Syntheses of ZnO with Different Morphologies: Catalytic Activity toward

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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*,†,‡,§ Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/09/19. For personal use only.

†

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. 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. 4125 mesoporous catalysts for the Knoevenagel condensation. These catalysts exhibit good catalytic activities with recyclability 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 ZnO can easily be altered26−28 as well as activities. Thus, ZnO is considered to be a potential replacement for existing heterogeneous catalysts. For instance, the Kumar group reported the use of ZnO nanoparticles in coumarin synthesis;29 they focused on establishing a general synthetic strategy for various coumarins using microwave-derived ZnO nanoparticles. The properties of ZnO depend on various morphologies,26,28 which change with synthesis processes.27,28 Among the various properties, textural properties (morphologies, surface areas, and surface defects) are crucial for tailoring the activity of the materials in heterogeneous catalysis.30,31 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

1. INTRODUCTION Coumarins are heterocyclic compounds having a benzo-αpyrone moiety in their structure. These compounds have been widely studied because of their importance as antidepressants,1 antimicrobials,2 antioxidants,3 anti-inflammatories,4 antinociceptives,4 antitumor agents,5 antiasthmatics,6 antivirals (including anti-HIV),7 and anticoagulants.8 There are several methods utilized to prepare coumarins. Some examples are Knoevenagel condensation,9,10 Perkin reaction,11 Pechmann reaction,12,13 and Wittig reactions.14 Among the various available methods, the Knoevenagel condensation is the most useful method for synthesizing coumarins and their derivatives.10 The Knoevenagel condensation is usually catalyzed by weak acids and bases,15,16 and most of those homogeneous 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. A variety of heterogeneous catalysts have been used for the Knoevenagel condensation reaction.10,17−23 Besides these, the Al-Lohedan group has recently developed melamine-immobilized MCM-4116 and urea-functionalized SBA-15,24 and the Hamid group synthesized DL-alanine-functionalized MCM© XXXX American Chemical Society

Received: January 8, 2019

A

DOI: 10.1021/acs.inorgchem.9b00053 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

an Ar+ ion laser (514.5 nm). A NOVA 2000e instrument (Quantachrome) was used for N2 sorption analysis using N2 gas as 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 2oxo-2H-chromene-3-carboxylate (coumarin) from o-hydroxybenzaldehyde 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.

synthesized different morphologies of ZnO with different solvents and investigated the photocatalytic activities of the materials.26 They discussed the influence of solvents in obtaining different ZnO morphologies and the photodegradation activity of the obtained materials. Because the crystallographic morphology of ZnO depends on the precursor, solvent, pH, and time and temperature of a reaction,32−34 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(CH3COO)2·2H2O, ACS reagent, ≥98%], commercial ZnO powder (ACS reagent, ≥99.0%), diethylamine (DEA) [40% (w/w) aqueous solution], and 1propanol (Anhydrous, 99.7%) were purchased from Sigma-Aldrich. Sodium hydroxide (ACS reagent, ≥98%), methanol (HPLC grade, 99.9%), and acetonitrile (liquid chromatography/mass spectrometry reagent) were purchased from J. T. Baker. Ethanol (ACS grade, 99.98%) was purchased from Pharmco-Aaper. 2.2. ZnO Preparation. Approximately 1.097 g (0.1 M) of Zn(CH3COO)2·2H2O and 0.400 g (0.4 M) of NaOH were added to two separate beakers. Zn(CH3COO)2·2H2O was dissolved in 50 mL 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 spectroscopy, and room-temperature photoluminescence (PL) spectroscopy. 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

3. RESULTS 3.1. Structural Characterization. Structural characterization of ZnO nanocrystals was performed using PXRD techniques. X-ray diffraction patterns (Figure 1) indicate that

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 B


Article

Inorganic Chemistry

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), (100), (002), (100), and (002) planes, respectively. 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 of ZnO.26 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. 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 cm−1 (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.

with acetonitrile (ZnO in acetonitrile), 1-propanol (ZnO in 1propanol), 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 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 ZnO in DEA ZnO in acetonitrile ZnO in 1-propanol Comm ZnO (commercial ZnO)

10 12 31 38 45 56 81

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. 3.2. 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 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 (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

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 1propanol. C


Article

Inorganic Chemistry

Figure 3. continued

D


Article

Inorganic Chemistry

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.

