Enhanced catalytic activity and recyclability for oxidation of cinnamaldehyde catalysed by β-cyclodextrin cross-linked with chitosan

Article abstract of DOI:10.1080/10610278.2012.758367

Selective oxidation of cinnamaldehyde to benzaldehyde under mild conditions has been carried out for the first time in aqueous phase with β-cyclodextrin-chitosan polymer (β-CD-CTS) as a heterogeneous catalyst. Compared with β-cyclodextrin polymer (β-CDP) catalyst, β-CD-CTS showed significant promotion for the oxidation of cinnamaldehyde to benzaldehyde, in which 78% yield of benzaldehyde could be obtained. In addition, the mechanism of the oxidation was proposed according to the experimental results and theoretical calculation. The results indicated that chitosan had played a key role, and the significant promotion of conversion and selectivity for the oxidation could be attributed to inclusion complex formed via intermolecular weak interactions, e.g. hydrogen bonding between cinnamaldehyde and β-CD-CTS. The heterogeneous catalyst could be reused, and its catalytic efficiency remained unchanged, which suggests that the catalyst is efficient for the oxidation of cinnamaldehyde. The results reported herein may be a promising method for the production of benzaldehyde from natural cinnamaldehyde.

Full text of DOI:10.1080/10610278.2012.758367

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Enhanced catalytic activity and recyclability for
oxidation of cinnamaldehyde catalysed by β-
cyclodextrin cross-linked with chitosan
Zu-Jin Yang a , Hui Zeng a , Xian-Tai Zhou a & Hong-Bing Ji a
a School of Chemistry and Chemical Engineering, The Key Laboratory of Low-carbon
Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University,
Guangzhou, 510275, P.R. China
Published online: 16 Jan 2013.
To cite this article: Zu-Jin Yang , Hui Zeng , Xian-Tai Zhou & Hong-Bing Ji (2013) Enhanced catalytic activity and recyclability
for oxidation of cinnamaldehyde catalysed by β-cyclodextrin cross-linked with chitosan, Supramolecular Chemistry, 25:4,
233-245, DOI: 10.1080/10610278.2012.758367
To link to this article: http://dx.doi.org/10.1080/10610278.2012.758367
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Supramolecular Chemistry, 2013
Vol. 25, No. 4, 233–245, http://dx.doi.org/10.1080/10610278.2012.758367
Enhanced catalytic activity and recyclability for oxidation of cinnamaldehyde catalysed by
b-cyclodextrin cross-linked with chitosan
Zu-Jin Yang, Hui Zeng, Xian-Tai Zhou and Hong-Bing Ji*
School of Chemistry and Chemical Engineering, The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong
Province, Sun Yat-sen University, Guangzhou 510275, P.R. China
(Received 9 August 2012; final version received 7 December 2012)
Selective oxidation of cinnamaldehyde to benzaldehyde under mild conditions has been carried out for the first time in
aqueous phase with b-cyclodextrin–chitosan polymer (b-CD–CTS) as a heterogeneous catalyst. Compared with b-
cyclodextrin polymer (b-CDP) catalyst, b-CD–CTS showed significant promotion for the oxidation of cinnamaldehyde to
benzaldehyde, in which 78% yield of benzaldehyde could be obtained. In addition, the mechanism of the oxidation was
proposed according to the experimental results and theoretical calculation. The results indicated that chitosan had played a
key role, and the significant promotion of conversion and selectivity for the oxidation could be attributed to inclusion
complex formed via intermolecular weak interactions, e.g. hydrogen bonding between cinnamaldehyde and b-CD–CTS.
The heterogeneous catalyst could be reused, and its catalytic efficiency remained unchanged, which suggests that the
catalyst is efficient for the oxidation of cinnamaldehyde. The results reported herein may be a promising method for the
production of benzaldehyde from natural cinnamaldehyde.
Keywords: selective oxidation; b-cyclodextrin–chitosan polymer; benzaldehyde; mechanism; kinetics
1. Introduction
However, a large amount of solvent was required for
recycling b-CD, making this process less economic for
industrial application. b-CD polymer (b-CDP)-catalysed
oxidation of cinnamaldehyde has been tried owing to its
easy recyclability from the reaction mixture (16, 17).
Unfortunately, the decrease in catalytic selectivity was
observed for the immobilised b-CD.
In order to overcome the problem, our newly developed
immobilised b-CD, a b-cyclodextrin–chitosan polymer
(b-CD–CTS, as shown in Figure 1) as a recyclable
heterogeneous catalyst, exhibits much higher catalytic
selectivity and stability for the oxidation of cinnamalde-
hyde to benzaldehyde compared with b-CDP. The
improvement mechanism for the reaction selectivity was
well explained. It should be mentioned that its catalytic
mechanism proceeded in different ways. In addition, the
process can be run in a recirculating batch reactor under
mild conditions (Scheme 1). The work should be significant
for the production of natural benzaldehyde.
In recent years, the use of efficient catalysts for the
manufacture of fine chemicals has attracted much attention
dueto environmentalsafety. Theheterogenisationofcatalytic
systems has been a major area of research for many years due
to easy separation of catalysts from products and substrates in
industrial processes (1–4). Cyclodextrins (CDs) are a torus-
shaped cyclic oligosaccharide composed of 6–8 glucopyr-
anose units linked by glycosidic bonds (named a, b and
g-CD, respectively). It is known that CDs can form inclusion
complexes with different guest molecules in aqueous solution
or in the solid state through host–guest interactions (5, 6).
b-CD is the most abundant and most widely used among
them, and has been widely used as homogeneous catalysts in
various organic reactions, e.g. oxidation, reduction, ring
opening and hydrolysis in aqueous solution (7–11).
Benzaldehyde is the second most important perfume
(after vanillin), and has been widely used in cosmetics,
perfumery, food and pharmaceutical industries (12). With
increasing concern on food quality, the demand for natural
benzaldehyde is growing quickly (13). Because of its
limited supply and high price, synthesis of benzaldehyde
from natural cinnamon oil, which contains more than 80%
of cinnamaldehyde, has been used as a substitute of natural
benzaldehyde (14). Recently, b-CD has been used as a
homogeneous catalyst to improve the reaction selectivity
for the oxidation of cinnamaldehyde to benzaldehyde (15).
2. Results and discussion
2.1 Structure determination
2.1.1 FT-IR characterisation
The Fourier-transform infrared spectroscopy (FT-IR)
spectra of chitosan, b-CD, activated chitosan resins and
b-CD–CTS are shown in Figure 2. The spectrum of
*Corresponding author. Email: jihb@mail.sysu.edu.cn
q 2013 Taylor & Francis

