Ultra-large-scale Synthesis of Fe3O4 Nanoparticles and Their

Apr 1, 2014 - ... Fe3O4 Nanoparticles and Their Application for Direct Coal Liquefaction .... strategy displays a great potential for practical DCL ap...
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Ultra-large-scale Synthesis of Fe3O4 Nanoparticles and Their Application for Direct Coal Liquefaction Yizhao Li,† Fengyun Ma,† Xintai Su,*,† Longjiao Shi,† Beibei Pan,† Zhiqiang Sun,† and Yanglong Hou*,‡ †

Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China ‡ Department of Advanced Materials and Nanotechnology, College of Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Ultra-large-scale synthesis of iron oxide nanoparticles (875 g) has been achieved in a single reaction via a facile solution-based dehydration process. The obtained nanoparticles capped with hydrophobic oleic acid ligands are magnetite with the average size of 5 nm. The synthesized samples exhibit a higher catalytic activity toward the direct coal liquefaction (DCL) than the commercial Fe3O4 powders. The conversion, oil yield, and liquefaction degree with the synthesized Fe3O4 nanoparticles are 89.6, 65.1, and 77.3%, respectively. The excellent catalytic performance of the synthesized Fe3O4 nanoparticles can be attributed to their extremely small size and high dispersity. This facile approach to prepare highly active nanocatalyst for the DCL will be applicable for future industrial processes.

1. INTRODUCTION Catalytic cracking of coal to obtain clean liquid fuels by using a catalyst has been the subject of research recently in order to solve the problem of petroleum shortage.1−9 Various catalysts have been used for the direct coal liquefaction (DCL) process, such as Fe, Mo, Co, and Ni.10−16 Among them, despite relatively low activity, iron-based catalysts have received extensive research interest because of their low cost and environmentally friendly behaviors.17−19 An ideal small-sized iron-based catalyst is highly desirable for its high catalytic activity associated with a large surface area.20 Sharma et al.21,22 have reported that the 3−20 nm sized ferric sulfide showed a high selectivity to oil formation. Matson et al.23 have demonstrated the 10−50 nm sized iron-based catalysts with a significant C−C bond scission activity. Liu et al.24 reported that the ∼18 nm sized Fe2S3 showed a high activity for the DCL. All of the above reports indicate that the small-sized Fe-based nanomaterials will possess an improved catalytic property in the DCL. Therefore, an appropriate method for the synthesis of iron-based nanocatalysts with small size is required. The mass production of catalyst is necessary for the DCL. In the past several years, different methods for the large-scale synthesis of iron-based nanomaterials have been reported by several groups.25−28 Iron-based nanoparticles have been obtained on a scale of tens of grams through a sol−gel route in one reaction.29,30 High-yield carbon-encapsulated Fe3C nanoparticles have been prepared via a cocarbonization process in high temperature.31 Magnetite nanocrystals have been synthesized on a large scale of 40 g by thermal decomposition of the iron−oleate precursors in high boiling point solvent.32 However, these methods have some disadvantages, such as high cost, high energy consumption, and relatively low output. Therefore, a large-scale and facile way to produce iron-based nanomaterials for the DCL is indispensable. Herein, we report an ultra-large-scale synthetic method for Fe3O4 nanoparticles through a facile solution-based dehydra© 2014 American Chemical Society

