Effect of citric acid on hydrotreating activity of NiMo catalysts

Abstract

With depleting reserves of sweet crude oil, there is an increasing demand world over to process heavy crudes. Canada’s Athabasca oil sands basin is a rich source of this heavy feedstock derived from bitumen. However, the heavy gas oil feedstock derived from this bitumen has very high quantities of S (4 wt %) and N (0.4 wt %). In order to meet today’s strict environmental regulations, design of hydrotreating catalysts with increased active site density and improved intrinsic catalytic activity per site is critical. This work, in the first phase deals with the modification of the Al2O3 support to increase its acidity and in subsequent phases, citric acid has been used as an additive to enhance the formation of Type II NiMoS sites. Mesostructured alumino-silicate (MAS) materials were synthesized from ZSM-5 nanoclusters and used as catalysts supports for the hydrotreatment of a model compound and real feed stock. The alumino-silicate materials exhibited different acid strengths and textural properties depending on the duration of hydrothermal treatment of the zeolite seeds. The acidity of the MAS materials were found significantly higher than Al-SBA-15 and Al2O3 materials. The activity of NiMo catalysts supported on MAS materials were evaluated using the hydrodesulfurization of dibenzothiophene (DBT). The NiMo catalyst supported on mesoporous alumino silicate obtained after 16 hrs of hydrothermal treatment of the ZSM-5 precursors was found to be the most active in the HDS of DBT. Similarly, hydrotreating tests revealed that the composite made of 5 wt % MAS-16 and 95 wt % γ-Al2O3 was best suited for hydrotreating reactions with real feedstock. This support showed optimum acidity and excellent dispersion of the active species and was selected as the support of choice for all further hydrotreating reactions with real feed stock. In the second stage of this work citric acid (CA) in varying ratios was used to prepare NiMo catalysts supported on a composite of γ-Al2O3 and mesoporous alumino-silicates. Citric acid was found to form a complex with both Ni and Mo simultaneously. However, the promoting effect of citric acid in the hydrotreating was observed mostly due to the formation of a stable nickel-citrate complex. The hydrotreating activity of the synthesized catalysts is evaluated using heavy gas oil (HGO) derived from Athabasca bitumen, in a trickle bed continuous reactor. The Mo loading, Ni loading and the citric acid to nickel molar ratios were optimized on the basis of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activity of HGO. The best performing catalyst for the hydrotreating of heavy gas oil was obtained for a Mo loading of 13 wt %, Ni loading of 7 wt % and CA/Ni molar ratio of 1. The sulfur and nitrogen conversions for the best catalyst were found to be 93 wt % and 74 wt % respectively.