Shape-selective behavior

A peculiar shape-selective behavior has been observed in a Cu-based MOF containing one-dimensional tunnels with narrow necks at regular distances of... 

A (Figure 4.9). The diameter of such a neck, 2.3 A, is sufficiently large for a linear C-C chain to pass, but too small to also be an equilibrium adsorption position. The largest compound allowed inside the pores is a linear molecule limited in length to four carbon atoms due to the distance between two subsequent necks [103]. Another example of shape-selective behavior is found in a Zn-based MOF able to encapsulate linear hexane while branched hexanes are blocked [104]. 

Although the commercial benefits of ZSM-5 in catalytic cracking have frequently been discussed, there has not been significant discussion of the shape selective behavior of ZSM-5 in catalytic cracking. Rajagopalan et. al. (5) claim based on gas oil studies in a fixed bed reactor, that both the straight chain and mono-methyl branched paraffins have equivalent reactivity. Biswas et. al. (6) also report a large reduction in... 

Our work on the alkylation of meta-diisopropylbenzene with propene over the acid form of various 12-member ring zeolites and molecular sieves shows that these catalysts can reveal shape selective behavior (39). As the effective size of the voids increases, the ratio of the formed 1,3,5- to 1,2,4-triisopropylbenzene increases e.g., mordenite and zeolite Y give 1.1 and 2.5, respectively. Additionally, an amorphous Si02/Al203 catalyst yields a ratio of 3.5. Thus, the smaller 12-ring materials show shape selective behavior. Based on these results, extra-large pore materials such as VPI-5 may show some shape selectivity for this reaction, if acid sites can be incorporated into the material. 

Although diffusivity is often important in zeolite catalysis, other factors may also be crucial in determining shape selectivity. Recent work by Post 15a), for example, has shown that the shape selectivity behavior observed for the relative cracking rates of hexane isomers over H-ZSM 5 zeolite (see Section VIII) could not be understood on the basis of their measured diffusivities. Spatial restrictions imposed on transition-state species formed within the zeolite pores provide a possible explanation for the observed results. 

Rollmann and Walsh (266) have recently shown that for a wide variety of zeolites there is a good correlation between shape-selective behavior, as measured by the relative rates of conversion of n-hexane and 3-methyl-pentane, and the rate of coke formation (see Fig. 24). This correlation was considered to provide good evidence that intracrystalline coking is itself a shape-selective reaction. Thus, the rather constrained ZSM-5 pore structure exhibits high shape selectivity, probably via a restricted transition-state mechanism (242b), and therefore has a low rate of coke formation. Zeolite composition and crystal size, although influencing coke formation, were found to be of secondary importance. This type of information is clearly... 

The observed increase in the shape selectivity behavior of the zeolite, indicates a small but significant decrease in the effective channel diameter of zeolite. This is expected due to the formation of larger amounts of non-ffamework Ga-oxide species in the zeolite channels for the zeolite catalyst containing the binder. Earlier studies [12] also showed an increase in shape selectivity of H-GaMFI zeolite because of its degalliation during hydrothermal treatments. 

Heterogeneous catalysts, in particular zeolites with their various properties contribute extensively to the environmental protection in the synthesis of fine chemicals. For that a broad and very impressive range of acidic and basic catalysts is already available having all levels of properties between super acidity and super basicity. Furthermore the zeolitic materials may have shape-selective behavior. Also the possibility to prepare bi-functional catalysts will gain in importance. 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

Difference In surface concentrations required for achieving the shape-selectivity indicates fornatlon of silica with different surface conditions. The relationship between shape-selectivity and surface silicon concentration, however, does not largely depend on the Included cation, proton or sodium, but rather on the composition of zeolites. Strong dependence on the composition was confirmed on the dealumlnated mordenlte, since the behavior was not in agreement with those on the native species but with those expected from the composition. Therefore, growth of silica and pore size enclosure can be summarized. 

Their main advantage over organic solvents for applications in analytical chemistry is their low volatility coupled with high thermal stability up to 260°C, which make them useful as solvents for working at high temperatures, and also as stationary phases in GC.101102 They provide symmetrical peak shapes, and because their ranges of solvation-type interaction are different for anions and cations, they exhibit a dual selectivity behavior. 

Primary Shape Selectivity. There are several types of shape and size selectivity in zeolites. First, the reactant molecules may be too large to enter the cavities. A particularly good illustration of this behavior is given by Weisz and co-workers (5). Zeolites A and X were ion exchanged with calcium salts to create acid sites within the zeolite. These acid sites are formed as the water of hydration around the calcium ions hydrolyzes. When these zeolites are contacted with primary and secondary alcohols in the vapor phase, both alcohols dehydrate on CaX but only the primary one reacts on CaA. Since the secondary alcohol is too large to diffuse through the pores of CaA, it can not reach the active sites within the CaA crystals. This kind of selectivity is called reactant shape selectivity and is illustrated in Figure 3. 

The choice among the variety of different types of zeolites and related materials in a practical situation will depend on the characteristics of the reacting system and the types of selectivity effects to be expected. The pore size, the deactivation behavior and the chemical and thermal stability of the zeolite material determine whether or not a particular catalyst is attractive. The necessary condition for shape-selectivity effects to occur is that the pore size has to meet the dimensions of the reacting molecules. The radius of the crystallites as well as the strength and the number of the acid sites may then be adapted to the actual requirements during synthesis. 

The values of the three parameters can be obtained by fitting the experimental data to Equation 2. The form of (R) is S-shaped, and the location of the inflexion point (the maximum "rate" of coke deposition) plays an important role in the coke / activity / selectivity behavior of a catalyst in general (4). The coke level, at the inflexion point of the curve can be obtained from the fitted values of the three parameters. 

When pubhshed reports of the diffusivity of paraffins in ZSM-5 catalysts obtained from uptake rate measurements appeared grossly inconsistent with catalytic behavior. Werner participated in resolving the problem by determining diffusivities from catalytic behavior of catalysts of very different particle sizes. The analysis not only confirmed the many orders of magnitude higher true diffusivities but also allowed Werner to extend the technique to demonstrate that shape selectivity could occur due to lack of fit of a reactant (e.g., diffusion of a dimethyl paraffin) in the structure or lack of fit of a reaction complex (transition state) that must be created on the active site (e.g., the methyl paraffin/propyl cation complex). 

Finally, the use of computer modeling is seen to rapidly increase. This growth is likely to continue or accelerate and chemical synthesis strategies should benefit markedly from it. It can be expected that at first especially shape selective application will profit and it will still be largely a visualization technique to understand how a reactant /product molecule adapts and fits into the microporous environment. The challenge on theoretical chemistry will be how to predict reactivity patterns and molecule transformation inside these pores in order to be able to model the chemical behavior. 

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. 

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