Quantification of Brønsted Acid Sites in Zeolites by Water Desorption Thermogravimetry

The many studies that have been conducted on water adsorption on zeolites were carried out in “adsorption” mode, from adsorption isotherms or single‐point measurements, with the samples being first degassed at high temperature, and then exposed to water vapor at variable partial pressure. Here, we report Thermogravimetric Analysis (TGA) studies of HZSM‐5 saturated with water at room temperature, thus studying water adsorption in “desorption” mode. TGA was used to ascertain the amount of water remaining as a function of the pretreatment temperature, and to identify the temperature at which one water molecule per framework Al atom persists. Solid‐state 1H NMR spectroscopy, combined with molecular modeling, provided evidence for water molecules hydrogen bonded to Brønsted acid sites and formation of oxonium species for distinct water per framework Al atom. TGA is shown to be a simple and expeditious technique to measure the number of Brønsted acid sites in aluminosilicate zeolites, with advantages over the traditional methods of IR or TPD of dangerous adsorbed ammonia or pyridine. TGA of adsorbed water also provides information on the water clusters formed at the acid sites of HZSM‐5, which are proportional to the number of acid sites. An expeditious method for counting Brønsted acid sites using thermogravimetric analysis of water saturated zeolites is proposed. The size of the H2O cluster formed at the Brønsted acid sites in HZSM‐5 zeolites is a function of the temperature. At 100 °C, one H2O molecule per Al site remains, allowing the quantification of the acid sites.