Discrimination of the Synergistic Effect of Different Zinc Active Sites with a Brønsted Acid in Zeolite for Dehydrogenation Cracking of n-Octane and Ethane Dehydroaromatization

Abstract

The synergetic effect of different zinc active sites with a Brønsted acid site (BAS) in Zn-MCM-22 for n-octane dehydrogenation cracking and ethane dehydroaromatization was investigated. Zn-MCM-22 catalysts containing ZnO were prepared via incipient wetness impregnation (IM) using liquid ion grafting, whereas those containing [ZnO_x]²⁺ were prepared via atom-planting (AP) using the gas dechlorination reaction. The synergetic effects of BAS with micropore incorporated [ZnO_x]²⁺ and external surface ZnO species on the dehydrogenation of different molecule size reactants n-octane and ethane were compared. The synergistic effect of ZnO and BAS can improve ethane dehydrogenation through aromatization, whereas [ZnO_x]²⁺ as the introduced Lewis acid site (LAS) can override the bridge Si-OH-Al hydroxyl group BAS to inhibit the generation of benzene-toluene-xylene (BTX) and is more favorable for ethane dehydrogenation. The AP method can effectively regulate the n-octane dehydrogenation cracking product distribution by adjusting the volatilization time of ZnCl₂ vapors to regulate the ratio of LAS/BAS in zeolites. The kinetic analysis was used to correlate the roles of the [ZnO_x]²⁺ site and BAS in the dehydrogenation, hydrogen transfer, and cyclization reactions of n-octane and ethane, respectively. Moreover, the hydroxyl group grafted [ZnO_x]²⁺ sites have better activity and stability for higher temperature dehydrogenation.