Nickel-Passivating element selection in FCC process and mechanistic study on the passivation of nickel by lanthanum and phosphorus

Motivated by the dehydrogenation activity of nickel (Ni) and the toxicity of free nickel oxide (NiO) in the fluid catalytic cracking (FCC) catalysts, this study aims to reveal the appropriate elements to in-situ passivate nickel during the cracking reactions of oil by conducting hydrocarbon dehydrogenation tests of Y-zeolite loaded with Ni and passivating elements in a Pyro-probe micro-reactor coupled with GC-TCD and MS. The conditions were chosen as 550 °C with a catalyst-to-hydrocarbon mass ratio of 5:1. Additionally, hydrogen-temperature programmed reduction (H2-TPR) was conducted. In supporting the experimental observation, thermodynamic calculations were also performed to assist the selection of possible Ni-passivating elements in terms of the free Gibbs energy for their reactions with NiO. Lanthanum (La) and phosphorus (P) were found to exert the most effective effects on nickel passivation. The use of twice the quantity of La or P more than Ni, or the combined use of La and P each in equimolar ratio to Ni is able to decrease the dehydrogenation extent by 97%. La can passivate the free NiO species by chemically bonding with NiO which, otherwise, would bond with alumina in zeolite. Instead, P is preferentially coated on the Ni-bearing particles, which leads to the formation of phosphate that is inactive in the dehydrogenation of the model organic compounds when compared to the free NiO.