Al-TUD-1: Introducing tetrahedral aluminium

Article abstract of DOI:10.1016/j.molcata.2012.11.018

Tetrahedral aluminium was synthesized inside the TUD-1 structure with a low Si/Al ratio. The Al ion in its tetrahedral structure was stabilized by sodium as a counter cation. However, the tetrahedral aluminium species in the TUD-1 structure reverted to a mixture of hexa-, penta- and tetrahedral coordination sites after the exchange of sodium cations with protons. The Al-TUD-1 materials were found to be catalytically active for the acetal protection of benzaldehyde.

Full text of DOI:10.1016/j.molcata.2012.11.018

Journal of Molecular Catalysis A: Chemical 368–369 (2013) 88–94
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Al-TUD-1: Introducing tetrahedral aluminium
Selvedin Telalovic´ a,1, Sanjib Kumar Karmeea, Anand Ramanathanb,2, Ulf Hanefelda,∗
a Gebouw voor Scheikunde, Afdeling Biotechnologie, Technische Universiteit Delft, Julianalaan 136, 2628BL Delft, The Netherlands
b Center for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, KS 66047, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Tetrahedral aluminium was synthesized inside the TUD-1 structure with a low Si/Al ratio. The Al ion in its
tetrahedral structure was stabilized by sodium as a counter cation. However, the tetrahedral aluminium
species in the TUD-1 structure reverted to a mixture of hexa-, penta- and tetrahedral coordination sites
after the exchange of sodium cations with protons. The Al-TUD-1 materials were found to be catalytically
active for the acetal protection of benzaldehyde.
Received 22 October 2012
Accepted 23 November 2012
Available online 1 December 2012
Keywords:
© 2012 Elsevier B.V. All rights reserved.
Solid acid catalyst
Mesoporous materials
Tetrahedral aluminium
Aldol condensation
Acetal formation
1. Introduction
SBA-15 (Santa Barbara Amorphous material) [3]. More recently,
TUD-1 (Technische Universiteit Delft) was described, which has
In 1874 van’t Hoff and Le Bel independently from each other
described the tetrahedral structure of the carbon atom [1]. With
this decisive step three dimensional structures became a mainstay
in chemistry. The possibility of a tetrahedral structure of aluminium
hence also was recognized. The importance of this tetrahedral
structure for catalysis was thoroughly recognized with the advent
of zeolites as highly versatile, crystalline and nano-porous catalysts.
Tetrahedral aluminium induces Brønsted acidity, since the nega-
tively charged aluminium needs a counter ion, such as a proton.
On the other hand, calcination leads to loss of water and conse-
quently aluminium does not fulfil the octet rule and displays Lewis
acidity (Scheme 1). The crystalline character of zeolites allows for a
good tunability of this acidity, however due to their small pore-
size zeolites cannot be applied as catalysts for larger molecules
[2].
When preparing mesoporous silicates acidity can again be
induced by incorporating aluminium, thus allowing catalytic
conversion of larger molecules [3–5]. Furthermore, they can
function as carriers for covalent or non-covalent immobilisation
of chiral catalysts again due to incorporation of metals such as
aluminium [6–9]. The most prominent examples of mesoporous
molecular sieves are MCM-41 (Mobil Composition of Matter) and
several advantages: tuneable pore size (5–50 nm) and surface
area (500–1000 m2 g−1) and a three-dimensional pore structure,
leading to less mass-transfer limitations than experienced in the
one-dimensional pores of MCM-41 [5,10]. In addition, the material
is much more stable. Its synthesis requires complexing reagents
which are less expensive than the surfactants employed in the
synthesis of MCM-41 or tri-block polymers for synthesis of SBA-15.
Moreover, the synthesis of TUD-1 is very flexible and even allows
the use of environmentally benign SiO2 instead of commonly
employed tetraethyl orthosilicate (TEOS) [11].
The pore size and surface area determining step during the syn-
thesis of TUD-1 is the hydrothermal treatment. By varying the
duration, textural properties can be varied. The introduction of
metals into the amorphous siliceous TUD-1 structure is straight-
forward. However, due to differences in ion radius and/or valence
compared to silicon, metals often display different types of coordi-
nation in the framework. A typical example is the incorporation of
aluminium into the TUD-1 matrix giving rise to tetra-, penta- and
hexa-coordinated aluminium sites [4,12–14].
