Shape Selective Reactions over Zeolites

Since zeolites have small and uniform pores and most of the active sites are located inside this pore system, the selectivities of the catalytic reactions often greatly depend upon the relative dimensions of the molecules and the pore openings. Actually, an infrared spectroscopic study of ZSM-5 zeolite revealed that only 5—10 percent of Bronsted sites are located on the external surface of the zeolite and the rest inside the pore systems. The first report on shape-selective catalysis by Weisz and Frilette appeared in 1960. Many applications are found in the petroleum and chemical industries for catalytic cracking and hydrocracking and aromatic alkylation. 

In Table 3.33 the activities of CaX and CaA for dehydration of butyl and isobutyl alcohol are compared.Over CaX, both alcohols are dehydrated rapidly in the temperature range of 503 — 533 K, with the isobutyl alcohol showing somewhat greater activity. This behavior is compatible with the fact that both are primary alcohols and should resemble each other. Both CaX and CaA show litde difference in activity with butyl alcohol which can penetrate both crystals. However, the isobutyl alcohol, which 

Shape-selective reactions are widely applied in the chemical industry. Selectform- 

In the MTG (methanol-to-gasoline) process, methanol is converted effectively into hydrocarbons with gasoline-range boiling points over ZSM-5. The reaction of methanol over ZSM-5 yields hydrocarbons with six to ten carbon atoms and Cio aromatic compounds only in small or trace amounts. This can be explained by assuming that the upper size of the aromatic products is imposed by the ZSM-5 pore structure. 

Skeels, Proc. 6th Intern. Congr. Catalysis, 1976, The Chemical Society, London, 1977, p.645. 

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Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-25] on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 - unlike on H-mordenite or HY - the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 25]. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel s experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. 

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. 

It has been concluded that, in most cases, catalytic reactions over zeolites occur within their intracrystalline cages and channels. Zeolite catalysts can therefore be considered as a succession of nano or molecular reactors. The consequence is that the activity, selectivity, but also the stability of all the reactions carried out over zeolite catalysts, depend (slightly or significantly) on the shape and size of cages, channels and of their apertures, hence that shape selectivity is a general characteristic of zeolite catalyzed reactions. 

One of the most discussed cases of shape selectivity involving transition state selectivity or product diffusional constraints is the production of p-xylene over chemically modified MFI zeolites [248]. Several processes exist which utilize the shape selectivity of these zeolites, for example the alkylation of toluene with methanol [249], xylene isomerization [250] and selective toluene disproportionation [251]. The first two of these examples shall be used to describe in detail the principal possibilities to tailor the reaction pathway by shape selectivity. 

At higher temperature (>560K), over poisoned or unpoisoned catalyst, increased rates of diffusion of hydrocarbon products allow the shape-selectivity of the catalyst to become apparent. The isomerization reactions within the channels which lead to linear and singly branched hydrocarbons may also occur in reverse on the active sites of the external surface (most active sites at the zeolite surface occur at the mouth of the channels), which may thus mask some of the shape-selectivity of the zeolite. 

The authors [13] also proposed a multiphase mass transport model for toluene nitration, the details of, which are given in Fig. 2.4. It is based on the formation of a thin aqueous film aroimd the hydrophilic catalyst particles, which are dispersed in toluene medium. The model also accounted for the existence of vapor phase over the liquid-liquid-solid reaction medium. The major mass transfer resistances are offered by the liquid film around the catalyst particles and in the catalyst pores. The aqueous film and the liquid in the pores constitute the micro environment necessary to facilitate the desired level of lattice transformation in the catalyst particles. Figure 2.4 also shows the concept of the microenvironment within and around the catalyst particle. These studies have demonstrated that shape selectively effect of zeolite Beta catalyst is significantly enhanced by the specific microenvironment created within and around the catalyst particles. This has significantly enhanced the para-selectivity from 0.7 to 1.5. The microenvironment has also improved the accessibility of reactant molecules to the catalyst active sites. 

