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Synthesis of Ni-Mo Binary Transition Metal Complex and Application in Hydrodesulfurization.

Byline: Jilei Liang, Mengmeng Wu, Hongmei Cai, Yiyang Cao, Xiaorong Lu, Yuhang Wang and Min Zhu

Summary: The study showed the synthesis of nickel-molybdenum binary metal complex Ni(en)3MoO4 with high purity from the impregnating solution. The complex was characterized and confirmed by single-crystal XRD, FT-IR and TGA. The binary transition metal complex was impregnated to -Al2O3 to prepare hydrodesulfurization catalyst. The catalyst was characterized by low temperature N2 adsorption-desorption isotherms, XRD and HRTEM. No molybdenum and nickel species were observed over the hydrodesulfurization catalyst surface, and the average slab length and layer number of the MoS2 crystallites in the catalyst are 2.6 nm and 2.3, respectively. The results indicated the high dispersion of active metal species over support. The DBT conversion of the catalyst can reach 91.4%, and the overall pseudo-first order rate constant kDBT is 3.1x10-4 gcat-1 s-1. The result showed the catalyst possessed good catalytic activity and Ni(en)3MoO4 can make efficient precursor to produce hydrotreating catalyst.

Keywords: Transition metal complex; Hydrodesulfurization; Hydrothermal synthesis; Precursor.

Introduction

Hydrodesulfurization (HDS) is the most effective process to remove the refractory sulfur-containing molecules, such as dibenzothiophene (DBT), in petroleum-derived fuels [1, 2]. Lately, environmental laws have been enforced, leading to the development of more efficient catalysts for sulfur removal. In general, there are two routes to improve the HDS catalyst performance. One is using new supports, such as MCM-41 [3, 4], SBA-15 [5, 6] and modified alumina [7]. The other is selecting more efficient metal precursors, for instance, various polyoxometallates (Keggin, Anderson, Waugh-type) [8-11].

Recently, chelating agents were used to modify metal precursors in the preparation of HDS catalysts [12]. Under the specific conditions, chelating agents can coordinate promoting metals Ni to promote the sulfidation of Mo and retard that of Ni. Therefore, the promoter Ni can be anchored to the reactive edges of the MoS2 slabs, and subsequently form NiMoS active phase. Inspired by the idea, we select rational organic ligands to coordinate Ni in the impregnating solution and produce a binary Ni-Mo transition metal complex with specific formula to provide direct precursor for the HDS catalyst.

On the basis of the above consideration, ethylenediamine (en), an excellent ligand employed for the construction of transition metal complex, was added to the impregnating solution and then hydrothermally treated. A simple binary transition metal complex Ni(en)3MoO4 was obtained. The complex was structurally characterized and a preliminary study was carried out to explore the use of the complex as catalyst precursor for the HDS of DBT.

Experimental

Synthesis of Ni(en)3MoO4

All chemicals are analytical-grade regents, purchased from Sinopharm Chemical Reagent Co., Ltd and used as received. En was added to a solution of (NH4)6Mo7O24 (0.5 mmol), Na2B4O7 (1.5 mmol) and Ni(NO3)2 (1.5 mmol) with stirring until the pH was adjusted to 10.5. Then the solution was sealed in a Teflon-lined autoclave and heated at 140 AdegC for 24 h to get violet block-shaped crystals (90% yield, based on molybdenum).

Preparation of HDS catalyst and catalytic test

The HDS catalyst with 10 wt% MoO3 was prepared by incipient wetness impregnation of -Al2O3 in Ni(en)3MoO4 solution. Before evaluation, the catalyst was pre-sulfided at 350 AdegC for 4 h in a stream (100 mL*min-1) of 10 vol% H2S in H2. The pre-sulfided catalyst was transferred into a batch reactor in an inert atmosphere with 60 mL of n-hexadecane solution containing DBT (850 ppm of S). Finally, the catalytic activity test was performed at 300 AdegC and 7.3 MPa total pressures for 4 h with stirring. The liquid product was collected and analyzed by an Agilent 6890 chromatograph installed with an MS 80 mass spectrometer.

