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Applied Catalysis A: General 336 (2008) 54–60

A DFT–ONIOM study on the effect of extra-framework aluminum

on USY zeolite acidity

Nilton Rosenbach Jr., Claudio J.A. Mota *

Universidade Federal do Rio de Janeiro, Instituto de Quımica, Av. Athos da Silveira Ramos 149, CT Bloco A, Rio de Janeiro 21941-909, Brazil

Received 2 July 2007; received in revised form 26 September 2007; accepted 27 September 2007

Available online 5 October 2007

Abstract

The effect of Al(OH)2+ EFAL (extra-framework aluminum) species in the acid strength of USY zeolite was studied by the ONIOM scheme, at

PBE1PBE/6-31G(d,p) level. The model consists of a hexagonal prism and the sodalite cage (T30), representing a real part of the zeolite Y system

that allows calculation of different acid sites (O1 and O3). The acid strength was calculated by computing the deprotonating energy and comparing

the results with the data obtained for an isolated acid site. The calculations indicated that there occurs an increase in acid strength, depending on the

relative position between the EFAL and the acid site. However, the role of Al(OH)2+ EFAL is not to increase the energy of the acid form, through

Brfnsted/Lewis synergism, but to stabilize the conjugated base, formed upon deprotonation, by hydrogen bonding and nucleophilic interaction

with the framework oxygen atom.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Zeolite; Acidity; Extra-framework aluminum; DFT

1. Introduction

Ultrastable Y zeolites (USY) are the main components of

cracking catalysts. These zeolites are usually produced [1–3] by

hydrothermal treatment of the NH4Y at elevated temperatures

(500–700 8C). During this treatment, the aluminum atoms are

removed from the crystalline structure and remain inside the

pores and cavities as extra-framework aluminum (EFAL)

species. The chemical nature of these species is not entirely

known, but it is accepted that oxoaluminum cations, such as

AlO+, Al(OH)2+, AlOH2+, as well as neutral compounds, such

as AlOOH and Al2O3, are the most common EFAL species

[4,5]. It is worth mentioning that after hydrothermal treatment,

the prepared USY zeolite must be ammonium exchanged to

restore the catalytic activity, since many Brfnsted acid sites

remain neutralized.

The catalytic activity of USY zeolites is usually higher than

the parent, non-dealuminated, Y zeolite [6]. As the framework

silicon to aluminum (Si/Al) ratio goes up, the Brfnsted acid

sites become more isolated having an increased strength.

* Corresponding author. Fax: +55 21 25627106.

E-mail address: cmota@iq.ufrj.br (C.J.A. Mota).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.09.048

However, the presence of EFAL plays a more important role in

governing the catalytic activity of USY zeolites. Acid leaching

of steam-dealuminated zeolite Y leads to a decrease in the

catalytic activity of n-heptane cracking [7] and has been

interpreted in terms of extraction of the EFAL species.

Dealumination with an aqueous solution of (NH4)2SiF6 [8]

replaces the framework aluminum atoms with silicon atoms of

the fluorosilicate reagent. The aluminum species released from

the framework is dissolved in the aqueous solution and carried

off from the zeolite pores, forming a dealuminated Y zeolite

without EFAL species. The catalytic activities of the zeolites

dealuminated with (NH4)2SiF6 are normally significantly lower

than steam-dealuminated zeolites of comparable framework Si/

Al ratio [9–11]. The increased catalytic activity of USY zeolites

has been attributed [7,12–16] to a synergism between EFAL

species, which act as Lewis acid sites, and the framework

hydroxyl group, which is the Brfnsted acid site in zeolites

(Fig. 1). This would increase the acid strength of the zeolite in

the same manner as in liquid superacid solutions, where

synergism between Brfnsted and Lewis acids does occur.

The determination of the acidity of solid acids is a matter of

intense debate. There is not a universal accepted method to

determine the acid strength of solids, and the results depend

upon the technique employed [17]. Solids are not homogeneous

Fig. 1. Superacid site in zeolites.