Shape-selective zeolites can also be used to discriminate among potential products of a chemical reaction, a property called product shape selectivity. In this case, the product produced is the one capable of escaping from the zeolite pore structure. This is the basis of the selective conversion of methanol to gasoline over... 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

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. 

The alkylation of phenol investigated over H-MCM-22, H-ITQ-2 and H-MCM-36 showed that the delamelation and pillaring did not improve the catalytic activity and this was explained on the secondary processes taking place during the preparation of the corresponding materials, and which strongly affect the total acidity and the acidity on the external surface. Also, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. This seems to show that the tert-butylation reaction preferentially proceed at (or close to) the external surface of the zeolite layers. 

As was shown here in some examples, the field of catalysis over zeolites, although marnre, is still very much alive. The chemists who work with the synthesis zeolites continue to be very creative, the focus now being placed on the synthesis of materials that can catalyze reactions other than the acidic ones and/or reactions of bulkier molecules, that is, synthesis of zeolites with larger micropores or with a very large external surface, such as nanosize and delaminated zeolites. New concepts related to the mode of action of zeolite catalysts continue to emerge, as shown here with the shape selectivity of the external surface. These concepts are particularly useful to scientifically design selective and stable catalysts. 

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). 

The effect of the dealumination of HM on encapsulated products inside the catalysts used for the reaction is shown in Figure 5.28 29 The selectivity for 4,4 -DIPB inside the pores was almost constant over all HM zeolites although the selectivity for 4,4 -DIPB in bulk reaction products varied with the Si02/Al203 ratio. These results suggest that the shape-selective isopropylation occurs inside the pores even over HM with the low ratio. The low catalytic activity and the low... 

Xylenes. Because of the practical significance of xylenes, isomerization of xylenes over zeolites is frequently studied.348 The aim is to modify zeolite properties to enhance shape selectivity, that is, to increase the selectivity of the formation of the para isomer, which is the starting material to produce terephthalic acid. In addition, m-xylene isomerization is used as a probe reaction to characterize acidic zeolites.349,350... 

Such results lead us to assume that shape selectivities may occur even in easy reactions over modified zeolites. The study of the influence of other zeolites on the reactivity of styrene oxide and hindered analogs is in progress. 

Another example of secondary shape selectivity is shown by John and co-workers (77,73). They found that the hydroisomerization/hydroeracking of n-hexane over Pt/H-mordenite is significantly inhibited by the presence of benzene. They also found a correlation between the aromatic size relative to zeolite pore size on the inhibition of the hexane reaction and the changes in isomer selectivities. Figure 4 illustrates the relation between the various aromatics co-fed and the n-hexane isomerization rates on H-mordenite. From Figure 4, it is shown that as the kinetic diameter of the aromatics is increased, the isomer formation rate appears to pass through a minimum. This result can be explained by considering the size of the zeolite pore and the kinetic... 

Our work on the alkylation of meta-diisopropylbenzene with propene over the acid form of various 12-member ring zeolites and molecular sieves shows that these catalysts can reveal shape selective behavior (39). As the effective size of the voids increases, the ratio of the formed 1,3,5- to 1,2,4-triisopropylbenzene increases e.g., mordenite and zeolite Y give 1.1 and 2.5, respectively. Additionally, an amorphous Si02/Al203 catalyst yields a ratio of 3.5. Thus, the smaller 12-ring materials show shape selective behavior. Based on these results, extra-large pore materials such as VPI-5 may show some shape selectivity for this reaction, if acid sites can be incorporated into the material. 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

A review of this field has been given by Haw (1999). Reactions can be followed either in sealed glass ampoules or flow-through cells constructed within the spectrometer. The formation of intermediates can be studied in real time. An elegant example of this was shown in an early study of methanol to gasoline conversion over HZSM-5 zeolites. As a result of the shape selectivity of the catalyst, spectroscopic evidence of reaction intermediates, which were not seen as reaction products, was observed (Anderson and Klinowski, 1990). 

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