Bulk hydrotreating MonW12-nS2 catalysts based on SiMonW12-n heteropolyacids prepared by alumina elimination method
Introduction
Due to stricter environmental standards, the oil industry is under pressure to provide clean fuels. Therefore, in recent years, special attention has been focused on improving the activity and stability of hydrotreating catalysts for petroleum fractions. Mixed NiMoW sulfides based on alumina [[1], [2], [3], [4]], silica [5] or mesostructured silica [6] demonstrated excellent catalytic activity in hydrodesulfurization (HDS) of sulfur compounds. The superiority of mixed NiMoW systems over traditional bimetallic Ni(Co)Mo(W) catalysts was attributed to the synergistic effect when using Ni, Mo and W. With the density functional theory (DFT) calculations, it was found that NiMoWS catalysts have a more optimal metal-sulfur bond energy compared to NiMoS and NiWS catalysts [2].
The use of Ni(Co)MoWS systems, in which both tungsten and molybdenum atoms are simultaneously present, allowed increasing HDS and HYD activities [[1], [2], [3],[7], [8], [9], [10]]. Thomazeau et al. [2] reported that the formation of mixed MoWS2 crystallites is possible only from a precursor which contains both closely related metals in the structure at once. The structure of the mixed active phase is greatly influenced by sulfidation conditions. Previously, for unpromoted catalysts based on mixed H4SiMo3W9O40 heteropolyacid (HPA), we found that mixed MoWS2 active species with a core-shell structure, in which smaller islands of Mo were surrounded by W atoms, were formed in the gas-phase sulfidation, while in the liquid-phase sulfidation a structure with random distribution of molybdenum and tungsten atoms was formed, as visualized by HAADF [11]. Moreover, it was found that the use of a mixture of two monometallic H4SiMo12O40 and H4SiW12O40 HPAs led to the preferential formation of corresponding monometallic MoS2 and WS2 particles. In the co-hydrotreatment of DBT and naphthalene, catalysts with an ordered core-shell structure of the MoWS2 active phase had highest rate constants for both HDS and HYD reactions.
In addition, the possibility of using bulk Mo(W) sulfide catalysts, which do not contain a support, in hydrotreatment processes on stationary catalyst beds is also being investigated. The concentration of active phase in these catalysts can reach 80–100 %. With the same composition of the active phase, the activity of bulk catalysts in hydroprocessing can be 1.5–1.7 times higher than that of their supported analogs. Thus, industrial bulk NiMoW NEBULA catalysts have higher catalytic activity compared to traditional alumina supported catalysts. The increase in catalytic activity can be explained by the formation of highly active trimetallic NiMoW sulfides [12]. In the middle of last year, the ExxonMobil jointly with Albemarle proposed a new catalyst, Celestia™, the successor of the NEBULA catalyst. Industrial implementation makes promising the development of new catalytic systems based on bulk mixed NiMoW sulfides.
Recently, due to the development of technologies for deep hydroconversion of heavy oil residues in three-phase suspension-type reactors (slurry-reactors) [[13], [14], [15], [16]], in which nanoscale (Ni)Mo(W)S2 particles are formed in situ [[14], [15], [16]], the interest to bulk catalysts based on transition metal sulfides, has been increasing.
Various methods for preparing bulk catalysts such as comaceration [17], homogeneous sulfide precipitation [18], thiosalt decomposition [19], hydrothermal [20,21] and solvothermal [22,23] syntheses, as well as the method of fluoric acid (HF) etching of the substrate of supported catalysts [24,25] have been described. Previously, we reported that unsupported catalysts synthesized via etching of alumina support, exhibited higher catalytic properties compared to those prepared by other methods [26], due to high dispersion of active sulfide particles and good accessibility to active sites.
