Al2O3 catalysts prepared using citric acid as chelating agent

Accepted Manuscript Title: Effect of thermal treatment on morphology and catalytic performance of NiW/Al2 O3 catalysts prepared using citric acid as chelating agent Authors: V. Yu. Pereyma, O.V. Klimov, I.P. Prosvirin, E. Yu. Gerasimov, S.A. Yashnik, A.S. Noskov PII: DOI: Reference:

S0920-5861(17)30502-3 http://dx.doi.org/doi:10.1016/j.cattod.2017.07.019 CATTOD 10933

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

16-4-2017 5-7-2017 15-7-2017

Please cite this article as: V.Yu.Pereyma, O.V.Klimov, I.P.Prosvirin, E.Yu.Gerasimov, S.A.Yashnik, A.S.Noskov, Effect of thermal treatment on morphology and catalytic performance of NiW/Al2O3 catalysts prepared using citric acid as chelating agent, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.07.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of thermal treatment on morphology and catalytic performance of NiW/Al2O3 catalysts prepared using citric acid as chelating agent V.Yu. Pereyma*, O.V. Klimov, I.P. Prosvirin, E.Yu. Gerasimov, S.A. Yashnik, A.S. Noskov Boreskov Institute of catalysis SB RAS pr. Lavrentieva 5, Novosibirsk, Russia (*) corresponding author: [email protected] Graphical Abstract

Highlights:   

1) Pre-treatment at 300°C results in maximum HDS, HDN and hydrogenation activity 2) Increasing NiWS stacking degree with the increase of pre-treatment temperature 3) Decreasing W and Ni sulfidation degree with the increase of pre-treatment temperature

Abstract In the present work, NiW/Al2O3 catalyst was prepared with the use of citric acid and thermally treated at temperatures of 120, 220, 300 and 450°C. It was shown that the thermal treatment at temperatures of 220-300°C leads to decomposition of citrate ligands, 450°C – to the almost complete removal of organic carbon. HRTEM of the sulfided catalysts showed that stacking number of WS2 increases with the increase of the thermal treatment temperature while the WS 2 slab length has minimum at 220°C. The results of XPS indicated that the sulfidation degree of tungsten and nickel decreases with the increase of the thermal treatment temperature. The catalyst thermally treated at 300°C resulted to be most active in DBT HDS, quinoline HDN and naphthalene hydrogenation reactions. The higher activity

of this catalyst in comparison with the samples thermally treated at 120°C and 220°C can be explained by an increased stacking degree of the sulfide WS2 slabs while maintaining a small length of sulfide slabs and by a smaller carbon content that can impede access to the active sites. The calcination at 450°C leads to low activity due to the low dispersion of the active component and a decrease in the degree of sulfidation of tungsten and nickel.

Keywords: NiW/Al2O3; citric acid; thermal treatment; dibenzothiophene hydrodesulfurization; hydrodenitrogenation.

1. Introduction New more stringent standards for sulfur and polyaromatic compounds in diesel fuels were introduced in recent years. For this reason, it is extremely urgent to develop hydrotreating (HDT) catalysts with increased HDS and hydrogenating activity. Mo or W sulfides promoted by Co or Ni and deposited on the surface of the oxide supports are usually used as HDT catalysts. It is known that NiW HDT catalysts exhibit higher hydrogenation activity than CoMo and NiMo catalysts [1– 4]. High hydrogenation activity allows to reduce the content of polyaromatic hydrocarbons and to increase the cetane number of diesel fuels. High hydrogenating activity also favors HDS of sterically hindered sulfur-containing compounds such as 4,6-alkylsubstituted dibenzothiophenes, the removal of which is necessary for the production of diesel fuel with ultra-low sulfur content. Various modifying agents are usually used to improve the activity of hydrotreating catalysts. One of the most commonly used modifiers are chelating agents such as citrate, NTA, EDTA and glycols [5–11]. The effect of the chelating agents is associated with a number of factors, such as prevention of early sulfidation of the promoter

