Acid Catalysis in High-temperature and Supercritical Fluids

1 Acid Catalysis in High-temperature and Supercritical Fluids 

Data taken from ref 28 and 29 (b) The variation in the autodissociation constant of water K with temperature. Data taken from ref. 28 and 31. 

As the kTw value for water at 250 °C is three orders of magnitude higher than under ambient temperatures, this makes HTW and SCW attractive solvents for performing acid- and base-catalysed reactions and reduces the need to add external acids or bases to the solution. Furthermore, cooling serves to neutralise the solution, which provides a relatively straightforward approach to reaction quenching. This is an attractive feature for industry, which often relies upon the addition of mineral acids or solid-state acids to catalyse reactions. The circumvention of the need for external acid catalysts therefore reduces the quantity of hazardous byproducts that require neutralisation. Examples of acid-catalysed reactions in HTW and SCW include acylations, condensations, cyclisations, eliminations and hydrolyses , which have been recently reviewed elsewhere.  

These results highlight that for some aeid-eatalysed reactions in HTW and sew, the increase in the observed rate constant at weakly acidic and neutral pHs can be a consequence of an increase in general acid catalysis by water, as opposed to an increase in specific acid catalysis by H . This is a consequence of the substantive increase in thermal energy available in HTW and sew. The resulting increase in the heterolysis of water manifests itself more significantly in general acid catalysis than in the increase of K, for specific acid catalysis. 

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