Catalytic properties of: N -hydroxyphthalimide immobilized on a novel porous organic polymer in the oxidation of toluene by molecular oxygen

Article abstract of DOI:10.1039/c6ra14936g

By molecular design, using a micro-sized macroporous silica mold, N-hydroxyphthalimide (NHPI) was synthesized and immobilized on a novel porous organic polymer (POP) polystyrene (pSt). The POP pSt was prepared using surface-initiated graft polymerization

Full text of DOI:10.1039/c6ra14936g

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Page 1 of 16
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Catalytic Property of N-hydroxyphthalimide Immobilized on A Novel Porous
Organic Polymer in Oxidation of Toluene by Molecular Oxygen
Meng Jian, Cui Jianlan*, Luo Dongmei, Xie Meina
(Department of Chemical Engineering, North University of China, Taiyuan, China, 030051)
Abstract: By molecule designing, based on microꢀsized macroporous silica mold, Nꢀhydroxyphthalimide (NHPI) was
synthesized and immobilized on a novel porous organic polymers (POPs) polystyrene (pSt). The POPs pSt was
prepared by surfaceꢀinitiated graft polymerization and etched away the silica matrix. Subsequently, based on
1
,4ꢀdichloro methoxy butane used as chloromethylation reagent, chloromethylation POPs was obtained through
FriedelꢀCrafts alkylation reaction. Next phthalic anhydride was bonded onto the POPs through the esterification
reaction, which was carried out between the chloromethyl groups on POPs and the carboxyl group of trimellitic
anhydride (TMA), obtaining the products POPsꢀPA. And then the bonded phthalic anhydride (PA) reacted with
hydroxylamine hydrochloride, NHPIꢀimmobilized heterogeneous material pSt/NHPI was obtained. The products
obtained were fully characterized. Finally, a coꢀcatalyst system was constituted by solid catalyst pSt/NHPI and a little
of Co(OAc) , which was used in the oxidation reaction processes of toluene with molecular oxygen as oxidant at lower
2
temperature and ordinary pressure of oxygen. The experimental results showed that the coꢀcatalyst system can make
toluene deeply oxidized into benzoic acid, and this combination catalyst also possessed high catalytic activity, excellent
catalytic selectivity and recyclability.
Keywords: Silica gel particles; Porous organic polymer; Nꢀhydroxyphthalimide; Immobilization; molecular oxygen;
catalytic oxidation
Benzoic acid is an important feedstock，which has been widely used in various areas, such as an
intermediate for caprolactam, dyes and perfume, an auxiliary agent for rubber industry, and also an
[1ꢀ3]
agent for preservative or intermediate reaction . Up to now, the production of benzoic acid is
[4,5]
heavily relied on the oxidation of toluene, which requires expensive reagents and harsh conditions
.
Therefore, the direct green oxidation of toluene to benzoic acid with air or molecular oxygen has
attached extensive attention. At present, several sophisticated oxidation catalyst systems which are
[6ꢀ10]
able to use molecular oxygen as the end oxidant have been introduced
. The combination of
Nꢀhydroxyphthalimide (NHPI) and some mediators is attractive and promising for the oxidation of
[11ꢀ14]
alcohols in homogeneous catalyst systems
. Moreover, it is difficult to separate and recover NHPI
after the reaction, whereas immobilizing NHPI catalyst on solid supports can overcome this
shortcoming. Now the heterogenization of homogeneous catalysts has become a hot subject, whereas
[15ꢀ18]
the research in immobilized NHPI is scarce
. In this study, NHPI is immobilized on a new POPs
pSt.
MENG Jian (1990ꢀ ): Male, Doctor Graduate, Shanxi Huairen person, mainly research functional polymer materials
Communication email: mengjian9067@163.com.

