P. Bhanja et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
Alumina is a widely studied metal oxide, which gained a consider-
2.3. Instrumentation
able attention in chemical industry as adsorbent, catalyst, catalyst
support, drying agent either alone or in the presence of other oxides
[34–36]. γ-alumina, the most well-known aluminum oxide phase has
been studied intensively as catalyst in organic chemistry [37]. Com-
pared to clays and zeolites, alumina does not have accessible reactive
groups and cavities. However, due to the presence of electron deficient
Al(III) sites together with surface basicity alumina are responsible for
its broad spectrum of applications, viz. purification of ground water,
automobile emission control, catalytic support in petroleum refining
and so on [38]. Due to their high specific surface area with narrow pore
size distribution, uniformity of pore channels and tunable pore dia-
meter, mesoporous alumina [39,40] could be an ideal candidate in
heterogeneous catalysis for the synthesis of various value added organic
fine chemicals. Innovation of ordered mesoporous alumina with
amorphous wall by a sol-gel approach under rigid hydrolysis process
and condensation of reagents are documented by Somorjai et al. [41].
Zhang et al. recently proposed a new route of obtaining mesoporous
alumina where they have innovated an ordered crystalline mesoporous
alumina molecular sieves with CMK as hard template [42]. With the use
of ionic liquids both as solvent and template Lian et al. evolved [43] a
newer method, ionothermal synthesis to produce γ-Al2O3 mesoporous
nanoflakes. A ligand-assisted solvent evaporation induced co-assembly
route is developed by Wei et al. for synthesizing the ordered meso-
porous alumina (OMA) with an ultra large pore size using a high mo-
lecular-weight poly(ethylene oxide)-b-polystyrene (PEO-b-PS) as a soft
template, aluminum acetylacetonate as a precursor and tetrahydrofuran
as a solvent [44]. Weinberger et al. reported the synthesis of meso-
porous alumina using photo-cross-linked polydimethylacrylamide hy-
drogels as porogen matrices [45].
Small and wide angle powder X-ray diffraction patterns of MPA-1
and MPA-2 were recorded using Bruker D8 Advance SWAX dif-
fractometer operated at voltage of 40 kV and current 40 mA. With a
standard silicon sample the instrument was calibrated, using Ni-filtered
Cu Kα (λ = 0.15406 nm) radiation. By employing Quantachrome
Autosorb 1-C surface area analyzer nitrogen adsorption-desorption
isotherms were obtained at 77 K. For gas adsorption purpose, the
samples were degassed for 6 h at 453 K under high vacuum analysis.
NLDFT (non local density functional theory) method has been em-
ployed for estimation of the pore size distributions from the nitrogen
sorption isotherm. Using
a Perkin-Elmer spectrum 100 spectro-
photometer FTIR spectrum of the sample was recorded. Very small
amount of solid sample was allowed to grind finely with a specially
purified salt KBr to get rid of scattering effects from large crystals. Then
the solid mixture was pressed by using mechanical press to obtain the
translucent pellet and the pellet was subjected to keep inside the
spectrophotometer for which the beam of the spectrometer can pass
through. Thermogravimetric (TGA) and differential thermal analysis
(DTA) of the mesoporous alumina samples were carried out in a TGA
instrument thermal analyzer TA-SDT Q-600 under air flow. The tem-
perature-programmed desorption of ammonia and CO2 (NH3-TPD, CO2-
TPD) experiments were performed on a flow apparatus (Micrometrics,
ChemiSorb 2720). For the both experiments before taking that material
in the U-type glass cell the sample was allowed to keep for outgassing at
130 °C temperature under inert atmosphere (helium) for 3 h. In case of
NH3-TPD, after cooling down to room temperature, the ammonia (NH3)
gas flow was started for 30 min at 30 mL/min to get the saturation
condition and helium gas was purged again for 45 min to flush out the
additional amount of NH3 gas from the cell. Similar procedure was
followed in case of CO2-TPD only after replacing the NH3 gas flow by
CO2 through the samples. Then NH3-TPD and CO2-TPD desorption
profile of this material are obtained using a thermal conductivity de-
tector (TCD) while increasing the sample temperature at 5 °C/min
ramp.
