Ozone initiated oxidation of 1,2-dichlorobenzene catalyzed by manganese loaded gamma alumina and silica

Article abstract of DOI:10.1016/j.cattod.2020.06.025

The ozone-initiated oxidation of 1,2-dichlorobenzene catalysed by manganese supported on metal oxide (γ-Al₂O₃ and SiO₂) at ambient temperature and pressure conditions is reported in this study. Wet impregnation method was used to synthesise various percentages of Mn loading on γ-Al₂O₃ and SiO₂ supports. The catalysts were characterised by FT-IR, SEM, EDX, TEM, ICP-OES, BET and XRD techniques. All the reactions were conducted in an impinger glass reactor using 25 mL pure 1,2-dichlorobenzene and 0.25 g of the catalysts. The reaction products were characterised by GC–MS and FT-IR for quantitative and qualitative identification of the products. The 2.5, 5, 7.5 and 10 % of Mn impregnated on γ-Al₂O₃ and SiO₂ were found to be more active than γ-Al₂O₃ and SiO₂ supports. The 5 % Mn/SiO₂ catalyst was found to be the most active catalyst in the ozonation reaction of 1,2-dichlorobenzene with a percentage conversion of 44 % and percentage selectivity of 88 % towards the main product mucochloric acid. Whereas, 5 % Mn/γ-Al₂O₃ catalyst resulted in the percentage conversion of 40 % and percentage selectivity of 86 % towards the main product mucochloric acid. The activity of the catalysts is attributed to manganese loaded on γ-Al₂O₃ and SiO₂ supports.

Full text of DOI:10.1016/j.cattod.2020.06.025

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Ozone initiated oxidation of 1,2-dichlorobenzene catalyzed by manganese
loaded gamma alumina and silica
Nomthandazo Mkhizea, Prabal Pratap Singhb, Deepak Kumar Dasb,
Viswanadha Srirama Rajasekhar Pullabhotlaa,*
a Department of Chemistry, University of Zululand, Private Bag X1001, Kwa-Dlangezwa, 3886, South Africa
b Department of Chemistry, GLA University, Mathura, UP, 281406, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oxidation
1,2-Dichlorobenzene
Mn/γ-Al2O3 and Mn/SiO2 catalysts
Ozone and Mucochloric acid
The ozone-initiated oxidation of 1,2-dichlorobenzene catalysed by manganese supported on metal oxide (γ-
Al2O3 and SiO2) at ambient temperature and pressure conditions is reported in this study. Wet impregnation
method was used to synthesise various percentages of Mn loading on γ-Al2O3 and SiO2 supports. The catalysts
were characterised by FT-IR, SEM, EDX, TEM, ICP-OES, BET and XRD techniques. All the reactions were con-
ducted in an impinger glass reactor using 25 mL pure 1,2-dichlorobenzene and 0.25 g of the catalysts. The
reaction products were characterised by GC–MS and FT-IR for quantitative and qualitative identification of the
products. The 2.5, 5, 7.5 and 10 % of Mn impregnated on γ-Al2O3 and SiO2 were found to be more active than γ-
Al2O3 and SiO2 supports. The 5 % Mn/SiO2 catalyst was found to be the most active catalyst in the ozonation
reaction of 1,2-dichlorobenzene with a percentage conversion of 44 % and percentage selectivity of 88 % to-
wards the main product mucochloric acid. Whereas, 5 % Mn/γ-Al2O3 catalyst resulted in the percentage con-
version of 40 % and percentage selectivity of 86 % towards the main product mucochloric acid. The activity of
the catalysts is attributed to manganese loaded on γ-Al2O3 and SiO2 supports.
1. Introduction
aromatics under different degradation methods [5]. The common
characteristics of the model compounds and PCAs are their chemical
There is an increase in global pollution and some of the factors that
causes the increase are large number of organic impurities and pollu-
tants that are found in the biosphere, particularly chlorine-based pol-
lutants which are widely known for their non-degradable nature and
their high toxicity [1,2]. There are many types of chlorinated organic
compounds that are found in the environment, but a certain class of
chlorinated organic compounds have received much attention than
others due to their bioaccumulation in living tissues and their high
toxicity, this class of organic compounds is known as polychlorinated
aromatics (PCAs). PCAs have high persistence in the environment
which make them immune to degradation by biological processes [3].
These organic compounds are allowed as commercial compounds due
to their wide range of applications in agricultural and industrial sectors
[4].
behavior and structural properties. In the present study 1,2-di-
chlorobenzene is used as a model compound so as to predict degrada-
tion of PCAs. Since orthodichlororbenzene is also non-biodegradable,
its continuous and excessive use as an insecticide is hazardous to en-
vironment [6]. This has prompted the researchers to work towards the
development of the degradation methods that can be used to convert
1,2-dichlorobenzene into less toxic value-added products [7].
Chlorination process is one of the processes that have been used to
inactivate pathogenic microorganism that are found in aqueous en-
vironment but it has a disadvantage of initiating the development of by-
products that are less oxidizable and more poisonous than the parent
material [8]. The studies show that there is a need for process of
breaking down organic compounds that is more preferable than
chlorination; the process is called ozonation [9,10]. Ozonation is in-
creasingly becoming the important last treatment innovation that is
used to improve the removal of color and furthermore the disposal of
persevering toxins that are available in the bio-treated effluents [11].
