Keywords

1 Introduction

Metal–Organic Frameworks (MOFs) are keenly structured with the coordination bonds between inorganic metal nodes and organic ligands (Fig. 1). Pronounced by high porosity, tunable functionality, and concentrated metal sites, they are convincing enough to act as the adsorbent and catalytic materials for the generation to come. Specifically, the existence of unsaturated metal centers and electron-deficient groups make them competent of acting as Lewis acid sites, which has further made MOFs highly potential candidates in catalysis applications [43].

Fig. 1
figure 1

Basic structural unit of MOF

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These materials rely on Werner’s coordination chemistry, wherein metal ions (or clusters) are coordinately connected by bridging ligands, to form infinite porous frameworks. Prior to the mid-1990s, researchers mainly focused on two types of porous materials, either purely inorganic materials or carbon materials. MOF is an unprecedented class of porous material that combines the feature of both two types. Similar to zeolites, MOFs are generally prepared via hydrothermal or solvothermal methods, which permits for steady nucleation of crystal seed from hot solutions. However, MOFs use organic linkers to bridge metal clusters and form two or three-dimensional lattices, which offers enhanced structural diversity than classical porous materials. Zeolites have found broad application for catalysis in the chemical industry owing to their uniform porosity, but the types of reactions are limited to acid-catalyzed reactions [26].

With their outstanding, unique set of properties, MOFs form an intriguing class of porous materials, distinguishing them from other materials such as mesoporous silica, porous carbon, and microporous fully inorganic zeolites. Precisely tunable pore size, high surface area which provides a more available surface for interaction with guest species, chemical tailorability, and synthetic flexibility are some of the noteworthy properties of MOFs that set them apart. Most importantly, the structure and functionality of MOFs can be tuned during their synthesis, following the isoreticular principle [61]. MOFs can be functionalized by the addition of substituent functional groups on the backbone of the bridging ligands during synthesis. However, this strategy is limited as many of the desired functionalities cannot withstand the reaction conditions of MOF synthesis [4]. Additionally, even a small change in the reaction parameters, chemical nature of the linker, sterics, electronic configuration of the metal ion and the ligand, results in drastic changes in the MOF thus formed and we may not get the target MOF [61]. To overcome these challenges and limitations of pre-synthetic functionalization, an alternative strategy, known as post-synthetic modification, can be employed. It is an approach that leads to diverse functionalities without affecting the structural stability, porosity, and crystallinity of the as-synthesized framework [4, 36]. Owing to their unique set of properties and highly ordered crystalline structure, it is of paramount significance to mention that the position, as well as the degree of functionalization of MOFs, are controllable.

In this chapter, we will present how imparting functionality can further accelerate the reaction and elaborate the array of reactions catalyzed by MOFs. Furthermore, from the application perspective, we have highlighted their use as lewis acid catalyst, wherein we cite examples from recent research works to illuminate the concept.

2 Potential of Metal–Organic Frameworks (MOFs) Over Other Conventional Catalysts

MOFs are a new class of organic–inorganic hybrid crystalline porous materials composed of metal ions/clusters as nodes and organic ligands as linkers, first synthesized in 1999 [24]. These materials possess a number of significant advantages over conventional catalysts (e.g., zeolites, silicates, organometallic complexes, transition metals, activated carbon, another homogeneous catalyst, etc.). The fascinating characteristics of MOFs that make them ideal for their application in various reactions are shown in Fig. 2.

Fig. 2
figure 2

The fascinating properties of MOFs

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This is worth noting that the presences of a large surface area and porous structure functionalize the guest species to be introduced into the pores and permit the substrates to approach the internal active sites. This facilitates mass transport and enables the encapsulation of precursors at the atomic level into the pores of MOFs to design functional materials with enhanced morphology [26].

The MOFs acts as active centers in catalysis that makes them capable of a range of organic transformation that could not be successfully catalyzed by the traditional catalyst. Besides other conventional catalysts, MOFs bridge the metal clusters with the utilization of organic linkers and this provide a broad range of MOFs configurations including nanocubes, nanoframes, nanowires with a precise combination of atomic metal ligands, it reflects the flexibility in the design and increases the structural assortment of MOFs than traditional materials. Also, theoretical studies indicate that thousands of MOFs are possible, to date more than 88,000 have been reported and the possibility to create new structures is even greater [35].

