Synthesis gas, conversion

The catalytic reaction between CO and H2 to hydrocarbons and methanol is mechanistically related to the ammonia synthesis. Many different products, ranging from methane to the longer hydrocarbons encountered in diesel oil or in waxes, as well as oxygenated products such as methanol, ethanol, and acetaldehyde can be formed depending on the choice of the catalyst and the reaction conditions. 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

CO dissociation is an activated reaction step, so the reactive surface is covered with CO. Whereas adsorption of CO requires one surface site, dissociation of CO requires empty surface sites (6.5). Upon dissociation of a molecule, two adatoms are generated that now will occupy two surface sites. When a surface is completely covered by CO, at least one CO molecule beside the reacting CO has to be removed in order to make dissociation of the other CO molecules possible. The overall effect is that the CO hydrogenation reactions have a negative reaction order in CO. The order in hydrogen is positive, however. Many promoters added to Fischer-Tropsch catalysts enhance the rate by an increase of the rate of CO dissociation (Section 6.6.5). 

In spite of the factors that have a negative effect on the probability that the CO bond breaks, CO dissociation is not the rate-determining step. If this were the case, then the rate would not exhibit a positive order in hydrogen. If the reaction between an adsorbed carbon and the first hydrogen atom were rate determining, and all subsequent reactions fast, it would be 

In this model, the order in CO falls between -3/2 and 1/2, while the order in hydrogen would be 1/2 at most, corresponding to the limit of weak adsorption. 

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Dimethyl Ether. Synthesis gas conversion to methanol is limited by equiUbrium. One way to increase conversion of synthesis gas is to remove product methanol from the equiUbrium as it is formed. Air Products and others have developed a process that accomplishes this objective by dehydration of methanol to dimethyl ether [115-10-6]. Testing by Air Products at the pilot faciUty in LaPorte has demonstrated a 40% improvement in conversion. The reaction is similar to the Hquid-phase methanol process except that a soHd acid dehydration catalyst is added to the copper-based methanol catalyst slurried in an inert hydrocarbon Hquid (26). 

A similar process to SMDS using an improved catalyst is under development by Norway s state oil company, den norske state oHjeselskap AS (Statod) (46). High synthesis gas conversion per pass and high selectivity to wax are claimed. The process has been studied in bubble columns and a demonstration plant is planned. 

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. 

Status of Indirect Liquefaction Technology The only commercial indirect coal liquefaction plants for the production of transportation fuels are operated by SASOL in South Africa. Construction of the original plant was begun in 1950, and operations began in 1955. This plant employs both fixed-bed (Arge) and entrained-bed (Synthol) reactors. Two additional plants were later constructed with start-ups in 1980 and 1983. These latter plants employ dry-ash Lurgi Mark IV coal gasifiers and entrained-bed (Synthol) reactors for synthesis gas conversion. These plants currently produce 45 percent of South Africa s transportation fuel requirements, and, in addition, they produce more than 120 other products from coal. 

Coproduction of electricity along with synthesis gas conversion offers the potential for significant cost savings. The once-through liquid-phase methanol technology was developed specifically for this... 

In the following we will concentrate on three important cases, i.e. CO oxidation on alkali doped Pt, ethylene epoxidation on promoted Ag and synthesis gas conversion on transition metals. We will attempt to rationalize the observed kinetic behaviour on the basis of the above simple rules. 

A finely ground physical mixture of 5% Ru (as RuC )/ZSM-5 which was subsequently pelletized was used in the study of the effects of process variables on synthesis gas conversion. 

Ru, RuCo, and Co carbonyl cluster-derived catalysts, 38 362-363 structural model, 38 366-367 synthesis gas conversion, 38 364-365 ZSM-5-supported Pd catalysts, 39 207-208... 

Figure 19.3 Synthesis gas conversion as a function of time for the precipitated iron oxide catalyst pretreated with CO (weight = 72.7 g, Sg = 32 m2 g ). O, CO , H2 O, CO + H2.

