Enhanced rates of enzymatic saccharification and catalytic synthesis of biofuel substrates in gelatinized cellulose generated by trifluoroacetic acid

Tânia M Shiga,Jiliang Liu,Bryon S Donohoe,Ximing Zhang,Haibing Yang,Lee Makowski,Weihua Xiao,Anna T Olek,Nathan S Mosier,Tao Hou,Maureen C Mccann,Nicholas C Carpita

doi:10.1186/s13068-017-0999-2

Abstract

BackgroundThe crystallinity of cellulose is a principal factor limiting the efficient hydrolysis of biomass to fermentable sugars or direct catalytic conversion to biofuel components. We evaluated the impact of TFA-induced gelatinization of crystalline cellulose on enhancement of enzymatic digestion and catalytic conversion to biofuel substrates.ResultsLow-temperature swelling of cotton linter cellulose in TFA at subzero temperatures followed by gentle heating to 55 °C dissolves the microfibril structure and forms composites of crystalline and amorphous gels upon addition of ethanol. The extent of gelatinization of crystalline cellulose was determined by reduction of birefringence in darkfield microscopy, loss of X-ray diffractability, and loss of resistance to acid hydrolysis. Upon freeze-drying, an additional degree of crystallinity returned as mostly cellulose II. Both enzymatic digestion with a commercial cellulase cocktail and maleic acid/AlCl3-catalyzed conversion to 5-hydroxymethylfurfural and levulinic acid were markedly enhanced with the low-temperature swollen cellulose. Only small improvements in rates and extent of hydrolysis and catalytic conversion were achieved upon heating to fully dissolve cellulose.ConclusionsLow-temperature swelling of cellulose in TFA substantially reduces recalcitrance of crystalline cellulose to both enzymatic digestion and catalytic conversion. In a closed system to prevent loss of fluorohydrocarbons, the relative ease of recovery and regeneration of TFA by distillation makes it a potentially useful agent in large-scale deconstruction of biomass, not only for enzymatic depolymerization but also for enhancing rates of catalytic conversion to biofuel components and useful bio-products.

Highlights

The crystallinity of cellulose is a principal factor limiting the efficient hydrolysis of biomass to fermentable sugars or direct catalytic conversion to biofuel components
We report here that partial dissolution of cellulose by subzero temperatures is sufficient to substantially enhance both enzymatic digestion and catalysis to fuel substrates
trifluoroacetic acid (TFA) solubilization and generation of gelatinized cellulose Cotton linter cellulose (50 mg mL−1) maintained at − 20 °C for 15 h formed a thick, semi-solid slurry that melted into a clear solution between 2 and 5 h of subsequent incubation at 55 °C (Fig. 1a)

Summary

Introduction

The crystallinity of cellulose is a principal factor limiting the efficient hydrolysis of biomass to fermentable sugars or direct catalytic conversion to biofuel components. Treatments with dilute acids improve enzymatic yields of fermentable sugars, but hydrolysis and loss of non-cellulosic sugars, Shiga et al Biotechnol Biofuels (2017) 10:310 decomposition of sugars at high temperatures, problems with acid recovery, and other environmental considerations prompted a search for alternatives [12,13,14,15]. Steam expansion at neutral temperatures reduced decomposition, and the separation of lignin and cellulose improved subsequent enzymatic digestion to fermentable sugars [16, 17]. The ammonia treatment extracts xylans and redistributes lignin to the surface of the cell walls, enhancing access of enzymes to cellulose through creation of large, porous networks [21]. Biomass treated with steam at high pressure and temperature alone is sufficient to enhance the yields of sugars from enzymatic digestion [22, 23]

Methods

Results

Discussion

Conclusion