Reconstructed Mg/Al hydrotalcite as a solid base catalyst for synthesis of jasminaldehyde

Article abstract of DOI:10.1016/j.apcata.2010.07.021

Reconstructed hydrotalcites (Mg/Al molar ratio = 3.5) of varied reconstruction time were synthesized and used as catalysts for solvent free condensation of 1-heptanal with benzaldehyde. Maximum conversion of 1-heptanal with higher selectivity to jasminaldehyde was obtained using reconstructed hydrotalcites of 8-12 h reconstruction time. Catalytic activity of reconstruction hydrotalcite was compared with as-synthesized and activated hydrotalcite of Mg/Al molar ratio 3.5 and significantly higher conversion of 1-heptanal was observed in case of reconstructed hydrotalcite of 8 h reconstruction time as a catalyst. Similar to the conversion, higher selectivity to jasminaldehyde was also obtained using reconstructed hydrotalcite. Effect of reconstruction time on conversion and selectivity to jasminaldehyde was studied by varying the reconstruction time of hydrotalcite from 0.5 to 72 h. Kinetic experiments were carried out to study the effect of stirring speed, benzaldehyde to 1-heptanal molar ratio, amount of catalyst and reaction temperature on the rate of reaction using reconstructed hydrotalcite as a catalyst.

Full text of DOI:10.1016/j.apcata.2010.07.021

Applied Catalysis A: General 386 (2010) 34–42
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Reconstructed Mg/Al hydrotalcite as a solid base catalyst for synthesis of
jasminaldehyde
Sumeet K. Sharmaa,1, Parimal A. Parikh , Raksh V. Jasraa,∗
b
a
Discipline of Inorganic Materials & Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR), G.B. Marg, Bhavnagar – 364 021, Gujarat, India
Department of Chemical Engineering, SV National Institute of Technology, Surat 395 007 Gujarat, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 March 2010
Received in revised form 9 July 2010
Accepted 12 July 2010
Available online 14 August 2010
Reconstructed hydrotalcites (Mg/Al molar ratio = 3.5) of varied reconstruction time were synthesized and
used as catalysts for solvent free condensation of 1-heptanal with benzaldehyde. Maximum conversion
of 1-heptanal with higher selectivity to jasminaldehyde was obtained using reconstructed hydrotalcites
of 8–12 h reconstruction time. Catalytic activity of reconstruction hydrotalcite was compared with as-
synthesized and activated hydrotalcite of Mg/Al molar ratio 3.5 and significantly higher conversion of
1
-heptanal was observed in case of reconstructed hydrotalcite of 8 h reconstruction time as a catalyst.
Keywords:
Similar to the conversion, higher selectivity to jasminaldehyde was also obtained using reconstructed
hydrotalcite. Effect of reconstruction time on conversion and selectivity to jasminaldehyde was studied
by varying the reconstruction time of hydrotalcite from 0.5 to 72 h. Kinetic experiments were carried
out to study the effect of stirring speed, benzaldehyde to 1-heptanal molar ratio, amount of catalyst and
reaction temperature on the rate of reaction using reconstructed hydrotalcite as a catalyst.
1
-Heptanal
Jasminaldehyde
Condensation
Reconstructed hydrotalcite
Kinetics
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
di-methyl acetal was mixed with benzaldehyde in second step.
Then, the mixture was again heated at desired temperature in the
presence of a catalyst to obtain jasminaldehyde. In our previous
reports [9,10], catalytic activity of amine/diamine functionalized
synthetic talc (MOS) and hydrotalcites (as-synthesized and acti-
vated) of varied divalent metal cations [M(II)] and divalent to
trivalent metal cations molar ratio [M(II)/Al; M(II) = Mg, Ni, Zn] was
studied in detail for synthesis of jasminaldehyde by solvent free
condensation of 1-heptanal with benzaldehyde. Jasminaldehyde
selectivity is a crucial parameter in this reaction because by-
product (2-n-pentyl-2-nonenal) is also formed by base catalyzed
self-condensation of 1-heptanal. Highest selectivity to jasminalde-
hyde (86%) was obtained using as-synthesized hydrotalcite of
Mg/Al molar ratio 3.5 as compared to activated hydrotalcite (75%)
of similar molar ratio [10]. No significant difference in the conver-
sion of 1-heptanal was observed on activation of hydrotalcite. The
Jasminaldehyde is a commercially important chemical in the
cosmetic and fragrance industries. Apart from its natural source,
jasminaldehyde is also synthesized by condensation of 1-heptanal
