Restricted transition state selectivity zeolites

Transition-state selectivity is sometimes difficult to distinguish from product shape selectivity. A recent study by Kim et al. (8) shows that the high para-selectivity for the alkylation of ethylbenzene with ethanol in metallosilicates (MeZSM-5) is not due to product selectivity alone. They conclude that the primary product of the alkylation on ZSM-5 type metallosilicates is p-diethylbenzene which isomerizes further inside the cavity of ZSM-5 to other isomers. As the acid sites of zeolites becomes weaker (achieved by substituting different metals into the framework of the zeolite), the isomerization of the primarily produced p-isomer is suppressed. Although Kim et al. attribute this suppression of the isomerization activity to restricted transition-state selectivity, it is more likely that this suppression is due to the decrease in acid strength. 

H. van Bekkum et al. (72) studied a number of catalysts in the Fischer synthesis starting from l-phenyl-2-butanone 40 (with R, = Ph, R2 = CH3) and phenylhydrazine. The isomeric products are the bulky 2-ethyl-3-phenylindole 45 (with R, = Ph, R2 = CH3) and the linear 2-benzyl-3-methylindole 46 (with R, = Ph, R2 = CH3). Catalysis of the inolization of 40 by soluble as well as solid (e.g. Amberlyst 15) catalysts typically yielded a mixture of the two isomers in a bulky/linear ratio of about 75/25. Zeolite BEA reverses this bulky/linear ratio giving 75% of the linear isomer 46, a result interpreted in terms of restricted transition-state selectivity. Although in zeolite BEA the intraporous formation of 45 is largely suppressed, it is in fact probably not completely inhibited. 

The selective Fischer synthesis of 2-benzyl-3-methylindole from l-phenyl-2-butanone and phenylhydrazine and catalyzed by zeolite BEA can be considered as an interesting example of restricted transition state selectivity. 

One of the most in ortant properties of zeolites is their ability to cany out shqie selective reactions [5]. These can be cl sified as, firstfy, product shape selective reactions in which the only products formed are those which can diffiise out of e pores of die zeolite, second, reactant shape selective reactions which occur when some of the molecules in a reactant mbcture are too large to diffiise through the catalyst pores, and, thirdfy, restricted transition-state selective reactions in which the only reactions which occur are those in which qiace exists in the pores or cavities to allow the formation of the activated transition state con lex. In some cases where the zeolite is three dimensional the gze of the channel intersections will also be a determining ictor. This unique catalytic property is related to the pore size of the zeolite and has led to the synthesis of zeohtes with a very w e range of pore gzes. 

Restricted transition state selectivity is observed when the transition state leading to one product is more bulky than that leading to another. In this case the product from the less-restricted transition state will be favored. As an example, the disproportion of o-xylene to trimethyl benzene and toluene requires a bulky diaryl species as the transition state but isomerization to m- or p-xylene simply requires successive 1-2 methyl shifts, a process having a smaller transition state. As the size of the zeolite cavity decreases, the ratio of the rate of disproportionation to the rate of isomerization decreases because the larger transition state is not as easily accommodated in the smaller cavities. 2... 

Running the Fisher indole synthesis on an unsymmetrical phenyl hydrazone gives a mixture of 2,3-disubstituted indoles. For example, reaction of the phenyl hydrazone, 34, with acid can give both 35 and 36 (Eqn. 22.26). Soluble acids and Amberlyst-15 give these two products in a 75 25 ratio at 100% conversion. With an H-M catalyst they are formed in a 65 35 ratio but over a dealuminated H-beta zeolite, the selectivity is reversed and 36 is produced in an 82% yield at 100% conversion.62 n was proposed that the preferential formation of 36 over the H-beta catalyst was the result of a restricted transition state selectivity. ... 

The most important consequence of restricted transition state selectivity is that ZSM-5 and many other medium-pore zeolites deactivate much slower than most other crystalline and amorphous catalysts. The difference is not trivial. In most acid catalyzed reactions large-pore zeolites deactivate within minutes or in hours, whereas the activity of ZSM-5 ranges from weeks to years. Most of the coke in large-pore zeolites is formed within the pores. In ZSM-5 most of the coke is deposited on the outer surface of the crystals like an eggshell over an egg [23] because coke precursors cannot form in the pores of pentasil molecular sieves. The resistance of ZSM-5 to coking makes a number of industrial processes economical. 