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 N2 sorption analysis. Figures 7 and 8

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 E


Article

Inorganic Chemistry

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 the ZnO prepared by other synthetic methods.35,36 The hysteresis loops of ZnO in acetonitrile, ZnO in ethanol, and ZnO in methanol were somewhat larger than those of ZnO in DEA, ZnO in H2O, and ZnO in 1-propanol. The hysteresis 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 consistent with the reported values.35,36 The pore size distributions (Figure 8) of the prepared materials were broad, and most of them are centered at around 30 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 m2/g) > ZnO in acetonitrile (53 m2/g) > ZnO in methanol (35 m2/g). Besides surface area, pore diameters and 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).

Figure 5. Raman spectra of the synthesized ZnO samples.

Figure 6. PL spectra of synthesized ZnO materials.

present N2 sorption isotherms and BJH pore size distributions for the materials, respectively. Figure 7 demonstrates that the F


Article

Inorganic Chemistry

catalytic reaction. The catalytic activity of the materials toward coumarin synthesis was determined from the calculated conversion (%) and selectivity (%) of the reactants and products. 3.6.1. Optimal Conditions for Catalytic Reaction. To 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 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 8). If the amount of catalyst is smaller (entry 11) or larger (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 10). The conversion increased with an increase in reaction time until 24 h (entries 2 and 6−8) and then decreases (entry 9). An experiment was performed using ethanol (5 mL) as a 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 condensation) 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

Table 2. Summary of N2 Sorption Analysis of Prepared ZnO Materials material ZnO in methanol ZnO in ethanol ZnO in water ZnO in DEA ZnO in acetonitrile ZnO in 1propanol commercial ZnO

pore diameter (nm)

pore volume (cm3/g)

surface area (m2/g)

29 16.8 1.2 4.2 16.6

0.40 0.42 0.07 0.07 0.84

35 61 7 3 53

1.8

0.10

10

3.2

0.02

3

3.5. Basicity Analysis of ZnO Catalysts. The basicity of the ZnO catalyst was estimated from CO2 adsorption analyses. The CO2 adsorption curves for the synthesized ZnO catalysts are shown in Figure 9. The CO2 adsorption capability depends

Figure 9. CO2 adsorption curves (at 25 °C) of synthesized ZnO materials.

on the basic sites present in a material. The catalyst with high basicity (having highly basic sites) adsorbs large amounts of CO2. The basicity of the ZnO samples based on the CO2 uptake (Figure 9) decreases in the following order: ZnO in 1propanol > 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 condensation) 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

G


Article

Inorganic Chemistry Table 3. Optimization of the Coumarin Synthesis Reaction (Scheme 1)a entry

reaction temperature (°C)

time (h)

catalyst amount (mg)

conversionb (%)

selectivityb (%) of coumarin

TONc

1 2 3 4 5 6 7 8 9 10 11 12 13 14d

80 100 120 140 160 100 100 100 100 100 100 100 100 100

3 3 3 3 3 9 18 24 30 24 24 24 24 24

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 17 43 46 45 65 74 62 0 24 34 31 61

58 >99 96 92 65 97 92 94 90 0 95 93 93 95

0.60 1.59 3.38 8.53 9.13 8.91 12.90 14.69 12.31 0.00 9.53 3.37 1.53 12.10

a Reaction conditions: o-hydroxybenzaldehyde (2 mmol), ZnO (synthesized in methanol), diethyl malonate (2.5 mmol), solvent free, and a closed vessel reaction. bConversion and selectivity were estimated from the concentration of the reactants and products detected using GC/MS. cThe turnover numbwer (TON) is the moles of o-hydroxybenzaldehyde converted per mole of catalyst. dEthanol was used as a solvent.

