Condensation reactions assisted by acidic hydrogen bonded hydroxyl groups in solid tin(ii)hydroxychloride

Article abstract of DOI:10.1039/c3ra41152d

Tin(ii)hydroxychloride is reported as a heterogeneous Bronsted acid catalyst. Sn(OH)Cl was synthesized by a precipitation method and characterized by XRD, FT-IR, ¹H MAS NMR, SEM, TG-DTA and N₂ sorption. The acidity measurements of tin(ii)hydroxychloride by FT-IR pyridine adsorption and ¹H MAS NMR showed the presence of Bronsted acidity. The Bronsted acidity can be attributed to strong hydrogen bonding between the -OH and Cl groups. Sn(OH)Cl showed high activity for important condensation reactions such as the Prins condensation, Claisen-Schmidt condensation and ketalization. The catalytic activity of Sn(OH)Cl was compared with that of SnO, SnO₂ and Sn₂(OH)₂O. The catalyst was recycled three times with a negligible decrease in activity. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41152d

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Condensation reactions assisted by acidic hydrogen
bonded hydroxyl groups in solid tin(II)hydroxychloride
Cite this: RSC Advances, 2013, 3,
10795
Vijaykumar S. Marakatti, Ganapati V. Shanbhag* and Anand B. Halgeri
Tin(II)hydroxychloride is reported as a heterogeneous Brønsted acid catalyst. Sn(OH)Cl was synthesized by a
precipitation method and characterized by XRD, FT-IR, 1H MAS NMR, SEM, TG-DTA and N2 sorption. The
acidity measurements of tin(II)hydroxychloride by FT-IR pyridine adsorption and 1H MAS NMR showed the
presence of Brønsted acidity. The Brønsted acidity can be attributed to strong hydrogen bonding between
the –OH and Cl groups. Sn(OH)Cl showed high activity for important condensation reactions such as the
Prins condensation, Claisen–Schmidt condensation and ketalization. The catalytic activity of Sn(OH)Cl was
compared with that of SnO, SnO2 and Sn2(OH)2O. The catalyst was recycled three times with a negligible
decrease in activity.
Received 10th March 2013,
Accepted 26th April 2013
DOI: 10.1039/c3ra41152d
www.rsc.org/advances
hols.4 The Prins reaction of b-pinene with paraformaldehyde
results in the formation of nopol, an unsaturated primary
Introduction
There has been an increasing interest in recent years in
developing novel solid acid catalysts and modify them for the
selective synthesis of fine and speciality chemicals. The major
types of heterogeneous acid catalysts include zeolites (ZSM-5,
H-Beta), mesoporous materials (MCM-41 and SBA-15), clays
(montmorillonite K-10), heteropoly acids (HPW and HSiW),
anion modified oxides (WO3–ZrO2, SO422–ZrO2), functiona-
lized resins (Amberlyst-15) and mixed oxides (SiO2–Al2O3). The
above foremost catalysts and their modified forms have been
well applied in a large number of organic transformations. But
still there is a thirst for the chemist to design and develop new
classes of catalysts which are comparable to the leading ones.
It is well known that metal chlorides and their hydroxides are
applied as acid and base catalysts respectively.1,2 However,
metal hydroxychlorides, which fall in between metal chlorides
and their hydroxides, are not well established as catalysts.
Metal hydroxychlorides are well known as minerals found in
various parts of the world. One such example is
tin(II)hydroxychloride, a mineral known by the name abhur-
ite.3 The crystallographic and mineralogical properties of
tin(II)hydroxychloride are well understood but its application
has been overlooked so far. Its insolubility in water and
organic solvents stimulated our curiosity to study its proper-
ties and application as a heterogeneous catalyst.
