Amino functionalized chitosan as a catalyst for selective solvent-free self-condensation of linear aldehydes

Article abstract of DOI:10.1016/j.molcata.2010.10.012

An aminopropyltrimethoxysilane functionalized chitosan was found to be an efficient solid base catalyst for the self-aldol condensation of linear aldehydes under solvent-free conditions. The modified catalyst was characterized using physical techniques, elemental analysis, FT-IR, and TGA. The modified chitosan was evaluated for the aldol condensation of C₃-C₇ linear aldehydes in which the selective formation was obtained for α,β-unsaturated aldehydes. A decreasing trend in the conversion from propanal to heptanal was observed. Propanal and pentanal were subjected for detail investigations to study the effect of parameters like amount of catalyst and aldehyde, and temperature on the conversion and selectivity. Kinetic performance of the modified chitosan investigated for a representative aldehyde, pentanal showed that the rate was increased with the catalyst amount, pentanal and temperature. The catalyst was reused up to six cycles without significant loss in its activity and selectivity.

Full text of DOI:10.1016/j.molcata.2010.10.012

Journal of Molecular Catalysis A: Chemical 333 (2010) 158–166
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Amino functionalized chitosan as a catalyst for selective solvent-free
self-condensation of linear aldehydes
Tharun Jose, N. Sudheesh, Ram S. Shukla∗
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR),
G. B. Marg, Bhavnagar 364021, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An aminopropyltrimethoxysilane functionalized chitosan was found to be an efficient solid base catalyst
for the self-aldol condensation of linear aldehydes under solvent-free conditions. The modified catalyst
was characterized using physical techniques, elemental analysis, FT-IR, and TGA. The modified chitosan
was evaluated for the aldol condensation of C3–C7 linear aldehydes in which the selective formation was
obtained for ␣,␤-unsaturated aldehydes. A decreasing trend in the conversion from propanal to heptanal
was observed. Propanal and pentanal were subjected for detail investigations to study the effect of param-
eters like amount of catalyst and aldehyde, and temperature on the conversion and selectivity. Kinetic
performance of the modified chitosan investigated for a representative aldehyde, pentanal showed that
the rate was increased with the catalyst amount, pentanal and temperature. The catalyst was reused up
to six cycles without significant loss in its activity and selectivity.
Received 15 February 2010
Received in revised form
17 September 2010
Accepted 11 October 2010
Available online 16 October 2010
Keywords:
Chitosan
Solid base catalyst
Aldol condensation
Aldehyde
© 2010 Elsevier B.V. All rights reserved.
Kinetics
1. Introduction
important aldol product is aldol intermediate of pentanal which is
commercially important product and finds use in the synthesis of
Aldol condensation is an important reaction in organic syn-
thetic chemistry due to the formation of carbon–carbon bonds
during the reaction process. This type of reaction may occur via
self-condensation between two of the same aldehyde or ketone
molecules or via cross-condensation between two molecules of dif-
ferent aldehydes or ketones [1–3]. Such a reaction usually proceeds
over basic or acidic catalysts such as sodium hydroxide or sulfuric
acid in the liquid phase. However, the disadvantages of such a pro-
cess include corrosion, safety hazards, separation procedures, and
environmental problems due to the use of sodium hydroxide or
sulfuric acid. Therefore, the use of solid bases or acids has drawn
increasing interest during the past three decades [4–9]. The solid
base catalysts generally used for aldol condensations include MgO
[10], hydrotalcites [11,12] and synthetic talc [13].
plasticizer and detergent alcohols [15].
The aim of present investigation is to develop a suitable solid
base catalyst for aldol condensation of linear aldehydes in solvent-
free environment. Chitosan is a polyaminosaccharide, normally
obtained by alkaline deacetylation of chitin [16]. Chitosan, even
having a large amount of –NH2 group, is not found to be effective
for base catalyzed reactions [17]. The generally used method is to
treat with HCl and then to form chitosan beads by adding it into
the NaOH solution. It will be a better choice to functionalize the
chitosan avoiding the use of acids and alkalis to obtain active base
catalyst. Therefore, modified chitosan was synthesized by modifi-
cation with aminopropyltrimethoxysilane (APTMS), characterized
and investigated as solid base catalyst for self-aldol condensation
of C3–C7 linear aldehydes.
Self-condensations of aldehydes are important in the view
of industrial applications. These products find applications in
the fields of pharmaceuticals, fragrances, plasticizers, detergents
and cosmetics. The synthesis of 2-ethylhexenal and its hydro-
genated product 2-ethylhexanol from self-condensation of butanal
find its use in soap, detergent, plasticizers, coatings, adhesives,
dioctyl-phthalate and specialty chemical industries [14]. Another
2. Experimental
2.1. Materials
The aldehydes propanal (98%) from Loba chemie, India, butanal
(99%) from s.d. Fine Chemicals, India, and pentanal (97%), hexanal
(98%), heptanal (95%) from Sigma–Aldrich, USA, were obtained.
