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

No abstract

The role of Co in sulfidized Co-Mo hydrodesulfurisation catalysts supported on carbon and alumina

The thiophene hydrodesulphurisation activities of cobalt sulphide catalysts supported on activated carbon have been measured in a flow microreactor operating at atmospheric pressure. The cobalt content was varied between 1.3 and 13.3 wt% Co, and the oxidic precursor catalysts were dried by three different procedures. Structural characteristics and the degree of Co dispersion in the oxidic precursor and in the sulphidised state of the catalysts were obtained by X-ray photoelectron spectroscopy. The oxidic cobalt phase present in the precursor catalysts was found to be inhomogeneously dispersed over the carbon carrier surface. Dispersion decreased significantly during catalyst sulphidation. XPS results showed that the cobalt was fully sulphidised, although sulphur was found to be present in excess of the amount corresponding to stoichiometric Co9S8. The catalysts demonstrated very high hydrodesulphurisation activities, which were by far superior to those of corresponding carbon- or aluminasupported molybdenum catalysts. By extrapolation of the inverse activities to zero loading the activities of optimally dispersed cobalt and molybdenum sulphide were determined. Comparison of the activity of carbon-supported Co-Mo with the activities of the optimally dispersed catalyst supported on carbon demonstrated that the so-called promoter effect in sulphidised Co-Mo catalysts can be explained completely by the exceptionally high catalytic activity of cobalt sites and showed that the role of MoS2 in these catalysts is mainly to function as a support for optimally dispersed cobalt ions

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

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

Periodic trends in the hydrodenitrogenation activity of carbon-supported transition metal sulfide catalysts

No abstract

Chromia and chromium sulfide pillared clays differing in pillar density

Chromia pillared clays differing in pillar density were synthesized from the Li-form of montmorillonite calcined in a controlled manner. These materials were characterized by XRD, TGA, AAS, nitrogen adsorption and their thiophene hydrodesulfurization activity. It is found that clays varying in cation exchange capacity pillared with Cr-12 pillar precursor ions can provide a family of catalysts that differ in surface area, porous structure and concentration of active sites. Sulfidation of the heat-treated chromia pillared clay resulted in the formation of chromium sulfide pillared derivatives with high surface area and well-developed porous structure. The activities for thiophene hydrodesulfurization and consecutive butene hydrogenation over chromium sulfide pillared montmorillonite are highest on clays with the largest pillar concentration and correlated with the total surface area. However, the activity per number of pillars increased with decreasing pillar density. Thiophene HDS predominantly results in the formation of butane and butenes, irrespective of the pillar population. The rate of deactivation is somewhat larger for catalysts that have a larger lateral pillar spacing. The chromium sulfide pillared clays exhibit a relatively high stability in thiophene HDS compared to zeolitic catalysts which is most likely due to their low protonic acidit

Effect of gallium ions and of preparation methods on the structural properties of cobalt-molybdenum-alumina catalysts

Small amounts of gallium ions were added to y-alumina and their influence on the structural properties of the system Co-(Mo)/y-Al203 was studied. It is shown that, due to the presence of Ga3+ ions, a surface spinel CoAl204 is formed with a larger amount of Co2+ in tetrahedral sites as compared to the spinel formed on gallium-free alumina. A decrease of the segregated Co304 is also observed. A possible effect of gallium ions on molybdenum is discussed. It is also reported that different preparation methods (single or double impregnation) lead to the formation of different surface species. Cobalt aluminate, molybdate monolayer, and Co3O4, depending on the Co content, are formed on doubly impregnated specimens. Cobalt aluminate and cobalt molybdate are the main species formed on singly impregnated specimens. Finally brief consideration is given to how the Co and Mo species, present in the oxide form, change in the sulfided form

Influence of phosphate on the structure of sulfided alumina supported cobalt-molybdenum catalysts

