Chemical Control of Thermal Expansion in Cation-Exchanged Zeolite A

Variable-temperature powder X-ray diffraction studies have been used to monitor dramatic changes in the thermal expansion properties of zeolites with the LTA topology on changing the pore contents. Detailed structural analysis was performed on dehydrated Li-, Na-, K-, Rb-, and Cs-exchanged zeolite A and a comparison made with their purely siliceous analogue ITQ-29. Mean thermal expansion coefficients were also determined for the hydrated alkali-metal-exchanged forms. Thermal expansion behavior ranging from negative to positive was observed as different monovalent cations were included in the zeolite pores. Cation-induced strain to the zeolite framework has been shown to play a significant role in the thermal expansion mechanism of LTA-zeolites. Atomic-scale mechanisms behind the thermal expansion behavior have been deduced for ITQ-29, dehydrated Ag-A, and dehydrated Na-A systems.