Microporous and Mesoporous Materials 302 (2020) 110204

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Microporous and Mesoporous Materials
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Adsorptive properties in toluene removal over hierarchical zeolites
Shushu Huang a, b, Wei Deng b, Long Zhang b, Dengyao Yang b, c, d, Qiang Gao a, **,
Zhengfang Tian e, ***, Limin Guo b, *, Tatsumi Ishihara c, d
a

Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, 430074, China
School of Environmental Science and Technology, Huazhong University of Science and Technology, Wuhan, 430074, China
c
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 8190395, Japan
d
International Institute for Carbon-Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 8190395, Japan
e
Hubei Key Laboratory of Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang, 438000, China
b

A R T I C L E I N F O

A B S T R A C T

Keywords:
Adsorptive properties
Toluene removal
Hierarchical zeolites

Three kinds of hierarchical Mordenite Framework Inverted (MFI)-type nanozeolites including Si-MFI, Al-MFI and

Ti-MFI were successfully synthesized by two-stage varying temperature hydrothermal treatment. Taking toluene
as the probe molecule, a series of methods were used to evaluate the adsorption properties of as-synthesized
hierarchical zeolites samples including the adsorption breakthrough curves, simulation of adsorption iso­
therms as well as kinetics models and toluene-TPD (the temperature programmed desorption of toluene). The
dynamic adsorption results showed that Al-MFI exhibited the highest adsorption capacity for toluene (58 mg/
gads) under dry gas condition while Ti-MFI had the optimal toluene adsorption performance (45 mg/gads) under
the wet gas condition (the relative humidity was 50%). The simulation results indicated that the adsorption
behavior of toluene on hierarchical MFI nanozeolites conformed to the Freundlich principle, and the pseudo-firstorder adsorption model was suitable to elucidate the adsorption process. Moreover, the fitting result of intra­
particle diffusion model indicated that the adsorption process was affected by multiple adsorption steps and the
intraparticle diffusion was not the control step.

1. Introduction

usually produces some toxic byproducts [13]. Among them, adsorption
has been still considered as one of the simplest and most effective
technologies for VOCs removal due to its low cost and easy operation
[12,13]. Additionally, the adsorbents are favored to be reused by
adsorption/desorption recycling. Up to now, different kinds of porous
materials (such as carbon-based materials, zeolites, organic polymers
and composites) are practically used [14–19]. As its non-flammable
property, zeolites with large surface area and peculiar microporous
channels have been considered as the most potential complementary
adsorbents for activated carbons [7,20–23]. For the conventional zeo­
lites, the diffusion of adsorbate molecules is usually restricted due to its
small pore size [24,25]. Then, the synthesis of hierarchical zeolites has
been used to overcome drawbacks of the conventional zeolites [26–29].
And the construction of nanosized zeolites is another good method to
facilitate the mass diffusion [30,31]. For example, the nanosized zeolite
Y was used for various protein adsorption [32], and nanozeolites with
increased external surface area showed the higher uptake of protein.


Volatile Organic Compounds (VOCs), which are widely used in in­
dustries such as petrochemicals, printing, pharmaceuticals and painting,
is regarded as one of the major resources to air pollution including the
photochemical smog and particulate matter. In addition, VOCs itself has
harmful effects on human health [1–3]. Nowadays, VOCs removal on the
end-of-pipe control is thus of great importance for protecting the envi­
ronment as well as public health. Various technologies, such as
adsorption, absorption, catalytic combustion and photocatalytic degra­
dation and so on, have been developed to remove VOCs [4–11]. The
VOCs absorption/adsorption can be achieved by dissolving VOCs in
chemical solvents or adsorbing on the adsorbents, which is simple for
practical application but sometimes not favorable due to its limited
absorption and followed additional separation process [12]. The incin­
eration method consumes large amount of energy to generate high
temperature for the reaction and photocatalytic degradation method

* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: (Q. Gao), (Z. Tian), (L. Guo).
/>Received 24 January 2020; Received in revised form 17 March 2020; Accepted 19 March 2020
Available online 8 April 2020
1387-1811/© 2020 Elsevier Inc. All rights reserved.


