Effects of the Ni-Mo ratio on olefin selective hydrogenation catalyzed on Ni-Mo-S active sites: A theoretical study by DFT calculation

The hydrogenation saturation of olefin in heavy naphtha components is significant for hydrocracking production. The substantial aromatic compounds in the products severely hinder the hydrogenation saturation of olefins due to the strong competitive adsorptions on the active sites. To identify a suitable Ni-Mo ratio for weakening the competitive adsorption, a series of Ni-Mo-S model nanoclusters with various Ni-Mo ratios on the edges are established. Coordinatively unsaturated active sites (CUSs) are created; competitive adsorption data of 3-ethyl-1-hexene (3-E-1-H), 3-ethyl-2-hexene (3-E-2-H), meta-xylene (m-xylene), 2,5-dimethylnaphthalene (2,5-DMA) and anthracene (ANT) are obtained; and the hydrogenation saturation of olefins on these edges is calculated. The results demonstrate that the lowering of the H2S desorption energy by the Ni atoms on the edges is favorable for CUS creation. Ni atoms also modify the structure and the orbital properties of the CUS such that the hydrogenation reactants are easier to adsorb. A moderate Ni-Mo ratio will enhance the adsorption of the olefin, whereas an excessively large Ni-Mo ratio will lead to stronger aromatic competitive adsorptions on both the S-edge and the Mo-edge. Moderate Ni addition could effectively promote hydrogen activation and transportation, which are the essential elementary reactions of the olefin saturation. It is inferred that a lower Ni-Mo ratio for the posthydrotreating catalyst may be favorable for olefin selective saturation.