Kinetic Study of the Selective Hydrogenation of Acetylene over Supported Palladium under Tail-End Conditions

International audienceThe kinetics of the selective hydrogenation of acetylene in the presence of an excess of ethylene has been studied over a 0.05 wt. % Pd/α-Al 2 O 3 catalyst. The experimental reaction conditions were chosen to operate under intrinsic kinetic conditions, free from heat and mass transfer limitations. The data could be described adequately by a Langmuir-Hinshelwood rate-equation based on a series of sequential hydrogen additions according to the Horiuti-Polanyi mechanism. The mechanism involves a single active site on which both the conversion of acetylene and ethylene take place

Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water

International audienceThe kinetic stability of any material in water relies on the presence of surface weak spots responsible for chemical weathering by hydrolysis. Being able to identify the atomistic nature of these sites and the first steps of transformation is therefore critical to master the decomposition processes. This is the challenge that we tackle here: combining experimental and modeling studies we investigate the stability of alumina in water. Exploring the reactivity of shape-controlled crystals, we identify experimentally a specific facet as the location of the weak spots. Using biased ab initio molecular dynamics, we recognize this weak spot as a surface exposed tetra-coordinated Al atom and further provide a detailed mechanism of the first steps of hydrolysis. This understanding is of great importance to heterogeneous catalysis where alumina is a major support. Furthermore, it paves the way to atomistic understanding of interfacial reactions, at the crossroad of a variety of fields of research

Periodic reactor operation for parameter estimation in catalytic heterogeneous kinetics. Case study for ethylene adsorption on Ni/Al2O3

Periodic operation of chemical reactors has usually been analyzed for a potential gain in performance. However, at the laboratory scale, periodic operation of a catalytic reactor can be used for kinetic studies with better parameter estimation, which is inherent to transient kinetic studies. Different mathematical methods are developed and compared in this study to simulate and model concentration cycling in a fixed bed catalytic reactor. Different cases including dissociative and molecular adsorption are considered with or without external and internal mass transfer limitations. The analysis of these cases gives information on the contribution of the different parameters on the phase lag and gain loss. This information can be used to better plan experimental kinetic studies. Ethylene adsorption over a Ni/Al2O3 catalyst has been studied experimentally in a dedicated set-up with a high-speed Infrared analyzer capable of analyzing the outlet gas mixture with a data acquisition frequency up to 30 Hz. The data are modeled with one of the analytical expressions developed in this work to estimate the equilibrium adsorption constant

Hydrolysis of oligosaccharides over solid acid catalysts: a review

Mild fractionation/pretreatment processes are becoming the most preferred choices for biomass processing within the biorefinery framework. To further explore their advantages, new developments are needed, especially to increase the extent of the hydrolysis of poly- and oligosaccharides. A possible way forward is the use of solid acid catalysts that may overcome many current drawbacks of other common methods. In this Review, the advantages and limitations of the use of heterogeneous catalysis for the main groups of solid acid catalysts (zeolites, resins, carbon materials, clays, silicas, and other oxides) and their relation to the hydrolysis of model soluble disaccharides and soluble poly- and oligosaccharides are presented and discussed. Special attention is given to the hydrolysis of hemicelluloses and hemicellulose-derived saccharides into monosaccharides, the impact on process performance of potential catalyst poisons originating from biomass and biomass hydrolysates (e.g., proteins, mineral ions, etc.). The data clearly point out the need for studying hemicelluloses in natura rather than in model compound solutions that do not retain the relevant factors influencing process performance. Furthermore, the desirable traits that solid acid catalysts must possess for the efficient hemicellulose hydrolysis are also presented and discussed with regard to the design of new catalysts

Traitements et conseils à l'officine des affections cutanées les plus couramment rencontrées

MONTPELLIER-BU Pharmacie (341722105) / SudocSudocFranceF

Influence des propriétés texturales et de la méthode de préparation sur les performances hydrogénantes de catalyseurs Pd/C

Nous nous sommes attachés à étudier l influence des propriétés physicochimiques et texturales du charbon sur les caractéristiques et les performances catalytiques de catalyseurs 5% Pd/C dans des réactions d hydrogénation (cinnamaldéhyde et cyclododécatriène). La première voie de préparation de catalyseurs est l échange cationique H+ Pd2+. Cette technique requiert une fonctionnalisation oxydante (HNO3, NaOCl, H2O2) afin de créer des fonctions d ancrage. Les matériaux ainsi préparés sont très dispersés (dPd = 1,5 nm). Cependant leur stabilité thermique est restreinte du fait de la décomposition des fonctions stabilisantes dès 200C. Une technique alternative est la préparation par déposition-précipitation (DP) qui ne requiert aucun prétraitement oxydant mais un contrôle strict des conditions opératoires est nécessaire pour assurer une reproductibilité parfaite. La préparation de catalyseurs Pd/C à partir de deux lots commerciaux (L3S) selon les techniques précédentes conduit à des matériaux aux caractéristiques et aux performances hydrogénantes identiquesMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF