Methoxy species

At high alkali coverages (near monolayer coverage), when the adsorbed alkali overlayer shows a metal-like character, alkali-methoxy species are formed. As shown by TPD experiments in the system K/Ru(001) these alkali-methoxy species are more stable than the methoxy species on clean Ru(001) and adsorbed methanol on 0.1K/Ru(001). On metal surfaces inactive for methanol decomposition, e.g. Cu(lll), these alkali-methoxy species are formed even at low alkali coverages, due to the weaker interaction of the alkali atoms with the metal surface. The formation of these species stabilizes the methoxy species on the metal surface and enhances the activity of the metal surface for methanol decomposition. 

In 1978, Wachs and Madix34 drew attention to the role of oxygen in the oxidation of methanol being not completely understood at copper surfaces. They established the role of methoxy species as the favoured route to the formation of formaldehyde and that to a lesser extent some methanol was... 

Figure 4.8. v(OC) bands for methoxy species adsorbed on CZ-50/50 sample reduced at 673 K, then reoxidized at room temperature by adding successive doses of 02 (spectra a-n) [72]. 

Finocchio, E., Daturi, M., Binet, C. et al. (1999) Thermal Evolution of the Adsorbed Methoxy Species on CexZr, x02 Solid Solution Samples A FT-IR study, Catal. Today, 52, 53. 

REAPDOR method to measure the C-Al distances by numerical simulations of the time-dependence of the REAPDOR evolution effect. An intemuclear C-Al distance of 3.1 A was determined for a signal at 57.3 ppm in ZSM-5, which is in excellent agreement with quantum chemical calculations of surface methoxy species [232]. Larger C-Al distances (>4 A) were determined for nearby signals at 60.0 and 61.7 ppm. 

Within the literature, FTIR has been used to identify the various intermediate species formed over Au/Ti02 under both methanol decomposition and methanol reforming conditions.65 In the case of methanol decomposition, unlike methanol reforming, only methoxy species associated with Ti02 were observed. 

In 1999, Binet et al.395 published a review on the response of adsorbed molecules to the oxidized/reduced states of ceria. In light of recent infrared studies on ceria, the assignments for OH groups, methoxy species, carbonate species, and formates are highly instructive. The OH and methoxy species have been briefly discussed. Characteristic band assignments of carbonate and formate species are provided below, the latter formed form the dissociative adsorption of formic acid, the reaction of CO with H2-reduced ceria surface, or via selective oxidation of methanol. Formate band intensities were a strong function of the extent of surface reduction of ceria. 

The simultaneous desorption peaks observed at 560-580 K in TPR are of reaction-limited desorption. The peak temperatures of these peaks do not depend on the coverage of methoxy species, indicating that the desorption rate (reaction rate) on both surfaces has a first-order relation to the coverage of methoxy species. Activation energy (Ea) and the preexponential factor (v) for a first-order process are given by the following Redhead equation [12] ... 

Therefore, above 480 K only methoxy species are left on the surface. The major reaction path with 50% selectivity is formation of formaldehyde ... 

When the hydrogen atoms released from methoxy species in step 8.14 are trapped with extra oxygen atoms (step 8.10), step 8.11, and step 8.12 cannot be discriminated from each other. Alternatively, the hydrogen atoms react with OeH(a) to produce H20(g) ... 

Similarly, a double functionalization can be reached when an activating group is present in close vicinity to the triple bond. Tsuji et al. have discovered that with a diphosphine palladium(O) complex, a carbonate function in the a-position of the alkyne provides by decarboxylation a palladium methoxy species on which the alkyne moiety can be isomerized into an al-lenyl a -bonded group. CO insertion in the Pd - C bond, reductive elimination with the methoxy group and further cyclization with incorporation of a second CO molecule give rise to the corresponding cyclopentenone as shown in Scheme 21 [127]. 

The carbomethoxy cycle starts with the attack of a methoxy group at a coordinated carbonyl group or a migratory insertion of CO in a palladium methoxy bond. Any type of methoxy species will have a low concentration in the acidic medium of the reaction. In Figure 12.20 many details of these reactions, discussed above in section 12.2, have been omitted and only a shorthand notation is presented. Subsequently insertion of ethene takes place. It is known from stoichiometric experiments that both reactions are relatively slow. In the final step a formal protonation takes place, which as we saw before, may actually involve enolate species. 

The spectra obtained from the chemisorption of methanol onto catalyst above 100°C indicated the progressive oxidation of methoxy species to formate via dioxymethylene/HCHO and finally to CO, CO2, and H2. Phenol adsorbed on the surface Lewis acid-base pair site and dissociated to phenolate anion and proton. The formation of phenolate anion and proton were discerned from the strong intense C-0 stretching vibration and the disappearence of phenolic 0-H stretching vibration, respectively. Importantly, there were series of definite low intensity bands between 2050 and 1780 cm" that were identified as the out-of-plane aromatic C-H bending vibrations [79, 84-85]. These bending vibrations are possible only if the phenyl ring of phenol is perpendicular to the catalyst surface. 

A detailed investigation of aniline N-methylation on Cui xZnxFc204 was carried out through in situ FTIR spectroscopy. The reactants (aniline and methanol) and possible products (NMA, DMA and o-toluidine) were adsorbed on the catalysts and analyzed [106,107]. Adsorption of methanol indicated a dissociative chemisorption as methoxy species on catalyst surface at 100°C. As the temperature increased, oxidation of methoxy species to formaldehyde to dioxymethylene to formate species was observed, and above 300°C complete oxidation takes place to CO, CO2 and H2. Indeed methanol alone on Cui xZnxFc204 and Cui.xCoxFc204 behaves in a similar way [79,107]. 

The Sm silylamide surface species [(=SiO)Sm N(SiHMe2)2 (THF)] (28a-c, 29 and 30) (Scheme 12.11) readily undergo silylamido methanol and silylamido indenyl exchange reactions, affording materials [Sm(OMe)(THF)] MCM-41LP.280 (33c) and [Sm lnd(SiHMe2) (THF)J MGM-41LP.28o (34b) [126]. Notably, discrete Sm methoxy species are unknown in molecular chemistry due... 

Interestingly, the protodelithiation enthalpy of 2-lithio-l,3-dimethoxybenzene is very nearly the same as that for the single methoxy species o-lithioanisole. If the stabilization of lithium by an ortho ether group is due mainly to intramolecular complexation or... 

The most active and selective catalysts are Ni supported on LaGa03 and Rh supported on AI2O3. The reaction was suggested to proceed through the oxidation of methyl (-GH3) or decomposition of methoxy species (-OCH3) formed by dissociative adsorption of DME [229]. 

The methoxy species was detected by a reaction with acetic acid, ethyl iodide or water (or ethanol) giving methyl acetate, methyl ethyl ether or methanol, respectively. Formates were determined using dimethylsulfate (DMS). 

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