Enhanced Catalytic Efficiency in the Epoxidation of Alkenes for

Article pubs.acs.org/IECR

Enhanced Catalytic Efficiency in the Epoxidation of Alkenes for Manganese Complex Encapsulated in the Hydrophobic Interlayer Region of Layered Double Hydroxides Yuqing Liu, Zhe An, Liwei Zhao, Hui Liu, and Jing He* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Inspired by the crucial role of nature hydrophobic regions in highly efficient enzymatic catalysis, this work proposes a simple but valid catalyst-design strategy by simply employing the interlayer region of layered double hydroxides (LDHs) to mimic the bilayer structure of nature phospholipids. The interlayer region of LDHs intercalated with dodecyl sulfonate, an amphiphilic species, provides bidimensional hydrophobic environments for encapsulated Mn(TPP)OAc. The Mn centers encapsulated in the hydrophobic interlayer region of LDHs has been found to exhibit superior catalytic activity to homogeneous counterpart in the epoxidation of a variety of alkenes, including cyclohexene, heptylene, phenylethylene, 3-methyl3-buten-1-ol, and even functional alkenes such as ethyl cinnamate and chalcone. It has also been found that TOF increases with decreasing polarity of the catalyzed substrate while TON depends on the polarity difference between the catalyzed alkene and produced epoxide.

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INTRODUCTION Selective catalytic oxidation, which converts bulk chemicals into high value-added products, has been recognized as one of atomeconomical technologies. Metalloporphyrin complexes, such as Mn(III)-porphyrin, Co(II)-porphyrin, Fe(III)-porphyrin, and Ru-porphyrin, are predominant catalysts for catalytic oxidation reactions.1 Because of the available multivalence states of ruthenium center, ruthenium compounds show superior catalytic performance in a diversity of redox reactions.2,3 As a relatively scarce metal, however, limited production and high price restrict the large-scale application of ruthenium as catalyst. Efforts have thus been devoted to the valid substitution of ruthenium compounds for wider commercialization.4 Unfortunately, most nonprecious metal compounds as catalysts showed inferior catalytic activity to ruthenium-based catalysts.5,6 The development of highly efficient catalysts is yet of great challenge. A few examples of high catalytic efficiency have been reported.7−9 But more efforts are desired. To develop highly efficient catalysts for selective catalytic oxidations, this work proposes a simple but valid catalyst-design strategy to mimic the bilayer structure of nature phospholipids10 (Scheme 1), which is inspired by the crucial role of nature hydrophobic regions11−13 in the high efficacy of enzymatic catalysis. The strategy simply employs the interlayer region of layered double hydroxides (LDHs) intercalated with amphiphilic species as flexible bidimensional hydrophobic environments for catalytic sites. The amphiphilic species used here is dodecyl sulfonate (AS), which can be easily introduced into the interlayer region by ion exchange or homogeneous precipitation approach. Previous efforts devoted to mimicing the nature hydrophobic environment for catalytic sites in heterogeneous catalysts include the usage of cyclodextrins, dendrimers, and self-assembled supramolecular systems.14−24 Enhancement of reaction rates by accumulating substrates has been observed in the selective oxidation, aldol addition, and nazarov cyclization reactions, etc.14−22 But rigid geometry and © 2013 American Chemical Society

restricted space of cavities limit the access of substrates and inhibit the escape of products.23,24 Relatively, the bilayer structure of phospholipids provides 2D accessible hydrophobic regions, which not only enhances the affinity to substrates but also has less diffusion hindrance.25−29 Mimicking the bilayer phospholipids has thus attracted much attention, but their application in catalysis28,29 wants more concern.

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EXPERIMENTAL SECTION Preparation. The nitrate-intercalated LDH (Zn/Al-NO3LDHs) sample was prepared by the ion exchange of Zn/AlCO3-LDHs with NO3− in acidic solution,30 and dodecyl sulfonate (AS) intercalated Zn/Al-LDH (Zn/Al-AS-LDHs) sample was prepared from Zn/Al-NO3-LDHs following the ion exchange approach as reported previously.31 0.07 mol of Zn(NO3)2·6H2O (2.0824 g), 0.0035 mol of Al(NO3)3·9H2O (1.31295 g), and 0.0245 mol of urea (1.4715 g) in 700 mL of deionized water were loaded quickly under thorough agitation. After the resultant mixture solution was crystallized at 105 °C for 27 h, Zn/Al-CO3-LDH powdery product was collected by filtration; 0.04 g of Zn/Al-CO3-LDHs was suspended in 400 mL of acidic NaNO3 aqueous solution (0.62 mol of NaNO3 and 0.062 mol of HNO3). The suspension was stirred at ambient temperature for 24 h under N2 atmosphere. The resultant slurry was centrifuged. The solid was collected after being extensively washed with decarbonated deionized water and dried at 40 °C. 0.01 mol of sodium dodecyl sulfonate (SAS, 2.7238 g) was dissolved in 200 mL of deionized water at 70 °C to produce a transparent solution. Then, 0.50 g of Zn/Al-NO3LDHs was suspended in the solution, and the suspension was slowly stirred at 70 °C for 48 h. The solid was washed with Received: Revised: Accepted: Published: 17821

