Solvothermal synthesis of MoS₂ nanospheres in DMF–water mixed solvents and their catalytic activity in hydrocracking of diphenylmethane

Abstract

MoS₂ nanospheres were successfully prepared via a solvothermal process using 1-ethyl-3-methylimidazolium bromide ([EMIM]Br) in a mixed solvent of DMF–water. A probable [EMIM]Br aggregation in different mixed solvents and the formation mechanism of the MoS₂ nanospheres are presented. The MoS₂ nanospheres delivered high catalytic activity in hydrocracking of diphenylmethane.

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Molybdenum disulfide (MoS₂) with a layered and hexagonal structure is used as a catalyst,¹ solid lubricant,² electrode,³ and in hydrogen storage media.⁴ As a catalyst, the layered anisotropic structure of MoS₂ provides a massive “edge area” at which the catalytic interaction of reactants with MoS₂ edges and defects occurs.^5,6 During recent decades, a variety of catalysts based on MoS₂ have been used in processes such as the hydrocracking and hydrodesulfurization of petroleum,^7,8 the hydrodeoxygenation of bio-oil,⁹ and the production of chemicals.^10,11 Compared with supported catalysts, unsupported catalysts have the advantages of high dispersion and avoidance of pore-plugging in the residue hydrocracking process. As a consequence, dispersed MoS₂ catalysts have been extensively employed in ENI-EST, SOC, and (HC)₃ CASH hydrorefining technologies of residue.^12–14

MoS₂ with different micro/nano structures has been synthesized by multiple methods, such as a solvo-hydrothermal method,^15–17 sonochemical method,^18,19 chemical vapor deposition,^20,21 a chemical solution route,²² microwave synthesis,²³ and a thermal sulfurization method.²⁴ The solvo-hydrothermal approach is an effective and simple way to prepare micro/nanostructured materials at low temperature.²⁵ In recent years, ionic liquids (ILs) have been used as templates in hydrothermal synthesis of various micro/nano-structured inorganic materials.^16,26,27 Ma et al. obtained MoS₂ microspheres with average diameter about 2.1 μm via a hydrothermal method assisted by [BMIM][BF₄].¹⁶ Hollow MoS₂ microspheres with diameters ranging from 1.8 μm to 2.1 μm were prepared in [BMIM]Cl/water emulsions.²⁸ In the same ionic liquids/water medium, hollow vesicle-like MoS₂ microspheres (1–2 μm) were synthesized with (NH₄)₂MoS₄ as precursor and hydrazine hydrate as reductant.²⁹ However, to our knowledge, there are no reports on preparation of MoS₂ nanospheres by a solvo-hydrothermal route assisted by ionic liquid.

In this research, we present a solvothermal route for synthesis of MoS₂ nanospheres using a small molecule ionic liquid (1-ethyl-3-methylimidazolium bromide, [EMIM]Br) as a template in dimethyl formamide (DMF)–water mixed solvents. A probable [EMIM]Br aggregation in different mixed solvents and the formation mechanism of MoS₂ nanospheres are presented. Additionally, the catalytic activity of the as-prepared MoS₂ products was investigated in diphenylmethane hydrocracking.

Detailed experiments from the ionic liquid-assisted solvothermal synthesis of MoS₂ products in DMF–water mixed solvents to their catalytic activity in hydrocracking of diphenylmethane are described in the ESI.† XRD patterns of the as-prepared MoS₂ products are demonstrated in Fig. 1. The major diffraction peaks can be indexed to the hexagonal 2H MoS₂ (JCPDS 37-1492). As shown in Fig. 1a–c, the absence of a relatively strong diffraction peak at around 2θ = 14° (002) reveals that the well-stacked layered structure of MoS₂ did not form during the solvothermal process. Meanwhile, the diffraction peaks at 2θ = 8.6° and 17.6° are considered to be the (002) and (004) peaks shifting to lower angle range, which has been confirmed by other researchers.^17,29 This is a consequence of the enlarged interlayer spacing caused by the ILs between MoS₂ layers. Compared with Fig. 1a, the weak peaks in Fig. 1b and c show that the crystallinity of MoS₂ products becomes poor in the presence of DMF. After annealing at 700 °C for 2 h in the mixed atmosphere of N₂/H₂ (9/1, v/v), the MoS₂ products exhibit a strong diffraction peak at 2θ = 14.2° (Fig. 1d), implying that the crystallinity of MoS₂ products is improved by annealing.

Fig. 1 XRD patterns of MoS₂ samples synthesized at 200 °C for 24 h. (a) V₁/V₂ (DMF/water) = 0/9; (b) V₁/V₂ = 3/6; (c) V₁/V₂ = 5/4; and (d) after annealing of (b) in the mixed atmosphere of N₂/H₂ (9/1, v/v) at 700 °C for 2 h. *Shifted peaks of (002) and (004).

