Elsevier

Fuel

Volume 277, 1 October 2020, 118136
Fuel

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

https://doi.org/10.1016/j.fuel.2020.118136Get rights and content

Highlights

  • Build and analyzed the thermodynamic property of Ni-Mo-S active sites with various Ni-Mo atomic ratio.

  • Analyzed the orbital property of Ni-Mo-S active sites with various Ni-Mo atomic ratio.

  • Analyzed the competitive adsorption between olefins and aromatics on Ni-Mo-S active center.

  • Analyzed the hydrogenation saturation process of olefin on Ni-Mo-S active center.

Abstract

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.

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).

References (43)

  • S. Ding et al.

    Oxygen effects on the structure and hydrogenation activity of the MoS2 active site: A mechanism study by DFT calculation

    Fuel

    (2017)
  • S. Ding et al.

    Catalytic characteristics of active corner sites in CoMoS nanostructure hydrodesulfurization-A mechanism study based on DFT calculations

    J Catal

    (2017)
  • S. Ding et al.

    Niobium modification effects on hydrodesulfurization of 4,6-DMDBT catalyzed on Ni-Mo-S active sites: A combination of experiments and theoretical study

    Fuel

    (2019)
  • S. Ding et al.

    Substituent effects of 4,6-DMDBT on direct hydrodesulfurization routes catalyzed by Ni-Mo-S active nanocluster-A theoretical study

    Catal Today

    (2018)
  • T. Weber et al.

    A density functional theory study of the hydrodesulfurization reaction of dibenzothiophene to biphenyl on a single-layer NiMoS cluster

    Catal Today

    (2008)
  • Y. Zhu et al.

    Location of Co and Ni promoter atoms in multi-layer MoS2 nanocrystals for hydrotreating catalysis

    Catal Today

    (2016)
  • J.V. Lauritsen et al.

    Location and coordination of promoter atoms in Co- and Ni-promoted MoS 2 -based hydrotreating catalysts

    J Catal

    (2007)
  • L.S. Byskov et al.

    DFT calculations of unpromoted and promoted MoS2-based hydrodesulfurization catalysts

    J Catal

    (1999)
  • A.D. Gandubert et al.

    Optimal promoter edge decoration of CoMoS catalysts: A combined theoretical and experimental study

    Catal Today

    (2008)
  • E. Krebs et al.

    Mixed sites and promoter segregation: A DFT study of the manifestation of Le Chatelier's principle for the Co(Ni)MoS active phase in reaction conditions

    Catal Today

    (2008)
  • C. Dupont et al.

    Hydrodeoxygenation pathways catalyzed by MoS2 and NiMoS active phases: A DFT study

    J Catal

    (2011)
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