The advantage of ozonation process over chlorination is that it does not
Due to the high risk of exposure, the laboratory studies of such
compounds are usually undertaken using model compounds, the model
compounds include chlorophenols and chlorobenzenes. The model
compounds are used to predict the destruction of polychlorinated
⁎ Corresponding author.
E-mail address: PullabhotlaV@unizulu.ac.za (V.S.R. Pullabhotla).
https://doi.org/10.1016/j.cattod.2020.06.025
Received 4 March 2020; Received in revised form 27 May 2020; Accepted 5 June 2020
0920-5861/©2020ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:NomthandazoMkhize,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2020.06.025

N. Mkhize, et al.
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require some additional processes in order to remove excess disin-
fectant from water but higher doses of ozone can be used [12].
Ozone is a powerful oxidant that has been increasingly drawing
attention as an alternative oxidant because of its strong capability to
degrade hydrocarbons even at low temperature [13,14]. Ozonation
reaction involves the direct molecular reactions of ozone with the
compounds dissolved and involves the transformation of ozone into
secondary oxidants like hydroperoxyl radicals and hydroxyl radicals
[11]. Ozonation alone lead to partial mineralization, contrast to this
catalytic ozonation can be used due to its higher effectiveness in the
degradation of organics [15,16]. Catalytic ozonation is divided into
homogeneous catalytic ozonation and heterogeneous catalytic ozona-
tion [17]. In homogenous catalytic ozonation, the decomposition of
molecular ozone is catalyzed by the transition metal ions and this
process has disadvantages, for example, it results in reaction products
that are very hard to separate and the process is less active [16].
In heterogeneous catalytic ozonation, ozone decomposition is cat-
alyzed by metals on metal oxides as supports or metal oxides. In recent
years, heterogeneous catalytic process has been receiving an increasing
attention because of its high potential effectiveness in the oxidation as
well as in the mineralization of stubborn organic pollutants [18]. There
are various metal oxide catalysts that have been used in the hetero-
geneous catalytic ozonation and now they are developed because of
their catalytic durability as well as their catalytic performance [19].
Among the catalysts that have been used, alumina has been successfully
used in the ozonation of pollutants. Thus V2O5/Al2O3, V2O5/SiO2,
V2O5/TiO2, CuCO3/Fe2O3 and so on. Vanadium loaded on titania that is
prepared by using sol-gel method showed selectivity and stability that
are very high for oxidation of orthodichlorobenzene but vanadium
oxide was toxic, however it leads to the formation of secondary pollu-
tion [20].
The oxidation reactions where V2O5/Al2O3, V2O5/SiO2, V2O5/TiO2
were used as catalysts showed high selectivity (98 %–100 %) but little
mineralisation [8]. There is no study that has shown the use of man-
ganese loaded on alumina and silica support as catalysts for the oxi-
dation of orthodichlorobenzene. Manganese has a facile redox behavior
which facilitates the diffusing of oxygen and high adsorption capacity
of oxygen [21]. The main reasons for the wide usage of gamma alumina
in many applications as an adsorbent and as a catalyst are its me-
chanical strength, acid-base properties, thermal stability and high sur-
face area [22]. Silica has also gained a considerable interest due to it
high thermal, mechanical and chemical stability, controlled surface
area properties and its ability to maintain the dispersion of metals
during the reaction [23].
2.5 % Mn loaded on γ-Al2O3 (2.5 % Mn/γ-Al2O3) catalyst, 0.9025 g of
manganese chloride tetrahydrate (MnCl2.4H2O) was dissolved in
100 mL of distilled water. The prepared solution was slowly dispensed
into a 250 mL beaker containing 9.098 g of γ-Al2O3 (and beaker con-
taining SiO2) with vigorous stirring for homogeneous dispersion of Mn
onto γ-Al2O3 and SiO2 supports. This was then followed by evaporation
of water by placing the beaker on a hot plate at 70 °C until thick paste is
obtained and thereafter, this step was completed by drying the catalyst
precursor overnight in an oven at 90 °C. The as-synthesized 2.5 % Mn/
γ-Al2O3 and 2.5 % Mn/SiO2 catalysts were calcined at 300 °C for 5 h
[24]. The similar procedure was followed by using adequate amounts of
the MnCl2.4H2O to synthesize the 5.0, 7.5 and 10.0 Wt% Mn/γ-Al2O3
and Mn/SiO2 catalysts.
2.1.2. Oxidation of 1,2-dihlorobenzene
The oxidation of 1,2-dichlobenzene was carried out in an impinger
unit with porous bubbler of porosity 2. Orthodichlorobenzene was
added into the impinger before ozone is fed. Ozone was fed into the
impinger unit via a porous bubbler. Initially blank ozonation of 1,2-
dichlorobenzene was studied, where ozonation reaction was studied in
the absence of the catalyst. Then ozonation reactions was then cata-
lyzed with bare gamma alumina, silica and various percentages of
manganese loaded on gamma alumina and silica supports. In all the
oxidation reactions 25 mL of 1,2-dichlorobenzene and 0.25 g of cata-
lysts were used.