tion reaction. The synthesized Fe3O4 nanoparticles show a higher catalytic activity than the commercial Fe3O4 powders in the DCL. The superior catalytic activity of the nanocatalyst could be attributed to the smaller size and high dispersion of the particles. Significantly, the current ultra-large-scale synthetic strategy displays a great potential for practical DCL applications.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were analytical and used as starting materials without further purification. The commercial Fe3O4 powders were obtained from Taijihuan Nanotechnology Co., Ltd. The raw coal used in this study is a jet coal from the Heishan mine in Xinjiang, China. Its proximate, ultimate, and petrographical analysis results are given in Table 1. The Heishan coal sample was ground to a particle size of less than 200 mesh prior to its utilization in the DCL. 2.2. Preparation of the Fe3O4 Nanoparticles. The synthesis was carried out in a 100 L autoclave reactor. Oleic acid (7.52 kg, 26.63 mol) and NaOH (1.07 kg, 26.63 mol) were dissolved in a mixture of toluene (35 L), ethanol (20 L), and deionized water (15 L) to form a homogeneous pale yellow sodium oleate solution with vigorous stirring. Then FeCl3· 6H2O (1.80 kg, 6.66 mol) and FeCl2·4H2O (0.67 kg, 3.35 mol) were added into the reaction system in sequence, and a black color instantaneously appeared. The mixture was refluxed at 70 °C for 4 h with vigorous stirring. After the reaction, the upper organic phase containing the nanoparticles was separated, and excess ethanol was added to the toluene dispersions to make the nanoparticles precipitate. The resulting powder was washed Received: Revised: Accepted: Published: 6718

January 15, 2014 March 26, 2014 April 1, 2014 April 1, 2014 dx.doi.org/10.1021/ie500216c | Ind. Eng. Chem. Res. 2014, 53, 6718−6722

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Table 1. Proximate, Ultimate, and Petrographical Analyses of a Heishan Coal Sample proximate analysis (wt %)

a

ultimate analysis (wt %, daf)

petrographical analysis (vol %)

Mad

Ad

Vdaf

C

H

Oa

N

S

vitrinite

inertinite

exinite

H/C

3.08

1.12

32.58

81.27

4.54

12.02

0.76

1.37

57.8

41.4

0.8

0.67

Obtained by difference.

3. RESULTS AND DISCUSSION The XRD patterns of the synthesized nanoparticles and commercial powders are shown in Figure 1. All of the

and centrifuged with the water and the ethanol and dried at 60 °C for 2 h. 2.3. Characterization. The X-ray diffraction (XRD) patterns were recorded with a Rigaku D/max-ga X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) from 20 to 80° at a scanning speed of 2° min−1. X-ray photon spectroscopy (XPS) measurement was performed by an ESCALAB 250 Xi XPS system of Thermo Scientific, where the analysis chamber was 1.5 × 10−9 mbar and the X-ray spot was 500 μm. Transmission electron microscope (TEM) images were taken on a FEI Tecnai T20 with an accelerating voltage of 200 kV. Particle size distribution was measured by dynamic light scattering (DLS) using a Malvern Zeta-Sizer Nano-S90. Fourier transform infrared (FT-IR) spectra were obtained on a Bruker EQUINOX55 spectrophotometer in the wave interval between 4000 and 400 cm−1. The mass fraction of iron in the Fe 3 O 4 nanoparticles was determined by flame atomic absorption spectrophotometry (Hitachi Z-2000) after microwave digestion (Milestone ETHOS microwave system). The weight of oleic acid ligands in the synthesized Fe 3 O 4 nanoparticles was calculated by subtracting the weight of Fe3O4 from the weight of the synthesized catalysts. 2.4. Reaction of the Direct Coal Liquefaction. In a typical experiment, 30.0 g of Heishan coal sample was mixed with 60.0 g of tetralin as a solvent, 0.76 g (1.4 wt % dry and ashfree (daf) as Fe) of synthesized Fe3O4 nanoparticles as a catalyst, and 0.32 g of sulfur (1.4 S/Fe mole ratio) as a cocatalyst under ultrasonication for 30 min to produce a viscous suspension. The reaction mixture was charged into a 1.0 L autoclave reactor. Then the autoclave was pressurized with hydrogen to 6 MPa at room temperature and heated to 430 °C. After stirring at 300 rpm for 60 min, the resulting product was extracted in sequence with n-hexane, toluene, and tetrahydrofuran (THF) in a Soxhlet extractor. The n-hexane-soluble (HS), n-hexane-insoluble but toluene-soluble, toluene-insoluble but THF-soluble, and THF-insoluble substances were defined as oil and solvent, asphaltene, pre-asphaltene, and residue, respectively. The direct liquefaction of Heishan coal with commercial Fe3O4 powders was also processed under a similar procedure. In addition, the conversion, oil yield, and liquefaction degree of coal were determined using the following equations:

