Development of MOF-Derived Carbon-Based ... - ACS Publications

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Development of MOF-Derived Carbon-Based Nanomaterials for Efficient Catalysis Kui Shen, Xiaodong Chen, Junying Chen, and Yingwei Li* Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ABSTRACT: Carbon-based nanomaterials have been widely used as catalysts or catalyst supports in the chemical industry or for energy or environmental applications due to their fascinating properties. High surface areas, tunable porosity, and functionalization are considered to be crucial to enhance the catalytic performance of carbon-based materials. Recently, the newly emerging metal−organic frameworks (MOFs) built from metal ions and polyfunctional organic ligands have proved to be promising self-sacrificing templates and precursors for preparing various carbon-based nanomaterials, benefiting from their high BET surface areas, abundant metal/organic species, large pore volumes, and extraordinary tunability of structures and compositions. In comparison with other carbon-based catalysts, MOF-derived carbon-based nanomaterials have great advantages in terms of tailorable morphologies and hierarchical porosity and easy functionalization with other heteroatoms and metal/metal oxides, which make them highly efficient as catalysts directly or as catalyst supports for numerous important reactions. In this perspective, we intend to give readers a survey of the research advances in the use of MOFs as self-sacrificing templates and precursors to prepare carbon-based nanomaterials, mainly including heteroatom-doped porous carbons and metal/metal oxide decorated porous carbons for applications as catalysts in energy and environment-related electrocatalysis and traditional heterogeneous catalysis. Finally, some perspectives are provided for future developments and directions of MOF-derived carbon-based materials for catalysis. KEYWORDS: metal−organic frameworks, carbon, catalysis, pyrolysis, nanomaterials

1. INTRODUCTION Catalysts can be viewed as entities added into reaction mixtures to speed up the rate of reaction and change the path of a chemical reaction without themselves being consumed.1 Among all existing catalyst systems, porous carbon-based materials, as a huge family of porous materials, have been viewed as the most important and common catalysts or catalyst supports over the past decades as a result of their specific characteristics, which are mainly (1) excellent chemical and mechanical stability, (2) tunable porosity and surface chemistry, (3) good electrical and thermal conductivity, (4) high specific surface area, (5) excellent diversity of their structure, and (6) easy handling and low cost of manufacture.2−6 Generally, porous carbons with different nanoscaled pores can be obtained by simple thermal decomposition of various organic precursors under controlled atmospheres.7−10 Although many porous carbons prepared from the above methods also possess high surface areas and show outstanding performances over commercially available activated carbons in a wide range of applications, their pores often display disordered and noninterconnected characteristics, which greatly limits their applications in some specific fields. To introduce a relatively ordered porosity, a template can be applied to guide the formation of pores during the pyrolysis process. Ordered porous carbons can be successfully prepared by pyrolysis of appropriate carbon precursors within the rigid © 2016 American Chemical Society

nanocasting molds which already possess ordered porosity (labeled as hard-templated methods)11−13 or decomposition of a mesophase pitch carbon precursor and organic polymers formed by cooperative self-assembly between the structuredirecting agents and organic precursor species in solution (labeled as soft-templated methods).14−17 Although porous carbons with ordered micro-, meso-, and macropores could be effectively prepared by hard-templated methods, their ordered structures are often unstable due to their being composed of nanoarrays connected by small nanorods. Meanwhile, the use of hard templates as scaffolds also makes the process complicated, high-cost, time-consuming, and consequently unsuitable for large-scale industrial production. Nanoporous carbons with relatively stable structures can be obtained by soft-templated methods; however, only a few samples have been reported until now due to the difficulties in finding a suitable organic template that would not be decomposed before the complete carbonization of organic precursors. Recently, metal−organic frameworks (MOFs) built from metal ions and polyfunctional organic ligands have been proved to be ideal sacrificial templates for fabricating various carbonbased nanomaterials, including porous carbons, heteroatomReceived: April 30, 2016 Revised: July 3, 2016 Published: July 26, 2016 5887

DOI: 10.1021/acscatal.6b01222 ACS Catal. 2016, 6, 5887−5903

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ACS Catalysis doped porous carbons, and metal/metal oxide decorated porous carbons, via thermal decomposition under controlled atmospheres.18−20 As an alternative method for the hard- or soft-template methods, the carbon-based nanomaterials prepared by this novel MOF-templated route possess many advantages such as high specific surface area, tailorable porosity, and easy functionalization with other heteroatoms or metal/ metal oxides. On account of these advantages, there are now a growing number of reports on the preparation of various MOFtemplated carbon-based nanomaterials via a choice of suitable MOF templates, pyrolysis atmosphere and temperature, loading of additional precursors, and further postsynthetical functionalization, as well as their applications in the energy and environmental areas, including batteries (e.g., lithium ion,21−23 lithium−sulfur,24−26 and lithium−air batteries27,28), supercapacitors,29−31 and gas adsorption and separation,32−34 which have been systematically reviewed recently by several groups.35−38 Different from these reviews, this perspective, as shown in Figure 1, will focus on the recent developments of

Figure 2. Components and structure of MOFs. Reproduced with permission from ref 39. Copyright 2012, Royal Society of Chemistry.

nanohybrids (abbreviated as MMC). (1) There is a large variety of MOFs with different metal ions, organic linkers, morphologies, and structures, which can be correspondingly transformed to various MMCs with extraordinary tunability of structures and compositions, indicating versatile features. (2) The periodic alternation of the metal ions or metallic clusters with organic ligands in MOF structures can play an extremely important role in preventing the aggregation of metal NPs or metal oxide nanostructures in pyrolysis processes. (3) Most MOFs have very high pore volumes, which allow the entry and polymerization of additional precursors inside the pores. (4) The preparation of MOFs has the advantages of mild conditions and simple processes. Many frequently used MOFs such as ZIF-8, ZIF-67, and HKUST-1 can even be synthesized under ambient temperature and pressure. (5) While the framework topology is maintained, MOFs with various morphologies such as nanocubes, hollow spheres, hollow polyhedrons, nanowires, nanorods, and so on can be easily fabricated, which can produce a series of MMCs with their original shapes maintained. Due to these advantages, a great number of MMCs with different compositions and structures have been prepared and used as promising catalysts in many important reactions. It was found that the structures of MOF precursors and the pyrolysis conditions had significant effects on the microstructure and physicochemical properties of the obtained MMCs, which eventually influenced their catalytic properties. Here we will discuss in detail the progress in this field. 2.1. MOF-Derived MMC for Electrocatalysis. 2.1.1. Oxygen Reduction Reaction (ORR). ORR has been viewed as one of the most fundamentally and technologically important electrochemical reactions for fuel cells and lithium−air batteries.41 Although Pt-based catalysts are regarded as the dominant and best electrocatalysts for promoting the ORR, their large-scale commercialization is impeded by their poor stability, CO deactivation, scarcity, and high cost of Pt.42 To reduce or replace the Pt-based electrodes in fuel cells, extensive efforts have been devoted to developing some alternative ORR catalysts, such as transition-metal chalcogenides,43 transitionmetal NPs and oxides loaded on various supports,44,45 pyrolyzed metal porphyrins,46 and heteroatom-doped carbon materials.47,48 In recent years, with the development of carbon catalysis and nanotechnology, researchers have found that the MOF-derived MMC could provide a good alternative to the Ptbased electrocatalysts. A considerable number of MMCs with different structures and compositions have been prepared and used as potential electrocatalysts for the ORR. The first example of using porous MOFs as a new class of precursors for preparing non-platinum-group-metal (nonPGM) ORR catalysts was reported by Liu and co-workers.49 In their study, they chose the cobalt imidazolate frameworka subclass of MOF materialas the precursor with the potential to produce high active-site density and a uniformly distributed

Figure 1. Various carbon-based nanomaterials prepared by the pyrolysis of MOFs and their applications in numerous important reactions.

various MOF-derived carbon-based nanomaterials (M/MOx decorated carbons and doped porous carbons) for catalytic applications, including their preparation, characterization, and catalytic performance for reactions involved in heterogeneous catalysis such as oxidation, hydrogenation, dehydrogenation, bio-oil refining, and Fischer−Tropsch synthesis and also for reactions involved in electrocatalysis including the ORR, HER, and OER. We hope that our perspective can be of assistance for future progress in developing advanced MOF-derived carbonbased nanomaterials with the desired structure and composition for boosting catalysis.

