Development of Covalent Triazine Frameworks As Heterogeneous Catalytic Supports

polymers

Review Development of Covalent Triazine Frameworks as Heterogeneous Catalytic Supports

Norini Tahir †, Chidharth Krishnaraj † , Karen Leus * and Pascal Van Der Voort * Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281 (S3), 9000 Ghent, Belgium * Correspondence: [email protected] (K.L.); [email protected] (P.V.D.V.); Tel.: +32-926-444-46 (P.V.D.V.) Both authors contributed equally to this work. † Received: 27 May 2019; Accepted: 5 August 2019; Published: 9 August 2019

Abstract: Covalent triazine frameworks (CTFs) are established as an emerging class of porous organic polymers with remarkable features such as large surface area and permanent porosity, high thermal and chemical stability, and convenient functionalization that promotes great potential in heterogeneous catalysis. In this article, we systematically present the structural design of CTFs as a versatile scaﬀold to develop heterogeneous catalysts for a variety of chemical reactions. We mainly focus on the functionalization of CTFs, including their use for incorporating and stabilization of nanoparticles and immobilization of molecular complexes onto the frameworks.

Keywords: covalent triazine frameworks; heterogeneous catalysis; catalytic supports; metal catalysis; organic synthesis

1. Introduction to Covalent Triazine Frameworks (CTFs) Covalent triazine frameworks (CTFs) represent a class of highly stable porous organic materials based on triazine linkages. CTFs are generally prepared through a simple trimerization of aromatic nitriles in the presence of a catalyst. The formation of triazine rings from nitrile compounds catalyzed by molten metal salts has been applied since the early 1960s [1]. G.H Miller (Texaco Inc.) extended this approach by introducing the potential of metal chloride as a good catalyst for the trimerization of a variety of aromatic nitriles [2]. The formation of thermally stable, insoluble and infusible polymeric materials occurs at elevated temperatures of 410 to 550 ◦C. These triazine containing polymers possess rich electron-deficient structures and have gained attention in supramolecular chemistry because of their strong tendency for hydrogen bonding, metal chelation and π-π interactions [3]. In 2008, Thomas and co-workers adapted the concept and introduced the synthesis of CTFs catalyzed by ZnCl2 under ionothermal reaction conditions which produces permanent porosity and crystallinity. They first reported CTF-1 with a surface area of 790 m2 g 1 and a pore volume of 0.40 cm3 g 1 which was · − · − obtained from the trimerization of 1,4-dicyanobenzene in molten ZnCl2 in a closed quartz ampule at 400 ◦C for 40 h (Scheme1a) [4]. Structurally, most of the reported CTFs are amorphous because of the harsh synthesis conditions and high stability of the triazine linkage, yet they exhibit high specific surface area and permanent porosities [5]. In addition, the chemical structures and functionalities of this porous material can be adjusted by varying the structure of the monomer used as the building block. The introduction of heteroatoms or functional groups into the frameworks enhance the potential of CTFs for various applications especially gas storage, adsorption and catalysis. The utilization of CTFs for various applications have been reviewed in several reports. Jin et al. reported the synthesis and applications of CTFs for energy and environmental purposes [6]. Tan et al.

Polymers 2019, 11, 1326; doi:10.3390/polym11081326 www.mdpi.com/journal/polymers Polymers 2019, 11, 1326 2 of 24 reviewed the development and challenges in the synthesis and potential applications of CTFs in gas adsorption and separation, energy storage and conversion, photocatalysis as well as heterogeneous catalysis [7]. In addition, our group has reported the functionalization of CTFs with nitrogen rich functionalities, (pyridine-4,2,6-triyl)-based CTF, and fluorine-containing hydrophobic CTF. The synthesized materials effectively enhanced the adsorption properties and selectivity towards CO2 and H2 due to stronger interaction between the gas molecules and the functionalized CTF materials [8,9]. Furthermore, we also demonstrated the potential of CTFs as a platform for the encapsulation of γ-Fe2O3 for implementing it as an excellent and novel adsorbent toward remediation of inorganic contaminants in water [10]. The presence of stoichiometric and well-defined triazine moieties in the framework makes CTFs very attractive materials as scaffold for metals, especially in heterogeneous catalysis [11]. They are particularly interesting owing to their high basicity due to the nitrogen-containing triazine moieties, exceptional chemical inertness, especially in acidic and basic media, and outstanding stability over a broad range of temperatures and pressure. Furthermore, their high surface area and porosity with rigid pore structures enables the facile diffusion of substrates and products during catalysis. The application of CTFs in catalysis is a rapidly developing interdisciplinary research field. A number of reviews have been reported toward the applications of triazine-based polymers in heterogeneous catalysis [12,13]. This paper will present an in-depth review on the recent advances in preparation and functionalization of CTF materials for heterogeneous catalysis, and focuses on the development of CTFs as catalyst supports with an emphasis on the application in synthetic organic chemistry.

2. Design and Synthesis Methods of CTFs Over the last decade, several procedures for the synthesis of CTFs have been developed. Porous CTF materials having a desired topology and functionality can be constructed from various monomers with specific functional groups, under suitable reaction conditions. For instance, CTFs can be prepared through the trimerization of aromatic nitriles in the presence of acidic catalysts [4,14], polycondensation reaction of monomer containing specific functional groups [15,16], and cross-coupling reaction of triazine containing building units [17,18]. However, for catalytic applications, the surface area and pore size, as well as the stability during the catalytic reaction conditions and reusability are important criteria for the material assessment. Especially for catalytic purposes, the accessibility of immobilized metal species, specific surface area and porosity play a major role especially in terms of mass transfer throughout the catalyst and eventually the overall catalytic activity. The following sections contain various methods used to synthesize CTFs and their effect on the end-product.

2.1. Trimerization of Aromatic Nitriles Diﬀerent conditions can be used for the trimerization of nitrile compounds leading to a triazine unit. The most common trimerization of nitriles is conducted under ionothermal conditions in molten ZnCl2 at 400 ◦C for 40–48 h in a closed quartz ampule [4]. Molten ZnCl2 is a good catalyst for the trimerization reaction and has a highly soluble reaction medium for the nitrile compounds. Harsh reaction conditions are applied to ensure the reversibility of the cyclotrimerization reaction [19]. However, the high stability of the triazine linkage causes low reversibility of the polymerization process. Hence, most of the obtained CTFs have a low crystallinity or show a limited long-range order to which results into the formation of amorphous structures. To date, only CTF-0, CTF-1 and CTF-2 have been successfully synthesized as crystalline CTFs using the ZnCl2 route [4,20,21]. An excess amount of ZnCl2 as the molten solvent as well as an increase in the synthesis temperature, results in an increased speciﬁc surface area and porosity of CTFs. However, partial decomposition occurs at elevated temperatures causing a loss of nitrogen and the creation of structural defects and thus results in mainly amorphous materials. Besides, synthesis at a temperature below 350 ◦C using extended reaction times up to 168 h leads to the formation of oligomeric products without any porosity [22]. Polymers 2019, 11, 1326 3 of 24

An advanced synthetic protocol involves the addition of a salt template during the polymerization of CTFs to introduced mesopores into a microporous material without the loss of nitrogen content in the framework. In this approach, binary mixtures of ZnCl2 with various alkali chlorides such as LiCl, NaCl, and KCl were used as a reaction medium for the trimerization of nitriles [23]. The synthesis was performed at 300 ◦C during 60 h to initiate the formation of oligomers and subsequently, the samples were heated to 450 ◦C for an additional 5 h for complete polymerization. The salt-templated materials exhibit a significantly increased specific surface area and lead to an enormous increase of the mesopore volume up to ten times compared to the conventional method for CTF-1. However, the solubility of the used organic species in the corresponding salt melt turned out to be crucial for the generation of porous CTFs. Another approach to reduce the polymerization reaction time is by using microwave (MW)-assisted ionothermal synthesis [24]. ZnCl2 is a good MW absorber, and high porosity CTFs were obtained easily within 10–60 min. However, the fast heating rate and high reaction temperature led to carbonization and rather low nitrogen contents. The trimerization of aromatic nitriles can also be catalyzed by CF3SO3H in CHCl3 at room temperature or even within a microwave-assisted condition [14]. Despite mild reaction conditions that avoid the incipient carbonization of the materials, this approach cannot be applied to all functional nitriles building blocks and also the CTFs produced using this method exhibited low to moderate Brunauer–Emmett–Teller (BET) surface area compared to the ionothermal synthesis route. Hence, the ZnCl2 catalyzed trimerization of aromatic nitriles is the most practical approach due to the ease of synthesis and feasibility for a variety of functional building units, which in turn can be utilized for heterogeneous catalytic applications. However, the presence of residual Zn in the CTF even after a thorough cleaning cannot be ignored. Usually, at the end of the synthesis, the CTF is formed as a monolith which is crushed into a powder before carrying out the cleaning treatments. Firstly, the CTF is stirred in diluted hydrochloric acid and water to remove the residual Zn and Cl ions. Hereafter, washing steps in organic solvents such as tetrahydrofuran or acetone are used to remove the unreacted moieties from the material. In most of the reported papers, these washing steps are used directly, or a slightly modified procedure is followed. Nevertheless, it is important to note that a complete removal of ZnCl2 remains a challenge.

2.2. Polycondensation Synthesis Route The triazine-based frameworks can also be synthesized through direct condensation of aromatic amides catalyzed by phosphorus pentoxide (P2O5) (Scheme1b) [ 16]. The polymerization of amides catalyzed by P2O5 was conducted in a degassed ﬂame-sealed ampule at a temperature up to 400 ◦C. The resulting CTFs exhibit not only a high speciﬁc surface area and stability but also a high crystallinity. Because of the lower sublimation temperature of the amide monomers, an appropriate temperature for the synthesis of CTFs is crucial in order to ensure better structural properties of the frameworks. Polymers 2019, 11, x FOR PEER REVIEW 2 of 24

gas adsorption and separation, energy storage and conversion, photocatalysis as well as heterogeneous catalysis [7]. In addition, our group has reported the functionalization of CTFs with nitrogen rich functionalities, (pyridine-4,2,6-triyl)-based CTF, and fluorine-containing hydrophobic CTF. The synthesized materials effectively enhanced the adsorption properties and selectivity towards CO2 and H2 due to stronger interaction between the gas molecules and the functionalized CTF materials [8,9]. Furthermore, we also demonstrated the potential of CTFs as a platform for the Polymersencapsulation2019, 11, 1326 of γ-Fe2O3 for implementing it as an excellent and novel adsorbent toward remediation4 of 24 of inorganic contaminants in water [10].

Scheme 1. (a) Trimerization of 1,4-dicyanobenzene catalyzed by ZnCl2 under ionothermal reaction Scheme 1. (a) Trimerization of 1,4-dicyanobenzene catalyzed by ZnCl2 under ionothermal reaction conditions for the formation of covalent triazine framework (CTF)-1; (b) Schematic representation of conditions for the formation of covalent triazine framework (CTF)-1; (b) Schematic representation the P2O5-catalyzed direct condensation of terephthamide (TA) to a discrete pCTF-1; (c) Scheme of the P2O5-catalyzed direct condensation of terephthamide (TA) to a discrete pCTF-1; (c) Scheme showing reaction mechanism for CTF-HUST synthesis. (Modified and reproduced with permission showing reaction mechanism for CTF-HUST synthesis. (Modiﬁed and reproduced with permission from [4], [15] and [16] respectively. Copyright 2008, 2018 and 2017, Wiley-VCH). from [4,15,16] respectively. Copyright 2008, 2018 and 2017, Wiley-VCH). The presence of stoichiometric and well-defined triazine moieties in the framework makes CTFs Another approach is by a polycondensation reaction of aldehydes and amidines in DMSO at very attractive materials as scaffold for metals, especially in heterogeneous catalysis [11]. They are 120 C in the presence of cesium carbonate (Cs CO ) as a base (Scheme1c) [ 15]. The condensation particularly◦ interesting owing to their high basicity2 due3 to the nitrogen-containing triazine moieties, reaction involves a Schiﬀ base formation followed by a Michael addition. Furthermore, this one-pot polymerization of CTFs can be scaled up to multigram level because of the relatively low synthesis temperature. An advanced condensation approach can be utilized for the formation of crystalline CTFs by in situ formations of the aldehyde monomers through the controlled oxidation of alcohols [25]. This occurs by the slow oxidation of the alcohol into the aldehyde monomer in the dimethyl sulfoxide (DMSO) solution, and the reaction temperatures were subsequently increased to boiling point. The reaction temperature plays a critical role in this polycondensation reaction. By performing the polymerization reaction at a high temperature, the alcohol will oxidize rapidly to form an aldehyde and generates a high concentration of nuclei to form small particles, and thus only soluble oligomers or fragments Polymers 2019, 11, 1326 5 of 24 are formed, while polymerization reaction at a temperature below boiling point mainly yielded a low degree of crystallinity of CTFs. Therefore, it is very crucial to keep the polymerization reaction at a lower temperature in the initial stage to control the in-situ formation of aldehyde and then maintain a higher temperature to boiling point to enhance the polymerization rate to improve the crystallization of the CTFs.

2.3. Cross-Coupling of Triazine-based Building Blocks Microporous triazine-based polymers can also be derived from triazine containing building blocks through a relatively simple cross-coupling reaction such as Friedel-Crafts and Yamamoto homo-coupling. In the Friedel-Crafts reaction, cyanuric chloride acts as the triazine monomer which reacts with aromatic compounds in the presence of AlCl3 as Lewis acid catalyst (Scheme2)[ 17,26,27]. Generally, the cross-coupling reaction is performed under reflux in dichloromethane (DCM) and is feasiblePolymers for 2019 various, 11, x FOR aromatic PEER REVIEW components. This method produces microporous CTFs with a5 specific of 24 surface area of up to 1668 m2 g 1, possessing a low crystallinity due to the irreversible polymerization · − linkage.linkage. Also, Also, this this synthetic synthetic approach approach is is unfavorable unfavorable from an an of of economic economic and and ecological ecological perspective perspective andand is notis not a scalablea scalable reaction. reaction. InIn a similara similar manner, manner, CTFs CTFs can can also also be be derivedderived byby aa Ni-catalyzed Yamamoto Yamamoto reaction reaction between between 2,4,6-tris-(4-bromo-phenyl)-(1,3,5-triazine)2,4,6-tris-(4-bromo-phenyl)-(1,3,5-triazine) andand Ni(cod)Ni(cod)22 catalystcatalyst in in the the presence presence of of 2,2 2,2′-bipyridyl0-bipyridyl in in dehydrateddehydrated dimethylformamide dimethylformamide (DMF)(DMF) underunder inertinert conditions at at 105 105 °C◦C[ [18].18]. Porous Porous CTFs CTFs with with a a specificspecific surface surface area area of of up up to to 2015 2015 m m2 2g·g−11 and a pore pore volume volume of of 1.36 1.36 cm cm3·g3 −g1 were1 were obtained. obtained. · − · −

SchemeScheme 2. 2.Friedel-Crafts Friedel-Crafts synthesis synthesis for for microporousmicroporous covalent triazine triazine polymer polymer formation. formation. Reproduced Reproduced withwith permission permission from from [17 [17],], Copyright Copyright 2015, 2015, Royal Royal SocietySociety ofof Chemistry.Chemistry.

