Kinetic investigations on the modified chitosan catalyzed solvent-free synthesis of jasminaldehyde

Article abstract of DOI:10.1016/j.molcata.2011.02.016

Kinetic investigations were performed for the modified chitosan catalyzed solvent-free synthesis of jasminaldehyde. The rates of formation for both jasminaldehyde and 2-pentyl nonenal were calculated. Kinetic investigations were performed as a function of the amount of the catalyst, heptanal, benzaldehyde and temperature. The rate of formation of jasminaldehyde was first order with respect to the lower amount of catalyst and showed saturation at higher amounts. A critical amount of heptanal was needed for the formation of jasminaldehyde. The maximum rate was showed for heptanal-benzaldehyde ratio of 1:4. The Langmuir-Hinshelwood rate model used for the formation of jasminaldehyde was found to fit with R² value of 0.95. The computational studies were indicative for the rate determining steps, as the reaction of benzaldehyde with deprotonated heptanal on surface for jasminaldehyde, and proton abstraction for the formation of 2-pentyl nonenal. Mass transfer effects for the condensation reaction were studied.

Full text of DOI:10.1016/j.molcata.2011.02.016

Journal of Molecular Catalysis A: Chemical 339 (2011) 86–91
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Kinetic investigations on the modified chitosan catalyzed solvent-free synthesis
of jasminaldehyde
N. Sudheesh, Sumeet K. Sharma, Munir D. Khokhar, 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 364 021, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Kinetic investigations were performed for the modified chitosan catalyzed solvent-free synthesis of jas-
minaldehyde. The rates of formation for both jasminaldehyde and 2-pentyl nonenal were calculated.
Kinetic investigations were performed as a function of the amount of the catalyst, heptanal, benzalde-
hyde and temperature. The rate of formation of jasminaldehyde was first order with respect to the lower
amount of catalyst and showed saturation at higher amounts. A critical amount of heptanal was needed
for the formation of jasminaldehyde. The maximum rate was showed for heptanal–benzaldehyde ratio
of 1:4. The Langmuir–Hinshelwood rate model used for the formation of jasminaldehyde was found to
fit with R2 value of 0.95. The computational studies were indicative for the rate determining steps, as
the reaction of benzaldehyde with deprotonated heptanal on surface for jasminaldehyde, and proton
abstraction for the formation of 2-pentyl nonenal. Mass transfer effects for the condensation reaction
were studied.
Received 16 December 2010
Received in revised form 19 February 2011
Accepted 22 February 2011
Available online 2 March 2011
Keywords:
Jasminaldehyde
Chitosan
Kinetics
Aldol condensation
Mass transfer
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
catalyst for the condensation of heptanal and benzaldehyde. The
catalyst was active by giving >99% conversion with major selectiv-
␣-Pentyl cinnamaldehyde is a perfumery chemical of jasmine
odor with commercial name jasminaldehyde [1]. It is synthesized
by the condensation of heptanal with benzaldehyde in the pres-
ence of a catalyst. The used catalysts are liquid alkalis like NaOH
and KOH [2]. The liquid alkalis have drawbacks of environmental
pollution due to alkali wastes, lack of catalyst recycling and post
reaction treatment of the spent catalyst. Thus the development of
heterogeneous solid catalysts can be a substitute for overcoming
these drawbacks.
There are few reports where the heterogeneous solid catalysts
like H-beta zeolites, Al-MCM-41, ALPO, Na-ALPO [3], magnesium
organo silicate [4] and hydrotalcite [5] have been investigated for
the synthesis of jasminaldehyde. Development of a heterogeneous
catalyst based on biopolymer can gain a much attraction towards
the synthesis of jasminaldehyde. Chitosan, a biopolymer derived
from naturally occurring chitin have the potential to act as a solid
base catalyst. However the research in this regard is very scanty
[6–9]. Chitosan were extensively studied for its bio-activity related
to drug delivery and medical applications [10–12]. The investi-
gations on chitosan, a biopolymer, have promising potential to
develop eco-friendly catalyst for condensation reactions. A chitosan
based solid base catalyst [6] was recently reported to be an efficient
ity (84%) to jasminaldehyde by cross condensation along with small
formation of the self condensation product.