2. MOF-DERIVED METAL/METAL OXIDE−CARBON NANOHYBRIDS FOR CATALYSIS MOFs are a class of crystalline porous materials constructed by alternatively connecting the metal ions or small metallic clusters with organic ligands in a 3D porous structure (Figure 2).39 Thus, by pyrolysis of MOFs under an inert atmosphere at a suitable temperature, the metal ions can be transformed to metal nanoparticles (NPs), metal oxide nanostructures, or both depending on the reduction potential of metal atoms present in the MOFs,40 which are highly dispersed on or embedded in a ligand-derived carbon matrix. In comparison to conventional precursors such as transition-metal-containing phthalocyanines and porphyrins, there are a number of advantages in using MOFs as precursors to synthesize metal/metal oxide−carbon 5888

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3).60 As expected, the optimized Co/NC derived from BMZIF20 (the Zn/Co molar ratio was 20) exhibited excellent ORR

catalytic center of Co−N4, which was considered to be the active ORR site.50 Through pyrolysis under an inert atmosphere at 750 °C, this MOF was transformed into a highly efficient non-PGM ORR catalyst that showed an onset potential of 0.83 V vs a reversible hydrogen electrode (RHE), which was comparable to those of the best cobalt-based nonPGM catalysts. In addition, the electron numbers over this catalyst were found to range from 3.2 to 3.5, which was further improved to 3.3−3.6 after sonicating in sulfuric acid before preparing the catalyst, suggesting a dominative four-electrontransfer process with certain peroxide formation. These interesting electrocatalytic properties have motivated researchers to develop other MOF-derived MMCs as active ORR catalysts.51,52 ZIF-67, a typical subclass of ZIF materials composed of Co(MeIm)2 primary units (MeIm = 2-methylimidazole), is one of the most used precursors for the preparation of highly active Co−N−C ORR catalysts due to its high nitrogen/metal content and facile synthesis.53 Upon simple pyrolysis under an inert atmosphere, the Co atoms and 2-methylimidazoles of ZIF-67 can be turned into porous N-doped carbon and Co NPs, respectively, resulting in a highly dispersed Co/NC nanocomposite. It was reported that the Co cations coordinated by the aromatic nitrogen ligands in ZIF-67 might assist the formation of ORR-active Co−Nx−C sites in the derived catalyst.54 The best Co/NC catalyst was obtained by optimizing the pyrolysis temperature (900 °C) and the acid leaching process, which exhibited more positive onset and halfwave potentials as well as higher saturation current density than commercial Pt/C catalysts in alkaline electrolytes and comparable saturation current density in acidic electrolytes. In addition, the size of the MOF precursors also plays an important role in determining their ORR catalytic properties. Xia and co-workers first had this idea from the finding that a reduction in the size of the ZIF-67 precursors could significantly improve the catalytic activities of their derived ORR catalysts.55 The nanoelectrocatalyst derived from the smallest ZIF-67 nanocrystal showed a superior ORR performance in acidic solution with an onset potential of 0.86 V and a half-wave potential of 0.71 V (vs RHE). This excellent activity was attributed to the smaller particle size, which could provide more active sites and easy access to catalytic centers and thus promote a faster mass and electron transfer process. This study demonstrates that small MOFs can impressively make a great difference, which could open up new ways to design novel MOF-derived nanomaterials for highly active catalysts. Although ZIF-67 is a good candidate for preparing highly active Co/NC catalysts, the direct pyrolysis of ZIF-67 can only offer metal−carbon hybrids with low surface areas and porosity, which is not efficient for mass transport inside the samples.54−56 On the other hand, a Zn-based ZIF-8 with an isostructural structure to ZIF-67 has been well demonstrated to be able to afford high-surface-area carbons with high N contents, whereas it cannot provide highly active metal−Nx−C sites and well-graphitized carbon.40,57−59 In order to combine the merits of the above two MOFs, a series of bimetallic ZIFs (BMZIFs) based on ZIF-8 and ZIF-67 with varied Zn/Co ratios were prepared successfully by Jiang and co-workers, and the BMZIFs were further used as templates to prepare unprecedented porous carbons, inheriting both merits of carbons independently from ZIF-8 and ZIF-67. The obtained materials possessed large surface areas, high graphitization degree, and highly dispersed N and CoNx active species (Figure

Figure 3. Schematic illustration for the preparation of porous carbons from BMZIFs for highly efficient oxygen reduction reactions. Reproduced with permission from ref 60. Copyright 2015, WileyVCH.

activity, which approached that of the commercial Pt/C in alkaline media. Upon further doping with phosphorus, this catalyst possessed not only better ORR catalytic activity (onset potential of −0.04 V, half-wave potential of −0.12 V vs Ag/ AgCl, and diffusion-limited current density of 6 mA cm−2) but also superior stability and tolerance to methanol in comparison to Pt/C catalyst with the same loading. Subsequently, Sun and co-workers also reported almost the same idea that selfsupported porous Co−N−C nanopolyhedrons with excellent ORR catalytic properties could be prepared by pyrolysis of a Zn/Co bi-MOF without any post-treatments, confirming the great promise of this novel bi-MOF self-adjusted strategy for advanced electrodes substituting Pt/C in energy storage and conversion.61 Generally, the high surface area and periodic porous structure of MOFs are believed to be beneficial for fabricating good ORR catalysts. However, the ordered structures of MOFs are destroyed after the pyrolysis process at high temperatures, only resulting in disordered and noninterconnected pore structures that can slow down the mass transport in the catalyst. Recently, An and co-workers proposed a facile MOFinduced strategy to fabricate novel Co@Co3O4@C core@ bishell NPs encapsulated in situ into a highly ordered porous carbon matrix (CM) (denoted Co@Co3O4@C-CM) for ORR catalysis.62 As illustrated in Figure 4a, ZIF-9, a Co-based MOF, was first introduced into the confined space of a highly ordered CM that was functionalized beforehand with carboxyl groups. The as-prepared MOF-CM composite was then pyrolyzed under an inert atmosphere to obtain Co@C-CM, which was later transformed into core@bishell Co@Co3O4@C nanoparticles on the CM via a controlled oxidation. By combination of the porous matrix possessing interconnected ordered channels, good activity of Co@Co3O4@C NPs, and strong coupling/interaction between NPs and the CM, this sample showed excellent ORR activity with an onset potential as high as 0.93 V and a half-wave potential of 0.81 V (vs RHE), which was almost identical with those of the commercial Pt/C (Figure 4b). Furthermore, the ORR catalytic activity of an MOF-derived MMC can also be significantly enhanced through the introduction of second multidimensional porous nanostruc5889

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Figure 4. (a) Schematic illustration for the synthesis of Co@Co3O4@C-CM. (b) ORR polarization curves of the CMs Co@Co3O4-C (from pure ZIF-9) and , Co@Co3O4@C-CM and Pt/C. Reproduced with permission from ref 62. Copyright 2015, Royal Society of Chemistry. (c) Schematic illustration for the synthesis of LDH@ZIF-67-800 via a directed growth of MOF arrays followed by a subsequent pyrolysis process. (d) ORR polarization curves of CoAl-LDH-800 (derived from CoAl-LDH), ZIF-67−-800 (derived from ZIF-67), LDH@ZIF-67-800, and Pt/C catalyst. Reproduced with permission from ref 63. Copyright 2016, Wiley-VCH. (e−g) SEM and TEM images of hollow frameworks of nitrogen-doped carbon nanotubes derived from ZIF-67 (700 °C and H2). Reproduced with permission from ref 65. Copyright 2016, Nature.