The thermal and chemical stability of catalysts plays an essential role in catalysis. Thermal stability is the ability of MOF compounds to resist any change in their physical structure and chemical properties upon heating to relatively high temperatures. In particular, the key attributes for determining the thermal stability of MOFs are:

  • the arrangement of functional units

  • ligand/nodes stability

  • a coordinated solvent molecule's presence.

Generally, MOFs can only be heated up to 150–300 °C but according to a study by Ma et al. after interpenetration, MOFs are stable up to 400 °C maintaining their framework integrity, and after double interpenetration Yb MOF show thermal stability up to 500 °C with enhanced gas-adsorption feature [39]. In a recent study, the most extensively analyzed MOFs in catalysis are MOF-74, ZIF-8, CPO-27, UiO-66, CuBDC (HKUST-1), MIL-53, and MIL-101 which can withstand prolonged heating [56].

The chemical stability of MOFs, refers to their ability to resist the consequences of exposure to numerous chemicals in their environment, for example, moisture, solvents, acids, bases, and binary compound solutions. Additionally, the chemical stability of MOFs have been studied broadly in following three medium:

  • Acidic medium

  • Basic medium

  • Hydrolytic medium.

The chemical stability of MOFs is significantly influenced by their intrinsic structure, which includes metal ion charge density, ions/clusters connectivity, basicity, and orientation, as well as hydrophobicity of ligand. Despite the fact that many MOFs are susceptible to structural degradation even in the ambient atmosphere due to the lability of coordination bonds between metal ions and ligands, in recent years an increasing number of MOFs with excellent chemical stability have been discovered by primarily using two methods. The first is to create stable unknown MOFs by de novo synthesis, while the second is to increase the stability of existing MOFs [15].

As a result, MOFs are ideal for their application in gas separation, chemical sensing, and numbers of catalysis reactions. Thus, all these factors contribute to the advancement in broader applications of MOFs over zeolites and other conventional catalysts.

3 Fundamental Properties of MOFs Contributing Towards Catalysis

Metal–Organic Frameworks (MOFs) are suitably heterogeneous catalysts that belong to a class of porous crystalline materials featuring a series of unique properties demonstrated in Fig. 3.

Fig. 3
figure 3

Unique properties of MOFs

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Currently, there are several types of catalytic processes which include biocatalysis, chemical catalysis, photocatalysis, electrocatalysis, and much more heterogeneous catalysis, where it is generally observed that different structural uniqueness of MOFs is beneficial for its utilization in these processes.

One of the most prominent characteristics of MOFs is their high porosity which is the pore space at micro-and mesoscale in frameworks, even sufficiently large pores allow access to harness the pore chemistry. In MOFs the pore spaces range from a few Angstroms to tens of nanometers, also a variety of pore sizes can be observed within the same material. MOFs can achieve size and shape selectivity due to their adjustable and well-defined pores as the voids facilitate the mass transfer from the exterior to the interior of materials which significantly also increases the surface area [35].

Furukawa et al. synthesized four MOFs, named MOF-180, MOF-200, MOF-205, MOF-210 respectively using Zn4O(CO2)6 units with one or two types of organic linkers. Their study highlighted the defining porosity property of MOFs, which is an important considering factor in gas storage application as MOF-200 possess the lowest known crystal density (0.22 g/cm3) with 90% of the total volume is empty and this makes them porous material [21]. Porosity information of various MOFs is given in Table 1.

Table 1 Porosity information of several MOFs
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Metal–Organic Frameworks (MOFs) exhibits ultra-high surface area due to their highly organized pore structure and these offer more space for molecular adsorption as well as in chemical reactions. Also, various catalytic active sites/centers of MOFs such as open metal sites, bifunctional acid–base sites, and introduction of multiple pores inside the MOFs by post-synthetic modification (PSM), increases the surface area which is conducive for the improved properties of materials [14]. According to a study by Li and co-workers, Th-MOFs were prepared by is reticular synthesis method without changing the comprehensive topology of MOFs. The synthesized product Th-SINAP-13 exhibited the largest surface area of 3396.5 m2/g with a void space of 74% among thorium materials [35].

