Prevalence of the external surface over the internal pores in the spontaneous generation of tetrathiafulvalene radical cation incorporated in the novel delaminated ITQ-2 zeolite

Article abstract of DOI:10.1039/b009304l

ITQ-2 is a novel delaminated zeolite whose crystalline sheets (2.5 nm depth) define a large external surface (～720 m² g^-1) having 12-membered ring (MR) open cups together with a reduced microporosity due to a 10 MR channel system. This feature is the reverse of that of classical zeolites (i.e., much larger internal area compared to the external surface). Herein, we have used tetrathiafulvalene and triphenylamine to probe the capability of layered ITQ-2 zeolite to generate persistent organic radical cations compared to conventional Y, mordenite and ZSM-5 zeolites as well as to Al/MCM-41 (3.2 nm diameter) and amorphous silica-alumina. By selective silylation of the external cups, it has been found that formation of a significant concentration of persistent organic radical cations in ITQ-2 is a property that occurs within the open cups of the external surface rather than inside the 10 MR pores.

Full text of DOI:10.1039/b009304l

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Prevalence of the external surface over the internal pores in the
spontaneous generation of tetrathiafulvalene radical cation
incorporated in the novel delaminated ITQ-2 zeolite
Avelino Corma,*a Vicente Fornes,a Mar•a Sales Galletero,a Hermenegildo Garc•a*a and
Carlos J. Gomez-Garc•ab
a Instituto de T ecnolog•a Qu•mica CSIC-UPV , Universidad Politecnica de V alencia, Avda. de los
Naranjos s/n, 46022 V alencia, Spain. E-mail: acorma=itq.upv.es
b Departamento de Qu•mica Inorganica, Universitat de V alencia, Av. Dr. Moliner 100, Burjassot,
V alencia, Spain
Received 20th November 2000, Accepted 15th January 2001
First published as an Advance Article on the web 9th February 2001
ITQ-2 is a novel delaminated zeolite whose crystalline sheets (2.5 nm depth) deÐne a large external surface
(D720 m2 g~1) having 12-membered ring (MR) open cups together with a reduced microporosity due to a 10
MR channel system. This feature is the reverse of that of classical zeolites (i.e., much larger internal area
compared to the external surface). Herein, we have used tetrathiafulvalene and triphenylamine to probe the
capability of layered ITQ-2 zeolite to generate persistent organic radical cations compared to conventional Y,
mordenite and ZSM-5 zeolites as well as to Al/MCM-41 (3.2 nm diameter) and amorphous silicaÈalumina. By
selective silylation of the external cups, it has been found that formation of a signiÐcant concentration of
persistent organic radical cations in ITQ-2 is a property that occurs within the open cups of the external
surface rather than inside the 10 MR pores.
medium, large and extra-large pore size depending on whether
Introduction
the number of TO (T \ Si or Al) of the smallest ring aperture
4
The ability of acid zeolites to act as single electron acceptors
and spontaneously generate signiÐcant amounts of radical
cations upon adsorption of low oxidation potential organic
compounds is well documented.1h17 Although it is known
that the oxidizing sites are related to the presence of Al and
acid sites, their exact structure and location still remains to be
fully understood. Both Bronsted and Lewis acid sites have
been proposed as the electron acceptor sites of the zeolite. The
activation treatment of the zeolite may play a role in the cre-
ation of these acid sites. In this regard, the higher ability of
zeolites to generate persistent (sometimes indeÐnitely long-
lived) organic radical cations compared to amorphous silicaÈ
alumina and mesoporous MCM-41 has been suggested to be
related to the microporous structure of zeolites and to the
internal location of the generated radical cations.13,14 It has
been proposed that the tight Ðt between the zeolite framework
and the radical cation is the major factor responsible for the
remarkable stability of these otherwise highly reactive
species.14 When the host/guest dimensions are very close, the
rigid zeolite framework protects the radical cation from the
attack of oxygen, water or any external reagent.7 In this
regard, zeolites will share with other solid acids the ability to
spontaneously generate organic radical cations, but they will
be unique in stabilizing these species by embedding them into
their micropores.
is 8, 10, 12 or more, respectively.22 However, even for extra-
large pore zeolites (over 12 TO aperture) the pore dimension
4
is below the nm range and obviously there are still numerous
reagents and products that are too large to di†use through
their internal voids.
