Nano-sized mesoporous phosphated tin oxide as an efficient solid acid catalyst

Article abstract of DOI:10.1039/C8RA08962K

Herein, we prepared a mesoporous tin oxide catalyst (mSnO₂) activated with phosphate species by the adsorption of phosphate ions from a phosphoric acid solution onto tin oxyhydroxide (Sn(OH)₄) surface. The phosphate content ranged from 3 to 45 wt%. The nonaqueous titration of n-butylamine in acetonitrile was used to determine the total surface acidity level. FTIR of chemically adsorbed pyridine was used to differentiate between the Lewis and Br?nsted acid sites. Thermal and X-ray diffraction analysis indicated that the addition of phosphate groups stabilized the mesostructure of mSnO₂ and enabled it to keep its crystalline size at the nanoscale. FTIR analysis indicated the polymerization of the HPO₄^2? groups into P₂O₇^4?, which in turn reacts with SnO₂ to form a SnP₂O₇ layer, which stabilizes the mesoporous structure of SnO₂. The acidity measurements showed that the phosphate species are distributed homogeneously over the mSnO₂ surface until surface saturation coverage at 25 wt% PO₄^3?, at which point the acid strength and surface acidity level are maximized. The catalytic activity was tested for the synthesis of hydroquinone diacetate, where it was found that the % yield of hydroquinone diacetate compound increased gradually with the increase in PO₄^3? loading on mSnO₂ until it reached a maximum value of 93.2% for the 25% PO₄^3?/mSnO₂ catalyst with 100% selectivity and excellent reusability for three consecutive runs with no loss in activity.

Full text of DOI:10.1039/C8RA08962K

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Nano-sized mesoporous phosphated tin oxide as
an efficient solid acid catalyst†
ab
Cite this: RSC Adv., 2019, 9, 810
ac
S. M. Hassan,a M. A. Mannaa
and Amr Awad Ibrahim
*
Herein, we prepared a mesoporous tin oxide catalyst (mSnO2) activated with phosphate species by the
adsorption of phosphate ions from a phosphoric acid solution onto tin oxyhydroxide (Sn(OH)4) surface.
The phosphate content ranged from 3 to 45 wt%. The nonaqueous titration of n-butylamine in
acetonitrile was used to determine the total surface acidity level. FTIR of chemically adsorbed pyridine
was used to differentiate between the Lewis and Bronsted acid sites. Thermal and X-ray diffraction
¨
analysis indicated that the addition of phosphate groups stabilized the mesostructure of mSnO2 and
enabled it to keep its crystalline size at the nanoscale. FTIR analysis indicated the polymerization of the
2ꢀ
HPO4 groups into P2O74ꢀ, which in turn reacts with SnO2 to form a SnP2O7 layer, which stabilizes the
mesoporous structure of SnO2. The acidity measurements showed that the phosphate species are
3ꢀ
distributed homogeneously over the mSnO2 surface until surface saturation coverage at 25 wt% PO4
,
at which point the acid strength and surface acidity level are maximized. The catalytic activity was tested
for the synthesis of hydroquinone diacetate, where it was found that the % yield of hydroquinone
Received 29th October 2018
Accepted 18th December 2018
3ꢀ
diacetate compound increased gradually with the increase in PO4 loading on mSnO2 until it reached
DOI: 10.1039/c8ra08962k
a maximum value of 93.2% for the 25% PO43ꢀ/mSnO2 catalyst with 100% selectivity and excellent
reusability for three consecutive runs with no loss in activity.
rsc.li/rsc-advances
problems.2 Oil rening and pharmaceutical industries utilize
Introduction
2ꢀ
solid catalysts with acidic properties.6–8 SO42ꢀ, PO43ꢀ, WO4
,
Clꢀ and Fꢀ supported on metal oxides have been used to
increase their thermal stability, mesoporosity, and more criti-
cally, the acidity level of such catalysts.9–12 Most of the published
works to date have been focused on SO42ꢀ- and WO42ꢀ-sup-
ported metal oxide systems,13–18 which have been found to be
suitable for specic acid-catalyzed reactions. The acidity of such
systems strongly depend on the preparation method.11,14,16
A template pathway for preparing mesoporous materials is
a novel process to produce materials with high activity for the
synthesis of industrially relevant chemicals.19–22 Mesoporous
materials are characterized by a high surface area, ordered pore
channels, and large pore volume value. These advantages can
facilitate active components achieving a highly dispersed state
on the support, which raises their activity for reacting mole-
cules.23,24 For better performance, an efficient heterogeneous
catalyst needs a higher acid strength level and density as well as
thermal stability to minimize catalyst poisoning and leaching.
