Asphaltenes were precipitated from a heavy residue and from its hydrotreated products at constant pressure, liquid-hourly space-velocity and hydrogen-to-oil ratio (100 kg/cm(2), 0.25 h(-1), and 5000 ft(3)/bbl, respectively). The effect of reaction temperature was studied in the range of 380-410 degrees C. The experiments were carried out in a two fixed-bed reactor unit. The first test used a NiMo/gamma-Al2O3 catalyst and the second test a NiMo/SiO2-Al2O3 catalyst. It was observed that the molecules of asphaltenes formed after reaction from the NiMo/SiO2-Al2O3 catalyst are smaller in size and less concentrated in metals than those originated from NiMo/gamma-Al2O3 catalyst, which are more concentrated in nitrogen and the asphaltenes structure is more aromatic. The NiMo/SiO2-Al2O3 catalyst enhances metal removal in liquid products. (C) 2014 Published by Elsevier Ltd.