Metal−Organic Frameworks: Versatile Materials for HeterogeneousPhotocatalysisLe 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
ABSTRACT: Photocatalysis is one of the most importantchemical methods to mitigate the energy and environmentalcrisis via converting inexhaustible solar energy into cleanchemical potential. The general history of the development ofphotocatalysis based on porous metal−organic frameworks(MOFs) is simply divided into three branches with a focusplaced on the distinct structural role of the photocatalyticcenter: the inorganic cluster nodes, the organic linkers, and theguests in the pores of MOFs. In each branch, thesephotocatalytic centers are considered to be monodispersedwithin the crystal lattices with the other two structure rolesregularly distributed to isolate the active centers andsometimes to provide more functions other than photoactivity.This distinctive nature has rendered MOFs as promising candidates for photocatalysis not only because they combine thebenefits of heterogeneous catalysis and homogeneous catalysis but also because they facilitate the possibility of mergingmultifunctional catalytic sites for concerted or cascade photocatalysis. The design strategy and improvement approaches forMOF-based photocatalysts are also introduced with an emphasis on structure. Our intention is for this comprehensive view ofMOFs-involved photocatalysts to inspire new ideas for designing heterogeneous photocatalysts toward the better utilization ofsolar energy.
In nature, green plants use solar radiation to convert water andcarbon dioxide into energy and biomolecules, which areessential for their survival.1 With the ever-increasing energyand environment demand to provide a high quality of life,humans are urged to learn from how nature obtains energy andmaterials through a sustainable approach called artificialphotosynthesis or simply photocatalysis.2−6 The classicalphotocatalytic process consists of three steps: first, a photo-sensitizer absorbs light to reach its excited state; second, theexcited state transforms into a charge separation state,producing a mobile electron and a hole; finally, the trans-portable electron or hole reacts with substrate. A photocatalystthat absorbs the energy of light and most importantly uses theenergy to produce a new redox active center is the heart ofphotocatalysis.Ever since Fujishima and Honda discovered in 1972 that
semiconducting TiO2 can drive water splitting under UVirradiation, extensive efforts have been devoted to investigatingsemiconductor-based photocatalysts.7−11 TiO2 is the flagship ofsemiconductor photocatalysts and has been extensively appliedto photodriven water splitting and organic contaminantdegradation.9−11 However, the insensitivity of TiO2 to a largeportion of the solar energy spectrum (44%)the visible light
portionhas forced researchers to develop photocatalystscapable of absorbing visible light.12 Transition-metal complexessuch as Ru(bpy)3
2+ are thus attracting increasing attention fortheir enormous potential to catalyze useful and unique organictransformations upon visible-light illumination.13−16
Generally, an excellent photocatalyst should own thefollowing features: strong absorption of visible light, longlifetime of excited state, high yielding of charge separationstates, and good charge mobility. Heterogeneous photocatalystsare in great demand for their recyclability and ease of separationfrom workup.17 Well known as a hybrid porous crystallinematerial linked by coordination bonds between organic linkersand metal or metal cluster nodes, metal−organic frameworks(MOFs) are ideal candidates for photocatalyst design.18−27
First, the three distinct components of MOFsmetal nodes,organic linkers and porescan all be easily tailored forphotocatalysis due to the modular nature of MOFs, combiningthe merits of organic and inorganic chemistry. Second, theuniform channels or pores are beneficial for substrate−catalystinteractions, and the well-defined crystalline structure of MOFs
Received: August 4, 2016Revised: September 19, 2016Published: October 13, 2016
is useful for elucidating structure−function relationships andgood for charge mobility. Third, as a solid material, MOFs areeasy to separate from reaction mixtures and can then be reusedfor the next run, possibly extending the lifetime of the catalystsand reducing waste and contamination.The exploiting of MOFs as photocatalysts has followed the
progress of photocatalysis and has been accompanied by thegrowth of MOF structural and functional complexity. To date,numerous reports demonstrating the unique merits of MOFs-based photocatalysis over purely organic or inorganic systemshave been reported, and excellent reviews are published almostyearly.28−34 In this perspective, MOF-based photocatalysts areclassified into three groups according to the intrinsic structuralroles of the photocatalytic centers (Figure 1). In type I MOFs,
the 0D inorganic cluster nodes act as semiconductor dotsphotocatalysts that are well-isolated by the organic linkers andsometimes also by the pores of the MOFs. These MOFs aredescribed as isolated semiconductor dots that are regularly anduniformly monodispersed in the lattice of the crystals. In type IIMOFs, the linkers are functional organic and metal−organicdye-based photocatalysts that are consolidated and separated bythe metal nodes. These MOFs are considered to beheterogenized dye-based photocatalysts that well positionedinto isolated fashion with ordered arrays. In type III MOFs, thephotocatalytic species that have suitable sizes and structures areencapsulated within the pores of the MOFs. These MOFs areconsidered as regular distributed supramolecular systems thatwere isolated by the MOF scaffold. Outstanding examples ofMOF photocatalysis are presented to reveal the design routesin terms of these three types of MOFs.
