New Insights into Manganese Local Environment in MnS‐1 Nanocrystals

Manganese plays an important role in redox catalysis using zeolites as inorganic support materials, but the formation of the preferred redox manganese species (framework or extraframework) is still not well understood. Herein, the influence of the amount of manganese together with conventional and microwave-assisted hydrothermal synthesis paths on the formation of manganese species within the zeolite silicalite-1 (S-1) with MFI structure was investigated. It was found out that both synthesis procedures led to the formation of framework and extraframework manganese species, but in different molar ratios. However, the conventional synthesis procedure with all Mn/Si molar ratios generates more framework Mn in comparison to the microwave procedure. Additionally, the diminution of the zeolite crystals to nanoscale from 100 to 200 nm was achieved via the conventional procedure for the first time. UV−vis, Raman, and X-ray absorption spectroscopic analyses revealed different local environments of manganese: Mn incorporated into the silicalite-1 framework as “framework manganese” and Mn present as “extraframework manganese” (Mn2O3, Mn3O4). TEM reveals the presence of Mn3O4 nanorods. Both framework manganese and extraframework manganese exhibit good catalytic activity for styrene epoxidation. Catalytic results suggest that, in oxidation reactions of hydrocarbons, framework manganese is more active at lower Mn contents (Mn/Si < 0.015), whereas extraframework manganese is more active at higher loadings (Mn/Si > 0.015).


■ INTRODUCTION
Interest in the development of environmentally benign, selective, and stable catalysts for specific purposes has been growing in the last decade. Manganese is one of the most intensively used elements for redox catalysis. 1 In order to obtain recyclable as well as stable catalysts, it is often preferred to isolate and stabilize the redox-active metal species by incorporation into inorganic matrices as framework or/and extraframework species. 2 Zeolites, i.e., microporous aluminosilicates with pore openings up to 2 nm, are already widely applied as inorganic matrices for stabilization of manganese redox sites in heterogeneously catalyzed conversion. 3 However, the optimal synthesis strategy which would lead to the preferred manganese redox species in certain framework types of zeolites remains a matter of debate due to the challenging characterization of the often very low concentration of manganese present (typically below 1 wt %). For example, hydrothermal synthesis and post-synthesis procedures can promote different forms of manganese species, from isolated ions in framework and extraframework positions to single oxides (MnO, Mn 2 O 3 , and MnO 2 ) or mixed oxides such as Mn 3 O 4 in Mn-containing zeolites. 4 Silicalite-1 is a high-silica zeolite with an MFI framework topology. Incorporation of manganese into micrometer-sized crystals of this zeolite with sizes of 100−200 μm generates framework manganese redox sites. 3 Meng reported a facile synthesis of a Mn-containing MFI-type zeolite by hydrothermal synthesis from a clear solution using a manganese organic precursor and silicates at 180°C for 2 days, preparing hexagonal platelets with particle sizes of 500−700 nm. 5 Recently, the incorporation of manganese into silicalite-1 was achieved by the hydrothermal conversion of manganeseexchanged magadiite under neutral conditions (crystals 2−10 μm in size), 6 while a two-step procedure containing mechanochemical pretreatment and hydrothermal synthesis led to the incorporation of a high content of manganese (up to 6 mol %) into the framework (crystals 10−30 μm in size). 7 The diminution of the particle size from the micrometer to the nanometer scale results in a larger specific surface area of the catalyst and, thus, a higher activity. Clear solutions with an excess of organic templates (SDAs) are generally used for the preparation of zeolite nanoparticles. These systems provide the necessary conditions for uniform nucleation and crystal growth. Consequently, the obtained nanocrystals are uniform in size. 8 Tetraethyl orthosilicate (TEOS) as a source of silica, the temperature, and the time of crystallization play important roles in the control of size of the obtained silicalite-1 nanocrystals. 9 To reduce the particle size, different heating technologies can be used: for example, microwave-assisted synthesis, which can provide an efficient control of particle size distribution. 8,10 In addition to controlling the particle size effect, microwave-assisted synthesis can provide also rapid volumetric heating with a shorter synthesis time and higher synthesis rate, selectivity, and yield in comparison to the conventional hydrothermal preparation method. 11 As a result, microwave heating can realize fast crystallization and rapid materials preparation in minutes, while hours or even days are needed by conventional heating methods. Therefore, microwave synthesis can lead to relatively low cost, energy savings, and high efficiency for materials preparation. 12 Recently, we have prepared a heterogeneous Fenton-type nanocatalyst by incorporation of manganese into silicate nanoparticles of mesostructured KIL-2. 13 Manganese was incorporated into the silicate framework with a molar Mn/Si ratio of up to 0.01; when the manganese concentration exceeded this value, manganese species were deposited on the surface of the materials as manganese oxide nanoparticles. In addition, in our preliminary synthesis using microwave irradiation, silicalite-1 nanoparticles with Mn 2 O 3 species, which were well distributed between silicalite-1 nanoparticles, served as active sites for polysulfide adsorption in lithium sulfur batteries. 14 In this work, the preparation of manganese-containing silicalite-1 nanocrystals by microwave-assisted and conventional hydrothermal synthesis with tunable manganese local environment is presented. Manganese local environments in the resulting materials were determined by UV−vis, Raman, and X-ray absorption (XAS) spectroscopic techniques supported by TEM analysis. The catalytic activity of the materials was tested in styrene epoxidation.

