Ana Primo

Basic zeolites containing palladium as bifunctional heterogeneous catalysts for the Heck reaction

Abstract K + and Cs + exchanged X zeolite containing PdCl 2 catalyzes the Heck reaction of PhI or PhBr with styrene in toluene or polymethylbenzenes without the need of extrinsic base. The reaction is truly heterogeneous and no leaching was observed. The activity of the fresh catalyst can be regained in a large extent upon reactivation with water washings. The unprecedented preferent formation of the 1,1-diphenylethene has been observed using PdCl 2 -CsX as catalyst, a fact that has been related to the smaller size of the transition state leading to this isomer compared to stilbene.

Nanosized vanadium, tungsten and molybdenum oxide clusters grown in porous chitosan microspheres as promising hybrid materials for selective alcohol oxidation

The ability of chitosan biopolymer to coordinate vanadium, tungsten and molybdenum metallic species and to control their mineralisation growth provides a new family of surface-reactive organic-inorganic hybrid microspheres. Drying the resulting materials under supercritical conditions allowed the gel network dispersion to be retained, thereby leading to a macroporous catalyst with surface areas ranging from 253 to 278 m(2) g(-1). On account of the open framework structure of these microspheres, the redox species entangled within the fibrillar network of the polysaccharide aerogels were found to be active, selective and reusable catalysts for cinamylalcohol oxidations.

Graphene from Alginate Pyrolysis as a Metal-Free Catalyst for Hydrogenation of Nitro Compounds.

Graphene obtained by pyrolysis of alginate at 900 °C under inert atmosphere and exfoliation is used as a metal-free catalyst for reduction of nitro to amino groups with hydrogen as a reagent. The process is general for aromatic and aliphatic, conjugated and isolated nitro groups, and occurs with low selectivity over hydrogenation of carbon-carbon double bonds.

One‐Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets Supported on Graphene and Six Orders of Magnitude Enhancement of the Resulting Catalytic Activity

Pyrolysis of chitosan films containing Au(3+) renders 1.1.1 oriented Au nanoplatelets (20 nm lateral size, 3-4 nm height) on a few layers of N-doped graphene (Au/fl-G), while the lateral sides were 0.0.1 oriented. Comparison of the catalytic activity of Au/fl-G films with powders of unoriented Au NPs supported on graphene showed that Au/fl-G films exhibit six orders of magnitude enhancement for three gold-catalyzed reactions, namely, Ullmann-like homocoupling, C-N cross coupling, and the oxidative coupling of benzene to benzoic acid. This enhancement is the result of the defined morphology, facet orientation of Au nanocrystals, and strong gold-graphene interaction.

A test reaction to assess the presence of Brönsted and the softness/hardness of Lewis acid sites in palladium supported catalysts

Cyclic ethylene acetal of α-bromopropiophenone gives rise to three distinctive products, each one being specific to Bronsted, hard Lewis and soft Lewis sites, respectively. In the presence of Bronsted sites, hydrolysis of cyclic acetal forms the corresponding α-bromophenone. Hard or soft Lewis sites promote selectively a rearrangement of the phenyl or ethylenedioxy group to form 2-phenylpropionate ester or dioxine, respectively. This test reaction has been used to assess the influence of the support on the catalytic activity of palladium in zeolites. Three series of supports including alkali-exchanged faujasite, Beta zeolites and sepiolites have been tested. It has been observed that the nature of the zeolite plays an important role on the palladium selectivity, basic supports increasing the softness of the palladium Lewis sites. The catalytic test correlates well with electrochemical data about the reduction potential of supported palladium, XPS characterization of the oxidation state of palladium supported on the surface and activity in the Suzuki reaction.

