CHEMICAL ENGINEERING TRANSACTIONS
VOL. 57, 2017
A publication of
The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet
Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš, Laura Piazza, Serafim Bakalis
Copyright © 2017, AIDIC Servizi S.r.l.
ISBN 978-88-95608- 48-8; ISSN 2283-9216
A Comparative Study of Toluene Oxidation on Different Metal
Oxides
Marina Duplančić*, Vesna Tomašić, Stanislav Kurajica, Iva Minga, Karolina
Maduna Valkaj
University of Zagreb, Faculty of Chemical Engineering and Technology, Marulićev trg 19, Zagreb, Croatia
marina.duplancic@fkit.hr
This work reports the results of experimental and theoretical investigation of toluene oxidation on different
metal oxide based catalysts (manganese oxide, MnOx, mixed manganese-iron oxide, MnFe, perovskite-type
manganese oxide, LaMnO3 and commercial Pt-Al2O3 catalyst). Particular attention was devoted to single and
mixed manganese based oxides and ceria based materials as alternatives to conventionally used noble metal
containing catalysts.
Toluene oxidation was performed under steady-state conditions in an integral fixed bed reactor operating over
a wider range of reaction temperatures and at various space times. The influence of reaction variables on the
rate of toluene oxidation was examined using the simple first-order kinetic model and the one-dimensional
(1D) pseudo-homogeneous model to describe the reaction system. The proposed model was verified
comparing the theoretical predictions with the experimental laboratory results.
The results of catalytic tests indicated that the mixed manganese-iron oxide (MnFe) exhibited remarkable
catalytic activity for the toluene oxidation, almost comparable with the activity of the commercial Pt-Al2O3. The
reaction temperature T50 corresponding to 50% of the toluene conversion was observed at 419 K for the MnFe
oxide and at 405 K for the Pt-Al2O3. A very good agreement of experimental data with the proposed 1D model
was obtained. Based on the shape of the light-off curve and the values of the apparent activation energies,
which decreased from 120.36 kJ/mol to 16.88 kJ/mol with reaction temperature increase, it was concluded
that the reaction rate was probably limited by the mass transfer, no matter the relatively small catalyst particle
size fraction employed in this study (315 - 400μm).
1. Introduction
Every year, large amounts of volatile organic compounds (VOCs) are emitted into the air, from both natural
and anthropogenic sources and harms human health and our environment. Therefore, control of VOCs
emission into the environment become one of the priorities of environmental catalysis. Catalytic oxidation is
the preferred technology for abatement of relatively low VOCs concentrations (~10 ppm v).
According to the literature, effective heterogeneous catalysts for the VOCs oxidation are either supported
noble metal or transition metal oxides. (Okal and Zawadski, 2009; Morales et al., 2006). Generally, noble
metals are more active, but more expensive than transition metals. Among the transition metal oxides,
manganese oxides (MnOx) are reported to be very efficient in catalytic oxidation, since they contain various
types of labile oxygen, which are necessary to complete the catalytic redox cycle (Morales et al., 2006).
Several metal oxides, especially ceria-based materials have been investigated and it has been observed that
may exhibit interesting activity for the VOCs and CO oxidation (Piumetti et al., 2016; Wang et al., 2016). High
activity of ceria (CeO2) mostly originates from the remarkable ability of ceria to store and release oxygen
depending on the formation of oxygen vacancies (Pérez et al., 2014). The multi-component catalysts are quite
often used in heterogeneous catalysis, since active phases (and promoters) may interact with each other. This
may have a beneficial effect on the structural and electronic properties of the catalyst, thus improving their
oxidation activity and thermal durability (Vedrine, 2014). Several authors have used perovskite-structured
mixed metal oxides with general formula ABO3 as catalysts for VOC oxidation (Zhang et al., 2014; Stege et
al., 2011).
DOI: 10.3303/CET1757149
Please cite this article as: Duplancic M., Tomasic V., Kurajica S., Minga I., Maduna Valkaj K., 2017, A comparative study of toluene oxidation
on different metal oxides, Chemical Engineering Transactions, 57, 889-894 DOI: 10.3303/CET1757149
889
In this work different metal oxides (MnOx, MnFe, perovskite LaMnO3 and nanocrystalline CeO2) were
synthesized and their catalytic activities were tested for the toluene oxidation. The most efficient catalyst for
toluene oxidation was chosen based on the exhibited activities of the prepared catalysts in comparison to the
commercial Pt-Al2O3 catalyst, taking into account the T50 (temperature corresponding to 50 % of toluene
conversion) and T90 (temperature corresponding to 90 % of toluene conversion) values. Finally, the one-
dimensional (1D) pseudo-homogeneous model was proposed to describe catalytic oxidation of toluene over
the most active catalyst prepared in this study.