Therefore, mesoporous materials, primarily metal oxides, have
attracted growing interest as a support in the eld of catalysis.25
Tin oxide is one of the most attractive functional materials
because of its potential applications, mainly as a catalyst and as
a carrier in supported catalysts.26 Besides its intrinsic non-
stoichiometry and crystal defects, tin oxide nanoparticles can
be used more efficiently as a catalyst due to the faster migration
of the oxide ions within the SnO2 nanoparticles to increase the
The replacement of liquid acid or acid halide catalysts, such as
H2SO4, H3PO4, HF and AlCl3, is a critical factor for the design of
cleaner processes for green catalytic action.1 The problems of
homogeneous liquid acid catalysis include toxicity, corrosion,
catalyst waste and difficulty in separation and recovery.2 The
economics of the separation cost majorly determine the
viability of such a process.3 Efforts have been made to design
environmentally safe processes and technologies to achieve
these goals. The best catalytic technology for solving the prob-
lems of the liquid acid catalyst is to replace it by a heteroge-
neous solid acid catalyst.4 The reuse and recycling of the catalyst
in heterogeneous processes are the main advantages of the
increased catalyst efficiency over homogeneous ones.5 The
presence of an interconnected network of a large number of
pores, acid sites of different strength and the hydrophobic
properties of solid catalyst surfaces support inhibiting the
processes of corrosion and separation and for limiting pollution
aChemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt.
E-mail: amr_awad@mans.edu.eg
bChemistry Department, Faculty of Education and Science, Amran University, Sa'dah,
Yemen
cDepartment of Chemistry, Virginia Commonwealth University, Richmond, VA 23284-
2006, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra08962k
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surface to volume ratio. Many authors have used high valence at 423 ꢁC corresponded to a weight loss of 3.1 wt%, which may
cations of metal oxide to promote the thermal stability and be due to the crystallization of tin oxyhydroxide into tetragonal
catalytic property of pure tin oxide.27,28 Therefore, it is of interest cassiterite SnO2.36 In the case of PO43ꢀ/Sn(OH)4 dried samples,
to study the novelties of phosphate-supported SnO2 as a solid the DTA in Fig. 2 shows exothermic peaks at 321 ꢁC, 334 ꢁC and
acid catalyst. Various chemical routes have been used to 342 ꢁC as the phosphate content increases for the 8%, 25% and
synthesize nanoparticles of mesoporous metal oxides, e.g., 45% PO43ꢀ/Sn(OH)4 samples, respectively. Also, the exothermic
precipitation, sol–gel, hydrothermal, microwave-assisted peaks of the crystallization of tin oxyhydroxide into mSnO2 were
syntheses and ultrasonic spray pyrolysis.29,30 The addition of also shied to higher temperatures as the phosphate content
anion groups to a mesostructure metal oxide during the prep- increased, reaching 427 ꢁC, 484 ꢁC and 504 ꢁC for the 8%, 25%
aration methods prevent the sintering process and thus lead to and 45% PO43ꢀ/mSnO2 samples, respectively, indicating that
improvements in their thermal stability and textural the addition of phosphate to mSnO2 may enhance the surfac-
properties.31,32
tant–tin oxyhydroxide surface interaction,37 thus increasing the
Further improvement is necessary, however, for the prepa- thermal stability of the tin oxyhydroxide phase and shiing its
ration of thermally stable mesoporous tin oxide. This process is crystallization to higher temperature. These results show no
3ꢀ
still a challenge to researchers.9,33 Unlike sulfated metal oxides, effect that could be assigned to the decomposition of PO4
phosphated metal oxides have received inadequate attention. species, which was attributed to the thermal stability when
Albeit, phosphate-based solid acids have proven quite effica- treated with phosphoric acid.38
cious in several industrially important acid-demanding
reactions.
Fig. 2S(A)† shows the low-angle XRD patterns of 3%, 25%,
and 35% PO43ꢀ/mSnO2 samples heated at 400 ꢁC. The XRD
The present study aimed to adopt a phosphate-modied curves exhibit one diffraction peak at 2q ¼ 2.4ꢁ, corresponding
mesoporous tin oxide (mSnO2) system as a catalyst. The modi- to the (100) reection, which was assigned to the presence of
ed catalytic systems (H3PO4/m-SnO2) were then applied to the hexagonal mesostructured SnO2.39,40 The same peak was
preparation of hydroquinone diacetate, which is widely used as observed for the calcined samples at 400 ꢁC, 550 ꢁC and 650 ꢁC
an ingredient for skin bleaching cream. DTA was used to (Fig. 2S†), which indicates that the presence of phosphate
systematically elucidate the physico-chemical properties of the groups maintain the mesoporous structure of mSnO2.
H3PO4/mSnO2 catalysts, while TGA, XRD, TEM and FTIR tech-
Fig. 3A illustrates the wide-angle XRD patterns of mSnO2,
3ꢀ
niques were applied for characterization, and the texture and for the 3%, 25% and 35% PO4 /mSnO2 calcined at 400 ꢁC.
properties were determined from N2 adsorption at ꢀ196 ꢁC. The The samples were found to display four well-distinguished
surface acidity was also examined.
