de Vhj Vincent Beer

Carbon-supported sulfide catalysts

The activities of sulfided MoC, WC, CoC, NiC, Co-MoC, and Ni-WC catalysts for thiophene hydrodesulfurization and butene hydrogenation were studied using a flow microreactor operating at atmospheric pressure. The following parameters were varied: type of carbon support, carbon pretreatment, catalyst preparation method, and content of active material. The results are compared with those obtained for series of sulfided Mo/γ-Al2O3, W/γ-Al2O3, MoSiO2, and WSiO2. Some samples, viz., MoC, CoC, and Co-MoC, were also studied by X-ray photoelectron spectroscopy (XPS). The carbon-supported catalysts demonstrated outstanding performance for thiophene hydrodesulfurization. XPS analysis showed the presence of low-valence-state sulfur, e.g., S− or (S-S)2−, in MoC and CoC catalysts with low molybdenum or cobalt content. These sulfurspecies are supposedly connected with the catalytic activity for hydrodesulfurization. CoC and NiC were found to have a hydrodesulfurization activity which was higher (Co) or the same (Ni) as that measured for MoC or WC. Therefore Co (Ni) ions in Co (Ni)-Mo (W)C catalysts are considered as promoters for the MoS2 (WS2) phase (low CoMo or NiW ratios) or as additional active species.

Effect of the support on the structure of Mo-based hydrodesulfurization catalysts: Activated carbon versus alumina☆

The structure of oxidic and sulhded MO catalysts supported on activated carbon was studied by means of X-ray photoelectron spectroscopy (XPS), temperature programmed sulfiding (TPS), and suIfur analysis measurements. In the oxidic state the MO phase was highly dispersed as isolated or polymerized monolayer species at MO loadings helow 3 wt% and as very tiny three-dimensional particles at higher loadings. Upon sulfiding particle growth took place, although the size of the sulfide particles remained below 4.6 nm even in the sample with the highest MO loading (14.1 wt%). TPS patterns showed that sulfiding proceeded via a mechanism of 0-S substitution reactions and was completed at temperatures below 560 K. In the suhided catalysts only MO(W) was detected by XPS and S/MO stoichiometries determined by XPS, TPS, and chemical sulfur analysis varied between 1.5 and 2.0, demonstrating that MO& was the major phase present after sulfidation. The higher catalytic activity for MO/C compared to Mo/A120, is explained by differences in the structure of the sulfide phases present and in the interaction between these phases and the respective supports.

The role of cobalt and nickel in hydrodesulfurization: Promoters or catalysts?

Theories of hydrodesulfurization have assumed that W and Mo are catalysts for the reaction and that Co and Ni are promoters which enhance or preserve the activity of the catalysts. To test this, samples of sulfided C-supported Co-, Ni-, W-, and Mo-containing samples were tested as catalysts for thiophene hydrodesulfurization. The results do not clearly define the role of Co and Ni; however, at low concentrations, the promoter function is probably an adequate explanation, but at higher concentrations, more study is indicated to clearly define the role of Co and Ni. Catalytic effects of other C-supported sulfided metals are now being studied. (BLM)

A Refinement on the Notion of Type I and II (Co)MoS Phases in Hydrotreating Catalysts

The influence of Mo content on thiophene hydrodesulfurization for alumina- and carbon-supported (Co)Mo catalysts is investigated by TPR, XPS and H2-D2 equilibration. Activity trends for alumina-supported catalysts are explained by a gradual change of the type I MoS2 active phase from a large to a small number of Mo-O-Al anchors. A pure type II phase without such anchors and a high HDS activity is only obtained employing chelating ligands or a carbon support. At low Mo loading a separate oxysulfidic Mo species in strong interaction with alumina exhibits an unexpectedly high HDS activity, which, however, is difficult to promote by Co.

The influence of surface functionality on the activity of carbon-supported catalysts

The aim of this research is to investigate how the presence of surface functional groups can influence the activity of carbon-supported MoS2 catalysts for coal asphaltene hydrogenation. Porous carbons were subjected to various chemical treatments in order to introduce oxygen and nitrogen surface functionalities prior to impregnation with ammonium tetrathiomolybdate. Supports and catalysts were examined by FTIR. Preoxidation of polymer-derived carbons lowered catalyst activity whereas preoxidation of a carbon black composite support increased it. Similar mixed results have been reported in the literature. The modified catalytic activity cannot be unequivocally attributed to surface oxygen. Qualitatively, it is considered that these groups exert some influence, but it is the structure of the carbon which is ultimately the controlling factor. In some cases, oxidation may introduce sites which promote interaction with metal species and, in others, oxidation may modify or destroy favourable sites which are already extant. Prenitriding was found to have a distinct influence in enhancing catalyst activity. The presence of nitrogen-containing surface groups may provide preferential sites for the adsorption of Mo species.

