Role of Nb2O5 Crystal Phases on the Photocatalytic Conversion of Lignin Model Molecules and Selectivity for Value‐Added Products

The selective photocatalytic conversion of guaiacol and phenol using Nb₂O₅ with different crystal phase under UV-VIS light irradiation in aqueous media was performed. Photoinduced ⁻⋅OH are the main responsible for phenol selective conversion to pyrogallol, while ⁻⋅OH and O₂⋅⁻ attack on guaiacol methoxy functional group was the main path for guaiacol conversion to salicylaldehyde and hydroquinone.

Abstract

The photocatalytic conversion in aqueous media of phenol and guaiacol as a lignin model compound using Nb₂O₅ with different crystal phases was studied. Nb₂O₅ particles were synthesized using hydrothermal methods, where it was observed that changes in the solvent control their morphology and crystal phase. Different photocatalytic behavior of Nb₂O₅ was observed with the selected model compounds, indicating that its selection directly impacts the resulting conversion and selectivity rates as well as the reaction pathway, highlighting the relevance of model molecule selection. Photocatalytic conversion of phenol showed conversion rate (C%) up to 25 % after 2 h irradiation and high selectivity (S%) to pyrogallol (up to 50 %). Orthorhombic Nb₂O₅ spheres favored conversion through free hydroxyl radicals while monoclinic rods did not convert phenol. Guaiacol photocatalytic oxidation showed high conversion rate but lower selectivity. Orthorhombic and monoclinic Nb₂O₅ favored the formation of resorcinol with S % ~0.43 % (C % ~33 %) and ~13 % (C % ~27 %) respectively. The mixture of both phases enhanced the guaiacol conversion rate to ~55 % with ~17 % of selectivity to salicylaldehyde. The use of radical scavengers provided information to elucidate the reaction pathway for these model compounds, showing that different reaction pathways may be obtained for the same photocatalyst if the model compound is changed.

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