Hydrogen activating carbon monoxide

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

Fe/Ir catalysts In situ Fe and Ir Mossbauer spectroscopy of silica-supported Fe/Ir catalysts with different iron to iridium ratios following pretreatment in hydrogen show that the reduction of the Fe component is enhanced by the presence of Ir metal. The presence of Ir was found to increase the catalytic activity in hydrogenation of carbon monoxide and also to influence selectivity... 

Rhodium and cobalt participate in several reactions that are of value in organic syntheses. Rhodium and cobalt are active catalysts for the reaction of alkenes with hydrogen and carbon monoxide to give aldehydes, known as hydroformylation,281... 

Solutions of Ru3(CO)i2 in carboxylic acids are active catalysts for hydrogenation of carbon monoxide at low pressures (below 340 atm). Methanol is the major product (obtained as its ester), and smaller amounts of ethylene glycol diester are also formed. At 340 atm and 260°C a combined rate to these products of 8.3 x 10 3 turnovers s-1 was observed in acetic acid solvent. Similar rates to methanol are obtainable in other polar solvents, but ethylene glycol is not observed under these conditions except in the presence of carboxylic acids. Studies of this reaction, including infrared measurements under reaction conditions, were carried out to determine the nature of the catalyst and the mechanism of glycol formation. A reaction scheme is proposed in which the function of the carboxylic acid is to assist in converting a coordinated formaldehyde intermediate into a glycol precursor. 

The use of Ru(acac)3 under very high temperature (268 °C) and pressure (1300 bar of H2/CO) in THF provides a catalyst for the hydrogenation of carbon monoxide to methanol and methyl formate [73]. The active species is derived from Ru(CO)5. 

In order to further assess the effect of parametrical changes to the Rh-3-SlLP hydroformylation system, the ratio between the partial pressures of hydrogen and carbon monoxide (pH2 pco ratio) has been varied between 0.25 and 4 (at constant total pressure) in reactions performed at 65 and 100 °C [31]. Increasing the hydrogen partial pressure had a profound effect on the catalyst activity for both temperatures, as depicted in Fig. 5. 

Table I. Effect of the Addition of Cesium Ions on the Catalytic Activity and Selectivity of the Ru/Rh/HOAc System for the Hydrogenation of Carbon Monoxide ...

To exhibit such an active and selective catalytic effect, the catalyst must be a fairly good hydrogenation catalyst that is able to activate molecular hydrogen. It must also activate carbon monoxide without dissociating it. A nondissociative chemisorption permits the hydrogenation of carbon monoxide to occur on both oxygen and carbon. Considering the formation of surface methoxide in the second mechanism [Eq. (3.43)], a further requirement is that the catalyst not form a too stable metal methoxide. 

Throughout these studies, no product other than propane was observed. However, subsequent studies by Sinfelt et al. [249—251] using silica-supported Group VIII metals (Co, Ni, Cu, Ru, Os, Rh, Ir, Pd and Pt) have shown that, in addition to hydrogenation, hydrocracking to ethane and methane occurs with cobalt, nickel, ruthenium and osmium, but not with the other metals studied. From the metal surface areas determined by hydrogen and carbon monoxide chemisorption, the specific activities of... 

It is noteworthy that the substrates or products are dissolved gases hydrogen, oxygen, carbon monoxide, carbon dioxide, methane, ammonia. However, the enzymes show no common pattern, either in the chemical state of nickel or in the type of reaction catalyzed. Their nickel-containing sites are remarkably diverse (Table 1), and in four enzymes the active center comprises groups in addition to the nickel ion. 

The use of solutions of dicobalt octaearbonyl under hydrogen and carbon monoxide pressure has been described in detail (23,24) as effective in adding hydrogen or hydrogen and carbon monoxide to unsaturated organic compounds. Thus in the oxo reaction hydrogen as well as carbon monoxide is activated by what is believed to be homogeneous solutions of dicobalt octaearbonyl. 

Alternative techniques do exist, however, for obtaining information regarding the distribution and number of catalytic components dispersed within or on the support. Selective gas adsorption, referred to as chemisorption, can be used to measure the accessible catalytic component on the surface indirectly by noting the amount of gas adsorbed per unit weight of catalyst. The stoichiometry of the chemisorption process must be known in order to estimate the available catalytic surface area. One assumes that the catalytic surface area is proportional to the number of active sites and thus reaction rate. This technique has found use predominantly for supported metals. A gas that will selectively adsorb only onto the metal and not the support is used under predetermined conditions. Hydrogen and carbon monoxide are most commonly used as selective adsorbates for many supported metals. There are reports in the literature of instances in which gases such as NO and O2 have been used to measure catalytic areas of metal oxides however, due to difficulty in interpretation they are of limited use. 

The hydrogenation of carbon monoxide over the amorphous Fe2oNi6oP20 and FejoZrio catalysts was carried out at atmospheric pressure and at temperatures from 220 to 370°C. The amorphous catalysts exhibited stable and high activities higher than the crystalline catalysts of the same compositions. 

---