2.2. Characterisation of catalysts
2.2.1. X-ray diffraction
X-ray diffraction is one the analysis that was performed to char-
acterise the catalysts. This technique was conducted to determine the
different phases of Mn (metal) and metal oxide (γ-Al2O3 and SiO2) that
is present in the catalyst and also crystal lattice of the support (γ-Al2O3
and SiO2). The powder diffraction patterns were recorded by using a
Bruker AXS-D8 with
a Cukα as radiation source of wavelength
0.15406 nm. Scan speed was 0.2/min over 10−90° scan range with
operating conditions of 40 kV and 40 mA.
2.2.2. Fourier transform-infrared spectroscopy (FT-IR)
In all calcined catalysts, the functional groups present were identi-
fied using FT-IR spectroscopy. The FT-IR spectroscopy analysis was
carried out using a Bruker Tensor 27 FT-IR spectrometer with a stan-
dard ATR cell. Acetone was used to clean the crystal’s surface prior to
every analysis. The force gauge adjusted to 22 gauge for proper contact
between the surfaces. The mid-IR region for catalyst was kept at the
The current study focuses on ozone-initiated oxidation of orthodi-
chlorobenzene catalyzed by manganese loaded on alumina and silica
supports at ambient reaction conditions.
range of 500−4000 cm−1
.
2.2.3. Scanning Electron Microscope (SEM)
2. Materials and methods
Scanning Electron Microscope was used to study the morphology
and topography of the both support and metal loaded calcined cata-
lysts. Carl Zeiss FE-SEM Sigma VP-03-67 instrument was used with
operating conditions of 20 kV over a working distance of 6−9 mm. All
the catalysts samples were dried and ground to fine powder with mortar
and pestle prior to the analysis. The analysis was then completed by
placing a small amount of the powdered sample on a piece of a two-way
carbon tape and then mounted on a stub (sample holder). The com-
positional analysis of the sample was then carried out on an Oxford
instrument X-MaxN 50 mode 54-XMX1003 EXD analyzer.
2.1. Materials
Aluminium oxide (γ-Al2O3, Aldrich, nanopowder < 50 nm particle
size, St. Louis, MO, Germany), 1,2-dichlorobenzene (Sigma Aldrich,
99.0 %), manganese(II) chloride tetrahydrate (MnCl2·4H2O, 99.0 %,
reagent grade, Aldrich St. Louis, MO, Germany) and silica gel (SiO2,
Merck, 70–230 mesh ASTM). These materials were purchased and used
without any further purification.
2.1.1. Catalyst preparation
2.2.4. Transmission Electron Microscopy (TEM)
Wet impregnation method was used to prepare metal (Mn) doped
metal oxide catalysts (Mn/γ-Al2O3 and Mn/SiO2). Adequate amount(s)
of manganese chloride tetrahydrate that was dissolved in 100 mL of
distilled water which was then calculated based on the percentage of
manganese (2.5, 5.0, 7.5 and 10.0 Wt%) that was supported on gamma
alumina and silica. For example, in this method to prepare 10 g of the
Transmission electron microscopy was used to study the mor-
phology, particle size and size distribution of all calcined catalysts. The
analysis was carried out on a JOEL JEM-2010 electron microscope with
an accelerating voltage of 200 kV. The images were captured with
Gatan camera and analyzed with Gatan imaging software. The samples
were prepared by dispensing fine powder of catalyst in toluene and
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sonication for 20 min. Then the drop of a catalyst sample was placed on
a holy carbon coated copper grid with mesh size of 150 and allowed to
dry at room temperature before TEM images were captured.
2.2.5. Inductively coupled plasma- optical emission spectroscopy (ICP-OES)
The metal content (Mn) of the catalyst was determined using an
Agilent 700 series ICP-OES with a 710 ICP-OES detector instrument. A
series of standard solutions 25, 50, 75, and 100 ppm were prepared
from manganese salt (MnCl2.4H2O) by dilution with distilled water.
The sample preparation was done by digestion of 0.15 g of 2.5, 5.0, 7.5
and 10.0 % Mn loaded catalysts in the mixture of 4 mL HF, 3 mL HCl
and 3 mL HNO3 at 60 °C and then heated until digestion was complete.
The resultant solution was diluted with distilled water and made upto
100 mL solutions of 2.5, 5.0, 7.5 and 10.0 % Mn loaded on γ-Al2O3 and
SiO2 catalysts.
2.2.6. Brunauer-Emmet-Teller (BET)
BET surface analyser was used to determine the surface areas of as-
synthesised catalyst samples. Firstly, the samples were degassed at
250 °C overnight under N2 flow in micrometrics flow prep 060 as a form
of pretreatment. Then the samples were analyzed in an automated and
multiple-point micrometrics Gemini 2360 BET surface area under liquid
nitrogen conditions.
2.3. Characterization of ozonation reaction product
2.3.1. Fourier transform-infrared spectroscopy (FT-IR)
The FT-IR spectroscopy, as discussed in Section 2.2.2 for the func-
tional group identification of catalysts, was also used to identify func-
tional groups present in the reaction product.