■ EXPERMENTAL SECTION
Catalyst Preparation. Manganese acetate (99%, Merck) solution, TEOS (98%, Acros), and TPAOH (40%, Alfa Aesar) were mixed at room temperature, respectively. A clear brown solution with molar composition 10 SiO 2 :x MnO:4 TPAOH:195 H 2 O (x = 0.05, 0.1, 0.2, 0.5) were aged with stirring for 1 day at room temperature. After aging, 20 mL of the clear solution was transferred into a Teflon-lined stainless steel autoclave for one-step hydrothermal conventional synthesis or into the Teflon autoclave for two-step microwave synthesis in an ETHOS 1600 microwave oven (Milestone). The onestep hydrothermal syntheses were performed at 175°C for 1 day, while the two-step microwave heating was performed by the following regime: the first step at a temperature of 80°C for 30 min and the second step at 150°C for 20 min. After the syntheses, the solid products were separated by rapid centrifugation, washed with distilled water, and dried at 60°C for several hours. The obtained products were calcined at 500°C for 5 h in an air flow.
Catalyst Characterization. The X-ray powder diffraction (XRD) patterns were recorded on a PANalytical X'Pert PRO high-resolution diffractometer with Alpha1 configuration using Cu Kα 1 radiation (1.5406 Å) in the 2θ range from 5 to 50°2θ using a step of 0.034°a nd a collection time of 100 s per step on a fully opened X'Celerator detector. Morphology and surface properties were observed by scanning electron microscopy (SEM) on a Zeiss Supra 3VP microscope. Elemental analysis was performed by the EDX method with an INCA Energy system attached to the Zeiss Supra 3VP microscope. The structures of 05MnS-1-MW and 01MnS-1-MW were investigated by high-resolution transmission electron microscopy (HRTEM) on a JEOL JEM 2100 200 kV field-emission gun (FEG) microscope. The sample was dispersed in ethanol and placed on a copper holey carbon grid. The specimen was additionally coated with carbon in order to prevent excessive charging of the samples under the electron beam. UV−vis diffuse reflectance (DR) spectra were recorded on a PerkinElmer Lambda 35 apparatus equipped with a Praying Mantis accessory. Background was recorded with Spectralon reference. Fine powdered samples were scanned in the spectral range between 200 and 1100 nm, with a slit set to 2 nm and a scanning speed of 480 nm/min. Raman spectra were recorded in the spectral range from 70 to 1600 cm −1 using a Witec Alpha 300 confocal microscope that employed a green laser with an excitation wavelength of 532 nm, an accumulation time of 50 s, and a resolution of 4 cm −1 at 10 mW laser power. Mn K-edge X-ray absorption spectra of MnS-  3 ) were measured in the energy region of the Mn K-edge in transmission mode at the XAFS beamline of the ELETTRA synchrotron facility in Trieste, Italy, and at the P65 beamline of PETRA III at DESY, Hamburg. Both beamlines provided a Si(111) double-crystal monochromator with about 1 eV resolution at the Mn K-edge. The samples were prepared as self-supporting pellets with an absorption thickness (μd) of about 2.5 above the Mn K-edge, while reference Mn samples were pressed from a homogeneous mixture of the Mn compound and boron nitride powder into self-supporting pellets with a total absorption thickness of about 1.5 above the Mn Kedge. At the XAFS beamline the absorption spectra were measured within the interval −250 to +1000 eV relative to the Mn K-edge. In the XANES region equidistant energy steps of 0.3 eV were used, while for the EXAFS region equidistant k steps of 0.03 Å −1 were adopted with an integration time of 1 s/step. At the P65 beamline the absorption spectra were measured in the energy region from −150 eV to +1000 eV relative to the Mn K-edge in continuous fast (3 min) scans and rebinned to the same energy and k steps for XANES and EXAFS regions. Four to eight repetitions were collected for each sample and superimposed to improve the signal to noise ratio. Exact energy calibration was established with the simultaneous absorption measurements on Mn metallic foil inserted between the second and third ionization cells. The analysis of XANES and EXAFS spectra was performed with the Demeter (IFEFFIT) program package, 15 in combination with FEFF6 program code 16 for ab initio calculation of photoelectron scattering paths.
