Carbon dioxide reduction kinetic process

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

The redox conditions in the environment will determine which carbon species are thermodynamically stable carbon dioxide under oxidative conditions, and elementary carbon and hydrocarbons under reductive conditions. The oxidation state of organic substances is always between these two thermodynamically stable species. Since these organic substances are thermodynamically not stable, they are driven to transform to carbon dioxide or to elementary carbon/hydrocarbons, which processes, however, are limited by kinetic barriers. 

To our knowledge, only one report exists in which the formation of carboxylic acid by radical carboxylation with carbon dioxide has been documented. Curran and co-workers observed the formation of 9-anthracenecarboxylic acid in 10% yield, together with 71% yield of anthracene, when the radical reduction of 9-io-doanthracene with the ethylene-spaced fluorous tin hydride was run using supercritical CO2 (90°C, 280 atm) as the reaction media (Scheme 4-41) [69], As demonstrated in this example, the CO2 trapping reaction by radicals is not an efficient process and therefore is of limited synthetic utility. The rate constant for the addition is yet to be determined, but kinetic studies to date indicate that generally the decarboxylation of acyloxyl radicals is a rapid process [70]. 

The reverse-flow chemical reactor (RFR) has been shown to be a potentially effective technique for many industrial chemical processes, including oxidation of volatile organic compounds such as propane, propylene, and carbon monoxide removal of nitrogen oxides sulfur dioxide oxidation or reduction production of synthesis gas methanol formation and ethylbenzene dehydration into styrene. An excellent introductory article in the topic is given by Eigenberger and Nieken on the effect of the kinetic reaction parameters, reactor size, and operating parameters on RFR performance. A detailed review that summarizes the applications and theory of RFR operation is given by Matros and Bunimovich. 

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