Methanol oxidation formaldehyde

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

Oxidation of methanol to formaldehyde with vanadium pentoxide catalyst was first patented in 1921 (90), followed in 1933 by a patent for an iron oxide—molybdenum oxide catalyst (91), which is stiU the choice in the 1990s. Catalysts are improved by modification with small amounts of other metal oxides (92), support on inert carriers (93), and methods of preparation (94,95) and activation (96). In 1952, the first commercial plant using an iron—molybdenum oxide catalyst was put into operation (97). It is estimated that 70% of the new formaldehyde installed capacity is the metal oxide process (98). 

A third possible route is to produce formaldehyde from methyla1 that is produced from methanol and formaldehyde (112,113). The incentive for such a process is twofold. Eirst, a higher concentrated formaldehyde product of 70% could be made by methyla1 oxidation as opposed to methanol... 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

However, this advance has an important shortcoming the lack of context. More than one idea is expressed in a document a patent on oxidation catalysts, for example, could include examples of the oxidation of methanol to formaldehyde and of 2-propanol to acetone. A simple coordinate search for conversion of methanol to acetone would retrieve such a document from a file that provides no context. 

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equihbrium Ethane to ethylene, propane to propylene, water-gas shirt reaction CO -I- H9O CO9 + H9, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

Liver alcohol dehydrogenase (ADH) is relatively nonspecific and will oxidize ethanol or other alcohols, including methanol. Methanol oxidation yields formaldehyde, which is quite toxic, causing, among other things, blindness. Mistaking it for the cheap... 

Since the dichromate ion on the left side of the equation has been reduced to chromic ion, Cr+ on the right side, the conversion of methanol to formaldehyde must involve oxidation. To show more clearly that methanol has been oxidized, let us balance this reaction by the method of half-reactions. We have encountered the halfreaction involving dichromate and chromic ions before (Problem 20b in Chapter 12). It is... 

This equation shows that the methanol molecule has lost electrons and thus has been oxidized. Formaldehyde is the second member in the oxidation series of methane. 

From this completed half-reaction we see that the conversion of methanol to formic acid involves the loss of four electrons. Since the oxidation of methanol to formaldehyde was only a two-electron change, it is clear that formic acid is a more highly oxidized compound of carbon than formaldehyde or methanol. 

Methanol oxidation on Pt has been investigated at temperatures 350° to 650°C, CH3OH partial pressures, pM, between 5-10"2 and 1 kPa and oxygen partial pressures, po2, between 1 and 20 kPa.50 Formaldehyde and C02 were the only products detected in measurable concentrations. The open-circuit selectivity to H2CO is of the order of 0.5 and is practically unaffected by gas residence time over the above conditions for methanol conversions below 30%. Consequently the reactions of H2CO and C02 formation can be considered kinetically as two parallel reactions. 

MoVW-mixed oxide as a partial oxidation catalyst for methanol to formaldehyde... 

In this paper, the preparation, characterization and the catalytic performance of the Moo.esVoasWo.ioOx-mixed oxide as a partial oxidation catalyst for the methanol to formaldehyde reaction was studied. 

Conversion of methanol into formaldehyde by methanol dehydrogenase. A complex array of genes is involved in this oxidation and the dehydrogenase contains pyrroloquinoline quinone (PQQ) as a cofactor (references in Ramamoorthi and Lidstrom 1995). Details of its function must, however, differ from that of methylamine dehydrogenase that also contains a quinoprotein—tryptophan tryptophylquinone (TTQ). 

Redox reactions with metal porphyrins (MPs) as photocatalysts. A spectacular example here is the reaction that couples upon illumination with the sunlight, methanol oxidation to formaldehyde with the formation of hydrogen peroxide in be nzene-methanol mixture (90 10)... 

The oxidative dehydrogenation of methanol to formaldehyde is a model reaction for performance evaluation of micro reactors (see description in [72]). In the corresponding industrial process, a methanol-air mixture of equimolecular ratio of methanol... 

Figure 3.36 Arrhenius plot for the oxidative dehydrogenation of methanol to formaldehyde performed in a micro reactor [72].

The oxidative dehydrogenation of methanol to formaldehyde was choosen as model reaction by BASF for performance evaluation of micro reactors [1, 49-51, 108]. In the industrial process a methanol-air mixture of equimolecular ratio of methanol and oxygen is guided through a shallow catalyst bed of silver at 150 °C feed temperature, 600-650 °C exit temperature, atmospheric pressure and a contact time of 10 ms or less. Conversion amounts to 60-70% at a selectivity of about 90%. 

The oxidation of an undisclosed methanol derivative to the corresponding formaldehyde compound is a large-scale BASF process which was established in recent years, whereas the similar methanol-to-formaldehyde process, performed on a much larger scale, has been practised at BASF for more than 100 years [1,49-51, 108]. The exact nature of the substituent(s) was not disclosed by BASF for reasons of confidentiality, although many publications on that topic appeared. The nature of the substituent makes the derivative, as the results of the investigations show, more labile to temperature. 

A so-called direct pathway involving a more weakly adsorbed perhaps even partially dissolved intermediate. Likely candidates for such intermediates are formaldehyde and formic acid. The oxidation mechanism of formic acid is discussed in Section 6.3. The idea is that the formation of a strongly adsorbed intermediate is circumvented in the direct pathway, though in practice this has appeared difficult to achieve (the dashed line in Fig. 6.1). Section 6.4 will discuss this in more detail in relation to the overall reaction mechanism for methanol oxidation. 

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]). 

In this section, we present results of potentiodynamic DBMS measurements on the continuous (bulk) oxidation of formic acid, formaldehyde and methanol on a Pt/ Vulcan catalyst, and compare these results with the adsorbate stripping data in Section 13.3.1. We quantitatively evaluate the partial oxidation currents, product yields, and current efficiencies for the respective products (CO2 and the incomplete oxidation products). In the presentation, the order of the reactants follows the increasing complexity of the oxidation reaction, with formic acid oxidation discussed first (one reaction product, CO2), followed by formaldehyde oxidation (two reaction products) and methanol oxidation (three reaction products). 

Figure 13.3 Potentiodynamic electrooxidation of (a) formic acid, (b) formaldehyde, and (c) methanol on a Pt/Vulcan thin-film electrode (7 xgpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH (a), HCHO (b), or CH3OH (c). The potential scan rate was 10 mV s and the electrolyte flow rate was 5 p-L s at room temperature). The top panels show the faradaic current (solid lines), the partial currents for Ci oxidation to CO2 (dashed lines) and for formic acid formation (dash-dotted line), calculated from the respective ion currents, and the difference between the measured faradaic current and the partial current for CO2 oxidation (formic acid oxidation (a), formaldehyde oxidation (b)), or the difference between faradaic current and the sum of the partial currents for CO2 formation and formic acid oxidation (methanol oxidation, (c)) (dotted line). The solid lines in the lower panels in...

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

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