peaks at 2q ¼ 26.64ꢁ, 33.91ꢁ, 38.10ꢁ and 51.74ꢁ, corresponding
to the (110), (100), (200) and (211) reection planes of a tetrag-
onal cassiterite phase of mSnO2, respectively. The addition of
Results and discussion
3ꢀ
3ꢀ
PO4 to mSnO2 until 25 wt% PO4 shows an XRD pattern
3ꢀ
The thermograms of DTA and the TGA of dried PO43ꢀ/Sn(OH)4
samples are shown in Fig. 1. The sample showed endothermic
peaks at low temperature, which are assigned to the evolution of
physically adsorbed water. The sample also showed two
exothermic peaks. The rst exothermic effect with a weight loss
similar to that of the pure mSnO2, indicating that the PO4
species exists as amorphous phosphate groups on the mSnO2
3ꢀ
surface.41 When the loading of PO4 increased to 35 wt%,
a new reection peak appears at 2q ¼ 22.61ꢁ, which can be
assigned to a layer-structured SnP2O7.42 On the other hand, it is
clear from Fig. 3B that the intensity of the diffraction peaks of
the 3% PO43ꢀ/mSnO2 sample increases with the increase in the
ꢁ
of 8.9 wt% at around 312 C was attributed to the decomposi-
tion of the surfactant template and partial removal of the OH
groups (dehydroxylation),34,35 while the second exothermic peak
ꢁ
ꢁ
calcination temperature of the sample from 400 C to 650 C,
Fig. 1 DTA and TGA curves of dried Sn(OH)4.
Fig. 2 DTA curves of (a) 8%, (b) 25%, (c) 45% PO43ꢀ/m-SnO2 samples.
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Fig. 3 (A) Wide-angle XRD pattern of (a) m-SnO2 (400 ꢁC) and the
samples of PO43ꢀ/m-SnO2 (400 ꢁC) at (b) 3%, (c) 25%, (d) 35%, (B)
wide-angle XRD patterns of the sample 3% PO43ꢀ/m-SnO2 at (a)
400 ꢁC, (b) 550 ꢁC, (c) 650 ꢁC.
Fig. 4 TEM images of (a) mSnO2 calcined at 550 ꢁC, (b) 3% PO43ꢀ/m-
3ꢀ
SnO2 calcined at 400 ꢁC, (c) 3% PO4 /m-SnO2 calcined at 550 ꢁC, (d)
8% PO43ꢀ/m-SnO2 calcined at 400 ꢁC and (e) 25% PO43ꢀ/m-SnO2
calcined at 400 ꢁC.
which may be due to the increase in the dispersion of PO43ꢀ on
the mSnO2 surface, which in turn increase the thermal stability
3ꢀ
is apparent that mSnO2 has a signicant void fraction, which
may be due to the mesoporous structure of SnO2, and
concomitantly a rather low density. Fig. 4c–e show the TEM
images of the 3%, 8% and 25% PO43ꢀ/mSnO2 catalysts, which
indicate that when PO43ꢀ species are loaded on mSnO2 calcined
at 400 ꢁC, the lattice retains the ordered mesoporous channels
and the PO43ꢀ groups are embedded uniformly into the mSnO2
framework without disordering the mesostructured framework.
The selected area electron diffraction (SAED) pattern images
(insets) of the samples exhibit diffraction rings attributed to the
polycrystalline behaviour of the mSnO2, while the crystallite
degrees of the 3%, 8% and 25% PO43ꢀ/mSnO2 catalysts decrease
of mSnO2, arising from the interaction of PO4 and mSnO2.
The degree of crystallization of cassiterite mSnO2 gradually
decreases with the increase in the phosphate content, while the
width of the reections is considerably broadened, indicating
a small crystalline domain size (Table 1). The addition of
phosphate anions to the template synthesis reaction medium of
mSn(HO)4, substantially slows down the crystallization process
via retarding the crystal size growth by occupying the defects on
the surface of mSn(HO)4,43 thus increasing its thermal stability
and thus protecting the mesoporous structure from
collapsing.41 The crystallite size data were determined from X-
ray diffraction and are listed in Table 1, showing that the crys-
tallite size of PO43ꢀ/mSnO2 is smaller in comparison to the pure
mSnO2 nanocrystalline (13.70–11.30 nm). Furthermore, the
crystallite size of 3% PO43ꢀ/mSnO2 increases from 12.17 to
16.66 nm with the increase in calcination temperature (400–650
ꢁC), which indicates that pure mSnO2 and PO43ꢀ/mSnO2 are
present in the nanoscale. In the phosphate sample annealed
between 400 ꢁC and 650 ꢁC, the crystallite size slightly changes
from 13.70 to 16.66 nm, suggesting that the PO4 species remain
bonded to the mSnO2 surface, stabilizing it against sintering. It
was reported that the order of the mesostructured materials
strongly depends on the texture properties of the pores.44 An
analogous phenomenon was observed with TiO2, ZrO2 and
Fe2O3.44,45
3ꢀ
with the increase in PO4 content.46,47 The TEM images of 3%
PO43ꢀ/mSnO2 calcined at 550 ꢁC (Fig. 4b) illustrate that the
mesoporous structure remains even aer calcination at 550 ꢁC,
which reects the role of the phosphate groups in improving the
thermal stability of the tin oxide.47 It is clear from the TEM
images that the dark colour and the area occupied by the
phosphate group both increase with both the phosphate
content and the calcination temperature, indicating the great
dispersion of phosphate groups on the mesoporous SnO2
surfaces. Also, the side images (inset) show an increase in the
crystalline size with increasing calcination temperature, which
is in a good agreement with the results ꢁfrom the XRD analysis.
