Acetalization of heptanal over Al-SBA-1 molecular sieve

Article abstract of DOI:10.1016/j.catcom.2010.09.019

Al-SBA-1 (Si/Al = 40, 80 and 120) and Al,Mg-SBA-1 (Si/(Al + Mg) = 40 and 80) molecular sieves were synthesized and characterized. Acetalization of n-heptanal with methanol was studied under autogenous pressure between 80 and 150 °C. Since protonation of n

Full text of DOI:10.1016/j.catcom.2010.09.019

Catalysis Communications 12 (2010) 299–303
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Catalysis Communications
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Short Communication
Acetalization of Heptanal over Al-SBA-1 molecular sieve
⁎
K. Venkatachalam, M. Palanichamy, V. Murugesan
Department of Chemistry, Anna University, Chennai 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 June 2010
Received in revised form 14 September 2010
Accepted 16 September 2010
Available online 21 September 2010
Al-SBA-1 (Si/Al=40, 80 and 120) and Al,Mg-SBA-1 (Si/(Al+Mg)=40 and 80) molecular sieves were
synthesized and characterized. Acetalization of n-heptanal with methanol was studied under autogenous
pressure between 80 and 150 °C. Since protonation of n-heptanal was fast, addition of methanol to the same
formed hemiacetal slowly whereas conversion of hemiacetal to acetal was fast. The catalysts exhibited nearly
similar conversion irrespective of their difference in acidity, and all of them showed more than 80%
conversion either at 80 or 100 °C. Hence, it is evident that the difference in acidity is not so important in
differentiating the activity of the catalysts. The large pore size and hydrophilic and hydrophobic properties are
suggested to be the main factors that control acetalization.
Keywords:
SBA-1
Acetalization
Hemiacetal
Acetal
© 2010 Elsevier B.V. All rights reserved.
Vinyl ether
1. Introduction
different zeolites such as HY, Hß, HZSM-5 and HMOR by Thomas et al.
[19]. But zeolites exhibited mass transport limitations.
The development of one-pot preparative procedure offers signif-
icant advantages due to reduction in the number of synthetic steps,
significant increase in efficiency and prevention of isolation of inter-
mediates [1]. Acetalization is a one-pot process and it is a specific
requirement to protect the carbonyl group during manipulation of
multifunctional organic molecules, since the product acetals display
higher stability towards strong bases, Grignard reagents, lithium
aluminium hydride, strong oxidants and esterification reagents than
their parent carbonyl compounds [2]. Besides the interest of acetals
as protecting groups, many of them found direct applications
in fragrances and cosmetics [3,4], as food and beverage additives
[5,6] and in the synthesis of enantiomerically pure compounds
[7,8], steroids [9], pharmaceuticals [3,10] and polymers [11]. Since
their demand is high in the market, mild and eco-friendly acetaliza-
tion procedures with selective catalysts are important for acetaliza-
tion. Traditionally, acetalization of aldehydes and ketones is
performed using trimethyl orthophosphate in the presence of acid
catalysts such as HCl, H2SO4, p-toluene sulphonic acid and ferric
chloride [12,13]. Eco-friendly solid acid catalysts such as sulphated
zirconia and titania [14], Ce-exchanged Montmorillonite clay [15],
acidic zeolites [3,16,17] and siliceous mesoporous materials [18] were
also reported to be active for acetalization. One-pot acetalization of
cyclohexanone, acetophenone and benzophenone was carried out
using methanol over H-montmorillonite clay, silica, alumina and
Mesoporous solid acid catalysts are advantageous as they are non-
hazardous and free from mass transport limitations, possess high
surface area and provide easy catalyst recovery. Based on the
advantages of mesoporous materials, acetalization of n-heptanal
with methanol was attempted for the first time over SBA-1 molecular
sieves. The product, acetal, is used as a flavouring agent for beverages,
chewing gum and cordimonts. Acetalization of n-heptanal has not
been reported so far over solid acid catalysts in general and SBA-1 in
particular.
2. Experimental
2.1. Preparation of the catalysts
Al-SBA-1 molecular sieves were synthesized under acidic condi-
tions using cetyltriethylammonium bromide as the template. Tetra-
ethyl orthosilicate (TEOS, Aldrich, 98%) and aluminium hydroxide
were used in aqueous solution of hydrochloric acid (4.5 N) Cetyl-
triethylammonium bromide (CTEABr) was synthesized by reacting
equimolar amount of 1-bromohexadecane and triethylamine in
ethanol under reflux for 2 days. The resultant surfactant was purified
by recrystallization with chloroform and ethyl acetate mixture [20].
