Fischer-Tropsch conditions

FIGURE 9.10 Thermodynamic view of cobalt surface segregation in the presence of CO (and H2). Activating catalyst restructuring under Fischer-Tropsch conditions. 

As a check to confirm that no extraneous non-polymer-attached catalytic species were present, the following experiment was performed. Polystyrene without attached cyclopentadiene was exposed to Co2(C0)e, extracted using a Soxhlet extractor and dried in vacuo in exactly the same manner as was used to synthesize 5. When used under the above Fischer-Tropsch reaction conditions, these treated, white polystyrene beads did not discolor, release any detectable species into solution, cause a CO/H2 pressure drop, or result in the formation of any detectable amounts of methane. These observations argue against the presence of small amounts of occluded Co2(C0)e or C04 (CO) 12 which could conceivably have been active or precursors to active species. It should be noted that the above clusters were reported to be essentially inactive under Fischer-Tropsch conditions (140°C, toluene, 1.5 atm., 3/1 H2/CO, three days) leading to mere traces of methane (11). The lack of products under our conditions also indicates that, at least in the absence of resin-bound CpCo(C0)2 or its derivatives, the polystyrene support did not degrade. 

The conversion of iron catalysts into iron carbide under Fischer-Tropsch conditions is well known and has been the subject of several studies [20-23], A fundamentally intriguing question is why the active iron Fischer-Tropsch catalyst consists of iron carbide, while cobalt, nickel and ruthenium are active as a metal. Figure 5.9 (left) shows how metallic iron particles convert to carbides in a mixture of CO and H2 at 515 K. After 0.5 and 1.1 h of reaction, the sharp six-line pattern of metallic iron is still clearly visible in addition to the complicated carbide spectra, but after 2.5 h the metallic iron has disappeared. At short reaction times, a rather broad spectral component appears - better visible in carburization experiments at lower temperatures - indicated as FexC. The eventually remaining pattern can be understood as the combination of two different carbides -Fe2.2C and %-Fe5C2. 

When the Fe-MnO catalyst is analyzed after use in the Fischer-Tropsch reaction (the synthesis of hydrocarbons from CO and H2), the XRD pattern in Fig. 6.2 reveals that all metallic iron has disappeared. Instead, a number of weak reflections are visible, which are consistent with the presence of iron carbides, as confirmed by Mossbauer spectroscopy [7]. The conversion of iron to carbides under Fischer-Tropsch conditions has been studied by many investigators and has been discussed in more detail in Chapter 5 on Mossbauer spectroscopy. 

R. S. Albal, Y. T. Shah, N. L. Carr and A. T. Bell, Mass transfer coefficients and solubilities for hydrogen and carbon monoxide under Fischer-Tropsch conditions, Chem. Eng. Sci., 1984, 39, 905-907. 

An important subsequent observation seemed to indicate that carbides are not reactive under Fischer-Tropsch conditions.235 When carbon was deposited on a surface by the decomposition of l4CO, labeled carbon was not incorporated into the products. This and other evidence accumulated against the carbide mechanism by the 1950s led to the formulation of other mechanisms. The hydroxymethylene or enolic mechanism191 assumes the formation via the hydrogenation of carbon monoxide [Eq. (3.13)] of a surface-bound hydroxymethylene species (2) ... 

The conversion of iron catalysts into iron carbide under Fischer-Tropsch conditions is well known, and has been the subject of several studies [21-25]. A funda-... 

Wilson and de Groot (8) performed elegant experiments with singlecrystal cobalt surfaces. During exposure to s)mthesis gas under Fischer-Tropsch conditions, large areas were found to restructure specifically, corrugated islands a few nanometers in diameter appeared on the initially present surface terraces. 

Where there are very strong M—C interactions, carbide formation can occur. For example, reduced iron is converted into Fe Cy under the Fischer-Tropsch conditions. The carbide is characterized by a lower M—C bond energy than pure metallic iron. The high chain-growth probability observed with iron-containing catalysts is a result of the relatively weak M—C bond on the iron carbide surface. 

Laboratory Fischer-Tropsch synthesis tests were performed in a slurry-phase Constant Stirred Tank Reactor. The pre-reduced catalyst (20-30 g) was suspended in ca 300 ml molten Fischer-Tropsch wax. Realistic Fischer-Tropsch conditions were employed, i.e. 220 °C 20 bar commercial synthesis gas feed 50 vol% H2, 25 vol% CO and 25 vol% inerts synthesis gas conversion levels in excess of 50%. Use was made of the ampoule sampling technique as the selected on-line synthesis performance monitoring method [23]. 

The unmodified alumina-supported cobalt catalyst (catalyst A) was tested in the pilot plant slug-flow reactor under realistic Fischer-Tropsch conditions. The produced wax was... 

Carbon monoxide is hydrogenated over ruthenium zeolites in both methanation and Fischer-Tropsch conditions. is exchanged in the zeolite as the amine complex. The zeolites used are Linde A, X, Y, and L, natural chabazitey and synthetic mordenite from Norton. The zeolites as a support for ruthenium were compared with alumina. The influence of the nature of the zeolite, the ruthenium metal dispersion and the reaction conditions upon activity and product distribution were investigated. These zeolites are stable methanation catalysts and under the conditions used show a narrow product distribution. The zeolites are less active than other supports. Sintering of ruthenium metal in the zeolite supercages shows only minor effects on methanation activity, although under our Fischer-Tropsch conditions more C2 and C3 are formed. 

In this work, experiments at ambient pressure were carried out under methanation and Fischer-Tropsch conditions. The zeolites as a support for ruthenium were compared with a more conventional one (alumina). The influence of the nature of the zeolite, the dispersion of the ruthenium metal and the reaction conditions upon activity and product distribution were investigated. 

Ruthenium zeolites are active and stable methanation catalysts. Under the Fischer-Tropsch conditions used here they show a narrow product distribution. When the size of the ruthenium particles enclosed in the zeolite cages is increased, there is hardly any eflFect found on the methanation activity. Under F. T. conditions a higher amount of C2 and C3 products are formed. Zeolites are generally less active than other supports. In the class of zeolite supports, the less acidic zeolites act as promoters of the CO hydrogenation under methanation conditions the... 

Table 2. Activities and selectivites measured in a fixed-bed reactor at Fischer-Tropsch conditions (T = 473 K, P = 20 bar, Hj/CO = 2.1).

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