Alkali promoters, Fischer-Tropsch catalysts

It is interesting to note that the addition of ruthenium to iron produces effects on selectivity similar to those observed when alkali promoters are added to iron that is, methane production is reduced and olefin formation is enhanced. However, alkali metals also tend to. enhance the formation of oxygenated compounds which results in a less favorable selectivity and in product separation problems. Under our reaction conditions, we see no evidence for the formation of any oxygenated compounds. It will be interesting to see if this olefin. paraflBn ratio over Ru-Fe catalysts can be enhanced at higher pressures, as is presently achieved with typical, promoted Fischer-Tropsch catalysts, without the concomitant production of less desirable oxygenated compounds. It appears from this study that alloy catalysts can provide favorable shifts in selectivity, and future studies should provide further evidence of this capability. 

Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

An XPS Investigation of iron Fischer-Tropsch catalysts before and after exposure to realistic reaction conditions is reported. The iron catalyst used in the study was a moderate surface area (15M /g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. 

Ngantsoue-Hoc, W., Zhang, Y., O Brien, R.J., Luo, M., and Davis, B.H. 2002. Fischer-Tropsch synthesis Activity and selectivity for Group I alkali promoted iron-based catalysts. Appl. Catal. 236 77-89. 

Iron is the first bcc metal to be studied using SCAC, and the results for CO adsorption on the stepped Fe 211 surface have revealed a complex adsorption mechanism [14]. Iron is a potential Fischer-Tropsch catalyst, having the ability to dissociate CO without the assistance of an alkali metal promoter. Heat of adsorption... 

Formally, ammonia synthesis is closely related to Fischer-Tropsch synthesis. Industrial operation involves the use of an iron catalyst promoted with calcium and potassium oxides. However, the reason we consider this process here is not directly in connection with alkali promotion of the catalyst. We are concerned with a remarkable achievement reported by Yiokari et al. [15], who use a ton-conducting electrolyte to achieve electrochemical promotion of a fully promoted ammonia synthesis catalyst operated at elevated pressure. Specifically, they make use of a fully promoted industrial catalyst that was interfaced with the proton conductor CaIno.iZro.903-a operated at 700K and 50 bar in a multipellet configuration. It was shown that under EP the catalytic rate could be increased by a factor of 13 when... 

Liquid-phase Fischer-Tropsch synthesis has been investigated using a slurry-bed reactor. The catalytic activity of ultrafine particles (UFP) composed of Fe was shown to be greater than that of a precipitated Fe catalyst. The difference was interpreted as caused by the different nature of surface structure between these catalysts, whether porous or not. The obtained carbon number distributions over alkali-promoted Fe UFP catalysts were simulated by a superposition of two Flory type distributions. It is ascertained that the surface of alkali-promoted UFP catalysts possesses promoted and unpromoted sites exhibiting different chain growth probabilities. 

Amorphous alumincphosphates have been used as catalysts or catalyst supports for a number of years. Canpelo and his coworkers have shewn that they can be promoted by alkali metals or F ions (Ref. 25-26), but they behave like the amorphous aluminosilicates in being non-selective. The unsubstituted aluminophosphates have essentially no catalytic activity although they do dehydrate methanol to dimethyl ether (ref. 11). They can, hewever, be used as catalyst supports, and Coughlin and Rabo (ref. 27) have considered irtpregnated AlP04 s as supports for Fischer-Tropsch catalysts. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

The TPR-XAFS technique confirmed that doping Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) remarkably promotes the carburization rate relative to the undoped catalyst. The EXAFS results suggest that either the Hagg or e-carbides were formed during the reduction process over the cementite form. A correlation is observed between the a-value of the product distribution and the carburization rate. 

The readsorption and incorporation of reaction products such as 1-alkenes, alcohols, and aldehydes followed by subsequent chain growth is a remarkable property of Fischer-Tropsch (FT) synthesis. Therefore, a large number of co-feeding experiments are discussed in detail in order to contribute to the elucidation of the reaction mechanism. Great interest was focused on co-feeding CH2N2, which on the catalyst surface dissociates to CH2 and dinitrogen. Furthermore, interest was focused on the selectivity of branched hydrocarbons and on the promoter effect of alkali on product distribution. All these effects are discussed in detail on the basis... 

The carbon number distribution of Fischer-Tropsch products on both cobalt and iron catalysts can be clearly represented by superposition of two Anderson-Schulz-Flory (ASF) distributions characterized by two chain growth probabilities and the mass or molar fraction of products assigned to one of these distributions.7 10 In particular, this bimodal-type distribution is pronounced for iron catalysts promoted with alkali (e.g., K2C03). Comparing product distributions obtained on alkali-promoted and -unpromoted iron catalysts has shown that the distribution characterized by the lower growth probability a, is not affected by the promoter, while the growth probability a2 and the mass fraction f2 are considerably increased by addition of alkali.9 This is... 