August 15, 2013 October 19, 2013 November 22, 2013 November 22, 2013 dx.doi.org/10.1021/ie4026693 | Ind. Eng. Chem. Res. 2013, 52, 17821−17828

Industrial & Engineering Chemistry Research

Article

Scheme 1. (A) Phospholipids Bilayer in Nature and (B) Schematic Structure of Proposed Biomimetic Catalysta

a

The amphiphilic species applied here in biomimetic catalyst (B) is dodecyl sulfonate.

corrected STEM coupled with a modified INCA-450 spectrometer. The high-resolution SEM was operated at an acceleration voltage of 30 kV. 1H NMR spectra were recorded on Bruker Avance 400 MHz NMR spectrometer (Bruker, Bremen, Germany) and recorded in CDCl3 as solvent at ambient temperature. Catalytic Test. Weighed amount of catalyst and 1.5 mL of acetonitrile were added in an airtight reaction flask. O2, as oxidant, was filled in the system by using a dioxygen balloon. Then, 0.006 mol of alkene and 0.009 mol of isobutylaldehyde were added. The reaction mixture was stirred at 25 °C for 24 h. The catalytic process was monitored by a Shimadzu GC-2010 with a DB-5 column. The products were identified by gas chromatography−mass spectrometry (GC/MS), and quantified on Shimadzu GC-2010 with a DB-5 capillary column (30 m × 0.25 mm id, 0.5 μm film thickness) with N2 as the carrier gas at 15.4 cm/sec linear velocity, if not specially indicated. The pressure of GC injector is 48.0 kPa and the split ratio is 50. Calibration of GC peak areas of the products was done using solutions with known amounts of their standard compounds purchased from suppliers (the epoxide of cyclohexene, phenylethylene, heptylene, and ethyl cinnamate) or purified from the reaction mixture (the epoxide of 3-methyl-3 -buten-1ol). The epoxide of 3-methyl-3-buten-1-ol, 2-oxiraneethanol, was identified by 1H NMR. 1H NMR (400 MHz, CDCl3): δ = 3.66−3.77 (m, 2H), 2.80 (d, 1H, J1 = 4 Hz), 2.64 (d, 1H, J1 = 4 Hz), 3.69 (s, 6H), 1.82−1.97 (m, 2H), 1.37 (s, 3H). The content of substrate and product in the system was calculated by an internal standard method using toluene or decane as an internal standard. The epoxide of chalcone was determined by HPLC using a DAICEL Chiralcel OD column, λ = 254 nm, iPrOH/hexane = 10:90, 0.8 mL/min. The standard compound for identification of the epoxide of chalcone, phenyl(3phenyloxiran-2-yl)methanone, was prepared according to general procedure and identified by 1H NMR. 1HNMR (400 MHz, CDCl3): δ = 8.04 (d, 2H, J = 8.0 Hz), 7.65 (t, 1H, J = 8.0 Hz), 7.52 (t, 2H, J = 8.0 Hz), 7.43 (m, 5H), 4.33 (s, 1H), 4.11 (s, 1H). The TON was calculated according to the plateau conversion, and TOF according to the initial conversion.

