Selective photo-oxidation of glucose

Biomass transformation into high-value chemicals has attracted attention according to the ���green chemistry��� principles. Low price and high availability make biomass one of the most interesting renewable resources as it provides the means to create sustainable alternatives to the oil-derived building blocks of the chemical industry In recent year, the need for alternative environmentally friendly routes to drive chemical reactions has in photocatalytic processes an interesting way to obtain valuable chemicals from various sources using the solar light as energy source. The purpose of this work was to use supported noble metal nanoparticles in the selective photo-oxidation of glucose through using visible light. Glucose was chosen as model molecule because it is the cheapest and the most common monosaccharide. Few studies about glucose photo oxidation have been conducted so far, and reaction mechanism is still not totally explained. The aim of this work was to systematically analyze and assess the impact of several parameters (eg. catalyst/substrate ratio, reaction time, effect of the solvent and light source) on the reaction pathway and to monitor the product distribution in order to draw a general reaction scheme for the photo oxidation of glucose under visible light. This study regards the reaction mechanism and the influence of several parameters, such as solvent, light power and substrate concentration. Furthermore, the work focuses on the influence of gold and silver nanoparticles and on the influence of metal loading. The glucose oxidation was monitored through the mass balance and the products selectivity. Reactions were evaluated in terms of glucose conversion, mass balance and selectivities towards arabinose and gluconic acid. In conclusion, this study is able to demonstrate that the photo oxidation of glucose under visible light is feasible; the full identification of the main products allows, for the first time, a comprehensive reaction mechanism scheme.

Functionalization of gold and silver nanoparticles with diphenyl dichalcogenides probed by surface enhanced Raman scattering

This paper describes a surface-enhanced Raman scattering (SERS) systematic investigation regarding the functionalization of gold (Au) and silver (Ag) nanoparticles with diphenyl dichalcogenides, i.e. diphenyl disulfide, diphenyl diselenide, and diphenyl ditelluride. Our results showed that, in all cases, functionalization took place with the cleavage of the chalcogenchalcogen bond on the surface of the metal. According to our density functional theory calculations, the molecules assumed a tilted orientation with respect to the metal surface for both Au and Ag, in which the angle of the phenyl ring relative to the metallic surface decreased as the mass of the chalcogen atom increased. The detected differences in the ordinary Raman and SERS spectra were assigned to the distinct stretching frequencies of the carbonchalcogen bond and its relative contribution to the ring vibrational modes. In addition, the SERS spectra showed that there was no significant interaction between the phenyl ring and the surface, in agreement with the tilted orientation observed from our density functional theory calculations. The results described herein indicate that diphenyl dichalcogenides can be successfully employed as starting materials for the functionalization of Au nanoparticles with organosulfur, organoselenium, and organotellurium compounds. On the other hand, diphenyl disulfide and diphenyl diselenide could be employed for the functionalization of Ag nanoparticles, while the partial oxidation of the organotellurium unit could be detected on the Ag surface. Copyright (C) 2011 John Wiley & Sons, Ltd.

Selective oxidation of 5-hydroxymethylfurfural using monometallic and bimetallic supported nanoparticles

This work deals with the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) using metal supported catalysts. Catalysts were prepared from the immobilisation of preformed monometallic (Au, Pd) and bimetallic (AuCu, AuPd) nanoparticles on commercial oxides (TiO2, CeO2). Au-TiO2 catalyst was found to be very active for HMF oxidation; however, this system deactivated very fast. For this reason, we prepared bimetallic gold-copper nanoparticles and an increase in the catalytic activity was observed together with an increase in catalyst stability. In order to optimise the interaction of the metal active phase with the support, Au and AuCu nanoparticles were supported onto CeO2. Au-CeO2 catalyst was found to be more active than the bimetallic one, leading to the conclusion that in this case the most important feature is the interaction between gold and the support. Catalyst pre-treatments (calcination and washing) were carried out to maximise the contact between the metal and the oxide and an increase in the FDCA production could be observed. The presence of ceria defective sites was crucial for FDCA formation. Mesoporous cerium oxide was synthesised with the hard template method and was used as support for Au nanoparticles to promote the catalytic activity. In order to study the role of active phase in HMF oxidation, PdAu nanoparticles were supported onto TiO2. Au and Pd monometallic catalysts were very active in the formation of HMFCA (5-hydroxymethyl-2-furan carboxylic acid), but Pd was not able to convert it, leading to a low FDCA yield. The calcination of PdAu catalysts led to Pd segregation on the particles surface, which changed the reaction pathway and included an important contribution of the Cannizzaro reaction. PVP protected PdAu nanoparticles, synthesised with different morphologies (core-shell and alloyed structure), confirmed the presence of a different reaction mechanism when the metal surface composition changes.

