Unveiling the importance of reactant mass transfer in environmental catalysis: Taking catalytic chlorobenzene oxidation as an example

Article abstract of DOI:10.1016/j.cclet.2020.09.001

To date, investigations onto the regulation of reactants mass transfer has been paid much less attention in environmental catalysis. Herein, we demonstrated that by rationally designing the adsorption sites of multi-reactants, the pollutant destruction efficiency, product selectivity, reaction stability and secondary pollution have been all affected in the catalytic chlorobenzene oxidation (CBCO). Experimental results revealed that the co-adsorption of chlorobenzene (CB) and gaseous O₂ at the oxygen vacancies of CeO₂ led to remarkably high CO₂ generation, owning to their short mass transfer distance on the catalyst surface, while their separated adsorptions at Br?nsted HZSM-5 and CeO₂ vacancies resulted in a much lower CO₂ generation, and produced significant polychlorinated byproducts in the off-gas. However, this separated adsorption model yielded superior long-term stability for the CeO₂/HZSM-5 catalyst, owning to the protection of CeO₂ oxygen vacancies from Cl poisoning by the preferential adsorption of CB on the Br?nsted acidic sites. This work unveils that design of environmental catalysts needs to consider both of the catalyst intrinsic property and reactant mass transfer; investigations of the latter could pave a new way for the development of highly efficient catalysts towards environmental pollution control.

Full text of DOI:10.1016/j.cclet.2020.09.001

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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Unveiling the importance of reactant mass transfer in environmental
catalysis: Taking catalytic chlorobenzene oxidation as an example
Kexin Caoa,b, Xiaoxia Daia,b, Zhongbiao Wua,b, Xiaole Wenga,b,
*
a
Key Laboratory of Environment Remediation and Ecological Health, Ministry of Education, College of Environmental and Resource Sciences, Zhejiang
University, Hangzhou 310058, China
b
Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 310058, China
A R T I C L E I N F O
A B S T R A C T
Article history:
To date, investigations onto the regulation of reactants mass transfer has been paid much less attention in
environmental catalysis. Herein, we demonstrated that by rationally designing the adsorption sites of
multi-reactants, the pollutant destruction efficiency, product selectivity, reaction stability and secondary
pollution have been all affected in the catalytic chlorobenzene oxidation (CBCO). Experimental results
revealed that the co-adsorption of chlorobenzene (CB) and gaseous O2 at the oxygen vacancies of CeO2 led
to remarkably high CO2 generation, owning to their short mass transfer distance on the catalyst surface,
while their separated adsorptions at Brönsted HZSM-5 and CeO2 vacancies resulted in a much lower CO2
generation, and produced significant polychlorinated byproducts in the off-gas. However, this separated
adsorption model yielded superior long-term stability for the CeO2/HZSM-5 catalyst, owning to the
protection of CeO2 oxygen vacancies from Cl poisoning by the preferential adsorption of CB on the
Brönsted acidic sites. This work unveils that design of environmental catalysts needs to consider both of
the catalyst intrinsic property and reactant mass transfer; investigations of the latter could pave a new
way for the development of highly efficient catalysts towards environmental pollution control.
© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Received 23 June 2020
Received in revised form 23 July 2020
Accepted 2 September 2020
Available online xxx
Keywords:
VOCs oxidation
Chlorinated organics
Reactant mass transfer
Secondary pollution
Catalyst design
Environmental catalysis
Environmental catalysis is of great importance in air pollution
control, which converts the air pollutants into harmless products
via a range of heterogeneous catalytic reactions [1]. Typical
examples include selective catalytic reduction (SCR) of NOx [2–4],
catalytic destruction of organic wastes [5], the methane catalytic
reforming with carbon dioxide [6], etc. Recent development in the
environmental catalysis has been greatly accelerated by the
increasingly stringent emission standards, while numerous
techniques have been oriented to industrial-scale applications,
making an important contribution to the improvement of air
quality in China. As the core of environmental catalysis, rational
design of environmental catalysts with an aim to maximize their
catalytic activities have been extensively explored, which yields
significant outcomes in terms of increasing the number of active
sites [7,8] and enhancing the redox ability of catalysts [9–11].
