Green Chemistry Series
A catalyst is a substance that affects the rate or the direction of a chemical reaction, but is not consumed or altered during the process. This definition is, however, valid only theoretically in terms of chemistry, because the catalyst may undergo physical and sometimes chemical changes. A catalyst therefore affects only the rate of a chemical reaction and not its equilibrium, i.
Heterogeneous catalysis enables the development of large-scale continuous processes, whereas homogeneous catalysis is mainly used in fine chemicals manufacturing. Catalysis is mainly developed through advances in three areas: Chemical kinetics is an important tool for studying catalytic reactions, and is concerned primarily with chemical changes and the energy and mass fluxes associated with them.
Kinetics is simply the measurement and analysis of reaction rates. The rate of a chemical reaction is defined as where V , C , and t are the system volume, extent of reaction, and time, respectively. Usually, integral methods based on integration of the reaction rate expressions and differential methods based on differentiation of experimental concentration versus time data are the techniques used to determine empirical reaction rate functions.
The rate equations of simple reactions for which the concentrations of reactants, intermediates, and products depend on time can be solved analytically.
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However, for complex parallel—consecutive reactions, the rate expression becomes unmanageable and closed-form solutions cannot be easily obtained, especially for the time dependence of various species concentrations in the system. There are also numerous chemical reactions that show this parallel—consecutive behavior; examples include HYD, oxidative dehydrogenation, hydrodesulfurization HDS , and selective oxidation Babich et al. The term consecutive reactions refer to those reactions in which one or more of the products formed initially primary product undergoes a subsequent reaction to give another product.
A secondary product may therefore be produced by various routes. In handling such reactions, it is necessary to deal with the problems of determining the orders and rate constants for each individual reaction.
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Many catalysts can stimulate one reaction with various activities and selectivities. Appropriate and reliable kinetic investigations are crucial for ranking catalysts in order of activities and selectivities. The product distribution as a function of contact time between catalyst and feed can help to identify the reaction network.
Silver and gold-catalyzed multicomponent reactions
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Green Chemistry Series
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Heterogeneous Catalysis for Today's Challenges: