10 th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved. The superior activity of the CoMo hydrotreating catalysts, prepared using citric acid: what’s the reason? A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova, M.A. Fedotov, D.I. Kochubey, Yu.A. Chesalov, V.I. Zaikovskii, I.P. Prosvirin, A.S. Noskov Boreskov Institute of Catalysis SB RAS, Novosibirsk,630090, Russia Abstract It was demonstrated, that the main positive role of citric acid during the hydrotreating catalysts preparation is consist in the formation of bimetallic complex Co 2 [Mo 4 (C 6 H 5 O 7 ) 2 O 11 ]nH 2 O, that is a good precursor for selective formation of catalyst active phase, so called Co-Mo-S phase type II. The preparation method for this bimetallic complex using different precursor is described. The catalysts prepared by the complex deposition onto alumina support were studied during the different stages of the catalyst genesis. Applicability of these catalysts for ultra low sulfur diesel production was shown. 1. Introduction The impregnated solutions containing cobalt and molybdenum compounds and citric acid (CA) is widely used for high active hydrotreating catalysts preparation, including industrial catalysts. An obvious positive effect of citric acid on the activity of catalysts is explained in variety of ways by different authors. Thus, Fujikawa et al. [1] prescribes the superior catalyst activity to the formation of cobalt citrate complexes, which are thermally stable at temperatures up to 200ºC and contributes the selective formation of the catalyst active component, so called Co-Mo-S phase of type II, described in [2]. Bergwerff et al. [3] reported the formation in a solution of molybdenum citrate complexes. Deposition of these Mo complexes onto Al 2 O 3 support provides uniform molybdenum distribution along the carrier granule and ensures a high dispersion of MoS 2 particles, that increases the relative activity of catalyst. In the cited and other papers of mentioned authors, the complex formation between CA and the individual components of the impregnating solution - cobalt or molybdenum was detected, while the formation of bimetallic Co-Mo-CA complexes was not revealed. Earlier Van Veen [4] reported that the positive role of chelating organic ligands consists in the formation of CoMoL compounds in the impregnating solution, whose structure remains unchanged after the drying of the catalyst; moreover the ligands screen metals from the interaction with the carrier. During the sulfidation stage of the CoMoL containing catalysts the Co-Mo-S phase type II is mainly formed, whereas in the catalysts prepared without using of chelating agents the low active Co-Mo-S phase type I is formed. Concerning our experimental results, the reasonable Van Veen’s conclusions should be supplemented by the followed statements: 1. Chelating ligands promotes the coordination of Co towards molybdenum containing anion forming a bimetallic compound with a close proximity of Keywords: hydrotreating catalysts, citric acid, bimetallic CoMo complex