Pbx(OH)y cluster formation in STI framework-type zeolites: anomalous thermal behaviour and increased thermal stability

The structural modifications occurring in zeolites upon heating are of interest because of technological and industrial applications. In this study, we report the anomalous behaviour of a Pb-exchanged zeolite (Pb13.4(OH)10Al17.4Si54.6O144 ∙38H2O) with STI framework type. For the first time, we observed a switch forom negative to positive thermal expansion during continuous heating. The dehydration was tracked in situ from 25 to 450 °C by single crystal X-ray diffraction, infrared, and X-ray absorption spectroscopy. Furthermore, toTo assist interpretation of the experimental results, molecular dynamics simulations were performed on a series of different theoretical models. Initially, Pb-STI unit-cell volume contracts (ΔV = -3.5%) from 25 to 100°C. This is in line with the trend observed in STI zeolites. Surprisingly, at 125°C, the framework expanded (ΔV = +2%), adopting a configuration, which resembles that of the room temperature structure. Upon heating, the structure loses H2O but no de-hydroxylation occurred. This behaviour is explained via the formation of Pbx(OH)x (x= 2,4) clusters, which prevent the shrinking of the channels, rupture of the tetrahedral bonds and occlusion of the pores. This zeolite has therefore an increased thermal stability with respect to other STI metal-exchanged zeolites, with important consequences foron its applications.