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Porous organic polymers (POPs) have attracted tremendous attention in both academia and
industry owing to their unique functionalization and relatively excellent stability, and have widely
[19ꢀ22]
applied in gas storage, separation and catalysis
. As is known to all, silica gel (SiO ) exhibiting
2
excellent mechanical property and fine thermal and chemical stability, such as relatively good stability,
[23,24]
a great large pore surface area, good adsorption capacity and an ordered pore arrangement
, has
emerged as an inorganic solid support and has been widely used in polymer composites. However,
[25ꢀ27]
many researches focus on its external surface but neglect to utilize its abundant pore structure
,
herein, a new porous organic frameworks have been designed and synthesized through novel and
convenient method base on silica gel particles.
Constructing mercapto group/AIBN(ꢀSH/AIBN) redox initiating system, the mercapto groups on
the surfaces of modified particles MPMS/SiO with the decomposition of initiator AIBN in DMF
2
solution, on which the hydrogen atom transferred to one free radical of benzoic acid so that sulfur free
radicals were obtained on the surfaces of silica gel particles, and the graft polymerization of St was
caused on the surface of silica gel particles, preparing the grafted particles pSt/SiO , then etched away
2
the silica matrix and formed the POPs pSt. And then 1,4ꢀbis(chloromethoxyl) butane (BCMB),
trimellitic anhydride (TMA) and hydroxylamine hydrochloride were used as the reaction reagent,
respectively, through three macromolecular reactions, chloromethylation reaction, esterification and
acyl imidization, and eventually immobilized material pSt/NHPI was obtained. The above entire
chemical process can be showed schematically in Scheme 1.

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Scheme 1. Chemistry of preparing the heterogeneous catalyst pSt/NHPI
In this communication, the aerobic oxidation of toluene into benzoic acid working together with
the coꢀcatalyst Co(OAc) was conducted, which used molecular oxygen as final oxidant by
2
lowꢀtemperature and normalꢀpressure of oxygen operation, and the possible catalytic oxidation
mechanism was discussed. The results show that it has a certain application value in the field of solid
supported catalysis.
2
Experimental Section
2
.1 Chemicals and materials
Silica gel (about 125 ꢁm, Haiyang Chemical Limited Company, Qingdao, China).
γꢀmercaptopropyl trimethoxysilane (MPMS, Nanjing Chuangshi Chemical Aux Ltd., Jiangsu, China).
Styrene (St, Suzhou Nanhang Chemical Ltd., China), purification by vacuum distillation before use.
2
,2’ꢀazobisisobutyronitrile (AIBN, 98% purity, Shanghai Zhongli Chemical Plant, Shanghai, China),
purification by recrystallization from ethanol before use. Trimellitic anhydride (TMA, Tianjin
Guangfu Fine Chemical Ltd., China), Toluene, Aceticacid, N,Nꢀdimethylformamide(DMF),
Dimethylacetamide(DMAC)(Tianjin Shentai Chemical Ltd., China). Transition metal salts:
Co(OAc) 、CuCl 、Fe(acac) 、MnSO (Shanghai Jingchun Biochemical Ltd., China). Hydroxylamine
2
2
2
4
hydrochloride (Ruishuo Chemical Limited Company, Shanghai, China). Other reagents were of
analytical pure grade and were used without further purification.
2
.2 Apparatus

