Biorefining of Spruce Wood for Nanocellulose and Reducing Sugars Production: Exploring Greener Alternatives

Abstract

Lignocellulosic biomass, available in the form of agriculture and forest residues is an abundant renewable source of energy and products. Cellulose nanocrystals and fermentable reducing sugars are amongst the major bioproducts derived from woody biomass. Herein, the thesis explores developing non-conventional approaches of extraction cellulose nanocrystals (CNCs) and fermentable reducing sugars from Spruce wood. First, acetosolv pulping-alkaline hydrogen peroxide bleaching of spruce wood were employed to produce bleached wood pulp (BWP) for the derivation of CNCs. Treatment of BWP using sulfuric acid at concentration of 65.0 wt.% and 62.0 wt.% led to CNCs yield of 8.0±3.2 wt.% of dry wood and 25.1±0.7 wt.%, respectively. The ultrasonic treatment of acid hydrolyzed pulp at 80.0 % amplitude for 5.0 min resulted in obtaining good quality CNCs with high aspect ratios (length/width) up to 48.1 and crystallinity index of 80.8±1.7%. For fermentable reducing sugars production, different pretreatments of spruce wood were explored to improve the enzymatic digestibility of spruce wood. Acetosolv pulping-alkaline peroxide bleaching was the most effective pretreatment method, leading to total reducing sugar (TRS) yield of ~95.0 wt.% of total hydrolysable sugars (THS) in 144.0 hours of enzymatic hydrolysis process. Additionally, the impact of enzyme pretreatment on enzymatic hydrolysis yield was further investigated. It was found that prolonged scCO2 pretreatment (16.0 MPa, 46.0 °C, 24.0 h) of enzyme cocktail decreased sugars yield in the subsequent enzymatic hydrolysis from 44.6±1.3 wt.% (untreated enzyme) to 6.3±1.1 wt.% (pretreated enzyme) at 24.0 h of enzymatic hydrolysis of BWP. The optimized enzyme cocktail was immobilized into silica oxide aerogel by sol-gel synthesis followed by scCO2 drying. The cocktail aerogel at 144.0 mg enzyme/g aerogel retained a residual yield of >50.0 % after fourth cycle of reuse compared to first cycle under atmospheric pressure. The activity was decreased due to leaching of enzyme and reduction in surface area and porous volume of aerogel enzyme over the cycles. TRS yield by the cocktail aerogel was <10.0 wt.% in second cycle of reuse under scCO2 due to enzyme leaching from aerogel to hydrolysate showing unfavorable conditions for cocktail aerogel in high pressure system.