Fundamental and practical differences with homogeneous reactions

1 FUNDAMENTAL AND PRACTICAL DIFFERENCES WITH HOMOGENEOUS REACTIONS 

The use of solid catalysts, especially zeolites, introduces large differences in the reaction scheme and mechanisms as well as in the set-up of the desired transformation. 

In addition to the chemical steps, which are the only steps involved in stoichiometric or in homogeneous catalysis reactions, heterogeneous catalysis reactions involve also physical steps, i.e. transport (transfer) of organic molecules (and heat) from the reaction mixture to the active sites of the solid catalyst and vice versa.113-151 Another difference deals with the chemical steps, which do not occur in the fluid phase, but for part of them involve both fluid and solid phases (chemisorption and desorption), the other part occurring at the surface of the catalyst.113-151 

Heat transfer limitations could affect significantly the rate and selectivity of endothermic and especially of exothermic reactions.[13 15] Whereas the external thermal limitations could be minimized, this is much more difficult for the internal ones. Indeed the heat is produced or consumed inside the micropores and its transfer to the external surface is particularly slow because of the well-known insulator properties of zeolites. 

The use of solid catalysts and especially zeolites in Fine Chemical synthesis introduces another complication with respect to homogeneous reactions. There is always a progressive decrease of the catalyst activity with increasing reaction time.1191 In some reactions, this deactivation can be due to irreversible chemical transformation of the zeolite catalyst, e.g. reactions with acid reactants causing dealumination and sometimes collapse of the framework. However, in most cases, deactivation results from poisoning of the active sites by the desired reaction 

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Fundamental and practical differences with homogeneous reactions. 42... 

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