ThoughThough several several synthesis synthesis methods methods have have been been developed developed for for the the preparation preparation of CTFs,of CTFs, it is it veryis very hard to makehard to a decisionmake a decision on which on ofwhich these of methodsthese method is thes is preferred the preferred option. option. The ionothermalThe ionothermal synthesis synthesis route usesroute harsh uses conditions harsh conditions which result which in materials result in thatmaterials are mainly that are amorphous mainly amorphous due to partial due carbonization to partial andcarbonization side reactions. and This side makes reactions. the characterization This makes the of characterization the material cumbersome. of the material However, cumbersome. as this is a straightforwardHowever, as this method is a straightforward it has been the method most appliedit has been method the most for applied the synthesis method of for CTFs the synthesis for several applicationsof CTFs for such several as catalysisapplications and such gas as adsorption. catalysis and Particularly gas adsorption. in catalysis, Particularly these in CTFscatalysis, have these been usedCTFs as highlyhave been stable used supports, as highly providing stable supports, a high recyclabilityproviding a high with recyclability limited loss with of activity. limited However, loss of activity. However, it is important to note here that it is difficult to implement the ionothermally synthesized CTFs in photocatalysis. Due to carbonization, the end- product is a black colored powder which is mostly not photoactive. For this purpose, CTFs synthesized through more milder routes (Bronsted acid, amidine method and in situ oxidation) are better. The Bronsted acid assisted method has however a few restrictions, as it does not produce crystalline materials and it is not compatible with all functional groups. Particularly for monomers containing heteroatoms, this method will often not work. Hence for photocatalysis, the amidine method and in situ oxidation method are the preferred options. The in-situ oxidation method provides additional benefits as materials with a higher crystallinity are obtained which is essential for an efficient charge transfer in photocatalysis. Therefore, in conclusion, the choice of the ideal synthetic route to produce a CTF depends to a large extent on the envisioned application.

2.4. Characterization of CTFs

Polymers 2019, 11, 1326 6 of 24 it is important to note here that it is difficult to implement the ionothermally synthesized CTFs in photocatalysis. Due to carbonization, the end- product is a black colored powder which is mostly not photoactive. For this purpose, CTFs synthesized through more milder routes (Bronsted acid, amidine method and in situ oxidation) are better. The Bronsted acid assisted method has however a few restrictions, as it does not produce crystalline materials and it is not compatible with all functional groups. Particularly for monomers containing heteroatoms, this method will often not work. Hence for photocatalysis, the amidine method and in situ oxidation method are the preferred options. The in-situ oxidation method provides additional benefits as materials with a higher crystallinity are obtained which is essential for an efficient charge transfer in photocatalysis. Therefore, in conclusion, the choice of the ideal synthetic route to produce a CTF depends to a large extent on the envisioned application.

2.4. Characterization of CTFs Underneath an overview is given of the most common and essential methods required to characterize CTFs. However, it is important to note that the presented list is not limited to only these methods but also other methods such as Raman spectroscopy, atomic force microscopy (AFM) etc. have been used in literature. Firstly, powder X-Ray diffraction (PXRD) is currently the most frequently used characterization method to confirm the structure of covalent organic frameworks (COFs). However, in the case of CTFs, this is difficult since most of the CTFs are obtained as amorphous powders. Nevertheless, in some reports CTFs have been obtained as crystalline powders from which a crystal model could be built [4,20,21]. Since the structure of the CTFs can be predicted based on the selected monomers, an initial crystal model can be built easily. This model can be optimized in a following step in correspondence to the best experimental PXRD pattern. A second essential characterization method for CTFs is Fourier-transform infrared (FT-IR) spectroscopy. FT-IR allows to determine the characteristic stretching and vibrating modes of the different functional groups present in the applied monomer during the CTF synthesis. Apart from the functional groups present in the monomer, the 1 triazine group has characteristic bands around ~1510 and 1350 cm− that need to be verified in the end-product. In the ionothermally obtained CTFs, it is important that the -CN stretching peak at 1 2260–2222 cm− is not present anymore in the final product as this confirms the complete conversion of nitrile into triazine. Apart from FT-IR, elemental analysis provides essential information on the overall elemental content in the material. An ideal crystal model is required to determine the unit cell for comparing the theoretical elemental content to the experimental values. However, for most of the CTF structures, the crystal model is not available due to the absence of resolvable PXRD patterns. Nevertheless, researchers use in many cases, the predicted ideal model to calculate the theoretical elemental content to compare it to the experimentally obtained values. An important value to calculate is the C/N ratio. A comparison of this value with the theoretical C/N ratio allows to determine the degree of carbonization that occurred during the synthesis. This also allows to estimate the loss of nitrogen in the CTFs. Solid-state nuclear magnetic resonance (ss-NMR) spectroscopy has been regularly used to characterize insoluble materials. In case of CTFs, ss-NMR can provide details on the chemical composition and exhibit characteristic peaks for the functional groups present within the material. In addition, nitrogen/Argon sorption measurements are the most important technique to characterize the porous properties of any material. The surface area, pore size distribution and pore volume can be obtained by using this method. It also may provide details on the micro/meso-porosity of the material. Especially during catalysis, porosity is important to allow a fast substrate diffusion and for the anchoring of homogeneous metal complexes or nanoparticles. After the anchoring step, this method can be applied to give an indirect indication on the success of the anchoring and the guest-framework interactions. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are also commonly used techniques to determine the morphology of the CTFs. TEM provides details on the inner surface of the material, whereas SEM can give information on the surface composition. Often TEM and SEM combined with energy dispersive X-ray (EDX) spectroscopy is used to obtain details on the elemental composition. X-ray photoelectron spectroscopy (XPS) is also used to determine Polymers 2019, 11, 1326 7 of 24 the elemental composition along with the bonding nature of the elements present in the CTF. It is, however, important to note that one must be careful in using the elemental composition obtained from XPS as it is only a surface technique and does not provide information on the bulk material itself. Thermogravimetric analysis (TGA) provides information on the stability of the material at elevated temperatures. In addition, the stability of CTFs is often examined under strong acidic, basic and solvent treatments. The PXRD or porosity measurements before and after the treatment are a good indicator for the stability of the materials.

3. CTFs as Support for Heterogeneous Catalysis CTFs have emerged as a novel platform for high-performance heterogeneous catalysis owing to their rigid structures, thermal stability, as well as their high acid-base resistivity in comparison Polymersto other 2019 porous, 11, x FOR organic PEER frameworks.REVIEW The most significant advantages of CTFs are the well-defined7 of 24 nitrogen species in their triazine units and the ease of functionalization of the organic ligand in order toto post-modify post-modify the the CTF CTF with with active active metal metal species species to to obtain heterogeneous catalysts. Furthermore, Furthermore, theirtheir high high specific specific surface surface area area and and porosity porosity provide provide a a good good accessibility accessibility of of the the active active sites, sites, and and heteroatomsheteroatoms contribute contribute to to the the stabilization stabilization of of the the active active species. species. The The following following sections sections are are divided divided intointo two two—metal – metal nanoparticle nanoparticle and and molecular molecular metal-based metal-based catalysis. catalysis. Table Table 1 summarizes various CTFs usedused for for different different catalytic catalytic reactions. reactions. Though Though CTFs CTFs have have been been utilized utilized for for several several types types of of catalysis catalysis (electrocatalysis,(electrocatalysis, photocatalysis, photocatalysis, organocatalysis), organocatalysis), we we have have focused focused this this review review towards towards liquid liquid phase phase organicorganic transformations. transformations.

3.1.3.1. Support Support for for Metal Metal Nanoparticles Nanoparticles N-heterocyclicN-heterocyclic moieties moieties in in the the CTFs CTFs are are beneficia beneficiall for for improving improving the the stability stability of of nanoparticles, nanoparticles, especiallyespecially for for liquid liquid phase phase catalytic catalytic reactions. reactions. Also, Also, the the porous porous surface surface of of the the support support can can offer offer steric steric restrictionrestriction to to prevent prevent the the growth growth of of metal metal clusters clusters which which causes causes agglomeration agglomeration and and thus thus deactivation deactivation ofof the the active active catalyst catalyst species. species. ThomasThomas et et al. al. immobilized immobilized Pd Pd nanoparticles nanoparticles (Pd (Pd NPs) NPs) onto onto the the CTFs CTFs materials materials to to develop develop an an activeactive and eefficientfficient catalystcatalyst forfor the the oxidation oxidation of of glycerol glycerol in in a liquid a liquid phase phase reaction reaction system system [28]. [28]. The The CTF CTFsupport support was was synthesized synthesized through through the the trimerization trimerization of of 1,4-dicyanobenzene 1,4-dicyanobenzene using using ZnCl ZnCl22 atat 400 °C◦C 2 1 andand then then heated heated up up to to 600 600 °C◦C for for local local expansio expansionn of ofthe the network. network. Surface Surface areas areas up to up 2814 to 2814 m2·g m−1 andg− · poreand porevolume volume of 1.79 of 1.79 cm3 cm·g−31 wereg 1 were obtained obtained (Scheme (Scheme 3).3 ).Further Further on, on, the the Pd Pd NPs-supported CTF · − (Pd(PdPVAPVA/CTF)/CTF) were were prepared prepared by by a a sol sol immobilization immobilization technique technique (NaBH (NaBH44/polyvinyl/polyvinyl alcohol alcohol (PVA)) (PVA)) to to ensureensure better better dispersion dispersion of of the the small NPs on the solid support. Pd PdPVA/CTF/CTF exhibited exhibited a a higher higher activity activity andand selectivity selectivity than than Pd-supported Pd-supported activated activated carbon carbon (Pd/AC) (Pd/AC) toward toward the the formation formation of of glycerate, glycerate, and and showedshowed a a high high stability stability up up to to three three cycles. cycles. Furthermore, Furthermore, Pd PdPVAPVA/CTF/CTF was was more more resistant resistant to to deactivation deactivation whichwhich was was attributed attributed to to the the stabilizing stabilizing effect effect achiev achieveded through through the the nitrogen nitrogen fu functionalitiesnctionalities present present in in thethe CTF CTF materials. materials.

SchemeScheme 3.3. ReactionReaction scheme scheme for for the dynamicthe dynamic trimerization trimerizat ofion 1,4-dicyanobenzene of 1,4-dicyanobenzene toward thetoward formation the formationof CTF (Adapted of CTF (Adapted with permission with permission from [28]. from Copyright [28]. Copyright 2010, American 2010, Americ Chemicalan Chemical Society). Society).

The applied preparation technique of immobilizing Pd NPs onto the CTFs was observed to influence the activity and stability of the Pd/CTF catalyst in this catalytic reaction. The preparation of PdIMP/CTF through the impregnation method led to the confinement of the Pd particles within the pores of the CTF (Figure 1) [29]. A slight decrease was observed in the activity and selectivity for the PdIMP/CTF compared to the PdPVA/CTF which could be attributed to the lower Pd exposure in the PdIMP/CTF sample. Remarkably, owing to the physical confinement of the Pd NPs within the CTF materials, PdIMP/CTF showed superior durability up to nine cycles.

Polymers 2019, 11, 1326 8 of 24

The applied preparation technique of immobilizing Pd NPs onto the CTFs was observed to Polymers 2019, 11, x FOR PEER REVIEW 8 of 24 influence the activity and stability of the Pd/CTF catalyst in this catalytic reaction. The preparation of PdIMP/CTF through the impregnation method led to the confinement of the Pd particles within the PdIMP/CTF through the impregnation method led to the confinement of the Pd particles within the pores of the CTF (Figure1)[ 29]. A slight decrease was observed in the activity and selectivity for the pores of the CTF (Figure 1) [29]. A slight decrease was observed in the activity and selectivity for the PdIMP/CTF compared to the PdPVA/CTF which could be attributed to the lower Pd exposure in the PdIMP/CTF compared to the PdPVA/CTF which could be attributed to the lower Pd exposure in the PdIMP/CTF sample. Remarkably, owing to the physical confinement of the Pd NPs within the CTF PdIMP/CTF sample. Remarkably, owing to the physical confinement of the Pd NPs within the CTF materials, PdIMP/CTF showed superior durability up to nine cycles. materials, PdIMP/CTF showed superior durability up to nine cycles.

(a) (b)

Figure 1. TEM images ofof PdPdPVAPVA/CTF/CTF ( a) and PdIMPIMP/CTF/CTF ( (bb).). Insets: Insets: Particle Particle size size distributions distributions of the Pd NPs (Reproduced withwith permissionpermission fromfrom [[29].29]. Copyright 2015, Wiley-VCH).

Prati etet al. al. examined examined the the influence influence of theof N-containingthe N-containing support support on the on catalytic the catalytic activity activity and stability and ofstability the Pd of system the Pd in system benzyl in alcohol benzyl oxidation alcohol oxidation [30,31]. The [30,31]. Pd NPs-supported The Pd NPs-supported CTF (Pd/CTF), CTF (Pd/CTF), activated carbonactivated (Pd carbon/AC), carbon(Pd/AC), nanotubes carbon (Pdnanotubes/CNTs) and(Pd/CNTs) nitrogen-doped and nitrogen-doped carbon nanotubes carbon (Pd nanotubes/N-CNTs) were(Pd/N-CNTs) prepared were by the prepared sol immobilization by the sol method.immobili Indeed,zation method. the triazine-rich Indeed, CTFthe materialtriazine-rich with CTF the highestmaterial nitrogen with the content highest significantly nitrogen content increased sign theificantly Pd-catalyst increased performance the Pd-catalyst compared performance to AC- and CNT-basedcompared to support, AC- and which CNT-based can be ascribedsupport, towhich the presence can be ascribed of N-functionalities to the presence which of N-functionalities limit the growth ofwhich the NPslimit and the contributes growth of tothe a positiveNPs and eff contributesect toward particleto a positive dispersion effect avoiding toward aggregationparticle dispersion of NPs. Moreover,avoiding aggregation He et al. highlighted of NPs. Moreover, the enhanced He et catalytic al. highlighted activity ofthe Pd enhanced/CTF in the catalytic hydrogenation activity of NPd/CTF-heterocyclic in the compoundshydrogenation compared of N-heterocyclic to the traditional compounds Pd/AC compared (Figure2a) to [ 32 the]. They traditional also indicated Pd/AC that(Figure the well2a) [32]. dispersed They also and indicated uniform distribution that the well of disp Pd NPsersed onto and the uniform CTF and distribution excellent catalytic of Pd NPs activity onto werethe CTF attributed and excellent to the electron-donation catalytic activity were from theattributed N-moieties to the in electron-donation the CTF to the Pd NPsfrom which the N-moieties exhibited strongin the CTF electronic to the metal-support Pd NPs which interaction. exhibited Zhangstrong etelectronic al. also reportedmetal-support the high interaction. activity and Zhang selectivity et al. ofalso Pd reported/CTF for the the high carbonylation activity and of selectivity aryl iodides of withPd/CTF amines for the (Figure carbonylation2b) [ 33]. of The aryl Pd iodides/CTFs werewith preparedamines (Figure by the impregnation-reduction2b) [33]. The Pd/CTFs methodwere prepared (PdCl2/NaBH by the4) and impregnation-reduction they catalyzed the synthesis method of α(PdCl-ketoamides2/NaBH4) under and they an atmosphericalcatalyzed the synthesis pressure of of α CO-ketoamides without any under specific an atmospherical additives, for pressure which the of typicalCO without reaction any of Pd-catalyzed specific additives, double carbonylationfor which the of aryl typical halides reaction required of additives Pd-catalyzed such as aminesdouble andcarbonylation high pressures of aryl of COhalides to drive required the catalytic additives reaction. such as amines and high pressures of CO to drive the catalytic reaction.

Polymers 2019, 11, x FOR PEER REVIEW 8 of 24

(a) (b)

Prati et al. examined the influence of the N-containing support on the catalytic activity and stability of the Pd system in benzyl alcohol oxidation [30,31]. The Pd NPs-supported CTF (Pd/CTF), activated carbon (Pd/AC), carbon nanotubes (Pd/CNTs) and nitrogen-doped carbon nanotubes (Pd/N-CNTs) were prepared by the sol immobilization method. Indeed, the triazine-rich CTF material with the highest nitrogen content significantly increased the Pd-catalyst performance compared to AC- and CNT-based support, which can be ascribed to the presence of N-functionalities which limit the growth of the NPs and contributes to a positive effect toward particle dispersion avoiding aggregation of NPs. Moreover, He et al. highlighted the enhanced catalytic activity of Pd/CTF in the hydrogenation of N-heterocyclic compounds compared to the traditional Pd/AC (Figure 2a) [32]. They also indicated that the well dispersed and uniform distribution of Pd NPs onto the CTF and excellent catalytic activity were attributed to the electron-donation from the N-moieties in the CTF to the Pd NPs which exhibited strong electronic metal-support interaction. Zhang et al. also reported the high activity and selectivity of Pd/CTF for the carbonylation of aryl iodides with amines (Figure 2b) [33]. The Pd/CTFs were prepared by the impregnation-reduction method (PdCl2/NaBH4) and they catalyzed the synthesis of α-ketoamides under an atmospherical pressure of CO without any specific additives, for which the typical reaction of Pd-catalyzed double Polymerscarbonylation2019, 11, 1326 of aryl halides required additives such as amines and high pressures of CO to drive9 of 24 the catalytic reaction.

