碳酸钠催化氯化胆碱/乳酸低共熔溶剂改性碱木质素及其表征

Modification and Characterization of Alkali Lignin by Choline-chloride/Lactic Acid Deep Eutectic Solvents Catalyzed by Sodium Carbonate

以季铵盐氯化胆碱(ChCl)与乳酸(Lac)、尿素(U)、草酸(OA)、甲酸(FA)分别合成低共熔溶剂(DES)并对工业碱木质素(AL)进行改性,研究了由不同氢键供体与ChCl合成的DES体系、反应条件及催化剂对碱木质素改性的木质素提取率的影响。研究发现使用氯化胆碱/乳酸低共熔溶剂(ChCl/Lac)时,木质素的提取率最高。通过单因素试验得到ChCl/Lac改性的最优条件为:在120℃时,Lac与ChCl物质的量比值(n_(Lac)/n_(ChCl))为12,DES添加量为碱木质素质量的20倍(m_(DES)/m_(AL)=20),反应时间12 h,木质素提取率达95.37%;当反应温度降低到100℃,无催化剂时,木质素提取率为40.39%,使用8%碳酸钠为催化剂时,木质素提取率提高至74.87%。采用FT-IR、^(13)C NMR、TG和DTG对木质素样品进行表征,由FT-IR、^(13)C NMR结果可得,改性中β-O-4键断裂并引入羟甲基和甲氧基,改性后木质素主要结构单元为紫丁香基结构。从TG和DTG分析得到碳酸钠催化改性后木质素放热峰向高温移动,热稳定性提高。

The deep eutectic solvents(DES)were synthesized by choline chloride(ChCl)and lactic acid(Lac),urea(U),oxalic acid(OA)and formic acid(FA),respectively.The industrial alkali lignin(AL)was modified by these DESs.The effects of different DES systems,reaction conditions and catalysts on the extraction yield of the modified alkali lignin were studied.It was found that the extraction yield of lignin was the highest when ChCl/Lac was used.The optimal conditions of ChCl/Lac modification were obtained by single factor test.And the optimal conditions were 120℃,the mole ratio of Lac to ChCl(n_(Lac)/n_(ChCl))12∶1,mass ratio of DES to alkali lignin(m_(DES)/m_(AL))20 and reaction time 12 h,under these conditions,the extraction yield of lignin was 95.37%.When the reaction temperature was reduced to 100℃,the lignin extraction yield was 40.39%without catalyst.While 8%sodium carbonate was used as catalyst,the lignin extraction yield increased to 74.87%.The lignin samples were characterized by FT-IR,^(13)C NMR,TG and DTG.The results of FT-IR and^(13)C NMR showed thatβ-O-4 bond was broken and hydroxymethyl and methoxyl groups were introduced during modification,and the modified lignin was mainly composed of syringyl unit.From TG and DTG results,the exothermic peak of lignin moved to high temperature after sodium carbonate catalytic modification,and the thermal stability was improved.