Characterization

The crystal structure was determined by single-crystal X-ray diffraction (XRD) analysis. The data were collected on a Bruker-AXS Smart CCD diffractometer (Mo K[alpha] radiation, I>>= 0.71073 A) at room temperature with I-scan mode, and the structures were solved by direct methods and refined by full-matrix least squares on F2 using SHELXTL97 software. The sample phase was identified by means of powder XRD (Panalytical X'Pert PRO MPD) analysis with Cu-K[alpha] radiation (I>>=1.5406 A) operated at scaning rate of 0.05Ao s-1. Thermogravimetric Analysis (TGA) was carried out on a SDT Q600 Thermal Analyzer in N2 atmosphere with a heating rate of 10 AdegC min-1. Fourier transform infrared spectroscopy (FT-IR) was recorded in the range 4500-400 cm-1 with a NEXUS FTIR. The textural characteristics of the catalyst were determined by low temperature N2 adsorption-desorption isotherms on a Micromeritics ASAP 2020 instrument.

Before the measurement, each sample was degassed in vacuum at 200 AdegC. High-resolution transmission electron microscopy (HRTEM) images of the pre-sulfided catalyst were obtained with a JEOL 2010 transmission electron microscope operating at 200 kV with 1.9 A point to point resolutions.

Results and Discussion

Structure characterization of synthesized Ni(en)3MoO4

The crystal structure was confirmed, as shown in Fig. 1, and it was seen that the crystal is composed of tetrahedral [MoO4]2-anions and octahedral [Ni(en)3]2+ cations. The Mo atom, on a twofold axis, is in a MoO4 tetrahedral environment.

The Ni atom, which also lies on a twofold axis, exhibits an octahedral coordination geometry completed by six N atoms from three en ligands. The crystal structure implies that Ni(en)3MoO4 was successfully synthesized [13].

The phase and purity of the crystal was examined by powder XRD measurement (shown in Fig. 2). It can be seen that powder XRD pattern of the synthesized complex is in completely consistent with the simulated pattern based on the result from single-crystal XRD via Mercury 1.4.2 software. Besides, no other peaks are found, suggesting the product is in high purity.

The FT-IR spectrum of Ni(en)3MoO4 was performed in Fig. 3. Peaks in the range of 3500-1413cm belong to en and crystal water in Ni(en)3MoO4, and 964, 814 cm are attributed to Mo=O, Mo-O-Mo of [MoO4], respectively.

TGA was performed to illustrate the thermo stability of the complex. As shown in Fig. 4, the TGA curve displays an initial weight loss of 14.85% between room temperature and 120 AdegC, suggesting the loss of one en molecule (calcd 14.2%). The second weight loss of 30.1% covering a temperature range from 200 AdegC to 450 AdegC is very close to the loss of other two en molecules (calcd 29.7%). The TGA curve is in good agreement with the differential thermal analysis (DTA), in which two strong endothermic processes appear at the temperature of 103 AdegC and 340 AdegC, attributing to the two weight loss stages of en respectively.

Characterization of the HDS catalyst

The isotherms of N2 adsorption-desorption and the pore size distribution profiles of both the -Al2O3 support and HDS catalyst are shown in Fig. 5, and Table-1 summarizes the BET surface area, average pore volume and average pore diameter. The -Al2O3 support and catalyst exhibit type IV N2 adsorption isotherms, which is typical for mesoporous materials [14]. Compared with the -Al2O3 support, the catalyst showed a small decrease in the specific surface area from 272.1 to 232.5 m2/g, average pore diameter from 8.8 to 5.4 nm and pore volume from 0.62 to 0.51 cm3/g, respectively. The reason is that the incorporation of Ni(en)3MoO4 to the -Al2O3 support results in the partial blockage of the pores.

