Al2O3 hydrotreating catalysts prepared from single Co2Mo10-heteropolyacid at extremely high metal loading

Catalysis Communications 127 (2019) 51–57

Contents lists available at ScienceDirect

Catalysis Communications journal homepage: www.elsevier.com/locate/catcom

Short communication

CoMo/Al2O3 hydrotreating catalysts prepared from single Co2Mo10heteropolyacid at extremely high metal loading M. Nikulshinaa, A. Kokliukhina,b, A. Mozhaeva,b, P. Nikulshina,b, a b

T

⁎

Samara State Technical University, 244, Molodogvardeyskaya st., Samara 443100, Russia All-Russia Research Institute of Oil Refining, 6/1 Aviamotornaya st., Moscow 111116, Russia

ARTICLE INFO

ABSTRACT

Keywords: Hydrodesulfurization Hydrogenation Co2Mo10HPA CoMoS Metal loading

A series of alumina supported CoMo catalysts with different molybdenum surface density was prepared using incipient wet impregnation method and applying H6[Co2Mo10O38H4] (Co2Mo10HPA) as a single oxidic precursor. Dried and sulfided catalysts were characterized by a variety of techniques, such as N2 adsorption, microRaman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). Catalytic properties have been examined in co-hydrotreating of dibenzothiophene and naphthalene using a fixed bed high-pressure flow reactor. It was found that using Co2Mo10HPA as a single precursor enables more than the double increase of maximum metal loading compared to conventional compounds. This also contributed to a significant increase in catalytic activity in hydrotreating.

1. Introduction

difficulty in impregnation of several salts is caused by anionic nature of base elements which require acidic conditions to promote adsorption on aluminum oxide, while impregnation with cobalt or nickel salts is preferable in the basic solution. Heteropolycompounds (HPCs) of Dawson, Anderson [9–11], Keggin [12] and Strandberg [13] structures are attractive alternatives to conventional precursors. The use of HPCs as the precursor of CoMo catalysts led to higher activity than that of the catalysts prepared from ammonium heptamolybdate (AHM) and cobalt salts produced through homogeneous distribution of active phase particles on the support surface [8,14]. Moreover, HPCs of Anderson type contain both Co and Mo atoms in their molecular structure [8,15]. Cobalt plays the role of promoter in the formation of the catalyst's active phase, and its origin is extremely important for catalytic activity. Recently Huang et al. [16] have proposed a heteropoly-compound hydrothermal deposition method for preparing a Co4Mo12/Al2O3 HDS catalyst via Mo12O30(μ2-OH)10H2[Co(H2O)3]4 HPC. Intimate contact between Co and Mo atoms and the high Co/Mo ratio in Co4Mo12 resulted in a high concentration of active CoMoS phase, as well as dibenzothiophene (DBT) HDS performance. Cabello et al. [17] proposed to use [Co2Mo10O38H4]6− heteropolyanion (Co2Mo10HPA) as an oxide precursor that has some advantages over conventional precursors, such as suppression of calcination steps during the activation process. Co2Mo10HPA used with a high Co/Mo ratio enhanced promoting effect of cobalt during thiophene hydrodesulfurization (HDS) [15,17,18]. However, HPAs containing the optimal Co/Mo ratio = 0.5 have not

Heavy oils with a high content of undesirable components, such as S-, N-containing compounds, and polyaromatics, being involved in feedstock make the hydroprocessing task more complicated. For this reason, numerous studies aimed at designing improved hydrotreating catalysts are ongoing [1]. However, most of novel catalysts are not commercial due to their sophisticated synthesis and high cost [2,3]. Increasing the active metal content is a simple and reliable way to improve catalytic activity of catalysts. The molybdenum content varies from 8 to 15% (by weight) of Mo in industrial Co(Ni)Mo catalysts [4]. Several studies have been focused on investigating the influence of metal loading on catalytic activity in hydroprocessing. Platanitis et al. [5] found out that the relationship between catalytic activity in thiophene hydrodesulfurization (HDS) and the molybdenum content in the catalyst has a maximum at 3.5–4.0 Mo at/nm2. Further Mo loading increasing led to decrease in MoS2 dispersion and, as a result, catalytic activity reduction [4,6]. Moreover, metal content increase in the catalyst may cause pore blocking and makes the sample more prone to sintering of the active phase species [7]. The amount of deposited metals and their distribution on the carrier surface primarily depend on the nature of oxide precursors. This factor should be considered at the top place since the first difficulties can arise from the stage of catalyst synthesis. Using most of the molybdenum salts as oxidic precursors is limited due to their poor solubility [4]. Also,

⁎

Corresponding author at: Samara State Technical University, 244, Molodogvardeyskaya st., Samara 443100, Russia. E-mail address: [email protected] (P. Nikulshin).

https://doi.org/10.1016/j.catcom.2019.05.003 Received 1 March 2019; Received in revised form 15 April 2019; Accepted 5 May 2019 Available online 07 May 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.

