Metal–Organic Frameworks: Versatile Materials for Heterogeneous

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Metal−Organic Frameworks: Versatile Materials for Heterogeneous Photocatalysis Le Zeng,† Xiangyang Guo,† Cheng He,† and Chunying Duan*,†,‡ †

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China

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ABSTRACT: Photocatalysis is one of the most important chemical methods to mitigate the energy and environmental crisis via converting inexhaustible solar energy into clean chemical potential. The general history of the development of photocatalysis based on porous metal−organic frameworks (MOFs) is simply divided into three branches with a focus placed on the distinct structural role of the photocatalytic center: the inorganic cluster nodes, the organic linkers, and the guests in the pores of MOFs. In each branch, these photocatalytic centers are considered to be monodispersed within the crystal lattices with the other two structure roles regularly distributed to isolate the active centers and sometimes to provide more functions other than photoactivity. This distinctive nature has rendered MOFs as promising candidates for photocatalysis not only because they combine the benefits of heterogeneous catalysis and homogeneous catalysis but also because they facilitate the possibility of merging multifunctional catalytic sites for concerted or cascade photocatalysis. The design strategy and improvement approaches for MOF-based photocatalysts are also introduced with an emphasis on structure. Our intention is for this comprehensive view of MOFs-involved photocatalysts to inspire new ideas for designing heterogeneous photocatalysts toward the better utilization of solar energy. KEYWORDS: metal−organic framework, heterogeneous photocatalysis, structure−function relationship, porosity, monodisperse, multifunctional, cascade or concerted photocatalysis

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INTRODUCTION In nature, green plants use solar radiation to convert water and carbon dioxide into energy and biomolecules, which are essential for their survival.1 With the ever-increasing energy and environment demand to provide a high quality of life, humans are urged to learn from how nature obtains energy and materials through a sustainable approach called artificial photosynthesis or simply photocatalysis.2−6 The classical photocatalytic process consists of three steps: first, a photosensitizer absorbs light to reach its excited state; second, the excited state transforms into a charge separation state, producing a mobile electron and a hole; finally, the transportable electron or hole reacts with substrate. A photocatalyst that absorbs the energy of light and most importantly uses the energy to produce a new redox active center is the heart of photocatalysis. Ever since Fujishima and Honda discovered in 1972 that semiconducting TiO2 can drive water splitting under UV irradiation, extensive efforts have been devoted to investigating semiconductor-based photocatalysts.7−11 TiO2 is the flagship of semiconductor photocatalysts and has been extensively applied to photodriven water splitting and organic contaminant degradation.9−11 However, the insensitivity of TiO2 to a large portion of the solar energy spectrum (44%)the visible light © 2016 American Chemical Society

portionhas forced researchers to develop photocatalysts capable of absorbing visible light.12 Transition-metal complexes such as Ru(bpy)32+ are thus attracting increasing attention for their enormous potential to catalyze useful and unique organic transformations upon visible-light illumination.13−16 Generally, an excellent photocatalyst should own the following features: strong absorption of visible light, long lifetime of excited state, high yielding of charge separation states, and good charge mobility. Heterogeneous photocatalysts are in great demand for their recyclability and ease of separation from workup.17 Well known as a hybrid porous crystalline material linked by coordination bonds between organic linkers and metal or metal cluster nodes, metal−organic frameworks (MOFs) are ideal candidates for photocatalyst design.18−27 First, the three distinct components of MOFsmetal nodes, organic linkers and porescan all be easily tailored for photocatalysis due to the modular nature of MOFs, combining the merits of organic and inorganic chemistry. Second, the uniform channels or pores are beneficial for substrate−catalyst interactions, and the well-defined crystalline structure of MOFs Received: August 4, 2016 Revised: September 19, 2016 Published: October 13, 2016 7935

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ACS Catalysis is useful for elucidating structure−function relationships and good for charge mobility. Third, as a solid material, MOFs are easy to separate from reaction mixtures and can then be reused for the next run, possibly extending the lifetime of the catalysts and reducing waste and contamination. The exploiting of MOFs as photocatalysts has followed the progress of photocatalysis and has been accompanied by the growth of MOF structural and functional complexity. To date, numerous reports demonstrating the unique merits of MOFsbased photocatalysis over purely organic or inorganic systems have been reported, and excellent reviews are published almost yearly.28−34 In this perspective, MOF-based photocatalysts are classified into three groups according to the intrinsic structural roles of the photocatalytic centers (Figure 1). In type I MOFs,

Scheme 1. (Left) Elementary Steps of Semiconductor Photocatalysis: (i) Light Absorption, (ii) Electron Promotion from the VB to the CB, (iii) Charge Migration to the Surface of the Particle, Oxidation of the Substrate (S) by the Positive Hole (h+), and Reduction of the S by an Electron (e−). Reprinted with Permission from Ref 32. Copyright 2016 Wiley-VCH. (Right) Scheme Presentation of Type I MOF as Monodispersed Semiconductor Dots Distributed on the Networks of the Lattice

producing an electron in the CB and a hole in the VB. The generated charges (holes or electrons) then migrate to the surface of the semiconductor to react with reductive or oxidizing substrates. The long journey of MOF-based photocatalysis began with the semiconducting property because of the inherent connection between MOFs and semiconductors.35−37 In the case of the metal−oxygen clusters as the nodes of the MOFs, these clusters can be simplified as semiconductor dots that are isolated and distributed regularly within the networks of MOFs (Scheme 1).19 Compared to traditional semiconductors, semiconductor dots are more efficient as photocatalysts because the detrimental charge recombination is no longer a problem. The MOF scaffold can exploit the full potential of semiconductor dots as photocatalysts because (1) the high porosity allows the substrate to be close to semiconductor dots so that the produced active electrons or holes do not need to travel to the surface of the whole material; (2) the density of semiconductor dots can be higher than the homogeneous systems, while no quenching will occur thanks to the isolation of inorganic clusters by organic linkers; (3) the organic linkers can serve as the photon antennae to increase the visible light absorption of semiconductors, besides the isolators for the semiconductor dots. This rational deduction is confirmed by many examples. In 2007, Garcia and co-workers were the first to provide experimental evidence supporting the behavior of MOF-5 as a semiconductor (Scheme 2).35 A charge-separated state of MOF-5 was observed to decay at the microsecond time scale after irradiation, and the band gap was estimated to be 3.4 eV. Thereafter, many MOFs, including the UiO-66 and MIL series, have been reported to behave as semiconductors.36 These semiconductor-like MOFs have been successfully applied to photocatalysis systems involving the photodegradation of organic pollutants and solar energy conversion, the foundation of semiconductor photocatalysis.37−40 Moreover, the tunability and diversity of MOF structure provide numerous methods for MOFs to function even better than real semiconductors.36,44,45 For example, the organic linker can be adjusted to be a better photon antenna via the facile introduction of −NH2 group. At the same time, noble-metal nanoparticles (NPs) can be encapsulated into the MOF pores as electron reservoirs,

