked through strong bonds throughout the crystalline network [36, 40, 41]. MOFs can be classified as (i) first generation, (ii) second generation and (iii) third generation depending upon the stability and flexibility of frameworks [42]. The description of different generations of MOFs is discussed in Fig. 16.4. The details of underlying principles involved in the design and synthesis of MOFs are discussed in a separate section.
Fig. 16.4
Classification of MOFs depending upon the nature of framework
Full size image

Due to open framework nature, porosity and tunable functionalities, MOFs find extensive use in gas storage, separation, catalysis, energy storage and various other important applications which will be discussed in a separate section.

Despite the exciting properties, MOFs also suffer from few but serious disadvantages which are as follows:

(i)
MOFs lack sufficient thermal and chemical stability, since they are held by relatively weak coordinate bond [43].
(ii)
Most of the MOFs are sensitive towards the presence of water [44].
(iii)
The porosity of MOFs is difficult to extend to mesoporous region, thus making it inefficient for certain application like storage of bigger sized molecules (e.g. bio-molecules) [44].

(c)
Organic porous frameworks

Porous materials which are completely built out of organic building blocks have always attracted the scientific community. Organic porous materials are exclusively prepared from organic building blocks linked through strong covalent bond. Materials of this class possess some unique advantages like:
1. (i)
  Organic frameworks show high thermal and chemical stability, since they are prepared by connecting organic building blocks through strong covalent bonds.
2. (ii)
  Unlike other frameworks, organic framework materials are made up of only lightweight elements (C, B, O, H, etc.) which appreciably reduce their density and make them suitable for gas storage purpose.

Depending upon the crystallinity of framework, organic framework materials are classified as:

(i)
Porous polymeric framework (POPs): These are class of heavily cross linked microporous polymers synthesized by irreversible C–C coupling reaction like Sonogashira–Hagihara coupling, Suzuki coupling, Friedel–Crafts reaction, acetyl cyclotrimerization, oxidative coupling reaction and phenazine ring fusion reaction [45,46,47,48,49]. POPs are amorphous in nature due to irreversible nature of bond formation mechanism (Fig. 16.5). These materials possess high thermal and chemical stability with high surface area. Examples of this class include CMP-5 and CMP-0 with surface area in the range from 512 to 1018 m²g⁻¹ as reported by Cooper and co-workers [50].
Fig. 16.5
Schematic representations of POPs and COFs by irreversible and reversible reactions, respectively
Full size image
(ii)
Covalent organic frameworks (COFs): Covalent organic frameworks are relatively new class of crystalline organic porous materials generated through reticular chemistry. In simple words, COFs can be explained as the geometric constructs of molecules which are positioned in a specific spatial orientation to enclose space into which reactivity of the atom and molecules are expressed in ways not possible in discrete molecules [51].

G. N. Lewis, a pioneer in the field of chemical bonding, had beautifully explained how atoms combine to form molecules via strong covalent bonds. With advances in the field of synthetic chemistry, researchers have developed “Retro-synthesis” which discusses the principles to synthesize pre-designed molecules from readily available small molecules. With untiring efforts, researchers learnt how to link the small organic molecules by covalent bonds to form macro molecular structures (polymers). Researchers were able to generate well-defined 2D and 3D supra-molecular structures through self-assembly of building blocks which are held together by weak interactions like van der Waals forces, intra- and inter-molecular hydrogen bonding and C···H interactions. However, any modifications in supra-molecular assemblies without destroying the structure are difficult to achieve because [51]:

(i)
Any modification of the constituent building blocks will alter the interaction leading to formation of different assemblies.
(ii)
Any chemistry operation on these assemblies can destroy their structural integrity.

Moreover, supra-molecular assemblies have poor thermo-chemical robustness as constituent building blocks are held by weak chemical interactions. Thus, it was considered essential to develop strategies that assemble molecular building blocks through strong covalent interactions instead of weak interactions. In 2005, Yaghi et al. have successfully demonstrated the formation of organic crystalline frameworks by linking the organic molecules in a precise manner through covalent bonds while maintaining their molecular integrity. They named these frameworks as covalent organic frameworks (COFs) [25]. COFs are mainly constructed by organic reactions which are reversible in nature. The reversible bond formation in COFs synthesis provides error checking and proof reading characteristic to the system. This creates an auto repair mechanism through multiple reversible bond formation cycles leading to the formation of stable COFs as final product [52]. The general approach for the synthesis of COFs can be described in following steps [51]:

(i)
Target network topology is identified which is subsequently deconstructed into its fundamental units.
(ii)
Evaluation of these units according to their connectivity and geometry (e.g. tetrahedral vs square-planar for the connectivity of four).
(iii)
Identification of organic molecules (linkers) called building blocks equivalent to these geometric units.
(iv)
Construction of COF by the formation of strong covalent bonds between the building blocks using the principle of reticular chemistry.

The widely used reactions for COFs synthesis were:

(i)
Boronic acid trimerization [25]
(ii)
Boronate ester formation [52]
(iii)
Nitrile group trimerisation [26, 53]
(iv)
Schiff base reaction [53].

It is worth mentioning that any attempt to synthesize COFs using irreversible reactions always leads to the formation of amorphous porous polymeric frameworks (POPs) because formation of amorphous material using irreversible reaction is thermodynamically favourable. Thus, reversibility during bond formation is an essential criterion for the formation of extended crystalline COFs. Microscopic reversibility can be achieved by controlling the concentration of the by-product (generally water) and pressure.

COFs exhibit high physico-chemical stability and low density due to presence of strong covalent bonds and lightweight constituent atoms. COFs can be pre-designed and modified without destruction of structure. Moreover, their pores can be functionalized and the size can be tuned through proper selection of the building blocks. As a result, COFs open up a new dimension in the material chemistry and are used in the various fields like gas storage, separation, catalysis, photo-conducting materials and sensors.

COFs can be realized in two dimensions (2D) or three dimensions (3D) depending upon the symmetric combinations of organic molecules used for the preparation. 3D COFs are rather rare due to scarcity of higher symmetric organic building units, and most of the COFs reported so far possess 2D structure [25, 53, 54]. In 2D COFs, the covalently bound framework proceeds in horizontal direction and thus exhibits homogeneous porosity in their extended 2D sheets. Stacking of these 2D sheets occurs via π-π interaction in eclipsed conformation which results in the formation of ordered columnar channels. Such channels in 2D COFs could facilitate charge carrier transport within the column, which implies that 2D COFs have potential for wide range of applications in fields such as sensing, separation, storage and catalysis. In contrast, 3D COFs possess extended three-dimensional framework which could led to high specific surface areas comparable to MOFs and are likely ideal candidates for gas storage and separation applications.

Despite such promising properties, real-life applications of COFs are yet to be realized due to the issues related to their chemical stability and scalability. Moreover, in the presence of moisture COFs generally get hydrolyzed to the starting materials. This is because COFs are generally formed by condensation reaction where water is excluded as by-product and its presence leads to reversible back reaction causing hydrolysis of COFs.

The following sections will discuss the design and synthesis strategy, synthetic protocols and applications of MOFs and COFs.

16.2 Design and Synthesis Strategy

The performance of the porous materials depends to a great extent on the proper design and synthesis of the materials with well-defined pore size and controlled functionality. In a broader view, MOFs may be considered as a combination of two central components: connectors and linkers [55, 56]. These two components are considered as starting reagents with which the principle framework of the MOFs is constructed. Apart from them, there are other auxiliary components, such as blocking ligands, counter anions, and non-bonding guests or template molecules. Figures 16.7 and 16.8 give an overview of the different types of connectors and linkers and their combination in the formation of different geometries of the MOFs.

The stability of the frameworks plays a very important role in the successful synthesis of frameworks. The coordination numbers as well as the coordination geometries of the metal ions are two very crucial parameters in deciding the stability of the framework. The nature of the metal ions and its oxidation state has a strong influence on the shape of the pore. Thus, depending on the nature of metal ions and its oxidation state various geometries like linear, T-shaped, Y-shaped, tetrahedral, square-planar, square-pyramidal, trigonal–bipyramidal, octahedral, trigonal-prismatic, and pentagonal-bipyramidal (Fig. 16.6) are possible.

Transition metal ions are commonly used as versatile connectors in the construction of MOFs. Linkers afford a wide variety of linking sites with tuned binding strength and directionality. The role of connectors and linkers in the formation of stable framework is well-explained by Kitagawa et al. [55] and Zhou et al. [56].

Depending upon the charge on the linker molecules, linkers are broadly classified into four categories: (a) inorganic ligands (e.g. halides, CN⁻, SCN⁻, cyanometallate ([M(CN)_x]ⁿ⁻), (b) neutral organic ligands like pyridine (py), 4,4̍-bipyridine (bpy), (c) anionic organic ligands like various carboxylates with suitable spacer and (d) cationic organic ligands like poly pyridinium ions. Figure 16.7 gives a schematic representation of the various types of linkers. The suitable combination of connector(s) and linker(s) gave different structural motif.