surface area, and rich pore structure. Consequently, the resulting Co,N-CNF exhibited superior ORR activity in comparison with the sample prepared in the absence of mSiO2, which was also better than that of the commercial Pt electrocatalyst at the same loading in alkaline media and comparable to that of the commercial Pt electrocatalyst in acidic media. Although a number of studies on the carbonization of MOF precursors have been carried out in the past few years, few of them have taken into account the influence of annealing atmosphere on the structures of the resulting MMC. Recently, Wang and co-workers found the direct pyrolysis of ZIF-67 particles under an Ar/H2 atmosphere could afford N-doped carbon nanotube frameworks (NCNTFs).65 As observed from Figure 4e−g, the resultant NCNTFs not only retained the similar size and polyhedral morphology of the initial ZIF-67 particles but also possessed hierarchical shells of interconnected crystalline NCNTs. It was demonstrated that the H 2 atmosphere played a critical role in the formation of NCNTFs by favoring the formation of metallic Co NPs, which could catalyze the growth of NCNTs. This particular discovery was remarkably different from all previous reports of the formation of microporous nanocarbons and/or metal oxide nanocomposites by direct carbonization of MOFs under an inert atmosphere. With the advantageous features of composition, structure, optimum graphitic degree, and N-doping level, the NCNTFs exhibited higher electrocatalytic activity and stability for oxygen reduction and evolution than the commercial Pt/C

tures, which provide enough effective space for the rapid diffusion of reactant and electrolyte into the active sites. In this context, Wei and co-workers fabricated a carbon-based network with controlled morphology and unique micro-/mesoporous structure by in situ growth of MOF arrays on the surface of LDH nanoplatelets and a subsequent pyrolysis process (Figure 4c).63 The as-prepared MOF arrays were strongly and uniformly anchored onto the surface of the LDH scaffold, which ensured the structural transformation from MOFs to carbon-based arrays with fine control over composition and morphology. Taking advantages of its desirable features, including highly effective active sites (e.g., N−C, Co−N−C), high surface area, and unique micro/mesoporous structure, the resulting 2D carbon-based network (designated LDH@ZIF-67800) showed excellent ORR activity (with an onset potential of 0.94 V and a half-wave potential of 0.83 V vs RHE) and stability (∼99% current retention over 20000 s), which were superior to those of the commercial Pt/C material (Figure 4d). Recently, still more handsome progress in preparing MOF-derived MMC 3D nanoframes has also been made by Zhang’s group, who developed a mesoporous-silica-protected calcination strategy to synthesize a novel hierarchically porous Co,N codoped carbon nanoframework (Co,N-CNF) by using a Zn,Co zeolitic imidazolate framework (Zn,Co-ZIF) as the precursor.64 This protection strategy could effectively prevent the irreversible fusion and aggregation of MOF-derived MMCs under hightemperature pyrolysis and thus produce a hierarchically porous Co,N-CNF electrocatalyst with high dispersity, large specific 5890

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Figure 5. (a) Principle of high-performance electrocatalysts for oxygen reduction reaction in alkaline solution. (b) LSV curves of MIL-88B-NH3 NPs, CNPs, and Pt/C at a rotation rate of 1600 rpm. (c) ADMFC single-cell performance constructed with CNPs and Pt/C catalysts at 60 °C under the same conditions (the loading amount of both CNPs and Pt/C was set to 3 mg/cm2). Reproduced with permission from ref 72. Copyright 2014, American Chemical Society. (d) Schematic illustration for the synthesis process of N-doped Fe/Fe3C@C/RGO. Reproduced with permission from ref 77. Copyright 2014, Wiley-VCH.

nitrogen, iron, and cobalt, was reported to be an ideal precursor for the synthesis of bimetal-doped carbon.73 On direct carbonization under a nitrogen flow at 700 °C, the PDAencapsulated Fe3[Co(CN)6]2 nanocube (PDA = polydopamine) could be converted to new types of core−shell structured (Fe,Co)@nitrogen-doped graphitic carbon (NGC) nanocubes (NCs). It was found that the carbon sources come from the cyanide ligand and PDA had been successfully pyrolyzed into graphitic carbon rather than amorphous carbon because of the catalysis of Fe and Co. As high-performance ORR catalysts, (Fe,Co)@NGC NCs exhibited not only catalytic activity comparable to that of the Pt/C catalyst but also much more outstanding catalytic selectivity and superior durability due to the protective effect of the NGC shell deposited on the surface of (Fe,Co) NCs. To fully utilize the potential advantages of MOF-derived MMCs and further enhance their ORR catalytic performance, the combination of MMC with other nanomaterials for the fabrication of MMC-based hybrids has been proven to be an effective and practical strategy.74 2D graphene sheet is a wonderful choice for building highly active graphene-based MOF-derived electrocatalysts, considering its unique twodimensional structure, excellent electron collection and transport properties, and high stability.75,76 Chen and co-workers prepared a novel Fe−N−C catalyst, in which the nitrogendoped core−shell-structured porous Fe/Fe3C@C nanoboxes were supported on reduced graphene oxide sheets (denoted as N-doped Fe/Fe3C@C/RGO), through the direct pyrolysis of GO-based Prussian blue (PB) nanocubes (Figure 5d).77 In the pyrolysis process, the PB nanocubes not only acted as templates/precursors but also provided nitrogen sources for the formation of N-doped Fe/Fe3C@C. Because of the synergistic effect between nitrogen-doped Fe/Fe3C@C and nitrogen-doped reduced graphene oxide (NRGO) sheets, the hybrid exhibited much better electrocatalytic activity (with an onset potential of 1 V and a half-wave potential of 0.93 V vs RHE), long-term stability, and methanol tolerance ability in comparison with those of the commercial Pt/C catalyst (10%

catalyst in alkaline solution. Futhermore, unlike most non-Pt electrocatalysts, the NCNTF catalyst also exhibited high ORR activity in 0.5 M H2SO4 solution, with an onset potential of ∼0.85 V, indicating its high tolerance to acidic media. In another similar study, Muhler and co-workers also revealed the critical role of the H2 atmosphere in the reductive carbonization of MOF for grafting porous carbons with highly graphitic CNTs.66 They successfully synthesized core−shell Co@Co3O4 NPs embedded in CNT-grafted N-doped carbon polyhedra by the pyrolysis of ZIF-67 under a reductive H2 atmosphere and subsequent controlled oxidative calcination. Furthermore, as a bifunctional catalyst for both ORR and OER (oxygen evolution reaction), the optimal catalyst (oxidative in O2 flow at 250 °C for 2 h) exhibited OER activity higher than that of Pt/C and IrO2, activity very similar to that of the benchmark OER catalyst (RuO2), and ORR activity comparable to that of the benchmark ORR catalyst (Pt/C). Apart from the above extensively investigated Co-based ZIFs, some other transition-metal-containing MOFs, such as CPM24,67 CPM-99,68 Fe-MIL-100,69 MIL-101 (Fe),70 Fe-BTT,71 etc., can also be used as excellent precursors to prepare highly active ORR electrocatalysts. As a typical example, Tang and coworkers reported that the direct pyrolysis of monodisperse nanoscale MIL-88B-NH3 (Fe3O(H2N-BDC)3, H2N-BDC = 2aminoterephtalic acid) with controllable size and shape could produce a novel Fe and N codoped nanocarbon (denoted as CNPs).72 The resultant sample exhibited promising catalytic ORR activity in an alkaline system with an onset potential of 1.03 V and a half-wave potential of 0.92 V (vs RHE), which were better than those of the commercial Pt/C catalysts (Figure 5a,b). Furthermore, the CNPs have also been successfully used as the cathode of real alkaline direct fuel cells, exhibiting a high output power density of 22.7 mW/cm2, which was 1.7 times higher than that of the commercial Pt/C catalyst (Figure 5c). In addition to the above monometal-doped nanocarbons, multimetal-doped carbons can also be synthesized by employing some multimetal-MOFs as the precursors. In this regard, Fe3[Co(CN)6]2, which is rich in carbon, 5891

