Catalysis of Organic..

210 Fructose Hydrogenationis not the case, and this could indicate that there is 3-dimensional carbonaceousstructure being created that leaves a certain amount of the surface clean for reactions.In conclusion, fructose preferably adsorbs in a flat manner where the adsorptionand hydrogenation of the least sterically hindered α-furanose is favored over themore abundant β-furanose. This adsorption involves both the ring oxygen and thehydroxyl oxygen on the anomeric carbon that gives up its hydrogen atom to form anadsorbed ketal to hydroxy ketose equilibrium via a 1,3 intramolecular hydrogen atomshift. This equilibrium intermediate could have more ketose hydrogenation or ketalhydrogenolysis behavior as determined by the metal’s ability to stabilize thedeveloping π-bond character where the shift towards more ketal hydrogenolysisleads to lower activity and higher activation energies. This adsorbed species ishydrogenated with retention at the anomeric carbon so that α-furanose producesmannitol and β-furanose gives sorbitol. All of our catalysts preferentially adsorbedand hydrogenated the sparser less sterically hindered α-furanose over the β-furanosefructose and this tendency was enhanced by weaker fructose adsorption, less firstorder behavior, a higher activation energy (more ketal hydrogenolysis behavior) anda lower hydrogen content. For these reasons, Cu was found to be less active andmore mannitol selective than Ni. The addition of Mo to Ni not only enhanced theadsorption strength of fructose to increase the amount of adsorbed β-furanose, but italso stabilized the ketose form of the equilibrium to give the highest hydrogenationactivity and sorbitol selectivity at the lowest activation energy measured here.Experimental SectionThe hydrogenation of 500 grams of a 40 wt.% aqueous fructose solution was carriedout in a 1L steel autoclave stirred at 1015 rpm under 50 bars of hydrogen with either2.4 wt.% of an activated Ni catalyst (a.k.a., sponge-type) at 100°C or 7.2 wt.% of anactivated Cu catalyst at 110°C. The hydrogen uptake was monitored during thereaction and the samples of the product mixture were analyzed by HPLC. Thecatalysts used here were products of Degussa AG and their formaldehyde treatmentswere performed according to the patent literature (14).References1. N. Borgeson, B. Brunner and K. Sakota, Food Additives, A Specialty Chemicals /SRI International Report, Dec. 1999.2. M. Makkee, A.P.G. Kieboom, H. van Bekkum, Starch/Stärke, 37, 136-141(1985).3. A.A. Unver and T. Turkey, U.S. Patent 3,632,656 to Atlas Chemical Industries,Inc. (1972).4. A. Heinen, J. Peters, and H. van Bekkum, Carbohydr. Res., 328, 449-457(2000).5. S.J. Angyal, Advances in Carbohydrate Chemistry and Biochemistry, 42 (1984)15-68.