Improved Saccharification and Fermentation of Rice Straw Using Various Solvents for Pretreatment Step

Increasing demand for energy increases the importance of renewable resources to meet this great amount of demands. Among different biofuels, bioethanol has been taking the most attention. Various resources could be used to biologically produce ethanol, but lignocelluloses are of special interest because of having no conflict with food resources. Lignocellulosic materials considered as one of the best renewable resources for biofuel production, are cheap and available in all over the world. These materials are found in cell wall of plants. Lignocelluloses are composed of three major polymers including cellulose, hemicelluloses, and lignin. Packed and complex structure of these materials reduces their digestibility and therefore, intensifies the significance of a pretreatment step. Rice straw, among other lignocelluloses, is the most available feedstock for bioethanol production. Solvent based pretreatment by N-methyl morpholine N-oxide (NMMO), 1-buthyl-3-methylimidazolium acetate ([BMIM][OAc]), and 1-ethyl-3-methylimidazolium acetate [EMIM][OAc] was done prior to enzymatic saccharification of rice straw. N-methyl morpholine N-oxide is an industrial solvent now used in fiber making process in Lyocell technology. Ionic liquids are low melting point salts which are viscose liquids in ambient temperature. Treatment was performed in an oil bath at 120°C for 1, 3, and 5 h with 5% solid loading. Treated and untreated straws were subjected to enzymatic hydrolysis at 45°C for 72 h by 20 FPU/g of substrate cellulase and 30 IU/g of substrate β-glucosidase. Hydrolysis was performed in a shaker incubator with the speed of 100 rpm. Glucose released was meatured using glucose HK assay kit. Simultaneous saccharification and fermentation was carried out at 38°C for 48 h using Saccharomyces cerevisiae as fermenting organism in an incubator with the speed of 80 rpm. Produced ethanol was determined by HPLC. Hydrolysis results indicated relatively complete conversion of glucan to glucose for all treated straws. Glucose yields were more than 96% for all treated samples, while this value was only 27.7% for untreated straw. Ethanol yield compared to maximum theoretical yield was improved from 35.4% for untreated straw to 93.3%, 79.7%, and 79.7% after pretreatment with NMMO, [EMIM][OAc], and [BMIM][OAc], respectively. FTIR analysis indicates reduction in crystalline matrix of straw. Crystallinity Index of straw reduced from 0.46 for untreated straw to less than 0.37 for all pretreated straws. In addition, confirmed by SEM images of various samples, structural destruction is one of the main reasons for observed improvements. NMMO was the best solvent among applied solvents in term of ethanol production per initial straw. 20.8 g bioethanol could be produced per 100 g of initial straw using a pretreatment step by NMMO at 120°C for 5 h, while this value was only 8.3 g for untreated straw. It means potential annual production of 250 billion liter ethanol of 955 million ton produced rice straw in all over the world.

Key Words:

Rice straw, NMMO, [BMIM][OAc], [EMIM][OAc], lignocelluloses, ionic liquid