Figure 1. Scheme presentation of the three types of MOF photocatalysts showing the distinct locations of photoinitiated redox centers: type I, the inorganic cluster nodes as monodispersed nanosemiconductors photocatalysts; type II, the photocatalytic organic or metal−organic dyes as linkers to be heterogenized; type III, the photocatalytic units as guests encapsulated in the pores of MOFs.

the 0D inorganic cluster nodes act as semiconductor dots photocatalysts that are well-isolated by the organic linkers and sometimes also by the pores of the MOFs. These MOFs are described as isolated semiconductor dots that are regularly and uniformly monodispersed in the lattice of the crystals. In type II MOFs, the linkers are functional organic and metal−organic dye-based photocatalysts that are consolidated and separated by the metal nodes. These MOFs are considered to be heterogenized dye-based photocatalysts that well positioned into isolated fashion with ordered arrays. In type III MOFs, the photocatalytic species that have suitable sizes and structures are encapsulated within the pores of the MOFs. These MOFs are considered as regular distributed supramolecular systems that were isolated by the MOF scaffold. Outstanding examples of MOF photocatalysis are presented to reveal the design routes in terms of these three types of MOFs.

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TYPE I MOFS-BASED PHOTOCATALYSTS WITH 0D INORGANIC CLUSTERS AS METAL NODES Semiconductors, especially inorganic semiconductors, have been the leading players in photocatalysis for decades.7−11 Scheme 1 shows the elementary steps that occur in a photocatalysis event mediated by semiconductors. Upon the absorption of photons with the energy greater than the band gap between the conduction band (CB) and the valence band (VB), charge separation occurs in the semiconductor, 7936

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ACS Catalysis Scheme 2. Photophysical Processes That Occur after the Irradiation of the MOF-5 Solid Material, Suggesting Semiconductor Behavior. Reprinted with Permission from Ref 35. Copyright 2007 Wiley-VCH

Figure 2. Cartoon showing the step-by-step nearest neighbor hopping (NNH) and long-distance jumping pathways of excitation migration in a network of chromophores. Reprinted with permission from ref 57. Copyright 2016 American Chemical Society.

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TYPE III MOFS WITH PHOTOREDOX SPECIES ENCAPSULATED IN PORES In addition to the metal nodes and the organic or metal− organic linkers, pores can be another origin of photoredox activity for MOFs, which enrich MOFs as photocatalysts while also providing additional possibilities for photocatalysis. Photoredox species of appropriate size can be introduced into the pores of MOFs as guests with sustained and often improved photoactivity owning to the isolation from each other and the mutual effect with the MOF scaffold.58,59 Interestingly, the microenvironment of MOFs’ pores can be finely adjusted for energy and electron transfer, thus making the MOF-based photocatalysis more similar to enzymatic catalysis.60−62 Importantly, the high porosity provides not only sufficient space for the catalyst−substrate interaction but also sizeselectivity for photocatalysis.65,68 Polyoxometalates (POMs), because of their outstanding performance as acid catalysts and limited surface area as a solid, are continually introduced into MOF pores to obtain stable porous catalysts with POMs dispersed at the molecular level.61−67 Su and co-workers were the first to report welldefined crystalline POM/MOF composites and the first to use them as true heterogeneous acid catalysts (Figure 3).63 Keggintype POMs were incorporated into HKUST-1 scaffolds through a one-step hydrothermal reaction, resulting in a POM/MOF series. One of these POM-MOF composites exhibited high activity and reusability in the hydrolysis of esters in excess

which can increase the charge mobility by orders of magnitude. Recently, Grzybowski and co-workers demonstrated that adding Ag nanoclusters to a Rb-CD(cyclodextrin)-MOF not only imparts moderate electrical conductivity to an otherwise insulating material but also renders it photoconductivity, with conductivity increasing by up to 4 orders of magnitude upon light irradiation.46 Lanthanide (Ln) ions are a unique type of photoactive metal node in MOFs and have been widely used to construct Ln-MOFs that function as luminescent sensors and light-emitting materials.41−43 However, the Laporte-forbidden f−f transition inherent to Ln ions limits the application of LnMOFs as photocatalysts.

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TYPE II MOFS WITH DYE MOLECULES AS ORGANIC LINKERS Dye molecules, particularly metallo-organic dye Ru(bpy)32+, have been extensively studied as photocatalysts in homogeneous systems for their beneficial photophysical properties, which include strong visible-light absorption and long-lived excited states.12−16,47,48 Building on the seminal results obtained using photoinduced electron transfer (PET), researchers have demonstrated various dye molecules to be very efficient in photoredox reactions through excitation.12−16 Incorporation of these photoredox-capable dye molecules into MOFs broadens and deepens the MOF photocatalysis to achieve more sophisticated and meaningful photoconversions. Meanwhile, MOFs enable the high-density and orderly distribution of isolated dye-photocatalysts along the heterogeneous support; by contrast, in homogeneous systems, the same concentration of dyes often induces detrimental aggregation that suppresses activity or causes self-quenching.49−52 Most importantly, the crystalline nature of the MOF structure provides other possibilities for the transport of excited states or photoinduced electrons and holes.53−56 For example, Lin and co-workers recently reported that the “through space” energy jumping of singlet excited states beyond the nearest neighbor can account for up to 67% of the energy transfer rate in truxene-based materials (Figure 2).57 This phenomenon can be observed only in highly ordered networks and may provide an additional pathway for energy transfer and exciton migration, which are in favor of photocatalysis.

