Oxidation of: P -toluic acid to terephthalic acid via a bromine-free process using nano manganese and manganese-copper mixed oxides

Article abstract of DOI:10.1039/c7nj04007e

The industrial process of terephthalic acid (TPA) production suffers from many environmental and economical drawbacks. Liquid-phase catalytic oxidation of alkylbenzene by oxygen is usually initiated by bromine in a corrosive acetic acid solvent. A recent

Full text of DOI:10.1039/c7nj04007e

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p-Toluic Acid Oxidation to Terephthalic Acid via Bromine-Free Process Using
Nano Manganese and Manganese-Copper Mixed Oxides
Mohamed A. Betihaa,b, Nadia G. kandilec, A. M. Badawib, Sanaa M. Solymanb,d,
and Ahmed S. Afifyb
a Egypt nanotechnology center (EGNC) Cairo university, El-Shiekh Zayed, 12588, Egypt.
b Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt.
c Faculty of women for Arts, Science and Education, Ain Shams University, Cairo, Egypt
d AL-Ghad International College for Applied Medical Science, Kingdom of Saudi Arabia.
The corresponding authors:
Ahmed S. Afify : ahmedsamyas@hotmail.com
Mohamed A. Betiha : mohamed_betiha@hotmail.com
1

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Abstract
The industrial process of terephthalic acid (TPA) production suffers from many
environmental and economical drawbacks. Liquid phase catalytic oxidation of
alkylbenzene by oxygen usually is initiated by bromine in a corrosive acetic acid
solvent. A recent catalytic system used environmentally benign nꢀhydroxysuccinimide
(NHSI) as a free radical promoter rather than more hazardous bromine molecules.
Manganese dioxide and CuꢀMn mixed oxide nanoparticles with different ratios
(CuMn11, CuMn12, and CuMn13) were prepared and examined as oxidation catalysts
for pꢀtoluic acid (pꢀTA) to produce TPA in an oxygen atmosphere. These catalyst
samples were characterized by HRTEM, XRD, and N2ꢀadsorptionꢀdesorption isotherm
techniques. The catalytic activity of prepared nanocatalysts for TPA was studied at
different conditions (reaction time, catalyst dosages, reaction solvent and varied oxygen
pressure and reaction temperature) to maximize the yield (%) and selectivity (%) toward
TPA synthesis. Catalyst characterization indicates that CuMn11 sample has a new phase
of the spinal structure. Furthermore, it is the most active catalyst that produces 99.5 and
99.6% in acetonitrile and ethylacetate solvents respectively under oxygen pressure 20
atm. and 150 °C.
Keywords:
Mixed metal oxides; Liquid phase oxidation; terephthalic acid (TPA)
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Introduction
Catalytic oxidation of methylbenzenes to corresponding alcohols, aldehydes and
carboxylic acids by molecular oxygen is an important technique to fabricate useful
oxygenated products 1-3. For example, selective catalytic oxidation of pꢀxylene (PX) is a
major industrial process for the production of TPA, which is an essential monomer for
4
strong fabric polyesters production . The commercial production of TPA is based on
the liquid phase catalytic oxidation of PX by air or oxygen in the presence of
(Co(OAc)2.4H2O), (Mn(OAc)2.4H2O) as homogeneous catalyst and Br2 or HBr as a
promoter in acetic acid as a solvent at temperature regime of 190–205 °C, pressures of
15 ꢀ 30 atm and reaction time of 8–24 h, acquiring a conversion of 95% and a selectivity
of 90% 5. However, acetic acid and Br2 cause severe corrosions in equipment when used
especially at high temperatures and give different unwanted byꢀproducts that complexed
the separation process. Also, diluted acetic acid resulted from the catalytic process
requires additional recovery and recycling units, and large amounts of acetic acid suffer
from combustion in real plants to carbon oxides 6, 7. Furthermore, Br2 is very harmful to
the human being, and cause serious environmental pollution.
The most promising promoters, Nꢀhydroxyimides family as Nꢀhydroxyphthalimide
(NHPI), when mixed with transition metal salts in aerobic oxidation of organic
compounds showed good activity reported by Ishii and coworkers 8-11. Using this
promoter in a catalytic process such as oxidation of alkylbenzenes by oxygen is a very
important reaction for the production of bulk and fine chemicals such as benzoic acid
and TPA 12. Practically, the oxidation of toluene was carried out under normal oxygen
pressure at room temperature by the use of a NHPI as promoter and Co(II) species as
catalyst for the oxidation of PX to yield 85% pꢀTA (not TPA) after 20 h at room
temperature by Ishii et al 9-11, 13-15 according to (Scheme 1).
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Scheme 1: Oxidation of alkylbenzenes with NHPI by oxygen
Various studies were reported including different transition metal ions like
(Co(II), Mn(II), V(III) and Ce(IV), etc.) as homogeneous catalysts 16-20. For example,
the oxidation of PX with oxygen gas (1 atm) in presence of NHPI (20 mol%) and
Co(OAc)2 (0.5 mol%) as catalysts in acetic acid as solvent at 100 °C for 14 h to
produced TPA in 67% yield and PTA (15%) associated with small amounts of (4ꢀ
carboxybenzyl alcohol), 1,4ꢀdiacetoxymethylbenzene and 4ꢀacetoxymethylbenzoic acid
as well as several unidentified compounds in 1–2% yields, at over 99% conversion 21, 22
.
