Syntheses of ZnO with Different Morphologies: Catalytic Activity toward Coumarin Synthesis via the Knoevenagel Condensation Reaction

Article abstract of DOI:10.1021/acs.inorgchem.9b00053

Heterogeneous catalysts are preferred in fine chemical industries due to their easy recovery and reusability. Here, we report an easily scalable method of ZnO catalysts for coumarin synthesis. Nanocrystalline ZnO particles with diverse morphologies and crystallite sizes were prepared using different solvents. The change in morphology results in changes in band gaps, defects, basicity, and textural properties (surface areas, pore volumes, and pore sizes). The catalytic performances of the synthesized ZnO materials were tested using coumarin synthesis via the Knoevenagel condensation. The catalyst synthesized using methanol shows the highest activity and selectivity (conversion of 74%, selectivity of 94%) with a turnover number of 14.69. The increased activity of the ZnO synthesized in methanol is attributed to the combined effects of moderate basicity and relatively high textural properties of the sample.

Full text of DOI:10.1021/acs.inorgchem.9b00053

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Syntheses of ZnO with Different Morphologies: Catalytic Activity
toward Coumarin Synthesis via the Knoevenagel Condensation
Reaction
†
‡
†
†
Md. R. Shakil, Andrew G. Meguerdichian, Habiba Tasnim, Alireza Shirazi-Amin,
†
,†,‡,§
Mohammad S. Seraji, and Steven L. Suib*
†Department of Chemistry, University of Connecticut, U-3060, 55 North Eagleville Road, Storrs, Connecticut 06269-3060, United
States
‡
Institute of Materials Science, University of Connecticut, U-3136, 97 North Eagleville Road, Storrs, Connecticut 06269-3136,
United States
§
Department of Chemical & Biomolecular Engineering, University of Connecticut, U-3222, 191 Auditorium Road, Storrs,
Connecticut 06269-3222, United States
ABSTRACT: Heterogeneous catalysts are preferred in fine chemical
industries due to their easy recovery and reusability. Here, we report an
easily scalable method of ZnO catalysts for coumarin synthesis. Nanocrystal-
line ZnO particles with diverse morphologies and crystallite sizes were
prepared using different solvents. The change in morphology results in
changes in band gaps, defects, basicity, and textural properties (surface areas,
pore volumes, and pore sizes). The catalytic performances of the synthesized
ZnO materials were tested using coumarin synthesis via the Knoevenagel
condensation. The catalyst synthesized using methanol shows the highest
activity and selectivity (conversion of 74%, selectivity of 94%) with a turnover
number of 14.69. The increased activity of the ZnO synthesized in methanol is
attributed to the combined effects of moderate basicity and relatively high
textural properties of the sample.
25
1
. INTRODUCTION
41 mesoporous catalysts for the Knoevenagel condensation.
These catalysts exhibit good catalytic activities with recycla-
bility for the reaction. However, synthetic processes of those
catalysts are complex and tedious. ZnO, being basic and
abundant, has the potential to be a promising heterogeneous
catalyst for coumarin synthesis. ZnO is an environmentally
benign and inexpensive catalyst. Moreover, the properties of
Coumarins are heterocyclic compounds having a benzo-α-
pyrone moiety in their structure. These compounds have been
1
widely studied because of their importance as antidepressants,
2
3
4
antimicrobials, antioxidants, anti-inflammatories, antinoci-
ceptives, antitumor agents, antiasthmatics, antivirals (in-
cluding anti-HIV), and anticoagulants. There are several
methods utilized to prepare coumarins. Some examples are
4
5
6
7
8
26−28
ZnO can easily be altered
as well as activities. Thus, ZnO
9
,10
11
is considered to be a potential replacement for existing
heterogeneous catalysts. For instance, the Kumar group
Knoevenagel condensation,
reaction,
Perkin reaction, Pechmann
and Wittig reactions. Among the various
1
2,13
14
29
reported the use of ZnO nanoparticles in coumarin synthesis;
available methods, the Knoevenagel condensation is the most
useful method for synthesizing coumarins and their deriva-
they focused on establishing a general synthetic strategy for
various coumarins using microwave-derived ZnO nano-
particles.
1
0
tives. The Knoevenagel condensation is usually catalyzed by
15,16
weak acids and bases,
and most of those homogeneous
The properties of ZnO depend on various morpholo-
catalysts are toxic and harmful chemicals. In addition,
homogeneous catalysts are often very hard to separate from
the reactants and products. On the other hand, heterogeneous
catalysts are easy to separate from the reaction mixture,
reusable in nature, inexpensive, and nontoxic. Hence,
heterogeneous catalysts are preferred in industry.