decarbonated deionized water and anhydrous ethanol, and then vacuumed at ambient temperature for 12 h. To prepare Mn(TPP)OAc (TPP = meso-tetraphenylporphyrin dianion)-encapsulated Zn/Al-AS-LDHs, we first exfoliated Zn/Al-AS-LDH by refluxing its suspension in butanol, isooctane, or hexane, until a colloidal solution was produced. Typically, 0.1 g of Zn/Al-AS-LDHs was refluxed in 200 mL of butanol, isooctane, or hexane for 36 h. Then, 0.02 g (0.03 mmol) of Mn(TPP)OAc was added in the system and the suspension was stirred for another 12 h. Then butanol (or isooctane, or hexane) was evaporated and the resultant slurry was centrifuged. The restored solid was collected after being extensively washed with ethanol and methylene dichloride to remove excess Mn(TPP)OAc, and then dried at 40 °C. The resulting sample was denoted Zn/Al-AS-Mn(TPP)OAc-LDHsbutanol, Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane, or Zn/AlAS-Mn(TPP)OAc-LDHs-hexane. As a control sample, the exfoliated Zn/Al-AS -LDHs was restored by simple evaporation of butanol without introduction of Mn(TPP)OAc. The resulting sample was denoted Zn/Al-AS-LDHs-restoration. The control sample, with Mn(TPP)OAc just adsorbed on the hydrophilic surface of Zn/Al-LDHs, was prepared by refluxing Zn/Al-CO3-LDHs in a solution of Mn(TPP)OAc in butanol. Ru(TPP)CO as the homogeneous control catalyst was synthesized following a procedure reported previously.32,33 160 mg of H2TPP (meso-tetraphenylporphyrin) and 160 mg of Ru3(CO)12 were refluxed in 40 mL of decalin at 200 °C for 48 h under N2 atmosphere. The resulting red solution was loaded on an alumina column. Decalin and some other impurities were removed with hexane as the eluent. The brick red band containing the desired product was then eluted with dichloromethane and the eluent was collected. The eluent was evaporated by rotatory evaporator. The solid was purified by recrystallization from dichloromethane/n-hexane (1:6 v/v), and then Ru(TPP)CO (the red-purple crystal) was obtained. Characterization. The powder X-ray diffraction (XRD) patterns were recorded with a Shimadzu XRD-6000 diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 0.15405 nm). Fourier transform infrared (FTIR) spectra in the range of 400−4000 cm−1 were measured on a Bruker Vector 22 infrared spectrophotometer using the KBr pellet technique with a resolution of 1 cm−1. The content of Mn, Zn or Al of the LDH samples was determined by inductively coupled plasma (ICP) atomic emission spectrophotometry (Shimadzu ICPs-7500) after a weighed amount of sample was dissolved in an aqueous HNO3 solution. The morphologies and dimensions of the particles were examined with a Zeiss supra 55 scanning electron microscope. A Hitachi S-5500 scanning electron microscope equipped with EDS was used to identify the element distribution in nanoscale. The EDS measurements were performed with the dedicated aberration-

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RESULTS AND DISCUSSION Figure 1 shows the powder X-ray diffraction (XRD) patterns of Mn(TPP)OAc encapsulated Zn/Al-AS-LDHs and as-prepared Zn/Al-AS-LDHs. A set of (00l) reflections typical of hydrotalcite-like structure is clearly observed in each case. Zn/Al-ASMn(TPP)-LDHs-butanol gives a basal spacing (d003) of 2.15 nm (Figure 1a), whereas Zn/Al-AS-Mn(TPP)OAc-LDHsisooctane (Figure 1b) and Zn/Al-AS-Mn(TPP) OAc-LDHshexane (Figure 1c) give similar basal spacing, which is 1.95 and 1.94 nm, respectively. The basal spacing is 1.97 nm for Zn/AlAS-LDHs-restoration (Figure 1d), and 2.66 nm for as-prepared 17822

dx.doi.org/10.1021/ie4026693 | Ind. Eng. Chem. Res. 2013, 52, 17821−17828


Article

lyst, the Mn amount adsorbed on the hydrophilic surface of Zn/Al-CO3-LDHs is 0.06 wt %. In the line-scanning EDS image (Figure 2), an evendistribution of Mn/Al ratio is observed throughout the whole

Figure 1. XRD patterns of (a) Zn/Al-AS-Mn(TPP)OAc-LDHsbutanol, (b) Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane, (c) Zn/AlAS-Mn(TPP)OAc-LDHs-hexane, (d) Zn/Al-AS-LDHs-restoration, and (e) as-prepared Zn/Al-AS LDHs.

