Mercedes Alvaro

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.

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 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 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.

Enhancement of the Catalytic Activity of Supported Gold Nanoparticles for the Fenton Reaction by Light

Laser flash photolysis of supported gold nanoparticles exciting at the surface plasmon band (532 nm) has allowed in the case of Au/CeO(2) and Au/OH-npD (OH-npD: Fenton-treated diamond nanoparticles) detection of transients decaying in the microsecond time scale that have been attributed as indicating photoinduced electron ejection from gold based on N(2)O quenching and the observation of the generation of methyl viologen radical cations. This photochemical behavior has led us to hypothesize that there could be assistance to the catalytic activity of these materials by irradiation in those cases wherein the mechanism involves electron transfer to or from a substrate to the gold. This hypothesis has been confirmed by observing that the catalytic activity of Au/OH-npD for the Fenton degradation of phenol with hydrogen peroxide can be increased over 1 order of magnitude by irradiation at 532 nm. Moreover, there is a linear relationship between the initial reaction rate and the incident photon flux. This photoenhancement allows promoting Fenton activity at pH 8 in which the catalytic activity of Au/OH-npD is negligible. The same photo enhancement activity for the Fenton degradation of phenol was observed for other supported gold catalysts including those that do not exhibit microsecond transients in the nanosecond laser flash photolysis (Au/TiO(2) and Au/SiO(2)) due to their lifetime shorter than microseconds. It is proposed that the photo enhancement should be a general phenomenon in gold catalysis for those reaction mechanisms involving positive and/or negative gold species.

Synthesis and catalytic activity of a chiral periodic mesoporous organosilica (ChiMO)

A chiral vanadyl salen complex having two peripheral trimethoxysilyl groups has been used to obtain a chiral periodic mesoporous organosilica having MCM-41 periodicity and the two Si-CH2 groups anchored on the framework; this solid induces 30% enantioselectivity in the cyanosilylation of benzaldehyde.

Graphene oxide as an acid catalyst for the room temperature ring opening of epoxides

The minute amount of hydrogen sulfate groups introduced into the graphene oxide (GO) obtained by Hummers oxidation of graphite renders this material as a highly efficient, recyclable acid catalyst for the ring opening of epoxides with methanol and other primary alcohols as nucleophile and solvent.

Catalytic activity of unsupported gold nanoparticles

This article reviews some of the compelling evidence showing that colloidal gold nanoparticles without any solid support exhibit intrinsic catalytic activity for some of the typical gold-catalyzed reactions including CO oxidation, aerobic oxidation of alcohols and diols, borohydride reductions and carbon–carbon cross coupling reaction among other reactions. A critical view of the state-of-the-art indicating open issues such as the role of the nature and concentration of the ligand and the possibility of preparing colloidal samples with preferential crystallographic planes is provided.