N-hydroxyphthalimide incorporated onto Cu-BTC metal organic frameworks: An novel catalyst for aerobic oxidation of toluene

Article abstract of DOI:10.1007/s11164-015-2384-8

N-hydroxyphthalimide (NHPI) incorporated onto a Cu-based metalorganic framework [NHPI/Cu-BTC (BTC:1,3,5-benzenetricarboxylate)] was prepared and used as a heterogeneous catalyst for aerobic oxidation of toluene under solvent-free conditions. Cu-BTC was sy

Full text of DOI:10.1007/s11164-015-2384-8

Res Chem Intermed
DOI 10.1007/s11164-015-2384-8
N-hydroxyphthalimide incorporated onto Cu-BTC
metal organic frameworks: An novel catalyst
for aerobic oxidation of toluene
Li Bao1 Xingxing Li1 Zhaowei Wu1
•
•
•
Xia Yuan1 He’an Luo1
•
Received: 25 July 2015 / Accepted: 30 November 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract N-hydroxyphthalimide (NHPI) incorporated onto a Cu-based metal–
organic framework [NHPI/Cu-BTC (BTC:1,3,5-benzenetricarboxylate)] was pre-
pared and used as a heterogeneous catalyst for aerobic oxidation of toluene under
solvent-free conditions. Cu-BTC was synthesized through a hydrothermal and a
solvothermal method with modification. The NHPI/Cu-BTC catalysts were syn-
thesized by an impregnation method and characterized by scanning electron
microscopy, X-ray diffractometry, Fourier transformed infrared spectroscopy,
thermogravimetric analysis, and nitrogen adsorption. Results show NHPI was
successfully incorporated onto Cu-BTC metal organic frameworks. The NHPI/Cu-
BTC showed high catalytic performance in the aerobic oxidation of toluene. In the
presence of 1 mol% NHPI/Cu-BTC and absence of any co-catalyst or co-solvent,
the toluene oxidation was conducted at 110 °C for 2 h with 7.6 % conversion and
more than 50 % benzaldehyde and benzyl alcohol selectivity. The structure stability
of the catalysts after recovery was also studied.
Keywords Metal–organic framework Á N-hydroxyphthalimide (NHPI) Á Aerobic
oxidation of toluene Á Solvent-free
Introduction
The toluene oxidation is an important transformation in the chemical industry
because the major products including benzaldehyde, benzyl alcohol, and benzoic
acid are widely used to synthesize a diversity of useful chemicals, such as
pharmaceuticals, dyestuffs, perfume, foodstuffs, preservatives, and rubber auxiliary
&
Xia Yuan
yxiamail@163.com
1
School of Chemical Engineering, Xiangtan University, Xiangtan City 411105, Hunan, People’s
Republic of China
123

L. Bao et al.
agents [1, 2]. Nowadays, benzaldehyde and benzyl alcohol are more valuable than
benzoic acid, and are especially needed in the industry. Currently, benzaldehyde is
commercially prepared from the chlorination of toluene and then hydrolysis, which
leads to some serious problems such as equipment corrosion and environmental
pollution. Moreover, the resulting product is limited to synthesizing some high-
quality compounds such as perfumes or pharmaceuticals for the inevitable produc-
tion of chlorine [3, 4]. As reported, the direct oxidation from toluene to
benzaldehyde is an environment-friendly method, and the resulting chlorine-free
product can be used in perfumes and pharmaceuticals. In industry, caprolactam is
synthesized from the SNIA process. In this process, toluene oxidation, as the first
step, is carried out at 165 °C and 1.0 MPa catalyzed by cobaltous salts [5]. In
addition, toluene is a starting material with the conversion of 14–15 %, which is
mostly oxidized to benzoic acid with only a trace amount of benzaldehyde and
benzyl alcohol (BD ? BA) formed as by-products [6]. Therefore, the controlled
oxidation from toluene to BD ? BA using new catalytic materials under mild
conditions has attracted much attention.
