Reaction pathways of glucose and fructose on Pt nanoparticles in subcritical water under a hydrogen atmosphere

The reactivity profiles of glucose (Glc), fructose (Fru), and possible intermediate compounds treated with Pt nanoparticles protected by polyethyleneimine (Pt-PEI) were studied at 403–543 K in subcritical water under a hydrogen atmosphere of 5 MPa. Isomerization between Glc and Fru proceeded at the temperature as low as 403 K and was accelerated by Pt-PEI and H2. The rate of isomerization from Glc to Fru in the presence of Pt-PEI and H2 was approximately three times greater than the rate of the reverse reaction. Yields of sorbitol and mannitol were maximized at 443 K and decreased at higher temperatures. Under reaction temperatures of 483–543 K, Glc produced mainly 1,2-propanediol, 1,2-hexanediol, and ethylene glycol, while Fru yielded 1,2-propanediol, 1,2-hexanediol, and glycerol. The molar ratios of C3 compounds to the sum of C2 and C4 compounds in this temperature range were approximately 2:1–1.5:1 in the former and 3:1–4:1 for Fru, indicating that C3 compounds can be preferentially produced from Fru. The reactivity profiles of sorbitol, mannitol, and 1,2-hexanediol were examined in separated experiments at 483–543 K, and it was found that these linear C6 compounds were not converted to the previously mentioned C2–C4 diols and triols. This suggests that there is little possibility for Glc or Fru to form C2–C4 compounds via sorbitol, mannitol, or 1,2-hexanediol on the Pt catalyst, and it is likely that retro-aldol reactions of Glc and Fru are the major routes to these molecules. The temperatures necessary to conduct these reactions on Pt were much lower than those without Pt.