26,28
27,28
gies,
which change with synthesis processes.
Among
the various properties, textural properties (morphologies,
surface areas, and surface defects) are crucial for tailoring the
3
0,31
activity of the materials in heterogeneous catalysis.
Hence,
the investigation of the catalytic activity of ZnO with their
morphologies and other surface properties (surface areas and
surface basicity) is very interesting. Xu et al. previously
A variety of heterogeneous catalysts have been used for the
10,17−23
Knoevenagel condensation reaction.
Besides these, the
Al-Lohedan group has recently developed melamine-immobi-
1
6
24
lized MCM-41 and urea-functionalized SBA-15, and the
Hamid group synthesized DL-alanine-functionalized MCM-
Received: January 8, 2019
©
XXXX American Chemical Society
A
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an Ar+ ion laser (514.5 nm). A NOVA 2000e instrument
synthesized different morphologies of ZnO with different
(
Quantachrome) was used for N sorption analysis using N gas as
solvents and investigated the photocatalytic activities of the
2
2
2
6
the adsorbate at 77 K (−196 °C). To calculate the specific surface
area of the materials, the Brunauer−Emmett−Teller (BET) method
was utilized, whereas the Barrett−Joyner−Halenda (BJH) desorption
method was used for the pore size distributions and pore volumes. To
investigate the basicity of the materials, room-temperature CO2
adsorption experiments were performed using Quantachrome
Autosorb-1-1C automated adsorption. All samples were degassed at
150 °C for 3 h before the sorption analysis was performed.
2.4. Catalytic Activity Measurements. To investigate the effects
of morphology on the catalytic activity of the ZnO samples, a
coumarin synthesis reaction was carried out. The synthesis of ethyl 2-
oxo-2H-chromene-3-carboxylate (coumarin) from o-hydroxybenzal-
dehyde and diethyl malonate via the Knoevenagel condensation
reaction was utilized as a model reaction. In a typical coumarin
synthesis, the reactants o-hydroxybenzaldehyde (2 mmol) and diethyl
malonate (2.5 mmol) and catalyst ZnO (8.2 mg) were placed in a
closed reactor. The reactor with this mixture was then heated at 100
°C for 24 h while being vigorously stirred. When the reaction time
elapsed, the product was collected from the system by filtration. The
products were analyzed using an Agilent Technologies gas
chromatography/mass spectrometry (GC/MS) instrument. The
GC/MS instrument consisted of a 7820 GC system and a 5975
series MSD mass detector. The products were separated from the
reactants in the GC system using a nonpolar cross-lined methyl
siloxane column with dimensions of 12 in. × 0.200 mm × 0.33 μm.
The conversion was calculated from the concentration of reactants
and products. The selectivity was estimated on the basis of the
coumarin and 2-propenoic acid as the only products.
materials. They discussed the influence of solvents in
obtaining different ZnO morphologies and the photo-
degradation activity of the obtained materials. Because the
crystallographic morphology of ZnO depends on the
precursor, solvent, pH, and time and temperature of a
3
2−34
reaction,
in this study a modified reaction system (in
terms of precursors, solvents, and reaction times) is utilized to
tailor ZnO morphology, crystalline quality, and textural
properties. The correlation between the catalytic activities for
coumarin synthesis and structural properties of synthesized
ZnO is also explored. This study could be significant in
selecting and designing an effective catalyst for coumarin
synthesis based on low-cost ZnO materials.
Herein, an easy, scalable preparation of ZnO with various
morphologies using a modified solvothermal method is
described. The diverse morphologies of ZnO were obtained
using different solvents in the reaction. The crystalline
qualities, textural properties, surface basicity, and defects of
the prepared ZnO samples were investigated. The variation of
the catalytic activity of ZnO samples with morphology was
examined using the Knoevenagel condensation as a model
reaction. The correlation between the activity of ZnO and
surface properties was explored.
2
. EXPERIMENTAL SECTION
2
.1. Materials. Zinc acetate dihydrate [Zn(CH COO) ·2H O,
3 2 2
3
. RESULTS
ACS reagent, ≥98%], commercial ZnO powder (ACS reagent,
99.0%), diethylamine (DEA) [40% (w/w) aqueous solution], and 1-
propanol (Anhydrous, 99.7%) were purchased from Sigma-Aldrich.