Table 1

Relative stability of the Brfnsted acid sites in Y zeolite (kcal mol�1)

Theoretical level O1H O2H O3H O4H

Empirical shell model potential 0.0 3.5 �1.3 4.4

Ab initio shell model potential 0.0 2.2 �2.1 4.6

DFT shell model potential 0.0 1.4 �2.7 3.3

Embedded cluster 0.0 6.9 1.1 5.4

Periodic DFT/VWN//DNP/DFT/VWN/DNP 0.0 2.6 0.8 2.1

Periodic DFT/GGA/DNP//DFT/VWN/DNP 0.0 2.3 1.2 1.9

Occupation (experimental) 0.4 0 0.6 0

N. Rosenbach Jr., C.J.A. Mota / Applied Catalysis A: General 336 (2008) 54–60 55

and possess Brfnsted and Lewis acid sites of different acid

strengths on the surface. The accessibility to the acid sites also

plays an important role and might explain, in some cases, the

differences observed in the measurements of acid strength.

Base thermo-desorption is one of the most used [18] technique

to determine the acid strength distribution of solids, but it is not

specific for Brfnsted sites and is subject to severe critics.

Spectroscopic observations of adsorbed probe molecules

are also used to estimate the acid strength of solid materials

[19–24], but some of these techniques are not accessible for

ordinary, day-to-day studies. Linear free energy relationship of

H/D exchange with substituted benzene derivatives has recently

been used to assess the acid strength of different solid acid

materials [25]. The results showed that proton transfer on

zeolites involves a considerable symmetric transition state, with

little degree of charge separation.

Despite small discrepancies, most of the results indicate that

the acid strength of zeolites is comparable to 90% sulfuric acid

solutions, and they cannot be classified as superacids [26].

However, many reactions that occur in superacid media, and are

believed to proceed through dicationic intermediates, are also

observed within the zeolite pores [27–30], suggesting that

under some circumstances they behave as superacids. On the

other hand, a microcalorimetric study [31] of amine adsorption

showed that steam-dealuminated and (NH4)2SiF6-dealumi-

nated Y zeolites have virtually the same acid strength, in spite

of the large difference in catalytic activity. This result suggests

that other effects, besides acid strength, might be responsible

for the catalytic activity of zeolites.

Theoretical methods have gained widespread use in the

study of catalytic systems. Using a T6 cluster (T = Si or Al) to

represent a part of the zeolite Y structure, we showed that

oxoaluminum cations, representing monomeric EFAL species,

are preferentially located near the framework aluminum atom,

being bi-, tri- and tetracoordinated with the structure,

depending on the nature of the oxoaluminum cation [32].

We were able to show that some cationic species, such as Al3+,

are hydrolyzed to form Al(OH)2+ cation and a Brfnsted acid

site, indicating that coordination of high valence cations with

the framework structure is difficult. We also investigated [33]

the role of cationic oxoaluminum cations, as model of EFAL

species, in the deprotonation energy of the HT6 cluster, finding

that most of the species studied do not increase the acid

strength. In some cases, there occurred an intramolecular

proton migration from the Brfnsted acid site to the oxygen

atom of the EFAL, resulting in a new acid site with reduced acid

strength. Only in the case of Al(OH)2+ EFAL the deprotonation

energy was lower than the value calculated for an isolated acid

site, indicating that the EFAL had increased the acid strength of

the zeolite. Notwithstanding, the effect is not associated with

the Brfnsted/Lewis acid synergism, but to the stabilization of

the zeolite-conjugated base by hydrogen bonding with the

EFAL hydroxyl groups.

In spite of providing a qualitative description of many

zeolites properties, the cluster approach is limited in describing

the different crystallographic environments of the Brfnsted

acid sites in many zeolite systems.

Periodic calculations can circumvent some of these

limitations. In this approach, the size of the zeolite structure

can be reduced to the size of a unit cell of the solid. Periodic

density functional calculations have become feasible for

zeolites. However, the computational costs are still high,

which limit the scope of the studies with this technique.

Moreover, zeolites present large unit cell size, involving many

atoms, and the presence of the framework aluminum atom

causes an asymmetry on the crystalline structure.

The combined quantum mechanics/molecular mechanics

(QM/MM) approach is one of the most currently used methods

to study complex reaction systems, such as heterogeneous

catalysts. Table 1 shows the relative stability of the Brfnsted

acids sites in zeolite Y, considering the four crystallographi-

cally distinct oxygen atoms (O1–O4), at different levels of

calculation [34]. The results with QM/MM approach gives the

correct ordering obtained experimentally and is also in good

agreement with periodic calculations.