Toluene (99.5%) was obtained from Fisher Scientific, India. Amino-
propyltrimethoxysilane (97%) and chitosan were purchased from
Sigma–Aldrich, USA. All the chemicals were used as such.
∗
Corresponding author. Tel.: +91 278 2567760; fax: +91 278 2566970.
E-mail address: rshukla@csmcri.org (R.S. Shukla).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.012

T. Jose et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 158–166
159
2.2. Catalyst synthesis
Amino functionalization of the chitosan was done by treating
chitosan with APTMS. In a typical procedure 1.5 g chitosan and 1 g
APTMS was taken in 25 ml toluene in a 50 ml round bottom flask
(RBF). The RBF was connected with a water condenser and an inert
atmosphere was created by nitrogen. The RBF under an inert atmo-
sphere was refluxed in an oil bath at 110 ◦C. The refluxing was
continued for 24 h with stirring at 450 rpm. After 24 h, the flask
was cooled to room temperature. The catalyst was filtered, dried
at 100 ◦C and powdered to get the amino functionalized chitosan
(CHM).
2.3. Characterization of the catalysts
Fourier transform infrared spectra (FT-IR) were recorded with
Perkin-Elmer, GX-FTIR using KBr pellet. Elemental (C, H, N) anal-
ysis of chitosan and modified chitosan was carried out with
Perkin-Elmer CHNS/O analyzer (Series II, 2400). Thermogravimet-
ric analysis (TGA) was done using Mettler Toledo TGA/SDTA 851e
equipment in flowing nitrogen (flow rate, 50 mL/min), at a heating
rate of 10 ◦C/min. N2 sorption analysis was carried out at 77.4 K in a
sorptometer (ASAP 2010, Micromeritics). The sample was degassed
at 120 ◦C for 4 h prior to the sorption analysis.
Fig. 1. FT-IR spectrum of CHM.
3. Result and discussion
2.4. Aldol condensation
3.1. Characterization of the catalysts
Weighed amount of aldehyde and catalyst was taken in an oven
dried 50 ml double necked round bottom flask. One neck of the
flask was fitted with refluxing condenser having spiral tube inside
and another neck of the flask was blocked with silicon rubber sep-
tum. The top of the refluxing condenser was connected to balloon
filled with nitrogen. The entire experimental setup was kept in an
oil bath equipped with temperature and agitation speed control-
ling units. The water at 15 ◦C was circulated in refluxing condenser
throughout the course of reaction from a water chiller at the flow
rate of 6 L/min. The reaction was carried out at 100 ◦C for 8 h. The
silicon grease was used in all joints to prevent the vapor loss of
reaction mixture and progress of the reaction was monitored in
terms of consumption of aldehyde. The analysis of product mixture
was carried out by gas chromatography (GC) (Shimadzu 17A, Japan)
and GC–MS (mass spectrometer, Shimadzu-QP2010, Japan). The GC
has a 5% diphenyl and 95% dimethyl siloxane universal capillary
column (60 m length and 0.25 mm diameter) and a flame ioniza-
tion detector (FID). The initial column temperature was increased
from 40 to 200 ◦C at the rate of 10 ◦C/min. Nitrogen gas at a flow
rate of 100 mL/min was used as the carrier gas. The temperatures
of the injection port and FID were kept constant at 200 ◦C during
the product analysis. The retention times of different compounds
were determined by injecting pure compounds under identical GC
conditions.
3.1.1. FT-IR analysis
The FT-IR spectrum of CHM (Fig. 1) showed distinctive absorp-
tion band at 1542 cm−1 for –NH2 bending. The absorption bands at
1154 cm−1 (anti-symmetric stretching of the C–O–C Bridge), 1093
and 1026 cm−1 (skeletal vibration involving the C–O stretching)
are characteristics of its saccharide structure [18]. The absorption
bands at 3441, 2921 cm−1 are attributed to O–H and methylene
(–CH2) group respectively.
3.1.2. Thermogravimetric analysis
The thermal behavior of CHM is shown in Fig. 2. Chitosan has two
main weight losses with one starting at 80 ◦C and another starting
at around 260 ◦C. The first weight loss of 5% is attributed to the
2.5. Kinetics
Kinetic experiments were carried out in an oven dried double
necked round bottom flask in which desired amounts of aldehyde
and catalyst were taken. The flask was kept in an oil bath having
provision for control of temperature and agitation speed. During
the course of the reaction aliquots were taken out at different
time intervals and the analysis of product mixture was carried
out by GC. The kinetics was investigated in detail as a function
of temperature and the catalyst amount and aldehyde. To ensure
the reproducibility of condensation reaction, repeated experiments
were carried out under identical reaction conditions. The results
obtained, including conversion and selectivity data were found to
be reproducible within 5% variation.
