JOURNAL OF CATALYSIS 171, 467–475 (1997) ARTICLE NO. CA 971796 Effect of Phase Composition of the Oxidic Precursor on the HDS Activity of the Sulfided Molybdates of Fe(II), Co(II), and Ni(II) Joaqu´ ın L. Brito 1 and A. Liliana Barbosa 2 L aboratorio de Fisicoqu´ ımica de Superficies, Centro de Qu´ ımica, Instituto Venezolano de Investigaciones Cient´ ıficas, IVIC, A partado 21827, Caracas 1020-A , Venezuela Received March 5, 1997; revised June 16, 1997; accepted June 17, 1997 The catalytic HDS activities of unsupported sulfided molybdates of Fe(II), Co(II), and Ni(II) have been examined measuring the con- version of thiophene at 400 ◦ C under atmospheric pressure. The ox- idic precursors employed included the hydrates AMoO 4 · H 2 O and the α- and β -AMoO 4 polymorphs (A = Fe, Co, or Ni). The previ- ous finding that sulfided β -NiMoO 4 is a better HDS catalyst than α-NiMoO 4 is now generalized to the other two molybdate sys- tems, suggesting that the tetrahedral environment of Mo in the β -isomorphs provides a more active A–Mo–S phase than the oc- tahedral one in the α-molybdates. The reduced (nonpresulfided) molybdate samples showed lower HDS activities than those sul- fided in pure H 2 S. Prereduction followed by sulfiding seems to be an optimum procedure for the highest HDS activity of the nickel molybdates and also of β -FeMoO 4 . It was found that NiMoO 4 · H 2 O renders a more active sulfided catalyst than the β -phase synthesized by calcination at 760 ◦ C, and this seems to be related to the in situ generation of β -NiMoO 4 with highersurface area during the activa- tion of the hydrate at 400 ◦ C. The hydrated phase of cobalt behaved similarly, but that of iron, on the contrary, was a poorercatalytic precursorthan β -FeMoO 4 . It is suggested that decomposition of the hydrated compound in the case of Fe could generate a more crys- talline material upon sulfiding due to the possibility of changes in the oxidation state of the metals (i.e., Fe 2+ ⇄ Fe 3+ , Mo 6+ ⇄ Mo 5+ ) dur- ing thermal transformations in presence of evolved water. c  1997 Academic Press INTRODUCTION Research in the field of hydrodesulfurization (HDS) catalysishasbeen carried out duringthe past 50years.Cata- lyst formulations consisting of “cobalt molybdate” on alu- mina were first empiricallydeveloped and optimized by trial and error approaches.Later,nickel–molybdenum formula- tions were found to be active and also empirically opti- 1 To whom correspondence should be addressed at IVIC/BAMCO CCS-199-50, P.O. Box 025322, Miami, FL 33102-5322. Fax: x-58-2-504- 1350. E-mail: joabrito@ivic.ivic.ve. 2 Current address:Universidad de Cartagena,Campusde Zaragocilla. Cartagena, Colombia. mized. This type of work was pursued almost exclusively in the industrial laboratories (1). After about 1965, groups in academic institutions became increasingly involved in re- search on these catalytic systems, focusing their attention in the oxidic precursors of the catalysts, as it was shown early that the active (sulfided) state is very sensitive to ex- posure to ambient conditions and consequently difficult to handle without special in situ techniques. With the devel- opment of such techniques, it became feasible to study the active sulfides, with the result that now most emphasis in re- search on the catalytic systems is put on the sulfided state. However, catalysts in the oxidic state are important ma- terials themselves, as is evident considering that the cata- lysts are fully oxidized not only when originally prepared, but also after regeneration procedures that play an im- portant role in the technology of HDS processes (2). In addition, Fe, Co, and Ni molybdates are important compo- nents of industrial catalysts for partial oxidation of hydro- carbons (3). The molybdates of divalent Fe, Co, and Ni of general for- mula A MoO 4 exist in three distinct phases. Two of them occur under ambient pressure, while the other one ( A MoO 4 -II) is observed at higher pressures. In the most used nomenclature system, the normal pressure isomorphs are designated α- and β -A MoO 4 (4), where the α prefix ac- counts for the STP stable phase and β corresponds to the high-temperature modification. In the case of Fe, Co, and Ni, the three α-phases are isotypic, with Mo and the second metalboth in octahedralcoordination;the β -isomorphsare also isotypic and present Mo in tetrahedral coordination, the group VIII metalbeingagain octahedrallycoordinated. While pure β -NiMoO 4 is unstable below about 250 ◦ C (5), β -CoMoO 4 and β -FeMoO 4 can be stabilized at room tem- perature, depending on their thermal history (4, 6). Of rele- vance for catalyticapplicationsand modelstudies,thisfam- ily of compounds can be prepared by soft chemical routes (coprecipitation of A MoO 4 · n H 2 O phases followed by cal- cination at relatively low temperatures), as opposed to the high-temperature ceramic-type preparationsgenerallyem- ployed for structural studies. 467 0021-9517/97 $25.00 Copyright c  1997 by Academic Press All rights of reproduction in any form reserved.