Figure 1. XRD patterns of (a) the synthesized nanoparticles and (b) the commercial powders.

reflection peaks that appeared can match well with the database of the cubic magnetite (JCPDS Card No. 19-0629). No other impurity peaks are detected, suggesting that the compositions of the above samples are Fe3O4. As observed in Figure 1a, the broader diffraction peaks of the synthesized Fe3O4 nanoparticles indicate the small crystallite size. As shown in Figure 1b, the narrow diffraction peaks of the commercial Fe3O4 powders indicate the big crystallite size. The crystallite sizes of the synthesized nanoparticles and commercial powders are 4.6 and 46.0 nm calculated by Scherrer equation. Furthermore, the XPS spectrum of the synthesized nanoparticles exhibits two peaks at 710.8 and 724.1 eV, corresponding to the peaks of Fe 2p3/2 and Fe 2p1/2 in Fe3O4, respectively (Figure S1, Supporting Information).33,34 There are no obvious shakeup satellite structures at the higher binding energy side of both main peaks, which is the characteristic of Fe3O4. Together, XRD and XPS results confirm that the synthesized product is Fe3O4. Figure 2 shows a representative TEM image of the synthesized Fe3 O4 nanoparticles and commercial Fe 3O 4 powders. From Figure 2a, self-assembly of spherical shape nanoparticles into one-dimensional rods can be clearly seen. A photograph of 875 g of nanoparticles obtained in a single reaction is shown in the inset of Figure 2a. The yield was 82.3%, which was calculated by substituting the iron mass fraction that was 52.7% obtained by flame atomic absorption spectrophotometry. Figure 2b exhibits that the diameters of the synthesized Fe3O4 nanoparticles are about 5 nm. In addition, the wide size distribution of synthesized nanoparticles is also observed according to the result of DLS (Figure S2, Supporting

conversion/% = [1 − (Wr − Wash − Wc)/Wdaf ] × 100 (1)

oil yield/% = [(WHS − Ws − Wo)/Wdaf ] × 100

(2)

liquefaction degree/% = (oil yield/%) + [(WA + WPA)/ Wdaf ] × 100

(3)

where Wdaf is the dry and ash-free weight of coal; Wr is the weight of the residue; Wash is the weight of ash; Wc is the total weight of Fe3O4 and sulfur; WHS is the weight of HS; Ws is the weight of solvent, i.e., the 60.0 g of tetralin; Wo is the weight of oleic acid in the catalyst; WA is the weight of asphaltene; and WPA is the weight of pre-asphaltene. 6719

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Fe3O4 nanoparticles have been coated with hydrophobic oleic acid ligands. In the inset of Figure 3, it can be observed that the synthesized nanocatalysts are dispersed in the organic phase, whereas the commercial Fe3O4 powders are dispersed in the aqueous phase. The hydrophobic surface of the synthesized Fe3O4 nanoparticles is beneficial to the DCL process. The catalytic activities of the synthesized nanoparticles and the commercial powders toward the DCL were investigated. As shown in Figure 4, it can be seen that the conversion, oil yield,

Figure 2. TEM images of the synthesized Fe3O4 nanoparticles and the commercial Fe3O4 powders. (a) Low-magnification TEM image of the synthesized Fe3O4 nanoparticles; inset, a photograph of 875 g of the synthesized Fe3O4 nanoparticles in a single reaction. (b) Highmagnification TEM image of the synthesized Fe3O4 nanoparticles. (c) Low-magnification TEM image of the commercial Fe3O4 powders. (d) High-magnification TEM images of the commercial Fe3O4 powders.