[5,7,9,11], increase in the dispersion and homogeneity of

distribution of the deposited metal compounds [6,10], prevention of strong interaction of the supported metals with the carrier surface and the increase in the sulfidation degree of Ni(Co) and W(Mo) [8–10]. All these factors favor selective formation of the type II Ni(Co)-W(Mo)-S phase, which is the most active in the HDS, HDN and hydrogenation reactions [12,13]. Carbon formed from the organic ligands during the sulfidation stage also can have an important role. Thus, carbon can act as an insulating agent and contribute to the increase in the dispersion of Ni(Co)-W(Mo)-S sulfide particles [8]. The thermal treatment is an important step in the preparation of HDT catalysts. Firstly, after the impregnation, the catalysts undergo drying stage which is usually carried out at temperatures of 80-120°C. In the case of preparation of the catalysts from inorganic salts (for example, nickel nitrate and ammonium

metatungstate) without the use of chelating agents, the catalysts usually undergo additional calcination stage. The calcination allows removing counterions (nitrate and ammonium ions) from the catalyst and leads to an increase in the dispersion of the deposited metals and formation of the oxide bimetallic precursor of the active sulfide phase. A side negative effect of the calcination is the formation of oxide metal species with the strong interaction with the support, which difficultly undergo sulfidation. Also the formation of low-dispersed particles of molybdenum or tungsten oxide is possible during the calcination of the catalysts with high metal loadings. When chelating agents are used as modifiers in the preparation of HDT catalysts the calcination of the catalysts is usually not carried out, and thermal treatment in this case is limited by drying at temperatures of 100-120°C. However, it was shown in [14,15], that CoMo/Al2O3 catalyst prepared using citrate Co-Mo bimetallic complexes shows the maximum activity when the drying temperature is 220°C. Partial decomposition of Co-Mo citrate complexes takes place at this temperature, which facilitates the sulfiding of Co and Mo in comparison with the catalyst dried at 120°C. At the same time the calcination of such catalyst at the temperature above 300°C leads to the more significant destruction of complex compounds and decreases the activity of the catalyst. For CoMo/Al 2O3 catalysts prepared using ethylenediamine as the chelating agent the optimum calcination temperature was reported to be about 400°C [16]. Such high temperature is necessary to remove nitrogen compounds from the catalyst. In work [17] a comparison of dried and calcined NiMo/SBA-15 catalysts prepared using citrate as the chelating agent was carried out. It was shown that calcination at 500°C leads to a strong increase in the selectivity phenylcyclohexane/biphenyl in DBT HDS reaction and to a significant increase in the activity of the catalyst in HDS of 4,6DMDBT. In the literature there is very little data on the effect of thermal treatment on the properties of supported NiW catalysts prepared using chelating agents. Y.Yoshimura [18] showed that calcination at 500°C of the NiW/Al 2O3 catalyst prepared using citrate leads to a significant increase in activity in the hydrogenation of 1-methylnaphthalene compared to the catalyst dried at 120°C. Thus, a more

detailed study of the effect of heat treatment of supported NiW catalysts prepared using chelating agents is of undoubted interest. The aim of this work was to study the influence of the thermal treatment temperature on the structure and catalytic properties of NiW/Al 2O3 catalysts prepared using citrate as a chelating agent. NiW/Al2O3 catalysts thermally treated at temperatures of 120, 220, 300 and 450°C were studied in the oxide and sulfided form and tested in the reactions of DBT HDS, quinoline HDN and hydrogenation of naphthalene. 2. Experimental 2.1 Preparation of the alumina support A pseudoboehmite powder TH-70 manufactured by Sasol was used to prepare the alumina support. The pseudoboehmite powder was peptized with an aqueous solution of nitric acid and then the resulting paste was extruded. The resulting extrudates were dried in air stream at 120 °C and calcined at 550°C for 4 hours. 2.2 Preparation of NiW/Al2O3 catalysts The catalyst NiW/Al2O3 containing 2.7 wt.% Ni and 17.0 wt.% W was prepared by incipient wetness impregnation. The impregnating solution was prepared by successively dissolving citric acid, C6H8O7·H2O (98%), nickel (II) hydroxide, Ni(OH)2 (Aldrich), and ammonium paratungstate, (NH4)10H2 (W12O42)·4H2O (98%) in water. pH of the final impregnating solution was 2.91. The preparation procedure is also described in [19]. The catalyst after impregnation was dried at 120°C and then divided into 4 parts: the first part was not subjected to further thermal treatment and was designated as NiW-120, the three remaining parts were subjected to calcination in static air at 220, 300 and 450°C for 4 hours. The samples obtained were designated as NiW-220, NiW-300 and NiW-450 respectively. The molar ratio of nickel to citrate was 1:2. According to the literature data this ratio is close to the optimal [20]. 2.3 Study of impregnating solutions and catalysts