The morphology and structure of MoS₂ nanospheres, synthesized in mixed solvents of V₁/V₂ = 3/6, before and after annealing were characterized by SEM and TEM. Fig. 2a and b shows that the MoS₂ products are nanospheres with a mean diameter of 400 nm. The MoS₂ nanospheres with rough surface are made up of numerous petal-shaped nanoflakes, as can be seen from the clear view of a MoS₂ nanosphere in Fig. 2a. From the BET measurement, the specific surface area of MoS₂ nanospheres is 67.9 m² g⁻¹. In addition, the HRTEM image (Fig. 2c) clearly reveals that the MoS₂ nanoflakes were poorly stacked with fewer layers (3–8 layers) and that the average spacing of MoS₂ layers is about 0.91 nm. ILs, inside MoS₂ nanospheres and between MoS₂ layers, would vanish during annealing at 700 °C. Therefore, the MoS₂ nanospheres become irregular in shape, and the size decreases to 300–400 nm (as shown in Fig. 2d and e). Fig. 2f shows that the petal-shaped MoS₂ nanoflakes were straightened by annealing, and that the number of layers in each flake increases to dozens with a characteristic MoS₂ layer spacing of 0.65 nm (002). The HRTEM results are consistent with the XRD patterns.

Fig. 2 SEM, TEM and HRTEM images of MoS₂ samples synthesized at 200 °C for 24 h in V₁/V₂ = 3/6. (a–c) before annealing, (d–f) after annealing in the mixed atmosphere of N₂/H₂ (9/1, v/v) at 700 °C for 2 h.

The ratio of DMF to water has a significant influence on the morphology of MoS₂ products. As shown in Fig. 3a, hollow MoS₂ microspheres with diameter of 1.7–3.8 μm were synthesized in the absence of DMF. However, irregular and agglomerated MoS₂ nanoflakes were obtained in the mixed solvents of V₁/V₂ = 5/4 (Fig. 3c). Compared with the nanoflakes in MoS₂ nanospheres (Fig. 2c), the shape of these MoS₂ nanoflakes (Fig. 3d) was straighter during the solvo-hydrothermal process. The specific surface areas of the hollow MoS₂ microspheres and MoS₂ nanoflakes are 35.1 m² g⁻¹ and 49.7 m² g⁻¹, respectively. Both the products in Fig. 3 were poorly stacked with the average spacing of MoS₂ layers being greater than 0.9 nm. The full width of half maximum (FWHM) values of the main peaks of MoS₂ samples synthesized in different solvents are listed in Table 1. The FWHM values of (002) and (004) reflections increase with increasing DMF content of mixed solvents. The smallest FWHM value implies the largest crystalline grain of MoS₂ samples synthesized without DMF. As can be seen from the HRTEM images of MoS₂ samples, the flakes in MoS₂ samples synthesized without DMF have a more straight structure and MoS₂ layers.

Fig. 3 TEM and HRTEM images of as-prepared MoS₂ samples synthesized in V₁/V₂ (DMF/water) = 0/9 (a and b), V₁/V₂ = 5/4 (c and d).

Table 1 FWHM of the main peaks of MoS₂ samples synthesized in different solvents

V1/V2 (DMF/water)	FWHM
	(002)	(004)
0/9	0.47°	0.40°
3/6	0.94°	1.16°
5/4	1.87°	2.38°

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According to the literature,²² the reaction involved in the solvothermal synthesis process is as follows:

4(NH₄)₆Mo₇O₂₄ + 63CS(NH₂)₂ + 136HCl + 58H₂O → 28MoS₂ + 7(NH₄)₂SO₄ + 136NH₄Cl + 63CO₂

It has been reported that ILs act as a template in solvo-hydrothermal synthesis of MoS₂.^16,28,29 [EMIM]⁺ can form vesicles in certain solvents under appropriate conditions. Then, Mo₇O₂₄⁶⁻ anions can adsorb onto the vesicle surfaces by electrostatic attraction,³⁰ and react with the H₂S in situ produced by hydrolysis of CS(NH₂)₂. Based on the experimental results, the probable IL aggregation in different solvents and the formation mechanism of MoS₂ nanospheres synthesized in V₁/V₂ = 3/6 is presented in Scheme 1. ILs could form multilamellar vesicles with diameters of a few micrometers in pure water. With gradual addition of DMF, the multilamellar vesicles break because of higher energy, then small multilamellar vesicles or unilamellar vesicles are formed to obtain a low energy state.³¹ MoS₂ nanospheres will be generated gradually on the surface of IL vesicles (V₁/V₂ = 3/6). However, if the addition of DMF continues, the stable aggregation structure of ILs could not form in the mixed solution (V₁/V₂ = 5/4).

Scheme 1 Schematic illustration of IL aggregation in different solvents and the formation mechanism of MoS₂ nanospheres synthesized in V₁/V₂ = 3/6.