This has motivated an intense search for zeolitic materials
that can overcome the pore size limitations of conventional
microporous zeolites. In this regard, the synthesis of meso-
porous MCM aluminosilicates has constituted a signiÐcant
breakthrough.23h25 However, the poor hydrothermal
stability26h29 and the weak strength of the acid sites30 have
been two important drawbacks against the widespread use of
mesoporous MCM aluminosilicates as catalysts. Recently, we
have reported the preparation of a layered zeolitic material
termed ITQ-2, obtained by delamination of the layered pre-
cursor of MCM-22 zeolite.31,32 ITQ-2 encompasses a large
well structured accessible external surface area above 700 m2
g~1 formed by 12 MR open cups, with a microporous system
formed with a residual microporosity and small internal
surface by a sinusoidal 10-membered ring channel (ca. 0.55 nm
in diameter) parallel to the main plane of the layer (Fig. 1).
Obviously, the reports on the ability to generate organic
radical cations are limited to the classical zeolitic families and
they focus on the characterization, persistence and reactivity
of the embedded organic radical cation. The aim of this article
is to determine if the external surface of this material with
0.7 ] 8.2 nm cups represents a well of potential and whether
molecules adsorbed there can generate organic radical cations
analogously to their generation in the channels of zeolites.
The behavior of ITQ-2 has been compared with that of Y,
mordenite MCM-22 and ZSM-5 zeolites as well as meso-
porous Al/MCM-41 (3.2 nm diameter) and amorphous silicaÈ
alumina (25 wt.%). We have found compelling evidence for
Zeolites are widely used as solid acids in some important
processes of the petrochemical industry and are being increas-
ingly introduced for the production of specialty and Ðne
chemicals.18,19 Furthermore, the use of zeolites as heter-
ogeneous catalysts in general organic reactions is an active
area of research.20,21 However, one of the major limitations
that restricts the use of zeolites in catalysis is the narrow range
of available pore sizes. Zeolites are classiÐed as small,
1218
Phys. Chem. Chem. Phys., 2001 3, 1218È1222
This journal is ( The Owner Societies 2001
DOI: 10.1039/b009304l

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Fig. 2 Di†use reÑectance UVÈVis spectra (plotted as the inverse of
the reÑectance, R) of zeolite ZSM-5 (a), mordenite (b) and HY (c) after
incorporation of TTF by stirring a dichloromethane solution at reÑux
temperature for 2 h.
Fig. 1 Crystal structure of ITQ-2 showing the 2.5 nm layered struc-
ture encompassing open cups and the 10-membered ring channels
running between the major planes of the layer.
TTF~` radical cation, based on the coincidence with the
reported characteristic optical spectrum of this radical cation
in solution.40 The formation of dimeric species reported in
other media and conditions was conclusively ruled out by the
absence of a signiÐcant absorption at 715 nm that is charac-
teristic of this dimer.40
the prevalence of accessible open cups on the external surface
over internal micropores in the generation of persistent
organic radical cations, provided that the organic donor can
experience the presence of these potential wells.
Interestingly, in the case of HY zeolite, even though the
solids developed a light colour during the incorporation pro-
cedure, the colour immediately vanished upon Ðltration of the
solids. This observation suggests that some TTF~` could have
been formed in this tridirectional, large pore zeolite imparting
its characteristic colour, but apparently it is not persistent
enough to survive exposure to the ambient. Analysis of the
organic material by GC after incorporation in HY indicates
that TTF has remained unaltered and no reaction product
could be detected. The most reasonable interpretation is that
the initial oxidation of TTF to TTF~` reverts rapidly back to
the neutral molecule in HY zeolite. In support of this is the
fact that TTF~` generated on medium pore sized, ZSM-5 and
MCM-22 exhibits some partial decay over the time that can
be conveniently followed by the decrease in the intensity of
TTF~` characteristic bands in the di†use reÑectance UVÈVIS
spectra for periods longer than weeks.
