Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks

Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks Keywords Metal–organic frameworks · Thin films · Single-site catalysts · Infrared spectroscopy · Defects · Active sites 1 Introduction Metal–organic frameworks (MOFs, also known as porous * Yuemin Wang coordination polymers or PCPs) are an emerging class of yuemin.wang@kit.edu porous materials of a hybrid organic/inorganic nature that * Christof Wöll combine the properties of both organic and inorganic porous christof.woell@kit.edu materials [1–7]. MOFs are typically stable up to tempera- Institute of Functional Interfaces, Karlsruhe Institute tures above 250 °C (in some cases, e.g., ZIF-8 [8], maximum of Technology, 76344 Eggenstein-Leopoldshafen, Germany Vol.:(0123456789) 1 3 2202 Y. Wang, C. Wöll temperatures as high as 550  °C can be tolerated). They vacancies, play a crucial role in the catalytic process [32]. exhibit a high degree of crystallinity and have large surface Recently, it has become evident that MOFs may feature a areas. The maximum degree of porosity and the size of the variety of intrinsic structural defects and the correspond- pores clearly exceed those of zeolites. In early 2017, the ing (intentional) defect engineering is a powerful strategy number of characterised MOFs was estimated to be 70,000 for advanced control of MOF chemical properties (Fig. 1) [9]. [33, 34]. The defect engineered MOFs (DEMOFs), synthe- Due to these unique chemical and physical properties sized by tailoring of linkers and/or metal ions, have shown MOF materials have opened up new perspectives in a variety modified physical and chemical properties [33– 48]. Both of different fields, ranging from gas storage and separation to local, isolated defects [modified CUS (mCUS)] and large- chemical sensing, catalysis, and drug delivery [10–21]. With scale defects (e.g. missing nodes, mesopores, Fig. 1) can be respect to applications in heterogeneous catalysis, different formed in DEMOFs and these defects can have important aspects must be considered. First, after adding additional consequences for their applications in catalysis. Further- coupling units (e.g., carboxylic acids or pyridine-units), more, MOF materials can serve as a host matrix (support) molecules active in homogeneous catalysis can be incorpo- for loading of catalytic components such as metal or metal rated into MOFs, thus providing a strategy to unite homo- oxide nanoparticles. These confined NPs inside MOFs may geneous and heterogeneous catalysis. Second, even without show unique properties which are significantly different further functionalization, many MOFs already show inter- from conventional supported catalysts. esting catalytic properties originating from the presence of In addition to MOF powders, the standard form of MOFs coordinatively unsaturated metal sites (CUS) at the nodes. obtained by the conventional solvothermal synthesis, a num- Such CUS sites are reactive for various chemical reactions. ber of methods have been developed to fabricate monolithic, A particular advantage of such sites in MOFs is that, com- crystalline, and highly oriented MOF thin films (SURMOFs) pared to oxide-supported metal nanoparticles (NPs), these [49–58]. Among them, the kinetically controlled layer-by- metal cations are highly dispersed within the framework of layer (lbl) or liquid phase epitaxy (LPE) growth process MOFs and thus serve as isolated, “single atom” catalytic has been extensively applied to produce well-defined SUR - sites. Accordingly, the interest in porous MOF materials as MOFs with a high degree of crystalline order, both vertical potential catalysts (in particular single-site catalysts [22]) and parallel to the substrate surface [59–64]. The highly- has intensified [22– 31]. ordered SURMOFs not only retain the intrinsic properties While in some MOFs CUS-sites are an intrinsic property of the corresponding MOFs, but also allow the design of of the MOF lattice itself, in other cases such undercoordi- architectures that cannot be achieved using MOF powders nated sites are absent in the ideal structure and only occur and to supplement their applicability as chemical sensors, as a result of defect formation within the framework mate- smart membranes, electronic and optoelectronic devices, rial. In the case of heterogeneous catalysis, e.g., on oxide as well as in catalytic coatings [65–68]. More importantly, surfaces, it is well known that defect sites, such as oxygen SURMOFs deposited on conducting substrates (e.g., Au) Fig. 1 Defect-engineered MOFs (DEMOFs). The modulation of the represent perfect and defect metal sites, respectively; the green high- defect structure on the micro and meso-scale by defect linker doping lighted unit indicates the parent micropores. Reproduced with per- of the framework is shown. The blue and short red sticks represent mission from Ref. [33]. Copyright 2014 American Chemical Society perfect and defective linkers, respectively; the yellow and black balls 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2203 can serve as model systems for a thorough study of MOF structure characterization of catalysts. However, a reliable properties by employing virtually all surface-sensitive tech- characterization of metal species in MOFs, and particularly niques developed in Surface Science because the charging those at defect sites in DEMOFs, is a challenge. Characteri- problems often hampering the analysis of bulk MOF sam- zation with electron microscopy, for example, is extremely ples are largely reduced. difficult since MOF materials contain organic ligands Overall, the extremely high structural and compositional which can be easily damaged by the high energy electrons design ability of MOF materials holds promise for their employed in TEM. Only very recently, Zhang et al. [83] have application in catalysis, including photocatalysis and electro- reported the high-quality atomic-resolution TEM images catalysis. The reactive properties of MOFs can vary signifi- of UiO-66 by developing a suite of methods to overcome cantly depending on the modification of both metal centers the experimental obstacles. Alternatively, MOFs can be and organic linkers as well as loading of metal/oxide NPs. characterized by spectroscopic techniques such as electron In this short report, we will highlight recent advances paramagnetic resonance, X-ray spectroscopy (XPS, EXAFS, in the field of MOF catalytic applications, with the main XANES) and infrared spectroscopy (FTIR). Furthermore, emphasis being on isolated, single active sites in perfect theoretical modeling of MOFs is required to properly inter- and defect-engineered MOFs, as well as for MOF thin films pret the experimental results. and SURMOFs. We will focus on structural and electronic properties as well as chemical reactivity of the selected MOF 2.1 Pristine MOFs systems based on fundamental investigations conducted primarily by our group and our collaboration partners. For The Cu-based HKUST-1 ([Cu btc ], btc = benzene-1,3,5- 3 2 other more general information about applications of MOFs tricarboxylate) is a prototypical MOF and has been used in catalysis and other fields we refer the reader to numerous in many studies [84]. This MOF contains so-called paddle- excellent review papers published recently (see e.g., [22, wheel units, where 4 carboxylate groups are bound to a 65–82]). Cu -dimer. In this interesting catalytic unit, the metal ions We will initially discuss the chemical reactions catalyzed are under-coordinated and can bind additional molecular by perfect and defect-engineered MOFs (e.g., Cu-HKUST-1, species (e.g., CO, H O, pyridine) at the axial position of the Ru-HKUST-1) using a combined experimental and theoreti- paddle-wheel units. Indeed, the CO oxidation reaction can cal approach. This section will be followed by a brief review be catalyzed by HKUST-1 at ambient pressure and elevated of the chemical nature and catalytic activity of SURMOF temperatures [85, 86]. In order to gain deeper insights into thin films. The next sections will focus on homochiral MOFs the active sites and reaction mechanisms, this system was as well as on metal and metal–oxide NPs embedded inside systematically investigated by using high-resolution ultra- MOFs. Finally, the enormous potential of SURMOF-based high vacuum infrared spectroscopy (UHV-FTIRS) in con- materials with respect to electrocatalysis will be highlighted. junction with density functional theory (DFT) calculations [87]. The sophisticated UHV-FTIRS apparatus combines a vacuum FTIR spectrometer (Bruker Vertex 80v) with a 2 Coordinatively Undercoordinated multi-chamber UHV system (Prevac), which not only ena- Single Active Sites in Pristine bles in situ transmission IR experiments on MOF powders and Defect‑Engineered MOFs (DEMOFs) supported on an inert metal mesh, but also allows the record- ing of IR reflection absorption spectroscopy (IRRAS) data To date, MOF materials have been extensively investigated, using a grazing incidence geometry on SURMOF thin films. and tremendous effort has been dedicated to their synthesis, This methodology has been demonstrated to be an invaluable properties and applications. However, a thorough atomic- tool to monitor chemical and photochemical reactions on the level understanding of structural and electronic properties surface of metal oxide powders [32]. 2+ −1 of the isolated, single active metal sites (CUS and mCUS) As shown in Fig. 2, the intrinsic Cu CUS (2179 cm ) as well as the structure–activity relationship continues to be was identified as the predominant species in HKUST-1, a major challenge; many crucial issues remain unanswered. while a small amount of native Cu defects were detected −1 This lack of information is due to the great complexity of as minor species (2125 cm , a few percent), in line with nanostructured MOFs, especially for DEMOFs in the pres- the observation for HKUST-1 thin films (see below in the ence of the different types of defects described below. A section of SURMOFs) [88]. Upon exposure to O , the UHV- comprehensive and fundamental understanding of active FTIRS data provide direct spectroscopic evidence for a sur- metal sites requires state-of-the-art analytical techniques prisingly high catalytic activity of Cu–MOFs (HKUST-1 that are suitable to probe the local chemical environments. and MOF-14, [Cu btb ], btb = 1,2,3-benzenetrisbenzoate). 3 2 Microscopic techniques [e.g., high-resolution transmis- Clearly, the presence of dioxygen leads to CO oxidation sion electron microscopy (HRTEM)] are powerful tools for even at temperatures as low as 105 K. The spectroscopic 1 3 2204 Y. Wang, C. Wöll Fig. 2 a UHV-FTIR spectra obtained after exposing [Cu btc ] cules adsorb on opposing paddle-wheel units in the iso configuration; 2 3 (HKUST-1) first to CO and subsequently to O at 105  K for differ -the O molecule is located (symmetrically) between these fragments 2 2 ent times (top). Intensity of the IR bands as a function of time for (top). Resulting structure: two C O molecules adsorb on different different CO species in the presence of molecular oxygen at 105  K paddle-wheel units (bottom). Reproduced with permission from Ref. (bottom). b Structure of HKUST-1. c Schematic representation of the [87]. Copyright 2012 John Wiley and Sons hypothetical reaction mechanism. Starting structure: two CO mole- information demonstrated that this reaction takes place only very interesting. Such modifications, of the general type 2+ on the intrinsic Cu CUS, whereas, rather unexpectedly, [M (btc) ], can be obtained by incorporating Mo [94], 3 2 the Cu defect sites are inactive for the low-temperature CO Cr [95], or Zn [96] into the parent MOF structure. These oxidation. On the basis of the high-level quantum chemical HKUST-1 analogues possess coordinatively unsaturated calculations, a concerted mechanism was proposed, whereby metal dimers at axial positions of paddle-wheel units where the impinging O molecule is activated in the presence of adsorption and chemical reactions may take place. More pre-adsorbed CO and interacts simultaneously with two iso- challenging is the synthesis of HKUST-1 analogues where carbonyl species at neighboring CUS to yield two C O mol- the central pair consists of metal ions in different charge ecules (see Fig. 2). Notably, this mechanism differs entirely states, thus yielding heterovalent paddle-wheel units. Such from those reported to occur in the case of CO oxidation mixed-valence MOFs are expected to show redox activities on metal oxides or on oxide-supported metal NPs (see e.g., [97, 98] and electric conducting properties [99, 100] that [89–93]). could be applicable to a porous electrode for batteries, fuel From a catalytic point of view, isostructural MOF vari- cells, capacitors, etc. ants of HKUST-1 where Cu is replaced by other metals are 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2205 In contrast to all other known [M (btc) ] frameworks that energies (see Fig.  3). These findings suggest that similar 3 2 II,II II,III are based on M units, the Ru–MOF, [Ru (btc) Cl ], complexity must be taken into explicit account for many 2 3 2 1.5 II,III exhibits mixed-valence R u paddle-wheel units with two other mixed valence MOFs with intrinsically low-coordi- 2+ 3+ differently charged metal centers (Ru and Ru ), which nated metal sites (CUS), which has not yet been accom- are stabilized by additional counter ions Cl (or in general plished