A new approach to construct a hydrodesulfurization catalyst from a crystalline precursor: ligand-induced self-assembly, sulfidation and hydrodesulfurization

Jilei Liang ab, Mengmeng Wu ab, Jinjin Wang a, Pinghe Wei b, Bingfeng Sun b, Yukun Lu *a, Daofeng Sun a, Yunqi Liu *a and Chenguang Liu a
aState Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, P. R. China. E-mail: lyk@upc.edu.cn; liuyq@upc.edu.cn
bJiangsu Key Laboratory of Chiral Pharmaceuticals Biosynthesis, College of Pharmacy and Chemistry & Chemical Engineering, Taizhou University, Taizhou 225300, P. R. China

Received 27th September 2018 , Accepted 26th October 2018

First published on 26th October 2018


This paper proposes a new approach for investigating the mechanism of the formation of the active phase of a hydrodesulfurization (HDS) catalyst via crystalline polyoxometalate (POM) precursors. The proposed strategy induces the crystallization of small Ni–Mo–O clusters in an impregnating solution by the coordinate bonding and supramolecular interaction of organic ligands to form POMs. By exploiting the “ligand-induced self-assembly” strategy, two Ni–Mo binary POMs with different frameworks, namely, Mo2Ni and PMo11Ni, were isolated from the impregnating solution by means of 4,4′-bpy. The sulfidation process of the precursors and the formation mechanism of the NiMoS active phase were fully characterized by a multi-technique approach that comprised, in particular, in situ FT-IR spectroscopy, XRD and Raman spectroscopy for different degrees of sulfidation. The results of the characterization revealed the structure-directing effects (framework effect, promoting effect and ligand effect) of the POM precursors on the structure of the active phase and even its HDS performance. MoS2 was formed at 200 °C from Mo2Ni, and the Ni species interacted with the edges of MoS2 to form the NiMoS active phase, whereas PMo11Ni formed MoS2 at 300 °C. The structure-directing effects enabled a higher content and better dispersion of the NiMoS active phase, which explains the higher HDS reactivity of sulfided Mo2Ni. The bottom-up self-assembly approach not only provides a better understanding of the composition of the impregnating solution and the formation mechanism of the NiMoS active phase but also sheds light on the rational design and controllable preparation of NiMoS catalysts with high performance.


1. Introduction

Increasingly strict fuel standards and environmental legislation motivate petroleum refineries to provide green fuels with an ultra-low sulfur content.1–4 To reach such a low level, hydrodesulfurization (HDS) is considered to be one of the most effective methods of removing sulfur-containing compounds from fuels.5–8 The key to improving HDS performance lies in the preparation of highly effective catalysts, which has attracted considerable attention in recent years. Currently, industrial HDS catalysts commonly contain Mo-based sulfides as the active components and Ni as a promoter supported on γ-Al2O3, i.e., the so-called synergistic NiMoS active phase.9,10 These catalysts are traditionally fabricated in a simultaneous step or a succession of steps (incipient wetness impregnation in both cases), which are then usually followed by calcination to obtain oxidic catalysts and subsequent sulfidation to produce NiMoS catalysts. In general, one of the most important steps is the impregnation step owing to the dramatic effects of deposition of the oxidic precursor on the final NiMoS active phase.11

Regarding the P- and Mo-containing impregnating solution, this is actually a polyoxometalate (POM)-based mixture consisting of PMo9O31(OH)36−, PMo11O397−, PMo12O403−, P2Mo5O236−, and P2Mo18O626−.12,13 These POMs are metal–oxygen clusters with well-defined structures, sizes and shapes and have attracted great attention in many fields.14,15 Under appropriate conditions, these clusters can be transformed into each other,16 as shown in Fig. 1. Although the identification of P-containing POMs has been performed for decades via31P NMR spectroscopy in the liquid as well as the solid state, the separation of these POMs from the impregnating solution and their structural determination remain a challenge, which also means that it is difficult to establish correlations between the precursor and the active phase. This embarrassing situation therefore hinders the development of more active HDS catalysts. Hence, the investigation of innovative molecular platforms other than conventional precursors for establishing correlations between the precursor and the active phase is required.


image file: c8cy02007h-f1.tif
Fig. 1 Equilibria between various POMs in the impregnating solution.

Researchers have tested many methods for the rational design and fabrication of potential molecular platforms, in particular, the addition of organic chelating agents to prepare precursors with novel chemical compositions. In these studies, Co2+ and Ni2+ cations were bound to chelating agents to form stable complexes and then linked to different Mo-containing heteropolyanions. On the one hand, the chelating agents provide close contact between Ni2+/Co2+ ions and Mo-containing heteropolyanions over the surface of the support, especially in the stages of drying and sulfidation. On the other hand, the stable complexes formed between the chelating agents and Ni2+/Co2+ ions do not collapse after drying but decay during the stage of sulfidation via the selective attachment of Ni/Co atoms to the edges of MoS2 slabs. Citric acid (CA),17,18 ethylenediaminetetraacetic acid (EDTA),19,20 and nitrilotriacetic acid (NTA)21 are the chelating agents commonly used to obtain these novel HDS catalyst precursors. In fact, the methodology prepares POMs as molecular platforms for the design and preparation of HDS catalysts. However, it is difficult to determine the formula and molecular structure of POMs prepared in this way, which results in an unsolved question regarding the relation between the HDS catalyst precursor and the active phase. Moreover, in the abovementioned precursors there is no guarantee that the Ni promoter atoms will disperse homogeneously and connect closely with Mo clusters via an Ni–O–Mo coordination interaction, which leads to an inefficient Ni-Mo synergistic effect.

