Hierarchical ZSM-5 zeolite with uniform mesopores and improved catalytic properties

Article abstract of DOI:10.1039/c5nj02856f

Hierarchical ZSM-5 with uniform mesoporosity was synthesized by the sequential coupling of two strategies: generation of secondary porosity by the crystallization of silanized protozeolitic units and a subsequent treatment with a basic surfactant-containing solution. The ZSM-5 zeolite obtained exhibited high crystallinity and contained two levels of uniform porosities within the micro- and mesopore ranges, respectively. The uniform mesoporosity is the result of the reorganization of irregular mesopores, created initially from the silanized protozeolitic units, via the local rearrangement of zeolitic fragments, and promoted by contact with a cationic surfactant under mild basic conditions. Interestingly, this second treatment was less effective when it is applied to a non-hierarchical ZSM-5 sample, showing that the presence of initial secondary porosity is essential for allowing the surfactant/ammonia solution to modify the zeolite and form uniform mesopores. Characterization of the zeolite samples using different techniques showed that the crystallinity, Al coordination and acidic features of the zeolite do not change significantly after the mesopore narrowing treatment, even though significant variations in the textural properties are observed as expected. The effects of the occurrence of regular mesoporosity on the catalytic properties of the hierarchical ZSM-5 zeolite were proven using the acylation of 2-methoxynaphthalene as a test reaction. The material with uniform mesopores showed both the highest activity and selectivity towards 6-acetyl-2-methoxynaphthalene, which was interpreted as the result of the presence of a more regular and less rough mesopore surface, which in turn facilitated the interaction between the reactant molecules and the active sites located and distributed over the mesopores.

Full text of DOI:10.1039/c5nj02856f

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Page 1 of 33
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Hierarchical ZSM-5 zeolite with uniform mesopores and improved
catalytic properties
D. P. Serrano1,2,*, J. M. Escola2, R. Sanz2, R.A. Garcia2, A. Peral2, I. Moreno2 and M. Linares2
1IMDEA Energy Institute, Avda Ramón de la Sagra, nº 3, 28935, Móstoles (Spain)
2Environmental and Chemical Engineering Group, ESCET, Rey Juan Carlos University, c/
Tulipán s/n, 28933, Móstoles (Spain)
* to whom correspondence should be addressed
eꢀmail: david.serrano@urjc.es
Tel: +34 91 488 74 50; Fax: +34 91 488 70 68
1

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Abstract
Hierarchical ZSMꢀ5 with uniform mesoporosity has been here synthesized through the sequential
coupling of two strategies: generation of a secondary porosity by crystallization of silanized
protozeolitic units and subsequent treatment with a basic surfactantꢀcontaining solution. The
ZSMꢀ5 zeolite so obtained exhibits a high crystallinity and contains two levels of uniform
porosities within the microꢀ and mesopore ranges, respectively. The uniform mesoporosity is the
result of the reorganization of irregular mesopores, created initially from the silanized protozeolitic
units, by means of the local rearrangement of zeolitic fragments, promoted by the contact with a
cationic surfactant under mild basic conditions. Interestingly, this second treatment is little
effective when it is applied to a nonꢀhierarchical ZSMꢀ5 sample, denoting that the presence of an
initial secondary porosity is essential for affording the surfactant/ammonia solution to modify the
zeolite and, consequently, for the formation of uniform mesopores. Characterization of the zeolite
samples by means of different techniques pointed out that the crystallinity, Al coordination and
the acidic features of the zeolite do not change meaningfully after the mesopore narrowing
treatment, although significant variations in the textural properties are observed as it was
expected. The effects of the occurrence of a regular mesoporosity on the catalytic properties of
the hierarchical ZSMꢀ5 zeolite were proved using the acylation of 2ꢀmethoxynaphthalene as test
reaction. The material having uniform mesopores showed both the highest activity and selectivity
towards 6ꢀacetylꢀ2ꢀmethoxynaphthalene, which was interpreted as a result of the presence of a
more regular and less rough mesopore surface, which in turn facilitates the interaction between
the reactant molecules and the active sites located and distributed over the mesopores.
Keywords: hierarchical ZSMꢀ5; protozeolitic unit silanization; surfactantꢀmediated reorganization;
uniform mesoporosity.
2

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1. INTRODUCTION
The preparation of hierarchical zeolites has been a booming research field along the last
decade1ꢀ11. Hierarchical zeolites are characterized by the presence of a bimodal porous
distribution, having a combination of microꢀ and mesopores. The occurrence of mesoporosity
allows the reagents to have a better accessibility towards the active sites, removing steric
limitations and shortening additionally the intracrystalline diffusion pathways12. These features
have resulted in several advantages such as enhanced activities13, better dispersion of metal
active phases14,15 and lower deactivation rates16. Different procedures have been proposed for
the preparation of hierarchical zeolites, such as dealumination17, carbon templating11,
desilication18, protozeolitic units silanization6,9,10, use of amphiphile organosilanes7 and dual
templating with surfactants8. Although these methods enable to obtain hierarchical zeolites, the
nature of the attained mesoporosity differs meaningfully among them since the underlying
principles of the different procedures are very diverse. Thus, the BET surface area values
exhibited by hierarchical zeolites prepared by the protozeolitic units silanization and desilication
methods were distinctly higher than those of the hierarchical zeolites obtained by the carbon
templating route11,12. Moreover, the contribution and properties of the mesoporosity so generated
are difficult to be adjusted, usually showing a wide pore size distribution. The importance of
mesopore quality and pore connectivity in the enhanced catalyst lifetime of hierarchical zeolites
has been recently highlighted19,20. In this way, a high contribution of the hierarchical porosity and
mesopore connectivity has been recently achieved from disordered MFI nanosheets with 2.5 nm
thickness, giving rise to increased conversion in the Pechmann condensation of pyrogallol and
resorcinol21.