N-hydroxyphthalimide (NHPI) used in autoxidation reactions was first reported
in 1986 [7]. Later it was reported that NHPI can be combined with transition metal
salts in the presence of oxygen to efficiently oxidize alkanes [8], alcohols [9], and
alkylaromatics [10] into the corresponding carbonyl compounds and/or carboxylic
acids. Naturally, the new catalytic system combining NHPI with metal salts [11,
12], which served as radical catalysts, was used in the oxidation of toluene. It was
proposed that a phthalimide N-oxyl (PINO) radical can be generated from NHPI in
the presence of metal salts under mild conditions [13], while the carbon radicals
derived from toluene under dioxygen led to oxygenation of compounds into
benzaldehyde, benzyl alcohol, and benzoic acid as well as some by-products. This
novel catalytic system with high conversion and suitable selectivity in oxidation of
toluene would produce benzoic acid as the major final product. As reported, 2 %
benzaldehyde and 81 % benzoic acid were produced in the presence of 10 mol%
NHPI and 0.5 mol% Co(OAc)2 in acetic acid under an oxygen atmosphere at 25 °C
for 20 h [10]. Moreover, the oxidation of toluene after 12 h at 90 °C showed[95 %
high conversion and [99 % benzoic acid selectivity, in the presence of 1 mol%
NHPI, 1 mol% CoCl2Á6H2O, and 0.5 mol% didecyldimethyl-ammonium bromide
(DDAB) with a continuous flow of oxygen [14]. Toluene was efficiently
transformed with 70.9 % conversion and 93.5 % selectivity to benzoic acid at
90 °C within 2 h in the presence of 5 mol% N-hydroxyquinolinimide (NHQI),
2.5 mol% CuCl2 combined with hydroxypyrrolo [3,4-b]pyrazine-5,7-dione
(HPPDO), and 7 mL of acetonitrile under oxygen [15].
Metal–organic frameworks (MOFs) are porous crystalline materials, and their
structures are composed of transition metal ions (or clusters) that are connected to
bi- or tri-podal rigid organic linkers [16–18]. MOFs have attracted wide attention
owing to the highest porosities, world-record surface areas, and wide chemical
inorganic–organic composition [19]. MOFs are widely used in gas storage [20], gas
purification and separation applications [21], magnetism [22], sensors [23], as well
as heterogeneous catalysis [24–26] owing to their special structural characteristics.
As reported, a highly-active Fe-containing MIL-101 material was prepared and used
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N-hydroxyphthalimide incorporated onto Cu-BTC metal organic…
as a heterogeneous catalyst for oxidation of alcohols and alkenes [27]. A metal
organic framework encapsulated with gold nanoparticles (noted as Au/UiO-66) was
developed and used for selective oxidation of benzyl alcohol [28]. Palladium
chloride was successfully immobilized on MOF-253 and proved to be a highly-
active heterogeneous catalyst for Suzuki–Miyaura and Ullmann-type coupling
reaction [29]. As an example of this classical material, Cu-BTC [BTC:1,3,5-
benzenetricarboxylate Cu3(BTC)2, or HKUST-1], which is generated by intercon-
nection of [Cu2(O2CR)4] units with a three-dimensional system, was first
synthesized in 1999 [30]. Since then, the improved Cu-BTC synthesis method
was reported [31–35]. Meanwhile, Cu-BTC was widely used in gas storage and gas
separation [36–39] owing to the high porosity. Currently, owing to the large pore
size, large surface area, and high metal content with active sites, the attention on
application of Cu-BTC was transferred in catalysis. When cyanosilylation of
benzaldehyde was used as a test reaction to study the catalytic properties of Cu-
BTC, the removal of the Cu-bound water molecules allowed accessing the Lewis
acid copper sites, which activated the reaction with a reasonable yield of 50–60 %
after 72 h (313 K) [31]. Later, with the ethylene ketal of 2-bromopropiophenone as
a test substrate, Alaerts et al. [40] proved that the active sites in Cu-BTC were hard
Lewis acid sites and studied the adsorption CO and acetonitrile in IR spectroscopy
supported as further evidence. Then Cu-BTC was used as a catalyst in the
hydroxylation of toluene [41], Aldol condensation [42], chemical reduction of
nitrophenol [43], oxidation of cyclooctene [44], and even oxygen reduction [45].
