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Confinement of Mn3+ redox sites in Mn-KIT-6 and its catalytic activity for styrene epoxidation
Accepted Manuscript 3+
Confinement of Mn epoxidation
redox sites in Mn-KIT-6 and its catalytic activity for styrene
Nilamadanthai Anbazhagan, Gaffar Imran, Ahsanulhaq Qurashi, Arumugam Pandurangan, Shanmugam Manimaran PII:
S1387-1811(17)30226-3
DOI:
10.1016/j.micromeso.2017.03.049
Reference:
MICMAT 8237
To appear in:
Microporous and Mesoporous Materials
Received Date: 28 December 2016 Revised Date:
8 March 2017
Accepted Date: 27 March 2017
Please cite this article as: N. Anbazhagan, G. Imran, A. Qurashi, A. Pandurangan, S. Manimaran, 3+ Confinement of Mn redox sites in Mn-KIT-6 and its catalytic activity for styrene epoxidation, Microporous and Mesoporous Materials (2017), doi: 10.1016/j.micromeso.2017.03.049. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Confinement of Mn3+ Redox Sites in Mn-KIT-6 and
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its Catalytic Activity for Styrene Epoxidation Nilamadanthai Anbazhagan a, Gaffar Imran a, Ahsanulhaq Qurashi b, Arumugam Pandurangan *, Shanmugam Manimaran a
a
Department of Chemistry, Anna University, Chennai – 600025, India
b
Department of Chemistry and Centre of Excellence in Nanotechnology, King Fahd University
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a
Of Petroleum and Minerals, Dhahran- 31261, Saudi Arabia
*Corresponding author: E-mail: [email protected], Fax: +9144 22200660; Tel: +9144
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Abstract In the present study, a series of manganese (Mn) containing 3D cubic mesoporous KIT-6 materials with different Si/Mn ratio (100, 50 25 & 10) having Ia3d space group were synthesized
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for the first time by a facile one-pot hydrothermal process. The synthesized materials were methodically characterized by various analytical techniques including XRD, N2 sorption, HRTEM, diffuse reflectance UV-vis (DRS-UV-vis), EPR and FT-IR. The well ordered mesoporous
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nature of Mn incorporated KIT-6 materials are realized by XRD, N2 adsorption-desorption isotherm and HR-TEM analysis. Surface area, pore volume and pore size of Mn-KIT-6 with
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different Si/Mn ratio are in the range of 451-786 m2/g, 0.57-0.9 cm3/g and 4.5–4.9 nm respectively. The incorporation of Mn3+ in the framework and fine dispersion of Mn2+ species over silica matrix of Mn-KIT-6 are confirmed by the results of UV-DRS, FT-IR and EPR. Further, the presence of higher amount of Mn2+ extra-framework species in Mn-KIT-6(10) is
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realized by the presence of high amplitude signal at 2.009 (g value) and the contraction in H1 hysteresis loop as shown in EPR and BET isotherm respectively. Catalytic behavior of Mn-KIT6 material was evaluated for the epoxidation of styrene with various oxidants in which t-butyl
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hydro peroxide (TBHP) promotes the desired formation of styrene oxide under mild liquid phase conditions. Mn-KIT-6 shown to be highly stable and active which mainly depends on the
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framework substituted Mn3+ sites.
Keywords: Mn3+, isomorphous substitution, KIT-6, epoxidation
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1. Introduction Since the early 1990’s both academics and industries initiated research on mesoporous silicates (M41S, SBA-n) as a support for catalysis [1] and various other applications. Among
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these, mesoporous materials with three dimensional porous structure have attracted a significant attention owing to their excellent textural characteristics such as inter-linkage pore structure with narrow pore size distribution, high specific surface area, pore volume and unique 3D channel
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network which provide a highly opened porous host with easy and direct access for guest species, thus facilitating inclusion or diffusion throughout the pore channels without pore
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blockage [2]. Mesoporous silica, KIT-6 with Ia3d-type [3] are of particular interest over other materials like MCM-41 and MCM-48 because of its three dimensional structure with a large mesopores of diameter and easy synthesis method. More importantly it has high thermal and hydrothermal stability. Further, it possesses high surface area, large pore volume and tunable
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pore size [4].
Recently, research is being focused on designing constructing effective redox / acidic type catalysts by incorporation of various transition metals. Metals such as Sn, W, Zr, Nb, Al, Ce, Fe,
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Ti and Mo [5, 6] on 3D mesoporous silica, KIT-6 as support have been studied as catalysts for various catalytic conversions to useful products in greener way. It was found that manganese
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containing homogeneous catalyst shows high catalytic activity towards epoxidation of alkenes [7]. However, major drawback of using homogeneous Mn based material is catalyst recovery, which arises due to complexation of catalysts with substrate molecules used for catalytic reactions. Mn-incorporated heterogeneous type catalysts have received much attention as oxidation type catalyst. For example, highly crystalline manganese incorporated mesoporous materials such as Mn-MCM-41 and Mn-MCM-48 and microporous Mn-S-1 and Mn-SAPO-34
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are widely investigated in which predominantly exposed Mn3+ sites selectively act as a catalyst for catalyzing the oxidation of alkyl aromatics and hydrocarbon conversion respectively [8, 9]. Further, these Mn3+ sites in Mn-SBA-12 & Mn-SBA-16 and Mn-SBA-15 catalysts were also
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found to play a key role for selective formation of amides [10] and trans-stilbene oxide [11] respectively. Recently, a facile synthesis of Mn coupled with an active mesoporous material namely FDU-5 has been reported for trans-stilbene epoxidation [12]. In addition, manganese
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incorporated TUD-1 has been found to be active for various reacting molecules such as styrene, trans-stilbene [13] ethylbenzene [14] and cyclohexane [15]. Mn-incorporated SBA-1 is effective
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for oxidation of ethylbenzene [16]. All these results clearly indicate the promising role of manganese in various mesoporous silicate materials for various catalytic reactions. On the other hand, the method of incorporation of manganese into silicate materials plays an important role for the confinement of manganese species inside the silica matrix as support.
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Manganese incorporated silicate prepared by sol-gel and EISA (Evaporation Induced Self Assembly) techniques resulted in the formation of high amount of manganese species as large aggregations of metal oxides on the surface [12, 14]. Furthermore, it was reported that 30% of
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confinement of manganese sites [16] were observed for n-MnSBA-1 material synthesized at 0oC by cooperative assembly method. The isolated tetrahedral Mn3+ sites in the resulting material
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shows promising efficiency for the formation of selective product in comparison with extraframework Mn2+ sites [10, 11, 14, 16]. Though manganese incorporated silica materials exhibit promising catalytic activity for various reactions, the challenging aspect is to synthesize Mn incorporated materials with high manganese content without agglomeration by a cost-effective method. Herein, a series of manganese (Mn) containing 3D cubic mesoporous KIT-6 materials with different Si/Mn ratio (100, 50 25 & 10) have been synthesized for the first time by a facile
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one-pot hydrothermal process. The characterizations results show that manganese ions are uniformly incorporated in the framework of silica. Further, well ordered mesoporous nature of Mn incorporated KIT-6 materials are realized by XRD, N2 adsorption-desorption isotherm, UV-
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DRS, EPR and HR-TEM analysis. The catalytic efficiency of resulting Mn-KIT-6 has been evaluated for styrene epoxidation using TBHP as oxidant. Further, the cyclability and chemical
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stability of Mn-KIT-6 have been carried out and the results are discussed.
2. Experimental
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2.1. Materials required
Manganese nitrate (Aldrich) and Tetraethyl orthosilicate (TEOS, Aldrich) were used as the source for manganese and silica respectively. Non-ionic surfactant Tri-block co-polymer (PluronicP123, EO20–PO70–EO20, with an average molecular weight 5800, Aldrich) and co-
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solvent such as n-butanol (Aldrich) were used without any further purification. 2.2. Synthesis of Mn-KIT-6
Three-dimensional cubic mesoporous Mn-KIT-6 with different nSi/nMn ratio ranging from 100 -
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10 were synthesized by following the procedure as reported [17, 18] previously. Pluronic P123 (5 g) dissolved in water was added to 0.5 M HCl (190 g) solution in a polypropylene bottle and
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stirred at 308 K. After complete dissolution, 5 g of n-butanol is added at once and stirred for another 1 h. Then, 10.6g of TEOS and required amount of manganese nitrate was added at once to the homogeneous clear solution. This mixture is left under vigorous and constant stirring at 308 K for 24h. The mixture is aged at 373 K for 24 h under static conditions. The white precipitated product is filtered hot without washing and dried at 373 K for 12 h in air and the mixture was stirred for 24 h at 308 K. The mixture was then transferred to a Teflon-lined
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autoclave and was aged at 100 ◦C for 24 h under static hydrothermal conditions. The suspension was filtered off and the collected precipitate was dried at 100 ◦C for 12 h in air. The dried sample was then powdered and calcined at 823 K for 5 h in air atmosphere to remove the template. In
different concentration of TEOS and manganese nitrates were used. 2.3. Characterization of Mn-KIT-6
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order to prepare mesoporous KIT-6 materials with different Si/Mn ratio (100, 50 25 & 10),
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Powder XRD (low and high angle) patterns were recorded using a Phillips Xpert X-ray diffractometer with Cu-Kα radiation (ࣅ= 0.1548 nm) in the 2ᅎ range of 0.5–5o, and 10- 80o. The
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manganese content in Mn-KIT-6 was recorded using ICP-OES Optima 5300 DV (Perkin Elmer) instrument. The specific surface area, total pore volume and average pore diameter were measured by N2 sorption studies which were conducted using Micromeritics ASAP 2020 porosimeter instrument. Pore size distribution was obtained by applying the BJH analysis to the
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adsorption curve of the nitrogen adsorption–desorption isotherms. The morphology of Mn-KIT-6 was observed using scanning electron microscope (SEM, Hitachi-S3400N instrument). Transmission electron microscopy (HR-TEM) was carried out with HRTEM JEOL 3010
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operating at an accelerating voltage of 300 KV. The FT-IR spectra of Mn-KIT-6 diluted with KBr were recorded using a Bruker Spectrometer Tensor with a resolution of 4 cm-1. Diffuse
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reflectance UV-Vis spectra of Mn-KIT-6 samples were collected using a Shimadzu 2100 spectrophotometer equipped with an integrating sphere assembly in the range of 200-800 nm using BaSO4 as reference. Electron spin resonance (ESR) spectroscopy studies were carried out by loading samples in a Suprasil quartz EPR tube and the spectra were recorded using a Bruker X-band CW (EMX 102.7) spectrometer with variable temperature capability.