and benzaldehyde in the presence of liquid sodium hydroxide
(
NaOH) or potassium hydroxide (KOH) in more than stoichiomet-
ric amounts [1]. This homogeneous process has several drawbacks
like reusability of catalyst, use of hazardous and corrosive liquid
base like KOH or NaOH, separation and disposal of effluent con-
taining NaOH or KOH. Therefore, it is desirable to develop a solid
base catalyst to provide a green catalytic route for the synthesis of
jasminaldehyde. Catalytic activities of large pore HY and beta zeo-
lites, amorphous aluminophosphate (ALPO), hydrotalcite, synthetic
talc and MCM-41 aluminosilicates (Al-MCM-41) based materials
have been studied by several researchers for the synthesis of jasmi-
naldehyde [1–10]. Corma and coworkers [2–4] reported a process
in which 1-heptanal was heated with excess methanol to obtain
heptanal di-methyl acetal in the first step and formed heptanal
−
catalyst having Brønsted (OH groups) basic sites was observed
to be more selective for jasminaldehyde as compared to the cata-
lyst having Lewis (O2 ) basic sites [10]. These results indicate that
the proper tuning of acidic and basic properties of hydrotalcite
can enhance the jasminaldehyde selectivity. The proper combi-
nation of acidio-basic properties of hydrotalcite required for the
reaction can be achieved by intercalation of hydroxyl groups in a
control way by reconstruction process at desired reconstruction
time. When calcined hydrotalcite is stirred with water vapours
or de-carbonated water under inert atmosphere, a highly active
solid base catalyst is obtained by restoration of the original lay-
ered structure (memory effect) of hydrotalcite having hydroxyl
−
∗ Corresponding author. Present address: R&D Centre, VMD, Reliance Industries
Limited, Vadodara 391 346, Gujarat, India. Tel.: +91 265 6696313;
fax: +91 265 6693934.
E-mail addresses: sumi 51@yahoo.com, sks25@bath.ac.uk (S.K. Sharma),
rakshvir.jasra@ril.com (R.V. Jasra).
Present address: Department of Chemical Engineering, University of Bath,
Claverton Down, Bath BA2 7AY, UK.
1
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.07.021

S.K. Sharma et al. / Applied Catalysis A: General 386 (2010) 34–42
35
Scheme 1. Synthesis of jasminaldehyde.
groups in major amounts as charge balancing anions in the inter-
layer space instead of carbonate anions [11]. The Brønsted (OH )
trate reached to 7. Washed precipitate was dried in an oven
−
◦
at 80 C for 14 h and ground to convert it into powder form.
◦
basic sites formed by reconstruction method are reported to be
more active than Lewis (O2−) sites for aldol condensation reac-
tions [12–15]. The reconstruction time and degree of reconstruction
have pronounced effect on the structural and catalytic activity of
hydrotalcite. Effect of reconstruction time on the structural prop-
erties of hydrotalcite is well documented in the literature [16–21].
However, limited literature is available for the effect of recon-
struction time on catalytic activity of reconstructed hydrotalcite
Activation of hydrotalcite was carried out at 450 C in muffle fur-
nace for 4 h in the presence of air and further the material was
reconstructed in liquid phase. For the synthesis of reconstructed
hydrotalcite samples, freshly activated hydrotalcite (2.5 g) was
mixed in 1 M NaOH solution prepared in 100 mL de-carbonated
deionized water under nitrogen atmosphere. Suspension was
stirred at 800 rpm for desired reconstruction time under N2 atmo-
◦
sphere at 30 C. Then, the suspension was filtered in vacuum and
[
12,13].
The present work is taken up with as objective to study the effect
obtained solid cake was washed thoroughly to remove sodium ion.
Finally, solid cake was dried at 80 C for 14 h, ground and store in
◦
of reconstruction time on catalytic activity of reconstructed hydro-
talcite of Mg/Al molar ratio 3.5 for the synthesis of jasminaldehyde
by condensation of 1-heptanal with benzaldehyde in the solvent
free conditions (Scheme 1). The reconstructed hydrotalcite sam-
ples of varied reconstruction time were synthesized, characterized
to study the structural changes during reconstruction period and
used as catalysts for the synthesis of jasminaldehyde. Kinetic exper-
iments were also carried out to study the effect of stirring speed,
benzaldehyde to 1-heptanal molar ratio, amount of catalyst and
reaction temperature on the rate of condensation reaction using
reconstructed hydrotalcite as a catalyst.
vacuum.