In our previous study, we prepared Strandberg-type P–Mo–Ni POMs (Ni(NH4)2P2Mo5O21 and Ni2P2Mo5O21), which are essential components of the P- and Mo-containing impregnating solution, and employed them as molecular platforms for the rational design and controllable fabrication of NiMoS catalysts.22 Multiple directing effects of the Strandberg-type P–Mo–Ni POM precursors on the composition and morphology of the NiMoS active phase were found. However, the effects of other POMs in the impregnating solution on the NiMoS active phase were still not known. Inspired by the strategy of the ligand-induced self-assembly of POMs in coordination and structural chemistry, we designed a new approach for preparing an HDS catalyst precursor with a specific formula and molecular structure. In this approach, a rationally selected organic ligand was added to the impregnating solution to disrupt the pre-established equilibria of the POMs. Then, the ligand induced the nucleation and self-assembly of clusters via coordination and weak interactions to form crystalline POMs with a specific formula and molecular structure, which we separated from the solution. With the help of modern analytical instruments, the molecular and crystal structure of the POMs were clearly revealed, including the cluster frameworks, Ni–Mo relative positions, and bond distances and angles, etc. The employment of well-defined POMs as active materials or precursors is beneficial for studying the active site and activation mechanism of the HDS reaction, and it is therefore an ideal model for studying the structure and formation mechanism of the NiMoS active phase in an HDS catalyst.

In the present study, 4,4′-bpy was selected as the organic ligand owing to its various types of bonding, such as coordinate bonding and hydrogen bonding via the N atoms and stacking interactions of the pyridine rings (Fig. 2). Two interesting Ni–Mo binary transition metal POMs, namely, Ni2(4,4′-bpy)Mo4(4,4′-bpy)2O14 (denoted as Mo2Ni, 4,4′-bpy = 4,4′-bipyridine) and [HPMo11Ni(4,4′-bpy)O39][Ni(4,4′-bpy)(H2O)3]·2(4,4′-Hbpy)·3H2O (denoted as PMo11Ni) were successfully synthesized from the impregnating solution.23,24 In these POMs, Ni and Mo atoms are connected to each other via bridging O atoms in a two-dimensional layered structure. In Mo2Ni, these bimetallic layers are interlinked via 4,4′-bpy molecules, which produce a three-dimensional pillared layer framework. In contrast, a three-dimensional zipper-like framework is formed in PMo11Ni. These POMs were obtained from the solution with a specific formula and molecular structure, and the proximity of the Ni and Mo atoms was established. Hence, as HDS catalyst precursors, these POMs may provide potential molecular platforms for the study of the correlation between the precursor and the active phase.


image file: c8cy02007h-f2.tif
Fig. 2 Schematic representation of the role of 4,4′-bpy: various modes of interaction in self-assembly.

On the basis of our recent study of NiMoS catalysts,22,25 in this paper we studied the evolution of these POMs during sulfidation and characterized the sulfidation process of POMs in detail at each step by a multi-technique approach that combined temperature-programmed sulfidation, Raman spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, in situ Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, N2 physisorption, CO temperature-programmed desorption, and X-ray photoelectron spectroscopy. This study not only opens new avenues for understanding the composition of the impregnating solution and the formation mechanism of the NiMoS active phase but also sheds light on the rational design and controllable preparation of NiMoS catalysts with high performance.

2. Experimental section

2.1. Materials

Nickel nitrate (Ni(NO3)2·6H2O, AR), 4,4′-bpy (C10H8N2, AR), phosphoric acid (H3PO4, AR), decalin (C10H18, AR), dibenzothiophene (DBT, C12H8S, AR), and ammonium heptamolybdate [(NH4)6Mo7O24·4H2O, AR] were obtained from Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation of PMo11Ni and Mo2Ni

PMo11Ni and Mo2Ni were prepared as described in our previous work. A mixture of (NH4)6Mo7O24 (0.272 g for PMo11Ni, 0.374 g for Mo2Ni), Ni(NO3)2 (0.145 g), 4,4′-bpy (0.165 g), and H2O (30 mL) was stirred evenly, and the pH was adjusted to 4.3 (for PMo11Ni) or 5.8 (for Mo2Ni) with H3PO4. The suspension that was obtained was sealed in an autoclave and heated at 140 °C for 3 days, and green (PMo11Ni) or cyan (Mo2Ni) crystals were separated, washed and air-dried.

2.3. Catalytic test

A test of HDS reactivity for DBT was carried out in a batch reactor, and the temperature and pressure used were 300 °C and 2.0 MPa, respectively. Before the evaluation, 0.1 g Mo2Ni or 0.078 g PMo11Ni (containing the same content of Mo) was pre-sulfided at a certain temperature in a gas mixture of H2S (10 vol%)/H2. Then, the sulfidation product was put into the batch reactor with 50 g of a solution of DBT (2 wt%) in decalin. The sulfide particles had a diameter of 0.25 mm and the stirring rate was 1000 rpm, which can overcome the limitations of inter- and intra-particle heat and mass diffusion.26 The product distribution was determined using an Agilent 6890 chromatograph with a 50 m OV-101 capillary column.

Eqn (1) was employed to calculate the proportion of Mo atoms at the edges of MoS2, which is denoted as fMo.27

 
image file: c8cy02007h-t1.tif(1)
where Moedge is the number of Mo atoms at the edge of the metal sulfide, Mototal is the total number of Mo atoms in the metal sulfide, ni is the total number of Mo atoms along one edge of an MoS2 slab calculated viaeqn (2), and t is the number of MoS2 slabs.
 
