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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 13889−13894

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Layered Double Hydroxide Method for Preparing Ni−Mo/γ-Al2O3 Ultradeep Hydrodesulfurization Catalysts Linyi Lv,† Yawen Bo,† Dekun Ji,‡ Wei Han,† Honghai Liu,§ Xionghou Gao,‡ Chunyan Xu,† and Hongtao Liu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China Beijing Institute of Petrochemical Technology, Beijing 102617, P. R. China § Petrochemical Research Institute, Petrochina Company Limited, Beijing 100195, P. R. China

Ind. Eng. Chem. Res. 2018.57:13889-13894. Downloaded from pubs.acs.org by NORTH CAROLINA A&T STATE UNIV on 10/17/18. For personal use only.

‡

ABSTRACT: This article reports a novel strategy for preparing NiMo/γ-Al2O3 ultradeep hydrodesulfurization (HDS) catalysts via thermal decomposition of layered double hydroxides (LDHs). Ni−Al−[C 6 H 4 (COO) 2 ] 2− -LDHs/ γ-Al2O3 (terephthalate-pillared Ni−Al-LDHs/γ-Al2O3) composites were first obtained by in situ crystallization of Ni−Al− [C6H4(COO)2]2−-LDHs on the surface of γ-Al2O3. Then Ni−Al−Mo7O246−-LDHs/γ-Al2O3 composites were synthesized from the above composites by the anion exchange of [C6H4(COO)2]2− with Mo7O246−. Finally, bimetallic Ni−Mo/γ-Al2O3 catalysts were obtained by the subsequent decomposition of Ni−Al−Mo7O246−-LDHs/γ-Al2O3. The LDHs strategy imposes significant effects on improving the Ni/Mo dispersion with weak interaction of active metal and support and thereby greatly improves the HDS properties of the resulting catalysts. The HDS assessment results revealed that the catalyst obtained by LDHs procedure exhibits outstanding HDS activities for 4,6-dimethyldibenzothiophene and Dagang FCC diesel. The novel strategy employed in this article sheds light on the industrial preparation and application of NiMo/γ-Al2O3 catalysts.

1. INTRODUCTION Dispersion, which can be defined as the proportion of the molar number of active component on the surface to the total molar number of active component, plays a important role in determining the catalytic properties of hydrodesulfurization catalysts, such as Ni−Mo/Al2O3 catalyst.1,2 To enhance the catalytic property of the supported catalysts, a great deal of efforts have been made to improve the dispersion of active metals. It was widely accepted that the dispersion could be improved by reducing the particle size of the active components. For this purpose, different methods have been developed. Impregnation and precipitation are the two typical methods to decrease the particle size of active components. However, these strategies would lead to the agglomeration of active metals, lowering the dispersion degree of the active metals on the surface of support.3−5 Although equilibrium deposition is a facile route to enhance the dispersion, this strategy can meanwhile enhance the interaction between the active metals and the support and thereby lead to the incomplete sulfidation of active phases. Another recognized method to obtain materials with controllable particle size is hydrothermal deposition method. Bao et al. have prepared highly dispersed Ni−W/Al2O3 catalyst under mild hydrothermal conditions.6,7 However, this method needs to introduce the organic surfactants, which makes the synthesis procedure costly. Therefore, there continues to be a challenging goal to explore a novel route to meet the need for improving the dispersion without strengthening the active phase−support interaction. © 2018 American Chemical Society

Emerging as a fruitful route for obtaining mixed metal oxides, layered double hydroxide (LDH) method had been well developed, and via this method various metal oxides had been obtained, such as Mg−Al oxides,8 Ni−Al oxides,9−11 Cu−Ce oxide,12 Ce(Eu)-Zn oxides,13,14 and Pt−Mo(W) oxides.15 The structure of layered double hydroxides (LDHs) could be expressed as the following format: [MII1−xMIIIx(OH)2]x+(An−x/n)· mH2O. In this formula, An− is the interlayered anion, and MII/MIII are divalent and trivalent metals, respectively.16−24 It is wellknown that, through being thermally decomposed, LDHs can be transformed into highly dispersed metal oxide (such as Mg−Al oxides, Ni−Al oxides, and Cu−Ce oxide). These composites were characterized with good thermal stability and high surface area, all of which were crucial to the catalytic activity of supported catalysts. Based on this, it was believed that the LDH method could be a facile strategy to obtain hydrodesulfurization catalysts with excellent activity. Duan et al. prepared Ni−Al−CO3-LDHs/γ-Al2O3 composite in which Ni−Al−CO3-LDHs are located on the pore surface of γ-Al2O3 by in situ synthesis.25 On the basis of this, alumina with NiO particles highly dispersed on its surface was obtained by subsequent calcination. Due to the excellent dispersion, the resulting materials exhibit improved catalytic properties. However, the introduction of Mo (the main active component Received: Revised: Accepted: Published: 13889

