Efficient catalytic oxidation of methyl aromatic hydrocarbon with: N -alkyl pyridinium salts

Article abstract of DOI:10.1039/c9ra08185b

A series of N-alkyl pyridinium salts were synthesized and employed as metal-free catalyst for the selective oxidation of methyl aromatic hydrocarbon with molecular oxygen. The electronic effect of the substitutes was found to be an important factor for the catalytic performance. With the introduction of electron-donating substitute -N(CH₃)₂, the conversion of p-xylene and selectivity of p-toluic acid could be simultaneously increased. 1-Benzyl-4-N,N-dimethylaminopyridinium salt showed the highest catalytic activity, and 95% conversion with 84% of selectivity to p-toluic acid could be obtained for the selective oxidation of p-xylene. Several methyl aromatic hydrocarbons could all be efficiently oxidized with the reported catalyst at the absence of any metal species.

Full text of DOI:10.1039/c9ra08185b

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Efficient catalytic oxidation of methyl aromatic
hydrocarbon with N-alkyl pyridinium salts†
Cite this: RSC Adv., 2019, 9, 38891
a
a
a
b
a
*
*
Qiaohong Zhang, Honghao He, Huibin Wang, Zhan Zhang and Chen Chen
A series of N-alkyl pyridinium salts were synthesized and employed as metal-free catalyst for the selective
oxidation of methyl aromatic hydrocarbon with molecular oxygen. The electronic effect of the substitutes
was found to be an important factor for the catalytic performance. With the introduction of electron-
donating substitute –N(CH3)2, the conversion of p-xylene and selectivity of p-toluic acid could be
simultaneously increased. 1-Benzyl-4-N,N-dimethylaminopyridinium salt showed the highest catalytic
activity, and 95% conversion with 84% of selectivity to p-toluic acid could be obtained for the selective
oxidation of p-xylene. Several methyl aromatic hydrocarbons could all be efficiently oxidized with the
reported catalyst at the absence of any metal species.
Received 9th October 2019
Accepted 7th November 2019
DOI: 10.1039/c9ra08185b
rsc.li/rsc-advances
showed better lipophilicity and good catalytic activity was ob-
tained at lower temperatures.18–21
Introduction
Compared with those heterogeneous catalysts, homoge-
neous organic catalysts own better solubility and could be used
under mild conditions. For example, metal-complex or the
metal-complexation promoted organic N-hydroxyl catalytic
systems were reported for the conversion of hydrocarbon.3,4,22–24
For the above mentioned catalytic system, metal species were all
involved. Development of the metal-free catalytic system for the
selective oxidation of hydrocarbon was a challenge for the
chemists. Pure organic catalyst could show better interaction
with the organic substrates. Under mild reaction conditions,
higher catalytic activity is expected to be obtained and coking or
over-oxidation caused by the metal-species might also be avoi-
ded. Encouraging progresses have been achieved on the metal-
free organocatalysis such as the oxidation of alcohol to
aldehyde/ketone catalyzed by 2,2,6,6-tetramethyl-piperidyl-1-
oxy (TEMPO), and selective oxidation of hydrocarbon by N-
hydroxyphthalimide (NHPI) system, or the other organic mole-
cules.25–29 Besides these systems, it was found that simple
nitrogen cation compounds were useful in facilitating the
oxidation of hydrocarbon. For example, quaternary ammonium
salts could catalyze the oxidation of different hydrocarbon such
as cyclohexene, tetralin, and ethylbenzene.30–32 Nitrogen-cation
compounds such as methyl violet could be used as the
electron-promoter to combine with NHPI in promoting the
metal-free oxidation of aromatic hydrocarbon.33,34
Selective oxidation of methyl aromatic hydrocarbon is one of
the key technologies in transforming petrochemical raw mate-
rials into useful oxygenates such as alcohol, aldehyde, ketone,
and carboxylic acid, which are valuable products in agricultural,
pharmaceutical, polymer industries, etc.1–5 For example, oxida-
tion of toluene to benzoic acid and oxidation of p-xylene to p-
