Facile preparation of efficient oil-soluble MoS2 hydrogenation nanocatalysts

Journal of Natural Gas Chemistry 20(2011)408–412

Facile preparation of efficient oil-soluble MoS2 hydrogenation nanocatalysts Shutao Wang, Changhua An∗ ,

Jie He,

Zongxian Wang

State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum, Qingdao 266555, Shandong, China [ Manuscript received January 11, 2011; revised March 24, 2011 ]

Abstract Oil-soluble MoS2 nanoparticles with narrow size distribution have been synthesized by a facile composite-surfactants-aided-solvothermal process. The as-prepared nanoparticles can be directly used as hydrogenation nanocatalysts or as precursors to achieve efficient supported nanocatalysts. The surfaces of these nanoparticles are proposed to be encapsulated within a layer of organic modifiers, which are responsible for the enhancement of their solubility in organic solvents. The activated-carbon supported MoS2 nanocatalysts exhibit higher activity than the unsupported ones towards hydrogenation reactions of naphthalene, owing to the synergistic effects between nanoparticles and supports. The advantages of the present nanocatalysts, such as removal of conventional presulfiding requirements and reduction of nanoparticle aggregations, make them become promising applications in related petroleum chemical industry. Key words hydrogenation; catalyst; nanoparticles; MoS2 ; preparation; solubility

1. Introduction

The hydrotreating or hydrorefining process in the field of petroleum chemical industry concludes a series of reactions, primarily consisting of removing sulfur, nitrogen, oxygen, and hydrogenation (HYD) of polyaromatics [1,2]. The HYD reaction is an especially important step towards the hydrodesulfurization (HDS) of inactive compounds with large steric effect, such as 4, 6-dimethyldibenzothiophene (4, 6-DMDBT). Performance of a catalytic HYD reaction is mainly determined by the quality and property of the catalyst, which can be acquired via various methods. The most intensively studied HYD catalysts are noble metals. However, the issues associated with the costliness, aggregation, and easy sulfidation to lose activity [1,3,4] hindered their widely use in real applications. Therefore, the design of efficient and cost-effective HYD catalysts is of paramount importance and essentially urgent. Nanoparticles (NPs) of transition metal sulfides, i.e. molybdenum disulfide (MoS2 ), have been attracted much at-

tention for potential applications as useful HYD catalysts [2,5−7]. HYD activity of MoS2 can be attributed to the steps and other structural defects presented at the edges of the MoS2 nanoclusters [8,9], which are intimate along the onedimensional electronic edge states according to the prediction of density functional theory (DFT) [6]. A variety of methods including gas-solid [10], solvothermal [11−13], electrochemical [14], microwave [15] and pressing assisted processes [16] have been developed to synthesize MoS2 NPs. Nevertheless, high surface energy of NPs makes them have a strong tendency to aggregate, leading to a significantly decrease in catalytic activity, especially when experiencing high temperature environment. To ensure high HYD catalytic activities, the achievement of high quality MoS2 NPs with large surface areas, good solubility in proper solvents and more reactive sites is highly desirable. As part of our continuing efforts to synthesize monodispersed NPs, herein we report on the synthesis of oil-soluble MoS2 NPs by a composite-surfactants-aided-solvothermal process [13,17]. The obtained NPs can be used as precursors

∗

Corresponding author. Tel: +86-546-7807562; Fax: +86-532-86981787; E-mail: [email protected] This work was supported by the Shandong Provincial Natural Science Foundation (ZR2009BQ008) and the Fundamental Research Funds for the Central Universities (10CX03008A). Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60207-1

Journal of Natural Gas Chemistry Vol. 20 No. 4 2011

to achieve good HYD nanocatalysts in different forms, such as unmodified MoS2 NPs, oil-soluble MoS2 NPs, and activated carbon (AC) supported oil-soluble MoS2 NPs (MoS2 /AC). The results of catalytic hydrogenation reaction of naphthalene (NAT, C10 H10 ) indicate that the supported MoS2 NPs exhibit higher catalytic activity than unsupported ones, owing to the existence of synergistic effects between NPs and the supports and the grant of exposing more active sites.

409

2. Experimental All the analytical-grade chemicals were purchased from Shanghai Chemical Reagents Company and were used as received without further purification. The strategy for the synthesis of MoS2 NPs and the use of them as precursors to produce efficient nanocatalysts are schematically shown in Scheme 1.

