Thermally Assisted Photocatalytic Dehydrogenation of Methanol in Continuous Steady-State Operation.

The direct dehydrogenation of methanol to produce formaldehyde has long been considered a dream reaction, because it promises higher product value, lower safety hazards, and lower CO2 emissions compared to state-of-the-art methanol oxidation. However, thermodynamic equilibrium constraints prevent competitive yields under purely thermocatalytic conditions. This study presents thermally assisted photocatalytic dehydrogenation of methanol as a method enabling formaldehyde yields far beyond the limits of pure thermocatalysis. In continuous, stable operation over 70 h, formaldehyde selectivity of 95% and methanol conversions up to 60 times above thermodynamic equilibrium are achieved. The setup consists of a flow reactor with a catalytically active Pt/TiO2-coated plate, which is irradiated with UV light from the top, heated from the backside, and continuously fed with methanol. Variation of temperature (78-120°C), residence time (6.1-23.5 s), and irradiation intensity (118.1-255.8 mW cm- 2) shows that high values of all three parameters are required to maximize formaldehyde and hydrogen yields. Continuous co-dosing of oxygen at very low concentrations (≤1 mol%) significantly increases conversion and shifts selectivity toward methyl formate. Our findings suggest that oxygen microdosing allows replenishment of surface oxygen sites on TiO2, which act as key adsorption centers in the thermally assisted photocatalytic process.