234
Z.-J. Yang et al.
d
c
3100
O
O
3400
OH
HO
OH
b
a
O
O
HO
O
NH
890
HO
O
NH2
O
C
HO
NH
C
CH3
O
CH2
CH2
CH2
1650
1599
2869
1030
2938
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm–1
)
O
C
NH
Figure 2. FT-IR spectra of chitosan (a), b-CD (b), activated
chitosan resin (c) and b-CD–CTS (d).
O
O
HO
HO
Figure 1. Structural representation of b-CD–CTS.
2.1.2 X-ray powder diffraction (XRD) results
The XRD patterns of chitosan and b-CD–CTS are shown in
Figure 3. Characteristic intense peaks at 2u ¼ 108 and 208
were observed for chitosan due to its crystalline nature (20).
The peaks at 2u ¼ 108 disappeared, and the peak at
2u ¼ 208 decreased greatly for b-CD–CTS. The results
demonstrated that chemical modification of chitosan and
grafting with b-CD destroyed its original crystallinity to
some extent.
b-CD–CTS shows peaks ascribable to both b-CD (OZH,
3400 cm21; CZOZC, 1030 cm21) and the chitosan
skeleton (NZH, 3100–3400 cm21; CvO, 1650 cm21
and NZH, 1564 cm21), which is in agreement with the
previously reported data (18).
When chitosan is cross-linked by glutaraldehyde, it
can be seen that the two peaks of activated chitosan beads
at 2938 and 2869 cm21 are stronger than those of chitosan,
indicating that a number of methyl groups are substituted
into chitosan. The peak observed at 1599 cm21 decreased
greatly, and a sharp peak observed at 1650 cm21 is
attributed to the CvN vibrations of Schiff base imines.
When b-CD is immobilised on activated chitosan beads
using epichlorohydrin as a cross-linker, the peak at
2938 cm21 is much stronger, which shows that more
methyl groups are formed within the beads. The peak at
1030 cm21 for b-CD–CTS is assigned to the CZOZC
stretching. The increase in the band intensity at around
1030 cm21 can be attributed to b-CD. In addition, the peak
at 890 cm21 is the characteristic bands of a-(1,4)
glucopyranose in b-CD, which provides further evidence
that b-CD was immobilised on chitosan beads through
cross-linking with epichlorohydrin (19).
2.1.3 TG results
Thermogravimetry (TG) curves of chitosan, activated
chitosan beads and b-CD–CTS are shown in Figure 4. It
can be seen that chitosan exhibits three mass loss steps.
The mass loss of 2% at 808C is caused by dehydration. The
second stage starts at about 2608C with mass loss of 65%
due to heat decomposition of chitosan. The third stage is
a
b
O
O
β-CD-CTS, H2O2
O
O
+
+
0
10
20
30
40
50
60
70
80
NaHCO3, H2O, 333K
2θ/˚
Scheme 1. Oxidation of cinnamaldehyde to benzaldehyde
catalysed by b-CD–CTS.
Figure 3. XRD of chitosan (a) and b-CD–CTS (b).

Supramolecular Chemistry
235
100
80
60
40
20
0
a
c
b
100
200
300
400
500
600
Temperature (˚C)
Figure 4. TGA curves of chitosan (a), activated chitosan resins
(b) and b-CD–CTS (c).
the carbonisation process above 4608C with weight loss of
31%. Activated chitosan beads and b-CD–CTS show two
degradation stages. The first mass loss step, from about 50
to 1208C, concerns mainly the loss of water. Thermal
decompositions of activated chitosan beads and b-CD–
CTS were observed in 230–5808C and 250–5808C,
respectively. The second degradation stage of chitosan
exhibits higher temperature (2608C) than that of activated
chitosan beads (2308C) and b-CD–CTS (2508C). The
result indicates that the modified chitosan is thermally less
stable than chitosan due to the weak cross-linkage.
Figure 5. Solid-state NMR CP/MAS of b-CD (a), chitosan (b)
and b-CD–CTS (c).
However, weak resonances at d 174 and 24 ppm are also
observable in b-CD–CTS, respectively. It is clear that the
acetylated units are not involved in the cross-links.
2.2 Reaction studies
2.2.1 Catalytic activities of b-CD–CTS
As shown in Scheme 1, the oxidation of cinnamaldehyde
was carried out at 608C under 1 atm. From Figure 6, b-CD–
CTS showed a significant high activity Turnover
2.1.4 13C NMR results
Solid-state 13C NMR spectroscopy has been considered as
the most useful tool for the structural analysis of insoluble
polymers (21).
-1
TOF = 256 h
100
80
60
40
20
0
Conversion of cinnamaldehyde
Yield of benzaldehyde
The 13C NMR spectrum of b-CD (a), chitosan (b) and
b-CD–CTS (c) is shown in Figure 5, respectively. Figure
5(a) shows four 13C signals between 50 and 110 ppm due
to spectral overlap of some of the six carbon resonance
lines from the seven a-D-glucose units. As shown in
Figure 5(b), 13C signals appear at d 105.5 (C1), 79.6 (C4),
77.5 (C5), 73.0 (C3), 62.9 (C6), 59.2 (C2), 174 (CvO) and
23 ppm (CH3). b-CD–CTS is observed to be quite similar
to b-CD and chitosan in the solid state, indicating that the
structure characteristics of b-CD and chitosan were well
maintained in b-CD–CTS (Figure 5(c)). However, only
three broad signals in the b-CD–CTS, assigned to an
overlap between 50 and 110 ppm, cause a decrease in
overall structural and packing order. As previously
reported (22), the signals of epichlorohydrin cross-linked
with chitosan are completely hidden by C-2,3,4 and 5 b-
CD peaks. By comparing with 13C NMR spectrum of
chitosan and b-CD–CTS, an additional peak in the range
of 135–150 ppm was observed, indicating that cross-
linking reaction with glutaraldehyde occurred (23).
-1
TOF = 0.33 h
a
b
c
d
e
Figure 6. Catalytic activity of various catalysts: 1 mmol
cinnamaldehyde, 25 ml H2O, 608C, 2.5 h, (a) 4 ml 30% H2O2
(w/w); (b) 4 ml 30% H2O2 (w/w), 2.5 mmol NaHCO3; (c)
4 ml 30% H2O2 (w/w), 2.5 mmol NaHCO3, 2 g chitosan;
(d) 4 ml 30% H2O2 (w/w), 2.5 mmol NaHCO3, 1 mmol
b-CD; (e) 4 ml 30% H2O2 (w/w), 2.5 mmol NaHCO3, 0.1 g
b-CD–CTS (0.0015 mmol b-CD). TOF: mmol oxidised
cinnamaldehyde/(mmol b-CD in the catalyst £ time).
Selectivity: mmol produced benzaldehyde/ mmol oxidised
cinnamaldehyde.

