N-Hexane aromatization on synthetic gallosilicates with MFI structure

Abstract

Various gallosilicates having an MFI structure, [H-(Ga)ZSM-5], with controlled content of nonframework and framework gallium were hydrothermally synthesized from gels of the following composition: 15 Na 2O· x Ga 2O 3·100 SiO 2·13 TPABr·4400 H 2O. Varying the synthesis times revealed that at nearly unchanged total Si/Ga ratio the catalytic activity in the conversion of n-hexane is directly linked to the degree of crystallization, i.e. the framework gallium content. Non-framework gallium alone is not found to be active. IR spectroscopy and TPD of NH 3 revealed that 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 increase with increasing total gallium content. Remarkably, this increase continues when only the content of non-framework gallium increases. This is explained by a synergetic cooperation of framework gallium as Brønsted sites and non-framework gallium as Lewis sites. The distribution of aromatics in the product supports the known concept of dehydrogenating Lewis sites and cracking Brønsted sites. ESR data of the coke formed revealed a simultaneous existence of olefinic and aromatic coke, observed on gallosilicates only.