Catalysis Communications 127 (2019) 51–57

M. Nikulshina, et al.

been discovered yet. The prospects of Co2Mo10HPA as an initial precursor for the preparation of highly active HDS catalysts were also described previously [15,18–21]. High solubility in water allows using the Co2Mo10HPA precursor for the preparation of high-percentage metal sulfide catalysts. The objective of this study was to determine a maximum degree of metal loading on the surface of synthesized alumina-based catalysts using single Co2Mo10HPA. Dependence of the CoMoS active phase species characteristics and catalytic activity in HDS and hydrogenation (HYD) reactions on the surface density of molybdenum, varied from 2 to 17 at Mo/nm2, has been investigated.

number of Mo atoms at the edge surface by the total number of Mo atoms using crystallite sizes determined from HRTEM:

D=

Moe + Moc = Mo T

6n i

6

i = 1.. t i = 1.. t

3ni 2

3n i + 1

(1)

where ni is the number of Mo atoms along one side of the MoS2 slab, as determined by its average slab length L (nm), t is the total number of slabs and MoE, MoC and MoT are numbers of Mo atoms located on the edges, corners and the total number, respectively, in an average MoS2 crystallite (evaluated from HRTEM). An average number of slabs per stack (N ) was calculated using the following equation:

2. Experimental

ni Ni

2.1. Support and catalyst preparation

N =

Decamolybdodicobaltate heteropolyacid H6[Co2Mo10O38H4] (Cо2Mo10HPA) was synthesized according to the procedure described in [22]. d-CoMo/Al2O3 catalysts with the surface density of molybdenum d(Mo) from 2 to 17 at/nm2 were prepared by the incipient wetness technique via impregnation of γ-Al2O3 extrudates with an aqueous solutions containing the required Cо2Mo10HPA amounts. After the impregnation, the catalysts were dried at room temperature for 12 h, at 60 °C for 2 h, at 80 °C for 2 h, and at 110 °C for 5 h without calcination. The content of metals in oxide samples was monitored by X-ray fluorescence spectroscopy using EDX800HS analyzer. For further studies of physicochemical properties of the prepared catalysts, the oxidic samples were sulfided in a fixed-bed reactor at 400 °C for 4 h in a flow of 10 vol% of H2S in H2 under atmospheric pressure. The removal of the catalysts and their further operation were carried out in a glove box in an inert Ar medium.

i = 1.. t

n

(2)

i = 1.. t

where ni is the number of stacks with Ni layers. 2.2.4. X-ray photoelectron spectroscopy (XPS) The X-ray photoelectron spectra (XPS) of the catalysts were recorded with a Kratos Axis Ultra DLD apparatus provided with a hemispherical electron analyzer applying AlKaa (hnn = 1486.6 eV, 150 W) X-ray source. The samples were degassed at the t = 150 °C and P = 10−5 mbar and then transferred to the ion-pumped analysis chamber, where residual pressure during the data acquisition process was kept below 7 × 10−9 mbar. Binding energies (BEs) were referred to the C 1 s peak (284.9 eV) to account for charging effects. Binding energy scale was preliminarily calibrated by the position of the Au 4f7/2 peak at 83.96 eV and the Cu 2p3/2 peak at 932.62 eV core levels. Decompositions of the S 2p, Co 2p and Mo 3d XPS spectra were performed using Casa XPS software package. Appropriate oxidic and sulfided references were used as supported monometallic catalysts [22]. The surface atomic concentration for each species was calculated according to the equation:

2.2. Characterization of the catalysts 2.2.1. Raman spectroscopy Analysis of dried oxide catalysts by Raman spectroscopy was carried out using an InVia Basis system Renishaw (UK) for Raman spectral analysis with excitation wavelength of 532 nm, using 1800 line/mm diffraction grating and a CCD-matrix (1024 × 256 pixels) as a detector which provided spectral resolution approximately 2 cm−1. The Raman spectra were recorded from the sample surface using a monochromatic radiation source 2 μm in diameter.

Aj / Sj

C (j ) T (at. %) =

i = 1.. n

Ai / Si

× 100 (3)

where Aj is the measured area of the species j, Sj is the sensitivity factor of the atom, related to the species i (provided by the manufacturer), and C(j)T is the absolute amount of the species j. Relative concentrations of each species, cobalt oxide Co2+, separate cobalt sulfide Co9S8, and CoMoS, molybdenum oxide Mo6+, oxysulfide MoSxOy and MoS2 were determined for every sulfided catalyst. For example, a relative amount of Co in the CoMoS phase was determined by the equation:

2.2.2. Textural characteristics The textural properties of the alumina support and sulfided dCoMo/Al2O3 catalysts were determined by the method of low-temperature N2 adsorption on a Quantachrome Autosorb-1 porosimeter. Prior to taking the measurements, all samples and support were heat treated at 110 and 350 °C, respectively, for 4 h and p < 10−1 Pa. The specific surface area (SSA) was measured using the BET method under relative partial pressures (P/P0), ranging from 0.05 to 0.3. The total pore volume and pore size distribution were calculated from the isotherms at P/P0 = 0.99 and employing Barret-Joyner-Halenda method (BJH) respectively.

