Cobalt-molybdate catalysts

Alternative means for removal of carbonyl sulfide for gas streams iavolve hydrogenation. For example, the Beavon process for removal of sulfur compounds remaining ia Claus unit tail gases iavolves hydrolysis and hydrogenation over cobalt molybdate catalyst resulting ia the conversion of carbonyl sulfide, carbon disulfide, and other sulfur compounds to hydrogen sulfide (25). 

When the Claus reaction is carried out in aqueous solution, the chemistry is complex and involves polythionic acid intermediates (105,211). A modification of the Claus process (by Shell) uses hydrogen or a mixture of hydrogen and carbon monoxide to reduce sulfur dioxide, carbonyl sulfide, carbon disulfide, and sulfur mixtures that occur in Claus process off-gases to hydrogen sulfide over a cobalt molybdate catalyst at ca 300°C (230). 

Carbon monoxide has been found to poison cobalt molybdate catalysts. It causes not only instantaneous deactivation but a cumulative deactivation as well. It should be removed from treat gas entirely or at least reduced to a very low value. Carbon dioxide also must be removed since it is converted to CO in the reducing atmosphere employed in Hydrofining. Liquid water can damage the structural integrity of the catalyst. Water, in the form of steam does not necessarily hurt the catalyst. In fact 30 psig steam/air mixtures are used to regenerate the catalyst. Also, steam appears to enhance the catalyst activity in... 

Effect of Catalyst The catalysts used in hydrotreating are molybdena on alumina, cobalt molybdate on alumina, nickel molybdate on alumina or nickel tungstate. Which catalyst is used depends on the particular application. Cobalt molybdate catalyst is generally used when sulfur removal is the primary interest. The nickel catalysts find application in the treating of cracked stocks for olefin or aromatic saturation. One preferred application for molybdena catalyst is sweetening, (removal of mercaptans). The molybdena on alumina catalyst is also preferred for reducing the carbon residue of heating oils. 

Lapidus (LI) described liquid residence-time distribution studies for air-water and air-hydrocarbon in cocurrent, downward flow through a column of 2-in. diameter and 3-ft height. Spherical glass beads of 3.5. mm diameter and cobalt molybdate catalyst cylinders of -in. diameter were used as packing materials. 

An iron-promoted cobalt molybdate catalyst (Fe0 03Co0.9 7MoO4) was studied by Maksimov et al. [195,196] with respect to the role of iron in the transfer of charge. Iron strongly enhances the catalytic activity and at the same time increases the conductivity by a factor of 100. Mossbauer spectroscopy reveals that 4% of the iron ions are present as Fe2+ impurity . This fraction is doubled at steady state reaction conditions, and indicates participation of iron in the charge transfer process. 

Firsova et al. (136) also investigated a cobalt molybdate catalyst containing a small amount of Fe3+, after exposure to a reaction mixture of propylene and oxygen. The authors observed the valence change of Fe3+ to Fe2+ and the formation of a surface complex between the hydrocarbon and the iron (Fe—O—C—). In contrast to pure iron molybdate which also forms a surface Fe—O—C— complexes, the electronic transitions in the cobalt iron molybdate were reversible. The observed valence change showed that iron ions increase the electronic interaction between ions in the catalyst and the components of the reaction mixture. 

The so-called cobalt molybdate catalyst has been used much in the petroleum industry for hydrotreating and hydrodesulfurization. More recently, these catalysts have been employed in coal liquefaction and synthoil upgrading. The latter probably accounts for the recent rash of publications on this very interesting catalyst system. Indeed, of the papers surveyed for this review, the majority have been published in the past 5 years with no letup in sight. 

The cobalt molybdate catalyst is also suitable for the reduction of organic nitrogen bases (30). From Colorado-shale-oil fractions containing 2% N2 and 0.7% sulfur, jet and Diesel fuels have been obtained with 0.01 to 0.1% N2 and 0.03 to 0.04% sulfur. 

Various polar and chemical compounds reportedly are capable of poisoning or deactivating disproportionation catalysts if present in the feed or allowed to contact the catalyst after activation. For example, propylene conversion over cobalt-molybdate catalyst was reduced when 300—2000 ppm of oxygen, water, carbon dioxide, hydrogen sulfide, ethyl sulfide, acetylene, or propadiene... 

Use of triethylaluminum to maintain the activity of cobalt molybdate catalyst for disproportionation of 1-butene is reported in a patent issued to Shell International 41 Tributylphosphine was used with tungsten oxide-silica cata-... 

Clark and Cook 71) disproportionated [1-14C] propylene and [2-14C] propylene over cobalt oxide-molybdate-alumina catalyst. At 60 °C their results were consistent with those reported Mol and coworkers, confirming the four-center mechanism. At temperatures above 60 °C, double-bond isomerization activity of the cobalt-molybdate catalyst became a factor and at 160 °C nearly one-half of [l-l4C] propylene had isomerized to [3-14C] propylene prior to disproportionation. The authors note that at temperatures where isomerization does not occur, the possibility of a jr-allyl intermediate appears to be excluded however, at higher temperatures, the 77-allyl mechanism cannot be so easily dismissed. 

Cobalt Molybdate Catalyst. Sequence of Reactions. Similar reactions were observed over the more active commercial cobalt molybdate catalyst. This effected 82% conversion of a 5-/d. thiophene shot at 400° C. 

Effect of Temperature on Reactions. The hydrocarbon products given by single shots or slugs of thiophene on cobalt molybdate catalyst were analyzed at various temperatures in the range 400° to 274° C. There was little change in percentage unsaturation or in the relative amounts of the various C4 olefins pro-... 

Adsorption Studies on Cobalt Molybdate Catalyst. Thiophene and C4 Hydrocarbons. The peak delay due to slow desorption of thiophene and its C4 product from the catalyst in single-shot experiments was investigated further, as it seemed probable that the desorption was slow enough to have a considerable effect on the over-all rate of reaction. The delay in appearance of the peak maximum... 

Figure 8. Effect of sample size on peak delay 3.6 grams of cobalt molybdate catalyst, 54 mm. deep, 5.8 liters of H per hour...

Examination of the reaction products indicated that the primary products of reaction were probably butadiene and H2S. The rates of hydrogenation of butadiene and butene were found to be consistent with the amounts appearing in the reaction products (provided, in the case of cobalt molybdate catalyst, that H2S was present to simulate reaction conditions). The results support the view that C-S bond cleavage is the first step in thiophene desulfurization, rather than hydrogenation of the ring. 

The flow reaction over cobalt molybdate catalyst had an apparent activation energy of 25 kcal. per mole. Shot reactions showed comparatively high conversions at low temperatures, due possibly to the absence of blocked sites. 

Lapidus4 Spherical glass beads. 3.5 mm diameter Cobalt molybdate catalyst cylinders. 0.32 cm diameter 5.1 cm 91.5 cm... 

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