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2.4. Epoxidation of styrene Epoxidation of styrene over Mn-KIT-6 catalysts was carried out under liquid phase conditions. In a typical run, styrene (1 mmol, SRL), t- butyl hydrogen peroxide (TBHP, 70% in H2O,
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Spectochem), acetonitrile (15g, Merck) and chlorobenzene (0.03ml, internal standard, Merck) were taken in a 50 mL RB flask. Then 0.2g of pre-activated Mn-KIT-6 catalyst was added to the reaction mixture and the resulting suspension was stirred and refluxed for 24h at static
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temperature of 333K. After completion of each reaction, samples were collected systematically after centrifugation. Then 15µl of the filtrate (liquid samples) was diluted with 50 µl of
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acetonitrile and later was analyzed by gas chromatograph (GC) (Model: Shimadzu) having a capillary column RTX-5 (30m x 0.25 mm x 0.25 mm) and flame ionisation detector (FID). The product was also analysed by gas chromatograph equipped with mass spectrophotometer (GCMS) (Model: Agilent 6890NMS- JEOL).
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3. Results and Discussion:
Manganese acetate was initially used for the synthesis of Mn-SBA-1 [16] with slight modification of pH. The reaction proceeded as follows in equation (1).
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Mn (CH3COO) 2 + H+
Mn2+ + CH3COOH
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However, acetic acid being a weak acid does not result in the formation of a low pH
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and as a result hinders the formation of mesoporous KIT-6 material by lowering condensation of silica which results in a loss of catalyst. Further, manganese nitrate is a better choice as the source of manganese [10] on account of the formation of HNO3 which is a strong acid and lowers the overall pH which leads to the formation of ordered mesophase structure as depicted in below scheme.
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3.1. Characterization:
The structural ordering of the Si-KIT-6 and Mn-KIT-6 materials was investigated by
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powder x-ray diffractometer and the pattern is shown in Fig.1. The XRD pattern of Si- KIT-6 (Fig.1a) gives rise to well ordered XRD reflections which can be assigned to (211) (220) (332) and (420) diffractions of the cubic Ia3d symmetry [2], which are characteristic for highly ordered mesoporous structures. Mn-incorporated KIT-6 with different concentration of Mn content
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(Fig.1b-1e) also exhibit similar XRD pattern with well resolved peak in the low 2θ region at 2θ= 0.84o-0.910 with slight decrease in higher order peaks, revealing that silica possesses well ordered structure even after Mn incorporation in to the frame work of silica and the structural
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ordering of the mesoporous material, KIT-6 is replicated in Mn-incorporated KIT-6 materials. Though XRD pattern for (211) plane was retained for all Mn-incorporated KIT-6 materials, the
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peak corresponding to (220), (332) and (420) plane decreases upon increase in metal content upto Si/Mn ratio of 25 and the pattern disappears when the concentration of Mn-ion further increases. This may be due to either partial structure collapse or the formation of metal oxide species inside the mesopores upon calcination. These results are consistent with the results obtained for various metal doped KIT-6 materials reported previously [6, 19]. The unit cell parameter (ao) and d211 values of Mn-KIT-6 are presented in Table 1. It has been reported that the
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unit cell parameter increases with increasing metal content [20, 21]. In the present study, we observe a slight increase in the unit cell value and lattice constant for Mn-incorporated KIT-6 in comparison with Si-KIT-6, demonstrating that the incorporation of manganese ions into frame-
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work of silica leads to increase in lattice constant and unit cell values. The nSi/nMn molar ratios of all the samples under investigation are summarised in Table 1. It can be seen from the Table 1 that Si, Mn in the gel incorporated into the solid is almost in close agreement with the input gel
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composition. In the case of Mn-KIT-6 (100) and Mn-KIT-6 (50), the Mn content in the solid phase is higher than the input gel, suggesting preferential incorporation of Mn ions in to the
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framework of silica. Similar observations were previously reported in the case of zeolites [22]. High angle XRD patterns of Mn-KIT-6 samples are shown in Figure 2 (a-d). The reflections indexed to amorphous silica were obtained at 2θ values of 23o for all Mn-KIT-6 samples. However no reflections were found corresponding to any MnOx (Mn2O3, MnO and Mn3O4)
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species, indicating that Mn-ion is incorporated into silica without forming any impurities. These results concluded that Mn-ions have been successfully incorporated into the silica framework without forming any impurities and retains the ordered mesoporous structure similar to pure
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siliceous KIT-6 material, which is consistent with the results obtained for manganese incorporated SBA-1[16].
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The nitrogen adsorption–desorption isotherms and pore size distribution of Si-KIT-6 and
Mn-KIT-6 with different Si/Mn ratios are shown in Fig. 3 and Fig. 4. Nitrogen adsorptiondesorption isotherm of Si-KIT-6 and Mn-KIT-6 showed Type IV isotherm with capillary condensation between 0.45-0.6 P/P0 of nitrogen, which is characteristic of mesoporous material. The H1-type hysteresis loop of Si-KIT-6 is typical of uniform mesoporous solids with narrow pore size distribution. However, the slight changes in H1 hysteresis loop is observed for Mn
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incorporated Si-KIT-6 materials, indicating the possible formation of amorphous MnOx clusters in the mesoporous channels of Mn-KIT-6. The incorporation of Mn into the walls of Si-KIT-6 has a significant effect on the specific surface area, specific pore volume and pore diameter of
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the materials (Figure 4 and Table 1). The pore volume is reduced from 0.96 to 0.57 cm3/g and the specific surface area declines from 845 to 451m2/g for Mn- incorporated Si-KIT-6 materials with Si/Mn ratio of 100 to 10, indicating slight shrinkage in the mesoporous nature of Si-KIT-6
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with increasing metal content. It is observed that the pore diameter of Si-KIT-6 decreases with increasing Mn-ion content from 6.6 nm to 4.5 nm. The pore diameter of Mn-KIT-6 (10) and Mn-
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Si-KIT-6 (25) is smaller than of Mn-KIT-6 (50) and Mn-KIT-6 (100) and much smaller than SiKIT-6, which could be attributed to the formation of more metal oxide clusters in the mesopores of Mn-KIT-6 (10) and Mn-KIT-6 (25). The morphology and the presence of elements in Mnincorporated Si-KIT-6 (50) were analyzed by TEM and SEM, and SEM-EDAX analysis (Fig. 5).
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The existence of spherical nature of silica particles and the presence of Mn in addition to Si and O have been observed. Further, the well interconnected ordered mesoporous structure of MnKIT-6 (50) is evident from HR-TEM images, which is consistent with the results obtained from
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XRD analysis.
The results of FT-IR spectra for Si-KIT-6 and Mn-KIT-6 materials with different Si/Mn ratio are
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shown in Fig. 6 and it is found that the results of Mn-KIT-6 in the present study is in good agreement with the results obtained for Mn incorporated M41S materials [23, 24]. FT-IR spectra of Mn-KIT-6 materials display bands at 1650, 1225, 1084, 964 & 803 cm-1. The band at 1650 cm-1 corresponds to the presence of bending vibration of O-H band which is due to the presence of moisture on Si-KIT-6 and Mn-KIT-6 materials. The bands at 1225 & 1084 cm-1 and 803 cm-1 are assigned to υas of Si-O-Si band and υs of Si-O-Si structural siloxane group respectively.
