The high thermal stability of the synthetic zeolite K-L: dehydration mechanism by in-situ SR-XRPD experiments

Thermally induced structural modifications of a synthetic zeolite L [K8.46 (Al8.35 Si27.53) O7217.91H2O, framework type LTL, s.g. P6/mmm, a = 18.3367(1) and c = 7.5176(1) Å] were studied by temperatureresolved synchrotron X-ray powder diffraction. In the investigated temperature range (RT-814 C), neither structure breakdown nor phase transitions occurred. The largest unit cell deformation was observed between 100 and 240 C, accompanied by an increase and decrease of the a and c cell parameters, respectively. After complete water release, an inversion of the a and c parameter behavior was observed, while the cell volume continued to increase, although following a more flattened curve. Overall, in the investigated T range, a small cell volume increase of 0.7% was observed. The release of the five water molecules present in zeolite L started with the most weakly bonded one and occurred between 80 and 240 C. During dehydration the framework underwent minor rearrangements, which facilitated water release: the apertures of the main 12-ring and the 8-ring channels became more circular and the 6-membered rings became more hexagonal. The thermal expansion of zeolite L, very unusual for a non-siliceous zeolite, was interpreted and compared with previous data reported in literature for this porous material, and with the behavior of the synthetic phases ITQ-4 and CIT-5.