Alkylation and Transalkylation Reactions of Aromatics - ACS

Chapter 24

Alkylation and Transalkylation Reactions of Aromatics 1

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S. R. Blaszkowski and R. A . van Santen 1

DSM Research, P.O. Box 18, 6160 MD Geleen, Netherlands Schuit Institute of Catalysis, Laboratory for Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands

Downloaded by PENNSYLVANIA STATE UNIV on August 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch024

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Density Functional Theory calculations have been performed to analyse the reaction energy of solid acid catalyzed methyl transfer reactions. Different mechanistic routes for the alkylation of benzene and toluene by methanol have been compared. An associative reaction path via an intermediate complex of methanol and benzene or toluene is found to be the preferred route. The activation energy is 123 and =120 kJ/mol for benzene and toluene, respectively. A methoxy-mediated path involves very high activation barriers compared to the associative route. However coadsorbated water gives a large reduction of the activation energy for this reaction. Different mechanisms for toluene transalkylation: involving biphenyl methane as an intermediate, directly via methyl transfer, and methoxy mediated have been compared. When the reaction proceeds via a biphenyl methane intermediate the preferred route is the one where the reaction chain of elementary reactions is propagated via hydride transfer. The limiting step is the initial dehydrogenation, with an activation energy of +277 kJ/mol, which is present only in the very first step of the reaction chain. In the following steps, instead of initial dehydrogenation is replaced by proton assisted cracking of biphenyl methane becomes the step with the highest activation barrier. The direct mechanisms via methyl transfer or via intermediate methoxy do present activation barriers that are lower than the dehydrogenation step but higher than via biphenyl methane/hydride transfer mediated reaction. For small pore zeolites, where large molecules like biphenyl methane cannot be formed, they should be considered as optional routes for the transalkylation reaction.

The discovery that the medium-pore zeolite ZSM-5 can alkylate or disproportionate mono-substituted benzene compounds to achieve with nearly 100% selectivity parasubstituted xylene has generated significant fundamental (1-9) as well as practical

© 1999 American Chemical Society

In Transition State Modeling for Catalysis; Truhlar, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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308 interest (10). Here we will present an analysis of the possible intermediates and elementary reaction steps leading to (trans)alkylation of aromatics. In the zeolite catalyzed alkylation of benzene or toluene a first question is whether the carboncarbon bond formation proceed via an associative reaction mechanism (11) or in consecutive reaction steps. Consecutive reaction steps require the formation of intermediate methoxy species, generated by dissociation of methanol or toluene. Associative reactions proceed via coadsorption of the corresponding molecules and no intermediate methoxy formation is necessary. Until some time ago it was generally accepted that the alkylation of aromatics proceeds via intermediate methoxy formation (4-6,12). However recently Ivanova and Corma (73) by using C NMR have shown that an associative reaction path is more likely. Just as for alkylation reactions, transalkylation reactions can be proposed to occur according to a consecutive reaction scheme via intermediate methoxy formation or according to an associative reaction mechanism with direct CH transfer. Additionally for the transalkylation a third option is the formation of a biphenyl methane intermediate. This reaction route appears to dominate (1,2,7). As we will show in the latter case adsorbed benzyl-cation intermediates have to be proposed and the reaction has to be considered a reaction chain propagated by hydride transfer. For both reactions, alkylation and transalkylation of aromatics, there is now ample evidence that in the zeolite not a primary reaction step (4,11) but consecutive reactions and difrusional limitations result in the high selectivity for p-xylene (14). Isomerisation has also been found to proceed via a bimolecular route similar to the transalkylations. Here we will analyze several reaction paths and their corresponding reaction energy diagrams based on the results of DFT calculations. The zeolite will be represented by a lT-atom cluster. Earlier we successfully demonstrated the use of this approach to analyze dimethyl ether and carbon-carbon bond formation mechanisms (//). There are two limitations to the use of the cluster approximation to study proton activated reactions in zeolites. Firstly the acidity of the cluster can be different from the acidity of the real zeolitic site and it may depend on cluster size. Secondly no information on the size or shape of the cavities is provided for by the cluster calculations. Elsewhere we have extensively discussed the effect of cluster size on computed transition state energies (75). Deprotonation of the zeolite requires very high activation energies (15). Nevertheless, the breaking of the proton-cluster bond is compensated for by a co-interacting change in the electrostatic interaction between positively charged transition state and negatively charged cluster, making the energy involved in protonation reactions to become much lower. Also by comparing computed transition states energies using different clusters one can obtain converged values using the Brcmsted-Polanyi (75) relation between activation energies and reaction energies of reactions with the same reaction-mechanism. According to this relation there is a linear relationship between activation energy and reaction energy. The latter is in our case proportional to the deprotonation energy. We have shown earlier (75) that for cracking reaction this lowers the activation energies of transition states on a 1TH cluster by 25 kJ/mol more than it would in a low aluminum contents zeolite. Clearly when the micropores of the zeolites are smaller in size than the transition states considered in the model calculations, the size of the micropore will


