Delignification Mechanism And Chemical Reactions In The Formic Acid Pulping Process Of Wheat Straw

The non-wood fiber materials will still play a very important role in the foreseeable future in the pulp and paper industry of China. The conventional pulping technology of the annual plants caused serious environmental problems due to the water pollution. It will be of practical significance to develop a renovative pulping technology of non-wood materials to China paper industry.Due to the different dissolvability in formic acid, the components in wheat straw can be separated with the solvet. Compared to cellulose, lignin can be readily dissolved in formic acid and, therefore, it can be easily applied to the separation of lignin from the raw materials. The volatility of formic acid is good enough to be simply evaporated and recycled in the loop.The delignification and degradation of carbohydrates are considered the most important reactions in the cooking process, In this paper, the milled wheat straw lignin (MWL) and lignin carbohydrates complex (LCC) was employed in the study of delignification mechanism. The wheat straw pulping was conducted with aqueous formic acid (F), formic acid-hydrogen peroxide (FP), formic acid-hydrogen peroxide-sulfuric acid and formic acid-sulfuric acid (FS). The element analysis, gel permeation chromatography (GPC), FTIR, proton nuclear magnetic resonance (¹H -NMR), carbon nuclear magnetic resonance (¹³C -NMR) and GC/MS are also used in the structure analysis. in wheat straw before and after the aforesaid treatment. The mechanisms of delignification and carbohydrates degradation, chemical reaction kinetics were studied in this research. The following results are obtained:According to the ¹H -NMR and ¹³C -NMR results, it can be found that the lignin in wheat straw is GSH type. Based on the element analysis results, the empirical formula of lignin is C₉H_8.35O_3.61(OCH₃)_1.342. The element of oxygen content in wheat straw is rather high, and the degree of unsaturation is low. The lignin molecular of wheat straw consist about 30 of C₉ units.The reactions of degradation and condensation of lignin occurred simultaneously in the process of the F, FP and FPS. The distribution of the lignin molecular weight becomes wider than MWL after F, FP,FPS treatments, the number of C₉ unit in lignin macromolecue can be as high as 50. There exists a little amount of carboxyl and aldehyde groups in the original lignin. The phenolic hydroxyl is about 27% of the total hydroxyl group contents and can increase readily to 1.6-2.8 times of the initial phenolic hydroxyl concentration after F, FP and FPS treatment. After the F treatment the element of hydrogen and oxygen and aliphatic hydroxyl groups decreased, the degree of unsaturation increased; after FP and FPS treatment, the oxidation reaction occurred in the side chains, the conjugated carbonyl decreased. Thefragmentations of the side chains include the C_β-C_γ, C_α-C_β and Ar-C_α.The chemical reactions in the cooking process are: esterification reactions of the hydroxyl group with formic acid, which may cause the increase of the unconjugated carboxyl groups; the Baeyer-Villiger reactions, the oxidation of the side chains; reverse Prins reactions, resulting in the C_γ elimination and the increase of the degree of unsaturation in side chains; the electrophilic substitution of aromatic hydogen with the HO⁺, resulting in the increase of the phenolic hydroxyl groups; the Prilezhaev epoxidation reaction, leading to the increment of the carbonyl groups and esterification in the side chains; the condensation of lignin, in the result of the increment of molecular weight of lignin.The carbohydrates in LCC are mainly xylan, the bond type in lignin include the β-O-4, β-β, 5-5, β-1 and β-5. The α-C=O and unsaterated α-β structure can also be found in the lignin of LCC. The linkage between lignin and carbohydrates include ester bond, ether bond as well as acetal structure.The ester bond and the β-O-4 structures in LCC are stable in formic acid treatment. The LCC can be esterified.The new bonding can be found after the co-treatment of MWL and xylose with formic acid.There exist three stages in the delignification process according to the rate. In the first delignification stage, about 70% of the total lignin was delignified in about 20 min. In the second stage about 20% of the total lignin was removed in 30-40 min; in the third stage the residual lignin, less than 10% of the total lignin was removed in the following cooking time. In the four process tested, the sequence of the delignification rate is FPS>FP>F and FS.The carbohydrates degradation rate can also be divided into three stages. In the first stage(fast degradation stage), about 80% of the total carbohydrates degraded are removed in about 23-35 min; the second stage(mild degradation stage), there are only about 10% of the total carbohydrates removed are eliminated in 32-42 min; in the third stage the carbohydrates eliminated are less than 10% of the totally removed from the straw. In the four process tested, the sequence of the carbohydrates degradation rate is FS>F>FP and FPS.the results showed that the coefficients of delignification selectivity of F,FP,FPS are all above 1.5. The delignification rate is higher than the degradation of carbohydrates. The delignification selectivity of the whole process is FPS>FP>F>FS. The addition of hydrogen peroxide can increase both the delignification selectivity and the amount of delignification. The cooking process can be accelerated by the addition of small amount of sulfuric acid. The FP process can be carried out at high temperature with a very good delignification selectivity. FPS should be conducted under lower temperature to avoid the mass degradation of carbohydrates.The kinetics analysis of the cooking process showed that the delignification reactions in the three observed process of F, FP, FPS are all in the first order, and the active energy are E_aF>E_aFP>E_aFPS. The addition of the peroxide and sulfuric acid facilitates the delignification process. The carbohydrates degradation reactions in the three process observed are all in the first order too, but the active energy are almost the same in different process with the variations of 5%. The rising of the temperature is favorite forthe delignification in the F process while the FPS should not conducted under a high temperature due to the carbohydrates degradation. The chemical thermodynamics analysis showed that the cooking process is a endothermic process. Due to the entropy production in the cooking process the reaction can occurr spontaneously. In the first cooking stage, the out-side diffusion is the controlling process, in the second cooking stage the in-side diffusion is the controlling process, and in the final cooking stage the chemical reactions are the controlling process.