The catalytic properties of MnS-1-MW from microwave-assisted hydrothermal synthesis and of MnS-1-K prepared by conventional hydrothermal synthesis were studied in the epoxidation of styrene in a batch reactor in liquid acetonitrile under reflux at 1 bar and 343 K. The catalytic experiments were carried out as described ithe n literature, 17 except that an overall reaction volume V reaction = 20 cm 3 was used.
■ RESULTS AND DISCUSSION Structural Properties. The method of preparation either using the microwave-assisted hydrothermal (products denoted MnS-1-MW) or the conventional synthesis (products denoted MnS-1-K) with different temperatures and times of crystallization can influence the local environment of the resulting manganese species within the silicalite-1 framework. Before hydrothermal synthesis, a light pink clear solution was prepared by mixing an aqueous manganese(II) acetate solution  20 The maximum crystallization temperature in microwave synthesis was set to 150°C in order to avoid overheating. Namely, MnO 2 can efficiently absorb microwaves, causing a rapid increase in the temperature and pressure. The advantage of microwave irradiation over the conventional method is in raising the temperature of the whole volume simultaneously, whereas in the heated autoclaves used for conventional hydrothermal synthesis, the reaction solution in contact with the autoclave wall is heated first. 12 In our study, the solution was warmed by microwaves to the desired temperature (150°C ) in a few minutes and the syntheses were completed in 50 min. On the other hand in conventional hydrothermal synthesis, the reaction solution needed several hours to warm up to 175°C, and the reaction was completed in 24 h. Figure 1 shows XRD patterns of the obtained products after (A) two-step microwave-assisted hydrothermal synthesis and (B) one-step conventional hydrothermal synthesis. All products correspond to the fully crystalline MFI structure with no observable impurities: e.g., manganese oxides. It can be seen that the samples with the highest amount of manganese (05MnS-1) prepared by both synthesis paths show lower intensities of diffraction peaks in comparison to the 005MnS-1 samples.
The manganese content in the investigated samples was determined by EDAX analysis ( Table 1). The trend of Mn content within the samples is consistent with the amount of initially added Mn 2+ precursor.
The morphology of all products is shown in Figure 2. A comparison of the synthesis method and the morphology of the nanoparticles with the same amount of manganese in the reaction solution (Mn/Si = 0.02) shows spherical shapes formed during conventional heating, while hexagonal prisms were obtained by microwave heating. Size differences can be observed as well: round disks prepared by microwave heating are larger (500 nm) than particles (200 nm) obtained by conventional synthesis from the same reaction solution. Nanocrystals 100−200 nm in size were observed for the first time on comparison to the literature. 5  The SEM observations in Figure 2A indicate that MnS-1-MW products with lower manganese amounts (0.005 and 0.01) are phase pure, while the product with the highest amount of manganese (0.05) shows the presence of an additional phase in the form of nanorods.
HR-TEM analysis was performed in order to identify the accompanying phase of MnS-1 zeolite crystals. Figure 3a shows typical hexagonal prismatic crystals of silicalite-1 (marked with S) with an estimated size of 500 nm in the 05MnS-1-MW sample. The surfaces of zeolite crystals are covered with nanorods with an average size of 10 × 50 nm (indicated with Mn on Figure 3a). These nanoparticles exhibit a polycrystalline nature, as indicated by selected area electron diffraction (SAED) performed on the agglomerate shown on Figure 3b. Measured distances of the Scherrer rings on the SAED pattern nicely correspond to hausmannite Mn 3 O 4 lattice distances (Figure 3c). A comparison of the measured and referenced d values is also shown in Table 2. An additional confirmation of the hausmannite presence as the extraframework Mn oxide phase in the 05MnS-1 sample was determined by HR-TEM observations on individual rodlike nanocrystals. Figure 3d shows such crystallites oriented on the [101]-zone axis, where   Figure 4 indicate a much lower occurrence of additional Mn oxide species than in the case of the 01MnS-1-MW sample. An additional phase occurs only as individual crystallites on the surface of silicalite crystals (indicated as Mn in Figure 4a), which made their identification by SAED impossible. However, morphological similarity with the rodshaped crystals that occur in higher concentrations in the 01MnS-1-MW sample clearly identified as Mn 3 O 4 (Figure 4b) gives a strong indication that the additional Mn oxide phase in 01MnS-1-MW mostly belongs to the hausmannite phase as well.