alcohol. Nopol finds wide applications in the agrochemical
industry as a pesticide, and as a fragrance in the soap/
detergent industries.5 Nopol was traditionally prepared using
homogenous catalysts.6 However, these methods suffer from
drastic reaction conditions, low yield and unwanted side
products. To overcome these problems several heterogeneous
catalysts like Sn–MCM-41,7a Sn–kenyaite,7b Sn–SBA-15,7c sul-
fated zirconia,7d Fe–Zn metal cyanides,7e Zn–MCM-41,7f
mesoporous iron phosphate,5 ZnCl2 impregnated on mon-
tmorillonite7g and Zr–SBA-157h were designed for use in the
Prins reaction. Most of the above catalysts indicate the
requirement of Lewis acidic sites for the selective synthesis
of nopol. The use of a strong Brønsted acid catalyst usually
results in isomerized products.8
The condensation of ethyl acetoacetate with ethylene glycol
to produce fructone is an example of ketalization.9a Fructone
has an apple scent and is used as a flavouring agent. The
ketalization reaction is well catalyzed by acid catalysts like
ZSM-5,9a WO3–ZrO2,9b USY9c and silica functionalized –SO3H
groups.9d The Claisen–Schmidt condensation of 29-hydroxy
acetophenone with benzaldehyde is another condensation
reaction and results in the formation of flavanone. A large
number of acid and base catalysts have been applied for this
reaction.10a–c The flavanoids are widely used in the pharma-
ceutical industry for their anti-cancer, anti-inflammatory, anti-
bacterial and anti-AIDS pharmacological activities.
The Prins reaction is an important and well-known carbon–
carbon bond forming reaction in organic synthesis. The
condensation of an olefin with an aldehyde leads to the
formation of 1,3-dioxanes, 1,3-diols and unsaturated alco-
In the present work, we report, for the first time, the
catalytic properties of tin(II)hydroxychloride. To establish its
catalytic properties, important condensation reactions such as
the Prins condensation, Claisen–Schmidt condensation and
ketalization have been studied as model reactions (Scheme 1).
The uniqueness of tin(II)hydroxychloride as an acid catalyst
Materials Science Division, Poornaprajna Institute of Scientific Research (PPISR),
Bidalur Post, Devanahalli, Bangalore-562110, India.
E-mail: shanbhag@poornaprajna.org; Fax: +9123611836; Tel: +9127408552
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analyzed in the 2h range of 10 to 55 with a scanning rate of
0.02 degrees per second. The FT-IR (ATR-technique–Bruker a-T
model) spectra of the tin based catalysts were obtained in the
range of 4000 to 500 cm21. TGA and DTA curves were recorded
simultaneously using a Universal V 2.4F-TA analyzer by
heating the sample at the rate of 10 uC min21 in flowing
nitrogen gas. The 1H MAS NMR spectra of Sn(OH)Cl and
Sn2(OH)2O were measured using a Bruker DSX-300 wide-bore
solid state spectrometer operating at a resonance frequency of
300 MHz with a pulse length of 5 ms and a recycle time of 5 s.
N2 sorption measurements were carried out at 77 K in a Nova
1000 Quantachrome instrument and specific surface areas
were determined by the Brunauer–Emmett–Teller (BET)
equation.
Catalytic activity
Scheme 1 a) Prins condensation b) Ketalization and c) Claisen–Schmidt
condensation reactions.
In a typical reaction procedure, b-pinene (10 mmol), paraf-
ormaldehyde (20 mmol), catalyst (0.23 g) and solvent, toluene
(5 ml) were added to a 100 ml glass reactor equipped with a
reflux condenser and a magnetic stirrer. The mixture was
stirred vigorously at atmospheric pressure at the desired
temperature for a specific time. After the reaction, the product
mixture was analysed by gas chromatography (Shimadzu-2041,
FID detector) using a RTX-5 column. The products were
identified by GC-MS using standards. Reaction products were
quantified using a multi-point calibration curve through the
external standard method. Similarly, Claisen–Schmidt and
ketalization reactions were carried out without solvent with
the required amount of reactants and catalyst as mentioned in
Table 2.
was determined by establishing its physicochemical properties
and comparing it with tin hydroxide and tin oxide.
Experimental
Materials
The chemicals SnCl2?2H2O, ethylene glycol, ethyl acetoacetate,
paraformaldehyde, benzaldehyde and toluene were procured
from Merck, India. b-Pinene and 29-hydroxyacetophenone
were purchased from Sigma Aldrich, USA. All chemicals were
used directly without further purification.
Catalyst preparation
Results and discussion
Tin(II)hydroxychloride [Sn(OH)Cl] was prepared using
a
method reported elsewhere.11,12 In a typical catalyst prepara-
tion, 25 g of tin(II)chloride dihydrate was dissolved in
concentrated HCl (20 ml) and water (10 ml). The solution
was stirred until it was clear (step 1). To the solution, 1 : 1 (v/v)
ammonia–water was added until the pH of the solution
reached 1.0 (step 2). Then 2 N ammonia solution was added
until the pH reached 2.5 (step 3). The precipitate obtained was
well stirred, filtered and washed successively with water,
ethanol and diethyl ether. Finally the catalyst was dried at 120
uC for 12 h, followed by drying for 1 h at 180 uC to obtain
Sn(OH)Cl. For the preparation of Sn2(OH)2O, the above
procedure was modified: in step 3 the precipitate was obtained
at pH 7.5. Due to the possibility of aerobic oxidation of tin(II),
an inert atmosphere was maintained in all of the preparation
steps. The calcination of Sn(OH)Cl at 400 uC resulted in the
formation of SnO2 [designated as SnO2-400]. The calcination of
Sn2(OH)2O at 300 and 500 uC gave rise to SnO and SnO2
[designated as SnO2-500], respectively. Calcination was per-
formed in a muffle furnace under static air for 2 h.
