Green synthesis of zinc oxide particles with apple-derived compounds and their application as catalysts in the transesterification of methyl benzoates

Article abstract of DOI:10.1039/d0dt01069c

ZnO nanoparticles (ZnONPs) were successfully synthesized using bravo-de-esmolfe apple extract in aqueous medium at room temperature. ZnO microparticles, prepared with a pure apple phytochemical, quercetin (ZnOq), or without phytochemicals (ZnO) were studied for comparative purposes. The re-use of apple waste for highly efficient catalyst production, based on green synthetic routes, can be added to the concept of a circular economy. The synthesized ZnO particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N₂adsorption/desorption Brunauer-Emmett-Teller (BET) theory. The XRD patterns indicated the formation of a hexagonal wurtzite phase with high purity and SEM and TEM analyses revealed the morphology of the particles. The apple extract produced spherical ZnONPs composed of round lamina-like structures, similar to the micro sized lamina-like shape of ZnOq and dissimilar to the flower-like shape of ZnO. The green synthesized ZnO nanoparticles (ZnONPs) led to a high product yield ofca. 96% within 24 h of reaction time in the transesterification reaction of different carboxylic esters.

Full text of DOI:10.1039/d0dt01069c

Dalton
Transactions
View Article Online
View Journal
PAPER
Green synthesis of zinc oxide particles with apple-
derived compounds and their application as catalysts
in the transesterification of methyl benzoates†
Cite this: DOI: 10.1039/d0dt01069c
Mohamed M. A. Soliman, a,b Elisabete C. B. A. Alegria,
*
a,c Ana P. C. Ribeiro,*a
Marta M. Alves, *a Marta S. Saraiva,b,d M. Fátima Montemora and
a
Armando J. L. Pombeiro
ZnO nanoparticles (ZnONPs) were successfully synthesized using bravo-de-esmolfe apple extract in
aqueous medium at room temperature. ZnO microparticles, prepared with a pure apple phytochemical,
quercetin (ZnOq), or without phytochemicals (ZnO) were studied for comparative purposes. The re-use of
apple waste for highly efficient catalyst production, based on green synthetic routes, can be added to the
concept of a circular economy. The synthesized ZnO particles were characterized by X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption/
desorption Brunauer–Emmett–Teller (BET) theory. The XRD patterns indicated the formation of a hexag-
onal wurtzite phase with high purity and SEM and TEM analyses revealed the morphology of the particles.
The apple extract produced spherical ZnONPs composed of round lamina-like structures, similar to the
micro sized lamina-like shape of ZnOq and dissimilar to the flower-like shape of ZnO. The green syn-
thesized ZnO nanoparticles (ZnONPs) led to a high product yield of ca. 96% within 24 h of reaction time
in the transesterification reaction of different carboxylic esters.
Received 21st March 2020,
Accepted 20th April 2020
DOI: 10.1039/d0dt01069c
rsc.li/dalton
produce novel “green-based” materials is essential and is in
line with the concept of a circular economy, towards the re-
1. Introduction
According to the Food and Agriculture Organization of the cycling of waste as a useful resource.