L = 3.2(2ni − 1) Å(2)

The calculated value of fMo is an indicator of the turnover frequency (TOF, s−1) in HDS performance. We assumed that all edge sites of the catalyst are reactive in the HDS performance for DBT, and the TOF value determined by eqn (1) is slightly smaller than the real TOF value.28,29

2.4. Characterization

Crystal data were collected using a Bruker SMART Apex CCD single-crystal X-ray diffractometer at room temperature. The crystal data and cell refinement of the two POMs were solved by direct methods and refined by full-matrix least-squares on F2 with the SHELXTL crystallographic package.30,31 The data are summarized in Table S1 in the ESI.

Temperature-programmed sulfidation (TPS) was performed with a continuous flow of H2S (10 vol%)/H2, and the evolution of H2S was monitored by UV-vis spectroscopy. TPS was performed at a heating rate of 5 °C min−1 up to 650 °C, and this temperature was then maintained for 30 min.

In situ FT-IR spectra were recorded using a Bruker FT-IR spectrometer with a liquid-N2-cooled MCT detector. Mo2Ni/PMo11Ni was pelletized with KBr and put in the in situ cell. Then, the sample was heated in a flow of H2S (10 vol%)/H2 at a rate of 2 °C min−1, and the FT-IR spectra were recorded at different temperatures.

Raman spectra were recorded using an inVia Reflex laser Raman spectrometer (Renishaw, UK) at room temperature. The laser wavelength and wavenumber accuracy were 532 nm and 2 cm−1, respectively. To avoid decomposition of the sample during recording, the power of the laser beam was decreased using a filter.

Powder XRD profiles were obtained with a PANalytical XRD apparatus (X'Pert PRO MPD) using Cu Kα radiation (λ = 0.15406 nm) with a voltage of 40 kV. The samples were scanned from 5° to 75° at a scanning rate of 0.05° s−1.

High-resolution transmission electron microscopy (HRTEM) images of the POM sulfides were obtained with a JEM-2100UHR microscope. Before measurement, the samples were ultrasonically dispersed in ethanol. Then, drops of the suspension were used, and the sulfide was well dispersed on a copper grid coated with carbon.

The textural properties of the sulfided POMs were determined from low-temperature N2 adsorption–desorption isotherms recorded using a Micromeritics ASAP 2020 instrument. Prior to the measurements, the samples were outgassed in a vacuum. The specific surface area was determined via the Brunauer–Emmett–Teller method, whereas the total pore volume and pore size distribution were calculated via the Barrett–Joyner–Halenda method.

X-ray photoelectron spectroscopy (XPS) experiments on the fresh Mo2Ni and PMo11Ni sulfides were performed using a PHI 5000 VersaProbe instrument with monochromatic Al Kα radiation. The binding energy was calibrated with the C 1s peak as an internal standard. To quantify the contents of Mo and Ni species, XPSPEAK software was used to fit the XPS spectra.

CO temperature-programmed desorption (CO-TPD) of samples was monitored with a Micromeritics AutoChem II chemisorption analyzer. The samples were heated at 500 °C for 1 h and exposed to CO for 0.5 h at 100 °C. Then, CO was desorbed in an He atmosphere for 1 h. Finally, the TPD profile was obtained at a heating rate of 10 °C min−1.

3. Results and discussion

3.1. Characterization of Mo2Ni and PMo11Ni

The structures of the POMs that were obtained were determined from the single-crystal XRD data, and their frameworks were determined by Diamond software and are presented in Fig. 3. In Mo2Ni, Ni is coordinated by 4,4′-bpy molecules and the O atoms of {MoO6} octahedra to form {N(NiO4)N} octahedra. These {N(NiO4)N} octahedra and Mo polyhedra are connected alternately to form 6-membered {Mo4Ni2} rings in a corner-sharing pattern, as shown in Fig. S1. These {Mo4Ni2} rings are linked via corner-sharing to form a three-dimensional pillared layer framework. In PMo11Ni, Ni is coordinated by 4,4′-bpy molecules and discrete H2O molecules, which form {[Ni(4,4′-bpy)3(H2O)3]2+}n chains. With the existence of π⋯π interactions, these chains and the Keggin-type PMo11NiO395− fragments are connected to form a two-dimensional supramolecular layer in the ab plane and are further interlinked along the c axis to form a three-dimensional zipper-like framework.
image file: c8cy02007h-f3.tif
Fig. 3 Polyhedron representations of Mo2Ni (a) with a pillared layer framework and PMo11Ni (b) with a zipper-like framework.

POMs of the formulae Mo2O72− and PMo11O397− exist in the P–Mo impregnating solution. With the addition of Ni2+, interactions between Mo2O72−, PMo11O397−, and Ni2+ occur to form {Mo4Ni2} rings and PMo11NiO395− fragments. Then, 4,4′-bpy ligands induce these {Mo4Ni2} rings or PMo11NiO395− fragments to self-assemble by π⋯π stacking and hydrogen-bonding interactions to form the pillared layer framework of Mo2Ni and zipper-like framework of PMo11Ni shown in Fig. 4.


image file: c8cy02007h-f4.tif
Fig. 4 From small Ni–Mo–O clusters in solution to solid-state coordination frameworks of Mo2Ni and PMo11Ni via a ligand-induced self-assembly strategy.

