Two-dimensional manganese oxide on ceria for the catalytic partial oxidation of hydrocarbons
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Abstract
Although the rational synthesis of catalysts with strong oxide-support interactions to modulate the geometric and electronic structures and achieve unusual catalytic performance is challenging in heterogeneous catalysis, it is in significant demand for the efficient and sustainable transformation of chemicals. Here, we report the synthesis and performance of a ceria-supported two-dimensional manganese oxide catalyst with strong metal oxide-support interactions, which help to produce well-dispersed and amorphous MnOx layers on the CeO2 matrix
Keywords
INTRODUCTION
The oxidation of sp3-hybridized C-H bonds has emerged as a facile and industrially important process for the sustainable production of alcohols, ketones and epoxides from petroleum hydrocarbons[1-4]. Generally, the inorganic salts of ClO4- and NO3- are used as oxidants, which are costly and environmentally unfriendly[5]. Although molecular oxygen is regarded as a favorable oxidant, harsh reaction conditions are necessary for the activation of strong C-H bonds, which generally leads to significant energy consumption and uncontrollable selectivities[6-9]. In addition, over-oxidation occurs in many cases to produce carbon dioxide and other byproducts, particularly during vapor-phase oxidation[3,10]. As an attractive process in industry, the liquid-phase oxidation of C-H bonds has attracted tremendous attention, but the conversion of hydrocarbon substrates is always unsatisfactory. Subsequently, noble metal catalysts, peroxide initiators, organic solvents, supercritical CO2 and ionic liquid additives are used for enhancing the substrate conversion[11-16], but new problems arise, such as increased costs and extra product purification. Therefore, the efficient catalytic oxidation of C-H bonds with molecular oxygen over non-noble metal catalysts, as well as avoiding the use of solvents and initiator additives, remains challenging.
Mn-Ce oxides have been extensively investigated as efficient catalysts for the total oxidation of NO[17], CO[18-23] and organic molecules for pollutant removal[24-28], because of their high activity toward the activation of molecular oxygen. The features of Mn-Ce oxides in molecular oxygen activation have also motivated research into the selective oxidation of C-H bond and excellent success has been achieved. For example, Mn-Ce oxides were synthesized with surfactants or ionic liquids to maximize the solid-solution phase, which readily exhibited superior activity and selectivity for the selective oxidation of hydrocarbons[18]. However, organic solvents are still necessary in most of these cases[18]. Furthermore, the expensive surfactants and ionic liquids still limit their practical applications. Developing strategies for the synthesis of efficient Mn-Ce catalysts therefore remains a vital area of research in this field.
Here, we report a ceria-supported two-dimensional manganese oxide catalyst that is highly efficient for the selective oxidation of hydrocarbons without the use of solvents and initiator additives. The key to this success is anchoring the two-dimensional and amorphous manganese oxide layers on the cerium matrix (MnOx/CeO2) via strong oxide-support interactions (SOSIs). Owing to these features, the catalyst readily reacts with molecular oxygen to give a high capacity of active oxygen species at 46.1%. In the solvent- and initiator-free oxidation of C-H bonds on a series of hydrocarbons with molecular oxygen at low temperatures, the MnOx/CeO2 catalyst exhibits high activities and selectivities, as well as good stabilities in the recycling tests (Scheme 1). For example, in the aerobic oxidation of ethylbenzene, it gives a conversion of 63.5% and a selectivity of 95.7% to the corresponding alcohol/ketone products, thereby outperforming highly efficient heterogeneous catalysts under solvent- and initiator-free systems reported in the literature.
Scheme 1. Synthetic and catalytic strategies for ceria-supported two-dimensional manganese oxide catalyst with strong oxide-support interactions.