Fig. 2. TGA of CHM.

160
T. Jose et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 158–166
OH
R
Base catalyst
+
R
CHO
R
CHO
R
CHO
2
1
-H2O
R
R
R
CHO
R
-H2O
R
CHO
R
CHO
4
3
R = CH3, CH3CH2, CH3CH2CH2, CH3(CH2)2CH2, CH3(CH2)3CH2
Scheme 1. Pathways of self-aldol condensation of linear aldehydes.
removal of adsorbed water molecules. The second stage weight loss
of 60% is due to the decomposition of polysaccharide chain. Hence
the catalyst is thermally stable up to 250 ◦C.
of accessible amine group was found to be 32%. The functionalized
APTMS and chitosan itself are capable enough to contribute for the
observed 32% of accessible amino group. In the modified chitosan,
the grafted APTMS will have most of the amino groups exposed on
the surface whereas the amino groups of the chitosan itself being in
bulk will not have that much exposure. The salicylaldehyde adsorp-
tion onto –NH2 group is done in the presence of ethanol which
is a protic solvent. The use of protic solvent ethanol leads to the
swelling of polysaccharide framework of chitosan [19,20]. In protic
solvents the chitosan swells and the salicylaldehyde can enter into
the polymer chain to have access to the bulk amino groups. This
resulted in the higher accessible basic sites though the catalyst has
lower surface area.
3.1.3. C, H, N analysis
C, H, N analysis of chitosan and CHM were recorded and found
to be: chitosan (C: 39.32, H: 6.55, N: 8.37); CHM (C: 41.04, H: 6.71,
N: 10.96). From these results it is clear that the chitosan was func-
tionalized by APTMS.
3.1.4. Surface area and number of accessible amine group
It is known that the chitosan normally has a very low surface
area. The surface area was found to be 1.58 m2/g. The number of
accessible amino groups on the catalyst was determined by treat-
ing 50 mg catalyst. The catalyst (50 mg) was mixed with 3 ml of
0.16 M solution of salicylaldehyde in ethanol with nitrobenzene
as GC internal standard. The salicylaldehyde forms salicylidimine
Schiff base with the accessible amino group present on the catalyst.
The residual salicylaldehyde was calculated by GC from which the
accessible amino groups were determined. The percentage amount
3.2. Catalytic activity
The catalytic activity of CHM was evaluated for self-
condensation of linear aldehydes from propanal to heptanal as
represented in Scheme 1 and the corresponding results are listed
in Table 1. The experiments were carried out in detail with
Table 1
Aldol condensation of linear aldehydes.
Entry
Aldehyde
% conversion with time (h)
% selectivity with time (h)
␣,␤-Unsaturated aldehyde*
TOF
2
4
6
8
␤-Hydroxy aldehyde$
Further aldol product#
2
4
6
8
2
4
6
8
2
4
6
8
1
2
3
4
5
Propanal
Butanal
Pentanal
Hexanal
Heptanal
58 (98)
88
78
75
73
65
94
90
89
88
82
98
96
95
93
90
83 (96)
84
85
89
87
89
90
92
94
92
93
94
94
95
95
97
17 (1)
15 (4)
– (7)
– (5)
– (7)
3
3
–
–
–
2
–
–
–
–
–
–
–
–
–
– (3)
– (2)
17 (3)
14 (4)
12 (5)
13
12
11
13
11
8
8
6
8
7
6
6
5
5
3
43
39
39
37
33
42 (98)
38 (96)
35 (93)
30 (92)
85 (94)
83 (90)
86 (91)
88 (88)
Reaction conditions: aldehyde = 2.32 × 10−2 mol, catalyst = 50 mg, temperature = 100 ◦C, rpm = 450. TOF = (mmol of ␣,␤-unsaturated aldehyde/g catalyst/h).
In parenthesis, reaction conditions: aldehyde = 1 g, catalyst = 100 mg, temperature = 100 ◦C, rpm = 450, time = 1 h.
1 – *2-methyl pentenal, $3-hydroxy-2-methyl-pentanal, #2,4-dimethyl hepta-2,4-dienal.
2 – *2-ethyl hexenal, $3-hydroxy-2-ethyl-hexanal, #2,4-diethyl octa-2,4-dienal.
3 – *2-propyl heptenal, #2,4-dipropyl nona-2,4-dienal.
4 – *2-butyl octenal, #2,4-dibutyl deca-2,4-dienal.
5 – *2-pentyl nonenal, #2,4-dipentyl undeca-2,4-dienal.

T. Jose et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 158–166
161
Table 2
Effect of catalyst amount on the condensation of propanal.