In heterogeneous catalysis and catalysis in general it is impor-
tant to have well defined catalytic sites [15]. The aim of this study
was therefore to increase the amount of tetrahedrally coordinated
aluminium inside the siliceous TUD-1 matrix. Tetrahedrally coor-
dinated aluminium prepared from a highly defined and reactive
precursor gives rise to Brønsted acid sites that can be employed
in catalysis such as the FCC [16–18]. Additionally they can also
act as anchoring sites for different ionic chiral catalysts employing
non-covalent ion-exchange techniques [9,19]. In that respect two
different aluminosilicates, namely H-Al-TUD-1 and Na-Al-TUD-1
∗
Corresponding author. Tel.: +31 15 278 9304; fax: +31 15 278 1415.
E-mail address: u.hanefeld@tudelft.nl (U. Hanefeld).
Current address: Anorganische Chemie en Katalyse, Universiteit Utrecht, Nobel-
1
straat 2a, 3512EN Utrecht, The Netherlands.
2
Tel.: +1 785 864 1631; fax: +1 785 864 6051.
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.11.018

S. Telalovi´c et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 88–94
89
dissolved in absolute ethanol (27.65 g, 0.60 mol) and anhydrous
2-propanol (27.04 g, 0.45 mol).
After the addition of demineralised water the mixture was
stirred for additional 0.5 h at room temperature followed by aging
for 6 h. The resulting wet gel was dried at 70 ◦C for 21 h and 2 h
at 98 ◦C. Hydrothermal treatment was performed in an autoclave
with Teflon insert at 160 ◦C for 19 h. Finally the solid was calcined
in air (with 1 ◦C min−1 to 550 ◦C, 4 h at 550 ◦C, with 1 ◦C min−1 to
600 ◦C, 10 h at 600 ◦C).
Scheme 1. Models for Brønsted and Lewis acid sites.
were synthesized and the impact of different cations on the amount
of tetrahedrally coordinated aluminium as well as the textural
properties was evaluated.
2.2.2. Synthesis of Na-Al-TUD-1 (Si/Al = 4)
Two Na-Al-TUD-1’s were prepared with different Na/Al ratios.
Samples were denoted as Na-Al-TUD-1.4 and Na-Al-TUD-0.7. The
last number standing for the final Na/Al ratio determined by ICP.
The synthesis of Na-Al-TUD-1.4 was as follows: To aluminium iso-
propoxide (4.24 g, 0.02 mol) anhydrous 2-propanol was added. The
entire mixture was stirred for 2 h, then tetraethyl orthosilicate
(17.30 g, 0.08 mol) was added as silica source followed by drop-
wise addition of a solution of triethanolamine (12.50 g, 0.08 mol)
and demineralised water (5.03 g, 0.28 mol). Finally, to the clear
mixture NaOH (0.97 g, 0.02 mol) dissolved in demineralised water
(11.46 g, 0.64 mol) was added dropwise under vigorous stirring. The
obtained molar composition was 1 SiO2:0.125 Al2O3:1 TEA:0.29
NaOH:11 H2O.
2. Experimental
2.1. Instruments
Powder X-ray diffraction (XRD) patterns were obtained on
a Philips PW 1840 diffractometer equipped with a graphite
monochromator using CuK␣ radiation.
The textural properties of the materials were characterized by
volumetric N2 physisorption at 77 K using Micromeritics ASAP 2010
equipment. Prior to the physisorption experiment, the samples
were dried overnight at 573 K (P ≤ 10−2 Pa). From the nitrogen sorp-
tion isotherms, the specific surface area SBET, the pore diameter
dP,BJH and the pore volume Vp,BJH were calculated. The latter two
were determined from the adsorption branch of the nitrogen sorp-
tion isotherms as typically applied for TUD-1-type materials [5,10].
Chemical analysis of Si, Al and Na were performed in duplet by
dissolving the samples in 1% HF and 1.25% H2SO4 solution and mea-
suring them with Inductively Coupled Plasma – Optical Emission
Spectroscopy (ICP-OES) on a Perkin-Elmer Optima 3000dv instru-
ment.
