Improved large mesoporous ordered molecular sieves - Stabilization and acid/base functionalization

Article abstract of DOI:10.1016/j.cattod.2010.03.005

The preparation of nanoporous materials with enhanced stability using an improved synthesis route using reactive inorganic silica and alumina species is reported. This way improved mesoporous molecular sieves were obtained. The synthesized aluminum substituted mesoporous molecular materials (Al-MMS) contain very large pores of 50-200? size combined with an improved pore wall thickness. Increased wall thickness and Al substitution lead to an improved chemical stability against alkaline solution. The textural, structural and acid properties are investigated by physico-chemical methods. The catalytic performance acidic materials was tested in the benzoylation reaction; amino functionalized materials were studied in the base catalyzed Michael addition.

Full text of DOI:10.1016/j.cattod.2010.03.005

Catalysis Today 152 (2010) 54–60
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Improved large mesoporous ordered molecular sieves—Stabilization and
acid/base functionalization
Hendrik Kosslick , Irene Pitsch , Jens Deutsch , Marga-Martina Pohl , Axel Schulza,b,
a,∗
a
a
a
c
c
d
e
Vu Anh Tuan , Nguyen Dinh Tuyen , Ligia Frunza , Christian Jaeger
Leibniz Institute for Catalysis e.V. at the University of Rostock Albert-Einstein-Str. 29a, D-18059 Rostock, Germany
Institute of Chemistry, University of Rostock, Albert-Einstein-Str. 3a, D-18059 Rostock, Germany
Institute of Chemistry, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam
National Institute of Material Physics, Atomistilor Str. 105, R-077125 Magurele, Romania
a
b
c
d
e
BAM – Federal Institute for Materials Research and Testing, Division I.3, Working Group NMR Spectroscopy, Richard-Willstätter Str. 11, D-12489 Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Keywords:
The preparation of nanoporous materials with enhanced stability using an improved synthesis route
using reactive inorganic silica and alumina species is reported. This way improved mesoporous molecular
sieves were obtained. The synthesized aluminum substituted mesoporous molecular materials (Al-MMS)
contain very large pores of 50–200 Å size combined with an improved pore wall thickness. Increased wall
thickness and Al substitution lead to an improved chemical stability against alkaline solution. The textural,
structural and acid properties are investigated by physico-chemical methods. The catalytic performance
acidic materials was tested in the benzoylation reaction; amino functionalized materials were studied in
the base catalyzed Michael addition.
Nanoporous molecular sieve
Al substitution
Stability
Synthesis
Acidity
Catalysis
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
and Brønsted acidic materials have been obtained [9,12,19]. In some
cases formation of strong Lewis acid sites [20] or pure Brønsted
acidic materials [21,22] has been emphasized. The Si/Al ratio of
mesoporous molecular sieves could be extended down to 7 and or
5.5, respectively [9,19]. Silylation of surface silanol groups allow
functionalization of mesoporous molecular sieves with sulfonic
acid and base amino groups [23,24].
In this contribution the preparation of nanoporous materials
with enhanced stability will be reported. An improved synthe-
sis route using reactive inorganic silica and alumina species is
presented. This way synthesized Al-MMS materials contain very
large pores of 50–200 Å size combined with an improved pore wall
thickness. Increased wall thickness and Al substitution lead to an
improved chemical stability against alkaline solution. The textural,
structural and acid properties are investigated by physico-chemical
methods. The catalytic performance of amino functionalized mate-
rials has been studied in the base catalyzed Michael addition. The
results show, that the large pore sizes combined with interconnec-
tivity of the pores allow the formation of large molecules.
Ordered mesoporous materials attract growing attention
because of their wide potential of application in catalysis, sep-
aration, biotechnology, etc. [1–7]. The accessibility of acid/basic
sites, the diffusivity of larger molecules or the immobilization of
catalytic complexes and enzymes are crucial for the desired appli-
cation. Despite of the achieved progress, the preparation of large
porous ordered aluminosilicates is still challenge. The pore struc-
ture of ordered mesoporous materials like SBA-15 is rather complex
[
8]. Synthesis conditions strongly influence the textural properties
as pore size, shape and pore volume, pore connectivity and parti-
cle morphology [9–11]. Hard-templating allows the formation of
nano-macroporous materials [12]. Pore design influences the cat-
alytic properties by access to active sites [13]. Also hydrothermal
stability [14] may enhanced by presence of some micropores in the
pore walls [15]. The incorporation of aluminum into the silicate
network is of special importance for creation of acid sites. Beside
direct hydrothermal synthesis of aluminium substituted meso-
porous materials [9,10] post-synthesis modification by treatment
of pure siliceous mesoporous materials with aluminium chloride
and other aluminium sources has been investigated [16–19]. Lewis
2. Experimental
2.1. Synthesis
∗ Corresponding author. Tel.: +49 381 4986384; fax: +49 381 4986382.
E-mail address: Hendrik.Kosslick@catalysis.de (H. Kosslick).
Ordered aluminum substituted silicates (Al-MMS) were pre-
pared by hydrothermal treatment starting with template con-
0
920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.03.005

