Study On Oxiammonolysis Of The Industrial Alkaline Lignin

Industrial alkaline lignin is the most important by-product produced by alkaline pulping enterprises.The traditional recovery method is mainly based on combustion,which is contrary to the concept of biomass refining and leading to a waste of valuable resources.The alkaline lignin has not been utilized rationally,the utilization rate of agricultural fertilizer in China is low and the subsequent pollution is large.Based on that,this experiment will be used to purify,classify and oxiammonoly modify the industrial alkaline lignin to provide a useful reference for the application of industrial alkaline lignin fertilizer.On the one hand,it can improve environmental pollution,on the other hand,it can bring additional economic benefits.This paper is mainly divided into three parts.The first part is purifying the recovered industrial alkaline lignin by acid precipitation method,to obtain the alkaline lignin with higher purity.Under optimizing the experimental conditions,the yield of purified alkaline lignin is 53.78%.The composition of alkaline lignin before and after purification is analyzed by NREL method.The results show that the lignin content in the purified alkaline lignin increased significantly,reaching 72.87%,which could be used as a further oxiammonolysis modification.The second part is to use the ethanol dissolution fractionation method to separate the purified alkaline lignin into three fractions with different molecular weights based on the difference of solubility.The results are:the yield of fraction 1 is 34.14%,the average molecular weight is the smallest,which is 2194;the yield of fraction 2 is 16,78%,the average molecular weight is 2294;the yield of fraction 3 is 47.92%,the average molecular weight is 4237.The Karl Fischer method is used to determine the total hydroxyl content of the purified alkaline lignin and the fractions.The results show that the total hydroxyl content in the purified alkaline lignin is 1.84%,and the total hydroxyl content in fraction 1 is 2.55%.The total hydroxyl content is 2.01%,and the total hydroxyl content in fraction 3 is 0.90%.Fraction 1 has a high yield,a small average molecular weight,and higher total hydroxyl content,so fraction 1 is optimal.Infrared spectroscopy and nuclear magnetic resonance spectroscopy of industrial alkaline lignin,purified alkaline lignin and three fractions showed that the infrared spectrum curves of the important characteristic functional groups of industrial alkaline lignin and purified alkaline lignin basically overlap,indicating the purification method used in this paper.It is effective and does not cause damage to the structure of alkaline lignin.The characteristic absorption peaks on the infrared spectrum and the chemical shifts on the nuclear magnetic resonance spectrum indicate that the content of hydroxyl groups decreases with the increase of the relative molecular mass of the lignin,which is consistent with the trend of the Karl Fischer method.The third part is oxiammonolysis modification of purified alkaline lignin under the conditions of normal temperature and pressure or high temperature and pressure,which H2O2 is used as oxidant and ammonia is used as ammoniating agent,to explore the optimal process conditions.Then,the three fractions were treated with oxiammonolysis under the optimal process conditions to compare the effect of molecular weight on the modification reaction.The changes of product structure,nitrogen content and carboxyl content during the reaction were detected by means of characterization,such as nitrogen determination,infrared spectroscopy,conductometric titration and elemental analysis.The purpose of oxiammonolysis modification is to reduce the carbon to nitrogen ratio(C/N)and increase the nitrogen content.Under normal temperature and pressure,the best process conditions for modification treatment of the purified alkaline lignin are:10%purified alkaline lignin,30%ammonia water,20%H2O2,reaction temperature 45℃,reaction time 1.5 h.Under the optimal process conditions,fraction 1 with lower molecular weight has the highest total nitrogen content after ammonia oxidation(up to 13.06%),and carbon and nitrogen are relatively low,which meets the requirement of microbial degradation.Under high temperature and pressure,the best process conditions for modification treatment of the purified alkaline lignin are:10%purified alkaline lignin,20%ammonia water,20%H2O2,reaction temperature 120℃,reaction time 45 min.Under the optimal process conditions,fraction 1 with lower molecular weight has the highest total nitrogen content after ammonia oxidation(up to 15.77%),and carbon and nitrogen are relatively low,which meets the requirement of microbial degradation.In addition,under high temperature and pressure conditions,the nitrogen content of the modified product is higher,and the oxiammonolysis of industrial alkaline lignin is more complete,but it is not as mild and safe as normal temperature and pressure.The structure of the alkaline lignin after oxiammonolysis has changed greatly.In the infrared spectrum,the hydroxyl absorption peak is partially shifted;the N-H bending vibration absorption peak of the amide and the N-H in-plane bending vibration absorption peak of-NH2 appear,as well as the C-N stretching vibration absorption peak of primary,secondary and tertiary amines.The peak intensity of the aromatic ring changes and the aromatic nucleus is damaged to some extent.The hydroxyl group in alkaline lignin structure may be oxidized to the carboxyl group during the reaction,the results showed that under the two reaction conditions,the content of carboxyl groups increased first and then decreased during the reaction.And the carboxyl groups increases when average molecular weight decreases.