Research Article pubs.acs.org/journal/ascecg
Catalytic Epoxidation of cis-Cyclooctene over Vanadium-Exchanged Faujasite Zeolite Catalyst with Ionic Liquid as Cosolvent Linlu Bai, Kaixin Li, Yibo Yan, Xinli Jia, Jong-Min Lee,* and Yanhui Yang* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore637459, Singapore S Supporting Information *
ABSTRACT: Introducing ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][NTf2]) into dimethylformamide (DMF) as a cosolvent remarkably promoted the catalytic epoxidation of cis-cyclooctene over vanadiumexchanged faujasite zeolite catalysts (V-X), using tert-butyl hydroperoxide (TBHP) as the oxidant. The XRD spectra and SEM images show a certain destructive effect of IL on the zeolite support, while the UV-vis and UV-Raman results revealed the preserved local coordination of vanadium sites of the V-X catalyst during the reaction. The 29Si MAS NMR along with ICP and XPS characterizations further illustrated the specific interaction between [emim][NTf2] and the zeolite support as well as the vanadium site. The enhanced catalytic performance was mainly attributed to the ability of [emim][NTf2] to form hydrogen bonds with byproduct t-butanol and consequently free the vanadium active sites for substrate adsorption during the reaction. It was also proposed that the activation energy of the kinetically relevant step was decreased in the weak acidic environment offered by the imidazolium cation of [emim][NTf2]. KEYWORDS: Epoxidation, cis-Cyclooctene, [emim][NTf2], Ionic liquid, Hydrogen bonding
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Ionic liquids (ILs) as “green” solvents have attracted rapidly increased attention during the past few years. Interests in conducting catalytic processes in ILs have experienced a tremendous growth, and a variety of catalytic reactions have been successfully carried out in such neoteric media.9 Indeed, many advantages of applying ILs as solvents are mainly due to their physical properties such as extremely low vapor pressure. In addition, IL is often referred to as a “designable solvent” because of its physical properties, such as melting point, viscosity, density, and solubility, and that it can be precisely tuned. More and more studies highlighted the importance of the chemical nature of these unusual IL solvents that obviously affects the catalytic reactions through interaction with the catalyst or the components in the liquid phase. Srinivasan et al. proposed the formation of hydrogen bonding between the IL cation and the carbonyl group of anhydrides as reactant.10 The anions of IL possessing coordination atoms, such as N, O, F, were also proposed to be able to coordinate with the metallic active sites.11 Moreover, it is generally accepted that the ionic characters of ILs provide the catalysts a unique ionic
INTRODUCTION
Epoxidation of alkenes plays an important role in synthetic chemistry, allowing the hydrocarbon substrate to be converted to valuable precursors in the synthesis of fine chemicals.1 Formulating highly efficient catalysts for alkene epoxidation therefore becomes a vital area in catalysis research. In recent years, a number of transition metal complexes have been reported to be effective in the homogeneous catalytic epoxidation of alkenes.2−4 Nevertheless, homogeneous catalytic processes suffer drawbacks such as the complication of metal complex synthesis and metal leaching during the reaction. In addition, it is rather difficult to retract the catalyst from the reaction mixture for recycling purposes, and the catalyst/ product isolation generates large amounts of organic/inorganic wastes. Therefore, heterogeneous catalysts that can overcome the above-mentioned problems of homogeneous catalysts are strongly desired. Tang et al. reported the epoxidation of styrene catalyzed by cobalt(II)-containing molecular sieves.5 Iron-based solid catalysts were also attempted.6 Our group showed the liquid phase trans-stilbene epoxidation over catalytically active cobalt-substituted TUD-1 mesoporous materials (Co-TUD-1) using molecular oxygen.7 Recently, a readily synthesized vanadium-exchanged faujasite zeolite catalyst (V-X) has been reported by our group, which exhibited a reasonably good catalytic activity in the epoxidation of alkenes using tert-butyl hydroperoxide (TBHP) as the oxidant and dimethylformamide (DMF) as the solvent.8 © XXXX American Chemical Society
Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: August 26, 2015 Revised: December 11, 2015
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DOI: 10.1021/acssuschemeng.5b00854 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Epoxidation of cis-Cyclooctene over V-X Catalyst in Different Solventsa selectivityc (%) entry 1 2 3 4 5 6 7 8 9 10 11
solvent d
DMF DMF [emim][NTf2] DMF DMF DMF DMF DMFd DMF DMFe [emim][NTf2]e
b
cosolvent
R
− − − [emim][NTf2] [emim][NTf2] [emim][NTf2] [emim][NTf2] [emim][NTf2] [bmpy][NTf2] − −
− − − 4:1 3:2 2:3 1:4 2:3 2:3 − −
conversion (%)
cyclooctene oxide
2-cycloocten-1-one
5.3 44.0 94.1 54.4 66.8 90.0 94.6 12.7 85.4 8.3 92.6
34.6 94.0 94.6 96.8 96.5 93.0 91.0 90.4 90.9 80.0 93.7
− 4.8 2.6 2.7 1.8 3.2 2.2 3.8 2.0 9.7 2.1
Reaction conditions: 1 mmol cis-cyclooctene, 0.2 g of V-X catalyst, 10 mmol TBHP, 10 mL of solvent, 80 °C, 24h. bMolar ratio between DMF and [emim][NTf2] in the mixed solvent. cMain byproduct is 2-cycloocten-1-one. dNo catalyst. e1 mL of t-butanol is added. a
background pressure in the analysis chamber was lower than 1 × 10−7 Pa. Measurements were performed using 20 eV pass energy, 0.1 eV step, and 0.15 dwelling time. Energy correction was carried out using the C1s peak of adventitious C at 284.6 eV. 29Si MAS NMR spectra were recorded on a 400 MHz Bruker spectrometer. Inductively coupled plasma (ICP, Dual-view Optima 5300 DV ICP-OES system) was employed to measure the elemental content of sodium. Catalytic Epoxidation Reactions. The epoxidation of ciscyclooctene was carried out in a batch reactor operated under atmospheric conditions. Experiments were conducted using a roundbottomed flask (25 mL of capacity) precharged with 1 mmol of ciscyclooctene and 0.2 g of V-X catalyst. The IL was added into DMF and mixed thoroughly. After adding 10 mL of mixed solvent, the reaction mixture was stirred vigorously using a magnetic stirrer (1000 rpm of stirring speed) and heated in a silicon oil bath to the desired temperature. By adding 1.374 mL of TBHP solution (70% in water, Fluka), whose molar amount is 10 times that of the reactant, as the oxidant in the batch reactor, the epoxidation reaction was initiated. After the allowed reaction time, the catalyst powder was filtered off, and the liquid organic products were analyzed using an Agilent gas chromatograph 6890 equipped with a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) and a FID detector. Dodecane was the internal standard to calculate cis-cyclooctene conversion and epoxide selectivity. The byproducts were identified by a gas chromatography−mass spectrometry (GC−MS, Agilent, GC 6890N, MS 5973 inert).
environment that plays an important role in stabilizing the catalytically active species or the reaction intermediates.12,13 Up to now, the study of alkene epoxidation with ILs as solvent/cosolvent is mainly focused on the homogeneous catalytic processes. For instance, Anil et al. studied the tungstate complex in ILs.14 Rhenium(VII) and molybdenum(VI) complexes in IL were investigated in the alkene epoxidation.15 Manganese porphyrins in IL was also reported.16 Nonetheless, the heterogeneous catalytic process with ILs as solvent/cosolvent for alkene epoxidation is rarely investigated; no detailed mechanistic study on the specific interaction between ILs and active sites/support/substrate was reported. In this work, the epoxidation of cis-cyclooctene over the V-X catalyst with TBHP as oxidant and a representative IL 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][NTf2]) as the cosolvent (along with DMF) was studied. In addition, the IL 1-butyl-3-methylpyrrolidium bis(trifluoromethylsulfonyl)imide ([bmpy][NTf2]) was also examined as cosolvent for comparison.
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EXPERIMENTAL SECTION
Materials and Methods. cis-Cyclooctene (95% stabilized, Acros Organic), DMF (A.C.S. Reagent, J.T. Baker), and TBHP (70% in water, Fluka) were employed as received without further purifications. ILs, [emim][NTf2] and [bmpy][NTf2], were purchased from Merck. The V-X catalyst was synthesized as follows: 2 g of NaX faujasite zeolite (Molecular sieves 13X, powder
