Klaas Keizer

Analysis and theory of gas transport in microporous sol-gel derived ceramic membranes

Sol-gel modification of mesoporous alumina membranes is a very successful technique to improve gas separation performance. Due to the formed microporous top layer, the membranes show activated transport and molecular sieve-like separation factors. This paper concentrates on the mechanism of activated transport (also often referred to as micropore diffusion or molecular sieving). Based on a theoretical analysis, results from permeation and separation experiments with H2, CO2, O2, N2, CH4 and iso-C4H10 on microporous sol-gel modified supported ceramic membranes are integrated with sorption data. Gas permeation through these membranes is activated, and for defect-free membranes the activation energies are in the order of 13?15 kJ.mol?1 and 5?6 kJ.mol?1 for H2 and CO2 respectively. Representative permeation values are in the order of 6×10?7 mol.m?2.s?1.Pa?1 and 20×10?7 mol.m?2.s?1.Pa?1 for H2 at 25°C and 200°C, respectively. Separation factors for H2/CH4 and H2/iso-butane are in the order of 30 and 200 at 200°C, respectively, for high quality membranes. Processes which strongly determine gas transport through microporous materials are sorption and micropore diffusion. Consequently, the activation energy for permeation is an apparent one, consisting of a contribution from the isosteric heat of adsorption and the activation energy for micropore diffusion. An extensive model is given to analyse these contributions. For the experimental conditions studied, the analysis of the gas transport mechanism shows that interface processes are not rate determining. The calculated activation energies for micropore diffusion are 21 kJ.mol?1 and 32 kJ.mol?1 for H2 and CO2, respectively. Comparison with zeolite diffusion data shows that these activation energies are higher than for zeolite 4A (dpore=4A), indicating that the average pore size of the sol-gel derived membranes is probably smaller.

Formation and characterization of supported microporous ceramic membranes prepared by sol-gel modification techniques

The formation is described of supported microporous membranes (by IUPAC definition rpore < 1 nm), prepared by modification of mesoporous γ-alumina membranes with polymeric sols. The mesoporous γ-alumina membranes, with a top-layer thickness in the order of 7–10 μm, and with pore radii of 2–2.5 nm, have a very high surface finish (mean roughness 40 nm). The amorphous microporous top-layer thickness is in the order of 60–100 nm. Gas transport properties are effectively improved as is shown by activated permeation and molecular sieve-like separation factors in the order of 50–200 for H2/CH4. These microporous top-layers can be prepared from a relatively wide range of sol structures; from inorganic oligomers which are too small to result in significant scattering with SAXS, to polymeric structures with fractal dimensions in the range: 1

Transport properties of alkanes through ceramic thin zeolite MFI membranes

Polycrystalline randomly oriented defect free zeolite layers on porous -Al2O3 supports are prepared with a thickness of less than 5 μm by in situ crystallisation of silicalite-1. The flux of alkanes is a function of the sorption and intracrystalline diffusion. In mixtures of strongly and weakly adsorbing gases and a high loadings of the strongly adsorbing molecule in the zeolite poze, the flux of the weakly adsorbing molecule is suppressed by the sorption and the mobility of the strongly adsorbing molecule resulting in pore-blocking effects. The separation of these mixtures is mainly based on the sorption and completely different from the permselectivity. At low loadings of the strongly adsorbing molecules the separation is based on the sorption and the diffusion and is the same as the permselectivity. Separation factors for the isomers of butane (n-butane/isobutane) and hexane (hexane/2,2-dimethylbutane) are respectively high (10) and very high (> 2000) at 200°C. These high separation factors are a strong evidence that the membrane shows selectivity by size-exclusion and that transport in pores larger than the zeolite MFI pores (possible defects, etc) can be neglected.