United Nations (FAO),1,2 approximately one-third of all food in
In this context, green-biosynthesis using agricultural waste
the world is wasted, which is translated into a loss of 1.3 for the fabrication of relevant metal-derived catalysts is of
billion tons of food per year. In the EU alone, 88 million tons utmost importance. Zinc oxide (ZnO) possesses great scientific
of food per year are wasted, with fruits and vegetables repre- and technological importance, not only due to its unique
senting 59% of the most commonly wasted foods. This has a physicochemical properties but also due to its potential appli-
huge impact on the economy, and mainly on the environment cations in several fields, being an interesting candidate for this
since up to 8% of greenhouse gas emissions associated with green-biosynthesis approach.3,4
global warming are derived from food waste. Therefore, the
Plant phytochemicals are believed to have a dual purpose,
utilization of vegetable and fruit waste as a prime matter to one related to the reduction of metallic ions during the syn-
thesis process and the other related to the stabilization of the
synthesized ZnO particles.5,6 In the fruit sector a large variety
aCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,
Portugal. E-mail: apribeiro@tecnico.ulisboa.pt, martamalves@tecnico.ulisboa.pt;
Tel: +351-21-8317-163
bCentro de Química e Bioquímica, Departamento de Química e Bioquímica,
Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
cÁrea Departamental de Engenharia Química, ISEL, Instituto Politécnico de Lisboa,
Portugal. E-mail: ebastos@deq.isel.ipl.pt
dBioISI – Biosystems & Integrative Science Institute, Departamento de Química e
Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Lisboa,
Portugal
†Electronic supplementary information (ESI) available: Fig. S1. 1H-NMR spec-
trum of ethyl benzoylformoate (Table 2, entry 2) and Fig. S2. 1H-NMR spectrum
of ethyl 4-nitrobenzoate (Table 2, entry 16). See DOI: 10.1039/D0DT01069C
of products are offered, with apples (12.7 million tons) being
one of the most important fruits in terms of the size of harvest
in the EU,7 and therefore considerably contributing to fruit
waste. Despite this, only a few studies have focused on the syn-
thesis of ZnO particles using this natural resource. Fatemi
et al. reported the synthesis of biomorphic porous ZnO nano-
structures via an aqueous sol–gel soaking route using pieces of
apple pulp and skin as templates for glucose detection.8 Alves
et al. reported the synthesis of ZnO particles using pulp and
skin extracts of apple var. Starking and derived phytochemicals
proposed for bioimaging applications.9 Another apple derived
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
phytochemical, pectin, was used to mediate the synthesis of Fig. 1b, f and j) and TEM (transmission electron microscopy,
ZnO hierarchical superstructures and apple juice was recently see Fig. 1c, d, g, h, k and l).
used for the fabrication of reduced graphene oxide–ZnO nano-
The successful synthesis of ZnO particles was confirmed by
composites, both showing high photocatalytic activity XRD analysis (Fig. 1a, e and i). The diffractograms reveal the
a
towards hazardous dye degradation under ultraviolet (UV) characteristic peaks of ZnO, at 31.7, 34.4, 36.3, 47.5, 56.6, 62.8
light.10,11 To the authors’ best knowledge, there are no other and 67.8 degrees, corresponding to the crystal planes (1 0 0),
reports addressing ZnO particles synthesized with apple- (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2), respectively,
derived extracts for catalytic applications, making these in agreement with the JCPDS No. 00-036-1451 card for the ZnO
materials worthy of dedicated research effort.
crystalline structure. In the XRD diffractogram of ZnO particles
The transesterification reaction has been widely used in synthesized with quercetin (Fig. 1e), besides the crystal planes
organic synthesis, namely in the synthesis of various impor- assigned to ZnO, the additional peaks belonging to pure quer-
tant chemical intermediates for the preparation of complex cetin crystal planes suggest that this compound could have
natural products, paint additives and pheromones.12
been trapped within these crystal structures.9,16–18 As shown in
It is also a recognized method for the modification of com- Fig. 1(b–d) the nanosized particles (ZnONPs) were successfully
pounds and for the introduction of functional groups. synthesized using apple extract. The microsized ZnO particles,
Although a few novel and practical transesterification reactions obtained using pure polyphenol, quercetin (ZnOq, Fig. 1f–h),
have been reported,13,14 a highly efficient and environmentally or without phytochemicals (ZnO, Fig. 1j–l)9, were studied for
friendly catalyst, able to get into the circular economy trend, comparative purposes. The carbon element was detected in
for transesterification is still required to overcome limitations both ZnONPs and ZnOq particles, confirming the presence of
such as environmental pollution, the difficulty of separation organic species in these crystals (Table S1†).
and low substrate applicability.15
The use of the apple extract yielded nano quasi-spherical
In this work, ZnO nanoparticles synthesized using a novel structures composed of round lamina-like structures (Fig. 1b–
apple extract (bravo-de-esmolfe) (ZnONPs) were used for the first d). These round structures, synthesized with apple pulp var.
time in the transesterification of methyl benzoates and compared bravo-de-esmolfe, are similar to the previously described oval
with ZnO microparticles derived from phytochemicals (quercetin) structures produced using apple pulp var. Starking.9 A round
(ZnOq) or prepared without these organic templates (ZnO).
