分子筛分步催化降解碱木素制备芳香化学品

Producing aromatic chemicals from stepwise degradation of alkali lignin over zeolites catalysts

本研究采用分步催化的方式对碱木素进行降解,即先利用氢型β分子筛(H-BEA)催化液化碱木素,再采用氢型超稳Y分子筛(H-USY)对液化后的产物进行催化降解。结果表明,相较于非催化条件下的直接降解,分步催化降解将产物中芳香化学品含量从30.6%提升到51.2%。质谱分析(MALDI-TOF)结果表明,降解途径是碱木素中的芳香五聚体被液化为三聚体再进一步降解为芳香二聚体和单体。1H NMR结果表明,第1步催化液化过程破坏了大部分C—O键,而第2步催化降解过程主要以破坏C—C键为主。气相色谱检测到的芳香单体占芳香化学品的16.2%。其中,H-BEA分子筛在液化阶段催化部分酚类烷基化,使得酚羟基的邻位或对位活性降低,防止酚类在第2步催化过程中发生缩合。

Lignin is the second most abundant natural polymer and consists of methoxylatedphenylpropane units cross-linked by C—C and C—O bonds. This renewable resource is of interest as an alternative to petroleum for the production of valuable chemicals like phenols, benzene, and toluene. In the present study, aromatic chemicals were produced from stepwise degradation of alkali lignin over zeolites. Namely, the lignin was first liquefied over H-BEA zeolite, thus the obtained oil was catalytic refined over H-USY zeolite. The results showed that the yield of aromatic chemicals increased from 30.6% to 51.2% compared to single step reaction. In the process, parts of the phenolic hydroxyl groups were transformed into aromatic ethers over H-BEA at first stage and the C—C linkage were broken over H-USY at second stage. According to molecular weight analysis results by matrix assisted laser desorption/ionization time of flight（MALDI-TOF）, it could be concluded that lignin first degradated to 4 or 5 C9 units followed by further degradation to aromatic trimmers, and the aromatic trimmer then degraded to aromatic dimmer or monomers. ^1H NMR demonstrated that the C—O linkages were broken during the liquefaction stage, and the refining stage only broke some C—C linkage. Finally, 16.2% monomers of aromatic chemicals were detected by gas chromatography. A possible explanation for the improved output of aromatic species over stepwise degradation is that the alkylation of phenolic hydroxyl groups can facilitate lignin depolymerization; the resulted aromatic ethers are less active toreact with lignin in the second catalytic degradation stage.