Mechanism Of Horseradish Peroxidase Catalyzed Polymerization Of Sulfomethylated Lignin And Application In The Preparation Of Polypyrrole Conductive Paper

Alkali lignin?AL?is a by-product from Kraft pulping process and lacks of strong hydrophilic functional groups.The introduction of sulfonic acid groups by sulfonation is the most common method for imparting water solubility and surface physicochemical properties to AL.Horseradish peroxidase?HRP?can catalyze the polymerization of lignin efficiently to increase the molecular weight which significantly increases its application performance as dispersant,etc.Research on the effect of structural characteristics of lignin on enzyme catalyzed polymerization is not detailed.In order to provide theoretical guidance for the biological modification of lignin,we use lignin model compounds with different structure to study the mechanism of HRP catalyzed polymerization.Polypyrrole conductive paper can be widely used as electromagnetic shielding materials,anti-static packaging materials and electrochemical materials.Using HRP catalyzed-polymerized lignosulfonate as a dispersant and dopant for the preparation of polypyrrole conductive paper is expected to solve the difficult processing of conductive polymer and weak bonding with fibers,and further improve the conductivity,strength and stability,which provides new ideas for the high value utilization of industrial lignin.In the present study,horseradish peroxidase?HRP?was used to catalyze the polymerization of lignin model compounds and their sulfomethylated products in the presence of hydrogen peroxide?H₂O₂?.With the use of gas chromatography?GC-MS?,hydrogen nuclear magnetic resonance spectrometer?¹H-NMR?,this work systematically investigated the effect of structural characteristics such as functional groups and linkages of lignin model compounds on the polymerization activity.And the reaction enthalpy,free energy,steric hindrance energy and electron cloud density,etc.were calculated by molecular simulation technique to reveal the catalytic polymerization mechanism of HRP for lignin substrate.In order to widen the application of lignin,lignosulfonate-polypyrrole conductive papers were prepared by in-situ polymerization and self-assembly methods using lignosulfonate?SAL?and HRP catalyzed-polymerized lignosulfonate?HSAL?as a dopant and dispersant.The effect of dopants and assembled layers on the performance of the conductive paper were investigated by infrared spectroscopy?FT-IR?,tensile strength,electrochemical,static contact angle and scanning electron microscopy?SEM?,and the mechanism of the preparation process was proposed.The main conclusions are as follows:?1?The effect of the structural characteristics of lignin on the reactivity of HRP catalyzed polymerization mainly includes the following aspects:phenolic hydroxyl group is necessary for HRP catalyzed polymerization of lignin,the presence of methoxy group inhibits the polymerization,the introduction of sulfonic group can promote the polymerization,and?-O-4type lignin model compounds have higher reactivity than?-O-4 type lignin model compounds.During the polymerization,?-5 linkage was most easily formed followed by?-?linkage,and?-O-4 linkage formed lastly.Higher charge density on O₄ position of lignin model compounds facilitates the formation of more phenoxy radicals and thereby promotes polymerization.HRP catalyzed polymerization is also related to the steric hindrance of lignin model compounds,and lower steric hindrance leads to higher reactivity.The mechanism of HRP modification was as follow:HRP initiated the reaction by oxidizing phenol hydroxyl group to phenoxy radicals in the presence of H₂O₂.Subsequently,phenoxy radicals were easily delocalized and activated the C₅,C₃ and C?positions of lignin.Finally,the lignin polymer chains propagated by radical-radical coupling.?2?Polypyrrole conductive paper was prepared by in-situ polymerization.Results showed that lignosulfonate not only played a role as dispersant and dopant but also better improved the comprehensive performance of conductive paper than hydrochloric and polystyrene sulfonic.More importantly,since lignosulfonate had a three-dimensional network structure and strong interaction with the fibres,the conductive stability under various bending states and the tensile strength of conductive paper were remarkably improved.The oxidation resistance and waterproof of lignosulfonate could also increase the conductive stability of conductive paper in air and moisture.Compared with lignosulfonate?SAL?,the molecular weight of HRP catalyzed-polymerized lignosulfonate?HSAL?is significantly increased,thereby HSAL could better disperse polypyrrole.Furthermore,HSAL polymerized into more stretched spherical structure,resulted in the ordering growth of polypyrrole and the improvement the conductivity.The spherical structure of HSAL also increased its adsorption sites and strength on fibers,and further improved the conductive stability and tensile strength of the conductive paper.?3?Multi-layer polypyrrole conductive paper was prepared by self-assembly method.Results showed that compared with in-situ polymerization,conductive paper prepared by self-assembly method has better properties.The loading,conductivity and stability of lignosulfonate-polypyrrole paper were improved with the increase of layers.Lignosulfonate can improve the conductive stability and tensile strength of the conductive paper more significantly than hydrochloric and polystyrene sulfonic,because multi-layer self-assembly is more favorable for the loading of lignosulfonate with three-dimensional network structure.At the same time,polypyrrole could be uniformly dispersed by lignosulfonate thereby adsorbed on the paper surface in porous form with no effect on the permeability of conductive paper.HSAL can further improve the comprehensive performance of polypyrrole conductive paper.The primary analysis considering the mechanism of self-assembly method preparing lignosulfonate-polypyrrole conductive paper was as follows.In the process of self-assembly,lignosulfonate preferentially combined with the fibers through hydrogen bonding,and then adsorpted the cationic conductive polypyrrole by electrostatic interation.Finally,a conductive coating with adjustable loading and conductivity was formed on the paper.