The synthesis of Na-Al-TUD-0.7 was slightly adjusted. Before
the final addition of NaOH dissolved in water tetraethyl ammo-
nium hydroxide (TEAOH, 35 wt% in H2O, 4.37 g, 0.01 mol) was
added. The molar composition was 1 SiO2:0.125 Al2O3:1 TEA:0.175
NaOH:0.125 TEAOH:11 H2O.
The obtained gels were left to stir for 0.5 h and poured into a
porcelain bowl to age overnight. After drying the gels at room tem-
perature they were dried at 98 ◦C for 21 h followed by hydrothermal
treatment at 180 ◦C for 21 h in an autoclave equipped with Teflon
inserts. Finally solids were calcined in air at 600 ◦C for 10 h with a
heating rate of 1 ◦C min−1
.
Solid state 27Al MAS NMR experiments were performed at 9.4 T
on a Bruker Avance-400 MHz spectrometer operating at 104.2 MHz
with a pulse width of 1 ms. 4-mm zirconia rotors with a spinning
speed set to 11 kHz were used. The chemical shifts are reported
with respect to Al(NO3)3 as an external standard at ı = 0 ppm.
Transmission electron microscopy (TEM) was performed by
using a Philips CM30T electron microscope with a LaB6 filament
as the source of electrons operated at 300 kV.
Temperature-programmed desorption of ammonia (NH3-TPD)
measurements were carried out on a Micromeritics TPR/TPD 2900
instrument equipped with a thermal conductivity detector (TCD).
About 30 mg sample was pre-treated at 550 ◦C in a flow of He
2.2.3. Ion exchange
2.5 g of Na-Al-TUD-1 sample was stirred with 100 mL portions
of 1 M NH4Cl for five times, first two exchanges were performed at
room temperature and last three at 80 ◦C. After each ion-exchange
step, catalyst was washed with 50 mL portions of demi water. After
ion exchange catalyst was placed in the oven to be dried at a tem-
perature of 75 ◦C overnight followed by calcination in air at 550 ◦C
for 5 h at heating rate of 1 ◦C min−1
.
2.2.4. Synthesis of 2-phenyl-1,3-dioxolane
(10 mL min−1) for 1 h. Then 9.98 vol% NH3 in He (10 mL min−1
)
Freshly distilled benzaldehyde and dry ethylene glycol were
used for all the reactions. The reactions were carried out under N2
in a 10 mL Schlenk flask. Al-TUD-1 samples were calcined at 550 ◦C
and used in the reactions. Prior to its use the ion exchanged Na-
Al-TUD-1 (viz. H-Al-TUD-1, H-Al-TUD-0.7-IE and H-Al-TUD-1.4-IE)
and Na-Al-TUD-1 catalysts were activated at 120 ◦C under high
vacuum for 1 h.
was adsorbed at 100 ◦C for 30 min. Subsequently, a flow of He
(10 mL min−1) was passed through the reactor for 30 min to remove
any weakly adsorbed NH3 from the sample. Desorption of NH3
was monitored in the range of 100–600 ◦C at a heating rate of
10 ◦C min−1
.
2.2. Materials
Benzaldehyde (106 mg, 1 mmol), was added to ethylene glycol
(5 mL) and catalyst (50 mg). Then the reaction was stirred at 78 ◦C
for 24.5 h. Aliquots 30 ␮L were taken at different time intervals and
the samples were centrifuged for 5 min. From this 20 ␮L of samples
were diluted in 980 ␮L of mobile phase (H2O:CH3CN: 95:5) and
analysed by HPLC.
All chemicals were obtained from commercial sources viz.
Sigma–Aldrich, Acros and Alfa Aesar.
