Abstract
The effect of different acid sulfite pretreatment conditions on released components in the hydrolysates and the pretreated solid residues’ response to enzymatic hydrolysis for Eucalyptus globulus chips was investigated. Sodium bisulfite (0–15%), and sulfuric acid (0–5%) were used to pretreat chips at 170 °C and 190 °C, for as long as 30 min. The hydrolysates were analyzed through high-performance liquid chromatography (HPLC) and spectrophotometry. Overall porosity and pores larger than 2.65 nm (size of a typical cellulase) on the solid residues were estimated using glucose and two dextrans with different hydrodynamic radii as probes. The external specific surface area was analyzed by dynamic light scattering. The solid residues underwent enzymatic hydrolysis with an enzymatic cocktail. Very high (84–95%) carbohydrate conversion was achieved for either an extensively delignified biomass or a biomass with very high content of sulfonated residual lignin (23.4%), since internal porosity enables enzymes accessibility. At least 5% sodium bisulfite and 1% sulfuric acid was required to attain a carbohydrate release above 90% in the enzymatic hydrolysis. Results suggest that the presence of sulfonated lignin does not impair the enzymatic hydrolysis rate and extent. The increase of pretreatment temperature had a positive effect mainly on the initial rate of carbohydrates release in the enzymatic hydrolysis. The increase of the wood material dimensions from pins to conventional chips significantly decreased the hemicellulose removal in acid sulfite pretreatment but had a small effect on the enzymatic yield.
Highlights
The development of second-generation biofuels has been lately receiving a lot of attention due to the limitations of using crops for producing first-generation bioethanol [1,2]
The percentages of glucose, combined XMG, products of carbohydrate degradation, formic and acetic acids and the combined soluble lignin and all other degradation products absorbing at 205 nm (SLY) that were released into the hydrolysates, concerning the initial biomass weight, were calculated following the high-performance liquid chromatography (HPLC) determination of the corresponding compounds’ concentrations in the pretreatment hydrolysates
The increase of temperature from 170 ◦ C to 190 ◦ C, even during a short period of time (30 min), had an important impact on carbohydrates extraction; for mild chemical reaction conditions (e.g., 2s_1ac) the XMG removal increased from 14.5% to 18.5% on wood, which practically is the XMG content in the wood
Summary
The development of second-generation biofuels has been lately receiving a lot of attention due to the limitations of using crops for producing first-generation bioethanol [1,2]. Cellulose and hemicelluloses from lignocellulosic biomass are hydrolysable polysaccharides that give way to carbohydrates whose fermentation results in ethanol, the cell walls of lignocellulosic materials form a barrier that has a natural resistance to microbial and enzymatic deconstruction [6,7,8,9] Several factors such as lignin content, lignin distribution and its structure, cellulose crystallinity and degree of polymerization, pore size and internal porosity, end-product inhibition, need for synergism, and non-productive enzyme adsorption on lignin have been suggested to account for the recalcitrance of lignocellulosic materials to pretreatment and notably to enzymatic hydrolysis [3,6,10,11,12,13,14].
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