Catalytic fixation

Catalytic fixation of carbon dioxide by metal complexes. S. Inoue and H. Koinama, Rev. Inorg. Chem., 1984, 6,291 (53). 

The difficulty to transform CO2 into other organic compounds lies in its high thermodynamic stability. Typical activation energies for the dissociation and recombination ofC02 are of 535 and 13 kJ/mol, respectively [5], The activation can occur by photochemical or electrochemical processes, by catalytic fixation or by metal-ligand insertion mechanisms. As documented in different reviews, organometallic compounds, metallo-enzyme sites and well defined metallic surfaces are able to activate carbon dioxide [6-16],... 

Catalytic fixation of molecular ni- 14 Pol. trogen by complexes of transi- (28) tion metals... 

Tlte catalytic fixation of carbon dioxide to formic acid is possible, using a combination of Group Vlil transition metal complexes and bases in the presence of water. Typical catalysts are Pdfdppe) and RuH (PPh3)4, but nickel, rhodium. 

Formic acid salts can also be prepared by the reaction of CO2 and II2 with an alkali metal manganese pcntacarbonyK such as NaMn(CO). in an inert solvent [164]. Magnesium format was synthesized directly from CO2, II3 and Mg under mild conditions 165], The catalytic fixation of Hj and CO2 was carried out in the TiCU 2THF/Mg/THF system and the following mechanism has been proposed (Scheme 4). 

Nature provides very interesting examples of catalytic fixation of both the entire carbon dioxide molecule and its reduced forms. The utilisation of either biosystems or mimetic complexes is very challenging for chemists. 

Reduction of carbon dioxide to graphite carbon via methane by catalytic fixation with membrane reactor... 

Fig. 1 Catalytic fixation of CO into carbon via methane with conventional fixed bed reactor, temperature at CO2 methanation was fixed at...

Carbon dioxide can effectively be added to the epoxide ring of GVE to produce the corresponding cyclic carbonate, OVE. Quaternary ammonium salt catalysts showed good catalytic activity even at atmospheric pressure of carbon dioxide. Since the blends of poly(OVE-co-AN) and SAN showed good miscibility, catalytic fixation of carbon dioxide to polymer blends via cyclic carbonate could be one of choice for the reduction and utilization of the greenhouse gcis. 

Several light-induced electron transfer cycles using transition metal polypyridyl complexes that lead to catalytic fixation of CO2 or CO have been identified [69,70]. Visible light photolysis of C02-saturated aqueous acetonitrile solutions containing Ru(bpy)32+ (as photosensitizer), Co(II) ions (as the catalyst), 4,7-Me2-phenanthroline (as the ligand to complex the Co(II) in situ), triethanolamine (as donor) yields a mixture of CO and H2 (synthetic gas). The syn gas mixture is produced by simultaneous occurrence of two reduction reactions ... 

Catalytic Incinerators. Catalytic incinerators, often used to remove hydrocarbons from exhaust gas streams, are more compact than direct-flame incinerators, operate at lower temperatures, often require Htfle fuel, and produce Httle or no NO from atmospheric fixation. However, the catalytic bed must be preheated and carefliUy temperature controlled. Thus these are generally unsuited to intermittent and highly variable gas flows. 

Figure 4.8 The active site in all a/p barrels is in a pocket formed by the loop regions that connect the carboxy ends of the p strands with the adjacent a helices, as shown schematically in (a), where only two such loops are shown, (b) A view from the top of the barrel of the active site of the enzyme RuBisCo (ribulose bisphosphate carboxylase), which is involved in CO2 fixation in plants. A substrate analog (red) binds across the barrel with the two phosphate groups, PI and P2, on opposite sides of the pocket. A number of charged side chains (blue) from different loops as welt as a Mg ion (yellow) form the substrate-binding site and provide catalytic groups. The structure of this 500 kD enzyme was determined to 2.4 A resolution in the laboratory of Carl Branden, in Uppsala, Sweden. (Adapted from an original drawing provided by Bo Furugren.)...

The chemical reactivity of coordinated N2 has been extensively studied because of its potential relevance to the catalytic and biological fixation of N2 to NH3 (p. 1035). For other recent work on the reactions of coordinated dinitrogen see refs. 41-44... 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

Paoletti et al. used a mixed aza oxo macrocycle (53) to form (yu3-C03) carbonate species on absorption of atmospheric C02. The crystal structure showed a trimer with threefold symmetry and six-coordinate zinc centers.461 This was described as C02 fixation however, three equivalents of zinc complex are required for each C02 molecule and so it is not a catalytic process. 

As described above, many reports published to date indicate that metal complexes are promising catalysts for C02 fixation. The catalytic activity is considered basically to be due to a C02-catalyst complex formation. Thus, the complexes have to provide a binding site for C02, and this can be realized for some catalysts by losing a ligand on reduction of the catalyst at the electrode. Also, the C02 molecule is not linear but is rather a bent structure155,156 in the activated state of the C02-catalyst complexes. Theoretical calculations of C02-catalyst bonding157 and general ideas about activation of C02 by metal complexes have been summarized in several recent articles.158,159... 

S. Ikeda, M. Yoshida, and K. Ito, Bull. Chem. Soc. Jpn. 58 (1985) 1353 S. Ikeda and K. Ito, Abstracts of the Symposium on Electrochemistry and Catalytic Process for Carbon Dioxide and Nitrogen Fixation, held at the Institute for Molecular Science, Okazaki, 1986, p. 9 (in Japanese). 

The kinetics of reaction of DABCO (7.66) and nicotinic acid (7.67 R = COOH) with the aminochlorotriazine dye Cl Reactive Red 3 (7.2) were studied under neutral conditions at temperatures in the range 100-130 °C. Quaternisation by DABCO was much more rapid than by nicotinic acid under these conditions. Neutral exhaust dyeing tests at 130 °C using the bis(aminochlorotriazine) analogue Cl Reactive Red 120 (7.48 X = Cl) with the two catalysts confirmed these trends, in that the degree of fixation was greatly increased by DABCO but nicotinic acid showed no appreciable catalytic effect [60]. This difference may be attributable to steric strain of the C-N+ bond in the quaternised triazine structure by the non-planar DABCO substituent. 

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