2.3. Characterization of the catalyst
Powder X-ray diffraction (P-XRD) patterns of hydrotalcite sam-
ples were recorded on a Philips X’Pert MPD system equipped
with XRK 900 reaction chamber, using Ni-filtered Cu-K˛ radiation
◦
ꢀ = 1.54056 A˚ ) over a 2ꢁ range of 5–70 . Crystallinity of hydrotalcite
(
samples was calculated by the summation of integral intensities of
0 0 3) and (0 0 6) planes. Fourier transform infra red (FT-IR) spectra
(
of hydrotalcite samples were recorded with a Perkin-Elmer Spec-
trum GX-Fourier transform infrared spectrometer (FT-IR) system in
the region of 400–4000 cm using KBr pellets. Thermogravimet-
ric analysis (TGA) of hydrotalcite samples was carried out using
−
1
2
. Experimental
2.1. Materials
a Mettler Toledo TGA/SDTA 851e equipment in flowing nitrogen
◦
(flow rate = 50 mL/min) at a heating rate of 10 C/min. Surface area
Magnesium nitrate (Mg(NO ) ·6H O; 98.9%), aluminum nitrate
measurements of hydrotalcite samples were carried out using ASAP
3
2
2
◦
(
Al(NO ) ·9H O; 99.1%), sodium carbonate (Na CO ; 99.9%) and
2010 Micromeritics, USA. Samples were activated at 130 C for 4 h
3
3
2
2
3
−
2
sodium hydroxide (NaOH; 99.9%) were purchased from s.d. Fine
Chemicals, India for the synthesis of hydrotalcite samples. The
double distilled milli-pore deionized water was used in the
present study. 1-Heptanal and benzaldehyde were purchased from
Sigma–Aldrich and used as received.
under vacuum (5 × 10 mm Hg) prior to N2 adsorption measure-
ments.
2.4. Synthesis of jasminaldehyde
Desired amount of catalyst, benzaldehyde and 1-heptanal was
2.2. Catalyst synthesis
taken in an oven dried double-necked round bottom flask with
.01 g tetradecane as an internal standard. One neck of flask was
0
Hydrotalcite of Mg/Al molar ratio 3.5 was synthesized by co-
precipitation method at constant pH [22]. In a typical synthesis
fitted with refluxing condenser having spiral tube inside for cold
water circulation (6 L/min) at 10 C. The nitrogen was purge into
◦
procedure, an aqueous solution (A) of Mg(NO ) ·6H O (0.0518 mol)
flask from top of the refluxing condenser to avoid oxidation of 1-
heptanal and benzaldehyde. A silicon rubber septum was inserted
in second neck of flask to withdraw samples at different time inter-
val for GC analysis. Flask fitted with refluxing condenser was kept
in an oil bath equipped with temperature and agitation speed
controllers. Reaction mixture was analyzed by gas chromatogra-
phy (GC) (Shimadzu 17A, Japan) and GC–MS (mass spectrometer,
Shimadzu-QP2010, Japan). GC has a 5% diphenyl and 95% di-methyl
siloxane universal capillary column (60 m length and 0.25 mm
diameter) and a flame ionization detector (FID). Conversion of 1-
heptanal and selectivity to jasminaldehyde were calculated by the
3
2
2
and Al(NO ) ·9H O (0.0149 mol) was prepared in 150 mL double
3
3
2
distilled deionized water. Solution A was added drop wise into
a second solution (B) containing Na CO3 (0.079 mol) in 150 mL
2
double distilled deionized water, in around 45 min under vigor-
◦
ous stirring at 25 C. Constant pH (10 ± 0.1) of the mixture was
maintained by adding 1 M NaOH solution. The content was then
transferred into teflon coated stainless steel autoclave and aged
◦
at 60 C for 16 h under autogenous pressure. Then formed precip-
itate was cool down to room temperature, filtered and washed
thoroughly with hot deionized distill water until pH of the fil-

3
6
S.K. Sharma et al. / Applied Catalysis A: General 386 (2010) 34–42
following formula:
ꢀ
ꢁ
moles of 1-heptanal reacted
moles of 1-heptanal fed
%
%
Conversion =
× 100
Selectivity of jasminaldehyde
ꢀ
ꢁ
moles of jasminaldehyde
moles of (jasminaldehyde + 2-n-pentyl-2-nonenal
=
× 100
2.5. Kinetic analysis and reproducibility
Reaction parameters which might have pronounced effect on
the rate of condensation of 1-heptanal with benzaldehyde include,
degree of reconstruction of hydrotalcite, stirring speed, benzalde-
hyde to 1-heptanal molar ratio, amount of catalyst and reaction
temperature. Therefore, kinetic experiments were carried out by
varying these parameters. Sample (0.1 mL) was taken using glass
syringe at different time interval and change in concentration of
Fig. 2. Degree of reconstruction of hydrotalcite with respect to time.
in liquid phase using de-carbonated water under inert atmo-
sphere. All characteristic peaks of as-synthesized hydrotalcite were
observed in the XRD of reconstructed hydrotalcite sample within
1
-heptanal with respect to time was determined by gas chromatog-
raphy in each case. Total reaction mass loss due to sampling for
kinetic analysis was not more than 2% during the course of reac-
tion. Initial rate of reaction was calculated in the lower conversion
1
h reconstruction time which showed that the magnesium and
aluminum coordination modes were restored to the original octa-
hedral hydrotalcite structure [17]. Peaks intensities and sharpness
of (0 0 3) and (0 0 6) planes, which are directly proportional to the
crystallinity of material, were observed to increase on increasing
the reconstruction time of hydrotalcite. The degree of reconstruc-
tion (˛) was calculated by comparing the peak intensities of (0 0 3)
and (0 0 6) planes of each sample with the peak intensities of sam-
ple at 72 h reconstruction time. The 72 h sample was assumed as a
reconstructed hydrotalcite of highest crystallinity. Degree of recon-
structionwas observedtoincrease almostlinearlyupto 12 h(Fig. 2).