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PerkinꢀElmer 1700 Infrared Spectrometer (FTIR, PerkinꢀElmer Company, USA); Unicꢀ2602 UV
spectrophotometer (Unic Company, Shanghai); LEOꢀ438VP scanning electronic microscope (SEM,
LEO Company, UK); STA449 thermogravimetry analyzer (TGA, Netzsch Company, German);
ASPA2000 Physisorption (Micromeritics Company, USA); GCꢀ2010 gas chromatograph (GC,
Shimadzu Company, Japan). PEꢀ2400II Elemental Analyzer (EA, PerkinꢀElmer Company, USA).
2
.3 Preparation and characterization of pSt/NHPI
2
.3.1 General procedure for preparation of POPs pSt
According to the operation steps of Ref. [28,29], the graft polymerization of St was caused on the
surface of silica gel particles, and the specific operation steps were as follows. Firstly, silica gel
particles were treated for activation by using aqueous solution of hydrochloric acid as activation
reagent. Secondly, activated silica gel particles were surfaceꢀmodified with coupling agent MPMS,
obtaining the modified products MPMS/SiO , whose mercapto group content was 1.08mmol/g. Using
2
AIBN as initiator and DMF as solvent, the graft polymerization of St on the surfaces of MPMS/SiO2
particles were carried out, obtained the grafting particles pSt/SiO , then transferred them into
2
polyethylene beaker and added 30ml of 40% hydrofluoric to etch away the silica gel, and followed by
washing and drying, finally, the porous organic polymers pSt was prepared. The grafting degree of the
pSt/SiO particles was determined by TGA.
2
2
.3.2 Immobilization of NHPI on the POPs pSt
1) Chloromethylation of the POPs
According to the procedure ascribed in Ref.[30], using selfꢀmade 1,4ꢀbis(chloromethoxyl) butane
BCMB) as the chloromethylation reagent and adopting SnCl as Lewis acid catalysts, the reaction
(
(
4
was performed at room temperature in a dichloromethane solvent for 9h. The amount of the chlorine
content on the product CMꢀpSt was determined by Calorimeter and Volhard method.
(2) Bonding of phthalic anhydride on CMꢀpSt
0
.5g of CMꢀpSt were placed in a fourꢀnecked flask equipped with a mechanical agitator and a
reflux condenser, followed by adding 30mL of DMF as solvent and soaking for 5h. And then 1.3g of
TMA was dissolved in 10mL of DMF, and the mixture was added into the flask. Afterwards, 0.5mL of
triethylamine (Et N) as catalyst was added into the reaction system, the esterification reaction between
3
chloromethyl groups on CMꢀpSt and the carboxyl group of TMA was performed at 85℃ for 5h. After
reaction finished, the concentrations of TMA in the supernatants were measured by ultraviolet
spectrophotometry at 291 nm, which showed that the conversion of chloromethyl of CMꢀpSt can
reaches 70%. The product pSt/PA was fully washed with DMF and aether in turn for several times,
and then filtrated and dried under vacuum.

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(3) Synchronously synthesizing and immobilizing NHPI on pSt/PA
In a fourꢀnecked flask, 1g of pSt/PA was dipped in 30 mL of pyridine for 5h in order to fully
swell the polymeride, followed by adding 0.8g of hydroxylamine hydrochloride as reaction reagent.
The reaction between the bonded phthalic anhydride (PA) and hydroxylamine hydrochloride was
conducted at 90℃ with stirring for 15 h. After the reaction, the product was acidized by adding
diluted hydrochloric acid. The resultant product was washed repeatedly with distilled water and
ethanol, collected by filtering and dried under vacuum. Finally, the heterogeneous catalyst pSt/NHPI
was obtained, on which the immobilization amount (Q) of NHPI was measured by weighing method,
and it was 2.25mmol/g.
Q = (m ꢀm )/M*1000
2
1
In this formula, m and m are the quality of pSt/PA and pSt/NHPI (g), and M is the sum of molar
1
2
mass of N and H atoms, which is about 15 g/mol.
2
.4 Catalytic oxidation of toluene with molecular oxygen
The catalytic oxidation of toluene at the presence of molecular O was conducted in a reactor
2
equipped with a mechanical stirrer, thermometer, O inlet and reflux condenser, then 40 mL of
2
aceticacid and 1.0 g of pSt/NHPI were added and dissolved for 5h, and followed by adding 2ml of
toluene and 0.04g of Co(OAc) as the coꢀcatalyst. Oxygen at the fixed flow rate which was 16 mL/min
2
was passed into the reaction system. The oxidation reaction was carried out at 80℃ and continuously
stirred for 30 h. Samples of the reaction mixture were taken at fixed time intervals, and then measured
by a gas chromatograph with the internal standard method (N as carrier gas, PEGꢀ20M Capillary
2
Column and FID detection), showing that benzoic acid was the main product and a small amount of
byproduct benzaldehyde. After the oxidation reaction, the heterogeneous catalyst pSt/NHPI was
soaked, washed with distilled water and ethanol in turn to completely remove the organic matter
physically attached on the particles, and dried under vacuum. The recovered particles were reused in
the oxidation reaction of toluene under the same conditions to examine their recycling performance.
3
3
. Results and Discussion
.1 Synthesis of NHPI catalyst
Constructing mercapto group/AIBN(ꢀSH/AIBN) redox initiating system, and the graft
polymerization of St was caused on the surface of silica gel particles, preparing the grafted particles
pSt/SiO , followed by adding hydrofluoric acid to dissolve the silica gel, and then synchronously
2
synthesized and immobilized NHPI on the POPs via several steps of polymer reaction, resulting in the
heterogeneous catalyst particles pSt/NHPI.
3
.1.1 Effects of main factors on the immobilization of NHPI