Table 4. Synthesis of Coumarin from o-Hydroxybenzaldehyde and Diethyl Malonate (Scheme 1)a catalyst

conversionb (%)

selectivityb (%) of coumarin

TONc

TOFd

yield of coumarine (%)

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 9.51

0.612 0.579 0.620 0.288 0.288 0.381 0.396

70 66 45 34 34 47 48

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. bConversion and selectivity calculated from the concentration of the reactants and products detected by GC/MS. cThe turnover number (TON) is the moles of o-hydroxybenzaldehyde converted per mole of catalyst. dThe turnover frequency (TOF) = TON/reaction time (hours). eYield = (conversion × selectivity)/100.

a

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 peaks are clear evidence of good crystallinity.37 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 growth.26 ZnO nucleation and growth kinetics are influenced by the pH of the reaction mixture.26,32,38 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 the morphology of the ZnO samples.26,38 In addition, solvent polarity and saturated vapor pressure have a profound effect on the nucleation and preferred orientation of crystals.26,38−40 Hence, solvents with different boiling points and polarities can produce different vapor pressures and interface−solvent interactions during synthesis that might affect the growth

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.

4. DISCUSSION 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. H


Article

Inorganic Chemistry

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).

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 nm may be due to intrinsic defects of the crystals.31,39 The phenomenon of the green emission of ZnO due to the intrinsic defects (zinc vacancy, oxygen vacancy, zinc interstitials, oxygen interstitials, and antisite oxygen) has been reported previously.31,33,49−51 The amounts of intrinsic defects and green emission intensity of ZnO crystals are correlated with each other.39 The variation of the morphology of ZnO is responsible for different levels of defects present in the ZnO samples.39 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. 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

kinetics of ZnO crystals, leading to different morphologies. The morphology and crystallinity are connected to the band gap.41 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. The representation of ZnO optical phonons is reported as A1 + E1 + 2B1 + 2E2.42−44 The observed strong peak at 438 cm−1 in the Raman spectra is assigned to the E2(high) mode, which characterizes the wurtzite structure of ZnO.43,44 The peak positioning around 583 cm−1 is ascribed to A1(LO)/ E1(LO) modes of ZnO, indicating the presence of oxygen vacancies.31,45 Hence, ZnO in 1-propanol has good crystallinity and a smaller number of defects (oxygen vacancies). The peaks positioned around 331 and 379 cm−1 are associated with E2(high) − E2(low) and A1(TO) modes, respectively. However, two other common Raman peaks for ZnO samples that usually appear around 410 and 657 cm−1, which are ascribed to E 1 (TO) and A 2 (LO) + E 2 (low) modes, respectively, are missing here.43,46 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 and holes.43,47,48 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 I


Article

Inorganic Chemistry

(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.

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. There are two factors that primarily control the basicity of a material. These are the negative charges on the anions and the coordination of the anions. The coordination of anions is affected by the imperfections and/or defects in the materials.52 Hence, the surface basicity changes depending on the defects of the materials. Therefore, the observed basicity of the ZnO catalysts may be due to different intrinsic defects, which are governed by the different morphologies. The variation of exposed planes (TEM data) with morphology may also contribute to the number of basic sites of the ZnO catalysts. Additional studies are required to correlate the basicity of the catalyst to the amount of defects and/or types of defect present in the materials. According to the literature, the Knoevenagel condensation is usually catalyzed by mild acids and bases.53,54 Xu et al. suggested that the Knoevenagel condensation prefers the weak strength of acids and bases.20 Hence, the strength of the basicity or acidity of a catalyst plays a significant role in the reaction. The intermediate basicity of ZnO in methanol may favor coumarin formation. However, further studies are required to determine the exact correlation of the observed activity (Table 4) and basicity (Figure 9).

