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Article

Novel Effect of Zinc Nitrate/Vanadyl Oxalate for Selective Catalytic Oxidation of α-Hydroxy Esters to α-Keto Esters with Molecular Oxygen: An In Situ ATR-IR Study

by
Yongwei Ju
1,
Zhongtian Du
2,*,
Chuhong Xiao
2,
Xingfei Li
2 and
Shuang Li
1,*
1
School of Chemical Engineering, Northwest University, Xi’an 710069, China
2
School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China
*
Authors to whom correspondence should be addressed.
Molecules 2019, 24(7), 1281; https://doi.org/10.3390/molecules24071281
Submission received: 5 March 2019 / Revised: 26 March 2019 / Accepted: 29 March 2019 / Published: 2 April 2019
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Abstract

:
Selective oxidation of α-hydroxy esters is one of the most important methods to prepare high value-added α-keto esters. An efficient catalytic system consisting of Zn(NO3)2/VOC2O4 is reported for catalytic oxidation of α-hydroxy esters with molecular oxygen. Up to 99% conversion of methyl DL-mandelate or methyl lactate could be facilely obtained with high selectivity for its corresponding α-keto ester under mild reaction conditions. Zn(NO3)2 exhibited higher catalytic activity in combination with VOC2O4 compared with Fe(NO3)3 and different nitric oxidative gases were detected by situ attenuated total reflection infrared (ATR-IR) spectroscopy. UV-vis and ATR-IR results indicated that coordination complex formed in Zn(NO3)2 in CH3CN solution was quite different from Fe(NO3)3; it is proposed that the charge-transfer from Zn2+ to coordinated nitrate groups might account for the generation of different nitric oxidative gases. The XPS result indicate that nitric oxidative gas derived from the interaction of Zn(NO3)2 with VOC2O4 could be in favor of oxidizing VOC2O4 to generate active vanadium (V) species. It might account for different catalytic activity of Zn(NO3)2 or Fe(NO3)3 combined with VOC2O4. This work contributes to further development of efficient aerobic oxidation under mild reaction conditions.

Graphical Abstract

1. Introduction

α-keto esters, which have two-functional groups, are valuable precursors in a variety of organic transformations [1,2,3]. For example, phenylglyoxylate and its derivatives are used as key intermediates for synthesis of herbicide metamitron [2], and pyruvate and its derivatives can be employed for the preparation of pharmaceutical pindolol and pesticides thiabendazole [3]. In the past several decades, several methods for synthesis of α-keto esters have been reported, such as esterification of α-keto acids or α-carbonyl aldehydes, oxidative cleavage of β-diketones, oxidative esterification of aryl-ketones and so on [4,5,6]. Considering the excellent atom efficiency, there is a growing interest to develop efficient catalytic oxidation of α-hydroxy esters into their corresponding α-keto esters [7,8].
Oxidation of α-hydroxy esters into α-keto esters is essentially selective oxidation of a secondary alcohol adjacent to a carboxylic ester group. Due to the inherent steric hindrance and electronic effect, complete conversion of α-hydroxy esters is usually difficult, while oxidative C-C bond cleavage could easily occur between two adjacent carbonyl carbon atoms under harsh oxidative reaction conditions [7,8]. Recently, several efficient aerobic catalytic systems have been developed under mild liquid reaction conditions [9,10,11,12,13], such as Pd-, Au-, and Ru-based catalysts, AZADO-NaNO2 and TEMPO-NaNO2. We are encouraged by all these methods, although some drawbacks still exist, such as side reactions, and use of expensive organic or metal catalysts. Therefore, it is still rather challenging for efficient catalytic oxidation of α-hydroxy esters into α-keto esters with molecular oxygen under mild reaction conditions.
As a base transitional metal, vanadium catalysts have been uncovered for aerobic oxidation of alcohols to carbonyl compounds recently [14,15]. Kirihara et al. achieved excellent yields of α-keto esters using fuming and corrosive VOCl3 [16]. Hanson et al. described the key role of pyridine during the alcohol oxidation by dipicolinate vanadium (V) [17,18]. Xu et al. also reported efficient catalytic oxidation of 5-hydroxymethylfurfural to furan-2,5-dicarbaldehyde, and C-C bond cleavage could be tuned using different vanadium-based catalysts [19,20]. These reports have provided us with important information concerning the selective oxidation of α-hydroxy esters using vanadium-based catalysts under mild reaction conditions. We used a highly efficient catalytic system consisting of Zn(NO3)2/VOC2O4 for selective oxidation of α-hydroxy esters into α-keto esters with molecular oxygen under mild reaction conditions, and employed infrared spectroscopy to investigate the unique catalytic activity of Zn(NO3)2 compared with other metal nitrates such as Fe(NO3)3.

