S.C. Patankar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 171–182
173
2 cm−1 between 4000 cm−1 and 400 cm−1. The surface properties of
catalyst prepared were measured by the Brunauer–Emmett–Teller
(BET) method by using ASAP 2010 (Micromeritics, USA) instrument.
The catalyst samples were first degassed under vacuum at 350 ◦C
for 4 h. The measurements were made at liquid nitrogen temper-
ature using N2 gas as the adsorbent with a multipoint method.
Micrographs of surface morphology of the catalyst samples were
captured with Camera SU 30 microscope, JEOL, Japan equipment.
The samples were mounted on specimen studs and coated with
platinum by sputtering to prevent charring of samples during anal-
ysis.
hydroxide [NH4OH], 35% w/w aqueous hydrochloric acid, mesity-
lene and hexadecylamine were procured from M/s S.D. Fine
Chemical Ltd., Mumbai. Tetraethyl orthosilicate (TEOS) and tita-
nium isopropoxide [Ti(isoOC3H7)4] were procured from M/s Merck
Chemicals Ltd., and M/s Spectrochem Ltd., Mumbai, respectively.
2.2. Catalyst preparation
Titanium modified hexagonal mesoporous silica (HMS), des-
ignated as HMS (Ti), and was prepared as reported earlier [16].
Hexadecylamine was used supramolecular template/surfactant
and mesitylene was used as pore expander. The process of calci-
nation was necessary to remove the organic template. Molar ratios
of SiO2/swelling agent = 1.66 and swelling agent/surfactant = 2.33
were applied for the preparation of HMS (Ti). Required amount
of HMS (Ti) was then added to aqueous solution of magnesium
nitrate [Mg(NO3)2·6H2O] and aluminum nitrate [Al(NO3)3·9H2O].
The quantities of magnesium nitrate and aluminum nitrate were
fixed such that the Mg/Al molar ratio was 2:1. The mass ratio
of calcined hydrotalcite (CHT):HMS (Ti) was fixed at 2:10. The
resulting solution was stirred vigorously for 30 min at 60 ◦C to
form slurry. The slurry was added drop-wise to 35% (w/w) aque-
ous ammonium hydroxide [NH4OH] under vigorous stirring while
maintaining pH at 9–10. The slurry was aged at 90 ◦C for 3 h. The
precipitate was filtered off and put into a container of distilled
water for decantation. The decanted precipitate was recovered by
filtration and dried at 100 ◦C for 2 h in oven. This served as the
host structure for loading palladium by nanocolloidal method. The
colloidal palladium suspension of particles was obtained by hydrol-
ysis of aqueous solution of palladium nitrate [Pd(NO3)2·2H2O] and
sodium citrate [Na3(C6O7H3)] with 0.5 M NaOH. The molar ratio
of [citrate]/[Pd] was 1. The exchange process of the host structure
was performed in air at room temperature for 12 h. The required
amount of host structure was dispersed in the required amount of
0.003 M aqueous suspension of colloid Pd-hydroxy citrate particles.
The Pd-colloid-Mg/Al/HMS(Ti)-layered double hydroxide was then
recovered and washed by dispersion and centrifugation in deion-
ized water and then dried at 80 ◦C for 4 h. The final catalyst was
obtained by calcination at 500 ◦C in air for 3 h. The prepared cata-
lyst consisted of palladium loaded on calcined hydrotalcite which
was supported on titanium modified hexagonal mesoporous silica.
The catalyst was also made by sol–gel and impregnation methods.
A series of catalysts designated as xPd-CH-Z were prepared, where
x% w/w loading of palladium (Pd), calcined hydrotalcite supported
on titanium modified hexagonal mesoporous silica (CH) and Z is
the method of synthesis (Z = N, S or I; N = nanocolloidal, S = sol–gel,
I = impregnation).
2.4. Reaction procedure and analysis
All experiments for synthesis of ethyl benzyl acetoacetate and
MIBK were carried out in a batch autoclave (Amar Equipments,
Mumbai) of 100 mL capacity. The reactor was equipped with four
blade 45◦ inclined pitched turbine impeller, temperature controller
(
1 ◦C), pressure indicator and speed regulator ( 5 rpm). Reac-
tants and the catalyst were charged into the reactor. The reaction
mixture was flushed with nitrogen to remove traces of air, pres-
surized with hydrogen to 5 atm and then heated to the desired
temperature. In a standard reaction for synthesis of ethyl benzyl
acetoacetate, equimolar quantities of benzaldehyde and ethylace-
toacetate were used in such a way that the reaction mixture was
35 cm3 with n-decane as internal standard. For MIBK synthesis,
30 mL acetone was used with n-decane as internal standard. Sam-
ples were withdrawn periodically after the desired temperature
had reached and were analyzed using GC equipped with a capillary
column BP-50 (0.25 m film thickness × 0.25 mm column ID × 25 m
column length) and FID detector. The products were confirmed by
GC–MS (QP2010, Shimadzu) and by matching the residence time
of pure samples.
3. Results and discussion
3.1. Efficacy of various catalysts
A variety of catalysts synthesized in the laboratory, accord-
ing to a well thought strategy, were evaluated for their efficacy
and robustness in the synthesis of twoindustrially relevant and
academically interesting compounds, namely, ethyl benzyl ace-
toacetate and MIBK synthesis. The strategy behind studying these
two reactions was to understand the relative rates of condensa-
tion, dehydration and hydrogenation and thereby to design and
mum selectivity of the desired products. In the case of formation of
ethyl benzyl acetoacetate, the aldehyde and ester react whereas in
the case of MIBK synthesis, there is a self-condensation of acetone.
Thus, the relative strength of basic, acidic and metal sites could
be tuned properly. Table 1 lists the various catalysts prepared by
different methods having different site densities, distribution and
strengths of Pd, MgO and Al2O3. The catalyst made by impregna-
tion method was not found to give high selectivity towards ethyl
benzyl acetoacetate and MIBK. The catalyst made by nanocolloidal
method had a comparable activity and selectivity towards ethyl
benzyl acetoacetate and MIBK with the catalyst made by sol–gel
method. However, the catalyst made by nanocolloidal method was
found to be reusable as the active phase in the catalyst made by
sol–gel method leached in reaction mixture on reuse (Tables ES1
and ES2). Hence further studies were done with the catalyst made
by nanocolloidal method. The loading of palladium determines
the catalytic strength of metal sites and the Mg/Al ratio decides
the distribution of the catalytic strength of base and acid catalytic
sites. In the case of ethyl benzyl acetoacetate synthesis, increasing
2.3. Catalyst characterization
Temperature programmed desorption (TPD) using NH3 and CO2
as probe molecules was used for acidity and basicity measurement,
respectively (Autochem II 2910, Micromeritics, USA) to understand
the nature of active sites generated on the catalyst surface. For
NH3−TPD runs, the catalyst sample was degassed under nitrogen
flow upto 300 ◦C in a quartz tube. Ammonia in helium (5% v/v) was
adsorbed on the catalyst at room temperature. The physisorbed
gas was then removed by flow of nitrogen. Chemisorbed ammo-
nia was measured using TPD in conjunction with TCD detector.
Similar procedure was followed for CO2–TPD measurements using
10% v/v carbon dioxide in helium gas. The TPR study was performed
using H2 as probe molecule. Powder XRD (MinislexRegako, Japan)
was used to study the multifunctional nature of catalyst with Cu
K˛ radiation with beam current of 40 kV and 100 mA. The data
were collected by varying 2ꢀ from 0–80◦. Infrared spectra of the
samples pressed in KBr pellets were obtained at a resolution of