The uniform and crystalline structure of MOFs makes the reactant accessible to active sites via open channels, which increases the selectivity of the ligands and modifies the components, this provides the possibility to synthesis a limitless number of MOFs materials. As a result, fundamental materials can be tuned according to their specific application to provide unique structural features and desired chemical compositions. They offer better opportunities to explore structure–activity correlations and undergo computational investigations. Recently, the multi-metal catalyst has been developed to provide a substantial degree of freedom in composition design, further promoting the synergistic effect between components to improve its chemical performance [42].

Versatile functionalities of MOFs include the following:

  • Catalysis

  • Gas and liquid adsorption

  • Gas storage and treatment

  • Conductivity

  • Food

  • Sensoring and detection

  • Textile reform.

The term catalysis describes a process wherein the reaction rate and results are altered by the presence of catalysts which usually don’t get consumed during the reaction and are often removed after reaction to avoid the existence of impurity in the end product. Whereas the process of adsorption refers to the binding of the molecules on the surface, basically it is used to separate mixtures (liquid or gas) into their constituent components. Also, MOFs improve the effectiveness of membranes and filters by increasing the accessible surface area that binds molecules [35].

It is widely used in the storage of compressed gases, separation, removal of impurities and odors. It also, open new horizons for addressing conductivity challenges by developing electrodes and electrolyte composites. In the food industry, MOFs can be utilized in a variety of applications such as quality assurance, management of the shelf life, agrochemical distribution, etc. MOFs present the possibility for sensing and detecting small molecules, medical diagnostics, explosives, etc. because of the presence of conciliated porous structure. The fabrics reformed by MOFs decrease the odor and protect the wearer from hazardous chemicals [35].

4 MOFs as Robust Host for Various Nanoparticles in Heterogeneous Catalysis

The various unique properties possessed by MOFs as discussed earlier enable them to show the potential in heterogeneous catalysis. However, in order to further enhance their catalytic activity, MOFs can be made to undergo controlled integration with functional materials like metal nanoparticles (MNPs), polyoxometallates (POMs), quantum dots (QDs), enzymes, etc. Metal Nanoparticles are rapidly gaining researchers’ interest as one of the most promising guest species. Because of the high surface energies of MNPs, they are thermodynamically unstable and tend to aggregate during the catalytic reaction, which generally leads to loss of catalytic activity [1, 63].

In this regard, the use of various surface capping agents has been recognized as a solution but such kind of surface contamination has a negative impact on the catalytic activity of MNPs. To this end, the immobilization of NPs inside porous materials like zeolites, porous silica, has been demonstrated as an effective approach. Amongst the porous materials, MOFs have been found to be the most promising candidates, because of the various reasons which are reported below [1, 7, 63]:

  • They possess unique properties like diverse chemical composition, ultra-high surface area, and permanent porosity.

  • They are versatile with tunable pore structures for meeting the desired requirements of MNPs.

  • They are highly adaptable to catalytic design.

  • Aggregation and leaching issues are eventually minimized due to confinement and electronic effects offered by MOFs.

  • The inner pore surfaces of MOFs can be easily modified.

Broadly, there are three strategies for the preparation of MNP@MOF composites, as listed in Fig. 4 and discussed below in brief:

Fig. 4
figure 4

Preparation of MNP@MOF composites

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  1. 1.

    Ship-in-bottle method: In this approach, for the synthesis of MNPs, the preformed MOF is placed into a metal precursor followed by further treatment in order to reduce the metal precursor into its metallic state [24]. It includes procedures such as:

    1. (i)

      Solution impregnation

    2. (ii)

      Chemical vapor deposition

    3. (iii)

      Double solvent approach (DSA)

    4. (iv)

      Thermal decomposition.

  2. 2.

    “Bottle around ship” method: In this approach, the preformed MNPs are added during the synthesis of MOF. Due to this fact, better location and size control of MNPs can be obtained, as compared to the ship-in-bottle process [24]. However, such procedures are not suitable for all types of MOFs, UiO-66 and ZIF-8 have been mostly reported so far.

  3. 3.

    One-step synthesis: As the name suggests, this method involves one-step procedure where both MNPs and MOF precursors are mixed together to form the corresponding MNP@MOF composite.

Chen et al. developed an advanced Pd-MOF catalyst using a novel and efficient strategy. They encapsulated palladium precursors within the pores of UiO-67 prior to the MOF assembly [7]. This strategy allowed the Pd nanoparticles to evenly distribute within the cavities of MOFs. Moreover, the obtained composite showed significantly enhanced catalytic activity and stability compared to those prepared by the traditional impregnation method.

