Estimation of Potential and Effective Brønsted Acid Site Concentrations in Acidic Mordenites by Conductometric Titration Method

Quantitative determination of Brønsted acid sites in acidic mordenite samples with varying proton concentrations has been studied using aqueous conductometric titration method. Conductometric titration of the ammonium form of mordenite with sodium hydroxide is feasible and the results indicate potential number of Brønsted acid sites. However, Brønsted acid sites concentration in acidic mordenites, measured by this method, is less than that evaluated by ammonium-mordenite titration. Calcination procedure, to produce acidic mordenites from its ammonium form, reduces Brønsted acid sites by dealumination and dehydroxylation processes. In conductometric titration, only accessible Brønsted acid sites are titrated and extra-framework aluminum species, which are generated during calcination and reside in the structure, are not titrated. This finding is supported by observing more reduction in the Brønsted acidity when higher temperatures of calcination are employed. Also, comparing diffuse reflectance spectra of the acidic mordenite before and after titration does not show extra-framework aluminum species reacting with added base. A negative linear correlation was observed between concentration of Brønsted acid sites measured by conductometric titration and the temperature of calcination within 450-750°C. * E-mail: [email protected] Introduction Solid acids have found a wide range of uses as catalysts for many important reactions [1-3]. Zeolites