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The NHPIꢀimmobilized heterogeneous catalyst was prepared via three macromolecular reaction
steps. The chloromethylation of the phenyl on the polymer has been fully examined by our group, and
the optimum reaction conditions were determined, the reaction of the bonded phthalic anhydride (PA)
with hydroxylamine hydrochloride was also a mature chemical reaction, however, the reaction of
CMꢀpSt with TMA was a key step. So in this study the effect of reaction parameters on the high
bonded amount of phthalic anhydride (PA) was investigated, the optimum reaction conditions were
established.
By fixing the other reaction conditions, Figure 1 gives the rate of concentration chloromethyl
group conversion (CGC) change versus time at different temperatures. The results clearly showed that
the esterification reaction rate increased with temperature, because the chloromethyl group conversion
at 85℃ was very close to that at 90℃, a suitable reaction temperature was 85℃.
60
45
30
15
0
5
5
℃
℃
℃
70
85
℃
℃
8
0
95
1
2
3
4
5
Time, h
Figure 1. Variation of chloromethyl group conversion rate of CMꢀpSt at different temperatures
Table 1. Dielectric constant data for various solvents
a
a
DMAC/Dioxane
DMF/Dioxane
(V/V=1:1)
Solvent
DMAC
DMF
36.7
(V/V=1:1)
Dielectric constant ε
3
7.8
20.0
19.5
(25℃)
a
The dielectric constant of the mixied solvent was calculated according to molar ration of the two solvents.
Table 2. Effect of reaction conditions on the esterification reaction
Reaction
b
Entry
Temperature(℃)
Catalyst
Solvent
CGC(%)
time(h)
a
1
2
3
85
85
85
Et N
DMF/Dioxane
5
5
5
57.3
59.5
65.5
3
a
Et N
3
DMAC/Dioxane
Et N
3
DMF