Entry
Catalyst
amount (mg)
% conversion with
time (h)
% selectivity with time (h)
2-Methyl pentenal
TOF
4
8
3-Hydroxy-2-methyl-pentanal 2,4-Dimethyl hepta-2,4-dienal
4
8
4
8
4
8
1
2
3
4
25
50
75
6 (27)
42 (72)
51 (100)
78 (100)
25
82
93
95
36 (39)
45
96
83
80
64 (57)
53
2
5
– (4)
2
2
12
16
4
19
20
35
49 (78)
64 (86)
98 (93)
50 (19)
30 (6)
2 (–)
1 (3)
6 (8)
– (7)
100
4
Reaction conditions: propanal = 2 g, temperature = 100 ◦C, rpm = 450.
In parenthesis, reaction conditions: propanal = 1 g, temperature = 100 ◦C, rpm = 450, time = 2 h. TOF = (mmol of ␣,␤-unsaturated aldehyde/g catalyst/h).
2.32 × 10−2 mol of aldehyde and 50 mg catalyst with time. No reac-
tion was observed when chitosan as such was used. The modified
chitosan was found to be an efficient catalyst for the self-aldol
condensation with appreciable conversion and selectivity to major
formation of ␣,␤-unsaturated aldehyde with a small formation of
␤-hydroxy aldehyde and further aldol products. A decreasing trend
in the conversion from propanal to heptanal was observed. As the
reaction time increased to 8 h the conversion reached up to 98%
for propanal. It is decreased to 90% for heptanal. The reason for
decrease intheconversionwithincreasein chainlengthis the larger
chain length in which the removal of H+ ion by the basic catalyst
becomes difficult due to the +I effect of alkyl group.
In order to see the effect of lower time and higher catalyst
amount, 100 mg catalyst was used for the condensation of 1 g
aldehydes for 1 and 2 h reaction times. In 1 h time both the conver-
sion and selectivity to ␣,␤-unsaturated aldehyde were increased
(Table 1). On increasing the time for 2 h, the conversion was
increased to 100% for all the aldehydes and selectivities for ␣,␤-
unsaturated aldehyde for propanal, butanal, pentanal, hexanal and
heptanal were 93, 90, 88, 84 and 85% respectively. A compari-
son of the results of conversion, selectivity and time associated
with 100 and 50 mg of the catalyst indicated that both conver-
sion and selectivity were more for 100 mg catalyst in less time.
The TOF was calculated in terms of the product ␣,␤-unsaturated
aldehyde (mmol of product/g catalyst/h) at 4 h of reaction time. A
decreasing trend in the TOF was observed on increasing the chain
length. The TOF for propanal was 43 mmol/(gcat h) and dropped to
33 mmol/(gcat h) for heptanal. Propanal and pentanal were inves-
tigated in detail to observe the effects of the catalyst amount and
aldehyde, and temperature on conversion and selectivity.
1, 8 h) to 2-methylpentenal and 3-hydroxy-2-methyl-pentanal
was 45 and 53% respectively with minor formation (2%) of 2,4-
dimethyl hepta-2,4-dienal. The conversion (82%) and selectivity to
2-methylpentenal (96%) were increased (Entry 2, 8 h) with 50 mg
catalyst.
On further increasing the amount (Entry 3 and 4, 8 h), the
conversion was increased but the selectivity got decreased.
The decrease in the selectivity of 2-methylpentenal at higher
catalyst amount may be mainly due to the condensation of
2-methylpentenal with propanal to higher condensation prod-
uct. At lower amount (1 g) of propanal higher conversions were
obtained at lower time of 2 h. The conversion (Entry 1) at the
low catalyst amount (25 mg) was 27% for 1 g of propanal. The
selectivity (Entry 1) to 2-methylpentenal and 3-hydroxy-2-methyl-
pentanal was 39 and 57% respectively with minor formation
(4%) of 2,4-dimethyl hepta-2,4-dienal. On further increasing the
catalyst amount the conversion became 100% and selectivity to
2-methylpentenal was obtained up to 93% with 100 mg (Entry 4)
catalyst.
In the studied effect of the catalyst amount on the self-
condensation of two different amounts of pentanal (1 and 2 g) given
in Table 3, for 1 g pentanal, 51% conversion was obtained with 25 mg
(Entry 1) catalyst, which increased up to 100% on increasing the
amount of catalyst to 75 mg (Entry 3) at 2 h. For the higher amount
of pentanal of 2 g, 80% conversion of pentanal was obtained at 25 mg
(Entry 1) catalyst, which increased up to 95% on increasing the
amount of catalyst to 50 mg (Entry 2) at 8 h. The TOF showed an
increasing trend with the catalyst amount for propanal. For pen-
tanal TOF was initially increased and then got decreased.