2.3.2. Gas chromatography–mass spectrometry (GC–MS)
GC analysis was carried out on an Agilent 7890A GC system coupled
with 5975Vl (Triple axis) Mass Selective Detector (MSD). The column
type fitted was an Agilent Hp-5MS, 5 % phenyl methyl siloxane with
column dimensions of 30 m x 250μm × 0.25μm. The carrier gas that
was used at a flow rate of 0.7 mL/min is helium. The detector tem-
perature and the injection source temperature were 230 and 250 °C
respectively. The initial oven temperature was held constant for 2 min
then ramped-up to the final temperature of 300 °C at a rate of 6 °C/min.
A 6 μL sample was injected with the split ratio 100:1.
Fig. 1. XRD patterns of (a) γ-Al2O3 support, 2.5 % Mn/γ-Al2O3, 5 % Mn/γ-
Al2O3; 7.5 % Mn/γ-Al2O3 and 10 % Mn/γ-Al2O3 catalysts and (b) SiO2 support,
2.5 % Mn/SiO2, 5 % Mn/SiO2, 7.5 % Mn/ SiO2 and 10 % Mn/SiO2 catalysts.
does show a well-defined peak showing amorphous nature of silica. The
literature reveals that x-ray diffractograms of metals or metal oxide
catalyst containing silica mostly shows only one strong broad peak
which is the silica peak thus it the characteristic amorphous nature of
silica [28]. Silica support and various percentages of manganese loaded
on silica catalysts shows only intense peak at 2-theta = 22.6ᵒ (100)
which is the peak that is assigned to silica.
3. Results and discussion
3.1. Catalyst characterization results
3.1.1. X-Ray diffraction (XRD)
Fig. 1a shows X-ray diffractograms of γ-Al2O3 support and various
loading of % Mn/γ-Al2O3 catalysts. The XRD peaks that are observed at
2-theta = 33.5ᵒ, 37ᵒ, 39.8ᵒ, 46.9ᵒ, 62.2ᵒ and 67ᵒ are assigned to γ-Al2O3
belonging to these hkl values (220), (311), (222), (400), (500) and
(440), respectively, with cubic crystal structure (JCPDS 00-010-0425)
[25]. There were no diffraction peaks that distinguished the metal or
their metal oxide that were detectable for 2.5–7.5 % wt loading of
manganese. However, at 10 % Mn/ γ-Al2O3 there are two peaks that are
observed at 2-theta = 17.5ᵒ (101) and 23.2ᵒ (111) which can be as-
signed to tetragonal hausmsnnite Mn3O4 (JCPDS No. 24-0734) and pure
orthorhombic phase of cubic Mn2O3 (JCPDS No. 65-7467) respectively
[26]. The X-ray diffractogram shows a decrease in the stability of
crystalline of γ-Al2O3 as manganese is incorporated into γ-Al2O3. This is
observed by broadening of the peaks and decrease in the intensity of the
peaks as shown in Fig. 1a.
3.1.2. Fourier Transform-infrared Spectroscopy (FT-IR)
Fig. 2a shows the FT-IR spectra of various % Mn/γ-Al2O3 catalysts
and γ-Al2O3 support. At around 3450 cm−1 there is broad adsorption
band which can be ascribed to the vibrational stretching frequency of
hydroxyl groups that are present on Lewis acid sites of gamma alumina.
These hydroxyl groups result from the dissociative chemisorption of
water to Lewis acids sites and also modification of surface of aluminium
coordination by protonation of Lewis basic sites [29]. As the percentage
of the manganese loaded on gamma alumina increases, these bands are
significantly observed. At 1095 cm-1 there is a shoulder that is re-
presenting Al-O bond. The inter-vibrational mode frequency was ob-
served in the FT-IR spectrum of gamma alumina support and the ab-
sorption shoulder within the β range between 950−500 cm-1, it is
assigned to the octahedral AlO6 and tetrahedral AlO4 that are known to
reflect at infrared spectrum of gamma alumina [30].
Fig. 1b shows the X-ray diffractograms of SiO2 support and various
% Mn/SiO2 catalysts. Calcination at 300 ᵒC causes the diffraction peaks
to be sharper, as a result the heat treatment at this temperature made
gradual crystallization of the material [27]. The X-ray diffractogram
Fig. 2b displays the FT-IR analysis results of SiO2 and Mn loaded
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and 5 % Mn/SiO2 catalysts, these results also suggest that the manga-
nese metal is evenly distributed on the surface of γ-Al2O3 and SiO2
supports.
3.1.4. Transmission Electron Microscopy (TEM)
Fig. 5(a–d) display the TEM images of γ-Al2O3 support and various
% of Mn loaded γ-Al2O3 catalysts. γ-Al2O3 shows spherical shape but
some of the particles are rod shaped (γ-Al2O3 is more spherical shaped
than rod shaped) with a particle size ranging from 11 to 15 nm. How-
ever, after doping manganese on the γ-Al2O3 support rod shaped par-
ticles are obtained with varying particle size. The measured average
particle size for 2.5 % Mn/γ-Al2O3 has a range of 37–63 nm while that
of 5 %, 7.5 % and 10 % Mn/γ-Al2O3 (Fig. S5) differed only slightly with
a particle size range of 30–63 nm, 13–16 nm, and 13–60 nm respec-
tively.