2.2.1. Synthesis of Al-TUD-1 (Si/Al = 4)[4]
Aluminium isopropoxide (6.12 g, 0.03 mol) was added to a
mixture of absolute ethanol (27.65 g, 0.60 mol) and anhydrous 2-
propanol (27.04 g, 0.45 mol) at 45 ◦C; followed by the addition of
tetraethyl orthosilicate (24.99 g, 0.12 mol) and tetraethylene gly-
col (29.17 g, 0.15 mol). The entire mixture was stirred for an hour
before dropwise addition of demineralised water (5.41 g, 0.30 mol)
2.2.5. Reaction of benzaldehyde with n-butanol
The reaction of benzaldehyde with n-butanol was carried out
under N2 in a 10 mL Schlenk flask. Benzaldehyde (106 mg, 1 mmol)
was added to a mixture of n-butanol (296, 4 mmol) and H-Al-TUD-
1 catalyst (50 mg). Then the reaction mixture was stirred at 78 ◦C

90
S. Telalovi´c et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 88–94
A
H-Al-TUD-1
Na-Al-TUD-0.7-IE
Na-Al-TUD-1.4-IE
Na-Al-TUD-0.7
Na-Al-TUD-1.4
Na-Al-TUD-0.7
H-Al-TUD-1
Na-Al-TUD-1.4
0.0
B
0.2
0.4
0.6
0.8
1.0
0
1
2
3
4
2
10 20 30 40 50 60 70
degrees
Relative Pressure (p/p )
θ
0
5
4
3
2
1
0
Fig. 1. XRD of H-Al-TUD-1, Na-Al-TUD-1.4, Na-Al-TUD-0.7, Na-Al-TUD-1.4-IE and
Na-Al-TUD-0.7-IE, Si/Al ratio of 4.
Na-Al-TUD-0.7
Na-Al-TUD-1.4
H-Al-TUD-1
for 24 h. After this the reaction mixture was washed with ace-
tonitrile (1 mL). Then it was transferred to a 1.5 ml eppendorf and
centrifuged for 2 min. The organic layer was separated from the
catalyst and analysed by HPLC.
2.2.6. TLC and HPLC analysis
Thin-layer chromatography (TLC) was performed using pre-
coated silica gel SIL G/UV 254 plates. HPLC separation was carried
out using following instrument and method: Pump: waters 590,
UV detector: waters 486, UV, flow: 1 mL min−1. A CHROMOLITH
Speed ROD, RP-18e, 50–4.6 mm, column is used for this separation
with mobile phase H2O/CH3CN (95v:5v). The retention time of ben-
zyl alcohol, benzaldehyde and 2-phenyl-1,3-dioxolane was found
to be 3.2, 6.4 and 11.8 min. The amount of 2-phenyl-1,3-dioxolane
was determined by using a calibration curve.
10
100
1000
Pore Diameter (Å)
Fig. 2. N2-physisorption isotherms (A) and pore size distribution (B) of Na-Al-TUD-1
and H-Al-TUD-1 samples.
3. Results and discussion
this amount of tetrahedrally coordinated aluminium is lower
(Table 1, entries 1 and 2) [12]. Tetrahedrally coordinated alu-
minium is responsible for Brønsted acid sites essential for catalysis
or as anchoring sites for anionic chiral catalysts. H-Al-TUD-1
possesses differently coordinated aluminium sites which can
induce undesired side reactions in the various applications of the
material. Hexa-coordinated aluminium often recognized as extra
framework species can affect adsorption and catalytic properties
[21]. Earlier attempts to remove the penta- and hexa-coordinated
Al from Al-TUD-1 with a Si/Al ratio of 4 by a post synthesis washing
step with HCl solution was only partially successful [19]. Therefore
here we aimed at a solution to increase the amount of tetrahedrally
coordinated aluminium by an adjustment of previously published
synthesis method [12].
For TUD-1 materials with a Si/Al ratio 30 it was described in
the literature that complete framework incorporation as tetrahe-
dral Al was achieved. However, this was due to crystallization and
the formation of ZSM-5 in the TUD-1 structure [22]. In essence the
material was ZSM-5 with a TUD-1 superstructure. Consequently a
new approach had to be found. It is generally known that when
aluminosilicates are synthesized in the presence of Na+ aluminium
becomes mainly tetrahedrally incorporated [23]. For mesoporous
MCM-41 structural aluminium is thermally less stable than, for
example, in zeolite Y, because the MCM-41 structure lacks strict
crystallographic order at the atomic level and very small H+ cations
cannot satisfy the framework charge balance as efficiently as Na+
3.1. Material preparation and characterisation
Amorphous aluminosilicate of TUD-1 structure, with a low Si/Al
ratio of 4 (H-Al-TUD-1) was synthesized in order to ensure high
ion-exchange capability. Tetraethylene glycol was used as a com-
plexing reagent, since it ensures the best results in the synthesis of
TUD-1 based materials with low Si/Al ratios [9,12,20]. The meso-
porous character of H-Al-TUD-1 was determined by XRD as well
as N2 physisorption measurements. The sample possesses a peak
at low angles, situated at 0.5 2ꢀ degrees, characteristic for meso-
porous materials (Fig. 1), however with much lower intensity than
obtained in other samples. At higher degrees no crystalline alu-
mina, ı- or ꢀ-Al2O3, was detected. Minor peaks found at 38, 44 and
65 2ꢀ degrees are due to aluminium from the sample holder. The
mesoporous character was further confirmed by N2 physisorption
measurements characteristic for mesoporous materials (Fig. 2). A
type IV isotherm with a type H1 hysteresis loop characteristic for
mesoporous samples with a pore size distribution around 16 nm,
pore volume of 1.2 cm3 g−1 and surface area around 485 m2 g−1
(Table 1) is clearly detectable.
