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Catalytic oxidation of cyclic hydrocarbons with hydrogen peroxide using Fe complexes immobilized into montmorillonite
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Catalytic oxidation of cyclic hydrocarbons with hydrogen peroxide using Fe complexes immobilized into montmorillonite Syuhei Yamaguchi*, Daijiro Ihara, Yuki Yamashita, Yudai Uemoto, Hidenori Yahiro Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron complex Montmorillonite Oxidation Cyclic hydrocarbons Hydrogen peroxide
[Fe(Ligand)x]2+ complexes immobilized into montmorillonite ([Fe(Ligand)x]2+@Mont, Ligand: 2,2′;6′,2′'-terpyridine (x = 2), 1,10-phenanthroline (x = 3), and 2,2′-bipyridine (x = 3)) were prepared by an ion-exchange method whereby cations present within montmorillonite (probably Ca2+) were replaced by [Fe(Ligand)x]2+ complexes provided by [Fe(Ligand)x](ClO4)2. The catalytic properties of the species obtained were then investigated in the oxidation of cyclic hydrocarbons, such as cyclohexane and benzene, with H2O2. The catalytic oxidation of cyclohexane and benzene with H2O2 over [Fe(Ligand)x]2+@Mont catalysts proceeded, and the desired alcohols were obtained as the main reaction products, whereas no catalytic activity was displayed by neat montmorillonite. Regardless of the reaction conditions, [Fe(terpy)2]2+@Mont (terpy stands for 2,2′;6′,2′'terpyridine) exhibited the best catalytic performance in the oxidation of the mentioned substrates by H2O2, among all the catalysts tested.
1. Introduction
several researchers; however, to the best of our knowledge, the oxidation of benzene to phenol with H2O2 as the oxidant and in the presence of the mentioned catalysts has not been reported. Recently, we reported the catalytic activity of Fe-bipyridine complexes encapsulated into Ytype zeolites ([Fe(bpy)3]2+@Na-Y), which are able to catalyze the oxidation of benzene and cyclohexene with H2O2 in CH3CN and/or H2O as solvents [49–53]. The maximum catalytic activity of [Fe(bpy)3]2+@ Na-Y in the oxidation of benzene with H2O2 was achieved when the volume ratio of the solvents (CH3CN and H2O) was 1:1 [51,52]. We also found that [Fe(terpy)2]2+@Na-Y and [Fe(phen)3]2+@Na-Y (terpy = 2,2′;6′,2′'-terpyridine and phen = 1,10-phenanthroline) exhibited a higher catalytic activity than [Fe(bpy)3]2+@Na-Y in the oxidation of benzene with H2O2 to phenol [52]. Montmorillonite is a clay mineral considered part of the smectite group, and it is characterized by a 2:1 structure with two tetrahedral silica sheets sandwiching a central octahedral alumina sheet whereby Al3+ ions have been partially replaced by Mg2+ ions. Since montmorillonite has a large cation exchange capacity [54], several cations, including individual metal ions and cationic metal complexes, can be easily immobilized in montmorillonite by an ion-exchange method. In this study were prepared inorganic–organic hybrid materials whereby [Fe(Ligand)x]2+ complexes were immobilized into montmorillonite: [Fe(terpy)2]2+@Mont, [Fe (phen)3]2+@Mont, and [Fe(bpy)3]2+@Mont. The catalytic activity of these materials for the oxidation of cyclic hydrocarbons like
The catalytic direct hydroxylation of inert CeH bonds in hydrocarbons under mild conditions is a major challenge in synthetic chemistry, whether it applies to the chemical industry or academic research [1–5]. In particular, the catalytic hydroxylation of benzene to phenol using environmentally friendly oxidants, such as hydrogen peroxide (H2O2) [6–14], O2 in combination with reducing agents [15–21], and H2O with electrochemical [22] or photochemical reaction systems [23,24] has attracted a great deal of attention. One of the most attractive areas in catalysis research is the development of inorganic–organic hybrid materials that catalyze oxidation reactions [25–36]. Many researchers have reported inorganic–organic hybrid materials based on montmorillonite, which is a type of clay, and several transition metal complexes, which can be used to catalyze the oxidation of hydrocarbons by H2O2 [25–30], O2 [31–33], t-butyl hydrogen peroxide (TBHP) [28,30,34], iodosylbenzene (PhIO) [27,29,35], and sodium periodate (NaIO4) [36] as oxidants, trapping of volatile sulfur derivatives [37,38], adsorption of phenols [39–43], clay-modified electrodes [39–43], improvement of optical purity [44–46], ethylene polymerization [47], adsorption of phenols [48], and so on. The oxidation of organic substrates like cyclohexane [25–29], n-octane [30], and others [29] with H2O2 acting as an oxidant over transition metal complexes immobilized into montmorillonite have been reported by
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Corresponding author. E-mail address: [email protected] (S. Yamaguchi).
https://doi.org/10.1016/j.cattod.2019.12.023 Received 30 June 2019; Received in revised form 28 November 2019; Accepted 16 December 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Syuhei Yamaguchi, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.12.023
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parentheses after the sample name, for example the notation [Fe (terpy)2]2+@Mont (0.8) represents a [Fe(terpy)2]2+-based inorganic–organic hybrid material whereby the Fe/Mg molar ratio is 0.8.
cyclohexane and benzene with H2O2 wasinvestigated. 2. Experimental 2.1. Materials and instruments
2.3. Catalytic oxidation
Montmorillonite (Montmorillonite K10) was purchased from SIGMA-ALDRICH. All chemicals were used as received. The actual elemental compositions of the prepared samples were measured by Xray fluorescence (XRF) analysis using a Rigaku ZSX Primus instrument. The Fe/Mg or Ca/Mg molar ratios of the samples were calculated based on the intensity of the peak attributed to each element. The powder Xray diffraction (XRD) patterns of the catalysts were collected on a Rigaku MiniFlex II diffractometer using CuKα radiation. UV–vis spectra were recorded on a Hitachi U-4000 spectrometer, in the case of solid samples, or on a Shimadzu U-1200 spectrometer, in the case of liquid samples. Gas chromatography (GC) analysis was performed on a Shimadzu GC-2014 instrument with a flame ionization detector equipped with a DB-1MS capillary column (internal diameter =0.25 mm and length =30 m) at the nature of non-polar liquid phase. Gas chromatography–mass spectrometry (GC–MS) spectra were recorded on a Shimadzu GCMS-QP5050A (ionization voltage =70 eV) instrument equipped with a DB-1MS capillary column (internal diameter =0.25 mm, length =30 m) at the nature of non-polar liquid phase.
