Metal Organic Frameworks as Heterogeneous Catalysts in Oleﬁn Epoxidation and Carbon Dioxide Cycloaddition

: Metal–organic frameworks (MOFs) are a family of porous crystalline materials that serve in some cases as versatile platforms for catalysis. In this review, we overview the recent developments about the use of these species as heterogeneous catalysts in oleﬁn epoxidation and carbon dioxide cycloaddition. We report the most important results obtained in this ﬁeld relating them to the presence of speciﬁc organic linkers, metal nodes or clusters and mixed-metal species. Recent advances obtained with MOF nanocomposites were also described. Finally we compare the results and summarize the major insights in speciﬁc Tables, outlining the major challenges for this emerging ﬁeld. This work could promote new research aimed at producing coordination polymers and MOFs able to catalyse a broader range of CO 2 consuming reactions.


Introduction
International Union of Pure and Applied Chemistry (IUPAC) defines MOFs as a coordination network with an open framework containing potential voids [1]. This emerging class of porous coordination polymers are formed by metal ion or cluster nodes and functional organic ligands, all connected through coordination bonds to form 1D, 2D o 3D networks ( Figure 1) [2][3][4][5][6]. MOFs can be easily obtained by several different synthetic methods, such as electrochemical [7], solvothermal [8] and mechanochemical [9], slow diffusion [10], and more recently also by microwave-assisted heating [11].
The crystal structures of MOFs can be customized depending on the metal and ligand choice as also on the solvents and reaction conditions employed. [12] Due to the high surface areas [13] and ultrahigh porosity they are attractive for CH 4 , CO 2, and H 2 sorption and storage. Most MOFs have higher volumetric H 2 and CH 4 storage capacities concerning traditional porous materials.
In recent years nanoscale MOFs have been also investigated for their potential applications in biomedicine, for example for drug delivery [14] and biological imaging [15], mainly for the possibility to use biocompatible building blocks. MOFs were employed as electrode materials for supercapacitors using Co-based coordination polymers [16], for magnetic and electronic devices [17], for water harvesting where H 2 O is extracted from the air by solar energy [18], and finally also for non-linear optics [19].
The use of MOFs as a catalyst has been widely explored and several applications have been developed, for example in the production of fine chemicals [20], or the definition of possible new green protocols replacing non-eco-friendly catalysts [21]. Differences in activity and selectivity toward specific organic reactions are significantly dependent on the MOFs structure [22]. The main MOFs advantage, when we consider their use in catalysis, is in the possibility to design and predict the structural properties based on of linker features, coordination number and geometry of the metal. the MOFs structure [22]. The main MOFs advantage, when we consider their use in catalysis, is in the possibility to design and predict the structural properties based on of linker features, coordination number and geometry of the metal. The presence of coordinatively unsaturated metal sites, the variety of basic linkers available, the stability to solvents and to reaction conditions, the possibility to host guest molecules within the pores makes MOFs perspective materials for heterogenous catalysis. They have also a lot of advantages concerning other inorganic systems as zeolites and aluminophosphates, i.e., they can be modified using organic synthesis, being possible to decorate their pores with catalytic sites. MOFs can be tailored by a simple change in the initial synthetic conditions or by using post-synthetic reactions. These modifications make MOFs excellent candidates for designing functional materials to allow the attachment of different catalysts [23].
While the characterization of deposited species upon conventional catalyst supports, such as metal oxides, tends to be challenging due to the non-uniform surface and pore structures of the support, the crystalline nature of MOFs enables visualization of the catalytically active species within the framework, which leads to a detailed characterization of active catalytic sites and provides insight into structure−activity relationships.
In this review we want to focus on the most recent progress in two reactions MOFscatalyzed, i.e., the olefin epoxidation and the cycloaddition of CO2 to epoxides to yield organic carbonates as a final product, by performing a rigorous analysis of the best MOFs in terms of conversion and selectivity. Specifically, we examined MOF-based catalytic materials producing epoxide and cyclic carbonates with percentages of conversion and selectivity exceeding 70, in the 2015 to 2021 period. Moreover, few relevant papers on the heterogeneous MOFs catalysts published before 2015, for a useful comparison have been considered.
Epoxides are important species and intermediates in the production of pharmaceuticals, agrochemicals, and relevant industrial chemicals. In the global market, the production of propylene oxide achieves 8 million tons per year with an expected annual increase of 5% [24]. Due to the industrial relevance of catalytic oxidation of olefins to fine chemicals, numerous studies have been devoted to the development of efficient homogeneous [24] and heterogeneous catalysts [24]. However, high selectivity and enantioselectivity in epoxidation reactions remain a challenge. While recovery and product separation are the The presence of coordinatively unsaturated metal sites, the variety of basic linkers available, the stability to solvents and to reaction conditions, the possibility to host guest molecules within the pores makes MOFs perspective materials for heterogenous catalysis. They have also a lot of advantages concerning other inorganic systems as zeolites and aluminophosphates, i.e., they can be modified using organic synthesis, being possible to decorate their pores with catalytic sites. MOFs can be tailored by a simple change in the initial synthetic conditions or by using post-synthetic reactions. These modifications make MOFs excellent candidates for designing functional materials to allow the attachment of different catalysts [23].
While the characterization of deposited species upon conventional catalyst supports, such as metal oxides, tends to be challenging due to the non-uniform surface and pore structures of the support, the crystalline nature of MOFs enables visualization of the catalytically active species within the framework, which leads to a detailed characterization of active catalytic sites and provides insight into structure−activity relationships.
In this review we want to focus on the most recent progress in two reactions MOFscatalyzed, i.e., the olefin epoxidation and the cycloaddition of CO 2 to epoxides to yield organic carbonates as a final product, by performing a rigorous analysis of the best MOFs in terms of conversion and selectivity. Specifically, we examined MOF-based catalytic materials producing epoxide and cyclic carbonates with percentages of conversion and selectivity exceeding 70, in the 2015 to 2021 period. Moreover, few relevant papers on the heterogeneous MOFs catalysts published before 2015, for a useful comparison have been considered.
Epoxides are important species and intermediates in the production of pharmaceuticals, agrochemicals, and relevant industrial chemicals. In the global market, the production of propylene oxide achieves 8 million tons per year with an expected annual increase of 5% [24]. Due to the industrial relevance of catalytic oxidation of olefins to fine chemicals, numerous studies have been devoted to the development of efficient homogeneous [24] and heterogeneous catalysts [24]. However, high selectivity and enantioselectivity in epoxidation reactions remain a challenge. While recovery and product separation are the main drawbacks for homogenous catalysts, MOFs used as heterogeneous catalysts in the oxidation of olefins have attracted significant attention ( Figure 2) [25].  CO2 is the primary greenhouse gas in the atmosphere, and it is the cause of environmental and energy-related problems in the world. Nowadays, the development of new methods is fundamental to capture and convert CO2 into useful chemical products to improve the environment and promote sustainable development. Several studies have been carried out on MOF's efficiency to capture CO2. The linkers that connect the MOFs metal nodes are the major sites for CO2 binding. The linkers that connect the MOFs metal nodes are the major sites for CO2 binding, and they can be chemically modified with functional groups to increase their interaction with CO2. Moreover, unsaturated metals ions can be introduced in the MOFs structure. A significantly benefit generated from the possibility to have adequate quantities of CO2 in concentrated form within a MOF is the possible use of CO2 as a chemical reagent ( Figure 3). A significant number of MOFs has been recently reported to catalyse the CO2 cycloaddition reaction to epoxides to give cyclic organic carbonates (OCs) and several papers describe the potential and effectiveness of MOFs in this important process, so it is necessary to identify better strategies to build new advanced materials as MOFs or MOFbased species to grow selectivity, capacity, and conversion of this catalytic reaction.