Figure 2. Schematic picture of (a) Pd/CTF catalyzed hydrogenation of N-heterocycles (Reproduced Figure 2. Schematic picture of (a) Pd/CTF catalyzed hydrogenation of N-heterocycles (Reproduced with with permission from [32]. Copyright 2015, Royal Society of Chemistry); (b)Pd/CTF catalyzed Polymerspermission 2019, 11, x fromFOR PEER [32]. REVIEW Copyright 2015, Royal Society of Chemistry); (b)Pd/CTF catalyzed carbonylation9 of 24 carbonylation of aryl iodides with amines (Reproduced with permission from [33]. Copyright 2016, of aryl iodides with amines (Reproduced with permission from [33]. Copyright 2016, Royal Society of Royal Society of Chemistry). PalkovitsChemistry). et al. developed Ru/CTF catalysts for the selective oxidation of 5- hydroxymethylfurfural (HMF) [34,35]. Ru NPs were distributed onto various CTFs using the coordination-reduction Palkovits et al. developed approach Ru in/CTF which catalysts the RuCl for the3·xH selective2O precursor oxidation in ethanol of 5-hydroxymethylfurfural was refluxed in the presence(HMF) [34 of,35 the]. RuCTF NPs materials, were distributed and the ontoresultin variousg coordinated CTFs using Ru-species the coordination-reduction were reduced under approach H2 atmospherein which the (Scheme RuCl xH 4). OThe precursor reported in study ethanol showed was refluxed that the in enhanced the presence catalytic of the performance CTF materials, of Ru- and 3· 2 supportedthe resulting CTF coordinated not only depended Ru-species on were the surface reduced area under and H porosity2 atmosphere of the (Scheme prepared4). materials The reported but alsostudy was showed based on that the the N-content enhanced and catalytic the inherent performance polarity of of Ru-supported the catalyst. Furthermore, CTF not only the depended CTF-based on supportthe surface catalysts area and were porosity far more of theactive prepared compared materials to the butconventional also was based Ru/C, on and the Ru/Al N-content2O3 catalysts and the andinherent additionally, polarity the of theN-functionalit catalyst. Furthermore,ies provided the stabilizing CTF-based effect support for the catalysts Ru species, were enabling far more an active easy recyclingcompared of to the the catalysts. conventional Ru/CTFs Ru/C, were and also Ru/Al utiliz2O3 edcatalysts as a selective and additionally, catalyst for the the N-functionalities conversion of xylitolprovided to glycols stabilizing with e yieldsffect for up the to Ru80% species, [36]. The enabling catalytic an results easy recycling revealed of that the the catalysts. increased Ru /N-contentCTFs were inalso CTF-supported utilized as a selectiveRu catalysts catalyst suppressed for the conversion the decarbonylation of xylitol toreactions glycols withand hence yields resulted up to 80% into [36 a]. highThe catalyticselectivity results toward revealed the desired that the products. increased N-content in CTF-supported Ru catalysts suppressed the decarbonylationZhang et al. synthesized reactions a andnovel hence porous resulted CTF-based into a highon di-(4-cyanophenyl)ethyne selectivity toward the desired building products. units (CTF-DCE)Zhang to et anchor al. synthesized Ag species a novelfor the porous terminal CTF-based alkyne carboxylation on di-(4-cyanophenyl)ethyne reactions under atmospherical building units pressure(CTF-DCE) [37]. to The anchor CTF-DCE Ag species was for synthesized the terminal under alkyne the carboxylation ionothermal reactionssynthesisunder strategy atmospherical through a sequencepressure of [37 the]. The heat-resistant CTF-DCE oligomer was synthesized formatio undern at moderate the ionothermal temperature synthesis to a highly strategy porous through CTF a atsequence high temperature of the heat-resistant with a BET oligomer surface formationarea of 1355 at moderatem2·g−1 and temperature pore volume to of a highly0.93 cm porous3·g−1 (Scheme CTF at 5).high The temperature CTF-DCE supported-silver with a BET surface (CTF-DCE-Ag) area of 1355 m was2 g 1preparedand pore by volume treating of 0.93the CTF cm3 gmaterial1 (Scheme with5). · − · − AgNOThe CTF-DCE3 in hot supported-silverDMSO at 80 °C. (CTF-DCE-Ag)FTIR, PXRD and was XPS prepared analysis by confirmed treating the the CTF deposition material with of metallic AgNO3 0 Agin0 hot onto DMSO the triazine at 80 ◦C. moiety FTIR, PXRDof the andCTF-DCE. XPS analysis The CTF-DCE-Ag confirmed the exhibited deposition a six-fold of metallic higher Ag TONonto comparedthe triazine to moiety the Ag@MIL-101 of the CTF-DCE. catalyst The for CTF-DCE-Ag CO2 capture exhibited and conversion a six-fold into higher propiolic TON acids compared under to similarthe Ag@MIL-101 catalytic reaction catalyst conditions. for CO2 capture The CTF-DCE- and conversionAg could into be propiolicrecovered acids and reused under similarup to five catalytic runs withoutreaction any conditions. significant The decrease CTF-DCE-Ag in activity could and, be th recoveredus indicated and the reused stability up toand five recyclability runs without of the any catalyst.significant decrease in activity and, thus indicated the stability and recyclability of the catalyst.

SchemeScheme 4. 4. SchematicSchematic representation representation of of the the coordination coordination and and stabilization stabilization of of Ru Ru NPs NPs onto onto CTF CTF material material (Reproduced(Reproduced with with permission permission from from [34]. [34]. Copyright Copyright 2015, 2015, Wiley-VCH). Wiley-VCH).

Polymers 2019, 11, x FOR PEER REVIEW 9 of 24

Palkovits et al. developed Ru/CTF catalysts for the selective oxidation of 5- hydroxymethylfurfural (HMF) [34,35]. Ru NPs were distributed onto various CTFs using the coordination-reduction approach in which the RuCl3·xH2O precursor in ethanol was refluxed in the presence of the CTF materials, and the resulting coordinated Ru-species were reduced under H2 atmosphere (Scheme 4). The reported study showed that the enhanced catalytic performance of Ru- supported CTF not only depended on the surface area and porosity of the prepared materials but also was based on the N-content and the inherent polarity of the catalyst. Furthermore, the CTF-based support catalysts were far more active compared to the conventional Ru/C, and Ru/Al2O3 catalysts and additionally, the N-functionalities provided stabilizing effect for the Ru species, enabling an easy recycling of the catalysts. Ru/CTFs were also utilized as a selective catalyst for the conversion of xylitol to glycols with yields up to 80% [36]. The catalytic results revealed that the increased N-content in CTF-supported Ru catalysts suppressed the decarbonylation reactions and hence resulted into a high selectivity toward the desired products. Zhang et al. synthesized a novel porous CTF-based on di-(4-cyanophenyl)ethyne building units (CTF-DCE) to anchor Ag species for the terminal alkyne carboxylation reactions under atmospherical pressure [37]. The CTF-DCE was synthesized under the ionothermal synthesis strategy through a sequence of the heat-resistant oligomer formation at moderate temperature to a highly porous CTF at high temperature with a BET surface area of 1355 m2·g−1 and pore volume of 0.93 cm3·g−1 (Scheme 5). The CTF-DCE supported-silver (CTF-DCE-Ag) was prepared by treating the CTF material with AgNO3 in hot DMSO at 80 °C. FTIR, PXRD and XPS analysis confirmed the deposition of metallic Ag0 onto the triazine moiety of the CTF-DCE. The CTF-DCE-Ag exhibited a six-fold higher TON compared to the Ag@MIL-101 catalyst for CO2 capture and conversion into propiolic acids under similar catalytic reaction conditions. The CTF-DCE-Ag could be recovered and reused up to five runs without any significant decrease in activity and, thus indicated the stability and recyclability of the catalyst.

PolymersScheme 2019, 11 5.5., Polymerizationx FORPolymerization PEER REVIEW of di-(4-cyanophenyl)ethyneof di-(4-cyanophenyl)eth (DCE)yne under(DCE) ionothermalunder ionothermal synthesis conditionssynthesis10 of 24 (Reproducedconditions (Reproduced with permission with permission from [37]. Copyright from [37]. 2018,Copyright American 2018, Chemical American Society). Chemical Society). 3.2. Immobilization of Molecular Metal Catalysts 3.2. Immobilization of Molecular Metal Catalysts CTFs have been targeted as catalyst support for molecular complexes because of their CTFs have been targeted as catalyst support for molecular complexes because of their exceptional exceptional chemical inertness toward acidic and basic media, combined with their outstanding chemical inertness toward acidic and basic media, combined with their outstanding stability over a stability over a broad range of temperatures and pressures enable CTFs to be particularly interesting broad range of temperatures and pressures enable CTFs to be particularly interesting for sustainable for sustainable development in heterogeneous catalysis. To date, the aim to design CTF materials development in heterogeneous catalysis. To date, the aim to design CTF materials with strong with strong electron-donating based ligands to provide high efficiency, good selectivity, and robust electron-donating based ligands to provide high efficiency, good selectivity, and robust stability have stability have been targeted by several research groups. The design of organic functionalized ligand- been targeted by several research groups. The design of organic functionalized ligand-based CTF based CTF supported transition metal complexes for specific catalytic applications are summarized supported transition metal complexes for specific catalytic applications are summarized below. below. Nitrogen-based ligands such as pyridine and bipyridine have been widely applied as homogeneous Nitrogen-based ligands such as pyridine and bipyridine have been widely applied as active transition metal catalyst supports. Functionalization of strong binding sites to a framework homogeneous active transition metal catalyst supports. Functionalization of strong binding sites to a leads to various catalytic applications through efficient and stable heterogeneous based catalysts. framework leads to various catalytic applications through efficient and stable heterogeneous based Nitrogen-rich microporous CTFs derived from pyridine containing building units have been reported catalysts. Nitrogen-rich microporous CTFs derived from pyridine containing building units have by Thomas et al. These materials were synthesized by a trimerization reaction under ionothermal been reported by Thomas et al. These materials were synthesized by a trimerization reaction under conditions at temperatures up to 600 C (Scheme6)[ 19]. The resulting pyridine-based CTF with a ionothermal conditions at temperatures◦ up to 600 °C (Scheme 6) [19]. The resulting pyridine-based specific surface area of 1061 m2 g 1 and a pore volume of 0.93 cm3 g 1, contained numerous bipyridyl CTF with a specific surface area· of− 1061 m2·g−1 and a pore volume ·of− 0.93 cm3·g−1, contained numerous moieties that are accessible and enable anchoring of a transition-metal complex. bipyridyl moieties that are accessible and enable anchoring of a transition-metal complex.

SchemeScheme 6.6. SchematicSchematic representationrepresentation ofof thethe synthesissynthesis ofof thethe pyridine-basedpyridine-based CTFCTF [[19].19].

Schüth et al. reported the potential of pyridine-based CTF material as a solid matrix for Pt(II) complex catalyzed methane oxidation [38]. The platinum species were introduced onto the CTF using two different routes; by an in-situ reaction of CTF and the Pt(II) precursor in the catalytic reaction mixture (K2[PtCl4]-CTF), or by pre-coordination of the Pt(II) complex onto the CTF (Pt-CTF). Both Pt- modified CTF catalysts (K2[PtCl4]-CTF and Pt-CTF) exhibited a high activity and selectivity for the oxidation of methane to methanol under extremely harsh reaction conditions at 215 °C in concentrated sulfuric acid (30% SO3) and high pressure of CH4 (40 bar). The Pt-CTF catalyst showed a high stability over several runs with a TON above 250. The K2(PtCl4)-CTF catalyst showed a deactivation after the fifth run. The platinum sites were analyzed by XPS before and after catalysis, revealing the Pt(II) to be the predominant species. It was observed that the K2(PtCl4)-CTF material possesses a lower amount of incorporated Pt species than Pt-CTF. In a follow-up study, the local environments of the Pt sites within the Pt-CTF catalyst were identified to be an analogue of the molecular Periana catalyst, Pt(bpym)Cl2 (Figure 3) [39]. The analysis by EXAFS, XANES, XPS, and the surface characterization methods, provided evidence for the coordination of Pt(II) onto the pyridinic nitrogen sites in the Pt-CTF catalyst which are similar to one of the two coordination environments in crystalline Pt(bpym)Cl2. In addition, this study provides

Polymers 2019, 11, x FOR PEER REVIEW 11 of 24

Polymers 2019, 11, 1326 11 of 24 detailed insights into the underlying molecular compositions, structures, and distribution of a heterogenized Periana catalyst onto the CTF materials. GasconSchüth et al. reported reported the the immobilization potential of pyridine-based of an (IrCp*Cl CTF2)2 complex material on as the a solid pyridine matrix based for Pt(II)CTF materialcomplex catalyzedas a base methanefree catalyst oxidation for the [38 ].transfer The platinum hydrogenation species werereaction introduced [40]. The onto Ir(III) the CTF complex using coordinatedtwo different to routes; the bipyridyl by an in-situ moieties reaction within of the CTF framework, and the Pt(II) resulted precursor in an air- in theand catalytic moisture-stable reaction Ir@CTFmixture catalyst. (K2[PtCl XPS4]-CTF), analysis or by revealed pre-coordination that the Ir ofspecies the Pt(II) in the complex system ontoare in the the CTF oxidation (Pt-CTF). state Both 3+ andPt-modified remained CTF unchanged catalysts (Kprior2[PtCl to 4and]-CTF after and catalysis. Pt-CTF) Under exhibited the a optimized high activity reaction and selectivity conditions, for the the Ir@CTFoxidation (0.4 of mol methane % Ir toused methanol in reaction) under catalyzed extremely the harsh transfer reaction hydrogenation conditions at of 215 1-octene-3-ol◦C in concentrated with a 98%sulfuric conversion acid (30% and SO 82%3) and yield high to pressure the targeted of CH 3-octa4 (40 bar).none, The in the Pt-CTF presence catalyst of isopropanol showed a high as stabilitysolvent atover 120 several °C under runs an with inert a atmosphere TON above 250.(N2, 2 The bar) K 2for(PtCl 234 h,)-CTF without catalyst the showedneed fora additives. deactivation In a after typical the catalyticfifth run. reaction The platinum of allylic sites alcoho werels, analyzed an additive by XPS such before as bases andafter or hydrogen catalysis, acceptors revealing are the Pt(II)needed to to be promotethe predominant the reaction. species. Most It was likely, observed the CTF that material the K2(PtCl is not4)-CTF just material an inert possesses support afor lower the amountIr(III)Cp* of complexincorporated dispersion, Pt species but than rather Pt-CTF. the presence of the pyridine molecules plays an active role as co- catalystIn aby follow-up assisting study, the deprotonation the local environments and coordination of the Pt sites of withinthe initial the Pt-CTFalcohol catalyst onto the were Ir identifiedcenter to formto be anthe analogue initial metal of the enolate molecular for Perianathe isomerizatio catalyst, Pt(bpym)Cln reaction. 2The(Figure turn-over3)[ 39]. frequency The analysis (TOF) by EXAFS, of the Ir@CTFXANES, catalyst XPS, and was the 24 surface min− characterization1, and outperformed methods, other provided Ir(III)-based evidence porous for thematerial coordination catalysts. of Pt(II)The recyclabilityonto the pyridinic of the nitrogen Ir@CTF sitesmateri in theal was Pt-CTF performed catalyst up which to six are consecutive similar to one runs. of the However, two coordination a slight decreaseenvironments of activity in crystalline was observed Pt(bpym)Cl over 2the. In first addition, three thisruns, study while provides a more detailedpronounced insights decrease into theof activityunderlying was molecularobserved compositions,afterwards. The structures, catalyst anddeactivation distribution was of mainly a heterogenized attributed Perianato build-up catalyst of adsorbedonto the CTF products materials. on the catalyst surface, progressively blocking the active sites.

