Porous silica beads supported TEMPO and adsorbed NOx (PSB-TEMPO/NOx): An efficient heterogeneous catalytic system for the oxidation of alcohols under mild conditions

Article abstract of DOI:10.1002/adsc.201000876

The combination of NO_x gas which is stored in the pore canals of porous silica beads (PSB) with a heterogeneous catalyst, PSB-supported 2,2,6,6-tetramethylpiperdine 1-oxyl (PSB-TEMPO, 1), afforded a highly efficient, widely applicable, and efficiently recyclable approach for the selective aerobic oxidation of alcohols. This novel catalytic system (PSB-TEMPO/NO _x) can be employed in the oxidation of a wide range of alcohols to their corresponding aldehydes and ketones with selectivities as high as 99% at complete conversions under mild conditions. O₂ is the terminal oxidant. PSB-TEMPO can be recycled for more than 10 times without significant loss of activity.

Full text of DOI:10.1002/adsc.201000876

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DOI: 10.1002/adsc.201000876
Porous Silica Beads Supported TEMPO and Adsorbed NOx
(PSB-TEMPO/NOx): An Efficient Heterogeneous Catalytic
System for the Oxidation of Alcohols under Mild Conditions
Lei Dia and Zhang Huaa,*
a
Department of Applied Chemistry, Dalian University of Technology, No.139, No.158, Zhongshan Road, 116012 Dalian,
Liaoning Povince, Peopleꢀs Republic of China
Fax : (+86)-0411-3989-3808; phone: (+86)-0411-3989-3808; e-mail: zhanghua@dlut.edu.cn
Received: November 22, 2010; Revised: February 22, 2011; Published online: May 17, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000876.
Abstract: The combination of NOx gas which is
stored in the pore canals of porous silica beads
(PSB) with a heterogeneous catalyst, PSB-support-
ed 2,2,6,6-tetramethylpiperdine 1-oxyl (PSB-
TEMPO, 1), afforded a highly efficient, widely ap-
plicable, and efficiently recyclable approach for the
selective aerobic oxidation of alcohols. This novel
catalytic system (PSB-TEMPO/NOx) can be em-
ployed in the oxidation of a wide range of alcohols
to their corresponding aldehydes and ketones with
selectivities as high as 99% at complete conversions
under mild conditions. O2 is the terminal oxidant.
PSB-TEMPO can be recycled for more than 10
times without significant loss of activity.
hypervalent iodineACTHNGUTRENNUG ACHTUNGTRENNUNG
(III) complexes,[16] [BMIm][PF6][17]
and laccase[18]. Our previous study focused on the cat-
alytic system TEMPO/t-BuONO which is highly effi-
cient and economically competitive. t-BuONO was
used as an efficient NO equivalent in this system.
However, this system cannot satisfy the need of cost
efficiency from the viewpoints of product separation
and catalyst recycle. Obviously, the homogeneous re-
action conditions have limited its industrial applica-
tions. To get over this disadvantage, many supported
TEMPO catalysts have been synthesized based on or-
ganic[19] as well as inorganic[20] supports. Some of
them were applied as catalysts in alcohol oxidations
according to the Aneli protocol.[21] But to the best of
our knowledge none of them directly make use of
NOx (x=1, 2) gas as co-catalyst. Investigations of
Keywords: aerobic oxidation; heterogeneous cataly-
sis; immobilization; NOx adsorption; 4-oxo-
TEMPO; porous silica beads; 2,2,6,6-tetramethylpi-
perdine 1-oxyl
NOx storage in the ample pore canals of activated
carbon,[22] zeolite[23] and Al2O3
have been men-
[24]
tioned in a number of publications. Silica gel has the
potential capacity to be an NOx storage medium be-
cause of its abundant pore canal structure. Besides,
À
the plentiful Si OH groups around the surface of
silica gel are commonly used to immobilize TEM-
The oxidation of alcohols to carbonyl compounds and PO.[20a–c,f–h] Potentially it could be a promising method
carboxylic acids is of importance both in fundamental for the oxidation of alcohols (Scheme 1) if one com-
research and in industrial manufacturing.[1] In the past bined together the two processes mentioned above
decade, great efforts have been made to develop effi- (the storage of NOx gas and the immobilization of
cient catalytic oxidation systems[2] using TEMPO TEMPO). And the most crucial point will be the se-
(2,2,6,6-tetramethylpiperdine 1-oxyl) and molecular lection of the catalyst support, viz., the gas storage
oxygen as catalyst and terminal oxidant, respectively. medium.