The combination of MOFs and MNPs for enhanced catalytic activity has attracted immense interest in recent years. This is because, in MNP@MOF composites, the multiple advantages of both the components are integrated due to the synergistic effect between MOFs and MNPs while the shortcomings of MNPs and MOFs are mitigated [58, 63].

The combination of MOFs and MNPs for enhanced catalytic activity has attracted immense interest in recent years. This is because, in MNP@MOF composites, the multiple advantages of both the components are integrated due to the synergistic effect between MOFs and MNPs while the shortcomings of MNPs and MOFs are mitigated [58, 63].

MOFs are one of the most excellent supports of noble metal NPs, especially for the application in heterogeneous catalysis. It is worth noting that the catalytically active metal ions are immobilized by the functional groups present on the organic linkers of MOFs, therefore, there is no loss of expensive metals [38]. Zhang et al., for the first time, reported a versatile strategy for the incorporation of noble metal NPs into MOFs containing carboxylic acid-based ligands. The resulting NP/MOF composites showed excellent shape selectivity in various reactions like olefin hydrogenation and aqueous reaction in the reduction of 4-nitrophenol. Also, it exhibited a higher molecular diffusion rate in CO oxidation [67]. As discussed earlier, MOFs consist of organic linkers and inorganic nodes. Due to the presence of organic linkers, the interactions between metal NPs and the inorganic nodes are weaker which results in unsatisfying catalytic activity. To overcome this issue, Tsumori et al. developed a metal/quasi-MOF composite, Au/MIL-101(Cr), through a controlled deligandation process in order to expose the inorganic Cr–O nodes to the guest Au NPs. This composite showed significantly enhanced catalytic performance in the low-temperature oxidation of CO [53].

Noble metal NPs have been extensively used in the field of heterogeneous catalysis. However, the practical use of these precious metals is limited due to their high costs and global reserve scarcities. Therefore, the development of non-noble MNP@MOF composites is important which possess similar catalytic activity as noble MNP@MOF composite [56]. To this, transition metals come up as a sustainable alternative owing to their low cost and abundance. In a recent study, Habib and co-workers have discussed the applications of non-noble MNP@MOF composites in heterogeneous catalysis. These composites are found to be useful in CO and CO2 conversion reactions [24].

The field of application of MOF-based composites in heterogeneous catalysis is still in its developing stage. Some large-scale synthetic strategies need to be developed in order to synthesis MNP@MOF composites at affordable costs for their practical applications. It is expected that these composites will have a bright future if persistent efforts are made towards such challenges [63].

5 Reactions Catalyzed by MOFs and Prospects for Applications

In recent years, MOFs have gained tremendous attention as potential catalysts. Catalysis is one of the most successful implementations of these materials. Earlier studies were focused mainly on demonstrating the catalytic sites possessed by MOFs which are required to catalyze a certain process. Now, with the advancement in the research, the catalytic applications of MOFs range from conventional catalytic implementations to photocatalysis, and electrocatalysis too [2]. Different MOFs have been used to catalyze a wide variety of chemical transformations. Some of the important reactions catalyzed by MOFs are discussed below.

5.1 Polymerization Reactions

Catalysts enable the rapid, and efficient formation of desired polymers from the corresponding monomers. Though various molecular catalysts have been found effective for these reactions, their separation from the reaction products is quite difficult. To overcome this issue, the use of heterogeneous catalysts has been suggested. MOFs have attracted attention in this field owing to their fundamental desirability of chemical and structural uniformity, high porosity, and well-defined crystallography [23].

Polyalkenes are among the most ubiquitous polymers. They are used in electronic connections, building, and construction, toys, etc. Li and co-workers prepared a new class of PCP catalysts for ethylene polymerization, using Zr4+ ions and a tritopic phenoxy-imine linker. These PCP catalysts were found to exhibit moderate to high activity. The study nicely demonstrated that a prospective catalyst can be used as a building block for developing a porous network, which is functional for ethylene polymerization [34].