H. Kosslick et al. / Catalysis Today 152 (2010) 54–60
55
taining reaction gels. The composition and treatment of prepared
reaction mixtures were similar to SBA-15 synthesis [25]. Pluronic
P123 was used as template. Mesitylene acted as pore size enlarging
spacer. Chemicals were of p.a. grade and purchased from Aldrich
and Fluka, respectively. In contrast to usual SBA-15 synthesis pro-
cedures, a reactive inorganic silica precursor and Al-oxo-hydroxo
species were used as alumina source. The reactive silica was pre-
pared by ion exchange of water glass over an acidified organic resin.
The obtained fresh silica solution was immediately mixed with
the alumina source. The Si/Al ratio of the synthesis gel was var-
ied between 1 and 25. Thereafter, the template was added under
vigorous stirring. The obtained reaction gels were hydrothermal
treated at elevate temperature for 2–4 days under autogeneous
pressure. The recovered reaction product was washed, dried and
◦
calcined at 550 C. Exchange of residual sodium ions by ammonium
was carried out by stirring the calcined Al-MMS material in diluted
◦
ammonium nitrate solution at 60 C.
Fig. 1. X-ray diffraction pattern of calcined Al-MMS.
2.2. Functionalization
For acid functionalization of MMS materials 1 g of calcined
3. Results and discussion
3.1. Synthesis
and pre-dried samples were treated with 3.5 mmol of tri-
ethoxy(mercaptopropyl) silane in toluene under reflux for 2 h. The
obtained mercaptosilylated material was oxidized to sulfonic acid
using hydrogen peroxide.
The new synthesis approach, using reactive inorganic silica and
alumina precursors as starting materials and Pluronic P123 as tem-
plate yields highly ordered mesoporous aluminosilicate materials
as indicated by the XRD pattern (Fig. 1). By using mesitylene as addi-
tive the pore size could be extended up to 200 Å. This extra large
porous material shows, however, a foam-like structure. The well
ordering of the pore structure is also confirmed by the TEM images
(Fig. 2). They show ordered parallel arranged interconnected pores
of uniform size (50–100 Å). Interestingly, the Al-MMS materials
have increased thickness of pore walls of 40–50 Å. This is a pre-
requisite for an improved stability of large porous materials. For
comparison, the wall thickness of MCM-41 type materials is usually
in the range of 10–20 Å. Alumina by-products have not be found.
EDX analysis revealed a homogeneous distribution of aluminum
throughout the samples. Scanning electron micrographs of selected
samples show that the ordered structured materials may crystallize
as in well shaped particles. In has been reported that the morphol-
ogy of ordered mesoporous molecular sieves depend strongly on
the synthesis conditions [10]. Al-MMS-100 crystallized in form of
intergrown disc-like particles with sizes of 5–10 ␮m (Fig. 3).
For grafting of basic function, calcined Al-MMS samples were
stirred with aminopropyl- and N-dimethylaminopropyl silane of
different basicity in toluene under reflux.
2.3. Characterization
Samples were characterized by TEM, XRD, nitrogen sorption,
ammonia TPD and pyridine FTIR and 13C, Al and Si MAS solid
state NMR spectrometry as reported elsewhere [26,27]. The chem-
ical compositions of mesoporous materials were determined by
CHNS and ICP AES analysis.
27
29
2.4. Catalysis
Catalytic testing of acid and base functionalized mesoporous
molecular sieves was performed in the liquid phase in organic
solvents under batch conditions using glassy reaction vessels
under stirring. The catalytic performance of acidic materials was
tested in the benzoylation reaction. Reaction conditions: 12 ml
anisol, 0.6 g benzoic anhydride, 200 mg catalyst, reaction time; 3 h,
◦
temperature: 50 C. 0.1 g benzoic acid-n-butylester was added as
3.2. Characterization
internal standard for HPLC-analysis. The Michael addition reaction
of methyl-vinyl-ketone and 2-methyl-cyclohexane-1,3-dione was
used for catalytic testing of base functionalized of MMS materials.
Textural properties like pore size and distribution, spe-
cific surface areas and volume were determined by nitrogen
Fig. 2. TEM images of nanoporous aluminosilicate having pore size ca. 100 Å along (left) and perpendicular (right) to the pore direction.

