Direct crystallisation of hydroxysodalite and MFI membranes on α-alumina supports
Van Niekerk, Anelia
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Hydroxysodalite and MFI are zeolites that consist of crystalline (tecto)aluminosilicates and silicates with specific aperture sizes. Zeolite membranes are suited for any process requiring separation on a molecular level (molecular-sieving effect) or based on their selective sorption properties due to their hydrophilic/hydrophobic nature. Zeolites are stable in harsh physical (pressure and temperature) and chemical environments. By synthesising a zeolite membrane, the unique characteristics can be combined and used for specific separations in a continuous process. The objective of this study was to synthesise thin hydroxysodalite and high silica MFI membranes by crystallising the zeolite as a continuous and defect-free intergrown layer onto the surface of a tubular a-alumina support using a conventional oven by varying different synthesis parameters and to characterise these membranes with SEM, elemental analysis, XRD and single gas permeations. An in situ hydrothermal technique was used to crystallise hydroxysodalite and MFI layers onto the tubular a-alumina support using a conventional oven. With hydroxysodalite three molar oxide ratios were investigated, while with MFI two silica sources, namely fumed silica and TEOS, were investigated. The crystallisation of these two zeolites was related to various synthesis parameters such as the water content, precursor ageing periods, synthesis time and temperature. After obtaining closed membranes, the membranes were characterised. A study of the structure, chemical composition and the compactness of the zeolite layer was necessary to determine the quality of the membrane in terms of its structure related parameters using SEM, elemental analysis and XRD. Single gas permeations were used to determine the permeation related parameters, thus obtaining the selectivity of the membrane. The synthesis parameters had different effects on the zeolite membranes, depending on the molar oxide ratio and chemicals used during the synthesis. With hydroxysodalite, the amount of water had an influence on the degree of contamination by other zeolite phases, while for the MFI membrane it had an influence on the degree of intergrowth between individual zeolite crystals, and on nucleation. For both zeolite membranes, an increase in the synthesis time lead to increased crystal growth. By varying the ageing time of the precursor solution, it was possible to control the ratio of homogeneous versus heterogeneous nucleation and limit secondary nucleation when synthesising MFI membranes. Decreasing the synthesis temperature for MFI limited secondary nucleation. For the MFI membranes the template combination of TPAOH and TPABr lead to an increased crystal growth. The silica source had a significant influence on the crystal size and crystal size distribution. Fumed silica had a single crystal size distribution, while TEOS had a wide crystal size distribution. The hydroxysodalite and the two high silica MFI membranes synthesised had good integrity. Hydroxysodalite had an ideal selectivity for He/N2 above Knudsen selectivity, even though there were intercrystalline pores due to the high aluminium content. The high silica MFI (fumed silica) membrane had an ideal selectivity for n/i-butane of 433 at 333 K and 1.0 bar transmembrane pressure, while the high silica MFI (TEOS) membrane exhibited an ideal selectivity for n/i-butane of 327 at 378 K and 0.5 bar transmembrane pressure. The ideal selectivities of the high silica membranes MFI membranes compared well with to literature values. The permeances of all the gases through the hydroxysodalite membrane were higher than through the two MFI membranes. This higher permeance was due to the higher aluminium content of the hydroxysodalite membrane, which lead to a higher concentration of intercrystalline pores. Regarding the MFI membranes, the permeances of the gases through the TEOS membrane were higher than through the fumed silica membrane. Due to the higher ideal selectivity of fumed silica membrane, it was clear that it had a better integrity due to a lower aluminium content. The permeance through the hydroxysodalite membrane was higher than that reported in literature, while the permeance through the two high silica MFI membranes correlated well with that in literature. Two framework type zeolite membranes were synthesised on tubular a-alumina supports, namely hydroxysodalite and high silica MFI. The membranes had good integrity, which was reflected in the high average selectivity and flux values obtained for different gases, under various conditions.
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