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ACS Catalysis Pt on Vulcan XC-72). Subsequently, the same group also reported the direct pyrolysis of graphene oxide supported ZIF67 to afford a novel nitrogen-doped graphene/cobaltembedded porous carbon polyhedron hybrid.78 Benefitting from the porous carbon structure, N/Co-doping effect, introduction of NRGO, and good contact between N/Codoped PCP and NRGO, this hybrid exhibited excellent ORR electrocatalytic activity in basic media with an onset potential of 0.97 V (vs RHE), which was close to that of the commercial Pt/C catalyst, as well as superior durability, a four-electron pathway, and excellent methanol tolerance. In addition to these graphene-involved porous nanocomposites, the combination of MOF-derived carbon with carbon nanotube was also proposed.79 Co3O4 NPs embedded in N-doped mesoporous graphitic carbon layer/multiwalled carbon nanotube (MWCNT) hybrids were synthesized by a facile carbonization and subsequent oxidation process of MWCNT-based ZIF-9, which can serve as a high-performance and low-cost bicatalyst for both the OER and ORR. Obviously, these studies open up new ways to prepare efficient and low-cost electrocatalysts to replace precious-metal-based catalysts by the rational design of MOF-derived hybrids. 2.1.2. Hydrogen/Oxygen Evolution Reactions. Electrochemical splitting of water into hydrogen and oxygen has been considered as a highly desirable approach to produce hydrogen to store light or electric energy in the form of chemical bonds.80,81 The water-splitting reaction can be divided into two half-reactions: the hydrogen evolution reaction (HER; 2H+ + 2e− → H2) and the oxygen evolution reaction (OER, H2O + 2(h+) → 1/2O2 + 2H+), both of which are crucial for the overall efficiency of water splitting.82,83 Efficient water splitting requires high-performance electrocatalysts to promote the reaction rate of the HER and OER. In spite of the intense efforts devoted to this field, state-of-the-art hydrogen evolution electrocatalysts still rely on platinum-based materials.84,85 However, the high cost and scarcity of these materials greatly hinder their large-scale industrial applications, and thus the search for and development of low-cost non-noble-metal electrocatalysts with high performance remains a great challenge. In research to address this challenge, it was found that some MOF-derived MMCs could exhibit strong potential for use as electrocatalysts to producie hydrogen or oxygen from water splitting. Similar to its applications in the ORR, ZIF-67 can also be used as a precursor and self-sacrificial template to prepare promising non-precious-metal electrocatalysts for the HER and OER. As a supplemental work on ZIF-derived MMCs, Sun and co-workers developed a facile MOF-derived route to synthesize porous Co-P/NC nanopolyhedrons composed of CoPx nanoparticles embedded in N-doped carbon matrices by direct carbonization of ZIF-67 followed by phosphidation (Figure 6a).86 The as-prepared Co-P/NC exhibited remarkable catalytic performance for both the HER and OER in 1.0 M KOH, affording a current density of 10 mA cm−2 at low overpotentials of −154 mV for the HER and 319 mV for the OER, respectively (Figure 6b,c). Moreover, the Co-P/NC-based alkaline electrolyzer approached 165 mA cm−2 at 2.0 V, which was much greater than that of Pt/IrO2 (89 mA cm−2). The 3D interconnected mesoporosity with high specific surface area, high conductivity, and synergistic effects of CoPx encapsulated within N-doped carbon matrices were responsible for these superior performances. Subsequently, a similar MOF-templated strategy was also proposed for the preparation of N and B

Figure 6. (a) Illustration of the MOF-derived route to synthesize porous Co-P/NC nanopolyhedrons. (b) HER and (c) OER polarization curves of Co/NC (without phosphidation step), Co-P/ NC, Pt/C, and IrO2 in 1.0 M KOH at 2 mV s−1 and 2000 rpm (solid and dotted red lines represent samples with loading amounts of 0.283 and 1.0 mg cm−2, respectively). Insets give the expanded regions around the onset of catalysis. Reproduced with permission from ref 86. Copyright 2015, American Chemical Society.

codoped graphitic carbon cages that encapsulate Co nanoparticles (denoted as Co@BCN) as highly active HER electrocatalysts by a two-step pyrolysis of H3BO3-containing ZIF-67.87 It was demonstrated that the synergetic effect between the Co NPs and the N,B codoped carbon shell played a key role in enhancing the HER catalytic performance of Co@ BCN. Furthermore, in order to enhance the conductivity and electrocatalytic activity of ZIF-67-derived nanocomposites, Jiang and co-workers incorporated reduced graphene oxide as an excellent conductor to prepare a layered CoP/reduced graphene oxide (rGO) composite.88 The as-prepared nanohybrid not only exhibited superior HER catalytic performance in acid solution but also could behave as an electrocatalyst for both the HER and OER in alkaline solution with great efficiency and durability. In addition to ZIF-67-derived Co-based electrocatalysts, other MOF-derived single-metal 89 and alloy NPs 90−92 encapsulated in a carbon matrix were also investigated for the HER and OER recently. For example, Chen and co-workers synthesized FeCo alloy nanoparticles encapsulated in nitrogendoped graphene layers by pyrolyzing Fe3[Co(CN)6]2 spheres in N2 (Figure 7a).92 The optimal catalyst (obtained at 600 °C) exhibited promising HER catalytic performance with a small onset overpotential of about 88 mV, an overpotential of only 262 mV at 10 mA cm−2, and especially long-term stability after 10000 cycles (Figure 7b,c), which, as the DFT calculations indicated, originated from the decreasing ΔGH* values of nitrogen dopants and the metal−graphene composite structure. Group VI transition-metal carbides, such as tungsten carbide93 and molybdenum carbide,94,95 are very promising electrocatalysts for the HER due to their corrosion resistance, high stability, high activity, and low cost. However, preparing nanostructured metal carbides with small nanocrystallites and desirable porosity for high electrocatalytic activity is still a great 5892

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pyrolysis of Mo-based POMs containing HKUST-1. The porous MoCx nano-octahedrons were finally obtained after removal of metallic Cu particles by Fe3+ etching. Benefiting from the porous and robust structure, as well as the ultrafine primary nanocrystallites, the as-prepared MoCx-based catalyst exhibited remarkable electrocatalytic activity for the HER with an onset potential of ∼25 mV (vs RHE) in acidic electrolyte (Figure 8b) and ∼80 mV (vs RHE) under basic conditions (Figure 8c). The above MOF-assisted strategy has been developed subsequently by Tang and co-workers to prepare a porous nanoMoC@GS with ultrafine MoC nanoparticles confined in GS (graphitized carbon shell) simply via the direct pyrolysis of a pure Mo-based metal−organic framework (Mo3(BTC)2).97 In this pyrolysis process, the ligand-derived carbon species could confine and stable the MoCx nanocrystallites and accelerate the charge transfer kinetics in the electrochemical process due to its excellent conductivity. As a result, the optimized porous nanoMoC@GS (obtained at 700 °C) showed excellent HER performance with a low overpotential (η10 = 124 and 77 mV at a current density of −10 mA cm−2), a small Tafel slope (43 and 50 mV dec−1), and a high exchange current density (j0 = 0.015 and 0.212 mA cm−2) in 0.5 M H2SO4 and 1.0 M KOH, respectively. These works indicated that the MOF-templated strategy was indeed a feasible way to design and prepare highperformance nanostructured metal carbide based electrocatalysts for the OER and HER. Generally, most of the reported OER catalysts were prepared in the forms of thin films or particle agglomerates, which could be later coated onto glassy carbon, nickel foam, or other conductive species.98,99 However, these time-consuming filmcasting or -coating procedures often resulted in uncontrolled microstructure of the obtained electrodes, featuring limited catalytically active surface areas that are unfavorable for electron conductivity and multiphase reactant/product transport during the OER. Recently, Qiao and co-workers reported an advanced 3D electrode based on hybrid Co3O4-carbon porous nanowire arrays directly grown on Cu foil (Figure 9b).100 As illustrated in Figure 9a, a Co-naphthalene dicarboxylate was directly grown on Cu foil through a hydrothermal process at 80 °C. Next, the above MOF was converted into Co3O4-C composite via carbonization under an N2 atmosphere, which retained the original nanowire morphology of the parent MOF with simultaneously generated pores inside. Because of the strong binding between the Co3O4-C nanowires and the Cu foil current collector, the as-prepared hybrid could be directly used as the working electrode without employing extra substrates or binders. In the OER electrochemical test, this novel electrode showed an outstanding activity with a low onset potential of ∼1.47 V (vs RHE), which was slightly lower than the value of ∼1.45 V for the IrO2/C noble-metal catalyst (Figure 9c). However, it exhibited an OER current much higher than that of IrO2/C even at the same loading amount and carbon content. The high OER performance of this novel electrode, as the authors asserted, could be attributed to the porous nanowire array electrode configuration and the in situ carbon incorporation, which led to enlarged active surface area, strong structural stability, and improved mass/charge transport. 2.2. MOF-Derived MMC for Heterogeneous Catalysis. 2.2.1. Catalytic Oxidation Reaction. The oxidation of alcohols using molecular oxygen is an attractive process for the synthesis of fine chemicals, and thus a variety of efficient catalysts have been developed. However, to date, most of the heterogeneous

Figure 7. (a) Synthetic route and model of the FeCo alloys encapsulated in nitrogen doped graphene layers by using Fe3[Co(CN)6]2 spheres as the template. (b) Polarization curves of various samples. (c) Polarization curves of the optimal catalyst (obtained at 600 °C) after the 1st, 2000th, 5000th, and 10000th cycles. Reproduced with permission from ref 92. Copyright 2015, Royal Society of Chemistry.

