Competition for adsorption influence on reaction rate, stability and selectivity

4 COMPETITION FOR ADSORPTION INFLUENCE ON REACTION RATE, STABILITY AND SELECTIVITY 

Competition between reactant product and solvent molecules for adsorption within the zeolite micropores and on the internal active sites plays a significant role in the liquid phase transformation of functionalized compounds over zeolites. 

The Fries rearrangement of PA over H-BEA zeolites, which is a simple reaction, was chosen to introduce the competition for adsorption on the zeolite catalysts and its role on the reaction rate. Ortho- and para-hydroxyacetophenones (o- and p-HAP), para-acetoxyacetophenone (p-AXAP) and phenol (P) are the main products o-HAP, P and p-AXAP, which are directly formed (primary products), 

Kinetic studies A kinetic study of PA transformation was carried out in a batch reactor at 433 K in the presence of sulfolane (very polar) or dodecane (nonpolar) as solvent.[48] The initial reaction rates, the decrease in rate with time and the product distribution depend very much on the solvent polarity. The initial rates are lower in the polar sulfolane than in the nonpolar dodecane solvent. In the latter solvent, but not in sulfolane, there is a rapid decrease in the reaction rates, this decrease affecting preferentially the bimolecular formation of p-AXAP and p-HAP. As a consequence, these products are much more favoured with respect to o-HAP in sulfolane than in dodecane. The (p-AXAP + p-HAP)/o-HAP ratio is equal to 7 with sulfolane and 1 with dodecane. 

This means a reaction order with respect to PA between zero and 1. The value of 0.5 which is found indicates a relatively strong adsorption of PA (K3Cpa non-negligible with respect to 1). On the other hand, the very polar sulfolane solvent is more strongly adsorbed than the PA reactant (KdCs 1 + KaCpA) and thus  

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