Aqueous phase reforming of glycerol using doped graphenes as metal-free catalysts

Boron-doped graphene obtained by pyrolysis at 900 °C of the boric acid ester of alginate was found to be the most active graphene among a series of doped and co-doped graphenes to promote the aqueous phase reforming of glycerol at 250 °C. This reaction is of interest in the context of valorization of the aqueous wastes of carbohydrate syrups. Control experiments adding to undoped graphene 1 wt% of triphenylborane, tris(pentafluorophenyl)borane or bis(pinacolyl)diborane as models of possible boron atom types present in B-doped graphene, and boric acid that could be present in a residual amount after pyrolysis, show in all cases an increase in the catalytic activity of graphene. B-doped graphene has also activity for glucose aqueous phase reforming. B-doped graphene undergoes deactivation upon reuse, probably due to B leaching. The results show that graphenes are promising metal-free catalysts for aqueous phase reforming and are alternatives to those containing platinum.

Iron Nanoparticles Embedded in Graphitic Carbon Matrix as Heterogeneous Catalysts for the Oxidative C−N Coupling of Aromatic N−H Compounds and Amides

Fe or Co nanoparticles (NPs) and two nanoparticulate Fe‐Co alloys having different Fe/Co atomic ratio with average particle size ranging from 10.9 to 26.5 nm embedded in turbostratic graphitic carbon matrix have been prepared by pyrolysis at 900 °C under inert atmosphere of chitosan powders containing Fe2+ and Co2+ ions in various proportions. The resulting Fe/Co NP@C samples have been evaluated as heterogeneous catalysts for the oxidative C−N coupling of amides and aromatic N−H compounds. It was observed that sequential addition of two aliquots of tert‐butyl hydroperoxide (TBHP) in an excess of N,N‐dimethylacetamide (DMA) as solvent affords the corresponding coupling product in high yields, and the most efficient catalyst was Fe NP@C. Fe NP@C is reusable and exhibits a wide scope. The catalytic activity of Fe is supported by using highly pure Fe salt and by the observation that purposely addition of Cu2+ impurities even plays a detrimental effect on the catalytic activity. Mechanistic studies by quenching with 2,2,6,6‐tetramethylpiperidyl‐1‐oxyl (TEMPO) have shown that the amide radical is the key reaction intermediate, and the role of Fe NP@C is to generate the first radicals by TBHP decomposition.

Defective graphene as a metal-free catalyst for chemoselective olefin hydrogenation by hydrazine

A series of defective graphenes containing or not containing N, B, S and other heteroatoms exhibited general activity as metal-free catalysts for the hydrogenation of CC double bonds by hydrazine in the presence of oxygen. The best-performing graphene was the one obtained from the pyrolysis of alginate and subsequent exfoliation by sonication. The material was reusable in three consecutive runs without decay in its catalytic activity, and it exhibited 99% chemoselectivity for CC double bonds vs. nitro group hydrogenation in contrast with conventional Pd supported on carbon, which was almost unselective. Theoretical calculations using a model for defective graphene for styrene hydrogenation showed adsorption of the substrate by π–π stacking, resulting in activation of the double bond and direct interaction of cis-diimide with the CC group.

Selective photocatalytic benzene hydroxylation to phenol using surface-modified Cu2O supported on graphene

J. H. thanks the Chinese Scholarship Council for a graduate scholarship. Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa, CTQ2015-69653-CO2-R1 and Grapas) and Generalitat Valenciana (Prometeo 2017-083) is gratefully acknowledged. This work was also supported by NSFC (21872031, U1705251) and 973 Program (2014CB239303) of P. R. China.

Engineering active sites on reduced graphene oxide by hydrogen plasma irradiation: mimicking bifunctional metal/supported catalysts in hydrogenation reactions

H2 plasma has been used to generate carbon vacancies on reduced graphene oxide to increase its catalytic activity as a hydrogenation catalyst. A relationship between the power of the plasma treatment and the exposure time with the activity of the material was observed for CC double bond hydrogenation. The activity data in the case of 1-octene, showing skeletal isomerization besides hydrogenation, indicate that H2 plasma treatment can introduce hydrogenating and acid sites rendering a bifunctional catalyst that is reminiscent of the activity of noble metals supported on acid supports.