lamina-like structure, but at the microscale level (in the order of
1–5 µm), was obtained when using the pure apple-phytochem-
ical, quercetin; besides this predominating morphology, other
microsized flower shapes (around 500 nm), with triangular sub-
units, were also obtained (Fig. 1f–h).9 In the absence of phyto-
chemicals, the microsized particles (2–3 µm) evidenced flower-
like shapes, composed of pyramidal subunits (Fig. 1j–l).9
2. Results and discussion
2.1. Characterization of the as-synthesized green ZnO
particles
A deeper analysis of the as-synthesized particles revealed
that in the presence of the apple phytochemical, the round-
lamina subunits of ZnONPs are composed of nanometric
bumpy structures (Fig. 1d), whereas in the presence of querce-
tin, the subunits are composed of bumpy structures with sub-
micrometric sizes (Fig. 1h). In contrast, the absence of phyto-
chemicals (ZnO) induced a flattened surface (inset of Fig. 1l).
These observations suggest that in the presence of quercetin,
specific voids and interspaces can be formed within the crys-
talline structure of ZnOq particles.
The ZnO particles were analyzed by XRD (X-ray diffractometry,
see Fig. 1a, e and i), SEM (scanning electron microscopy, see
Moreover, other texture properties, such as surface area,
pore volume and pore size were obtained by BET analysis.
Besides this, the particle size of the prepared ZnO particles
was estimated. The surface areas obtained for ZnO, ZnONPs
and ZnOq were 5, 11 and 17 m2 g−1, respectively (Table S2†).
These results suggest a type-II isotherm, the most common
classification for non-porous or macroporous adsorbents,
according to the IUPAC classification. The type II isotherm rep-
resents
unrestricted
monolayer-multilayer
adsorption
(Fig. S3†).19,20
Fig. 1 Physicochemical characterization of ZnO particles synthesized
with (a–d) apple phytochemicals, ZnONPs, with (e–h) a pure phyto-
chemical – quercetin, ZnOq, or (i–l) without phytochemicals, ZnO; (a, e
and i). X-ray diffractograms, where (*) refers to the crystalline planes of
ZnO; scanning electron microscopy (b, f and j) and transmission elec-
tron microscopy (c, d, g, h, k and l) images.
Overall, the presence of polyphenols affected the ZnO
crystal morphology, in a way that the same lamina-like mor-
phology (nano or microsize) or bumpy vs. flattened structures
could be explored to compare the catalytic performances.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Paper
2.2. Transesterification reaction catalysed by ZnO particles
The catalytic activity of the ZnO particles obtained with
different precursors, namely those obtained from the apple
extract (ZnONPs) and for comparative purposes from quercetin
(ZnOq) and particles with no added templates (ZnO), was
investigated for the transesterification reaction of different
methyl benzoates in ethanol, namely methyl benzoylformate
(Scheme 1), methylbenzoate, methyl 4-iodobenzoate, methyl
4-aminobenzoate, methyl 4-nitrobenzoate, methyl 3-nitro-
benzoate and methyl 3-hydroxybenzoate (Table 1 and Fig. 2),
following a procedure reported elsewhere.21,22 Reaction para-
meters such as temperature, reaction time, and the amount of
catalyst were explored to optimize the catalytic process. After
each catalytic experiment, the ZnO particles were separated by
Fig. 2 Transesterification reaction using different substrates (methyl
benzoylformate, methyl 4-nitrobenzoate, methyl 4-iodobenzoate or
methyl 4-aminobenzoate). Reaction conditions: reaction time = 24 h;
temperature = 75 °C; 2 mol% catalyst/substrate.
centrifugation and the solvent evaporated under vacuum. The
1
obtained crude mixture of products was analyzed by H NMR
spectroscopy (Tables 1 and 2). Under optimized conditions
(catalyst/substrate molar ratio of 2%, 2 mL of ethanol, 24 h,
100 °C), yields up to 96–98% were achieved by the ZnONPs
and ZnOq/ethanol systems (catalyst/substrate molar ratio of
2%), respectively, in the transesterification reaction of methyl
benzoylformate (Table 2).
Scheme 1 Transesterification reaction of methyl benzoylformate with
ethanol.