[CoMoS] (%) =

ACoMoS × 100 ACoMoS + ACo9S8 + ACo2 +

(4)

where AX represents the peak area of the species x. The effective Co content in the CoMoS phase was determined using the equation: CoMoS

2.2.3. High-resolution transmission electron microscopy (HRTEM) HRTEM studies of the fresh sulfided catalysts were carried out using Tecnai G2 20 electron microscope with an accelerating voltage of 200 kV and 0.14 nm lattice-fringe resolution. The samples were prepared for HRTEM on a perforated carbon film mounted on a copper grid, and 15–20 representative micrographs were obtained in high-resolution mode for each catalyst. The length of at least 500 slabs was measured for each sample using ImageJ software developed for direct qualitative analysis of images. MoS2 dispersion was calculated on the assumption that the MoS2 slabs were perfect hexagons [23]. MoS2 dispersion (D) was statistically evaluated by dividing the total

= [CoMoS] × C (Co) T

(5)

where C(Co)T represents the effective concentration of cobalt determined by XPS (at.%). The promoter ratio in the active phase slab was determined using the relation:

(Co/Mo)slab =

CCoMoS CMoS2

(6)

where CX is the absolute concentration of Co (Mo) in the CoMoS (MoS2) species (at. %). The promoter ratio in the edge of the active phase slab was calculated as follows: 52

Catalysis Communications 127 (2019) 51–57

M. Nikulshina, et al.

(Co/Mo)edge =

(Co/ Mo)slab (Co/ Mo)slab × MoT = Moe + Moc D

to those of the initial support. According to elemental analysis (XRF), the prepared catalysts contained different amount of Co (from 0.7 to 3.5 wt%) and Mo (from 5.8 to 28.5 wt%) corresponding to Mo surface density varying from 2 to 17 at/nm2. Mo/Co atomic ratios were close in all the samples and equal to ca. 5, which corresponds to stoichiometry of metals in initial Co2Mo10HPA precursor. The molybdenum loading increase led to a slow decrease in the surface area and pore volume, which is in agreement with data reported in [7]. Pore size distributions of the sulfided catalysts are shown in Table 1. The initial alumina support and d-CoMo/Al2O3 catalysts with d(Mo) until 4 at/nm2 displayed mesoporous structures predominantly of 4–8 and 8–17 nm. The molybdenum density increase up to 9 at/nm2 resulted in the increase in the proportion of small 4–8 nm pores and reduction of large mesopores due to a consecutive uniform deposition of Co2Mo10HPA into main pores of alumina, which in turn resulted in reducing the mean diameter of pores of the sulfided catalyst. After higher precursor loading (13–17 at/nm2), the mesopores 4–8 nm in diameter significantly decreased due to further deposition of the precursors into small pores, and, in contrast, only large mesopores are present in the 17- CoMo/Al2O3 catalyst.

(7)

where D is active phase dispersion determined from the HRTEM measurements. 2.3. Examination of catalytic activity and selectivity HDS and HYD activities of the prepared catalysts were evaluated in a fixed bed high-pressure flow reactor. The 0.2 g d-CoMo/Al2O3 catalyst 0.25–0.50 mm in size was diluted with carborundum (0.2–0.4 mm) in a ratio of 1/2 by volume and loaded into the isothermal region of the reactor. Before testing, the catalysts were in situ sulfided by the gas phase method according to the procedure described above. Catalytic tests were carried out at the temperature of 275 °C, the pressure of 3.0 MPa, a 500 NL/L volume ratio of H2 to feedstock, and 40 h−1 liquid hourly space velocity (LHSV). A model feedstock was a mixture of DBT (1500 ppm S), naphthalene (3 wt%) and hexadecane (as an internal standard, 1 wt%) in toluene (as a solvent). The reaction products were detrmined by gas-liquid chromatography on a Crystal5000 instrument equipped with a 30 m OV-101 quartz capillary column with identification by matching retention times to commercially available standards and by a GC/MS analysis using a Finnigan Trace DSQ. The rate constants of the pseudo-first-order reactions of DBT HDS and naphthalene HYD were calculated using the equations:

kHDS =

FDBT ln(1 W

xDBT ) and kHYD =

FNaph W

ln(1

xNaph )

3.2. Raman spectroscopy Fig. 1 shows the Raman scattering curves for d-CoMo/Al2O3. Prior to analysis, the catalysts were dried in an air flow at the temperature of 110 °C. All spectra exhibit a characteristic peak at the 950 and 906 cm−1, which refers to symmetric and anti-symmetric vibrations of the Mo]O2t bond in polymolybdate entities [25–27]. As previously mentioned [25,28], the Mo oxide species may weakly interact with the support resulting in higher reducibility and activity in the HDS reaction. The Raman band at 564 cm−1 corresponds to ν(CoeOeMo) stretching vibrations [29]. The bands around 216–218 and 340–350 cm−1 were assigned, respectively, to bending of δ(MoeO2t) and δ(MoeOaeMo) vibrations [30]. No specific band at 570 cm−1 corresponding to ν(AleOeMo) stretching vibrations as well as strong bands at 940 and 952 cm−1 and a less intense band at 819 cm−1 belonging to the bulk βCoMoO4 phase. Therefore, it can be concluded that good dispersion of oxidic polymolybdate entities remains on the surface of the catalysts even at high Mo loading (up to 17 at/nm2).