Zn/Al-AS-LDHs (Figure 1e) that is consistent with a bilayer arrangements34 of AS anion in the interlayer region. It can be clearly observed that, Zn/Al-AS-Mn(TPP)-LDHs-butanol shows a basal spacing different from either the as-prepared ZnAl-AS-LDHs as precursor or the Zn/Al-AS-LDHs swollen/ restored without Mn(TPP)OAc introduced (Zn/Al-AS-LDHsrestoration), whereas Zn/Al-AS-Mn (TPP)OAc-LDHs-isooctane or Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane gives the same basal spacing as Zn/Al-AS-LDHs-restoration. In the restoration to encapsulate Mn(TPP)OAc into the interlayer region via the hydrophobic interactions between tetraphenylporphyrin moiety and carbon chains of interlayer AS, a rearrangement of interlayer AS anions occurred in each case. But also, the AS rearrangement for Zn/Al-AS-Mn(TPP)-LDHs-butanol was induced by the encapsulated Mn(TPP)OAc. As reported previously,35 taking the chain length of AS anion as 1.85 nm36 and the thickness of LDH layer as 0.48 nm,37 the basal spacing is calculated to be 2.33 nm when the interlayer AS is arranged in a perpendicular monolayer. A basal spacing smaller than 2.33 nm is attributed to a tilted monolayer arrangement of interlayer AS anions, and a basal spacing higher than 2.33 nm originates from a bilayer arrangement with two SO3− groups linked to the adjacent layers of LDH while hydrocarbon chains interpenetrated or overlapped. So in the restoration of exfoliated Zn/Al-AS-LDHs with or without Mn(TPP)OAc introduced in this work, the interlayer AS anions can be deduced to be rearranged from bilayer to tilted monolayer. The encapsulated Mn(TPP)OAc probably reduced the tilted degree of interlayer AS anions, resulting in an enlarged interlayer gallery height in comparison to without Mn(TPP)OAc encapsulated. According to the ICP analysis, the Mn loading in Zn/Al-AS-Mn(TPP)-LDHs-butanol is 0.48 wt %, which is higher than the Mn content in the natural enzyme and other heterogeneous Mn catalysts.38−40 The Mn loading is 0.25 and 0.08 wt % for Zn/Al-AS-Mn(TPP)OAc- LDHs-isoocatane and Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane. For the control cata-

Figure 2. Line-scanning EDX images of (a) Zn/Al-AS-Mn(TPP)OAcLDHs-butanol, (b) Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane, and (c) Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane.

hexagonal LDH crystallite for Zn/Al-AS-Mn(TPP)- LDHsbutanol (Figure 2a). However, Zn/Al-AS-Mn(TPP)OAcLDHs-isooctane (Figure 2b) affords a higher Mn/Al ratio at the edge of hexagonal LDH crystallite, and Zn/Al-ASMn(TPP)OAc-LDHs-hexane (Figure 2c) affords a more scattered Mn distribution than Zn/Al-AS-Mn(TPP)-LDHsbutanol through the hexagonal LDH crystallite. The results indicate, the Mn moiety has been encapsulated in homogeneous distribution along the bidimensional interlayer region in Zn/Al-AS-Mn(TPP)-LDHs-butanol, whereas the Mn(TPP) is mainly distributed at the edges of interlayer regions in Zn/AlAS-Mn(TPP)OAc-LDHs-isooctane. The Mn distribution in Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane in less homogeneous than in Zn/Al-AS-Mn(TPP)-LDHs-butanol, hinting that Mn(TPP)OAc in Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane has mainly been adsorbed on the outside of Zn/Al-AS LDHs hydrophobic layers. The deduction about the distribution of Mn moiety from EDS image is consistent with the Mn loading determined from ICP analysis. In the FT-IR spectra (Figure 3), the absorption assigned to SO stretching vibration appears at 1206, 1157, and 1048 cm−1 for Zn/Al-AS-Mn(TPP)-LDHsbutanol (Figure 3a). The SO vibration appears at 1185 and 17823



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butanol, Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane, and Zn/ Al-AS-Mn(TPP)OAc-LDHs-hexane solids are denoted Mn(TPP)OAc between, around, and on the hydrophobic layers, respectively in the following discussion. The resulting solids were first used as the catalysts for the epoxidation of cyclohexene with O2 as oxidant. With the Mn(TPP)OAc encapsulated between the hydrophobic layers (Zn/Al-AS-Mn(TPP)-LDHs-butanol), a turnover frequency (TOF) of as high as 4108 h−1 and turnover number (TON) of 48974 (Table 1, entry 1) have been obtained, which are Table 1. Catalytic Epoxidation of Cyclohexene with Various Catalystsa

entry

catalysts

TOF (h−1)b

TONb

selectivity (%)b

1

Mn(TPP)OAc between the hydrophobic layers Mn(TPP)OAc on the hydrophobic layers surface Mn(TPP)OAc around the hydrophobic layers Mn(TPP)OAc on the hydrophilic layers Mn(TPP)OAc Ru(TPP)CO Zn/Al-AS-LDHsc isobutylaldehydec Zn/Al-AS-LDHs, isobutylaldehydec

4108 ± 31

48974 ± 244

80 ± 1

4188 ± 19

26252 ± 212

79 ± 0

884 ± 32

13514 ± 192

84 ± 2

1506 ± 25

5028 ± 86

83 ± 1

520 ± 21 699 ± 15

3227 ± 72 6882 ± 96

78 ± 2 81 ± 2

2 ± 0d 4 ± 0d

31 ± 0 63 ± 0

Figure 3. FT-IR spectra of (a) Zn/Al-AS-Mn(TPP)OAc-LDHsbutanol, (b) Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane, (c) Zn/AlAS-Mn(TPP)OAc-LDHs-hexane, (d) Zn/Al-AS-LDHs-restoration, and (e) as-prepared Zn/Al-AS LDHs.