Figure 3. View of a (001) sheet with two types of pores, A and B, in NENU-n (n = 1−6). The Cu-BTC framework and Keggin polyanions are represented by wireframe and polyhedral models, respectively. Reprinted with permission from ref 63. Copyright 2009 American Chemical Society. 7937

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methanol mixture under UV irradiation. The introduction of an amino group into the organic linker added another absorption band beyond 300 nm to NH2−UiO-66 and enabled it to perform better in the photocatalytic HER than UiO-66. The presence of Pt nanoparticles as cocatalysts can promote the photoconversion to nearly five times the yield. The long-lived charge separation states recorded by the laser flash photolysis spectra of UiO-66 and NH2−UiO-66 confirmed the semiconductor role of MOFs for this photodriven HER (Scheme 3).

water. Furthermore, selectivity based on substrate size and accessibility to the pore surface was observed, which highlighted the heterogeneity of these POM-MOF catalysts. Inspired by this work, researchers have devoted much effort to the research of catalytic POM-MOF assemblies, and photoactive POM-MOF systems have recently become a topic of great interest.61,62,64−67 The three types of MOF-based photocatalysts offer plenty of ideas for researchers to explore and have been successfully applied in photocatalysis.28−34 One more particular thing to be noted regarding the superiority of MOFs as photocatalysts is that multiple functional components can be organized and integrated into a single material in an ordered and hierarchical manner for facile energy and electron transfer, thus providing infinite possibilities for synergistic or cooperative catalysis.69,70 In the following section, we provide an overall profile of MOF photocatalysis, with a special focus on the design aspect. Meaningful and successful application examples of the three types of MOFs will be introduced.

Scheme 3. Mechanistic Proposal to Rationalize the Photophysics of Zr-MOF Used To Photo-Drive the Hydrogen Evolution Reaction (HER). Reproduced with Permission from Ref 79. Copyright 2010 Wiley-VCH

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PHOTOCATALYSIS WITH TYPE I MOFS Inorganic semiconductors were the first and remain the most widely studied heterogeneous photocatalysts.7−9 When integrated into a MOF scaffold, the metal clusters maintain their semiconducting properties, and the porosity of the MOF structure helps make full use of the metal clusters as photocatalysts. As a traditional photocatalyst that has received lasting attention, semiconductors are of particular interest in the photodegradation of organic pollutants and in solar energy conversion processes such as water splitting.8−11 Naturally, the application of type I MOFs as photocatalysts is primarily focused on these fields. Nowadays, there are reviews on the photoinduced decomposition of organic pollutants, especially organic dyes, the photochemical reduction of protons and carbon dioxide, and the photooxidation of alcohols using MOFs as photocatalysts.37−40 Almost all the photoactive MOFs in these aforementioned reviews consist of nanosemiconductors, the metal−oxygen clusters. The development of MOFs for the photoreduction of protons and carbon dioxide will be discussed below, whereas the photodegradation of organic pollutants and the photooxidation of alcohols using MOFs will not be discussed in this perspective because these applications share the same design and improvement strategies. The reduction of protons to hydrogen and the reduction of carbon dioxide to carbon monoxide or simple carboncontaining organics are regarded as two powerful methods to solve the energy and environment dilemma.3−6 Various inorganic semiconductors and noble metal complexes have been studied to achieve these two valuable reduction reactions with the assistance of photoenergy.3−11 Many of the MOFs capable of photocatalytic hydrogen evolution or carbon dioxide reduction contain metal−oxygen clusters, taking advantage of their semiconducting properties.37,39,40 Only a few examples involve the incorporation of proton/carbon dioxide reduction catalysts into MOF scaffolds or pores.71−78 However, although rare, these materials can also exhibit favorable catalytic efficiency. After observing the semiconductor behavior of MOF-5, Garcia and co-workers continued to explore the potential of MOFs as photocatalysts, leading to the first example of a MOFbased photocatalyst for the hydrogen evolution reaction (HER).79 The photocatalytic HER was driven by highly water-stable Zr-MOFs UiO-66 and NH2−UiO-66 in a water/

Ti-MOFs attract considerable interest as photocatalysts because they contain titanium-oxo species. MIL-125(Ti) has been intensively studied as a photocatalyst since Sanchez, Serre, and co-workers synthesized this intriguing MOF and observed its photochromic behavior, revealing the photoinduced conversion from Ti(IV) to Ti(III).80 Inspired by this work, Li and co-workers added an −NH2 group into MIL-125(Ti), generating NH2-MIL-125(Ti), which was active in the photocatalytic reduction of CO2 to HCOO− under visible light.81 The multinuclear Ti-centers in NH2-MIL-125(Ti) are both photo- and redox-active. The amino functionality not only transfers the absorption band for MIL-125(Ti) from the UV region to the visible region but also results in a higher absorption capability toward carbon dioxide. The photodriven carbon dioxide reduction of NH2-MIL-125(Ti) was performed in acetonitrile (MeCN) with triethanolamine (TEOA) as the electron donor (Scheme 4). The generation of Ti3+ upon Scheme 4. Proposed Mechanism for the Photocatalytic Carbon Dioxide Reduction over NH2-MIL-125(Ti) under Visible-Light Irradiation. Reprinted with Permission from Ref 81. Copyright 2012 Wiley-VCH

visible-light illumination was observed as a color change from bright-yellow to green when NH2-MIL-125(Ti) was irradiated under nitrogen in the absence of carbon dioxide. When carbon dioxide was pumped in, the color turned back to yellow and the carbon dioxide was reduced by Ti3+ to yield HCOO−. TEOA 7938

