Aromatization and isomerization of 1-hexene over alkali-treated HZSM-5 zeolites: Improved reaction stability

The HZSM-5 zeolites with micro-mesopore hierarchical porosity have been prepared by the post-synthesis of alkali treatment, and tested in the aromatization and isomerization reactions of 1 -hexene in a continuous flow fixed bed. Effects of several variables upon the controlled formation of mesopores have been thoroughly investigated. The results showed that new mesopores could be created and enlarged on ZSM-5 zeolite by increasing the NaOH concentration, temperature and treatment time employed in the alkali treatment, among which the NaOH concentration was the most effective variable in the studied extent. However, excessive high NaOH concentration induced the collapse of zeolite framework, while high temperature and long treatment time induced the backward deposition of amorphous Si onto the zeolite, which would decrease the amount of formed mesopores. When the properly alkali-treated HZSM-5 zeolites were used as catalysts, the reaction activity of 1-hexene was enhanced, and the stabilities of aromatization and isomerization reactions were improved while that of the cracking reaction was deteriorated. These variations of reaction stabilities were associated with the introduction of mesopores over the zeolites, which shortened the diffusion pathlength of the feed/ products and reduced the channel blockage. (c) 2008 Elsevier B.V. All fights reserved.