toluic acid or terephthalic acid are all important commercial
processes in the chemical industry.6,7 For these processes,
owing to the inert reactivity of the C–H bond, metal-containing
catalytic systems are oen utilized as catalysts to facilitate the
conversion of aromatic hydrocarbons at elevated temperature
or pressure.8–10 One typical example is the Co–Mn–Br homoge-
neous catalytic system which was rstly developed in the middle
of last century and is still being utilized in the present.7,11,12
Metal-containing catalytic systems such as Co, Cu, or Mn-based
oxides, etc. showed good catalytic activity at higher reaction
temperatures.3,4,13–17 For their surface hydrophilicity, however,
oxygenated products such as organic acid are easily adsorbed on
their surface, and over-oxidation or coking were likely to happen
at higher temperatures. The surface organic modication
strategy was illustrated for inorganic heterogeneous catalysts in
our previous work to modulate the surface hydrophilicity/
hydrophobicity; the hydrophobic or superhydrophobic catalyst
aSchool of Material Science and Chemical Engineering, Ningbo University, 818
Fenghua Road, Ningbo, 315211, PR China. E-mail: chenchen@nbu.edu.cn; Fax: +86
574 87609836; Tel: +86 574 87609836
In nature, it was found that pyridinium cation analogues
(e.g. NAD+) played an important role in the redox processes.35
Compared with the conventional quaternary ammonium salts,
quaternary pyridinium salts posses some advantages including
better thermal stability and simpler recycling from the reaction
mixture.36,37 Seldom studies, however, have focused on their
utilization in the selective oxidation of methyl aromatic
bChina Tobacco Henan Industrial Co. Ltd, No.8 The 3rd Avenue, Zhengzhou, 450001,
PR China. E-mail: zhangzhan2059729@126.com
† Electronic supplementary information (ESI) available: Inuence of temperature
on the catalytic oxidation of p-xylene, optimization of reaction temperature,
optimization of catalyst concentration, and correlative FT-IR and NMR
spectrums. See DOI: 10.1039/c9ra08185b
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conrmed by FT-IR measurement, 1H NMR and 13C NMR
spectroscopy.
General procedure for the catalytic oxidation of aromatic
hydrocarbon
Catalytic oxidation reactions were carried out in a 20 ml scale
Teon-lined autoclave equipped with magnetic stirrer and
automatic temperature controller. In a typical experiment,
10 mmol of p-xylene and 5 ml of acetonitrile were added into the
autoclave followed by 0.5 mmol of c as catalyst. And 0.2 mmol of
p-tolualdehyde (TALD) was also introduced as initiator to
reduce the induction period. Aer the autoclave was sealed, the
air inside was replaced with O2 for three times, then the auto-
Scheme 1 Structure of different N-alkyl pyridinium salts.
hydrocarbon. In this study, a series of N-alkyl pyridinium salts
with different type of substitutes were designed and synthesized
(as shown in Scheme 1). These compounds are 1-benzylpyridin-
1-ium bromide (a), 1-benzyl-4-cyanopyridin-1-ium bromide (b),
1-benzyl-4-(dimethylamino)pyridin-1-ium bromide (c), 1-decyl-
4-(dimethylamino)pyridin-1-ium bromide (d), 1-dodecyl-4-
(dimethylamino)pyridin-1-ium bromide (e), respectively, and
their catalytic performance in the oxidation of methyl aromatic
hydrocarbon was investigated.
ꢀ
clave was heated to 160 C. The pressure was kept at 1.5 MPa
during the reaction by continuously recharging of oxygen. Aer
reaction, the autoclave was cooled to room temperature and
carefully depressurized. 5 ml of dimethyl formamide (DMF) was
used to dissolve the products for analysis. Different products
were identied by GC-MS spectrometer and comparing reten-
tion time of authentic samples. 4-Carboxylic benzaldehyde (4-
CBA) and terephthalic acid (TPA) were quantied by HPLC
using prevail organic acid column and oxalic acid as the
internal standard. The mobile phase consisted of acetonitrile
and water (30 : 70, V/V) by adjusting the pH ¼ 2–3 with phos-
phoric acid. Other products were quantied by GC using HP-5
column and 1,3-dichlorobenzene as the internal standard.
