【摘要】 <正> 草类是中国造纸工业的重要原料,而麦草在草类原料中又占有很大比重。对于麦草的碱法制浆机理研究表明,在生产条件下蒸煮温度升到100℃时,就有60％的木素被溶出…。因此麦草在制浆过程中的主要脱木素阶段在100—110℃以前,这与木材有很大不同,这种差异可能是由于麦草的生物特性、木素结构和组成以及局部化学与木材不同所造成。早在1944年,Creighton和Hibbert对竹和黑麦草进行了硝基苯氧化的研究,产物中除了香草醛和紫丁香醛外,还得到大量的对羟基苯甲醛。草类木素中对羟基苯丙烷单元的存在也由分离得到相应的Hibbert酮得到证实。在这以前,Smith和Higuchi等发更多还原

【Abstract】 In order to get a better understanding of the characteristics of wheat straw lignin and its delignification mechanism during pulping, milled wood lignin isolated from wheat straw was investigated through physical and chemical methods and compared with milled wood lignin from birch. Results obtained indicated that wheat straw lignins differed greatly from wood lignins in their properties. According to elemental analyses, wheat straw MWL contained 0.17% nitrogen and its C₉ formula was C₉H_7·39O_3·0 （OCH₃）_1·09, with methoxyl groups much similar to those in softwood lignin. The total hydroxyls of wheat straw MWL were determined to be 1.33 OH/C₉ by means of acetylation. Differences in phenolic hydroxyl contents were particularly conspicuous. The phenolic hydroxyl contents of wheat straw MWL and birch MWL estimated by UV ionization differential spectra （Asi） were 2.25% and 1.34%, the former containing 70% more phenolic hydroxyls than the latter. The higher content of phenolic hydroxyls indicated that wheat straw lignin could possibly be more easily removed during pulping. Using two different systems: Sephadex LH—60 with 0.1 MLiCl in DMF and μ-styragel with THF, molecular weight distribution of wheat straw MWL and birch MWL were determined by gel permeation chromatography. Both of them led to the same conclusion that the molecular weight of wheat straw lignin was much lower than that of birch lignin. The molecular weight of wheat straw MWL: Mn 2953 Mw8854, accounted for about one-half as much as that of birch MWL. This might be considered as one of the major causes for the easy dissolution of wheat straw lignin during pulping. Wheat straw MWL was treated with NaOH solution and alkaline hydrolytic products were determined by HpLC. p-Coumaric acid, 2.08% （based on MWL Wt,） was found to be the major product. A small amount of ferulic acid, about onefifteenth of p-coumaric acid was identified, HPLC results also showed the presence of traces of p-hydroxybenzonic, vanillic and syringic acids Alkaline nitrobenzene oxidation gave the mole ratios V:S:H of 1:0.77:0.31 for wheat straw MWL This indicated that wheat straw lignin was rich in guaiacyl groups. On the other hand, the amount of p-hydroxybenzaldehyde （H） in saponified wheat straw MWL was only 58% of that in unsaponified samples. This suggested that this portion of p-hydroxyphenyl could have been linked via unsaponifiable bonds to the macro-structure of lignin, possibly through phenolic ether linkages. Differences in UV spectra were found among lignin samples. Compared with birch MWL, wheat straw MWL gave a shoulder around 315nm, and was almost unaltered even after NaBH₄ reduction and completely dissappeared after treatment with NaOH solution. It was suggested that this absorption be due to the presence of p-coumaric acid and ferulic acid esterified to lignin. Besides, wheat straw MWL gave an absorption at 280 nm and its absorption coefficient was 20.4 1. g^-1. cm^-1 being much similar to softwood lignins. These results seemed to be consistent with the fact that a relatively large amout of guaiacyl groups existed in wheat straw lignin. The bands at 1715 and 1170 cm^-1 of IR spectra were responsible for the absorption of ester linkages. These absorption peaks were not found in birch MWL, and almost completely dissappeared in wheat straw MWL treated with NaOH solution, This result further confirmed the occurence of ester linkages in wheat straw lignin. On the other hand, compared with bagasse lignin, relatively weaker absorption intensities of 1715 and 1170 cm^-1 were obtained for wheat straw lignin. This led to the came conclusion drawn from Chemical analyses, that the amout of ester linkages of wheat straw lignin was less than those in bagasse lignin. The functional group contents of wheat straw MWL were calculated by¹H-NMR spectra to be 1.10 OCH₃/C₈ and 1.35 OH/C₉. much similar to results from chemical analyses. In the range between 6.28—8.00ppm. three signals corresponding to the protons of p-hydroxyphenyl, guaiacyl and syringyl units were observed. On the basis of peak area ratio, the amount of the three units of wheat straw MWL was estimated to be G>S>H, confirming with results obtained through nitrobenzen oxidation. ¹³C-NMR spectra showed that wheat straw lignins were GSH-lignins. By comparing relative intensity of signals, high phenolic hydroxyl content of wheat straw lignin was suggested. This result further confirmed results from chemical analyses. Therefore it might be concluded that the removed of wheat straw lignin being relatively easy during pulping may be attributed to its higher phenolic hydroxyl contents, lower molecular weight and the occurrence of ester linkages.更多还原