Methanation reaction, Fischer-Tropsch

These activities were based on activities per unit metallic area rather than on activities per gram, as in the results of Fischer and Tropsch. Rostrup-Nielsen suggests that the low activity of the cobalt may be due to the fact that the reaction was carried out under conditions when the cobalt can be oxidized by the steam of the reaction mixture. For the methanation reaction, Fischer, Tropsch, and Dilthey49 give the following order of activities for the Group VIII metals ... 

Moreover, alkanes can also be produced from the reaction of reforming products, H2 and CO/CO2, via methanation and Fischer-Tropsch processes. 

Group II The activity drops more than the Ni surface concentration (Fig. 13), i.e., at least about 20 times. However, for several reactions this drop is two or more orders of magnitude. The reactions included in this group are methanation and Fischer-Tropsch synthesis, isomerization, de-hydrocyclization or hydrogenolysis of alkanes, ether formation from alcohols, metathesis of alkylamines, and possibly other reactions. 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

Metal-support interactions have been recently reviewed by Bond (93), who drew special attention to catalysts that gave evidence for strong metal-support interactions (SMSI). This condition was first observed in 1978 by Tauster et al. (94) for Pt on titania catalysts. The catalysts seemed to lose their capacity for H2 and CO chemisorption but nevertheless retained and enhanced their activity for only two types of reaction methanation and Fischer-Tropsch synthesis. Since then a considerable number of papers devoted to SMSI studies have been published all over the world. 

Conversion of Carbon Monoxide to Hydrocarbons Methanation and Fischer-Tropsch Synthesis. - CO and H2S over CoS, M0S2, and sulphided Co-Mo and Ni-W catalysts (603-723 K) gave CH4, H2, CO2, COS, CS2, and S. The initial reaction gave COS and H2 the latter then reacted with CO to give... 

Although work has not focused on the reactions of carbon to the same extent, there seems good evidence that similar species may be present on iron, cobalt, ruthenium, and iridium. So far, the studies have been concerned with active carbons as intermediates in the methanation and Fischer-Tropsch reactions. Extension of the arguments to the gasification reactions of carbons may well give interesting information as to the exact nature of species involved under different conditions. 

A case has been (X2.) made for a mechanistic commonality between gas/solid and electrocatalytic approaches to similar reactions such as the interaction of hydrogen molecules or CO with Pt surfaces. Unsupported Ru has exceptional activity for methanation and Fischer-Tropsch type gas/solid reactions (121. The electrochemical formation of CH has only been observed on Ru and not with other materials such as Pt, Mo, C, Pd, Ag, Os, Ni, GaAs, GaP, and Si (14.) Evidently the exceptional character of Ru in gas phase reactions is carried over in electrochemical systems. It is useful to discuss our electrochemical results vis it vis what is known about the gas/solid methanation reaction. However the formation of CH from CO (15.) rather than CO2 (16.), is much better characterized. 

Transient Isotopic Kinetic Studies of Methanation. - The Fischer-Tropsch reaction results in the formation of a wide distribution of hydrocarbons containing different numbers of carbon atoms. In contrast, the related reaction of methanation of CO/H2 mixtures involves only one product and is easier to study using isotope transient kinetic techniques. The results of the methanation reaction have a direct relevance to the Fischer-Tropsch reaction and are reviewed below. 

Isotopic Transient Kinetic Studies of the FTS. - Unlike methanation, the Fischer-Tropsch reaction produces a variety of hydrocarbon products having multiple carbon... 

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. 

During and subsequent to the reduction process, it is important to avoid overreduction of the magnetite active material to lower oxides, carbides, or metallic iron species. Metallic iron species are active catalysts for methanation and Fischer-Tropsch reactions. These are particularly undesirable since (7) hydrogen is consumed and (2) the exothermic nature of these reactions is such that hot spots can develop in the reactor ... 

Further reaction of CO and/or CO2 with H2 would lead to methanation or Fischer-Tropsch reactions [125]. The selectivity towards hydrogen or alkanes can be adjusted by the choice of the active metal and the supports of the catalysts. In this way, higher selectivities towards hydrogen could be achieved if Pt, Ni, and Ni-Sn are used on alumina and titania as support material, whereas Ru, Rh, and Ni and also Si02—AI2O3 as support revealed higher alkane selectivities [124,126]. 

Scheme 16.3 Comparison of kinetic expression for methanation versus Fischer-Tropsch reaction.

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

The mechanism of the Fischer-Tropsch reactions has been the object of much study (note Eqs. XVI11-55-XV111-57) and the subject of much controversy. Fischer and Tropsch proposed one whose essential feature was that of a metal carbide—patents have been issued on this basis. It is currently believed that a particular form of active adsorbed carbon atoms is involved, which is then methanated through a series of steps such as... 

Fischer-Tropsch Process. The Hterature on the hydrogenation of carbon monoxide dates back to 1902 when the synthesis of methane from synthesis gas over a nickel catalyst was reported (17). In 1923, F. Fischer and H. Tropsch reported the formation of a mixture of organic compounds they called synthol by reaction of synthesis gas over alkalized iron turnings at 10—15 MPa (99—150 atm) and 400—450°C (18). This mixture contained mostly oxygenated compounds, but also contained a small amount of alkanes and alkenes. Further study of the reaction at 0.7 MPa (6.9 atm) revealed that low pressure favored olefinic and paraffinic hydrocarbons and minimized oxygenates, but at this pressure the reaction rate was very low. Because of their pioneering work on catalytic hydrocarbon synthesis, this class of reactions became known as the Fischer-Tropsch (FT) synthesis. 

The Fischer-Tropsch reaction is highly exothermic. Therefore, adequate heat removal is critical. High temperatures residt in high yields of methane, as well as coking and sintering of the catalyst. Three types of reac tors (tubular fixed bed, fluidized bed, and slurry) provide good temperature control, and all three types are being used for synthesis gas conversion. The first plants used tubular or plate-type fixed-bed reactors. Later, SASOL, in South Africa, used fluidized-bed reactors, and most recently, slurry reactors have come into use. 

Fischer Tropsch synthesis is catalyzed by a variety of transition metals such as iron, nickel, and cobalt. Iron is the preferred catalyst due to its higher activity and lower cost. Nickel produces large amounts of methane, while cobalt has a lower reaction rate and lower selectivity than iron. By comparing cobalt and iron catalysts, it was found that cobalt promotes more middle-distillate products. In FTS, cobalt produces... 

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