Active phase of dispersed MoS2 catalysts for slurry phase hydrocracking of vacuum residue
Graphical abstract
Introduction
Heavy crude oil is expected to play a decisive role in oil production in the near future as the global supply of crude oil moves to higher levels with the depletion of existing light crude oil [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Heavy oils, however, are less economically viable than conventional crude oils due to the presence of substantial amounts of asphaltenes and metal impurities [11], [12]. Vacuum residue (VR), which is the heaviest and the most contaminated part of the refinery process, contains high-boiling-point hydrocarbons (above 813 K) with heteroatoms of 3–5% sulfur, 0.2–1.0% nitrogen, and 0.1% metals in the form of heteroorganic compounds. Moreover, the VR includes 5–30% asphaltenes, which are rich in polyaromatic compounds and heteroatoms, inducing precipitation or coke formation, depending on the reaction conditions [13], [14], [15], [16].
Conversion of VR to light oil is achieved by pyrolysis, such as coking and visbreaking, or hydrocracking processes [8], [11], [17]. Hydrocracking is a more efficient method of producing light fraction oils from heavy oils than thermal cracking, and thus much effort has been devoted to the conversion of heavy oils in the hydrocracking process [11], [13], [18], [19], [20], [21], [22], [23], [24], [25]. Due to severe coke formation arising in conventional fixed-bed hydrocrackers of heavy oils, moving-bed or ebulliated-bed processes have been used as alternatives for residue hydrocracking [26], [27], [28]. More recently, a slurry-bed hydrocracking process using oil-dispersible catalysts has been demonstrated and has revealed its superiority over conventional VR hydrocrackers, showing full conversion of VR into lighter fuels [3], [29], [30], [31], [32], [33], [34]. This has also been reviewed [2], [9], [35], [36], [37]. Among commercial slurry-phase hydrocrackers [2], [9], [35], [36], [37], Veba Combi Cracking (VCC) [3] has been operating for many years, and ENI Slurry Technology (EST) [11] is currently being demonstrated.
The catalysts for slurry-phase hydrocracking are usually submicrometric particles of metal sulfides of Mo or W, which are applied either ex situ by synthesis of nanoparticles followed by dispersion in oils or in situ by oil-soluble metal precursors mixed with the feed. The in situ catalysts are sulfided by thermally decomposed S species in feedstocks during temperature ramps [3]. The catalyst precursor can be a water-soluble salt, an oil-soluble metal complex, or a finely powdered solid. Sulfided transition metals including Mo, Ni, Co, V, and Ru have been reported to be active for hydroconversion, MoS2 being the most commonly used catalyst [17], [38], [39], [40]. Oil-soluble Mo precursors have been found superior to water-soluble ones, with better dispersion and higher activity [13]. The ligand stabilization method was useful to synthesis monodisperse nano-MoS2 particles, i.e., ex situ catalysts. Yu et al. first introduced nanosized MoS2 synthesis techniques using Mo(CO)6 and trioctylphosphine oxide (TOPO) as a capping agent for spherical, onion-shaped, and tube-shaped MoS2 particles under 5 nm [41].
The catalytic activity of MoS2 is related to the structure of the catalyst. Daage and Chianelli proposed the “rim-edge” model of MoS2 for hydrotreating, in which two kinds of exterior sites are responsible for hydrogenation (HYD) and hydrodesulfurization (HDS) [42]. For example, the rim sites around the exposed basal planes contribute to both HYD and HDS, and the edge sites around the interior layers of MoS2 stacks activate only HDS. Hensen et al. reported that the number of stacking layers of MoS2 can increase the hydrogenation activity by promoting planar adsorption of aromatic rings on the edge sites [43]. Lauritsen et al. recently proposed the “brim-site” model as the active site of HDS, based on observations of MoS2 nanoparticles using scanning tunneling microscopy (STM) [44]. The brim sites belong to Mo edges that are fully sulfided, and are known to exhibit electron-exchange capability like that of typical metal catalysts.
Although the structure and activity of supported MoS2 catalysts have been well established in conventional hydrotreating, a detailed understanding of unsupported MoS2 catalysts in terms of the active phase in hydrocracking of VR is still lacking. This study focuses on the synthesis of MoS2 nanoparticles using trioctylphosphine oxide as a capping agent to obtain different sizes and morphologies of MoS2 and the demonstration of catalytic activity for hydrocracking of VR. Moreover, MoS2 catalysts were also prepared in situ to examine the difference in structure and activity from ex situ MoS2 catalysts. The goals of this work are thus to elucidate the active site of the dispersed MoS2 catalysts with different morphologies in the hydrocracking of VR. Measurements of hydrocracking were carried out at 673 K and 9.5 MPa H2 in an autoclave batch reactor using VR. Emphasis will be placed on the use of EXAFS analysis to verify the structure of the dispersed MoS2.
Section snippets
Materials
A VR as a feed was provided from a refinery in Korea, and the properties of the feedstock are reported in Table 1. Molybdenum hexacarbonyl (Mo(CO)6, Alfa Aesar, 98%) was employed as precursor to prepare MoS2 catalysts. Bulk MoS2 (Alfa Aesar, 99%) was also used as a reference sample.
Preparation of dispersed MoS2 catalysts
Oil-dispersed MoS2 catalysts were prepared in situ during the temperature ramping step of VR HCK tests using a Mo precursor of Mo(CO)6 combined with the feedstock VR [3]. To examine the effect of Mo concentration on
Formation and structure of MoS2 nanoslabs
To investigate the effects of dispersed MoS2 catalyst morphology on VR HCK performance, different morphologies of catalysts were obtained using the ligand stabilization method in ex situ synthesis of the catalyst. For example, synthesis conditions of time and temperature in the nucleation and sulfidation steps were varied as summarized in Table 2. Depending on the preparation conditions, the MoS2 catalysts exhibited different morphologies such as small, medium, and large monoslab MoS2 and
Conclusions
Monolayered MoS2 nanoparticles were successfully synthesized ex situ by a ligand stabilization method using Mo(CO)6 as a Mo precursor and trioctylphosphine oxide (TOPO) as a coordinating agent, where nucleation and sulfidation steps played an important role in the formation of nanosized MoS2 slab with different size distributions ranging from 5.6 nm (MoS2-ES) to 10.5 nm (MoS2-EL). A high sulfidation temperature gave rise to the formation of a four-layer MoS2 stack. In addition, monolayered MoS2
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