Characterization of acid sites in alumina and in zeolites: A FTIR study of adsorption and of the condensation of acetone

A series of ultrastable Y zeolites with different amounts of framework and nonframework aluminum were prepared by fluorination of H-USY zeolite. The samples were characterized by chemical analysis, physical adsorption of nitrogen at low temperature and of pentane at room temperature, X-ray powder diffraction, 29Si and 27Al magic angle spinning nuclear magnetic resonance. n-Pentane isomerization has been studied mainly at 285°C, while the activation energy was measured on several samples.; The catalytic activity appears to be a direct function of both the number of Lewis and of the number of Brönsted acid sites. It might be that the “acid site” is in fact a complex resulting from the interaction between the proton donor and an electron acceptor.; The interactions between the acetone carbonyl and the carbonyl and double bond of mesityl oxide with Brönsted and Lewis acid sites of acid catalysts have been studied by Furrier transform infrared spectroscopy. The acetone carbonyl interaction with bridging acidic OH and with bridging weakly acidic OH can be distinguished from one another semi-quantitatively.; The Brönsted and Lewis sites are not easily distinguished with acetone while they are with mesityl oxide. The system acetone-mesityl oxide absorbed on acid catalysts is rather unique, since the reagent and the reaction product of the condensation reaction are qualitatively and quantitatively detectable by FTIR spectroscopy. This permits the study of the reaction from different points of view.; The condensation reaction of acetone on alumina and acid zeolites has been followed by FTIR. Under identical conditions, the reaction rate is faster on alumina and the condensation goes beyond the formation of mesityl oxide. Zeolites without nonframework aluminum are poor catalysts. It appears that Lewis sites are responsible for the activation of the molecule. On alumina the reaction would take place between gas phase acetone and acetone adsorbed on Lewis sites. On zeolites the reaction would involve acetone molecules adsorbed on Brönsted and Lewis sites.; Poisoning of the acetone condensation reaction with H₂O was examined on USY, HY and alumina. The experiments confirmed the observation that the Lewis acid sites are responsible for the acetone activation. The mechanism of the poisoning is different on alumina and zeolites. The stoichiometry of poisoning is one Lewis site per one water molecule dissociatively adsorbed on alumina and two Lewis sites per one H₂O molecule chemisorbed on the Lewis acid site of USY.