Properties In Oxidation Reactions Research Articles

Nanostructured silica-supported gold: Effect of nanoparticle size distribution and electronic state on its catalytic properties in oxidation reactions

Gold nanocatalysts, active in several oxidation reactions, suffer of insufficient time-on-stream stability. The easiest way to solve this problem is modifying the support, due to metal-support interaction. This study compares modifying effects of MgO and La2O3 on textural, electronic, and catalytic properties of Au nanoparticles (NPs) supported on inert nanostructured SiO2 in CO oxidation and liquid phase 1-octanol oxidation. Modification of the silica support surface with La and Mg increased metal support interaction, leading to gold particles with primary size of 1 nm but with different stability: stable under different pretreatment conditions on Mg-modified samples but highly sensible to the pretreatments on La-modified samples. Both modifiers changed electronic properties of supported gold favoring formation and stabilization of Auδ+ states, which are probable gold active sites in catalytic redox processes. Modification with La and Mg oxides changed catalytic properties in CO oxidation before and after pretreatment in H2 at 300 °C for 1 h. Gold catalysts supported on La- and Mg-modified silica showed similar performance in 1-octanol oxidation with higher conversion than unmodified Au/SiO2. La and Mg showed better promoting effects of catalytic properties in this reaction than redox modifiers (Fe and Ce) supported on small SiO2 particles.

Alloy oxidation as a route to chemically active nanocomposites of gold atoms in a reducible oxide matrix.

While nanoparticles are being pursued actively for a number of applications, dispersed atomic species have been explored far less in functional materials architectures, primarily because composites comprising dispersed atoms are challenging to synthesize and difficult to stabilize against sintering or coarsening. Here we show that room temperature oxidation of Au-Sn alloys produces nanostructures whose surface is terminated by a reducible amorphous oxide that contains atomically dispersed Au. Analysis of the oxidation process shows that the dispersal of Au in the oxide can be explained by predominant oxygen anion diffusion and kinetically limited metal mass transport, which restrict phase separation due to a preferential oxidation of Sn. Nanostructures prepared by oxidation of nanoscale Au-Sn alloys with intermediate Au content (30-50%) show high activity in a CO-oxidation probe reaction due to a cooperative mechanism involving Au atoms as sites for CO adsorption and reaction to CO2 embedded in a reducible oxide that serves as a renewable oxygen reservoir. Our results demonstrate a reliable approach toward nanocomposites involving oxide-embedded, atomically dispersed noble metal species.

Mn–Co–Al–Mg mixed oxides by auto-combustion method and their use as catalysts in the total oxidation of toluene

Manganese oxides were prepared by the auto-combustion method and while employing variable Mn/Mg ratios from 0.1 to 1.0 with glycine as a combustible and a glycine/nitrate=1 ratio. The Mn/Mg ratio that showed the best activity in the oxidation of toluene was maintained in order to synthesize the hydrotalcite structure by the co-precipitation method, which by calcination, allows the formation of a mixed oxide. Cobalt oxides were prepared by the two synthesis methods while maintaining the best metal charge (corresponding to the best catalytic behavior).The obtained solids were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), temperature programmed reduction (H2-TPR) and surface area measurements, and the catalytic activity for the complete toluene oxidation was evaluated. The results revealed that the auto-combustion catalysts are obtained in very short times and that their physico-chemical and catalytic properties are the same or higher than those of the co-precipitation catalysts. The solid with the Mn/Mg=1 ratio is the most active and selective catalyst and shows more activity than the oxide obtained by the co-precipitation method. The redox behavior in the manganese catalysts depends on the preparation method, which, in the case of auto-combustion, avoids the segregation of phases. The manganese reacts faster than the cobalt with the glycine and produces oxides with better properties in oxidation reactions. On the contrary, the cobalt oxides prepared by the two synthesis methods with a Co/Mg=1 ratio presented similar results in toluene oxidation.

Alkene epoxidation by manganese(III) complexes immobilized onto nanostructured carbon CMK-3

