Catalysts carbon dioxide reactions

Large amounts of styrene are commercially produced by dehydrogenation of ethylbenzene (EB) in the presence of steam using iron oxide-based catalysts. Carbon dioxide, small amounts of which are formed as a by-product in the ethylbenzene dehydrogenation, was known to depress the catalytic activity of commercial catalyst [7,8]. However, it has been recently reported that several examples show the positive effect of carbon dioxide in this catalytic reaction [5,9,10]. In this study, we investigated the effect of carbon dioxide in dehydrogenation of ethylbenzene over ZSM-5 zeolite-supported iron oxide catalyst. 

Although the reaction requires no external catalyst, carbon dioxide is activated by the interaction of its electrophilic carbon atom and the negatively polarized carbon in the ortho (or para) position of the phenolate ring. This mechanism is supported by the fact that even under high CO2 pressure no salicylic acid is formed from phenol. The product is stabilized via an a > y proton migration. The free acid is obtained from the sodium salt in reaction with an external proton (usually from sulfuric acid). Formally, this reaction can be regarded as the insertion of CO2 into an aromatic C-H bond however, the above mechanism disproves this idea. 

However, more recent investigations with CO/CO2/H2 mixtures have shown that the active catalyst is finely divided copper on the smface of the catalyst and that ZnO plays no particular role in the industrial catalyst. Carbon dioxide plays a key role here (Scheme 8-4) [T22]. All steps take place on copper surfaces. The hydroge-nolysis of formate on copper surfaces has been proven. It is now assumed that the function of CO is to remove atomically boimd oxygen from the surface with formation of CO2, which then act as the primary reactant. This example shows once again that it is not possible to formulate a mechanism simply by combining apparently plausible reaction steps. 

Unfortunately, the use of ethanol in fuel cells is connected with a problem not yet solved. Experimental data show that during anodic oxidation of ethanol on platinum catalysts, carbon dioxide is not the only reaction product, but also appreciable... 

S.2 Removal of carbon monoxide and dioxide. Carbon monoxide and carbon dioxide must be removed before the synthesis reaction because of their effects on the synthesis reaction and on the catalyst. Carbon dioxide can be removed by physical or chemical means. Physical means are normally used when the partial oxidation process is employed (section 3.3.3.4). In the steam reforming process we normally adopt chemical methods of carbon dioxide removal. The chemical reagents involved are ... 

This condition is most damaging under open circuit conditions when no load is applied and the cell voltage is close to one volt Under high potentials, the catalyst carbon support experiences oxidizing conditions, initially resulting in an increase in oxygen surface species and associated increase in hydrophilicity. As the carbon surface further oxidizes, carbon is converted to carbon dioxide (reaction 6.4), the Pt particle cormections are severed, the catalyst layer collapses and the catalyst layer dramatically thins. 

Carbon monoxide and carbon dioxide cause the temporary deactivation of ammonia catalysts. Carbon dioxide can also lead to further problems because it forms ammorrittm carbonate in the make-up gas corrrpressor and the synthesis loop. The removal of these impurities is, therefore, a vital step in the prrrification of synthesis gas. Removal of carbon dioxide has generally been via absorption in some srritable solvent, whereas at the present time, the concentration of carbon monoxide is redtrced to a low level by reaction with steam in the water gas shift reactiorr, prior to almost complete removal by an additional procedtrre. 

The complete assembly for carrying out the catalytic decomposition of acids into ketones is shown in Fig. Ill, 72, 1. The main part of the apparatus consists of a device for dropping the acid at constant rate into a combustion tube containing the catalyst (manganous oxide deposited upon pumice) and heated electrically to about 350° the reaction products are condensed by a double surface condenser and coUected in a flask (which may be cooled in ice, if necessary) a glass bubbler at the end of the apparatus indicates the rate of decomposition (evolution of carbon dioxide). The furnace may be a commercial cylindrical furnace, about 70 cm. in length, but it is excellent practice, and certainly very much cheaper, to construct it from simple materials. 

Decomposition Reactions. Minute traces of acetic anhydride are formed when very dry acetic acid is distilled. Without a catalyst, equiUbrium is reached after about 7 h of boiling, but a trace of acid catalyst produces equiUbrium in 20 min. At equiUbrium, about 4.2 mmol of anhydride is present per bter of acetic acid, even at temperatures as low as 80°C (17). Thermolysis of acetic acid occurs at 442°C and 101.3 kPa (1 atm), leading by parallel pathways to methane [72-82-8] and carbon dioxide [124-38-9] and to ketene [463-51-4] and water (18). Both reactions have great industrial significance. 

Formaldehyde is readily reduced to methanol by hydrogen over many metal and metal oxide catalysts. It is oxidized to formic acid or carbon dioxide and water. The Cannizzaro reaction gives formic acid and methanol. Similarly, a vapor-phase Tischenko reaction is catalyzed by copper (34) and boric acid (38) to produce methyl formate ... 

The carboaylatioa of methanol to give formic acid is carried out ia the Hquid phase with the aid of a basic catalyst such as sodium methoxide. It is important to minimi2e the presence of water and carbon dioxide ia the startiag materials, as these cause deactivatioa of the catalyst. The reactioa is an equHibrium, and elevated pressures are necessary to give good conversions. Typical reaction conditions appear to be 80°C, 4.5 MPa (44 atm) pressure and 2.5% w/w of catalyst. Under these conditions the methanol conversion is around 30% (25). 

In addition to the processes mentioned above, there are also ongoing efforts to synthesize formamide direcdy from carbon dioxide [124-38-9J, hydrogen [1333-74-0] and ammonia [7664-41-7] (29—32). Catalysts that have been proposed are Group VIII transition-metal coordination compounds. Under moderate reaction conditions, ie, 100—180°C, 1—10 MPa (10—100 bar), turnovers of up to 1000 mole formamide per mole catalyst have been achieved. However, since expensive noble metal catalysts are needed, further work is required prior to the technical realization of an industrial process for formamide synthesis based on carbon dioxide. 

Butane-Based Fixed-Bed Process Technology. Maleic anhydride is produced by reaction of butane with oxygen using the vanadium phosphoms oxide heterogeneous catalyst discussed earlier. The butane oxidation reaction to produce maleic anhydride is very exothermic. The main reaction by-products are carbon monoxide and carbon dioxide. Stoichiometries and heats of reaction for the three principal reactions are as follows ... 

Because of the delay in decomposition of the peroxide, oxygen evolution follows carbon dioxide sorption. A catalyst is required to obtain total decomposition of the peroxides 2 wt % nickel sulfate often is used. The temperature of the bed is the controlling variable 204°C is required to produce the best decomposition rates (18). The reaction mechanism for sodium peroxide is the same as for lithium peroxide, ie, both carbon dioxide and moisture are required to generate oxygen. Sodium peroxide has been used extensively in breathing apparatus. 

---