The above described reactor is useful for the measurements of heat of reaction as well as thermal behavior of gas-liquid or gas- liquid-solid, high-pressure, high-temperature reactions. Since the reactor can be operated under adiabatic conditions, it simulates the commercial operation. The reactor was successfully utilized by Bhattacharjee et al. (1986) for investigating thermal behavior of slurry phase, catalytic synthesis gas conversion. 

In a recent communication (250), deviations from Schulz-Flory kinetics were observed for a RuNaY synthesis gas conversion catalyst (see Fig. 24). A comparative catalyst, prepared by impregnating silica with ruthenium, i.e., Ru/SiOz, and tested under the same conditions, yielded a product distribution which gave a good fit to Schulz-Flory kinetics. The sharp decrease in chain growth probability for Cf0 products over RuNaY is perhaps surprising for such a relatively large-pore zeolite. Further studies (251-253) on this system indicated that there was a correlation between the ruthenium particle size in the zeolite and the product distribution. 

Evaluation of Some New Zeolite-Supported Metal Catalysts for Synthesis Gas Conversion... 

The efficiency and selectivity of a supported metal catalyst is closely related to the dispersion and particle size of the metal component and to the nature of the interaction between the metal and the support. For a particular metal, catalytic activity may be varied by changing the metal dispersion and the support thus, the method of synthesis and any pre-treatment of the catalyst is important in the overall process of catalyst evaluation. Supported metal catalysts have traditionally been prepared by impregnation techniques that involve treatment of a support with an aqueous solution of a metal salt followed by calcination (4). In the Fe/ZSM-5 system, the decomposition of the iron nitrate during calcination produces a-Fe2(>3 of relatively large crystallite size (>100 X). This study was initiated in an attempt to produce highly-dispersed, thermally stable supported metal catalysts that are effective for synthesis gas conversion. The carbonyl Fe3(CO) was used as the source of iron the supports used were the acidic zeolites ZSM-5 and mordenite and the non-acidic, larger pore zeolite, 13X. 

Catalytic Evaluation In order to investigate support effects in these iron/zeolite catalysts prepared from Fe3(C0)12 by the extraction technique, three catalysts of similar weight percent iron loading were evaluated for their ability to catalyze synthesis gas conversion these catalysts were 15.0% Fe/ZSM-5, 16.4% Fe/Mordenite andl5.0% Fe/13X. All catalysts were evaluated under similar conditions as described in the experimental section. Catalytic data is presented in the accompanying figures in each figure the first three points for each catalyst are data obtained at 280°C, the second three points are data at 300°C. 

The catalysts evaluated are active for synthesis gas conversion the percent conversion of H2 and CO is shown for each catalyst in Figure 1 as a function of time under evaluation conditions and temperature. At 280°C the percent conversion of synthesis gas increases with time for the acidic zeolite-supported catalysts, Fe/ZSM-5 and Fe/Mordenite, but decreases for the larger pore, non-acidic zeolite-supported catalyst Fe/13X. The percent conversion increases for all catalysts at 300°C for Fe/ZSM-5 and Fe/Mordenite the conversions remain constant at this temperature for several days, although for Fe/13X the conversion increases with time. The trends in % synthesis gas conversion, particularly % CO, are reflected in the weight % hydrocarbons, carbon dioxide and water obtained in the reactor effluent over the period of evaluation, see Figure 2. It is apparent that the catalysts are effective for the production of hydrocarbons from synthesis gas, but also catalyze the water gas shift reaction the % hydrocarbons and%C02 obtained are greater at the higher temperature (300°C) whereas the % H2O is less at this temperature than at 280°C. 

This study was initiated in an attempt to produce highly-dispersed, thermally-stable, zeolite-supported metal catalysts and to investigate the effect of acidity and pore size of the zeolite on the products obtained from synthesis gas conversion. As a result of this study, several conclusions can be made. 

Second, the iron oxide readily forms the iron carbide X-Fe5C2 after reduction in hydrogen and subsequent carbiding in synthesis gas. The thermally stable, long-lived catalysts that are obtained are active for synthesis gas conversion. 

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