In situ sulfonic acid-functionalized MIL-101(Cr) catalyzed liquid-phase Beckmann rearrangement of cyclohexanone oxime

https://doi.org/10.1016/j.micromeso.2020.110031Get rights and content

Highlights

  • The sulfonic acid-functionalized MIL-101(Cr) was prepared and used for catalyzing Beckmann rearrangement for the first time.

  • The sulfonic-acid groups were highly dispersed and facilitate contact with the reaction substrate.

  • The sulfonic-acid functionalized MIL-101(Cr) catalysts were easy to synthesize, recyclable and stable.

Abstract

We prepared in situ sulfonic-acid-functionalized MIL-101(Cr) catalysts using different chromium sources and mineralizers. We characterized these catalysts by X-ray diffraction, N2-physical adsorption, Fourier-transform infrared, scanning electron microscopy, X-ray photoelectron spectroscopy, NH3 temperature-programmed desorption, and inductively coupled plasma measurements. We subsequently used the solid acid catalysts to catalyze the liquid-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam for the first time. The results showed that when the reaction was carried out at 130 °C for 4 h with benzonitrile as the solvent, the MIL-101(Cr)-S1, prepared using chromium trioxide (CrO3) as the chromium source and hydrochloric acid as the mineralizer, showed the highest conversion of 35.1% and selectivity of 65.9%. This was better than that of MIL-101(Cr)-S2, which was prepared using chromium (III) nitrate nonahydrate [Cr(NO3)3·9H2O] as the chromium source and hydrofluoric acid as the mineralizer. This occurred because the MIL-101(Cr)-S1 had a higher acid content and strength than those of the MIL-101(Cr)-S2. Additionally, the MIL-101(Cr)-S1 could be reused three times in succession without loss of its catalytic activity.

Introduction

Caprolactam is an important intermediate for the manufacture of synthetic fibers and engineering plastics [1]. The current industrial process for caprolactam production invokes the Beckmann rearrangement of cyclohexanone oxime catalyzed by concentrated sulfuric acid. Although the conventional method is known to yield caprolactam with a high selectivity, this conventional method is ecologically and economically questionable for various reasons, including the high oleum consumption rate and large amount of ammonium sulfate formed as a by-product [2]. Furthermore, several environmental and safety concerns are considered to be major shortcomings. To overcome these problems, a preferable alternative method is to carry out the Beckmann rearrangement using solid catalysts. Several potential materials, such as zeolites [3,4], mixed oxides [5,6], and mesoporous molecular sieves [[7], [8], [9]], are of great interest because of their environmentally friendly properties. Nevertheless, this reaction usually is carried out at temperatures higher than 250 °C, and such high temperatures have negative effects, resulting in the fast deactivation of the catalyst resulting from coke formation. Moreover, compounds that hardly vaporize or become unstable at high temperatures are not suitable for vapor-phase reactions. Therefore, the liquid-phase catalytic system appears to be more promising, in which the reaction can proceed at a moderate temperature, and the catalyst deactivation is decreased because of the presence of solvent.

To date, various solid catalysts, such as sulfonic acid resins, heteropoly acids, molecular sieves, and sulfonic-acid-functionalized mesoporous materials, have been used in the liquid-phase Beckmann rearrangement of cyclohexanone oxime. For example, FDU-14–SO3H exhibited a conversion of 91.4% and selectivity of 85.8% for the liquid-phase Beckmann rearrangement of cyclohexanone oxime and could be reused several times with a slight decrease in conversion [10]. H-PDVB–SO3H exhibited a high yield of caprolactam (75%) during the heterogeneous Beckmann rearrangement of cyclohexanone oxime [11]. By optimizing the reaction conditions, 100% cyclohexanone oxime conversion and 100% caprolactam selectivity could be achieved over Bi-SBA-15 [1]. Most of these catalysts exhibited good yields of caprolactam at moderate temperatures. They possessed inherent limitations, however, including long reaction times, poor stabilities, easy deactivation, and lack of recoverability.

