Reaction selectivity restricted transition state

For this reason, recent attention has been dedicated to micro- and meso-porous materials (MMM), and also for their possible use as shape-selectivity controlled reactions (space-restricted transition states, preferential diffusion and back-diffusion). Even if the latter aspects are better known regarding the performances of microporous materials, other aspects can be evidenced. In fact, also in mesoporous materials, where the dimensions of the channels are larger, as required for shape-selectivity effects, a change of reactivity of molecules inside the channels could be present due to confinement effects [281]. Therefore, the catalytic reactivity shown by basic sites located inside mesoporous channels could differ from that of the same sites located instead on the external surface of the mesoporous ordered material. 

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-... 

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. 

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. 

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... 

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

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. 

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. 

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... 

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. 

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 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. 

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. 

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]. 

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]. 

A third type of control, called spatiospectffeity, occurs when both reactants and products pass the opening but reaction intermediates or transition states are restricted by the size of the cavity. In xylene isomerization processes, selectivity is lost through disproportionation to toluene and trimethylbenzene. Diphenylroethane intermediates are too large for ZSM-5... 

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. 

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. 

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. 

---