Sodium hydroxide (ACS reagent, ≥98%), methanol (HPLC grade,
9.9%), and acetonitrile (liquid chromatography/mass spectrometry
≥
3.1. Structural Characterization. Structural character-
ization of ZnO nanocrystals was performed using PXRD
techniques. X-ray diffraction patterns (Figure 1) indicate that
9
reagent) were purchased from J. T. Baker. Ethanol (ACS grade,
9.98%) was purchased from Pharmco-Aaper.
.2. ZnO Preparation. Approximately 1.097 g (0.1 M) of
Zn(CH COO) ·2H O and 0.400 g (0.4 M) of NaOH were added to
9
2
3
2
2
two separate beakers. Zn(CH COO) ·2H O was dissolved in 50 mL
3
2
2
of water, whereas NaOH was dissolved in 25 mL of water [distilled
deionized (DDI)]. Both solutions were slowly mixed together while
being stirred to prepare ZnO in a water catalyst. The reaction mixture
was heated in a 100 mL Teflon-lined stainless-steel autoclave for 6 h.
After the reaction, the mixture was cooled and the precipitates were
collected from the reaction mixture by centrifugation with deionized
water (five times) and ethanol (once). The collected precipitates were
then dried in a laboratory oven at 60 °C for 12 h. The same procedure
was repeated using different solvents to prepare ZnO in methanol,
ZnO in ethanol, ZnO in propanol, ZnO in DEA, and ZnO in
acetonitrile.
2
.3. Characterization of Materials. X-ray powder diffraction
(
PXRD) was performed using a Rigaku Ultima IV powder X-ray
diffractometer with Cu Kα radiation (λ = 1.54178 Å) to study the
structures of the materials. Images of the materials’ surface were
recorded using an FEI Quanta 250 FEG field emission scanning
electron microscope. Transmission electron microscopy (TEM)
analysis was carried out with an FEI Talos F200X microscope to
investigate the exposed faces of the catalysts. The optical properties,
crystallinity, and defects of the materials were studied using
ultraviolet−visible (UV−vis) absorption spectroscopy, Raman spec-
troscopy, and room-temperature photoluminescence (PL) spectros-
copy. For analysis via UV−vis absorption spectroscopy, dispersions of
samples in distilled deionized water (DDW) were used. UV−vis
absorption measurements were conducted using a Shimadzu UV 2450
spectrophotometer. PL studies were carried out directly on the
pelletized solid sample using a Horiba Fluorolog III PL instrument
with an excitation wavelength of 325 nm. Raman analysis was
performed using a Renishaw Raman System 2000 Ramanscope with
Figure 1. XRD patterns of all ZnO samples synthesized in different
solvents.
all of the ZnO samples have a hexagonal crystal structure. All
observed lines in the XRD data were indexed as ZnO with a
hexagonal crystal structure. No additional peaks from other
phases were observed. The positions of the diffraction lines
and their relative intensities matched with the standard JCPDS
card (36-1451) for the ZnO hexagonal crystal structure. The
relative intensities of (002) planes of the ZnO sample
synthesized in ethanol (ZnO in ethanol) are slightly different
from the JCPDS file. The XRD lines for the ZnO prepared
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with acetonitrile (ZnO in acetonitrile), 1-propanol (ZnO in 1-
propanol), and DEA (ZnO in DEA) are relatively sharp,
whereas those of ZnO prepared in water (ZnO in water),
methanol (ZnO in methanol), and ZnO in ethanol are
relatively broader. These line broadenings indicate that those
materials have smaller crystallite sizes. The calculated
crystallite sizes (calculated using the Scherrer equation) of
the samples are listed in Table 1. The crystallite sizes of the
rough and bumpy. The size of the flakes was fairly uniform and
in the range of 80−100 nm. ZnO in acetonitrile presents the
same morphology as ZnO in DEA. However, the size of the
flakes varies from 50 to 550 nm. ZnO in water has a granular
morphology. The granules are well-separated from each other.
The size of the granules varies from 100 to 500 nm. The
granules seem to be made of fused 10−20 nm particles. ZnO in
methanol displays a spherical particle morphology.
The growth faces or most exposed faces of ZnO materials
were studied by TEM analysis. The TEM/HR-TEM data of
the ZnO materials are presented in Figure 3.