236
Z.-J. Yang et al.
Table 1. Effect of the different amounts of b-CD–CTS on the oxidation of cinnamaldehyde.a
b-CD–CTS Conversion of cinnamaldehyde Yield of benzaldehyde Yield of epoxide Yield of phenylacetaldehyde
(%)
Entry
(g)
(%)
(%)
(%)
1
2
3
4
5
0
35
78
87
96
99
18
62
69
78
71
11
8
5
2
2
3
4
5
6
5
0.05
0.075
0.10
0.20
a Reaction conditions: 1 mmol cinnamaldehyde, 4 ml 30% H2O2 (w/w), 2.5 mmol NaHCO3, 25 ml H2O, 608C, 2.5 h.
Frequency (TOF) of 256 h21) than that of b-CD (TOF of
0.33 h21) for the reaction, and 96% conversion of
cinnamaldehyde and 78% yield of benzaldehyde were
obtained in the presence of H2O2, NaHCO3 and b-CD–
CTS. The catalytic performance was greatly influenced by
the deficiency of the above components. Only 4%
cinnamaldehyde was converted into benzaldehyde in the
absence of NaHCO3. Benzaldehyde (18%) was given with
the addition of H2O2 and NaHCO3, and the addition of
chitosan could not significantly enhance the catalytic
performance. The above results showed that synergistic
action between b-CD and the functional group of chitosan
played a crucial role for the oxidation of cinnamaldehyde.
b-CD–CTS exhibits much better catalytic ability than that
of b-CD, which indicates that the support associated with
b-CD has a significant positive influence for the reaction
system owing to the intermolecular weak interactions, e.g.
hydrogen bond between cinnamaldehyde and b-CD–CTS.
Figure 7. The temperature range of 30–708C has been
studied. The results indicated that the rising temperature
could improve the conversion of cinnamaldehyde and the
yield of benzaldehyde significantly, suggesting that the
reaction was highly sensitive to the temperature.
Cinnamaldehyde conversion (96%) and benzaldehyde
yield (78%) were obtained at 608C. However, when the
temperature was over 608C, the yield would decrease
resulting in the polymerisation and disproportionative
condensation of benzaldehyde (24). In addition, lower
temperature is especially advantageous for keeping the
natural essence of benzaldehyde. Thus, the optimal
temperature for the oxidation of cinnamaldehyde to
benzaldehyde is 608C.
2.2.4 Effect of the amount of H2O2
Hydrogen peroxide, with high active oxygen content, is an
environmentally friendly oxidant for which water is the
sole by-product (25). The amount of H2O2 used in this
system is another important factor influencing the
oxidation of cinnamaldehyde. The H2O2 amount from 0
to 5 ml was investigated, as shown in Table 2. The
conversion increased with an increase in H2O2 amount. In
the absence of H2O2, only 17% conversion of cinnamal-
dehyde and 9% yield of benzaldehyde were obtained due
2.2.2 Effect of b-CD–CTS amount
The effect of b-CD–CTS amount on the oxidation of
cinnamaldehyde was also investigated by varying its
amount from 0 to 0.20g, as shown in Table 1. The results
indicated that the conversion of cinnamaldehyde and yield
of benzaldehyde increased with an increase in the catalytic
amount, coupling with a slight enhancement of selectivity
for benzaldehyde. Maximal conversion (ca. 99%) was
obtained with 0.10g of catalyst. It should be noted that the
oxidation reaction in aqueous solution at 608C for 2.5 h gave
78% yield of benzaldehyde, which is significantly higher
than that of the previous report (63%) (16), indicating that b-
CD–CTS can significantly improve the yield of benzal-
dehyde than that from the oxidation of cinnamaldehyde with
b-CDP as the phase-transfer catalyst. On the other hand,
excessive b-CD–CTS would lead to unwanted side
reactions and cause decrease in benzaldehyde yield. It
could be ascribed to further oxidation of benzaldehyde into
benzoic acid catalysed by the excess amount of b-CD–CTS.
100
90
80
70
60
80
70
60
50
40
30
Benzaldehyde
30
40
50
60
70
Reaction temperature (˚C )
2.2.3 Effect of reaction temperature
Figure 7. Effect of reaction temperature on cinnamaldehyde
oxidation: 1 mmol cinnamaldehyde, 0.1 g b-CD–CTS, 4 ml
H2O2 (w/w), 2.5 mmol NaHCO3, 25 ml H2O, 2.5 h.
The effect of reaction temperature on the oxidation of
cinnamaldehyde catalysed by b-CD–CTS is shown in

