MoS2 catalysts were firstly prepared over three different alumina supports γc-Al2O3, γT-Al2O3, and δ-Al2O3 with the same Mo loading per nm2. Then, using various deposition methods and different Co-precursors as Co(NO3)2, Co(acac)2 and Co(CO)3NO, Co-promoted MoS2 catalysts were obtained. The different catalytic systems were tested in hydrodesulfurization (HDS) reactions of thiophene and 4.6-dimethyldibenzothiophene (4,6-DMDBT) under atmospheric and high pressure (4 MPa), respectively. For the non-promoted catalysts, we observed for δ-alumina supported system a higher intrinsic catalytic activity than for γc-Al2O3 and γT-Al2O3. For the Co-promoted systems, the use of cobalt organometallic complex allowed obtaining catalysts which exhibit higher intrinsic activity than the catalysts prepared by impregnation of Co inorganic salts on supported Mo sulfide. Furthermore, the intrinsic catalytic activities of promoted systems obtained with δ-alumina support were always higher than for the classical γ-alumina. This support effect can be partially explained and supported by physicochemical characterization results of XPS and IR of adsorbed CO at low temperature (77 K).

CoMo additive-impregnated dried catalysts are studied, exploring the "CoMoS" active phase features by combining X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) and catalytic tests. Starting from different polyoxomolybdate precursors, additive-impregnated dried, additive-free dried and calcined catalyst performances are compared. TEM reveals that the mean particle sizes are about 3.1 nm and do not depend on the catalytic precursors except for the additive-free dried catalysts exhibiting higher lengths: 3.7 nm. XPS quantification of the Mo species shows that 75 mol% of the Mo species are present in the MoS2 phase whatever the preparation route. This value is slightly enhanced (ca. 85%) with additive impregnation. The molybdenum to aluminium surface coverage ratio (Mo/Al) ranking is found to be as follows: additive-free dried < additive-impregnated dried < calcined. However, this ranking is not significantly modified by the impregnating solution used, and the behaviour is similar for the cobalt to aluminium ratio (Co/Al). A geometrical model combining XPS quantification of the crystallite's Co/Mo ratio and DFT calculations is used to establish a correlation with the catalytic results obtained in toluene hydrogenation. It is shown that the catalytic performances of additive-free dried, additive-impregnated dried and calcined catalysts directly correlate the number of mixed Co-Mo sites present at the MoS2 edges. DFT calculations highlight that the adsorption step of toluene is thermodynamically favored on the mixed Co-Mo site located at the M-edge. As a consequence, this study suggests that the various routes of preparation leading to different catalytic performances would not lead to new types of active sites or morphology but rather to a different number of mixed sites present at the edges.

The aim of this work was to determine the physico-chemical parameters governing impregnation of H4Co2Mo10O386- Anderson-like heteropolyanion (HPA) solutions on gamma-alumina. This species was first synthesized in solution and found to be stable in acidic medium and large molybdenum concentration range, as determined by Raman spectroscopy and Principal Component Analysis (PCA). Impregnation of various solutions on gamma-alumina showed that H4Co2Mo10O386- dimers are kept only at high molybdenum concentration.

The molecular knowledge of nitrogen compounds in diesel feedstocks has become a key issue in the development of hydrotreatment processes, especially for ultra-low sulfur diesel production. Indeed, nitrogen species have a strong impact on the hydrodesulfurization (HDS) pathway, since basic nitrogen is known to poison acidic sites of HDS catalysts.Since conventional methods only allow a poor degree of information, the increased separation power of comprehensive two-dimensional gas chromatography (GC x GC-NCD) was used in this study to obtain a detailed overview of nitrogen compounds by type (basic/neutral), by family and by carbon breakdown in diesel and liquefied coal samples. Partially hydrogenated compounds such as tetrahydrocarbazole and tetrahydroquinoline derivatives could even be detected in liquefied coal samples as well as diesel from ebullated bed conversion units. Comparison of GC-NCD with GC x GC-NCD for quantitative determination of nitrogen compounds by family was achieved in a first step. These results demonstrate the superiority of GC x GC to allow for a comprehensive characterization of nitrogen compounds in diesel and related samples in one injection. Furthermore, nitrogen speciation by GC x GC-NCD technique allows identifying most nitrogen species in conventional diesel or liquefied coal samples, with no use of mass spectrometry.GC x GC-NCD was also applied to a wide range of diesel feedstocks obtained from distillation, cokefaction, FCC, ebullated bed hydroconversion units to correlate nitrogen species to the origin of the feedstocks or distillation end points, giving interesting indications on reaction mechanisms involved in the processes. (c) 2008 Published by Elsevier Ltd.

