Aldol Condensation of Benzaldehyde and Heptanal Over Zinc Modified Mixed Mg/Al Oxides
an open alumina crucible. Quadrupole mass detector Omni-
2.2 Characterization
Star GSD320 (Pfeiffer Vacuum Austria GmbH) was used
for detection of fragments. Measurements were carried out
by SCAN mode with voltage 1450 V of electron multiplier.
The chemical composition was verified by using an ICP-
EOS Agilent 725 (Agilent Technologies Inc.). Before analy-
sis, a 200 mg sample was dissolved in 10 cm3 of H2SO4 (1:1)
and heated. After dissolution, the sample was cooled down,
diluted with demineralized water and heated to 100 °C for a
few minutes. Finally, the solution of sample was transported
to volumetric flask and measured.
The crystallographic structure of the precursors and cata-
lysts was determined by examining the X-ray diffraction
(XRD) patterns of the powder samples obtained by using
D8 Advance ECO (Bruker) applying CuKα radiation
(λ=1.5406 Å). The step size of 0.02° and the step time of
0.5 s were used. The patterns were collected over a 2θ range
of 5°–70° and evaluated by using the Diffrac.Eva software
with the Powder Diffraction File database (PDF 2–2002,
International Centre for Diffraction Data).
The specific surface areas (BET) of the catalysts were
determined by N2 adsorption/desorption at − 196 °C by
using Autosorb iQ (Quantachrome Instruments). All sam-
ples were dried before the analysis in a glass-cell at 110 °C
(200 °C for calcined materials) under vacuum for 16 h.
SEM images were obtained by using a scanning electron
microscope (SEM) JSM-7500F (JEOL Ltd.) with a cold
cathode—field emission SEM (parameters of measurements:
1 kV, GB high mode).
2.3 Aldol Condensation
For the studied aldol condensation, benzaldehyde and hep-
tanal (both Sigma-Aldrich, 97%) were used. Heptanal was
freshly distilled before the reaction. The solvent used for the
reaction was N,N’-dimethylformamide (DMF, Penta, p.a.).
The catalysts were activated before the reaction by calcina-
tion (air, 450 °C, 15 h).
In a typical experiment, a 25 ml round-bottomed flask
was equipped with condenser and filled up with solvent
(when used, 1 ml), catalyst and benzaldehyde, and the
reaction mixture was stirred vigorously and heated to the
desired temperature (80–100 °C). Then heptanal was added
in three portions—in minutes 0, 30 and 60 of the reaction (to
suppress autocondensation of heptanal). The reactant ratio
was as follows: 20 wt% of catalyst compared to heptanal
amount, heptanal:benzadehyde molar ratio 1:2. A typical
experiment involved: benzaldehyde (0.87 ml, 8.50 mmol),
heptanal (0.60 ml, 4.25 mmol), 0.097 g of catalyst and DMF
as solvent (1 ml).
Acid–base properties of materials were characterized by
means of CO2 and NH3 temperature programmed desorption
(TPD) using Autochem 2950 HP (Micromeritics Instrument
Corporation). Typically, a 100 mg sample in a quartz U-tube
reactor was pretreated in He to 450 °C with temperature
ramp of 10°C/min. In the case of NH3-TPD, the sample
was cooled to 50 °C and then saturated by ammonia (flow
25 ml/min, 10 vol% NH3/He for 30 min). Subsequently, the
gas was changed to helium (25 ml/min) in order to remove
physically/weakly adsorbed ammonia [flushing out until the
baseline was constant (60 min)]. After this procedure the
temperature was increased to 450 °C at a rate of 15°C/min
to obtain NH3-TPD curves. In the case of CO2-TPD, the
pretreatment was the same as for NH3-TPD. After the pre-
treatment the sample was cooled to 50 °C and the gas was
switched to gas mixture of 10 vol% CO2/He (25 ml/min).
The sample was saturated by CO2 for 30 min. Then the gas
was changed to helium and left for 1 h at 50 °C in order
to remove weakly adsorbed molecules. TPD curves were
obtained by increasing the temperature from 50 to 450 °C
with the ramp of 15 °C/min. The changes of gas concentra-
tion were monitored by a TCD detector.
Prior to analysis, the samples (4× 0.1 ml) were centri-
fuged, diluted with ethanol, and then analyzed using Shi-
madzu GC 17A chromatograph fitted with nonpolar column
ZB-5 (60 m, 0.32 mm diameter, 0.25 µm film) and FID.
3 Results
3.1 Characterization
Chemical composition of all laboratory prepared samples
was determined by the ICP method. The results presented in
Table 1 show chemical composition, i.e. molar ratio of zinc,
specific surface area (BET) of dried and calcined Zn–Mg–Al
samples decreased with the increasing Zn content in the
sample in range of zinc content 0.80–2.11. As for zinc modi-
fied samples, the largest specific surface area is in the sample
prepared by homogenous precipitation, which is caused by
contribution of hydrozincite precipitated on the LDH sur-
face. The low specific surface area of impregnated sample
Diffuse Reflectance Infrared Fourier Transform Spectros-
copy (DRIFT) of dried and calcined samples was obtained
using Nicolet iS 10 (Thermo Scientific). The sample (15 mg)
was mixed with KBr (300 mg) and measured with the fol-
lowing parameters: number of scans 128 and resolution
2 cm−1.
Thermogravimetric analyses (TGA) of dried LDH cata-
lysts were obtained using TA Instruments TGA Discovery
series operating at heating ramp 10°C/min from the tem-
perature of 40–900 °C in flowing of nitrogen (20 ml/min,
Linde 5.0). Approximately 15 mg of sample was heated in
1 3