Introduction of aluminium into the TUD-1 matrix generally
creates hexa-, penta- and tetrahedrally coordinated aluminium.
According to 27Al-MAS-NMR measurement the amount of tetra-
hedrally coordinated aluminium (54 ppm) in H-Al-TUD-1 is 28%
(Table 1, Fig. 3). Compared to previously synthesized H-Al-TUD-1

S. Telalovi´c et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 88–94
91
Table 1
Data from ICP, N2-physisorption and NH3-TPD measurements of H-Al-TUD-1 and Na-Al-TUD-1 samples.
b
b
M-TUD-1
nSi/Al
nNa/Al
Tetrahedral aluminium (%)
SBET (m2 g−1
)
dP,BJH (nm)
VP,BJH (cm3 g−1
)
NH3-TPD (mmol g−1
)
H-Al-TUD-1
5.4
4
4.2
3.6
3.3
6.1
–
–
1.4
0.7
0.06
0.07
28
43
>95
>95
33
485
600
255
384
165
301
16
15
9.1
2.9
16
1.2
1.1
0.8
0.3
0.71
0.32
1.03
–
0.85
0.91
0.37
1.07
H-Al-TUD-1a
Na-Al-TUD-1.4
Na-Al-TUD-0.7
Na-Al-TUD-1.4-IE
Na-Al-TUD-0.7-IE
36
3.7
a
Ref. [12].
After calcination.
b
cations [24]. In the synthesis of Al-MCM-41 sodium aluminate was
employed as source of Al. This ensured a high degree of struc-
tural order and all of the aluminium was in tetrahedral position
[25–27]. However, our attempts to use either triethanolamine or
tetraethylene glycol as a template with sodium aluminate in the
TUD-1 synthesis, led to the synthesis of greyish material with a low
surface area. We therefore turned our attention to NaOH to intro-
duce Na+ instead of protons as charge balance for the tetrahedral
coordination of trivalent aluminium. In the synthesis of MCM-41,
application of NaOH allowed the synthesis of MCM-41 with Si/Al
ratio close to the Loewenstein limit (Si/Al ratio of 1) with all alu-
minium in tetrahedral coordination [28–30].
Applying NaOH during the synthesis two different samples Na-
Al-TUD-1 were successfully synthesized. Triethanolamine rather
than tetraethylene glycol (which is commonly used for synthesis
of H-Al-TUD-1 materials with low Si/Al as it is easier to remove
by calcination than triethanolamine) was employed as complex-
ing reagent and aluminium isopropoxide as source of aluminium.
The mesoporous character was demonstrated by XRD and N2-
physisorption measurements. XRD measurements revealed that
both samples synthesized display a strong peak at low angles char-
acteristic for mesoporous samples (0.8 2ꢀ degrees, Fig. 1).
The type IV isotherm obtained from N2-physisorption measure-
ments further confirmed the mesoporous characteristics of these
materials (Fig. 2). Their hysteresis loops are different from those
found in the H-Al-TUD-1. For the sample having a higher amount
of Na (Na-Al-TUD-1.4) the adsorption and desorption branches are
almost vertical and nearly parallel belonging to H1 hysteresis loop
characteristic for uniform, near cylindrical pores. In contrast Na-Al-
TUD-0.7 displays a H4 hysteresis loop where the desorption branch
is much steeper than the adsorption branch due to non-uniform
pores.
A
H-Al-TUD-1
Na-Al-TUD-1.4
Na-Al-TUD-0.7
The Na-Al-TUD-1 samples have a Si/Al ratio of around 4 (Table 1).