2. Results and Discussion

Initially, oxidation of methyl DL-mandelate with molecular oxygen was selected as a model reaction, and performed in acetonitrile under mild reaction conditions (80 °C, 0.2 MPa O2). Table 1 shows the results on catalytic oxidation of methyl DL-mandelate with different catalysts. Only 5–6% conversion of methyl DL-mandelate was obtained using Zn(NO3)2 or VOC2O4 (Table 1, Entries 1–2). In contrast, 99% methyl phenylglyoxylate selectivity with 99% methyl DL-mandelate conversion was obtained within 1.5 h when both Zn(NO3)2 and VOC2O4 (molar ration = 1) were employed (Table 1, Entry 3). Moreover, no C-C bond cleavage of methyl phenylglyoxylate was observed even by prolonging the reaction time from 1.5 h to 6 h, which suggests that methyl phenylglyoxylate could remain stable under such oxidative environment (Table 1, Entry 4). Furthermore, nearly quantitative yield of methyl phenylglyoxylate could be facilely obtained at room temperature (25 °C) under normal pressure of molecular oxygen (Table 1, Entry 5). Compared with VOCl3 and other vanadium organic complexes [17,18,21,22,23], Zn(NO3)2/VOC2O4 is inexpensive and halogen free. All these results indicate that the catalytic system of Zn(NO3)2/VOC2O4 was efficient for catalytic oxidation of methyl DL-mandelate into methyl phenylglyoxylate with dioxygen under mild reaction conditions.
Further, we also compared different nitrates for catalytic oxidation of methyl DL-mandelate under the same reaction conditions (Table 1, Entries 6–10). When Co(NO3)2, Ce(NO3)3, Fe(NO3)3, Ni(NO3)2 and NaNO3 were employed instead of Zn(NO3)2, only 8–33% conversion of methyl DL-mandelate was observed. Zn(NO3)2 showed the best performance among those metal nitrates tested in this work. As a typical transition metal nitrate with multi-valences, Fe(NO3)3 was often used as a key component for selective oxidation of alcohols in previous reports [24,25]. Unexpectedly, Fe(NO3)3 displayed much lower catalytic activity than Zn(NO3)2, and only 16% methyl DL-mandelate conversion was obtained using Fe(NO3)3/VOC2O4 as the catalyst under the same reaction conditions. Thus, the nitrates could affect the catalytic activity significantly in this work. This issue is discussed in the following section.
Besides methyl DL-mandelate, lactate esters and benzoin were also oxidized (Table 2). Methyl lactate and ethyl lactate are typical aliphatic α-hydroxy esters, which are more difficult to be oxidized than methyl DL-mandelate. When they were oxidized, 21% and 59% conversion was obtained, respectively, with 99% pyruvate esters selectivity within 1.5 h. To our delight, 99% methyl lactate and ethyl lactate conversion with high selectivity of pyruvate esters was achieved after prolonging the reaction time to 4 h. Furthermore, good yields of pyruvate esters could also be attained at normal pressure of molecular oxygen at room temperature (Table 2, Entries 1–2). Moreover, the selectivity of byproducts (mainly acetic acid) derived from C-C bond cleavage was no more than 3%. As described above, it seemed difficult to obtain high yields of pyruvate directly from lactate via oxidation because of the side formation of CO2 through C−C bond fission [7,8]. For example, only 70% selectivity towards ethyl pyruvate was obtained when TiO2 was used for catalytic oxidation of