Olefin Epoxidation
C=C bond epoxidation is an attractive reaction for industrial process to obtain raw materials for epoxy resin, polymers, and pharmaceutical intermediates. Although homogenous catalysts in the epoxidation of alkene have been largely studied in the past few decades, the separation from the reaction mixture and its subsequent reusability remain open challenges [26,27]. CO 2 is the primary greenhouse gas in the atmosphere, and it is the cause of environmental and energy-related problems in the world. Nowadays, the development of new methods is fundamental to capture and convert CO 2 into useful chemical products to improve the environment and promote sustainable development. Several studies have been carried out on MOF's efficiency to capture CO 2 . The linkers that connect the MOFs metal nodes are the major sites for CO 2 binding. The linkers that connect the MOFs metal nodes are the major sites for CO 2 binding, and they can be chemically modified with functional groups to increase their interaction with CO 2 . Moreover, unsaturated metals ions can be introduced in the MOFs structure. A significantly benefit generated from the possibility to have adequate quantities of CO 2 in concentrated form within a MOF is the possible use of CO 2 as a chemical reagent ( Figure 3).  CO2 is the primary greenhouse gas in the atmosphere, and it is the cause of environmental and energy-related problems in the world. Nowadays, the development of new methods is fundamental to capture and convert CO2 into useful chemical products to improve the environment and promote sustainable development. Several studies have been carried out on MOF's efficiency to capture CO2. The linkers that connect the MOFs metal nodes are the major sites for CO2 binding. The linkers that connect the MOFs metal nodes are the major sites for CO2 binding, and they can be chemically modified with functional groups to increase their interaction with CO2. Moreover, unsaturated metals ions can be introduced in the MOFs structure. A significantly benefit generated from the possibility to have adequate quantities of CO2 in concentrated form within a MOF is the possible use of CO2 as a chemical reagent ( Figure 3). A significant number of MOFs has been recently reported to catalyse the CO2 cycloaddition reaction to epoxides to give cyclic organic carbonates (OCs) and several papers describe the potential and effectiveness of MOFs in this important process, so it is necessary to identify better strategies to build new advanced materials as MOFs or MOFbased species to grow selectivity, capacity, and conversion of this catalytic reaction.

Olefin Epoxidation
C=C bond epoxidation is an attractive reaction for industrial process to obtain raw materials for epoxy resin, polymers, and pharmaceutical intermediates. Although homogenous catalysts in the epoxidation of alkene have been largely studied in the past few decades, the separation from the reaction mixture and its subsequent reusability remain open challenges [26,27]. A significant number of MOFs has been recently reported to catalyse the CO 2 cycloaddition reaction to epoxides to give cyclic organic carbonates (OCs) and several papers describe the potential and effectiveness of MOFs in this important process, so it is necessary to identify better strategies to build new advanced materials as MOFs or MOF-based species to grow selectivity, capacity, and conversion of this catalytic reaction.

Olefin Epoxidation
C=C bond epoxidation is an attractive reaction for industrial process to obtain raw materials for epoxy resin, polymers, and pharmaceutical intermediates. Although homogenous catalysts in the epoxidation of alkene have been largely studied in the past few decades, the separation from the reaction mixture and its subsequent reusability remain open challenges [26,27].
Very recently, taking advantage of the tunability of MOFs, several transition metalbased epoxidation catalysts have been developed using MOFs synthesis in combination with post-synthetic modification. Several literature reports require utilization of expensive transition metals, but in the last period also metals like Cu, a classic non-noble transition metal, abundant, inexpensive, and non-toxic, become appealing catalyst sources.

Metal Nodes/Clusters as Catalytically Active Sites
MOFs can be applied as ideal platforms for heterogeneous catalysis towards olefin epoxidation thanks to several structural features with intrinsic catalytic activity such as the coordinatively unsaturated sites on MOFs nodes, defects, and catalytically active organic linkers.
Three pillared-layered Co 6 -MOFs were utilized as heterogeneous catalysts for the selective oxidation of styrene using air, and benzyl alcohol with oxygen. The hexaprismatic [Co 6 (µ 3 −OH) 6 ] cluster with different variable valences activate the oxygen molecule for aerobic epoxidation of alkenes [28]. In Co6-MOF-3, large pores facilitated the mass transfer giving the fastest reaction rate with high conversion and good selectivity for oxidation of both styrene and benzyl alcohol [29].
The presence of a Co(II) open site on the surface maximizes the catalytic productivity, giving 82.56% of conversion and 71% of ter-butyl-2-cyclohexenyl-1-peroxide. Moreover, a Co(II)-based catalyst exhibits similar activity over five cycles without metal leaching [30] ( Table 1).
The static and rotary hydrothermally synthetic method could affect significantly both the process of crystallization and heterogeneous catalytic activity of MOFs in the epoxidation reaction. For example, Co-MOF-150-2, hydrothermally synthesized by rotary crystallization at 150 rpm for 2 h, has reached 95.7% yield of 2,3-epoxypinane from α-pinene in aerobic conditions. The high catalytic activity of Co-MOF-150-2 is due to the better exposure of the metal active in the high crystalline structure, where the lamellar layer was more homogenous. The thinner Co-MOF-150-2 was also investigated in the epoxidation of the other olefins. Additionally, the catalytic activity was relevant for cyclic olefins like cyclooctene (78.5% of conversion after 5h of reaction) and for linear olefins (after 12 h 87.2% of 1-decene was transformed into the epoxide) [31].
A Co-MOF has been prepared under surfactant-thermal condition: NTUZ30 has been obtained with two different secondary building units (SBU), i.e., the unusual trinuclear [Co 3 (µ 3 -OH)(COO) 7 ] and [Co(COO) 4 ]. The cobalt sites onto the surface can convert transstilbene into the corresponding epoxide with excellent selectivity and high conversion [32].
In Cu 4 O(OH) 2 (Me 2 trz-pba) 4 , the Cu 4 (µ 4 -O)(µ 2 -OH) tetrahedral node possesses two Cu 2+ ions bridged by hydroxyl group, which take part in the activation of oxidating agent TBHP, promoting a quicker formation of tert-butoxyl and tert-butylperoxyl radicals, whereas in Cu(Me-4py-trz-ia) the asymmetric unit contains two crystallographically independent Cu 2+ ions. One of them possesses two unsaturated sites that could cause a change in Lewis acidity and generate different redox properties [36].
Epoxidation  2 ]} n ribbon-like 1D chains by intermolecular H-bonding between hydrogen in the imidazolium ion and the free carboxylate oxygens of pdc 2− , this 2D supramolecular structure being crucial to ensure the reaction heterogeneity. Likewise, 1-hexene showed almost complete conversion but increasing the chain length of alkene, the double bond becomes sterically hindered limiting the approach to the active site, and the catalytic activity decreases [40] (Table 1).
