The water gas shift reaction

Reaction of coal and steam generates water gas (eq. 3.17) in which the proper ratio of carbon monoxide and hydrogen is adjusted by making use of the water gas shift reaction (WGSR) (eq. 3.18, 3.19). 

Based on the study of the chemistry of metal carbonyls under WGSR conditions, it is generally accepted that the reaction involves the following steps. 

The best known catalysts of the water gas shift reaction are the binary metal carbonyls ([Fe(CO)5], [M(CO)6] M = Cr, Mo, W [342,345-347]) and carbonyl clusters ([Ru3(CO)i2], [Rh fCO) ], [Ir4(CO)i2]) [347], In the basic solutions under 35-120 bar CO, elevated temperature (100-125 °C) is needed for reasonable catalytic activity. Often these complexes are used in solutions of nitrogen-containing bases (amines, pyridine or picolines) or the precursors are themselves complexes of N-donor ligands (substituted bipyridines and phenantrolines [348], pyridine, 2-, 3- or 4-picoline [349], 

The basis of this unusual behaviour is in that with CO and trifluoroacetate [Ru2(p.-r 2-02CCF3)2(CO)6] undergoes disproportionation to [Ruj(CO)i2] and to [/ac-Ru(02CCF3)3(C0)3]+ and these two complexes act jointly, but on separate ways to yield Y2H2 and C02 + A H2, respectively. What makes this mechanistic suggestion sound (and the investigations elegant) is in that all intermediates were isolated or characterized, and all steps of the reaction were studied individually, too. 

The Water-gas Shift Reaction.—This reaction is catalysed by M(CO) (activity M = W Mo Cr) in the presence of base and under phase-transfer conditions these carbonyls, in common with MS(CO)i2 (M =Ru or Os), are also active in the presence of sodium sulphide. The most active catalysts reported are Fe(CO)6 in basic methanol (turnover No. 2000 per day at 180 °C ) and Rh6(CO)i6 with diamine co-catalysts (e.g., en, turnover No. a 25 h at 100 C). Photolysis of [RuCl(CO)(bipy)a]Cl in water under CO produces COa and catalytically the CO2 is produced in a thermal step, whereas the formation of Ha is photo-initiated. Water-gas has also been used to hydroformylate pent-1-ene in the presence of ruthenium complexes similarly, water-gas is used in reaction (9), which is catalysed by a variety of Group VIII metal complexes 

Two stepwise methods for effecting the water-gas shift reaction at atmospheric pressure are outlined below [Equations (10), (II), and (12), X=C1 or Br, pnp=2,6-bis(diphenylphosphinomethyl)pyridine]  

Heterogenized Catalysts.—When a catalyst derived from FeafCOIia supported on magnesia is treated with either CO-t-HgO or ethylene, propylene is formed selectively.  

Other studies in this area are summarized in Table 2. 

6 Carbon Monoxide Reduction, The Water Gas Shift Reaction and Reactions of Carbon Dioxide 

1 Carbon Monoxide Reduction.- Books describe the catalytic conversions of synthesis gas and alcohols to chemicals , and the chemistry of the catalysed hydrogenation of carbon monoxide . 

The role of metal hydrides in the catalytic reduction of CO via both intra- and intermolecular hydride transfer pathways has been discussed . The activation of CO by reaction with a secondary amine and subsequent reduction to the methylamine is catalysed by Group VIII carbonyls albeit at very low rates . 

2 The Water Gas Shift Reaction.- A review of transition metal carbonyls as precursors for water gas shift catalysts has appearedise, [HRU3(CO)ji] anchored to silica via ammonium or pyridinium groups is active at low temperatures 100-150°C and [Rhe(CO)i6] chemisorbed on 11-AI2O3 or [RhCl(00)2)2 chemisorbed on n-AljOg, NaY zeolite and HY zeolite are active between 25-l00 C , 

The ratio of CO to H2 in syngas can be controlled by the water-gas shift reaction (WGSR, Equation 7) and it is possible to make either hydrogen or carbon monoxide, or any ratio of the two, by suitable adjustment of conditions. 

The WGSR is normally practised as a heterogeneously metal-catalyzed reaction Fe is the most commonly used catalyst. However other metals are also active, for example the homogeneous Rh/H catalyst in the Monsanto acetic acid process (Section 4.2.4) concurrently catalyzes the WGSR via a Rh(I)/ Rh(III) cycle (Equations 8 and 9), 

In these reactions, CO coordinated to a transition metal (particularly in a high oxidation state) becomes susceptible to nucleophilic attack by water, leading to release of CO2 [mechanism E. C. Baker, D. E. Hendricksen, R. Eisenberg, J. Am. Chem. Soc., 1980, 102, 1020]. 

after removing CO2 and H2O, hydrogen or hydrogen mixtures adequate for different applications can be obtained. 

The water-gas shift reaction may be carried out at 360 °C by using the metal oxide Fe304-Cr203 as a heterogeneous catalyst. However, because of the thermodynamic parameters of the reaction - AG =—6.82 kcal mole AH = — 9.84 kcalmole and AS = —10.1 eu at 298 K-its efficiency and thermal input requirements are more favorable at lower temperatures. 

Some organometallic compounds and among them some metal cluster carbonyls have proved to be interesting homogeneous catalysts for the water-gas shift reaction at relatively low temperatures. Selected examples of catalysis of this reaction by cluster are described in Table 2.30. 

Ungerman C, Landis V, Moya SA, Cohen H, Walker H, Pearson RG, Rinker RG, Ford PC (1979) J. Am. Chem. Soc. 101 5922. 

However, the intermediate concentration in the reaction mixture is always low, probably due to its high reactivity with carbon monoxide. 

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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]. 

Again with platinized Ti02, ultraviolet irradiation can lead to oxidation of aqueous CN [323] and to the water-gas shift reaction, CO + H2O = H2 + CO2 [324]. Some mechanistic aspects of the photooxidation of water (to O2) at the Ti02-aqueous interface are discussed by Bocarsly et al. [325]. 

Methanol (qv) is one of the 10 largest volume organic chemicals produced in the wodd, with over 18 x 10 t of production in 1990. The reactions for the synthesis of methanol from CO, CO2, and H2 are shown below. The water gas shift reaction also is important in methanol synthesis. 

Study of the mechanism of this complex reduction-Hquefaction suggests that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the ceUulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (46). In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur will not be developed. However, the Hquefaction mechanism probably involves catalytic hydrogenation because carbon monoxide would be expected to form at least some hydrogen by the water-gas shift reaction. 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

Reactions of Synthesis Gas. The main hydrogen manufacturing processes produce synthesis gas, a mixture of H2 and CO. Synthesis gas can have a variety of H2-to-CO ratios, and the water gas shift reaction is used to reduce the CO level and produce additional hydrogen, or to adjust the H2 to-CO ratio to one more beneficial to subsequent processing (69) ... 

In the next step, the CO is converted to CO2 and hydrogen by the water gas shift reaction step ... 

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

These reactions show that the synthesis gas stoichiometry is dependent on both the nature of the feedstock as well as the generation process. Reactions 4 and 5, together with the water gas shift reaction 3, serve to independently determine the equiUbrium composition of the synthesis gas. 

Synthesis gas, a mixture of CO and o known as syngas, is produced for the oxo process by partial oxidation (eq. 2) or steam reforming (eq. 3) of a carbonaceous feedstock, typically methane or naphtha. The ratio of CO to may be adjusted by cofeeding carbon dioxide (qv), CO2, as illustrated in equation 4, the water gas shift reaction. 

The overall process for producing a 1 1 CO to ratio by partial methane oxidation and the water gas shift reaction is represented by equation 5. 

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

This is the reverse of the water-gas shift reaction in the production of hydrogen and ammonia (qv). Carbon dioxide may also be reduced catalyticaHy with various hydrocarbons and with carbon itself at elevated temperatures. The latter reaction occurs in almost all cases of combustion of carbonaceous fuels and is generally employed as a method of producing carbon monoxide. 

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

Metal coordination compounds may also provide alternatives to the heterogeneous catalysts used for the water gas shift reaction. In fact, Ru, Rh, Ir, and Pt coordination compounds have all shown some promise (27). 