The nature of bonding and overall charge plays a very important role in deciding the stability of frameworks [55]. Overall framework has to be neutral. Since in MOFs, most of the connectors are cationic metal ions, the positive charge of the metal ions got balanced by negatively charged linkers such as carboxylates. Though the above-mentioned four types of linkers are used in metal–organic framework synthesis, it has been observed that organic ligands with carboxylate functional groups with suitable spacer (type C) are most commonly used in MOFs synthesis. It is also seen that apart from the anionic linkers some inorganic anions, like BF₄⁻, NO₃⁻, PF₆⁻, SiF₆⁻ and N₃⁻, are also used in MOFs synthesis as a counter-anion of the metal salts, and these inorganic anions exist either as free guests or as counter-ions mostly to neutralize the cationic connector. Besides charge neutralization they also help in increasing the stability of the frameworks and modify the channel shape through hydrogen bonding with their O and F atoms.

In general, the bonding interaction in MOFs can be divided into three classes (Fig. 16.8), namely (a) coordination bond (CB) where a pure coordination bonding interaction takes place between the connectors and the linkers with donation of electron pair from linker to the connector, (b) combination of coordination bond and hydrogen bond (CB + HB): both coordinate and H-bonding are present in the framework. The presence of H-bonding in addition to coordination bonds imparts additional stability in the framework compare to coordinate bonding only, (c) coordination bonding along with other interaction like d_π-p_π, d_π-d_π or δ-bonding.

Sometimes, the mutual orientation of the two aromatic rings may undergo π-π interaction or the π-cloud of one aromatic ring interacts with CH-moieties of other unit. It has been observed that 1D and 2D motifs often aggregate through these additional weak bonds to give 3D frameworks. This type of weak interaction plays a very important role in the interaction of the guest molecules with the motifs.

Unlike MOFs, in covalent organic frameworks (COFs) the symmetry of the building blocks governs the topological structures of the pores [26]. For example, COFs with hexagonal pores can be generated by the combination of a C3-symmetrical building blocks or a C3- and C2-symmetrical building block, while the combination of C4- and C2- or C6- and C2-symmetrical building blocks results tetragonal and trigonal pores, respectively. Figure 16.9 represents the formation of various topological structures of the pores in combination of various building blocks.

In COFs, there are no metal ions, and hence, the possibility of frameworks extension via the coordination bonding of the organic ligands with the metal ions which is one of the main modes of interaction in MOFs is not possible [52]. Thus, in COFs the extension of the frameworks takes place via various condensation reactions. Depending upon the type of functional groups present in the building blocks, various condensation reactions such as boronic acid trimerization, boronate ester formation, Schiff base reaction and nitrile trimerization are reported for the construction of COFs (Figs. 16.10 and 16.11).

Most of the afore-mentioned chemical reactions, being kinetically irreversible in nature lead to the formation of polymeric compound instead of crystalline framework material [25, 26]. COFs are highly porous crystalline and stable materials which can be prepared by reversible cross-linking of rigid organic building blocks. In this aspect, concept of dynamic covalent chemistry (DCC) pertaining to reversible bond formation (reticular chemistry) find relevance for the synthesis of crystalline COFs. Contrary to conventional covalent bond formation, DCC regulates the thermodynamic equilibrium during bond formation via self-correction and thus leading to the formation of the most thermodynamically stable crystalline structures. Figure 16.12, gives the various steps involved in the formation of crystalline COFs.

However, reversibility is not the sole factor that can assure the long range ordering. The topology of the building blocks has important role to play in deciding the crystallinity of COFs. The symmetry of building blocks meets the requirement of constructing the regular pores while DCC accounts for the reversible formation of covalent bonds through self-correction. Thus, choice of building block should be in accordance with the reactivity of the functional group that triggers dynamic covalent bond formation. In addition, different types of organic linkers on basis of symmetry should be selected so that the topology of the pore of the framework materials can be controlled. For instance, Schiff base condensation reaction is the more profuse chemistry used for the synthesis of imine-based COFs. This is attributed to the control provided over bond formation, breaking and reformation of bond which ultimately facilitate crystallization process in COFs formation [57].

Apart from DCC strategy and topology of building blocks, several other factors such as reaction conditions, temperature, amount of the reactants, water and catalyst amount influence the reaction. These important parameters needed to be concerned during synthesis of thermodynamically stable highly porous and crystalline COFs. Moreover, the solvent combination is also vital parameter to control the porosity and crystallinity of the COFs. To sum up, there are handful reversible reactions available that fulfils the criteria for the formation of thermodynamically stable crystalline architectures.

16.2.1 Synthetic Methods

As already mentioned, the important as well as the crucial step in MOFs/COFs synthesis is to find out the suitable conditions that lead to the formation of building blocks without decomposition of the organic linker. Shape and size of the channel of the frameworks depend to a large extent on the synthetic methods employed. Figure 16.13 gives an overview of the different synthetic methods conventionally used for MOFs synthesis.

The parameters affecting the framework synthesis can be divided mainly into two categories: compositional parameters (molar ratios of starting materials, pH, solvent, etc.) and process parameters (reaction time, temperature, pressure, etc.). Temperature is one of the main parameters in the preparation of porous frameworks. Various synthetic routes like solvothermal, non-solvothermal, microwave heating, electrochemistry, mechanochemical and sonochemistry (Fig. 16.14) are frequently used. A brief knowledge about the various synthetic routs is mentioned below.

16.2.1.1 Solvothermal Synthesis

This is one of the highly popular and commonly used techniques for the synthesis of porous crystalline frameworks. Here the reaction takes place in close vessels under autogeneous pressure above the boiling point of the solvent. Generally, this is a comparatively low temperature and high pressure reaction and the reaction is carried out in auto-clave. The method involves mixing of the reactants in a single solvent or combination of solvents in different ratios. The solvent or mixture of solvents of different polarity has been identified as one of the important parameters directly related to induce crystallinity in the frameworks [59,60,61]. The organic precursor employed is usually soluble in organic solvents which are one of the reasons to make this method quite popular in synthesis of highly porous crystalline frameworks. This synthetic route is widely used in the synthesis of porous MOFs and COFs. Since COFs are mainly prepared by condensation reactions, water is generated as side product and occupies the space above the reaction mixture. The water further cools down and again mixed with the organic solvent which the system can utilize during the process of self-healing and thus governs the reversibility of the reaction. Though the technique is most widely used one, the major drawbacks are long reaction time (several hours), temperature and solvent selection.

16.2.1.2 Non-solvothermal Synthesis

The non-solvothermal reaction can further be classified in two categories: (1) taking place at room temperature and (2) taking place at elevated temperatures. The synthesis at low temperature takes place either by precipitation followed by recrystallization or by slow evaporation of the solvent. The methods are well known to grow simple molecular or ionic crystals because of the possibility of tuning the reaction conditions and hence the nucleation rate and crystal growth. For crystal growth, concentration of reactants plays an important role. Reactants concentration has to be optimized such that it should not exceed critical nucleation concentration, and it can be done by varying temperature or more commonly by slow evaporation of solvent. Other methods routinely employed to obtain crystalline MOFs are layering of solutions or slow diffusion of reactants. Some prominent MOFs obtained at room temperature by just mixing the starting materials are MOF-5, MOF-74, MOF-177, HKUST-1 or ZIF-8. There is also an increasing interest in creating protocols for COFs synthesis under mild conditions such as room temperature and hence improve COFs processability on surfaces which is necessary for their use in practical applications. Bein et al. [62] have synthesized thin films of COF-5 and BTD-COF via vapour-assisted conversion method. This method involves the drop casting of the boronic acid precursors on glass slide, followed by incubation in a desiccators containing mesitylene–dioxane solvent mixture (1:1) in a separate glass vials. With time, the solvent vapours diffuse out slowly from the glass vials. As a result, the solvent molecule gets in contact with the reactant molecules drop casted over the glass substrate and COF thin films get formed on the glass slide. Due to slow diffusion kinetics of the solvent vapours, the COF crystallite formation reaction was complete in 72 h. Top view scanning electron microscopy (SEM) reveals continuous coverage of inter-grown particles on the substrate. Surface area of the synthesized COF thin film is commensurate to that of its solvothermal counterpart. On a similar ground, Ballesté et al. [63] have established micro-fluid-based synthetic method where the reaction between constituent building blocks leading to imine-based COFs takes place under controlled diffusion conditions at room temperature. This method involves the mixing of droplets of reactants and acetic acid injected through the separate nozzles inside the channel. COFs will get formed within few seconds and can be collected simultaneously from the outlet. Material obtained using this method is observed to have crystalline fibre like morphology. The dynamic nature of the protocol allows the direct printing of the COFs onto the surfaces. Variation of the reaction temperature has a strong influence on the product formation, crystallinity, reaction rates as well as morphology of crystals. A prolonged reaction time may lead to degradation of the framework.

16.2.1.3 Microwave-Assisted Synthesis

Microwaves (MW) are also used in the synthesis of porous frameworks [64]. This route is considered as more efficient than other conventional routes for small-scale synthesis because of less reaction time, clean products and high yield. When a sample (solid/polar solvent) is subjected to microwave, an electric current is generated due to the mobile electron or ions. In solid sample, this electric current leads to the heating of the sample due to the resistance of the solid sample. In case of solution, the presence of polar solvent and hence the polar molecules orient themselves according to the electromagnetic field. Since under microwave an oscillating field is produced, the molecules change their orientation accordingly. Thus by applying suitable frequency, collision between the molecules takes place. As a result, the kinetic energy of the molecule and hence the temperature of the sample increase. Due to the direct interaction of the radiation with the sample, MW-heating presents a very high energy-efficient method of heating. Cooper et al. [65] have first introduced the microwave heating method for rapid synthesis of crystalline porous frameworks, and the framework was purified easily with few washing steps only. At present, different synthetic protocols are under development for utilizing the microwave heating for synthesis of framework materials on bulk scale. The bottle neck of this technique is the choice of appropriate solvents and selective energy input.