Table 1 Transesterification reaction of various methyl esters in ethanol
catalyzed by different ZnO particlesa
The transesterification reaction of various methyl benzoates
in ethanol was investigated for ZnONPs, ZnOq and ZnO (with
no phytochemicals) (Table 1 and Fig. 2), which were converted
with yields up to 78%, under similar reaction conditions (2 mL
of ethanol, 24 h, 75 °C). The substrates with the strongest elec-
tron-withdrawing substituent groups (iodo and nitro) show
higher product yields (Table 1, entries 10–12, 18–20, and
22–24 for iodo and nitro, respectively), while those with the
electron-donating groups (NH2 and OH) show the lowest ones
(Table 1, entries 14–16 and 26–28, for NH2 and OH, respect-
ively). We could also observe that in the case of methyl nitro-
benzoate, the position of the nitro group (para or meta) seems
to influence the reaction yield, affording a better conversion
when this functional group is in the para position, probably
due to the lower stereochemical hindrance in this case, relative
to the meta position.
The transesterification reaction of methyl benzoates per-
formed in the absence of any catalyst (blank reaction) gives
only 2% yield for the methyl benzoylformate substrate and no
products for the other substrates (Table 1, entries 1, 5, 9, 13,
17, 21 and 25).
Throughout this study, it was possible to confirm that for
some of the tested substrates (those with the nitro or iodo sub-
stituent), the ZnO nanoparticles obtained using the apple
extract (ZnONPs) or the microparticles obtained using querce-
tin (ZnOq) showed higher conversions compared to the ZnO
particles obtained without phytochemicals. These results show
that the presence of organic compounds on the surface of the
ZnO particles can make a positive contribution (e.g. stabilizing
action) to the yield of the transesterification reaction, whereas
Time
Catalyst (h)
Yieldb
(%)
Entry Substrate
Product
1
2
3
4
5
6
7
8
—
24
2
ZnONPs 24
77
78
77
—
0.2
0.2
0.2
—
7
ZnOq
ZnO
—
24
24
24
ZnONPs 24
ZnOq
ZnO
—
24
24
24
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
ZnONPs 24
ZnOq
ZnO
—
24
24
24
6
3.2
—
0.3
0.3
1.3
—
6
9
5
—
5
5
ZnONPs 24
ZnOq
ZnO
—
24
24
24
ZnONPs 24
ZnOq
ZnO
—
24
24
24
ZnONPs 24
ZnOq
ZnO
—
24
24
24
4
—
0.4
0.4
0.5
ZnONPs 24
ZnOq
ZnO
24
24
a Reaction conditions (unless stated otherwise): 2 mol% of catalyst/sub-
strate, 1.0 mmol of methyl benzoylformate, methyl 4-iodobenzoate,
methyl 4-aminobenzoate, methyl 4-nitrobenzoate, methyl 3-nitro-
benzoate or methyl 3-hydroxybenzoate, 2 mL of ethanol, 75 °C. b Molar
yield (%) based on the substrate, i.e. moles of product per 100 mol of
substrate determined by 1H NMR.
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Table 2 Transesterification reaction of methyl benzoylformate and methyl 4-nitrobenzoate catalyzed by different ZnO particlesa
Entry
Substrate
Catalyst
ZnONPs
Time (h)
24
Temp. (°C)
Product
Yieldb (%)
1
2
3
4
5
6
7
8
75
100
75
100
75
100
75
100
75
100
75
100
100
100
75
100
100
100
75
77
96
78
98
77
97
6
14
16
25
9
19
11
11
29
45
44
31
5
ZnOq
ZnO
24
24
24
72
24
ZnONPs
9
10
11
12
13c
14d
15
16
17c
18d
19
20
21
22
ZnOq
24
24
72
72
72
24
ZnO
100
75
100
14
35
45
72
a Reaction conditions (unless stated otherwise): 2 mol% of catalyst/substrate, 1.0 mmol of methyl benzoylformate or methyl 4-nitrobenzoate and
2 mL of ethanol at 75 or 100 °C, during 24 or 72 h. b Molar yield (%) based on the substrate, i.e. moles of product per 100 mol of substrate deter-
mined by 1H NMR. c Second catalytic cycle. d Third catalytic cycle.
the size of the particles does not seem to have a significant did not exceed 7% even after 72 h of reaction. At 100 °C the
influence on it.
