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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Targeted Development of Sustainable Green Catalysts for Oxidation of Alcohols via Tungstate-Decorated Multifunctional Amphiphilic Carbon Quantum Dots Masoumeh Mohammadi,† Aram Rezaei,*,‡ Ardeshir Khazaei,*,‡ Shu Xuwei,§ and Zheng Huajun§ †

Faculty of Chemistry, Bu-Ali Sina University, Hamedan P.O. Box 38695-65178, Iran Nano Drug Delivery Research Center, Health Technology Institute, Kermanshah University of Medical Sciences, Kermanshah 67145-1673, Iran § Department of Applied Chemistry, Zhejiang University of Technology, Hangzhou 310032, China

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‡

S Supporting Information *

ABSTRACT: Achieving green and sustainable chemical processes by replacing organic solvents with water has always been one of the green chemistry goals and a challenging topic for chemists. However, the poor solubility of organic materials is a major limitation to achieving this goal, especially in alcohol oxidation. In this contribution, the development and design of amphiphilic catalysts via abundant, safe, cheaper, and more biocompatible sources have received notable attention. To this purpose, herein, our group successfully synthesized a new multifunctional amphiphilic carbon quantum dot (CQD) composed of 1-aminopropyl-3-methyl-imidazolium chloride ([APMim][Cl]), dodecylamine (DDA), and citric acid (CA) (denoted as CQDs@DDA-IL/Cl) using a one-pot hydrothermal route. The CQDs@DDA-IL/Cl was then utilized as an amphiphilic stabilizer for anchoring tungsten ions using an anion-exchange method (marked as CQDs@DDA-IL/W). The CQDs@DDA-IL/W as a reusable catalyst selectivity mediated the oxidation of alcoholic substrates with stoichiometric H2O2 in water solvent. The extraordinary performance of our catalyst was attributable to the coexistence of ionic liquid (IL) and DDA upon the surface of the CQDs@DDA-IL/W, which plays a main duty in the hydrophobic/hydrophilic balance, and significantly increase the catalyst compatibility in the aqueous medium with the purpose of removing organic solvents. As a result, the great mass transfer occurs in the two-phase medium using this amphiphilic nanocatalyst without any phase transfer catalyst (PTC) or other additives. The 100% selectivity, excellent turnover number (TON) and turnover frequency (TOF), high yield, almost complete and fast conversion of alcohol to the desired aldehydes and ketones without more oxidation, and easy and no-trouble isolation of product and catalyst are outstanding features of this catalytic system. KEYWORDS: amphiphilic catalyst, multifunctional carbon quantum dot, selective alcohol oxidation, supported-ionic liquid, tungsten species

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INTRODUCTION The high selective and controllable oxidation of alcohols is one of the predominant and challenging reactions in synthetic chemistry because carbonyl compounds derived from them are widely used as starting precursor in the synthesis and design of high-value chemicals. For economic and environmental reasons, to avoid the generation and accumulation of toxic waste from traditional expensive, toxic, and highly corrosive reagents, H2O2-based catalytic oxidation as a green alternative has received growing attention.1−3 In this regard, the idea of supported-ionic liquid phase (SILP) catalytic systems has become considerably the focus of attention of researchers, because of the amalgamation the advantages of ILs with various solid supports.4−7 Therefore, the use of ILs grafted on solid supports for the immobilization of catalytic sites has © XXXX American Chemical Society

received widespread attention. Because tungsten species are catalytically very active, and they have a high potential to active of H2O2, therefore, their immobilization via SILP technology to reach top level of efficiency and selectivity toward oxidation reactions have been extensively utilized.8−18 However, a major limitation of these heterogeneous supported catalysts is that the reaction speed of at least one degree lower compared with the best homogeneous counterparts. This decrease in the rate of aforementioned reactions in the presence of H2O2 as cocatalyst is attributed to the formation of a water/oil twophase system, which is the main factor in reducing the mass Received: May 7, 2019 Accepted: August 14, 2019