July 22, 2018 September 27, 2018 October 2, 2018 October 2, 2018 DOI: 10.1021/acs.iecr.8b03383 Ind. Eng. Chem. Res. 2018, 57, 13889−13894

Article

Industrial & Engineering Chemistry Research

impregnation procedure. γ-Al2O3 was impregnated with Ni(NO3)3 and (NH4)6MoO6 solution in the proper proportion. The catalyst was obtained by calcination of previous resulting solids at 500 °C for 4 h, which is denoted as Cat-IM. The weight contents of Cat-LDH and Cat-IM are 2.9% NiO and 19% MoO3 measured by X-ray fluorescence analysis (XRF). 2.2. Characterizations. A Rigaku D/Max 2500VB2+/PC diffractometer was used to obtain XRD patterns. HITACHI S4700 scanning electron microscope (SEM) was used to study the morphology of the samples. A Micromeritics ASAP 2405N system was used to measure the isotherms of nitrogen. XRF was processed on a PANalytical B.V. X’Pert PRO X-ray fluorescence spectrometer (XRF). 2.3. Catalyst Activity Assessment. HDS activities were studied in a tubular fixed-bed continuous flowing microreactor (10 mL) using 4,6-DMDBT and Dagang FCC diesel as feedstocks, respectively. The catalysts were presulfied with a CS2 (3 wt %)/cyclohexane mixture for 8 h. The operation conditions are liquid hourly space velocity (LHSV) 8.0 h−1, 300 °C, 6.0 MPa, and H2 to hydrocarbon (HC) volumetric ratio 500. Sulfur content in the reactants and products was measured by WK-2C microcoulombmeter (provided by Jiangsu Jiangfeng Electroanalytical Instrument Co. Ltd.). The HDS reaction of Dagang FCC diesel (total sulfur content 1068 μg/mL) was carried out at the following conditions: 360 °C, (LHSV) 1.5 h−1, 6.0 MPa, and H2 to hydrocarbon (HC) volumetric ratio 500. HDS activity was assessed with a 1 wt % 4,6-DMDBT/ cyclohexane mixture at the following condtions: 300 °C, 6.0 MPa, and H2 to hydrocarbon (HC) volumetric ratio 500.

for HDS) into the layers of LDHs is still not feasible due to the significant mismatching of Mo6+ and Al3+. Based on the previous studies, it is expected that the incorporation of Mo into Ni-containing LDHs precursors, followed by the controlled thermal decomposition, will be an exciting approach for the preparation of NiO−MoO3/γ-Al2O3 catalysts with highly enhanced activity of HDS. However, the introduction of a significant quantity of Mo6+ species into the LDHs layers was not feasible due to the radius-mismatching of the larger Mo6+ and the smaller Al3+ and Ni2+. Based on the previous studies, it is expected that the introduction of Mo into Ni-containing LDHs, followed by the well-controlled decomposition, will be an exciting route for the preparation of NiO−MoO3/γ-Al2O3 catalysts with highly enhanced activity of HDS. In spite of this, the introduction of large quantity of Mo6+ species into the LDHs layers is still a challenge due to the radius-mismatching of the larger Mo6+ and the smaller Al3+ and Ni2+. However, the introduction of Mo6+ into the Ni-LDHs interlayer galleries can be considered as an alternative route for obtaining Ni−Mo-containing LDHs. As we know, urea will undergo hydrolysis slowly when the temperature is above 60 °C. For this reason, urea was chosen as a precipitant. Urea slowly hydrolyzed to produce OH− with increasing temperature, providing the alkaline environment for in situ growth of LDHs on γ-Al2O3. As a result of this, the content of hydrotalcite can be modulated to control the NiO content in the catalyst accordingly. In this study, terephthalate-pillared Ni−Al−[C6H4(COO)2]2LDHs/γ-Al2O3 composites were first synthesized by in situ crystallization, and then through the anion exchange of [C6H4(COO)2]2− with Mo7O246−, the Ni−Al−Mo7O246−LDHs/γ-Al2O3 composites were obtained. Bimetallic NiMo/ γ-Al2O3 catalysts were finally obtained by the thermal decomposition of the above NiMo-contained composites, and their catalytic performances were studied.