One successful approach affording hierarchical zeolites is the method based on the
crystallization of protozeolitic units, previously functionalized with organosilanes, which prevents
partially their aggregation into conventional zeolite crystals. After calcination, the space
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previously occupied by the organosilane is released, giving rise to the appearance of supermicroꢀ
and mesopores. This method has shown generality since it has been applied successfully to a
variety of zeolites such as ZSMꢀ5, TSꢀ1, Beta or mordenite1. Additionally, the amount and nature
of the organosilane is a key element for tailoring the attained mesoporosity22 – 25. However, as in
other strategies, the hierarchical zeolites so obtained usually present a broad mesopore
distribution, encompassing the 2 ꢀ 8 nm range. Other silanizationꢀbased methods, which employ
silylated polymers as organosilanes, also share this drawback and lead to hierarchical zeolites
with nonꢀuniform mesopores6. The use of amphiphile organosilanes has afforded the preparation
of hierarchical zeolites with narrower pore size distributions7, although in some cases it has been
detected the presence of a portion of the initial amorphous phase, not completely dissolved and
converted, in the final zeolitic material26,27
.
Desilication is other treatment extensively investigated for developing zeolitic materials
with mesoporosity. This method follows a really simple approach consisting of treating the zeolite
in a strong alkaline medium under controlled conditions, which causes the extraction and removal
of a part of the silica from the zeolite framework. Consequently, significant variations are
observed between the Si/Al ratio of the raw zeolite and the final hierarchical one. The desilication
route has proved to be a versatile procedure capable of leading to the synthesis of different
mesoporous zeolites18,28, although having usually broad mesopore size distributions. Moreover,
this route is strongly dependent on the Si/Al atomic ratio of the starting zeolite18 and often
requires the use of different treatments, such as acid washing or the addition of
tetraalkylammonium cations (e.g. TPAOH) to the alkaline solution, which act as “pore growth
moderators”29. A remarkable result has been obtained by the group of Fajula when applying the
desilication method to USY zeolites previously dealuminated, which has led to the development
of hierarchical zeolites with trimodal porosity30.
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Other interesting alternative for the preparation of hierarchical zeolites is the one based
on the treatment of the zeolite with a surfactantꢀcontaining alkaline solution31ꢀ32. The alkaline
medium is provided by an ammonia solution, having a mild basicity, which seems to be an
important aspect to avoid the occurrence of desilication phenomena. GarciaꢀMartínez et al.31 has
proved that this treatment is very effective when applied to dealuminated zeolite Y, showing the
generation of uniform mesopores as a result of the treatment with the surfactant/ammonia
solution. This important finding has been later confirmed by other researchers following a similar
procedure32.
In this context, the present work reports the preparation of hierarchical ZSMꢀ5 following a
route that has been envisaged as the sequential combination of two of the approaches above
described. Firstly, a secondary porosity is induced in the zeolite by crystallization of silanized
protozeolitic nanounits. The irregular mesopores so obtained are then reorganized by treatment
with a surfactantꢀcontaining alkaline solution, leading to materials with a narrow mesopore size
distribution. The results here reported prove the need of the existence of a previous amount of
mesoporosity as an essential aspect for the surfactant reorganization treatment to be effective. In
a recent work, our group has reported the combination of these strategies for the preparation of
hierarchical TSꢀ1 zeolite with uniform mesoporosity33, showing now that it can be successfully
applied for ZSMꢀ5 zeolite. Additionally, in the present work a general mechanism is proposed to
explain the generation of the observed uniform mesoporosity. Other relevant conclusion derived
from the results here reported is the observation that narrowing the mesopore size distribution
induces significant improvements in the catalytic properties exhibited by the hierarchical ZSMꢀ5 in
the acylation of 2ꢀmethoxynaphthalene.
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2. EXPERIMENTAL
2.1. Catalysts Preparation
a) Materials synthesis
Hierarchical ZSMꢀ5 was synthesized by crystallization of silanized protozeolitic units,
according to a method earlier published in the literature9. Initially, a precursor ZSMꢀ5 zeolite
solution was prepared with the following molar composition: 1 Al2O3 : 60 SiO2 : 11 TPAOH : 1500
H2O. Tetraethoxysilane (TEOS, 98%; Aldrich), aluminum isopropoxide (AIP; Aldrich), tetrapropylꢀ
ammonium hydroxide (TPAOH, 40%; Alfa), and distilled water were used as starting reagents.
Firstly, the precursor solution was aged at ambient temperature for 44 h and then it was
precrystallized under reflux and stirring (100 rpm) at 90 ºC for 22 h. Then, the resulting zeolite
protozeolitic units were functionalized with phenylaminopropyltrimethoxysilane (PHAPTMS,
Aldrich). The organosilane was added in a proportion of 8 mol% with respect to the silica content
in the initial gel and the silanization reaction was performed at 90 ºC for 6 h. Finally, crystallization
was carried out in Teflon®ꢀlined stainlessꢀsteel autoclaves under static conditions and
autogenous pressure at 170 ºC for 7 days. The solid product obtained was separated by
centrifugation, washed several times with distilled water, dried overnight at 110 ºC and calcined in
air at 550 ºC for 5 h. This sample was denoted as hꢀZSMꢀ5.
For comparison purposes, conventional ZSMꢀ5 was synthesized as reference following
the same procedure above described, but omitting the precrystallization and organosilane
functionalization steps. Likewise, a sample of AlꢀMCMꢀ41 was prepared for catalytic comparison
according to a procedure published elsewhere34 from a solution with the following molar
composition: Al2O3 : 60 SiO2 : 9 CTABr : 165 NH3 : 8000 H2O.
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2.2. Mesopore Narrowing Treatment
The synthesized zeolites were calcined and subsequently subjected to the mesopore
narrowing treatment by contacting with a surfactantꢀcontaining ammonia solution. In this
procedure, 1.00 g of calcined zeolitic material was dispersed in 62.8 g of a 0.37 M NH4OH
aqueous solution containing 0.7 g of hexadecyltrimethylammonium bromide (Aldrich). The
resultant mixture was stirred at room temperature for 30 min, being then loaded into an autoclave
and subjected to a hydrothermal treatment for 20 h at 150 ºC. The solid obtained was recovered
by centrifugation, washed several times with distilled water, dried overnight at 110 ºC and, finally,
calcined at 550 ºC for 5 h. The samples thus prepared from the conventional and hierarchical
ZSMꢀ5 samples were denoted as ZSMꢀ5 (mnt) and hꢀZSMꢀ5 (mnt), respectively.