As most of the acid sites are located in pores of molecular size the rate and the selectivity of catalytic reactions depend not only on the intrinsic properties of the sites but also on the pore structure. A zeolite catalyst selects the reactant or the product by their ability to diffuse to and from the active sites (reactant and product selectivity). Steric constraints in the environment of the sites limit or inhibit the formation of intermediates or transition states (restricted transition state selectivity) [24,25]. The strong polarizing interaction between zeolite crystallites and adsorbed molecules leads to an unusually high concentration of the reactants in the pores. This concentration effect causes an enhancement of the rates of bimolecular reaction steps over monomolecular reaction steps [26]. 

Summary Meerwein-Ponndorf-Verley and Oppenauer reactions (MPVO) are catalysed by metal oxides which possess surface basicity or Lewis acidity. Recent developments include the application of basic alkali or alkaline earth exchanged X-type zeolites and the Lewis-acid zeolites BEA and [Ti]-BEA. The BEA catalysts show high stereoselectivity, as a result of restricted transition state selectivity, in the MPV reduction of substituted alkylcyclohexanones with i-PrOH. 

Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BEA (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions. 

Shape Selectivity. One of the most important features of zeolite catalysts is their ability to act as a molecular sieve because the channels have molecular dimensions. Three types of shape selectivity can be distinguished reactant, product, and restricted transition state selectivity, depending on whether reactants can enter, products can leave, or intermediates can be formed in the zeolite catalyst, respectively. Medium-pore zeolites have been shown to have excellent restricted transition state selectivity. The high resistance toward coke formation on medium-pore zeolites has also been attributed to this type of shape selectivity. Transition state selectivity and product selectivity have been observed directly in the methanol conversion on ZSM-5 by means of magic-angle-spinning NMR. ... 

It was found that zeolite beta is a highly shape-selective catalyst for the Fischer indole synthesis of 2-benzyl-3-methylindole from phenylhydrazine and 1 -phenyl-2-butanone. A selectivity of 83 % for this isomer was obtained at full conversion. Combined results from catalytic experiments and sorption measurements indicated that the formation of the isomeric 2-ethyl-3-phenylindole is suppressed as a consequence of restricted transition state selectivity. 

The sequence is exactly opposite to that of conventional acid catalysis The reactants that are best able to form carbenium ions in solution are the least reactive with zeolite catalysis. The restricted transition state selectivity suppresses cracking of the more highly branched hydrocarbons in the cavities [T25]. 

In early work on molecular shape-selectivity [1-3], three kinds of mechanisms were envisaged. Reactant selectivity occurs when some molecules of the feed are too bulky to diffuse through the zeolite pores and are prevented from reacting. Product selectivity occurs when among all the product molecules formed within the pores, only those with the proper dimensions can diffuse out and appear as products in the bulk. Restricted transition-state selectivity occurs when certain reactions are... 

Significant differences in the acetylation of naphthalene with acetic anhydride (2 1 molar ratio) over HBEA are observed with decalin or sulfolane as solvents the diffoent hydrophilicities of these solvents dramatically inflnence the resulting naphthalene conversion. The hydrophilic sulfolane into-acts more strongly with the zeolite surface, thus blocking the acid sites that are less available for the acylation reaction (naphthalene conversion = 14%) on the contrary, the hydrophobic decalin enables the adsorption of acetic anhydride and increases the rate of acylation reaction (naphthalene conversion=25%). Due to the defined structure of the HBEA, the selective formation of isoma-15 is probably achieved via a restricted transition state selectivity (15 13=81 19 at 35% naphthalene conversion). It must be underlined that different secondary products are, in general, produced on the catalyst surface due to consecutive reaction of the products. 