Fig. 6(a–d) shows the TEM images of SiO2 support and various % of
Mn loaded SiO2 catalysts. SiO2 has a narrow size distribution (particle
size ranging from 19 to 35 nm) with a spherical shape. However, after
doping manganese on the SiO2 support the spherical shape of silica is
maintained but when 7.5 % and 10 % Mn/SiO2 the images show that
some of the particles present are rod shaped. The measured average
particle size for 2.5 % Mn/SiO2 has a narrow range of 21–38 nm while
that of 5 %, 7.5 % and 10 % Mn/SiO2 (Fig. S6) differed only slightly
with a particle size range of 8–16 nm, 18 −64 nm and 15–75 nm re-
spectively.
3.1.5. Inductively coupled plasma- optical emission spectroscopy (ICP-OES)
Table 1 below show the percentage weight of manganese on each of
the Mn/γ-Al2O3 which were analyzed using ICP-OES and EDX (Fig. S1
(a–d)). The experimental Mn wt% are close to anticipated theoretical
percentages from the results obtained from both ICP and EDX, which
suggests that manganese was incorporated successfully onto the surface
of gamma alumina.
Table 2 below shows the percentage weight of manganese on each
of the Mn/SiO2 which were analyzed using ICP-OES and EDX. The re-
sults from ICP show that actual Mn wt% for the Mn/SiO2 catalysts is
much less than the theoretically calculated wt% for all the catalysts
while EDX (Fig. S2 (a–d)) results shows closer values of Mn actual wt%
to the theoretically calculated values for 2,5, 5.0 and 7.5 % Mn/SiO2
catalysts.
ICP-OES data can be affected by spectral interferences which is
characterized by an overlap of the analyte (metal loaded metal oxide
catalyst) by an interfering element. The background signal for de-
termination of analyte signal can also be interfered. These interferences
can result in enhancement/suppression of signals and this can lead to
negative or positive results which ultimately lower the accuracy and
precision of the method. The reduction of manganese content in ICP-
OES data maybe due to uneven dispersion of manganese over the metal
oxide supports (γ-Al2O3 and SiO2) and also by significant aggregation of
manganese, which can be supported by the SEM-EDX data.
Fig. 2. FT-IR spectra of (a) γ-Al2O3 support, 2.5 % Mn/γ-Al2O3, 5 % Mn/γ-
Al2O3, 7.5 % Mn/γ-Al2O3 and 10 % Mn/γ-Al2O3 catalysts and (b) SiO2 support,
2.5 % Mn/ SiO2, 5 % Mn/ SiO2, 7.5 % Mn/ SiO2 and 10 % Mn/ SiO2 catalysts.
SiO2 catalysts. The spectrum of silica was observed to entail the ab-
sorption peaks at 1100 cm−1 and 790 cm−1, which corresponds to the
stretching of Si-O-Si and bending vibrations of Si-O respectively. The
Mn loaded silica catalysts at 2.5 % loading, attained the similar infrared
spectra to that of pure silica support. The vibrational stretching fre-
quency of hydrogen atom in hydroxide catalyst appeared at 3445 cm-1
for 5 %–10 % nickel loaded on silica [31,32].
3.1.3. Scanning Electron Microscope (SEM)
Fig. 3 below shows the SEM images of γ-Al2O3 support and various
percentages of Mn loaded γ-Al2O3 catalysts. γ-Al2O3 support shows the
porous nature which appear as fluffy particles with poorly defined
structure. The morphology of γ-Al2O3 show a rough surface with high
surface kinks and defects which belong to metal attached to the sup-
port. The surface of gamma alumina becomes fine and aggregated as
the Mn content increases. The existence of manganese on γ-Al2O3 was
further confirmed by EDX images shown in Fig. S1(a–d).
Fig. 4 shows SEM images of SiO2 support and various percentages of
Mn loaded on SiO2 catalysts. Silica support shows the existence of small
black spot that correspond to the pore spaces like cylindrical holes.
Silica support is a very fine gel compound aggregate that is surrounded
by great number of pores that give the impression of a sponge. Its wide
porous texture is said to have well adsorption capacity. At % Mn/SiO2
catalysts there are small traces of manganese (large white spots) that
are distributed on the surface of the catalyst. The existence of manga-
nese on SiO2 was further confirmed by EDX images shown in Fig.
S2(a–d). Figs. S3 and S4 shows the SEM EDX images of 5 % Mn/γ-Al2O3
3.1.6. Brunauer-Emmet-Teller (BET)
Table 3 displays the BET results of pure γ-Al2O3 support, 2.5 % Mn/
γ-Al2O3 and 7.5 % Mn/γ-Al2O3 catalysts. The BET surface area of γ-
Al2O3 was found to be 295.0 m2/g. However, the surface area tends to
decrease with metal loading, the BET surface areas of 2.5 % Mn/γ-
Al2O3 and 7.5 % Mn/γ-Al2O3 were found to be 167.6 m2/g and
107.8 m2/g respectively confirming the gradual filling of the pore vo-
lume of the γ-Al2O3 support. Furthermore, the pore volumes of 2.5 %
Mn/γ-Al2O3 and 7.5 % Mn/γ-Al2O3 catalysts were found to be 0.3826
and 0.3022 cm3/g respectively attributing the incorporation of the Mn
into the pore volume of the γ-Al2O3 support. The BET results showed
the Mn loading into γ-Al2O3 support results in decrease of both pore
volume and surface area. However, the increase in pore size as surface
area decreases was observed. This is caused by pores that are filled and
also blocked by manganese particles, which increased the surface of Mn
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Fig. 3. SEM images of (a) γ-Al2O3 support, (b) 2.5 % Mn/γ-Al2O3, (c) 5 % Mn/γ-Al2O3, (d) 7.5 % Mn/γ-Al2O3; and (e) 10 % Mn/γ-Al2O3 catalysts.
