Preparation Of Carboxymethylated Lignin And Its Mechanism Of Action In Graphitc/H₂O Dispersion System

Nowadays, the gradual shortage of natural resources and the rapid worsening of theenvironment have already become the severe problems for the survival and continuousdevelopment of human being. How to utilize the renewable biomass resource with highefficiency has attracted worldwide concern. Lignin, the second most abundant biopolymer onearth, is one of renewable natural biomass resources. In china, industrial lignin mainly existedin the waste liquor from the paper and pulping industry has the properties of rich sources, lowprice and poor water solubility. Using industrial lignin as raw material, producingenvironmentally friendly green fine chemical products via chemical modification anddeveloping green chemical processes not only reduce the pollution damage to theenvironment caused by the pulping waste liquor, but also increase the economic value addedof industrial lignin, which has double significance in environment and resource.Using wheat straw alkali lignin （WAL） as raw material, the research on preparation ofcarboxymethylated lignin （CML） was carried out via organic solvent-medium method andwater-medium method, respectively. Results showed that the productivity ofcarboxymethylation and carboxylic group content of CML prepared in organic solventmedium were94.8%and2.76mmol/g under the optimum conditions of ethanol and waterwith the volume ration of80:20as solvent, reaction temperature of70°C and two-stageNaOH addition; the productivity and carboxylic group content of CML prepared in watermedium were93.0%and2.82mmol/g under the optimum conditions of NaOH massconcentration of20.0%, monochloroacetic acid mass concentration of37.5%, reactiontemperature of70°C and reaction time of90min.The structures of CML samples were characterized and determined by means of theautomatic potentiometric titration, FTIR,¹H,¹³C–NMR and¹H-¹³C HSQC–NMRspectroscopy, respectively. For the CML sample prepared in organic solvent medium, theabsorption peaks at1604and1422cm-1were assigned to the characteristic absorption bandsof craboxylate （–COO-）;¹³C–NMR spectrum showed that many strong peaks within the rangeof179¹70ppm were originated from the C atom of–COOH, and the appearances of somesignals in the range of73⁶6ppm were attributed to the C atoms of methylene groups of carboxymethyl groups （–CH2COOH）. Moreover, there was a marked increase in the relativequantity of guaiacyl unit with α-ether, and the increase in carboxylic group content of WALwas much larger than the decrease in phenolic group content, which illustrated that thesubstitution reaction of carboxymethylation in organic solvent medium occurredsimultaneously in phenolic hydroxyl group and the α-position aliphatic hydroxyl group ofWAL. For the CML sample prepared in water medium, the absorption peaks at1601and1421cm-1were assigned to the characteristic absorption bands of–COO-, and a strong peak at1725cm-1in H-form CML originated from unconjugated carbonyl stretch; In¹H–NMR spectrum, abroad peak at12.47ppm was assigned to proton of the hydroxylic group, and the appearancesof two new signals at4.57ppm and4.35ppm were attributed to protons of methylene groupsof–CH2COOH; In¹³C–NMR spectrum, a very strong signal at170.62ppm was originatedfrom the C atom of–COOH, and two new signals at69.28ppm and65.74ppm wereattributed to C atoms of methylene groups of–CH2COOH; Moreover, in¹H-¹³C HSQC–NMRspectrum, there were also two new crosspeaks at δ4.57/65.07ppm and4.35/68.6ppmattributed to H/C signals of methylene groups in substituents of-CH2COOH. By calculatingthe relative quantity of the main lignin units shown in¹³C–NMR spectra, it was found thatthere was obviously an increase in relative quantity of guaiacyl unit with α-ether, but therelative quantity of syringyl unit has hardly any change at all after carboxymethylationreaction in comparison with that before reaction. Results showed that the carboxymethylationreaction in water medium also occurred simultaneously in phenolic hydroxyl group and theα-position aliphatic hydroxyl group. Finally, a possible mechanism of carboxymethylationreaction of WAL in water medium was proposed.The effects of mass concentration, solution pH and ionic strength on the viscositybehavior and aggregation of CML in aqueous solution were studied by means of viscometer,Zeta potential analyzer, static fluorometer and dynamic light scattering technique. It wasfound that the reduced viscosity of CML solution increased dramatically with decreasingCML concentration, which confirmed the obvious characteristic of polyelectrolyte of CML indilute aqueous solution; moreover, the reduced viscosity of CML solution decreased rapidlywith increasing solution pH, while