Dienes also play an important part in everyday life, since they are widely utilized in the automotive and roofing industries due to their resistance to ozone, ultraviolet light, and heat. Russell et al. demonstrated that MOF compounds containing two sets of different lanthanide elements (Nd3+, Tb3+/Eu3+) can be used for luminescent polymer production. Neodymium was used because of its well-known catalytic properties for dienes polymerization. With the aim to provide luminescent properties, a second transition element, terbium or europium, was added to the MOF structure. Various MOFs satisfying the aforementioned criteria were prepared and used for the catalysts for the polymerization of isoprene [49].

5.2 Oxidation Reactions

Recently, various research groups have attempted to use MOFs for oxidation reactions. Torbina et al. investigated chromium-based MOFs, MIL-100, and MIL-101, for the oxidation of propylene glycol, using tert-butyl hydroperoxide (TBHP) as an oxidant. Hydroxyacetone was obtained as the main oxidation product while acetaldehyde and acetic acid were the minor oxidation products. MIL-101 catalyst could preserve its structure during a minimum of three catalytic cycles. Moreover, it could be recycled without much deterioration of activity and selectivity [52].

Kholdeevaa et al. prepared Fe, and Cr containing MOFs, MOF-100, and MOF-101, and compared their catalytic activities in allylic oxidation of alkenes with molecular oxygen and oxidation of anthracene with tert-butyl hydroperoxide (TBHP). In the oxidation of anthracene, 100% selectivity was observed for both Cr-MOFs and Fe-MIL-101, with 92–100% anthracene conversion. In the case of oxidation of alkenes, Cr-based MOFs gave unsaturated alcohols while Fe-containing MOFs produced unsaturated alcohols. The stability of MOFs was found to increase in the following order: Fe-MIL-101 < Fe-MIL-100 < Cr-MIL-100, Cr-MIL-101 [32].

It is worth noting that the polyoxometallate (POM)-based MOFs are efficient catalysts for the oxidation of various sulfides into sulfoxides, with high conversion (90–100%) and selectivity (95–100%). Heterogeneous catalysts like PW-MOF are more active catalysts as compared to H2O2, Also, these could be recovered easily and reused five times, without much loss of catalytic activity [25].

5.3 Coupling and Cross-Coupling Reactions

Coupling reactions are widely opted by organic chemists for carrying out various organic transformations and chemical synthesis. Nguyen and co-workers developed a metal–organic framework Fe3O4(BPDC)3 and used it to catalyze the direct C–N coupling of azoles with ethers by oxidative C–H activation, to produce azole derivatives. In the study, it was demonstrated that the contribution of the leached active iron species to the production of the preferred azole product was negligible. The catalyst could be reused several times without a significant change in the catalytic efficiency [41]. Some other MOFs which have been used to catalyze coupling reactions are summarized in Table 2.

Table 2 Various MOF catalysts used for coupling reactions
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Cross-coupling reactions, which produce new carbon–carbon, and carbon-heteroatom bonds, are performing well in the pharmaceutical industry. Homogeneous catalysts are popular in this field because of their relative stability in air and water. However, their industrial applications are limited because these are:

  • prone to aggregation

  • difficult to recycle.

MOFs have been gradually applied in such organic transformations, taking advantage of being a heterogeneous catalytic material for easy catalyst recovery [30]. Li et al. prepared a Pd(II)@UiO-67 composite, using a mixed ligand approach. They used a direct incorporation strategy to immobilize the organic palladium complex Pd (H2bpydc)Cl2 on a porous MOF, UiO-67. The obtained composite showed high efficiency for the catalytic conversion of aryl chlorides. High yields were obtained for the Heck and Suzuki–Miyaura coupling reactions of aryl chlorides. Moreover, the catalytic efficiency of the catalyst remained almost unaffected after at least five cycles [6].

5.4 CO2 Conversion Reactions

The rising quantity of CO2 in the atmosphere is mostly due to the combustion of fossil fuels which leads to the emission of greenhouse gases and this is well known to have a substantial influence on climate change causing global warming. Consequently, the reduction of CO2 to commodity products is a recent critical study in green chemistry. But, the conversion of CO2 into valuable chemicals is a major challenge because of the thermodynamic stability of CO2. Considering the major challenge in reduction due to the complexity of the multielectron step involved, MOFs have attracted intense research interest as it could become a replacement for currently used commercial techniques of CO2 sorption due to their selectivity, structural features for efficient formation of desirable products [56].