TEM analysis shows that exposed planes of the materials
vary with morphology. The most exposed planes for ZnO in
ethanol, ZnO in acetonitrile, ZnO in water, ZnO in methanol,
ZnO in DEA, and ZnO in 1-propanol are the (101), (102),
Table 1. Crystallite Sizes of the Synthesized ZnO Samples
synthesized ZnO in various solvents
calculated crystallite size (nm)
ZnO in ethanol
ZnO in methanol
ZnO in water
10
12
31
38
45
56
81
ZnO in DEA
(
100), (002), (100), and (002) planes, respectively.
ZnO in acetonitrile
ZnO in 1-propanol
Comm ZnO (commercial ZnO)
3.3. Optical Analysis. The band gaps, crystallinity, and
defects of the ZnO samples were examined using UV−vis
absorption, Raman, and PL spectroscopies. Figure 4 shows
UV−vis absorption data for the prepared ZnO samples. A
typical absorption band positioned from 360 to 387 nm was
found to be common for all samples, indicating the absorption
ZnO samples prepared in different solvents varied from 10 nm
smallest) to 56 nm (largest). The calculated crystallite sizes
support the XRD line broadening data shown in Figure 1.
.2. Morphology and Growth Face Analysis of ZnO
Catalysts. The scanning electron microscopy (SEM) images
shown in Figure 2) were taken to analyze the morphology of
(
26
of ZnO. The positions of the absorption peak changed with
the solvent. ZnO in methanol exhibits an absorption band at
around 365 nm. A blue shift (from 362 to 353 nm) compared
to ZnO in methanol was observed for ZnO in ethanol. ZnO in
acetonitrile, ZnO in DEA, ZnO in 1-propanol, and ZnO in
water demonstrate a red shift relative to ZnO in methanol. The
red shift (from 362 to 377 nm) is largest for ZnO in water
followed by those of ZnO in 1-propanol, ZnO in acetonitrile,
and ZnO in DEA. In addition, ZnO in methanol and ZnO in
ethanol exhibit greater absorption than ZnO in acetonitrile,
ZnO in DEA, and ZnO in 1-propanol.
3
(
the prepared ZnO samples. The studies reveal that all
synthesized ZnO is nanocrystalline. ZnO in ethanol exhibits
a sandlike surface morphology. The sandlike particles are
interconnected, having a nanosize finger-like shape. When
ZnO was synthesized with methanol (ZnO in methanol), a flat
but rough morphology was observed having fairly uniform
spherical particles. The particles were attached to each other
and formed a compacted surface of spherical particles. The
sizes and shapes of the particles were discernible. The particle
sizes varied from 10 to 15 nm. ZnO synthesized using DEA
Raman spectroscopy was used to determine the crystallites
and the purity of the crystal phases. Figure 5 shows the data
obtained from the Raman spectroscopy analyses. All of the
spectra exhibit prominent peaks at 331, 438, 583, and 379
(
ZnO in DEA) as a solvent displays a flowerlike morphology.
The flower consisted of flakes oriented in different angles and
directions like flower petals. The lengths of the flakes range
from 0.2 to 1.5 μm. ZnO in 1-propanol also forms a flakelike
morphology. These flakes combine to form a large aggregate.
The aggregation of flakes is irregular, making the surface very
−
1
cm (very weak).
PL spectroscopy was employed further to study any possible
structural defects in the synthesized materials. Figure 6
presents the PL spectra of the prepared ZnO nanocrystals.
Figure 2. SEM images of (a) ZnO in ethanol, (b) ZnO in acetonitrile, (c) ZnO in water, (d) ZnO in methanol, (e) ZnO in DEA, and (f) ZnO in 1-
propanol.
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Figure 3. continued
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Figure 3. TEM images of synthesized ZnO samples with different morphologies: (x) TEM image and (y) HR-TEM image. The insets show FFT
and inverse FFT images of the selected area.
All samples (except ZnO in ethanol and ZnO in acetonitrile)
show a strong peak positioned at 360−380 nm indicating a UV
emission peak. ZnO in water and ZnO in DEA demonstrate a
broad peak positioned at 520−550 nm (green emission peak).
ZnO in DEA exhibits a clear blue shift (from 378.6 to 364.4
nm), while ZnO in water shows a red shift (from 378.6 to
383.4 nm) as compared with ZnO in methanol. The intensities
of the UV emission for ZnO in water and ZnO in DEA were
weaker than the green emission.
3.4. Analysis of Textural Properties (surface area and
porosity). The textural properties of the synthesized materials
were determined by N sorption analysis. Figures 7 and 8
2
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Figure 7. N2 sorption isotherms of ZnO prepared in different
solvents.