Sn(OH)Cl, Sn2(OH)2O, SnO and SnO2 prepared under different
pH and calcination conditions were characterized by powder
XRD. The XRD patterns confirmed the formation of the above
Catalyst characterization
The formation and phase purity of the tin based catalysts were
determined by X-ray diffractometer instrument (Bruker-D2
Phaser) with Cu-Ka source (l = 1.542 Å). The samples were
Fig. 1 XRD patterns of tin based catalysts synthesized at different pH and
calcination temperatures.
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Table 1 Catalytic activity of tin catalysts in the Prins reactiona
Catalyst
Calcination temperature
(uC)
Surface area
Conversion of b-pinene
(mol%)
Selectivity for nopol
(mol%)
(m2 g21
)
Blank
—
—
8
9
18
13
16
—
—
—
0
98
32
3.8
35
32
91.4
91
0
99
94
83
96
92
96
97
94
Sn(OH)Cl
SnO2-400
Sn2(OH)2O
SnO
SnO2-500
Sn(OH)Cl–R1b
Sn(OH)Cl–R2b
Sn(OH)Cl–R3b
180
400
180
300
500
—
—
—
88
a
Reaction conditions: b-pinene = 10 mmol, paraformaldehyde = 20 mmol, solvent toluene = 5 ml, catalyst weight = 0.23 g, T = 90 uC, Time =
b
12 h. R = Catalyst recycle.
tin compounds by identifying the respective pure crystalline
phases.12 (Fig. 1)
Sn2(OH)2O but with a less intense peak. On the other hand,
SnO and SnO2 showed minor Lewis acidity.
The Prins reaction of b-pinene with paraformaldehyde was
carried out in the liquid phase at 90 uC, toluene was used as
the solvent. The catalytic activities of all the tin based catalysts
are tabulated in Table 1. Sn(OH)Cl showed high catalytic
performance with 98% conversion of b-pinene and 99% nopol
selectivity. On the other hand, SnO and SnO2 showed a low
conversion of y35%. The use of Sn2(OH)2O led to 3.8%
conversion and 83% nopol selectivity, much lower compared
to Sn(OH)Cl. FT-IR and solid state 1H MAS NMR of the
catalysts were used to correlate the catalytic activity with the
active sites. An FT-IR pyridine adsorption study was used to
determine the type of acid sites, by the method reported
elsewhere.4 Initially, we assumed that Sn(OH)Cl possessed
Lewis acidity due to the presence of tin. However, the pyridine
FT-IR spectra of Sn(OH)Cl clearly indicated the absence of a
peak due to Lewis acidity at y1445 cm21 (Fig. 2). Instead, to
our surprise, it showed a peak at 1532 cm21 due to Brønsted
acidity. A similar observation was made in the case of
These FT-IR results indicate that the active sites of
Sn(OH)Cl and Sn2(OH)2O are Brønsted acidic, whereas for
tin oxides they are Lewis acidic. The area of the peak at 1532
cm21 indicated that Sn2(OH)2O possessed about half the
amount of Brønsted acidity compared to Sn(OH)Cl. However,
Sn2(OH)2O exhibited very low activity in the Prins reaction
(3.8% conversion). This indicated that the strength of the acid
sites might be different in Sn(OH)Cl and Sn2(OH)2O. The
presence of these Brønsted acid sites could be due to the
hydrogen bonded protons, which were confirmed by FT-IR and
1H MAS NMR studies.
FT-IR spectra of Sn(OH)Cl and Sn2(OH)2O were used to
understand the difference in the strength of the acid sites. The
FT-IR spectra of Sn(OH)Cl showed broad peaks in the region of
3200 to 3600 cm21 due to hydrogen bonded –OH groups, as
shown in Fig. 3. The peaks at 975 and 928 cm21 are assigned
to the deformation of –OH groups, and the peak at 623 cm21 is
due to Sn–O stretching.13 The peaks at 3571 and 3409 cm21
indicate the presence of non- or weakly hydrogen bonded –OH
Fig. 2 Acidity measurements of tin based catalysts by pyridine FT-IR.
