Mass-Transfer Limitation in Mesopores of Ni–MgO Catalyst in Liquid-Phase Hydrogenation

Abstract

Liquid-phase hydrogenation of cyclohexanone, acetone, 2-butanone, 3-pentanone, and 4-heptanone to the corresponding secondary alcohols was investigated over various porous Ni catalysts at 0°C under a hydrogen pressure of 1.1 MPa. Pore size distributions as well as Ni surface area of Ni–MgO catalysts, 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 calcination temperature of the precursors. For the hydrogenation of acetone, reaction rate constants were directly proportional to the Ni surface areas of the catalysts, and Raney nickel which had the largest Ni surface area showed the highest reaction rate. For the hydro genation of other reactants larger than acetone in molecular size, however, rate constants do not have a simple linear correlation with Ni surface area. Ni–MgO catalysts with large mesopores exhibited reaction rates higher than those of Raney nickel catalysts with the largest Ni surface areas. Assuming that diffusion of both reactants 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 transfer in porous materials by a novel approach analyzing reaction rate data coupled with pore size distribution and hydrogen chemisorption data. Cumulative Ni surface areas were calculated by multiplying the Ni surface area by a fraction 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 that the rate constants for the hydrogenation of 2-butanone, cyclohexanone, 3-pentanone, and 4-heptanone were proportional to cumulative Ni surface areas in pores larger than critical sizes of 2.0, 2.3, 3.2, and 3.7 nm in radius, respectively. It has been consequently elucidated that the mass transfer of the reactants is restricted in pores smaller than a critical size that depends on the size of the reactants.