Slow increase in the degree of reconstruction was observed on fur-
ther increase in the reconstruction time from 12 to 72 h.
(
[
∼10%) range of 1-heptanal by the method reported in literature
23]. The mass transfer resistance was analyzed by performing
experiments at varied amount of catalyst and stirring speed. To
ensure reproducibility of the reaction, repeated experiments were
carried out under identical reaction conditions. Obtained results
including rate of reaction, conversion and selectivity were found to
be reproducible within ± 2.5% variation.
3
. Results and discussions
3.1. Characterization of catalyst
FT-IR spectra of as-synthesized, calcined and reconstructed
hydrotalcite at different reconstruction time are shown in Fig. 3.
P-XRD patterns of as-synthesized, calcined and reconstructed
−1
Absorption bands at about 3465–3497 cm
were attributed to
hydrotalcite at varied reconstruction time are shown in Fig. 1. P-
XRD pattern of as-synthesized hydrotalcite sample was obtained
similar to the pure hydrotalcite [21]. Basal spacing of 0 0 3 plane
dspacing = 0.765 nm confirmed the presence of carbonate anions in
the stretching vibrations of structural hydroxyl groups in the
brucite-like layer. Hydrogen bonding between water molecules
2−
−1
and interlayer CO3
anions was confirmed by the presence of a
−1
shoulder at 3000 cm . The bands at around 1640 and 1370 cm
the interlayer space of hydrotalcite. Calcination of hydrotalcite at
showed the presence of interlayer water molecules and carbon-
◦
4
50 C gave a uniform mixed oxide phase Mg(Al)Ox, with diffrac-
−1
ate anions, respectively. The band appeared at about 555 cm was
tion lines very similar to that of MgO. As-synthesized hydrotalcite
structure was recovered via reconstruction of calcined hydrotalcite
ascribed to the translation mode of hydroxyl groups mainly influ-
enced by the aluminum cations [16,19]. Disappearance of peaks
−
1
−1
(water bending vibrations) and 3000 cm
at around 1640 cm
interaction of H O–CO3 in the interlayer) was observed in the
2−
(
2
IR spectrum of calcined hydrotalcite due to removal of water
−
1
molecules. Intensity of the bands at around 3497 and 1370 cm
also decreased significantly. Reconstruction of calcined hydrotal-
cite in the de-carbonated water induced a recovery of pristine
hydrotalcite structure. Presence of structural hydroxyl groups in
the brucite-like layer was confirmed by the appearance of band at
−
1
around 3468 cm in all reconstructed hydrotalcite samples. The
−
1
appearance of strong bands at about 3000 and 1640 cm
also
confirmed the presence of water molecules in reconstructed hydro-
talcite. On increasing the reconstruction time, the intensity and
−
1
broadening of bands at around 3468 and 3000 cm were observed
to increase which shows that the amount of water/hydroxyl groups
increased on increasing reconstruction time. Translation modes of
hydroxyl groups, mainly influenced by the aluminum and magne-
sium cations, were observed in the FT-IR spectra of reconstructed
−
1
hydrotalcite. The peak at about 1370 cm
indicated that the
carbonate ions are still present after reconstruction. Similar obser-
vations on the presence of carbonate anions were reported in the
literature also [16].
Fig. 1. P-XRD patterns of as-synthesized, calcined and reconstructed hydrotalcite
at different reconstruction time.

S.K. Sharma et al. / Applied Catalysis A: General 386 (2010) 34–42
37
Fig. 3. FT-IR spectra of as-synthesized, activated and reconstructed hydrotalcite at different reconstruction time.
3468 cm 1 in the FT-IR spectra of reconstructed hydrotalcite
−
Two steps weight loss, similar to that of as-synthesized
hydrotalcite at respective temperatures, was observed in the
thermogravimetric analysis (TGA) of reconstructed hydrotalcite
samples (Table 1). Weight loss in first step was attributed the
loss of physically adsorbed water molecules with relatively smaller
samples.