ethyl benzoylformate yield increased significantly over time, to
The transesterification of methyl benzoylformate and a maximum (at 24 h) of 96, 98 and 97% for ZnONPs, ZnOq
methyl 4-nitrobenzoate gave the highest yields, and therefore and ZnO, respectively (Table 2, entries 2, 4 and 6, respectively).
these substrates were selected to evaluate other reaction para-
To make the catalytic process cost-effective, the reusability
meters, such as reaction time and temperature (Table 2 and of the ZnO particles (Fig. 4) was also explored. Since the ZnOq
Fig. 3). The reaction was carried out at 75 or 100 °C, tempera- particles showed the best yield (Table 2 and Fig. 3), they were
tures at which the polyphenols’ integrity is maintained, for 24 isolated by centrifugation after catalyzing the methyl-4-nitro-
or 72 hours. The transesterification of methyl benzoylformate benzoate transesterification reaction for 24 or 72 h at 100 °C
was also performed at room temperature; however, the yield (Table 2, entries 12 and 16, respectively) and reused in 2 more
cycles (Table 2, entries 13, 14, 17 and 18). It was found that for
the reaction performed for 24 h, there was a reduction of ca.
42% yield in the 2nd cycle compared to that in the first one
Fig. 3 Effect of the temperature and the reaction time on the transes-
terification reaction of methyl 4-nitrobenzoate. Reaction conditions:
time = 24 or 72 h; temperature = 75 or 100 °C; 2 mol% catalyst/methyl
benzoylformate.
Fig. 4 Recycling studies of the transesterification reaction of methyl
4-nitrobenzoate in the presence of ZnOq particles. Reaction conditions:
time = 24 or 72 h; temperature = 100 °C; 2 mol% catalyst/methyl
benzoylformate.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Paper
(Table 2, entry 13 relatively to entry 12). Between the 2nd not show any morphological changes after the transesterifica-
(Table 2, entry 13) and 3rd (Table 2, entry 14) cycles, there was tion reaction.
no significant change.
To have a deeper insight into the involvement of quercetin-
For the reaction performed at 100 °C, it was confirmed that derived molecules in the transesterification reaction catalyzed
there was no significant change in the yield between the first by ZnOq, the FTIR-ATR technique was used. As expected, the
and second cycles (Table 2, entry 17 relatively to entry 16), but spectra (Fig. 5c) can be related to quercetin-derived molecular
it decreased by ca. 30% from the second to the third cycle bonding: the bands in the region between 1550 and 1660 cm−1
(Table 2, entry 18 relatively to entry 17).
can be assigned to the stretching vibration modes of C–C,
The loss of activity in both cases, more pronounced in the CvC, and C–O, and those in the region below 1300 cm−1
first one, could indicate the occurrence of partial catalyst correspond to the C–O and C–C stretching modes and/or to
deactivation.
the C–O, O–H and C–H bending modes, including those
It should be noted that the behavior of the biosynthesized within the aromatic rings of quercetin.9 These quercetin-
ZnO nanoparticles resembles that of ZnONPs obtained by related peaks are no longer visible in the ZnOq′ spectrum
another sustainable method, involving precipitation using zinc (Fig. 5c); the main peaks belonging to O–H bonding around
nitrate and sodium hydroxide in aqueous medium at room 3400 cm−1, two prominent peaks in the region of 1500 cm−1
,
temperature.22
stretching vibration modes of C–C, CvC, and C–O, and those
in the region below 1300 cm−1 corresponding to the C–O and
C–C stretching modes and/or to C–O, O–H and C–H bending
modes can be related to the alteration of the quercetin chemi-
cal environment by the solvent, ethanol and methanol and/or
their derived ketones (Fig. 5d).23 We believe that some mole-
cules of the solvent, which also have OH groups, have reacted
with quercetin and in this way changed the detected organic
species.
