A comparative study of the catalytic performance of Co-Mo and Co(Ni)-W carbide catalysts in the hydrodenitrogenation (HDN) reaction of pyridine
´ ´ Hamid A. Al-Megren a,*, Sergio L. Gonzalez-Cortes b,*, Tiancun Xiao c, Malcolm L.H. Green c
Petroleum and Petrochemicals Research Institute, King Abdulaziz City forScience and Technology, P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia b ´ ´ ´ Laboratorio de Cinetica y Catalisis, Departamento de Quımica, Facultad de Ciencias, La Hechicera, Universidad de Los Andes, ´ Merida 5101, Venezuela c Wolfson Catalysis Center, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK Received 24 March 2007; received in revisedform 3 June 2007; accepted 13 June 2007 Available online 23 June 2007
Abstract In this contribution the HDN catalytic behaviour of Co-Mo carbide catalysts and Co(Ni)-W carbide catalysts is compared in order to establish a rational effect of cobalt (or nickel) over Mo and W carbide HDN catalysts. The bimetallic and trimetallic catalysts were characterized by using elemental analysis, X-raydiffraction (XRD), infrared spectroscopy, Raman spectroscopy, thermo-gravimetric analysis and measurements of BET speciﬁc surface area. The catalytic performance was evaluated in a continuous ﬂow reactor using hydrodenitrogenation of pyridine as model reaction. The incorporation of cobalt onto the structure of Mo2C reached an optimal Co/Mo ratio of 0.43 (i.e. Co4Mo6Cx catalyst), whose HDN activity andstability was markedly higher than industrial catalysts (i.e. CoMoS/Al2O3 and NiMoS/Al2O3). Higher molar ratios facilitated the segregation of promoter. This was reﬂected in a poor catalytic stability not only on Co-Mo carbide catalysts, but also on the Co(Ni)-W carbide catalysts. The CoWCx bimetallic catalyst was more active in the steady state than Ni-containing catalysts. Two modes of pyridineadsorption may occur in the HDN reaction, the end-on mode appears to be the more favourable at low temperatures whereas the side-on mode is more favourable at higher temperatures. Further increasing reaction temperature over 400 8C leads to an increase in the hydrogenolysis reaction so more methane is produced, while the percentage of other hydrocarbon products decreased. # 2007 Elsevier B.V. Allrights reserved.
Keywords: Pyridine hydrodenitrogenation; Carbide catalysts; Co-promoted Mo carbides; Co(Ni)-promoted W carbides
1. Introduction Hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) are very important processes in oil reﬁning, reducing nitrogen and sulphur content of petroleum feed stocks [1–5]. These processes are often carried out over g-Al2O3 supported molybdenum ortungsten sulphide, using cobalt or nickel as promoters [3–5]. However, increasingly stringent environmental regulations require deep removal of sulfur and nitrogen
* Corresponding authors. E-mail addresses: firstname.lastname@example.org (H.A. Al-Megren), ´ ´ email@example.com (S.L. Gonzalez-Cortes). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.06.013
fromdiesel and gasoline, which cannot be reached over conventional Mo or W sulphide catalysts. In order to meet this requirement, the development of more robust catalytic materials that are selective and stable to C–N cleavage is an important objective. Among the efﬁcient HDN catalysts, transition metalcarbides and -nitrides have shown excellent HDN activity, with high selectivities for the formationof aromatic products [6–14]. Schlatter et al.  found that molybdenum nitride and molybdenum carbide catalysts were more active than a commercial Ni-Mo/Al2O3 sulﬁde catalyst, and they required less hydrogen for the HDN of quinoline. Ramanathan and Oyama showed that HDN activity for quinoline activity using early transition metal carbides followed the order, group
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