Synthesis, Characterization And Catalytic Performance Of Modified Mesoporous MCM-41 Molecular Sieves

The M41s mesoporous molecular sieves successfully synthesized by Mobil researchers in 1992 have attracted great interest due to their large specific surface area, uniform pore size distribution and rich surface functional groups. However, the nature of the neutral framework with few lacuna, weak acid intensity, poor ion-exchange ability and hydrothermal stability of M41s materials, limits their potential in catalysis, adsorption and separation, environmental protection and so on. Molecular sieves with transition metals incorporated into the framework have become the hot topic. It has been demonstrated that V-MCM-41 materials has poor hydrothermal stability and thermal stability, which is considered to be a serious problem in any practical application of this material that requires structural integrity. The materials with high hydrothermal and thermal stability become the aim of several scientists. The preparation of mesoporous materials with tunable pore size is the efficient method for its application as novel catalytic materials.In this work, the industrial inorganic silicate Na2O·3.3SiO2 was selected as a cheaper source of silica instead of expensive organic precursors, using CH3(CH2)15·(CH3)3NBr (CTAB) as surfactant, and the supported mesoporous materials was prepared by direct hydrothermal synthesis. The framework structure , texture, thermal stability and hydrothermal stability of V-MCM-41 materials had been investigated; the X-ray diffraction (XRD), N2 adsorption/desorption, Fourier-transformed infrared spectroscopy(FT-IR), diffuse reflectance UV-Vis spectra, inductively coupled plasma technique (ICP), scanning electron microscopy (SEM),Transmission electron microscopy (TEM) and differential thermogravimetric analysis(TG/DTG) were utilized for the characterizations of the supported mesoporous materials. The aim of the current work was to synthesize and characterize the mesoporous molecular sieves, investigate the thermal and hydrothermal stabilities of the samples, discuss the synthesis mechanism of the materials and probe the method of the tunable pore size of the materials. This dissertation mainly was consisted by the following several aspects.1. V-MCM-41 mesoporous materials were synthesized by hydrothermal method. The results indicated that several factors affected the synthesis. The crystallizing temperature, pH value and ratio of Si and CTAB were the most important factors. The texture parameters of the materials were affected by the following factors, active component V amounts, crystallizing time, aging time, calcination atmosphere, the order of materials adding and so on. The proper parameters were selected: n (Si): n (CTAB): n (V)=1: 0.2: 0.08, pH=9.5, crystallizing at 110℃for 48 h, no aging, adding vanadium source first and then silica source, calcination at 550℃for 6 h in air.2. V-MCM-41 mesoporous materials with different silica source were synthesized by hydrothermal method with Na2SiO3·9H2O, colloidal silica, and Na2O·3.3SiO2 as silica source, which was noted as VMC, VMS and VMH, respectively. V-MCM-41 mesoporous materials prepared by tetraethylorthosilicate.(TEOS)in acid condition and alkaline condition were noted as VMT-ac and VMT-al, respectively. The results indicated that .the synthesis processes were different with different silica sources, especially in the crystallizing temperature and pH value. The specific surface area of materials prepared by inorganic silica is lager than that of organic silica. It was due to the different speed of hydrolyzation and polymerization of different silica sources, which resulted in different polymer form. The different polymer would make difference with the reciprocity between the surfactants and silica source. The materials were prepared by inorganic silica in alkali condition, but the materials obtained by organic silica were synthesized in acid conditions. The optimal parameters of synthesis for the four silica were listed in following: crystallizing temperature at 110℃and pH value≈5.5 for VMT-ac materials; crystallizing temperature at 110℃and pH value≈10.5 for VMT-al materials; crystallizing temperature at 110℃and pH value≈9.5 for VMH materials; crystallizing temperature at 125℃and pH value≈10.5 for VMC materials; crystallizing temperature at 125℃and pH value≈11.5 for VMS materials.3. The hydrothermal and thermal stability of V-MCM-41 materials using Na2O·3.3SiO2 as silica source were investigated. The surface area, pore volume and pore size of the V-MCM-41 materials decreased with augmentation of hydrothermal treatment time. The mesoporous structure of V-MCM-41 materials was retained, the