(8)

where kHDS and kHYD are pseudo-first-order reaction constants for DBT HDS and naphthalene HYD (mol g−1 h−1), respectively; xDBT and xNaph are the conversion (%) of DBT and naphthalene, respectively; FDBT and FNaph are the reactant flow (mol h−1), and W is the catalyst weight (g). It is known that HDS of DBT occurs via two routes: direct desulfurization (DDS), leading to the formation of biphenyl (BP) and the hydrogenation route (HYD), which implies the formation of tetrahydrodibenzothiophene, then cyclohexylbenzene (CHB) and finally, bicyclohexyl (BCH). The HYD/DDS selectivity ratio for DBT HDS was estimated according to:

SHYD/DDS =

kHYD C + CBCH = CHB kDDS CBP

3.3. HRTEM analysis

(9)

where CCHB, CBCH, and CBP are concentrations (mol. %) of CHB, BCH, and BP in the reaction products, respectively. In order to gain a better understanding of the diffrences in catalytic activities of the catalysts depending on the composition and characteristics of the (Co)MoS active phase species, turnover frequencies (TOF, s−1) [21,24] for HDS of DBT and HYD of naphthalene were calculated as follows:

TOFHDS =

FNaph xNaph ArMo FDBT xDBT ArMo and TOFHYD = W CMoS2 D 3600 W CMoS2 D 3600

The active phase morphology of the sulfided catalysts was studied by HRTEM. Fig. S1 shows representative micrographs of the CoMo catalysts, and Table 2 displays the average length and stacking number of CoMoS slabs. The increase in molybdenum loading from 2 to 17 Mo at/nm2 led to a simultaneous rise of both values – the average length (from 3.3 to 4.8 nm) and the stacking number of CoMoS active phase particles (from 2.0 to 3.2) in the CoMo catalysts. As a consequence, the 13-CoMo/Al2O3 and 17-CoMo/Al2O3 catalysts with the highest loading of metals had the lowest dispersion of the active phase species among the examined samples. Such dependence on the metal content is in good agreement with the literature data [31].

(10)

−1

where FDBT and FNaph are the reactant flow (mol h ); xDBT and xNaph are the conversions (%) of DBT and naphthalene, respectively; ArMo is the atomic weight of molybdenum (95.9 g/mol); W is the catalyst weight (g); СMoS2 is the effective Mo content in MoS2 or CoMoS species (wt%).

3.4. XPS analysis The surface composition of the sulfided catalysts was identified by XPS. Fig. S2 shows the Co 2p and Mo 3d XPS spectra of the sulfided dCoMo/Al2O3 catalysts. Spectral region of Co 2p3/2 contains three peaks with their respective satellites. The peak at the binding energy (BE) 778.6 eV corresponds to cobalt in the CoMoS species. The signals at 778.1 and 781.5 eV are related to the Co9S8 species and to Co2+ in the oxidic environment, respectively [19–22]. The Mo 3d spectra exhibit three contributions attributed to Mo6+ in the oxidic environment (Mo3d5/2 BE = 232.1 and Mo3d3/2 BE = 235.3 eV); the peaks at BE = 230.0 eV (Mo3d5/2) and BE = 233.2 eV (Mo3d3/2) characterize

3. Results and discussion 3.1. Textural properties The textural properties of the prepared catalysts are given in Table 1. Prior to the characterization, all d-CoMo/Al2O3 samples were sulfided. After impregnation of Co2Mo10HPA and sulfidation, the specific surface area (SBET) and specific pore volume decreased compared 53

Catalysis Communications 127 (2019) 51–57

M. Nikulshina, et al.

Table 1 Composition and textural characteristics of support and sulfided d-CoMo/Al2O3 catalysts. Catalyst

Al2O3 2-CoMo/Al2O3 4-CoMo/Al2O3 9-CoMo/Al2O3 13-CoMo/Al2O3 17-CoMo/Al2O3

a

2

−1

Mo

Co

SBET (m g

– 5.8 10.5 19.7 24.7 28.5

– 0.7 1.3 2.4 3.0 3.5

205 193 158 129 107 62

Vpb

)

Pore size distribution (%) 3

−1

(cm g

)

0.58 0.53 0.29 0.24 0.22 0.20

c

D (nm)

< 4 nm

4–8 nm

8–17 nm

> 17 nm

7.8 7.6 7.6 4.1 9.4 12.2

1 2 2 1 3 3

54 50 49 61 37 10

34 36 39 27 50 75

11 10 10 11 10 12

SBET is the surface area. Vp is the pore volume. D is the pore diameter.