16.2.1.4 Mechanochemical Synthesis

Mechanical grinding has been identified as another efficient methodology where mechanical force is used for the preparation of frameworks at room temperature [66]. Mechanical force helps in breaking the intra-molecular bonds and forming new chemical bonds. The method can be used in a solvent free condition and hence is environment friendly and is taking place at room temperature. The first example of such synthesis was reported in 2006 [67] where three-dimensional microporous [Cu(INA)₂] (INA = isonicotinic acid) was obtained by grinding mixture of copper acetate and isonicotinic acid for 10 min. In contrast to conventional synthesis, here metal salts can be replaced by metal oxides as starting material and in that case water is formed as the only side product. One such example is the synthesis of pillared-layered MOFs using ZnO. The success of the technique depends on various parameters like counterpart of the metal ions, ligands basicity and ligands melting points. Figure 16.14 gives an overview of the various favourable conditions for this approach.

From Fig. 16.14, it is seen that ligands having low melting point and metal salt with basic counterpart like acetate, combination of reactants where internal solvent like acetic acid, water of crystallization will be released during the course of the reaction favour the success of the technique. Ligand with high melting point and metal ions with less basic counter-ions like nitrate and the combination of ligand and metal system where no solvent will be released during the milling, the chance of the reaction decreases. The technique is also used for the synthesis of COFs. In a typical synthesis, the reactants are mixed and ground in mortar–pestle in the presence of very small amount of solvent, typically water. The paste so obtained is further subjected to high temperature (90–120 °C) typically for 12–24 h followed by several washing with suitable solvent in order to obtain highly porous and crystalline framework materials. The use of mechanical force for the synthesis of highly crystalline and porous imine-based COFs was reported by Banerjee et al. [66]. Further, they have demonstrated the synthesis of COFs in bulk scale as well as the processability of COFs in different shape and size without compromising the porosity and crystallinity of the materials. One of the major limitations with this methodology is the exfoliation of the frameworks into sheets during the grinding process which ultimately affect the porosity and crystallinity of the frameworks.

16.2.1.5 Sonochemical Synthesis

Sonochemical technique is also used for the synthesis of MOFs. This is a fast, energy-efficient, environment friendly, room temperature method. A fundamental principle of sonochemistry and its use for the synthesis of nanomaterials is well-documented in the literatures [68, 69]. Here a high energy ultrasound with a frequency between 20 kHz and 10 MHz is applied to the reaction mixture.

16.2.1.6 Electrochemical Synthesis

This is also well-established and extensively used technique for the synthesis of MOFs with the exclusion of anions, such as nitrate, perchlorate or chloride. In this process rather than using metal salts, metal ions are continuously introduced through anodic dissolution to the reaction medium containing dissolved linker molecules and a conducting salt. This technique was successfully utilized in the synthesis of Zn₂⁺ , Cu₂⁺ and Al₃⁺-based MOFs by Gascon et al. [70].

16.2.1.7 Ionothermal Synthesis

In this method, the ionic liquids simultaneously serve the purpose of both solvent and template or structure directing agent in the formation of solids [71]. This strategy is well known to prepare COFs containing triazine core with high porosities and surface areas by trimerization reaction of simple, cheap and abundant aromatic nitriles [72]. The triazine-based materials synthesized by this methodology are often observed to have nearly same properties as that of zeolites and metal organic framework (MOFs). Typically, it is high-temperature reaction where solid reactant mixed with ZnCl₂ was taken in a quartz tube followed by heating at 400 °C to afford the COFs. The molten ZnCl₂ acts as a catalyst for trimerization reaction which is otherwise reversible at this temperature. Aromatic nitriles show good solubility in this ionic melt due to strong Lewis acid base interactions. The trimerization reaction can be monitored simply by recording FTIR spectra of the reaction mixture at different reaction times and temperature. The disappearance of strong absorption of carbonitrile band at 2218 cm⁻¹ and appearance of intense band at 1352 cm⁻¹ corresponding to the formation of triazine ring are indicative of completion of trimerization reaction.

16.2.1.8 Synthesis of Mono Layers on Surface

Owing to their robust structures, accessible functionalization sites, tunable optical and electronic properties, the thin layer of self-organized molecular networks (2D) is ideally suited for many high end applications [54, 61(a),73]. However, the growth of thin film on desirable substrate is highly challenging task as the frameworks are inherently insoluble and thus inhibit the processability. Being unprocessable powders, the porous frameworks cannot be interfaced to electrodes or fabricated into device form unless there is a way of synthesis of oriented 2D layered framework on desirable surface. The thin layer of the porous framework could provide better opportunity of understanding the structural details. Porte et al. [74] have reported the first example of surface covalent organic frameworks (SCOFs) by growing SCOF-1 (dehydration of 1,4-benzenediboronic acid (BDBA) with boronic acid) and SCOF-2 (condensation of BDBA with 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP)) on the clean Ag(111) surface. Later, Dichtel et al. [74c] have demonstrated a simple solvothermal condensation method where oriented COFs thin films were directly grown over single layer graphene (SLG) supported over various substrates. The thickness, crystallinity and morphology of the film were controlled with the reaction time. In due course, Wan et al. [75] have emerged with an innovative idea where highly ordered surface COFs layer was fabricated over 2D materials. To achieve this, they introduced a small amount of CuSO₄.5H₂O as a water regulator which further acts as an equilibrium manipulating agent in a dehydration reaction of BDBA into a closed system. The product obtained was highly ordered molecular network which otherwise known to form disordered network at high temperature [74a, 76]. The reversible release of water from CuSO₄.5H₂O during the heating cooling cycles controls the reversibility of the reaction, thus improving the crystallinity of surface covalent organic framework (SCOFs) drastically.

16.2.1.9 Interfacial Synthesis

Interfacial growth of COFs thin-film methodology is a unique approach. Adopting this methodology, a self-standing 2D COFs membrane can be easily produced. The idea behind this interfacial synthesis is reaction at the interface formed by two different immiscible solvent containing respective reactants. Likewise other methods, the thin films obtained are not defect-free but this limitation can be minimized through varying the different parameters such as concentration of the reactants and temperature. Recently, Dey et al. [77] have demonstrated the synthesis of highly crystalline and porous thin films of COFs via interfacial synthetic method.

16.2.1.9.1 Synthesis via Precursor Approach

Apart from the above-mentioned various techniques, precursor approach is also extensively used for MOFs preparation. In this approach, pre-built polynuclear coordination complexes having structure and functions similar to or identical with the inorganic bricks are used and the ligands were substituted with new ligands to form the desired frameworks. The steps involved in the process may well be understood with the below-mentioned example (Fig. 16.15), where [Zr₆] methacrylate oxocluster was used as the precursor and the new framework was synthesized by replacing monocarboxylate by dicarboxylate [78]. Another way of such synthesis is the replacement of coordinating solvent with linker. [Cu₂(CDC)₂(DMA)(EtOH)]₆(CDC = 9H-carbazole-3,6-dicarboxylate; DMA = N, N-dimethyl acetamide) has two axial ethanol molecule per copper centre. This axial ethanol molecule may be replaced by some other linkers. Figure 16.16 represents one such example where new MOFs are formed by replacing the axial ethanol [79]. The direct syntheses of these MOFs are rather difficult.

16.2.2 Methods for Post-Synthetic Functionalization of MOFs

In general, the presence of functional group dictates the application of the framework. The direct synthesis of MOFs with functionalized organic linkers is limited because of the direct coordination of the functional group with the metal centre and hence prevents the formation of the frameworks. Moreover, MOFs contain organic linkers, the frameworks can be functionalized even after their formation, and the method is known as post-synthetic modification (PSM). There are a variety of ways by which the framework can be functionalized. Based on the nature of the bond formed or broken during PSM, the process is broadly classified into three categories, namely (A) covalent modification; (B) dative modification and (C) post-synthetic de-protection as discussed below.

In covalent modification, a new covalent bond is formed (Fig. 16.17). This method of PSM is the most powerful and versatile ways to introduce a large number of functional group. One such example is schematically shown in Fig. 16.19 [80] where acetylation takes place due to the reaction of acetic anhydride with IRMOF-3 via the breakage of N–H bond and formation of new N–C bond.

In dative PSM, a new metal–ligand coordination boned is formed (Fig. 16.18). Therefore, in dative PSM, either a ligand is added to the framework that coordinate to the SBU of the MOF or a metal source is added which then bound to the organic linker of the MOF via dative bonds.

Examples of dative PSM are given in Fig. 16.20. In both the cases, on heating the frameworks under vacuum, the axially coordinated solvents (DMF and H₂O) could be removed and on subsequent treatment of the coordinatively unsaturated frameworks with pyridine derivatives new frameworks could be formed [82, 83]. It has been observed that the direct synthesis by solvothermal technique was not as fruitful as by dative PSM due to a significant loss of pyridine during synthesis in case of Zn(II) and not reactivity of pyridine in case of KHUST-1.

