Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

The Chemistry and Applications of Metal-Organic Frameworks

Polymeric graphitic carbon nitride materials (for simplicity: g-C(3)N(4)) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. In contrast to graphenes, g-C(3)N(4) is a medium-bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the "polymer chemistry" of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g-C(3)N(4) and its modifications have a high thermal and chemical stability and can catalyze a number of "dream reactions", such as photochemical splitting of water, mild and selective oxidation reactions, and--as a coactive catalytic support--superactive hydrogenation reactions. As carbon nitride is metal-free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.

Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis

This review summarizes the use of metal–organic frameworks (MOFs) as a versatile supramolecular platform to develop heterogeneous catalysts for a variety of organic reactions, especially for liquid-phase reactions. Following a background introduction about catalytic relevance to various metal–organic materials, crystal engineering of MOFs, characterization and evaluation methods of MOF catalysis, we categorize catalytic MOFs based on the types of active sites, including coordinatively unsaturated metal sites (CUMs), metalloligands, functional organic sites (FOS), as well as metal nanoparticles (MNPs) embedded in the cavities. Throughout the review, we emphasize the incidental or deliberate formation of active sites, the stability, heterogeneity and shape/size selectivity for MOF catalysis. Finally, we briefly introduce their relevance into photo- and biomimetic catalysis, and compare MOFs with other typical porous solids such as zeolites and mesoporous silica with regard to their different attributes, and provide our view on future trends and developments in MOF-based catalysis.

Applications of metal–organic frameworks in heterogeneous supramolecular catalysis

For the first time, this work presents Au@Ag core−shell nanoparticles (NPs) immobilized on a metal−organic framework (MOF) by a sequential deposition−reduction method. The small-size Au@Ag NPs reveal the restriction effects of the pore/surface structure in the MOF. The modulation of the Au/Ag ratio can tune the composition and a reversed Au/Ag deposition sequence changes the structure of Au−Ag NPs, while a posttreatment process transforms the core−shell NPs to a AuAg alloy. Catalytic studies show a strong bimetallic synergistic effect of core−shell structured Au@Ag NPs, which have much higher catalytic activities than alloy and monometallic NPs.

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Synergistic Catalysis of Au@Ag Core−Shell Nanoparticles Stabilized on Metal−Organic Framework

Gold nanoparticles (Au NPs) were directly deposited on commercially available poly(methyl methacrylate) (PMMA) beads by the deposition reduction method by adding dropwise of NaBH4 into the aqueous solution of Au(en)2Cl3. The average diameter of Au NPs was 6.9 nm and, in contrast to the case of rapid addition, there were no aggregated particles larger than 20 nm. Catalytic activity of Au/PMMA was tested for the reduction of 4-nitrophenol to 4-aminophenol with an excess amount of NaBH4. Gold NPs on PMMA showed the highest catalytic activity among polymer supported Au NPs previously reported. The reaction rate is the pseudo-first-order with respect to 4-nitrophenol. The pseudo-first-order rate constant and the activation energy were estimated to be 7.2–7.9 × 10−3 s−1 at 295 K and 38 kJ mol−1, respectively.

Reduction of 4-nitrophenol to 4-aminophenol over Au nanoparticles deposited on PMMA

