Controlled synthesis of hexagonal mesostructure silica and macroporous ordered siliceous foams for VOCs adsorption

Article abstract of DOI:10.1039/c4ra12553c

In this study, ordered hexagonal mesostructure siliceous materials and macroporous ordered siliceous foams were synthesized in pH = 5 buffer solution by simply adjusting the reaction temperature with P123 (EO₂₀PO₇₀EO₂₀) as a structure-directing agent and tetramethoxysilane (TMOS) as the silica source. Increasing the reaction temperature from 15 to 35 °C led to a structural evolution from ordered hexagonal mesostructure siliceous material with a pore size of 10 nm to macroporous ordered siliceous foam with a pore size of ～100 nm. The adsorption and desorption performance of hexagonal mesostructure and macroporous ordered siliceous samples (HMS and MOSF) were investigated under static (water vapor, n-hexane and 93# gasoline) and dynamic (n-hexane) conditions. Compared with commercial silica gel (SG), activated carbon (AC) and HMS, MOSF show higher static adsorption capacity of n-hexane and 93# gasoline, higher dynamic n-hexane adsorption capacity, and more stable breakthrough time under dynamic adsorption conditions. The designed MOSF with highest volatile organic compounds (VOCs) removal capacity and recyclability show great potential for VOCs controlling. This journal is

Full text of DOI:10.1039/c4ra12553c

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Controlled synthesis of hexagonal mesostructure
silica and macroporous ordered siliceous foams for
VOCs adsorption†
Cite this: RSC Adv., 2015, 5, 5695
Hongning Wang,a Xiao Rong,a Lu Han,d Mei Tang,a Meihua Yu,c Jun Zhang,c
b
a
*
Weiqiu Huang and Ruoyu Chen
In this study, ordered hexagonal mesostructure siliceous materials and macroporous ordered siliceous
foams were synthesized in pH ¼ 5 buffer solution by simply adjusting the reaction temperature with
P123 (EO20PO70EO20) as a structure-directing agent and tetramethoxysilane (TMOS) as the silica source.
Increasing the reaction temperature from 15 to 35 ꢀC led to a structural evolution from ordered
hexagonal mesostructure siliceous material with a pore size of 10 nm to macroporous ordered siliceous
foam with a pore size of ꢁ100 nm. The adsorption and desorption performance of hexagonal
mesostructure and macroporous ordered siliceous samples (HMS and MOSF) were investigated under
static (water vapor, n-hexane and 93# gasoline) and dynamic (n-hexane) conditions. Compared with
commercial silica gel (SG), activated carbon (AC) and HMS, MOSF show higher static adsorption capacity
of n-hexane and 93# gasoline, higher dynamic n-hexane adsorption capacity, and more stable
breakthrough time under dynamic adsorption conditions. The designed MOSF with highest volatile
organic compounds (VOCs) removal capacity and recyclability show great potential for VOCs controlling.
Received 17th October 2014
Accepted 11th December 2014
DOI: 10.1039/c4ra12553c
www.rsc.org/advances
formed in different regions in the phase diagram of surfactants
or block copolymers. The phase transition from liquid crystals
1 Introduction
Since the discovery of mesoporous materials,1,2 diverse siliceous
porous structures have been developed due to the availability of
various surfactants used as structure-directing agents, such as
ordered mesostructures, hierarchical mesoporous and macro-
porous materials, and vesicle-like hollow particles, etc.3 The
pore structure, such as channel connectivity and pore
symmetry, of porous materials is one of the most important
physical parameters for their applications.4–6 Therefore, the
controlled synthesis of porous materials with tunable structures
and pore sizes has attracted increasingly attention.7
By using liquid crystal templating (LCT)1,8 or cooperative self-
assembly,2,9,10 ordered mesostructured organic–inorganic
composites have been synthesized and this eld has attracted
thousands of documents. Vesicles11–14 and liquid crystals15,16 are
to uni- or multi-lamellar vesicular structures is theoretically
predictable in the process of surfactant self-assembly by using
the concept of surfactant packing parameter (g),12 and has also
been experimentally achieved through decreasing the
hydrophilic/hydrophobic block ratio,17 or decreasing reaction
temperature.18 Moreover, the structure transformation of
organic–inorganic hybrid materials is also achieved by tuning
the reaction temperature,19,20 or adding organic additives,19,21–25
or using mixed surfactants,26–29 or mixed silica sources.30–32
Nevertheless, such structure transition has rarely been achieved
in surfactant-templated organic–inorganic composites by
adjusting a single parameter. A cooperative vesicle templating
(CVT) approach was proposed to synthesize siliceous uni-
lamellar vesicles and foams using nonionic block copoly-
mers,33–35 where the synthesis condition (pH of ꢁ5) was
different from that of SBA-15 type ordered mesoporous mate-
rials (at strong acidic condition).10 Compared to the enthusiasm
for the study of ordered mesostructured materials, much less
attention has been paid in the synthesis and applications of
vesicular materials directed by organic templates.
aJiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou
University, 1 Gehu Road, Changzhou, Jiangsu 213164, P. R. China. E-mail: cxdcry@
163.com
bJiangsu Provincial Key Laboratory of Oil and Gas Storage and Transportation
Technology, Changzhou University, Xingyuan Road, Changzhou, Jiangsu, 213016,
P. R. China
Herein, we report a structure evolution from ordered
hexagonal mesostructure siliceous materials (HMS) with pore
diameter z 10 nm to macroporous ordered siliceous foams
(MOSF) with pore diameter z 118 nm in pH ¼ 5 buffer solution
by nely tuning reaction temperature, with a ꢁ10 times pore
expansion. In this approach, the structure directing agent
cAustralian Institute for Bioengineering and Nanotechnology, The University of
Queensland, Brisbane, QLD 4072, Australia
dResearch and Development Center, China Tobacco Anhui Industrial Co., Ltd., 9
Tianda Road, Hefei, Anhui, 230028, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra12553c
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used is EO20PO70EO20 (denoted P123, BASF), where EO is (BET) method was utilized to calculate the specic surface areas
poly(ethylene oxide) and PO is poly(propylene oxide) and tet- (SBET) based on the linear part of the BET plot (P/P0 ¼ 0.05–0.25).
ramethyl orthosilicate (TMOS) used as silica source. The By using the Barrett–Joyner–Halenda (BJH) model, the pore
obtained HMS and MOSF can be used for VOCs adsorption. The volumes and pore size distributions were derived from the
adsorption and desorption performance of HMS and MOSF was adsorption branches of isotherms, and the total pore volumes
studied under static (water vapor, n-hexane and 93# gasoline) (Vt) were estimated from the adsorbed amount at a relative
and dynamic (n-hexane) conditions, with commercial silica gel pressure P/P0 of 0.992. The micropore volumes (Vm) and
(SG) and activated carbon (AC) as references.
micropore surface areas (Sm) were calculated from the t-plot
method.
2 Experimental methods
2.1 Materials
2.4 Static adsorption and desorption measurements
Static adsorption equilibrium measurements at room temper-
ature were carried out using a digital microbalance (Model YP/
1002) with a sensitivity of 0.1 mg.36 The adsorption–desorption
properties of the VOCs or water were determined in a desiccator
with a series of VOCs or distilled water under the plate. The
Triblock copolymers Pluronic P123 was purchased from Sigma-
Aldrich. TMOS was purchased from BASF Corp. 93# gasoline
was purchased from Sinopec. AC was bought from Xinsen
Carbon Industry Co., Ltd., Shaowu, Fujian, China. SG is a gi
from Qingdao Xinli trade limited company. Other chemicals
were purchased from Shanghai Chemical Corp. All chemicals
were used as received without any further purication. Milli-
pore water was used in all experiments.