High stable zirconium-based MOFs are largely used as active and recyclable catalysts for a variety of catalytic transformations. The catalytic activity of UiO-66 and other Zr-MOFs can be greatly attributed to the presence of random defects in their crystalline structure [41][42][43]. These accessible Lewis acid centers, sometimes in conjunction with Lewis basic sites (e.g., amine groups) in functionalized linker, lead to a significant increase in the catalytic activity [44,45].
Recently, the reaction mechanism underlying both thioether oxidation in nonprotic solvents and epoxidation of electron-deficient C=C bonds in α,β-unsaturated ketones, catalysed by UiO-66 and UiO-67 has been exhaustively investigated [46]. This study suggests the formation of hydroperoxo zirconium species as an oxidant. As already known, the oxidation of less-reactive α,β-unsaturated carbonyl compounds was accompanied by oxidation of MeCN solvent and H 2 O 2 under basic conditions [47], but this nucleophilic peroxo species derived from H 2 O 2 and Zr-MOF can contribute to the epoxidation of the electron-deficient C=C bonds because the reaction readily proceeds even in ethyl acetate.

Mixed-Metal Species
Many efforts have been made to improve the catalytic performance of MOFs, and one possible way is the construction of bimetallic clusters by functionalization of metal nodes/clusters with active transition metals to afford MOF-based catalysts with high performance.
A hydrothermal reaction has been used to synthesise Cu x -Co y -MOF, where Co(NO 3 ) 2 ·6H 2 O and Cu(NO 3 ) 2 ·3H 2 O inorganic metal salts have been one-pot added to a ligand solution in different molar ratios. In addition to the high catalytic activity by doping Cu-MOF with Co, a better selectivity to produce styrene oxide is achieved. At the optimal reaction conditions, the conversion and the selectivity of styrene to styrene oxide increased to 97.81% and 83.04%, respectively, by using Cu 0.25 -Co 0.75 -MOF, the catalyst of this series with higher content of Co 2+ [48]. Another study showed how the conversion of styrene-to-styrene oxide increased rapidly when Mn ions were introduced into a Cu-MOF with the two ligands 2,5-dihydroxyterephthalic acid (H 4 DHTA) and 2-picolinic acid (PCA). Mn 0.1 Cu 0.9 -MOF exhibits interesting catalytic activity for the epoxidations of various aromatic and cyclic olefins and a weak activity on decomposition of H 2 O 2 . Styrene can be oxidized by H 2 O 2 , through peroxybicarbonate-assisted catalysis, the styrene oxide yield achieving 85% in the presence of Mn 0.1 Cu 0.9 -MOF at 0 • C for 6 h [49].
To increase conversion and selectivity in the solvent-free aerobic oxidation of olefins, MOF catalysts based on 3d metal copper (II), cobalt (II) and H 2 ODA (oxydiacetic acid) 6 ]·12H 2 O} n (LaCoODA) were employed. Catalytic studies pointed out the difference in aerobic oxidation of cyclohexene performances due to different physicochemical properties, surface area and redox properties of the metals ( Table 2). [50] LaCoODA, based on Co(II), showed better conversion and selectivity for 2-cyclohexen-1one. This is due to the structural differences between the square planar LaCuODA and the octahedral LaCoODA, in the latter case the water molecules could easily leave the channels to foster interaction between the active sites and the oxidant/catalyst. Moreover, the acid properties of the copper(II) ions are less effective than the redox properties of cobalt(II) ones, as far as the catalytic performances [34,51].
In NU-1000 single-ion-based iron(III) species have been incorporated using solutionphase post-synthetic metalation with two different iron(III) precursors. The resulting NU-1000-Fe-NO 3 and NU-1000-Fe-Cl frameworks show two crystallographically independent Fe sites (Fe1 resides in the c-pore and Fe2 in the hexagonal mesopore), coordinated to the bridging and terminal oxygens of the Zr6 node, with Fe−O distances in NU-1000-Fe-Cl being much longer than those of NU-1000-Fe-NO 3 ( Figure 5) [52]. Epoxidation of cyclohexene in vapour H 2 O 2 with NU-1000-Fe-NO 3 as catalysts initially yields cyclohexene epoxide derived from heterolytic activation of H 2 O 2 , which in turn hydrolyzes rapidly to trans-cyclohexanediol. Otherwise, NU-1000-Fe-Cl yields a mixture of products and by-products, derived from the radical oxidation products due to homolytic activation of H 2 O 2 [53,54]. This behaviour is probably due to the difference in the metal−node distance between the frameworks, the active site rearranging differently. One-step template-free synthesis of ultrathin (∼5 nm) mixed-valence {V16} clustersbased MOF nanosheets [Ni(4,4′-bpy)2]2 [V7 IV V9 V O38Cl]·(4,4′-bpy)·6H2O (NENU-MV-1) has been also reported. A large number of vanadium catalytically active sites in the NENU-MV-1 nanosheet allowed excellent cyclohexene oxidation under air exhibiting a conversion of 95%. Moreover, the nanometer scale of the catalyst increased the catalytic activity 2.7 times compared to the bulk crystal (0.25 mm) for olefin epoxidation. Excellent catalytic performances have been shown for different olefin substrates [55].

Organic Linkers with Functional Catalytically Active Sites
Functional groups such as amino, pyridyl, amide, sulfonic acid, etc. present in organic linkers serve as active sites for catalysis and strongly influence the intrinsic catalytic activity of the MOFs through inductive effects. In addition, organic linkers can be catalytically active when organic functional groups and/or functional molecular catalysts (e.g., metalloporphyrins, salen and related ligands, chiral molecules, Schiff-base complexes, etc.) are introduced by post-synthetic ways. Alternatively, the same functional molecular catalysts can also be used as building units to fabricate MOFs.
Molybdenum complexes have been widely applied as homogeneous catalysts for the epoxidation of alkenes by H2O2 and organic hydroperoxide, a complete conversion and One-step template-free synthesis of ultrathin (∼5 nm) mixed-valence {V16} clustersbased MOF nanosheets [Ni(4,4 -bpy 38 Cl]·(4,4 -bpy)·6H 2 O (NENU-MV-1) has been also reported. A large number of vanadium catalytically active sites in the NENU-MV-1 nanosheet allowed excellent cyclohexene oxidation under air exhibiting a conversion of 95%. Moreover, the nanometer scale of the catalyst increased the catalytic activity 2.7 times compared to the bulk crystal (0.25 mm) for olefin epoxidation. Excellent catalytic performances have been shown for different olefin substrates [55].

Organic Linkers with Functional Catalytically Active Sites
Functional groups such as amino, pyridyl, amide, sulfonic acid, etc. present in organic linkers serve as active sites for catalysis and strongly influence the intrinsic catalytic activity of the MOFs through inductive effects. In addition, organic linkers can be catalytically active when organic functional groups and/or functional molecular catalysts (e.g., metalloporphyrins, salen and related ligands, chiral molecules, Schiff-base complexes, etc.) are introduced by post-synthetic ways. Alternatively, the same functional molecular catalysts can also be used as building units to fabricate MOFs.
Molybdenum complexes have been widely applied as homogeneous catalysts for the epoxidation of alkenes by H 2 O 2 and organic hydroperoxide, a complete conversion and selectivity being reported. To overcome the recoverability and reusability issues correlated to the use of homogeneous molybdenum catalysts, molybdenylacetylacetonate has been supported on TMU-16-NH 2 [Zn 2 (NH 2 -BDC) 2 (4-bpdh)]·3DMF, an amine-functionalized two-fold interpenetrated MOF via dative and combined covalent and dative post-synthetic modification [57].
