Hydrocarbon formation from methanol

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

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... 

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 ... 

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. 

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 ... 

Original scheme of hydrocarbon pool mechanism. Adapted from Dahl IM, Kolboe S. On the reaction mechanism for propene formation in the MTO reaction over SAPO-34. Catal Lett 1993 20 329-36 Dahl IM, Kolboe S. On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34 1. Isotopic labeling studies of the co reaction ofethene and methanol. J Catal 1994 149 458—64. 

Hutchings GJ, Hunter R. Hydrocarbon formation from methanol and dimethyl ether a review of the experimental observations concerning the mechanism of formation of the primary products. Catal Today 1990 6 279-306. 

N. Govind, J. Andzelm, K. Reindel and G. Fitzgerald. Zeohte-catalyzed hydrocarbon formation from methanol Density functional simulations. Int. J. Mol. Sci. 3, 2002, 423-134. 

Hydrocarbon formation from methyl chloride can be catalyzed by ZSM-5482 483 or bifunctional acid-base catalysts such as W03 on alumina.420,447 The reaction on ZSM-5 gives a product distribution (43.1% aliphatics and 57.1% aromatics at 369°C) that is very similar to that in the transformation of methanol, suggesting a similar reaction pathway in both reactions.483 W03 on A1203 gives 42.8% C2-C5 hydrocarbons at 327°C at 36% conversion.447 When using methyl bromide as the feed, conversions are comparable. However, in this case, HBr can be very readily air-oxidized to Br2 allowing a catalytic cycle to be operated. Since bromine is the oxidant, the reaction is economical. The one step oxidative condensation of methane to higher hydrocarbons was also achieved in the presence of chlorine or bromine over superacidic catalysts.357... 

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. 

Copper-containing mordenite catalysts have also been reported to be active for carbonylation of vapor-phase methanol [170]. Initially, the predominant reaction products were hydrocarbons resulting from methanol-to-gasoline chemistry, but after about 6 h on stream at 350 °C the selectivity of the catalyst changed to give acetic acid as the main product. A recent investigation was carried out with in situ IR and solid-state NMR spectroscopies to probe the mechanism by detecting surface-bound species. The rate of carbonylation was found to be enhanced by the presence of copper sites (compared to the metal-free system), and formation of methyl acetate was favored by preferential adsorption of CO and dimethyl ether on copper sites [171],... 

In the beginning of 90th, Tan and Davis [136] investigated the coreaction of ethylene and methanol over silicalite S-115 by the isotopic tracer method ( "C labeled or unlabeled methanol and unlabeled or labeled ethylene) and concluded that ethylene was converted by adding a Cj specie derived from methanol. However, the relative "C in the hydrocarbon products revealed that the alkylation of alkenes is more rapid than the formation of Cj" and alkenes from methanol oidy. For the conversion of ethylene only (in the absence of MeOH), the dimerization to form butenes was the dominant reaction. Adding a flow of water in an amount equimolar to ethylene significantly decreased the conversion of ethylene. The addition of methanol to the feed stream, in an amount equal to that of ethylene, increased the total conversion and altered the product distribution so that propylene is formed in about twice the amount of butenes. The labels on the C3-C5 number products were similar to that of ethylene. The authors concluded therefore that the C3-C5 products were formed by the successive addition of an unlabeled Cj species derived from methanol to labeled ethylene. The data clearly show that the formation of ethylene from methanol is a slow reaction compared to the addition of the Cj species to the products. Thus, the formation of ethylene is an important issue only for the reaction initiation. In those processes, where a small amount of alkenes are added to the methanol feed, the formation of ethylene directly from methanol represents a small part of the hydrocarbons produced from methanol. 

Benefits depend upon location. There is reason to beheve that the ratio of hydrocarbon emissions to NO has an influence on the degree of benefit from methanol substitution in reducing the formation of photochemical smog (69). Additionally, continued testing on methanol vehicles, particularly on vehicles which have accumulated a considerable number of miles, may show that some of the assumptions made in the Carnegie Mellon assessment are not vahd. Air quaUty benefits of methanol also depend on good catalyst performance, especially in controlling formaldehyde, over the entire useful life of the vehicle. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

The catalyst used for the conversion of methanol to gasoline is based on a new class of shape-selective zeolites (105-108), known as ZSM-5 zeolites, with structures distinctly different from other well-known zeolites. Apparently, the pore dimensions of the ZSM-5 zeolites are intermediate between those of wide-pore faujasites (ca. 10 A) and very narrow-pore zeolites such as Zeolite A and erionite (ca. 5 A) (109). The available structural data indicate a lattice of interconnecting pores all having approximately the same diameter (101). Hydrocarbon formation... 

The role of the much discussed "primary reaction" of formation of a C2-hydrocarbon from methanol is then limited to producing a very small amount chemisorbed ethene during the incubation period. This C2 will react easily to C3 via alkylation with methanol. 

Klemm (16) and Lee and coworkers (17) have examined the effect of various solvents on the photochemistry of cyclobutanone. By monitoring the quantum yields for formation of ethylene (B-cleavage product) and cyclopropane (decarbonylation product) in different solvents, they were able to demonstrate a significant reduction in the quantum yields for product formation in methanol as compared to other hydrocarbon solvents. Whereas the quantum yield of ethylene formation was found to be essentially solvent insensitive, that for cyclopropane formation was found to be somewhat solvent sensitive. This suggested that B-cleavage and decarbonylation do not result from the same immediate precursor. Since ring-expansion derivatives have not been isolated from photolyses carried out in saturated hydrocarbon solvents, the importance of this process under these conditions remains to be determined. Irradiation of cyclobutanone in the presence of 1,3-penta-diene (17,59) or 1,3-cyclohexadiene (16) did not appear to affect the quantum yields for ketone disappearance or product appearance. 

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