in accurate detail. − − other X species such as OH ) to obtain an overall charge- neutral framework [101]. The combined results of UHV- 2.2 DEMOFs: Organic Linker Engineering FTIRS measurements using CO and CO as probe molecules and of accurate DFT calculations revealed that the structural The above discussion of MOF catalytic properties focuses and electronic properties of mixed-valence Ru–MOFs were mainly on reactant adsorption and reactive transformations much more complex than expected, and a straightforward occurring at CUS exposed at the metal ion nodes of the assignment of the observed IR bands was impossible. Two framework. However, the restricted specificity and the con- kinds of CO species bound to Ru cation sites were iden- fined coordination space of native CUS impose significant tified based on the isotopic substitution and temperature- limitations, and many MOF-types (e.g., layer-pillared MOFs 16 −1 dependent IR data (Fig. 3). The C O band at 2171 cm where Cu-paddlewheel-bound, 2D planes are stacked using, −1 18 (2120 cm for C O) was attributed to a weakly bonded e.g., bi-pyridine units) do not contain coordinatively unsatu- 16 −1 CO species, whereas the low-lying C O band at 2137 cm rated sites. In analogy to heterogeneous catalysts (e.g., metal −1 18 (2085 cm for C O) originated from a CO species with an oxides and supported metal NPs), where defects are known enhanced binding energy. A reasonable assignment, leading to be the active sites in catalysis [102], it is interesting to to the best-match between DFT calculations on both model also consider defects in MOFs. As shown in Fig. 2, reduced systems and experimental data, is based on the assumption Cu species were observed to occur as intrinsic defects (with that one of the Cl counter ions is transferred to a neighbor- a density of a few percent) in HKUST-1. ing paddle-wheel, forming an anionic secondary building Such defects should be absent in the perfect MOF lattice, unit blocked by two Cl counterions. As a consequence, the and, indeed, more sophisticated MOF fabrication methods II,III other positively charged paddle-wheel with a Ru dimer have been shown to reduce the concentrations of structural exposes two “free” CUS, where two different Ru–CO spe- imperfections [103], which can act as color centers [104]. cies could be formed with different frequencies and binding For “real” MOFs, however, the concentration of defects may Fig. 3 Temperature-dependent UHV-FTIR spectra obtained after and CO coordinated benzoate Ru-dimer paddle wheel systems as 16 18 exposing the clean Ru–MOF to a C O and b C O at 90 K and then models for local structures of Ru–MOF (atom coloring: ruthenium, 16 18 elevating the temperatures. a, b, (A) exposure to CO/ C O at 90 K; green; chlorine, magenta; carbon, black; oxygen, red; hydrogen, and heated to (B) 100 K, (C) 110 K, (D) 120 K, (E) 130 K, (F) 140 K, white). Reproduced with permission from Ref. [101]. Copyright 2013 (G) 150 K, (H) 160 K, (I) 170 K, (J) 180 K, and (K) 190 K. c Com- American Chemical Society puted model systems of CO adsorption for different scenarios of Cl 1 3 2206 Y. Wang, C. Wöll be sizeable, yielding different types of unsaturated sites numbers. Consequently, more CO molecules can adsorb to which strongly affect the chemical and physical properties of Cu mCUS with a higher binding energy with respect to 2+ 2+ these porous materials [105–112]. In this context, the inten- the parent Cu /Cu nodes, as confirmed by temperature- tional and controlled introduction of various defects into dependent IR analysis [33]. MOF frameworks is of great importance for rational design In addition to the local coordinatively undercoordinated of MOF materials with desired specific properties [33– 48]. metal sites created by integrating fragmented linkers with A particularly elegant way to introduce defects into MOFs one coupling unit missing (type A: mCUS, see Fig. 5), lattice is by use of “defective” linkers [33, 38, 45]. By adjusting defects corresponding to the complete absence of metal ions the concentration of these modified linkers relative to the (type B: node vacancies) could be formed by raising the dop- regular ones the defect concentration can be precisely tuned. ing concentration of defect linkers in DEMOFs. The coex- Such DEMOFs are of a more complex nature compared to istence of defects of both types A and B in Cu–DEMOFs the (more or less) “defect-free” reference materials due to was demonstrated by the combined UHV-FTIRS and XPS the structural heterogeneity resulting from the incorpora- approach [113]. The node vacancies are directly related to tion of the defect linkers or metal ions. A comprehensive the formation of functionalized mesopores, or large-scale experimental characterization in conjunction with theory is defects, in DEMOFs (Fig.  1) [33]. Hupp and coworkers required for a fundamental understanding of the defects in have reported that the introduction of node vacancies (type DEMOFs. In the case of HKUST-1 ([Cu btc ]; Cu-BTC), B defects) into HKUST-1 can finely tune the sorption prop- 3 2 Marx et al. demonstrated defect engineering of CUS via the erties of MOFs [114]. These results revealed the structural solvothermal synthesis with carefully chosen fragmented modulations in DEMOFs at two different length scales in a linkers [38]. In the resulting framework, the trivalent single step, which overcame restrictions of active site speci- 3− btc linker was partially replaced by divalent pyridine- ficity and the confined coordination spaces at the isolated, 2− 3,5-dicarboxylate (pydc ). Such linker substitution was single metal centers. expected to yield reduced Cu CUS at the defect-modified The multivariate nature of DEMOFs represents a new paddlewheel unit. However, the expected change of the oxi- dimension of tailoring functions. In catalysis, both pristine dation state for the copper species was not detected based on and mixed-linker Cu–MOFs showed high catalytic activity the XANES and EXAFS results in Ref. [38]. It is worth not- toward low-temperature hydroxylation of aromatic com- ing that the pristine HKUST-1, in the form of both powders pounds, while the product selectivity was significantly modi- 2− [33, 87] and thin films (SURMOFs) [88], features intrinsi- fied in the presence of pydc linkers [38]. We have further cally reduced Cu sites as the minority species, the amount investigated CO oxidation and alcohol oxidation reactions of which depends on the synthesis, oxidative or reductive within defect-engineered HKUST-1 by IR spectroscopy. treatment, and the activation conditions. Whereas the reduced Cu species are inactive for the low- Fischer and coauthors reported a series of defect-engi- temperature CO oxidation, methanol oxidation occurs at + 2+ neered HKUST-1 via systematic and controlled framework mixed-valence Cu /Cu metal nodes (mCUS), indicating incorporation of various types of defect linkers Lx (L1-L4, special catalytic properties of DEMOFs due to the coexist- see Fig. 4a) using the mixed-linker solid-solution approach ence of low-coordinated Cu CUS (i.e., more electron-rich 2− as a novel synthesis strategy [33]. To gain more detailed sites) and the adjacent defect linkers (pydc ) as functional- insights into the local environments of the mCUS, UHV- ized groups [33]. FTIR spectroscopy was employed to monitor the chemisorp- The same kind of defect engineering has been reported for tion and thermal desorption of CO on different DEMOFs the mixed-valence [Ru btc ] structural analogue of HKUST- 3 2 (Fig.  4). A large number of Cu-related CO bands were 1, where defects were introduced in a controlled manner by a observed which varied in shape and intensity depending on mixed-component (native btc and defect linkers) solid-solu- the nature and density of the defect linkers L1–L4. These tion approach to yield Ru–DEMOFs [36, 37]. The high-level findings revealed the existence of copper species with vari- spectroscopic characterization using primarily UHV-FTIRS ous chemical and structural environments. The assignment and X-ray based techniques (XPS, XANES, XRD) demon- of the CO vibrations was assisted by quantum mechanical/ strated the successful incorporation of various defect linkers molecular mechanical (QM/MM) calculations. The com- that, however, did not reduce the overall integrity and robust- bined results from UHV-FTIRS, XPS, and theory provided ness of the framework in a substantial way [36, 37]. Depend- solid evidence for the simultaneous and controllable modi- ing on the nature and concentration of fragmented linkers, fication of the electronic properties and the proximate coor - both defects of type A and type B (Fig. 5) were detected. δ+ dination space at the metal centers (mCUS) upon incorpo- The type A defects feature reduced R u (0 < δ < 2) sites at ration of defect linkers (see Fig.  4c) [33]. The doping of the modified metal nodes resulting from the fragmented or HKUST-1 with Lx led to the formation of reduced Cu / missing linkers. The formation of node vacancies (defects 2+ Cu paddlewheel units (nodes) with lowered coordination B) was facilitated by the doping with 5-X-isophthalic acids 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2207 + 2+ Fig. 4 A Defect Linker concept for DEMOFs: illustration of increas- mixed valence defect Cu /Cu (btc) (pydc) being typical for sample 2− 3− 2− ing defect degree by Lx incorporation, i.e. btc /Lx exchange. DEMOFs (L4) is shown and energetically feasible binding modes for The defect linkers L1–L3 were chosen as benzene-1,3-dicarboxylates one to three coordinated (adsorbed) CO molecules are given (only 2− with various functional groups at 5-position (L1: nidc , –NO ; L2: the QM system is shown for clarity, Cu, brown; C, black; O, red; N, 2− 2− cydc ; –CN; L3: h ydc , –OH) and L4 was pyridine-3,5-dicarbox- blue; H, white) together with computed (scaled) CO stretching nor- 2− −1 ylate (pydc ). B UHV-FTIR spectra obtained after exposing repre- mal mode frequencies (cm ). (e) For comparison the defect is shown sentative DEMOF samples ([Cu (btc) (Lx) ]) with different defect in a close-up, indicating the embedding of the QM system in the MM 3 2−d d linkers L1–L4 to various amounts of CO at 90  K. C QM/MM com- environment. Reproduced with permission from Ref. [33]. Copyright puted binding modes of CO. (a–d) The QM/MM model for a local 2014 American Chemical Society 1 3 2208 Y. Wang, C. Wöll work [36, 37] revealed that the Ru–DEMOFs, synthesized by controlled incorporation of different kinds of defect link - ers, exhibited unusual reactivity for the CO conversion to CO in a dark environment. This reaction does not occur at the “defect-free” Ru-HKUST-1, as supported by the UHV- FTIRS data (Fig.  6) which showed only C O -related IR −1 12 −1 13 bands at 2335 cm ( CO ) and 2272 cm ( CO ). How- 2 2 2− ever, the introduction of pydc defect linkers led to the −1 appearance of low-frequency bands at 2040 and 2000 cm δ+ assigned to CO bound to modified Ru CUS, which were accompanied by a gradual decrease of CO signals. These findings led to the conclusion that the low-temperature (90 K) conversion of C O to CO is driven by strong interac- δ+ tions with reduced R u CUS (i.e., more electron-rich sites) in Ru–DEMOFs. As a result, the enhanced charge transfer from Ru3d to the C O 2π antibonding orbital can facilitate 2 u δ− the formation of chemisorbed C O species that may act as an intermediate to finally give rise to CO. The reactivity Fig. 5 Scheme of different defect types in the [M (BTC) ] (HKUST- of Ru–DEMOFs toward the C O conversion depended not 3 2 1, M = Cu, Ru) family as well as the regular paddlewheel units. The δ+ only on the concentration of reduced Ru , but also on the purple sphere in defect type B stands for a vacancy of the missing nature of defect linkers. The pydc linkers further promoted metal node. Reproduced with permission from Ref. [113]. Copyright the activation of CO due to the presence of basic pyridyl 2017 John Wiley and Sons δ+ N sites in proximity to the reactive Ru mCUS (possible formation of pyridyl N O species [121]), while the 5-OH- (5-X-ip, X = OH, H, NH , Br) linkers where the functional ip-doped Ru–DEMOFs showed much lower reactivity as groups X are much smaller than carboxylate or non-coordi- compared to pydc-doped materials [37]. nating groups (e.g., H). The presence of type B defects in Furthermore, the pydc-modified Ru–DEMOFs, pre- Ru–DEMOFs led to enhanced porosity yielding mesopores, treated with hydrogen at 423 K, exhibited dramatically as was observed for Cu–DEMOFs (see Fig. 1) [33]. enhanced activity and selectivity for olefin hydrogenation MOF-based materials have shown great competency for versus the competing isomerization side reaction [36]. the photocatalytic or electrocatalytic reduction of CO to This outcome was attributed to the efficient formation of CO and other value-added chemicals [115–120]. Our recent Ru–H species via a heterolytic, base-assisted activation δ+ Fig. 6 a UHV-FTIR spectra (the regions of C O and CO vibrations) ing reduced and lower-coordinated Ru reactive centers and func- obtained after exposing the parent and defect-engineered Ru–MOFs tionalized defect linkers. Reproduced with permission from Ref. [36]. to CO at 90 K. b Structure of the defect-engineered Ru–MOFs show- Copyright 2014 John Wiley and Sons 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2209 of dihydrogen at the cooperative active centers including potential to be similarly modified