Mass-transfer limitation in mesopores of Ni-MgO catalyst in liquid-phase hydrogenation

Liquid-phase hydrogenation of cyclohexanone, acetone, 2-butanone, 3-pentano ne, and 4-heptanone to the corresponding secondary alcohols was investigate d over various porous Ni catalysts at 0 degrees C under a hydrogen pressure of 1.1 MPa. Pore size distributions as well as Ni surface area of Ni-MgO c atalysts, which were prepared from a melt of the corresponding nitrates and citric acid, with high Ni contents of 60-80 wt% were controlled by the cal cination temperature of the precursors. For the hydrogenation of acetone, r eaction rate constants were directly proportional to the Ni surface areas o f the catalysts, and Raney nickel which had the largest Ni surface area sho wed the highest reaction rate. For the hydrogenation of other reactants lar ger than acetone in molecular size, however, rate constants do not have a s imple linear correlation with Ni surface area. Ni-MgO catalysts with large mesopores exhibited reaction rates higher than those of Raney nickel cataly sts with the largest Ni surface areas. Assuming that diffusion of both reac tants and products is restricted in small pores such as in Raney nickel, we tried to evaluate an effective pore size for the liquid-phase mass transfe r in porous materials by a novel approach analyzing reaction rate data coup led with pore size distribution and hydrogen chemisorption data. Cumulative Ni surface areas were calculated by multiplying the Ni surface area by a f raction of cumulative surface area located in pores larger than a specific size to the total surface area, and relationship between the cumulative Ni surface areas and the reaction rate constants were examined. It was found t hat the rate constants for the hydrogenation of 2-butanone, cyclohexanone, 3-pentanone, and 4-heptanone were proportional to cumulative Ni surface are as in pores larger than critical sizes of 2.0, 2.3, 3.2, and 3.7 nn in radi us, respectively. It has been consequently elucidated that the mass transfe r of the reactants is restricted in pores smaller than a critical size that depends on the size of the reactants. (C) 2000 Academic Press.