Homogeneous Sulfide Precipitation Research Articles

UNSUPPORTED NiMo CATALYSTS. INFLUENCE OF THE SULFINDING TEMPERATURE AND EVOLUTION OF THE UNSUPPORTED NiMoS PHASE DURING REACTION

AbstractA series of unsupported NiMo catalysts, containing 30% Ni atoms, were prepared by the homogeneous sulfide precipitation method (HSP) and sulfided at different temperatures in the range 673‐1073 K. XPS, XRD, microelectrophoresis and BET surface area measurements were used to characterize the samples. The catalytic activity of the various catalysts for the hydrodesulfurization of thiophene and the hydrogenation of cyclohexene in a mixture containing both compounds was measured in mild conditions (573 K, 3 MPa). Both the evolution of the “NiMoS phase” during catalytic reaction and the influence of the sulfidation temperature on the properties of the unsupported NiMo catalysts were investigated.The sample sulfided at 673 K gave all the characteristic signals of the “NiMoS phase”. As the sulfidation temperature increases, larger amounts of nickel segregate from “NiMoS phase” to form nickel sulfide. The “NiMoS phase” was found unstable under reaction conditions. The maximum catalytic activity for both HDS and HYD reactions was obtained when both “NiMoS phase” and bulk Ni3S2 were present in the catalyst. It thus appears as a precursor of a very active mixture of phases rather than of the stable catalyst itself. These results are discussed in relation to some models attempting to explain the synergy in the NiMo catalysts. They are compatible with the remote control mechanism.

Characterization and Activity of Unsupported Ni-Mo Sulfide Catalysts in HDN/HDS Reactions

Sulfided Ni-Mo catalysts prepared by homogeneous sulfide precipitation (HSP) and impregnated thiosalt decomposition (ITD) methods were used in thiophene HDS, pyridine HDN, and simultaneous HDS/HDN reactions. All catalysts were characterized by BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. The effect of various reaction parameters such as catalyst composition, method of catalyst synthesis, reaction temperature, and reaction pressure on catalytic behavior has been investigated. The inhibition/enhancement effects in simultaneous HDS/HDN reactions were also examined. The studies conducted over the unsupported catalysts present trends very similar to those obtained over the supported catalysts and suggest the intrinsic nature of active sites to be essentially independent of the support

Characterisation of unsupported FeMoS catalysts: Stability during reaction and effect of the sulfiding temperature

Unsupported iron, molybdenum and iron-molybdenum sulfides were prepared by the Homogeneous Sulfide Precipitation (HSP) method. They were investigated by surface area, XRD, MAS (Mössbauer Absorption Spectroscopy), electrophoretic migration, and XPS measurements; various temperature-programmed methods have also been used. Catalytic activity in the simultaneous hydrodesulfurisation of thiophene and hydrogenation of cyclohexene at 3 MPa and 573 K has been measured. It has been verified that the HSP method gives samples containing a high proportion of the Fe species giving the signal attributed to the so called “FeMoS” species, together with a noncrystalline phase possibly corresponding to FeS 2 (pyrite) and some pyrrhotite (Fe 1− x S). When the final reduction- sulfidation (15% H 2S in H 2) temperature of the HSP precipitate is increased (from 673 to 1073 K), FeS2 and the iron of FeMoS transform to troilite (FeS). Samples sulfided above 873 K no longer contain FeMoS. The FeMoS “phase” decomposes extensively during the catalytic reaction. Catalytic activity does not correlate with the quantity of FeMoS present in the samples, but seems to vary in parallel with the amount of FeS 2 remaining in the working catalyst.

Hydrodesulfurization catalysts prepared by two methods analyzed by transmission electron microscopy

Samples of molybdenum sulfide, cobalt sulfide and mixtures in atomic ratios r = Co/(Co + Mo) of 0.0, 0.3, 0.5, 0.7, and 1.0 were prepared by two different methods, homogeneous sulfide precipitation (HSP) and impregnated thiosalt decomposition (ITD). Samples were observed by high-resolution electron microscopy using imaging and diffraction modes. Both preparation methods present the “rag structure” typical of MoS 22H with some structural differences between them. The average number of layers (n) in molybdenum disulfide crystals is about the same in both preparation methods, while the average length (L) of the molybdenum disulfide crystallites obtained by HSP is larger than that of those obtained by ITD. The particle size is smaller for ITD samples. The presence of cobalt does not greatly modify the number of layers of the MoS 22H stacks in mixed samples. An increase in the intracrystallite disorder is observed.