2.3. Characterization of the catalysts
All samples were characterized by a number of techniques in order to determine their
main physicochemical properties. Powder XꢀRay Diffraction (XRD) patterns were collected
employing CuK α radiation on a Philips X’PERT MPD diffractometer. Diffraction data were
recorded in the range 5 ꢀ 50°, using a step size of 0.02° and a counting time of 10 s, respectively.
The Si and Al contents of the catalysts were determined by Inductively Coupled Plasma Atomic
Emission Spectroscopy (ICPꢀAES) using a Varian Vista AX instrument. Highꢀresolution 27Al MASꢀ
NMR spectra of the zeolite samples were recorded at 104.1 MHz using a Varian Infinity 400
spectrometer. All the measurements were carried out at ambient temperature with a spinning
frequency of 11 kHz and pulse width of 2.5 s. Transmission Electron Microscopy (TEM) images
were obtained with a PHILIPS TECHNAI 20 microscope operating at 200 kV. Thereby, the
samples were crushed, dispersed in acetone and deposited on a carbonꢀcoated copper grid.
Argon adsorptionꢀdesorption isotherms were obtained using a Micromeritics ASAP 2010
instrument at ꢀ186 ºC (87.5 K). The samples were evacuated at 300 °C under vacuum for 12 h
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prior to the analyses. The textural properties of the synthesized materials were determined from
the adsorptionꢀdesorption isotherms. Specific surface areas were calculated using the BET
equation, while the cumulative pore volume curves and pore size distributions (PSD) were
estimated by applying the NLDFT method to the adsorption branch of the isotherms, assuming
cylindrical pore geometry. This method has been also employed for determining the relative
contribution of microꢀ and mesopores to both surface area and pore volume.
FTIR spectra of previously adsorbed pyridine on the zeolite samples were recorded on a
Jasco 6300 FTIR spectrophotometer, working with a resolution of 4 cmꢀ1. Selfꢀsupporting wafers
of the zeolites with a thickness of ca. 6 mg cmꢀ2 were prepared by pressing (7 x 103 kg cmꢀ2) the
powdered sample for 10 min. Before measurements, the wafers were degassed at 500 ºC for 4 h
in a vacuum cell fitted with greaseless stopcocks and KBr windows under a residual pressure of 2
x 10ꢀ4 mbar. Then, the sample temperature was decreased to 150 ºC and exposed to pyridine
vapour (1.5 mbar pressure) for 5 min. The physically adsorbed pyridine fraction was removed by
degassing the sample for 30 min at 150 ºC. Once the spectrum was recorded, the sample was
degassed again at increasing temperatures in the range 250ꢀ450 ºC while recording the spectra
at each degassing temperature. For quantitative characterization of the acid sites, the following
bands and absorption coefficients were used: pyridine PyH+ band at 1543 – 1548 cmꢀ1 (ε = 1.67
cm ꢁmolꢀ1) for the Brönsted acid sites and pyridine PyL bands at 1455 – 1446 cmꢀ1 (ε = 2.2 cm
ꢁmolꢀ1) for the Lewis acid sites35.
2.4. Catalytic tests
The catalytic properties of the samples were tested in 2ꢀmethoxynaphthalene (2ꢀMN) acylation
using acetyl chloride as acylating agent (Scheme 1). The catalytic tests were carried out under
atmospheric pressure in a 100ꢀmL threeꢀnecked batch reactor equipped with a reflux condenser,
a thermometer and a magnetic stirrer. In a typical reaction, 0.75 g of 2ꢀMN in 20 mL of solvent
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(nitrobenzene) was loaded into the reactor. Subsequently, 0.200 mL of sulpholane, used as
internal standard, and 0.150 g of fresh catalyst (catalyst/2ꢀMN = 0.20 w/w) were added to the
reaction mixture, being kept magnetically stirred under nitrogen. Once the reaction temperature
(180 ºC) was reached, 0.330 mL of acylating agent (acetyl chloride) was added to the flask.
Periodically, aliquots of the reaction mixture were collected in order to assess the time evolution
of the reaction, the mixture being allowed to react for 180 min. The products were analyzed by
capillary gas chromatography in a Varian 3900 Series instrument equipped with a 30 m x 0.25
mm capillary column (CP SIL 8 CB) and a FID detector.
O
O
CH3
H3C
O
CH3
O
O
O
H C
Cl
3
CH3
CH3
T
A
T
1A-2MN
8A-2MN
2-MN
A
T
O
CH3
O
6A-2MN
CH3
Scheme 1. Acylation of 2-MN
3. RESULTS AND DISCUSSION
The present work reports a synthesis strategy for the preparation of hierarchical ZSMꢀ5
having a narrow mesopore size distribution and how this feature affects and improves its
physicochemical and catalytic properties. The method here outlined consists of the combination
of two different approaches:
•
The generation of a secondary porosity by the addition of a silanization agent to the
zeolite precursor gel, which has been earlier demonstrated to be very effective for hindering
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partially the aggregative growth of zeolitic nanounits and, therefore, leading to the appearance of
superꢀmicro and mesopores once the organic compounds are removed by calcination9. This
secondary porosity is not uniform and shows typically a relatively broad pore size distribution.
•
The capability of surfactant/ammonia solutions, under hydrothermal conditions, to
promote the local rearrangment of zeolitic fragments. This effect has been recently reported by
GarcíaꢀMartínez et al.31 applied to dealuminated zeolite Y. However, as it will be shown here, the
effectiveness of this treatment alone is quite limited if the parent material contains solely the
typical zeolitic micropores.