Experimental
General
4-Dimethylamino pyridine (DMAP, 99%), 4-cyano pyridine
(99%), 4-tert-butyl toluene (99%), 4-chloro toluene (>98%), p-
tolualdehyde (TALD, 99%), p-toulic acid (TA, 99%), 4-carboxylic
benzaldehyde (4-CBA, 99%), terephthalic acid (TPA, 99%) were
commercial products from J&K Chemical Co. Ltd. n-Decyl
bromide (99%) and n-dodecyl bromide (98%) were purchased
from Aladdin Chemical Reagents Co. Ltd., China. Other mate-
rials and reagents were purchased from Tianjin Kermel
Chemical Reagent Co. Ltd., China. Different methyl aromatics
General procedure for the decomposition of tert-butyl
hydroperoxide
In a typical reaction procedure, 7 mmol of TBHP and 1 mmol of
N-alkyl pyridinium salt were placed in a 10 ml ask and stirred
were used aer removing water by adding 5 wt% of 4 A molec- under 80 ꢀC for 1 h in the dark. Then the reaction mixture was
ular sieve and being treated for 24 hours. The gas chromatog- rapidly cooled to room temperature and the conversion of TBHP
raphy (GC) was Agilent-7890 equipped with a ame ionization was determined by iodometric titration.
detector. High-performance liquid chromatography (HPLC)
measurements were performed on Waters e2695 with UV/Vis
Results & discussion
2489 spectrometer. FT-IR spectrums were recorded on Bruker
1
TENSOR 27 spectrometer using KBr pellet (Fig. S1†). H NMR
Initially, the selective oxidation of p-xylene was chosen as
and 13C NMR spectrums were recorded on a Bruker AVANCE III
a model reaction to test the catalytic performance of these N-
alkyl pyridinium salts in the absence of solvent (Scheme 2). And
NMR spectrometer in DMSO-d6 with tetramethylsilane (TMS) as
the internal standard. GC-MS were recorded by Agilent 6890N
the reaction results were listed in Table 1. It can be observed
GC/5973 mass spectrometer (Fig. S2–S6†).
that the investigated N-alkyl pyridinium salts could all catalyze
the oxidation of p-xylene.
The main product is p-toluic acid (TA). Small amount of p-
tolualcohol (TOAL) and p-tolualdehyde (TALD) were also ob-
tained as by-product. The distribution of the product is different
with those results obtained with metal-containing catalyst
General procedure for the synthesis of N-alkyl pyridinium
salts
Different N-alkyl pyridinium salts were synthesized by reuxing
corresponding pyridines with alkyl bromide in a procedure
refereeing to the previously published report.36,38 The synthesis
was carried out under the following procedure: 1 mol of pyri-
dine and 80 ml of toluene were placed in the ask, respectively.
1.1 mol of alkyl bromide were added dropwise into the ask at
70 ꢀC and reuxed for 24 hours. Then, the pyridinium salts
formed as solid and they were washed by acetone for three times
to remove residual materials. Finally they were evaporated and
dried at high vacuum under 70 ꢀC overnight. The structure was Scheme 2 Oxidation of p-xylene.
