Molecular Heterogeneous Catalysis, Wiley (2006), 352729662X

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The proton–oxygen bond is covalent, but also highly polarizable. The zeolite proton does not transfer completely to the adsorbing molecule upon adsorption. The interaction is largely comprised of the two hydrogen bonds that form between methanol and the zeolite. The first is the bond between the now weakened zeolite proton and the basic oxygen atom of methanol, the second is between the acidic methanol proton and a second basic oxygen atom of the Al tetrahedron. The large interaction between the polar groups on the molecule and proton site in the zeolite is indicative of the hydrophilic nature of the protonic sites.

In order to activate reactant C–O or C–C bonds, the zeolitic Brønsted OH bond must dissociate. To dissociate this bond into an H+ and a negatively charged zeolite costs 1250 kJ/mol. This energy cost is lowered by the energy gain of binding the proton to the reactant hydrocarbon. The low dielectric constant of the hydrophobic zeolite framework

(εsilicalite = 2, compared with εH2O = 80) disfavors charge separation. The fragments of opposite charge therefore tend to attract one another. Positively charged intermediates

are, therefore, rarely the lowest energy ground state. Protonated molecular fragments will more readily adsorb to form as an alkoxy intermediate. This is illustrated in Fig. 4.5.

Figure 4.5. Comparison of the DFT-calculated structures of the reactant, transition and product states of the protonation reaction of propylene by a zeolitic proton. (a) Results of calculations using zeolite clusters. (b)Results from periodic DFT calculations on the structure and the resulting energy for the protonation of propylene by the protonated form of chabazite. Al values are in kJ/mol.

Figure 4.5 shows the energies of the initial weak hydrogen-bonded adsorbed state of propylene, the proton-activated transition state and the final alkoxy product state of the protonated propylene. The structures and energies are established from DFT cluster calculations using the model structure shown in Fig. 4.5a and periodic DFT calculations using the unit cell of chabazite and the zeolitic protons (Fig. 4.5b). The cluster used in Fig.

4.5a was created by “cutting” it out from the periodic crystalline framework and capping the terminating oxygen atoms with hydrogen atoms so as to neutralize the cluster. One notes the relatively small di erence in energies of the adsorbed propylene before proton transfer (initial state) and the protonated propylene bound to the zeolite as an alkoxy species (product state). The transition state for this reaction requires a significant stretch (activation) of the OH bond to give the near formation of a protonated propyl cation and this is high in energy. This high barrier is due to the large energy cost required to cleave the zeolite OH bond heterolytically . This is only partially compensated by the formation of a new C–H bond and the electrostatic stabilization between the protonated propylene and negatively charged zeolite. The comparison of the results from cluster and periodic DFT calculations shows only small di erences for the neutral-bonded state and the alkoxy state. The di erence in energy between the relative energies for the protonated carbenium-like transition states, however, is significantly greater. A considerable degree of charge separation occurs in the transition state, which is partially screened by the polarization of the large oxygen atoms of the zeolite cavity. This screening of the dipole between the positively charged transition state and negatively charged zeolite framework reduces the activation barrier for protonation from 105 kJ/mol in the cluster model to 55 kJ/mol[8] in the periodic approach.

Figure 4.6. Transition states and their energies with respect to the reactant state of adsorbed toluene.

(a) Toluene activated by cluster; (b) toluene activated by a proton in the mordenite structure.

This concept of electrostatic screening of the charge separation in the transition state is illustrated in detail in Fig. 4.6 for the proton-activated methyl carbon bond cleavage of toluene. The calculated transition-state structure and the energies for the protonated cleavage of the C–C bond of toluene in the pore of the mordenite channel that result from periodic DFT calculations, are shown in Fig. 4.6b for comparison with the cluster results shown in Fig. 4.6a. The similarity in predicted transition-state structures for both the cluster and periodic results is noteworthy. In the transition state, a proton attaches to the aromatic ring. The CH3 + group that forms subsequently tilts out of the plane of the molecule and binds at an angle which is close to 90◦. The transition state structures are quite similar to the free protonated toluene cation and do not experience a steric constraint due to the occlusion in the zeolite channel. The energetic di erence is due

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Figure 4.6c. Polarization of the electronic density in the shift isomerization transition state of toluene catalyzed by an acidic mordenite.

to the electrostatic interaction of the protonated toluene transition state with the negatively charged zeolite framework which is more realistically represented in the periodic simulations.

The charges that form on the transition-state complex are screened by the oxygen atoms. The changes in charges that occur on the oxygen atoms in the zeolite channel are shown in Fig. 4.6c. This screening of charged intermediates results in a significant reduction in the energy of charge separation. This is a consequence of the match between the size and shape of the positively charged reaction intermediates and the size and shape of the cavity. The protonated intermediates in the transition states shown in Figs. 4.5 and

4.6 can be considered to be the analogues of a classical carbonium ion formed in superacid solutions[11] (see also Chapter 5, page 237). Carbonium-ion and carbenium chemistry is

well understood in solution media. Carbonium ions contain protonated saturated C–C or C–H bonds, whereas carbenium ions result from the protonation of alkenes. The carbonium ion is classically defined as containing carbon atoms with a coordination number of five. The carbenium ion, on the other hand, contains carbon atoms with classical valencies with a coordination number of three. The carbenium ion typically takes on a planar configuration.