Diversified co-catalysts[3] were found to be good in
promoting the yields of products. The co-catalysts can
be generally divided into two groups: one is transition
metal compounds including FeACHTNUGRTNEUNG
(NO3),[4] FeCl3,[5] Cu(I)
salts,[6] Cu(II) salts,[7] VOSO4,[8] Cu-Mn oxide,[9] Cu-
Mn nitrates,[10] Mn-Co nitrates,[10b] TiO2,[11] and Ru
complexes[12], the other is non-transition metal com-
Scheme 1. General protocol for the oxidation of alcohols
pounds such as NaNO2,[13] t-BuONO,[14] m-CPBA,[15] using PSB-TEMPO/NOx.
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Scheme 2. The synthesis procedure of catalyst 1 from 2.
Table 1. The elemental analysis results of catalyst 1 and 2.
Entry Catalyst C [%][a] H [%][a] N [%][a] df [mmol·gÀ1]
1
2
2
1
3.117
7.156
0.923
1.445
0.967
1.227
0
0.186
[a]
Mass percentage.
Herein, we report a novel heterogeneous catalytic
system, PSB-TEMPO/NOx, which uses porous silica
beads (PSB) as the catalyst support and the gas stor-
age material.
The catalyst PSB-TEMPO (1) was obtained in a
one-step reductive amination procedure starting from
aminopropyl-functionalized porous silica beads (PSB-
NH2, 2) which are commercially available
(Scheme 2).[20b] The elemental analysis (C, H, N) re-
sults of catalysts 1 and 2 are summarized in Table 1.
The degree of functionalization (df) of 1 (Table 1,
entry 2) was calculated on the basis of the increase of
nitrogen content in comparison to the non-modified
support 2 (Table 1, entry 1), and the final value is
0.186 mmol·gÀ1. Figure 1 shows the nitrogen adsorp-
tion/desorption isotherms of catalyst
1 (top), 2
(middle) and 3 (PSB-TEMPO-10: the recycled PSB-
TEMPO which has been used 10 times) (bottom). The
isotherms of catalyst 1 and 2 fitted the type IV[25] iso-
therm which is generally obtained for mesoporous
materials with a uniform pore structure. According to
the classification of hysteresis loops given by
IUPAC,[26] the hysteresis loops of the catalyst 1 and 2
are H1 type which is often associated with porous ma-
terials exhibiting a narrow distribution of relatively
uniform cylindrical-like pores. Similarly, the BJH
(Barret–Joyner–Helenda) pore size distributions of
each catalyst (Figure 2) are representatively narrow at
the mean value of 11.2029 and 9.8081 nm (Table 2),
respectively, also suggesting an open channel struc-
ture. The BET (Brunauer–Emmett–Teller) surface
areas and the pore volumes of catalyst 1 and 2 are
also collected in Table 2. It can be seen that the im-
mobilization of 4-oxo-TEMPO results in a decrease in
Figure 1. The nitrogen adsorption/desorption isotherms of
catalysts 1 (top), 2 (middle) and 3 (bottom).
NOx temperature programmed desorptions (NOx-
the BET surface area from 228.8248 to TPD) were employed to measure the adsorbed
207.9908 m2·gÀ1. A decrease of pore volume from amount of NOx. The NOx-TPD curves of catalyst
0.753433 to 0.617040 cm3·gÀ1 is also observed after the PSB-TEMPO, before (Figure 3, top) and after
immobilization process. The average pore diameters (Figure 3, bottom) the adsorption of NOx, are shown
(4V/A) decrease from 11.2029 to 9.8081 nm for the below. The amount of desorbed NOx of samples was
same reason.
calculated from the area under their corresponding
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Porous Silica Beads Supported TEMPO and Adsorbed NOx (PSB-TEMPO/NOx)
Figure 2. The BJH adsorption pore size distribution of cata-
lysts 1, 2 and 3.
Table 2. The physical data of catalysts 1, 2 and 3.
Physical data
2
1
3
BET surface area [m2·gÀ1]
Pore volume[a] [cm3·gÀ1]
228.8248 207.9908 208.8081
0.753433 0.617040 0.628208
average pore diameter[b] [nm] 11.2029 9.8081
9.9334
[a]
BJH adsorption cumulative volume of pores, the diame-
ters of the pores are between 1.7000 nm and
300.0000 nm.
[b]
BJH adsorption average pore diameter (4V/A).
Figure 3. The NOx-TPD curves of catalyst 1, before (top)
and after (bottom) the adsorption of NOx.
concentration curves, and the value is 0.720 mmol·gÀ1
(Table 3, entry 2).