desiccator was placed in
a constant-temperature water
container and equipped with an analytical balance. First, the
powder sample was loaded on a weighting bottle (50 mm ꢃ 30
mm) without lid. Under a high-vacuum of 0.9 mbar, the sample
was heated to 100 ꢀC for at least 2 h to remove all adsorbed
impurities. Aer that, the weighting bottle (with lid) was cooled
to the room temperature in drying apparatus. Then the weigh-
ing bottle (without lid) with adsorbent inside keep in contact
with the VOCs or water for sufficient time at 25 ꢀC. Aer the
adsorption became saturated, desorption procedure was
carrying out by heating the sample to 100 ꢀC in a vacuum
environment for 75 min (until the mass of the sample do not
change any more). All the static adsorption experiments were
performed in triplicate (each including 5 adsorption–desorp-
tion cycles) to ensure the reproducibility of the data. According
to the mass change of the sample before and aer the adsorp-
tion and desorption procedure, the adsorption capacity and
desorption percentage of the sample could be calculated as
follows:
2.2 Synthesis
The synthesis includes two steps. The rst step involves reactant
mixing and ripening, while the second step is ageing. In a
typical synthesis, 1.0 g of P123 and 1.7 g of Na2SO4 (0.40 M) were
dissolved in 30 g of pH ¼ 5.0 NaAc–HAc buffer solution [Ct ¼
0.02 M, where Ct ¼ C(NaAc) + C(HAc)] to form a homogenous
ꢀ
solution under stirring at 15 C. 1.52 g of TMOS was added to
this mixture solution under stirring. Aer 5 minutes, the stir-
ring was stopped. The resultant mixture was kept at static
condition for 24 h. During the above reactant mixing and
ripening step, the temperature was xed at 15 ꢀC by using a
constant temperature thermostatic water bath (ꢂ0.1 ꢀC). Then,
all the ꢀreactants were transferred into an autoclave and heated
at 100 C for 24 h. The white precipitates were ltered, repeat-
edly washed with water to remove the inorganic salts, and then
dried at room temperature. The nal products were obtained by
calcination at 550 ꢀC for 5 h. The other reaction conditions are
exactly the same in all synthesis except that the temperature in
the reactant mixing and ripening step was changed systemati-
m2 ꢄ m1
X ¼
(1)
m1
m3 ꢄ m4
m2 ꢄ m1
Y ¼
ꢃ 100%
(2)
ꢀ
cally in the range of 5–35 C.
where X and Y are the adsorption and desorption rate of the
sample, m1 and m2 are the mass of the sample before and aer
the adsorption, m3 and m4 are the mass of the sample before
and aer the desorption, respectively.
2.3 Characterization
Powder X-ray diffraction (XRD) patterns were recorded with a
Bruker D4 powder X-ray diffractometer using Cu Ka radiation
(40 kV, 40 mA). Scanning electron microscopy (SEM) images
were recorded on a Philips XL30 electron microscope (Nether-
lands) operating at 20 kV. A thin gold lm was sputtered on the
sample before characterization. Transmission electron micros-
copy (TEM) images were taken with a JEOL 2011 microscope
(Japan) operating at 200 kV. For the TEM measurements, the
samples were dispersed in ethanol and then dried on a holey
carbon lm Cu grid. Nitrogen sorption isotherms were
measured at 77 K with a Micromeritics Tristar 3000 analyzer
(USA). Before measurements, the samples were degassed in a
vacuum at 180 ꢀC for at least 5 h. The Brunauer–Emmett–Teller
2.5 Titration of surface hydroxyl
The density of surface hydroxyl was determined according to
Boehm titration.37–40 Typically, 0.50 g of sample was dispersed in
100 mL mixture of ethanol and NaCl and then sealed and
shaken thoroughly for 24 h. The pH value of the resulting
suspension was rst adjusted to 4.0 by 0.01 M HCl or 0.018 M
NaOH and then the pH was further titrated from 4.0 to 9.0 with
0.018 M NaOH. The density of surface hydroxyl was calculated
as follow:
D ¼ CVNA ꢃ 10ꢄ3
(3)
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which D (ꢃ 1020 gꢄ1) was the density of surface hydroxyl of the
sample, C, V and NA were the concentration of NaOH (M), which
was calibrated by C8H5KO4, the titration volume of NaOH (mL)
during the pH value of the resulting suspension varied from 4.0
to 9.0 and the Avogadro constant, respectively.
2.6 Dynamic adsorption and desorption measurements
The dynamic adsorption was carried out by a ow method
reported by Hu et al.41 Adsorbent was loaded in a xed-bed
reactor (internal diameter of 0.80 cm, length of 8.00 cm).
Before adsorption measurements, samples were degassed at
ꢀ
100 C overnight under vacuum condition to remove the phys-
ically adsorbed water molecules and small organic impurities.