2.3.1 Nitrogen adsorption-desorption and carbon content analysis The textural properties of the catalysts and supports were determined by nitrogen physisorption using an ASAP 2400 (USA) instrument. Prior to analysis, samples were subjected 150-300°C for 2 h in N2 flow. The BET surface areas were calculated from the nitrogen uptakes at relative pressures ranging from 0.05 to 0.30. The total pore volume was derived from the amount of nitrogen adsorbed at a relative pressure close to unity (in practice, P/P0 = 0.995) by assuming that all accessible pores had been filled with condensed nitrogen in the normal liquid state. The pore size distribution was calculated using the BJH method using the desorption branch of the isotherm. The carbon content of the catalysts was determined using a Vario EL Cube element analyzer. 2.3.2 FTIR spectroscopy IR spectra were acquired in the range of 4000–250, with 4 cm−1 resolution in a Bomem MB-102 FTIR spectrometer. The solid specimens were generated via the conventional procedure, i.e. by the tableting 1.5 mg of the probe with 500 mg of KBr. The liquid aqueous specimens were placed inside a KRS-5 capillary cuvette. In this case, the recorded spectra were subtracted from the normalized spectra of distilled water at 700 cm−1 (the broad absorbance band of water molecule vibrational stretching). 2.3.3. Raman spectroscopy Raman spectra were recorded at room temperature in the range of 3600–100 cm−1 using a Bruker RFS 100/S FT-Raman spectrometer (Germany). The excitation source used was the 1064 nm line of a Nd-YAG laser operating at power level of 100 mW. 2.3.4. Thermal analysis The thermogravimetric (TGA), differential thermogravimetric (DTG) analysis and differential scanning calorimetry (DSC) of pre-dried samples was conducted using a NETZSCH STA 449C-Jupiter apparatus. TG and DTG curves were recorded in flowing air in the range from room temperature to 600°C (heating

rate 10°C/min). For analysis, 30 mg of a sample was loaded in a corundum crucible. Calcined alumina was used as a reference sample. 2.3.5. H2-TPR The temperature-programmed reduction experiments were conducted using a flow reactor connected with a thermal conductivity detector. A catalyst sample (100 mg, fraction 0.25-0.50 mm) was placed in the reactor, pretreated in the argon flow (50 cm3/min) at thermal treatment temperature (120, 220, 300 and 450 C) for 0.5 h and cooled to room temperature. Then, a hydrogen-argon mixture (10 vol.% H2) was passed through the catalyst sample as the reducing agent at a feed rate of 50 cm3/min. The temperature range and heating rate were 25-1000C and 10C/min, respectively. Water, CO and CO2 formed during the reductive decomposion of the citrate ligands as well as during the reduction of nickel-oxygen and tungsten-oxygen compounds were frozen out in a trap at –70C. Besides, the thermal conductivity of H2 is 20times higher than other gases (CO, CO2), therefore the H2-TPR peaks corresponded to hydrogen consumption. The hydrogen consumption was calibrated against the reduction of CuO (Reachem, very-high-purity) under similar conditions, assuming a complete one-step CuO reduction to zero-valent copper. 2.3.6. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectra (XPS) were recorded using a SPECS spectrometer (Germany) with a PHOIBOS-150 hemispherical energy analyser and Al Kα irradiation (hν = 1486.6 eV, 200 W). The binding energy scale was preliminarily calibrated using the peak positions of the Au 4f 7/2 (84.0 eV) and Cu 2p 3/2 (932.67 eV) core levels. The samples were supported using conductive scotch tape. The internal reference method was used for the correct calibration of the photoelectron peaks. Al 2р (Еb = 74.6 eV) and Al 2s (Еb = 119.4 eV) lines were used for calibration. A low-energy electron gun (FG-15/40, SPECS) was used for the sample charge neutralization. 2.3.7. High-resolution transmission electron microscopy (HRTEM) HRTEM images were obtained by a JEM-2010 electron micro-scope (JEOL, Japan) with a lattice-fringe resolution of 0.14 nm at an accelerating voltage of 200

kV. The high-resolution images of the periodic structures were analysed by the Fourier method. Samples for HRTEM examination were prepared on a perforated carbon film mounted on a copper grid. Stacking number and the slab length of the sulfide active component were defined using the average data for at least 1000 particles. The dispersion of the active component (fW) was calculated in the approximation of the ideal hexagonal shape of tungsten disulfide slabs [21] using Eq. (1) and (2): 𝑓𝑊 = ∑ 𝑛𝑖 =

∑𝑖=1..𝑡 6𝑛𝑖 −6

2 𝑖=1..𝑡 3𝑛𝑖 −3𝑛𝑖 +1

𝐿 0.64

+ 0.5

(1) (2)

