TEMPO radical polymer grafted silicas as solid state catalysts for the oxidation of alcohols

Article abstract of DOI:10.1039/c3ra41823e

TEMPO polymer-grafted silicas were synthesized by "grafting from" and "grafting to" methods using RAFT polymerization, and their catalytic activities as a new class of solid state catalyst for oxidation reactions with alcohols demonstrated.

Full text of DOI:10.1039/c3ra41823e

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TEMPO radical polymer grafted silicas as solid state
catalysts for the oxidation of alcohols3
Cite this: DOI: 10.1039/c3ra41823e
Kei Saito,a Koji Hirose,b Teruyuki Okayasu,b Hiroyuki Nishideb and Milton T.
W. Hearn*a
Received 5th November 2012,
Accepted 29th April 2013
TEMPO polymer-grafted silicas were synthesized by ‘‘grafting from’’ and ‘‘grafting to’’ methods using
RAFT polymerization, and their catalytic activities as a new class of solid state catalyst for oxidation
reactions with alcohols demonstrated.
DOI: 10.1039/c3ra41823e
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One of the most important desiderata in the field of green and
sustainable technological processes is improvement in catalyst
performance.1 Although heterogeneous catalysts are favoured
for many chemical reactions within industrial settings, the
ability to recycle the catalyst often remains a challenge after
the chemical reaction is completed, or when improvements to
the overall economic efficiency of the process have to be
realised.2 One of the most significant tasks in organic
synthesis is the oxidation of alcohols.3 However, such
oxidation reactions have frequently been performed with
stoichiometric amounts of inorganic oxidants, many of which
are extremely hazardous to use or toxic.4 The availability of
easy-to-handle heterogeneous catalysts, which can easily
recycled after use in liquid phase oxidation reactions, and
which have low toxicity, good stability and high efficiency is
thus an essential requirement.
The compound 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO)
is known to be a stable organic radical that can be stored for
long periods of time without decomposition. A wide variety of
derivatives and analogues of this type of stable free organic
radical has been reported.5 Since TEMPO can work as a redox
reagent, it has been used in oxidation reactions under a variety
of conditions{.6 Several TEMPO functionalized solid catalysts,
prepared by immobilising individual TEMPO molecules onto
solid support materials as a covalently anchored mono-
molecular layer, have been previously studied for their
potential in oxidation reactions.7 However, these reactions
usually require more than stoichiometric amounts of these
mono-molecular layer solid state catalysts to achieve accep-
table product yields due to the low loading of TEMPO
molecules onto the solid support. To overcome this constraint,
attention has turned to the generation of TEMPO functiona-
lized polymers allowing the introduction of higher densities of
TEMPO. For example, our group has reported the preparation
of TEMPO functionalized polyvinyl polymers for use as new
materials in organic batteries, which can be charged in
seconds via the oxidation of the nitroxide radical and
reduction of the corresponding oxoammonium moiety of the
TEMPO molecule.8 However, polymeric TEMPO functionalized
solid state catalysts based on multiply-displayed TEMPO units
introduced into a polymeric backbone, which has been
concomitantly immobilized onto a carrier surface, have not
been previously reported for use in oxidation reactions.
Based on the hypothesis that a multiply-displayed TEMPO
polymer grafted solid state catalyst of high radical density
would lead to recyclable heterogeneous catalysts with
improved catalytic activity and efficiency due to the increased
number and accessibility of the active sites of the catalyst to
the substrate, we describe herein methods to prepare a new
type of TEMPO polymer grafted silica materials and describe
their application as novel solid state catalysts for oxidation
reactions. The intent of this study was to evaluate the catalytic
performance of this new class of oxidative heterogeneous
catalyst when used in loco multiple times, thus circumventing
the constraints found with single cycle heterogeneous cata-
lysts.
aCentre for Green Chemistry, Monash University, Clayton, Victoria 3800, Australia.