FigureFigure 3. (a(a) )Schematic Schematic diagram diagram of of the the molecular molecular Pt-CTF Pt-CTF catalyst catalyst having having a a similar similar coordination coordination

environmentenvironment as the the molecular molecular Periana Periana catalyst (Pt(bpym)Cl 2)) for for the the oxidation oxidation of of methane methane to to methanol methanol inin concentrated concentrated sulfuric sulfuric acid; acid; ( (b)) Structural model of Pt(bpym)Cl 2;;( (cc)) EXAFS EXAFS analysis analysis of of Pt-CTF Pt-CTF and and thethe fitted fitted theoretical theoretical Pt(bpym)Cl Pt(bpym)Cl2 model;model; ( (dd)) XPS XPS N N 1s 1s spectra spectra of of Pt-modified Pt-modified 2,2 2,2′0-bypyrimidine-bypyrimidine ligand ligand andand ( (ee)) XPS XPS N N 1s 1s spectra of Pt-modified Pt-modified CTF (Adapted with with permission permission from from [39]. [39]. Copyright Copyright 2016, 2016, AmericanAmerican Chemical Society).

InGascon another et catalytic al. reported application the immobilization of the pyridine of-based an (IrCp*Cl CTF as2) 2catalyticcomplex support, on the the pyridine same group based reportedCTF material on the as immobilization a base free catalyst of a Ni(II) for the complex transfer onto hydrogenation the CTF which reaction catalyzed [40]. the The oligomerization Ir(III) complex ofcoordinated ethylene [41]. to the Ni(II) bipyridyl was coor moietiesdinated within onto thethe framework,CTF support resulted by the inimpregnation an air- and moisture-stableof an excess of Nickel(II)Ir@CTF catalyst. bromide XPS ethylene analysis gl revealedycol dimethyl that the ether Ir species (DME*NiBr in the2) system under aremild in conditions. the oxidation The state TEM 3+ micrographsand remained revealed unchanged a good prior dispersion to and after of catalysis. Ni throughout Under thethe optimizedsolid materials reaction with conditions, no metallic the nanoparticleIr@CTF (0.4 molformation % Ir used and inXPS reaction) analysis catalyzed indicated thethe transferpreferential hydrogenation coordination of of 1-octene-3-ol Ni to the pyridinic with a N98% species conversion within and the 82% CTF. yield The to catalytic the targeted performanc 3-octanone,e and in active the presence site accessibility of isopropanol influenced as solvent the selectivityat 120 ◦C underof the catalyst an inert in atmosphere the ethylene (N oligomerization.2, 2 bar) for 23 h, The without microporous the need Ni(II) for additives. supported In pyridine- a typical CTFcatalytic catalyst reaction showed of allylic higher alcohols, selectivity an to additive C8 than such to C as6. The bases slower or hydrogen diffusion acceptors of olefins are throughout needed to thepromote pores the of reaction. the materials Most likely, causes the CTFa re-adsorpt material ision not justof the an inertproducts, support thus forthe leading Ir(III)Cp* to complexfurther oligomerizations.dispersion, but rather the presence of the pyridine molecules plays an active role as co-catalyst by

Polymers 2019, 11, 1326 12 of 24 assisting the deprotonation and coordination of the initial alcohol onto the Ir center to form the initial metal enolate for the isomerization reaction. The turn-over frequency (TOF) of the Ir@CTF catalyst 1 was 24 min− , and outperformed other Ir(III)-based porous material catalysts. The recyclability of the Ir@CTF material was performed up to six consecutive runs. However, a slight decrease of activity was observed over the first three runs, while a more pronounced decrease of activity was observed afterwards. The catalyst deactivation was mainly attributed to build-up of adsorbed products on the catalyst surface, progressively blocking the active sites. In another catalytic application of the pyridine-based CTF as catalytic support, the same group reported on the immobilization of a Ni(II) complex onto the CTF which catalyzed the oligomerization of ethylene [41]. Ni(II) was coordinated onto the CTF support by the impregnation of an excess of Nickel(II) bromide ethylene glycol dimethyl ether (DME*NiBr2) under mild conditions. The TEM micrographs revealed a good dispersion of Ni throughout the solid materials with no metallic nanoparticle formation and XPS analysis indicated the preferential coordination of Ni to the pyridinic N species within the CTF. The catalytic performance and active site accessibility influenced the selectivity of the catalyst in the ethylene oligomerization. The microporous Ni(II) supported pyridine-CTF catalyst showed higher Polymers 2019, 11, x FOR PEER REVIEW 12 of 24 selectivity to C8 than to C6. The slower diffusion of olefins throughout the pores of the materials causes a re-adsorptionPalkovits et of theal. reported products, that thus the leading porosity to further and oligomerizations.polarity of CTF supported Rh(III) catalysts significantlyPalkovits influence et al. reported the catalytic that activity the porosity in hydr andoformylation polarity of reaction CTF supported [42]. Rhodium(III) Rh(III) catalysts chloride significantlyhydrate was influence immobilized the catalytic onto various activity CTF in hydroformylation materials having reaction different [42]. porosities Rhodium(III) and chloridesurface hydratepolarities. was Superior immobilized results onto were various obtained CTF materials with the having Rh(III) diff erentcatalyst porosities supported and surfaceonto the polarities. highly Superiormesoporous results and were low obtainedpolarity of with 4,4′ the-dicyanobiphenyl Rh(III) catalyst based supported CTF, ontowhilst the the highly microporous mesoporous and high and lowsurface polarity polarity of 4,4 of0 -dicyanobiphenylthe 2,6-dicyanopyridine based based CTF, whilst CTF support the microporous showed the and lowest high activity. surface polarityThis study of thedemonstrates 2,6-dicyanopyridine that the mesoporosity based CTF support and accessibility showed the lowestof the activity.active sites This in study combination demonstrates with thatthe thepolarity mesoporosity of the andsurface accessibility support of theoffers active high sites activity in combination and selectivity with the polarityfor the of solvent-free the surface supporthydroformylation offers high of activity 1-octene. and selectivity for the solvent-free hydroformylation of 1-octene. OneOne ofof thethe greatgreat advantagesadvantages ofof CTFsCTFs isis thatthat theirtheir solid-statesolid-state propertiesproperties cancan bebe adjustedadjusted byby varyingvarying the starting building blocks. blocks. Gascon Gascon et etal. al. designed designed a mesoporous a mesoporous CTF CTF constructed constructed of 2,6- of 2,6-dicyanopyridinedicyanopyridine and and 4,4 4,4′-dicyanobiphenyl0-dicyanobiphenyl with with the the ratio ratio of of 1:2 1:2 (Scheme (Scheme 77))[ [43].43]. The biphenylbiphenyl monomermonomer introducedintroduced thethe mesoporositymesoporosity into thethe CTFCTF whilewhile thethe presencepresence ofof thethe pyridinepyridine sitessites inin thethe frameworkframework notnot onlyonly providedprovided anan inherentinherent basicitybasicity butbut alsoalso introducedintroduced bipyridinebipyridine moietiesmoieties thatthat actedacted asas metalmetal supportsupport thatthat assistedassisted thethe activationactivation ofof smallsmall molecules.molecules.

SchemeScheme 7.7. SynthesisSynthesis of ofthe the mesoporous mesoporous CTF CTF from from trim trimerizationerization of 2,6-dicyanopyridine of 2,6-dicyanopyridine and and4,4′- 4,4dicyanobiphenyl0-dicyanobiphenyl under under ionothermal ionothermal synthesis synthesis cond conditionsitions (adapted (adapted with with permission permission from from [[43].43]. CopyrightCopyright 2015,2015, Wiley-VCH).Wiley-VCH).

ToTo bebe moremore practical,practical, inin termsterms ofof catalystcatalyst recycling,recycling, thethe mesoporousmesoporous CTFCTF waswas shapedshaped intointo aa sphericalspherical form,form, withoutwithout losinglosing itsits properties.properties. Th Thee CTF-based spheres were prepared byby aa phasephase inversion method using polyimide Matrimid® as a binder (Figure 4) [44]. A phase inversion method refers to a process in which solvents are removed from a liquid-polymer mixture to produce a porous solid. In this case, the CTF-Matrimid suspension was loaded into a syringe and was pumped into water where the anti-solvent causes the phase inversion. The resulting CTF spheres showed a high thermal stability, porosity, and the possibility of coordinating metal clusters. In contrast to the powder-based materials, the spheres-based catalyst is more easily handled, and is fully recycled without loss of material through at least four consecutive runs. A highly efficient organometallic complex, Ir(III) Cp* was immobilized through coordination within the CTF spheres to render a molecular yet heterogeneous and stable catalyst, which is easy to handle and to recycle in the CO2 hydrogenation to formic acid. Moreover, utilization of shaped particles carries the advantage of facile catalyst recycling and improved reproducibility. However, the porosity of the CTFs could not be completely preserved upon formulation. To address this issue, and the diffusion limitations derived from heterogenization, Gascon and co- workers introduced a well-defined CTF coated on cordierite monoliths, prepared by quasi chemical vapor deposition (Figure 5) [45]. These coatings are stable, easy to handle, and can be used in the same way as the parent material to coordinate metal complexes. Extensive characterization demonstrates that these coatings contain similar properties as the powder-based materials. The monolith-supported CTF showed superior performance for the selective partial oxidation of methane

Polymers 2019, 11, 1326 13 of 24 inversion method using polyimide Matrimid® as a binder (Figure4)[44]. A phase inversion method refers to a process in which solvents are removed from a liquid-polymer mixture to produce a porous solid. In this case, the CTF-Matrimid suspension was loaded into a syringe and was pumped into water where the anti-solvent causes the phase inversion. The resulting CTF spheres showed a high thermal stability, porosity, and the possibility of coordinating metal clusters. In contrast to the powder-based materials, the spheres-based catalyst is more easily handled, and is fully recycled without loss of material through at least four consecutive runs. A highly efficient organometallic complex, Ir(III) Cp* was immobilized through coordination within the CTF spheres to render a molecular yet heterogeneous and stable catalyst, which is easy to handle and to recycle in the CO2 hydrogenation to formic acid. Moreover, utilization of shaped particles carries the advantage of facile catalyst recycling and improved reproducibility. However, the porosity of the CTFs could not be completely preserved upon formulation. To address this issue, and the diffusion limitations derived from heterogenization, Gascon and co-workers introduced a well-defined CTF coated on cordierite monoliths, prepared by quasi chemical vapor deposition (Figure5)[ 45]. These coatings are stable, easy to handle, and can be used in the same way as the parent material to coordinate metal complexes. Extensive characterization demonstrates that these coatings contain similar properties as the powder-based materials. The monolith-supported CTF Polymers 2019, 11, x FOR PEER REVIEW 13 of 24 showed superior performance for the selective partial oxidation of methane to methanol, attributed to the better mass transport properties of the thin layer coated onto the monolith channels compared to to methanol, attributed to the better mass transport properties of the thin layer coated onto the the powder-form CTF. monolith channels compared to the powder-form CTF.

FigureFigure 4.4. (A) SEM of a cut CTF-based CTF-based sphere; (B) synthesis conditions forfor thethe shapingshaping ofof CTF;CTF; ((CC)) SEMSEM imageimage andand itsits molecularmolecular structuresstructures ofof immobilizedimmobilized IrIr complexcomplex onon thethe CTF-basedCTF-based sphere;sphere; ((DD)) XPSXPS analysisanalysis of iridium iridium species species in in the the powder powder and and shap shapeded CTF CTF catalysts catalysts (Reprinted (Reprinted with withpermission permission from from[44]. Copyright [44]. Copyright 2016, 2016,Wiley-VCH). Wiley-VCH).

Figure 5. (Left) Representation of the coating procedure step. (Right) SEM micrograph of (A) CTF powders (B–E), CTF-coated monolith (F) view on the wall of a CTF-coated monolith (Adapted with permission from [45]. Copyright 2017, American Chemical Society).

Polymers 2019, 11, x FOR PEER REVIEW 13 of 24

to methanol, attributed to the better mass transport properties of the thin layer coated onto the monolith channels compared to the powder-form CTF.

Figure 4. (A) SEM of a cut CTF-based sphere; (B) synthesis conditions for the shaping of CTF; (C) SEM image and its molecular structures of immobilized Ir complex on the CTF-based sphere; (D) XPS Polymersanalysis2019, 11 of, 1326iridium species in the powder and shaped CTF catalysts (Reprinted with permission from14 of 24 [44]. Copyright 2016, Wiley-VCH).

Figure 5.5. (Left) RepresentationRepresentation of thethe coatingcoating procedureprocedure step.step. ( Right) SEM micrograph of ((A)) CTFCTF powderspowders (B–E),), CTF-coatedCTF-coated monolithmonolith (F) viewview onon thethe wallwall ofof aa CTF-coatedCTF-coated monolithmonolith (Adapted(Adapted withwith permission from [45]. Copyright 2017, American Chemical Society). Polymerspermission 2019, 11, x from FOR [PEER45]. CopyrightREVIEW 2017, American Chemical Society). 14 of 24

Lotsch et al.al. developed a CTF based on a bipyridine-basedbipyridine-based building block with hierarchicalhierarchical microporosity and high specific surface areas of up to 1100 m2 g 1 in which the bipyridine units acts microporosity and high specific surface areas of up to 1100 m·2·g−−1 in which the bipyridine units acts as scascaffoldffold to site-specificallysite-specifically coordinate thethe molecularmolecular metals including Co, Ni, Pt and Pd which are anchored on the CTFs byby usingusing wetwet impregnationimpregnation methodmethod (Scheme(Scheme8 8))[ 46[46].].

Scheme 8.8.Schematic Schematic representation representation of the of synthesis the synthesi of the bipyridine-baseds of the bipyridine-based CTF (bpyCTF) CTF (Reproduced (bpyCTF) with(Reproduced permission with from permission [46]. Copyright from [46]. 2012, Copyright Royal Society 2012, Royal of Chemistry). Society of Chemistry).

Yoon etet al.al. utilized the bipyridine-based CTF (bpyCTF)(bpyCTF) as support for Ir- and Ru-complexes in the hydrogenationhydrogenation ofof COCO22 to formate [[47–49].47–49]. The XPS measurements revealed the complexationcomplexation ofof both Ir and Ru Ru ions ions onto onto the the bpyCTF bpyCTF and and showed showed that that the the coordination coordination environment environment was was similar similar to tothat that of oftheir their homogeneous homogeneous counterparts. counterparts. Both Both catalysts catalysts converted converted CO CO2 2intointo formate formate under mildmild reaction conditions in the presence of triethylamine as a base and demonstrated excellent recyclability over consecutive runs. The turnover number (TON) and turnover frequency (TOF) outperformed the conventional heterogeneous catalysts. Owing to the high stability of CTFs in acidic and basic media, Yoon et al. extended the study of the bpyCTF as catalytic support by incorporating the Ir and Rh half-sandwich complexes as active heterogeneous catalysts for the aqueous-phase hydrogenation of carbonyl compounds (Figure 6) [50]. Most of the MOFs and other transition metal-based oxide catalysts are unstable in acidic reaction conditions. Interestingly, these CTF supported heterogeneous catalysts (bpy-CTF-(Cp*IrCl))Cl and (bpy-CTF-(Cp*RhCl))Cl demonstrated to be efficient, recyclable, and have superior activities at acidic medium reaction (pH 3.5) for a broad substrate scope. In addition, these are also industrially viable catalysts for the exclusive production of alcohols in water.