The ZnO-catalyzed transesterification reaction may involve
the coordination of the carbonyl oxygen of the methyl ester to
the ZnO solid base catalyst. This interaction promotes the
nucleophilic attack of the alcohol producing an intermediate
2.3. Characteristics of the ZnOq particles post the
transesterification reaction
To characterize the ZnOq particles that showed the best trans-
esterification reaction, the initial (ZnOq) and final catalyst
(ZnOq′) were analysed. After the transesterification reaction,
the phenolic content, which was of 0.12 0.01 µg polyphenols
per mg of particles in the as-synthesised ZnOq particles, was
not changed (0.120 0.004 µg polyphenols per mg of particles)
by the conditions of the reaction that ZnOq catalyzed. The
XRD diffractogram of ZnOq′ (Fig. 5a), when compared to that
of ZnOq (Fig. 1e), shows that the ZnO and quercetin peaks
were retained, but with different intensity ratios.
which, upon elimination of
a methanol molecule and
rearrangement, leads to the formation of
(Scheme 2).21,24,25
a new ester
When ZnOq′ was analysed by SEM (Fig. 5b) and compared
to the as-synthesised ZnOq particles (Fig. 1f), the catalyst did
Overall, the presence of polyphenols affected the ZnO
crystal morphology (Fig. 1). While the NPs are well acknowl-
edged for their particularly high specific area, and therefore
potential catalysts with high conversion rates, the presence of
a specific phytochemical, quercetin, gave rise to the design of
micro sized catalysts with a slightly higher performance than
Fig. 5 Physicochemical characterization of the ZnO particles syn-
thesised with quercetin after the transesterification reaction (ZnOq’); (a)
X-ray diffractogram where (*) refers to the crystalline planes of ZnO; (b)
scanning electron microscopy (SEM) image; (c) FTIR-ATR spectra of
ZnOq before and after catalysis (ZnOq’); (d) schematic representation of
the ZnO particles before (ZnOq) and after catalysis (ZnOq’).
Scheme 2 Proposed mechanism for the transesterification reaction of
methyl benzoylformate catalyzed by ZnO.
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
that of the ZnONPs. The same was true when this ZnOq cata-
lyst was compared with the ZnO devoid of phytochemicals.
The different performances achieved can be primarily related
to the morphology of the subunits composed of the distinct
ZnO particles (Fig. 1d, h, l), with the presence of sub-micro
sized voids in ZnOq, likely resulting in a better combination of
the surface area for product conversion (Scheme 1).
The work presented herein has two main novelties: (i) the
inclusion of apple waste into the concept of the circular
economy towards the re-use of this disposal for the separation
and purification of quercetin; (ii) natural organic compounds,
by allowing the fabrication of micro-sized ZnO particles with
excellent catalytic activity, can overcome the hazardous effects
associated with the manipulation of NPs, which is especially
relevant in the context of the industrial application of transes-
terification reactions.
Scheme 3 Synthesis of ZnO particles with apple phytochemicals,
ZnONPs, with a pure phytochemical – quercetin, ZnOq, or without phy-
tochemicals, ZnO.
crystallinity was identified by X-ray diffraction (XRD) using a
D8 Advance Bruker AXS θ–2θ diffractometer with a copper radi-
ation source (Cu Kα, λ = 1.5406 Å) and a secondary monochro-
mator operated at 40 kV and 40 mA. FTIR-ATR spectral
measurements were performed using a Nicolet 6700 in the
400–4000 cm−1 with 4 cm−1 resolution. For the BET analysis,
the samples were degassed in two stages at 90 °C for 1 h and
130 °C for
6 h prior to the sample mounting on a
Micromeritics ASAP 2060 gas sorption instrument.21,26 The
elemental analysis had been performed at the MicroLab in
Instituto Superior Técnico, Lisbon, Portugal. To determine the
total phenolic content, the Folin–Ciocalteu method was
used.27 Briefly, to each well of a microplate water (235 µL), the
sample (or solvent, in the control, 5 µL), Folin–Ciocalteu
reagent (15 µL, Fluka®) and saturated Na2CO3 aqueous solu-
tion (45 µL) were added. The microplate was incubated for
30 min at 40 °C and the absorbance at 765 nm was measured.
Tannic acid was used as the standard.
3. Materials and methods
All the chemicals were obtained from commercial sources and
used as received. Zinc nitrate hexahydrate and sodium hydrox-
ide were of analytical reagent grade from Merck. The synthesis
of ZnO particles was performed using zinc nitrate (Sigma-
Aldrich), quercetin (Roth) and potassium hydroxide (Sigma-
Aldrich).
