Silica gel-supported TEMPO with adsorbed NOx for selective oxidation of alcohols under mild conditions

Article abstract of DOI:10.1016/S1872-2067(12)60657-3

A column padding silica gel was used to support 2,2,6,6- tetramethylpiperdine-1-oxyl (TEMPO) and to adsorb NO_x to provide an oxidant system for the selective oxidation of alcohols. A wide range of alcohols were oxidized to the corresponding aldehydes and ketones with high selectivities and conversions using this heterogeneous catalyst under mild conditions.

Full text of DOI:10.1016/S1872-2067(12)60657-3

Chinese Journal of Catalysis 34 (2013) 1848–1854
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Silica gel‐supported TEMPO with adsorbed NOx for selective
oxidation of alcohols under mild conditions
Hua Zhang*, Luoling Fu, Hongmin Zhong
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A column padding silica gel was used to support 2,2,6,6‐tetramethylpiperdine‐1‐oxyl (TEMPO) and
to adsorb NOx to provide an oxidant system for the selective oxidation of alcohols. A wide range of
alcohols were oxidized to the corresponding aldehydes and ketones with high selectivities and
conversions using this heterogeneous catalyst under mild conditions.
Received 26 May 2013
Accepted 10 July 2013
Published 20 October 2013
© 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Silica
2,2,6,6‐tetramethylpiperdine‐1‐oxyl
Nitrogen oxide
Heterogeneous catalysis
Alcohol oxidation
1. Introduction
the aerobic oxidation of various organic compounds under mild
reaction conditions have become increasingly attractive [12].
As one of the most important oxidation reactions, the selec‐
tive oxidation of alcohols to the corresponding carbonyl com‐
pounds is used in the synthesis of organic intermediates and
fine chemicals [1,2]. Many efficient methods have been devel‐
oped for the selective oxidation of alcohols using a variety of
oxidizing agents and catalysts, including stoichiometric oxygen
donors [3,4], metal catalysts, and N‐oxide catalysts [5–10].
However, these methods for the oxidation of alcohols suffer
from drawbacks such as the use of at least a stoichiometric
amount of oxidant, giving a large quantity of harmful byprod‐
ucts and requiring high temperature and high pressure condi‐
tions in the oxidation process. For decreasing environmental
and economic stress, another approach to construct a clean
catalytic system for the oxidation of alcohols, such as using
molecular oxygen [11] (H2O2, O2, O3, etc.) as oxidant has be‐
come popular in recent years.
2,2,6,6‐tetramethylpiperdine‐1‐oxyl (TEMPO), which can oxi‐
dize various kinds of alcohols into their corresponding alde‐
hydes and ketones, is more stable and efficient [13] than other
N‐oxides. It has the advantages of high efficiency, high selectiv‐
ity, and easy handling and is environmentally friendly for the
oxidation of alcohols. TEMPO also expressed nearly quantita‐
tive oxidation [14–16] and has been applied in industry suc‐
cessfully [17]. In particular, TEMPO in combination with an
active molecular oxygen co‐catalyst as a catalytic system to
oxidize alcohols has become attractive recently [18–21]. Fol‐
lowing Anelli's TEMPO/NaBr/NaOCl system, a series of TEMPO
in combination with active molecular oxygen co‐catalysts as
terminal oxidant systems such as TEMPO/NaBr/NaNO2 [19],
H2O2/HBr/NaNO2 [20], and TEMPO/HBr/TBN [21] have been
developed as efficient methods for the oxidation of alcohols.
However, some of these systems use Br, which is not consid‐
ered friendly to the environment, some need demanding condi‐
In the past two decades, N‐oxides as valuable catalysts for
*Corresponding author. Tel/Fax: +86‐411‐84986205; E‐mail: zhanghua@dlut.edu.cn
This work was partially supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT0711).