Gold has turned out to be one of the most attractive elements in catalysis research since the discovery of CO oxidation at 70 8C over Au nanoparticles (NPs) supported on base metal oxides. During the last decade, Au NPs supported on metal oxides and activated carbons (AC) have been widely studied for liquid phase reactions. In liquid phase reactions, catalytic performance of Au particles is mainly defined by two major factors: i) the nature of supports and ii) the size of Au particles. In particular, the size of 2 nm appears to be a critical point, where the Au particles dramatically change their catalytic and physicochemical properties. However, the conventional deposition–precipitation method is not applicable to AC due to the acidic nature of AC. Mixing Au colloids with AC could hardly give Au clusters with a diameter smaller than 2 nm. A latest trend in the selection of supports is the use of organic polymers. Recently, some organic polymers were found to be effective to stabilize colloidal Au in a cluster size, which showed high catalytic activity for aerobic oxidation of alcohols at room temperature. However, a constraint is that sophisticated synthetic techniques are usually required to obtain Au clusters. On the other hand, porous coordination polymers (PCPs) consisting of metal ions and organic ligands with highly regular nanometer-sized cavities or channels are an emerging class of porous materials. They are expected to be efficient supports for metal clusters to control size and shape by means of their cavities. In addition, PCPs have a wide variety of porous structures, various kinds of components, and surface properties which would lead to tailor-made catalysts for the desired reactions. Thus, investigations of the preparation methods for PCP supported Au clusters and the support effect of PCPs would offer a new frontier in catalysis by Au. Fischer and co-workers have reported the preparation of Pd, Cu, Ru clusters, and Au NPs stabilized by Zncontaining PCP, MOF-5 ([Zn4O ACHTUNGTRENNUNG(bdc)3]n (bdc=benzene-1,4dicarboxylate) by chemical vapour deposition (CVD). Although Au particles could be hardly deposited in cluster size, they were obtained as NPs in the range of 5 to 20 nm on MOF-5 due to weak interaction. 12] Therefore, the generation of Au clusters that fit in and/or on the cavities of PCPs is still a challenging research target. Herein we report a very simple but the most effective method for the direct deposition of Au clusters onto several kinds of PCPs including MOF-5 by solid grinding with a volatile organogold complex without using organic solvents. We also investigated their catalytic properties for the liquid phase alcohol oxidation with molecular oxygen. To the best of our knowledge, catalysis of PCP supported Au clusters in liquid phase has not yet been studied. The PCP supports used were one-dimensionally channelled PCPs such as CPL-1 ([Cu2ACHTUNGTRENNUNG(pzdc)2 ACHTUNGTRENNUNG(pyz)]n, pzdc=pyrazine-2,3-dicarboxylate, pyz=pyrazine), CPL-2 ([Cu2ACHTUNGTRENNUNG(pzdc)2ACHTUNGTRENNUNG(bpy)]n, bpy=4,4’-bipyridine), [14] Al-MIL53 ([Al(OH) ACHTUNGTRENNUNG(bdc)]n) [15] with pores of 4 @6, 6 @ 8, 8.5 @8.5 A, respectively, and three-dimensional PCPs such as MOF-5 and Cu-BTC ([Cu3ACHTUNGTRENNUNG(btc)2]n (btc=benzene-1,3,5-tricarboxylate) [16] with pores of 15 @ 15 and 11 @11 A, respectively. Volatile organogold complex, Me2AuACHTUNGTRENNUNG(acac) (acac= acetylacetonate) and PCPs were ground in an agate mortar in air for 20 min at room temperature. Then the mixture was treated in a stream of 10 vol% H2 in N2 at 120 8C for 2 h to obtain Au/ [a] Dr. T. Ishida, M. Nagaoka, Prof. Dr. M. Haruta Department of Applied Chemistry Graduate School of Urban Environmental Sciences Tokyo Metropolitan University, 1-1 Minami-osawa Hachioji, Tokyo 192-0397 (Japan) Fax: (+81) 42-677-2851 E-mail : haruta-masatake@center.tmu.ac.jp [b] Dr. T. Ishida, Dr. T. Akita, Prof. Dr. M. Haruta Japan Science and Technology Agency (JST) CREST, 4-1-8 Hon-cho, Kawaguchi Saitama 322-0012 (Japan) [c] Dr. T. Akita Research Institute for Ubiquitous Energy Devices National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka Ikeda, Osaka 563-8577 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200800980.

Deposition of Gold Clusters on Porous Coordination Polymers by Solid Grinding and Their Catalytic Activity in Aerobic Oxidation of Alcohols

Gold catalysts: towards sustainable chemistry.

Since the discovery of catalysis by Au nanoparticles (NPs), unique catalytic features of Au have appeared that are greatly different from those of Pd and Pt. In this Review, we aimed to disclose how the unique catalytic abilities of Au are generated with respect to (a) the contact structures between Au and its supports and (b) the size of the Au particles. For CO oxidation, the catalytic activity of Au on reducible metal oxides (MOx) is strongly correlated with the amount of oxygen vacancies of the MOx surface, which play a key role in O2 activation. Single atoms, bilayers of Au, sub-nm clusters, clusters (1-2 nm), and NPs (2-5 nm) have been proposed as the active sizes of the Au species, which may depend on the type of support. For propylene epoxidation, the presence of isolated TiO4 units in SiO2 supports is important for the production of propylene oxide (PO). Au NPs facilitate the formation of Ti-OOH species, which leads to PO in the presence of H2 and O2, whereas Au clusters facilitate propylene hydrogenation. However, Au clusters can produce PO by using only O2 and water, whereas Au NPs cannot. For alcohol oxidation, the reducibility of the MOx supports greatly influences the catalytic activity of Au, and single Au atoms more effectively activate the lattice oxygen of CeO2. The basic and acidic sites of the MOx surface also play an important role in the deprotonation of alcohols and the activation of aldehydes, respectively. For selective hydrogenation, heterolytic dissociation of H2 takes place at the interface between Au and MOx, and the basic sites of MOx contribute to H2 activation. Recent research into the reaction mechanisms and the development of well-designed Au catalysts has provided new insights into the preparation of high-performance Au catalysts.

Tamao Ishida

Papers

Synergistic Catalysis of Au@Ag Core−Shell Nanoparticles Stabilized on Metal−Organic Framework

Reduction of 4-nitrophenol to 4-aminophenol over Au nanoparticles deposited on PMMA

Deposition of Gold Clusters on Porous Coordination Polymers by Solid Grinding and Their Catalytic Activity in Aerobic Oxidation of Alcohols

Gold catalysts: towards sustainable chemistry.

Importance of Size and Contact Structure of Gold Nanoparticles for the Genesis of Unique Catalytic Processes.