Tamás I. Korányi

Activation of unsupported CoMo catalysts in thiophene hydrodesulfurization

Abstract A series of unsupported oxidic CoMo catalysts with different mole fractions r = Co (Co + Mo) was prepared by coprecipitation of solutions of (NH 4 ) 6 Mo 7 O 24 and Co(NO 3 ) 2 . The calcined catalysts contain a-CoMoO 4 ( r = 0.5) mixed with MoO 3 ( r 3 O 4 ( r > 0.5) according to X-ray diffraction (XRD) and electron diffraction. The a-CoMoO 4 transforms partially into b-CoMoO 4 upon grinding. The higher the cobalt ( r ) and b-CoMoO 4 contents, the higher are the surface area increase and the degree of reduction of calcined catalysts during hydrogen treatment at 673 K. Electron microscopy (EM) data agree well with the surface area increase observed after reduction. X-ray photoelectron spectroscopy (XPS) shows the reduction of molybdenum rather than that of cobalt. Reduced crystalline phases cannot be identified by these techniques. Sulfidation with a mixture of H 2 /thiophene following reduction caused a drastic drop in surface area but the particle size seen by EM does not increase. Weak oxythiomolybdate XRD bands appeared after slight sulfidation, most XRD signals disappeared after massive sulfidation of samples with r = 0.5. Cobalt promotes sulfidation of molybdenum in the bulk, but the maximum sulfidation degree was about half of the stoichiometric value. XPS shows surface cobalt enrichment, XRD and EM traces of Co 9 S 8 in samples with r = 0.38 and 0.50. A pronounced maximum was observed in initial hydrodesulfurization (HDS) activity and hydrogenation (HYD) selectivity at medium Co content. On used catalysts, this synergism disappeared. We attribute the highest HDS activity of short-living cobalt-oxythiomolybdate(s) formed initially during sulfidation. HYD was promoted by sulfided molybdenum and by less surface Cobalt.

Preparation and characterization of candidate catalysts for deep hydrodesulfurization of gasoils. Sulfidation and acidity characteristics of supported Ni/W and Ni/Mo catalysts

Abstract The uptake of 35 S -labelled H 2 S, the thiophene hydrodesulfurization (HDS) activity in pulse and flow system and the acidity measured by infrared (IR) band intensities of adsorbed pyridine were studied after various treatments over NiW/amorphous silica–alumina (ASA), NiW/Al 2 O 3 and NiMo/Al 2 O 3 catalysts. The Fourier Transformation Infrared (FTIR) measurements indicated that the NiW/ASA catalyst contained some Bronsted and much more Lewis acid sites, whereas less Bronsted acid sites were observed on the alumina supported samples. The concentration of Bronsted and Lewis acid sites decreased after sulfidation. The amount of total sulfur uptake of NiMo/Al 2 O 3 catalyst was about twofold of the one of NiW samples. The sulfidation level of the catalysts was much lower in the pulse system than in the flow one and also the thiophene conversion was much lower on NiW catalysts in the former setup than in the latter system. A band was detected in the radiochromatogram of NiW/ASA catalyst during desorption of H 2 S in the pulse system which was attributed to weakly adsorbed H 2 S on the Bronsted acidic sites. This weakly adsorbed H 2 S hindered the isomerization of n -butane in the flow system but not in the pulse system.

High-resolution electron microscopy of cobalt-molybdenum catalysts: structural changes and activity

Abstract Unsupported cobalt—molybdenum oxide catalyst (atomic ratio of Co: Mo = 1:1) was calcined in air at 723 K, reduced in hydrogen at 673 K and sulphided at 673 K with hydrogen—thiophene mixtures for 10 and 70 min and with hydrogen—hydrogen sulphide for 120 min. High-resolution electron micrographs of the calcined sample show the presence of CoMoO4; this is also the main component detected after reduction, but its crystallinity is poor in the latter instance. MoS2 layers appear after even brief sulphidation. Sulphidation with hydrogen sulphide results in a catalyst covered almost entirely with MoS2, with traces of Co9S8 among its layers. Prolonged sulphidation with hydrogen—thiophene results in segregation of MoS2 and Co9S8 into separate crystallites; also, elemental sulphur appears. The catalytic activity at these stages has been evaluated.

