Shape selectivity origin

However, the reactivities of primary alcohols are much lower than the reactivities of secondary alcohols. While an increase in reactivity of 2-alcohols with increasing chain length can be expected on the basis of chemical reactivity, the decrease beyond C8 must have another origin, which may be reactant shape selectivity in the TS-1 catalyst. The 2-alcohol generally react faster than the 3-alcohol (Van der Pol et al., 1993b). 

Asymmetric hydrogenations have been reported with palladium on silk 123>, palladium on modified cellulose 124> and on modified ion exchange resins 125 >. Also with Raney Nickel modified with amino adds 126> and peptides, 27>. Platinum-carbon catalysts exhibiting shape selectivity have been made by coating them with a thermosetting resin, which is carbonized. In such a way an organic molecular sieve skin is formed over the original catalyst 128>. 

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

Whereas the acetylation of phenyl ethers over zeolite catalysts leads to the desired products, acetylation of 2-MN occurs generally at the very activated C-l position with formation of l-acetyl-2-methoxynaphthalene (l-AMN). A selectivity for l-AMN close to 100% can be obtained over silicoaluminate MCM-41 mesoporous molecular sieves[22] and FAU zeolites,133 341 whereas with other large pore zeolites with smaller pore size (BEA, MTW, ITQ-7), 2-AMN (and a small amount of l-acetyl-7-methoxynaphthalene, 3-AMN) also appears as a primary product. Average pore size zeolites, such as MFI, are much less active than large pore zeolites. These differences were related to shape selectivity effects and a great deal of research work was carried out over BEA zeolites in order to specify the origin of this shape selectivity the difference is either in the location for the formation of the bulkier (l-AMN) and linear (2-AMN) isomers (only on the outer surface for l-AMN, preferentially within the micropores for 2-AMN)[19 21 24 28 381 or more simply in the rates of desorption from the zeolite micropores.126 32 33 351... 

Figure 16. Reconstruction of an occluded almost symmetric shape. The original shape is shown as a dashed line. The reconstructed shape is shown as a solid line, (a) The closest symmetric shape following angular selection about the centroid, (b) The closest symmetric shape following angular selection about the center of symmetry, (c) The closest symmetric shape following angular selection about the centroid and altered symmetry evaluation (see text).

Similar behavior was discovered in subsequent studies for ZSM-5 (772,174) and ZSM-11 (173) zeolites synthesized with aluminum and boron in the zeolite lattice and for boron-synthesized ZSM-11 zeolites (173). The modification of the ZSM-5 and ZSM-11 samples produced a minor improvement in shape selectivity and a large decrease in acidity and hence activity. The initial heat for the B-ZSM-11 sample decreased from 160 kJ mol" for Al-ZSM-11 to 65 kJ mor , and the acidity decreased to 10% of the original value. The q-d curve also showed a maximum at high coverages, which was attributed to the formation of a NH NHa) complex on reacting B—OH—NH3 with NH3. Dehydroxylation at 1073 K increased the initial heat to 170 kJ mol", a value comparable to the initial heat of 185 kJ mol" on Al-ZSM-11, and it sharpened the maximum in the q-9 curve. This behavior is apparently due to the formation of a few strong Lewis acid sites. The sample synthesized with both boron and aluminum behaved differently than those with only aluminum or boron. The q-6 curve for this sample showed maxima at about 145-175 kJ mol" and at about 60-70 kJ mol for 673 and 1073 K dehydroxylation temperatures, respectively. The acidity of this sample was 30% lower than an Al-ZSM-11 sample with similar Si/Al ratio. The initial heat for the aluminum zeolite was 170 to 190 kJ mol". It was shown, with IR spectroscopy of adsorbed ammonia, that the boron-modified samples showed little or no Brpnsted acidity. 

Enzymes not only catalyze reactions, but they do so with a high degree of specificity that originates from the shapes of their active sites. Shape selectivity can also be a property of other supramolecular hosts which, therefore, are said to be enzyme mimics.52 If two products stem from a single reactant, i.e., R->PA + PB, then an enzyme mimic can bias the competition in favor of one product over the other. 

A similar shape selective effect was observed in the liquid phase. Those catalysts with the smaller pore and channel openings were more selective for para-nitrotoluene. However, in the liquid phase, no induction period was observed. Rather, all catalysts exhibited significant deactivation throughout time on stream and after 5 hrs. little of the original activity remained. As shown in Fig. 3, the para selectivity was found to decrease with time on stream. This would indicate that deactivation occurs within the pore channels effectively reducing the preferential capacity of the catalyst to generate the para isomer. The decrease in para selectivity was not evident on Beta zeolite, which has larger pores and may allow for a more uniform production of... 

J. P. Collman, J. 1. Brauman, B. Mcunicr, T. Hayashi, T. Kodadek, S. A. Raybuck, Epoxidation of olefins by cytochrome P-450 model compounds Kinetics and stereochemistry of oxygen atom transfer and origin of shape selectivity, J. Am. Chem. Soc. 107 (1985) 2000. 

The importance of the desorption step in the reaction network may originate from the specific interaction of the 5-ring hydrocarbon molecules with the erionite pore system. The shape-selectivity of the erionite component strongly influences the equilibrium reactions n-heptane o 5-iing naphthenes methyl-cydohexane toluene because only n-heptane and the cyclopentane derivatives are able to reach the active Bronsted sites. In addition, the mobility of these hydrocarbons in the erionite should be quite different. 

The good selectivity obtained in the iGioevenagel product (95% at 90% conversion) versus the Michael ones could be explained either by a lower basicity of the encaged caesium oxide on zeolite compare to Mg-Al hydrotalcites [26] or by a shape selective properties of this modified zeolites. Complementary work is needed to determine the origin of this last result. 

The Role of Pore Structure The Origin of Shape Selectivity... 

In the conversion of a mixture of n-alkanes in absence of secondary shape-selectivity, the molecules with the largest carbon number inhibit the reaction of the less strongly adsorbed shorter molecules. Dauns and Weitkamp observed this phenomenon in the conversion of decane and dodecane on LaY type zeolite catalysts [28]. In this 12-MR zeolite, the conversion of dodecane is not affected by the presence of the lower boiling decane (it decreases from 64 to 60%), while the decane conversion drops from 47 to 20% (Table 1). Santilli and Zones studied the conversion of hexane and hexadecane on I M-5 and SSZ-16 zeolites [26]. The 10-MR zeolite ZSM-5 converts hexane and hexadecane to a similar extent in separate experiments (Table 1). In mixtures, the presence of hexane has little effect on the hexadecane conversion, but the latter inhibits the hexane conversion. However, on the 8-MR zeolite SSZ-16, hexadecane causes a slight increase in the hexane conversion, and hexane keeps hexadecane from reacting (Table 1). In the original work [26], no explanation for this secondary shape-selectivity effect has been... 

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