Acid hydrolysis of O-acetyl-galactoglucomannan

Abstract

Acid hydrolysis of O-acetyl-galactoglucomannan (GGM), the main non-cellulosic polysaccharide in coniferous wood, was studied at 90 °C in the pH range of 0.5–2.0 using different acids (hydrochloric acid, sulfuric acid, trifluoroacetic acid, oxalic acid, and formic acid) as homogeneous catalysts and cation exchange resins (Smopex 101 and Amberlyst 15) as heterogeneous catalysts. The hydrolysis rate was exclusively dependent on the pH of the solution, but independent of the concentration and type of acid. Analysis of the formation rates of oligomers with DP 2–5 revealed that GGM hydrolysis occurs rather randomly. The final products were mannose, galactose, and glucose. In the beginning of the reaction, the formation of galactose was slightly faster than those of other sugars, since galactose is a side substituent to the glucose–mannose main chain of GGM. However, with longer reaction times, the formation rates of sugar monomers were correlated with their initial abundances in the polysaccharide. Autocatalytic kinetics was observed in the presence of the heterogeneous catalysts: an induction time was observed, after which the reaction proceeded with the same rates as with homogeneous acids. The autocatalytic effect is probably explained by the increase of the hydrolysis rate constant, as the GGM chain length decreases and the molecule becomes more accessible for the active sites on the outer surface of the catalyst.