BET surface area of as-synthesized and activated hydrotalcite
samples was obtained as 106 and 214 m /g, respectively (Table 1)
while surface area of reconstructed hydrotalcite sample of 1 h
reconstruction time increased to 231 m /g. Increase in the sur-
face area of reconstructed hydrotalcite sample in the liquid phase
has also been reported by other researchers [16,24]. The observed
higher surface area of reconstructed hydrotalcite may be due to
the effect of mechanical stirring during the reconstruction process
that can break and exfoliate the hydrotalcite-like platelets. Further
increase in the reconstruction time led to the significant decrease
in surface area of samples.
2
2
amount of condensed water molecules and CO without collapse of
2
◦
the hydrotalcite structure in the temperature range of 115–125 C.
The weight loss in second step was observed due to removal of
condensed water molecules and carbon dioxide from the carbon-
ate anions present in the interlayer space of hydrotalcite in the
◦
temperature range of 405–410 C. Weight loss in the reconstructed
hydrotalcite sample was observed to increase on increasing the
reconstruction time. For example, 18% total weight loss in the
reconstructed hydrotalcite sample was calculated for 0.5 h recon-
struction time, which increased to 32% for 4 h and finally 51%
for the 72 h reconstruction time. Increment in the weight loss
on increasing reconstruction time of the hydrotalcite sample is
due to increased amount of hydroxyl groups/water molecules
present in the reconstructed hydrotalcite samples, which is fur-
ther supported by the broadening of peak intensity at around
3
.2. Catalytic activity for the synthesis of jasminaldehyde
Effect of reconstruction time on the catalytic activity of
reconstructed hydrotalcite for synthesis of jasminaldehyde was
investigated at 130 C and benzaldehyde to 1-heptanal molar ratio
(Fig. 4). Hydrotalcite of Mg/Al molar ratio 3.5 gave maximum
◦
5
Table 1
Weight loss and BET surface area of as-synthesized, activated and reconstructed hydrotalcite at different reconstruction time.
◦
◦
2
Reconstruction time (h)
T1 ( C)
T2 ( C)
W1 (%)
W2 (%)
Total weight loss (%)
BET surface area (m /g)
As-synthesized
Activated
125
400
450–550
410
410
405
405
405
410
405
10
0
6
7
9
10
12
12
14
13
16
26
4
36
4
106
214
226
231
187
162
131
118
110
101
97
0
1
2
4
8
1
2
4
7
.5
115
115
115
120
125
125
120
125
125
12
18
20
22
22
26
29
33
35
18
25
29
32
34
38
43
46
51
2
4
8
2
405
410
W1 = first stage weight loss at temperature T1.
W2 = second stage weight loss at temperature T2.

3
8
S.K. Sharma et al. / Applied Catalysis A: General 386 (2010) 34–42
Fig. 4. Effect of reconstruction time of hydrotalcite on conversion and selectivity
for the synthesis of jasminaldehyde. Reaction conditions: 1-heptanal = 8.05 mmol,
◦
benzaldehyde = 40.25 mmol, catalyst = 0.09 g, T = 130 C, t = 3 h at 800 rpm.
selectivity to jasminaldehyde in our earlier reports [10] therefore,
hydrotalcite of 3.5 Mg/Al molar ratio was taken as a precursor
for reconstruction experiments in the present study and recon-
struction time was varied from 0.5 to 72 h. Both, conversion of
1
-heptanal and selectivity to jasminaldehyde were observed to
increase on increasing the reconstruction time from 0.5 to 12 h.
At 0.5 h reconstruction time, 79% conversion of 1-heptanal was
achieved with 70% selectivity to jasminaldehyde in 180 min reac-
tion time. Conversion of 1-heptanal increased to 94% with 82%
selectivity to jasminaldehyde on increasing the reconstruction time
of hydrotalcite to 6 h. On further increasing the reconstruction time
to 12 h, conversion reached to 98%. Slight increase in the conversion
of 1-heptanal was observed in the range of 8–12 h reconstruc-
tion time. Conversion of 1-heptanal decreased to 87 and 84% for
reconstructed hydrotalcite having reconstruction time of 24 and
Fig. 5. Conversion of 1-heptanal (A) and selectivity to jasminaldehyde (B)
using as-synthesized, activated and reconstructed hydrotalcite (reconstruction
time = 8 h) as catalysts for the synthesis of jasminaldehyde. Reaction conditions:
◦
1-heptanal = 8.05 mmol, benzaldehyde = 40.25 mmol, catalyst = 0.09 g, T = 130 C at
800 rpm.
