New Methods for Quantitative Determination of Brønsted Acid Sites on Solid Acids: Applicability and Limits for Al2O3-Promoted SO42−/ZrO2 Catalysts

Two quantitative methods, both based on H/D exchange, have been developed for the Brønsted acid sites titration on solid acids. The agreement between the results obtained with the two methods on unpromoted sulfated zirconia (SZ) is remarkable: 0.061 and 0.064 mmol of H/g of catalyst were respectively found with the first (anhydride method) and the second (isobutane method) method. Furthermore, the use of a small alkane as the probe molecule with the isobutane method gives us information about the capacity of the solid to activate an alkane. Nevertheless, this last method is hard to apply to very active catalysts such as 3% alumina-promoted sulfated zirconia (SZA3). Indeed, it was not possible in this case to find optimal temperature conditions, low enough to minimize secondary reactions such as isomerization or cracking and, at the same time, sufficiently high for an accurate quantitative H/D exchange measure. On the other hand, the anhydride method was always applicable to SZAn catalysts and the obtained results were very reproducible.