EXPERIMENTAL
Synthesis
Synthesis of MnOx/CeO2 catalysts
As a typical synthesis of the MnOx/CeO2(0.05) [where 0.05 is the theoretical Mn/(Mn + Ce) molar ratio] catalyst, 6 g of cerium nitrate [Ce(NO3)3·6H2O] and 0.26 g of a manganese nitrate aqueous solution
Synthesis of acid-treated MnOx/CeO2(0.05)
First, the acid-treated MnOx/CeO2(0.05) catalyst was synthesized by washing the as-synthesized
Synthesis of MnOx/CeO2 and MnOx/SiO2
For comparison, MnOx/CeO2 with an Mn/(Mn + Ce) molar ratio at 0.05 was also prepared by an incipient wetness co-impregnation method. In a typical run, an Mn(NO3)2 aqueous solution (50 wt.%) was used as a precursor and CeO2 was used as a support (CeO2 was prepared by the citric acid-assisted method discussed above). After impregnation, the catalysts were placed in the atmosphere statically overnight and then dried in an oven and calcined at 600 °C for 4 h in air. The resulting catalyst was denoted as MnOx/CeO2.
Catalysis
Catalytic oxidation of hydrocarbons
The oxidation of hydrocarbons in the liquid phase was performed with a high-pressure autoclave reactor equipped with a magnetic stirrer (900 rpm). In a typical run, the catalyst and hydrocarbon were mixed in the reactor and pure oxygen was used for the reaction. The autoclave was purged with oxygen three times to remove the air in the autoclave before the reaction. Oxygen was then introduced to the desired pressure. Next, the reactor was rapidly heated to the desired temperature (the reaction temperature was measured by a thermocouple in the autoclave and the pressure was measured at the reaction temperature). After the reaction, the reactor was placed in an ice bath to stop the reaction, and bromobenzene or biphenyl was used as the internal standard. In the stability test, the catalyst was removed from the reactor and washed with ethanol after each reaction run, dried overnight at 150 °C and then used in the next experiment to evaluate its recyclability.
The products were analyzed by gas chromatography (GC-14C, Shimadzu, with a flame ionization detector and ethanol or acetonitrile used to dilute the liquor before the analysis) with a flexible quartz capillary column (OV-17). The gas-phase products (e.g., CO or CO2 produced from over-oxidation) were analyzed using a Fu Li-9790 gas chromatograph equipped with a thermal conductivity detector (TCD). The as-synthesized catalyst was used each time to make sure the reactions were conducted under identical conditions. High-pressure oxygen has been extensively used in aerobic oxidations and the reaction systems in this work were out of the explosion limits of the reactants. For example, the explosion limit of ethylbenzene is 1.0%-7.1% in oxygen and the concentration of ethylbenzene in the gaseous phase in this work is out of the explosion limit. Fire and static electricity were therefore avoided for safety reasons.
Catalytic oxidation of CO
The oxidation of CO was performed in a continuous-flow fixed-bed quartz vertical reactor (length of
Characterization
Temperature-programmed desorption
The temperature-programmed desorption of O2 (O2-TPD) was measured in a BELCAT II catalyst analyzer equipped with a TCD. As a typical run, 30 mg of solid catalyst were pretreated at 600 °C for 1 h in air before the measurement and then cooled to 50 °C. The sample was then heated to 800 °C at a ramp rate of
In-situ Fourier transform infrared spectroscopy
The in-situ Fourier transform infrared (FTIR) spectra were recorded using a Nicolet is50 FTIR spectrometer equipped with an MCT/A detector, ZnSe windows and a high temperature reaction chamber. As a typical run, 50 mg of MnOx/CeO2(0.05) were localized in the sample chamber and pretreated at 120 °C for 1 h in flowing Ar (30 sccm) and then cooled to room temperature. The steam containing ethylbenzene (partial pressure in the range of 9-16 mbar) was introduced into the system with a flow of Ar carrier gas (30 sccm). After adsorption for 1 h at room temperature, a pure Ar gas was introduced to purge the sample for another 1 h and the FTIR spectrum of ethylbenzene adsorbed on the solid sample was then recorded. After increasing the reaction temperature to 160 °C in flowing Ar and maintaining it for 30 min, the FTIR spectra were recorded to understand the interaction between ethylbenzene and the active oxygen species on the catalyst surface. Whilst maintaining the temperature of the chamber at 160 °C, oxygen was introduced to further promote the interaction, where the FTIR spectra were recorded to detect the formation of carbon-oxygen species.
