Dawit Beyene | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Dawit Beyene. This seminar is open to the general public to attend.

Thesis Topic: Biorefinery strategies for co–generation of cellulose nanocrystals and value–added products from wood pulp

PhD with Dr. David Bressler

Seminar Abstract:

Cellulose nanocrystals (CNCs) are sustainable and renewable nanoparticles derived from cellulose with desirable reinforcement, rheological, absorbant and barrier properties for various applications. CNCs can be isolated from purified cellulose such as wood pulp by concentrated (64% wt) sulfuric acid hydrolysis. The acid degrades non–crystalline cellulose and hemicelluloses, while the highly crystalline CNC precursors are fragmented to CNCs. This process has limitations due to low CNC yield (10 %) and the loss of valuable sugars in the acid stream that decreases the acid recovery efficiency. In this thesis, efforts were made to develop biorefinery strategies to integrate cellulase and hydrothermal treatments with acid hydrolysis to transform the non–crystalline constituents in wood pulp to value–added co–products and improve CNC recovery and yield.

In the first strategy, cellulase treatment 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 yields and (b) improve CNC recovery from acid hydrolysis reaction. 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 % feedstock) for filter paper with significant increase in the crystallinity index 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 % feedstock, respectively), with surprisingly no changes in crystallinity. CNC recovery per acid hydrolysis reaction 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 yield, estimated by accounting the mass loss from cellulase treatment, significantly decreased for filter paper even at 2 h. This implies simultaneous degradation of CNC precursors, which was more abundant in filter paper. CNC yields were constant for wood pulp from 2–8 h. Hence a biorefinery strategy with efficient saccharification and CNC recovery was achieved from 8 h cellulase treatment mediated CNC isolation. 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 recovery and yield 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 % pulp at 175–225 °C treatment while significant cellulose hydrolysis was only apparent at 225 °C (7.0 ± 1.5 wt % pulp). 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 the CNC recovery and yield improved by up to 4 and 2 folds, respectively, relative to the untreated pulp. These improvements can be translated to reduced CNC production cost 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 color was imparted on CNC from hydrothermally treated pulp at 225 °C due to carbonization, which was evident from elemental analysis.

These studies demonstrated cellulase and hydrothermal treatment mediated acid hydrolysis biorefinery strategies that can efficiently generate fermentable sugar and furfural co–products and improve CNC recovery and yield, respectively.