Polymers 2019, 11, 1326 15 of 24 reaction conditions in the presence of triethylamine as a base and demonstrated excellent recyclability over consecutive runs. The turnover number (TON) and turnover frequency (TOF) outperformed the conventional heterogeneous catalysts. Owing to the high stability of CTFs in acidic and basic media, Yoon et al. extended the study of the bpyCTF as catalytic support by incorporating the Ir and Rh half-sandwich complexes as active heterogeneous catalysts for the aqueous-phase hydrogenation of carbonyl compounds (Figure6)[ 50]. Most of the MOFs and other transition metal-based oxide catalysts are unstable in acidic reaction conditions. Interestingly, these CTF supported heterogeneous catalysts (bpy-CTF-(Cp*IrCl))Cl and (bpy-CTF-(Cp*RhCl))Cl demonstrated to be eﬃcient, recyclable, and have superior activities at acidic medium reaction (pH 3.5) for a broad substrate scope. In addition, these are also industrially viable catalysts for the exclusive production of alcohols in water. Polymers 2019, 11, x FOR PEER REVIEW 15 of 24

FigureFigure 6. StructuralStructural representation representation of (a of) homogeneous (a) homogeneous Ir- and Ir- Rh-based and Rh-based catalysts, catalysts, and (b) CTF- and (supportedb) CTF-supported catalysts catalysts (Adapted (Adapted with permission with permission from from[50]. [50Copyright]. Copyright 2016, 2016, Royal Royal Society Society of ofChemistry). Chemistry).

TheThe highhigh specificspecific surfacesurface areaarea andand goodgood stabilitystability ofof thethe bpyCTFbpyCTF materialmaterial inin variousvarious catalyticcatalytic conditionsconditions waswas furtherfurther utilizedutilized byby thethe groupgroup ofof YoonYoon etet al.al. toto immobilizedimmobilized aa bimetallicbimetallic (Al(OTf)(Al(OTf)22) (Co(CO)(Co(CO)44) based catalyst catalyst onto onto the the bpyCTF bpyCTF for for the the carbonylation carbonylation of of propylene propylene oxide oxide (PO) (PO) into into β- βbutyrolactone-butyrolactone [51]. [51 ].The The catalyst catalyst was was prepared prepared by by the the immobilization immobilization of of Al(OTf) Al(OTf)33 ontoonto the the bpyCTF, - usingusing aa hydrothermal hydrothermal method. method. Subsequently, Subsequently, the OTfthe OTfanions- anions were exchangedwere exchanged with (Co(CO) with (Co(CO)4)− anions.4)− Theanions. nitrogen The adsorptionnitrogen adsorption measurements measurements proved that theproved presence that ofthe the presence anions did of notthe completely anions did block not thecompletely pores of block bpyCTF, the topores allow of small bpyCTF, molecules to allow such small as PO molecules and CO such to reach as PO the and active CO catalytic to reach sites. the XPSactive analysis catalytic revealed sites. aXPS similar analysis coordination revealed environment a similar coordination in the developed environment heterogeneous in the developed catalyst in comparisonheterogeneous to thecatalyst homogeneous in comparison catalyst. to the The homo activegeneous heterogeneous catalyst. The (bpy-CTF-Al(OTf) active heterogeneous2)(Co(CO) (bpy-4) catalystCTF-Al(OTf) converted2)(Co(CO) PO into4) catalystβ-butyrolactone converted PO with into a high β-butyrolactone selectivity of with 90%, a whichhigh selectivity is comparable of 90%, to thosewhich of is the comparable homogeneous to those catalyst. of Moreover,the homogeneou the catalysts catalyst. was readily Moreover, separated the fromcatalyst the productwas readily and repeatedlyseparated from used the for severalproduct runs. and repeatedly The selectivity used was for maintained,several runs. although The selectivity a slight was reduction maintained, in the activityalthough was a slight also observed reduction which in the may activity originate was from also partial observed discharge which of may the (Co(CO)originate4) −fromanion partial from thedischarge catalyst of upon the (Co(CO) product4) separation.− anion from the catalyst upon product separation. VeryVery recently,recently, ourour groupgroup reportedreported thethe post-functionalizationpost-functionalization ofof bpyCTFbpyCTF withwith (Ir(OMe)(cod))(Ir(OMe)(cod))22 complexcomplex towardstowards thethe developmentdevelopment of aa highlyhighly stablestable andand eefficientfficient catalystcatalyst forfor thethe CHCH borylationborylation ofof aromaticaromatic compoundscompounds (Figure(Figure7 )[7) 52[52].]. TheThe synthesizedsynthesized Ir(I)@bpyCTFIr(I)@bpyCTF catalystcatalyst exhibitedexhibited excellentexcellent catalyticcatalytic activity for for CH CH borylation borylation of ofaren areneses and and heteroarenes heteroarenes in the in thepresence presence of B2 ofPin B2 2asPin a2 boronas a boronsource. source. Ir(I)@bpyCTF Ir(I)@bpyCTF also exhibits also exhibitshigh stabilit highy stabilityand reusability and reusability for at least for five at least cycles five without cycles significant loss of activity. Physical characterizations combined with density functional theory (DFT) calculations elucidated the local geometry of the immobilized Ir(I)-complex onto the bipyridine- moieties in the framework. A similar coordination environment in comparison to its homogeneous counterpart was observed. More interestingly, Ir(I)@bpyCTF also showed similar catalytic trend as the homogeneous Ir-based catalysts and exhibited a better performance than the conventional Ir- based heterogeneous catalysts.

Polymers 2019, 11, x FOR PEER REVIEW 16 of 24 Polymers 2019, 11, 1326 16 of 24 without signiﬁcant loss of activity. Physical characterizations combined with density functional theory (DFT) calculations elucidated the local geometry of the immobilized Ir(I)-complex onto the bipyridine-moieties in the framework. A similar coordination environment in comparison to its homogeneous counterpart was observed. More interestingly, Ir(I)@bpyCTF also showed similar catalytic trend as the homogeneous Ir-based catalysts and exhibited a better performance than the conventional Ir-based heterogeneous catalysts. Polymers 2019, 11, x FOR PEER REVIEW 16 of 24

Figure 7. Structural representation of the Ir(I)@bpyCTF catalyzed CH boryation of aromatic compound in the presence of B2Pin2 as a boron source (reproduced with permission from [52]. Copyright 2019, Elsevier).

The N-heterocyclic carbene (NHC) or imidazolium-based ligand possess strong σ-donating and poor π-accepting characters which can improve the electron density on the central metal ions that would enhance the efficiency of transition metal complexes heterogenized catalysts. Hence, Yoon and co-workers introduced the NHC-based CTF as catalyst support and employed it as an efficient heterogeneous catalyst for the CO2 hydrogenation to formate (Scheme 9) [53]. The Ir(III) complex was immobilizedFigure 7.7. Structuralonto Structural the NHC-functionalized representation representation of theof Ir(I)@bpyCTFthe CTF Ir(I)@bp through catalyzedyCTF N^C catalyzed CHcoordination boryation CH boryation of site aromatic via aof compoundpost-synthetic aromatic compound in the presence of B2Pin2 as a boron source (reproduced with permission from [52]. metalationin the presence strategy. of Remarkably, B2Pin2 as a boron the source Ir-NHC-CTF (reproduced system with exhibited permission excellent from [52]. activity Copyright with 2019, better TONElsevier).Copyright and TOF 2019, in comparison Elsevier). to the values observed for the Ir-based bpyCTF and sphere-shaped Ir@CTF mentioned above for the same catalytic reactions. However, the activity of the catalyst decreasedThe N-heterocyclic over three successive carbene runs (NHC) which or imidazolium-based is attributed to the ligandIr leaching possess during strong each σ-donating cycle. and poor Theπ-accepting-accepting positive charged characterscharacters imidazolium-CTF which can improve was also the electronemployed density as a support on the for centralcentral Rh-based metalmetal molecular ions that wouldcatalysts enhance in the carbonylationthe the efficiency eﬃciency of of of transition methanol transition metal [54]. metal Theco complexesmplexes characterization heterogenized heterogenized of the catalysts. catalyst catalysts. Hence,revealed Hence, Yoon that Yoon and the andco-workersRh-complex co-workers introducedwas introduced incorporated the theNHC-based as NHC-based a single-site CTF CTF catalystas ascata catalystlyst throughout support support theand and support employed employed by ligationit it as as an an of eefficient ﬃthecient Rh heterogeneouscomplex to the catalystcatalyst abundant forfor theN-atomthe COCO22 hydrogenationsites of the CTF to (Figure formate 8 (Scheme). The Rh-bpim-CTF9 9))[ [53].53]. The Ir(III) system complex showed was a immobilizedhigher efficiency onto and the long-term NHC-functionalized stability during CTF CTF throughthe through methanol N^C NˆC coordinationcarbonylationcoordination site in plug-flow viavia aa post-syntheticpost-synthetic gas phase metalationreaction compared strategy.strategy. to Remarkably, Remarkably, the Rh-bpy-CTF the the Ir-NHC-CTF catalyst.Ir-NHC-CTF The system elsystemectron exhibited donationexhibited excellent effectexcellent activityof the activity abundant with with better N-atom better TON andTONsites TOFin and the in TOFCTF comparison enhancedin comparison to the the nucleophilicity valuesto the observedvalues ofobserv th fore central theed Ir-basedfor metal the Ir-based bpyCTFion and bpyCTFhence and sphere-shaped increased and sphere-shaped the catalytic Ir@CTF mentionedIr@CTFactivity. mentionedThe above strong for aboveion-pair the same for interaction catalyticthe same reactions. betweencatalytic However, thereac Rhtions. centers the However, activity and the of thethe positive activity catalyst charged of decreased the supportcatalyst over threedecreaseddramatically successive over increa three runssed successive which the stability. is attributed runs which to theis attr Ir leachingibuted to during the Ir leaching each cycle. during each cycle. The positive charged imidazolium-CTF was also employed as a support for Rh-based molecular catalysts in the carbonylation of methanol [54]. The characterization of the catalyst revealed that the Rh-complex was incorporated as a single-site catalyst throughout the support by ligation of the Rh complex to the abundant N-atom sites of the CTF (Figure 8). The Rh-bpim-CTF system showed a higher efficiency and long-term stability during the methanol carbonylation in plug-flow gas phase reaction compared to the Rh-bpy-CTF catalyst. The electron donation effect of the abundant N-atom sites in the CTF enhanced the nucleophilicity of the central metal ion and hence increased the catalytic activity. The strong ion-pair interaction between the Rh centers and the positive charged support dramatically increased the stability.

Scheme 9.9. Schematic representation of synthesis of thethe Ir-NHC-CTFIr-NHC-CTF (Reproduced(Reproduced withwith permissionpermission from [[53].53]. Copyright 2017, AmericanAmerican Chemical Society).

Polymers 2019, 11, 1326 17 of 24

The positive charged imidazolium-CTF was also employed as a support for Rh-based molecular catalysts in the carbonylation of methanol [54]. The characterization of the catalyst revealed that the Rh-complex was incorporated as a single-site catalyst throughout the support by ligation of the Rh complex to the abundant N-atom sites of the CTF (Figure8). The Rh-bpim-CTF system showed a higher efficiency and long-term stability during the methanol carbonylation in plug-flow gas phase reaction compared to the Rh-bpy-CTF catalyst. The electron donation effect of the abundant N-atom sites in the CTF enhanced the nucleophilicity of the central metal ion and hence increased the catalytic activity. The strong ion-pair interaction between the Rh centers and the positive charged support dramatically increased the stability. Polymers 2019, 11, x FOR PEER REVIEW 17 of 24

FigureFigure 8.8.Structural Structural representation representation of theof Rh-bpim-CTFthe Rh-bpim-CTF catalyst catalyst in resting in resting state (1 ),state and (2(1)), the and suggested (2) the structuresuggested of structure the in situ of generated the in situ active generated state (Reprinted active state with (Reprinted permission with from permission [54]. Copyright from 2018,[54]. RoyalCopyright Society 2018, of Chemistry).Royal Society of Chemistry).

InIn anotheranother catalyticcatalytic study,study, thethe samesame groupgroup ofof YoonYoonet et al.al. reportedreported onon thethe potentialpotential ofof thisthis charged − imidazolium-basedimidazolium-based CTF CTF to to formform complexescomplexes with with (Co(CO) (Co(CO)44))− by maintaining the homonuclear ion pair andand eefficientlyfficiently catalyzedcatalyzed thethe carbonylationcarbonylation ofof epoxideepoxide [[55].55]. TheThe imidazolium-CTFimidazolium-CTF waswas treatedtreated withwith KCo(CO)KCo(CO)4 in methanol under a COCO atmosphereatmosphere andand subsequentlysubsequently aa facilefacile exchangeexchange ofof endogenousendogenous -- − ClCl anions withwith exogenousexogenous (Co(CO)(Co(CO)44))− anionsanions occurred. XPS XPS analysis showed that thethe coordinationcoordination environmentenvironment ofof Co Co anions anions present present in the in heterogenized the heterogenized catalyst wascatalyst similar was to thatsimilar of the to homogeneous that of the systemhomogeneous and further system characterization and further by characterization ICP-OES, SEM and by elemental ICP-OES, analysis SEM collectivelyand elemental confirmed analysis the − existencecollectively of (Co(CO)confirmed4)− inthe the frameworks.existence of The(Co(CO) heterogenized4) in the (imidazolium-CTF)(Co(CO) frameworks. The heterogenized4) catalyst exhibited(imidazolium-CTF)(Co(CO) a much better selectivity4) catalyst in comparison exhibited toa itsmuch homogeneous better selectivity counterpart in comparison (Bmim)(Co(CO) to its4) catalysthomogeneous for the counterpart production (Bmim)(Co(CO) of methyl 3-hydroxybutyrate4) catalyst for the (MHB) production from propylene of methyl oxide 3-hydroxybutyrate (PO) and CO. Typically,(MHB) from the propylene formation ofoxide side (PO) products and CO. in this Typically, catalytic the reaction formation are inevitable of side products due to the in this nucleophilic catalytic attackreaction of methanolare inevitable on PO. due Furthermore, to the nucleophilic the catalyst attack stability of methanol was revealed on PO. via Furthermore, the recycling experimentsthe catalyst − whichstability shower was revealed that a small via the amount recycling of (Co(CO) experiments4)− ions which leached shower in every that successivea small amount cycle. of (Co(CO)4) ions Theleached leaching in every of the successive metal active cycle. sites from the catalytic support resulted into a decrease in selectivity and recyclabilityThe leaching of of the the catalyst. metal Thus,active Yoonsites andfrom co-workers the catalytic designed support a bis-imidazolium-basedresulted into a decrease CTF in asselectivity catalytic and support recyclability to enhance of the the catalyst. stability Thus, of theYoon (Co(CO) and co-workers4)− ions duringdesigned the a catalyticbis-imidazolium- process ofbased the hydroesterificationCTF as catalytic support of epoxides to enhance (Figure the9 )[stability56]. The of the catalytic (Co(CO) results4)− ions demonstrated during the catalytic that the bis-imidazolium-functionalizedprocess of the hydroesterification framework of epoxides introduced (Figure 9) [56]. intramolecular The catalytic dication-anion results demonstrated interaction that withinthe bis-imidazolium-functionalized the framework and literally framework activated more introd epoxideuced intramolecular with better selectivitydication-anion and interaction decreased thewithin (Co(CO) the framework4)− ions leaching and literally from activated the CTF. more This significantlyepoxide with increasedbetter selectivity the catalytic and decreased stability forthe epoxide(Co(CO) carbonylation.4)− ions leaching from the CTF. This significantly increased the catalytic stability for epoxide carbonylation.

Table 1. Summary of various catalytic reactions performed by utilizing CTFs.