Selective oxidation of cyclohexene through gold functionalized silica monolith microreactors

Two simple, reproducible methods of preparing evenly distributed Au nanoparticle containing mesoporous silica monoliths are investigated. These Au nanoparticle containing monoliths are subsequently investigated as flow reactors for the selective oxidation of cyclohexene. In the first strategy, the silica monolith was directly impregnated with Au nanoparticles during the formation of the monolith. The second approach was to pre-functionalize the monolith with thiol groups tethered within the silica mesostructure. These can act as evenly distributed anchors for the Au nanoparticles to be incorporated by flowing a Au nanoparticle solution through the thiol functionalized monolith. Both methods led to successfully achieving even distribution of Au nanoparticles along the length of the monolith as demonstrated by ICP-OES. However, the impregnation method led to strong agglomeration of the Au nanoparticles during subsequent heating steps while the thiol anchoring procedure maintained the nanoparticles in the range of 6.8 �� 1.4 nm. Both Au nanoparticle containing monoliths as well as samples with no Au incorporated were tested for the selective oxidation of cyclohexene under constant flow at 30 ��C. The Au free materials were found to be catalytically inactive with Au being the minimum necessary requirement for the reaction to proceed. The impregnated Au-containing monolith was found to be less active than the thiol functionalized Au-containing material, attributable to the low metal surface area of the Au nanoparticles. The reaction on the thiol functionalized Au-containing monolith was found to depend strongly on the type of oxidant used: tert-butyl hydroperoxide (TBHP) was more active than H2O2, likely due to the thiol induced hydrophobicity in the monolith.

Redox-controlled crotyl alcohol selective oxidation:in situ oxidation and reduction dynamics of catalytic Pd nanoparticles via synchronous XANES/MS

In-situ, synchronous MS/XANES reveals the Pd catalyzed selective aerobic oxidation of crotyl alcohol is regulated by the balance between the oxidation state and reducibility. Dynamic XANES measurements provide a new, rapid method to determine redox kinetics of nanoparticles and identify important parameters to optimize catalyst design. �� 2012 American Chemical Society.

Frequency upconversion luminescence from Yb(+3)-Tm(+3) codoped PbO-GeO(2) glasses containing silver nanoparticles

Infrared-to-visible and infrared-to-infrared frequency upconversion processes in Yb(3+)-Tm(3+) doped PbO-GeO(2) glasses containing silver nanoparticles (NPs) were investigated. The experiments were performed by exciting the samples with a diode laser operating at 980 nm (in resonance with the Yb(3+) transition (2)F(7/2)->(2)F(5/2)) and observing the photoluminescence (PL) in the visible and infrared regions due to energy transfer from Yb(3+) to Tm(3+) ions followed by excited state absorption in the Tm3+ ions. The intensified local field in the vicinity of the metallic NPs contributes for enhancement in the PL intensity at 480 nm (Tm(3+) :(1)G(4)->(3)H(6)) and at 800 nm (Tm(3+) : (3)H(4) -> (3)H(6)). (C) 2009 American Institute of Physics. [doi:10.1063/1.3211300]

Preparation and characterization of the novels terpolymers of poly-{trans-[RuCl(2)(vpy)(4)]-styrene-divinylbenzene} and styrene-divinylbenzene-vinylpiridine impregnated with silver nanoparticles

This paper reports the preparation and characterization of poly-{trans-[RuCl(2)(vpy)(4)]-styrene-divinylbenzene} and styrene-divinylbenzene-vinylpiridine filled with nanosilver. Theses materials were synthesized by non aqueous polymerization through a chemical reaction using benzoyl peroxide as the initiator. The nanosilver was obtained from chemical reduction using NaBH(4) as reducing agent and sodium citrate as stabilizer. The nanometric dimension of nanosilver was monitored by UV-visible and confirmed through TEM. The morphology was characterized by SEM and the thermal properties were done by TGA and DSC. The antimicrobial action of the polymers impregnated with nanosilver was evaluated using both microorganisms, Staphylococcus aureus and Escherichia coli. The antimicrobial activity of the poly-{trans-[RuCl(2)(vpy)(4)]-styrene-divinylbenzene} filled with nanosilver was confirmed by the presence of an inhibition halo of the bacterial growth in seeded culture media, but was not confirmed to the styrene-divinylbenzene-vinylpiridine. The present work suggest that trans - [RuCl(2)(vpy)(4)] complex facilitate the release of silver ion from the media.

Synthesis of silver nanoparticles using DL-alanine for ESR dosimetry applications

The potential use of alanine for the production of nanoparticles is presented here for the first time. Silver nanoparticles were synthesized using a simple green method, namely the thermal treatment of silver nitrate aqueous solutions with in-alanine. The latter compound was employed both as a reducing and a capping agent. Particles with average size equal to 7.5 nm, face-centered cubic crystalline structure, narrow size distribution, and spherical shape were obtained. Interaction between the silver ions present on the surface of the nanoparticles and the amine group of the DL-alanine molecule seems to be responsible for reduction of the silver ions and for the stability of the colloid. The bio-hybrid nanocomposite was used as an ESR dosimeter. The amount of silver nanoparticles in the nanocomposite was not sufficient to cause considerable loss of tissue equivalency. Moreover, the samples containing nanoparticles presented increased sensitivity and reduced energetic dependence as compared with pure DL-alanine, contributing to the construction of small-sized dosimeters. (C) 2011 Elsevier Ltd. All rights reserved.

