Studies on the Surface Property of Pt/KL Zeolite Catalysts

Pt/KL catalysts prepared by ion exchange (IE), ion exchange followed by the addition of KCl (IE-KCl), incipient wetness impregnation (IWI), and coimpregnation with KCl (IWI-KCl) were compared regarding the aromatization of hexane, and were characterized by transmission electron microscopy, the chemisorption of H2 and NH3, the competitive hydrogenation of toluene and benzene, and the infrared spectroscopy of adsorbed CO. The majority of Pt particles on the various Pt/KL catalysts seem to be highly dispersed inside the zeolite channels. The presence of KCl in the Pt/KL catalysts resulted in a decrease in the acidity of zeolite, giving rise to a reduction in the C1–C5 selectivity, regardless of the method of catalyst preparation. The competitive reaction of toluene/benzene and FT-IR results consistently show that the Pt particles on the IE-KCl catalyst have the most electron-rich sites among the Pt/KL samples examined. The high specific activity for benzene production of the IE-KCl catalyst should be attributed to these Pt particle sites with high electron-donating ability. It is proposed that the terminal adsorption of hexane, resulting in the formation of primary alkyl species, is favorable on electron-rich Pt particles, leading to a 1,6-ring closure, and eventually benzene.