However, since most of environmental heterogeneous reactions
involve two or more reactants, the pollutant destruction efficiency
is not only dependent on the intrinsic properties of applied
catalysts, but also on the mass transfer and collision probability of
these multi-reactants. Current works put great efforts on modify-
ing the catalyst intrinsic properties, while investigations onto how
to regulate the reactants mass transfer rate has been paid much
less attention; the latter is believed to play crucial role in
determining the pollutant conversion efficiency and reaction
selectivity.
Chlorinated volatile organic compounds (Cl-VOCs) are well-
known with inherent bioaccumulation and potential carcinoge-
nicity, many of which have been listed as priority control pollutants
worldwide [12,13]. Catalytic destruction of chlorinated organics
remains a great challenge in environmental catalysis, owning to it
encounters problems of catalyst deactivation [14] and secondary
pollution (i.e., abundant more toxic byproducts) [15,16], which
severely hinders this technique towards industrial scale applica-
tion [17–19]. This process is initiated by the scission of C–Cl bond at
acidic (Brönsted/Lewis) sites or superficial oxygen vacancy and the
activation of gaseous O2 at oxygen vacancy, followed by the
reaction between multi-adsorbates to convert the Cl-VOCs into
CO2, H2O, HCl/Cl2 and intermediates [20,21]. The involvement of
Cl-VOCs and O2 adsorptions at various active sites and the
abundant reaction byproducts make the catalytic destruction of
* Corresponding author at: Key Laboratory of Environment Remediation and
Ecological Health, Ministry of Education, College of Environmental and Resource
Sciences, Zhejiang University, China.
E-mail address: xlweng@zju.edu.cn (X. Weng).
https://doi.org/10.1016/j.cclet.2020.09.001
1001-8417/© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Cao, et al., Unveiling the importance of reactant mass transfer in environmental catalysis: Taking catalytic
chlorobenzene oxidation as an example, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.09.001

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Cl-VOCs much ideal for exploring the importance of mass transfer
in determining the pollutant destruction efficiency and product
selectivity.
Herein, we choose CeO2 nanorods as a model catalyst, because it
has abundant superficial oxygen vacancies [22,23] that could
provide sufficient adsorption sites for both of the Cl-VOCs and
gaseous O2. Chlorobenzene (CB) was selected as typical Cl-VOCs,
the oxidation of which has shown to easily generate reaction
intermediates [24], and can be used to evaluate the reaction
selectivity.
Furthermore, to get a contrasted catalyst, a HZSM-5 zeolite
with abundant Brönsted acidic sites was introduced by using a
dry-mixing route in a ball miller. This catalyst was expected to
provide separated adsorption sites for Cl-VOCs and gaseous O2, as
the Cl-VOCs were shown to preferentially adsorb on the Brönsted
HZSM-5 sites. The separated adsorptions of Cl-VOCs and O2 and the
poor-mixing of CeO2 and HZSM-5 effectively increased the mass
transfer distance of their adsorbates, which should yield varied
catalytic performance in comparison with their co-adsorption on
the CeO2 vacancies.