■ TYPE I MOFS-BASED PHOTOCATALYSTS WITH 0DINORGANIC CLUSTERS AS METAL NODES
Semiconductors, especially inorganic semiconductors, havebeen the leading players in photocatalysis for decades.7−11
Scheme 1 shows the elementary steps that occur in aphotocatalysis event mediated by semiconductors. Upon theabsorption of photons with the energy greater than the bandgap between the conduction band (CB) and the valence band(VB), charge separation occurs in the semiconductor,
producing an electron in the CB and a hole in the VB. Thegenerated charges (holes or electrons) then migrate to thesurface of the semiconductor to react with reductive oroxidizing substrates. The long journey of MOF-based photo-catalysis began with the semiconducting property because ofthe inherent connection between MOFs and semiconduc-tors.35−37 In the case of the metal−oxygen clusters as the nodesof the MOFs, these clusters can be simplified as semiconductordots that are isolated and distributed regularly within thenetworks of MOFs (Scheme 1).19 Compared to traditionalsemiconductors, semiconductor dots are more efficient asphotocatalysts because the detrimental charge recombination isno longer a problem.The MOF scaffold can exploit the full potential of
semiconductor dots as photocatalysts because (1) the highporosity allows the substrate to be close to semiconductor dotsso that the produced active electrons or holes do not need totravel to the surface of the whole material; (2) the density ofsemiconductor dots can be higher than the homogeneoussystems, while no quenching will occur thanks to the isolationof inorganic clusters by organic linkers; (3) the organic linkerscan serve as the photon antennae to increase the visible lightabsorption of semiconductors, besides the isolators for thesemiconductor dots. This rational deduction is confirmed bymany examples. In 2007, Garcia and co-workers were the firstto provide experimental evidence supporting the behavior ofMOF-5 as a semiconductor (Scheme 2).35 A charge-separatedstate of MOF-5 was observed to decay at the microsecond timescale after irradiation, and the band gap was estimated to be 3.4eV. Thereafter, many MOFs, including the UiO-66 and MILseries, have been reported to behave as semiconductors.36
These semiconductor-like MOFs have been successfully appliedto photocatalysis systems involving the photodegradation oforganic pollutants and solar energy conversion, the foundationof semiconductor photocatalysis.37−40 Moreover, the tunabilityand diversity of MOF structure provide numerous methods forMOFs to function even better than real semiconductors.36,44,45
For example, the organic linker can be adjusted to be a betterphoton antenna via the facile introduction of −NH2 group. Atthe same time, noble-metal nanoparticles (NPs) can beencapsulated into the MOF pores as electron reservoirs,
Figure 1. Scheme presentation of the three types of MOFphotocatalysts showing the distinct locations of photoinitiated redoxcenters: type I, the inorganic cluster nodes as monodispersednanosemiconductors photocatalysts; type II, the photocatalytic organicor metal−organic dyes as linkers to be heterogenized; type III, thephotocatalytic units as guests encapsulated in the pores of MOFs.
Scheme 1. (Left) Elementary Steps of SemiconductorPhotocatalysis: (i) Light Absorption, (ii) ElectronPromotion from the VB to the CB, (iii) Charge Migration tothe Surface of the Particle, Oxidation of the Substrate (S) bythe Positive Hole (h+), and Reduction of the S by anElectron (e−). Reprinted with Permission from Ref 32.Copyright 2016 Wiley-VCH. (Right) Scheme Presentationof Type I MOF as Monodispersed Semiconductor DotsDistributed on the Networks of the Lattice
which can increase the charge mobility by orders of magnitude.Recently, Grzybowski and co-workers demonstrated thatadding Ag nanoclusters to a Rb-CD(cyclodextrin)-MOF notonly imparts moderate electrical conductivity to an otherwiseinsulating material but also renders it photoconductivity, withconductivity increasing by up to 4 orders of magnitude uponlight irradiation.46 Lanthanide (Ln) ions are a unique type ofphotoactive metal node in MOFs and have been widely used toconstruct Ln-MOFs that function as luminescent sensors andlight-emitting materials.41−43 However, the Laporte-forbiddenf−f transition inherent to Ln ions limits the application of Ln-MOFs as photocatalysts.
■ TYPE II MOFS WITH DYE MOLECULES ASORGANIC LINKERS
have been extensively studied as photocatalysts in homoge-neous systems for their beneficial photophysical properties,which include strong visible-light absorption and long-livedexcited states.12−16,47,48 Building on the seminal resultsobtained using photoinduced electron transfer (PET),researchers have demonstrated various dye molecules to bevery efficient in photoredox reactions through excitation.12−16
Incorporation of these photoredox-capable dye molecules intoMOFs broadens and deepens the MOF photocatalysis toachieve more sophisticated and meaningful photoconversions.Meanwhile, MOFs enable the high-density and orderlydistribution of isolated dye-photocatalysts along the heteroge-neous support; by contrast, in homogeneous systems, the sameconcentration of dyes often induces detrimental aggregationthat suppresses activity or causes self-quenching.49−52 Mostimportantly, the crystalline nature of the MOF structureprovides other possibilities for the transport of excited states orphotoinduced electrons and holes.53−56 For example, Lin andco-workers recently reported that the “through space” energyjumping of singlet excited states beyond the nearest neighborcan account for up to 67% of the energy transfer rate intruxene-based materials (Figure 2).57 This phenomenon can beobserved only in highly ordered networks and may provide anadditional pathway for energy transfer and exciton migration,which are in favor of photocatalysis.
■ TYPE III MOFS WITH PHOTOREDOX SPECIESENCAPSULATED IN PORES
In addition to the metal nodes and the organic or metal−organic linkers, pores can be another origin of photoredoxactivity for MOFs, which enrich MOFs as photocatalysts whilealso providing additional possibilities for photocatalysis.Photoredox species of appropriate size can be introduced intothe pores of MOFs as guests with sustained and often improvedphotoactivity owning to the isolation from each other and themutual effect with the MOF scaffold.58,59 Interestingly, themicroenvironment of MOFs’ pores can be finely adjusted forenergy and electron transfer, thus making the MOF-basedphotocatalysis more similar to enzymatic catalysis.60−62
Importantly, the high porosity provides not only sufficientspace for the catalyst−substrate interaction but also size-selectivity for photocatalysis.65,68
Polyoxometalates (POMs), because of their outstandingperformance as acid catalysts and limited surface area as a solid,are continually introduced into MOF pores to obtain stableporous catalysts with POMs dispersed at the molecularlevel.61−67 Su and co-workers were the first to report well-defined crystalline POM/MOF composites and the first to usethem as true heterogeneous acid catalysts (Figure 3).63 Keggin-type POMs were incorporated into HKUST-1 scaffolds througha one-step hydrothermal reaction, resulting in a POM/MOFseries. One of these POM-MOF composites exhibited highactivity and reusability in the hydrolysis of esters in excess
Scheme 2. Photophysical Processes That Occur after theIrradiation of the MOF-5 Solid Material, SuggestingSemiconductor Behavior. Reprinted with Permission fromRef 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 ina network of chromophores. Reprinted with permission from ref 57.Copyright 2016 American Chemical Society.
