Borosilicates: synthesis, properties and applications

Borosilicate zeolites are synthesized via isomorphous substitution of Al by B in the framework of zeolites. The first attempts for the incorporation of boron into the framework of zeolites were not successful; the crystallization of borosilicate zeolite did not take place or the substitution of Al by B was negligible. The successful replacement of Al by B in the framework of zeolites was reported only after the discovery of high silica zeolites [1]. Because of differences between the chemical properties of Al and B, borosilicate zeolites exhibit different properties in comparison with aluminosilicate counterparts. For instance, the borosilicate zeolites have intrinsically weak acidity and thus they are not considered as useful catalysts in acid catalysis processes. However, they find their applications as efficient catalysts in reactions requiring low acidity and as precursors for the preparation of other metallosilicates by post-synthesis method [2].In recent years, we have studied the synthesis of ZSM-5 type borosilicate zeolites using different methods (including hydrothermal and dry gel conversion methods) that here we mainly focus on the hydrothermal synthesis. According to our findings, the synthesis condition such as gel composition, pH, crystallization time and temperature can influence the morphology, particle size, boron content and crystallization degree of the synthesized zeolites [3]. The synthesized borosilicate zeolites were used as drug delivery systems (DDSs) for loading and release of doxorubicin (DOX) and their cytotoxicity was tested by MTT assay. The results proved that the borosilicate zeolites are biocompatible and can serve as pH-sensitive drug carriers. In order to study the effect of framework composition on the performance of these systems as DDSs, the incorporation of Fe3+ ions into the framework of borosilicates was studied. By incorporation of Fe3+ in zeolite framework, the efficiency of zeolites as DDSs falls drastically. However, the formation of extra-framework iron species (such as iron oxides) during the synthesis of zeolite leads to the higher loading and release efficiency of drug which shows the iron oxide- borosilicate composites are proficient DDSs [3, 4].