However, the presence of both Mn 3 O 4 and Mn 2 O 3 in the products was expected because of the hydrothermal synthesis performed under basic conditions, necessary to obtain an MFI structure, and calcination in an air flow at 500°C, respectively. This is in accordance with UV−vis results, which show the presence of the mixture of manganese oxides. Diffuse reflectance UV−vis spectroscopy is used very commonly for the determination of the oxidation and coordination state of metal complexes in solid materials. However, in the case of manganese, the possibility of Mn species with various oxidation states occurring causes difficulties in the interpretation of the experimental results. 4 UV−vis DR spectra of the samples prepared by microwave and conventional synthesis ( Figure 5) are very complex. A band between 253 and 263 nm can be assigned to the framework tetrahedrally coordinated    5 This band is intense in all samples with Mn/Si higher than 0.01, while for both 005MnS-1 samples it is observed as a shoulder. The broad band around 360 nm with a tail extending to 800 nm, present in 02MnS-1-MW and 05MnS-1-MW, could be due to the extraframework Mn 3+ . 5,23 The tail extension could indicate also the presence of Mn oxide clusters on the external surface of the zeolite. 24 The band (01MnS-1-MW) and shoulder (02MnS-1-MW, 01MnS-1-K, and 02MnS-1-K) around 320 nm indicate the presence of Mn 3 O 4 . 25,26 Other bands around 380 and 500 nm due to Mn 2 O 3 can be observed. 23,26 The broad shoulder could indicate the existence of a wide range of distorted-octahedral Mn 3+ sites that have transitions with a slightly different energy. 27,28 According to both graphs, higher amounts of manganese (≥0.01 MnO) increase the incorporation on framework sites on one side, while on the other side they also promote the formation of oxides. The last trend is especially observed for conventionally prepared samples. Among the samples prepared by microwave heating, the spectra of the 01MnS-1-MW and 05MnS-1-MW show the most intense band at 260 nm, suggesting the largest amount of framework Mn 3+ in these samples, while this band is even more intense in the samples 01MnS-1-K, 02MnS-1-K, and 05MnS-1-K prepared by conventional synthesis. A more intense broad band at 360 nm can be seen for conventionally prepared samples, indicating that more manganese oxides may be present in these samples. The advantage of microwaveassisted synthesis over conventional synthesis is shown toward the formation of framework Mn 3+ when the reaction solution contains 0.01 MnO according to UV−vis spectra.
Although Raman is a sensitive technique which can detect subtle phase information, e.g. framework metal sites in zeolite and extraframework species, it is difficult to resolve between bands due to overlapping. It can be observed that the spectra in Figure 6 can be divided into three main well-resolved peaks. The first peak can be assigned to the characteristic vibrations of the Si−O bonds in the five-membered ring of the MFI structure (372−383 cm −1 ) 5,29 and of manganese oxides. 30 The second peak, which appears between 550 and 700 cm −1 , is typical for Mn−O stretching of octahedral [MnO 6 ] from manganese oxides, 30,31 while the last vibrations around 800 cm −1 are characteristic (Si−O−Si or Mn−O−Si) for an MFI structure. 5 An additional vibration is present around 876 cm −1 , which may be assigned to μ-oxo-bridged [Mn 2 O] 2+ species. 5,32 The most intense vibrations arose from the 05MnS-1-K sample with the highest amount of manganese. Higher amounts of manganese oxides are present in the samples prepared by conventional hydrothermal synthesis, which is in accordance with UV−vis DR spectra. These spectroscopic results reveal Mn 2+ and Mn 3+ incorporated in the framework with the coexistence of the manganese oxides placed on the silicalite surface.