A high porous NU-1000 MOF has been post-modified with the chiral L-tartaric acid, by SALI (solvent-assisted ligand incorporation) to build a chiral Zr-based MOF [C-NU-1000] [58]. Moreover, another active catalytic site, molybdenyl acetylacetonate, MoO 2 (acac) 2, was incorporated on chiral NU-1000 to explore catalytic performance in the asymmetric epoxidation of olefins (Figure 6a) [59]. When olefins approach by pro-Sor R-face to the catalytic active center, they interact with the OH group of the tartrate through H-bond which induces chirality generating two chiral intermediates. The [C-NU-1000-Mo] catalyst, used in the epoxidation of styrene and 1-decene, can discriminate the S configuration in epoxides (Figure 6b).
Molybdenum tricarbonyl complexes are known to be effective catalysts for the epoxidation of olefins. They form an oxomolybdenum (VI) species in the presence of tert-butyl hydroperoxide (TBHP) as an oxidant which acts as highly active catalytic sites for the epoxidation of olefin. M(CO) 6 was deposited on UiO-66 and UiO-67 by chemical vapor deposition (CVD) treatment, UiO-66-Mo(CO) 3 , and UiO-67-Mo(CO) 3 heterogeneous catalysts being fabricated. Herein, the larger tetrahedral and octahedral cavities of UiO-67 enable more accessibility of cyclooctene to catalytically active sites showing higher catalytic activity for the cyclooctene conversion than UiO-66-M(CO) 3 [33].
Metallosalen-based crystalline porous materials have been realized for heterogeneous catalytic applications towards cyclopropanation, alkene epoxidation and hydrolytic kinetic resolution of epoxides with interesting enantioselectivities [5]. UiO-68-Me has been modified via post-synthetic exchange (PSE) with single-and mixed-M(salen) linker (M = Cu, Fe, Cr, V, Mn) to fabricate UiO-66(NH 2 ) attractive species for heterogeneous asymmetric catalysis, useful to overcome the problem of metal leaching. It was found that the single-M(salen) chiral MOFs (R)-UiO-68-Mn and (R)-UiO-68-Fe catalyse the epoxidation of alkenes to epoxides with up to a 98% ee of epoxide and 97% ee, respectively. The different catalytic metal centers in the mixed-(M)salen species UiO-68-Mn-Cr gave consecutive reactions starting from the epoxidation of alkene followed by ring-opening reaction of epoxide to produce the desired amino alcohol in 80−85% yields with 80−99.5% ee. Catalytic activity and enantioselectivity of all chiral UiO-68 catalysts remain unchanged for 10 cycles [68].
Encapsulation of Cu-or Ni-salen species in NH 2 -MIL-101(Cr) through one-pot method gave a series of effective heterogeneous catalysts in the styrene oxidation under mild conditions. Specifically, the styrene conversion obtained using TBHP was 98.78%. The concentrated electronic density around Cu(II) in the Cu salen@NH 2 -MIL-101(Cr) catalyst promoted the formation of tBuOOCu(III)-salen enhancing the selectivity to epoxide [69].
A recent synthetic strategy resides in the incorporation of different functionality in one single framework to generate a multivariate MOF (MTV-MOFs). On this basis, a chiral MOF based on multiple metallosalen bridging ligands has been synthesised. Firstly, M(salen)-derived dicarboxylate ligands H 2 L5 M [M = Cu, VO, CrCl, MnCl, Fe(OAc), and Co(OAc)] were synthesized by reactions of N,N -bis(3-tert-butyl-5-(carboxyl)salicylide (H 4 L5) and the corresponding metal salts in MeOH at room temperature. Secondly, the crystals of binary or ternary MTV-MOFs (CuV, CuMn, CuCr, CuFe, and CuCo) were obtained by heating a 1:1 or 1  3 ], the combination of Mn 3+ and Co 3+ promotes the epoxidation of alkene followed by enantioselective hydrolysis of epoxide to afford diols [70].
The achiral Zr-MOF, [PCN-224(Mn(Cl)], based on tetratopic ligand [manganese (chloride) tetrakis(4-carboxyphenyl)porphyrin [Mn(Cl)-TCPP], has been post-synthetically modified with tartrate anion, as a chiral auxiliary. The final chiral PCN-224-Mn(tart) contains two active metal sites (Zr and Mn) as Lewis acid centers and the chiral tartrate counterion, as Brønsted acid sites (OH functional group) has been investigated as chiral nucleophile catalyst towards both asymmetric epoxidation and CO 2 fixation. Asymmetric epoxidation of several aromatic and aliphatic olefins like styrene, trans-stilbene, 4-methylstyrene, αmethylstyrene, 1-phenyl-1-cyclohexene, 1-decene, and 1-octene has been tested by using PCN-224-Mn(tart) with aldehyde as co-catalyst, CH 3 CN, and O 2 . The all-reaction conversions were completed with an optimum range of epoxide selectivity 83-100% and high ee (84−100%). Several factors allow high enantioselectivity in the formation of the epoxide: the framework porosity, the active Mn center, a preferred face of the olefin (pro-S or -R face) close to produce the more stable configuration, and the noncovalent interactions between H atom of the olefinic double bond of the preferred face and chiral centers (Table 3) [71].

Epoxidation with MOF-Based Composites
One possible way to improve the chemical and mechanical stability of MOFs as potentially heterogeneous catalysts is their immobilization onto/into supports. In this contest, solid polymer, graphene, and inorganic particles [72] or inorganic polymers [73] are largely employed as supports.
To overcome the poor hydrostability of [Cu 3 -BTC 2 ] [74], a porous dendrimer-like porous silica nanoparticles (DPSNs) has been utilized as a carrier to support Cu-BTC Nps. The nanocomposites DPSNs@Cu-BTC were prepared by growing Cu 2 O NPs in the centerradial porous channels of DPSNs. After that, Cu 2 O NPs were dissolved in the presence of acid, oxidant and 1,3,5-benzenetricarboxylic acid (H 3 BTC) [75]. The obtained Cu-BTC NPs have shown limited growth and a uniform distribution without agglomeration. The small size of Cu-BTC NPs (40 ± 25 nm) is useful in the aerobic epoxidation of various cyclic olefins achieving high catalytic activity without by-products. Good yield and selectivity were detected with inert terminal linear alkenes. Otherwise, epoxidation of styrene only achieved 65% of conversion due to the kinetic instability of styrene oxide (Table 4) [76].
The amphiphilic MIL-101-GH, a porous hierarchical material, has been explored as catalyst for the biphasic epoxidation reaction of 1-octene with H 2 O 2 . MIL-101-GH hydrogel was obtained by dispersing MIL-101 nanoparticles homogeneously in aqueous graphene oxide (GO) solutions. The TS-1 catalyst, commercially used in this biphasic reaction, was then introduced in MIL-101-GH. The resulting system, MIL-101-GH-TS-1, overcame the lower activity toward olefin epoxidation of TS-1, and the amphiphilic MIL-101-GH increased the contact areas of TS-1 with both H 2 O 2 and 1-octene. The catalytic performance of MIL-101-GH-TS-1 has been much higher than that of single TS-1 and the 1,2-epoxyoctane was obtained without other by-products [77].