Alkalization of iron catalysts causes two different effects. The selectivities of 1-alkenes are raised and both the growth probability a2 and the fraction f2 are markedly increased, as already shown in Figure 11.2. Detailed studies on the promoter effect of alkali have revealed the effect on 1-alkene selectivity to saturates at 1 mass% of K2C03, while the effect on f2 already begins at 0.2 mass% of K2C03.1213 This difference points to specific active sites in Fischer-Tropsch syn-... 

But how ubiquitous actually are alkalis in the promotion of reactions catalyzed at metal surfaces An examination of recent authoritative sources [6,7] shows that the majority of medium-to large-scale processes do not employ alkali promoters, even when one includes nonmetallic (i.e., metal oxide) catalysts. In a number of cases (e.g., steam reforming of naphtha) it seems clear that the role of alkali is simply to reduce the acidity of the oxide support. There are famous cases, of course, where the presence of alkali species on the catalytically active metal surface is critically important to the chemistry. Notable are ethene epoxidation (Ag-Cs), ammonia synthesis (Fe-K), acetoxylation of ethene to vinyl acetate (Pd, Pd/Au-K), and Fischer-Tropsch synthesis (Fe, Co, Ru-K). The first three are major industrial... 

The reactions of carbon dioxide and water with carbonaceous species are well known reactions and have been employed in various chemical processes to control or remove unwanted carbonaceous material. The most important application is found in the gasification of coal, forming a mixture of hydrogen and carbon monoxide (syngas), which is also used in important industrial processes such as methanol synthesis and in the production of hydrocarbons (Fischer-Tropsch synthesis). In these reactions, alkali metal catalysts and promoters have been used extensively for their ability to activate water and carbon dioxide species, thus speeding up their reaction with carbon. Alkali metals are therefore a clear candidate for this study. A number of mechanisms have been suggested... 

Deactivation and Regeneration of Alkali Metal Promoted Iron Fischer-Tropsch Synthesis Catalysts... 

The oldest information on the subject is in early patents of BASF on alkahzed Co catalysts, and the patents by Fischer and Tropsch on the so called synthol process. More recently, several papers by the Bureau of Mines Laboratory S have demonstrated that an interesting amount of oxygenates can be produced with standard alkali-promoted Fe catalysts, when proper reaction conditions are chosen and when the catalyst is properly run in . 

After generation of the synthesis gas, conversion to liquid hydrocarbons, waxes, alcohols, and ketones is achieved using an iron or a cobalt catalyst (Table 19.10) in fixed-bed or entrained-bed reactors. A variety of catalysts, among them magnetite (iron oxide), have been proposed and used for the Fischer-Tropsch synthesis (Kugler and Steffgen, 1979 Cooper et al., 1984 Hindermann et al., 1984 Mirodatos et al, 1984 Moser and Slocum, 1992). Magnesium oxide (MgO) is frequently added as a structural, or surface, promoter, and potassium oxide (or other alkali metal oxide) is often added as a chemical promoter (Dry and Ferreira, 1967,1968). 

The structural promoter functions to provide a stable, high-area catalyst, while the chemical promoter alters the selectivity of the process. The effectiveness of the alkali metal oxide promoter increases with increasing basicity. Increasing the basicity of the catalyst shifts the selectivity of the reaction toward the heavier or longer-chain hydrocarbon products (Dry and Ferreira, 1967). By the proper choice of catalyst basicity and ratio, the product selectivity in the Fischer-Tropsch process can be adjnsted to yield from 5% to 75% methane. Likewise, the proportion of hydrocarbons in the gasoline range ronghly can be adjnsted to produce 0%-40% of the total hydrocarbon yield. 

Ribeiro MC, Jacobs G, Davis BH, Cronauer DC, Kropf AJ, Marshall CL (2010) Fischer — Tropsch synthesis an in-situ TPR-EXAFS/XANES investigation of the influence of Group I alkali promoters on the local atomic and electronic structure of carburized iron/ silica catalysts. J Phys Chem C 114 7895-7903... 

In 1923, Franz Fischer and Hans Tropsch reported the synthesis of long chain hydrocarbons from synthesis gas over alkali-promoted iron catalysts. The Fischer-Tropsch process became operational in 1936, and made Germany independent of imported fuels in World War II. 

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