Pot catalytique

Le pot catalytique ou catalyseur est un élément de l'ensemble du pot d'échappement des moteurs à combustion interne qui vise à réduire la nocivité des gaz d'échappement. Constitution thumb|Vue « en écorché » d'un pot catalytique à enveloppe en inox.Dès la toute fin du , alors que seulement quelques milliers de voitures « à pétrole » sont en circulation, sont conçus en France des premiers prototypes de pots catalytiques automobiles, constitués d'une matière inerte « de contact » imprégnée de platine, d'iridium et de palladium, enfermée dans un cylindre métallique à double paroi isolante traversé de part en part par les gaz d'échappement.thumb|Vue de la structure interne d'un pot catalytique. Les versions modernes sont constituées d'une chambre d'acier inoxydable dans laquelle sont conduits les gaz d'échappement, lesquels traversent les conduits d'une structure en nid d'abeille généralement faite en céramique. L'intérieur des conduits est recouvert d'une fine couche de cris

Rational Design of Pd-Based Catalysts for Selective Alkyne Hydrogenations

Rocio Micaela Crespo Quesada

Traditionally, the search for active and selective catalysts involved a rather tedious "hit-and-miss" approach in which hundreds, if not thousands catalysts were tested until the optimal substance was identified. With the advancing theoretical understanding of catalysis and the development of computational power, a new era of rational catalyst design is dawning. This approach, grounded on first principles, is based on new advances in synthesis, characterization and modeling with the ultimate aim of predicting the expected behavior of a catalyst based on chemical composition, molecular structure and morphology. The rational design of a catalyst is a complex process which spans across several levels of scale. In this thesis, the pursuit of a rational design for Pd-based catalysts effective in alkyne hydrogenations is presented. A multi-level integrated approach was thus applied ranging from the nano-scale design of the active sites for a specific reaction, taking into account its structure sensitivity, to the micro-scale design of the supported Pd nanoparticles including metal-additive and metal-support interactions as well as mass and heat transfer phenomena. In order to rationally design a catalyst at a nano-scale, i.e. a catalyst's active site, several methodologies have been hitherto applied, such as the study on single crystals or model catalysts. Here we present the use of metal nanoparticles with tuned sizes (5-30 nm) and shapes (cubes, octahedra and cube-octahedra) prepared via colloidal techniques. These nanoparticles can be tested per se and represent a new generation of model catalysts, complementing single crystal studies, which inherently lack the complexity of industrial catalysis. However, metal nanoparticles, especially those prepared in a controlled manner in order to tailor their shape and size, require the use of stabilizing and/or capping agents capable of directing their growth. These substances can mask the true catalytic behavior of the nanoparticles. Thus, it is of great importance to study the interactions of the active phase with the substances that are in close contact with it, in the so-called meso-scaled level of rational catalyst design. Therefore, the effect of the nature of the stabilizing agent on the catalytic response was studied, as well as the promoting effect of some of these substances. Finally, a methodology was developed capable of eliminating organic stabilizing agents from the surface of nanoparticles without compromising their morphological stability. The knowledge gathered in the first two levels can be applied further to reach the microscale rational catalyst design. In this thesis, a final catalyst consisting on well-defined stabilizer-free supported Pd nanoparticles was used in the hydrogenation of acetylene. This deep study of the catalytic behavior of well-defined Pd catalysts throughout several levels of scale and complexity have given us the tools needed to perform a rational catalyst design for alkyne hydrogenations. Depending on the specific reaction, the active phase can be optimized in terms of the desired activity and selectivity and can be tuned even further with the use of specific additives. Finally, the appropriate support also exerts a promoting effect on the nanoparticles and ensures their anchoring in addition to avoiding heat and mass transfer artifacts.

EPFL2011

Hydrogénation sélective du 2-butyne-1,4-diol avec des catalyseurs de palladium supportés sur des matériaux filamenteux structurés

