Selectivity restricted transition state

This third form of shape selectivity depends on the fact that chemical reactions often proceed via intermediates. Owing to the pore system, only those intermediates that have a geometrical fit to the zeolite cavities can be formed during catalysis. 

This selectivity occurs preferentially when both monomolecular and bimolecular rearrangements are possible. In practice, it is often difficult to distinguish restricted transition state selectivity from product selectivity. 

An example is the disproportionation of /w-xylene to toluene and trimethyl-benzenes in the wide-pored zeolite Y (Fig. 7-5 c). In the large zeolite cavity, bulky diphenylmethane carbenium ion transition states can be formed as precursors for methyl group rearrangement, whereby the less bulky carbenium ion B is favored. Thus the reaction product consists mainly of the imsymmetrical 1,2,4-trimethylben-zene rather than mesitylene (case A). In contrast, in ZSM-5, with its medium sized pores, monomolecular xylene isomerization dominates, and the above-mentioned disproportionation is not observed as a side reaction. 

Restricted transition state selectivity is also of importance in the alkylation of benzene with ethylene to give ethylbenzene. High selectivities for ethylbenzene are achieved on H-ZSM-5 owing to suppression of side reactions. These high selectivities were also explained by the fact that the possible bimolecular disproportionation of ethylbenzene is suppressed. 

H-ZSM-5 is also used as catalyst in the large-scale MTG (methanol to gasoline) process. The products are hydrocarbons, aromatics in the benzene range, and water. The reaction is based on the dehydration of methanol to dimethyl ether, followed by numerous reactions that proceed via carbenium ion intermediates. The largest molecules observed, e.g., durene (1,2,4,5-tetramethylbenzene), correspond to the high-boiling components of gasoline. The favorable product distribution in this process can be attributed to restricted transition state selectivity. 

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Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

Restricted transition-state selectivity occurs when certain reactions are pre-... 

Restricted transition state selectivity Figure 1 Mechanisms of shape-selective catalysis. 

Although a clear distinction among these mechanisms is difficult, there is an important difference between product selectivity and restricted transition-state selectivity mechanisms. In the former mechanism, the product composition inside the pores should either be close to equilibrium, or the selectivity for the products inside the pores should be lower than that for bulk products. However, the selectivity for the narrowest isomer of the encapsulated products should be as high as that of bulk products in the latter mechanism. 

Pentanol reacts much faster than 3-pentanol. The ratio of reactivities calculated from data at 50% H202 conversion is 12 1. Because in term of diffusion rates and chemical behavior these two alcohols are similar to each other, the results are explained by restricted transition-state selectivity, a steric influence of the catalyst pores. Cyclohexanol is oxidized at a very low rate, and this is best... 

It is reasonable to consider that in titanium silicate-catalyzed reactions the oxidizing species also acts as an electrophile. The different order of reactivity of the C4 olefins in the presence of titanium silicates relative to that observed with soluble catalysts must therefore arise from the fact that alkyl substitution at the double bond is responsible not only for inductive effects, but also for increases in the size and the steric requirements of the molecules. Since the rates of diffusion of the different butenes cannot be the cause of the different reaction rates, a restricted transition-state selectivity must be operating. 

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. ... 

Restricted transition-state selectivity occurs when certain reactions are prevented because the corresponding transition state would require more space than available in the cavities or pores. Neither reactant nor product molecules are prevented from diffusing through the pores. Reactions requiring smaller transition states proceed unhindered. [21,22]. 

Reactant and product selectivities are mass-transfer related phenomena and therefore depend on particle size. Intrinsic properties of the crystal structure (but not diffusion or crystal size) affect restricted transition-state selectivity. Thus, we may distinguish reactant and product type selectivities from restricted transition-state selectivity by observing particle-size effects. 

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. 

Disprop ortionation of m-xylene Al-M Isomerization to other p- and o-xylenes, disproportionation to toluene and trimethyl benzenes were the main reactions. For both reactions, activity increased with increase in the number of pillars. Selectivity for disproportionation increased with decreasing number of pillars due to restricted transition state selectivity. 67 68... 

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. 

Restrictive transition-state selectivity in the reaction of l-ethyl-2-methylbenzene over mordenite (a) the transition state can be formed (b) the isomerization of l-ethyl-2-methylbenzene to l-ethyl-3-methylbenzene means that the geometric and volume requirements of the transition state are incompatible with the available pore space. Note that, in this diagram, Me = CH3 (methyl) and Et = C2H5 (ethyl)... 

When phenol is alkylated over ZSM-5 with bulkier reagents, such as propene, propan-l-ol or propan-2-ol, substitution takes place to the exclusion of almost all other reactions an ortholpara ratio of 0.08 is observed (Figure 4.4 overleaf). The formation of bulky disubstituted phenols is also suppressed. It is believed that this is an example of restricted transition-state selectivity. 

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