The FTIR spectra of the mSnO2 550 C, xPO43ꢀ/mSnO2 and
3PO43ꢀ/mSnO2 samples calcined at different temperatures are
shown in Fig. 5A and B. The band at 1625 cmꢀ1 appearing in the
TEM images of mSnO2 show an ordered structure with
a uniform mesoporous arrangement into hexagons (Fig. 4a). It
Table 1 Comparison of the structural parameters of mSnO2 and various PO43ꢀ/mSnO2 samples
Samples
D (nm)
d100 (nm)
dpore,BJH (nm)
a0 (nm)
twall (nm)
mSnO2-400
13.70
12.17
11.87
11.30
14.50
16.66
3.62
3.63
3.66
3.69
3.60
3.57
3.22
3.15
2.66
2.52
3.41
3.82
4.18
4.19
4.23
4.26
4.16
4.13
0.96
1.04
1.57
1.74
0.75
0.31
3%PO43ꢀ/mSnO2-400
25%PO43ꢀ/mSnO2-400
35%PO43ꢀ/mSnO2-400
3%PO43ꢀ/mSnO2-550
3%PO43ꢀ/mSnO2-650
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spectra of the samples even aer calcination at different
temperatures corresponds to the OH bending vibrations. The
FTIR spectra of phosphate mSnO2 generally show absorption
bands at 1035 and 1150, 1360, 1410 and 1520 cmꢀ1. The
absorption bands at 1360, 1410 and 1520 cmꢀ1 could be
assigned to P–O stretching frequency bonds in which their
order is close to two P]O, phosphoryl groups.48 By increasing
the temperature to 650 ꢁC, the broadband at 1550–1300 cmꢀ1 is
reduced to one sharp band and one shoulder, due to the
formation of bidentate-bound phosphate ions (C2V point
group). The band at 1035 and the shoulder at 1150 cmꢀ1 are
Scheme 1 Formation of PO43ꢀ/mSnO2.
with the increase in phosphate content from 3.15 to 2.52 nm
(Table 1). The approximate thickness of the porous wall (twall
3ꢀ
frequencies for PO4 and the position of such a fundamental
)
band was observed in phosphated zirconia.49 The fading of the
shoulder and the presence of a broad and sharp band located at
calculated from the difference between the hexagonal unit cell
parameter (a0 ¼ 2d100/O3) and dpore,BJH (Table 1), aer calcina-
1055 cmꢀ1 when the PO4 content was increased to 45 wt%
3ꢀ
tion at 400 ꢁC twall increased with increasing the phosphate
3ꢀ
3ꢀ
may be due to the dropping of the symmetry in the free PO4
content from 0.96 nm for mSnO2 to 1.74 nm for the 35% PO4
/
(Td point group) to either C3V or C2V.48 This may be due to the
mSnO2 samples. The dpore,BJH and twall values decreased with
the calcination temperature. On the other hand, the values of
d110-spacing and a0 were found to decrease with the increase in
calcination temperature, which may indicate that some changes
occur on the mesostructure as the calcination temperature of
PO43ꢀ/mSnO2 increases. The values of SBET, as shown in Table
3ꢀ
PO4 groups being bound to the tin oxide surface and being
distributed homogeneously.50,51 Here, the 1055 cmꢀ1 band is
divided into two bands at 1035 and 1150 cmꢀ1 at 650 ꢁC, which
could be assigned to the bidentate bound phosphate ions (C2V
2ꢀ
point group), which motivate the polymerization of the HPO4
groups into P2O74ꢀ, which is in a good agreement with the
appearance of a new peak due to SnP2O7 in the X-ray pattern
(Fig. 3A).
3ꢀ
2, increase gradually with the rise of PO4 content to reach
a maximum for 25% PO43ꢀ/mSnO2. This amount was optimal to
form a monolayer, and the values of SBET decreased with further
3ꢀ
These results indicate that phosphate groups are chemically
adsorbed on the surface of mSnO2 and still exist on the surface
even aer calcination of the samples at high temperatures. The
mechanism, as explained in Scheme 1, follows the dehydrox-
ylation reaction of P/mSnO2 samples, with amorphous
Sn(HPO4)2 formed on the surface, which is further heated to
form tin pyrophosphate, SnPꢁ2O7, which is in a good agreement
with the XRD pattern at 650 C.
Fig. 6 shows the nitrogen adsorption–desorption isotherms
of some representative samples of PO43ꢀ/mSnO2, in which the
isotherms exhibit a type IV feature, which is a characteristic
feature of mesoporous materials.52 Barrett–Joyner–Halenda
(BJH) analysis demonstrated that the average pore diameter
(dpore,BJH) of mSnO2 calcined at 550 ꢁC was 13.21 nm (Table 1).
The dpore,BJH of PO43ꢀ/mSnO2 samples was found to decrease
increases of the PO4 content.
Moreover, the SBET values were found to decrease when the
calcination temperature increased up to 650 ꢁC.50,51 The
increase in the surface area values of PO43ꢀ/mSnO2 over that of
mSnO2 may be due to the formation of two-dimensional surface
layers from Sn-PO43ꢀ, which stabilize the defects and inhibit the
collapsing of the mSnO2 surface,53 which also explains the role
3ꢀ
of PO4 in enhancing the porous structure of PO43ꢀ/mSnO2.