The reaction characteristics and byproducts generation of CeO2
and CeO2/HZSM-5 catalysts in the catalytic CB oxidation (CBCO)
were evaluated using a range of analytical techniques, including
powder X-ray diffraction (XRD), transmission electron microscopy
(TEM), temperature program reduction of hydrogen (H2-TPR),
temperature program desorption of oxygen (O2-TPD), fourier
transform infrared spectroscopy (FT-IR), gas chromatography mass
spectrometry (GC-MAS), etc. XRD indicated the dry-mixing did not
change the crystal structure of CeO2 and HZSM-5 (Fig. S1 in
Supporting information). The former exhibited characteristic
patterns at 28.7ꢀ, 33.1ꢀ, 47.4ꢀ, 56.3ꢀ, 69.7ꢀ and 76.9ꢀ with a cubic
fluorite structure (JCPDS No. 89-8436), and the latter revealed an
MFI type framework at 7.9ꢀ, 8.8ꢀ, 23.0ꢀ, 23.9ꢀ, 29.8ꢀ, 45.5ꢀ and 55.1ꢀ
(JCPDS No. 44-0002). Scanning electron microscope (SEM)
revealed that CeO2 was composed of monodispersed nanorods
(200À500 nm in length) and in the CeO2/HZSM-5, these nanorods
were much shorter (50À200 nm) and showed certain agglomer-
ations (Fig. S2 in Supporting information). Energy dispersive X-ray
spectroscopy (EDX) mapping indicated the HZSM-5 and CeO2 were
not well mixed, owning to the use of dry mixing method (Fig. S3
in Supporting information). The Brunauer-Emmet-Teller (BET)
surface area measurements showed the CeO2 with a surface area
of 96.0 m2/g, which was lower than that of CeO2/HZSM-5
(120.1 m2/g), attributing to the HZSM-5 with a high BET surface
area of 180.1 m2/g.
introduction of HZSM-5 greatly enhanced the Brönsted acidity of
the catalyst. This acidity was mainly derived from the proton H on
the surface of HZSM-5. The amounts of acidic sites were also
greatly increased by introducing the HZSM-5. In the NH3-TPD
profile, the type of acids can be divided into weak acid (below
200 ꢀC), medium strong acid (200À400 ꢀC), and strong acid (above
400 ꢀC) based on the NH3 desorption temperature. As shown in
Fig. 1b, the CeO2 exhibited two broad NH3 desorption peaks
centered at 99 ꢀC and 464 ꢀC, both of which were resulted from the
Ce4+/Ce3+ (dominant) and the surface acidic hydroxyl group
(bridged OHad) [27]. After loading the HZSM-5, the intensity of
NH3 desorption peaks were significantly enhanced, and shifted to
74 ꢀC and 351 ꢀC, respectively, suggesting that enriched weak and
medium strong acidities were introduced to the CeO2/HZSM-5
catalyst, consistent with the Py-IR results.
The selective adsorption of CB on the CeO2 and CeO2/HZSM-5
catalysts were confirmed using in situ FT-IR analyses. The spectra
were collected at 150 ꢀC in a stream of 500 ppm CB and 10 vol% O2
within 30 min. As shown in Fig. 2a, the bands at 1591, 1479 and
1444 cmÀ1 are assigned to C=C degenerate stretching vibrations of
the aromatic ring [28]. According to the literature [29], on the
dehydroxylated defect-free CeO2 surface, CB adsorption was
mainly through Ce4+ÁÁÁ
p-electron type interaction, while on the
hydroxylated surface, this preceded via a dual-site interaction
(OHÁÁÁ -electron and OHÁÁÁCl). During the preparation of CeO2
p
nanorods, a large number of hydroxyl groups remained on the
catalyst surface after alcohol washing. As a result, the CB was
shown to initially adsorb on the Ce–OH site. This is confirmed by
the changes of ÀOH vibration, which exhibited inverted peaks in
the range of 3750À3625 cmÀ1 after CB adsorption. The appearance
of 3600 cmÀ1 band is considered as the result of the migration of
these inverted peaks, owning to the disturbance of adsorbed
species [29]. The bands in the range of 2000À1700 cmÀ1 can be
attributed to the out of plane distortion harmonics (combination
and overtones) of the C–H bond [30], which are derived from the
interaction of
p electron cloud of benzene ring and electron center
of oxide surface [31]. The characteristic bands at 3068 and
2829 cmÀ1 are derived from the vibration of C–H on benzene ring
[32]. These bands increased gradually in the first 10 min, and then
decreased, suggesting that the OH groups on the CeO2 surface were
gradually consumed by CB adsorption.