Metal organic frameworks (MOFs), an important family of crystalline materials constructed by organic linkers and metal nodes, currently are receiving considerable attention for their potential applications in the fields of gas storage, separation, and catalysis [[12], [13], [14], [15], [16], [17], [18], [19]]. This interest in MOFs has arisen because of their extremely high surface areas, large porosities, tunable pore sizes, flexible functionalities, and high fraction of uniform and accessible metal sites. Although many MOFs have been discovered, only a few have been tested in catalytic reactions, because of their low thermal, chemical, and hydrolytic stabilities compared to completely inorganic zeolites.

MIL-101(Cr) is one of the most widely investigated and applied MOFs in a variety of fields, not only because its pores and pore windows are large enough to allow access for even large reactant molecules to diffuse into the pores but also for its sufficiently stable framework. It can remain in air for several months without collapse, and in water and common organic solvents, it maintains excellent stability. MIL-101(Cr) contains numerous unsaturated metal sites, providing Lewis acid sites. The use of MIL-101(Cr) in applications, such as separation and catalysis, are largely hindered by the lack of functional sites, especially those that require strong acid catalysis. The sulfonic-acid-functionalized MIL-101(Cr) presented in this study features highly dense and uniform acidic sites and can be used as a solid acid for various organic transformations, including the deacetalization nitroaldol reaction [20], esterification [21], acetalization [22], and alcoholysis of epoxides [23]. To the best of our knowledge, the Beckmann rearrangement reaction is an acid-catalyzed reaction. Thus, we explored two synthesis routes for the production of sulfonic acid-functionalized MIL-101(Cr) and applied them to catalyze the liquid-phase Beckmann rearrangement for the first time. Furthermore, we systematically investigated the optimization of reaction conditions and catalytic reusability.

Section snippets

Chemicals

All of the reagents and reactants were obtained from commercially available sources and used without further purification. Chromium nitrate nonahydrate and chromium oxide (analytical reagent (AR), 98%) were obtained from Macklin (China). Hydrofluoric acid (AR, 40%), fuming hydrochloric acid (AR, 37%), concentrated sulfuric acid (AR, 98%), chlorobenzene (AR, 99%), toluene (AR, 99.5%), benzonitrile (AR, 99%), ethanol, and methanol were purchased from the Sinopharm Chemical Reagent Co., Ltd

Characterization of catalysts

The FT-IR spectra confirmed the successful introduction of the -SO3H group. Fig. 1 shows a comparison of the prototypical MIL-101(Cr) and sulfonic-acid- functionalized MIL-101(Cr). The new bands that appeared at 1185 and 1225 cm−1 were attributed to the Odouble bondSdouble bondO symmetric and asymmetric stretching vibrations [26]. The peak at 1026 cm−1 resulted from the S–O stretching vibrations [20]. The band at 1080 cm−1 was ascribed to the in-plane skeletal vibrations of benzene rings substituted by sulfonic acid

Conclusions

We synthesized sulfonic-acid-functionalized MIL-101(Cr) in one step using different chromium sources and a mineralizer, and for the first time, it was used to catalyze the liquid-phase Beckmann rearrangement of cyclohexanone oxime. FT-IR, XPS, XRD, and N2-adsorption characterization suggested that –SO3H groups could be successfully introduced into the framework of the MIL-101(Cr), and the crystal structure remained almost unchanged. In addition, the synthesized sulfonic-acid-functionalized

CRediT authorship contribution statement

Qiaoqiao Niu: Writing - original draft. Mengjiao Liu: Formal analysis. Zhiying Xiao: Data curation. Xia Yuan: Writing - review & editing. Jian Wu: Project administration.

Declaration of competing interest

We would like to submit the enclosed manuscript entitled “In situ sulfonic acid-functionalized MIL-101(Cr) catalyzed liquid-phase Beckmann rearrangement of cyclohexanone oxime”, which we wish to be considered for publication in “Microporous and Mesoporous Materials”. No conflict of interest exists in the submission of this manuscript, and the manuscript has been approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original

Acknowledgment

The authors acknowledge the financial supports from National Natural Science Foundation of China (No. 21776237, No.21376201) and Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization.

References (36)

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