Cobalt-molybdenum catalysts preparation

We begin with the structure of a noble metal catalyst. The emphasis is on the preparation of rhodium on aluminum oxide and the nature of the metal-support interaction. Next we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here we are concerned with the composition and structure of the catalytically active... 

The first description of a synergetic effect due to a mixed cobalt-molybdenum catalyst (oxides and sulfides) was in 1933 by Pease and Keithon (11) at the Princeton University. Their catalytic system was active for the HDS of a mixture of benzene and thiophene. However, difficulty in reproducing their results already pointed out the complexity of this promotion effect, highly dependent on the conditions of preparation and pretreatment and the experimental conditions. 

Another SIMS study on model systems concerns molybdenum sulfide catalysts. The removal of sulfur from heavy oil fractions is carried out over molybdenum catalysts promoted with cobalt or nickel, in processes called hydrodesulfurization (HDS) [17]. Catalysts are prepared in the oxidic state but have to be sulfided in a mixture of H2S and H2 in order to be active. SIMS sensitively reveals the conversion of Mo03 into MoSi, in model systems of MoCf supported on a thin layer of Si02 [21]. 

TOF-SIMS images (Figs. 13.5 and 13.6) illustrate the ability to detect changes in the dispersion (uniform or presence of metal clusters) of the active phase in supported-oxide catalysts. Figure 13.5 shows nearly uniform distribution of molybdenum. The surface contamination with NH4+ ions coming from a precursor, which were not removed during the catalyst preparation process, is also observed. Cobalt clusters in the range of several micrometers are clearly visible in Fig. 13.6. 

Impregnation of cobalt and molybdenum (without sodium) increases largely the isomerizing activity of the catalyst the /3-pinene is then completely converted. The catalysts prepared with sodium molybdate and sodium hydroxide (Co-Mo-Na and Na-Co-Mo-Na) have lower isomerizing activities while their HDS activities are significantly increased. As in the case of alumina supported catalysts the sulfided CoMo phase protected by a double layer of alkaline ions on the carbon support gives the best results in HDS of /3-pinene. The behaviour of this catalyst was examined in desulfurization of the turpentine oil (40% a-pinene, 25% /3-pinene, 25% A -carene and 10% camphene + dipentene + myrcene, 1500 ppm S). The results are recorded in Table 6. 

Hydrogenation tests made on the 600°-1000°F heavy gas oil from in situ crude shale oil showed that a nickel-molybdenum-on-ahimina catalyst was superior to either cobalt-molybdenum-on-alumina or nickel-tungsten-on-alumina catalysts for removing nitrpgen from shale oil fractions. This nickel-molybdenum-on-alumina catalyst was used in the preparation of the synthetic crude oil. A high yield of premium refinery feedstock whose properties compared favorably with those of a syncrude described by the NPC was attained by hydrogenating the naphtha, light... 

Catalyst Preparation. The catalysts were prepared by impregnation of -alumina extrudates ( SA=253 m /g ). Each impregnation was followed by drying overnight at 120°C and calcination at the indicated temperatures during one hour. Molybdenum was brought on the support as an ammonium molybdate solution cobalt and nickel as nitrate solutions. Each component was impregnated separately. 

Hutchings (170) plotted (Figure 31) the activity against the surface area for a number of promoted catalysts and deduced that most of the catalysts conform to a linear correlation. The only enhancement of the specific activity was observed for the cerium-promoted catalyst. This result shows that care must be taken in the interpretation of the catalyst performance data, particularly when catalysts prepared by different methods are compared. In a separate study, Hutchings and Higgins (171) found that chromium, niobium, palladium, antimony, ruthenium, thorium, zinc, and zirconium each had very little effect on the specific activity of (VO)2P207. A significant increase in surface area was observed with zirconium, zinc, and chromium, which could be of use as structural promoters. Iron-, cesium-, and silver-doped catalysts decreased the specific activity, and cobalt and molybdenum were the only promoters found to increase the specific activity. 

Theories and principles of the characterization techniques are not described here. For consistenc), all the catatysts described in this review are referred to with the same nomenclature, although a different nomenclature is sometimes used in the cited publications. Each catalyst component (element) separated by the symbol indicates the sequence of its introduction into the catalyst formulation from right to left. Those separated by the symbol 7 between right and left belong to the support material and the elements on the support, respectively. For example, NiMo-P/Al refers to a catalyst prepared such that the phosphorus-containing precursor is loaded on the alumina support first, followed by nickel and molybdenum, which are introduced simultaneously. CoMo/Al — P refers to a catalyst in which cobalt and molybdenum are introduced simultaneously onto an alumina support doped with phosphorus-containing species. Each element may represent its oxide or sulfide forms. In all cases, A1 refers to the alumina-based support or to its hydroxide precursor. 

Higher activity catalyst can be achieved by increasing the metal content up to the limit of the support capacity, although the molybdenum efficiency decreases. Consequently, we have worked on the different steps of a catalyst preparation (carrier selection and shaping, Co/Mo ratio, molybdenum and cobalt introduction methods, promotor, thermal and hydrothermal treatments...) and examined the activity of the resulting catalyst at each step. 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

Three Co2Moio(Co) catalysts were prepared one on gamma alumina and two on supports modified by TMSiOMe respectively at 0.5 TMSi/nm and 0.9 TMSi/nm. Catalysts thus obtained on all three supports have a similar cobalt/molybdenum molar ratio of 0.5. Their respective Mo loadings expressed as wt% of M0O3 are 16.7 for catalyst prepared on alumina, 15 for catalyst prepared on 0.5-TMSi-alumina and 15.7 for catalyst prepared on 0.9-TMSi-alumina. Raman spectroscopy was used to characterize the nature of the supported oxomolybdate phase. In both cases, the spectra (not reported here) show the features of a well dispersed polymolybdate phase. In particular, no M0O3 nor C0M0O4 was evidenced. 

Other Early Developments. In addition to the breakthrough by Ziegler, two other discoveries of ethylene polymerization catalysts were made in the early 1950s. A patent by Standard Oil of Indiana, filed in 1951, disclosed reduced molybdenum oxide or cobalt molybdate on alumina (13). At the same time, Phillips discovered supported chromium oxide catalysts, prepared by impregnation of a silica-alumina support with Cr03 (14 16). Both the Phillips catalyst and titanium chloride based Ziegler catalysts are widely used in the production of high density polyethylene (HDPE). 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

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