Synthesis of fibrous nano-silica-supported TEMPO and its application in selective oxidation of alcohols

Full text of DOI:10.1246/cl.130788

doi:10.1246/cl.130788
Published on the web September 21, 2013
1505
Synthesis of Fibrous Nano-silica-supported TEMPO and Its Application
in Selective Oxidation of Alcohols
Jie Zhu, Xue-jing Zhao, Peng-cheng Wang, and Ming Lu*
Nanjing University of Science and Technology, 200# Xiaolingwei Road, Nanjing 210094, P. R. China
(Received August 25, 2013; CL-130788)
the fibrous silica nanospheres, a pseudo-homogeneous system was
formed during the reaction process, and both improved reaction
efficiency and simple separation of the catalyst were achieved.
The nanosize and large surface area of KCC-1 make it an
excellent choice for TEMPO grafting. As opposed to normal
nanoparticles with pores to maintain the large surface area, the
fibrous KCC-1 ensured that TEMPO was grafted on the external
surface, which could dramatically increase the accessibility
between the substrate and the catalyst. The nanosized catalyst
provided a pseudo-homogeneous system rather than a heteroge-
neous one, which also greatly facilitated the reaction process. The
first step in synthesizing this catalyst was to functionalize KCC-1
with amino groups, which was achieved with 3-aminopropyltri-
ethoxysilane to generate amino-functionalized KCC-1, denoted
as KCC-1-NH2. Then, the obtained aminopropyl-functionalized
nanoparticles were subjected to reductive amination with
1-hydroxy-4-oxo-2,2,6,6,-tetramethylpiperidine in the presence
of NaBH3CN to prepare the corresponding KCC-1-supported
TEMPO (KCC-1/TEMPO).
A fibrous nanosized catalyst with TEMPO supported on silica
nanospheres was synthesized and used in the selective oxidation
of alcohols into the corresponding aldehydes or ketones. The
fibrous morphologies of the catalyst allowed easy accessibility
between the substrate and the catalyst, and thus, improved
reaction efficiency was obtained. A pseudo-homogeneous system
was formed during the reaction process, with easy recovery of the
catalyst through filtration.
Selective oxidation of primary and secondary alcohols into
the corresponding aldehydes or ketones is undoubtedly one of
the most important and challenging transformations in organic
chemistry.1 Many oxidation reagents such as Ru-, Se-, Cr-, and
Mn-based oxides2 and hypervalent iodine3 have been traditionally
used to accomplish this transformation. These oxidants, however,
tend to be expensive and generate large amounts of toxic heavy
metal waste, which inhibit their application. Recently, utilization
of the stable nitroxyl radical 2,2,6,6-tetramethylpiperidine 1-oxyl
(TEMPO) for the oxidation of alcohols, employing a cooxidant
such as NaOCl,4 trichloroisocyanuric acid,5 m-chloroperbenzoic
acid,6 oxone,7 or a hypervalent iodine compound8 appears very
appealing in view of catalytic efficiency and green chemistry
concerns. Irrespective of the cooxidant is used, however, product
isolation and TEMPO recovery remain key issues.
The IR spectrum (Figure S1)16 of KCC-1-NH2 showed
detectable changes that were characteristic of the -NH2 group,
between 3000 and 3500 cm¹1, C-H stretching vibrations in APTS
around 1500 cm¹1, which clearly differed from that of the bare
KCC-1 nanoparticles. Some characteristic peaks due to TEMPO
¹1
at around 1300 cm were also exhibited when comparing the IR
spectrum of the 4-hydroxy-2,2,6,6-tetramethylpiperdine-1-oxyl
raw material (4-oxo-TEMPO) and the catalyst KCC-1/TEMPO.
A minor difference could be detected when comparing the IR
spectrum of catalyst KCC-1/TEMPO with that of KCC-1-NH2.