Kinetics of such reactions play an important role to address
the extent of the contribution of the catalyst towards the cross
and self condensation products. The kinetics of condensation reac-
tions were reported for the both self and cross condensations. The
kinetics of self condensation of acetaldehyde using NaOH as cat-
alyst was investigated by Bell and McTigue [13,14] and Guthrie
[15]. Casale et al. have reported the kinetics of C2–C8 aliphatic
aldehydes [16]. Cross condensation of benzaldehyde–acetone
[17,18], benzaldehyde–propanal [19], citral–acetone [20,21] and
butyraldehyde–formaldehyde [22] are kinetically investigated.
Detail kinetic investigations are paid less attention for the syn-
thesis of jasminaldehyde by condensation of benzaldehyde and
heptanal. The possible products during the condensation are the
cross and self aldol products, jasminaldehyde and 2-pentyl none-
nal respectively. The endeavor of the present study is to investigate
the reaction kinetics and mechanism of the synthesis of jasminalde-
hyde using heterogeneous chitosan a biopolymer as catalyst.
2. Experimental
2.1. Materials and methods
∗
Benzaldehyde and heptanal were procured from Sigma–Aldrich
and used without further purifications. NaOH and HCl were pur-
Corresponding author. Tel.: +91 278 2567760; fax: +91 278 2566970.
E-mail address: rshukla@csmcri.org (R.S. Shukla).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.02.016

N. Sudheesh et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 86–91
87
Table 1
Physical properties of the catalyst.
Physical parameter
Value
Dimension
m2 g−1
BET surface area
Accessible amine group
Particle size
1.04
45%
0.411 × 10−3
m
chased from s.d. Fine Chemicals, India for the synthesis of catalyst.
The double distilled milli-pore water was used during the synthe-
sis of the catalyst. The catalyst was synthesized according to the
method available in literature [6]. The physical properties of the
synthesized catalyst are given in Table 1.
2.2. Aldol condensation and kinetic measurements
Benzaldehyde and heptanal were taken in the desired molar
ratio with the required amount of the chitosan catalyst, in a two
neck round bottom flask and 0.01 g decane was added to this mix-
ture as an internal standard. One neck of the flask was connected
with a condenser through which water at 18 ◦C was circulated.
Other neck was blocked with silicon rubber septum. The flask was
kept in an oil bath equipped with temperature controller and a
magnetic stirring unit. The reaction was conducted under inert
atmosphere (N2) to inhibit the formation of corresponding acids
from aldehyde. The product mixture was cooled down to room
temperature after completion of the reaction and was filtered. The
analysis of product mixture was carried out by GC (Shimadzu 17A,
Japan) and GC–MS (Schimadzu QP-2010, Japan). The GC has 5%
diphenyl and 95% dimethyl siloxane universal capillary column and
flame ionization detector (FID). Repeated experiments were carried
out under identical reaction conditions to ensure the reproducibil-
ity of the reaction.
Fig. 2. Dependence of rate on the amount of catalyst: heptanal = 7.9 mmol, ben-
zaldehyde = 39.5 mmol, temperature = 140 ◦C and 600 rpm.
almost linear decrease in the amount of heptanal and the formation
of jasminaldehyde and self condensation product 2-pentyl nonenal
were observed. Similar types of kinetic profile plots were made for
all parametric variations and the rates were calculated from the
slopes of early linear portions of the corresponding plots.
Kinetic investigations were performed as a function of the
amount of the catalyst, heptanal, benzaldehyde and temperature
by varying these parameters in a wide range. While varying a
parameter, other parameters were kept constant under identical
conditions. The effects of these parameters were investigated on
the rates of the formation of jasminaldehyde and 2-pentyl nonenal.
The rates of the formation of jasminaldehyde (Eq. (1)) and 2-pentyl
nonenal (Eq. (2)) were calculated from the plots (Fig. 1) of their
increasing concentration with time.
