Kinetic investigation of the hydrolytic hydrogenation of oligosaccharides to sorbitol

• Kinetische Untersuchung der hydrolytischen Hydrierung von Oligosacchariden zu Sorbitol

Negahdar, Leila; Palkovits, Regina (Thesis advisor); Liauw, Marcel (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2015)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2014

Abstract

The rare characteristics of sorbitol as a promising intermediate in biomass to biofuel conversion have attracted much research. Nevertheless, adequate understanding of the mechanism and kinetics of its reactions is still missing, mostly because of the complex molecular structure of the polysaccharides that are involved. This dissertation is the tale of our research on the kinetics and mechanism of sorbitol production through hydrolytic hydrogenation of oligosaccharides. Preceding research on this topic is almost entirely based on the controversial hypothesis that conversion of polysaccharides to sorbitol passes through a consecutive hydrolysis to monosaccharides followed by hydrogenation to sorbitol. Our research, on the other hand, reveals two competing reaction pathways, namely hydrolysis of oligosaccharides, and its hydrogenation to reduced form. More interestingly, at lower reaction temperatures the hydrogenation pathway becomes considerably dominant which is contrary to the widely accepted premise. To overcome the molecular complexity of polysaccharides, we settled for model-molecules such as disaccharide and trisaccharide which have simple structure and sufficiently resemble the polysaccharides. Most of our effort has been directed towards selective hydrolysis-hydrogenation of these model molecules over a catalytic system composed of molecular acids and supported metal catalysts. Kinetic study of disaccharide showed that at lower reaction temperatures, the hydrogenation pathway is dominant whereas at higher reaction temperatures, direct hydrolysis of disaccharide becomes favorable. Analysis of kinetic data confirmed the hydrolysis reaction as the rate determining step. Kinetic investigation of trisaccharide also indicated that the hydrogenation proceeds faster than hydrolysis. At the same time, a facilitated hydrolysis of reduced trisaccharide compared with non-reduced counterpart was observed. The study was extended to include oligosaccharides with longer chains, up to heptasaccharides, using the same underlying kinetic model. Despite growing complexity of the reaction network, the same kinetic selectivities i.e. the hydrogenation over hydrolysis as well as facile hydrolysis of reduced compounds were confirmed. Overall, a direct hydrogenation of oligosaccharides to reduced forms followed by hydrolysis appears as a superior sorbitol production pathway.