However, there are only a few reports about incorporating organic substrates onto
Cu-BTC, which proved to be an effective catalyst in esterification [46] and
knoevenagel condensation [47]. Herein, we report the combination of NHPI with
Cu-BTC (NHPI/Cu-BTC) by an impregnation method and use the prepared catalyst
as an effective heterogeneous catalyst into the solvent-free oxidation of toluene.
Experimental methods
Materials
Copper(II) nitrate trihydrate [Cu(NO3)23H2O (99.5 %)] was purchased from
GuangFu Technology Development Co,. Ltd. (Tianjin, China). 1,3,5-benzene-
tricarboxylic acid [H3BTC, 98 %] and NHPI (97 %) were purchased from
Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Other reagents and
solvents used here were all analytical-grade reagents obtained from the vendors and
used without further purification.
Characterization techniques
The morphologies of the synthesized products were investigated on a Jeol JSM-
6610LV scanning electron microscope (SEM) equipped with a tungsten filament.
Samples were sputtered with gold prior to analysis and spread on a carbon disc
mounted to SEM aluminum pin stubs. Fourier transformed infrared spectroscopy
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L. Bao et al.
(FTIR) was recorded in KBr disks with a Thermo Nicolet 380 FT-IR spectrometer.
Elemental microanalysis (EA) was determined using a Vario EL analyzer.
A Thermo Jarrell Asch IRIS Advantage 1000 inductively coupled plasma (ICP)-
atomic spectroscopy was used to analyze metal contents. X-ray diffraction (XRD)
patterns were detected on a Rigaku D/MAX 2500 diffractometer at 45 kv, 250 mA
˚
for CuKa (k = 1.5418 A) radiation, with a step size of 0.02° in 2h. Thermogravi-
metric analysis (TGA) was performed on a TGA/SDTA 851e device under N2
atmosphere from 303 to 1073 K, with a heating rate of 10 K min-1. The Brunauer,
Emmett and Teller (BET) surface area and pore volume were measured by
analyzing the results of nitrogen adsorption at 77 K in a NOVA-2200e Quanta
chrome apparatus. Prior to measurements of N2 adsorption, all samples were out-
gassed at 423 K for 24 h.
Preparation of Cu-BTC
Two synthesis methods were selected from a reported synthesis route [48]. The
details of the synthesis procedure are as follows.
H3BTC (1.9664 g, 0.009 mol) was dissolved in ethanol (20 mL) and Cupric
nitrate trihydrate (4.48 g, 0.018 mol) was dissolved in deionized water (10 mL) in
different beakers. Prior to transfer into a Teflon-lined stainless steel autoclave, the
two solutions were mixed and stirred at 298 K for 16 h. Then the autoclave was
placed into an oven and heated at 413 K for 48 h. After cooling to room temperature
normally, the resulting blue crystals were filtered, washed thoroughly with
deionized water, and finally dried at 358 K for 12 h. The obtained product is
named as Cu-BTC-A.
H3BTC (1.0 g, 4.8 mmol) was dissolved in 30 mL of a 1:1 mixture of ethanol/
N,N-dimethylformamide (DMF). Cu(NO3)23H2O (2.077 g, 8.6 mmol) was dis-
solved in 15 mL of water in another flask. The two solutions were then mixed and
stirred for 10 min before transferring into a Teflon-lined stainless steel autoclave.
Then the autoclave was placed into an oven and heated at 373 K for 10 h. Finally,
the reaction vessel was cooled to room temperature normally. The resulting blue
crystals were isolated by filtration and extracted with methanol for 12 h using a
Soxhlet extractor to remove solvated DMF. The product was then dried at 303 K for
12 h and named as Cu-BTC-B.