Air was taken as a carrier gas and adjusted to keep a total ow
rate of 0.03 L minꢄ1, and the concentration of n-hexane was
controlled at 0.25 g Lꢄ1. The adsorbed amount of adsorbate was
determined by the concentration change before and aer
adsorption measurements tested by using a GC equipped with a
ame ionization detector. The equilibrium dynamic adsorption
capacity (q) of the adsorbents was calculated from the break-
through curves according to the equation as follows:
ꢀ
ꢁ
ð
CA
C0
FA 1 ꢄ
W
dt
Fig. 1 SEM images of siliceous materials (a)–(f) synthesized at 15, 25,
26, 27, 30 and 35 ꢀC, respectively.
q ¼
(4)
where CA and C0 were the outlet and inlet concentrations (g Lꢄ1
)
of the stream through the xed bed column, respectively. FA
(L minꢄ1) was the mixed gas ow rate of n-hexane and air, and W
(g) was the amount of the adsorbent loaded in the adsorption
bed. The time rang of integration was from 0 to te (min), where
te was adsorption equilibrium time.
sample (a) conrms the highly ordered hexagonal mesostruc-
ture (Fig. 3a). As the temperature increased to 25 ꢀC, three
diffraction peaks at 2q ¼ 0.81, 1.42, and 1.65ꢀ can be observed in
ꢀ
the XRD pattern of sample (b) (25 C) (Fig. 2b), which can be
indexed to a two-dimensional hexagonal structure in general.
However, TEM image of sample (b) reveals that the hexagonal
mesostructure co-exists with spherical vesicular structure
(Fig. 3b). When the temperature was further increased to 26 ꢀC,
one weak broad diffraction peak is observed, indicating a
disordered mesostructure (Fig. 2c). The vesicular structures are
the major phase in the TEM image of sample (c) (Fig. 3c). A
further slight increase of temperature to 27 ꢀC leads to the
3 Results and discussion
3.1 Characterization of the materials
Fig. 1 shows scanning electron microscopy (SEM) images of the
calcined materials synthesized at 15, 25, 26, 27, 30, 35 ꢀC,
respectively. Sample (a) (synthesized at 15 ꢀC) has a rod-like
morphology with a length of ꢁ1 mm and a diameter ranging
from 150 to 300 nm (Fig. 1a), similar to that of SBA-15 materials
synthesized at acidic conditions in the presence of inorganic
salts.42 Sample (b) (25 ꢀC) shows a mixed rod-like and spherical
morphologies (Fig. 1b). The rod-like morphology disappears in
samples (c) and (d) synthesized at 26 and 27 ꢀC, respectively
(Fig. 1c and d). When the reaction temperature is further
increased to 30 ꢀC, the macroporous foam-like structure can be
observed for sample (e) (Fig. 1e). Macroporous ordered foam
ꢀ
structure can be obtained within the temperature of 30–35 C,
and the structure synthesized at 35 ꢀC (f) is most ordered
(Fig. 1f).
The X-ray diffraction (XRD) patterns and transmission elec-
tron microscopy (TEM) images of samples (a)–(f) are shown in
Fig. 2 and 3, respectively. The XRD pattern of sample (a) (15 ꢀC)
shows three well-resolved reections at 2q ¼ 0.94, 1.62, and
1.86ꢀ, which can be attributed to the (100), (110), and (200)
diffractions of the two-dimensional p6mm symmetry with a
Fig. 2 XRD patterns (a)–(d) of siliceous materials synthesized at 15, 25,
lattice parameter of a ¼ 10.9 nm (Fig. 2a). The TEM image of 26 and 27 ꢀC, respectively.
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Fig. 4 Nitrogen sorption isotherms (A) and pore size distribution
curves (B) of siliceous materials (a)–(f) synthesized at 15, 25, 26, 27, 30
and 35 ꢀC, respectively. The Y-axes value is raised 500, 1000, 1500,
2000 and 2500 cm3 gꢄ1 (A) and 0.2, 0.4, 0.6, 0.8 and 1.0 cm3 gꢄ1 nmꢄ1
(B) for (b), (c), (d), (e) and (f), respectively.
The hexagonal structure can also be obtained at 5 and 20 ꢀC
(Fig. S1–3†). However, careful TEM investigation indicates that
the formation of vesicular structures commences at 20 ꢀC
(Fig. S3†). On the basis of these observations, the structural
evolution as a function of reaction temperature is shown in
Scheme S1:† the hexagonal structure transformed to vesicular
structures over a temperature range of 20–26 ꢀC, while the pore
ꢀ
size expands ꢁ10 times from 15 to 35 C. Our results demon-
strate that, porous materials with different pore morphology
can be achieved by simply changing the reaction temperature.