in a controlled manner δ+ 2− reduced Ru mCUS and with the adjacent p ydc serving by the choice of the functionalized defect linkers. as a suitable base ligand. The Ru–H species were iden- tified by the characteristic vibrations at 1956–1975 and 2.3 Mixed‑Metal DEMOFs: Metal Node Engineering −1 2057–2076 cm [36]. The proposed reaction mechanism is described in Fig.  7, where the formation of Ru–H is Along with the linker modification in DEMOFs as discussed most likely the rate-determining step [36]. above, the chemical nature of MOF materials can also be The simultaneous presence of two types of defects A precisely tuned by partial metal substitution at framework and B markedly affected the catalytic activities of 5-X- nodes. The latter approach has been employed to synthe- ip (X = OH, H, NH , Br) engineered Ru–DEMOFs [37]. size mixed-metal DEMOFs with the partial substitution of δ+ 2+ 2+ The reduced Ru sites (type A) were responsible for the intrinsic Cu centers in HKUST-1 by Zn and other met- enhanced catalytic performance for ethylene dimerization als of 3d-row (Mn, Fe, Co) that have similar effective ionic 2+ due to the redox properties of Ru mCUS, as observed for radii and are thus closely related with Cu in coordination RhCa-X Zeolite catalysts [122]. Regarding the Paar–Knorr chemistry [123, 124]. The corresponding DEMOFs showed pyrrole synthesis, the Ru–DEMOFs showed much higher enhanced selective sorption of O resulting from the incor- catalytic activity for the conversion of phenylamine to poration of second metal ions into the framework [123]. The pyrrole, as compared to the parent Ru–MOF. Again, this doping with metals of the Pd group is of special interest transformation was attributed to the presence of type A due to their novel catalytic activity for numerous reactions. δ+ defects, where the reduced Ru mCUS (single active However, the introduction of metals of the 4d or 5d row is sites) are bound to the O atom of the carbonyl group and more challenging because the presence of these metal ions facilitate nucleophilic attack by the ion-pair of the amino makes 3D crystal formation difficult due to kinetic reasons group of the phenylamine [37]. Interestingly, an increase [125, 126]. in the density of incorporated 5-OH-ip led to a decrease Recently, the mixed metal Pd@[Cu Pdx(btc) ] with 3−x 2 n of the yield of pyrrole [37]. This result could be explained various levels of doping with Pd were obtained via one-pot in terms of the gradual dominance of defects B at higher synthesis [127]. The XPS results provided evidence for the 2+ doping levels; the formation of node vacancies eliminated simultaneous introduction of Pd -doped framework nodes δ+ 0 part of the reactive Ru centers, thus accounting for the and Pd NPs embedded into MOFs. As shown in Fig.  8, reduced catalytic activity of Ru–DEMOFs. three Pd 3d doublets (3d and 3d ) were resolved in the 5/2 3/2 Overall, our results demonstrated the controlled incor- deconvoluted Pd3d core-level spectra. The two doublets poration of various defect linkers into isoreticular Cu- and at 338.9/344.2 eV (Pd1) and 337.9/343.2 eV (Pd2) were 2+ Ru–MOFs (HKUST-1). The structural, electronic, and ascribed to Pd species, revealing the successful incorpo- 2+ reactive properties of Cu–DEMOFs and Ru–DEMOFs ration of Pd into the framework of Cu-HKUST-1 leading varied strongly depending on the density and nature of to the formation of Cu–Pd and/or Pd–Pd paddlewheels. The the fragmented linkers. Other [M (btc) ] compounds have doublet at 335.8/341.0 eV (Pd3) is characteristic for metallic 3 2 Fig. 7 Olefin hydrogena- tion involving base-assisted heterolytic splitting of H over defect-engineered Ru–MOFs. Note that the pydc linker in DEMOFs offers a basic pyridyl- N atom in the proximity of the reactive Ru centers. Reproduced with permission from Ref. [36]. Copyright 2014 John Wiley and Sons 1 3 2210 Y. Wang, C. Wöll Fig. 8 a Deconvoluted XPS data of Pd@[Cu Pd (btc) ] 3−x x 2 n MOFs with various doping levels of Pd in HKUST-1. b Simultaneous incorporation 2+ 2+ 0 of Pd /M nodes and Pd NPs dispersion into MOF. The 2+ Pd sites in such designed MOFs play an important role in enhancing the catalytic activity of the hydrogenation of p-nitrophenol with NaBH to p-aminophenol. Reproduced with permission from Ref. [127]. Copyright 2016 Royal Society of Chemistry Pd species; its relative concentration increased as the dop- predominantly responsible for the observed high catalytic ing level of Pd increased, indicating the simultaneous load- activity [127]. ing of Pd NPs into the framework. Along with the DEMOFs in HKUST-1 topology, intrin- 2+ 0 Both Pd -containing MOFs [128–130] and Pd @MOFs sic and intentionally created atomic-level defects in other [131–134] exhibited superior catalytic performance for typi- types of MOFs (e.g., Zr–MOFs (UiO-66, UiO-67) [45, 47, cal palladium-catalyzed reactions such as the Suzuki C–C 135–138], NU-125 [139], MOF-69 [35], MIL-53 [35]) have coupling and hydrogenation. The Pd@[Cu Pd (btc) ] been the subject of numerous experimental investigations. 3−x x 2 n 2+ MOFs featuring both incorporated Pd nodes and loaded The defects of type A (reduced metal centers with more open Pd NPs show substantially enhanced catalytic activity coordination environments) were generated either by a direct toward the aqueous-phase hydrogenation of p-nitrophenol synthesis strategy or by post-synthesis approaches. In addi- with NaBH to p-aminophenol, as contrasted with the pris- tion, Brozek and Dinca reported the fabrication of a series of tine Cu–MOFs (HKUST-1). Furthermore, it was evident mixed-metal MOF-5 analogues via isomorphous substitution 2+ 2+ that the Pd species play a key role in this reaction and are at the Zn O cornerstone (see Fig. 9), where the Zn species 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2211 varied depending on the quality of MOF thin films that could be controlled in a straightforward fashion [142]. As discussed above for MOF powders, the defect-engineered SURMOFs can be fabricated by controlled introduction of defects using different strategies such as fragmented linker incorporation [33, 38], or thermal treatment [39, 143]. It is expected that the intentional creation of defects inside SURMOFs has important consequences for tuning the electronic structure and catalytic properties of MOF materials. 2+ Fig. 9 Isomorphous substitution of Zn by other metal cations at the The lbl method can also be used to fabricate MOF mem- Zn O cornerstone of MOF-5. Reproduced with permission from Ref. branes. The ability to separate different molecules allows [140]. Copyright 2013 American Chemical Society the integration of size-exclusion principles to MOF-based catalysts. The first SURMOF-based monolithic membrane 2+ was fabricated by Shekhah et al. [144] and was reported to were partially replaced by metal cations with the same (Cr , 2+ 2+ 3+ 3+ 3+ Mn, Fe ) or different oxidation states (Ti, V, Cr be well-suited for the separation of small molecules, e.g., H, N CO, CO, CH , as well as other small hydrocarbons. with Cl as counter-ion) [140]. Furthermore, the approach 2 2, 2 4 of partial post-synthetic metal exchange was employed to A more recent study has demonstrated the use of MOF- based membranes for the natural gas purification and high- produce mixed Al/Fe-MIL-53, Zr/Ti-UiO-66 as well as Zr/ Hf-UiO-66 MOFs featuring mixed metal nodes in the frame- value industrial separations such as butane isomers [145]. Furthermore, it has been shown that the unique opportuni- work [141]. Overall, these different types of defects have been shown to account for the high reactivity of MOF cata- ties to functionalize MOFs permits numerous interesting, membrane-related functionalities, including membranes lysts for a number of catalytic reactions [40]. The controlled incorporation of MOFs with defects of different types and where the permeability and selectivity can be switched on by light [146, 147]. As shown in Fig. 11, the photoswitch- concentrations represents a novel approach for the predic- tive rational design of MOF-based single-site catalysts at able SURMOF membrane was fabricated by assembling linkers decorated with photoresponsive azobenzene-side- the atomic level. groups into the framework, where the precise control of the cis/trans azobenzene ratio by controlled irradiation times 3 MOF Thin Films Grown with ultraviolet or visible light allows for a continuous tun- ing of the separation of molecular mixtures [146]. Since this by the Layer‑by‑Layer Method approach can also be applied to fabricate large-area (larger than 20 cm × 20 cm) membranes [148], and also by apply- Employing the lbl or LPE method, established by Wöll and coworkers [59], to fabricate crystalline, monolithic MOF ing spray-methods, it is, in principle, suited for a continuous coating process. thin films, or SURMOFs [65] affords interesting oppor - tunities for MOF applications in catalysis. In addition to With respect to catalysis, a particularly interesting aspect of SURMOFs is the possibility to combine two different providing well-defined SURMOF substrates which can be applied for chemical transformations as observed for MOF types of chemically active MOFs into one multilayer thin film or membrane by using heteroepitaxy. This approach bulk powders, the lbl method can be used to introduce differ - ent types of defects in SURMOFs, e.g. at internal interfaces allows the creation of tandem catalysts where two different catalytically active components can be incorporated into one in hetero-multilayer structures [65]. In addition, interstitial sites can be created by loading guest species, including single, monolithic thin film (or membrane). The close proximity of two different catalytically active metal or oxide NPs or nanoclusters (NCs), inside the parent SURMOF materials. species which can be realized by a programmed lbl approach is important in the context of reaction cascades with short- An instructive example are HKUST-1 SURMOFs grown on an MHDA/Au substrates (see Fig. 10). The experimen- lived intermediates. In addition, a number of different cata- lysts may interfere, e.g., one catalyst affects the action of tal data from UHV-FTIRS and XPS were interpreted using electronic structure calculation and allowed to derive a the other, or leads to decomposition of the second catalyst. Avoiding these unwanted effects can be achieved by anchor - rather consistent picture [88]. The results showed consist- ently the presence of a small amount (~ 4%) of reduced ing the active species within a three-dimensional porous network. As opposed to just mixing the two catalysts in a Cu species in the pristine HKUST-1 thin film. Upon heat- ing, the temperature-induced creation of Cu defects was liquid, embedding the two different catalysts in a MOF will maintain their individual activities. clearly observed (Fig. 10). The density of intrinsic defects 1 3 2212 Y. Wang, C. Wöll Fig. 10 a Schematic drawing of HKUST-1 grown on an MHDA/ Au substrate. b XP spectra of HKUST-1 SURMOF at dif- ferent temperatures. c UHV- IRRAS spectra of CO adsorbed on activated HKUST-1 SURMOF at 110 K. The sample was annealed at 350 and 420 K. Reproduced with permission from Ref. [88]. Copyright 2012 John Wiley and Sons One of the first demonstrations of such tandem catalysts catalyzed by the Mn-porphyrin to yield epoxide as the realized by the lbl approach are porphyrin-based SUR- reactive intermediate. In a second step, the proximally MOFs. In the paper by Hupp and coworkers [149], a two- sited Zn-porphyrin facilitates the insertion of C O into component SURMOF was grown on a correspondingly the epoxide, giving rise to the cyclic carbonate as the final functionalized substrate (see Fig. 12). This hybrid system product (Fig. 12d). comprising two different metallo-porphyrins (ZnMn-RPM) The technological impact of such tandem catalysts can was shown to be active toward the epoxidation of olefin be improved substantially by realizing them in the form of substrates (Mn-porphyrin) and for epoxide opening (Zn- thin membranes through which the reactants are passed. porphyrin). Although this system could not yet be realized This technology will combine size-exclusion properties in the form of a membrane, this tandem SURMOF thin with catalytic activities. A particular advantage of the layer yielded catalytic turnover numbers which were sub- molecular framework-based approach for catalyst design stantially higher than the corresponding bulk MOF materi- is that the influence of the introduction of additional side- als [149]. The reaction studies in this case was a tandem groups to the MOF–ligands on the chemical activities can reaction consisting of first methoxy-styrene epoxidation be explored in a relatively straightforward fashion. In most 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2213 Fig. 11 a Schematic illustra- tion of tunable, remote-con- trollable molecular selectivity by a photoswitchable MOF membrane. b SEM cross- section image of the SURMOF membrane on the mesoporous a-Al O . c The structure of 2 3 Cu (AzoBPDC) (AzoBiPyB) 2 2 with the azobenzene groups. The transition between trans and cis states can be tuned by irradiation with 365 and 455 nm light, respectively. Reproduced with permission from Ref. [146]. Copyright 2016 Springer Nature cases, adding additional functionalities (e.g. OH, –NH , We would like to conclude this section on MOF thin films –CH , etc.) to a ligand will not change the overall structure