A new solid acid SO42?/TiO2 catalyst modified with tin to synthesize 1,6-hexanediol diacrylate

Article abstract of DOI:10.1016/S1872-2067(16)62474-9

A new solid acid catalyst, SO₄^2?/TiO₂ modified with tin, was prepared using a sol-gel method and its physicochemical properties were revealed by nitrogen adsorption-desorption, X-ray powder diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, infrared spectroscopy of adsorbed pyridine, temperature-programmed desorption of ammonia and thermal gravimetric analysis. The structure, acidity and thermal stability of the SO₄^2?/TiO₂-SnO₂ catalyst were studied. Incorporating tin enlarged the specific surface area and decreased crystallite size of the SO₄^2?/TiO₂ catalyst. The total acid sites of the modified catalyst increased and Br?nsted acid strength remarkably increased with increasing tin content. The decomposition temperature of sulfate radical in the modified catalyst was 100 °C greater and its mass loss was more than twice that of the SO₄^2?/TiO₂ catalyst. The SO₄^2?/TiO₂-SnO₂ catalyst was designed to synthesize 1,6-hexanediol diacrylate by esterification of 1,6-hexanediol with crylic acid. The yield of 1,6-hexanediol diacrylate exceeded 87% under the optimal reaction conditions: crylic acid to 1,6-hexanediol molar ratio = 3.5, catalyst loading = 7%, reaction temperature = 130 °C and reaction time = 3 h. The modified catalyst exhibited excellent reusability and after 10 cycles the conversion of 1,6-hexanediol was above 81%.

Full text of DOI:10.1016/S1872-2067(16)62474-9

Chinese Journal of Catalysis 37 (2016) 1469–1476
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
2−
A new solid acid SO4 /TiO2 catalyst modified with tin to synthesize
1,6‐hexanediol diacrylate
Xiaxia Baia, Liuyi Panb, Peng Zhaoc, Daidi Fana, Wenhong Lia,*
a School of Chemical Engineering, Northwest University, Xi’an 710069, Shaanxi, China
b School of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, Shaanxi, China
c School of Pharmacy, Shaanxi University of Chinese Medicine, Xianyang 712046, Shaanxi, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A new solid acid catalyst, SO42‒/TiO2 modified with tin, was prepared using a sol‐gel method and its
physicochemical properties were revealed by nitrogen adsorption‐desorption, X‐ray powder dif‐
fraction, scanning electron microscopy, Fourier transform infrared spectroscopy, infrared spec‐
troscopy of adsorbed pyridine, temperature‐programmed desorption of ammonia and thermal
gravimetric analysis. The structure, acidity and thermal stability of the SO42‒/TiO2‐SnO2 catalyst
were studied. Incorporating tin enlarged the specific surface area and decreased crystallite size of
the SO42‒/TiO2 catalyst. The total acid sites of the modified catalyst increased and Brönsted acid
strength remarkably increased with increasing tin content. The decomposition temperature of sul‐
fate radical in the modified catalyst was 100 °C greater and its mass loss was more than twice that
of the SO42‒/TiO2 catalyst. The SO42‒/TiO2‐SnO2 catalyst was designed to synthesize 1,6‐hexanediol
diacrylate by esterification of 1,6‐hexanediol with crylic acid. The yield of 1,6‐hexanediol diacrylate
exceeded 87% under the optimal reaction conditions: crylic acid to 1,6‐hexanediol molar ratio = 3.5,
catalyst loading = 7%, reaction temperature = 130 °C and reaction time = 3 h. The modified catalyst
exhibited excellent reusability and after 10 cycles the conversion of 1,6‐hexanediol was above 81%.
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Received 11 March 2016
Accepted 20 May 2016
Published 5 September 2016
Keywords:
Solid acid catalyst
Tin
Sol‐gel method
1,6‐Hexanediol diacrylate
Esterification reaction
Published by Elsevier B.V. All rights reserved.
1. Introduction
acrylates for radiation curing has caused a large market de‐
mand to improve the productivity and yields of acrylate syn‐
Acrylate monomers, including universal acrylates and spe‐
cial acrylates [1], are basic reaction monomers of high‐molecu‐
lar weight compounds and raw materials for organic reactions.