DOI: 10.1016/S1872‐2067(12)60657‐3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 34, No. 10, October 2013

Hua Zhang et al. / Chinese Journal of Catalysis 34 (2013) 1848–1854
2 g of SPE column packing silica gel (Dalian Sipore Co. Ltd., the
0.5 g silica-gel-TEMPO-NO
OH
x
O
particle and pore size of the SPE padding silica were 40 µm
diameter and 10 nm, respecitvely) was suspended in the mix‐
ture with gentle agitation. After 24 h reaction, the catalyst was
filtered, washed in a Soxhlet apparatus with CH2Cl2 for 20 h,
and then dried in a vacuum oven at 45 °C for 3 h. After evapo‐
ration of the solvent, 2.130 g of silica gel‐supported TEMPO
was obtained.
The NOx adsorption procedure was as follows. A 5 mL of
H2SO4 solution (H2SO4:H2O = 1:2, v:v) was placed in a separa‐
tory funnel, and 5 g of NaNO2 was placed in a jar. The acid solu‐
tion was dropped into the sodium nitrite over 30 min. After the
silica gel–supported TEMPO (0.5 g) was placed in an adsorp‐
tion tube (10 cm length and 15 mm inside diameter), NOx gas,
which was first dried by a CaCl2 tube, was fed into the adsorp‐
tion tube and finally introduced into a tail gas absorption de‐
vice. The adsorption was allowed to continue for 1 h. Then the
connection was shut down between the calcium chloride tube
and the adsorption tube. Meanwhile the connection between
the CaCl2 tube and tail gas absorption device was opened for 2
h to release physisorbed NOx from the surface of the silica
gel‐supported TEMPO. Finally, silica gel‐supported TEMPO/
NOx was obtained.
air, CH Cl , r.t.
2
2
R
1
R
2
R
R
2
1
Scheme 1. Alcohol oxidation with the silica gel‐TEMPO‐NOx system in
air.
tions, and some need multistep product separation. Efforts to
develop a new oxidant system are required to overcome these
inherent shortcomings. TEMPO immobilized on a mesoporous
material [22,23] combined with an active molecular oxygen
co‐catalyst in the field of alcohol oxidation has become more
attractive.
Silica gel consists of many pores and a large surface area,
which provide sites for the immobilization of TEMPO and the
storage of NOx. Since it was reported recently that NO2 oxidized
TEMPO into the TEMPO cation to initiate alcohol oxidation and
NOx was removed by silica‐supported aminoxyls [24,25], we
envision that NOx can be used as the direct electron bridge be‐
tween TEMPO and molecular oxygen. Thus the use of silica
gel‐supported TEMPO with adsorbed NOx as a catalytic system
for the oxidation of alcohols may be efficient. Our previous
work reported the PSB‐TEMPO‐NOx system [26]. Here our in‐
terest was in designing NOx storage in padding silica
gel‐TEMPO material as a heterogeneous oxidation system for
the selective oxidation of alcohols (Scheme 1) under mild con‐
ditions.
2.2. Catalyst characterization
The supported TEMPO catalyst was obtained by direct
grafting of silica gel with 3‐(triethoxysilyl) propamino‐TEMPO,
which was prepared following the procedure described by Ci‐
riminna et al. [27], as silylating agent [28] (Scheme 2). Func‐
tionalized and unfunctionalized silica gel were characterized by
N2 adsorption and Fourier transform infrared spectroscopy
(FT‐IR). The functional group contents of the surface were ob‐
tained from elemental analysis. The quantitative determination
of NOx adsorbed content was measured by NOx tempera‐
ture‐programmed desorption (NOx‐TPD), which was also com‐
pared with the elemental analysis (C, H, N) values.
All materials were degassed for 4 h at 100 °C before using a
Micromeritics ASAP 2020 physical adsorption apparatus at
–196 °C to measure the N2 adsorption isotherms. The specific
surface area of the samples was calculated by the BET equation.
Pore volume was obtained by the BJH method for adsorption
cumulative volume of pores between 1.7 and 300 nm diameter.
The BJH model was used to derive the pore diameter distribu‐
tions from the adsorption branch of the isotherm. NOx absorp‐
tion was analyzed by elemental analysis on an Elemental Vario
EL III and by FT‐IR spectroscopy. NOx‐TPD was conducted by
the method of putting the catalyst (0.05 g) in a homemade de‐
sorption tube with temperature‐programmed heating of the
tube from 25 to 500 °C in a 80 mL/min of N2 flow at 20 °C/min.
