One-step synthesis of solid sulfonic acid catalyst and its application in the acetalization of glycerol: Crystal structure of cis-5-hydroxy-2-phenyl-1,3- dioxane trimer

Article abstract of DOI:10.2478/s11696-012-0203-x

A one-pot method was employed to immobilize sulfonic acid onto silica obtained from rice husk ash using 3-(mercaptopropyl)trimethoxysilane to form a solid catalyst denoted as RHASO₃H. BET measurements of the catalyst showed the surface area to be 340 m² g^-1 with the average pore volume of 0.24 mL g^-1 and the pore diameter of 2.9 nm. Acidity test of cation exchange capacity and pyridine adsorption studies revealed the presence of Bronsted acid sites on the catalyst surface. The catalyst was used in the acetalization reaction of glycerol with benzaldehyde. Under optimized conditions, the reaction showed the maximum conversion of 78 % after 8 h with 67 % selectivity towards the five membered ring isomer. Variation in the glycerol concentration had a significant effect on the reactants conversion. A single crystal X-ray study of one of the products proved the existence of a unique trimer formed by hydrogen bonding by the six-membered cis-isomer. The catalyst was several times recycled without any loss of its catalytic activity.

Full text of DOI:10.2478/s11696-012-0203-x

Chemical Papers 66 (11) 1048–1058 (2012)
DOI: 10.2478/s11696-012-0203-x
ORIGINAL PAPER
One-step synthesis of solid sulfonic acid catalyst and its application
in the acetalization of glycerol: crystal structure
of
-5-hydroxy-2-phenyl-1,3-dioxane trimer
a
a
b
Farook Adam*, Muazu Samaila Batagarawa, Kasim Mohammed Hello,
c
Salih S. Al-Juaid
aSchool of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
bChemical Department, College of Science, Al-Muthanna University, P.O. Box 47077, Baghdad, Iraq
cChemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80200, Jeddah, Saudi Arabia
Received 16 December 2011; Revised 14 March 2012; Accepted 30 March 2012
A one-pot method was employed to immobilize sulfonic acid onto silica obtained from rice husk
ash using 3-(mercaptopropyl)trimethoxysilane to form a solid catalyst denoted as RHASO
3
H. BET
2
−1
measurements of the catalyst showed the surface area to be 340 m
g
with the average pore
−
1
volume of 0.24 mL g and the pore diameter of 2.9 nm. Acidity test of cation exchange capacity
and pyridine adsorption studies revealed the presence of Brønsted acid sites on the catalyst surface.
The catalyst was used in the acetalization reaction of glycerol with benzaldehyde. Under optimized
conditions, the reaction showed the maximum conversion of 78 % after 8 h with 67 % selectivity
towards the five membered ring isomer. Variation in the glycerol concentration had a significant
effect on the reactants conversion. A single crystal X-ray study of one of the products proved the
existence of a unique trimer formed by hydrogen bonding by the six-membered cis-isomer. The
catalyst was several times recycled without any loss of its catalytic activity.
ꢀ
c 2012 Institute of Chemistry, Slovak Academy of Sciences
Keywords: surface modification, sol–gel, sulfonic acid, glycerol, acetalization, cis-5-hydroxy-2-
phenyl-1,3-dioxane
Introduction
tion times and hazardous chemicals leading to ineffi-
cient preparation techniques which result in low yields,
mainly because of the heterogeneous reaction condi-
tions. Traditionally, homogeneous acid catalysts such
as AlCl3, HF, and H2SO4 are commonly used in the
industry. However, the production of large volumes of
toxic waste which poses environmental risk and the
cost inefficiency are some of the drawbacks of these
catalysts. Therefore, the development of solid acid cat-
alysts for liquid phase reactions to create a cleaner, re-
newable, and recyclable catalyst is desired (Eskandar
et al., 2011). In the view of this development, there is
a need to design new pathways of the preparation of
these catalysts.