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Whereas the band at 460 cm-1 is due to the presence of υb of Si-O-Si and Si-O-Mn band in mesoporous silica framework [25]. Further, it is interesting to note that the band at 964 cm-1 is assigned to stretching vibration band of Si-O-Mn linkage in Mn-Si-KIT-6, which increases with
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increase in the concentration of Mn content, confirming the incorporation of Mn-ion into the silica matrix of KIT-6. These results are consistent with the results obtained from XRD analysis. The co-ordination geometry of Mn incorporated Si-KIT-6 is analysed by the UV-Vis absorption
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spectra and the absorption spectra of calcined Mn incorporated Si-KIT-6 with different Si/Mn ratio (10, 25, 50 and 100) is shown in Fig. 7. It shows absorption bands at 220, 260 nm and two
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broad bands at 310-350 nm and 500 nm. A distinct individual absorption bands around 220 and 260 nm observed for all Mn-KIT-6 (100, 50, 25 &10) samples. These bands can be assigned to O2-→Mn3+, a charge transfer transition of trivalent manganese ions in tetrahedral coordination geometry [10, 13-16]. This band could be assigned to the strong bonding of oxygen ligands to
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Mn3+ ions. The intensity of this band increases monotonically with increasing Mn content. Absorption band at 310-350 nm is also assigned to the electron transfer of O2-→ Mn3+ as reported previously for the UV-vis. spectrum of Mn3O4 [26, 27 - 29]. The broad absorption band
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which appears at 500 nm is due to 6A1g →4T2g transition of the octahedral coordinated divalent manganese species [17, 25] confirming the presence Mn in extra framework species. A similar
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band was also reported in Mn incorporated SBA-15[30]. All these results revealed that the majority of the manganese ions in the Mn-Si-KIT-6 samples occupy framework position in the surface layer of the Si-KIT-6 channel walls. In order to study the coordination of high spin Mn ions in Si-KIT-6, electron paramagnetic resonance (EPR) analysis was carried out at room temperature for calcined Mn-KIT-6 samples with different Si/Mn ratio. Mn-KIT-6 (100, 50 & 25) samples exhibited low resolved hyperfine
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structure (shown in Fig. 8) centered at g value of 2.009 with hyperfine coupling constant A= 93 G which is attributed to Mn2+ in octahedral geometry [24]. It is noted that EPR signals of Mn2+ in Mn-KIT-6(10) is different to the other ratio of Mn-Si-KIT-6 materials indicating decrease in
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Mn2+-Mn2+ band distance in Mn-KIT-6. These results demonstrate the presence of extraframework Mn2+ with increase of manganese content and is in well consistence with BET results, i.e., the contraction of hysteresis loop as observed in nitrogen adsorption-desorption isotherm
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with increase in manganese content is due to the presence of Mn2+-ions in extra framework. 3.2 Catalytic activity:
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In order to evaluate the catalytic activity of Mn-KIT-6 materials, liquid phase epoxidation of styrene and TBHP with ratio 1:1 was carried out at 60oC for 24h. The selective products obtained during the epoxidation reaction are styrene oxide, phenyl acetaldehyde (PA), benzaldehyde (BZ) and benzoic acid (BZOOH). The effect of various parameters such as
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different catalysts (Si/Mn ratio), oxidant (TBHP): Styrene and temperature were studied systematically and will be discussed in the following section. 3.2.1. Effect of Si/Mn ratio varying 100-10 content in Mn-KIT-6 samples with various
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Oxidants
Epoxidation of styrene was carried out using various oxidants such as air, H2O2 and
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TBHP. No characteristic change was seen when air and H2O2 were used as oxidants. However, TBHP proved to be a good oxidant due to the generation of active oxygen species, an effect which was earlier observed by Selvaraj et al. [15, 31]. Hence TBHP was chosen as the oxidant for all Si/Mn ratios of KIT-6 catalysts (Table 2). In contrast, it is noted that pristine Si-KIT-6 mesoporous silicates showed very less catalytic conversion inferring that isomorphous substituted manganese plays a major role in the catalytic conversion of styrene. With increase in
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manganese content, conversion of styrene was increased from 73% of Mn-KIT-6 (100) to 89% of Mn-KIT-6 (25) with an increased selectivity of styrene oxide from 24% to 47% as presented in Table 2. However, considerably lower conversion results were obtained for Mn-KIT-6(10)
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owing to its lower surface area, contracted hysteresis with lower pore volume and also due to presence of higher range of extra-framework species (Mn2+) as evidenced out from DRS-UV-Vis
25 > 50 > 100 > 10 of Mn-KIT-6 catalysts. 3.2.2. Mechanism of styrene epoxidation:
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and EPR spectra. The order of the activity of the catalysts with different Si/Mn ratios followed as
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Styrene oxide formation could be possibly explained by the formation of metalhydroperoxide intermediates. Frame-work substituted Mn3+ ions are the primary reason for the activation of TBHP which chemisorbs on the active Mn sites of KIT-6 and forms the metalhydroperoxide as intermediate. Mimoun et al. also suggested similarly that the vanadium peroxo
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radical is the active species for transferring oxygen to reactants [32]. Subsequently, in metalhydroperoxide intermediate complex the O-O bond gets polarized and gets transferred to the nucleophilic C-C (double bond) finally forming styrene oxide as product.
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3.2.3. Effect of TBHP: Styrene
Styrene was studied by varying the molar ratio of TBHP from 0.5 to 2 (Table 3) over Mn-
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KIT-6 (100). Styrene conversion increases from 54% to 73% with increase in the TBHP concentration from 0.5 to 1. The selectivity of styrene oxide also increased simultaneously from 19% to 24%. Decrease in conversion and selectivity was obtained when ratio was doubled which could be due to the possible blocking of active site and excess of H2O present in 70% of TBHP with Mn-KIT-6 catalysts. Similar results were obtained when Ce-MCM-48 and MnOx grafted on SBA-15 was used as catalysts for styrene conversion [33 - 37]. 1:1 molar ratio of both
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TBHP and styrene gave the exact production of reactive species for the formation of the desired product. 3.2.4. Effect of Temperature
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The effect of reaction temperature with Mn-KIT-6(100) is presented in Table 4. With increasing temperature from 40-70o C, the conversion of styrene drastically increases from 32% to 84% indicating temperature plays a key role in the conversion of styrene epoxidation. On the other
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hand, the drastic decrease in selectivity (16%) of styrene oxide was observed when reaction was carried out at about 70o C. Though reaction at high temperature shows good styrene conversion,
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selectivity of styrene oxide decreases due to the formation of various by-products in the reactions. Thus the temperature of 60oC has been optimized to carry out epoxidation of styrene with Mn-KIT-6 catalysts with various Si/Mn ratios. 3.2.5. Reusability and Hot-Filtration
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To study the stability of Mn-KIT-6 materials during catalytic reaction, the used catalyst in the reaction was recovered by filtration; washed and followed by activation at 300o C. This catalyst was used for styrene conversion again for five cycles and the results are shown in figure 9. The
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results displayed clearly showed that there was no significant loss in the conversion concluding that Mn-KIT-6 proved to be active and highly recyclable catalyst for the epoxidation reactions.
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To account the issue of leaching and further verify the heterogeneous nature, Mn-KIT-6 (100) samples were also subjected to hot filtration study (result shown in Fig. 10). After 1h of reaction, the mixture was hot-filtered and catalyst was removed. Without any solid materials (catalyst) hot-filtered reaction mixture was further continued for stirring at 60o C for 24h, and later the same was analysed by atomic absorption spectroscopy (AAS) in order to understand the Mn leaching from the catalyst. The results show negligible concentration of manganese ions in the
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solution and this confirms the truly heterogeneous behavior of Mn-KIT-6 catalysts synthesized in the present study. 4. Conclusions
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3D mesoporous materials with Mn incorporation into the framework of KIT-6 has been successfully realized by a direct one-step hydrothermal process using pluronic P123 and nbutanol as the structural directing agent and co-solvent respectively. Manganese nitrate was
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considered to be the best choice for the preparation of well ordered Mn-KIT-6. The presence of ordered mesoporous structure of Mn substituted KIT-6 materials has been confirmed by XRD,
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HR-TEM and N2 adsorption-desorption analysis. DRS UV-Vis spectra reveal that Mn3+ species substituted uniformly in the framework of silica for all Mn substituted KIT-6 with different Si/Mn ratios. Mn2+ species predominate in the case of Si/Mn = 10 ratio which was confirmed by DRS-UV-vis and EPR technique. Catalytic activity of Mn-KIT-6 for styrene epoxidation using
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TBHP as oxidant shows that conversion of styrene increases from 73% to 89% with increase in the concentration of Mn in the framework of silica. An increased selectivity of styrene oxide from 24% to 47% was observed for Mn-KIT-6 (25). The order of catalytic activity of Mn-KIT-6
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catalysts with different Si/Mn ratios are 25 > 50 > 100 > 10. On the other hand, pristine Si-KIT-6 mesoporous silicates showed very less catalytic conversion for oxidation of styrene. These
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results demonstrate that isomorphously substituted manganese in framework plays a crucial role for the improved catalytic reaction for the conversion of styrene. Further, cyclability and hotfiltration studies reveal that Mn-KIT-6 is highly stable and recyclable catalyst for many cycles for the epoxidation reactions.
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Acknowledgement Authors acknowledge the financial support from DST (FIST) and UGC (SAP) to Department of Chemistry, CEG, Anna University for providing necessary facilities. Authors thanks IIT-SAIF,
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CLRI and PSG Institute of Advanced studies for providing low angle- XRD, EPR and HR-TEM analysis respectively.
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[10] A. Kumar, D. Nepak, D. Srinivas, Catal. Commun. 37 (2013) 36– 40. [11] M. Selvaraj, T. G. Lee, J. Phys. Chem. B, 110 (2006) 21793-21802. [12] G. Imran, V. Srinivasan, R. Maheswari, R. Anand, S. Bala, J. Porous Mater. 23 (2016) 57– 65.