An intermediate situation between Y and medium pore zeo-
lites occurs in the case of the large pore mordenite, wherein an
intense optical absorbance due to TTF~` is initially recorded
after the incorporation procedure, but the intensity progres-
sively decreases within hours leading to a residual, stationary
di†use reÑectance spectrum attributable to a minor propor-
tion of persistent TTF~`. This decay of TTF~` radical cation
within mordenite can also be adequately followed with a con-
ventional spectrophotometer. The initial disappearance
kinetics Ðts reasonably well to a Ðrst-order decay with a half-
life under our experimental conditions of 2 h, approximately.
The previous observations during TTF adsorption as well as
the di†use reÑectance UVÈVis spectra study are compatible
with a rationalization in which a spontaneous generation of
TTF~` due to the acid sites of the solid occurs. Once TTF~` is
generated by interaction with the solid acid sites its sub-
sequent persistence will be a function of the TTF~`/zeolite
tight Ðt. This interpretation assumes that TTF~` is located
totally or predominantly inside the zeolite micropores,
wherein geometrical restrictions can apply. We have per-
formed molecular modeling at the semiempirical level of the
best docking of TTF~` inside the straight channels of ZSM-5
that shows that this molecule Ðts tightly inside the pentasil
zeolite pores. As reported in other cases,41 we have also found
here a relationship between the presence of acidity on the
solid and its ability to generate TTF~`. Zeolite Y and morde-
nite in its neutral Na`-form give negligible absorption bands
Results and discussion
The main analytical and textural characteristics of the solids
used in this work are collected in Table 1. Given that ITQ-2
micropores are 10-membered rings (D5.4 A diameter), in this
work we have used small tetrathiafulvalene (TTF) and large
triphenylamine (TPA) as probe molecules. TTF would experi-
ence the electron acceptor sites located either in the channels
or in the open cups. On the other hand, TPA (D12 A) cannot
be accommodated within the cups and, therefore, TPA would
probe the external surface of ITQ-2 as a ““ÑatÏÏ surface without
potential wells and experience only a loose interaction.
TTF (E ca. 0.5 V vs. SCE in acetonitrile) has been exten-
ox
sively used as the electron donor in numerous charge transfer
complexes33h39 and in some of them the corresponding
TTF~` radical cation has been characterized by EPR and
optical spectroscopy.33,40 However, in spite of this, as far as
we know, this is the Ðrst report that uses TTF to probe the
oxidizing sites in zeolites. Upon incorporation of TTF from
dichloromethane solutions into thermally dehydrated morde-
nite, ZSM-5 and MCM-22 zeolites in their H`-form, the
solids become brownish red. Di†use reÑectance UVÈVis spec-
troscopy of the powders after Ðltration indicates the formation
of a new species characterized by two absorption bands at 440
and 580 nm (Fig. 2). This species was identiÐed as monomeric
Table 1 Some relevant physicochemical parameters of the solid acids
used in this work
BET
surface
area/m2 g~1 /lm
Micropore
volume
/cm3 g~1
Si/Al
ratio
Crystal size
ZSM-5
B40
386
423
0.5È1
0.111
0.170
0.155
0.303
0.240
0.46
0.059
0.009
0.121
H-mordenite
Na-mordenite
NaY
10
0.05È0.3
0.1È0.5
0.2È1
0.2È1
0.1È0.3
È
6.5 356
2.6 654
2.6 554
HY
Al/MCM-41
SiO /Al O
ITQ-2
MCM-22
13
2.5 289
58
50
874
2
2 3
806a
400b
\0.5
B1
a External surface area measured by N isotherm adsorption of 706
2
m2 g~1. b External surface area measured by N isotherm adsorption
2
of 82 m2 g~1.