To investigate the crucial role of the active oxygen species on the catalyst, we eliminated the oxygen by H2 reduction during the in-situ FTIR study. In a typical run, 50 mg of the MnOx/CeO2(0.05) sample were pretreated at 200 °C with 10% H2/Ar for 2 h and then cooled to room temperature. The steam containing ethylbenzene was then introduced into the system with a flow of Ar carrier gas (30 sccm). After adsorption for 1 h at room temperature, a pure Ar gas was introduced to purge the sample for another 1 h. The FTIR spectrum of ethylbenzene adsorbed on the solid sample was then recorded. After that, the MnOx/CeO2(0.05) catalyst was treated at 200 °C in O2 for 1 h and then cooled to room temperature. Equivalently, ethylbenzene was introduced into the sample again and the FTIR spectrum was collected for understanding its adsorption behavior on the oxidized MnOx/CeO2(0.05) sample. The chamber temperature was then increased to 80 °C with an oxygen flow and the FTIR spectra were recorded to characterize the interaction of ethylbenzene with the catalyst.
In-situ Raman spectroscopy
Raman spectra were recorded using an HR800 Raman spectrometer equipped with an Ar excitation source
To further confirm the active surface oxygen species, in-situ oxidation of CO was conducted with 40 mg of the CeO2 or MnOx/CeO2(0.05) sample dried in a vacuum overnight to remove the water species. The Raman spectra of CeO2 were recorded at room temperature in air. The Raman spectra of MnOx/CeO2(0.05) were recorded at room temperature in air, room temperature in O2, 140 °C in CO, 250 °C in CO, 350 °C in CO and 350 °C in CO and O2.
Catalytic data analysis
Carbon balance during oxidation of hydrocarbons
The carbon balance before and after the reaction was calculated based on the number of carbon atoms in all the liquid reactants and products. For example, in the oxidation of ethylbenzene, acetophenone and phenylethyl alcohol are the detectable liquid products, while the CO2 from over-oxidation was not included in calculating the carbon balances. The carbon balance values are calculated according to the following equation:
where C% is the carbon balance, Mf is the final number of moles of ethylbenzene in the reactor after reaction, 8 is the number of carbon atoms in a single ethylbenzene molecule, M1(M2, M3, ……, Mx) is the number of moles of liquid product 1 (product 2, 3, ……, x) in the reactor after the reaction, n1(n2, n3, ……, nx) is the number of carbon atoms in a single molecule of product 1 (product 2, 3, ……, x) and Me is the number of moles of ethylbenzene in the feed mixture before the reaction.
In the kinetic study, the average reaction rates were calculated from the number of moles of substrate converted per gram of the catalyst in 1 h (mmol gcat-1 h-1) and the conversion of the reactants was controlled to be lower than 20%, which approximated the true reaction rates.
RESULTS AND DISCUSSION
Synthesis and structural characterization
The CeO2 support was synthesized via a citric acid-assisted method [Figure 1A]. Typically, Ce(NO3)3 and citric acid were dissolved in a mixture of water, ethanol and glycerol, where the citric acid can chelate with metal ions to form the metal citrate precursor [Supplementary Figure 1]. After removing the solvent and forming a fluffy material by stirring at 90 °C, followed by calcination to remove the organic species, the CeO2 was finally obtained [Supplementary Figure 2]. The CeO2-supported manganese oxide catalysts were synthesized following the same procedures by adding Mn(NO3)2 in the starting solution and denoted as MnOx/CeO2(y) [where y is the theoretical molar ratio of Mn/(Mn + Ce)] [Supplementary Figures 3-5].
Figure 1. (A) Procedures for synthesizing MnOx/CeO2(y) catalyst with MnOx species anchored on the surface of ceria matrix. (B) XRD patterns of commercial CeO2 and MnOx/CeO2(y) catalysts with different Mn loadings. (C) Zoomed-in views of diffractions peaks in (A) from 30° to 40° for MnOx/CeO2(y) catalysts with different Mn loadings. (D) O1s X-ray photoelectron spectra of MnOx/CeO2(0.05), CeO2 and MnO2. XRD: X-ray diffraction.