A chemiluminesence detector (Ecophysis CLD 60) was applied
to analyze the outlet gas composition online.
2. Experimental
2.1. Catalyst preparation
4‐oxo‐TEMPO was synthesized according to the literature
[29]. 4‐oxo‐TEMPOH (0.704 g, 4 mmol), APTES (0.884 g, 4
mmol), and methanol (8.5 mL) were added into a 100 mL flask
with vigorous stirring for 2 h. Then NaBH3CN (0.25 g, 6 mmol)
was added, and the mixture was stirred at ambient tempera‐
ture for 12 h. The pH was adjusted to 3–4 with HCl (38%), and
2.3. Aerobic oxidation
All the alcohols were purchased from Acros, Alfa Aesar, and
Energy Chemical Companies and used without further purifica‐
tion. A solution of alcohol substrate (1.0 mmol) in CH2Cl2 (6
mL) was prepared in a 50 mL long‐necked, round‐bottom flask
O
N
O
O
O
O
O
O
O
Silica gel
r.t., 48 h
O
Si
NH2
O
Si
NH
NH
Si
NaBH3CN, MeOH
r.t., 12 h
O
N
O
N
O
Scheme 2. Synthesis of silica gel‐supported TEMPO.

Hua Zhang et al. / Chinese Journal of Catalysis 34 (2013) 1848–1854
The elemental analysis of catalysts is shown in Table 1. The
Table 1
degree of functionalization of silica gel‐TEMPO was calculated
on the basis of the increase of nitrogen content in comparison
to that of the non‐modified support. The final value was 0.292
mmol/g. As shown in Table 1, the amount of absorbed NOx was
0.794 mmol/g, and even after one usage, there was 0.337
mmol/g NOx left. This indicates that this immobilization meth‐
od gave a good degree of functionalization and the prepared
material also adsorbed NOx well.
Elemental analysis of catalysts.
Ca
Ha
Na Catalyst
NOx
Catalyst
(%) (%) (%) (mmol/g) (mmol/g)
Silica
Silica gel‐TEMPO
Silica gel‐TEMPO‐NOx
0.055 2.341 0.010
0
5.430 2.670 0.819 0.292
5.013 2.387 1.868 0.292
0
0
0.749
0.337
Silica gel‐TEMPO‐NOx(1) b 5.077 2.057 1.292 0.292
a Quality percentage.
b Silica gel‐TEMPO‐NOx that was used once.
FT‐IR spectrum changes were observed for the samples of
silica gel‐TEMPO‐NOx and silica gel‐TEMPO. It was reported
that NO2 oxidized TEMPO into TEMPO+, and NO2 changed to
equipped with a ground glass elbow to be in contact with the
atmosphere. Then silica gel‐supported TEMPO/NOx (0.5 g) was
added, and after that the resulting mixture was magnetically
stirred at room temperature and ambient pressure. TLC and GC
(GC Agilent 6890N) were applied to monitor the reaction.
When the reaction was over, the mixture was vacuum filtered
–
–
NO3 , which has been studied with the V3(NO3 ) band seen near
1388 cm–1 [30]. Our system also contained these reactions. The
sharp peak at 1384 cm–1 corresponds to NOx adsorbed by silica
gel‐TEMPO‐NOx and silica gel‐TEMPO‐NOx (1) (Fig. 1). Obvi‐
ously, the FT‐IR spectra showed that the NOx storage was suc‐
cessful in the preparation. In addition, the 1384 cm–1 peak over
silica gel‐TEMPO‐NOx(1) spectrum also gave clear evidence
that there was still NOx left in the catalyst after it was used
once.
to get the product, which was examined by H and 13C NMR
1
analysis (Bruker spectrometer, 400 MHz) with CDCl3 as the
solvent and TMS as internal standard. The area normalization
method was applied to calculate conversion and selectivity of
the oxidation from GC results.