In a typical catalytic reaction, 1 mmol of substituted aromatic al-
dehyde and 1.1 mmol of malononitrlie were dissolved into 25 mL round
bottom (RB) flask containing 1 mL absolute ethanol. Then the reaction
mixture was allowed to continuous stir for 20 min at room temperature
when p-chlorobenzaldehyde was used as a model reaction. For different
substrates reaction time varied from 7 to 120 min. For the acid hy-
drolysis of intermediate Knoevenegal product, 200 mg of 2-(2-hydro-
xybenzylidene)-malononitrile was taken into RB flask containing 10 mL
water and three drops of 98% H2SO4. Then the reaction mixture was
stirred continuously in preheated oil bath at 50 °C temperature for
1.5 h. The progress of the reaction was monitored through TLC and the
product was confirmed through 1H NMR and 13C NMR in CDCl3 solvent
without any column purification. After catalytic reaction the products
were identified using 1H and 13C NMR experiments using a Bruker DPX-
300/500 NMR spectrometer.
Herein, we report the synthesis of ordered mesoporous γ-Al2O3
using P123 as a structure directing agent via evaporation-induced self-
assembly (EISA) method [46] followed by calcination and explored its
catalytic activity. The material exhibits high surface acidic and basic
sites together, thermal stability and considerably good surface area.
Being amphoteric in nature the mesoporous γ-Al2O3 material acts as a
heterogeneous bi-functional catalyst for the Knoevenagel condensation
reactions and for the synthesis of coumarin-3-carboxylic acid with ex-
cellent product yields.
2. Experimental section
2.1. Chemicals
Pluronic P123 (Poly(ethylene glycol))-block-poly(propylene glycol)-
block-poly(ethylene glycol) (Mnv = ∼5800) and aluminum isoprop-
oxide (Mw = 204.24 g/mol) were obtained from Sigma Aldrich, India.
Citric acid (Mw = 192.123 g/mol) and hydrochloric acid were pur-
chased from Merk, India. All other organic solvents were used in cat-
alytic reactions without further purification.
2.2. Synthesis method of mesoporous alumina (MPA-1)
3. Results and discussions
In particular synthetic procedure, at first 2.0 g of structure directing
agent P123 was dissolved in 40 mL absolute ethanol. 3.2 g of 37 wt%
hydrochloric acid was added to the reaction mixture and followed by
addition of 1.0 g of citric acid. After continuous stirring for 1 h 4.0 g of
aluminum isopropoxide was added to the solution and allowed to vig-
orous stirring for 6 h until the white coloured gel was formed. Then the
resulting viscous solution was subjected to keep inside the 60 °C oven
for 60 h for slow evaporation reaction method. After drying the sample
white solid was calcined at 500 °C and 1000 °C temperature in aerobic
condition for 4 h at temperature ramp of 10 °C/min, denoted by MPA-1
and MPA-2 respectively. The solid samples were subjected for thorough
characterization. A schematic illustration for formation of MPA-1 has
3.1. Nanostructure analysis
The small angle powder X-ray diffraction patterns of calcined
samples MPA-1 and MPA-2 are shown in Fig. 1. The very sharp peak at
2θ value of 0.97° and low intense peak at 2θ value of 1.66° and 1.93°
have been observed in small angle pattern of MPA-1 sample of Fig. 1a
can be attributed to the 100, 110 and 200 planes of the 2D-hexagonally
ordered mesophase. It is noticed that only one diffraction peak appears
at 2θ value of 1.09° in Fig. 1b, suggesting the partial loss of periodicity
of the mesopores in sample MPA-2 upon calcination of the as-synthe-
sized material at 1000 °C. On the other hand the wide angle powder X-
ray diffraction patterns of MPA-1 and MPA-2 samples are shown in
2