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the Ti-oxygen clusters. This Co@MOF composite is a recyclable catalyst free of noble metals producing hydrogen from water under visible light. It displays a stable TOF of 0.8 h−1 even after 65 h of operation. The introduction of the cobalt molecular catalyst into the cavities of the framework resulted in a 20-fold enhancement of photocatalytic activity compared to the pristine NH2-MIL-125(Ti), whereas the cobalt molecular catalyst could not produce hydrogen under the same conditions because of the lack of a photosensitizer. This example reveals the potential of MOFs as an ideal modular functional material for photocatalysis via the elaborate design and integration of well-defined molecular building blocks. The higher density of the semiconductor nodes within the crystal lattices features a new fashion to enhance the catalytic efficiency. The assembly of a functional unit within MOF scaffold can trigger new properties beyond the simple merging of two parts. Jiang and co-workers found that a deep electron trap state occurred after the integration of porphyrin into Zr-MOF, PCN222, and the photocatalytic conversion of carbon dioxide into formate anion with PCN-222 was significantly improved than the corresponding porphyrin ligand.86 The semiconductor character of PCN-222 was illustrated by a Mott−Schottky plot that showed the possibility for photoreduction of CO2 by PCN222. As expected, the HCOO− anion was continuously produced with PCN-222 as the photocatalyst and TEOA as the electron donor in CH3CN under visible-light irradiation. ESR studies revealed the generation of ZrIII ions in this photoreduction, indicating the electron transfer from the excited porphyrin ligand to the Zr6 metal nodes. In this case, the pristine homogeneous photocatalyst porphyrin served as antenna to sensitize Zr-oxo clusters, which then transfer electron to substrate CO2, making PCN-222 a type I MOF. When H2TCPP was employed as the photocatalyst under similar conditions, clearly weaker performance was observed: only 2.4 μmol of HCOO− was produced after 10 h (30 μmol for PCN-222). This result shows that the photocatalytic activity of the porphyrin was greatly enhanced by the formation of the MOF structure, inspiring a series of experiments to unveil the reason. Steady-state photoluminescence (PL) measurement of PCN222 indicated a pronounced PL emission quenching from H2TCPP, suggesting greatly suppressed radiative electron−hole recombination in PCN-222 relative to that in H2TCPP. Moreover, ultrafast transient absorption (TA) spectroscopy of PCN-222 revealed that the ΔA recovery converges to an

offered an electron to regenerate the pristine NH2-MIL125(Ti). Subsequently, Li and co-workers observed that NH2−UiO66(Zr) catalyzes the photocatalytic reduction of carbon dioxide under visible light with TEOA as a sacrificial agent, and the photogenerated ZrIII was first confirmed and revealed.82 The partial substitution of the organic linker improved the efficiency of the photocatalytic reduction of carbon dioxide. Furthermore, the metal Zr can also be partially replaced by Ti through a postsynthetic exchange method to obtain a more capable MOFphotocatalyst for both the carbon dioxide reduction and hydrogen evolution under visible light.83 These works definitely show the enormous potential of MOFs, which benefit from the facile adjustment of structure and functionality, for developing better catalysts for photocatalytic carbon dioxide reduction and hydrogen evolution.84,85 Very recently, another useful strategy to improve the performance of MOFs as a semiconductor-photocatalyst was reported by Gascon, Vlugt, Reek and co-workers.77 The pores of NH2-MIL-125(Ti) were decorated with the Co-based electrocatalyst Co-dioxime-diimine via a “ship-in-a-bottle” strategy (Figure 4), and the resulting Co@MOF composite

Figure 4. Concept of photocatalytic hydrogen production using a Co@MOF composite with Co-dioximediimine encapsulated into the pores of NH2-MIL-125(Ti). Reprinted with permission from ref 77. Copyright 2015 Royal Society of Chemistry.

outperformed all previously reported MOF-based analogues for photochemical hydrogen production. Though the redox catalytic center for substrate transformation is inside the MOF pores, this Co@MOF composite still belongs to the type I MOF-catalyst because the photoinduced redox center is still

Figure 5. (a) Transient TA spectra of PCN-222 registered at different probe delays (pump at 500 nm). (b) Representative TA kinetics of PCN-222 taken at the probing wavelength of 430 nm. In (a) and (b), the TA signal is given as absorbance change (ΔA) with the unit of mOD. Reprinted with permission from ref 86. Copyright 2015 American Chemical Society. 7939

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Figure 6. Perspective view of the 3D porous framework of Zn−SnIV−TPyP down the c axis (left) and a schematic representation of the photooxidation of phenol and sulfides (right). Reprinted with permission from ref 50. Copyright 2014 American Chemical Society.

Figure 7. Schematic presentation of the Ir and Ru complexes-doped UiO-67 MOFs for the photocatalysis of three organic transformations. Reproduced with permission from ref 71. Copyright 2011 American Chemical Society.

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PHOTOCATALYSIS WITH TYPE II MOFS Organic photocatalysts are gaining increasing attention and renewed enthusiasm in regard to photosynthesis.12−16 The poor absorption of light prevents type I MOFs from being powerful photocatalysts for more sophisticated and useful organic transformations. The incorporation of rigid photocatalyst as linkers into MOF scaffolds opens a new avenue for photocatalytic organic conversions. In fact, the immobilization of active photocatalysts onto a suitable matrix is a long-pursued goal for researchers to avoid self-quenching between photocatalysts and the contamination of the product, especially now that the most widely commercialized photocatalysts are all heterogeneous in nature for easy workup and relatively low cost.17,49 In this context, MOFs are the perfect heterogeneous platform for photoactive linkers because they are orderly dispersed as single-site catalysts in a MOF scaffold, and the distance between them can be finely adjusted for better performance; the porous nature of MOFs provides sufficient space for substrate activation and product departure.18−27 In the meantime, the strong coordination bonds ensure the stability and recyclability of MOFs. More and more researchers have demonstrated that the pristine homogeneous photocatalyst can drive photocatalysis more efficiently or completely after being incorporated or embedded into MOFs.28−34 The homogeneous star-photocatalysts such as porphyrin and [Ru(bpy)3]2+ were the first batch of photofunctional organic linkers used to construct efficient MOF photocatalysts for

asymptote (dashed line in Figure 5b) with a nonzero value of approximately −1.2 mOD within the probe-delay limit of the pump/probe spectrometer (∼3 ns). Notably, the nearly perfect parallelism between the asymptote and the ΔA = 0 line suggests that the eventual recovery to ΔA = 0 features an extremely long lifetime (τ3), possibly indicating a third trap state. The detrapping of electrons from this deep trap state was so slow that its corresponding radiative electron−hole recombination was dramatically suppressed, which is in good line with the steady-state PL spectra. In summary, as a direct result of framework formation, a deep electron trap state emerged in PCN-222, and this trap state could boost electron−hole separation and then supply long-lifetime electrons for the photoreduction of carbon dioxide via effective suppression of the detrimental electron−hole recombination. This work reinforces the advantage of MOF structures for the integration of functional units, supplying surprises rather than common mixing. Although type I MOFs are typical MOF-based catalysts for photocatalytic HER and carbon dioxide reduction, other types of MOF-photocatalysts for these two key transformations also exhibit satisfactory activity.71−78 Homogeneous photocatalysts such as Re or Ir complexes, porphyrin derivatives and POMs have been built into MOF structures as linkers to obtain type II MOF photocatalysts for these two reactions.71−75 Excellent examples will be introduced in detail in the following section. 7940