S. Huang et al.

Microporous and Mesoporous Materials 302 (2020) 110204


Voung et al. reported the nanozeolites with small particle size and high
surface area demonstrated the improved activity in the standard gas oil
cracking [33]. Li and the co-workers also found nanosized zeolite Y
exhibited the higher reaction rate in the selective catalytic reduction of
NO2 with urea due to its more silicon hydroxyl groups resulting from its
nanosized morphology and larger external surface area [34]. Up to now,
the hierarchical zeolites consisted of nanosized zeolites have been rarely
reported for adsorptive removal of VOCs [30,31,35].
Herein, we firstly synthesized three kinds of hierarchical zeolites
consisted of nanosized MFI zeolites. Then toluene was used as the probe
molecule to evaluate the adsorptive properties of the as-synthesized
hierarchical zeolites by the dynamic breakthrough experiments.
Furthermore, the adsorption simulation combined with toluene-TPD
were adopted to elucidate the adsorption mechanism. According to
our knowledge, this is the first time to report hierarchical zeolites con­
sisted of nanosized MFI zeolites for the toluene adsorption.
2. Experimental section
2.1. Chemicals and regents
All chemicals were used without further treatments: tetraethyl
orthosilicate (TEOS, >99%), aluminum isopropoxide (AIP, >98%), tet­
rabutyl titanate (TBOT, >99%) and sodium hydroxide (NaOH, analyt­
ical reagent) were from Sinopharm Chemical Reagent Co., Ltd.
Tetrapropylammonium hydroxide solution (TPAOH, 25% in H2O) was
from Shanghai Cainorise Chemical Co., Ltd. Deionized water was used in
the experiment.
2.2. Materials synthesis
Al and Ti-containing MFI nanozeolite aggregates were synthesized
using two-stage varying temperature hydrothermal treatment [36].
Typically, 0.31 g aluminum isopropoxide or 0.51 g tetrabutyl titanate
was mixed with 15.62 g tetraethyl orthosilicate and then dissolved in

25.92 g deionized water. The water solution of TPAOH (10.98 g, 25 wt%
TPAOH diluted with 30 g deionized water) with 0.45 g NaOH was
dropwise added into the above-mentioned solution. After stirring for
3–5 h at 313 K, the mixed solution was further stirred for 2 days at 373 K
to obtain the precursor solution. After cooling down, 60 g deionized
water was added into the precursor solution and then the solution was
treated at 423 K in Teflon-lined stainless-steel autoclaves for a second
hydrothermal treatment. After crystallization for 24 h, the samples were
centrifuged, washed by deionized water three times, overnight dried at
373 K and finally calcined at 823 K for 7 h. The as-prepared samples
were donated as Al-MFI or Ti-MFI.
The preparation of Si-MFI was similar with the above-mentioned
procedure and without the addition of aluminum isopropoxide or tet­
rabutyl titanate.
2.3. Sample characterization
The powder XRD (X-Ray Diffraction) patterns of as-prepared samples
were recorded on a Shimadzu XRD-7000 diffractometer using Cu Kα
radiation (30 mV and 40 mA) from 5 to 50� at a rate of 5� /min. The
actual Si/M (M ¼ Al or Ti) ratios were determined by X-ray Fluorescence
(XRF) analysis on Shimadzu XRF-1800XRF. Fourier transform infrared
spectroscopy (FTIR) was conducted on a Bruker Tensor II spectrometer.
Sample disks were made by mixing dried samples with KBr and then
pressed them into tablet form. All infrared spectra were recorded over
400 to 4000 cmÀ 1 region with a resolution of 4 cmÀ 1. The crystallinity of
as-prepared hierarchical zeolite samples was estimated from (I550/I450)
� 100%/0.72 where I550 and I450 were the intensities of the infrared
bands at 550 and 450 cmÀ 1 [37]. The Ultraviolet–visible diffuse
reflection spectroscopy (UV–Vis) was conducted on a Shimadzu
UV-3600 spectrometer from 200 to 800 nm and BaSO4 was employed as


Fig. 1. XRD patterns (A), FTIR spectra (B) of the as-prepared hierarchical
zeolite samples and UV–vis diffuse reflectance spectra (C) of Ti-MFI.