■

(1) Sashidhara, K. V.; Kumar, A.; Chatterjee, M.; Rao, K. B.; Singh, S.; Verma, A. K.; Palit, G. Discovery and Synthesis of Novel 3Phenylcoumarin Derivatives as Antidepressant Agents. Bioorg. Med. Chem. Lett. 2011, 21, 1937−1941. (2) Chimenti, F.; Bizzarri, B.; Bolasco, A.; Secci, D.; Chimenti, P.; Granese, A.; Carradori, S.; Rivanera, D.; Zicari, A.; Scaltrito, M. M.; et al. Synthesis, Selective Anti-Helicobacter Pylori Activity, and Cytotoxicity of Novel N-Substituted-2-Oxo-2H-1-Benzopyran-3Carboxamides. Bioorg. Med. Chem. Lett. 2010, 20, 4922−4926. (3) Kostova, I.; Bhatia, S.; Grigorov, P.; Balkansky, S.; S. Parmar, V.; K. Prasad, A.; Saso, L. Coumarins as Antioxidants. Curr. Med. Chem. 2011, 18, 3929−3951. (4) Bansal, Y.; Sethi, P.; Bansal, G. Coumarin: A Potential Nucleus for Anti-Inflammatory Molecules. Med. Chem. Res. 2013, 22, 3049− 3060. (5) Lacy, A.; O’Kennedy, R. Studies on Coumarins and CoumarinRelated Compounds to Determine Their Therapeutic Role in the Treatment of Cancer. Curr. Pharm. Des. 2004, 10, 3797−3811. (6) Sánchez-Recillas, A.; Navarrete-Vázquez, G.; Hidalgo-Figueroa, S.; Rios, M. Y.; Ibarra-Barajas, M.; Estrada-Soto, S. Semisynthesis, Ex Vivo Evaluation, and SAR Studies of Coumarin Derivatives as Potential Antiasthmatic Drugs. Eur. J. Med. Chem. 2014, 77, 400−408. (7) Hwu, J. R.; Lin, S. Y.; Tsay, S. C.; De Clercq, E.; Leyssen, P.; Neyts, J. Coumarin-Purine Ribofuranoside Conjugates as New Agents against Hepatitis c Virus. J. Med. Chem. 2011, 54, 2114−2126. (8) Peng, X.-M.; Damu, G. L. V; Zhou, C.-H. Current Developments of Coumarin Compounds in Medicinal Chemistry. Curr. Pharm. Des. 2013, 19, 3884−3930. (9) González-González, A.; Aparicio-Solano, D. M.; AguilarMariscal, H.; Gómez-Rivera, A.; Roa, L. F.; Alvarado-Sánchez, C.; Lobato-García, C. E.; Romero-Ceronio, N. Synthesis of 3-Carboxylated Coumarins by Knoevenagel Condensation and Exploratory AntiInflammatory Activity Evaluation by in Vivo Model. Am. J. Org. Chem. 2016, 6, 17−28. (10) Vekariya, R. H.; Patel, H. D. Recent Advances in the Synthesis of Coumarin Derivatives via Knoevenagel Condensation: A Review. Synth. Commun. 2014, 44, 2756−2788. (11) Vilar, S.; Quezada, E.; Santana, L.; Uriarte, E.; Yánez, M.; Fraiz, N.; Alcaide, C.; Cano, E.; Orallo, F. Design, Synthesis, and Vasorelaxant and Platelet Antiaggregatory Activities of CoumarinResveratrol Hybrids. Bioorg. Med. Chem. Lett. 2006, 16, 257−261. (12) Khaligh, N. G. Synthesis of Coumarins via Pechmann Reaction Catalyzed by 3-Methyl-1-Sulfonic Acid Imidazolium Hydrogen Sulfate as an Efficient, Halogen-Free and Reusable Acidic Ionic Liquid. Catal. Sci. Technol. 2012, 2, 1633−1636. (13) Bouasla, S.; Amaro-Gahete, J.; Esquivel, D.; López, M. I.; Jiménez-Sanchidrián, C.; Teguiche, M.; Romero-Salguero, F. J. Coumarin Derivatives Solvent-Free Synthesis under Microwave Irradiation over Heterogeneous Solid Catalysts. Molecules 2017, 22, 2072. (14) Belavagi, N. S.; Deshapande, N.; Sunagar, M. G.; Khazi, I. A. M. A Practical One-Pot Synthesis of Coumarins in Aqueous Sodium Bicarbonate via Intramolecular Wittig Reaction at Room Temperature. RSC Adv. 2014, 4, 39667−39671. (15) Zhou, J.-F.; Song, Y.-Z.; Lv, J.-S.; Gong, G.-X.; Tu, S. Facile One-Pot, Multicomponent Synthesis of Pyridines Under Microwave Irradiation. Synth. Commun. 2009, 39, 1443−1450. (16) Appaturi, J. N.; Jothi Ramalingam, R.; Al-Lohedan, H. A. Synthesis, Characterization and Catalytic Activity of Melamine