The ONIOM method [35] is similar to QM/MM hybrid

methods. In this approach, the atoms of the active site can be

described with a more sophisticated theoretical level and the

rest of the zeolite structure can be treated with a less computer

demanding method. This approach is able to correctly

reproduce the properties of the Brfnsted acid sites in ZSM-

5 zeolite [36]. Other studies show that the ONIOM method can

also differentiate crystallographically different acid sites,

indicating that lattice effects have a significant role [37–41].

In this work, we used the ONIOM method to investigate the

acid properties of USY zeolite and better understand the role of

the Al(OH)2+ EFAL species.

2. Computational details

The T30 molecular system used in this work, where T stands

for framework Al or Si atoms, represents a part of the zeolite Y

structure (the sodalite cage and hexagonal prism) and is formed

by 102 atoms. The border silicon atoms were saturated with

hydrogen atoms, to avoid dangling bonds. Before any

calculation, the geometry of the fully silica T30 mode was

optimized at semi-empirical MNDO level. The proton and the

Table 2

Relative and deprotonation energies of the Brfnsted acid sites in Y zeolite

Site Relative energy Deprotonation energy

DH (kcal mol�1) DS (cal mol�1 K�1) DG (kcal mol�1)

O1H 0.0 306.5 24.8 299.1

O2H – – –

O3H �2.4 308.9 25.8 301.2

O4H 2.4 – – –

N. Rosenbach Jr., C.J.A. Mota / Applied Catalysis A: General 336 (2008) 54–6056

Al(OH)2+ EFAL were then added and one or more framework

silicon atoms were replaced by aluminum atoms, obeying the

Lowenstein rule, to keep charge neutrality. All calculations

were performed at the ONIOM scheme available in Gaussian

98 package [42].

In the ONIOM approach, the molecular system can be divided

in to several layers. The high layer, a T6 cluster containing three

of the four crystallographically distinct oxygen atoms (O1, O3

and O4), were described at PBE1PBE/6-31G(d,p) level, whereas

the low layer, describing the rest of the zeolite system, was

described with the semi-empirical MNDO method.

The energy of the framework system (Ereal) is determined

only classically, whereas the energy of the model (Emodel),

formed by the atoms in the T6 cluster and the atoms used in the

saturation of the dangling bonds, is determined by quantum

mechanical methods (QM) and by classical methods (MM).

The position of the link atoms is defined by the positions of the

atoms in the framework system.

EONIOMðQM:MMÞTOTAL ¼ EQM

model þ EMMreal � EMM

model

¼ Ehighmodel þ Elow

real � Elowmodel

The geometry of all species were fully optimized and

characterized as minima on the potential energy surface by the

absence of imaginary frequencies, after vibrational analysis.

Zero-point energies (ZPE) and thermal corrections at 298.15 K

were calculated, using the frequencies obtained at the same

level. Unless otherwise stated, energy calculations refer to the

enthalpic term at 298.15 K and 1 atm.

3. Results and discussion

Table 2 shows the relative stability of the topological

different Brfnsted acid sites and the correspondent deprotona-

tion energy, at ONIOM (PBE1PBE/6-31G(d,p):MNDO) level.

The relative stability is similar to that obtained by other

theoretical methods (see Table 1). However, the absolute values

are not equivalents. The O1 site, which is directed to the

supercage, is slightly more acidic than the O3 site, which is

pointed to the sodalite cage, as can be seen by the lower

Fig. 2. Optimized structures of the Brfnsted acid site O1H (1) in the presence of th

(PBE1PBE/6-31G(d,p):MNDO) level.

deprotonation energy at this level of calculation. This is

consistent with the higher stability of the proton attached to O3,

which is 2.4 kcal mol�1 lower in energy with respect to the

attachment in the O1 site. It should be stressed that, at the

present level of calculation, the 2.4 kcal mol�1 energy

difference is small and might be within the errors of the

method. Thus, the results should be taken more as a general

trend, rather than as absolute values.