Figure 3. View of a (001) sheet with two types of pores, A and B, inNENU-n (n = 1−6). The Cu-BTC framework and Keggin polyanionsare represented by wireframe and polyhedral models, respectively.Reprinted with permission from ref 63. Copyright 2009 AmericanChemical Society.
water. Furthermore, selectivity based on substrate size andaccessibility to the pore surface was observed, which high-lighted the heterogeneity of these POM-MOF catalysts.Inspired by this work, researchers have devoted much effortto the research of catalytic POM-MOF assemblies, andphotoactive POM-MOF systems have recently become atopic of great interest.61,62,64−67
The three types of MOF-based photocatalysts offer plenty ofideas for researchers to explore and have been successfullyapplied in photocatalysis.28−34 One more particular thing to benoted regarding the superiority of MOFs as photocatalysts isthat multiple functional components can be organized andintegrated into a single material in an ordered and hierarchicalmanner for facile energy and electron transfer, thus providinginfinite possibilities for synergistic or cooperative catalysis.69,70
In the following section, we provide an overall profile of MOFphotocatalysis, with a special focus on the design aspect.Meaningful and successful application examples of the threetypes of MOFs will be introduced.
■ PHOTOCATALYSIS WITH TYPE I MOFSInorganic semiconductors were the first and remain the mostwidely studied heterogeneous photocatalysts.7−9 When inte-grated into a MOF scaffold, the metal clusters maintain theirsemiconducting properties, and the porosity of the MOFstructure helps make full use of the metal clusters asphotocatalysts. As a traditional photocatalyst that has receivedlasting attention, semiconductors are of particular interest inthe photodegradation of organic pollutants and in solar energyconversion processes such as water splitting.8−11 Naturally, theapplication of type I MOFs as photocatalysts is primarilyfocused on these fields. Nowadays, there are reviews on thephotoinduced decomposition of organic pollutants, especiallyorganic dyes, the photochemical reduction of protons andcarbon dioxide, and the photooxidation of alcohols using MOFsas photocatalysts.37−40 Almost all the photoactive MOFs inthese aforementioned reviews consist of nanosemiconductors,the metal−oxygen clusters. The development of MOFs for thephotoreduction of protons and carbon dioxide will be discussedbelow, whereas the photodegradation of organic pollutants andthe photooxidation of alcohols using MOFs will not bediscussed in this perspective because these applications sharethe same design and improvement strategies.The reduction of protons to hydrogen and the reduction of
carbon dioxide to carbon monoxide or simple carbon-containing organics are regarded as two powerful methods tosolve the energy and environment dilemma.3−6 Variousinorganic semiconductors and noble metal complexes havebeen studied to achieve these two valuable reduction reactionswith the assistance of photoenergy.3−11 Many of the MOFscapable of photocatalytic hydrogen evolution or carbon dioxidereduction contain metal−oxygen clusters, taking advantage oftheir semiconducting properties.37,39,40 Only a few examplesinvolve the incorporation of proton/carbon dioxide reductioncatalysts into MOF scaffolds or pores.71−78 However, althoughrare, these materials can also exhibit favorable catalyticefficiency.After observing the semiconductor behavior of MOF-5,
Garcia and co-workers continued to explore the potential ofMOFs as photocatalysts, leading to the first example of a MOF-based photocatalyst for the hydrogen evolution reaction(HER).79 The photocatalytic HER was driven by highlywater-stable Zr-MOFs UiO-66 and NH2−UiO-66 in a water/
methanol mixture under UV irradiation. The introduction of anamino group into the organic linker added another absorptionband beyond 300 nm to NH2−UiO-66 and enabled it toperform better in the photocatalytic HER than UiO-66. Thepresence of Pt nanoparticles as cocatalysts can promote thephotoconversion to nearly five times the yield. The long-livedcharge separation states recorded by the laser flash photolysisspectra of UiO-66 and NH2−UiO-66 confirmed the semi-conductor role of MOFs for this photodriven HER (Scheme 3).
Ti-MOFs attract considerable interest as photocatalystsbecause they contain titanium-oxo species. MIL-125(Ti) hasbeen intensively studied as a photocatalyst since Sanchez, Serre,and co-workers synthesized this intriguing MOF and observedits photochromic behavior, revealing the photoinducedconversion 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 thephotocatalytic reduction of CO2 to HCOO− under visiblelight.81 The multinuclear Ti-centers in NH2-MIL-125(Ti) areboth photo- and redox-active. The amino functionality not onlytransfers the absorption band for MIL-125(Ti) from the UVregion to the visible region but also results in a higherabsorption capability toward carbon dioxide. The photodrivencarbon dioxide reduction of NH2-MIL-125(Ti) was performedin acetonitrile (MeCN) with triethanolamine (TEOA) as theelectron donor (Scheme 4). The generation of Ti3+ upon
visible-light illumination was observed as a color change frombright-yellow to green when NH2-MIL-125(Ti) was irradiatedunder nitrogen in the absence of carbon dioxide. When carbondioxide was pumped in, the color turned back to yellow and thecarbon dioxide was reduced by Ti3+ to yield HCOO−. TEOA
Scheme 3. Mechanistic Proposal to Rationalize thePhotophysics of Zr-MOF Used To Photo-Drive theHydrogen Evolution Reaction (HER). Reproduced withPermission from Ref 79. Copyright 2010 Wiley-VCH
Scheme 4. Proposed Mechanism for the PhotocatalyticCarbon Dioxide Reduction over NH2-MIL-125(Ti) underVisible-Light Irradiation. Reprinted with Permission fromRef 81. Copyright 2012 Wiley-VCH
offered an electron to regenerate the pristine NH2-MIL-125(Ti).Subsequently, Li and co-workers observed that NH2−UiO-
66(Zr) catalyzes the photocatalytic reduction of carbon dioxideunder visible light with TEOA as a sacrificial agent, and thephotogenerated ZrIII was first confirmed and revealed.82 Thepartial substitution of the organic linker improved the efficiencyof the photocatalytic reduction of carbon dioxide. Furthermore,the metal Zr can also be partially replaced by Ti through apostsynthetic exchange method to obtain a more capable MOF-photocatalyst for both the carbon dioxide reduction andhydrogen evolution under visible light.83 These works definitelyshow the enormous potential of MOFs, which benefit from thefacile adjustment of structure and functionality, for developingbetter catalysts for photocatalytic carbon dioxide reduction andhydrogen evolution.84,85
