Study Of Horseradish Peroxidase Catalyzed Polymerization Of Sulfomethylated Lignin

Alkali lignin (AL) is a by-product from kraft pulping process, which is a complicatedcross-linked network macromolecule. It is also an important raw material to acquire aromatichydrocarbon compounds from the natural resources. The modification of AL is of greatpositive significance to achieve substantial development and industrialized utilization of AL.That’s also beneficial to reduce the use of fossil fuels so that it can ensure national energysecurity and environmental protection. AL has broad molecular weight distribution, poorwater solubility and single function, which restricts its application. It is necessary to have itmodified to improve its performance. Among the chemical modification methods, sulfonationis the most convenient method to improve the water solubility and surface activity of AL.With the development of biotechnology and the enhancement of people’s environmentalprotection consciousness, biological method has recently become hotspot in ligninmodification. It has advantages of high efficiency, high selectivity, mild reaction condition, nopollution and it will become one of the major directions of lignin modification in the future.In the present study, pine alkali lignin and wheat straw alkali lignin were chosen as rawmaterials. Different akali lignins were first sulfomethylated with sulphite to obtainsulfomethylated alkali lignin (SAL), and then further incubated with HRP to acquire watersoluble lignin macromolecule polymer HSAL. With the use of Elemental Analyzer, GelPermeation Chromatography, Ultraviolet Spectrograph, Infrared Spectrometer and HydrogenNuclear Magnetic Resonance Spectrometer, this research preliminarily revealed the effect ofsulfomethylation, kinds and molecular weight of SAL on HRP catalytic polymerization bydetecting the molecular weight, the content of functional groups and structure characteristicsof SAL and HSAL. Furthermore, the molecular simulation calculation was also used to revealthe mechanism of SAL polymerized by HRP. With the use of Particulate Charge Analyzer,Atomic Force Microscopy and Dispersion Stability Instrument, this work systematicallystudied the surface physicochemical properties of SAL and HSAL, which is to providetheoretic basis for the improvement of industrial lignin’s application performance byenzymatic modification and to provide a new technique to widen the application of lignin.Pine alkali lignin was used as raw material to study the effect of HRP catalytic polymerization on SAL. Results showed HRP modification could markedly increase themolecular weight and the sulfonation degree of SAL. The increase of the concentration ofHRP could improve the catalytic polymerization reactivity of SAL. HRP modification didn’tbreak the main structure of lignin, but caused the increase of the amount of sulfonic andcarboxyl group and the drop of the amount of phenolic and methoxyl group in SAL. Theprimary analysis considering the mechanism HRP modification was as follows. HRP initiatedthe reaction by oxidizing phenol hydroxyl group to phenoxy radicals in the presence of H2O2.Subsequently, phenoxy radicals were easily delocalized and activated the C5, C3and βpositions of SAL. By radical-radical coupling, the lignin polymer chains propagated andcaused the free radicals sulfonation of SAL. On the other hand, owing to the strong oxidizingproperty, HRP oxidized hydroxyl and aldehyde groups to carboxyl group and oxidizedmethoxy benzene structures to benzoquinones of SAL.By changing the sulfonation degree of SAL, we studied the effect of sulfomethylation ofSAL on reactivity of further HRP catalytic polymerization. It showed that thesulfomethylation could improve the polymerization of HRP incubation, and vice versa. Theincreasing sulfonation degree improved the water solubility of PAL, which in turns made SALeasier to combine with the active center of HRP. The results of molecular simulation alsoshowed that the increase of SAL’s sulfonic group contents increased the G in HRP catalyticpolymerization of lignin monomer, which made it easier for SAL to form β-O-4’、β-β’ andβ-5’linkages.This study also compared reactivity in sulfomethylation and HRP catalyticpolymerization of pine sulfomethylated lignin and wheat straw sulfomethylated lignin.Results showed that wheat straw sulfomethylated lignin had a higher reactivity than pinesulfomethylated lignin, which revealed that the sulfomethylation reactivity of alkali ligninwas mainly related to its molecular weight and methoxyl group content but had nothing to dowith its phenol hydroxyl content. During HRP catalytic polymerization, pine sulfomethylatedlignin showed higher reactivity than wheat straw sulfomethylated lignin. On one hand, thephenolic hydroxyl content of pine sulfomethylated lignin was higher than wheat strawsulfomethylated lignin’s and the molecular weight was lower, which made it more reactive.On the other hand, sulfomethylation mainly occurs on C5site of alkali lignin. Pine sulfomethylated lignin mainly formed β-O-4’ and β-β’ linkages during HRP catalyticpolymerization, which was not reacting on the same site of sulfomethylation and caused lesscompetitive effect. However, wheat straw sulfomethylated lignin mainly formed β-5’ linkagesduring HRP catalytic polymerization, which was reacting on the same site of sulfomethylationand caused more competitive effect.The ultrafiltration method was adopted to molecular weight classification of pine alkalilignin to study the effect of alkali lignin’s molecular weight on HRP catalytic polymerizationof SAL. Results showed that different alkali lignin fractions had similar phenolic hydroxylcontent and that the reactivity of SAL in HRP catalytic polymerization was mainly decided bythe molecular weight of it. Moreover, low molecular weight fraction of SAL had higher watersolubility, lower methoxyl content, more H and G structures and more β-β’ linkage than highmolecular weight fraction, which was advantageous to improve the reactivity of SAL insulfonation and HRP catalytic polymerization.The effect of HRP catalytic polymerization on the surface chemical properties of SALwas also studied. It can be proved that HRP catalytic polymerization process can obviouslyincrease the surface charge density and adsorptive property and dispersion stability on solidparticles of SAL.In conclusion, HRP catalytically polymerized alkali lignin provided a green andconvenient technique for the preparation of high molecular weight and high sulfonationdegree modified lignin products. This technique achieved a breakthrough on improving AL’smolecular weight and sulfonation degree at the same time compared with the traditionalchemical modification. This can not only widen the application of lignin but also replace thedwindling lignosulfonate, which has great significance on solving the problem of energyshortage and environmental pollution.