Carbon as a reductant

Test-10 was done to demonstrate the technical feasibility of volatilizing zinc by feeding zinc oxide into a pre-melted Fe-S-O matte. A solid mixture consisting of 81% ZnO and 19% charcoal was injected into the bath at a feed rate of 50 g/min by air. The zinc solubility of the matte was about 0.9 wt% Zn at Pzn=0-1 atm and 1350°C. This test successfiilly demonstrated the feasibility of treating zinc oxide materials in the Fe-S-O matte by using carbon as a reductant and oxygen as the diluting gas. 

Otsuka, K., Murakoshi, S., Morikawa, A. (1983). Potential metal oxides for the production of hydrogen from water by a reduction-oxidation cycle using carbon as a reductant. Fuel Process Technology, 7, 203—211. 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

Hydroxylamines ordinarily do not accumulate in the reduction of aromatic nitro compounds for, with some exceptions, most systems in competition will reduce the hydroxylamine function preferentially. Nonetheless, systems have been found that afford the intermediate aromatic hydroxylamine in excellent yield. With hydrogen gas as a reductant and platinum-on-carbon or -on-alumina and about I wt % of DMSO based on nitro compound as a modifier, aromatic hydroxylamines can be formed in 90% yield under mild conditions. The reduction slows markedly after absorption of the second mole of hydrogen and should be stopped at this stage (80). 

This was also accomplished with BaRu(0)2(OH)3. The same type of conversion, with lower yields (20-30%), has been achieved with the Gif system There are several variations. One consists of pyridine-acetic acid, with H2O2 as oxidizing agent and tris(picolinato)iron(III) as catalyst. Other Gif systems use O2 as oxidizing agent and zinc as a reductant. The selectivity of the Gif systems toward alkyl carbons is CH2 > CH > CH3, which is unusual, and shows that a simple free-radical mechanism (see p. 899) is not involved. ° Another reagent that can oxidize the CH2 of an alkane is methyl(trifluoromethyl)dioxirane, but this produces CH—OH more often than C=0 (see 14-4). ... 

The structure of the products is determined by the site of protonation of the radical anion intermediate formed after the first electron transfer step. In general, ERG substituents favor protonation at the ortho position, whereas EWGs favor protonation at the para position.215 Addition of a second electron gives a pentadienyl anion, which is protonated at the center carbon. As a result, 2,5-dihydro products are formed with alkyl or alkoxy substituents and 1,4-products are formed from EWG substituents. The preference for protonation of the central carbon of the pentadienyl anion is believed to be the result of the greater 1,2 and 4,5 bond order and a higher concentration of negative charge at C(3).216 The reduction of methoxybenzenes is of importance in the synthesis of cyclohexenones via hydrolysis of the intermediate enol ethers. 

Butterworth, S.L. Granular activated carbon as a toxicity reduction technology for wastewater treatment. Proc. Am. Chem. Soc. Spring Natl. Meet. Fuel Chem. Div. 1996, 41 ), 466. 

The reaction of complex hydrides with carbonyl compounds can be exemplified by the reduction of an aldehyde with lithium aluminum hydride. The reduction is assumed to involve a hydride transfer from a nucleophile -tetrahydroaluminate ion onto the carbonyl carbon as a place of the lowest electron density. The alkoxide ion thus generated complexes the remaining aluminum hydride and forms an alkoxytrihydroaluminate ion. This intermediate reacts with a second molecule of the aldehyde and forms a dialkoxy-dihydroaluminate ion which reacts with the third molecule of the aldehyde and forms a trialkoxyhydroaluminate ion. Finally the fourth molecule of the aldehyde converts the aluminate to the ultimate stage of tetraalkoxyaluminate ion that on contact with water liberates four molecules of an alcohol, aluminum hydroxide and lithium hydroxide. Four molecules of water are needed to hydrolyze the tetraalkoxyaluminate. The individual intermediates really exist and can also be prepared by a reaction of lithium aluminum hydride... 

Fig. 28 Reductive electrochemistry data for (72). Cyclic voltammetric curves for a 0.1-mM CH2CI2 solution of (72) at 100 mV s , glassy carbon as a working electrode, Pt-mesh as a counter electrode, and a Ag wire as a quasi-reference electrode, T = 25 °C, TBAPFs (0.1 M) was used as supporting electrolyte.

Production. Silicon is typically produced in a three-electrode, a-c submerged electric arc furnace by the carbothermic reduction of silicon dioxide (quartz) with carbonaceous reducing agents. The reductants consist of a mixture of coal (qv), charcoal, petroleum coke, and wood chips. Petroleum coke, if used, accounts for less than 10% of the total carbon requirements. Low ash bituminous coal, having a fixed carbon content of 55—70% and ash content of <4%, provides a majority of the required carbon. Typical carbon contribution is 65%. Charcoal, as a reductant, is highly reactive and varies in fixed carbon from 70—92%. Wood chips are added to the reductant mix to increase the raw material mix porosity, which improves the SiO (g) to solid carbon reaction. Silica is added to the furnace in the form of quartz, quartzite, or gravel. The key quartz requirements are friability and thermal stability. Depending on the desired silicon quality, the total oxide impurities in quartz may vary from 0.5—1%. 

In pyrometallurgy1 7 12 13 (high temperature dry smelting), however, hydrogen is much less effective as a reductant, but carbon becomes important. For reasons we discuss in Section 17.8, the concentrate to be reduced is usually in the form of a metal oxide, so the reductant has to be something that has a greater affinity for the available oxygen atoms than has the... 

Figure 17.13 shows immediately that carbon is useless as a reductant for chlorides, since CCU is barely thermodynamically stable even at room temperature and becomes endergonic above about 400 °C. Similarly, CF4 is the least exergonically formed (per F atom) of all the fluorides covered by Fig. 17.13, and so carbon cannot reduce them. In principle, hydrogen could reduce titanium tetrachloride to titanium, but in reality titanium hydrides (Section 5.7) would be obtained. The only satisfactory reductants for metal chlorides and fluorides are magnesium and (less practically, on account of their extreme reactivity) sodium or calcium. 

---