Tungsten Trioxide-Catalyzed The Mild Oxidative Depolymerization Of Lignin And Its Model Compounds

Recently,depolymerization of lignin rich in aromatic rings to fuel and high value-added chemicals has attracted the attention.In this paper,lignin model compounds and lignin were oxidized by one-pot method using Nano Tungsten trioxide（Nano-WO₃）as catalyst.The structure,morphology and morphology of commercial Nano-WO₃ were characterized by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）and field emission transmission electron microscopy（TEM）.Additionally,8 kinds of lignin model compounds withβ-O-4 structure were synthesized,in which 2-（2-methoxyphenoxy）-1-（4-methoxyphenyl）-ethanol（1a）was selected as the probe to optimize the reaction conditions.Finally,organic solvent lignin was used as substrate to investigate the effect of solvent,polymerization inhibitor,ethylene glycol dosage,dioxane dosage,temperature,time,catalyst dosage and the ratio of alkali/oxidant to lignin.XRD analysis showed that Nano-WO₃ catalyst was monoclinic crystal.XPS analysis further confirmed that W exists in the form of W⁶⁺and O exists in the form of W-O-W in monoclinic WO₃.Based on SEM and TEM images,Nano-WO₃ is lamellar structure with particle size of 50-100 nm.The 1a of optimal reaction conditions were as follows:substrate（1a）0.5 mmol,Nano-WO₃ 0.1mmol,70%tert-butyl hydroperoxide（TBHP）3.9 mmol,24%sodium hydroxide 4.8 mmol at 80 ^oC for 2h.With itsconversion rate was 98.0%,1a was converted into guaiacol,p-methoxyacetophenone,and p-methoxybenzoic acid,and the corresponding yield was 16.9%,14.5%and 81.8%,respectively.Studies on other model compounds showed that the reaction systems could effectively break theβ-O-4 bond,with a conversion rate of 96%-99%.Ketones or carboxylic acids,main products derived from cleavage ofβ-O-4 bond,depends on the methoxy group in the model compound.The main product yield was 82.3%-98.9%.The yield of phenol was much lower than that of ketone or acid since phenol could be further oxidized and polymerized.The yield of phenol could be 98%-99.4%in the absence of methoxy group on the aromatic rings.Further deuteration experiments,kinetic isotope effect experiments and intermediate verification experiments provided evidence for the reaction mechanism.The catalyst cycle experiment showed that the Nano-WO₃ catalyst possessed good recyclability,and the gram-level reaction further proved the practicability of the developed scheme.On the basis of the model compound,using Nano-WO₃ catalyst,adding 1.6 m L of dioxane and90μL ethylene glycol reaction temperature 100 ^oC and reaction time 4 h,the conversion of orgaosolv lignin catalytic oxidation was 93.3%,and the product were aromatic compounds,and selectivity to vanillic acid in the liquid product was 90%.Results of the study showed theoretical basis for the catalytic system under temperature and conditions.There are 60 figures,27 tables and 104 references in this paper.