The yield of TPA is improved to 82% when Mn(OAc)2 (0.5 mol%) is added to the
NHPI/Co(OAc)2 system. The synergistic effect of Co and Mn salts in the aerobic
oxidation of alkylbenzenes has been well documented 23-25. The current situation is that
the aerobic oxidation of PX is performed under air (30 atm ) by the NHPI/Co/Mn
system within 3 h at 150 °C to form TPA in 84% yield 26, 27 (Scheme 2).
Scheme 2: Oxidation of PX by O2 with NHPI/Mn(OAc)2/Co(OAc)2
This method has some advantages such as the use of environmentally benign, Nꢀ
hydroxysuccinimide (NHSI), as a free radical promoter rather than a more hazardous
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bromine compound, coupled with the use of cheap and available oxidizing agent,
oxygen gas, but keeps many disadvantages such as the difficulty of catalyst recovery
and corrosion problems due to the presence of acetic acid. From an economic point of
view, the use of nano transition metal oxides as heterogeneous catalysts that has
considerable surface area rather than the soluble transition metals salts to overcome
separation and recycling problems with a more environmentally friendly approach is
one of the future solutions to this problem 28.
Bulk and supported mixed metal oxides are extensively used in industry as
4
heterogeneous oxidation catalysts . These catalysts distinguished by variable oxidation
states with redox properties, the contribution of a surface lattice oxygen atom and
30
oxygen vacancies or defect sites in oxidation reactions 29. Sabina Nicolae et al.
prepared mixed manganese iron oxides as heterogeneous catalysts by hydrothermal
treatment and citrate methods to be studied in the oxidation of PX using different
oxidizing agents. The results were negligible with molecular oxygen while conversion
of PX of 98% and yield only in pꢀTA of 93% (not TPA) were obtained using tertꢀbutyl
hydroperoxide. At present, the AMCO process produced terephthalic acid using a
31
homogeneous system of cobalt and manganese in the presence of dilute acetic acid
.
Despite the high efficiency of cobalt and manganese catalyst used in Amoco 32, some
alternative green technologies based on the use of carbon dioxide 33, water as a solvents
35
34, the use of heterogeneous catalysts such as Pd/ zeoliteꢀY that enable recyclability
are still need more development to meet the economic need for terephthalic acid
production.
Herein, CuꢀMn mixed metal oxide, which is commercially available, is prepared
with different ratios using coꢀprecipitation technique and compared with single MnO2ꢀ
nanorods as liquidꢀphase oxidation catalysts in the presence of NHSI as promoter to
36
oxidize pꢀTA, the main intermediate of TPA production
, which is more difficult
25, 37-
stage of terephthalic acid production and formed with high yield rather than TPA
39,with molecular oxygen and trying to replace the corrosive acetic acid with other
solvents such as acetonitrile or ethylacetate, taking into account the environmental
precautions of this process. Also, a detailed study was conducted to identify the degree
of stability of the prepared catalysts. Various reactivity indexes have been analyzed
including metal ratio, reaction time, catalyst dosages, pressure, and temperature in the
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presence of different solvents to provide an industrially applicable catalyst for the
production of terephthalic acid.
2. Experimental
2.1. Materials
All chemicals and solvents were purchased and used without any further
purification. pꢀtoluic acid (pꢀTA), nꢀhydroxysuccinimide (NHSI), cobalt (II) acetate
(Co(OAc)2.4H2O), manganese(II) acetate (Mn(OAc)2.4H2O), potassium permanganate
(KMnO4) were purchased from SigmaꢀAldrich. All solvents like acetonitrile, glacial
acetic acid, and ethyl acetate were obtained from Merck Chemical Co. The chloride
salts of copper and manganese and sodium hydroxide were from El Nasr Chemical Co.,
Cairo, Egypt.
2.2. Preparation of nanocatalysts
2.2.1. Manganese dioxide (MnO2) nanorods
KMnO4 (6.5 gm) was charged into 150ꢀml round flask preꢀcharged with 150 ml
water/ethanol (1:1). After complete dissolution, the flask is transferred to ultrasonic bath
operated at 35 kHZ for 3 h. After that, 1 ml of H2O2 (30%) was added to complete the
precipitation. The black precipitate of manganese oxide was collected by filtration and
washed several times thoroughly with water to remove any residual of K+ ions. The
solid product was vacuum dried overnight at 60°C and was calcined at 400 °C for 2 h 40.