New heterogeneous catalysts were prepared through immobilization of Mn(III) complexes onto CMK-3 with different surface treatments. The [Mn(TF5PP)Cl] and [Mn(5-HOsalhd)Cl] complexes were anchored by a microwave-assisted protocol (CAT I and CAT III, respectively) and the Jacobsen catalyst was immobilized using conventional synthesis conditions (CAT II). XPS and FTIR revealed that the CMK-3 target functionalities for immobilization were the surface oxygen groups. XRD showed that the support treatments led to a decrease of its 2D-hexagonal order, but the complex grafting did not induce changes on the microstructure.CAT I was active in the epoxidation of cis-cyclooctene by H2O2, with substrate conversions and epoxide selectivities in the ranges of 38–30% and 100–97%, respectively, in three cycles with almost no complex leaching. CAT II was active in the epoxidation of 6-cyano-2,2-dimethylchromene using m-CPBA/NMO system with a substrate conversion similar to that of the free complex and higher ee %, but with some Mn leaching in the catalytic runs. CAT III was active in the epoxidation of indene, by H2O2, in ethanol with substrate conversions, epoxide selectivities and ee % values in the ranges of 45–37%, 83–86% and 10–12%, respectively. A correlation between the structural properties of the supports and the reaction times could be proposed. These results highlight the potentiality of CMK-3 to act as support for metal complexes with catalytic properties in oxidation reactions.

New cup-shaped α-cyclodextrin derivatives and a study of their catalytic properties in oxidation reactions

A series of new α-cyclodextrin derivatives with a substituted propylene bridge attached to the 6-O's of the A,D-glucose units are reported. The compounds were prepared from the known 6 A,6 D-di- O-(prop-2-methylidene-1,3-dienyl)-hexadeca- O-benzyl-α-cyclodextrin, which was transformed into 6 A,6 D-di- O-(prop-2-methyl-1,3-dienyl)-α-cyclodextrin, 6 A,6 D-di- O-(prop-2-formyl-2-hydroxy-1,3-dienyl)-α-cyclodextrin, 6 A,6 D-di- O-(prop-2-aminomethyl-2-hydroxy-1,3-dienyl)-α-cyclodextrin, 6 A,6 D-di- O-(prop-2-hydroxymethylidene-1,3-dienyl)-α-cyclodextrin, 6 A,6 D-di- O-(prop-2-formyl-1,3-dienyl)-α-cyclodextrin, 6 A,6 D-di- O-(prop-2-carboxy-1,3-dienyl)-α-cyclodextrin and 6 A,6 D-di- O-(prop-2-methoxycarbonyl-1,3-dienyl)-α-cyclodextrin. The new compounds were evaluated for their ability to affect amine and alcohol oxidations in the presence of hydrogen peroxide.

Effect of the synthesis conditions on the redox and catalytic properties in oxidation reactions of LaCo 1− xFe xO 3

The aim of this work was to study the effect of the synthesis conditions of LaCoO 3 on the redox properties and the catalytic activity for the CH 4 and CO oxidation reactions. Five LaCoO 3 samples were prepared by different methods: solid state (SS), coprecipitation (COP), citrate complexation (CIT) and reactive grinding of the single oxides (RG) and of an amorphous precursor (COPRG). TPD-O 2 experiments showed three kinds of oxygen. First, two surface oxygen species (designated as α-oxygens) were distinguished. The amounts of oxygen desorbing in the two α-oxygen desorption peaks were found to be mainly dependant on the specific surface area of the samples and stay lower than one monolayer (0.7–0.8). The last desorption peak, which is generally attributed to the desorption of some lattice oxygens (β-oxygens), was found to be more dependant on the morphology of the sample. H 2 reduction experiments (TPR-H 2) showed the usual two-step reduction. The reduction temperatures were found to be directly related to the calcination temperatures. H 2 uptakes and XRD analysis suggest a reduction of slightly more than one e − in the first step and the formation of some Co 0 before the beginning of the second reduction step. Kinetic analysis for the CH 4 oxidation reaction showed that the specific reaction rate is significantly affected by the synthesis procedure. Grinding an amorphous precursor results in a perovskite which combines a high specific surface area (made possible by the use of grinding) and a high activity. The RG sample presents a lower activity than COPRG, due to a higher iron contamination. Nevertheless, the differences observed in the oxidation reaction rate could not be correlated to the redox properties, measured by TPD-O 2 and TPR-H 2, of the catalysts. Then, these methods are not adequate to evaluate the reactivity of the solids. When tested in the CO oxidation reaction, the samples present different comportments than in the CH 4 oxidation reaction. These differences are discussed on the basis of the oxidation mechanisms involved in the two reactions.

