Methanol, hydrocarbon formation

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... 

Perform "wet"-methanol-hydrocarbon flashes to estimate the liquid water plus methanol and hydrocarbon phases the methanol concentration is adjusted to satisfy the Hammerschmidt equation prediction for the desired hydrate formation temperature depression. 

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... 

Metal molybdates421 and cobalt-thoria-kieselguhr422 also catalyze the formation of hydrocarbons. It is believed, however, that methanol is simply a source of synthesis gas via dissociation and the actual reaction leading to hydrocarbon formation is a Fischer-Tropsch reaction. Alumina is a selective dehydration catalyst, yielding dimethyl ether at 300-350°C, but small quantities of methane and C2 hydrocarbons423 424 are formed above 350°C. Heteropoly acids and salts exhibit high activity in the conversion of methanol and dimethyl ether.425-428 Acidity was found to determine activity,427 130 while hydrocarbon product distribution was affected by several experimental variables.428-432... 

The mechanism of the conversion of methanol to hydrocarbons has been the subject of substantial studies 450-455 Despite the intensive research, however, many details of this very complex transformation remain unsolved. It now appears generally accepted that methanol undergoes a preliminary Brpnsted acid-catalyzed dehydration step and that dimethyl ether, or an equilibrium mixture of dimethyl ether and methanol, acts as the precursor to hydrocarbon formation 433... 

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... 

Hydrocarbon formation is more interesting in the electrochemical reduction of COj, since multielectron transfer is required in this process. In the electrochemical reduction of concentrated COj in the COj-methanol medium, the major products are still the two-electron transfer products, CO and methyl formate, at the Cu electrode, when tetrabutylammonium salts are used (Table 3). However, when tetraethylammonium salt was used as the supporting electrolyte, efficient formation of methane and ethylene was observed with good reproducibility. We defined the hydrocarbon selectivity as the ratio of the... 

It is well known that the product distribution also depends on the electrode material used [13, 14]. We have examined the effect of various electrode materials on the product distribution in the CO -methanol system. The current efficiencies of the reduction products are shown in Figure 6. The production of formate was fairly favorable at all electrodes, in comparison with that in aqueous systems. For example, the production of formate was more than 20% on Pt and Ni electrodes, which is much higher than that observed in aqueous systems [14]. At the metals Sn and Sb, formate production was favoured, as in the aqueous systems, but CO formation was also somewhat favored. This is due to the effect of supporting electrolyte. The electrodes Ag, Zn and Pd showed similar activities for CO production, as in aqueous systems. The efficiency of hydrocarbon formation at the Cu electrode was found to be lower, whereas that at the Ni electrode was found to be higher than that in aqueous systems. The balance of hydrogen and carbon atom concentrations on the electrode surface may explain this difference. 

It has been reported that methanol is formed from C0-H2 reaction over silica-supported Pd, Pt, Ir catalysts (5). The behaviour of the metal was found to be influenced by the carrier (6,7,8, 9). The selectivity in methanol was discussed in terms of acid-base properties of the support which influenced the non dissociative adsorption of CO on the metal required for oxygenated hydrocarbon formation, in terms of electronic interaction between the metal and the support or in terms of stabilization by the carrier of oxidized metal cations which would adsorb CO non dissociatively. We have studied the C0-H2 reaction at 553 K, 30 atmospheres over Pt supported on a variety of oxides. The characteristics of the catalysts are given in table 2... 

The particle size of gold was less than 4 nm. Product selectivity is greatly affected by the support oxides. Au/Fc203 and Au/Ti02 were shown to be more active for hydrocarbon formation and for both forward and reverse water-gas shift reactions. Over all the catalysts tested by Haruta s group, carbon dioxide produced methanol at lower temperatures and produced methane more selectively than carbon monoxide [469]. 

The catalysts used in alcohol synthesis hold the key to selectivity for methanol, for higher oxygenates, and to the control of hydrocarbon formation. Of interest are the mechanisms and the structure-function relationships in the catalysis of the C-H bond formation in reactions (1) and (2), C-C bond formation in reaction (5), and C-O bond formation in reactions (4), (6) and (7), as well as of reactions utilizing the synthesis intermediates as building blocks for organic syntheses such as amine (refs. 9-11) and aldol (refs. 12-14) syntheses. Further, the mechanistic roles of CO2 and water are of importance to understanding the... 

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. 

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 ... 

Figure 4 reports the catalytic activity for the Cat 1. It is worth noting that H.M.A. were obtained also with the undoped catalyst, even if an increase of activity both in methanol and H.M.A. synthesis was observed by doping with a low amount of potassium (up to 0.4% ca.). These data are in good agreement with those reported in the literature (16,33), taking into account the different ways to express the amount of potassium added. Furthermore, a decrease of the hydrocarbon formation was observed in this range. 

---