The above results show that a simple approach for the
synthesis of mesoporous or macroporous siliceous materials
with different structures (hexagonal and vesicular) and
morphologies was achieved by simply changing the reaction
temperature. Because the other reaction conditions are exactly
the same, the difference should be mainly attributed to the
inuence of temperature on the cooperatively self-organized
block copolymer/silicate composite structures. In the self-
Fig. 3 TEM images (a)–(f) of siliceous materials synthesized at 15, 25,
26, 27, 30 and 35 ꢀC, respectively. Scale bar is 100 nm.
formation of dominant vesicular structures in the TEM image of assembly of surfactants, the structure of supra-molecular
sample (d) (Fig. 3d). No diffraction peak is observed for sample aggregates is predicted mainly by the g factor of organic
(d) (Fig. 2c), in accordance with the structure revealed by TEM surfactant molecules.12,43–45 While in the case of mesostruc-
observation. The disordered (sample (e)) and ordered (sample tured1,2,8–10 and vesicular organic–inorganic composites,34,35 the
(f)) foam-like structures shown in SEM images (Fig. 1e and f) are self organized structures are determined by not only the
also conrmed in their TEM images (Fig. 3e and f).
surfactant templates, but also the inorganic species that have
The N2 sorption isotherms and pore size distribution curves interaction with the surfactants head groups. Therefore, it is the
of samples (a)–(f) are shown in Fig. 4. For sample (a), only one hydrophilicity/hydrophobicity ratio of both templates and
capillary condensation step occurs at P/P0 ¼ 0.7–0.8 (Fig. 4A-a), inorganic precursors that decide the nal structure of organic–
and the pore size is calculated to be 10 nm (Fig. 4B-a) (for other inorganic composites.
physicochemical properties, see Table 1). The isotherm of
In this synthesis system, nonionic surfactant P123 has both
sample (b) shows two steps of capillary condensation in the the hydrophilic poly(ethylene oxide) (PEO) chains and hydro-
adsorption branch, occurred at P/P0 ¼ 0.7–0.8 and P/P0 > 0.9, phobic poly(propylene oxide) (PPO) chains, which drives the
respectively (Fig. 4A-b). The existence of bimodal pores should formation of micelles with PPO as the core and PEO chains as
come from the mesoporous and vesicular structures in sample the corona.22,46–48 Aggregation of P123 in water is sensitive to the
(b). When the reaction temperature is further increased in the additives. The addition of “salting out” electrolyte Na2SO4,
range of 26–35 ꢀC (sample (c)–(f)) (Fig. 4A-b–f), the volume would decrease critical micelle concentration temperature, thus
adsorbed in smaller pores (ꢁ10 nm) is even less, while the micellization is favored, and highly ordered mesoporous sili-
capillary condensation in macropores (>50 nm) is dominant cates with hexagonal structure can be therefore prepared at 5–
(Fig. 4B-b–f). Sample (f) has a uniformly distributed macropores 15 C, which is consistent with the reported by Yu et al.49 The
ꢀ
of about 118 nm (Fig. 4B-f), in agreement with its ordered amphiphilic behavior of (PEO)x–(PPO)y–(PEO)x polymers is
macrostructure.19
temperature dependent. At low temperature, PEO is more
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Table 1 Physicochemical properties of siliceous materials synthesized in pH ¼ 5 buffer solution at different temperatures
Temperature
Pore diameter
(nm)
Sample
(ꢀC)
SBET (m2 gꢄ1
)
Sm (m2 gꢄ1
)
Vt (cm3 gꢄ1
)
Vm (cm3 gꢄ1
)
a
b
c
d
e
f
15
25
26
27
30
35
532
512
495
508
450
293
137
42
40
52
50
32
1.27
2.11
2.09
1.45
2.77
1.99
0.060
0.014
0.013
0.019
0.018
0.011
10
12, 100a
12, 115a
65
90
118
a
Not the distance of d100
.
hydrophilic.23 As the temperature increases, the PEO chains selective synthesis of vesicular or mesostructured silica mate-
dehydrate and become more hydrophobic.50 Therefore PEO rials by the CVT or LCT approach can be rationally designed and
groups are less available for interaction with the silica source. successfully achieved for further application.
This tends to increase the hydrophobic volumes of the surfac-
tant aggregates. At the same time, temperature also affects the
hydroxylation and condensation behaviors of TMOS. As the
temperature increased, the hydrolysis and condensation rate of
TMOS increased.51 This in turn changes size of the headgroup,
length of the hydrophobic tail, and the ratio between hydro-
philic and hydrophobic volumes,30,52 nally led to increasing g,
realizing the transformation from hexagonal to vesicular silica.
