Experimental and DFT analysis of the acid and reduction properties of Fe-Cu/ZSM-5

Abstract

Methane is a well-established feedstock in the chemical industry with a wide range of sources from natural gas to methane hydrates. In previous works, Fe based catalysts impregnated with Cu or Zr in ZSM-5 were tested for the non-oxidative aromatization of methane to benzene, the observed results regarding the induction period and coke deposition were promising. In this work, the interactions between Fe and Cu that could promote these properties were studied by synthesizing Fe-Cu catalysts supported in ZSM-5 with varying Cu/Fe ratios and carrying characterizations by experimental methods (XRD, BET, TPR-H2, TPD-NH3, Raman and TEM) and computational methods (DFT calculations). The TPR results show a promotive effect between Cu and Fe which increases the H2 consumption. Results from TPD tests show that the acid strength increases with Cu loading, and a new peak is observed above 600 °C. The DFT calculations were carried out for a system consisting of a [M(μ−O)2M]2+ (M = Fe, Cu) cation deposited in two Brønsted acid sites in the γ site of the zeolite. Computational results indicate an increased stability of the reduced form of Fe when bonded with Cu, corroborating with the TPR results. Moreover, results for charge calculation show that the presence of Cu leads to a charge depletion in Fe when bonded in the same site, which explains the increased acid strength. The acid and reduction properties of the catalyst can be controlled using different Cu/Fe ratios and tuned for use in methane non-oxidative aromatization and other reactions.