The self-condensation is catalyzed by the active basic sites avail-
able on the surface of catalyst. As the amount of catalyst decreases,
the amount of active basic sites available for condensation reaction
on the surface of catalyst also decreases and hence lower con-
version was observed at lower catalyst amount. Another possible
reason for lower activity of the catalyst is strong adsorption of reac-
tant molecules (or slow diffusion) on the surface of CHM at lower
amount of catalyst, which could block the active basic sites present
on the surface of catalyst. On increasing the amount the number of
the active basic sites increases significantly.
3.3. Effect of catalyst amount
The effect of catalyst amount on conversion of propanal and
selectivity of 2-methylpentenal was studied by varying the amount
of catalyst from 25 to 100 mg at two different amounts of propanal
(1 and 2 g) and at 100 ◦C (Table 2). The conversion was increased
with increase in the catalyst amount. The conversion (Entry 1, 8 h)
at the low catalyst amount (25 mg) was 25%. The selectivity (Entry
Table 3
Effect of catalyst amount on the condensation of pentanal.
Entry
Catalyst amount (mg)
% conversion with time (h)
% selectivity with time (h)
2-Propyl heptenal
TOF
2
4
6
8
2,4-Dipropylnona-2,4-dienal
2
4
6
8
2
4
6
8
1
2
3
4
25
50
75
18 (51)
38 (91)
49 (100)
59 (100)
30
75
80
92
60
89
95
95
80
95
97
98
92 (82)a
83 (86)b
85 (85)c
94 (84)d
94
89
94
97
96
94
95
98
97
95
96
97
8 (1)
17 (5)
15 (10)
6 (11)
6
11
6
4
6
5
2
3
5
4
3
34
41
31
27
100
3
Reaction conditions: pentanal = 2 g, temperature = 100 ◦C, rpm = 450.
In parenthesis, reaction conditions: pentanal = 1 g, temperature = 100 ◦C, rpm = 450, time = 2 h, 3-hydroxy-2-propyl-heptanal was also obtained with a17%, b9%, c5% and d5%
selectivity.

Table 4
Effect of amount of propanal.
Entry
Amount of
propanal (g)
Catalyst/propanal
ratio (w/w)
% conversion with time (h)
% selectivity with time (h)
2-Methyl pentenal
TOF
4
8
3-Hydroxy-2-methyl-pentanal
2,4-Dimethylhepta-2,4-dienal
4
8
4
8
4
8
1
2
3
4
5
6
1
1.5
2
2.5
3
4
0.050
0.033
0.025
0.020
0.017
0.013
92
81
42
47
17
9
100
98
82
77
55
14
93
92
49
44
46
43
97
99
96
95
90
86
5
2
50
56
54
57
–
1
2
2
10
14
2
6
1
–
–
–
3
–
2
3
–
–
40
52
19
24
11
7
Reaction conditions: catalyst = 50 mg, temperature = 100 ◦C, rpm = 450.
Table 5
Effect of amount of pentanal.
Entry
Amount of
pentanal
Catalyst/pentanal
ratio (w/w)
% conversion with time (h)
% selectivity with time (h)
2-Propyl heptenal
TOF
2
4
6
8
2,4-Dipropylnona-2,4-dienal
2
4
6
8
2
4
6
8
1
2
3
4
1
2
3
4
0.050
0.025
0.017
0.013
91
38
12
13
95
75
25
22
95
89
34
31
97
95
44
42
86a
83
72
63
96
89
69
71
98
94
72
71
97
95
79
74
5
4
2
6
28
29
3
5
21
26
28
41
16
19
17
28
37
11
31
29
Reaction conditions: catalyst = 50 mg, temperature = 100 ◦C, rpm = 450.
a
9% 3-hydroxy 2-propylheptanal.

Table 6
Effect of temperature on the condensation of propanal.
Entry
Temperature (◦C)
% conversion with time (h)
% selectivity with time (h)
2-Methyl pentenal
TOF
4
8
3-Hydroxy-2-methyl-pentanal
2,4-Dimethylhepta-2,4-dienal
4
8
4
8
4
8
1
2
3
4
5
25
60
100
140
160
15
32
42
47
56
28
56
82
89
94
26
34
49
52
55
38
74
96
94
90
74
66
50
45
42
62
26
2
–
–
–
–
1
3
3
–
–
2
6
8
4
10
19
23
29
Reaction conditions: catalyst = 50 mg, propanal = 2 g, rpm = 450.
Table 7
Effect of temperature on the condensation of pentanal.
Entry
Temperature (◦C)
% conversion with time (h)
% selectivity with time (h)
2-Propyl heptenal
TOF
2
4
6
8
2,4-Dipropylnona-2,4-dienal
2
4
6
8
2
4
6
8
1
2
3
4
5
25
60
100
140
160
6
8
10
60
89
95
98
11
77
95
97
98
89
81
83
91
92
77
95
89
92
94
85
98
94
94
96
87
98
95
94
97
11
19
17
9
23
5
11
8
15
2
6
6
4
13
2
5
6
3
4
24
41
52
55
14
38
47
64
41
75
92
96
8
6
Reaction conditions: catalyst = 50 mg, pentanal = 2 g, rpm = 450.