The catalytic oxidations of benzene and cyclohexane were carried out as follows: the catalyst (7.9 μmol), solvent CH3CN and/or H2O (10 mL), substrate (7.9 mmol), and 30 % aqueous hydrogen peroxide (7.9 mmol) were transferred into a glass tube reactor. The reaction was carried out at 50 °C under Ar atmosphere. Subsequently, triphenylphosphine (1.1 mmol) was added to the glass tube reactor as a quencher and o-dichlorobenzene (0.34 mmol) was added as an internal standard. The reaction products were analyzed by GC and identified by the comparison of their mass spectra with those of authentic samples. The turnover number (TON) was calculated as the number of product molecules obtained per iron center (TON [‒] = Product [mol]/Fe [mol]). 3. Results and discussion 3.1. Characterization of [Fe(Ligand)x]2+@Mont catalysts In Fig. 1 are shown the values of the Ca/Mg ratios as a function of the Fe/Mg values for the three [Fe(Ligand)x]2+@Mont catalysts. Notably, the Mg/Al ratios were almost constant (0.18–0.23) for all prepared catalysts and raw montmorillonite. In the case of [Fe(Ligand)x]2+@Mont catalysts, the Ca/Mg ratios decreased as the Fe/Mg ratios increased, suggesting that Ca2+ ions in the original montmorillonite were exchanged for [Fe(Ligand)x]2+ ions. The UV–vis diffuse reflectance spectra of [Fe(Ligand)x]2+@Mont (0.8) catalysts are shown in Fig. 2A. In the case of untreated montmorillonite, no absorption was observed between the ultraviolet and visible regions (Fig. 2A(d)). The absorption spectra of [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@Mont, and [Fe(bpy)3]2+@Mont (Fig. 2A(a)–(c)) displayed two sets of bands in the 400–650 nm and 250–400 nm wavelength ranges, which can be assigned to a metal-to-ligand (d–π*) charge-transfer (MLCT) and a π–π* transition within the ligand (bpy, phen, and terpy), respectively, similar to those observed for [Fe(bpy)3](ClO4)2, [Fe(phen)3](ClO4)2, and [Fe (terpy)2](ClO4)2 (Fig. S1). For all [Fe(Ligand)x]2+@Mont catalysts, the intensity of the peak at 550 nm due to the MLCT increased alongside the value of the Fe/Mg ratio, and it reached a maximum at Fe/ Mg = 0.8–1.2 (Figs. 2B and S2), suggesting that the [Fe(Ligand)x]2+ ions were coming close in montmorillonite. XRD patterns of [Fe(Ligand)x]2+@Mont (0.8) catalysts are shown in Fig. 3A. The peaks of
2.2. Preparation of iron-containing catalysts 2.2.1. Preparation of [Fe(Ligand)x](ClO4)2 As ion-exchange agents, [Fe(terpy)2](ClO4)2 [55], [Fe(phen)3] (ClO4)2 [56,57], and [Fe(bpy)3](ClO4)2 [58] were prepared by the method reported elsewhere. Concisely, [Fe(terpy)2](ClO4)2 was prepared by the following method: 2,2′,6′,2′'-terpyridine (0.28 g, 1.2 mmol) was added into a H2O solution (100 mL) of FeSO4∙7H2O (0.13 g, 0.48 mmol) at about 80 °C, and the resulting mixture was stirred at the same temperature for 1 h. Upon the addition of sodium perchlorate (0.37 g, 3.0 mmol) into the solution thus obtained, a reddish-purple precipitate formed immediately. After filtration, recrystallization of the precipitate from CH3CN/diethyl ether produced a needle-like reddish-purple crystalline solid. Needle-like reddish-purple solid; C30H22Cl2FeN6O8 (721.28) calcd. C 49.95, H 3.07, N 11.65; found C 49.53, H 3.11, N 11.60. In a similar way, [Fe(phen)3](ClO4)2 and [Fe(bpy)3](ClO4)2 were prepared using 1,10-phenanthroline and 2,2′-bipyridine as ligands, respectively. A needle-like orange crystalline solid and a needle-like red crystalline solid were obtained as the syntheses of the former and latter complexes were being performed, respectively. Needle-like orange solid; C36H24Cl2FeN6O8 (795.36) calcd. C 54.36, H 3.04, N 10.57; found C 54.02, H 3.23, N 10.45. Needle-like red solid; C30H24Cl2FeN6O8 (723.30) calcd. C 49.81, H 3.34, N 11.62; found C 49.90, H 3.43, N 11.76. 2.2.2. Preparation of [Fe(Ligand)x]2+@Mont catalysts [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@Mont, and [Fe(bpy)3]2+@ Mont were prepared by the ion-exchange method of montmorillonite with Fe(terpy)2](ClO4)2, [Fe(phen)3](ClO4)2, and [Fe(bpy)3](ClO4)2, respectively. The typical conditions for [Fe(Ligand)x]2+@Mont catalyst preparation are as follows: montmorillonite was added into an aqueous solution of [Fe(Ligand)x](ClO4)2 at an appropriate concentration, and the resulting mixture was stirred for 24 h at room temperature. The solid present in the mixture following the stirring process was collected by suction filtration, washed with deionized water, and then dried in ambient air at room temperature to obtain the [Fe(Ligand)x]2+@Mont catalyst. The values for the M/Mg (M = Ca or Fe) molar ratios of the resulting [Fe(Ligand)x]2+@Mont catalysts are summarized in Table S1. Hereinafter, the Fe/Mg molar ratio for each sample is denoted in
Fig. 1. Plots of the Ca/Mg ratio values versus Fe/Mg ratio values for [Fe (Ligand)x]2+@Mont catalysts. [Fe(terpy)2]2+@Mont (▲), [Fe(phen)3]2+@ Mont (⬤), and [Fe(bpy)3]2+@Mont (⬛). 2
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Fig. 2. (A) UV–vis diffuse reflectance spectra of [Fe(terpy)2]2+@Mont (a), (b), [Fe [Fe(phen)3]2+@Mont (bpy)3]2+@Mont (c), and montmorillonite (d). (B) Intensity of metal-toligand (d–π*) charge-transfer peaks as a function of the Fe/Mg ratio in [Fe (▲), [Fe (terpy)2]2+@Mont (phen)3]2+@Mont (⬤), and [Fe (bpy)3]2+@Mont (⬛).
(001) plane of [Fe(Ligand)x]2+@Mont (0.8) catalysts (Fig. 3A(a)–(c)) were shifted to a lower angle side than that of montmorillonite (Fig. 3A(d)). This observation indicates that, regardless of the identity of the ligand, the interlayer distance of (001) plane derived from montmorillonite increased as the Fe/Mg value increased (Figs. 3B and S3). Based on the results of XRF, XRD, and UV–vis experiments, it was concluded that the three iron complexes, [Fe(bpy)3]2+, [Fe(phen)3]2+, and [Fe(terpy)2]2+, were immobilized into montmorillonite.