3. Results and discussion
400
3.1. Synthesis of immobilized TEMPO
Adsorption
(a)
Desorption
300
Silica gel‐supported TEMPO was achieved by the direct
functionalization of the silica gel surface with a silylating agent
(Scheme 2), which was prepared with 4‐oxo‐TEMPO as the
starting material. Our immobilization method involved just
stirring in methanol and Saxhlet extraction with CH2Cl2. All the
reagents were used without any further treatment. This meth‐
od is green and time‐saving and has the same positive effects as
other immobilization methods that need treatment in dry tol‐
uene and dry methanol for several days.
200
100
0
Adsorption
Desorption
400
300
200
100
0
(b)
3.2. Characterization of the materials
3.2.1. Elemental analysis and FT‐IR study
400
300
200
100
0
Adsorption
Desorption
(c)
Silica-gel-TEMPO-NOx(1)
802.16
470.10
1636.79
1384.47
3448.48
3447.23
0.0
0.2
0.4
0.6
0.8
1.0
(d)
Silica-gel-TEMPO-NOx
1637.14
1097.30
1097.39
Relative pressure (p/p0)
0.25
0.20
0.15
0.10
0.05
0.00
802.57
1384.42
Silica-gel
469.85
Silica-gel-TEMPO
Silica-gel-TEMPO
Silica-gel-TEMPO(1)
1635.36
802.30
3450.53
470.76
1100.40
0
20
40
60
80
100
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm1)
Pore diameter (nm)
Fig. 2. N2 adsorption isotherms of silica gel (a), silica gel‐TEMPO (b),
silica gel‐TEMPO(1) (c), and their pore size distribution (d).
Fig. 1. FT‐IR spectra of the silica gel‐TEMPO sample.

Hua Zhang et al. / Chinese Journal of Catalysis 34 (2013) 1848–1854
Table 2
Physical data of catalysts.
600
(a)
NOx
NO2
NO
BET surface Pore volume a
Average pore
500
400
300
200
100
Catalyst
area (m2/g)
(cm3/g)
0.71
diameterb (nm)
Silica gel
Silica gel‐TEMPO
Silica gel‐TEMPO(1)
348
477
486
6.9
4.5
4.3
0.66
0.67
a Pores with diameters between 1.7 and 300 nm.
b BJH adsorption average pore diameter (4V/A).
0
3.2.2. Textural and morphological characteristics and
adsorption characterization
600
(b)
NOx
NO2
NO
500
400
300
200
100
The N2 adsorption isotherms of the materials confirmed the
structural quality of the supported silica gel‐TEMPO as com‐
pared to that of its parent silica gel (Fig. 2). The isotherms were
type IV [31], which exhibited the characteristics of mesoporous
materials with a disordered pore structure. The hysteresis
loops of the samples are H1 type according to the classification
of hysteresis loops by IUPAC [32], which is associated with
porous materials exhibiting a narrow distribution of relatively
uniform cylindrical pores. These uniform pores provide room
for NOx adsorption and alcohol oxidation. The specific surface
area, mesopore diameter, and average cumulative pore volume
(VBJH, 4V/A) of the samples are presented in Table 2. After the
silica gel was immobilized with TEMPO, the pore volume of
silica gel‐grafted TEMPO decreased from 0.71 to 0.66 cm3/g,
and the average pore diameter decreased from 6.9 to 4.5 nm.
These changes suggested that TEMPO was immobilized onto
silica successfully. Conversely, the surface areas of the samples,
which followed an opposite trend (ranging from 348 to 477
m2/g), can be explained by that silica gel particles were broken
into smaller sizes, yielding an increase in surface area. The pore
volume of silica gel‐TEMPO and silica gel‐TEMPO(1) almost did
not change, which showed that very little TEMPO was lost from
the silica gel and the catalyst physical characteristics were also
not affected by the oxidation reaction. The average pore diam‐
eter of silica gel‐TEMPO and silica gel‐TEMPO(1) was 4.5 and
4.3 nm, respectively, which was evidence that there was NOx
residue in the one‐time used catalyst.