[13] A. Ramanathan, T. Archipov, R. Maheswari, U. Hanefeld, E. Roduner, R. Glaser, J. Phys. Chem. C, 112 (2008) 7468-7476. [14] G. Imran, M. P. Pachamuthu, R. Maheswari, A. Ramanathan, S. S. Basha, J. Porous Mater. 19 (2012) 677.
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[15] R. Maheswari, R. Anand, G. Imran, J. Porous Mater. 19 (2012) 283. [16] G. Imran, R.Maheswari, Mater. Chem. Phy. 161 (2015) 237-242. [17] R. Anand, R. Maheswari, D. H. Barich, S.Bala, Micropor. Mesopor. Mater.190 (2014) 240– 247.
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[18] Q. Pan, R. Anand, W. Kirk Snavely, R.V.Chaudhari, S. Bala , Ind. Eng. Chem. Res. 52 (2013) 15481 −15487.
[19] R. Anand, S. Bala, R. Maheswari, Micropor. Mesopor. Mater.167 (2013) 207–212. [20] E.G. Derouane, M. Mestdagh and L.J. Vielvoye, J. Cat. 33 (1974) 169.
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[21] C.F. Cheng and J. Klinowski, J. Chem. Soc., Faraday Trans. 92 (1996) 289.
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Mater. 78 (2005) 139-149.
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[23] M. Selvaraj, P. K. Sinha, K. Lee, I. Ahn, A. Pandurangan, T. G. Lee, Micropor. Mesopor.
[24] S. Vetrivel, A. Pandurangan, J. Mol. Catal. A: Chem. 246 (2006) 223-230. [25] R. Merkache, I. Fechete, M. Maamache, M. Bernard, P. Turek, K. Al-Dalama, F. Garin, Appl. Catal. A: Gen. 504 (2015) 672–681.
[26] S. Velu , N. Shah , T.M. Jyothi, S. Sivasanker, Micropor. Mesopor. Mater. 33 (1999) 61–75.
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[27] Q. Zhenping, B. Yibin, Y. Qin, Y. Wang, F. Qiang, Chem. Eng. J. 209 (2012) 163–169. [28] T. Tsoncheva, J. Rosenholm, M. Linden, F. Kleitz, M. Tiemann, L. Ivanova, M. Dimitrov, D. Paneva, I. Mitov, C. Minchev, Micropor. Mesopor. Mater. 112 (2008) 327–337. [29] N. Stamati, K. Goundani, J. Vakros, K. Bourikas, Ch. Kordulis, Appl. Catal. A: Gen. 325
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(2007) 322–327.
[30] G. Satish Kumar, M. Palanichamy, M. Hartmann, V. Murugesan, Catal. Commun. 8 (2007)
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493–497.
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[33] R. Anand, R. Maheswari, Prem. S. Thapa, S. Bala, ACS Symp. Series; Am.Chem. Soc. 1132 Chap. 8, 213-228. [34] Q. Tang, H. Shuangquan , Y. Chen, Z. Guo, Y. Hu, Y. Yang, Micropor. Mesopor. Mater. 132 (2010) 501–509.
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[35] H. Shuangquan, D. Liu, C. Wang, Y. Chen, Z. Guo, A. Borgna, Y. Yang, App. Catal. A: Gen. 386 (2010) 74–82. [36] Z. Guo, C. Zhou, H. huangquan, Y. Chen, X. Jia, R. Lau, Y. Yang, , Appl. Catal. A: Gen. 419–420 (2012) 194–202.
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[37] Q.H. Zhang, Y. Wang, S. Itsuki, T.Shishido, K.Takehira, J. Mol. Catal. A: Chem. 188 (2002)
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189 – 200.
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Scheme1: Epoxidation of styrene with TBHP oxidant over Mn-KIT-6 catalysts
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Scheme2: Plausible mechanisitic scheme for epoxidation of styrene over Mn-KIT-6
O OH
-t
O Mn3+
TBHP
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H
O
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tBu
(Mn-KIT-6)
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O
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Mn3+
O
Mn
Bu OH
O
Mn
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Table 1: Physical parameters of various Si/Mn ratio of KIT-6
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b
d211 (nm)
ao (nm)
(cm3/g)
Dp, BJH f (nm)
Wg (nm)
Si-KIT-6
/
9.83
24.0
845
0.96
6.6
Mn-KIT-6 (100)
174
9.95
24.3
786
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Si/Mn
(m2/g)
Vtp,BJH d
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ICP
SBET c
XRD
a
4.9
4.5
Mn-KIT-6 (50)
69
9.95
Mn-KIT-6 (25)
31
10.0
Mn-KIT-6 (10)
15
10.12
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0.93
717
0.81
4.9
4.1
24.4
578
0.61
4.5
4.6
24.7
451
0.57
4.5
5.0
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24.3
Values obtained by ICP-OES in final synthesis gel
b
ao = Unit cell parameter (d211 * √6)
c
SBET = Surface area determined using Brunauer–Emmett–Teller (BET) equation
d
Vtp,BJH = Total pore volume at relative pressure of 0.99 (P/Po) by BJH method
e
Vmp= micropore volume by HK method
f
Dp, BJH = Pore diameter by BJH adsorption
g
W = Wall thickness (ao /2 - Dp, BJH )
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Table 2: Catalytic Evaluation of styrene with various Mn-KIT-6 catalysts X styrene (%)
Selectivity (%) SBZ %
SPA %
SBZOOH %
Others%
79
8
/
/
8
/
3
/
2
/
13
6
5
Mn-KIT-6(100)
73
24
63
5
Mn-KIT-6(50)
84
38
52
7
Mn-KIT-6(25)
89
47
49
2
Mn-KIT-6(10)
81
21
56
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Si-KIT-6
S Styrene Oxide % 13
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Catalyst
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Reaction condition: nStyrene / nTBHP = 1:1 (each 1mmol); Catalyst = 0.2 g, Acetonitrile = 15g, Internal standard- Chlorobenzene -0.03ml, Temperature = 60oC, time = 24 h.
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Table 3: Variation of mole ratio of TBHP: Styrene with Mn-KIT-6(100):
S Styrene Oxide %
SBZ %
SPA %
0.5
54
19
67
7
1
73
24
63
5
2
61
17
71
SBZOOH %
Others%
6
/
8
/
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Xstyrene (%)
5
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TBHP :TS
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Selectivity (%)
4
3
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Reaction condition: Mn-KIT-6(100) = 0.2 g, Acetonitrile = 15g, Internal standardChlorobenzene -0.03ml, Temperature = 60oC, time = 24 h
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Table 4: Variation of Temperature with Mn-KIT-6(100) catalyst:
Selectivity (%) Xstyrene (%)
S Styrene Oxide %
SBZ %
SPA %
SBZOOH %
Others%
32
8
79
3
10
/
50
57
12
68
9
11
/
60
73
24
63
5
8
/
70
84
16
71
3
9
2
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Temperatu re (oC) 40
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Reaction condition: nStyrene / nTBHP = 1:1 (each 1mmol); Mn-KIT-6(100) = 0.2 g, Acetonitrile = 15g, Internal standard- Chlorobenzene -0.03ml, Time = 24 h
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Figure Captions Figure 1: Low Angle X-ray diffraction pattern of (a) Si-KIT-6, (b) Mn-KIT-6(100), (c) Mn-KIT6(50), (d) Mn-KIT-6(25), and (e) Mn-KIT-6(10).
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Figure 2: High Angle X-ray diffraction pattern of (a) Mn-KIT-6(100), (b) Mn-KIT-6(50), (c) Mn-KIT-6(25), and (d) Mn-KIT-6(10).
Figure 3: N2 Adsorption – Desorption isotherms of Mn- (a) Si-KIT-6, (b) Mn-KIT-6(100), (c)
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Mn-KIT-6(50),(d) Mn-KIT-6(25), and (e) Mn-KIT-6(10).
Figure 4 : Pore Size distribution of (a) Si-KIT-6, (b) Mn-KIT-6(100), (c) Mn- KIT-6(50), (d)
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Mn-KIT-6(25), and (e) Mn-KIT-6(10).
Figure 5: SEM, SEM-EDAX, TEM Images of Mn-KIT-6 (50).
Figure 6: FT-IR spectra of Mn-KIT-6 (a) Si-KIT-6, (b) Mn-KIT-6(100), (c) Mn-KIT-6(50), (d) Mn-KIT-6(25), and (e) Mn-KIT-6(10).
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Figure 7: Diffuse reflectance UV-Vis spectra of (a) Mn-KIT-6(10), (b) Mn-KIT-6(25), (c) MnKIT-6(50), and (d) Mn-KIT-6 (100).
Figure 8: EPR Spectra of (a) Mn-KIT-6(100), (b) Mn-KIT-6(50), (c) Mn-KIT-6(25), (d) Mn-
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KIT-6 (10).
Figure 9: Reusability of Mn-KIT-6(100) for epoxidation of styrene with TBHP.
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Figure 10: Hot filtration study of styrene epoxidation over Mn-KIT-6(100).
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Graphical Abstract
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Confinement of Mn3+ Redox Sites in Mn-KIT-6 and its Catalytic Activity for Styrene Epoxidation
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Highlights: •
KIT-6 with different manganese loading was achieved by one pot synthesis.
•
Mn3+ in tetrahedral and Mn2+ in octahedral coordination observed.