However, the characteristic peaks belonging to TEMPO were
not too obvious. There are probably two main reasons for this
observation. First is the overlap of characteristic bands for the
amino group and TEMPO. Second, is the lower amount of
TEMPO compared with the bulk silica, and the fibrous morphol-
ogies hindered its detection.
Hence, to further confirm the immobilization of TEMPO,
thermogravimetric (TG) studies for KCC-1, KCC-1-NH2, and
KCC-1/TEMPO were also carried out, as shown in Figure S2.16
KCC-1 showed only negligible (about 1.2%) weight loss from
50 to 800 °C. As for KCC-1-NH2, after an initial weight loss of
absorbed moisture up to 110 °C, the nanocomposite was stable
until 450 °C, after which a weight loss of 12% was observed up
to 650 °C, which can be attributed to the loss of the covalently
bound aminopropyl groups. For KCC-1/TEMPO, two-step
weight losses could be observed. 4-Oxo-TEMPO decomposed
completely before 200 °C, as shown in curve d (Figure S2).
Hence, the weight loss of KCC-1/TEMPO in the same range of
To address these problems, several supported TEMPO
catalytic systems have been developed, including silica-supported
TEMPO,9 MCM-41-supported TEMPO,10 sol-gel-entrapped
TEMPO,11 and polyamine-immobilized piperidinyloxyl (PIPO),12
poly(vinyl alcohol)-graft-poly(ethylene glycol) resin-supported
TEMPO,13 to afford heterogeneous catalysts, are readily separated
from the reaction mixtures but are usually far less versatile than
the homogeneous TEMPO. Immobilization of TEMPO onto ionic
liquids to realize a homogeneous process has also been reported.
However, the disadvantages are also obvious, including the
complexity of preparation and storage, difficulty in separation,
and loss of activity during recycles.
The past decade has seen significant advances in the
fabrication of new porous solids with ordered structures from a
wide range of different materials, with silica being the most
common one. In particular, Polshettiwar reported a silica nano-
sphere with fibrous morphologies, KCC-1, with the high surface
area attributed to the fibers but not to the pores, which
dramatically increases its accessibility.14 The unique property
should be useful in the design of silica-supported catalyst, but
only limited research has been carried out in this area.15
Herein, in this paper, a nanosized catalyst with TEMPO
supported on silica nanospheres was synthesized, KCC-1/
TEMPO. The nanosized catalyst with a fibrous morphology
offered a large surface area and was proved to be effective in the
selective oxidation of various alcohols into their corresponding
carbonyl compounds. Thanks to the combination of TEMPO and
temperature was proposed to be the loss of TEMPO. A loading
¹1
of TEMPO at approximately 0.3 mmol g
was confirmed.
There was still some weight loss around 650 °C for KCC-1/
TEMPO, which was attributed to the remained aminopropyl
groups.
Chem. Lett. 2013, 42, 1505-1507
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1506
proceed smoothly with the help of O2. The reaction between NO2
and hydroxylamine led to the completion of the recycling of
KCC-1/TEMPO. The reaction efficiency increased directly along
with the amount of catalyst and was almost free from the effect of
the silica support. A catalytic amount of KCC-1/TEMPO (as less
as 5 mol % that of substrate) was enough to obtain a good yield,
and further increase in the catalyst amount would be meaningless.
Reaction termination could be observed with the removal of it.
In the view of TOF (turnover frequency), 4-oxo-TEMPO
performed much better compared with the supported ones,
probably because of the nature of the homogeneous process.
The TOF of the pseudo-homogeneous TEMPO-catalyzed process
would be similar under the same reaction conditions. With
temperature being the most important influence factor on TOF,
high temperatures were favorable for the reaction, and excellent
catalytic performance was achieved at 80 °C. After the supporting
process, the activity (reflected as TOF) decreased from 13.8 to
9.1, probably because of the transformation from the homoge-
neous to the pseudo-homogeneous system. TEMPO with KCC-1
support (with a surface area of 641 m2 g¹1 15) showed both a
higher TOF and yield compared with nano-SiO2 and Al2O3 (with
a surface area of 500 and 150 m2 g¹1, respectively). This was
attributed to the larger surface area and greater accessibility of
fibrous KCC-1 compared with both nano-SiO2 and Al2O3. The
bad dispersion of TEMPO on support with smaller surface area
would lead to less active site than that in theory.
