High acid catalytic activity of aluminosilicate molecular sieves with MCM-41 structure synthesized from precursors of colloidal faujasite

Article abstract of DOI:10.1039/b209983g

The synthesis of micro/mesoporous aluminosilicate with a hexagonal arrangement of pores has been achieved by cationic surfactant-templating in a tetramethylammonium-containing solution precursor of colloidal faujasite; this material is zeolite nanocrystal-free and exhibits high catalytic activity in m-xylene conversion.

Full text of DOI:10.1039/b209983g

High acid catalytic activity of aluminosilicate molecular sieves with
MCM-41 structure synthesized from precursors of colloidal faujasite†
Javier Agúndez, Isabel Díaz, Carlos Márquez-Álvarez, Joaquín Pérez-Pariente* and Enrique Sastre
Instituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, 28049, Madrid, Spain.
E-mail: jperez@icp.csic.es; Fax: 34 91585 4760; Tel: 34 91585 4784
Received (in Cambridge, UK) 11th October 2002, Accepted 18th November 2002
First published as an Advance Article on the web 4th December 2002
The synthesis of micro/mesoporous aluminosilicate with a
hexagonal arrangement of pores has been achieved by
cationic surfactant-templating in a tetramethylammonium-
containing solution precursor of colloidal faujasite; this
material is zeolite nanocrystal-free and exhibits high cata-
lytic activity in m-xylene conversion.
This solution was mixed at room temperature with 101.85 g
of a 20 wt% hexadecyltrimethylammonium (CTA) bromide
aqueous solution, and heated at selected temperatures for 3 h in
polypropylene bottles (if T < 100 °C) or in Teflon-lined,
stainless-steel 60 ml sized-autoclaves (if T 4 100 °C). The solid
was then filtered off, washed with deionized water and dried at
60 °C overnight. The total yield of oxides averaged 60%. The
samples were calcined at 550 °C under continuous flow of N2
(130 cm3 min21) for 1 h, followed by air (130 cm3 min21) for
6 h. For comparison purposes, two aluminium-rich Al-MCM-41
samples were prepared according to refs. 6 and 7.
The X-ray diffraction pattern of the as-made sample
synthesized at 150 °C (Fig. 1) is characteristic of a hexagonal
arrangement of pores with a unit cell size (a0) of 4.3 nm. No
reflections at angles higher than that corresponding to the 210
reflection of the hexagonal symmetry are observed (see inset in
Fig. 1). Upon calcination at 550 °C in N2/air flow, a single peak
is observed in the XRD pattern (Fig. 1), shifted to a d value (d
= 2.3 nm) much lower than that corresponding to the most
intense reflection of the as-made material. We observed a
decrease in intensity and the broadening of the XRD profile for
samples synthesized at temperatures below 150 °C, whereas at
175 °C, well-ordered MCM-41-type materials are obtained.
TEM images of a calcined sample synthesized at 150 °C (Fig.
2) show the hexagonal array of channels characteristic of the
MCM-41 structure. By Fast Fourier Transform (FFT) of the
TEM images (Fig. 2, inset) we estimate a unit cell dimension of
2.9 nm, in good agreement with the 2.8 nm value obtained by
XRD assuming that the peak at d = 2.3 nm corresponds to the
100 reflection of the hexagonal p6mm symmetry. Therefore, a
unit cell contraction of ca. 1.4 nm takes place upon calcination.
This severe shrinkage is not totally unexpected, taking into
account that the as-made material contains as much as 49 wt%
organic material, which suggests that it possesses a large
amount of structural defects.
The low acidity and hence moderate catalytic activity of
aluminium-containing ordered mesoporous materials, partic-
ularly Al-MCM-41, is generally attributed to the well-known
absence of structural order in the aluminosilicate matrix.
A number of strategies have been reported in the last few
years to increase the activity of mesoporous aluminosilicates in
acid-catalyzed reactions.1–3 One of such approaches consists in
mimicking the physicochemical conditions prevalent in the
synthesis gels from which zeolite materials are obtained, and
promoting the surfactant-assisted assembly of the zeolite
precursors present in the gel. Mesostructures characterized by
enhanced activity have been obtained from ZSM-5 and Beta
seeds.2
The synthesis of ordered mesoporous materials possessing a
faujasite-like framework would be particularly appealing, due
to the importance of this zeolite in the oil refining industry.
Steam-stable mesoporous materials have been synthesized
through the assembly of Y zeolite seeds mediated by hexadecyl-
trimethylammonium (CTA) cations, but the activity that these
aluminosilicates exhibited, before steaming, in the cracking of
cumene was close to that of Al-MCM-41 synthesized by
conventional methods.3 This process made use of a pure
inorganic seeding gel, whereas tetraalkylammonium cations,
TPA and TEA, are present in the synthesis gels from which
mesoporous materials of enhanced catalytic activity are ob-
tained.2 Therefore, it could be possible that the presence of
organic cations in the zeolite gel precursor is required to obtain
materials possessing high acid strength.
We report in this work the synthesis and catalytic activity of
aluminosilicates with hexagonal pore arrangement from tetra-
methylammonium-containing solutions that are precursors of
faujasite crystals of colloidal size.
The synthesis procedure involves the preparation of a
solution precursor of colloidal faujasite, with the following
molar composition:4
The small unit cell suggests that the pore size of the material
should be unusually small as well, and close to the border
between micro- and mesopores (2.0 nm), in order to avoid
unreasonable wall thickness. The small pore size is reflected in
the N2 isotherm (ESI†), where the conventional, sharp step of
pore filling at p/p0 values in the range 0.2–0.4 is not observed.
Instead, microporosity (0.24 cm3 g21) is detected by t-plot
Al2O3+1.53 (TMA)2O+0.088 Na2O+3.62 SiO2+246 H2O
which has been obtained following the procedure reported in
ref. 5: 47.81 g of wet freshly precipitated aluminium hydroxide
(87 wt% H2O) was added under stirring to a solution containing
44.82 g of a 25 wt% TMAOH aqueous solution (Aldrich), 0.29
g NaOH (Prolabo) and 78.15 g deionized water, until complete
dissolution of the Al(OH)3. Then, 28.94 g Ludox SM30 (30%
SiO2 Aldrich, previously exchanged with Dowex HCR-S ion
exchange resin) was added and the mixture stirred for 45 min.
The opalescent solution obtained was poured into poly-
propylene bottles and heated at 100 °C for 24 h.
† Electronic supplementary information (ESI) available: N2 isotherm of
sample ZPM-150. FTIR spectra of samples ZPM-150, Al-MCM-41-15 and
USY. See http://www.rsc.org/suppdata/cc/b2/b209983g/
Fig. 1 XRD patterns of sample synthesized at 150 °C, as-made (a) and
calcined (b).
150
CHEM. COMMUN., 2003, 150–151
This journal is © The Royal Society of Chemistry 2003