Figure 4. UV−vis absorption spectra of the prepared ZnO samples.
Figure 8. BJH desorption pore size distributions of the prepared ZnO.
synthesized ZnO samples have type IV isotherms followed by a
hysteresis loop. The obtained sorption isotherms for the
synthesized ZnO materials were similar to those reported for
35,36
the ZnO prepared by other synthetic methods.
The
Figure 5. Raman spectra of the synthesized ZnO samples.
hysteresis loops of ZnO in acetonitrile, ZnO in ethanol, and
ZnO in methanol were somewhat larger than those of ZnO in
DEA, ZnO in H O, and ZnO in 1-propanol. The hysteresis
2
loop of commercial ZnO was the smallest among all of those of
the materials being studied.
These observations confirm that ZnO materials synthesized
in ZnO in ethanol, ZnO in methanol, and ZnO in acetonitrile
were porous and ZnO synthesized in other solvents (ZnO in
DEA, ZnO in water, ZnO in 1-propanol, and commercial
ZnO) were relatively less porous or almost nonporous. The
hysteresis loops for all of the materials occur at a range of high
relative pressures from 0.80 to near to their saturation point,
but for commercial ZnO, a hysteresis loop occurs from 0.5 to
0
.95 relative pressure. The hysteresis loops of ZnO in DEA are
35,36
consistent with the reported values.
The pore size distributions (Figure 8) of the prepared
materials were broad, and most of them are centered at around
3
0 nm. However, these distributions for commercial ZnO,
ZnO in DEA, and ZnO in water were so broad that there are
no observable centers. The BET data (Table 2) illustrate the
materials that have relatively large surface areas: ZnO in
ethanol (61 m /g) > ZnO in acetonitrile (53 m /g) > ZnO in
methanol (35 m /g). Besides surface area, pore diameters and
2
2
2
Figure 6. PL spectra of synthesized ZnO materials.
pore volumes of the materials change significantly with
solvents. ZnO in methanol has the largest pore diameter (29
nm) followed by ZnO in ethanol (16.8 nm).
present N sorption isotherms and BJH pore size distributions
for the materials, respectively. Figure 7 demonstrates that the
2
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Table 2. Summary of N Sorption Analysis of Prepared ZnO
Materials
catalytic reaction. The catalytic activity of the materials toward
coumarin synthesis was determined from the calculated
conversion (%) and selectivity (%) of the reactants and
products.
2
pore diameter
(nm)
pore volume
surface area
3
2
material
(cm /g)
(m /g)
3.6.1. Optimal Conditions for Catalytic Reaction. To
ZnO in methanol
ZnO in ethanol
ZnO in water
ZnO in DEA
ZnO in
acetonitrile
29
16.8
1.2
4.2
16.6
0.40
0.42
0.07
0.07
0.84
35
61
7
3
53
determine the optimal reaction conditions, a number of
control experiments were performed using ZnO in methanol as
a catalyst. Table 3 shows the results of the experiments. The
conversion (%) of the coumarin synthesis fluctuates with
different temperatures, heating times, and catalyst loadings.
The conversion (%) is favored at high temperatures (entries
ZnO in 1-
propanol
commercial ZnO
1.8
3.2
0.10
0.02
10
3
3
(
−5). However, a high temperature disfavors the selectivity
entries 4 and 5) of coumarin. The optimal amount of catalyst
for the coumarin synthesis reaction was 8.2 mg (5 mol %, entry
). If the amount of catalyst is smaller (entry 11) or larger
3.5. Basicity Analysis of ZnO Catalysts. The basicity of
8
the ZnO catalyst was estimated from CO adsorption analyses.
2
(entries 12 and 13) than 8.2 mg, the conversion decreases.
There was zero conversion (%) when the reaction was run
without a catalyst (ZnO) even for 24 h of reaction time (entry
The CO adsorption curves for the synthesized ZnO catalysts
2
are shown in Figure 9. The CO adsorption capability depends
2
1
0). The conversion increased with an increase in reaction
time until 24 h (entries 2 and 6−8) and then decreases (entry
). An experiment was performed using ethanol (5 mL) as a
9
solvent, and the conversion (%) of the reaction is lower than
that of the solvent-free condition (entry 14). Therefore, the
optimal conditions for the coumarin synthesis using ZnO in
methanol as a catalyst were a temperature of 100 °C, a reaction
time of 24 h, no solvent, and 8.2 mg of ZnO catalyst.