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DOI: 10.1021/acsami.9b07961 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces transfer during reactions.19,20 Hence, amphiphilic heterogeneous catalysts with a tunable hydrophilicity/hydrophobicity property were developed that act as an interlink between the organic substrate in the oil layer and H2O2 in the aqueous layer.21−24 Catalyst support plays a substantial influence on the performance of supported catalysts. Because of unique properties such as excellent specific surface area, great porosity, excellent electrical conductivity, easy availability, and low toxicity, carbon-based materials are a popular choice as catalyst support and carrier for delivery of catalytic active centers to accelerate the chemical process.25−28 Carbon quantum dots (CQDs) as splendid carbon nanomaterials with a size below 10 nm have received appreciable interest in recent years. A surface enriched with the oxygen of these NPs contributes greatly to the easy functionalization/ modification of these zero-dimensional (0D) nanomaterials.29−34 On the basis of the tunability of surface functionalization of these nanomaterials, the design of amphiphilic CQDs can create a revolution in the carbon nanomaterials world and lead to their broader application in various fields compared with those only used in the aqueous or oil phase. However, most of the reported CQDs in previous works are suspended in polar solvents because of the abundant of hydroxyl and carboxylate groups on their surfaces, which is enhance the hydrophilicity and stability of CQDs in an aqueous medium. Also, diminution of oxygen functional groups with hydrophobic long-chain molecules can also lead to the formation of CQDs with hydrophobic characteristic. Therefore, simultaneous attendance of both hydrophilic and hydrophobic groups on the CQDs surfaces to prepare amphiphilic CQDs is a key prerequisite. Suitable surface modification using amidation reaction between amine modifiers and surface −COOH functional groups is a routine procedure for the fabrication of amphiphilic CQDs.35−38 However, there are a few reports in the field of amphiphilic CQDs synthesis and their application, especially their modification using ILs as a surface modifier.30,31,39−41 On the basis of these unique susceptibilities of CQDs, as well as easy and diverse synthesis methods from inexpensive and available sources, and also their excellent performance as a metal-free catalyst or catalyst support in the oxidation reactions especially alcohols oxidation,42−46 amphiphilic CQDs can be attractive candidates as the catalyst basis for the stabilization of catalytic species. Considering these preconceptions and to develop amphiphilic CQD-supported catalysts, our research team was encouraged to design a heterogeneous catalyst based on multifunctional amphiphilic CQDs as catalyst support and tungstate ions (WO4=) as a catalytically active center. In this research work, we designed a new heterogeneous catalyst based on a novel multifunctional amphiphilic carbon quantum dots (CQDs) using CA as a carbon source and DDA and [APMim][Cl] as surface modifying agents. First, CQDs functionalized with [APMim][Cl] and DDA (denoted as CQDs@DDA-IL/Cl) were successfully fabricated by a one-pot hydrothermal procedure. Then, the CQDs@DDA-IL/Cl was utilized as a new amphiphilic support to immobilize the WO4= ions by an anion-exchange technique. The catalytic activity of the CQDs@DDA-IL/W as a highly selective catalyst was investigated in the oxidation of various alcoholic substrates to carbonyl compounds without overoxidation to acid in the presence of H2O2 as a safe oxidant in water medium. The prominent benefits of this new support are that it creates excellent dispersity in both aqueous and organic media and

reduces the metal leaching during the reaction process. Consequently, its catalytic activity is maintained after six successive cycles. Therefore, the design of carbon-based multifunctional amphiphilic catalyst can create the revolution in the green chemistry field and catalysts world via eliminating pollutant species and increasing the yield and efficiency of the chemical reaction.

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EXPERIMENTAL SECTION

Materials and Apparatus. Used solvents and chemicals were supplied from Fluka (Switzerland) or Merck (Germany), and used without any further purification. Deionized (DI) water was applied in all tests. To measure X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, USA), we used an ESCALab MKII spectrometer with Al Kα (1.4866 keV) as the X-ray source. To collect survey and high-resolution spectra, we used an energy analyzer (SPECS PHOIBOS 150 MCD) and examined the spectra by an Aspen Energy analyzer 8.8. Using C 1s level at 284.8 eV as an internal standard, we obtained binding energy shifts. The 1H NMR and 13C NMR (300 MHz) analyses were performed with a Bruker Avance DPX NMR spectrometer using tetramethylsilane (TMS) as an internal standard. The actual contents of W in the catalysts was examined using inductively coupled plasma mass spectrometry (ICPOES, Vista-MPX). X-ray diffraction (XRD) spectra were recorded on an Siefert XRD 3003 PTS diffractometer with Cu Kα radiation (λ = 1.54 Å).TGA analysis was measured using a TGA Q 50 analyzer under N2 flow and scan rate of 10 °C min−1. A Shimadzu UV 2100 PC UV−visible spectrophotometer was used to record UV−vis absorption spectra. EDX analysis and FE-SEM measurements were carried out on a SIGMA VP 500 (Zeiss) microscope. Transmission electron microscopy (TEM) was investigated on a Philips EM10C 200 kV microscope. FT-IR spectra were measured using a PerkinElmer PE-1600-FTIR spectrometer. A 400 MHz Bruker spectrometer with a maximum z-gradient strength of 34 G/cm, was applied to gain diffusion ordered spectroscopy (DOSY) observations. Usingan LED with bipolar gradient pulse pair, we examined 2 spoil gradients program with PFGs (ledbpgp2s), water self-diffusion coefficient, and tracer diffusion coefficient. Pulsed gradients of duration d 1/4 4.40 ms were applied and the diffusion time (D) was 49.90 ms. The gradients, G, were changed in 16 steps up to the gradient of 32.35 G/cm. The self-diffusion of IL and CQDs@DDAIL/Cl were achieved in water (D2O, T = 25 °C) using obtained NMR spectra at different gradients (G), Top Spin software, and plotting the ratio of I/I0 versus G2. GC experiments of samples were obtained using a 6890 Agilent gas chromatograph with a HP-5 capillary column (phenylmethyl siloxane 30 mm × 320 μm × 0.25 μm) equipped with the flame-ionization detector. Synthesis Procedure of CQDs Functionalized with the [APMim][Cl] and DDA. The knownprecursor of [APMim][Cl] synthesized based on previous works.47 First, a mixture of citric acid (3 mmol) as a carbon fountain, DDA (4.5 mmol) and [APMim][Cl] (4.5 mmol) as surface modifier were dissolved in 15 mL of H2O:CH3CN (2:1) by sonication for 5 min and a milky mixture was obtained. Afterward, it was transferred into an autoclave. The closed autoclave was then subjected to a conventional one-pot hydrothermal treatment under 200 °C for 4h. After cooling to room temperature, the [APMim][Cl] and DDA-functionalized CQDs were obtained as a reddish-brown dispersion. The resulting material was designated as CQDs@DDA-IL/Cl. Synthesis of Tungstate Ions (WO4=) Immobilized on [APMim][Cl] and DDA-Functionalized CQDs (denoted as CQDs@DDA-IL/W). A simple anion exchange technique was used for the procurement of the CQDs@DDA-IL/W. First, the assynthesized CQDs@DDA-IL/Cl dispersion sonicated. Then, the Na2WO4.2H2O (2 mmol) was separately dissolved in DI water (2 mL) and was slowly injected to the aforementioned CQDs@DDAIL/Cl dispersion and stirred at room temperature for overnight. The color of the mixture was changed from reddish-brown to light brown. B


