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

Article abstract of DOI:10.1039/c5ra08424e

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.

Full text of DOI:10.1039/c5ra08424e

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Solvothermal Synthesis of MoS2 Nanospheres in DMF-Water
Mixed Solvents and Their Catalytic Activity in Hydrocracking of
Diphenylmethane
Received 00th January 20xx,
Accepted 00th January 20xx
Hui Du,a Dong Liu,*a Min Li,a Raja L. AL Otaibi,*b Renqing Lv c and Yadong Zhang a
DOI: 10.1039/x0xx00000x
www.rsc.org/
MoS2 nanospheres were successfully prepared via solvothermal synthesis
process assisted by 1-ehtyl-3-methylimidazolium bromide materials.16,26,27 Ma et al. obtained MoS2 microspheres with average
([EMIM]Br) in mixed solvent of DMF-water. probable diameter about 2.1 µm via hydrothermal method assisted by
of
various
micro/nano-structured
inorganic
a
A
[EMIM]Br aggregation in different mixed solvents and the [BMIM][BF4].16 Hollow MoS2 microspheres with diameter ranging
formation mechanism of MoS2 nanospheres were presented. from 1.8 µm to 2.1 µm were prepared in [BMIM]Cl/water
Additionally, the MoS2 nanospheres delivered a high catalytic emulsions.28 In the same ionic liquids/water medium, hollow
activity in the hydrocracking of diphenylmethane.
vesicle-like MoS2 microspheres (1-2µm) were synthesized with
(NH4)2MoS4 as precursor and hydrazine hydrate as reductant.29
However, to our knowledge, there were no reports on the
preparation of MoS2 nanospheres through solvo-hydrothermal
route assisted by ionic liquid.
Molybdenum disulfide (MoS2) with layer and hexagonal structure
was wildly used as catalyst,1 solid lubricant,2 electrode,3 and
hydrogen storage media,4 etc. As a catalyst, the layered anisotropic
structure of MoS2 provides massive “edge area” where the catalytic
interaction of reactants with MoS2 edges and defects occurs.5,6
During the past decades, a variety of catalysts based on MoS2 were
wildly used in the catalytic 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, the unsupported catalysts have the advantages
of high dispersion and avoiding pore-plugging problem in residue
hydrocracking process. As a consequence, dispersed MoS2 catalysts
have been extensive employed in ENI-EST, SOC, and (HC)3 CASH
hydrorefining technologies of residue.12-14
MoS2 with different micro/nano structures have been
synthesized through multiple methods, such as solvo-hydrothermal
method,15-17
sonochemical
method,18,19
chemical
vapor
deposition,20,21 chemical solution route,22 microwave synthesis,23
and thermal sulfurization method.24 It is well known that solvo-
hydrothermal approach was an effective and simple way to prepare
micro/nanostructured materials at low temperature.25 In recent
years, ionic liquids (ILs) were used as templates in hydrothermal
V1/V2 (DMF/water)
FWHM
(002)
0.47°
0.94°
1.87°
(004)
0.40°
1.16°
2.38°
0/9
3/6
5/4
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In this research, we present a solvothermal route for the products are nanospheres with the mean diameter of 400 nm. The
synthesis of MoS2 nanospheres using a small molecule ionic liquid MoS2 nanospheres with rough surface are constituted by numerous
(1-ethyl-3-methylimidazolium bromide, [EMIM]Br) as template in petal shaped nanoflakes, which can be seen from the clear view of
dimethyl formamide (DMF)-water mixed solvents.
A probable MoS2 nanosphere in Fig. 2a. From the BET measurement, the
[EMIM]Br aggregation in different mixed solvents and the specific surface area of MoS2 nanospheres is 67.9 m2·g-1. In
formation mechanism of MoS2 nanospheres were presented. addition, the HRTEM image (Fig. 2c) clear reveals that the MoS2
Additionally, the catalytic activity of as-prepared MoS2 products in nanoflakes were poorly stacked with fewer layers (3-8 layers) and
diphenylmethane hydrocracking were investigated.
the average spacing of MoS2 layers is about 0.91 nm. ILs, inside
Detailed experiments from the ionic liquid-assisted solvothermal MoS2 nanospheres and between MoS2 layers, would be vanished
synthesis of MoS2 products in DMF-water mixed solvents to their during annealing at 700 oC. Therefore, the MoS2 nanospheres
catalytic activity in hydrocracking of diphenylmethane are described become irregular in shape, and the size decreases to 300-400 nm
in the ESI. XRD patterns of the as-prepared MoS2 products are (as shown in Fig. 2d and e). Fig. 2f shows that the petal shaped
demonstrated in Fig. 1. The major diffraction peaks can be indexed MoS2 nanoflakes straightened by annealing, and the number of
to the hexagonal 2H MoS2 (JCPDS 37-1492). As shown in Fig. 1a-c, layers in each flake increases to dozens with a characteristic MoS2
the absence of a relative strong diffraction peak at around 2θ = 14° layer spacing of 0.65 nm (002). The HRTEM results are consistent
(002) reveals that the well-stacked layered structure of MoS2 did with the XRD patterns.
not form during 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.
Comparing with Fig. 1a, the weak peaks in Fig. 1b and c show that