5
6
H. Kosslick et al. / Catalysis Today 152 (2010) 54–60
Fig. 3. SEM image of calcined Al-MMS.
Fig. 5. Pore size distributions derived from nitrogen sorption measurements of Al-
MCM-41, Al-MMS-50, Al-MMS-100 and Al-MMS-200 (from left to right).
lary condensation of nitrogen in the meso- or nano-pores. The
small hysteresis in the adsorption–desorption curve point to large
pore openings and free accessibility of the internal pore volume.
The pore size maxima of considered Al-MMS samples are ca. 50,
1
00 and 200 Å, respectively. The width of pore size distribution
increases with growing pore size (Fig. 5). The specific pore volumes
3
range from ca. 0.5 to 2 cm /g and depend strongly on the synthesis
conditions like temperature, thermal pre-altering, crystallization
temperature and time as well as on the chemical compositions.
Using this method, mesoporous materials with Si/Al ratios down
to 1.4 could be prepared. With growing Si/Al ratio, the specific
surface areas and pore volume increase (Table 1). This is inline
with earlier findings with MCM-41 tape materials. It might be
that the aluminum influences network formation due by restric-
tion the flexibility of T–O–T bridging angles between the AlO4 and
SiO4 tetrahedra. The synthesis conditions for considered different
pore sized Al-MMS-50, Al-MMS-100 and Al-MMS-200 are given in
Table 2.
Fig. 4. Nitrogen adsorption isotherms of Al-MMS-50, Al-MMS-100 and Al-MMS-200
from bottom to top).
(
In the 27Al MAS solid state NMR spectra of as-synthesized Al-
MMS samples appear two main signals at ca. 60 ppm and near
adsorption–desorption measurements (Fig. 4). The sorption
isotherms show the typical course observed with mesoporous
materials. The first up-take at low a relative pressure p/p0 is due
to surface layer adsorption followed by multi layer adsorption.
The second adsorption step in the isotherm is related to capil-
IV
0 ppm. They correspond to tetrahedral coordinated framework Al
VI
and extra-framework octahedral coordinated Al units (Fig. 6).
After calcinations, the Al content decreases. Simultaneously, a
IV
V
signal for fivefold coordinated Al appears. Interestingly, treat-
Table 1
Influence of the crystallization conditions on the specific surface area and pore volume proper of AL-MMS materials.
2
3
Sample
Si/Al ratio gel
Si/Al ratio material
Crystallization conditions
Surface area BET [m /g]
Pore volume BJH [cm /g]
◦
Al-MMS-50
Al-MMS-100
Al-MMS-200
11.3
11.1
11.2
12.3
13.3
12.1
1 d/60
C
473
595
588
0.42
1.01
2.01
◦
◦
◦
20 h/35 C;4 d/100
20 h/35 C;4 d/100 C organic additive
C
◦
Table 2
Influence of of the Si/Al ratio in the synthesis gel on the concentration of cation sites, i.e. potential Brønsted sites and textural propoerties.
NH4+ exchange capacity [mmol/g]
BET surface area [m /g]
2
Specific pore volume [cm /g]
3
Si/Al ratio gel
Si/Al ratio material
1
4
7
.0
.6
.3
1.4
5.8
9.8
1.5
21.9
∞
0.10
0.39
0.33
0.39
0.32
0.0
317
505
627
602
900
820
0.19
1.12
1.09
0.95
1.41
1.10
1
1
0.7
7.7
∞