NOx‐TPD was employed to measure the NOx adsorption
quantity. The amount of desorbed NOx of the samples was cal‐
culated based on desorption peak area. As shown in Fig. 3, the
desorption curve of silica gel‐TEMPO exhibited its desorption
capacity is 0. The desorption curve of silica gel‐TEMPO‐NOx was
flat in the purge phase at room temperature Fig. 3(b)), which
suggested that physically adsorbed NOx had been discharged.
The adsorbed NOx amount was 0.357 mmol/g(Table 3), which
agreed with the result from the elemental analysis in order of
magnitude. The NOx‐TPD curve of silica gel‐TEMPO(1) (Fig.
3(c)) shows that even when once used, 0.160 mmol/g NO2 (Ta‐
ble 3, parts of the gas was lost before entering the detector)
was still left in the channels. This observation is important for
interpreting the catalytic mechanism of the alcohol oxidation
reaction.
0
600
(c)
NOx
NO2
NO
500
400
300
200
100
0
0
500
1000
Time (s)
1500
2000
Fig. 3. NOx‐TPD profiles of silica gel (a), silica gel‐TEMPO‐NOx (b), and
silica gel‐TEMPO(1) (c).
To test the catalytic activity of the silica gel‐supported
TEMPO‐NOx system in the oxidation of alcohols (Table 4), the
reaction conditions were first optimized using benzyl alcohol as
a model substrate. Benzyl alcohol was readily oxidized to ben‐
zyldehyde with 100% conversion and > 99% product selectivi‐
ty in the presence of 0.5 g silica gel‐supported TEMPO/NOx.
This catalytic system was applied to oxidize various types of
benzylic aliphatic alcohols into their corresponding aldehydes
and ketones, and the results are listed in Table 4. Most benzylic
alcohols can be oxidized to their corresponding aldehydes
within 2 h with 99% conversion and selectivity as high as 99%,
while a little difference was observed between some elec‐
tron‐rich and electron‐deficient benzylic alcohols. (Table 4,
entries 2–4, 2.5 h; entry 9, 3.5 h). It is worth mentioning that
Table 3
NOx amount from different samples.
NO
(mmol/g)
0
NO2
(mmol/g)
0
0.120
0.039
NOx
(mmol/g )
0
0.357
0.160
Catalyst
Silica gel‐TEMPO a
Silica gel‐TEMPO‐NOx
Silica gel‐TEMPO(1)
a Before NOx adsorption.
b After NOx adsorption.
b
0.237
0.121
3.3. Aerobic oxidation of alcohols by silica gel‐supported
TEMPO‐NOx

Hua Zhang et al. / Chinese Journal of Catalysis 34 (2013) 1848–1854
Table 4
Aerobic oxidation of alcohols using silica gel‐supported TEMPO‐NOx
a
.
Time Convertionb Selectivityc
Entry
Substrate
Product
(h)
(%)
(%)
(a) (b) (c)
(d)
(e)
1
2
3
4
5
6
7
2
>99
>99
CH2OH
CH2OH
CH2OH
CHO
CHO
2.5
2.5
2.5
2
>99
>99
>99
>99
>99
>99
98
>99
>99
>99
>99
>99
Fig. 4. Pictures of silica gel (a), silica gel‐TEMPO (b), silica gel‐TEMPO‐
NOx (c), and the reaction mixture of started (d) and completed (e) reac‐
tion.
CHO
CH2OH
Cl
CHO
Cl
CH OH
2
CHO
We also found an interesting phenomenon in the oxidation
process. The transparent white silica gel (Fig. 4(a)) turned light
yellow when grafted with TEMPO (Fig. 4(b)), and then changed
to orange after adsorbing NOx (Fig. 4(c)). When the catalyst was
added into an alcohol CH2Cl2 solution, it turned light yellow
again (Fig. 4(d)). However, as the reaction proceeded, the color
of the silica gel changed to be orange gradually. In general, the
catalyst in the reaction mixture after the oxidation reaction
looked like that before it was added into the flask (Fig. 4(e)).