On comparing the structures of Mo2Ni and PMo11Ni with those of HDS catalyst precursors reported in the literature,32,33 we observed two advantages of the POMs reported in this paper. Firstly, Mo2Ni and PMo11Ni are single crystals and have a homogeneous distribution of Ni and Mo atoms in the crystal lattice, which can prevent the formation of a catalyst with an eggshell structure. Secondly, in the HDS catalyst precursors reported in the literature the Ni and Mo atoms are in a separate cation and anion, respectively. In contrast, the Mo and Ni atoms in our POMs are connected by bridging O atoms to form Mo–O–Ni coordinate bonds with Mo⋯Ni (intramolecular) distances of 3.705 Å and 3.403 Å in Mo2Ni and PMo11Ni, respectively. The Mo–O–Ni bonds in Mo2Ni and PMo11Ni can provide the required atomic-scale contact between Ni and Mo during the sulfidation stage.34

In addition, some differences in structure and composition were found between Mo2Ni and PMo11Ni, as follows. (1) Different frameworks. The planar Ni–Mo–O clusters form a pillared layer framework in Mo2Ni, whereas the Keggin-type Ni–Mo–O clusters form a zipper-like framework in PMo11Ni. The pillared layer networks of Mo2Ni are linked by 4,4′-bpy molecules, which results in micropores with a size of 11.43 × 11.40 Å. In contrast, no pores are formed in PMo11Ni. (2) Different Ni/Mo ratios. The Ni/Mo ratios in Mo2Ni and PMo11Ni are 0.5 and 0.18, respectively. (3) Different contents of 4,4′-bpy molecules. The contents of 4,4′-bpy in Mo2Ni and PMo11Ni are 39% and 24%, respectively. These 4,4′-bpy molecules can isolate adjacent structural units, which prevents agglomeration of the NiMoS active phase formed during sulfidation. (4) Different degrees of agglomeration and dispersion of Mo clusters. In Mo2Ni, the Mo atoms are arranged in {Mo2O7} dimers and interconnected to form a monolayer pattern, and are then linked by 4,4′-bpy molecules to generate the pillared layer structure with an interlayer distance of 9.121 Å. On the other hand, PMo11Ni comprises {Mo11O35} undecamers, and the average distance between adjacent PMo11NiO395− fragments is only 5.452 Å. These results indicate that the degree of agglomeration is higher and the dispersion of Mo atoms is poorer in PMo11Ni.

3.2. Sulfidation of Mo2Ni and PMo11Ni

The TPS results for bulk Mo2Ni and PMo11Ni are shown in Fig. S2. Three H2S consumption regions are found in the TPS traces of Mo2Ni and PMo11Ni, and fully sulfided states were achieved, as both traces returned to the baselines.35 For Mo2Ni, the first H2S consumption region occurred at about 140 °C, whereas that for PMo11Ni occurred at 204 °C. The fact that the H2S consumption temperature for Mo2Ni was lower indicates that it is more easily sulfided than PMo11Ni. Three explanations can account for this phenomenon, as follows. (1) The Mo–Oa bond length in Mo2Ni (1.725 Å) is slightly greater than that in PMo11Ni (1.682 Å) (Tables S2 and S3), which suggests that the Mo–Oa bonds in Mo2Ni can be more easily sulfided. (2) As shown in Fig. S2, a large number of hydrogen bonds and π⋯π interactions constitute the POM-based supramolecular synthon in PMo11Ni, which makes this compound more stable and thus results in a higher sulfidation temperature. (3) Nikulshin et al.36 reported that the planar structure of Anderson-type POMs was beneficial for sulfidation. Hence, it is also inferred that Mo2Ni with its pillared layer framework undergoes faster sulfidation than PMo11Ni with its zipper-like framework. In addition, the micropores between the pillared layer networks in Mo2Ni can adsorb H2S, which is probably another reason that accounts for the easy sulfidation of Mo2Ni.

Because the order of sulfidation of Ni and Mo in HDS catalysts is still under debate, no conclusion can be drawn. Some researchers believe that the sulfidation temperature of Ni is lower and hence that Ni is sulfided earlier than Mo.37,38 However, other researchers think that chelating agents can promote the sulfidation of Mo and retard that of Ni.39,40 Thus, an Ni promoter can be anchored to the active edges of the MoS2 slabs and subsequently form the NiMoS active phase. The structural analysis of Mo2Ni and PMo11Ni showed that Ni is coordinated by the N atoms of 4,4′-bpy to form an Ni-4,4′-bpy complex. Therefore, it is inferred that 4,4′-bpy can serve as a chelating agent and may retard the sulfidation of Ni. The first H2S consumption peak is related to simple O–S exchange to produce MoOxSy species, whereas the remaining consumption peaks are associated with the sulfidation of MoOxSy to form MoS2 and the sulfidation of Ni.41

In situ FT-IR spectra were recorded to reveal the evolution of Mo2Ni and PMo11Ni during the sulfidation process (Fig. 5). For Mo2Ni, the peaks at 870–900 cm−1, 830–860 cm−1, and 1400–1700 cm−1 are ascribed to vibrations of Mo–Oa, Mo–Ob–Mo, and 4,4′-bpy, respectively. The coordination environment and bond length of Mo-O in Mo2Ni are shown in Fig. S4 and Table S2. As indicated above, the Mo–Oa bond length in Mo2Ni is greater than the typical Mo–Oa bond length, which results in a decrease in the force constant k. According to eqn (3), the peak due to Mo–Oa in Mo2Ni undergoes a red shift to a lower frequency. The peak intensity decreased with an increase in the sulfidation temperature, which indicated that Mo2Ni was gradually sulfided. However, the intensity of the Mo–Ob–Mo peak declined more quickly than that of the Mo–Oa peak, which indicated that O–S exchange mainly occurs on Mo–Ob bonds and later on Mo–Oa bonds during sulfidation. The reason is the greater bond length and lower bond energy of Mo–Ob, which make it more reactive.42 At 200 °C, the peak due to Mo–Ob–Mo disappeared, which was probably a sign of the formation of Mo–S. As the temperature increased to 400 °C, the peaks due to Mo2Ni disappeared, which indicated that carbonization was accompanied by sulfidation and that a highly sulfided state was probably reached.43

 
image file: c8cy02007h-t2.tif(3)


image file: c8cy02007h-f5.tif
Fig. 5 In situ FT-IR spectra recorded during the sulfidation of bulk Mo2Ni (a) and PMo11Ni (b).

