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-H 2 , TPD-NH 3 , Raman and TEM) and computational methods (DFT calculations). The TPR results show a promotive effect between Cu and Fe which increases the H 2 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) 2 M] 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.

Nature of the interactions between Fe and Zr for the methane dehydroaromatization reaction in ZSM-5

“…The values of x ¼ 5 and y ¼ 0e2.5 were chosen to explore the effect of increasing Zr presence in the 5Fe catalyst, while the values of x ¼ 3.3, y ¼ 1.7 and x ¼ 2.5, y ¼ 2.5 were chosen to explore the effects of increasing the Zr/Fe ratio whilst keeping the total metal amount (5%) constant. The pure iron in ZSM-5 catalyst was used as a reference with data from our previous work [30] and denoted 5Fe (5% w/w).…”

“…The computational approach and modeling of bimetallic sites in ZSM-5 is based in a previous work [30]. DFT calculations were carried out using the Quantum ESPRESSO software.…”

“…The Bader algorithm [37,38] was used to calculate the atomic charges. The NH 3 adsorption energy and carbon substitution energy were calculated as in a previous work [30] using Eqs. S1-S6, described in the supplementary material.…”

“…2b. The peaks for the iron oxide reduction are clearly visible for 5Fe at 390, 520 and 715 C. These peaks correspond respectively to the Fe 2 O 3 reduction to Fe 3 O 4 , reduction of more easily accessible Fe 3 O 4 oxides and the reduction of Fe 3 O 4 oxides in clusters to FeO with a small fraction reducing to Fe 0 [30]. The increase in the Zr content (5Fe0$5Zr, 5Fe1Zr and 5Fe2$5Zr) did not change considerably the first two reduction peaks.…”

“…All subsequent calculations utilized these fixed spin values in their respective systems. In our previous work [30], we have calculated the oxygen removal formation energy, the NH 3 adsorption energy in FeeCu bimetallic surfaces and compared the results with TPD-NH 3 and TPR-H 2 results. In this work, these energies were calculated in a similar manner to study the interactions between Fe and Zr.…”