ethyl lactate into ethyl pyruvate under mild reaction conditions [26]. Our results are competitive when compared with that using VOCl3 as a catalyst [27]. More than 95% benzil selectivity with 99% benzoin conversion could be achieved within 4 h. Oxidation of benzoin into benzil could also proceed smoothly under normal pressure of molecular oxygen at room temperature (Table 2, Entry 3). Only 5% benzoic acid/benzaldehyde byproducts were detected. Therefore, Zn(NO3)2/VOC2O4 is effective for catalytic oxidation of α-hydroxy compounds even under normal pressure and room temperature.
Operando attenuated total reflection infrared (ATR-IR) spectroscopy has been demonstrated as a powerful method to monitor real-time chemical reactions [28].To explore the oxidation process catalyzed by Zn(NO3)2/VOC2O4, oxidation of methyl DL-mandelate was monitored by online ATR-IR (Figure 1). The bands at 1694 and 1742 cm−1 were assigned to C=O stretching vibrations of carbonyl (methyl phenylglyoxylate) and esters functional group, respectively. The band at 1096 cm−1 was assigned to C-O stretching vibration of methyl DL-mandelate [29]. The signal at 1694 cm−1 rose gradually in the beginning, while the peak height at 1096 cm−1 (C-O stretching vibration of methyl DL-mandelate) diminished. Methyl DL-mandelate was converted to methyl phenylglyoxylate, and no other significant intermediates were detected by ATR-IR during the oxidation reaction. Moreover, the peaks at 1742 and 1694 cm−1 remained stable as the reaction time prolonged, and no other significant signals appeared, indicating that few byproducts were generated under such oxidative conditions. These observations were in accordance with GC analysis (Table 1).
To explore the different catalytic activities of nitrates, we selected Zn(NO3)2 and Fe(NO3)3 to study the interaction of catalyst components using FT-IR. The reaction of Zn(NO3)2 or Fe(NO3)3 with VOC2O4 was carried out in acetonitrile without methyl DL-mandelate (80 °C, 0.2 MPa O2, 1.5 h). After the pre-treatment, a red brown powder was obtained after removing acetonitrile. The FT-IR spectra are presented in Figure 2. For the FT-IR spectrum of VOC2O4 (Figure 2, Line a), the band at 977 cm−1 was attributed to be characteristic of V=O stretching vibration and the 826 cm−1 band was due to O-C=C bending and M-O stretching vibration [30]. The shoulder bands at 1390 and 1356 cm−1 were assigned to N-O stretching vibrations of Zn(NO3)2 (Figure 2, Line b), while the sharp band at 1386 cm−1 was attributed to N-O stretching vibration of Fe(NO3)3 (Figure 2, Line c) [31]. It should be noted that the FT-IR spectrum of reaction mixture derived from Zn(NO3)2/VOC2O4 (Figure 2, Line d) was quite different from those of VOC2O4 or Zn(NO3)2 (Figure 2, Lines a and b). Two new bands at 1326 and 816 cm−1 (Figure 2, Line d) appeared while no similar new absorption bands were generated when VOC2O4 was reacted with Fe(NO3)3 under the same reaction conditions (Figure 2, Line e). These results suggest that the reaction mixtures derived from Zn(NO3)2 and Fe(NO3)3/VOC2O4 were quite different, especially the form of nitric species. These phenomena may be related to the generation of different NOx gases, which might account for catalytic activity difference by using Zn(NO3)2 or Fe(NO3)3 with VOC2O4, respectively.
