| Ordered mesoporous materials possess regular pore structure and tunable pore size of 1.5-50nm, breaking the pore size constraint (＜1.5 nm) of microporous zeolites. Instead of the mostly used single molecules, the M41S materials brought a new approach in the synthesis of molecular sieves, which used self-assembled molecular aggregates as templates. Beside this, mesoporous materials are currently under intense investigation with respect to their utilities as industrial catalysts, adsorbents, biological catalysts, photocatalysis, environmental protection, drug carriers, optics, electrode materials and host-guest chemistry.Well-ordered mesoporous molybdenum and tungsten oxides were synthesized using quaternary ammonium Gemini surfactant [CmH2m+1N+(CH3)2-(CH2)2-N+(CH3)2 CmH2m+1]·2Br-(denoted as Cm-2-m, m=12, 14 and 16) as templating agents. The effects of different experimental conditions on the mesostructure and surface properties of products were investigated by X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), nitrogen adsorption-desorption technique and temperature programmed analysis (TGA). Interactions between Gemini surfactants and disodium ethylenediamine tetraacetic acid (Na2EDTA) were investigated by surface tension technique, rheological measurements, transmission electron microscopy (TEM), electrical conductivity and proton nuclear magnetic resonance (1H NMR). Highly-ordered mesoporous molybdenum and tungsten oxides were prepared using Gemini/Na2EDTA co-assemblies as templates.A series of well-ordered lamellar mesoporous molybdenum oxides were prepared using Gemini surfactant Cm-2-m(m=12, 14 and 16) as templating agents and ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24·4H2 as precursor. Results showed that contrary to complete structure collapse after removing tetradecyltrimethylammonium bromide (TTAB, which could be thought of the monomer of C14-2-14) from molybdenum oxide-TTAB composite, the lamellar mesostructure was retained after removal of C14-2-14 from corresponding composite. SEM images showed that the sample was mainly consisted of aggregated particles. The intercalated surfactants adopted bilayer arrangement with a tilt angle between the molybdenum oxide layers. The layer spacing of the sample increased with the increase of relative content and length of alkyl chain of surfactant. Concentration of surfactant, pH value of reaction system and hydrothermal temperature paid important effects on the ordering of the obtained mesostructures. Stronger self-assembly ability and stronger electrical interaction with molybdenum oxide made C14-2-14 more effective than TTAB in the synthesis of mesoporous molybdenum oxide material. Mesostructured molybdenum oxides prepared by sodium molybdate showed much poorer ordering compared with that of sample obtained from ammonium molybdate. Ammonium molybdate was a better precursor in synthesis of mesoporous molybdenum oxides.Well-ordered hexagonal mesoporous tungsten oxides were synthesized using Cm-2-m as templating agents and sodium tungstate as precursor. SEM images revealed the belt morphology of the obtained material. XRD results indicated that the pore walls of the material were consisted of hexagonal/orthorhombic WO3 mixed crystals. Molar ratio of W/saa, concentration of surfactants, pH value of reaction system and hydrothermal temperature paid important effects on the ordering of the mesopores and the crystallinity of the pore walls. Sample prepared by TTAB showed much lower hydrothermal stability than that of sample obtained from C14-2-14. C14-2-14 showed distant advantages over TTAB in synthesis of mesoporous tungsten oxides.Interactions between C14-2-14 and Na2EDTA were investigated in detail. Na2EDTA could penetrate into the close layer of the C14-2-14 micelles and replace parts of the Br-. Stronger interactions with C14-2-14 comparing to Br" made EDTA2- more sensitive to surfactants. Shielding effects of EDTA2- reduced the repulsion between head groups and facilitated the aggregation of Gemini surfactants. Increasing the concentration of Na2EDTA led to the strengthening of this facilitation. However, further increase of the surfactant concentration paid no further effect on the properties of the solution after a certain concentration value.Highly-ordered mesoporous molybdenum and tungsten oxides were synthesized using Gemini surfactant/Na2EDTA co-assemblies as templates. The addition of Na2EDTA made an appreciable impact on the ordering and hydrothermal stability of the products. The ordering of the samples increased with the increase of the molar ratio of Na2EDTA/saa. The stability of the mesostructure improved due to the coordination between Na2EDTA and Mo(W), which supported a scaffold for the material during the hydrothermal treatments. This EDTA referred approach might provide a new approach to improve the stability of other transition metal oxide materials.
|
PDF Full Text Request