N-HEXANE AROMATIZATION ON SYNTHETIC GALLOSILICATES WITH MFI STRUCTURE

Various gallosilicates having an MFI structure, [H-(Ga)ZSM-5], with co ntrolled content of non-framework and framework gallium were hydrother mally synthesized from gels of the following composition: 15 Na2O.x Ga 2O3.100 SiO2.13 TPABr.4400 H2O. Varying the synthesis times revealed t hat at nearly unchanged total Si/Ga ratio the catalytic activity in th e conversion of n-hexane is directly linked to the degree of crystalli zation, i.e. the framework gallium content. Non-framework gallium alon e is not found to be active. IR spectroscopy and TPD of NH3 revealed t hat gallium is incorporated into the framework up to a maximum of 2 Ga /u.c. The conversion of n-hexane and the selectivity for aromatics inc rease with increasing total gallium content. Remarkably, this increase continues when only the content of non-framework gallium increases. T his is explained by a synergetic cooperation of framework gallium as B ronsted sites and non-framework gallium as Lewis sites. The distributi on of aromatics in the product supports the known concept of dehydroge nating Lewis sites and cracking Bronsted sites. ESR data of the coke f ormed revealed a simultaneous existence of olefinic and aromatic coke, observed on gallosilicates only.