Liquid-phase hydrogenation of ketones in the mesopores of nickel catalysts

The liquid-phase hydrogenation of ketones was investigated over Raney nickel and Ni–MgO catalysts with known pore structures: the effects of the sizes of the reactant and solvent on the catalytic activity were examined at 0 °C at a hydrogen pressure of 1.1 MPa to discuss the mass transfer of reactants/products and solvents within the mesopores. The catalytic activity decreases linearly with increasing reactant molecular size in the hydrogenation of both aliphatic and aromatic ketones with an acetyl group, such as acetone, butan-2-one, heptan-2-one, nonan-2-one, undecan-2-one, acetophenone, 4′-methylacetophenone, 4′-ethylacetophenone, and 4′-n-propylacetophenone. In the hydrogenation of ketones such as acetone, cyclohexanone, and heptan-4-one in linear alkane solvents, the catalytic activity decreases with the increasing length of the carbon chain of the solvent, and a zigzag phenomenon, i.e. the catalytic activity in alkane solvents with an odd number of carbons in the chain is higher than in those with an even number of carbons, is observed for larger reactants in smaller pores. Neither the activity drop nor the zigzag tendency appears for smaller reactants reacted in sufficiently large mesopores. It can be concluded that strong resistance for the mass transfer of larger reactants in smaller mesopores explains the difference in the catalytic hydrogenation reactivity of reactant diffusing through solvent in a mesoporous catalyst.