On the Nature of Nitrogen-Containing Carbonaceous Deposits on Coked Fluid Catalytic Cracking Catalysts

The regeneration of FCC catalysts leads to significant NO_x emissions. Hence, the identification of surface deposits and reaction intermediates is important for understanding the mechanisms by which nitrogen-containing species are converted into NO_x or N₂. Characterization of the feed and of carbonaceous deposits on spent FCC catalysts by IR and NMR spectroscopy as well as by (MA)LDI-TOF mass spectrometry showed that polyaromatic pyrrole derivatives (alkylcarbazoles, alkylbenzocarbazoles, alkylindoles) are the main source of nitrogen in the feed of FCC units. Consequently, (poly-)aromatic compounds (m/z = 350-850) such as carbazole and quinoline derivatives are the main nitrogen-containing components in the coke. During oxidative regeneration, these pregraphitic species are converted into smaller aromatic compounds similar to the nitrogen molecules originally present in the feedstock. With increasing temperature, nitrogen-containing coke species accumulate on the catalyst surface and can only be removed by oxidative regeneration at 550-700 °C.