Study On Lignin Catalytic Conversion To Prepare Syringaldehyde

As a renewable resource existing intensively in nature, lignin has complex three-dimensional network structure, which restricts its large-scale high valued applications to some degree. Recently, preparing phenolic compounds and aromatic aldehydes by lignin degradation has become a hotspot. In order to realize green and high efficient process of lignin conversion, the present study elaborated the following aspects in detail:(1)For sake of avoiding secondary pollution during the lignin conversion procedure, ionic liquids (ILs) were introduced into catalytic conversion process in consideration of their structural stabilities and thermal stabilities. The effects of types and dosage of both lignin and ILs, reacting temperature, reacting period and pH value on syringaldehyde yield were investigated. Results showed that the optimal reacting condition was:wheat straw sodium lignosulfonate 0.20 g, ethanolamine tetrafluoroborate ([MEA][BF4]) 0.20 g, solid to liquid ratio=1:150(g/mL), temperature 160℃, reacting 6 h, syringaldehyde yield was 9.54%. Adjusting pH value to 4.35 before reaction, syringaldehyde yield could increase 55.80% in comparison with the contrast group.(2)Considering that ILs were not so easily separated with the reacting system and relative higher costs, perovskite-type oxides LaB1-xCuxO3(B=Fe, Ni) with easier separation and lower costs were introduced into the lignin catalytic oxidation process. Effects of types of lignin and catalyst, dosage of catalyst, reacting temperature and time, alkali concentration and O2 pressure on syringaldehyde yield were discussed. The optimal reaction condition was as follows:de-alkali lignin 0.60 g, solid-liquid ratio=1:50 (g/mL),5 wt% dixon supported LaFe0.2Cu0.8O3 0.60 g,1.0mol·L-1 NaOH 30 mL, temperature 160℃, O2 pressure 0.80 MPa, reacting time 2.5 h, syringaldehyede yield was 10.00%.(3)In the interest of improving lignin quality, three pretreatment technologies were introduced to pretreat industrial lignin, including microwave pretreatment (MP), hydrothermal pretreatment (HP) and alkali treatment. The solid recovery (Rs), weight-average molecular weight (Mw), poly-dispersity (Pd) and functional group contents were compared before and after the pretreatment. Results showed that Rs of MP was average higher than 80.00%, while Rs of HP was just higher than 65.00%. Mw of pretreating lignin was bigger than the un-treated one, but the Pd became better. Contents of phenolic hydroxyl group were larger in HP samples than in MP ones. On the other hand, carboxyl group contents were lower than un-treated industrial lignin, which could be attributed to the decarboxylation reaction. FT-IR spectrum showed the characteristic peaks in 1220 cm-1 and 1122 cm-1 were stronger after pretreatment, which provided more possibilities for preparing syringyl products.This study testified catalytic performance of both ionic liquids and perovskite-type oxides, revealing the feasibility to prepare high-value added chemicals by these two kind of catalysts. Results exhibited effects of different pretreatment ways on the structure and properties of lignin, providing theory evidence and data support for the high valued transformation of lignin.