Subnanometric Gold Clusters on CeO2 with Maximized Strong Metal

KEYWORDS: C-H oxidation, Subnanometric gold clusters, Metal-support interaction,. Heterogeneous catalyst. We report the ..... the oxidation of CO, whi...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Subnanometric Gold Clusters on CeO2 with Maximized Strong Metal−Support Interactions for Aerobic Oxidation of Carbon− Hydrogen Bonds Shaodan Xu,*,† Huanxuan Li,† Jia Du,† Junhong Tang,*,† and Liang Wang‡ †

College of Materials & Environmental Engineering, Hangzhou Dianzi University, No. 1158, Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China ‡ Key Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Xixi Campus, No. 148, Tianmushan Road, Hangzhou 310028, China S Supporting Information *

ABSTRACT: We report the synthesis of subnanometric Au clusters with average diameters at 1.2 nm on a low-crystallinity CeO2 support (sub-Au/LC-CeO2), which exhibit high activities and selectivities in the aerobic oxidation of C−H bonds on a series of petroleum-derived hydrocarbons under solvent-free conditions. This phenomenon is strongly related to the maximized strong metal−support interactions between subnanometric Au and the LC-CeO2 support. KEYWORDS: C−H oxidation, Subnanometric gold clusters, Metal−support interaction, Heterogeneous catalyst

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INTRODUCTION The oxidation of sp3-hybrid carbon−hydrogen (C−H) bonds has been regarded to be one of the most important routes for producing valuable alcohols, ketones, and aldehydes from hydrocarbons in the petroleum industry.1−9 Generally, these reactions were performed under unsustainable reaction conditions (e.g., harsh temperature >400 °C) for the activation of C−H bonds with high bonding energy, which is consumes energy and leads to serious problems of overoxidation to form carbon dioxide and coke to deactivate the catalysts.10−12 In order to overcome these issues, the liquid-phase oxidation of hydrocarbons has been utilized.13−15 A well-known example is the liquid-phase oxidation of cyclohexane, an important reaction for the production of K-A oil (mixture of cyclohexanone and cyclohexanol) as feedstocks of Nylon-6 and Nylon-66.16−18 This industry process was performed over homogeneous catalysts of cobalt or manganese salts with air as the oxygen donor, where the catalysts were difficult to separate and regenerate from the reaction liquor.19 In addition, it is still difficult to control the product selectivities over the homogeneous catalysts, and the cyclohexane conversions have to be limited at 99.5 98.0 >99.5 >99.5 98.1

a

Reaction conditions: 1.2 MPa of O2, 50 mg of catalyst, 27 mmol of substrate, 0.8 mmol of TBHP, 10 h. Conv = [consumed cyclohexane]/ [initial cyclohexane] × 100%. Sel = [cyclohexanol + cyclohexanone]/ [consumed cyclohexane] × 100%. K/A = [cyclohexanone]/[cyclohexanol] × 100%. The byproducts are CO2 and some others. b Undetectable. c150 mg of catalyst was used. dIn nitrogen without oxygen. eWith 100 mg of hydroquinone as additive. f1.2 MPa of air was used as oxidant.

95.2% at 150 °C. Increasing the reaction temperature to 170 °C leads to the slightly increased conversion, but the selectivity of K-A decreased to 70.2% due to the overoxidation to form CO2. In contrast, the other supported Au catalysts all give relatively lower activity. For example, the cyclohexane conversion over 1.1Au/CeO2-C and 2.9Au/CeO2-C are at 7.0 and 2.1%, respectively. The SiO2 and Al2O3 supported Au catalysts failed to catalyze the cyclohexane oxidation (Figure S4). These data confirm the superior catalytic performances of sub-Au/LCCeO2 in the aerobic oxidation of cyclohexane. Furthermore, we calcined the sub-Au/LC-CeO2 sample in air, which resulted in catalysts with aggregated Au nanoparticles on LC-CeO2 (subAu/LC-CeO2-500, Figure S5). In the aerobic oxidation of cyclohexane, sub-Au/LC-CeO2-500 gave cyclohexane conversion at 6.9%, which is much lower than 14.0% over the assynthesized sub-Au/LC-CeO2 but still higher than 2.1% over commercial CeO2 with similar Au loading. These data suggest the important role of both subnanometric Au clusters and amorphous CeO2 for achieving good performances in the reaction. A further test was performed using nitrogen instead of oxygen in the reactor, where the conversion of cyclohexane was undetectable, demonstrating that the molecular oxygen was the principal oxygen donor for the catalytic oxidations. Additionally, if the hydroquinone was added into the reaction liquor, the reaction would be switched off, suggesting that the oxidation of the C−H bond proceeds through a radical chain mechanism. Notably, when the reaction was performed using air as an oxidant instead of pure oxygen, the sub-Au/LC-CeO2 still gave

