Pb_x(OH)_y cluster formation and anomalous thermal behaviour in STI framework-type zeolites

For the first time, the structural investigation of a Pb-exchanged zeolite (Pb_13.4(OH)₁₀Al_17.4Si_54.6O₁₄₄ ∙38H₂O) with STI framework type, revealed a highly unusual and intriguing sudden volume increase under continuous heating. Understanding the fundamental mechanisms leading to such an unusual behaviour is essential for technological applications and interpretation of chemical bonding in zeolites. The dehydration was tracked in situ from 25 to 450 °C by single crystal X-ray diffraction, infrared and X-ray absorption spectroscopy. Further interpretation of the experimental observations was supported by ab initio molecular dynamics simulations. Initially, Pb-STI unit-cell volume contracts (ΔV = - 3.5%) from 25 to 100 °C. This agrees 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. The key mechanism leading to the sudden volume increase was found to be the formation of Pb_x(OH)_y 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 on its applications.