M.M.A. Soliman, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
toxic, low corrosion, waste minimization and recycling nature. Al-
though a considerable amount of work has been devoted on such a
system, the use of nanocatalysts is still understudied, mainly due to the
cost involved in their fabrication.
catalyst (0.43 mg, 0.5 mol%) in 2 mL EtOH was added into a capped
glass vessel. The mixture was heated at 75 °C for the desired time (ty-
pically 24 h). The solution was then centrifuged to isolate the catalyst.
The solvent was evaporated in vacuum, leading to a crude product. The
product mixture was analysed by 1H-NMR in CDCl3. The yield of the
ethyl ester product (relatively to the methyl ester) was established ty-
pically by taking into consideration the relative amounts of these
compounds, as given by 1H-NMR. The same method was applied to the
transesterification with other alcohols by using the appropriate alcohol
instead of EtOH. Selected 1H-NMR spectra (transesterification reaction
of methyl benzoylformate with different alcohols, entries 1–3, 5 and 7,
Table 3) are presented in Supporting Information (Figures S5-S9).
The potential of ZnO nanoparticles to adopt different forms, sizes
and their high stability, associated with the intrinsic Lewis acidity of the
zinc ion [16], are the key components for their choice for this work. To
our knowledge, there has been no report on transesterification of α-keto
carboxylic ester (methyl benzoylformate) using ZnO nanoparticles.
Thus, the main objectives of the current study are as follows: (i) design,
preparation and characterization of pure ZnO nanoparticles by a low-
cost, simple and nontoxic precipitation method, using water as the
solvent and performed at room temperature and (ii) study of the cata-
lytic activity of the synthesized ZnO nanoparticles in the transester-
ification reaction of different esters especially α-keto carboxylic esters.
3. Results and discussion
3.1. Synthesis of the ZnO nanoparticles
2. Experimental
The synthesis of the ZnO nanoparticles (ZnO NPs) was achieved by
mixing aqueous solutions of zinc nitrate hexahydrate Zn(NO3)2·6H2O
and sodium hydroxide (NaOH) (pH ≈10-12) (Eq. (1)).
All the chemicals were obtained from commercial sources and used
as received. Zinc nitrate hexahydrate and sodium hydroxide were of
analytical reagent grade from Merck.
2+
Zn
+ 2OH−(aq) → ZnO(s) + H2O
(1)
(aq)
The pH of the reaction was changed by addition of NaOH. At basic
pH, the Zn(OH)2 microcrystals are dissolved in alkaline solution to form
2.1. Synthesis of ZnO nanoparticles
2−
Zn(OH)4
ions that are the dominant species in solution. These ions
At room temperature, zinc nitrate hexahydrate Zn(NO3)2.6H2O
(0.60 g, 2 mmol) was dissolved in distilled water (5 mL) under con-
tinuous magnetic stirring. Sodium hydroxide aqueous solution (1 M)
was added dropwise, under magnetic stirring, until pH ≈10-12. After
completion of the addition, the solution was stirred for 30 min at room
temperature whereafter a white suspension was formed. The white
precipitate was filtered off, washed with distilled water and dried
overnight in a hot air oven at 80 °C (Scheme 1).
serve as growth units for the following nucleation and crystal growth of
ZnO in the bulk solution. (Eqs. (2) and (3)) [17].
Zn2+ + 4OH− → Zn(OH)4
(2)
(3)
2-
2−
Zn(OH)4
→ ZnO + 2OH- + H2O
After the filtration, the sample is washed with water. From the de-
crease in the pH, there is no longer an electrostatic driving force and all
the particles are converted to ZnO.
2.2. Characterization of ZnO nanoparticles
The choice of sodium hydroxide (NaOH) as alkalinizing agent was
based on the high pH values conferred by this base to the precursor
solution, promoting the formation of more defined nanostructures (fa-
vour larger crystallite size and higher relative crystallinity) [17,18].
The synthesized ZnO NPs were analysed in detail in terms of their
morphological, structural and catalytic properties. Rai et al. [18] re-
ported that the morphology of the zinc oxide powder is highly influ-
enced by the mineralization agent (alkalinizing agent).
The synthesized ZnO nanoparticles were characterized in terms of
structural and optical properties by scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDX) using a scanning
electron microscope JEOL 7001 F with Oxford light elements EDX de-
tector. FTIR spectra measurements were obtained on a Nicolet 6700 in
the 400-4000 cm−1 using 4 cm−1 resolution in KBr pellets. The powder
diffuse reflectance spectra (UV–vis-DRS) were recorded in the wave-
length range of 300–700 nm using an ISR 2600 plus integration sphere
in diffuse reflectance mode using BaSO4 as reference. Spectra were
recorded at room temperature, and the data were transformed through
the Kubelka-Munk function. Powder X-ray diffraction data were col-
lected with D8 Advance Bruker AXS θ-2θ diffractometer, with a copper
radiation source (Cu Kα, λ =1.5406 Å) and a secondary mono-
chromator, operated at 40 kV and 40 mA. Thermogravimetric analyses
were carried out with a Perkin-Elmer Instrument system (STA6000) at a
heating rate of 10 °C min−1 under a dinitrogen atmosphere, in the
range of room temperature up to ca. 800 °C. Nitrogen adsorption iso-
therm was collected on a Micromeritics ASAP 2060 gas sorption in-
strument at 77 K; the sample was pre-treated in a vacuum oven at
130 °C for 24 h before analysis. The Brunauer-Emmett-Teller (BET)
surface area was obtained from the N2 adsorption isotherm.
3.2. Characteristics of the synthesized ZnO nanoparticles
To obtain the morphological properties of the produced ZnO, a
characterization by scanning electron microscopy (SEM) was per-
formed. SEM images show ZnO to be in nanostructure form and with
flakes which are agglomerated and have a homogenous distribution
(Fig. 1). The temperatures used in their synthesis were not sufficiently
high to turn them into separated and more dispersed flakes. They are
generally random and not uniform, very similar to those described in
the literature [19]. The flakes have also been analysed by EDX (Figure
S1, SI) and proven to be pure ZnO.
To examine the crystallinity and crystal phases, the synthesized ZnO
NPs were examined by X-ray powder diffraction (Fig. 2). The crystalline
nature of ZnO was confirmed by the intense diffraction peaks indexed
to common ZnO hexagonal phase, i.e., wurtzite structure, consistent
with the standard JCPDS Data 36-1451 (Fig. 2). X-Ray diffraction pat-
tern shows 2θ at values of 31.8, 34.4, 36.3, 47.5, and 56.6 that could be
attributed to the categorized ZnO wurtzite hexagonal crystal planes
(100), (002), (101), (102) and (110), respectively. The peak broadening
in the XRD pattern clearly indicates that small nanocrystals are present
in the sample. There is no evidence of bulk materials and impurity.
Thus, the lack of impurity-related peaks and the highly intense dif-
fraction peak for ZnO reinforces the evidence of the high purity and
2.3. Transesterification reaction catalysed by ZnO nanoparticles
A mixture of methyl benzoylformate (164.2 mg, 1 mmol) and ZnO
Scheme 1. Synthetic procedure for the synthesis of ZnO nanoparticles.
2