Synthesis of a new ordered mesoporous NiMoO4 complex oxide and its efficient catalytic performance for oxidative dehydrogenation of propane

Journal of Energy Chemistry 23(2014)171–178

Synthesis of a new ordered mesoporous NiMoO4 complex oxide and its efficient catalytic performance for oxidative dehydrogenation of propane Xiaoqiang Fan,

Jianmei Li, Zhen Zhao∗ ,

Yuechang Wei, Jian Liu∗ , Aijun Duan, Guiyuan Jiang

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China [ Manuscript received September 10, 2013; revised November 14, 2013 ]

Abstract Highly ordered mesoporous NiMoO4 material was successfully synthesized using mesoporous silica KIT-6 as hard template via vacuum nanocasting method. The structure was characterized by means of XRD, TEM, N2 adsorption-desorption, Raman and FT-IR. The mesoporous NiMoO4 with the coexistence of α-NiMoO4 and β-NiMoO4 showed well-ordered mesoporous structure, a bimodal pore size distribution and crystalline framework. The catalytic performance of NiMoO4 was investigated for oxidative dehydrogenation of propane. It is demonstrated that the mesoporous NiMoO4 catalyst with more surface active oxygen species showed better catalytic performance in oxidative dehydrogenation of propane in comparison with bulk NiMoO4 . Key words ordered mesoporous structure; NiMoO4 complex oxide; vacuum nanocasting; propane; oxidative dehydrogenation

1. Introduction Highly ordered mesoporous metal oxides with large surface area, narrow pore size distribution and large pore volume have attracted much attention in the fields of catalysis, separation, magnetism, chemical sensing and optoelectronics in recent years [1−4]. These mesoporous materials can be prepared via soft [5] and hard template pathways [6]. So far, some well-ordered mesoporous monometal oxides have been prepared by soft template methods [7,8]. However, in this case, a strong interaction between structure-directing agents and precursors is necessary to assemble ordered target materials and avoid the macroscale phase separation [9]. Meanwhile, the metal oxides prepared by soft template methods often possess poorly crystallized walls and thermostability, which limits their applicatons. Ryoo and co-workers have introduced the nanocasting route to synthesize ordered mesoporous materials using mesoporous silica hard template [10]. Nanocasting is an efficient approach for the synthesis of highly ordered crystalline mesoporous materials, because mesoporous silica templates can provide stable supports for high temperature crystallization [11]. Following this

method, a number of mesoporous monometal oxides have been successfully prepared, including NiO, MoO2 , Co3 O4 , CoO, Cr2 O3 , CuO and Fe2 O3 , etc [12−17]. However, compared with the mesoporous monometal oxides, the synthesis of mesoporous binary or complex metal oxides is very difficult, even if these binary metal oxides have exhibited many unusual catalytic performances in many chemical reactions. Ni-Mo oxide catalysts have drawn considerable interest for their potential uses as heterogeneous catalysts in a variety of catalytic processes such as methanol oxidation [18−20]. Typically, these catalysts exhibit high catalytic activities for oxidative dehydrogenation (ODH) of light alkanes [21−27]. However, the catalytic activities of bulk Ni-Mo oxide catalysts are still low due to their low surface areas and diffusion limitation for the products. The ordered mesoporous structure with narrow pore size distribution is favorable for the diffusion of products so as to avoid deep oxidation and coke formation [28]. The large surface areas offer more surface active sites for the reaction [29]. Therefore, it is very significant to synthesize ordered mesoporous Ni-Mo oxide catalysts for ODH of light alkanes. The difficult points to synthesize binary metal oxides are

∗ Corresponding authors. Tel: +86-10-89731586; Fax: +86-10-69724728; E-mail: [email protected] (Zhen Zhao); Tel: +86-10-89732326; Fax: +86-10-69724728; E-mail: [email protected] (Jian Liu) This work was supported by NSFC (21073235, 21173270, 21177160, 21376261), 863 Program (2013AA065302) and PetroChina Innovation Foundation (2011D-5006-0403).

Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60132-7

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how to improve the filling degree in mesopores for the precursor solution and how to change the precursors to consecutive mixed oxide with long-range mesostructure ordering instead of isolated nanoparticles. There are some approaches to improve the filling degree: introduce functional groups to the wall of the hard template [30]; use appropriate precursors. Herein, we employed a vacuum nanocasting method to improve the impregnation of precursors into mesopores of KIT-6 template. The calcination process was carried out in a quartzcovered bottle to improve the long-range ordering [31]. Probe reaction by oxidative dehydrogenation of propane (ODHP) has been employed to determine the catalytic performance of mesoporous and bulk NiMoO4 . 2. Expermental 2.1. Synthesis of materials 2.1.1. Preparation of mesoporous NiMoO4 Mesoporous NiMoO4 sample was synthesized via a vacuum nanocasting procedure using KIT-6 as hard template. Cubic mesoporous KIT-6 silica template was prepared according to the literature [32]. A typical synthesis of mesoporous NiMoO4 was as follows: 5 mmol Ni(NO3 )2 ·6H2 O and 5 mmol phosphomolybdic acid hydrate were dissolved in 20 mL ethanol to form a homogeneous solution. Then, the solution was added to a beaker containing 1 g mesoporous silica KIT-6 template. This mixture was placed in a closed container which was connected with the vacuum pump and stirred for 12 h. The vacuum pump was used to remove the air from the container and keep high vacuum. After 12 h stirring, the mixture was then transferred to a vacuum drying oven (<10−3 bar) to let the solvent evaporate completely at room temperature. After that the sample was calcined at 500 ◦ C at the rate of 1 ◦ C/min and held at this temperature for 6 h. The resulting sample was treated with 4% HF to remove the silica template, followed by washing with water several times and then drying at 60 ◦ C. For comparison, NiMoO4 sample was also prepared by nanocasting method. And the whole procedure was under atmospheric pressure. 2.1.2. Preparation of bulk NiMoO4 Ni(NO3 )2 ·6H2 O and phosphomolybdic acid hydrate with molar ratio of 1 were dissolved in distilled water and stirred 1 h for full mixing. Then the mixture was dried at 40 ◦ C for 12 h. The obtained powder was calcined at 500 ◦ C for 6 h. 2.2. Catalyst characterization XRD patterns were recorded on a powder X-ray diffractometer (Shimadzu XRD 6000) using Cu Kα (λ = 0.15406 nm) radiation with a Nickel filter operating at 40 kV and 10 mA. The surface area and pore size distribution

of the catalysts were characterized by a nitrogen sorption technique (autosorb iQ, USA). The sample was degassed under a vacuum at 300 ◦ C for 4 h prior to automatic analyzer analysis at 77 K. The surface morphology of the catalyst was observed by field emission scanning electron microscopy (FESEM) on a Quanta 200F instruments using an accelerating voltage of 5 kV. The TEM and HRTEM images were carried out using a JEOl JEM 2100 electron microscope equipped with a field emission source at an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI-1600 ESCA spectrometer using Mg Kα (hν = 1253.6 eV, 1 eV = 1.603×10−19 J) X-ray source. The binding energies were calibrated using C 1s peak of contaminant carbon (BE = 284.6 eV) as an internal standard. Fourier transform infrared spectra (FT-IR) of the samples in the form of KBr powder-pressed pellets were measured on a FTS-300 FT-IR spectrophotometer (USA) under ambient conditions. Raman analysis was performed at room temperature under ambient conditions on a Renishaw inVia Reflex Raman spectrometer with a 532 nm laser. 2.3. Activity measurements The catalytic activity tests were performed in a fixed-bed reactor at atmospheric pressure. The catalyst (0.1 g) with particle size of 40−60 mesh was used to test the activity of propane oxidative dehydrogenation. The feed consisted of a mixture of propane/oxygen/helium with a molar ratio of 1/2/7. The total gas flow rate was 20 mL/min. The axial temperature profile was monitored by a thermocouple inside a tube which was inserted into the catalyst bed. Experiments were carried out in the temperature interval of 500−600 ◦ C. Feed and products were analyzed on-line by a gas chromatograph (SP-2100), equipped with FID detector. A Porapak Q column was used for the seperation of CO, CO2 , C2 H4 , C3 H6 , C3 H8 . A methanizer with Ni catalyst was used to convert CO and CO2 to methane at 380 ◦ C. 3. Results and discussion 3.1. Structural characterization of mesoporous NiMoO4 catalyst The morphology of the mesoporous NiMoO4 sample prepared under vacuum condition was investigated by means of FESEM. Figure 1 shows the SEM image of mesoporous NiMoO4 . As shown, the morphology of the mesoporous sample was sphere and the size of the nanosphere was submicrometer scale (50−300 nm), which is much smaller than the KIT-6 template particle. To investigate the regularity of the mesopores, transmission electron microscopy (TEM) measurements were carried out. The TEM images of KIT-6/NiMoO4 composite, silicafree mesoporous NiMoO4 and element mapping are present in Figure 2. As shown in Figure 2(a), NiMoO4 particles distributed over the silica matrix and tended to be spherical.