by pointing out that SURMOFs are also well-suited to study of the MOF. transport phenomena occurring in these framework materi- This approach has been used to improve the efficiency als in a systematic fashion. One example are so-called sur- of a UiO-66 based catalyst for phosphate ester hydrolysis, face barriers, which are relevant for most porous materials. where a rate enhancement of up to 20 times was observed for By using a quartz-crystal microbalance (QCM) based setup, UiO-66 after modification by amino (NH ) moieties that act Heinke et al. could demonstrate that in case of HKUST-I, as a proton-transfer agent during the catalysis cycle [150]. these surface-barriers are not an intrinsic property of MOFs The particular advantage of MOFs in this context is the but result from surface imperfections originating from water- ability to integrate functional units into a porous framework induced corrosion [152]. material without changing the overall architecture of the MOF, which allows tuning of the chemical activity without affecting diffusivities, etc.. Additionally, the catalytic perfor - 4 Homochiral MOFs and Enantioselective mance of MOF materials could also be tuned via the incor- Asymmetric Catalysis poration of inorganic groups containing metal centers as side-groups [22]. This procedure was termed post-synthetic Kim and coworkers reported the first homochiral MOF metalation (PSMet) as the additional catalytically-active (POST-1) that catalyzes a transesterification reaction in an metal moieties can only be added post-synthetically [151]. enantioselective manner [153]. Since this seminal work, The same decoration strategies of MOF–ligands also apply homochiral MOFs have been extensively investigated with to MOF thin films, which enable the multifunctional proper - the aim to rationally fabricate and engineer MOFs mate- ties of SURMOFs to be adjusted in a controlled manner (see rials for heterogeneous asymmetric catalysis. These kinds e.g. the photoswitchable SURMOF membrane depicted in of MOFs can be obtained via distinct strategies such as Fig. 11 [146]). introduction of achiral active centers during synthesis, 1 3 2214 Y. Wang, C. Wöll Fig. 12 a The porphyrinic dipyridine pillar used in the construction of the ZnMn- RMP MOF containing a Mn atom, which can be used as an epoxidation catalyst, and b the porphyrinic tetracarboxylic acid strut used to make the 2D sheets of the ZnMn-RPM MOF containing a Zn atom, which can act as an epoxide-opening catalyst. c Crystallographically- derived representation of a unit cell of the ZnMn-RPM framework. d Schematic repre- sentation of tandem catalysis of ZnMn-RPM for the synthesis of cyclic carbonate. Reproduced with permission from Ref. [149]. Copyright 2016 John Wiley and Sons post-synthetic modification of homochiral MOFs, or incor - in Fig.  13, nanosized homochiral titanium oxo-clusters poration of asymmetric catalysts directly into the framework (Ti–MOCs) were embedded into HKUST-1 frameworks by [154–160]. The substantial potential of homochiral MOFs in using the LPE lbl method [170]. The resulting Ti–MOC@ enantioselective asymmetric catalytic reactions has been dis- HKUST-1 metacrystal was quite efficient regarding enan - cussed by a number of different groups [25, 155, 161–166]. tiomer recognition and separation. Although the combina- Tremendous efforts have been dedicated to homochiral tion of enantiomer-selectivity with catalytically activities MOF powder materials. However, investigations of the cor- in MOF thin films has not been explored, we consider the responding MOF membranes and thin films (SURMOFs) potential of this direction to be enormous, in particular, are few. Only recently, Wöll, Fischer and coworkers reported when combined with membranes. the fabrication of a series of enantiopure metal-camphorate frameworks (MCamFs) deposited on a quartz crystal micro- balance (QCM) substrate via an in situ LPE lbl approach by 5 Metal and Oxide Nanoparticles Embedded changing the metal nodes and/or linker molecules in succes- within MOFs sive deposition cycles [167–170]. Enantioselectivity with regard to the diffusion of different enantiomers into a MOF In the previous paragraphs, we have demonstrated the great thin film can be modulated by using linkers of different chi- potential of MOFs for catalytic performance and it is far rality [167, 168]. In addition, a thorough study of isoreticu- from being fully exploited. But even more possibilities exist lar chiral SURMOFs with identical stereogenic centers but to add further dimensions to the application of MOFs in different pore dimensions demonstrated that the pore sizes catalysis. A particularly important one is the impregnation must be adjusted to achieve the highest enantioselectivity of MOFs with NCs or NPs. Under mild conditions (tem- in chiral nanoporous materials [169]. Furthermore, chiral perature below 250–550 °C, depending on type of MOF), metal–organic nanoclusters (MOCs) can be loaded into the the porous molecular frameworks are sufficiently stable to achiral MOFs, again, achieving a high selectivity between prevent sintering or agglomeration of embedded, catalyti- the diffusivity of different enantiomers [170]. As shown cally active particles, the most severe problems encountered 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2215 inside MOFs accounted for the high catalytic activity for liq- uid-phase aerobic oxidation of alcohols (see Fig. 14d). The nano-sized bimetallic alloys are known to show enhanced catalytic performance in numerous reactions as compared to their monometallic counterparts. However, the exclusive encapsulation of bimetallic NPs with tunable com- positions into MOFs is challenging [174–179]. More recently, the embedding of core–shell PdPt and RuPt nano-alloys into Zr-based MOFs (UiO-66 and its derivatives) was realized by template synthesis [180]. The resulting bimetallic core–shell NPs exhibited substrate specic fi size-selectivity and signifi - cantly enhanced catalytic activity for the hydrogenation of nitrobenzene compared to pure Pt loaded UiO-66 [181]. The catalytic performance of MOF materials can also be tuned in a controllable way by encapsulation of vari- ous metal oxide NPs [182–185]. The surface structure and reactivity of nanostructured ZnO particles, embedded into ZIF-8 via chemical vapor infiltration followed by oxidative annealing, were characterized by UHV-FTIRS using C O as a probe molecule [185]. In contrast to pure ZnO NPs exposing mainly non-polar (10–10) surfaces, the confined ZnO NPs inside ZIF-8 were dominated by polar O–ZnO and Zn–ZnO facets as well as defect sites, which were highly reactive for CO activation. The isolated metal oxides (e.g., ZnO, T iO, Fe O ) stabilized within the MOF matrix 2 2 3 showed enhanced multifunctional (catalytic, magnetic, opti- cal) properties and have promising applications in catalysis, photocatalysis, and other fields such as electronic devices and sensors [182–185]. In comparison to MOF bulk powders, much less informa- tion is available for the loading of MOF thin films (SUR - Fig. 13 a Structure of the R–Ti–MOC clusters. b Schematic presenta- MOFs) with metal or metal oxide NPs. Recently, Wöll and tion of in  situ lbl growth of enantiopure Ti–MOC-loaded HKUST-1 coworkers reported the first fabrication of Bi O NPs encap- 2 3 thin film using LPE approach. Reproduced with permission from Ref. sulated into HKUST-1 thin films via a novel approach, in [170]. Copyright 2016 American Chemical Society which bismuth-triphenyl was used to create small bismuth oxide particles into the MOF pores [186]. Also in this case the size of the largest Bi O clusters slightly exceeded that when exploiting the high chemical activity of such small 2 3 particles for chemical transformations. of the MOF pores. The size distribution could be narrowed down substantially and at the same time shifted to lower Several strategies exist with regard to loading metal, metal oxide or other chemically active NPs into the MOFs. One of values by adding amino groups acting as nucleation centers to the MOF linkers. Without changing the MOF architecture, the first papers in this area used metal containing precursors to realize small Pd-clusters embedded in the MOF [171]. In this lattice constant, and topology, these additional amino groups acted as nucleation centers, thus achieving a much narrower case, the liberation of the metal atoms was achieved by either exposure to high pressures of H or by irradiation with light. size distribution. Such bismuth oxide particles are highly active in photocatalysis, as demonstrated by the photodegra- It is noteworthy that the palladium clusters were substantially larger than the pores of the MOFs (Fig. 14 case b) [171]. In a dation of nuclear fast red (NFR, C H NO SNa) dye [186]. 14 8 7 similar manner, Au NPs were loaded into different MOF mate- rials (e.g., ZIFs [172], MOF-5 [173]) and distributed homo- 6 SURMOFs and Electrocatalysis geneously over the MOF matrix matching with the cavities (Fig. 14 case c) as conr fi med by HRTEM observations. The Thin MOF films deposited on an electrode also exhibit size distribution and shape of embedded NPs was controlled by the framework structure and the functional groups at the interesting properties in electrochemistry and electroca- talysis [187, 188]. In particular, monolithic, pinhole-free linkers. The homogeneous distribution of confined Au NPs 1 3 2216 Y. Wang, C. Wöll Fig. 14 Top: three characteristic cases of microstructures for NPs narrow size distribution matching with the cavities and homogene- supported by MOFs. a Particles typically larger than the cavity size ously distributed over the volume of MOF. Bottom: d schematic view with a preferred anchoring close to the outer surface of the MOF. b of liquid phase alcohol oxidation with the Au@ZIF-8 material; Both Particles evenly distributed throughout the volume of the MOF crys- benzyl alcohol (BA) and methyl benzoate (MB) are able to access the tal but still exhibiting a broad size distribution with an average par- pores and can diffuse through the network. Reproduced with permis- ticle size exceeding the dimensions of the pores. c Particles with a sion from Ref. [172]. Copyright 2010 American Chemical Society SURMOFs, MOF thin films prepared using the lbl-process In addition to the demonstration of the suitability of SUR- (see Sect. 3, above) exhibited a great potential with respect MOFs for electrochemistry (see above), exciting properties to electrochemistry [189–191]. Although the application of toward photovoltaics (construction of MOF thin film based SURMOFs, both empty and after loading with electroactive solar cells [193, 194]) have been demonstrated. compounds such as ferrocene, has been quite successful, applications in electrocatalysis have been less common. In a recent paper by Liu, Wöll, Sun, and coworkers [192], 7 Summary and Outlook a monolithic, pinhole-free Re-based SURMOF was grown on a conductive, transparent substrate (fluorinated tin oxide, The selected examples of MOF chemical activity discussed or FTO) and exhibited well-defined electrochemical proper - in this short review clearly demonstrate the great potential ties. As demonstrated by the X-ray diffraction (XRD) data, of these porous framework materials for catalysis. Active the MOF films were highly oriented, with the [001] direc- sites can be introduced into these crystalline coordination tion perpendicular to the substrate. These SURMOFs were networks using a broad variety of strategies, and MOFs can shown to be highly effective in the electrocatalytic conver - be used for catalysis either as powders or in the form of sion of CO to CO. The faradaic efficiency found in these thin films (SURMOFs), with the possibility to also fabricate experiments was astonishingly high, amounting to 93 ± 5%. catalytically active membranes. The combined experimental Furthermore, the current densities which could be achieved and theoretical results presented above for selected systems for these high-quality monolithic coatings were found to provided deep, fundamental insights into the structural, elec- −2 be larger than 2  mA  cm , thus exceeding the densities tronic and reactive properties of active sites in pristine and recorded for MOF thin films prepared using other methods defect-engineered MOFs (DEMOFs). Depending on the sys- by at least one order of magnitude. tem, isolated, coordinatively unsaturated metal sites (CUS) Although few applications have so far been reported are either an intrinsic component of the perfect system or for photo-electrocatalysis of MOF thin films (e.g., MOFs can be introduced by e.g. defect engineering or by loading as photosensitizers on T iO nanowires for water splitting with suitable guest species, thus creating the potential to fab- [193]), we also foresee enormous potential in this direction. ricate single-site catalysts [22]. The catalytic performance of 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2217 CuI[4,4′,4″,4‴-tetracyanotetraphenylmethane]BF .xC H NO . MOFs can be significantly enhanced by tailoring of organic 4 6 5 2 J Am Chem Soc 112:1546–1554 linkers and/or metal cations via defect-engineering strategies 2. Kitagawa S, Matsuyama S, Munakata M, Emori T (1991) in a controlled manner. 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Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks

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Chemistry; Catalysis; Industrial Chemistry/Chemical Engineering; Organometallic Chemistry; Physical Chemistry