Information). It should be that the self-assembly of the Fe3O4 nanoparticles widens the size distribution. The TEM images of the commercial sample shown in Figure 2c,d demonstrate an aggregation of irregular cubic shape particles with the size range of 50−200 nm. The FT-IR spectra of the synthesized Fe3O4 nanoparticles and the commercial Fe3O4 powders are shown in Figure 3. The

Figure 4. Results of the direct liquefaction of Heishan coal with two kinds of catalysts.

and liquefaction degree with the commercial Fe3O4 catalysts are 84.3, 51.2, and 66.2%, respectively. When the synthesized Fe3O4 nanoparticles were used in the reaction system, the conversion, oil yield, and liquefaction degree reach 89.6, 65.1, and 77.3%, respectively. It should be pointed out that the conversion, oil yield, and liquefaction degree of the synthesized Fe3O4 nanoparticles are 5.3, 13.9, and 11.1% higher than that of the commercial Fe3O4 nanocatalysts. These results indicate that decreasing the particle size of catalysts is an effective approach for enhancing their catalytic activity in the DCL. In this work, the oleic acid capped Fe3O4 nanoparticles show an excellent catalytic performance for the DCL, which can be attributed to their extremely small size and high dispersity. The general mechanism of DCL is described as a free-radical process, in which coal is thermally decomposed into free radicals, which are then stabilized by abstraction of a hydrogen atom from the donor molecule.8,37 In a typical DCL reaction, the prepared Fe3O4 nanoparticles can be well-dispersed in the hydrogen−donor solvent (i.e., tetralin) owing to the oleic acid ligands.38 Therefore, the catalyst will fully contact with coal samples. This facilitates the generation of the activated hydrogen and the hydrogenation of the cracked coal fragments.39 Consequently, the synthesized catalyst shows excellent performance in the DCL process. In addition, it should be noted that the catalytic performance of the Fe3O4 nanoparticles is inferior slightly to that of the Fe3O4 nanocrystals with wellordered crystal lattice reported in our previous study.38 However, in the present work, the ultra-large-scale synthesis of Fe3O4 nanoparticles is carried out under a mild condition with inexpensive and low toxic raw materials, which is beneficial for the practical applications of the nanocatalysts.

Figure 3. FT-IR spectra of (a) the synthesized Fe3O4 nanoparticles and (b) the commercial Fe3O4 powders. Inset: Photograph of synthesized nanoparticles dispersed in hexane (left) and commercial powders dispersed in water (right).

absorption peaks at 593 cm−1 in Figure 3a and 576 cm−1 in Figure 3b are assigned to the Fe−O vibration.35 As shown in Figure 3a, the absorption peaks at 2922 and 2852 cm−1 are attributed to the stretching vibration of the -CH2 group in oleate. The bands at 1527 and 1433 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of COO− originating from the oleate.36 The above results reveal that 6720

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4. CONCLUSION In conclusion, oleic acid capped Fe3O4 nanoparticles with 5 nm in diameter have been synthesized on an ultra-large-scale. The prepared nanoparticles exhibited an excellent catalytic performance for the DCL. With the synthesized Fe3O4 nanopaticles, the conversion, oil yield, and liquefaction degree reached 89.6, 65.1, and 77.3%, respectively. The catalytic activity of the synthesized nanocatalysts was higher than that of the commercial Fe3O4 powders. The current synthetic procedure is very simple and can readily be scaled up to industrial applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1 and S2 showing the XPS spectrum and the size distribution of the synthesized Fe3O4 nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(X.S.) Tel. and fax: +86 991 8581018. E-mail: suxintai827@ 163.com. *(Y.H.) Tel. and fax: +86 10 6275 3115. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support of the National Natural Science Foundation of China (Grant Nos. 21266031, 51125001, and 51172005).

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