3. RESULTS AND DISCUSSION 3.1. XRD. Figure 1 shows the XRD pattern of Cat-LDH. The peaks at 37.6°, 45.8°, and 67.2° are attributed to Al2O3 phase.26

2. EXPERIMENTAL SECTION 2.1. Synthesis. (1) A Ni−Al−[C6H4(COO)2]2−-LDHs/γ-Al2O3 composite was synthesized by a coprecipitation method. Terephthalic acid (3.32 g) was added into 50 mL of deionized water and dissolved by the addition of ammonia. Then, 2.91 g of Ni(NO3)2·6H2O, 1.8 g of urea, and 0.8 g of NH4NO3 were added into the solution under stirring. After that, γ-Al2O3 was added to the mixture. The resultant suspension was aged for 18 h and subsequently crystallized at 80 °C for 24 h. After filtration and washing, the resulting powder was dried at 100 °C for 12 h to obtain Ni−Al−[C6H4(COO)2]2−-LDHs/γ-Al2O3. (2) A Mo7O246−−Ni−Al-LDHs/γ-Al2O3 composite was synthesized according to the following procedures. The above TAMA−Ni−Al-LDHs/γ-Al2O3 composite was added to a solution containing 0.01 mol of Na2MoO4 in 20 mL of water under stirring, and the pH of the system was maintained at 4.5 by dropwise adding 4 mol/L nitric acid. After 10 min, the green solids were processed with filtration, washing, and drying at 373 K for 12 h to obtain the Ni−Al−Mo7O246−-LDHs/γ-Al2O3 composite. (3) A Ni−Mo/γ-Al2O3 catalyst was prepared by calcination of the Ni−Al−Mo7O246−-LDHs/γ-Al2O3 composite under 773 K for 4 h, which is denoted as Cat-LDH. Besides, for comparison purpose, a counterpart oxidic catalyst with similar metal loadings was obtained by the conventional

Figure 1. XRD pattern of Cat-LDH.

NiO and MoO3 were highly dispersed in the pores and surfaces of supports to get the highly dispersed hydrogenation catalyst. 3.2. XPS. It has been generally accepted that the dispersion and crystallite size can be determined by XPS characterization. Figure 2 and Figure 3 show the XPS spectra of Mo 3d and Ni 2p for the two catalysts obtained by different methods. Table 1 listed the relevant electron-binding energies of Mo 3d and Ni 2p calculated from XPS spectra. The values of binding energies of Mo 3d5/2 in the two catalysts are 236.2 eV (Cat-IM) and 236.0 eV (Cat-LDH), respectively. Those were attributed to Mo6+, which had the binding energy Mo 3d5/2 = 236.0 ± 0.2 ev. 13890