Special acrylates have been used in numerous areas such as
leather tanning, papermaking, spinning, coating, adhesives, ink
and especially radiation curing [2,3], despite their small scale
production with poor yields from industrial syntheses. Radia‐
tion curing materials consist of reactive diluents, photoinitia‐
tors, oligomers and additives, and a main component com‐
prised of a polymer of acrylate monomers such as polyester
acrylate, epoxy acrylate and urethane acrylate [4,5]. The use of
theses.
The largest obstacle to industry synthesis of acrylates is the
catalysts used. Post‐reaction separation of catalysts is difficult
and cause corrosion to equipment and environmental pollution
[6,7]. The catalysts used are generally not recyclable and re‐
quire rigorous reaction conditions [8]. Traditional acid cata‐
lysts (sulfuric acid and p‐toluene sulfonic acid [9]) are gradual‐
ly being replaced by solid acids such as metal salts [10], sulfat‐
ed metal oxides [11,12] and heteropoly acids [13]. The induc‐
tive effect of the S=O group in sulfated metal oxides
(SO42−/MOx) is so strong that the Lewis acidic strength of the
* Corresponding author. Tel: +86‐13186102519; E‐mail: liwenhong@nwu.edu.cn
DOI: 10.1016/S1872‐2067(16)62474‐9 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 9, September 2016

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Xiaxia Bai et al. / Chinese Journal of Catalysis 37 (2016) 1469–1476
metal cations increases, and if water is adsorbed on the cata‐
lyst, a hydrogen atom is readily ionized to generate a Brönsted
acid. These two acid types transform each other and coexist so
that SO42−/MOx are superacid catalysts [14]. This type of solid
acid catalysts has the following advantages: (i) strong acidic
strength; (ii) nontoxic, non‐corrosive and environmentally be‐
nign; (iii) liable to separate from the reaction system allowing
catalyst reuse. However, further application of SO42−/MOx cata‐
lysts is slightly restricted because of their low surface area,
poor thermal stability and it is especially difficulty to adjust
their acidity [15,16]. A common approach to enhance the cata‐
lytic activity of SO42‒/MOx solid acids is to combine frequently
used supports such as TiO2, ZrO2 or Fe2O3 with Si, Al, Mo or Sn
and some rare earth metals. Yan et al. [17] prepared SO42‒/ZrO2
and SO42‒/ZrO2‐Al2O3 catalysts by impregnation. Li’s group [18]
successfully designed a SO42‒/Fe2O3‐SiO2 catalyst with varied
acidic strength and Brönsted acid sites. Zhao et al. [19] ob‐
tained a SO42‒/ZrO2‐La2O3 catalyst by a surfactant‐assisted
co‐precipitation/hydrothermal crystallization with subsequent
impregnation method. Esterification of 1,6‐hexanediol with
crylic acid using other modified catalysts results in poor
1,6‐hexanediol diacrylate yields and the catalysts were not
reusable. These catalysts have been discounted for this esteri‐
fication reaction because of insufficient activity and severe de‐
activation of the esterification reaction. In this work, we modify
SO42‒/TiO2 catalysts by addition of tin (Sn) and evaluate their
use in the synthesis of 1,6‐hexanediol diacrylate.
A series of SO42‒/TiO2‐x%SnO2 catalysts were prepared us‐
ing a sol‐gel method and applied to catalyze the esterification
reaction of 1,6‐hexanediol with crylic acid. The structure of the
catalysts was analyzed using the nitrogen adsorption‐desorp‐
tion isotherms, X‐ray powder diffraction (XRD) patterns, scan‐
ning electron microscope (SEM) images, Fourier transform
infrared spectrogram (FT‐IR), infrared spectrogram of ad‐
sorbed pyridine, temperature‐programmed desorption profiles
of ammonia (NH3‐TPD) and thermogravimetric (TG) curves.