3.1. Changes induced in the physicochemical properties of ZSM-5 zeolite
The first experiments were designed to establish whether the secondary porosity
generated by the protozeolitic silanization route might be turned into a uniform mesoporosity
using a mesopore narrowing treatment consisting of the addition of a solution containing
ammonia plus the surfactant (hexadecyltrimethylammonium bromide). Additionally, another point
of interest was to ascertain whether the presence of this initial mesoporosity is an essential
aspect to promote the formation of uniform mesopores, comparing the results so obtained with
those corresponding to the use of a nonꢀhierarchical ZSMꢀ5 material as parent sample. Thereby,
four ZSMꢀ5 samples were synthesized using different procedures. A nonꢀhierarchical reference
sample (ZSMꢀ5) was prepared without including any precrystallization nor silanization step during
its synthesis. A second reference ZSMꢀ5 sample, having hierarchical porosity, was synthesized
according to the protozeolitic silanization route, being denoted as hꢀZSMꢀ5. Both samples were
subsequently subjected to the mesopore narrowing treatment leading to the materials named as
ZSMꢀ5 (mnt) and hꢀZSMꢀ5 (mnt), respectively.
Figure 1 shows the XRD patterns corresponding to the ZSMꢀ5 samples. They display the
typical reflections of the MFI structure and no broad bottom reflection in the range 2θ ∼ 20 ꢀ 25º,
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proceeding from amorphous material, was appreciated. This fact indicates the highly zeolitic
nature of the four ZSMꢀ5 samples, showing that the surfactant/ammonia treatment does not affect
their crystallinity. While the intensity of the XRD peaks decreases for the nonꢀhierarchical zeolite
after this treatment, the opposite is observed when using the hierarchical ZSMꢀ5 as parent
sample.
h-ZSM-5 (mnt)
h-ZSM-5
ZSM-5 (mnt)
ZSM-5
5
10 15 20 25 30 35 40 45 50
2 θ (º)
Figure 1. Highꢀangle XRD patterns of the ZSMꢀ5 samples.
These variations suggest that some changes have occurred in the effective size of the
crystalline domains after the surfactant/ammonia treatment. For the nonꢀhierarchical ZSMꢀ5, the
decrease in the intensity could be indicative of a lower crystallinity, although no amorphous phase
is detected in this sample whereas desilication does not seem to be produced, at least
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significantly, since the Si/Al ratio remains almost constant (see Table 1). In contrast, for the hꢀ
ZSMꢀ5 sample the situation is different since it is made up of aggregates of crystalline nanounits,
so it is feasible that the surfactant/ammonia treatment could induce their partial merge, leading
towards some growth in the size of the crystalline domains.
Table 1. Physicochemical properties of the ZSM-5 samples.
c)
c)
c)
c)
SBET
(m2/g)
420
SZMP
SSP+EXT
VTOT
(cm3/g)
0.403
0.447
0.504
0.483
VZMP
VSP+EXT
SA
Sample
Si/Ala)
contentb)
(m2/g)
(m2/g)
(cm3/g)
0.198
0.159
0.149
0.151
(cm3/g)
0.205
0.288
0.355
0.332
ZSM-5
ZSM-5 (mnt)
h-ZSM-5
28
27
30
32
0.0
0.0
344
76
395
276
119
303
202
0.08
0.08
562
259
h-ZSM-5 (mnt)
464
262
a) Calculated by ICPꢀAES; b) Ratio silicon from silanization agent PHAPTMS / total silicon; c) ZMP: zeolite micropores; SP+EXT:
secondary+external porosity.
The yields of solids attained after the surfactant/ammonia treatment was in both cases
over 85 wt%. Taking into account that in practice is not possible to recover by centrifugation all
the zeolitic material due to the small particle size of the samples, this means that the real yields
are even higher. In this way, as shown in Table 1, the Si/Al of the samples after the
surfactant/ammonia treatment are very similar to those of the parent zeolites. This result
suggests that, if any dissolution of the zeolite has occurred during the surfactant/ammonia
treatment, it has affected in a very similar extent to both silicon and aluminium atoms, so the
overall zeolite Si/Al ratio is little changed. These findings are clear indications that the ZSMꢀ5
zeolite is quite stable against bulk dissolution and desilication when hydrothermally treated with
the surfactant/ammonia solution, which can be related with the mild basicity provided by the
ammonia solution.
Two main tools have been used to investigate the porosity of the ZSMꢀ5 samples: Ar
adsorptionꢀdesorption isotherms and TEM measurements. Figure 2 shows the cumulative pore
volumes (left side) and the corresponding pore size distributions (PSD) (right side), calculated by
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application of the NLDFT model to the adsorption branch of the Ar adsorption isotherms. This
model has the advantage of providing a continuous pore size distribution that covers the whole
range of microꢀ and mesopores, hence it is very well suited for being applied to materials with
hierarchical porosity.
0.6
4
ZSM-5
3
2
1
0.4
0.2
0.0
0.4
0.2
0.0
0.4
0.2
0.0
0.4
0.2
0.0
0
4
ZSM-5 (mnt)
h-ZSM-5
3
2
1
0
4
3
2
1
0
4
h-ZSM-5 (mnt)
3
2
1
0
10
100
1000
10
100
Pore diameter (Å)
Figure 2. Ar adsorptionꢀdesorption isotherms at 87.5 K of the ZSMꢀ5 samples: a) cumulative pore
volumes and b) pore size distributions, determined by application of the NLDFT model.
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For the parent ZSMꢀ5 sample just a main peak is observed in the PSD at about 0.56 nm,
which matches well with the zeolitic micropores. It should be mentioned that a small step is
appreciated around 0.8 ꢀ 1 nm in the cumulative pore volume curve, which corresponds to the
peak, located in the same range, in the pore size distribution. This step has been attributed in the
literature to a fluidꢀtoꢀcrystalline transition of the Ar adsorbed in the MFI micropores36 or to
crystalline phase transitions37, hence being not associated to any real porosity. The adsorption
observed in the parent ZSMꢀ5 sample for pores larger than 20 nm is really originated by the
external surface area and intercrystalline voids, as this material is formed by nanocrystals with
sizes in the range 20 – 80 nm (see TEM images in Figure 3). When this sample is subjected to
the surfactant/ammonia treatment, the material so obtained (ZSMꢀ5 (mnt)) exhibits lower
microporosity, whereas a very broad mesopore distribution appears within the 3 – 20 nm range.
This result indicates that in this case the surfactant/ammonia treatment is capable of generating
some mesoporosity but with little pore size uniformity.
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Figure 3. TEM micrographs of the ZSMꢀ5 samples: a) ZSMꢀ5, b) ZSMꢀ5 (mnt), c) hꢀZSMꢀ5 and
d), e) and f) hꢀZSMꢀ5 (mnt).
