Catalysis of Organic..

Ostgard et al . 199not the ionic saltlike hydrides (13) that one would need for an S N 2 reaction. Hence,the goal of this work was to clear up the inconsistencies of the currently proposedmechanisms.Results and DiscussionSince the tautomeric species’ concentrations at different temperatures can influencefructose hydrogenation, we reviewed the literature to find the data in Table 1 (5).Although the listed maximum temperature was 80°C and our reactions were at100°C (for Ni and Mo doped Ni) and 110°C (for Cu and its doped varieties), thesedata suggest that our reaction solution had > 42% furanose forms, the overall β-to-αratio was < 7 and the furanose β-to-α ratio was close to ~3. None of our catalystswere 100% selective for either sorbitol or mannitol and their mannitol selectivitiesranged from 44.1% for the Mo doped Ni to 67.7% for the 80°C formaldehyde treated(FT) Cu catalyst. The catalyst’s properties clearly affect the interactions of thetautomeric forms with its surface to give it a very specific activity and selectivityprofile. The catalysts used here were chosen to observe the affects of coordinating,blocking and chemisorption assisting promoters as well as the type of active metaland its ensemble size. We used the deposition of formaldehyde to decrease theensemble size of the active metal as described in the literature (14). It is known thatformaldehyde disproportionates over Ni (110) as low as 95 K to give methanol andCO (15). This CO adsorbs strongly on the metal as a site-blocker and conceivablyas an electronic modifier that can form bridged and linear species whose relativeamounts depend on the surface coverage and temperature. This strongly held speciesdoesn’t desorb from Ni until 170°C (16,17,18,19), and this agrees with thetemperature programmed oxidation (TPO) of this FT catalyst (14) that gave off ameasured amount of CO 2 from 200 to 370°C. The TPO data also indicated that FTdid not change the catalyst's other attributes (20). The Ni adsorbed CO is clearlystable enough to survive the conditions used here for fructose hydrogenation and thistechnique helped us to evaluate the influence of the adsorbed species on the reaction.As seen in Figure 2, the Mo doped Ni is clearly the most active of the not FTcatalysts followed closely by Ni, and all the Cu catalysts are more than an order ofmagnitude less active (albeit at a higher reaction temperature) than the Ni ones. Feand Pt dopants do not change the activity of the Cu catalysts very much and this is instark contrast to the overwhelmingly positive affects these promoters exert on thedehydrogenation of aminoalcohols (21). Increasing the level of FT steadilydecreases the activity of the Ni and Cu catalysts in a predictable fashion and theactivity of the lowest level FT Mo doped catalyst is very similar to the Ni catalystwithout FT. The drop in activity is strongly dependent on treatment temperature forthe Cu catalyst, where 80°C is more effective than 25°C for the decomposition offormaldehyde on this surface. As seen in Figure 3, the not FT Cu catalyst produceda ~2:1 mannitol:sorbitol ratio where promotion with Pt and Fe led to slightly lessmannitol. The FT Cu catalysts show a very shallow maximum at 32.3 mmolformaldehyde per mol of Cu for the 25°C treatment, and the FT of Cu at 80°C led tothe most mannitol selective surface. The FT also improved the mannitol selectivity