Sunlight-driven and gram-scale vanillin production via Mn-defected γ-MnO2 catalyst in aqueous environment

Abstract

The production of vanillin from biomass offers a sustainable route for synthesizing daily-use chemicals. However, achieving sunlight-driven vanillin synthesis through H₂O activation in an aqueous environment poses challenges due to the high barrier of H₂O dissociation. In this study, we have successfully developed an efficient approach for gram-scale vanillin synthesis in an aqueous reaction, employing Mn-defected γ-MnO₂ as a photocatalyst at room temperature. Density functional theory calculations reveal that the presence of defective Mn species (Mn³⁺) significantly enhances the adsorption of vanillyl alcohol and H₂O onto the surface of the γ-MnO₂ catalyst. Hydroxyl radical (˙OH) species are formed through H₂O activation with the assistance of sunlight, playing a pivotal role as oxygen-reactive species in the oxidation of vanillyl alcohol into vanillin. The Mn-defected γ-MnO₂ catalyst exhibits exceptional performance, achieving up to 93.4% conversion of vanillyl alcohol and 95.7% selectivity of vanillin under sunlight. Notably, even in a laboratory setting during the daytime, the Mn-defected γ-MnO₂ catalyst demonstrates significantly higher catalytic performance compared to the dark environment. This work presents a highly effective and promising strategy for low-cost and environmentally benign vanillin synthesis.

Fig. 1. (a) Schematic synthesis process of γ-MnO₂ catalysts with Mn defects (Mn_d). (b) PXRD pattern, (c and d) SEM images, (e) TEM image, (f) SAED pattern, and (g) HRTEM image (left) and corresponding intensity plot (right) of γ-MnO₂ catalyst. (h) EDX elemental maps showing the distributions of Mn and O. Scale bars: (c) 2 μm, (d) 500 nm, (e) 50 nm, (g) 2 nm, (h) 1 μm.

Fig. 2. Mn 3s XPS spectra for (a) γ-MnO₂ with different amounts of urea added in the synthesis and (b) MnO₂ with different crystal structures. The molar ratio of Mn³⁺/Mn⁴⁺ and percentage of oxygen defects (O_d) in different (c) γ-MnO₂ and (d) α-MnO₂.

Fig. 3. Catalytic performance of γ-MnO₂ and comparison catalysts for oxidation of vanillyl alcohol to vanillin. (a) AOS-yield of vanillin on γ-MnO₂, (b) AOS-yield of vanillin on MnO₂ with various crystal structures.

Fig. 4. Exploring the role of oxygen species in the oxidation of vanillyl alcohol over γ-MnO₂ catalyst. (a) Oxidation of vanillyl alcohol under different atmospheres or with various quenches. (b and c) EPR spectra of DMPO-˙OH and DMPO-O₂˙⁻ over various γ-MnO₂ catalysts. (d) Proposed the role of oxygen species in the oxidation of vanillyl alcohol. *O₂ refers to the adsorbed oxygen.

Fig. 5. In situ diffuse reflectance IR spectra (DRIFTS) of activation of O₂/H₂O over γ-MnO₂ catalyst. (a) Activation of O₂/H₂O under various conditions. (b) Activation of O₂/H₂O under sunlight illustration.

Fig. 6. (a) DFT calculations for vanillyl alcohol adsorbed on γ-MnO₂ catalyst with/without defects. (b) DFT calculations for H₂O adsorbed on γ-MnO₂ catalyst with/without defects. (c) A plausible mechanism of oxidation of vanillyl alcohol to vanillin over γ-MnO₂ catalyst under air atmosphere.