A SPECTROPHOTOMETRIC STUDY OF THE ACIDITY OF SOME SOLID ACIDS

Weakly basic Hammett and HR indicators were used to characterize the strongest sites of some solid acids. A spectrophotometric method was devised because it was demonstrated that conclusions reached by observing color changes can be very misleading. The method provided a means to measure the amounts of indicator adsorbed and the rates and efficiency of the adsorption. The data suggested that adsorption equilibrium with all of the surface cannot be achieved and that in some cases the pore mouths of zeolites become blocked by irreversibly adsorbed molecules. Nevertheless the results obtained ranked the catalysts correctly in order of increasing acidity as judged by catalytic measurements. The strongest acid found was an H-mordenite showing sites with Ho < -12.4. Some of the problems encountered in making meaningful acidity measurements and in their interpretation are discussed. © 1990.