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ACS Applied Materials & Interfaces Scheme 1. Total Illustration of the Synthesis Procedure of the CQDs@DDA-IL/W Catalyst

To eliminate the large dots and agglomerated particles, the filtrate was centrifuged at 16 000 rpm for 10 min. To delete the unreacted molecular precursors, we subjected the supernatant to the dialysis against DI water through a dialysis membrane (500 Da) for 48 h. The dialysate was lyophilized and the target catalyst (denote as CQDs@ DDA-IL/W) was obtained as a brown powder. Catalytic Test of theCQDs@DDA-IL/W for the Oxidation of the Alcohol in H2O. To investigate the catalytic performance of the CQDs@DDA-IL/W, a two-necked round-bottomed flask fitted with a reflux condenser immersed in an oil bath equipped with a thermometer was used. Without the use of any organic solvent or additive, this reactor was charged with 1 mmol of alcohol, 8 mg of catalyst, and 2 mL of H2O. Then 3 mmol of H2O2 (30 wt % in water) was added drop by drop into the reaction container and the catalyzed mixture was vigorously stirred at 70 °C for a defined time as reported in Table 3. After detecting the completion of the oxidation via analyzing the aliquots of reaction at regular intervals using TLC, the reaction flask was automatically chilled to room temperature. The carbonyl products and unreacted reactant in the reaction mixture were isolated using a facile extraction with water/ethyl acetate. The organic phase was then concentrated by full evaporation of the solvent and used for GC and 1H NMR analyses. Conversion and yields of each sample have been analyzed using GC. For the GC quantitative analysis of these extracted samples, GC condition with column Hp-5 was first set as following: inlet temp 250 °C, first column temp 90 °C, final column temp 190 °C, sleep 10 °C/min, carrier gas N2, flow = 0.7 mL/min. Also, chlorobenzene as an internal standard was selected and reproducibility of each catalytic run was tested using GC at least three times and the average value was finally reported. In this test, the oxidation products retention times with those of authentic samples were compared and by injecting standards solutions of reactants and products, the calibration curves were obtained. Finally, conversion and yield were calculated on the basis of these calibration curves.

immobilized on DDA and IL-functionalized CQDs (details in the Experimental Section). To validate and characterize various surface functional groups of the CQDs@DDA-IL/Cl and CQDs@DDA-IL/W samples, we used FT-IR spectroscopy along with a more accurate technique of XPS. In the FT-IR pattern of both CQDs (Figure 1A, B), there are a variety of signals in the range of 400

Figure 1. FT-IR diagrams of the (A) CQDs@ DDA-IL/Cl and (B) CQDs@ DDA-IL/W.

to 4000 cm−1. The broad peak near 3423 cm−1 appointed to N−H and O−H stretching modes. The absorption bands at 1769, 1699 , 1571, and 1384 cm−1 generated by CO stretching vibrations from carboxyl, ketone, CC stretching and −OH bending vibrations, respectively. These observations display the accumulation of the oxygen-containing functional groups on the surface of as-prepared CQDs as a result of the carbonization of CA. The existence of characteristic absorption bands in the range of 2960 to 2840, and the peak at 1441 cm−1 are mainly due to symmetric and asymmetric stretching modes, and scissoring vibration of the −CH2− and −CH3− groups from DDA and [APMim][Cl], respectively. Also, two peaks

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RESULTS AND DISCUSSION Synthesis and Identification of the Pristine Catalyst. The CQDs@DDA-IL/W was prepared in two tandem steps which are clearly shown in Scheme 1. First, the amphiphilic CQDs@DDA-IL/Cl as support was synthesized by the hydrothermal method of starting materials. In the second step, the CQDs@DDA-IL/Cl subjected to ion-exchange technique with Na2WO4.2H2O to give tungsten(VI) ions C


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ACS Applied Materials & Interfaces

Figure 2. High-resolution XPS spectra of the CQDs@DDA-IL/W sample: (A) overall XPS spectrum, (B) W 4f XPS spectra, (C) C 1s XPS spectra, (D) N 1s XPS spectra, and (E) O 1s XPS spectra.