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Abstract

Keywords Metal–organic frameworks · Thin films · Single-site catalysts · Infrared spectroscopy · Defects · Active sites 1 Introduction Metal–organic frameworks (MOFs, also known as porous * Yuemin Wang coordination polymers or PCPs) are an emerging class of yuemin.wang@kit.edu porous materials of a hybrid organic/inorganic nature that * Christof Wöll combine the properties of both organic and inorganic porous christof.woell@kit.edu materials [1–7]. MOFs are typically stable up to tempera- Institute of Functional Interfaces, Karlsruhe Institute tures above 250 °C (in some cases, e.g., ZIF-8 [8], maximum of Technology, 76344 Eggenstein-Leopoldshafen, Germany Vol.:(0123456789) 1 3 2202 Y. Wang, C. Wöll temperatures as high as 550  °C can be tolerated). They vacancies, play a crucial role in the catalytic process [32]. exhibit a high degree of crystallinity and have large surface Recently, it has become evident that MOFs may feature a areas. The maximum degree of porosity and the size of the variety of intrinsic structural defects and the correspond- pores clearly exceed those of zeolites. In early 2017, the ing (intentional) defect engineering is a powerful strategy number of characterised MOFs was estimated to be 70,000 for advanced control of MOF chemical properties (Fig. 1) [9]. [33, 34]. The defect engineered MOFs (DEMOFs), synthe- Due to these unique chemical and physical properties sized by tailoring of linkers and/or metal ions, have shown MOF materials have opened up new perspectives in a variety modified physical and chemical properties [33– 48]. Both of different fields, ranging from gas storage and separation to local, isolated defects [modified CUS (mCUS)] and large- chemical sensing, catalysis, and drug delivery [10–21]. With scale defects (e.g. missing nodes, mesopores, Fig. 1) can be respect to applications in heterogeneous catalysis, different formed in DEMOFs and these defects can have important aspects must be considered. First, after adding additional consequences for their applications in catalysis. Further- coupling units (e.g., carboxylic acids or pyridine-units), more, MOF materials can serve as a host matrix (support) molecules active in homogeneous catalysis can be incorpo- for loading of catalytic components such as metal or metal rated into MOFs, thus providing a strategy to unite homo- oxide nanoparticles. These confined NPs inside MOFs may geneous and heterogeneous catalysis. Second, even without show unique properties which are significantly different further functionalization, many MOFs already show inter- from conventional supported catalysts. esting catalytic properties originating from the presence of In addition to MOF powders, the standard form of MOFs coordinatively unsaturated metal sites (CUS) at the nodes. obtained by the conventional solvothermal synthesis, a num- Such CUS sites are reactive for various chemical reactions. ber of methods have been developed to fabricate monolithic, A particular advantage of such sites in MOFs is that, com- crystalline, and highly oriented MOF thin films (SURMOFs) pared to oxide-supported metal nanoparticles (NPs), these [49–58]. Among them, the kinetically controlled layer-by- metal cations are highly dispersed within the framework of layer (lbl) or liquid phase epitaxy (LPE) growth process MOFs and thus serve as isolated, “single atom” catalytic has been extensively applied to produce well-defined SUR - sites. Accordingly, the interest in porous MOF materials as MOFs with a high degree of crystalline order, both vertical potential catalysts (in particular single-site catalysts [22]) and parallel to the substrate surface [59–64]. The highly- has intensified [22– 31]. ordered SURMOFs not only retain the intrinsic properties While in some MOFs CUS-sites are an intrinsic property of the corresponding MOFs, but also allow the design of of the MOF lattice itself, in other cases such undercoordi- architectures that cannot be achieved using MOF powders nated sites are absent in the ideal structure and only occur and to supplement their applicability as chemical sensors, as a result of defect formation within the framework mate- smart membranes, electronic and optoelectronic devices, rial. In the case of heterogeneous catalysis, e.g., on oxide as well as in catalytic coatings [65–68]. More importantly, surfaces, it is well known that defect sites, such as oxygen SURMOFs deposited on conducting substrates (e.g., Au) Fig. 1 Defect-engineered MOFs (DEMOFs). The modulation of the represent perfect and defect metal sites, respectively; the green high- defect structure on the micro and meso-scale by defect linker doping lighted unit indicates the parent micropores. Reproduced with per- of the framework is shown. The blue and short red sticks represent mission from Ref. [33]. Copyright 2014 American Chemical Society perfect and defective linkers, respectively; the yellow and black balls 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2203 can serve as model systems for a thorough study of MOF structure characterization of catalysts. However, a reliable properties by employing virtually all surface-sensitive tech- characterization of metal species in MOFs, and particularly niques developed in Surface Science because the charging those at defect sites in DEMOFs, is a challenge. Characteri- problems often hampering the analysis of bulk MOF sam- zation with electron microscopy, for example, is extremely ples are largely reduced. difficult since MOF materials contain organic ligands Overall, the extremely high structural and compositional which can be easily damaged by the high energy electrons design ability of MOF materials holds promise for their employed in TEM. Only very recently, Zhang et al. [83] have application in catalysis, including photocatalysis and electro- reported the high-quality atomic-resolution TEM images catalysis. The reactive properties of MOFs can vary signifi- of UiO-66 by developing a suite of methods to overcome cantly depending on the modification of both metal centers the experimental obstacles. Alternatively, MOFs can be and organic linkers as well as loading of metal/oxide NPs. characterized by spectroscopic techniques such as electron In this short report, we will highlight recent advances paramagnetic resonance, X-ray spectroscopy (XPS, EXAFS, in the field of MOF catalytic applications, with the main XANES) and infrared spectroscopy (FTIR). Furthermore, emphasis being on isolated, single active sites in perfect theoretical modeling of MOFs is required to properly inter- and defect-engineered MOFs, as well as for MOF thin films pret the experimental results. and SURMOFs. We will focus on structural and electronic properties as well as chemical reactivity of the selected MOF 2.1 Pristine MOFs systems based on fundamental investigations conducted primarily by our group and our collaboration partners. For The Cu-based HKUST-1 ([Cu btc ], btc = benzene-1,3,5- 3 2 other more general information about applications of MOFs tricarboxylate) is a prototypical MOF and has been used in catalysis and other fields we refer the reader to numerous in many studies [84]. This MOF contains so-called paddle- excellent review papers published recently (see e.g., [22, wheel units, where 4 carboxylate groups are bound to a 65–82]). Cu -dimer. In this interesting catalytic unit, the metal ions We will initially discuss the chemical reactions catalyzed are under-coordinated and can bind additional molecular by perfect and defect-engineered MOFs (e.g., Cu-HKUST-1, species (e.g., CO, H O, pyridine) at the axial position of the Ru-HKUST-1) using a combined experimental and theoreti- paddle-wheel units. Indeed, the CO oxidation reaction can cal approach. This section will be followed by a brief review be catalyzed by HKUST-1 at ambient pressure and elevated of the chemical nature and catalytic activity of SURMOF temperatures [85, 86]. In order to gain deeper insights into thin films. The next sections will focus on homochiral MOFs the active sites and reaction mechanisms, this system was as well as on metal and metal–oxide NPs embedded inside systematically investigated by using high-resolution ultra- MOFs. Finally, the enormous potential of SURMOF-based high vacuum infrared spectroscopy (UHV-FTIRS) in con- materials with respect to electrocatalysis will be highlighted. junction with density functional theory (DFT) calculations [87]. The sophisticated UHV-FTIRS apparatus combines a vacuum FTIR spectrometer (Bruker Vertex 80v) with a 2 Coordinatively Undercoordinated multi-chamber UHV system (Prevac), which not only ena- Single Active Sites in Pristine bles in situ transmission IR experiments on MOF powders and Defect‑Engineered MOFs (DEMOFs) supported on an inert metal mesh, but also allows the record- ing of IR reflection absorption spectroscopy (IRRAS) data To date, MOF materials have been extensively investigated, using a grazing incidence geometry on SURMOF thin films. and tremendous effort has been dedicated to their synthesis, This methodology has been demonstrated to be an invaluable properties and applications. However, a thorough atomic- tool to monitor chemical and photochemical reactions on the level understanding of structural and electronic properties surface of metal oxide powders [32]. 2+ −1 of the isolated, single active metal sites (CUS and mCUS) As shown in Fig. 2, the intrinsic Cu CUS (2179 cm ) as well as the structure–activity relationship continues to be was identified as the predominant species in HKUST-1, a major challenge; many crucial issues remain unanswered. while a small amount of native Cu defects were detected −1 This lack of information is due to the great complexity of as minor species (2125 cm , a few percent), in line with nanostructured MOFs, especially for DEMOFs in the pres- the observation for HKUST-1 thin films (see below in the ence of the different types of defects described below. A section of SURMOFs) [88]. Upon exposure to O , the UHV- comprehensive and fundamental understanding of active FTIRS data provide direct spectroscopic evidence for a sur- metal sites requires state-of-the-art analytical techniques prisingly high catalytic activity of Cu–MOFs (HKUST-1 that are suitable to probe the local chemical environments. and MOF-14, [Cu btb ], btb = 1,2,3-benzenetrisbenzoate). 3 2 Microscopic techniques [e.g., high-resolution transmis- Clearly, the presence of dioxygen leads to CO oxidation sion electron microscopy (HRTEM)] are powerful tools for even at temperatures as low as 105 K. The spectroscopic 1 3 2204 Y. Wang, C. Wöll Fig. 2 a UHV-FTIR spectra obtained after exposing [Cu btc ] cules adsorb on opposing paddle-wheel units in the iso configuration; 2 3 (HKUST-1) first to CO and subsequently to O at 105  K for differ -the O molecule is located (symmetrically) between these fragments 2 2 ent times (top). Intensity of the IR bands as a function of time for (top). Resulting structure: two C O molecules adsorb on different different CO species in the presence of molecular oxygen at 105  K paddle-wheel units (bottom). Reproduced with permission from Ref. (bottom). b Structure of HKUST-1. c Schematic representation of the [87]. Copyright 2012 John Wiley and Sons hypothetical reaction mechanism. Starting structure: two CO mole- information demonstrated that this reaction takes place only very interesting. Such modifications, of the general type 2+ on the intrinsic Cu CUS, whereas, rather unexpectedly, [M (btc) ], can be obtained by incorporating Mo [94], 3 2 the Cu defect sites are inactive for the low-temperature CO Cr [95], or Zn [96] into the parent MOF structure. These oxidation. On the basis of the high-level quantum chemical HKUST-1 analogues possess coordinatively unsaturated calculations, a concerted mechanism was proposed, whereby metal dimers at axial positions of paddle-wheel units where the impinging O molecule is activated in the presence of adsorption and chemical reactions may take place. More pre-adsorbed CO and interacts simultaneously with two iso- challenging is the synthesis of HKUST-1 analogues where carbonyl species at neighboring CUS to yield two C O mol- the central pair consists of metal ions in different charge ecules (see Fig. 2). Notably, this mechanism differs entirely states, thus yielding heterovalent paddle-wheel units. Such from those reported to occur in the case of CO oxidation mixed-valence MOFs are expected to show redox activities on metal oxides or on oxide-supported metal NPs (see e.g., [97, 98] and electric conducting properties [99, 100] that [89–93]). could be applicable to a porous electrode for batteries, fuel From a catalytic point of view, isostructural MOF vari- cells, capacitors, etc. ants of HKUST-1 where Cu is replaced by other metals are 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2205 In contrast to all other known [M (btc) ] frameworks that energies (see Fig.  3). These findings suggest that similar 3 2 II,II II,III are based on M units, the Ru–MOF, [Ru (btc) Cl ], complexity must be taken into explicit account for many 2 3 2 1.5 II,III exhibits mixed-valence R u paddle-wheel units with two other mixed valence MOFs with intrinsically low-coordi- 2+ 3+ differently charged metal centers (Ru and Ru ), which nated metal sites (CUS), which has not yet been accom- are stabilized by additional counter ions