路易斯酸复合Pt/ZrO2协同催化碱木质素还原降解制备酚类低聚物

Synergetic Effects of Pt/ZrO2 and Lewis Acid on Reductive Depolymerization of Alkali Lignin to Phenolic Oligomers

以路易斯酸FeCl3、CuCl2和NiCl2复合Pt/ZrO2催化碱木质素还原降解制备高酚羟基含量活性低聚物,考察了催化剂组成和用量、反应温度、反应时间、氢气压力等对碱木质素还原降解反应的影响。采用GPC、FT-IR、31P NMR、13C NMR及2D-HSQC NMR等表征分析了碱木质素及其降解产物,探讨了降解前后碱木质素的化学结构、组成及化学键变化。研究结果表明:与Pt/ZrO2相比,路易斯酸复合Pt/ZrO2具有较高的协同催化反应活性,可有效提高降解产物的产率及生成酚类化合物的选择性,Pt/ZrO2-FeCl3具有最优催化活性。适宜的催化反应条件为:催化剂Pt/ZrO2-FeCl3与碱木质素质量比为1∶15、催化剂中FeCl3与Pt/ZrO2质量比值为0.2、反应温度300℃、反应时间3 h、氢气压力4 MPa,该条件下碱木质素催化还原降解得到的低聚物数均相对分子质量(Mn)为998,产率为89%,且具有较高的酚羟值(362 mg/g)和较低的固体残渣率(2.70%)。对碱木质素、Pt/ZrO2催化降解产物(DL1)和Pt/ZrO2-FeCl3催化降解产物(DL2)的结构表征表明:在木质素还原降解反应过程中,紫丁香基(S)型结构单元比愈创木基(G)型结构单元更易被破坏,且Pt/ZrO2-FeCl3催化剂对碱木质素分子结构中—OCH3、β-O-4及β-β键断裂表现出较高的选择性。

A series of Pt/ZrO2 and Lewis acid co-catalytic systems,such as Pt/ZrO2-FeCl3,Pt/ZrO2-CuCl2 and Pt/ZrO2-NiCl2,were prepared for selective depolymerization of alkali lignin to generate oligomers with high content of phenolic hydroxyl groups and low content of solid residue.The effects of reaction parameters such as the component and dosage of catalysts,hydrogen pressure,reaction temperature and reaction time on the reductive depolymerization of alkali lignin were investigated.GPC,FT-IR,31P NMR,13C NMR and 2D-HSQC NMR were conducted to reveal the changes of structure and linkage in alkali lignin and its depolymerized products.The results showed that the synergetic catalysis of Pt/ZrO2 and Lewis acid(FeCl3,CuCl2,NiCl2)exhibited higher catalytic activity and better selectivity for phenolic compounds than Pt/ZrO2.Pt/ZrO2-FeCl3 exhibited the best catalytic activity among Pt/ZrO2-FeCl3,Pt/ZrO2-CuCl2 and Pt/ZrO2-NiCl2.Under the optimal conditions of mass ratio of Pt/ZrO2-FeCl3 to lignin 1∶15,mass ratio of FeCl3 to Pt/ZrO20.2,reaction temperature 300℃,reaction time 3 h,hydrogen pressure 4 MPa,the given products with higher content of phenolic hydroxyl groups(362 mg/g)and lower content of solid residue(2.70%)were obtained,as well as,the conversion rate of lignin and the number average molecular mass(Mn)of the product were 89.0%and 998,respectively.Furthermore,the comparative structure characterization of the alkali lignin,Pt/ZrO2 catalytic depolymerization product(DL1)and Pt/ZrO2-FeCl3 catalytic depolymerization product(DL2)demonstrated that S units were more prone to be damaged during the reductive depolymerization of alkali lignin,the cleavage of methoxy groups and intramolecularβ-O-4 andβ-βlinkages were more effective facilitated by Pt/ZrO2-FeCl3 than Pt/ZrO2.