Characterization of spent catalysts from hydrotreating of different feedstocks in batch reactor

In this work, a commercial NiMoP/γ-Al2O3 catalyst was tested in the hydrotreating of different refinery feedstocks (light cycle oil, heavy cycle oil, light coker gas oil, and straight-run gas oil) using the same reaction conditions in a batch reactor (T = 350 °C, P = 50 kg/cm2, stirring rate = 750 rpm, H2/oil ratio = 32 Nm3/m3, reaction time = 6 h) in order to analyze its deactivation due to coke formation. Different characterization techniques were carried out, such as N2 physisorption, elemental and sulfur analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray fluorescence (XRF), metal contents, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM). The outcomes revealed that of the four hydrotreated refractory feedstocks, light coker gas oil led to a higher carbon deposition on the catalyst, decreasing its specific surface area drastically and thus increasing its degree of deactivation compared with the other spent catalysts, as follows: S-LCGO (22.6 %) > S-LCO (17.3 %) > S-HCO (13.9 %) > S-SRGO (7.2 %), this correlates linearly with the content of aromatics in feedstock (vol.%), like so: LCGO (98.9 %) > LCO (91.7 %) > HCO (85.5 %) > SRGO (33.2 %), indicating that a higher content of aromatics in feedstock (coke precursor) leads to a higher degree of deactivation. The surface carbon deposited on the catalysts corresponds to a "soft" or "young" type of coke according to the results obtained by TGA and XRD due to a coke dehydrogenation process, which was deposited on the active sites and the catalyst support matrix. On the other hand, by means of compositional analysis, XRF, and XPS, it was possible to identify that the coke formed has a structural chemical composition of CwHxNySz, whereby the H/C atomic ratio confirmed its soft nature due to the hydrogenation reactions that occur. Even so, the S-LCGO showed the highest degree of coke accumulation due to its low H/C value, observing the same trend: S-LCGO (0.76) < S-HCO (1.18) < S-LCO (1.24) < S-SRGO (1.34). Finally, XPS, NMR, and STEM studies have revealed that coke deposits are composed of poly-aromatic rings with high electron density, containing aromatic, paraffinic, and pregraphitic carbon.