Figure 1B shows the X-ray diffraction (XRD) patterns of the commercial CeO2 and MnOx/CeO2(y) samples with different Mn loadings, which all exhibit peaks at 28.7°, 33.3°, 47.7° and 56.5°, assigned to the (111), (200), (220) and (311) diffraction peaks of a typical CeO2 crystal, respectively[18,22]. Notably, the shift of these diffraction peaks was undetectable for the MnOx/CeO2(y) samples compared to the pure CeO2, suggesting a lack of the Mn-Ce solid-solution phase[18,22]. The high dispersion and/or amorphous features of MnOx on the CeO2 matrix were evidenced by the undetectable XRD peaks associated with manganese oxide for MnOx/CeO2(0.025), MnOx/CeO2(0.05) and MnOx/CeO2(0.1) [Figure 1C]. However, the diffraction patterns of higher Mn content samples showed the crystallization of a Mn3O4 hausmannite phase (JCPDS 071841)[28], demonstrating the formation of sintered MnOx species in these samples.
The N2 sorption demonstrates that the surface areas of CeO2 and MnOx/CeO2(0.05) are lower than 10 m2/g, which should be due to the high-temperature calcination of the catalysts (800 °C for 4 h) that lead to the aggregation of the CeO2 support and the collapse of the pore structure. The surface geometry of
Figure 2. Electron microscopy and structural characterization. (A, E) BF-STEM images and the corresponding (B-D, F-H) Mn, Ce and O elemental maps of MnOx/CeO2(0.05). (E) Enlarged view of the yellow square in (A). (I-L) ABF-STEM and EDS analysis of
Figure 2E shows an enlarged view of the region in the yellow square in Figure 2A. Notably, the crystalline feature of the Mn species is completely unobservable. However, the elemental maps decidedly confirmed the existence of the MnOx species [Figure 2F-H], demonstrating that the structural feature of
The high-resolution HAADF-STEM image of the MnOx/CeO2(0.05) catalyst directly shows that the CeO2 matrix is partially coated with layers with an undetectable lattice [Figure 2M]. In particular, in this case, the catalyst calcined at 800 °C for 4 h precluded the carbon-containing species on the surface of CeO2. Considering that isolated Ce sites can be found in the amorphous region [Figure 2N], it is reasonable to assign the amorphous layers to the MnOx species. In addition, the Mn/(Mn + Ce) ratio appears at 0.22 on the catalyst surface by XPS analysis, which is distinctly different from the ratio of 0.05 in the whole oxides, indicating that MnOx mainly exists on the CeO2 surface. These results suggest that the structural feature of MnOx/CeO2(0.05) is dominated by amorphous MnOx layers on the surface of the CeO2 matrix rather than a monophasic solid solution. The unique structure of MnOx/CeO2(0.05) can be reasonably assigned to the citric acid-assisted method [Figure 1A], because the remarkably different decomposition temperature of cerium citrate (~161 °C) and manganese citrate (~296 °C) would benefit the phase separation to hinder the formation of the solid solution.
The MnOx structure was further characterized by Raman and EPR measurements. As shown in Figure 2O and Supplementary Figure 7, the as-synthesized MnOx/CeO2(0.05) catalyst shows a blue shift of the MnOx mode (654 cm-1) compared with that of various manganese oxide and supported MnOx catalysts prepared by the conventional impregnation method (≤ 640 cm-1), which is due to the SOSIs between MnOx and the CeO2 support and the formation of defect-rich MnOx layers[29-31]. In contrast, the EPR spectrum of the MnOx/CeO2(0.05) catalyst shows sharp sextuple signals with a g value at 2.00 [Figure 2P], indicating the formation of abundant oxygen vacancies by loading of the MnOx species in the MnOx/CeO2(0.05) catalyst[32]. For comparison, the bulky manganese oxide (MnO2), MnOx/CeO2 and MnOx/SiO2 prepared by the conventional impregnation method [Supplementary Figure 8] exhibit broad and weak signals in the EPR spectra [Figure 2P], which can be attributed to the Mn-Mn coupling (i.e., aggregation)[33-35]. These results reasonably confirmed the successful formation of well-dispersed and amorphous MnOx layers on the surface of the ceria matrix and the construction of abundant oxygen vacancies/defective sites in the MnOx/CeO2(0.05) catalyst, which are beneficial for the oxygen activation.