For kinetic measurements, a series of kinetic experiments were
carried out for the determination of the rates. Aliquots were with-
drawn from the reaction mixture at different intervals and were
analyzed by GC.
d[jasminaldehyde]
Ratejasminaldehyde, v1
=
(1)
(2)
dt
3. Results and discussion
d[2-pentyl nonenal]
Rate2-pentyl nonenal, v2
=
dt
3.1. Kinetic profile
The rate was expressed in terms of mmol of product per unit time
per gram of the catalyst [23].
The kinetic profiles in term of the consumption of heptanal and
formations of condensation products are given in Fig. 1. With time
3.2. Dependence of the rate on catalyst amount
The effect of the catalyst amount on rates of aldol condensa-
tion was studied between 10 and 200 mg of the catalyst and the
corresponding results are given in Fig. 2. The rate of formation of
jasminaldehyde was increased linearly indicating first order depen-
dence with the catalyst amount. The increase in the amount of
the catalyst increases the total available active sites and thus will
increase the rate of reaction. When the amount of the catalyst was
increased to higher amounts the rate tends to attain saturation. On
the other hand the rate of formation of self condensation product,
2-pentyl nonenal, was increased slowly upto 100 mg of the catalyst
and further the rate showed a saturation kinetics.
3.3. Dependence of the rate on heptanal
The dependence of rate on heptanal was studied between hep-
tanal amounts of 0 and 31.6 mmol and was shown in Fig. 3. It was
observed that the rate of formation of jasminaldehyde was very
low at lower heptanal concentration and also a critical amount
of the heptanal is needed for the cross aldol condensation to
yield jasminaldehyde. The rate of formation of jasminaldehyde
Fig. 1. Kinetic profile: catalyst = 100 mg, heptanal = 7.9 mmol, benzalde-
hyde = 39.5 mmol and temperature = 140 ◦C.

88
N. Sudheesh et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 86–91
Fig. 3. Dependence of rate on heptanal: catalyst = 100 mg, benzalde-
Fig. 5. Dependence of rate on temperature: catalyst = 100 mg, heptanal = 7.9 mmol,
benzaldehyde = 39.5 mmol and 600 rpm.
hyde = 39.5 mmol, temperature = 140 ◦C and 600 rpm.
was increased after this critical amount and reaches a maximum
and then decreased. Heptanal showed inhibition kinetics towards
higher amounts. At higher amounts of the heptanal the self con-
densation was highly favored due to more availability of heptanal
on the catalytic sites leading to the self condensation. The rate of
formation of 2-pentyl nonenal was linearly increased with increase
in the amount of heptanal.
decreasing the rate of formation of both jasminaldehyde and 2-
pentyl nonenal.
3.5. Dependence of rate on the temperature
The dependence of rate on the temperature was studied
between 100 and 180 ◦C, and was shown in Fig. 5. The rate of forma-
tion of jasminaldehyde was increased with temperature. The rate
of self condensation product was not much affected by the temper-
ature. The activation energy was calculated from the Arrhenius plot
and found to be 20.1 kJ/mol for the formation of jasminaldehyde.
3.4. Dependence of the rate on benzaldehyde
The dependence of rate on benzaldehyde was studied between
benzaldehyde amounts of 0 and 79 mmol, and the result was shown
in Fig. 4. The rate of formation of jasminaldehyde was increased
with increase in the amount of the benzaldehyde. The highest rate
was observed for heptanal to benzaldehyde ratio of 1:4, i.e. hep-
tanal 7.9 mmol and benzaldehyde 31.6 mmol. The rate of formation
of 2-pentyl nonenal was decreased with increase in the amount of
benzaldehyde. The inhibition kinetics shown for formation of jas-
minaldehyde at higher benzaldehyde amount was due to higher
adsorption of benzaldehyde onto the active sites of the catalyst
leading to lower availability of the same for heptanal. This results
in the formation of lower amount of carbanion from heptanal thus
3.6. Dependence of rate on the particle size and stirring speed
The catalytic experiments on the effects of the variation of parti-
cle size (0.211–0.853 mm) and stirring speed (200–800 rpm) were
conducted and the corresponding results are given in Figs. 6 and 7
respectively. The particle size lower than 0.5 mm had no effect on
the rates (Fig. 6) showing that the reactions were under kinetic
regime with negligible internal mass transfer towards lower parti-
cle size. The particle size of 0.411 mm was taken for studying the
effect of rate on stirring speed, Fig. 7. The rate of formation of jasmi-
Fig. 4. Dependence of rate on benzaldehyde: catalyst = 100 mg, heptanal = 7.9 mmol,
Fig. 6. Dependence of rate on particle size: catalyst = 100 mg, heptanal = 7.9 mmol,
temperature = 140 ◦C and 600 rpm.