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Microporous and Mesoporous Materials 302 (2020) 110204

Table 1
Textural properties of the as-prepared adsorbents.
Samples
Si-MFI
Al-MFI
Ti-MFI
a
b
c
d
e
f

Average lattice constantsa
a

b

c


18.4672
20.0588
18.6905

19.3771
28.5813
19.7030

13.1867
13.4596
13.3906

SBETb (m2/g)

Vtotalc (cm3/g)

Vmicrod (cm3/g)

RCe (%)

Si/Mf

294
362
312

0.21
0.26
0.21


0.10
0.07
0.04

67
69
61

/
24.3
25.7

Calculated from the results of XRD.
Surface area are calculated by Brunauer-Emmett-Teller (BET) method.
Total pore volume is obtained from the single point adsorption volume at P/P0 ¼ 0.985.
Micropore volume is obtained by the t-plot method.
Calculated by [(peak I550/peak I450) � 100%/0.72] from FTIR.
Measured by XRF.

Fig. 2. N2 sorption isotherms (A) and pore-size distributions (B) of the asprepared hierarchical zeolite samples.

internal standard. The porosity of as-prepared samples was analyzed by
N2 adsorption/desorption measurements using Micromeritics Tristar
3020 at 77 K. Before measurement, all samples were degassed under N2
at 423 K for 6 h. The specific surface area was calculated using the
Brunauer-Emmett-Teller (BET) method. And the pore size distribution
was calculated from the adsorption branch of N2 sorption isotherms

Fig. 3. SEM images of as-prepared hierarchical zeolite samples Si-MFI (A), AlMFI (B) and Ti-MFI (C).


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Microporous and Mesoporous Materials 302 (2020) 110204

Fig. 5. The breakthrough curves of as-prepared hierarchical zeolite samples for
toluene adsorption under dry condition (A) and humid condition (B, RH ¼ 50%)
at 298 K.

using Barrett-Joyner-Halenda (BJH) methods. Field emission scanning
electron microscopic (FE-SEM) images were obtained with a ZEISS
SIGMA 300 field emission scanning electron microscopy and the sample
power was put onto the carbon tape for direct observation. Transmission
electron microscope (TEM) images were obtained on Hitachi HF5000 at
an accelerating voltage of 200 kV.
2.4. Dynamic adsorption measurements of toluene
The toluene dynamic breakthrough curves were measured in a
continuous flow fixed-bed quartz reactor at atmospheric pressure. All
the feed gases used in this work were of high-purity grade (99.99%).
Gaseous toluene was generated by bubbling N2 through liquid toluene at
a thermostatic water bath (the concentration of toluene was controlled
by adjusting the flow rate of N2). The simulated exhaust gas consisted of
100 ppm toluene, 20% O2 and N2 (as balance gas). Vapor was introduced
by bubbling through water at a thermostatic water bath (the vapor was
controlled by adjusting the flow rate of N2) to maintain the atmosphere
with the relative humidity of 50%. Before adsorption experiments, the
adsorbent was firstly degassed in 573 K under N2 atmosphere. After
being cooled to room temperature, the simulated exhaust gas was


Fig. 4. TEM images of as-prepared hierarchical zeolite samples Si-MFI (A), AlMFI (B) and Ti-MFI (C).

