Pulping and Bleaching PSE 476/Chem E 471

Pulping and BleachingPSE 476/Chem E 471 Lecture #10 Kraft Pulping Carbohydrate Reactions PSE 476: Lecture 10

Agenda • Carbohydrate Reaction Mechanisms • Glycosidic Cleavage • Peeling • Stopping • Glucomannan Reactions • Xylan Reactions • Other Hemicellulose Reactions • Cellulose Reactions PSE 476: Lecture 10

Carbohydrate Reactions • Carbohydrates react slower than lignin under alkaline conditions. Overall, however, just as much carbohydrates react as does lignin. • The main alkaline reactions of carbohydrates are: • Glycosidic cleavage. • Peeling. • Stopping. PSE 476: Lecture 10

Alkaline Pulping : CarbohydratesGeneral Aspects • There are considerable carbohydrate losses during kraft pulping due to alkaline degradation reactions. • Acetyl groups are very quickly cleaved. • Carbohydrates undergo “peeling” • Peeling is the process in which sugars are removed one by one from the reducing end of the polymer. • Hemicelluloses highly degraded through “peeling” • Monosaccharide fragments from peeling are highly degraded to acidic compounds. • This reaction is stopped by “stopping” reactions. • Glycosidic linkages in carbohydrates are cleaved through hydrolysis reducing overall molecular weight and creating new reducing ends. PSE 476: Lecture 10

“Peeling Reaction” Formation of new reducing end group (C) (D) (B) (E) (A) A. Isomerization B enediol formation C. b-alkoxy elimination D. Tautomerization E. Benzilic acid rearrangement Notes PSE 476: Lecture 10

Benzilic acid rearrangement Bond rotation Nuclephilic addition α-hydroxy-carboxylic acid Proton transfer PSE 476: Lecture 10

“Stopping Reaction” (B) (C) (D) (A) A. 1,2 Enediol formation B. b-hydroxy elimination C. Tautomerization D. Benzilic acid rearrangement (will not “peel”) Notes PSE 476: Lecture 10

Cleavage of Glycosidic Bonds (B) (A) (C) A. Inversion of ring confirmation, C2 OH ionized B. Ionized hydroxyl groups attacks C1 eliminating methoxyl group forming 3 membered epoxide (oxirane) • Opening of epoxide forms new reducing end or if steric conditions are correct a 1,6 anhydride. This compound is opened by alkali. This reaction requires elevated temperatures Notes PSE 476: Lecture 10

Cleavage of Glycosidic Bonds Notes PSE 476: Lecture 10

Alkaline Reactions of Glucomanans/Cellulose • Glucomannans: • Very unstable to peeling reactions • Galactose side chains fairly resistant • Cellulose • Large Dp means that glycosidic cleavage more important than peeling • Cellulose loss small but viscosity (Mw) significantly reduced PSE 476: Lecture 10

Loss of Glucomannans During Kraft Pulping Notes PSE 476: Lecture 10

Loss of Xylans During Kraft Pulping Notes PSE 476: Lecture 10

Xylan peeling Reactions • Xylans much more resistant to peeling than glucomannans. • Large % of xylans are dissolved during pulping instead of peeled. Much of these xylans precipitate on the fiber surface at the end of the cook as alkali is consumed. • There appears to be two different temperature dependent mechanisms responsible for the protection from peeling. PSE 476: Lecture 10

Resistance of Xylan to Peeling Below 100°C • Low temperature stability (below 100°C) is due to the resistance of galacturonic acid to peeling at this temperature. • Galacturonic acid groups are found in unique end group. (see below) • Above 100°C, galacturonic acid groups undergo peeling. --Xly-14---D-Xly3--L-Rha-12--D-GalA-14-D-Xly PSE 476: Lecture 10

4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly       4-O-Me--D-Glc   -L-Araf  Resistance of Xylan to Peeling above 100°C • A theory has been presented that above 100°C glucuronic acid side chains on xylans slow the peeling reaction. • Above 120°C, glucuronic acids are somewhat converted to hexenuronic acids which are much more stable to peeling. Hexenuronic acid formation is discussed in the next slide. PSE 476: Lecture 10

Formation of Hexenuronic Acids • Hexenuronic acids are formed from uronic acids under alkaline conditions. • Method for identifying these compounds in pulps just developed. • Interfere with Kappa (lignin concentration) determination. • Attract metals (color problem/cause problems in bleaching) PSE 476: Lecture 10

Loss of Other Hemicelluloses During Kraft Pulping • The minor hemicelluloses such as pectins, starches, etc. are supposedly completely destroyed during kraft pulping. • This happens through dissolution and peeling. • Many of these carbohydrates are water soluble so removal is easy. Once they are in the hot alkali solution they are easily destroyed. PSE 476: Lecture 10

Cellulose Reactions During Kraft Pulping • Cellulose undergoes peeling and glycosidic cleavage reactions during kraft pulping. • Because cellulose molecules are so long, peeling reactions only cause small yield losses. • Glycosidic cleavage is more of a problem because of molecular weight losses that cause strength problems. This reaction also increases the rate of peeling through generation of new reducing end groups. • Because cellulose molecules are so large dissolution is not an issue. PSE 476: Lecture 10

Pulping and Bleaching PSE 476/Chem E 471