The observation above was further confirmed by investigating the interaction of Zn(NO3)2 or Fe(NO3)3 with VOC2O4 using online ATR-IR (Figure 3). Firstly, in the absence of substrate, VOC2O4 was stirred at 80 °C in acetonitrile under an oxygen atmosphere, and no apparent variation of spectrum was observed. This observation suggests that VOC2O4 alone remained stable under oxidative reaction conditions. When Zn(NO3)2 was introduced, new bands at 1326 and 816 cm−1 were generated simultaneously (Figure 3, top). On the contrary, when Fe(NO3)3 was added under the same reaction conditions, the ATR-IR spectrum (Figure 3, bottom) was distinct from that derived from Zn(NO3)2/VOC2O4. The bands at 1326 and 816 cm−1 were not observed (Figure 3, bottom). The online ATR-IR results were in good agreement with FT-IR results (Figure 2). These results demonstrated that different nitric species occurred in liquid phase by adding Zn(NO3)2 or Fe(NO3)3 under such oxidative conditions. This difference might be associated with the generation of NOx gases. It might account for lower catalytic activity for Fe(NO3)3 than that of Zn(NO3)2.
To verify the hypothesis mentioned above, we employed ATR-IR spectroscopy to detect composition of the gas phase, which was generated from the reaction of Zn(NO3)2/VOC2O4 or Fe(NO3)3/VOC2O4 (Figure 4). Firstly, Zn(NO3)2/VOC2O4 was stirred without the substrate under an oxygen atmosphere at 80 °C. In these experiments, the React IR™ 15-diamond probe was immersed in gas phase rather than liquid phase. The ATR-IR spectrum of gas phase demonstrated that new species which IR bands at 1671, 1305 and 936 cm−1 generated were attributable to HNO3 vapor (Figure 4, Line a) [32]. This observation suggests that HNO3 vapor probably was involved as one of key components during the oxidation using Zn(NO3)2/VOC2O4. In contrast, when the gas derived from Fe(NO3)3/VOC2O4 was monitored under the same conditions, IR bands at 1756, 1383, 1211 and 1003 cm−1 were detected. These bands were related to N2O3 and N2O4 mixture gas (Figure 4, Line b) [33,34], which were quite different from those derived from Zn(NO3)2/VOC2O4 (Figure 4, Line a). All these results above indicate that different NOx gases were involved during catalytic oxidation using Zn(NO3)2/VOC2O4 or Fe(NO3)3/VOC2O4. The obvious difference in NOx gases generated by using Zn(NO3)2/VOC2O4 or Fe(NO3)3/VOC2O4 was further investigated.
UV-vis spectroscopy was employed to detect coordination state of Zn(NO3)2/Fe(NO3)3 in CH3CN. An intense and broad peak at 293 nm was observed in Zn(NO3)2 in CH3CN solution, which was assigned to the n–π* transition, while no obvious peak was found on Fe(NO3)3 in CH3CN solution (Figure S2). It is proposed that the observed variation in the UV-vis spectrum should be interpreted as a coordination interaction taking place between Zn(NO3)2 and CH3CN solvent [35]. ATR-IR spectra in the C-N region of Zn(NO3)2/Fe(NO3)3 in CH3CN solution are shown in Figure S3. The bands at 2253 and 2292 cm−1 were assigned to C-N stretching vibrations of CH3CN, a new band at 2314 cm−1 appeared as Zn(NO3)2 was added to CH3CN, while no new band at 2314 cm−1 appeared as Fe(NO3)3 was added to CH3CN. This difference could be attributed to the unique complete filled 3d electronic orbitals of Zn2+ (3d10), which is quite different from Fe3+ (3d5) causing vibrations of the coordinated CH3CN. Previous studies also report a similar [Zn(CH3CN)2](NO3)2 coordination complex forming in Zn(NO3)2 in CH3CN solvent [36]. It can be inferred that charge-transfer from Zn2+ to coordinated nitrate groups might account for the generation of different NOx gases using Zn(NO3)2 and Fe(NO3)3, respectively [37].