Polyoxometalate-based (POMs) heterogeneous catalysts are attractive species in the catalytic epoxidation of olefin. They have got great catalytic activity, selectivity, and easy separation but their leaching mainly due to the strong complexing capability of solvent and H 2 O 2 oxidants, represents the major obstacle in the possible applications [78,79]. To overcome the stability issue of POMs, the polyoxomolybdic cobalt (CoPMA) and polyoxomolybdic acid (PMA) species were incorporated into UiO-bpy, a Zr-based MOFs, through self-assembly process under solvothermal condition [80]. CoPMA@UiO-bpy showed the highest catalytic activity for cyclooctene oxidation with H 2 O 2 and also for the oxidation of styrene and 1-octene with O 2 as oxidant and tert-butyl hydroperoxide (t-BuOOH) as initiator. This is due to the uniform distribution and better immobilization of POM clusters within the size-matched cages of Zr-MOFs owing to the presence of bipyridine groups in the UiO-bpy framework. It is noteworthy that CoPMA@UiO-bpy shows excellent recyclability and stability against the leaching of active POM species.
Composite material has been obtained by encapsulating H 5 -PMo 10 V 2 O 40 polyoxometalates (POMs) and 1-octyl-3-methylimidazolium bromide, ionic liquids (ILs), in the mesoporous cages and large surface area of MIL-100 (Fe). The synergic effect of ILs, Lewis and Brønsted acid sites in both PMo 10 V 2 species and MOF created a PMo10V2-ILs@MIL-100(Fe) hybrid with significant catalytic properties in cycloolefins epoxidation. Indeed, the PMo10V2 was activated by the imidazolium cations originated from ILs and the incorporation on MIL-100(Fe) prevented the leaching of POMs [81]. This composite is easily regenerated for 12 cycles without loss catalytic performance [82].
MIL-100(Fe) combined with the polyoxometalate (C 16 H 36 N) 6 K 2 [γ-SiW 10 O 36 ] has been reported to catalyse epoxidation of 3Z,6Z,9Z-octadecatriene to the corresponding 6,7-epoxide with high site selectivity (82.35%). The conversion catalysed by POM/MIL-100(Fe) exhibits a greater performance when the MOF contains unsaturated Lewis acid iron ions [83]. The main product of this epoxidation is a sex pheromone of E. obliqua Prout and can be potentially used in pest insect control with environmental friendliness.
Two POMs-based MOFs, [Cu 6 (bip) 12 6 ]·6H 2 O (H 2 bip = 1,3-bis(imidazolyl)propane; bib = 1,4-bis(imidazol)butane)), have been fabricated using a flexible N-containing bidentate ligands via hydrothermal condition. They have been employed in the catalytic processes for selective alkene epoxidation and recycled four times without loss of quality ( Figure 7) [84]. Brønsted acid sites in both PMo10V2 species and MOF created a PMo10V2-ILs@MIL-100(Fe) hybrid with significant catalytic properties in cycloolefins epoxidation. Indeed, the PMo10V2 was activated by the imidazolium cations originated from ILs and the incorporation on MIL-100(Fe) prevented the leaching of POMs [81]. This composite is easily regenerated for 12 cycles without loss catalytic performance [82]. MIL-100(Fe) combined with the polyoxometalate (C16H36N)6K2[γ-SiW10O36] has been reported to catalyse epoxidation of 3Z,6Z,9Z-octadecatriene to the corresponding 6,7epoxide with high site selectivity (82.35%). The conversion catalysed by POM/MIL-100(Fe) exhibits a greater performance when the MOF contains unsaturated Lewis acid iron ions [83]. The main product of this epoxidation is a sex pheromone of E. obliqua Prout and can be potentially used in pest insect control with environmental friendliness.
A dually functionalized catalytic system for the tandem H2O2-generation/alkene-oxidation reaction has been realized. A microcrystal of UiO-66-NH2 has been used as a platform to encapsulate Au and Pd metal NPs and later Pd/Au@UiO-66-NH2 surfaces have been post-synthetically modified with a (sal)Mo VI (sal = salicylaldimine) molecular epoxidation catalyst. The porosity of Pd@UiO-66-sal(Mo) allows H2 and O2 gases to come into contact with the encapsulated NPs to generate H2O2. The synergic effect of the generated Metal nanoparticles can grow without agglomeration in a porous matrix to produce a stable and active heterogeneous catalyst. Pd NPs have been loaded on the pre-synthesized UiO-66-NH 2 using a simple solution impregnation method and NaBH 4 reduction. The amino groups in the linkers allow a strong interaction with Pd (II) ions which is essential to yielding well-dispersed Pd/UiO-66-NH 2 catalyst. The experiments suggest that the best catalytic activity for styrene epoxidation has been found under Pd NPs loadings of 3.69 wt% [85].
A dually functionalized catalytic system for the tandem H 2 O 2 -generation/alkeneoxidation reaction has been realized. A microcrystal of UiO-66-NH 2 has been used as a platform to encapsulate Au and Pd metal NPs and later Pd/Au@UiO-66-NH 2 surfaces have been post-synthetically modified with a (sal)Mo VI (sal = salicylaldimine) molecular epoxidation catalyst. The porosity of Pd@UiO-66-sal(Mo) allows H 2 and O 2 gases to come into contact with the encapsulated NPs to generate H 2 O 2 . The synergic effect of the generated H 2 O 2 and (sal)Mo VI in a MOF enhanced epoxide productivity reducing alkene hydrogenation side reaction. This study showed that (sal)Mo moieties in Pd@UiO-66-NH 2 epoxidize cis-cyclooctene substrate faster, leading to the more effective usage of the H 2 O 2 oxidant [86].
Systems composed of a magnetic uniform Fe 3 O 4 (PAA) microspheres core and of a copper-doped MOF shell demonstrated an easily catalyst recovery approach improving turnover number and turnover frequency. In addition, these magnetic core-shell heterogeneous catalysts improve both stability of the metal active site and dispersity of catalyst materials reducing the metal leaching. Two interesting magnetic core-shell copper-doped catalysts, Fe 3 O 4 @P4VP@ZIF-8 and Fe 3 O 4 /Cu 3 (BTC) 2 have been prepared by combining the solvothermal method with layer-by-layer assembly. Initially, monodispersed PAAmodified Fe 3 O 4 particles were synthesized by solvothermal methods [87]. In the case of Fe 3 O 4 /Cu 3 (BTC) 2 , Fe 3 O 4 particles were alternately immersed in solutions containing Cu(CH 3 COO) 2 ·H 2 O and H 3 BTC such that Cu 3 (BTC) 2 nanocrystals grow layer-by-layer on the surface of PAA-modified Fe 3 O 4 particles. This nanosized porous structure increases the contact between the Cu(II) active sites present in the Cu 3 (BTC) 2 shell and the catalytic substrates [88]. In Fe 3 O 4 @P4VP@ZIF-8 catalyst, on the other hand, the Fe 3 O 4 (PAA) core has been coated with P4VP middle layer to adsorb a large number of Zn 2+ for the growth of the ZIF-8 shell thickness on the surface of the core-shell Fe 3 O 4 (PAA)@P4VP. Then, the Zn 2+ ions were partially substituted by Cu 2+ ions in the ZIF-8 shell framework. The ions exchange allowed a well-dispersed copper active site in the resulting copper-doped ZIF-8 structure, avoiding their leaching [89].
