Defect Sites in Zeolites: Origin and Healing

This paper deals with the synthesis conditions-defect formation relationship in zeolites. Silicalite-1 (MFI-type) is used as a model material. Samples synthesized from a system with high basicity (at 100 degrees C), a system with moderate basicity (at 150 degrees C), and a fluoride-containing system in neutral medium (at 170 degrees C) are compared. Well-crystallized materials with sizes approximate to 0.1, 1-10, and 30-40 mu m are obtained. The samples are analyzed by complementary methods providing information on the short- and long-range order in the zeolite framework. A strong correlation between the number of point defects in the zeolite framework and preparation conditions is established. Silicalite-1 synthesized under mild synthesis conditions from a highly basic system exhibits a larger number of framework defects and thus low hydrophobicity. Further, the calcined samples are subjected to aluminum and silicon incorporation by postsynthesis treatment. The Al/Si incorporation in the zeolite framework and its impact on the physicochemical properties is studied by XRD, TEM/SEM, solid-state NMR, FTIR, and thermogravimetric analyses. The defects healing as a function of the number of point defects in the initial material and zeolite crystal size is evaluated. The results of this study will serve for fine-tuning zeolite properties by in situ and postsynthesis methods.