Elsevier

Applied Catalysis A: General

Volume 592, 25 February 2020, 117417
Applied Catalysis A: General

Efficient selective oxidation of benzyl alcohol with oxygen in a continuous fixed bed reactor over NiGa hydrotalcite derived catalyst

https://doi.org/10.1016/j.apcata.2020.117417Get rights and content

Highlights

  • Aerobic oxidation of benzyl alcohol in a fixed-bed reactor was realized over NiGa catalysts.

  • Calcination has a key effect on the composition of the catalyst and the surface property.

  • Surface basicity has an important effect on the catalytic performance.

  • Formation of coke should be responsible for the deactivation of catalyst.

  • Regeneration of catalyst was realized with only slight reduction of the performance.

Abstract

A series of NiGa catalysts have been prepared by calcining Ni5Ga-LDH under different temperatures, detailedly characterized, and investigated in the aerobic oxidation of benzyl alcohol to benzaldehyde in a fixed-bed reactor. The results indicated that calcination has a key effect on the composition of the catalyst and the catalytic performance, and the NiGa hydrotalcite calcined at 250 ℃ (Ni5Ga-LDH-250) exhibited the highest activity, which probably was due to the highest amount of surface basic site and the composition of Ni species. Under the optimized conditions, an excellent 91.5 % yield of benzaldehyde could be continuously obtained. Moreover, the catalytic performance kept almost steady in 170 h, and the catalyst could be regenerated and reused with slight reduction of the catalytic performance. According to the analysis of structure-activity relationship and the results of controlled experiments, possible reaction pathways for the aerobic oxidation of benzyl alcohol over Ni5Ga-LDH-250 are proposed.

Introduction

Benzaldehyde is an important and commonly used raw material for the production of fine chemicals for such applications as fragrances and botanical agents [1,2]. Hydrolyzation of benzyl chloride and oxidation of toluene are the two main commercial processes for the benzaldehyde production. However, these processes suffer from the production of chlorine-containing waste water and quite low-efficiency. In the current laboratory, benzyl alcohol oxidation by stoichiometric amounts of oxidant is the main method for the synthesis of benzaldehyde. However, the application of stoichiometric reagents, including manganese oxide or chromium salts, results in many drawbacks in cost and environment. Development of catalytic process for the production of benzaldehyde from benzyl alcohol is highly desirable.

To date, many homogeneous or heterogeneous catalytic protocols have been studied for the reaction using various oxidants, such as H2O2 [3,4], TBHP [5], NaIO4 [6], molecular oxygen [7,8], etc. It is obvious that aerobic oxidation with heterogeneous catalyst is the most attractive from the economic, environmental and practical perspectives [[9], [10], [11]]. Various catalysts based on Au [[12], [13], [14]], Ag [15], Pd [[16], [17], [18]], Ni [[19], [20], [21]] and Mn [[22], [23], [24]], have been prepared and investigated. However, most of these processes are performed in tank reactor and in discontinuous pattern, which make the procedure complex and sometimes influence the stability of the catalyst for the formation and accumulation of by-product. In this sense, development of efficient catalyst and continuous process for the production of benzaldehyde still is an important industrial challenge. Some catalytic systems have been developed for the continuous process [[25], [26], [27], [28], [29], [30]], but they always suffer the drawbacks of using precious metals, high temperature or low catalytic efficiency. Mesoporous ceria-zirconia solid solution has been developed by Pudukudy and coworkers for the vapour phase oxidation of benzyl alcohol, however, the maximum benzyl alcohol conversion was only 80 % with about 90 % selectivity to benzaldehyde [31].

Layered double hydroxides (LDHs) is commonly known as hydrotalcites [32], have been widely studied as functional materials for its special properties [[33], [34], [35], [36], [37], [38]]. In our previous work, we have found that the Ni-containing hydrotalcites can effectively accelerate the selective oxidation of alcohols to aldehydes using molecular oxygen as the sole oxidant [20,21] in carousel reaction tube. Some features have been observed for the catalytic system: (I) the catalytic efficiency was low, 0.5 g Ni-containing LDHs was required for the full conversion of 0.5 mmol benzyl alcohol; (II) the activity was high at the initial stage, and then reduced significantly, which probably was due to the formation and accumulation of H2O; (III) increment of the reaction temperature had little effect on the catalytic selectivity below 140 ℃, which makes it possible to conduct the reaction under high temperature to improve the efficiency. To develop efficient and practical process for the benzyl alcohol oxidation, the above observations prompted us to investigate the reaction in a vapor-phase continuous reactor, although sometimes it has a lower productivity than condensed phases. We speculated that higher temperature can be adopted and the produced H2O can be removed during the reaction, subsequently the catalyst can be stabilized and the catalytic performance will be improved.

On the other hand, although hydrotalcites has been widely studied as catalysts in various reactions, very few report deals with the preparation of macroscopically structured particulate catalyst based on hydrotalcites, which is very vital for the application in a continuous process. In this context, a series of Ni-containing catalysts derived from hydrotalcites have been prepared and investigated in the aerobic oxidation of benzyl alcohol in a vapor-phase fixed bed process. The effects of various reaction conditions, the stability of the catalyst and the regenerated method have been detailedly studied. The present LDHs-derived catalysts for the production of benzaldehyde from benzyl alcohol oxidation may have interest in a prospective industrial application.

Section snippets

Experimental section

The chemical reagents used in the experiments are all analytically pure and were purchased from Energy-Chemical.

Catalyst screening

In the present study, the Ni-containing hydrotalcites catalysts were firstly screened. For the samples with different trivalent metallic ions, the characteristic hydrotalcite reflections can be obviously observed from the powder XRD patterns (Fig. S1). The sharp (003), (006) and (009) planes can be found at about 11°, 22°, and 35°, respectively, indicating the formation of a layered structure [[39], [40], [41]]. Moreover, the diffraction peaks of the (110) and (113) crystal faces appearing

Conclusions

In the present research, various Ni-containing hydrotalcites with different trivalent metal ions have been investigated in the benzyl alcohol oxidation using molecular oxygen as the oxidant in a continuous fix-bed process, and Ni5Ga-LDH showed the highest catalytic activity. Moreover, a series of catalysts have been prepared using Ni5Ga-LDH as the precursor by calcination under different temperatures, and systematically characterized. The results indicated that calcination has significantly

Author contributions

Weiyou Zhou: Ideas, analysis of experimental data, writing the initial draft, revision.

Guanhu Chen: Catalyst synthesis and catalytic experiments.

Binxun Yu: Analysis of experimental data.

Jiacheng Zhou: Catalyst synthesis and characterization.

Junfeng Qian: Analysis of experimental data.

Mingyang He: Oversight and leadership responsibility for the research activity planning and execution.

Qun Chen: Design and direction of the project.

Acknowledgements

This work was supported by Advanced Catalysis and Green Manufacturing Collaborative Innovation Center of Changzhou University, Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), Natural Science Foundation of Jiangsu Province of China (BK20181461).

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