Electron Microscopy in hydrodesulfurization catalysts

Applications of transition metal sulfides for hydroprocessing catalysts have included a variety of reactions. It is generally believed that an interaction between the active phase (Mo or W) and the promoter (Co or Ni) takes place. Several models have been suggested to explain the enhanced catalytic activity. The catalytic properties of the unsupported sulfides are dependent on the catalyst preparation methods . In this work we study by electron microscopy two sets of unsupported samples ranging from molybdenum sulfide to cobalt sulfide. The specimens were prepared by the following methods, a slight variation of the classical homogeneous sulfide precipitation (HSP) method, and a new method called impregnated thiosalt decomposition (ITD).

Homogeneous sulfide precipitation catalysts characterized by X-ray diffraction

Samples from molybdenum sulfide to cobalt sulfide were prepared with mixtures in atomic ratios r= Co (Co + Mo) of 0.0,0.2, 0.3, 0.5, 0.7 and 1.0. We used the common method of homogeneous sulfide precipitation (HSP). Measurements obtained by X-ray diffraction (XRD) show the presence of the MoS 2-2H phase and segregation of cobalt in two phases, Co 9S 8 and CoS 1.035.

Catalytic activity of hydrodesulfurization catalysts prepared by two methods

Two sets of samples, ranging from molybdenum sulfide to cobalt sulfide, were prepared by two different methods: homogeneous sulfide precipitation (HSP), and a new preparation method of CoMo and NiMo bulk sulfides, called impregnated thiosalt decomposition (ITD). Hydrodesulfurization of thiophene was measured at temperatures between 533 K and 593 K in a conventional flow system. The products were analyzed by gas chromatography. Both sets of samples (HSP and ITD) present the typical volcano curve of specific activity with larger values for catalysts prepared by HSP than those by ITD. Samples prepared by ITD present comparable intrinsic activities than those prepared by HSP. Activation energies for both methods of preparation are larger for promoted catalysts with a value of 23 for HSP and 12.6 kcal/mol for ITD.

Characterization by XRD High Resolution Electron Microscopy of Hydrodesulfuration Catalysts

AbstractRecently a lot of work has attempted to correlate the activity of Co and Mo sulfide catalysts with observations of high resolution electron microscopy. Hydrodesulfuration (HDS) bulk sulfide catalysts are specially suited for this type of study. We prepared unsupported molybdenum sulfide by three methods: the homogeneous sulfide precipitation (HSP) (1), co-maceration (CM) (2), and impregnated thiosalt decomposition (ITD) (3). Pure cobalt sulfide was prepared only by HSP, and two Co-Mo sulfides in the ratios of 0.3 and 0.5 at. % were prepared by HSP and ITD. The samples were treated with a mixture of 20% H2S/H2 for 4 hrs. at 400 C. In all Co/Co+Mo mixtures the presence of Co9S8 and MoS2 was identified in both, ESP and ITD treatments. Characterization was done by high resolutibn electron microscopy. The ITD method can be considered a better method, as opposed to ESP for the preparation of mixed catalysts. This method produces poorly crystalline specimens that are usually associated to higher chemical activity.

Mössbauer emission spectroscopy of an unsupported cobalt-molybdenum hydrodesulfurization catalyst

A57Co doped unsupported CoMo sulfide catalyst with atomic composition ratio r=Co/(Co+Mo)=0.3 was prepared by the homogeneous sulfide precipitation method and exposed to a series of reduction-sulfidation treatments. The treated samples were analyzed by Mossbauer emission spectroscopy using very long accumulation times. Computer decomposition of the spectra revealed the presence of five different cobalt species which were identified as Co9S8, CoS1+x, and three species related to the Co−Mo−S structure. Reduction of the sample under atmospheric pressure of H2 at (and above) 573 K causes an increase of the amount of Co9S8 at the expense of all other species. These results afford a new insight into the stability of the Co−Mo−S structure and of the sulfur rich CoS1+x phase under hydrotreating conditions.

Hydrodesulfurization catalysts: Electrophoretic study of Mo(or W)CO, Mo(or W)Ni, and Mo(or W)Ca sulfided phases

When CoMoO 4, NiMoO 4, CaMoO 4, CaWO 4, NiWO 4, and CaWO 4 are treated at 400 °C for 3.5 h in a 10% H 2S H 2 flow, which are experimental conditions commonly used for the sulfidation of hydrodesulfurization catalysts, the products obtained are the sulfides MoS 2, WS 2, Co 9S 8, NiS, and CaS, respectively. The increase of the sulfidation temperature of unsupported CoMo samples prepared by the homogeneous sulfide precipitation procedure, leads to a displacement of the cobalt located at the surface of the CoMoS phase.