challenge because of the easy agglomeration of nanoparticles at a high reaction temperature. Recently, Yao and co-workers proposed a MOF-assisted strategy for synthesizing nanostructured MoCx nano-octahedrons as electrocatalysts for efficient hydrogen production.96 As illustrated in Figure 8a, a Cu-based MOF (HKUST-1) was chosen as the host and template, and the MoCx-Cu hybrid could be prepared by

Figure 8. (a) Schematic illustration of the synthesis procedure for porous MoCx nano-octahedrons by an MOF-assisted strategy. (b, c) Polarization curve of MoCx nano-octahedrons and Pt/C at 2 mV s−1 in 0.5 M H2SO4 (b) and 1 M KOH (c). Reproduced with permission from ref 96. Copyright 2015, Nature. 5893

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N-doped porous carbon (Co/C-N), which were prepared by one-pot thermal decomposition of a Co-containing MOF (Co9(btc)6(tpt)2(H2O)15]·(solvent) (btc = 1,3,5-benzenetricarboxylate, tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine). In the catalytic test, it was found that the optimized sample (pyrolyzing at 700 °C) exhibited excellent catalytic performance for the oxidation of 1-phenylethanol to acetophenone (98% conversion and almost 100% selectivity after 24 h of reaction time) and also a wide substrate scope for both aryl and alkyl alcohols. In addition, due to the magnetism of Co NPs, this catalyst system could be easily separated and reused at least five times without any significant loss in catalytic efficiency under the investigated conditions. Analogous aerobic oxidation of alcohols to the corresponding aldehydes was also achieved by another copper−carbon nanocomposite (Cu@C), which was prepared by simple pyrolysis of Cu3(BTC)2 at 800 °C for 6 h under an inert atmosphere.114 With the assistance of TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and NMI (N-methylimidazole), this Cu-based catalyst not only exhibited outstanding performance for the aerobic oxidation of primary alcohols with about 90% yield but also revealed a broad scope of substrates and much better reusability in comparison to the Cu3(BTC)2/TEMPO/NMI catalyst system. Beyond alcohols, the oxidation of hydrocarbons has also been catalyzed by using MOF-derived MMCs as potential catalysts. Hydrocarbons have been selectively oxidized, with molecular oxygen or H2O2 as the oxidant, by Co/N-C,115 TiO2/C,116 FeCo/C,117 or Cu/C,118 which were obtained by simple pyrolysis of Co-MOF, MIL-125(Ti), MIL-45b, or HKUST-1(Cu3(BTC)2) under an inert atmosphere, respectively. For example, Li and co-workers found that the pyrolysis of MIL-45b at 500 °C for 6 h could produce a novel hollow magnetically separable FeCo/C nanocomposite material (denoted as FeCo/C(500)), which showed high specific activity and selectivity for direct transformation of 5hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF).117 The unique hollow structure of FeCo/C(500) favored the adsorption of HMF and quick desorption of the formed DFF from the catalyst surface, leading to a high yield of DFF (over 99%) that could be comparable to that of noble-metal catalysts (such as Pd/C,119 Pt/C,119 and Ru-based catalysts120,121) under similar or even milder reaction conditions. In addition to the oxidations of alcohols and hydrocarbons, MOF-derived MMCs have also been used as efficient catalysts for the catalytic oxidation of CO to the less toxic CO2, which is always one of the hottest topics in the field of heterogeneous catalysis due to its great value in academic and industrial applications.122−124 Generally, the supported noble-metal catalysts, e.g. Au,125−127 Pd,128,129 and Pt,130 could efficiently catalyze this reaction, but the high cost of these noble-metalbased catalysts still inhibited their application. Hence, the study of CO oxidation catalyzed by other low-cost transition metals has been a very important and practical topic. Recently, Li and co-workers found that the ZIF-67-derived Co/NC catalysts could efficiently catalyze the low-temperature oxidation of CO.131 In particular, Co/C-600, which was obtained from pyrolyzing ZIF-67 at 600 °C under an argon atmosphere, not only exhibited high catalytic activity for CO oxidation even at a temperature as low as −30 °C but also showed good long-term stability with unchanged 100% conversion of Co after 24 h time on stream at room temperature (Figure 10a,b). Increasing the MOF pyrolysis temperature could lead to the aggregation of the Co NPs (Figure 10c−f), resulting in lower catalytic activity

Figure 9. (a) Schematic illustration for the fabrication of Co3O4-C nanowire arrays. (b) SEM image of Co3O4-C nanowire arrays. (c) Polarization curves of Co3O4-C nanowire arrays, IrO2/C, Co3O4 nanowire arrays (prepared by calcination of Co3O4-C nanowire arrays in air to eliminate carbon species), and the MOF in an O2-saturated 0.1 M KOH solution (scan rate 0.5 mV s−1). Reproduced with permission from ref 100. Copyright 2014, American Chemical Society.

catalyst systems for this reaction that have been identified are based on noble metals (such as gold,101−105 ruthenium,106,107 and palladium108−110) or have required the addition of base additives. Therefore, the development of cost-effective, highly efficient, and recyclable heterogeneous catalysts for such transformations under mild conditions is of great interest and pivotal importance. Recently, Li and co-workers prepared a series of carbon-supported cobalt catalysts by pyrolyzing ZIF67 at 600−900 °C under an Ar atmosphere, which could effectively catalyze the aerobic oxidation of alcohols to esters at room temperature (25 °C) under base-free and atmospheric conditions.111 These materials were composed of uniform Co NPs which were embedded in N-doped graphite with average diameters of 9−30 nm. Using the esterification of p-nitrobenzyl alcohol and methanol as a model reaction, the Co@C-N(800) (the pyrolysis temperature was 800 °C) exhibited the highest efficiency with respect to both conversion and selectivity, affording the desired product methyl p-nitrobenzoate in >99% yield. Furthermore, this catalytic system could also be extended to a broad substrate scope for aromatic and aliphatic alcohols as well as diols, providing good to excellent yields of the target products with high selectivities. Subsequently, Jiang and coworkers also reported the excellent catalytic performance of ZIF-67-derived N-doped porous carbon incorporating Co and CoO nanoparticles for the direct oxidation of alcohols to esters under similar reaction conditions.112 On the basis of these interesting results, Li and co-workers recently further developed another MOF-derived Co-based catalyst system for the highly efficient oxidation of alcohols to carbonyl compounds in neat water under an atmospheric pressure of air and base-free conditions.113 This catalyst system featured uniform magnetic Co nanoparticles being stabilized by 5894

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so on, which were stabilized on various supports. In the last two years, due to their unique properties, some MOF-derived MMCs have also been applied as heterogeneous catalysts for these reactions, including transfer hydrogenation of unsaturated bonds,144 reduction of 4-nitrophenol,145 and hydrogenation of diverse nitro compounds.146 Transition-metal-catalyzed transfer hydrogenation protocols have received much interest in the past because of the immense number of opportunities that exist to provide high atom efficiencies and generate advantageous economics. The use of hydrogen donor reagents such as alcohols in transfer hydrogenation reactions can avoid the use of autoclaves and high-pressure hydrogen. Recently, Li and co-workers developed a novel non-noble-metal Co@C−N system obtained by pyrolyzing a Co-containing MOF with the structural formula of [Co(bdc)(ted)0.5]·2DMF·0.2H2O (bdc = 1,4-benzenedicarboxylic acid, ted = triethylenediamine) for catalytic transfer hydrogenation reactions.144 The as-prepared Co@C-N catalysts were proved to be highly active and selective in the hydrogenation of a variety of unsaturated bonds, including CC, CN, NO, and CO bonds, with isopropyl alcohol in the absence of any base additives. Moreover, the catalysts are magnetically separable and reusable under the investigated conditions. More recently, Lotsch et al. also developed a porous carbon supported Cu/Cu2O composite architecture (denoted as Cu/ Cu2O/C) for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4.145 The Cu/Cu2O/C, which was composed of Cu/Cu2O NPs of 40 nm diameter that distributed uniformly both on the surface of the porous carbon, was prepared by an MOF-pyrolyzed method with Cu3(BTC)2 as both sacrificial template and copper precursor and phenol formaldehyde resin as the carbon precursor. Interestingly, this composite was competitive with some noble-metal-based catalysts and other copper-based catalysts in the aspect of required reaction time and turnover rate under similar reaction conditions.147,148 Later, Jiang and co-workers developed another highly efficient γ-Fe2O3@porous carbon nanocatalyst for the similar hydrogenation of nitro compounds, via one-step pyrolysis of Fe-MIL-88A at 500 °C.146 This metal oxide based nanocatalyst showed excellent catalytic performance for the hydrogenation of a variety of substituted aromatic or aliphatic nitro compounds to their corresponding amines. Furthermore, in addition to general groups, this catalyst also exhibited high chemoselectivity to the reducible groups, including aldehyde, nitrile, ketone, etc. 2.2.3. Fischer−Tropsch Synthesis. Fischer−Tropsch synthesis (FTS) has been recognized as an important technology in the production of liquid fuels and chemicals from natural gas based synthesis gas (a mixture of CO and H2). In FTS process technology, a high-performance catalyst plays an essential role in industrial applications.149,150 Among several options, Febased FTS catalysts have attracted much attention due to their high olefin selectivity, easy accessibility, low cost, and high activity, but they often suffered from rapid deactivation because of carbon deposition, sintering, and iron phase changes under typical FTS conditions. To minimize catalyst deactivation, Gascon and co-workers proposed a novel MOF-templated strategy for preparing highly dispersed iron carbides embedded in a matrix of porous carbon via the pyrolysis of the Fe-based MOF Basolite F300 (Fe(BTC)) at 773 K under an N2 atmosphere for 8 h (denoted as Fe@C, Figure 12a).151 The obtained Fe@C featured a high dispersion of Fe nanoparticles