The yield of 1,6‐hexanediol diacrylate was used as an evalua‐
tion index of catalytic activity to attain the optimal reaction
conditions by varying acid to alcohol molar ratio, catalyst
amount, reaction temperature and time. Finally, the reusability
of the SO42‒/TiO2‐SnO2 catalyst was evaluated.
for 12 h and then ground to obtain the catalyst support
TiO2‐SnO2. Finally, the support was impregnated by sulfuric
acid (1 mol/L) for 1 h and calcined at 500 °C for 4 h.
2.2. Catalyst characterization
The textural properties of samples were measured from ni‐
trogen adsorption‐desorption isotherms at −196 °C on a TriStar
II 3020V instrument and the specific surface area was calcu‐
lated using the Brunauer‐Emmett‐Teller (BET) equation at a
relative pressure of 0.03–0.4. Crystal structural features were
determined from XRD patterns acquired on
a Rigaku
D/max‐2400 diffractometer over the range of 2θ = 10°−80° at a
scan rate of 8°/min using Cu Kα radiation (λ = 0.15406 nm). The
microscopic surface features of the catalysts were investigated
by SEM on a QUANTA‐200 at 1.0 kV. The characteristic func‐
tional groups of the samples were detected by FT‐IR spectros‐
copy on a Bruker EQUIN‐55 over the range of 400 to 4000 cm−1
with a resolution of 0.2 cm−1. The thermal properties of the
samples were recorded by TG curves on an American TA Q500
thermoanalyzer in nitrogen at a heating rate of 10 °C/min to
800 °C. The acidity of samples were determined by FT‐IR of
adsorbed pyridine on a Nicolet Nexus 476 FT‐IR spectrometer.
Samples were pretreated at 400 °C for 4 h under vacuum and
then cooled to room temperature to saturate adsorbed pyri‐
dine. Samples were heated to 150 °C and excess pyridine evac‐
uated for 1 h before acquiring their infrared spectrogram. The
concentration of each type of acid sites was calculated accord‐
ing to equation C = IAπR2/(Wε) [20,21], where C = concentra‐
tion of Lewis or Brönsted acid sites (µmol/g), IA = integrated
absorbance of the Lewis or Brönsted bands (cm‒1), R = radius
of catalyst disk (cm), W = mass of catalyst disk (g), and ε = mo‐
lar extinction coefficient (εL = 2.22 cm/μmol and εB = 1.67
cm/μmol). The acidity of recycled catalysts was analyzed by
NH3‐TPD on a Micromeritics ChemiSorb 2720 auto‐adsorption
apparatus. Samples were activated at 300 °C for 2 h in a N2 flow
of 40 mL/min. After cooling to 100 °C, ammonia was adsorbed
by samples for 0.5 h. Samples were heated up to 700 °C at a
rate of 10 °C/min in order to desorb ammonia.
2.3. Catalyst performance
2. Experimental
The modified SO42‒/TiO2‐SnO2 catalysts were applied to the
synthesis of 1,6‐hexanediol diacrylate by esterification of
1,6‐hexanediol with crylic acid (Scheme 1). Reactants were
charged to a 100‐mL flask equipped with a water separator and
magnetic stirring, followed by addition of moderate amount of
the catalyst, azeotropic agent and polymerization inhibitor. The
reaction system was heated, driving the water produced in the
esterification reaction from the mixture. This process inevitably
removed some acid because the azeotropic agent also formed
weak interactions with organic molecules in the reaction sys‐
2.1. Catalyst preparation
TiCl4 and a mass fraction of SnCl4·5H2O were dissolved in
deionized water and the pH adjusted to 9 by addition of con‐
centrated ammonia. The reaction mixture was aged for 24 h at
a low temperature, the precipitate was separated by filtration
and washed repeatedly using deionized water until the filtrate
was free of chloride ions. The isolated solid was dried at 110 °C
Scheme 1. Synthesis of 1,6‐hexanediol diacrylate by esterification of 1,6‐hexanediol with crylic acid.

