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.