Effect of water treatment on Sn-BEA zeolite: Origin of 960 cm−1 FTIR peak

Abstract

The correlation of the ∼960 cm−1 IR absorption band, frequently observed in Sn-BEA zeolite, to the incorporation of Sn in the zeolite framework has been investigated. IR and NMR spectra of freshly calcined and water treated zeolites combined with first principle calculations indicate that the 960 cm−1 band is not a vibration involving Sn but rather a result of isolated internal silanol groups. Thermogravimetric analysis and temperature-programmed desorption showed the silanol groups condense to form water at temperatures between 400 and 700 °C. These silanol groups form slowly at mild relative humidity (∼30–40%) and 25 °C, but in less than an hour at 98 °C in liquid water: they are expected to form in the course of an aqueous phase reaction. The water uptake observed following a one-hour liquid water treatment is 1.2 wt% in Sn-BEA with Si/Sn = 125, equal to 4–5 water molecules per Sn or 1 water molecule per 40 Si, too high for all of it to be directly linked to interaction with Sn atoms. Instead this water uptake can be explained by a site that can be hydroxylated at relatively low temperatures and dehydrated at temperatures typically used for zeolite calcination.