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ACS Catalysis meaningful organic syntheses.71,90−97 In early 2011, Wu and coworkers reported a SnIV−porphyrin-based MOF (Zn−SnIV− TPyP) for the efficient photooxygenation of phenol and sulfides (Figure 6).91 The metalloporphyrin building block SnIV−TPyP presents distinct photocatalytic activity in the homogeneous phase. Upon excitation, the SnIV−TPyP reached to its triplet excited state, which transfer energy with oxygen to produce 1O2, one of the famous reactive oxygen species (ROS). 1 O2 then oxidizes substrate to corresponding product. However, this photoactivity was heavily influenced by reaction with singlet oxygen. After immobilization of SnIV−TPyP into Zn−SnIV−TPyP, the quenching process was overcome efficiently thanks to the enough space between photocatalytic units SnIV−TPyP and the easy interaction between 1O2 and substrate. Thus, Zn−SnIV−TPyP provided remarkable activity for the photooxygenation of phenol and sulfides. Almost simultaneously, Lin and co-workers demonstrated the doping of catalytically competent Ir and Ru complexes into a highly stable and porous UiO-67 and applied the resultant phosphorescent [Ru(bpy)3 ]2+-derived MOF 5 and [Ir(ppy)2(bpy)]+-derived MOF 6 to three photocatalytic organic transformations (aza-Henry reaction, aerobic amine coupling, and aerobic oxidation of thioanisole), with good results (Figure 7).71 [Ru(bpy)3]2+and [Ir(ppy)2(bpy)]+ have been extensively investigated as photoredox catalysts in various photocatalytic organic reactions.16,47 The incorporation of these preciousmetal-containing photocatalysts into porous MOFs is highly desirable for a prolonged catalytic lifetime because of the stabilization effect of MOFs and their recyclability. The importance of MOF permanent porosity was demonstrated by the poor catalytic performance of amorphous nanoparticles in the photodriven aza-Henry reaction (18% conversion, corresponding to the background reaction). The same strategy was also applied for the immobilization of the CO2-reduction photocatalyst, [ReI(dcbpy) (CO)3Cl], obtaining MOF 4. When irradiated in CO2-saturated MeCN with TEA as sacrificial agent, MOF 4 turned from orange to green, and CO and H2 were detected by gas chromatography (GC). During the first 6 h, The CO-TONs reached 5.0 and the molar ratio of the CO and H2 production was around 10. Unfortunately, MOF 4 became inactive in CO generation after two 6 h reaction runs, due to the detachment of Re−carbonyl moieties from the dcbpy group in the MOF 4 framework. Even so, the total COTON of the MOF 4 are still 2 times higher than the homogeneous system for 20 h reaction, presumably owing to the catalyst stabilization by the MOF framework. Following this work, Lin and co-workers developed a new approach for the photocatalytic HER with Ru/Ir complexderived MOFs.87,61,62 Pt NPs and various POMs were introduced to improve electron transfer during the HER; these Ru/Ir complex-derived MOFs are among the most efficient MOF-based photocatalysts for HER. For example, Lin and co-workers synthesized two Pt@MOF composites by loading Pt nanoparticles into the pores of [Ir(ppy)2(bpy)]+based MOFs 1 and 2 (Figure 8).87 The photochemical hydrogen evolution with Pt@MOF composites was operated with TEA as the electron donor and with the Ir-phosphor absorbing visible light and transferring electrons to Pt nanoparticles for the final reduction step. A total Ir-TON of 7000 was obtained for MOF 2 via 48 h hydrogen evolution experiments, which is approximately 5 times the TON value afforded by the homogeneous control. The better cooperation between photoexcitation of the MOF frameworks and electron

Figure 8. Phosphorescent Zr-carboxylate MOFs (1 and 2) and subsequent loading of Pt NPs inside MOF cavities to form the Pt@1 and Pt@2 assemblies for the synergistic photogeneration of hydrogen. Reprinted with permission from ref 87. Copyright 2012 American Chemical Society.

injection into the entrapped Pt NPs was responsible for the markedly improved efficiency. Moreover, these Pt@MOF composites could be readily recycled and reused. The modular nature of MOF scaffold endows MOFs with infinite potential for the construction of multifunctional materials, which is also the case for the photoactive speciesinvolved MOF. Various functionalities can be added into photoactive MOFs, being the metal nodes or the second linkers. Therefore, distinct capable units in a photoactive MOF can operate simultaneously and even synergistically toward substantial utilization of solar energy. Farha, Hupp, and coworkers described a dual-function MOF that simultaneously detoxified two chemical warfare agent (CWA) stimulants at room temperature (Figure 9).92 Efficient deactivation of CWAs

Figure 9. Dual-function MOF constructed from a phosphortriesteraselike Zr6-containing node and a photoactive porphyrin linker for the simultaneous degradation of simulants of two CWAs. Reproduced with permission from ref 92. Copyright 2015 American Chemical Society.

requires multiple detoxification pathways, such as hydrolysis and oxidation, to occur simultaneously because predicting which CWAs will need to be deactivated is difficult. The multifunctional nature of MOFs makes them a perfect material to incorporate distinct catalytic moieties to generate broadspectrum detoxification materials. The dual-function MOF was constructed from a Zr6-containing node and a porphyrin organic linker. Upon visible-light (LED) irradiation, a simulant of mustard gas, 2-chloroethyl ethyl sulfide, was oxidized to a nontoxic product by singlet oxygen, which was generated from 7941

