Hermenegildo Garcia

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.

Mixed-metal or mixed-linker metal organic frameworks as heterogeneous catalysts

Metal organic frameworks (MOFs) are among the most studied heterogeneous catalysts that have been applied to promote a wide range of reactions. Most of the initial studies on the catalytic activity of MOFs were based on the use of materials containing a single metal and a single linker. However, the most recent trend in the field is to exploit the synthetic flexibility offered by MOFs to obtain new MOFs possessing two different metals in their structure, or the same metal in different oxidation states (“mixed metals”) or different linkers (“mixed linkers”), resulting in materials with a superior catalytic activity over the corresponding single metal or single linker MOFs. This review is aimed to address the possible advantages of the use of mixed metal or mixed linker strategies to increase the activity of MOFs in some selected reactions. After some general sections introducing the structural features of MOFs, the nature of possible active sites, different ways to characterize mixed-metal or mixed-ligand MOFs and good practices, the main body of the review describes the current state of the art in the use of this type of MOF as heterogeneous catalysts, classified depending on the presence of more than one metal or more than one ligand. The final concluding remarks include some future targets in the area.

Metal-Organic Frameworks as Catalysts for Oxidation Reactions.

This Concept is aimed at describing the current state of the art in metal-organic frameworks (MOFs) as heterogeneous catalysts for liquid-phase oxidations, focusing on three important substrates, namely, alkenes, alkanes and alcohols. Emphases are on the nature of active sites that have been incorporated within MOFs and on future targets to be set in this area. Thus, selective alkene epoxidation with peroxides or oxygen catalyzed by constitutional metal nodes of MOFs as active sites are still to be developed. Moreover, no noble metal-free MOF has been reported to date that can act as a general catalyst for the aerobic oxidation of primary and secondary aliphatic alcohols. In contrast, in the case of alkanes, a target should be to tune the polarity of MOF internal pores to control the outcome of the autooxidation process, resulting in the selective formation of alcohol/ketone mixtures at high conversion.

Copper Nanoparticles Supported on Doped Graphenes as Catalyst for the Dehydrogenative Coupling of Silanes and Alcohols

Copper nanoparticles (NPs) supported on a series of undoped and doped graphene materials (Gs) have been obtained by pyrolysis of alginate or chitosan biopolymers, modified or not with boric acid, containing Cu(2+) ions at 900 °C under inert atmosphere. The resulting Cu-G materials containing about 17 wt % Cu NPs (from 10 to 200 nm) exhibit high catalytic activity for the dehydrogenative coupling of silanes with alcohols. The optimal material consisting on Cu-(B)G is more efficient than Cu NPs on other carbon supports.

MIL-101 promotes the efficient aerobic oxidative desulfurization of dibenzothiophenes

MIL-101 promotes aerobic oxidation in n-dodecane of dibenzothiophene (DBT) and its methyl-substituted derivatives to their corresponding sulfones with complete selectivity, without observation of the sulfoxide. DBT sulfones can be completely separated from n-dodecane by water extraction. MIL-101(Cr) without the need of pre-activation was found to be more convenient than the also-active MIL-101(Fe) analog. The reaction exhibits an induction period due to the diffusion inside the pore system of the solvent or oxygen and it is not observed if the MIL-101 sample is first in contact with the solvent at the reaction temperature for a sufficiently long time. MIL-101 is reusable for at least five times without any sign of deactivation according to the time-conversion plots. Evidence by electron paramagnetic resonance spectroscopy detecting the hydroperoxide radical adduct with a spin trapping agent and Raman spectroscopy detection of superoxide supports that the process is an auto-oxidation reaction initiated by MIL-101 following the expected radical chain mechanism inside the MIL-101 cages.

Graphenes as Efficient Metal‐Free Fenton Catalysts

Reduced graphene oxide exhibits high activity as Fenton catalyst with HO(.) radical generation efficiency over 82 % and turnover numbers of 4540 and 15023 for phenol degradation and H2 O2 consumption, respectively. These values compare favorably with those achieved with transition metals, showing the potential of carbocatalysts for the Fenton reaction.

Reduced Graphene Oxide as a Metal-Free Catalyst for the Light-Assisted Fenton-Like Reaction

Natural sunlight irradiation increases the catalytic activity of reduced graphene oxide (rGO) as metal‐free Fenton‐like catalyst for phenol degradation. Generation of hydroxyl radical intermediates was confirmed by EPR spectroscopy using a spin‐trapping agent.

Chitosan‐Templated Synthesis of Few‐Layers Boron Nitride and its Unforeseen Activity as a Fenton Catalyst

A novel procedure for the preparation of few-layers boron nitride (BN), either as films or as suspended BN platelets, is presented, based on the pyrolysis of chitosan, which serves both as a matrix to embed ammonium borate and as a template for BN synthesis. The resulting BN samples are characterized by XRD, Raman and X-ray photoelectron spectroscopy, and by TEM and AFM imaging. The samples exhibit deep UV emission, which is characteristic of high quality BN. This template synthesis and the easy exfoliation of BN platelets facilitate the use of BN as an extremely high-efficiency Fenton catalyst for the generation of highly aggressive hydroxyl radicals in water.

Graphenes as Metal‐free Catalysts for the Oxidative Depolymerization of Lignin Models

Graphene oxide promotes the oxidative degradation of α‐guaiacylglycerol‐β‐guaiacyl ether, a lignin model compound, in different solvents including acetonitrile, toluene and water mainly to guaiacol, 2‐methoxyquinone, vanillic acid and coniferyl aldehyde. Reduced graphene oxide and B‐doped graphene exhibit similar activity as graphene oxide. Control experiments indicate that metal impurities should play a marginal role in the observed catalytic activity.

Deactivation of Cu3(BTC)2 in the Synthesis of 2-Phenylquinoxaline

Cu3(BTC)2 deactivates by deterioration of its crystal structure during the use of this metal organic framework as catalyst in the synthesis of 2-phenylquinoxaline from phenacylbromide and o-phenylenediamine at room temperature. The material resulting from the use of Cu3(BTC)2 as catalyst was characterized by powder XRD, UV–Vis diffuse reflectance spectra and EPR showing that the crystal structure collapses and Cu2+ becomes reduced under the reaction conditions.Graphical Abstract