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4
5
6
7
85
85
85
85
Et N
DMAC
DMAC
DMAC
DMAC
5
5
5
5
68.2
61.9
46.2
34.6
3
Bu N
3
NaOH
Na CO3
2
a
The volume ratio of two solvents was 1:1.
b
The used amount of catalyst in this reaction were 15mmol. Et
3
N (Triethylamine), Bu
3
N (Triꢀnꢀbutylamine).
In the presence of HCl agent, the reaction between chloromethyl group of CMꢀpSt and carboxy
of TMA is a nucleophilic substitution reaction. Carboxy hydroxyl loses hydrogen proton and translates
into a carboxylic acid anion as a nucleophilic attack group, while chloromethyl loses chlorine
translates into carbenium ion which is closely associated with the polarity of the solvent. A higher
solvent polarity gives a stronger dipoleꢀdipole interaction of solvent and nucleophilic reagent, which is
helpful for removing the chlorine atom and attacking the carboxylic acid anion. Obviously, the
polarity of the solvents can promote the nucleophilic substitution reaction. Based on this, DMAC
should be selected as the suitable solvent. At the same time, the suitable catalyst was Et N which can
3
be well dissolved in the reaction system.
3
.1.2 Characterization of functional particles pSt/NHPI
As shown in Figure 2, the chemical modification of SiO was confirmed by IR spectroscopy.
2
POPsꢀpSt
POPsꢀpSt
CMꢀpSt
pSt /SiO2
pSt/PA
pSt/NHPI
SiO2
4000
3500
3000
2500
2000
1500
1000
4000 3500 3000 2500 2000 1500 1000
−
1
−1
Wavenumber (cm )
Wavenumber (cm )
Figure 2. IR spectra of the reactants
ꢀ1
ꢀ1
ꢀ1
In the spectrum of SiO at 3438 cm , 1126 cm , around 800 cm are observed, these bands are
2
assigned to the silanol groups as well as the adsorbed water, the antisymmetric stretching vibration
absorption of SiꢀOꢀSi bond and the symmetrical stretching vibration absorption of SiꢀO bond. In the
spectrum of pSt/SiO , the characteristic absorption of the methylene groups of the main chain of the
2
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
polymer appeared at 2928 cm , while the peaks at 1610 cm , 1500 cm , 1450 cm and 674 cm are
ascribed to skeleton vibration of benzene ring on polystyrene. In the spectrum of POPsꢀpSt, it can be

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ꢀ1
ꢀ1
clearly observed that the absorption peak at 800 cm and 1126 cm has disappeared after silica gel
dissolved. In the spectrum of CMꢀpSt, except for the characteristic absorption bands of POPsꢀpSt, the
ꢀ1
ꢀ1
appearance of two absorption bands at 1421 cm and 670 cm was due to the chloromethyl group
–
CH Cl. In the spectrum of pSt/PA, the adsorptions of chloromethyl group were much weakened,
2
ꢀ1
whereas the characteristic absorption band of ether group CꢀOꢀC appeared at 1230 cm , also two
ꢀ1
ꢀ1
characteristic absorption bands of cyclic anhydride appeared at 1782 cm and 1724 cm , which
attributed to asymmetry and symmetry coupling vibration adsorption of anhydride carbonyl,
respectively. In the spectrum of pSt/NHPI, the characteristic absorption band of free radicals group
ꢀ1
ꢀ1
NꢀO appeared at 1365 cm , the strong absorption band at 3443 cm represented the ꢀOH stretching
vibration absorption, all of the above changes of spectrum data sufficiently confirmed that pSt/NHPI
particles were prepared.
Figure 3. SEM images of four kinds of particles
(
a) Silica particles SiO ; (b) Graft particles pSt/SiO2
2
(c) POPs pSt; (d) Functional POPs pSt/NHPI
Figure 3 demonstrated the change of structure of particles in the course of chemical modification
of SiO . The surfaces of raw silica gel particles(a) were rough and scraggy, whereas the surfaces of the
2
grafted particles pSt/SiO (b) became smoother and flatter. It was caused by the coating and filling up
2
action of the functional macromolecule. It can be observed from (c) and (d) that with the corrosion of

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silica gel, the obtained polymeric network samples were composed of loosely agglomerated particles
with irregular shape and a large number of hollow apertures.
0.03
SiO2
pSt/NHPI
pSt
0.02
0.01
0.00
1
10
Pore Width (nm)
100
Figure 4. Pore size distribution for pSt and pSt/NHPI
The porosity parameters of the POPs were summarized in Tab.1, the porous skeleton of SiO2
mainly composed of macropores, whereas the porous skeleton of the two POPs mainly composed of
mesopores, the surface area and pore volume of the POPs decreased with NHPI immobilized. It is
mainly due to the reagent NHPI bonded to the pores through several steps of polymer reaction.
Table 3. Textural properties of samples
Pore size distribution/%
2
3
Sample
ABET(m /g)
Vtotal(cm /g)
microꢀ mesoꢀ macroꢀ
SiO2
325
343
249
0.35
0.43
0.38
1.3
5.7
9.9
15.5
91.2
85.8
83.2
3.1
POPs pSt
POPs pSt/NHPI
4.3
The thermal stabilities of the graft particles were evaluated by thermogravimetric analysis (TGA)
from room temperature to 800℃ under a nitrogen atmosphere.
1
00
9
8
7
6
5
0
0
0
0
0
pSt /SiO2
1
00
200
300
400
T /
500
600
700
℃
Figure 5. TGA curves of pSt/SiO particles
2
The results were presented in Figure 5. It can be found that the graft polymer pSt/SiO exhibited
2