Where ni is the number of W atoms along one side of a WS2 slab, L is the slab length and t is the total number of slabs shown by the TEM micrographs. 2.3.8. Catalyst sulfidation The catalysts were crushed to a 0.25–0.5 mm particle size and sulfided with H2S (200 h−1) at atmospheric pressure. Sulfidation was performed in two steps – 2 h at 220°С and 2 h at 400°С. After sulfidation the samples were cooled to room temperature in nitrogen flow. Sulfided catalysts were labeled as NiW-T-S, where «T» is the temperature of thermal treatment. All of the sulfide catalysts were kept in inert atmosphere (N2) after sulfidation. 2.3.9. Catalytic activity tests The tests of the sulfided catalysts in the HDS of DBT, HDN of quinoline and hydrogenation of naphthalene were carried out in the fixed-bed reactor in the down flow regime. For testing, 0.300 g of pre-sulphided catalyst was diluted with silicon carbide (0.125-0.25 mm) up to volume of 4 ml and placed in the isothermal zone of the reactor between two layers of silicon carbide. The reactor i.d. was of 12 mm and its length was of 400 mm. Conditions of the catalytic tests: temperature - 280 °C, pressure - 35 bar, LHSV - 40 h-1, ratio hydrogen:feed - 500 m3/m3. The model feed contained 5.0 wt.% of naphthalene, 1.44 wt.% of DBT (equivalent to 2500 ppm sulfur), 0.184 wt.% of quinoline (200 ppm nitrogen) in n-undecane. The reaction products were analyzed by gas chromatograph Clarus 580 (Perkin Elmer) equipped

with a flame ionization detector and a Restek RTX-DHA capillary column. The nitrogen content of the reaction products was determined using the Xplorer NS analyzer. 3. Results and discussion 3.1. Textural properties and carbon content According to the elemental analysis data samples NiW-120, NiW-220, NiW300 contain significant amounts of carbon (Table 1) due to the presence of citrate ligands or its decomposition products in the catalysts. With increasing thermal treatment temperature the carbon content in the catalysts decreases as a result of the decomposition of citrate fragments. The content of carbon in NiW-450 catalyst is relatively low, which indicates an almost complete decomposition of citrate. Sulfided catalysts also contain carbon but its content is reduced by 30-40% compared to the oxide catalysts. The texture characteristics of the support and catalysts in oxide and sulfided forms were studied using low-temperature nitrogen physisorption. Nitrogen adsorption-desorption isotherms are shown in Fig. 1. All of the studied samples show type IV isotherms according to the IUPAC classification with a H2 hysteresis loop, which are typical for mesoporous materials with "ink-bottle" pores. Fig. 2 shows the pore size distributions for the alumina support and the catalysts. The deposition of metals on the alumina surface leads to a significant reduction in the pore volume and the specific surface area. In NiW-120 and NiW-220 catalysts a significant decrease in specific surface area and pore volume compared to the support while maintaining the average pore diameter may indicate blocking of the support pores. As the thermal treatment temperature increases to 300 and 450°C the specific surface area and pore volume increase and the average pore diameter decreases which may indicate the unblocking of small mesopores. After sulfidation of NiW-300 and NiW-450 samples, the specific surface area and pore volume decrease significantly while for NiW-120 sample these parameters practically do not change.

3.2. Thermogravimetric analysis The TGA and DTG curves of the samples NiW-120, NiW-220, NiW-300 and NiW-450 (Fig.3) show a decrease in weight in the range of 50-150°C due to the removal of physically adsorbed water from the surface of the catalysts. For the NiW120 catalyst intensive mass loss occurs at the temperature interval 200-300°C with a maximum at 265°C on the DTG curve and is accompanied by an endothermic effect (maximum at 257°C on the DSC curve). The DSC curve of sample NiW-220 also shows an endothermic peak with a maximum at 267°C. The endothermic effect during the decomposition of citric acid and citrates is due to the decarboxylation and dehydroxylation reactions with the formation of acetonedicarboxylate and aconitate (from citric acid dehydroxylation and decarboxylation, respectively) [22,23]. From the comparison of the DTA curves and the elemental analysis data of the NiW-120 and NiW-220 catalysts it can be concluded that thermal treatment at 220°C results in partial dehydroxylation and decarboxylation of citrate ligands. Intensive weight loss for samples NiW-120 and NiW-220 at 280-400°C is associated with the further destruction of organic fragments and is accompanied by an exothermic effect with a maximum at 350-360°C. For the NiW-300 sample the maximum of weight loss on the DTA curve and the exothermic peak on the DSC curve are shifted toward higher temperatures (410 and 390°C, respectively). Comparing the curves of DTA catalysts NiW-220 and NiW-300 it can be concluded, that thermal treatment at 300°C leads to partial destruction of aconitate and acetonedicarboxylate fragments. The DTG curve of the NiW-450 sample contains only a peak corresponding to the desorption of physically adsorbed water, which indicates the almost complete removal of citrate ligands during the thermal treatment at 450°C. 3.3. Infrared and Raman spectroscopy The impregnating solution and the catalysts in oxide form were studied using FTIR and Raman spectroscopy (Fig. 4 and 5). In our previous work with the use of FTIR and Raman spectroscopy we obtained results indicating that in the impregnating solution prepared by dissolution of nickel hydroxide, citric acid and