2. Experimental
2.1 Catalyst synthesis and characterization
The powder MnOx and mixed MnFe oxide based catalysts were synthesized by the (co)precipitation method,
starting with aqueous solutions of the transitional metal nitrates (e.g. Mn(NO3)2·4H2O, Fe(NO3)3·9H2O) using
modified procedure described previously in the literature (Morales et al., 2006). Nanocrystalline ceria, CeO2
was prepared by means of hydrothermal process using Ce(SO4)2×3H2O and NaOH as precursors, the details
of the procedure have been described previously (Kurajica et al., 2016). Perovskite-type of catalyst, LaMnO3
was prepared by the citrate method using stoichiometric amounts of an aqueous solution of the metal nitrates
(e.g. La(NO3)3·6H2O and Mn(NO3)2·4H2O) in the presence of citric acid (C6H8O7·H2O) (Zhang et al, 2014.). All
catalysts were calcined at 773 K before catalytic activity evaluation in toluene oxidation.
The prepared catalysts were characterized by X-ray powder diffraction (XRD) and Fourier transform infrared
(FTIR) spectroscopy. The powder X–ray diffraction (XRD) was accomplished using Shimadzu diffractometer
XRD 6000 with CuKα radiation. Data were collected between 10 and 70 °2θ, in a step scan mode with steps
of 0.02° and counting time of 0.6 s. The morphology of the catalytic samples was analyzed on Vega 3 Tescan
scanning electron microscope. FTIR spectroscopy experiments were performed on a PerkinElmer Spectrum
One FTIR spectrometer. Sample scans were collected with a spectral range from 400 to 4000 1/cm with step
of 4 1/cm.
2.2 Catalytic activity tests
Catalytic oxidation of a model VOC pollutant, toluene, over 0.05 g of each catalyst described in Section 2.1
was carried out in a conventional fixed bed reactor in the temperature range of 373 K to ca. 700 K and at
various space times. The gas mixture consisted of toluene in nitrogen (242 ppm of toluene in nitrogen, DOL
Group, Monza, Italy) and air as an oxidant (Messer). Space times were changed varying the total flow rate of
the gas mixture (20-140 cm3/min) over a constant amount of catalyst. The catalyst was placed in the middle of
the reactor between two quartz wool plugs. The reaction temperature was regulated by a thermo-controller
(TC208 Series) connected to the K-type thermocouple placed within a concentric thermowell inside the reactor
and the heaters around the reactor. Reaction was carried out at constant inlet concentration of toluene and at
constant ratio of toluene and oxidant (air). The gas mixture was controlled using the mass flow controllers
(Brooks). The reactor effluent was analysed before and after reaction using an on-line gas chromatograph
(Shimadzu GC-2014) equipped with a flame ionization detector (FID) and a Carbowax 20M column.
2.3 One dimensional mathematical model
In the second part of this study a one dimensional (1D) pseudohomogeneous model was applied to represent
the steady-state operation of the fixed-bed reactor used for the low temperature toluene oxidation over MnFe
catalyst. Under conditions of the high flow velocity of the reaction mixture and small catalyst particles size (315
- 400 μm) the following assumptions are taken into account in the model development: the plug flow and
steady-state conditions, negligible resistance to the interphase mass transfer and chemical oxidation of
toluene that can be approximated by the first order reaction (Everaert and Baeyens, 2004). Isothermal
conditions were adopted as additional hypothesis, because of the high dilution of toluene in the inlet stream
and a small amount of heat generated by the reaction.
Based on these assumptions the reactor and the first order kinetic model were represented by the following
equations:
- sA A A b
dc
u f c r
dz
s ss b A Ar f c k c (1)
where the rate of reaction per unit mass of catalyst, sAr , is associated with the bulk density of the catalyst, b ,
in order to ensure the consistency of the dimensions.