molybdenum atoms in the oxysulfide species and Mo3d5/2 and Mo3d3/2 peaks at about 228.8 eV and 232.0 eV, respectively, Mo4+ is characteristic of MoS2 [19–22]. A peak at BE = 226.1 eV was assigned to sulfur (S 2 s). Decomposition of the XPS spectra indicates metal distribution for cobalt and molybdenum species present on the surface of the sulfided dCoMo/Al2O3 catalysts (Table S1). All studied catalysts exhibited a relatively high (> 70 rel. %) molybdenum sulfidation degree. With d(Mo) growth from 2 to 9 at/nm2 in d-CoMo/Al2O3, the MoS2 amount built up from 72 to 85 rel. %. The 13-CoMo/Al2O3 and 17-CoMo/Al2O3 samples had a slightly lower molybdenum sulfidation degree ~82 rel. % compared to 9-CoMo/Al2O3. A different tendency was observed for cobalt sulfidation. The d(Mo) increase from 2 to 4 at/nm2 led to increase in the CoMoS active phase amount by 10 rel. %. However, further increasing of the metal content resulted in a significant decrease of the CoMoS phase amount and in the growth of the Co9S8 phase content. The Co/Mo ratio at the CoMoS edges increased from 0.54 to 1.21 with metal loading growth, alongside with the effective Co concentration increase from 0.12 to 1.02 at. % detected in the CoMoS phase species. The highest promotion degrees (Co/Mo)edge corresponded to high-loading catalysts with d(Mo) from 9 to 17 at/nm2 (Table 2). Effective Co content in the CoMoS phase increased with metal loading growth. It should be noted that, when proceeding from 4 to 9 at/nm3, the ratio of CoMoS and Co9S8 particles changes, possibly, due to the full promotion degree of MoS2 crystallites and the formation of II type CoMoS [4,32]. An excess of cobalt, not included in the CoMoS phase, in the process of sulfidation forms low-active cobalt sulfide. The calculated values (Co/Mo)slab and (Co/Mo)edge (Table 2) show that, when the surface molybdenum concentration is > 9 at/nm2, the number of mixed centers per crystallite or one Mo atom gains its maximum [33].

3.5. Catalytic properties

170

270

370

909

568

470

4-CoMo/Al2O3 9-CoMo/Al2O3

904

565

905

565

907

2-CoMo/Al2O3

564 566

349 348

218 217

346

216

352

216

352

218

906

Table S2 shows the results of co-HDT of DBT and naphthalene over the d-CoMo/Al2O3 catalysts. The reagent conversion ranged from 12.5 to 66.6% and from 1.7 to 7.9% for DBT HDS and naphthalene HYD, respectively. HDS as well as HYD rate constants progressively increased with d(Mo) increasing from 2 to 9 at/nm2. The kHDS and kHYD values over the 9-CoMo/Al2O3 catalyst reached the maximum among all studied catalysts. The loss in HDS and HYD activities with further growth of the metal content to d(Mo) = 17 at/nm2 was about 2.3 times (rate constants decreased from 26.7 × 10−4 to 11.6 × 10−4 mol h−1 g−1) and 1.9 times (rate constants decreased from 9.9 × 10−4 to 5.2 × 10−4 mol h−1 g−1), respectively. As expected, all catalysts demonstrated strong selectivity toward the BP formation via the DDS pathway [34] with an average SHYD/DDS selectivity ratio 0.09. Fig. 2 shows the changes in DBT HDS and naphthalene HYD activities and selectivity in the HDS reaction as a function of molybdenum loading in the d-CoMo/Al2O3 catalysts. As molybdenum loading increased, the activity in both HDS and HYD passed through the maximum. In both reactions, the highest activity was achieved over the 9CoMo/Al2O3 catalyst. Rate constants in HDS and HYD decreased where the content of metals in the 13-CoMo/Al2O3 and 17-CoMo/Al2O3 samples was higher. The TOF values normalized on the edge sites of CoMoS slabs for DBT HDS and naphthalene HYD were also calculated (Table S2). The relationship between the TOF numbers, Co/Mo ratio in the CoMoS edges and average particle size of the active phase is plotted in Fig. 3. Increase of promoter ratio in catalysts with d(Mo) = 2–9 at/nm2 led to rising TOF number for HDS of DBT that coincides with CoMoS model of Topsøe and Clausen [4]. The highest TOF value for HDS of DBT was achieved at the average length of CoMoS slabs equal to 4.4 nm with the

951 949

c

– 2.0 4.0 9.2 13.1 16.9

Textural characteristics

952 952

b

Content (wt%)

951

a

d(Mo) at/nm2

13-CoMo/Al2O3 17-CoMo/Al2O3

570

670

770

870

970

Raman Shift [1/cm]

Fig. 1. Raman spectra of oxidic d-CoMo/Al2O3 catalysts dried at 110 °C. 54

1070

Catalysis Communications 127 (2019) 51–57

M. Nikulshina, et al.

Table 2 Morphological characteristics of CoMoS active phase species and their promotion degree in sulfided d-CoMo/Al2O3 catalysts. Catalyst

2-CoMo/Al2O3 4-CoMo/Al2O3 9-CoMo/Al2O3 13-CoMo/Al2O3 17-CoMo/Al2O3

Average length L̅ (nm)

