Transparent aqueous dispersions of ultrasmall ZnO nanoparticles with enhanced oxygen vacancy as efficient pseudohomogeneous photocatalysts for the depolymerization of lignin

[Display omitted] •Transparent aqueous dispersions of ultrasmall ZnO nanoparticles are prepared.•KH550 as modifier sensitizes ZnO NPs to increase oxygen vacancy concentration.•It is firstly used as pseudohomogeneous photocatalyst for lignin depolymerization.•Lignin is converted into value-added vanillic acid with high yield of 17.6%•A possible photocatalytic mechanism emphasizes the importance of h+ and •O2– Lignin represents the most abundant aromatic biomass resource in nature. The development of efficient catalysts opens an avenue for photocatalytic depolymerization of lignin waste into value-added products. Herein, transparent aqueous dispersions of ultrasmall ZnO nanoparticles (4 nm) were firstly adopted as pseudohomogeneous photocatalysts for highly efficient lignin depolymerization. The proper addition of NH2(CH2)3Si(OC2H5)3 (KH550) as a modifier not only contributes to the formation of smaller ZnO nanoparticles, better dispersibility in water and better transparency of nanodispersions, but also sensitizes ZnO nanoparticles to increase the oxygen vacancy concentration, thereby enhancing the photocatalytic performance. Therefore, the as-prepared catalyst exhibits excellent activities for photocatalytic depolymerization of sodium lignosulphonate with a depolymerization of 96.2 % (UV) and 87.3 % (solar light), with a very low catalyst amount in 120 min, far surpassing most reported catalysts. Furthermore, lignin could be converted into high-value-added vanillic acid with yields of 17.6 % (UV, 30 min) and 3.7 % (solar light, 60 min), respectively. This study provides an available practice for photocatalytic depolymerization and valorization of lignin and an opportunity to efficiently utilize the entire lignocellulosic biomass.