Figure 1D shows the O1s X-ray photoelectron spectra of the MnOx/CeO2(0.05), CeO2 and MnO2 samples. All samples show three peaks at 529.3, 531.4 and 532.7 eV, which can be assigned to the lattice oxygen of O2- (Olat), surface oxygen of O2-, O22- or O- (Osur) and adsorbed oxygen species from water/carbonate on the solid surface (Oads), respectively[18,19,28]. In these multiple oxygen species, the surface oxygen species from the defective site with an unsaturated structure displays a key role in the oxidation, which is denoted as active oxygen[18,26]. The atomic ratio of active oxygen to the total oxygen atoms on the oxide surface of CeO2 and MnO2 is 26.8% and 28.4%, respectively, while the MnOx/CeO2(0.05) exhibited a remarkably improved active oxygen capacity of 46.1% [Supplementary Table 1], which is even comparable to that of Mn-Ce solid solutions (44.1%) with a high Mn loading (50%)[18]. Considering that the MnOx/CeO2(0.05) has a low Mn loading (5%) and is mainly presented as amorphous MnOx layers on the CeO2 surface, the high capacity of active oxygen should be due to the maximized interfacial effect, causing the rich Ce3+ and Mn2+/Mn3+ species [Supplementary Table 1, Figure 2P, Supplementary Figure 9] on the surface to benefit the formation of unsaturated oxygen.
Redox investigation
The redox properties of the active oxygen species on MnOx/CeO2(0.05) were firstly identified by the O1s X-ray photoelectron spectrum in a temperature-programmed redox cycle [Figure 3A, Supplementary Figure 10, Supplementary Table 2]. Hydrogen treatments at low temperature (room temperature to 80 °C) partially reduced the surface oxygen sites, as evidenced by the decreased intensity of the O1s peak of Osur (531.4 eV) (Figure 3A and its inset), indicating that active oxygen species were available at low temperature. The subsequent reduction at the higher temperature of 120 °C led to further removal of the active oxygen. Very importantly, the active oxygen can be regenerated after treatment in air at 50 °C, displaying a similar O1s X-ray photoelectron spectrum to that of the as-synthesized sample (Figure 3A and its inset). Therefore, the high activity and remarkable stability of MnOx/CeO2(0.05) in the catalytic oxidation at low temperatures are reasonably expected.
Figure 3. Redox study. (A) In-situ O1s X-ray photoelectron spectra of MnOx/CeO2(0.05) under different redox conditions. Inset: the dependences of Osur capacities on redox conditions. (B) In-situ Raman spectra of MnOx/CeO2(0.05) and CeO2 under different conditions.
Further evidence regarding the redox properties of the active oxygen species is provided by in-situ Raman spectroscopy. As shown in Supplementary Figure 11, MnOx/CeO2(0.05) gives the F2g mode of the CeO2 matrix at 460 cm-1[23,30,36]. Compared to the spectrum of CeO2, MnOx/CeO2(0.05) shows an additional band at 656 cm-1 [Figure 3B], which is associated with the Mn-O-Mn stretching mode of the surface amorphous MnOx species with abundant defects[29-31], corresponding to the active oxygen in the X-ray photoelectron spectra [Figure 3A]. Notably, broadening, a decrease in intensity and a red shift of the band at 656 cm-1 occurred during the in-situ treatment of MnOx/CeO2(0.05) in atmospheric Ar at 140 °C. The intensity of the band at 645 cm-1 was further decreased when the temperature increased to 250 and 350 °C in Ar. This can be well explained by the desorption of the active oxygen species associated with MnOx species to form Mn sites with more defects[20,23]. Considering the well-dispersed, amorphous and layered structure of the MnOx species, it can be reasonably inferred that the desorption of the interfacial oxygen species accounts for the signal changes. To probe the reversibility of the active oxygen at low temperature, we exposed the sample to molecular oxygen at room temperature, resulting in a very similar Raman spectrum to the as-synthesized MnOx/CeO2(0.05) sample, thereby confirming the reversibility of active oxygen species [Figure 3B, Supplementary Figure 12], in good agreement with the in-situ XPS results [Figure 3A]. The active oxygen on MnOx/CeO2(0.05) was further evidenced by the O2-TPD test [Supplementary Figure 13]. The active and reproducible oxygen species over MnOx/CeO2(0.05) make it a potentially efficient catalyst for aerobic oxidation reactions.