In zeolite catalysis, carbeniumor carbonium-ion intermediates are energetically located at the top of the reaction energy barriers. In contrast, in superacid solutions, these protonated intermediates are ground-state reactants. The zeolite carboniumand carbenium-ion transition state concepts are illustrated for C–C activation and olefin isomerization reactions below.

The transition states for the proton-activated cleavage of propane and butane are shown in Fig. 4.7. These transition-state intermediates can be considered as protonated propane

and butane and hence are the analogues of classical carbonium ions. The structures shown in Fig. 4.7 have been computed in the chabazite cavity [12]. The calculated activation

energies for proton activation are quite high, between 170 and 200 kJ/mol. The n-butane

molecule does not fit well in the small pore of the chabazite framework. Therefore, it must adopt a gauche conformation.

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The charged carbocations produced after the C–H and C–C bond cleavage can be considered protonated alkenes or carbenium ions, in which the hybridization around the positively charged carbon is sp2. The carbenium ions subsequently adsorb to form alkoxy intermediates on the zeolite lattice, as illustrated in Fig. 4.5.

Reaction steps that proceed through the formation of primary carbenium ions, i.e. with the positive charge developing on the terminal carbon atom of the hydrocarbon chain, are energetically unfavorable and considered to be forbidden in classical liquidphase carbenium ion chemistry. In zeolites, however, the formation of a primary terminal carbenium ion can be stabilized by its interaction with the negatively charged zeolite framework. For instance, the isomerization of n-butene to isobutene is catalyzed by the zeolite ferrierite and occurs initially as a monomolecular reaction. This isomerization reaction has to proceed through the formation of a primary carbenium ion intermediate. Butene adsorbs and subsequently isomerizes in the zeolite, thus leading to the formation of the cyclopropyl cationic intermediate (I) sketched in Fig. 4.9. The ring opening of the cyclopropyl intermediate that follows leads to the formation of the primary carbenium ion (II). Intermediate (I), is formed in the transition state. Intermediate (II), on the other hand, is stabilized as an alkoxy intermediate.

Figure 4.9. Isomerization of n-butene to isobutene via the formation of a primary carbenium ion intermediate (schematic).

The analogous states involved in pentene isomerization in mordenite [15] are illustrated in Fig. 4.10a and b. Pentene isomerization occurs via the secondary rather than the primary carbenium ion.

Figure 4.10a. Protonated pentene adsorbed as a secondary alkoxy species to mordenite zeolite framework[15].

Figure 4.10b. The calculated structure of the adsorbed dimethylcyclopropyl intermediate[15].

Figure 4.10c. The calculated energy diagram for the isomerization of pentene to adsorbed isobutene[15].

The corresponding computed reaction energy diagram which is shown in Fig. 4.10c[15] proceeds through the formation of the three adsorbed intermediates shown in Fig. 4.10a and b. The first transition state (TSI) is a secondary n-carbenium ion-like state that forms as the result of protonation of pentane (Fig. 4.10a) and leads to the formation of the adsorbed n-pentyl intermediate. The second transition state (TSII) corresponds to the state that leads to cyclopentyl formation (structure 4.10b). The third transition state (TSIII) leads to the formation of isobutyl through C–C cleavage of the cyclopentyl ring. Let us return now to the question of whether the stabilization by a zeolite makes protonation reactions via primary carbenium ions possible. We analyze this here for the two protonation options of isobutene shown in Fig. 4.11.

The computed reaction energy diagrams for the two di erent reaction paths in two di erent zeolites which have cylindrical micropores are calculated here. The mordenite zeolite has a one-dimensional 12-ring channel and ferrierite (TON; TON is the nomenclature according to the International Zeolite Association) a one-dimensional 10-ring channel. Hence there are di erences in the curvature of the channels between the two zeolites. The reaction energy changes are shown for the reaction paths proceeding through a primary carbenium or tertiary carbenium ion in Fig. 4.12[16a].

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Figure 4.11. Protonation options for isobutane (schematic).

Figure 4.12. Protonation reaction energies of isobutene. A comparison of the formation of primary and tertiary carbenium ions. (DFT–VASP calculations). (a) Mordenite. (b) Ferrierite (DFT–VASP calculations) (TON)[16b .

First it is important to note the small di erence in energy of the protonated groundstate primary (n-butoxy) and tertiary (isobutoxy) alkoxy species in mordenite. The transi- tion-state energies of the two corresponding intermediate carbenium ions, however, demonstrate a much larger energy di erence.