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Microporous and Mesoporous Materials 302 (2020) 110204

Table 2
Toluene dynamic adsorption capacity under different relative humidity and the fitting parameters of different isotherm models for various adsorbents at 298 K.
Samples
Si-MFI
Al-MFI
Ti-MFI

Dynamic adsorption capacity Q (mg/gads)

Langmuir isotherm model

RH ¼ 0

RH ¼ 50%

Qwet/Qdry

qmax

KL


R

44
58
53

44
42
45

1.00
0.72
0.85

0.09
0.08
0.11

0.008
0.026
0.007

0.83
0.59
0.58

introduced to flow through the adsorbent bed (75 mg, 40–60 mesh) at a
rate of 100 mL/min giving the GHSV of 80,000 mL/(g⋅h). The dynamic
adsorption capacity (Q (mg/gads)) was calculated by the following

equation:
Q¼

Freundlich isotherm model
2

Kf

1/n

R2

0.007
0.025
0.007

0.38
0.17
0.42

0.99
0.99
0.96

shown in Fig. 1C and Fig. S1. Compared with the spectra of as-prepared
sample Si-MFI and Al-MFI (Fig. S1), the obvious absorption peak
appeared at 210 nm for as-prepared sample Ti-MFI (Fig. 1C), which was
ascribed to the charge transfer phenomenon of O2 (2p) to tetrahedral
coordination Ti4ỵ (3d) in Ti(OSi)4 structure and characteristic peak for
isolated, tetrahedral coordinated Ti(OSi)4 species in hydrophobic zeolite

frameworks [41]. Therefore, the Ti species were successfully incorpo­
rated into the tetrahedral coordination framework of the as-prepared
sample Ti-MFI. In addition, the absence of the broad absorption bands
at 260–280 nm indicated no six-coordinate titanium species among the
structure of Ti-MFI. No absorption peak was observed at 330 nm indi­
cating the absence of isolated TiO2 within as-prepared sample Ti-MFI
[42]. XRF results indicated that the Si/M (M ¼ Al or Ti) ratios of the
Al-MFI and Ti-MFI were about 24.3 and 25.7, respectively (Table 1).
The N2 adsorption-desorption technique was used to understand the
porous and nanostructure of as-prepared hierarchical zeolite samples
and the results were shown in Fig. 2. The nitrogen sorption isotherms of
the as-prepared samples (Fig. 2A) exhibited two hysteresis loops. The
hysteresis loop corresponding to the relative pressure P/P0 > 0.4 was H4
type, which implied the narrow lit-like pores existing in the as-prepared
samples [43]. And the hysteresis loop existed at lower relative pressure
(P/P0 was around 0.1–0.3) was ascribed to the crystalline-like phase
transition of N2, which was also observed for MFI topology with high
Si/Al ratios [44]. The phase transition of the adsorbed N2 predominantly
occurred in a non-reversible manner and led to low-pressure hysteresis
[45]. The corresponding pore size distribution curves were shown in
Fig. 2B and demonstrated the presence of mesopores between 2 and 3
nm. In addition, the wide pore size distribution more than 10 nm was
also observed for the as-prepared samples. The detailed textual prop­
erties of as-prepared samples were summarized in Table 1. The BET
surface area of the as-synthesized zeolites distributed in 294–362 m2/g.
The total volume obtained from the single point adsorption volume at
P/P0 ¼ 0.985 for Si-MFI was 0.21 cm3/g, for Al-MFI was 0.26 cm3/g and
for Ti-MFI was 0.21 cm3/g, respectively.
In order to understand the morphology, the as-prepared samples
were characterized by electron microscopy and the corresponding SEM

images were shown in Fig. 3. The spherical nanozeolite crystal aggre­
gation with smooth surface could be well observed and the mesoporous
structure resulted from the interparticle of nanozeolite crystal aggre­
gation, which was also well reflected by the results of N2 sorption iso­
therms. The size of aggregated spherical nanozeolite of Si-MFI, Al-MFI
and Ti-MFI were around 500–700, 200–450 and 350–550 nm, respec­
tively. The TEM images of the as-prepared samples were shown in Fig. 4
and Fig. S2. And the observation revealed that all the as-prepared SiMFI, Al-MFI and Ti-MFI possessed the mesoporous structure originated
from the aggregation of nanosized zeolite particles. In addition, the
samples had a spherical morphology with a diameter of about 300–500
nm, which was also consistent with the SEM observations.

