Research NoteA surfactant-assisted hydrothermal deposition method for preparing highly dispersed W/γ-Al2O3 hydrodenitrogenation catalyst
Introduction
Hydrodenitrogenation (HDN) is one of the most important reactions performed during hydrotreating processes of petroleum-refining products [1]. It is widely accepted that the key points to prepare highly active HDN catalysts are the control of the structure and nature of the active sites [2], [3]. One approach to achieving this goal is to use different support materials with high surface areas, such as activated carbons [4], mesoporous alumina [5], and mesoporous molecular sieves (e.g., MCM-41) [6]. Despite many promising laboratory results, these supports suffer from poor thermal/hydrothermal stability and thus are inapplicable to industry. Another approach is to increase the loading of active species while maintaining their high dispersion on support. However, further increasing the loading of active species by the conventional impregnation method yields limited improvement in catalyst HDN activity due to the increasing agglomeration tendency of the active species [7]. The equilibrium deposition filtration method reported by Karakonstantis et al. [8] could improve the dispersion of active species at high loadings, but this improvement results from the strong active phase–support interaction, which may limit its application due to the resulting incomplete sulfidation of active phases [9]. So far, there is no report on how to highly disperse active species on support without increasing the metal–support interaction. The present investigation addresses this issue.
Emerging as a powerful method for preparing various nanosized metal oxides [10], the hydrothermal deposition method (HDM) has gained wide application because of its capability to control the size, morphology, and surface chemistry of particles due to the unique hydrothermal conditions. It was noted that under the hydrothermal conditions, aqueous solutions also provide ions involved with high diffusivity and thus facilitate the deposition of nanoparticles on the internal surface of porous substrates [11]. These advantages of the HDM perfectly suit the requirements to highly disperse active metal species on porous supports; thus, the application of the HDM to the preparation of supported catalysts with higher metal loadings is expected to be an exciting approach to enhancing the activity of hydrotreating catalysts. However, to the best of our knowledge, this approach has not yet been examined in the literature.
In this study, a highly dispersed W/γ-Al2O3 catalyst (designated as catalyst HD) was prepared by a surfactant (cetyltrimethylammonium bromide, CTABr)-assisted hydrothermal deposition method developed in the present investigation, and its acidity, its morphology, and the state of tungsten species were compared with those of a catalyst (denoted as catalyst IM) prepared by the conventional impregnation method. In addition, the HDN performance of the two catalysts was evaluated using 1.0 wt% pyridine in cyclohexane as a model compound.
Section snippets
Catalyst preparation
Preparation of the oxidic catalyst HD involved the following steps. First, 6.0 g γ-Al2O3 particles (20–40 meshes, Sasol GmbH, Germany) were added into a 0.1 M sodium tungstate (99 wt%, Shantou Xilong Chemical Co., PR China) solution of 77.0 mL. Then, a 0.5 M ionic surfactant CTABr (99 wt%; Beijing Chemical Co., PR China) solution of 7.7 mL was slowly added to the above mixture and the dropwise addition of a 1 M HCl (37 wt%; Beijing Chemical Co., PR China) solution of 15.4 mL under stirring was
XRD and XPS
The XRD patterns of the calcined catalysts are shown in Fig. 1. The peaks at , 45.9°, and 67.1° are attributed to the pure γ-Al2O3 phase, and those at , 23.6°, and 24.4° are assigned to the bulk WO3 crystallites formed due to the higher loading of WO3 compared with its dispersion capacity on alumina [8]. Obviously, there exists a characteristic triplet of the bulk WO3 crystallite in the pattern of the oxidic catalyst IM, whereas it disappears in the pattern of the oxidic
Conclusions
Significantly different from various methods for improving the dispersion of active metal species by strengthening the metal–support interaction, the surfactant-assisted hydrothermal deposition method developed in the present investigation could realize the higher dispersion of active metal species while yielding the weaker metal–support interaction via the adjusting and stabilizing effects of the surfactant CTABr on the structure of active species. Compared with the catalyst prepared by
Acknowledgements
This work was supported by the National Basic Research Program of China (Grant 2004CB217807), the Natural Science Foundation of China (Grant 20606037), and the CNPC Innovation Foundation (Grant 05E7020).
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