ammonium paratungstate in water, tungsten is present in the form of complex anions [W2O5(Cit)2]6- with Ni2+ and ammonium ions present as counterions [19]. IR spectrum of the impregnation solution in the frequency range 1100-1800 cm-1 shows absorption bands related to asymmetric vibrations νas(C=O) of free β-carboxyl groups (1717 cm-1) and coordinated carboxyl groups (1635 and 1571 cm-1) [24]. Absorption bands related to the symmetric vibrations νs(C=O) in the range of 14001450 cm -1 overlap with bands of deformation ν4 vibrations of ammonium ions. When the complex compounds are supported on the alumina surface, the free carboxyl groups interact with the surface of the support, which leads to the disappearance of the 1717 cm-1 absorption band in the IR spectrum of the NiW-120 catalyst. For catalysts dried at 220 and 300°C FTIR spectra show absorption bands with a frequency of 1710-1715 cm -1. These absorption bands seems to be formed as a result of the decomposition of the citrate complex and are related to the νas(C=O) vibrations in the aconitate and acetonedicarboxylate fragments. The increase of the calcination temperature to 450°C leads to the almost complete decomposition of citrate ligands and the removal of ammonium ions, which is accompanied by the disappearance of corresponding absorption bands in the IR spectrum. Interpretation of Raman spectra of the catalysts is complicated by strong fluorescence of the samples. Nevertheless, in the Raman spectra of catalysts, absorption bands in the range 900-1050 cm-1 related to the vibrations of terminal oxygen atoms ν(W=O) can be distinguished. Raman spectrum of NiW-120 catalyst shows absorption band ν(W=O) 946 cm-1, which is close to ν(W=O) band in the impregnating solution (943 cm-1) which indicates that the structure of [W2O5(Cit)2]6does not change significantly. Some broadening of this band can be attributed to the interaction of [W2O5(Cit)2]6- with the support surface. The broadening of the absorption band ν(W=O) with a shift of the absorption maximum up to 958 cm-1 in Raman spectrum of NiW-220 catalyst can be associated with the formation of polytungstates [25]. The absorption bands ν(W=O) in the range 985-1000 cm -1 in

the NiW-450 Raman spectrum can be associated with the formation of surface tungsten oxide [26,27]. 3.4. Temperature-programmed reduction (H2-TPR) H2-TPR profiles for the catalysts in the oxide form are shown in Fig. 6. The low temperature TPR signals (260-310°C) for NiW-120 and NiW-220 samples could be due to evolution of products from decomposition of loosely bound citric acid. [28]. TPR profiles of NiW-120, NiW-220 and NiW-300 catalysts are quite similar. TPR profiles of these samples show an intense peak with a maximum of about 455°C in the range of 300-600°C which can be attributed to the reduction of nickel compounds. In work [9] the complex compound [Ni(CyDTA)]2− deposited on γ-Al2O3 showed reduction peak at 452°C, while for nickel nitrate deposited on γAl2O3 the maximum on the TPR profile was observed at about 300°C. Massive NiO particles deposited on γ-Al2O3 also undergo reduction at 300°C [29] however, their formation at heat treatment temperatures of 300°C or lower can be ruled out. Thus, the maximum at 455°C observed for NiW-120, NiW-220 and NiW-300 samples is characteristic for nickel (II) cations in the complex compound. The peak symmetry may indicate a homogeneous distribution of nickel compounds. Table 2 shows data on the hydrogen consumption in the temperature ranges of 300-600 and 600-1000°C. For NiW-120, NiW-220 and NiW-300 samples, the molar ratio of H2 consumption in the interval 300-600°С to the nickel content in the catalyst may indicate a complete reduction of nickel. For the NiW-450 catalyst, the hydrogen consumption at 300-600°C is significantly lower, which indicates an incomplete reduction of nickel in this temperature range. Reduction peaks in the range of 600 to 1000°C can be related to the reduction of tungsten oxide species and products of strong interaction of nickel with the support, such as NiAl2O4. In this temperature range a peak with a maximum at 655°C and a superposition of peaks of 830 and 940°C were observed in the TPR spectra of NiW-120, NiW-220 and NiW-300 catalysts. The first peak (655°C), apparently, is associated with the reduction of nickel-polytungstate complexes containing

