Ni-Fe Alloying Enhances the Efficiency of the Maltose Hydrogenation Process: the Role of Surface Species and Kinetic Study

Unlike the conversion of monosaccharides to the corresponding polyols, the production of maltitol by hydrogenation of maltose has been seldom investigated in the literature, despite its industrial importance. Monometallic Ni catalysts are known for their lack of stability, and the objective of the present paper is to determine through a kinetic study, to what extent a Ni-Fe/SiO2 bimetallic catalyst would outperform a Ni/SiO2 catalyst in the aqueous phase hydrogenation of maltose, as they have been reported to do for monosaccharides. The effect of reaction parameters (T = 80-150 ?, P-H2 = 20-40 bar, maltose mass fraction in water = 4.4-17.5 wt%) on activity, selectivity, and stability was examined. In all cases, maltitol was the major product, with a carbon balance higher than 98%, but maltose hydrolysis to glucose occurred in the upper range of temperature. In order to preserve both the catalyst selectivity and stability, a temperature of 80 C was selected for the kinetic study. A first order model including an inhibiting term based on maltose concentration could fit the evolution of the conversion of maltose as a function of time. The adsorption constant of maltose and the apparent hydrogenation rate constant for the Ni-Fe catalyst were both larger by a factor 2-3 compared with the Ni catalyst, indicating a stronger interaction of maltose with the Ni-Fe surface. Another major difference was a reaction order of 0.5 with respect to the hydrogen pressure on Ni-Fe/SiO2 compared with a near zero-order on Ni/SiO2, stressing significant differences in coverage of the bimetallic surface. The activity of the Ni-Fe catalyst remained constant for three runs of reaction without major structural changes, while the Ni catalyst deactivated by transforming to a phyllosilicate phase. As far as activity, selectivity and stability are concerned; Ni-Fe/SiO2 appeared as a better suited catalyst than Ni/SiO2 for the aqueous phase hydrogenation of maltose at 80 C, with a more pronounced benefit than formerly reported for xylose on the same catalysts.