Al-SBA-1 was synthesized as follows: solution “A” was prepared
by adding CTEABr (2.46 g) to dil. HCl (4.5 N), cooled to 0 °C and
homogenized for 30 min. The pre-cooled (0 °C) TEOS and aluminium
hydroxide was then added to solution A under vigorous stirring which
continued for 5 h at 0 °C. The mixture was then heated at 100 °C for
1 h. The solid product was recovered by filtration and dried in an oven
overnight at 100 °C. The gel composition was: 1 TEOS: 0.0025–0.025
⁎
Corresponding author. Tel.: +91 44 22357023/22358645; fax: +91 44 2220066/
22201213.
E-mail address: v_murugu@hotmail.com (V. Murugesan).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.09.019

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K. Venkatachalam et al. / Catalysis Communications 12 (2010) 299–303
Al(OH)3: 0.2 CTEABr: 10-56 HCl: 125-700 H2O. The as-synthesized
material was calcined in air at 550 °C for 6 h. Al,Mg-SBA-1 (Si/(Al+
Mg)=40 and 80) molecular sieves were synthesized by adopting
the same procedure. Magnesium acetate was used as the precursor
for magnesium.
These patterns matched well with the previous reports [21,22].
However, the intensity of patterns decreased with increase in the Si/Al
ratio. Hence, isomorphic substitution of silicon by aluminium is
suggested to decrease the orderly arrangement of pores. The same
observation was also noted in Al,Mg-SBA-1 catalysts. The intensity
of the patterns of Al,Mg-SBA-1 (Si/(Al+Mg)=40) was less than Al,
Mg-SBA-1 (Si/(Al+Mg)=80). In addition, the position of the pat-
terns in both Al-SBA-1 and Al,Mg-SBA-1 are shifted to lower 2θ with
the decrease in aluminium content. Hence, the interplanar spacing of
the planes might increase with a decrease in the aluminium content.
2.2. Characterization
The X-ray diffraction (XRD) patterns of calcined Al-SBA-1 and Al,
Mg-SBA-1 catalysts were recorded on a PANalytical X'Pert Pro X-ray
diffractometer using CuKα radiation. The low angle diffractograms
were recorded in the 2θ range 1.0–10° with a 2θ step size of 0.01°
and a step time of 10 s at each point. Surface area was measured
by nitrogen adsorption at 77 K on a ASAP-2010 porosimeter from
Micromeritics Corporation (Norcross, GA, USA). The density and
strength of acid sites were determined by temperature programmed
desorption of ammonia (TPD NH3) on a Micromeritics chemisorb
2750 pulse chemisorption system. FT-IR spectra of the materials
were recorded on a Nicolet (Avatar 360) FT-IR spectrometer. Scanning
electron microscopic (SEM) images were obtained from a SEM
(Hitachi S-4500LV) instrument.
3.2. BET
The nitrogen adsorption-desorption isotherms of Al-SBA-1 (Si/
Al=40, 80 and 120), Al,Mg-SBA-1 (Si/(Al+Mg)=40, and 80) are
shown in Fig. 2. The isotherms are quite similar and a steep increase in
all the isotherms due to pore condensation occurred below 0.1 (p/p0)
for all the samples. Similar isotherms were also reported by Dai et al.,
Che et al. and Balasubramanian et al. [23–25]. The pore condensation
reached the maximum for Al-SBA-1 (40) close to relative pressure
(p/p0) of 0.2 whereas the same is shifted to relative pressure (p/p0)
of 0.3 for Al-SBA-1 (80) and Al-SBA-1 (120). Hence, the pore size of
these catalysts is larger than Al-SBA-1 (40). Al,Mg-SBA-1 (Si/(Al+
Mg)=40 and 80) showed nearly similar p/p0 values for pore
condensation and the curves are nearly similar to Al-SBA-1 (40).
Based on the smooth rise in pore condensation, it could be inferred
that there may be uniform distribution of pores. The surface area
varied from 1050 to 1360 m2/g and pore size from 2.48 to 2.78 nm
(Table 1). Though the relative pressure for pore condensation is low,
it cannot be presumed that materials are microporous. As the surface
area and pore size are high and they show XRD patterns at low
angles which are characteristics of mesoporous materials.