•
Confinement of incorporated Mn3+ framework species are active for styrene
Mn-KIT-6 catalysts are re-usable and recyclable upto 5 cycles.
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•
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epoxidation
Confinement of Mn epoxidation
redox sites in Mn-KIT-6 and its catalytic activity for styrene
Nilamadanthai Anbazhagan, Gaffar Imran, Ahsanulhaq Qurashi, Arumugam Pandurangan, Shanmugam Manimaran PII:
S1387-1811(17)30226-3
DOI:
10.1016/j.micromeso.2017.03.049
Reference:
MICMAT 8237
To appear in:
Microporous and Mesoporous Materials
Received Date: 28 December 2016 Revised Date:
8 March 2017
Accepted Date: 27 March 2017
Please cite this article as: N. Anbazhagan, G. Imran, A. Qurashi, A. Pandurangan, S. Manimaran, 3+ Confinement of Mn redox sites in Mn-KIT-6 and its catalytic activity for styrene epoxidation, Microporous and Mesoporous Materials (2017), doi: 10.1016/j.micromeso.2017.03.049. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Confinement of Mn3+ Redox Sites in Mn-KIT-6 and
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its Catalytic Activity for Styrene Epoxidation Nilamadanthai Anbazhagan a, Gaffar Imran a, Ahsanulhaq Qurashi b, Arumugam Pandurangan *, Shanmugam Manimaran a
a
Department of Chemistry, Anna University, Chennai – 600025, India
b
Department of Chemistry and Centre of Excellence in Nanotechnology, King Fahd University
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a
Of Petroleum and Minerals, Dhahran- 31261, Saudi Arabia
*Corresponding author: E-mail: [email protected], Fax: +9144 22200660; Tel: +9144
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22358653
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Abstract In the present study, a series of manganese (Mn) containing 3D cubic mesoporous KIT-6 materials with different Si/Mn ratio (100, 50 25 & 10) having Ia3d space group were synthesized
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for the first time by a facile one-pot hydrothermal process. The synthesized materials were methodically characterized by various analytical techniques including XRD, N2 sorption, HRTEM, diffuse reflectance UV-vis (DRS-UV-vis), EPR and FT-IR. The well ordered mesoporous
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nature of Mn incorporated KIT-6 materials are realized by XRD, N2 adsorption-desorption isotherm and HR-TEM analysis. Surface area, pore volume and pore size of Mn-KIT-6 with
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different Si/Mn ratio are in the range of 451-786 m2/g, 0.57-0.9 cm3/g and 4.5–4.9 nm respectively. The incorporation of Mn3+ in the framework and fine dispersion of Mn2+ species over silica matrix of Mn-KIT-6 are confirmed by the results of UV-DRS, FT-IR and EPR. Further, the presence of higher amount of Mn2+ extra-framework species in Mn-KIT-6(10) is
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realized by the presence of high amplitude signal at 2.009 (g value) and the contraction in H1 hysteresis loop as shown in EPR and BET isotherm respectively. Catalytic behavior of Mn-KIT6 material was evaluated for the epoxidation of styrene with various oxidants in which t-butyl
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hydro peroxide (TBHP) promotes the desired formation of styrene oxide under mild liquid phase conditions. Mn-KIT-6 shown to be highly stable and active which mainly depends on the
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framework substituted Mn3+ sites.
Keywords: Mn3+, isomorphous substitution, KIT-6, epoxidation
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1. Introduction Since the early 1990’s both academics and industries initiated research on mesoporous silicates (M41S, SBA-n) as a support for catalysis [1] and various other applications. Among
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these, mesoporous materials with three dimensional porous structure have attracted a significant attention owing to their excellent textural characteristics such as inter-linkage pore structure with narrow pore size distribution, high specific surface area, pore volume and unique 3D channel
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network which provide a highly opened porous host with easy and direct access for guest species, thus facilitating inclusion or diffusion throughout the pore channels without pore
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blockage [2]. Mesoporous silica, KIT-6 with Ia3d-type [3] are of particular interest over other materials like MCM-41 and MCM-48 because of its three dimensional structure with a large mesopores of diameter and easy synthesis method. More importantly it has high thermal and hydrothermal stability. Further, it possesses high surface area, large pore volume and tunable
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pore size [4].
Recently, research is being focused on designing constructing effective redox / acidic type catalysts by incorporation of various transition metals. Metals such as Sn, W, Zr, Nb, Al, Ce, Fe,
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Ti and Mo [5, 6] on 3D mesoporous silica, KIT-6 as support have been studied as catalysts for various catalytic conversions to useful products in greener way. It was found that manganese
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containing homogeneous catalyst shows high catalytic activity towards epoxidation of alkenes [7]. However, major drawback of using homogeneous Mn based material is catalyst recovery, which arises due to complexation of catalysts with substrate molecules used for catalytic reactions. Mn-incorporated heterogeneous type catalysts have received much attention as oxidation type catalyst. For example, highly crystalline manganese incorporated mesoporous materials such as Mn-MCM-41 and Mn-MCM-48 and microporous Mn-S-1 and Mn-SAPO-34
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are widely investigated in which predominantly exposed Mn3+ sites selectively act as a catalyst for catalyzing the oxidation of alkyl aromatics and hydrocarbon conversion respectively [8, 9]. Further, these Mn3+ sites in Mn-SBA-12 & Mn-SBA-16 and Mn-SBA-15 catalysts were also
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found to play a key role for selective formation of amides [10] and trans-stilbene oxide [11] respectively. Recently, a facile synthesis of Mn coupled with an active mesoporous material namely FDU-5 has been reported for trans-stilbene epoxidation [12]. In addition, manganese
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incorporated TUD-1 has been found to be active for various reacting molecules such as styrene, trans-stilbene [13] ethylbenzene [14] and cyclohexane [15]. Mn-incorporated SBA-1 is effective
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for oxidation of ethylbenzene [16]. All these results clearly indicate the promising role of manganese in various mesoporous silicate materials for various catalytic reactions. On the other hand, the method of incorporation of manganese into silicate materials plays an important role for the confinement of manganese species inside the silica matrix as support.
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Manganese incorporated silicate prepared by sol-gel and EISA (Evaporation Induced Self Assembly) techniques resulted in the formation of high amount of manganese species as large aggregations of metal oxides on the surface [12, 14]. Furthermore, it was reported that 30% of
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confinement of manganese sites [16] were observed for n-MnSBA-1 material synthesized at 0oC by cooperative assembly method. The isolated tetrahedral Mn3+ sites in the resulting material
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shows promising efficiency for the formation of selective product in comparison with extraframework Mn2+ sites [10, 11, 14, 16]. Though manganese incorporated silica materials exhibit promising catalytic activity for various reactions, the challenging aspect is to synthesize Mn incorporated materials with high manganese content without agglomeration by a cost-effective method. Herein, a series of manganese (Mn) containing 3D cubic mesoporous KIT-6 materials with different Si/Mn ratio (100, 50 25 & 10) have been synthesized for the first time by a facile
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one-pot hydrothermal process. The characterizations results show that manganese ions are uniformly incorporated in the framework of silica. Further, well ordered mesoporous nature of Mn incorporated KIT-6 materials are realized by XRD, N2 adsorption-desorption isotherm, UV-
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DRS, EPR and HR-TEM analysis. The catalytic efficiency of resulting Mn-KIT-6 has been evaluated for styrene epoxidation using TBHP as oxidant. Further, the cyclability and chemical
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stability of Mn-KIT-6 have been carried out and the results are discussed.
2. Experimental
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2.1. Materials required
Manganese nitrate (Aldrich) and Tetraethyl orthosilicate (TEOS, Aldrich) were used as the source for manganese and silica respectively. Non-ionic surfactant Tri-block co-polymer (PluronicP123, EO20–PO70–EO20, with an average molecular weight 5800, Aldrich) and co-
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solvent such as n-butanol (Aldrich) were used without any further purification. 2.2. Synthesis of Mn-KIT-6
Three-dimensional cubic mesoporous Mn-KIT-6 with different nSi/nMn ratio ranging from 100 -
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10 were synthesized by following the procedure as reported [17, 18] previously. Pluronic P123 (5 g) dissolved in water was added to 0.5 M HCl (190 g) solution in a polypropylene bottle and
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stirred at 308 K. After complete dissolution, 5 g of n-butanol is added at once and stirred for another 1 h. Then, 10.6g of TEOS and required amount of manganese nitrate was added at once to the homogeneous clear solution. This mixture is left under vigorous and constant stirring at 308 K for 24h. The mixture is aged at 373 K for 24 h under static conditions. The white precipitated product is filtered hot without washing and dried at 373 K for 12 h in air and the mixture was stirred for 24 h at 308 K. The mixture was then transferred to a Teflon-lined
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autoclave and was aged at 100 ◦C for 24 h under static hydrothermal conditions. The suspension was filtered off and the collected precipitate was dried at 100 ◦C for 12 h in air. The dried sample was then powdered and calcined at 823 K for 5 h in air atmosphere to remove the template. In
different concentration of TEOS and manganese nitrates were used. 2.3. Characterization of Mn-KIT-6
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order to prepare mesoporous KIT-6 materials with different Si/Mn ratio (100, 50 25 & 10),
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Powder XRD (low and high angle) patterns were recorded using a Phillips Xpert X-ray diffractometer with Cu-Kα radiation (ࣅ= 0.1548 nm) in the 2ᅎ range of 0.5–5o, and 10- 80o. The
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manganese content in Mn-KIT-6 was recorded using ICP-OES Optima 5300 DV (Perkin Elmer) instrument. The specific surface area, total pore volume and average pore diameter were measured by N2 sorption studies which were conducted using Micromeritics ASAP 2020 porosimeter instrument. Pore size distribution was obtained by applying the BJH analysis to the
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adsorption curve of the nitrogen adsorption–desorption isotherms. The morphology of Mn-KIT-6 was observed using scanning electron microscope (SEM, Hitachi-S3400N instrument). Transmission electron microscopy (HR-TEM) was carried out with HRTEM JEOL 3010
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operating at an accelerating voltage of 300 KV. The FT-IR spectra of Mn-KIT-6 diluted with KBr were recorded using a Bruker Spectrometer Tensor with a resolution of 4 cm-1. Diffuse
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reflectance UV-Vis spectra of Mn-KIT-6 samples were collected using a Shimadzu 2100 spectrophotometer equipped with an integrating sphere assembly in the range of 200-800 nm using BaSO4 as reference. Electron spin resonance (ESR) spectroscopy studies were carried out by loading samples in a Suprasil quartz EPR tube and the spectra were recorded using a Bruker X-band CW (EMX 102.7) spectrometer with variable temperature capability.