After the characterization of catalyst PSB-TEMPO, the average pore diameter (from 9.8081 to
benzyl alcohol (1 mmol) was used to test the activity 9.9334 nm). Besides, the type of isotherm (Figure 1,
of the catalytic system PSB-TEMPO/NOx (Table 4). bottom) and the shape of pore size distribution curve
In all 15 h were needed to complete the reaction (Figure 2) of catalyst 3 are almost identical with those
when the catalyst loading was 0.3 g (Table 4, entry 1). of catalyst 1, indicating that the impact of the 10 con-
When the loading was increased to 0.5 g, the reaction secutive oxidation reactions on the catalyst structure
time shrank to 4 h (Table 4, entry 2). Another 1.5 h was not obvious. Thus we conjectured that the NOx
was saved when the loading was increased to 0.7 g storage capacity of catalyst 3 which is largely affected
(Table 4, entry 3). Comparing the results, 0.5 g was by the pore canal structure has remained intact.
seen to be the optimal quantity of catalyst loading.
Finally, the optimal catalytic system was applied to
In order to investigate the recyclable performance various alcohols as summarized in Table 5. Aliphatic
of catalyst 1, the oxidation of substrate benzyl alcohol secondary alcohols including 2-octanol, cyclohexanol
in 10 consecutive runs under the optimal oxidation gave good conversions of the corresponding ketones
conditions was carried out. The results appeared to be (entries 13 and 16). Analiphatic primary alcohol, iso-
excellent, complete conversions into benzaldehyde in octanol, showed low activity under these conditions,
4 h, indicating that PSB-TEMPO is a sufficiently recy- but the complete oxidation could also be achieved
clable heterogeneous catalyst.
with a longer oxidation time, and the selectivity could
Subsequently, the nitrogen adsorption/desorption reach up to 96%. The aliphatic primary alcohol penta-
analyses were conducted on catalyst 3 (PSB-TEMPO- nol was completely overoxidized into pentanoic acid.
10: the recycled PSB-TEMPO which has been used Benzylic alcohols including primary alcohols (enries
for 10 times) to determine any changes in the pore 2–9) and secondary alcohols (entries 10 and 11) can
canal structure. And the results are listed in Table 2, be oxidized into their corresponding aldehydes and
they showed that there is no significant change in the ketones in complete conversions within 5 h. But these
surface area (from 207.9908 to 208.8081 m2·gÀ1), the aromatic aldehydes were not stable, they were oxi-
pore volume (from 0.617040 to 0.628208 cm3·gÀ1) and dized to the carboxylic acid quickly when exposed to
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COMMUNICATIONS
Table 3. The desorbed amount of NOx.
wide range of alcohols can be converted into their
corresponding carbonyl compounds with almost com-
plete conversions and high selectivities in just 5 h with
the loading of catalyst set to 0.5 g. Moreover, this het-
erogeneous catalyst was shown to be stable during the
oxidation reactions, thus allowing it to be recycled for
10 consecutive runs without any significant loss of ac-
tivity. Besides, the simple product separation process
makes this catalytic system more attractive. This is an
environmentally friendly catalyst instead of the noble
metal and inorganic compounds which will produce
tons of contamination. Further studies will focus on
Entry Catalyst
NO
[mmol·gÀ1]
NO2
[mmol·gÀ1]
NOx
[mmol·gÀ1]
G
U
ACHTUNGTRENNUNG
1
2
1 (before)[a]
1 (after)[b]
0
0
0
0.402
0.318
0.720
[a]
[b]
Before the adsorption of NOx.
After the adsorption of NOx.
the air. The heteroaromatic alcohol furfuryl alcohol the improvement of the catalyst support and the ad-
was oxidized into 2-furaldehyde completely within 4 h sorption method of NOx.
(entry 12).
In our case, the reaction occurs in the pore canals
of PSB where the catalyst 4-oxo-TEMPO is immobi- Experimental Section
lized. And the co-catalyst NOx is also stored in the
pore canals through adsorption. The oxidation reac- Preparation of catalyst
tion under this catalytic system can be divided into 3
4-Oxo-TEMPO was prepared according to the literature
processes (Scheme 3): (i) the alcohols diffuse into the
pore canal of PSB; (ii) the alcohols are converted into
their corresponding carbonyl compounds by the cata-
lysts in the pore canals; (iii) the carbonyl compounds
diffuse out of the pore canal. We know that 4-oxo-
TEMPO and NO2 are stored in the pore canals al-
ready, once the alcohol molecules diffuse into the
pore canal and the 3 molecules come together, the ox-
idation reaction occurs. As for homogeneous condi-
tions of the TEMPO/NOx system, the encountering
probability of the three molecules is lower than that
in the pore canals. So we speculate that the catalytic
efficiency under homogeneous conditions of the
TEMPO/NOx system should be lower than that of the
PSB-TEMPO/NOx system. The reaction mechanism
can be depicted as follows (Scheme 3), the oxoammo-
nium cation (PSB-TEMPO+) which is initially oxi-
dized from PSB-TEMPO is reduced to PSB-
TEMPOH in the oxidation process of alcohols (cycle
I). NO2 is responsible to the oxidation of PSB-
TEMPOH to PSB-TEMPO+, at the same time NO2 is
reduced to NO (cycle II) which will be reoxidized by
O2 which diffuses into the pore canals from the air.