Characterization of Fe and Ni-promoted unsupported MoS2 catalysts and their precursors via thermal effects in temperature-programmed reactions

Precursors of unsupported NiMo and FeMo sulfide hydrodesulfurization catalysts with concentration ratiosr=Ni(Fe)/(Ni(Fe) + Mo) ranging from 0.1 to 0.3 were prepared by three methods: homogeneous sulfide precipitation (HSP), inverse HSP and coprecipitation. Differential thermal analysis was used to study the decomposition under argon, and the reduction/sulfidation under 15% H2S—H2 of the precursors and the subsequent oxidation under air of the samples obtained after these reactions. The reactivity of the solids varies as a function of the preparation method, the nature of the promoter and the concentration ratio. The degree of sulfidation of the precursor and the presence of either NH4NO3 or NH2Cl formed from group VIII metal salts and (NH4)2S may affect the thermal behaviour of samples during DTA.

The reactivity and stability of mixed-sulfide structures in unsupported MoS 2-based hydrodesulfurization catalysts promoted by group VIII metals

Unsupported CoMo, NiMo and FeMo catalysts have been prepared by the homogeneous sulfide precipitation method and the coprecipitation method. The presence of a group VIII metal (Me) causes a decrease in the reactivity with O 2 and an increase in the reducibility in H 2. These effects are correlated with the increase in the catalytic activity. The stability of cobalt and iron sulfide species in CoMo and FeMo catalysts was studied by Mössbauer spectroscopy. A significant amount of MeMoS structure remains stable after strong reducing treatment. Catalysts contain also sulfur-rich group VIII metal sulfide species which appear stabilized by MoS 2. The role of these species in promoting HDS activity is evaluated.

The role of group VIII metal promoter in MoS 2 and WS 2 hydrotreating catalysts : II. Catalytic and physicochemical properties of nickel-promoted catalysts

The promotional effects of nickel on the catalytic activities of MoS 2 and WS 2 have been investigated. A series of unsupported NiW sulfides with a composition ratio, r = Ni Ni + W (atom), varying from 0.0 to 1.0, and two unsupported and fully sulfided nickel-molybdenum catalysts (one prepared by comaceration method and the other by homogeneous sulfide precipitation method) with r = 0.25 have been tested for cyclohexane isomerization, cyclohexene hydrogenation, and thiophene hydrogenolysis at 305 °C and 30 bars of hydrogen pressure. Surface area, X-ray diffraction, and ESR measurements on NiW samples show that in the low composition range, r < 0.05, the surface area decrease is associated with the formation of large crystallites. The ESR study shows the presence of W(V), W(III), and sulfur-containing paramagnetic species in the sample with r = 0.00. At the synergetic composition range, r = 0.15 to 0.40, the surface area versus composition plot shows a wide maximum, while the ESR results indicate the presence of certain ferromagnetic nickel-containing phase. It is shown that a strong electronic interaction takes place between two sulfide phases.

Electrophoretic characterization of unsupported sulfide hydrodesulfurization catalysts

The zeta potential of pure sulfides (MoS 3, MoS 2, Co 9S 8, NiS and Fe 1-xS), unsupported CoMo, NiMo, and FeMo mixed sulfide catalysts, and elemental sulfur is studied by electrophoretic migration measurements. Zeta potential and isoelectric point (IEP) are affected by the presence, on the surface, of oxidic species and/or elemental sulfur. A standardized evacuation procedure (673K, 1 h) was designed in order to guarantee reproducible results. The IEP of MoS 2 samples prepared by decomposition of either MoS 3 or ammonium thiomolybdate was 1.4 and 2.0, respectively. The IEP of MoS 3, Co 9S 8, NiS, Fe l-xS and S (rhombic) are 1.9, 1.9, 2.8, 3.5 and 3.1, respectively. The IEP of CoMo (r = Co/(Co+Mo) = 0.3) sulfide catalysts prepared by the homogeneous sulfide precipitation (HSP), “inverse” HSP and coprecipitation methods are identical (IEP = 1.5). The nature and the concentration of promoter (up to Me/(Me+Mo) = 0.3) do not influence the IEP of HSP catalysts. Thus, an IEP value of 1.5 appears a characteristic of compound sulfides Me-Mo-S. In the CoMo sulfide catalyst prepared by the comaceration method, two families of particles are observed, with IEP = 2.9 and 1.9. They were identified as MoS 2 covered by S and Co 9S 8. Electrophoretic migration measurements thus allow an easy characterization of surface species in unsupported hydrodesulfurization catalysts.