Figure 10. (a) Conversion as a function of reaction temperature for Co oxidation over various catalysts under dry gas conditions. (b) Variation of the catalytic activity for CO oxidation as a function of time over Co/C-600 at room temperature under dry gas conditions. (c−f) TEM images of the ZIF-67-derived Co/C catalysts obtained at different pyrolysis temperatures: (c) 600 °C; (d) 700 °C; (e) 800 °C; (f) 900 °C. Reproduced with permission from ref 131. Copyright 2015, Royal Society of Chemistry.

at high pyrolysis temperatures. The results obtained after the introduction of moisture into the feed gas showed that the ZIF67-derived Co/C-600 was also tolerant of wet conditions, showing an unusual temperature-dependent catalytic behavior. Subsequently, Huo and co-workers developed another welldispersed and size-controllable CuO/TiO2 catalyst for this reaction by the pyrolysis of Cu-BTC/TiO2 composite (Figure 11).132 In the evaluation test for Co oxidation, CuO/TiO2 with

Figure 11. Schematic illustration of the proposed synthetic strategy of CuO/TiO2 catalyst. Reproduced with permission from ref 132. Copyright 2015, Wiley-VCH.

2.5 wt % copper species was found to give the most active performance with the full conversion of CO at 175 °C, which outperformed some supported noble metals (such as Pd/COP4,133 Au@SnO2,134 Au@UIO-66,135 and Pt/CeO2136) in terms of the temperature needed to completely oxidize CO. 2.2.2. Catalytic Hydrogenation Reaction. Catalytic hydrogenation is one of the most important reactions extensively employed in industry. Traditionally, hydrogenation reactions are catalyzed by Au,137,138 Pd,139 Ru,140 Pt,141 Ni,142 Co,143 and 5895

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nitrogen-containing precursor (such as melamine-based polymers, polyacrylonitrile, vinylpyridine resin, and silk fibroin), which is known as in situ doping,157−159 or through lowtemperature postdoping of preformed carbon nanomaterials with other N-containing precursors (such as urea, nitric acid, and especially ammonia).160,161 Postdoping of porous carbon often leads to surface functionalization only without alteration of their bulk properties, while in situ doping not only is a simple and efficient route but also can incorporate N atoms into the entire structure homogeneously, which has led to it being preferred for preparing N-doped porous carbon. Due to the open-framework structures and high surface areas, some N-containing MOFs were also applied as excellent precursors for derivation of in situ N-doped carbons with exceptionally high BET surface areas and total pore volumes. Currently, ZIF-8 is the most widely used self-sacrificed precursor for preparing N-doped porous carbons due to its facile synthesis methods,162,163 large porosity,164,165 and exceptional thermal stability.164 Another great advantage of using ZIF-8 as the precursor is that the Zn content can be easily vaporized during the carbonization process, which is the very feature that distinguishes them from common hard templates. The first example of MOF-derived N-doped porous carbon was reported by Xu et al.57 By elaborately choosing an N-rich and highly porous ZIF-8 as both a precursor and a template and furfuryl alcohol (FA) as a second precursor, they successfully prepared N-doped porous carbon with an exceptionally high surface area and pore volume (3405 m2 g−1 and 2.58 cm3 g−1) via polymerization and carbonization at 1000 °C (Figure 13).

Figure 12. (a) MOF-mediated strategy for the preparation of Fe-based FTS catalyst. (b) HAADF STEM micrograph of Fe@C (scale bar, 20 nm). (c) HRTEM micrograph of Fe@C (scale bar, 5 nm). (d) Product distribution after 10 h TOS for the unpromoted and promoted Fe@C catalysts. FTO reaction conditions: 613 K, 20 bar, H2/CO = 1, and GHSV of 30000 h−1. Reproduced with permission from ref 151. Copyright 2015, Nature.

(average dFe = 3.6 nm, Figure 12b,c) confined within a porous carbon matrix, despite the high Fe loading content (38 wt %). When being used in FTS, the Fe@C exhibited an excellent catalytic activity that was comparable to that of commercial benchmark catalysts and 1 order of magnitude higher than that of previously reported Fe catalysts.152 Furthermore, the authors found that the Fe@C promoted with 0.6 wt % of K displayed an optimal selectivity to C2−C5 olefins (20.5% carbon selectivity and 44.6% CO2-free selectivity), an increased activity, and a reduced methane selectivity (5%) (Figure 12d). These interesting results demonstrate that the MOF-mediated synthesis strategy is a promising route for the preparation of exceptionally dispersed Fe nanoparticles in a porous carbon matrix with outstanding FTS performance.

Figure 13. Schematic illustration of the preparation procedure of Ndoped porous carbon by using ZIF-8 as both a precursor and a template and furfuryl alcohol (FA) as a second precursor (the cavity in ZIF-8 is highlighted in yellow). Reproduced with permission from ref 57. Copyright 2011, American Chemical Society.

3. MOF-DERIVED HETEROATOM-DOPED POROUS CARBON FOR CATALYSIS Porous carbons have been widely used as metal-free catalysts in the fields of heterogeneous catalysis and electrocatalysis with structural defects and functional groups on the surface as the active sites.153,154 In particular, the introduction of heteroatoms (such as N, P, B, and I) into carbon nanomaterials could cause electron modulation to tune their optoelectronic properties and/or chemical activities and thus further enhance their catalytic properties in many reaction systems.155,156 The catalytic performance of heteroatom-doped porous carbon strongly depends on their microstructure/morphology, doping properties, and electrical conductivity, which are determined by the synthesis methodology. Generally, N-doped carbon nanomaterials can be prepared either by the direct pyrolysis of the

For a comparison, the direct carbonization of ZIF-8 at 1000 °C led to the N-doped porous carbon with a surface area of 3184 m2 g−1, demonstrating the positive influence of the second FA precursor on improving the pore texture of the resultant carbon. Following their step, a great number of MOF-derived porous carbons with different structures and doping properties have been prepared by pyrolyzing various MOF structures and used in various applications including hydrogen storage,166 CO2 capture,32,167 supercapacitor electrodes,168,169 and toluene vapor adsorption,169 and some systematic reviews have appeared on these topics.35,37 Especially, with the merits of high surface area and porosity, adjustable doping properties, and controllable structural characteristics, these MOF-derived heteroatom-doped porous carbons have also been proven to be 5896

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Figure 14. (a) ZIF-7/glucose composite derived nitrogen-doped porous carbons as metal-free electrocatalysts for the ORR. Reproduced with permission from ref 170. Copyright 2014, Royal Society of Chemistry. (b) Schematic illustration of the synthesis of highly graphitized nitrogendoped porous carbon nanopolyhedrons by using nanoscale ZIF-8 as a self-sacrificing template. Reproduced with permission from ref 172. Copyright 2014, Royal Society of Chemistry.