Infrared spectroscopic investigation of CO adsorption on SBA-15- and KIT-6-supported nickel phosphide hydrotreating catalysts.

The infrared (IR) spectra of CO adsorbed on 10, 20, and 30 wt % nickel phosphide-containing reduced SBA-15 and KIT-6 mesoporous silica-supported catalysts have been studied at 300-473 K. On the catalysts containing a stoichiometric amount of phosphorus with 20 wt % loading, the most intense IR absorption band was observed at 2097-2099 cm(-1), which was assigned to CO terminally bonded to coordinatively unsaturated Ni(delta+) (0 < delta < 1) sites. The frequency of this band was 15 cm(-1), higher than that in the spectrum of a reduced Ni2P/SiO2 catalyst, indicating a modified Ni-P charge distribution. This band shifted to lower wavenumbers, and its intensity decreased, while the relative intensity of another band at 2191-2194 cm(-1) assigned to CO terminally bonded to P increased going to catalytically less active, excess-P-containing SBA-15-supported catalysts. CO also adsorbed as a bridged carbonyl (1910 cm(-1)) and as Ni(CO)4 (2050 cm(-1)) species, and the formation of surface carbonates was also identified. The nature of the surface acidity was studied by temperature-programmed desorption of ammonia (NH3-TPD). Weak and strong acid sites were revealed, and the high excess-P-containing catalyst released the highest amount of ammonia, indicating that a high concentration of strong acidity can be disadvantageous for reaching high hydrotreating catalytic activity. The modified Ni-P charge distribution, the mode of CO adsorption on surface nickel phosphide sites, as well as the acidity can be directly connected to the catalytic activity of these mesoporous silica-supported catalysts.

Identification of SH groups in zeolite-supported HDS catalysts by FTIR spectroscopy

Abstract NiNaY zeolite (prepared by ion exchange of NaY) and CoHY zeolite (prepared by impregnation of HY) were studied by Fourier Transform Infrared (FTIR) spectroscopy and by the thiophene hydrodesulfurization (HDS) catalytic reaction. The metal- (Ni or Co) loading in the nonsulfided zeolites correlated with the Lewis acidity (determined by pyridine adsorption) indicating a high dispersion level of Ni or Co species. Following sulfidation of the zeolites by H 2 H 2 S at 373 K two kind of SH band were observed in the IR spectra: the broad band at 2520 cm−1 was assigned to H2S hydrogen bonded to the strong Bronsted acid sites (SiOHAl groups), the sharp band at 2580 cm−1 was assigned to H2S coordinatively bound to the Lewis acidic cationic sites. These SH bands were not identified during sulfidation above 473 K, but the similar catalytic activities and product selectivities after sulfidation either at 373 K or at 673 K indicate that SH groups bonded to strong Bronsted acid sites are present and should play a role in HDS catalysts working under industrial conditions.

Structural consequences of mild oxidative template removal in the synthesis of modified MCM-41 silicates

Abstract Results concerning the structural consequences of template removal from MCM-41 mesoporous materials are described using ozone, N 2 O and NO 2 as oxidants in comparison with the conventional method applying oxygen. Si, TiSi-, VSi and ZrSi-MCM-41 samples were synthesized by the usual methods. For characterization of the as-synthesized and treated samples XRD, nitrogen adsorption, 29 Si MAS NMR-, IR- and UV–Vis spectroscopic methods were used. The catalytic activity of the samples was tested in the Friedel–Crafts alkylation of toluene by benzyl chloride. The comparison of template removal agents showed that ozone was the most active at low temperature (423 K), and the treatment was less destructive than burning off the template in oxygen. Nitrogen oxide treatment (NO 2 and N 2 O) resulted in template removal at relatively low temperature (573–623 K), and structure deterioration was small. Si NMR spectroscopic data and IR spectra taken in the framework vibration range revealed that more original –SiOH groups remained as hydroxyl nests, furthermore, the heteroatom remained in tetrahedral coordination after ozone and nitrogen oxide treatment compared to burning off the template by oxygen. The results proved the advantages of ozone or nitrogen oxide treatments: (i) gentler to heteroatoms situated in the framework of the materials, probably leaves them intact, (ii) does not result in the formation of secondary micropores, which would decrease the uniform arrangement of the original pore systems, (iii) by preserving the active centers in their original coordination more uniform product distribution may be expected in catalytic reactions.