H. Kosslick et al. / Catalysis Today 152 (2010) 54–60
57
ment in diluted aqueous ammonium nitrate solution leads to a
IV
markedly increase of the intensity of the Al signal, usually related
to Brønsted acid sites. It is now dominating. The intensity changes
VI
are not due to dissolution of extra-framework Al species. Cal-
cination of the ammonium exchanged molecular sieve to get the
IV
Brønsted acidic H-form leads again to a decrease of Al and increase
VI
V
IV
of Al and Al species. However, the relative amount of Al
units is improved in comparison to the material obtained after the
first calcinations. After contacting with moisture or water treat-
IV
ment again the Al signal is dominating. Hence, the framework
is very flexible and undergoes structural re-arrangements during
different treatments. Therefore, differences in sample pretreat-
IV
ments may also contribute to the observation of different Al to
VI
Al ratios for similar mesoporous materials reported in the lit-
erature [16,20,21]. However, Al-rich materials (Si/Al–1.4) contain
penta- and octahedral coordinated aluminum beside tetrahedral Al
species.
Structural re-arrangements during the activation procedure
are evidenced by solid state 29Si MAS NMR spectra especially
for Al-MMS with Si/Al = 21.9 (Fig. 7). The spectrum of the as-
synthesized material is well structured showing three overlapping
lines corresponding to twofold, threefold and fourfold connected
SiO -tetrahedron centered at ca. −92, −102 and 107 ppm, also
4
denoted as Q , Q and Q4 silicon sites, respectively [28,29]. After
2
3
calcination, signal lines are broadened and not resolved. Obviously,
condensation of SiO -tetrahedron and corresponding formation of
4
Si–O–Si brides increases the width of the Si–O–Si bridging angle
distribution. After aqueous ammonium exchange the network
relaxes or partially hydrolysis resulting again in more resolved
29
Si NMR lines. However, the relative intensity, and hence concen-
tration, of fourfold connected SiO -tetrahedron is increased. The
4
lattice connectivity and stability is enhanced. Increasing Al-content
results in broadened spectra and an overall low field shift. The low
filed shift is mainly due to the substitution of Silicon by aluminum
in tetrahedral network sites. The number of Si(1Al), Si(2Al) and
Si(3Al) units increases. A clear discrimination between bond angle,
connectivity and substitution effects on the chemical shifts and
Fig. 6. 27Al MAS NMR spectra of Al-MMS-100 after different steps of activation. From
bottom to top: as-synthesized and dried at 70 C, calcined at 550C, NH4-exchanged,
and the calcined NH4-form.
◦
Fig. 7. 29Si MAS NMR spectra of Al-MMS of with different Si/Al ratio (from left to right: dried as-synthesized, calcined at 550 ◦C and re-hydrated).

5
8
H. Kosslick et al. / Catalysis Today 152 (2010) 54–60
Fig. 9. Comparison of NH3-TPD curves of ammonia loaded and ammonium
exchanged Al-MMS-100: with Si/Al = 13.5 and Si/Al = 21.9.
higher site concentration especially at low temperature between
◦
+
1
20 and 300 C. Desorption of ammonia from NH exchanged
4
◦
Al-MMS start at 350 C for AL-MMS (Si/Al = 13.5). The desorption
temperature increases with growing Si/Al ratio to ca. 400 C for
◦
Al-MMS (Si/Al = 21.9). Hence, it can be concluded, that the Si/Al
ratio has a great influence on the acidic strength of BS even in
large mesoporous networks with amorphous wall structures. The
acid strength of sites is relatively strong as indicated by high
desorption temperature of ammonia from ammonium exchanged
◦
samples starting at 350–400 C in the NH -TPD spectra. The high-
3
◦
temperature desorption branch above 500 C is most likely due to
desorption of re-adsorbed ammonia from strong Lewis sites.
The chemical stability of Al-MMS materials was investigated by
treatment of calcined and silylated samples in propanol, water and
◦
sodium carbonate solution (pH = 11.4) at 60 C. temperature. Textu-
Fig. 8. Temperature-programmed FTIR spectra of pyridine desorption from H,
ral changes were studied by nitrogen sorption measurements. For
comparison, a Si-SBA-15 was included. After treatment in alkaline
solution, a 30% loss in specific surface area was found with Si-SBA-
Al-MMS-100 (above) and (below) Si-MMS (top—pyridine adsorbed at room tem-
◦
perature; thereafter, from top to bottom, starting desorption of pyridine 100 C and
◦
to 400 C in steps of 50 K).
1
5. In contrast, the Al-substituted MMS samples showed only minor
appearance of the 29Si NMR spectra requires more sophisticated
investigations.
loss by ca. 5%. This result shows the advanced stability of Al con-
taining MMS. Organosilylation stabilizes generally the materials.
However, a loss of grafted silanes of 10–20% based on N-analysis
was observed for both types of materials after treatment in sodium
carbonate solution.
FTIR spectra of adsorbed pyridine the presence of Brønsted
(
1
BS) and Lewis sites (LS) indicated by the absorbance at 1548 and
445 cm , respectively (Fig. 8). The concentration of both the types
−1
of sites increases with growing Al content. Some BS are strong.
◦
−1
Even at 400 C some intensity of the 1548 cm absorbance related
BS sites remains in the spectrum. In contrast, pure Si-MMS is not
acidic. Neither a BS band nor strong LS giving rise to absorbance at
3.3. Catalysis
3.3.1. Acid catalysis
−1
1
445 and ca. 1630 cm are observed. Coordinative bound pyridine
is desorbed below 200 C.
The acidity of Al-MMS samples was additionally inves-
tigated by temperature-programmed ammonia desorption
experiments—NH -TPD (Fig. 9). To distinguish between LS and
The catalytic performance of Al-MMS samples was tested in the
benzoylation of anisole with benzoic anhydride (Scheme 1).
This reaction is sensitive to Brønsted acid sites. The reactiv-
ity observed with Al-MMS samples confirms the Brønsted acidic
character of acid sites. The conversion increases with Al-content
of Al-MMS samples (Fig. 10). The Al-content is related to the con-
centration of Brønsted sites. This confirms conclusion derived from
◦
3
+
BS bound ammonia, ammonia loaded H, Al-MMS as well as NH4
exchanged Al-MMS were studied. Ammonia loaded samples show
Scheme 1.

