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Montserrat Rodriguez Delgado, Speaker at Chemical Engineering Conferences
University of the Balearic Islands, Spain
Title : Infrared spectroscopy for ranking zeolite acidity: The VTIR method


Bronsted-acid zeolites, which contain the structural unit [Si(OH)Al], are currently used as catalysts in a wide range of technological processes, spanning from the petrochemical industry to biomass upgrade and the production of fine chemicals, to name only a few examples. For many of the chemical processes involved in those applications appropriate acid strength is a key factor determining catalytic performance, and hence the convenience to have a reliable method for evaluating relative Bronsted acidity. On account of its easy implementation, IR spectroscopy (at a fixed low temperature) of an adsorbed weak base, which interacts with the zeolite OH groups by hydrogen bonding, is a very frequently used instrumental technique to rank zeolite acidity, and carbon monoxide is the probe molecule of choice. The H-bonded species ZO-H•••CO (where Z stands for the zeolite framework) can easily be monitored by IR spectroscopy, as hydrogen bonding brings about a characteristic red shift, ΔνOH, of the O-H stretching frequency. The magnitude of such a shift is frequently used to rank Brønsted acidity of protonic zeolites. Nonetheless, the enthalpy change ΔH0 involved in the hydrogen-bonding interaction should be a more reliable indicator. In fact ΔνOH and ΔH0 are often found to correlate among themselves; but that is not always the case. We report herein on the application of variable-temperature IR (VTIR) spectroscopy to determine, simultaneously, ΔνOH and ΔH0; and revise recent experimental results showing how the usual practice of ranking Brønsted acid strength of zeolites by the corresponding O-H frequency shift probed by an adsorbed weak base can sometimes be misleading.


Montserrat Rodriguez Delgado obtained her BSc and PhD degrees in Chemistry at the University of the Balearic Islands (Spain), and she carried out pre-doctoral research at the University of Caen (France) and postdoctoral research at the Universities of Turin (Italy) and Madrid (Complutense). Currently, she is Senior Lecturer of Inorganic Chemistry and Materials Science at the University of the Balearic Islands; where her main research interests are in several areas of surface science (with a focus on phenomena occurring at the solid–gas interface), zeolites, and nanostructured materials for advanced environmental and technological applications.