Figs. 2–4 show the structures of the O1H and O3H Brfnsted

acid sites in the presence of the AlO+ and Al(OH)2+, as well as

the structure of the conjugated bases formed upon deprotona-

tion. For AlO+, all the calculations ended with the proton

attached to the EFAL oxygen atom, yielding a new acid site

(structure 1). This behavior has already been observed with

cluster calculations [33] and confirmed the point that such an

EFAL species may not exist in a real zeolite environment, being

protonated by the Brfnsted acid site of the zeolite. The same

situation was found for Al(OH)2+, when considering the proton

initially attached to O1 and the EFAL in the four T-atom ring

located in the sodalite cage (structure 3). There occurs a

migration of the proton to one of the hydroxyl groups of the

Al(OH)2+ EFAL, forming a strong hydrogen bond (1.49 A)

with the O1 oxygen atom. This intramolecular hydrogen bond is

similar in strength to the hydrogen bonds found in some

dicarboxylic acids [43] and sterically congested diamine,

known as proton sponges [44,45]. This situation was not

observed in the cluster calculations [33], probably because in

the real zeolite Y system the geometric changes upon

interaction with the EFAL are more restricted and cannot be

properly described with cluster calculations.

e AlO+ EFAL and of the respective conjugated base (2) determined at ONIOM

Fig. 3. Optimized structures of the Brfnsted acid site O1H (3) and O3H (5) in the presence of the Al(OH)2+ EFAL located in the four T-atom rings of the sodalite cage

and the respective conjugated base (4) at ONIOM (PBE1PBE/6-31G(d,p):MNDO) level.

N. Rosenbach Jr., C.J.A. Mota / Applied Catalysis A: General 336 (2008) 54–60 57

When the proton occupies the O3 site (structure 5) or when

the Al(OH)2+ EFAL occupies the four T-atoms ring located in

the hexagonal prism (structures 6 and 8), no migration of the

proton to the Al(OH)2+ hydroxyl groups was observed. Energy

calculations showed that structure 3 is the most stable among

the isomeric structures of the Al(OH)2+/zeolite systems

(Table 3). Structure 6 has the highest energy, lying

21.1 kcal mol�1 above structure 3 at ONIOM (PBE1PBE/6-

31G(d,p):MNDO level). Structures 8 and 5 are 3.6 and

11.8 kcal mol�1 higher in energy than 3, respectively. The

higher stability of structure 3 may be associated to the strong

hydrogen bonding between the Al(OH)2+ hydroxyl group and

the framework oxygen atom.

Table 4 presents the deprotonation energies of the EFAL/

zeolite systems studied. Compared to an isolated acid site the

Table 3

Relative energy of isomeric Al(OH)2+/zeolite systems

Al(OH)2+/zeolite Structure Relative energy (kcal mol�1)

Acid form 3 0.0

5 11.8

6 21.1

8 3.6

Conjugated base 4 8.1

7 0.0

AlO+/zeolite system showed a weaker acid strength, with

significant increase of the deprotonation energy. This may be

ascribed to the internal proton migration, which attaches the

proton in the AlO+ oxygen atom. Therefore, the acidity is

significantly reduced, as already observed in clusters calcula-

tions [33]. The Al(OH)2+/zeolite system showed two different

behaviors, depending on the relative position of the proton and

the EFAL. Structures 5, 6 and 8 presented a deprotonation

energy lower than the value computed for the isolated acid site,

whereas structure 3 presented a slightly higher deprotonation

energy, indicating a decrease in acid strength relative to the

isolated site. The conjugated base 4, which is associated with

deprotonation of 3 and 5, is higher in energy than structure 7,

associated with deprotonation of 6 and 8, by 8.1 kcal mol�1.

This difference and the fact that structure 6 is the highest energy

Al(OH)2+/zeolite system might explain the higher acid strength

of structure 6, compared with the other isomeric Al(OH)2+/

zeolite systems.

The synergism between the Lewis and Brfnsted acid sites is

frequently considered to explain the higher activity of USY

zeolites, when EFAL species are present. The calculations

indicated that species 5, 6 and 8 showed a deprotonation energy

significantly lower than an isolated acid site, with either the

proton attached to O1 or O3 and, by consequence, an increased

acid strength. However, the explanation is not due to a

synergism between the Brfnsted and Lewis sites, but to

Fig. 4. Optimized structures of the Brfnsted acid site O1H (6) and O3H (8) in the presence of the Al(OH)2+ EFAL located in the four T-atom ring of the hexagonal

prism and respective conjugated base (7) at ONIOM (PBE1PBE/6-31G(d,p):MNDO) level.

