Methoxy intermediate

Figure 13.47 Mechanism for disproportionation of meto-xylene to toluene and trimethylben-zene via methoxy intermediate.

Research activity on methanol has been vigorous because of its commercial importance as an alternative feedstock in fuel cells. When CH3OH is chemisorbed on a catalytic surface at ambient temperatures, it is usually present as a methoxy intermediate the latter then undergoes extensive decomposition to yield a product distribution that depends upon the temperature. A tabulation of products generated under various experimental conditions such as metal catalyst and decomposition temperature is given in Table 1 HREELS... 

The partial oxidation of methanol to formaldehyde over metal oxides is important industrially, and the results of experiments done in the transient regime are summarized for M0O3 (249). The results in Fig. 41 show that the conversion of methanol and the selectivity toward formaldehyde in a Mars-van Krevelen process are both favored by a relatively oxidized surface. Weber (250) has shown by theoretical calculations that the surface methoxy intermediate should lose one of its hydrogens as a hydride ion attracted to Mo and not as a hydrogen atom or proton attracted to a doublely bonded or bridging oxygen. 

Surface-bound methoxy, CH3O, is an intermediate in a variety of surface processes in catalysis and electrocatalysis involving methanol. The chemistry of methoxy on Pt(lll) and the Sn-alloys had been elusive because of the difficulty of cleanly preparing adsorbed layers of methoxy. One approach is to use the thermal dissociation of an adsorbed precursor, methyl nitrite (CH O-NO), to produce methoxy species on such surfaces at temperatures lower than required for methoxy formation from methanol [58, 59]. The methoxy intermediate is strongly stabilized (to 300 K) against thermal decomposition on both Sn/Pt(lll) alloys, whereas on Pt(lll), dissociation occurs below 140 K. There is a high selectivity to formaldehyde, CHjO, on both alloys, i.e., methoxy disproportionates to make equal amounts of formaldehyde and methanol. The two Sn/Pt(lll) alloys do not form CO and products characteristic of methoxy decomposition on Pt(l 11). 

Peck JW, Beck D, Mahon D, Panja C, Saliba N, Koel BE (1998) Methyl nitrite adsorption as a novel route to the surface methoxy intermediate. J Phys Chem 102 3321... 

Under UHV conditions, methanol can be desorbed dissociatively as carbon monoxide and molecular hydrogen at temperatures between 200 and 300 K. A stable surface methoxy intermediate (CH30-) is formed by the scission of the O-H bond [115]. The Pt(110) (1 x 2) reconstructed surface is the only plane to show the stable methoxy species co-adsorbed with the oxygen, whereas on the (111) and (100) platinum planes the surface combination leads to the formation of carbon dioxide [138]. 

The TPRS spectra reveal that the decomposition of the surface methoxy intermediates... 

The kinetics of methanol oxidation over metal oxide catalysts were elegantly derived by Holstein and Machiels [16], The kinetic analysis demonstrated that the dissociative adsorption of water must be included to obtain an accurate kinetic model. The reaction mechanism can be represented by three kinetic steps equilibrated dissociative adsorption of methanol to a surface methoxy and surface hydroxyl (represented by K,), equilibrated dissociative adsorption of water to two surface hydroxyls (represented by K ), and the irreversible hydrogen abstraction of the surface methoxy intermediate to the formaldehyde product and a surface hydroxyl (the rate determining step, represented by kj). For the case of a fully oxidized surface, the following kinetic expression was derived ... 

In the above kinetic expression, the Arrhenius rate constant, 1, is modified by the two adsorption equilibrium constants of methanol and water during the steady state kinetic studies. During the transient TPRS experiments, however, only the Arrhenius rate constant, kj, is measured since there is no vapor phase methanol or water to be equilibrated. The similar TPRS results for the oxidation of the surface methoxy intermediate to formaldehyde over the different supported vanadia catalysts reveal that all the catalysts possess the same kj. Consequently, the dramatic differences in the steady state TOFs during methanol oxidation over the different supported vanadia catalysts must be associated with the methanol and water equilibrium adsorption constants. Both methanol and water can be viewed as weak acids that will donate a proton to a basic surface site, but methanol is more strongly adsorbed than water on oxide surfeces, K, > [16]. Furthermore, the methanol oxidation TOFs... 

Methanol-like or methoxy intermediates undergo a CO insertion and are thereby transformed into C2 intermediates. (4) Two molecules of CO on the same site combine into C2 species. (5) Methanol undergoes a consecutive condensation reaction in the adsorbed state with another methanol molecule, or with another alcohol, to produce higher alcohols. 

In the case study for methanol dehydrogenation, ab initio electrochemical calculations identified the presence of a dual reaction pathway that proceeds through the formation of the hydroxymethyl intermediate as the dominant path to form CO and the formation of the methoxy intermediate as the path to form formaldehye. Differences in the electronic structure of the adsorbed methoxy and hydroxymethyl ultimately control the onset of the dual path at potentials lower than about 0.5 V. While these results are encouraging, theory has only just begun to tackle the complexity of the electrocatalytic systems. 

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