Cl (or in general plished in accurate detail. − − other X species such as OH ) to obtain an overall charge- neutral framework [101]. The combined results of UHV- 2.2 DEMOFs: Organic Linker Engineering FTIRS measurements using CO and CO as probe molecules and of accurate DFT calculations revealed that the structural The above discussion of MOF catalytic properties focuses and electronic properties of mixed-valence Ru–MOFs were mainly on reactant adsorption and reactive transformations much more complex than expected, and a straightforward occurring at CUS exposed at the metal ion nodes of the assignment of the observed IR bands was impossible. Two framework. However, the restricted specificity and the con- kinds of CO species bound to Ru cation sites were iden- fined coordination space of native CUS impose significant tified based on the isotopic substitution and temperature- limitations, and many MOF-types (e.g., layer-pillared MOFs 16 −1 dependent IR data (Fig. 3). The C O band at 2171 cm where Cu-paddlewheel-bound, 2D planes are stacked using, −1 18 (2120 cm for C O) was attributed to a weakly bonded e.g., bi-pyridine units) do not contain coordinatively unsatu- 16 −1 CO species, whereas the low-lying C O band at 2137 cm rated sites. In analogy to heterogeneous catalysts (e.g., metal −1 18 (2085 cm for C O) originated from a CO species with an oxides and supported metal NPs), where defects are known enhanced binding energy. A reasonable assignment, leading to be the active sites in catalysis [102], it is interesting to to the best-match between DFT calculations on both model also consider defects in MOFs. As shown in Fig. 2, reduced systems and experimental data, is based on the assumption Cu species were observed to occur as intrinsic defects (with that one of the Cl counter ions is transferred to a neighbor- a density of a few percent) in HKUST-1. ing paddle-wheel, forming an anionic secondary building Such defects should be absent in the perfect MOF lattice, unit blocked by two Cl counterions. As a consequence, the and, indeed, more sophisticated MOF fabrication methods II,III other positively charged paddle-wheel with a Ru dimer have been shown to reduce the concentrations of structural exposes two “free” CUS, where two different Ru–CO spe- imperfections [103], which can act as color centers [104]. cies could be formed with different frequencies and binding For “real” MOFs, however, the concentration of defects may Fig. 3 Temperature-dependent UHV-FTIR spectra obtained after and CO coordinated benzoate Ru-dimer paddle wheel systems as 16 18 exposing the clean Ru–MOF to a C O and b C O at 90 K and then models for local structures of Ru–MOF (atom coloring: ruthenium, 16 18 elevating the temperatures. a, b, (A) exposure to CO/ C O at 90 K; green; chlorine, magenta; carbon, black; oxygen, red; hydrogen, and heated to (B) 100 K, (C) 110 K, (D) 120 K, (E) 130 K, (F) 140 K, white). Reproduced with permission from Ref. [101]. Copyright 2013 (G) 150 K, (H) 160 K, (I) 170 K, (J) 180 K, and (K) 190 K. c Com- American Chemical Society puted model systems of CO adsorption for different scenarios of Cl 1 3 2206 Y. Wang, C. Wöll be sizeable, yielding different types of unsaturated sites numbers. Consequently, more CO molecules can adsorb to which strongly affect the chemical and physical properties of Cu mCUS with a higher binding energy with respect to 2+ 2+ these porous materials [105–112]. In this context, the inten- the parent Cu /Cu nodes, as confirmed by temperature- tional and controlled introduction of various defects into dependent IR analysis [33]. MOF frameworks is of great importance for rational design In addition to the local coordinatively undercoordinated of MOF materials with desired specific properties [33– 48]. metal sites created by integrating fragmented linkers with A particularly elegant way to introduce defects into MOFs one coupling unit missing (type A: mCUS, see Fig. 5), lattice is by use of “defective” linkers [33, 38, 45]. By adjusting defects corresponding to the complete absence of metal ions the concentration of these modified linkers relative to the (type B: node vacancies) could be formed by raising the dop- regular ones the defect concentration can be precisely tuned. ing concentration of defect linkers in DEMOFs. The coex- Such DEMOFs are of a more complex nature compared to istence of defects of both types A and B in Cu–DEMOFs the (more or less) “defect-free” reference materials due to was demonstrated by the combined UHV-FTIRS and XPS the structural heterogeneity resulting from the incorpora- approach [113]. The node vacancies are directly related to tion of the defect linkers or metal ions. A comprehensive the formation of functionalized mesopores, or large-scale experimental characterization in conjunction with theory is defects, in DEMOFs (Fig.  1) [33]. Hupp and coworkers required for a fundamental understanding of the defects in have reported that the introduction of node vacancies (type DEMOFs. In the case of HKUST-1 ([Cu btc ]; Cu-BTC), B defects) into HKUST-1 can finely tune the sorption prop- 3 2 Marx et al. demonstrated defect engineering of CUS via the erties of MOFs [114]. These results revealed the structural solvothermal synthesis with carefully chosen fragmented modulations in DEMOFs at two different length scales in a linkers [38]. In the resulting framework, the trivalent single step, which overcame restrictions of active site speci- 3− btc linker was partially replaced by divalent pyridine- ficity and the confined coordination spaces at the isolated, 2− 3,5-dicarboxylate (pydc ). Such linker substitution was single metal centers. expected to yield reduced Cu CUS at the defect-modified The multivariate nature of DEMOFs represents a new paddlewheel unit. However, the expected change of the oxi- dimension of tailoring functions. In catalysis, both pristine dation state for the copper species was not detected based on and mixed-linker Cu–MOFs showed high catalytic activity the XANES and EXAFS results in Ref. [38]. It is worth not- toward low-temperature hydroxylation of aromatic com- ing that the pristine HKUST-1, in the form of both powders pounds, while the product selectivity was significantly modi- 2− [33, 87] and thin films (SURMOFs) [88], features intrinsi- fied in the presence of pydc linkers [38]. We have further cally reduced Cu sites as the minority species, the amount investigated CO oxidation and alcohol oxidation reactions of which depends on the synthesis, oxidative or reductive within defect-engineered HKUST-1 by IR spectroscopy. treatment, and the activation conditions. Whereas the reduced Cu species are inactive for the low- Fischer and coauthors reported a series of defect-engi- temperature CO oxidation, methanol oxidation occurs at + 2+ neered HKUST-1 via systematic and controlled framework mixed-valence Cu /Cu metal nodes (mCUS), indicating incorporation of various types of defect linkers Lx (L1-L4, special catalytic properties of DEMOFs due to the coexist- see Fig. 4a) using the mixed-linker solid-solution approach ence of low-coordinated Cu CUS (i.e., more electron-rich 2− as a novel synthesis strategy [33]. To gain more detailed sites) and the adjacent defect linkers (pydc ) as functional- insights into the local environments of the mCUS, UHV- ized groups [33]. FTIR spectroscopy was employed to monitor the chemisorp- The same kind of defect engineering has been reported for tion and thermal desorption of CO on different DEMOFs the mixed-valence [Ru btc ] structural analogue of HKUST- 3 2 (Fig.  4). A large number of Cu-related CO bands were 1, where defects were introduced in a controlled manner by a observed which varied in shape and intensity depending on mixed-component (native btc and defect linkers) solid-solu- the nature and density of the defect linkers L1–L4. These tion approach to yield Ru–DEMOFs [36, 37]. The high-level findings revealed the existence of copper species with vari- spectroscopic characterization using primarily UHV-FTIRS ous chemical and structural environments. The assignment and X-ray based techniques (XPS, XANES, XRD) demon- of the CO vibrations was assisted by quantum mechanical/ strated the successful incorporation of various defect linkers molecular mechanical (QM/MM) calculations. The com- that, however, did not reduce the overall integrity and robust- bined results from UHV-FTIRS, XPS, and theory provided ness of the framework in a substantial way [36, 37]. Depend- solid evidence for the simultaneous and controllable modi- ing on the nature and concentration of fragmented linkers, fication of the electronic properties and the proximate coor - both defects of type A and type B (Fig. 5) were detected. δ+ dination space at the metal centers (mCUS) upon incorpo- The type A defects feature reduced R u (0 < δ < 2) sites at ration of defect linkers (see Fig.  4c) [33]. The doping of the modified metal nodes resulting from the fragmented or HKUST-1 with Lx led to the formation of reduced Cu / missing linkers. The formation of node vacancies (defects 2+ Cu paddlewheel units (nodes) with lowered coordination B) was facilitated by the doping with 5-X-isophthalic acids 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2207 + 2+ Fig. 4 A Defect Linker concept for DEMOFs: illustration of increas- mixed valence defect Cu /Cu (btc) (pydc) being typical for sample 2− 3− 2− ing defect degree by Lx incorporation, i.e. btc /Lx exchange. DEMOFs (L4) is shown and energetically feasible binding modes for The defect linkers L1–L3 were chosen as benzene-1,3-dicarboxylates one to three coordinated (adsorbed) CO molecules are given (only 2− with various functional groups at 5-position (L1: nidc , –NO ; L2: the QM system is shown for clarity, Cu, brown; C, black; O, red; N, 2− 2− cydc ; –CN; L3: h ydc , –OH) and L4 was pyridine-3,5-dicarbox- blue; H, white) together with computed (scaled) CO stretching nor- 2− −1 ylate (pydc ). B UHV-FTIR spectra obtained after exposing repre- mal mode frequencies (cm ). (e) For comparison the defect is shown sentative DEMOF samples ([Cu (btc) (Lx) ]) with different defect in a close-up, indicating the embedding of the QM system in the MM 3 2−d d linkers L1–L4 to various amounts of CO at 90  K. C QM/MM com- environment. Reproduced with permission from Ref. [33]. Copyright puted binding modes of CO. (a–d) The QM/MM model for a local 2014 American Chemical Society 1 3 2208 Y. Wang, C. Wöll work [36, 37] revealed that the Ru–DEMOFs, synthesized by controlled incorporation of different kinds of defect link - ers, exhibited unusual reactivity for the CO conversion to CO in a dark environment. This reaction does not occur at the “defect-free” Ru-HKUST-1, as supported by the UHV- FTIRS data (Fig.  6) which showed only C O -related IR −1 12 −1 13 bands at 2335 cm ( CO ) and 2272 cm ( CO ). How- 2 2 2− ever, the introduction of pydc defect linkers led to the −1 appearance of low-frequency bands at 2040 and 2000 cm δ+ assigned to CO bound to modified Ru CUS, which were accompanied by a gradual decrease of CO signals. These findings led to the conclusion that the low-temperature (90 K) conversion of C O to CO is driven by strong interac- δ+ tions with reduced R u CUS (i.e., more electron-rich sites) in Ru–DEMOFs. As a result, the enhanced charge transfer from Ru3d to the C O 2π antibonding orbital can facilitate 2 u δ− the formation of chemisorbed C O species that may act as an intermediate to finally give rise to CO. The reactivity Fig. 5 Scheme of different defect types in the [M (BTC) ] (HKUST- of Ru–DEMOFs toward the C O conversion depended not 3 2 1, M = Cu, Ru) family as well as the regular paddlewheel units. The δ+ only on the concentration of reduced Ru , but also on the purple sphere in defect type B stands for a vacancy of the missing nature of defect linkers. The pydc linkers further promoted metal node. Reproduced with permission from Ref. [113]. Copyright the activation of CO due to the presence of basic pyridyl 2017 John Wiley and Sons δ+ N sites in proximity to the reactive Ru mCUS (possible formation of pyridyl N O species [121]), while the 5-OH- (5-X-ip, X = OH, H, NH , Br) linkers where the functional ip-doped Ru–DEMOFs showed much lower reactivity as groups X are much smaller than carboxylate or non-coordi- compared to pydc-doped materials [37]. nating groups (e.g., H). The presence of type B defects in Furthermore, the pydc-modified Ru–DEMOFs, pre- Ru–DEMOFs led to enhanced porosity yielding mesopores, treated with hydrogen at 423 K, exhibited dramatically as was observed for Cu–DEMOFs (see Fig. 1) [33]. enhanced activity and selectivity for olefin hydrogenation MOF-based materials have shown great competency for versus the competing isomerization side reaction [36]. the photocatalytic or electrocatalytic reduction of CO to This outcome was attributed to the efficient formation of CO and other value-added chemicals [115–120]. Our recent Ru–H species via a heterolytic, base-assisted activation δ+ Fig. 6 a UHV-FTIR spectra (the regions of C O and CO vibrations) ing reduced and lower-coordinated Ru reactive centers and func- obtained after exposing the parent and defect-engineered Ru–MOFs tionalized defect linkers. Reproduced with permission from Ref. [36]. to CO at 90 K. b Structure of the defect-engineered Ru–MOFs show- Copyright 2014 John Wiley and Sons 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2209 of dihydrogen at the cooperative active centers including potential