In conclusion, a highly efficient heterogeneous cata-
lytic system, PSB-TEMPO/NOx, for the oxidation of
procedure.[23] PSB-TEMPO was prepared according to the
procedure depicted in Scheme 2.[18b] The adsorption proce-
dures of NOx are as follows: sulfuric acid solution
(H2SO4:H2O=1:2 [v:v], 8 mL) was placed in a separatory
funnel and dropped onto sodium nitrite (3.45 g) which was
placed in a jar at the average rate of 3 drops per minute.
And the NOx gas that came into being was firstly dried by a
calcium chloride tube, then fed into a adsorption tube
(15 mm inside diameter and 10 cm length) in which the
PSB-TEMPO (0.5 g) was placed and finally introduced into
a tail gas adsorber. The overall adsorption time was 1 h.
When the NOx adsorption was finished, the connection be-
tween the calcium chloride tube and the adsorption tube
was shut down, while keeping the connection between the
adsorption tube and tail gas adsorber open for 2 h to let the
physisorbed NOx be released from the surface of PSB-
TEMPO.
Characterization of Catalysts
The nitrogen adsorption/desorption isotherms were mea-
sured with a Micromeritics ASAP2020 physical adsorption
apparatus at 77 K. The BET (Brunauer–Emmett–Teller)
equation was used to calculate the specific surface area of
sample, in an N2 relative pressure (P/P0, P0: the saturated
vapor pressure of N2 at 77 K) between 0.05 and 0.3. The
pore diameter distributions were derived from the adsorp-
alcohols in air at ambient temperature is reported. A tion branches of the isotherms using the Barrett–Joyner–
Table 4. The oxidation of benzyl alcohol[a] over different catalyst loadings.
Entry
1 (after)[b] [g]
Time [h]
Product
Conversion [%][c]
Selectivity [%][c]
1
2
3
0.3
0.5
0.7
15
4
2.5
benzaldehyde
benzaldehyde
benzaldehyde
>99
>99
>99
>99
>99
>99
[a]
The quantity of benzyl alcohol is 1 mmol.
After the adsorption of NOx.
Conversions and selectivities are based on gas chromatography (GC) with area normalization.
[b]
[c]
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Porous Silica Beads Supported TEMPO and Adsorbed NOx (PSB-TEMPO/NOx)
Table 5. The oxidation of alcohols over optimal reaction conditions.[a]
Entry
1
Substrate
Product
Time [h]
4
Conversion [%][b]
Selectivity [%][c]
>99
>99
2
3
4
4
>99
>99
91
>99
4
5
4
4
>99
>99
>99
>99
6
4
>99
>99
7
8
9
4
5
5
>99
>99
>99
>99
>99
>99
10
11
4
4
>99
>99
>99
>99
12
13
4
>99
>99
>99
10
89
14
20
>99
98
15[d]
16
5
5
>99
>99
0
>99
[a]
All reactions were carried out according to the general procedure, see Experimental Section.
Conversions and selectivities are based on gas chromatography (GC) with area normalization.
Selectivity refers to the product aldehydes and ketones.
[b]
[c]
[d]
Complete conversion to pentanoic acid was detected.
Halen (BJH) model. The pore volumes were determined ac-
cording to the BJH adsorption cumulative volume of pores
between 1.7000 nm and 300.0000 nm diameter. Elemental
analyses (C, H, N) were carried out on an Elemental vario
EL III. The NOx temperature-programmed desorption
(NOx-TPD) was conducted on a home-made set-up in the
laboratory of Professsor X. P. Wang from Dalian University
of Technology. The catalyst (0.05 g) was placed in the de-
sorption tube and heated from 258C to 5008C in an N2 flow
(50 mLminÀ1) at 108C·minÀ1, the outlet gas composition was
analyzed online with a chemiluminesence detector (Ecophy-
sis CLD 60).
General Procedure for the Alcohol Oxidation
The alcohol substrate (1.0 mmol), PSB-TEMPO/NOx (0.5 g)
and dichloromethane (6 mL) were placed in a 50-mL long-
necked, round-bottom flask equipped with a ground-glass
elbow to communicate with the atmosphere. The resulting
mixture was magnetically stirred at room temperature and
ambient pressure. The reaction was monitored by TLC and
GC (Agilent 6890N). After completion of the reaction, the
reaction mixture was vacuum filtered to separate the cata-
lyst, which was washed 3 times with dichloromethane
(10 mL), dried and sent to adsorb NOx gas before reuse.
The organic layers were combined, and the solvent was re-
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Supporting Information
The specification of the chemicals and reagents and the gen-
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Acknowledgements
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