Catalysis of Organic..

202 Fructose Hydrogenationmmol formaldehyde per mol of Ni. This extraordinary jump in reaction orderconfirms the concept that formaldehyde selectively decomposed on the Mo of thiscatalyst and what remains is a templated active Ni surface. This is the first situationwhere the 49.2 FT Mo doped catalyst does not look exactly like a fresh Ni catalystwithout Mo or FT, and this may be due to the affect of Mo’s spatial surfacearrangement on the deposition of formaldehyde and its resulting surface structure.Figure 9 presents the trend between mannitol selectivity and the amount of zeroorder behavior, where the lower levels of zero order behavior (as indicated by itstopping at higher fructose concentrations) produce more mannitol for the not FTcatalysts. It is obvious that the catalysts with the lower FT levels also follow theleast squares fit of Figure 9 and that the higher FT amounts produce catalysts that nolonger fit this trend. The reaction order data demonstrate unambiguously that zeroorder activity resulting from the strong adsorption of fructose will enhance thehydrogenation of the most abundant active species in the reaction solution (βfuranose)to form more sorbitol, while less zero order activity created from the weakadsorption of fructose will favor the least sterically hindered species (α-furanose) togive more mannitol.The kinetics of catalytic reactions are usually treated with the Langmuir-Hinshelwood model (6,9), however this is not as effective as the Michaelis-Mentenequation for the description of catalytic surfaces where the desorption of thesubstrate can influence the reaction rate as portrayed in Figure 10. These data can beanalyzed graphically with Lineweaver-Burk (LB) double reciprocal plots (please seeFigure 11, Figure 12, Figure 13 and Figure 14) and the details of these analyses aredescribed thoroughly in the literature (22). The changes in the slopes and the y-intercepts of these graphs can visually show a change from one type of surfacebehavior to another. These values can also be used to calculate the maximumactivity (V max ) under zero order conditions without diffusional constraints and theMichaelis constant (K m ) as seen with the equations in Figure 12. K m can also bedescribed as the apparent dissociation constant of the adsorbed fructose (K –1 / K 1 ofFigure 10). Although this is a bisubstrate reaction, the hydrogen pressure was keptconstant and we will simply consider the effects of formaldehyde deposition (thesource of the irreversibly adsorbed inhibitor) on the conversion of fructose in ourinterpretation of the data. The visual inspection of Figure 11 shows that the additionof low levels of formaldehyde (from 31.5 to 62.9 mmol formaldehyde per mol metal)results in an incrementally directly proportional drop in activity as the slopes of theselines gradually increase with higher FT levels. At these levels the residues offormaldehyde act as non-competitive inhibitors (i.e., they don’t occupy the activesites) that don’t impede adsorption but they do slow down the conversion of theadsorbed species into product via electronic and/or steric factors. There is a largedifference in both the slopes and the y-intercepts of the plots as the FT levelincreases from 62.9 to 81.8 mmol of formaldehyde per mol of Ni and this correlatesto both the departure from the optimal FT area for mannitol selectivity and thetransformation from non-competitive to something similar to uncompetitiveinhibition for the FT. Uncompetitive inhibition occurs when both the inhibitor and