CoNiMo/Al2O3 sulfide catalysts for dibenzothiophene hydrodesulfurization: Effect of the addition of small amounts of nickel

https://doi.org/10.1016/j.micromeso.2020.110574Get rights and content

Highlights

  • The influence of incorporating low amounts of nickel into cobalt-promoted MoS2 catalysts was herein evaluated.

  • HDS optimum efficiency is reached for the CoNi0.05Mo/Al2O3 catalysts.

  • The CoNi0.1Mo/Al2O3 catalyst with the richest Ni content shows the lowest HDS activity.

  • TOF values allowed us to analyze the incorporation of nickel into Co-promoted MoS2 slabs.

Abstract

Trimetallic CoNiMo hydrodesulfurization (HDS) catalysts were herein synthesized on Al2O3 support through an incipient wetness co-impregnation procedure using ammonium heptamolybdate and cobalt (and nickel) acetate as precursors. CoNiMo catalysts were obtained by adding in a small proportion of nickel with a molar amount representing between 1 and 10% of the total Co atomic loading. This addition of low amounts of nickel allows keeping the (Co + Ni)/(Co + Ni + Mo) molar ratio close to the optimum value of 0.3. The as-formed catalysts were labeled as CoNixMo/Al2O3 with x the relative stoichiometry of Ni compared to Co. Catalysts were then characterized by N2 adsorption-desorption isotherms, X-ray diffraction, ICP-OES spectroscopy, Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Catalysts were tested in the HDS of dibenzothiophene (DBT). Results show a significant impact of adding small amounts of nickel to alumina-supported Co-promoted MoS2 on the final HDS activity. While up to CoNi0.03Mo/Al2O3, the resulting HDS activity is hardly influenced by the addition of nickel, a strong beneficial effect is observed for CoNi0.05Mo/Al2O3 with activity reaching a value of 7.2 × 10−7 mol.gcat−1.s−1. Increasing further, the amount of nickel abruptly decreased the DBT HDS activity. Interpretation of these results is then provided considering the additional role of nickel for improving the intrinsic activity of the HDS sites.

Introduction

In recent years, environmental legislation to improve fuel quality has become more and more strict [1,2]. The first regulation enforcement was issued in the United States in 1993 to limit the sulfur oxides emission tolerance of transportation vehicles through a reduction from 0.2 to 0.5 wt % to 500 ppm in sulfur allowed in diesel fuels [3]. Since then, the Environmental Protection Agency (EPA) has decreased twice the maximum sulfur amount authorized in diesel and gasoline. For instance, the current specification of the sulfur amount allowed for on-road diesel fuel is 15 ppm [4,5]. This specification has been extended to non-road engine diesel fuels in 2010. This has triggered an increasing request for high-quality diesel fuels in developed countries. In this respect, in the US market, demand for high-quality diesel has increased from 3.16 Mb/d (million barrels per day) in 2005 to 3.31 Mb/d in 2010. Moreover, light cycle oil (LCO) cuts from fluid catalytic cracking (FCC) units are more and more often used in the diesel pool [6]. This higher LCO proportion increases the difficulty for producing clean diesel products since LCO feeds contain a higher amount of highly refractory sulfur compounds such as alkyldibenzothiophenes.

Nowadays, traditional hydrodesulfurization catalysts cannot produce ultra-low sulfur diesel (ULSD) as required by the latest regulations [7] except if using high temperatures and pressure conditions harmful for catalytic life cycle longevity. To improve deep hydrodesulfurization ability, it is necessary to design new catalysts with enhanced desulfurization activity able to efficiently remove sulfur from highly refractory compounds. New more efficient hydrodesulfurization processes can be obtained through the implementation of new HDS technologies [8,9] or through new catalytic systems using either new supports [[10], [11], [12], [13], [14], [15], [16], [17]] or new active phases [[18], [19], [20], [21], [22], [23], [24], [25], [26], [27]], the latter approach being the most promising way to enhance the catalytic HDS efficiency [28]. Traditionally, bimetallic HDS catalysts are composed of Mo(W)S2-based solids supported on Al2O3 and promoted by cobalt or nickel. Bimetallic HDS catalysts are susceptible to thermal, chemical, or mechanical degradation and metal or coke poisoning, which often leads to inefficient ways to treat feedstocks [[29], [30], [31]]. To increase the versatility of catalysts to work in various harmful conditions, investigations have tried to develop trimetallic catalysts such as NiMoW, CoNiW, and CoNiMo. In this respect, some studies have been performed considering CoNiMo catalysts for hydrodesulfurization applications [[32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. However, contradictory results were obtained about the interest of adding a second promoter for increasing HDS activity. While some contributions found that CoNiMo catalysts are less active than traditional bimetallic systems for the HDS of thiophene [37,38] and of 4,6-DMDBT [41], other studies have reported higher HDS activity for the CoNiMo combination in the HDS of thiophene [39], dibenzothiophene [35,40], vacuum gas oil [34] or heavy gasoil [33]. However, up to now, a rational interpretation of the positive (or negative) role devoted to the addition of a second promoter to MoS2-based catalysts has not been proposed yet.