2.2.2. Copper- manganese mixed metal oxides
They were prepared using a coprecipitation method 41. Aqueous solutions of
CuCl2 with definite concentration (0.2 mol/l) was mixed vigorously with different
concentrations of xMnCl2.6H2O (where x = 0.2, 0.4, 0.6 mol/l) at 80 °C. Aqueous
sodium hydroxide solution (NaOH) (0.25 mol/l) was added to the mixed solution
slowly, drop by drop, with vigorous stirring while the temperature was maintained at 80
°C until pH = 9 was achieved. This procedure took approximately 2 h and the
temperature continued at 80°C for further 3 h. The solution was then allowed to cool
down to room temperature. The precipitate was then filtered, washed several times with
warm distilled water for removal of Na+ ions. The precipitate was then dried at 120 °C
overnight and then was calcined at 300 °C in air for 4 h, giving the final catalysts which
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are dark black colored. They donated as CuMn11, CuMn12, and CuMn13 where the
digits represented the molar ratios of Cu to Mn.
2.3. Characterization
Wideꢀangle Xꢀray diffraction pattern was recorded on an XPERT Xꢀray
diffractometer in the angular range from 10 to 70 (2θ). The BET surface area, average
pore diameter, and pore volume were measured at ꢀ196 °C on a NOVA 3200 system
(USA). The catalysts were outgassed for 3 h at 300 °C under vacuum in the degassing
port of the adsorption analyzer. The specific surface area was calculated using the BET
model. The pore size distributions were obtained from the desorption branch of the
nitrogen isotherms using BarrettꢀJoynerꢀHalenda (BJH) method. Highꢀresolution
transmission electron microscope (HRTEM) images were obtained using a JEOL JEMꢀ
2100 (Japan) operating at 200 kV. The Xꢀray photoelectron spectra were obtained using
an ESCALAB MK II spectrometer provided with a hemispherical electron analyzer and
Al anode Xꢀray exciting source (Al Kα =1487.6 eV).
All the oxidation reaction components (reactants, intermediates, promoter and
products) were analyzed by using an HPLC (Waters model 2489) consists of an
isocratic pump conducted through XBridge C18 column with an internal diameter of 4.6
mm, a length of 250 mm and a particle size of 5 µm equipped with ultraviolet detection
(254 nm). The mixture of water/acetonitrile has a flow rate of 1mL/min with the
injected volume of 20 µl was used in the isocratic mode at ambient temperature. Each
sample analysis took about 12 min
2.4. Catalytic test
The catalytic activity was screened at refluxing and 1 atm, and then the catalytic
reaction with the optimum conditions is carried out at different pressures and
temperatures.
2.4.1. Catalytic oxidation of p-TA at atmospheric pressure
The oxidation of pꢀTA was performed in a 3ꢀneck flask equipped with a magnetic
stirrer and a septum for withdrawing samples at intervals of 1 h. The system was also
equipped with a condenser to ensure complete condensation and recycling of the
evaporated compounds. In a typical run, 80 mmol of pꢀTA, 16 mmol of NHSI, 0.4
mmol of Co(OAc)2, 0.4 mmol of Mn(OAc)2 and 250 ml of selected solvent
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(acetonitrile, ethylacetate or mixture of acetonitrile and acetic acid 9:1) were mixed in a
glass batch flask. An appropriate weight of selected MnO2 or CuꢀMn mixed oxide
catalysts was added, and the stirring speed was set at 600 rpm. When the mixture was
refluxed, pure oxygen was continuously bubbled into the solution mixture.
2.4.2. Catalytic oxidation of p-TA at high pressure (autoclave)
The highꢀpressure reactor is a 450 ml inconel autoclave. The system is also
equipped with a condenser, a gasꢀliquid separator and a liquid recycle tank to ensure
complete condensation and recycling of the evaporated compounds. The liquidꢀphase is
continuously stirred at 193 rpm. The reactor temperature is regulated by an electric
heating jacket with a thermocouple within ±0.2 °C, and a regulating valve controlled the
reactor pressure.
The reactor was charged with the same concentrations of previous constituents (pꢀ
TA, NHSI, Co(OAc)2, Mn(OAc)2), selected solvent and catalyst weight from the best
results obtained at 1 atm. After purging with pure oxygen, the reactor was closed during
heating and agitated to attain the required pressure and temperature.
In atmospheric and pressurized reaction, the solid and mother liquor were
separated by filtration. The clear liquid containing organic substrate, intermediates, and
products were analyzed using an HPLC with an Ultraviolet detector with a suitable
analytical method to calculate the yield, conversion, and selectivity percents for all
compounds Fig. 1.
Fig. 1: HPLC chromatogram of all reaction components with retention time.
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3. Results and Discussion
In this work,
reaction conditions (about ≈87 °C and atmospheric pressure) at fixed concentration
PꢀTA and NHSI as a promoter in presence heterogeneous catalysts as a recyclable catalyst
The discussion concentrates on the approach of green chemistry including (1) utilization
of heterogeneous nanocatalysts with different style, (2) study the effect of NHSI (instead
of Br2) as a promoter, and (3) replacement of corrosive acetic acid solvent by acetonitrile
or ethyl acetate solvents
t
he liquidꢀphase oxidation of pꢀTA was carried out under mild
s
of
.
a
.