This journal is ß The Royal Society of Chemistry 2013
Fig. 3 FT-IR spectra of tin catalysts.
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Fig. 4 1H MAS NMR spectra of a) Sn(OH)Cl and b)Sn2(OH)2O.
groups, whereas the band at 3296 cm21 is due to –OH groups
involved in strong hydrogen bonding. This difference in the
strength of hydrogen bonding might have weakened the –O–H
bond leading to Brønsted acidity in these catalysts. Similar
hydrogen bonded –OH groups were also found in the FT-IR
spectra of Sn2(OH)2O and therefore it was difficult to
distinguish the difference in the strength of hydrogen-bonding
between the two.
Fig. 5 Thermal analysis of Sn(OH)Cl.
Sn(OH)Cl showed the presence of thin plates arranged in
spherical particles with an average size of 15 mm (Fig. 6). The
N2 adsorption measurements of all the tin based catalysts
showed a low specific BET surface area (,20 m2 g21) as shown
in Table 1.
Hence, further differences in hydrogen bonded protons
were evaluated by 1H MAS NMR. Sn(OH)Cl exhibited three
different types of protons at 6.94, 5.43 and 0.12 ppm whereas
Sn2(OH)2O showed only two peaks at 6.35 and 5.0 ppm (Fig. 4).
The chemical shift region of 3–7 ppm is generally assigned
to hydrogen bonded protons.14 The increase in chemical shift
(from 6.35 to 6.94 and 5.0 to 5.43) for Sn(OH)Cl compared with
Sn2(OH)2O can be related to the stronger hydrogen bonding
between the electronegative chlorine atom and the vicinal –OH
groups. Halogens directly bonded to metals are known to be
good hydrogen bond acceptors.15 The strongly polarized
character of the metal–halogen bond results in an enhanced
partial negative charge on the halogen. Such a halogen atom
can attract the hydrogen atom of an –OH group resulting in
To establish general applicability, Sn(OH)Cl was also
studied for the ketalization and Claisen–Schmidt condensa-
tion reactions. The ketalization reaction of ethyl acetoacetate
(EAA) with ethylene glycol was carried out in liquid phase at
100 uC. Sn(OH)Cl showed 63% EAA conversion with 98%
selectivity for fructone. Sn2(OH)2O and tin oxides were less
active for this reaction compared to Sn(OH)Cl (Table 2). The
higher activity of Sn(OH)Cl is due to the higher Brønsted
acidity compared with the other Sn catalysts. Solvent free
Claisen–Schmidt condensation of 29-hydroxyacetophenone (2-
HAP) with benzaldehyde was carried out at 140 uC under an N2
1
weakening of the O–H bond. The peak broadening in the H
MAS NMR spectra of Sn(OH)Cl further confirms that the
hydrogen atoms are loosely held due to strong hydrogen
bonding. This hydrogen bonding between –OH and Cl could
be responsible for generating the required amount of Brønsted
acidity necessary to catalyze the Prins reaction with almost
complete conversion.
To gain more insight into the physicochemical properties of
Sn(OH)Cl, the thermal and morphological properties were
evaluated by TG-DTA, SEM and N2 sorption techniques. The
TG-DTA profile showed the weight loss in three steps as shown
in Fig. 5. The initial weight loss below 210 uC is due to
dehydration, and the region between 200–400 uC can be
attributed to a slow dehydroxylation. The substantial weight
loss in the range of 400–550 uC is due to volatilization of tin as
SnCl2 and the formation of oxide.12 It is evident that Sn(OH)Cl
is thermally stable up to around 200 uC and hence can be used
as a catalyst below this temperature. The SEM images of
Fig. 6 SEM images of Sn(OH)Cl.
