The development of bioplastics, currently derived from corn, is an important field of study because of the need to reduce the use of petrochemicals. However, corn usage generates controversy because it's in direct competition with human feed. The solution involves converting lignocellulose, derived from agricultural or forestall residues, into D-lactic acid which could be further used for poly-lactic acid (PLA) production. The conventional method for developing poly-lactic acid involves pretreating the lignocellulose with dilute acid to separate the compounds and then performing enzymatic hydrolysis to further separate the cellulose from the unwanted products. Lignocellulose contains hemicellulose, cellulose, and lignin. Lactic acid bacteria converts glucose, a constituent of cellulose, into D-lactic acid, the main precursor of PLA. In a recent study, cellulose, as a major compound of biomass, was hydrolyzed and the obtained sugars were fermented by Lactobacillus delbrueckii (Gavilà et al., 2015). Finally, D-lactic acid might be separated and purified for conversion into PLA. The process produces poly-lactic acid for industrial purposes to replace petrochemicals as a potential approach to limit the effects of human impact on the environment.
The goal of this experiment was to explore the potential methods to make the production of poly-lactic acid a viable option. To eliminate the need for expensive enzymes and slow production times, the use of an autoclave device was investigated during the hydrolysis of cellulose to compare with the already established microwave hydrolysis (established at 120ºC). This experiment focused on optimizing the hydrothermal hydrolysis of cellulose to determine if it would yield either increased conversion of cellulose or higher selectivity towards glucose. To do the optimization, cellulose was inserted with 3% weight of sulfuric acid in an autoclave reactor at different temperatures (120, 150, 180 and 200 ºC) for three hours. The investigation also tested both the microwave and autoclave with the use of pre-sonication in an effort to further breakdown the cellulose structure. The samples were characterized using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Environmental Surface Electron Microscope (ESEM).
The results revealed that the microwave reaction had a higher conversion, at 120 ºC, of cellulose compared to the autoclave reaction. The results also showed that sonication had no effect on the cellulose hydrolysis regardless of performing the hydrolysis in the autoclave or microwave. However, the results did show that at 200 ºC, the main product of the hydrothermal hydrolysis in the autoclave was levulinic acid, which is an important building-block obtainable from biomass (Holladay et al., 2004). At 200 ºC, 77% of the cellulose was hydrolyzed in the autoclave. Similar conversions were achieved using the microwave and the autoclave. However, different products can be obtained from the different hydrolysis methods. Using the microwave, glucose was mainly obtained while in the autoclave, levulinic acid was the main product. This work proved that microwave assisted hydrolysis is more energy efficient and also showed the different value-added compounds that can be obtained from biomass.
References
Gavilà, L., Constantı, M., Medina, F., "D -Lactic acid production from cellulose : dilute acid treatment of cellulose assisted by microwave followed by microbial fermentation." Cellulose, vol. 22, no. 5, October 2015. [Online]. Available: http://link.springer.com. [Accessed: Sept. 10,2015]
Holladay, J.E., White, J.F., Bozell, J.J., Johnson, D., Werpy, T., Petersen, G., Aden, A. (2004). Top value-added chemicals from biomass. Pacific Northwest National Laboratory, National Renewable Energy Laboratory, Office of Biomass Program. Richland, WA. Available: http://www.nrel.gov/docs/fy04osti/35523.pdf
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