Then, siliceous materials with different structures can be
obtained with the cooperation assembly of P123 and TMOS
under the experimental conditions varying the synthetic
temperature, and the transformation between the LCT and CVT
was realized.
Our observations provide direct support for the competition
between CVT and LCT pathways in the same organic templating
system. In the case of P123 as a template while TMOS as the
silica source, it is the change in the hydrophilicity/
hydrophobicity ratio of both templates and inorganic precur-
sors induced by temperature that nally leads to a great struc-
tural variation form conventional ordered mesostructures to
vesicles in cooperatively organized organic–inorganic compos-
ites. Our results demonstrate that the LCT and CVT approach
can be adjusted by nely control reaction temperature, a
3.2 Static adsorption and desorption properties
Sample synthesized at 15 (a) and 35 ꢀC (f) (specied as HMS and
MOSF, respectively) with different morphologies and structures,
were selected to estimate their adsorption properties for water
vapor and VOCs, comparing with commercial SG and AC. The
study of static adsorption–desorption capacity of water vapor
and VOCs was based on the mass change of the samples before
and aer the adsorption or desorption procedure. The histo-
grams of water vapor adsorption capacity are presented in
Fig. 5a. The water vapor desorption capacities and the titrated
surface hydroxyl density are listed in Table 2. The HMS presents
the highest water vapor adsorption capacity of about 0.706 g gꢄ1
owing to high density of surface hydroxyl (2.34 ꢃ 1020 gꢄ1),
which is higher than that of traditionaꢄl 1SBA-15 and hollow
spheres calcined at 550 ꢀC (0.643 g g
and 0.264 g gꢄ1
,
respectively, unpublished data). MOSF adsorbs a smaller
amount of water vapor (0.632 g gꢄ1) as compared to HMS, but
higher than that of SG (0.445 g gꢄ1) and AC (0.604 g gꢄ1). The
results conrm that HMS and MOSF are somewhat hydrophilic
in nature due to a certain amount of hydrophilic silanol groups
on the surface. The small pore size of HMS favors hydrogen
bonding of the surface hydroxyls. On the contrary, the silanols
Fig. 5 The histogram of static water vapor adsorption capacities from triplicate measurements on different materials (a) and the relationship
between static water vapor adsorption capacities and the densities of surface hydroxyl group (b).
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Table 2 The water vapor adsorption capacities, desorption efficien-
cies and the densities of surface hydroxyl group of different samples
Adsorption
Desorption
(%)
Density of surface
Samples
(g gꢄ1
)
hydroxyl (ꢃ1020 gꢄ1
)
SG
0.445
0.706
0.632
0.604
97.6
99.8
99.4
98.1
1.77
2.34
2.21
2.13
HMS
MOSF
AC
are too far apart in the wide pores of MOSF, so with larger pore
size of MOSF, an increased distance between silanol groups on
the pore surfaces would improve the hydrophobicity. The rela-
tionship between water vapor adsorption capacities of all the
samples and the corresponding densities of surface hydroxyl
groups is linear, where the linear tting coefficient R2 ¼ 0.995
(as shown in the inset of Fig. 5b), which reveals that the amount
of adsorbed water molecules on the adsorbents is greatly
inuenced by the accessible hydroxyl groups.
Fig. 6 shows the FT-IR patterns of calcined HMS, MOSF, and
commercial SG. The almost absence of peaks corresponding to
the strong C–H stretching and bending vibrations in the 2971,
2933, 2886, 1464, and 1384 cmꢄ1 implied P123 was almost
completely removed from HMS and MOSF through calcination
process. The band in the region of 3420–3440 cmꢄ1 is from the
Si–OH groups while the peak at 1633 cmꢄ1 can be attributed to
the O–H bending vibration mode of the adsorbed water mole-
cules.53 The intensities of the bands related to the Si–OH and
O–H groups for HMS were stronger than that of MOSF and SG,
which was correspond to the corresponding titrated results of
the densities of surface hydroxyl groups.