octahedrally coordinated cations of W(VI) [30]. Significant differences between NiW-300 sample and samples NiW-120 and NiW-220 are observed only in the range 730-940°C. This temperature range corresponds to the reduction of tungsten oxide compounds interacting with the support and containing W(VI) cations in the tetrahedral environment [31,32]. Apparently, thermal treatment at 300°C somewhat improves the ability to reduction of tungsten species. The TPR profile of NiW-450 sample differs significantly from NiW-120, NiW-220 and NiW-300 and shows a wide asymmetric peak with a maximum at 740°C. The large width of the peak indicates that this peak contains contributions from a set of components including tungsten and nickel oxide compounds with different interaction degrees with the carrier and with each other. In any case, the temperature range of the reduction of nickel compounds is shifted to higher temperatures in comparison with the NiW-120, NiW-220 and NiW-300 catalysts, which indicates the presence of nickel in the form of compounds strongly interacting with the carrier and/or tungsten oxide. The maximum at 740°C according to the literature data may correspond to the reduction of the surface tungsten oxide interacting with alumina [26,33] and/or reduction of NiWO4 [34]. The shoulder on the TPR profile of about 900°C can also be associated with the reduction of NiAl 2O4 [33]. 3.5. XPS data In order to determine the state of Ni and W on the surface of the sulfided catalysts XPS spectroscopy was used. W 4f and Ni 2p XPS spectra of the sulfide catalysts and their decomposition by components are shown in Fig. 7 and 8. According to the data given in the literature [35], the decomposition of the Ni 2p XPS spectrum was carried out on 3 components: Ni in NiWS phase (Ni 2p3/2: 853.7 eV, Ni 2p1/2: 870.9 eV), sulfide NixSy (Ni 2p3/2: 852.6 eV, Ni 2p1/2: 869.8 eV) and nickel oxide species (Ni 2p3/2: 856.4 eV, Ni 2p1/2: 873.8 eV). The decomposition of W 4f XPS was carried out on two components: sulfided W4+ (W 4f7/2: 32,2 eV, W

4f5/2: 34.3 eV) and oxide W6+ (W 4f7/2: 35.8 eV, W 4f5/2: 38.0 eV). The relative contents of nickel and tungsten species are given in Table 3. The sulfidation degree of tungsten in the catalysts is from 48 to 53%, which is typical for NiW catalysts as these catalysts are difficult to sulfide. The degree of tungsten sulfidation decreases as the drying (calcination) temperature increases, which is due to the removal of citrate ligands, which can perform a screening function. At the same time, this effect is not very pronounced, which can be explained by the high sulfidation temperature (400°C). The proportion of nickel oxide compounds also increases with the increase of the thermal treatment temperature, which can also be associated with a stronger interaction with the carrier. The relative content of NiWS is from 59 to 62%. With an increase in the thermal treatment temperature there is a slight decrease in Ni content in the NiWS phase and in sulfide NixSy. 3.6. HRTEM On TEM micrographs (Fig.9) WS2-like sulfide slabs oriented parallel to the direction of the electron beam are observed as dark lines. The size of the sulfide layers and the degree of stacking correspond to the literature data for NiW/Al 2O3 catalysts [8,9,36]. Length and stacking number distributions for WS2–like crystallites are shown in Fig.10. With increasing thermal treatment temperature, an increase in the degree of stacking of WS2 sulfide layers is observed. Single-layer sulfide particles predominate in the NiW-120-S catalyst; single- and double-layered - in the NiW-220-S and NiW-300-S catalysts; two- and three-layer - in the NiW-450 catalyst. An increase in the thermal treatment temperature from 120°C to 220°C leads to a decrease in the average length of the sulfide slabs, giving a minimum, while the further increase in the treatment temperature leads to an increase in the size of the sulfide layers. The calcination at 450°C leads to a significant increase in the sulfide slabs length, leading to a significant decrease in the number of edge tungsten atoms (Table 4).