E-mail: milton.hearn@monash.edu; Fax: +61-3-9905-8501; Tel: +61-3-9905-4547
bDepartment of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan.
E-mail: nishide@waseda.jp; Fax: +81-3-3209-5522; Tel: +81-3-3200-2669
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra41823e
{ Typical conditions for the oxidation of alcohols: The alcohol (4 mmol) was
mixed with the catalyst (0.02 mmol) in 10 mL of CH2Cl2 at 5 uC. An aqueous
solution of KBr (0.5 M, 0.8 mL) and NaOCl (0.35 M, 14.3 mL with the pH
adjusted to pH 9 by NaHCO3) was added to the reaction mixture, which was then
stirred for 10 min at 5 uC. After this time, the catalyst was removed by filtration,
the organic layer recovered and the CH2Cl2 removed by gentle rotary evaporation
to yield the product. Analogous procedures were employed with water and the
other solvent systems. The oxidation reaction of benzylalchol was carried out in
a large scale and the product conversion of the reaction was calculated by
isolated yield.
To this end, a series of multiply-displayed TEMPO polymer
grafted silicas has been synthesized by grafting poly(2,2,6,6-tetra-
methylpiperidinyl-oxy methacrylate) (PTMA)9 onto silica. In initial
experiments, a ‘‘grafting from’’ method was employed using the
RAFT chain transfer agent, S-methoxycarbonyl-phenyl-methyl
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Scheme 1 TEMPO polymer grafted silica synthesis by ‘‘grafting from’’ method.
Scheme 2 Synthesis of the TEMPO polymer grafted silica by the ‘‘grafting to’’
method.
S9-trimethoxysilylpropyltrithiocarbonate, introduced onto the
silica surface to form a silica-supported chain transfer agent,
1.10 Then, 2,2,6,6-tetramethylpiperidine methacrylate was graft
polymerized using 1 by RAFT polymerization and the so-formed
precursor, poly(2,2,6,6-tetra-methylpiperidine methacrylate)
grafted silica treated with 3-chloroperoxybenzoic acid to yield
the TEMPO polymer grafted silica 2 (Scheme 1).
cyano-4-[(phenyl-thioxomethyl)thio]-1-(2-carboxyethyl)-1-cya-
noethylbenzodithio-ate),13 and treated with 3-chloroperoxy-
benzoic acid to form poly(PTMA), 3. By tuning the amount of
the chain transfer agent present, different 39s of various
molecular weights were synthesized. The molecular weights of
these polymers were again measured by GPC. The number of
radical groups introduced onto the poly(PTMA) was measured
by SQUID, demonstrating that all of these polymers had very
high radical concentrations, typically above 90% of the
theoretical value.
The prepared 39s were then grafted onto synthesized
amino-functionalized silica (ca. 2.0 mmol g21) of particle size
of ca. 10 nm using a condensation agent to yield a series of
different TEMPO polymer grafted silicas, 49s (Scheme 2).
N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was
used as the condensation agent. EEDQ was selected for this
condensation reaction since this reagent is readily available at
low cost and allows the coupling in high yield in a single one-
pot operation.14 The molecular weights of the poly(PMTA)s
and the other characterization data of the different 49s are
summarised in Table 1. All samples showed a high level of
TEMPO immobilization (up to 2.1 mmol g21) and a radical
concentration greater than 90% of the theoretical value The
physical form and size of the derived TEMPO immobilization
particles was consistent with the presence of an extensively
grafted layer.
Illustrative of these results, the morphology of the synthetic
material 4-2 (entry 2, in Table 1) was determined by scanning
electron microscope (SEM) and dynamic light scattering (DLS).