The metal complexes in an SIB catalyst are confined to separate supercages. Consequently, the formation of inactive dimers is no longer possible. Shape-selectivity is another feature of SIB catalysts that follows from the restricted space inside the zeolite pore system. This can be simply due to discrimination in size of the reactant molecules (a large reactant molecule is excluded from the zeolite) or to a constrained orientation of the reactant at the catalytic site (transition state selectivity). 

Intermediate pore zeolites typified by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes in the petroleum and petrochemical industry (2-4). This paper describes the selectivity characteristics of two different aromatics conversion processes Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two different principles (5,j6) are responsible for their high selectivity a restricted transition state in the first, and mass transfer limitation in the second. 

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

The use of zeolites can overcome many of these limitations and provide new controlled entries into these oxidized hydrocarbons and new materials. For example, some of the most valuable industrial intermediates are terminally oxidized hydrocarbons, snch as n-hexanol or adipic acid, that are not readily available in free-radical chain processes. The ability of zeolites to function as shape-selective catalysts can, in principle, be used to restrict access, by reactant or transition state selectivity, to sites not normally attacked by oxidants [3]. 

In general the selectivity in toluene methylation found with MFI molecular sieve catalysts is proposed to be caused either by a restricted transition state to form m- and o-xylene [10,11,12] and/or diffusional constraints of the bulkier isomers, o- and m-xylene, in the pores of zeolite ZSM5 [3,4], Recent results on the methylation of toluene based on in situ analysis of the working catalyst showed that all three isomers were primary products in toluene methylation [13]. The high p-selectivity was explained to be due to transport constraints of the bulkier... 

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... 

A very high stereoselectivity was observed in the reduction of 4-tert-butylcyclohexanone to the m-alcohol (> 95%), which is the industrially relevant product. The observed high selectivity to the thermodynamically unfavorable cis-alcohol was explained by a restricted transition-state for the formation of the trans-alcohol within the pores of the zeolites (Scheme 5). This reaction was found not only to be catalysed by Al-Beta, van der Waal et al. reported the catalytic activity of aluminum-free zeolite titanium beta (Ti-Beta) in the same reaction.74 Again, a very high selectivity to the cis-alcohol was observed indicating similar steric restrictions on the mechanism. Kinetically restricted product distributions were also reported for the 2-,3- and 4-methylcyclohexanone the cis, trans- and ds-isomers being the major products, respectively. In this case the tetrahedrally coordinated Ti-atom was assumed to behave as the Lewis acid metal center. Recent quantum-chemical calculations on zeolite TS-1 and Ti-Beta confirm the higher Lewis acidic nature of the latter one.75... 

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... 

In the alkylation of benzene with long-chain a-olefins (Ce, Cg, C12, etc ), the large-pore zeolites mordenite, beta-zeolite, and ZSM-12 favor the less bulky 2-phenyl isomers. HY and rare-earth-Y produce a mixture of other -alkylbenzene isomCTS. Shiqie selectivity is attributed here to both product- and restricted transition state type selectivities [76]. 

Recently Creyghton et al. [6,7] reported the use of zeolite beta in the MPVO reduction of 4-t-butylcyclohexanone. ITie high selectivity towards the thermodynamically less favoured ds-alcohol is explained by a restricted transition-state around a Lewis-acidic aluminium in the zeolite pores. When using an aluminium-free zeolite, titanium beta, in the epoxidation of olefins, we have shown that Ti-beta has acidic properties when alcoholic solvents were employed [8], This was ascribed to the Lewis-acidic character of titanium in the zeolite framework. As we reported very recently [9], Ti-beta is found to be an excellent catalyst in MPVO reactions with a tolerance for water. Here, results are presented on the high selectivity, stability and low by-product formation of the catalyst, Ti-beta, in both the liquid-phase and gas-phase MPVO reactions. 

Both Ti-beta and TS-1 show a high activity towards the oxidation of small size substrates such as ethyl sulfide in methanol as solvent, which can be the proof that the efficiency of the active Ti sites is the same for the two catalysts. In this context, the poor activity of the TS-I catalyst in the case of the large molecules can be attributed both to a restricted transition state shape selectivity and to a difliisivity effect of reagents and products. These effects are not as important for a large pore zeolite, such as Ti-beta. A similar result was... 

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