5. CONCLUSIONS ZnO morphologies are synthesized by changing solvents with the solvothermal method. Crystallinities of the prepared ZnO samples vary with morphologies that lead to changes in band gaps and intrinsic defects (oxygen vacancies) of the ZnO materials. The catalytic activity of the ZnO samples for coumarin synthesis via the Knoevenagel condensation varies with morphology. Different morphologies of ZnO provide different textural properties and basicities of the catalyst, which govern the activity of the materials. ZnO synthesized using methanol shows the highest activity in terms of yield with a TON of 14.69. The enhanced activity of the ZnO in methanol is attributed to the combined effects of its textural properties (relatively large surface area, pore volume, and pore size) and intermediate basic strength.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +1 860 486 2797. Fax: +1 860 486 2981. ORCID

Steven L. Suib: 0000-0003-3073-311X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Biological and Geological Sciences, for support of this research J


Article

Inorganic Chemistry Immobilized MCM-41 for Condensation Reactions. J. Porous Mater. 2018, 25, 629−641. (17) Sarmah, B.; Srivastava, R. Highly Efficient and Recyclable Basic Ionic Liquids Supported on SBA-15 for the Synthesis of Substituted Styrenes, Carbinolamides, and Naphthopyrans. Mol. Catal. 2017, 427, 62−72. (18) Aider, N.; Smuszkiewicz, A.; Pérez-Mayoral, E.; Soriano, E.; Martín-Aranda, R. M.; Halliche, D.; Menad, S. Amino-Grafted SBA15 Material as Dual Acid-Base Catalyst for the Synthesis of Coumarin Derivatives. Catal. Today 2014, 227, 215−222. (19) Parvin, M. N.; Jin, H.; Ansari, M. B.; Oh, S. M.; Park, S. E. Imidazolium Chloride Immobilized SBA-15 as a Heterogenized Organocatalyst for Solvent Free Knoevenagel Condensation Using Microwave. Appl. Catal., A 2012, 413−414, 205−212. (20) Xu, L.; Wang, C.; Guan, J. Preparation of Acid-Base Bifunctional Mesoporous KIT-6 (KIT: Korea Advanced Institute of Science and Technology) and Its Catalytic Performance in Knoevenagel Reaction. J. Solid State Chem. 2014, 213, 250−255. (21) Calvino-Casilda, V.; Olejniczak, M.; Martín-Aranda, R. M.; Ziolek, M. The Role of Metallic Modifiers of SBA-15 Supports for Propyl-Amines on Activity and Selectivity in the Knoevenagel Reactions. Microporous Mesoporous Mater. 