Scheme 1. The preparation of different forms of MoS2 nanocatalyst using as-synthesized nanoparticles as precursors

2.1. Synthesis of oil-soluble MoS2 NPs and AC supported MoS2 nanocatalyst In a typical procedure, 0.2 mmol of ammonium heptamolybdate tetrahydrate (AHM, (NH4 )6 Mo7 O24 ·4H2 O) together with 6.6 mmol of thiourea ((NH2 )2 CS) were added to 40 mL of ethylene glycol (EG, HOCH2 CH2 OH) solution containing the composite surfactants comprising 0.84 mmol of sodium oleate (SO, CH3 (CH2 )7 )CH=CH(CH2 )7 COONa) and 0.45 mmol of cetylamine (CA, CH3 (CH2 )15 NH2 ) in a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 ◦ C for 24 h, and then subsequently cooled to room temperature naturally. The precipitates were centrifuged, washed with distilled water, absolute ethanol, and extracted with ligarine. The obtained ligarine solution was then filtered, concentrated, and dried in a vacuum at room temperature. In the case of unmodified MoS2 NPs preparation, the procedure is similar to the oil-soluble NPs as described above without using any surfactant. In order to introduce useful oxygen-containing groups on the surfaces of AC, activation pretreatment is a prerequisite, usually conducted with concentrated HNO3 at room temperature [18]. Then the supported nanocatalysts were prepared through impregnating AC into hexane solution of MoS2 NPs at 40 ◦ C for 30 min followed by heat treatment in atmosphere at 120 ◦ C and calcination at 500 ◦ C for 4−5 h. 2.2. Characterization The morphology of the products was examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using a Hitachi H-800 transmission electron microscope and a Hitachi X-650 scanning electron microscope, respectively. X-ray photoelectronic spectra (XPS)

were recorded on a Perkin-Elmer PHI-5300 spectrometer, using nonmonochromatized Mg Kα radiation as the excitation source in vacuum of 10−7 Pa. The charge shift correction was made and binding energy of C 1s electron with 284.6 eV was used for calibration of the instrument. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet FTIR Magna-750 spectrometer. Crystallographic information of the product was investigated with X-ray powder diffraction (XRD) on a Rigaku D/Max-c¸A diffractometer using a graphite-filter Cu Kα radiation at a scanning rate of 2 o /min. The diffraction data were collected in the 2θ range from 10o to 70o . 2.3. Evaluation of catalytic performance The catalytic performance of the MoS2 NPs was examined via a hydrogenation reaction of NAT. The products of HYD reactions were analyzed by gas chromatography (GC), using a Varian 3400 chromatograph (GS-GasPro GC/PLOT) with a flame ionization detector (FID) for 25 min. The split ratio was 100 : 1 and the amount of feed was 1 μL. The increasing temperature rate was 10.0 ◦ C/min. The temperatures of feed entrance, initial column, and final column were 300 ◦ C, 100 ◦ C, and 300 ◦ C, respectively. When supported catalyst was used, the loadings of NPs was maintained at 10 wt% so as to guarantee the same quantity of unsupported nanocatalysts in the system. In a typical process, 0.2 g of NAT, 1 wt% of freshly produced NPs, and 2.0 g of dispersion medium of clay-refinedlube-base oil were loaded into an autoclave with the capacity of 50 mL. The autoclave was blown three times with N2 prior to filling H2 to 6.0 MPa. Then the HYD reaction was proceeded at 360 ◦ C for 1−2 h. The reaction products were cooled with cold water, and dissolved in toluene. For unmodified MoS2 NPs, a presulfurization in the presence of H2 S

410

Shutao Wang et al./ Journal of Natural Gas Chemistry Vol. 20 No. 4 2011

was carried out at 300 ◦ C for 30 min with tin bath before the start of reactions. The other parameters were kept identical to those of the modified MoS2 NPs. GC analyses show that the catalytic HYD products of NAT comprise tetrahydronaphthalene (4HN) and decahydronaphthalene (10HN) without the presence of ring-open ones. The HYD activity is measured as the percentage of the peak areas (A) for 4HN and 10HN compared with those of the feed NAT. The conversion (C) is used to evaluate the catalytic efficiency of the nanocatalysts via the following equation: C (%) =