3.1. Synthesis of ZnO particles
For the first time, the extract of commercial apples (Malus
domestica var. bravo-de-esmolfe) was used for the synthesis of
ZnO nanoparticles (ZnONPs). To minimize the alterations due
to (i) apple development, only fruits with commercial maturity
were used, and due to (ii) preservation treatments, only pulp
was used. After washing, the apples were peeled, and the pulp
was cut into small pieces. An aqueous apple extract was
obtained after boiling 0.2 g of fresh pulp pieces per 100 mL of
water for 20 min. The final extract for particle synthesis was
obtained after filtering using a no. 1 filter (Whatman). The syn-
thesis of ZnO particles was carried out as previously described
by the authors.9 Briefly, [Zn(NO3)2·6H2O] 2% (w/v) was added
to the apple extract (ZnONPs), to a pure apple phytochemical,
quercetin (0.15 mg mL−1) (ZnOq) solution, or to that without
phytochemicals (pure water) (ZnO). Upon the complete dis-
solution of Zn(NO3)2·6H2O, a solution of KOH (1 M) was added
[1 : 6 (v/v)] to promote ZnO precipitation. The particles were
collected with a 0.45 µm nylon filter (Magna) and dried over-
night in an oven (Memmert) at 80 °C (Scheme 3).
3.3. Transesterification of methyl benzoylformate catalyzed
by ZnO nanoparticles
A mixture of methyl benzoylformate (164.2 mg, 1 mmol) and
ZnO catalyst (1.6 mg, 2 mol%) in 2 mL EtOH was added into a
capped glass vessel. The mixture was heated at 75 °C for the
desired time (typically 24 or 72 h). The solution was then cen-
trifuged to isolate the catalyst. The catalyst was washed with
ethanol and acetone, and then dried at 80 °C prior to re-use.
The solvent was evaporated in a vacuum, yielding a crude
1
product. The product mixture was analysed by H-NMR spec-
troscopy in CDCl3. The yield of the ethyl ester product (rela-
tively to the methyl ester) was established typically by taking
into consideration the relative amounts of these compounds,
as given by 1H-NMR. Selected 1H-NMR spectra are presented
in the ESI (Fig. S1 and S2†).
3.2. Characterization of the ZnO particles
4. Conclusions
The ZnO particles were characterized by scanning electron
microscopy (SEM) using
a
JEOL-JSM7001F equipment. The use of eco-friendly approaches to synthesize ZnO particles
Whenever required, the conductivity of the ZnO particles was resulted in particles with distinct sizes and morphologies. The
increased by adding a thin coating of conductive gold/palla- use of an aqueous apple extract resulted in nanoparticles with
dium (Polaron E-5100). For the transmission electron round-like structures with nanosized bumpy features, whereas
microscopy (TEM) analysis, a Hitachi H-9000-NA microscope the use of a natural compound, quercetin, rendered micro-
operated at 200 kV with supporting copper grids was used. The sized round-like structures with submicron voids. In the
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Paper
absence of phytochemicals, flower-like structures with flat-
tened surfaces were obtained.
table_sector_in_the_EU_-_a_statistical_overview
(Data
extracted in July and August 2019, Planned article update:
September 2023).
The green synthesized ZnO particles (ZnONPs and ZnOq)
have proved to be efficient catalysts for the transesterification
of methyl 4-nitrobenzoate and methyl benzoylformate with the
ZnOq microparticles obtaining the highest yields (up to 98%)
under mild conditions. The surface morphology and organic
species of the ZnO particles, rather than their size, seem to
8 H. Fatemi, A. A. Khodadadi, A. A. Firooz and Y. Mortazavi,
Curr. Appl. Phys., 2012, 12, 1033.
9 M. M. Alves, S. M. Andrade, L. Grenho, M. H. Fernandes,
C. Santos and M. F. Montemor, Mater. Sci. Eng., C, 2019,
101, 76.
affect their catalytic performance. This cheap and easily avail- 10 A.-J. Wang, Q.-C. Liao, J.-J. Feng, P.-P. Zhang, A.-Q. Lia and
able phytochemical obtained from food waste is an important J.-J. Wang, CrystEngComm, 2012, 14, 256.
achievement to promote a green approach to transesterifica- 11 P. Kumbhakar, A. Pramanik, S. Biswas, A. K. Kole,
tion reactions in the industrial context.