benzaldehyde = 39.5 mmol, temperature = 140 ◦C and 600 rpm.

N. Sudheesh et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 86–91
89
Table 2
Physico-chemical data for mass transfer calculations.
Symbol Quantity
Value
Dimension
T
Temperature
Concentration of heptanal
Reaction volume
Catalyst weight
Particle diameter
413
1580
K
C0
Vr
w
dp
ꢀp
ꢂs
ꢀs
Ms
ꢁs
N
mol m−3
m3
5 × 10−6
0.1 × 10−3
0.411 × 10−3
860
kg
m
Particle density
kg m−3
Pa s−1
kg m−3
g mol−1
Viscosity of benzaldehyde
Density of benzaldehyde
Molecular weight of benzaldehyde
Association factor for benzaldehyde
Stirring revolutions
7.901 × 10−4
980
106.12
1
10
s−1
m
Sd
VA
Characteristic length of magnetic bead 2 × 10−2
Molar volume of heptanal at normal
boiling point
163.99
cm3 mol−1
D
Diffusivity
1.8685 × 10−9
4961.4
431.35
m2 s−1
Re
Sc
Sh
kf
Robs
Ca
ꢃ
Reynolds number
Schmidt number
Sherwood number
Mass transfer coefficient
Experimental rate
Carberry number
Porosity
321.24
14.608 × 10−4
0.398 × 10−6
0.01 × 10−2
0.0142
m s−1
mol s−1
Fig. 7. Dependence of rate on stirring speed: catalyst = 100 mg, particle
size = 0.411 mm, heptanal = 7.9 mmol, benzaldehyde = 39.5 mmol and tempera-
ture = 140 ◦C.
␶
Tortuosity factor
4.5
Deff
ꢄꢁ2
Effective diffusivity
Wheeler–Weisz group
0.5895 × 10−11 m2 s−1
0.361
naldehyde was lower at 200 rpm which increased at 400 rpm and
remained almost same. The rate of formation of 2-pentyl nonenal,
the self condensation product, was higher at lower stirring speed of
200 rpm and then decreased at 400 rpm and remained almost same.
This shows that the aldol reactions were under kinetic regime with
negligible mass transfer.
where ꢀs is the density of solvent (benzaldehyde), N is the rota-
tional speed (revolutions per second), Sd is the stirrer length.Sc is
the Schmidt number (Eq. (8)):
ꢂs
Sc =
(8)
3.7. Mass transfer effects
ꢀsD
Reactionengineeringparametershavebeenstudied for the mass
transfer effects during the reaction to have an insight into exter-
nal (liquid/solid interface) and internal diffusions (pore) using the
Carberry number and Wheeler–Weisz criterion respectively [24].
The Carberry number (Ca) defined as the ratio of observed exper-
imental reaction rate to mass transfer rate at liquid/solid interface
is given as Eq. (3):
The Wheeler–Weisz criterion is given by Eq. (9):
Rateobsdp2
ꢄꢁ2
=
(9)
4VrC0Deff
Effective diffusivity Deff is given by Eq. (10):
Dꢃ
Deff
=
(10)
ꢅ
Rateobsdpꢀp
Ca =
(3)
6kfC0w
where ꢃ is the porosity and ꢅ is the tortuosity factor.