3.1. Textural and Structural Characterization of different Mn/Cu-Mn oxides
3.1.1. X-ray diffraction analysis (XRD)
The crystalline phases of prepared catalysts were studied by Xꢀray diffraction
analysis. As shown in Fig. 2, The XRD pattern of MnO2 confirms the formation of its
exact phase. The characteristic peaks at 12.5°, 18.07°, 28.65°, 37.3°, 41.95°, 49.85°,
59.74° and 65.15°, are well matched with the diffraction peaks (1 1 0), (2 0 0), (3 1 0),
(2 1 1), (3 0 1), (4 1 1), (6 0 0) and (5 2 1) reflection planes of MnO2 (JCPDS card PDF
# 00ꢀ044ꢀ0141). The lattice parameters of caddiceꢀclewꢀlike MnO2 are a = 9.7875 and c
= 2.8600, are highly identical to the standard values (JCPDS card PDF file no. 44ꢀ0141,
a = 9.7847, c = 2.863).
Fig. 2: XRD pattern of prepared MnO2 nanocatalyst
Also, Fig. 3 shows XRD patterns of the CuMn11, CuMn12, and CuMn13 mixed
oxides catalysts. The pattern of CuMn11 exhibits new peaks compared to MnO2 with
high intensity (2θ = 30.89°, 35.7°) besides some characteristic peaks for CuO (2θ =
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35.77° and 48.5o) and MnO2 (2θ = 28.03o, 41.49o). These observations suggested the
formation of new specific phase with spinel structure of copperꢀmanganese is formed
CuMn2O4 in CuMn11 sample due to the interaction between the two oxides during the
coꢀprecipitation at pH of 9 CuMn2O4 (ICDDꢀPDF no. 74–2422) 42. The patterns of
CuMn12 and CuMn13 have separated phases of crystalline MnO2 and CuO. The
intensity of the main peak of MnO2 is enhanced, indicating the increase of the Mn
concentration in the CuMn13 catalyst. Noticeably, the Mn3O4 phase was not detected in
all samples.
Fig. 3: XRD pattern of prepared Cuꢀ Mn mixed oxides
3.1.2. Surface area analysis (BET)
Fig. 4 shows the corresponding N2ꢀadsorptionꢀdesorption isotherm, and pore size
distributions. It is clear that the MnO2 exhibits Type IV isotherm with the H1 hysteresis
loop, which is a characteristic of open cylindrical pores. Pore size distribution driven by
desorption branch confirms the MnO2 nanorods possesses wellꢀdefined mesopore
diameter. It should be noted that the hysteresis loop for MnO2 is narrower due to the
influence of mesopore interconnectivity on N2 vaporization. The surface area of MnO2
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was calculated to be 39.2 m2/g using the BET method, and the total pore volume was
0.068 cm3/g.
Fig. 4: (a) N2ꢀ adsorptionꢀdesorption isotherm and (b) pore size distributions of
MnO2 nanocatalyst
The isotherms of CuMn11, CuMn12, and CuMn13 mixed oxide catalysts are
43ꢀ45
typical of mesoporous solids according to IUPAC classification
with slow
steepness adsorption of N2 gas (Fig. 5). In all catalyst CuMn11, CuMn12 and CuMn13,
the N2ꢀadsorption branch present an enhanced N2ꢀadsorption at P/P0 in the regime of
0.8ꢀ0.98, which denotes the presence of an additional porosity. The absence of plateau
is also giving an indication for the presence of voids between particles. The SBET of
CuMn11, CuMn12, and CuMn13 catalysts was 36, 30 and 22 m2/g, respectively. The
high surface of the CuMn11 catalyst may be due to the resultant spinal framework,
which is in good agreement with XRD results. The pore size distribution corresponding
to the CuMn11, CuMn12 and CuMn13 mesopores is centered around 3.77, 2.47 and
2.71 nm, respectively besides a relatively broad peak placed in the range of 3.8 nm,
corresponding to the secondary porosity. The reduction in pore size accompanied with
decreasing copper ratio may be due to the presence of copper in the outer surface of
manganese pore. The pore volume of CuMn11, CuMn12, and CuMn13 was 0.067,
0.045 and 0.029 cm3/g, respectively.
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Fig. 5: (a) N2ꢀ adsorptionꢀdesorption isotherm and (b) pore size distributions of
Cuꢀ
Mn mixed oxide nanocatalysts
3.1.3. HRTEM of prepared nanocatalysts
HRTEM is a widely used technique in catalysis research field for collecting
information about metal particle sizes and its morphology. The HRTEM analysis of
nano MnO2 catalyst is shown in Fig. 6(a). It reveals the formation of uniform rodꢀlike
morphology with different aspect ratios. The average dimensions calculated from the
HRTEM micrographs ranged from ≈11 ± 3 nm widths and ≈ 58 ± 8 nm length. The high
crystallinity of MnO2 particles may result from ultrasonication effect during
preparation. In case of CuMn11, HRTEM verifies the formation of the distorted oval
shape with a particle size of 32 nm Fig. 6(b) [26]. Also, plateꢀlike particles may be an
ordered MnO2 which intercalated through CuO particles and/or hydrogen bonding.
