Shape Selectivity transition state

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

Conclusive evidence has been presented that surface-catalyzed coupling of alcohols to ethers proceeds predominantly the S 2 pathway, in which product composition, oxygen retention, and chiral inversion is controlled 1 "competitive double parkir of reactant alcohols or by transition state shape selectivity. These two features afforded by the use of solid add catalysts result in selectivities that are superior to solution reactions. High resolution XPS data demonstrate that Brpnsted add centers activate the alcohols for ether synthesis over sulfonic add resins, and the reaction conditions in zeolites indicate that Brpnsted adds are active centers therein, too. Two different shape-selectivity effects on the alcohol coupling pathway were observed herein transition-state constraint in HZSM-5 and reactant approach constraint in H-mordenite. None of these effects is a molecular sieving of the reactant molecules in the main zeolite channels, as both methanol and isobutanol have dimensions smaller than the main channel diameters in ZSM-S and mordenite. 

It is often difficult to distinguish restricted transition state shape selectivity from product shape selectivity due to the lack of clear experimental evidence that the pore geometry and local spatial environment are actually influencing the reaction rate [63]. The following test reactions are more likely be impacted by transition state selectivity effects. 

Dewaxing is the final example of a reaction illustrated here with possibly multiple restricted transition state shape selectivity effects. Bifunctional zeolitic catalysts... 

Computational elucidation of the transition state shape selectivity phenomenon. J. Am. Chem. Soc., 126 (3), 935-947. 

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. 

Transition-state shape-selective catalysis certain reactions are prevented because the... 

Both H2O2 and hydroperoxides are industrially important oxidants. An accurate evaluation of advantages and disadvantages requires an accurate analysis of every specific case, in view of the different technical problems and economic constraints that the use of one or the other entails. The reactivity of H202 is so high that it can easily oxidize many primary reaction products, and these reactions become more likely as the reaction temperature is increased. Some of these reactions are influenced by reactant shape selectivity and by restricted transition-state shape selectivity. 

Evidence of variables that influence the relative rates of reaction of olefins and alcohols was obtained from experiments with compounds that have both olefinic and alcoholic functions and by the competitive oxidation of mixtures of olefins and alcohols. The data of Table VI show that when the double bond has no substituents, as in allyl alcohol, but-3-en-l-ol, or 2-methylbut-3-en-l-ol, only the epoxide is formed but when the double bond has substituents, the epoxida-tion rate is decreased and ketone and aldehyde products from the oxidation of the OH group are formed. This effect is more pronounced with a greater degree of substitution. Since the double bond and the OH group are part of the same molecule, the difference must arise from the different abilities of the reactants to coordinate and react at the titanium center restricted transition-state shape selectivity is a possibility. The terminal double bond, sterically less hindered, interacts strongly with titanium, preventing coordination of the competing OH... 

Reactant shape selectivity effects related to the dimensions of reactant molecules and catalyst pores, including restricted transition-state shape-selectivity effects as well as chemical and stereochemical selectivity. 

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 ... 

Shape selective reactions are typically carried out over zeolites, molecular sieves and other porous materials. There are three major classifications of shape selectivity including (1) reactant shape selectivity where reactants of sizes less than the pore size of the support are allowed to enter the pores to react over active sites, (2) product shape selectivity where products of sizes smaller than the pore dimensions can leave the catalyst and (3) transition state shape selectivity where sizes of pores can influence the types of transition states that may form. Other materials like porphyrins, vesicles, micelles, cryptands and cage complexes have been shown to control product selectivities by shape selective processes. 

For these simulations, the primary isomer distribution is chosen according to the thermodynamic equilibrium (sec Table 6). Such a situation would be encountered in practice when neither the reaction mechanism kineti-cally favors a particular isomer nor restricted transition state shape-selectivity effects occur. The disproportionation reaction is assumed to be unaffected by diffusion (i.e. y < 0.01). The effective diffusivities of the ortho and meta isomers are fixed, and assumed to be equal, but by a factor of Ro smaller than the effective diffusivity of the para isomer. 

Transition state shape selectivity (or spatioselectivity) occurs when the formation of reaction intermediates (and/or transition states) is sterically limited by the space available near the active sites. This spatioselectivity depends on the size and shape of cages, channels and channel intersections. This type of selectivity was first proposed by Csicsery (34) to explain the absence of 1, 3, 5- trialkylbenzenes in the disproportionation products of dialkyl-benzenes transformation over H-mordenite although these trialkylbenzenes could diffuse in the zeolite channels. The space available in these channels was not sufficient to accommodate the diphenylmethane intermediates involved in the formation of 1, 3, 5-trialkyl benzenes they are bulkier than those involved in the formation of 1, 2, 3 and 1, 2, 4 trialkylbenzenes (Figure 1.5 c). 

Clear-cut examples of effects of zeolite pore architecture on the selectivity of Diels Alder reactions are not easily found. For instance, 4-vinylcyclohexene is formed with high selectivity from butadiene over a Cu -Y zeolite however, the selectivity is intrinsically due to the properties of Cu1, which can be stabilized by the zeolite, and not to the framework as such (30-31). A simple NaY has been used in the cycloaddition of cyclopentadiene and non-activated dienophiles such as stilbene. With such large primary reactants, formation of secondary products can be impeded by transition state shape selectivity. An exemplary reaction is the condensation of cyclopentadiene and cis-cyclooctene (32) ... 

Transition state shape selectivity is invoked when a reaction path involves a bulky transition state that is not compatible with the size of the pores (Figure 7). 

However, since epoxidation occurs within pores of cross-section comparable to that of the olefin, steric restrictions generally prevail over inductive effects, leading to anomalous reactivity orders. They result from restrictions to diffusion in the pores (reactant shape selectivity) and to the approach of the double bond to the active species (transition state shape selectivity). The first is sufficient to explain... 

I he recent literature related to selective skeletal isomerization of -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, o-butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of (he ciitalysl that leads to poisoning of (he acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective. 

The previous results underline the importance of shape selectivity effects even for the transformation of small olefins such as butene. The results are in agreement with the early, related work by Haag et al. 59). who investigated cracking of olefins and paraffins catalyzed by the zeolite HZSM-5 and distinguished between restricted transition state shape selectivity and mass transport shape selectivity, ft is clear that the effects discussed here are best described in terms of restricted transition state shape selectivity. 

It has been shown by using a methodology combining molecular dynamics and an energy minimization technique 60) that in the PER pores (cavities and channels), the formation of Cti olefin intermediates is inhibited. These theoretical results agree well with the experimental indications of restricted transition state shape selectivity. Indeed, for materials such as zeolites and MeAPOs, most of the sites are expected to be located inside the micro-pores. Molecular sieves with large mesopores and/or large external surface... 

The main factor governing the isobutylene selectivity is restricted transition-state shape selectivity. The space available around the acid site governs the isobutylene selectivity by allowing the reaction to proceed mainly through the monomolecular and not the bimolecular mechanism. 

The observed improvement in stability for ZSM-5 containing catalysts (Figure 1 and Table 1), is mainly due to the unique structure and novel configuration of ZSM-5. The well-known transition-state shape selectivity restricts the formation of aromatic hydrocarbons with carbon number higher than 10 [17], decreasing the rate of formation of heavier aromatics that are believed to be the precursors of coke, that mainly cause catalyst deactivation by occupying or blocking the way to active sites. 

HL but less selective for pyrrolidine. Other partially exchanged HL zeolites were both less active and less selective than HL itself but de-aluminated HL had enhanced activity and selectivity. The high selectivity for ring transformation was attributed to transition-state shape-selective catalysis in the straight channels of L zeolite. 

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