Temperature-Induced Structural Changes in Acidic L-Zeolite: An In Situ Synchrotron Time-Resolved Powder Diffraction Study

Zeolite L (K6Na3Al9Si27O72·21H2O, the synthetic counterpart of the mineral perlialite) has important catalytic properties, the efficiency of which derives from the high acidity due to the presence of Brønsted sites (BAS), enhanced by the synergistic activity with Lewis sites (LAS) with increasing temperature. In this work, we carried out a detailed structural characterization of the acid form of this zeolite (H-LTL) by in situ time-resolved synchrotron X-ray diffraction from room temperature up to 800 °C on a powdered sample in order to obtain the structural response to heating. Despite the fact that the crystallinity is maintained when the maximum temperature is reached, thermal pathways show significant distortions of channel aperture as well as differences in host-guest interactions. The distortion of the T2 polyhedron and the progressive widening of the O3-T2-O5 angle suggest the formation of a Lewis acid site from the progressive conversion of BAS on O5 at 550 °C during heating. T...