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Why does NiMo catalyst has more hydrogenation potential than CoMo? What nature of it is enabling it to do so?

Why does Nitrogen molecules in hydrotreating units takes the path of hydrogenation followed by hydrogenolysis instead of direct hydrogenolysis unlike sulphur molecules which gets hydrogenolysis directly?

Between CoMo and NiMo, Which catalyst has more deactivation rate and why?

Sep-2023

Answers

Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

The behaviour of NiMo and CoMo catalysts is strictly related to the chemical interaction between the metals and carrier (Type I and Type II catalysts) in the catalyst. The hydroprocessing reactions takes place in the active sites of the catalyst which is generally accepted to be located in the sulfur vacancies of the on the edges of MoS2 crystallites, these vacancies is significantly increased when the catalyst is promoted with Co or/and Ni. The Co-Mo-S phase is similiar to MoS2 structures with promoter atoms located in the edges of a tretragonal pyramidal geometry at the edge planes of the MoS2 while to Ni promoted catalysts, Ni can be present in three forms after the sulfidation: Ni3S2 crystallites over the support, nickel atoms on the edges of MoS2 structures, and nickel cations at octahedral or tetrahedral sites in the alumina. These different arrangement and interaction between the promoters (Ni and Co) with the MoS2 structures and the support leads to the different behaviour for CoMo and NiMo for hydrotreating reactions, being the CoMo more selective for sulfur removal under relatively low hydrogen consumption while the NiMo catalyst is more selective for hydrogenation and hydrodenitrogenation under higher hydrogen consumption rates.

The reactivity of sulfur compounds to the hydrotreating reactions tend to be higher than the nitrogen compounds once nitrogen in generally concentrated in the cracked and heavier fractions of the crude oil and great part of these nitrogen compounds have six or five pyridinic ring which are unsaturated, for remove nitrogen from these heterocyclic compounds it's necessary to hydrogenate the ring containing the nitrogen before to broke the carbon-nitrogen bond (hydrogenolysis), this is necessary due to the high energy of the carbon-nitrogen bonds in these rings. In the sulfur compounds case, the most part of the sulfur atoms are concentrated in thiophenic molecules that present relatively low energy bonds carbon-sulfur and can directly result in sulfur removal without necessity to saturate the heteroatom ring. By this reason, in hydroprocessing units treating heavier feeds which can concentrated refractory sulfur compounds like dimethyldibenzothiophene, the catalyst blending requires to rely on NiMo bed aiming to promote the hydrogenation function of the catalyst in order to minimize the steric hindrance of the sulfur molecules and improve the reactivity and consequently the efficiency of the hydrotreatment.

Related to the deactivation rate, this depends on the feed quality and severity of the processing unit, but is expected than NiMo catalysts tends to have a higher deactivation rate than the CoMo catalysts once this catalyst (NiMo) is applied to treat heavier and cracked feeds which is notable refractories to hydroprocessing reactions.

Sep-2023