centered at 1170 cm−1, 622 cm−1 are ascribed to C−N stretching, and C−H bending vibrations of imidazolium ring, respectively. Besides these, the absorption band at 720 cm−1 correspond to the long-chain vibrations of the alkyl groups’ derivated from DDA. The stretching vibrations of the amide CO at 1653 cm−1 and the broad peak at 3423 cm−1 that assigned to N−H stretching vibration, indicating covalent functionalization of CQDs with DDA and [APMim][Cl] through chemical interaction between −COOH groups at CQDs surface and −NH2 group at [APMIm][Cl] and DDA.30,41,47,48 Also, positive zeta potential (+22.3 mV for CQDs@DDA-IL/Cland +15.0 mV for CQDs@DDA-IL/W) clearly provided a powerful reason for the abundance of amine groups in the CQDs surface by amid linkages (Figure S1).30 In the spectrum of the CQDs@DDA-IL/W (Figure 1B), the presence of the characteristic band around 879 cm−1 after an exchange of chloride with tungstate that is assigned to WO stretching frequency in WO4=, confirming the success of this exchange.15,16,47 Also, to calculate the accurate loading of WO4= ions in the CQDs@DDA-IL/W sample, we applied ICP-OES analysis. The above test report indicated that the loading amount of W in the CQDs@DDA-IL/W was 121 ppm (equal to 6.05% W, or 0.329 mmol g−1 WO4=). The XPS analyses were used to gain further insight into the surface configuration and oxidation state of the W in the CQDs@DDA-IL/W sample. All spectra from this analysis are shown in Figure 2A−D. The content of Na has introduced in the process of immobilization the WO4= ions, and Si was caused by monocrystalline silicon (Table S1). As evident from the overall XPS spectrum (Figure 2A), the CQDs@DDA-IL/ W contains C, O, N, and W elements with atomic percentages 63.24, 21, 12.36, and 1.17, respectively (Table S1). These results patently indicate that carbon is a central element in synthesized CQDs. The high-resolution W 4f spectra are clearly shown in Figure 2B, which is divided into the W 4f 7/2 and W 4f 5/2 peaks in the range of 34 to 38 eV. The binding

energies of W 4f 7/2 and W 4f 5/2 are 34.9 and 37.1 eV, respectively. From these results, the oxidation mood of the W element can be deduced and verified as W6+.49,50 Three main peaks of C 1s (Figure 2C) are located at 284.8, 286.1, and 288.1 eV, corresponding to C−C/CC, CO, and C−N groups, respectively. In the high-resolution N 1s spectra (Figure 2D), two peaks at 400.1 and 401.6 eV attributed to N−H and C−N/C = N. Figure 2E displays the high-resolution O 1s binding signals. Two typical peaks around 531.4 and 532.3 eV can be assigned to the C−O/CO, O−H species, respectively. The extra peak at 530.7 eV is attributed to the W−O because of the introduction of WO4= ions.30,41,47 Combination of the FT-IR and XPS results assures us that the aromatic sp2 carbon network is successfully formed and the carbon quantum dots surface has been functionalized with DDA and [APMIm][Cl]. Also, WO4= ions as catalytic species are anchored on the as-prepared CQDs surface through electrostatic interactions. The 1H and 13C NMR spectra (Figure 3) show the presence of different kinds of H atoms and the sp2 and sp3 hybridized C atoms in the different regions as argued below. In the 1H NMR spectrum (Figure 3A), a set of peaks situated at 1−3 ppm are assigned to sp3 saturated protons of alkyl groups resulting from DDA and [APMIm][Cl]. The 3−4.5 ppm peaks can also be considered for the protons connected to an electron-withdrawing (EWG) O or N atoms, such as C−OH and C−N. In addition, peaks in the range of 7−9 ppm as distinguished peaks of imidazole ring were detected. With respect to the 13C NMR spectrum (Figure 2B), the presence of imidazole ring carbons in the range of 123−140 ppm is clearly confirmed. Also, signals at 122 and 180 ppm are ascribed to CO groups and aromatic CC carbon atoms, respectively. Furthermore, peaks at 20−60 ppm range can be allocated to sp3 carbons of the alkyl chain from DDA and [APMIm][Cl]. The sp3 C atoms attached to EWG were inhabited between 60 and 80 ppm.30,41,48,51 The results from NMR provide another proof D


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Figure 3. (A) 1H NMR and (B) DDA-IL/Cl.

