Hydrothermal conversion of zeolites: An in situ synchrotron X-ray powder diffraction study

Hydrothermal conversion of zeolite LTA by LiCl(aq) is an efficient synthesis route for zeolite Li-A(BW), LiAlSiO4 . H2O, resulting in highly crystalline materials. In the present work hydrothermal crystallization of zeolite Li-A(BW) was followed in situ by time resolved synchrotron X-ray powder diffraction. These experiments were combined with ex situ scanning electron microscopy on partially converted samples. Fitting of the crystallization curves with rate expressions indicated limited nucleation and showed that intermediate amorphous or solute phases were not present in any significant amount. Partially converted samples were prepared in situ, making it possible to monitor the progress of crystallization and to determine the degree of conversion in each sample. Scanning electron microscope images of 40 and 60% converted samples showed that the parent zeolite LTA crystallites to a large degree serve as nucleation centers for the crystallization of zeolite Li-A(BW). The zeolite Li-A(BW) crystals grow radially from the nucleation site, forming clusters of flat needle-like crystals. The information obtained from the combined experiments clearly shows that the hydrothermal conversion of zeolite LTA into zeolite Li-A(BW) is a solution-mediated process.