Figure 16.19 represents one example where both covalent and dative post-synthetic modification take place. The first step represents the covalent modification, and the second step represents the dative modification.

Sometimes during framework synthesis functional group needs to be protected by converting it into some suitable derivative so that it does not coordinate the metal centre during synthesis. After framework formation, the deprotection of the functional group is carried out and the modification is known as post-synthetic deprotection modification.

In principle, both the covalent or dative bonds can be broken during post-synthetic deprotection. Figure 16.20 reflects one of the serendipitous finding by Yamada and Kitagawa, where in-situ deprotection of an organic linker was observed, resulting in a functionalized MOF [84]. The protection of hydroxyl group was carried out via acetylation of 2,5-dihydroxyterephthalic acid with acetic anhydride and deprotection takes place during MOF formation.

16.2.3 Activation

The final step before utilizing the porosity of MOFs is the removal of guest molecules from the pore keeping the structural integrity and porosity. The guest molecules may be solvents or any other chemicals used during synthesis. The process is known as “activation”. High boiling solvents like N, N-dimethylformamide (DMF), N, N-diethylformamide (DEF) or dimethyl sulfoxide (DMSO) are conventionally used in MOFs synthesis. These high boiling solvents generate high capillary force and high surface tension during activation. This high surface tension leads to the partial or full collapse of the frameworks in many cases during activation. In many cases, it has been observed that the measured surface area is less than the one calculated from single crystal X-ray data and is due to incomplete activation. Therefore, the activation is a very crucial step in MOFs chemistry. The following four techniques are the most common and widely used strategies for MOFs activation: (a) conventional heating under vacuum; (b) solvent exchange; (c) supercritical CO₂ (scCO₂) processing; and (d) freeze-drying [85]. Figure 16.21 illustrates the various physical phenomena occured during these steps.

Conventional activation: Activation by heating under vacuum which is similar to the technique used for activation of zeolites and carbons is the simplest technique. The essential criteria for applying this technique are that the frameworks have to be thermally stable. Though this strategy for activation has been successfully applied in some cases like the activation of Cr-MIL-101 [86] and UIO-66 [87], this strategy finds minimal utility for accessing the full porosity of many MOFs. It has been observed that in many cases the framework losses the crystallinity and porosity upon activation by this technique. This observation can be well-understood based on the diagram of Fig. 16.21. As the framework passes through liquid-to-gas phase, a significant amount of surface tension and capillary forces generates and this force breaks the weak coordination bonds resulting the loss of crystallinity and porosity.

Activation by solvent exchange: The most commonly used activation technique is the activation by solvent exchange. Normally during synthesis of MOFs, higher boiling solvents like DMF and DEF are used and remain trapped in the pores. These higher boiling solvents are replaced with lower boiling solvents like acetone and methanol which are removed by mild heating under vacuum. One such example of showing activation by solvent exchange is the activation of MOF-5. The framework was synthesized in DMF–chlorobenzene mixture. The framework could be activated by exchanging DMF–chlorobenzene with comparatively low boiling CHCl₃. The importance of the selection of suitable activation technique is reflected in the activation of IRMOFs as shown by Nelson et al. [88]. When conventional activation technique was employed, IRMOF-3 showed a BET surface area of 10 m²g⁻¹ while no N₂ uptake was observed for IRMOF-16. When DMF was exchange with CHCl_3, the surface areas were increased to 1800 m²g⁻¹ for IRMOF-3 and 470 m²g⁻¹ for IRMOF-16.

Activation by supercritical CO₂ (scCO₂) processing: Activation by supercritical CO₂ is the relatively new strategy for activation of MOFs. In this process, solvents which are miscible with liquid CO₂ are exchanged with liquid CO₂ at high pressure (>73 atm). Then the sample is brought above the supercritical temperature of CO₂ (i.e. 31 °C) so that the framework gets occupied with supercritical CO₂. Finally, the supercritical CO₂ apparatus is slowly vented while holding the temperature above the critical point. As a result of which the system is transforming directly from the supercritical phase to the gas phase without liquid-to-gas phase transition and hence avoiding the capillary forces.

Activation by freeze-drying: Another newly developed method for MOFs activation is the activation by benzene freeze-drying. Here first the solvent is exchanged with benzene and left in benzene. The MOF is then frozen to 0 °C and brought back to room temperature. The procedure is repeated several times. Upon the final freeze cycle, the MOF is placed under vacuum at a temperature and pressure below the solvent’s triple point. Since in the final step the sample is warmed under reduced pressure, benzene directly sublimes which is a direct solid-to-gas phase transition and hence avoids the liquid-to-gas phase transition and hence associated capillary forces. Recent advances show that instead of benzene which is carcinogenic, cyclohexane also can be used.

16.3 Application of MOFs and COFs

MOFs and COFs possess some desirable properties like inherent porosity, tunable pore size, large surface area and ordered channel structure, low density, relatively high thermo-chemical stability, and designable functionality affirms the candidature of these materials for wide range of application (Fig. 16.22) like gas storage and separation, heterogeneous catalysis, optoelectronics, energy storage, chemical sensing and drug delivery. This section presents a detail discussion of some selected application of MOFs and COFs.

16.3.1 Gas Storage Application

Gases like H₂ and CH₄ are important from view point of energy production. Thus, medium for their effective storage is considered as an essential requirement. Various options are available for storing gases but these often require high pressure and multi-stage compressors. Moreover, these options are not viable from economic point of view. Thus, there is a need to substitute these materials by simple, easy to handle and cheaper options. In this regard, porous framework materials especially MOFs and COFs are preferable choice because of their easy synthetic procedures, high surface area, well-defined pores and channels with structural and compositional tunability. This section deals with the utility of MOFs and COFs for storage application of various important gases from energy and environment point of view with brief discussion over the factors responsible for effective storage.

16.3.1.1 Hydrogen Storage

To reduce the dependence upon the fossil fuels, there is an ongoing quest for clean energy sources with high energy density. Hydrogen gas (H₂) upon combustion produces water along with enormous amount of energy due to its high calorific value (120 MJ Kg⁻¹) and thus considered as clean and ideal source of energy. However, low volumetric storage capacity of H₂ due to its low density in gaseous state (0.08 kg m⁻³) hinders its use as an efficient fuel. Thus, effective storage materials need to be investigated. The goal set by US Department of Energy (DOE) for hydrogen storage is 1.5 kg of H₂/Kg in gravimetric capacity and 1.0 kg of H₂/L of volumetric capacity at operating temperature of 233–333 K with a pressure of 100 atm by the year 2020 [89].

Several favourable characteristics of MOF like large surface areas, functionalized polar groups, open metal centres facilitate the storage of H₂ in it. Many MOFs have been tried so far for H₂ uptake capacity, and results showed that MOF possess the required characteristics to act as H₂ uptake material and one of the best options to meet the targets set by US department of Energy in near future. For example, MOF-177 derived from zinc acetate unit [Zn₄O(CO₂)₆] and the tri-topic linker 4,4′,4′′- benzene-1,3,5-triyltribenzoate (BTB) to form a (6, 3) net exhibit remarkable H₂ uptake on a gravimetric basis (7.5 wt%). The performance of MOF-177 for H₂ uptake is attributed to its large surface area (5000 m²g⁻¹) [90]. Similarly, IRMOF-20 derived from same metallic SBU and linear di-topic link thieno[3,2-b]thiophene-2,5-dicarboxylic acid (TTDC) showed substantial H₂ uptake on volumetric basis (34 g/L) [90]. The measurements were carried out at 70 bar and 77 K. The important thing to note from the performance of these MOFs that they dispels the common misconception that porous materials will inherently have poor volumetric storage capacity. There are several other MOFs which showed appreciable H₂ uptake capacity like MOF-74, HKUST-1, IRMOF-11, MIL-101, NOTT-102 and MOF-205 [91,92,93,94]. From studies, it is evident that MOFs with large surface area and open metal sites provide stronger interactions between metal nodes and H₂ which is the principle reason behind the success of MOFs for H₂ uptake. Theoretical studies suggest that doping of MOFs with certain specific metal ions can boost their H₂ uptake capacity.

On the other hand, COFs also pose as promising candidates for H₂ uptake due to their chemical robustness, high water stability and crystallinity along with high surface area and tunable pore size. Three-dimensional COFs showed better gas storage properties compared to two dimensional COFs. Various COFs have been explored for H₂ storage. For example, 3D COFs like COF-102 and COF-103 exhibit reasonably high H₂ uptake of 7.24 and 7.05 wt% at 77 K and 85 bar, which is at par with MOF-177 and IRMOF-20 [26]. COF-10 with BET surface area (1760 m²g⁻¹) showed H₂ uptake of 3.9 wt% at 77 K and 85 bar which is highest among 2-D COFs [95].

From various studies, it is inferred that H₂ uptake capacity of COFs can be improved in two ways:

(i)
Increasing the surface area of COFs while maintaining their microporous nature.
(ii)
Metallation of the COFs with metal ions such as Li⁺, Ca²⁺ and Pd²⁺ which can strongly interact with H₂.