Characterization of unsupported CoMo sulfide catalysts and their precursors by temperature-programmed reactions

Precursors of unsupported CoMo sulfide catalysts with concentration ratios, r = Co/(Co + Mo), varying from 0 to 1 were prepared by four different methods: comaceration (CM): homogeneous sulfide precipitation (HSP); inverse HSP (IHSP); and coprecipitation (CP). Differential thermal analysis (DTA) was used to investigate successively the thermal decomposition (under argon) or the reduction/sulfidation (under 15% H 2SH 2 mixture) of these catalyst precursors and the air oxidation of the decomposed or sulfided samples. Various proportions of (NH 4) 2MoS 4 and (NH 4) 2MoO 2S 2 were found in the precursors, depending on the preparation method. The presence of cobalt markedly affected the reaction under H 2S/H 2 mixture. Upon oxidation, the CM catalysts behave like pure MoS 2, indicating the biphasic nature of these samples. In contrast, the HSP (IHSP, CP) catalysts appear more difficult to oxidize, owing to the presence of finely dispersed cobalt species. The amount of a-CoMoO 4 detected in the oxidized catalysts by X-ray diffraction may be used as a convenient estimation of the dispersion of cobalt in the precursor sulfides.

A Comparison of Some Catalytic Properties of Unsupported MOS2 and WS2 Catalysts Promoted by Group VIII Metals

AbstractUnsupported sulfided CoMo, NiMo, CoW and NiW catalysts prepared by comaceration (CM) method and homogeneous sulfide precipitation (HSP) method have been studied in hydrodesulfurization of dibenzothiophene and hydrogenation of biphenyl.Comparison of intrinsic rates shows that, for both reactions, the HSP catalysts are more efficient than the corresponding CM samples. Moreover, the promoting effect of Ni in hydrogenation is substantially higher than that of Co.From decomposition of the precursors, followed by differential thermal analysis, a parallelism appeared between all mixed catalysts prepared by the same procedure. For CM precursors, the decomposition features show the persistence of the character of Mo and W compounds and confirm the limited interaction existing between Co (or Ni) and Mo (or W) in the precursor state. On the contrary, for HSP precursors the exothermic peak is always recorded at a much lower temperature and a comparison with previous results obtained on CoMo (CM) and CoMo (HSP) catalysts suggests that a mixed phase probably exists in all the HSP samples.

Unsupported cobalt molybdenum sulfide catalysts Part II: Characterization and evolution of physicochemical properties during catalytic reaction

Cobalt molybdenum sulfide catalysts were prepared by two different methods, comaceration (CM) and homogeneous sulfide precipitation (HSP). Both types o catalysts were characterized by several techniques, DTA, XPS, AEM, conductivity measurements and temperature programmed reduction. Physicochemical properties were determined for different states of the catalyst:precursor, fully sulfided and catalytically tested. It is confirmed t

Influence of Reduction / Sulfidation Treatments on the Physico‐Chemical and Catalytic Properties of Unsupported Cobalt‐Molybdenum/Hydrodesulfurization Catalysts

AbstractUnsupported CoMo sulfide catalysts prepared by the homogeneous sulfide precipitation method were submitted to 3 MPa 4% H2S‐H2 at 573K, and 0.1 MPa H2 at 573 and 723K. The samples were characterized by MES, XPS, XRD and temperature‐programmed sulfur extraction. Their activity was measured in the HDS of thiophene and hydrogenation of cyclohexene. MES reveals the presence of Co9S8, CoS1+x, Co‐Mo‐S and two unidentified species. Treatment under H2 at 573K caused an increase of activity which was correlated with an increase of the Co9S8 concentration at the expense of all other species. Treatment under H2 at 723K caused a further increase of activity which was correlated with the sintering of the Co9S8 phase. Part of the sulfur extracted by treatment under H2 up to 733K cannot be recovered by subsequent treatment under H2S/H2. These results do not provide straightforward evidence for deciding between the current models used for explaining the synergetic effect, namely the existence of a Co‐Mo‐S phase or the remote control hypothesis. The sulfur deficiency of the catalyst appears to play a major role in the activity.

On the Role of Promoter Atoms in Unsupported Hydrodesulfurization Catalysts: Influence of Preparation Methods

AbstractUnsupported Co‐Mo catalysts were prepared by two different methods:by the co‐maceration (CM) method and by a new homogeneous sulfide precipitation (HSP) method. Structural examinations by means of XRD and Mössbauer Emission Spectroscopy (HES) show that the CM catalysts contain several phases with Co9S8 being the dominant cobalt containing phase. On the other hand, the HSP catalysts are shown to contain only one CO phase within an extended range of Co/Mo atomic ratios and this phase is the Co‐Mo‐S phase. Like in the case of supported catalysts, the Co‐Mo‐S phase is shown to be the phase responsible for the activity, whereas Co9S8 is shown to have little or no influence on the activity. The differences in activity observed between the HSP and CM catalysts as well as other unsupported catalysts studied in the literature all seem to be related to differences in the preparation procedures resulting in different promotional efficiencies for the formation of Co‐Mo‐S.