Surface disproportionation catalysts

Surface areas reported for heterogeneous disproportionation catalysts were generally greater than 100 m2/g and the promoter concentrations reported... 

Thermal cracking tends to deposit carbon on the catalyst surface which can be removed by steaming. Carbon deposition by this mechanism tends to occur near the entrance of the catalyst tubes before sufficient hydrogen has been produced by the reforming reactions to suppress the right hand side of the reaction. Promoters, such as potash, are used to help suppress cracking in natural gas feedstocks containing heavier hydrocarbons. Carbon may also be formed by both the disproportionation and the reduction of carbon monoxide... 

Disproportionation. Carbon monoxide readily disproportionates into elemental carbon and carbon dioxide [124-38-9] on a catalyst surface... 

This decomposition is thermodynamically favored by decreasing temperature and increasing pressure (28). Decomposition is extremely slow below 673 K in the absence of a catalyst however, between 673—873 K many surfaces, particularly iron (29), cobalt, and nickel (30), promote the disproportionation reaction. 

The application of these catalysts in the initial state (without any special treatment of the surface organometallic complexes of such cata-lysts) for ethylene polymerization has been described above. The catalysts formed by the reaction of 7r-allyl compounds with Si02 and AUOj were found to be active in the polymerization of butadiene as well (8, 142). The stereospecificity of the supported catalyst differed from that of the initial ir-allyl compounds. n-Allyl complexes of Mo and W supported on silica were found to be active in olefin disproportionation (142a). 

Similar disproportionation is likely to occur during catalytic hydrocarbon oxidation since the Bl2Mo20g catalyst is subjected to continuous redox cycling under such conditions. Therefore, any kinetic or catalytic information about Bi2Mo20n is suspect unless the absence of surface restructuring can be confirmed. 

Sojka, Z., Pietrzyk, P., Martra, G. et al. (2006) EPR and DFT study of NO interaction with Ni/Si02 catalyst Insight into mechanistic steps of disproportionation process promoted by tripodal surface nickel complex, Catal. Today, 114, 154. 

This situation is termed pore-mouth poisoning. As poisoning proceeds the inactive shell thickens and, under extreme conditions, the rate of the catalytic reaction may become limited by the rate of diffusion past the poisoned pore mouths. The apparent activation energy of the reaction under these extreme conditions will be typical of the temperature dependence of diffusion coefficients. If the catalyst and reaction conditions in question are characterized by a low effectiveness factor, one may find that poisoning only a small fraction of the surface gives rise to a disproportionate drop in activity. In a sense one observes a form of selective poisoning. 

Experiments carried out by feeding TBPE only over H-MWW, showed that the O-alkylated product do not rearrange to C-alkylated phenol derivatives in our conditions, but it is hydrolysed to phenol. So, TBPE is not a reaction intermediate and perforce O-and C-alkylation are parallel reactions. Also o-TBP and p-TBP were fed each of them alone over our catalysts. As it could be observed in Fig. lb, o-TBP convert to p-TBP (by transalkylation) and in minor extent 2,4-DTBP (by disproportionation), while p-TBP (results not shown here) convert to 2,4-DTBP (by disproportionation). Because the transalkylation and disproportionation are bimolecular reactions and need large spaces, it is plausible to suppose that the alkylation could not take place in the pores, but on the external surface of H-MWW zeolites. 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

The catalytic isomerization of 1-methylnaphthalene and all lation of 2-methylnaphtha-lene with methanol were studied at ambient pressure in a flow-type fixed bed reactor. Acid zeolites with a Spaciousness Index between ca. 2 and 16 were found to be excellent isomerization catalysts which completely suppress the undesired disproportionation into nwhthalene and dimethylnaphthalenes due to transition state shape selectivity. Examples are HZSM-12, H-EU-1 and H-Beta. Optimum catalysts for the shape selective methylation of 2-methylnaphthalene are HZSM-5 and HZSM-li. All experimental finding concerning this reaction can be readily accounted for by conventional product shape selectivity combined with coke selectivation, so there is no need for invoking shape selectivity effects at the external surface or "nest effects", at variance with recent pubhcations from other groups. 

Very little is known about the behaviour of different catalysts only a few comparisons of alumina and silica—alumina have been made. On Al2-03—Si02, the disproportionation of diethylamine is more rapid by one order of magnitude than its deamination on A1203, the rates are comparable [149], The activity of alumina for aniline disproportionation is higher than of silica—alumina [150]. The steric demands of the alumina surface are higher than those of silica—alumina as the comparison of the chemisorption of pyridine and 2,6-dimethylpyridine has shown [158]. 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

The interaction between the catalyst s solid surface and the organic compound is considered to be responsible for the disproportionation of chlorofluorocarbons (CFCs).1 The solid phase itself takes part in this process. In a concerted reaction cycle, a metal —Cl bond is exchanged with aC-F bond, and also a metal —F bond with a C —Cl bond. The average number of exchanged metal —Cl and metal —F bonds in the exchange equilibrium between the gas and solid phases is constant over time. 

The individual mass transfer and reaction steps outlined in Fig. 4.15 will now be described quantitatively. The aim will be firstly to obtain an expression for the overall rate of transformation of the reactant, and then to examine each term in this expression to see whether any one step contributes a disproportionate resistance to the overall rate. For simplicity we shall consider the gas to consist of just a pure reactant A, typically hydrogen, and assume the reaction which takes place on the interior surface of the catalyst particles to be first order with respect to this reactant only, i.e. the reaction is pseudo first-order with rate constant A ,. In an agitated tank suspended-bed reactor, as shown in Fig. 4.20, the gas is dispersed as bubbles, and it will be assumed that the liquid phase is well-mixed , i.e. the concentration CAL of dissolved A is uniform throughout, except in the liquid films immediately surrounding the bubbles and the particles. (It will be assumed also that the particles are not so extremely small that some are present just beneath the surface of the liquid within the diffusion film and are thus able to catalyse the reaction before A reaches the bulk of the liquid.)... 

In the absence of a catalyst, the disproportionation is too slow to be observed at room temperature. Rapid, exothermic, and potentially explosive decomposition of hydrogen peroxide is initiated, however, by heat and by a broad range of catalysts, including transition metal ions, certain anions (such as I-), metal surfaces, blood (Figure 14.9), and even tiny particles of dust. Because decomposition is accelerated by light, hydrogen peroxide is stored in dark bottles. It is best handled in dilute aqueous solutions concentrated solutions and the pure liquid are extremely hazardous materials. 

Lewis and Wills83) obtained initial differential rate data for the disproportionation of propylene over a C0O-M0O3 -Ala 03 catalyst. Temperatures of 394-478 °K and pressures of 1 to 9 atmospheres were used. The authors reported the experimental data were well correlated when it was assumed that a dual site surface reaction was the controlling step in the mechanism. 

Moffat and Clark 84> found that a Langmuir-Hinshelwood model applied to a heterogeneous surface can be used to describe both the general kinetics and the rate-temperature maxima reported by Banks and Bailey (Fig. 2) for olefin disproportionation on cobalt molybdate-alumina catalyst. They conclude that the rate-temperature maximum was caused by the reversible deactivation of sites superimposed on the irreversible poisoning of sites. 

The surface concentration of carbon in the used catalysts is consistent with the bulk carbon analysis, while surface carbon on the fresh and regenerated samples seems disproportionately high. Surface sulfur is consistently lower than the bulk values of Table I indicating the presence of metallic sites on the surface, since these levels on the used catalysts are incomplete for total sulfiding of Co and Mb. 

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