16.3.1.2 CH₄ Storage

Methane gas is another most convenient, inexpensive and relatively clean source of energy. Natural gas also contains around 95% of methane. Gravimetric heat of combustion of methane (50 MJ Kg⁻¹) is much lower than that of hydrogen (120 MJ Kg⁻¹) but comparable to that of gasoline (44 MJ Kg⁻¹). Therefore, methane can replace gasoline and coal which are often responsible for air pollution. Cars powered by CH₄ produced relatively less carbon dioxide than gasoline but they require costly tanks and compressors to store CH₄. Therefore, for practical use of CH₄ as a fuel, an efficient storage material is required. US-DOE has set a target of 263 cm³ of CH₄ per cm³ of adsorbent [95].

MOFs and COFs have the ability to store methane at lower pressures because methane molecules can pack tightly in their pores. The basic criteria for high methane uptakes in porous materials are:

(i)
High sorption capacity.
(ii)
Good adsorption enthalpy.
(iii)
An efficient charge–discharge rate.

First methane storage studies in MOFs were reported by Noro et al. [96]. They utilized [{CuSiF₆(4,4′‐bipyridine)₂}_n] stabilized in 3D microporous network to store 0.21 g/ml of methane. Various other MOFs like [PCN-14 Cu₂(adip)] (16 wt% at 35 bar), HKUST-1 (15.7 wt% at 150 bar), MIL-101(14.2 wt% at 125 bar), IRMOF-1 [228 cm³(STP) g⁻¹ at 36 bar) showed substantial methane uptake capacity [97,98,99]. MOF-177 and Ni-MOF-74 exhibit 22 wt% at 100 bar and 190 cm³ (STP)g⁻¹ at 35 bar methane uptake capacity, respectively, which is the highest storage among the dry samples [100]. In case of MOFs, open metal sites and microporous nature along with high pore volume are the contributing factors responsible for high methane uptake.

Among COFs, COF-102 and COF-103 exhibit highest methane uptake of 187 mg g⁻¹ and 175 mg g⁻1,respectively, at 35 bar and 298 K which is comparable to best previously reported materials: Ni-MOF-74 [90, 101]. Theoretically, it was predicted that Li ion doping in these COFs can double their methane uptake capacity due to increased induced dipole interactions and London dispersion forces between methane molecules and doped lithium ions. From various studies, it is concluded that like H₂ uptake, CH₄ uptake in COFs is related to the surface area of the COFs and the concentration of doped ions which improves weak interactions.

16.3.1.3 CO₂ Storage

Every year, the uncontrolled combustion of fossil fuel for transportation and running industries is increasing the concentration of greenhouse gases such as carbon dioxide and carbon monoxide in the atmosphere. Sea level rising and dramatic change of the climatic conditions are some of the environmental sector which are under direct impact of air pollution due to increase amount of carbon dioxide in the atmosphere. According to one article published in the OECD Environmental Outlook to 2050 released at the 2011 United Nations Climate Change Conference, discussed the need for negative emissions, stating “Achieving lower concentration targets up to 450 ppm for clean environment to sustain existence of life”. Thus, the major quest in this regard is to develop a material for CO₂ storage and sequestration.

Various porous materials like activated carbon and zeolites have been explored for CO₂ adsorption. MOFs have also shown great potential for CO₂ storage due to their high internal surface area and the presence of polarity due to functional groups inside the pores. Variety of MOFs based on the functionality and surface area has been explored for CO₂ uptake. For example, MOF-177 and MIL-101 exhibit nearly 60 wt% of uptake capacity [101a, 103]. NU-100 (69.8 wt%, 40 bar), Mg-MOF-74 (68.9 wt%, 36 bar), MOF-5 (58 wt%, 10 bar) and HKUST-1 (19.8 wt%, 1 bar) are other well-known MOFs which show considerably high CO₂ uptake. MOF-210, due to its ultra-high surface area (10,450 m²g⁻¹) exhibit very high CO₂ uptake (74.2 wt% at 50 bar) which is higher than that of any other porous material. Also, it has been established through theoretical as well as experimental studies that the presence of polar groups such as –NH₂, or free N containing heterocyclic residues facilitates the CO₂ uptake [102, 103].

Likewise, COFs have also shown great potential for CO₂ uptake. Yaghi and co-workers for the first time studied a series of COFs for CO₂ capture. Studies reveal that COF-102 exhibits highest uptake (27 mmol g⁻¹ at 35 bar) which is at par with the uptake shown by MOF-5 [104]. Theoretical calculations have predicted that doping in COFs can improve their CO₂ uptake. For example, lithium doping in COF-102 and COF-105 improves their CO₂ storage capacity to 409 and 344 mg g⁻¹, respectively, which is manifolds higher than their pristine COFs [105]. Studies reveal that triazine-based, azine-based and imine-based COFs show high CO₂ uptake due to dipole-induced dipole interactions between polar groups and CO₂ molecules inside the framework pores [72]. It has also been observed that the presence of polar functional groups in the pores introduces the selectivity in adsorption of gases.

16.3.1.4 Ammonia Storage

Ammonia is widely used in chemical industries (for the production of nitrogeneous fertilizers) and pharmaceutical industry. It is also explored as an energy source for fuel cells [106]. However, its handling, storing and shipping are costly affair, and also, it requires special precautions due to its corrosive nature and toxicity. The US Occupational Safety and Health Administration (OSHA) sets a 15-min. exposure limit for 35 ppm gaseous ammonia [107]. Various porous materials like activated carbons have been explored for NH₃ uptake, but low ammonia affinity limits their wider utility [108]. MOFs, due to the presence of open metal sites and pores decorated with functional groups, show great potential for NH₃ uptake. For example, Dinca and co-workers [109] have explored three triazolate-based MOFs, among which Mn-MOF exhibit highest NH₃ uptake at 298 K and 1 bar. The study showed that uptake of NH₃ by MOF is better than activated carbons currently used commercially. The large NH₃ uptake is ascribed to high open metal sites in the frameworks while their excellent resistance towards ammonia is attributed to its triazolate-based ligands. Since NH₃ is a bronsted base, it has great affinity towards acidic group (such as sulphonic acid). The presence of such functional group besides open metal site can also act as strong ammonia capture site. Yaghi and co-workers modified UiO-66-NH₂ with anhydrous HCl which exhibits considerably high NH₃ uptake compared to pristine MOF at ambient temperature [110]. Fe-MIL-101 decorated with sulphonic acid group also exhibits great ammonia uptake due to the presence of sulphonic acid groups [107].

Similar to MOFs, various COFs have also been explored for NH₃ uptake. Yaghi and co-workers have synthesized boroxine and boronated ester-based COFs for storage of NH₃ gas. Boronate ester-based COFs (COF-10) having high density Lewis acid boron sites which can strongly interact with Lewis base (NH₃) exhibit an ammonia storage capacity of 15 mol kg⁻¹ at 1 bar and 298 K the highest ammonia uptake showed by any porous materials [111]. The exceptional high uptake of ammonia by COF-10 is due to the formation of a classical ammonia borane coordination bond. Adding more to the advantage, the ammonia gas adsorbed by the COFs at room temperature can be easily recovered by application of heat which makes the process reversible in nature.

16.3.2 Heterogeneous Catalysis

MOFs and COFs are currently attracting considerable interest as heterogeneous catalysts at moderate temperatures. MOFs with exotic topologies, versatile chemical composition, organic and inorganic building units have shown outstanding catalytic performance in various organic transformation reactions such as oxidation, acetylation, hydroxylation, epoxidation, coupling, hydrogenation, condensation, alkylation and cyclization. In MOFs, both metal centres and organic ligands contribute to catalytic activities while pores provide space for small molecules. The metal centres act as Lewis acid site for catalytic activity are obtained by removing coordinated water/solvent while terminal ligands act as Lewis basic site. Various MOFs have been explored for variety of organic transformation reactions. For example, Cu-BDC has been used for the acetylation of alcohols with 80% efficiency, HKUST-1 for cyanosilylation of aldehydes, MOF-199 for oxidative C–C coupling, IRMOF-9-NH₂ for Knoevenagel condensation Allylic N-alkylation, NUGRH-1 for Friedel Craft reaction, etc. [112].

On the other hand, COFs have also emerged as an efficient catalyst owing to the presence of well-defined active sites such as N, O and S in the framework. Utilizing the pre- or post-synthetic modification approach, interior of the framework can be easily modified for catalysis. The metal ion can be introduced either in-situ or ex-situ approach in the framework for catalysis. Due to polymeric nature, COFs are insoluble in organic solvent and thus can be easily recovered after catalysis without significant loss in their catalytic performance. COFs can be further utilized to carry out the catalysis where organic molecules of large size can easily diffuse inside the pores. Wang et al. have incorporated Pd²⁺ ions in the imine-based COF (COF-LZU1) pore walls, bonding with the imine’s nitrogen and present in between the two adjacent COFs layers. In Suzuki–Miyaura coupling reaction this Pd@COF-LZU-1 has shown high catalytic activity [113]. Likewise, cobalt loaded porphyrin-based COF (COF-366 Co) was utilized for the electrochemical reduction of CO₂ with decent activity and selectivity to a competing reaction (H₂ formation) [114]. Banerjee et al. utilized the ex-situ approach, to immobilize palladium (Pd) and gold (Au) nanoparticles inside the framework of TpPa-1 COF [115]. These metal loaded COFs are remarkable water stable even under acidic as well basic conditions. The resulting Pd loaded COF has showed an excellent activity towards C-H activation and C–C coupling reaction where Au nanoparticles loaded TpPa-1 was used for the reduction reaction of nitro compounds. In such metal loaded COFs, the COF skeleton imparted an extra chemical robustness to the active metal centres during catalysis [115].