3.7. Results of testing in DBT HDS, quinoline HDN and naphthalene hydrogenation The catalysts were tested in simultaneous HDS DBT, quinoline HDN and hydrogenation of naphthalene (Table 5). The conversion of DBT proceeded almost exclusively by the direct desulfidation route with the formation of biphenyl, which is due to the presence of quinoline in the feed mixture, which inhibits the hydrogenation route of the conversion of DBT [37]. Tetralin was the only product of the hydrogenation of naphthalene. The formation of decalin was not observed, since monoaromatic compounds are much more difficult to undergo hydrogenation in comparison with polyaromatic compounds [38,39]. Quinoline under the conditions of the hydrotreating reaction is relatively easily converted to 1,2,3,4tetrahydroquinoline, further breaking of the C-N bonds and complete removal of nitrogen from the molecule proceeds much more difficultly [40]. For this reason, the total nitrogen content of the reaction products was measured in this work and the degree of denitrogenation was calculated instead of quinoline conversion. The tests of the catalysts showed correlation between the activity in HDS of DBT, HDN of quinoline and hydrogenation of naphthalene. NiW-300-S catalyst was the most active in all of these reactions. An increase in the thermal treatment temperature from 120°C to 300°C leads to a gradual increase in the activity of NiW/Al2O3 catalysts in HDS DBT, HDN quinoline and hydrogenation of naphthalene. Calcination of the catalyst at 450°C leads to a sharp decrease in activity. 3.8. Discussion In order to explain the differences in the activity of the catalysts, let us consider the data obtained by physicochemical methods. According to the XPS data, the Ni content in NiWS phase and the degree of sulfidation of tungsten differ slightly for the catalysts NiW-120-S, NiW-220-S and NiW-300-S. At the same time, a significant decrease in the length of sulfide slabs and an increase in the WS 2 stacking degree are observed if we move from NiW-120-S catalyst to NiW-220-S. It was

shown earlier in the literature that increasing the degree of stacking can significantly increase the activity of NiW catalysts [41]. The WS2 multi-layered particles have a higher density of vacancies compared to the single-layered particles and the particles with a smaller number of sulfide layers. Thereby higher WS2 stacking degree promotes π-adsorption of aromatic molecules and increases the hydrogenating activity of catalysts containing multilayered sulfide particles. The formation of particles with a higher degree of stacking may be due to a weaker interaction with the support. Citrate complexes interact with the surface of the support through the carboxyl groups of citrate ligands. Thermal treatment at 220 and 300°C leads to the decarboxylation and decomposition of citrate ligands, which can weaken the interaction of the metals with the support and favor formation of the sulfide particles with a higher stacking degree during the sulfidation. Weakening the interaction of the oxidic precursors of the active phase with the support can significantly improve desulfurizating properties of HDT catalysts [28,42,43]. At the same time, carbon formed during decomposition of citrate ligands can perform an isolating function and prevent an increase in the length of sulfide slabs. Comparing NiW-220-S and NiW-300-S catalysts slight increase in the average size of the sulfide layers and a slight decrease in the sulfidation degree of nickel and tungsten for NiW-300-S are observed. However, the NiW-300-S exhibits higher activity compared to the NiW-220-S catalyst. One explanation may be a further increase in the degree of stacking of the sulfide phase with the increase of the treatment temperature from 220 to 300°C. On the other hand, when the thermal treatment temperature is increased from 220 to 300°C the carbon content in the catalyst is significantly reduced with the increase of specific surface area and pore volume. Thus, thermal treatment at 300°C can increase the availability of the active component for the reacting molecules. The increase of the thermal treatment temperature to 450°C leads to a noticeable decrease in the degree of nickel and tungsten sulfidation, as well as to a significant increase in the size of the sulfide slabs, which is expressed in a decrease

in the number of edge tungsten atoms. In comparison with NiW-300-S, the number of edge tungsten atoms in NiW-450-S is lower by approximately 30%. Apparently, this effect is due to the fact that the NiW-450 catalyst contains practically no carbon, which can have an insulating function and contribute to an increase in the dispersion of the sulfide component. 4. Conclusions The thermal treatment temperature in the preparation of NiW/Al 2O3 hydrotreating catalysts is an important factor that significantly influences their structure and activity. It is shown that an increase in the thermal treatment temperature of a NiW/Al2O3 catalyst prepared using citric acid as a chelating agent from 120°C to 300°C leads to a significant increase in the activity of the catalyst in DBT HDS, quinoline HDN and hydrogenation of naphthalene. The increase in activity is due to the increase in the stacking degree of WS2 while maintaining small length of sulfide slabs and to the decrease in the carbon content, which can impede access to active centers. Thermal treatment at a higher temperature (450°C) results in a low catalyst activity due to a decrease in the sulfidation degree of tungsten and nickel and a decrease in the dispersion of the sulfide component. Acknowledgement The work was supported by Ministry of Education and Science of the Russian Federation: Project No. 14.610.21.0008, identification number of the project RFMEFI61015X0008. References [1] S.P. Ahuja, M.L. Derrien, J.F. Le Page, Prod. RD 9 (1970) 272–281. [2] B.H. Cooper, A. Stanislaus, P.N. Hannerup, ACS Prepr. Div Fuel Chem. 37 (1992) 6. [3] A. Stanislaus, B.H. Cooper, Catal. Rev. 36 (1994) 75–123. [4] H.R. Reinhoudt, R. Troost, S. van Schalkwijk, A.D. van Langeveld, S.T. Sie, H. Schulz, D. Chadwick, J. Cambra, V.H.J. de Beer, J.A.R. van Veen, J.L.G. Fierro, J.A. Moulijn, in:, B.D. and P.G. G.F. Froment (Ed.), Stud. Surf. Sci. Catal., Elsevier, 1997, pp. 237–244.

