Methanol oxidation pathways

Blume R, Havecker M, Zafeiratos S, Teschner D, Vass E, Schnorch P, Knop-Gericke A, Schlogl R, Lizzit S, Dudin P, Barinov A, Kiskinova M (2007) Monitoring in situ catalytically active states of Ru catalysts for different methanol oxidation pathways. Phys Chem Chem Phys 9 3648-3657... 

Respiratory, or oxidative, metaboHsm produces more energy than fermentation. Complete oxidation of one mol of glucose to carbon dioxide and water may produce up to 36 mol ATP in the tricarboxyHc acid (TCA) cycle or related oxidative pathways. More substrates can be respired than fermented, including pentoses (eg, by Candida species), ethanol (eg, by Saccharomjces), methanol (eg, by Hansenu/a species), and alkanes (eg, by Saccharomjces lipoljticd). 

Ab initio methods allow the nature of active sites to be elucidated and the influence of supports or solvents on the catalytic kinetics to be predicted. Neurock and coworkers have successfully coupled theory with atomic-scale simulations and have tracked the molecular transformations that occur over different surfaces to assess their catalytic activity and selectivity [95-98]. Relevant examples are the Pt-catalyzed NO decomposition and methanol oxidation. In case of NO decomposition, density functional theory calculations and kinetic Monte Carlo simulations substantially helped to optimize the composition of the nanocatalyst by alloying Pt with Au and creating a specific structure of the PtgAu7 particles. In catalytic methanol decomposition the elementary pathways were identified... 

The elementary reaction energies and thermodynamics for methanol dehydrogenation have been shown to be significantly influenced by electrode potential. The oxidation pathways become much more favorable at higher potentials. The relative barriers of O—H to C—H bond activation decrease with increasing potential, which decreases the overall selectivity to CO and CO2 and increases the yield of formaldehyde. This is consistent with experimental studies. The oxidation of CO intermediates appears to occur via adsorbed hydroxyl intermediates. The hydroxyl intermediates are more weakly held to the surface than atomic oxygen, and thus have significantly lower barriers for the oxidation of CO. 

Perhaps the most important paradigm in research on the mechanism of the electrocatalytic oxidation of small organic molecules is the dual pathway mechanism introduced in Capon and Parsons [1973a, b], and reviewed in Parsons and VanderNoot [1988]. In terms of methanol oxidation, the dual pathway may be summarized in a simplified way by Fig. 6.1. The idea is that the complete oxidation of methanol to carbon dioxide may follow two different pathways ... 

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. 

Batista EA, Malpass GRP, Motheo AJ, Iwasita T. 2003. New insight into the pathways of methanol oxidation. Electrochem Commun 5 843-846. 

For both methanol oxidation and formic acid oxidation, a dual-pathway mechanism has been proposed (for methanol oxidation, see Lamy et al. [1983] Jarvi and Stuve [1998] Cuesta [2006] Housmans et al. [2006] Iwasita [2003] for formic acid oxidation, see Parsons and VanderNoot [1988] Sun et al. [1988] Willsau and Heitbaum [1986] Miki et al. [2002] Samjeske and Osawa [2005] Chen et al. [2006a, b, c] Samjeske et al. [2005, 2006] Miki et al. [2004], Chang et al. [1989]), in which one reaction pathway proceeds via formation and subsequent oxidation of COad (P, indirect pathway ), while the other leads, via one or more reaction intermediates RI, directly to CO2 ( direct pathway ) (Fig. 13.8a). 

Whereas in the indirect pathway, COad is clearly identified as a reaction intermediate, the specific nature of the intermediate(s) in the direct pathway is under debate. For methanol oxidation, species such as COH [Xia et al., 1997 Iwasita et al., 1987, 1992 Iwasita and Nart, 1997], CHO [Zhu et al., 2001 Willsau and Heitbaum, 1986 Wilhehn et al., 1987], COOH [Zhu et al., 2001], and adsorbed formate species [Chen et al., 2003] have been proposed. Adsorbed formate species were identified during formaldehyde oxidation [Samjeske et al., 2007], methanol oxidation [Nakamura et al., 2007 Chen et al., 2003, unpublished], and fornfic acid oxidation [Miki et al., 2002, 2004 Samjeske and Osawa, 2005 Chen et al., 2006a, b, c Samjeske et al., 2005, 2006]. 