16.3.3 Energy Storage

The porous framework materials containing redox-active centre can be easily converted into energy storage materials. MOFs have the natural advantage because the metal ion present in the framework can easily undergo different redox states on application of the potential. MOFs with high surface area and low density are promising electrode material for rechargeable batteries and super-capacitors of next generation. MOFs can be tailor made to suit the final application by choosing the specific metal site and tuning their pore sizes. The chemical interaction between the metal sites and functional linkers with polysulphides improves the cycling performance of Li–S batteries. Moreover, metal centres in MOFs are the active site for redox reactions while open framework structure supports reversible insertion and extraction of ions [116]. MIL-53(Fe) acts as a cathode material to reversibly insert Li⁺. Studies reveal that 0.6 Li⁺ per Fe³⁺ could intercalate into MIL-53(Fe) at C/40 with no structural alteration [117]. The result reveals that MOFs are material of choice for lithium ion battery. Apart from being cathode material, MOFs can also perform the function of anode material. For example, Mn-BTC MOFs (Mn-1,3,5-benzenetricarboxylate) exhibit high specific capacity of 694 mAh g⁻¹ and approximately 83% capacity retention over 100 cycles at 103 mA g⁻¹. The COO– groups in Mn-BTC MOFs play a significant role for the Li⁺ insertion/extraction [118].

On the other hand, owing to their ultra-high surface area COFs have the unique ability of integrating redox-active groups which pose them superior candidates as electrochemical capacitors. Different redox-active organic linkers such as quinone, naphthalene diimide (NDI) and pyridine were incorporated to serve the purpose. Dichtel et al. have synthesized an anthraquinone moiety containing Schiff-based COF (DAAQ-TFP COF) by the condensation reaction between 1,3,5-triformylphloroglucinol and 2,6-diaminoanthraquinone. The anthraquinone moiety itself is redox-active. The DAAQ-TFP COF displays reversible redox processes. Due to extra chemical stability from COFs framework, the DAAQ-TFP COF has displayed an excellent supercapacitance performance even after 5000 charge–discharge cycles. Taking the advantage of redox-activity of anthraquinone moiety, they have fabricated DAAQ-TFP COF as oriented thin films. The improved capacitance values during the capacitance measurement of thin films, in comparison with the randomly oriented COFs powder, have been observed [119]. Later on, Xu et al. came up with post-synthetic approach and introduced redox-active characteristics to NiP-COFs with organic radicals such as TEMPO which has displayed a high capacitance value of 167 Fg⁻¹ [120]. In another interesting approach, COFs was fabricated over the amine-reduced graphene oxide. Adopting this approach, the stacking or aggregation between the graphene sheets was reduced. This has provided more exposed electrode surface area for energy storage application. Moreover, COFs can also act as host material for sulphur to extract positive effect on Li–S batteries. The electrode constructed by impregnation of sulphur into the pores of COFs exhibits stable cycling performance.

16.3.4 Drug Delivery

Design and development of bio-compatible drug delivery system with high drug uptake and administrable drug release are of prime importance to minimize the side effects and in turn enhance treatment efficacy [121]. Many different kinds of nano-carriers have been explored for this purpose such as mesoporous silica, metal nanoparticles, quantum dots, dendrimers and organic micelles. However, these nano-carriers suffer from either low loading capacity or unacceptable degradability and toxicity [122]. In next generation drug delivery system, MOFs and COFs have shown great potential to be used as effective drug carriers. The advantages of using MOFs as drug carriers are [122,123,124]:

(i)
High surface area and porosity powered MOFs with high drug loading capacity.
(ii)
MOFs can adopt diverse morphologies, composition and chemical properties owing to the versatility in their structures which favours them with multifunctionalities and stimuli responsive drug controlled release.
(iii)
Any modification in MOFs structure will not alter their desirable physico-chemical properties.
(iv)
Relatively weak coordinate bonds make MOFs biodegradable material.

Above favourable features of MOFs enable them as promising material for drug delivery. For example, MIL-100(Cr) and MIL-101(Cr) constructed from di- and tri-carboxylates show considerably high ibuprofen uptake of 0.35 g/g for dehydrated MIL-100(Cr) and 1.4 g/g for dehydrated MIL-101(Cr). The release kinetics of ibuprofen under physiological condition indicates a total release of ibuprofen from body in 3 days from MIL-100(Cr) and 6 days from MIL-101(Cr). Compared to MCM-41, MIL-100(Cr) exhibits similar ibuprofen dosage and kinetics while MIL-101(Cr) showed four times larger drug content and slower drug release kinetics. Although this study is based on chromium-based MOFs which are known for their toxicity, this work opens new avenue for MOFs-based drug delivery system [125]. Subsequently, MIL-53(Fe), a less toxic iron analogue, was developed which shows 20 wt% ibuprofen loading and total release took 21 days under physiological condition [126]. This work highlights the flexible nature of MOFs to optimize drug-matrix interaction. Likewise, MIL-100(Fe) has been used as drug delivery system for an antitumor drug doxorubicin (9 wt%, 2 weeks release time) [127]. Zn-based MOFs, Zn-TATAT (TATAT = 5,5′ 5′′-(1,3,5-triazine-2,4,6-triyl) tris (azanediyl) triisophthalate) and Zn-CDDB (CDDB = 4,4′-(9-H carbazole-3,6-diyl) dibenzoic acid) were used as carrier for anticancer drug, 5-fluorouracil, which showed 33.3 wt% and 53.3 wt% drug loading capacity [128, 129]. It is found that hydrogen bonding interactions between the drug and MOFs are responsible for such high uptake of drug. Zirconium-based MOFs UiO-66 have attracted considerable interest as a drug carrier because of their two octahedral and tetrahedral cages, bio-compatibility and stability. It is synthesized from zirconium-oxo-clusters and terephthalate anions bearing different functional groups. Studies revealed that UiO-66 and its NH₂ functionalized analogue MOFs are effective carrier for caffeine and 5-fluorouracil. In another study, UiO MOF constructed from ZrCl₄ and aminotriphenyldicarboxylic acid (amino-TPDC) bridging ligands were used for the co-delivery of cis-platin and pooled small interfering RNAs (siRNAs) to enhance their therapeutic efficacy by overcoming drug resistance genes and resensitizing-resistant ovarian cancer cells to cis-platin treatment. The drugs cis-platin and siRNAs were sequentially loaded by encapsulating and coordinating to metal sites on the MOFs surface [122, 130]. Moreover, there are stimuli responsive MOFs which exhibited regulated delivery of loaded drugs upon in response to variety of stimuli such as pH, magnetic field, ions, temperature, light and pressure [122, 131,132,133,134].

Likewise, COFs also possess following excellent properties which enable this class of porous materials to be used in drug delivery application [135]:

(i)
High surface area and porous volume with tunable pore structures for large drug uptake and controlled release.
(ii)
π-π conjugated system which facilitates to load aromatic group-based drug through π-π stacking. Moreover, this π conjugation and laminated structure impart excellent photoelectric properties in COFs which enables them for bio-sensing and bio-imaging.
(iii)
Dynamic covalent linkages for stimuli responsiveness: COFs are constructed from dynamic covalent linkages instead of weak interactions and coordinate bond, which makes them stable enough in normal conditions and degrade to release the drug by the application of stimuli such as change in pH [136].
(iv)
Unique tailorable characteristics and outstanding modifiability: Reports show that surface modification in COFs can improve their bio-compatibility and targeting ability [137]. COFs can be attached to biological probe and drugs through post-synthetic modifications.

Due to such exciting properties of COFs, attempts were made to insert large drug molecule inside the framework for drug delivery applications. Yan et al. have utilized a 3D polyimide-based COF (PI-COF-4 and PI-COF-5) to load ibuprofen (IBU) drug molecule inside the pore. The drug loading was confirmed from the UV–Vis studies while a thermo-gravimetric study reveals the drug loading of 24 wt% in PI-COF-4 and 20 wt% in PI-COF-5 with respect to the COF [138]. In addition to IBU, PI-COFs were able to deliver captopril and caffeine too. Interestingly, this was the first example of applying COF as drug delivery system. This result set an example and demands a further development of COFs for pharmaceutical applications. Later on, Zhao et al. have demonstrated the loading of three different drug molecules, 5-fluorouracil, captopril and ibuprofen, inside the pore of the two-dimensional PI-2-COF and PI-3-COF [139]. Lotsch and co-workers have designed imine-based TTI-COF for the loading of Quercetin used to boost immunity. The drug was bound on the wall of pores through weak interactions [140]. In another report, a photo-responsive single layer COF was fabricated in which the azo-benzene group was introduced in the organic linker. Application of UV radiation leads to reversible photo-induced decomposition-recovering of COFs and exhibits controlled loading and release of copper phthalocyanine. Furthermore, doxorubicin loaded covalent triazine nanopolymer (CTNP) synthesized via the Friedel–crafts reaction acts as a potential nanocarrier for cancer therapy and imaging [141]. Banerjee and co-workers explored imine-based COFs for drug loading and release study. They also utilize COFs for targeted drug delivery. Post-synthetic modified covalent organic sheets with folic acid were used to load and deliver 5-fluorouracil [142]. Although the loading capacity of this modified COF was low, it possesses good anticancer activity compared to other reported COFs.