3ꢀ
The increase in PO4 content beyond 25 wt% led to the
production of more phosphate layers and/or the phosphate
groups attacking the pore mouths blocking them, and conse-
quently the surface area values decreased.54,55 The above results
3ꢀ
suggest that the interaction between PO4 and mSnO2 crys-
tallites not only stabilizes the crystallite structure in lower
dimensions, but also it keeps phosphate groups at the SnO2
surfaces, inhibiting the crystallite aggregation and acting as
a structure progeny director mediating nanoparticle growth and
assembly and stabilizing the mesostructure walls and
Fig. 5 (A) FTIR spectra of (a) m-SnO2 (550 ꢁC) and (b) 3%, (c) 25%, (d)
35%, (e) 45% PO43ꢀ/m-SnO2 catalysts calcined at 550 ꢁC. (B) FTIR Fig. 6 (A) Adsorption–desorption isotherms of nitrogen at ꢀ196 ꢁC (B)
spectra of 3% PO43ꢀ/m-SnO2 calcined at: (a) 450 ꢁC, (b) 550 ꢁC, (c) pore size distributions for PO43ꢀ/m-SnO2 at (a) 3%, (b) 8%, (c) 18%, (d)
650 ꢁC.
25%, (e) 35%, (f) 45% calcined at 400 ꢁC.
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Table 2 Surface characteristics of PO43ꢀ/m-SnO2 catalysts
a bridging phosphate group on the surface, as shown in Scheme
1, which causes a reduction in acidic hydrogen and a decrease
in the total acidity.
Catalyst
T (ꢁC) SBET m2 gꢀ1 dpore,BJH (nm) VT ml gꢀ1
Pyridine adsorption on the PO43ꢀ/mSnO2 samples was used
m-SnO2
550
400
400
29
65
95
130
155
113
75
63
56
41
3.22
3.15
2.96
2.81
2.66
2.52
2.46
3.35
3.41
3.82
0.13
0.16
0.17
0.18
0.18
0.14
0.14
0.15
0.15
0.14
3% PO43ꢀ/m-SnO2
8% PO43ꢀ/m-SnO2
¨
to specify the Bronsted (B) and Lewis (L) acid sites on the
sample surfaces. The FTIR spectra of pyridine adsorbed over the
18% PO43ꢀ/m-SnO2 400
25% PO43ꢀ/m-SnO2 400
35% PO43ꢀ/m-SnO2 400
45% PO43ꢀ/m-SnO2 400
¨
Bronsted (B) and Lewis (L) acid sites of the samples are shown
in Fig. 7A and B. Bands at around 1444 and 1452 cmꢀ1 were
characteristic of pyridine adsorbed on Lewis acid sites.59 The
bands at 1536 and 1553 cmꢀ1 were due to the pyridinium ion
3% PO43ꢀ/m-SnO2
3% PO43ꢀ/m-SnO2
3% PO43ꢀ/m-SnO2
450
550
650
adsorbed on Bronsted acid sites.60 Moreover, other FTIR bands
¨
appear at 1484 and 1503 cmꢀ1, attributed to the overlap of Lewis
61
¨
and Bronsted acid sites. The integrated areas of the bands at
1536 and 1452 cmꢀ1 were used to calculate the number of
¨
Bronsted and Lewis acid sites. It was evident from Fig. 7C that
increasing the thermal stability.56 Again, the increase in surface
area and pore volume and the decrease in the average pore
diameter of the 3% to 25% PO43ꢀ/mSnO2 samples over pure
mSnO2 with increasing the phosphate content indicate a rela-
tively high thermal stability of the prepared samples. The
decrease in the pore volume and dpore,BJH upon increasing the
phosphate content over 25 wt% may be due to the formation of
multi-phosphate layers, as proposed in Scheme 1, whereby the
surface hydroxyl groups of Sn(OH)4 are replaced by phosphate
groups and therefore there is an increased energy barrier and
less crystal growth sites, consequently inhibiting the crystal
growth.
The samples showed a bimodal pore size distribution con-
sisting of smaller and larger interparticle pores, as illustrated in
Fig. 6B. The addition of PO43ꢀ to mSnO2 was associated with an
increase in the number of mesopores and micropores, as
evident from the increase in the height of the distribution
maxima and slight shi to lower rH values until a maximum at
25 wt% phosphate was reached, where it begins to decrease.
Upon increasing the calcination temperature, (Fig. 3S†), both
the smaller and larger interparticle pores decreased, which is in
line with the decrease in the surface (Table 2). The width of the
¨
the samples have Bronsted and Lewis acid sites and these
increase with the increase in phosphate loading up to 25%
PO43ꢀ/mSnO2, and then decrease with further increases in
3ꢀ
¨
PO4 loading above 25 wt%. Moreover, the number of Bronsted
and Lewis acid sites decreases gradually with the increase in
calcination temperature over 400 ꢁC (Table 3). The surface
distribution and interaction of phosphate species with the SnO2
¨
surface increase the number of Bronsted and Lewis acid sites.
In analogy with sulfated tin oxide,62 the increase in Lewis acidity
by phosphate loading may be due to the formation of a tin oxide
surface phosphorus complex containing P]O covalent bonds,
and so the inductive effect of these covalent bonds of P]O
strengthen the Lewis acid character of the sample, as suggested
in Scheme 2.63,64 Fig. 5S† shows the NH3-TPD results for the pure
ꢁ
m-SnO2 calcined at 400 C, which shows a broad peak of NH3
desorption at 240 ꢁC, which indicates the presence of weak acid
sites. Aer modifying the mSnO2 with PO43ꢀ, thiꢁs peak starts to
ꢁ
disappear and two peaks start to appear at 334 C and 440 C,
corresponding to moderate and strong acid sites, respectively.