The peaks at 1079, 1049, 942, 876, and 810 cm−1 due to PMo11Ni are attributed to vibrations of P–Od, Mo–Oa, Mo–Ob–Mo, and Mo–Oc–Mo, respectively, which are typical characteristics of Keggin-type POMs.44 The coordination environment and bond length of Mo–O in PMo11Ni are presented in Fig. S5 and Table S3. The Mo–Oa bond length is 1.682 Å, which is obviously shorter than those of Mo–Ob (1.946 Å) and Mo–Oc (1.933 Å). Therefore, with an increase in the sulfidation temperature the peak intensity decreases as a result of O–S exchange, which mainly occurs on Mo–Ob/Oc bonds and then on Mo–Oa bonds. At 250 °C, the band due to Mo–O–P disappeared, which suggested the decomposition of the Keggin-type structure of PMo11Ni. It is inferred that three groups of edge-sharing Mo3O(S)13 triplets (trimetallic clusters) and one NiMo2O(S)13 triplet were formed. When the temperature increased to 300 °C, these triplets decomposed and sulfides were formed. In contrast, the sulfidation temperature of PMo11Ni is higher than that of Mo2Ni. It is noted that the peaks due to Mo–Oa (942 cm−1) and P–Od (1049 cm−1) underwent blue shifts to higher wavenumbers of 957 and 1063 cm−1, respectively, at 250 °C. The location of an FT-IR absorption peak that arises from vibrations of chemical bonds is mainly influenced by electronic effects (chemical microenvironment), such as the inductive effect, conjugative effect, and hydrogen bonding effect. Here, the decomposition of the Keggin-type structure in PMo11Ni disrupts the delocalized system (Keggin structure), and the electron cloud of the double bonds is further localized on Mo[double bond, length as m-dash]Oa. For P–Od, the breaking of Mo–O–P bonds changes the electron density or delocalizes the charge of P–Od, which would have a considerable effect on the frequencies in the FT-IR spectrum. These factors caused the blue shift in the peaks due to Mo–Oa and P–Od. When the temperature increased to 450 °C, the peaks due to PMo11Ni disappeared completely, which suggested that a highly sulfided state was reached.

FT-IR spectroscopy cannot detect vibrations of Mo–S in the region below 500 cm−1. Hence, we recorded the Raman spectra of Mo2Ni and PMo11Ni at different sulfidation temperatures. The results shown in Fig. 6 present the peaks, which prove the formation of MoS2.


image file: c8cy02007h-f6.tif
Fig. 6 Raman spectra of Mo2Ni (a) and PMo11Ni (b) at different sulfidation temperatures.

At room temperature, several peaks due to Mo–O–Mo appeared in the Raman spectra of Mo2Ni and PMo11Ni.45,46 With an increase in the sulfidation temperature, the intensity of the Mo–O–Mo peaks declined, which indicated the evolution of Mo2Ni and PMo11Ni. For Mo2Ni, when the temperature increased to 200 °C vibrations of MoS2 were observed at 399 and 370 cm−1, which are attributed to the stretching of S–Mo–S bonds perpendicularly to the basal plane or along the c-axis (A1g) and the stretching of Mo–S bonds along the basal plane (E12g), respectively.47,48 In addition, a weak peak at 340 cm−1 was found, which indicated the formation of superlattices in single-layer MoS2.49,50 For PMo11Ni, peaks due to MoS2 at 398, 373, and 339 cm−1 appeared at 300 °C. These results also show that Mo2Ni is more easily sulfided than PMo11Ni, which is in accordance with the in situ FT-IR and TPS results. As the temperature continued to increase, the intensities of the MoS2 peaks increased, which was most probably a result of a lower degree of distortion and disorder of MoS2 slabs and the preformed NiMoS active phase.51

XRD can give information for the bulk during the sulfidation of Mo2Ni and PMo11Ni. The XRD patterns of the POMs at different sulfidation temperatures are presented in Fig. 7. The XRD patterns of Mo2Ni and PMo11Ni changed with an increase in the sulfidation temperature. For Mo2Ni, the XRD intensities declined at about 100 °C, which indicated that the crystal structure of Mo2Ni was changing and that sulfidation was occurring. As the temperature increased to 200 °C diffraction peaks due to MoS2 appeared, which suggested that the crystal structure of Mo2Ni was destroyed and sulfidation was achieved. At 250 °C, diffraction peaks due to Ni3S2 were found.52 As the temperature increased, the intensities of the peaks due to MoS2 and Ni3S2 increased. The XRD patterns prove that the Ni-4,4′-bpy complex can retard the sulfidation of Ni. In this way, Ni atoms can move to the active edges of MoS2 to form a well-dispersed type II NiMoS active phase, which is in accordance with the results reported by Badoga et al.53 For PMo11Ni, a similar trend was found. As the temperature increased, the XRD peaks due to the (101), (005), (110), and (040) crystal planes disappeared as a result of the exchange of O for S to form MoOxSy. At 300 °C, the crystal structure of PMo11Ni was destroyed and weak diffraction peaks due to MoS2 appeared. In contrast to Mo2Ni, only MoS2 was found after the sulfidation of PMo11Ni, and no NixSy was found. This must have been due to the very low content of Ni in PMo11Ni.


image file: c8cy02007h-f7.tif
Fig. 7 XRD patterns of Mo2Ni (a) and PMo11Ni (b) at different sulfidation temperatures.

