Pyrolysis—gas chromatography/mass spectrometry of wheat straw fractions obtained by alkaline treatments used in pulping processes

Abstract

Different wheat straw fractions representative of industrial and laboratory-scale alkaline pulping processes have been studied by pyrolysis—gas chromatography/mass spectrometry (Py-GC/MS). For comparative purposes, a milled straw lignin from wheat straw was also studied. The pyrolytic patterns were suitable for estimating the extent to which lignin-derived material is present in the hemicellulose preparations, or hemicellulose-derived products contributed to the alkalilignin fractions. The results reflect the different efficiency of the treatments to remove lignin and carbohydrate during alkaline pulping of Gramineae straws. The virtual lack of carbohydrate-derived pyrolysis products was observed only in the alkalilignin preparation obtained with the highest NaOH concentration studied (1 M) at 30°C for 12 h. The pyrolysis data showed that the guaiacyl fragments were dominant in the lignin associated to hemicellulose fractions, suggesting structural relationships between hemicellulose and lignin through guaiacyl type units. This was complementary to the large amounts of syringyl units that remained in the alkalilignin preparations. Although Py-GC/MS provides markers for the major classes of cell wall components, such as lignin and structural polysaccharides, it does not yield fragments which could be ascribed with certainty to cinnamic acids, which are abundant and play an important role in cereal straws as lignin—carbohydrate linkages.