Low temperature synthesis and properties of ZSM-5 aggregates formed by ultra-small nanocrystals

ZSM-5 formed by aggregates of nanocrystals with ultra-small crystal size (10-20 nm) has been prepared by hydrothermal crystallization of clear synthesis mixtures at low temperatures (70-90 degreesC) under atmospheric pressure. Compared with the conventional autoclave treatment, this method leads to a significant reduction in the crystal size, although with lower zeolite yields, which has been assigned to the mild conditions used (low temperature and atmospheric pressure), as they may prevent the crystal growth by aggregation/densification of primary units in the nanometer range. Characterization of the samples obtained at long synthesis times by both X-ray and electron diffraction show they are highly crystalline materials, indicating that the ultra-small crystal size is compatible with the detection of crystallinity. After calcination, most of the Al atoms remain tetrahedrally coordinated, which is a proof of their stability. The nanosized ZSM-5 exhibits a very high external surface area (typically around 200 m(2)/g), as a consequence of its small crystallites, making this material a potentially interesting catalyst for the conversion of bulky molecules. Likewise, a mathematical model has been developed which correlates the variation of the textural properties of the ZSM-5 zeolite with its crystal size. (C) 2004 Elsevier Inc. All rights reserved.