Promoter-induced synthesis of MCM-41 type mesoporous materials including Ti- and V-MCM-41 and their catalytic properties in oxidation reactions

The addition of small amounts of certain oxyanions of group VA, VIA and VIIA (such as nitrate, phosphate, arsenate, sulfate, bromate, iodate etc.) to the synthesis gel mixture of MCM-41 type mesoporous materials reduces the synthesis time considerably. The hydrothermal syntheses of Si-MCM-41, Al-MCM-41, Ti-MCM-41 and V-MCM-41 were carried out in the presence and absence of promoters by refluxing the gel mixture at atmospheric pressure. Studies on the effect of different promoters in the synthesis of Si-MCM-41 show different degrees of enhancement in the long-range ordering process. It has been observed that the higher the charge/radius ratio ( Z/ r, Å −1) of the central cation of the promoter, the higher is the ability of the promoter to speed up the synthesis. The concept of promoter-induced synthesis of MCM-41 type mesoporous materials is applicable not only to Si-MCM-41 but also to Al-MCM-41, Ti-MCM-41 and V-MCM-41. The materials were characterized by XRD, N 2 adsorption, diffuse reflectance UV–vis spectroscopy, FTIR, solid state MAS NMR, SEM, EDX and TEM. The XRD patterns and the quite high BET surface areas with isotherms of type IV of all the samples are typical for hexagonal MCM-41 type ordered mesoporous materials. EDX analysis, solid state 27 Al and 51 V MAS NMR spectra and diffuse reflectance UV–vis spectra of the M-MCM-41 (M = Ti and V) samples confirm the incorporation of Al, Ti and V into the framework. The catalytic properties of the two Ti-MCM-41 samples and the two V-MCM-41 samples, prepared in the presence and absence of a promoter, were compared by means of the epoxidation of cyclohexene and the hydroxylation of 1-naphthol in the presence of different peroxide sources. Comparable catalytic properties indicate that the samples, prepared in the presence and absence of a promoter, are similar in nature and the metal ions are present in the framework, presumably in a tetrahedral coordination.

Sn-ZSM-12, a new, large pore MTW type tin-silicate molecular sieve: synthesis, characterization and catalytic properties in oxidation reactions

A new large pore tin-silicate analogue of zeolite ZSM-12 (MTW topology) with Si/Sn molar ratio >70 has been synthesized hydrothermally using a new template, hexamethylene bis(benzyl dimethyl ammonium hydroxide). This material exhibits an expansion in unit cell volume (XRD), an IR band at 970 cm−1 and a charge transfer band at 205 nm in the UV-Vis region indicating the presence of Si-O-Sn units with Sn4+ centers in Td configuration. Sn-ZSM-12 catalyzes the oxidation of phenol,m-cresol andm-xylene using dilute H2O2 as an oxidizing agent.

Low temperature, efficient synthesis of new As(V)-silicate molecular sieves with MFI topology and their catalytic properties in oxidation reactions

New MFI type arseno-silicate molecular sieves (As(V)-MFI), synthesized for the first time, can be crystallized at lower temperature (70–90‡C) in 1–3 days under hydrothermal conditions. Interestingly, these As(V)-MFI molecular sieves can be synthesized even faster than the As-free silicalite-1 under identical conditions. Further, the crystallization becomes faster with the increase in As content of the reaction mixture, contrary to the observations made on all the other high silica molecular sieves, so far. These microporous As(V)-silicates exhibit significant catalytic activity in (i) phenol hydroxylation using 30% aq. H2O2 (ca. 55% H2O2 efficiency) and (ii) oxidative dehydrogenation of 2-butanol and benzyl alcohol using air under fixed bed vapor phase reaction conditions exhibiting ca. 75–85% selectivity towards 2-butanone and benzaldehyde+benzoic acid, respectively.

A mechanism for sensing reducing gases with vanadium pentoxide films

Vanadiium pentoxide is generally a non-stoichiometric material, which is known for its catalytic properties in oxidation reactions. As it is an electric conductor when oxygen vacancies are present, the purpose of this investigation is to determine the influence that reducing gases have on the conductance of vanadium pentoxide films. These films are prepared from crushed powder and by evaporation. It is found that a reducing gas such as hydrogen increases the conductance of the film in air, If a platinum catalyst is added. For the recovery process, the presence of oxygen is necessary, as demonstrated by the responses to hydrogen and oxygen in a nitogen flow. Additionally, vanadium pentoxide powder loses wieght when exposed to 5000 ppm hydrogen in air, and regains it when exposed to air alone. To explain the observed response, a model is proposed that involves a surface reaction of atmospheric reducing gas and atmospheric oxygen, and fifusion of lattice oxygen in response to changes at the surface.

Synthesis, characterization and catalytic properties of vanadium silicates with a ZSM-48 structure

A new crystalline inicroporous vanadium silicate with ZSM-48 structure has been synthesized. On the basis of physico-chemical characterization, it is shown that vanadium atoms can be incorporated into the lattice and that the maximum incorporation that can be achieved following the synthesis method is about 0.8 V/u.c. These materials have interesting catalytic properties in oxidation reactions involving peroxides at low temperatures.