the crystallinity of MoS2 products becomes poor with the presence
o
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.
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 show that the MoS2
2 | J. Name., 2012, 00, 1-3
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The ratio of DMF to water plays a significant influence on the 4 (NH4)6Mo7O24·+ 63 CS(NH2)2 + 136 HCl + 58 H2O → 28 MoS2 + 7
morphology of MoS2 products. As shown in Fig. 3a, the hollow MoS2 (NH4)2SO4 + 136 NH4Cl + 63 CO2
microspheres with diameter of 1.7-3.8 µm were synthesized in the
It has been reported that ILs act as template in solvo-
absence of DMF. However, the irregular and agglomerated MoS2 hydrothermal synthesis of MoS2.16,28,29 [EMIM]+ could form vesicles
6-
nanoflakes were obtained in the mixed solvents of V1/V2 = 5/4 (Fig. in certain solvent under proper conditions. Then, Mo7O24 anions
3c). Compared with the nanoflakes in MoS2 nanospheres (Fig. 2c), adsorb on the vesicle surface through electrostatic attraction,30
the shape of MoS2 nanoflakes (Fig. 3d) grew straighter during the which react with the H2S in situ produced by hydrolysis of CS(NH2)2.
solvo-hydrothermal process. The specific surface areas of the Based on the experimental results, the probable ILs aggregation in
hollow MoS2 microspheres and MoS2 nanoflakes are 35.1 m2·g-1 and different solvents and the formation mechanism of MoS2
49.7 m2·g-1, respectively. Besides, both the two products in Fig. 3 nanospheres synthesized in V1/V2 = 3/6 is presented in Scheme 1.
were poorly stacked with the average spacing of MoS2 layers ILs could form multilamellar vesicles with diameter of a few
greater than 0.9 nm. The full width of half maximum (FWHM) of the micrometres in pure water. With the gradually addition of DMF, the
main peaks of MoS2 samples synthesized in different solvents are multilamellar vesicles break because of higher energy, then small
listed in Table 1. The FWHM values of (002) and (004) reflections multilamellar vesicles or unilamellar vesicles are formed to obtain a
increase with increasing the DMF content of mixed solvents. The low energy state.31 MoS2 nanospheres generated gradually on the
smallest FWHM value implies the largest crystalline grain of MoS2 surface of ILs vesicles (V1/V2 = 3/6). However, if the addition of DMF
samples synthesized without DMF. As can be seen from the HRTEM continues, the stable aggregation structure of ILs could not form in
images of MoS2 samples, the flakes in MoS2 samples synthesized the mixed solution (V1/V2 = 5/4).
without DMF show more straight structure and MoS2 layers.
According to literature,22 the reaction involved in the of different MoS2 products at various temperatures. It can be seen
solvothermal synthesis process was shown as follows: that DPM barely hydrocracked when the reaction was performed at
Table 2 shows the results of DPM hydrocracking in the presence
Catalyst
None
Temperature (oC) Conversion of DPM (%)
Selectivity (mol%)
Benzene
100
Toluene
99.9
75.3
88.6
90.1
69.5
83.2
83.8
71.8
86.2
87.5
BCH
0
DHM
0
450
0.7
MoS2 microsphere 350
15.1
39.6
52.5
18.9
51.5
72.8
17.8
48.1
66.3
75.3
88.6
90.1
69.5
83.1
83.8
71.9
86.2
87.6
23.8
9.7
0.9
1.7
2.6
1.1
2.9
5.4
0.8
2.0
4.2
400
450
7.3
MoS2 nanosphere
MoS2 nanoflake
350
400
450
350
400
450
29.4
13.9
10.8
27.4
11.8
8.3
BCH, benzylcyclohexane; DHM, dicyclohexylmethane.
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o
450 C without catalyst, while all of the MoS2 products significantly catalytic activity in the hydrocracking of diphenylmethane. The
facilitate the conversion of DPM. Under the hydrocracking scission of Car–Calk bond dominates the DPM conversion at high
conditions, the coordinatively unsaturated sites can be readily temperature. Meanwhile, the hydrogenation saturation
formed at the edge and corner positions of MoS2, which are the abilities of MoS2 products were enhanced by high
sites for hydrogen activation.32 Hydrogen molecules split to yield temperature, which of MoS2 nanosphere was higher than
the hydrogen free-radicals on the coordinatively unsaturated sites, MoS2 nanoflake and MoS2 microsphere at same temperature.
then the produced hydrogen free-radicals addition to the ipso
position of DPM to trigger the scission of Car–Calk bond.33 Besides,
Acknowledgement
the produced hydrogen free-radicals also can help to saturate the
aromatic rings of DPM. Benzene and toluene are the main products
This work was financially supported by the National Natural
of DPM hydrocracking, while small amounts of benzylcyclohexane
Science Foundation of China (21176259), Shandong Provincial
and dicyclohexylmethane are also identified in the hydrocracked
Natural Science Foundation, China (ZR2015BM003) and the
products.
Fundamental Research Funds for the Central Universities
As demonstrated from Table 2, under the same hydrocracking
(15CX05009A).
conditions, the order about the 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
Notes and references
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4 | J. Name., 2012, 00, 1-3
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