MOFs offer multiple advantages such as active surface area, robust pore distribution, crystallinity, etc. These materials are considered to mitigate the harmful impact of CO2 through fixation and conversion of CO2 using different chemical reduction reaction approaches. Also, the careful selection of linkers as well as by adjustment of pores shape/size and surface area considerably increases the gas-adsorption efficiency [11].

Various catalytic processes such as photocatalytic or electrocatalytic reduction as well as hydrogenation are used for the conversion of CO2 into value-added products. General products of catalytic conversion of CO2 are carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), ethanol (C2H5OH), methane (CH4), and others. Therefore, CO2 reduction occurs through various processes and reduction conditions to develop targeted products using appropriate MOFs [67].

5.4.1 Photoreduction

Interestingly, photocatalytic CO2 reduction is well known as artificial photosynthesis which is used to convert CO2 to carbon-based fuels through the utilization of solar energy. This is a straightforward and ecologically beneficial method of converting solar energy into chemical energy. The aforementioned process was first discovered by Inoue et al. in 1979, combining a suspended semiconductor photocatalyst with Xe lamp irradiation [28].

It’s important to note that photocatalytic reduction of CO2 can not only reduce the atmospheric CO2 concentration but also alleviate the energy constraints. However, the traditional photocatalysts endure some inevitable defects as these materials are inert in chemical reactivity and are difficult to recollect and recycle for reuse. Hence MOFs are ideal for utilization in photoreduction reactions as they inherit excellent adsorption capability toward CO2 transformation and unique structural characteristics [10]. As MOFs are formed from metal or metal cluster nodes linked to multi-dented organic components, the metal nodes can initiate photocatalysis. Based on the studies of controlling the modifications on the metal ions or organic linkers, it is easier to tune the light adsorption capacity of MOFs.

In this regard, for the very first time, Li et al. reported that a Ti-containing MOF material (NH2-MIL-125(Ti)) was prepared on replacing the linker in MIL-125(Ti). It was revealed that HCOO− was the only produced product under visible light irradiation [19].

5.4.2 Electroreduction

The major by-product of fuel burning is CO2 which is a relatively low energy molecule, so its reduction necessitates large reduction potentials. Electrocatalytic CO2 reduction occurs in the cathode, which uses MOFs as an electrode, and involves the absorption of CO2 on the cathode's surface, electron transfer from the cathode to the absorbed CO2, and product desorption from the cathode's surface. Simultaneously, the oxygen evolution process takes place in the anode compartments, releasing H+ that migrates to the cathode to facilitate CO2 reduction. Copper (Cu) materials are recommended for this process as they advance strong electrochemical activity towards multi-carbon products [37].

Han and co-workers reported the first paper on the reduction of CO2 electrochemically by utilizing MOFs as cathode and ionic liquid (ILs) as an electrolyte to produce multielectron reduction products, i.e., methane. In another study by Wang et al. zeolitic imidazolate framework (ZIF-8) nanomaterial was synthesized and used as a framework for electrocatalytic CO2 reduction and yielded 65% CO [57].

It’s worth noting that in a comparative study by Yadav and co-workers, differently shaped tin(Sn) catalysts were synthesized using the new solar electro-deposition method. The selective formation of HCOOH with faradaic efficiency of 94.5% at 1.6 V versus Ag/AgCl using the prepared Sn electrocatalyst [60]. It is proposed that Sn-based catalysts are generally used in CO2 electroreduction to produce HCOOH is because of their intrinsic activity. To enhance this activity, a strategy of node doping in MOFs for CO2 electroreduction was introduced by Geng et al. Herein, an active tin (Sn) node was doped into zeolitic imidazolate framework-8 (ZIF-8) using an ion-exchange method, resulting in the highest faradaic efficiency of 74% with a current density of 27 mA cm−2 at −1.1 V versus RHE for production of HCOOH [22].

5.4.3 Hydrogenation

Hydrogenation is a reduction process in which molecular hydrogen is added to an element, compound, or molecule, usually in the presence of a catalyst. The most commonly used catalysts are metal nickel, platinum, and palladium [4]. Using a renewable source of H2 to capture CO2 and convert it to methanol is a potential technique to minimize net CO2 emissions while providing valuable fuels. In a study by Ye et al. a catalyst consisting of microporous metal–organic framework (UiO-67) further functionalized with catalytically active lewis pair, the functional group was synthesized by density functional theory. This catalyst was capable of producing methanol from CO2 and H2 [65].

