Full Length ArticleEffects of the Ni-Mo ratio on olefin selective hydrogenation catalyzed on Ni-Mo-S active sites: A theoretical study by DFT calculation
Graphical abstract
Introduction
Hydrocracking is the currently utilized industrialized process for producing clean fuel and chemical raw materials [1], [2]. The hydrocracking product is complicated mixture with a wide distillation range [3], and the heavy naphtha fraction is an important feedstock for catalytic reforming, of which aromatics are the ideal components [4]. In many cases, the hydrogenation degree is expected to be moderate in the hydrocracking process to avoid excessive saturation of aromatics [5], [6]. However, this treatment will leave behind considerable olefins, which could partially convert to mercaptan with hydrogen sulfide in the reactors and the pipelines [7]. Therefore, the olefin content must be limited within a reasonable level.
An effective approach for controlling the olefin content is to use a small amount of posthydrotreating catalyst in the end of the hydrocracking reactor. Limited by the loading volume, this posthydrotreating catalyst is expected to have high hydrogenation saturation activity. The olefins in heavy naphtha are easy to saturate on the hydrotreating catalyst [8], [9], [10]. However, during the hydrocracking process, the olefins are accompanied by massive monocyclic aromatics and even polycyclic aromatics, which could exhibit severe competitive adsorption on the hydrogenation active sites [8]. Therefore, the active sites of the posthydrogenation catalyst are expected to have high selectivity for olefin adsorption and hydrogenation saturation.
For the active metal of the hydrotreating catalyst, the bimetal Ni-Mo combination performs prominently in terms of hydrogen activation and saturation [11], [12], [13], [14], [15]. Ni atoms, as promoters, preferentially locate on the edge of the MoS2 framework and form Ni-Mo-S active sites. [16], [17], [18], [19], [20]. Several exposed atom pairs, such as the S = S, S-Mo and Ni-Mo on the edges, could dissociate hydrogen molecules into active hydrogen [21], [22]. Meanwhile, the weak interaction between Ni and S favors H2S desorption and the generation of coordinatively unsaturated active sites (CUSs); hence, the catalytic cycle can proceed smoothly on the active sites [23], [24]. The Ni-Mo ratio on the edge could affect the atomic combination and arrangement on the active sites [20]. Facilitated by quantum chemistry calculation, the changes in the catalytic properties that are caused by the Ni-Mo ratio could be studied in detail theoretically, especially the key processes in the complicated reactant system. Hopefully, this study could provide strategies for improving the olefin hydrogenation selectivity by controlling the Ni content in the preparation of catalysts. Table 1
Section snippets
Modeling
The framework of the Ni-Mo-S active nanoclusters is determined by scanning tunneling microscopy (STM) characterization [16], [25], [26], [27] and the edge structures of the Mo-edge and S-edge are based on the following reports [28], [29], [30].To comparatively investigate the catalytic differences among Ni-Mo-S nanoclusters with various Ni-Mo ratios, four Ni-Mo-S models with Ni percentages that range from 0 to 100 (denoted as Ni-X, where X is the number of conjoint Ni atoms on the edges) are
Effects of Ni-Mo ratio on CUS
The CUS is the prime active structure with exposed unoccupied d orbitals that participate in the adsorption of reactants and in the hydrogenation saturation reactions. One important effect of the Ni promoter on the edge is weakening of the adsorption to the H2S so that the metal atoms are easier to expose [26]. etailed information regarding H2S desorption on the S-edges and Mo-edges with various Ni-Mo ratios is presented in Table 2. On Ni-S-0-edge, each S atom bonds with two Mo atoms and one H
Conclusions
The Ni-Mo ratio on the edges of the Ni-Mo-S nanoclusters effects the hydrogenation saturation of heavy naphtha olefin. The Ni atoms that substitute the Mo atoms on the edge weaken the interactions between S and the metal atoms on the edge, which enhances the H2S desorption and the creation of CUS sites via the hydrogenation reaction. Meanwhile, the Ni atoms provide more space and orbitals with suitable morphology for the adsorption of the reactants. The low Ni-Mo ratio on the edge favors the
CRediT authorship contribution statement
Sijia Ding: Writing - original draft. Shujiao Jiang: Methodology. Jifeng Wang: Writing - review & editing. Xinlu Huang: Software. Zhanlin Yang: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financially supported by the Scientific Research Fund of SINOPEC (Grant No. 116017).
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