2016, 224, 201−207. (22) Wach, A.; Drozdek, M.; Dudek, B.; Szneler, E.; Kustrowski, P. Control of Amine Functionality Distribution in Polyvinylamine/SBA15 Hybrid Catalysts for Knoevenagel Condensation. Catal. Commun. 2015, 64, 52−57. (23) Parida, K. M.; Mallick, S.; Sahoo, P. C.; Rana, S. K. A Facile Method for Synthesis of Amine-Functionalized Mesoporous Zirconia and Its Catalytic Evaluation in Knoevenagel Condensation. Appl. Catal., A 2010, 381, 226−232. (24) Appaturi, J. N.; Johan, M. R.; Ramalingam, R. J.; Al-Lohedan, H. A. Highly Efficient Green Mesostructured Urea Functionalized on SBA-15 Catalysts for Selective Synthesis of Benzlidenemalononitrile. Microporous Mesoporous Mater. 2018, 256, 67−74. (25) Appaturi, J. N.; Selvaraj, M.; Abdul Hamid, S. B.; Bin Johan, M. R. Synthesis of 3-(2-Furylmethylene)-2, 4-Pentanedione Using DLAlanine Functionalized MCM-41 Catalyst via Knoevenagel Condensation Reaction. Microporous Mesoporous Mater. 2018, 260, 260− 269. (26) Xu, L.; Hu, Y. L.; Pelligra, C.; Chen, C. H.; Jin, L.; Huang, H.; Sithambaram, S.; Aindow, M.; Joesten, R.; Suib, S. L. ZnO with Different Morphologies Synthesized by Solvothermal Methods for Enhanced Photocatalytic Activity. Chem. Mater. 2009, 21, 2875− 2885. (27) Ö zgür, Ü .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doǧan, S.; Avrutin, V.; Cho, S. J.; Morkoç, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301. (28) Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter 2004, 16, R829−R858. (29) Kumar, B. V.; Naik, H. S. B.; Girija, D.; Kumar, B. V. ZnO Nanoparticle as Catalyst for Efficient Green One-Pot Synthesis of Coumarins through Knoevenagel Condensation. J. Chem. Sci. 2011, 123, 615−621. (30) Kim, D.; Huh, Y. Morphology-Dependent Photocatalytic Activities of Hierarchical Microstructures of ZnO. Mater. Lett. 2011, 65, 2100−2103. (31) Wang, Y.; Li, X.; Wang, N.; Quan, X.; Chen, Y. Controllable Synthesis of ZnO Nanoflowers and Their Morphology-Dependent Photocatalytic Activities. Sep. Purif. Technol. 2008, 62, 727−732. (32) Wang, H.; Xie, J.; Yan, K.; Duan, M. Growth Mechanism of Different Morphologies of ZnO Crystals Prepared by Hydrothermal Method. J. Mater. Sci. Technol. 2011, 27, 153−158. (33) Yin, J.; Gao, F.; Wei, C.; Lu, Q. Water Amount Dependence on Morphologies and Properties of ZnO Nanostructures in DoubleSolvent System. Sci. Rep. 2015, 4, 3736. (34) Boz, I.; Kaluza, S.; Boroǧlu, M.; Muhler, M. Synthesis of High Surface Area ZnO Powder by Continuous Precipitation. Mater. Res. Bull. 2012, 47, 1185−1190.