Xiaxia Bai et al. / Chinese Journal of Catalysis 37 (2016) 1469–1476
1471
tem. Feeds and products were analyzed by a GC2060 gas
Table 1
Structural parameters of the SO42‒/TiO2‐x%SnO2 catalysts.
chromatograph with an OV‐17 capillary column (30 m × 0.53
mm × 1.0 μm) and a flame ionization detector at 350 °C. The
column temperature was initially held at 70 °C for 2 min, then
heated to 260 °C at a rate of 10 °C/min, and held at 260 °C for
10 min. A standard curve of 1,6‐hexanediol at different concen‐
trations was prepared to allow the content in samples to be
measured by an external standard method. The content of
1,6‐hexanediol diacrylate in samples was based on the purity
test of acrylate monomer with polyfunctional groups using the
industrial standard for radiation curing [22]. The conversion of
1,6‐hexanediol (X) and the yield of 1,6‐hexanediol diacrylate
(Y) were used as proxies for the catalytic activity of the
SO42‒/TiO2‐SnO2 samples:
Catalyst
SO42‒/TiO2
A
BET (m2/g)
63
Vp (cm3/g)
0.094
DBJH (nm)
10.52
9.98
SO42‒/TiO2‐3%SnO2
SO42‒/TiO2‐6%SnO2
SO42‒/TiO2‐9%SnO2
124
0.223
126
102
0.229
0.193
9.95
10.06
in the quantity of pores from 5 nm to 10 nm in the catalysts
doped with tin, especially for (3–6)%SnO2. This phenomenon is
explained by the fact that an appropriate amount of tin weak‐
ens the diffusion coefficient of the material surface and indi‐
rectly hinders the agglomeration of the crystallites [27]. Pore
volume (VP) was approximately obtained from the adsorbed
volume of nitrogen at the maximum relative pressure, while
specific surface area (ABET) was calculated by the BET equation
at lower pressure, and pore diameter (DBJH) was calculated by
the Barrett‐Joiner‐Halenda (BJH) method at medium pressure
(Table 1). The specific surface area of the SO42–/TiO2‐6%Sn
catalyst was 126 m2/g, which was twice as large as that of the
unmodified catalyst. Both the specific surface area and pore
volume increased and then decreased with increasing amounts
of tin. The reason for the decrease may be that excessive tin
integrated into bulky particles that covered the pores of cata‐
lyst, resulting in lower nitrogen adsorbing capacity.
X = ((mA)feed ‒ (mA)product)/(mA)feed × 100%
Y = (118 × (mC)product)/(226 × (mA)feed ‒(mA)product) × 100%
where mA is the mass of 1,6‐hexanediol, and mC is the mass of
1,6‐hexanediol diacrylate.
Solid catalyst was recovered from the reaction mixture at
the end of the reaction and recycled for 10 experiments using
optimal reaction conditions in order to examine the reusability
of the SO42‒/TiO2‐SnO2. The recovered catalyst was washed
with toluene and dried at 100 °C before the next cycle. The
amount of reactant was decrease in proportion to the lost cata‐
lyst mass.
3. Results and discussion
3.1.2. Crystalline structure
The crystal structure of the SO42‒/TiO2‐x%SnO2 catalysts
was analyzed by XRD (Fig. 2). The prominent anatase crystal
peak of titanium oxide was observed in the SO42‒/TiO2 samples
at around 2θ = 25.3°, which is assigned to the crystallographic
plane (1 0 1). A series of weak peaks at around 2θ = 37.8°,
48.0°, 53.9°, 62.7° are assigned to the (0 0 4), (2 0 0), (1 0 5)
and (2 0 4) planes, respectively. The characteristic peaks of
rutile titanium oxide are absent, indicating that the catalyst
effectively inhibits the phase transformation of titanium oxide
from anatase to rutile [28,29]. The crystalline phase of tin oxide
is not observed in the XRD patterns of SO42‒/TiO2‐SnO2 until
the loading value of tin increases to 9%. For this sample, there
is a new peak at 2θ = 27.2° which is assigned to the prismatic
crystal of tin oxide. The XRD results reveal that tin oxide was
3.1. Structural analysis of catalysts
3.1.1. Structural parameters
The nitrogen adsorption‐desorption isotherms and corre‐
sponding pore diameter distribution curves of the tin doped
SO42‒/TiO2 catalysts are shown in Fig. 1. All of nitrogen adsorp‐
tion‐desorption isotherms are classical type IV according to
IUPAC classification and present a H1 hysteresis loop in a p/p0
range of 0.4–0.9, which is a common characteristic of mesopo‐
rous material (Fig. 1(a)) [23–26]. All of the SO42‒/TiO2‐x%SnO2
catalysts exhibited a narrow distribution of small sized pores in
the range of 5–15 nm, with the pore diameter mainly distrib‐
uted around 10 nm (Fig. 1(b)). There was a prominent increase
160
0.8
(3)
(2)
(b)
(a)
0.6
0.4
0.2
0.0
120
80
40
0
(2)
(4)
(3)
(4)
(1)
(1)
10
0.0
0.2
0.4
0.6
0.8
1.0
0
20
30
40
50
60
Relative pressure (p/p0)
Pore diameter (nm)
Fig. 1. (a) N2 adsorption‐desorption isotherms and (b) pore diameter distribution curves of SO42‒/TiO2‐x%SnO2. (1) x = 0; (2) x = 3; (3) x = 6; (4) x = 9.