Table-1: Texture properties of the -Al2O3 support and HDS catalyst.

Samples###SBET(m2/g) Pore volume (cm3/g) Pore diameter (nm)

-Al2O3###272.1###0.62###8.8

HDS catalyst###232.5###0.51###5.4

After pre-sulfidation, the HDS catalyst was examined by powder XRD measurement (shown in Fig. 6). There were only peaks corresponding to -Al2O3 found, and no XRD peaks corresponding to molybdenum and nickel species were observed. This result could be due to the high dispersion of Mo and Ni species over support surface and no aggregation after pre-sulfidation.

HRTEM is considered to be the most effective technique to characterize the dispersion of the active phase via visualizing of the slabs of MoS2 particles [15]. Representative HRTEM image of the sulfided HDS catalyst is shown in Fig. 7. The black fringes in the HRTEM micrograph correspond to the layers of MoS2 crystallites which are evenly distributed on the surface of the -Al2O3 support. These bent black fringes present a spacing of 0.62 nm which is the characteristic of (002) basal planes of MoS2 crystalline. To quantitatively compare the slab length and stacking number of the MoS2 slabs on the catalyst, statistical analysis was made based on at least 20 images including 500-600 slabs taken from the different parts of the catalyst, and the result is shown in Fig. 8. The average slab length and layer number of the MoS2 crystallites in the catalyst are 2.6 nm and 2.3, respectively. The smaller average length and less stacking number imply that the HDS catalyst may have more active sites exposed to reactants.

Catalytic Activities

The catalytic results of DBT HDS showed that the DBT conversion can reach 91.4% at 4 h, and the value of overall pseudo-first order rate constant kDBT is 3.1x10-4 gca -1 s-1. The catalytic data is higher than the reported one [16], indicating the higher catalytic activity of the catalyst with Ni(en)3MoO4 as precursor. In Ni(en)3MoO4, Ni is coordinated by en, in hence, both Ni and Mo are converted into sulfides at similar temperature, leading to the formation of more active NiMoS phase. Concluded by the XRD and HRTEM analysis, the good dispersion of NiMoS phase can account for the higher catalytic activity.

There are two pathways in HDS of DBT: one is the direct desulfurization (DDS) route to yield biphenyl, and the other is through the hydrogenation (HYD) route to produce cyclohexylbenzene with tetrahydrodi-benzothiophene and hexahydrodibenzothiophene as intermediates [17]. The experiment also showed the reaction product distributions obtained at 50% of DBT conversion to distinguish the individual contributions of the HYD and DDS routes to the overall HDS performance of the catalyst. The HYD/DDS was 2.2, indicating that the catalyst is more favorable for the HYD pathway.

Conclusion

By adding en to impregnating solution and hydrothermally treated, a binary transition metal complex Ni(en)3MoO4 was prepared with high purity. The complex was impregnated to -Al2O3 to prepare HDS catalyst. The characterizations showed that no molybdenum and nickel species were observed over the catalyst surface. The average slab length and layer number of the MoS2 crystallites in the catalyst are 2.6 nm and 2.3, respectively, indicating the high dispersion of active metal species in catalyst. The DBT conversion can reach 91.4%, and the overall pseudo-first order rate constant kDBT is 3.1x10-4 gca -1 s-1. The result showed the catalyst possessed good catalytic activity and Ni(en)3MoO4 can make efficient precursor to produce hydrotreating catalyst.

Acknowledgment

Financial support from Natural Science Foundation of Jiangsu Higher Education Institutions of China (17KJB530009), Taizhou City Science and Technology Supporting Program (TS201627, 201602) and Jiangsu Province College Students Innovation and Entrepreneurship Training Program (201612917016Y, 201612917025X) are gratefully acknowledged.

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Author:Liang, Jilei; Wu, Mengmeng; Cai, Hongmei; Cao, Yiyang; Lu, Xiaorong; Wang, Yuhang; Zhu, Min
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Dec 31, 2017
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