Supramolecular Chemistry
237
Table 2. Effect of the amount of H2O2 on the oxidation of cinnamaldehyde.a
Amount of
H2O2 (ml)
Conversion of
cinnamaldehyde (%)
Yield of
benzaldehyde (%)
Yield of
epoxide (%)
Yield of
phenylacetaldehyde (%)
Entry
1
2
3
4
5
6
0
1
2
17
70
80
88
96
93
9
51
61
70
78
71
0
13
10
6
2
2
0
3
4
5
6
5
2 þ 1
2 þ 2
2 þ 2 þ 1
a Reaction conditions: 1 mmol cinnamaldehyde, 0.10 g b-CD–CTS, 2.5 mmol NaHCO3, 25 ml H2O, 608C, 2.5 h.
Table 3. Effect of the different amounts of NaHCO3 on the oxidation of cinnamaldehyde.a
NaHCO3
(mmol)
Reaction
time (h)
Conversion of
cinnamaldehyde (%)
Yield of
benzaldehyde (%)
Yield of
epoxide (%)
Yield of
phenylacetaldehyde (%)
Entry
1
2
3
4
5
6
0
4
3
2.5
2.5
2.5
2
5
70
81
90
96
99
4
51
62
72
78
70
0
15
13
10
2
0
2
5
6
5
5
0.5
1.5
2.0
2.5
3.5
2
a Reaction conditions: 1 mmol cinnamaldehyde, 0.10 g b-CD–CTS, 4 ml 30% H2O2 (w/w) 2.5 mmol NaHCO3, 25 ml H2O, 608C.
to hydrolysis of cinnamaldehyde catalysed by bicarbonate,
also indicating that alkaline hydrolysis via retro-Aldol
condensation is not predominant in the present reaction
system. Conversion of cinnamaldehyde (96%) and yield of
benzaldehyde (78%) were obtained when the amount of
H2O2 reached 4 ml. Further increase in H2O2 amount did
not improve the conversion.
Similarly, excessive NaHCO3 would decrease the yield
of benzaldehyde due to increase in side reactions.
2.3 Kinetic studies
The oxidation rate of cinnamaldehyde in the presence of
b-CD–CTS is represented in Figure 8(A). Linear
relationship between ln(C0/Ct) and time was observed,
indicating that the reactions followed pseudo-first-order
kinetics:
2.2.5 Effect of the amount of NaHCO3
Hydrogen peroxide is a weak oxidant in the absence of
activators. According to the previous reports (26, 27), the
bicarbonate–H2O2 system was a simple and efficient
method for the activation of hydrogen peroxide. The effect
of the amount of sodium bicarbonate from 0 to 3.5 mmol
on the oxidation of cinnamaldehyde is summarised in
Table 3. The result indicated that the amount of NaHCO3
plays an important role for the oxidation of cinnamalde-
hyde. In the absence of NaHCO3, the conversion of
cinnamaldehyde and the yield of benzaldehyde were only
5% and 4%, respectively. With an increase in the NaHCO3
amount, the conversion of cinnamaldehyde and the yield
of benzaldehyde increased significantly. Yield of benzal-
dehyde (78%) was obtained when the amount of NaHCO3
reached 2.5 mmol. As previously reported (26), it has been
confirmed that a key aspect of such reactions is that
hydrogen peroxide and bicarbonate combine in an
equilibrium process to produce peroxymonocarbonate
(HOOCO22 ). Peroxymonocarbonate is an active oxidant,
which efficiently promotes the epoxidation of cinnamal-
dehyde, which is further oxidised to benzaldehyde.
lnðC0=CtÞ ¼ kt:
ð1Þ
Here, C0, Ct, t and k are the initial cinnamaldehyde
concentration, cinnamaldehyde concentration at t, reaction
303 K
313 K
323 K
333 K
3.0
A
2.4
1.8
1.2
0.6
0.0
0
20
40
60
80
2.4
1.8
1.2
0.6
0.0
B
303 K
313 K
323 K
333 K
0
20
40
t / min
60
80
Figure 8. Pseudo-first-order plots of cinnamaldehyde oxidation
by H2O2–NaHCO3 system at different reaction temperatures in
the presence and absence of b-CD–CTS.

