Ferrous chelates

A polymer-supported lipoamide-ferrous chelate system was used as catalyst for the reduction of diphenylacetylene to cis-stilbene with sodium borohydride the dithiol-iron(II) (1 1) complex formed was suggested to be the active species. The chitosanlipoamide system has the highest activity among various insoluble polymers investigated 95,96). 

The technical and economic aspects of wet flue gas simultaneous desulfurization and denitrification systems are presented so that their practicality for utilization by utility industry can be assessed. The emphasis is on the kinetics of the systems based on the employment of ferrous chelates to promote the solubility of NO and the reactivity of NO with SO2 in scrubbing liquors. Analytical techniques are developed for characterizing reaction intermediates and products. Alternative approaches and novel ideas that could develop into a more efficient and cost-effective scrubber system employing metal chelate additives are discussed. 

The development of more efficient ferrous chelates that can increase the binding rate and equilibrium constant with NO, and also the reaction rate of ferrous nitrosyl chelates with sulfite/bisulfite ion, would allow the employment of smaller absorbers, reducing tanks, and L/G (flow rate ratio of scrubbing liquors to flue gas) to achieve the same scrubbing efficiencies. The determination of optimum scrubbing conditions and chemistry such that the formation of undesirable products can be depressed or eliminated would allow the reduction of cost in the area of scrubbing liquor regeneration. This paper addresses the kinetics and thermodynamics of important reactions in-... 

Reversible Reaction of NO with Ferrous Chelates. The binding of NO to Fe +(L) can be expressed by the following equation ... 

Table I. Kinetic and Thermodynamic Data for Reversible NO Coordination to Ferrous Chelates...

While there is uncertainty in and for Fe2+(NTA)(NO) and Fe(II)(EDTA)(NO), the rate constants for reaction (1), Littlejohn and Chang (9), and Teramoto et al. (17) indicate that is on the order of 107 M-l sec l. The equilibrium constants Keq = k /k-i are fairly well established (10) as being about 10 at 25 °C, indicating that k i is about 10 sec . From this approximate value of k i and the consumption rate equation for NO + S032 , we can calculate a consumption rate for Fe2+(L)(NO). However, the calculated rate is considerably faster than the observed rate. The calculated rate was obtained assuming that the nitric oxide released by the ferrous chelate reacts at the rate for hydrated nitric oxide. 

A more likely assumption would be that the nitric oxide is not hydrated until it is released by the ferrous chelate. Since the hydration rate is so slow (18) (k 0.14 at 25 °C), the rate of reaction of Fe2+(L)(N0) + S03 no longer depends on k i, but on Keq and the hydration rate constant. Using this methodology, the calculated rates agree well with the observed rates for both Fe +(EDTA)(NO) and (NTA) (NO). For example, with initial concentrations of... 

Measurements of the absorption of nitric oxide by Fe2+(EDTA) solutions done by Teramoto et al. (17) allowed them to estimate k to be on the order of 10 M-1 sec for the EDTA complex. This indicates that NO does not need to become hydrated to bind to the ferrous chelate. This explains why Sada et al. (22) saw much more toO from the Fe2+(EDTA) + S032 solutions than from solutions of S032 alone. In the limited time available to extract NO from a gas bubble, the... 

Oxidation of Ferrous Chelates to Ferric Chelates. Flue gas contains about 5% oxygen. When dissolved, oxygen can oxidize ferrous ions to ferric ions which are inactive for coordination with NO. The oxidation rate of ferrous ions is accelerated in the presence of chelating agents, e.g., EDTA and NTA. This acceleration may be ascribed to the stabilization of the oxidized form by the chelation. 

The improvement over the existing Japanese processes can be made by developing a more efficient ferrous chelate such that it can provide better absorption efficiency for NO, faster reaction rates between NO and S02> and better stability for the ferrous chelate toward oxidation, compared to Fe +(EDTA) or Fe +(NTA) employed in Japanese processes. The development of an efficient and cost-effective method for the reduction of ferric chelate to ferrous chelate without producing dithionate ions could make the process attractive. In addition to these areas, the study of several alternative approaches and novel ideas could develop into a much more efficient and cost-effective scrubber system employing metal chelate additives. 

Immobilization of the ferrous chelate onto a solid substrate (27). If an efficient Immobilized ferrous-ion catalyst can be found, it could provide important Improvements over the presently available Japanese processes. These improvements include simplification in processes design for species separation, reduction of water, energy, and catalyst consumption, and reduction of operating costs. 

Demmink, J.F., van Gils, I.C.F, and Beenackers, A.C.M., 1997, Absorption of nitric oxide into aqueous solutions of ferrous chelates accompanied by instantaneous reaction, Ind. Eng. Chem. Res., 36, 4914-4927... 

The very sharp crystalline formation of the C. acidi-urici ferredoxin relative to that from C. pasteurianum has been correlated with their different amino acid compositions, in particular with the extra proline at site 16 (294). It is hence interesting that the kinetics of heat-inactivation at 70 °C indicates a higher stability for the former. Similarly, the aerobic reaction of the C. acidi-urici and C. tartarivorum ferredoxins with o-phenanthroline is faster for the most thermolabile of the two. This was reflected clearly in the temperature dependence of the reaction (285). Along these lines, Gillard et al. (295) have reported that iron is removed from native Peptostreptococcus elsdenii and from C. pasteurianum ferredoxins by o-phenanthroline while this ferrous chelator was found inactive on the C. acidi-urici protein. The magnitude of these differences needs further substantiation since other authors have claimed that o-phenanthroline can sequester iron from the C. acidi-urici protein as well (296). 

A number of processes currently undergoing development for simultaneous removal of SO2 and N0X have been based on either the oxidation of relatively insoluble NO to more soluble NO2 or the employment of a water-soluble ferrous-chelating compound as a catalyst to aid in the absorption of the insoluble NO. These ferrous compounds have the ability to form complexes with the NO and thus promote the absorption of the NO. Once in solution N0X can be reduced by the absorbed S02 to form molecular N2, N2O, or reduced nitrogen compounds such as N0H(S03)... 

Addition of a ferrous sequestering agent (e.g., porphyrins) can drastically shift the reduction potential toward more positive values and into the range of biological reductants, favoring reduction of the ferric ion. Addition of a ferrous chelator with formation constant greater than 10 theoretically allows transferrin (E TvTxjt -i. 9i... 

Co-factor studies have been performed for all the enzymes listed, except for dihydroxy benzoate extradiol dioxygenase. Tests for dependence on ferrous ions and reducing agents and of inhibition by ferrous chelators and oxidizing agents indicate the presence of iron in the ferrous state in the active forms of metapyrocatechase 116), protocatechuate 83,117), dihydroxykynurenate (2), homoprotocatechuate 118), carboxy-... 

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