Aerobic epoxidation of cyclic olefins (e.g., cyclohexene, norbornene) using both magnetic core-shell copper-doped Fe 3 O 4 @P4VP@ZIF-8 and Fe 3 O 4 /Cu 3 (BTC) 2 as heterogeneous catalyst achieved high conversion and selectivity (99%) in the formation of the epoxide under mild reaction conditions. Epoxidation of styrene by using Fe 3 O 4 @P4VP@ZIF-8 as a catalyst has brought only 54% selectivity of the desired epoxide owing to the kinetic instability of styrene oxide and its oxidation into benzaldehyde [90].
A series of Zr-based core-shell MOF composites with mesoporous cores and microporous shells have been synthesized by solvothermal under kinetic control. PCN-222(Fe) crystals have been synthesized and used as seed crystals to grow the Zr-BPDC(UiO-67) crystals. Meso-and micro-porosity inside of PCN-222(Fe)@Zr-BPDC(UiO-67) drives the catalytic performances for olefin epoxidation reaction [91]. Indeed, the core MOF with Fe-porphyrin moieties represents the catalytic center, while the shell controls the selectivity of the substrate through tuneable pore size. This size-selective catalyst showed almost complete conversions for small olefins.

CO 2 Epoxide Cycloaddition to Cyclic Carbonates
Cycloaddition reaction of CO 2 with epoxides represents one of the most economically efficient approaches in the production of cyclic organic carbonates with relevant applications ranging from raw materials in the pharmaceuticals industry, polar aprotic solvents, electrolytes in lithium batteries, lubricants, precursors for polycarbonate materials, and other fine chemicals. CO 2 , being a C1 feedstock has, in fact, a high potential from the chemical point of view [92]. CO 2 can be employed in the highly atom-economical acid-catalysed epoxides cycloaddition to give cyclic organic carbonates, relevant species for industrial applications [93]. The cyclic carbonates (OCs) have been also used as intermediates for engineered polymers, as a lubricant (in 1987 Agip Petroli added dialkylcarbonates as lubricant in a formulation of semisynthetic gasoline engine oil components), and more recently found application in varnish production, green solvents or electrolytes in lithium-ion batteries.
The CO 2 cycloaddition mechanism involves an acid catalyst (Lewis or Brønsted acid) that coordinates to the epoxide substrate activating it toward nucleophilic attack by the co-catalyst (e.g., typically a tetraalkylammonium halide). The resulting halo-alkoxide intermediate reacts with carbon dioxide to generate the cyclic carbonate and subsequently regeneration of both catalyst and co-catalyst [93].
The CO 2 fixation reaction catalysed by homogeneous or heterogeneous catalysts has been extensively investigated, however some drawbacks remain. Differently from the homogeneous, heterogeneous catalysts (e.g., ionic liquid-supported solids [94][95][96], polymers [96,97], and porous organic frameworks [98]) have the advantages of easy separation and regeneration of the catalyst, but they often required rough conditions (high temperature, pressure, and time) due to a lack of accessible surface area for accelerating interactions of CO 2 and reagents with active sites. Therefore, the high surface area, tunability, and CO 2 sorption capacity of MOFs can be beneficial for improving the efficiency of the CO 2 cycloaddition reaction. Lewis acid metal centers and Brønsted acid groups in MOFs can promote the activation of the epoxide ring, while the functional groups in the ligands can act as Lewis/basic sites improving not only the CO 2 affinity inside the pore but also can fulfil the role of co-catalyst (Figure 8) [99]. Inorganics 2021, 9,   The Hf-cluster-based NU-1000 (Hf-NU-1000) demonstrated excellent catalytic activity, greater than the Zr-cluster-based NU-1000 under the same mild reaction conditions [100]. Indeed, the presence of high density stronger acidic Brønsted sites, due to stronger M−O bonds, gave a complete and quantitative conversion of styrene oxide and propylene oxide to form cyclic carbonates. Moreover, high yields have been detected for the cycloaddition reaction of CO2 with industrially important epoxide divinylbenzene dioxide (DVBDO) [101].
Large pores in the MOFs, easily functionalized by polar groups, can promote CO2 fixation in a short reaction time under ambient CO2 pressure and moderate temperature without the use of solvent. Within the mesoporous M-MOF-184 series (M = Co, Ni, Mg, Zn), Zn-MOF-184 achieved efficient catalysis performances to convert CO2 to cyclic carbonates under ambient conditions for several epoxy substrate, due to the presence of high concentration of accessibly acidic metals, basic 2-oxidobenzoate anion sites and to the high polarity induced by C≡C bonds and π systems from the phenyl rings in the linkers. Low conversion has been detected for larger epoxides due to limit diffusion into the MOF pores of reactants toward the active sites [102]. The hydrothermally synthesized flexible Zn-based {[Zn2(TBIB)2(HTCPB)2]·9DMF·19H2O}n, has been synthesized employing two types of large linkers 1,3,5-tri(1H-benzo[d]imidazol-1-yl)benzene (TBIB) and 1,3,5tris(4′-carboxyphenyl-)benzene (H3TCPB). A porous structure with 1D channels was generated via noncovalent supramolecular interactions between the layers. The presence of free protonated carboxylic acid groups(−COOH), carbonyl groups (−C=O), and the presence of Lewis basic sites from the rich N-containing TBIB on the surface pores enhance the selectivity toward CO2. Moreover, the COOH group helps in catalysing the CO2 cycloaddition reaction efficiently through noncovalent interaction with the epoxide substrate, followed by ring-opening upon nucleophilic attack of co-catalyst [103].
Zeolitic imidazolate frameworks are known for their high CO2 solubility and capture ability [104], especially the chloro-functionalized ZIF-95 [105]. The CO2 cycloaddition to propylene oxide by using ZIF-95 and a quaternary ammonium salt as cocatalyst procured over 99% selectivity to the desired propylene carbonate product under moderate conditions [106]. Also the imidazolate-containing species Im-UiO66(Zr)MOF reacts with methyl iodine to produce (I − )MeIm-UiO-66 that demonstrate efficiency in the CO2 cycloaddition reaction toward a broad range of substrates, in this case without the addition of co-catalyst [107].
Conversely, imidazolium-based IL units were grafted and immobilized into UiO-67 via direct ligand functionalization that, considering the post synthetic approach, is a quantitative method. The obtained species show a high density of IL sites. UiO-67-IL converts epichlorohydrin substrate in 95% yield under co-catalyst and solvent-free conditions. The yield increases to 99% in a shorter time when TBAB was employed (TBAB = tetrabutylammonium bromide) [108] (Table 5).
UiO-66-NH2 pores were modified with ILs such as methylimidazolium bromide and methylbenzimidazolium bromide by coupling reactions, to generate ILA@U6N and ILB@U6N MOFs. The Lewis acid sites (for activation of the epoxide) and the IL functional The Hf-cluster-based NU-1000 (Hf-NU-1000) demonstrated excellent catalytic activity, greater than the Zr-cluster-based NU-1000 under the same mild reaction conditions [100]. Indeed, the presence of high density stronger acidic Brønsted sites, due to stronger M−O bonds, gave a complete and quantitative conversion of styrene oxide and propylene oxide to form cyclic carbonates. Moreover, high yields have been detected for the cycloaddition reaction of CO 2 with industrially important epoxide divinylbenzene dioxide (DVBDO) [101].