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higher decomposition temperature about 380℃, indicating excellent thermal resistance to be used as
supported materials.
Contents of nitrogen, carbon and hydrogen in pSt/NHPI were determined by the PEꢀ2400II EA.
The experimental conditions were as follows: The dosage of sample was 1.5mg, the temperature of
combustion tube was set at 950℃, acetanilide was used as the standard sample.
Table 4. Elemental analysis
Element
C
H
N
Content (wt%)
72.84
5.10
3.11
The immobilization amount of NHPI was determined by the content of nitrogen in Table 4, and
the determination result 2.23mmol/g was coincident with that of the above chemical analysis.
3
.2 Catalytic characteristics of pSt/NHPI
The oxidation of toluene was catalyzed by the combination catalysts of pSt/NHPI and Co(OAc)2
using molecular oxygen as final oxidant, Figure 6 showed that it can make toluene to be highly
effectively transformed into benzoic acid with a yield of 48% at 80℃ in 30 h. In order to make
comparison, the catalytic experiment was also carried out only using pSt/NHPI, Co(OAc) and
2
nothing at all, and results showed that the oxidation of toluene almost didn’t occur in the three
systems.
6
0
5
6
4
3
1
0
5
0
5
0
Toluene
Toluene
pSt/NHPI+Co(OAc)2
4
pSt/SiO /NHPI+Co(OAc)
pSt/NHPI+Co(OAc)2
2
2
50
40
30
20
Benzoic acid
30
50
40
30
20
10
Benzoic acid
15
Benzaldehyde
pSt/NHPI
10
Co(OAc)2
1
0
15 20 25 30 35
Without Cat.
0
5
10 15 20 25 30 35
.
5
10
15
20
25
30
35
5
10
15
20
Time, h
25
30
35
Time, h
Figure 6. Variation curves of toluene conversion with time
using different catalyst systems
Figure 7. Effect of two particles on toluene
conversion with time
(Temperature: 80℃, Solvent: Aceticacid)
(Temperature: 80℃, Solvent: Aceticacid)
The catalytic experiments also were conducted with the particles pSt/SiO /NHPI and Co(OAc)2
2
in Figure 7. It also can be clearly seen that the pSt/NHPI catalyst showed higher activities than the
pSt/SiO /NHPI catalyst, because the reactant concentrations between the catalysts were different. The
2
former catalyst had more abundant pore structure, thus creating a much higher reactant concentration

