Direct observation of a new aluminum Lewis acid site in a zeolite

Abstract

We report the formation of a new Lewis acid Al site in H-FAU zeolite upon 650 ⁰C thermal treatment, unknown for any zeolite. Spectroscopy and DFT calculations reveal this site is a naked Al+3 ion which is charge balanced by a triplet of adjacent framework oxygens with net charge of -1 for each Si-O-Al moiety. This is the first reported observation of a +3 cation stabilized in a zeolite and the first confirmation of existence of aluminum triplets (as opposed to Al pairs) in that can stabilize such cations in siliceous zeolites. This site forms a thermally stable carbonyl O3Al-CO complex with the highest known frequency at 2252 cm-1 for a carbonyl complex on any solid material. These findings open new horizons in zeolite chemistry and expand our understanding of polyvalent metals’ interactions with zeolites.