Acid Site Type, Concentration and Strength

In general terms, Bronsted acidity is defined in terms of the equilibrium between an acid, HA and a base, B  

The strength of acids in non-aqueous solution can be determined by investigating their tendency to donate protons to bases of known pK values which have different colours in the protonated and unprotonated forms. UV-visible spectroscopy of the solution is then able to give the ratio of these forms of the indicator molecules. 

For the indicator bases the equilibrium B + //+ BH is established. The Hammett acidity value, Ho, is then defined  

Nevertheless, the concept of Hammett addity ean be of some use. Bases with the values of — 11.4 adsorbed on the large-pore zeolite H-Y, for example, are 

The most obvious differences between solid and liquid acids are in their physical properties. Solids can be heated, which enhances the rate of proton transfer reactions which are slow at room temperature, can be used in solid-liquid and solid-gas reactions and can readily be separated from reactants and products. One of their limitations, however, is that the catalyst can become covered in strongly adsorbed by-product, or at high temperatures by carbonaceous residue, coke , resulting in deactivation. In this case, the utility of the catalyst may ultimately be determined by how readily it can be regenerated. 

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