Quantification of Bronsted 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 H-1 NMR spectroscopy, combined with molecular modeling, provided evidence for water molecules hydrogen bonded to Bronsted 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 Bronsted 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.



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Bornes, C; Amelse, J; Peacock, M; Marshall, CL; Schwartz, M; Geraldes, CFGC; Rocha, J; Mafra, L

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J. Amelse thanks BP Amoco Chemical Company Petrochemicals Technology, Naperville, IL USA, and M. Peacock thanks BP Petrochemicals Technology, Hull, UK management. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. We also thank FCT for funding the project PTDC/QEQ-QAN/6373/2014. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project No. 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). C. Bornes acknowledges FCT for the doctoral fellowship PD/BD/142849/2018 integrated in the PhD Program in NMR applied to chemistry, materials and biosciences (PD/00065/2013). Work at Argonne National Laboratory (Marshall/Schwartz) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. We also acknowledge Zheng Lu for assistance in TGA measurements. Keywords: Zeolites

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