164
T. Jose et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 158–166
3.4. Effect of amount of aldehyde
Amount of aldehyde has significant effect on the aldol conden-
sation. The results of the variation of the amounts of propanal and
pentanal are given in Tables 4 and 5 respectively. At lower amount
of propanal (Table 4. Entry 1, 1 g, 8 h) conversion was 100% with
97% selectivity of 2-methylpentenal. On increasing the amount of
propanal (Entry 3, 2 g), both conversion (82%) and selectivity of 2-
methylpentenal (86%) were decreased following similar decreasing
trend on further increasing the amount to 4 g. The conversion and
selectivity got increased on increasing the catalyst/propanal ratio.
The highest selectivity (99%) was found at the catalyst/propanal
ratio of 0.033 (Table 4, Entry 2, 8 h).
With 1 g of pentanal (Table 5, Entry 1), 97% conversion was
achieved. On further increasing the amount of pentanal, conversion
and selectivity of 2-propylheptenal were decreased. The lower con-
version at higher amount of aldehyde is due to the less availability
of the active basic sites of the catalyst. On decreasing the ratio of the
catalyst/aldehyde the TOF for the substrates propanal and pentanal
were firstly increased. On further decreasing the ratio the TOF was
decreased.
Fig. 3. Kinetic profile for pentanal condensation reaction; pentanal = 2.32 ×
10−2 mol, catalyst amount = 50 mg, temperature = 140 ◦C.
3.5. Effect of temperature
The effect of temperature was studied in the range of 25–160 ◦C
for the self-condensation of propanal and the results are given in
Table 6. With an increase in the temperature the conversion and
selectivity to 2-methylpentenal were increased where as the selec-
tivity to 3-hydroxy-2-methyl-pentanal was decreased. At 25 ◦C
(Entry 1) conversion was 28%, on increasing the temperature to
60 ◦C (Entry 2) conversion increased to 56%. On further increasing
the temperature (Entry 4) conversion increased to 94% at 160 ◦C
but selectivity to 2-methylpentenal was decreased. The condensa-
tion of lower aldehydes having low boiling points will not get much
affected by increasing the temperature which is much above their
boiling points. The temperature was much effective only towards
the lower temperature of 25–100 ◦C for propanal. The conversion
for 8 h was doubled from 25 (Table 6, Entry 1, 28%) to 60 ◦C (Table 6,
Entry 2, 56%) and became 82% at 100 ◦C after that on further increas-
ing the temperature the conversion could reached to 94% at 160 ◦C,
indicating that higher temperature is not much influential. On
increasing the temperature TOF was increased.
slope of the early linear portion of the decrease in the amount of
pentanal. Variation of the catalyst amount (Fig. 5) showed that
as the catalyst amount increased, rates were increased, and at
higher catalyst amount the reaction rate tended to approach satu-
ration.
The effect of amount of pentanal varied in the range of
1.7–4.6 × 102 mol (Fig. 6) indicated that the rates were increased
on increasing the amount of pentanal. The effect of reaction tem-
perature shown in Fig. 7 showed that rate was increased with
temperature. The rate, 11 × 10−4 mol/min/gcat at 60 ◦C increased
to 20 × 10−4 mol/min/gcat at 100 ◦C. The rate was increased to
40 × 10−4 mol/min/gcat at 140◦ C. The low activity of the catalyst
for aldol condensation of pentanal at lower temperature may be
due to the strong adsorption of pentanal molecules on the sur-
face of chitosan. Therefore, high temperature is required to avoid
this undesirable adsorption which leads to decreased catalytic
activity.
The results of the effect of temperature on the self-condensation
ofpentanalare given in Table 7. Theconversion(Entry1) ofpentanal
was increased from 11 to 95% on increasing the temperature from
at 25 to 100 ◦C. The selectivity of 2-propylheptenal was increased
up to 100 ◦C (98%) and after that started decreasing.
3.6. Kinetic studies
In order to have an insight into the kinetic performance of the
CHM catalyzed aldol condensation reactions a representative alde-
hyde, pentanal was subjected for detailed kinetic investigations as
the function of temperature, catalyst amount and pentanal. While
varying a parameter, other parameters were kept constant under
identical conditions. The effects were evaluated in terms of the
rates of the decreasing amount of pentanal with time during the
reaction. Kinetic profile for the pentanal condensation reaction is
shown in Fig. 3. The consumption of pentanal was 55 × 10−4 mol
in 1st h and after 1st h to 6th h a regular decrease was observed
in which 151 × 10−4 mol was consumed in the next 5 h indicating
that the initial rate of consumption was faster. The aldol con-
densation product 2-propylheptenal, was increased with time and
further aldol product 2,4-dipropylnona-2,4-dienal was observed in
very low amounts. The rates of condensation were calculated from
the consumption of pentanal in moles with time (Fig. 4) from the
Fig. 4. Plot of decreasing amount of pentanal with time for the variation of catalyst
amount; pentanal = 2.32 × 10−2 mol, temperature = 100 ◦C.