238
Z.-J. Yang et al.
Table 4. The rate orders and rate constants at different reaction temperatures in the presence and absence of b-CD–CTS.
Presence of b-CD–CTS
Correlation coefficient (R)
Absence of b-CD–CTS
Correlation coefficient (R)
T (K)
k (min21
)
k (min21
)
303
313
323
333
0.00931
0.01756
0.02729
0.03930
0.9964
0.9979
0.9951
0.9980
0.00451
0.00927
0.01941
0.02875
0.9986
0.9991
0.9976
0.9983
time (min) and the globe reaction apparent rate (min21),
respectively. The slope of ln(C0/Ct) versus t was used to
calculate k.
The rate constant k (min21) and correlation coefficient
R of oxidation process are shown in Table 4. The rate
constant of pseudo-first-order kinetics increases with
rising reaction temperature. The rate constant was found to
increase from 0.00931 to 0.03930 min21 when the
temperature increased from 303 to 333 K.
energy of the reaction (kJ mol21), R is the gas constant
(8.314 J mol21 K21) and T is the absolute temperature (K).
According to the rate constants in Table 4, Arrhenius
plots of ln k and 1/T are shown in Figure 9. The activation
energies Ea in the presence and absence of b-CD–CTS
were calculated as 40.05 and 52.95 kJ mol21, respectively.
It is obvious that b-CD–CTS could reduce the energy
barrier for the oxidation of cinnamaldehyde.
Figure 8(B) shows the fitting results of the first-order
kinetic model, which also indicated that the oxidation of
cinnamaldehyde to benzaldehyde in the absence of b-CD–
CTS followed pseudo-first-order kinetic model. The
corresponding parameters are shown in Table 4. The rate
constant increased from 0.00451 to 0.02875 min21 with
the temperature rise from 303 to 333 K.
2.4 Reusability of catalyst
The insoluble b-CD–CTS could be easily recycled by
centrifugation after the reaction. The filtered b-CD–CTS
was washed with ethanol and deionised water. After
drying, the catalyst was reused for the next run under the
same conditions. As shown in Table 5, similar conversion
and selectivity were obtained when the recovered b-CD–
CTS was reused for four times. The catalytic activity was
not affected.
As shown above, the rate constants in the presence of
b-CD–CTS were higher than those without b-CD–CTS,
suggesting that b-CD–CTS could significantly promote
the oxidation of cinnamaldehyde. The relationship
between rate constant and the temperature can be
described by:
2.5 Effect of hydrogen bonds
k ¼ k0expð2Ea=RTÞ:
Here, k is the rate constant of reaction (min21), k0 is
the constant-frequency factor (min21), E is the activation
ð2Þ
2.5.1 FT-IR analysis of the inclusion complex
The IR spectra of b-CD–CTS (a), physical mixture of b-
CD–CTS and cinnamaldehyde (b), b-CD–CTS–cinna-
maldehyde complex (c) and cinnamaldehyde (d) were
recorded from 400 to 4000 cm21, as shown in Figure 10.
The spectra of physical mixture (b) and inclusion complex
(c) are similar to that of b-CD–CTS (a), indicating that the
main structure between the inclusion complex or physical
mixture and b-CD–CTS is the same. The characteristic
peaks of b-CD–CTS (a) and cinnamaldehyde (d) were
–3.0
Ea = 40.05 KJ/mol
–3.5
–4.0
–4.5
Ea = 52.95 KJ/mol
Table 5. Reusability of b-CD–CTS for the oxidation of
cinnamaldehyde.a
–5.0
–5.5
Entry
b-CD–CTS
Conversion (%)
Selectivity (%)
1
2
3
4
5
Fresh
96
95
94
94
93
81
79
78
79
78
Reuse 1
Reuse 2
Reuse 3
Reuse 4
3.00
3.05
3.10
3.15
3.20
)
3.25
3.30
1000/T (K–1
Figure 9. Arrhenius plots of the rate constants for
cinnamaldehyde oxidation. (X: in the presence of b-CD–CTS;
B: in the absence of b-CD–CTS).
a Reaction conditions: cinnamaldehyde (1 mmol), b-CD–CTS (0.10 g),
30% H2O2 (4 ml, w/w), NaHCO3 (2.5 mmol), H2O (25 ml), 608C, 2.5 h.

Supramolecular Chemistry
239
CZNH2 was shifted to 285.8 eV from 285.3 eV, while
the shift of the other C1s BEs of CZC (or CZH), CZO
and CvO (or OZCZO) was not obvious (Figure 11(e)).
The N1s BEs further evidenced the change, and the N1s
BE of NH2ZC in the inclusion complex shifted from
402.2 to 401.6 eV (Figure 11(d)). Moreover, new peaks
at 398.8 and 533.8 eV were detected, which should be
assigned to the N1s BE of NZH· · ·O and the O1s BE of
OZH· · ·O group in the inclusion complex, respectively.
The results showed that the C1s BE of CZNH2 increased
and that the N1s BE of b-CD–CTS decreased after
inclusion of cinnamaldehyde. It could be attributed to the
formation of hydrogen bond between CvO of cinna-
maldehyde and OH or NH2 groups of b-CD–CTS,
respectively. Therefore, the changes of O1s of CvO
indicate the formation of hydrogen bond among CvO of
cinnamaldehyde, which confirms the formation of
hydrogen bond in the complex.
1610
1688
2750
3030
2820
d
c
b
a
1250
1335
1668
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm–1
)
Figure 10. FT-IR spectra of b-CD–CTS (a), physical mixture
of b-CD–CTS and cinnamaldehyde (b), b-CD–CTS–
cinnamaldehyde (c) and cinnamaldehyde (d).
2.5.3 Theoretical results
Geometries and energies of the most stable inclusion
complexes of glucopyranoside unit in isolated b-CD–CTS
with cinnamaldehyde or benzaldehyde optimised by
the program package DMol3 method are displayed in
Figure 12. The negative BEs further demonstrate that
glucopyranoside unit in the isolated b-CD–CTS can
interact with cinnamaldehyde or benzaldehyde to form
stable inclusion complexes. For the hydrogen bond
interaction of OZH· · ·O, as previously reported (29, 30),
the cut-off criteria of hydrogen bond interaction are the
observed at the same position for the physical mixture.
However, for the spectrum of inclusion complex (c),
obvious difference of characteristic absorption peaks for
benzene ring of the guest could be found: (1) sCvC peak
(1610 cm21) vanished; (2) the intensity of sCvO peak
(1688 cm21) decreased and shifted to 1668 cm21 for
the inclusion complex. These changes indicate that the
aromatic part in cinnamaldehyde was included into the
hydrophobic cavity of b-CD–CTS. (3) sC2O 2 H peaks at
1335 and 1250 cm21 for the inclusion complex have much
higher intensity compared to that of b-CD–CTS. These
evidences indicate the formation of hydrogen bonds
between the hydroxyl groups of b-CD–CTS and the
˚
O· · ·O distance from OZH· · ·O hydrogen bond (#3.2 A)
and the bond angle ($908). The bond lengths of hydrogen
˚
bonds of OZH· · ·O (a) and (b)are inturn2.457 and 2.728 A,
and the corresponding bond angles are in turn 110.5398 and
131.9088, demonstrating that the hydrogen bond between
cinnamaldehyde or benzaldehyde and the isolated b-CD–
CTS plays a vital role in the reaction processes. Hydrogen
bond, whichleadstomorecinnamaldehyde inthecavitiesof
b-CD, effectively expands the interface and significantly
promotes the oxidation of cinnamaldehyde to benzal-
dehyde. According to the previous report (30, 31),
difference between the two BEs of cinnamaldehyde and
benzaldehyde is the base for selective separation of the two
aldehydes in b-CD–CTS via b-CD inclusion. The more
negative the BE, the more stable the corresponding b-CD–
CTS/guest complex. It should be noted that different BEs
lead to difference in stability between cinnamaldehyde or
benzaldehyde and glucopyranoside unit in the b-CD–CTS.
Herein, the BE for the glucopyranoside unit/cinnamalde-
hyde (280.76 kJ/mol) is negative, which is much lower
than that of the glucopyranoside unit/benzaldehyde
(247.55 kJ/mol) in the isolated b-CD–CTS. From an
energetic point of view, it can be concluded that
cinnamaldehyde prefers to penetrate the hydrophobic
carbonyl groups of cinnamaldehyde. (4) sCZH (3030 cm21
)
in the benzene ring and sCZH at the 2820 and 2750 cm21 in
the aldehyde group for cinnamaldehyde disappeared in the
IR spectrum of the inclusion complex. This is probably
attributed to the fact that cinnamaldehyde enters into
the cavity of b-CD–CTS as well, respectively. Based on the
above discussion, it is reasonable to conclude that the host–
guest inclusion complex was formed (28).
2.5.2 X-ray photoelectron spectroscopy (XPS) analysis
of the inclusion complex
In order to explore the high selectivity of b-CD–CTS
catalyst, the difference in binding energy (BE) of b-CD–
CTS before and after cinnamaldehyde inclusion was
investigated and evaluated by XPS. The corresponding
spectra of N1s, C1s and O1s are shown in Figure 11.
Comparing the binding energies of each element between
b-CD–CTS and the inclusion complex, the C1s BE of