Figure 7. (A) TEM image of sub-Au/LC-CeO2 after the 6th run in the aerobic oxidation of cyclohexane. The yellow cycles highlighted the Au nanoparticles. (B) Dependences of cyclohexane conversion on time in the beginning of the reaction in cyclohexane oxidation over sub-Au/ LC-CeO2. The reaction conditions are the same as those in Table 1. The catalyst was separated from the reaction liquor at 2 h in the red curve.

degree at 53% (Table S2), which are comparable to the assynthesized catalyst. Furthermore, characterization by XP spectroscopy showed that the used sub-Au/LC-CeO2 still gives very similar XP spectra to the fresh sample (Figures 5A− C), confirming the high stability. Moreover, when the catalyst was removed from the reaction liquor during the reaction, the reaction was immediately switched off (Figure 7B). ICP analysis of the reaction liquor shows that the concentration of Au is lower than 5 ppb, demonstrating that the Au leaching is negligible. All these data confirm the high stability and good recyclability of sub-Au/LC-CeO2 catalyst, which is crucial for the potential practical application. E

DOI: 10.1021/acssuschemeng.8b00202 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Aerobic Oxidation of Various Substrates. More catalytic tests of sub-Au/LC-CeO2 were performed in the aerobic oxidation of various petroleum-derived molecules, including the ethylbenzene, toluene, n-propylbenzene, diphenylmethane, 1,2,3,4-tetrahydronaphthalene, and 1,2-diphenylethane, where the satisfactory substrate conversions and selectivities to the corresponding ketones/aldehydes/alcohols were always achieved (Table 2). For example, in the oxidation ethylbenzene,

LC-CeO2 catalyst exhibits good catalytic performances in the aerobic oxidation C−H bonds on various petroleum-derived hydrocarbons, giving satisfactory yields to the corresponding aldehydes/ketones/alcohols. The good performance of subAu/LC-CeO2 is related to the subnanometric Au, which has maximized metal−supported interactions with the CeO2 support, leading to the formation of rich active oxygen species. Importantly, the sub-Au/LC-CeO2 is stable and exhibits constant performances in the recycle tests. This work not only reports an active catalyst for C−H oxidation but also presents a size effect of Au catalysts in constructing metal− support interactions. The strategy in this work would be helpful for developing highly active catalysts by strengthening the metal−support interactions.

Table 2. Catalytic Data in Aerobic Oxidation of Various Hydrocarbons over sub-Au/LC-CeO2 Catalysta

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00202. XRD, TEM, TG, Raman, and more catalytic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.X.). *E-mail: [email protected] (J.T.). ORCID

Shaodan Xu: 0000-0002-7215-8689 Liang Wang: 0000-0002-5826-1866 Author Contributions

S.X. performed the catalyst preparation, characterizations, and catalytic tests. H.L. and J.D. performed the sample characterizations. S.X. and J.T. designed this study, analyzed the data, and wrote the paper. L.W. participated in the discussion and offered helpful suggestions.

selectivity (%) substrate

conversion (%)

P1

P2

ethylbenzene toluene n-propylbenzene diphenylmethaneb 1,2,3,4-tetrahydronaphthalene 1,2-diphenylethaneb

33.2 6.5 44.1 45.9 59.3 36.5

96.0 81.2 84.3 76.2 70.2 67.2

4.0 18.8 10.0 2.1 22.8 19.9

Notes

P3

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (No. 41373121).

5.7

a Reaction conditions: 1.2 MPa of O2, 160 °C, 50 mg of catalyst, 27 mmol of substrate, 0.8 mmol of TBHP, 10 h. bThe byproducts are benzyl alcohol, benzaldehyde, benzene, and some others.

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the sub-Au/LC-CeO2 gives ethylbenzene conversion at 33.2% with 96.0% selectivity to acetophenone, outperforming the other supported Au catalysts with relatively lower ethylbenzene conversions.43 Even in the oxidation toluene, which is more challenging to oxidize, the sub-Au/LC-CeO2 still gives the toluene conversion at 6.5%, giving phenyl alcohol and benzaldehyde as major products. These data confirmed the universality of sub-Au/LC-CeO2 for the C−H bond oxidation in various molecules.

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CONCLUSION In summary, an alternative catalyst of subnanometric Au clusters with average diameters at 1.2 nm supported on CeO2 support has been prepared (sub-Au/LC-CeO2). The sub-Au/ F

DOI: 10.1021/acssuschemeng.8b00202 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.8b00202 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX