Structural characteristics of the lignins from sugarcane bagasse and straw

Abstract

Sugarcane (Saccharum spp.) is a perennial monocotyledonous plant from the Gramineae family that is the main feedstock for the production of sugar and also ethanol. Two major residues are produced by the sugarcane industry, the fibrous fraction following juice extraction (named bagasse), and the harvest residue (straw). Sugarcane bagasse and straw are lignocellulosic resides basically composed of cellulose, hemicelluloses, and lignin, and are attractive feedstocks for the production of second-generation ethanol and other bio-based products, in the context of the so-called lignocellulosic biorefinery. Converting bagasse and straw to ethanol requires an enzymatic saccharification of the polysaccharides to reducing sugars and their subsequent fermentation to ethanol. However, the presence of recalcitrant lignin reduces the accessibility of the enzymes to the cellulose, and decreases the efficiency of the hydrolysis. Costly and harsh pretreatment processes are therefore needed to circumvent cell-wall recalcitrance by removing, or cleaving and redistributing, the lignin polymer. The efficiency of the pretreatment process is highly dependent on both the lignin content and structure, and therefore the knowledge of the structure of the lignin is important to develop appropriate pretreatment methods for delignification, as well as for potentially higher-value utilization of the lignin. In this paper, we report a detailed characterization of the lignins in sugarcane bagasse and straw by using different analytical techniques (Py-GC/MS, 2D-NMR, DFRC). The analyses indicate that the lignin from sugarcane bagasse is S-rich (H:G:S molar ratio of 2:38:60) whereas the lignin from sugarcane straw is G-rich (4:68:28). The compositional differences are also reflected in differences in the relative abundances of the various interunit linkages. Thus, the lignin from bagasse is mostly made up by ß¿O¿4¿ alkyl-aryl ether units (83% of all linkages), followed by minor amounts of phenylcoumarans (6%) and other condensed units. The lignin from straw, on the other hand, has lower amounts of alkyl-aryl ether units (75% of all linkages) and has higher amounts of condensed structures such as phenylcoumarans (15%) and dibenzodioxocins (3%). The differences observed in the lignins of the two sugarcane residues suggest that the bagasse material will pretreat more easily (because of its higher S-content and lower condensation degree) than the straw (with a higher content of condensed structures). In addition, the two residues contain significant levels of p-coumarate, ferulate, and tricin that might also be valuable to extract as commodity chemicals.