In order to support this conclusion, we further evaluated the catalytic performance of the MnOx/CeO2(0.05) catalyst for the CO oxidation reaction, where the activation of oxygen molecules is regarded as a key step[37,38]. Figure 4A shows the dependence of CO conversion on temperature over the MnOx/CeO2(0.05) and CeO2 catalysts. The positive role of the interfacial effect on MnOx/CeO2(0.05) is evidenced by the full conversion of CO over MnOx/CeO2(0.05) at a lower temperature (270 °C) than that over CeO2 (370 °C). By correlating the inverse of the absolute temperature (1/T) with the initial reaction rates, the apparent activation energies of MnOx/CeO2(0.05) and CeO2 appeared at 51.4 and 69.2 kJ/mol, respectively [Figure 4B]. The relatively lower apparent activation energy of MnOx/CeO2(0.05) indicates that CO oxidation is easier. Based on the understanding that the activation energy is sensitive to the different active oxygen[37,39], a lower apparent activation energy of MnOx/CeO2(0.05) than that of CeO2 should be reasonably attributed to the abundant Mn-O-Ce interfacial sites to activate O2, rather than simply increasing the number of active oxygen species. Figure 4C shows the dependence of the reaction rate on oxygen partial pressure over different catalysts, where MnOx/CeO2(0.05) always displays higher reaction rates than CeO2 at the same oxygen pressure, thereby demonstrating its higher activities. Interestingly, under relatively low oxygen pressure (1.0-3.5 kPa), the reaction rates (r0) over CeO2 increased constantly with oxygen pressure, showing the reaction kinetic order at 0.37 [Figure 4D]. In contrast, the influence of oxygen pressure on the
Figure 4. CO oxidation. (A) Dependence of CO conversion on temperature over MnOx/CeO2(0.05) and CeO2 catalysts. (B) Arrhenius plots of CO oxidation over MnOx/CeO2(0.05) and CeO2 catalysts. (C) Dependence of reaction rate on oxygen pressure in CO oxidation over MnOx/CeO2(0.05) and CeO2 catalysts. (D) Data characterizing the kinetic reaction order of oxygen in CO oxidation over MnOx/CeO2(0.05) and CeO2 catalysts.
Aerobic oxidation of hydrocarbons
Initial attempts to evaluate the catalysts were performed in the selective oxidation of ethylbenzene to acetophenone and 1-phenylethanol at a low temperature of 110 °C for 6 h without solvent and initiator additives [Table 1]. A higher molar ratio of acetophenone and 1-phenylethanol indicates the deep dehydrogenation of 1-phenylethanol to acetophenone. The blank run without catalysts showed undetected reactivity (entry 1). The commercial nanosized CeO2, MnO2 and CeO2 were active for the reaction, giving ethylbenzene conversion at 3.0%, 1.3% and 6.8%, respectively (entries 2-4). The Mn salts of MnCl2 and KMnO4, which are well-known homogeneous catalysts in oxidation, gave ethylbenzene conversion at 8.2% and 1.9%, respectively (entries 5 and 6). Interestingly, interfacing MnOx on CeO2 significantly influenced the catalytic activity, where enhanced conversions were achieved over the MnOx/CeO2 catalysts (entries 7-11). By optimizing the Mn loadings on the catalysts, the best performance was achieved over MnOx/CeO2(0.05) with the highest capacity of active surface oxygen, displaying ethylbenzene conversion at 12.9% (entry 8). The decreased activity for the MnOx/CeO2 catalysts with an Mn/(Mn + Ce) ratio higher than 0.05 is due to the aggregation of the MnOx species [Figure 1C], leading to limited Mn-O-Ce interfacial sites for the oxidation of substrates. Very interestingly, the oxidation of ethylbenzene can be performed at a low temperature at 70 °C (entry 14), demonstrating the superior catalytic activity of MnOx/CeO2(0.05).