Eric Joannet

The principal objective of this thesis is the development of a palladium catalyst supported on woven fabrics of activated carbon fibers (ACF) for the reactions of hydrogenations in liquid phase and the design of multiphase reactors using the characteristics of these structures. This work was focused on Preparation, characterization and optimization of catalysts supported on the woven fabrics of activated carbon fibers; The intrinsic kinetics of the selective hydrogenation of 2-butylene-1,4-diol to 2-butyne-1,4-diol as model reaction; The comparison of the activity of our structured catalyst with that of commercial catalysts; Increase the catalyst concentration in the multiphase reactors with the design of a loop reactor containing a bubble column staged with the fiber fabrics supporting the catalyst, its characterization and the startup of this loop reactor which can operate in continuous or in batch mode. Chapter 4 presents the methods of manufacture of our palladium catalysts supported on woven structures of activated carbon fibers, their reproducibility and their homogeneous deposition. The characteristics of the support such as its handiness, its resistance to mechanical forces, the opening of its surface to liquid flow, its great specific surface area and its homogeneous distribution of micropores are defined there. The deposition of nanoparticules of active phase (0.5-5%mass) is characterized by metal surface obtained (280-87 m2/g), the dispersion of this surface (62.8-19.6%) and the average diameter of the nanoparticles (1.78-5.72 nm). The total degree of initial reduction (before reaction) was confirmed. Chapter 5 is devoted to the study of the reaction of selective hydrogenation of the 2-butyne-1,4-diol (B3) to 2-butylene-1,4-diol (B2) with a commercial palladium catalyst supported on activated carbon powder. A zero value even slightly negative was found for B3 partial order and equal to one for hydrogen. The activity obtained for this catalyst is large (37.1 mol(H2)/h gPd) but its initial selectivity is particularly low (SB3,0 = 76.8 %). Effect of support was tested and highlighted. Two supports (CaCO3 and Al2O3) gave reaction rates approximately half the size of those of the carbon support, but an initial selectivity close to 99 %. In chapter 6, selective hydrogenation in an autoclave with an agitator containing structured catalyst is tested. This reactor is first of all evaluated like a reactor working in kinetic mode for the selected reaction. Kinetics was studied and showed that the fibrous support does not change the orders of reaction compared to the commercial catalyst. Energy of activation of 30 kJ/mol was found in the studied configuration. A variation of the catalytic amount on woven structure was studied and gave a reaction rate of 19.4 (mol(H2)/h gPd) for an initial selectivity of 98 % for the best catalyst (Pd(2%)/ACF). The reuse of this catalyst with an in situ reactivation in hydrazine allowed a stabilization of the activity at 50% of the initial activity after four experiments. A constant Turn Over Frequency (TOF) for catalysts with high deposits (2-5%) was given. An increase in the TOF for the low deposits (0.5% and 1%) was found. This increase in the TOF agrees with a fall of the initial selectivity. The effect of several variables of the reaction such as pressure, temperature and pH of the solution were examined. Within the framework of the variation of pH an initial selectivity of 100% and selectivity during reaction remaining higher than 95% until a conversion of 90 % were obtained. This profit of selectivity was achieved only with the detriment to the reaction rate which decreased by a factor 2.5. Finally, experiments with even less harm to the environment were made without solvent thus allowing the avoidance of purification. The rate of hydrogenation in this configuration is very high (79.2 mol(H2)/(h gPd)). But in this configuration, this increase in activity is not done at the expense of the initial selectivity (98 %). In chapter 7, the increase in the catalyst concentration in the reactor was required for higher productivity. For that purpose, a new concept of reactor was studied. This new reactor is a loop reactor containing a bubble column staged with woven catalytical structure. The pressure drop, the emulsion crossing the bubble column and the RTD were characterized. The reactor, functioning in batch or continuous mode, was studied. The productivity in batch was improved by an order of magnitude due to the increase in the catalytic concentration. A unit worked during 50 hours continuously without deactivation of catalyst providing 160 g of B2 per hour (> 1 tons/year). The reaction led to a conversion of 85% with a selectivity of 96 %.

EPFL2005

Rational Design of Pd-Based Catalysts for Selective Alkyne Hydrogenations

Rocio Micaela Crespo Quesada

EPFL2011

Hydrogénation sélective du 2-butyne-1,4-diol avec des catalyseurs de palladium supportés sur des matériaux filamenteux structurés

Eric Joannet

EPFL2005

Catalytic potential of transition metal carbonyl compounds containing ferrocenyl-based phosphine ligands in asymmtric synthesis

Rational Design of Pd-Based Catalysts for Selective Alkyne Hydrogenations

Hydrogénation sélective du 2-butyne-1,4-diol avec des catalyseurs de palladium supportés sur des matériaux filamenteux structurés

Rational Design of Pd-Based Catalysts for Selective Alkyne Hydrogenations

Catalytic potential of transition metal carbonyl compounds containing ferrocenyl-based phosphine ligands in asymmtric synthesis

Hydrogénation sélective du 2-butyne-1,4-diol avec des catalyseurs de palladium supportés sur des matériaux filamenteux structurés