N. Rosenbach Jr., C.J.A. Mota / Applied Catalysis A: General 336 (2008) 54–6058

hydrogen bonding stabilization of the conjugated base. This is

clear for structure 4, where a hydrogen bond distance of

1.89 A was computed for the interaction between the

Al(OH)2+ hydroxy groups with the framework oxygen atom

(O1). The same behavior was found in cluster calculations to

explain the increased acidity of the Al(OH)2+ T6 cluster [33].

On the other hand, the high acidity of structures 6 and 8 might

be partly explained by a Brfnsted/Lewis acid synergism,

because Al(OH)2+ interacts with the framework oxygen atoms

adjacent to the acid site in these protonated structures. In fact,

the interaction in 6 leads to a highly energetic structure, as

shown in Table 3. One may infer that such a structure may not

exist in a real zeolite environment, migrating the EFAL to a

Table 4

Deprotonation energy of EFAL/zeolite systems at ONIOM (PBE1PBE/6-31G(d,p)

Reaction EFAL Site of the proton DH

1 ? 2 + H+ AlO+ O1b 369

3 ? 4 + H+ Al(OH)2+ O1

b 311

5 ? 4 + H+ Al(OH)2+ O3

b 299

6 ? 7 + H+ Al(OH)2+ O1

c 282

8 ? 7 + H+ Al(OH)2+ O3

c 299

a Calculated at 298 K, 15 K and 1 atm.b EFAL in the four T-atom rings of the sodalite cage.c EFAL in the four T-atom rings of the hexagonal prism.

more favorable position. However, the conjugated base 7,

associated with deprotonation of 6 and 8, shows a

nucleophilic interaction of the deprotonated oxygen atom

(O1) with the EFAL aluminum atom, expressed by the 1.88 A

computed for the Al–O bond length. This leads to a strong

stabilization of the conjugated base, reflected in the

8.1 kcal mol�1 lower energy than structure 4. Hence,

calculations suggest that the decrease of the deprotonation

energy of 6 is probably due to a synergism between the

Brfnsted and Lewis acid sites, which increase the energy of

the system by 21.1 kcal mol�1 relative to 3, as well as to the

stabilization of the conjugated base 7, by interaction of the

EFAL aluminum atom with the framework oxygen atom. On

:MNDO) level

(kcal mol�1) DS (cal mol�1 K�1) DGa (kcal mol�1)

.8 28.6 361.3

.2 27.9 302.9

.4 27.3 291.3

.1 22.7 275.3

.6 18.5 294.1

N. Rosenbach Jr., C.J.A. Mota / Applied Catalysis A: General 336 (2008) 54–60 59

the other hand, since structure 6 is the less stabilized isomeric

form of the Al(OH)2+/zeolite system, it would be improbable

that it exists within the zeolite cages, but 8, which is only

3.6 kcal mol�1 higher than 3, may exist in a real zeolite USY

environment, suggesting that stabilization of the conjugated

base plays a more important role in increasing the acid

strength, than Brfnsted/Lewis acid synergism.

The acid strength of superacid solutions is in great part due

to the stabilization of the conjugated anion, with formation of

Sb2F11��(SbF5)n species. The negative charge is widely

dispersed through interactions between the fluorine atoms

of the Sb2F11� moiety with the antimony atom of the SbF5

[46–51]. In zeolites, the stabilization of the conjugated base is

mostly concentrated on the atoms in the neighborhood of the

acid site. The calculations show that in many instances, the

EFAL hydroxyl groups stabilize the conjugated base, as well as

the acid form, through hydrogen bonds. This is the classical

model of anion stabilization in protic solvents [52], but this

mechanism is not possible in a completely isolated acid site. In

addition, the calculations have shown that anion stabilization

through nucleophilic interactions between the framework

oxygen atoms and the EFAL aluminum atom is also possible,

and contributes to increase the acid strength of structures 6 and

8 with respect to an isolated acid site. This behavior is similar to

the anion stabilization in superacid solutions, where nucleo-

philic interactions between the fluorine and the antimony atom

do occur.

