Synthesis of NiMo catalysts supported on mesoporous Al-SBA-15 with different morphologies and their catalytic performance of DBT HDS

doi:10.1016/j.apcatb.2014.10.034

Applied Catalysis B: Environmental

Volume 165, April 2015, Pages 269-284

https://doi.org/10.1016/j.apcatb.2014.10.034 Get rights and content

Highlights

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SBA-15 with different morphologies was synthesized via a facile method.
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Al-SBA-15 with a Si/Al molar ratio of 10 was synthesized via different Al resources.
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The supported NiMo catalysts with different morphologies show different HDS activity.
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The morphology of Al-SBA-15 plays a vital role in the DBT HDS performance.
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A reaction network of DBT HDS over Al-SBA-15-supported NiMo catalysts was proposed.

Abstract

Mesoporous SBA-15 materials with different morphologies were successfully synthesized through different methods by using pluronic P123 triblock copolymer (EO₂₀PO₇₀EO₂₀) and cosurfactant (CTAB) as structure-directing agent. Moreover, highly ordered mesoporous Al-SBA-15 with a Si/Al molar ratio of 10 has been prepared with three different Al resources using post-synthesis methods. The characterization results of X-ray fluorescence spectroscopy, X-ray power diffraction, N₂ adsorption–desorption and ²⁷Al nuclear magnetic resonance spectroscopy revealed that the chemical grafting of aluminum chloride on the surface of SBA-15 was the best post-synthesis route for the preparation of Al-SBA-15 with different morphologies. The corresponding Al-SBA-15-supported NiMo hydrodesulfurization (HDS) catalysts with different morphologies were prepared and evaluated using DBT as a probe reactant. The sphere NiMo/AS-SP catalyst has higher dispersion degree of MoS₂ slabs and more acid sites than other catalysts. The catalytic results showed that the morphologies and mesochannels of SBA-15 played an important role in the catalytic performance of DBT HDS over NiMo catalysts, especially in the diffusivity of DBT molecules in the catalysts, while they have little effect on the selectivity of DBT HDS. The kinetics investigations confirmed that the activity of DBT HDS over these catalysts followed the order: long-rod catalyst (NiMo/AS-LR) < hexagonal prism catalyst (NiMo/AS-HP) < short-rod catalyst (NiMo/AS-SR) < sphere catalyst (NiMo/AS-SP). The high activity of the last one was attributed to the superior diffusion of the supports and the better dispersion of the active components.

Graphical abstract

Introduction

The sulfur compounds of the exhaust emissions from gasoline and diesel fuels combustion processes are the major components of air pollutants, which are very harmful to human health and the environment, and therefore, sulfur contents in transportation fuels should be limited to near zero in the stringent fuel specification [1], [2]. Various methods to remove the S in oil distillates have been proposed in many developed countries, such as hydrogenation, adsorption, biological desulfurization, etc. Among these technologies, the most effective process in large-scale industrial production is the catalytic hydrotreating [3]. However, the traditional hydrodesulfurization technique has been confronted a great challenge since the S limitation in the EU-V fuel specification was restricted lower than 10 ppm, meaning that the highly refractory molecules such as dibenzothiophene and 4,6-dimethyldibenzothiophene need to be desulfurized in the hydrotreating process [4], [5]. In order to meet the demands of ultraclean fuels, novel HDS catalysts with high selectivity and activity have been paid more attention. Compared with the conventional HDS-support γ-Al₂O₃, hierarchical porous materials can not only offer space for dispersing the catalytic active species, but also provide acid sites for the HDS reactions. Recently, the synthesis of novel supports becomes the special issue of the design and development of hydrotreating catalysts.

Soni et al. [6] reported that KIT-6-supported Mo, CoMo, and NiMo catalysts exhibited higher activities for the conversion of thiophene than γ-Al₂O₃-supported catalysts. Zhang et al. [2] prepared a novel micro–mesoporous composite material Beta-KIT-6 (BK) and used as the catalyst support for hydrodesulfurization of dibenzothiophene. The result showed that NiMo/Beta-KIT-6 exhibited a higher activity than the NiMo/γ-Al₂O₃ catalyst due to the superior mass transfer ability of the cubic Ia3d mesoporous structure. Li et al. [7] prepared NiW/Al-SBA-15 catalyst and used in hydrodesulfurization. They found that it showed a higher hydrodesulfurization activity than the NiW/γ-Al₂O₃ catalyst because of the perfect combination of the better dispersion and higher stacking of Ni-W-S phases on the support with the suitable distributions of B and L acid sites simultaneously. Mesoporous silica materials with high surface areas and well-ordered pore structures have potential applications in the catalytic conversion processes of large molecules because their large pore sizes can mitigate the diffusion barrier of the reactants and the products. Mesoporous SBA-15 [8], [9] material synthesized using pluronic P123 triblock copolymer (EO₂₀PO₇₀EO₂₀) as SDA has relatively large pore and shows high hydrothermal stability due to its special configuration. Many researchers have employed SBA-15-type materials as the supports of hydrodesulfurization catalysts [10], [11], [12], [13]. However, compared with the conventional microporous zeolites [14], SBA-15 has much weaker acidity and lower hydrothermal stability because of its neutral silicon frameworks. Thus, how to substantially improve the hydrothermal stability and the acidity of SBA-15 attracts more interests. Al-containing SBA-15 material prepared by partial replacement of silicon ions with aluminum ions can improve the hydrothermal and acidity of SBA-15 [15], [16], [17], [18]. Luan et al. [19] reported that Al could not be constructed into the SBA-15 structure by direct incorporation method, because most of the Al sources can dissolve in the strong acidic media. Therefore, the post-synthetic method that can graft Al³⁺ into the SBA-15 surface seems to be an alternative choice to prepare Al-SBA-15.

