Methanol methane-formaldehyde mechanism

A new mechanism, called the methane-formaldehyde mechanism, has been put forward for the transformation of the equilibrium mixture of methanol and dimethyl ether, that is, for the formation of the first C-C bond.643 This, actually, is a modification of the carbocation mechanism that suggested the formation of ethanol by methanol attaching to the incipient carbocation CH3+ from surface methoxy.460,462 This mechanism (Scheme 3.3) is consistent with experimental observations and indicates that methane is not a byproduct and ethanol is the initial product in the first C-C bond formation. Trimethyloxonium ion, proposed to be an intermediate in the formation of ethyl methyl ether,447 was proposed to be excluded as an intermediate for the C-C bond formation.641 The suggested role of impurities in methanol as the reason for ethylene formation is highly speculative and unsubstantiated. 

In the proposal for the methane-formaldehyde mechanism, methane and formaldehyde are formed from the surface methoxy intermediates and react to form ethanol [83-86]. Tajima et al. [83] used theoretical calculations to indicate that CHyHCHO are also potential reaction intermediates, van Santen with Blaszkowski [86] have theoretically found a transition state for hydride transfer between methanol and surface methoxy to form methane and potentially formaldehyde. Because methane is very slow to react, almost all conventional experimental and theoretical studies have supposed methane to be independent of the first C-C bond... 

The carbon dioxide photoreduction upon visible light has been performed using Pt-CdS-Ru02 powder (particles <1 pm) and K[Ru(H-edta)Cl] 2H20 in C02 saturated water solution [93], Formic acid, formaldehyde as the main products and trace amounts of methanol, methane, and carbon monoxide have been identified. Also hydrogen and oxygen as products of water decomposition were evolved. The proposed mechanism of the process is given below (L = H-edta) ... 

The oxidation of methane and other light alkanes in the gas phase at high temperature gives alcohols, aldehydes and ketones and occurs via a chain radical mechanism (for certain stages of the process, see recent publications [30]). The methane oxidation produces methanol and formaldehyde as main oxygenates. Due to the fact that both products are much more reactive than hydrocarbon, the yield of alcohol and aldehyde is only a few percents. Usually the reactions were... 

Based on the above results, it may be concluded that selective oxidation of methane to formaldehyde with hydrogen peroxide is implemented by a mechanism different from that in which methanol is formed as the intermediate oxidation product [115]. 

A kinetic study has been carried out on the partial oxidation of methane to formaldehyde over a silica-supported vanadia catalyst. The results indicate that oxygen was adsorbed on the catalyst and took part in the reaction in an Eley-Rideal or Mars-van Krevelen maimer. The nature of the interaction with the catalyst was dependent on whether the reaction took place in methane rich (pcH4 = 80 kPa) or lean (po,4 = 4 kPa) conditions. A reaction mechanism for the partial oxidation of methane to formaldehyde is proposed, which is consistent with the data reported here. Methanol oxidation experiments over this catalyst suggested that it was not an intermediate under the conditions employed during this study. 

An Eley-Rideal/Mars-van Krevelen type of mechanism was found for the partial oxidation of methane to formaldehyde. The differences in the rate equations were due to differences in the amounts of oxygen present in methane rich and lean conditions. Methanol was not an intermediate in the reaction. Methanol oxidation experiments indicated that methanol was oxidised sequentially to formaldehyde, carbon monoxide and hence to carbon... 

INITIAL STEPS IN METHANOL CONVERSION AND AN ALTERNATIVE HOMOLOGATION MECHANISM A small amount of methane (ca. 1C%) is formed in methanol conversion, and appears to be one of the first products formed (ref. 11). When a small amount of methanol is sorbed onto ZSM-5 zeolite, the lattice is methylated (ref. 5). Subsequent temperature-programmed desorption gives dimethyl ether and desorbed methanol first, then (at 250-300°C) methane (stable) and formaldehyde (unstable), and finally aromatic products (ref. 22-23). 

The hydroxylation theory has been criticized also by Callendar 16 on the basis of the necessity for splitting of the oxygen molecule, a step not likely to occur readily at the temperatures at which the slow oxidations are conducted. The lack of experimental evidence to support any mechanism involving the ionization of oxygen prior to or at the time of oxidation of a hydrocarbon is an additional factor in opposition to the idea that an alcohol is the primary oxidation product. At explosion temperatures, however, atomic oxygen may be present and effective as such. Actually most of the experimental work on the direct oxidation of methane with elemental oxygen lias shown that water and formaldehyde are among the first reaction products, whereas methanol is not, and several processes 17 claim this reaction to fonn formaldehyde industrially. 

