Membrane Materials

Nanoporous

In the mid-nineties ECN started with the development of nanoporous (pore size < 1 nanometer) silica membranes on tubular supports for gas separation and pervaporation applications. ECN was among the first to make high quality inorganic membranes on a square meter scale. The low resistance against moisture rendered these materials unsuitable for the demanding conditions found in the water gas shift reactors. The implementation of end standing methyl groups in the silica structure resulted in a stability improvement and these so-called MeSi membranes can be applied in the dehydration of alcohols up to about 100°C (Chem. Commun. 2004, 834). More recently, a new material was discovered in which organic moieties as integral part of the network play an essential role. Now, the maximum application temperature has been increased to over 150°C (Chem. Commun. 2008, 1103). These membranes combine superior separation performances with unprecedented stability even in the presence of acids (J. Membr. Sci. 2008, 324, 111). This development is currently in a pre-commercial phase. For more information of these so-called HybSi® membranes please visit www.HybSi.com.

Noble metal alloys

Palladium based alloys hold a great promise as a hydrogen selective membrane material. This family of materials can take up a lot of hydrogen in a very short period of time and is able to transport it rapidly as well. The thin metal layers (<5µm) are supported by an alumina support. We are studying various alloys that prevent the issues of phase segregation under the influence of hydrogen uptake, and brittleness (Acta Mater. 2008, 56, 6132). Currently, we are scaling up these materials and are testing the long term performance under conditions met in industrial processes such as methane steam reforming and the water gas shift reaction. To speed up the acceptance of this technology we are making small scale separation modules available to third parties through www.HYSEP.com.

Perovskites

ECN has a long standing experience in research related to perovskite materials. These complex oxides have found use in e.g. solid oxide fuel cells as developed in the SOFC group. In the Membrane Technology group we have been developing membranes for the large scale production of oxygen with a very high purity as a straight replacement of conventional cryogenic distillation technologies. The performance of these materials were determined of flat (un)supported functional membranes of ~20cm2 (J. Membr. Sci. 2006, 276, 178). Both flux and selectivity criteria were met. An extensive study on the various membrane modules in which the membrane played a key role, demonstrated to us that a supported tubular shape was the most promising geometry (J. Membr. Sci. 2006, 278, 66). On this we are currently concentrating our efforts.

Ceramic Supported Polymers

Over the recent years the development of high temperature polymers has been very rapid. Especially, a large number of polyimides have become commercially available. Still, the poor mechanical properties of these materials at higher temperatures prevent the application at elevated temperatures. For this reason, we use cheap ceramic supports as an effective mechanical reinforcement. This approach has been followed for example in the search for a robust membrane system for high temperature pervaporation (J. Membr. Sci. 2008, 319, 126). The same approach can be used for composite membranes in which the functionality of the polymer is enhanced through the introduction of metal or metal oxide nanoparticles, or zeolites.

Supported Ionic Liquid Membranes

Recently, ionic liquids (ILs) have attracted a lot of attention as environmental friendly alternatives to hazardous organic solvents for the application as reaction media. In addition, the selective uptake of gases gives these materials also a potential for separation processes. Some ionic liquids exhibit high affinity for CO₂ or certain hydrocarbons. Once these liquids are contained in a support, a supported liquid membrane can be formed. The challenge is to prepare pressure resistant thin liquid films contained in a porous support. This put severe constraints on the nature and pore size of the support. We have used multilayer ceramic tubes with a tailored pore size as support. The ionic liquid is retained in these pores by capillary action.