All these facts indicated that it was still TEMPO that served
as the active species of oxidation reactions. The immobilization
process would not change the reaction mechanism and the activity
decrease was not obvious either, probably because of the pseudo-
homogeneous process and fibrous morphologies, which provide
the large surface area and ready access to the substrate molecules.
The merits of introducing KCC-1 nanoparticles contribute to both
good catalytic performance and easy recycling.
To examine the utility and generality of this methodology for
the oxidation of alcohols, we applied the present catalyst system
to various alcohols. The substrate scope was extended to benzylic,
allylic, heterocyclic, alicyclic, and aliphatic alcohols. In all cases,
ketones or aldehydes were the only detected reaction products.
The oxidation results of aromatic alcohols are shown in Table 1.
Obviously, all the primary benzylic alcohols were converted into
their corresponding aldehydes in high yields. Secondary alcohols
such as benzhydrol and 2-phenylethanol also gave a yield of 53%
and 58%, respectively. It is noteworthy that a type of heterocyclic
alcohol, which was less active in many reported systems, worked
well in this system.
Furthermore, oxidations of some aliphatic alcohols were
tested, as listed in Table 2. With 2 mol % of the catalyst, the
activities exhibited by secondary alcohols were not too low in the
system. It was found that the oxidative efficiency was not affected
by the existence of the double bond and that the double bond
remained stable during the oxidation process. Unfortunately,
in case of aliphatic alcohols such as 1-C8H17OH and isooctyl
alcohol, the results were unsatisfactory even after increasing the
reaction time.
Usually, leaching is a major problem hindering the recycling
process. In order to further investigate the stability of the catalyst
during our catalytic process, TG and IR measurements for freshly
prepared KCC-1/TEMPO and the catalyst reused 5 times with the
same mass were carried out, as shown in Figure S3.16 The TG
curve for the reused catalyst revealed a weight loss process similar
Figure 1. (a) TEM images of KCC-1/TEMPO; (b) SEM images of
KCC-1/TEMPO.
Transmission electron microscope (TEM) and scanning
electron microscope (SEM) images (Figure 1) indicated that the
material consisted of colloidal spheres of uniform size, with
diameters that range from 200 to 300 nm (the length of fibers was
not included). The catalyst also exhibited excellent dispersivity
without assembly or scattered SiO2. It confirmed that the coating
of amino groups and grafting of TEMPO did not destroy the
morphologies and that KCC-1/TEMPO retained the fibrous
structure of the original KCC-1. The dendrimeric fibers could
be observed clearly on these materials to form spheres, which
allowed easy access to the available large surface area.
The size of KCC-1/TEMPO could tend to the nanoscale, as
also reflected in Figure 1. During the oxidation process, nano-
sized KCC-1, carrying TEMPO, was well distributed in the
solvent to form a pseudo-homogeneous system rather than a
heterogeneous one, which alleviated the disadvantage of hetero-
geneous immobilization. Furthermore, as opposed to the homo-
geneous catalytic system of TEMPO, there was an advantage of
recycling since the catalyst was on the order of hundreds of
nanometers and could easily be recovered through filtration.
