Amarajothi Dhakshinamoorthy

Catalysis by metal nanoparticles embedded on metal–organic frameworks

The present review describes the use of metal-organic frameworks (MOFs) as porous matrices to embed metal nanoparticles (MNPs) and occasionally metal oxide clusters, which are subsequently used as heterogeneous catalysts. The review is organized according to the embedded metal including Pd, Au, Ru, Cu, Pt, Ni and Ag. Emphasis is also given in the various methodologies reported for the formation of the NPs and the characterization techniques. The reactions described with this type of solid catalysts include condensation, hydrogenations, carbon-carbon coupling, alcohol oxidations and methanol synthesis among others. Remaining issues in this field have also been indicated.

Carbocatalysis by graphene-based materials

Financial support by the Spanish Ministry of Economy and Competitiveness (MINECO, Severo Ochoa program, CTQ 2012-32315 and CTQ2010-18671) and Generalitat Valenciana (GV/2013/040) is gratefully acknowledged. A.D.M. thanks University Grants Commission, New Delhi for the award of Assistant Professorship under its Faculty Recharge Programme.

Photocatalytic CO2 reduction by TiO2 and related titanium containing solids

The shortage of fossil fuels and the need to find alternative renewable and sustainable fuels for transportation are triggering an increasing interest in the photocatalytic reduction of CO2. In this review, we have focused on titanium containing photocatalysts that effect the reduction of CO2 to fuels. The various products that are more generally formed are CH4, CH3OH, CO as well as HCOOH. This review has been organized primarily depending on the type of titanium material used as the photocatalyst. The list includes pure TiO2 as well as metal- and non metal-doped titania, noble metals supported on titania and micro-/mesoporous titanosilicates or porous matrices containing titania clusters. In a general introduction we comment on the limitations of the current approaches and the various possibilities and conditions for performing the irradiation. In a final section, we also give our view on future developments and open issues to be addressed in this field.

Commercial metal–organic frameworks as heterogeneous catalysts

Metal-organic frameworks (MOFs) are porous crystalline materials that have promising applications as heterogeneous catalysts. After describing the composition, textural properties and crystal structure of four commercially available MOFs, we summarize organic transformations for which these commercial MOFs exhibit higher catalytic activity than the corresponding soluble metal salts or metal ion-exchanged zeolites. In the present article, we have focused on reactions requiring Lewis-acid sites or redox centers to illustrate the potential applications and limitations of commercial MOFs. In a final section, we provide our views on future developments whose ultimate target will be the use of a commercial MOF as a heterogeneous catalyst for a real industrial process in fine chemistry, thus, realizing the advantages of these materials with respect to zeolites or other solid catalysts in liquid-phase reactions.

Metal–Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production

Metal-organic frameworks (MOFs) are crystalline porous materials formed from bi- or multipodal organic linkers and transition-metal nodes. Some MOFs have high structural stability, combined with large flexibility in design and post-synthetic modification. MOFs can be photoresponsive through light absorption by the organic linker or the metal oxide nodes. Photoexcitation of the light absorbing units in MOFs often generates a ligand-to-metal charge-separation state that can result in photocatalytic activity. In this Review we discuss the advantages and uniqueness that MOFs offer in photocatalysis. We present the best practices to determine photocatalytic activity in MOFs and for the deposition of co-catalysts. In particular we give examples showing the photocatalytic activity of MOFs in H2 evolution, CO2 reduction, photooxygenation, and photoreduction.

Metal–organic frameworks as heterogeneous catalysts for oxidation reactions

In this Perspective, we describe the use of metal–organic frameworks (MOFs) as heterogeneous catalysts for oxidations using hydroperoxides or molecular oxygen. These two types of oxidants fulfill the requirements of green chemistry in terms of environmental benignity and sustainability. For the sake of clarity and to illustrate the possibilities of using MOFs as oxidation catalysts, we have constrained ourselves to present the results obtained for the oxidation of cycloalkanes, oxidation of benzylic positions, oxidation of cycloalkenes and aerobic oxidation of alcohols. The use of MOFs as catalysts for enantioselective oxidations has been dealt with in a separate section in which the peculiarity of the required oxidants and the advantages of having homo chiral MOFs have been discussed. The key features of MOFs as catalysts, the similarities with inorganic porous solids and future developments in this field have been discussed in separate sections.