For 45% PO4 , only one peak at 440 ꢁC is seen, corresponding
to the strong acid sites due to the saturation of the surface with
the PO4 groups.26
Fig. 8A and B show the effect of the phosphate content and
consequently, the number of Bronsted and Lewis acid sites on
the % formation of hydroquinone diacetate compound. As can
be seen, the % yield of hydroquinone diacetate compound
3ꢀ
3ꢀ
mesopore type increased with phosphate content, which
3ꢀ
conrmed the role of PO4
mSnO2.56
in increasing the porosity of
¨
The nonaqueous potentiometric titration curves of n-butyl-
amine of Fig. 4S† were used to calculate the total number of acid
3ꢀ
increases gradually with the increase in PO4
loading on
3ꢀ
sites and the acid strength of the PO4 /mSnO2 catalysts.57,58 As
mSnO2 until it reaches a maximum value of 93.2% for the 25%
PO43ꢀ/mSnO2 catalyst with a selectivity of 100%. The surface
evident from Fig. 4S† and the data in Table 3, the acidity and the
acid strength values of PO43ꢀ/mSnO2 changed with the phos-
phate loading and reached their maximum value for 25%
acidity and acid strength of PO43ꢀ/mSnO2 are enhanced and
3ꢀ
¨
attain a maximum at 25 wt% PO4 . Moreover, the Bronsted
and Lewis acid sites change with the same trend as the forma-
tion of hydroquinone diacetate compound, which may indicate
3ꢀ
ꢁ
PO4 /mSnO2 (Ei ¼ +480.6 mV) calcined at 400 C, which may
be due to the completion of a bidentate phosphate monolayer
on the tin oxide surface, as evident from the FTIR data. The
decrease in the total acidity and the acid strength upon
¨
that Lewis and/or Bronsted acid sites can catalyse the formation
of hydroquinone diacetate compound. No activity was observed
for pure mSnO2, which may be due to its low acidity and site
strength (+74 mV). As evident from Table 3 and Scheme 2,
increasing the calcination temperature from 400 C to 650 C
lead to a decrease in the catalytic activity, which is in line with
the decrease in surface area and the decrease of the number of
3ꢀ
increasing the PO4 loading may be due to the formation of
more phosphate layers on the mSnO2 surface. The calcination
ꢁ
temperature 400 C was the most suitable calcination temper-
ꢁ
ꢁ
ature for the generation of strong acidic sites.
The loss of surface acidity at higher calcination temperatures
+
ꢁ
up to 650 C may be due to the loss of H by dehydration with
the surface OH or OH of the next phosphate groups, forming
¨
Bronsted and Lewis acid sites. The calcination temperature is
an essential aspect in the synthesis of phosphated metal oxide
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Table 3 Acidity characteristics of PO43ꢀ/m-SnO2 catalysts
Catalyst
T (ꢁC)
Ei (mV)
No. of acid sites/g ꢂ 10ꢀ19
B/L
Conversion%
TOF hꢀ1
m-SnO2
400
400
400
400
400
400
400
450
550
650
74
0.84
0.96
1.08
1.20
1.33
0.96
0.72
0.90
0.84
0.78
0.54
1.04
0.82
1.12
1.68
1.14
1.48
1.07
1.52
1.36
0
8.6
0
3% PO43ꢀ/m-SnO2
8% PO43ꢀ/m-SnO2
18% PO43ꢀ/m-SnO2
25% PO43ꢀ/m-SnO2
35% PO43ꢀ/m-SnO2
45% PO43ꢀ/m-SnO2
25% PO43ꢀ/m-SnO2
25% PO43ꢀ/m-SnO2
25% PO43ꢀ/m-SnO2
320.4
372.6
450.7
480.6
421.8
382.3
295.6
274.6
203.5
222.88
1158.76
1818.31
1743.47
2083.70
2180.45
2360.83
1824.53
972.87
50.3
87.7
93.2
80.4
63.1
85.4
61.6
30.5
3ꢀ
Fig. 8 (A) Effect of PO4 content wt%/mSnO2 calcined at 400 ꢁC on
the number of Bronsted acid sites and % hydroquinone diacetate. (B)
¨
3ꢀ
Effect of PO4
content wt%/mSnO2 calcined at 400 ꢁC on the
number of Lewis acid sites and % hydroquinone diacetate.