The catalytic system consisting of nitrates and vanadium compounds has been reported, and the cascade electron transfer among nitric, vanadium species and oxygen was proposed [19,20]. vanadium (IV) species could be easily oxidized to vanadium (V) species by nitric oxidative gas rather than oxygen and after that the active vanadium (V) species oxidized alcohol group to ketone. We also believe that similar redox cycles existed in this work. To verify whether this nitric oxidative gas derived from the interaction of Zn(NO3)2 with VOC2O4 could promote the oxidation of VOC2O4 to generate active vanadium (V) species. XPS measurements were employed to detect the active vanadium (V) species after the interaction of Zn(NO3)2 or Fe(NO3)3 with VOC2O4. Firstly, the reaction of Zn(NO3)2 or Fe(NO3)3 with VOC2O4 was carried out in acetonitrile (80 °C, 0.2 MPa O2, 1.5 h). The reaction mixture was then obtained after removing acetonitrile. V 2p XPS spectra of the reaction mixture was shown in Figure 5. The peak of vanadium (IV) locates at 516.8 eV and that of vanadium (V) at 517.9 eV, which are in good accordance with previous report (516.5 eV for vanadium (IV) and 517.6 eV for vanadium (V)) [38]. It can be seen that vanadium (V) species appeared after the interaction of Zn(NO3)2 with VOC2O4 (Figure 5, Line a), while no vanadium (V) species existed in the reaction mixture of Fe(NO3)3/VOC2O4 under the same reaction conditions (Figure 5, Line b). V 2p XPS spectra of the reaction mixture provided strong evidence for the generation of vanadium (V) species in the Zn(NO3)2/VOC2O4 catalytic system. It is known that oxidizing ability of nitric oxidative gas was quite different [39]. Combining ATR-IR with XPS characterization results suggests that HNO3 gas derived from the interaction of Zn(NO3)2 with VOC2O4 could be in favor of oxidizing vanadium (IV) species to active vanadium (V) species, thus explaining why Zn(NO3)2 combined with VOC2O4 exhibited unique catalytic activity, compared with other tested metal nitrates such as Fe(NO3)3 under the same reaction conditions.
Based on the above results and previous reports [19,20], the reaction mechanism for Zn(NO3)2/VOC2O4 catalyzed aerobic oxidation of α-hydroxy esters is proposed in Scheme 1. Firstly, the interaction of VOC2O4 and Zn(NO3)2 occurred under such oxidative conditions. In the presence of water, the oxidative gas of HNO3 was generated after the interaction of VOC2O4 with Zn(NO3)2. Then, vanadium (IV) species derived from VOC2O4 underwent oxidation to vanadium (V) species by HNO3 gas, and after that the active vanadium (V) species oxidized α-hydroxy esters to α-keto esters with the formation of vanadium (IV) species. In this oxidation reaction, the oxidative gas of HNO3 generated in this catalyst system is expected to initiate and sustain vanadium (IV)/vanadium (V) redox cycle, and molecular oxygen plays an important role in NOx redox cycle. Such proposed reaction mechanism could explain the above experimental phenomena.

