Conversion of Synthesis Gas to Hydrocarbons

Initial studies using cobalt-, nickel-, and iron-modified zeolites X and Y (241, 242) were, however, not particularly encouraging with relatively poor activities, selectivities, and stabilities. This situation has now changed dramatically with the discovery by Mobil Oil Corporation of a new series of synthetic high-silica zeolites. The so-called ZSM-5 zeolite (in the H form) is capable of converting methanol quantitatively to hydrocarbons and water (239), i.e., 

Jacobs has described (249a) these two different approaches in terms of secondary (physical mixtures) and primary (FT function in zeolite matrix) effects. In the former case the results obtained can be quite well understood in terms of the separate behavior of each component. However, in the latter case the results may be different since the primary FT products are formed inside the spatially restricting pores of the zeolite. 

In a recent communication (250), deviations from Schulz-Flory kinetics were observed for a RuNaY synthesis gas conversion catalyst (see Fig. 24). A comparative catalyst, prepared by impregnating silica with ruthenium, i.e., Ru/SiOz, and tested under the same conditions, yielded a product distribution which gave a good fit to Schulz-Flory kinetics. The sharp decrease in chain growth probability for Cf0 products over RuNaY is perhaps surprising for such a relatively large-pore zeolite. Further studies (251-253) on this system indicated that there was a correlation between the ruthenium particle size in the zeolite and the product distribution. 

Clearly, this field of research is in its infancy and the high level of current activity should lead to considerable further development. 

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The first demonstration of catalytic conversion of synthesis gas to hydrocarbons was accompHshed ia 1902 usiag a nickel catalyst (42). The fundamental research and process development on the catalytic reduction of carbon monoxide was carried out by Fischer, Tropsch, and Pichler (43). Whereas the chemistry of the Fischer-Tropsch synthesis is complex, generalized stoichiometric relationships are often used to represent the fundamental aspects ... 

In formal terms, the addition of formaldehyde across the double bond has taken place. The reaction was discovered by Roelen while working on the conversion of synthesis gas to hydrocarbons. The process was first patented in 1938 and a commercial plant was briefly operated at the end of the war after which it was dismantled. In 1948 a commercial plant was put on stream by Exxon in the U.S. 

Shortly after World War I, Badische Amlin patented the catalytic conversion of synthesis gas to methanol, and Fischer and Tropsch (F-T) announced a rival process in which an iron catalyst converted synthesis gas into a mixture of oxygenated hydrocarbons. Later,... 

F. Fischer and H. Tropsch, who first described the conversion of synthesis gas into hydrocarbons and oxygen-containing compounds ( oxygenates ) over heterogeneous transition metal catalysts such as iron/zinc oxide. This reaction was developed into a process for the conversion of coal into gasoline. At present such a process is economically feasible only where coal is plentiful and cheap while access to oil products is limited. Currently only South Africa operates plants using the Fischer-Tropsch process. 

Rytter, E. Eri, S. Schanke, D., 2002a, Catalyst and process for conversion of synthesis gas to essentially paraffinic hydrocarbons , WO/GB03/04873, priority. 

Dimethyl Ether. Synthesis gas conversion to methanol is limited by equiUbrium. One way to increase conversion of synthesis gas is to remove product methanol from the equiUbrium as it is formed. Air Products and others have developed a process that accomplishes this objective by dehydration of methanol to dimethyl ether [115-10-6]. Testing by Air Products at the pilot faciUty in LaPorte has demonstrated a 40% improvement in conversion. The reaction is similar to the Hquid-phase methanol process except that a soHd acid dehydration catalyst is added to the copper-based methanol catalyst slurried in an inert hydrocarbon Hquid (26). 

Notably, once the oxygenated hydrocarbons have been converted into synthesis gas, it is then possible to carry out the subsequent conversion of synthesis gas into a variety of liquid products by well-established catalytic processes, such as the production of long-chain alkanes by Fischer-Tropsch synthesis and/or the production of methanol. 