Metal Material Monomer Type of Reaction Activity Ref content Rate of glycerol 1 wt% Pd Oxidation of glycerol consumption: 982 Pd/CTF [28] −1 −1 (nanoparticles) into glyceric acid mol·h ·mol Selectivity = 81% Turn-over frequency 1 wt% Pd Oxidation of Benzyl Pd/CTF (TOF) = 1453 h−1 [30] (nanoparticles) alcohol Selectivity = 98% Pressure/Selectivity = 30 Hydrogenation of N- 4 wt% Pd bar/98.9%; 20 bar/97.9% Pd/CTF heterocycles (1,10- [32] (nanoparticles) for 8H-phen phenanthroline) TOF = 47.6 h−1 (1st cycle)

PolymersPolymers 20192019 2019,,, 1111, 11,,, xxx, FORFORFORx FOR PEERPEERPEER PEER REVIEWREVIEWREVIEW REVIEW 181818 ofof of 2424 24 Polymers 2019,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 18 of 24 Polymers 2019, 11, x FOR PEER REVIEW 18 of 24 Polymers 2019, 11, 1326 18 of 24 TableTable 1. 1.SummarySummary Summary ofof of variousvarious various catalyticcatalytic catalytic reactionsreactions reactions performedperformed performed byby by utilizingutilizing utilizing CTFs.CTFs. CTFs. Polymers 2019, 11, xTable FOR PEER 1. Summary REVIEW of various catalytic reactions performed by utilizing CTFs. 18 of 24 Polymers 2019, 11, xTable FOR PEER 1. Summary REVIEW of various catalytic reactions performed by utilizing CTFs. 18 of 24 Polymers 2019, 11, x FOR PEER REVIEW Metal 18 of 24 Material Monomer MetalMetal Type of Reaction Activity Ref MaterialMaterial MonomerTableMonomer 1. Summary of variousMetal catalytic reactionsTypeType of ofperformed Reaction Reaction by utilizing Activity Activity CTFs. Ref Ref Material Monomer contentcontentMetalcontent Type of Reaction Activity Ref Material MonomerTable 1. Summary of variouscontent catalytic reactionsType of performed Reaction by utilizing Activity CTFs. Ref Table 1. Summary of variouscontent catalytic reactions performedRate by of glycerol utilizing CTFs. Metal RateRate of ofglycerol glycerol Material Monomer Type of Reaction Rate Activity of glycerol Ref 11 Metalwtwt1 wt %% % PdPd Pd OxidationOxidation of ofglycerol glycerol consumption:consumption:Rateconsumption: of glycerol 982982 982 MaterialPd/CTF Monomer 1content wt % Pd OxidationType of Reactionof glycerol consumption: Activity 982 Ref[28] MaterialPd/CTFPd/CTFPd/CTF Monomer 1 wt % Pd OxidationType of Reactionof glycerol consumption: Activity−1 982−1 Ref[28][28][28] MaterialPd/CTF Monomer(nanoparticles) Metal1content wt % Pd content Oxidationinto glyceric of Type glycerol acid of Reactionconsumption:mol·h−1·mol−1 982−1 −1 Activity[28] Ref Pd/CTF (nanoparticles)(nanoparticles)(nanoparticles)content intointointo glycericglyceric glyceric acidacid acid mol·hmol·h−−11·mol·mol·mol−−11 [28] Pd/CTF (nanoparticles)(nanoparticles) intointo glycericglyceric acidacid Ratemol·h of −glycerol1·mol·mol−1 [28] −1 −1 (nanoparticles) into glyceric acid SelectivitySelectivityRatemol·hSelectivity of glycerol·mol == 81%81%= 81% Rate of glycerol 1 wt % Pd Oxidation of glycerol consumption:Selectivity = 81% 982 Pd/CTF 1 wt % Pd Oxidation of glycerol consumption:Selectivity = 81% 982 [28] 1 1 1 wt1 wt% Pd % Pd OxidationOxidation of glycerol of glycerolTurn-overconsumption:Turn-over into− 1frequency frequency 982−1 mol h− mol− Pd/CTF Pd/CTF (nanoparticles)1 wt % Pd Oxidationinto glyceric of Benzyl acid Turn-overmol·h frequency·mol [28] [28] Pd/CTF 11 wtwt1 wt %% % PdPd Pd OxidationOxidation of ofBenzyl Benzyl Turn-over− 1frequency−1− 1 [28]· · Pd/CTF (nanoparticles)1 wt % Pd Oxidationinto glyceric of Benzyl acid Turn-over(TOF)mol·h =− 14531frequency·mol h−1− 1 −1 [30] Pd/CTFPd/CTFPd/CTF (nanoparticles)(nanoparticles)1 wt % Pd Oxidationinto glyceric of Benzyl glycericacid acid(TOF)(TOF)mol·h(TOF) == 14531453=·mol 1453 hh− −1h1 consumption:[30][30][30] 982 Pd/CTF (nanoparticles)1 wt % Pd Oxidationalcohol of Benzyl (TOF)Selectivity(TOF) == 14531453 = 81% hh−1 [30][30] (nanoparticles)(nanoparticles)(nanoparticles) alcoholalcoholalcohol −1 Pd/CTF (nanoparticles) alcohol (TOF)SelectivitySelectivitySelectivity = 1453 == 81%98%98%=h 98% Selectivity [30] = 81% (nanoparticles) alcohol Turn-overSelectivity frequency = 98% 1 wt % Pd Oxidation of Benzyl Pressure/SelectivityTurn-overSelectivity frequency = 98% = Pressure/SelectivityTurn-overPressure/SelectivityPressure/Selectivity frequency−1 Turn-over == = frequency Pd/CTF 1 wt % Pd Oxidation of Benzyl Pressure/Selectivity(TOF) = 1453 h = [30] (nanoparticles)1 wt1 wt% Pd % Pd OxidationalcoholOxidation of Benzyl of Benzyl −1 Pd/CTF Pressure/Selectivity(TOF)30(TOF) bar/98.9%; == 14531453 h h20−1 = [30][30] 1 Pd/CTF (nanoparticles) Hydrogenationalcohol of N- 30Selectivity3030 bar/98.9%;bar/98.9%; bar/98.9%; = 98% 2020 20 (TOF) = 1453 h− [30] (nanoparticles)(nanoparticles) HydrogenationHydrogenationalcohol of alcoholofN --N- 30 bar/98.9%; 20 4 wt4 wt % %Pd Pd Hydrogenation of N- bar/97.9%30Selectivitybar/97.9% bar/98.9%; for = for 98% 8H-20 8H- Selectivity = 98% Pd/CTF 44 wtwt %% PdPd Hydrogenationheterocycles (1,10- of N- Pressure/Selectivitybar/97.9%bar/97.9% forfor 8H-8H- = [32] Pd/CTFPd/CTFPd/CTF 4 wt % Pd heterocyclesheterocyclesheterocycles (1,10-(1,10- (1,10- bar/97.9% for 8H- [32][32][32] Pd/CTF (nanoparticles)(nanoparticles)(nanoparticles)4 wt % Pd heterocycles (1,10- Pressure/Selectivitybar/97.9%phenphenphen for 8H- = [32] Pd/CTF (nanoparticles)(nanoparticles) heterocyclesphenanthroline) (1,10- 30 bar/98.9%;phen 20Pressure /[32]Selectivity = 30 phenanthroline)phenanthroline)phenanthroline) −1 (nanoparticles) HydrogenationHydrogenation of N- TOF30 ofbar/98.9%; =phen 47.6 h−−11 20(1st−1 4 wt % Pd phenanthroline) bar/97.9%TOF30TOF bar/98.9%; = 47.6= 47.6 for h − h18H- 20(1st(1st (1st 4 wt % Pd Hydrogenationphenanthroline) of N -- TOF = 47.6 h−1 (1stbar /98.9%; 20 bar/97.9% Pd/CTF Pd/CTF 4 wt % Pd heterocyclesN (1,10--heterocyclesbar/97.9%TOF =cycle) 47.6 for h −1 8H- (1st [32] [32] (nanoparticles)(nanoparticles)4 wt % Pd bar/97.9%cycle)cycle)phencycle) for 8H- for 8H-phen Pd/CTF heterocycles (1,10- cycle) [32][32] (nanoparticles) phenanthroline)(1,10-phenanthroline) Severalcycle)phen substrates 1 (nanoparticles) SeveralSeveralSeveralphen substratessubstrates substrates −1 TOF = 47.6 h (1st cycle) Selectivephenanthroline) double TOFSeveral = 47.6 substrates h (1st − phenanthroline)SelectiveSelectiveSelective doubledouble double −1 2.05 wt % Pd Selective double TOFSeveral =tested 47.6 substrates h −1 (1st(1st Pd/CTF 2.052.052.05 wtwt wt %% % PdPd Pd carbonylationSelective double of aryl testedcycle)testedtested [33] Pd/CTFPd/CTFPd/CTF 2.05 wt % Pd carbonylationcarbonylationcarbonylationSelective ofof of arylaryl aryl double tested Several substrates[33][33][33] tested Pd/CTF (nanoparticles)(nanoparticles)2.05(nanoparticles)2.05 wt % wt Pd % Pd carbonylation of aryl YieldYield uptestedcycle)cycle) up to to90% 90% with with [33] Pd/CTF Pd/CTF (nanoparticles) carbonylationiodidescarbonylation of aryl Yield ofSeveral aryl up tosubstrates 90% withYield up[33] to 90% with [33] (nanoparticles) Selectiveiodidesiodidesiodides double Yield up to 90% with (nanoparticles)(nanoparticles) iodides YieldSeveralselectivityselectivity up tosubstrates 90% = 95%= 95%with 2.05 wt % Pd Selectiveiodides double iodides selectivityselectivitytested == 95%95% selectivity = 95% Pd/CTF carbonylationSelective double of aryl selectivity = 95% [33] 2.05 wt % Pd Oxidation of 5- selectivitytestedtested = 95% Pd/CTF (nanoparticles) carbonylationOxidationOxidation of of of5- aryl 5- Yield up to 90% with [33] Pd/CTF carbonylationOxidationiodides of ofOxidation 5- aryl of [33] (nanoparticles)(nanoparticles)3.91 wt % Ru hydroxymethylfurfuralOxidation of 5- YieldConversion up to 90% > 99% with Ru/CTF-c 3.913.913.91 wtwt wt %% % RuRu Ru hydroxymethylfurfuralhydroxymethylfurfuralhydroxymethylfurfuraliodides ConversionselectivityConversion = >95% 99%> 99% [35] Ru/CTF-cRu/CTF-c 3.913.91 wt wt% Ru % Ru hydroxymethylfurfuraliodides5-hydroxymethylfurfural Conversion > 99% Conversion[35][35][35] > 99% Ru/CTF-c Ru/CTF-c (nanoparticles)(nanoparticles)3.91(nanoparticles) wt % Ru hydroxymethylfurfuraltoto to 2,5-2,5- 2,5- ConversionselectivityselectivityYieldYield = 41.4% = == 41.4%> 95%95% 99% [35] [35] Ru/CTF-c (nanoparticles) Oxidationto 2,5- of 5- Yield = 41.4% [35] (nanoparticles)(nanoparticles) to 2,5- 2,5-furandicarboxylicYield = 41.4% Yield = 41.4% (nanoparticles) furandicarboxylicfurandicarboxylicfurandicarboxylicOxidationto 2,5- of 5- acidacid acid Yield = 41.4% 3.91 wt % Ru hydroxymethylfurfuralfurandicarboxylicfurandicarboxylic acidacidacid Conversion > 99% Ru/CTF-c 3.915 wt% wt % Ru Ru hydroxymethylfurfuralfurandicarboxylic acid FullConversion conversion > 99% with [35] (nanoparticles)3.9155 wt%wt%5 wt wt% % RuRu RuRu hydroxymethylfurfuralto 2,5- FullFullConversionFullYield conversionconversion conversion = 41.4% > 99% withwith with Ru/CTF-c 5 wt% Ru Full conversion with [35][35] (nanoparticles)(nanoparticles)coordination5 wt%5 wt Ru % Ru Hydrogenolysistoto 2,5-2,5- of Full15%Yield conversion selectivity = 41.4% Fullwith to conversion with 15% Ru/CTF-c coordinationcoordinationcoordination furandicarboxylicHydrogenolysisHydrogenolysis acidof of 15%15%15% selectivityselectivity selectivity toto to [36] Ru/CTF-c Ru/CTF-cRu/CTF-c coordinationcoordination usingHydrogenolysisHydrogenolysis of of15% xylitol selectivity toselectivity [36][36][36] to propylene [36] Ru/CTF-c usingcoordination (RuCl2(p- furandicarboxylicHydrogenolysisxylitol acidof 15%propylene selectivity glycol to [36] usingusingusing5 wt% (RuCl(RuCl (RuCl Ru22(p-(p- 2(p- furandicarboxylicxylitolxylitolxylitol acid Fullpropylenepropylenepropylene conversion glycolglycol glycol with Ru/CTF-c (RuClusing (RuCl(p-cymene))2(p- xylitol propylene glycol [36]glycol usingcymene))5 wt% (RuCl2 Ru22(p- 2 xylitol Fullpropylene conversion glycol with coordinationcymene))cymene))5 cymene))wt% Ru22 2 Hydrogenolysis of Full15% conversion selectivity with to Ru/CTF-c cymene))2 [36] CTF-DCE-CTF-DCE- coordinationcoordination4.3cymene))4.3 wt wt wt% %Ag2% Ag Ag HydrogenolysisCarboxylationCarboxylationCarboxylation of of of of15% terminal TON:selectivityTON: 247 247 to TON: 247 CTF-DCE-Ru/CTF-c using4.34.3 wtwt(RuCl %% AgAg2(p- Carboxylationxylitol of propyleneTON: 247glycol [36][37] CTF-DCE-AgCTF-DCE-Ru/CTF-c 4.3 wt % Ag Carboxylation of TON: 247 [36][37][37][37] [37] CTF-DCE-Ag using(nanoparticles)4.3(nanoparticles) wt (RuCl % Ag22(p- Carboxylationterminalxylitol alkynes ofalkynes propyleneYieldTON: > 247 98.9glycol Yield[37] > 98.9 AgAg using(nanoparticles)(nanoparticles)(nanoparticles)cymene)) (RuCl 2(p- terminalterminalterminalxylitol alkynesalkynes alkynes propyleneYieldYield > glycol98.9> 98.9 Ag (nanoparticles) terminal alkynes Yield > 98.9 [37] Ag (nanoparticles)cymene))2 Lowterminal temperature alkynes Yield > 98.9 CTF-DCE- 4.3cymene)) wt % Ag2 LowCarboxylationLow temperature temperatureLow of temperature TON: 247 Low temperature SelectivitySelectivity > 75%> 75% CTF-DCE-Pt-CTF 4.3 wt- % Ag oxidationLowCarboxylation temperature of methane of SelectivitySelectivityTON: 247 >> 75%75% Selectivity[37][38] > 75% Pt-CTF CTF-DCE-Pt-CTFPt-CTFPt-CTFAg (nanoparticles)4.3 wt-- % - Ag- oxidationoxidationoxidationCarboxylationterminaloxidation ofof alkynes of methanemethane methane of of methaneSelectivityYieldTON: to > 24798.9 > 75% [38][38][38] [38] Pt-CTF - oxidation of methane SelectivityTONTON = 246=> 24675% TON[37][38][37] = 246 Pt-CTFAg (nanoparticles)- oxidationterminalto methanol of alkynes methane YieldTON >= 98.9246 [38] Ag (nanoparticles) Lowterminaltoto to temperaturemethanolmethanol methanol alkynes methanol YieldTON >= 98.9246 to methanol TON = 246 IsomerizationLowto methanoltemperature of 1- Selectivity > 75% IsomerizationIsomerizationLowIsomerization temperature ofof of 1-1- 1- −1 Pt-CTF - oxidationIsomerization of methane of 1- SelectivityTOF = 24 min> 75%−1 − 1 [38] IsomerizationIsomerization of 1- SelectivityTOF ofTOF = 24= 24 min> 75%min−−11 Ir@CTFPt-CTF 2.4 wt- % Ir oxidationIsomerizationocten-3-ol of methane to of 3- 1- TOFTON = 24 = 246min −1 [38][40] 1 Ir@CTF Ir@CTFPt-CTFIr@CTFIr@CTF 2.42.42.4 wtwtwt wt%% % IrIr % Ir Ir oxidationocten-3-olocten-3-oltoocten-3-ol methanol of methane toto to 3-3- 3- −1 TOF[38][40][40]=[40]24 min− [40] Ir@CTF 2.4 wt % Ir octen-3-ol1-octen-3-ol to 3- to 3-octanoneTOFTON = 24 = 246min [40] Ir@CTF 2.4 wt % Ir octen-3-oltooctanone methanol to 3- [40] Isomerizationtooctanoneoctanone methanoloctanone of 1- octanone −1 Isomerizationoctanone of 1- 62%TOF conversion = 24 min Isomerization of 1- 62%62%62% conversionconversion conversion−1 62% conversion Ir@CTF 2.4 wt % Ir Hydroformylationocten-3-ol to 3- of 62%TOF conversion= 24 min−1 [40] HydroformylationHydroformylationHydroformylation of of 62%TOFof conversion= 24 min−1 Ir@CTF 2.4 wt % Ir Hydroformylationocten-3-ol to 3- of 62% conversion −1 − 1 [40] 1 Rh@CTF-cRh@CTF-cIr@CTFRh@CTF-c 3.53.52.43.5 wtwtwt wt wt %%% % RhRhIr% Rh Rh Hydroformylationocten-3-ol to 3- of TOFTOF = 600h= 600h−−11 TOF[40][42][42]=[42] 600h [42] Rh@CTF-c 3.5 wt % Rh Hydroformylationcrudeoctanone 1-octene of TOF = 600h−1 [42][42] − crudecrudecrude 1-octene1-octene 1-octenecrude 1-octene −1 Rh@CTF-c 3.5 wt % Rh crudeoctanone 1-octene TOFTONTON = = 600h 12000= 12000 TON[42]= 12,000 crude 1-octene 62%TON conversion = 12000 Hydroformylation of 62%TON conversion = 12000 Rh@CTF-c 3.5 wt % Rh 62%TOF conversion = 600h−1 [42] Hydroformylation of Rh@CTF-c 3.5 wt % Rh crude 1-octene TOF = 600h−−11 [42] Rh@CTF-c 3.5 wt % Rh TOF = 600h −1 [42] Dehydrogenation of TOFTON = 27000= 12000 h −1 DehydrogenationDehydrogenationcrudecrude 1-octene1-octene of of TOFTOF = 27000= 27000 h−− 1h1 1 CTF-Ir 16 wt % Ir DehydrogenationDehydrogenation of TOFTON of = 27000= 12000 h− −11 TOF =[43]27,000 h− CTF-Ir CTF-IrCTF-Ir 161616 wtwt wt wt%% % IrIr% Ir Ir Dehydrogenation of TOFTON = 27000= 12000 h− 1 [43][43][43] [43] CTF-Ir 16 wt % Ir formicformicformic acidacid acid TONTON = 1060000= 1060000 [43][43] CTF-Ir 16 wt % Ir formicformic acidacidformic acidTON = 1060000 TON[43]= 1,060,000 formic acid TON = 1060000 Dehydrogenation of TOF = 27000 h−1