A SEM image of 4-2 (Fig. 1) demonstrated the presence of
largely spherical objects in the size range of 80–110 nm. The
presence of a small percentage of particles of ca. 10 nm
average diameter can be attributed to the un-reacted fumed
silica material. The average size of 4-2 was 108 nm from DLS
which supported the result from SEM. The size of 4-2 was
significantly higher than the size of the original amino
functionalized silica support, which had an average particle
size of 9 nm. As noted above, the size difference between the
synthesized 4-2 and amino functionalized silica confirms the
existence of an extensively grafted TMEPO polymer layer on
the surface of the silica.
The product, 2, was characterized by IR, elemental analysis,
and thermogravimetric analysis (TGA). The polymer graft ratio
present for 2 was 57 wt% as calculated from the weight loss
observed by TGA from 200 uC to 600 uC due to the
decomposition and removal of TEMPO polymer from the
surface of the silica. Depending on the polymerization
conditions, immobilized densities of the TEMPO units on 2,
determined from the graft ratio, up to 1.6 mmolg21 could be
achieved. This density level is considerably higher than that of
commercially available silicas partially functionalized with a
mono-layer of TEMPO molecules (e.g. mono-TEMPO-Si, 0.6
mmol g21).11 The molecular weight of the optimally grafted
TEMPO polymer was determined by cleaving the polymer from
2 using an aminolysis reaction.12 The molecular weight of the
cleaved TEMPO polymer was measured by gel permeation
chromatography (GPC) and found to be Mn = 1.9 6 104, Mw/Mn
= 2.6. This value is close to the theoretical value (Mn = 2.3 6
104) calculated from the monomer and chain transfer agent
concentration, indicating that the molecular weight of the
grafted polymer can be controlled by the RAFT polymerization
conditions. The most important value, the number of radical
groups introduced onto the silica surface, was estimated by
superconducting quantum interference device (SQUID) mea-
surements. The amount of radical groups on 2 was found to be
only ca. 45% of the theoretical value. This result suggests that
under the ‘‘grafting to’’ conditions, treatment of the TEMPO
precursor molecule of 2 with 3-chloroperoxybenzoic acid to
form the radical grafted polymer species was mainly asso-
ciated with surface modification, with the intra-porous regions
of 2 not readily accessed due to steric restriction.
Based on these results, the synthesis of a series of TEMPO
polymer grafted silicas with a radical densities approaching
100% of the theoretical value, was undertaken using a
‘‘grafting to’’ method. 2,2,6,6-Tetramethylpiperidine metha-
crylate was RAFT polymerized with the chain transfer agent, (4-
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Table 1 TEMPO polymer grafted silica 4 synthesis by grafting to method
3
entry
Mn (x 104)
Mw/Mn
graft ratioa (wt%)
immobilized TEMPOa (mmol g21
)
radical conc.b (%)
1 (4-1)
2 (4-2)
3 (4-3)
4 (4-4)
1.0
1.7
3.0
7.4
1.2
1.2
1.1
1.2
54
64
66
91
1.56
1.71
1.76
2.11
92
91
90
99
a
b
Calculated by TGA. Estimated by SQUID.
a greatly enhanced capability as oxidation catalysts compared
to monomer TEMPO functionalized silicas due to the higher
radical density that can be achieved by the grafted TEMPO
polymerisation procedure.
Interestingly, the oxidative capacity of the catalysts, 4,
appeared to follow a parabolic dependency on the molecular
weight of the grafted TEMPO polymer. For example, the
catalytic performance of 4 initially increased with polymer
molecular weight (4-1 A 4-2). This can be explained by the
number of immobilized TEMPO units on the catalyst increas-
ing along with the molecular weight of the grafted TEMPO
Fig. 1 SEM image of TEMPO Catalyst 4-2.