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in the catalyst pores when compared with the latter. According to the Arrhenius equation, high reactant
concentrations leaded to strong catalytic activity.
5
4
3
2
1
0
0
0
0
0
0
Co(OAc)2
Table 5. Standard electrode potential of relevant redox couples
MnSO4
Fe(acac)2
CuCl2
Entry
φ /V
3
+
2+
1
2
3
4
Co +e
Co
1.84
1.51
0.77
0.15
3
+
2+
Mn +e
Mn
3
+
2+
Fe +e
Fe
2
+
+
Cu +e
Cu
10
15
20
25
30
35
Time, h
Figure 8. Variation curves of the yield of benzoic acid
with time using different coꢀcatalyst
(Temperature: 80℃, Solvent: Aceticacid)
The catalytic activity of several different coꢀcatalysts was displayed in Figure 8 and its order was
Co(OAc) >MnSO >Fe(acac) >CuCl . For the four coꢀcatalysts, the yield of benzoic acid increased
2
4
2
2
with the advance of the reduction potential of the coꢀcatalysts. As described in scheme 2 in the
catalytic oxidation of toluene, that high valence metal ions triggers the generation of the nitrogen
carbonyl cation which was a critical step.
Table 6. Effect of reaction conditions on catalytic reaction
m(combination
Reaction
time(h)
30
Entry
Temperature(℃) n(NHPI):n(Co(OAc)2)
Yield (%)
catalyst)/g
1.0
1
2
3
4
5
6
7
8
9
80
80
80
80
80
90
70
60
80
80
80
10:1
12:1
14:1
16:1
17:1
14:1
14:1
14:1
14:1
14:1
14:1
39.5
41.6
45.6
43.3
38.3
45.9
36.3
21.1
38.5
47.8
48.1
1.0
30
1.0
30
1.0
30
1.0
30
1.0
30
1.0
30
1.0
30
0.95
1.04
1.06
30
1
0
30
11
30

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As shown in Table 6, by fixing the added amount of the combination catalysts (1.0g), the mole
ratio of the immobilized NHPI to Co(OAc) was changed in series. When the molar ratio is equal to
2
1
4:1, it can make toluene to be highly effectively transformed into benzoic acid with a yield of 45.6%
in 30 h. The effect of the amount of the combination catalyst in the oxidation reaction was examined.
With increasing amount of coꢀcatalysts, the yield of benzoic acid increased, so for this reaction system,
the suitable amount of the combination catalyst was 1.04g in which the mole ratio of the immobilized
NHPI to Co(OAc) is 14:1. The results also clearly showed that the catalytic reaction rate increased
2
with temperature, because the immobilized amount of NHPI at 80℃ was very close to 90℃, a
suitable reaction temperature is 80℃.
3
.2.1 Catalytic Selectivity.
As described above, for the catalytic oxidation of toluene with the coꢀcatalyst system in this
study, the obtained product was mainly composed of benzoic acid, and a small amount of byproduct
benzaldehyde. For the oxidation system containing pSt/NHPI and Co(OAc) , Figure 9 demonstrated
2
the changing curves of the yields of benzoic acid and benzaldehyde with time, respectively.
50
40
30
20
10
Benzoic acid
90
8
0
9
8
8
7
.0
.5
.0
.5
70
Benzaldehyde
7.0
6
5
0
0
6
6
.5
.0
5.5
5
10 15 20 25 30 35
Time, h
5
10
15
20
25
30
35
Time, h
Figure 9. The yields of benzoic acid and benzaldehyde as well as selectivity of benzoic acid with time
Temperature: 80℃, Solvent: Aceticacid)
(
As is shown in Figure 9. During the oxidation process of toluene, both the yield and selectivity of
benzoic acid increase with time in the process. After 30 h, the yield and selectivity of benzoic acid
level off, the selectivity is 86% and the yield is 48%. For the byproduct benzaldehyde, the yield
increases at beginning and then declines with time, the reason for that is probably associated with the
further oxidation of benzaldehyde to benzoic acid. In summary, in the oxidation of toluene by
molecular oxygen, the heterogeneous catalyst pSt/NHPI in combination with a small amount of the
coꢀcatalyst Co(OAc) has high catalytic selectivity for benzoic acid.
2
3
.2.2 Mechanistic studies