Mn K-edge XANES analysis was used to determine the average Mn valence state of Mn cations in the MnS-1 samples with different amounts of Mn from 0.01 to 0.05 Mn/Si, synthesized via a hydrothermal conventional or microwave procedure. No spectra of 005MnS-1-MW or 005MnS-1-K samples were obtained due to the low amount of manganese present in both samples. The normalized XANES spectra of the samples are plotted in Figure 7   Better insight into the average Mn coordination and chemical state in the MnS-1 samples can be obtained by Mn K-edge EXAFS analysis, which can directly probe the average local structure around Mn cations in the samples. One representative EXAFS spectrum, i.e. the EXAFS spectrum of the 01MnS1-MW sample synthesized via a hydrothermal microwave procedure, was selected for quantitative EXAFS analysis. The Fourier transform magnitude of the EXAFS spectrum ( Figure 8) exhibits two dominant peaks in the R range between 1 and 4 Å, representing the contributions of photoelectron scattering on the nearest shells of neighbors around the Mn atom. A strong peak in the R range between 1 and 2.2 Å can be attributed to photoelectron backscattering on the nearest oxygen neighbors around Mn. The following composed peak in the R range between 2.5 and 3.4 Å represents the contributions from more distant Mn coordination shells.
The EXAFS spectrum was quantitatively analyzed for the coordination number, distance, and Debye−Waller factor of the nearest coordination shells of neighboring atoms. A quantitative analysis was performed with the IFEFFIT program packages 18 using FEFF6 code, 19 in which the photoelectron scattering paths were calculated ab initio from a presumed distribution of neighboring atoms. The atomic species of neighbors are identified in the fit by their specific scattering factor and phase shift. A good fit is obtained in the k range of 3.5−12 Å −1 and an R range from 1 to 4 Å (Figures 8). A complete list of best-fit parameters obtained by EXAFS fit is given in Table 3.
EXAFS analysis shows that manganese cations of the 01MnS-1-MW sample are octahedrally coordinated to six oxygen atoms in the first coordination shell; three of them are at a shorter distance of 1.91 Å, and three of them at a longer distance of 2.27 Å. Mn neighbors are found in the second coordination sphere at the distance of 3.45 Å and oxygen neighbors at 2.9 and 4.1 Å. The obtained structural parameters cannot be assigned to any pure manganese oxide. The EXAFS result indicates that the sample contains a mixture of manganese oxides in the form of nanoparticles or amorphous forms and framework manganese (typically 3-fold coordination     (Figure 9). Thus, we can conclude that all MnS-1 spectra are composed of slightly different mixtures of manganese oxides, most probably in the form of nanoparticles or amorphous forms.
Catalytic Perfomance. Catalytic styrene epoxidation was performed over MnS-1-MW and MnS-1-K samples with different Mn/Si molar ratios of 0.005, 0.01, 0.02, and 0.05. Figure 10 shows the conversion of styrene after 24 h for materials prepared by microwave-assisted synthesis (MnS-1-MW) and a conventional hydrothermal method (MnS-1-K) with different Mn/Si molar ratios.
For MnS-1-K samples with Mn/Si molar ratios 0.01, 0.02, and 0.05 essentially the same conversion after 24 h reaction time was observed. Evidently, the conversion of styrene is independent of the Mn/Si molar ratio and there is no difference between MnS-1 containing framework Mn 3+ and the extraframework Mn 3+ . In contrast, the conversion over the materials obtained by microwave-assisted synthesis increases with increasing Mn/Si molar ratio. Over the material with Mn/ Si = 0.05, the conversion is even higher than that over the material from the conventional hydrothermal synthesis. In other words, the materials from conventional hydrothermal synthesis and with Mn/Si ≤ 0.02 are catalytically more active than the materials from microwave-assisted synthesis with the same Mn/Si molar ratio, possibly due to the lower size of the crystals and different molar ratio of Mn-containing species. Figure 11 shows the initial reaction rates of the styrene epoxidation over the MnS-1-MW and MnS-1-K materials with different Mn/Si molar ratios, respectively. For both materials, the initial reaction rate increases with increasing Mn/Si molar ratio as expected. However, the increase for the MnS-1-MW materials is more pronounced than for the MnS-1-K materials. The amplitude reduction factor (S 0 2 = 0.8) was determined on MnO and kept fixed during the fit. Uncertainties in the last digit are given in parentheses. The best fit is obtained with the shift of the energy origin ΔE o = −1.0 ± 0.6 eV. The goodness of fit parameter (R factor) is 0.012.