Catalytic data in oxidation of ethylbenzenea
| Entry | Catalyst | Temp. (°C) | Time (h) | Conv. (%) | Sel. (%) | A/Pb |
| 1 | Blank | 110.0 | 6.0 | < 0.1 | n.d. | n.d. |
| 2 | Commercial CeO2 | 110.0 | 6.0 | 3.0 | > 99.0 | 0.8 |
| 3 | MnO2 | 110.0 | 6.0 | 1.3 | > 99.0 | 0.6 |
| 4 | CeO2 | 110.0 | 6.0 | 6.8 | > 99.0 | 1.1 |
| 5c | MnCl2 | 110.0 | 6.0 | 8.2 | 97.6 | 2.6 |
| 6c | KMnO4 | 110.0 | 6.0 | 1.9 | 94.7 | 2.0 |
| 7 | MnOx/CeO2(0.025) | 110.0 | 6.0 | 9.2 | > 99.0 | 1.2 |
| 8 | MnOx/CeO2(0.05) | 110.0 | 6.0 | 12.9 | > 99.0 | 1.9 |
| 9 | MnOx/CeO2(0.1) | 110.0 | 6.0 | 7.1 | > 99.0 | 0.7 |
| 10 | MnOx/CeO2(0.2) | 110.0 | 6.0 | 5.6 | > 99.0 | 0.4 |
| 11 | MnOx/CeO2(0.3) | 110.0 | 6.0 | 4.1 | > 99.0 | 0.5 |
| 12 | MnOx/CeO2(0.05) | 110.0 | 16.0 | 27.1 | > 99.0 | 2.1 |
| 13 | MnOx/CeO2(0.05) | 110.0 | 24.0 | 29.4 | > 99.0 | 2.6 |
| 14 | MnOx/CeO2(0.05) | 70.0 | 6.0 | 0.7 | > 99.0 | 0.8 |
| 15 | MnOx/CeO2(0.05) | 90.0 | 6.0 | 4.3 | > 99.0 | 1.5 |
| 16 | MnOx/CeO2(0.05) | 130.0 | 6.0 | 24.2 | > 99.0 | 2.0 |
| 17 | MnOx/CeO2(0.05) | 130.0 | 24.0 | 34.7 | > 99.0 | 1.9 |
| 18 | MnOx/CeO2(0.05), 200 mg | 130.0 | 24.0 | 52.8 | > 99.0 | 3.4 |
| 19 | MnOx/CeO2(0.05), 400 mg | 130.0 | 24.0 | 59.2 | > 99.0 | 7.0 |
| 20 | MnOx/CeO2(0.05), 400 mg, 3 MPa of O2 | 130.0 | 24.0 | 63.5 | 95.7 | 5.8 |
| 21 | MnOx/CeO2(0.05), 600 mg, 3 MPa of O2 | 130.0 | 24.0 | 65.6 | 77.6 | 4.7 |
| 22d | MnOx/CeO2 | 130.0 | 24.0 | 19.2 | > 99.0 | 16 |
| 23 | MnOx/CeO2(0.05), 2 MPa of N2 | 110.0 | 6.0 | < 0.1 | n.d. | n.d. |
| 24 | MnOx/CeO2(0.05), 50 mg of hydroquinone | 110.0 | 6.0 | < 0.1 | n.d. | n.d. |
To realize the best performance, we optimized the reaction temperature, time, catalyst amount and oxygen pressure in the reactor (entries 12-21). The ethylbenzene conversion of 59.2% with > 99.0% selectivity to acetophenone and 1-phenylethanol was achieved (entry 19). This result, obtained under solvent- and initiator-free conditions, even outperforms a series of superior catalysts for C-H bond selective oxidations reported in the literature [Supplementary Table 3], such as the Au nanosheet with undetectable reactivity under solvent- and initiator-free conditions[12]. The MnOx/CeO2 catalyst synthesized from the conventional impregnation method with the same Mn loading to MnOx/CeO2(0.05) exhibited poor activity (entry 22), which might be due to the aggregation of the MnOx particles and the limited interfacial sites, as confirmed by the Raman and EPR analysis [Figure 2O and P]. Increasing the MnOx/CeO2(0.05) catalyst amount led to enhanced ethylbenzene conversion but unsatisfactory carbon balance values because of the over-oxidation to CO and CO2 (entries 20 and 21, Supplementary Figure 14). The molecular oxygen indeed acted as oxygen donors in the reaction, as evidenced by the undetectable conversion of ethylbenzene when using nitrogen instead of oxygen in the reaction (entry 23). When the free radical scavenger was added to the reaction system, the reaction was quenched, demonstrating that the reaction follows a radical chain mechanism (entry 24).
