Dynamic behaviour of carbocations on zeolites: Mobility and rearrangement of the C4H7+ system

Article abstract of DOI:10.1039/c3cc40627j

The mobility and rearrangement of the C₄H₇ ⁺ system over Chabazite were studied using ab initio molecular dynamics. The results indicated the high mobility of the cations, which can rearrange within picosecond time interva

Full text of DOI:10.1039/c3cc40627j

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Dynamic behaviour of carbocations on zeolites:
+
mobility and rearrangement of the C4H7 system
Cite this: Chem. Commun., 2013,
49, 4480
Diego P. Kling,a Erick S. A. Machado,a Henrique C. Chagas,a Alex P. A. dos Santos,b
Received 24th January 2013,
Accepted 15th March 2013
d
abe
Nilton Rosenbach Jr.,ac Jose Walkimar Carneiro and Claudio J. A. Mota*
´
DOI: 10.1039/c3cc40627j
www.rsc.org/chemcomm
The mobility and rearrangement of the C4H7+ system over Chabazite
were studied using ab initio molecular dynamics. The results indi-
cated the high mobility of the cations, which can rearrange within
picosecond time intervals. Experimental studies of nucleophilic
substitution supported the theoretical findings.
Scheme 1 Product distribution of nucleophilic attack on the three centre bond
Zeolites are crystalline aluminosilicates widely used as catalysts in
the petrochemical industry due to their acidic and shape selective
properties.1 The true nature of the intermediates in the acid-
of the bicyclobutonium ion.
catalyzed zeolite reactions is a matter of debate, in which covalent Isotopic perturbation experiments have supported these
alkoxides and ionic carbocations can play a role.2 We have studied conclusions.7 The cyclopropylcarbinyl and the bicyclobutonium
the nature of these intermediates by adsorbing alkylhalides over cations were considered as the most likely species involved in
metal-exchanged zeolites.3 In these systems, the metal cation this equilibrium. The interconversion between them involves a
coordinates with the alkylhalide to form a metal-halide species, barrier of less than 16.7 kJ molꢀ1 at 270 K in the gas phase. The
leaving a carbocation or an alkoxide on the zeolite structure. structural equilibration occurs within a time interval of less than
Using this technique we were able to show4 the rearrangement 10ꢀ10 s.8 Such results are also supported by theoretical methods
and nucleophilic substitution of cyclopropylcarbinyl halides over used to elucidate the potential energy surface of C4H7+ in the gas
NaY at room temperature. The results were interpreted in terms of phase9 and in solution.10 Most of the calculations indicate that
+
the formation of the bicyclobutonium cation (C4H7 ), which may the cyclopropylcarbinyl cation is slightly less stable than the
be nucleophilically attacked at three different positions, giving bicyclobutonium ion in the gas phase. In addition, the potential-
rise to cyclobutyl and allylcarbinyl halides, as well as the parent energy surface of C4H7+ is nearly flat and might involve different
cyclopropylcarbinyl halide (Scheme 1).
carbocations as, for example, the allylcarbinyl cations.
The nature of the C4H7+ cation has been extensively discussed
There are few computational studies on the dynamics of
in the last 50 years. Roberts suggested the bicyclobutonium cation, carbocations in solution, most of them involving fluxional
which behaves as a set of charge-delocalized rapidly equilibrating systems11 or ion pairing.12 The studies of molecular dynamics
carbonium ions, as the main intermediate in the rearrangement of on zeolites mostly involve hydrocarbon interactions with the active
the cyclopropylcarbinyl system.5 However, 13C NMR studies per- site,13 without explicitly studying the dynamics of carbocation
formed in superacid solution pointed to a set of rapidly equili- formation or reaction. We wish to show ab initio molecular
brating ions of lower symmetry.6 The temperature dependence dynamics results of the C4H7 system, which may help in under-
+
of the 13C NMR chemical shifts indicated a rapid equili- standing the time scale of carbocation rearrangement within the
brium between two cations with a low rearrangement barrier. zeolite cavity and the role of alkoxides and ion pairing.
The simulations were carried out using a trigonal unit cell of
a
Chabazite (a = b = c = 9.421 Å and a = b = g = 94.21) containing
´
Universidade Federal do Rio de Janeiro, Instituto de Quımica,
Av Athos da Silveira Ramos 149, CT Bloco A, 21941-909, Rio de Janeiro, Brazil.