We might conclude from the present calculations that, the

main role of EFAL species in USY zeolites is to stabilize the

conjugated base, formed upon deprotonation, rather than

increase the energy of the acid form, through Brfnsted/Lewis

synergism. Such an interaction would lead to a structure of high

energy, which would probably rearranges to a more stable one,

moving the EFAL to a more favorable location. On the other

hand, the delocalization of the negative charge on the zeolite-

conjugated base demands stabilization from the atoms in the

neighborhood of the acid site, and plays a central role in the

development of more active zeolite catalysts.

4. Conclusions

ONIOM calculations of Al(OH)2+/zeolite Y systems, at

PBE1PBE/6-31G(d,p):MNDO level, showed that the acid

strength is affected by the location of the EFAL species. A

significant decrease in the acid strength was verified when there

occurs an intramolecular proton migration, from the framework

oxygen atom to the EFAL oxygen atom. This behavior was

found for AlO+/zeolite systems, as well as for Al(OH)2+ EFAL

when the proton is attached to the O1 site and the EFAL located

in the four T-atom ring of the sodalite cage. For the other

calculated Al(OH)2+/zeolite systems, with proton attached to

O3 or the EFAL located in the four T-atom ring of the hexagonal

prism, the deprotonation energy was lower than the value found

for an isolated acid site, showing an increase in the acid strength

of the zeolite. Nevertheless, calculations showed that the main

role of the EFAL is not to increase the energy of the acid form,

through Brfnsted/Lewis synergism, but to stabilize the

conjugated base by hydrogen bonding or nucleophilic

interaction.

Acknowledgements

Authors thank financial support from CAPES, FAPERJ and

CNPq.

References

[1] Q.L. Wang, G. Giannetto, M. Torrealba, G. Perot, C. Kappenstein, M.

Guisnet, J. Catal. 130 (1990) 459.

[2] M. Guisnet, Q.L. Wang, G. Giannetto, Catal. Lett. 4 (1990) 299.

[3] B. Xu, F. Rotunno, S. Bordiga, R. Prins, J.A. van Brokhoven, J. Catal. 241

(2006) 66.

[4] R.D. Shannon, K.H. Gardner, R.H. Staley, G. Bergeret, P. Gallezot, A.

Arnoux, J. Phys. Chem. 89 (1985) 4778.

[5] M.J. Remy, D. Stanica, G. Poncelet, E.J.P. Feijen, P. Grobet, J. Phys.

Chem. 100 (1996) 12440.

[6] J.R. Sohn, S.J. DeCanio, P.O. Fritz, J.H. Lunsford, J. Phys. Chem. 90

(1986) 4847.

[7] Q.L. Wang, G. Giannetto, M. Guisnet, J. Catal. 130 (1991) 471.

[8] G. Garralon, V. Fornes, A. Corma, Zeolites 8 (1988) 268.

[9] R.A. Beyerlein, G.B. McVicker, L.N. Yacullo, J. Ziemiak, J. Phys. Chem.

92 (1988) 1967.

[10] R. Carvajal, P.J. Chu, J.H. Lunsford, J. Catal. 125 (1990) 121.

[11] C.J.A. Mota, R.L. Martins, L. Nogueira, W.B. Kover, J. Chem. Soc.,

Faraday Trans. 90 (1994) 2297.

[12] C. Mirodatos, D. Barthomeuf, J. Chem. Soc., Chem. Commun. (1981) 39.

[13] P.O. Fritz, J.H. Lunsford, J. Catal. 118 (1989) 85.

[14] S. Beran, J. Phys. Chem. 94 (1990) 335.

[15] A. Corma, V. Fornes, F. Rey, Appl. Catal. 59 (1990) 267.

[16] F. Lonyi, J.H. Lunsford, J. Catal. 136 (1992) 566.

[17] W.E. Farneth, R.J. Gorte, Chem. Rev. 95 (1995) 615.

[18] R. Gorte, Catal. Lett. 62 (1999) 1.

[19] B.S. Umansky, W.K. Hall, J. Catal. 124 (1990) 97.

[20] B.S. Umansky, J. Engelhadt, W.K. Hall, J. Catal. 127 (1990) 128.

[21] T. Xu, E. Munson, J. Haw, J. Am. Chem. Soc. 116 (1994) 1962.

