Catalysis of Organic..

Ostgard et al . 19723. The Effects of Various CatalystModifiers on the Hydrogenation ofFructose to Sorbitol and MannitolAbstractDaniel J. Ostgard, Virginie Duprez, Monika Berweiler, Stefan Röder andThomas TackeDegussa AG, Rodenbacher Chaussee 4, 63457 Hanau, Germanydan.ostgard@degussa.comFructose hydrogenations have been performed with various sponge-type Ni and Cucatalysts and the reaction data have been correlated to the catalysts’ properties. Cu isless active than Ni and it favors the production of mannitol over sorbitol by a ~2:1ratio, while Ni generates them on a ~1:1 basis. Promoting Ni with Mo increased therate of hydrogenation, and decreased the mannitol selectivity to 44.1%. The datashow that the least active catalysts adsorb fructose weaker, have less zero orderbehavior and higher activation energies leading to higher mannitol selectivities fromthe preferred hydrogenation of the sparser less sterically hindered α-furanose withretention at the anomeric carbon. Highly active catalysts adsorb fructose stronger fora more competitive, but still not favored, adsorption of the more abundant β-furanoseleading to more sorbitol. Depositing carbonaceous residues onto the catalyst prior tothe reaction showed that smaller ensembles favor mannitol, thereby confirming itssource to be the readily adsorbed α-furanose.IntroductionMannitol is widely used as a dusting, bulking and anticaking agent in the food andpharmaceutical industries due to its nonhygroscopicity (1). It is made by reducinginvert sugar (1:1 glucose:fructose) over sponge-type Ni (2) to give a ~25%:~75%mannitol:sorbitol mix that is separated by fractional crystallization (3). Glucosegives almost exclusively sorbitol, and fructose generates roughly a 50:50mannitol:sorbitol mix. Although mannitol’s market is much smaller than sorbitol’s,its price is 3.9 (1) to 4.4 (4) times higher meaning that an in-depth understanding offructose hydrogenation could help to optimize this process for the marketplace.Unlike most organic compounds, aqueous reducing sugars can form up to sixdifferent tautomers (α-pyranose, β-pyranose, α-furanose, β-furanose, acyclic andhydrated forms) (5). It is the relative concentrations, activities and adsorptionstrengths of these tautomers on the catalyst that determine the activity and selectivityof the sugar’s hydrogenation (4,6,7). Figure 1 shows the mutarotation of fructosewith the expected hydrogenation products when assuming retention at the anomericcarbon. The rate of mutarotation is much faster than the rate of hydrogenation for