When MnO2 percent increased, these particles increased with enlargement. This clearly
appears in Fig. 5(c) of CuMn12 catalyst photo also with the formation of small oval
form with high extent of intramolecular spaces. Nevertheless, CuMn13 nanocatalyst
appeared in parallel squared plates combined to each other. Their diameter is range
between 28 ꢀ 40 nm as shown in Fig. 6(d). Agglomeration was clearly observed in Fig.
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6(b, c and d) due to hydrogen bonds between metal hydroxides and water molecules
during calcinations that form bridges between adjacent particles 46, 47
.
Fig. 6: HRTEM image of (a) MnO2 nanorods, (b) CuMn11, (c) CuMn12
and (d) CuMn13
The Xꢀray photoelectron spectroscopy (XPS) of CuMn11 is shown in Fig. 7a-c.
The XPS of Cu 2p core level exhibited peaks at 933.96 and 953.75 eV, which can be
attributed to the BE (binding energy) of Cu 2p3/2 and Cu 2p1/2, indicating the presence of
Cu2+ and Cu(I) species. Moreover, the ꢀCu of split spinꢀorbit components is 19.79 eV,
suggesting the presence of Cu–O species 43, 48. In addition, the shakeꢀup peak is
observed at BE of 943.28 eV is due to an unfilled Cu3d9 shell, implying further
43
indication for the presence of CuO species . Two peaks can be observed at a binding
energy (BE) about 642.1 and 653.8 eV, which are assigned to Mn 2p1/2 and Mn 2p3/2,
respectively 49 (Fig. 7b). These peaks are curveꢀfitted into three peak components at BE
of ~640.8, 641.7 and 642.9 eV, which assigned to Mn (II), Mn (III) and Mn (IV),
respectively. The atomic ratio of Mn(II)/Mn(III) was of 1.1, reflecting the presence of
Mn3O4 (MnO. Mn2O3). The CuMn11 exhibited a high ratio of Mn(III)/Mn(II) due to the
presence of copper species inhibited the transformation of Mn(III) to Mn(II). The
possible reason for the disappearance of Mn (IV) in XRD is that Mn3O4 composed of
Mn (III) and Mn (II). The XPS of O 1s spectrum showed BE of 529.85, 531.37 and
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532.9 eV, corresponding to Mn–O–Mn, Mn–OH and crystalline water (H–O–H),
50
respectively
(Fig. 7c). The Mn–OH species contribute about 12%, suggesting the
CuMn11 offer high performance than Mn(II) due to the presence of hydroxyl group
facilitate the redox property
Cu
Mn
O2
Fig. 7: The XPS spectra of CuMn11 nanocatalyst
3.2. The catalytic activity of prepared catalysts in TPA preparation
In this work, we evaluate the role of prepared MnO2 nanorods and CuꢀMn mixed
oxides as heterogeneous catalysts for the liquid phase oxidation of pꢀTA as a model
reactant for highly pure TPA production Scheme 3. The reaction was also preceded via
soluble metal acetates as homogenous ones by oxygen gas in the presence of NHSI
(instead of Br2) as a promoter. The synergetic effect of homogeneous and heterogeneous
systems will be studied at different reaction time, catalyst weight, solvent types,
pressures and reaction temperature for optimizing pꢀTA conversion (%) and TPA yield
(%) according to the following scheme:
Scheme 3: Steps of PX oxidation with intermediates to form TPA
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3.2.1. Effect of reaction time
Many experiments were done to study the catalytic performance of a single metal
oxide (MnO2) and mixed metal oxides (CuꢀMn) for terephthalic acid production. The
oxidation reaction is carried out at 1 atm oxygen pressure with the constant weight of
catalyst (0.2 g) and NHSI by 1 h interval time up to 12 h in acetonitrile solvent.
Fig. 8 shows a diagram of TPA yield (%) against reaction time. It illustrates that
all tested nanocatalysts donate low to a moderate yield of TPA ranged from ~ 16.8% to
46.6%. The most effective catalyst was CuMn11 while CuMn13 was the lowest one.
CuMn11 initiated the oxidation reaction significantly after reaction time 2 h while
MnO2 and CuMn12 after 4 h, and the most delayed one was CuMn13, 6 h. The unique
activity of CuMn11 relative to other catalysts may be due to its spinal structure which
mostly has more oxygen vacancies promote the reaction 51-53. Besides, the activity of
MnO2 and CuMn12 is more close to each other, this may be due to the formation of
separated MnO2 and CuO phases, and therefore MnO2 component is mostly the
effective part in CuMn12. These results coincided with HRTEM and XRD analysis
(peak at 2θ = 27.9◦).
Fig 8: Effect of reaction time on TPA yield (%) using MnO2 & CuꢀMn
nanocatalysts
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Fig. 9 shows the yield of 4ꢀCBA during 12 h by using MnO2 and CuꢀMn
nanocatalysts. It is clearly observed that formation of 4ꢀCBA and TPA start at the same
time after generating free radicals of NHSI for each catalyst. Lower initiation period is
able to predict the expected higher catalyst performance. All tested catalysts resulted in
fixed activity towards 4ꢀCBA after 5 h of reaction time. After 12 h, MnO2, CuMn11 and
CuMn12 exhibit nearly the same results about 15% while CuMn13 tends to form lower
yield (9.3%).
