CO, hydrogenation

The carbonyls in [Ru3(CO)i2]/NaY are eliminated by heating under vacuum at 593 K, leaving the Ru particles with a size of 15-20 A. No further aggregation occurs under the CO + H2 reaction conditions and the resulting catalysts yields a product spectrum centered on C4 hydrocarbons with a sharp cut-off at Ballivet- 

Tkatchenko et al. also reported the similar observation that Fe3(CO)i2 incorporated in NaY decomposed to give highly dispersed Fe particles which chemisorbed CO to form the carbonyl species characterized by IR spectra. The resulting catalyst provided higher selectivities toward lower olefins with an upper limit of Cg-Cio, 

HM3(CO)i2 cations (M = Ru, Os) were formed inside the galleries of pillared clay (alumina montmorillorite) which afford Ru (or Os) particles ( 50 A) embedded within the elay sheets after H2 reduction. The Ru clusters/clay catalysts exhibited a marked selectivity for branched hydrocarbons in CO hydrogenation. 

The practical value of the Fischer-H opsch reaction is limited by the unfavorable Schulz-Flory distribution of hydrocarbon products that is indicative of a chain growth polymerization mechanism. In attempts to increase the yields of lower hydrocarbons such as ethylene and propylene (potentially valuable as feedstocks to replace petrochemicals), researchers have used zeolites as supports for the metals in attempts to impose a shape selectivity on the catalysis [114] or to control the performance through particle size effects. [IIS] These attempts have been partially successful, giving unusual distributions of products, such as high yields of C3 [114] or C4 hydrocarbons. [116] However, the catalysts are often unstable because the metal is oxidized or because it migrates out of the zeolite cages to form crystallites, which then give the Schulz-Flory product distribution. 

Zeolite entrapped metal carbonyl clusters prepared by the methods described above are potentially more stable and selective CO hydrogenation catalysts 

Tible 4-7. Reactions involving CO which are catalyzed by zeolites containing metal carbonyl clusters. The clusters are regarded as catalyst precursors the catalytically active species are generally not known. 

Quster/Zeolite Reaction Conditions Comment Refs. 

Zhou et al. [52] showed that strongly basic NaY zeolites containing entrapped osmium carbonyl clusters (described above) catalyzed CO hydrogenation and gave a non Schulz-Flory distribution of Ci-Q hydrocarbons with high alkene to alkane ratios. The catdyst was stable, operating at 300°C and 19 bar with a CO/H2 molar ratio of 1, for more than 20 days with only a small loss in activity and only a moderate loss in selectivity. However, the activity was markedly lower than that of conventional supported metal catalysts. 

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The M. W. Kellogg Co., Hydrogen Chloride to Chlorine, The Eel-ChlorProcess, 20th Chlorine Plant Managers Seminar, New Orleans, La., Eeb. 9, 1977. 

The previous volume measurement was done by methane because this does not react and does not even adsorb on the catalyst. If it did, the additional adsorbed quantity would make the volume look larger. This is the basis for measurement of chemisorption. In this experiment pure methane flow is replaced (at t = 0) with methane that contains C = Co hydrogen. The hydrogen content of the reactor volume—and with it the discharge hydrogen concentration— increases over time. At time t - t2 the hydrogen concentration is C = C2. The calculation used before will apply here, but the total calculated volume now includes the chemisorbed quantity. 

The effeet of integration method and stepsize must be eheeked for every application where temperature runaway is possible. Those will be mostly oxidations, but other reaetions ean be very exothermie, too. During the 1973/74 oil erisis, when synthetie natural gas projeets were in vogue, one of the CO hydrogenation teehnologies was found to be very exothermie and prone to runaway also. 

Amine and caustic solutions are used to remove these impurities. The amine solvents known as alkanolomines remove both H2S and CO, Hydrogen sulfide is poisonous and toxic. For refinery furnaces and boilers, the maxinaum HjS concentration is normally about 160 ppm. 

In this investigation (Table VIII), it was found that Kw values for CO hydrogenation depend on the 0.9 power of the reciprocal of particle diameter. In view of this and the literature, a linear (first power) dependence on the reciprocal of particle diameter was used in the Kw expression. Accuracy of measurement is certainly insufficient to distinguish between a 0.9 and a 1.0 power dependence. 

It is evident that the equation for Ref. 13 has broken down completely for CO hydrogenation. The other equations (II, 14) for CO hydrogenation gave correlations similar to those obtained by the simple kinetics. These equations are all, however, of relatively simple form. They use low activation energies and in general show an activity dependence on the square root of the pressure, similar to that of the simple kinetics. 