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This is an indication that the growth of the mesoporous crystal follows the channel of KIT-6 template outward from the nucleation center with a uniform speed [33]. After the silica was removed, mesoporous NiMoO4 was obtained. The TEM image of it is shown in Figure 2(b). NiMoO4 material showed a highly ordered pore structure, confirming the high level of infiltration achieved by the solution precursors under vacuum nanocasting condition even only one step impregnation was used. Figure 2(c) shows the high-angle annular dark-field scaning TEM (HAADF-STEM) image of mesoporous NiMoO4 , which was well consistent with the image of the original KIT-6. Thus, mesoporous NiMoO4 corresponded to a perfect inversion replica of the original highly ordered KIT-6. To confirm the uniform presence of molybdenum and nickle elements in the mesoporous NiMoO4 product, the distributions of molybdenum and nickle in NiMoO4 matrix were investigated by STEM-EDX elemental mapping. The region of the larger pore was chosen to make the image easier to be distinguished. As shown in Figure 2(d), the elemental mapping images clearly show the presence of Ni, Mo and O, which were all homogeneously dispersed in the whole

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domains. These results indicate that the ordered mesoporous NiMoO4 complex oxide can be successfully synthesized by vacuum nanocasting method. In contrast, the TEM image of NiMoO4 under atmospheric pressure showed inferior ordered mesopores (Figure 3) and existed in the form of nanoparticles.

Figure 1. SEM image of mesoporous NiMoO4

Figure 2. TEM images of (a) KIT-6/NiMoO4 composite and (b) mesoporous NiMoO4 ; (c) HAADF-STEM image of mesoporous NiMoO4 and (d) Energydispersive spectroscopy element mapping of mesoporous NiMoO4

The ordered mesoporous structure was further examined by small angel X-ray diffraction (SAXRD). The SAXRD patterns of mesoporous NiMoO4 prepared under vacuum and atmospheric pressure are shown in Figure 4(a). The pattern of mesoporous NiMoO4 prepared by vacuum nanocasting showed well defined diffraction peaks which can be indexed to the reflections in cubic Ia 3d space groups of KIT-6. The intense diffraction peak at 2θ = 0.7o with a weak shoul-

der peak can be assigned to (211) and (220) reflection, and the broad peak between 1.5o and 2.3o is caused by strong overlapping of the reflections for Ia 3d symmetry. SAXRD pattern clearly confirms that the mesoporous NiMoO4 has been successfully replicated from 3D cubic KIT-6 silica template. On the other hand, there was no apparent peak in pattern (2), indicating inferior long-range order structure, which is in good agreement with the TEM results. Wide angle XRD patterns

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of bulk NiMoO4 and mesoporous NiMoO4 before and after reaction are shown in Figure 4(b). The diffraction peaks of mesoporous NiMoO4 at 2θ = 14.3o, 25.4o, 28.8o , 32.7o, 43.9o and 47.4o are attributed to α-NiMoO4 [34], which is in good agreement with the corresponding bulk material. In contrast to the information from XRD patterns, Raman and FT-IR spectroscopies are particularly powerful techniques for detecting small amounts of amorphous phases which are not detectable by XRD. Figure 5 displays the Raman spectra of mesoporous and bulk NiMoO4 . The bands at 962, 913, and 706 cm−1 attributing to octahedral coordination molybdenum and oxygen vibration, are assigned to α-NiMoO4 [35]. And the bands at 945, 896 and 825 cm−1 due to tetrahedral coordination molybdenum and oxygen vibration, are assigned to β-NiMoO4 [36]. Figure 6 shows FT-IR spectra of mesoporous and bulk NiMoO4 . α-NiMoO4 is characterized by bands at 934 and 955 cm−1 [37]. There were also two more peaks at 881 and 802 cm−1 in mesoporous NiMoO4 sample, which correspond to the β-NiMoO4 phase. These peaks are used to distinguish the two phases as a result of the structural change from six-coordinated molybdenum in α-NiMoO4 to four-coordinated molybdenum in β-NiMoO4 [38]. Moreover, there was no monometallic oxide (NiO, MoO3 ) phase existing in the mesoporous NiMoO4 sample, indicating highly misci-

