Selectivity Fischer-Tropsch catalysts

In the very active field of unmodified nanoparticles recent discoveries have been made on size-selective Fischer-Tropsch catalysts that convert selectively CO and H2 into hydrocarbons there is a strong dependence of activity, selectivity and Hfetime on Co particle size. This topic of unmodified, supported or unsupported, nanoparticles is outside the scope of this chapter [74, 75]. Nevertheless, we mention discoveries made by Degussa, who have patented a process for H2O2 synthesis from molecular oxygen and molecular hydrogen with nanosized Pd particles (6 A) [76]. 

The application of the method for the synthesis of MTBE, TAME and other octane improvers for gasoline is proposed. An application of the method to branch alkenes synthesised by a selective Fischer-Tropsch catalyst is demonstrated. 

FIG. 6. Product distribution from an alkene selective Fischer-Tropsch catalyst before and after passage over 6% WOx/HT-alumina isomerization catalyst. 

KangJ, Cheng K, Zhang L, Zhang Q, DingJ, Hua W Mesoporous zeoHte-supported ruthenium nanoparticles as highly selective Fischer-Tropsch catalysts for the production of C5-C11 isoparaffins, Angew Chem Int Ed 50(22) 5200—5203, 2011. 

Synthetic Fuels. Hydrocarbon Hquids made from nonpetroleum sources can be used in steam crackers to produce olefins. Fischer-Tropsch Hquids, oil-shale Hquids, and coal-Hquefaction products are examples (61) (see Fuels, synthetic). Work using Fischer-Tropsch catalysts indicates that olefins can be made directly from synthesis gas—carbon monoxide and hydrogen (62,63). Shape-selective molecular sieves (qv) also are being evaluated (64). 

The two important discoveries in the search for iron-based Fischer-Tropsch catalysts were (a) the finding that the addition of alkali yielded significant improvements in the activity and selectivity (to liquid products) of iron catalysts (15), and (b) the development of the medium-pressure synthesis (16). In 1943 a pilot plant was constructed at Schwarz-Leide in Germany for the comparative testing of iron-based catalysts. However, the outcome of World War II curtailed its activities. After 1945 many of the plants were destroyed and, for those remaining, recommencement of operation was forbidden for several years. Of the three plants restarted, the last at Bergkamen was closed in 1962. 

Finding ethane to be the major product is a significant result, because for all the other systems described above, and indeed for Fischer-Tropsch catalysts in general, methane is the major hydrocarbon product. Furthermore, as is discussed in Section III, high selectivity to C2 could have important mechanistic as well as commercial implications. 

Borg, S. Storsaeter, S. Eri, H. Wigum, E. Rytter and A. Holmen, The effect of water on the activity and selectivity for gamma-alumina supported cobalt Fischer-Tropsch catalysts with different pore sizes, Catal. Lett., 2006, 107, 95-102. 

X. Huang and C. B. Roberts, Selective Fischer-Tropsch synthesis over an A1203 supported cobalt catalyst in supercritical hexane, Fuel Process Technol., 2003, 83, 81-99. 

Cobalt-based Fischer-Tropsch catalysts are the subject of continuing interest as large-scale Gas-to-Liquids plants come on line. Fernando Morales and Bert Weckhuysen (Utrecht University, the Netherlands) look specifically at the effects of various promoters for these catalysts, particularly Mn. The effect of these promoters in controlling the activity and selectivity of the overall reaction can be critical in the overall process economics. This chapter also looks at new spectroscopic techniques that have recently been used to study the effects of these promoters. 

Mn-promoted Fe-based Fischer-Tropsch Catalysts. 4.1.1 Unsupported Fe-Mn Fischer-Tropsch Catalysts. Iron-based F-T catalysts possess both hydrogenation and WGS activity, imposing a flexible option as a working catalyst for typically coal-derived CO-rich syngas conversion. Iron-based catalysts often contain small amounts of K and some other metals/metal oxides as promoters to improve their activity and selectivity. Mn has been widely used as one of the promoters for unsuppported Fe-based F T catalysts, particularly in promoting the production of C2 C4 olefins. ... 

Supported Fe-Mn Fischer-Tropsch Catalysts. A much more limited number of studies have dealt with supported Mn-promoted Fe F-T catalysts. In this respect, it is worthwhile to mention the work of Xu et al These authors added MnO to a Fe/silicalite catalyst and observed an enhanced selectivity towards light olefins. Meanwhile the yields for methane as well as for CO2 formation were almost unaffected by MnO addition. Moreover, the conversion of CO was also insensitive to the addition of the MnO promoter. 

Mechanism of Hydrocarbon Synthesis over Fischer Tropsch Catalysts P. Biloen and W. M. H. Sachtler Surface Reactions and Selectivity in Electro-catalysis... 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

Isoalkanes can also be synthesized by using two-component catalyst systems composed of a Fischer-Tropsch catalyst and an acidic catalyst. Ruthenium-exchanged alkali zeolites288 289 and a hybrid catalyst290 (a mixture of RuNaY zeolite and sulfated zirconia) allow enhanced isoalkane production. On the latter catalyst 91% isobutane in the C4 fraction and 83% isopentane in the C5 fraction were produced. The shift of selectivity toward the formation of isoalkanes is attributed to the secondary, acid-catalyzed transformations on the acidic catalyst component of primary olefinic (Fischer-Tropsch) products. 

Silveston, P. L., Hudgins, R. R., Adesina, A. A., Ross, G. S. Feimer, J. L. 1986 Activity and selectivity control through periodic composition forcing over Fischer-Tropsch catalysts. Chem. Engng Sci. 41(4), 923-928. 

The electron microscope offers a unique approach for measuring individual nano-sized volumes which may be catalytically active as opposed to the averaging method employed by spectroscopic techniques. It is just this ability of being able to observe and measure directly small crystallites or nano-volumes of a catalyst support that sets the microscope apart from other analyses. There have been many studies reported in the literature over the past fifteen years which emphasize the use of analytical and transmission electron microscopy in the characterization of catalysts. Reviews (1-5) of these studies emphasize the relationship between the structure of the site and catalytic activity and selectivity. Most commercial catalysts do not readily permit such clear distinction of physical properties with performance. The importance of establishing the proximity of elements, elemental distribution and component particle size is often overlooked as vital information in the design and evaluation of catalysts. For example, this interactive approach was successfully used in the development of a Fischer-Tropsch catalyst (6). Although some measurements on commercial catalysts can be made routinely with a STEM, there are complex catalysts which require... 

FIGURE 1 Influence of inverse space velocity on the selectivity of CO conversion for formation of CO2 on iron-containing Fischer-Tropsch catalysts the data show that formation of CO2 is a secondary reaction (possibly accompanied to a small degree by a primary reaction forming this product). Reaction conditions T = 523 K, p = 20 bar, (Hj/COjiniet = 2 (molar). 

The inhomogeneity of the surface sites on a typical Fischer-Tropsch catalyst implies that the observed overall kinetics may vary because the selectivity of the reaction depends strongly on structure. 

These trends are consistent with observations made to characterize the chain growth of surface carbon that was deposited by methane decomposition. In a row of the periodic table, the selectivity to hydrocarbon formation was foimd to increase from right to left for example, palladium shows a lower selectivity than ruthenium 111,112). Metals such as platinum and iridium are characterized by higher selectivities for chain growth initiated from "Cl" species than other metals because of their relatively high M—C bond energies. However, platinum and iridium are unsuitable as Fischer-Tropsch catalysts because the dissociation of CO is too slow. 

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