Impact of Metal Content on the Deactivation of a Bifunctional Hydrocracking Catalyst

International audienceThe catalytic performance of a hydrocracking bifunctional catalyst and consequently its activity loss depend on the balance between the metal (M) and acid (A) functions. Commonly, the metal function is reputed to stabilize catalyst activity and should therefore alleviate deactivation. In order to test this hypothesis, this study aimed to assess the impact of metal oxide (MoO3) content on the deactivation of a vacuum gasoil hydrocracking catalyst (nickel-molybdenum sulfide dispersed on a carrier containing USY zeolite). To do so, two bifunctional catalysts with different metal oxide contents were submitted under specific operating conditions promoting catalyst deactivation, i.e., high temperature (T = 418 °C) and high space velocity (LHSV = 3 h–1). Experiments were carried out in a pilot unit with a hydrotreated VGO feedstock containing 120 ppmw of organic nitrogen. The loss in activity was measured by following the decrease of both the VGO conversion (370 °C+) and the apparent kinetic constants for the main hydrocracking reactions (cracking, hydrodenitrogenation, and aromatics hydrogenation) with time on stream. The aged catalysts, sampled from three different reactor locations, were characterized by elemental and textural analyses and thermogravimetry to investigate the quantity and nature of the coke deposits. To determine the residual activity of the hydrogenation and acid catalyst functions, catalytic tests with different model compounds (toluene and n-heptane) were carried out. It was observed that in the hydrocracking of feedstocks with high nitrogen concentration, the decrease of the metal content causes a substantial increase of the nitrogen compounds’ concentration in the catalytic bed. This increase significantly reduces the available acid sites and hence the hydrocracking activity. However, at the same time, the M/A ratio is modified in favor of the metal function. The better balance of the catalyst with low metal loading results in a lower coke deposition and consequently a lower deactivation rate than with the catalyst with higher metal content