(35) Mikhail, R. S.; Guindy, N. M.; Ali, I. T. Variation of the Specific Surface Area and Porosity of Zinc Oxide Prepared in Various Atmospheres. J. Appl. Chem. Biotechnol. 1976, 26, 199−206. (36) Jadhav, N. A.; Singh, P. K.; Rhee, H. W.; Bhattacharya, B. Effect of Variation of Average Pore Size and Specific Surface Area of ZnO Electrode (WE) on Efficiency of Dye-Sensitized Solar Cells. Nanoscale Res. Lett. 2014, 9, 1−8. (37) Shakil, M. R.; El-Sawy, A. M.; Tasnim, H.; Meguerdichian, A. G.; Jin, J.; Dubrosky, J. P.; Suib, S. L. Single-Doped and Multidoped Transition-Metal (Mn, Fe, Co, and Ni) ZnO and Their Electrocatalytic Activities for Oxygen Reduction Reaction. Inorg. Chem. 2018, 57, 9977−9987. (38) Cheng, B.; Samulski, E. T. Hydrothermal Synthesis of OneDimensional ZnO Nanostructures with Different Aspect Ratios. Chem. Commun. 2004, 986−987. (39) Zhang, J.; Sun, L.; Yin, J.; Su, H.; Liao, C.; Yan, C. Control of ZnO Morphology via a Simple Solution Route. Chem. Mater. 2002, 14, 4172−4177. (40) Kunjara Na Ayudhya, S.; Tonto, P.; Mekasuwandumrong, O.; Pavarajarn, V.; Praserthdam, P. Solvothermal Synthesis of ZnO with Various Aspect Ratios Using Organic Solvents. Cryst. Growth Des. 2006, 6, 2446−2450. (41) Foo, K. L.; Kashif, M.; Hashim, U.; Liu, W. W. Effect of Different Solvents on the Structural and Optical Properties of Zinc Oxide Thin Films for Optoelectronic Applications. Ceram. Int. 2014, 40, 753−761. (42) Millot, M.; Gonzalez, J.; Molina, I.; Salas, B.; Golacki, Z.; Broto, J. M.; Rakoto, H.; Goiran, M. Raman Spectroscopy and Magnetic Properties of Bulk ZnO:Co Single Crystal. J. Alloys Compd. 2006, 423, 224−227. (43) Yousefi, R.; Zak, a. K.; Jamali-Sheini, F. The Effect of Group-I Elements on the Structural and Optical Properties of ZnO Nanoparticles. Ceram. Int. 2013, 39, 1371−1377. (44) Karamat, S.; Rawat, R. S.; Lee, P.; Tan, T. L.; Ramanujan, R. V. Structural, Elemental, Optical and Magnetic Study of Fe Doped ZnO and Impurity Phase Formation. Prog. Nat. Sci. 2014, 24, 142−149. (45) Pradhan, A. K.; Zhang, G. B.; Loutts, U. N.; Roy, Y. C.A. B.; Cui, Y.; Burger, A. Structural and Speectroscopic Characteristics of ZnO and ZnO: Er3+ Nanostructures. J. Phys.: Condens. Matter 2004, 16, 7123−7129. (46) Kuriakose, S.; Satpati, B.; Mohapatra, S. Enhanced Photocatalytic Activity of Co Doped ZnO Nanodisks and Nanorods Prepared by a Facile Wet Chemical Method. Phys. Chem. Chem. Phys. 2014, 16, 12741. (47) Ashokkumar, M.; Muthukumaran, S. Electrical, Dielectric, Photoluminescence and Magnetic Properties of ZnO Nanoparticles Co-Doped with Co and Cu. J. Magn. Magn. Mater. 2015, 374, 61−66. (48) Sebastian, K. C.; Chawda, M.; Jonny, L.; Bodas, D. Structural, Magnetic and Optical Studies of (Zn0.90Co0.05Ni0.05O) DMS. Mater. Lett. 2010, 64, 2269−2272. (49) Pandey, S. K.; Awasthi, V.; Verma, S.; Mukherjee, S. Blue Electroluminescence from Sb-ZnO/Cd-ZnO/Ga-ZnO Heterojunction Diode Fabricated by Dual Ion Beam Sputtering. Opt. Express 2014, 22, 30983. (50) Xu, C.; Xu, G.; Liu, Y.; Wang, G. A Simple and Novel Route for the Preparation of ZnO Nanorods. Solid State Commun. 2002, 122, 175−179. (51) Guo, L.; Yang, S.; Yang, C.; Yu, P.; Wang, J.; Ge, W.; Wong, G. K. L. Synthesis and Characterization of Poly(Vinylpyrrolidone)Modified Zinc Oxide Nanoparticles. Chem. Mater. 2000, 12, 2268− 2274. (52) Choudhary, R. V.; Rane, H. V. Acidity/Basicity of Rare-Earth Oxides and Their Catalytic Activity in Oxidative Co. J. Catal. 1991, 130, 411−422. (53) Bigi, F.; Chesini, L.; Maggi, R.; Sartori, G. Montmorillonite KSF as an Inorganic, Water Stable, and Reusable Catalyst for the Knoevenagel Synthesis of Coumarin-3-Carboxylic Acids. J. Org. Chem. 1999, 64, 1033−1035. K


Article

Inorganic Chemistry (54) Heravi, M.; Bakhtiari, K.; Taheri, S.; Oskooie, H. A Straightforward Method for the Synthesis of Functionalized Trisubstituted Alkenes through Na2S/Al2O3 Catalyzed Knoevenagel Condensation. J. Chin. Chem. Soc. 2007, 54, 1557−1560.

L


Syntheses of ZnO with Different Morphologies: Catalytic Activity toward

Recommend Documents