| Lignin is an important renewable resource and can be used to produce the valuable chemicals-single phenolic compounds by the oxidation degradation reaction which were widely used in fields of medicine, food, and paint, etc. Now, the wet oxidation catalytized by copper sulphate in the heterogeneous phase is often used in the oxidation of lignin degradation. It exists some shortcomings, such as low reaction rate, much waste liquid discharge, poor product selectivity, serious equipment corrosion. It is well knowen that ionic liquids have the characteristics of the good solubility and thermal stability, reusability, and structure designability. Therefore, this paper synthezes and characterizes some new ionic liquids, and ueses in the homogeneous oxidation degradation reaction of liginin to produce the high value chemicals single phenolic compounds.Five triazolyl ionic liquids(1-butyl-3-methylimidazolium triazole [Bmim]tr, Nn-butylmorpholinium triazole [Bmmo]tr, N-n-butylpyridinium triazole [Bpy]tr, 1-hydrogen-4-methyltriazolium phosphate [Hnmtr]DMP, 1-hydrogen-3-methylbenzotriazolium phosphate [Hnmbt]DMP) were synthesized and characterized by FT-IR and NMR. The results indicated that the synthesized compounds were the target products. Otherwise, the synthesized ionic liquids showed the good thermal stability with more than 180 thermal decomposition temperature. Ionic liquids [Bmim]tr、[Bmmo]tr and [Bpy]tr were alkaline, in which [Bmim]tr had the highest alkalinity(4.48 mmol/g).While ionic liquids [Hnmtr]DMP and [Hnmbt]DMP showed acidic, in which [Hnmtr]DMP had the highest acidity(4.02 mmol/g). Otherwise, all ionic liquids were easily soluble in polar organic solvents.The synthesized ionic liquids were used to dissolve lignin under oil bath and microwave heating. It was found that ionic liquid [Bmim]tr had good solubility performance,and the solubility of lignin was 58.61 g at 55 ℃ under microwave heating. The chemical structure, molecular weight, crystalline and morphology of re-generated lignin from microwave treatment were characterized by HSQC, FT-IR, GPC, XRD and SEM. The results indicated that microwave heating was more favourable for the destruction of hydrogen bond and β-O-4’ linkages, which accelerated the dissolution of lignin. And the basic structural unit of lignin was not destroyed in the dissolution process. The dissolution mechanism of lignin in ionic liquid was the hydrogen bond damage and fracture of β-O-4’ connection structure.The alkaline ionic liquids([Bmim]tr, [Bmmo]tr and [Bpy]tr) were used as solvents for the lignin degradation to prepare vanillyl alcohol. The results showed that lignin was more easily degradable and highly selective production of vanilla alcohol in the solvent ionic liquid [Bmim]tr. Under the better reacation conditions: [Bmim]tr 10 g,lignin 0.30 g, CuSO4 0.012 g, DMSO 1 g, oxygen pressure 4.0 MPa, reaction temperature 170 ℃, reaction time 70 min, the conversion of lignin and the selectivity of vanillyl alcohol were 100% and 23.5%, respectively. Furthermore, the recovery ionic liquid had a good reusability in the reaction.The acidic ionic liquids([Hnmtr]DMP and [Hnmbt]DMP) were used as solvents for lignin degradation to prepare eugenol. It was found that lignin was efficiently degradated to form eugenol in the presence of [Hnmtr]DMP. The better reaction conditions were obtained as followes: [Hnmtr]DMP 10 g,lignin 0.30 g, CuSO4 0.012 g, oxygen pressure 4.0 MPa, reaction time 60 min, reaction temperature 160 ℃. Under the above reaction conditions, the conversion of lignin and the selectivity of eugenol were 100% and 31.6%, respectively. Furthermore, the recovery ionic liquid had a good reusability in the reaction.In conclusion, novel triazolyl ionic liquids were synthesized and used as solvents in the homogeneous oxidative degradation of lignin to prepare monophenol aromatic compounds, and the high conversion of lignin and high selectivity of the products were obtained. This study can provide the guidance for the high value conversion and utilization of lignin. |
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