Kinetics of CO hydrogenation

More recently Walker and Vannice have examined iron carbon molecular sieves, Fe-CMS, for the hydrogenation of carbon monoxide. The report was particularly thorough and rich in detail regarding the preparation of the Fe-CMS materials. Chemisorption of CO and H2, as well as the kinetics of CO hydrogenation and methanation were described (25.) In other investigations Walker and coworkers have examined the effect of... 

Goodman D W, Kelley R D, Madey T E and Yates J T Jr 1980 Kinetics of the hydrogenation of CO over a single crystal nickel catalyst J. Catal. 63 226... 

Muetterties has suggested that the dimeric hydride [RhH(P OiPr 3)2]2 catalyzes alkene and alkyne hydrogenation via dinuclear intermediates [91]. However, no kinetic evidence has been reported to prove the integrity of the catalysts during the reactions. On the other hand, studies of the kinetics of the hydrogenation of cyclohexene catalyzed by the heterodinuclear complexes [H(CO) (PPh3)2Ru((u-bim)M(diene)] (M = Rh, Ir bim=2,2 -biimidazolate) suggested that the full catalytic cycle involves dinuclear intermediates [92]. 

Very early, from the analysis of ignition, flame speed, and detonation velocity data, investigators realized that small concentrations of hydrogen-containing materials would appreciably catalyze the kinetics of CO—02. The H20-catalyzed reaction essentially proceeds in the following manner ... 

Phosphinecarbonyl complexes of cobalt have long been known to act as hydrogenation catalysts. In a recent study involving cyclohexene the kinetics of its hydrogenation by the complex [CoH(CO)2(PBun3)2] were studied. Unlike the systems described above, the carbonyl complexes generally require elevated temperature and pressure. The proposed mechanism is given in Scheme 6. 

Anionic chromium hydride complexes proved to be efficient hydrogen atom donors. Newcomb determined PPN+ HCr(CO)5 to be an efficient radical initiator and reducing agent for radicals and determined the kinetics of the hydrogen abstraction reaction [214]. In line with the observation that 3d metal complexes are much more prone to radical pathways than the corresponding 4d and 5d complexes, an increase of the extent of competing S -pathways for the bromide abstraction was found for molybdenum and tungsten complexes compared to the chromium complex. 

Interestingly, the free energies (Box 7) and the enthalpies of formation of the products of CO hydrogenation are also reflected in the rates (kinetics) of the various reactions thus hydrocarbons are formed relatively easily. While there is... 

Table III. Kinetic Results of CO Hydrogenation over Nickel Catalysts...

B. Chen and J.G. Goodwin, Jr. Isotopic Transient Kinetic Analysis of CO Hydrogenation on Cu-Modified Ru/Si02, J. of Catalysis 158,228 (1998). 

A plot of In M P)/P) versus P yields a value of a from either the slope or the ordinate intercept. Agreement between the slope and intercept is used as a criterion of the soundness of Schulz-Flory fit. A typical product distribution of CO hydrogenation that reflects the polymerization kinetics is shown in Figure 7.34. 

Brandt and co-workers studied the kinetics of the hydrogenation of 1,2-diphenylpropene by B (Ar = Ph, R = Bu), as the B(3,5-FjCCgH4)4" salt apparently. They found the rate to be first-order in the concentrations of both and catalyst. These authors also did a theoretical study of a modified system with Ar = R = Me and with ethylene as the substrate. The... 

In the case of dejin hydrogenation the oxidative addition of hydrogen happens directly to the Rh-alkyl complexes n-II or iso-II. Hydrogen transfer and reductive ehmination results in the same saturated alkane product from both regioisomers of the catalyst. Thus, olefin hydrogenation is a favored side reaction for all catalyst complexes that favor the kinetics of oxidative hydrogen addition over CO association and insertion. Olefin hydrogenation is, for example, much more relevant for Co hydroformylation catalysts compared to their Rh counterparts. 

Kinetics. Extensive studies of the kinetics of methane synthesis were reported by White and co-workers (10,11, 12, 13, 14, 15). They studied the reaction between CO and hydrogen over a reduced nickel catalyst on kieselguhr at 1 atm and 300°-350°C (10). They correlated the rate of methane formation by the equation ... 

As particle size decreases, hydrogen leakage decreases and hot spot temperature in the bed is higher. Thus the smaller particle size has greater activity (see Table VI). A kinetic system which defines the reaction in terms of CO and C02 methanation and CO shift conversion was used to determine the activity (see last column of Table VI). 

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. 

The kinetics of NO reduction by hydrogen and CO was studied by Ayen and Peters. Hydrogen reduction of NO over oxides of copper, zinc, and chromium was studied at 375-425°C. The products formed include... 

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... 

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