Ingolf Voigt

Preparation of zeolite membranes on the inner surface of ceramic tubes and capillaries

After many experiences with ZSM-5 layers on flat ceramic discs the investigation now was focused on the preparation of ZSM-5 membranes on the inner surface of ceramic tubes and capillaries to get useful membrane shapes with a high membrane area/module volume ratio for industrial applications. The influence of two different types of seeds, the tube diameter and the movement of the synthesis solution during the crystallization step was investigated. By using nanosized MFI seeds and by pumping the synthesis solution through the support tubes homogeneous ZSM-5 membranes of 30 μm thickness and a high H2 permeance of 4500 l/(m2 h bar) and a H2/SF6 single gas permselectivity above the Knudsen factor were achieved.

TiO2-NF-membranes on capillary supports

Abstract Starting from stress simulations suitable shapes of ceramic capillaries (outer diameter, OD; inner diameter, ID) were calculated. Capillaries were prepared by conventional extrusion down to an OD/ID 1.1 mm/0.7 mm. Capillaries with an 2.9 mm/1.9 mm were selected as the most favorite shape. A special coating technique was developed to prepare NF-membranes inside these types of capillaries. Single capillaries were tested and compared with monochannel tubes with OD/ID 10 mm/7 mm. The same cut-off of 450 g/mol measured with PEG standards and a similar water flux of 17 l/(m 2 hbar) compared with 20 l/(m 2 hbar) were achieved by dilution of the polymeric TiO 2 -sol. A new quality of ceramic NF-membrane with a cut-off of 250 g/mol and a water flux of 10 l/(m 2 hbar) was developed by a two step coating process. Test modules with approximately 50 capillaries were built in order to prove the module performance.

High-temperature stability and saturation magnetization of superparamagnetic nickel nanoparticles in microporous polysilazane-derived ceramics and their gas permeation properties.

Superparamagnetic Ni nanoparticles with diameters of about 3 nm are formed in situ at room temperature in a polysilazane matrix, forming Ni/polysilazane nanocomposite, in the reaction between a polysilazane and trans-bis(aceto-kO)bis(2-aminoethanol-k(2)N,O)nickel(II). The thermolysis of the Ni/polysilazane nanocomposite at 700 °C in an argon atmosphere results in a microporous superparamagnetic Ni/silicon oxycarbonitride (Ni/SiCNO) ceramic nanocomposite. The growth of Ni nanoparticles in Ni/SiCNO ceramic nanocomposite is totally suppressed even after thermolysis at 700 °C, as confirmed by HRTEM and SQUID characterizations. The analysis of saturation magnetization of Ni nanoparticles in Ni/polysilazane and Ni/SiCNO nanocomposites indicates that the saturation magnetization of Ni nanoparticles is higher than expected values and infers that the surfaces of Ni nanoparticles are not oxidized. The microporous superparamagnetic Ni/SiCNO nanocomposite is shaped as a free-standing monolith and foam. In addition, Ni/SiCNO membranes are fabricated by the dip-coating of a tubular alumina substrate in a dispersion of Ni/polysilazane in THF followed by a thermolysis at 700 °C under an argon atmosphere. The gas separation performance of Ni/SiCNO membranes at 25 and 300 °C is assessed by the single gas permeance (pressure rise technique) using He, H2, CO2, N2, CH4, n-propene, n-propane, n-butene, n-butane, and SF6 as probe molecules. After hydrothermal treatment, the higher increase in the hydrogen permeance compared to the permeance of other gases as a function of temperature indicates that the hydrogen affinity of Ni nanoparticles influences the transport of hydrogen in the Ni/SiCNO membrane and Ni nanoparticles stabilize the structure against hydrothermal corrosion.

High-Flux Carbon Molecular Sieve Membranes for Gas Separation

Carbon membranes have great potential for highly selective and cost-efficient gas separation. Carbon is chemically stable and it is relative cheap. The controlled carbonization of a polymer coating on a porous ceramic support provides a 3D carbon material with molecular sieving permeation performance. The carbonization of the polymer blend gives turbostratic carbon domains of randomly stacked together sp2 hybridized carbon sheets as well as sp3 hybridized amorphous carbon. In the evaluation of the carbon molecular sieve membrane, hydrogen could be separated from propane with a selectivity of 10 000 with a hydrogen permeance of 5 m3 (STP)/(m2 hbar). Furthermore, by a post-synthesis oxidative treatment, the permeation fluxes are increased by widening the pores, and the molecular sieve carbon membrane is transformed from a molecular sieve carbon into a selective surface flow carbon membrane with adsorption controlled performance and becomes selective for carbon dioxide.

Determination of the oxygen ion diffusion coefficients in mixed conductors by the volumetric measurement of the reoxidation kinetics

Abstract Mixed conducting ceramics (ionic and electronic conducting oxides) have a high application potential for the oxygen separation from oxygen containing gas mixtures (mostly air). Because the ion diffusion controls frequently the rate of the total permeation flux, a simple and fast determination of the oxygen diffusion coefficient is the prerequisite for an efficient material screening. In this work, the volumetric measurement of the reoxidation of chemically reduced mixed conductors was used for the determination of oxygen ion diffusion coefficients. The described method has advantages compared to the frequently used mass relaxation technique, e.g. a higher measuring effect, the suitability for fast processes (by use of large samples) and a high resistance to abrupt pressure changes. The checkup of the measurement method on the reference materials SrFe 1− x Co x O z ( x =0.6) and La 0.79 Sr 0.2 MnO 3+ z indicates a sufficient agreement with previous published values. The experimental results show a high internal consistency, as it is essential for a screening method. Diffusion coefficients are independently on the oxygen concentration over a wide range of the oxygen stoichiometry for all tested materials. Low activation energies in the range 0.10–0.5 eV were found. These latter results are opposed to the majority of publications on mixed conducting oxides. Additionally, new promising perovskite compositions in the system Sr 1− x Ca x Mn 1− y Fe y O 3− z were presented.