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simple mixing of the corresponding MOFs with the chiral adduct. For typical photocatalysis with a PET process, an excited state of photosensitizer is obtained after the absorption of a photon. Specially, some photosensitizers may produce active radicals right after the absorption. Xing and co-workers utilized anthracene, one photosensitizer of this particular type, to construct a visible light responsive MOF, NNU-35, for the promising photocatalytic atom-transfer radical polymerization (ATRP) reaction of methacrylate monomers.94 The incorporation of the anthracene-based ligand L1 into NNU-35 resulted in absorption and emission bands that were broader and redshifted because of energy transfer and/or charge transfer interactions in the MOF structure. The resonance EPR signal after exposure to visible light was assignable to the anthracenebased ligand, and it can sustain for about several minutes even without light irradiation, clearly suggesting its free-radical nature. The broad visible-light absorption and desirable photoinduced charge separation in NNU-35 are intriguing for light-switchable ATRP. ATRP is an efficient method for controlled radical polymerization, which allows nonexperts facile access to functionalized polymer materials with welldefined structures and architectures. More recently, photoinduced ATRP has received intensive attention because of its unique features of temporal and spatial control of the chain-extension process. A typical ATRP reaction system of MMA with a copper complex as the catalyst and EBiB as the initiator was chosen to test the photoactivity of NNU-35. 48% monomers were polymerized after 8 h of irradiation with 520 nm light, and a relatively narrow molecular weight distribution (Mw/Mn) of 1.12 was observed. No polymerization occurred in the absence of NNU-35. More importantly, the reaction could be triggered easily through light switching. When the visible light was periodically turned on and off, the polymerization also occurred periodically, exhibiting the characteristics of living radical polymerization. Figure 11 clearly

the photosensitized porphyrin moieties. Meanwhile, the Zr6 nodes containing Zr−OH−Zr in the same MOF, mimicking the Zn−OH−Zn active site in PTE, hydrolyzed another simulant of nerve agents (such as GD or VX), dimethyl 4nitrophenyl phosphate, to a nontoxic complex. The structure of this dual-function MOF remained intact after catalysis, and the degradation efficiency for fresh CWAs was the same as for the first run. The versatility and tunability of MOFs suggests that suitably engineered successors may eventually prove useful for air-filtration equipment and for the destruction of stockpiles or spills of chemical warfare agents. In addition, reducing the size of MOF crystals to the nanoregime leads to acceleration of the catalysis. The combination of chiral catalysis and photocatalysis is a hot topic in chemistry because chiral complexes are highly desirable as medical materials but their traditional chemical synthesis is often very tedious. Through careful selection and design, Duan and co-workers realized the first integration of a triphenylamine photoactive unit and the chiral component L- or D-pyrrolidin-2-ylimidazole (PYI) in a MOF and successfully applied these chiral photoactive MOFs (Zn−PYIs) in the lightdriven asymmetric α-alkylation of aldehydes (Figure 10).60 The

Figure 10. Integration of the stereoselective organocatalyst L- or D-PYI (blue) and a triphenylamine photoredox group (red) into one single MOF to promote the asymmetric α-alkylation of aliphatic aldehydes in a heterogeneous manner. Reprinted with permission from ref 60. Copyright 2012 American Chemical Society.

targeted Zn−PYIs could not be obtained through direct solvothermal synthesis due to the instability of PYI. A tertbutoxycarbonyl (Boc) group was thus introduced to protect the catalytically active N−H site of pyrrolidine, and the resulting precursor MOF (Zn−BCIP) was converted into the active Zn−PYI simply by heating in DMF solution. The asymmetric α-alkylation of aldehydes with Zn−PYI1 started with phenylpropylaldehyde and diethyl 2-bromomalonate as the coupling partners and a common fluorescent lamp (26 W) as the light source. A high reaction efficiency (74% yield) and excellent enantioselectivity (92% ee) were achieved, demonstrating the successful execution of photoactive chiral MOF design. Absorption and luminescence experiments suggested that the quenching process was typically attributed to the photoinduced electron transfer (PET) process from Zn−PYI1* to diethyl 2bromomalonate. The chiral PYI moieties acted as cooperative organocatalytic active sites to further induce the PET-generated active intermediate in a MOF channel with remarkable stereoselectivity. Therefore, Zn−PYI1 represents the first example of a MOF-based heterogeneous asymmetric photocatalyst for this important reaction. Because of the restricted movement of the substrates within the MOF’s interior and multiple chiral inductions as well, the integration of both the photocatalyst and asymmetric organocatalyst into a single MOF makes the enantioselection superior to that achieved through

Figure 11. Proposed mechanism for NNU-35 mediated ATRP under visible light. Reprinted with permission from ref 94. Copyright 2016 Royal Society of Chemistry.

illustrates the successful use of MOF NNU-35 for the lightswitchable ATRP reaction via continuous regeneration of the copper catalyst. The heterogeneous photocatalyst NNU-35 first transfers an electron to an oxidized Cu(II)/PMDETA complex after the visible-light-induced formation of a radical. The generated Cu(I) complex then reacts with the alkyl halide (R− X) initiator, forming radicals (R•) to initiate the ATRP. The implementation of NNU-35 for the ATRP reaction successfully expanded the application range of photoactive MOFs to radical chemistry. A breakthrough in photocatalysis involving consecutive photoinduced electron transfer (conPET) processes was 7942

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ACS Catalysis reported by König in 2014.48 Before that, the energy conferred by visible-light excitation for subsequent redox chemistry was limited to the one single absorbed photon. Therefore, useful reactions requiring high energy are either performed under UV irradiation or with highly active substrate precursors under strict conditions, both of which are obstacles to wide application. Through the integration of perylene diimide (PDI) into MOF Zn-PDI, the conPET process for the efficient visible-light-driven reduction of aryl halides was introduced into the heterogeneous catalysis field, as accomplished by Duan and co-workers (Figure 12).95 Perylene diimide-derivative H2PDI

cannot proceed without highly sensitive and active donor molecules, UV-A irradiation and strictly inert reaction conditions. The integration of conPET-active PDI bricks into the Zn-PDI architecture thus represents a critical step toward more efficient solar energy utilization via a conPET process. In addition to direct incorporation into MOF scaffold, photocatalysts can be introduced to MOFs through postsynthetic modification (PSM), benefiting from the porosity and the well-defined structure.76,87−89 Xu described the synthesis of photoactive MOF-253-Pt for the reduction of water to form hydrogen under visible-light irradiation through PSM of a 2,2′bipyridine-based MOF with platinum ions.76 The visible-light induced hydrogen evolution of the MOF-253-Pt was examined in the presence of 15 vol % TEOA as a sacrificial electron donor in water at pH 8.5; the amount of hydrogen produced from MOF-253-Pt was nearly 5 times greater than that produced from the Pt(bpydc)Cl2 complex. For many capable molecular photocatalysts, the direct solvothermal synthesis often failed to produce the wanted MOFs, while PSM are more feasible. This PSM strategy combines the advantages of molecular catalysts with a highly ordered and stable inorganic support, and it is sure to be widely applicable for MOFs as photocatalysts.89