After introducing dimensionless variables, given by Eq. (2):
890
0
* *
max
/
/
A A A
y c c
(2)
the model equations were transformed into dimensionless form:
*
max-
A
b A
dy
k y
d
s
s b A
r k y (3)
and the appropriate boundary conditions at the reactor inlet were given by the following equations:
0, 0 1,Ay 1
s
A
y (4)
Parameter estimation (the rate constant, k) was performed using a modified differential method of data
analysis and the Nelder-Mead method of non-linear optimization. The mean square root of differences
between the experimentally measured concentrations of toluene and theoretical values provided by the model
was used as the correlation criteria (SD).
3. Results and discussion
3.1 Physico-chemical properties of the prepared catalyst
Figure.1. shows XRD patterns of MnOx, MnFe, CeO2 and LaMnO3 catalysts. Diffraction pattern of MnOx
catalyst reveals the presence of two manganese oxides, α-Mn2O3 (ICDD PDF No. 73-1826) and α-MnO2
(ICDD PDF No. 44-0141). It is obvious that α -Mn2O3 is the major phase while α -MnO2 could be classified as
a minor phase. Narrow α -Mn2O3 diffraction peaks point out to large crystallite size while broad and faint α-
MnO2 are consequence of poorly crystalized phase having small crystallites.
Figure 1: XRD patterns of the prepared catalysts thermally treated at 773 K
Diffraction pattern of MnFe catalyst is the most complex one displaying at least three and possibly four
phases. There is no doubt that α-MnO2 and Fe2O3 (ICDD PDF No. 72-0469) phases are present, but
additional peaks observed in diffraction pattern could be attributed to both, α-Mn2O3 or FeMnO3 (ICDD PDF
No. 75-0894). Both of those phases have diffraction peaks at similar angles having similar intensities, which is
a consequence of the same structure and close Mn3+ and Fe3+ radii. Additionally, peaks that can be attributed
to those phases are weak and occasionally overlapped with the peaks of the main phases. Also, the possibility
of the solid solution formation also can’t be ruled out. Therefore, based on displayed pattern it could be stated
that at least one of them and possibly both, or their solid solution are present in the sample. Diffraction pattern
of the CeO2 catalyst is the simplest one since this sample is pure ceria (ICDD PDF No. 34-0394). The peaks
are extremely broad pointing out to very small crystallite size. Finally, LaMnO3 catalyst is characterized with
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very weak and poorly resolved diffraction peaks. Based on this pattern it could be speculated that the sample
is composed of at least three poorly crystallized phases, α-La2O3 (ICDD PDF No. 74-1144), MnO (ICDD PDF
No. 75-1090) and LaMnO3 (ICDD PDF No. 72-0840).
Fourier Transform Infrared Spectroscopy (FTIR) analysis was performed to compare the chemical composition
of the fresh (unused) and used MnFe catalyst. The spectrum (Figure 2.) confirmed that there was no
significant adsorption of the reactants or possible reaction intermediates (e.g. benzaldehyde or benzoic acid)
on the surface of the catalyst during the long-term toluene oxidation over MnFe catalyst. The peak at 1368
1/cm (M-O rocking in plane) implies the presence of metal-oxide group in the catalyst and the peaks at 1736
1/cm (C=O or M-H stretching) and at 1228 1/cm (C-O stretching) can be attributed to some nitrogen or carbon
containing molecules probably adsorbed during the catalyst preparation.
Figure 2: FTIR spectra: a) fresh (unused) MnFe catalyst, b) MnFe catalyst after long-term usage.
3.2 Catalytic performance for toluene oxidation
Toluene, used in this study as a model VOC pollutant, is quite often studied owing to its chemical stability and
highly toxic potential (da Silva et al., 2016).
Figure 3: Light-off curve of toluene oxidation as a function of reaction temperature over different catalysts.