DOI: 10.1021/acs.iecr.8b03383 Ind. Eng. Chem. Res. 2018, 57, 13889−13894

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atomic ratios of active components to aluminum in the aluminum surface. Thus, the ratios of Ni/Al and Mo/Al of the two catalysts Cat-IM and Cat-LDH are obtained from XPS data (Table 1). Moreover, the Ni/Al atomic ratio in the aluminum surface for the Cat-LDH is 0.0335, 2.6 times that for Cat-IM. Moreover, the surface Mo/Al atomic ratio for the Cat-LDH is 0.156, 1.5 times that for Cat-IM. Despite the same NiO and MoO3 contents over the two oxidic catalysts obtained from XRF results (NiO%(wt) = 2.9%, MoO3%(wt) = 19%), Cat-LDH has enhanced surface atomic ratio of Ni/Al and Mo/Al, indicating the much smaller size of active species in it. These results illustrate that the NiO and MoO3 dispersion on alumina surface can be enhanced significantly by LDH method, and thus more active centers can be formed on Al2O3 surface.6,7 On the other hand, the formation of platelike structures by the decomposition of LDHs during the calcinations can prevent sintering of Ni and Mo oxides species. 3.3. N2 Adsorption−Desorption Characterization. The textural properties of Cat-IM and Cat-LDH are also shown in Table 1. It is clear that the BET surface area and pore volume of Cat-LDH are larger than those of γ-Al2O3 and Cat-IM, although Cat-IM exhibit lower BET surface area and pore volume than Al2O3. Vakros et al.28 and Bao6 reported that larger crystallinities of active metals obtained by the conventional impregnation could plug into the alumina support pores, thus causing specific surface area decrease. Bao et al. have reported a novel hydrothermal deposition method (HDM) to obtain NiW/Al2O36 and W/Al2O37 catalysts. They found that HDM could form Ni and W with smaller particles, and thus the resultant catalysts had higher BET surface area and pore volume than the catalysts obtained by impregnation method.29,30 The present results of oxidic CatLDH are similar to those of Bao’s investigations. For example, oxidic Cat-LDH has much larger surface area and pore volume than the support γ-Al2O3. These results suggested that the LDHs method was effective in increasing pore volume and surface area of the resulting catalysts. 3.4. H2-TPR. The TPR curves of the Cat-IM and Cat-LDH are shown in Figure 4. It can be seen that Cat-LDH and Cat-IM

Figure 2. XPS spectra of Ni 2p in two catalysts: (a) Cat-IM and (b) Cat-LDH.

Figure 3. XPS spectra of Mo 3d in two catalysts: (a) Cat-IM and (b) Cat-LDH.

Table 1. Properties of γ-Al2O3, Cat-IM, and Cat-LDH Ni 2p3/2, eV Mo 3d5/2, eV Ni/Al Mo/Al surface area (BET), m2/g pore volume, cm3/g

γ-Al2O3

Cat-IM

Cat-LDH

272 0.55

856.6 236.2 0.0129 0.105 265 0.52

856.1 236.0 0.0335 0.156 286 0.66

For comparison, Cat-IM and Cat-LDH had the binding energies of Ni 2p3/2 856.1and 856.6 eV, respectively, which correspond to oxidic Ni2+ (this species has the binding energy Ni 2p3/2 = 856.6 ± 0.2 eV). The results of XPS studies showed that the chemical states of the two active metals (Mo and Ni) were similar in the two catalysts. XPS could exhibit crucial information regarding the chemical states (especially the dispersion) of the supported transition metal oxides due to its high surface sensitivity.27 XPS data can be employed to measure the Ni(Mo) phases dispersion in HDS catalysts. Interestingly, the dispersion could be obtained by

Figure 4. TPR curves of (a) Cat-LDH and (b) Cat-IM.

have a temperature peak at 355 and 370 °C, respectively, corresponding to the reduction of Ni species. It is generally accepted that the peak temperature in TPR curve reflects the interaction strength of the metal and aluminum support. Since 13891

DOI: 10.1021/acs.iecr.8b03383 Ind. Eng. Chem. Res. 2018, 57, 13889−13894

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the increased surface area and pore volume. Duan et al. prepared Ni−Al−CO3-LDHs/Al2O3 composite in which Ni−Al−CO3LDHs are located on both the surface and the pores of support (γ-Al2O3) by in situ synthesis.25 Well-dispersed NiO isolated by Al2O3 can be obtained by calcinations. These advantages of LDHs precursors suited most of the requirements to welldispersed active components on porous aluminum supports. These results suggest the effectiveness of LDHs method in enhancing the active metals dispersion due to the formation of isolate obtained from calcined Al2O3. 3.6. Catalytic Activity. The catalytic perfermance of 4, 6-DMDBT conversion on sulfided Cat-LDH and Cat-IM is shown in Figure 7. The sulfided Cat-LDH exhibited higher

