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Synthesis, Characterization and Formation Mechanism of Mesoporous Molybdenum and Tungsten Oxides Based on Gemini Surfactants

Author: YuXiaoJuan
Tutor: XuZhengHe；HanShuHua
School: Shandong University
Course: Physical and chemical
Keywords: Mesoporous materials Gemini Surfactants Aluminum oxide Tungsten oxide EDTA
CLC: TB383.4
Type: PhD thesis
Year: 2009
Downloads: 213
Quote: 1
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Abstract


Nanoscale pore structure of ordered mesoporous materials with regular adjustable aperture range between 1.5-50nm, broke through the the the microporous zeolites aperture limit (<1.5 nm). Template synthesis of ordered mesoporous materials based on molecular self-assembly to form aggregates, changing the methods of the past, a small molecule synthetic molecular sieves as the template, and brings new opportunities for molecular sieve class materials synthesis. Mesoporous material in the petrochemical, bio-catalysis, adsorption and separation, photocatalysis, environmental protection, medicine carrier, the field of light-induced light-emitting materials, electrode materials and host-guest chemistry attractive prospect, by physics, chemistry and materials science other field researchers highly concerned, to become one of the hot spots of interdisciplinary research. In this paper, a series of quaternary ammonium Gemini surfactants [C m H 2m 1 N (CH 3 ) 2 - (CH 2 ) 2 -N (CH 3 ) 2 the C m H 2m 1 ] · 2Br - (abbreviated as C m-2-m , m = 12,14 and 16) as template synthesis of ordered mesoporous molybdenum oxide and tungsten oxide. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption - desorption, thermal gravimetric analysis (TGA) technology to study a variety of response factors (surfactant / the ratio of the inorganic precursor, surfactant concentration, chain length, the synthesis system pH (s), water and heat the reaction conditions, etc.) the synthesized mesoporous molybdenum, tungsten oxide structure and the surface properties of. By surface tension, rheological measurements, transmission electron microscopy (TEM), electrical conductivity, and proton nuclear magnetic resonance characterization by means of a dilute solution the Gemini surfactant Disodium EDTA (Na 2 EDTA) interaction Gemini surfactant / Na 2 EDTA composite system as a template synthesis of highly ordered mesoporous molybdenum, tungsten oxide materials. C 14-2-14 as template agent, ammonium molybdate inorganic precursors prepared by surfactant - molybdenum oxide composite material has a highly ordered layered mesoscopic structures, solvent extraction after removal of the surfactant, the mesoscopic structures remain formation of mesoporous materials, the material presented flaky appearance. The surfactant is inserted by way of ion exchange, double-like arrangement between the molybdenum oxide layer having a certain inclination angle to the of polymolybdate root layered voids. The relative content of the surfactant is higher, the alkyl chain length, the greater the layer spacing of the resultant layered mesoscopic structure. Surfactant concentration, pH and temperature of the hydrothermal reaction system are ordering on the of molybdenum oxide mesoscopic structure. Higher template concentration, moderate pH and temperature of the hydrothermal conducive to the formation of channels in order high mesoscopic materials. The molybdenum oxide orderly use of tetradecyl trimethyl ammonium bromide (TTAB Molecules under the same conditions, can be regarded as monomer C 14-2-14 ) as a template to prepare obtained mesoscopic lower removed by extraction the TTAB after layered structure completely collapsed. The Gemini surfactant strong self-assembly ability and the strong interaction of the precursor to become superior than TTAB performance more templating agent. Compared with ammonium molybdate, sodium molybdate as precursor obtained surface-active agent - molybdenum oxide mesoscopic materials bore less orderly. Ordered mesoporous materials synthesis, ammonium molybdate is than sodium molybdate performance more superior precursor. C 14-2-14 template agent sodium tungstate inorganic precursor tungsten oxide materials prepared by a highly ordered two-dimensional hexagonal mesoporous structure, materials have regular ribbon The appearance, the hole wall by a hexagonal / orthogonal in WO 3 mixing crystalline composition. The relative content of the surface active agent, concentration, pH value and the temperature of the hydrothermal pore ordered and the crystallinity of the bore wall have a major impact. Under the same conditions using TTAB as template to prepare mesoporous tungsten oxide hydrothermal stability is low, and the hole wall composed of amorphous material. Compared with the TTAB, Gemini surfactant is the superior performance of mesoporous tungsten oxide template agent. Gemini surfactant dilute solution the of Na 2 EDTA interaction by surface tension, rheological measurements, transmission electron microscopy (TEM), electrical conductivity, and proton nuclear magnetic resonance characterization means. EDTA 2 - can penetrate the to enter C 14-2-14 micelle dense layer close layer Br - swapped out. The relative Br - , EDTA 2 - and C stronger interaction between 14-2-14 , which makes of Na < sub> 2 EDTA ratio of NaBr more significant impact on the surfactant solution. EDTA 2 - added can shield the positive charge of the head group of the surfactant, to weaken the electrostatic repulsion between the head groups, to promote surfactant Ordering more closely. This role with EDTA 2 - concentration increased and enhanced, but after more than a certain limit value continues to increase the EDTA 2 - concentration is no longer on the system obvious impact. To C 14-2-14 / Na 2 EDTA composite system as a template to be synthesized at low surfactant concentrations of highly ordered mesoporous molybdenum, tungsten oxide physical materials, and Na 2 EDTA join the pores of mesoporous materials orderly and hydrothermal stability. Na 2 EDTA / surfactant molar ratio of the greater, the higher the order of the pores of the resulting material. The the na 2 EDTA with molybdenum, tungsten atom strong ligand effect so that the material after the high-temperature hydrothermal treatment process has a structural support, its structural strength is relatively not added the of Na 2 EDTA increased significantly. EDTA with a lot of metal elements can ligand can be inferred that the addition of EDTA may provide new ideas to improve the hydrothermal stability of other transition metal oxide materials.

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