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Table 2 Catalytic activity of tin catalysts in ketalization and Claisen–Schmidt condensation reactions
Ketalizationa
Claisen–Schmidt condensationb
Conversion of
ethyl acetoacetate
(mol%)
Selectivity for
fructone
(mol%)
Conversion of 29-hydroxy
acetophenone
(mol%)
Selectivity for
flavanone
(mol%)
Selectivity for
chalcone
(mol%)
Entry
Catalyst
1
2
3
4
5
6
7
8
9
Blank
4
63
5.5
20
6.3
5.2
63
62
63
98
98
80
86.5
80
78
96
97
94
2
93
13
94
68
11
89
87
86.3
65
77
25
74
75
20
74
73
70
35
23
75
26
25
80
26
27
30
Sn(OH)Cl
SnO2-400
Sn2(OH)2O
SnO
SnO2-500
Sn(OH)Cl–R1c
Sn(OH)Cl–R2c
Sn(OH)Cl–R3c
a
b
Reaction conditions: ethyl acetoacetate = 12 mmol, ethylene glycol = 24 mmol, catalyst weight = 0.15 g, T = 100 uC, time = 2 h. Reaction
c
conditions: 29-hydroxyacetopheneone = 5 mmol, benzaldehyde = 10 mmol, catalyst weight = 0.09 g, T = 140 uC, Time = 10 h. R = Catalyst
recycle.
atmosphere. This reaction is known to be catalyzed by both
acidic and basic sites, and with hydrogen bonded protons.16
The conversion of 2-HAP and selectivity for flavanone were
almost the same for both Sn(OH)Cl and Sn2(OH)2O after a 10 h
reaction (Fig. 7). However, Sn2(OH)2O showed a high initial
activity of 86% after 2 h, whereas Sn(OH)Cl exhibited only 10%
conversion after 2 h which gradually increased to 93% at 10 h.
This could be due to the presence of the basic –OH groups in
Sn2(OH)2O which catalyze the Claisen–Schmidt reaction more
effectively compared with Sn(OH)Cl. SnO gave 68% 2-HAP
conversion with 75% flavanone selectivity, whereas SnO2
showed very low activity (11% 2-HAP conversion with 20%
flavanone selectivity) (Table 2).
conversion remained the same (25%) even after 10 h of
reaction time, indicating that there was no leaching of active
sites into the reaction media. This confirmed that the
Sn(OH)Cl catalyst is truly heterogeneous in nature.
A recyclability study was carried out to check the efficiency
of the catalyst. The catalyst was separated by filtration after the
reaction, washed with acetone and dried at 120 uC for 12 h
followed by activation at 180 uC for 1 h. The dried catalyst was
reused for the next cycle. Sn(OH)Cl exhibited good recyclability
for the Prins reaction with minimal decrease in activity after 3
recycles. The XRD pattern of Sn(OH)Cl after 3 recycles matched
that of the fresh catalyst indicating the structural integrity of
the catalyst (Fig. 8).
For the leaching test, the Prins reaction was carried out
under the conditions mentioned in Table 1. The reaction was
stopped after 1 h (25% conversion and 99% selectivity) and the
hot reaction mixture containing the catalyst was filtered. To
the filtrate, the required amount of paraformaldehyde was
added and the reaction was continued up to 10 h. The
Investigations into the application of the Sn(OH)Cl catalyst
for several other acid catalyzed organic transformations, and
modifications to improve the catalyst’s properties are in
progress. The evaluation of other metal hydroxychlorides like
Zn(OH)Cl, Pb(OH)Cl and Cu(OH)Cl for suitable organic
transformations are also under investigation.
Fig. 8 XRD pattern of Sn(OH)Cl, a) fresh catalyst and b) catalyst after 3 recycles.
RSC Adv., 2013, 3, 10795–10800 | 10799
Fig. 7 Effect of reaction time on the Claisen–Schmidt condensation reaction.
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The present study describes the synthesis, characterization
and application of Sn(OH)Cl as a heterogeneous catalyst. The
characterization by FT-IR pyridine adsorption and 1H MAS
NMR showed the presence of Brønsted acidity in the catalyst.
This Brønsted acidity in Sn(OH)Cl is attributed to a strong
hydrogen bonding between the –OH and Cl groups. Sn(OH)Cl
is a novel and highly efficient acid catalyst for the selective
synthesis of nopol, flavanone and fructone by appropriate
condensation reactions. The higher activity of Sn(OH)Cl
compared with Sn2(OH)2O, SnO and SnO2 is due to the
presence of Brønsted acidity. The catalyst is truly hetero-
geneous and can be used in up to 3 recycles with minimal
decrease in activity.
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Acknowledgements
Vijaykumar S.M acknowledges Admar Mutt Education
Foundation (AMEF) for a fellowship. The authors are thankful
to NMR Centre IISc, Bangalore for NMR analysis and BIT,
Bangalore for the surface area measurements. VSM is thankful
to the Manipal University for permitting this research as a part
of the Ph.D programme.
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