remaining ratio of surface area, pore volume and pore diameter were 84.3%, 81.6% and 91.9%, respectively, for the sample under hydrothermal treatment for 8 d. Clearly, well-resolved (100), (110) and (200) diffraction peaks of V-MCM-41 were observed from the XRD picture, which indicated that the mesopore-structure was kept almost totally even after 8 d aging in boiling water. The diffraction peaks became slightly broad and peak intensities decreased slightly with the augmentation of aging time in the boiling water, which indicated that the order of the materials was weaken with the increasing of aging in the boiling water. Furthermore, after 10d hydrothermal treatment, the mesoporous structure of this material was destroyed. However, the remaining ratio of specific surface area was 80.7%. The hydrothermal treatment indicated the mesoporous materials V-MCM-41 was of good hydrothermal stability. The sample was thermal treated at 900℃for 12 h, the mesopore-structure of this materials was retained. The remaining ratio of specific surface area and pore diameter of the sample were 83.71 % and 68.2 %. No remarkable changes were observed for the isotherms of the V-MCM-41 sample calcined at air and nitrogen/air atmosphere, which indicated that the samples could resist the released heat of template combustion and desorption when the samples were calcined in the air, which farther indicated that the samples were of high thermal stability.4. The different formation mechanism in the different system would explain the synthesis of the mesoporous materials. The formation mechanism of the materials prepared by the Na2O·3.3SiO2 was investigated. The effect of the crystallizing temperature could be explained by the Cooperative Formation Mechanism. The effect of the mol ratio of surfactants and silica source could be attributed to the Liquid Crystal Template Mechanism. The effect of the pH value was ascribed as Lamellar Transformation Hexangular Mechanism. The formation mechanism of this material could be explained by the cooperative results of several formation mechanisms, not only by one formation mechanism.5. The different surfactant amounts in the mesoporous materials synthesis can result in the different pore size of the materials. The pore size of the materials changed from 3.52 nm to 5.26 nm with the CTAB amounts shift; the amounts of CH3(CH2)11·(CH3)3NBr (DTAB) could change the pore size of the materials: from 2.21 to 4.15 nm; the pore size of the materials changed from 4.21 nm to 10.91 nm using P123 surfactant. The pore size of the materials was linear with length of long-chain alkyl of surfactants. The longer alkyl of surfactants, the larger pore size of the materials was. The pore size shifted from 2.21 to 3.94 nm with less DTAB and more CTAB. The organic amine was selected to expand the pore size of these materials. the (C2H5)3N (TEA) was used as surfactants, the pore size can be enlarged to 18.4 nm and specific surface area was 189 m2/g. the pore size could be changed with the different ratio of CTAB and CH3(CH2)11N(CH3)2 (DMDA), the range of pore size was 3.94~5.49 nm. The pore size didn't increase with the increasing of amounts of organic amine. The organic amine could destroy the expansion of pore size with the condition of adding excess amounts of amine. Ammonia hydrothermal treatment couldn't expand the pore size which indicated the V-MCM-41 materials remained good crystal structure. The application of excessive amounts of DMDA was somewhat less effective for expansion of mesopore pore size. It was shown that the use of DMDA and TEA to restructure MCM-41 with 3.94 nm pores prepared under conventional hydrothermal synthesis afforded mesoporous silica with up to 6.62 and 9.30 nm pores, respectively. 2.36 times expansion of pore size for TEA and 68% improvement for DMDA was achieved successfully.6. The catalytic performance of catalysts using MCM-41 supports was investigated.①the selective oxidation of styrene using hydrogen peroxide as oxidant over V-MCM-41 samples was investigated; the phenylacetic acid was the principal most important product. The selectivity of phenylacetic acid was 49.44% and the conversion of H2O2 was 55.9% for the sample of V-MCM-41, which was calcined at 550 ?C in flowing nitrogen at first, then in air. The sample of V-MCM-41, which was calcined only in air at 550℃gave 63% H2O2 conversion and 38.74% selectivity in phenylacetic acid.②Mesoporous MCM-41 zeolite supported HRh(CO)(PPh3)3 complex for isobutene hydroformylation was prepared. Compared with homogeneous catalysts, the catalysts were of high catalytic performance and high selectivity of isovaleraldehyde besides the advantage of easy separation of catalyst from the product mixture.