Synthesis of a green catalyst for economically vi-
able application is certainly an important step towards
hazard-free environment. Although most chemical
transformations are greatly influenced by catalysts;
nowadays, attention is not only focused on the viabil-
ity of such catalysts, but also environmental consider-
ations and cost-effectiveness are taken into considera-
tion. Silica modified with 3-(trimethoxysilyl)propane-
1
-thiol (MPTMS) has been previously reported (Ka-
rimi & Khalkhali, 2005; Gupta et al., 2007; Bossaert
et al., 1999; Adam et al., 2010). Most of the reported
preparations of silica-MPTMS involve long prepara-
*Corresponding author, e-mail: farook@usm.my

F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
1049
Glycerol is a by-product of biodiesel production
Experimental
from vegetable oils and animal fats. Its increasing pro-
duction calls for a viable alternative for its utilization
instead of the disposal by incineration or discharge
into the water body thereby polluting the environ-
ment. There are many uses for glycerol including: pro-
duction of glycerin acetate which can be used as a fuel
additive (Melero et al., 2007), in the synthesis of es-
ters, polyethers, and alkyl resins (Behr et al., 2008).
One important application of glycerol is in the con-
densation reaction with aldehydes and ketones to form
cyclic acetals. These acetals are fine chemicals useful
in cosmetic, pharmaceutical, food, and beverage in-
dustry (Behr et al., 2008; Climent et al., 2002). The
process is also considered as an important method of
carbonyl groups protection in the presence of other
functional groups during the manipulation with mul-
tifunctional organic molecules (Karimi & Khalkhali,
Chemicals used in this study were used as pur-
chased: glycerol (99 %, QReC, Malaysia), dodecane
(99 %, ACROS, Belgium), benzaldehyde (≥ 99 %,
Sigma–Aldrich, Belgium), toluene anhydrous (99.5 %,
QReC, Malaysia), 3-mercaptoprophyltrimethoxysila-
ne (MPTMS) (≥ 95 %, Merck, Germany), ethanol
(99.8 %, BDH, France), hydrogen peroxide (35 %,
Systerm, Malaysia), nitric acid (65 %, Systerm,
Malaysia), hexane (99 %, QReC, Malaysia), ethyl
acetate (99.5 %, QReC, Malaysia), acetone (99 %,
QReC, Malaysia), and triethylamine (99 %, Sigma–
Aldrich, Germany). Rice husk was collected from a
rice mill in Penang, Malaysia.
The rice husk ash (RHA) was chosen as a source
of silica (Adam & Chua, 2004) as it was available
in abundance. Silica was extracted from RH using a
previously reported method (Ahmed & Adam, 2007).
Briefly, rice husk was washed with a copious amount
of water and rinsed with distilled water. After drying
at room temperature, RH was stirred with 1.0 M nitric
acid at room temperature for about 24 h. Cleaned RH
was pyrolyzed in a muffle furnace (Model AAF 11/7,
2
007).
Until recently, most catalysts employed in acetal-
ization and ketalization reactions were generally pro-
tonic acids, Lewis acids (Umbarkar et al., 2009), and
several types of transition metal complexes such as
Rh, Pd, and Pt (Cataldo et al., 1999). Although the
use of these types of catalyst provides good results,
separation of the catalyst from the reaction mixture
is a strong drawback in addition to the high costs of
these noble metals.
◦
Carbolite, UK,) at 600 C for 6 h. The resulting white
ash (RHA) was washed with distilled water and used
as the source of silica in this study.
Silica supported sulfonic acid catalysts can be
a better alternative to commercially available sul-
fonic acid resins and zeolites. Sulfonic acid resins
such as Amberlyst-15 suffer from low surface area
and thermal stability (Shimizu et al., 2004) while
zeolites provide some reaction products with high
conversion and selectivity. However, these catalysts
also suffer from diffusion restrictions due to small
pore diameter (Hermida et al., 2008) which makes
them unsuitable for liquid phase reactions, especially
with bulky molecules such as 1,3-dioxane and 1,3-
dioxolanes. To overcome this drawback, a number
of silica supported catalysts such as SBA-15 sulfonic
acid (Margolese et al., 2000) or MCM-41 sulfonic acid
One-step preparation of solid silica sulfonic
acid
About 3.0 g of RHA were stirred in 100 mL of
◦
1.0 M NaOH in a plastic container at 80 C for 30
min. MPTMS (6.0 mL, 0.024 mol) was added to the
sodium silicate solution. The solution was then slowly
titrated with 3.0 M nitric acid under constant stirring.
