Traits of milled wood lignin isolated from different crop straw based on FT-IR

Abstract: As vast agricultural byproduct and an important source of lignocellulosic biomass in China, straws of maize, cotton and wheat are now being developed as renewable energy resources to address a serious energy shortage and environmental issues related to other energy sources. It is necessary to improve the comprehensive utilization efficiency of the three types of straw. Although maize (monocotyledon), cotton (dicotyledon) and wheat (monocotyledon) belong to therophyte herbage from the perspective of phytology, but their straw forms are totally different. For the sake of the difference among them, the utilization cannot be generalized without regard to the effect of chemical composition, in particular the characteristics of lignin. It is composed of phenyl propane monomers connected by the ether and carbon-carbon bonds, containing a variety of active groups. There are three types of phenylpropane, p-hydroxy phenyl, guaiacyl and syringyl, which correspond to hydroxy phenyl lignin (H-lignin), syringyl lignin (S-lignin) and guaiacyl lignin (G-lignin), respectively. Lignin within plants has different shares of the three constitutional units. The heterogeneity of linkage types among the phenyl propane monomers has different effects on the pretreatment for the utilization and removal of lignin. Previous researches have shown that the relative ratio of G-lignin/S-lignin (G/S value) is relevant to the ease or complexity of delignification. Therefore, the higher the ratio, the harder the delignification is. In this paper, three samples of each kind of straw were collected, coarse grinded, degreased, dewaxed, and fine grinded, in sequence. Then the milled wood lignin (MWL) was isolated from the straws of cotton, corn and wheat. After the all procedures above, MWL was studied with FT-IR, by potassium bromide pellet technique. Normalization was conducted with 9 spectra obtained, at the wavelength of 1 506 cm-1 (to make A1 506=1) as spectral pretreatment. Then the spectra were compared in 3 aspects: the relative G/S ratio represented by A1 265/A1 329, the identification of lignin types based on plant taxonomy, and the differences and similarities comparing with the MWL from eucalyptus. The results indicate that there is more guaiacyl and less syringyl in the MWL isolated from the samples, the relative ratio of G/S among the three kinds of straws was not significant difference, and no evident regularity is found. Secondly, the identification results based on plant taxonomy show that the MWL from cotton straw is the type of GS1, while that from corn and wheat straw belong to the type of HGS; the content of basic units of lignin in mole percent in these straws have following laws: for H-lignin, corn straw＞wheat straw＞＞cotton straw; for G-lignin, cotton straw＞corn straw＞wheat straw; for S-lignin, corn straw＞wheat straw＞cotton straw. Thirdly, the MWL isolated from straw of corn and wheat share the generality of gramineous plants, while the MWL from cotton straw is closer to ligneous plants, which have distinguish peak at 1 265 and 1 226 cm-1.