Formate from methanol

The FTIR studies revealed that the formation of CO2 is only detected when the CO starts to be oxidized (Fig. 6.18). Therefore, it was proposed that the mechanism has only one path, with CO as the C02-forming intermediate [Chang et al., 1992 Vielstich and Xia, 1995]. This has two important and practical consequences. First, methanol oxidation will be catalyzed by the same adatoms that catalyze CO oxidation, mainly ruthenium. Second, since the steric requirements for CO formation from methanol are quite high, the catalytic activity of small (<4nm) nanoparticles diminishes [Park et al., 2002]. 

A different mechanism seems to operate in the case of poison formation from methanol [Herrero et al., 1993]. In this case, modification of the Pt(lll) surface by Bi deposition only causes a linear decrease in the amount of poison formed, indicating the existence of a mere third-body effect. Complete inhibition of the poisoning reaction is achieved for > 0.23, i.e., before the surface is completely covered. This suggests the existence of ensemble requirements for this reaction, which need enough free contiguous Pt sites to take place. 

With regard to the second question, while COad formation from methanol is slow at potentials in the H pd region, formaldehyde adsorption transients on a Pt film electrode showed rapid COad formation under these condition [Chen et al., to be published]. Furthermore, Korzeniewski and Childers [1998] reported increasingly... 

Figure 3. The effect of pH on the average rate of methane formation from methanol. In 0.2 M Na2S04 at 60 °C and at constant over potential (see table 3).

The two catalyst components are rhodium and iodide, which can be added in many forms. A large excess of iodide may be present. Rhodium is present as the anionic species RhI2(CO)2. Typically the rhodium concentration is 10 mM and the iodide concentration is 1.5 M, of which 20% occurs in the form of salts. The temperature is about 180 °C and the pressure is 50 bar. The methyl iodide formation from methanol is almost complete, which makes the reaction rate also practically independent of the methanol concentration. In other words, at any conversion level (except for very low methanol levels) the production rate is the same. For a continuous reactor this has the advantage that it can be operated at a high conversion level. As a result the required separation of methanol, methyl acetate, methyl iodide, and rhodium iodide from the product acetic acid is much easier. 

Dahl, l.M. and Kolboe, S. (1996) On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34 2. Isotopic labeling smdies... 

Initial methane formation from methanol on the fresh catalyst is proposed to proceed on Bronsted acid sites as a reaction with a hydride donor - in... 

Hanst, P. L., and E. R. Stephens, Infrared Analysis of Engine Exhausts Methyl Nitrite Formation from Methanol Fuel, Spectroscopy, 4, 33-38 (1989). 

Methanol can be converted to hydrocarbons over acidic catalysts. However, with the exception of some zeolites, most catalysts deactivate rapidly. The first observation of hydrocarbon formation from methanol in molten ZnCl2 was reported in 1880, when decomposition of methanol was described to yield hexamethylbenzene and methane.414 Significant amounts of light hydrocarbons, mostly isobutane, were formed when methanol or dimethyl ether reacted over ZnCl2 under superatmo-spheric pressure.415 More recently, bulk zinc bromide and zinc iodide were found to convert methanol to gasoline range (C4-C13) fraction (mainly 2,2,3-trimethyl-butane) at 200°C with excellent yield (>99%).416... 

Jain and Pillai (345) have shown that the ether formation from methanol, n-propanol, and isopropanol is inhibited when phenol and acetic acid were... 

Parera and his co-workers (359-362) have studied the poisoning effect of amines, pyridine, phenol, and acetic acid. A reduced rate of ether formation from methanol at the standard temperature of 230°C was observed when the poisons were present in the feed. In most cases the original activity was recovered, although rather slowly. Most probably the poisons were either displaced by alcohol and/or water or removed from the surface by chemical transformations. 

Figueras Roca and co-workers (346) have used preadsorbed TCNE to poison the basic sites specifically. The rate of ether formation from methanol and ethanol responded very sensitively to the poisoning with TCNE, so that the participation of basic sites in the bimolecular alcohol dehydration seems to be proved. 

Wang, C.Y., J. Rabani, D.W. Bahnemann and J.K. Dohrmann (2002b). Photonic efficiency and quantum yield of formaldehyde formation from methanol in the presence of various Ti02 photocatalysts. Journal of Photochemistry and Photobiology A-Chemistry, 148(1-3), 169-176. 

Fig. 4.68. Proton catalysed ethylene formation from methanol.

The background theory for estimating free energy barriers using constrained dynamics is covered in more detail in a similar study of dimethyl ether formation from methanol in zeolites Hytha, M., Stich, L, Gale, J.D., Terakura, K. 

