Surface chain initiation

Detailed studies by Patzlaff et al.918 have shown that addition of ethene causes an increased fraction I, of the distribution characterized by a and a small increase of ctj. This indicates that ethene mainly acts as a chain initiator of hydrocarbons formed according to distribution 1, and to a very small extent as a surface intermediate for insertion into a growing chain. Concurrent experimental results were obtained by Schulz and Claeys.19 Distribution 2 and also a2 are not affected by co-feeding of ethene. Figure 11.4 shows that ethene changes the ASF plot only in the range of low carbon numbers. 

The heterogeneous catalyst accelerates hydrocarbon oxidation. The rate of oxidation increases with increasing concentration of the catalyst. However, this increase in the oxidation rate with the catalyst concentration is not unlimited. The oxidation rate reaches a maximum value and does not increase thereafter. Moreover, the cessation of the reaction was observed and very often at a very small increase in the catalyst concentration. Such phenomenon was named critical phenomenon. The basis of critical phenomenon lies in the chain mechanism of oxidation and the dual ability of the catalyst surface to initiate and terminate chains. Numerous observations and studies of critical phenomenon in catalytic liquid-phase oxidations were performed [271 283]. Here are a few examples. 

Critical phenomena, observed in heterogeneous catalysis (see Chapter 10), proved that the surface of a catalyst possesses two kinds of action on the liquid-phase oxidation chain initiation and chain termination (see Chapter 10). 

In addition to hydroperoxide decomposition, the copper surface was found to initiate the chains through activation of dioxygen. The rate of chain initiation in the presence of the copper powder was found to be... 

Different explanations have been proposed to explain this effect, including the influence of water on the adsorbed carbon species on the surface and the reduction of secondary hydrogenation of primary olefins by water, thereby facilitating olefin readsorption and chain initiation.8... 

Mechanism I. Following work on the slow oxidation of acetaldehyde, during which the surface-volume ratio of the reaction vessel was varied, Cullis and Khokhar (8) interpreted the mechanism in terms of the ability of amines to be absorbed onto the surface of the reaction vessel. Thus, if the chain-initiating processes between the fuel and oxygen occur on the surface, the fuel will not be oxidized until the amine is burned off. 

The effects of solvents on polymeric materials are usually physical rather than chemical in nature. The primary polymer chains remain intact, but the molecular structure is changed, the magnitude of the change typically decreasing as one moves from the material surface (the initial point of solvent-polymer contact) into the bulk portion of the sample. 

Most examples of catalysis can be readily characterised as homogeneous or heterogeneous but there are examples of catalysis which overlap the two types. Consider a system in which intermediates are formed at the surface and then are desorbed into the gas phase and react there. Such intermediates might generate a chain reaction in the gas phase, i.e., chain initiation and chain termination occur at the surface but chain propagation occurs in the gas phase. 

The results of studies of this reaction in a hollow reactor show high selectivity up to 640 °C in the range of 4-EP volume rate from 0.065 to 0.78 h 1 and at 4-EP 20% aqueous H202 = 1 3 [94], Under optimal conditions at 620 °C, 4-VP yield equals 20.9% with 92% selectivity. Injection of quartz granules to the reactor raises the yield to 44.3% and selectivity to 96%. This is because the total surface on which, probably, the chain initiation reaction ... 

The surface chemistry involved in the conversion of isobutane on solid acids is shown schematically in Fig. 17. The conversion takes place according to a surface chain reaction scheme involving initiation, propagation, and... 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

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