木质素种类和催化剂添加量对热解产物的影响

Effects of lignin species and catalyst addition on pyrolysis products

木质素是一种天然可再生的芳烃类高聚物,可通过催化热解技术制取苯、甲苯和二甲苯(简称"三苯")等高附加值的平台化学品。该文选取4种木质素,分别为磨木木质素(milled wood lignin, MWL)、碱木质素(alkali lignin, AL)、Klason木质素(Klasonlignin,KL)和溶剂型醇解木质素(organosolvethanollignin,OEL),采用热重-红外光谱联用仪(TGA-FTIR)和热解-气质谱联用仪(Py-GC/MS),研究木质素种类和HZSM-5(Si/Al=25)催化剂添加量(1:1、1:2、1:3和1:5)对木质素催化热解产物的影响。结果表明:1)通过催化剂结构表征可知,HZSM-5属于密排六方晶相结构,孔径分布主要以微孔为主,弱酸含量高于强酸,该结构特征易于发生择形催化反应,增加BTX的产率;2)通过木质素的元素分析及其相关结构表征可知,在4种木质素中,MWL的C和H元素含量最高,分别为62.96%和7.24%,MWL的有效氢碳比值最高,达到0.67。AL的O元素含量最高,达到44.25%,并且其高位热值最低仅为19.90MJ/kg,说明AL中含有更多的含氧官能团和β-O-4连接键;3)由TGA-FTIR分析可知,由于MWL的重均分子量(MW)和多分散系数数值较大,其催化降解失质量温度范围最宽,失质量峰数量最多,但是AL残炭率最低,表明AL的热稳定性最差,AL中的挥发份更多的转化为热解气体和液体产物;并且随着催化剂添加量的增加,H2O、CH4、CO2和CO释放量增加;4)由Py-GC/MS分析可知,随着HZSM-5的加入,导致木质素热解过程中含氧的酚类化学组分含量显著降低,酚类中间产物通过脱羟基、脱甲氧基、脱羰基等脱氧反应逐步转化为BTX,表现出优异的择形催化能力。由于MWL的有效氢碳比最高,使得MWL催化热解产生的芳烃产率最高,并且苯、二甲苯及其甲苯的含量在木质素/催化剂添加量分别为1:5、1:3和1:3时达到最大值,其绝对峰面积分别4.51×107、1.26×108和8.58×107。该文研究可为木质素催化热解制取高附加值化学品提供理论指导。

Benzene, toluene, and xylene(BTX), are value-added aromatic platform chemicals that have been widely used in the synthetic resin, rubber, dye, agrochemical, and pharmaceutical industries. Owing to the natural aromatic structure, lignin has a great potential to serve as starting material for the production of BTX by catalytic fast pyrolysis(CFP) process. In this work, four types of lignin isolated from palm kernel shell with different severities, namely milled wood lignin(MWL), alkali lignin(AL), Klason lignin(KL), and organosolv ethanol lignin(OEL) were used in lignin CFP process. First, the characteristics of zeolite catalyst(HZSM-5 with Si/Al ratio of 25) were analyzed by X-ray diffraction(XRD), automatic specific surface area and pore size distribution analyzer(BET) and automatic chemical adsorption instrument(NH3-TPD), etc. Then, the effects of lignin types and lignin/catalyst ratios(1:1,1:2,1:3, and 1:5) on the product distribution during lignin catalytic fast pyrolysis process were investigated by using thermogravimetric analyzer coupled with Fourier transform infrared spectrometry(TGA-FTIR) and pyrolyzer coupled with gas chromatography/mass spectrometer(Py-GC/MS). Results were showed as followed: 1) Based on the structural characterization analysis of the catalyst, the crystal form of HZSM-5 was dense hexagonal crystal structure. The dominant pore size distribution in HZSM-5 was microporous, and the content of weak acid in HZSM-5 was higher than strong acid. 2) Based on the ultimate analysis of four types of lignin, MWL has the highest content of C and H elements(62.96 % and 7.24 % respectively)and highest effective hydrogen/carbon ratio of 0.67, while AL had lowest effective hydrogen/carbon ratio of 0.29 and highest O content of 44.25%, indicating that AL contains more oxygen-containing functional groups and β–O–4 linkages. 3) Based on the TGA-FTIR analysis, due to the larger weight-average molecular weight(Mw) and polydispersity coefficient(PDI), MWL had widest temperature range of weight loss and maximum number of weight loss peaks. But the AL residual carbon rate is the lowest, indicating that AL has the worst thermal stability and the volatiles in the AL are more converted into pyrolysis gases and liquid products. Among the four types of lignin, the release amount of small molecular weight components(H2 O, CH4, CO2 and CO) showed an increase tendency with the increase of lignin/catalyst ratio. 4) Based on the Py-GC/MS analysis, the bio-oil of lignin CFP was mainly composed of S-typed phenols, G-typed phenols, P-typed phenols, C-typed phenols, and aromatics. With the increase of lignin/catalyst ratio, the content of total phenols was gradually decreased, and it was converted into aromatics by a series of deoxygenation reactions, such as the decarboxylation reaction, the decarbonylation reaction, and the breakage of ether bond. Among the four types of lignin, the highest value of effective hydrogen-carbon ratio was obtained in MWL(0.69), which made MWL a more favorable starting material for the production of BTX. MWL had the highest content of aromatics during CFP process because of its highest effective hydrogen/carbon ratio. The contents of benzene, xylene, and toluene of MWL CFP process reached their maximum values of 4.51×107, 1.26×108, and 8.58×107, when the lignin/catalyst ratios were 1:5,1:3, and 1:3, respectively. The results of this work could provide basic data for the production of high value-added chemicals(BTX) from lignin by CFP.