High surface area cobalt-on-alumina catalyst

Synthesis of High Surface Area Cobalt-on-Alumina Catalysts by Modification with Organic Compounds... 

The reactions are catalyzed by transition metals (cobalt, iron, and ruthenium) on high-surface-area silica, alumina, or zeolite supports. However, the exact chemical identity of the catalysts is unknown, and their characterization presents challenges as these transformations are carried out under very harsh reaction conditions. Typically, the Fischer-Tropsch process is operated in the temperature range of 150°C-300°C and in the pressure range of one to several tens of atmospheres [66], Thus, the entire process is costly and inefficient and even produces waste [67]. Hence, development of more economical and sustainable strategies for the gas-to-liquid conversion of methane is highly desirable. 

Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m g ) alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. 

Group 1 alkali metals (including potassium) are poisons for cobalt catalysts but are promoters for iron catalysts. Catalysts are snpported on high-surface-area binders/supports such as silica, alumina, and zeolites (Spath and Dayton, 2003). Cobalt catalysts are more active for FTS when the feedstock is natnral gas. Natnral gas has a high H2 to carbon ratio, so the water-gas-shift is not needed for cobalt catalysts. Iron catalysts are preferred for lower quality feedstocks such as coal or biomass. 

TMS catalyst, used in refineries, are mixed sulfides of cobalt-molybdenum (CoMo) or nickel-molybdenum (NiMo) with a promoter atomic ratio Co(Ni)/[Co(Ni) -I- Mo(W)] between 0.2 and 0.4 (2), supported on high surface area materials such as alumina to increase dispersion of the active component of the catalyst. Although CoMo sulfide is the favorite catalyst for HDS reactions, the use of NiMo sulfide is preferred in HDN reactions and hydrogenation processes, to treat feedstock with a high concentration of unsaturated compounds. 

It is well known that alumina, titania [10,11,12] and magnesium oxide [13,14] dissolve in acidic aqueous solutions and even at pH values close to the isoelectric point [15,16], In this study, it will be shown that these support surfaces were modified with promoters to increase the inertness thereof to acidic/aqueous environments, and not to stabilise the support against sintering and loss in surface area at high temperatures [17,18], This paper will deal with the modification of alumina and titania supports for cobalt based slurry phase Fischer-Tropsch catalysts to ensure the successful operation of slurry phase bubble column reactors on commercial scale,... 

The results of this work provide evidence that catalysts consisting of cobalt supported on conventional silica carriers deactivate due to support collapse and formation of cobalt-silicates. Indeed, several previous studies provide evidence that steam can react with silica and alumina carriers at high reaction temperatures and high PhjO to decrease their BET surface areas while increasing their pore diameters [14, 18, 19]. The results of this work for Co/silica are consistent with those of Hilman et al. [20, 21] for Co/alumina, both showing that high concentrations of steam during reaction cause a loss of activity, loss of chemisorption surface area and formation of a stable cobalt silicate or aluminate. 

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