polymer. However, above
performance of 4 decreased with the molecular weight of the
grafted polymer (4-2 A 4-4) with 4-4, which corresponded to
a certain value, the catalytic
As an initial evaluation of the catalytic performances of the
TEMPO polymer grafted silicas 4-1, 4-2, 4-3, and 4-4 (entry 1–4
in Table 1, respectively), the oxidation of benzyl alcohol as the
substrate was examined (Table 2). The efficiency of the TEMPO
polymer grafted silica was also compared to a commercially
available mono-TEMPO Si. The reactions can be carried out in
water or in a CH2Cl2/water bi-phasic system. As evident from
the results, 4-2 showed the highest product conversion and
reaction rate constant compared to the other synthetic
materials. In addition, the conversion and the reaction rate
the highest molecular weight grafted TEMPO polymer (Mn
=
7.4 6 104), yielding the lowest conversion and rate constant of
the four different types of 4. Although dispersion of the
catalyst, and thus its efficiency, will be affected by the
molecular weight of the grafted polymer, an alternative
explanation for the decrease in catalytic performance of the
catalyst containing the highest molecular weight polymer
species would be steric crowding of the TEMPO moieties and
lower solvation propensity of the larger polymeric chains.
In order to establish the reusability of the catalyst, the
catalyst was recovered by filtration and reutilised in subse-
quent cycles (Fig. 2). The catalyst was found to retain its
activity for at least five cycles of reuse. The performance of the
catalyst remained almost constant above 90% for all of these
recycle experiments.
constant of 4-2 (>95%, 1.01 6 1022 L mol21
higher than that of the mono-TEMPO Si (58%, 0.23 6 1022
mol21 21). No evidence for the oxidative modification of the
s
21) was much
L
s
aromatic ring was obtained as has previously been noted15
with other types of TEMPO-mediated oxidation reactions. The
need to use a co-catalyst, such as transition metal or nitric
oxide derivatives,16,17 or to employ surface coating with an
ionic liquid,18 is also circumvented with this new class of
immobilised polyTEMPO silica-based catalysts. Moreover, the
above results indicate that TEMPO polymer grafted silicas have
These oxidation reactions with 4 were extended to various
other alcohols. The oxidation of 1-octanol and 1-decanol
yielded the corresponding aldehydes in high yield (ca. 98–
Table 2 Product conversion and reaction rate constant of the oxidation benzyl alcohol to benzaldehyde with TEMPO catalystsa
Catalyst
4-1
4-2
4-3
4-4
mono-TEMPO-Si
conversion (%)b
90
0.91
95
1.01
89
0.62
80
0.45
58
0.23
rate constant k1 (61022 Lmol21 s21
)
a
b
Reaction time: 10 min, reaction temp: 5 uC, catalyst/alcohol: 0.5 mol%, solvent CH2Cl2/water. Conversions were determined by NMR and
GC.
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Fig. 3 Time vs. conversion of the oxidation of benzyl alcohol with 4-2 (N =
CH2Cl2/water, # = ethyl acetate/water, & = toluene/water, % = N,N-
dimethylformamide/water, m= MeTHF/water, e = CPME/water).
Fig. 2 Recycling of the catalyst 4-2 in the oxidation of benzyl alcohol (reaction
time: 10 min, reaction temp: 5 uC, catalyst/alcohol: 0.5 mol%, solvent CHCl3/
water).
1
formation of carboxylic acids was not detected by H NMR or
13C NMR in the products from these reactions. These results
suggest that these new types of polyTEMPO grafted catalysts
have potential as effective and selective solid oxidation
catalysts for both primary and secondary alcohols.
As noted above, these reactions can be carried out using a
CH2Cl2/water solvent biphasic condition. In order to replace
CH2Cl2/water (Fig. 3) with a greener solvent for the reaction,
ethyl acetate/water, 2-methyltetrahydrofuran (MeTHF)/water,
cyclopentyl methyl ether (CPME)/water were examined as
alternatives for the oxidation reaction of benzyl alcohols. As
a result, ethyl acetate/water resulted in comparable yields as
CH2Cl2/water, and would thus be a preferred option for these
oxidative reactions including those for the more hydrophobic
alcohols.
