A series of mesoporous materials modified by manganese were prepared using surfactants with different alkyl chain length by hydrothermal method. X-ray diffraction (XRD), TEM imaging, nitrogen adsorption-desorbtion isotherm, UV-VIS diffuse reflectance spectroscopy, X-ray Absorption Fine Structure and Temperature Programmed Reduction (TPR), were employed to characterize the synthesized catalysts, and the results indicate the successful incorporation of Mn ions in the silica matrix of MCM-41. X-ray powder diffraction showed the hexagonal mesopore structure for Mn-MCM-4 and the N2 adsorption–desorption isotherms give as important information about the pore size distribution and BET surface obtained by using BJH method. UV-VIS spectra confirmed the presence of Mn2+/Mn3+ species in the framework of the mesorporous materials, and X-Ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) were performed to determine the oxidation state and local symmetry of transition metal ions incorporated in the MCM-41 molecular sieve. The aim of our investigations was to study the influence of the pore size of catalytic centers (Mn ions) on their redox properties.
Mesoporous molecular sieve materials have opened new perspectives for catalytic processes, based on novel principles. In 1969 the synthesis of an ordered mesoporous material, was recorded in literature but the remarkable properties of this material were not observed1. The discovery in the 1990s of MCM-41 by researchers from the Mobil Oil Corporation2,3 open a new way for the preparation, characterization and application of mesoporous molecular sieves. These materials present remarkable features such as: high surface area, narrow pore size distribution, flexible framework structure, which make these materials interesting for their use in different applications. Catalysts efficiency for a specific reaction depends of the design of the active site. It was shown that the characteristics of MCM-41 are strongly affected by the synthesis conditions such as: mole ratio of the components in the synthesis solution, autoclaving time, pH, silica source, etc4.
One of the most important properties of MCM-41 is the pore diameter, which can be independently changed from the chemical composition of the pore walls by changing the length of the organic template molecules used during synthesis.
Highly ordered Mn-MCM-41 with the metal ions incorporated in the silica matrix has been prepared for different types of reactions5-7. We can synthesize highly ordered Mn-MCM-41 samples with the manganese ions dispersed on an atomic scale in the silica matrix of the mesoporous material.
In this work we report a systematic investigation of the properties of modified silica mesoporous materials of different pore dimensions. Nitrogen adsorption-desorbtion isotherm and X-ray diffraction (XRD) was used for investigating surface area, structure and pore size distribution, this property was confirmed by TEM images. UV-VIS spectroscopy and X-ray Absorption Fine Structure is important in our investigation for determining the oxidation state and local symmetry. Temperature Programmed Reduction (TPR) is an useful technique for investigating the redox properties of synthesized materials. The effect of alkyl chain length on the Mn-MCM-41 structure is compared to the similar Mn-MCM-41 results published elsewhere8.
Silica sources used in the synthesis of the surfactant was Cab-O-Sil from Riedel-deHaen and tetramethylamonium silicate form Aldrich, and the manganese source was MnSO4 ● H2O from Aldrich. The surfactant solution was prepared by ion exchanging of different quantities of cetil trimethyl ammonium bromide aqueous solution with ion exchange resin Amberjet 4400 (Aldrich). The pH was adjusted from the synthesis solution using acetic acid.
For obtaining our catalysts we follow the recipe described in the literature9,10,11. Two series of materials have been prepared and here we describe the preparation process for the sample C14 Mn-MCM-41 with 2wt% Mn. The silica matrix of MCM-41 has been synthesized using surfactants with different alkyl chain length C14 MCM-41 and C16 MCM-41 respectively. The manganese ions were incorporated into the framework using the hydrothermal method. The pH was adjusted at 11.5 by adding acetic acid in the synthesis solutions, and the final solution was thoroughly mixed and then poured intro a recipient autoclave resistant, and placed for 6 days at 1000C in an autoclave. After this time the solution was cooled at room temperature, filtered and washed with deionized water. The solid obtained by filtration was dried for 12h at 750C. The resulting solid was finally calcinated at a constant heating rate of 320/h to 5400C for 18h under He flow and 5h under air at the same temperature to remove the surfactant residue.
3. Results and Discussion
Characterization of the catalysts
Nitrogen physisorption is used to provide the primary information about the structure of our materials. Comparatively with XRD or TEM witch give as information about limited part of the samples, the nitrogen physisorption it is used to compare the structure of each sample because it shows the volume averaged value. Nitrogen adsorbtion-desorbtion isotherms were measured at 77K whit a ASAP Analyzer from Micrometrics. The BET surface area, average pore diameter, pore volume, were determined by multipoint N2 adsorbtion - desorption isotherms. Figure 1 shows the distribution of the pore for as calcinated samples. It can be observed that the as synthesized mesoporos materials C14 and C16 Mn-MCM-41 have uniform pore size, and that their pore size decrease with the decreasing of alkyl chain lengths. The diameter distribution of the pore can be independently changed from the chemical composition of the pore walls by changing the length of the organic template molecules during synthesis. From the literature it is well know that the longer surfactant chain length results the higher relative pressure of the capillary condensation13. All samples show structural order regardless of the surfactant chain length. The higher the slope, the pores are more uniform. This can be further translated into pore size distributions obtained using the Barret-Joyner-Halenda method. From the pore size distribution were determined the full width at half maximum (fwhm) for each sample. The corresponding textural parameters calculated by N2 adsorbtion desorption isotherms are listed in Table 1. It can be see that the catalysts maintained characteristic type IV isotherms with hysteresis loop, witch are the characteristic of mesoporous materials with uniform mesopores. Longer surfactant chain lengths result in narrower fwhm and steeper slopes for the capillary condensation. Pore volume also agrees with the decrease in surface area. The sample obtained using C16 surfactant present a good structure comparatively with the samples using C14 surfactants in the synthesis solution.