3.2.1. Oxidation of cyclohexane In Fig. 4 are reported the catalytic activities and alcohol selectivity of [Fe(Ligand)x]2+@Mont (0.8) catalysts in the oxidation of cyclohexane with H2O2 performed in CH3CN and CH3CN–H2O (1:1). The selectivity of the alcohol product in CH3CN solvent (82–84 %) was higher than that in CH3CN–H2O (1:1) mixed solvent (60–66 %). Unmodified montmorillonite displayed no catalytic activity in the oxidation of cyclohexane with H2O2, under the same experimental conditions utilized for [Fe(Ligand)x]2+@Mont (0.8) catalysts. In CH3CN (Fig. 4A), the order of the catalytic activity for the oxidation of cyclohexane with H2O2 was observed to be as follows: [Fe(terpy)2]2+@Mont > [Fe (phen)3]2+@Mont > [Fe(bpy)3]2+@Mont. In CH3CN–H2O (1:1) (Fig. 4B), the catalytic activity of [Fe(terpy)2]2+@Mont was higher than those of [Fe(phen)3]2+@Mont and [Fe(bpy)3]2+@Mont. Overall, [Fe(Ligand)x]2+@Mont catalysts displayed higher catalytic activities in the oxidation of cyclohexane with H2O2 when utilized in CH3CN–H2O (1:1) than in CH3CN solution. Table S2 shows that catalytic activity for the oxidation of cyclohexane with H2O2 using unsupported and supported Fe complexes. The alcohol selectivity of supported Fe complex, [Fe(Ligand)n]2+@Mont (0.8), was higher than that of unsupported one, [Fe(Ligand)n](ClO4)2, although the catalytic activity of the former catalyst was lower than that of the latter one. A similar result was reported by Machado et al. [35]; they found that Fe(III) and Mn(III) cationic porphyrins immobilized in montmorillonite exhibits high selectivity to cyclohexanol for the catalytic oxidation of cyclohexane with iodosylbenzene. Such a high alcohol selectivity of supported metal complex may come from the prompt desorption of alcohol produced from active site before further oxidation; that is, in the limited space like the interlayer of
3.2. Catalytic activity of the three [Fe(Ligand)x]2+@Mont catalysts in the oxidation of cyclic hydrocarbons with H2O2 In order to confirm the effect of the solvent on the H2O2-driven benzene oxidation, the oxidation has been conducted in CH3CN and in CH3CN–H2O mixed solvent using as catalysts: Fe complexes encapsulated into a cation-exchanged Y-type zeolite ([Fe(bpy)3]2+@M-Y (M = Na+, Cs+, Ca2+, NH4+, N(CH3)4+, etc.) [51,52]. Results from these investigations indicated that use of the CH3CN–H2O mixed solvent (CH3CN: H2O = 1: 1) was associated with the best performance for the oxidation of benzene with H2O2 catalyzed by [Fe(bpy)3]2+@M-Y. It was suggested that hydrophobic interaction between ligand and substrate against benzene oxidation may act strongly in narrow spaces such as zeolite. In order to confirm whether the abovementioned hydrophobic interaction occurring in zeolite also takes place in layered compounds like montmorillonite, the effect of the solvent has on the oxidation of cyclic hydrocarbons (e.g., cyclohexane and benzene) with H2O2 catalyzed by [Fe(Ligand)x]2+@Mont catalysts was investigated conducting the experiments detailed in the following sections.
Fig. 3. (A) X-ray diffraction patterns of (a), [Fe [Fe(terpy)2]2+@Mont (phen)3]2+@Mont (b), [Fe(bpy)3]2+@ Mont (c), and montmorillonite (d). (B) Interplanar distance of (001) plane as a function of the Fe/Mg ratio in [Fe (▲), [Fe (terpy)2]2+@Mont (phen)3]2+@Mont (⬤), and [Fe (bpy)3]2+@Mont (⬛).
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Fig. 4. Catalytic activity and alcohol selectivity of [Fe(Ligand)x]2+@Mont catalysts (Fe/Mg = 0.8) in the oxidation of cyclohexane with H2O2 performed in CH3CN (A), and CH3CN–H2O (1:1) mixed solvent (B). Reaction condition: Fe in catalysts (7.9 μmol), cyclohexane (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), solvent (10 mL), 50 °C, 24 h and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
was higher than the corresponding parameters for [Fe(phen)3]2+@ Mont and [Fe(bpy)3]2+@Mont. In CH3CN–H2O (1:1) (Fig. 5B), the order of catalytic activity in the oxidation of benzene with H2O2 was observed to be as follows: [Fe(terpy)2]2+@Mont > [Fe(phen)3]2+@ Mont > [Fe(bpy)3]2+@Mont. In particular, [Fe(terpy)2]2+@Mont and [Fe(phen)3]2+@Mont exhibited especially high catalytic activity. The catalytic activities of [Fe(Ligand)x]2+@Mont catalysts in the oxidation of benzene with H2O2 were much higher in CH3CN–H2O (1:1) than in CH3CN. A similar solvent effect was observed for the oxidation of benzene with H2O2 to phenol over [Fe(bpy)3]2+@Y catalyst [51]. CH3CN or H2O solvent has each problem inhibiting the catalytic reaction; benzene hardly diffuses in H2O, while H2O2 hardly diffuses in CH3CN. These problems were demonstrated to be offset for using the mixed solvent, CH3CN+H2O; [Fe(bpy)3]2+@Y exhibited the better catalytic performance for oxidation of benzene with H2O2 to phenol in the mixed solvent, CH3CN+H2O, than in CH3CN or H2O solvent [51]. From the concept of hard and soft acids and bases (HSAB) [59], Ca2+ ion in montmorillonite as well as Na+ ion in Y-zeolite are hard acid, H2O and alcohols are hard base, and CH3CN is soft base. Because Na+ and Ca2+ ions have a higher affinity with H2O and alcohols than that with CH3CN, Na+ and Ca2+ ions can easily capture alcohols in CH3CN
montmorillonite and the pore of zeolite, the produced alcohol on active site is attracted to counter cation such as Na+ and Ca2+ by electrostatic interaction, resulting in the inhibition of further oxidation to ketone, as reported previously [49,50]. The catalytic activities of [Fe(Ligand)n]2+@Y catalysts reported previously [52] are also shown in Table S2. The catalytic activity of [Fe (bpy)3]2+@Mont was comparable to that of [Fe(bpy)3]2+@Y catalyst which was reported to exhibit the highest catalytic activity among [Fe (Ligand)n]2+@Y [52]. 3.2.2. Oxidation of benzene In Fig. 5 are reported the catalytic activities and phenol selectivity of [Fe(Ligand)x]2+@Mont (0.8) catalysts in the oxidation of benzene with H2O2 performed in CH3CN and CH3CN–H2O (1:1). The phenol selectivities for reactions conducted in CH3CN (almost 100 %) were slightly higher than those measured for reactions conducted in CH3CN–H2O (93–100 %). Also in this case, unmodified montmorillonite displayed no catalytic activity in the oxidation of benzene with H2O2, under the same experimental conditions employed for [Fe(Ligand)x]2+@Mont (0.8) catalysts. In CH3CN (Fig. 5A), the catalytic activity of [Fe(terpy)2]2+@Mont in the oxidation of benzene with H2O2
Fig. 5. Catalytic activity and phenol selectivity of [Fe(Ligand)x]2+@Mont catalysts (Fe/Mg = 0.8) in the oxidation of benzene with H2O2 performed in CH3CN (A) and CH3CN–H2O (1:1) mixed solvent (B). Reaction condition: Fe in catalysts (7.9 μmol), benzene (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), solvent (10 mL), 50 °C, 24 h and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
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of the catalytic activities of [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@ Mont, and [Fe(bpy)3]2+@Mont depends on the substrate. Therefore, the catalytic activities of the catalysts prepared in the present study were compared to each other in pairs. The results of these experiments are reported in Fig. 8. The value of the ratio {TON of [Fe(phen)3]2+@ Mont}/{TON of [Fe(bpy)3]2+@Mont} increased as cyclohexane was replaced by benzene as the substrate (Fig. 8(A)); the value of the ratio {TON of [Fe(terpy)2]2+@Mont}/{TON of [Fe(bpy)3]2+@Mont} increased as cyclohexane was replaced by benzene as the substrate (Fig. 8(B)); the values of the ratios {TON of [Fe(phen)3](ClO4)2}/{TON of [Fe(bpy)3](ClO4)2} and {TON of [Fe(terpy)2](ClO4)2}/{TON of [Fe (bpy)3](ClO4)2} for the oxidation of benzene were quite similar to those for the oxidation of cyclohexane (Fig. S4). Generally, the spread of πelectrons over phen and terpy ligands is larger than that over bpy ligand, and the spread of π-electrons over substrates increases between cyclohexane and benzene. The observed trends suggest that the spread of π-electrons over phen and terpy ligands in [Fe(phen)3]2+@Mont and [Fe(terpy)2]2+@Mont, respectively, increases the catalyst’s uptake ability of π-electron-possessing substrates like benzene and cyclohexene, as a consequence of π–π stacking interactions between the ligands and such substrates.