36 atoms, in which a silicon atom of the structure was replaced
by an aluminium atom (Si–Al ratio equal to 11). In such a
molecular model, the shortest distance between atoms of the
reactive domain and their repeated images is greater than 9 Å in all
trajectories, minimizing the possibility of undesired interactions
between the organic moieties. The bicyclobutonium cation was
E-mail: cmota@iq.ufrj.br; Fax: +55-21-25927106; Tel: +55-21-25900990
b
c
´
Universidade Federal do Rio de Janeiro, Escola de Quımica, Brazil
´
Centro Universitario Estadual da Zona Oeste, Avenida Manuel Caldeira de
Alvarenga, 1203, 23070-200, Campo Grande, Rio de Janeiro, Brazil
d
´
´
Universidade Federal Fluminense, Instituto de Quımica, Niteroi, Brazil
e INCT Energia e Ambiente, UFRJ, Rio de Janeiro, Brazil
c
4480 Chem. Commun., 2013, 49, 4480--4482
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methanol, 10ꢀ4 minꢀ1 for ethanol and 5 ꢁ 10ꢀ5 minꢀ1 for
n-propanol. These values are at least 100 orders of magnitude slower
than the kinetic constant observed in the absence of alcohols. The
explanation may be related to the competition between the alcohol
and the cyclopropylcarbinyl chloride for coordinating with the
sodium cations on the zeolite. Besides the rearranged chlorides,
formed upon internal return of the halide, we were able to observe
the respective ethers, formed upon the nucleophilic attack of the
alcohols on the carbocation. The distribution of the ethers is similar
to the distribution found in solution, favouring the cyclopropyl-
carbinyl and cyclobutyl isomers, which were formed in approxi-
mately 48% and 47% selectivity, over the allycarbinyl ether, which
was produced in about 5% selectivity at room temperature. This
+
Fig. 1 Mean square displacement (MSD) of the C4H7 species adsorbed on the
Chabazite zeolite at 50, 150, 300 and 500 K.
inserted into the main channel near the aluminium atom. Periodic distribution can be explained by the activation energy10 for nucleo-
potential was restricted to the second Brillouin zone. The calcula- philic attack at the bicyclobutonium cation and expresses the
tions were performed using the Vienna Ab initio Simulation Package kinetic control. Since the distribution observed on zeolites is similar
(VASP)14 using generalized gradient approximation (GGA) formu- to the distribution found in solution,5 the results support the
+
lated by Perdew and Wang for the exchange–correlation functional, mobility of the C4H7 cation inside the zeolite cavity, allowing the
the plane-wave basis with a cut-off energy of 300 eV for electron carbocation to be attacked at the most favourable positions. These
valence contributions and ultra soft pseudopotentials to describe results are in agreement with MD calculations that indicated the
the electron–core interactions. The molecular dynamics trajectories high mobility of the C4H7+ cation within the zeolite cavity at room
of 3.0 ps (after 1.0 ps of equilibration) were obtained using the temperature, thus reinforcing the lower significance of ion pairing
Verlet algorithm with a time step of 0.5 fs within the NVT processes in zeolite-catalyzed reactions at higher temperatures.
(canonical) ensemble at different temperatures (50, 150, 300
The theoretical MD results also revealed the fluxionality of the
C4H7 cation, indicating that the energy barrier for the inter-
+
´
and 500 K) controlled by the Nose–Hoover thermostat.
To more deeply understand the mobility of the C4H7+ cation conversion between the bicyclobutonium and cyclopropylcarbinyl
within the zeolite cavity, molecular dynamics (MD) simulations cations is relatively small on the zeolite surface, as observed in
were performed at different temperatures. According to the superacid solution. In fact, it was frequently observed along the
analysis of the NVT trajectories, the C4H7+ cation is mobile inside trajectories calculated at 50, 150, 300 and 500 K, the back and
the Chabazite cavity over all trajectories calculated at 50, 150, 300 forth interconversion between the bicyclobutonium and cyclo-
and 500 K. However, such behaviour is more pronounced above propylcarbinyl cations, as depicted in Scheme 2.
room temperature, as shown using the mean square displace-
The C2–C4 distance is indicative of the bicyclobutonium–cyclopro-
+
ment (MSD)† of the C4H7 cation at different temperatures pylcarbinyl interconversion along the MD trajectories. Fig. 2 shows
evaluated by means of the classical equation (Fig. 1).
The theoretical results of the mobility of C4H7+ inside the zeolite
cavity can be supported by experiments of nucleophilic substitution
of the cyclopropylcarbinyl chloride in the presence of alcohols. We
have previously observed that the product distribution of nucleo-
philic substitution of the cyclopropylcarbinyl chloride over NaBr
impregnated on NaY does not follow the kinetic distribution
observed in solution, which favours the cyclic cyclobutyl and
cyclopropylcarbinyl products.4 However, this result could be either
Scheme 2 Bicyclobutonium–cyclopropylcarbinyl cation interconversion (the positive
due to successive ionization of the formed bromides, yielding the charge was not shown).
