Study On The Formation Of Pseudo-lignin In The Pretreatment Of 72 Poplar Bark With Dilute Acid

Lignocellulosic materials are the most abundant,green and renewable biological resources,mainly composed of carbohydrates（cellulose and hemicellulose）and aromatic-based biopolymers（lignin）,which can be used to produce various fuels and fine chemicals to replace fossil energy.In the process of lignocellulosic utilization,the bark is produced in huge quantities,but it is often discarded,which is easy to breed diseases and insect pests and cause environmental pollution.Among the discarded bark,the thickness of the phloem in the 72 poplar bark is not only thicker than that of other poplars,but also 3-5times thicker than the phloem of coniferous wood.Direct discarding is a great waste of forest plant resources.The phloem of 72 poplar has loose structure,large cavity and thin wall,low degree of lignification,low anti-degradation barrier,and can significantly reduce production costs when used in enzymatic hydrolysis.In the enzymatic hydrolysis of lignocellulosic to produce fermentable sugar,dilute acid is the most common pretreatment method to destroy the natural anti-degradation barrier of plant cell wall,but it has no obvious effect on the separation of lignin,but will make the content of acid-insoluble lignin higher than that of raw materials,resulting in pseudo lignin.Pseudo-lignin is considered to be the aromatic family substance formed by dehydration condensation and aromatization of carbohydrates during dilute acid pretreatment.Pseudo-lignin binds to cellulase unproductively,which affects the efficiency of enzymatic hydrolysis.In this study,72 poplar phloem was used as raw material to explore the effect of pretreatment conditions on the production of pseudolignin and the production mechanism of pseudolignin during the utilization of lignocellulose.The 72 poplar phloem was delignified to prepare holocellulose.The holocellulose was pretreated,and the model compound was used as a comparison to analyze the relationship between pseudo-lignin yield and temperature,time,and the content of components in the acid hydrolysate.Using a new separation method,double ball mill and enzymatic hydrolysis methods and deep eutectic solvents to separate pseudo-lignin.And the structure of the separated pseudo-lignin was compared with that of lignin from different raw materials,in order to study the efficiency separation method and formation mechanism of pseudo-lignin and to better explore its structure.The main conclusions of this paper are as follows:（1）The phloem of 72 poplar was simple in structure,mainly composed of phloem parenchyma cells and sieve tube molecules[81.9±1.8%of the total cell area],with low density and porosity,many parenchyma cells and low lignin content.In addition,phloem was less lignified,and its density and crystallinity were lower,which were 8.7%,13.4%and 23.4%lower than xylem of 72 poplar,Chinese fir and moso bamboo,respectively.The special structure of 72 poplar phloem is particularly favorable for mechanical or chemical degradation and physical and chemical modification to achieve high value and efficient utilization of 72 poplar bark.（2）The holocellulose was pretreated with 1 wt%H₂SO₄ at different temperatures for 1,2 and 4 h.The color of hydrolysate increased with time and temperature,and the relative content of pseudo-lignin increased to 56.8%at 220℃for 4 h,which was 55.6%higher than that in raw materials.HPLC detection of the components in the hydrolyzate showed that during the pretreatment process,the cellulose and hemicellulose in the cellulose were hydrolyzed into glucose and xylose.With the extension of time,formic acid,acetic acid,levulinic acid,HMF,furfural and other by-products were produced in the hydrolysate.Moreover,the content of by-products in the solution was inversely proportional to the content of pseudo-lignin.The results showed that in this process,these small molecule compounds as intermediate products of the reaction,are further transformed into pseudo-lignin during the pretreatment process.The crystallinity of the samples increased from 40.6%to 73.5%with the increase of the pretreatment intensity.In addition,the thermal stability of the sample increased with the increase of the pretreatment temperature,indicating that with the increase of the pretreatment intensity,the pseudo-lignin was condensed into macromolecular substances that were difficult to be thermally degraded.When a simulated compound composed ofα-cellulose and xylose was pretreated,a phenomenon similar to that of holocellulose appears.（3）The chemical structure of the chemical structures of lignin and pseudo-lignin extracted by double ball mill and enzymatic digestion and DES was compared by FTIR and 2D-HSQC NMR techniques,and the macromolecular structure of pseudo-lignin was proposed.It was found that pseudo-lignin had similar functional groups as lignin,such as hydroxyl group,aromatic ring,carbonyl group and ether bond.The formation mechanism of pseudo-lignin was speculated based on the experimental results of HPLC,FTIR,and 2D-HSQC NMR:The cellulose and hemicellulose in the holocellulose were hydrolyzed by acid catalysis to obtain glucose and xylose.The monosaccharides were degraded to 5-hydroxymethylfurfural and furfural,which were further converted into intermediates such as humins substances,polymers containing hydroxyl and furan rings（FPHG）and polymers containing carbonyl,furan rings and C=C double bonds（FPCD）to form the backbone of pseudo-lignin.and then 5-hydroxymethylfurfural,furfural and their derivatives were linked by esterification reactions,hydroxylaldehyde condensation,aldehyde addition and acetal reactions under pretreatment conditions to form pseudo-lignin macromolecules.