Figure 3. summarizes the toluene conversion values over different catalysts as a function of the reaction
temperature. The characteristic sigmoidal light-off curves are obtained, which usually appears in the catalytic
systems related to the oxidation of CO, hydrocarbons and similar reaction systems. Among the set of the
prepared catalysts, the mixed manganese-iron oxide, MnFe outperforms the other oxides, exhibiting catalytic
performance comparable to the performance of the well-known commercial Pt-Al2O3 catalyst. Comparison of
the results observed for pure MnOx and mixed MnFe oxide indicates a beneficial effect produced by the
incorporation of iron into manganese oxide to activate toluene at lower temperatures during the catalytic
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oxidation. To compare catalytic activities of different catalysts, the T50 (temperature corresponding to 50 % of
toluene conversion) and T90 (temperature corresponding to 50 % of toluene conversion) of toluene over the
catalysts were estimated from the Figure 3. and displayed in Table1. In terms of T50 the following increasing
order of activity can be drawn: CeO2 < LaMnO3<MnOx<MnFe<Pt-Al2O3. According to the literature, cerium
improves the catalytic role of manganese in toluene oxidation (Pérez et al., 2014; Kim et al., 2008). Although
the catalytic performance of CeO2 in toluene oxidation was less than that of the manganese based catalysts,
our future studies will be focused on preparation and more detailed examination of ceria-manganese based
mixed oxides and related materials deposited on the metallic substrate with controlled structures and
morphologies for enhanced performance in the catalytic oxidation of toluene and other VOCs.
Table 1: T50 and T90 values for studied catalysts
CeO2 MnOx MnFe LaMnO3 Pt-Al2O3
T50 [K] 540 455 419 479 405
T90 [K] 619 535 433 498 415
3.3 Mathematical model results
A one dimensional (1D) pseudohomogeneous model described in Section 2.3 was applied to model the low
temperature toluene oxidation over MnFe catalyst. Based on the described assumptions the developed
mathematical model was applied and the results are presented as comparison of values obtained by the
proposed model and experimental data obtained in the fixed bed reactor for toluene oxidation at different
temperatures, shown in Figure 4a)., and as the estimated values of the rate constant, k and the mean square
deviations, SD, given in Table 2.
Figure 4: Comparison between experimental data (points) and the values predicted by proposed model (lines)
over MnFe oxide at different temperatures (a) and Arrhenius plots for determination of the apparent activation
energies, Ea (b).
Table 2: Estimated values of the rate constants, the mean square deviations, SD, the apparent activation
energies, Ea and frequency factors, Ar.
T [K] k [1/min] SD*103
413 1,044.48 12.19 Ar= 1.863*1018 1/min
Ea= 120.36 kJ/mol 423 2,904.76 10.72
433 5,256.96 6.89
448 7,901.82 3.39 Ar= 760,704.31 1/min
Ea= 16.88 kJ/mol 473 10,909.25 2.73
488 12,368.80 1.33
503 12,725.25 0.62
As can be seen at Figure 4(a)., a satisfactory degree of correlation was established. The agreement between
the predicted and experimental data supports the ability of the proposed model to describe the experimental
system used in this work. Figure 4(b). shows Arrhenius plots used for determination of the apparent activation
energies. As it can be seen, unusual Arrhenius plots are obtained corresponding to the apparent energies of
893
activation, which decreased from 120.36 kJ/mol in the lower temperature range (413 - 433 K) to 16.88 kJ/mol
in the higher temperature range (448 - 503 K). The explanation of such observations is usually the change in
the reaction mechanism or the transition from kinetic to mass-transfer limited reaction regime.
4. Conclusions
A set of metal oxide catalysts prepared by different methods were tested for the toluene oxidation reaction and
compared to well-known commercial Pt-Al2O3 catalyst. Temperatures at which 50 and 90 % toluene
conversion occurs were taken as indices of the catalytic activity (T50 and T90). The following increasing
oxidation activity order (measured in terms of T50 values) was observed: CeO2 < LaMnO3 < MnOx < MnFe <
Pt-Al2O3 indicating that the mixed manganese-iron based catalyst, MnFe exhibits much better catalytic activity
for toluene oxidation than the MnOx and, most importantly, its catalytic activity was almost comparable to the
activity of the commercial Pt-Al2O3. The activity of MnFe was found to be superior to single MnOx, highlighting
the promoting effect of iron. A 90 % toluene conversion was observed at 433 K (T90) and complete oxidation
was obtained at about 475 K over MnFe catalyst. The agreement between the values predicted by 1D
pseudohomogenous model and experimental data supports the ability of the proposed model to describe the
experimental system used in this work. The values of the apparent activation energies were determined, which
decreased from 120.36 kJ/mol to 16.88 kJ/mol with increasing reaction temperature, indicating that the
reaction rate was probably limited by the mass transfer at higher reaction temperatures (448 - 503 K).
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