Fischer-Tropsch primary products

The primary product from Fischer-Tropsch synthesis is a complex multiphase mixture of hydrocarbons, oxygenates, and water. The composition of this mixture is dependent on the Fischer-Tropsch technology and considerable variation in carbon number distribution, as well as the relative abundance of different compound classes is possible. The primary Fischer-Tropsch product has to be refined to produce final products, and in this respect, it is comparable to crude oil. The primary product from Fischer-Tropsch synthesis can therefore be seen as a synthetic crude oil (syncrude). There are nevertheless significant differences between crude oil and Fischer-Tropsch syncrude, thus requiring a different refining approach.1... 

Dancuart, L. P., De Haan, R., and De Klerk, A. 2004. Processing of primary Fischer-Tropsch products. Stud. Surf. Sci. Catal. 152 482-532. 

The Fischer-Tropsch synthesis, which may be broadly defined as the reductive polymerization of carbon monoxide, can be schematically represented as shown in Eq. (1). The CHO products in Eq. (1) are any organic molecules containing carbon, hydrogen, and oxygen which are stable under the reaction conditions employed in the synthesis. With most heterogeneous catalysts the primary products of the reaction are straight-chain alkanes, while the secondary products include branched-chain hydrocarbons, alkenes, alcohols, aldehydes, and carboxylic acids. The distribution of the various products depends on both the type of catalyst and the reaction conditions employed (4). 

In view of the size of operation being contemplated, it is unlikely that homogeneous catalysts will play a primary role in the production of synthetic oil. However, from the standpoint of the chemical industry, the complex mixture of products obtained from the classical Fischer-Tropsch process is generally unattractive owing to the economic constraints imposed by costly separation/purification processes. What is needed is a catalyst system for the selective conversion of CO/H2 mixtures to added-... 

The existence of such a growth step is consistent with the high proportion of C2 products found in the Ir4(CO)12/NaCl-2AlCl3 system (59). Furthermore, in systems where dimerization is less favored, hydrogenation of the primary carbene fragment could explain the considerable amounts of methane formed in many heterogeneous Fischer-Tropsch systems. 

Gas-to-liquids (GTL) is the chemical conversion of natural gas into petroleum products. Gas-to-liquid plants use Fischer-Tropsch technology, which first converts natural gas into a synthesis gas, which is then fed into the Fischer-Tropsch reactor in the presence of a catalyst, producing a paraffin wax that is hydro-cracked to products (see also Chapter 7). Distillate is the primary product, ranging from 50% to 70% of the total yield. 

Current interest in synthetic fuels production by Fischer-Tropsch (FT) reactions have created a need for removal of byproduct oxygenates, formed by the FT reaction. The oxygenates consist of primary and internal alcohols, aldehydes, ketones, esters and carboxylic acids. The hydrocarbon products derived from the FT reaction range from methane to high molecular weight paraffin waxes containing more than 50 carbon atoms. 

The objective of tiie research described here is to explore synthesis gas generation by direct oxidation of CH4 (reaction 3). A reactor giving complete conversion to a 2/1 mixture of H2 and CO would be the ideal upstream process for the production of CH3OH or for the Fischer-Tropsch process. As discussed above, currently implemented or proposed processes utilize a combination of oxidation and reforming reactions to generate synthesis gas from CH4 and O2. In this work, we seek a faster, more efficient route of syngas generation in which H2 and CO are the primary products of CH4 oxidation. It is expected that this may be difficult because... 

The oxo reaction (31) is carried out in the liquid phase at high pressure using a cobalt catalyst. A mixture of aldehyde isomers is always produced, each isomer being one carbon number higher than the starting olefin. As a group the oxygenated products of the hydrocarbon synthesis (Fischer-Tropsch) process and the oxo process are primary compounds and thus (except, of course, the methyl and ethyl derivatives) differ fundamentally from the products based on alcohols made by the hydration of olefins, which are always secondary or tertiary in structure. 

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. 

Modification of the zeolite appears to have affected the selectivity of Ru in these hydrogenation reactions. Exchange of K cations for Na cations in Y zeolite increases the basicity of the support (ref. 9). In Fischer-Tropsch reactions over similar catalysts, Ru/Y catalysts so modified yielded significant increases in the olefinic product fraction at the expense of paraffins. Olefins are believed to be primary products in F-T synthesis, with paraffins being produced from olefins in secondary hydrogenation reactions. In an analogous fashion, the Ru/KY catalyst used in the present study might also be expected to... 

The hydroformylation (or 0x0 ) reaction was discovered in 1938 by Roelen who was working on the formation of oxygenates as by-products of the Fischer-Tropsch reaction over cobalt catalysts. It soon became clear that the aldehydes and alcohols found were the products of secondary reactions undergone by the 1-alkenes (which are the primary products of the Fischer-Tropsch reaction, Section 4.7.2) with syngas. Further work showed that Roelen had discovered a new reaction, in which the elements of H and CHO were added to an olefin (hence hydroformylation), and which was catalyzed by cobalt. It was later found that the true precatalyst was not cobalt metal but derivatives of dicobalt octacarbonyl, such as the hydride, CoH(CO)4. 

In this review, we limit ourselves to the mechanisms of primary product formation, which are fimdamental to Fischer-Tropsch chemistry. Using new information mainly from computational studies, we focus on two coidlicting hypotheses regarding the key reaction steps that lead to chain growth. 

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). 

Available reaction-transport models describe the second regime (reactant transport), which only requires material balances for CO and H2. Recently, we reported preliminary results on a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) and interparticle (convection) transport processes (4, 5). The model clearly demonstrates how diffusive and convective restrictions dramatically affect the rate of primary and secondary reactions during Fischer-Tropsch synthesis. Here, we use an extended version of this model to illustrate its use in the design of catalyst pellets for the synthesis of various desired products and for the tailoring of product functionality and molecular weight distribution. 

We conclude that linear and branched olefins and paraffins can be formed after one surface sojourn by termination of growing surface chains. Therefore, they are primary Fischer-Tropsch products (4,14). a-Olefins readsorb and initiate chains in a secondary reaction. Thus, olefins reenter the primary chain growth process and continue to grow. These chains ultimately terminate as olefins or paraffins, in a step that can resemble a secondary hydrogenation reaction because it leads to the net consumption of olefins and to the net formation of paraffins, but which proceeds via primary FT synthesis pathways. 

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

Recent studies indicate that a-olefins, the major primary products formed during Fischer-Tropsch synthesis, participate in secondary reactions [12], Chains can terminate either by P-hydrc en abstraction to form an a-olefin or by H-addition to form a paraffin [13,14], Olefins can undergo secondary reactions by subsequent readsorption leading to isomerization or hydrogenation. We observe selectivity relationships that are consistent with Egiebor s proposal that significant secondary hydrogenation reactions can occur on iron catalysts [12],... 

Any mechanistic proposal must comply with the following observations. (1) The Fischer-Tropsch hydrocarbon synthesis follows the formalism of polymerization kinetics with a Schulz-Flory distribution of the molecular weights. (2) a-Olefins and alcohols occur as the primary products. (3) The aliphatic final products are formed consecutively by hydrogenation of the olefins according to " C-labeling experiments [4 f, 30 b]. (4) Chain termination processes do not deactivate the catalyst centers because the chain-growth velocity stays constant for weeks. 

The high selectivities for propylene which can be as high as 45 % (12) and the low selectivities for ethylene suggest that ethylene could be a primary product in Fischer-Tropsch which could undergo a secondary reaction leading selectivity to propylene. 

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