2.3. Catalytic studies
A mixture of n-heptanal (1 mol), methanol (3 mol) and a freshly
activated catalyst (0.05 g) was taken in a 50 ml autoclave. The
autoclave was then placed in an oven and the temperature kept at 80,
100, 120 or 150 °C for 12 h. It was then cooled and the sample cen-
trifuged. The centrifugate was analyzed with a gas chromatograph
(Shimadzu GC-17A) using DB-5 capillary column (30 m×0.25 m×
0.25 μm) equipped with a FID detector. The identification of prod-
ucts was also made using GC-MS Perkin Elmer Auto System XL gas
chromatograph (Perkin-Elmer elite series PE-5 capillary column,
30 m×0.25 mm×1 μm) equipped with a Turbo mass spectrometer
(El, 70 eV) with helium as carrier gas at a flow rate of 1 ml/min.
3.3. TPD—ammonia
The acidity of mesoporous materials was determined by TPD
(NH3) and the results are presented in Table 2. All the materials
possess weak and medium acid sites, and this classification is mainly
based on the desorption of ammonia between 150 and 250 °C for
weak acid sites and 250 and 350 °C for medium acid sites. This is the
common distribution observed for all mesoporous materials. There is
no systematic variation of total acidity with the variation of Si/Al ratio.
It is an indirect evidence for non-framework alumina in them. But
further attempts were made to prove such non-framework species.
3. Results and discussion
3.1. XRD
The XRD patterns of Al-SBA-1 (Si/Al=40, 80 and 120), Al,Mg-SBA-
1 (Si/(Al+Mg)=40 and 80) are shown in Fig. 1. Al-SBA-1 molecular
sieves showed characteristic patterns corresponding to (200), (210)
and (211) of a three-dimensional cubic structure (space group Pm3n).
Fig. 1. XRD patterns of calcined (a) Al,Mg-SBA-1 (40), (b) Al,Mg-SBA-1 (80), (c) Al-SBA-1
Fig. 2. Adsorption isotherms of (a) Al,Mg-SBA-1 (40), (b) Al,Mg-SBA-1 (80), (c) Al-SBA-
(40), (d) Al-SBA-1 (80), and (e) Al-SBA-1 (120).
1 (40), (d) Al-SBA-1 (80) and (e) Al-SBA-1 (120).

K. Venkatachalam et al. / Catalysis Communications 12 (2010) 299–303
301
Table 1
Textural properties of the catalysts.
Catalyst
BET surface area
Pore volume
(cm3/g)
Pore size
(nm)
(m2/g)
Al-SBA-1 (40)
Al-SBA-1 (80)
Al-SBA-1 (120)
Al,Mg-SBA-1 (40)
Al,Mg-SBA-1 (80)
1220
1305
1360
1050
1230
0.70
0.73
0.74
0.67
0.71
2.56
2.75
2.78
2.53
2.48
3.4. FT-IR
The FT-IR spectra of calcined Al-SBA-1 (Si/Al=40, 80 and 120), Al,
Mg-SBA-1 (Si/(Al+Mg)=40, and 80) are shown in Fig. 3. All the
spectra showed similar features irrespective of aluminium or mag-
nesium content. The presence of defective –OH groups in all the
catalysts are confirmed by its stretching vibration, which occurred
around 3740 cm−1. The broad envelope between 3000 and 3740 cm−1
is due to –OH stretching vibration of water. The broadening is due to
hydrogen bonding. The asymmetric stretching vibrations of Si-O-Si
and Si-O-Al (or Mg) bonds gave intense broad bands between 1000
Fig. 3. FT-IR spectra of calcined (a) Al,Mg-SBA-1 (40), (b) Al,Mg-SBA-1 (80), (c) Al-SBA-
1 (40), (d) Al-SBA-1 (80) and (e) Al-SBA-1 (120).
SBA-1 catalysts, Al-SBA-1 (40) showed higher conversion than
the others. But the difference between the conversion of Al-SBA-1
(Si/Al=40, 80 and 120) catalysts is not significant. Hence the acidity
of the catalysts alone is not the important factor to account for their
activity. Similar results were also reported by Rabindran Jermy et al.
