Effects of lignin species and catalyst addition on pyrolysis products

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 (H2O, 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.