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Table 1 p-Xylene oxidation in the absence of metalsa
Selectivity (%)
Entry Catalyst
Conv. (%) TA TALD TAOL Othersb
1
2
3
4
5
6
a
b
c
d
39
27
52
48
50
4
72
70
86
75
74
19
54
73
14
13
15
6
14
12
43
20
24
49
10
13
2
5
4
31
19
—
33
5
2
6
6
10
7
7
3
4
e
TBAB
MgCO3
T (p-Cl)PPMnCl 12
Co(II) (DPDME) 15
7c
8d
9e
28
Fig. 1 Decomposition of TBHP with different N-alkyl pyridinium salts.
a
Reaction conditions: 10 mmol of p-xylene, 0.25 mmol of c, 0.2 mmol of
b
p-tolualdehyde as initiator, 1.0 MPa O2, 160 ꢀC, 2 h. Mainly (4-
c
methylbenzyl)-p-toluates, 4-CBA, and TPA. 190 ꢀC, 2 h (ref. 41).
d
180 ꢀC, 3 h, air pressure, 2.0 mp, with acetic acid as solvent (ref. 42).
e
150 ꢀC, 3.5 h, (ref. 43).
being used, in which deep oxidation happened and then
considerable amount of 4-carboxylicbenzaldehyde (4-CBA) and
terephthalic acid (TPA) produced.39 For organic catalysts,
introduction of substitute was an efficient method to modulate
their catalytic activity.28,40 Effect of different substituted groups
was studied in the present work. For the oxidation of p-xylene,
with catalyst a being employed, the conversion reached 39%
(Table 1, Entry 1). Replacing of hydrogen with cyano group, the
obtained catalyst b gave a p-xylene conversion of 27% (Table 1,
Entry 2). While for catalyst c the p-xylene conversion increased
to 52% (Table 1, Entry 3). The conversion of p-xylene increased
in the order of –CN < –H < –N(CH3)2 with the modulation of the
substituted group on the pyridine ring. And the same order was
followed for the changing of the selectivity of TA. The highest
selectivity of 86% for TA could be obtained with catalyst c being
used. In literature, MnCO3, metalloporphyrin T (p-Cl)PPMnCl,
or metallo-deuteroporphyrin Co(II) (DPDME) was reported as
good catalyst for the selective oxidation of p-xylene, and the
conversion was 28%, 12%, and 15%, respectively, which was
obtained even at a much higher reaction temperature of 190 ꢀC
(Table 1, Entry 7–9).41–43 By comparison, the reported catalyst in
the present work showed better catalytic activity and higher
selectivity to TA. In addition, it could also be observed that the
introduction of electron-donating group was positive while the
introduction of electron-withdrawing group was negative for the
obtaining of high catalytic activity and selectivity. Owning to
a resonance stabilization of the positive charge delocalization,
the introduction of electron-donating group could increase the
stability of the pyridinium salts, while the introduction of
electron-withdrawing cyano-group acted the contrary effect.44–46
A free-radical mechanism usually involved during the catalytic
oxidation of hydrocarbon with the organic catalyst, in which
organic hydroperoxide is the primary product which could be
decomposed to the free-radical intermediate and the nal
oxygenated products.31,32 Then, the decomposition of tert-butyl
hydroperoxide (TBHP) was used to primarily evaluate the cata-
lytic performance of different N-alkyl pyridinium salts in the
present work (Fig. 1). It could be found that dimethylamino
Fig. 2 Influence of solvents on p-xylene oxidation. Reaction condi-
tions: 10 mmol of p-xylene, 0.5 mmol of c as catalyst, 0.2 mmol of p-
tolualdehyde as initiator, 5 ml of organic solvent, 1.5 MPa O2, 160 ꢀC,
3 h.
group substituted N-alkyl pyridinium salts c exhibited higher
activity than a and b, which exhibited similar trends with that in
the catalytic oxidation of p-xylene. Similar results were obtained
in the process of the decomposition of CHHP.32 The catalytic
ability of decomposing organic hydroperoxide is an important
factor for the catalytic performance. The above two factors
probably led to the difference on the catalytic performance in
the present work. Furthermore, with the variation of N-alkyl
substituted group, no remarkable differences were obtained.
When catalyst d and e were used, comparable p-xylene conver-
sion of 48% and 50% were obtained, respectively (Table 1, Entry
4 and 5). Control experiment using conventional quaternary
ammonium salts of tetrabutyl ammonium bromide (TBAB) was
also carried out. Under the same reaction conditions, only 4%
of p-xylene was converted (Table 1, Entry 6).