Fig. 7 The histograms of static adsorption capacities (a), (b) and
desorption efficiencies (c), (d) from triplicate measurements of
n-hexane and 93# gasoline on different adsorbents.
for hexane and 0.256 for 93# gasoline, respectively, unpublished
ꢀ
data) calcined at 550 C, far lower than that of MOSF (1.80 and
1.24 g gꢄ1, respectively). The n-hexane and 93# gasoline adsorp-
tion capacity of commercial SG in the rst cycle is 0.449 g gꢄ1 and
0.434 g gꢄ1, respectively. As to commercial AC, n-hexane and 93#
gasoline adsorption capacities are 0.619 g gꢄ1 and 0.668 g gꢄ1
,
respectively. As can also be seen from Fig. 7a and b, all the
siliceous samples (HMS, MOSF and SG) have stable n-hexane
and 93# gasoline adsorption capacities. However, the n-hexane
and 93# gasoline adsorption capacity of AC decreased in the
second adsorption cycle and kept constant during the following
adsorption process.
N-Hexane and 93# gasoline have been used as model
adsorbates to evaluate the static VOCs adsorption capacity of
different materials. As shown in Fig. 7a and b, the rst n-hexane
and 93# gasoline adsorption capacity of HMS is 0.840 and 0.958
g gꢄ1, respectively, higher than that of traditional SBA-15 (0.481
g gꢄ1 for n-hexane and 0.493 g gꢄ1 for 93# gasoline, respectively,
unpublished data) and hollow silicious spheres (HSSs) (0.641
The desorption behavior of the adsorbents is very important
for their reuse. It would be ideal for practical applications if
VOCs can be recovered using mild heat treatment and simul-
taneously the adsorbents can be regenerated. As for the water
vapor desorption efficiency of all the samples, a nearly complete
desorption can be found (Table 2). All the siliceous samples
(MOSF, HMS and SG) also show almost 100% desorption of
n-hexane and 93# gasoline (Fig. 7c and d). As to AC, the
desorption efficiencies are low (72.3% for n-hexane and 56.6%
for 93# gasoline, respectively) during the rst desorption
process, and the desorption efficiencies can reach 100% during
the second and the following desorption process. And the N2
sorption results of AC aer 3rd repetitive adsorption–desorp-
tion processes show that the surface area and pore volume
decreased distinctly (Table. S1†), which is called the degenera-
tion of AC. The main reasons may be the adsorbed large
molecule hydrocarbons in the micropores of AC cannot be
easily desorbed under experimental conditions, thus the
Fig. 6 FT-IR patterns of calcined HMS, MOSF and SG.
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microspores will cease to be effective during next cycle Compared with MOSF and SG, the concentrations of n-hexane
adsorption. For the chemical adsorption because of the thermal for HMS and AC increase more rapidly aer the breakthrough,
effect during VOCs adsorption, or the unsaturated hydrocarbon, implying less diffusion resistance of HMS and AC during the
sulphur and so on in oil vapor, the surface of AC will be covered adsorption process because of their higher micropores (Table
by carbon, polymer and other compounds, leading to the partly SI†). However, the rst equilibrium dynamic adsorption
disappearing of micropores.54,55 The remarkable stable and high capacities (q) of n-hexane are in the order of MOSF (1.63 g gꢄ1) >
adsorption capacity of n-hexane and 93# gasoline, along with HMS (0.773 g gꢄ1) > AC (0.483 g gꢄ1) > SG (0.334 g gꢄ1).
the complete desorption capacity can reveal that the synthe-
sized HMS and MOSF are excellent VOC absorbents.
Adsorption experiments with three consecutive cycles were
performed to investigate the regeneration and re-use properties
of four adsorbents. Aer each adsorption cycle, the saturated
ꢀ
adsorbent was heated to 100 C under vacuum for 75 min so
3.3 Dynamic adsorption and desorption behaviors
that the adsorbed n-hexane can be desorbed. The desorbed
adsorbents were then re-used for the next adsorption proce-
dure. The corresponding desorption efficiencies of n-hexane on
four adsorbents for three consecutive cycles are shown in
Fig. S4† for comparison. As can be seen from Fig. S4,† the rst
desorption efficiency of n-hexane on HMS, MOSF and SG are
99.1, 99.7 and 98.6%, respectively. The almost complete
desorption may be due to the relatively weak adsorption of n-
hexane on the external surface or the open mesopores. MOSF,
SG, and HMS have stable breakthrough time, equilibrium
adsorption time, and dynamic capacity aer three consecutive
adsorption–desorption cycles. However, AC can only be des-
orbed partially (59.6%) during the rst desorption process
under the same desorption conditions. The result indicates that
regeneration under the chosen experimental conditions was
unsuccessful in desorbing the adsorbed n-hexane from some of
the adsorption sites of AC. It may be due to that AC has rich
micropores, the n-hexane (with large molecular weight) adsor-
bed in the micropores of AC is strongly held by adsorption
forces and the strongly adsorbed molecules cannot be easily
desorbed efficiently under this mild desorption conditions.54,55
The breakthrough time, equilibrium adsorption time and
dynamic adsorption capacity of AC decrease dramatically
(Fig. 8c and d) and keep stable aer 2nd repetitive dynamic
adsorption–desorption cycles.