Gas transport and separation with ceramic membranes. Part II. Synthesis and separation properties of microporous membranes

Non-supported microporous silica (amorphous) and titania thin films were made by the polymeric gel route. The titania system consisted of particles smaller than 5 nm. Reproducible modification of supported γ-alumina films with silica demands a strict control of every modification step. Silica films of 30–60 nm thickness on top of and presumably partly inside the γ-alumina film were realised. The permeabilities of helium and hydrogen through this film are activated, while the propylene permeability was below the detection limit. Separation factors of a H2---C3H6 mixture are larger than 200 at 200 °C with a flux of the preferentially hydrogen of 1.6 × 10−6 mol/m2-sec-Pa. The pores must be of molecular dimensions to realise this (< 1 nm diameter). Preliminary research shows that changes in the synthesis parameters result in higher activation energies and improved separation properties. The relation between synthesis, resulting microstructure and gas separation properties, however, is not yet fully understood.

Gas separation mechanisms in microporous modified γ-al2o3 membranes

The transport of pure gases and of binary gas mixtures through a microporous composite membrane is discussed. The membrane consists of an alumina support with a mean pore diameter of 160 nm and an alumina top (separation) layer with pores of 2-4 nm. The theory of Knudsen diffusion, laminar flow and surface diffusion is used to describe the transport mechanisms. It appears for the composite membrane that Knudsen diffusion occurs in the toplayer and combined Knudsen diffusion/laminar flow in the support at pressure levels lower than 200 kPa. For the inert gas mixture H2/N2 separation factors near 3 could be achieved which is 80% of the theoretical Knudsen separation factor. This value is shown to be the product of the separation factor of the support (1.9) and of the top layer (1.5). The value for the top layer is rather low due to the relatively small pressure drop across this layer. This situation can be improved by using composite membranes consisting of three or more layers resulting in a larger pressure drop across the separation layer. CO2 surface diffusion occurs on the internal surface of the investigated alumina membranes. At 250-300 K and a pressure of 100 kPa the contribution of surface diffusion flow measured by counterdiffusion is of the same order of magnitude as that resulting from gas diffusion. The adsorption energy amounts —25 kJ/mol and the surface coverage is 20% of a monolayer at 293 K and 100 kPa. The calculated surface diffusion coefficient is estimated to be 2-5 x 10-9 m2/sec. Modification of the internal pore surface with MgO increases the amount of adsorbed CO2 by 50-100%. Modifications with finely dispersed silver are performed to achieve O2 surface diffusion.

Textural evolution and phase transformation in titania membranes: Part 1.—Unsupported membranes

Textural evolution in sol–gel derived nanostructured unsupported titania membranes has been studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermal gravimetry (TG), X-ray diffraction (XRD) and N2 adsorption. The anatase-to-rutile phase transformation kinetics were studied using the Avrami model. The precursor gel had a surface area of ca. 165 m2 g–1, which after heat treatment at 600 °C for 8 h reduced to zero. Undoped titania-gel layers transformed to more than 95% rutile after calcination at 600 °C for 8 h. The causes of surface-area reduction and pore growth were anatase crystallite growth and the enhanced sintering of rutile during transformation. Lanthanum oxide was identified as a suitable dopant for shifting the transformation temperature to ca. 850 °C. Lanthanum oxide doped titania showed an improved stability of porous texture compared to that of the undoped titania membranes.

Gas and surface diffusion in modified γ-alumina systems

The transport of pure gases through a microporous membrane is described. The alumina-based membrane (pores 2.5-4 nm) is suitable for Knudsen diffusion separation. To improve the separation factor, interaction with and mobility on the pore wall of one of the gases of a mixture is necessary. To introduce surface diffusion of oxygen and hydrogen, a γ-alumina membrane was impregnated with silver. If temperature and atmosphere are controlled carefully, finely dispersed silver up to 17% by weight can be introduced. At higher loads and under oxidizing conditions, particle growth occurs. In adsorption experiments, little oxygen adsorption on the silver-modified γ-alumina could be detected. This is due to a decrease in accessible surface area of the silver because of particle growth of silver under oxygen. The mobility of hydrogen on the surface was tested by counterdiffusion experiments, of which the theory is given. Hydrogen shows a considerable mobility on the surface at 293 K. At low pressures the flux ratio of hydrogen to nitrogen improved from 3.8 to 8.8. Magnesia was introduced into the γ-alumina membrane to enhance the adsorption and mobility of CO2. It is known that 30% of the CO2 transport on non-modified γ-alumina is surface diffusion. The highest achievable magnesia load was 2.2% by weight. Introduction of magnesia into the γ-alumina surface gives more strong base sites and fewer weak base sites. This results in stronger bonding of CO2 on the surface, but the amount adsorbed is comparable with the amount of CO2 adsorbed on non-modified γ-alumina. The contribution of surface diffusion to the total transport decreases with the introduction of magnesia, as is shown by counterdiffusion. The more strongly bonded CO2 is less mobile, resulting in a smaller surface flux.

