In-situ preparation of molybdenum trioxide-silver composites for the improved photothermal catalytic performance of cyclohexane oxidation

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Abstract

The selective catalytic oxidation of cyclohexane has important theoretical and practical application value. However, high conversion rate and high selectivity are difficult to achieve simultaneously by conventional catalytic system. In this work, blue molybdenum trioxide (MoO3) nanorods with oxygen vacancies were prepared by hydrothermal method using hydrated molybdic acid as a precursor under the reduction of formic acid, and in-situ produced MoO3-silver (MoO3-Ag) composites were further used in the photothermal catalytic oxidation of cyclohexane with high conversion and high selectivity using dry air as oxidant. The results showed that the best conversion rate of cyclohexanone and cyclohexanol (KA oil) could reach 8.6% with the selectivity of 99.0%. The excellent catalytic performance of MoO3-Ag composites can be attributed to the significantly increased visible and near-infrared light absorption caused by the plasma resonance effect of Ag nanoparticles and oxygen vacancies, and the prevented charge recombination by MoO3-Ag Schottky heterojunction. This work provides new reference solutions for the design and preparation of high-performance photothermal catalysts for the selective oxidation of hydrocarbons.

Introduction

Cyclohexane selective catalytic oxidation plays an important role in the chemical industry [1], [2]. However, the current catalytic systems are difficult to achieve both high activity and high selectivity, which limits their further application in the oxidation reactions of cyclohexane. The activation and the selective oxidation of saturated hydrocarbon bonds typified by cyclohexane have always been difficult in the field of industrial catalysis [3]. Because Carbon-Hydrogen (Csingle bondH) bonds were very stable, they usually need to be activated under harsh conditions such as high temperature and high pressure. Moreover, their oxidation products of cyclohexanone and cyclohexanol are more easily oxidized than cyclohexane, resulting in the low selectivity of cyclohexanone and cyclohexanol (KA oil) at high conversion rates. For example, TiO2/reduced Graphene Oxide, Co3O4/Al2O3, NH2-MIL125/TiO2 and other composite materials have been used as catalysts for cyclohexane oxidation, but it is difficult to obtain high selectivity and high activity at the same time, which is not fully suitable for practical industrial applications [4], [5], [6], [7], [8], [9].Therefore, it is very important and valuable to develop a new type of high activity and stable recyclable catalyst for cyclohexane oxidation process using cheap air as oxidant under mild conditions.

Because traditional thermal catalytic process can selectively activate Csingle bondH bonds and effectively promote the decomposition of intermediate products, it was widely applied in the industrial production of KA oil. In order to prevent excessive oxidation of cyclohexane into CO2 and H2O, the conversion rate of cyclohexane oxidation is generally controlled to only about 5%, and the selectivity of KA oil is only 80 ~ 85% [10]. However, thermal catalytic oxidation of cyclohexane has the problems of higher reaction temperature, more side reactions, and difficulty in achieving high conversion rate and high selectivity simultaneously. Therefore, it is imperative to explore a new reaction process for the selective oxidation of cyclohexane under mild conditions. Photocatalytic technology can effectively utilize solar energy and cheap oxygen to oxidize cyclohexane under mild conditions [11]. Compared with traditional thermal catalytic systems, photocatalytic systems are able to generate more reactive free radicals. The photocatalytic cyclohexane oxidation process can adjust the types of active radicals by changing the energy levels of conduction bands and valence bands, thereby effectively regulating the type of its oxidation products and improving the selectivity of KA oil. However, due to the low photocatalytic activity caused by the easy recombination of photogenerated electrons and holes, a longer reaction time is required to obtain high conversion rate. Therefore, there is an urgent need to further improve the photocatalytic activity and conversion rate [12].

Photothermal synergistic catalytic oxidation of cyclohexane can combine the advantages of both thermal catalysis and photocatalysis, and obtain better activity and selectivity than photocatalysis and thermal catalysis alone. As an efficient photothermal catalytic material, it must meet the following conditions: first, it should have strong light absorption, good charge separation and transmission, and strong photothermal effects; second, it should have good thermal catalytic activity [13], [14]. In our early work, WO3 was used in the photothermal synergetic catalytic oxidation of cyclohexane with generally good catalytic activity. However, there were still problems of limited light absorption and low thermal catalytic activity [15], [16], [17]. Compared with WO3 nanomaterials, MoO3 with oxygen vacancies has higher thermal catalytic performance in the cyclohexane oxidation because the existence of pentavalent molybdenum (Mo5+) and Mo5+-O-Mo6+ can promote the decomposition of cyclohexyl hydroperoxide [18], [19], [20], [21].