240
Z.-J. Yang et al.
399.4
398.8
(a)
(d)
N1s
N1s
399.4
402.6
402.2
394 396 398 400 402 404 406
Binding energy (eV)
394 396 398 400 402 404 406
Binding energy (eV)
284.6
(e)
C 1s
(b)
C 1s
286.6
284.6
285.3
286.6
287.7
285.8
287.7
280 282 284 286 288 290 292
Binding energy (eV)
280 282 284 286 288 290 292
Binding energy (eV)
O1s
O1s
(c)
(f)
532.6
533.3
532.6
533.3
533.8
532.2
532.0
528
530
532
534
536
538
528 530 532 534 536 538
Binding energy (eV)
Binding energy (eV)
Figure 11. XPS of N(1s) (a), C(1s) (b) and O(1s) (c) core-level spectra of b-CD–CTS, and N(1s) (d), C(1s) (e) and O(1s) (f) core-level
spectra of b-CD–CTS/cinnamaldehyde inclusion complex.
cavity rather than staying in the bulk of the aqueous phase.
The produced benzaldehyde tends to escape the cavities of
b-CD in b-CD–CTS owing to the much higher BE.
Therefore, b-CD–CTS can be used to enhance the mass
transfer significantly, that is why the optimal ratio
(cinnamaldehyde:b-CD–CTS) is much lower than the
previouslyreporteddata(1:1)(24). Therefore, masstransfer
of b-CD–CTS significantly improves the catalytic ability.
naphthalene as an internal standard. The dependence of the
conversion of cinnamaldehyde and the yields of the
products on reaction time are shown in Figure 13. The
conversion of cinnamaldehyde was greatly increased by
extending the reaction time. The yield of epoxide almost
simultaneously reached to 70% at the time of 0.5 h, and
then gradually decreased to 2%. The yield of benzal-
dehyde increased to 78% at the time of 2.5 h. On the other
hand, the yield of phenylacetaldehyde was obtained at the
yield of 6% at the same time and kept constant despite
extending the time. The above results suggest that epoxide
is intermediate in the reaction process. Peroxymonocar-
bonate can efficiently promote epoxide to benzaldehyde in
the system.
2.6 Plausible mechanism for the reaction
The oxidation of cinnamaldehyde to benzaldehyde
catalysed by b-CD–CTS in the presence of H2O2/-
NaHCO3 in a batch was investigated by GC–MS with

Supramolecular Chemistry
241
BE = –80.76 KJ/mol
BE = –47.55 KJ/mol
Figure 12. Optimised structure of glucopyranoside unit/cinnamaldehyde (a), glucopyranoside unit/benzaldehyde (b) in the isolated b-
CD–CTS obtained by DMol3 method.
Based on the above experimental data, together with
XPS and calculation results, a plausible reaction mechan-
ism has been proposed for b-CD–CTS-catalysed oxidation
of cinnamaldehyde by H2O2–NaHCO3. Firstly, b-CD in
the b-CD–CTS and cinnamaldehyde can form the
inclusion complex with the intermolecular hydrogen bond
OZH· · ·O at the second rim of b-CD (Figure 14). The host–
guest interaction between cinnamaldehyde and parent
b-CD has been verified by 1H NMR and ROESY as
previously reported (24). Secondly, due to nucleophilic
attack of hydroxide ion via the hydrolysis of sodium
bicarbonate on the substrate, small amount of benzaldehyde
was directly produced via hydrolysis, and the result was in
accordance with the previously reported result (24). On the
other hand, hydrogen peroxide and bicarbonate can
combine in an equilibrium process to produce peroxymo-
nocarbonate HOOCO22 . It is well known that bicarbonate–
H2O2 system can epoxidise alkenes through the nucleo-
philic attack of HOOCO22 species to the CvC double bond
(26). The non-covalent intermolecular interaction between
100
80
60
40
20
0
a
b
c
d
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Reaction time/h
Figure 13. Plots of conversion and yields of the products versus
reaction time for the cinnamaldehyde/b-CD–CTS/H2O2 –
NaHCO3 system at 608C; (a, conversion of cinnamaldehyde; b,
yield of benzaldehyde; c, yield of epoxide; d, yield of
phenylacetaldehyde).