6 Significance of MOFs as Lewis Acid Catalysts

As mentioned earlier in this chapter, MOFs are a class of hybrid crystalline materials, where metal ions/clusters are erected as nodes and organic ligands as linkers. Generally, these nodes are eliminated from crystal lattice by heating at higher temperatures to form an’ Open metal site’ (OMS). This OMS mainly constitutes transition metals that can serve as Lewis acid [27]. In the year 1994, for the very first time, Fujita et al. reported a Cd-bipyridine MOF that acts as a lewis acid catalyst [20]. Thereafter, researchers have widely studied various reactions where MOFs act as lewis acid. Some of the examples of these reactions are condensation reactions, cyanosilylation reactions, and Friedel–crafts reactions.

6.1 In Condensation Reactions

Dhakshinamoorthy et al. reported the Claisen–Schmidt Condensation catalyzed by Metal–Organic Frameworks wherein in particular they take Metal–organic framework [Fe(BTC) (BTC = 1,3,5-benzenetricarboxylic acid)] to prepare selectively different chalcone derivatives bearing various functionalities [13].

In another similar report, N-Hydroxyphthalimide (NHPI) was assimilated on a Fe III-based metal–organic framework [NHPI/Fe(BTC), BTC: 1,3,5-benzenetri-carboxylate)]. This obtained structure is further utilized as a heterogeneous catalyst for the aerobic oxidation of benzylamine and its derivatives to synthesize the corresponding benzyl imines under typical conditions with molecular oxygen as the only oxidant at 1008 °C. The catalytic activity of NHPI/Fe (BTC) compared with other heterogeneous catalysts (supported gold nanoparticles) proves to be superior in terms of percentage conversion and selectivity [12].

In a recent pH-sensitive study of different structural materials, 3D MOF materials stand out for aldol condensation reactions of various aromatic aldehydes with acetone under heterogeneous conditions. pH-controlled synthesis of MOFs involving magnesium and pyrazole-3,5-dicarboxylic acid (H3L) was done, which permits a progressive increase of dimensionality from zero-dimensional to three-dimensional frameworks. The porous 3D compound has been found to effectively catalyze aldol condensation reactions [50].

6.2 In Cyanosilylation Reaction

Cyanosilylation is a nucleophilic addition reaction in which nitrile and silyl groups are added across double or triple bonds of carbonyl compounds to produce cyanohydrins using Lewis acids as catalysts. It's noteworthy, during the reaction procedure, the reagent trimethylsilylcyanide (TMSCN) is preferred because it is easy to handle and poses low dissociation energy for Si@C bond. Also, it has remarkable conversion efficiency without causing adverse effects [59].

  • The mechanism of cyanosilylation reaction involves three steps

    1. 1.

      Aldehyde oxygen coordinates onto Lewis acid sites.

    2. 2.

      Nitrile groups attack carbonyl groups.

    3. 3.

      Silyl groups isomerize to form cyanohydrin.

The cyanosilylation reaction indicates certain important characteristics for effective catalytic activity in MOFs. They are reported below:

  • Pore size

  • Active sites number

  • Dimension of MOFs

  • Size of particle

  • Condition of solvent.

The large pore size of the MOF allows the easy accessibility of the substrates to the exposed metal sites in the cages or channels, which is very important for the reaction to proceed smoothly. In MIL-101(Cr), the presence of wider pores makes it possible to perform product separation and incrementation relative to MIL-47, MIL-53(Al), and UiO-66 [51].

The mass transfer capability and open metal sites play a prominent role in cyanosilylation reaction. In a view to enhancing these properties, Hu et al. used the strategy of mixed ligand to synthesize HP-CuBTC framework, developed by integrating the BTC ligand with defective linkers. In comparison to pristine Cu-BTC, the latter exhibits significantly improved lewis acid catalytic activity in cyanosilylation reaction for the conversion of benzaldehyde to cyanohydrins [51].

Recently, for practical application of chiral MOFs as asymmetric cyanation catalysts has been studied. To this, Zhu and co-workers reported VO-MOF complex which demonstrated improved stereoselectivity and cyanation of aldehyde [69].