the catalyst background, carry a 2:1 quota of imidazolium cations and WO4= as counteranions; and (iii) the Na2WO4 was not physically absorbed onto the support surface. To further verify the results of EDS analysis and distribution of the constituent elements of the CQDs@DDA-IL/Cl and CQDs@ DDA-IL/W, we applied the mapping technique for each sample. The EDS elemental mapping spectra of the CQDs@ DDA-IL/Cl (Figure S2A) indicates that the elements of C, N, O, and Cl have uniformly distributed. Likewise, uniform distribution of the O, C, N, and W in the CQDs@DDA-IL/W mapping images can be seen (Figure S2B).16,49,54−56 Finally, the results of these two analyses are in close overlap with each other and with FT-IR, NMR, and XPS observations. To analyze the surface morphology, size, and shape of the CQDs@ DDA-IL/W as a target material, we performed TEM and FE-SEM analyses (Figure 5A−C). TEM image (Figure 5A) indicates that hydrothermally prepared CQDs@DDA-IL/ W nanoparticles are spherical, very tiny, and uniformly dispersed without any evident aggregation. Also, the size distribution histogram of these nanoparticles confirms that their size on average below 10 nm in diameter (Figure 5B). The CQDs@DDA-IL/W nanoparticles display the average size around 2.67 nm according to the size distribution graph (Figure S3). These results are in considerable accordance with reported earlier research works in this area.30,41,48In addition, the field-emission scanning electron microscopy (FE-SEM) analysis validates the TEM results. As shown in FE-SEM images (Figure 5B, C), the geometric shape of these CQDs NPs is spherical-like with an average size below 10 nm. On the basis of the above-mentioned analyses, the size of the CQDs@ DDA-IL/W NPs is located in the range of CQDs size, which is a strong sign of their successful synthesis.47 To check the thermal stability and exchange of chloride ions with WO4= active ions, thermogravimetric analysis (TGA) was carried out in the range 30−700 °C on the CQDs@DDA/IL/ Cl and CQDs@DDA-IL/W (Figure 6). The TGA diagram of the CQDs@DDA-IL/Cl and CQDs@DDA-IL/W show a total weight loss of 81.88 and 62.20% in this temperature range, respectively. From 30 to 120 °C for both samples an approximate weight loss of ∼2% can be seen, which was related to the evaporation of remaining water molecules during incomplete lyophilization. After this, the TGA profile of the CQDs@DDA-IL/Cl and CQDs@DDA-IL/W weight decrease shown in the range of 120−240 °C (∼8%) and 120−183 (∼6%), respectively. This weight loss, indicating that the decomposition of oxygen-containing functional groups is done. At TGA curve of the CQDs@DDA-IL/Cl, the main weight loss starts about 240 and goes to 530 °C, whereas in the case of the CQDs@DDA-IL/W, it occurs in the range of 183−540. In this case, the weight loss can be a sum of two concomitant processes: thermal degradation of the DDA and the [APMIm][Cl] connected to the surface of CQDs. Because of the existence of catalytic WO4=, decomposition of the organic fragments in the CQDs@DDA-IL/W occurs at a lower temperature than in the CQDs@DDA-IL/Cl, which is confirmed by the exchange of chloride with tungstate.16,30,31,42 These findings indicate that the CQDs@DDA-IL/W as a target material possess thermal stability up to near 200 °C. On the whole, all these analyses confirm the successful formation of the novel amphiphilic nanocatalyst. The XRD pattern of the CQDs@DDA-IL/Cl and CQDs@ DDA-IL/W samples is depicted in Figure 7a. In the case of the CQDs@DDA-IL/W sample, there is a broad diffraction peak

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C NMR images of the CQDs@

for the successful synthesis of the CQDs@DDA-IL/Cl with the proposed structure. DOSY NMR technique52,53 would be a good way of showing that the ligands are attached to the surface of support versus free. On the basis of this anslysis, in the DOSY NMR spectrum of both [APMIm][Cl] and CQDs@DDA-IL/Cl samples (Figure 4B, C), the peak at 4.3 ppm with logarithm of mass diffusion coefficient (Log D) of −8.8 to −9.0 assigned to water (H2O/HOD), while the Log D of [APMIm][Cl] and the CQDs@DDA-IL/Cl is located at −9.25 and −9.6, respectively. In this regard, the overlaid 2D-DOSY spectrum (Figure 4A), clearly shows that there is a sensible difference between the mass diffusion coefficient of IL and modified carbon dots, supporting the covalent binding of IL and DDA to the CQD surface. For deeper elucidating the constituent elements and successful multifunctionalization of the CQDs@DDA-IL/Cl and CQDs@DDA-IL/W samples, we also carried out EDS and elemental mapping analyses (Figure S2). From weight percent results of EDS analysis of both samples (Table S1, Supporting Information), it can be understood that the C element is the main constituent of the as-prepared CQDs skeleton (49.46% for the CQDs@DDA-IL/Cl and 44.85% for the CQDs@DDAIL/W). The presence of the N atom (20.26% for the CQDs@ DDA-IL/Cl and 18.91% for the CQDs@DDA-IL/W) from the DDA and [APMIm][Cl] structures confirms that surface functionalization has happened. The increased O content in the CQDs@DDA-IL/W compared with CQDs@DDA-IL/Cl (30.17% versus 23.98%, Table S1) was due to the exchange of Cl− with WO4=. The presence of only a trace of Na and Cl in the EDS and mapping profile of the CQDs@DDA-IL/W (Figure S2B) clearly confirms that (i) successful and almost complete exchange of Cl− with WO4= occurred; (ii) CQDs, as E


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Figure 4. (A) Overlaid 2D-DOSY NMR spectra of the CQDs@ DDA-IL/Cl and [APMIm][Cl] in water; (B) 2D-DOSY NMR spectrum of the CQDs@DDA-IL/Cl in water; and (C) 2D-DOSY NMR spectrum of the [APMIm][Cl] in water.