Very recently, another useful strategy to improve theperformance of MOFs as a semiconductor-photocatalyst wasreported by Gascon, Vlugt, Reek and co-workers.77 The poresof NH2-MIL-125(Ti) were decorated with the Co-basedelectrocatalyst Co-dioxime-diimine via a “ship-in-a-bottle”strategy (Figure 4), and the resulting Co@MOF composite
outperformed all previously reported MOF-based analogues forphotochemical hydrogen production. Though the redoxcatalytic center for substrate transformation is inside theMOF pores, this Co@MOF composite still belongs to the typeI MOF-catalyst because the photoinduced redox center is still
the Ti-oxygen clusters. This Co@MOF composite is arecyclable catalyst free of noble metals producing hydrogenfrom water under visible light. It displays a stable TOF of 0.8h−1 even after 65 h of operation. The introduction of the cobaltmolecular catalyst into the cavities of the framework resulted ina 20-fold enhancement of photocatalytic activity compared tothe pristine NH2-MIL-125(Ti), whereas the cobalt molecularcatalyst could not produce hydrogen under the same conditionsbecause of the lack of a photosensitizer. This example revealsthe potential of MOFs as an ideal modular functional materialfor photocatalysis via the elaborate design and integration ofwell-defined molecular building blocks. The higher density ofthe semiconductor nodes within the crystal lattices features anew 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 stateoccurred after the integration of porphyrin into Zr-MOF, PCN-222, and the photocatalytic conversion of carbon dioxide intoformate anion with PCN-222 was significantly improved thanthe corresponding porphyrin ligand.86 The semiconductorcharacter of PCN-222 was illustrated by a Mott−Schottky plotthat showed the possibility for photoreduction of CO2 by PCN-222. As expected, the HCOO− anion was continuouslyproduced with PCN-222 as the photocatalyst and TEOA asthe electron donor in CH3CN under visible-light irradiation.ESR studies revealed the generation of ZrIII ions in thisphotoreduction, indicating the electron transfer from theexcited porphyrin ligand to the Zr6 metal nodes. In this case,the pristine homogeneous photocatalyst porphyrin served asantenna to sensitize Zr-oxo clusters, which then transferelectron to substrate CO2, making PCN-222 a type I MOF.When H2TCPP was employed as the photocatalyst undersimilar conditions, clearly weaker performance was observed:only 2.4 μmol of HCOO− was produced after 10 h (30 μmolfor PCN-222). This result shows that the photocatalytic activityof the porphyrin was greatly enhanced by the formation of theMOF structure, inspiring a series of experiments to unveil thereason.Steady-state photoluminescence (PL) measurement of PCN-
222 indicated a pronounced PL emission quenching fromH2TCPP, suggesting greatly suppressed radiative electron−holerecombination in PCN-222 relative to that in H2TCPP.Moreover, ultrafast transient absorption (TA) spectroscopy ofPCN-222 revealed that the ΔA recovery converges to an
Figure 4. Concept of photocatalytic hydrogen production using aCo@MOF composite with Co-dioximediimine encapsulated into thepores of NH2-MIL-125(Ti). Reprinted with permission from ref 77.Copyright 2015 Royal Society of Chemistry.
Figure 5. (a) Transient TA spectra of PCN-222 registered at different probe delays (pump at 500 nm). (b) Representative TA kinetics of PCN-222taken 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 withpermission from ref 86. Copyright 2015 American Chemical Society.
asymptote (dashed line in Figure 5b) with a nonzero value ofapproximately −1.2 mOD within the probe-delay limit of thepump/probe spectrometer (∼3 ns). Notably, the nearly perfectparallelism between the asymptote and the ΔA = 0 line suggeststhat the eventual recovery to ΔA = 0 features an extremely longlifetime (τ3), possibly indicating a third trap state. Thedetrapping of electrons from this deep trap state was so slowthat its corresponding radiative electron−hole recombinationwas dramatically suppressed, which is in good line with thesteady-state PL spectra. In summary, as a direct result offramework formation, a deep electron trap state emerged inPCN-222, and this trap state could boost electron−holeseparation and then supply long-lifetime electrons for thephotoreduction of carbon dioxide via effective suppression ofthe detrimental electron−hole recombination. This workreinforces the advantage of MOF structures for the integrationof functional units, supplying surprises rather than commonmixing.Although type I MOFs are typical MOF-based catalysts for
photocatalytic HER and carbon dioxide reduction, other typesof MOF-photocatalysts for these two key transformations alsoexhibit satisfactory activity.71−78 Homogeneous photocatalystssuch as Re or Ir complexes, porphyrin derivatives and POMshave been built into MOF structures as linkers to obtain type IIMOF photocatalysts for these two reactions.71−75 Excellentexamples will be introduced in detail in the following section.
■ PHOTOCATALYSIS WITH TYPE II MOFS
Organic photocatalysts are gaining increasing attention andrenewed enthusiasm in regard to photosynthesis.12−16 Thepoor absorption of light prevents type I MOFs from beingpowerful photocatalysts for more sophisticated and usefulorganic transformations. The incorporation of rigid photo-catalyst as linkers into MOF scaffolds opens a new avenue forphotocatalytic organic conversions. In fact, the immobilizationof active photocatalysts onto a suitable matrix is a long-pursuedgoal for researchers to avoid self-quenching between photo-catalysts and the contamination of the product, especially nowthat the most widely commercialized photocatalysts are allheterogeneous in nature for easy workup and relatively lowcost.17,49 In this context, MOFs are the perfect heterogeneousplatform for photoactive linkers because they are orderlydispersed as single-site catalysts in a MOF scaffold, and thedistance between them can be finely adjusted for betterperformance; the porous nature of MOFs provides sufficientspace for substrate activation and product departure.18−27 Inthe meantime, the strong coordination bonds ensure thestability and recyclability of MOFs. More and more researchershave demonstrated that the pristine homogeneous photo-catalyst can drive photocatalysis more efficiently or completelyafter 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 organiclinkers used to construct efficient MOF photocatalysts for
Figure 6. Perspective view of the 3D porous framework of Zn−SnIV−TPyP down the c axis (left) and a schematic representation of thephotooxidation 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.
