Changes in Cellulose Crystalline Structure in Lignocellulosic Biomass During Delignification Studied By Sum Frequency Generation (SFG) Spectroscopy and X-Ray Diffraction (XRD)

Changes in cellulose crystalline structure in lignocellulosic biomass during delignification studied by sum frequency generation (SFG) spectroscopy and x-ray diffraction (XRD)

Kabindra Kafle¹, Christopher Lee¹, Heenae Shin^2,3, Justin O. Zoppe², David K. Johnson⁴, Seong H. Kim^1,*, Sunkyu Park^2,3,*

¹ Department of Chemical Engineering and Material Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

² Department of Forest Biomaterials, North Carolina State University, Raleigh, NC 27695

³ Department of Forest Sciences, Seoul National University, Seoul, Korea

⁴ National Renewable Energy Laboratory, Golden, CO 80401

* Corresponding authors: shkim@engr.psu.edu, sunkyu_park@ncsu.edu

Abstract

Bioethanol production from renewable sources such as lignocellulosic has been an attractive way to reduce dependence on traditional petroleum fuels. However, ethanol yields are far from efficient because of the presence of hemicelluloses and lignins in the lignocellulosic material. Thus, removal of lignin from intact lignocellulose is necessary in order for enzymes to access cellulose and hydrolyze them. Thermo-chemical pretreatment methods are commonly used to delignify the lignocellulose, during which the native cellulose structure and assembly is likely altered. In this work, the structural changes in cellulose crystalline structure during oxygen and sodium chlorite delignification were studied using sum frequency generation vibration (SFG) spectroscopy and X-ray diffraction (XRD). SFG can selectively detect the ordering of crystalline cellulose without any interference from other cell wall polymers. The removal of lignin was higher during sodium chlorite delignification compared to oxygen. The ordering of crystalline cellulose microfibrils in hardwood (lignocellulose source), over optical length scale, increased drastically at an elevated temperature in presence of water. This also corresponded to an increase in crystal size from XRD analyses. The crystal size increase was pronounced with the elevation of temperature without using reagents or removing discernible amount of lignin. Quantification of crystalline cellulose based on calibration curves from α -cellulose showed a non-linear relation between glucan amount present and SFG intensity. Structural changes in cellulose in terms of crystal size and better aggregation/ordering are suggested to play an important role during the delignification of lignocellulosic material. The ability of SFG to selectively detect crystalline cellulose structure and organization in biomass from native and delignified wood samples could help engineer better methods in biomass utilization.