The performance of the fibrous catalyst was first tested in the
oxidation of benzyl alcohol with O2 as the oxidant and NaNO2
as the cooxidant; the comparable results are summarized in
Table S1.16 It was found that almost no product was obtained in
pure acetonitrile without any catalyst or with KCC-1, KCC-1/
NH2 only without TEMPO. As the oxidant and cooxidant, both
O2 and NaNO2 were essential to the oxidation process. KCC-1/
TEMPO (nitrosonium) oxidized alcohols into their corresponding
aldehydes or ketones and was itself converted into hydroxylamine
¹
(KCC-1/TEMPOH). NO and NO2 were released from NO2
under acidic conditions and the oxidation of NO into NO2 could
Chem. Lett. 2013, 42, 1505-1507
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1507
Table 1. Oxidation of aromatic alcoholsa
hundreds of nanometers, the catalyst could be separated simply by
filtration. More than 90% of the catalyst could be recovered from
the reaction system, with only a trace amount remaining in
CH3CN, which was also proved by comparing the solution state
before and after separation, as shown in Figure S4. This also
ensured the possibility of reuse of the catalyst.
OH
OH
OH
OH
OMe
OH
OMe
99%
(98%)
OMe
96%
(95%)
96%
(94%)
CH3
3 h
94%
(92%)
4 h
4 h
98%
(98%)
With benzyl alcohol as the substrate, the reusability of the
catalyst was further studied. As shown in Figure S4, the
recovered KCC-1/TEMPO can be reused at least 5 times without
any notable loss of activity. High selectivity was maintained as
well during the recycling, without any apparent decline. The
leaching process was not obvious, as mentioned above, and the
recovered catalyst was sufficient to achieve 5 rounds of recycling.
In conclusion, a novel catalyst that was efficient for selective
alcohol oxidation was prepared with TEMPO supported on silica
nanospheres. The catalyst maintained the fibrous morphology of
the KCC-1 support, which dramatically increased the accessibility
between the substrate and the catalyst. A wide set of aliphatic,
allylic, heterocyclic, and benzylic alcohols could be oxidized into
the corresponding carbonyl compounds with good-to-excellent
yields. As the catalyst was on the order of hundreds of
nanometers, a pseudo-homogeneous system was realized during
the reaction, and both improved reaction efficiency and simple
separation of the catalyst could be achieved.
3 h
2 h
OH
OH
OH
OH
NO2
OH
CH3
Ph
CH3
Ph
Ph
NO2
54%
(53%)
99%
(98%)
59%
83%
(81%)
10 h
10 h
6 h
3 h
85%
(82%)
(58%)
7 h
OH
OH
OH
O
S
N
93%
(91%)
84%
(83%)
5 h
5 h
74%
(72%)
4 h
aReaction conditions: alcohol 2 mmol, CH3CN 15 mL, catalyst
0.02 mmol, NaNO2 0.1 mol, 10% H2SO4 0.1 mL, O2 was imputed
of 0.004 m3 h¹1, 80 °C. Conversion was in parenthesis and GC-
yield out of parenthesis.
Table 2. Oxidation of aliphatic alcoholsa
References and Notes
OH
OH
1
2
3
a) D. Romano, R. Villa, F. Molinari, ChemCatChem 2012, 4, 739. b) S.
Caron, R. W. Dugger, S. G. Ruggeri, J. A. Ragan, D. H. B. Ripin,
Chem. Rev. 2006, 106, 2943.
a) Y. Hirai, T. Kojima, Y. Mizutani, Y. Shiota, K. Yoshizawa, S.
Fukuzumi, Angew. Chem., Int. Ed. 2008, 47, 5772. b) H. Sun, Y.
Zhang, F. Guo, Z. Zha, Z. Wang, J. Org. Chem. 2012, 77, 3563.
a) Z. Liu, Z.-C. Chen, Q.-G. Zheng, Org. Lett. 2003, 5, 3321. b) C.
Zhu, A. Yoshimura, Y. Wei, V. N. Nemykin, V. V. Zhdankin,
Tetrahedron Lett. 2012, 53, 1438.
OH
80%
(79%)
88%
(87%)
10 h
65%
(63%)
6 h
12 h
OH
7%
OH
OH
6
4
5
a) X.-E. Wu, L. Ma, M.-X. Ding, L.-X. Gao, Synlett 2005, 607. b) A.
