Maximizing Active Fe Species in ZSM-5 Zeolite Using Organic-Template-Free Synthesis for Efficient Selective Methane Oxidation

Abstract

The selective oxidation of CH4 in the aqueous phase to produce valuable chemicals has attracted considerable attention due to its mild reaction conditions and simple process. As the most widely studied catalyst for this reaction, Fe-ZSM-5 demonstrates high intrinsic activity and selectivity; however, Fe-ZSM-5 prepared using conventional methods has a limited number of active Fe sites, resulting in low CH4 conversion per unit mass of the catalyst. This study reports a facile organic-template-free synthesis strategy that enables the incorporation of more Fe into the zeolite framework with a higher dispersion degree compared to conventional synthesis methods. Because framework Fe incorporated in this way is more readily transformed into isolated extra-framework Fe species under thermal treatment, the overall effect is that Fe-ZSM-5 prepared using this method (Fe-HZ5-TF) has 3 times as many catalytically active sites as conventional Fe-ZSM-5. When used for the selective oxidation of CH4 with 0.5 M H2O2 at 75 °C, Fe-HZ5-TF produced a high C1 oxygenate yield of 109.4 mmol gcat –1 h–1 (a HCOOH selectivity of 91.1%), surpassing other catalysts reported to date. Spectroscopic characterization and density functional theory calculations revealed that the active sites in Fe-HZ5-TF are mononuclear Fe species in the form of [(H2O)3Fe(IV)O]2+ bound to Al pairs in the zeolite framework. This differs from conventional Fe-ZSM-5, where binuclear Fe acts as the active site. Analysis of the catalyst and product evolution during the reaction suggests a radical-driven pathway to explain CH4 activation at the mononuclear Fe site and subsequent conversion to C1 oxygenates.

Acid-Promoted Selective Oxidation of Methane to Formic Acid over Dispersed Rhodium Catalysts under Mild Conditions

“…As a promising feedstock of chemicals and liquid fuels, methane has attracted much attention due to its high reserve and cheap supply. Direct oxidation of methane into value-added oxygenates under mild conditions becomes an ideal transformation route, − but it remains a challenge because activation of the strong C–H bond of methane (bond energy = 104 kcal/mol) is difficult. In recent years, relatively strong oxidants of sulfur trioxide and hydrogen peroxide were used to oxidize methane into value-added oxygenates under mild conditions. ,− For example, sulfur trioxide was used to prepare methanesulfonic acid from methane, and excellent activity was reached at 50 °C .…”

Effect of CO2 Co-Feeding on the Stabilization of Atomically Dispersed Iron Species over MgAl2O4 During Ethane Dehydrogenation Reactions

“…1.4 Å. Meanwhile, the signal of Fe–Fe path significantly decreased, suggesting the atomic dispersion of Fe species over 3Mg7Al support. − …”

Solar-Driven Reforming of Methane and Nitrogen to Methanol and Ammonium on Iron-Modified Zeolite under Ambient Conditions in Water

“…Nowadays, heterogeneous photocatalysts for methane oxidation are mainly semiconductors, such as TiO 2 , ZnO, C 3 N 4 , BiVO 4 , Bi 2 WO 6 , and WO 3 . Meanwhile, graphene or ZSM-5-confined Fe-based catalysts are reported as one of the most promising thermal catalysts for selective methane oxidation by H 2 O 2 under ambient conditions. − Considering both superior performance of Fe-based catalysts in selective methane oxidation and the excellent N 2 reduction performance of the Fe-mediated semiconductor photocatalyst, , it is plausible to synthesize an Fe-based photocatalyst for both methane oxidation and N 2 reduction so that methanol and ammonia are produced simultaneously. Such photocatalysts have not been reported, to the best of our knowledge.…”