Methanol Synthesis. The transformation of synthesis gas to methanol [Eq. (3.3)] is a process of major industrial importance. From the point of view of hydrocarbon chemistry, the significance of the process is the subsequent conversion of methanol to hydrocarbons (thus allowing Fischer-Tropsch chemistry to become more selective). 

Historical Development and Future Perspectives The Fischer-Tropsch process dates back to the early 1920s when Franz Fischer and Hans Tropsch demonstrated the conversion of synthesis gas into a mixture of higher hydrocarbons, with cobalt and iron as a catalyst [35, 36], Some 20 years earlier, Sabatier had already discovered the reaction from synthesis gas to methane catalyzed by nickel [37]. The FTS played an important role in the Second World War, as it supplied Germany and Japan with synthetic fuel. The plants used mainly cobalt catalysts supported on a silica support called kieselguhr and promoted by magnesia and thoria. 

The use of bifunctional metal/zeolite catalysts for the conversion of synthesis gas (carbon monoxide and hydrogen) to gasoline range hydrocarbons has recently attracted much attention. For example, the combination of metal oxides with the medium pore ( 6A) zeolite ZSM-5 and the use of a metal nitrate impregnated ZSM-5 catalyst have been shown to produce gasoline range hydrocar-... 

Hydrocarbon Research Inc., elected partial oxidation for the Carthage Hydrocol plant at Brownsville. After initial experiments that Hydrocarbon Research conducted at Olean, New York, The Texas Company assumed responsibility for further development of partial oxidation at its Montebello, California, laboratory, under duBois ( Dubie ) Eastman. For conversion of natural gas to gasoline by Fischer-Tropsch synthesis, partial oxidation s advantage over steam-methane reforming lay in its ability to operate at a pressure approximating that of the synthesis, thereby essentially eliminating need for compression of synthesis gas. 

Steam gasification followed by conversion of the synthesis gas to hydrocarbons or a mixture rich in these. 

The concept of chemisorption is a key to the understanding of catalytic reactions. Catalytic events consist of elementary reactions on the catalyst surface in which chemical bonds are formed between surface atoms and an adsorbing molecule. These interactions cause rupture of chemical bonds within the adsorbing molecule and formation of new bonds between the fragments. We will discuss explanations of the selective behavior of metals mainly with respect to three important types of reactions the conversion of synthesis gas, hydrocarbon conversion and selective (metal-catalyzed) oxidation. When particularly relevant, reference to other reactions will be made. We wish to relate proposed reaction intermediates and their chemical change to the electronic properties of the surface site where the surface reaction occurs. One then is interested in the strength of adsorbate-metal chemical bonds before and after chemical change of the reaction intermediate. These values affect the thermodynamics of the elementary steps and hence enable an estimate of the equilibria that exist between different surface species. It is the primary information a chemist requires to rationalize chemical reaction rates. In order to estimate rates, one needs information on transition states. Advanced quantum-chemical calculations can provide such information. 

Although conversion of synthesis gas into a wide range of long-chain hydrocarbons and oxygenates by the Fischer-Tropsch synthesis is already known since the 1920s and applied on the industrial scale, the reaction mechanism is still not fully understood, more probably due to complex spectrum of reaction products. 

Post et al. (1989) prepared a series of iron and cobalt-base catalysts. The studies were performed in a fixed-bed micro reactor system at temperatures in the range 473—523 K. Variation of catalyst particle size in the range 0.2-2.6 mm showed that the conversion of synthesis gas decreases considerably when the average particle size was increased. Under reaction conditions, the major part of the hydrocarbon product would be the liquid. The liquid would fill the pores of the catalyst so that transport of hydrogen and carbon monoxide to the reactive sites occurred by diffusion of these reactants through this Hquid medium inside the pores. The apparent effective diffusivity D, can be related to the molecular diffusivity, (H2) and solubility, H (H2) of hydrogen in the paraffinic liquid by Eq. (49) ... 

The FT reactions, named after the original inventors, are essentially the conversions of synthesis gas (CO -h H ) to a mixture of hydrocarbons and to a lesser extent oxygenated hydrocarbons. The commercial FT reactions such as the one practiced by SASOL use potassium- and copper-promoted heterogeneous iron catalysts. 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

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