Large pores in the MOFs, easily functionalized by polar groups, can promote CO 2 fixation in a short reaction time under ambient CO 2 pressure and moderate temperature without the use of solvent. Within the mesoporous M-MOF-184 series (M = Co, Ni, Mg, Zn), Zn-MOF-184 achieved efficient catalysis performances to convert CO 2 to cyclic carbonates under ambient conditions for several epoxy substrate, due to the presence of high concentration of accessibly acidic metals, basic 2-oxidobenzoate anion sites and to the high polarity induced by C≡C bonds and π systems from the phenyl rings in the linkers. Low conversion has been detected for larger epoxides due to limit diffusion into the MOF pores of reactants toward the active sites [102]. The hydrothermally synthesized flexible Zn-based {[Zn 2 (TBIB) 2 (HTCPB) 2 ]·9DMF·19H 2 O} n, has been synthesized employing two types of large linkers 1,3,5-tri(1H-benzo[d]imidazol-1-yl)benzene (TBIB) and 1,3,5-tris(4carboxyphenyl-)benzene (H 3 TCPB). A porous structure with 1D channels was generated via noncovalent supramolecular interactions between the layers. The presence of free protonated carboxylic acid groups(−COOH), carbonyl groups (−C=O), and the presence of Lewis basic sites from the rich N-containing TBIB on the surface pores enhance the selectivity toward CO 2 . Moreover, the COOH group helps in catalysing the CO 2 cycloaddition reaction efficiently through noncovalent interaction with the epoxide substrate, followed by ring-opening upon nucleophilic attack of co-catalyst [103].
Zeolitic imidazolate frameworks are known for their high CO 2 solubility and capture ability [104], especially the chloro-functionalized ZIF-95 [105]. The CO 2 cycloaddition to propylene oxide by using ZIF-95 and a quaternary ammonium salt as cocatalyst procured over 99% selectivity to the desired propylene carbonate product under moderate conditions [106]. Also the imidazolate-containing species Im-UiO66(Zr)MOF reacts with methyl iodine to produce (I − )MeIm-UiO-66 that demonstrate efficiency in the CO 2 cycloaddition reaction toward a broad range of substrates, in this case without the addition of co-catalyst [107].
Conversely, imidazolium-based IL units were grafted and immobilized into UiO-67 via direct ligand functionalization that, considering the post synthetic approach, is a quantitative method. The obtained species show a high density of IL sites. UiO-67-IL converts epichlorohydrin substrate in 95% yield under co-catalyst and solvent-free conditions. The yield increases to 99% in a shorter time when TBAB was employed (TBAB = tetrabutylammonium bromide) [108] (Table 5).
UiO-66-NH 2 pores were modified with ILs such as methylimidazolium bromide and methylbenzimidazolium bromide by coupling reactions, to generate ILA@U6N and ILB@U6N MOFs. The Lewis acid sites (for activation of the epoxide) and the IL functional sites (for epoxide ring-opening) efficiently catalyse the epichlorohydrin conversion under mild conditions [109].
A linear ionic polymer was inserted inside the MIL-101(Cr) via in situ polymerization to form polyILs@MIL-101(Cr) stable heterogeneous composites. This polyILs@MIL-101 is able to catalyse the CO 2 cycloaddition reaction with various epoxides with good to excellent conversions, including terminal epoxides with both electron-withdrawing and electron-donating substituents without the need of co-catalyst [110].
A new multimodal catalytic system has been designed via two steps post-synthetic modification of the metal nodes in the NU-1000 framework. A tandem functionalization was performed starting from the incorporation of ortho-, meta-, and para-pyridinecarboxylic acids into the framework of NU-1000(M), then the pyridine moieties were alkylated with various haloalkanes (CH 3 I, C 4 H 9 I, C 4 H 9 Br, and C 6 H 4 F 9 I) to introduce co-catalyst moieties near to the inorganic node [111]. Among catalysts, NU-1000(Zr) functionalized with 4-PyCOOH and CH 3 I, i.e., SALI-4-Py-I-(Zr), showed the highest styrene carbonate yield without co-catalyst, the epoxy ring being activated upon coordination to Zr 4+ center (Lewis acid site) and the halogen anion opening the epoxy ring by nucleophilic attack on the less sterically hindered carbon atom [111].
Another strategy developed to increase the catalytic performances is based on the incorporation of an amine group in MOFs. Essentially, the amino group has the dual advantage of acting as an electron donor (Lewis base) toward CO 2 and increasing the local concentration of CO 2 near catalytic centres through a high CO 2 adsorption [113,114].
The amine-functionalized NH 2 -MIL-101(Al) has been synthesized using a solvothermal or microwave method and its catalytic activity in the solvent-free cycloaddition of CO 2 to styrene oxide achieved nearly total conversion and selectivity in 96% yield, with a TOF of 23.5 h −1 [115]. The coordinatively unsaturated aluminium centers present in the SBUs (Lewis acidic sites) bind the epoxides and activate them toward ring-opening, this step is immediately followed by the attack of the bulky bromide ions of TBAB. The pendant amino groups polarize the CO 2 molecules, through the nucleophilic attack at the carbon atom, and facilitate CO 2 insertion and cycloaddition (Figure 9). During the catalytic reaction, the micro and mesoporous of the framework facilitate the diffusion of substrates and reactants to enhance their interactions [116].
Recently, the acid-base pair UiO-66-NH 2 has been used to synthesize bio-based fivemembered cyclic carbonate from vegetable oil methyl ester by CO 2 fixation. At first, 95% of double bonds in the O-acetyl methyl ricinoleate starting material were converted to epoxide through an enzyme-catalyzed process. Then, the cycloaddition of epoxy fatty acid methyl esters was performed in the presence of UiO-66-NH 2 as catalyst and TBAB as co-catalyst for CO 2 fixation. At 120 • C under 3 MPa CO 2 pressure for 12 h, the reaction conversion reached 94.4% [117] (Table 5).
A series of diamino-tagged zinc bipyrazolate MOFs have been investigated as heterogeneous catalyst in the reaction of CO 2 with the epoxides epichlorohydrin and epibromohydrin to give the corresponding cyclic carbonates at 393 K and pCO 2 5 bar under relatively mild conditions (solvent and co-catalyst-free) [118]. The presence of amino group in the MOFs pores increased the CO 2 storage capacity as well as the catalytic performances. The epoxide has been activated through halogen-amine interaction which was observed in structure of the [epibromohydrin@Zn(3,3 -(NH 2 ) 2 BPZ)] adduct. The isomeric Lewis basic site (NH 2 ) in Zn(3,5 NH 2 -Bpz) (64% yield) improves more than twice the catalytic transformation of epichlorohydrin compared to its mono(amino) parent Zn(BPZNH 2 ) (32% yield) [118]. Inorganics 2021, 9, x FOR PEER REVIEW 18 of 29 Figure 9. Proposed mechanism for the cycloaddition of styrene oxide and CO2 using tetrabutylammonium bromide.
Recently, the acid-base pair UiO-66-NH2 has been used to synthesize bio-based fivemembered cyclic carbonate from vegetable oil methyl ester by CO2 fixation. At first, 95% of double bonds in the O-acetyl methyl ricinoleate starting material were converted to epoxide through an enzyme-catalyzed process. Then, the cycloaddition of epoxy fatty acid methyl esters was performed in the presence of UiO-66-NH2 as catalyst and TBAB as cocatalyst for CO2 fixation. At 120 °C under 3 MPa CO2 pressure for 12 h, the reaction conversion reached 94.4% [117] (Table 5).