ble of Ni(NO3 )2 ·6H2 O and phosphomolybdic acid hydrate in nanocasting procedure. Both FT-IR and Raman characterization results demonstrated that β-NiMoO4 is formed in mesoporous catalyst sample. And compared with bulk NiMoO4 , the relative intensities of β-NiMoO4 in IR and Raman spectra for mesoporous sample were larger than those of bulk sample, indicating higher content of β-NiMoO4 in mesoporous NiMoO4 sample.

Figure 3. TEM image of NiMoO4 synthesized under atmospheric pressure

Figure 4. (a) SAXRD patterns of mesoporous NiMoO4 prepared under vacuum condition (1) and atmospheric pressure (2); (b) Wide angle XRD patterns for (1) bulk, (2) mesoporous NiMoO4 and (3) mesoporous NiMoO4 after reaction

The pore structures and surface area of mesoporous NiMoO4 were examined by N2 adsorption-desorption measurement and the results are displayed in Figure 7(a). A typical type-IV isotherm was obtained, indicating a characteristic of mesoporous material. The pore size distribution calculated from the desorption isotherm by BJH method is shown in Figure 7(a) inset. The first peak centered around 3.5 nm, which is in good agreement with the wall thickness of KIT-6. There was also another peak at 12.9 nm, which is caused from the half replicated of KIT-6 [12]. It seems that half replicated frameworks have been mostly formed. This may be due to a low interconnectivity within the gyroid structure of the silica template, resulting in the precursor filling in one of the two

enantiomeric subframeworks [39]. In addition, the pore size larger than 25 nm is related to the interspace among spherical NiMoO4 secondary particles. The isotherm and pore size distribution of NiMoO4 prepared under atmospheric pressure is shown in Figure 7(b). The pore size of this material had a extensive distribution, which may be caused from the space between the nanoparticles. The BET surface area of mesoporous NiMoO4 was 99.6 m2 /g, which is much higher than that of bulk NiMoO4 (8.7 m2 /g). 3.2. Catalytic performance for ODHP to propene As for the catalytic performance of NiMoO4 oxide which

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is concerned, Jibril et al. [40] have reported that Ni-Mo mixed oxide/MCM-41 catalysts show good catalytic performance for ODHP. ODHP to propene is employed as the probe reaction, giving its economic significance as to upgrading cheaper paraffin feedstock into value-added olefin. It should be pointed out that both mesoporous and bulk NiMoO4 catalysts were directly used for ODHP without heat treatment transforming α-NiMoO4 to β-NiMoO4 . The reaction conditions and catalytic performance of NiMoO4 catalysts are shown in Table 1. Propane conversion of 11.8% and

Figure 5. Raman spectra of NiMoO4 : (1) mesoporous NiMoO4 before reaction, (2) bulk NiMoO4 before reaction, and (3) mesoporous NiMoO4 after reaction

selectivity to propene of 60.1% over mesoporous NiMoO4 catalyst were obtained. Although the conversion of 11.8% was not as high as that of Mo-NiMCM41 [40], the space-time yield of 3.8 mmol/(g·h) was much higher than that obtained on Mo-NiMCM-41 catalyst (1.8 mmol/(g·h)). The catalytic performance of mesoporous and bulk NiMoO4 at different reaction temperatures are shown in Figure 8. For both mesoporous and bulk NiMoO4 catalysts, propane conversion increased, while selectivity to propylene decreased with the increase of reaction temperature, resulting in a volcano plot of propene yield (Figure 9) with a maximum value of 15.5% at 560 ◦ C for mesoporous NiMoO4 .