Cin FMt
W

where Cin is the concentration of toluene in the feed gas, F (mL/min) is
the total flow rate, M is the relative molecular mass of toluene (92 g/
mol), W (g) is the weight of the loaded adsorbent, t (min) is the
adsorption time.
2.5. Temperature programmed desorption of toluene
The temperature programmed desorption of toluene (Toluene-TPD)
experiments were conducted at different ramping rates of temperature.
Before each experiment, 75 mg adsorbent was degassed at 573 K for 1 h
under high-purity Ar flow at 100 mL/min. After being cooled to room
temperature under flowing Ar, the sample was exposed to a mixture gas
containing 1000 ppm toluene in balance with Ar for 30 min. Then, the
gas flow was switched to pure Ar to remove the residual toluene within
the measurement system. Finally, the TPD-toluene profiles were
measured as a function of temperature from 303 to 600 K with various
heating rate from 4.5 to 18 K/min under Ar flow (100 mL/min). The

signal of toluene was monitored by an online MS (mass spectrum, Hiden
HPR 20) at the m/z ¼ 92.
3. Results and discussions
3.1. Textual properties of samples
The crystal structure of as-prepared samples was characterized by
XRD and the results were shown in Fig. 1A, which demonstrated the
three samples had the typical MFI microporous zeolite structure (JCPDS,
PDF#44–0003) without any additional diffraction peaks. And the cell
parameters, which were calculated based on the XRD results, of Al-MFI
and Ti-MFI slightly increased (the detailed data were shown in Table 1)
compared with those of Si-MFI, implying that both Al and Ti were suc­
cessfully doped into the lattice of zeolite. In order to further estimate the
framework crystallinity, the as-prepared samples were characterized by
FTIR and the results were shown in Fig. 1B. It was reported that the peak
at near 450 cmÀ 1 was attributed to the stretching of Si–O bonds and the
bands around 550 cmÀ 1 were the characteristic bands of double fivemembered rings of MFI zeolites [38]. Meanwhile, the IR bands near
1100 cmÀ 1 and 1230 cmÀ 1 belonged to the asymmetric stretching vi­
bration of Si–O–Si bonds [39]. An obvious peak at 550 cmÀ 1 could be
detected on as-prepared samples, indicating that all samples were MFI
structure zeolite, which was consistent with the results of XRD charac­
terization. The broadened peaks near 3500 cmÀ 1 were attributed to
hydroxyl groups of water molecules adsorbed on as-prepared samples
[40]. Among the prepared zeolites, the Si-MFI sample exhibited lower
intensity of the adsorption peak, indicating the Si-MFI possessed better
hydrophobicity. The crystallinity of as-prepared Si-MFI, Al-MFI, and
Ti-MFI were 67%, 69%, and 61%, respectively (Table 1). In order to
further understand framework, the as-prepared samples were charac­
terized by UV–vis diffuse reflectance spectrum and the results were

3.2. Dynamic adsorption capacities

The dynamic adsorption behaviors of toluene on Si-MFI, Al-MFI and
Ti-MFI under dry and humid condition were measured and the break­
through curves were presented in Fig. 5. The breakthrough time of
toluene adsorption over as-synthesized samples under dry gas (Fig. 5A)
was 30, 54 and 51 min and the corresponding saturated adsorption ca­
pacity was 44, 58 and 53 mg/gads for Si-MFI, Al-MFI and Ti-MFI,
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Microporous and Mesoporous Materials 302 (2020) 110204

Fig. 6. Nonlinear fits of Langmuir and Freundlich isotherm model for toluene
adsorption on as-prepared hierarchical zeolite samples Si-MFI (A), Al-MFI (B)
and Ti-MFI (C).

Fig. 7. Nonlinear fits of pseudo-first-order, pseudo-second-order kinetics model
and Elovich model for toluene adsorption by as-prepared hierarchical zeolite
samples Si-MFI (A), Al-MFI (B) and Ti-MFI (C).
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Si-MFI and Ti-MFI well maintained the adsorption capacity during the
reusing process and the values were 42 and 41 mg/gads, which corre­
sponded to 95.5% and 91.1% of the adsorption capacity during the first

process for Si-MFI and Ti-MFI, respectively.