In order to gain a better understanding of the morphology and promoter edge content of the active phase of industrial HDT NiMoP catalysts in working conditions, a multi-technique study has been undertaken on a series of NiMoP catalysts with various Ni/Mo ratios. The combination of X-ray Photoelectron Spectroscopy (XPS), Transition Electron Microscopy (TEM), Density Functional Theory (DFT) modeling and catalytic testing (toluene hydrogenation) provided data to build a morphological model of NiMoS nanocrystallites. A parallel has been established with their CoMoS counterparts obtained in our previous work in order to emphasize differences arising from the promoter atom. This study confirms the importance of the presence of mixed Ni-Mo sites on the edges of the NiMoS nanocrystallites, and especially on the M-edge for reactions involving hydrogenation. These results provide new guidelines for future and ever more active catalysts.

CoMoS phases supported on gamma-Al2O3, TiO2 and SiO2 were prepared using various synthesis methods and Co(NO3)(2)center dot 6H(2)O or Co(acac)(2)center dot nH(2)O as precursors of the promoting agent. The catalysts obtained were tested in the thiophene hydrodesulfurization (HDS) reaction at atmospheric pressure. These experiments showed that the catalysts prepared by the impregnation of supported MoS2 with cobalt precursors exhibit better HDS properties than the catalysts prepared by successive impregnation of Mo and Co salts followed by a sulfidation step. Furthermore, with gamma-Al2O3 and SiO2, the use of Co(acac)(2)center dot nH(2)O, without any further calcination after the impregnation, allowed even better catalytic activities. With TiO2, the promoting effect is identical whatever the Co precursor used. To cite this article: C Roukoss et al., C R. Chimie 12 (2009). (C) 2009 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

The introduction of a glycol-type additive in hydrotreating catalysts is an efficient procedure to improve catalytic activity. Nevertheless, controversial explanations about the activity enhancement mechanism exist in the literature. This may be due to different catalyst preparation procedures, different location of the additive impregnation step, or simply because several phenomena are implied in this improvement. The aim of this work is thus to rationalize the roles of these additives with respect to (i) species present in the impregnation solution as well as on the catalyst surface and (ii) the preparation step where the additive impregnation is performed, i.e. after drying or after calcination. Different impregnation solutions have been used containing (a) ammonium heptamolybdate or cobaltomolybdate heteropolyanions for CoMo catalysts and (b) phosphomolybdate heteropolyanion with different P/Mo molar ratio for CoMoP catalysts. Surface species have been thoroughly characterized for dried and calcined catalysts prior to and after the additive impregnation using triethyleneglycol. For all dried and calcined CoMo and CoMoP catalysts, a redissolution phenomenon has been evidenced after the additive impregnation, leading to the formation of the Anderson heteropolyanion AlMo6O24H63−. This redissolution phenomenon is however limited by the low solubility of AlMo6O24H63−. Moreover, in the case of CoMoP dried catalysts (P/Mo molar ratio ≥ 0.4), characterization of additive-containing catalysts evidenced PCoMo11O407− formation. Redissolution and redispersion due to the additives are thus enhanced because phosphomolybdic species have a much higher solubility than AlMo6O24H63−. Similar observations, although less pronounced, may be drawn for calcined catalysts. Indeed, a stronger precursor–support interaction has been created during calcination. Catalysts performances were evaluated in toluene hydrogenation and activities obtained match perfectly.