R. Sarkar and P. Kumbhakar, J. Hazard. Mater., 2018, 360,
193.
12 A. Solhy, J. H. Clark, R. Tahir, S. Sebti and M. Larzek, Green
Chem., 2006, 8, 871.
13 M. Hatano and K. Ishihara, Chem. Commun., 2013, 49,
1983.
Conflicts of interest
There are no conflicts to declare.
14 K. Ishihara, M. Niwa and Y. Kosugi, Org. Lett., 2008, 10, 2187.
15 B. Teng, J. Shi and C. Yao, Green Chem., 2018, 20, 2465.
16 X.-Y. Li, Y.-C. Li, D.-G. Yu, Y.-Z. Liao and X. Wang,
Int. J. Mol. Sci., 2013, 14, 21647.
17 A. Zeynep, S. Ipek, E. Durgun and U. Tamer, J. Mater. Sci.,
2018, 53, 1527.
18 C. Li, Z.-H. Wang and D.-G. Yu, Colloids Surf., B, 2014, 114,
404.
19 M. D. Donohue and G. L. Aranovich, Adv. Colloid Interface
Sci., 1998, 76, 137.
Acknowledgements
This work was supported by the Fundação para a Ciência e
Tecnologia (FCT), project UIDB/00100/2020 of Centro de
Química Estrutural, fellowship PD/BD/114400/2016 to
M. M. A. S. and PTDC/QUI-QIN/29778/2017 project and by
Instituto Politécnico de Lisboa (IPL/2019/STREAM_ISEL
project). APCR and MMA thank Instituto Superior Técnico for
Scientific Employment contracts IST-ID/119/2018 and IST-ID/
154/2018, respectively, under Decree-Law no. 57/2017.
20 M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier,
F. Rodriguez-Reinoso, J. Rouquerol and K. S. W. Sing, Pure
Appl. Chem., 2015, 87, 1051.
21 A. Karmakar, G. M. D. M. Rúbio, M. F. C. G. Silva,
A. P. C. Ribeiro and A. J. L. Pombeiro, RSC Adv., 2016, 6,
89007.
References
1 FAO, Food Wastage Footprint: Impacts on Natural
Resources (Summary Report), 2013, pp. 1–63, http://www. 22 M. M. A. Soliman, A. Karmakar, E. C. B. A. Alegria,
fao.org/docrep/018/i3347e/i3347e.pdf.
2 T. H. O’Donnell, J. Deutsch, C. Yungmann, A. Zeitz and
S. H. Katz, Food Nutr. Sci., 2015, 6, 883.
A. P. C. Ribeiro, G. M. D. M. Rúbio, M. S. Saraiva,
M. F. C. G. Silva and A. J. L. Pombeiro, Catal. Today, 2019,
DOI: 10.1016/j.cattod.2019.08.053.
3 S. Ahmed, Annu, S. A. Chaudhry and S. Ikram, 23 X. Hu, R. J. M. Westerhof, D. Dong, L. Wu and C. Z. Li, ACS
J. Photochem. Photobiol., B, 2017, 166, 272. Sustainable Chem. Eng., 2014, 2, 2562.
4 P. Vishnukumar, S. Vivekanandhan, M. Misra and 24 H. E. Hoydonckx, D. E. Vos and S. A. Chavan, Top. Catal.,
A. K. Mohanty, Mater. Sci. Semicond. Process., 2018, 80, 143. 2004, 27, 83.
5 H. Agarwal, S. V. Kumar and S. Rajeshkumar, Resour.-Effic. 25 S. Yan, S. O. Salley and K. Y. Simon Ng, Appl. Catal., A,
Technol., 2017, 3, 406.
2009, 353, 203.
6 S. Iravani, Green Chem., 2011, 13, 2638.
7 The fruit and vegetable sector in the EU - a statistical over-
26 R. A. Abdol Aziz, S. F. Abd Karim and N. A. Rosli, Key Eng.
Mater., 2019, 797, 271.
view, Eurostat. 2015. Available online: https://ec.europa.eu/ 27 V. L. Singleton and J. A. Rossi, Am. J. Enol. Vitic., 1965, 16,
eurostat/statisticsexplained/index.php/The_fruit_and_vege-
144.
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.