Conversion of Methanol into Hydrocarbons

Svelle, S., Joensen, F., Nervlov, J., Olsbye, U., lillerud, K.-P., Kolboe, S., and Bjorgen, M. (2005) Conversion of methanol into hydrocarbons over the zeolite H-ZSM-5 ethene formation is mechanistically separated from the formation of higher alkenes. /. Am. Chem. Soc., 128,14770-14771. 

Ono et al. (759) reported that heteropolyacids such as H3PW12O40 and H4SiW 2O40 catalyze the conversion of methanol into hydrocarbons, although the activities are less than that of HZSM-5. In contrast to HZSM-5, the main products observed with heteropolyacids are aliphatic C -C6 hydrocarbons, the selectivities for aromatic hydrocarbons being small (Table XIX). 

Product Distribution in Conversion of Methanol into Hydrocarbons (189)... 

Y. Ono, T. Baba, J. Sakai, T. Keii, J., Conversion of methanol into hydrocarbons catalysed by metal salts of heteropolyacids, Chem. Soc., Chem. Commun., 1981, 400-401. 

Conversion of Methanol into Hydrocarbons 255 Table 4.12 Yields from methanol for two reactor systems... 

After combining all these equations, the overall conversion of biomass into hydrocarbon or methanol adopts the stoichiometry of Reactions (6) and (7) ... 

The effect of the Si/Al ratio of H-ZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption microcalorimetry of ammonia and tert-butylamine. The observed increase in light olefins selectivity and decrease in methanol conversion with increasing Si/Al ratio was explained by a decrease in total acidity [237]. 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

Afterwards, the synthesis of various new zeolites, especially ZSM5 (MFI, 1967), the discovery of new shape selective transformations such as the (accidental) discovery of the remarkably stable and selective conversion of methanol into gasoline range hydrocarbons over HZSM5 (7), the development of post-synthesis treatments of zeolites,. .. combined to make them the single most important family of catalysts used all other the world. 

Molecular sieve catalysts that have been used for the conversion of methanol to hydrocarbons fall into two general classifications. Most of the initial research was done using ZSM-5 (MFI), a medium-pore size zeolite with a three dimensional pore system consisting of straight (5.6 x 5.3 A) and sinusoidal channels (5.5 x 5.1 A). While most of this work was directed at the conversion of methanol to liquid hydrocarbons for addition to gasoline, it was found that the product slate could be shifted toward light olefins by the use of low pressure and short contact times. 

Chang and Silvestri [lb], reporting the conversion of methanol to hydrocarbons over zeolite ZSM-5, also favored a carbene mechanism for initial C-C bond formation. However, the complicity of free carbenes was considered unlikely from an energetics viewpoint, and a concerted mechanism was proposed for carbene generation with concurrent sp3 insertion into MeOH or DME. Later, the possibility of a sequential mechanism was considered [14]. 

Methanol Conversion to Olefins. - Chabazite, erionite, zeolite T, and ZK-5 have been used by Chang et al. for the conversion of methanol into olefins. The C2-C4 olefin concentration in the hydrocarbon fraction was always less than 60 wt% at 100% methanol conversion. It follows from Table 3 that the hydrocarbon fraction becomes richer in Cj-C olefins as the conversion of methanol decreases. That is because the conversion of olefins to paraffins is lower. Hydrocarbon fractions with more than 80 wt% of Cj-C olefins were attained with a dealuminated H-erionite, but the conversion of methanol was very low. 

The first of these new cobalt catalysts were made in 1986 by coprecipitation techniques using aqueous solutions with ammonium bicarbonate as the precipitant in a similar way to the methods used for methanol synthesis catalysts. The new catalysts were immediately found to be very active and selective catalysts for the conversion of syngas into hydrocarbons. A particularly attractive feature was their low methane make and tolerance of CO2 The CO2 tolerance was ascribed to the interplay between the support and the cobalt phase both in the oxidized and reduced forms. The general belief is that the support stabilizes the cobalt phase such that the catalyst can be operated at the higher temperatures, required to maintain activity despite competitive adsorption by CO2, without any loss in stability. Other investigators e.g. Shell have used similar strategies [2]. 

As indicated, the process can be directed towards ethylene production and hence chemical synthesis. The use of ZSM-5 catalysts for direct conversion of syngas into hydrocarbons (i.e., without the need to produce methanol first) and selective preparation of benzene, toluene, and xylene aromatics only are already being actively investigated. 

Methanol has been studied as a possible precursor of gasoline. For example, certain zeolite catalysts (Section 3-3) allow the conversion of methanol into a mixture of hydrocarbons. 