3. Materials and Methods

3.1. Catalyst Preparation

VOC2O4·2H2O was synthesized according to literature procedure [40]. Typically, vanadium pentoxide (2.0 g), oxalic acid dihydrate (4.3 g) and glacial acetic acid (16.8 mL) were added to a flask (50 mL). The reaction mixture was stirred for 2 h at 110 °C. The resulting slurry was cooled to room temperature, filtered, and the filtrate was discarded. The solid was dried at 80 °C under a vacuum for 5 h. The FT-IR spectrum of VOC2O4·2H2O is presented in Figure S1.

3.2. Typical Procedures of Reaction

In a typical procedure for catalytic oxidation α-hydroxy esters, α-hydroxy esters (5 mmol), VOC2O4·2H2O (0.25 mmol), Zn(NO3)2·6H2O (0.25 mmol), and CH3CN (5 mL) were placed in autoclave (25 mL). Oxygen was then added (0.2 MPa). The mixture was stirred at 80 °C for 1.5 h. After the reaction was completed, the resulting mixture was filtered and determined by gas chromatography (GC-7900 equipped with FID detector and PEG-20 capillary column, Techcomp Ltd, Shanghai, China) Conversion and selectivity were determined based on area normalization method. Products were verified by GC-MS, NMR and ATR-IR.

3.3. Typical Procedures of Analysis Method

GC-MS was performed on a Thermo Fisher Trace 1300s-ISQ LT instrument (Thermo Fisher, Waltham, MA, USA) with electron ionization (EI) mass spectrometry. NMR spectra were recorded at 500 MHz with Bruker Avance III HD instrument (Bruker, Billerica, MA, USA) using CDCl3 as the solvent with tetramethylsilane (TMS) as the internal standard.
The FT-IR spectra were recorded within a 4000–500 cm−1 region on a Thermo Fisher Nicolet iN10 MX & iS10 infrared spectrometer (Thermo Fisher, Waltham, MA, USA) using KBr pellets.
The UV–vis spectra was recorded within a 200–800 nm region on an Agilent Cary 60 spectrophotometer (Agilent, Palo Alto, CA, USA) with quartz cells at room temperature.
XPS measurements were performed on a Thermo Fisher ESCALAB 250Xi spectrometer (Thermo Fisher, Waltham, MA, USA).
The situ ATR-IR spectra were recorded within a 3000–600 cm−1 region on a Mettler Toledo React IR15 spectrometer (Mettler Toledo, Zurich, Switzerland) equipped with a liquid-nitrogen-cooled MCT (mercury cadmium telluride) detector and a diamond probe. The typical experiments were carried out in a closed flask. The IR probe was immersed in liquid phase reaction mixture or gas phase.

4. Conclusions

An efficient catalytic system consisting of Zn(NO3)2/VOC2O4 was developed for aerobic oxidation of α-hydroxy esters. Methyl DL-mandelate or methyl lactate could be facilely oxidized into its corresponding α-keto ester with high yields (up to 99% conversion of α-hydroxy esters) even under normal pressure at room temperature. Zn(NO3)2 combined with VOC2O4 exhibited unique catalytic activity compared to other tested metal nitrates such as Fe(NO3)3. Different nitric species were generated after interaction of VOC2O4 with Zn(NO3)2 or Fe(NO3)3. The coordination complex was identified by UV-vis and ATR-IR indicated the unique reaction route in Zn(NO3)2/VOC2O4, which gave diverse nitric oxidative gas with different oxidizing ability. The XPS result suggested that HNO3 gas derived from the interaction of Zn(NO3)2 with VOC2O4 could promote the oxidation of VOC2O4 to generate active vanadium (V) species. Further development of efficient vanadium-based catalysts for oxidation of alcohols is currently underway.

Supplementary Materials

The supplementary materials are available online, Figure S1: FT-IR spectrum of vanadyl oxalate, Figure S2: UV–vis spectra of Zn(NO3)2 or Fe(NO3)3 in CH3CN solvent, Figure S3: ATR-IR spectra in the C-N region of Zn(NO3)2 or Fe(NO3)3 in CH3CN solvent, Figures S4 and S5: NMR spectra of products, Figures S6–S9: MS spectra of products, Figures S10–S17: Original spectra of GC. Table S1: The effect of catalyst loading and Zn(NO3)2 loading for catalytic oxidation of methyl DL-mandelate, Table S2: Catalytic oxidation of various alcohols.

Author Contributions

Y.J., C.X. and X.L. investigated literature; Y.J. performed the experiments and analyzed the data; and Z.D. and S.L. wrote and approved the paper.