Figure 15. (a) Scheme of alveoli-inspired porous carbon materials for electrochemical energy applications. (b) TEM image of alveoli-inspired porous carbon materials. Reproduced with permission from ref 173. Copyright 2015, Wiley-VCH. (c) Illustration of the tellurium nanowire-directed templating synthesis of ZIF-8 nanofibers and derived N-doped carbon nanofibers. (d) TEM image of ZIF-8-derived N-doped carbon nanofibers (annealed at 1000 °C). Reproduced with permission from ref 174. Copyright 2014, American Chemical Society. (e) Schematic illustration of the stepwise structural evolution from MOF-5 to NGPC/NCNTs. (f) TEM image of NGPC/NCNTs-900 (annealed at 900 °C). Reproduced with permission from ref 177. Copyright 2015, American Chemical Society.

potential candidates to replace noble-metal-based catalysts in the field of catalysis. For this class of novel doped carbons, the most investigated metal-free catalysis was the electrochemical ORR, where their excellent electrocatalytic activity, long durability, and environmental friendliness have aroused extensive attention among researchers. The first example of the utilization of ZIF-derived N-doped porous carbon as a metal-free catalyst for the ORR was reported by Cao’s group.170 They chose ZIF-7, a member of the ZIFs, as the self-sacrificed precursor with glucose as the second carbon source to produce N-doped porous carbon (Figure 14a). The addition of the environmentally friendly glucose not only improved the graphitization degree of the resulting carbons but also facilitated the removal of Zn species impurities, which led to the formation of metal-free in situ Ndoped porous carbons. Taking advantage of the high SSA and the high electrical conductivity and content of pyridinic N, the carbon-L (the sample was derived from glucose/ZIF-7 composites that were synthesized under liquid conditions) exhibited not only excellent ORR activity (the onset and half-

wave potentials were 0.86 and 0.70 V vs RHE, respectively) and nearly four-electron selectivity (the electron transfer number was 3.68 at 0.3 V), which were close to those of commercial 20% Pt/C, but also better stability and increased tolerance to the methanol crossover effects, which were also superior to those of the Pt/C catalyst. Subsequently, Xu and co-workers successfully prepared a series of N-decorated nanoporous carbons with high surface areas (e.g, 3268 m2 g−1 at the carbonization temperature of 1000 °C) and good electrochemical properties as Pt-free ORR catalysts by employing commercial ZIF-8 as both the precursor and template along with furfuryl alcohol and NH4OH as the secondary carbon and nitrogen sources via an easily handled method.171 The N-decorated NC900 (annealed at 900 °C) showed an ORR onset potential at 0.83 V (vs RHE), which was comparable with 0.95 V for the commercial Pt/C catalyst. The authors attributed the high ORR catalytic activity of the ZIF-8derived N-doped porous carbon to its large number of active basic N sites and its high surface area, which provided proper channels with easy mass diffusion. Subsequently, Hong and co5897

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of each unique hybrid structure.175 Recently, Wang and coworkers proposed a MOF-templated route for the synthesis of a series of highly active hybrids of nitrogen-doped graphitic porous carbon and carbon nanotubes (denoted as NGPC/ NCNTs) by using MOF-5, urea, and nickel as the carbon and nitrogen precursors and graphitization catalyst, respectively, with a successive carbonization, catalytic graphitization, and nitridation (Figure 15e,f).177 These NGPC/NCNT hybrids demonstrated prominent catalytic activities toward the oxygen reduction reaction (ORR) in alkaline medium. In comparison to the benchmark Pt/C catalyst, the optimized NGPC/NCNT900 (annealed at 900 °C) exhibited superior catalytic activity, durability, and methanol tolerance, which made it one of the best ORR electrocatalysts derived from MOFs. This study not only showed that NGPC/NCNT-900 might serve as a promising alternative to Pt-based ORR catalysts in alkaline fuel cells but also opened up new possibilities for the synthesis of a variety of more active carbon-based ORR catalysts derived from MOFs. Apart from ORR applications, the MOF-derived heteroatomdoped porous carbons have also been shown to be promising heterogeneous catalysts for some liquid-phase oxidation178 and hydrogenation reactions.179,180 In this content, Li and coworkers demonstrated the efficiency of MOF-derived N-doped porous carbon in catalyzing the selective oxidation of hydrocarbons to prepare the corresponding oxygen-containing derivatives.178 This new kind of carbon-based catalyst with high surface areas and pore volumes, as well as high contents of sp2bonded carbons, was prepared from the simple carbonization of ZIF-67 and the subsequent removal of the Co species in aqua regia. As metal-free catalysts, these nitrogen-doped carbon materials exhibited excellent catalytic performances and robust stabilities in the aerobic oxidations of cyclohexane and toluene as well as the oxidative coupling of amines. Detailed characterization results indicated that the accessible mesopores generated by chemical etching and the homogeneous distribution of doped graphitic-type nitrogen should be responsible for the unprecedented performance of these carbon catalysts. In another work, several types of MOF-templated Ndoped porous carbon were found to be active for the catalytic reduction of 4-nitrophenol to 4-aminophenol.180 MOF-5, ZIF8, and PCN-224 were chosen as templates and carbon/nitrogen sources to obtain porous carbon materials with various nitrogen dopant forms and contents, degrees of graphitization, porosities, and surface areas. Among these metal-free catalysts, the PCN-224-templated porous carbon material optimized by pyrolysis at 700 °C exhibited a lower activation energy and higher activity (the reaction rate constant is up to k = 5.3 × 103 s−1) in comparison to the reported metal-free catalysts181−183 and even many metallic catalysts,184,185 due to its high surface area, hierarchical pores, and high nitrogen content (mainly pyrrolic nitrogen species).

workers also reported the preparation of ZIF-8-derived Ndoped porous carbon as an active metal-free ORR catalyst (Figure 14b).172 However, the obtained nanocarbons prepared at a similar pyrolysis temperature but using the nanoscale ZIF-8 as precursor showed much lower BET surface areas (e.g., 973 m2 g−1 at a carbonization temperature of 1000 °C for 5 h). This indicated that the properties of the parent MOFs played a critical role in the textural properties of the resulting carbonaceous materials. Despite the relatively lower surface area, these N-doped nanocarbons also exhibited outstanding ORR performance. The optimized NGPC-1000-10 (carbonized at 1000 C for 10 h) exhibited comparable ORR activity via an efficient four-electron-dominant ORR process coupled with superior methanol tolerance as well as cycling stability in alkaline media. Although N-doped porous carbons with high surface area and promising electrocatalytic properties could be constructed from MOFs by facile carbonization in several ways with or without post-treatment, many of them were microporous carbon with an sp3-dominant structure, which suppressed the mass transport and exhibited low electrical conductivity.57 An efficient approach to address this issue was recently proposed by Chung and co-workers, who prepared an alveoli-inspired metal-free N-doped porous carbon with high transport motivated structures by pyrolyzing an polydopamine modified ZIF, where the polydopamine coating led to the creation of sp2 carbon structures, which in turn could help overcome the mass transport and low electron conductivity problems that sp3 carbons possess (Figure 15a,b).173 Just as expected, the obtained N-doped porous carbon prepared by this method demonstrated superior ORR activity, which was comparable to that of the Pt/C commercial electrocatalyst, and the proper mass transport of oxygen and ions via an optimum pore size. Furthermore, the electrocatalytic performance of MOFderived materials could be effectively enhanced by nanostructuring. In particular, MOF-derived metal-free electrocatalysts with ordered nanostructures and optimized morphologies can favor the exposure of more active sites, enhance electron transfer, and facilitate faster reactant mass transfer, which together endow them with better catalytic activity. In this context, a very interesting ZIF-8 nanofiber was prepared by using ultrathin tellurium nanowires (TeNWs) as the hard templates, which could induce the attachment of ZIF-8 nuclei and nanocrystals onto the active surface of the TeNWs. This ZIF-8 nanofiber was then used as a self-sacrificed template to prepare one-dimensional carbon materials (Figure 15c).174 In comparison with bulk porous carbon by direct carbonization of ZIF-8 crystals, these novel carbon nanofibers exhibited a complex network structure, hierarchical pores, and high surface area (Figure 15d). On further doping with phosphorus species, the P-Z8-Te-1000 (pyrolyzed at 1000 °C) showed high ORR activity in an alkaline medium with an onset potential of ∼−0.07 V and half-wave potential of ∼−0.161 V (vs Ag/AgCl), which were even better than those of the benchmark Pt/C catalyst. N-doped carbon nanotubes (CNTs), typical one-dimensional materials, are attractive for a wide range of applications, including their use as metal-free catalysts for the ORR due to the peculiar property changes induced by doping.47,175,176 Especially, it was reported that the combination of CNTs with other types of carbon-based materials such as porous carbon and graphene was very likely to further enhance their ORR catalytic performance because of the synergistic contributions

4. MOF-DERIVED CARBON-BASED NANOMTERIALS AS CATALYST SUPPORTS It is well-known that the support plays a critical role in the catalytic performance of a solid catalyst.186 As a new member of the carbon family, the outstanding properties of MOF-derived porous carbon such as large surface area and good electrical and high electrochemical stability also make them ideal catalyst supports of metal-based catalysts. There were several reports on the use of MOF-derived porous carbon as catalyst supports for immobilizing various precious-metal NPs187,188 and alloy 5898

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Figure 16. (a) Schematic illustration showing the synthetic strategy of ZIF-8-derived hydrophilic N-doped carbon and its selective catalysis in biofuel upgrade reaction. Reproduced with permission from ref 187. Copyright 2016, Royal Society of Chemistry. (b) Schematic illustration of the process used for the synthesis of Co@Pd/NC. Reproduced with permission from ref 199. Copyright 2015, American Chemical Society.