3.6.2. Catalytic Performance of the ZnO Samples. The
catalytic performance of the prepared materials for coumarin
synthesis is presented in Table 4. ZnO in methanol and ZnO
in ethanol exhibit 74% and 70% conversion, respectively, with
a 94% selectivity toward coumarin. ZnO in water, ZnO in
DEA, and ZnO in acetonitrile exhibit a lower conversion (in
the range of 35−49%) with a good selectivity (96%) compared
to those of ZnO in methanol and ZnO in ethanol. In
comparison, commercial ZnO shows a conversion of 51%,
which is lower than those of ZnO in methanol and ZnO in
ethanol but higher than those of ZnO in water and ZnO in
DEA. All of these data indicate that the catalytic activity
toward coumarin synthesis (via the Knoevenagel condensa-
tion) varies based on the ZnO morphology.
3.7. Reusability, Stability, and Heterogeneity of the
Catalysts. The reusable nature of ZnO in methanol (the best
catalyst) was studied by recycling the catalyst several times for
the reaction (Scheme 1). After each cycle, the catalyst was
regenerated from the mixture of reactants and products by
being filtered, washed (with ethanol using a centrifuge), and
dried. The catalyst was then heated at 400 °C for 4 h to
reactivate the catalyst’s reacting sites. An approximately 20%
loss of conversion was observed after the fourth cycle of the
Figure 9. CO adsorption curves (at 25 °C) of synthesized ZnO
2
materials.
on the basic sites present in a material. The catalyst with high
basicity (having highly basic sites) adsorbs large amounts of
CO . The basicity of the ZnO samples based on the CO
2
2
uptake (Figure 9) decreases in the following order: ZnO in 1-
propanol > ZnO in acetonitrile > ZnO in methanol > ZnO in
ethanol > ZnO in water > commercial ZnO > ZnO in DEA.
3.6. Studies of the Catalytic Activity of the Prepared
ZnO Samples. To investigate the effects of different
morphologies (and their associated changes in texture, defects,
band gaps, and basicity of ZnO materials) on catalytic activity,
coumarin synthesis reactions (using the Knoevenagel con-
densation) were studied using synthesized ZnO samples as
catalysts. Scheme 1 shows the schematic illustration of the
Scheme 1. Synthesis of Coumarin from o-Hydroxybenzaldehyde and Diethyl Malonate
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a
Table 3. Optimization of the Coumarin Synthesis Reaction (Scheme 1)
b
b
c
entry
reaction temperature (°C)
time (h)
catalyst amount (mg)
conversion (%)
selectivity (%) of coumarin
TON
1
2
3
4
5
6
7
8
9
1
1
1
1
1
80
100
120
140
160
100
100
100
100
100
100
100
100
100
3
3
3
3
3
8.2
8.2
8.2
8.2
8.2
8.2
8.2
8.2
8.2
no catalyst
4.1
16.4
32.8
8.2
3
8
58
>99
96
92
65
97
92
94
90
0
0.60
1.59
3.38
8.53
9.13
17
43
46
45
65
74
62
0
24
34
31
61
9
8.91
18
24
30
24
24
24
24
24
12.90
14.69
12.31
0.00
9.53
3.37
0
1
2
3
4
95
93
93
95
1.53
12.10
d
a
Reaction conditions: o-hydroxybenzaldehyde (2 mmol), ZnO (synthesized in methanol), diethyl malonate (2.5 mmol), solvent free, and a closed
b
c
vessel reaction. Conversion and selectivity were estimated from the concentration of the reactants and products detected using GC/MS. The
turnover numbwer (TON) is the moles of o-hydroxybenzaldehyde converted per mole of catalyst. Ethanol was used as a solvent.
d
a
Table 4. Synthesis of Coumarin from o-Hydroxybenzaldehyde and Diethyl Malonate (Scheme 1)
b
b
c
d
e
catalyst
conversion (%)
selectivity (%) of coumarin
TON
TOF
yield of coumarin (%)
ZnO in methanol
ZnO in ethanol
ZnO in 1-propanol
ZnO in water
ZnO in DEA
ZnO in acetonitrile
commercial ZnO
74
70
75
35
35
49
51
94
94
60
96
96
96
95
14.69
13.90
14.90
6.95
6.95
9.15
0.612
0.579
0.620
0.288
0.288
0.381
0.396
70
66
45
34
34
47
48
9.51
a
Reaction conditions: o-hydroxybenzaldehyde (2 mmol), diethyl malonate (2.5 mmol), ZnO (8.2 mg), solvent free, 100 °C, 24 h, and a closed
b
c
vessel reaction. Conversion and selectivity calculated from the concentration of the reactants and products detected by GC/MS. The turnover
number (TON) is the moles of o-hydroxybenzaldehyde converted per mole of catalyst. The turnover frequency (TOF) = TON/reaction time
d
e
(
hours). Yield = (conversion × selectivity)/100.