Apart from loading anticancer drugs, many functional COFs themselves possess anticancer activity. EDTFP-1 COF constructed from 2,4,6-triformylphloroglucinol and 4,4’-ethylenedianiline could accelerate ROS generation and caused the apoptosis of cancer cells [143].

Overall, it may be concluded that MOFs and COFs have shown potential to be used as next generation drug delivery system. However, challenges still exist regarding the targeted drug delivery and efficient clearing of framework once they have finished their job in vivo. Therefore, modification in design according to the need and detailed examination of their in vivo behaviours is of paramount importance to take this class of nano-carriers from bench to bed.

16.3.5 Separation

Separation is a process that splits the mixture into its components. It is opposite to the process of mixing, which is thermodynamically favoured process, and separation is generally not a spontaneous process. Separation of the components of a mixture is often based on selective adsorption. Adsorptive separation by a porous material is usually achieved in following ways [144]:

(i)
Size and shape exclusion also known as molecular sieving effect: Based on size and shape, certain components are stopped from entering the pores of an adsorbent while others are allowed to enter where they are adsorbed. This is steric separation and is common in zeolites and molecular sieves.
(ii)
Thermodynamic equilibrium effect: There is a different adsorbate packing interactions for different components over the surface of adsorbent which leads to selective adsorption.
(iii)
Kinetic effect: Difference in the diffusing rates of the components also leads to selective adsorption.
(iv)
Quantum sieving effect: Light molecules differ in their rate of diffusion in narrow micropores which assist in separating them.

MOFs are ideal candidate for gas separation applications. Structures and properties of MOFs can be well-designed and customized by the choice of metallic SBUs and organic linkers. This remarkable feature of MOFs is quite different from zeolites where customization of structure according to need is not possible. In addition to this, high porosity with tailorable pore size, diverse scope of functionalities, thermal and chemical stability is some of the attractive properties of MOFs which are elementary for separation applications. Owing to such excellent properties, various MOFs have been explored for the gas separation and purification purpose. For example, ytterbium-based PCN-17 MOF comprising of large cages linked by small aperture was able to separate H₂ and O₂ over N₂ and CO [145]. Zn₂(cnc)₂(dpt) and MIL-96 were found to suitable for the separation of CO₂ and CH₄ [146, 147]. The separation is due to size/shape exclusion of MOFs leading to selective adsorption of CO₂ over CH₄. In another study, utility of Cu-based MOF [Cu₂(pzdc)₂(pyz)] was demonstrated for the separation of C₂H₂ and CO₂, and the task for which traditional porous materials like activated carbon and zeolites is of no use as these two molecules are very similar in size. The sorption isotherms of both gases show that MOFs binds preferably with C₂H₂ than CO₂ at ambient temperature and low pressure. The selectivity is due to strong binding interaction between surface O atoms and C₂H₂ through hydrogen bonding. Guo et al. have synthesized copper net supported HKUST-1 (Cu₃(BTC)₂) membranes and utilized them for the separation of H₂/CO₂, H₂/CH₄ and H₂/N₂ [148]. The separation of H₂ is due to its high permeation flux through MOF membrane compared to other gases. H₂ molecules being small can pass through HKUST-1 membrane more easily compared to CO₂, CH₄ and N_2, and this can be attributed to the structural and chemical feature of the MOF which favours stronger interaction with CO₂, CH₄ and N₂ than H₂. The selective separation/adsorption in some MOFs is due to the steric effects and the interactions between adsorbate molecules and surface of adsorbent. For example, ZIF-95 and ZIF-100 showed high CO₂ storage capacity compared to CH₄, CO and N₂ which was attributed to the cooperative effects of the pore apertures (similar to CO₂) and the strong quadrupolar interactions of CO₂with N atoms present of the pore walls [149].

From the above discussion, it can be concluded that MOFs showed great promise in gas separation because both their pore size/shape and their surface properties can be easily tuned by the choice of metal node and organic linkers. In addition to this, open metal site in MOFs also assists in the separation of polar and non-polar gas pairs such as CO₂/CH₄ [150].

However due to relatively low adhesion of MOFs-based membranes to a polymeric support and possible defects between crystals‚ limits the full potential of MOFs to be realized for separation applications. Therefore, COFs-based membrane was explored for the gas separation, heavy metal ion separation, nanofilteration and water treatment because of their exciting characteristics such as

(i)
Tunable pore size: Size exclusion depending upon the pore size is an important criterion for porous membrane-based separation. The pore size of the COFs depends upon the geometry and connectivity of the linkers which can be modified either by changing the length and structure of organic linkers or by post-synthetic modification to introduce large side groups and functional groups [76, 151].
(ii)
Chemical stability: COFs which can sustain their crystallinity and porosity in humid, organic solvents and strongly acidic conditions are of great utility in gas separation and water treatment. The stability of COFs can be improved by the rational selection of organic linkers for the COFs synthesis and by introducing intra-molecular and inter-molecular hydrogen bonding [115, 152].
(iii)
Hydrophilicity: Hydrophilic COFs find extensive use in desalination, dye extraction and pervaporation [153,154,155].
(iv)
Surface charge: Charge present over the surface of COFs plays a significant role in desalination and organic solvent nanofilteration.

Apart from above-mentioned characteristics of COFs, large surface area and adaptable functionality are also appreciated for their application in gas separation and water treatment. A computational study revealed that the monolayer CTF-0-based membrane can deliver a very high separating factor at room temperature for separation of H₂ from H₂/CO₂, H₂/N₂, H₂/CO and H₂/CH₄ mixture [156]. It is interesting to note that COFs membranes exhibit high H₂ permeability with highest being recorded for ultrathin continuous 2D-CTF-1 membrane. With increase in the thickness of COFs, selectivity of H₂/CO₂ improved while the permeance of gas is reduced [157]. Similarly, Gao et al. have synthesized 3D COF membrane (COF-320) on porous alumina ceramic support under solvothermal condition and utilized it for H₂/CH₄ and N₂/H₂ separation application. The COF-320 membrane exhibits high hydrogen permeation flux as compared to other gases leading to H₂ selectivity of COF membrane [158]. Banerjee et al. recently have prepared mixed matrix membranes (MMMs) of COF over PBI polymers support [159]. This membrane is flexible in nature and displays high degree of thermo-chemical stability which further enhances its potential in separation application. The observed moderate selectivity in CO₂/CH₄ and CO₂/N₂ separation is attributed to the existence of polymeric chains trapped inside during synthesis. The presence of these polymeric chains reduces the pore size and thus restricts it in achieving high selectivity for separation. In another report, self-standing thin films of COFs have been prepared via interfacial crystallization process. Depending upon the concentration of the reagents, the thickness of the films was tuned. In addition, it is easy to handle, flexible in nature and can be grown in different diameters. Although these self-standing thin films are not defect free but still, they have demonstrated remarkable solvent-performance and solute-rejection performance.

Not just limited to separation of gases, COFs can also be used for the separation and extraction of elements and nanofilteration of dyes, salts and other organics from wastewaters. For instance, benzimidizole-based 2D COF functionalized with carboxylic acid has been employed as a solid-state matrix for the separation and enrichment of uranium [160]. Wang et al. have demonstrated the utility of COF-based membranes for the removal of dyes from water. The membrane showed high pure water permeability (50 L m² h⁻¹ bar⁻¹) and a high Congo rejection rate (99.5%) which was better than MOFs-based membranes [161]. With COFs (LZU1) membrane still higher water permeability (75 L m² h⁻¹ bar⁻¹) and approximately similar rejection rate of methylene blue (99.2%) and Congo dye (98.6%) was achieved [154]. Another application of COFs membranes was in desalination. Tunablity of pore size, surface charge and hydrophilicity of COFs membranes improves selectivity and reduces membrane fouling. Simulation studies have revealed that water desalination through seven TpPa-X membranes with various functional groups showed over 95% NaCl rejection while the water permeance was three orders of magnitude higher than typical commercial seawater reverse osmosis (RO) [153]. COFs-based extraction systems have also been developed for enrichment and analysis of molecules such as sudan dyes which are present in chilli powder below detection limit.

Despite so many applications, challenges such as long-term stability of COF-based membrane in realistic separation, high-cost and time-consuming fabrications methods might hinder their industrial use.