Fig. 6. Plots of the catalytic activity of each [Fe(Ligand)x]2+@Mont catalyst in the oxidation of benzene with H2O2 performed in CH3CN–H2O (1:1) mixed solvent versus the value of the Fe/Mg ratio in the individual catalyst. [Fe (terpy)2]2+@Mont (▲), [Fe(phen)3]2+@Mont (⬤), and [Fe(bpy)3]2+@Mont (⬛). Reaction condition: Fe in catalysts (7.9 μmol), benzene (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), CH3CN (5 mL), H2O (5 mL), 50 °C, 24 h and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
4. Conclusion Catalysts obtained via the immobilization of iron complexes [Fe (terpy)2]2+, [Fe(phen)3]2+, and [Fe(bpy)3]2+ into montmorillonite to produce [Fe(Ligand)x]2+@Mont were prepared by an ion-exchange process whereby cations present within montmorillonite (probably Ca2+) were replaced by [Fe(Ligand)x]2+ complexes obtained as a result of the dissolution in water of [Fe(Ligand)x](ClO4)2. The [Fe (Ligand)x]2+ complexes are assumed to have been inserted between the layers of montmorillonite, since the interplanar distances of (001) plane derived from montmorillonite in [Fe(Ligand)x]2+@Mont catalysts increased with the amount of immobilized [Fe(Ligand)x]2+ ions. The [Fe (Ligand)x]2+@Mont catalysts did catalyze the oxidation of cyclohexane and benzene with H2O2, whereas no oxidation products were obtained when unmodified montmorillonite was utilized as catalyst. Regardless of the reaction conditions, among the catalysts tested, [Fe(terpy)2]2+@ Mont exhibited the best catalytic performance in substrate oxidation with H2O2. Based on the results of experiments performed to compare the catalytic activities of [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@Mont, and [Fe(bpy)3]2+@Mont, we suggest that the interaction between ligands in [Fe(Ligand)x]2+ ions characterized by a high degree of πelectron delocalization for phen and terpy and substrates possessing πelectrons for benzene is emphasized between the montmorillonite layers.
solvent. In the mixed solvent, since H2O molecules may be coordinated with Ca2+ions in montmorillonite, it is difficult for Ca2+ ions to capture alcohols, resulting in low alcohol selectivity; further study is needed to clarify the reaction mechanism. 3.2.3. Effect of the Fe/Mg ratio on the activity of [Fe(Ligand)x]2+@Mont catalysts In Fig. 6 is shown the plots of the catalytic activity of each [Fe(Ligand)x]2+@Mont catalyst in the oxidation of benzene with H2O2 performed in CH3CN–H2O (1:1) mixed solvent as a function of the Fe/Mg ratio in the relevant catalyst. The catalytic activity order for the mentioned reaction is as follows: [Fe(terpy)2]2+@Mont > [Fe(phen)3]2+@ Mont > [Fe(bpy)3]2+@Mont. Regardless of the specific catalyst, the catalytic activity increased as the Fe/Mg value in the catalyst increased. This result indicates that the catalytic activity of the catalysts increased as the concentration of the iron complex in montmorillonite increased. As the concentration of the iron complex in montmorillonite increases, the hydrophobicity between the layers is expected to increase because of making intermolecular interaction network of metal complexes as seen in molecular crystal of [Fe(bpy)3](ClO4)2 [50]. If one can assume that the uptake of benzene into interlayer become easier due to hydrophobicity in interlayers, the probability of collision between the iron complex and the substrate increases, resulting in high catalytic activity.
Author contributions S.Y and H.Y. conceived and designed the experiments on synthesis and catalytic application of iron compounds; D.I., Y.Y., and Y.U. performed synthesis of the iron catalysts; D.I. performed experiments on oxidation and analysis of products by GC; S.Y. and H.Y wrote the paper.
3.2.4. Time course of the catalytic oxidation of cyclic hydrocarbons performed over [Fe(terpy)2]2+@Mont (0.8) In Fig. 7 are shown data reflecting the time course of the catalytic oxidation of cyclohexane and benzene with H2O2 over [Fe(terpy)2]2+@ Mont (0.8) in CH3CN–H2O (1:1) mixed solvent. The alcohol selectivity of benzene oxidation (Fig. 7B) was remarkably higher than that of cyclohexane oxidation (Fig. 7A). Furthermore, regardless of substrate, the TON values associated with the use of [Fe(terpy)2]2+@Mont (0.8) gradually increased with time. Among the catalysts we tested, [Fe (terpy)2]2+@Mont is the most effective in the oxidation of benzene with H2O2 to phenol.
CRediT authorship contribution statement Syuhei Yamaguchi: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing - original draft, Writing review & editing. Daijiro Ihara: Data curation, Formal analysis, Investigation. Yuki Yamashita: Data curation, Formal analysis, Investigation. Yudai Uemoto: Data curation, Formal analysis, Investigation. Hidenori Yahiro: Conceptualization, Investigation, Methodology, Supervision, Writing - original draft, Writing - review & editing.
3.2.5. Effect of the ligand in [Fe(Ligand)x]2+@Mont catalysts As can be evinced from the data shown in Figs. 4B and 5 B, the order 5
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Fig. 7. Time course of the catalytic oxidation of cyclohexane (A) and benzene (B) with H2O2 performed over catalyst [Fe(terpy)2]2+@Mont (0.8) in CH3CN–H2O (1:1) mixed solvent. Reaction condition: Fe in catalysts (7.9 μmol), substrate (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), CH3CN (5 mL), H2O (5 mL), 50 °C, and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
Fig. 8. Comparison of catalytic activity of pairs of [Fe(Ligand)x]2+@Mont catalysts (Fe/Mg = 0.8) in the oxidation of cyclic hydrocarbons (cyclohexane and benzene) by H2O2 in CH3CN–H2O (1:1) mixed solvent. (A) {TON of [Fe of [Fe (phen)3]2+@Mont}/{TON (bpy)3]2+@Mont}, (B) {TON of [Fe of [Fe (terpy)2]2+@Mont}/{TON (bpy)3]2+@Mont}. TON [‒] = Product [mol]/Fe [mol].
