Inelastic Neutron Scattering Study on the Location of Brønsted Acid Sites in High Silica LTA Zeolite

The presence of Brønsted acid sites (SiOHAl) is crucial in many of the applications of zeolites in petroleum refining and related industries. This acidity relies on the number and location of acid sites, but the latter is difficult to know accurately in spite of a large number of experimental techniques available. Zeolites usually contain many types of acid sites depending on the crystallographic different O-sites and their chemical environment. In the present work we show how inelastic neutron scattering (INS) can be employed for an accurate determination of the location of Brønsted acid sites in zeolites. This is achieved using a sample of LTA zeolite with few defects of connectivity, few (O1H, O2H, O3H) crystallographically different oxygens, and high Si/Al ratio. The narrow and intense peaks corresponding to SiOH bending modes ca. 1100 cm–1 have been shown, using periodic DFT calculations, to strongly depend on the type of oxygen to which the proton is bonded. A comparison between calculations and INS experiments shows that a large majority of the protons in the sample are of the type O2H, which vibrate close to the small cavity of LTA.