The thermal and catalytic conversion of different hydrocarbon fractions, often with hydrotreating and other reaction steps, is characterized by a broad variety of feeds and products (Table 1, entry 4). New processes starting from natural gas are currently under development these are mainly based on the conversion of methane into synthesis gas, further into methanol, and finally into higher hydrocarbons. These processes are mainly employed in the petrochemical industry and will not be described in detail here. Several new processes are under development and the formation of BTX aromatics from C3/C4 hydrocarbons employing modified zeolite catalysts is a promising example [10],... 

Complete Methanol Conversion - The major products of the MTG conversion are hydrocarbons and water. Consequently, any unconverted methanol will dissolve into the water phase and be lost unless a distillation step to process the very dilute water phase is added to the process. Thus, essentially complete conversion of methanol is highly preferred. 

Table IV summarizes the results of methanol conversion over the catalyst samples employed in the TPD study 30). The reaction was conducted in a conventional fixed-bed flow reactor under the conditions given in the table. The results are in agreement with those of the TPD measurement. Na - and H -TSMs are inactive for the methanol conversion, whereas Ti -TSM promotes dehydration, converting 50% of the fed methanol into dimethyl ether and a small amount of methane. The negligible activity of Li -Hect is improved slightly by exchanging the Li ion with and dramatically by exchanging Li with Ti. Na -Bent is an acidic clay. All of the three Bent catalysts, even Na -Bent, show higher activity than Ti -TSM, and the hydrocarbon yield reflects this difference in catalytic activity. Na -Bent is sufficiently active to give 60% conversion but has no ability subsequently to dehydrate dimethyl ether into hydrocarbons. The activity of H -Bent is higher than that of Na" -Bent, but the hydrocarbon yield is as low as 9%. As expected from the results of TPD measurement, the activity of Ti -Bent is remarkably high and converts 60% of fed methanol into hydrocarbons that are a mixture of methane, C2-5 olefins, and a small amount of Cs hydrocarbons.

Another synthesis process proposed to receive benefits from operating with monolith catalysts is the conversion of methanol for gasoline production [16,17J. The catalyst used was the ZSM-5 zeolite. However, rather than binding the catalyst onto the wall by use of a washcoat, it was uniformly crystallized on the cordierite honeycomb (62 cells/cm ) wall surfaces (up to 30% by weight), similar to the method described in the patent assigned to Lachman and Patil [18]. The effects of methanol partial pressure on conversion and temperature on hydrocarbon selectivity were determined. Three regimes of mass transfer resistances are experienced in this reaction reactant transfer to the reactor walls within the monolith channels through the laminar flow, diffusion resistance at the surface between zeolite crystals on the walls, and diffusion into the zeolite molecular-size pores to the active sites within the crystals, where the reaction rate limit is anticipated. 

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). 

Recently, interest has grown in the modification of zeolites to effect the conversion of methanol (refs. 1-4) or toluene (ref. 1,5) into specific types of hydrocarbons. The introduction of modifiers, such as P and B, increased the yield of olefins in the former case and the relative distribution of para aromatics in both cases. However, the fundamental question remains open as to whether the role of modification consists only of blocking the channels and thus creating diffusional hindrances, or also in altering the concentration and strength of acid sites. In this work, Mo-zeolites (ZSM-5, mordenite and Y) were used to investigate the modification effect of Mo on these reactions. 

The basic process flow of the MTG reaction section is shown in Fig. 1. Conversion of methanol to gasoline occurs in two steps. First, the methanol is vaporized and is partly dehydrated to an equilibrium mixture of dimethyl ether, methanol and water over an alumina catalyst in a dehydration reaction. About 1536 of the reaction heat is released in this first step. The equilibrium mixture is then combined with recycle gas of light hydrocarbon reaction products and passed to a conversion reactor where the second set of reactions take place over ZSM-5 catalyst to form gasoline. The major and substantial amount of the reaction heat (85%) is released in the Conversion Reactor where the heat is removed by the recycle gas. Reactor temperature is controlled to limit the temperature rise in the catalyst bed. Hot reactor effluent is used to preheat the recycle gas and to vaporize the methanol feed to the DME reactor. The gasoline is separated from the recycle gas and water formed and sent to fractionation, treatment and blending into finished stock. 

The direct conversion of methane into methanol, hydrocarbons, or oxygenated fuels has attracted a great deal of interest. At the beginning of the 20th century, one of the first patents was granted in 1905 to Lance and Elworthy [165]. These inventors claimed that oxidizing methane with hydrogen peroxide in the presence of ferrous sulfate could form methanol, formaldehyde, and formic acid. 

Ono et al. ° observed an abrupt jump in the conversion of methanol on HZSM-5 at constant space time when the temperature was increased from 280 to 300 C. Because the apparent activation energy associated with it is too high for ordinary chemical reactions and because this "jump" was also observed when feeding dimethylether, autocatalytic effects in the transformation of dimethylether into hydrocarbons were invoked. The temperature at which the "jump" occurred depended on the Si/Al-ratio. Such a sudden... 

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