Photocatalytic CO2 Reduction using Non‐Titanium Metal Oxides and Sulfides

Titanium dioxide (TiO2 ) is by far the most widely used photocatalyst both for the degradation of pollutants and in the field of renewable energies for the production of solar fuels. However, TiO2 has strong limitations in CO2 reduction, particularly under visible light irradiation. The flat-band potential of electrons in the conduction band of TiO2 is lower than that required for CO2 reduction and, therefore, it seems appropriate to develop and validate materials other than TiO2 . In addition, the photoresponse of TiO2 requires photons of wavelengths in the UV range shorter than 380 nm and strategies to implement a visible-light photoresponse on TiO2 by doping have not been completely satisfactory particularly because of problems in reproducibility and stability of the materials. For these reasons, we focus in this Review on semiconductors other than TiO2 that show photocatalytic activity in CO2 reduction. Attention has been paid to the irradiation conditions to put the productivity data into context. The role of co-catalyst and heterojunctions to increase the efficiency of charge separation is also discussed. Our aim is to describe the state of the art in the field of photocatalytic CO2 reduction using materials other than TiO2 , trying to trigger further research in this area.

Metal organic frameworks as heterogeneous catalysts for the production of fine chemicals

This review focuses on the use of metal organic frameworks (MOFs) as catalysts for the synthesis of fine chemicals. While petrochemistry is characterized by gas phase reactions, in which MOFs cannot compete with robust zeolites, MOFs are better suited for liquid phase reactions performed at moderate temperatures. These are the conditions typically employed for the production of fine chemicals characterized by being more complex and diverse molecules of low volatility, but with high added value. For the preparation of this type of compound, MOFs offer the advantage of wide open porosity in the nanometer scale and a large void volume. In the present review we have summarized the reports that appeared up to early 2013 on the use of MOFs as catalysts in the liquid phase for the production of fine chemicals, primarily classified according to the type of active site and the functional group formed in the reaction. Prospects for future development in this field are provided in the last section.

Metal nanoparticles as heterogeneous Fenton catalysts.

The Fenton reaction (the generation of hydroxyl radicals from hydrogen peroxide) is the most useful method for degradation of organic pollutants in aqueous solution at moderate concentrations. In this Review we summarize the use of metal nanoparticles, either unsupported or deposited on large-surface-area solids, as Fenton catalysts. The Review complements two other reviews in the field of heterogeneous Fenton catalysis using aluminosilicates and carbonaceous materials. Herein, particular emphasis is given to the reaction conditions in which these catalysts are used, highlighting the operating mechanism and the relative efficiency of the materials. Aspects such as leaching of the metal to the solution, reusability, and the concentration of hydrogen peroxide used are analyzed in detail. Besides a critical description of the present status of the field, future trends and the need to establishing valid comparisons to assess the relative efficiencies of the materials are commented on.

Metal–Organic Frameworks as Efficient Heterogeneous Catalysts for the Regioselective Ring Opening of Epoxides

An iron-based metal-organic framework, [Fe(BTC)] (BTC: 1,3,5-benzenetricarboxylate) is an efficient catalyst in the ring opening of styrene oxide with alcohols and aniline under mild reaction conditions. Out of the various alcohols tested for ring opening of styrene oxide, methanol was found to be the most reactive in terms of percentage conversion and reactivity. The rate of the ring-opening reaction of styrene oxide decreases as the size of the alcohol is increased, suggesting the location of active sites in micropores. [Fe(BTC)] was a truly heterogeneous catalyst and could be reused without loss of activity. The analogous compound [Cu(3)(BTC)(2)] was also found to be effective, although with somewhat lower activity than [Fe(BTC)]. The present heterogeneous protocol is compared with a homogeneous catalyst to give an insight into the reaction mechanism.