to be similarly modified in a controlled manner δ+ 2− reduced Ru mCUS and with the adjacent p ydc serving by the choice of the functionalized defect linkers. as a suitable base ligand. The Ru–H species were iden- tified by the characteristic vibrations at 1956–1975 and 2.3 Mixed‑Metal DEMOFs: Metal Node Engineering −1 2057–2076 cm [36]. The proposed reaction mechanism is described in Fig.  7, where the formation of Ru–H is Along with the linker modification in DEMOFs as discussed most likely the rate-determining step [36]. above, the chemical nature of MOF materials can also be The simultaneous presence of two types of defects A precisely tuned by partial metal substitution at framework and B markedly affected the catalytic activities of 5-X- nodes. The latter approach has been employed to synthe- ip (X = OH, H, NH , Br) engineered Ru–DEMOFs [37]. size mixed-metal DEMOFs with the partial substitution of δ+ 2+ 2+ The reduced Ru sites (type A) were responsible for the intrinsic Cu centers in HKUST-1 by Zn and other met- enhanced catalytic performance for ethylene dimerization als of 3d-row (Mn, Fe, Co) that have similar effective ionic 2+ due to the redox properties of Ru mCUS, as observed for radii and are thus closely related with Cu in coordination RhCa-X Zeolite catalysts [122]. Regarding the Paar–Knorr chemistry [123, 124]. The corresponding DEMOFs showed pyrrole synthesis, the Ru–DEMOFs showed much higher enhanced selective sorption of O resulting from the incor- catalytic activity for the conversion of phenylamine to poration of second metal ions into the framework [123]. The pyrrole, as compared to the parent Ru–MOF. Again, this doping with metals of the Pd group is of special interest transformation was attributed to the presence of type A due to their novel catalytic activity for numerous reactions. δ+ defects, where the reduced Ru mCUS (single active However, the introduction of metals of the 4d or 5d row is sites) are bound to the O atom of the carbonyl group and more challenging because the presence of these metal ions facilitate nucleophilic attack by the ion-pair of the amino makes 3D crystal formation difficult due to kinetic reasons group of the phenylamine [37]. Interestingly, an increase [125, 126]. in the density of incorporated 5-OH-ip led to a decrease Recently, the mixed metal Pd@[Cu Pdx(btc) ] with 3−x 2 n of the yield of pyrrole [37]. This result could be explained various levels of doping with Pd were obtained via one-pot in terms of the gradual dominance of defects B at higher synthesis [127]. The XPS results provided evidence for the 2+ doping levels; the formation of node vacancies eliminated simultaneous introduction of Pd -doped framework nodes δ+ 0 part of the reactive Ru centers, thus accounting for the and Pd NPs embedded into MOFs. As shown in Fig.  8, reduced catalytic activity of Ru–DEMOFs. three Pd 3d doublets (3d and 3d ) were resolved in the 5/2 3/2 Overall, our results demonstrated the controlled incor- deconvoluted Pd3d core-level spectra. The two doublets poration of various defect linkers into isoreticular Cu- and at 338.9/344.2 eV (Pd1) and 337.9/343.2 eV (Pd2) were 2+ Ru–MOFs (HKUST-1). The structural, electronic, and ascribed to Pd species, revealing the successful incorpo- 2+ reactive properties of Cu–DEMOFs and Ru–DEMOFs ration of Pd into the framework of Cu-HKUST-1 leading varied strongly depending on the density and nature of to the formation of Cu–Pd and/or Pd–Pd paddlewheels. The the fragmented linkers. Other [M (btc) ] compounds have doublet at 335.8/341.0 eV (Pd3) is characteristic for metallic 3 2 Fig. 7 Olefin hydrogena- tion involving base-assisted heterolytic splitting of H over defect-engineered Ru–MOFs. Note that the pydc linker in DEMOFs offers a basic pyridyl- N atom in the proximity of the reactive Ru centers. Reproduced with permission from Ref. [36]. Copyright 2014 John Wiley and Sons 1 3 2210 Y. Wang, C. Wöll Fig. 8 a Deconvoluted XPS data of Pd@[Cu Pd (btc) ] 3−x x 2 n MOFs with various doping levels of Pd in HKUST-1. b Simultaneous incorporation 2+ 2+ 0 of Pd /M nodes and Pd NPs dispersion into MOF. The 2+ Pd sites in such designed MOFs play an important role in enhancing the catalytic activity of the hydrogenation of p-nitrophenol with NaBH to p-aminophenol. Reproduced with permission from Ref. [127]. Copyright 2016 Royal Society of Chemistry Pd species; its relative concentration increased as the dop- predominantly responsible for the observed high catalytic ing level of Pd increased, indicating the simultaneous load- activity [127]. ing of Pd NPs into the framework. Along with the DEMOFs in HKUST-1 topology, intrin- 2+ 0 Both Pd -containing MOFs [128–130] and Pd @MOFs sic and intentionally created atomic-level defects in other [131–134] exhibited superior catalytic performance for typi- types of MOFs (e.g., Zr–MOFs (UiO-66, UiO-67) [45, 47, cal palladium-catalyzed reactions such as the Suzuki C–C 135–138], NU-125 [139], MOF-69 [35], MIL-53 [35]) have coupling and hydrogenation. The Pd@[Cu Pd (btc) ] been the subject of numerous experimental investigations. 3−x x 2 n 2+ MOFs featuring both incorporated Pd nodes and loaded The defects of type A (reduced metal centers with more open Pd NPs show substantially enhanced catalytic activity coordination environments) were generated either by a direct toward the aqueous-phase hydrogenation of p-nitrophenol synthesis strategy or by post-synthesis approaches. In addi- with NaBH to p-aminophenol, as contrasted with the pris- tion, Brozek and Dinca reported the fabrication of a series of tine Cu–MOFs (HKUST-1). Furthermore, it was evident mixed-metal MOF-5 analogues via isomorphous substitution 2+ 2+ that the Pd species play a key role in this reaction and are at the Zn O cornerstone (see Fig. 9), where the Zn species 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2211 varied depending on the quality of MOF thin films that could be controlled in a straightforward fashion [142]. As discussed above for MOF powders, the defect-engineered SURMOFs can be fabricated by controlled introduction of defects using different strategies such as fragmented linker incorporation [33, 38], or thermal treatment [39, 143]. It is expected that the intentional creation of defects inside SURMOFs has important consequences for tuning the electronic structure and catalytic properties of MOF materials. 2+ Fig. 9 Isomorphous substitution of Zn by other metal cations at the The lbl method can also be used to fabricate MOF mem- Zn O cornerstone of MOF-5. Reproduced with permission from Ref. branes. The ability to separate different molecules allows [140]. Copyright 2013 American Chemical Society the integration of size-exclusion principles to MOF-based catalysts. The first SURMOF-based monolithic membrane 2+ was fabricated by Shekhah et al. [144] and was reported to were partially replaced by metal cations with the same (Cr , 2+ 2+ 3+ 3+ 3+ Mn, Fe ) or different oxidation states (Ti, V, Cr be well-suited for the separation of small molecules, e.g., H, N CO, CO, CH , as well as other small hydrocarbons. with Cl as counter-ion) [140]. Furthermore, the approach 2 2, 2 4 of partial post-synthetic metal exchange was employed to A more recent study has demonstrated the use of MOF- based membranes for the natural gas purification and high- produce mixed Al/Fe-MIL-53, Zr/Ti-UiO-66 as well as Zr/ Hf-UiO-66 MOFs featuring mixed metal nodes in the frame- value industrial separations such as butane isomers [145]. Furthermore, it has been shown that the unique opportuni- work [141]. Overall, these different types of defects have been shown to account for the high reactivity of MOF cata- ties to functionalize MOFs permits numerous interesting, membrane-related functionalities, including membranes lysts for a number of catalytic reactions [40]. The controlled incorporation of MOFs with defects of different types and where the permeability and selectivity can be switched on by light [146, 147]. As shown in Fig. 11, the photoswitch- concentrations represents a novel approach for the predic- tive rational design of MOF-based single-site catalysts at able SURMOF membrane was fabricated by assembling linkers decorated with photoresponsive azobenzene-side- the atomic level. groups into the framework, where the precise control of the cis/trans azobenzene ratio by controlled irradiation times 3 MOF Thin Films Grown with ultraviolet or visible light allows for a continuous tun- ing of the separation of molecular mixtures [146]. Since this by the Layer‑by‑Layer Method approach can also be applied to fabricate large-area (larger than 20 cm × 20 cm) membranes [148], and also by apply- Employing the lbl or LPE method, established by Wöll and coworkers [59], to fabricate crystalline, monolithic MOF ing spray-methods, it is, in principle, suited for a continuous coating process. thin films, or SURMOFs [65] affords interesting oppor - tunities for MOF applications in catalysis. In addition to With respect to catalysis, a particularly interesting aspect of SURMOFs is the possibility to combine two different providing well-defined SURMOF substrates which can be applied for chemical transformations as observed for MOF types of chemically active MOFs into one multilayer thin film or membrane by using heteroepitaxy. This approach bulk powders, the lbl method can be used to introduce differ - ent types of defects in SURMOFs, e.g. at internal interfaces allows the creation of tandem catalysts where two different catalytically active components can be incorporated into one in hetero-multilayer structures [65]. In addition, interstitial sites can be created by loading guest species, including single, monolithic thin film (or membrane). The close proximity of two different catalytically active metal or oxide NPs or nanoclusters (NCs), inside the parent SURMOF materials. species which can be realized by a programmed lbl approach is important in the context of reaction cascades with short- An instructive example are HKUST-1 SURMOFs grown on an MHDA/Au substrates (see Fig. 10). The experimen- lived intermediates. In addition, a number of different cata- lysts may interfere, e.g., one catalyst affects the action of tal data from UHV-FTIRS and XPS were interpreted using electronic structure calculation and allowed to derive a the other, or leads to decomposition of the second catalyst. Avoiding these unwanted effects can be achieved by anchor - rather consistent picture [88]. The results showed consist- ently the presence of a small amount (~ 4%) of reduced ing the active species within a three-dimensional porous network. As opposed to just mixing the two catalysts in a Cu species in the pristine HKUST-1 thin film. Upon heat- ing, the temperature-induced creation of Cu defects was liquid, embedding the two different catalysts in a MOF will maintain their individual activities. clearly observed (Fig. 10). The density of intrinsic defects 1 3 2212 Y. Wang, C. Wöll Fig. 10 a Schematic drawing of HKUST-1 grown on an MHDA/ Au substrate. b XP spectra of HKUST-1 SURMOF at dif- ferent temperatures. c UHV- IRRAS spectra of CO adsorbed on activated HKUST-1 SURMOF at 110 K. The sample was annealed at 350 and 420 K. Reproduced with permission from Ref. [88]. Copyright 2012 John Wiley and Sons One of the first demonstrations of such tandem catalysts catalyzed by the Mn-porphyrin to yield epoxide as the realized by the lbl approach are porphyrin-based SUR- reactive intermediate. In a second step, the proximally MOFs. In the paper by Hupp and coworkers [149], a two- sited Zn-porphyrin facilitates the insertion of C O into component SURMOF was grown on a correspondingly the epoxide, giving rise to the cyclic carbonate as the final functionalized substrate (see Fig. 12). This hybrid system product (Fig. 12d). comprising two different metallo-porphyrins (ZnMn-RPM) The technological impact of such tandem catalysts can was shown to be active toward the epoxidation of olefin be improved substantially by realizing them in the form of substrates (Mn-porphyrin) and for epoxide opening (Zn- thin membranes through which the reactants are passed. porphyrin). Although this system could not yet be realized This technology will combine size-exclusion properties in the form of a membrane, this tandem SURMOF thin with catalytic activities. A particular advantage of the layer yielded catalytic turnover numbers which were sub- molecular framework-based approach for catalyst design stantially higher than the corresponding bulk MOF materi- is that the influence of the introduction of additional side- als [149]. The reaction studies in this case was a tandem groups to the MOF–ligands on the chemical activities can reaction consisting of first methoxy-styrene epoxidation be explored in a relatively straightforward fashion. In most 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2213 Fig. 11 a Schematic illustra- tion of tunable, remote-con- trollable molecular selectivity by a photoswitchable MOF membrane. b SEM cross- section image of the SURMOF membrane on the mesoporous a-Al O . c The structure of 2 3 Cu (AzoBPDC) (AzoBiPyB) 2 2 with the azobenzene groups. The transition between trans and cis states can be tuned by irradiation with 365 and 455 nm light, respectively. Reproduced with permission from Ref. [146]. Copyright 2016 Springer Nature cases, adding additional functionalities (e.g. OH, –NH , We would like to conclude this section on MOF thin films –CH , etc.) to a ligand will not