centered at a 2θ value of around 22.42° corresponding to the interlayer spacing 0.364 nm, whereas for the CQDs@ DDAIL/Cl, a diffraction peak is located at a 2θ value of around 19.8° corresponding to the interlayer spacing of 0.448 nm. The results obtained from this test indicate that, first, the synthesized CQDs show the poor crystallinity, and they are often composed of amorphous carbon phases and (002) graphite facet. Second, because of the injection of nitrogencontaining groups, the magnitude of the interlayer spacing is slightly higher than that for graphite (0.364 nm for the CQDs@DDA-IL/W and 0.448 nm for the CQDs@DDA-IL/ Cl vs 0.34 for graphite). Also, on the basis of the Scherrer equation (D = Kλ/(β cos θ)), the size of the CQDs@DDA-IL/ Cl and CQDs@DDA-IL/W NPs was calculated as 1.455 and 0.29 nm, respectively.30,37,56 To further ensure the successful synthesis of the CQDs@ DDA-IL/Cl and CQDs@DDA-IL/W NPs, we investigated optical properties of their aqueous solution using UV−vis absorption and PL spectroscopy. The aqueous dispersion of both samples emitted severe blue light under 365 nm UV irradiation (as shown in Figure 7b). This blue emission further confirmed that the particle size is in the range between 2 and 3 nm. In the UV−vis absorption

spectrum of the CQDs@ DDA-IL/Cl and CQDs@DDA-IL/W (Figure 7b), two absorption peaks at 253 and in the range of 300−400 nm were illustrated. The weak band at 253 nm is assigned to the π−π* electronic transition of graphitic sp2 domains, whereas typical broad absorption at 300−400 nm belongs to the n−p* transition of CO bonds. Figure 7c presents a photoluminescence (PL) spectrum of the CQDs@ DDA-IL/W catalyst with a variation of excitation wavelength (370−450 nm). Under 450 nm excitation, the highest PL emission was observed at 527 nm. On the whole, all these tests confirm the successful formation of the CQDs@DDA-IL/ W.41,47,48 Before the catalytic performance of the CQDs@DDA-IL/W catalyst in the oxidation of alcohols was examined, its amphiphilicity as a main and outstanding character was checked in a broad range of common organic solvents and in water at room temperature. The results of this study showed that because of the presence of both hydrophobic and hydrophilic groups on the CQDs@DDA-IL/W surface, it shows good amphiphilicity with high dispersity in water, ethanol, acetonitrile, dimethylformamide, dimethyl sulfoxide, chloroform, and dichloromethane. As shown in Figure 8, the F


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Figure 5. (A) TEM image of the CQDs@DDA-IL/W; (B) the size distributions of the CQDs@DDA-IL/W; and (C, D) FE-SEM images of the CQDs@DDA-IL/W.

H2O2 and preferably in pure water as a safe solvent. To obtain the optimal reaction conditions, we selected oxidation of benzyl alcohol as a model reaction and explored the role of CQDs@DDA-IL/W amount, the molar ratio of H2O2, reaction temperature, time, and additives. The findings of these experiments are listed in Table 1. As the first parameter of these set, the role of tungstate catalyst was investigated. In the absence of any catalyst and only in the presence of 3 mmol H2O2, when the reaction temperature was set at 90 °C, no obvious conversion of benzyl alcohol is virtually achieved (Table 1, entry 1). Under the same conditions used in entry 1 (1 mmol benzyl alcohol, 3 mmol H2O2, and 90 °C temperature), the catalytic activity of the CQDs@DDA-IL/ W was then studied in comparison to the CQDs@DDA-IL/Cl and Na2WO4 as an unsupported catalyst (8 mg of each). Among them, the CQDs@DDA-IL/W obviously showed the maximum catalytic activity with remarkable TON and TOF (conversion of 95% and 100% selectivity) (Table 1, entry 4) and the CQDs@DDA-IL/Cl exhibited minimum activitiy. The very low conversion of oxidation of benzyl alcohol in the presence of the CQDs@DDA-IL/Cl and unsupported Na2WO4 is mainly ascribed to the absence of tungstate ions as the reason for this conversion and low mass transfer of hydrophobic substrate in the biphasic system, respectively (Table 1, entries 2 and 3). In addition, no significant improvement in benzaldehyde yield was achieved by increasing the CQDs@DDA-IL/W catalyst amount (Table 1, entry 5) and even leads to a decrease in the TON/TOF values.

Figure 6. Thermogravimetric curve (TGA) for the CQDs@DDA-IL/ Cl (pink) and CQDs@DDA-IL/W (yellow).

blue emission of the CQDs@DDA-IL/W dissolved in each of the above solvents under 365 nm UV irradiation confirm this claim. Hence, on the basis of the good dispersity/miscibility of the CQDs@DDA-IL/W in both water and alcohol, it can mediate alcohol oxidation in water as a green solvent.35 Catalytic Assessment of the CQDs@ DDA-IL/W Catalyst. After the physicochemical characterization of the CQDs@DDA-IL/W, its catalytic abilities were assessed in the oxidation of the alcohols as a fundamental reaction using 30% G


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Figure 7. (a) XRD image of the CQDs@DDA-IL/Cl (pink) and CQDs@DDA-IL/W (green); (b) UV−vis absorbance spectra of the CQDs@ DDA-IL/Cl (pink) and CQDs@DDA-IL/W (green); and (c) emission diagrams of the CQDs@DDA-IL/W with increasing excitation wavelengths from 370 to 450 nm in 10 nm increments.

Figure 8. Photographs of emission of the CQDs@DDA-IL/W dissolved in a range of common laboratory solvents under 365 nm UV irradiation.