The change in pH was monitored by a pH meter. A
white gel started to form when pH reached the value
of 10, after which 10 mL of a mixture of H O (ϕ
2
2
r
= 0.3) in ethanol were added. When pH reached the
value of 7, further 10 mL of the mixture of H O in
2
2
ethanol were added. Titration was continued until pH
of the solution reached the value of 3; the last 10 mL
of the mixture of H O in ethanol were added. The
(Van Rhijn et al., 1998) have been employed in ac-
etalization, esterification, ketalization, and other re-
lated processes. This is partly because these catalysts
are non-volatile, recoverable, and easy-to-synthesize,
and they are generally regarded as “green catalysts”.
However, the tedious work-up procedure (several
hours of refluxing), use of hazardous chemicals such
as tetraethoxysilane (TEOS), poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene gly-
col) (Pluronic 123), cetyltrimethylammonium bromide
2
2
gel was aged for 24 h at room temperature and sepa-
rated by a centrifuge (Rotina 38, Hettich Zentrifugen,
UK). The process was repeated six times, and the fi-
nal washing was done using acetone. The sample was
◦
then dried at 110 C for 24 h and finally it was ground
to a fine powder, which weighed 6.5 g. It was labelled
as RHASO H.
3
The prepared sample was characterized by elemen-
(
CTAB) etc., either as the silica source or template,
tal analysis (Perkin–Elemer, USA, Series II, 2400 Mi-
cro elemental analyser). Thermo gravimetric analysis
are considered the main drawbacks of this type of cat-
alysts. In the present work, a one-step process was
used to synthesize a silica supported acid catalyst us-
ing rice husk ash as a cheap silica source in a short
preparation time (32 min) under ambient conditions.
e
(TGA) was carried out on a TGA/DTA SDTA 851
◦
◦
(Perkin–Elmer, USA) analyzer from 30 C to 900 C
◦
−1
at the heating rate of 20 C min . Powder X-ray
diffraction pattern was obtained using a Diffractome-

1
050
F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
ter (D5000 Kristalloflex, Siemens, Germany), nitrogen
adsorption–desorption analysis was carried out using
an Automatic physiosorption Autosorb-1 CLP (Quan-
tachrom, USA), FTIR spectroscopy was carried out
29
using a (Perkin–Elmer, USA) System 2000. Si and
1
3
C MAS NMR were obtained from a Bruker Avance
TM
(
400 MHz Ultrasheild , Germany) equipped with a
Fig. 1. Reaction sequence for the one-step synthesis of
RHASO3H. Possible structures of the catalyst showing
◦
magic angle-spinning (MAS) inclined at 57.4 . The
sample was spun at 8.0 kHz in a 4.0 mm outer diam-
eter zirconia ceramic rotor spinner. Scanning electron
microscopy (SEM) was done using a Leica Cambridge
S360 (Cambridge, UK) equipped with energy disper-
sive spectrometry, EDX (Edax Falcon System). TEM
micrograph was obtained using a Philips CM12 (Mun-
ster, Germany).
1
2
3
the T , T , and T silicon centres.
benzaldehyde : glycerol was studied by the experiment
described above.
In order to isolate the products, the catalyst was
filtered off and the mixture was concentrated using a
rotary vacuum evaporator, it was later subjected to
silica column chromatography using an hexane : ethyl
acetate mixture (ϕr = 2 : 3) as the eluent. The prod-
uct from one of the fractions was recrystallized to
obtain a single crystal (37.4 % yield) suitable for
the X-ray crystallographic analysis. The crystal was
found to have the following approximate dimensions:
0.04 mm × 0.05 mm × 0.40 mm.
The sample together with a beaker of pyridine were
placed in a desiccator equipped with a valve connec-
tion to a membrane vacuum pump (AMB Greiffen-
berger Antriebstechnik, model MZ2C, CE 2002/06,
Germany). The system was evacuated for 1 h at the
rate of 1.7 m h . The system was then closed and
the desiccator was kept under vacuum for 48 h for the
pyridine vapour to equilibrate. The atmosphere in the
desiccator was evacuated again for 1 h at the same
pumping rate. The sample was then removed and an-
alyzed by FTIR using a KBr disc. The spectrum was
recorded in the absorption mode (Adam et al., 2011a,
3
−1
Results and discussion
2
011b).
Simple and direct modification of sodium sili-
cate with MPTMS and in situ oxidation of the mer-
capto group with hydrogen peroxide in an acidic
medium was carried out using a sol–gel technique
at room temperature and pressure to form silica
with the —CH2—SO3H group on its surface in good
yield. The synthesis of RHASO3H is presented in
Fig. 1. Successful formation of RHASO3H was con-
firmed by the following physical and chemical analy-
ses.
