Cr-Doped Alumina as Support for NiMo Vacuum Residue Hydroprocessing Catalyst
Mesoporous alumina was doped with Cr using a coprecipitation method in order to prepare a support for hydrocracking catalysts. NiMo catalysts were prepared by impregnation on alumina (NiMo/Al2O3) and Cr-doped alumina (NiMo/Al2O3-Cr) and characterised. The presence of Cr in the support led to a better dispersion of NiMo and lower metal reduction temperatures as determined by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). As a consequence, a larger number of metal active sites was available in reduced form at operating conditions. The textural properties remained relatively unaffected by the coprecipitation of Cr together with the alumina. The catalysts were tested in their activity towards hydrodeasphaltenisation reactions using vacuum residue as feed in a batch reactor in two successive one-hour experiments involving reutilisation of the catalysts with fresh feed. It was found that Cr not only aided metal dispersion in catalyst synthesis but also coke dispersion during reaction, leading to sustained catalytic activity despite NiMo/Al2O3-Cr having a larger amount of coke deposits. Spent catalysts had a reduction in surface area and pore volume when compared to the fresh materials. Spent NiMo/Al2O3 catalysts had a decrease in average pore diameter (APD) whereas NiMo/Al2O3-Cr maintained the fresh material APD. NiMo/Al2O3-Cr could accommodate higher coke yields without significant variation in its textural properties.