Jose J. Fripiat

Physical Chemistry
Distinguished Professor Emeritus
Laboratory for Surface Studies
PhD, University of Louvain, Belgium
e-mail: jose@uwm.edu
Selected Publications

Many finely divided solids catalyze chemical reactions, which means that they enhance the rate of the reaction by several orders of magnitude. The catalytic effect depends upon the nature of the reaction and fundamental properties of the solid.

For instance, alumina (Al₂O₃) obtained by calcining Al(OH)₃ catalyzes the isomerization of 1-butene into 2-butene whereas Al(OH)₃ doesn't. In order to understand the reason of this difference of behavior, consider the nuclear magnetic resonance (NMR) frequency of an Al nucleus as observed by exciting the transition between energy levels with a high field radio frequency.

ppm Aluminum NMR Spectra of Al(OH)3 showing one resonance frequency, AlIV. ppm Aluminum NMR Spectra of Al2O3 showing two resonance frequencies, AlIV and AlVI.

In the figure, there are two NMR spectra. The top one is observed for Al(OH)₃: it contains one resonance line whose frequency is taken as reference. This resonance is characteristic of an aluminum coordinated to six oxygens Al^VI. The bottom spectrum is that observed for Al₂O₃. In addition to a peak due to Al^VI, it contains a resonance line due to aluminum with four oxygen neighbors (tetrahedral Al or Al^IV): The catalytic activity of the aluminum in the aforementioned reaction is due to the presence on the surface of aluminum with an incomplete coordination shell Al^IV instead of Al^VI: incomplete coordination creates Lewis acid sites which catalyze the isomerization of 1-butene.

This example illustrates the major goal of the laboratory, that is the study of the surface and of the catalytic properties of disordered or imperfect solids. A serious effort is made to study the nature of surface acidity in dealuminated zeolites. In this class of catalysts the dealumination results in the formation of acid Lewis sites associated with non-framework Al nano-particles. The nature of Lewis acidity as well as the influence of the interactions between Lewis and Brönsted acid sides on the catalytic performance of zeolites is still poorly understood.

Thus a part of our activity is devoted to the synthesis of solids with controlled defects while the other part is to study solid state nuclear magnetic resonance, electron paramagnetic resonance, X-ray diffraction, infrared spectroscopy, adsorption measurements, calorimetry, etc. The chemical techniques aim to analyze the reaction kinetics, with gas chromatography and mass spectroscopy as analytical tools.

• 
• 
• 
• 

• 
• 
• 
• 

• 
• 
• 
• 

• 
• 
• 
• 

• 
• 
• 
•