After 10 min, a new band appeared at 1667 cmÀ1, which
gradually increased with the measuring time. This band has been
assigned to the CB adsorption on Ce3+-Vo sites [33], which could
result in the cleavage of C–Cl band, leaving the Cl at oxygen
vacancies (Vo). The dissociated Cl at the Vo is inclined to attack the
C+ of phenyl, leading to an electrophilic chlorination and the
formation of (poly) chlorinated byproducts [34]. The continued
growth of this peak indicated that after the complete consumption
of surface hydroxyls in the CeO2, the CB was mainly adsorbed on
surface Vo sites. Additionally, the vibration bands at 1534 and
1174 cmÀ1 are assigned to the intermediate products of maleic acid
[28] and the inverted bands at 2935 and 2845 cmÀ1 can be
attributed to methylene (ÀCH2À) and methyl (ÀCH3) [29]. Fig. 2b
illustrates the adsorption of CB on the CeO2/HZSM-5 catalyst. It
was noted that loading of the HZSM-5 effectively changed the
adsorption model of CB on the catalyst surface, where the CB was
found to mainly adsorb on the hydroxyls of HZSM-5, revealing the
characteristic bands at 1578, 1478, 1444 and 1253 cm-1 [35]. The in
situ FT-IR analyses confirmed our assumption that the CB was
preferentially adsorbed on the HZSM-5, which effectively separat-
ed the adsorption site with O2, while this separated adsorption
model made the two adsorbates have a comparatively larger mass
transfer distance than co-adsorbed on the CeO2.
To confirm the existence of Brönsted acidity in the CeO2/HZSM-
5 catalyst, pyridine adsorption infrared spectroscopy (Py-IR) and
NH3 temperature programmed desorption (TPD) were conducted.
As shown in Fig. 1a, the pyridine desorption peaks mainly located
at 1595,1545, and 1490 cmÀ1, which correspond to the Lewis acidic
site, Brönsted acidic site and the combination of them, respectively
[25,26]. In comparison with CeO2, the CeO2/HZSM-5 catalyst
exhibited a very intense peak at 1545 cmÀ1, suggesting that the
To investigate the reaction characteristics of CeO2 and CeO2/
HZSM-5 in the CBCO reaction, a CB-TPSR experiment involving a
Fig. 1. (a) pyridine-IR and (b) NH3-TPD profiles of CeO2 and CeO2/HZSM-5 catalysts.
Please cite this article in press as: K. Cao, et al., Unveiling the importance of reactant mass transfer in environmental catalysis: Taking catalytic
chlorobenzene oxidation as an example, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.09.001

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3
Fig. 2. In situ FT-IR spectra of (a) CeO2 and (b) CeO2/HZSM-5 catalysts at 150 ꢀC in a stream of 500 ppm CB and 10 vol% O2 within 30 min.
flow of 500 ppm CB and 10 vol% O2 was conducted. The dynamic
and timely generation of CO2 from this reaction were in situ
monitored. As shown in Fig. 3a, the different mass transfer distance
in the CeO2 and CeO2/HZSM-5 indeed resulted in a distinct change
in CO2 generation, where the CeO2 with short transfer distance
yielded an intense CO2 desorption peak in the temperature range of
225À450 ꢀC. In comparison, the CeO2/HZSM-5 with separated
absorption sites of CB and O2 exhibited a much lower and
postponed CO2 desorption peak. This result verifies that the mass
transfer distance between the reactant adsorbates plays a crucial
role in determining the CB destruction efficiency and CO2
selectivity, where short distance yielded much higher destruction
efficiency and CO2 selectivity than the longer one. However, it was
noted that the co-adsorption of CB and O2 at the Vo resulted in
severe Cl poisoning of the catalyst, where in a 250 ꢀC stability test
(Fig. 3b), the CeO2 was shown to be rapidly deactivated, but the
CeO2/HZSM-5 displayed a much better long-term stability. Since
the introduction of HZSM-5 was shown to not significantly alter
the redox properties of CeO2 catalyst (as confirmed by H2-TPR and
O2-TPD analyses in Figs. S4-S5 in Supporting information), we
believed that the higher long-term stability of CeO2/HZSM-5
should be attributed to the preferential adsorption of CB at HZSM-5
that hindered the Cl occupation at Vo and ensured the continuous
O2 activation for CBCO reaction.