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[13,31,32]
A mechanism for this catalytic oxidation is based on previous studies.
It can be described
as a cascade of redox reactions containing three cycles which is shown in Scheme 2.
Scheme 2. Proposed mechanism for the catalytic aerobic oxidation of toluene
During the redox process, with the help of the complexation of Co(II) of Co(OAc) , a series of
2
electron transfer is initiated with dioxygen in Cycle I. The key step in the total oxidation process is
that the PINO radicals generate via the homolytic cleavage of the hydroxyl group of NHPI (Cycle I).
Subsequently, the hydrogen atom of the substrate toluene is taken away by the PINO radical and
formed a carbon free radical, so as to open the free radical chain reaction which includes chain
initiation, propagation and termination steps. Meantime, hydroperoxide will be formed, and with its
subsequent decomposition, eventually toluene is oxidized to benzoic acid and benzaldehyde. With
repeating of the three cycles, toluene is continuously transformed into benzoic acid.
3
.2.3 Reuse property of the catalyst
The cyclic repeated experiments were carried out by using the combination catalyst, which
contained pSt/NHPI and Co(OAc) , and the supported catalyst pSt/NHPI was used in each catalytic
2
oxidation which should be separated from the reaction mixture for reuse.
70
60
50
40
30
20
1
2
3
4
5
6
7
Number of cycle
Figure 10. Effect of cycle number on catalyst activity

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(Temperature: 80℃, Solvent: Aceticacid, Reaction time: 30 h)
The experimental results were shown in Figure 10. The catalytic activity declined a little at the
second recycle during the reuse of 7 times (toluene conversion fallen obviously from 56% to 43%),
after that, the catalytic activity was quite stable (toluene conversion was at about 44%). In addition,
the appearance of pSt/NHPI was kept well during the recycling. Therefore, it can be concluded that
this immobilized catalyst has excellent reuse performance.
4
. Conclusions
Via several steps of polymer reaction in this investigation, a novel porous organic polymeric
support pSt for the immobilization of NHPI has been prepared. The experimental results show that the
reaction of CMꢀpSt with TMA is a key step, the results indicate that DMAC is a suitable solvent, and
the optimal reaction temperature is 80℃ in the esterification reaction. The supported catalyst
pSt/NHPI could oxidate toluene into benzoic acid with the coꢀcatalyst Co(OAc) using molecular
2
oxygen as final oxidant, the superiority of this catalyst over others includes a very higher catalytic
activity. In summary, an efficient and convenient method is designed to support NHPI, and
demonstrated the effectiveness of the heterogeneous catalyst pSt/NHPI in the catalytic oxidations of
hydrocarbons.
Acknowledgements
We really appreciate the National Natural Science Foundation of China(Project No. 21404093),
Shanxi Province Youth Science and Technology Foundation(Project No. 2013021009ꢀ1) and Fund for
Postgraduate of North University of China (Project No. 20151230).
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Graphical abstract
Catalytic Property of Nꢀhydroxyphthalimide Immobilized on A Novel Porous
Organic Polymer in Oxidation of Toluene by Molecular Oxygen
MENG Jian, CUI Jianlan*, Luo Dongmei，Xie Meina
In this work, a novel porous organic polymers(POPs) pSt has been prepared based on
micro-sized macroporous silica gel and was used for the immobilization of NHPI, the obtained
heterogeneous catalyst pSt/NHPI could adsorb toluene and oxidate it into benzoic acid working
together with the co-catalyst Co(OAc) by molecular oxygen. The experimental results show that
2
this combination catalysts possess high catalytic selectivity and catalytic activity, and transforming
toluene into benzoic acid with a yield of 48% in 30h.
OCH3
CH3
C
CH3
OCH3
O
Si CH CH CH S
SiO2
SiO2
O
Si CH CH CH SH H3C
2
2
2
+
N
N
N
C
C
CH3
2
2
2
C
N
OCH3
OCH3
AIBN
CH2
HC
CH2 CH
SiO2
n
St
pSt/SiO2
SnCl4
R
CH CH2
+ ClCH OꢀCH ꢀOCH Cl
2
2
2
R
CH CH2
n
4
n
(POPs) pSt
O
CH Cl
2
O
C
CMꢀpSt
OH Et N
3
OH
O
R
CH CH2
n
O
O
O
C
pSt/PA
H C
2
O
R
CH CH2
O
n
O
O
C
N OH
O
H C
2
O
pSt/NHPI