A series of diamino-tagged zinc bipyrazolate MOFs have been investigated as heterogeneous catalyst in the reaction of CO2 with the epoxides epichlorohydrin and epibromohydrin to give the corresponding cyclic carbonates at 393 K and pCO2 5 bar under relatively mild conditions (solvent and co-catalyst-free) [118]. The presence of amino group in the MOFs pores increased the CO2 storage capacity as well as the catalytic performances. The epoxide has been activated through halogen-amine interaction which was observed in structure of the [epibromohydrin@Zn(3,3′-(NH2)2BPZ)] adduct. The isomeric Lewis basic site (NH2) in Zn(3,5 NH2-Bpz) (64% yield) improves more than twice the catalytic transformation of epichlorohydrin compared to its mono(amino) parent Zn(BPZNH2) (32% yield) [118].
Vanadium chlorides have been used to produce the post-metalated Zr-based MOF-VCl3 and MOF-VCl4, with biphenyl-4,4′-dicarboxylic acid, and 2,2′-bipyridine-5,5′dicarboxylic acid, respectively, which provide Lewis basic sites. Their high catalytic activity in the CO2 cycloaddition to various organic epoxides was attributed to the accessible Cu 2+ /Ni 2+ metal active sites in the metallocyclams and the presence of coordinatively unsaturated Zr 4+ sites in the equatorial plane of the Zr6 cluster in VPI-100 MOFs [112]. Post-synthetic metalation of organic linkers is employed strategically to tailor the MOFs' properties. In Hf-Bipy-UiO-67, the 2,2-bipyridine-5,5-dicarboxylate ligand was grafted with Mn(OAc) 2 and the resulting Hf-Bipy-UiO-67(Mn(OAc) 2 showed that synergy of the binary Lewis acid function significantly enhances the CO 2 uptake capacity and catalytic performance of the cycloaddition reaction under mild conditions [119].
Vanadium chlorides have been used to produce the post-metalated Zr-based MOF-VCl3 and MOF-VCl4, with biphenyl-4,4 -dicarboxylic acid, and 2,2 -bipyridine-5,5 -dicarboxylic acid, respectively, which provide Lewis basic sites. Their high catalytic activity in the CO 2 cycloaddition to various organic epoxides was attributed to the accessible Cu 2+ /Ni 2+ metal active sites in the metallocyclams and the presence of coordinatively unsaturated Zr 4+ sites in the equatorial plane of the Zr 6 cluster in VPI-100 MOFs [112].
UiO-type MOFs become susceptible to water and alkaline solution when the length of the carboxylic linker increase. A series of UiO-type MOF named ZSF, incorporating chiral metallosalen as linker has been produced. The chemically stable ZSF-1 MOF, synthesized by dissolving a mixture of ZrCl 4 , Cy-salen-Ni, and modulators (trifluoroacetic acid), showed excellent catalytic performance for the conversion of CO 2 with epoxides into cyclic carbonates. The tetrahedral cages of ZSF-1 decorated with salen-Ni moieties entrap efficiently CO 2 and activate the substrate. ZSF-1 catalyses efficiently the asymmetric cycloaddition of CO 2 with styrene oxide giving 94% yield of the resulting cyclic carbonate [120]. With other epoxides, specifically epichlorohydrin, the catalytic activity of ZSF-1 increases until to 99% of conversion thanks to the presence of electron-withdrawing Cl group, which promotes the nucleophilic attack of Br − during the ring-opening process.
The chiral PCN-224-Mn(tart) (see Section 2.3) has been used in asymmetric CO 2 cycloaddition to styrene epoxide, its derivative showing conversions of 96% and 87%, respectively. The missing-linker defects in the Zr cluster and in the Mn center are Lewis acids inducing catalytic ability into the framework for CO 2 chemical fixation. In addition, the auxiliary chiral tartrate anions, and the co-catalyst (Bu 4 NBr) act as nucleophiles generating a chiral epoxide, semi-intermediate, starting from prochiral styrene substrate. The CO 2 addition leads asymmetrically to cyclic carbonate with a high ee, and it is related to the interaction of the chiral centers and substrate pro R/S face. Moreover, catalytic reactions with PCN-224-Mn(tart) were performed at low energy and ambient pressure and temperature [71].

Conclusions
MOF-based catalysts are now a very promising class of compounds as they merge relevant characteristics of both homogeneous and heterogeneous catalysts. They can be easily modified by changing linkers substituents to increase affinity for reactants, or by growing the number of active catalytic sites.
In this review, we have explored the ability of MOFs, MOF nanocomposites and mixed metal species toward olefin epoxidation and carbon dioxide cycloaddition.
We have observed that the olefin conversion and the epoxide selectivity are strongly dependent on the metal nodes/clusters, Co and Cu species being the most efficient, in some cases as for the epoxidation of a-pinene by Co-MOF-150-2 a conversion and an epoxide selectivity close to 100% being found.
Mixed metal MOFs can be also successfully employed in styrene and cyclohexene epoxidation, the best results being obtained with Cu/Co, Mn/Cu, and Ni/V species.
Selected functional groups introduced in organic linkers can also act as catalytically active sites. Amino, pyridyl, amide and sulfonic acid groups, but also metalloporphyrins, vanadium and molybdenum acetylacetonate, tartaric acid, salen and analogous molecules can be inserted or deposited to obtain also greater selectivity. UiO-66, UiO67, and PCN-224, appropriately functionalized can induce a complete conversion and selectivity as in the case of the geraniol epoxidation.
MOF-based composites are often employed to increase the hydrostability of selected MOFs or to perform epoxidation also of specific substrates as norbornene or octadecatriene. Specifically, a porous dendrimer-like porous silica nanoparticles (DPSNs) used as a carrier to support Cu-BTC NPs overcame the poor hydrostability of [Cu 3 -BTC 2 ] MOF achieving high catalytic activity without by-products under mild reaction conditions.
Finally, MOFs and MOF-based composites show a great efficiency toward CO 2 cycloaddition to epoxides, conversion being generally in the range 70-100% and selectivity close to 100%. The use of chiral ligands and amine-functionalized ligands seems to be very promising. The CO 2 binding mode can in fact open new strategies for activation of CO 2 and its transformation.
However, the low reactivity and inert nature of CO 2 make its incorporation and activation into organic substrates still a challenge. Currently, the heterogeneous MOFsbased catalysts, as well as the technical system, remain at the laboratory scale and that makes the costs of productions of these materials extremely pricey. It is desirable that the improvement of MOFs-based catalysts might lead to technically viable efficiencies to industrial production to allow their large-scale application, in the next future. This review clearly shows that MOFs are now perspective materials and valid candidates for catalytic epoxidation and CO 2 cycloaddition reactions.
Author Contributions: Conceptualization, A.T. and C.P.; methodology, A.T. and C.P.; software, A.T. and C.P.; validation, A.T. and C.P.; data curation, A.T. and C.P.; writing-original draft preparation, A.T. and C.P.; writing-review and editing, A.T. and C.P.; funding acquisition, C.P. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by University of Camerino.

Conflicts of Interest:
The authors declare no conflict of interest.