thermodynamically more stable allycarbinyl halide, or due to a
preferential attack on the bicyclobutonium cation due to ion pairing
with the zeolite framework, which may result in preferential orienta-
tion of the cation toward the nucleophile. To answer this question
we performed the rearrangement of cyclopropylcarbinyl chloride on
NaY in the presence of methanol, ethanol and n-propanol as
external nucleophiles. The procedure was basically the same as
reported elsewhere,4 just introducing two-fold molar excess of the
alcohol in relation to the halide at room temperature. Control
experiments showed that in the absence of the zeolite, no reaction
takes place between the chloride and the alcohols at room tem-
perature. The rearrangement of cyclopropylcarbinyl chloride on NaY
at room temperature is slower in the presence of alcohols, with
pseudo first order kinetic constants of 4 ꢁ 10ꢀ4 minꢀ1 for Fig. 2 Probability density of the C2–C4 distance as a function of temperature.
c
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Table 1 Relative stability of the C4H7+ species adsorbed on Chabazite (values in
Based on the relative stability it is possible to explain the
dominance of certain C4H7+ isomers in the MD trajectories. The
bicyclobutonium cation is the most stable C4H7 ion, which
kJ molꢀ1
)
+
Species
DU (kJ molꢀ1
)
justifies its predominance in the trajectories calculated at 50 K.
In addition, the small difference in stability between the
bicyclobutonium and cyclopropylcarbinyl cations explains the
fast interconversion between them in the trajectories calculated
at room temperature and above. This result indicates that
rearrangement of carbocations on zeolites is fast near room
Allylcarbinyl cation
75.3
69.1
66.6
0.0
Cyclopropylcarbinyl cation
Bicyclobutonium cation
Allylcarbinyl alkoxide
the probability density of the C2–C4 distance along the MD temperature and occurs within the picosecond time scale. The
trajectories. As the interconversion takes place, with formation allylcarbinyl cation is rarely observed in the trajectories calcu-
of the cyclopropylcarbinyl cation, this distance may stretch up lated at 300 K, but is more frequently observed in the trajec-
to 2.8 Å. At 50 K, due to the low thermal energy available, the tories calculated at 500 K. These data indicate that less stable
bicyclobutonium ion prevails, as observed in the C2–C4 dis- carbocations can be formed inside the zeolite surface at higher
tance within 1.2 and 1.8 Å. At higher temperatures, there is temperatures, having enough time to be converted into pro-
significant probability of the C2–C4 distance to be within 2.1 ducts, before going to alkoxides. This may explain several
and 2.8 Å, indicative of the interconversion into the cyclopro- hydrocarbon processes on zeolites, such as cracking, which
pylcarbinyl cation.
occur at high temperatures.
In addition to the bicyclobutonium and the cyclopropyl-
carbinyl cations, the allylcarbinyl isomer was also observed in
some of the trajectories calculated at 300 K and in all trajec-
tories calculated at 500 K. This result is consistent with
previous calculations10 that point out the higher energy of the
allylcarbinyl cation relative to the bicyclobutonium and cyclo-
propylcarbinyl isomers. In one of the trajectories calculated at
500 K, the allylcarbinyl cation interacted with the framework
oxygen atom near the aluminium atom to yield the respective
alkoxide, which persisted till the end of the path.
Notes and references
N
N
ꢀ
ꢁ
ꢂ
ꢃ
P
P
1
1
2
† MSDðtÞ ¼ Drj2ðtÞ
¼
Drj2
¼
rjðtÞ ꢀ rjð0Þ
N j¼1
N j¼1
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Geometry optimizations of selected structures along the MD
trajectory calculated at 500 K were performed in order to check
if they correspond to minima on the potential energy surface
(PES). Most of the optimizations systematically yielded the
bicyclobutonium cation as the local minimum, being the most
+
stable C4H7 ion. However, some of the structures corre-
sponded to the cyclopropylcarbinyl and allylcarbinyl cations.
+
Table 1 shows the relative stability of the C4H7 species on
Chabazite calculated from the electron energy differences with-
out considering thermal and vibrational contributions. The
bicyclobutonium cation is more stable than the cyclopropylcar-
binyl isomer by 2.5 kJ molꢀ1 and by 8.7 kJ molꢀ1 in relation to
the allylcarbinyl cation. On the other hand, the allylcarbinyl
alkoxide is the calculated most stable species, lying
66.6 kJ molꢀ1 below the bicyclobutonium cation. Nevertheless,
we were able to observe a trajectory for the formation of the
alkoxide of the carbocation only at 500 K, when the allycarbinyl
cation was formed. This result suggests that, although the
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´
´
cation is favourably orientated near the framework oxygen
atom. With the increase in temperature and the consequent
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c
4482 Chem. Commun., 2013, 49, 4480--4482
This journal is The Royal Society of Chemistry 2013