2 3

1049 cm−1 for Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane (Figure 3b), at 1224 and 1052 cm−1 for Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane (Figure 3c), and at 1219 and 1064 cm−1 for Zn/Al-AS-LDHs-restoration (Figure 3d). For the as-prepared Zn/Al-AS-LDHs (Figure 3e), the SO absorption appears at 1185 and 1048 cm−1. The rearrangement of interlayer AS anions from overlapped bilayer to tilted monolayer causes a blue shift of SO absorption (comparing Figure 3d and e). It is clear that, for Zn/Al-AS-Mn(TPP)-LDHs-butanol (Figure 3a), the encapsulation of Mn(TPP)OAc causes the SO absorption at 1185 cm−1 to split to 1206 and 1157 cm−1, indicative of the effects of Mn(TPP)OAc on the interlayer AS anions, which is consistent with the homogeneous distribution of Mn(TPP)OAc in the interlayer region as deduced from EDX characterization. Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane (Figure 3b) presents the same SO absorption as Zn/AlAS-LDHs-restoration (Figure 3d), while Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane (Figure 3c) as as-prepared Zn/Al-AS-LDHs (Figure 3e). The results indicate the Mn(TPP)OAc introduction has no visible effects on the chemical environments of the interlayer AS anions for Zn/Al-AS-Mn(TPP)OAc-LDHsisooctane and Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane, which supports the deduction from EDX characterization that the Mn(TPP)OAc is distributed mainly at the edges of interlayer regions in Zn/Al-AS-Mn(TPP)OAc-LDHs-isooctane or just adsorbed on the surface of LDH layers in Zn/Al-ASMn(TPP)OAc-LDHs-hexane. On the basis of the above characterizations, it can be concluded that the Mn moiety in Zn/Al-AS-Mn(TPP)-LDHsbutanol has been encapsulated between the hydrophobic layers in homogeneous distribution along the bidimensional interlayer region with the highest Mn loading, while mainly distributed at the edges of interlayer regions in Zn/Al-AS-Mn(TPP)OAcLDHs-isooctane and on the hydrophobic layers in Zn/Al-ASMn(TPP)OAc-LDHs-hexane. The Mn moiety in Zn/Al-ASMn(TPP)OAc-LDHs-hexane is more accessible although in lower loading. So the resulting Zn/Al-AS-Mn(TPP)-LDHs-

4 5 6 7 8 9 a

Reaction condition: 1.0 mg (0.09 mmol Mn) of Zn/Al-ASMn(TPP)OAc-LDHs-butanol, 1.5 mg (0.07 mmol Mn) of Zn/AlAS-Mn(TPP)OAc-LDHs-isooctane, or 1.5 mg (0.02 mmol Mn) of Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane, 6 mmol of cyclohexene, 9 mmol of isobutyl-aldehyde, and 1.5 mL of acetonitrile. All data were reproduced at least twice and reported as an average. bDetermined on a Shimadzu GC-2010 with a DB-5 column by using toluene as an internal standard. cControl experiments: reaction time = 24 h. d Conversion of cyclohexene.

much higher than not only homogeneous Mn(TPP)OAc (TOF = 520 h−1, TON = 3227) but also Ru(TPP)CO (TOF = 699 h−1, TON=6882) (Table 1, entries 5 and 6). With the Mn(TPP)OAc absorbed on the outside of Zn/Al-AS-LDHs hydrophobic layers (Zn/Al-AS-Mn(TPP)OAc-LDHs-hexane), a TOF of 4188 h−1 was observed (Table 1, entry 2), similar to what is observed with the Mn(TPP)OAc encapsulated in the hydrophobic interlayer region (Table 1, entry 1). The results demonstrate that the hydrophobic microenvironment is the key for achieving high TOF whether Mn(TPP)OAc is located in the interlayer or adsorbed on the layers. However, without the bidimensional hydrophobic region, which not only aggregates substrates but also stabilizes the transition state,41 a lower TON (26252) was obtained with Mn(TPP)OAc absorbed on hydrophobic LDH layers. With the Mn(TPP)OAc around the hydrophobic interlayer (Zn/Al-AS-Mn(TPP)OAc-LDHsisooctane), inferior TOF (884 h−1) and TON (13514) were observed (Table 1, entry 3), hinting that the catalytic activity is facilitated with the homogeneous distribution of Mn centers. With the Mn(TPP)OAc adsorbed on the hydrophilic LDH 17824



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Table 2. Catalytic Epoxidation of Alkenes with Different Hydrophobic Performancea

a

Reaction condition: 1 mg of catalyst, 6 mmol of alkenes, 9 mmol of isobutylaldehyde, and 1.5 mL of acetonitrile. All data were reproduced at least twice and reported as an average. The conversion and selectivity were determined on a Shimadzu GC-2010 with a DB-5 column by using toluene as an internal standard.

