Effect of metal oxide composite method on catalytic oxidation performance of aerogel supported Pd catalysts in oxidative carbonylation

https://doi.org/10.1016/j.jorganchem.2020.121110Get rights and content

Highlights

  • Catalysts were prepared by the in-situ method, the precipitation method and the impregnation method.

  • The composite method affect the particle size and specific surface area.

  • The composite method affect the surface oxygen species.

Abstract

A series of metal manganese-cerium silicon composite aerogel supported Pd catalysts were prepared by the in-situ method, the precipitation method and the impregnation method. The catalysts were applied to synthesize diphenyl carbonate (DPC) by oxidative carbonylation. The effects of different preparation methods and composite metal oxide contents on catalytic activities were studied. The prepared catalysts were characterized by XRD, FTIR, BET, TEM, H2-TPR and XPS. The results showed that the metal composite method had a great influence on the catalyst particle size and specific surface area; the low temperature oxidation performance and surface oxygen species content of the catalysts prepared by different methods were different. The catalyst prepared by the impregnation method has a large specific surface area and particle size, good low-temperature oxidation performance and more surface adsorption oxygen, which helps to improve the multi-step electron transfer efficiency, to promote the regeneration of the active component Pd2+ and to increase catalytic activity.

Introduction

Diphenyl carbonate (DPC) is an important intermediate for the preparation of polycarbonate (PC), which is the second largest engineering thermal plastic. The main methods of its preparation are phosgene, transesterification and oxidative carbonylation method [1,2], among which the oxidative carbonylation method is to synthesize DPC directly with CO, O2 and phenol in the presence of catalyst. Because of its simple process, cheap raw materials, only a single by-product H2O, and a high atom utilization, this method is a green environmental protection method with bright prospect in industrial application [3]. The catalysts for the synthesis of DPC by the oxidative carbonylation are divided into two main types: homogeneous complex catalysts and heterogeneous supported catalysts. The former has a big disadvantage-its product is very hard to separate. As a consequence of good performance of easy product separation, the heterogeneous catalytic oxidation method becomes the most active research direction of synthesizing DPC by carbonylation [4,5]. After more than a decade of active research and development, a variety of supported catalysts have been attempted such as the activated carbon, molecular sieves, polymers, carbon nanotubes [[6], [7], [8], [9]] and so on. Unfortunately, the active component Pd is very easy to aggregate, and Pd2+ is very poor to be recycled, which led to the low yield of DPC. This kind of research has been confronted with a bottleneck, with no industrialized report.

Metal silicon composite aerogel has many excellent carrier properties of aerogel such as low density, high specific surface area, high porosity, 3D network structure and so on [[10], [11], [12]]. Silicon aerogel with a mesoporous structure limits the grain size of the metal oxide, and its high specific surface area promotes the active ingredient and the metal cocatalyst to be highly dispersed on the carrier, thereby providing more catalytic active sites; electron transfer between metal and silicon-based aerogels, ie metal-carrier strong interaction (SMSI effect), can regenerate high-valence metals and improve the oxidation capacity of metal cocatalyst; in addition, silicon aerogels have chemical stability and thermal stability, so it is destined to have a good application prospect in the oxidative carbonylation of phenol to DPC. The preparation process of metal/silicon-based composite aerogel is simple, the reaction raw materials is easy to get and the reaction process is easy to control. At present, the mature preparation methods include in-situ method, precipitation method and impregnation method [13,14].

Surface modification of Pb catalysts with rare earth oxides to improve the oxidation activity and to enhance catalytic performance is one of the hotspots [[15], [16], [17], [18]]. Among them, the most important rare earth in industrial catalysis is a composite oxide of Ce. Its special catalytic performance is manifested in the following three aspects: 1. The ionic radius of rare earth is large, with a high charge, and easy to form a stable new crystal structure; 2. The ionic radius of rare earth is large, with high coordination, and between different coordination structures, the conversion energy barrier is so low, that it is suitable for catalytic reaction; 3. Cerium is easy to convert between +3 valence state and +4 valence state, and it is easy to form flowing oxygen vacancies in the structure, so it has better oxygen storage performance and transmission capacity on surface, and can produce a synergistic effect with the metal interface [19], which will contribute to the multistep electron transfer process of the oxidative carbonylation. Based on the above three reasons, the metal Ce is inevitably a good promoter in the carbonylation catalytic oxidation process. According to the theory of redox potential, the metal oxides, which can play an intermediate role in the redox chain of the oxidative carbonylation, generally have an oxidation-reduction potential between the following two states, that is, a high-valent cation can oxidize Pd0 to form an active component Pd2+, and the reduced state, low-valent ions are rapidly oxidized by oxygen to high-valent ions. While the redox potential of MnO2/Mn2+ (1.224 V) just lies between Pd2+/Pd0 (0.99 V) and O2/H2O (1.23 V). Theoretically, Mn oxides can implement the redox cycle regeneration of the catalytic active component Pd2+. Therefore, the composite oxide of Ce–Mn is a promising cocatalyst for oxidative carbonylation of phenol to DPC.

In this paper, Ce–Mn oxide silicon-based composite aerogel supported Pd catalysts were prepared with the low-energy atmospheric pressure drying technology and the catalytic carrier reconstruction technology, which fully utilized the synergistic promotion of the cocatalyst, Ce–Mn oxide and the carrier, silicon-based aerogel. It investigated the different effects of in-situ, precipitation and impregnation methods on the synthesis of DPC by oxidative carbonylation of phenol.

Section snippets

In-situ method

According to the sum of the mass of Ce and Mn (in which the molar ratio was 1:1) and the mass of silica (calculated as the source of silicon in ethyl orthosilicate), that is 10%, 20%, 40% and 60% respectively, manganese nitrate (Mn(NO3)2) and cerous nitrate (Ce(NO3)3) were dissolved in ethanol, and added stepwise to the precursor of silica-based aerogel (the final molar ratio of ethanol, TEOS, deionized water, N, N-dimethylformamide was 3:1:6:0.8), and adjusted to the pH of 3 with nitric acid

Characterization of the catalysts

The XRD pattern of the catalysts prepared by the three methods is shown in Fig. 1. Clear diffraction peaks appear respectively at 36.5°, 41.2°, and 58.7° of the three XRD curves, which matches the MnO standard spectra (JCPDS 07–0230) in the PDF card library. So it describes that the samples have a metal oxide with aggregated phase MnO. At 28.60°, 47.64°, 56.48°, etc. there are also clear diffraction peaks, and it matches CeO2 standard spectra (JCPDS 34–0394) in the PDF card library, so it

Conclusion

A series of metal cerium manganese silicon-based composite aerogel supported Pd catalysts were prepared by the in-situ method, precipitation method and impregnation method respectively, and these catalysts were applied to the synthesis of DPC by oxidative carbonylation of phenol. The effects of different preparation methods and the content of cerium manganese oxide on the catalytic activity were investigated. The results show that the catalyst prepared by the impregnation method has relatively

Declaration of competing interest

The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

We are grateful for the financial support from Hubei Provincial Natural Science Foundation of China (Grant No.2019CFB318) and Scientific Research Fund of Wuhan Institute of Technology (Grant No.K201941).

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