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2.4. Epoxidation of styrene Epoxidation of styrene over Mn-KIT-6 catalysts was carried out under liquid phase conditions. In a typical run, styrene (1 mmol, SRL), t- butyl hydrogen peroxide (TBHP, 70% in H2O,
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Spectochem), acetonitrile (15g, Merck) and chlorobenzene (0.03ml, internal standard, Merck) were taken in a 50 mL RB flask. Then 0.2g of pre-activated Mn-KIT-6 catalyst was added to the reaction mixture and the resulting suspension was stirred and refluxed for 24h at static
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temperature of 333K. After completion of each reaction, samples were collected systematically after centrifugation. Then 15µl of the filtrate (liquid samples) was diluted with 50 µl of
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acetonitrile and later was analyzed by gas chromatograph (GC) (Model: Shimadzu) having a capillary column RTX-5 (30m x 0.25 mm x 0.25 mm) and flame ionisation detector (FID). The product was also analysed by gas chromatograph equipped with mass spectrophotometer (GCMS) (Model: Agilent 6890NMS- JEOL).
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3. Results and Discussion:
Manganese acetate was initially used for the synthesis of Mn-SBA-1 [16] with slight modification of pH. The reaction proceeded as follows in equation (1).
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Mn (CH3COO) 2 + H+
Mn2+ + CH3COOH
1
However, acetic acid being a weak acid does not result in the formation of a low pH
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and as a result hinders the formation of mesoporous KIT-6 material by lowering condensation of silica which results in a loss of catalyst. Further, manganese nitrate is a better choice as the source of manganese [10] on account of the formation of HNO3 which is a strong acid and lowers the overall pH which leads to the formation of ordered mesophase structure as depicted in below scheme.
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3.1. Characterization:
The structural ordering of the Si-KIT-6 and Mn-KIT-6 materials was investigated by
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powder x-ray diffractometer and the pattern is shown in Fig.1. The XRD pattern of Si- KIT-6 (Fig.1a) gives rise to well ordered XRD reflections which can be assigned to (211) (220) (332) and (420) diffractions of the cubic Ia3d symmetry [2], which are characteristic for highly ordered mesoporous structures. Mn-incorporated KIT-6 with different concentration of Mn content
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(Fig.1b-1e) also exhibit similar XRD pattern with well resolved peak in the low 2θ region at 2θ= 0.84o-0.910 with slight decrease in higher order peaks, revealing that silica possesses well ordered structure even after Mn incorporation in to the frame work of silica and the structural
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ordering of the mesoporous material, KIT-6 is replicated in Mn-incorporated KIT-6 materials. Though XRD pattern for (211) plane was retained for all Mn-incorporated KIT-6 materials, the
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peak corresponding to (220), (332) and (420) plane decreases upon increase in metal content upto Si/Mn ratio of 25 and the pattern disappears when the concentration of Mn-ion further increases. This may be due to either partial structure collapse or the formation of metal oxide species inside the mesopores upon calcination. These results are consistent with the results obtained for various metal doped KIT-6 materials reported previously [6, 19]. The unit cell parameter (ao) and d211 values of Mn-KIT-6 are presented in Table 1. It has been reported that the
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unit cell parameter increases with increasing metal content [20, 21]. In the present study, we observe a slight increase in the unit cell value and lattice constant for Mn-incorporated KIT-6 in comparison with Si-KIT-6, demonstrating that the incorporation of manganese ions into frame-
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work of silica leads to increase in lattice constant and unit cell values. The nSi/nMn molar ratios of all the samples under investigation are summarised in Table 1. It can be seen from the Table 1 that Si, Mn in the gel incorporated into the solid is almost in close agreement with the input gel
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composition. In the case of Mn-KIT-6 (100) and Mn-KIT-6 (50), the Mn content in the solid phase is higher than the input gel, suggesting preferential incorporation of Mn ions in to the
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framework of silica. Similar observations were previously reported in the case of zeolites [22]. High angle XRD patterns of Mn-KIT-6 samples are shown in Figure 2 (a-d). The reflections indexed to amorphous silica were obtained at 2θ values of 23o for all Mn-KIT-6 samples. However no reflections were found corresponding to any MnOx (Mn2O3, MnO and Mn3O4)
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species, indicating that Mn-ion is incorporated into silica without forming any impurities. These results concluded that Mn-ions have been successfully incorporated into the silica framework without forming any impurities and retains the ordered mesoporous structure similar to pure
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siliceous KIT-6 material, which is consistent with the results obtained for manganese incorporated SBA-1[16].
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The nitrogen adsorption–desorption isotherms and pore size distribution of Si-KIT-6 and
Mn-KIT-6 with different Si/Mn ratios are shown in Fig. 3 and Fig. 4. Nitrogen adsorptiondesorption isotherm of Si-KIT-6 and Mn-KIT-6 showed Type IV isotherm with capillary condensation between 0.45-0.6 P/P0 of nitrogen, which is characteristic of mesoporous material. The H1-type hysteresis loop of Si-KIT-6 is typical of uniform mesoporous solids with narrow pore size distribution. However, the slight changes in H1 hysteresis loop is observed for Mn
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incorporated Si-KIT-6 materials, indicating the possible formation of amorphous MnOx clusters in the mesoporous channels of Mn-KIT-6. The incorporation of Mn into the walls of Si-KIT-6 has a significant effect on the specific surface area, specific pore volume and pore diameter of
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the materials (Figure 4 and Table 1). The pore volume is reduced from 0.96 to 0.57 cm3/g and the specific surface area declines from 845 to 451m2/g for Mn- incorporated Si-KIT-6 materials with Si/Mn ratio of 100 to 10, indicating slight shrinkage in the mesoporous nature of Si-KIT-6
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with increasing metal content. It is observed that the pore diameter of Si-KIT-6 decreases with increasing Mn-ion content from 6.6 nm to 4.5 nm. The pore diameter of Mn-KIT-6 (10) and Mn-
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Si-KIT-6 (25) is smaller than of Mn-KIT-6 (50) and Mn-KIT-6 (100) and much smaller than SiKIT-6, which could be attributed to the formation of more metal oxide clusters in the mesopores of Mn-KIT-6 (10) and Mn-KIT-6 (25). The morphology and the presence of elements in Mnincorporated Si-KIT-6 (50) were analyzed by TEM and SEM, and SEM-EDAX analysis (Fig. 5).
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The existence of spherical nature of silica particles and the presence of Mn in addition to Si and O have been observed. Further, the well interconnected ordered mesoporous structure of MnKIT-6 (50) is evident from HR-TEM images, which is consistent with the results obtained from
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XRD analysis.
The results of FT-IR spectra for Si-KIT-6 and Mn-KIT-6 materials with different Si/Mn ratio are
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shown in Fig. 6 and it is found that the results of Mn-KIT-6 in the present study is in good agreement with the results obtained for Mn incorporated M41S materials [23, 24]. FT-IR spectra of Mn-KIT-6 materials display bands at 1650, 1225, 1084, 964 & 803 cm-1. The band at 1650 cm-1 corresponds to the presence of bending vibration of O-H band which is due to the presence of moisture on Si-KIT-6 and Mn-KIT-6 materials. The bands at 1225 & 1084 cm-1 and 803 cm-1 are assigned to υas of Si-O-Si band and υs of Si-O-Si structural siloxane group respectively.