Table 1. Optimization Results of the Oxidation of Benzyl Alcohola

entry

catalyst

amount (mg)

time (h)

H2O2 (mmol)

T (°C)

1 2 3 4 5 6 7 8 9 10 11f

Na2WO4 CQDs@DDA-IL/Cl CQDs@DDA-IL/W CQDs@DDA-IL/W CQDs@DDA-IL/W CQDs@DDA-IL/W CQDs@DDA-IL/W CQDs@DDA-IL/W CQDs@DDA-IL/W CQDs@DDA-IL/W

8 8 8 15 8 8 8 8 8 8

1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 24 24 1.5

3 3 3 3 3 3 3 3 3 5 3

90 90 90 90 90 70 50 r.t. r.t. 70 70

conversion (%)b

yield (%)c

TONd

TOFe (h−1)

95 94 97 85 81 82 97 96

trace trace 95 94 97 85 81 82 97 96

719 380 734 643 613 621 734 727

479 253 489 429 409 25 30 484

a

Benzyl alcohol (1 mmol). bConversions were calculated based on initial mmol of benzyl alcohol. cCatalytic reaction yields were analyzed and identified by GC analysis. dTON = mmol products/mmol (WO4= anions). eTOF = TON/reaction time (h). fReaction condition: benzyl alcohol (1 mmol), H2O:toluene (1:1).

temperature was optimized and it was found that decreasing the reaction temperature to 70 °C enhanced the yield up to 97% with the highest values of TON and TOF (Table 1, entry

Remarkably, in all tests in which the CQDs@DDA-IL/W is used as a target catalyst, 100% selectivity toward benzaldehyde was observed. As the second parameter, the reaction H


Research Article


Table 2. Investigation of Additives Effect over Catalytic Activity of the CQDs@ DDA-IL/W Catalyst under Optimum Conditionsa

entry

additive (mmol %)

conversion (%)b

yield (%)c

1 2 3 4 5 6 7 8

NaCl NaHSO4 H2SO4 H3PO4 ClCH2COOH Pyridine KOH NEt3

95 90 94 91 92 95 94 96

95 90 94 91 92 95 94 96

a

Benzyl alcohol (1 mmol), H2O2 (3 mmol). bConversions were determined based on initial mmol of benzyl alcohol. cYields were determined by GC analysis.

Table 3. Selective Oxidation Using CQDs@ DDA-IL/W Catalysta

a Reaction conditions: substrate (1 mmol), CQDs@ DDA-IL/W (8 mg), 30% H2O2 (3 mmol), H2O (2 mL), 70 °C. bConversions calculated based on initial mmol of substrates. cYields were determined by GC analysis. dTON = mmol aldehyde or ketone/mmol (WO4= anions). eTOF = TON/ reaction time (h).

1, entry 11) with entry 6 shows that water is the best media for this oxidation because of its safe and green nature. Consistent with these results, using 3 equiv of 30% H2O2, 8 mg of the CQDs@DDA-IL/W in water as a reaction media under 70 °C for at least 90 min, maximum catalytic activity in the benzyl alcohol oxidation was clearly observed with notable TON/ TOF values, and these conditions as an optimum condition was considered. Under the obtained optimum condition, benzyl alcohol was selectively oxidized into the corresponding benzaldehyde without overoxidation to benzoic acid. This excellent catalytic performance of the CQDs@DDA-IL/W in an organic/aqueous biphasic system can be attributed to hydrophilic/hydrophobic balance from surface multifunctionalization of CQDs using DDA and the [APMIm]+ moiety.

6), and the reduction of temperature (down to room temperature) dramatically reduced the oxidation yield to 81% (Table 1, entries 7 and 8). In this case, by increasing the reaction time up to 24 h, not only was significant improvement in aldehyde yield not observed but it also results in a sharp drop in the TOF value (Table 1, entry 9). The effect of the molar ratio of H2O2 as the third parameter was also probed; it was found that increasing the molar ratio of the H2O2 even with increasing reaction time (up to 24 h) does not have any effect on the yield, but rising temperature led to a sharp decrease in TOF (Table 1, entry 10). In the end, to achieve the highest conversion, at least 90 min of reaction time is required. Also, the water/toluene solvent mixture (1:1) was screened in the catalytic assessment of the CQDs@DDA-IL/W under the same condition. The similar results of this catalytic run (Table I


