Catalysis of Organic..

Ostgard et al . 205Max Act., mol/(min·mol metal)1,81,61,41,210,80,60,40,20Figure 15NiMo doped NiCuPt doped CuFe doped CuFT - NiFT - Mo doped NiFT - Cu at 25°CFT - Cu at 80°C0 0,2 0,4 0,6 0,8 1 1,2Vmax, mole/(min·mol metal)Figure 15. The correlation between the experimental max activity and V max .Figure 16. The correlation between %mannitol and V max .%Mannitol Selectivity66625854504642without FT or Pty = -22,088x + 69,542R 2 = 0,9914Figure 16NiMo doped NiCuPt doped CuFe doped CuNi - FTMo doped Ni - FTCu - FT at 25°CCu - FT at 80°C0 0,2 0,4 0,6 0,8 1 1,2Vmax, mol/(min·mol metal)mol/(min·mol metal)Max Act.,1,81,61,41,210,80,60,40,20Figure 17NiMo doped NiCuPt doped CuFe doped CuFT - NiFT - Mo doped NiFT - Cu at 25°CFT - Cu at 80°Cfor without FT and Pty = -0,0614x + 2,4107R 2 = 0,98160 10 20 Km 30 40 50Figure 17. The correlation between the experimental max activity and K m .Figure 18. The correlation between %mannitol and K m .itol Selectivity%Mann66625854504642for without FT and Pty = 0,6862x + 38,234R 2 = 0,995Figure 18NiMo doped NiCuPt doped CuFe doped CuNi - FTMo doped Ni - FTCu - FT at 25°CCu - FT at 80°C0 10 20 Km 30 40 50formaldehyde does push this relationship to higher mannitol levels for the same K mvalues at the lower FT levels. However the higher FT amounts inhibit the adsorptionso strongly that the K m is actually reduced, thereby implying that if fructose doesmanage to adsorb on this heavily treated surface it will actually have a longerresidence time than even on a clean surface. On possibility would be the creation of3-dimentional carbonaceous residues on the surface that not only inhibit fructoseadsorption, but it also inhibits its desorption via potential steric effects.The results show us that mannitol formation is favored by a less active catalystwith a higher activation energy that has as little as possible zero order behavior, aweak fructose adsorption strength (i.e., with a higher K m value) and an optimallysomewhat smaller ensemble size. The production of sorbitol is logically favored byjust the opposite trends. These tendencies allow us to formulate a fructosehydrogenation mechanism with the possible intermediates shown in Figure 19. It hasbeen shown that one needs an abstractable hydrogen atom on the hydroxyl groupattached to the anomeric carbon for this reaction to occur (7) and we propose that theadsorbed reaction intermediate must have both the ring oxygen and the hydroxylgroup of the anomeric carbon chemisorbed onto the catalytic metal surface in orderfor this reaction to occur. It will then be possible for the catalyst to homolytically