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Whereas the band at 460 cm-1 is due to the presence of υb of Si-O-Si and Si-O-Mn band in mesoporous silica framework [25]. Further, it is interesting to note that the band at 964 cm-1 is assigned to stretching vibration band of Si-O-Mn linkage in Mn-Si-KIT-6, which increases with
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increase in the concentration of Mn content, confirming the incorporation of Mn-ion into the silica matrix of KIT-6. These results are consistent with the results obtained from XRD analysis. The co-ordination geometry of Mn incorporated Si-KIT-6 is analysed by the UV-Vis absorption
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spectra and the absorption spectra of calcined Mn incorporated Si-KIT-6 with different Si/Mn ratio (10, 25, 50 and 100) is shown in Fig. 7. It shows absorption bands at 220, 260 nm and two
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broad bands at 310-350 nm and 500 nm. A distinct individual absorption bands around 220 and 260 nm observed for all Mn-KIT-6 (100, 50, 25 &10) samples. These bands can be assigned to O2-→Mn3+, a charge transfer transition of trivalent manganese ions in tetrahedral coordination geometry [10, 13-16]. This band could be assigned to the strong bonding of oxygen ligands to
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Mn3+ ions. The intensity of this band increases monotonically with increasing Mn content. Absorption band at 310-350 nm is also assigned to the electron transfer of O2-→ Mn3+ as reported previously for the UV-vis. spectrum of Mn3O4 [26, 27 - 29]. The broad absorption band
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which appears at 500 nm is due to 6A1g →4T2g transition of the octahedral coordinated divalent manganese species [17, 25] confirming the presence Mn in extra framework species. A similar
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band was also reported in Mn incorporated SBA-15[30]. All these results revealed that the majority of the manganese ions in the Mn-Si-KIT-6 samples occupy framework position in the surface layer of the Si-KIT-6 channel walls. In order to study the coordination of high spin Mn ions in Si-KIT-6, electron paramagnetic resonance (EPR) analysis was carried out at room temperature for calcined Mn-KIT-6 samples with different Si/Mn ratio. Mn-KIT-6 (100, 50 & 25) samples exhibited low resolved hyperfine
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structure (shown in Fig. 8) centered at g value of 2.009 with hyperfine coupling constant A= 93 G which is attributed to Mn2+ in octahedral geometry [24]. It is noted that EPR signals of Mn2+ in Mn-KIT-6(10) is different to the other ratio of Mn-Si-KIT-6 materials indicating decrease in
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Mn2+-Mn2+ band distance in Mn-KIT-6. These results demonstrate the presence of extraframework Mn2+ with increase of manganese content and is in well consistence with BET results, i.e., the contraction of hysteresis loop as observed in nitrogen adsorption-desorption isotherm
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with increase in manganese content is due to the presence of Mn2+-ions in extra framework. 3.2 Catalytic activity:
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In order to evaluate the catalytic activity of Mn-KIT-6 materials, liquid phase epoxidation of styrene and TBHP with ratio 1:1 was carried out at 60oC for 24h. The selective products obtained during the epoxidation reaction are styrene oxide, phenyl acetaldehyde (PA), benzaldehyde (BZ) and benzoic acid (BZOOH). The effect of various parameters such as
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different catalysts (Si/Mn ratio), oxidant (TBHP): Styrene and temperature were studied systematically and will be discussed in the following section. 3.2.1. Effect of Si/Mn ratio varying 100-10 content in Mn-KIT-6 samples with various
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Oxidants
Epoxidation of styrene was carried out using various oxidants such as air, H2O2 and
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TBHP. No characteristic change was seen when air and H2O2 were used as oxidants. However, TBHP proved to be a good oxidant due to the generation of active oxygen species, an effect which was earlier observed by Selvaraj et al. [15, 31]. Hence TBHP was chosen as the oxidant for all Si/Mn ratios of KIT-6 catalysts (Table 2). In contrast, it is noted that pristine Si-KIT-6 mesoporous silicates showed very less catalytic conversion inferring that isomorphous substituted manganese plays a major role in the catalytic conversion of styrene. With increase in
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manganese content, conversion of styrene was increased from 73% of Mn-KIT-6 (100) to 89% of Mn-KIT-6 (25) with an increased selectivity of styrene oxide from 24% to 47% as presented in Table 2. However, considerably lower conversion results were obtained for Mn-KIT-6(10)
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owing to its lower surface area, contracted hysteresis with lower pore volume and also due to presence of higher range of extra-framework species (Mn2+) as evidenced out from DRS-UV-Vis
25 > 50 > 100 > 10 of Mn-KIT-6 catalysts. 3.2.2. Mechanism of styrene epoxidation:
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and EPR spectra. The order of the activity of the catalysts with different Si/Mn ratios followed as
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Styrene oxide formation could be possibly explained by the formation of metalhydroperoxide intermediates. Frame-work substituted Mn3+ ions are the primary reason for the activation of TBHP which chemisorbs on the active Mn sites of KIT-6 and forms the metalhydroperoxide as intermediate. Mimoun et al. also suggested similarly that the vanadium peroxo
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radical is the active species for transferring oxygen to reactants [32]. Subsequently, in metalhydroperoxide intermediate complex the O-O bond gets polarized and gets transferred to the nucleophilic C-C (double bond) finally forming styrene oxide as product.
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3.2.3. Effect of TBHP: Styrene
Styrene was studied by varying the molar ratio of TBHP from 0.5 to 2 (Table 3) over Mn-
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KIT-6 (100). Styrene conversion increases from 54% to 73% with increase in the TBHP concentration from 0.5 to 1. The selectivity of styrene oxide also increased simultaneously from 19% to 24%. Decrease in conversion and selectivity was obtained when ratio was doubled which could be due to the possible blocking of active site and excess of H2O present in 70% of TBHP with Mn-KIT-6 catalysts. Similar results were obtained when Ce-MCM-48 and MnOx grafted on SBA-15 was used as catalysts for styrene conversion [33 - 37]. 1:1 molar ratio of both
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TBHP and styrene gave the exact production of reactive species for the formation of the desired product. 3.2.4. Effect of Temperature
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The effect of reaction temperature with Mn-KIT-6(100) is presented in Table 4. With increasing temperature from 40-70o C, the conversion of styrene drastically increases from 32% to 84% indicating temperature plays a key role in the conversion of styrene epoxidation. On the other
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hand, the drastic decrease in selectivity (16%) of styrene oxide was observed when reaction was carried out at about 70o C. Though reaction at high temperature shows good styrene conversion,
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selectivity of styrene oxide decreases due to the formation of various by-products in the reactions. Thus the temperature of 60oC has been optimized to carry out epoxidation of styrene with Mn-KIT-6 catalysts with various Si/Mn ratios. 3.2.5. Reusability and Hot-Filtration
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To study the stability of Mn-KIT-6 materials during catalytic reaction, the used catalyst in the reaction was recovered by filtration; washed and followed by activation at 300o C. This catalyst was used for styrene conversion again for five cycles and the results are shown in figure 9. The
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results displayed clearly showed that there was no significant loss in the conversion concluding that Mn-KIT-6 proved to be active and highly recyclable catalyst for the epoxidation reactions.
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To account the issue of leaching and further verify the heterogeneous nature, Mn-KIT-6 (100) samples were also subjected to hot filtration study (result shown in Fig. 10). After 1h of reaction, the mixture was hot-filtered and catalyst was removed. Without any solid materials (catalyst) hot-filtered reaction mixture was further continued for stirring at 60o C for 24h, and later the same was analysed by atomic absorption spectroscopy (AAS) in order to understand the Mn leaching from the catalyst. The results show negligible concentration of manganese ions in the
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solution and this confirms the truly heterogeneous behavior of Mn-KIT-6 catalysts synthesized in the present study. 4. Conclusions
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3D mesoporous materials with Mn incorporation into the framework of KIT-6 has been successfully realized by a direct one-step hydrothermal process using pluronic P123 and nbutanol as the structural directing agent and co-solvent respectively. Manganese nitrate was
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considered to be the best choice for the preparation of well ordered Mn-KIT-6. The presence of ordered mesoporous structure of Mn substituted KIT-6 materials has been confirmed by XRD,
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HR-TEM and N2 adsorption-desorption analysis. DRS UV-Vis spectra reveal that Mn3+ species substituted uniformly in the framework of silica for all Mn substituted KIT-6 with different Si/Mn ratios. Mn2+ species predominate in the case of Si/Mn = 10 ratio which was confirmed by DRS-UV-vis and EPR technique. Catalytic activity of Mn-KIT-6 for styrene epoxidation using
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TBHP as oxidant shows that conversion of styrene increases from 73% to 89% with increase in the concentration of Mn in the framework of silica. An increased selectivity of styrene oxide from 24% to 47% was observed for Mn-KIT-6 (25). The order of catalytic activity of Mn-KIT-6
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catalysts with different Si/Mn ratios are 25 > 50 > 100 > 10. On the other hand, pristine Si-KIT-6 mesoporous silicates showed very less catalytic conversion for oxidation of styrene. These
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results demonstrate that isomorphously substituted manganese in framework plays a crucial role for the improved catalytic reaction for the conversion of styrene. Further, cyclability and hotfiltration studies reveal that Mn-KIT-6 is highly stable and recyclable catalyst for many cycles for the epoxidation reactions.
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Acknowledgement Authors acknowledge the financial support from DST (FIST) and UGC (SAP) to Department of Chemistry, CEG, Anna University for providing necessary facilities. Authors thanks IIT-SAIF,
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CLRI and PSG Institute of Advanced studies for providing low angle- XRD, EPR and HR-TEM analysis respectively.
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References:
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[6] R. Rajalakshmi, R. Maheswari, R. Anand, Mate. Res. Bulletin, 75 (2016) 224–229. [7] R. Ben-Daniel, L. Weiner, R. Neumann, J. Am. Chem. Soc., 124 (2002) 8788 – 8780. [8] N. Novak Tusar, S. C. Laha, S. Cecowski , I. Arcon , V. Kaucic , R. Glaser, Micropor.
