Chemically modified ceramic membranes – Study of structural and transport properties
Prof. Dr. Ing. D.H.A. Blank
Dr. Ir. J.E. ten Elshof
This PhD thesis describes the development and separation properties of the composite membrane system α-alumina(macroporous)/γ-alumina(mesoporous)/hybrid silica (microporous). The influence of chemical modification of the surface and pores of the mesoporous and microporous layers of the membrane by covalently bonded organic groups was studied.
Two synthesis strategies to make more hydrophobic layers have been followed in this regard. The first is grafting of the surface of the intermediate mesoporous layer with organochlorosilanes, the second one is the in situ hydrolysis and condensation of organosilane precursors to prepare microporous top layers.
The grafting of γ-alumina powders and membranes with organochlorosilanes was addressed. The effects of covalently bonded bulky long chain organosilanes and multifunctional organosilanes on the efficiency of the grafting process were investigated. Unsupported powders were characterized by nitrogen and CO2 adsorption techniques, thermogravimetric analysis and SEM. The effect of modification of γ-alumina membranes was studied by XPS, permporometry, and solvent permeation experiments. Characterization of unsupported modified γ-alumina powders and results obtained on modified γ-alumina membranes substantiate each other. The multifunctional precursors formed a polymerized network inside the mesopores which imparted greater resistance to flow of solvents as well as a more hydrophobic character.
A more hydrophobic microporous material than silica was developed from 1,2-bis(triethoxysilyl)ethane and methyltriethoxy silane precursors. This yielded hybrid nanosized sols of 2-8 nm in ethanol, which were characterized by dynamic light scattering and mass spectrometry. The particle size of the sol could be tuned to meet the requirements for defect-free thin film formation by varying the preparation parameters of the sol like hydrolysis ratio, pH, and molar ratio of the two precursors. In situ 29Si NMR of the sols was carried out to follow the development during sol-gel synthesis. The pore sizes and porosity of unsupported microporous powders derived from these sols were characterized by sorption of nitrogen, CO2 and acetylene.
The use of hybrid nanosols with sufficiently large particles that could not penetrate into the γ-alumina layer resulted in defect-free continuous films. The resulting hybrid silica membranes were characterized by using SEM, XPS, permporometry, gas permeation and pervaporation. The hydrophobicity of the membrane was also observed in gas permeation experiments. The membrane pores appear to provide a hydrophobic environment, as the permeance of H2 containing water vapour at various partial pressures remained unaffected by the presence of water, unlike standard silica.
These hybrid silica membranes exhibited enhanced hydrothermal stability in pervaporation of a 97.5 wt% n-butanol / 2.5 wt% water mixture. It was possible to carry out this process with an organosilica membrane coated on a 30 cm long tube of 14 mm outer diameter at the high temperature of 150°C for an extended period (>9 months) without deterioration of the membrane. In comparison with the maximum working temperature of 95°C of standard silica membranes, this presents a significant improvement.