Visible light selective photocatalytic conversion of glucose by TiO2

- TiO2 Visible light photocatalytic conversion of glucose to selective products.
- Effect of crystalline phase, solvent and substrate concentration is critical.
- Visible light results in higher selectivity to gluconic acid and less mineralisation compared to UVA.
- Formation of TiO2-glucose complex charge transfer responsible for visible light activity.

The visible-light mediated selective photo-oxidation of glucose using unmodified TiO2 as a catalyst is reported for the first time. The effect of the catalyst to substrate ratio, lamp power and TiO2 crystalline phases on the conversion and product distribution under both visible and UVA light is explored. Higher conversions were obtained under UVA light but as a result of substantial mineralization through an unselective pathway. Optimization of the reaction conditions resulted in 42% glucose conversion under visible light with 7% selectivity to gluconic acid and 93% selectivity to other partial oxidation products with the total suppression of a mineralization pathway to CO2. It is also shown that selective glucose conversion can occur under natural sunlight light after 7 h exposure. In this systematic study, we prove that it is indeed possible to use TiO2 as a photocatalyst to upgrade biomass derivatives selectively by tailoring the reaction conditions. Most importantly, we find the ligand-to-metal charge-transfer effect as a result of glucose adsorption to the surface of TiO2 determinant in the photo-activity observed under visible light irradiation. Knowledge of this effect along with the careful control of reaction conditions means that the selective photo-catalytic conversion of other biomass derived carbohydrates under visible light is a viable route to higher value chemicals.

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