Short CommunicationPreparation of copper (II) ion-containing bisimidazolium ionic liquid bridged periodic mesoporous organosilica and the catalytic decomposition of cyclohexyl hydroperoxide
Graphical abstract
Highlights
► Cu2 +-containing bisimidazolium ionic liquid bridged PMO was prepared. ► The catalyst showed high activity in the decomposition of cyclohexyl hydroperoxide. ► The catalyst showed prominent stability.
Introduction
The liquid-phase autoxidation of cyclohexane is an important industrial process, with producing cyclohexanone and cyclohexanol (K/A oil) about 6 × 1010 t/year. K/A oil are precursors for production of caprolactam and adipic acid, which are the building blocks for nylon-6 and nylon-6,6 [1]. To minimize the yield of by-products such as organic acids and polymer, industrially, two step processes were employed. The cyclohexyl hydroperoxide (CHHP) is generated in an uncatalyzed step, followed by its catalytic decomposition. The decomposition step plays a very important role in the overall oxidation process [2], [3]. In order to obtain high yield of K/A oil, catalyst was usually utilized in the CHHP decomposition, such as cobalt (II) species with additive NaOH, chromium (VI) species [2]. These catalysts, however, own both economic and environmental drawbacks. Due to easy recyclability, exploring new heterogeneous catalysts for efficient decomposition of CHHP in absence of NaOH and toxic metal species is of great importance [4], [5], [6], [7], [8].
Recently, periodic mesoporous organosilicas (PMOs) have attracted much attention due to large surface areas, well-defined nanoporous structures, and structural diversity of the organosilica frameworks [9]. PMOs could be prepared by condensation of organo-bridged silsesquioxane precursors in the presence of soft organic templates [10]. Organic groups are distributed uniformly inside the framework and will not block the pore, which is favorable for the guest molecule diffusion [11]. Furthermore, the bridging organic groups can be easily functionalized. These characteristics make PMOs showing potential application in catalysis [9], [12], [13], [14]. Since the reports of PMOs in 1999 [15], [16], [17], various organic species have been introduced into the pore walls of PMOs [18].
Metal ion-containing ionic liquids prepared by combing of imidazolium derivative ionic liquid with metal halide can be used as catalysts in many reactions [19], [20], [21], [22]. The consumption of ionic liquids in practical processes and the relatively high viscosity of the ionic liquids limited their application [23]. Immobilization of ionic liquids is an efficient way to overcome these problems. Post-graft method was utilized to introduce metal ion-containing ionic liquids on the surface of the solid materials [24], [25], [26]. But for the grafted ionic liquids, it also has some drawbacks, such as leaching of active component from the support and the low loading of the ionic liquids [23]. The bridge of metal ion-containing ionic liquids inside the framework will not block the pore and may possess high stability and high loading, which is more attractive [23].
In continuation with our research to develop high-efficiency heterogeneous catalysts for the decomposition of CHHP [4], [5], [6], herein, we report a novel copper (II) ion-containing bisimidazolium ionic liquid bridged PMO (Cu-BIM-PMO) as heterogeneous catalyst for the decomposition of CHHP (Scheme 1). Through the electrostatic binding between bisimidazolium cation and CuCl42 − complex, the stability of catalyst improved apparently and metal leaching was negligible under the reaction conditions. In the decomposition of CHHP, this catalyst showed high catalytic activity and can be reused at least five times without apparent loss of activity.
Section snippets
Synthesis of BIM-PMO
The synthesis of PMO precursor was summarized in the supporting information. BIM-PMO was prepared according to a reported procedure [27]. Pluronic P123 (4.02 g) and KCl (22 g) were added to a solution of HCl (2 M, 100 mL) and distilled water (30 g) at 40 °C. Then, 1,1′-di(3-propyltrimethoxysilane)-3,3′-propylenediimidazolium dichloride (1.15 g) and tetraethoxysilane (7.50 g) in absolute methanol (2 mL) were added and stirred for 24 h. The resulting mixture was then transferred into a teflon-lined
Characterization of the materials
Low-angle powder X-ray diffraction (XRD) patterns of BIM-PMO and Cu-BIM-PMO are presented in Fig. S5. Both BIM-PMO and Cu-BIM-PMO show typical three diffraction peaks at low angle, suggesting these materials possess a highly ordered hexagonal pore structure. Compared with that of BIM-PMO, the diffraction intensity of Cu-BIM-PMO decreased and the position of these diffraction peaks shifted toward higher angle. It was believed that the interaction between copper ion and two imidazole cycles
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (21103175, 21103206 and 21233008) and the Doctor Startup Foundation of Liaoning Province.
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