Reaction mechanism., production formaldehyde from methanol

In the following, after a brief description of the experimental setup and procedures (Section 13.2), we will first focus on the adsorption and on the coverage and composition of the adlayer resulting from adsorption of the respective Cj molecules at a potential in the Hup range as determined by adsorbate stripping experiments (Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants and the contribution of the different reaction products to the total reaction current under continuous electrolyte flow, first in potentiodynamic experiments and then in potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V, which was chosen as a typical reaction potential. The results are discussed in terms of a mechanism in which, for methanol and formaldehyde oxidation, the commonly used dual-pathway mechanism is extended by the possibility that reaction intermediates can desorb as incomplete oxidation products and also re-adsorb for further oxidation (for the formic acid oxidation mechanism, see [Samjeske and Osawa, 2005 Chen et al., 2006a, b Miki et al., 2004]). 

A simplified scheme of the dual pathway electrochemical methanol oxidation on Pt resulting from recent advances in the understanding of the reaction mechanism [Cao et al., 2005 Housmans et al, 2006] is shown in Fig. 15.10. The term dual pathway encompasses two reaction routes one ( indirect ) occurring via the intermediate formation of COads. and the other ( direct ) proceeding through partial oxidation products such as formaldehyde. 

Thus, many groups have sought alternative oxidants. A polyoxometaUate (POM) has been shown to act as a mediator of oxidation by 0 (Equation 18.9). In this case, the reaction of methane with O in the presence of Periana s catalyst supported on HjPVjMOjjO j as acid and mediator of oxidation has been reported to form a mixture of methanol and acetaldehyde. The mechanism of the formation of the acetaldehyde product from methane is not firm, but is proposed to occur by oxidative coupling of methane with formaldehyde, which would be generated from methanol. These reactions occur with modest turnover numbers of about 30, but the use of and a POM is a clear advance over the original Shilov process with platinum(IV) as the stoichiometric oxidant. 

Methanol homologation to hi er alcohols, in which the carbon being added to the alcohols comes from methanol, has been claimed in a noncatalytic reaction with metal acetylides [100]. For example, the reaction of methanol and CeC2 at 400°C and 0.1 MPa yielded alcohols up to pentanols, with a maximum selectivity for 2-methyl-1-propanol of 77%. The product distribution included a mixture of alcohols, CO, H2, and CH4. Depending on the contact time in the laboratory-scale test reactor, ethanol selectivities ran d from 1.3% (C atom) to 12.5% and 2-methyl-1-propanol selectivities ranged from 58 to 86%. Methanol conversion was <2%. Using 13C-labeled methanol. Fox et al [101] showed that methanol rather than the metal acetylide was the source of carbon in the higher alcohols. A formaldehyde condensation reaction mechanism has been invoked to e q)lain the 13C distribution in the product. 

Similar ideas can be applied to formaldehyde oxidation. For bulk formaldehyde oxidation, we found predominant formic acid formation under current reaction conditions rather than CO2 formation. Hence, it cannot be ruled out, and may even be realistic, that formaldehyde is first oxidized to formic acid, which can subsequently be oxidized to CO2. The steady-state product distribution at 0.6 V is much more favorable for such a mechanism as in the case of methanol oxidation. On the other hand, because of the high efficiency of COad formation from formaldehyde, this process is likely to proceed directly from formaldehyde adsorption rather than via formation and re-adsorption of formic acid. Alternatively, the second oxygen can be introduced via formaldehyde hydration to methylene glycol, which could be further oxidized to formic acid and finally to CO2 (see the next paragraph). 

The nitrophenyl radical can react with the iodide ion and solvent, methanol, as well. Transference of hydrogen radical from methyl alcohol to nitrophenyl radical gives rise to nitrobenzene and formaldehyde (CHjOH —> CH2O). Though carefully sought among the products of the reaction, 3-iodonitro-benzene and 4-nitroanisole were lacking. This completely rejects another possible mechanism of the reaction, cine-substitution, which involves the formation of dehydrobenzene as described earlier. 

The results from the infrared studies and from the GC analysis show that the reaction of methane with the ferric molybdate catalysts gives methanol, formaJdehyde, carbon dioxide, and carbon monoxide as final products. The IR spectra also indicate the formation of methoxy, surface dioxymethylene, surface formate species, and adsorbed formaldehyde. Based on these observations, a mechanism was proposed to account for all intermediates and final products and is shown in Figure 5. Since the surface structure of the catalysts is not known, the surface is represented by a straight line in the scheme. 

Both diazonium salts and iodonium salts can be effectively used as arylating agents. The aryl group is transferred either heterolyt-ically or homolytically, depending on the choice of salt and reaction conditions. In acid solution diazonium salts decompose relatively cleanly to give products consistent with a polar mechanism. In basic solution the product is a complex mixture resulting from free radical intermediates (6). When either benzenediazonium chloride or fluoborate is decomposed in acidic methanol, the major product, formed in 93% yield, is anisole, and less than 1% biphenyl is isolated. In the same solvent with an acetate buffer, the product contains 85-90% benzene, 4.5% biphenyl, 0.6% azobenzene, some anisole, and 80-90 mole % formaldehyde per mole of diazonium salt decomposed (7). 

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