Surface methoxy mechanism

Surface methoxy mechanism. Adapted from Stocker M. Methanol-to-hydrocarbons catalytic materials and their behavior. Microporous Mesoporous Mater 1999 29 3—48. 

The surface methoxy mechanism appears quite probable since the activation barriers are not so high compared to the other routes. 

Based on in situ 13C NMR data, surface methoxy groups are reported to form hydrocarbons at temperatures of 523 K and above [273]. The authors have suggested that these hydrocarbons may contribute to the hydrocarbon pool that is established to participate in the catalytic reaction mechanism to form higher hydrocarbons from methanol. Other reactions with amines or halides have also been published [276]. 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

The initial activation of methanol by a Br0nsted acid proton is merely the first of many stages in the MTG process (213). Many mechanisms have been proposed on the basis of experimental studies of these the surface methoxonium ion pathway of Hutchings and Hunter (240) has received a partial experimental justification. The first stage of this mechanism involves the dehydration of adsorbed methanol to give water and a surface methoxy species. This reaction has been investigated theoretically by a number of authors (221, 222, 241). 

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. 

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. 

The mechanism for the oxidation of methanol to formaldehyde on iron molybdate catalysts (illustrated in Figure 4) envisages the H-abstraction and electron transfer of surface methoxy species desorption of the products and reoxidation of metal sites complete the cycle. 

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 ... 

C MAS NMR has been performed in-situ under batch conditions to investigate the mechanism of aniline alkylation with methanol on acidic H-Y and basic CsNa-Y impregnated with cesium hydroxide. On acidic zeolite H-Y, methanol reacts with zeolitic hydroxyl groups to give surface methoxy groups which, in turn, play a role as alkylating species in aniline methylation. On the basic zeolite Y, meth ol is converted into formaldehyde, which is responsible for the N-alkylation. Both on acidic and basic zeolites, N-methylaniline is the primary alkylation product. Toluidines and N-methyltoluidines are formed only on the acidic zeolite at elevated temperatures after complete conversion of methanol into N-methylaniline. 

After the injection of a flow of CHsOH at 433 K for l.Oh (Fig. 34c), all signals disappeared except that of the DMF at 63.5 ppm. The results of this experiment demonstrate that surface methoxy groups prepared on zeolite HY are reactive and contribute to the formation of DMF by the mechanism described in Fqs. (27a, b). 

The dehydration of MeOH to DME, the first step in the reaction mechanism, is widely described in the literature [57-59]. As long as only DME and water are observed as the products in the effluent, no detectable coke amount is formed on the catalyst. The reaction proceeds via surface methoxy groups produced by dehydration of MeOH, which fiirther undergo a nucleophilic attack by another MeOH molecule in order to form DME (Fig. 8) [57-59]. 

Reaction mechanism for the formation of DME by dehydration of methanol over H-ZSM-5 catalyst. (H" ) acidic site, MeOH (+) protonated methanol (methoxonium ion), /R, ) surface methoxy species, DMO" ) protonated DME (dimethyloxonium ion). Adapted from Park T-Y, Froment CF. Analysis of fundamental reaction rates in the methanol-to-olefins process on ZSM-5 as a basis for reactor desigfi and operation. Ind Eng Chem Res 2004 43 682-9 Park F-Y, Froment CF. Kinetic modelingofthe methanol to olefins process. 2. Experimental results, model discrimination, and parameter estimation. Ind Eng Chem Res 2001 40 4187-96 Park F-Y, Froment CF. Kinetic modelingofthe methanol to olefins process. 1. Model formulation. Ind Eng Chem Res 2001 40 4172-86. 

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 mechanism is thought to involve dissociation of hydrogen, which reacts with molecularly adsorbed CO2 to form formate adsorbed on the surface. The adsorbed formate is then further hydrogenated into adsorbed di-oxo-methylene, methoxy, and finally methanol, which then desorbs. The reaction is carried out under conditions where the surface is predominately empty and the oxygen generated by the process is quickly removed as water. Only the forward rate is considered and the process is assumed to go through the following elementary steps ... 

---