Preparation of NHPI/Cu-BTC
This method is a modification from a previous work [49]. Cu-BTC (1.0 g) was
activated under vacuum at 473 K for about 2 h. NHPI (210 mg) was dissolved in a
mixed solution of 7 mL of dichloromethane and 3 mL of methanol or 10 mL of
tetrahydrofuran (THF). The activated Cu-BTC was added to this solution, and the
resulting suspension was heated at 313 or 338 K (the temperature was selected
depending on the solvent) for 4 h. After the required time, the mixture was cooled,
and the solvent was removed by rotary evaporation. The resulting NHPI/Cu-BTC
was dried at 353 K for 2 h. Finally, we acquired four catalysts: NHPI/Cu-BTC-A-1,
NHPI/Cu-BTC-A-2, NHPI/Cu-BTC-B-1, and NHPI/Cu-BTC-B-2, using different
123

N-hydroxyphthalimide incorporated onto Cu-BTC metal organic…
Cu-BTC samples and solvents. The letters A or B present different Cu-BTC
samples. The number 1 means that dichloromethane and methanol are chosen as
solvents to dissolve NHPI, and the number 2 indicates that THF is used.
Catalytic procedure for oxidation of toluene
In a typical reaction, toluene (5 g, 55 mmol) together with an appropriate amount of
catalyst NHPI/Cu-BTC [0.43 g, 1 mol%, mol%: n (catalyst)/n (toluene)] were
placed into a 50 mL Teflon-lined magnetically stirred autoclave equipped with a
block heater and a thermometer. The reactions were carried out at the required
temperature under a molecular oxygen atmosphere (1.6 MPa) for hours. After that,
the catalyst was filtered and washed with chloroform thoroughly, and the reaction
mixture analyzed on an Agilent gas chromatography (GC)-6820 device equipped
with a 30 m 9 0.32 mm 9 0.5 mm HP-Innowax capillary column, with N2 used as
carrier gas.
Results and discussion
Characterization
NHPI was incorporated onto Cu-BTC by adsorption from solutions as described in
the Experimental section, and the resulting NHPI/Cu-BTC materials were
characterized by SEM, FT-IR, TGA, XRD, and N2 adsorption.
We acquired light blue Cu-BTC-A and bright Prussian blue Cu-BTC-B, which
were in accordance with a reported article [48]. The SEM images (Fig. 1) show that
the two different Cu-BTC samples (1a–1b) are both octahedral crystals. Moreover,
Cu-BTC-A (1a) is smoother and a few other small spherical particles are present,
whereas Cu-BTC-B (1b) is cubic-shaped with sharp edges without any spherical-
shaped particle. The results indicate that Cu-BTC-A synthesized at higher
temperature is impurity for the formation of Cu2O as by-product. The changes in
color and morphology may be partly attributed to the differences in synthesizing
temperature and post-treatment procedures [30, 32, 48]. The morphological
structures of the synthesized four NHPI/Cu-BTC (1c–1f) materials are also shown
in Fig. 1. Compared with the single Cu-BTC, the crystal structure of NHPI/Cu-BTC
materials become less irregular, which may be attributed to the mixing and stirring
during the synthesized procedure. Unexpectedly, the images show few distinctions
using different solvents as reaction media.
The XRD patterns of both Cu-BTC products are shown in Fig. 2. All samples
shows high crystallinity, and especially the diffraction peaks at 2h = 6.6°, 9.4°,
11.6°, 13.4°, 17.4°, 19.0°, 25.9° can be indexed to crystalline Cu-BTC (Fig. 2). The
strong diffraction peaks at 2h = 36.4° are present in the products (2a–2c), but not in
Samples (2d–2f), indicating that Cu-BTC-A contained a small amount of Cu2O
impurity [50]. It was proved that Cu2O as a by-product was formed easier at high
temperature [30]. In our cases, Cu-BTC-A was synthesized at a high temperature of
413 K. Neither sample showed the peaks at 2h = 35.5°, 38.7°, indicating the
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L. Bao et al.