1 3

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309 prevent their formation. For the ZSM-5 zeolites this implies that the bimolecular reactions will only occur at the channel cross sections. Embedding procedures using empirical potentials will have to be used to actually analyze the increases in transition state energies that occur in narrow pores. The use of Car-Parinello approaches (16) that enable full consideration of the three dimensional structures of zeolites offers no solution. This is because the quantum mechanical method applied is also based on the electronic Density Functional Theory, that cannot be properly used to compute the weak Van der Waals interactions that dominate the interaction of the adsorbate with the zeolite cage. Cluster calculations give relevant results when applied to a system where steric constraints do not play an important role and the only dominant effect of the zeolite cage is a weak stabilization of the complex by Van der Waals interactions. The reference state for the reacting molecule within the cluster approximation is not the molecule in the gas-phase and empty zeolite, but the molecule (or molecules) physically adsorbed in the micropores of the zeolite (17). The cluster approximation implies that the attractive Van der Waals interaction-energies do not change during reaction. For a first order reaction with a proton activated elementary reaction step as rate limiting step this leads to the following relation between apparent and true activation energy (18): E ds(app)=Eact(true) - (1-0) E ds a

a

E ds is the absolute energy value of reactant adsorption and 9 is its coverage. The energies predicted by the cluster calculations are the true activation energies. In view of the relatively high adsorption energies of aromatic molecules in medium pore zeolites,«80 kJ/mol (19), the corrections to the apparent activation energies due to adsorption are considerable. This is even more the case for bimolecular reactions, with a more complex dependence of the apparent activation energy on adsorption energies. After a short summary of the computational details, we will start the results section with a discussion of benzene and toluene methylation by reaction with methanol. Addition of a surface methoxy species is compared with associative methylation from methanol. The addition of methoxy to benzene proceeds via a transition state close to the transition state obtained earlier by Beck et al. (20) for H/D exchange between deuterated zeolite and benzene. A characteristic of such transition states is that several oxygen atoms of the protonic sites are involved. Protonation is to be considered as Lewis base/Brensted acid assisted reaction. In the second part of the results section we will compare the different transalkylation routes. a

Computational details All calculations in this work are based on Density Functional Theory, DFT (21). The Dgauss program (versions 2.1 and 3.0) part of the UniChem package from Cray research Inc. (22) was used. The local density approximation (LDA) with the exchange-correlation potential given in the form parameterized by Vosko et al. (23) is used to obtain transition state geometries. The LDA without non-local correction has


310 been found to be inadequate for the calculation of accurate binding energies for reactions which involve hydrogen transfer (24,25). Because of this, non-local exchange and correlation corrections (NL) due to Becke (26) and Perdew (27), respectively, are included to the final total LDA energy. This level of correction is found to be excellent for binding energies. The basis sets are of double-zeta quality and include polarization functions for all non-hydrogen atoms, DZPV (28). They were optimized for use in density functional calculations in order to minimize the basis set superposition error (29), BSSE, as has been demonstrated by Radzio et al. (30) in studies of the Cr molecule. A second set of basis functions, thefittingbasis set (57) is used to expand the electron density in a set of single particle Gaussian-type functions. Total LDA energy gradients and second derivates are computed analytically (32). No symmetry constrains have been used in the optimisation of any of the studied structures. Several different approaches can be used to represent the zeolitic/adsorbate system: i) the cluster approach, used in the present work, ii) to embed the cluster in a set of point charges chosen to reproduce the bulk potential (33) and, more recently, iii) first principle calculations where the full zeolite structure is calculated using periodic boundary conditions (17,34). We commented on the limitations and also use of our approach in the introductory part of this paper. The molecular systems considered are the zeolitic cluster interacting with methanol and/or aromatic molecules as benzene, toluene and xylene or other reaction intermediate. The single HOHAl(OH) cluster has been used to represent the acidic zeolite. The main reason for choosing this small cluster is the very large size of the organic molecule what results in a very large system to be optimized. The transition states obtained are of course very useful as initial structure when computer resources become available for calculations on larger systems. Elsewhere (75) we have shown that the estimated error in the computed interaction energies due to the DFT approximation is of the order of 25 kJ/mol.