To elucidate the above observations clearly, log Po/w, defined as the log10 of the partitioning coefficient between 1-octanol and water42 and standing for the polarity of substrate, was introduced. Relating log Po/w with TOF (Figure 4) reveals, the less polar the substrate, the higher TOF is obtained, which is not hard to understand because the lower polarity of substrate favors the substrate accumulation into the bidimensional hydrophobic region. But TON depends on the polarities of both substrate and product. So Δlog Po/w, the difference

layers (Mn(TPP)OAc adsorbed on the surface of Zn/Al-CO3LDHs), a TOF of as low as 1506 h−1 and a TON of 5028 were observed (Table 1, entry 4). The Mn center on the hydrophilic surface of Zn/Al-CO3-LDHs is as accessible as that on the hydrophobic surface of Zn/Al-AS-LDHs, while the hydrophilic surface results in a much lower catalytic activity (Table 1, entry 4) than hydrophobic surface (Table 1, entry 2), further indicating the significant role of hydrophobic environment in the improvement of TOF and TON. The control experiments (Table 1, entries 7−9) prove that all the catalysis originates from the Mn sites. To demonstrate the validity of the catalyst designed here in a broad range, we performed the epoxidation of other three alkenes, 3-methyl-3-buten-1-ol, phenylethylene, and heptylene (Table 2). Encouragingly, the catalytic activity of the Mn(TPP)OAc encapsulated between hydrophobic layers is significantly improved for all the three substrates in comparison to homogeneous Mn(TPP)OAc and even Ru(TPP)CO. With 3-methyl-3-buten-1-ol as catalyzed substrate, a TOF of 1504 h−1 and a TON of 24192 are observed. With phenylethylene as catalyzed substrate, a TOF of 4235 h−1 and a TON of 34365 are achieved. With heptylene as catalyzed substrate, the most hydrophobic among the alkenes employed in this work, a TOF of 4040 h−1 and a TON of 18664 are observed. The TOF of heptylene on the Mn(TPP)OAc encapsulated between hydrophobic layers is 14-fold as high as on homogeneous Mn(TPP)OAc and 7.5 fold as high as on homogeneous Ru(TPP)CO.

Figure 4. The log Po/w and Δlog Po/w of heptylene, phenylethylene, cyclohexene, and 3-methyl-3-buten-1-ol, and their TOF and TON on the Mn(TPP)OAc encapsulated in hydrophobic interlayer region of Zn/Al-AS LDHs. 17825



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Table 3. Catalytic Recycle Performance of Mn(TPP)OAc Encapsulated between the Hydrophobic Layers in the Epoxidation of Cyclohexene catalyst

leaching of Mn in solid (%)

leaching of Mn detected in filtrate (%)

conversion in 6 h (%)

selectivity (%)

TOF (h−1)

80 ± 0 79 81

4038 ± 15 3822 4009

80 ± 1 79 80

4204 ± 8 3908 3547

83 ± 0 80 81

875 ± 6 1138 1873

Mn(TPP)OAc between the hydrophobic layers fresh 1st recycle run 2nd recycle run 3rd recycle run

5.1 4.2 9.6

fresh 1st recycle run 2nd recycle run 3rd recycle run

6.5 7.3 11.9

fresh 1st recycle run 2nd recycle run 3rd recycle run

12.6 13.5 14.8

40 ± 1 4.7 37 4 33 9.4 Mn(TPP)OAc on the hydrophobic layers surface (fresh) 22 ± 0 6.4 18 7.3 13 11.8 Mn(TPP)OAc around the hydrophobic layers (fresh) 19 ± 0 12.5 21 13.3 16 14.6

Table 4. Catalytic Epoxidation of Functional Alkenesa

a

Reaction condition: 1 mg of catalyst, 6 mmol of alkenes, 9 mmol of isobutylaldehyde, and 1.5 mL of acetonitrile. All data were reproduced at least twice and reported as an average. The conversion and selectivity for the epoxidation of ethyl cinnamate were determined on a Shimadzu GC-2010 with a DB-5 column by using toluene as an internal standard. The conversion and selectivity for the epoxidation of chalcone were determined by HPLC analysis using a DAICEL Chiralcel OD column, i-PrOH/hexane = 10:90, λ = 254 nm, flow rate = 0.8 mL/min.