Fig. 1 SEM micrographs of products: a Cu-BTC-A; b Cu-BTC-B; c NHPI/Cu-BTC-A-1; d NHPI/Cu-
BTC-B-1; e NHPI/Cu-BTC-A-2; f NHPI/Cu-BTC-B-2
product was CuO purity. Meanwhile, the diffraction peaks of NHPI/Cu-BTC
materials in Fig. 2 were well consistent with Cu-BTC samples. It is clear that the
incorporation of NHPI onto Cu-BTC did not change the crystal structure.
The thermal stability of synthesized Cu-BTC and NHPI/Cu-BTC products is
presented in Fig. 3. The thermogram of Cu-BTC samples shows three regions. As
for Cu-BTC-A (3e), the first weight loss (21 %) region between 303 and 433 K
indicates the loss of moisture, the second region between 433 and 577 K shows a
steady weight, and the third region starts at 577 K, where the structure of Cu-BTC
collapses and the organic linker is buried. As for Cu-BTC-B (3f), the first region is
between 303 and 443 K, with a weight loss about 33 % higher than that of Cu-BTC-
123

N-hydroxyphthalimide incorporated onto Cu-BTC metal organic…
*
Cu O impurity
2
∗
a
∗
b
∗
c
d
e
f
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2θ/deg
Fig. 2 XRD pattern of Cu-BTC and catalysts: a Cu-BTC-A; b NHPI/Cu-BTC-A-1; c NHPI/Cu-BTC-A-
2; d NHPI/Cu-BTC-B-1; e NHPI/Cu-BTC-B-2; f Cu-BTC-B
397
473
a
b
593
c
d
433
577
e
443
577
f
300
400
500
600
700
800
900
Temperature/K
Fig. 3 TGA curve of Cu-BTC and catalysts: a NHPI/Cu-BTC-A-1; b NHPI/Cu-BTC-A-2; c NHPI/Cu-
BTC-B-1; d NHPI/Cu-BTC-B-2; e Cu-BTC-A; f Cu-BTC-B
A, indicating the different contents of moisture or volatile solvent. The differences
were attributed to post-treatment procedures used in preparation. Also, there is a
steady weight from 443 to 577 K and the structure decomposed at above 577 K,
finally forming CuO as the product. The same decomposition temperature at 577 K
indicated the same thermal stability of the two samples. Obviously, NHPI/Cu-BTC
catalysts had an additional weight loss, which contributed to the decomposition of
NHPI at 473–593 K (Fig. 3a–d). The results undoubtedly indicate that NHPI was
successfully incorporated onto the carrier Cu-BTC.
123

L. Bao et al.
The BET surface area and pore volume of all samples are determined. The Cu-
BTC-A has a surface area of 766 m2g-1 and a pore volume of 0.345 cm3g-1, while
Cu-BTC-B has a surface area of 1404 m2g-1, which is twice as large as sample A,
and a pore volume of 0.634 cm3g-1. After incorporation of NHPI, the surface area
of Cu-BTC samples sharply decreased (\30 m2g-1 in all NHPI/Cu-BTC samples),
which was a significant evidence for the internal location of NHPI inside the carrier
Cu-BTC.
The FT-IR patterns of all compounds are shown in Fig. 4. For both Cu-BTC
samples, the presence of a broad and strong peak at 3454 cm-1 is assigned to m
(H2O). The peak at 1641 cm-1 is assigned to m (C=O) of the deprotonated H3BTC.
The peaks at 1373 and 1589 cm-1 are due to the msym (C–O2) and masym (C–O2)
stretching modes, respectively. The IR bands around 729 and 1450 cm-1 are due to
(C–H) bending mode and a combination of benzene ring stretching and deformation
modes. The absorption bands around 500 cm-1 are assigned to m (Cu–O) stretching
modes [51]. The absence of the peak at 2970 cm-1 for sample B indicated the
solvent DMF inside the channel of the compound was removed effectively by
methanol as an extracted solvent [52]. As shown in Fig. 4, especially the bands at
1790, 1722, 1185, 976, and 882 cm-1 of NHPI/Cu-BTC indicated that NHPI was
successfully incorporated onto Cu-BTC again. Theoretically, it is reasonable to
˚
assume that NHPI, with molecular size around 6.5 A, can be accommodated inside
the intracrystalline space of Cu-BTC [49].