meaningful organic syntheses.71,90−97 In early 2011, Wu and co-workers reported a SnIV−porphyrin-based MOF (Zn−SnIV−TPyP) for the efficient photooxygenation of phenol andsulfides (Figure 6).91 The metalloporphyrin building blockSnIV−TPyP presents distinct photocatalytic activity in thehomogeneous phase. Upon excitation, the SnIV−TPyP reachedto its triplet excited state, which transfer energy with oxygen toproduce 1O2, one of the famous reactive oxygen species (ROS).1O2 then oxidizes substrate to corresponding product.However, this photoactivity was heavily influenced by reactionwith singlet oxygen. After immobilization of SnIV−TPyP intoZn−SnIV−TPyP, the quenching process was overcomeefficiently thanks to the enough space between photocatalyticunits SnIV−TPyP and the easy interaction between 1O2 andsubstrate. Thus, Zn−SnIV−TPyP provided remarkable activityfor the photooxygenation of phenol and sulfides.Almost simultaneously, Lin and co-workers demonstrated
the doping of catalytically competent Ir and Ru complexes intoa highly stable and porous UiO-67 and applied the resultantphosphorescent [Ru(bpy)3]
2+-derived MOF 5 and [Ir-(ppy)2(bpy)]
+-derived MOF 6 to three photocatalytic organictransformations (aza-Henry reaction, aerobic amine coupling,and aerobic oxidation of thioanisole), with good results (Figure7).71 [Ru(bpy)3]
2+and [Ir(ppy)2(bpy)]+ have been extensively
investigated as photoredox catalysts in various photocatalyticorganic reactions.16,47 The incorporation of these precious-metal-containing photocatalysts into porous MOFs is highlydesirable for a prolonged catalytic lifetime because of thestabilization effect of MOFs and their recyclability. Theimportance of MOF permanent porosity was demonstratedby the poor catalytic performance of amorphous nanoparticlesin the photodriven aza-Henry reaction (18% conversion,corresponding to the background reaction). The same strategywas also applied for the immobilization of the CO2-reductionphotocatalyst, [ReI(dcbpy) (CO)3Cl], obtaining MOF 4. Whenirradiated in CO2-saturated MeCN with TEA as sacrificialagent, MOF 4 turned from orange to green, and CO and H2were detected by gas chromatography (GC). During the first 6h, The CO-TONs reached 5.0 and the molar ratio of the COand H2 production was around 10. Unfortunately, MOF 4became inactive in CO generation after two 6 h reaction runs,due to the detachment of Re−carbonyl moieties from thedcbpy group in the MOF 4 framework. Even so, the total CO-TON of the MOF 4 are still 2 times higher than thehomogeneous system for 20 h reaction, presumably owing tothe catalyst stabilization by the MOF framework.Following this work, Lin and co-workers developed a new
approach for the photocatalytic HER with Ru/Ir complex-derived MOFs.87,61,62 Pt NPs and various POMs wereintroduced to improve electron transfer during the HER;these Ru/Ir complex-derived MOFs are among the mostefficient MOF-based photocatalysts for HER. For example, Linand co-workers synthesized two Pt@MOF composites byloading Pt nanoparticles into the pores of [Ir(ppy)2(bpy)]
+-based MOFs 1 and 2 (Figure 8).87 The photochemicalhydrogen evolution with Pt@MOF composites was operatedwith TEA as the electron donor and with the Ir-phosphorabsorbing visible light and transferring electrons to Ptnanoparticles for the final reduction step. A total Ir-TON of7000 was obtained for MOF 2 via 48 h hydrogen evolutionexperiments, which is approximately 5 times the TON valueafforded by the homogeneous control. The better cooperationbetween photoexcitation of the MOF frameworks and electron
injection into the entrapped Pt NPs was responsible for themarkedly improved efficiency. Moreover, these Pt@MOFcomposites could be readily recycled and reused.The modular nature of MOF scaffold endows MOFs with
infinite potential for the construction of multifunctionalmaterials, which is also the case for the photoactive species-involved MOF. Various functionalities can be added intophotoactive MOFs, being the metal nodes or the secondlinkers. Therefore, distinct capable units in a photoactive MOFcan operate simultaneously and even synergistically towardsubstantial utilization of solar energy. Farha, Hupp, and co-workers described a dual-function MOF that simultaneouslydetoxified two chemical warfare agent (CWA) stimulants atroom temperature (Figure 9).92 Efficient deactivation of CWAs
requires multiple detoxification pathways, such as hydrolysisand oxidation, to occur simultaneously because predictingwhich CWAs will need to be deactivated is difficult. Themultifunctional nature of MOFs makes them a perfect materialto incorporate distinct catalytic moieties to generate broad-spectrum detoxification materials. The dual-function MOF wasconstructed from a Zr6-containing node and a porphyrinorganic linker. Upon visible-light (LED) irradiation, a simulantof mustard gas, 2-chloroethyl ethyl sulfide, was oxidized to anontoxic product by singlet oxygen, which was generated from
Figure 8. Phosphorescent Zr-carboxylate MOFs (1 and 2) andsubsequent loading of Pt NPs inside MOF cavities to form the Pt@1and Pt@2 assemblies for the synergistic photogeneration of hydrogen.Reprinted with permission from ref 87. Copyright 2012 AmericanChemical Society.