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Fig. 1. Nitrogen adsorption–desorption isotherms of alumina support, oxide catalysts (a) and sulfided catalysts (b) Fig. 2 Pore size distributions of the alumina support, oxide catalysts (a) and sulfided catalysts (b) (obtained from the desorption branch of the isotherms) Fig. 3. Profiles of TGA (a), DTA (b) and DSC (c) of the NiW/Al2O3 catalysts in oxide form Fig. 4. FTIR spectra of the impregnating solution (a) and samples NiW-120 (b), NiW-220 (c), NiW-300 (d), NiW-450 (e) Fig. 5. Raman Spectra of the impregnating solution (a) and samples NiW-120 (b), NiW-220 (c), NiW-300 (d), NiW-450 (e) Fig. 6. H2-TPR profiles of NiW-120, NiW-220, NiW-300 and NiW-450 samples Fig. 7. W 4f XPS spectra of NiW-120-S (a), NiW-220-S (b), NiW-300-S (c), NiW-450-S (d) samples Fig. 8. Ni 2p XPS spectra of NiW-120-S (a), NiW-220-S (b), NiW-300-S (c), NiW-450-S (d) samples Fig. 9. HRTEM images of catalysts NiW-120-S (a), NiW-220-S (b), NiW-300-S (c), NiW-450-S (d) Fig. 10. Length (a) and stacking number (b) distributions for WS2–like crystallites in the sulfided catalysts

Table 1. Carbon content in oxide and sulfided NiW/Al2O3 catalysts and their texture characteristics Sample

Support

Carbon content, wt.%

BET surface area, m2/g

Pore volume, cm3/g

Average pore diameter, Å

-

182

0.75

109

Catalysts in oxide form NiW-120

5.72

72

0.27

108

NiW-220

5.43

88

0.32

111

NiW-300

3.09

143

0.44

97

NiW-450

0.27

155

0.53

102

Sulfided catalysts NiW-120-S

3.56

75

0.27

108

NiW-220-S

3.24

76

0.26

108

NiW-300-S

2.18

95

0.33

107

NiW-450-S

0.16

104

0.35

107

Table 2. H2 TPR data Catalyst

Hydrogen consumption, mmol/g

300-600°C

600-1000°C

Molar ratio of H2 consumption in temperature interval 300-600°С to the nickel content in the catalyst,

Molar ratio of H2 consumption in interval 6001000°С to the tungsten content in the catalyst

(mol/g)/(mol/g)

(mol/g)/(mol/g)

NiW-120

0,48

0,34

1,27

0,45

NiW-220

0,51

0,34

1,29

0,42

NiW-300

0,60

0,47

1,40

0,54

NiW-450

0,31

1,19

0,68

1,29

Table 3. W 4f and Ni 2p XPS data Sample

Relative contents, % WS2

WOx

NiOx

NiWS

NixSy

NiW-120-S

53

47

25

62

13

NiW-220-S

53

47

29

61

10

NiW-300-S

51

49

28

61

11

NiW-450-S

48

52

31

59

10

Table 4. Properties of sulfide phase obtained from HRTEM

Sample

Average slab length, Å

fW

Average stacking number

NiW-120-S

38.2

0.25

1.8

NiW-220-S

32.4

0.29

2.2

NiW-300-S

35.5

0.26

2.5

NiW-450-S

45.7

0.18

3.2

Table 5. Results of activity tests in DBT HDS, quinoline HDN and hydrogenation of naphthalene Catalyst

DBT conversion, %

Naphthalene conversion, %

Quinoline HDN degree, %

NiW-120-S

81.0

16.9

17.6

NiW-220-S

84.7

19.7

18.8

NiW-300-S

92.6

22.7

21.4

NiW-450-S

63.4

13.6

16.7