Photocatalytic Degradation of β-O-4 Lignin Model Compound by In2S3 Nanoparticles Under Visible Light Irradiation

Cleaving the abundant β-O-4 linkages in lignin is a key issue for producing value-added products by controlled lignin depolymerization. Herein, hydrothermally synthesized In2S3 nanoparticles were primarily used to photodegrade guaiacylglycerol-β-guaiacyl ether, a β-O-4 lignin model compound, under visible light irradiation. The as-synthesized In2S3 nanoparticles are found to be typical β-In2S3 nanocrystals of cubic phase and composed of large plate-like particles and small granular particles by using X-ray diffraction technique and field-emission scanning electron microscopy. The bandgap energy of the In2S3 nanoparticles is estimated to be 1.78 eV using an UV-visible diffuse reflectance spectroscopy. The photodegradation and structure variation of lignin model compound were evaluated by the variation of its UV-vis absorption spectrum, Fourier transform infrared spectrum, and X-ray photoelectron spectroscopy, while its degradation products were identified by using the gas chromatography-mass spectrometry. The results show that the as-synthesized In2S3 nanoparticles can photocatalytically break the β-O-4 linkage and oxidize the hydroxyl/methoxyl groups of lignin model compound under visible light irradiation although the lignin model compound is photo-resistant even under UV irradiation. The photodegradation products of lignin model compound consist of various aromatic monomers including value-added acetovanillone, vanillin, and coniferyl aldehyde. A possible pathway is proposed for photodegrading lignin model compound in the presence of the as-synthesized In2S3 nanoparticles under visible light irradiation.