Funding

This research was funded by the National Natural Science Foundation of China (21473184 and 21878244), the Fundamental Research Funds for the Dalian University of Technology (DUT15RC(3)094) and the Provincial Key Laboratory Program of Shaanxi Provincial Education Department (16JS105).

Acknowledgments

We acknowledge School of Petroleum and Chemical Engineering, Dalian University of Technology for situ ATR-IR analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds are not available from the authors.
Molecules 24 01281 g001 550
Figure 1. The real-time profile of methyl DL-mandelate oxidation monitored by online ATR-IR. Reaction conditions: 10 mmol methyl DL-mandelate, 1 mmol Zn(NO3)2, 1 mmol VOC2O4, 10 mL CH3CN, 80 °C, O2 balloon.
Figure 1. The real-time profile of methyl DL-mandelate oxidation monitored by online ATR-IR. Reaction conditions: 10 mmol methyl DL-mandelate, 1 mmol Zn(NO3)2, 1 mmol VOC2O4, 10 mL CH3CN, 80 °C, O2 balloon.
Molecules 24 01281 g001
Molecules 24 01281 g002 550
Figure 2. FT-IR investigation on the interaction of nitrates and VOC2O4: (a) VOC2O4; (b) Zn(NO3)2; (c) Fe(NO3)3; (d) Zn(NO3)2 + VOC2O4, Zn(NO3)2:VOC2O4 = 1:1 (molar ratio), 0.2 MPa O2, 5 mL CH3CN, 80 °C, 1.5 h; and (e) Fe(NO3)3 + VOC2O4, Fe(NO3)3:VOC2O4 = 1:1 (molar ratio), 0.2 MPa O2, 5 mL CH3CN, 80 °C, 1.5 h.
Figure 2. FT-IR investigation on the interaction of nitrates and VOC2O4: (a) VOC2O4; (b) Zn(NO3)2; (c) Fe(NO3)3; (d) Zn(NO3)2 + VOC2O4, Zn(NO3)2:VOC2O4 = 1:1 (molar ratio), 0.2 MPa O2, 5 mL CH3CN, 80 °C, 1.5 h; and (e) Fe(NO3)3 + VOC2O4, Fe(NO3)3:VOC2O4 = 1:1 (molar ratio), 0.2 MPa O2, 5 mL CH3CN, 80 °C, 1.5 h.
Molecules 24 01281 g002
Molecules 24 01281 g003a 550Molecules 24 01281 g003b 550
Figure 3. In situ ATR-IR investigation on interaction of nitrates and VOC2O4, nitrates:VOC2O4 = 1:1 (molar ratio): Interaction of Zn(NO3)2 and VOC2O4 (top); and interaction of Fe(NO3)3 and VOC2O4 (bottom).
Figure 3. In situ ATR-IR investigation on interaction of nitrates and VOC2O4, nitrates:VOC2O4 = 1:1 (molar ratio): Interaction of Zn(NO3)2 and VOC2O4 (top); and interaction of Fe(NO3)3 and VOC2O4 (bottom).
Molecules 24 01281 g003aMolecules 24 01281 g003b
Molecules 24 01281 g004 550
Figure 4. NOx gas detected by in situ ATR-IR. Reaction conditions: 1 mmol Zn(NO3)2 or Fe(NO3)3, 1 mmol VOC2O4, 10 mL CH3CN, 80 °C, O2 balloon.
Figure 4. NOx gas detected by in situ ATR-IR. Reaction conditions: 1 mmol Zn(NO3)2 or Fe(NO3)3, 1 mmol VOC2O4, 10 mL CH3CN, 80 °C, O2 balloon.
Molecules 24 01281 g004
Molecules 24 01281 g005 550
Figure 5. V 2p XPS spectra of the reaction mixture: V 2p XPS spectra of the Zn(NO3)2/VOC2O4 reaction mixture (a); and V 2p XPS spectra of the Fe(NO3)3/VOC2O4 reaction mixture (b).