The values calculated for physico-chemical data for mass trans-
fer is listed in Table 2. The mass transfer is negligible and does not
affect the reaction kinetics at Ca < 0.05 and ꢄꢁ2 < 1. For present sys-
tem, the calculated Ca (0.01 × 10−2) and ꢄꢁ2 (0.361) both being
very low, indicated that the mass transfer is negligible and has no
effect on reaction rate.
where Rateobs = total rate (v1 + v2) in mol s−1, dp = the particle
diameter, ꢀp = particle density, C0 = initial heptanal concentration,
w = catalyst weight and kf = mass transfer coefficient (obtained from
Eq. (4)).
kfdp
Sh =
(4)
D
where Sh is the Sherwood number and D is the diffusivity calculated
using Wilke–Chang correlation (Eq. (5)),
3.8. Reaction mechanism
7.4 × 10−12(ꢁsMs)1/2T
The proposed reaction mechanism for the condensation of hep-
tanal and benzaldehyde for the synthesis of jasminaldehyde on the
chitosan catalyst has been shown in Fig. 8. The reaction gets initi-
ated on the –NH2 group present on the chitosan 1. As the initiation
step, heptanal loses its acidic hydrogen to the amine group form-
ing a carbanion, benzaldehyde gets adsorbed on the other –NH2
and form an imine species 2. The carbanion of heptanal attacks
on the C N species 3 and forms an unstable intermediate 4 which
releases the jasminaldehyde and the active sites are again reused
for the further catalytic cycles. The mechanism is equivalent to the
reported [4] synthesis of jasminaldehyde over amino functional-
ized magnesium organosilicate. Self condensation of the heptanal
is possible by the reaction between the heptanal carbanion with
another molecule of heptanal to yield the product, 2-pentyl none-
nal.
D =
(5)
ꢂs(VA)0.6
where Ms is the molecular weight of solvent (benzaldehyde), ꢁs is
the association factor for solvent, ꢂs is the viscosity of solvent at
temperature T and VA is the molar volume of solute (heptanal) at
its normal boiling point. The VA and ꢂs in Eq. (5) are expressed in
cm3 mol−1 and centi Poise respectively, to obtain D in m2 s−1
.
Sherwood number (Eq. (6)) is obtained from the correlation,
Sh = 2 + 0.6Re1/2Sc1/3
(6)
(7)
where Re is the Reynolds number (Eq. (7)) for stirred vessel,
ꢀsNSd2
Re =
ꢂs

90
N. Sudheesh et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 86–91
Fig. 8. Proposed reaction mechanism for the synthesis of jasminaldehyde over modified chitosan.
3.9. Kinetic modelling
These surface adsorbed heptanal carbanions can also undergo sur-
face reaction with adsorbed heptanal and a proton to yield self
condensed product 2-pentyl nonenal (step (5)). Finally jasminalde-
hyde (step (6)), 2-pentyl nonenal (step (7)) and water (step (8)) get
desorbed and the catalyst surface becomes available.
The calculation of Ca and ꢄꢁ2 showed that the reaction was not
diffusion controlled. The RDS for jasminaldehyde may be proton
abstraction (Fig. 9, step (3)) or the surface reaction (Fig. 9, step (4)).
A kinetic model for the proton abstraction as RDS is given as Eq.
(11):
The reaction steps for the condensation of heptanal with ben-
zaldehyde are presented in Fig. 9. It is important to find the rate
determining step (RDS) for the formation of aldol condensation
products. The heptanal (step (1)) and benzaldehyde (step (2)) get
adsorbed on the catalyst surface [᭹]. Heptanal carbanion forms
by abstraction of proton from adsorbed heptanal in step (3). On
the surface, heptanal carbanion reacts with adsorbed benzalde-
hyde and a proton yielding jasminaldehyde and water in step (4).
kjKHCH
Ratejasminaldehyde, v1
=
(11)
1 + KHCH + KBCB
where kj is the reaction rate constant for the formation of jas-
minaldehyde, CH and CB are the concentrations of heptanal and
benzaldehyde respectively and KH and KB are the adsorption coef-
ficients of heptanal and benzaldehyde, respectively. The model (Eq.