99%). The reaction with 1-undecanol, 1-dodecanol, and
4-hydroxy-methylbiphenyl also gave similar results (ca. 99%,
97%, 84%, respectively). In contrast, the reaction with
secondly alcohols, e.g. 2-octanol and 2-decanol, only gave
trace amounts of the corresponding aldehydes (ca. 4% and
2%, respectively) which suggesting a high level of substrate
specificity of the catalyst. The catalyst was also evaluated for
the oxidation of the diol. 2-methyl-1,3-hexanediol. The reac-
tion yielded 54% (rate constant k1 = 1.1 6 1022 Lmol21 s21) of
the corresponding 1-hydroxy-2-methyl-3-ketohexane and a
trace amount of the 2-methyl-3-hydroxyhexanaldehyde from
the reaction (Table 3). Compared to conventional alternatives,
the conversion was much higher than that of, for example,
mono-TEMPO Si (product conversion = 5%, rate constant k1 =
0.45 6 1022 L mol21 s21). The oxidation of another diol, 2,2,4-
trimethyl-1,3-pentanediol, did not proceed with either 4 or the
mono-TEMPO-Si and this observation can be explained by the
steric effect of access to the alcohol sites. Moreover, the
Conclusions
This study documents the synthesis and properties of multi-
ply-displayed TEMPO polymer grafted silicas prepared by the
‘‘grafting to’’ method. The route that was followed involved the
synthesis of several TEMPO polymers by the RAFT polymeriza-
tion of PMTA and then immobilising these precursors onto the
carrier silica. The derived TEMPO polymer silicas have been
found to be very effective as a heterogeneous catalyst for the
oxidation of various alcohols. In this study, aqueous sodium
hypochlorite was used as the terminal oxidant for the
regeneration of these polyTEMPO polymer-grafted solid state
catalysts. Although these conditions lead to a reduction in
atom efficiencies vis-a-vis the use of molecular oxygen within a
single cycle, the described reactions can nevertheless be
readily and efficiently employed in a recycling mode as facile,
transition metal free liquid phase oxidations. In comparison to
alternative procedures based on the use of free or mono-
functionalised TEMPO materials with molecular oxygen for the
aerobic oxidation reactions of a variety of compound classes,19
although superficially of higher atom efficiency, these alter-
native conditions are based on the use of (i) highly flammable
Table 3 Conversion (%) of alcohols to aldehydes with TEMPO catalysta
conversion (%)c
Entry Substrate
product
4b
mono-TEMPO-Si
1
2
3
98
99
53
51
34
5
0.1 0.3
a
Reaction time: 10 min, reaction temp: 5 uC, catalyst/alcohol: 0.5
b
mol%, solvent CH2Cl2/water. 4-3 for entry 1 and 2, 4-2 for entry 3.
c
Conversions were determined by NMR and GC.
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solvents such as a,a,a-trifluorotoluene,20 known also to be very
toxic towards aquatic organisms; (ii) high mole percentages of
TEMPO, typically21 in excess of 10 mole%; or (iii) the need for
various organometallic complexes, e.g. Fe(III) or oxamide Cu(II)
co-catalysts,19,22 in the presence of ionic liquids such as
[bmim]PF6. As a consequence, assessment of the ‘‘green-ness’’
of a particular chemical reaction should take into account
considerations that extend beyond just the atom efficiency of
the reactants themselves, but also incorporate the ‘‘whole-of-
reaction’’ attributes of the various other chemicals and
materials that are employed as part of the synthetic protocol.
As such, oxidative reactions carried out with these new
multiply-displayed TEMPO solid state heterogeneous catalysts
provide a useful addition to the green chemical panel of
catalysts. Moreover, these alternative conditions do not
capture the advantages of densely immobilised TEMPO
polymers as solid state catalysts with very high radical
concentrations for the rapid high yield oxidation of alcohols
as described in this investigation. This study thus documents
useful routes to the preparation and application of a new class
of solid phase oxidation catalysts for use in synthetic reactions
based on more benign and green chemical procedures.
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