A number of measurements made on the methanation of C02 may be correlated by using Equation 5 with the same values of AH as for the CO hydrogenation. On the basis of diffusion considerations, the value of KwS for C02 hydrogenation was taken as 80% of that for CO hydrogenation. Attempts were made to correlate data when both CO and C02 were methanated by using simple diffusion for both with the C02 rate set at 80% of the CO rate. In order to get good agreement with experimental data, it is necessary to introduce a variable water-gas shift reaction activity. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

Figure 2.41. X-ray photoemission spectra of Fe foil after CO hydrogenation at 548 K in CO/H2=1 20 at 1 bar total pressure and varying reaction times, (a) C Is spectra from K-free Fe. (b) K 2p and C Is spectra from K-covered Fe, Ok -OA 28 Reprinted with permission of the American Chemical Society.

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

Figure 8.57. Effect of catalyst potential on the rates of formation of C2H6, C2H4) HzCO, CH3OH and CH3CHO during CO hydrogenation on Pd/YSZ. The rate of CH4 formation is of the order 10 9 mol/s and is only weakly affected by UWr Single pellet design P=12.5 bar, T=350°C. pH2/pco= -8, flowrate 85 cm3 STP/min.5 59...

Systematic assessment of alumina-supported cobalt-molybdenum nitride catalyst Relationship between nitriding conditions, innate properties and CO hydrogenation activity... 

Figure 7. Nitridation at different H2 NH3 Figure 8. CO hydrogenation reaction rate...

The activity of all catalysts were evaluated for the CO hydrogenation reaction. The histogram shown in Fig. 8 reveals that the bimetallic Co-Mo nitride system has appreciable hydrogenation activity with exception of samples 2 and 4. This apparent anomaly was probably due to the relatively high heat of adsorption for these two catalysts, which offered strong CO chemisorption but with imfavourable product release. 

The present study revealed effects of various rutile/anatase ratios in titania on the reduction behaviors of titania-supported cobalt catalysts. It was found that the presence of rutile phase in titania could facilitate the reduction process of the orbalt catalyst. As a matter of fact, the number of reduced cobalt metal surface atoms, which is related to the overall activity during CO hydrogenation increased. 

Hydrogen chemisorption Static H2 chemisorption at 100°C on the reduced cobalt catalysts was used to determine the number of reduced surface cobalt metal atoms. This is related to the overall activity of the catalysts during CO hydrogenation. Gas volumetric chemisorption at 100°C was performed using the method described by Reuel and Bartholomew [6]. The experiment was performed in a Micromeritics ASAP 2010 using ASAP 2010C V3.00 software. 

Another example of potassium as a promoter is in the hydrogenating of CO to give methanol directly, as mentioned earlier [M. Maack, H. Friis-Jensen, S. Sckerl, J. H. Larsen and I. Chorkendorff Top. Catal. 22 (2003) 161]. Here it works as a promoter for CO hydrogenation, but with conventional methanol synthesis great efforts are made to avoid the presence of alkalis in the catalyst as they tend to ruin the selectivity by promoting the production of higher alcohols, i.e. the surface becomes too reactive. Thus great care has to be exercised to achieve the optimal effects. 

Figure 9. Relative rate of CO hydrogenation as a function of copper coverage on a Ru(OOOl) catalyst Reaction temperature 575K. Results for sulfur poisoning from Figure 7 have been replotted for comparison.

CO conversion data relative to (N1 SI ) and (ThNl Fe, series were taken from ref. ( ) and (,9), respectively. Catalytic measurements were obtained for oxygen treated N1 Th Intermetallics. Prior to each run, a sample mixture (50 mg cata ys + 50 mg ground quartz) was reduced In H. at 275 C for 16 hours. CO hydrogenation was carried out at 275 C using H /C0 ratio 9. More experimental details are given elsewhere (10). 

This contribution Is concerned with the magnetic and Mossbauer characterization of (a) Fe/zeollte (mordenlte) systems, and that of (b) Fe and/or Ru on boron-doped carbon substrates. Some correlations between the characterization and CO hydrogenation parameters will be pointed out. Because of limitations of space, we shall present salient features of these Investigations. At the outset. It would be befitting to present a succinct background on the basic principles of magnetic and Mossbauer characterization. 

Vannice and P.L. Walker Jr. for their assistance in CO-hydrogenation with the various Fe/carbon catalysts and many helpful discussions. 

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