The mechanism of formation of methyl formate from methanol and carbon monoxide in the presence of DBU has been investigated (77NKK457). In the resulting equilibrium, the rate of formation of methyl formate was found to be first order with respect to the carbon monoxide pressure and to the concentrations of methanol and DBU. 

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). 

Whereas trimethyloxonium formation occurs in zeolites, they have been shown not to be the intermediates for C-C bond formation from methanol as has been believed for a long time. In the presence of coadsorbed methanol C-C bond formation occurs in a direct reaction between adsorbed methoxy and methanol [59]. 

MECHANISM OF HYDROCARBON FORMATION FROM METHANOL CLARENCE D. CHANG... 

Hydrocarbon formation from methanol has been intensively investigated in the past decade since the first reports from Mobil [1] describing the conversion of methanol to aromatic gasoline using zeolite catalysts. The general reaction pathway was elucidated in this early work and is represented by the sequence ... 

The mechanisms of acid-catalyzed DME formation from methanol and aromatiza-tion of olefins were widely investigated in the years before the discovery of the methanol-to-gasoline reaction. There is a consensus that the intermediate in DME formation from methan.ol over solid acid catalysts is a protonated surface methoxyl, which is subject to nucleophilic attack by methanol [2]. Aroma-tization of olefins is believed to proceed along classical carbenium pathways, with concurrent hydrogen transfer [3]. The mechanism of the crucial step of initial C-C bond formation from MeOH/DME is an unsolved problem, however, and is the subject of ongoing controversy. At last tally, there were some two dozen mechanistic proposals in the literature. It is not possible here to present a comprehensive review of the entire field. However, a number of common themes can be identified. This commonality is discussed and the concepts currently in vogue are critically reviewed. Another issue, whether ethylene is the "first" olefin, has been widely debated [2], but is beyond the scope of this survey. 

Carbenic mechanisms. Venuto and Landis [10] were the first to address the question of mechanism of hydrocarbon formation from methanol over zeolites, in this case zeolite X [11]. These workers proposed a scheme involving a-elimina-tion of water and polymerization of the resultant methylcarbenes to olefins. Swabb and Gates [12], elaborating on Venuto-Landis, proposed that concerted action of acid and basic sites in the zeolite (mordenite) facilitates a-elimina-tion of water from methanol. According to Salvador and Kladnig [13], carbenes are generated through decomposition of surface methoxyls (a-el imination of silanol) formed initially upon chemisorption of methanol on the zeolite (zeolite Y). Hydrocarbons are assumed to form, in the latter two schemes, also by carbene polymerization. 

Free radical methanisms. There has lately been renewed interest in the possibility that free radicals may play a role in the conversion of methanol to hydrocarbons. Zatorski and Krzyzanowskl [8] had earlier proposed a radical mechanism for hydrocarbon formation from methanol over natural mordenite. 

It is apparent that much resourceful, imaginative experimentation has been done to resolve the question of C-C bond formation from methanol. Although the answer remains elusive, these experiments tell us at least what is probably not involved in the bond formation, particularly in the presence of zeolite catalysts. The Stevens rearrangement of oxonium ylide can be ruled out, as well as the carbocationic route invoking hypervalent carbon transition states. Not excluded are surface-bound species such as carbenoids and ylides. Again there seems to be a consensus that surface methoxyls are precursors to these reactive C- intermediates, which seems somehow to be "coming full circle", since surface methoxyls were early shown to be intermediates in the formation of DME, which is itself an intermediate in hydrocarbon formation. Finally, if the free radical character of the initiation step proves correct, the implications to zeolite catalysis will be far-reaching. 

HYDROCARBON FORMATION FROM METHANOL USING W03/A1203 AND ZEOLITE ZSM-5 CATALYST ... 

Since it was first reported in 1976 that protonated ZSM-5 zeolites are excellent catalysts for conversion of methanol (and many other oxygenated compounds ) into hydrocarbons in the C - C q range the catalyst and the reactions have been intensely studied. Several aspects of the reactions leading to hydrocarbon formation from methanol or dimethyl ether over H-ZSM-5 or other protonated zeolites still remain unclear. In particular the first OC bond formation has been debated, and several mechanisms proposed (ref. 1). 

Hydrocarbon formation from methanol is catalyzed by Bronsted acids. The general reaction path for hydrocarbon formation from methanol over zeolite ZSM-5 [3], the catalyst of choice [4], was defined in early Mobil studies [lb], and is represented by ... 

Virtually every possible reactive C intermediate has been invoked to explain the crucial step of initial C-C bond formation from methanol/DME. Proposed mechanisms can be broadly classified as carbenic, carbocationic, ylide, and free radical. In some proposals several of these categories are combined. 

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