Shape-selective transformations

As zeohtes are known for their weU-defined micropore size and the associated ability to host shape-selective transformations, it is of distinct importance that the micropores in the hierarchical systems have the same size as those in the... 

Although shape-selective transformation usually infers enantiodiscrimina-tion, truly asymmetric transformations using imprinted polymers remain scarce... 

Afterwards, the synthesis of various new zeolites, especially ZSM5 (MFI, 1967), the discovery of new shape selective transformations such as the (accidental) discovery of the remarkably stable and selective conversion of methanol into gasoline range hydrocarbons over HZSM5 (7), the development of post-synthesis treatments of zeolites,. .. combined to make them the single most important family of catalysts used all other the world. 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

Conducting reactions in nanospace where the dimensions of the reaction vessel are comparable to those of the reactants provides a new tool that can be used to control the selectivity of chemical transformations.1 This dimensional aspect of nano-vessels has been referred to as shape selectivity.2 The effect of spatial confinement can potentially be exerted at all points on the reaction surface but its influence on three stationary points along the reaction coordinate (reactants, transition states, and products) deserve special attention.3,4 (1) Molecular sieving of the reactants, excluding substrates of the incorrect dimension from the reaction site can occur (reactant selectivity). (2) Enzyme-like size selection or shape stabilization of transition states can dramatically influence reaction pathways (transition state selectivity). (3) Finally, products can be selectively retained that are too large to be removed via the nano-vessel openings/pores (product selectivity). 

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

The performance of organic reactions in organized media, e.g. by zeolite confinement, and the use of zeolites as selective and green catalysts for organic transformations, have been popularized in recent years. The main advantage of the zeolites to be tested as media or catalysts for carrying out organic reactions is the so-called shape selectivity . ... 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

A unique titanium(IV)-silica catalyst prepared by impregnating silica with TiCLt or organotitanium compounds exhibits excellent properties with selectivities comparable to the best homogeneous molybdenum catalysts.285 The new zeolite-like catalyst titanium silicalite (TS-1) featuring isomorphous substitution of Si(IV) with Ti(IV) is a very efficient heterogeneous catalyst for selective oxidations with H2C>2.184,185 It exhibits remarkable activities and selectivities in epoxidation of simple olefins.188,304-306 Propylene, for instance, was epoxidized304 with 97% selectivity at 90% conversion at 40°C. Shape-selective epoxidation of 1- and 2-hexenes was observed with this system that failed to catalyze the transformation of cyclohexene.306 Surface peroxotitanate 13 is suggested to be the active spe-... 

The MFI class of channel zeolites, of which ZSM-5 is a member, are of enormous importance in the petrochemicals industry because of their shape-selective adsorption and transformation properties. The most well-known example is the selective synthesis and diffusion of p-xylene through ZSM-5, in preference to the o- and m-isomers. Calcined zeolites such as ZSM-5 are able to carry out remarkable transformations upon normally unreactive organic molecules because of super-acid sites that exist... 

Reactant shape selectivity was the basis of the Selectoforming process previously mentioned. The n-alkanes of light gasoline (essentially n-pentane, n-hexane) enter the pores of the erionite catalysts and are transformed into propane and n-butane, whereas the branched alkanes are excluded from the pores and do not react (Figure 1.5a). 

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). 

D DM catalysts have a new pore structure consisting of crystalline domains of 8- and 12-ring pores connected by mesopores (5-10 nm). The presence of the latter enhances accessibility to the micropore regions without seriously compromising the shape-selective character of the catalyst. This combination of changes in acidity and pore structure transforms synthetic mordenites into highly active, stable and selective alkylation catalysts. 

Figure 5 schematically illustrates the concepts of reactant shape selectivity. Only linear paraffins that are able to diffuse and are adsorbed inside the pores can undergo a chemical transformation, e.g., acid catalyzed cracking. The property is exploited in some chemical processes, such as the dewaxing of lubes and middle distillates, through the selective cracking or isomerization of the linear paraffin fraction. 

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. 

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. 

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