Fig 9: Effect of reaction time on 4ꢀCBA yield (%) using MnO2 and CuꢀMn
nanocatalysts
3.2.2. Effect of catalyst weight
The oxidation reaction is carried out using different catalyst weights at 1 atm
oxygen pressure in acetonitrile by 1 h interval time for 12 h. Catalyst weights were used
0.15, 0.20 and 0.40 g for the oxidation reaction. All results are collected in Table 1.
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Table 1: Oxidation of pꢀTA results with different catalysts and catalyst weights.
Conv. %
p-TA
36.4
Yield %
4-CBA
Selectivity %
TPA
Weight
(gm)
Catalyst
MnO2
TPA
19.8
35.5
34.1
20.5
47.6
48.1
14.3
39.3
28.8
10.5
16.8
23.5
4-CBA
45.5
32.0
35.1
34.7
23.7
18.2
40.5
28.6
29.0
53.2
35.6
34.0
0.15
0.20
0.40
0.15
0.20
0.40
0.15
0.20
0.40
0.15
0.20
0.40
16.6
16.7
18.4
10.9
14.8
10.9
9.8
54.5
68.0
64.9
65.3
76.3
80.3
59.5
71.4
71.0
46.8
64.4
66.0
52.2
52.6
31.5
CuMn 11
CuMn 12
CuMn 13
62.4
60.0
24.1
55.1
15.8
11.8
12.0
9.3
40.5
22.5
26.1
35.7
12.1
Reaction conditions: pꢀTA (80 mmol), NHSI (16 mmol) 0.4 mmol of each Co(OAc)2 and Mn(OAc)2 were dissolved in acetonitrile
with bubbled pure oxygen gas and refluxed for 12 h at atm. pressure
.
Table 1 displays the variation of catalyst weight effect on pꢀTA oxidation at the
same abovementioned conditions. The catalyst weight effect correlated with pꢀTA
conversion (%), TPA and 4ꢀCBA yield and selectivity (%). It has been found that by
increasing catalyst amount from 0.15 to 0.2 g, the conversion and yield (%)
significantly increased especially with CuMn11 and CuMn12 which doubled, while
excess weight to 0.4 g decreased the productivity except CuMn13. This may refer to
that the obtained acidic products in reaction medium may be adsorbed and poised the
catalyst active sites. On the other hand, CuMn13 sample has a higher concentration of
secondary wider pores (~3.8 nm) which may facilitate adsorption of reactants and
desorption of products on the catalyst surface, therefore, its catalytic activity increased
by increasing the sample weight. The results of the yield and selectivity (%) are
correlated together indicating that the optimum catalyst weight is 0.2 g of the best
catalyst CuMn11, so no need for extra catalyst amount to improve the reaction
outcomes.
3.2.3. Effect of solvent
The pꢀTA oxidation using prepared catalysts was carried out using different
solvents like acetonitrile (polar), ethyl acetate (nonpolar), and a mixture of acetonitrile /
acetic acid 9:1 (acidic). The last mixture was used to confirm the definite role of the
acidic medium in target reaction.
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New Journal of Chemistry
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The results of different solvents are collected in Table 2. These experiments were
carried out using optimum catalyst weight, 0.2 g, at 1 atm by refluxing for 12 h. It has
been found that acetonitrile solvent achieved the best pꢀTA conversion (%), TPA and 4ꢀ
CBA yield (%) and selectivity (%) values using all tested catalysts. Otherwise, the
acidic solvent has the lowest rate. In acetonitrile, CuMn11 has great yield value (47.6%)
rather than CuMn12 and MnO2 which have a moderate yield (%) of TPA 39.3% and
35.5%, respectively. While the TPA yield (%) ranged from 11.1% to 18% by using
ethyl acetate with all catalysts. This may occur due to the more solubility of oxygen gas
in acetonitrile rather than ethyl acetate. Additionally, highly polar solvents like
acetonitrile may facilitate the formation of active oxygen species and thereby enhance
the catalytic activity 54. CuꢀMn mixed oxides nanocatalysts exhibited nearly the same
yield of terephthalic acid in an acidic solvent of 9.3% ± 0.5. Consequently, using this
mixture as a solvent is excluded from our choices.
Table 2: Oxidation of pꢀTA results in different solvents with MnO2 and CuꢀMn mixed oxides.
