Maximizing methanol selectivity over the microporous FeS-1 catalyst via aqueous-phase partial oxidation of methane with H2O2†

The conversion of methane to methanol faces challenges in liquid-phase systems due to lower methanol selectivity, often resulting in higher formic acid production. Previous studies have shown that iron-exchanged MFI zeolites, i.e., Fe–ZSM-5, tend to favor formic acid over methanol due to the indiscriminate decomposition of the oxidant. To address this, our study aims to identify the active sites responsible for such over-oxidation and develop methods to suppress these sites, thereby enhancing methanol selectivity. Hence, we have utilized hydrothermally synthesized microporous iron silicalite-1 (FeS-1) with an MFI structure and conducted a systematic comparison of its catalytic performance with FeZSM-5 and Fe–ZSM-5, which contain framework and extra-framework iron sites, respectively. This comparison highlights the relationship between active site distribution and methanol selectivity. Additionally, the analysis using DRUV-VIS and EPR spectroscopic techniques suggests that the yield of methanol and formic acid is found to vary monotonically with the amount of iron in framework and extra-framework sites, respectively, in the zeolitic matrix of fresh FeS-1. Therefore, selectively removing extra-framework iron and/or partially dissolving framework iron in MFI-based catalysts results in a significant reduction in formic acid yield, with only a small effect on methanol yield. Interestingly, in contrast to Fe–ZSM-5, both FeZSM-5 and FeS-1 maintain a significant amount of framework iron in the framework sites which results in a prominent enhancement in methanol selectivity (65%). Further investigation into FeS-1, FeZSM-5, and Fe–ZSM-5 underscored the importance of framework Si–O–Fe linkages in enhancing methanol selectivity.