Zeolite Catalysts Prepared with Maximum Brønsted Acidity Reveal a Dominant Contribution from Inaccessible Sites

Abstract

Zeolite Y catalysts contain Brønsted acid sites in accessible large pores, known as supercages, and in sterically inaccessible small pores within sodalite cages. Access to acid sites within sodalite units is precluded due to their 0.26 nm cage windows, which are smaller than the critical diameters of relevant hydrocarbons. By preparing a series of HY catalysts with different acid site densities, including for the first time reported those with theoretical maximum acid site density, the number of acid sites in the inaccessible sodalite cages is shown to be a dominant factor in reactivity for catalysts in which the structural integrity of the sodalite unit is preserved. Using a combination of catalyst preparation, spectroscopy, isotopic exchange measurements with bulky hydrocarbons, and high-temperature isooctane cracking experiments on catalysts with fixed supercage proton amounts but varying sodalite proton concentrations, the dominant contribution of sodalite cage acid sites is quantified. DFT calculations suggest that a plausible mechanism involves framework flexibility in the form of site exchange via rotation of sodalite protons into supercages. In addition to revealing that commonly used acid site density measurements and turnover frequency calculations may exclude contributions from important sites, these data demonstrate that BASs in intact sodalite cages can define and control catalyst reactivity in HY catalysts. Large increases in catalyst activity are obtained by exchanging La3+ cations into sodalite cages with maximum acid site densities. These findings extend beyond HY catalysts, as they imply that other zeolite catalysts with sterically occluded acid sites may be tuned for improved performance.