Finally, the element analysis results are showed in Table 1. The element contents
of C and H both changed slightly between the two samples. However, Cu-BTC-A
contains higher Cu content than Cu-BTC-B, which may be the key factor to the
catalytic performance.
a
b
c
d
e
f
g
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber/cm-1
Fig. 4 FI-IR spectroscopy of materials: a NHPI/Cu-BTC-A-1; b NHPI/Cu-BTC-A-2; c NHPI/Cu-BTC-
B-1; d NHPI/Cu-BTC-B-2; e Cu-BTC-A; f Cu-BTC-B; g NHPI
123

N-hydroxyphthalimide incorporated onto Cu-BTC metal organic…
Table 1 The result of element analysis and inductively coupled plasma
Simple
C (%)
H (%)
Cu (%)
Cu-BTC-Aa
Cu-BTC-Bb
a
25.0
29.0
3.2
3.3
36.7
24.4
Sample synthesized at high temperature (413 K)
b
Sample synthesized at lower temperature (373 K)
Aerobic oxidation of toluene
NHPI in combination with metal salts was used as a heterogeneous catalyst for
oxidation of toluene by using acetic acid as a solvent [10, 11]. Considering the
solvent might cause environmental pollution, we used NHPI/Cu-BTC as a catalyst
for selective oxidation of toluene (shown in Scheme 1) under solvent-free
conditions and the catalytic performances are listed in Table 2.
Initially, the oxidation of toluene without catalyst or Cu-BTC (both Cu-BTC-A
and Cu-BTC-B) alone under oxygen did not result in conversion. When NHPI acts
as catalyst alone, the conversion of toluene is lower. However, NHPI adsorbed onto
Cu-BTC was an active catalyst in the presence of oxygen at 110 °C, and NHPI/Cu-
BTC catalysts showed different conversions of toluene and suitable BD ? BA
selectivity in 2 h in the absence of any co-solvent. When Cu-BTC-A was chosen as
the carrier with different impregnation solvents for NHPI, the conversion rates of
toluene were 7.7 and 7.6 %, respectively. On the contrary, the conversions of
toluene were 5.8 and 5.4 % with Cu-BTC-B as the carrier. The negligible change
may contribute to different contents of Cu, which was proved to be a co-catalyst in
the oxidation [53]. Interestingly, when THF worked as a solvent for the synthesis of
NHPI/Cu-BTC, the selectivity degrees of BD ? BA are higher than that of
dichloromethane and methanol used in the procedure (Table 2, Entry 4, 5, 6, 7). In
contrast to NHPI/Cu-BTC, two control experiments were performed using a
physical mixture of Cu-BTC and NHPI in the same ratio as an incorporated catalyst.
Cu-BTC-A results in an 8.2 % conversion with 57.3 % BD ? BA selectivity, and
Cu-BTC-B yielded a 7.9 % conversion with 52.9 % of BD ? BA selectivity.
O
NOH
NHPI/CuBTC
O
NHPI
CHO
CH2OH
COOH
+
+
O2
Solvent free
BzH
BzOH
BzA
Scheme 1 Aerobic oxidation of toluene using NHPI/Cu-BTC as catalyst
123