3.3 4.1 4.4 4.7 4.8

Average stacking number N̅

2.0 2.4 2.6 3.0 3.2

Dispersion of MoS2 particles Da

0.35 0.29 0.27 0.25 0.25

(Co/Mo)edge

0.54 0.68 1.06 1.16 1.21

Distribution of slab length (rel. %)

Distribution of stacking number (rel. %)

< 2 nm

2–4 nm

4–6 nm

6–8 nm

> 8 nm

1

2

3

>4

13 10 2 4 1

62 53 40 36 35

21 24 49 39 39

4 8 8 13 18

0 5 1 8 7

20 7 16 5 5

65 51 38 18 9

14 38 44 61 65

1 4 2 16 21

and short particles (3.0–3.2 nm). In contrast, the 4-CoMo/Al2O3 catalyst had particles with the highest HYD activity among the studied samples (Fig. 3), and the 9-CoMo/Al2O3 sample demonstrated the highest TOF number in DBT HDS. This observation may be explained by the difference in the morphology and composition of sulfided particles present on the surface of the d-CoMo/Al2O3 catalysts. According to the “rim-edge” model [35], rim sites are active in both HDS and HYD reactions, while edge sites are likely to participate only in the DDS route of HDS. Although the “rimedge” model was initially developed for unpromoted and unsupported catalysts, it may explain the experimentally observed relationships between the morphology of the active phase and catalytic activity (selectivity) for promoted bulk and supported Co(Ni)Mo(W)S2 catalysts [19,28,34]. Fig. 4 displays a direct dependence of the average stacking number on metal loading in the prepared catalysts as well as on the Co9S8 species content. The results shown in Fig. 5 indicate that naphthalene HYD activity decreased with increasing the staking number which in turn got higher as metal loading increased. Moreover, HYD activity of the catalysts is strongly influenced by the relative amount of CoMoS and Co9S8 phases. Thus, 4-CoMo/Al2O3 had 72 rel. % of Co in the CoMoS phase and only 4 rel. % in Co9S8. A proportion of Co9S8 particles became larger as loading of metals increased > 6- and 11-fold for the 9-CoMo/Al2O3 and 17-CoMo/Al2O3 samples, respectively (Fig. 4). Hence, the high concentration of metals resulted in the simultaneous reduction of the number of HYD sites and the increase of inactive Co-sulfide particles capable of blocking access to the active phase of the prepared catalysts. Fig. 5 shows dependences of rate constants in HDS of thiophene and DBT over the CoMo/Al2O3 catalysts prepared from Co2Mo10Co3 salt [18] as well as conventional Co(NO3)2 and AHM (NH4)6Mo7O24 [18] or monometallic Mo/Al2O3 samples synthesized using AHM only [5]. It is

Fig. 2. Correlation between catalytic activity and HYD/DDS selectivity in HDS of DBT and HYD of naphthalene over d-CoMo/Al2O3 catalysts and surface density of molybdenum.

edges fully covered by Co atoms. It was previously found in [19] that, for the CoMo/Al2O3 catalysts with d(Mo) = 4 at/nm2 and the Co/Mo ratio = 0.16–0.32 prepared from Co2Mo10HPA, the maximal TOF value for DBT HDS corresponded to the minimal (Co/Mo)edge ratio (0.3–0.4)

Fig. 3. 3D dependence of the TOF number in DBT HDS and naphthalene HYD over d-CoMo/Al2O3 catalysts on the average length of the (Co)MoS2 phase species and (Co/Mo)edge promotion degree. (Empty markers correspond to unpromoted 4-Mo/Al2O3 catalyst [20]; the values near the marker show the Mo loading (at/nm2)).

Fig. 4. Dependence of the amount of Co species (solid curves) and average stacking number (dotted curve) of active phase particles on the metal loading in d-CoMo/Al2O3 catalysts. 55

