O2 as initiator of autocatalytic degradation of hemicelluloses and monosaccharides in hydrothermal treatment of spruce

The influence of oxygen (0-50 bar) on the molar mass and composition of hemicelluloses after hydrothermal treatment of spruce chips was studied in a batch reactor setup at 130 degrees C-160 degrees C. Purified galactoglucomannan was studied as a reference. The dissolved oxygen enhanced significantly the depolymerization of hemicelluloses from over 15,000 g/mol to 180 g/mol (monomers) as well as promoted acids formation from the monosaccharides. About 2-3 times faster depolymerization was observed already with rather low amounts of oxygen (2-8 bar), while the kinetics of depolymerization was significantly slower in inert atmosphere. The decrease in pH from neutral to as low as 2.5 lead to an autocatalytic effect on the polysaccharide hydrolysis and the consecutive reactions of monosaccharides. The results emphasize the importance of performing hydrothermal treatment under inert atmosphere if large polymeric hemicellulsoses are desired or if monosaccharides are targeted in hydrolysis of hemicelluloses. If autocatalysis begins, it is difficult to stop.

Automated summary

The influence of oxygen on the depolymerization of hemicelluloses and the formation of acids from monosaccharides during hydrothermal treatment of spruce chips was studied. The presence of dissolved oxygen significantly enhanced the depolymerization process and acid formation. Lower amounts of oxygen resulted in faster depolymerization, while it was slower in an inert atmosphere. Decreasing pH had an autocatalytic effect on polysaccharide hydrolysis and monosaccharide reactions.