T. Jose et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 158–166
165
Fig. 5. Dependence of rate on catalyst amount; pentanal = 2.32 × 10−2 mol, temper-
Fig. 7. Dependence of rate on temperature; pentanal = 2.32 × 10−2 mol, catalyst
ature = 100 ◦C.
amount = 50 mg.
3.7. Comparison with closely related catalyst systems
It is of interest to have a comparative insight into the perfor-
mance of the modified chitosan as catalyst with closely related solid
base catalyst systems for self-condensation. Table 8 lists the per-
formance of the solid base catalyst systems for self-condensation of
propanal, butanal and hexanal. For propanal with CHM, 100% con-
version (Entry 1) was achieved in 2 h. In terms of conversion and
time the performance of CHM was found to be better than other
catalyst systems for propanal (Entries 2–5). For catalyst systems
(Entries 2–5) the conversions were less than 100% and also the time
taken was 10 h. Among other catalysts (Entries 2–5) hydrotalcite
(3.5) activated at 450 ◦C for 4 h was found to be better in terms of
conversion (97%) and selectivity (99%) in 10 h. For butanal (Entries
6–8) CHM showed 100% conversion in 2 h at 100 ◦C. The results
with ␥-Al2O3 (Entry 8) is shown at lower temperature 50 ◦C but
the conversion is very low (33%) in 2 h. The fixed bed catalyst sys-
tem Nb2O5/SiO2 (Entry 8) showed 51.3% conversion at 150 ◦C. The
comparison of CHM (Entry 9) and NH2-FSM (Entry 10) for hexanal
showed that 100% conversion was achieved in 2 h by CHM where as
NH2-FSM could give 86% conversion at higher temperature 110 ◦C
with very long time of 20 h.
Fig. 6. Dependence of rate on amount of pentanal; catalyst amount = 50 mg, tem-
perature = 100 ◦C.
Table 8
Comparison of some closely related catalyst systems for aldol condensation.
Entry Catalyst
Substrate Time (h) Temp. (◦C) % conv.
% selectivity
Ref.
␣,␤-Unsaturated aldehyde ␤-Hydroxy aldehyde
1
2
3
4
5
6
7
8
9
CHM
Propanal
Propanal
8 (2)
10
10
10
10
2
–
2
2
20
100 (100)
100
98 (100)
83
94 (93)
86
–
9
0
14
0
2
–
58.8
5
Present work
[21]
[21]
[13]
[13]
Present work
[22]
[23]
Present work
[3]
Hydrotalcite (3.5) without activation
Hydrotalcite (3.5) activated at 450 ◦C/4 h Propanal
100
100
100
100
150
50
100
110
97
70
32
100
99
82
99
90
Magnesium organo silicate
Natural talc
CHM
Propanal
Propanal
Butanal
Butanal
Butanal
Hexanal
Hexanal
5.9% Nb2O5/SiO2 co-precipitated
51.3
98.5
␥-Al2O3
CHM
33
100
86
19.4
84
10
NH2-FSMa
70b
–
a
APTMS fuctionalized FSM-16.
% yield.
b

166
T. Jose et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 158–166
Table 9
Reusability of the catalyst.
Run
% conversion
% selectivity
2-Methyl pentenal
3-Hydroxy-2-methyl-pentanal
2,4-Dimethyl hepta-2,4-dienal
Fresh catalyst
First recycle
Second recycle
Third recycle
Fourth recycle
Fifth recycle
82
82
80
78
77
75
96
96
95
94
95
93
2
2
3
2
2
6
2
2
2
4
3
1
Reaction conditions: catalyst = 50 mg, propanal = 2 g, temperature = 100 ◦C, rpm = 450.
3.8. Reusability of catalyst
tative aldehyde, pentanal significantly depended on the catalyst
amount, pentanal and temperature. The catalyst was separated and
effectively used up to six cycles under identical employed condi-
tions.
The spent catalyst was regenerated by washing with toluene.
It is then filtered and dried at 100 ◦C. The regenerated catalyst was
again used for the aldol condensation of the propanal under similar
conditions. From the data on conversion of propanal and selectivity
of 2-methylpentenal given in Table 9 it is observed that the cata-
lyst was reproducible up to six cycles without significant loss in
its activity for aldol condensation of propanal. Further more the
recycled catalyst was checked for elemental analysis to study the
leaching of the APTMS, if any. The obtained results were %C: 43.78,
%H: 6.82, %N: 10.59. This confirms the stable functionalization of
the CHM and the increase in %C may be attributed to the presence
of unreacted adsorbed aldehydes and products. The FT-IR spec-
trum of recycled catalyst also gave an intense stretch at 1649 cm−1
corresponding to the imine formed by the aldehyde during aldol
condensation.
