Activation conditions play a key role in the activity of zeolite cay: NMR and product studies of brønsted acidity

Abstract

CaY, activated under different conditions, was characterized with ¹H, ³¹P, and ¹H/²⁷Al double resonance MAS NMR. The 1H MAS NMR spectra of CaY, calcined in an oven at 500°C, shows resonances from H₂O (bound to Ca²⁺ and the zeolite framework), CaOH⁺, aluminum hydroxides, silanols, and Brønsted acid sites. No evidence for Lewis acidity is observed on adsorption of trimethylphosphine, and an estimate of ≈16 Brønsted acid sites per unit cell is obtained for this sample. CaY activated in an oven at higher temperatures contains less water, but all the other species are still present. In contrast, CaY activated by slow ramping of the temperature under vacuum to 500 or 600°C shows a much lower concentration of Brønsted acid sites (<1/unit cell). Again, no evidence for Lewis acidity was observed. These NMR results have been utilized to understand the very different product distributions that are observed for reactions of 1,1- and 1,2-diarylethylenes in zeolite CaY activated in an oven (in air) and under vacuum. Samples with high concentrations of Brønsted acid sites react stoichiometrically with these sites, yielding diarylalkanes. At low concentrations, the Brønsted acid sites can act catalytically resulting in isomerization reactions.