Regulation of the Lewis acidity on matrix and their performance in the catalytic cracking of light hydrocarbons

Abstract

The reasonable matching of zeolite and matrix is one of the most effective strategies to increase the yield of light olefins in naphtha catalytic cracking. However, the influence of the surface Lewis acidity within the matrix on the cracking reactions has remained ambiguous. Therefore, in present study, boron and zinc co-modified γ-Al₂O₃ and tin modified mesoporous silica KIT-6 with tuned surface Lewis acidity were applied to evaluate the cracking reactivity of n-heptane and 1-hexene to light olefins, in which the matrix was used alone and coupled with ZSM-5 zeolite in different packed modes. The effects of the modifiers on the textural properties and surface acidity of γ-Al₂O₃ and KIT-6 were investigated by XRD, TEM, N₂ physical absorption-desorption, and NH₃-TPD. The results showed that B doping reduced the Lewis acidity (both in the amount and acid strength) of γ-Al₂O₃, while the incorporation of Zn doping led to increased Lewis acidity. In addition, the Lewis acidity of ordered mesoporous KIT-6 increased as Sn doping rose. While for pure matrix, the ascend in conversions of n-heptane and 1-hexene was consistent with the increased Lewis acidity of the B and Zn co-modified γ-Al₂O₃ and xSn/KIT-6 rose, along with decreased activation energy. In contrast, when coupled with ZSM-5 zeolite, the highest conversion was achieved in the dual-bed manner of matrix and zeolite, and the conversion increased concomitantly with the increase in the Lewis acidity of the matrix. However, excessive Lewis acidity can accelerate the hydrogen transfer rate while diminishing the selectivity of light olefins.