Figure 9. Dual-function MOF constructed from a phosphortriesterase-like Zr6-containing node and a photoactive porphyrin linker for thesimultaneous degradation of simulants of two CWAs. Reproducedwith permission from ref 92. Copyright 2015 American ChemicalSociety.
the photosensitized porphyrin moieties. Meanwhile, the Zr6nodes containing Zr−OH−Zr in the same MOF, mimickingthe Zn−OH−Zn active site in PTE, hydrolyzed anothersimulant of nerve agents (such as GD or VX), dimethyl 4-nitrophenyl phosphate, to a nontoxic complex. The structure ofthis dual-function MOF remained intact after catalysis, and thedegradation efficiency for fresh CWAs was the same as for thefirst run. The versatility and tunability of MOFs suggests thatsuitably engineered successors may eventually prove useful forair-filtration equipment and for the destruction of stockpiles orspills of chemical warfare agents. In addition, reducing the sizeof MOF crystals to the nanoregime leads to acceleration of thecatalysis.The combination of chiral catalysis and photocatalysis is a
hot topic in chemistry because chiral complexes are highlydesirable as medical materials but their traditional chemicalsynthesis is often very tedious. Through careful selection anddesign, Duan and co-workers realized the first integration of atriphenylamine photoactive unit and the chiral component L- orD-pyrrolidin-2-ylimidazole (PYI) in a MOF and successfullyapplied these chiral photoactive MOFs (Zn−PYIs) in the light-driven asymmetric α-alkylation of aldehydes (Figure 10).60 The
targeted Zn−PYIs could not be obtained through directsolvothermal synthesis due to the instability of PYI. A tert-butoxycarbonyl (Boc) group was thus introduced to protect thecatalytically active N−H site of pyrrolidine, and the resultingprecursor MOF (Zn−BCIP) was converted into the activeZn−PYI simply by heating in DMF solution. The asymmetricα-alkylation of aldehydes with Zn−PYI1 started with phenyl-propylaldehyde and diethyl 2-bromomalonate as the couplingpartners and a common fluorescent lamp (26 W) as the lightsource. A high reaction efficiency (74% yield) and excellentenantioselectivity (92% ee) were achieved, demonstrating thesuccessful execution of photoactive chiral MOF design.Absorption and luminescence experiments suggested that thequenching process was typically attributed to the photoinducedelectron transfer (PET) process from Zn−PYI1* to diethyl 2-bromomalonate. The chiral PYI moieties acted as cooperativeorganocatalytic active sites to further induce the PET-generatedactive intermediate in a MOF channel with remarkablestereoselectivity. Therefore, Zn−PYI1 represents the firstexample of a MOF-based heterogeneous asymmetric photo-catalyst for this important reaction. Because of the restrictedmovement of the substrates within the MOF’s interior andmultiple chiral inductions as well, the integration of both thephotocatalyst and asymmetric organocatalyst into a single MOFmakes the enantioselection superior to that achieved through
simple mixing of the corresponding MOFs with the chiraladduct.For typical photocatalysis with a PET process, an excited
state of photosensitizer is obtained after the absorption of aphoton. Specially, some photosensitizers may produce activeradicals right after the absorption. Xing and co-workers utilizedanthracene, one photosensitizer of this particular type, toconstruct a visible light responsive MOF, NNU-35, for thepromising photocatalytic atom-transfer radical polymerization(ATRP) reaction of methacrylate monomers.94 The incorpo-ration of the anthracene-based ligand L1 into NNU-35 resultedin absorption and emission bands that were broader and red-shifted because of energy transfer and/or charge transferinteractions in the MOF structure. The resonance EPR signalafter exposure to visible light was assignable to the anthracene-based ligand, and it can sustain for about several minutes evenwithout light irradiation, clearly suggesting its free-radicalnature. The broad visible-light absorption and desirablephotoinduced charge separation in NNU-35 are intriguing forlight-switchable ATRP. ATRP is an efficient method forcontrolled radical polymerization, which allows nonexpertsfacile access to functionalized polymer materials with well-defined structures and architectures.More recently, photoinduced ATRP has received intensive
attention because of its unique features of temporal and spatialcontrol of the chain-extension process. A typical ATRP reactionsystem of MMA with a copper complex as the catalyst andEBiB as the initiator was chosen to test the photoactivity ofNNU-35. 48% monomers were polymerized after 8 h ofirradiation with 520 nm light, and a relatively narrow molecularweight distribution (Mw/Mn) of 1.12 was observed. Nopolymerization occurred in the absence of NNU-35. Moreimportantly, the reaction could be triggered easily through lightswitching. When the visible light was periodically turned on andoff, the polymerization also occurred periodically, exhibiting thecharacteristics of living radical polymerization. Figure 11 clearly
illustrates the successful use of MOF NNU-35 for the light-switchable ATRP reaction via continuous regeneration of thecopper catalyst. The heterogeneous photocatalyst NNU-35 firsttransfers an electron to an oxidized Cu(II)/PMDETA complexafter the visible-light-induced formation of a radical. Thegenerated Cu(I) complex then reacts with the alkyl halide (R−X) initiator, forming radicals (R•) to initiate the ATRP. Theimplementation of NNU-35 for the ATRP reaction successfullyexpanded the application range of photoactive MOFs to radicalchemistry.A breakthrough in photocatalysis involving consecutive
photoinduced electron transfer (conPET) processes was
Figure 10. Integration of the stereoselective organocatalyst L- or D-PYI(blue) and a triphenylamine photoredox group (red) into one singleMOF to promote the asymmetric α-alkylation of aliphatic aldehydes ina heterogeneous manner. Reprinted with permission from ref 60.Copyright 2012 American Chemical Society.
