Ultrastabilization of Zeolite Y Transforms Bronsted-Bronsted Acid Pairs into Bronsted-Lewis Acid Pairs

Two pairs of Bronsted acid sites have been identified in H,Na-Y zeolite, located in the supercage and in the sodalite cage, which upon ultrastabilization (dealumination) are transformed into pairs of Bronsted and Lewis acid sites. This mild postsynthetic modification step is an important process for converting this material into an active catalyst for large-scale commercial reactions. Pairing structures and their transformations have been investigated using H-1 double-quantum NMR spectroscopy experiments for dehydrated zeolite, H,Na-Y, and its ultrastabilized form, H,Na-USY. This approach enables the detection of pairs of Bronsted and Lewis acid sites with unprecedented H-1 resolution and distinguishing them from isolated acid sites. The dealumination is also detected by static Al-27 solid-state NMR experiments.