Table 3
Kinetics parameters for the adsorption of toluene on various adsorbents at 298 K.
Model

Si-MFI

Al-MFI

Ti-MFI

Pseundo-firstorder

Qe (mg/gads)
k1 (minÀ 1)
R2

62
0.012
0.9826

104
0.007
0.9890

100
0.007
0.9911


Pseundo-secondorder

Qe (mg/gads)
k2 (minÀ 1)
R2

102
7.4 �
10À 5
0.9803

182
2.2 �
10À 5
0.9881

178
2.1 �
10À 5
0.9905

Elovich

a (mg/gads∙minÀ 1)
b (gads/mg)
R2

2.07
0.062
0.926


2.24
0.049
0.902

2.00
0.057
0.898

Weber and Morris

K1 (mg/
gads∙minÀ 0.5)
C1 (mg/gads)
R21
K2 (mg/
gads∙minÀ 0.5)
C2 (mg/gads)
R22
K3 (mg/
gads∙minÀ 0.5)
C3 (mg/gads)
R23

4.347

4.514

4.180


À 7.66
0.986
7.005

À 7.95
0.986
8.502

À 7.36
0.986
7.542

À 21.12
0.992
À 0.398

À 29.78
0.996
1.965

À 25.40
0.997
1.714

45.00
0.613

33.81
0.831


28.68
0.640

3.3. Adsorption isotherm model
In order to understand the interactions between toluene molecules
and the as-prepared samples under dry condition, the adsorption models
of Langmuir and Freundlich [46] were applied to fit the experimental
data and to study the toluene adsorption behavior over Si-MFI, Al-MFI
and Ti-MFI. The detailed information on the adsorption equilibrium
models was enclosed in Supplementary Materials. The nonlinear fits of
the mentioned isotherm models were shown in Fig. 6, and the fitted
parameters were listed in Table 2. It was observed that the correlation
coefficients (R2 ¼ 0.99, 0.99 and 0.96 on Si-MFI, Al-MFI and Ti-MFI,
respectively) of the Freundlich model were much higher than those
obtained from the Langmuir model (R2 ¼ 0.83, 0.59 and 0.58 on Si-MFI,
Al-MFI and Ti-MFI, respectively). The results indicated that the
Freundlich model could well describe the toluene adsorption in the
as-prepared three adsorbents, implying that the distribution of adsorp­
tion sites on the surface of MFI zeolite molecular sieves was uneven and
toluene were adsorbed by the means of multi-layer adsorption. Ac­
cording to the Freundlich theory, the value of Kf reflects the adsorption
ability of the adsorbent. The larger the value of Kf, the stronger the
adsorption capacity [47]. The calculated Kf values of Si-MFI, Al-MFI, and
Ti-MFI were 0.007, 0.025, and 0.007 respectively, indicating that
Al-MFI possessed a better adsorption performance for toluene than
Si-MFI and Ti-MFI for toluene, which was consistent with the experi­
mental results. The value of 1/n is correlated with the adsorption
strength and the corresponding values obtained from the fitting were
less than 0.5 for the three as-synthesized samples, indicating that
toluene could be well adsorbed on the as-synthesized the MFI nano­