loaded on γ-Al2O3 catalysts.
assigned to C]O functional group. The C–H stretching is observed at
878 cm−1 and 1250 cm−1. The OH vibrational stretching appeared at
3498 cm−1. Functional groups that are observed in the IR spectrum
tend to be in good agreement with spectrum corresponding to muco-
chloric acid.
Table 4 shows the BET results for pure SiO2 support, 2.5 % Mn/SiO2
and 7.5 % Mn/SiO2 catalysts. The BET surface area of pure SiO2 support
was found to be 506.0 m2/g. However, the surface area of SiO2 an-
ticipated to decrease with Mn loadings and it was observed that when
2.5 wt% Mn was loaded on SiO2, the surface area was found to be
339.6 m2/g which further decreased to 279.9 m2/g upon 7.5 wt% of Mn
loading. We can observe from the Table 4 that BET surface area and
pore volume were decreased with the increase in the Mn metal loading
from 2.5 to 7.5 wt%. The decrease in surface area and pore volume can
be attributed to the tendency of metal particles to form aggregates
leaving the measurable internal volume as pores.
3.2.2. Gas chromatography– mass spectroscopy
The identification of the reaction products mixture was done by gas
chromatography which was coupled with mass spectrometry detector in
order to facilitate the identification of compounds that are in the re-
action mixture by elucidation of the molar mass. Fig. 8(a) show the
chromatogram of 1,2-dichlorobenzene before ozonation, while Fig. 8(b)
show the chromatogram of 1,2-dichlorobenzene after 24 h of ozonation.
The peak at retention time 17.8 min refers to mucochloric acid which
was identified as the main product of ozonation of 1,2-dichlorobenzene.
The peak at retention time 8.4 min and 8.2 min are attributed to 1,2-
dichlorobenzene (un-reacted) and 3,4-dichloro-2,5-furadione respec-
tively. Scheme 1 represents the oxidation products formed from the
ozonation of 1,2-dichlorobenzene.
The surface area analysis was done to relate the surface areas of the
catalysts to the percentage conversion and selectivities in the ozonation
of 1,2-dichlorobenzene. The catalysts with the higher surface area
showed an effect in its activity in the ozone initiated oxidation of 1,2-
dichlorobenzene, showing the higher conversions and selectivities to-
wards the main products.
3.2. Characterisation of ozonation reaction product
Fig. S7 shows the mass spectra of (a) 1,2-dichlorobenzene (b) 3,4-
dichloro-2,5-furandione and (c) mucochloric acid. The base peak at m/
z = 168 represent the M+% of mucochloric acid, while the base peak at
m/z = 133 represent the fragments due to the cleavage of one Cl− ion.
The series of ions are detected at m/z = 87, m/z = 29 and m/z = 60 is
3.2.1. Fourier transform-infrared spectroscopy (FT-IR)
Fig. 7 display the FT-IR spectrum for the main product (mucochloric
acid), the spectrum shows adsorption band at 1598 cm−1 which is
5

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Fig. 4. SEM images of (a) SiO2 support, (b) 2.5 % Mn/SiO2, (c) 5 % Mn/SiO2, (d) 7.5 % Mn/SiO2 and (e) 10 % Mn/SiO2 catalysts.
due radicals O = C-C(Cl)-C, C−OH and CH-C(Cl) respectively. In the
mass spectrum of 3,4-dichloro-2,5-furandione, the molecular ion of m/
z = 166 represents of M+% of 3,4-dichloro-2,5-furandione. The series of
base peaks at m/z = 122, m/z = 87, and m/z = 59 which represent M+
fragments of 3,4-dichloro-2,5-furandione are due to radicals C(Cl)
=C(Cl)C = O, O = C-C(Cl)-C and C-O respectively.
The presence of unreacted 1,2-dichlorobenzene was confirmed by
comparing Fig. 8(a) and (b) which shows the retention time of 1,2-
dichlorobenzene to be 8.4 min. The calculation of conversion and per-
centage selectivity was done by using the gas chromatogram. The
percentage conversion was calculated on peak area of substrate out of
total peak area in the chromatogram, while the selectivity was ex-
pressed as amount of individual product formed out of total amount of
substrate converted as percentage. The undefined products were
formed due to further oxidation of initial product into simpler com-
pounds that are said to be less toxic and more biodegradable.
