High-efficiency oxidation of methane to formic acid by mononuclear Fe3+ synergizing with acidic iron hydroxyl on Al-free Fe-Silicalite-1
The mildly selective oxidation of methane to oxygenates in the aqueous phase is of great significance for methane upgrade, while it suffers low conversion and multiple oxidation products. Fe-based zeolite, especially Fe/ZSM-5 with heteroatom Al is the most widely investigated catalyst for this reaction, while the Fe supported pure Si-based zeolite without Al is rarely reported. Herein, a series of Fe-S-1 catalysts without introducing heteroatom Al is synthesized via facial dissolution-crystallization method and applied for methane selective oxidation to formic acid. The optimized 0.5Fe-S-1 shows a superior performance with the formic acid space time yield of 49751 µmol·g cat −1 ·h −1 and a liquid-phase formic acid selectivity of 93 % at 70 °C with H 2 O 2 as the oxidant, and present an excellent stability without obvious deactivation after five cycles, which outperforms all previously reported Al-free Fe/Silicalite-1 catalysts, and is comparable with conventional Fe/ZSM-5. Combined with multifarious characterizations including TEM, UV-Vis, Raman, NH 3 -TPD, Py-IR and XAS etc., two kinds of Fe species including the mononuclear tetrahedrally coordinated framework Fe 3+ species and mononuclear octahedral coordinated extra-framework Fe 3+ species are highly dispersed over Fe-S-1 even with a high Fe loading of 1.5 wt%, in which the framework Fe 3+ species content is positively correlated with the catalyst acidity and could provide acid sites to adsorb reactants and then extra-framework Fe 3+ species could activate methane/H 2 O 2 effectively. And the further EPR, in-situ FT-IR characterizations and control experiments demonstrates that the methane selective oxidation to formic acid follows a radical mechanism, in which ·CH 3 combines with ·OH or ·OOH to form CH 3 OH, and then further oxidized to HCOOH and finally over-oxidized to CO 2 .
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