Biorefinery Strategies for Co–generation of Cellulose Nanocrystals and Value–added Products from Wood Pulp

• Author / Creator

  Beyene, Dawit

• Cellulose nanocrystals (CNCs) are sustainable and renewable nanoparticles derived from cellulose with desirable properties for various applications. CNCs can be isolated from purified cellulose such as wood pulp by concentrated sulfuric acid hydrolysis. The acid degrades non–crystalline cellulose and hemicelluloses, while CNC precursors representing the bulk highly crystalline celluloses are converted to CNC by fragmentation. This process has limitations due to low CNC yield (10 wt % initial feedstock) and loss of valuable sugars in the acid stream that decreases acid recovery efficiency. In this thesis, biorefinery strategies were developed to integrate cellulase and hydrothermal treatments with acid hydrolysis to (a) transform non–crystalline constituents of wood pulp to value–added co–products and (b) generate a crystalline feedstock to improve CNC yield from acid hydrolysis (wt % feedstock for acid hydrolysis, CNC yield1) and the CNC yield from the initial feedstock, accounting for the mass loss due to the treatments (wt % original feedstock, CNC yield2).
  In the first strategy, cellulase treatment (using a Novozymes enzyme preparation supplemented with xylanase) was introduced prior to acid hydrolysis of Whatman™ No. 1 filter paper (model feedstock) and wood pulp. The objectives were to: (a) identify a treatment period for efficient saccharification for ethanol production without compromising CNC yield2, and (b) improve CNC yield1. The hypothesis was that cellulase treatment period is a determining factor for preferential degradation of non–crystalline cellulose and xylan with linear hydrolysis rate and minimal loss to CNC precursors. Preferential hydrolysis will generate a feedstock with concentrated CNC precursors that improves acid hydrolysis efficiency. During 10 h cellulase treatment, glucose yield plateaued at 6 h (36.5 ± 0.3 wt % original feedstock) for filter paper with a significant increase in the crystallinity of the residual solid. Therefore, 2–6 h cellulase treatment of filter paper lead to an efficient saccharification. Steady hydrolysis rate was maintained even at 10 h for an efficient saccharification of wood pulp to glucose and xylose (44.2 ± 1.4 and 12.1 ± 0.3 wt % original feedstock, respectively), with surprisingly no changes in crystallinity. CNC yield1 significantly improved from cellulase–treated filter paper (up to 1.2 fold) and wood pulp (almost doubled). CNC precursors were likely accumulated due to hydrolysis of non–crystalline chains. Hence, cellulase treatment can reduce acid and water consumption upstream and operation costs downstream from improved throughput. Less sugars in the acid stream will also ease acid recovery processes. CNC yield2 significantly decreased for filter paper even at 2 h. This implies simultaneous degradation of CNC precursors, which was more abundant in filter paper. CNC yields2 were constant for wood pulp from 2–8 h. Hence a biorefinery strategy with efficient saccharification and CNC isolation was achieved from 8 h cellulase treatment mediated acid hydrolysis. CNCs of comparable quality were isolated from cellulase–treated feedstock based on particle size, zeta potential, thermal stability and crystallinity analysis.
  The second strategy was to integrate hydrothermal treatment and acid hydrolysis to (a) generate furfural as a value–added co–product, and (b) form new CNC precursors in wood pulp and improve CNC yields from acid hydrolysis. The hypotheses were that hydrothermal treatment can substantially degrade xylan to furfural and also re–orient para–crystalline chains to form new CNC precursors. Xylan degradation ranged from 19–90 (wt % original feedstock) at 175–225 °C treatment while significant cellulose hydrolysis was only apparent at 225 °C (7.0 ± 1.5 wt % original feedstock). Substantial furfural yields were generated at 200 °C and 225 °C (19 and 21% xylan conversion, respectively). Hydrothermal treatment significantly improved the crystallinity index of wood pulp and consequently CNC yield1 and yield2 improved by up to 4 and 2 fold, respectively, relative to the untreated pulp. These improvements can be translated to reduced cost of CNC production and increased capacity. The particle size, zeta potential and crystallinity of CNC generated from hydrothermally treated pulp were not affected due to the treatment. However, a dark brown colored CNC was generated from hydrothermally treated pulp at 225 °C possibly due to caramelization.
  These studies demonstrated that cellulase and hydrothermal treatment mediated acid hydrolysis biorefinery strategies can efficiently generate fermentable sugar and furfural co–products, respectively and improve CNC yields.

• Subjects / Keywords

  • cellulase
  • cellulose nanocrystals
  • crystallization
  • acid hydrolysis
  • furfural
  • hydrothermal treatment

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-x2mw-my08

• License

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