H. Kosslick et al. / Catalysis Today 152 (2010) 54–60
59
Fig. 11. 13C MAS NMR spectrum of N-(dimethyl) aminopropyl grafted Al-MMS-100
4- signal of methyl group C-atoms, signal 1, 2 and 3 belonging to the different
C-atoms of the propyl group).
(
Fig. 10. Influence of the Brønsted site concentration in Al-MMS samples, expressed
as ammonium ion exchange capacity N, on the catalytic activity (conversion). Test
reaction: benzoylation of anisole with benzoic anhydride.
limes and line intensity ratios of carbon nuclei at different positions
in the propylsilyl group.
27Al NMR spectra and FTIR and TPD experiments. The Maximum
conversion of ca. 23% was reached Al-MMS sample with Si/Al = 5.8.
Deviations from the straight line in Fig. 10 are probably due to
pore blockages. Indeed, propylsulfonic grafted Al-MMS with high
loadings (up to 1.3 mmol/g) were surprisingly not catalytic active.
Obviously, starting materials are adsorbed at acid sites at the low
temperature of reaction. Thereby mobility is restricted in the con-
fined space of the pores. Elemental analysis showed enhanced C:S
ratios for all sulfonic acid functionalized samples. The grafting of
propylsulfonic acid functions was proven by 13C MAS NMR spec-
troscopy. Due to partial pore filling, the specific pore volume and
pore diameter were decreased.
The catalytic activity of the base functionalized amino alky-
lamino grafted MMS was studied in the Michael addition of
methyl-vinyl-ketone and 2-methyl-cyclohexane-1,3-dione to the
bulky 2-(␥-oxybutyl)-2-methyl-cyclohexan-1,3-dione (Scheme 2).
The conversions over mesoporous catalysts of different pore
size are shown are shown in Fig. 13. Basic functionalized A-MMS
5
0 and Al-MMS-100 catalysts were used. For comparison, also a
small porous MCM-48 catalyst was tested. The on-set conversion is
highest with aminopropyl silyted MMS-100 and MCM-48 samples
(top curve in Fig. 13), whereas N-(dimethyl) aminopropyl modi-
fied MCM-48 (middle curve) and Al-MMS (bottom curve) are less
active. Possibly, the secondary amine sites are less accessible in
the confined space of nanopores. The high activity of functional-
ized MCM-48, which has a pore size of ca. 30 Å, tends to show
the importance of the pore connectivity besides the pore size.
MCM-48contains a pseudo three-dimensional pore system which
Y-shaped pore intersections providing concave and convex pore
surfaces. All catalysts work highly selective (97%). Nearly complete
conversion was achieved after 20 h.
3.3.2. Base catalysis
Base catalysts were obtained by grafting of aminopropyl and N-
(
dimathyl) aminopropyl functions. The grafting was controlled by
CHN analysis of samples, nitrogen sorption experiments (Fig. 11)
13
and C MAS NMR spectrometry (Fig. 12). All confirm the grafting by
nitrogen and carbon content and C:N ratio, decreased pore volume
and size as well by the appearance of the corresponding 13C NMR
Fig. 12. Decrease of nitrogen adsorption capacity and pore size caused by grafting of aminopropyl functions in the pores of Al-MMS-100 B compared to the unloaded sample,
Nitrogen sorption isotherms (left), pore size distribution (right).

6
0
H. Kosslick et al. / Catalysis Today 152 (2010) 54–60
Scheme 2.
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Acknowledgement
The authors thank Drs. M. Schneider and U. Bentrup for kind
providing X-ray difraction and FTIR measurements.