CTF-Ir 16 wt % Ir Dehydrogenation of TOF = 27000 h−−11 [43] Dehydrogenationformic acid of TOFTON = = 27000 1060000 h CTF-Ir 16 wt % Ir [43][43] formicformic acidacid TON = 1060000

Ir@meso-Ir@meso-Ir@meso- Ir@meso- 2 wt % Ir Hydrogenation of CO2 TON = 358 [44] PolymersIr@meso- 2019,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 22 wtwt2 wt %% % IrIr Ir Hydrogenation Hydrogenation Hydrogenation ofof of COCO CO22 2 TON TON TON == 358358= 358 19 [44] [44] [44]of 24 Ir@meso-CTFIr@meso-Ir@meso-CTF 2 wt % Ir Hydrogenation2 of CO TON = 358 [44] CTFCTF 2 wt % Ir Hydrogenation of CO2 TON TON2 == 358358 [44] [44] PolymersCTF 2019 , 11, x FOR PEER REVIEW 22 wtwt %% IrIr Hydrogenation Hydrogenation ofof COCO22 TON TON == 358358 19 [44] [44] of 24 PolymersCTFCTF 2019 ,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 19 of 24 PolymersIr@meso- 2019, 11, x FOR PEER REVIEW 19 of 24

Ir@meso- 2 wt % Ir Hydrogenation of CO2 TON = 358 [44] Ir@meso-CTF 2 wt % Ir Hydrogenation of CO22 TON TON == 358358 [44] [44] Ir@meso-CTF Ir@meso-CTF 0.23 μmol/0.045 Dehydrogenation of TOF = 207,200 h−1 / CTF@monoIr@meso-Ir@meso-Ir@meso- −1 [45] CTF@monoIr@meso-Ir@meso- 0.230.230.23 μmol/0.045 μmol/0.045 DehydrogenationDehydrogenation of of TOFTOF = 207,200= 207,200 h− −1h1 //− 1 / [45] Ir@meso-Ir@meso- 0.23 μmgmol/0.045 Ir Dehydrogenationformic acid of TOFTON = 207,200 = 2230 h−−11 / 1 CTF@monoCTF@mono 0.23 μmgµmol/0.045 Ir Dehydrogenationformic acid of TOFTON = 207,200 = 2230 h −1 // [45][45] CTF@monolithlith 0.230.230.23 μmgmol/0.045mol Ir /0.045 mgDehydrogenationformicDehydrogenation acid of TOFTON of= 207,200 = 2230 h −1 // TOF =[45][45]207,200 h− / Ir@meso-CTF@monolithCTF@mono mgmg Ir Ir formicformicformic acidacid acid TONTON = 2230= 2230 [45][45] [45] CTF@monolithlithlithlith mg Ir Ir formicformic acidacidformic acid TON = 2230 TON[45] = 2230 Ir@meso-lithlith mg Ir formicformic acidacid TON = 2230 lith 0.23 μmol/0.045 Dehydrogenation of TOF = 207,200 h−1 / Ir@meso-Ir@meso-lith CTF@mono 0.23 μmol/0.045 Dehydrogenation of TOF = 207,200 h−−11 / [45] 0.23 μmol/0.045 Dehydrogenation of TOF = 207,200 h−1 / CTF@monoBpy-CTF- mg Ir Hydrogenationformic acid of CO2 TOFTON = =5300 2230 h [45] CTF@mono lith 4.7 wt % Ir [45][47] Bpy-CTF- 4.7mg wt Ir % Ir Hydrogenationformicformic acidacid of CO2 TOFTON = =5300 2230 h −1 [47] (IrCp*Cl)Cl to formate TON = 5000− 1 1 (IrCp*Cl)ClBpy-CTF-lith Hydrogenationto formate of CO2 TOFTON = 5300= 5000 h −1 Bpy-CTF-lith 4.7 wt % Ir HydrogenationHydrogenation of CO2 of COTOF2 =to 5300 h−1 TOF[47]= 5300 h− Bpy-CTF-(IrCp*Cl)Cl(IrCp*Cl)Cl 4.74.7 wt wt% Ir % Ir to formate TON = 5000 [47] [47] (IrCp*Cl)Cl 4.7 wt % Ir to formate TON = 5000 [47] (IrCp*Cl)Cl(bpy-CTF- to formate formate TON = 5000 TON = 5000 Hydrogenation of CO2 TOF = 22,700 h−1 (bpy-CTF- Hydrogenation of CO2 TOF = 22,700 h (bpy-CTF-Ru(acac)2) 1.68 wt % Ru −1 [48] (bpy-CTF- Hydrogenationto formate of CO2 TOFTON = 22,700 = 21200 h −1 1 Hydrogenationto formate of CO2 TOFTON = 22,700= 21200 h −1 Ru(acac)Cl 2) 1.68 wt % Ru HydrogenationHydrogenation of CO2 ofTOF CO 2= to22,700 h−1 TOF =[48]22,700 h− (bpy-CTF-Ru(acac)Ru(acac)Cl)Cl 2) 1.681.68 wt wt% Ru % Ru [48] [48] Ru(acac)2 2) 1.68 wt % Ru to formate TON = 21200 [48] Cl to formate formate TON = 21200 TON = 21,200 Cl to formate TON = 21200 −1 (bpy-CTF-Cl Hydrogenation of CO2 TOF = 38,800 h−1 2.1 wt % Ru [49] (bpy-CTF- 2.1 wt % Ru Hydrogenation of CO2 TOF = 38,800 h−1 [49] RuCl3) to formate 2 TON = 20,000− 1 1 (bpy-CTF-RuCl ) 2.1 wt % Ru HydrogenationtoHydrogenation formate of CO2 ofTOF COTON 2= 38,800to= 20,000 h −1 TOF =[49]38,800 h− (bpy-CTF-RuCl(bpy-CTF-) 2.12.1 wt wt% Ru % Ru Hydrogenation of CO2 TOF = 38,800 h [49] [49] RuCl3 3) 2.1 wt % Ru to formate TON = 20,000 [49] RuCl3) to formate formate TON = 20,000 TON = 20,000 RuCl(bpy-3) to formate ConversionTON = 20,000 = 99% (bpy- Conversion = 99% CTF(RhCp*(bpy- RhConversion catalyst is = more 99% (bpy- Transfer Conversion = 99% Conversion = 99% CTF(RhCp* Transfer Rh catalyst is more CTF(RhCp*Cl)Cl) & 1.78 wt % Rh & activeRh catalyst than Ir iscatalyst. more CTF(RhCp* hydrogenationTransfer of Rh catalyst is moreRh catalyst[50] is more active (bpy-CTF(RhCp*Cl)Cl)Cl)Cl) & 1.781.78 wt wt % % Rh Rh & & 4.7hydrogenationTransfer hydrogenationof active thanof Ir catalyst. [50] Cl)Cl)(bpy- & 1.784.7 wtwt %% RhIr & Transfer activeBut, only than Ir Ir catalyst catalyst. Cl)Cl) & 1.78 wt % Rh & carbonylhydrogenation compounds of active than Ir catalyst.than Ir catalyst.[50] But, only [50] &(bpy-CTF(IrCp*Cl)Cl)(bpy- 4.7 wtwt % %Ir Ir carbonylhydrogenation compoundscarbonyl of compounds But, only Ir catalyst [50] CTF(IrCp*(bpy- 4.7 wt % Ir hydrogenation of maintainedBut, only Ir activitycatalystIr catalyst[50]maintained (bpy- 4.7 wt % Ir carbonyl compounds But, only Ir catalyst CTF(IrCp* carbonyl compounds maintained activity CTF(IrCp*Cl)Cl) carbonyl compounds maintainedduring recycling. activityactivity during recycling. CTF(IrCp* maintained activity Cl)Cl) during recycling. (bpy-CTF-Cl)Cl) Carbonylation of during recycling. (bpy-CTF-Al(OTf)Cl)Cl)) 3.763.76 wt % % Al Al & & 2.67 Carbonylation of epoxidesConversionduring recycling. > 99% Conversion > 99% (bpy-CTF-Al(OTf)2 2) Carbonylationepoxides into βof- [51] (bpy-CTF-Al(OTf)2) 3.76 wt % Al & Carbonylationepoxides into β of- Conversion > 99% [51] [51] (Co(CO)4)(bpy-CTF- 3.762.67 wtwtwt %% % AlCo Co& Carbonylationinto ofβ -lactonesConversionSelectivity = >90% 99% Selectivity = 90% (Co(CO)Al(OTf)24)) 3.76 wt % Al & epoxideslactones into β- Conversion > 99% [51] (Co(CO)Al(OTf)24)) 2.67 wt % Co epoxideslactones into β- Selectivity = 90% [51] Al(OTf)2) 2.67 wt % Co epoxides into β- Selectivity = 90% [51] (Co(CO)4) 2.67 wt % Co lactones Selectivity = 90% (Co(CO)4) lactoneslactones Ir(I)@bipyC(Co(CO)4) C-H borylationlactonesC-H of borylation1,2- ofTON = 64 TON = 64 8.26 wt % Ir [52] Ir(I)@bipyCTFIr(I)@bipyC 8.268.26 wt % wt Ir % Ir C-H borylation of 1,2- TON = 64 [52] [52] Ir(I)@bipyCTF C-Hdichlorobenzene borylation1,2-dichlorobenzene of 1,2- YieldTON = = 95% 64 Yield = 95% Ir(I)@bipyC 8.26 wt % Ir C-H borylation of 1,2- TON = 64 [52] TF 8.26 wt % Ir dichlorobenzene Yield = 95% [52] TF 8.26 wt % Ir dichlorobenzene Yield = 95% [52] TF dichlorobenzene Yield = 95%− 1 Ir0.68- Hydrogenation of CO2 TOF = 1600 h−1 0.68 wt % Ir −1 [53] NHC-CTFIr0.68- Hydrogenationto formate of CO2 TOFTON = =1600 24,300 h −1 NHC-CTFIr0.68- Hydrogenationto formate of CO2 TOFTON = = 1600 24,300 h−1 Ir0.68- 0.68 wt % Ir Hydrogenation of CO2 TOF = 1600 h−1 [53] NHC-CTF 0.68 wt % Ir to formate TON = 24,300 [53] NHC-CTF 0.68 wt % Ir to formate TON = 24,300 [53] NHC-CTFRh-bpim- Carbonylationto formate of ConversionTON = 24,300 = 93% 0.61 wt % Rh [54] Rh-bpim- 0.61 wt % Rh Carbonylation of Conversion = 93%−1 [54] Rh-bpim-CTF Carbonylationmethanol of ConversionTOF = 2100 = h93%−1 Rh-bpim- 0.61 wt % Rh Carbonylation of Conversion = 93% [54] CTF 0.61 wt % Rh methanol TOF = 2100 h−1 [54] CTF 0.61 wt % Rh methanol TOF = 2100 h−1 [54] (imidazoliuCTF Directmethanol Synthesis of TOF = 2100 h−1 (imidazoliu Direct Synthesis of (imidazolium- DirectMethyl Synthesis 3- of Conversion > 99% (imidazoliu 3.62 wt % Co Direct Synthesis of [55] m- 3.62 wt % Co Methyl 3- Conversion > 99% [55] CTF)(Co(Cm- HydroxybutyrateMethyl 3- from ConversionSelectivity = > 86% 99% m- 3.62 wt % Co Methyl 3- Conversion > 99% [55] CTF)(Co(C 3.62 wt % Co Hydroxybutyrate from Selectivity = 86% [55] CTF)(Co(CO)4) 3.62 wt % Co HydroxybutyratePropylene Oxide from Selectivity = 86% [55] CTF)(Co(C Hydroxybutyrate from Selectivity = 86% O)4) Propylene Oxide O)4) MannichPropylene reaction Oxide O)4) Propylene Oxide Mannich reaction betweenMannich 2-naphthol reaction TON = 213 Mannich reaction between 2-naphthol TON = 213 V@acacCTF 1.6 wt % V betweenand 2-naphthol N- ConversionTON = 213= 100% [57] between 2-naphthol TON = 213 V@acacCTF 1.6 wt % V and N- Conversion = 100% [57] V@acacCTF 1.6 wt % V methylmorpholineand N- N- ConversionYield = 95% = 100% [57] V@acacCTF 1.6 wt % V and N- Conversion = 100% [57] methylmorpholine N- Yield = 95% methylmorpholineoxide N- Yield = 95% methylmorpholine N- Yield = 95% oxide *For the annotation of the material, the annotation oxideused in the literature was applied. *For the annotation of the material, the annotation oxideused in the literature was applied. *For*For thethe annotationannotation ofof thethe material,material, thethe annoannotationtation usedused inin thethe literatureliterature waswas applied.applied. *For the annotation of the material, the annotation used in the literature was applied.