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Xiaxia Bai et al. / Chinese Journal of Catalysis 37 (2016) 1469–1476
(1)
1406
(1)
(2)
1627
(2)
(3)
1240
1139
1049
1047
995
995
1236
1234
1232
1137
(4)
(3)
(4)
1043
1137
1134
995
1043
10
20
30
40
2/(o )
50
60
70
80
1800 1700 1600 1500 1400 1300 1200 1100 1000 900
Wavenumber (cm1)
Fig. 2. XRD patterns of the SO42‒/TiO2‐x%SnO2 catalysts. (1) x = 0; (2) x
= 3; (3) x = 6; (4) x = 9.
Fig. 4. FT‐IR spectra of the SO42–/TiO2‐x%SnO2 catalysts. (1) x = 0; (2) x
= 3; (3) x = 6; (4) x = 9.
catalysts was detected by FT‐IR spectrometry (Fig. 4). Sulfate
groups can combine with metallic oxide into inorganic chelat‐
ing bidentate, bridge bidentate and covalent structures [31].
The common band at 1627 cm−1 is assigned to the bending
mode of water molecules associated with sulfate groups
[17,32]. The bands at 1049, 1139 and 1240 cm−1 are ν3 vibra‐
tions of bidentate sulfate groups in a C2ν symmetry, corre‐
sponding to the asymmetric stretching vibration of S–O, the
symmetric stretching vibration of S=O, and the asymmetric
stretching vibration of S=O, respectively [21,33–35]. The band
at 1406 cm−1 is attributed to the asymmetric stretching vibra‐
tion of S=O in a covalent sulfate structure and belongs to the
split band of ν3 vibrations, which is triggered by the interaction
between a sulfate group and metallic oxide in a severely dehy‐
drated condition [34,36]. A new sulfate group band appears at
995 cm‒1 for the catalysts with added tin and is considered to
be a ν1 symmetric stretching vibration of S–O in a C2ν sym‐
metry, suggesting the addition of tin decreased the symmetry
of bidentate sulfate groups [21,34]. The bidentate sulfate group
bands are slightly blue‐shifted with increasing tin content. This
phenomenon reveals that more and more bidentate sulfate
groups connect with metallic elements by chelating instead of
via a bridge structure because the frequency of ν3 splits in che‐
late structures are higher than those from bridged structures
[36]. The intensity of bands that correspond to the asymmetric
stretching frequency of S=O, which is directly related to the
acidic sites [18], increases with tin loading, and the catalyst
modified by 6% Sn exhibits the maximal acidity.
Table 2
Crystallite size of the SO42‒/TiO2‐x%SnO2 catalysts.
Catalyst
SO42‒/TiO2
SO42‒/TiO2‐3%SnO2
SO42‒/TiO2‐6%SnO2
SO42‒/TiO2‐9%SnO2
Crystallite size (nm)
13.6
12.2
11.5
10.3
excellently dispersed on the surface or within the channels of
the catalysts [30], indicating a large surface area for the tin
doped catalysts. The size of the crystal structure of each cata‐
lysts was calculated by Scherrer’s formula (Table 2). Crystallite
size decreased with increasing tin, which is explained by the
same principle as that used to explain the increases in pore
quantity.
3.1.3. Microscopic features
The SEM images of SO42‒/TiO2 and SO42‒/TiO2‐6%SnO2 cat‐
alysts show that tin makes a difference to the shape and ap‐
pearance of catalyst (Fig. 3). As expected, the appearance of the
SO42‒/TiO2 catalyst looks like tabulate, while the appearance of
the SO42‒/TiO2‐SnO2 catalyst looks like faveolate. The
SO42‒/TiO2‐SnO2 catalyst has better dispersity and polyporous
structure, which is in accordance with the results of the nitro‐
gen adsorption‐desorption and XRD analyses.
3.2. Acidity analysis of catalysts
3.2.1. Fourier transform‐infrared spectra
The change of chemical bonds in the SO42‒/TiO2‐x%SnO2
3.2.2. Infrared spectra of pyridine
The IR spectra of pyridine adsorbed on metal oxides is used
to determine the nature of acid sites. Of the four ring vibrations
(vCCN) modes (8a, 8b, 19a, and 19b), the 8a and 19b vibration
modes are the most sensitive to interactions of the pyridine
molecules nitrogen lone pair electrons [37]. Brönsted acid sites
are detected by reaction with pyridine, and pyridinium ion
bands appear at about 1637 cm‒1 (ν8a) and 1541 cm‒1 (ν19b);
whereas pyridine molecules coordinate with Lewis acid sites
and these are detected at around 1622 cm‒1 (ν8a) and 1454
cm‒1 (ν19b) [20,38–41]. Both Brönsted and Lewis acid site coex‐
(a) SO42-/TiO2
(b) SO42-/TiO2-6%SnO2
Fig. 3. SEM images of (a) SO42−/TiO2 and (b) SO42−/TiO2‐6%SnO2 cata‐
lysts.

Xiaxia Bai et al. / Chinese Journal of Catalysis 37 (2016) 1469–1476
1473
0
B
-3
(1) mass loss
5.2%
B+L
-6
L
-9
B
L
(3)
-12
-15
-18
-21
(4)
(2)
(2)
mass loss
11.0%
(1)
0
100 200 300 400 500 600 700 800
Temperature (oC)
1700
1650
1600
1550
1500
1450
1400
Wavenumber (cm1)
Fig. 6. TG profiles of SO42‒/TiO2 (1) and SO42‒/TiO2‐6%SnO2 (2) cata‐
Fig. 5. IR spectra of pyridine adsorbed on the SO42‒/TiO2‐x%SnO2 cata‐
lysts. (1) x = 0; (2) x = 3; (3) x = 6; (4) x = 9.
lysts.
3.4. Performance of catalyst in the esterification reaction
Table 3
The amount of acid sites in the SO42−/TiO2‐x%SnO2 catalysts.
3.4.1. Catalytic activity studies
Amount of acid sites (μmol/g)
Catalyst
The activity of the SO42‒/TiO2‐x%SnO2 catalyst was assessed
by the conversion of 1,6‐hexanediol and the yield of
1,6‐hexanediol diacrylate. The amount of tin doped in the
SO42‒/TiO2‐x%SnO2 influenced catalytic activity (Fig. 7). The
reaction conditions used were a crylic acid to 1,6‐hexanediol
molar ratio = 3.5, catalyst loading = 7%, esterification temper‐
ature = 130 °C and time = 3 h. The yield of 1,6‐hexanediol di‐
acrylate raised from 59.7% for the unmodified catalyst to
81.6% for the catalyst with 3% SnO2. Increasing the tin content
to 6% increased the consumption of 1,6‐hexanediol and the
yield of 1,6‐hexanediol diacrylate to 95% and 87%, respective‐
ly. The catalytic activity of the modified SO42‒/TiO2‐SnO2 was
superior to that of the conventional SO42‒/TiO2 catalyst.