Heterogeneous catalysis has a wide range of applications, and the catalyst can be easily separated from the reaction system and reused to effectively reduce industrial production costs [22], [23]. Among the heterogeneous catalysts, the metal catalysts supported on the carriers have higher catalytic activity, good thermal stability and recycling performance [22], [23], [24], [25]. Molybdenum trioxide (MoO3) semiconductors have excellent photocatalytic activity with the forbidden energy gap width of about 3.0 eV [26], [27]. Because the conduction band position of MoO3 located at about + 0.34 eV is more positive than the reduction potential of O2/O2 at −0.13 eV, the generation of superoxide radicals is unable to proceed, which can effectively prevent the excessive oxidation of cyclohexane [28].The introduction of oxygen vacancies can change the energy level structure of MoO3, improve its charge mobility, and increase the absorption capacity of visible and near-infrared light with higher photothermal conversion effects, thereby improving its catalytic activity [29], [30]. The loading of noble metal silver (Ag) can effectively inhibit the recombination of photogenerated electrons and holes to further improve the catalytic activity of MoO3 [31], [32]. Therefore, it is highly anticipated that the combination of MoO3 with oxygen vacancies and Ag nanoparticles can greatly improve the photothermal catalytic conversion rate of cyclohexane oxidation while maintaining high selectivity. Till now, the controlled synthesis of MoO3 nanorods with oxygen vacancies and its improved photothermal catalytic performance of cyclohexane oxidation have been hardly reported.

In this work, we use the reducibility of formic acid to controllably synthesize MoO3 nanorods with oxygen vacancies, and further prepare MoO3-Ag composites through in-situ reduction of Ag ions by Mo5+ in the MoO3 nanorods. The prepared MoO3-Ag composites with different Ag loading amounts were used in the photothermal catalytic oxidation of cyclohexane under dry air. The effects of oxygen vacancies and Ag loading amounts in the MoO3-Ag composite catalysts on the photocatalytic, thermal catalytic and photothermal catalytic performances of cyclohexane oxidation were investigated in detail. The photothermal synergetic catalytic mechanism was also proposed. Our work will have a beneficial impact on the design of catalysts and catalytic systems for the selective catalytic oxidation of alkanes and naphthenes.

Section snippets

Materials

Ammonium molybdate ((NH4)6Mo7O24·4H2O,99%), nitric acid (HNO3, 99%), acetic acid (absolute) , silver nitrate (AgNO3, 99%), ethanol (absolute) , acetone, ethylene glycol were purchased from Alfa Aesar company. Deionized water (DI water) was produced by the Millipore-Q water purification system. All the reagents were used without further treatment.

Synthesis of MoO3 nanorods and MoO3-Ag composites

The molybdenum trioxide nanorods were synthesized by a hydrothermal method (Scheme 1.). Typically, 500 mg of sodium molybdate was dissolved into 5 mL

Synthesis and characterization of MoO3 and MoO3-Ag composites

According to the synthesis method described above, by changing the type of acid, MoO3 nanorods with different morphologies will be prepared by a simple hydrothermal method as shown in Fig. 1. MoO3 short rods with a length of about 1 μm and a diameter of about 300 nm (Fig. 1a) were obtained without the use of acetic acid. After the addition of 2 mL acetic acid, white colored MoO3 nanowires with a length of about 5–10 μm and the diameter of about 400 nm (marked as L-MoO3) are shown in Fig. 1b. As

Conclusion

In summary, MoO3 nanorod-Ag composites were prepared for the improved photothermal catalytic performance in the cyclohexane oxidation with the oxidant of died air. The blue MoO3 nanorods with a smaller diameter than the literature and plenty of oxygen vacancies were synthesized by a hydrothermal method with the effect of reducing formic acid. The in-situ reduction reaction of silver ions by MoO3 nanorods was carried out to prepare MoO3-Ag composites with uniform size and distribution. TEM and

CRediT authorship contribution statement

Xiaoyu Wang: Data curation, Formal analysis, Methodology, Writing - original draft. Zhen Feng: Investigation, Methodology. Jincheng Liu: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Validation, Writing - review & editing. Zhilin Huang: Data curation, Formal analysis. Jinhong Zhang: Formal analysis, Methodology. Jijin Mai: Formal analysis. Yanxiong Fang: Viadiation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Authors would like to acknowledge the National Natural Science Foundation of China (Grant No.21978054 and 21776049) and the Natural Science Foundation of Guangdong Province (Grant No. 2018A030313174) support for this work.

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