Gheorghe, A. Matsuno, O. Reiser, Adv. Synth. Catal. 2006, 348, 1016.
a) L. De Luca, G. Giacomelli, S. Masala, A. Porcheddu, J. Org. Chem.
2003, 68, 4999. b) L. De Luca, G. Giacomelli, A. Porcheddu, Org.
Lett. 2001, 3, 3041.
97%
(96%)
7%
(7%)
3 h
24 h
24 h
(6%)
OH
OH
OH
6
7
a) J. A. Cella, J. A. Kelley, E. F. Kenehan, J. Org. Chem. 1975, 40,
1860. b) S. D. Rychnovsky, R. Vaidyanathan, J. Org. Chem. 1999, 64,
310.
a) C. Bolm, A. S. Magnus, J. P. Hildebrand, Org. Lett. 2000, 2, 1173.
b) P. L. Bragd, A. C. Besemer, H. van Bekkum, Carbohydr. Polym.
2002, 49, 397.
a) T. Yakura, A. Ozono, Adv. Synth. Catal. 2011, 353, 855. b) X.-Q. Li,
C. Zhang, Synthesis 2009, 1163.
a) L. Di, Z. Hua, Adv. Synth. Catal. 2011, 353, 1253. b) T. Fey, H.
Fischer, S. Bachmann, K. Albert, C. Bolm, J. Org. Chem. 2001, 66,
8154.
6
10
28%
(27%)
7%
(6%)
24 h
24 h
89%
(89%)
48 h
aReaction conditions: alcohol 2 mmol, CH3CN 10 mL, catalyst
0.04 mmol, NaNO2 0.1 mol, 10% H2SO4 0.1 mL, O2 was imputed
of 0.004 m3 h¹1, 80 °C. Conversion was in parenthesis and GC-
yield out of parenthesis.
8
9
to that of fresh KCC-1/TEMPO, demonstrating the stability of
the catalyst even after 5 times of recycling. However, a slight
decrease in weight loss at both periods of the TEMPO and -NH2
group were observed, which is attributed to the leaching process.
The leaching of a small amount of TEMPO or -NH2 group during
the reaction process led to an increased ratio of silica, so as to
reduce the weight loss in TG. The IR spectrum of the catalyst
reused 5 times also confirmed the conclusion.
10 a) M. J. Verhoef, J. A. Peters, H. van Bekkum, Stud. Surf. Sci. Catal.
1999, 125, 465. b) D. Brunel, F. Fajula, J. B. Nagy, B. Deroide, M. J.
Verhoef, L. Veum, J. A. Peters, H. van Bekkum, Appl. Catal., A 2001,
213, 73.
11 a) R. Ciriminna, J. Blum, D. Avnir, M. Pagliaro, Chem. Commun.
2000, 1441. b) R. Ciriminna, C. Bolm, T. Fey, M. Pagliaro, Adv. Synth.
Catal. 2002, 344, 159.
12 A. Dijksman, I. W. C. E. Arends, R. A. Sheldon, Chem. Commun.
2000, 271.
The advantage of the catalyst KCC-1/TEMPO lies in not
only the high catalytic activity attributed to TEMPO, but also the
pseudo-homogeneous process and ease of separation and recy-
clability provided by the KCC-1 support. As proved by TEM
images, the size of the catalyst reached the nanoscale and a
pseudo-homogeneous process could be achieved when mixing the
catalyst and the solvent, as shown in Figure S4.16 Further, with
13 G. Pozzi, M. Cavazzini, S. Quici, M. Benaglia, G. Dell’Anna, Org.
Lett. 2004, 6, 441.
14 V. Polshettiwar, D. Cha, X. Zhang, J. M. Basset, Angew. Chem., Int.
Ed. 2010, 49, 9652.
15 A. Fihri, M. Bouhrara, U. Patil, D. Cha, Y. Saih, V. Polshettiwar, ACS
Catal. 2012, 2, 1425.
16 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2013, 42, 1505-1507
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/