The influence of phosphate on the sulfiding rate of alumina-supported cobalt, molybdenum and cobalt-molybdenum catalysts has been studied by means of Temperature Programmed Sulfiding (TPS). Combination of the TPS results with recently published Temperature Programmed Reduction (TPR), Mössbauer spectrometry and nitrogen oxide adsorption measurements results in a detailed picture of the sulfidic catalyst. The sulfiding of cobalt ions in phosphorus-containing alumina-supported cobalt catalysts shifted to somewhat higher temperatures with increasing phosphorus content, most likely due to the presence of Co-AlPO4 phases. In the temperature region from 1000-1270 K, part of the cobalt sulfide species reacted with AlPO4 to cobalt phosphides. The amount of cobalt which reduced to phosphides increased strongly with increasing phosphorus content and became about 70% of the total amount of cobalt present. The sulfiding pattern of phosphorus-containing Co-Mo/Al catalysts is only slightly influenced by increasing phosphorus content below 1000 K. All components including the Co-Mo-O-P phase, present in the oxidic precursor, are relatively easy to sulfide. After sulfiding at 673 K, the Co-Mo-S phase is most likely still to be associated with the AlPO4 species. Analogous to cobalt catalysts, the major part of the cobalt in Co-Mo-P/Al catalysts ultimately reacted to cobalt phosphides above about 1000 K. The sulfiding rate of the molybdenum ions is not changed by the presence of phosphates. MoS2 does not react with the phosphates to molybdenum phosphide probably because this reaction is thermodynamically restricted. Based on the surface structures, deduced from a combination of TPR and TPS data, the striking difference between the effect of phosphates on the hydrodenitrogenation (HDN) and on the hydrodesulphurisation (HDS) reactions is discussed

On the formation of aluminum tungstate and its presence in tungsten oxide on gamma-alumina catalysts

No abstract

Characterization of silica-supported molybdenum oxide and tungsten oxide. Reducibility of the oxidic state versus hydrodesulfurization activity of the sulfided state

Thiophene hydrodesulfurization and butene hydrogenation have been studied for presulfided MoO3/SiO2 and WO3/SiO2 catalysts using a micro-flow reactor operating at atmospheric pressure. The catalysts have been prepared by dry as well as wet impregnation. The oxidic precursor catalysts were characterized by X-ray diffraction, surface area and pore volume measurements, and temperature-programmed reduction. Catalysts prepared by dry or wet impregnation are essentially the same. At low metal oxide contents the catalysts are of the monolayer type. At higher metal oxide contents also bulk compounds are present, which is demonstrated by means of X-ray diffraction as well as temperature-programmed reduction. The maximum concentration of monolayer-type compounds corresponds to approximately one transition metal atom per square nanometer of the carrier. A correlation could be established between reducibility of the oxidic monolayer-type catalysts and the activity for thiophene hydrodesulfurization. This correlation appears to be in good agreement with the one reported earlier for the analogous ¿-alumina-based catalysts. Butene hydrogenation activity of the sulfided MoO3/SiO2 and WO3/SiO2 monolayer-type catalysts also correlates with reducibility of the oxidic systems. Due to the presence of bulk compounds the turnover frequency in hydrodesulfurization as well as hydrogenation decreases at higher metal oxide contents

New sulfide catalysts for the hydroliquefaction of coal

Possibilities for the preparation of new metal sulfide catalyst systems based on carbon carriers having favourable textural and surface properties have been explored, and attention has been given to the characterization (structure) and evaluation (hydrosulfurization activity) of these catalysts. Two different types of carbon carriers were prepared viz., carbon black composite and carbon-covered alumina. The influence of various carbon carrier oxidative treatments on the activity of the supported sulfide phase has been studied. For the characterization of the structure of the active (Mo) sulfide phase deposited on an (activated) carbon carrier a combination of dynamic oxygen chemisorption, thiophene hydro desulfurization and X-ray photoelectron spectroscopy measurements was applied. Carbon black composites having outstanding textural properties for the application as support material for molybdenum sulfide catalysts could be prepared. The Inertness of their surface appeared to be a drawback for obtaining h'igh catalytic activity. However, catalytic activity was considerably improved after the reactivity of the carbon surface towards the molybdenum phase was Increased by means of an oxidative treatment. The carbon-covered aluminas prepared had promising textural properties. Covering the alumina surface by a layer of pyrolitlc carbon was found to improve the activity of supported (cobalt) sulfide phase. In none of the carboncovered aluminas prepared the alumina surface was completely covered since the carbon was not uniformly deposited. Therefore the maximum effect of the carbon coverage on the catalytic activity was never reached. However, heat treatment of the pyrolitic carbon appeared beneficial in this case. The results of dynamic oxygen chemisorption measurements showed that the molybdenum sulfide phase supported on active carbon had a high density of only one type of active site with a high turn over frequency. The alumina supported molybdenum phase had a lower active site density and different types of sites seemed to be present at low and high molybdenum concentration. For both the Mo/C and Mo/¿-Al2O3 catalysts, the decline in activity with run time was not reproduced by the oxygen chemisorption capacities. Thus Initial deactivation was neither caused by sintering of the active phase nor by pore blocking