242
Z.-J. Yang et al.
OH
O
–
CH3CHO
O
OH
1
O
HO
O
H
O
O
OH
O
O
δ
O
H
H
O
δO
O
O
β
OH
O
OH
CH3CHO
O
O
α
O
O
HO
O
HO
2
O
HO
O
HO
O
HO
O
HCO3
O
O
O
O
O
δ
O
δO
O
H
H
O O
β
O
O
OH
HCO4
HCO4
3
OH HCHO
HCO3
α
O
O
O
O
HO
HO
HO
O
O
O
Figure 14. The plausible mechanism of the oxidation of cinnamaldehyde catalysed by b-CD–CTS in the presence of H2O2 and NaHCO3.
b-CD and cinnamaldehyde also promoted the nucleophilic
oxidation. Then, a large amount of cinnamaldehyde was
converted to 3-phenyloxirane-2-carbaldehyde and a small
amount of 2-phenethyloxirane was obtained in the
epoxidation process, resulting from the nucleophilic attack
of peroxymonocarbonate on Ca which precedes Cb in the
CvC double bond. A further nucleophilic attack on the
epoxide by peroxymonocarbonate produces benzaldehyde
and phenylacetaldehyde, respectively. Finally, the catalyst
restored its initial state. It should be mentioned that alkaline
hydrolysis effect is weak in the present catalytic system,
and only 9% yield of the product is derived from the
contribution of hydrolysis.
of benzaldehyde from cinnamaldehyde using hydrogen
peroxide as an oxidiser under mild conditions. Interaction
between b-CD–CTS/cinnamaldehyde was investigated
by FT-IR, XPS and computation. The results indicate that
b-CD–CTS can form intermolecular weak interactions,
e.g. hydrogen bond with cinnamaldehyde, then efficiently
promote the oxidation of cinnamaldehyde and signifi-
cantly improve the selectivity for benzaldehyde. In
addition, the kinetic results show that the participation of
3
3
2.7 Larger scale continuous reaction experiment
5
Recirculating operation of batch reaction is of interest and
particularly useful in industrial-scale operation (32). The
yield of purified benzaldehyde was up to 72% in a
continuous reaction system with b-CD–CTS as the
catalyst (Figure 15). Comparing with other hydrolysis of
cinnamaldehyde (24, 33–35), this process realises clean
synthesis of natural benzaldehyde and provides excellent
yield for benzaldehyde under mild conditions, which is
especially important for practical application in industry.
This idea of recirculating operation also gives the
reference for other batch reactions.
6
3
1
4
2
3. Conclusions
Figure 15. Apparatus for recirculating reaction system. (1)
Mixing vessel; (2) pump; (3) valve; (4) sieve plate; (5) catalyst;
(6) heater exchanger.
In conclusion, the b-CD–CTS catalyst was found to be
recyclable, green and extremely efficient for the synthesis

Supramolecular Chemistry
243
b-CD–CTS decreased the activation energy (Ea) of the
oxidation, a pseudo-first-order reaction. This method is
bestowed with merits such as high yield, cost effective-
ness, weak alkaline aqueous phase conditions and
environmentally benign nature. These advantages of the
catalyst made this process very useful for the synthesis of
natural benzaldehyde in industrial process.
determined as 0.015 mmol/g according to the previously
reported method (37).
4.3 Preparation of the inclusion complex
b-CD–CTS 0.1 g and cinnamaldehyde 0.1362 g (1 mmol)
were dispersed in 25 ml deionised water. The mixture was
magnetically stirred at 608C for 0.5 h and then centrifuged.
Finally, the inclusion complex was dried under vacuum at
508C for 12 h to entirely remove the solvent, and then kept
in a desiccator until used.
4. Experimental
4.1 Chemicals
Chitosan with average molecular weight 50,000 and 95%
deacetylation degree was obtained from Nantong Xinch-
eng Biological Industrial Limited Co., Ltd, China. b-CD
(.99%) was purchased from Shanghai Boao Biotechnol-
ogy, China. Cinnamaldehyde (.99%) was obtained from
Sinopharmacy Chemical Reagent, China. All other
reagents and solvents were of analytical grade and used
without further purification unless indicated. Distilled
water was used throughout.
4.4 Characterisation of b-CD–CTS and the inclusion
complex
FT-IR spectra were measured by KBr pellets. All the
infrared spectra were recorded on a Bruker TENSOR 37
FT-IR spectrometer with the wavenumber ranging from
4000 to 400 cm21 and a resolution of 4 cm21. XRD patterns
were measured on a Rigaku Denki MAX III diffractometer
with Na-filtered Cu–Ka radiation. Step-scans were
recorded for all samples in the u range of 58–808. XRD
peaks were analysed using the Rigaku program ‘JADE-5’.
TG experiments were carried out on a Netzsch STA-449C
thermal analysis system. The flow rate of nitrogen was
about 40 ml/min, and a heating rate of 108C/min was used.
XPS analysis was carried out on a Vacuum Generator
ESCALAB 250 spectrometer using an Al–Ka monochro-
matic X-ray source and a hemispherical analyser. A fixed
analyser pass energy of 150 eV was used for survey scans,
and high-resolution scans of core-level regions were
recorded using a 20 eV pass energy. All core-level spectra
were referenced to the C1s neutral carbon peak at 284.5 eV
and obtained at a take-off 908 to the sample surface.
Single-contact 75.4 MHz 13C CP-MAS (cross-polaris-
ation-magic angle spinning) NMR spectra were recorded
on a Bruker Avance-400 spectrometer equipped with a
CP-MAS accessory. The cross-polarisation sequence was
utilised for all samples, which were spun at the magic at
6 kHz. A contact time of 1 ms and a pulse (repetition) time
of 5 ms were used, and more than 1000 scans were
accumulated for each spectrum. The shifts were measured
with respect to intensity, and approximately 300 mg of dry
sample was inserted into a ceramic rotor.
4.2 b-CD–CTS synthesis
Chitosan resin was prepared according to the method
previously reported with some modification (36). A typical
procedure was described as follows: 4.0 g chitosan was
mechanically stirred for 30 min in 50 ml 5% aqueous
acetic acid. A total of 100 ml chlorobenzene and 50 ml
toluene containing 0.5 g span 80 were added in a three-
necked flask. Then 7.0 g 25% glutaraldehyde was added
slowly. The mixture was magnetically stirred at 408C for
1 h and 608C for 3 h. The reaction was stopped, and the
obtained product was filtered and repeatedly washed with
water–ethanol and acetone. The beads were further dried
under vacuum at 608C for 24 h.
A total of 2.0 g of chitosan resins, 50 ml of water and
50 ml of dimethyl sulphoxide were introduced into a
250 ml three-necked flask, and stirred at 258C for 20 min.
Thereafter, 0.4 mol/l sodium hydroxide was added into the
reaction mixture to adjust the pH value to 10–12. The
mixture was then heated to 408C, and 45 ml of
epichlorohydrin was added slowly dropwise to the solution
for 90 min. The temperature was monitored during the
activation and maintained for 3 h while stirring at 200 rpm.
After 3 h, the mixture was filtered, and washed thoroughly
with large amount of water and acetone. The product was
then dried under vacuum at 408C for 12 h.
4.5 Density-functional theory calculation
Activated chitosan resins of 3.0 g were dispersed
in 100 ml 0.2 mol/l NaOH aqueous solution with stirring.
b-CD of 3.0 g was added into the reaction mixture and
heated gradually to 608C. After being stirred for 4 h at
608C, the mixture was filtered, and washed thoroughly
with large amount of water at room temperature. The
product was dried under vacuum at 508C for 24 h. The
amount of b-CD immobilised on the chitosan was
The inclusion complex was evaluated by the quantum-
chemical calculations, since it can provide some
information of BE and structure of the inclusion complex.
In order to rationalise the mechanism of interaction, a
basic assumption was proposed that the binding sites in the
polymer are glucopyranoside unit of the b-CD in the
isolated b-CD–CTS and that the global properties of these