Table 2 shows the results of diphenylmethane (DPM) hydrocracking in the presence of different MoS₂ products at various temperatures. It can be seen that DPM barely hydrocracked when the reaction was performed at 450 °C without a catalyst, while all of the MoS₂ products significantly facilitate conversion of DPM. Under the hydrocracking conditions, coordinatively unsaturated sites can be formed readily at the edge and corner positions of MoS₂, which are the sites for hydrogen activation.³² Hydrogen molecules split to yield hydrogen free-radicals on the coordinatively unsaturated sites, then the produced hydrogen free-radicals add to the ipso position of DPM to trigger scission of the C_ar–C_alk bond.³³ The produced hydrogen free-radicals also can help to saturate the aromatic rings of DPM. Benzene and toluene are the main products of DPM hydrocracking, while small amounts of benzylcyclohexane and dicyclohexylmethane are also identified in the hydrocracked products.

Table 2 Results of DPM hydrocracking in the presence of different MoS₂ catalysts under the initial hydrogen pressure of 5.0 MPa at different temperatures for 1 h^a

Catalyst	Temperature (°C)	Conversion of DPM (%)	Selectivity (mol%)
			Benzene	Toluene	BCH	DHM
a BCH, benzylcyclohexane; DHM, dicyclohexylmethane.
None	450	0.7	100	99.9	0	0
MoS2 microsphere	350	15.1	75.3	75.3	23.8	0.9
	400	39.6	88.6	88.6	9.7	1.7
	450	52.5	90.1	90.1	7.3	2.6
MoS2 nanosphere	350	18.9	69.5	69.5	29.4	1.1
	400	51.5	83.1	83.2	13.9	2.9
	450	72.8	83.8	83.8	10.8	5.4
MoS2 nanoflake	350	17.8	71.9	71.8	27.4	0.8
	400	48.1	86.2	86.2	11.8	2.0
	450	66.3	87.6	87.5	8.3	4.2

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As shown in Table 2, under the same hydrocracking conditions, the order of conversion of DPM in the presence of different MoS₂ products from high to low is MoS₂ nanosphere, MoS₂ nanoflake, MoS₂ microsphere, which is consistent with the order of specific surface area of the MoS₂ products. MoS₂ with high specific surface area can provide more coordinatively unsaturated sites and yield more hydrogen free-radicals to participate in the reaction. The MoS₂ nanosphere delivered a higher catalytic activity in DPM hydrocracking than a FeS₂ catalyst³³ and a MoS₂ catalyst produced by water-soluble Mo precursor.³⁴ Liu et al. used the ratio of gross products of hydrosaturation and DPM conversion to describe the “hydrogenation activity” of the catalyst.³⁴ Therefore, the prepared MoS₂ nanosphere presents higher hydrogenation activity than the MoS₂ microsphere and MoS₂ nanoflake. Moreover, the catalytic activity difference between the MoS₂ nanosphere and nanoflake was enhanced by high temperature. Table 2 also shows that the conversion of DPM increased with increasing reaction temperature in the presence of the same catalyst. This indicates that high temperature facilitates formation of hydrogen free-radicals to participate in DPM conversion.

The selectivity results of DPM hydrocracking in Table 2 show that the percentages of scission products (benzene and toluene) greatly increased with increasing temperature from 350 °C to 400 °C, but hardly increased with increasing temperature from 400 °C to 450 °C. However, the percentage of DHM continued to increase with the increasing temperature. These results reveal that scission of the C_ar–C_alk bond dominates the reaction at high temperature, meanwhile the hydrogenation saturation abilities of MoS₂ products were enhanced by high temperature. Additionally, at the same temperature, DPM conversion with MoS₂ nanospheres showed higher yields of BCH and DHM, which indicates that the hydrogenation saturation ability of MoS₂ nanospheres was higher than that of MoS₂ nanoflakes and MoS₂ microspheres.

Conclusions

In summary, MoS₂ nanospheres with an average diameter of 400 nm and specific surface area of 67.9 m² g⁻¹ were successfully prepared via a solvothermal process assisted by [EMIM]Br in a mixed solvent of DMF–water (V₁/V₂ = 3/6) at 200 °C for 24 h. The ratio of DMF to water has a significant effect on the size and morphology of MoS₂ products. The strategy described here could be extended to other transition metal sulfide nanomaterials. Additionally, the MoS₂ nanospheres delivered high catalytic activity in the hydrocracking of diphenylmethane. The scission of the C_ar–C_alk bond dominates the DPM conversion at high temperature. Meanwhile, the hydrogenation saturation abilities of MoS₂ products were enhanced by high temperature, with that of MoS₂ nanospheres being higher than those of MoS₂ nanoflakes and MoS₂ microspheres at the same temperature.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21176259), Shandong Provincial Natural Science Foundation, China (ZR2015BM003) and the Fundamental Research Funds for the Central Universities (15CX05009A).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures. See DOI: 10.1039/c5ra08424e

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