Shape-Selective Microporous Catalysts, the Zeolites 173

The activation energy for protonation through a primary carbenium ion is 60–80 kJ/mol higher than that through the tertiary carbenium ion. The height of this barrier is of the same order of magnitude as that for the isomerization of pentene, a reaction that readily occurs in a zeolite at 550 K. Hence it can be concluded that isomerization and other hydrocarbon conversion reactions via primary carbenium ions are possible at reasonable temperatures because of the stabilization of the carbenium ion transition state by the zeolite framework. The protonation via the tertiary carbenium ion, however, is substantially more favorable, which is in agreement with physical organic theory.

In the narrow pore ferrierite (TON), the energies of the two alkoxyspecies are quite different. For the reaction that proceeds via the tertiary carbenium ion, the free protonated isobutyl cation is even more stable than covalently bonded isobutoxy intermediate. The curvature of the ferrierite channel prevents the close approach of methyl groups on the isobutyl intermediate to oxygen atoms in the zeolite wall, hence the tertiary carbenium ion is a stable but freely moving intermediate.

These examples help to illustrate the importance of the shape and dimensions of the micropore and their influence on the electrostatic screening of the charges generated in the transition state. The previous discussion illustrates that the formation of the carbon– oxygen bond between protonated species and zeolitic oxygen atom is counteracted by repulsive interactions arising from the bulkiness of the protonated intermediate and the zeolite micropores. When the curvature of the cavity becomes significantly large, the bulkiness of the protonated intermediate prevents the formation of the corresponding alkoxy species, hence the free carbenium ion becomes a stable intermediate. If the reaction intermediates become larger than the micropore cavity, they will not be formed. The same holds for reactant molecules that are too large to enter a micropore. The suppression of the formation of intermediates larger than the micropore cavity can lead to a reduction in coke formation. For this reason, solid acid reactions carried out in zeolitic micropores are less susceptible to coke formation.

In the first part of this section we have shown for zeolite solid acids that carbenium or carbonium ion intermediates are typically present as transition states or unstable intermediates. The activation energies depend on the deprotonation energy of the zeolite, the stabilization of the charged cationic intermediates by screening e ects and by their interaction with the negative charge left on the zeolite lattice.

Three additional mechanistic aspects that are also essential to zeolite catalysis include:

–pre-transition state orientation

–associative versus alkoxy intermediate reactions

–sca olding e ects of coadsorbed polar molecules

We will illustrate the e ects of pre-transition state orientation for the dissociation of methanol. This reaction is essential for the formation of dimethyl ether, which is described below. This is followed by a discussion of the alkylation mechanisms.

Let us consider dimethyl ether formation from methanol, which proceeds through a consecutive reaction mechanism[17]. Figure 4.13a illustrates the reaction intermediates for the first reaction step in which the C–O bond in methanol is cleaved. The calculated reaction energy diagram for this reaction is shown in Fig. 4.13b. The reaction products that form are water and adsorbed methoxy.

C–O bond cleavage by protonation only occurs when methanol is rotated from its most stable adsorption mode (end-on), which has two hydrogen bonds, to the methanol side-on adsorbed mode which has only one hydrogen bridge between the zeolite proton and the

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Figure 4.13. (a) Reaction intermediates for the proton activated dissociation of CH3 OH, and in addition, (b) the corresponding reaction energy diagram. The structures and energies reported are from DFT calculations on small zeolitic cluster models[17].

adsorbed molecule and along with weak interaction between the framework oxygen atoms and the methyl group. The unsaturated fragment that is generated upon dissociation is stabilized by bonding to a surface oxygen atom. The side-on mode is now the only mode that enables the CH3 + cation to adsorb on the negatively charged oxygen atom attached to an aluminum atom in the lattice. This oxygen atom is di erent from the one to which the proton was initially bonded. The reaction energies shown in Fig. 4.13b suggest that under reaction conditions only a very small minority of the adsorbed molecules will be adsorbed in this so-called pre-transition state mode. The reaction energy diagram shown

Figure 4.14a. Reaction energy scheme for the consecutive direct reaction path towards dimethyl ether formation[16].

Figure 4.14b. Associative reaction path towards formation of dimethyl ether from methanol[16].

in Fig. 4.13b, however, has been computed for a cluster simulating the zeolitic proton. The transition-state value should be lowered by approximately 50 kJ/mol to correct for the absence of screening e ects by the zeolite lattice.

The methoxy species formed by the dissociation of methanol can react with a second methanol molecule to give dimethyl ether and a proton. This is termed the “alkoxy” intermediate for the consecutive direct reaction mechanism. Alternative reaction paths exist which can be described as “associative” reaction mechanisms in which the product of an association reaction is formed without the formation of an alkoxy intermediate species[16] . For two coadsorbed methanol molecules, the most stable adsorption mode does