the interaction between Ni−Al and Mo−Al in Cat-LDH is weak, the maximum reduction temperature of Cat-LDH is relatively low. Moreover, lower reduction temperature and weaker Ni−Al and Mo−Al interaction can significantly enhance the sulfidation of oxidic Ni and Mo species. The catalytic activity evaluation also confirmed this. It is well-known that low reduction temperature of active metals on Al2O3 supports results from the weaker active metal− supports interaction, and high dispersion of active phases is inherited from the stronger active metal−support interaction.6,7 Therefore, there is a paradox between the dispersion and the reduction temperature of active phases. Characterization results obtained in the present investigation indicate that LDH method can produce catalyst with simultaneous low reduction temperature and high dispersion of active components. The mechanism for this is under further investigation. 3.5. SEM. The morphologies of Cat-IM and Cat-LDH are shown in Figure 5 and Figure 6. SEM characterization shows

Figure 7. 4,6-DMDBT conversion on the two catalysts: (a) Cat-IM and (b) Cat-LDH.

activity of HDS, which showed an increase of 20% in 4,6-DMDBT conversion compared with the sulfided Cat-IM, although the two catalysts have similar content of active species (Ni and Mo) on the γ-Al2O3 supports. With the Dagang (Tianjin Petrochemical Company, CNPC) FCC diesel as feedstock, the activity stabilities of Cat-LDH and Cat-IM were studied, and sulfur compounds conversions are listed in Figure 8. During the continuous HDS process of 800 h, HDS efficiency of Cat-LDH remained stable (about 99%), much

Figure 5. SEM image of Cat-IM.

Figure 6. SEM image of Cat-LDH.

that platelike structures are preserved after the desulfurization. This implies that the plates are Al2O3 support but not MoS2 since topotactic transformation is very unlikely. It can be reasonably reduced that the platelike structures are formed from the LDHs precursors, and the active Ni−Mo phase is located at the edge of the plates. These active species located on the plates would be highly dispersed, and the sintering would be prevented by the plates. The geometry of those species also contributes to

Figure 8. Desulfurization efficiency of two sulfided catalysts: (a) Cat-LDH and (b) Cat-IM with different times. 13892

DOI: 10.1021/acs.iecr.8b03383 Ind. Eng. Chem. Res. 2018, 57, 13889−13894

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Figure 9. SEM image of Cat-LDH after HDS for 800 h.

higher than that of Cat-IM. The morphologies of the used CatLDH after 800 h are shown in Figure 9. It can be found that the form of platelike structures is retained, indicating the excellent stability of Cat-LDH, corresponding to the result in Figure 8. These results shed light on the industrial application of Cat-LDH. This difference of HDS activities for the two supported catalysts can be explained from two aspects. First, Ni and Mo active species on Cat-LDH have higher dispersion than Cat-IM, which are favorable for the conversion of sulfur-containing compounds. Second, the lower reduction temperature of Cat-LDH prepared by LDHs method may lead to the adequate sulfurization of MoO3 species located on γ-Al2O3, thus resulting in higher HDS activity.31 In summary, the LDHs method present an effective route to prepare supported catalysts with higher dispersion of active metals and lower reduction temperature, the combination of which leads to the excellent HDS activity of the resulting catalysts.

4. CONCLUSIONS Ni−Mo/γ-Al2O3 catalysts are prepared by thermal decomposition of Ni−Al−Mo7O246−-LDHs/γ-Al2O3 composites. The higher active metal dispersion while lowering the reduction temperature could be realized by the LDH method, which is greatly different from the conventional methods for enhancing the dispersion of active metals by elevating the reduction temperature. The as-prepared Ni−Mo/γ-Al2O3 catalyst exhibited greatly improved HDS activity compared with catalyst obtained by the conventional method of impregnation. These results demonstrate that HDS catalyst with moderate dispersion and lower reduction temperature can be realized by the LDH method. Thus, LDH strategy shed light on the facile preparation of ultradeep HDS catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongtao Liu: 0000-0002-4585-5980 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the PetroChina Company Limited (Grants Nos. 2016E-0701, 2016A-1801, and 2016A-1804). 13893

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DOI: 10.1021/acs.iecr.8b03383 Ind. Eng. Chem. Res. 2018, 57, 13889−13894