Effect of γ-alumina as active matrix added to HZSM-5 catalyst on the aromatization of methanol

摘要

HZSM-5-based catalyst is a recognized catalyst which is particularly selective towards the formations of aromatics in the methanol reaction. However, studies on HZSM-5-based catalyst were mainly focused on the addition of metallic or/and nonmetallic element. Quite few studies have reported the effect of active matrix such as γ-alumina on the aromatization of methanol. In this study, γ-alumina was introduced into HZSM-5-based catalyst for the purpose of investigating the effect of γ-alumina in methanol to aromatics reaction. The catalysts were characterized by X-ray diffraction, Temperature-programmed Desorption of NH3 (NH3-TPD), Pyridine adsorption FT-IR diffuse reflection spectroscopy and adsorption–desorption measurements of nitrogen, respectively. Characterizations showed that the introduction of γ-alumina increased the amount of mesopores and acid sites in the catalyst. The experimental mainly includes two parts. Firstly, separate reaction performances over the catalyst with/without γ-alumina and γ-alumina showed that γ-alumina could significantly promote the formations of aromatics. However, γ-alumina alone could merely convert methanol to dimethyl ether with a minor quantity of gaseous hydrocarbons. Acid properties showed that the introduction of γ-alumina increased the percentage of Lewis acid on catalyst surface and enhanced acid strength, as a result, promoted the production of active intermediates which was essential for aromatic formation. The rise of aromatics selectivity might be caused by the combined effect of acid site density and acid strength. Follow-up work was mainly focused on the effect of the loading amount and loading order of the catalyst with γ-alumina. Results indicated that the total aromatic yield increased gradually with the increasing amount of catalyst with γ-alumina regardless of the loading order of the catalyst with γ-alumina. Gasoline compositions showed that the increased aromatics were at the expense of paraffins, olefins, and naphthenes. Besides, all single aromatic hydrocarbons increased gradually with the increasing amount of catalyst with γ-alumina. And the aromatics had a larger variation change when methanol first passed through the catalyst with γ-alumina.