03.08.2015   Biomass Conversion

Organic Acid Fractionation

Formic and acetic acids are good solvents for lignin, and they can hydrolyze lignin in lignocellulosic material under elevated temperature thus resulting in delignifi — cation. At present, organic acids-based fractionations are preformed in formic or acetic acid solutions without and with the addition of catalysts (Table 11.2) [86-103]. As can be seen, the catalysts mainly used are inorganic acid such as hydrochloric acid, sulfuric acid and hydrogen peroxide.

11.4.1 Effect of Treatment on the Structure of Lignocellulosic Material

11.4.1.1 Reactions of Lignin

The chemical modifications of lignin during organic acid fractionation are mainly b-O-4 cleavage, lignin condensation, hydrolysis of LCC structures and the native ester structures, and esterification of the hydroxyl groups. By analyzing the dissolved lignin in a Milox pulping process with three stages, it was found that b-aryl ether bonds were mainly cleaved in the second stage. The precipitated lignin dissolved in the first stage contained a high amount of sugars, and its molecular weight increased with increased stages [104]. Lignin model compounds were investigated by reflux in 85% formic acid, and it was found that the compounds were completely consumed in 1 h. Primary, secondary and phenolic hydroxyl groups of the model compounds were partially converted into corresponding formates [105]. Compared to MWL, the dissolved lignin from acetic acid frac­tionation contained more acetyl groups in Ca and Cy, indicating that hydrolysis of native esters and acetylation (esterification) occurred simultaneously during the fractionation process. However, no acetylation was involved in the formic acid fractionation [106]. In addition, the cleavage of LCC structures was confirmed by the low contaminated sugars in the lignin fractionation precipitated from water [106].

Process

Raw material

Fractionation conditions

Results

Ref.

Acetosolv

Wheat straw

Acetic acid 90% (v/v), H0SO4 dosage 4% (w/w, on straw), liquor to solid ratio 10 (v/w), 105°C, 180 min

Pulp yield 50%, dissolved lignin yield 15%, and monosaccharides yield 35%

[96]

Acetosolv

Populus

Acetic acid 95% (v/v), H0SO4 dosage 1.5% (w/w, on straw), liquor to solid ratio 8 (v/w), 106°C, 180 min

Pulp: yield 52.1%, lignin content 6.79%

[97]

Acetosolv

Miscanthus x giganteus

Acetic acid 85%, HC1 dosage 0.10-0.15%, boiling point, 60-180 min

Pulp: yield 54.7-59.1%, lignin content 1.8-5.4%, viscosity 809-1151 ml/g

[98]

Acetosolv

Eucalyptus

Acetic acid 90%, F1C1 dosage 0.5%, liquor to solid ratio 10, boiling point, 180 min

Pulp: yield 46%, kappa number 31

[99]

Acetosolv

Beech

Acetic acid 90%, F1C1 dosage 0.2%, liquor to solid ratio 7, 130°C, 60 min

Pulp yield 50%, lignin content 7.5%, cellulose content 77.2%, xylan content 8.4%

[100]

Acetosolv

Marabou

Acetic acid 90%, F1C1 dosage 0.2%, 121 °С, 60 min

Delignification 84.8%, hemicelluloses degradation 78%

[101]

Formosolv

Miscanthus x giganteus

Formic acid 80%, F1C1 dosage 0.10-0.15%, boiling point, 60-180 min

Pulp: yield 47.1-53.3%, lignin content 3.2-5.0%, viscosity 838-1084 ml/g

[98]

Formosolv

Eucalyptus

Formic acid 80%, F1C1 dosage 0.2%, liquor to solid ratio 10, boiling point, 180 min

Pulp: yield 41.5%, kappa number 20.5

[99]

Formacell

Triticale straw

Formic acid 30%, acetic acid 50%, liquor to solid ratio 12, 107°C, 180 min

Pulp yield 48.5%, xylan content 14.3%, kappa number 33.8, viscosity 1181 ml/g

[102]

Formacell

Eucalyptus

Formic acid 8.5%, acetic acid 76.5%, liquor to solid ratio 5, 170°C, 90 min

Bleached pulp: brightness 92.2% ISO, viscosity 651 ml/g

[103]

11 Organosolv Fractionation of Lignocelluloses for Fuels, Chemicals and Materials 357

When hydrogen peroxide was added into the organic acid system, peroxyformic or peracetic acid was generated in situ through an equilibrium reaction between organic acid and hydrogen peroxide, and electrophilic HO+ ions were formed [107]. The HO+ ions reacted with lignin through ring hydroxylation, oxidative ring opening, substitution of side chains, cleavage of b-aryl ether bonds and epoxidation [108].