Polymers 2019, 11, x FOR PEER REVIEW 19 of 24 Polymers 2019, 11, x FOR PEER REVIEW 19 of 24 PolymersPolymers 2019 2019, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 1919 ofof 2424

Ir@meso- 0.23 μmol/0.045 Dehydrogenation of TOF = 207,200 h−1 / CTF@monoIr@meso- [45] Ir@meso-Ir@meso- 0.23 μmgmol/0.045 Ir Dehydrogenationformic acid of TOFTON = 207,200 = 2230 h −1 / CTF@monolith 0.230.23 μ μmol/0.045mol/0.045 DehydrogenationDehydrogenation of of TOFTOF = = 207,200 207,200 h h−−11 / / [45] CTF@monoCTF@mono mg Ir formic acid TON = 2230 [45][45] lith mgmg Ir Ir formicformic acid acid TONTON = = 2230 2230 lithlith

Bpy-CTF- Hydrogenation of CO2 TOF = 5300 h−1 4.7 wt % Ir [47] (IrCp*Cl)ClBpy-CTF- Hydrogenationto formate of CO2 TOFTON = 5300= 5000 h− 1 Bpy-CTF-Bpy-CTF- 4.7 wt % Ir HydrogenationHydrogenation of of CO CO22 TOFTOF = = 5300 5300 h h−−11 [47] (IrCp*Cl)Cl 4.74.7 wt wt % % Ir Ir to formate TON = 5000 [47][47] (IrCp*Cl)Cl(IrCp*Cl)Cl(bpy-CTF- toto formate formate TONTON = = 5000 5000 Hydrogenation of CO2 TOF = 22,700 h−1 (bpy-CTF-Ru(acac)2) 1.68 wt % Ru [48] (bpy-CTF-(bpy-CTF- 2 −1 Hydrogenationto formate of CO TOFTON = 22,700 = 21200 h− −11 Ru(acac)Cl 2) 1.68 wt % Ru HydrogenationHydrogenation of of CO CO22 TOFTOF = = 22,700 22,700 h h [48] Ru(acac)Ru(acac)22)) 1.681.68 wt wt % % Ru Ru to formate TON = 21200 [48][48] Cl toto formate formate TONTON = = 21200 21200 (bpy-CTF-ClCl Hydrogenation of CO2 TOF = 38,800 h−1 2.1 wt % Ru [49] −1 (bpy-CTF-RuCl3) Hydrogenationto formate of CO2 TOFTON = 38,800= 20,000 h (bpy-CTF-(bpy-CTF- 2.1 wt % Ru HydrogenationHydrogenation of of CO CO22 TOFTOF = = 38,800 38,800 h h−−11 [49] RuCl3) 2.12.1 wt wt % % Ru Ru to formate TON = 20,000 [49][49] RuClRuCl(bpy-33)) toto formate formate ConversionTONTON = = 20,000 20,000 = 99% CTF(RhCp*(bpy- RhConversion catalyst is = more99% (bpy-(bpy- Transfer ConversionConversion = = 99% 99% CTF(RhCp*Cl)Cl) & 1.78 wt % Rh & activeRh catalyst than Ir iscatalyst. more CTF(RhCp*CTF(RhCp* hydrogenationTransfer of RhRh catalyst catalyst is is more more [50] Cl)Cl)(bpy- & 1.784.7 wt wt % % Rh Ir & TransferTransfer activeBut, onlythan Ir Ir catalyst catalyst. Cl)Cl)Cl)Cl) & & 1.781.78 wt wt % % Rh Rh & & carbonylhydrogenation compounds of activeactive than than Ir Ir catalyst. catalyst. [50] CTF(IrCp*(bpy- 4.7 wt % Ir hydrogenationhydrogenation of of But,maintained only Ir catalystactivity [50][50] (bpy-(bpy- 4.74.7 wt wt % % Ir Ir carbonyl compounds But,But, only only Ir Ir catalyst catalyst Polymers 2019, 11CTF(IrCp*, xCl)Cl) FOR PEER REVIEW carbonylcarbonyl compounds compounds maintainedduring recycling. activity 19 of 24 CTF(IrCp*CTF(IrCp* maintainedmaintained activity activity Polymers 2019, 11,(bpy-CTF- 1326Cl)Cl) Carbonylation of during recycling. 19 of 24 Cl)Cl)Cl)Cl) 3.76 wt % Al & duringConversionduring recycling. recycling. > 99% (bpy-CTF-Al(OTf)2) Carbonylationepoxides into βof- [51] (bpy-CTF- Carbonylation of Ir0.68-NHC- (bpy-CTF- 3.762.67 wtwt %% AlCo & HydrogenationCarbonylation of of COConversionSelectivity2 = > 90% 99% TOF = 1600 h−1 (Co(CO)Al(OTf)24)) 3.763.76 wt wt % % Al Al & & epoxideslactones into β- ConversionConversion > > 99% 99% [51] Al(OTf)Al(OTf)22)) 0.682.67 wt%wt % Co Ir epoxidesepoxides into into β β-- Selectivity = 90% [51][51] [53] CTF (Co(CO)4) 2.672.67 wt wt % % Co Co lactonesto formate SelectivitySelectivity = = 90% 90% TON = 24,300 Ir(I)@bipyC(Co(CO)(Co(CO)44)) C-H borylationlactoneslactones of 1,2- TON = 64 8.26 wt %Table Ir 1. Cont. [52] Ir(I)@bipyCTF C-Hdichlorobenzene borylationCarbonylation of 1,2- of YieldTON = = 95% 64 Conversion = 93% Ir(I)@bipyCIr(I)@bipyC 8.26 wt % Ir C-HC-H borylation borylation of of 1,2- 1,2- TONTON = = 64 64 [52] Rh-bpim-CTF TF 0.618.268.26 wt% wt wt % % Rh Ir Ir dichlorobenzene Yield = 95% [52][52] [54] MaterialTFTF Monomer Metal contentdichlorobenzenedichlorobenzene Type of ReactionYieldYield = = 95% 95% Activity−1 Ref Ir0.68- Hydrogenationmethanol of CO2 TOF = 1600 h−1 TOF = 2100 h 0.68 wt % Ir [53] NHC-CTFIr0.68- Hydrogenationto formate of CO2 TOFTON = =1600 24,300 h−1 1 Ir0.68-Ir0.68- 0.68 wt % Ir HydrogenationHydrogenationDirectHydrogenation Synthesis of of CO CO22 ofof COTOFTOF2 = = to1600 1600 h h−−11 TOF[53]= 1600 h− Ir0.68-NHC-CTF 0.680.680.68 wt wt % wt% Ir Ir % Ir [53][53] [53] (imidazolium- NHC-CTF to formate formate TON = 24,300 NHC-CTFRh-bpim-NHC-CTF Carbonylationtoto formate formateMethyl of 3- ConversionTONTON = = 24,300 24,300 = 93%Conversion TON = 24,300 > 99% 0.61 wt % Rh [54] CTF)(Co(CO) Rh-bpim-CTF 3.62 wt% Co Carbonylationmethanol of ConversionTOF = 2100 = h93%−1 [55] Rh-bpim-Rh-bpim- 0.61 wt % Rh HydroxybutyrateCarbonylationCarbonylation of of fromConversionConversion = = 93% 93%Selectivity Conversion[54] = 86%= 93% Rh-bpim-CTF 0.610.610.61 wt wt % wt% Rh Rh % Rh Carbonylation of methanol −1 [54][54] [54] 4) CTF methanol TOF = 2100 h−−11 1 (imidazoliuCTFCTF DirectPropylenemethanolmethanol Synthesis of Oxide TOFTOF = = 2100 2100 h h TOF = 2100 h− (imidazolium- DirectMethyl Synthesis 3- of Conversion > 99% (imidazoliu(imidazoliu 3.62 wt % Co DirectDirectDirect Synthesis Synthesis Synthesis of of of Methyl [55] CTF)(Co(Cm- HydroxybutyrateMannichMethyl 3- fromreaction ConversionSelectivity => 86%99% Conversion > 99% (imidazolium-CTF)(Co(CO)4)m-m- 3.623.62 wt wt% Co % Co MethylMethyl3-Hydroxybutyrate 3- 3- ConversionConversion from > > 99% 99% [55] [55] CTF)(Co(CO)4) 3.623.62 wt wt % % Co Co HydroxybutyratePropylene Oxide from Selectivity = 86% Selectivity[55][55] = 86% CTF)(Co(CCTF)(Co(C HydroxybutyrateHydroxybutyratebetweenPropylene 2-naphthol from from OxideSelectivitySelectivity = = 86% 86% TON = 213 O)4) Propylene Oxide O)4) PropyleneMannich reaction Oxide V@acacCTF O)4) 1.6 wt% V PropyleneMannichand Oxide reactionN - between Conversion = 100% [57] betweenMannich 2-naphthol reaction TON = 213 TON = 213 MannichMannich reaction reaction2-naphthol and V@acacCTFV@acacCTF 1.61.6 wt wt% V % V betweenmethylmorpholineand 2-naphthol N- Conversion N-TON = 213 = 100% Conversion Yield[57] = 95%= 100% [57] betweenbetween N2-naphthol 2-naphthol-methylmorpholine TONTON = = 213 213 V@acacCTF 1.6 wt % V methylmorpholineand N- N- ConversionYield = 95% = 100% Yield[57] = 95% V@acacCTFV@acacCTF 1.61.6 wt wt % % V V andand N N-oxide- N-oxide ConversionConversion = = 100% 100% [57][57] methylmorpholineoxide N- Yield = 95% methylmorpholinemethylmorpholine N- N- YieldYield = = 95% 95% * For* the For annotation the annotation of ofthe the material, material, the the annotationannotationoxide used used in in the the literature literature was was applied. applied. *For the annotation of the material, the annotation oxideusedoxide in the literature was applied. *For the annotation of the material, the annotation used in the literature was applied. *For*For thethe annotationannotation ofof thethe material,material, thethe annoannotationtation usedused inin thethe literatureliterature waswas applied.applied.

Figure 9. (A) Comparison of the intramolecular stabilization of anion in (bis-imidazolium-CTF- Figure 9. (A) Comparison of the intramolecular stabilization of anion in (bis-imidazolium- Cl)(Co(CO)4) (right) over (imidazolium-CTF)(Co(CO)4) complexes (left); (B) Representation of the CTF-Cl)(Co(CO)4)(right) over (imidazolium-CTF)(Co(CO)4) complexes (left); (B) Representation synthesis of (bis-imidazolium-CTF-Cl)(Co(CO)4) (i) ZnCl2, 400 °C; 48 h (ii) KCo(CO)4, MeOH, 50 °C, of the synthesis of (bis-imidazolium-CTF-Cl)(Co(CO)4) (i) ZnCl2, 400 ◦C; 48 h (ii) KCo(CO)4, MeOH, 0.5 MPa of CO, 24 h (Adapted with permission from [56]. Copyright 2018, Royal Society of 50 ◦C, 0.5 MPa of CO, 24 h (Adapted with permission from [56]. Copyright 2018, Royal Society of Chemistry).

Acac-based transition metal complexes have shownshown signiﬁcantsignificant catalytic performance in a wide variety ofof organicorganic transformation transformation such such as as cross-coupling cross-coupling reaction, reaction, alcohol alcohol oxidation, oxidation, hydroxylation, hydroxylation, etc. Ouretc. Our group group designed designed CTFs functionalizedCTFs functionalized with acetylacetonate with acetylacetonate (acac) groups (acac) (acac-CTF) groups (acac-CTF) to support to a 2 VO(acac)support a2 complexVO(acac) which complex eﬃciently which catalyzed efficiently aMannich-type catalyzed a Mannich-type reaction (Figure reaction 10)[57 ].(Figure The acac-CTFs 10) [57]. wereThe acac-CTFs prepared fromwere the prepared trimerization from of the 4,4 0-malonyldibenzonitriletrimerization of 4,4′-malonyldibenzonitrile under ionothermal synthesis under conditions,ionothermal and synthesis BET surface conditions, areas upand to BET 1626 surface m2 g 1 areaswere up obtained. to 1626 Them2·g acac-CTFs−1 were obtained. showed The excellent acac- · − CTFs showed excellent CO2 and H2 storage capacity and good CO2/N2 selectivity. The V@acac-CTF

Polymers 2019, 11, 1326 20 of 24

COPolymers2 and H20192 storage, 11, x FOR capacity PEER REVIEW and good CO2/N2 selectivity. The V@acac-CTF catalyst was synthesized20 of 24 by a post-synthetic metalation of the acac-CTF material with VO(acac)2 in toluene. The strong metalation catalyst was synthesized by a post-synthetic metalation of the acac-CTF material with VO(acac)2 in of the vanadium ions toward OÔ coordination sites in the frameworks was confirmed by FTIR, 13C toluene. The strong metalation of the vanadium ions toward OÔ coordination sites in the MAS NMR, and XPS analysis. The in-situ formation of iminium ions from tertiary amine oxides in the frameworks was confirmed by FTIR, 13C MAS NMR, and XPS analysis. The in-situ formation of presence of V4+ anchored on acac-CTF facilitates the potential application in modified Mannich-type iminium ions from tertiary amine oxides in the presence of V4+ anchored on acac-CTF facilitates the reactions. The V@acac-CTF showed outstanding reactivity and reusability with a TON of 213 for potential application in modified Mannich-type reactions. The V@acac-CTF showed outstanding a widereactivity substrates and reusability scope. Thewithhigher a TON reactivityof 213 for anda wide reusability substrates of scope. the catalyst The higher is attributed reactivity and to the strongreusability coordination of the catalyst of the is vanadyl attributed ions to the to thestrong electron-donating coordination of the based vana acetylacetonatedyl ions to the electron- groups in thedonating framework. based acetylacetonate groups in the framework.

Figure 10. (A) Schematic representation of the synthesis of acac-CTF supported VO(acac)2 complex, Figure 10. (A) Schematic representation of the synthesis of acac-CTF supported VO(acac)2 complex, and (B)and V@acac-CTF (B) V@acac-CTF catalyzed catalyzed Mannich-type Mannich-type reactions reactions with di ﬀwitherent different substrates substrates (Adapted (Adapted with permission with frompermission [57]. Copyright from [57]. 2018, Copyright American 2018, Chemical American Society). Chemical Society).

4.4. Conclusions Conclusions and and Outlook Outlook CTFsCTFs have have attracted attracted a a great great dealdeal ofof attentionattention espe especiallycially in in heterogeneous heterogeneous catalysis catalysis due due to totheir their inherentinherent nitrogen nitrogen groups groups and and tunable tunable functionalitiesfunctionalities that that allow allow exquisite exquisite control control over over the the chemical chemical naturenature of of the the speciﬁc specific surface surface areas areas andand physicalphysical properties of of the the resulting resulting networks. networks. They They offer oﬀ era a numbernumber of of advantages advantages as as porous porous solid solid catalystscatalysts towards selected selected challenges challenges in in the the development development and and intensification of catalytic processes. CTFs can be utilized as a support for catalytically active metal

Polymers 2019, 11, 1326 21 of 24 intensification of catalytic processes. CTFs can be utilized as a support for catalytically active metal nanoparticulate species wherein it can provide a stabilization effect compared to conventional porous solid supports. Additionally, the organic building units can be tailored synthetically with defined functional moieties to enable the immobilization of molecular metal complexes. Despite the tremendous potential of CTFs as catalytic support, as demonstrated in this review, there are still challenges which need to be addressed in order to improve the application in organic synthesis and in industrial chemical production. Owing to their amorphous nature, their behavior in catalysis are often difficult to understand. The accessibility of active catalytic species in the framework is not always guaranteed, thus, the catalytic activity has always been determined by their TONs and TOFs calculated from the entire metal loading instead of accessible species. Although metal leaching is a limited issue, catalyst deactivation is still observed upon extended recycling tests. Further insight into the deactivation pathways is necessary for developing more robust catalysts with extended lifetime. CTFs are becoming an exciting new type of porous organic support for heterogeneous catalysis owing to their well-defined nitrogen contents and high thermal and chemical stability. Although numerous catalytic reactions catalyzed by metal supported CTFs have been presented in this review, further development of CTFs as metal support are still highly desired, especially toward industrially relevant reactions, such as C–H functionalization and asymmetric catalysis.

Funding: This work was supported by the Research Board of Ghent University (GOA010-17, BOF GOA2017000303). Acknowledgments: N.T. would like to acknowledge the Universiti Malaysia Sabah (UMS) and the Ministry of Higher Education of Malaysia (KPM) for PhD funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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