Isomerization reaction active-site control

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H2-D2 equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H2D)+ or (HD2)+ are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. 

Powders possessing relatively high surface area and active sites can be intrinsically catalytically active themselves. Powders of nickel, platinum, palladium, and copper chromites find broad use in various hydrogenation reactions, whereas zeolites and metal oxide powders are used primarily for cracking and isomerization. All of the properties important for supported powdered catalysts such as particle size, resistance to attrition, pore size, and surface area are likewise important for unsupported catalysts. Since no additional catalytic species are added, it is difficult to control active site location however, intuitively it is advantageous to maximize the area of active sites within the matrix. This parameter can be influenced by preparative procedures. 

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

A detailed examination of the reaction products of 2MP on both USHY catalysts indicates the modes of reaction are 1) isomerization, monomolecular and dimerization-cracking on pristine sites and 2) biinolecular chain processes between an adsorbed product carbocations and a gas phase reactant molecule. Figure 1 presents the experimental average conversion and the corresponding predicted conversion, with respect to time on stream, along lines of constant catalyst to reactant ratio. The sigmoidal behaviour exhibited at low conversions and short times on stream is consistent with the presence of the second type of reaction, one mediated by chain processes. Such behaviour contrasts sharply with that previously reported for linear paraffins on USHY (9) where 1 was observed to increase monotonically with reaction time. A kinetic model has been proposed (10) which accounts for all the mechanisms active in this system. The model assumes that the surface reaction is rate controlling in all cases and is ... 

In the presence of H2, there is not much difference in the conversion levels between Pt-SAPO-31 and SAPO-31, but the yield of the isomerization products are more over Pt-SAPO-31 than over SAPO-31. This can be due to the activated hydrogen from metallic (Pt) sites reacting with the carbocation intermediates and decreasing the rate of the disproportionation reaction." Also the spilled over hydrogen could lower the coke formation on Lewis acid sites by controlling the concentration of benzylic carbocations (coke precursors) while it does not affect the activity of Bronsted acid sites in the isomerization reaction. ... 

Corradini suggested a novel mechanism for syndiospecific chain-end control reactions in which the olefin migratory-insertion was assumed with 2,1-regiospecificity and in which the configuration of the last inserted monomer unit determines the configuration of the active sites in a subsequent rapid isomerization step. The satistical equations for this mechanism are equivalent to those for the chain-end control, Bernoullian model. 

Similarly as in (6.48), the coverage of the alkene on the acid sites is controlled by the equilibrium constant of adsorption. Therefore, at low coverage the effective activation energy of the isomerization reaction becomes (6.55)... 

Initially, the MCP adsorbs on the metal and dehydrogenates up to methylcy-clopentadiene, which then isomerize on an acid site to cyclohexadiene. Cyclohexa-diene migrates to the metal and finally gets dehydrogenated to benzene. Neither silica-alumina nor platinum supported on silica is active for this reaction, but a physical mixture of them is active to form benzene from MCP, which show the bifunctional character of this reaction. The metal content has no effect on the reaction rate, what indicate that the acid function is the one that controls the reaction rate. 

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