[26] for acetalization of cyclohexanone with different alcohols over
Al-MCM-41 materials. Obviously, the hydrophilic and hydrophobic
properties of the catalysts may play a prominent role in acetaliza-
tion. Hence, Al-SBA-1 (80) and Al-SBA-1 (120) with low acidity
and high hydrophobic property in comparison to Al-SBA-1 (40) can
better adsorb hydrophobic n-heptanal than Al-SBA-1 (40) in order
to exhibit high conversion. The hydrophobicity of the catalysts
decreases in the order: Al-SBA-1 (40)bAl-SBA-1 (80) b Al-SBA-1
(120). This order is based on the Si/Al ratio of the catalysts. The
intensity of –OH stretching vibration of water around 3400 cm−1
and its bending vibration at about 1600 cm−1 could be used for the
comparison of hydrophilic and hydrophobic property of the catalysts.
However, same amount of catalysts was not used for comparison.
The n-heptanal conversion over Al,Mg-SBA-1 also exhibited
similar trends as that of Al-SBA-1 catalysts. The n-heptanal conversion
decreased with increase in temperature, and both the catalysts
showed nearly similar conversion. Hence, a slight increase in the
acidity of Al-SBA-1 (40) may be compensated by a slight increase in
hydrophobicity of Al-SBA-1 (80) in order to exhibit nearly similar
conversion. Although these catalysts possess more density of acid
sites than the corresponding Al-SBA-1 catalysts, nearly similar
conversion of n-heptanal indicates that slightly high hydrophobicity
and 1300 cm−1. Their bending modes occurred just below 1000 cm−1
.
The –OH stretching vibrations of water in all the spectra were broad
and intense, indicating significant hydrophilic property.
3.5. SEM
The SEM image of Al-SBA-1 (40) is shown in Fig. 4. Most of the
particles revealed spherical morphology with different size, but larger
particles dominated over smaller particles. These larger particles have
a size range of 250–400 nm. Similar features were also shown by the
images of other molecular sieves (figure not shown). But the size of
the particles is slightly smaller than that of Fig. 4. The morphology
of the particles is nearly same as that reported earlier [25].
3.6. Catalytic studies
3.6.1. Effect of temperature
The results of acetalization over Al-SBA-1 (Si/Al=40, 80 and 120)
and Al,Mg-SBA-1 (Si/(Al+Mg)=40, 80) are presented in Table 3.
The major products were hemiacetal, acetal, and vinyl ether. The
n-heptanal conversion was less than 50% when the reaction was
carried out in the liquid phase at 1 atm. below the boiling point of
methanol (64.7 °C). Hence, the reaction was carried out under
autogenous pressure and above the boiling point of methanol. The
n-heptanal conversion decreased with the increase in temperature
over each of the catalysts. Hence, high temperatures well above the
boiling point of methanol are not favored for this reaction. Either the
chemisorption of n-heptanal on the Bronsted acid sites or diffusion
of methanol from the vapour phase into the liquid phase and into
the mesopores where the chemisorbed n-heptanal available might
be suppressed with the increase in temperature in order to show
decrease in conversion with the increase in temperature. Among Al-
Table 2
Acidity data of Al-SBA-1 catalysts with different Si/Al and Si/(Al+Mg) ratios.
Catalyst
Acid sites (mmol/g)
Weak (150-250 °C)
Medium (250-350 °C)
Al-SBA-1 (40)
Al-SBA-1 (80)
Al-SBA-1 (120)
Al,Mg-SBA-1 (40)
Al,Mg-SBA-1 (80)
0.38
0.20
0.09
0.42
0.30
0.08
0.05
0.04
0.12
0.05
Fig. 4. SEM images of Al-SBA-1 (40).

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K. Venkatachalam et al. / Catalysis Communications 12 (2010) 299–303
Table 3
Effect of temperature on n-heptanal conversion and product selectivity.
Catalyst
Temperature n-Heptanal Product selectivity (%)
(°C)
conversion
(%)
Acetal Hemiacetal Vinyl ether
Al-SBA-1 (40)
80
100
120
150
80
100
120
150
80
100
120
150
80
100
120
150
80
95
92
90
87
93
90
88
85
88
82
76
72
96
93
91
89
94
92
90
87
84
86
72
68
80
82
73
59
69
66
61
55
71
76
69
61
67
62
57
51
11
08
28
32
14
10
27
41
24
22
39
45
23
16
31
39
39
26
43
49
5
6
-
-
Al-SBA-1 (80)
6
8
-
-
Al-SBA-1 (120)
Al,Mg-SBA-1 (40)
Al,Mg-SBA-1 (80)
7
12
-
-
6
8
-
-
4
12
-
100
120
150
-
Reaction condition: catalysts amount 0.05 g; time 8 h; pressure autogenous pressure;
reactor 50 ml autoclave; Feed ratio 1:3 (n-heptanal: methanol).
of the latter catalysts plays a predominant role in equalizing
acetalization. It could be speculated that hydrophilic and hydrophobic
property could be very important for microporous materials but
not so for mesoporous materials. When the reactants freely diffuse
through the mesopores, there may be less hydrophobic force than in
zeolites. The free mass transport may also be considered as the main
cause for almost equal conversion over all the catalysts in addition to
hydrophobic force. Hence, the catalysts need not possess a very high
density of acid sites for acetalization under the given set of conditions,
provided there is no diffusional problem.