Mn/SiO2 catalysts in terms of percentage conversion in the ozonation
reactions of 1,2-dichlorobenzene. Reaction samples were drawn after 3,
6, 9, 12, 15, 18 and 24 h of ozonation time at ambient reaction con-
ditions. Blank ozonation, γ-Al2O3 and SiO2 catalyzed ozonation reaction
were initially done. The comparative results on the activity based on
percentage conversion of uncatalyzed, activated charcoal, pure γ-Al2O3,
pure SiO2, various Mn loaded γ-Al2O3, and various Mn loaded SiO2
catalysts are shown in Fig. 9. It can be seen from Tables S1–S4, the
ozonation reactions that were performed in the absence of catalyst and
in the presence of supports only (pure γ-Al2O3 and SiO2) show low
percentage conversions after 24 h ozonation reaction (25 %, 30 % and
31 % respectively).
Literature shows that activated charcoal (AC) can be a promising
alternative to the treatment of wastewaters which contains organic
contaminants. It is believed that AC promotes aqueous ozone decom-
position leading to the formation of active species, which are oxyge-
nated. The oxygenated species are responsible for enhancing organic
compounds oxidation/mineralization, making activated charcoal a
promising candidate in the ozonation reactions [33]. In the activated
charcoal catalyzed ozonation reaction, the selectivity towards muco-
chloric acid found as decreasing with increased ozonation time from 9
3.3. Oxidation of 1,2-dichlorobenzene
3.3.1. Percentage conversion
Tables S5–S12 display the catalytic performance of Mn/γ-Al2O3 and
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Fig. 5. TEM images of (a) γ-Al2O3 support, (b) 2.5 % Mn/γ-Al2O3, (c) 5 % Mn/γ-Al2O3, and (d) 7.5 % Mn/γ-Al2O3 catalysts.
to 24 h (Table S2). While in 5 % Mn/SiO2 catalyzed ozonation reaction,
the selectivity towards mucochloric acid was observed as increasing
with increased ozonation time from 3 to 24 h (Table S10). The ozona-
tion reaction catalyzed by 5 % Mn/SiO2 showed high percentage con-
version (44 %) towards the oxidation of 1,2-dichlorobenzene after 24 h
than ozonation reactions performed with the uncatalyzed, activated
charcoal, pure γ-Al2O3, pure SiO2, 2.5 %, 7.5 %, 10 %Mn/γ-Al2O3, 2.5
%, 7.5 %, 10 % Mn/SiO2 catalyzed reactions, followed by 5 % Mn/γ-
Al2O3 catalyzed reactions (Tables S1–S12).
From the Tables S1–S4, it can be observed that in the ozone initiated
oxidation of 1,2-dichlorobenzene results in the formation of the 3,4-
dichloro-2,5-furandione and mucochloric acid as products. In the un-
catalysed, activated charcoal, γ-Al2O3 and SiO2 catalysed reactions
there is no correlation or control over the product formation in terms of
conversion and selectivities (Tables S1–S4). Mn loaded γ-Al2O3 cata-
lysts resulted in the low concentration of mucochloric acid at the initial
stages and the concentration of the 3,4-dichloro-2,5-furandione was
higher (Tables S5–S8). Upon the increased reaction time the con-
centration of 3,4-dichloro-2,5-furandione was decreased and con-
centration of mucochloric acid was increased. In these reactions the
unidentified products only formed after 12 h of ozonation, this also
suggests that the 1,2-dichlorobenzene results in the initial formation of
3,4-dichloro-2,5-furandione and this further gets oxidized to produce
mucochloric acid as the main product.
unidentified products in addition to the 3,4-dichloro-2,5-furandione
and mucochloric acid (Tables S9–S12). The results from these tables
clearly indicating that in the ozonation of 1,2-dichlorobenzene, ozone
interacts with substrate and forms 3,4-dichloro-2,5-furandione as the
product. 3,4-Dichloro-2,5-furandione product upon further reaction
with ozone produces mucochloric acid.
3.3.2. Percentage selectivity
Initially 5 % Mn/SiO2 calcined catalyst resulted in the highest se-
lectivity towards mucochloric acid (62 %) and during 12 h of ozonation
5 % Mn/SiO2 and 5 % Mn/γ-Al2O3 catalysts resulted in the highest
selectivity of 72 and 73 % respectively as shown in Fig. 10(a). During
the 18 h of ozonation γ-Al2O3 support catalyst showed the lowest se-
lectivity (50 %) towards mucochloric acid. After 24 h of ozonation 5 %
Mn/SiO2 and 5 % Mn/γ-Al2O3 calcined catalysts showed a better se-
lectivity towards mucochloric acid (88 % and 86 % respectively). The
results show highest selectivity with 5 % Mn/SiO2, possibly due to more
surface area available for the selective oxidation of mucochloric acid.
Fig. 10(b) showed that as the ozonation time increases the selectivity
towards 3,4-dichloro-2,5-furandione decreases in the oxidation of 1,2-
dichlorobenzene (Tables S5–S12), suggesting the further oxidation of
3,4-dichloro-2,5-furandione to mucochloric acid.