The oxidation process can be illustrated as Scheme 3. White
silica gel with immobilized TEMPO turns light yellow because
of the color of TEMPO. After NO2 adsorption, the immobilized
TEMPO was oxidized to TEMPO+NO3– (cycle I). Meanwhile, NO2
was reduced to NO (cycle II), which was reoxidized by O2 that
diffused into the pores from the air in the atmosphere. Thus the
silica pores were filled with NOx, and the catalyst was orange.
When reaction began, alcohol was oxidized by TEMPO+NO3– to
the corresponding aldehyde or ketone, and at the same time,
TEMPO+NO3– was reduced to TEMPOH, which entered the next
cycle. When the oxidation process was completed, only orange
2
Cl
Cl
CH2OH
Cl
Cl
CHO
2
CH OH
2
CHO
CHO
F
F
8
9
2
>99
>99
>99
>99
CH2OH
O N
2
O2N
3.5
CH OH
2
CHO
OH
O
10
11
12
13
2
2
>99
>99
>99
97
>99
>99
>99
>99
OH
O
OH
O
2
Cl
Cl
OH
O
12
O
14
15
5
6
98
>99
92
OH
–
TEMPO+NO3 and NOx remained in the catalyst. The color
>99
CH OH
CHO
CHO
changes in the oxidation process can be explained that the silica
has enough mechanical strength and does not break under
magnetic stirring, so NOx will remain stored in the silica pores
after reaction, and the catalyst can be reused without a second
NOx adsorption. This is another advantage of this catalytic sys‐
tem.
2
CH OH
2
16d
2
>99
>99
S
S
a All reactions were carried out according to the procedure in Section
2.3.
b Conversion and selectivity are based on GC with area normalization.
c Selectivity refers to the products aldehyde and ketone.
d Only benzyl alcohol was oxidized.
the system also showed high activity in oxidizing various sec‐
ondary alcohols to their corresponding ketones (Table 4, en‐
tries 10–14). 2‐Octanol can be oxidized to 2‐octyl ketone with
98% of conversion in only 5 h. The oxidation of an aromatic
secondary alcohol with an electron‐donating group on the
benzene ring (Table 4, entry 12) was easier than that with an
electron‐withdrawing group (Table 4, entry 13) on the benzene
ring under the reaction conditions. The oxidation of benzyl
alcohol (1 mmol) together with sulfide (1 mmol) (Table 4, entry
16) was also studied. After 2 h, most of the benzyl alcohol
(>99%) was transformed into benzyl aldehyde, and only a little
amount of sulfide (<1%) was converted. This clearly indicates
that this catalytic system can selectively oxidize alcohols mixed
with sulfides.
Scheme 3. Proposed oxidation mechanism of alcohol oxidation cata‐
lyzed by silica gel‐supported TEMPO‐NOx.

Hua Zhang et al. / Chinese Journal of Catalysis 34 (2013) 1848–1854
Graphical Abstract
Chin. J. Catal., 2013, 34: 1848–1854 doi: 10.1016/S1872‐2067(12)60657‐3
Silica gel‐supported TEMPO with adsorbed NOx for selective
oxidation of alcohols under mild conditions
Hua Zhang*, Luoling Fu, Hongmin Zhong
Dalian University of Technology
The gas‐solid one body catalyst, silica gel‐TEMPO‐NOx, was pre‐
pared by immbolizing TEMPO on silica gel and then absorbing NOx.
Silica gel‐TEMPO‐NOx expressed good performance in the selective
oxidation of alcohols under mild conditions.
2010, 319: 119
4. Conclusions
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Silica gel‐supported TEMPO was prepared by grafting the
SPE padding silica gel surface with 3‐(triethoxysilyl) propa‐
mino‐TEMPO. The resulting material in combination with NOx
was used in alcohol oxidation. It was found that various kinds
of alcohol substrates, including benzylic alcohols, secondary
aliphatic, and aromatic alcohols, were oxidized to the corre‐
sponding aldehydes and ketones with high conversions and
selectivities at atmospheric atmosphere and room tempera‐
ture.
Acknowledgments
The authors thank Professor X. P. Wang at Dalian University
of Technology for the NOx‐TPD analysis assistance.
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