catalyst due to its role in the interaction between the metal
oxide surface and phosphate groups for the generation of active
acid sites. The optimum operating conditions for an anion-
metal oxide catalyst was found to depend on the type of metal
oxides, supported anions and the calcination temperature.52
It was observed that with increasing the molar ratio from
1 : 1 to 1 : 2 and 1 : 3, the % yield of hydroquinone diacetate
increased gently from 60.2%, to 77.5% to 93.2%, respectively,
with 100% selectivity. However, a further increase in the acetic
anhydride molar content was accompanied by a decrease in the
percentage yield of hydroquinone diacetate to 83.1% (molar
ratio 1 : 4). As acetic anhydride acts as both a solvent and
reactant, an excess of acetic anhydride is used to facilitate the
reaction until the molar ratio of hydroquinone to acetic anhy-
dride is 1 : 3. Many factors may affect the catalyst activity; the
dilution effect of acetic anhydride (solvent effect) and/or the
catalyst active sites may be blocked due to a higher acetic
anhydride concentration, thereby decreasing the yield of
hydroquinone diacetate. Because the activation of acetic anhy-
dride is the key step in the formation of hydroquinone diac-
etate, the decrease in % yield of hydroquinone diacetate may be
due to the presence of low (molar ratio 1 : 1 and 1 : 2) or excess
Fig. 7 (A) FTIR spectra of pyridine adsorbed on (a) 3%, (b) 8%, (c) 18%,
(d) 25%, (e) 35%, (f) 45% PO43ꢀ/m-SnO2 calcined at 400 ꢁC. (B) FTIR
spectra of pyridine adsorbed on the 25% PO43ꢀ/m-SnO2 sample
calcined at (a) 400 ꢁC, (b) 450 ꢁC, (c) 550 ꢁC and (d) 650 ꢁC. (C) Effect of
3ꢀ
PO4 content on the number of Bronsted and Lewis acid sites.
¨
¨
(molar ratio 1 : 4) activated acetic anhydride – Bronsted acid
3ꢀ
Scheme 2 Lewis acid on PO4 /mSnO2.
sites and/or activated acetic anhydride – Lewis acid sites
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¨
the next step. Finally, the proton of the Bronsted acid site is
eliminated to give hydroquinone acetate. The hydroquinone
¨
acetate reacts with another acetic anhydride – Bronsted acid site
to give the nal product. The mechanism of the formation of
hydroquinone diacetate on Lewis acid sites starts by the
chemisorption of the carbonyl group on the acetic anhydride,
which breaks down to form acylium ion. Then the OH of
hydroquinone attacks acylium ion to form an intermediate,
which lose a proton to give hydroquinone acetate, which
undergoes the same steps to give the product hydroquinone
diacetate, Scheme 3B.
Experimental
Synthesis of PO43ꢀ/mSnO2
For the synthesis of mesoporous tin oxide (mSnO2), 15 ml of
25 wt% NH4OH solution was added to 100 ml solution con-
taining 2.0 g CTAB dropwise and stirred for 15 min until the
solution became homogeneous. 4.6 g SnCl4$5H2O was dissolved
in 100 ml of distilled water and then added to NH4OH CTAB
solution slowly.65,66 The pH was xed at 8.5. The product was
ltered and washed with 2 wt% CH3COONa solution until
excess surfactant, and Clꢀ ions were removed. The sample was
Fig. 9 Reusability test for 25% PO43ꢀ/mSnO2.
intermediates. Therefore, 1 : 3 molar ratio of reactants was
considered optimum for the further studies.
The reusability of the 25% PO43ꢀ/mSnO2 catalyst was
checked with three consecutive experiments by using the
recovered catalyst. The catalyst was separated by ltration,
washed with acetone several times and dried overnight at
ꢁ
then dried overnight at 800 C.
ꢁ
A series of 3, 8, 18, 25, 35, and 45 wt% PO43ꢀ-supported
mSnO2 samples were prepared by suspending 3.0 g of dried
mSnO2 in 25 ml of distilled water; then the calculated amount
of 1 M H3PO4 solution was added dropwise to the suspension
during stirring for 2 h. Then the samples were calcined at
100 C. The reaction was reperformed on this reactivated cata-
lyst. It was observed that there was no signicant loss in activity,
as shown in Fig. 9, and the catalytic activity of the catalyst was
93.2%, 92.4% and 91.1%, respectively. Moreover Fig. 7SA and
B† show the TEM images of the 25% PO43ꢀ/mSnO2 aer the
third run, which showed no change in the morphology of the
reused catalyst; also the FTIR of the pyridine adsorbed on the
reused sample showed no signicant loss in the acidity. The
obtained results indicated that the catalyst had excellent reus-
ability over three consecutive runs.
400 ꢁC, 450 ꢁC, 550 ꢁC and 650 ꢁC for 4 h. 3, 8, 18, 25, 35 and 45
3ꢀ
are denoted for the wt% of PO4
.
Characterization
DTA and TGA of the uncalcined samples were carried out using
a Shimadzu thermal analyzer, type 50-H. The measurements
were carried out under N2 ow of 20 ml minꢀ1 with a heating
rate of 10 ꢁC minꢀ1. The XRD patterns were recorded using a PW
150 (Philips). The crystallite size (nm), unit cell d110 and a0 were
calculated from the strongest peak 110 of mSnO2 at 2q ¼ 26.54ꢁ
by Bragg's law.67
Reaction mechanism
The rst step in the formation of hydroquinone diacetate is
chemisorption of the carbonyl group of acetic anhydride on the
¨
Bronsted acid sites or Lewis acid sites of the catalyst, then on
¨
the adsorbed acetic anhydride on the Bronsted acid sites,
Scheme 3A shows a nucleophilic attack by OH of hydroquinone
to form an intermediate that breaks down to lose acetic acid in
nl ¼ 2d sin(q)
aꢁ ¼ 2d/O3
TEM images of the calcined PO43ꢀ/mSnO2 samples were
obtained using a Joel JEM-1230 operated at 120 KV. FTIR spectra
of the calcined samples were recorded by using MATTSON 5000
FTIR spectrophotometer (4 cmꢀ1 resolution and 16 scans) in
dried KBr (Merck). Physically adsorbed nitrogen at ꢀ196 ꢁC was
used for elucidation of the texture properties of the calcined P/
mSnO2 samples. The total acidity of the solid samples was
measured by suspending 0.05 g of solid catalyst in 10 ml of
acetonitrile for 3 h and titrating with 0.1 N n-butylamine base in
acetonitrile.35,57 An Orion 420 digital A pH-meter was used for
Scheme 3 Reaction mechanism for the formation of hydroquinone
diacetate on: (A) Bronsted acid sites of a solid acid catalyst, (B) Lewis
¨
¨
acid sites of a solid acid catalyst.
