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Direct and Selective Photocatalytic Oxidation of CH4 to Oxygenates with O2 on Cocatalysts/ZnO at Room Temperature in Water
journal contribution
posted on 2019-12-13, 18:33 authored by Hui Song, Xianguang Meng, Shengyao Wang, Wei Zhou, Xusheng Wang, Tetsuya Kako, Jinhua YeDirect conversion of methane into methanol and other
liquid oxygenates
still confronts considerable challenges in activating the first C–H
bond of methane and inhibiting overoxidation. Here, we report that
ZnO loaded with appropriate cocatalysts (Pt, Pd, Au, or Ag) enables
direct oxidation of methane to methanol and formaldehyde in water
using only molecular oxygen as the oxidant under mild light irradiation
at room temperature. Up to 250 micromoles of liquid oxygenates with
∼95% selectivity is achieved for 2 h over 10 mg of ZnO loaded
with 0.1 wt % of Au. Experiments with isotopically labeled oxygen
and water reveal that molecular O2, rather than water,
is the source of oxygen for direct CH4 oxidation. We find
that ZnO and cocatalyst could concertedly activate CH4 and
O2 into methyl radical and mildly oxidative intermediate
(hydroperoxyl radical) in water, which are two key precursor intermediates
for generating oxygenated liquid products in direct CH4 oxidation. Our study underlines two equally significant aspects
for realizing direct and selective photooxidation of CH4 to liquid oxygenates, i.e., efficient C–H bond activation
of CH4 and controllable activation of O2.