Catalysis PDF [PDF]

Zitiervorschau

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WHAT YOU KNOW ABOUT CATALYSTS  A substance that affects the rate of reaction but emerges from the process unchanged. HOW ELSE CAN IT BE DEFINED? ◦ Promotes a more efficient molecular path or mechanism. ◦ Changes only the rate of reaction and does not affect the equilibrium.

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Catalysts are the workhorses of chemical transformations in the industry. Catalysts are indispensable in ◦ Production of transportation fuels in one of the approximately 440 oil refineries all over the world ◦ Production of bulk and fine chemicals in all branches of chemical industry ◦ Prevention of pollution by avoiding formation of waste (unwanted byproducts) ◦ Abatement of pollution in end-of-pipe solutions (automotive and industrial exhaust)

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A catalyst offers an alternative, energetically favorable mechanism to the noncatalytic reaction

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Occurrence, study and use of catalysts and catalytic processes. Catalytic reaction is a cyclic event.

Ref: Concepts of Modern Catalysis and Kinetics by: I. Chorkendorff and J.W. Niemantsverdriet

Potential Energy Diagram comparing non catalytic and catalytic reaction

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The catalyst offers an alternative path for the reaction. The activation energy of the catalytic reaction is significantly smaller. The overall change in free energy for the catalytic reaction equals that of the uncatalyzed reaction. The catalyst accelarates both the forward and the reverse reaction to the same extent.

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If the bonding between reactants and catalyst is too weak. If bond between the catalyst and one species is too strong, the first will be mostly occupied by the latter. If product P is strongly bound to the catalyst for separation to occur, then it poisons the catalyst.

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The chemical nature of catalysts is as diverse as catalysis itself, although some generalizations can be made. Proton acids are probably the most widely used catalysts, especially for the many reactions involving water, including hydrolysis and its reverse. Multifunctional solids often are catalytically active, e.g. zeolites, alumina, higher-order oxides, graphitic carbon, nanoparticles, nanodots, and facets of bulk materials. Transition metals are often used to catalyze redox reactions (oxidation, hydrogenation). Examples are nickel, such as Raney nickel for hydrogenation, and vanadium(V) oxide for oxidation of sulfur dioxide into sulfur trioxide. Many catalytic processes, especially those used in organic synthesis, require so called "late transition metals", which include palladium, platinum, gold, ruthenium, rhodium, and iridium

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Catalysts can be either heterogeneous or homogeneous, depending on whether a catalyst exists in the same phase as the substrate

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Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. The total surface area of solid has an important effect on the reaction rate.

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Homogeneous catalysts function in the same phase as the reactants, but the mechanistic principles invoked in heterogeneous catalysis are generally applicable. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence of H+ on the esterification of esters, e.g. methyl acetate from acetic acid and methanol.

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In homogeneous catalysis, both the catalyst and the reactants are in the same phase. ◦ Example: Ozone reaction with chlorine atoms

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Industry uses a multitude of homogenous catalysts in all kinds of reactions to produce chemicals. The catalytic carbonylation of methanol to acetic acid

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Enzymes are nature’s catalysts. For the moment it is sufficient to consider an enzyme as a large protein, the structure of which results in a very shape-specific active site. ◦ Example: Enzyme catalase catalyzes the decomposition of hydrogen peroxide into water and oxygen

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In heterogeneous catalysis, solids catalyze reactions of molecules in gas or solution. Heterogeneous catalysts are the workhorses of the chemical and petrochemical industry.

Example: Cleaning automotive exhaust by catalytic reaction. Reaction Mechanism

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To accelerate reactions under relatively milder conditions Green Chemistry ◦ To eliminate the need for toxic reactants ◦ To minimize the production of waste or undesired by-products

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Atom Economy

ethylene epoxide

Microscopic (subnanometer): spectroscopy, computational chemistry, kinetics, mechanism mesoscopic (1-10 nanometer): catalyst preparation, characterization, mechanistic investigation (transport), Shaped catalyst (mm to cm): porosity, strength, attrition resistance Macroscopic level (reactors cm to m): mass/heat transport, catalyst activity, sensitivity, mechanical strength.

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Activation and breaking of a chemical bond inside a molecule: picosecond regime, completion of an entire reaction cycle from complexation between catalyst and reactants through separation from the product: microseconds for the fastest enzymatic reactions to minutes for complicated reactions on surfaces. On the mesoscopic level, diffusion in and outside pores, and through shaped catalyst particles may take between seconds and minutes, and the residence times of molecules inside entire reactors may be from seconds to, effectively, infinity if the reactants end up in unwanted byproducts such as coke, which stay on the catalyst.