Synthesis of hierarchically structured alumina support with adjustable nanocrystalline aggregation towards efficient hydrodesulfurization
Graphical abstract
Introduction
In recent decades, with increasing global concern on environment protection, strict regulations have been made to control the sulfur contents in gasoline and diesel in many countries, which accelerates the demand for high-quality petroleum products and also set high requirements for the industrial capacity of hydrodesulfurization (HDS) [[1], [2], [3], [4]]. Therefore, highly active HDS catalysts are urgently required. Conventionally, the HDS catalyst generally uses Ni (Co)-Mo (W) as the active component [[5], [6], [7]], with alumina or alumina composite oxide containing silica, titanium dioxide or zeolite used as the catalyst carrier [1,2,[8], [9], [10]]. To meet the increasing demand, new HDS catalysts have also been exploited, which mainly explore novel active components (such as NiP, MoP and WP) [[11], [12], [13], [14]]. However, the commercialization of these new catalysts is often limited by the high cost, the harsh preparation conditions and/or the poor catalyst stability. In this respect, exploitation of approaches towards highly efficient and stable Ni (Co)-Mo (W) catalysts is still necessary to upgrade the present HDS technologies.
Towards this end, it is crucial to develop desirable support materials because the HDS performance strongly depends on the physical properties (e.g., the specific surface area, the pore volume, and the pore size) and chemical properties (e.g., acidity) of the catalyst supports. The higher specific surface area facilitates the dispersion of the metal active component on the surface of the support. Until now, γ-Al2O3 support is the most widely used support for HDS catalysts [15]. However, the conventional alumina supports usually have an unimodal pore size distribution curve, most of which have only mesopores. As such, the reactions over such catalysts are susceptible to diffusion control; and the catalyst support could suffer from pore plugging easily, which leads to catalytic activity decline and short cycle life. In view of the above issues, hierarchically porous alumina materials, especially those with macro-meso-porosities, have aroused the interest of many scholars. The macropores can facilitate the diffusion of reactants with large molecule sizes from the outer surface of the catalyst to the active sites, whereas the mesopores provide the large surface area for the homogeneous dispersion of active sites, both of which work together to support high catalytic activity and durability.
To produce such hierarchical catalysts, template agents, such as carbon black [16], polystyrene (PS) [17], poly-(methyl methacrylate) (PMMA) [18], have been most commonly used. Pore size and volume of such prepared samples closely depend on the amount and/or the size of the templates. Maity et al. reported that the 10% carbon modified-alumina-supported P-containing CoMo catalysts (105 m2/g) which had larger pore diameter and higher pore volume exhibited higher HDS and HDM activities than the control samples [16]. Han et al. found that the coprecipitated Co–Mo/Al2O3 catalyst with both macro- and meso-pores whose surface area is 79 m2/g exhibited a superior durability for hydrodesulfurization of dibenzothiophene (DBT) compared to those without macropores [18]. However, it shall also be noted that, compared with the conventional mesoporous alumina, the macro-mesoporous alumina generally has relatively small specific surface area. Moreover, the acidity of the surface of the support which affects the interaction between the active metal components and the support also influence the dispersion and the active composition of the metal oxide [[19], [20], [21]]. Alumina carriers with suitable acidity are favorable for the hydrodesulfurization process. P, B and other elements were employed to further regulate the acidity of the catalytic support [16,[22], [23], [24]]. Therefore, the development of an ideal macro-mesoporous alumina carrier material with both high specific surface area and moderate acid sites is essentially important.
Herein, we reported citric-acid-mediated preparations of macro-mesoporous alumina with adjustable large specific surface area and moderate acid sites via a simple hydrothermal method without using any templates, and the role of citric acid on the formation of such hierarchical alumina was attempted. Thus-prepared alumina supports were loaded with Ni-Mo active components and tested for hydrodesulfurization reaction of DBT, which showed superior catalytic activity, clearly confirming their structural merits. On this basis, the properties of the alumina support, especially their mesoporous structure and acidic property which play a key role in improving the HDS catalytic efficiency, were characterized and discussed in detail.
Section snippets
Materials
Dibenzothiophene (DBT) was purchased from Aldrich. Al2(SO4)3·18H2O, urea, citric acid, (NH4)6Mo7O24·24H2O and Ni(NO3)2·6H2O were purchased from Sinopharm Chemical Reagent Co. All of these reagents were of analytical purity and were used without further purification. In addition, commercial alumina was purchased from Fushun Research Institute of Petroleum and Petrochemical, SINOPEC, Fushun, China.
Synthesis of macro-mesoporous alumina supports
The alumina precursors were synthesized by a hydrothermal method. Aluminum sulfate (5.1 g), urea
Formation and characterization of Al2O3-xCA support
Alumina precursor (AlOOH) microspheres were prepared by hydrothermal treatment of aluminum sulfate and urea together with citric acid, which was then easily converted to alumina microspheres, with the same size and shape via calcination. As shown in Fig. 1, SEM observation reveals that each microsphere consists of numerous primary nanocrystals; and the morphology and size of the primary crystals in the as-prepared alumina supports were found to be regulated by the dosages of citric acid. As
Conclusions
In this work, towards the pursuit of highly active hydrodesulfurization (HDS) catalysts, a citric-acid-mediated hydrothermal method has been developed, which generates hierarchically structured alumina hollow microspheres are assembled from numerous primary nanocrystals; such microspheres possess both macropores and mesopores, large specific surface area and pore volume, and desirable level of acidity; and such properties could be regulated by simply adjusting the dosage of citric acid. On this
Acknowledgements
This work is supported by the National Natural Science Foundation of China (21373168, 21473143, 21506107, 21773194, 21703182).
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