6.3 In Friedel–Crafts Reaction

The alkylation process has been widely employed by chemical and petrochemical industries to produce different commodities and fine products. Conventionally, this process was carried out with help of zeolites such as ZSM-5, Y-Zeolite at high temperatures. But this high temperature usually deactivates the catalyst leading to poor yields of product. To this instance, various MOFs such as MOF-5, IRMOF-3, or MOF-69 °C were employed for tert-butylation of toluene and biphenylene at 170 °C. Cu-MOF-74 is also used to perform the alkylation of anisole at low temperatures [45].

The catalytic behavior of Metal–Organic Frameworks of different structures (Fe(BTC), MIL-100 (Fe), MIL-100(Cr), and Cu3(BTC)2) was investigated in annulation reaction between 2-methyl-3- buten-2-ol and phenols differing in size (phenol, 2-naphthol). The highest conversions of phenols (45 and 75% after 1300 min of TOS for phenol and 2-naphthol, respectively) and selectivity (45 and 65% at 16% of phenol and 2-naphthol conversion, respectively) to target benzopyran were achieved over MIL-100 (Fe) possessing intermediate Lewis acidity, perfect crystalline structure, and the highest SBET surface area [3]. Table 3 summarizes the various MOF catalysts used for Friedel–crafts reactions.

Table 3 Various MOF catalysts used for Friedel–crafts reactions
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7 Conclusion and Future Perspectives

Progressing interest and development in the domain of MOF studies is continued which is justified enough by various applications of MOF material. With remarkable porosity and distinguishable functional properties, they have been identified as materials of high interest for the application of heterogeneous catalysis. Besides the large array of successful applications, there are pertinent challenges that MOFs face in their conventional aspects, and hence to further improving various strategies have been suggested. Functionalization is commonly utilized for exploiting the catalytic activity by many folds. In this chapter, we have thoroughly summarized various reactions that are accelerated by the MOF units.

Besides the edge of heterogeneous catalysts, various large-scale industrial processes still rely on homogeneous catalysis (e.g., Wacker oxidation, hydroformylation, ethylene oligomerization), owing to the lack of compositional and electronic control of heterogeneous catalysts. Collaborative efforts from industry experts and researchers from different fields can help in overcoming the obstacles and ensure advancement in the field of Metal–Organic Frameworks.

MOF:

Metal-Organic Framework

HKUST:

Hong Kong University of Science and Technology

IRMOFs:

Isoreticular Metal-Organic Framework

BDC:

1,4 Benzenedicarboxylic Acid

MIL:

Materials Institute Lavoisier

H4TPP:

5,10,15,20-tetra (1H-pyrazol-4-yp porphyrin)

ZIF:

Zeolitic Imidazolate Framework

BTC:

1,3,5benzenedicarboxylate

TCA:

Tricarboxy Triphenyl Amine

TBHP:

tert-butyl hydroperoxide

POM:

polyoxometallate

OMS:

Open metal site

NHPI:

N-Hydroxyphthalimide

TMSCN:

trimethylsilylcyanide

PCP:

Porous Coordination Polymers

CUS:

Coordination Unsaturated Sites

Bpy:

4,4′-bipyridyl

PyC:

Pyrrole-2-carboxylate

Bpdc:

4,4′-biphenyldicarboxylate

Bpydc:

2,2′-bipyridine-5,5′-dicarboxylate

NPs:

Nanoparticles

MNPs:

Metal Nanoparticles

UiO:

Universitetet i Oslo (University of Oslo)

CAL:

Coordinative Alignment

PCN:

Porous Coordination Network

PSM:

Postsynthetic Modifications

Important Links

  1. 1.

    https://www.google.com/amp/s/blog.novomof.com/blog/gasseparation-zeolites-vs-mofs%3fhs_amp=true

  2. 2.

    https://www.nanowerk.com/mof-metal-organic-framework.php

  3. 3.

    https://novomof.com/metal-organic-frameworks/?hsCtaTracking=ec8597ef-7650-4f00-9975-26a88ba24130%7C8d8099f6-9030-4abd-8c17-bc155c09fe0a

  4. 4.

    https://www.nature.com/articles/s42004-019-0184-6

  5. 5.

    https://www.sigmaaldrich.com/IN/en/applications/chemistry-and-synthesis/synthetic-methods/photocatalysis

  6. 6.

    https://www.nanowerk.com/news2/green/newsid=55082.php

  7. 7.

    https://www.thoughtco.com/definition-of-hydrogenation-604530

  8. 8.

    https://www.azom.com/article.aspx?ArticleID=16233.