Acetalization reaction
Acetalization of glycerol with benzaldehyde was
carried out in the liquid phase under argon atmo-
sphere in a 50 mL round bottom flask equipped with
a magnetic stirrer and a water condenser. Toluene
(15 mL, 0.14 mol) and glycerol (8 mL, 0.11 mol) were
separately transferred to a round bottom flask con-
taining 0.15 g of the catalyst (pre-dried at 110 C for
◦
2
4 h and cooled in a desiccator to minimize the mois-
Elemental analysis
ture content). After the reaction temperature reached
◦
1
20 C, benzaldehyde (5 mL, 0.1 mol) was added. The
Table 1 shows the chemical analysis of RHASO3H.
The analysis was carried out by a combination of
elemental and energy-dispersive X-ray spectroscopy
(EDX) methods. Due to the heterogeneous nature of
the silica samples, these values can only be treated
qualitatively. The combined analysis showed that car-
bon and sulfur are present in RHASO3H, while both
these elements are not present in RHA as it was ex-
pected.
reaction mixture was refluxed for 8 h under argon
atmosphere. Samples for the analysis (≈ 0.50 mL)
were withdrawn from the reaction mixture at regu-
lar intervals and 15 µL of dodecane were added as
an internal standard to the mixture and analyzed by
GC (Clarus 500, Perkin–Elmer, USA) the products
were identified by GC-MS (Clarus 600, Perkin–Elmer,
USA). A 30 m wax capillary column with an FID de-
tector was used in the GC. The column and the in-
jector port temperatures were maintained at 200 C.
In both cases, the oven temperature was programmed
from 50 C with a 2.50 min hold, then at 25 C min
to 160 C.
Elemental analysis showed that ca. 14 % of car-
bon is present, which represents the content of
◦
−
1
7.82 mmol g of the catalyst, while the sulfur con-
tent was 10.25 % representing the sulfur content of
◦
◦
−1
◦
−1
3.37 mmol g of the catalyst (Mizanur Rahman et
Catalytic activity differed with different mass of
the catalyst (0.05 g, 0.1 g, 0.15 g, and 0.2 g), different
temperatures (90 C, 100 C, 110 C, and 120 C), and
different mole ratios (1 : 1, 1 : 2, 1 : 3, and 1 : 4) of
al., 2008). Given that the elemental analysis has a 5–
7 % error margin, the C : S ratio in the catalyst is in
good agreement with the theoretical value of 3 : 1 (see
Fig. 1).
◦
◦
◦
◦

F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
1051
Nitrogen adsorption–desorption analysis
Fig. 3 shows the nitrogen adsorption isotherm ob-
tained for RHASO3H. The hysteresis loop was ob-
served in the range of 0.4 < P/Po < 1.0, which is
associated with capillary condensation according to
the IUPAC classification. The isotherm shown is of
type IV and exhibits an H2 hysteresis loop (Thommes,
2
010). Close observation of the program revealed that
the hysteresis loop did not close but it was rather open
ended. This indicates the presence of some degree of
micropores retaining the nitrogen and hence failing to
close the hysteresis loop. This is clearly seen in the
pore size distribution curve, where the maxima in the
micropore region (< 2 nm) and a major maximum
at ca. 4 nm can be observed. It can therefore be con-
cluded that RHASO3H consists of mesopores with the
narrow pore range of 3–4 nm.
Fig. 2. High angle XRD diffractogram for RHASO3H.
BET analysis revealed the specific surface area of
2
−1
RHA to be 347 m g (Adam & Ravendran, 2000).
X-ray diffraction analysis
However, the specific surface area of RHASO3H was
found to be 340 m2
g
−1
. Only a minimal decrease
XRD spectrum of RHASO3H is shown in Fig. 2. A
broad diffraction band at the 2θ angle of 22.80 was
observed. This is similar to that of pure silica obtained
by Radhika and Sugunan (2006), Adam et al. (2006),
and Saleh et al. (1990). The diffraction pattern was
typical of amorphous silica.
in the surface area was observed after the modifi-
cation which indicated that the method can lead to
the production of a high surface area catalyst. The
pore size distribution of RHASO3H (Fig. 3) showed
a minor narrow pore size range below 2 nm which is
in the microporous range and also a major pore size
◦
Table 1. Results of elemental and EDX analyses (C, H, and S contents) and BET analysis (surface properties) of RHASO3H
wi/%
Specific surface area
Average pore volume
Average pore diameter
Compound
m2
g
−1
mL g
−1
nm
2.9
C
H
S
Si
–
a
b
2.17a
–
10.25a
7.9
RHASO3H
13.9
5.9
340
0.24
b
b
2
23
a) Elemental analysis; b) EDX analysis.
