Effect of variations in pore structure and acidity of alkali treated ZSM-5 on the isomerization performance

Abstract

A series of alkali treated ZSM-5 samples were prepared and the effects of the alkali treatment conditions on the n-hexane isomerization performance of Pt/ZSM-5 were investigated. The characterization by XRD, N 2-adsorption, XRF, NH 3-TPD, Py-IR and NMR revealed that alkali treatment conditions had the effects on the crystallinity, textural property, Si/Al ratio and acidity of ZSM-5. Mild alkali treatment led to the reduction of the strong acid sites and did not distinctly change the pore structure, whereas severe alkali treatment resulted in the creation of the stronger acid sites and new mesopores. The correlation of the acidity and pore structure and isomerization behavior was established. It was proposed that the conversion of n-hexane was dependent on the amount of strong acid sites and irrelevant to the properties of acid sites and the pore structure. However, the selectivity for iso-hexanes was irrelevant to both the acidity and pore structure. The relative amount of dimethylbutanes (DMB) and methylpentanes (MP) in isomers was related not only to the pore structure, but to the acidity of ZSM-5. The suitable alkali treatment condition facilitated the high yield of isomers and the formation of more DMB.