Synthesis and characterization of mcm-41 containing ceo2

245 Journal of Chemistry,Vol. 45 (2), P. 245 - 249, 2007 SYNTHESIS AND CHARACTERIZATION OF MCM-41 CONTAINING CeO2 Received 10 July 2006 PHAM ANH SON, NGUYEN DINH BANG, NGO SY LUONG Faculty of Chemistry, HaNoi University of Science, VietNam National University SUMMARY MCM-41 and cerium containing MCM-41 mesoporous materials were obtained by hydrothermal method under atmospheric pressure (the molar ratio SiO2/CeO2= x, x = 160, 80, 40, 20). The characteristics of all samples were investigated by ThermoGravimetric - Differential Thermal Analysis (TG-DTA), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Energy-Dispersive X-ray spectrometry (EDX) and nitrogen adsorption-desorption isotherms. The results indicated that: particles are sphere, uniform and pore size is about 50 nm; the pore systems are hexagonal structure, ordered arrangement; the samples have high surface area (> 600 m2/g) with narrow pore size distribution curve. Results of EDX showed that the SiO2/CeO2 molar ratios of samples were very similar to the molar ratios in gel. I - INTRODUCTION Porous materials, which have high surface area, high thermal and hydrothermal stability, are widely used in many fields: catalyst for oil refining, petrochemistry and chemical synthesis; adsorbents[1, 2]. Since 1930, microporous materials, especially zeolites, have attracted strong attention as solid acids, base and redox- catalysts. However, zeolites present limitations when large molecules are involved. In 1992, researchers at Mobil Research and Development Corporation synthesized a type of mesoporous materials designated M41S. These of materials have highly ordered hexagonal array of unidimensional pores with a very narrow pore size distribution [3, 4]. They are interesting materials. In this paper, we report the results of studies concerning syntheses of MCM-41 and cerium containing MCM-41 mesoporous materials by hydrothermal method in which sodium silicate and cetyltrimethylammonium bromide were used as source of SiO2 and template, respectively. The characteristics of materials were determined by physical method (XRD, SEM, TEM, EDX, nitrogen adsorption isotherm). II - EXPERIMENTAL. 1. Syntheses of MCM-41 and cerium containing MCM-41 Synthesis of MCM-41: 30.2 g solution of sodium silicate and 30ml distillation water were taken to a flask. The solution was heated at 80oC with medium stir for 1 hour (solution A). Solution B, which included 5 g CTAB and 15 ml water, was heated at 70oC with vigorous stir for 2 hours. After that, two solutions were cooled naturally to room temperature. Step 1: Solution A was dropped slowly into solution B with violent stir. After that, the stir was continued for 1 hour at room temperature. Step 2: The gel was taken in a 300 ml flask and refluxed with vigorous stir for 24 hours at 100oC and atmospheric pressure. 246 Step 3: The flask was cooled to room temperature, pH of the solution was adjusted to 11 by CH3COOH 30% solution or NH3 25% solution. After that, the solution was refluxed for 24 hours at 100oC with vigorous stir. Step 2 and step 3 were repeated three times. The obtained materials were centrifugalized and washed with distillation water (8 - 10 times) until pH of washed water achieved ~7. The product was dried at 120oC in 24 hours and then calcined at 550oC in 10 hours in the presence of air. Synthesis of cerium containing MCM-41 (Ce-MCM-41): The above process was repeated with slight modification: After complete addition solution A into solution B, solution that contain calculated amount of Ce(SO4)2 was added. 2. Characterization - Thermogravimetric-Differential Thermal Analyses (TG-DTA) were carried out on DSC- SDT2960 TA (USA) under a flow of 2 l/h at a heating rate of 10oC/min to 800oC. - All X-Ray Diffraction patterns were recorded on Bruker D5005 (Germany) with CuK
radiation between 1 and 100 (step: 0.005, steptime: 1s). - Transmission Electron Macrographs (TEM) were obtained in JEOL JEM-1010 Electron Microscope (Japan) with 80 kV acceleration voltage. - Scanning Electron Micrographs (SEM) were obtained in JEOL JEM-5410LV Scanning Microscope (Japan). - The metal contents of samples were determined by EDX analyses with module of EDS ISIS 300 (Oxford, England) attached to JEOL JEM 5410LV Scanning Microscope. - The specific surface area and pore size distribution were determined by BET method from Nitrogen adsorption-desorption isotherms at 77K using Micromeritics ASAP-2010 (Micromeritics, USA). III - RESULTS AND DISCUSSION 1. Thermal Analysis Thermogravimetric-Differential Thermal analyses of MCM-41 and Ce-MCM-41 were carried out under the same experimental conditions. The patterns are very similar. The