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Results and discussion Alkylation reactions. For the alkylation reaction Table 1 summarizes elementary reaction steps and their respective activation energies. A comparison is made between the associative reaction mechanism and via intermediate methoxy species. In Figures la, lb, and lc the transition states of three different benzene alkylation mechanisms are shown. The transition states for toluene alkylation are similar, except that an extra methyl group appears in the o- m- or p- positions. The associative reaction path of the proton assisted reaction between methanol and benzene resulting in toluene (the transition state is shown in Figure la) presents an activation barrier of 123 kJ/mol. Alkylation by surface methoxy is compared in the presence (Figure lb) and absence (Figure lc) of coadsorbed water. The table shows that in presence of water the alkylation is much easier. The conclusion is that in the absence of water the direct associative reaction path is preferred. This agrees with the recent work of Ivanova et al. (75). However the presence of water makes the activation energies of both reaction mechanisms, associative and via surface methoxy (water 9



T (b)

AI

IO

O H

Figure 1. Transition state for (a) direct methylation of benzene by methanol (b) reaction of the surface methoxy and benzene, and (c) water assisted reaction of surface methoxy and benzene. Note the nearly 180° angle between O-CCCHsKCC^).


312 assisted) very close. This means that in the consecutive reaction steps the aromatic molecule reacts with methoxy species generated by dissociative adsorption of methanol.

Table 1. Activation energies for alkylation of benzene and toluene


figure

initial state

final

state

E (kJ/mol) act

123

la

benzene + methanol

toluene

lb

benzene + methoxy + water

toluene + water

lc

benzene + methoxy

toluene

(la) (lb) (lc)

toluene + methanol toluene + methoxy + water toluene + methoxy

o-xylene o-xylene + water o-xylene

118 123 183

(la) (lb) (lc)


m-xylene m-xylene + water m-xylene

121 128 190

(la) (lb) (lc)


p-xylene p-xylene + water p-xylene

119 124 183

133 180 192 240

forward backward forward backward

The transitions states for toluene alkylation are analogous to benzene alkylation, expect by the presence of a methyl group in the positions o-, m-, or p-. The corresponding benzene figure numbers are shown in parenthesis.

The activation energy of methoxy formationfrommethanol has been computed to be equals to 212 kJ/mol (11,35). With coadsorbed H2O this value is reduced to 160 kJ/mol (11). The lower reaction barrier for reaction of methoxy in the presence coadsorbed water is due to the nearly 180° angle between cluster oxygen methylcarbenium ion intermediate and benzene carbon atom in this particular transition state. The scaffolding effect that causes this has been notedfirstby Sinclair and Catlow (36) and is also operational on water promoted dissociative adsorption of methanol (77). For completeness Table 1 also collects the activation energies for the alkylation of toluene. As expected for an electrophylic substitution reaction, toluene is more reactive than benzene. The same preference for associative direct alkylation with methanol as for benzene is found. Ortho alkylation is slightly preferred over para and meta alkylation has the lowest rate. This agrees with the observed (37,38) preference for initial formation of ortho-xylene. As discussed in the introductory part, the p-


313 xylene will be the main product result of consecutive equilibration and diffusion reactions, as also confirmed by Monte Carlo calculations (7). The transalkylation reactions. Three reaction mechanisms for transalkylation can be distinguished: 1. Direct associative transalkylation:


+

R

C H ^ -

+

(1)

HP-CM

2. Indirect, methoxy mediated transalkylation:

Alkylation and Transalkylation Reactions of Aromatics - ACS

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