Alumina-grafted SBA-15 as a high performance support for Pd-catalysed cinnamyl alcohol selective oxidation

Ultrathin alumina monolayers grafted onto an ordered mesoporous SBA-15 silica framework afford a composite catalyst support with unique structural properties and surface chemistry. Palladium nanoparticles deposited onto Al-SBA-15 via wet impregnation exhibit the high dispersion and surface oxidation characteristic of pure aluminas, in conjunction with the high active site densities characteristic of thermally stable, high-area mesoporous silicas. This combination confers significant rate enhancements in the aerobic selective oxidation (selox) of cinnamyl alcohol over Pd/Al-SBA-15 compared to mesoporous alumina or silica supports. Operando, liquid-phase XAS highlights the interplay between dissolved oxygen and the oxidation state of palladium nanoparticles dispersed over Al-SBA-15 towards on-stream reduction: ambient pressures of flowing oxygen are sufficient to hinder palladium oxide reduction to metal, enabling a high selox activity to be maintained, whereas rapid PdO reduction and concomitant catalyst deactivation occurs under static oxygen. Selectivity to the desired cinnamaldehyde product mirrors these trends in activity, with flowing oxygen minimising CO cleavage of the cinnamyl alcohol reactant to trans-��-methylstyrene, and of cinnamaldehyde decarbonylation to styrene. �� 2013 Elsevier B.V.

Operando synchronous DRIFTS/MS/XAS as a powerful tool for guiding the design of Pd catalysts for the selective oxidation of alcohols

The selective aerobic oxidation of crotyl alcohol to crotonaldehyde was investigated by time-resolved synchronous DRIFTS/MS/XAS over silica and alumina supported Pd nanoparticles. Alcohol and oxygen reactant feeds were cycled through the catalyst bed while dynamic measurements of the palladium oxidation state, molecular adsorbates and evolved product distribution were made simultaneously on a sub-second timescale. Highly dispersed palladium nanoparticles remained in a partially oxidised state

In situ X-ray studies of crotyl alcohol selective oxidation over Au/Pd(1 1 1) surface alloys

The selective oxidation of crotyl alcohol to crotonaldehyde over ultrathin Au overlayers on Pd(1 1 1) and Au/Pd(1 1 1) surface alloys has been investigated by time-resolved X-ray photoelectron spectroscopy (XPS) and mass spectrometry. Pure gold is catalytically inert towards crotyl alcohol which undergoes reversible adsorption. In contrast, thermal processing of a 3.9 monolayer (ML) gold overlayer allows access to a range of AuPd surface alloy compositions, which are extremely selective towards crotonaldehyde production, and greatly reduce the extent of hydrocarbon decomposition and eventual carbon laydown compared with base Pd(1 1 1). XPS and CO titrations suggest that palladium-rich surface alloys offer the optimal balance between alcohol oxidative dehydrogenation activity while minimising competitive decomposition pathways, and that Pd monomers are not the active surface ensemble for such selox chemistry over AuPd alloys. Crown Copyright �� 2008.

Selective oxidation of allylic alcohols over highly ordered Pd/meso-Al2 O3 catalysts

Highly ordered mesoporous alumina was prepared via evaporation induced self assembly and was impregnated to afford a family of Pd/meso-Al2O3 catalysts for the aerobic selective oxidation (selox) of allylic alcohols under mild reaction conditions. CO chemisorption and XPS identify the presence of highly dispersed (0.9���2 nm) nanoparticles comprising heavily oxidised PdO surfaces, evidencing a strong palladium-alumina interaction. Surface PdO is confirmed as the catalytically active phase responsible for allylic alcohol selox, with initial rates for Pd/meso-Al2O3 far exceeding those achievable for palladium over either amorphous alumina or mesoporous silica supports. Pd/meso-Al2O3 is exceptionally active for the atom efficient selox of diverse allylic alcohols, with activity inversely proportional to alcohol mass.

Hierarchically ordered nanoporous Pd/SBA-15 catalyst for the aerobic selective oxidation of sterically challenging allylic alcohols

The utility of a hierarchically ordered nanoporous SBA-15 architecture, comprising 270 nm macropores and 5 nm mesopores (MM-SBA-15), for the catalytic aerobic selective oxidation of sterically challenging allylic alcohols is shown. Detailed bulk and surface characterization reveals that incorporation of complementary macropores into mesoporous SBA-15 enhances the dispersion of sub 2 nm Pd nanoparticles and thus their degree of surface oxidation. Kinetic profiling reveals a relationship between nanoparticle dispersion and oxidation rate, identifying surface PdO as the catalytically active phase. Hierarchical nanoporous Pd/MM-SBA-15 outperforms mesoporous analogues in allylic alcohol selective oxidation by (i) stabilizing PdO nanoparticles and (ii) dramatically improving in-pore diffusion and access to active sites by sesquiterpenoid substrates such as farnesol and phytol. �� 2013 American Chemical Society.