Catalysis of Organic..

Ostgard et al . 209product mixture. The Ni catalyst produced a ~1:1 mannitol:sorbitol ratio indicatingthat the α-furanose is only ~3 times more active than the β analog. The β-furanose ismore competitive, albeit still not preferred, on the Ni surface because its strongeradsorption strength increases the likelihood that this more concentrated tautomer willfind an adsorption site and the higher hydrogen content of Ni will decrease itsrequired residence time before it is hydrogenated. Doping the Ni catalyst with Moincreased both its activity and sorbitol selectivity. The literature suggests thatmolybdate species readily form complexes with the open keto and hydrated forms offructose (26). This means that the surface molybdate species on the Ni catalystassisted in the formation of reactive intermediates that have the most keto characterof all the catalysts used here resulting in the highest hydrogenation activity and thelowest activation energy. This increased activity led to the mannitol:sorbitol ratio of~0.8:1 meaning that α-furanose is now only ~2.4 times more active than β-furanosefor this reaction. This is attributed once more to the improved competitiveadsorption of the β-furanose on the Mo doped Ni surface.As discussed earlier, the FT deposits carbonaceous residues onto the surface andsplits it up into smaller ensembles. The smaller ensembles favor the adsorption ofthe least sterically hindered α-furanose leading to an increase in mannitol selectivity.This measurable increase in mannitol selectivity continues until the ensemble sizebecomes so small that it starts to inhibit adsorption and this leads to lower mannitolselectivity. This should force fructose to start adsorbing in more of a tilted fashionand then eventually to almost the edge adsorption modes shown in Figure 19. Asthis adsorption becomes more tilted, the steric interaction of the C5 methylenehydroxyl group should have a lower impact on the selectivity of the reaction and thiscould provide for the limited drop in the mannitol selectivity at the higher FT levelsseen here. One would also expect that the flat adsorbed α-pyranose will stopadsorbing and reacting at lower FT levels than the flat adsorbed α-furanose and β-furanose rings leading to fewer sources for mannitol and contributing to the drop inmannitol selectivity at the higher FT levels. The fastest increase in mannitolselectivity was after the FT of the Mo doped Ni, were the formaldehyde selectivelyblocks the Mo promoters and changes the activity/mannitol selectivity profile of thiscatalyst to what would be expected for a fresh Ni catalyst without Mo or FT. Thehighest mannitol selectivity was obtained with the 80°C FT Cu catalyst where thecombination of weak fructose adsorption and smaller ensemble sizes pushed thepreference for α-furanose adsorption high enough to give 67.7% mannitol.An increasingly tilted adsorption will also increase the ketal hydrogenolysischaracter of this reaction leading to the observed higher activation energies. It isgenerally thought that there is an optimal ensemble size with the right combinationof atoms for hydrogenolysis on Ni (27). Thus it is interesting that the catalyst'sactivity doesn’t decrease more as the reaction is forced further into a hydrogenolysismode while the average ensemble size becomes smaller. The Ni catalyst used heretypically has an average Ni crystal size of about 10 nm (28) and this should havebeen completely covered at the level of ~120 mmol formaldehyde per mole of activemetal when one assumes a formaldehyde:surface metal ratio of 1:1. Obviously this