zeolites [47].
3.4. Adsorption kinetics model
To further understand the adsorption mechanism of toluene on assynthesized hierarchical zeolites samples under dry condition, three
common models including pseudo-first-order adsorption, pseudosecond-order adsorption and Elovich adsorption models were used to
fit the experimental data. The detailed information was described in
Supplementary Materials. The fitting profiles of the experimental data
by these three models were shown in Fig. 7 and the corresponding ki­
netic parameters were listed in Table 3. The equilibrium adsorption
capacities of toluene on Si-MFI, Al-MFI and Ti-MFI obtained from the
fitting of pseudo-first-order adsorption model were 62, 104 and 100 mg/
gads, respectively. The corresponding first-order adsorption rate con­
stants were 0.012, 0.007 and 0.007 minÀ 1 with the correlation co­
efficients were 0.9826, 0.9890 and 0.9911. The fitting equilibrium
adsorption capacities of toluene on the as-prepared zeolite samples (SiMFI, Al-MFI and Ti-MFI) by the pseudo-second-order adsorption were
102, 182 and 178 mg/gads, accompanied by the adsorption rate con­
stants of 7.4 � 10À 5, 2.2 � 10À 5 and 2.1 � 10À 5 minÀ 1. The correlation
coefficients of this model were 0.9803, 0.9881 and 0.9905, respectively.
The initial sorption rates of toluene on as-synthesized Si-MFI, Al-MFI
and Ti-MFI obtained by Elovich adsorption model were 2.07, 2.24 and
2.00 mg/gads∙minÀ 1, the Elovich constants were 0.062, 0.049 and 0.057
gads/mg and the corresponding correlation coefficients were 0.926,
0.902 and 0.898, respectively. According to the fitting results, the Elo­
vich model over the experimental data of as-prepared samples had the
lowest correlation coefficients, which indicated that the Elovich model
was unsuitable to describe the adsorption of toluene on Si-MFI, Al-MFI
and Ti-MFI. Compared with the fitting results from pseudo-second-order
adsorption model, the equilibrium adsorption capacities obtained from
pseudo-first-order model matched better with the experimental results

Fig. 8. Linear fits of intraparticle diffusion for toluene adsorption on asprepared hierarchical zeolite samples.


respectively. The as-prepared Al-MFI and Ti-MFI exhibited better per­
formance than Si-MFI, which indicated the doping of Al and Ti within
the framework of the hierarchical zeolite could increase the toluene
adsorption capacity. When the relative humidity was fixed at 50%, the
corresponding saturated adsorption capacity was 44, 42 and 45 mg/gads
for Si-MFI, Al-MFI and Ti-MFI, respectively. The ratios of saturated
toluene adsorption capacity under humid and dry condition (Qwet/Qdry)
among the above three as-prepared samples were 1.00, 0.72 and 0,85, as
shown in Table 2. Compared the adsorption performances for toluene of
these three zeolites under dry and humid conditions, the toluene
adsorption performance of Si-MFI remained when the relative humidity
was fixed at 50% which could be ascribed to its high hydrophobicity. In
contrast, the H2O molecules in moisture gas were adsorbed on the
adsorption sites and thus hindered toluene adsorption to some extent on
the Al-MFI and Ti-MFI. Furthermore, the reusability of Si-MFI, Al-MFI
and Ti-MFI was also evaluated and the results were shown in Fig. S3. The
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Microporous and Mesoporous Materials 302 (2020) 110204

Fig. 9. Toluene-TPD under different βH from as-prepared hierarchical zeolites samples Si-MFI (A), Al-MFI (B) and Ti-MFI (C) and the linear dependence between In
(RT2p/βH) and 1/Tp for the desorption of toluene on the three samples (D).

samples were shown in Fig. 8 and the fitting kinetic parameters were
listed in Table 3. There were three sections shown in the fitting plot,
indicating that the adsorption was affected by multiple independent

steps besides intraparticle diffusion [48]. The three sections could be
ascribed to external diffusion, intraparticle diffusion and the final
adsorption equilibrium stage in turn [40,42]. The slopes of the curve
obtained in the second stage were the largest, indicating that the
adsorption of toluene on as-prepared samples was not limited by the
intraparticle diffusion. In other words, the porous structure resulted
from nanosized zeolite aggregation facilitated the mass diffusion during
the toluene adsorption procedure.