at j
460 and 540 nm and thus TTF~`, if formed (see below
max
Phys. Chem. Chem. Phys., 2001, 3, 1218È1222
1219

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Table 2 Normalized relative spin density obtained by double inte-
gration of the signal between 3340 and 3420 G observed in the EPR
spectra of samples of aluminosilicates containing TTF. The values
have been corrected to correspond to the same amount of adsorbed
TTF as measured by combustion chemical analyses
for EPR quantiÐcation), has to be in very low amounts com-
pared to the almost quantitative generation from TTF
observed for the H`-exchanged zeolites (compare IR present-
ed in Fig. 3). Given the lower strength of the acid sites of
Al/MCM-41 and silicaÈaluminas compared to acid zeolites,
these two solids should exhibit an ability to generate TTF~`
intermediate between that of H` and Na` zeolites, as was
indeed observed.
Normalized spin densitya
(%)
Sample
ZSM-5
ITQ-2
Na-mordenite
Na-Y
90
100
17
0.1
0.6
0.1
The results presented so far for TTF Ðt nicely with the
current paradigm of radical cation persistence in zeolites and
related microporous materials. The next step was to test the
behavior of ITQ-2 and compare it with those of MCM-22
(same 10-membered ring channel system, external surface
negligible) and Al/MCM-41 and SiO ÈAl O (large external
SiO /Al O
2
2 3
Al/MCM-41
a Estimated relative error ^20%.
2
2 3
surface, no cups or microporous channel system). Di†use
reÑectance UVÈVis spectroscopy shows that TTF~` radical
cation was generated upon adsorption of TTF on ITQ-2 from
CH Cl or cyclohexane solutions.
The most salient data from this Table are that the amounts
of TTF~` generated on ITQ-2 and ZSM-5 are the same within
the experimental error and over two orders of magnitude
higher than on Al/MCM-41 and SiO ÈAl O . This quantitat-
2
2
At the loading studied (10È20 mg TTF (g zeolite)~1) and
immediately after preparation essentially all the TTF was oxi-
dized to the radical cation. Although di†use reÑectance
UVÈVis spectroscopy indicates the formation of TTF~`
radical cation, it does not give any indication of how much
neutral TTF is adsorbed in the solid without being converted
into TTF~`. A quantitative indication that there is not a sig-
niÐcant amount of TTF left in the solid could be obtained
based on the conclusive information gained by IR spectros-
copy. TTF has a characteristic vibration band at about 1520
cm~1 that totally disappears upon incorporation on ZSM-5,
MCM-22 or ITQ-2. As an example, Fig. 3 compares the aro-
matic region of the IR spectrum of pure TTF in KBr disk with
that of a self-supported wafer of ITQ-2 after incorporation of
TTF. The absence of the 1520 cm~1 band in the IR spectrum
of TTF incorporated within zeolite indicates that no neutral
TTF is left and that the conversion to TTF~` is almost quan-
titative.
2
2 3
ive study clearly indicates that ITQ-2 behaves like a medium-
pore zeolite with respect to its ability to generate radical
cations of electron-rich aromatic compounds, which is notably
higher than those of silicaÈalumina and mesoporous solid
silicas. This is in spite of the reduced ITQ-2 microporosity
compared to conventional medium-pore zeolites. Therefore,
we wanted to discover whether TTF~` radical cation is gener-
ated only within the ITQ-2 channel system (as for MCM-22
and similar to ZSM-5, but with a signiÐcantly reduced area)
or also on the externally accessible cups.
To address this question, we started from ITQ-2 (Si/Al 50)
and proceeded to the selective silylation of the external surface
using hexamethyldisilazane. This procedure is analogous to
those used in the silylation of MCM-41 and other sili-
cates.42,43 In this way a new material ITQ-2sil was obtained
in which the external open cups have been blocked by the
bulky silyl groups while the channel still remains accessible.