measuring the potential variation. Lewis and Bronsted acid
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levels were determined from the exposure of the degassed solid
samples at 200 ꢁC to dried pyridine.68 Aer the removal of excess
pyridine, a MATTSON 5000 FTIR spectrophotometer was used
to record the spectra. For the NH3-TPD experiment, 200 mg of
4 X. Zhang, K. Wilson and A. F. Lee, Chem. Rev., 2016, 116,
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ꢁ
the samples were kept under ammonia gas for 4 h at 100 C.
Then the samples were ushed with helium, and then the
temperature was raised from 50 ꢁC to 700 ꢁC at a ramping rate of
ꢀ1
ꢁ
5 C min in the presence of helium, and the amount of NH3
desorbed was recorded using a TCD detector.
9 S. K. Samantaray and K. Parida, J. Mol. Catal. A: Chem., 2001,
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10 S. K. Bharadwaj, S. Hussain, M. Kar and M. K. Chaudhuri,
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13 S. Furuta, H. Matsuhashi and K. Arata, Catal. Commun.,
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Catalytic activity
The synthesis of hydroquinone diacetate (1,4-diacetoxybenzene)
was used to test the catalytic activity of the samples. 0.1 g of the
activated catalyst at 120 ꢁC was added to 3 ml (0.032 mol) of
acetic anhydride and 1.1 g (0.01 mol) of hydroquinone in
a 50 ml ask, and the mixture then stirred for 15 min. Aer
15 min, the clear solution was ltered to separate the catalyst,
and the ltrate was poured into 500 ml of crushed ice. The
crystalline solids were dried to constant weight. The product
was identied by FTIR, NMR spectroscopy and melting point
determination. The % yield of hydroquinone diacetate was then
calculated.
16 G. Lu, X. Li, Z. Qu, Q. Zhao, H. Li, Y. Shen and G. Chen,
Chem. Eng. J., 2010, 159, 242–246.
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18 R. Kourieh, S. Bennici, M. Marzo, A. Gervasini and A. Auroux,
Conclusions
Mesoporous SnO2 was successfully prepared through a simple
method and loaded with different amounts of phosphate
species. The XRD and TEM images showed that the samples
were mesoporous. Phosphate species were homogeneously
adsorbed on the surface of mesoporous SnO2 up to surface
saturation coverage at 25 wt% PO43ꢀ at which point the surface,
area surface acidity level and acid strength were at a maximum.
The acidity of the samples was found to decrease when the
phosphate loading exceeded 25 wt%, at which point poly-
phosphate multilayers were formed. Both Lewis and/or
Catal. Commun., 2012, 19, 119–126.
ˇ
´
19 R. M. Martın-Aranda and J. Cejka, Top. Catal., 2010, 53, 141–
153.
20 N. Pal and A. Bhaumik, RSC Adv., 2015, 5, 24363–24391.
¨
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22 J. Fan, Y. Dai, Y. Li, N. Zheng, J. Guo, X. Yan and G. D. Stucky,
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23 Q. Yuan, A.-X. Yin, C. Luo, L.-D. Sun, Y.-W. Zhang,
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24 L. Xu, H. Zhao, H. Song and L. Chou, Int. J. Hydrogen Energy,
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25 Z. Miao, H. Zhao, H. Song and L. Chou, RSC Adv., 2014, 4,
22509–22519.
¨
Bronsted acid sites were present on the catalyst surface. The
synthesis of hydroquinone diacetate was used as a model
reaction for testing the activity of the solid sample for the
formation of bulky molecules. The maximum formation of
hydroquinone diacetate and the surface acidities were found at
25 wt% phosphate loading and 400 ꢁC calcination temperature,
respectively. The catalyst could be successfully used many times
without signicant loss of its catalytic activity.
26 M. Srinivas, G. Raveendra, G. Parameswaram, P. S. Sai
Prasad, S. Loridant and N. Lingaiah, J. Chem. Sci., 2015,
127, 897–908.
Conflicts of interest
27 G. M. Maksimov, G. S. Litvak, A. A. Budneva, E. A. Paukshtis,
A. N. Salanov and V. A. Likholobov, Kinet. Catal., 2006, 47,
564–571.
There are no conicts to declare.
28 B. Mallesham, P. Sudarsanam, G. Raju and B. M. Reddy,
Green Chem., 2013, 15, 478–489.
29 J. Zhu, B. Y. Tay and J. Ma, Mater. Lett., 2006, 60, 1003–1010.
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