Carbon-covered alumina as a support for sulfide catalysts☆

Abstract Carbon-covered alumina carrier materials (10–35 wt.% carbon deposited) were prepared via pyrolysis (873–973 K) of cyclohexene or ethene on the surface of a γ-alumina and evaluated for their use as supports for cobalt sulfide hydrodesulfurization catalysts. Promising textural properties were obtained for the samples prepared: BET surface areas up to 334 m 2 g −1 , meso- and macropore surface areas reaching values of 190–270 m 2 g −1 , and narrow pore size distributions in the 2.5–10 nm pore radius range. XPS measurements showed that the alumina surface was not uniformly covered, probably due to diffusion limitations of the carbon forming hydrocarbons. The coverage could be improved (maximum value reached was 77%) by increasing the amount of carbon deposited as well as by an additional high-temperature (1073 K) treatment. The thiophene hydrodesulfurization activity of Co sulfide supported on the prepared carbon-covered aluminas was found to increase linearly with increasing alumina surface coverage by carbon. A threefold increase in activity compared to Co Al 2 O 3 catalysts was obtained, demonstrating the effective shielding by the carbon layer which reduces or eliminates the strong metal-alumina interactions. Oxidizing the carbon surface prior to the introduction of cobalt led to a further improvement of the catalytic activity.

The CoO-MoO3-gamma-Al2O3 : VI. Sulfur content analysis and hydrodesulfurization activities

The sulfur uptake of commercial and laboratory prepared catalysts of the type MoO 3 γ-Al 2 O 3 , CoO γ-Al 2 O 3 and CoO MoO 3 γ-Al 2 O 3 was studied at 400 °C using H 2 /SH 2 and thiophene/H 2 as sulfiding gases. The temperature, time, and H 2 S partial pressure of sulfiding were varied, and the fraction of sulfur removable by H 2 reduction at 400 °C was determined. The influence of the sulfur content on the activity for hydrodesulfurization of thiophene was also measured. Based on these findings the formation of MoS 2 and Co 9 S 8 as a result of the sulfidation is considered to be the most likely process, although the presence of small amounts of other sulfurcontaining species cannot be excluded. Experimental evidence is reported for the diffusion of Co 2+ ions from the bulk towards the surface of the γ-Al 2 O 3 support during the sulfiding process. The hydrogenolysis activity was found to decrease with increasing sulfur content for the MoO 3 γ-Al 2 O 3 catalyst, while on CoO MoO 3 γ-Al 2 O 3 the reverse effect was observed.

Influence of zeolite acidity on thiophene hydrodesulfurization activity

Abstract For H( x )NaY supported metal sulfide catalysts the intrinsic catalytic activity [thiophene hydro-desulfurization (HDS)] and the metal sulfide dispersion (dynamic oxygen chemisorption) are measured as a function of the acidity of the zeolite support (determined by ethylamine temperature-programmed desorption). The results show that the acidity can have a strong influence on both the conversion and the product selectivities. An increasing acidity results in an increase of the initial thiophene HDS activity. The increased activity is not caused by an increase in the metal sulfide dispersion, but, probably by a synergetic effect between the metal sulfide particles and the acidic zeolite support. The increase in initial thiophene HDS activity with increasing support acidity is also observed for cobalt, nickel and molybdenum sulfide catalysts prepared by impregnation using various zeolite supports (ZSM-5, Y, HUSY). The higher activity may be caused by an increase in thiophene adsorption in the zeolite pores, or by a direct effect of H + on the thiophene HDS reaction. Additionally, the acidic supports themselves also show a considerable initial HDS activity, indicating that the acid sites are able to desulfurize thiophene at reaction conditions.

Structure/metathesis activity relations of silica supported molybdenum and tungsten oxide

Abstract In this study the influence of metal content and impregnation technique (wet or dry) on struture and metathesis activity for silica supported molybdenum and tungsten oxide is described. Structural aspects are obtained from several techniques, in particular temperature programmed reduction and Raman spectroscopy. Catalytic activity has been measured in a six-channel microflow reactor. A combination of structural and metathesis activity data leads to the conclusion that in these systems the precursors for the catalytic sites in metathesis are surface compounds and not the bulk oxides. The results suggest that a prerequisite for catalytic activity is a combination of a high degree of dispersion and an easy reducibility.

Temperature-programmed reduction of Al2O3-, SiO2-, and carbon-supported Re2O7 catalysts

Temperature-Programmed Reduction (TPR) has been applied to characterize the reducibility of Al2O3-, SiO2-, and carbon-supported Re2O7 catalysts, over a wide range of transition metal content. Dried catalysts are found to contain a so-called monolayer-type Re7+ surface phase as well as crystalline NH4ReO4. Calcination at 575 or 825 K resulted in decomposition of NH4ReO4, formation of the Re7+ surface phase and Re2O7 clusters, and Re loss via sublimation of Re2O7. Differences in reducibility of the various catalyst samples are ascribed to variations in the strength and the heterogeneity of the Re7+-support interaction. The strength of the interaction was found to depend on the support material applied and decreased in the order: Al2O3 > SiO2 > carbon. The heterogeneity was essentially the same for all three supports. The largely varying literature data on the reducibility of Re2O7Al2O3 catalysts is supposedly related with the presence of additives, such as chlorides, which may increase the Re7+-support interaction.