Fig. 8 shows HRTEM micrographs of bulk Mo2Ni at different sulfidation temperatures. Lattice fringes of Mo2Ni can be observed in the image taken at a sulfidation temperature of 100 °C, and no obvious signs of sulfidation of Mo2Ni were found. When the temperature increased to 200 °C, obvious signs of sulfidation appeared from the exterior to the interior of the particles. Several fringes of MoS2 (as shown by the white arrows) were formed at the edge of Mo2Ni; however, no MoS2 was found in the bulk Mo2Ni. At a temperature of 250 °C, more MoS2 fringes were formed, and these MoS2 fringes were well dispersed without agglomeration. At a temperature of 400 °C, many MoS2 fringes were found, and both the slab length and the stacking number of MoS2 increased. The MoS2 slab length ranged from 5 to 10 nm, and the stacking number ranged from 4 to 8. Furthermore, Ni3S2 was found in the HRTEM image, which is in accordance with the XRD analysis. Grange and Vanhaeren concluded that a separate NixSy phase on MoS2 is beneficial for the adsorption and dissociation of H2 molecules.54 The resulting H species attack the MoS2 slabs and produce coordinatively unsaturated sites at the edges of MoS2 slabs. Notably, a large excess of Ni species may result in the formation of an amorphous structure, which was proved to be an inferior HDS catalyst precursor.55 In Mo2Ni, the Ni/Mo ratio is 0.5, which is very close to the optimum Ni/Mo ratio for conventional HDS catalysts and makes it a suitable precursor for HDS catalysts. On the basis of the above analysis, it is proposed that sulfidation progresses from the outer portion to the inner portion of the Mo2Ni particles. The pillared layer framework of Mo2Ni is beneficial for sulfidation. At 200 °C, MoS2 is formed. The result closely coincides with those of XRD and Raman analysis, in which weak peaks due to MoS2 in the XRD pattern and bands due to Mo–S in MoS2 in the Raman spectrum appeared at 200 °C.


image file: c8cy02007h-f8.tif
Fig. 8 HRTEM images of bulk Mo2Ni at different sulfidation temperatures.

Fig. 9 shows several representative HRTEM images of PMo11Ni at different sulfidation temperatures. Similar conclusions can be drawn about the sulfidation process of PMo11Ni. However, the temperature of formation of MoS2 (300 °C) is higher than that for Mo2Ni. In addition, no NixSy is formed during the sulfidation process, as a result of the lower content of Ni in PMo11Ni. The average stacking number N, length L, and proportion of Mo atoms at the edges of MoS2 in the sulfides of Mo2Ni and PMo11Ni are summarized in Table 1. The average slab length and stacking number of MoS2 are higher than those of traditional supported NiMoS catalysts. It is widely acknowledged that the HDS reactivity of a catalyst with shorter MoS2 slabs and a higher stacking number is higher.56,57 Because the sulfide of Mo2Ni possesses shorter MoS2 slabs and a higher stacking number than the sulfide of PMo11Ni, its HDS reactivity is believed to be higher.


image file: c8cy02007h-f9.tif
Fig. 9 HRTEM images of bulk PMo11Ni at different sulfidation temperatures.
Table 1 Morphological characteristics of samples determined via HRTEM and XPS characterization
Sample [L with combining macron] (nm) [N with combining macron] f Mo MoSulfidation NiMoS% S2−%
Mo2Ni sulfide-400 °C 5.4 5.6 0.21 92% 59 96.2
PMo11Ni sulfide-400 °C 6.8 4.2 0.15 83% 48 93.1


Moreover, EDS analysis and elemental mapping of Mo2Ni and PMo11Ni sulfided at 400 °C indicate a good distribution of Ni, Mo, and S (Fig. 10), which demonstrates the uniform distribution of the bulk sulfide and the possible formation of the NiMoS active phase. The EDS analysis shows that the sulfides of both Mo2Ni and PMo11Ni contained a small amount of carbonaceous species (16.7 and 11.4 wt%), which originated from the decomposition of 4,4′-bpy. Because these sulfides were prepared without calcination, carbonization is expected to have been accompanied by sulfidation.58


image file: c8cy02007h-f10.tif
Fig. 10 Results of EDS analysis and elemental mapping of Mo2Ni (a) and PMo11Ni (b) sulfided at 400 °C.

Alvarez et al.59 reported that the carbon content of the HDS catalyst precursor can strongly influence the textural and catalytic properties of the final catalyst. The reason is that carbon in the precursor significantly reduces the formation of an MoS2-like intermediate and leads to a final mesostructure of MoS2. Li et al.60 reported that carbonaceous species can improve the migration and growth of surface Mo species and modulate the orientation and morphology of surface sulfur species. The carbonaceous species in the sulfides of Mo2Ni and PMo11Ni that arise from the decomposition of 4,4′-bpy molecules can effectively prevent agglomeration of the NiMoS active phase, which may be conducive to an enhancement in HDS performance. The higher content of carbonaceous species in the sulfide of Mo2Ni arises from the greater number of 4,4′-bpy molecules in Mo2Ni, which may lead to a better isolating effect on the NiMoS active phase. Besides, higher contents of S (14.0 and 9.6 wt%) were also found in the EDS analysis of the sulfides of Mo2Ni and PMo11Ni, which suggested that the degree of sulfidation of Mo was higher.