Accessibility of the channel system was conÐrmed by pyridine
adsorption and observation by IR that the intensity of pyri-
dinium ions on the ITQ-2 after the silylation is very similar to
that recorded prior to the silylation procedure.
Importantly, ITQ-2 loses almost completely the capacity to
generate TTF~` radical ion upon silylation. The di†use reÑec-
tance UV-Vis spectra of silylated ITQ-2 containing TTF
exhibits an intensity of the 440 and 580 nm bands character-
istic of the radical cation more than one order of magnitude
lower than that of the original ITQ-2 before silylation under
the same conditions. This treatment leaves the acid sites and
Interestingly, adsorption of TTF onto Al/MCM-41 and
SiO ÈAl O also gives rise to the generation of TTF~` radical
2
2 3
cation, as assessed by di†use reÑectance UV-Vis spectroscopy.
The initial intensities of the TTF~` adsorption bands on Al/
MCM-41 and SiO ÈAl O are, however, weaker than that on
2
2 3
ITQ-2 and exhibit a relative decay with respect to the initial
intensity, 15 days after preparation, of 45 and 55% for Al/
MCM-41 and SiO ÈAl O , respectively, that is more pro-
2
2 3
nounced that the 20% decrease measured for ITQ-2.
To provide a better comparison of the ability of ITQ-2 and
the di†erent solids to generate TTF~`, an EPR quantiÐcation
of the relative number of spins for di†erent samples was
carried out. The results are summarized in Table 2. Selected
EPR spectra of some of the samples prepared are shown in
Fig. 4.
Fig. 3 1800È1300 cm~1 region of the FTIR spectra of TTF in KBr
disk (a) and after incorporation on ITQ-2 (b). Spectrum b was record-
ed at room temperature after outgassing self-supported zeolite wafers
at 200 ¡C under 10~2 Pa for 1 h to avoid interference of coadsorbed
water.
Fig. 4 EPR spectra recorded at room temperature of TTF~` gener-
ated upon adsorption of TTF onto ITQ-2 (a), ZSM-5 (b), SiO ÈAl O
(c) and Al/MCM-41 (d). Spectra c and d have been magniÐed by a
factor of 100.
2
2 3
1220
Phys. Chem. Chem. Phys., 2001, 3, 1218È1222

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Ðltration of the solid. ZSM-5 exhibits the same behavior as
ITQ-2. In contrast, mesoporous Al/MCM-41 can incorporate
TPA and it forms a signiÐcantly higher concentration of per-
sistent TPA~`, as seen by di†use reÑectance UVÈVis spectros-
copy. Fig. 6 compares the di†use reÑectance UVÈVis spectra
of TPA adsorbed onto ITQ-2 or Al/MCM-41. The results
observed using TPA as probe combined with those of ITQ-
2sil indicate that the major factor in the radical cation gener-
ation using zeolite ITQ-2 as host is the ability to experience
the potential wells of cups. When the organic electron donor
can Ðt inside the cups, then the novel layered zeolite ITQ-2
behaves like a conventional microporous zeolite.
Experimental
TTF (Aldrich), TPA (Aldrich), SiO ÈAl O 25 wt.% (BASF)
2
2 3
as well as HY (P.Q.), NaY (P.Q.) and Na` and H`-
mordenites (P.Q.) were commercial samples and used as
received. ZSM-5,46 MCM-22,47h49 Al/MCM-4123h25 and
ITQ-231,32 were prepared by hydrothermal solÈgel crys-
tallization according to the detailed procedures reported in
the literature. The crystal structure of the synthesized
aluminosilicates was conÐrmed by powder X-ray di†raction.