Fig. 3. Nitrogen adsorption–desorption isotherms of RHASO3H (a). Pore size distribution shows the presence of micropores and
mesopores in the range of 3–4 nm (b).

1
052
F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
Fig. 4. FTIR spectra of RHA, RHASO3H, and the differential spectrum.
Fig. 5. Solid state MAS NMR of RHASO3H: 29Si (a) and 13C (b).
range from 3 nm to 4 nm which is in the mesoporous
range (Margolese et al., 2000). The presence of narrow
mesopores results in the formation of a high surface
area catalyst. The results obtained for N2 adsorption–
desorption analysis of RHASO3H are summarized in
Table 1.
the differential spectrum can be caused by the methy-
lene group adjacent to the sulfate group; this also in-
dicates successful oxidation of the thiol group to sul-
−
1
fonic acid. The bands at 1472 cm in the differential
−
1
and 1413 cm in the RHASO3H spectra were both
assigned to the methylene group while the band at
−
1
1
231 cm was assigned to the Si—C bond as indi-
FTIR analysis
cated in the differential spectrum. The band at 1159–
−
1
1
115 cm in the RHASO3H spectrum is due to the
FTIR spectra of RHA and RHASO3H and the dif-
ferential spectrum are shown in Fig. 4. The FTIR
spectrum of RHA was described previously by Adam
and Chua (2004). Typically, the Si—O—Si vibrations
asymmetric and symmetric stretching bands of SO2
moieties, confirming the formation of sulfonic acid
(Melero et al., 2007).
−
1
29Si MAS NMR spectroscopy analysis
appear at ca. 1101 cm which is associated with the
condensed silica network. This vibration was shifted
−
1
Fig. 5 shows the 29Si and 13C MAS NMR spectra
to 1055 cm in RHASO3H. The broad bands around
3
−
1
−1
29
449 cm and 3457 cm are usually assigned to the
of RHASO3H. In Fig. 5a, the Si MAS NMR spec-
4
O—H vibration of Si—OH and H—OH groups of ab-
sorbed water. The sulfopropyl groups in RHASO3H
attached to the silica framework were identified by
tra of RHASO3H show the presence of Q (Si(OSi)4),
3
2
Q (Si(OSi)3(OH)), and Q (Si(OSi)2(OH)2) silicon
centers with chemical shifts (δ) at –112.8, –103.3, and
−
1
1
29
the methylene C—H stretching bands at 2922 cm
.
–94.1, respectively. The Si MAS NMR spectrum of
−
The disappearance of a weak band at 1413 cm in
RHA (not shown) showed similar chemical shifts at-

F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
1053
4
3
tributed to the Q and Q silicon atoms. These chem-
4
ical shifts are clearly shifted compared to the Q and
Q of RHA. This indicates successful immobilization
3
of propyl sulfonic acid onto silica followed by the
transformation of the thiol groups to sulfonic acid in
RHASO3H.
Three other chemical shifts, located at δ = –58.1,
29
–
52.6, and –50.6, were observed in the Si MAS NMR
spectrum of RHASO3H (Fig. 5a). The first chemi-
cal shift is attributed to the silicon atom of MPTMS
forming three siloxane bonds with the silicon of RHA
and one bond with the carbon of the propyl chain,
[
(—O—)3Si—CH2CH2CH2SO3H] (Fig. 1a). This is
Fig. 6. TGA/DTA curve of RHASO3H showing three-step
3
usually referred to as the T signal. The second one
was assigned to the formation of two siloxane bonds
and one OH group bonded to the silicon atom of
propyl sulfonic acid (as shown in Fig. 1b). This is
mass loss.
2
usually referred to as the T signal. The last one
was assigned to the formation of one siloxane bond
and two OH groups bonded to the silicon atom of
propyl sulfonic acid (as shown in Fig. 1c). This is the
1
29
T
signal. The Si MAS NMR spectrum of RHA,
as in our previous study (Hamoudi et al. 2004), did
m
m
not show the T [T = RSi(OSi)m(OH)3−m, m =
–1] silicon signal. Its appearance in the RHASO3H
3
spectrum indicates successful functionalization of sil-
ica with MPTMS which is consistent with the struc-
tures shown in Fig. 1.