Two component permeation through thin zeolite MFI membranes

Two component permeation measurements have been performed by the Wicke–Kallenbach method on a thin (3 μm) zeolite MFI (Silicalite-1) membrane with molecules of different kinetic diameters, dk. The membrane was supported by a flat porous -Al2O3 substrate. The results obtained could be classified in six regimes based on differences in: • Occupation degrees, θ, on the external surface. • Occupation degrees, θ, in the zeolite pores. • Mobilities in the zeolite pores. High separation factors are obtained for mixtures of “weakly/strongly adsorbing” gases and for mixtures of “small/large” gases. Separation by size-exclusion is demonstrated for 8 kPa n-C6H14/16 kPa 2,2-dimethylbutane (>600, T=298–473 K) and 0.31 kPa p-xylene/0.26 kPa o-xylene (>200, T=400 K). At low θ (high temperature, low pressures) the permeation of both components in a mixture equals the single-gas permeation of these components. At high θ (low temperature and high pressure) the permeation of both components in mixtures is different from single-gas permeation values. The single-gas permeance of “small” molecules with kinetic diameter dk <0.45 nm ranges from 1×10−8 to 15×10−8 mol m−2 s−1 Pa−1 in the temperature range 298–473 K. The permeation decreases with increasing dk of the molecules.

Gas transport and separation with ceramic membranes. Part I. Multilayer diffusion and capillary condensation

Multilayer diffusion and capillary condensation of propylene on supported γ-alumina films greatly improved the permeability and selectivity. Multilayer diffusion, occurring at relative pressures of 0.4 to 0.8 strongly increased the permeability of 6 times the Knudsen permeability, yielding permeabilities of 3.2 × 10−5 mol/m2-sec-Pa. The occurrence of a maximum in the permeability coincides with blocking of the pore by adsorbate (capillary condensation). This point could be predicted, employing adsorption data and the slit shape form of the pore. Separation factors of 27 were obtained with a N2---N3H6 mixture and a supported γ-alumina film, with C3H6 the preferentially permeating component. This very effective separation is due to pore blocking by adsorbate. The separation factor increased to 85 after modification of the system with magnesia by the reservoir method. However, the permeability of propylene decreased by a factor of 20 to 1.6 × 10−6 mol/m2-sec-Pa.

Preparation and characterization of thin zeolite MFI membranes on porous supports

Thin (2–7 μm) polycrystalline randomly-oriented zeolite MFI membranes were prepared on an -Al2O3 support by one or more subsequent hydrothermal treatments. Different particle sizes (275–700 nm) in the layer were achieved by changing the synthesis temperature (371–459 K). Zeolite membranes, prepared by two subsequent hydrothermal treatments have an optimum quality. Membranes prepared by a single hydrothermal treatment have defects and zeolite membranes obtained by three or more hydrothermal treatments become too thick and crack during the removal of the template. The zeolite MFI membranes were characterised by permeation measurements using single gases and mixtures of n-butane (n-C4H10) and iso-butane (i-C4H10). Permselectivities, F, and separation factors, , for n-C4H10/i-C4H10 decrease strongly with an increasing number of defects and hence provide a good indication of membrane quality. The separation, on the other hand, of a CH4/n-C4H10 mixture at high occupation of n-C4H10 is not much affected by small defects. This is ascribed to blocking of those defects by capillary condensation of the butane. Flux values decrease linearly with increasing zeolite crystallite size in the top layer indicating that the effective membrane thickness is only one particle thick. During hydrothermal treatment zeolite particles also grow at the other side of the support but do not decrease significantly the flux through the supported zeolite membrane.