Figure 11. Proposed mechanism for NNU-35 mediated ATRP undervisible light. Reprinted with permission from ref 94. Copyright 2016Royal Society of Chemistry.
reported by Konig in 2014.48 Before that, the energy conferredby visible-light excitation for subsequent redox chemistry waslimited to the one single absorbed photon. Therefore, usefulreactions requiring high energy are either performed under UVirradiation or with highly active substrate precursors understrict conditions, both of which are obstacles to wideapplication. Through the integration of perylene diimide(PDI) into MOF Zn-PDI, the conPET process for the efficientvisible-light-driven reduction of aryl halides was introduced intothe heterogeneous catalysis field, as accomplished by Duan andco-workers (Figure 12).95 Perylene diimide-derivative H2PDI
was chosen as the organic ligand because the conPET processwas discovered using a PDI derivative. However, the poorsolubility of PDIs and their strong tendency to aggregate,resulting from their large planar conjugated structure, severelyrestrict the development of this conPET process for othercatalytic transformations in homogeneous systems. Strong π···πinteraction between PDIs still existed in the Zn-PDI sheets;however, the coordination effect worked synergistically, makingthe aggregation controllable. Each of the three PDI moleculesformed a close J-aggregate group. The formation of J-aggregatesin Zn-PDI is beneficial for photocatalysis because of theexcellent ability of J-aggregates to delocalize and migrateexcitations. Moreover, the appropriate space between J-aggregates and the rigid framework of Zn-PDI helped reduceself-quenching, as concluded on the basis of the obviouslyweaker fluorescence intensity of H2PDI solutions with the samePDI 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 ofacetophenone, which was 3 times higher than the yieldafforded by the homogeneous counterpart, was detected after1 h of irradiation with blue LEDs and the use of 72 equiv of theelectron donor Et3N. The reason for the high performance isexplained on the basis of the aforementioned studies: the rigidZn-PDI framework isolated the active sites of PDI but allowedthem to be dense at the same time, which may be optimal forelectron or energy transfer. Moreover, the most inert arylchlorides were also effectively reduced using higher loadings ofZn-PDI. Normally, the photoreduction of inert aryl chlorides
cannot proceed without highly sensitive and active donormolecules, UV-A irradiation and strictly inert reactionconditions. The integration of conPET-active PDI bricks intothe Zn-PDI architecture thus represents a critical step towardmore efficient solar energy utilization via a conPET process.In addition to direct incorporation into MOF scaffold,
photocatalysts can be introduced to MOFs through post-synthetic modification (PSM), benefiting from the porosity andthe well-defined structure.76,87−89 Xu described the synthesis ofphotoactive MOF-253-Pt for the reduction of water to formhydrogen under visible-light irradiation through PSM of a 2,2′-bipyridine-based MOF with platinum ions.76 The visible-lightinduced hydrogen evolution of the MOF-253-Pt was examinedin the presence of 15 vol % TEOA as a sacrificial electron donorin water at pH 8.5; the amount of hydrogen produced fromMOF-253-Pt was nearly 5 times greater than that producedfrom the Pt(bpydc)Cl2 complex. For many capable molecularphotocatalysts, the direct solvothermal synthesis often failed toproduce the wanted MOFs, while PSM are more feasible. ThisPSM strategy combines the advantages of molecular catalystswith a highly ordered and stable inorganic support, and it issure to be widely applicable for MOFs as photocatalysts.89
■ PHOTOCATALYSIS WITH TYPE III MOFSBeyond the semiconducting metal-oxo clusters or photoactivelinkers, the inherent pores of MOFs can also be exploited torender MOF photoactivity. Suitable photocatalysts can bedispersed into MOF pores through one-pot synthesis duringsynthesis or through in situ deposition after the formation ofthe MOF scaffold.65,98Various photoactive species, includingperylene and DMASM (4-[p-(dimethylamino)styryl]-1-meth-ylpyridinium), have been introduced into MOF cavities asguests and have exhibited favorable properties resulting fromthe host−guest interaction.99−101 However, thus far, this type ofMOF-photocatalysts has remained in its infancy; metal NPsand POMs are the rare examples of photocatalysts encapsulatedinto MOF pores and applied to photoconversion.65−68,102,103
Metal NPs were initially introduced into photoactive MOFsas cocatalysts to facilitate energy transfer. Duan, Chen, and co-workers 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 byepitaxial growth or coalescence of nuclei depending on thedensity of ZIF-8 nuclei on Au NPs.102 The photodrivenoxidation of benzyl alcohol by these Au@ZIF-8 composites wasperformed in acetonitrile under visible light given the LSPR-related absorption of Au NPs (Figure 13). The conversion was25.8% for single-core Au@ZIF-8 and 51.6% for multicore Au@ZIF-8 after 24 h of irradiation. This difference in conversion isascribed to plasmonic coupling between Au NPs in themulticore structures. The size-selectivity of ZIF-8 that restrictsbenzyl alcohol from entering pore cavities too close to Au,according to the authors, is responsible for the conversionlower than that afforded by Au−SiO2 (56.9%). Later, the sameauthors 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 C-AFM, leading to nearly 6-fold greater conversion than thatprovided by NH2−UiO-66 for the photooxidation of benzylalcohol.103
MOFs, as excellent porous materials, have long beenintensively studied for gas storage and separation. Thisintriguing feature can be exploited in photocatalysis with gas-
Figure 12. Diagram illustrating the strategy of assembling insolublePDI into organized arrays in porous solid Zn-PDI to obtain anefficient photocatalyst for the visible-light-driven reduction of arylhalides and the oxidation of alcohols and amines. Reprinted withpermission from ref 95. Copyright 2016 American Chemical Society.
