Carbon dioxide catalytic reduction

Besides chemical catalytic reduction of carbon dioxide with hydrogen, which is already possible in the laboratory, we are exploring a new approach to recycling carbon dioxide into methyl alcohol or related oxygenates via aqueous eleetrocatalytic reduction using what can be called a regenerative fuel cell system. The direct methanol fuel cell... 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

Reduction of carbon dioxide takes place at various metal electrodes. The main products are formic acid in aqueous solutions and oxalate, CO, and formic acid in nonaqueous solutions. An indium electrode is the most potential saving for C02 reduction. Due to the difference in optimum conditions between those for C02 reduction to formic acid and those for formic acid reduction to further reduced products, direct reduction of C02 in aqueous solutions without a catalyst to highly reduced products seems to be difficult at metal electrodes. However, catalytic effects of metal electrodes themselves have recently become more clear for example, on Cu, methane was detected, while on Ag and Au, CO was produced effectively in aqueous solutions. Furthermore, at a Mo electrode, methanol was obtained. The power efficiency is, however, still low at any electrode. 

Catalysis of carbon dioxide reduction thus appears as a chemical catalysis process in which the most important step is stabilization of the catalyst-substrate adduct rather than its decomposition, which closes the catalytic loop. With divalent cations, Scheme 4.8 applies. 

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

Scheme 103 Catalytic reduction of carbon dioxide to methanol.

Scheme 144 Catalytic cycle of cathodic carbon dioxide reduction with rhenium complexes to carbon monoxide.

To study the effect of the Ru/Al Oj catalyst on hydrogen yield for refomung of glucose in supercritical water, the experiments were compared to reactions with and without catalytic runs imder identical conditions. Typical product distributions are shown in Table 6.9 for experiments with and without a Ru/Al Oj catalyst at 973 K with 1 wt.% glucose feed (Byrd et al., 2007). There was a significant reduction in carbon monoxide and methane yields in the presence of the catalyst. The main products of the reaction were hydrogen, methane, carbon dioxide, and carbon monoxide. The low carbon monoxide yield (0.1% by vol.) indicates that the water-gas shift reaction approaches completion. 

The catalytic cycles for reduction of alkyl and atyl halides using Ni(o), Co(i) or Pd(o) species are interrupted by added carbon dioxide and reaction between the first formed carbon-metal bond and carbon dioxide yields an alkyl or aryl car-boxylate. These catalyses reactions have the advantage of occuriiig at lower cathode potentials than the direct processes summarised in Table 4.14. Mechanisms for the Ni(o) [240] and Pd(o) [241] catalysed processes have been established. Carbon dioxide inserts into the carbon-metal bond in an intermediate. Once the carboxy-late-metal species is formed, a further electron transfer step liberates the carboxy-late ion reforming the metallic complex catalyst. 

The classical preparation of alkyllithium compounds by reductive cleavage of alkyl phenyl sulfides with lithium naphthalene (stoichiometric version) was also carried out with the same electron carrier but under catalytic conditions (1-8%). When secondary alkyl phenyl sulfides 73 were allowed to react with lithium and a catalytic amount of naphthalene (8%) in THF at —40°C, secondary alkyllithium intermediates 74 were formed, which finally reacted successively with carbon dioxide and water, giving the expected carboxylic acids 75 (Scheme 30) °. 

The reductive ring opening of six-membered nitrogen-containing heterocycles was studied with A-phenyltetrahydroisoquinoline (391). Its lithiation with lithium and a catalytic amount of DTBB (4.5%) afforded the benzylic intermediate 392, which was allowed to react with electrophiles giving, after hydrolysis, functionalized amines 393 (Scheme 110) . It is noteworthy that in the reaction with carbon dioxide, instead of the corresponding lactam, amino acid 393 with X = CO2H was exclusively isolated. 

Low-valent rhenium complexes are effective in the catalytic reduction of carbon dioxide. The conversion can be accomplished photolytically or electrochemically and is of interest with regard to fuel production and greenhouse gas remediation [9]. Electrocatalytic reduction of CO2 to CO is initiated by the reduction of fac-Re(bpy)(CO)3Cl or a related complex and can be accomplished in homogeneous solution [54, 55] or on a polymer-modified electrode surface [56]. Catalytic current... 

Experimentation with the catalytic reduction of carbon dioxide has stemmed from several important factors. Its presence as a greenhouse gas within the atmosphere has led to the need to find effective methods to lower its concentration. Also, as... 

Often intertwined with the catalytic reduction of carbon dioxide is the reduction of protons to form hydrogen gas. This competitive process takes place because of the relatively modest cathode potential required for hydrogen evolution. Moreover, some catalysts can shift the potential needed for the evolution of hydrogen into the region of the catalytic reduction of carbon dioxide, thereby decreasing the efficiency for the desired process involving carbon dioxide. This problem was... 

On the other hand, in two other papers, the formation of hydrogen gas was not mentioned, whereas carbon monoxide and formic acid were both observed as products. In studies carried out by Ogura and coworkers [123], electrogenerated [Co(PPh3)2L] (where L is a substituted quinoline, bipyridine, or phenan-throline moiety) was employed as a catalyst for the reduction of CO2 in anhydrous organic solvents, conditions for which the current efficiency for production of CO (the main product) was 83%. Similarly, in an investigation done by Behar et al. [124], who used cobalt porphyrins as catalysts in an acetonitrile medium, the formation of both carbon monoxide and formic acid was noted however, the catalytic species did not appear to contain cobalt(I), but rather a cobalt(O) species complexed with carbon dioxide. 

Numerous methods for the synthesis of salicyl alcohol exist. These involve the reduction of salicylaldehyde or of salicylic acid and its derivatives. The alcohol can be prepared in almost theoretical yield by the reduction of salicylaldehyde with sodium amalgam, sodium borohydride, or lithium aluminum hydride by catalytic hydrogenation over platinum black or Raney nickel or by hydrogenation over platinum and ferrous chloride in alcohol. The electrolytic reduction of salicylaldehyde in sodium bicarbonate solution at a mercury cathode with carbon dioxide passed into the mixture also yields saligenin. It is formed by the electrolytic reduction at lead electrodes of salicylic acids in aqueous alcoholic solution or sodium salicylate in the presence of boric acid and sodium sulfate. Salicylamide in aqueous alcohol solution acidified with acetic acid is reduced to salicyl alcohol by sodium amalgam in 63% yield. Salicyl alcohol forms along with -hydroxybenzyl alcohol by the action of formaldehyde on phenol in the presence of sodium hydroxide or calcium oxide. High yields of salicyl alcohol from phenol and formaldehyde in the presence of a molar equivalent of ether additives have been reported (60). Phenyl metaborate prepared from phenol and boric acid yields salicyl alcohol after treatment with formaldehyde and hydrolysis (61). 

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