Characterization of aged hydrotreating catalysts. Part I: Coke depositions, study on the chemical nature and environment

In this work various spent Ni(Co)MoP/Al2O3 catalysts from different origins were selected. The coke deposits of the spent catalysts were characterized in order to compare their behaviour in terms of quantity, molecular arrangement and reactivity between a cobalt and a nickel promoted catalyst. A thermo gravimetric analysis (TGA) was employed to evaluate the reactivity of the coke. Raman and NMR 13C spectroscopies were used to characterize the chemical nature of coke. All the characteristics observed are relevant with a three-stage deactivation scheme. In the first stage, the alumina is assumed to be rapidly covered by coke, while the active sites remain protected by their high hydrogenation activity. The coke forms relatively small graphite-like planes (10 Å). In the second stage, the nickel segregated from MoS2 particles during the aging and the graphite-like crystallite extends significantly (to 16 Å). In the final step of the coke evolution, it can be assumed that some organization occurs in the molecular structure of coke. The coke particles previously observed begin to stack, trapping the nickel segregated inside the coke matrix. Moreover, the coke deposit characterized on the spent CoMoP/Al2O3 is equally distributed between the active phase and the carrier whereas the coke seems to be preferentially deposited over the alumina surface on the spent NiMoP/Al2O3 catalysts. It suggests that the active sites on the CoMoP/Al2O3 catalysts are easily coked rather than the NiMoP/Al2O3 ones. It is also observed that the coke evolution is faster on the CoMoP/Al2O3 catalysts. Those differences, representing an important issue require solutions to prevent the coke deposit.

Graphical abstractFirstly, the coke forms graphite-like planes relatively small (10 Å). Then, the nickel segregates from MoS2 particles and the graphite-like crystallite extends significantly (to 16 Å). Finally, in the last step of the coke evolution, some organization occurs in the molecular structure of coke. The coke particles previously observed begin to stack, trapping the nickel segregated inside the coke matrix.Figure optionsDownload full-size imageDownload as PowerPoint slide