7
2 h, respectively. Selectivity to jasminaldehyde was also observed
to increase on increasing the reconstruction time of hydrotalcite
up to 12 h. However, no significant increase in the selectivity to
jasminaldehyde was observed on further increasing the reconstruc-
tion time of hydrotalcite to 72 h. These results clearly showed that
the reconstructed hydrotalcite of 8–12 h reconstruction time gave
highest conversion of 1-heptanal and maximum selectivity to jas-
minaldehyde, therefore, effect of other reaction parameters such
as, stirring speed, amount of catalyst, benzaldehyde to 1-heptanal
molar ratio and reaction temperature on conversion of 1-heptanal
and selectivity to jasminaldehyde was studied by taking recon-
structed hydrotalcite of 8 h reconstruction time as a catalyst. Kinetic
study was also carried out in detail to study the effect of above
parameters on the rate of reaction.
increased to 81% in case of reconstructed hydrotalcite. Calcined
hydrotalcite showed poor selectivity to jasminaldehyde (56%)
due to presence of Lewis basic sites, which assist faster self-
condensation of 1-heptanal to 2-n-pentyl-2-nonenal. Time on
stream data for the variation in conversion of 1-heptanal and
selectivity to main products, i.e. jasminaldehyde and 2-n-pentyl-
2
-nonenal is shown in Fig. 6 using reconstructed hydrotalcite of 8 h
reconstruction time as a catalyst.
To understand the effect of reconstruction of hydrotalcite
on the activity of catalyst, experiments were performed under
identical reaction conditions using as-synthesized, calcined and
reconstructed hydrotalcite samples as catalysts. Higher conver-
sion of 1-heptanal in shorter reaction time was observed using
reconstructed hydrotalcite of 8 h reconstruction time as a cata-
lyst (Fig. 5). 87% conversion of 1-heptanal was achieved in 60 min
reaction time using reconstructed hydrotalcite, while 30% and 43%
conversion of 1-heptanal were achieved using as-synthesized and
calcined hydrotalcite, respectively as catalysts. In case of the recon-
structed hydrotalcite, conversion of 1-heptanal reached 96% within
1
20 min, however, 51 and 60% conversion was obtained using
as-synthesized and calcined hydrotalcites, respectively. Recon-
structed hydrotalcite showed higher selectivity to jasminaldehyde
as compared to the as-synthesized and calcined hydrotalcite sam-
ples. For example, 68% selectivity to jasminaldehyde was found
in 120 min reaction time using as-synthesized hydrotalcite, which
Fig. 6. Variation of the conversion of 1-heptanal and selectivity to the main prod-
ucts (i.e. jasminaldehyde and 2-n-pentyl-2-nonenal) with time on stream. Reaction
conditions: 1-heptanal = 8.05 mmol, benzaldehyde = 40.25 mmol, catalyst = 0.09 g,
◦
T = 130 C at 800 rpm.

S.K. Sharma et al. / Applied Catalysis A: General 386 (2010) 34–42
39
Higher catalytic activity of reconstructed hydrotalcite as com-
pared to as-synthesized hydrotalcite is due to the presence of
−
higher number of Brønsted basic sites (OH ions) either on the
surface or near the edges of platelets. When calcined hydrotalcite
is stirred in de-carbonated water under inert atmosphere, Lewis
basic sites of higher basic strength are converted into Brønsted
basic sites of low/medium basic strength resulting into a highly
active solid base catalyst for condensation reactions. Basic acces-
−
sible sites (OH ) detected by the CO2 molecules were found to be
higher in case of liquid phase reconstructed hydrotalcite samples
[
16] and are probably located near the edges of platelets [16,25].
These sites can also be controlled by modifying the defects in the
structural framework of hydrotalcite. The hydroxyl groups, likely
to be bonded to low coordination sites (hydrotalcite obtained by
reconstruction method), result into more number of basic sites
as compared to those present in well ordered hydrotalcite struc-
ture obtained by ion-exchange process. Observed higher activity
of reconstructed hydrotalcite as compared to activated and as-
synthesized hydrotalcite concludes that the hydrotalcite structure
of irregular aggregates containing more hydroxyl anions has suffi-
cient basic capacity to catalyze faster condensation of 1-heptanal
with benzaldehyde.
3.2.1. Effect of stirring speed
To ensure pure kinetic regime and negligence of mass trans-
fer resistance for kinetic study, the reaction was carried out first
by varying the stirring speed and amount of catalyst. The stir-
ring speed was varied from 0 to 1000 rpm at benzaldehyde to
1
-heptanal molar ratio 5 (Fig. 7). In all cases 93–96% conversion
◦
of 1-heptanal was reached within 120 min reaction time at 130 C
temperature (Fig. 7A). However, significant difference in conver-
sion was observed in lower reaction time. Without stirring, 68%
conversion of 1-heptanal was observed in 60 min that increased to
7
5% on increasing the stirring speed to 300 rpm in similar reaction
time. On further increase in the agitation speed to 1000 rpm, no
significant effect on conversion was observed in 60 min. However,
reaction time and stirring speed have significant effect on the selec-
tivity to jasminaldehyde in the studied reaction conditions (Fig. 7B).
