Hydrophobic, Acid-Free Zeolite-Confined Pt-Cu Nanoalloys Break Activity-Selectivity Limits in Low-Temperature Methane-to-Methanol Oxidation.

The direct oxidation of methane (CH4) to methanol (CH3OH) remains a formidable challenge due to the inertness of CH4 and the tendency of CH3OH to overoxidize. Here, we report Pt-Cu nanoalloys encapsulated within hydrophobic, acid-free silicalite-1 (S-1) zeolite that breaks activity-selectivity limits in CH4 oxidation to CH3OH. The best catalyst exhibits a CH3OH productivity of 134 mol of CH3OH per mol of Pt per hour and a selectivity of 95% at 150 °C. Kinetic and spectroscopic studies revealed a sequential oxidation mechanism: CH4 is first oxidized to methyl hydroperoxide (CH3OOH) by in situ generated hydrogen peroxide, which subsequently converts to CH3OH. The catalytic reaction proceeds with an apparent activation energy of only 42 kJ/mol, the lowest reported to date. The outstanding performance arises from the synergy of the Pt-Cu alloy sites and the hydrophobic pore of S-1. Pt-Cu alloy sites specifically generate oxidizing species and selectively form CH3OH, which was not achieved by single metal catalysts and other bimetallic catalysts. A confined hydrophobic, acid-free environment enables rapid extraction of the CH3OH from the reaction field and thereby prevents overoxidation. These findings highlight how precious control over both the composition and the local environment of Pt-Cu nanoalloys can markedly enhance the catalytic oxidation of CH4 to CH3OH.