Control Of The Emission Of VOCs During Lignin Thermoplastic Processing By Laccase Treatment

With the continuous development of human society, the problems of environmental pollution and resource crisis have becoming more and more serious. Seeking a new clean energy has become the craze of research in recent years. Lignin as a by-product of the pulp and papermaking with the characteristics of wide source, low prices and biodegradable is a suitable new energy for contemporary society, its special molecular structure also makes the lignin has good mechanical properties and processing performance. Based on these advantages of lignin, lignin instead of petroleum feedstocks as applied materials has been widespread concern. However, there are still some difficulties in the application of lignin, one of the difficulties is the volatile organic compounds (VOCs) come from lignin and lignin thermoplastic process. These defects extremely limited the application of lignin.In this work, the study aims at reducing the volatile amount of VOCs come from lignin thermoplastic process by treated with laccase. We choose triploid populus tomentosa kraft lignin and larch kraft lignin as experiment raw materials, and detect the change in the volatile amount of VOCs before and after laccase treatment. In addition, the experimental materials were analyzed and the reaction mechanism between laccase and lignin was explored.The VOCs components and relative content of black liquor were detected by Solid Phase Micro-extraction (SPME) with Gas Phase and Mass Spectrometry (GC-MS). Test results show that the main volatile organic compounds in the black liquor are polyphenols, organic sulfides and pinene olefinic substances. In the triploid populus tomentosa kraft black liquor, the proportion of small molecule phenols to total volatile substances is 59.9%, while organic sulfides is 5.0%, pinene olefinic is 28.8%. In the larch kraft black liquor, the proportion of small molecule phenols, organic sulfides and pinene olefinic substances to total volatile substances is 44.8%,2.6% and 19.4% respectively.The VOCs volatilization of lignin thermoplastic process was tested by Headspace Gas Chromatography (HS-GC) and Tthermal Gravimetric Analysis (TGA) separately. HS-GC analysis shows that, VOCs volatilization of lignin decrease by 35.70% after treated by laccase. TGA results show that, the VOCs volatilization of lignin treated by laccase during the lignin thermoplastic process is less 0.9%(relative to the lignin content) than lignin control at the 190℃ thermoplastic temperature, however, the organic content is 0.7% (relative to the lignin content) higher than lignin control.The influence of laccase dosage on VOCs volatilization of lignin indicate that the VOCs volatilization of lignin during the thermoplastic process show a decreasing trend with the increase in the amount of laccase. In the conditions of 80 U/g of laccase dosage, phenolic compounds content of triploid populus tomentosa kraft lignin filtrate reduce by 32.6%, and phenolic compounds content of larch kraft lignin filtrate reduce by 24.0%.The reaction mechanism between laccase and lignin were explored by Gas Chromatograph Mass Spectrometer (GC-MS), Gel Permeation Chromatographer (GPC), Nuclear Magnetic Resonance Spectrometer (NMR) and Fourier Infrared Spectrum analyzer (FTIR). In summary the above test results, it demonstrate that, the main components of VOCs in the process of lignin thermoplastic are small molecules phenols which come from lignin degradation. By laccase catalytic oxidation, the phenolic hydroxyl groups of lignin and the free small molecule phenols are oxidized to phenoxy radicals, the generated phenoxy radicals converging mutuality makes the small molecule polyphenol content decrease, therefore, the small molecule polyphenol don’t easy volatilize. Simultaneously, the side chain of benzene also be oxidized during enzyme treatment process, more other carbon chain-containing substance are grafted on the benzene ring structure, it result to some others small molecule volatile substances do not easy volatilize during the lignin thermoplastic process.