For the sample with higher amount of sodium, Na-Al-TUD-1.4, the
surface area is lower (255 m2 g−1) compared with Na-Al-TUD-0.7
(384 m2 g−1) due to presence of micropores. In contrast, the pore
volume as well as pore size are larger for the sample Na-Al-TUD-1.4.
Most importantly 27Al-MAS-NMR proves beyond doubt that virtu-
ally all Al in the Na-Al-TUD-1 samples is tetrahedrally coordinated
(Fig. 3, Table 1).
The synthesis of amorphous aluminosilicate of TUD-1 struc-
ture with well-defined tetrahedrally coordinated aluminium was
achieved by in situ introduction of Na+ as NaOH. This leads to
decrease of surface area, pore volume as well as the pore size
due to larger radius of the sodium cations in Na-Al-TUD-1 than
is the case with protons in H-Al-TUD-1. Depending on the amount
of sodium introduced uniform, near-cylindrical pores can be cre-
ated yielding a more ordered TUD-1 based catalysts. Even with a
Na/Al ratio lower than 1 almost all Al is tetrahedrally coordinated
and a Na-Al-TUD-1 amorphous and mesoporous in nature as H-Al-
TUD-1 is obtained. Clearly the presence of Na+ during the synthesis
of amorphous mesoporous TUD-1 materials leads to more defined
heterogeneous catalysts. This approach leads to accessible tetrahe-
dral aluminium sites inside the mesoprous Al-TUD-1 in contrast to
micro/mesoporous TUD-1 composites containing tetrahedral alu-
minium sites in zeolitic channels that were described earlier [22].
Synthesis of amorphous three-dimensional Al-TUD-1 with nearly
all of aluminium in tetrahedral coordination was achieved. Clearly
all three types of Al-TUD-1 have a mesoporous structure and a
typical rather amorphous organisation (Fig. 4).
300
200
100
0
ppm
-100
-200
-300
Na-Al-TUD-0.7 IE
Na-Al-TUD-1.4 IE
B
For the catalytic activity the acidity of the materials is essential.
While for immobilisation via ion exchange it is equally possible to
utilize Na+ or H+ as a counter ion of the tetrahedral Al, in catalysis
this is expected to play an important role. Acidity of these samples
was therefore determined by NH3-TPD (Fig. 5) [31]. The H-Al-TUD-
1, as expected, displays higher total acidity than the Na-Al-TUD-1’s.
300
200
100
0
ppm
-100
-200
-300
Fig. 3. 27Al-MAS-NMR spectrum of H-Al-TUD-1 and Na-Al-TUD-1 samples (A) and
ion-exchanged Na-Al-TUD-1 samples (B).

92
S. Telalovi´c et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 88–94
Fig. 4. TEM images of Na-Al-TUD-1.4 (upper left), Na-Al-TUD-0.7 (upper middle), H-Al-TUD-1 (upper right), Na-Al-TUD-0.7-IE (lower left) and Na-Al-TUD-1.4-IE (lower
right). The bars represent 20 nm.
Again Na-Al-TUD-0.7 in which not all tetrahedral Al is balanced by
a sodium ion is more acidic than Na-Al-TUD-1.4.
Na-Al-TUD-1.4-IE seem to be accessible (compare NH3 TPD results,
Table 1, entries 5 and 6). Na 27Al-MAS-NMR reveal that both ion-
exchanged samples contained tetra-, penta- and hexa-coordinated
Al (Fig. 3B). Clearly, they had undergone a structural rearrangement
during ion-exchange. This is also visible in the TEM images (Fig. 4).
To impart Brønsted acidity to the Na-Al-TUD-1 materials an
ion exchange was performed. With ammonium chloride an almost
complete exchange of Na+ against H+ was achieved (Table 1). For
Na-Al-TUD-0.7-IE acidity similar to H-Al-TUD-1 was achieved in
terms on number of acid sites. In terms of acid strength Na-Al-
TUD-0.7-IE possesses slightly higher amount of medium to strong
acid sites (NH3 desorption above 300 ◦C) and lower amount of weak
acid sites compared to H-Al-TUD-1 (Fig. 5). For Na-Al-TUD-1.4-IE,
however, a loss of structural integrity, surface area and pore size
was observed, combined with a loss of acidity (Table 1, Fig. 5). Hav-
ing comparable number of tetrahedrally coordinated aluminium
as Na-Al-TUD-0.7-IE, and lower Si/Al ratio, not all of acid sites in
3.2. Catalysis
All Al-TUD-1 materials were studied for their catalytic activi-
ties. Two reactions viz. (i) reaction of benzaldehyde with n-butanol
and (ii) the acid catalysed acetal formation by the reaction of benz-
aldehyde with ethylene glycol were studied, in order to ensure
comparability with earlier studies. Earlier, Zhou et al. carried out
the reaction of benzaldehyde with n-butanol using ZSM-5, TUD-1
and Al-TUD-1 [22]. The authors reported that an aldol reaction lead
to the formation of an unusual condensation product (Scheme 2).