The selectivity of hemiacetal increased with an increase in tem-
perature. Hence adsorption of hemiacetal on the acid sites may be
hindered at high temperatures. At higher temperatures, the selectivity
of hemiacetal increased with increase in the hydrophobicity of
the catalysts. This observation supports our view that an increase
in hydrophobicity of the catalysts partly suppressed the effect of
temperature on adsorption. The selectivity of acetal decreased with an
increase in temperature is in accordance with the increase in the
selectivity of hemiacetal. The formation of vinyl ether was observed
at 80 and 100 °C but not at high temperatures. When hemiacetal
is protonated, two reactions are possible. After the formation of
carbonium ion, it can react with methanol to yield acetal or undergo
dehydration to form vinyl ether. The low selectivity of vinyl ether
indicates that it is not much favored in comparison to the reaction of
hemiacetal with methanol.
3.6.2. Effect of reaction time
In order to establish the sequence of steps involved in acetaliza-
tion, the effect of reaction time on n-heptanal conversion and the
products' selectivity was tested and the results are presented in
Fig. 5a. The n-heptanal conversion increased with an increase in
reaction time. The increase was significant up to 8 h. As the selectivity
to hemiacetal is less, its formation must be a slow process. In other
words, protonation of n-heptanal is a slow process. Similarly, the
formation of vinyl ether is also a slow process. However, protonation
of hemiacetal must be rapid as the selectivity of acetal was high at all
reaction intervals.
Fig. 5. Catalytic studies of acetalization of n-heptanal (a) effect of time, (b) effect of feed
ratio and (c) effect of the amount of catalysts on n-heptanal conversion and products
selectivity.
optimum feed ratio is 1:3. The slight increase in n-heptanal conversion
at 1:5 and a subsequent decrease at 1:7 and 1:10 are due to n-heptanal
dilution by excess methanol. The selectivity of hemiacetal increased
slightly with an increase in the content of methanol in the feed above
1:3. This is attributed to the dilution of hemiacetal by excess methanol.
The selectivity of acetal, therefore, increased from 1:2 to 1:3 and
decreased thereafter. All these observations are in support of the
proposed reaction pathway shown in Scheme 1.
3.6.3. Effect of feed ratio
The effect of feed ratio on n-heptanal conversion and the products'
selectivity was studied and the results are presented in Fig. 5b. The

K. Venkatachalam et al. / Catalysis Communications 12 (2010) 299–303
303
Scheme 1. Possible pathway for the formation of hemiacetal, acetal and vinyl ether.
3.6.4. Effect of catalyst loading
References
The effect of catalyst loading on n-heptanal conversion and the
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Fig. 5c. The optimum catalyst loading was found to be 0.05 g. Since the
feed ratio was 1:3, methanol content around the active site need
not be the same with an increase in the catalyst amount and hence
conversion decreased at higher loading. The selectivity of hemiacetal
clearly demonstrates the existence of unequal distribution of
methanol close to acid sites. The selectivity of acetal increased from
0.03 to 0.05 g of the catalyst but decreased at higher loading.
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4. Conclusion
This study concluded that SBA-1 catalysts exhibited similar n-
heptanal conversion irrespective of Si/Al and Si/(Al+Mg) ratios. This
conclusion elucidated that the reaction is mainly controlled by their
hydrophilic-hydrophobic property and free diffusion of reactants and
products rather the acidity of the catalysts. This study also revealed
that mesoporous materials are better than microporous materials for
acetalization of long-chain aldehydes due to free diffusion in the
mesoporous materials. This is a clean one-pot synthesis route for
acetal compared to mineral acid–catalyzed route.
Acknowledgement
The authors gratefully acknowledge the financial support from
the Council of Scientific & Industrial Research (CSIR), Government of
India, New Delhi, for this research work. One of the authors (Mr. K.
Venkatachalam) is grateful to CSIR for the award of Senior Research
Fellowship (SRF).
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