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PHOTOCATALYSIS WITH TYPE III MOFS Beyond the semiconducting metal-oxo clusters or photoactive linkers, the inherent pores of MOFs can also be exploited to render MOF photoactivity. Suitable photocatalysts can be dispersed into MOF pores through one-pot synthesis during synthesis or through in situ deposition after the formation of the MOF scaffold.65,98Various photoactive species, including perylene and DMASM (4-[p-(dimethylamino)styryl]-1-methylpyridinium), have been introduced into MOF cavities as guests and have exhibited favorable properties resulting from the host−guest interaction.99−101 However, thus far, this type of MOF-photocatalysts has remained in its infancy; metal NPs and POMs are the rare examples of photocatalysts encapsulated into MOF pores and applied to photoconversion.65−68,102,103 Metal NPs were initially introduced into photoactive MOFs as cocatalysts to facilitate energy transfer. Duan, Chen, and coworkers utilize the localized surface plasmon resonance (LSPR) of MOF-encapsulated NPs for photocatalysis.98,102,103 Au@ ZIF-8 single- or multicore−shell structures were obtained by epitaxial growth or coalescence of nuclei depending on the density of ZIF-8 nuclei on Au NPs.102 The photodriven oxidation of benzyl alcohol by these Au@ZIF-8 composites was performed in acetonitrile under visible light given the LSPRrelated absorption of Au NPs (Figure 13). The conversion was 25.8% for single-core Au@ZIF-8 and 51.6% for multicore Au@ ZIF-8 after 24 h of irradiation. This difference in conversion is ascribed to plasmonic coupling between Au NPs in the multicore structures. The size-selectivity of ZIF-8 that restricts benzyl alcohol from entering pore cavities too close to Au, according to the authors, is responsible for the conversion lower than that afforded by Au−SiO2 (56.9%). Later, the same authors reported that the introduction of Au NPs into NH2− UiO-66 favored the electron transfer from Au NPs to NH2− UiO-66 with a localized electronic state characterized by CAFM, leading to nearly 6-fold greater conversion than that provided by NH2−UiO-66 for the photooxidation of benzyl alcohol.103 MOFs, as excellent porous materials, have long been intensively studied for gas storage and separation. This intriguing feature can be exploited in photocatalysis with gas-

Figure 12. Diagram illustrating the strategy of assembling insoluble PDI into organized arrays in porous solid Zn-PDI to obtain an efficient photocatalyst for the visible-light-driven reduction of aryl halides and the oxidation of alcohols and amines. Reprinted with permission from ref 95. Copyright 2016 American Chemical Society.

was chosen as the organic ligand because the conPET process was discovered using a PDI derivative. However, the poor solubility of PDIs and their strong tendency to aggregate, resulting from their large planar conjugated structure, severely restrict the development of this conPET process for other catalytic transformations in homogeneous systems. Strong π···π interaction between PDIs still existed in the Zn-PDI sheets; however, the coordination effect worked synergistically, making the aggregation controllable. Each of the three PDI molecules formed a close J-aggregate group. The formation of J-aggregates in Zn-PDI is beneficial for photocatalysis because of the excellent ability of J-aggregates to delocalize and migrate excitations. Moreover, the appropriate space between Jaggregates and the rigid framework of Zn-PDI helped reduce self-quenching, as concluded on the basis of the obviously weaker fluorescence intensity of H2PDI solutions with the same PDI concentration. The conPET process with Zn-PDI was illustrated through the heterogeneous visible-light-driven reduction of aryl halides, starting from 4′-bromoacetophenone. The 87% yield of acetophenone, which was 3 times higher than the yield afforded by the homogeneous counterpart, was detected after 1 h of irradiation with blue LEDs and the use of 72 equiv of the electron donor Et3N. The reason for the high performance is explained on the basis of the aforementioned studies: the rigid Zn-PDI framework isolated the active sites of PDI but allowed them to be dense at the same time, which may be optimal for electron or energy transfer. Moreover, the most inert aryl chlorides were also effectively reduced using higher loadings of Zn-PDI. Normally, the photoreduction of inert aryl chlorides 7943

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the photothermal effects of metal nanocrystals with the favorable properties of MOFs for efficient and selective catalysis generates synergistic advantages and opens up an avenue to the prospects for MOF photocatalysis. POMs, in addition to NPs, are also frequently embedded into MOF cavities due to their excellent redox properties.61−67 However, most investigations have focused on thermal reactions or multielectron processes, with little attention devoted to the photoactivity of POMs. However, in 2015, Duan and co-workers reported a POM-MOF, CR−BPY1, that is active toward the photocatalytic oxidative coupling of lowreactive sp3 C−H bonds with environmentally benign and inexpensive oxygen as the oxidant (Figure 15).65 CR−BPY1 Figure 13. Schematic of the photooxidation of benzyl alcohol by Au@ ZIF-8 single- or multicore−shell structures under visible light. Reprinted with permission from ref 102. Copyright 2014 Royal Society of Chemistry.

phase compounds. Jiang and co-workers recently reported the synthesis of a Pd nanocubes@ZIF-8 composite material for the efficient and selective catalytic hydrogenation of olefins at room temperature under 1 atm H2 and light irradiation (Figure 14).68

Figure 15. Illustration of CR−BPY1 for the oxidative coupling of lowreactive sp3 C−H bonds through the synergistic catalysis of the photoredox polyoxometalate [SiW11O39Ru(H2O)]5− and the Cu2+ metal node. Reprinted with permission from ref 65. Copyright 2015 Royal Society of Chemistry.