244
Z.-J. Yang et al.
host, and cinnamaldehyde or benzaldehyde was selected as
model guests. The more negative the BE is, the more
thermodynamically favourable is the inclusion complex.
sites are not significantly different from the properties of
glucopyranoside unit in the isolated b-CD–CTS molecule
(as shown in Figure 16). Based on theoretical analysis,
hydrophobic interactions are postulated to be the main
driving forces in the inclusion complexation of cinnamal-
dehyde or benzaldehyde with the isolated b-CD–CTS in
b-CD–CTS.
4.6 Oxidation experiments
A total of 1.0 mmol cinnamaldehyde and 2.4 mmol sodium
bicarbonate were dissolved in 25 ml of deionised water at
608C, and 0.1 g b-CD–CTS was added into a 100 ml three-
necked flask fitted with a reflux condenser and magnetic
stirrer. The mixture was heated to 608C in an oil bath with
electric heater, and then 4 ml 30% H2O2 (35.0 mmol) was
added dropwise. The resulting system was performed at
608C under stirring for 3 h. After reaction, the mixture was
extracted by ethyl acetate and then centrifuged. The
extracted liquid mixture was analysed by GC–MS with
naphthalene as an internal standard. The reproducibility
for the data was within 5%.
The computations were carried out using the program
package DMol3 method (38–40) in Materials Studio
(version 4.0) of Accelrys Inc. All calculations were made
using the local density approximation in the Perden–Wang
(PWC) form at the DND basis set level.
The BE could be expressed as
BE ¼ Ecomplex 2 Eguest 2 Ehost
;
ð3Þ
Ecomplex was the total energy of inclusion complex, Eguest
was the sum of total energy of guests and Ehost was the
total energy of host. The isolated b-CD–CTS was used as
4.7 Kinetic experiments
Typical measurement was carried out as follows: 1.0 mmol
cinnamaldehyde and 1.2 mmol sodium bicarbonate were
dissolved in 100 ml of deionised water, and 0.1 g b-CD–
CTS was added into a 100 ml three-necked flask fitted with
a reflux condenser and magnetic stirrer. The mixture was
heated and kept at 303 K (or 313, 323 and 333 K). After
30 min, 4 ml H2O2 (30% aqueous) was added rapidly into
the mixture and the oxidation reaction occurred immedi-
ately. 0.5 ml of reaction mixture was sampled in a 1 ml
pipette at 20-min intervals. The sample was diluted to
100 ml with deionised water and analysed by UV–vis at
290 nm. The UV–vis absorbance data were conversed into
the product concentrations.
4.8 Large-scale experiments
The large-scale reaction experiments were carried out on a
recirculating reactor. Sodium bicarbonate (24.0 mmol) was
dissolved in 250 ml of deionised water, and 10.0 mmol
cinnamaldehyde was dissolved in aqueous solutions in the
mixing vessel. And 1 g b-CD–CTS was placed on the sieve
plate of reaction equipment. The mixture was heated to
608C in a heater exchanger, and then 40 ml of 30% H2O2
(350 mmol) was slowly dropped into the mixture, circling
for 2.5 h at 608C. After reaction, the solution was extracted
by ethyl acetate. The extracted liquid mixture was analysed
by GC–MS with naphthalene as an internal standard.
Acknowledgement
The authors thank the National Natural Science Foundation of
China (Nos 21036009 and 21176268) for providing financial
support to this project.
Figure 16. Optimised DMol3 isolated b-CD–CTS unit in the
b-CD–CTS.

Supramolecular Chemistry
245
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