The breakthrough measurement is a direct method designed to
make clear the dynamic performance of VOCs adsorption. In
many cases the actual adsorption processes of VOCs are asso-
ciated with adsorption using a xed bed in which adsorbents
are packed, into which air with VOCs ows. HMS and MOSF
with different morphologies and structures were adopted here
as representative samples to estimate the breakthrough
behaviors for n-hexane, comparing with AC and SG. To deter-
mine the dynamic adsorption performance of adsorbents, the
breakthrough time, the equilibrium adsorption capacity, and
the stability are important parameters. The breakthrough
curves of HMS, MOSF, SG and AC are shown in Fig. 8. The
typical breakthrough curves give the evolution of the CA/C0 ratio
as a function of time, where CA is the concentration of n-hexane
at the outlet of the adsorption bed and C0 is the concentration
of hexane at the inlet.56 The dynamic adsorption capacity was
obtained by measuring the area between the maximum baseline
and experimental curves.57
As shown in Fig. 8c, the breakthrough times of the four
samples are in the order of AC (30 min) > MOSF (26 min) > HMS
(19 min) > SG (16 min) for the rst dynamic adsorption process.
The stable breakthrough times of the four samples are in the
order of MOSF (27 min) > AC (25 min) > HMS (20 min) > SG (15
min) (Fig. 8c), and the stable q of n-hexane are in the order of
MOSF (1.62 g gꢄ1) > HMS (0.778 g gꢄ1) > AC (0.375 g gꢄ1) > SG
(0.326 g gꢄ1) (Fig. 8d). Although the adsorption capacity in the
dynamic adsorption was less than that in static one because of
limited and lower diffusion into compressed samples, the order
of adsorption capacity for the samples coincided with that in
static adsorption. However, it is worthy to note that aer 3 cycles
of adsorption for AC, the adsorption capacity of for n-hexane
remains constant and almost 77.6% of that of the correspond-
ing 1st cycle value. The dynamic desorption behavior of n-
hexane also shows a good agreement with that of the static
experiments.
Fig. S5† depicts the relationships between the rst dynamic
n-hexane adsorption capacities and the corresponding structure
parameters of different adsorbents. The dynamic n-hexane
adsorption capacities of different adsorbents are irrelevant to
the respective micro surface areas, micro pore volumes and
surface areas (Fig. S5a–c†). As can be seen in Fig. S5d,† a higher
pore volume directly leads to an increase in the adsorption
Fig. 8 The first breakthrough curves of HMS, MOSF, AC and SG. (a) is
the first time, (b) is the third time, (c) and (d) are the comparison of the
breakthrough times and equilibrium adsorption capacities of the three
times, respectively.
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capacities for n-hexane and is therefore an essential factor in
determining the adsorption capacity of adsorbents. However,
the macroscopic morphology also plays a great part in the
adsorption process. The slower adsorption behavior of the
MOSF and SG was the result of hindered diffusion,58 and the
MOSF with polyhedric cells similar to the Weaire–Phelan
structure33,35 and SG with smaller pore size39 show more diffu-
sion resistance under this dynamic adsorption conditions. It is
suggested that the adsorption behaviors of samples for VOC is
concerned with both pore structures and morphology. From the
dynamic adsorption data for four samples, it is concluded that
the dynamic adsorption performance for gaseous n-hexane
depends both on macroscopic morphology, pore characteristics
and the compositions. The synthesized MOSF exhibits an
excellent adsorption performance with higher dynamic
adsorption capacity than SG, AC and HMS.
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We have demonstrated a simple approach for the synthesis of
mesoporous or macroporous silica materials with different
structures (hexagonal and vesicular) and morphologies by using
triblock copolymers P123 as template and TMOS as silica
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Highly ordered mesoporous silica materials with hexagonal
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ꢀ
ꢀ
The adsorption and desorption performance of HMS and MOSF
under static (water vapor, n-hexane and 93#gasoline) and
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Acknowledgements
This work is supported by the National Science Foundations of
China (21101017), Jiangsu province science and technology
support program (BE2011651), Key University Science Research
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