reaction (Figure 10a). The catalyst stability was investigated
using PXRD data obtained before and after the reuse of the
catalyst (Figure 10b). The similarity of the obtained XRD
patterns indicates the structural stability of the catalyst at least
until the fourth cycle of the reaction. In addition, leaching of
the catalyst was checked to test the heterogeneity of ZnO. The
conversion of the reactants remains constant (Figure 10c) even
after the catalyst had been filtered off from the reaction
medium at a conversion of 52%. All of these results imply good
stability, remarkable reusability, and heterogeneity of the ZnO
in water catalyst.
The catalytic activity of synthesized ZnO materials is mostly
governed by the combined effects of textural properties and
basicity.
4.1. Structure of Synthesized ZnO Samples. The
presence of no additional peaks in XRD and Raman data is
evidence of pure hexagonal crystal structures of the prepared
ZnO samples. Peak intensities and narrow widths of XRD
37
peaks are clear evidence of good crystallinity. The observed
peak broadening for ZnO in methanol and ZnO in ethanol
may be due to the slow growth of ZnO crystals during
synthesis, which is related to the reduction of crystallite sizes
for corresponding ZnO samples. The morphologies of ZnO are
also related to the nucleation process and the kinetics of crystal
4
. DISCUSSION
26
growth. ZnO nucleation and growth kinetics are influenced
26,32,38
In this work, various morphologies of ZnO were prepared
using different solvents via a solvothermal method. Synthesized
ZnO materials were characterized to study structural
modulation (primarily in surface, crystallinity, and vacancies)
that occurs with the solvents. The effects of the structural
modifications of ZnO on catalytic activities for coumarin
synthesis via Knoevenagel condensation were investigated.
ZnO in methanol and ZnO in ethanol show 74% and 70%
conversation, respectively, with 94% selectivity. The activity of
ZnO in methanol and ZnO in ethanol in coumarin synthesis is
best compared to those of other synthesized forms of ZnO and
commercial ZnO. ZnO with different morphologies provides
the materials with different exposed faces, textures, and
basicities, and all of these can affect the catalytic activities.
by the pH of the reaction mixture.
Introducing different
solvents into the reaction mixture changes the pH, which may
affect the nucleation and growth rate of ZnO resulting in
different morphologies. Crystal growth rates of ZnO also
depend on internal factors such as intermolecular bonding
preferences and dislocations as well as external factors such as
the precursor, temperature, pressure, concentration, and
interface−solvent interaction. These factors may also effect
26,38
the morphology of the ZnO samples.
In addition, solvent
polarity and saturated vapor pressure have a profound effect on
26,38−40
the nucleation and preferred orientation of crystals.
Hence, solvents with different boiling points and polarities can
produce different vapor pressures and interface−solvent
interactions during synthesis that might affect the growth
H
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Article
Figure 10. (a) Reusability test of the ZnO in methanol catalyst [reaction conditions: o-hydroxybenzaldehyde (2 mmol), diethyl malonate (2.5
mmol), ZnO (8.2 mg), solvent free, 100 °C, 24 h, and a closed vessel reaction]. (b) PXRD patterns of ZnO in methanol before and after reuse of
the catalyst. (c) Test of homogeneity (leaching test/hot filtration test). The catalyst was removed after 52% conversion (reaction conditions were
the same as those of the reusability test).
kinetics of ZnO crystals, leading to different morphologies.
water is also evident from XRD data that may result in a
narrow band gap. Consequently, the UV emission band of
ZnO in water shifted toward a higher wavelength. The
emergence of a broad and weak emission band (for ZnO in
water and ZnO in DEA) in the green region around 520−550
The morphology and crystallinity are connected to the band
4
1
gap. The observed UV absorption peak shifts for different
ZnO samples may be due to changes in the band gaps of the
materials that can influence the catalytic activity of ZnO
materials.
31,39
nm may be due to intrinsic defects of the crystals.