Brönsted acid
Lewis acid
Total acid
59
SO42‒/TiO2
18
65
122
96
41
43
45
46
SO42‒/TiO2‐3%SnO2
SO42‒/TiO2‐6%SnO2
SO42‒/TiO2‐9%SnO2
108
167
142
ist in the series of SO42‒/TiO2‐x%SnO2 catalysts (Fig. 5). The
band at 1489 cm‒1 is ascribed to corporate vibrations of a co‐
ordinated pyridine molecule and a pyridinium ion. The inten‐
sity of the band at 1541 cm‒1 obviously increases with the
amount of tin in the catalysts and Brönsted acid sites range
from 18 μmol/gto a maximum of 122 μmol/g (Table 3), while
Lewis acid sites remain relatively stable at around 40 μmol/g.
The number of total acid sites for the catalyst with 6%SnO2 was
167 μmol/g, indicating that modifying the catalyst SO42‒/TiO2
with tin increased the number of acid sites.
The effects of crylic acid to 1,6‐hexanediol molar ratio, cata‐
lyst (SO42‒/TiO2‐6%SnO2) loading, reaction temperature and
reaction time on the esterification are depicted in Fig. 8. The
conversion of 1,6‐hexanediol monotonically increases with
increasing acid to alcohol molar ratio, while the yield of
1,6‐hexanediol diacrylate increased to a maximum value of
84.5% at a molar ratio of 3.5 and decreased suddenly at a high‐
er ratio. This trend is likely because the esterification reaction
is reversible and abundant crylic acid can drive the equilibrium
3.3. Thermal analysis of catalysts
TG curves analyzing the thermal properties of the
SO42‒/TiO2 and SO42‒/TiO2‐6%SnO2 catalysts are displayed in
Fig. 6. The weightlessness of catalysts mainly occurred at two
various temperature sections. The first weightlessness is
caused by the desorption of surface water below 200 °C [42].
The second weightlessness occurs above 500 °C and could be a
result of the loss of a sulfur source on the surface of the catalyst,
which is caused by the collapse of porous channels at high
temperature [31]. The decomposition temperature of sulfate
radicals is 630 °C in the SO42‒/TiO2‐SnO2 catalyst, approxi‐
mately 100 °C higher than the SO42‒/TiO2 catalyst, suggesting
that the bond between the sulfate radicaland titanium oxide is
more thermally stability in the catalyst with moderate tin dop‐
ing. The mass of sulfate radicallost in the SO42‒/TiO2‐SnO2 cat‐
alyst (11.0%) is more than twice that of the SO42‒/TiO2 catalyst,
which shows that the addition of tin contributes to the sulfate
radical site in place of hydroxide radical sites and produces
more acid sites.
110
Conversion of 1,6-hexanediol
Yield of 1,6-hexanediol diacrylate
100
90
80
70
60
50
9
3
6
0
Content of tin (%)
Fig. 7. Effects of tin content on the catalytic activity.

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Xiaxia Bai et al. / Chinese Journal of Catalysis 37 (2016) 1469–1476
100
90
80
70
60
50
40
30
(1)
(1)
(1)
(1)
(2)
(2)
(2)
(2)
(a)
(c)
(b)
(d)
2.0 2.5 3.0 3.5 4.0 0
Acid/alcohol molar ratio
2
4
6
8
10 100 110 120 130 140
Esterification temperature (oC)
1
2
3
4
5
Esterification time (h)
Catalyst amount (%)
Fig. 8. Reaction condition effects on 1,6‐hexanediol conversion (1) and 1,6‐hexanediol diacrylate yield (2). (a) Acid/alcohol molar ratio (7%, 3 h, 130
°C); (b) Catalyst amount (3.5, 3 h, 130 °C); (c) Esterification temperature (3.5, 7%, 3 h); (d) Esterification time (3.5, 7%, 130 °C).
towards products. However, excess crylic acid has more of a
chance to react with 1,6‐hexanediol diacrylate to form
side‐products, such as homopolymer and copolymer, when the
reaction reaches equilibrium, resulting in a decrease of ester
yield and raising the cost of product separation. The catalytic
effect is not significant when the amount of SO42‒/TiO2‐SnO2
used in the reaction is small because of a limited quantity of
available acid sites (Fig. 8(b)). The conversion of 1,6‐hexanediol
increases obviously from 71.4% for 1% catalyst to 93.8% for
7% catalyst. Increasing the catalyst amount further did not
increase the amount of 1,6‐hexanediol converted and the yield
of 1,6‐hexanediol diacrylate dropped. A large amount of cata‐
lyst increases catalytic activity, but the polyporous catalyst has
a strong adsorption function for products and a large amount of
1,6‐hexanediol diacrylate is adsorbed in the porous channel of
the catalyst. Esterification temperature and time also influence
the activity of catalyst in the reaction. The conversion of
1,6‐hexanediol and the yield of 1,6‐hexanediol diacrylate have
the same trend with increasing reaction temperature (Fig.