Figure 5. V 2p XPS spectra of the reaction mixture: V 2p XPS spectra of the Zn(NO3)2/VOC2O4 reaction mixture (a); and V 2p XPS spectra of the Fe(NO3)3/VOC2O4 reaction mixture (b).
Molecules 24 01281 g005
Molecules 24 01281 sch001 550
Scheme 1. Plausible reaction mechanism for Zn(NO3)2/VOC2O4 catalyzed aerobic oxidation of α-hydroxy esters.
Scheme 1. Plausible reaction mechanism for Zn(NO3)2/VOC2O4 catalyzed aerobic oxidation of α-hydroxy esters.
Molecules 24 01281 sch001
Table
Table 1. Catalytic oxidation of methyl DL-mandelate with different catalyst a.
Table 1. Catalytic oxidation of methyl DL-mandelate with different catalyst a.
Molecules 24 01281 i001
EntryMetal Nitratet (h)Conversion (%)Selectivity b (%)
1-1.56>99
2 cZn(NO3)21.55>99
3Zn(NO3)21.5>99>99
4Zn(NO3)26>99>99
5 dZn(NO3)248>99>99
6NaNO31.53395
7Ni(NO3)21.525>99
8Fe(NO3)31.51695
9Ce(NO3)31.511>99
10Co(NO3)21.58>99
a Reaction conditions: 5 mmol methyl DL-mandelate, 0.25 mmol metal nitrate, 0.25 mmol VOC2O4, 5 mL CH3CN, 80 °C, 0.2 MPa O2, 1.5 h. b Selectivity toward for methyl phenylglyoxylate. c No VOC2O4 was added. d Room temperature, normal pressure using O2 balloon.
Table
Table 2. Catalytic oxidation of lactate esters and benzoin with Zn(NO3)2/VOC2O4 a.
Table 2. Catalytic oxidation of lactate esters and benzoin with Zn(NO3)2/VOC2O4 a.
EntrySubstrateProductt (h)Conversion (%)Selectivity (%)
1 Molecules 24 01281 i002 Molecules 24 01281 i0031.55999
4>9997
72 b>9997
2 Molecules 24 01281 i004 Molecules 24 01281 i0051.52199
4>9998
72 b>9998
3 Molecules 24 01281 i006 Molecules 24 01281 i0071.58196
4>9995
48 b>9995
a Reaction conditions: 5 mmol substrate, 0.25 mmol Zn(NO3)2, 0.25 mmol VOC2O4, 5 mL CH3CN, 80 °C, 0.2 MPa O2. b Room temperature, normal pressure using O2 balloon.

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Ju, Y.; Du, Z.; Xiao, C.; Li, X.; Li, S. Novel Effect of Zinc Nitrate/Vanadyl Oxalate for Selective Catalytic Oxidation of α-Hydroxy Esters to α-Keto Esters with Molecular Oxygen: An In Situ ATR-IR Study. Molecules 2019, 24, 1281. https://doi.org/10.3390/molecules24071281

AMA Style

Ju Y, Du Z, Xiao C, Li X, Li S. Novel Effect of Zinc Nitrate/Vanadyl Oxalate for Selective Catalytic Oxidation of α-Hydroxy Esters to α-Keto Esters with Molecular Oxygen: An In Situ ATR-IR Study. Molecules. 2019; 24(7):1281. https://doi.org/10.3390/molecules24071281

Chicago/Turabian Style

Ju, Yongwei, Zhongtian Du, Chuhong Xiao, Xingfei Li, and Shuang Li. 2019. "Novel Effect of Zinc Nitrate/Vanadyl Oxalate for Selective Catalytic Oxidation of α-Hydroxy Esters to α-Keto Esters with Molecular Oxygen: An In Situ ATR-IR Study" Molecules 24, no. 7: 1281. https://doi.org/10.3390/molecules24071281

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