Acidity characterization of rare-earth-exchanged Y zeolite using 31P MAS NMR

Abstract

Detailed qualitative and quantitative information on the effects of rare-earth (RE) cations on the types (Brönsted and Lewis), strengths, and distributions of acid sites on Y zeolite was studied by solid-state 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, using adsorbed trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO) as probe molecules. A total of seven 31P resonance peaks, with 31P NMR/TMPO chemical shifts at δ = 78, 70, 65, 62, 58, 55, and 53, corresponding to sites with different acid strengths, were identified. The peaks at δ = 78 and 70 arose from external and internal acid sites, the peaks at δ = 65, 62, 58, and 53 were from internal Brönsted acid sites, and the peak at δ = 55 was from internal Lewis acid sites. With increasing RE content, the number of medium strength Brönsted acid sites (δ = 62 and 58) increased significantly, whereas those of strong Brönsted acid sites (δ = 65) and weak Lewis acid sites (δ = 55) decreased. These experimental results were explained in terms of the influence of framework Al, extra-framework Al, and RE cations on the Y zeolite acidity.