For the hꢀZSMꢀ5 sample, prepared from silanized protozeolitic units, the presence of a
hierarchical porosity with at least two levels of pore sizes is evidenced. The secondary porosity in
this parent material possesses a broad size distribution mainly within the 2 ꢀ 10 nm range,
whereas the adsorption occurring at larger pore sizes is really associated to the external surface
area located in the outer part of the zeolite particles. However, this picture drastically changes
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after the surfactant/ammonia treatment. Thus, for the hꢀZSMꢀ5 (mnt) sample, two wellꢀdefined
adsorption steps are observed in the cumulative pore volume curve, corresponding to microꢀ and
mesopores, respectively. This is confirmed by the pore size distribution of this material as it
presents two narrow peaks: the zeolitic micropore peak at about 0.56 nm and a second one
centered at ca. 4 nm. From these results, it can be concluded that the surfactant/ammonia
treatment is very effective in modifying the mesoporosity existing in the parent hierarchical ZSMꢀ
5, narrowing its pore size distribution and leading to the appearance of uniform mesopores. It is
particularly noteworthy that this uniform mesoporosity is mainly formed at the expense of the
irregular secondary porosity placed within the 2 ꢀ 10 nm range.
The main textural properties of the ZSMꢀ5 samples were derived also from the Ar
adsorption isotherms at 87 K, being summarized in Table 1. Seemingly, the mesopore narrowing
treatment decreases the BET surface values with regard to the reference parent samples. This
effect is more pronounced when using the hierarchical zeolite as raw material. The surface area
and pore volume associated to the micropores are preserved in the hꢀZSMꢀ5 (mnt) sample
compared to the parent one, while a significant reduction in these parameters is observed in the
case of the nonꢀhierarchical zeolite. Likewise, opposite trends between both types of samples are
denoted regarding the secondary porosity. In the case of the hierarchical ZSMꢀ5, the
surfactant/ammonia treatment causes a significant decrease of the secondary surface area,
which indicates that the new uniform mesopores are mainly formed by reorganization of those
previously existing in the parent zeolite. Nevertheless, even after this reduction, the contribution
of the secondary surface area in this material (hꢀZSMꢀ5 (mnt)) is quite higher than that in the
case of the sample prepared from the nonꢀhierarchical ZSMꢀ5 (ZSMꢀ5 (mnt)). The decrease in
the micropore volume for the latter after the surfactant/ammonia treatment is accompanied by
some enhancement of the external surface area. This is probably due to the surfactant causing
some changes in the micropore openings from the outer part of the zeolite crystals as its large
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size strongly hinders its penetration into the zeolite micropores. This fact implies that for this
material the formation of mesopores occurs from the outer surface of the zeolite crystals inwards
and with little possibility of being highly interconnected. However, the mesopore narrowing
treatment is not expected to alter the connectivity of the mesopores originally present in the
hierarchical ZSMꢀ5 sample.
Figure 3 illustrates TEM micrographs of the ZSMꢀ5 materials. The ZSMꢀ5 sample is made
up of cubicꢀlike crystals with a size within the 20 – 80 nm range, hence it can be considered a
typical nanocrystaline ZSMꢀ5 sample (Figure 3.a). After the surfactant/ammonia treatment, the
aspect of the crystals does not change significantly, and the presence of mesopores is not
evident (Figure 3.b). As shown in Figures 3.c, hꢀZSMꢀ5 consists of globular shape aggregates
formed by very small zeolite nanounits (5 – 10 nm size). The voids existing between these
nanounits are irregular and correspond with the secondary porosity identified in this sample from
the Ar adsorptionꢀdesorption measurements. After the surfactant/ammonia treatment (Figures
3.d, e and f), the similar globular shape aggregates of nanounits are appreciated for the hꢀZSMꢀ5
(mnt) sample. However, in this case, the voids existing within the aggregates are more regular in
both size and shape, which is in good agreement with the uniform mesopore size distribution
obtained from the Ar adsorptionꢀdesorption isotherms.
ZSMꢀ5 zeolite is conventionally employed as an acid catalyst due to the presence of acid
sites originated from the Al species. Accordingly, the coordination and environment of the Al
atoms is an important aspect influencing strongly its catalytic properties. In this way, the 27Al MAS
NMR spectra of the ZSMꢀ5 samples after calcination at 550 ºC are illustrated in Figure 4. The
relative shares of tetrahedral and octahedral aluminium have been approximated by their
respective peak area proportion in the 27Al MAS NMR. This procedure is a better choice than the
conventional method based on the direct estimation of the peak intensity shares38, since it allows
correcting to some extent the effect of strong quadrupolar line broadening leading to invisible
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aluminium. ZSMꢀ5 contains mostly tetrahedral aluminium (peak at about 50 ppm, AlIV = 91%),
incorporated to the zeolite lattice, whereas the proportion of extraframework aluminium is rather
low (peak at ca. 0 ppm, AlVI = 9%). After the surfactant/ammonia treatment, the relative shares of
these two types of Al species for ZSMꢀ5 (mnt) remain almost identical with values about 90 and
10%, respectively. Likewise, the values of the full width at half maximum (FWHM) of the
tetrahedral Al peak do not reveal any increase proceeding from the occurrence of distorted
aluminium environments, since they were fairly similar for both ZSMꢀ5 (7.1 ppm) and ZSMꢀ5
(mnt) (7.3 ppm) materials. These results confirm that, when using the parent ZSMꢀ5 sample, the
zeolite structure is little affected by the combined actions of surfactant and ammonia.
AlIV=78%
FWHM = 9.5 ppm
AlVI=22%
h-ZSM-5 (mnt)
AlIV=75%
FWHM = 10.1 ppm
AlVI=25%
h-ZSM-5
AlIV=90%
FWHM = 7.3 ppm
AlVI=10%
ZSM-5 (mnt)
AlIV=91%
FWHM = 7.1 ppm
AlVI=9%
ZSM-5
150 125 100
75
50
25
0
-25
-50
ppm
Figure 4. 27Al MAS NMR spectra of the calcined ZSMꢀ5 samples.
