What is the role of nitrogen compounds in crude oil in the context of catalyst deactivation during refining?

Nitrogen compounds in crude oil play a significant role in catalyst deactivation during refining processes. Catalyst deactivation refers to the loss of activity and selectivity of catalysts used in various refinery units, such as hydrotreating, hydrocracking, and catalytic cracking. Nitrogen compounds, including basic nitrogen compounds (like pyridines and quinolines) and non-basic nitrogen compounds (like pyrroles and indoles), contribute to catalyst deactivation through several mechanisms. One primary mechanism is by acting as a poison. Basic nitrogen compounds have a lone pair of electrons, making them Lewis bases. These bases can strongly adsorb onto the acidic sites of the catalyst, particularly on catalysts containing metals like nickel, molybdenum, cobalt, or platinum supported on alumina or silica-alumina. This adsorption blocks the active sites where desired reactions occur, thereby reducing the catalyst's activity. This is similar to how a key (nitrogen compound) gets stuck in a lock (active site), preventing the correct key (reactant) from working. Non-basic nitrogen compounds can also undergo reactions on the catalyst surface, forming coke precursors. Coke is a carbonaceous deposit that accumulates on the catalyst surface, physically blocking active sites and pores, thereby reducing access for reactants. Nitrogen-containing coke is often more difficult to remove than coke formed solely from hydrocarbons. Certain complex nitrogen compounds, such as porphyrins (which contain a metal atom coordinated to a nitrogen-containing ring), can also deposit metals onto the catalyst surface, leading to pore blockage and active site coverage. The level of catalyst deactivation caused by nitrogen compounds depends on several factors, including the concentration and type of nitrogen compounds in the crude oil, the operating conditions (temperature, pressure, space velocity), and the type of catalyst used. Therefore, hydrotreating processes are often employed upstream to remove nitrogen compounds (denitrification) and other impurities before the crude oil is sent to more sensitive catalytic units. This pretreatment extends the life of the more expensive and sensitive catalysts.