Fig. 11 presents the N2 adsorption–desorption isotherms and pore diameter distributions of the sulfides of Mo2Ni and PMo11Ni. These isotherms are type IV N2 adsorption isotherms with an H3-type hysteresis loop, which indicates that platelet aggregates with a mesostructure were formed.61 The sulfide of PMo11Ni exhibited a wider pore diameter distribution with two peaks at 7.2 and 17 nm, whereas the sulfide of Mo2Ni only displayed one peak at 7.4 nm with a distribution between 6 and 11 nm. Textural properties of the sulfides of Mo2Ni and PMo11Ni are presented in Table 2. The BET surface areas of the sulfides of Mo2Ni and PMo11Ni reached 59 and 50 m2 g−1, respectively. The formation of Ni3S2, which was detected by XRD and HRTEM, may account for the larger BET surface area of the sulfide of Mo2Ni, because Ni3S2 may help to increase the surface area of the sulfide and thus provide more active sites for the HDS of DBT.55,62 This result is similar to that of Bocarando et al.,63 who reported the pyrolysis of ammonium tetrathiomolybdate and nickel dithiocarbamate to prepare unsupported HDS catalysts. This indicates that Mo2Ni and PMo11Ni may represent good precursors for HDS catalysts.


image file: c8cy02007h-f11.tif
Fig. 11 N2 adsorption–desorption isotherms (a) and pore diameter distributions (b) of sulfides of Mo2Ni and PMo11Ni.
Table 2 Textural properties of the samples
Sample BET surface area (m2 g−1) Pore volume (cm3 g−1) Pore diameter (nm)
Mo2Ni sulfide-400 °C 59 0.18 7.4
PMo11Ni sulfide-400 °C 50 0.11 7.2, 17


It has been proved that POMs are suitable precursors for the NiMoS active phase,34,64–66 and three main advantages of POMs as precursors can account for this, as follows:67 (1) POMs incorporate all the elements required for an HDS catalyst at the molecular level simultaneously, which may be beneficial for the preparation of an oxidic precursor in a single impregnation step; (2) POMs provide the desired homogeneous spatial distribution and intimate interaction of the key elements on the atomic scale, which efficiently guarantees the synergetic effect of Mo and Ni in the HDS reaction; and (3) POMs that possess a well-defined molecular structure are perfect models for clarifying the transformation and formation mechanism of the HDS active phase.

The Mo 3d XPS spectra of the sulfides of Mo2Ni and PMo11Ni at 400 °C are shown in Fig. 12(a). The degree of sulfidation (Mosulfidation) of these samples was calculated and is shown in Table 1, in which the Mosulfidation value of the sulfide of Mo2Ni (92%) is higher than that of the sulfide of PMo11Ni (83%). Fig. 12(b) shows the Ni 2p XPS spectra of the sulfides of Mo2Ni and PMo11Ni used to detect the formation of the NiMoS active phase. Deconvolution of the Ni 2p XPS peaks was carried out. Three contributions attributed to Ni2+ are present, which were due to NixSy, NiMoS, and an oxidic phase.28,68 The binding energies corresponding to the three contributions to the Ni 2p peak closely coincide with those reported for NiMoS catalysts. The proportions of the NiMoS active phase (NiMoS% of the total Ni) in the sulfides of Mo2Ni and PMo11Ni were calculated and are presented in Table 1. For Mo2Ni sulfided at 200 °C, the proportion of the NiMoS active phase was 32%. When the temperature increased to 400 °C, the proportion of the NiMoS active phase increased to 59%. For PMo11Ni sulfided at 300 and 400 °C, the contents of the NiMoS active phase were 26% and 48%, respectively, which were slightly less than those of the sulfides of Mo2Ni. Fig. S6 shows the S 2p XPS spectra of the sulfides of Mo2Ni and PMo11Ni at 400 °C. The binding energy located at 162.8 eV is assigned to S22− (MoOxSy), whereas that at 161.5 eV indicates the presence of S2− (MoS2, NiMoS, and NiS phases).69 The higher proportion of S2− (S2−%) was calculated and is shown in Table 1, which indicates that the major fraction of sulfur comprised S2−. It also shows that the S2− content in the sulfide of Mo2Ni was higher than that in the sulfide of PMo11Ni, which indicates that more Mo oxide was sulfided to form MoS2 or NiMoS. The XPS results are in accordance with those of the TPS, in situ FT-IR, Raman, XRD, and HRTEM analyses and indicate that Mo2Ni and PMo11Ni are suitable precursors for the NiMoS active phase.


image file: c8cy02007h-f12.tif
Fig. 12 Deconvolution of Mo 3d (a) and Ni 2p (b) XPS spectra of sulfides of Mo2Ni and PMo11Ni.

The CO-TPD profiles of the sulfides of Mo2Ni (200 and 400 °C) and PMo11Ni (300 and 400 °C) are presented in Fig. 13, in which only one CO desorption temperature is found.70 The amounts of stacked CO desorbed from the sulfide of Mo2Ni (0.204 and 0.356 mmol g−1) were higher than those for the sulfide of PMo11Ni (0.182 and 0.325 mmol g−1), respectively, which suggested that there were more active sites in the sulfide of Mo2Ni. This result is consistent with that of the XPS analysis.


image file: c8cy02007h-f13.tif
Fig. 13 CO-TPD profiles of the sulfides of Mo2Ni (200 and 400 °C) and PMo11Ni (300 and 400 °C).

On the basis of the above analysis, we propose that the sulfidation process of Mo2Ni and PMo11Ni is as shown in Fig. 14. Mo2Ni initially decomposes at 100 °C, and simple O–S exchange occurs to produce MoOxSy. As the temperature increases, further sulfidation of MoOxSy is achieved, and MoS2 is formed at 200 °C. Then, the Ni species could interact with the edges of MoS2 slabs to form the NiMoS active phase. The 4,4′-bpy ligands exhibit a remarkable promoting effect on the sulfidation of Ni species and thus facilitate the redispersion of Ni species to form more NiMoS active sites. In the case of PMo11Ni, the Keggin-type structure begins to collapse at 250 °C, and three groups of edge-sharing Mo3O(S)13 triplets and one NiMo2O(S)13 triplet are formed. When the temperature increases, these triplets decompose and further O–S exchange occurs to form MoS2 at 300 °C. However, the contents of carbonaceous species and the NiMoS active phase in the sulfide of Mo2Ni are higher than those in the sulfide of PMo11Ni. The higher content of carbonaceous species can prevent agglomeration of the NiMoS active phase and bring about its homogeneous distribution during sulfidation.


image file: c8cy02007h-f14.tif
Fig. 14 Schematic representation of the sulfidation process of Mo2Ni and PMo11Ni.