More importantly, the catalyst is stable, reusable and can be easily filtrated and recycled after each reaction run. Consequently, it exhibited stable catalytic performance in the recycling tests
Mechanism investigation
Based on the aforementioned results, we conclude that MnOx/CeO2(0.05) is highly active for the selective oxidation of C-H bonds under solvent- and initiator-free conditions at low temperature based on its earth-abundant metals as catalysts and constructing SOSIs. In contrast to the existing understanding that highly active Mn-Ce catalysts should be dominated by the solid-solution phase, the greatest difference in our case is that MnOx/CeO2(0.05) has separated phases of two-dimensional and amorphous MnOx layers on CeO2. We conclude that the high efficacy of MnOx/CeO2(0.05) originates from the SOSIs and maximized interfacial effects, because a highly active solid-solution catalyst requires a high Mn concentration (~50%)[18].
In order to further address this conclusion, we performed acid treatment for MnOx/CeO2(0.05) to partially remove the surface MnOx species, as confirmed by the X-ray photoelectron spectra giving the surface Mn/(Mn + Ce) ratio decreased from 0.22 to almost undetectable [Figure 5A]. The acid-treated
Figure 5. Mechanism studies. (A) X-ray photoelectron spectra of MnOx/CeO2(0.05), acid-treated MnOx/CeO2(0.05) and CeO2. (B) Reaction rate and product selectivities in ethylbenzene oxidation over various catalysts. The triangles and stars represent the selectivities to 1-phenylethanol and acetophenone, respectively. (C) In-situ FTIR spectra of ethylbenzene adsorbed on
The aforementioned results demonstrate the important role of the active oxygen species on the
CONCLUSIONS
In summary, we have demonstrated a ceria-supported two-dimensional manganese oxide with a SOSI effect as an efficient catalyst for the selective oxidation of hydrocarbons, thereby breaking the normal principle of synthesizing Mn-Ce catalysts with an abundant solid-solution phase. The key to the success is to anchor well-dispersed and amorphous MnOx layers on the CeO2 matrix by SOSIs, forming abundant active oxygen species over MnOx/CeO2(0.05) because of the high interfacial efficacy, which displays crucial roles in the adsorption and activation of C-H bonds. In the selective oxidation of hydrocarbons to the corresponding alcohols and ketones, MnOx/CeO2(0.05) exhibited high activity, selectivity and recyclability under solvent- and initiator-free conditions at low temperature, outperforming the noble catalysts and the state-of-the-art Mn-Ce metal oxide catalysts. This work not only highlights the importance of SOSIs for the oxidation of
DECLARATIONS
Authors’ contributions
Carried out the catalyst preparation, characterization, catalytic tests, and prepared the draft manuscript: Wang H
Performed part of the catalyst characterization: Luo Q, Zhang J, Wang C
Performed the TEM characterization: Ge X, Zhang W
Planned the study, analysed the data and wrote the manuscript: Wang L, Xiao FS
Availability of data and materials
Experimental details on materials and characterization are available in the Supplementary Information, and the data supporting the findings of this study are available from the corresponding author upon reasonable request.
Financial support and sponsorship
This work is supported by National Natural Science Foundation of China (21822203 and 21932006), China National Postdoctoral Program for Innovative Talent (BX2021256), China Postdoctoral Science Foundation (2021M700119), and 2020 International Cooperation Project of the Department of Science and Technology of Jilin Province (20200801001GH).
Conflict of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
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Wang, H.; Wang, L.; Luo, Q.; Zhang, J.; Wang, C.; Ge, X.; Zhang, W.; Xiao, F. S. Two-dimensional manganese oxide on ceria for the catalytic partial oxidation of hydrocarbons. Chem. Synth. 2022, 2, 2. http://dx.doi.org/10.20517/cs.2022.02
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