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

The synergetic effect of different zinc active sites with a Brønsted acid site (BAS) in Zn-MCM-22 for -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] were prepared via atom-planting (AP) using the gas dechlorination reaction. The synergetic effects of BAS with micropore incorporated [ZnO] and external surface ZnO species on the dehydrogenation of different molecule size reactants -octane and ethane were compared. The synergistic effect of ZnO and BAS can improve ethane dehydrogenation through aromatization, whereas [ZnO] 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 -octane dehydrogenation cracking product distribution by adjusting the volatilization time of ZnCl vapors to regulate the ratio of LAS/BAS in zeolites. The kinetic analysis was used to correlate the roles of the [ZnO] site and BAS in the dehydrogenation, hydrogen transfer, and cyclization reactions of -octane and ethane, respectively. Moreover, the hydroxyl group grafted [ZnO] sites have better activity and stability for higher temperature dehydrogenation.