Catalysis Communications 127 (2019) 51–57

M. Nikulshina, et al.

Catal. A Chem. 412 (2016) 1–12. [6] J.A. Rob van Veen, E. Gerkema, A.M. van der Kraan, P.A.J.M. Hendriks, H. Beens, A 57 CO Mössbauer emission spectrometric study of some supported CoMo hydrodesulfurization catalysts, J. Catal. 133 (1992) 112–123. [7] M. Høj, K. Linde, T.K. Hansen, M. Brorson, A.D. Jensen, J.-D. Grunwaldt, Flame spray synthesis of CoMo/Al2O3 hydrotreating catalysts, Appl. Catal. A 397 (2011) 201–208. [8] P. Nikulshin, A. Mozhaev, C. Lancelot, P. Blanchard, E. Payen, C. Lamonier, Hydroprocessing catalysts based on transition metal sulfides prepared from Anderson and dimeric Co2Mo10-heteropolyanions. A review, C. R. Chimie 19 (2016) 1276–1285. [9] C.I. Cabello, I.L. Botto, H. Thomas, Anderson type heteropolyoxomolybdates in catalysis: 1. (NH4)3[CoMo6O24H6]·7H2O/γ-Al2O3 as alternative of Co-Mo/γ-Al2O3 hydrotreating catalysts, Appl. Catal. A 197 (2000) 79–86. [10] C.I. Cabello, I.L. Botto, F. Cabrerizo, M.G. Gonzales, H. Thomas, Γ-Al2O3-supported XMo6 anderson heteropolyoxomolybdates: adsorption studies for X = TeVI, AlIII, CoIII, CrIII and NiII by DR spectroscopy and TPR analysis, J. Adsorpt. Sci. Tech. 18 (2000) 591–608. [11] E. Puello-Polo, Y. Diaz, J.L. Brito, Effect of the structural modification by carbiding of alumina supported Anderson-type (NH4)4[NiMo6−xWxO24H6]·5H2O on hydrodesulfurization, hydrodechlorination and selective oxidation, Catal. Commun. 99 (2017) 89–93. [12] A. Alsalme, N. Alzaqri, A. Alsaleh, M.R.H. Siddiqui, A. Alotaibi, E.F. Kozhevnikova, I.V. Kozhevnikov, Efficient Ni–Mo hydrodesulfurization catalyst prepared through Keggin polyoxometallate, Appl. Catal. B 182 (2016) 102–108. [13] J. Liang, M. Wu, P. Wei, J. Zhao, H. Huang, C. Li, Y. Lu, Y. Liu, C. Liu, Efficient hydrodesulfurization catalysts derived from Strandberg P-Mo-Ni polyoxometalates, J. Catal. 358 (2018) 155–167. [14] J. Mazurelle, C. Lamonier, C. Lancelot, E. Payen, C. Pichon, D. Guillaume, Use of the cobalt salt of the heteropolyanion [Co2Mo10O38H4]6− for the preparation of CoMo HDS catalysts supported on Al2O3, TiO2 and ZrO2, Catal. Today 130 (2008) 41–49. [15] P.A. Nikulshin, N.N. Tomina, A.A. Pimerzin, A.V. Kucherov, V.M. Kogan, Investigation into the effect of the intermediate carbon carrier on the catalytic activity of the HDS catalysts prepared using heteropolycompounds, Catal. Today 149 (2010) 82–90. [16] T. Huang, S. He, J. Chang, J. Xu, Y. Fan, Preparation of a superior Co4Mo12/Al2O3 hydrodesulfurization catalyst by the hydrothermal deposition of heteropoly compounds on Al2O3, Catal. Commun. 122 (2019) 68–72. [17] C.I. Cabello, F.M. Cabrerizo, A. Alvarez, H.J. Thomas, Decamolybdodicobaltate(III) heteropolyanion: structural, spectroscopical, thermal and hydrotreating catalytic properties, J. Mol. Catal. A Chem. 186 (2002) 89–100. [18] C. Lamonier, C. Martin, J. Mazurelle, V. Harlé, D. Guillaume, E. Payen, Molybdocobaltate cobalt salts: new starting materials for hydrotreating catalysts, Appl. Catal. B 70 (2007) 548–556. [19] P.A. Nikulshin, D.I. Ishutenko, A.A. Mozhaev, K.I. Maslakov, A.A. Pimerzin, Effects of composition and morphology of active phase of CoMo/Al2O3 catalysts prepared using Co2Mo10–heteropolyacid and chelating agents on their catalytic properties in HDS and HYD reactions, J. Catal. 312 (2014) 152–169. [20] A. Kokliukhin, M. Nikulshina, A. Sheldaisov-Meshcheryakov, A. Mozhaev, P. Nikulshin, CoMo hydrotreating catalysts supported on Al2O3, SiO2 and SBA-15 prepared from single Co2Mo10-heteropolyacid: in search of self-promotion effect, Catal. Lett. 148 (2018) 2869–2879. [21] A.V. Mozhaev, P.A. Nikulshin, Al.A. Pimerzin, K.I. Maslakov, A.A. Pimerzin, Investigation of co-promotion effect in NiCoMoS/Al2O3 catalysts based on Co2Mo10heteropolyacid and nickel citrate, Catal. Today 271 (2016) 80–90. [22] P.A. Nikulshin, A.V. Mozhaev, K.I. Maslakov, A.A. Pimerzin, V.M. Kogan, Genesis of HDT catalysts prepared with the use of Co2Mo10HPA and cobalt citrate: study of their gas and liquid phase sulfidation, Appl. Catal. B 158-159 (2014) 161–174. [23] S. Kasztelan, H. Toulhoat, J. Grimblot, J.P. Bonnelle, A geometrical model of the active phase of hydrotreating catalysts, Appl. Catal. 13 (1984) 127–159. [24] T.K.T. Ninh, L. Massin, D. Laurenti, M. Vrinat, A new approach in the evaluation of the support effect for NiMo hydrodesulfurization catalysts, Appl. Catal. A 407 (2011) 29–39. [25] D.S. Kim, I.E. Wachs, K. Segawa, Molecular structures and reactivity of supported molybdenum oxide catalysts, J. Catal. 149 (1994) 268–277. [26] J.A. Bergwerff, T. Visser, B.M. Weckhuysen, On the interaction between Co- and Mo-complexes in impregnation solutions used for the preparation of Al2O3-supported HDS catalysts: a combined Raman/UV–vis–NIR spectroscopy study, Catal. Today 130 (2008) 117–125. [27] Y. Zhang, W. Han, X. Long, H. Nie, Redispersion effects of citric acid on CoMo/γAl2O3 hydrodesulfurization catalysts, Catal. Commun. 82 (2016) 20–23. [28] P.A. Nikulshin, A.V. Mozhaev, A.A. Pimerzin, V.V. Konovalov, A.A. Pimerzin, CoMo/Al2O3 catalysts prepared on the basis of Co2Mo10-heteropolyacid and cobalt citrate: effect of Co/Mo ratio, Fuel 100 (2012) 24–33. [29] C. Martin, C. Lamonier, M. Fournier, O. Mentré, V. Harlé, D. Guillaume, E. Payen, Evidence and characterization of a new decamolybdocobaltate cobalt salt: an efficient precursor for hydrotreatment catalyst preparation, Chem. Mater. 17 (2005) 4438–4448. [30] J. Moreau, O. Delpoux, E. Devers, M. Digne, S. Loridant, Decamolybdocobaltate heteropolyanions in aqueous solutions: chemistry driving their formation and domains of stability, J. Phys. Chem. A 116 (2012) 263–270.

