Pyridine hydrodenitrogenation (HDN) is more difficult than thiophene hydrodesulfurization (HDS), and there is a thermodynamic limitation on the first step of the HDN reaction mechanism which occurs, for example, at 5 to 11 bars, at temperatures above about 350°C. Pyridine inhibits the HDS reaction as previously reported, but sulfur compounds have a dual effect on HDN. At low temperatures, thiophene inhibits the reaction by competing with pyridine for hydrogenation sites on the catalyst. This retards the hydrogenation of pyridine to piperidine, reducing the overall reaction rate. At high temperatures the dominant effect is interaction of hydrogen sulfide, an HDS reaction product, with the catalyst to improve its hydrogenolysis (hydrocracking) activity. This increases the rate of piperidine hydrogenolysis, which is rate determining at the latter conditions, and enhances the overall rate of HDN.
CHARLES N. SATT'ERFIELD
SCOPEThe hydroprocessing of fuels containing relatively large amounts of organonitmgen compounds will become increasingly important in the future in the upgrading of synthetic fuels from oil shale and coal or processing of lower grades of crude petroleum. Organosulfur compounds are present, and the inhibiting effeet of organonitrogen compounds on catalytic hydrodesulfurization is established. However, very little has been published on the effect of organosulfur compounds on hydrodenitrogenation. These two groups of effects were explored by using as model compounds thiophene and PFidine which represent some of the less reactive organosulfur and organonitrogen compounds, respectively.Studies were made with a flow microreactor at temperatures of 200" to 500°C, 4.4 and 11.2 bars pressure, on commercial catalysts consisting of CoMo/Al,03, NiMo/ A1203, NiW/AI2O3 and NiW/SiO2-AlzO3.
CONCUSIONS A N D SIGNIFICANCEPyridine, a basic nitrogen compound, severely inhibits the hydrodesulfurization of thiophene on sulfided cobaltmolybdate type of catalysts, in agreement with previous studies. The pattern of this inhibition suggests there are two types of HDS sites involved. The first are postulated to be very active for HDS but very sensitive to nitrogen bases. Sufficient quantities of these bases will completely block these sites and render them inactive for HDS. The second type of sites are much less active for HDS, but they are also less susceptible to pyridine poisoning. These latter sites are responsible for HDS activity after the first type of sites have been blocked.The hydrodenitrogenation of pyridine is more difficult than thioohene HDS, and there appears to be a thermodynamic limitation to the first step in the reaction mechanism. Sulfur compounds have a twofold effect on HDN. At low temperatures they are postulated to compete with pyridine for hydrogenation sites on the catalyst, thereby inhibiting the rate of the hydrogenation step in the pyridine decomposition mechanism. At high temperatures the HDS reaction product hydrogen sulfide interacts with the catalyst to improve the hydrogenolysis activity. Thi...