change the overall structure by pointing out that SURMOFs are also well-suited to study of the MOF. transport phenomena occurring in these framework materi- This approach has been used to improve the efficiency als in a systematic fashion. One example are so-called sur- of a UiO-66 based catalyst for phosphate ester hydrolysis, face barriers, which are relevant for most porous materials. where a rate enhancement of up to 20 times was observed for By using a quartz-crystal microbalance (QCM) based setup, UiO-66 after modification by amino (NH ) moieties that act Heinke et al. could demonstrate that in case of HKUST-I, as a proton-transfer agent during the catalysis cycle [150]. these surface-barriers are not an intrinsic property of MOFs The particular advantage of MOFs in this context is the but result from surface imperfections originating from water- ability to integrate functional units into a porous framework induced corrosion [152]. material without changing the overall architecture of the MOF, which allows tuning of the chemical activity without affecting diffusivities, etc.. Additionally, the catalytic perfor - 4 Homochiral MOFs and Enantioselective mance of MOF materials could also be tuned via the incor- Asymmetric Catalysis poration of inorganic groups containing metal centers as side-groups [22]. This procedure was termed post-synthetic Kim and coworkers reported the first homochiral MOF metalation (PSMet) as the additional catalytically-active (POST-1) that catalyzes a transesterification reaction in an metal moieties can only be added post-synthetically [151]. enantioselective manner [153]. Since this seminal work, The same decoration strategies of MOF–ligands also apply homochiral MOFs have been extensively investigated with to MOF thin films, which enable the multifunctional proper - the aim to rationally fabricate and engineer MOFs mate- ties of SURMOFs to be adjusted in a controlled manner (see rials for heterogeneous asymmetric catalysis. These kinds e.g. the photoswitchable SURMOF membrane depicted in of MOFs can be obtained via distinct strategies such as Fig. 11 [146]). introduction of achiral active centers during synthesis, 1 3 2214 Y. Wang, C. Wöll Fig. 12 a The porphyrinic dipyridine pillar used in the construction of the ZnMn- RMP MOF containing a Mn atom, which can be used as an epoxidation catalyst, and b the porphyrinic tetracarboxylic acid strut used to make the 2D sheets of the ZnMn-RPM MOF containing a Zn atom, which can act as an epoxide-opening catalyst. c Crystallographically- derived representation of a unit cell of the ZnMn-RPM framework. d Schematic repre- sentation of tandem catalysis of ZnMn-RPM for the synthesis of cyclic carbonate. Reproduced with permission from Ref. [149]. Copyright 2016 John Wiley and Sons post-synthetic modification of homochiral MOFs, or incor - in Fig.  13, nanosized homochiral titanium oxo-clusters poration of asymmetric catalysts directly into the framework (Ti–MOCs) were embedded into HKUST-1 frameworks by [154–160]. The substantial potential of homochiral MOFs in using the LPE lbl method [170]. The resulting Ti–MOC@ enantioselective asymmetric catalytic reactions has been dis- HKUST-1 metacrystal was quite efficient regarding enan - cussed by a number of different groups [25, 155, 161–166]. tiomer recognition and separation. Although the combina- Tremendous efforts have been dedicated to homochiral tion of enantiomer-selectivity with catalytically activities MOF powder materials. However, investigations of the cor- in MOF thin films has not been explored, we consider the responding MOF membranes and thin films (SURMOFs) potential of this direction to be enormous, in particular, are few. Only recently, Wöll, Fischer and coworkers reported when combined with membranes. the fabrication of a series of enantiopure metal-camphorate frameworks (MCamFs) deposited on a quartz crystal micro- balance (QCM) substrate via an in situ LPE lbl approach by 5 Metal and Oxide Nanoparticles Embedded changing the metal nodes and/or linker molecules in succes- within MOFs sive deposition cycles [167–170]. Enantioselectivity with regard to the diffusion of different enantiomers into a MOF In the previous paragraphs, we have demonstrated the great thin film can be modulated by using linkers of different chi- potential of MOFs for catalytic performance and it is far rality [167, 168]. In addition, a thorough study of isoreticu- from being fully exploited. But even more possibilities exist lar chiral SURMOFs with identical stereogenic centers but to add further dimensions to the application of MOFs in different pore dimensions demonstrated that the pore sizes catalysis. A particularly important one is the impregnation must be adjusted to achieve the highest enantioselectivity of MOFs with NCs or NPs. Under mild conditions (tem- in chiral nanoporous materials [169]. Furthermore, chiral perature below 250–550 °C, depending on type of MOF), metal–organic nanoclusters (MOCs) can be loaded into the the porous molecular frameworks are sufficiently stable to achiral MOFs, again, achieving a high selectivity between prevent sintering or agglomeration of embedded, catalyti- the diffusivity of different enantiomers [170]. As shown cally active particles, the most severe problems encountered 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2215 inside MOFs accounted for the high catalytic activity for liq- uid-phase aerobic oxidation of alcohols (see Fig. 14d). The nano-sized bimetallic alloys are known to show enhanced catalytic performance in numerous reactions as compared to their monometallic counterparts. However, the exclusive encapsulation of bimetallic NPs with tunable com- positions into MOFs is challenging [174–179]. More recently, the embedding of core–shell PdPt and RuPt nano-alloys into Zr-based MOFs (UiO-66 and its derivatives) was realized by template synthesis [180]. The resulting bimetallic core–shell NPs exhibited substrate specic fi size-selectivity and signifi - cantly enhanced catalytic activity for the hydrogenation of nitrobenzene compared to pure Pt loaded UiO-66 [181]. The catalytic performance of MOF materials can also be tuned in a controllable way by encapsulation of vari- ous metal oxide NPs [182–185]. The surface structure and reactivity of nanostructured ZnO particles, embedded into ZIF-8 via chemical vapor infiltration followed by oxidative annealing, were characterized by UHV-FTIRS using C O as a probe molecule [185]. In contrast to pure ZnO NPs exposing mainly non-polar (10–10) surfaces, the confined ZnO NPs inside ZIF-8 were dominated by polar O–ZnO and Zn–ZnO facets as well as defect sites, which were highly reactive for CO activation. The isolated metal oxides (e.g., ZnO, T iO, Fe O ) stabilized within the MOF matrix 2 2 3 showed enhanced multifunctional (catalytic, magnetic, opti- cal) properties and have promising applications in catalysis, photocatalysis, and other fields such as electronic devices and sensors [182–185]. In comparison to MOF bulk powders, much less informa- tion is available for the loading of MOF thin films (SUR - Fig. 13 a Structure of the R–Ti–MOC clusters. b Schematic presenta- MOFs) with metal or metal oxide NPs. Recently, Wöll and tion of in  situ lbl growth of enantiopure Ti–MOC-loaded HKUST-1 coworkers reported the first fabrication of Bi O NPs encap- 2 3 thin film using LPE approach. Reproduced with permission from Ref. sulated into HKUST-1 thin films via a novel approach, in [170]. Copyright 2016 American Chemical Society which bismuth-triphenyl was used to create small bismuth oxide particles into the MOF pores [186]. Also in this case the size of the largest Bi O clusters slightly exceeded that when exploiting the high chemical activity of such small 2 3 particles for chemical transformations. of the MOF pores. The size distribution could be narrowed down substantially and at the same time shifted to lower Several strategies exist with regard to loading metal, metal oxide or other chemically active NPs into the MOFs. One of values by adding amino groups acting as nucleation centers to the MOF linkers. Without changing the MOF architecture, the first papers in this area used metal containing precursors to realize small Pd-clusters embedded in the MOF [171]. In this lattice constant, and topology, these additional amino groups acted as nucleation centers, thus achieving a much narrower case, the liberation of the metal atoms was achieved by either exposure to high pressures of H or by irradiation with light. size distribution. Such bismuth oxide particles are highly active in photocatalysis, as demonstrated by the photodegra- It is noteworthy that the palladium clusters were substantially larger than the pores of the MOFs (Fig. 14 case b) [171]. In a dation of nuclear fast red (NFR, C H NO SNa) dye [186]. 14 8 7 similar manner, Au NPs were loaded into different MOF mate- rials (e.g., ZIFs [172], MOF-5 [173]) and distributed homo- 6 SURMOFs and Electrocatalysis geneously over the MOF matrix matching with the cavities (Fig. 14 case c) as conr fi med by HRTEM observations. The Thin MOF films deposited on an electrode also exhibit size distribution and shape of embedded NPs was controlled by the framework structure and the functional groups at the interesting properties in electrochemistry and electroca- talysis [187, 188]. In particular, monolithic, pinhole-free linkers. The homogeneous distribution of confined Au NPs 1 3 2216 Y. Wang, C. Wöll Fig. 14 Top: three characteristic cases of microstructures for NPs narrow size distribution matching with the cavities and homogene- supported by MOFs. a Particles typically larger than the cavity size ously distributed over the volume of MOF. Bottom: d schematic view with a preferred anchoring close to the outer surface of the MOF. b of liquid phase alcohol oxidation with the Au@ZIF-8 material; Both Particles evenly distributed throughout the volume of the MOF crys- benzyl alcohol (BA) and methyl benzoate (MB) are able to access the tal but still exhibiting a broad size distribution with an average par- pores and can diffuse through the network. Reproduced with permis- ticle size exceeding the dimensions of the pores. c Particles with a sion from Ref. [172]. Copyright 2010 American Chemical Society SURMOFs, MOF thin films prepared using the lbl-process In addition to the demonstration of the suitability of SUR- (see Sect. 3, above) exhibited a great potential with respect MOFs for electrochemistry (see above), exciting properties to electrochemistry [189–191]. Although the application of toward photovoltaics (construction of MOF thin film based SURMOFs, both empty and after loading with electroactive solar cells [193, 194]) have been demonstrated. compounds such as ferrocene, has been quite successful, applications in electrocatalysis have been less common. In a recent paper by Liu, Wöll, Sun, and coworkers [192], 7 Summary and Outlook a monolithic, pinhole-free Re-based SURMOF was grown on a conductive, transparent substrate (fluorinated tin oxide, The selected examples of MOF chemical activity discussed or FTO) and exhibited well-defined electrochemical proper - in this short review clearly demonstrate the great potential ties. As demonstrated by the X-ray diffraction (XRD) data, of these porous framework materials for catalysis. Active the MOF films were highly oriented, with the [001] direc- sites can be introduced into these crystalline coordination tion perpendicular to the substrate. These SURMOFs were networks using a broad variety of strategies, and MOFs can shown to be highly effective in the electrocatalytic conver - be used for catalysis either as powders or in the form of sion of CO to CO. The faradaic efficiency found in these thin films (SURMOFs), with the possibility to also fabricate experiments was astonishingly high, amounting to 93 ± 5%. catalytically active membranes. The combined experimental Furthermore, the current densities which could be achieved and theoretical results presented above for selected systems for these high-quality monolithic coatings were found to provided deep, fundamental insights into the structural, elec- −2 be larger than 2  mA  cm , thus exceeding the densities tronic and reactive properties of active sites in pristine and recorded for MOF thin films prepared using other methods defect-engineered MOFs (DEMOFs). Depending on the sys- by at least one order of magnitude. tem, isolated, coordinatively unsaturated metal sites (CUS) Although few applications have so far been reported are either an intrinsic component of the perfect system or for photo-electrocatalysis of MOF thin films (e.g., MOFs can be introduced by e.g. defect engineering or by loading as photosensitizers on T iO nanowires for water splitting with suitable guest species, thus creating the potential to fab- [193]), we also foresee enormous potential in this direction. ricate single-site catalysts [22]. The catalytic performance of 1 3 Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks 2217 CuI[4,4′,4″,4‴-tetracyanotetraphenylmethane]BF .xC H NO . MOFs can be significantly enhanced by tailoring of organic 4 6 5 2 J Am Chem Soc 112:1546–1554 linkers and/or metal cations via defect-engineering strategies 2. Kitagawa S, Matsuyama S, Munakata M, Emori T (1991) in a controlled manner. 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Catalysis LettersSpringer Journals

Published: Jun 2, 2018

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