Table 1 Catalytic activity and selectivity at 400 °Ca
d
Sample
Si/Al ratioc R0
P/O ratioe I/D ratioe
USYb
Al-MCM-41-3
Al-MCM-41-15
ZPM-150
15
5.0
5.9
2.7
2.6
1.4
0.9
1.0
0.9
0.9
1.1
1
9
19
9
7.4 3 1023
7.5 3 1023
3.2 3 1021
1.8 3 1021
ZPM-175
12
a m-Xylene:N2 = 1+4. b a0 = 2.428 nm. c Bulk. d Reaction rate extrapolated
at zero time (mol g21 h21). e At ca. 5% m-xylene conversion (see text).
Fig. 2 TEM images and corresponding FFT patterns along the direction
parallel (a) and normal (b) to the channel axes in an MCM-41 type structure
of the sample synthesized at 150 °C and calcined.
Chemical analysis (TXRF) of the sample synthesized at 150
°C and calcined indicates that Na/Al = 0.1 and Si/Al = 2.7 (a
Si/Al ratio of 3.0 was obtained by XEDS in the electron
microscope).
analysis of the N2 isotherm. However, the pore size distribution
in the micropore region determined by Ar adsorption does not
show evidence of faujasite-type micropores, whereas a weak,
broad maximum centered at ca. 1.4 nm is present.
Catalytic activity of the calcined samples in m-xylene
conversion indicates that a deep modification of the T–O–T
connectivity exists in the vicinity of the protonic acid sites.
Table 1 collects the activity and selectivity of the calcined
samples synthesized at 150 °C (ZPM-150) and 175 °C (ZPM-
175), as well as those of two aluminium-rich Al-MCM-41
materials and an USY catalyst (CBV 720, a0 = 2.428 nm). The
activity of the samples derived from colloidal faujasite
precursors is nearly two orders of magnitude higher than that of
Al-MCM-41. Indeed, the activity of the new materials ap-
proaches that of USY. The para/ortho-xylene (P/O) ratio is 1,
the expected value for a large pore catalyst with no shape-
selectivity effects.11 It is remarkable that the isomerization/
disproportionation (I/D) ratio is close to 10, whereas I/D ratios
for faujasite zeolites are always below 2.12 The overall catalytic
behaviour indicates, in our opinion, the absence of zeolite
nanocrystals in the solid.
Taking into account that the I/D ratio decreases strongly as
the acid strength increases, our results suggest that this new
synthesis route gives rise to porous structured aluminosilicates
of enhanced acidity, their acid strength being nevertheless lower
than that of USY. These materials are suitable for many
catalytic processes that do not require strong acid sites, such as
isomerization or hydroisomerization, mild hydrocracking and a
number of hydroprocessing reactions, as well as cracking of
bulky feedstocks.
The 29Si MAS NMR spectrum of the calcined sample
synthesized at 150 °C (Fig. 3) shows a broad peak centered at
ca. 2102 ppm where prominent shoulders are observed.
Deconvolution of the spectrum suggests the presence of two
different sets of silicon environments, one of them comprising
four distinct Si locations centered at –107, –102 (the most
intense one), 297 and 292 ppm. The chemical shifts and
relative intensities of these signals are similar, respectively, to
Si(0Al), Si(1Al), Si(2Al) and Si(3Al) species in a zeolite-like
environment. The broad peaks centered at 2110 and 293 ppm