Conv. %
p-TA
25.4
52.2
9.5
Yield %
4-CBA
Selectivity %
TPA
Catalyst
MnO2
Solvent
TPA
18.5
35.5
4.5
4-CBA
27.3
32.0
52.8
38.4
23.7
61.3
49.2
28.6
48.4
45.5
35.6
59.3
ethyl acetate
acetonitrile
6.9
16.7
5.0
72.7
68.0
47.2
61.6
76.3
38.7
50.8
71.4
51.6
54.5
64.4
40.7
ACN/ AcOH 9:1
ethyl acetate
acetonitrile
25.2
62.4
27.2
23.9
55.1
18.1
20.4
26.1
21.7
15.5
47.6
10.5
12.1
39.3
9.3
9.7
CuMn 11
CuMn 12
CuMn 13
14.8
16.7
11.8
15.8
8.8
ACN/ AcOH 9:1
ethyl acetate
acetonitrile
ACN/ AcOH 9:1
ethyl acetate
acetonitrile
11.1
16.8
8.8
9.3
9.3
ACN/ AcOH 9:1
12.8
Reaction conditions: pꢀTA (80 mmol), NHSI (16 mmol), 0.2 gm catalyst and 0.4 mmol of each Co(OAc)2 and Mn(OAc)2 were
dissolved in selected solvent with bubbled pure oxygen gas and refluxed for 12 h at 1 atm.
.
3.2.4. Effect of oxygen pressure
One of the most vital parameter which is closely related to industrial processes is
the oxygen gas pressure 55. Therefore, the oxidation of pꢀTA was studied extensively at
different pressures, 10, 15 and 20 atm, by using the most efficient solvents acetonitrile
and ethyl acetate with different temperatures (110 and 150 °C) for reaction time 12 h
and the other conditions remained the same. The values of pꢀTA conversion (%), TPA
yield (%) and selectivity (%) related to pressure and temperature are shown in Table 3.
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Table 3: Oxidation of pꢀTA with MnO2 and CuꢀMn nanocatalysts in different oxygen pressure,
reaction temperature and solvents.
Conv. %
p-TA
74.6
78.6
76.4
86.7
88.6
90.1
82.0
85.0
84.6
99.5
99.1
99.6
81.7
82.2
84.4
81.8
97.0
96.2
87.5
88.0
96.5
97.3
97.9
98.3
Yield %
TPA
64.0
66.0
70.5
75.8
70.4
88.9
69.9
73.9
79.6
99.1
97.0
97.6
68.9
69.5
72.5
74.7
96.0
94.7
79.8
81.1
87.8
93.3
94.7
95.1
Selectivity %
TPA
Pressure
(atm)
Temperature
(°C)
Catalyst
MnO2
Solvent
10
15
20
20
20
20
10
15
20
20
20
20
10
15
20
20
20
20
10
15
20
20
20
20
110
110
110
150
110
150
110
110
110
150
110
150
110
110
110
150
110
150
110
110
110
150
110
150
acetonitrile
acetonitrile
acetonitrile
acetonitrile
ethyl acetate
ethyl acetate
acetonitrile
acetonitrile
acetonitrile
acetonitrile
ethyl acetate
ethyl acetate
acetonitrile
acetonitrile
acetonitrile
acetonitrile
ethyl acetate
ethyl acetate
acetonitrile
acetonitrile
acetonitrile
acetonitrile
ethyl acetate
ethyl acetate
85.8
84.0
92.3
87.4
79.5
98.7
85.2
86.9
94.1
99.6
97.9
98.0
84.3
84.5
85.9
91.3
99.0
98.4
91.2
92.2
91.0
95.9
96.7
96.7
CuMn 11
CuMn 12
CuMn 13
Reaction conditions: pꢀTA (80 mmol), NHSI (16 mmol), 0.2 gm catalyst and 0.4 mmol of each Co(OAc)2 and Mn(OAc)2 were
dissolved in a selected solvent with different pure oxygen pressures for 12 h
.
It describes that TPA yield (%) increases gradually with increasing the oxygen
pressure at 110 °C using all catalysts in acetonitrile. The conversion and yield (%)
values reach to 96.5 and 87.8 %, respectively, and the selectivity towards TPA was
major with CuMn11 at 110 °C, however, when reaction temperature raised to 150 °C,
these values were greatly improved especially in case of CuMn11 and MnO2. The best
oxidation activity was achieved with CuMn11 in acetonitrile at 20 atm and 150 °C as pꢀ
TA conversion (%) and TPA selectivity (%) 95.5 and 99.6 respectively.
By using ethyl acetate solvent, all tested catalysts achieved magnificent
conversion and yield percents at both 110 °C and 150 °C. Generally, CuMn11 was the
most efficient oxidation catalyst at different conditions. Also, CuMn12 achieved great
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improvement of conversion; yield and selectivity percents at 110 °C besides CuMn13
and MnO2 have distinguished results of conversion and yield at 150 °C.
Water molecules which are produced as a byꢀproduct of equimolar amounts in
oxidation with molecular oxygen can be strongly adsorbed on metals oxide surfaces via
hydrogen bonding which block catalysts active sites 56. So, the polarity of solvent is
recommended when the reaction was preceded at a temperature lower than 100°C but,
at > 100 °C, water molecules could be in the vapor phase with each solvent keeping free
active sites on catalyst surface dry 32, 57. The prominent effect of pressure may be due to
using closed system highꢀpressure reactor which allowed raising the reaction
temperature more than the boiling point of the selected solvent (≥88°C). Ethyl acetate as
a nonpolar solvent could obtain good catalytic oxidation activity.