3.3. Catalytic activities

To test the reactivity of the sulfides of Mo2Ni and PMo11Ni at different stages, their HDS performance for DBT was studied, and the conversions are presented in Fig. 15. Because the sulfidation temperature was lower than 150 °C, the conversions of DBT over the sulfides of Mo2Ni and PMo11Ni were very low. When the temperature increased to 200 °C, the conversion of DBT over the sulfide of Mo2Ni increased sharply, whereas for PMo11Ni this happened at 300 °C. The results for the reactivity are in close accordance with those of the TPS, in situ FT-IR, Raman, XRD, and HRTEM analyses, which showed that the NiMoS active phase appeared at 200 and 300 °C in the sulfidation of Mo2Ni and PMo11Ni, respectively. The HDS activity over Mo2Ni and PMo11Ni sulfided at 400 °C is shown in Table 3. The DBT conversions, overall HDS rate constants kDBT and TOFs for the sulfided POMs are consistent with results reported by Liu et al.,71 which further confirms the excellent reactivity of the sulfided POMs. Moreover, the DBT conversion was 10% higher for the sulfide of Mo2Ni than for the sulfide of PMo11Ni.
image file: c8cy02007h-f15.tif
Fig. 15 DBT conversions over Mo2Ni and PMo11Ni at different sulfidation temperatures.
Table 3 HDS activity for DBT over Mo2Ni and PMo11Ni sulfided at 400 °C
Sample DBT conversion (%) k DBT (10−7 mol s−1 gcat−1) TOF (10−3 s−1)
Mo2Ni sulfide-400 °C 82.4 6.6 6.5
PMo11Ni sulfide-400 °C 71.6 6.2 4.8


3.4. Structure-directing effects of POMs

The structure-directing effects of Mo2Ni can account for its higher HDS reactivity, as shown in Fig. 16, as follows. (1) The framework effect. Mo2Ni possesses a pillared layer framework, which makes it more beneficial for the formation of MoS2 slabs without undergoing great structural changes. In contrast, the Keggin-type PMo11Ni unit is spherical and has to undergo structural decomposition and reorganization to generate MoS2 slabs. In addition, the pillared layer framework of Mo2Ni brings about a lower degree of agglomeration of Mo. The distance between adjacent pillared layers of Mo in Mo2Ni is 9.121 Å, whereas that in PMo11Ni is 5.452 Å. The longer distance suggests that the degree of agglomeration of Mo in Mo2Ni is lower, which results in shorter MoS2 slabs and a higher stacking number and indicates a better dispersion of MoS2 than that in the sulfide of PMo11Ni. (2) The promoting effect. Mo2Ni has a high atomic Ni/Mo ratio. In our study, the Mo2Ni catalyst with an Ni/Mo ratio of 0.5 exhibited higher HDS reactivity than the PMo11Ni catalyst with an Ni/Mo ratio of 0.18, even though the latter catalyst contains P, which normally increases the HDS activity. The higher Ni content in Mo2Ni provides many more active sites for the HDS of DBT. Moreover, the separate Ni3S2 phase in the sulfide of Mo2Ni adsorbs and dissociates H2 and produces coordinatively unsaturated sites at the edges of MoS2. (3) The ligand effect. The ligand can serve as a chelating agent and retard the sulfidation of Ni to produce the NiMoS active phase and can create carbonaceous species in the final NiMoS catalyst. The carbon content in Mo2Ni is 30.2%, which is higher than that in PMo11Ni (18.7%). The presence of carbon in the precursor has a beneficial effect on the final NiMoS active phase and HDS performance. Zhang et al.72 reported that with an increase in the carbon content in the precursor the sulfidation temperature decreases greatly and that a higher degree of sulfidation and more active sites are obtained. Moreover, the higher content of carbonaceous species in the sulfide can prevent agglomeration of the NiMoS active phase and promote its homogeneous distribution during sulfidation. Hence, the higher content and better dispersion of the NiMoS active phase can explain the higher HDS reactivity of the sulfide of Mo2Ni.
image file: c8cy02007h-f16.tif
Fig. 16 Structure-directing effects of Mo2Ni and PMo11Ni on the final NiMoS active phase.

4. Conclusions

Two Ni–Mo binary transition metal POMs, namely, Mo2Ni and PMo11Ni, were prepared from an impregnating solution via a ligand-induced self-assembly strategy and used as molecular platforms to study the sulfidation process of POM precursors in the preparation of an HDS catalyst. The evolution was characterized in detail via multiple techniques. For Mo2Ni, MoS2 is formed at 200 °C, and Ni species interact with the edges of the MoS2 phase to form the NiMoS active phase, whereas the corresponding temperature for PMo11Ni is 300 °C. The structure-directing effects (framework effect, promoting effect and ligand effect) bring about a higher content and better dispersion of the NiMoS active phase in the sulfide of Mo2Ni, which can account for its higher HDS reactivity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Prof. Prins Roel (ETH Zurich) for the discussions of the manuscript. This work was financially supported by the Natural Science Foundation of China (Grants 21676300, 21878336, 21672243), Natural Science Foundation of Jiangsu Higher Education Institutions of China (Grant 17KJB530009, 18KJB580016), Shandong Provincial Natural Science Foundation (ZR2018MB035), Jiangsu Key Laboratory of Chiral Pharmaceuticals Biosynthesis (Grant SX1705), Fundamental Research Funds for the Central Universities (Grant 18CX07004A), and Taizhou City Science and Technology Supporting Program (Grants TS201627, 201602).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cy02007h
These authors contributed equally to this work.

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