it decreased first and then increased with increasing ionicstrength. The critical aggregation concentrations （CAC） of CML samples with different carboxylic group contents of2.82mmol/g,2.61mmol/g and1.99mmol/g in water were0.022g/L,0.025g/L and0.026g/L, respectively. When the concentration was higher than CAC,there existed aggregates with different particle sizes and distributions. With increasingsolution pH, the scattered light intensity of CML solution was weakening gradually, and ared-shift of the fluorescence excitation spectrum of CML occurred only under strong alkalinecondition. Results showed that the molecular aggregation of CML occurred and the Ï€-Ï€interaction among aromatic rings was strengthened under strong base condition. On the otherhand, the ionic strength had no obvious effect on the fluorescence excitation spectrum, but thescattered light intensity and particle size decreased first and then increased with increasingionic strength. The results indicated that when ionic strength was relatively lower, there onlyoccurred the intramolecular shrinkage; however, with further increasing ionic strength, themolecular aggregation was present. Both the carboxylic group content and urea had noobvious influences on the fluorescence excitation spectrum of CML.The effects of carboxylic group content, solution pH, ionic strength and urea on theadsorption characteristics of CML at the natural graphite （NG）/water interface wereinvestigated. It was found that the saturation adsorption capacity of CML on NG surfaceincreased with increasing carboxylic group content, and the adsorption isotherm was wellrepresented by the Langmuir Isotherm model. Solution ionic strength had no obviousinfluence on adsorption under neutral condition, while the amount adsorbed of CMLincreased with increasing solution ionic strength under acidic condition. With increasingsolution pH, the amount adsorbed of CML decreased first and then increased. The saturationadsorption capacity of CML was decreased with the addition of urea, moreover, the impactand extent on adsorption decreased with increasing solution pH, and could be ignored underalkaline condition.The effects of carboxylic group content, solution pH, ionic strength and urea on theadsorption characteristics of CML at the high-purity graphite （HPG）/water interface wereinvestigated. It was observed that the saturation adsorption capacity of CML on HPG surfaceincreased with increasing carboxylic group content, and the adsorption isotherm was also wellrepresented by the Langmuir Isotherm model. The amount adsorbed of CML increased withincreasing solution ionic strength under neutral condition, and reached the maximum when CML molar concentration was0.5mol/L; the uptake of CML decreased with increasingsolution pH; the saturation adsorption capacity of CML was also decreased with the additionof urea.The effects of solution pH and ionic strength on the adsorption characteristics of CML atthe gold/water interface were studied by using quartz crystal microbalance with dissipation.With increasing solution pH, the amount adsorbed of CML on gold surface decreased, and theviscoelasticity of adsorbed layer increased. The uptake of CML increased with increasingsolution ionic strength. Moreover, when CML molar concentration was1.0mol/L, theadsorbed layer turned into a rigid one, and a turning point was present with prolongingadsorption time.Using Turbiscan Lab analyzer, the influences of carboxylic group content of CML,dosage of dispersant and suspension pH on the dispersion stability of NG/water system andHPG/water system were investigated and compared with three common dispersants ofsodium lignosulfonate （SL）, sodium carboxymethyl cellulose （Na-CMC） and sodiumpolyacrylate （PAAS）, respectively. Results showed that the dispersion efficiency of CML withcarboxylic group content of2.82mmol/g was higher than those of three common dispersantsmentioned above. The dispersion stabilities of aqueous NG suspension prepared with CML of1.0%dosage at suspension pH6.7and aqueous HPG suspension prepared with CML of0.5%dosage at suspension pH7.3were optimal, respectively. Meanwhile, it was observed that theagglomerate of graphite particles was dramatically reduced and the particle size wasdecreased.The effects of carboxylic group content and dosage of CML, suspension pH on the zetapotential of HPG particle surface were investigated using Zeta potential analyzer. It was foundthat the zeta potential of HPG particle surface decreased with increasing carboxylic groupcontent and dosage of CML. When suspension pH was in the acidic range, with increasingsuspension pH, zeta potential decreased first and then increased; when suspension pH was inthe alkaline range, zeta potential decreased with increasing suspension pH.