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On the other hand, it is observed that the parent hierarchical sample (hꢀZSMꢀ5) presents
a higher contribution of octahedral species and a larger FWHM value compared to ZSMꢀ5. These
results agree with previous works and are a result of the remarkable contribution of the
secondary porosity in the hierarchical zeolite, which provokes a lower stability and a larger
variation of environments of the Al species. In particular, the lower connectivity of the aluminium
species located over the secondary porosity of the zeolite causes a higher flexibility in terms of
bond angles and lengths39,40. Interestingly, after the surfactant/ammonia treatment, the hꢀZSMꢀ5
(mnt) sample exhibits slightly higher proportion of tetrahedral aluminium species than the parent
hꢀZSMꢀ5, suggesting that a small amount of the extraframework Al species are reincorporated
into the zeolite structure, probably by healing some defect sites, such as silanol nests. It is also
noteworthy that the value of the FWHM parameter measured for the hꢀZSMꢀ5 (mnt) sample (9.5
ppm) is lower than that obtained for hꢀZSMꢀ5 (10.1 ppm). This result denotes that the mesopore
narrowing treatment here applied increases the uniformity of the aluminium environment in the
hierarchical zeolite. Other important aspect to be considered is whether the acid features of the
zeolite change after the surfactant/ammonia treatment. In this regard, FTIR measurements of hꢀ
ZSMꢀ5 and hꢀZSMꢀ5 (mnt) samples after pyridine adsorption were performed at several
evacuation temperatures (250, 350 and 450 ºC) in order to determine the concentration of both
Brönsted (band at 1545 cmꢀ1) and Lewis acid sites (band at 1455 cmꢀ1).
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0,30
0,25
0,20
0,15
0,10
0,05
0,00
0,30
T = 250 oC
T = 350 oC
T = 450 oC
0,25
0,20
0,15
0,10
0,05
0,00
h-ZSM-5 (mnt)
h-ZSM-5
h-ZSM-5 (mnt)
h-ZSM-5
Figure 5. Concentration of Brönsted and Lewis acid sites on hꢀZSMꢀ5 (mnt) and hꢀZSMꢀ5
samples.
As it can be appreciated in Figure 5, hꢀZSMꢀ5 contains a higher concentration of
Brönsted acid sites than Lewis ones, as it was expected for this zeolite structure possessing a
relatively high Si/Al ratio. Increasing the evacuation temperature provokes a reduction in the
amount of both types of acid sites being detected due to the progressive desorption of pyridine
molecules, which in turn depends on the strength of the acid sites. Comparing both materials, hꢀ
ZSMꢀ5 and ZSMꢀ5 (mnt), no significant variations are detected in the amount and the distribution
of Brönsted and Lewis acid sites. These results show that the surfactantꢀassisted reorganization
of the mesoporosity takes place preserving the acid properties of the zeolite. Moreover, this
finding is another indication that no amorphous phases are formed during the surfactant/ammonia
treatment since, in that case, the ratio of Brönsted/Lewis acid sites should have decreased
significantly according to the acidic features which are predominant in amorphous
aluminosilicates.
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3.2. Synthesis mechanism of hierarchical ZSM-5 zeolite with uniform mesopores
Based on the results above discussed, the scheme shown in Figure 6 is proposed for
summarizing the mechanism of the synthesis of hierarchical zeolites with uniform mesoporosity
following the approach here reported.
Figure 6. Mechanism of the synthesis of hierarchical ZSMꢀ5 zeolite with uniform mesoporosity.
The first step involves the preparation of a parent zeolite having hierarchical porosity by
means of the use of a silanization agent, which is anchored to the external surface of protozeolitic
nanounits. Accordingly, during the subsequent hydrothermal treatment, the growth of the
silanized protozeolitic nanounits is partially hindered leading to materials that contain, in addition
to the structureꢀdirecting agent, significant amounts of the silanization reagent. The secondary
porosity is generated once the organics have been removed by calcination, although the
materials so obtained possess a broad mesopore size distribution. Incorporation of silanization
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agents in the range 5 ꢀ 8 mol%, regarding the total silica content, is enough for leading to zeolites
with a high contribution of the secondary porosity to the overall textural properties.
In a second step, the hierarchical porosity is reorganized by hydrothermal treatment with
a surfactant/ammonia solution. The conditions of this treatment are conveniently selected to
avoid any damage to the zeolite, such as desilication, dealumination or amorphization. For those
employed in the present work, the Si/Al ratio of the zeolite framework remains almost unchanged,
hence it can be assumed that desilication does not take place in a significant extension. Likewise,
no amorphous phases are formed during the MNT treatment as concluded from the XRD
patterns. In fact, the XRD peak intensity of the hierarchical ZSMꢀ5 is increased after being
contacted with the surfactant/ammonia solution, which is indicative of some partial merge and
growth of the nanounits.
The main changes induced by the mesopore narrowing treatment are observed regarding
the textural properties. It is here proposed that the changes undergone by the mesopores occurs
mainly according to an Ostwald ripening mechanism, wherein the irregular surface of the
secondary porosity is converted into one with more uniformity and less roughness due to the
combined effect of the basic conditions and the surfactant. The treatment causes first the
breaking of some SiꢀOꢀSi or SiꢀOꢀAl bonds in the zeolite, partially releasing zeolitic fragments,
which are subsequently reorganized by the surfactant over the mesopore surface. Since mild
basic conditions are employed, these fragments keep their zeolitic nature, whereas overall
dissolution or desilication phenomena are avoided. Subsequently, reconstruction of SiꢀOꢀSi or Siꢀ
OꢀAl bonds allows the zeolitic fragments to be linked again to the zeolite nanocrystals.
Accordingly, the major role of the surfactant is to rearrange the zeolitic fragments over the
mesopore surface. These phenomena affect mainly to the secondary pores with sizes in the
range 2 – 10 nm, being transformed into mesopores with diameter about 4 nm. The mesoporosity
rearrangement is accompanied by changes in the textural properties, with a decrease of both the
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BET and secondary surface areas. Likewise, a more regular environment of the Al species is
achieved, as inferred from 27Al MAS NMR measurements.