16.3.6 Chemical Sensors

Highly sensitive and selective detection of gas, vapour phase analyte, heavy metal ions are of paramount importance for various applications such as chemical threat detection, medical diagnostic, occupational safety, environment and water monitoring. Highly porous, crystalline MOFs and COFs can detect gas and vapour phase analytes with high sensitivity as they can concentrate the analyte molecules at higher levels than are present in external atmosphere. This facilitates the direct detection of analytes in sorbent and eliminates the sample preparation step. Apart from the sorption capacity of MOFs/COFs, sensitivity of analyte detection depends upon the strength of analyte binding to MOFs/COFs, dynamics of analyte transport within the framework (slower transport will lead to long response time and poor signal) and pore size (it is observed that all else being equal, small pore will adsorb analytes more strongly compared to larger ones, leading to enhance sensitivity), while the selectivity of MOFs/COFs for specific analyte depends either on size/shape exclusion (molecular sieving) wherein analyte molecules smaller than pore aperture can be absorbed leaving the larger ones, or, on chemically specific interactions of analyte with pore surface [162]. MOFs/COFs surface or pore aperture can be tailored to host specific analyte by choice of building blocks or post-synthetic modification methods. This will increase the selectivity as well as the sensitivity of the detection of analytes. For instance, Eddaoudi and co-workers have constructed series of IRMOFs by varying the size and chemical functionality of linkers leading to the formation of iso-structural MOFs with pore aperture ranging from 30 to 3.8 Å [163].

MOFs/COFs-based sensors can be categorized on the basis of mode of signal transduction such as optical, electrical and electromechanical sensors. Based on different modes of signal transduction, many MOFs-based sensors have been explored. For example, luminescent Zn₃btc₂ exhibits size selective sensing of amines [164]. Smaller amines like ethylamine, dimethylamine and propylamine that can easily diffuse into the pores of MOFs showed decrease in the luminescence while aniline and butylamine showed no quenching, presumably due to size exclusion. Xi et al. [165] showed that MOFs decorated with phosphorescent iridium (III) complex exhibit emission quenching in the presence of O₂. The quenching occurs due to energy transfer to O₂ leading to the formation of triplet state. Same behaviour was found to be insensitive towards the presence of N₂. In addition to this, luminescent MOFs have used to detect nitro-aromatics (NAC), one of the major classes of secondary explosives. The detection of NACs is not easy because of their inferior vapour pressure and limited chemical reactivity. However, their electron deficient property is favourable for the formation of π-complexes with electron rich fluorophores which can be applied for their detection. The combination of porosity, luminescence and open metal site in MOFs makes them promising candidate for the sensing of NAC. For example, Li and co-workers have synthesized fluorescent [Zn₂(bpdc)₂(bpee)]·2DMF (bpdc = 4,4’-biphenyldicarboxylate; bpee = 1,2-bipyridylethene; DMF = dimethyl formamide) for the detection of DNT vapours. The MOF shows rapid response to analyte, the quenching percentage reached 85% within 10 s [166]. Lanthanide-based MOFs find great utility for the detection of NAC. Tb³⁺@NENU-522 displays high selectivity and recyclability in the detection of NAC explosives [167]. Interestingly, the sensing of TNT by the MOF can be easily distinguished by the naked eye.

Similarly to MOFs, COFs have also showed great potential in the field of sensing and detection. Recent studies have demonstrated the sensing applications of COFs which can be accredited to their diverse compositions and synergistic functionality which arises from the combination of well-defined porosity and semiconducting properties. Due to tunable porosity, COFs are capable of selectively hosting specific guest molecules, a prerequisite for sensing of ions or molecules. Various COFs-based sensors have been proposed for the detection of heavy metal ions [168], pH changes [169], organic explosives [170], etc. Jiang et al. [171] have demonstrated the synthesis of pyrene-based luminescent COF for sensing of nitro compounds. The resulted COF possess high sensitivity and selectivity for picric acid over other employed nitro compounds. Later on, Fang et al. [172] have reported the application of imine-based COFs as biosensor for bovine serum albumin and probe DNA immobilization owing to the strong electrostatic interactions. In another report, Wang et al. [168] have utilized thioether-based fluorescent COF (COF-LZU-8) for mercury, Hg²⁺ sensing and removal. In follow up, Liu et al. [173] utilized COF-JLU3 for selective sensing of Cu²⁺. Banerjee et al. [169] have synthesized new imide-based COFs (TpBDH and TfpBDH) which have been transformed into thin-layered covalent organic nanosheets (CONs) by simple liquid-based exfoliation method. These 2D CONs have been employed for fast and highly selective detection of nitro-aromatic analytes through luminescence-based turn off/on sensing mechanism. Compared to CONs, sensing ability of COFs is limited due to aggregated π stacked layers which render poor electron mobility and ineffective interaction with analytes. On the other hand, π-π interactions are considerably weakened in CONs which is the reason behind the superior sensing ability of CONs compared to the bulk COFs. Detection of harmful volatile organic compounds in workplace or water content of gas and solvent streams in industrial process is another important area in the field of chemical sensing. In this direction, Bein et al. [174] developed tetrakis(4-aminophenyl)pyrene-based COFs which can act as solid-state supra-molecular solvatochromic sensors that exhibit a strong colour change when exposed to humidity or solvent vapours. It has also been observed that solvatochromic response of sensors which is dependent on vapour concentration and solvent polarity.

16.3.7 Optoelectronics

Designing the material at molecular scale allows a fine adjustment of the energy gap as well as HOMO/LUMO levels of the semiconductor. MOFs and COFs built from aromatic building blocks with periodic arrays offers exciting semi-conductive and photo-conductive behaviours, making them suitable for various photo-electronic applications [175]. However, certain issues related to the fabrication of competitive electronic devices need to be addressed. Three-dimensional orderings at atomic scale enable COFs to perform important role in organic electronics such as light emission, charge transfer and separation. Yaghi et al. [176] for the first time noticed charge carrier mobility in imine-based 2D-COFs. In another example, COF-366 is a p-type semiconductor that exhibits one-dimensional hole mobility of 8.1 cm²V⁻¹ s⁻¹ superior to inorganic amorphous silicon (~1 cm²V⁻¹ s⁻¹). In addition, the hole mobility value is significantly higher than those of common conjugated polymers and ordered crystalline organic semiconductors. The electrical conductivity of COF-366 across a gap of 2 mm between two Au electrodes was determined. The electric current of 0.75 nA for COF-366 measured at the end of electrodes affirms that the COF-366 is conductive in nature. Studies indicated that pore volume of COFs is directly related to the electron conduction [177]. Banerjee et al. for the first time mechanochemically synthesized bipyridine-based COFs which surpassed its conventional solvothermal counterparts by exhibiting a stable open-circuit voltage of 0.93 V at 50 °C and a proton conductivity of 0.014 S/cm [178]. In 2014, Cai et al. [179] synthesized the TTF-COF by reaction between tetrathiafulvalene tetra benzaldehyde and p-phenylenediamine under solvothermal conditions. The TTF-COF attains planar sheet conformation while a distorted confirmation was observed in case of TTF-Py-COF. This contrast in the structural behaviour of two COFs originates from the difference of linkers between TTF units in the COFs. The TTF-COF having compact layer structure is responsible for its high carrier mobility which is found to 10^–5 to 1 cm²V⁻¹ s⁻¹. In 2013, Jiang et al. [180] published the results on the synthesis of conjugated organic framework having three-dimensionally ordered stable structure and delocalized π-clouds (CS-COFs) which is found to be hole conducting framework.

16.4 Conclusions

Great progress has been made in the synthesis and applications of metal organic and covalent organic frameworks (MOFs and COFs) in recent times. The chapter provides a clear in-depth understanding of structure–property relationships of MOFs and COFs. Various synthetic protocols summarized in this chapter could practically assist in design and synthesis of new MOFs and COFs. An important synthetic challenge in this field is the evolution of novel synthetic procedures that can be carried out at room temperature. The formative steps in this direction are the use simple yet important methods like mechanochemical grinding which leads to direct the formation of MOFs and COFs at room temperature. Second point that needs to be addressed is improving the stability of these porous materials. This can be done by the introduction of –OH functionality in the vicinity of Schiff base centres to establish intra-molecular hydrogen bonding or an irreversible enol to keto tautomerization following the Schiff base reaction leads to improved chemical stability of crystalline framework.

The aesthetic characteristics of MOFs and COFs, pore size tunability and chemical functionalization of the cavity are relevant from many perspectives, so much so that it enables MOFs and COFs as useful materials for virtually all aspects of storage, separation and catalysis. Based on various studies, it has been established beyond doubt that functionalized MOFs with open metal site and enhances H₂ and CO₂ uptake to meet desired standards.

In the culmination, MOFs and COFs provide infinite possibility to address the problem in energy, environment and health-related areas. The opportunity to efficiently utilize these architectures is limited only by imagination and our skills to prepare and characterize suitable well-defined structures.

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Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
Adish Tyagi & Siddhartha Kolay
Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, India
Adish Tyagi & Siddhartha Kolay

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Adish Tyagi
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Siddhartha Kolay
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Chemistry Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
A. K. Tyagi
Chemistry Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
Raghumani S. Ningthoujam

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Tyagi, A., Kolay, S. (2021). Synthesis of Metal Organic Frameworks (MOF) and Covalent Organic Frameworks (COF). In: Tyagi, A.K., Ningthoujam, R.S. (eds) Handbook on Synthesis Strategies for Advanced Materials . Indian Institute of Metals Series. Springer, Singapore. https://doi.org/10.1007/978-981-16-1807-9_16

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DOI: https://doi.org/10.1007/978-981-16-1807-9_16
Published: 17 August 2021
Publisher Name: Springer, Singapore
Print ISBN: 978-981-16-1806-2
Online ISBN: 978-981-16-1807-9
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