13C MAS NMR spectroscopy
13C MAS NMR of RHASO3H is shown in Fig. 5b,
where three chemical shifts which can be attributed
to the three carbon atoms of the propyl chain can
be clearly seen. These chemical shifts appeared at δ
Fig. 7. SEM micrograph of RHASO3H at magnification of
36000.
=
10.2 (C1), 22.2 (C2), and 47.6 (C3). In literature
associated with the decomposition of the propyl sul-
fonic acid fragments (Hamoudi et al., 2004). The third
continuous mass loss (ca. 14.8 %) occurred between
435 C and 901 C and it was associated with the de-
composition of the remaining organic groups and the
dehydration of the silanol groups (Zhao et al., 1997).
(Karimi & Khalkhali, 2005; Gupta et al., 2007) and
in our previous observation (Bossaert et al., 1999),
both C2 and C3 signals were found to be superim-
posed on each other giving only one signal instead of
the expected two. However, no overlap of the C2 and
C3 chemical shifts was observed after the transforma-
tion to RHASO3H. It can therefore be concluded that
propyl sulfonic acid was immobilized onto silica and
oxidized to RHASO3H.
◦
◦
Scanning electron microscopy
Fig. 7 shows the SEM micrograph of RHASO3H.
Close observation of the image at higher magnifica-
tion, 36000, further revealed the presence of aggre-
gate particles which may be associated with its highly
porous nature resulting in high surface area.
Morphology of the catalyst was studied by TEM
of RHASO3H (not shown) which did not show any
distinct structure which led to the conclusion that the
catalyst was generally composed of amorphous silica.
Thermogravimetric analysis (TGA/DTA)
In order to establish proper heat treatment, the
catalyst was analyzed by TGA as shown in Fig. 6.
The measurements were carried out in the range of
◦ ◦ −1
5
0–850 C at the heating rate of 10 C min . TGA
of RHASO3H showed three characteristic decomposi-
tion stages as shown in Fig. 6. The first mass loss (ca.
◦
◦
1
1.2 %, 1.18 mg) starting from 29 C to 172 C was
Pyridine adsorption FTIR spectra
due to the absorbed water (Margolese et al., 2000).
The second mass loss (ca. 14.9 %, 1.56 mg) occurred
between 172 C and 435 C and it is assumed to be
Surface acidity of RHASO3H was studied qualita-
tively by pyridine adsorption and desorption measure-
◦
◦

1
054
F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
Fig. 8. FTIR spectra of RHASO3H after (a) and before (b)
pyridine adsorption.
Fig. 10. Crystal structure of cis-5-hydroxy-2-phenyl-1,3-dioxa-
ne isomer, showing trimolecular H-bonding.
SH) and the abundant silanol groups within silica.
Fig. 9. Acetalization of glycerol with benzaldehyde over an
acidic catalyst. Six- and five-membered ring products
are shown.
This can lead to the formation of RHAPr—SO2—O—
—
Si— bonded structures instead of the usual RHAPr—
—
SO2—OH.
ments monitored by FTIR spectroscopy. FTIR spec-
Catalytic activity of RHASO3H in the acetal-
tra of RHASO3H obtained before and after pyridine
ization of glycerol
−
1
adsorption–desorption in the 1700–1400 cm region
−
1
are shown in Fig. 8. Strong bands at 1490 cm
1
,
The reaction of aldehyde and alcohol normally pro-
vides aldol as the product; however, when such reac-
tion is carried out with di- or trihydroxy alcohols, such
as glycerol, it usually results in the formation of cyclic
acetals (Behr et al., 2008; Climent et al., 2002; Um-
barkar et al., 2009; Deutsch et al., 2007; da Silva et
al., 2009) as shown in Fig. 9.
− −1
1
550 cm , and 1554 cm are due to the interaction
between the adsorbed pyridinium ions and the surface
Brønsted acid sites (Barzetti et al., 1996).
