Selective oxidation of olefins with aqueous hydrogen peroxide over phosphomolybdic acid functionalized knitting aryl network polymer

https://doi.org/10.1016/j.molcata.2015.12.012Get rights and content

Highlights

  • PMA/KAP was obtained by supporting phosphomolybdic acid on PPh3 functionalized KAP.

  • PMA/KAP is active and stable catalyst for olefin oxidation with H2O2/ethyl acetate.

  • Multi-weak interaction exits between PMA unit and PPh3 ligands knitted in the KAP.

  • Degradation of PMA to small species can be inhibited by the multi-weak interaction.

Abstract

A phosphomolybdic acid (PMA)-based heterogeneous catalyst, denoted as PMA/KAP, was prepared by immobilizing PMA onto a knitting aryl network polymer (KAP) based on triphenylphosphine (PPh3). The catalytic property of PMA/KAP was investigated for the selective oxidation of olefins with aqueous hydrogen peroxide (H2O2) as oxidant. When using ethyl acetate (EAC) as reaction medium, PMA/KAP performs higher activity and selectivity to epoxide for a variety of olefins, and it can be reused for several times without obvious loss of activity. When the reaction was carried out in acetonitrile (AN) medium, deactivation of PMA/KAP catalyst can be observed immediately. A variety of characterization results suggest that the degradation of PMA unit to (PO4[MoO(O2)2]4)3− occurs easily when the PMA/KAP catalyst is operated in H2O2/AN system, while such degradation behavior could be significantly inhibited when the catalyst is used in the system of H2O2/EAC. We proposed that the neighbouring P-containing ligands dispersed in the framework of KAP can produce a steric pocket with low electron density, which can promote the formation of multi-weak coordination interaction between PMA unit and several P ligands. Such multi-weak interaction can inhibit the degradation of PMA to (PO4[MoO(O2)2]4)3−, thus avoiding the leaching of active species from the KAP support, and resulting in the formation of relatively stable heterogeneous PMA supported catalyst for olefin epoxidation with H2O2 in the media of EAC.

Introduction

Porous organic frameworks (POFs) have received increasing attention due to their unique properties like large surface, low skeletal density, and high chemical stability [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Compared with traditional inorganic materials, one particular advantage of POFs is its ability to introduce various organic chemical functionalities for constructing novel advanced materials. Recently, a variety of POFs containing functional units have been reported, which have shown potential applications in adsorption, ion exchange, nanotechnology, and catalysis [11], [12], [13], [14], [15], [16].

Knitting aryl network polymers (KAPs) are a new family of POFs, which can be prepared though a simple one-step Friedel–Crafts reaction using aromatic compounds as the monomer and formaldehyde dimethyl acetal as the external cross-linker [17], [18], [19]. In the field of catalysis, a few recent works showed that a functionalized KAP material, obtained by knitting triphenylphosphine (PPh3) and benzene with an external cross-linker, could be used as suitable support to fabricate efficient heterogeneous catalysts. For instance, Pd nanoparticles supported PPh3-functionalized KAP exhibited excellent activity and selectivity for the Suzuki–Miyaura crossing coupling reaction of aryl chlorides [18]. Rh supported PPh3-based KAP showed higher activity and stability than Rh supported silica catalyst for the hydroformylation of higher olefins [20]. It was believed that the special coordinating ability of PPh3 ligands as well as the porous characteristics of KAP play critical role in stabilizing the nanoparticles of noble metal.