A4HN + A10HN ×100% ANAT + A4HN + A10HN

where, A is the peak area in GC spectra. 3. Results and discussion XRD pattern (Figure 1a) reveals the characteristics of wide diffraction peaks and weak intensity for the oil soluble products, implying their small sizes and low crystallinity. The

absence of the weakest (002) reflection at about 13o may verify the stacking disorder along the (001) direction of the 2H-MoS2 structure [16,19]. The pattern with the presence of broad peaks around 38o and 60o is in agreement with previous results [20,21]. Figure 1(b) shows a TEM image of the as-obtained oil-soluble products. It is clearly shown that the sample is composed of nearly monodispersed NPs with an average diameter of 40 nm. In contrast, the unmodified MoS2 in Figure 1(c) exhibits irregular shape with wide size distributions. Therefore, it can be concluded that the composite surfactants play a critical role in determining the microstructures such as shapes and sizes of the NPs. FTIR in Figure 2 provides further insights of the free molecules of SO, CA, and the oil-soluble MoS2 NPs. The adsorption of SO and CA on the surfaces of the NPs can be inferred, associated with that the stretching vibration of N−H bond in CA shifts from 1560 cm−1 to 1530 cm−1 , and the C=C double bond in SO (1480 cm−1 ) moves to 1460 cm−1 [13]. The occurance of stretching vibration for N−H (around 3330 cm−1 ) and C=O (1730 cm−1 and 1640 cm−1 ) reveals that the surfaces of the NPs are bonded with amides, which may be formed through the interaction between SO and CA. Therefore, the features of small sizes and rich structural curvatures of the as-synthesized oil-soluble MoS2 NPs enable them useful as HYD candidates [15]. Furthermore, the use of surface modifier can stabilize the NPs with the reduction of surface energy, and protect the sulfides from potential oxidation and deactivation in the catalytic reaction [19].

Figure 2. FTIR spectra of oil-soluble MoS2 nanoparticles, cetylamine, and sodium oleate

Figure 1. The XRD patterns of (a) oil-soluble MoS2 nanoparticles, TEM images of (b) oil-soluble MoS2 nanoparticles, and the (c) unmodified MoS2 nanoparticles

Figure 3 shows the XPS spectra of the as-synthesized sample. It can be seen that the binding energies for S 2p (161.65 eV) and Mo 3d (225.9 eV) are close to those in previous reports [16,19,22−24]. Analysis of the Mo 3d and S 2p peak intensities gives a S/Mo atomic ratio of 2.01 and precludes the possibility of the existence of polysulfide (164.1 eV for S 2p) in the product [19]. The enhancement C 1s signal demonstrates the adsorption of the organic surfactants on the surfaces of the NPs. The adsorption surface layer is very thin considering the rather weak signals for N 1s in CA molecular.

Journal of Natural Gas Chemistry Vol. 20 No. 4 2011

411

the improved activity of the oil-soluble MoS2 nanocatalyst is closely related with the surface adsorbed organic surfactants. Meanwhile, AC supported MoS2 nanocatalyst exhibits higher activity than unsupported one, which might be ascribed to a synergetic effect between the supports and the nanocatalyst [25−27]. Here, the in situ carbonization of the organic layer on the substrate surfaces during the catalytic process facilitates the integrations between the nanocatalyst and AC, and further enhances the synergetic effect between active component and support. As shown in Figure 5, SEM images of AC supported oil-soluble nanocatalyst before (Figure 5a) and after calcination (Figure 5b) illustrate that the catalyst after annealing features more porous microstructure and thus increased activity compared to that of the uncalcined one.