Hydrogen and carbon monoxide are produced by the gasification reaction, and they react with each other and with carbon. The reaction of hydrogen with carbon as shown in reaction (27-15) is exothermic and can contribute heat energy. Similarly, the methanation reaction (27-19) can contribute heat energy to the gasification. These equations are interrelated by the water-gas-shift reaction (27-18), the equilibrium of which controls the extent of reactions (27-16) and (27-17). 

The University of Veszprem in Hungary extended UCKRON with the water-gas shift reaction. This modification, which has six steps in its mechanism, is called the VEKRON test problem. (Arva and Szeifert 1989). 

CO2 can be readily obtained in small amounts by the action of acids on carbonates. On an industrial scale the main source Is as a byproduct of the synthetic ammonia process in which the H2 required is generated either by the catalytic reaction (a) or by the water-gas shift reaction (b) ... 

The carbon monoxide can then be further reacted with steam and/or hydrogen in the water gas shift reaction ... 

CASE STUDY FUEL AND CHEMICAL PRODUCTION FROM THE WATER GAS SHIFT REACTION BY FERMENTATION PROCESSES... 

The carbonyl complex [Ru(EDTAH)(CO)] has been reported to be a very good catalyst for reactions like hydroformylation of alkenes, carbonylation of ammonia and ammines as well as a very active catalyst for the water gas shift reaction. The nitrosyl [Ru(EDTA)(NO)] is an oxygen-transfer agent for the oxidation of hex-l-ene to hexan-2-one, and cyclohexane to the corresponding epoxide. 

An examination of some laboratory runs with diluted C150-1-02 catalyst can illustrate this problem. In one run with 304°C at inlet, 314 °C at exit, and 97,297 outlet dry gas space velocity, the following results were obtained after minor corrections for analytical errors. Of the CO present (out of an inlet 2.04 mole % ), 99.9885% disappeared in reaction while the C02 present (from an initial 1.96%) increased by over 30%. Equilibrium carbon oxides for both methanation reactions were essentially zero whereas the equilibrium CO based on the water-gas shift reaction at the exit composition was about one-third the actual CO exit of 0.03 mole %. From these data, activities for the various reactions may be estimated on the basis of various assumptions (see Table XIX for the effect of two different assumptions). 

Effects of Cold Gas Recycle and Approach to Equilibrium. Product gases resulting from various CGR ratios were analyzed (Table XI). For the experiments tabulated, a decrease in the cold recycle ratio resulted consistently in increases in the product gas concentrations of water vapor, hydrogen, and carbon dioxide and a decrease in methane concentration. These trends may be noted in experiment HGR-12 as the CGR ratio decreased from 8.7 1 to 1.2 1, in experiment HGR-13 as it increased from 1.0 1 to 9.1 1, and in experiment HGR-14 as it decreased from 3.0 1 to 1.0 1. These trends indicate that the water-gas shift reaction (CO + H20 —> C02 + H2) was sustained to some degree. Except for the 462-hr period in experiment HGR-14, the apparent mass action constants for the water-gas shift reaction (based on the product gas compositions in Table XI) remained fairly constant at 0.57-1.6. These values are much lower than the value of 11.7 for equilibrium conversion at 400°C. In... 

Oxidation of carbon monoxide by metal ions and homogeneous catalysis of the water gas shift reaction and related processes. J. Halpern, Comments Inorg. Chem., 1981,1, 3-15 (42). 

Alkali promoters are often used for altering the catalytic activity and selectivity in Fischer-Tropsch synthesis and the water-gas shift reaction, where C02 adsorption plays a significant role. Numerous studies have investigated the effect of alkalis on C02 adsorption and dissociation on Cu, Fe, Rh, Pd, A1 and Ag6,52 As expected, C02 always behaves as an electron acceptor. 

Write a balanced chemical equation for (a) the hydrogenation of ethyne (acetylene, C2H2) to ethene (C2H4) by hydrogen (give the oxidation number of the carbon atoms in the reactant and product) (b) the shift reaction (sometimes called the water gas shift reaction, WGSR) (c) the reaction of barium hydride with water. 

The reaction is endothermic, but large amounts of steam are used to minimize the temperature drop and, by way of the water-gas shift reaction, to prevent accumulation of coke on the catalyst. Ignore the reverse and competitive... 

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