Nevertheless, the structure of the condensation product reported
1.2
Na-Al-TUD-1.4
Na-Al-TUD-0.7
1.0
0.8
0.6
0.4
0.2
0.0
Na-Al-TUD-0.7-IE
H-Al-TUD-1
Na-Al-TUD-1.4-IE
100
200
300
400
500
600
Temperature (ºC)
Fig. 5. TPD-NH3 data of Na-Al-TUD-1.4, Na-Al-TUD-0.7, H-Al-TUD-1, Na-Al-TUD-
0.7-IE and Na-Al-TUD-1.4-IE.
Scheme 2. Reaction of benzaldehyde with n-butanol.

S. Telalovi´c et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 88–94
93
Even though the adjusted synthesis procedure of H-Al-TUD-
1 (introducing additional steps compared to earlier published
synthesis method [12]) does not lead to increased catalytic per-
formance in selected reactions, the presence of sodium instead of
protons as counter ions certainly provides interesting application
possibilities as a carrier instead of being a catalyst itself.
Scheme 3. Reaction of benzaldehyde with ethylene glycol.
4. Conclusions
by Zhou et al. is not similar to the aldol product (␤-hydroxy ketone)
[32–34]. Therefore, we have tested the H-Al-TUD-1 catalyst for
the reaction of benzaldehyde with n-butanol at 78 ◦C for 24 h. In
our case, no condensation product was observed and the HPLC
analysis revealed formation of benzyl alcohol (4%) (Scheme 2).
The formation of benzyl alcohol can easily be explained with a
Meerwein–Ponndorf–Verley reduction, a reaction that Al-TUD-1
was earlier shown to catalyse [35,36].
Furthermore, the catalysts were tested for the reaction between
benzaldehyde and ethylene glycol (Scheme 3). All reactions were
carried out at 78 ◦C for 24.5 h. It was found that, during the reaction
of benzaldehyde with ethylene glycol the two Na-Al-TUD cata-
lysts were virtually not active. Only Na-Al-TUD-0.7 having less
sodium than the Na-Al-TUD-1.4 showed some activity, a yield of
5.6% of 2-phenyl-1,3-dioxolane was obtained after 24.5 h. Acidic
H-Al-TUD-1 as a catalyst yielded 40% of 2-phenyl-1,3-dioxolane
after 0.5 h which remained constant up to 24.5 h (Fig. 6). Further-
more, ion-exchanged H-Al-TUD-1.4-IE and H-Al-TUD-0.7-IE were
used for the reaction of benzaldehyde with ethylene glycol under
similar reaction. The H-Al-TUD-1.4-IE and H-Al-TUD-0.7-IE catal-
ysed reactions gave a maximum yield of 51% and 53% after 4 h and
2 h respectively (Fig. 6).
We have successfully synthesized all tetrahedral aluminium
inside an amorphous mesoporous material of TUD-1 structure with
a low Si/Al ratio. Tetrahedral aluminium is effectively stabilized
only when sodium is employed as counter balancing cation. Upon
ion-exchange, replacing sodium cations by protons, aluminium
ions inside TUD-1 structure reverts back to combination of hexa-,
penta- and tetrahedral coordination sites as seen by synthesis of
H-Al-TUD-1 without the presence of a sodium salt. With sodium
as counter ion the materials displayed low acidity and thus low
catalytic activity. The synthesized Al-TUD-1 materials with pro-
tons as counter ions were catalytically active in the synthesis of
2-phenyl-1,3-dioxolane.
Acknowledgements
S.T. thanks NWO for generous support via a Mozaïek grant. A
senior research fellowship of the Technische Universiteit Delft to
S.K.K. is gratefully acknowledged.
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