Fig. 5. Dependences of the rate constants in HDS of thiophene (solid curves) and DBT (dotted curve) over (Co)Mo/Al2O3 catalysts on the molybdenum loading (■ and ● are adapted from [18]; ▲ is adapted from [5] and ◊ are series of catalysts from the present work).

obvious that loading of a single Co2Mo10HPA precursor is a convenient approach to prepare extremely high-percentage metal sulfide catalysts (> 9 at Mo/nm2) compared to the previously reported protocols where maximal HDS activity was shown for catalysts with 4–6 at Mo/nm2. 4. Conclusions In the present study, the CoMo/Al2O3 catalysts with the Mo surface density varied from 2 to 17 at/nm2 were prepared from the single Co2Mo10HPA precursor. The use of Co2Mo10HPA containing both Moand Co-promoter atoms allowed to prepare catalysts with high molybdenum loading (up to 28.5 wt%) by one-step impregnation. The CoMo/Al2O3 catalyst with d(Mo) of 9 at/nm2 demonstrated best catalytic performance in DBT HDS and naphthalene HYD reactions, which could be related to the maximal Co promotion degree and suitable morphology of CoMoS particles. This study suggests a convenient approach to the preparation of efficient hydrotreating catalysts. Acknowledgments Authors thank Russian Science Foundation for financial support of the investigation by Grant No. 17-73-20386. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.05.003. References [1] L. Yanga, C. Peng, X. Fang, Z. Cheng, Z. Zhou, Hierarchically macro-mesoporous NiMo/Al2O3 catalysts for hydrodesulfurization of dibenzothiophene, Catal. Commun. 121 (2019) 68–72. [2] L. Kaluža, M. Zdražil, Relative activity of Niobia-supported CoMo hydrodesulphurization catalyst prepared with NTA: a kinetic approach, Catal. Commun. 107 (2018) 62–67. [3] Y. Wang, C. Yin, X. Zhao, C. Liu, Synthesis of bifunctional highly-loaded NiMoW catalysts and their catalytic performance of 4,6-DMDBT HDS, Catal. Commun. 88 (2017) 13–17. [4] H. Topsøe, B.S. Clausen, F.E. Massoth, J.R. Anderson, M. Boudart (Eds.), Hydrotreating Catalysis, Springer-Verlag, Berlin–Heidelberg–N.Y, 1996. [5] P. Platanitis, G.D. Panagiotou, K. Bourikas, C. Kordulis, J.L.G. Fierro, A. Lycourghiotis, Preparation of un-promoted molybdenum HDS catalysts supported on titania by equilibrium deposition filtration: optimization of the preparative parameters and investigation of the promoting action of titania, J. Mol.

56

Catalysis Communications 127 (2019) 51–57

M. Nikulshina, et al. [31] P.D. Silva, N. Marchal, S. Kasztelan, Influence of high Mo loading on the HYD/HDS selectivity of alumina supported MoS2 catalysts, Stud. Surf. Sci. Catal. 106 (1997) 353–360. [32] M. Ramos, G. Berhault, D.A. Ferrer, B. Torresa, R.R. Chianelli, HRTEM and molecular modeling of the MoS2–Co9S8 interface: understanding the promotion effect in bulk HDS catalysts, Catal. Sci. Technol. 2 (2012) 164–178. [33] K. Marchand, C. Legens, D. Guillaume, P. Raybaud, A rational comparison of the

optimal promoter edge decoration of HDT NiMoS vs CoMoS catalysts, Oil Gas Sci. Technol. 64 (2009) 719–730. [34] P. Raybaud, H. Toulhoat, Catalysis by transition metal sulfides. From molecular theory to industrial applications, Technip Edition, 2013, p. 832 Paris. [35] M. Daage, R.R. Chianelli, Structure-function relations in molybdenum sulfide catalysts: the "rim-edge" model, J. Catal. 149 (1994) 414–427.

57