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

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.