Cation exchange capacity (CEC)
Concentrations of acid sites on RHASO3H was de-
+
termined in water by exchange with excess Na (from
Single crystal study of isolated products
+
NaCl) followed by titration of H released by stan-
dard NaOH solution. The cation exchange capacity
Reports on the products of this particular reac-
tion have been published by many authors, e.g. Cli-
ment et al. (2002), Umbarkar et al. (2009), Deutsch
et al. (2007), da Silva et al. (2009), and Farreira et
al. (2010). Various isomers were proposed on theoreti-
cal grounds with experimental support. In this study,
column chromatography was employed to isolate the
products which were later recrystallized to obtain suit-
able single crystals for X-ray crystallography analy-
sis. Crystal structure of the title compound is based
upon a network of trimolecular H-bonded molecules
as shown in Figs. 10 and 11. The structure was solved
and refined by a Bruker SHELXTL Software Package,
using the space group P –1, with Z = 6 for the for-
mula unit, C10H12O3. The final anisotropic full-matrix
(CEC/(equivalent per mass)) was found to be 0.62
meq per g of RHASO3H. Comparing the CEC results
with the sulfur content obtained from elemental anal-
ysis, a strong difference between the two methods was
found. However, the low CEC value can be attributed
to the poor accessibility of the sulfonic acid group by
the titrating base. Wang et al. (2006) and Marschal
et al. (2007) associated low CEC results with sulfonic
acid groups buried deep within the silica frame work.
Similarly, Li et al., (2007) attributed low CEC results
to crowdedness of arylsulfonic groups in the pore walls
of the catalyst thereby limiting the exchange capac-
ity between NaCl and the sulfonic acid group before
the titration. Another possible explanation is the co-
condensation between the mercapto group (RHAPr—
2
least-squares refinement on the F position with 307

F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
1055
Table 2. Intermolecular hydrogen bond distances and angles between three molecules
Donor–H
Acceptor–H
Donor–acceptor
Angle
Structural part
˚
◦
O1A—H1D· · ·O1B#2
O1B—H1F· · ·O1C
0.84
0.84
0.84
1.87
1.88
1.82
2.704(6)
2.684(5)
2.659(6)
172.2
160.0
173.2
O1C—H1L· · ·O1A#1
Fig. 11. Packing diagram of cis-5-hydroxy-2-phenyl-1,3-dioxa-
Fig. 12. Effect of time on the acetalization of glycerol with
benzaldehyde, with ( ) and without ( ) a catalyst.
ne in the crystal lattice.
•
Reaction conditions: mass of catalyst: 0.15 g, Benz to
◦
Gly mole ratio: 1 : 2, reaction temperature: 120 C.
variables converged at R1 = 14.23 % for the observed
data and wR2 = 33.92 % for all data. The goodness-
of-fit was 1.121. The largest peak in the final differ-
−
3
˚
ence electron density synthesis was 0.545 A and the
−
˚
3
largest hole was –0.589 A with the RMS deviation
−
˚
3
of 0.124 A . The unit cell constants were determined
based upon the refinement of the XYZ-centroids of
reflections above 20 σ(I). C—C distances were in the
˚
˚
range of 1.274–1.714 A (average of 1.503 A) and the
◦
C—C—C bond angles in the range of 108.3–126 (av-
erage of 116.01 ). Although, because of the electroneg-
◦
ativity differences, the distance between atoms and
their bond angles may vary, it was observed that
all C—C bond distances between the phenyl groups
◦
˚
were 1.39 A, and the bond angles were 120 , this
2
is consistent with normal C—C sp hybridized bond
distances and bond angles (Wade, 2006). Interest-
ingly, the intermolecular hydrogen bonding between
the three molecules shows a trimolecular association
with equal hydrogen bond distances of 0.84 nm as
shown in Table 2. Hydrogen bonded trimeric molecules
are rare in literature. Mendes et al. (2011) described
the hydrogen bonded trimer of luminol. However, no
crystal structure was reported.
Fig. 13. Selectivity of the reaction products; five-membered
ring isomer (HMPD) ( ) and six-membered ring iso-
•
mer (HPD) ( ), indicating the predominance of the
five-membered ring isomer in the reaction.
◦
120 C. Initial conversion of benzaldehyde during the
first hour was 16 % and increased to the maximum
of 78 % in 8 h. However, when the reaction was con-
ducted under similar conditions in the absence of a
catalyst, the maximum conversion of only 45.5 % was
achieved after 8 h (Fig. 12). This indicates the role of
the catalyst in accelerating the rate of the reaction.