Acknowledgments
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Catalytic oxidation of cyclic hydrocarbons with hydrogen peroxide using Fe complexes immobilized into montmorillonite Syuhei Yamaguchi*, Daijiro Ihara, Yuki Yamashita, Yudai Uemoto, Hidenori Yahiro Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron complex Montmorillonite Oxidation Cyclic hydrocarbons Hydrogen peroxide
[Fe(Ligand)x]2+ complexes immobilized into montmorillonite ([Fe(Ligand)x]2+@Mont, Ligand: 2,2′;6′,2′'-terpyridine (x = 2), 1,10-phenanthroline (x = 3), and 2,2′-bipyridine (x = 3)) were prepared by an ion-exchange method whereby cations present within montmorillonite (probably Ca2+) were replaced by [Fe(Ligand)x]2+ complexes provided by [Fe(Ligand)x](ClO4)2. The catalytic properties of the species obtained were then investigated in the oxidation of cyclic hydrocarbons, such as cyclohexane and benzene, with H2O2. The catalytic oxidation of cyclohexane and benzene with H2O2 over [Fe(Ligand)x]2+@Mont catalysts proceeded, and the desired alcohols were obtained as the main reaction products, whereas no catalytic activity was displayed by neat montmorillonite. Regardless of the reaction conditions, [Fe(terpy)2]2+@Mont (terpy stands for 2,2′;6′,2′'terpyridine) exhibited the best catalytic performance in the oxidation of the mentioned substrates by H2O2, among all the catalysts tested.
1. Introduction
several researchers; however, to the best of our knowledge, the oxidation of benzene to phenol with H2O2 as the oxidant and in the presence of the mentioned catalysts has not been reported. Recently, we reported the catalytic activity of Fe-bipyridine complexes encapsulated into Ytype zeolites ([Fe(bpy)3]2+@Na-Y), which are able to catalyze the oxidation of benzene and cyclohexene with H2O2 in CH3CN and/or H2O as solvents [49–53]. The maximum catalytic activity of [Fe(bpy)3]2+@ Na-Y in the oxidation of benzene with H2O2 was achieved when the volume ratio of the solvents (CH3CN and H2O) was 1:1 [51,52]. We also found that [Fe(terpy)2]2+@Na-Y and [Fe(phen)3]2+@Na-Y (terpy = 2,2′;6′,2′'-terpyridine and phen = 1,10-phenanthroline) exhibited a higher catalytic activity than [Fe(bpy)3]2+@Na-Y in the oxidation of benzene with H2O2 to phenol [52]. Montmorillonite is a clay mineral considered part of the smectite group, and it is characterized by a 2:1 structure with two tetrahedral silica sheets sandwiching a central octahedral alumina sheet whereby Al3+ ions have been partially replaced by Mg2+ ions. Since montmorillonite has a large cation exchange capacity [54], several cations, including individual metal ions and cationic metal complexes, can be easily immobilized in montmorillonite by an ion-exchange method. In this study were prepared inorganic–organic hybrid materials whereby [Fe(Ligand)x]2+ complexes were immobilized into montmorillonite: [Fe(terpy)2]2+@Mont, [Fe (phen)3]2+@Mont, and [Fe(bpy)3]2+@Mont. The catalytic activity of these materials for the oxidation of cyclic hydrocarbons like
The catalytic direct hydroxylation of inert CeH bonds in hydrocarbons under mild conditions is a major challenge in synthetic chemistry, whether it applies to the chemical industry or academic research [1–5]. In particular, the catalytic hydroxylation of benzene to phenol using environmentally friendly oxidants, such as hydrogen peroxide (H2O2) [6–14], O2 in combination with reducing agents [15–21], and H2O with electrochemical [22] or photochemical reaction systems [23,24] has attracted a great deal of attention. One of the most attractive areas in catalysis research is the development of inorganic–organic hybrid materials that catalyze oxidation reactions [25–36]. Many researchers have reported inorganic–organic hybrid materials based on montmorillonite, which is a type of clay, and several transition metal complexes, which can be used to catalyze the oxidation of hydrocarbons by H2O2 [25–30], O2 [31–33], t-butyl hydrogen peroxide (TBHP) [28,30,34], iodosylbenzene (PhIO) [27,29,35], and sodium periodate (NaIO4) [36] as oxidants, trapping of volatile sulfur derivatives [37,38], adsorption of phenols [39–43], clay-modified electrodes [39–43], improvement of optical purity [44–46], ethylene polymerization [47], adsorption of phenols [48], and so on. The oxidation of organic substrates like cyclohexane [25–29], n-octane [30], and others [29] with H2O2 acting as an oxidant over transition metal complexes immobilized into montmorillonite have been reported by
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Corresponding author. E-mail address: [email protected] (S. Yamaguchi).
https://doi.org/10.1016/j.cattod.2019.12.023 Received 30 June 2019; Received in revised form 28 November 2019; Accepted 16 December 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Syuhei Yamaguchi, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.12.023
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parentheses after the sample name, for example the notation [Fe (terpy)2]2+@Mont (0.8) represents a [Fe(terpy)2]2+-based inorganic–organic hybrid material whereby the Fe/Mg molar ratio is 0.8.
cyclohexane and benzene with H2O2 wasinvestigated. 2. Experimental 2.1. Materials and instruments
2.3. Catalytic oxidation
Montmorillonite (Montmorillonite K10) was purchased from SIGMA-ALDRICH. All chemicals were used as received. The actual elemental compositions of the prepared samples were measured by Xray fluorescence (XRF) analysis using a Rigaku ZSX Primus instrument. The Fe/Mg or Ca/Mg molar ratios of the samples were calculated based on the intensity of the peak attributed to each element. The powder Xray diffraction (XRD) patterns of the catalysts were collected on a Rigaku MiniFlex II diffractometer using CuKα radiation. UV–vis spectra were recorded on a Hitachi U-4000 spectrometer, in the case of solid samples, or on a Shimadzu U-1200 spectrometer, in the case of liquid samples. Gas chromatography (GC) analysis was performed on a Shimadzu GC-2014 instrument with a flame ionization detector equipped with a DB-1MS capillary column (internal diameter =0.25 mm and length =30 m) at the nature of non-polar liquid phase. Gas chromatography–mass spectrometry (GC–MS) spectra were recorded on a Shimadzu GCMS-QP5050A (ionization voltage =70 eV) instrument equipped with a DB-1MS capillary column (internal diameter =0.25 mm, length =30 m) at the nature of non-polar liquid phase.