NPs.189−191 These carbon-supporting catalysts exhibited promising catalytic properties in the hydrodeoxygenation of vanillin,187 dehydrogenation of hydrazine189,190 and formic acid,191 ORR,188 and HER.188 Indeed, enhancement in catalytic performance using MOF-derived porous carbon as supports has recently been reported. For example, Jiang and co-workers reported the stability of Pd NPs with very small sizes (∼1 nm) by MOF-derived N-doped porous carbon for catalysis in bio-oil refining.187 As illustrated in Figure 16a, ZIF-8 was first converted to hydrophilic N-doped porous carbon (NPC) with a surface area as high as 2184 m2 g−1 via a facile one-step pyrolysis. Then, a Pd precursor was introduced into the above NPC by impregnation method and reduced to tiny Pd NPs by H2. The resulting Pd/NPC exhibited excellent catalytic performance for the hydrodeoxygenation of vanillin with 100% conversion after 2 h of reaction time under mild conditions in water (0.2 MPa H2, 90 °C). This remarkable catalytic performance, as the authors expounded, could be mainly attributed to the unique properties of the NPC support, such as high surface area and hierarchical pores, good hydrophilicity, high content of graphitic-N doping, and high stability. Apart from MOF-derived porous carbon, some MOFdeived MMCs were also chosen as substrates to anchor metal NPs with good dispersion and enhanced catalytic performance.192−194 In particular, when the MMC supports contained some magnetic compositions (e.g., Co NPs194 and Fe2O3193), the obtained supported nanocatalysts could be easily separated and conveniently recycled from the reaction mixture due to their superparamagnetic behavior. There is a popular belief that the particle size of the NPs in metal-supported catalysts always plays the most important role in deciding their catalytic performance.195−197 Therefore, the high dispersion of metals as small nanoparticles onto a highsurface-area porous support is mainly required to optimize the amount of the surface of the expensive active phase exposed.198 Bearing this in mind, Li and co-workers recently proposed an MOF-templated strategy to prepare Co@Pd core−shell nanocatalysts, which could far surpass the traditional MOFs supporting Pd NPs in catalytic properties.199 As illustrated in Figure 16b, Co NPs embedded in N-doped carbon (Co/NC) were first synthesized by carbonizing ZIF-67 at 600 °C under an inert gas. The as-prepared Co/NC composite was then dispersed in an acetone solution, to which a required amount of Pd(NO3)2 was subsequently added. Because the standard reduction potential of the Co/Co2+ pair was much lower than that of the Pd2+/Pd pair, the Pd2+ could be reduced to Pd0 by a

galvanic displacement reaction, which deposited on the surface of Co NPs, resulting in a novel Co@Pd core−shell nanomaterial (Co@Pd/NC). The highly exposed Pd atoms on Co nanoparticles made Co@Pd/NC a highly efficient catalyst in the hydrogenation of nitrobenzene with up to 98% conversion after 45 min of reaction, which was overwhelmingly better than those of MIL-101- and ZIF-67-supported Pd NPs. This strategy thereby provides a great opportunity for developing a new class of MOF-templated core−shell nanocatalysts with potential applications in numerous catalytic reactions.

5. CONCLUSION AND PERSPECTIVE MOFs, as new kinds of porous materials, have gained particular attention in the last few decades due to their unique physicochemical properties and broad applications. Taking advantages of abundant metal/organic species, high BET surface areas, large pore volumes, and extraordinary tunability of structures and compositions, MOFs have been proved to be suitable self-sacrificing templates and precursors for the fabrication of various carbon-based nanomaterials. Using the MOF-templated approach, by altering the structural characteristics of the MOFs precursors and pyrolysis conditions, it is possible to synthesize metal NPs, metal oxide nanostructures, or both with highly refined textural characteristics and uniform and tunable particle size, which are highly dispersed on or embedded in a ligand-derived carbon matrix to form nonnoble-metal catalysts with high active site density. Meanwhile, metal-free porous carbons with high BET surface areas, unique electrical and thermal properties, controllable hierarchical structures, and adjustable doping properties can be also obtained from this simple approach. On the basis of these unique properties, the MOF-derived carbons have been shown to be promising catalysts or catalyst supports for heterogeneous catalysis and electrocatalysis. It is a pleasant surprise that some MOF-derived carbon-based nanomaterials with unique multidimensional structures or suitable functionalization properties even showed activity comparable to or better than that of the noble-metal-based catalysts in many important reactions, while they have reduced cost and enhanced durability. This indicates that the recently developed MOF-templated approach indeed offers new opportunities for designing advanced carbon-based catalysts with excellent catalytic performances. However, despite optimism for the preparations of MOFderived carbon-based nanomaterials and their applications as catalysts, there are still many key issues and challenges that need to be solved. First, in spite of great efforts dedicated to the 5899

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Science Foundation (2015M572323, 2016T90785), Fundamental Research Funds for the Central Universities (2015ZM045, 2015ZP002, and 2015PT004), and Guangdong Natural Science Foundation (2014A030310445, 2016A050502004 and 2013B090500027) for financial support.

investigation of MOF-derived M/N/C nanohybrids and doped porous carbons, researchers are still incapable of controlling the fine structures of these materials due to the limited knowledge of the precise mechanisms for the transformation process. Therefore, future work should pay more attention to a fundamental understanding of the formation mechanisms of MOF-derived carbon-based nanomaterials and find the key factors that influence the physicochemical properties of these materials. Second, almost all of the reported MOF-derived carbon-based catalysts exhibit disordered features with noninterconnected pores. This is obviously undesirable for their catalytic applications due to the limited reagent/product mass transfer. Thus, controlling the structural orders of these kinds of carbon-based materials will be an effective method to enhance their structural properties and catalytic efficiency. This will also be a great challenge in future studies. Third, although various MOF-derived carbon-based nanomaterials have exhibited excellent catalytic performance, so far, all of them have been obtained only at the laboratory level with very low yields and using time-consuming procedures. Therefore, developing practical approaches for the mass production of these promising catalysts is an urgent and pressing need in order to realize their commercialization. Fourth, for electrochemistry applications, further research is still needed to elucidate the relationship between the physicochemical properties of these MOF-derived carbon-based catalysts and their electrochemical properties. A greater knowledge of the nature of active sites and the reaction mechanisms over these catalysts are indispensable in guiding the design and preparation of future MOF-derived electrocatalysts. The combination of experimental and computational approaches and the exploration of new MOFderived carbon-based nanohybrids may offer good opportunities to elucidate these issues. Last but not least, the use of MOF-derived carbon-based nanohybrids in heterogeneous reactions is mainly limited to some simple liquid-phase reactions. There is clearly a requirement to expand their applications to other reaction systems to offer new directions for the applications of these new carbon-based catalysts. Overall, as this perspective indicates, the rapid development of carbon-based nanomaterials from the pyrolysis of MOFs presents many new opportunities for efficient catalysis. This novel approach should provide a simpler and more designable way to achieve advanced carbon-based catalysts, especially for energy and environment-related electrochemistry applications and traditional heterogeneous catalytic reactions. Although the use of MOF-derived carbon-based nanohybrids for catalysis is still far from real industrial application, the aforementioned advances and sustained research efforts in this field will trigger interest in scaling up their catalysis applications. Therefore, MOF-derived carbon-based nanomaterials, with no doubt, will continue to receive extensive attention in the field of nanomaterials and catalysis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21322606, 21436005, and 21576095), China Postdoctoral 5900

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