Though high selectivity for TA could be obtained for the
solvent-free oxidation of p-xylene, the conversion of p-xylene was
not satised. It was acknowledged that the solubility of TA in
the weak polar aromatic media is low. With the increasing of TA
solid products during the reaction, further reaction was difficult
to carry on. Then different organic solvents were introduced to
improve the reaction, and the results were shown in Fig. 2. To
our delight, higher catalytic activity was obtained with the polar
solvent such as acetonitrile, acitic acid, or dimethylformamide
being used, respectively. The oxidation efficiency was greatly
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product aer 4 h. Different from the data under solvent-free
conditions, the selectivity for TOAL was negligible in acetoni-
trile even at the initial time. Selectivity for TALD rapidly
decreased with the time manifesting that TALD was converted
to TA with the time. At the same time, the selectivity of deep
oxygenated products such as 4-CBA or TPA also increased
slowly. Under the optimized reaction time, the maximum yield
for TA was obtained aer 4 h with a p-xylene conversion of 95%
and TA selectivity of 84%. In addition, the effects of catalyst
concentration and reaction temperature on the catalytic
performance were also studied, respectively (Fig. S7 and S8†).
To extend the scope of this catalytic system, the oxidation of
different methyl aromatic hydrocarbon were investigated using
catalyst c under the optimized conditions (Table 2). o-Xylene
and p-tert-butyl toluene could also be oxidized with the
conversion of 96% and 93%, respectively (Table 2, Entry 1 and
2). While for the case of m-xylene and mesitylene, the conver-
sion was 63% and 56%, respectively (Table 2, Entry 3 and 4). In
the oxidation of methyl aromatics, electro-donating substitute
was favorable for the activation and conversion of methyl group.
This kind of promotion effect was weaker for the electro-
donating substitute when it emerges at the meta position,
which caused a lower conversion. Though toluene was even
more difficult to be oxidized, 15% of conversion could also be
obtained under the same reaction conditions (Table 2, Entry 5).
These results indicated that the reported catalyst in the present
Fig. 3 Influence of reaction time on p-xylene oxidation. Reaction
conditions: 10 mmol of p-xylene, 0.5 mmol of c, 0.2 mmol of p-tol-
ualdehyde as initiator, 5 ml of acetonitrile, 1.5 MPa O2, 160 ꢀC.
enhanced in acetonitrile, and 88% of p-xylene conversion was
obtained with a TA selectivity of 83%, which were higher than
those obtained in acetic acid or dimethylformamide. Then,
acetonitrile was selected as the solvent for the further study.
In order to better understand the reaction process, detailed
study with different reaction time was carried out in acetonitrile
using catalyst c (Fig. 3). p-Xylene conversion rapidly increased
during the rst 3 h, then it became steady aer 4 h. For the
distribution of oxygenated products, TA was still the dominant
Table 2 Catalytic oxidation of different methyl aromatics a
Entry
Substrate
Conv. (%)
Main product selectivity (%)
1b
96
2
93
3
4
63
56
15
5
a
Reaction conditions: 10 mmol of substrate, 0.5 mmol of c, 5 ml of acetonitrile, 0.2 mmol of p-tolualdehyde as initiator, 1.5 MPa O2, 160 ꢀC, 4 h.
The main by product is o-tolualdehyde.
b
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substituted N-alkyl pyridinium salts exhibited a better catalytic
activity. Especially with 1-benzyl-4-N,N-dimethylaminopyr-
idinium bromide being used as the catalyst, different methyl
aromatics were successfully oxidized in acetonitrile. The re-
ported results offered a simple organocatalytic system for the
metal-free oxidation of methyl aromatics. Intensive study on the
role of N-alkyl pyridinium salts and its application for the
catalytic oxidation of other substrate are underway.
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
This study was nancially supported by the National Natural
Science Foundation of China (21872075 and 21873052) and by
K. C. Wang Magna Fund in Ningbo University.
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