Figure 6. FT-IR spectra of mesoporous (1) and bulk NiMoO4 (2)

Figure 7. Nitrogen adsorption-desorption isotherms and pore size distributions (inset) for mesoporous NiMoO4 under vacuum (a) and atmospheric pressure (b)

Table 1. Reaction conditions and catalytic performance of mesoporous and bulk NiMoO4 for ODHP at 500 ◦ C Catalysts Meso-NiMoO4 Bulk NiMoO4 Mo-NiMCM-41 [37]

Weight of catalyst (g) 0.1 0.1 1

Space velocity of propane (mL/min) 2 2 5

C3 H8 conversion (%) 11.8 8.9 41.2

C3 H6 selectivity (%) 60.1 51.1 31.8

C3 H6 yield (%) 7.1 4.9 13.4

Space-time yield (mmol/(g·h)) 3.8 2.6 1.8

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NiMoO4 catalysts, XPS studies were conducted and O 1s XPS spectra of the mesoporous and bulk NiMoO4 are shown in Figure 10. Two peaks appeared on these two spectra. The peak at 530 eV is assigned to lattice oxygen species (O2− ), while the peak at 532.2 eV is attributed to adsorbed oxygen species [42]. As shown in Figure 10, the proportions of these two kinds of oxygen species on different catalysts were quite different. The mesoporous NiMoO4 catalyst had a higher proportion of adsorbed oxygen species than the bulk NiMoO4 . It indicates that the mesoporous NiMoO4 showed a large amount of surface active oxygen species and enhanced capability of propane activation, which gives rise to better catalytic property for ODHP. Figure 8. Propane conversion and propene selectivity as a function of reaction temperature

Figure 10. O 1s spectra of mesoporous (1) and bulk (2) NiMoO4

Figure 9. Yield of propene as a function of reaction temperature

It can be seen that the mesoporous NiMoO4 with higher ratio of β-NiMoO4 /α-NiMoO4 showed higher selectivity than that of bulk NiMoO4 . Mazzocchia et al. [41] have reported that the phase structures strongly affect the selectivity to propene and β-NiMoO4 is almost twice more selective in propene formation than α-NiMoO4 due to the different reactivity of O species. To investigate the nature of O species over

In general, large surface areas can offer more surface active sites for the reaction and the surface area of mesoporous catalyst is several times as that of bulk material. But it can be found that the catalytic activity of mesoporous NiMoO4 was not improved proportionally. In consideration of possible effect on the surface of mesoporous NiMoO4 caused by HF treatment, XPS characterization was carried out to study the surface composition of mesoporous NiMoO4 catalyst and the spectra are shown in Figure 11. Mo/Ni ratio was changed from 2.1 to 1.3 after HF treatment. Neiman et al. [43] have

Figure 11. X-ray photoelectron spectra of Mo 3d (a) and Ni 2p (b) levels for mesoporous NiMoO4 . (1) Before HF treatment, (2) After HF treatment

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found that a surface adsorption phase MoO3 is formed in MoO3 -NiO system which is consistent with our results of XPS. When NiMoO4 /KIT-6 composite was treated to remove KIT-6 template by HF, the surface MoO3 phase was also etched. In ODHP reaction, the existence of excess MoO3 is beneficial to catalytic performance [21]. The etching of surface MoO3 is an adverse factor which results in some negative effect on the catalytic performance of mesoporous NiMoO4 . To gain insight into the stability of the mesoporous NiMoO4 during the reaction, XRD, Raman and TEM characterizations were carried out. As shown in Figure 4 and Figure 5, there was no obvious phase structure change after the reaction, indicating good stability of the mesoporous NiMoO4 . The TEM image of mesoporous NiMoO4 after reaction (Figure 12) showed the existence of ordered mesopores, which indicates that the mesoporous catalyst still keep mesoporous structure after the reaction. The stability of mesoporous NiMoO4 catalyst with time on stream was also investigated and the result is shown in Figure 13. After 300 min reaction, no apparent deactivation performance was recorded, demonstrating that the mesoporous NiMoO4 possessed relative high stability.

4. Conclusions In summary, the present work shows a successful synthesis of highly ordered mesoporous NiMoO4 catalytic material using vacuum nanocasting method. It is demonstrated that under vacuum condition the regularity of mesopore can be improved efficiently. The mesoporous sample containing more β-NiMoO4 and surface oxygen species shows better catalytic performance for ODHP to produce propene than bulk NiMoO4 . Acknowledgements This work is supported by NSFC (21073235, 21173270, 21177160, 21376261), 863 Program (2013AA065302) and PetroChina Innovation Foundation (2011D-5006-0403).

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Figure 13. Catalytic behaviour of ODHP over mesoporous NiMoO4 catalyst as a function of time on stream at 560 ◦ C

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