Besides the acidity of SBA-15, the morphology, pore size, and pore structure of SBA-15 also have influence on the activity of catalytic reactions [7], [20], [21], [22]. The shape and size of mesoporous SBA-15 particles are crucial factors for the reactant and the product molecules diffusing in the particle channels, where the subsequent undesirable reactions might take place [23]. And the morphology of SBA-15 also plays an important role in dispersion of active metal species in the support mesopores which will improve the performance of the catalysts. Katiyar et al. [24] reported that the spherical SBA-15 particles had a higher saturation capacity than the fibrous particles on the adsorption of proteins. Chen [25] also synthesized a functionalized mesoporous SBA-15 with platelet morphology and short mesochannels. And the platelet SBA-15 materials were superior to the conventional SBA-15 materials with rod or fiber morphologies in facilitating the molecular diffusion and reducing the pore blockage when they were used in the adsorption or reactions of bulky molecules. Till now, many efforts have been devoted to the synthesis of mesoporous materials with uniform size and well-defined morphology. Mesoporous SBA-15 materials in the forms of rod, sphere, fiber, platelet, gyroid, discoid, sausage, and doughnut shapes have been obtained through various synthetic strategies [26], [27], [28], [29], [30], [31]. It has been well documented that the catalytic properties are related to the morphology and composition of material [5], [32], [33]. However, no detailed studies have been made to investigate the effect of morphology on the HDS activity over SBA-15-supported catalysts.

In this research, a series of SBA-15 materials with different morphologies were successfully synthesized using pluronic P123 triblock copolymer and cosurfactant (CTAB) as structure-directing agent. And three different post-synthesis routes to incorporate Al into SBA-15 were used to prepare Al-SBA-15 materials. The as-synthesized Al-SBA-15-supported NiMo catalysts with different morphologies were prepared through a two-step incipient-wetness impregnation method, and the corresponding HDS performances were evaluated using dibenzothiophene as probe molecules, while the conventional NiMo/γ-Al₂O₃ catalyst was taken as the reference. The effects of different morphologies of SBA-15 on the catalytic performances of DBT HDS were systematically investigated at different temperatures and weight times. Furthermore, the physicochemical properties of the supports and catalysts were characterized by various characterization techniques. The HDS results of DBT over these catalysts follow the activity order: long-rod catalyst (NiMo/AS-LR) < hexagonal prism catalyst (NiMo/AS-HP) < short-rod catalyst (NiMo/AS-SR) < sphere catalyst (NiMo/AS-SP). A possible reaction network of DBT HDS over NiMo supported on Al-SBA-15 catalysts with different morphologies was also proposed.

Section snippets

Synthesis of the supports

Mesoporous SBA-15 silicas with different morphologies were synthesized using Pluronic P123, cetyltrimethylammonium bromide (CTAB) as the structure-directing agent, and tetraethyl orthosilicate (TEOS) as the silicate source under acidic conditions. All commercial chemicals were used without further purifications. Three different morphologies of SBA-15, hexagonal prisms, short rods, and long rods were synthesized under different temperatures with the same compositions of the original reaction

Characterization of short-rod SBA-15 modified with Al by using different aluminum sources

The pure SBA-15 material has much weaker acidity and lower hydrothermal stability because of its neutral silicon frameworks. Thus, it is necessary to improve the hydrothermal stability and acidity of SBA-15 by partial replacement of silicon ions with aluminum ions. Klimova et al.[42] reported that NiMo catalysts supported on Al-containing SBA-15 materials with a Si/Al molar ratio between 30 and 10 show high activity in HDS of 4,6-dimethyldibenzothiophene. Suresh et al. [43] also prepared

Effect of the morphology of Al-SBA-15 on the catalytic performance for DBT HDS

The catalytic performances of DBT HDS over these catalysts are closely related to their physicochemical properties of the supports, the dispersion of the active metals, and the acid properties of the catalysts. Among these influence factors, the morphology and mesochannel of Al-SBA-15 mesoporous materials are the crucial factors for DBT HDS reaction. Simple morphologies with short unhindered path lengths, such as small spheres, crystal-like particles, and short straight rods, are beneficial for

Conclusions

Mesoporous SBA-15 silicas with spheres, hexagonal prisms, short rods, and long rods morphologies were synthesized via a facile method. Moreover, highly ordered mesoporous Al-SBA-15 with a Si/Al molar ratio of 10 had been synthesized via three different post-synthesis routes. The characterization results showed that alumination by reacting SBA-15 silica wall surfaces with alumina chloride in dry ethanol can maintain the surface area and porous structure of the SBA-15. After impregnation of Ni

Acknowledgements

The authors acknowledge the financial supports from the NNSFC (Nos. 21276277, 21173269 and 91127040), Specialized Research Fund for the Doctoral Program of Higher Education (20130007110003), and Opening Project of Guangxi Key Laboratory of PRPPIT (2012K01). The open funds of the State Key Laboratory of Chemical Resource Engineering and the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry.

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