The catalytic oxidations of benzene and cyclohexane were carried out as follows: the catalyst (7.9 μmol), solvent CH3CN and/or H2O (10 mL), substrate (7.9 mmol), and 30 % aqueous hydrogen peroxide (7.9 mmol) were transferred into a glass tube reactor. The reaction was carried out at 50 °C under Ar atmosphere. Subsequently, triphenylphosphine (1.1 mmol) was added to the glass tube reactor as a quencher and o-dichlorobenzene (0.34 mmol) was added as an internal standard. The reaction products were analyzed by GC and identified by the comparison of their mass spectra with those of authentic samples. The turnover number (TON) was calculated as the number of product molecules obtained per iron center (TON [‒] = Product [mol]/Fe [mol]). 3. Results and discussion 3.1. Characterization of [Fe(Ligand)x]2+@Mont catalysts In Fig. 1 are shown the values of the Ca/Mg ratios as a function of the Fe/Mg values for the three [Fe(Ligand)x]2+@Mont catalysts. Notably, the Mg/Al ratios were almost constant (0.18–0.23) for all prepared catalysts and raw montmorillonite. In the case of [Fe(Ligand)x]2+@Mont catalysts, the Ca/Mg ratios decreased as the Fe/Mg ratios increased, suggesting that Ca2+ ions in the original montmorillonite were exchanged for [Fe(Ligand)x]2+ ions. The UV–vis diffuse reflectance spectra of [Fe(Ligand)x]2+@Mont (0.8) catalysts are shown in Fig. 2A. In the case of untreated montmorillonite, no absorption was observed between the ultraviolet and visible regions (Fig. 2A(d)). The absorption spectra of [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@Mont, and [Fe(bpy)3]2+@Mont (Fig. 2A(a)–(c)) displayed two sets of bands in the 400–650 nm and 250–400 nm wavelength ranges, which can be assigned to a metal-to-ligand (d–π*) charge-transfer (MLCT) and a π–π* transition within the ligand (bpy, phen, and terpy), respectively, similar to those observed for [Fe(bpy)3](ClO4)2, [Fe(phen)3](ClO4)2, and [Fe (terpy)2](ClO4)2 (Fig. S1). For all [Fe(Ligand)x]2+@Mont catalysts, the intensity of the peak at 550 nm due to the MLCT increased alongside the value of the Fe/Mg ratio, and it reached a maximum at Fe/ Mg = 0.8–1.2 (Figs. 2B and S2), suggesting that the [Fe(Ligand)x]2+ ions were coming close in montmorillonite. XRD patterns of [Fe(Ligand)x]2+@Mont (0.8) catalysts are shown in Fig. 3A. The peaks of
2.2. Preparation of iron-containing catalysts 2.2.1. Preparation of [Fe(Ligand)x](ClO4)2 As ion-exchange agents, [Fe(terpy)2](ClO4)2 [55], [Fe(phen)3] (ClO4)2 [56,57], and [Fe(bpy)3](ClO4)2 [58] were prepared by the method reported elsewhere. Concisely, [Fe(terpy)2](ClO4)2 was prepared by the following method: 2,2′,6′,2′'-terpyridine (0.28 g, 1.2 mmol) was added into a H2O solution (100 mL) of FeSO4∙7H2O (0.13 g, 0.48 mmol) at about 80 °C, and the resulting mixture was stirred at the same temperature for 1 h. Upon the addition of sodium perchlorate (0.37 g, 3.0 mmol) into the solution thus obtained, a reddish-purple precipitate formed immediately. After filtration, recrystallization of the precipitate from CH3CN/diethyl ether produced a needle-like reddish-purple crystalline solid. Needle-like reddish-purple solid; C30H22Cl2FeN6O8 (721.28) calcd. C 49.95, H 3.07, N 11.65; found C 49.53, H 3.11, N 11.60. In a similar way, [Fe(phen)3](ClO4)2 and [Fe(bpy)3](ClO4)2 were prepared using 1,10-phenanthroline and 2,2′-bipyridine as ligands, respectively. A needle-like orange crystalline solid and a needle-like red crystalline solid were obtained as the syntheses of the former and latter complexes were being performed, respectively. Needle-like orange solid; C36H24Cl2FeN6O8 (795.36) calcd. C 54.36, H 3.04, N 10.57; found C 54.02, H 3.23, N 10.45. Needle-like red solid; C30H24Cl2FeN6O8 (723.30) calcd. C 49.81, H 3.34, N 11.62; found C 49.90, H 3.43, N 11.76. 2.2.2. Preparation of [Fe(Ligand)x]2+@Mont catalysts [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@Mont, and [Fe(bpy)3]2+@ Mont were prepared by the ion-exchange method of montmorillonite with Fe(terpy)2](ClO4)2, [Fe(phen)3](ClO4)2, and [Fe(bpy)3](ClO4)2, respectively. The typical conditions for [Fe(Ligand)x]2+@Mont catalyst preparation are as follows: montmorillonite was added into an aqueous solution of [Fe(Ligand)x](ClO4)2 at an appropriate concentration, and the resulting mixture was stirred for 24 h at room temperature. The solid present in the mixture following the stirring process was collected by suction filtration, washed with deionized water, and then dried in ambient air at room temperature to obtain the [Fe(Ligand)x]2+@Mont catalyst. The values for the M/Mg (M = Ca or Fe) molar ratios of the resulting [Fe(Ligand)x]2+@Mont catalysts are summarized in Table S1. Hereinafter, the Fe/Mg molar ratio for each sample is denoted in
Fig. 1. Plots of the Ca/Mg ratio values versus Fe/Mg ratio values for [Fe (Ligand)x]2+@Mont catalysts. [Fe(terpy)2]2+@Mont (▲), [Fe(phen)3]2+@ Mont (⬤), and [Fe(bpy)3]2+@Mont (⬛). 2
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Fig. 2. (A) UV–vis diffuse reflectance spectra of [Fe(terpy)2]2+@Mont (a), (b), [Fe [Fe(phen)3]2+@Mont (bpy)3]2+@Mont (c), and montmorillonite (d). (B) Intensity of metal-toligand (d–π*) charge-transfer peaks as a function of the Fe/Mg ratio in [Fe (▲), [Fe (terpy)2]2+@Mont (phen)3]2+@Mont (⬤), and [Fe (bpy)3]2+@Mont (⬛).
(001) plane of [Fe(Ligand)x]2+@Mont (0.8) catalysts (Fig. 3A(a)–(c)) were shifted to a lower angle side than that of montmorillonite (Fig. 3A(d)). This observation indicates that, regardless of the identity of the ligand, the interlayer distance of (001) plane derived from montmorillonite increased as the Fe/Mg value increased (Figs. 3B and S3). Based on the results of XRF, XRD, and UV–vis experiments, it was concluded that the three iron complexes, [Fe(bpy)3]2+, [Fe(phen)3]2+, and [Fe(terpy)2]2+, were immobilized into montmorillonite.
