The promoting effect of Pt on the co-aromatization of pentane with methane and propane over Zn-Pt/HZSM-5

Abstract

Abstract The co-aromatization of pentane, methane and propane over the Zn-Pt/HZSM-5 and Zn/HZSM-5 catalyst demonstrates the promoting effect of Pt in terms of higher pentane conversion, higher BTX selectivity and suppressed naphthalene formation, particularly in the presence of methane and propane. DRIFT spectroscopy is employed to probe the evolution of the reactant molecules on the catalyst surface, illustrating the catalytic sites that are not occupied by pentane but available to activate methane and propane molecules. The different adsorption and conversion behaviors of the reactant molecules over the surface catalytic sites are confirmed by isotopic labelling study. The synergetic effect between pentane, methane and propane in the co-aromatization reaction over Zn-Pt/HZSM-5 might be closely related to the variable aromatization performance observed under different reaction conditions. The chemical environment of Zn in the catalysts is probed by XAS technique. The XANES spectra of the catalysts reveal the increased coordination number compared with ZnO reference due to the bonding of Zn with the zeolite framework. The electron transfer from Zn to Pt would alter the electron distribution of the active catalytic sites and thus modify their catalytic activity. The acidity of the catalysts is also modified by the metal loading, which would impose a significant impact on the interaction between the catalyst and the reaction intermediates. The presence of Pt increases the strong acid site concentration that may enhance the activity of the catalyst in the aromatization process. The Zn and Pt particles of catalyst are uniformly dispersed on the catalyst surface as illustrated by TEM and element mapping, which is critical for the interaction between Zn and Pt species and the improved catalytic performance.