Acknowledgments
Authors thank Council of Scientific and Industrial Research
(CSIR), New Delhi, India for financial supports for Network Project
for the Development of Specialty Inorganic Materials for Diverse
Applications. NS acknowledges CSIR, New Delhi for the award of
Senior Research Fellowship.
References
[1] Y.C. Chang, A.N. Ko, Appl. Catal. A 190 (2000) 149–155.
[2] A. Molnár, B. Rác, Curr. Org. Chem. 10 (2006) 1697–1726.
[3] K. Shimizu, E. Hayashi, T. Inokuchi, T. Kodama, H. Hagiwara, Y. Kitayama, Tetra-
hedron Lett. 43 (2002) 9073–9075.
[4] G. Zhang, H. Hattori, K. Tanabe, Appl. Catal. 40 (1988) 183–190.
[5] T. Yashima, H. Suzuki, N. Hara, J. Catal. 33 (1974) 486–492.
[6] P. Chu, G.W. Kuehl, US Patent, 4,605,787 (1986).
[7] P.T. Weirzechowski, L.W. Zatarski, Catal. Lett. 9 (1991) 411–414.
[8] S.K. Sharma, P.A. Parikh, R.V. Jasra, J. Mol. Catal. A 286 (2008) 55–62.
[9] S.K. Sharma, H.A. Patel, R.V. Jasra, J. Mol. Catal. A 280 (2008) 61–67.
[10] V.K. Díez, C.R. Apesteguía, J.I. Di Cosimo, J. Catal. 240 (2006) 235–244.
[11] S. Abelló, F. Medina, D. Tichit, J. Pérez-Ramírez, J.E. Sueiras, P. Salagre, Y. Ces-
teros, Appl. Catal. B 70 (2007) 577–584.
To check for any loss of APTMS from the system the fresh cata-
lyst (CHM) was mixed in toluene and the supernatant solution was
separated. In the separated supernatant solution salicylaldehyde
was added and the solution was checked by UV–vis spectroscopy
in which no peak characteristic to imine was observed indicating
that there is no loss of APTMS.
4. Conclusions
[12] P. Kus´trowski, D. Sułkowska, L. Chmielarz, A. Rafalska-Łasocha, B. Dudek, R.
Dziembaj, Micropor. Mesopor. Mater. 78 (2005) 11–22.
Chitosan has been modified using APTMS avoiding the use of
HCl and NaOH. The modified chitosan was found to be an effi-
cient catalyst for the self-aldol condensation for linear aldehydes
under solvent-free conditions. All the aldehydes selectively gave
the desired dehydrated aldol formation as major product. The cat-
alyst evaluated for C3–C7 linear aldehydes indicated a decreasing
trend in the conversion from propanal to 1-heptanal. The conver-
sions were found to be increased with time. As the reaction time
increased to 8 h the conversion reached up to 98% for propanal and
90% for heptanal. With the amount of catalyst of 100 mg and alde-
hyde 1 g, the conversion was 100% for all aldehydes within 2 h.
Kinetic performance of the catalyst investigated for a represen-
[13] H.A. Patel, S.K. Sharma, R.V. Jasra, J. Mol. Catal. A 286 (2008) 31–40.
[14] S.K. Sharma, V.K. Srivastava, R.S. Shukla, P.A. Parikh, R.V. Jasra, New J. Chem. 31
(2007) 277–286.
[15] G.E. Barker, D. Forster, U.S. Patent 4,426,542 (1984).
[16] E.B. Denkba, M. Odaba, J. Appl. Polym. Sci. 76 (2000) 1637–1643.
[17] K.R. Reddy, K. Rajgopal, C.U. Maheswari, M.L. Kantam, New J. Chem. 30 (2006)
1549–1552.
[18] C. Peniche, W. Arguelles-Monal, N. Davidenko, R. Sastre, A. Gallardo, J.S. Roman,
J. Biomater. 20 (1999) 1869–1878.
[19] R. Valentin, K. Molvinger, F. Quignard, D. Brunel, New J. Chem. 27 (2003)
1690–1692.
[20] N. Sudheesh, S.K. Sharma, R.S. Shukla, J. Mol. Catal. A 321 (2010) 77–82.
[21] S.K. Sharma, P.A. Parikh, R.V. Jasra, J. Mol. Catal. A 278 (2007) 135–144.
[22] P. Moggi, G. Albanesi, Appl. Catal. 68 (1991) 285–300.
[23] H.E. Swift, J.E. Bozik, F.E. Massoth, J. Catal. 148 (1994) 759–770.