Comparative activity study in thiophene hydrodesulfurization of molybdena/alumina catalysts prepared by impregnation and solid/solid wetting

Abstract Molybdena/Al2O3 catalyst precursors have been prepared by impregnation from aqueous solution and by solid/solid wetting (spreading) in mixtures of MoO3 and Al2O3 in either humid or dry oxygen atmosphere. Thiophene HDS at atmospheric pressure was chosen as a test reaction. Those catalyst preparations which contained a surface heptamolybdate species in the oxide precursor state showed the highest HDS conversions.

Surface state, composition and catalytic activity of slightly sulfided Co-Mo/Al2O3 catalysts pretreated at different temperatures

Abstract γ-alumina samples precalcined at 773 or 1073 K, respectively, were coimpregnated with ∼ 10 w/w% Co and Mo in the molar ratio Co/(Co + Mo) = 0.65. These catalyst precursors were subjected to a second heat treatment at 723 or 1073 K. The catalytic activity of calcined, prereduced and presulfided (by H 2 S) samples was checked in the HDS reaction of thiophene model compound in H 2 . The “apparent surface percentages” of catalyst precursors and used catalysts - monitoring lines of Co, Mo, Al and also S and C - were determined by XPS. These values are not equal to the nominal bulk compositions and depend on the support and catalyst treatments. Possible correlations between the catalytic activity and the apparent surface percentage of Mo, Co and S are evaluated. High-temperature pretreatment of the support is advantageous for catalytic activity and so is a high-temperature calcination of the catalyst itself; highest activity is observed when these treatments are followed by presulfidation.

Investigation of unsupported CoMo catalysts in their oxidic state and during sulfidation by thiophene

Abstract Unsupported CoMo oxides of various compositions were investigated by IR, XRD and XPS techniques. They contain CoMoO4 plus the corresponding oxide with slight Mo enrichment on the surface. After reduction with H2, the changes in their catalytic activity in thiophene HDS were monitored with the reactant itself as the sulfiding agent. Relatively high activities with good C4-selectivities were observed when the degree of sulfidation of the catalysts was relatively low. Partially sulfided surface species possess the highest activity, especially in hydrogenation.

Correlation of surface composition and thiophene HDS activity of moderately sulfided CoMo/Al2O3 catalysts

Abstract Coimpregnated (Co)Mo/Al 2 O 3 catalysts ( r = Co/(Co + Mo) = 0, 0.29, 0.36 and 0.65) were studied by XPS in their calcined (oxidic), prereduced, presulfided form as well as after reacting thiophene over them at 673 K and atmospheric pressure, during which the catalytic activity was also monitored. The sulfidation of surface cobalt increased, that of surface molybdenum decreased with increasing nominal Co content ( r ) of the catalysts. On the basis of similar Co and Mo dispersions the existence of a surface cobalt-molybdenum interaction was proposed. Partial disruption of the Mo monolayer was suggested at high bulk Co content. Since maximum thiophene conversion is observed at a slight surface sulfidation and the surface ratio of Co to Mo is unity, cobalt-oxythiomolybdates — which were identified previously by XRD and IR in unsupported systems — are proposed to be the active phases at the moment of highest activity in our alumina-supported catalysts, similar to our unsupported systems.