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[10] A. Kumar, D. Nepak, D. Srinivas, Catal. Commun. 37 (2013) 36– 40. [11] M. Selvaraj, T. G. Lee, J. Phys. Chem. B, 110 (2006) 21793-21802. [12] G. Imran, V. Srinivasan, R. Maheswari, R. Anand, S. Bala, J. Porous Mater. 23 (2016) 57– 65.
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[15] R. Maheswari, R. Anand, G. Imran, J. Porous Mater. 19 (2012) 283. [16] G. Imran, R.Maheswari, Mater. Chem. Phy. 161 (2015) 237-242. [17] R. Anand, R. Maheswari, D. H. Barich, S.Bala, Micropor. Mesopor. Mater.190 (2014) 240– 247.
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[18] Q. Pan, R. Anand, W. Kirk Snavely, R.V.Chaudhari, S. Bala , Ind. Eng. Chem. Res. 52 (2013) 15481 −15487.
[19] R. Anand, S. Bala, R. Maheswari, Micropor. Mesopor. Mater.167 (2013) 207–212. [20] E.G. Derouane, M. Mestdagh and L.J. Vielvoye, J. Cat. 33 (1974) 169.
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[21] C.F. Cheng and J. Klinowski, J. Chem. Soc., Faraday Trans. 92 (1996) 289.
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[23] M. Selvaraj, P. K. Sinha, K. Lee, I. Ahn, A. Pandurangan, T. G. Lee, Micropor. Mesopor.
[24] S. Vetrivel, A. Pandurangan, J. Mol. Catal. A: Chem. 246 (2006) 223-230. [25] R. Merkache, I. Fechete, M. Maamache, M. Bernard, P. Turek, K. Al-Dalama, F. Garin, Appl. Catal. A: Gen. 504 (2015) 672–681.
[26] S. Velu , N. Shah , T.M. Jyothi, S. Sivasanker, Micropor. Mesopor. Mater. 33 (1999) 61–75.
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[27] Q. Zhenping, B. Yibin, Y. Qin, Y. Wang, F. Qiang, Chem. Eng. J. 209 (2012) 163–169. [28] T. Tsoncheva, J. Rosenholm, M. Linden, F. Kleitz, M. Tiemann, L. Ivanova, M. Dimitrov, D. Paneva, I. Mitov, C. Minchev, Micropor. Mesopor. Mater. 112 (2008) 327–337. [29] N. Stamati, K. Goundani, J. Vakros, K. Bourikas, Ch. Kordulis, Appl. Catal. A: Gen. 325
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493–497.
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[33] R. Anand, R. Maheswari, Prem. S. Thapa, S. Bala, ACS Symp. Series; Am.Chem. Soc. 1132 Chap. 8, 213-228. [34] Q. Tang, H. Shuangquan , Y. Chen, Z. Guo, Y. Hu, Y. Yang, Micropor. Mesopor. Mater. 132 (2010) 501–509.
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[35] H. Shuangquan, D. Liu, C. Wang, Y. Chen, Z. Guo, A. Borgna, Y. Yang, App. Catal. A: Gen. 386 (2010) 74–82. [36] Z. Guo, C. Zhou, H. huangquan, Y. Chen, X. Jia, R. Lau, Y. Yang, , Appl. Catal. A: Gen. 419–420 (2012) 194–202.
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[37] Q.H. Zhang, Y. Wang, S. Itsuki, T.Shishido, K.Takehira, J. Mol. Catal. A: Chem. 188 (2002)
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189 – 200.
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Scheme1: Epoxidation of styrene with TBHP oxidant over Mn-KIT-6 catalysts
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Scheme2: Plausible mechanisitic scheme for epoxidation of styrene over Mn-KIT-6
O OH
-t
O Mn3+
TBHP
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H
O
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tBu
(Mn-KIT-6)
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O
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Mn3+
O
Mn
Bu OH
O
Mn
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Table 1: Physical parameters of various Si/Mn ratio of KIT-6
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b
d211 (nm)
ao (nm)
(cm3/g)
Dp, BJH f (nm)
Wg (nm)
Si-KIT-6
/
9.83
24.0
845
0.96
6.6
Mn-KIT-6 (100)
174
9.95
24.3
786
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Si/Mn
(m2/g)
Vtp,BJH d
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ICP
SBET c
XRD
a
4.9
4.5
Mn-KIT-6 (50)
69
9.95
Mn-KIT-6 (25)
31
10.0
Mn-KIT-6 (10)
15
10.12
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0.93
717
0.81
4.9
4.1
24.4
578
0.61
4.5
4.6
24.7
451
0.57
4.5
5.0
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24.3
Values obtained by ICP-OES in final synthesis gel
b
ao = Unit cell parameter (d211 * √6)
c
SBET = Surface area determined using Brunauer–Emmett–Teller (BET) equation
d
Vtp,BJH = Total pore volume at relative pressure of 0.99 (P/Po) by BJH method
e
Vmp= micropore volume by HK method
f
Dp, BJH = Pore diameter by BJH adsorption
g
W = Wall thickness (ao /2 - Dp, BJH )
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Table 2: Catalytic Evaluation of styrene with various Mn-KIT-6 catalysts X styrene (%)
Selectivity (%) SBZ %
SPA %
SBZOOH %
Others%
79
8
/
/
8
/
3
/
2
/
13
6
5
Mn-KIT-6(100)
73
24
63
5
Mn-KIT-6(50)
84
38
52
7
Mn-KIT-6(25)
89
47
49
2
Mn-KIT-6(10)
81
21
56
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Si-KIT-6
S Styrene Oxide % 13
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Catalyst
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Reaction condition: nStyrene / nTBHP = 1:1 (each 1mmol); Catalyst = 0.2 g, Acetonitrile = 15g, Internal standard- Chlorobenzene -0.03ml, Temperature = 60oC, time = 24 h.
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Table 3: Variation of mole ratio of TBHP: Styrene with Mn-KIT-6(100):
S Styrene Oxide %
SBZ %
SPA %
0.5
54
19
67
7
1
73
24
63
5
2
61
17
71
SBZOOH %
Others%
6
/
8
/
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5
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Selectivity (%)
4
3
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Reaction condition: Mn-KIT-6(100) = 0.2 g, Acetonitrile = 15g, Internal standardChlorobenzene -0.03ml, Temperature = 60oC, time = 24 h
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Table 4: Variation of Temperature with Mn-KIT-6(100) catalyst:
Selectivity (%) Xstyrene (%)
S Styrene Oxide %
SBZ %
SPA %
SBZOOH %
Others%
32
8
79
3
10
/
50
57
12
68
9
11
/
60
73
24
63
5
8
/
70
84
16
71
3
9
2
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Temperatu re (oC) 40
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Reaction condition: nStyrene / nTBHP = 1:1 (each 1mmol); Mn-KIT-6(100) = 0.2 g, Acetonitrile = 15g, Internal standard- Chlorobenzene -0.03ml, Time = 24 h
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Figure Captions Figure 1: Low Angle X-ray diffraction pattern of (a) Si-KIT-6, (b) Mn-KIT-6(100), (c) Mn-KIT6(50), (d) Mn-KIT-6(25), and (e) Mn-KIT-6(10).
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Figure 2: High Angle X-ray diffraction pattern of (a) Mn-KIT-6(100), (b) Mn-KIT-6(50), (c) Mn-KIT-6(25), and (d) Mn-KIT-6(10).
Figure 3: N2 Adsorption – Desorption isotherms of Mn- (a) Si-KIT-6, (b) Mn-KIT-6(100), (c)
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Mn-KIT-6(50),(d) Mn-KIT-6(25), and (e) Mn-KIT-6(10).
Figure 4 : Pore Size distribution of (a) Si-KIT-6, (b) Mn-KIT-6(100), (c) Mn- KIT-6(50), (d)
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Mn-KIT-6(25), and (e) Mn-KIT-6(10).
Figure 5: SEM, SEM-EDAX, TEM Images of Mn-KIT-6 (50).
Figure 6: FT-IR spectra of Mn-KIT-6 (a) Si-KIT-6, (b) Mn-KIT-6(100), (c) Mn-KIT-6(50), (d) Mn-KIT-6(25), and (e) Mn-KIT-6(10).
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Figure 7: Diffuse reflectance UV-Vis spectra of (a) Mn-KIT-6(10), (b) Mn-KIT-6(25), (c) MnKIT-6(50), and (d) Mn-KIT-6 (100).
Figure 8: EPR Spectra of (a) Mn-KIT-6(100), (b) Mn-KIT-6(50), (c) Mn-KIT-6(25), (d) Mn-
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KIT-6 (10).
Figure 9: Reusability of Mn-KIT-6(100) for epoxidation of styrene with TBHP.
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Figure 10: Hot filtration study of styrene epoxidation over Mn-KIT-6(100).
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Graphical Abstract
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Confinement of Mn3+ Redox Sites in Mn-KIT-6 and its Catalytic Activity for Styrene Epoxidation
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Highlights: •
KIT-6 with different manganese loading was achieved by one pot synthesis.
•
Mn3+ in tetrahedral and Mn2+ in octahedral coordination observed.
•
Confinement of incorporated Mn3+ framework species are active for styrene
Mn-KIT-6 catalysts are re-usable and recyclable upto 5 cycles.
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•
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epoxidation