The
The representation of ZnO optical phonons is reported as
phenomenon of the green emission of ZnO due to the intrinsic
defects (zinc vacancy, oxygen vacancy, zinc interstitials, oxygen
42−44
A + E + 2B + 2E .
cm in the Raman spectra is assigned to the E (high) mode,
The observed strong peak at 438
1
1
1
2
−
1
interstitials, and antisite oxygen) has been reported pre-
2
43,44
31,33,49−51
which characterizes the wurtzite structure of ZnO.
The
viously.
The amounts of intrinsic defects and green
peak positioning around 583 cm− is ascribed to A (LO)/
1
emission intensity of ZnO crystals are correlated with each
1
39
E (LO) modes of ZnO, indicating the presence of oxygen
other. The variation of the morphology of ZnO is responsible
1
3
1,45
39
vacancies.
Hence, ZnO in 1-propanol has good crystallinity
for different levels of defects present in the ZnO samples.
and a smaller number of defects (oxygen vacancies). The peaks
positioned around 331 and 379 cm−1 are associated with
E (high) − E (low) and A (TO) modes, respectively.
4.2. Catalytic Activity of ZnO in Coumarin Synthesis
via the Knoevenagel Condensation Reaction. The order
of catalytic activity in terms of yield (conversion × selectivity)
is as follows: ZnO in methanol > ZnO in ethanol > ZnO in
acetonitrile > commercial ZnO > ZnO in 1-propanol > ZnO in
DEA or ZnO in water (Table 4). Although all of the
synthesized ZnO materials crystallize with the pure hexagonal
wurtzite crystal phase, they vary in their different textural and
structural properties due to synthesis with different solvents.
For example, crystallite sizes of the synthesized samples
increase in the following order: ZnO in ethanol < ZnO in
methanol < ZnO in water < ZnO in DEA < ZnO in acetonitrile
< ZnO in 1-propanol < commercial ZnO. However, the
activity is not correlated with the crystallite size (Table 1) of
the materials.
2
2
1
However, two other common Raman peaks for ZnO samples
−
1
that usually appear around 410 and 657 cm , which are
ascribed to E (TO) and A (LO) + E (low) modes,
1
2
2
4
3,46
respectively, are missing here.
The observed strong emission band around 360−380 nm
(
3.4−3.3 eV) in PL spectra of ZnO samples (except for ZnO
in ethanol and ZnO in acetonitrile) confirms the wide band
gaps of ZnO materials. The origin of UV emission in the ZnO
is believed to be due to the recombination of excited electrons
4
3,47,48
and holes.
The absence of the UV emission peak for
ZnO in ethanol may be due to low crystallinity, whereas that of
ZnO in acetonitrile is under investigation. The blue shift of the
UV emission peak for ZnO in DEA was indicative of a band
gap increase, whereas the red shift for ZnO in water could be
due to a band gap decrease. The low crystallinity of ZnO in
In some cases of heterogeneous catalysis, the surface area is
critical in influencing the catalytic performance. The large
surface area of a catalyst is associated with a large number of
I
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Inorganic Chemistry
Article
active sites, which enhances the catalytic activity of the
materials. Via comparison of the surface areas and percent
yields of coumarin, the surface areas of the ZnO samples are
found to be crucial and to play a vital role in the catalytic
activities. However, the catalytic activities (in terms of yield =
conversion × selectivity) are relatively high in the case of ZnO
in methanol and ZnO in ethanol, which have relatively large
surface areas, pore volumes, and pore diameters. This
observation suggests that the high activities of the synthesized
ZnO catalysts are due to the relatively large surface areas, pore
volumes, and pore sizes of the catalysts.
(Grant DE-FG02-86ER13622.A000). The authors thank Dr.
Md. Aman Uddin (Pasaogullari group member, University of
Connecticut) for helping with SEM measurements. The
authors also acknowledge Dr. Sourav Biswas (Suib group
member, University of Connecticut) for his suggestions about
these catalytic reactions.
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AUTHOR INFORMATION
Corresponding Author
E-mail: steven.suib@uconn.edu. Telephone: +1 860 486
797. Fax: +1 860 486 2981.
■
(
13) Bouasla, S.; Amaro-Gahete, J.; Esquivel, D.; Lop
́
́
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́
, C.; Teguiche, M.; Romero-Salguero, F. J.
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ORCID
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Steven L. Suib: 0000-0003-3073-311X
Notes
The authors declare no competing financial interest.
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