Synthesis in fluoride media and characterisation of aluminosilicate zeolite beta
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.