Elevating Photooxidation of Methane to Formaldehyde via TiO2 Crystal Phase Engineering

Photocatalytic conversion of methane to value-added products under mild conditions, which represents a long sought-after goal for industrial sustainable production, remains extremely challenging to afford high production and selectivity using cheap catalysts. Herein, we present the crystal phase engineering of commercially available anatase TiO2 via simple thermal annealing to optimize the structure-property correlation. A biphase catalyst with anatase (90%) and rutile (10%) TiO2 with the optimal phase interface concentration exhibits exceptional performance in the oxidation of methane to formaldehyde under the reaction conditions of water solvent, oxygen atmosphere, and full-spectrum light irradiation. An unprecedented production of 24.27 mmol g(cat)(-1) with an excellent selectivity of 97.4% toward formaldehyde is acquired at room temperature after a 3 h reaction. Both experimental results and theoretical calculations disclose that the crystal phase engineering of TiO2 lengthens the lifetime of photogenerated carriers and favors the formation of intermediate methanol species, thus maximizing the efficiency and selectivity in the aerobic oxidation of methane to formaldehyde. More importantly, the feasibility of the scale-up production of formaldehyde is demonstrated by inventing a pause-flow reactor. This work opens the avenue toward industrial methane transformation in a sustainable and economical way.

Automated summary

In this study, the crystal phase engineering of commercially available TiO2 catalyst was optimized to achieve high efficiency and selectivity in the oxidation of methane to formaldehyde under mild conditions. Experimental results and theoretical calculations revealed that crystal phase engineering prolonged the lifetime of photogenerated carriers and promoted the formation of intermediate methanol species, thus maximizing the efficiency and selectivity of formaldehyde production. Moreover, the feasibility of large-scale formaldehyde production was demonstrated by inventing a pause-flow reactor.