[22] P. Batamack, C. Doremieux-Morin, R. Vicent, J. Fraissard, J. Phys. Chem.

97 (1993) 9779.

[23] V. Semmer, P. Batamack, C. Doremieux-Morin, J. Fraissard, Top. Catal. 6

(1998) 119.

[24] J.F. Haw, Phys. Chem. Chem. Phys. 4 (2002) 5431.

[25] V.L.C. Goncalves, R.C. Rodrigues, R. Lorencato, C.J.A. Mota, J. Catal.

248 (2007) 158.

[26] G.A. Olah, G.K.S. Prakash, J. Sommer, Superacids, Wiley, NY, 1995.

[27] K.Y. Koltunov, S. Walspurger, J. Sommer, Chem. Commun. (2004) 1754.

[28] K.Y. Koltunov, S. Walspurger, J. Sommer, Catal. Lett. 98 (2004) 89.

[29] K.Y. Koltunov, S. Walspurger, J. Sommer, Tetrahedron Lett. 46 (2005)

8391.

[30] K.Y. Koltunov, S. Walspurger, J. Sommer, J. Mol. Catal. A 245 (2006) 231.

[31] A.I. Biaglow, D.J. Parrillo, G.T. Kokotailo, R.J. Gorte, J. Catal. 148 (1994)

213.

[32] D.L. Bhering, A. Ramirez-Solıs, C.J.A. Mota, J. Phys. Chem. B 107

(2003) 4342.

[33] C.J.A. Mota, D.L. Bhering, N. Rosenbach Jr., Angew. Chem. Int. Ed.

(2004).

[34] B. Delley, J.R. Hill, C.M. Freeman, J. Phys. Chem. A 103 (1999) 3772.

[35] K. Morokuma, F. Meseras, J. Comp. Chem. 16 (1995) 1170.

[36] K. Sillar, P. Burk, J. Phys. Chem. B 108 (2004) 9893.

[37] M. Sodupe, X. Solans-Monfort, V. Branchadell, J. Sauer, R. Orlando, P.

Ugliengo, J. Phys. Chem. B 109 (2005) 3539.

[38] W. Panjan, J. Limtrakul, J. Mol. Struct. (THEOCHEM) 654 (2003) 35.

[39] K. Bobuatong, J. Limtrakul, Appl. Catal. A 253 (2003) 49.

[40] X. Solans-Monfort, J. Bertran, V. Branchadell, M. Sodupe, J. Phys. Chem.

B 106 (2002) 10220.

N. Rosenbach Jr., C.J.A. Mota / Applied Catalysis A: General 336 (2008) 54–6060

[41] A. Damin, F. Bonino, G. Ricchiardi, S. Bordiga, A. Zecchina, G. Lamberti,

J. Phys. Chem. B 106 (2002) 7524.

[42] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.

Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C.

Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain,

O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.

Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala,

Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B.

Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu,

A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J.

Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez,

M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L.

Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople,

Gaussian 98, Rev. C.3, Gaussian Inc., Pittsburgh, PA, 1998.

[43] M. Garcia-Viloca, A. Gonzalez-Lafont, J.M. Lluch, J. Am. Chem. Soc.

119 (1997) 1081.

[44] H.A. Staab, T. Staupe, Angew. Chem. Int. Ed. 27 (1988) 865.

[45] R.W. Alder, Chem. Rev. 89 (1989) 1215.

[46] J.C. Culmann, M. Fauconet, R. Jost, J. Sommer, New J. Chem. 23 (1999)

863.

[47] R.J. Gillespie, K.C. Moss, J. Chem. Soc. A (1966) 1170.

[48] B. Bonnet, G. Mascherpa, Inorg. Chem. 19 (1980) 785.

[49] D. Mootz, K. Bartmann, Angew. Chem. Int. Ed. 27 (1988) 391.

[50] D. Kim, M.L. Klein, J. Phys. Chem. B 104 (2000) 10074.

[51] P.M. Esteves, A. Ramirez-Solıs, C.J.A. Mota, J. Am. Chem. Soc. 124

(2002) 2672.

[52] C. Reichardt, Solvent and Solvent Effects in Organic Chemistry, VCH–

Verlag, Weinheim, 1988.