4. Conclusions
Mn loaded SiO2 resulted in only the 3,4-dichloro-2,5-furandione and
mucochloric acid as products. Whereas, Mn loaded γ-Al2O3 yielded
The selective oxidation of 1,2-dichlobenzene has been challenging
7

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Fig. 6. TEM images for a) SiO2 b) 2.5 % Mn/SiO2 and c) 5 % Mn/ SiO2 and (d) 7.5 % Mn/ SiO2 catalysts.
Table 1
Table 4
Manganese wt% on each Mn/γ-Al2O3 catalyst analyzed using ICP-OES and EDX.
BET surface areas for the various Mn loaded SiO2 catalysts.
Theoretical Mn wt%
ICP Mn wt%
EDX Mn wt%
Catalyst sample
BET Surface Area
(m2/g)
Pore Volume
(cm3/g)
Pore Size
(nm)
2.5 % Mn/γ-Al2O3
5.0 % Mn/γ-Al2O3
7.5 % Mn/γ-Al2O3
10 % Mn/γ-Al2O3
2.50
5.00
7.50
10.0
1.95
6.46
2.48
4.26
1.47
4.08
5.60
11.93
Pure SiO2
2.5 % Mn/SiO2
7.5 %Mn/SiO2
506.0
339.6
279.9
–
–
7.28
8.24
0.009591
0.007381
Table 2
Manganese wt% on each Mn/SiO2 catalyst analyzed using ICP-OES and EDX.
Catalyst
Theoretical Mn wt%
ICP Mn wt%
EDX Mn wt%
2.5 % Mn/SiO2
5.0 % Mn/SiO2
7.5 % Mn/SiO2
10.0 % Mn/SiO2
2.50
5.00
7.50
10.0
0.33
1.75
2.63
3.44
2.12
5.10
8.17
19.83
Table 3
BET surface areas for the various Mn loaded γ-Al2O3 catalysts.
Catalyst sample
BET Surface Area
(m2/g)
Pore Volume
(cm3/g)
Pore Size
(nm)
Pure γ-Al2O3
2.5 % Mn/γ-Al2O3
7.5 % Mn/γ-Al2O3
295.0
167.6
107.8
–
–
0.3826
0.3022
9.13
11.21
Fig. 7. FT-IR spectrum of reaction product mucochloric acid.
8

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Fig. 8. Gas chromatograms of 1,2-dichlorobenzene a) before ozonation and b) after 24 h of ozonation using 5 % Mn/SiO2 catalyst.
as to identify the most suitable catalytic system for efficient oxidation of
chlorinated aromatics into biodegradable and less toxic organics at
ambient reaction conditions. The ozone initiated catalytic oxidation of
1,2-dichlorobenzene at ambient temperature and pressure conditions
was investigated. The main product mucochloric acid and the minor
product 3,4-dichloro-2,5-furandione were positively identified in the
24 h ozonation reaction. All the catalysts including activated charcoal
resulted in the similar ozonation products as it is observed in un-
catalyzed ozonation reaction. Bare γ-Al2O3 and SiO2 nano catalysts
were found to be active in the catalytic oxidation of 1,2-di-
chlorobenzene with moderate percentage selectivity toward formation
of mucochloric acid. The manganese doped γ-Al2O3 catalysts and also
manganese doped SiO2 catalysts exhibited high activity in the conver-
sion of 1,2-dichlorobenzene with high selectivity towards mucochloric
acid compared to that of bare γ-Al2O3 and SiO2 powder. This shows that
manganese acts as active site on the surface and within the pores of
gamma alumina and silica supports. The 5 % Mn/SiO2 catalyst was
found to be the most active catalyst towards the main product with
respect to % conversion (44 %) and also it was found to be the most
Scheme 1. Oxidation products formed from the ozonation of 1,2-di-
chlorobenzene.
due to the non-biodegradable nature and high resistance of the
chlorinated aromatic compounds towards chemical oxidation. The
studies have been conducted with a wide range of catalytic systems so
9

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dichlorobenzene. The catalysts with the higher surface area displayed
their catalytic activity by showing the higher conversions and se-
lectivities towards the main products. This study also revealed that the
activity of the catalysts in the catalytic ozonation of 1,2-di-
chlorobenzene is affected (lead to poor activity) by the increase in
manganese loading greater than 5 %.
CRediT authorship contribution statement
Nomthandazo Mkhize: Formal analysis, Investigation, Writing -
original draft. Prabal Pratap Singh: Formal analysis. Deepak Kumar
Das: Formal analysis. Viswanadha Srirama Rajasekhar Pullabhotla:
Conceptualization, Supervision, Methodology, Formal analysis, Writing
- original draft, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare no conflict of interest.
Fig. 9. Percentage conversions with uncatalyzed, activated charcoal, pure γ-
Al2O3, various Mn loaded γ-Al2O3 catalysts, pure SiO2 and various Mn loaded
SiO2 catalysts.
Acknowledgments
The authors acknowledge the EMU at the University of KwaZulu-
Natal, Westville campus, for providing us access to their TEM facility.
Rajasekhar Pullabhotla would like to acknowledge the Research and
Innovation Office, UZ, for the financial support in the form of Project S
451/12 and the National Research Foundation (NRF, South Africa) for
the financial support in the form of the Incentive Fund Grant (Grant No:
103691) and Research Developmental Grant for Rated Researchers
(112145).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.cattod.2020.06.025.
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