This suggests that the optimum reaction time was
8 h under the reaction conditions studied. Selectiv-
ity of the reaction products favored the formation of
dioxolane although at higher conversions, the selec-
Influence of reaction time on the acetalization
of glycerol
The effect of the reaction time on the conversion of
benzaldehyde over RHASO3H is shown in Fig. 12. The
reaction was carried out with 0.15 g of catalyst using
benzaldehyde: glycerol (Benz : Gly) ratio of 1 : 2 at

1
056
F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
Table 3. Effect of reaction conditions on the acetalization of
glycerol with benzaldehyde using RHASO3H as the
catalyst (0.15 g)
Temperature/ C Benz to Gly Cyclea
◦
Conversion/%
mole ratio
120
120
120
120
120
120
120
1 : 1
1 : 2
1 : 3
1 : 4
2 : 1
3 : 1
4 : 1
1
1
1
1
1
1
1
73
78
75
58
62
64
50
9
0
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1
1
1
1
1
2
3
4
65
70
74
78
78
75
74
74
Fig. 14. Acetalization of glycerol with benzaldehyde over
RHASO3H as a function of the catalyst mass. Re-
action conditions were: Benz to Gly mole ratio: 1 : 2,
1
1
1
1
1
1
1
00
10
20
20
20
20
20
◦
reaction time: 8 h, and temperature: 120 C.
the acetalization of glycerol with benzaldehyde.
a) Reusability test, fresh catalyst corresponds to cycle 1.
Influence of temperature
tivity changed to favor the formation of the dioxane
isomer, as shown in Fig. 13.
The effect of temperature on the conversion of glyc-
erol over the RHASO3H catalyst over the temperature
◦
◦
range of 90 C to 120 C is shown in Table 3. When
the temperature was increased, the conversion also in-
creased up to the maximum of 78 % in 8 h at the
highest temperature studied.
Influence of catalyst mass
Catalytic activity was investigated with different
amounts of RHASO3H in the range of 0.05 g and 0.20 g
keeping the molar ratio of Benz : Gly constant at 1 : 2
Reusability test
◦
for 8 h at 120 C. The effect of the catalyst mass on the
benzaldehyde conversion is shown in Fig. 14. When
the catalyst mass increased from 0.05 g to 0.15 g, the
conversion of benzaldehyde increased from 57 % to
The catalyst was regenerated after each run by
its separation by filtration; then, it was washed with
ethanol and dried at 120 C for 24 h before each reuse.
◦
7
8 %. Further increase of the mass of the catalyst
The FT IR results obtained are shown in Fig. 15. It
can be observed that the catalyst was stable and could
be reused several times without significant loss in its
catalytic activity.
showed a significant decrease (62 %). The presence
of an excess of the catalyst can cause the reactants
to adsorb evenly on the catalyst surface resulting in
low conversion of the reactants per unit of active sites.
This can lead to apparent decrease in the conversion.
Therefore, 0.15 g was taken as the optimum mass of
catalyst in this study.
Proposed mechanism for the acetalization of
glycerol with benzaldehyde over the RHASO3H
catalyst
Influence of molar ratio of reactants
Carbonyl oxygen of benzaldehyde can attack the
carbocation in glycerol to form an intermediate com-
pound (I ). The intermediate (I ) has two possible
paths, i.e. it either attacks the hydroxyl of the alpha
carbon in glycerol which leads to the formation of the
five membered ring isomer, or it can attack the hy-
droxyl of the beta carbon which leads to the formation
of the six membered isomer as shown in Fig. 16.
Influence of the mole ratio of reactants on the glyc-
erol conversion over the RHASO3H catalyst is shown
in Table 3. The catalytic experiment was conducted
◦
at 120 C using a fixed amount of catalyst (0.15 g).
First of all, the amount of glycerol was varied keeping
the benzaldehyde amount constant (1 : 1 to 1 : 4).
The maximum conversion of 78 % was obtained for
the molar ratio of 1 : 2 as seen in Table 3. Further
increase in the glycerol concentration decreased the
conversion to 58 %. When a fixed amount of glycerol
was used, the conversion was lower than 62 %. The
mole ratio of 1 : 2 seems to be the most suitable for
Conclusions
Many papers regarding the synthesis of solid sul-
fonic acid by a two-step process: immobilization of the
thiol group on a support and subsequent oxidation of

F. Adam et al./Chemical Papers 66 (11) 1048–1058 (2012)
1057
We also thank the Umaru Musa Yar’adua University, Katsina,
Nigeria for the scholarship to M S B.
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