Figure 4. The hydrogenation conversion of NAT catalyzed by different forms of MoS2 nanocatalysts. (1) Unmodified MoS2 nanocatalyst, (2) Unsupported oil-soluble MoS2 nanocatalyst, (3) Uncalcined oil-soluble MoS2 /AC nanocatalyst, (4) Calcined oil-soluble MoS2 /AC nanocatalyst

Figure 3. XPS spectra of oil-soluble MoS2 nanoparticles. (a) Survey profile, (b) High resolution spectrum of Mo 3d, (c) High resolution spectrum of S 2p

Figure 4 gives the conversion of NAT catalyzed by different types of MoS2 nanocatalysts as Scheme 1 presented. The HYD activity of the concerning nanocatalysts increases in the following order: unmodified MoS2
Figure 5. SEM images of active carbon supported oil soluble MoS2 . (a) Before calcination, (b) After calcination at 500 ◦ C

412

Shutao Wang et al./ Journal of Natural Gas Chemistry Vol. 20 No. 4 2011

4. Conclusions In summary, we have successfully synthesized oilsoluble MoS2 NPs via a facile composite-surfactants-aidedsolvothermal process. It is proposed that a layer of organic surfactants has been adsorbed on the surfaces of the MoS2 NPs, leading to a improved solubility in organic solvents and thus an increase in catalytic activity. For the HYD reactions of NAT, the activity of the obtained nanocatalysts increases in the following order: unmodified MoS2 nanocatalyst
References [1] Joo H S, Guin J A. Fuel Process Technol, 1996, 49(1-3): 137 [2] Farag H. J Colloid Interface Sci, 2010, 348(1): 219 [3] Niesz K, Koebel M M, Somorjai G A. Inorg Chim Acta, 2006, 359(9): 2683 [4] Klimova T, Vara P M, Lee I P. Catal Today, 2010, 150(3-4): 171 [5] Wentrcek P R, Wise H. J Catal, 1976, 45(3): 349

[6] Lauritsen J V, Nyberg M, Nørskov J K, Clausen B S, Topsøe H, Lægsgaard E, Besenbacher F. J Catal, 2004, 224(1): 94 [7] Kibsgaard J, Tuxen A, Knudsen K G, Brorson M, Topsøe H, Lægsgaard E, Lauritsen J V, Besenbacher F. J Catal, 2010, 272(2): 195 [8] Elizondo-Villarreal N, Vel´azquez-Castillo R, Galv´an D H, Camacho A, Yacam´an M J. Appl Catal A, 2007, 328: 88 [9] Bollinger M V, Jacobsen K W, Nørskov J K. Phys Rev B, 2003, 67(8): 085410 [10] Zak A, Feldman Y, Alperovich V, Rosentsveig R, Tennne R. J Am Chem Soc, 2000, 122(45): 11108 [11] Berntsen N, Gutjahr T, Loeffler L, Gomm J R, Seshadri R, Tremel W. Chem Mater, 2003, 15(23): 4498 [12] Shi H Q, Fu X, Zhou X, Wang D B, Hu Z S. J Solid State Chem, 2006, 179(6): 1690 [13] Zhang X Y, An C H, Wang S T, Wang Z X, Xia D H. J Cryst Growth, 2009, 311(14): 3775 [14] Li Q, Newberg J T, Walter E C, Hemminger J C, Penner R M. Nano Lett, 2004, 4(2): 277 [15] Vollath D, Szabo D V. Mater Lett, 1998, 35(3-4): 236 [16] Siadati M H, Alonso G, Torres B, Chianelli R R. Appl Catal A, 2006, 305(2): 160 [17] An C H, Wang S T, He J, Wang Z X. J Cryst Growth, 2008, 310(2): 266 [18] de la Puente G, Gil A, Pis J J, Grange P. Langmuir, 1999, 15(18): 5800 [19] Afanasiev P. J Catal, 2010, 269(2): 269 [20] Zelenski C M, Dorhout P K. J Am Chem Soc, 1998, 120(4): 734 [21] Afanasiev P, Bezverkhy I. Chem Mater, 2002, 14(6): 2826 [22] Brown N M D, Cui N Y, McKinley A. Appl Surf Sci, 1998, 134(1-4): 11 [23] Baker M A, Gilmore R, Lenardi C, Gissler W. Appl Surf Sci, 1999, 150(1-4): 255 [24] Iranmahboob J, Hill D O, Toghiani H. Appl Surf Sci, 2001, 185(1-2): 72 [25] H´edoire C E, Louis C, Davidson A, Breysse M, Maug´e F, Vrinat M. J Catal, 2003, 220(2): 433 [26] Joshi Y V, Ghosh P, Daage M, Delgass W N. J Catal, 2008, 257(1): 71 [27] Ferraz S G A, Zotin F M Z, Araujo L R R, Zotin J L. Appl Catal A, 2010, 384(1-2): 51