Nature and Catalytic Properties of In-Situ-Generated Bronsted Acid Sites on NaY

Aluminosilicate zeolites are widely used as acid catalysts, and different types of acid sites on these materials have been identified and characterized thoroughly. However, distinct acid sites on zeolites generated during reaction, such as the Bronsted acid sites on NaY upon exposure to organic acids and esters shown in our recent works (referred to as 0 extrinsic sites), remain largely unexplored. In this work, we determine the density of extrinsic Bronsted acid sites generated on NaY by acetic acid and methyl acetate by quantitative Fourier transform infrared (FTIR) spectroscopy. Kinetic investigations using isopropanol dehydration as a model reaction show that the catalytic characteristics of the extrinsic Bronsted acid sites on NaY are not only different from intrinsic Bronsted acid sites on proton-exchanged faujasites, but also are dependent on the precursor molecule (e.g., acetic acid and methyl acetate). The rate of isopropanol dehydration on NaY is found to scale linearly with the square root of the partial pressure of cofed acetic acid, rather than the quantity of spectroscopically measured Bronsted acid sites. We propose that the self-dissociation of acetic acid in the highly ionic zeolite pore network generates protons that are active for the dehydration reaction, and only fraction of these protons, i.e., those that are associated with the zeolite framework, can be identified spectroscopically. When cofeeding methyl acetate, the rate of isopropanol dehydration on NaY increases linearly with the quantity of spectroscopically measured Bronsted acid sites, which is attributed to the fact that the self-dissociation of methyl acetate does not generate active protons within the zeolite pore