The Activation Of Lignin In Ionic Liquid With Pd/C As Catalyst

The author gratefully acknowledge financial support provided by the National Natural Science Foundation of China.Lignin with aromatic characteristics is a natural macromolecular compound. Natural lignin behaved good reactivity for its high content active group, while industrial lignin treated with chemicals, has low reactivity, especially alkali lignin. High methoxy content, less hydroxyl and worse molecular weight distribution seriously limit the application of lignin. Hence the research on catalyst can improve lignin reactivity and molecular weight distribution is a key point to the application of lignin.The catalytic activation of was studied in this paper with Pd, carbon sufonic as catalysts, ionic liquid as catalyst and solvent. Evaluated the performance of catalysts by measuring hydroxyl group content and molecular weight distribution.13C-NMR spectrum and two-dimensional carbon HSQC NMR spectrum were used to characterized lignin to explore reaction mechanism. The antioxidant propertiy of lignin was also studied in this paper and the results are showed as follows:The Pd/C catalyst was prepared using impregnation method with formaldehyde as reducing agent and the carbon was treated by HNO3 and H2O2. The catalyst was characterized by atomic absorption spectrometry (AAS), scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and used to activate lignin. Research results show that the pretreated with HNO3 and H2O2, the carboxyl content is 1.31g improved six times; Pd load of 2.9%, a round ball evenly distributed on the surface of carbon, and exsisted cubic structure form, mainly for reduction state which is the main species on catalytic. After activation, the total hydroxy, phenolic hydroxyl and alcohol hydroxyl content of lignin increased by 158.74%,4.21% and 255.96%, respectively; the molecular weight and number average molecular weight is reduced, dispersity decreased.13C-NMR spectrum and two-dimensional carbon HSQC NMR spectrum showed that Pd/C was effectively to promote the S structure unit to G structure unit.Ionic liquid chloride (1-butyl-3-methyl imidazole) ([Bmim]C1) was used as catalyst and solvent on activation of lignin with H2 as reductant. The effect of reaction time, temperature, [Bmim]C1 concentration was explored and the optimum condition is:the reaction time is 4h, the temperature of 90℃, the concentration of [Bmim]C1 9wt%. The results showed that [Bmim]Cl/H2 reduction system was promising catalyst for activation of lignin, total hydroxyl content, phenolic hydroxyl and aliphatic hydroxyl increased by 87.20%,16.30% and 56.64%, respectively.13C-NMR spectrum and two-dimensional carbon HSQC NMR spectrum showed that [Bmim]Cl was efficient on cleavage the side chain but no contribution to the conversion of the S structure unit to G structure unit.Carbon sulfonic acid catalyst was prepared with larch powder and used as carrier to prepare Pd/C-SO3H catalyst and characterized by AAS and SEM. The results showed that carbon-based solid acid has kept the basic structure of larch and 1.73% Pd was load on surface. Acid density of Pd/C-SO3H is 3.81 mmol/g and mainly-SO3H form. After activation, the total hydroxyl content, phenolic hydroxyl and aliphatic hydroxyl were obviously improved, which always hydroxyl increased by 1.86 times, phenolic hydroxyl group increased by 0.49 times, aliphatic hydroxyl increased by 2.70 times. And the molecular weight and dispersion coefficient was decreased significantly. Pd/C-SO3H catalyst could encourage the S structure units to G structure units.The antioxidant propertiy of lignin was also evaluated. Before and after catalytic, the antioxidant properity of lignin was similar due to the catalytic system did not improve the content of phenolic hydroxyl. The oxidation resisitance increased with concentration increasing and weaker than BHT.