X-ray diffraction (XRD)
This method was employed to determine if the synthesized catalysts have the characteristic hexagonal pore structure for MCM-41 materials12. The measurement was performed using a Brooker X-ray diffractometer (Cu Kα, voltage 40 kV and 5mA intensity). XRD patterns of C14, C16 Mn-MCM-41 were taken in the 2θ range 1-100 at a rate 10/ min in steps of 0.01. The XRD for our catalysts is illustrated in figure 2 and we can say that our catalysts present a structure characteristic for mesoporous materials sieve well ordered. Like in the case of nitrogen physisorption results, the MCM-41 structure described by XRD is well ordered. When recorded with a regular powder X-ray diffractometer, the XRD pattern of a well ordered MCM-41 sample usually shows one main peak (100) and three other small peaks (110, 200, and 210) 9.
For our samples shows a sharp d100 reflection line in the 2θ range 1.9-30, and two broad peaks at 2θ range 3.6-5.50 for (110) and (200) planes in MCM-41. In case of C14 and C16 Mn-MCM-41 we can see three peaks (100, 110, 200) by using powder at the X-ray diffractometer, showing that the sample synthesized by this method has good structural order, consistent with the N2 adsorption isotherms and the resulting PSD.
Transmission Electron Microscopy (TEM)
Transmission electron microscopy is an useful and powerful techniques to image nanoscale materials and to investigating the pore structure of Mn-MCM-41 samples and diameter of metallic clusters. The TEM images of Mn-MCM-41 were obtained using on a Tecnai F20 200kV microscope. By using TEM we can observe modulations in chemical identity, crystal orientation and electronic structure of the samples as the regular absorption based imaging. Images collected for our samples are presented in , and shows long range order of the synthesized materials for C14 and C16 Mn-MCM-41 2wt% Mn loading presenting a good structure.
To investigate the local environment of manganese in the MCM-41 material, samples were analyzed by DR UV-vis spectroscopy and X-ray absorption spectroscopy. UV Vis spectra when Mn-MCM-41 samples present two different groups of peaks in the UV-VIS region. The UV-VIS spectra of the prepared catalysts show a main absorption band centered near 250 nm and a wide band about 500 nm which covers almost all the visible range of the spectrum. This absorption maximum at 500 nm is assigned to Mn2+ overlap with the literature reports, presented by Vetrivel and Parida respectively17. As literature papers absorption near 250 nm is associated with O2-→ Mn3+ charge transfer transition18,19. The sensitivities of the band at 250 and 500nm may be different, and thus the intensities of these bands cannot be used to evaluate the report Mn2+ / Mn3+ in the samples.
For X-ray absorbtion spectra we used the X18B beam line at the National Syncrotron Light Source, Brookhaven National Laboratory. The spectra were obtained at the Mn K edge (6535eV) at room temperature. X–Ray absorption spectroscopy is a technique based on the absorption of X-rays and extraction of photoelectrons that are scattered by neighboring atoms14. An EXAFS spectrum shows these interference effects and can be used to obtain the oxidation state of the element as well as detailed information on the interatomic distances and the number and type of neighbors of the absorbing atoms14. A Fourier transform of the characteristic EXAFS function yields a radial distribution function, which gives the distance from the absorbing atoms16. All data was processed with the IFEFFIT software. The oxidation state of Mn in the Mn-MCM-41 catalysts can be investigated in the derivative of the XANES spectra collected around the Mn K-edge
Mn (III) is a major component in all of the Mn incorporated MCM-41 catalysts as shown by the peak at 6547 eV characteristic of Mn2O3. The broad first peak in the catalyst spectra extended further to the left than in the spectra of Mn(III) oxide also suggests the presence of contributions from MnO.
Temperature programmed reduction (TPR)
The stability and reducibility of the transition metal ions incorporated in the MCM-41 silica framework was assessed by temperature programmed reduction using a CHEMBET QuantaChrome Instrument and the results are presented in figure 6. This technique is convenient for studying the reduction behavior of supported oxide catalysts and also gives as important information about material species, stability, reducibility and metal distribution. Approximately 100 mg of each sample was loaded into a quartz cell and heated to 5000C at a constant heating rate of 50/min in Ar flow, held at this temperature for 1h and subsequently cooled to room temperature. We followed this procedure to clean the surface of the catalysts before TPR investigation. TPR was take place in 5% H2 in the balance from 500C to 9000C at 50C/min in the temperature range from 250C to 9000C for investigating the reduction behavior of the manganese species in Mn-MCM-41. Manganese is completely reduced after we hold the temperature constant at 9000C for 1 h. The composition of the gas stream leaving the quartz cell was monitored using a thermal conductivity detector (TCD). If the sample is exposed to air can be reoxidized and can form manganese oxide at the surface of silica walls. If we tested again the sample we obtained different patterns compared to the first TPR run. The TPR method revealed direct interaction of transition oxide phases in the silica matrix.
The mesoporous materials synthesized with Mn isomorphously substituted were prepared by hydrothermal method. Like in case of Co-MCM-41 samples, the pore diameter can be controlled by changing the surfactant chain length. Mn was immobilized in a series of mesoporous material sieve with different pore sizes and the effect of pore size on the activity of the catalysts was studied. For Mn incorporated in MCM-41 materials, the catalytic activity was incorporated with the increase of pore size of catalytic support. Manganese was incorporated in the silica matrix and present good structure. X-ray diffraction results indicate that our sample is well ordered. UV-VIS spectroscopy supported that Mn2+ and Mn3+ in Mn-MCM-41 C14 and C16 framework are co-ordinated to silica surface. The sample we synthesized present high stability against reduction.
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