8(c)). Increasing the reaction temperature up to 130 °C im‐
proved both conversion and yield to maxima of 95.9% and
87.3%, respectively. The drop of 1,6‐hexanediol conversion
from 95.9% to 89.2% at higher temperature is likely caused by
the exothermal effect of the esterification reaction, with high
temperature suppressing reaction progress and shifting the
reaction equilibrium towards reactants. Meanwhile, polymeri‐
zation reactions occur more readily at high temperature, low‐
ering product yield. Conversion and yield are also dependent
on reaction time, with conversion of 1,6‐hexanediol increasing
drastically from 54.2% after 1 h to 94% after 3 h (Fig. 8(d)).
Reactions longer than 3 h did not convert substantially more
1,6‐hexanediol and the yield of the target decreased slightly.
This trend is probably caused by the polymerization of
1,6‐hexanediol diacrylate and crylic acid after esterification
equilibrium has been reached.
3.4.2. Catalyst reusability studies
The reusability of a catalystplays a crucial role in evaluating
its performance, and we conducted 10 repeat experiments to
determinate the stability of the SO42‒/TiO2‐SnO2 catalyst. The
conversion of 1,6‐hexanediol was above 93% for the first three
cycles because the catalyst removed some of the polar impuri‐
ties and water by accumulating at the acid sites (Fig. 9). The
conversion dramatically decreases from 93.7% to 82.0% for
the successive five cycles, which can be attributed to leaching of
sulfur sources from the surface of the titanium oxide. The acid
strength and the amount of acid sites on the surface of the
modified catalyst were determined after the third and eighth
reaction cycles by NH3‐TPD (Fig. 10). There are narrow desorp‐
tion peaks of NH3 at about 120 °C, corresponding to the weak
acids, and broad distributed peaks in the range of 200–450 °C,
corresponding to the medium and strong acids [8,17]. The
abundance of acid sites is listed in Table 4. The distribution of
100
95
90
85
80
75
70
65
60
(1)
(2)
100
200
300
400
500
600
700
1
2
3
4
5
6
7
8
9
10
Temperature (oC)
Cycle times
Fig. 10. NH3‐TPD profiles of the SO42‒/TiO2‐6%SnO2 catalyst after the
third (1) and eighth (2) cycles.
Fig. 9. Reusability of the SO42–/TiO2‐6%SnO2 catalyst.

Xiaxia Bai et al. / Chinese Journal of Catalysis 37 (2016) 1469–1476
1475
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Chin. J. Catal., 2016, 37: 1469–1476 doi: 10.1016/S1872‐2067(16)62474‐9
A new solid acid SO42−/TiO2 catalyst modified with tin to synthesize 1,6‐hexanediol diacrylate
Xiaxia Bai, Liuyi Pan, Peng Zhao, Daidi Fan, Wenhong Li*
Northwest University; Baoji University of Arts and Sciences; Shaanxi University of Chinese Medicine
110
Conversion of 1,6-hexanediol
Yield of 1,6-hexanediol diacrylate
HOCH2-(CH2)4-CH2OH + 2CH2=CH-COOH
CH2=CH-CO-O-(CH2)6-O-CO-CH=CH2
100
90
80
70
60
50
SO42-/TiO2-SnO2
3
6
9
0
Acidity
Structure
Stability
Content of tin (%)
This work aimed to modify the solid acid catalyst used to synthesize 1,6‐hexanediol diacrylate. The SO42−/TiO2 catalyst doped with tin is
superior to the unmodified catalyst, which is explained by the structural features, acidity and thermostability of the modified catalyst.

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