| Microwave heating has been attracting more and more attention for thermal treatment of solid fuels due to its advantages of fast heating rate and high heating efficiency over the traditional electrical heating method. In order to strengthen the understanding of catalytic degradation lignin assisted by microwave heating, acidic solvolysis of lignin (by formic acid) under microwave heating would be investigated without and with the addition of modified HUSY zeolite in this work.Microwave-assisted degradation of black-liquor lignin in formic acid without catalyst was studied, concerning the product yield and distribution of phenolic compounds against reaction temperature, reaction time, microwave power and liquid-solid ratio. The liquid product consisting of bio-oil 1 and bio-oil 2, achieved the maxima yield of 64.08% at 160℃ and 30 min (bio-oil 1:9.69% and bio-oil 2:54.39%). The GC-MS results revealed that the influence on the distribution regularities of monophenol by temperature and reaction time were obvious. It was found that bio-oil 2 was composed of lignin-related oligomers (mainly dimers and trimers) according to the distribution of the Mass-Average Molar Mass (Mw) of the molecules from 200 to 500,Aromatic oligomers with molecular weight of 274,290,318, 328,342,358,378,394 and 424 were characterized by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) analysis. The 13C-NMR results revealed that the carbonyl functional groups reduced after lignin degradation, methoxy was also decreased. Comparing with lignin, the β-O-4 keys、β-5 keys、β-β keys and 5-5 keys of the bio-oil 2 reduced to some extent, the β-O-4 keys gradually decreased with the increase of reaction temperature(from 0.27/C9 to 0.13/C9),5-5 keys also showed the decreased tendency with the increase of reaction temperature(from 0.4/C9 to 0.27/C9). Regarding GPC and FTIR results, both indicated that the depolymerization and repolymerization of lignin occurred simultaneously, the repolymerizaiton process played the dominant role when the temperature was higher than 160℃ and reaction time was more than 30 min. A possible mechanism was proposed for the microwave-assisted acidic solvolysis of lignin, specifying the kinetic relationship among the primary cracking of lignin and repolymerization of the oligomers. Microwave-assisted acidolysis lignin was divided into two stages, activation energy for the first stage as the depolymerization reaction was 33.67 kJ/mol, and the activation energy for the second stage as repolymerization of the products was 43.87 kJ/mol.Microwave-assisted catalytic solvolysis of lignin in formic acid was studied involving the addition of HUSY catalysts modified by oxalic acid.Characteristics of the modified catalysts were examined by XRD,NH3-TPD and BET. The highest yield of liquid product was achieved as 88.28 wt%(aromatic monomer fraction as bio-oil 1 of 15.36% and oligomer fraction as bio-oil 2 of 67.52%) with the addition of HUSY modified by 0.2 mol/L oxalic acid (HUSY-0.2M). The production of aromatic monomers in bio-oil 1 identified by GC/MS was enhanced with the addition of HUSY catalysts regardless of acidic treatment, and achieved the maximum value for HUSY-0.2M experiment. Aromatic oligomers with molecular weight of 328,342,358,378,394,424 and 454 were characterized by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) analysis. Formation of almost all identified oligomers was promoted with catalytic experiments with HUSYs except that of 328, extent of which is significantly determined by the pore size, Si/Al ratio and acidic site distribution of the modified catalysts.The study can provide experimental and theoretical guide for the comprehensive utilization of black liquor lignin and producing high value-added chemicals. |
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