could be attributed to a different phase and might arise from
amorphous silicate species. The 27Al MAS NMR spectrum of
the same calcined sample (Fig. 3) shows the presence of three
distinct signals centered at 1, 27 and 55 ppm. The high-field
signal corresponds to octahedral aluminium, and that at 55 ppm
is assigned to tetrahedral (structural) aluminium. Of interest is
the presence of the peak at 27 ppm, which has been assigned to
five-coordinate aluminium or, alternatively, aluminium in
highly distorted tetrahedral sites in dealuminated faujasite.8
This so-called pentacoordinated aluminium is in general not
observed in calcined aluminium-rich Al-MCM-41 materials
even for very low Si/Al ratios,9 although it has been identified
under certain synthesis conditions10
Both the 27Al and 29Si MAS NMR spectra suggest the
presence of Si–O–T (T = Si, Al) environments different from
those commonly present in Al-MCM-41 obtained by conven-
tional routes, which resemble those existing in faujasite.
However, no evidence of the presence of tiny, submicrometer-
sized crystals in the samples has been found by TEM or Ar
adsorption. Moreover, no bands corresponding to zeolite
framework vibrations are observed in the FTIR spectrum
(ESI†). Therefore, the structural and textural properties of the
material can be best explained by assuming that the T–O–T
connectivity of the tetrahedral sites located in the pore wall has
been substantially altered in comparison with that of Al-MCM-
41. However, no evidence on the presence of SBU precursors of
Y zeolite has been found in these materials.
We acknowledge the assistance of Dr Teresa Blasco in
collecting the solid state NMR spectra, and the financial support
of CICYT (project MAT2000-1167-C02-02).
Notes and references
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683; D. T. On and S. Kaliaguine, Angew. Chem., Int. Ed., 2001, 40,
3248.
2 Y. Liu, W. Zhang and T. J. Pinnavaia, Angew. Chem., Int. Ed., 2001, 40,
1255; Y. Liu, W. Zhang and T. J. Pinnavaia, Stud. Surf. Sci. Catal.,
2001, 135, 1337; Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D.
Zhao and F-S. Xiao, Angew. Chem., Int. Ed., 2001, 40, 1258.
3 Y. Liu, W. Zhang and T. J. Pinnavaia, J. Am. Chem. Soc., 2000, 122,
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4 Spain Pat. Appl., ES200201821.
5 B. J. Schoeman, J. Sterte and J. E. Otterstedt, Zeolites, 1994, 14, 110.
6 I. Díaz, J. Pérez-Pariente and E. Sastre, Stud. Surf. Sci. Catal., 1999,
125, 53.
7 R. B. Borade and A. Clearfield, Catal. Lett., 1995, 31, 267.
8 G. J. Ray and A. Samoson, Zeolites, 1993, 13, 410.
9 M. T. Janicke, C. C. Landry, S. C. Christiansen, S. Birtalan, G. D.
Stucky and B. F. Chmelka, Chem. Mater., 1999, 11, 1342.
10 S. Biz and M. G. White, J. Phys. Chem. B, 1999, 103, 8432.
11 J. A. Martens, J. Pérez-Pariente, E. Sastre, A. Corma and P. A. Jacobs,
Appl. Catal., 1988, 45, 85.
Fig. 3 27Al (a) and 29Si (b) MAS NMR spectra of the calcined sample
synthesized at 150 °C.
12 A. Corma, V. Fornés, J. Pérez-Pariente, E. Sastre, J. A. Martens and P.
A. Jacobs, ACS Symp. Ser., 1988, 368, 555.
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