(11)) did not fit with the experimental data and gave a linear trend
rather than the initial increase and decrease in rates on increasing
the benzaldehyde/heptanal ratio.
Hence a RDS based on surface reaction may be the model
suitable for jasminaldehyde formation. The rate of formation
of jasminaldehyde being dependent on the ratio of benzalde-
hyde to heptanal can be regarded as a reaction following typical
Langmuir–Hinshelwood (L–H) mechanism [17]. The experimental
rates were compared with the rates obtained from the L–H model
Fig. 9. The important reaction steps for the condensation of heptanal with benzalde-
hyde (catalyst surface [᭹], heptanal [Hep], benzaldehyde [Benz], jasminaldehyde
[Jas], 2-pentyl nonenal [Self]).

N. Sudheesh et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 86–91
91
mechanism. The rates were calculated in terms of formation of
jasminaldehyde and 2-pentyl nonenal. Kinetic investigations were
performed as a function of the amount of the catalyst, heptanal,
benzaldehyde and temperature. The rate of formation of jasmi-
naldehyde was first order at the lower amount of catalyst and
showed saturation at higher amounts. A critical amount of heptanal
was needed for the formation of jasminaldehyde. The maximum
rate was observed for heptanal to benzaldehyde ratio of 1:4. Based
on these observations a plausible reaction mechanism for chitosan
catalyzed synthesis of jasminaldehyde was proposed. The rate
model based on Langmuir–Hinshelwood mechanism for bimolec-
ular reactions was used to compare the experimental and model
rates for the formation of jasminaldehyde and found to have the
best fit with R2 value of 0.95. The rate constants for the for-
mation of jasminaldehyde (kj = 43.43 mmol/h/gcat) and 2-pentyl
nonenal (ks = 12.25 mmol/h/gcat) were determined. The modelling
studies were indicative of RDS for jasminaldehyde formation as the
bimolecular surface reaction and that for 2-pentyl nonenal as the
proton abstraction.
Fig. 10. Comparison of experimental and model rates vs. benzaldehyde/heptanal
ratio for the formation of jasminaldehyde and 2-pentyl nonenal.
Acknowledgments
for bimolecular reaction (Eq. (12)).
The authors acknowledge CSIR, New Delhi for the financial sup-
port through the Network Project on Catalysis. One of the authors,
NS acknowledges CSIR, New Delhi, for the award of Senior Research
Fellowship.
kjKHKBCHCB
(1 + KHCH + KBCB)2
Ratejasminaldehyde, v1
=
(12)
The parameters in the model rate equation (Eq. (12)) were
calculated by data regression of the experiments at different ben-
zaldehyde/heptanal ratios using Levenberg–Marquardt nonlinear
least-square method [20] and the values obtained for kj, KH and
KB are 43.43 mmol/h/gcat, 22.94 mmol−1 and 8.53 mmol−1 respec-
tively.
The experimental and model rates of formation of jasminalde-
hyde vs. benzaldehyde/heptanal ratio were plotted in Fig. 10. The
model fits with the experimental data with in the R2 value of 0.95.
These observations indicated that the RDS for jasminaldehyde is
the surface reaction of adsorbed benzaldehyde with heptanal car-
banion and a proton (Fig. 9, step (4)).
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For 2-pentyl nonenal also, the two possible RDS may be the pro-
ton abstraction (Fig. 9, step (3)) or surface reaction (Fig. 9, step (5)).
For self condensation, the surface reaction was observed to be not
RDS since it did not follow the L–H model.
The proton abstraction as RDS is given by Eq. (13):
ksK CH
1 + KHCHH + KBCB
Rate2-pentyl nonenal, v2
=
(13)
where ks is the rate constant for the formation of 2-pentyl nonenal.
By putting the KH and KB obtained from L–H model for jasmi-
naldehyde (Since the KH and KB is a constant for reactions under
similar conditions) in Eq. (13) (Fig. 10), a very good fitting (R2 = 0.97)
was observed and ks was found to be 12.25 mmol/h/gcat
.
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Modified chitosan catalyzed solvent-free synthesis of jasmi-
naldehyde was investigated in detail for its kinetics and reaction