3.2.1. Oxidation of cyclohexane In Fig. 4 are reported the catalytic activities and alcohol selectivity of [Fe(Ligand)x]2+@Mont (0.8) catalysts in the oxidation of cyclohexane with H2O2 performed in CH3CN and CH3CN–H2O (1:1). The selectivity of the alcohol product in CH3CN solvent (82–84 %) was higher than that in CH3CN–H2O (1:1) mixed solvent (60–66 %). Unmodified montmorillonite displayed no catalytic activity in the oxidation of cyclohexane with H2O2, under the same experimental conditions utilized for [Fe(Ligand)x]2+@Mont (0.8) catalysts. In CH3CN (Fig. 4A), the order of the catalytic activity for the oxidation of cyclohexane with H2O2 was observed to be as follows: [Fe(terpy)2]2+@Mont > [Fe (phen)3]2+@Mont > [Fe(bpy)3]2+@Mont. In CH3CN–H2O (1:1) (Fig. 4B), the catalytic activity of [Fe(terpy)2]2+@Mont was higher than those of [Fe(phen)3]2+@Mont and [Fe(bpy)3]2+@Mont. Overall, [Fe(Ligand)x]2+@Mont catalysts displayed higher catalytic activities in the oxidation of cyclohexane with H2O2 when utilized in CH3CN–H2O (1:1) than in CH3CN solution. Table S2 shows that catalytic activity for the oxidation of cyclohexane with H2O2 using unsupported and supported Fe complexes. The alcohol selectivity of supported Fe complex, [Fe(Ligand)n]2+@Mont (0.8), was higher than that of unsupported one, [Fe(Ligand)n](ClO4)2, although the catalytic activity of the former catalyst was lower than that of the latter one. A similar result was reported by Machado et al. [35]; they found that Fe(III) and Mn(III) cationic porphyrins immobilized in montmorillonite exhibits high selectivity to cyclohexanol for the catalytic oxidation of cyclohexane with iodosylbenzene. Such a high alcohol selectivity of supported metal complex may come from the prompt desorption of alcohol produced from active site before further oxidation; that is, in the limited space like the interlayer of
3.2. Catalytic activity of the three [Fe(Ligand)x]2+@Mont catalysts in the oxidation of cyclic hydrocarbons with H2O2 In order to confirm the effect of the solvent on the H2O2-driven benzene oxidation, the oxidation has been conducted in CH3CN and in CH3CN–H2O mixed solvent using as catalysts: Fe complexes encapsulated into a cation-exchanged Y-type zeolite ([Fe(bpy)3]2+@M-Y (M = Na+, Cs+, Ca2+, NH4+, N(CH3)4+, etc.) [51,52]. Results from these investigations indicated that use of the CH3CN–H2O mixed solvent (CH3CN: H2O = 1: 1) was associated with the best performance for the oxidation of benzene with H2O2 catalyzed by [Fe(bpy)3]2+@M-Y. It was suggested that hydrophobic interaction between ligand and substrate against benzene oxidation may act strongly in narrow spaces such as zeolite. In order to confirm whether the abovementioned hydrophobic interaction occurring in zeolite also takes place in layered compounds like montmorillonite, the effect of the solvent has on the oxidation of cyclic hydrocarbons (e.g., cyclohexane and benzene) with H2O2 catalyzed by [Fe(Ligand)x]2+@Mont catalysts was investigated conducting the experiments detailed in the following sections.
Fig. 3. (A) X-ray diffraction patterns of (a), [Fe [Fe(terpy)2]2+@Mont (phen)3]2+@Mont (b), [Fe(bpy)3]2+@ Mont (c), and montmorillonite (d). (B) Interplanar distance of (001) plane as a function of the Fe/Mg ratio in [Fe (▲), [Fe (terpy)2]2+@Mont (phen)3]2+@Mont (⬤), and [Fe (bpy)3]2+@Mont (⬛).
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Fig. 4. Catalytic activity and alcohol selectivity of [Fe(Ligand)x]2+@Mont catalysts (Fe/Mg = 0.8) in the oxidation of cyclohexane with H2O2 performed in CH3CN (A), and CH3CN–H2O (1:1) mixed solvent (B). Reaction condition: Fe in catalysts (7.9 μmol), cyclohexane (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), solvent (10 mL), 50 °C, 24 h and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
was higher than the corresponding parameters for [Fe(phen)3]2+@ Mont and [Fe(bpy)3]2+@Mont. In CH3CN–H2O (1:1) (Fig. 5B), the order of catalytic activity in the oxidation of benzene with H2O2 was observed to be as follows: [Fe(terpy)2]2+@Mont > [Fe(phen)3]2+@ Mont > [Fe(bpy)3]2+@Mont. In particular, [Fe(terpy)2]2+@Mont and [Fe(phen)3]2+@Mont exhibited especially high catalytic activity. The catalytic activities of [Fe(Ligand)x]2+@Mont catalysts in the oxidation of benzene with H2O2 were much higher in CH3CN–H2O (1:1) than in CH3CN. A similar solvent effect was observed for the oxidation of benzene with H2O2 to phenol over [Fe(bpy)3]2+@Y catalyst [51]. CH3CN or H2O solvent has each problem inhibiting the catalytic reaction; benzene hardly diffuses in H2O, while H2O2 hardly diffuses in CH3CN. These problems were demonstrated to be offset for using the mixed solvent, CH3CN+H2O; [Fe(bpy)3]2+@Y exhibited the better catalytic performance for oxidation of benzene with H2O2 to phenol in the mixed solvent, CH3CN+H2O, than in CH3CN or H2O solvent [51]. From the concept of hard and soft acids and bases (HSAB) [59], Ca2+ ion in montmorillonite as well as Na+ ion in Y-zeolite are hard acid, H2O and alcohols are hard base, and CH3CN is soft base. Because Na+ and Ca2+ ions have a higher affinity with H2O and alcohols than that with CH3CN, Na+ and Ca2+ ions can easily capture alcohols in CH3CN
montmorillonite and the pore of zeolite, the produced alcohol on active site is attracted to counter cation such as Na+ and Ca2+ by electrostatic interaction, resulting in the inhibition of further oxidation to ketone, as reported previously [49,50]. The catalytic activities of [Fe(Ligand)n]2+@Y catalysts reported previously [52] are also shown in Table S2. The catalytic activity of [Fe (bpy)3]2+@Mont was comparable to that of [Fe(bpy)3]2+@Y catalyst which was reported to exhibit the highest catalytic activity among [Fe (Ligand)n]2+@Y [52]. 3.2.2. Oxidation of benzene In Fig. 5 are reported the catalytic activities and phenol selectivity of [Fe(Ligand)x]2+@Mont (0.8) catalysts in the oxidation of benzene with H2O2 performed in CH3CN and CH3CN–H2O (1:1). The phenol selectivities for reactions conducted in CH3CN (almost 100 %) were slightly higher than those measured for reactions conducted in CH3CN–H2O (93–100 %). Also in this case, unmodified montmorillonite displayed no catalytic activity in the oxidation of benzene with H2O2, under the same experimental conditions employed for [Fe(Ligand)x]2+@Mont (0.8) catalysts. In CH3CN (Fig. 5A), the catalytic activity of [Fe(terpy)2]2+@Mont in the oxidation of benzene with H2O2
Fig. 5. Catalytic activity and phenol selectivity of [Fe(Ligand)x]2+@Mont catalysts (Fe/Mg = 0.8) in the oxidation of benzene with H2O2 performed in CH3CN (A) and CH3CN–H2O (1:1) mixed solvent (B). Reaction condition: Fe in catalysts (7.9 μmol), benzene (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), solvent (10 mL), 50 °C, 24 h and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
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of the catalytic activities of [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@ Mont, and [Fe(bpy)3]2+@Mont depends on the substrate. Therefore, the catalytic activities of the catalysts prepared in the present study were compared to each other in pairs. The results of these experiments are reported in Fig. 8. The value of the ratio {TON of [Fe(phen)3]2+@ Mont}/{TON of [Fe(bpy)3]2+@Mont} increased as cyclohexane was replaced by benzene as the substrate (Fig. 8(A)); the value of the ratio {TON of [Fe(terpy)2]2+@Mont}/{TON of [Fe(bpy)3]2+@Mont} increased as cyclohexane was replaced by benzene as the substrate (Fig. 8(B)); the values of the ratios {TON of [Fe(phen)3](ClO4)2}/{TON of [Fe(bpy)3](ClO4)2} and {TON of [Fe(terpy)2](ClO4)2}/{TON of [Fe (bpy)3](ClO4)2} for the oxidation of benzene were quite similar to those for the oxidation of cyclohexane (Fig. S4). Generally, the spread of πelectrons over phen and terpy ligands is larger than that over bpy ligand, and the spread of π-electrons over substrates increases between cyclohexane and benzene. The observed trends suggest that the spread of π-electrons over phen and terpy ligands in [Fe(phen)3]2+@Mont and [Fe(terpy)2]2+@Mont, respectively, increases the catalyst’s uptake ability of π-electron-possessing substrates like benzene and cyclohexene, as a consequence of π–π stacking interactions between the ligands and such substrates.