Moreover, the addition of nickel was systematically added in replacement of cobalt in very high amounts with HDS maxima observed at very different Ni/Co atomic ratios (from 0.66 for [34] to 1.54 for [35]). This situation is also worsened by the absence of direct experimental proof showing if separate CoMoS and NiMoS phases are separately formed or if new NiCoMoS sites are created, at least partly.

In this respect, one should also consider the affinity of Ni or Co for the two types of MoS2 edge sites, the so-called S- and M-edge sites. DFT calculations have indeed demonstrated that Co and Ni act differently when added as promoter sites of MoS2 edge planes. Under typical HDS conditions, Co prefers the S-edge and is only present partially on the M-edge resulting in Co coverages of 100% on the S-edge and 50% on the M-edge. On the opposite, Ni is present on both types of edges with a strong tendency to be incorporated first and only on M-edge sites at low loadings [[42], [43], [44], [45], [46]].

The objective of the present study was, therefore, to evaluate how the supplementary incorporation of low amounts of nickel in plus of cobalt, on available M-edge sites of MoS2 slabs as proposed by DFT calculations, can influence the textural, structural and catalytic properties of the resulting CoNiMo catalysts. Nickel promoter atoms have then been added in a small proportion in plus of cobalt atoms to form CoNiMo catalysts with precise control of the amount and nature of the promoters. The resulting trimetallic catalysts have then been fully characterized before being tested in the HDS of dibenzothiophene in order to determine the exact influence of the addition of low amounts of nickel to Co-promoted MoS2 on the final HDS catalytic response.

Section snippets

Materials

Commercial Al2O3 was purchased from Sasol Germany (SBET = 200 m2/g; Vp = 0.85 cm3/g; average pore diameter: 90 Å). Dibenzothiophene (98%), decahydronaphthalene (cis + trans) (98%), ammonium heptamolybdate tetrahydrate, (NH4)6Mo7O24.4H2O, cobalt acetate tetrahydrate (≥98%) and nickel acetate tetrahydrate (98%) were purchased from Sigma-Aldrich.

Catalysts synthesis

Al2O3-supported catalysts were prepared using an incipient wetness co-impregnation technique. One bimetallic CoMo catalyst supported on alumina and

N2 physisorption

Fig. 1A reports the N2 adsorption-desorption isotherms of the CoMo/Al2O3 reference catalyst and of the CoNixMo/Al2O3 solids. The isotherms present almost identical type IV profiles characteristic of a mesoporous distribution. Hysteresis loops are well-defined and do not differ from one sample to the next one exhibiting a type H1 shape corresponding either to particles crossed by nearly cylindrical channels or made of aggregates or agglomerates of spheroidal particles [48,49]. Table 1 reports

Conclusion

In the present study, the influence of incorporating low amounts of nickel into cobalt-promoted MoS2 catalysts was herein evaluated. Nickel was incorporated in relative proportions of 1, 3, 5, or 10 atomic percentages of the cobalt initially present. The resulting catalysts were characterized at both the oxide and sulfide states showing that optimum conditions of preparation were reached when about 5% of nickel was added in plus of cobalt. Before and after this optimum, substantial MoO3

CRediT authorship contribution statement

Juan A. Medina Cervantes: Investigation, writing. R. Huirache-Acuña: Conceptualization, Supervision, writing. J.N. Díaz de León: Resources. S. Fuentes Moyado: Funding acquisition. F. Paraguay-Delgado: Investigation. G. Berhault: Formal analysis, and writing. G. Alonso-Núñez: Supervision, writing, Project administration.

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.

Acknowledgments

We appreciate the support of: SENER-PEMEX through the project 117373, CONACYT project 182191 and CIC-UMSNH 2020. We also appreciate the technical support provided by F. Ruiz, M. Estrada, D. Dominguez, J.A. Díaz, I. Gradilla, E. Aparicio at CNyN-UNAM Mexico. Thanks to E. Guerrero L and W. Antunez for their technical help at NaNoTech –CIMAV Mexico.

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