was constructed from the incorporation of a rutheniumsubstituted POM, [SiW11O39Ru(H2O)]5−, which exhibited excellent photoactivity in various catalytic oxidation processes of organic substrates, into copper-BPY networks. Copper atoms were first connected by the BPY ligands to form a twodimensional square grid; adjacent sheets were then connected together via the coordination of Cu to a terminal oxygen atom of the deprotonated [SiW11O39Ru(H2O)]5− to generate a 3D framework. The catalytic potential of CR−BPY1 was examined via a model reaction using N-phenyl-tetrahydroisoquinoline and nitromethane as coupling partners under irradiation by an 18 W fluorescent lamp. A yield of 90% was observed after 24 h of irradiation, whereas less than half the conversions were obtained using the same equiv of copper(II) salts or/and K5[SiW11O39Ru(H2O)] as catalysts. These results suggested that the direct connection of copper(II) ions to [SiW11O39Ru(H2O)]5− anions was the key factor for improving the catalytic performance; this speculation was confirmed by a series of spectroscopic analyses and an elaborate control experiment that is discussed below. The significantly weaker (46% conversion) catalytic activity of CR−BPY2 than CR−BPY1 in the model C−C coupling reactions demonstrated that the direct CuII−O− W(Ru) bridges were the key factor in achieving the synergistic catalysis between the photocatalyst and the metal catalyst. Inspired by this work, the same author recently demonstrated the encapsulation of photosensitizing [W10O32]4− into MOF pores brought about a series of POM-MOFs as capable heterogeneous photocatalysts for the light-driven acceleration of the β- or γ-site C−H alkylation of aliphatic nitriles; these results suggest that the POM-MOF is a promising photocatalyst type.66,67

Figure 14. Self-assembly of Pd NCs@ZIF-8 and plasmon-driven selective catalysis of the hydrogenation of olefins. Reprinted with permission from ref 68. Copyright 2016 Wiley-VCH.

The encapsulation of Pd nanocubes in ZIF-8 solved several problems impeding catalysis, such as the propensity for aggregation and the random dispersion of Pd NCs under homogeneous conditions. In addition, the plasmonic photothermal effects of the Pd nanocube cores endowed the composite Pd NCs@ZIF-8 with photoactivity for the hydrogenation of olefins. The uniform pores of the ZIF-8 shell played important roles in the efficient hydrogenation: they accelerated the reaction by H2 enrichment and acted as a “molecular sieve” for olefins with specific sizes. Upon light irradiation, Pd NCs@ZIF-8 drove the hydrogenation of 1-hexene to almost complete conversion in 90 min, whereas only 58% conversion was achieved with Pd NCs. Moreover, the yields with Pd NCs@ZIF-8 remained approximately 100% after three consecutive runs, whereas a low yield of 21% with the Pd NCs was observed in the third run. Remarkably, the catalytic efficiency of hydrogenation with Pd NCs@ZIF-8 under 60 mW cm−2 of full spectrum or 100 mW cm−2 of visible-light irradiation at room temperature was comparable to that of a process driven by heating at 50 °C. Thus, hydrogenation with H2 may utilize solar energy instead of high-temperature heating, which is dangerous, energy-consuming, and environmentally damaging. The strategy of combining 7944

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CONCLUSIONS AND OUTLOOK After nearly a decade of development, photocatalysis with MOFs has reached its booming stage. Designing MOFs for photocatalysis is a feasible approach to obtain highly active, recyclable, and environmentally benign photocatalysts with simple workup, largely thanks to the tunable structure and high porosity of these crystalline materials. The photoactive species can be integrated into MOF scaffolds as inorganic cluster nodes (type I) or organic linkers (type II) or as an inclusion (type III). For type I MOF-based photocatalysts, the metal-oxo clusters can be regarded as semiconductor dots that are monodispersed and isolated in the frameworks to enhance the density of the catalytic sites meanwhile avoiding the aggregations of these nanosize dots, resulting in the high photoactivity toward proton or CO2 reduction as well as the degradation of organic contaminates in water and air. Introducing well-designed functional groups into the organic linkers to shift the absorption bands or using metal NPs as cocatalysts are the primary methods used to improve the performance of type I MOF photocatalysts. In the case of type II MOF-based photocatalysts, the structure of MOFs serves as an ideal platform for the immobilization of homogeneous photoactive organic and metal−organic dyes. In addition to the separation and stabilization of photoactive sites, MOFs also provide an opportunity for the photoactive unit to work with other functionalities simultaneously or synergistically, giving rise to broad-spectrum catalysts or the integration between photocatalysis and metal- or organo-catalysis. The photocatalysis of traditional organic synthesis, radical chemistry, and novel conPET processes have all been accomplished using this type of MOFs as photocatalysts. The photocatalysis with respect to type III MOF photocatalysts is indeed underdeveloped. Only metal NPs and POMs have been embedded into the pores of MOFs for photocatalysis, although the integration of MOF porosity and photocatalyst has led to the formation of high-performance photocatalysts. The future development of MOF-based photocatalysts requires deeper understanding of the advantages related to MOF-based materials over other systems. First, the excellent crystallinity of MOF-based materials can be further utilized to construct photocatalysts with inherent semiconducting property or even conducting property. Second, the postsynthetic modification approach can be widely used to obtain photoactive MOFs through metal- or ligand-exchange or photoactive species encapsulation, which will surely enlarge the pool of MOF photocatalysts and widen the application range of MOFmediated photocatalysis. Third, distinct functionality could be introduced into a single MOF for the construction of a MOFbased photocatalyst with cooperative and tandem catalysis. Fourth, the environment of pores within MOFs should be finely adjusted to promote enzymatic-type behavior of the encapsulated photocatalysts. Finally, MOFs could be scaled to nanosize for better interaction with the substrate and improved absorption of light. We are confident that the tunable and diverse MOFs are an excellent platform for the development of ideal photocatalysts toward a greener world.

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ACKNOWLEDGMENTS This work was supported by the NNSF (Nos. 21421005, 21231003, and 21501041) and the MOST “973 Project” (No. 2013CB733700).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7945

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DOI: 10.1021/acscatal.6b02228 ACS Catal. 2016, 6, 7935−7947

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DOI: 10.1021/acscatal.6b02228 ACS Catal. 2016, 6, 7935−7947