curves of TG-DTA are shown in Fig. 1. Total mass losses are about 39% including three steps. In the first step (50 - 120oC), the mass loss is about 4.2 - 4.8% due to desorption of physisorbed water held in the pores. In the second step (150 - 400oC), the mass loss is about 28 - 30% including two small steps which overlap each other: 1) decomposition of templates and oxidation of eliminated gases (according to exothermal peak on DTA curve); 2) remove of coke formed in the above step by the decomposition of templates. In the last step (400 - 500oC), the mass loss is about 5 - 7% due to the loss of water formed by the condensation of silanol groups. The TG-DTA curves show that almost all the template, including the water formed due to condensation of silanol groups, is lost completely from the pore system at 550oC. 2. The results of EDX and XRD All the samples of MCM-41 and Ce-MCM- 41 were analyzed EDX (Energy-Dispersive X- ray spectrometer) for determining elemental composition and the SiO2/CeO2 molar ratios of sample (table 1). The results in table 1 indicate that most of cerium were incorporated into the framework position or walls of silica network of MCM-41. All samples were recorded XRD. From 1 to 6o on the patterns, four peaks (d100, d110, d200, d210) which are characteristics of MCM-41 structure (hexagonal lattice symmetry) appear (Figs. 2a, 2b). When content of CeO2 increases, value of d100 increases lightly due to size of Ce 4+ is larger than size of Si4+ in the network. However, reflection from 10 - 50o showed that the pore walls are SiO2 amorphous (have no peak on the XRD pattern, Fig. 2c). Using data obtained from XRD, we can calculate the distance between two centres of pores by below formula [5]: 3 2 100 0 d a = 247 Figure 2: XRD patterns of MCM-41 (a), Ce-MCM-41 (40) (b) and XRD pattern with 2 = 10 - 50o (c) Table 2 indicates values of distance dhkl and distance between two centre of pores a0. Figure 1: TG-DTA curves of MCM-41 (a) and Ce-MCM-41 (b) Table 1: Results of elemental analysis of Ce-MCM-41 samples Sample % Element of Si % Element of Ce Ratio of (Si/Ce)sam (a) Ratio of (Si/Ce)gel (b) Ce-MCM-41 (160) 99.33 0.67 148.3 160 Ce-MCM-41 (80) 98.74 1.26 78.4 80 Ce-MCM-41 (40) 97.55 2.45 39.8 40 Ce-MCM-41 (20) 95.25 4.75 20.1 20 (a) SiO2/CeO2 molar ratios of samples obtained from EDX method; (b) SiO2/CeO2 molar ratios in gel. (a) (a) (b) (b) (c) 248 Table 2: Distances dhkl and a0 of MCM-41 and Ce-MCM-41 samples Samples d100, Å d110, Å d200,Å d210,Å a0, Å Si-MCM-41 42.57 24.44 21.17 15.95 49.2 Ce-MCM-41 (160) 41.79 24.05 20.94 15.65 48.3 Ce-MCM-41 (80) 42.32 24.21 21.05 15.70 48.9 Ce-MCM-41 (40) 41.94 24.10 21.03 15.70 48.4 Ce-MCM-41 (20) 41.95 24.12 20.91 15.80 48.4 3. SEM and TEM The SEM micrographs of MCM-41 and Ce-MCM-41 samples are shown in Fig. 3. Figure 3: Scanning Electron Micrographs of MCM-41 (a) and Ce-MCM-41 (b) These micrographs showed that particle morphology is sphere and uniform. The particle size is about 40 - 60 nm. Figure 4: Transmission Electron Micrographs of MCM-41 (a) and Ce-MCM-41 (b) Transmission Electron Micrographs revealed that arrangement of pores is ordered; pore structure is hexagonal; pore size is about 3 nm; thickness of pore wall is about 1 nm. 3. N2 adsorption - desorption isotherm All the samples showed that isotherms of type IV have inflection around P/P0 = 0.3 - 0.4. This is a characteristic of MCM-41, ordered mesoporous materials. These curves indicate the uniformity of the narrow pore size distribution. The specific BET surface area and average pore diameters are (a) (b) (a) (b) 249 calculated from a N2 adsorption isotherm using BJH model. All results are shown in table 3. Relative pressure, P/P0 Relative pressure, P/P0 Figure 5: N2 adsorption - desorption isotherm of MCM-41 (a) and Ce-MCM-41 (40) (b) Pore size distribution curves of MCM-41 (c) and Ce-MCM-41 (40) (d) Table 3: Nitrogen sorption pore diameter, BET surface area, distance between two pore a0, wall thickness of MCM-41 and Ce-MCM-41 samples Sample SBET, m 2/g Pore diameter d0, Å Wall thickness, Å Si-MCM-41 743.89 41.1 8.1 Ce-MCM-41 (160) 743.75 44.0 4.3 Ce-MCM-41 (80) 646.48 35.0 13.9 Ce-MCM-41 (40) 813.68 37.1 11.3 Ce-MCM-41 (20) 707.14 33.4 15.0 Wall thickness = a0 - d0 (The values of a0 are indicated in table 2). IV - CONCLUSIONS MCM-41 and Ce-MCM-41 mesoporous materials were prepared by hydrothermal method using sodium silicate and CTAB as sources of SiO2 and structure direction, respectively. The SEM micrographs indicate that particles are sphere and uniform and pore size is about 50 nm. The pore systems, which are hexagonal structure with ordered arrangement, are showed by XRD and TEM. 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