A great number of catalysts have been tried in the oxidation of methane at atmospheric pressure with the hope of obtaining intermediate products of oxidation. It appears, however, that catalysts tend to carry the reaction to equilibrium, at which state methanol, formaldehyde and formic acid are present in only extremely minute traces. This is well illustrated by the work of Wheeler and Blair," who studied the influence of catalysts in connection with their work on the mechanism of combustion. When methane was oxidized in the presence of metallic and metallic oxide catalysts, no formaldehyde could be detected even at very short times of contact. The formaldehyde produced in the circulation experiments was in a concentration much greater than that required for equilibrium in the reaction ... 

The hydroxylation theory of Bone2 and his co-workers has had wide acceptance as far as the oxidation of aliphatic hydrocarbons is concerned. The mechanism postulated involves the successive formation of hydroxyl compounds, which may add oxygen to form additional hydroxyl groups or which may lose water and decompose. In this way methane would first form methanol, then methylene glycol which would be decomposed to formaldehyde and water formaldehyde would be oxidized to formic acid or decomposed to carbon monoxide and hydrogen. The theory, however, is open to a number of criticisms. 

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. 

Using in situ FT-IR spectroscopy, the gas phase products and the principal intermediates involved in the catalytic conversion of methane over ferric molybdate catalysts were identified and the reaction mechanism was proposed. In the absence of an oxidizing agent, methane reacts with the oxygen of the catalyst to produce methoxy species, which is an important intermediate for methanol formation. Further oxidation of the methoxy groups results in the formation of surface dioxymethylene, adsorbed formaldehyde, and surface formate species. The decomposition of surface dioxymethylene and surface formate species will give carbon oxides and hydrogen. 

The very considerable research work of Bone and his associates led to his support of the hydroxylation mechanism for homogeneous oxidation of hydrocarbons with molecular oxygen. According to this mechanism, reaction between methane and oxygen takes place in steps methanol, formaldehyde, formic acid, and carbon dioxide, in the order named. That methanol has not been found among the products of methane oxidation under conditions where its presence could logically be expected does not necessarily preclude the possibility that it was the initial product. This is due to the thermal instability of methanol under the conditions and its tendency to decompose to hydrogen, carbon monoxide, and formaldehyde. 

Formaldehyde (lUPAC name methanal) quickly converts at daytime to CO (5.46-5.47) but also transfers into the aqueous phase where it hydrates (5.287) or reacts with S(IV) (reaction 5.286). In aqueous solution, methanol quickly oxidizes similar to the gas phase mechanisms to formaldehyde, which (together with scavenged HCHO) further oxidizes to formic acid (methane acid), likely via an OH adduct (the following reaction is speculative) ... 

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. 

The mechanism of methanol oxidation on Pt-based catalysts has been studied for several decades [1-14]. Complex parallel and series reaction pathways in which several adsorbed species and soluble intermediates were involved in methanol oxidation were proposed by Bagotzky et al. [2]. The in situ application of infrared spectroscopy during methanol oxidation showed that adsorbed CO is formed on the Pt surface [15]. However, other adsorbed intermediates are still not identified. Formaldehyde, formic acid, methyl formate, and dimethoxy methane have been identified as soluble intermediates [8, 10, 16-18]. The quantitative analysis of methanol oxidation products changing with various parameters can help us better understand the mechanism of methanol oxidation and identify reactirai pathways. This can be achieved by online quantitative differential electrochemical mass spectrometry (OEMS), which will be discussed in Sect. 3. 

Gierczak et al. (1997) observed methane, ethylene, acetylene, allene, and propyne as major products formaldehyde, methanol, formic acid, acetic acid, and hydrox-yacetone were identihed as minor products. The mechanism of formation of these products is unclear. Two expected products, observed by Raber and Moortgat (1996), propene and dimethylketene, were not observed, and CO and CO2 products observed by Raber and Moortgat, were not measured by Gierczak et al. (1997). In argon matrix-isolated studies of methacrolein photochemistry (4.2 K) and with A, >300 nm, Johnstone and Sodeau (1992) observed the isomerization of the original trans-methacrolein to di-methacrolein no HCO, CO, or propene could be detected. In similar matrix experiments at A >230 nm, dimethylketene, CO, and propene were observed together with other unidentified products. 

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