Silylation of ITQ-2 was carried out by stirring a suspension of
the dehydrated layered zeolite (1 g) with hexamethyldisilazane
(0.5 g) in toluene (25 ml) at reÑux temperature for 2 h. After
this time, the solid was Ðltered, exhaustively washed with
CH Cl and dried.42,43 Adsorption of TTF or TPA on the
Fig. 5 Visualization of the best docking of TTF inside the open cups
of ITQ-2. The molecular modeling shows that one of the rings of TTF
is totally inside the cup and that the sulfur atoms are well protected
by the zeolite lattice and they experience the internal electrostatic
Ðelds.
the accessibility to the channels unaltered. Based on this
result, we propose that TTF~` radical cations in ITQ-2 are
generated predominantly on the external cups. This is in sharp
contrast to the behavior of conventional microporous zeolites.
These cups will act as a well of potential energy and will sta-
bilize the encapsulated TTF~`. Molecular modeling at the
semiempirical level shows that TTF can penetrate partially
inside the external cups, and in this way the positive sulfur
atom will be protected, like a hand in a glove. A selected
visualization of this docking is shown in Fig. 5. The fact that
aromatic radical cations on the external surface of zeolites,
but partially penetrating inside the pores, are relatively pro-
tected from the attack of nucleophiles has been previously
assessed by electrochemical studies.
2
2
solids was carried out by stirring magnetically, at reÑux tem-
perature for 2 h, a suspension of the organic compound
(10È20 mg) in CH Cl or cyclohexane (20 ml) in the presence
2
2
of the corresponding solid (1 g) that had been previously
dehydrated by heating at 500 ¡C overnight. The solid was Ðl-
tered, washed with fresh solvent (20 ml), dried under vacuum
and stored in closed vials at the ambient. Di†use reÑectance
UVÈVis spectra were recorded using a Cary 5 spectrophotom-
eter adapted with a ““praying mantisÏÏ di†use reÑectance acces-
sory as reference. FTIR spectra of the zeolites containing TTF
were recorded in a Nicolet 710 FT spectrophotometer using
In order to provide some additional support to this novel
idea that external nests can protect adsorbed radical ions, tri-
greaseless quartz cells having CaF windows and a stopcock
2
valve that allowed them to be sealed under high vacuum. Self-
phenylamine (TPA) (E D 0.7 vs. SCE in acetonitrile)44,45 was
supported wafers (D10 mg) were prepared by pressing the
zeolite powder at 1È3 t cm~2 for 1È2 min. The samples were
outgassed at 100 ¡C under 10~2 Pa for 1 h before recording
the spectra at room temperature. Pyridine vapour at 10~1 Pa
was admitted at room temperature onto dehydrated ITQ-2
and ITQ-2sil samples placed in the IR cell. The cells were
evacuated at 250 ¡C under 10~2 Pa for 1 h and the intensity of
the IR spectra corresponding to the residual retained pyridine
compared for the two samples. EPR spectra were recorded at
[200 ¡C on a Brucker ER200D spectrometer working at the
X band (9.65 GHz) and using DPPH (g \ 2.0036) as reference.
Semiempirical calculation of the optimized geometry as well
as docking and molecular modeling was performed using the
Insight II program package working on a Silicon Graphics
workstation.
ox
also used as a ““roughÏÏ probe molecule of the ITQ-2 external
surface as if it were a Ñat surface since there is no possibility of
experiencing the cups due to the large TPA molecular size.
TPA is also too large (reported molecular diameter of 13 A) to
di†use inside the channel system of mordenite, ZSM-5 or
ITQ-2. Therefore, TPA~` (if formed) can only be located on
the open layers of ITQ-2 experiencing a loose interaction.
Upon stirring a dichloromethane solution of TPA in the pres-
ence of dehydrated ITQ-2 at reÑux temperature a weak green
coloration was developed that fades almost completely upon
Acknowledgements
Financial support by the Spanish DGES (MAT97È1016-C02-
01) is gratefully acknowledged. Dr. Lorenzo Fernandez and
Dr. German Sastre are thanked for their assistance in the
molecular modeling calculations.
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