Fig. 6. Plots of the catalytic activity of each [Fe(Ligand)x]2+@Mont catalyst in the oxidation of benzene with H2O2 performed in CH3CN–H2O (1:1) mixed solvent versus the value of the Fe/Mg ratio in the individual catalyst. [Fe (terpy)2]2+@Mont (▲), [Fe(phen)3]2+@Mont (⬤), and [Fe(bpy)3]2+@Mont (⬛). Reaction condition: Fe in catalysts (7.9 μmol), benzene (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), CH3CN (5 mL), H2O (5 mL), 50 °C, 24 h and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
4. Conclusion Catalysts obtained via the immobilization of iron complexes [Fe (terpy)2]2+, [Fe(phen)3]2+, and [Fe(bpy)3]2+ into montmorillonite to produce [Fe(Ligand)x]2+@Mont were prepared by an ion-exchange process whereby cations present within montmorillonite (probably Ca2+) were replaced by [Fe(Ligand)x]2+ complexes obtained as a result of the dissolution in water of [Fe(Ligand)x](ClO4)2. The [Fe (Ligand)x]2+ complexes are assumed to have been inserted between the layers of montmorillonite, since the interplanar distances of (001) plane derived from montmorillonite in [Fe(Ligand)x]2+@Mont catalysts increased with the amount of immobilized [Fe(Ligand)x]2+ ions. The [Fe (Ligand)x]2+@Mont catalysts did catalyze the oxidation of cyclohexane and benzene with H2O2, whereas no oxidation products were obtained when unmodified montmorillonite was utilized as catalyst. Regardless of the reaction conditions, among the catalysts tested, [Fe(terpy)2]2+@ Mont exhibited the best catalytic performance in substrate oxidation with H2O2. Based on the results of experiments performed to compare the catalytic activities of [Fe(terpy)2]2+@Mont, [Fe(phen)3]2+@Mont, and [Fe(bpy)3]2+@Mont, we suggest that the interaction between ligands in [Fe(Ligand)x]2+ ions characterized by a high degree of πelectron delocalization for phen and terpy and substrates possessing πelectrons for benzene is emphasized between the montmorillonite layers.
solvent. In the mixed solvent, since H2O molecules may be coordinated with Ca2+ions in montmorillonite, it is difficult for Ca2+ ions to capture alcohols, resulting in low alcohol selectivity; further study is needed to clarify the reaction mechanism. 3.2.3. Effect of the Fe/Mg ratio on the activity of [Fe(Ligand)x]2+@Mont catalysts In Fig. 6 is shown the plots of the catalytic activity of each [Fe(Ligand)x]2+@Mont catalyst in the oxidation of benzene with H2O2 performed in CH3CN–H2O (1:1) mixed solvent as a function of the Fe/Mg ratio in the relevant catalyst. The catalytic activity order for the mentioned reaction is as follows: [Fe(terpy)2]2+@Mont > [Fe(phen)3]2+@ Mont > [Fe(bpy)3]2+@Mont. Regardless of the specific catalyst, the catalytic activity increased as the Fe/Mg value in the catalyst increased. This result indicates that the catalytic activity of the catalysts increased as the concentration of the iron complex in montmorillonite increased. As the concentration of the iron complex in montmorillonite increases, the hydrophobicity between the layers is expected to increase because of making intermolecular interaction network of metal complexes as seen in molecular crystal of [Fe(bpy)3](ClO4)2 [50]. If one can assume that the uptake of benzene into interlayer become easier due to hydrophobicity in interlayers, the probability of collision between the iron complex and the substrate increases, resulting in high catalytic activity.
Author contributions S.Y and H.Y. conceived and designed the experiments on synthesis and catalytic application of iron compounds; D.I., Y.Y., and Y.U. performed synthesis of the iron catalysts; D.I. performed experiments on oxidation and analysis of products by GC; S.Y. and H.Y wrote the paper.
3.2.4. Time course of the catalytic oxidation of cyclic hydrocarbons performed over [Fe(terpy)2]2+@Mont (0.8) In Fig. 7 are shown data reflecting the time course of the catalytic oxidation of cyclohexane and benzene with H2O2 over [Fe(terpy)2]2+@ Mont (0.8) in CH3CN–H2O (1:1) mixed solvent. The alcohol selectivity of benzene oxidation (Fig. 7B) was remarkably higher than that of cyclohexane oxidation (Fig. 7A). Furthermore, regardless of substrate, the TON values associated with the use of [Fe(terpy)2]2+@Mont (0.8) gradually increased with time. Among the catalysts we tested, [Fe (terpy)2]2+@Mont is the most effective in the oxidation of benzene with H2O2 to phenol.
CRediT authorship contribution statement Syuhei Yamaguchi: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing - original draft, Writing review & editing. Daijiro Ihara: Data curation, Formal analysis, Investigation. Yuki Yamashita: Data curation, Formal analysis, Investigation. Yudai Uemoto: Data curation, Formal analysis, Investigation. Hidenori Yahiro: Conceptualization, Investigation, Methodology, Supervision, Writing - original draft, Writing - review & editing.
3.2.5. Effect of the ligand in [Fe(Ligand)x]2+@Mont catalysts As can be evinced from the data shown in Figs. 4B and 5 B, the order 5
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Fig. 7. Time course of the catalytic oxidation of cyclohexane (A) and benzene (B) with H2O2 performed over catalyst [Fe(terpy)2]2+@Mont (0.8) in CH3CN–H2O (1:1) mixed solvent. Reaction condition: Fe in catalysts (7.9 μmol), substrate (7.9 mmol), 30 % aqueous H2O2 (7.9 mmol), CH3CN (5 mL), H2O (5 mL), 50 °C, and Ar atmosphere. Turnover number (TON) [‒] = Product [mol]/Fe [mol].
Fig. 8. Comparison of catalytic activity of pairs of [Fe(Ligand)x]2+@Mont catalysts (Fe/Mg = 0.8) in the oxidation of cyclic hydrocarbons (cyclohexane and benzene) by H2O2 in CH3CN–H2O (1:1) mixed solvent. (A) {TON of [Fe of [Fe (phen)3]2+@Mont}/{TON (bpy)3]2+@Mont}, (B) {TON of [Fe of [Fe (terpy)2]2+@Mont}/{TON (bpy)3]2+@Mont}. TON [‒] = Product [mol]/Fe [mol].
Acknowledgments
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