Fig. 4. Quantitative analyses of polychlorinated byproducts collected in the
350 ꢀC off-gases of CeO2 and CeO2/HZSM-5 catalysts.
in Supporting information) that facilitated the electrophilic
chlorination reaction. This reaction was assumed to precede
through the electrophilic substitution of Cl over the Lewis acid sites
of CeCl4 [39] that attacked the accumulated CB at Brönsted HZSM-
5 sites, leading to the formation of dichlorobenzenes in the off-gas.
In summary, we have fabricated CeO2 and CeO2/HZSM-5
catalysts that were employed in the CBCO reaction to unveil the
importance of reactant mass transfer in environmental catalysis.
The co-adsorbed CB and O2 on the CeO2 surface resulted in a
remarkably high CO2 generation, while those separately adsorbed
on the CeO2/HZSM-5 yielded a much lower CO2 generation. This
verifies our assumption that rational design of the mass transfer
distance of reactant adsorbates can effectively regulate the
pollutant conversion efficiency and product selectivity. The co-
adsorption of CB and O2 was shown to cause severe deactivation of
the CeO2 catalyst, as the dissociated Cl occupied the surface oxygen
vacancy that hindered the O2 activation. While in the CeO2/HZSM-
5, the CB was preferentially adsorbed on the Brönsted acidic sites of
HZSM-5, which protected the oxygen vacancy from Cl poisoning,
leading to a high long-term stability in CBCO reaction. However,
the excessive adsorption of CB on the Brönsted sites distinctly
promoted electrophilic chlorination reaction, which generated
significant dichlorobenzenes in the off-gas, causing a severe
secondary pollution to the environment. The work conducted
herein unveils that the design of environmental catalysts needs to
consider both of catalyst intrinsic property and reactant mass
transfer, as they can both affect the pollutant conversion, product
selectivity, reaction stability and secondary pollution. To date,
modification of the reactant mass transfer has been paid much less
attention in environmental catalysis. Such an investigation could
pave a new way for the development of highly efficient catalysts for
environmental pollution control.
Reaction byproducts, particularly toxic polychlorinated organ-
ics in the off-gases were quantitatively analysed using a calibrated
GC–MS system. As shown in Fig. 4, the CeO2 and CeO2/HZSM-5
catalysts both generated certain polychlorinated byproducts,
including polychlorinated alkanes, polychlorinated alkenes and
dichlorobenzenes, amongst which, the dichlorobenzenes should
be paid the most concern as they are easily converted into dioxins,
leading to severe secondary pollution to the environment [36–38].
The CeO2 yielded approximately 2
while for the CeO2/HZSM-5, the amounts of m-dichlorobenzene
were measured at 5
g/m3, and the p-dichlorobenzene was shown
to be as high as 38
g/m3. Such a difference was show to originate
m
g/m3 of p-dichlorobenzene,
m
m
from the excessive adsorption of CB on the HZSM-5 surface (Fig. S6
Fig. 3. (a) CB-TPSR profiles of CO2 yield and (b) a stability test at 250 ꢀC on the CeO2
and CeO2/HZSM-5 catalysts; Reaction conditions: GHSV = 10 000 mL gÀ1 hÀ1
,
500 ppm CB, N2 flow rate =145 mL/min, O2 flow rate =15 mL/min.
Please cite this article in press as: K. Cao, et al., Unveiling the importance of reactant mass transfer in environmental catalysis: Taking catalytic
chlorobenzene oxidation as an example, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.09.001

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K. Cao et al. / Chinese Chemical Letters xxx (2020) xxx–xxx
[14] J. Liu, X. Dai, Z. Wu, X. Weng, Chin. Chem. Lett. 31 (2020) 1410–1414.
Declaration of competing interest
The authors report no declarations of interest.
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Please cite this article in press as: K. Cao, et al., Unveiling the importance of reactant mass transfer in environmental catalysis: Taking catalytic
chlorobenzene oxidation as an example, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.09.001