ABSTRACT
Abstract Fructose hydrogenations have been performed with various sponge-type Ni and Cu catalysts and the reaction data have been correlated to the catalysts’ properties. Cu is less active than Ni and it favors the production of mannitol over sorbitol by a ~2:1 ratio, while Ni generates them on a ~1:1 basis. Promoting Ni with Mo increased the rate of hydrogenation, and decreased the mannitol selectivity to 44.1%. The data show that the least active catalysts adsorb fructose weaker, have less zero order behavior and higher activation energies leading to higher mannitol selectivities from the preferred hydrogenation of the sparser less sterically hindered α-furanose with retention at the anomeric carbon. Highly active catalysts adsorb fructose stronger for a more competitive, but still not favored, adsorption of the more abundant β-furanose leading to more sorbitol. Depositing carbonaceous residues onto the catalyst prior to the reaction showed that smaller ensembles favor mannitol, thereby confirming its source to be the readily adsorbed α-furanose. Introduction Mannitol is widely used as a dusting, bulking and anticaking agent in the food and pharmaceutical industries due to its nonhygroscopicity (1). It is made by reducing invert 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). Glucose gives almost exclusively sorbitol, and fructose generates roughly a 50:50 mannitol: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 of fructose hydrogenation could help to optimize this process for the marketplace.