In the original proposal of the dual-pathway mechanism (for formic acid oxidation, see [Capon and Parsons, 1973a, b, c] for methanol oxidation, see [Parsons and VanderNoot, 1988 Jarvi and Stuve, 1998 Leung and Weaver, 1990 Lopes et al., 1991 Herrero et al., 1994, 1995]), both pathways lead to CO2 as the final product, as illustrated in the reaction scheme depicted in Fig. 13.8a [Jarvi and Smve, 1998]. In this mechanism, desorption of incomplete oxidation products was not included. The existence of a direct reaction pathway for methanol oxidation, following the dual-pathway mechanism, was justified by the observation of a methanol oxidation current at potentials where COad oxidation is not yet active [Sriramulu et al., 1998, 1999 Herrero et al., 1994, 1995]. The validity of this interpretation was questioned, however, by Vielstich and Xia (1995), who claimed that CO2 formation is observed only with the onset of COad oxidation and that the faradaic current measured at lower potentials is due to the formation of the incomplete oxidation products formaldehyde and formic acid. The latter findings were later confirmed by Wang et al. [2001], Korzeniewski and Childers [1998], and Jusys et al. [2001, 2003]. In more... 

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. 

Methanol still proceeds through an initial C H bond scission, but reacts with water before the OH bond breaks. Alternatively, formaldehyde formation likely occurs along the same pathway as CO formation. This is true if HCO is an intermediate in the decomposition pathway. Furthermore, the lack of a kinetic isotope effect for CH3OD indicates that formaldehyde is not the product of an initial O-H scission.94 Because formaldehyde and formic acid are not the thermodynamically favored products of methanol oxidation, they must be the result of kinetic limitations preventing the full oxidation to C02, analogous to the production of H202 for the reduction of oxygen (see next section). 

T. H. M. Housmans, A. H. Wonders, and M. T. M. Koper. Structure sensitivity of methanol electro-oxidation pathways on platinum An online electrochemical mass spectrometry study. Journal of Physical Chemistry B 110 (2006) 10021-10031. 

The third assumption above is that methanol chemisorbs at lower potentials essentially solely to give the chemisorbed intermediate(s), with only a negligible quantity of charge being associated with either any parallel oxidation pathway or with further conversion of the intermediate to C02. This problem was first tackled by Breiter [27-30], who suggested a parallel mechanism rather than a serial one at lower potentials, with the formation of a second active intermediate in a scheme of the form... 

Below 0.45 V the chemisorbed intermediates formed on methanol adsorption are stable on any smooth platinum surface, with the steady-state current for methanol oxidation being extremely small. Above this potential, oxidation of methanol takes place at a rate that increases exponentially with potential, with the product being primarily C02. In addition, above potentials of approximately 0.6 V, the surface is steadily stripped of adsorbed carbon-containing species, with the loss of such species being complete near 0.8 V. It would seem likely on most surfaces that it is oxidation of COads or =C-OH in a sequential reaction pathway that leads to C02, but more active intermediates, such as CO adsorbed at less stable sites, such as those at the edges... 

Bronkema and Bell (2007) analyzed the Raman bands of surface methoxy species and of supported vanadia to elucidate the mechanism of methanol oxidation to formaldehyde. In their detailed investigation, insight from Raman spectroscopy was combined with information from EXAFS and XANES spectroscopies. The authors discussed the reaction pathways in the presence and absence of 02, and identified the roles of various lattice oxygen sites. Formaldehyde was found to decompose to H2 and CO in the absence of 02 (Bronkema and Bell, 2007). Similar observations were reported by Korhonen et al. (2007) for methanol conversion on supported chromia catalysts. 

Fig. 1. The pathways of methanogenesis. Intermediates are abbreviated as in the text. The thick lines indicate the pathway for H2-CO2 methanogenesis, which is also in common to some extent with methanogenesis from one or more other substrates. The thin lines indicate specialized portions of pathways of methanogenesis from methanol, formate, and acetate, (a) Two different dehydrogenases have been reported, one dependent on H2F420, and one dependent on H2. (b) The source of these electrons may be H2F420 in some cases, but in other cases it is unknown see the text, (c) This is a possible alternative for methanol oxidation to the methylene-RjMPT level see the text for details.

Methanopterin (or a variant) is found in all methanogens, except possibly at very low levels in Methanosphaera stadtmanii[A6, %, 9]. This latter organism produces methane solely by the reduction of methanol to methane, with no methanol oxidation, and thus needs no methanopterin in the methanogenic pathway (however, it may need a low level for biosynthetic needs, e.g. in synthesis of serine or glycine [120]). 

Methenyl-H4MPT cyclohydrolase is essential for H2—CO2 methanogenesis. Pathway analysis and enzyme level studies suggest that this enzyme is also involved in methanol oxidation [224,346]. In acetate catabolism it does not have an obvious role, and in Methanosarcina growing on acetate this enzyme is down-modulated [224,346]. 

Liver is the principal site of chloroform metabolism which involves two major pathways, both of which are catalyzed by the cytochrome P-450 enzymes in the presence of NADPH. The oxidative pathway produces phosgene and the reductive pathway produces the dichloromethyl free radical. Other metabolites of chloroform include chloro-methanol, hydrochloric acid, hydrogen chloride, and digluathionyl dithiocarbonate, with carbon dioxide as the predominant end product of metabolism. 

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