Palladium catalysts loaded on bimodal porous silica were examined in the liquid-phase hydrogenation of 2-butenal at 0 degrees C and a hydrogen pressure of 1.1 MPa. The sizes of mesopores and macropores of the silica support are controllable in the range of 4-23 nm and 0.5-25 mu m, respectively. The macropores provide effective paths of mass transfer, and the mesopores present effective surfaces for dispersion of metals. The catalytic activity of Pd-silica with bimodal porous structure depends on the size of mesopores and macropores as well as on the particle 14 size of the support silica. The most active Pd-silica catalyst, with mesopores of 12 run and macropores of 2 mu m, shows much higher activity than does a commercial palladium carbon catalyst without macropores. The results indicate that the diffusion process inside the catalyst particles dominantly determines the reaction rate in the hydrogenation. (c) 2005 Elsevier B.V. All rights reserved.