Table 4
Desorption activation energy, Ed and peak temperature Tp of toluene on asprepared hierarchical zeolite samples.
Samples

Si-MFI
Al-MFI
Ti-MFI

Value of Tp (K) for peaks of TPD curve obtained at
heating rate, βH
4.5 (K/
min)

9 (K/
min)

13.5 (K/
min)

18 (K/
min)


346
345
344

353
355
354

363
359
362

369
366
366

Ed (kJ/
mol)

R2

55.2
65.4
58.7

0.93
0.97
0.99


3.5. Toluene-TPD

relatively. Moreover, the correlation coefficients of pseudo-first-order
adsorption model were larger than those from pseudo-second-order
adsorption model, indicating that the adsorption of toluene on asprepared samples could be better illustrated by pseudo-first-order
adsorption model.
In order to evaluate the intraparticle diffusion effect on adsorption
kinetics, Weber and Morris theory was proposed. The detailed infor­
mation was enclosed in Supplementary Materials. The linear fits of
intraparticle diffusion for toluene adsorption on as-synthesized zeolite

On the purpose of understanding the interaction strength between
toluene and the as-synthesized hierarchical zeolite samples, the tem­
perature programmed desorption of toluene (toluene-TPD) over assynthesized Si-MFI, Al-MFI and Ti-MFI was conducted at different
ramping rates. The detailed information on the procedure was enclosed
in Supplementary Materials and the corresponding toluene-TPD curves
of as-synthesized Si-MFI, Al-MFI and Ti-MFI at different heating rates
were shown in Fig. 9. The peaks related to toluene desorption could be
8


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Microporous and Mesoporous Materials 302 (2020) 110204

observed as the temperature increasing. In addition to the desorption
peaks of physically-adsorbed toluene at 340–370 K, the toluene
desorption peaks in the range of 450–500 K were also observed for AlMFI, indicating the stronger interaction between toluene and Al-MFI.
The desorption temperatures of toluene at different heating rates were
showed in Fig. 9 and listed in Table 4 and the gradual increase in the

heating rates led to the temperature increase of toluene desorption. The
desorption energy of toluene on Si-MFI, Al-MFI and Ti-MFI were
calculated according to toluene-TPD results from different heating rates
and corresponding desorption energy was 55.2, 65.4 and 58.7 kJ/mol,
respectively. Normally, the higher energy required for desorption
demonstrates the stronger the interaction between the adsorbate and the
adsorbent since desorption process is endothermic. Therefore, the
interaction strength of toluene with as-prepared zeolite samples fol­
lowed the order: Al-MFI > Ti-MFI > Si-MFI, which illustrated that the Al
and Ti doping within the framework of MFI zeolite could increase the
adsorption strength of toluene on MFI zeolites.

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4. Conclusions
Three kinds of spherical hierarchical zeolites consisting of nanosized
zeolite crystals were successfully synthesized. Using toluene as the
model VOCs molecule, the adsorptive properties of these samples were
evaluated by means of dynamic adsorption measurements, adsorption
simulations as well as the temperature programmed desorption of
toluene. The dynamic adsorption results showed that Al-MFI exhibited
the highest adsorption capacity to toluene under dry gas conditions
while Ti-MFI had the optimal toluene adsorption performance under the
wet condition (the relative humidity was 50%). The fitting of adsorption
isotherm results showed that the adsorption behavior of toluene on MFI
nanozeolites conformed to the Freundlich principle. The kinetic simu­
lation results showed that the pseudo-first-order adsorption model was
suitable to elucidate the adsorption process. And the toluene adsorption
process over as-prepared samples was affected by multiple steps and the
intraparticle diffusion was not the limited step. In addition, the Al and Ti
doping resulted in the increasing interaction strength between toluene
and the as-prepared samples and the interaction strength followed the
order: Al-MFI > Ti-MFI > Si-MFI. The findings herein will offer valuable
information for VOCs removal research and application by hierarchical
zeolites.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
The work was financially supported by National Key R&D program of
China (2017YFE0127400), Natural Science Foundation of Hubei Prov­
ince (2019CFA070), the Opening Project of Hubei Key Laboratory of

Processing and Application of Catalytic Materials (201829103) and
Program for Huazhong University of Science and Technology (HUST)
Academic Frontier Youth Team (2018QYTD03). The authors thank the
HUST Analysis and Testing Center of for analytical support. We really
appreciate Miss Linlin Zhang from Shanghai Institute of Ceramics Chi­
nese Academy of Sciences for her kindness and support on the TEM
measurements and suggestions during the special period of coronavirus
prevalence.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.micromeso.2020.110204.

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