Research Article

ACS Applied Materials & Interfaces According to the previously reported oxidation reactions using a combined system of sodium tungstate (Na2WO4)/30% H2O2/phase-transfer catalyst (PTC),57−59 the catalytic performance in the presence of the acidic and alkaline additives has been influenced. Thus, the effect of different additives was investigated (Table 2). In this case, improvement in the conversion and yield was not observed, but a slight decrease does occur. However, 100% selectivity was completely maintained. These results indicate that acidic and alkaline additives show no significant effect on amphiphilic CQD activity. Considering excellent results under optimum conditions, we then extended the scope of the presented catalyst in the oxidation of various alcoholic compounds. The results of this study have been summarized in Table 3. In this context, aromatic and aliphatic alcohol were selectivity oxidized to aldehydes and ketones in very good yields without any detectable overoxidation to acid. The fact that aliphatic alcohols compared with aromatic ones require longer time course and provide a lower yield of corresponding aldehyde or ketone (Table 3, entries 5 and 6). These results can be assigned to the easier adsorption and diffusion of aromatic alcohols via π−π interaction between the aromatic ring of reactants and CQDs support surface compared with aliphatic alcohols, which facilitates hydrogen bond formation of these substrates with the dangling − OH/−NH− moieties on the catalyst surface (see details in the proposed mechanism). In addition, in the case of benzylic alcohols, unlike aliphatic alcohols, the efficiency is extremely affected by electronic properties of substituents. On the basis of obtained results, electron-withdrawing groups reduce the conversion rate compared with benzyl alcohol. 4-nitro, 4-chlorobenzyl alcohol converted to the corresponding aldehyde after 2 h (Table 3, entries 2 and 3), whereas the benzyl alcohol was selectively oxidized to benzaldehyde after 1.5 h in high yield. Therefore, electron-withdrawing groups have a negative effect on the conversion and yield of the oxidation reaction.1,16,57 The FTIR and 1H NMR spectra from the oxidation mixture of each sample (Figure S4−S7) exhibited a characteristic peak of aldehyde product without the presence of acid peak. 1H NMR data clearly indicated the 100% selectivity of this nanocatalyst in the alcohol reaction. According to the above experiment results and previous studies in the area,16,42,59,60 we proposed a reaction mechanism (Scheme 2). As shown in Scheme 2, in the H2O2-containing water layer, W(O2)4 species is generated from the reaction of the WO4 anion with H2O2. Coordination of the dangling amide functional groups on the CQDs@DDA-IL/W surface at oil−water layer interface then stabilizes the W(O2)4 species. This coordination makes that these species are not easily decomposed to O2 and WO4. In fact, the CQDs@DDA-IL/W acts as a connection bridge because of its amphiphilic nature. Dangling hydrophilic and hydrophobic moieties on the CQDs@DDA-IL/W have a momentous impact in the diffusion of the alcoholic reactant into the water layer through creation of H-bonding between the dangling polar groups of the catalyst and alcohol. The ligand exchange is then carried out between alcohol and CQDs@DDA-IL/W and, ultimately, the desired carbonyl compound is produced in high yield as a result of dehydration of alkoxy ligand by the adjacent hydroperoxy ligand. We imagine that coordination is a crucial stage for this process because of catalytic species stabilization increment.

Scheme 2. Proposed Mechanism of Alcohol Oxidation to the Corresponding Aldehyde or Ketone by CQDs@DDAIL/W

Catalyst Reusability and Recyclability. The catalyst recyclability by trouble-free and fast methods is important as a key parameter of the heterogeneous catalyst. Therefore, the recyclability of the CQDs@DDA-IL/W was investigated in the selective oxidation of benzyl alcohol as a representative alcohol. After the completion of the first run, the CQDs@ DDA-IL/W catalyst was readily recovered from the mixture by extraction. The extraction process was followed by diluting the reaction mixture with water and ethyl acetate. Then, the bottom layer containing the CQDs@ DDA-IL/W was extracted and subsequently dried under vacuum overnight and reused in the next run under optimized conditions. As shown in Figure 9, after at least six consecutive runs, the above catalyst indicates only a negligible decrease in benzaldehyde yield compared with the used one-time catalyst (88 vs. 97%) but the selectivity was completely unchanged. These results confirm the catalytic stability of the CQDs@DDA-IL/W.

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CONCLUSION In summary, we have designed a novel amphiphilic CQDs multifunctionalized with [APMim][Cl] and DDA through one-step hydrothermal synthesis and it was then utilized as an amphiphilic stabilizer for the catalytically active WO4= ions. A variety of characterizations confirm that [APMim][Cl] and DDA have been successfully grafted on the CQDs surface. Excellent amphiphilicity nature of the CQDs@DDA-IL/W catalyst provides excellent dispersity and stability in various J



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solvents. This unique feature was used to remove the organic solvent and improve the mass transfer in biphasic catalysis. This “micelle-like” novel amphiphilic catalyst was mediated alcohols oxidation with 100% selectivity and high conversion without substantial loss in catalytic activity after six catalytic cycles. Moreover, rapid synthesis and simple recovery from the reaction medium make this catalyst very attractive for the synthesis of various aldehydes without overoxidation under environmentally benign reaction conditions. It is expected that the current study can provide the development basis of CQDs as tunable support for stabilizing catalytically active species. Overall, to modulate the adsorption, desired mass transfer of reactants, and desorption of products in a biphasic system and with the aim of removing organic solvents in the oxidation reactions, our research group designed a catalyst based on amphiphilic carbon quantum dots (CQDs).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07961. Zeta potential distribution graph of the CQDs@ DDAIL/Cl and CQDs@ DDA-IL/W; weight percent results of EDS, EDS profile, and corresponding elemental mapping images, FT-IR and 1H NMR patterns of the oxidation reaction mixture, and XPS analyses and size distribution graph of the CQDs@ DDA-IL/W aqueous solution (PDF)

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REFERENCES

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Figure 9. Recycling experiment.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected], [email protected]. Phone: +98 833 4276489. *Email: [email protected]. Phone: +98 811 8282807. ORCID

Aram Rezaei: 0000-0003-2408-7254 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful the Research Council of Research Council of Bu Ali Sina University and Kermanshah University of Medical Sciences for providing the facilities and financial support of this research. K


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