Effect of Synthesis Methods and Modification Treatments of ZSM-5 on Light Alkane Aromatization

Abstract

ZSM-5 samples were prepared by hydrothermal synthesis from template-free (TF) and templated (HZ) gel media. The HZ samples were hydrothermally dealuminated to different levels (from HZS-1 to HZS-4). The physicochemical properties of the samples were characterized by X-ray diffraction analysis (XRD), X-ray fluorescence, microcalorimetry, and 29Si and 27Al nuclear magnetic resonance, and their catalytic performance was evaluated for n-heptane aromatization. The samples exhibited considerable differences in catalytic properties such as acidity and aromatic product selectivities in n-heptane conversion. The TF sample exhibited slightly lower XRD crystallinity and surface area than HZ but possessed comparable aromatic selectivity, except for a slight variation in product distribution, including a greater selectivity for C9+ aromatics. Among the dealuminated samples, the sample steamed at 673 K (HZS-2), exhibited the presence of very strong acid sties, and enhanced aromatic yields when compared to parent HZ and other dealuminated catalysts. The performance of HZS-2 was also high in the aromatization of other n-paraffins, such as n-hexane, n-octane, and the feedstocks, natural gas liquids (NGL) and light naphtha (LN). The aromatization selectivity of the catalysts was found to be in the order, HZS-4 < HZS-3 < HZ < HZS-1 < HZS-2.