phase compounds. Jiang and co-workers recently reported thesynthesis of a Pd nanocubes@ZIF-8 composite material for theefficient and selective catalytic hydrogenation of olefins at roomtemperature under 1 atm H2 and light irradiation (Figure 14).68
The encapsulation of Pd nanocubes in ZIF-8 solved severalproblems impeding catalysis, such as the propensity foraggregation and the random dispersion of Pd NCs underhomogeneous conditions. In addition, the plasmonic photo-thermal effects of the Pd nanocube cores endowed thecomposite Pd NCs@ZIF-8 with photoactivity for the hydro-genation of olefins.The uniform pores of the ZIF-8 shell played important roles
in the efficient hydrogenation: they accelerated the reaction byH2 enrichment and acted as a “molecular sieve” for olefins withspecific sizes. Upon light irradiation, Pd NCs@ZIF-8 drove thehydrogenation of 1-hexene to almost complete conversion in90 min, whereas only 58% conversion was achieved with PdNCs. Moreover, the yields with Pd NCs@ZIF-8 remainedapproximately 100% after three consecutive runs, whereas a lowyield of 21% with the Pd NCs was observed in the third run.Remarkably, the catalytic efficiency of hydrogenation with PdNCs@ZIF-8 under 60 mW cm−2 of full spectrum or 100 mWcm−2 of visible-light irradiation at room temperature wascomparable to that of a process driven by heating at 50 °C.Thus, hydrogenation with H2 may utilize solar energy instead ofhigh-temperature heating, which is dangerous, energy-consum-ing, and environmentally damaging. The strategy of combining
the photothermal effects of metal nanocrystals with thefavorable properties of MOFs for efficient and selectivecatalysis generates synergistic advantages and opens up anavenue 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 thermalreactions or multielectron processes, with little attentiondevoted to the photoactivity of POMs. However, in 2015,Duan and co-workers reported a POM-MOF, CR−BPY1, thatis active toward the photocatalytic oxidative coupling of low-reactive sp3 C−H bonds with environmentally benign andinexpensive oxygen as the oxidant (Figure 15).65 CR−BPY1
was constructed from the incorporation of a ruthenium-substituted POM, [SiW11O39Ru(H2O)]
5−, which exhibitedexcellent photoactivity in various catalytic oxidation processesof organic substrates, into copper-BPY networks. Copper atomswere first connected by the BPY ligands to form a two-dimensional square grid; adjacent sheets were then connectedtogether via the coordination of Cu to a terminal oxygen atomof the deprotonated [SiW11O39Ru(H2O)]
5− to generate a 3Dframework. The catalytic potential of CR−BPY1 was examinedvia a model reaction using N-phenyl-tetrahydroisoquinolineand nitromethane as coupling partners under irradiation by an18 W fluorescent lamp. A yield of 90% was observed after 24 hof irradiation, whereas less than half the conversions wereobtained using the same equiv of copper(II) salts or/andK5[SiW11O39Ru(H2O)] as catalysts. These results suggestedthat the direct connection of copper(II) ions to [SiW11O39Ru-(H2O)]
5− anions was the key factor for improving the catalyticperformance; this speculation was confirmed by a series ofspectroscopic analyses and an elaborate control experiment thatis discussed below. The significantly weaker (46% conversion)catalytic activity of CR−BPY2 than CR−BPY1 in the modelC−C coupling reactions demonstrated that the direct CuII−O−W(Ru) bridges were the key factor in achieving the synergisticcatalysis between the photocatalyst and the metal catalyst.Inspired by this work, the same author recently demonstratedthe encapsulation of photosensitizing [W10O32]
4− into MOFpores brought about a series of POM-MOFs as capableheterogeneous photocatalysts for the light-driven accelerationof the β- or γ-site C−H alkylation of aliphatic nitriles; theseresults suggest that the POM-MOF is a promising photo-catalyst type.66,67
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 RoyalSociety of Chemistry.
Figure 14. Self-assembly of Pd NCs@ZIF-8 and plasmon-drivenselective catalysis of the hydrogenation of olefins. Reprinted withpermission from ref 68. Copyright 2016 Wiley-VCH.
Figure 15. Illustration of CR−BPY1 for the oxidative coupling of low-reactive sp3 C−H bonds through the synergistic catalysis of thephotoredox polyoxometalate [SiW11O39Ru(H2O)]
5− and the Cu2+
metal node. Reprinted with permission from ref 65. Copyright 2015Royal Society of Chemistry.
■ CONCLUSIONS AND OUTLOOKAfter nearly a decade of development, photocatalysis withMOFs has reached its booming stage. Designing MOFs forphotocatalysis is a feasible approach to obtain highly active,recyclable, and environmentally benign photocatalysts withsimple workup, largely thanks to the tunable structure and highporosity of these crystalline materials. The photoactive speciescan be integrated into MOF scaffolds as inorganic cluster nodes(type I) or organic linkers (type II) or as an inclusion (typeIII). For type I MOF-based photocatalysts, the metal-oxoclusters can be regarded as semiconductor dots that aremonodispersed and isolated in the frameworks to enhance thedensity of the catalytic sites meanwhile avoiding theaggregations of these nanosize dots, resulting in the highphotoactivity toward proton or CO2 reduction as well as thedegradation of organic contaminates in water and air.Introducing well-designed functional groups into the organiclinkers to shift the absorption bands or using metal NPs ascocatalysts are the primary methods used to improve theperformance of type I MOF photocatalysts. In the case of typeII MOF-based photocatalysts, the structure of MOFs serves asan ideal platform for the immobilization of homogeneousphotoactive organic and metal−organic dyes. In addition to theseparation and stabilization of photoactive sites, MOFs alsoprovide an opportunity for the photoactive unit to work withother functionalities simultaneously or synergistically, givingrise to broad-spectrum catalysts or the integration betweenphotocatalysis and metal- or organo-catalysis. The photo-catalysis of traditional organic synthesis, radical chemistry, andnovel conPET processes have all been accomplished using thistype of MOFs as photocatalysts. The photocatalysis withrespect to type III MOF photocatalysts is indeed under-developed. Only metal NPs and POMs have been embeddedinto the pores of MOFs for photocatalysis, although theintegration of MOF porosity and photocatalyst has led to theformation of high-performance photocatalysts.The future development of MOF-based photocatalysts
requires deeper understanding of the advantages related toMOF-based materials over other systems. First, the excellentcrystallinity of MOF-based materials can be further utilized toconstruct photocatalysts with inherent semiconducting prop-erty or even conducting property. Second, the postsyntheticmodification approach can be widely used to obtain photoactiveMOFs through metal- or ligand-exchange or photoactivespecies encapsulation, which will surely enlarge the pool ofMOF photocatalysts and widen the application range of MOF-mediated photocatalysis. Third, distinct functionality could beintroduced into a single MOF for the construction of a MOF-based photocatalyst with cooperative and tandem catalysis.Fourth, the environment of pores within MOFs should befinely adjusted to promote enzymatic-type behavior of theencapsulated photocatalysts. Finally, MOFs could be scaled tonanosize for better interaction with the substrate and improvedabsorption of light. We are confident that the tunable anddiverse MOFs are an excellent platform for the development ofideal photocatalysts toward a greener world.
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