Carbon monoxide hydrocarbon synthesis selectivity

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). 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

Selection of a process for hydrogen manufacture from hydrocarbons and coal therefore depends on the raw material and its cost, the scale of operation, the purity of the synthesis gas to be produced, the pressure level of the natural gas feed, and the number and type of processes that will consume the carbon monoxide and hydrogen. 

All the binary Cu/ZnO catalysts were found highly selective toward methanol without DME, methane, or higher alcohols and hydrocarbons detected in the product by sensitive gas chromatographic methods (59). Several of the composites were also found to be very active when subjected to a standard test with synthesis gas C0/C02/H2 = 24/6/70 at gas hourly space velocity of 5000 hr- pressure 75 atm, and temperature 250°C. The activities, expressed as carbon conversions and yields, are summarized in Table VIII. The end members of the series, pure copper and pure zinc oxide, were inactive under these testing conditions, and maximum activity was obtained for the composition Cu/ZnO = 30/70. The yields per unit weight, per unit area of the catalyst or the individual components, turnover rates per site titratable by irreversible oxygen and by irreversible carbon monoxide, are graphically... 

Third, and not least, the mechanistic features of the Fischer-Tropsch hydrocarbon synthesis mirror a plethora of organometallic chemistry. More precisely Molecular models have been invoked that could eventually lead to more product selectivity for eq. (1). Although plausible mechanistic schemes have been considered, there is no way to define precisely the reaction path(s), simply because the catalyst surface reactions escape detection under real process conditions (see Section 3.1.1.4). Nevertheless, the mechanism(s) of reductive hydrocarbon formation from carbon monoxide have strongly driven the organometallic chemistry of species that had previously been unheard of methylene (CH2) [7-9] and formyl (CHO) [10] ligands were discovered as stable metal complexes (Structures 1-3) only in the 1970s [7, 8]. Their chemistry soon explained a number of typical Fischer-Tropsch features [11, 12]. At the same time, it became clear to the catalysis community that molecular models of surface-catalyzed reactions cannot be... 

In conclusion, methane and carbon monoxide are not the most desirable products in those reactions. Hydrocarbons of medium (C4-C10) or very high (up to C40-C50) molecular weights have been produced in recent years from synthesis gas on various classical catalysts. Lanthanide-based intermetallic compounds do not seem to be very good candidates for that purpose, if one considers the poor selectivities obtained so far. 

Surface-science studies succeeded to identify many of the molecular ingredients of surface catalyzed reactions. Each catalyst system that is responsible for carrying out important chemical reactions with high turnover rate (activity) and selectivity has unique structural features and composition. In order to demonstrate how these systems operate, we shall review what is known about (a) ammonia synthesis catalyzed by iron, (b) the selective hydrogenation of carbon monoxide to various hydrocarbons, and (c) platinum-catalyzed conversion of hydrocarbons to various selected products. 

Catalysts not only accelerate a chemical reaction, but also help to channel a reaction to produce a desired product. This selectivity does not contradict the fact that the position of equilibrium itself cannot be influenced. It only means that under given circumstances, one of the many possible spontaneous parallel reactions will be considerably more accelerated than the others. For example, the process of hydrogenating carbon monoxide (Fischer-Tropsch synthesis) can produce methanol (catalysts ZnO, Cr203) or unsaturated hydrocarbons (catalyst Fe), depending upon the type of catalyst used and the reaction conditions. In contrast, we use the term specificity if a catalyst only affects certain substances. Very high selectivity and specificity can be found in reactions catalyzed by enzymes. These are very important reactions that will be gone into more detail in the next section. 

The Fischer-Tropsch s)mthesis is a process to convert synthesis gas (a mixture of carbon monoxide and hydrogen) to hydrocarbons that can be used as for instance transportation fuels. In the process all (straight chain) hydrocarbons fi om methane to heavy waxes are produced. In general this product distribution can be described by an Anderson-Schulz-Flory distribution based on a constant chain growth probability. As a consequence the selectivity towards diesel production is limited. When the diesel fraction is defined as CIO till C20, the maximum fraction of diesel that can be obtained is 39.4%, reached at a chain growth probability of 0.87. 

Synthesis from Carbon Monoxide. The carbonylation of aromatic hydrocarbons catalyzed by Pd(OCOCH3)2 in the presence of K2S2O8 has already been reported (53). C—H activation occurs electrophilically with ortho- and para-selectivities for electron-donating substituents. 

In the production of synthesis gases from hydrocarbons, the components hydrogen and carbon monoxide usually appear as complementary products, carbon dioxide can be obtained as a by-product as well. Apart from hydrocarbons and steam (steam reforming), some processes require carbon dioxide (CO2 reforming) as well as oxygen or air (partial oxidation and autothermal reforming) as feedstock. Usually, the process selection depends on two factors ... 

The partial oxidation of methane in catalytic monoliths at short contact-times is another example with several empty routes illustrating importance of thermodynamic consistency in selection of kinetic parameters. This reaction offers a promising route for the conversion of natural gas into more useful chemicals such as synthesis gas (syngas), a mixture of hydrogen and carbon monoxide. Syngas can subsequently be converted into methanol or higher hydrocarbons. The kinetic model for partial oxidation of methane on Rh includes 19 reversible reactions with six-gas phase species and 11 adsorbed species [5]. Presence of 19 steps, one balance equation (which relates coverage of surface species) and... 

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