It is concluded that homogeneous catalytic system is mostly helpful in pꢀTA
oxidation at 1 atm. Previous experiments at high pressure were also carried out using
the dual catalytic systems. Now, it is claimed to assign the role of prepared mixed metal
oxide, using the most efficient catalyst sample (CuMn11) without utilizing homogenous
catalyst (cobalt (II) acetate and manganese (II) acetate) at 20 atm and 150 °C for 12 h in
acetonitrile solvent. As shown in Table 4, CuMn11 achieved the same conversion (%)
of pꢀTA with a slight decrease in the yield of TPA by 1.4% only without homogeneous
systems in presence or absence of a homogeneous catalyst. Under the same conditions,
the reaction which proceeds without solid metal oxide in the presence of homogeneous
catalyst showed the conversion of pꢀTA, and TPA yield (%) decreased to 65%, and
54.1%, respectively. These results confirm the integrated activity of CuMn11 for
catalytic oxidation of pꢀTA, and there is no need for the homogeneous catalysis for the
oxidation reaction as shown in Table 4.
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Table 4: the role of CuMn11 for oxidation of pꢀtoluic acid without homogenous catalysts at
optimum conditions.
Conv. %
p-TA
Yield % Selectivity %
Metal
Pressure Temperature
Catalyst
Solvent
Acetates
(atm)
20
(°C)
150
150
150
TPA
99.1
97.7
0
TPA
99.6
98.1
0
+
-
acetonitrile
acetonitrile
acetonitrile
99.5
99.5
≈0
CuMn 11
20
20*
CuMn 11
Without
catalyst
+
20
150
acetonitrile
65.4
54.1
82.7
+
The symbols + and – means the reaction is carried out in the presence or absence of the homogeneous catalyst
* means the reaction is carried out using N2 gas
4. The proposed reaction mechanism
As shown in Scheme 4, the catalysis occurs by a free radical mechanism. The
oxidation reaction is initiated by generation of the succinimideꢀNꢀoxyl radical (SINO)
by abstraction of the (OꢀH) hydrogen in NHSI. Through homogenous metal ions, SINO
is created by highly active M3+ꢀdioxygen radical that formed by molecular oxygen in
addition to, mixed metal oxide surface with lattice oxygen and oxygen vacancies may
contribute in the formation of SINO radical by further two pathways. First, (M=O) and
adsorbed oxygen could abstract hydrogen of NHSI forming hydroxyl and hydroperoxyl
radicals. Second, these radicals could pick up other hydrogen atoms giving away water
molecules. It should be noted that active sites on mixed metal oxide are regenerated by
molecular oxygen. The SINO radicals then abstract a hydrogen atom from methyl group
of target pꢀTA, thus reverting NHSI and a carbonꢀcentered radical. This carbon radical
reacts with O2 to yield an alkylperoxy radical which subsequently abstracts the (OꢀH)
hydrogen from NHSI, forming a stable hydroperoxide. Then, hydroperoxide is
dissociated by metal ions and complete oxidation by oxygen to form carboxyl group.
The formation of excess SINO radicals by using metal oxide explains clearly the role of
mixed metal oxide with NHSI via oxidation reaction of pꢀTA.
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Scheme 4: Mechanism of pꢀTA oxidation reaction by O2 with NHSI/ mixed metal oxide.
5. Conclusion
The catalytic activity of MnO2 nanoꢀrods comparing with CuꢀMn mixed oxides
nanoparticles has been entirely examined for the oxidation of pꢀTA to produce TPA.
CuꢀMn mixed oxides nanoparticles achieved great activity at 20 atm in both acetonitrile
and ethyl acetate solvents rather than single metal oxide MnO2 with and without
homogeneous. The superior catalytic activity achieved with CuMn11 nanocatalyst at 20
atm oxygen pressure due to the high distribution of both Mn3+, Mn4+ ions, and Cu+,
Cu2+ ions in spinel structure containing as formed two Jahn–Teller ions (Cu2+ and Mn3+)
that enable the redox cycle of the two metals with distinctive surface properties as very
high conversion 99.5% and yield 99.1% in acetonitrile at 150°C. Deeply, this system
suppressed the presence of 4ꢀCBA intermediate (impurity) by using NHSI (instead of
Br2) as a promoter in a solvent rather than acetic acid. Therefore, the presence of copper
oxide combined with manganese oxide appends great influence on catalytic activity
towards TPA formation. Furthermore, it shows the effect of mixing two different metal
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ions with various oxidation states and their role in the excessive formation of free
radicals that push the rate of reaction.
conflict of interest
There are no conflicts of interest to declare
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Challenges and technological opportunities of nano-mixed oxide (Cu–Mn) allow green route to
valued terephthalic acid production from p-xylene.
HO
O
HO
NHSI
2
2
SINO
M
M
Mixed
Metal
Oxide
NHSI
2
SINO
2
O
O
O
O
M
M
COOH
M
M
CH3
Mixed
Metal
Oxide
Mixed
Metal
Oxide
O2
4
SINO
3 H2O
O2
COOH
COOH
O
O
N
OH
N
O
NHSI
SINO
=
=
O
O