Promoted Cobalt Catalysts

D.S. Atomic-Scale Structure of the Cobalt-Promoted Catalyst... 

Further evidence for the catalytic importance of amorphous material comes from experiments carried out with cobalt-doped catalysts. Hutchings et al. (217) found that doping of the catalysts with cobalt improved their performance. Moreover, Sajip et al. (148) found that the cobalt-promoted catalysts are far more disordered than the undoped catalysts. In the doped catalysts, the promoter is dispersed in the amorphous phase, and cobalt is not found in the vanadyl pyrophosphate crystals. It is thought that one of the properties of the cobalt promoter is to stabilize the disordered phase and V -containing phases in the final catalysts, which leads to improved performance. This suggestion implies that the disordered material is the catalytically active vanadium phosphate phase. 

As in the investigation with zirconium promoters carried out by Zeyss et al. (174), cobalt and iron were found to promote the formation of VOPO4 phases during the conversion of the precursor to the active catalyst. The difference in activity between the iron- and cobalt-promoted catalysts is considered to be a consequence of the different redox potentials of the promoters. As the ratio decreases, the butane conver-... 

This phenomenon has also been observed for catalysts prepared using an aqueous route (182). Both the iron and cobalt promoters led to an increase in selectivity. The iron-promoted catalyst was characterized by an increase in activity, but the cobalt-promoted catalyst was characterized by a decrease in activity. The decrease in activity of the cobalt-doped catalyst was attributed to the formation of VOPO4 in the final catalyst. The VOPO4 is formed by the oxidation of V0HP04 1 H20 during the introduction of the promoters in the incipient wetness technique. A similar effect was reported for catalysts doped with indium and tetraethy-lorthosilicate (TEOS) (181). The improved performance was observed only with both promoters in the catalyst. It was proposed that the... 

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

Catalysts used for preparing amines from alcohols iaclude cobalt promoted with tirconium, lanthanum, cerium, or uranium (52) the metals and oxides of nickel, cobalt, and/or copper (53,54,56,60,61) metal oxides of antimony, tin, and manganese on alumina support (55) copper, nickel, and a metal belonging to the platinum group 8—10 (57) copper formate (58) nickel promoted with chromium and/or iron on alumina support (53,59) and cobalt, copper, and either iron, 2iac, or zirconium (62). 

Catalyst choice is strongly influenced by the nature of the feedstock to be hydrotreated. Thus, whereas nickel-promoted and cobalt—nickel-promoted molybdenum catalysts can be used for desulfurization of certain feedstocks and operating conditions, a cobalt-promoted molybdenum catalyst is generally preferred in this appHcation. For denitrogenation and aromatics saturation, nickel-promoted molybdenum catalysts usually are the better choice. When both desulfurization and denitrogenation of a feedstock are required, the choice of catalyst usually is made so that the more difficult operation is achieved satisfactorily. 

Metals in the platinum family are recognized for their ability to promote combustion at lowtemperatures. Other catalysts include various oxides of copper, chromium, vanadium, nickel, and cobalt. These catalysts are subject to poisoning, particularly from halogens, halogen and sulfur compounds, zinc, arsenic, lead, mercury, and particulates. It is therefore important that catalyst surfaces be clean and active to ensure optimum performance. 

Fischer Tropsch synthesis is catalyzed by a variety of transition metals such as iron, nickel, and cobalt. Iron is the preferred catalyst due to its higher activity and lower cost. Nickel produces large amounts of methane, while cobalt has a lower reaction rate and lower selectivity than iron. By comparing cobalt and iron catalysts, it was found that cobalt promotes more middle-distillate products. In FTS, cobalt produces... 

The molybdenum on alumina catalyst was also tested for activity with and without arsenic. Although this catlyst has a much lower intrinsic activity for HDS, the results in Figure 4 show that 3.6% arsenic almost completely deactivates the catalyst. The small amount of activity remaining is that expected for AI2O3 alone. Thus arsenic also deactivates catalysts without cobalt promoters. 

MINIMIZATION OF CARBON DEPOSITS ON COBALT FTS CATALYSTS BY PROMOTION... 

Additives such as rare earth or noble metals are generally introduced into industrial cobalt FTS catalysts as structural or reduction promoters.92 The addition of various promoters to cobalt catalysts has also been shown to decrease the amount of carbon produced during the FTS.84 87 93 94 Also, the addition of promoter elements may decrease the temperature of regeneration, preventing the possible sintering of supported cobalt particles during such treatments.92... 

Das, T.K., Jacobs, G., Patterson, P.M., Conner, W.A., Li, J., and Davis, B.H. 2003. Fischer-Tropsch synthesis Characterization and catalytic properties of rhenium promoted cobalt alumina catalysts. Fuel 82 805-15. 

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... 

Holmen and coworkers15-17 also observed a loss in activity when water was introduced to un-promoted and Re-promoted cobalt deposited on >-Al203. In a recent paper similar results were reported for Co Re supported on both narrow-pore and wide-pore y-Al203,18 and permanent deactivation was observed when the inlet ratio H20 H2 was 0.7. The same group reported that the rhenium-promoted catalysts lost activity more rapidly than their un-promoted counterparts.14-1619... 

Whereas the effect of water on deactivation and on the overall activity of the FTS varies with the support, similar effects of water on the selectivity is reported for all catalysts, to a certain degree independent of the support, promoter and conditions. The effect can be summarized as an increase in C5 + selectivity, a decrease in methane selectivity, and in some instances a weak enhancement of the C02 selectivity is observed. Fig. 4 illustrates the effect on the C5 + and methane selectivity of adding water to cobalt catalysts supported on alumina, silica and titania, and both unpromoted and Re-promoted catalysts are shown. At the outset these selectivities are strong functions of the conversion, the C5 + selectivity increasing and the methane decreasing with increasing conversion, as illustrated by the trendlines in the figures. The points for methane are below, and C5 + -selectivity is above the line when water is added. Similar results were reported by many authors for alumina-supported catalysts,16-19 23 30 silica-supported catalysts,30 37 46-48 and titania-supported catalysts.19 30... 

T. K. Das, G. Jacobs, P. M. Patterson, W. A. Conner, J. Li and B. H. Davis, Fischer-Tropsch synthesis characterization and catalytic properties of rhenium promoted cobalt alumina catalysts, Fuel, 2003, 82, 805-815. 

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

The behaviour of the ruthenium catalysts is quite different from that previously reported for cobalt carbonyl catalysts, which give a mixture of aldehydes and their acetals by formylation of the alkyl group of the orthoformate (19). The activity of rhodium catalysts, with and without iodide promoters,is limited to the first step of the hydrogenation to diethoxymethane and to a simple carbonylation or formylation of the ethyl groups to propionates and propionaldehyde derivatives (20). 

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