Recently, numerous contributions have been made to develop highly active and stable polyoxometalates (POM)-based heterogeneous catalysts for the application in liquid-phase oxidation (or epoxidation) of olefins. By selecting different supports and preparation strategies, a variety of relatively active and stable POM-supported heterogeneous catalysts have been obtained [21], [22], [23]. For instance, Kholdeeva et al. immobilized [PW11CoO39]5− and [PW11TiO40]5− into MIL-101 through electrostatical interaction, and found that the resulting materials were active heterogeneous catalysts for the oxidation of olefins with H2O2 or O2 as oxidant [21]. Bordoloi et al. reported that molybdovanadophosphoric acids supported amine-modified mesoporous silicas, were highly active and stable catalysts for selective oxidation of anthracene with t-BuOOH as oxidant [22]. Kasai et al. immobilized [r-1,2-H2SiV2W10O40] on N-octyldihydroimidazolium cation-modified SiO2, to obtain an efficient catalyst for the oxidation of olefins and sulfides with H2O2 as oxidant [23]. Our recent work showed that phosphomolybdic acid supported COF-300 (a covalent organic framework material) exhibits high activity and stability for the epoxidation of cyclooctene and 1-octene with t-BuOOH as oxidant [24]. In spite of these considerable progresses, most of the POM supported catalysts commonly suffer from active species leaching from the supports, which is particularly serious when H2O2 is used as oxidant [25]. This can be mainly attributed to the strong complexing and solvolytic properties of H2O2 and solvents, which can usually transfer PMA cluster into dissolvable compound, or turn it to smaller species by oxidative degradation [26], [27]. Besides, it is quite hard to get truly heterogeneous supported POM catalysts, since the relatively large size of POM clusters brings serious difficulty to build stable linkages between supports and POM. Therefore, it is still a very significant subject to develop novel efficient POM-based heterogeneous catalysts for the oxidation of olefins with H2O2, and to reveal the key factors favoring the improvement of the stability of the POM supported catalysts.

In this work, we tried to use PPh3-based KAP as support to prepare phosphomolybdic acid (PMA) supported heterogeneous catalyst, and the catalytic performance of the resulting material (PMA/KAP) was studied for the oxidation of olefins with H2O2 as oxidant in different reaction media. Besides, a homogeneous catalyst named PMA-(PPh3)3 was also prepared by coordinating PMA with free PPh3 ligands for the purpose of comparison. By combining a variety of characterization results, it can be revealed that multi-interactions are present between one PMA unit and several P-containing ligands dispersed in the framework of KAP support, which can result in the formation of relatively active and stable PMA supported KAP catalyst for the epoxidation of olefins with H2O2 in the presence of EAC.

Section snippets

Materials

Benzene (PhH), triphenylphosphine (PPh3), FeCl3 (anhydrous), methanol, 1,2-dichloroethane (DCE), dimethylformamide (DMF), acetonitrile (AN), ethylacetate (EAC) and 30 wt% H2O2 aqueous solution were purchased from China National Medicines Corporation Ltd., all of which were of analytical grade and were used as received. Cyclooctene, cyclohexene, styrene, 1-hexene, 1-octene, a-piene and formaldehyde dimethylacetal (FDA) were purchased from Aldrich.

The preparation of catalysts

KAP was synthesized according to literature

Catalyst characterization

Fig. 1 shows the FT-IR spectra of KAP, PMA, PMA-(PPh3)3 and PMA/KAP. The characteristic bands of KAP are in agreement with the related literature results [18]. The benzene skeleton vibration peaks appear in the range of 1600–1450 cm−1, while the peaks at around 1250–950 and 900–650 cm−1 result from Csingle bondH bending vibrations of benzene ring. 1435 cm−1 could be attributed to the P-CH2 which shows that the phosphine ligands are embedded into the skeleton of KAP [28]. For PMA-(PPh3)3 and PMA/KAP, the

Conclusions

PMA functionalized KAP catalyst was synthesized successfully and applied into the oxidation/epoxidation of olefins with H2O2 as oxidant. It is found that the resultant PMA/KAP catalyst can perform relatively high activity and stability for the epoxidation of olefins when using EAC as the reaction media. The relative good stability of PMA/KAP catalyst can be mainly assigned to the presence of multiple bridge-linked phosphine ligands dispersed in the bulky KAP support, which can produce a steric

Acknowledgment

Financial support from the National Natural Science Foundation of China (No. 21173100 and 21320102001) is gratefully acknowledged.

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