between the log Po/w of alkene and corresponding epoxide, was introduced. With cyclohexene as substrate, which has the maximum Δlog Po/w, a TON of 14-fold higher than on homogeneous Mn(TPP)OAc was achieved (Figure 4). The significance of bidimensional hydrophobic region of Zn/Al-ASLDHs in enhancing the catalytic activity of encapsulated Mn centers in this work can be further confirmed by a comparison with the reports in literatures. With Mn (III)-porphyrin intercalated Mg/Al-LDH as the catalyst for the selective oxidation of 2,4,6-trichlorophenol (TCP), the reaction rate was estimated to be only 1-fold higher than that with the homogeneous catalyst (from 0.66 to 1.38 M−1 s−1).43 On the [Fe(TDFSPP)]-Zn2Al-LDH catalyst, with the iron porphyrins intercalated in the interlayer region of Zn/Al-LDHs, a lower TON than on the homogeneous counterpart was even observed in the selective catalytic oxidation of cyclohexene.44 In the epoxidation of a variety of alkenes, each heterogeneous Mn catalyst prepared in this work retains the selectivity similar to its homogeneous counterpart even with much higher TON

(conversion) than homogeneous catalytic system. The heterogeneous catalysts in this work were recovered by simple centrifugation, and then reused in the epoxidation of cyclohexene. Because of the noncovalent attachment of Mn(TPP)OAc, a minor part of Mn(TPP) was detected as being leached into the filtrate from solid catalyst (Table 3). But the leaching in three catalytic runs for the Mn(TPP)OAc encapsulated in the hydrophobic interlayer region is less than those observed for the Mn(TPP)OAc adsorbed on the hydrophobic layers and much less than for the Mn(TPP)OAc around the hydrophobic layers. As a result, the TOF with the Mn(TPP)OAc encapsulated in the hydrophobic interlayer region is retained better than with the Mn(TPP)OAc adsorbed on the hydrophobic layers. The TOF with the Mn(TPP)OAc around the hydrophobic layers is observed to increase with recycle time, probably because the Mn leached into the reaction fluid exhibited better activity than in solid, but which needs further confirmation. 17826