Interestingly, the surfactant/ammonia treatment is little effective when using as parent
zeolite a material free of secondary porosity. Although some changes have been observed in the
properties of the zeolite in this case, they are quite less significant and do not lead to a uniform
mesoporosity. This can be easily understood as for purely microporous zeolite samples the
interaction of the surfactant molecules occurs mainly over the external surface of the crystals,
while their access to the internal microporosity is strongly hindered.
3.3. Changes induced in the zeolite catalytic properties
The presence of uniform mesoporosity in hierarchical ZSMꢀ5 is expected to have deep
catalytic implications, both in terms of activity and selectivity. These effects should be especially
remarkable in those reactions involving bulky compounds and having, consequently, strong
diffusional and/or steric limitations for being catalysed by the active sites located within the
micropores of conventional zeolites. As it has been demonstrated above, the synthesis strategy
here reported allows the secondary porosity present in hierarchical zeolites to be modified,
leading to a narrow mesopore distribution. However, this change takes place at the expense of a
reduction in the contribution of the secondary porosity, with a decrease in the corresponding
surface area and pore volume. Accordingly, an important aspect to be clarified is how these
modifications in the share and features of the secondary porosity are going to affect the catalytic
behaviour of the hierarchical zeolites. In order to answer these questions, the different zeolite
samples were tested in this work as catalysts for the acylation of the bulky reagent 2ꢀ
methoxynaphthalene. This type of reaction was selected since in previous works it has been
found to be very sensitive to the presence of hierarchical porosity in zeolites39,40
.
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The desired product in the acylation of 2ꢀmethoxynaphthalene (2ꢀMN) is 6ꢀacetylꢀ2ꢀ
methoxynaphtalene (6Aꢀ2MN), which is an intermediate for the manufacture of the antiꢀ
inflammatory drug Naproxen. As shown in scheme 1, direct acylation reactions (A) results in the
formation of both 1ꢀacetylꢀ2ꢀmethoxynaphthalene (1Aꢀ2MN) and the desired 6Aꢀ2MN41. However,
the 1 position of 2ꢀmethoxynaphthalene is the most active, being kinetically favoured, so 1Aꢀ2MN
is the major product initially formed. As the reaction proceeds, the desired isomer 6Aꢀ2MN,
thermodynamically most stable, is also produced by intermolecular transacylation reactions (T)
involving both 2ꢀMN and 1Aꢀ2MN42,43. Additionally, 8ꢀacetylꢀ2ꢀmethoxynaphtalene (8Aꢀ2MN) may
be generated by means of intramolecular transacylation reactions of the 1Aꢀ2MN isomer.
Figure 7 illustrates the evolution of the 2ꢀmethoxynaphthalene conversion and the 6Aꢀ
2MN selectivity along the time obtained over the different zeolite samples here investigated. For
comparison, an AlꢀMCMꢀ41 sample has been included in the catalytic tests in order to determine
the behaviour of an amorphous aluminosilicate with ordered mesoporosity. This sample shows a
high BET surface area (893 m2 gꢀ1) consisting of uniform mesopores of 2.7 nm without any
microporosity, hence its performance as catalyst in reactions involving bulky molecules is not
limited by steric or diffusional factors as it usually occurs with conventional zeolites having just
micropores. Additionally, the AlꢀMCMꢀ41 Si/Al atomic ratio (28) is very close to that of the ZSMꢀ5
catalysts used in this work, although containing mostly Lewis acid sites of medium strength.
Therefore, this AlꢀMCMꢀ41 sample is a convenient reference material to compare its catalytic
performance with those of the ZSMꢀ5 samples here investigated and to assess the effects that
could be derived from the presence of any ordered mesophase after the surfactant/ammonia
mesopore narrowing treatment.
The reactions have been performed at 180 ºC using acetyl chloride as acylating agent.
In all cases, the conversion evolves increasing with time until a plateau is reached after 60 min of
reaction. This can be indicative of a catalyst deactivation or, as reported for other acylation
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reactions, to a product inhibition effect caused by the strong adsorption of the reaction products
on the active sites44. Comparing the two parent zeolite samples, it is evident that the hierarchical
ZSMꢀ5 presents a quite superior catalytic activity. Thus, the conventional ZSMꢀ5 sample showed
the poorest activity because of its inferior textural properties as it possesses the lowest
external/mesopore surface area (Sext+SP = 76 m2gꢀ1). This fact originates that conventional ZSMꢀ5
experiences strong steric hindrances for the acylation of the bulky 2ꢀMN, exhibiting quite lower
conversion compared to the hierarchical ZSMꢀ5 sample.
After the surfactant/ammonia treatment a significant enhancement of the conversion for
both materials is observed, the highest one corresponding again to that obtained from the
hierarchical zeolite. Thus, the improvement of the conversion for hꢀZSMꢀ5 (mnt) is more
pronounced, in particular at short reaction times when the product inhibition effect is less
significant. Therefore, in this case the presence of uniform mesoporosity has an important
positive effect on the catalytic activity. It is also noteworthy that the AlꢀMCMꢀ41 sample shows
worse results than the two hierarchical ZSMꢀ5 zeolites, reaching a plateau for a 2ꢀMN conversion
about 12%. Taking into account the superior textural properties of AlꢀMCMꢀ41 (very high surface
area and uniform mesopores), the cause of the lower activity observed for this sample compared
to the two hierarchical ZSMꢀ5 materials is to be ascribed to the occurrence of acid sites of lower
acid strength than those present over both zeolitic catalysts. This result is relevant as it confirms
the absence of amorphous ordered mesoporous phases in the hꢀZSMꢀ5 (mnt) sample since, in
that case, the 2ꢀMN conversion obtained with the latter should have been lower than that of
hierarchical ZSMꢀ5.
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a)
20
15
10
5
ZSM-5
ZSM-5 (mnt)
h-ZSM-5
h-ZSM-5 (mnt)
Al-MCM-41
0
0
50
100
150
200
Time (min)
b)
60
40
20
0
ZSM-5
ZSM-5 (mnt)
h-ZSM-5
h-ZSM-5 (mnt)
Al-MCM-41
0
50
100
150
200
Time (min)
Figure 7. Acylation of 2ꢀmethoxynaphthalene. Evolution along the reaction time of: a) 2ꢀMN
conversion and b) 6Aꢀ2MN selectivity.