Summarizing the above, it seems appropriate to combine the method of HF etching of a support and the use of mixed MoW oxide precursors to produce mixed bulk catalysts. That will allow purposefully creating catalysts with a given composition of mixed MoWS2 particles. In the present work, mixed MoWS bulk catalysts were synthesized by acid etching of alumina support from supported sulfide catalysts based on mixed H4SiMo1W11O40 and H4SiMo3W9O40 HPAs. Catalysts prepared from mixture of two monometallic H4SiMo12O40 and H4SiW12O40 HPAs with the same Mo/W molar ratio as in corresponding mixed HPAs were used as reference samples. The effects of catalyst composition on catalytic performance were studied in the hydrotreating reactions of DBT and naphthalene. Prepared bulk catalysts were characterized by methods such as nitrogen physisorption, extended X-ray absorption fine stricture spectroscopy (EXAFS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and powder X-ray diffraction (XRD). Moreover, a comparison was made with their alumina supported counterparts in order to reveal the effect of the support.
Section snippets
Catalyst preparation
A series of bulk MonW12-nS2 hydrotreating catalysts was synthesized by HF etching of alumina from MonW12-n/Al2O3 catalysts. First, supported samples were prepared by the incipient wetness method via impregnation of γ-Al2O3 extrudates with aqueous solutions of H4SiMonW12-nO40 HPAs [4,[9], [10], [11]]. These mixed HPAs are derived from the Keggin-type polyoxometallate H4SiW12O40, consisting of a regular SiO4 tetrahedron surrounded by 12 WO6 octaedra, which are connected by shared edges to form
Textural properties of catalysts
The textural properties of the prepared samples are summarized in Table 1. Sulfided alumina-based precursors displayed surface areas in the range of 164 to 218 m2/g and pore volumes around 0.53 cm3/g. The alumina removal resulted in a decrease of the surface area to 3−14 m2/g and pore volume to 0.01−0.04 cm3/g. These significant changes in the textural properties are related to the total removal of the porous support. X-ray fluorescence and XPS analysis confirmed the absence of aluminum and
Discussion
In order to gain a better understanding of the synergetic effect between molybdenum and tungsten, the rate constants in HDS and HYD were calculated by additive way based on the results obtained over monometallic MoS2 and WS2 references catalysts. Experimental values of the rate constants in DBT HDS and naphthalene HYD over Mo1W11S2 and Mo3W9S2 surpass the theoretical ones by 1.4 and 2.7 times, respectively. Catalytic activities of bimetallic reference samples are also higher than predicted,
Conclusions
We found that the HF etching of the alumina in MonW12-n/Al2O3 sulfided catalysts led to successful removal of the support. Further interaction between the particles of the etched sulfide active phase after resulfidation of bulk solids resulted to an increase in the average particle length and stacking number, which is independently confirmed by HRTEM, XRD and EXAFS data. The metal sulfidation degree was also raised especially for tungsten, due to removal of non sulfided species. The presence of
Declaration of competing interest
None.
CRediT authorship contribution statement
A. Kokliukhin: Investigation, Writing - original draft. M. Nikulshina: Validation, Writing - original draft. A. Mozhaev: Investigation. C. Lancelot: Investigation, Writing - review & editing. C. Lamonier: Conceptualization. N. Nuns: Investigation. P. Blanchard: Validation. A. Bugaev: Investigation, Methodology. P. Nikulshin: Conceptualization, Writing - review & editing.
Acknowledgments
Authors thank Russian Science Foundation for financial support of the investigation by Grant No. 17-73-20386. Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Nord – Pas de Calais and FEDER are acknowledged for supporting and funding partially this work (TEM and ToF-SIMS).
References (52)
- et al.
J. Mol. Catal. A
(2008) - et al.
Appl. Catal. A
(2007) - et al.
Appl. Catal. B
(2018) - et al.
J. Nat. Gas Chem.
(2010) - et al.
Appl. Catal. B
(2009) - et al.
Fuel
(2013) - et al.
Fuel
(2013) - et al.
Catal. Today
(2019) - et al.
Catal. Today
(2010) - et al.
J. Catal.
(2013)