J. Grimblot

Influence of sulphidation conditions on the properties of NiW/Al2O3 hydrotreating catalysts

Abstract The sulphidation of a nickel-tungsten catalyst has been studied by XPS and electron microscopy. These features are related to catalytic properties in test reactions characteristic of hydrotreatment (HDS, HDN, HN of aromatics) and to chemisorption measurements. It was observed that for a sulphidation temperature of 400°C, only 70% of the tungsten atoms are in a WS 2 - like form; this proportion increases to 95% at 650°C. High-resolution microscopy results indicate that a slight lateral growth of the WS 2 -like crystals occurs above 500°C. The activity in the HN of biphenyl does not vary with the sulphidation temperature in the same way as HDS reactions. These variations of catalytic activities support the existence of two different catalytic sites for hydrogenolysis and hydrogenation reactions. The crystallite length increase observed above 500°C leads to a decrease in the number of edge positions necessary for mixed site formation and therefore a decrease in the probe molecule uptake and HDS catalytic activity.

Industrial MoO3-promoter oxide-γ-Al2O3 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.

Nature and structure of tungsten surface species present on NiO-WO3/Al2O3 hydrotreating catalysts

Abstract XPS, ISS, LRS and FTIR have been used to characterize WO3/Al2O3 and NiO-WO3/Al2O3 catalysts. The W-like monolayer coverage limit was determined to be about 40% WO3. This saturation limit is in accordance with previous results on similar Mo-based catalysts if it is considered that adsorption concerns mainly the paratungstate ion. From XPS, ISS and LRS it is concluded that there is good nickel dispersion and it is proposed that nickel interacts with the paratungstate phase and prevents the formation of bulk WO3. The vibrational spectra (LRS and FTIR) are modified by introduction of different bases ((CH3)2O, NH3, C5H5N) which react with unsaturated sites on the W atom. A model of the supported calcined phase for low and high W loadings is proposed.

The sulphided state of nickel molybdenum catalysts supported on zirconia and aluminates

Abstract Characterization of the sulphided Ni-MoS 2 state supported on various carriers (aluminates, zirconia) has been undertaken in order to study the influence of the carrier on the genesis of the promoted active phase, refered to as the “NiMoS” phase. The influence of the sulphidation temperature on the stability of nickel associated in that phase has also been explored. The basis of comparison is a classical Ni-MoS 2 -alumina catalyst. In all these systems nickel interacts with molybdenum sulphide (decoration like position) but the “NiMoS” phase appears less stable on zirconia. On these aluminate and zirconia carriers, the charge transfer from nickel to molybdenum sulphide, as evidenced by the shifts of the active IR band of adsorbed carbon monoxide, has been found to be weaker than on the alumina supported catalyst. Such a carrier effect has been correlated with thiophene HDS activity results and discussed in terms of morphological effects as revealed by HREM.

In situ Laser Raman Spectroscopy of the sulphiding of WO3(MoO3/γAl2O3 catalysts

Abstract The surface chemistry of hydrated and dehydrated WO3(MoO3)/γAl2O3 hydrotreating catalysts during sulphiding has been investigated by in situ Laser Raman Spectroscopy (LRS). Two main intermediates have been identified after mild sulphiding : a WS3(MoS3) like phase and oxysulphide. After a complete sulphiding WS2(MoS2) cristallites are obtained. Evidence of the presence of SH group has also been found.

Hydrodemetallation of a vanadylporphyrin over sulfided NiMoγAl2O3, MoγAl2O3, and γAl2O3 catalysts - effect of the vanadium deposit on the toluene hydrogenation.

Abstract The toluene hydrogenation performances of Mo and NiMoAl2O3 hydrotreating catalysts are modified by vanadium deposited through HDM of the Vanadylporphyrin. Bulk deposited vanadium sulfide of mean VS1.1 stoichiometry does not greatly affect the intrinsic performances of the initial catalysts but it appears that a part of the deposited vanadium replaces Ni in the NiMoS phase and acts as a promoter of the supported MoS2

Preparation, characterization and catalytic properties of unsupported vanadium sulphides

Abstract This work concerns the preparation and the ability of unsupported vanadium sulphide to catalyze different reactions representative of hydrotreatment. Pure V2S3 was prepared by decomposition of ammonium thiovanadate at relatively low temperature. The reactivity of the decomposed solid towards oxygen confirms the high oxophilic character of this group V sulphide. The oxidation mechanism involves a superficial oxidation followed by an insertion of oxygen into the lacunar structure of the solid. The comparison of the catalytic properties of this chalcogenide with those of molybdenum or tungsten sulphides shows the superiority of vanadium sulphide for performing hydrogenation reaction of various cyclic molecules. In contrast, the hydrodesulphurization properties are of the same order.

Study of the reduction of MoO3γAl2O3 and WO3γAl2O3 catalysts by laser raman spectroscopy

Abstract Various pretreatements on those catalysts (oxide form) followed by H 2 reduction show that the primary effect of H 2 is a dehydration reaction. At high Mo loading, Two consecutive steps of reduction are evidenced: (i) formation of Mo (V) species as Mo(V)OH and (ii) formation of Mo(IV) species characterized by a (X =V, IV) bridge whose stretching mode is about 760 cm −1 . Low Mo loading samples are more difficult to reduce. By comparison with the Mo catalysts, the Wo 3 γAl 2 O 3 samples are less easily reducible but a similar initial reduced state has been found.

Oxidic precursors of molybdena supported on nickel and magnesium aluminate hydrotreating catalysts

The oxidic precursors of molybdena-based hydrotreating catalysts have been prepared using nickel and magnesium aluminates and characterized by laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). Three series of supports (∼80, 120 and 140 m2.g−1) have been investigated. Their aluminate spinel structure was confirmed both by X-ray diffraction (XRD) and LRS. Nickel oxide and magnesia have also been detected on the low surface area systems. In both series, a surface deficit of the foreign cation has been observed relative to the Al3+ of the aluminate phase. This deficit was more pronounced for Ni2+. Deposition of molybdate species does not modify the surface composition of these supports and a monolayer-like coverage up to 4.5 to 5 Mo at.nm−2 was obtained. Nickel added by impregnation was shown to interact with the supported oxomolybdate species, whereas the nickel of the support does not. These investigated supports closely resemble γ  Al2O3 in the preparation of hydrotreating catalysts.

Search for surface brönsted acidity over sulphided Ni-W/Al2O3 hydrotreating catalysts

Abstract A series of catalysts containing tungsten and nickel on a γ-alumina carrier were examined with respect to surface acidity. The studies focused on the behaviour of sulphided samples to which various techniques were applied. In agreement with prior studies on Co-Mo catalysts, infrared spectroscopy of adsorbed pyridine showed that both Bronsted and Lewis acidic sites are present on the calcined Ni-W samples, but Bronsted acidity can no longer be detected after sulphiding. Other probes (ammonia or piperidine) substantiated these conclusions. Moreover, gravimetric adsorption-desorption studies on ammonia confirmed that Bronsted acidity is not a major characteristic of the sulphide catalysts. The lack of strong Bronsted acidity was also demonstrated by measuring the rates of cyclohexanol dehydration or cyclohexylamine coupling carried out on the series of sulphides Ni-W catalysts. Therefore, it was proved that acidic mechanisms do not play any major role in catalysis by sulphides. In contrast, the sulphide catalysts show a better aptitude than typical cracking catalysts for isomerizing and cracking 3-methylpentane under hydrodesulphurization conditions but the product selectivity is different. The occurrence of a metallic-type catalysis is suggested, which could also operate in hydrodenitrogenation.