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On the other hand, significant differences are observed between the samples when
compared in terms of selectivity towards the desired product. The reference hꢀZSMꢀ5 shows
higher selectivity towards 6Aꢀ2MN compared to the conventional sample (ZSMꢀ5) and AlꢀMCMꢀ
41. In the case of AlꢀMCMꢀ41, it has been reported that the selectivity obtained in the 2ꢀMN
acylation is addressed mainly to the production of 1Aꢀ2MN because of the unrestrained nature of
its mesopores45,46
. Interestingly, for the hierarchical ZSMꢀ5 sample the 6Aꢀ2MN selectivity is
further improved after the mesopore narrowing treatment, increasing continuously along the
reaction time (Figure 7.b). As a result, the selectivity obtained over the hꢀZSMꢀ5 (mnt) sample is
very superior to that of the other catalysts. Thus, hꢀZSMꢀ5 (mnt) allowed obtaining 65.8%
selectivity towards 6Aꢀ2MN after 3 h of reaction, quite higher than that of the next catalyst (hꢀ
ZSMꢀ5, 41.5% selectivity). It is noteworthy that this enhancement in selectivity occurs despite the
above commented loss of secondary surface area after the surfactant/ammonia treatment, so its
origin must be related to the uniformity of the attained mesoporosity. These results denotes that a
more uniform mesoporosity enhances the activity of both the acylation and the intermolecular
transacylation reactions, resulting in an increase of the substrate conversion simultaneously with
a quite superior selectivity (over 60% at the longest reaction time) towards the target product over
the hꢀZSMꢀ5 (mnt) sample. Interestingly, the 6Aꢀ2MN selectivity value here obtained is quite
similar to the best ones so far published corresponding to Beta zeolite, that has been regarded as
the benchmark catalyst for this reaction due to the shape selectivity effect of its 3D 12 membered
ring channels47ꢀ49. Thus, when comparing Beta, USY, SSZꢀ33, SSZꢀ35 and ITQꢀ32 zeolites in the
acylation of 2ꢀmethoxynaphthalene with acetic anhydride50, Beta zeolite gave rise to the highest
selectivity towards 6Aꢀ2MN with values around 60%.
The improved catalytic properties observed after the surfactant/ammonia treatment are
ascribed mainly to the enhanced homogeneity of the mesopore features (size, geometry and
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surface). The presence of uniform mesopores involves an enhanced mass transfer of the reagent
molecules through channels with a lower tortuosity factor. On the other hand, a smooth uniform
mesopore surface is more amenable to adsorption of the reacting molecules than a rough one,
being less affected by steric limitations that could hinder the contact between bulky compounds
and some of the active sites located over the mesopore surface. In other words, a more uniform
mesoporosity increases the catalytic effectiveness of the mesopore surface area, since it favours
the participation in the reaction of all the active sites there located, giving rise to higher intrinsic
activities.
4. CONCLUSIONS
Hierarchical ZSMꢀ5 zeolite with uniform mesopore size distribution has been successfully
prepared by the combination of two approaches: crystallization of silanized protozeoltic units to
generate a secondary porosity and surfactant/ammoniaꢀmediated reorganization of the latter. The
application of the mesopore narrowing treatment to hierarchical ZSMꢀ5 allowed the previously
existing irregular secondary porosity (about 2 – 10 nm range) to be transformed into uniform
mesopores with sizes centered at 4 nm. In contrast, the surfactant/ammonia treatment had little
effect when applied to a zeolite sample free of the initial secondary porosity.
It is proposed that the uniform mesoporosity is generated by means of the local
rearrangement of zeolitic fragments according to an Ostwald ripening mechanism, wherein the
irregular surface of the secondary porosity is converted into one quite more uniform and with less
roughness due to the combined effect of the mild basic conditions and the surfactant. This picture
is consistent with the absence of amorphous phases and the negligible variations produced in the
zeolite framework composition after the mesopore narrowing treatment, showing the stability of
the ZSMꢀ5 structure against amorphization and desilication phenomena, probably due to the mild
basicity of the ammonia solution. The presence of a more regular mesoporosity in hierarchical
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ZSMꢀ5 is accompanied by a higher uniformity of the Al sites environment, whereas the zeolite
acid properties are preserved without any significant variation.
The effects of having uniform mesopores on the catalytic behaviour have been proved in
the acylation of 2ꢀmethoxynaphthalene. Higher activity, as well as superior selectivity towards the
desired 6ꢀacetylꢀ2ꢀmethoxynaphthalene isomer, was obtained over the hierarchical ZSMꢀ5 with
uniform mesopores. These effects are consequences of the enhanced homogeneity of both the
mesopore features (size, geometry and surface) and the environment of the corresponding active
sites, which in turn lead to an improved catalytic behaviour.
6. ACKNOWLEDGMENTS
The authors want to thank the funding by the Spanish government through the projects
ENE2011 – 29643 – C02 – 01 and CTQ 2011/22707. We want to thank Dr. J. L. García Fierro
from ICPꢀCSIC for performing the pyridine adsorption measurements shown in this work.
7. REFERENCES
1. D. P. Serrano, J. M. Escola, P. Pizarro, Chem. Soc. Rev. 2013, 42, 4004.
2. D. P. Serrano, J. Aguado, J.M. Escola, Catalysis, 2011, 23, 253.
3. J. PérezꢀRamírez, C. H. Christensen, K. Egeblad, C. H. Christensen, J. C. Groen, Chem. Soc.
Rev. 2008, 37, 2530.
4. K. Egeblad, C. H. Christensen, M. Kustova, C. H. Christensen, Chem. Mater. 2008, 20, 946.
5. J. Cejka, G. Centi, J. PérezꢀPariente, W. J. Roth, Catal. Today 2012, 179, 2.
6. H. Wang, T. J. Pinnavaia, Angew. Chem. Int. Ed. 2006, 45, 7603.
7. M. Choi, H. S. Cho, R. Srivastava, C. Verkatesan, D. H. Choi, R. Ryoo, Nat. Mater. 2006, 5,
718.
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