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High Activity and Selectivity Based on Nanoporous Metalloporphyrin Polymers. J. Am. Chem. Soc. 2010, 132, 9138. (9) Merlau, M. L.; Mejia, M. P.; Nguyen, S. T.; Hupp, J. T. Artificial Enzymes Formed through Directed Assembly of Molecular Square Encapsulated Epoxidation Catalysts. Angew. Chem., Int. Ed. 2001, 40, 4239. (10) Engelman, D. M. Membranes Are More Mosaic than Fluid. Nature 2005, 438, 578. (11) Zhou, D.; Cam, L. L.; Laughton, C. A. Mutagenesis Study at a Postulated Hydrophobic Region Near the Active Site of Aromatase Cytochrome P45. J. Biol. Chem. 1994, 269, 19501. (12) Zehmer, J. K.; Bartz, R.; Liu, P. Identification of a Novel Nterminal Hydrophobic Sequence that Targets Proteins to Lipid Droplets. J. Cell Sci. 2008, 121, 1582. (13) Singh, A. K.; Singh, N.; Tiwari, A. First Structural Evidence for the Mode of Diffusion of Aromatic Ligands and Ligand-induced Closure of the Hydrophobic Channel in Heme Peroxidases. J. Biol. Inorg. Chem. 2010, 15, 1099. (14) Dong, Z. Y.; Huang, X.; Mao, S. Z. Cyclodextrin-Derived Mimic of Glutathione Peroxidase Exhibiting Enzymatic Specificity and High Catalytic Efficiency. Chem.Eur. J. 2006, 12, 3575. (15) Chan, W. K.; Yu, W. Y.; Che, C. M. A Cyclodextrin-Modified Ketoester for Stereoselective Epoxidation of Alkenes. J. Org. Chem. 2003, 68, 6576. (16) Marinescu, L. G.; Bols, M. Very High Rate Enhancement of Benzyl Alcohol Oxidation by an Artificial Enzyme. Angew. Chem., Int. Ed. 2006, 45, 4590. (17) Astruc, D.; Boisselier, E.; Ornelas, C. Dendrimers Designed for Functions: From Physical, Photophysical, and Supramolecular Properties to Applications in Sensing, Catalysis, Molecular Electronics, Photonics, and Nanomedicine. Chem. Rev. 2010, 110, 1857. (18) Hayashi, Y.; Aratake, S.; Okano, T.; Takahashi, J.; Sumiya, T.; Shoji, M. Combined Proline-Surfactant Organocatalyst for the Highly Diastereo- and Enantioselective Aqueous Direct Cross-Aldol Reaction of Aldehydes. Angew. Chem., Int. Ed. 2006, 45, 5527. (19) Mase, N.; Nakai, Y.; Ohara, N.; Yoda, H.; Takabe, cK.; Tanaka, F.; Barbas, C. F., III Organocatalytic Direct Asymmetric Aldol Reactions in Water. J. Am. Chem. Soc. 2006, 128, 734. (20) Zhao, M.; Helms, B.; Slonkina, E.; Friedle, S.; Lee, D.; DuBois, J.; Hedman, B.; Hodgson, K. O.; Fréchet, J. M. J.; Lippard, S. J. Iron Complexes of Dendrimer- Appended Carboxylates for Activating Dioxygen and Oxidizing Hydrocarbons. J. Am. Chem. Soc. 2008, 130, 4352. (21) Yoshizawa, M.; Klosterman, J. K.; Fujita, M. Functional Molecular Flasks: New Properties and Reactions within Discrete, Self-Assembled Hosts. Angew. Chem., Int. Ed. 2009, 48, 3418. (22) Pluth, M. D.; Bergman, R. G.; Raymond, K. N. Proton-Mediated Chemistry and Catalysis in a Self-Assembled Supramolecular Host. Acc. Chem. Res. 2009, 42, 1650. (23) Hastings, C. J.; Pluth, M. D.; Bergman, R. G.; Raymond, K. N. Enzymelike Catalysis of the Nazarov Cyclization by Supramolecular Encapsulation. J. Am. Chem. Soc. 2010, 132, 6938. (24) Hu, S.; Li, J.; Xiang, J.; Pan, J.; Luo, S.; Cheng, J. Asymmetric Supramolecular Primary Amine Catalysis in Aqueous Buffer: Connections of Selective Recognition and Asymmetric Catalysis. J. Am. Chem. Soc. 2010, 132, 7216. (25) Berg, O. G.; Gelb, M. H.; Tsai, M. D.; Jain, M. K. Interfacial Enzymology: The Secreted Phospholipase A2-Paradigm. Chem. Rev. 2001, 101, 2613. (26) Weng, K. C.; Stålgren, J. J. R.; Duval, D. J.; Risbud, S. H.; Frank, C. W. Fluid Biomembranes Supported on Nanoporous Aerogel/ Xerogel Substrates. Langmuir 2004, 20, 7232. (27) Atanasov, V.; Atanasova, P. P.; Vockenroth, I. K.; Knorr, N.; Köper, I. A Molecular Toolkit for Highly Insulating Tethered Bilayer Lipid Membranes on Various Substrates. Bioconjugate Chem. 2006, 17, 631. (28) Groves, J. T.; Neumann, R. Regioselective Oxidation Catalysis in Synthetic Phospholipid Vesicles. Membrane-Spanning Steroidal Metalloporphyrins. J. Am. Chem. Soc. 1989, 111, 2900.

More interestingly, even when the Mn(TPP)OAc encapsulated in the hydrophobic interlayer region was used as catalyst in the epoxidation of functional alkenes, visibly improved activity was also observed while the selectivity of epoxide was well retained (Table 4). In the epoxidation of ethyl cinnamate, a TOF of 2778 h−1 and a TON of 28672 were achieved, much higher than not only homogeneous Mn(TPP)OAc but also Ru(TPP)CO. Similar enhancement of TOF and TON was also observed in the epoxidation of chalcone with the Mn(TPP)OAc encapsulated in the hydrophobic interlayer region in comparison to with homogeneous Mn(TPP)OAc or even Ru(TPP)CO.

4. CONCLUSIONS In conclusion, a heterogeneous Mn catalyst has been prepared through encapsulating Mn(TPP)OAc in the biomimetic flexible bidimensional hydrophobic region of layered double hydroxides, an anionic clay. Superior catalytic activity and comparable selectivity to homogeneous counterpart have been achieved in the epoxidation of a variety of alkenes. It provides a potential approach for highly efficient heterogeneous catalysts.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-64425280. Fax: +86-10-64425385. E-mail: [email protected] or [email protected]. Postal address: Box 98, 15 Beisanhuan Dong Lu, Beijing 100029, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank NSFC, PCSIRT (IRT1205), and 973 Project (2011CBA00504) for financial support. J. H. particularly appreciates the financial aid of China National Funds for Distinguished Young Scientists from the NSFC.

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REFERENCES

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Enhanced Catalytic Efficiency in the Epoxidation of Alkenes for

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