Mechanistic insight into the production of ethylene glycol and propylene glycol from biorenewable resources

The production of ethylene glycol and propylene glycol from biorenewable resources, i.e. sorbitol, has been parametrically examined numerous times. However, the current selectivity is too low to be economically feasible. In order to develop improved catalyst systems, a better understanding of the reaction mechanism is required. Sorbitol, due to its many functional groups is quite complex. Thus, in order to begin understanding of this system, analysis of the products is required. The study of the degradation of the product glycols and reaction of glycerol under hydrogenolysis conditions gives foundational insight. The effects of pH, competitive adsorption, and product degradation are determined for these smaller polyols. A mathematical model for the glycerol reaction and product degradation is developed which includes an instantaneous selectivity parameter, which is more accurate than an overall selectivity that does not account for product degradation. This foundation was built upon with an understanding of the effects sulfur has on the reaction. Sulfur, a known catalyst poison, lowers the reaction rate while increasing the selectivity toward propylene glycol. The limiting step in the reaction is proposed to be the catalytic diffusion of sulfur adatoms away from the adsorbed polyol during the dehydrogenation step. Finally, higher polyols are studied. The reaction rate of higher polyols is correlated to the configuration of the polyol. Analysis of the product distribution of various polyols shows that the decarboxylation reaction is occurring along with the retro-aldol reaction. A selectivity map is fitted to the product distribution to determine which hydroxyl groups are preferentially dehydrogenated. Sulfur decreases the dehydrogenation fraction of the primary alcohol group.