Synthesis in fluoride media and characterisation of aluminosilicate zeolite beta

Abstract

Zeolite beta has been synthesised for the first time in a wide range of initial Al/(Al+Si) ratios (0–0.14) in the presence of F^– and TEA⁺ at near neutral pH. The samples have been characterised by a number of techniques and compared to those synthesised in basic medium. An increase in crystal size and crystallinity as the Si/Al ratio increases together with an enhanced resolution of the XRD pattern and of the IR and ²⁹Si MAS NMR spectra are apparent. This is attributed to an enhanced local order as Si substitutes for Al in the zeolite framework. IR and ²⁷Al MAS NMR demonstrate dealumination is more severe the lower the Si/Al ratio. The presence of F^– in the as-made samples induces significant changes in the IR spectrum in the region of characteristic framework vibrations (500–650 cm^–1) probably due to a distortion of the framework. The determination of the micropore volume has been carried out using different methods, the t-plot method giving the more reliable results. Apparently, thermal analysis allows one to distinguish between TEA⁺ cations compensating for F^– anions from those compensating for framework Al, as the latter appear to be oxidised at a higher temperature.