Selective Formation of Light Olefin by n-Hexane Cracking Over HZSM-5: Influence of Crystal Size and Acid Sites of Nano- and Micrometer-Sized Crystals

A series of HZSM-5 catalysts with different characteristics (size, texture, surface acid site density) was prepared by hydrothermal method and their performances were investigated in catalytic cracking of n-hexane (as a model compound of light naphtha) at 370 and 470°C. Uniform nano-sized HZSM-5 (ca. ∼0.1μm) was found to be a very attractive catalyst, showing a selective formation of propylene and light olefin (about 83% at weight hourly space velocity (WHSV)=3h-1) and a high resistance to the deactivation. Under similar conditions micro-size HZSM-5 (ca. ∼2.0μm) produced higher amount of other hydrocarbons i.e., C5+) and coke. This is because nano-size crystals have a larger external surface area, which offers a higher number of pore entrances and enhances the reaction occurring inside the pore channels compared with large micro-sized crystals. In addition, it is found that the selectivity to light olefin increases by decreasing the number of acid sites on the external surface. This leads to the conclusion that in n-hexane cracking to light olefin, the activity and selectivity are related to the crystal size and the ratio of surface acid sites to total acid sites. As shown, reducing crystal size and decreasing acid sites on the external surface are remarkable ways to enhance catalytic activity and selectivity, respectively