Zeolites are crystalline, microporous materials with well-defined and tunable acid sites, and are used extensively as catalysts for a broad range of chemical transformations, especially in the petrochemical industry. Experimental techniques generally aim to characterize intrinsic zeolitic acid sites, i.e., those present on the material regardless of the reaction system, because these sites are generally assumed to be exclusively responsible for the catalytic properties of the zeolite. Our recent works demonstrate that Brønsted acid sites (BAS) can be generated on a solely Lewis acidic zeolite, NaY, upon the dissociative adsorption of organic acids. The acidic hydrogen of the organic species exchanges with surface Na+ cations, generating an adsorbed sodium salt and a BAS on the zeolite framework. The BAS generated via this ion exchange, referred to as extrinsic BAS in this study, with organic acids and esters produce an infrared band at 3645 cm-1 on the NaY sample, indistinguishable from the ν(AlO-H) mode of intrinsic BAS on faujasites. However, catalytic properties of the extrinsic BAS are entirely unknown because these sites are not observed by standard ex situ characterization techniques. In this work, we employ a combination of spectroscopic and kinetic techniques to demonstrate that the catalytic properties of the extrinsic BAS on NaY are not only fundamentally different from those of intrinsic BAS on faujasites, but also dependent on the identity of the organic acid or ester precursor. Acetic acid and methyl acetate are used as precursors to generate the extrinsic BAS on NaY, and the density of generated sites is determined using quantitative FTIR spectroscopy. The ion exchange is governed by a Langmuir-type adsorption equilibrium, where framework negative charges act as immobile adsorption sites. Employing isopropanol dehydration as a model reaction, the activation energies and TOF over both extrinsic and intrinsic BAS are measured, demonstrating the two types of sites have different catalytic properties.

Nature and catalytic properties of in situ generated Brønsted acid sites on NaY

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.