For example, selectivity to jasminaldehyde increased from 66% in
5
min reaction time to 78% in 240 min at 300 rpm. The selectivity to
jasminaldehyde was increased up to 82% in 240 min on increasing
the stirring speed to 500 rpm.
Initial rate of reaction was found to increase up to 500 rpm
Fig. 7. Effect of stirring speed on conversion of 1-heptanal (A), selectivity to
jasminaldehyde (B) and initial rate of reaction (C) using reconstructed hydro-
talcite as a catalyst for the synthesis of jasminaldehyde. Reaction conditions:
(
Fig. 7C), after that significant effect of stirring speed on rate of
reaction was not observed. Increase in the rate of reaction up to
00 rpm is due to the combined effect of the mass transfer resis-
◦
5
1-heptanal = 8.05 mmol, benzaldehyde = 40.25 mmol, catalyst = 0.09 g, T = 130 C.
tance. However, effect of mass transfer resistance is not prominent
after 500 rpm and reactionmoved intopurekineticregime. Absence
of mass transfer resistance was further confirmed by studying the
effect of catalyst amount on initial rate of reaction.
of basic site which aids faster self-condensation of 1-heptanal to
2-n-pentyl-2-nonenal, which is also a base catalyzed reaction.
Initial rate of reaction calculated from 1-heptanal consump-
tion data is shown in Fig. 8C. Initial rate of reaction was observed
to increase linearly up to 45 mg catalyst amount and thereafter,
slow increase in the rate of reaction was found up to 90 mg cat-
alyst amount. Significant effect on initial rate of reaction was not
observed in the range of catalyst amount from 90 to 250 mg, which
again confirmed the absence of mass transfer resistance. Therefore,
90 mg amount of catalyst at 800 rpm stirring speed was used for
further experiments to ensure pure kinetic regime.
3.2.2. Effect of catalyst amount
Conversion of 1-heptanal was significantly increased on increas-
ing the catalyst amount from 12 to 90 mg (Fig. 8A). Lower
conversion of 1-heptanal with higher selectivity to jasminalde-
hyde was obtained at lower amount of catalyst. At 12 mg catalyst
amount, only 37% conversion of 1-heptanal with 81% selectivity to
jasminaldehyde was observed in 60 min reaction time. Conversion
of 1-heptanal increased to 75 and 87% at 45 and 90 mg catalyst
amount, respectively in 60 min. On further increase in the amount
of catalyst from 90 to 250 mg, no significant difference in the con-
version of 1-heptanal was observed. Selectivity to jasminaldehyde
was observed to decrease significantly from 80 to 66% on increas-
ing amount of catalyst from 12 to 250 mg within 60 min reaction
time (Fig. 8B). Decrease in the selectivity to jasminaldehyde at
higher amount of catalyst is due to availability of higher number
3.2.3. Effect of benzaldehyde to 1-heptanal molar ratio
Benzaldehyde to 1-heptanal molar ratio has significant effect
not only on rate of reaction but also on the conversion and selec-
tivity to jasminaldehyde (Fig. 9). Higher conversion of 1-heptanal
with lower selectivity to jasminaldehyde was observed at lower
benzaldehyde to 1-heptanal ratio due to faster self-condensation

4
0
S.K. Sharma et al. / Applied Catalysis A: General 386 (2010) 34–42
Fig. 9. Effect of benzaldehyde to 1-heptanal molar ratio on conversion of 1-heptanal
A), selectivity to jasminaldehyde (B) and initial rate of reaction (C) using recon-
(
structed hydrotalcite as a catalyst for the synthesis of jasminaldehyde. Reaction
◦
conditions: 1-heptanal = 8.05 mmol, catalyst = 0.09 g, T = 130 C at 800 rpm.
Fig. 8. Effect of catalyst amount on conversion of 1-heptanal (A), selectivity to
jasminaldehyde (B) and initial rate of reaction (C) using reconstructed hydro-
talcite as a catalyst for the synthesis of jasminaldehyde. Reaction conditions:
banion of 1-heptanal) to the carbonyl group of another 1-heptanal
molecule is the most probable reason for higher selectivity of 2-n-
pentyl-2-nonenal at lower benzaldehyde to 1-heptanal molar ratio.
At higher benzaldehyde to 1-heptanal molar ratio, the concentra-
tion of benzaldehyde is higher which leads to increased interaction
probability of carbanions of 1-heptanal with activated benzalde-
hyde molecules on the active sites of catalyst.