L. Bao et al.
Table 2 Catalytic performance for aerobic oxidation of toluene
Entry
Catalysts
Conversion (%)
SBzH (%)
SBzOH (%)
SBzA (%)
1
None
–
–
–
–
2
Cu-BTC
NHPIa
–
–
–
–
3
2.9
7.7
7.6
5.8
5.4
8.2
7.9
5.2
4.3
9.2
66.3
41.1
44.1
45.7
54.2
43.3
42.4
53.8
65.1
37.3
18.5
8.9
11.0
6.6
8.0
14.0
10.5
3.9
3.0
8.7
15.2
50.0
44.9
47.7
37.8
42.7
47.1
42.3
31.9
43.3
4
NHPI/Cu-BTC-A-1e
NHPI/Cu-BTC-A-2
NHPI/Cu-BTC-B-1
NHPI/Cu-BTC-B-2
NHPI ? Cu-BTC-A
NHPI ? Cu-BTC-B
NHPI/Cu-BTC-B-1b
NHPI/Cu-BTC-B-1c
NHPI ? Cu(NO3)2Á3H2Od
5
6
7
8
9
10
11
12
Reaction conditions: toluene 5 g, catalyst 0.43 g (1 mol%), O2 1.6 MPa, 110 °C, 2 h
a
NHPI 0.09 g (1 mol%)
b
Reaction temperature 100 °C
Reaction temperature 90 °C
NHPI 0.09 g (1 mol%), Cu(NO3)2Á3H2O 0.34 g
c
d
e
The letters of A (synthesized at high temperature) or B (synthesized at lower temperature) presents
different Cu-BTC samples. The number 1 means that dichloromethane and methanol are chosen as
solvents to dissolve NHPI, and the number 2 indicates that tetrahydrofuran (THF) is used
a
b
c
d
e
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber/cm-1
Fig. 5 FI-IR spectroscopy of fresh and recycled catalysts: a R-NHPI/Cu-BTC-A-1; b R-NHPI/Cu-BTC-
A-2; c R-NHPI/Cu-BTC-B-1; d R-NHPI/Cu-BTC-B-2; e NHPI/Cu-BTC-A
To provide a qualitative assessment of catalytic performance of NHPI/Cu-BTC
with respect to the analogous unsupported catalyst, we also performed an additional
experiment with Cu(NO3)23H2O in combination with the same molar ratio of NHPI
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N-hydroxyphthalimide incorporated onto Cu-BTC metal organic…
a
b
c
d
e
f
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2θ/deg
Fig. 6 Powder XRD pattern of fresh and recycled catalysts: a NHPI/Cu-BTC-A-1; b R-NHPI/Cu-BTC-
A-1; c R-NHPI/Cu-BTC-A-2; d R-NHPI/Cu-BTC-B-1; e R-NHPI/Cu-BTC-B-2; f NHPI/Cu-BTC-B-1
as that of NHPI/Cu-BTC. As showed in Table 2, the reaction with Cu(NO3)23H2O
and NHPI resulted in 9.2 % conversion with 46.0 % of BD ? BA selectivity in 2 h.
In addition, some unidentified products were formed, which reduced the selectivity
of the desired oxidation products. When the temperature decreased to 100 and
90 °C, the conversion was reduced to 5.2 and 4.3 %, respectively.
The structural stability of NHPI/Cu-BTC for the oxidation of toluene using
molecular oxygen was investigated. After the first reaction, NHPI/Cu-BTC was
obtained by filtration, washing with chloroform and oven-drying at 80 °C for 2 h.
The structures of recycled catalysts are studied by FI-IR and XRD. As shown in
Figs. 5 and 6, the peaks of the used catalysts are well consistent with the fresh ones,
indicating the structures and perfect stability of the used NHPI/Cu-BTC catalysts
are maintained under the reaction conditions.
Conclusions
We successfully incorporated NHPI onto two different Cu-BTC samples (NHPI/Cu-
BTC), which were performed in controlled oxidation of toluene in the absence of
additional co-solvents or additives under mild reaction conditions for the first time.
The study shows that the reaction takes place with more than 50 % selectivity of
BD ? BA for 7.6 % conversion after 2 h at 110 °C in the presence of 1 mol%
NHPI/Cu-BTC as catalyst. The catalysts could be separated from the reaction
system by simple filtration and showed perfect stability. However, further studies
are still necessary to optimize the catalytic performance of the prepared catalysts for
toluene aerobic oxidation.
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L. Bao et al.
Acknowledgments This study was financially supported by the National Natural Science Foundation of
China (No. 21376200), Hunan Provincial Natural Science Foundation of China (No. 14JJ2065), Hunan
Provincial Colleges Innovation Platform Open Foundation (No. 13K044).
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