Industrial MoO 3-promoter oxide-γ-Al 2O 3 hydrotreating catalysts: genesis and architecture description

Abstract

The catalytic performances (activity, selectivity, deactivation by phase transformation …) of hydrotreating catalysts composed of supported Mo associated with a promoter (Co or Ni) and working in a sulphided state are greatly dependent on the sequences and conditions of the preparation process. A large number of works dealing with the description of one or several steps in the catalyst elaboration have been already published, specially in the previous Molybdenum Conference in which review articles can be found. However, in these works, all the preparations sequences are not always considered and some experimental conditions are far from those used in industrial catalyst preparation in the oxide or commercial form. In this paper we report new results about the architecture of oxide precursors of series of Mo-based catalysts prepared according to industrial conditions. These results will be compared and discussed with other descriptions already published. Starting with the support and commercial products chosen because they are very soluble in water, easily decomposed and less expensive [ammonium heptamolybdate (AHM), and cobalt or nickel nitrate], six steps, viz. molybdate impregnation, intermediate drying and calcination, impregnation of the promoter solution, drying and final calcination, are necessary to obtain the final or commercial catalyst. Moreover after the intermediate or final calcination, the samples prepared can be modified by aging during storage in the laboratory atmosphere. The nature and quantity of the supported species, their structure and size, the chemical state of the elements, their dispersion on the support, the interaction between the molybdate entities, with the support or with the promoter, are among the main aspects, considered in this investigation for which we used techniques dealing with surface characterization X-ray photoelectron spectroscopy (XPS) and ion-scattering spectroscopy (ISS)] and a vibrational technique such as laser Raman spectroscopy (LRS). During the first impregnantion step, an electrostatic interaction between the ionized surface species of the support and the molybdate ions in solution occurs. The subsequent drying and calcination steps induce formation of strong chemical bonds between the supported molybdate species and the carrier. After this step, the species are well dispersed on the support as proved by XPS and ISS. The second impregnation transforms the molecular aggregates previously created by (partial) hydrolysis and therefore gives back the initial species. The added promoter is then interacting with the polymolybdate or the support. Calcination can provoke a partial incorporation of the promoter into the support but bilayers in which the promoter is between the molybdate phase and the support are not detected. Aging of the catalyst after the calcination step is a hydration process. Reverse dehydration can be achieved by a new calcination step.