Initial rate of reaction was observed to decrease on increas-
ing the benzaldehyde to 1-heptanal molar ratio (Fig. 9C). At
lower molar ratio, for example 1, higher rate of reaction
◦
1
-heptanal = 8.05 mmol, benzaldehyde = 40.25 mmol, T = 130 C at 800 rpm.
of 1-heptanal. Almost 94% conversion of 1-heptanal was achieved
at benzaldehyde to 1-heptanal molar ratio in the range of 1–3
in 60 min reaction time (Fig. 9A). The conversion of 1-heptanal
decreased to 87% on increasing this ratio to 5 in similar reaction
time. Significant effect on the conversion within 60 min reaction
time was not observed in the range of benzaldehyde to 1-heptanal
molar ratio 5–10; however, some differences were observed in
lower reaction time (less than 30 min). Selectivity to jasminalde-
hyde was also observed to increase on increasing the benzaldehyde
to 1-heptanal molar ratio (Fig. 9B). At lower ratio (1), 44% selec-
tivity to jasminaldehyde was obtained that increased to 63% on
increasing the benzaldehyde to 1-heptanal molar ratio 3 in 60 min.
For benzaldehyde to 1-heptanal molar ratio 10, 83% selectivity
to jasminaldehyde was observed in 60 min reaction time which
increased to 88% at 240 min. Present data shows that the conversion
of 1-heptanal and selectivity to jasminaldehyde increases with the
reaction time. Faster addition of the ␣-carbon of enolate anion (car-
−
4
[
45 × 10 mol/(gcat min)] was calculated due to faster self-
condensation of 1-heptanal. The rate of reaction was found to
−
4
decrease to 42 × 10 mol/(gcat min) on increasing the molar ratio
to 3. On further increase in the benzaldehyde to 1-heptanal
molar ratio to 5, initial rate of reaction decreased rapidly to
−
4
2
5 × 10 mol/(gcat min). Similar observation was also reported in
the literature for condensation of benzaldehyde with acetone due
to competitive adsorption of reactants [13]. Slow decrease in the
rate of reaction was observed in the benzaldehyde to 1-heptanal
molar ratio range of 5–10.

S.K. Sharma et al. / Applied Catalysis A: General 386 (2010) 34–42
41
the formation of 2-n-pentyl-2-nonenal (Fig. 10B). Higher selec-
◦
tivity to jasminaldehyde (87%) was observed at 70 C in 60 min
◦
reaction time. On increasing the temperature to 100 C, similar
selectivity to jasminaldehyde was obtained in 240 min. Signifi-
cant decrease in the selectivity to jasminaldehyde was observed at
◦
1
70 C. Decrease in the selectivity to jasminaldehyde on increasing
reaction temperature is due to the faster self-condensation of 1-
heptanal to 2-n-pentyl-2-nonenal as compared to the condensation
of 1-heptanal with benzaldehyde.
Initial rate of reaction was observed to increase on increasing the
reaction temperature. Activation energy was calculated by plotting
◦
ln k versus 1/T in the temperature range of 130–170 C (Arrhenius
plot; Fig. 10C) and found to be 37.6 kJ/mol. The calculated activation
energy in the present study again confirmed that the reaction is far
away from the diffusional or mass transfer limitations.
4. Conclusions
Reconstruction of hydrotalcite was carried out by stirring a
calcined hydrotalcite sample in de-carbonated water under inert
atmosphere which results in the restoration of original layered
−
structure containing OH anions as major compensating anions
in the interlayer space. Reconstruction of hydrotalcite yielded a
highly active solid base catalyst for synthesis of jasminaldehyde.
Reconstructed hydrotalcite of 8 h reconstruction time showed 96%
conversion of 1-heptanal within 120 min as compared to 51 and
6
0% conversion using as-synthesized and calcined hydrotalcite,
respectively under identical reaction conditions. Higher conver-
sion of 1-heptanal with lower selectivity to jasminaldehyde was
observed at lower benzaldehyde to 1-heptanal ratio. 88% selec-
tivity to jasminaldehyde was obtained in 4 h reaction time using
1
2 mg reconstructed hydrotalcite of 8 h reconstruction time as a
catalyst at 8.05 mmol 1-heptanal and 40.25 mmol benzaldehyde
concentrations. Kinetic data showed that the initial rate of reaction
increased linearly up to 45 mg catalyst amount, thereafter, slow
increase in the rate of reaction was observed up to 90 mg catalyst
amount. Higher reaction temperature and catalyst amount favored
the faster self-condensation of 1-heptanal which results into lower
selectivity to jasminaldehyde. The activation energy was calculated
by Arrhenius plot and found to be 37.6 kJ/mol.
Acknowledgement
SKS thanks CSIR, New Delhi, for the award of a Senior Research
Fellowship.
Fig. 10. Effect of reaction temperature on conversion of 1-heptanal (A) and
selectivity to jasminaldehyde (B) using reconstructed hydrotalcite as a catalyst
for the synthesis of jasminaldehyde; Arrhenius plot (C). Reaction conditions: 1-
heptanal = 8.05 mmol, benzaldehyde = 40.25 mmol, catalyst = 0.09 g at 800 rpm.
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