Advanced Materials for Membrane Preparation

Perovskite oxides exhibit appreciable mixed oxygen ionic and electronic conductivity and have been paid much interests in the last three decades due to their potential applications as oxygen permeable membranes. The hollow fibre geometry can provide much larger areas per unit volume (500~9000 m²/m³) for oxygen permeation compared to the flat sheet or tubular membranes, making it possible to reduce the membrane system size and the operation cost remarkably. In this work, the progresses on the development of perovskite hollow fibre membranes and the application in oxygen production are summarized. The challenges associated with the commercialization of the technology are also presented and discussed.

Syngas is an important feedstock for the production of higher hydrocarbons or methanol. It can be produced via conversion of methane and the most extensively used process for this conversion is the methane steam reforming reaction carried out in large furnaces. On the other hand, hydrogen is nowadays produced via conversion of methane to syngas and successive water gas shift reaction and purification. Methane steam reforming is a highly endothermic reaction which is industrially operated under severe conditions resulting in several undesirable consequences: sintering of the catalyst, very high carbon deposition and the use of high-temperature resisting materials. These drawbacks for methane steam reforming can be overcome by using membrane reactors, systems able to combine the separation properties of membrane with the typical characteristics of catalytic reactions. By using for example Pd-based membrane reactors, the hydrogen produced can be continuously withdrawn from the reaction system circumventing the thermodynamic limitations and making the methane steam reforming feasible at lower temperatures than the traditional systems. A potential alternative technique to steam reforming processes for producing syngas is the partial oxidation of methane with oxygen, having the disadvantage (economical and technological) that pure oxygen is required. Using air instead of pure oxygen is beneficial only if it can be performed by using a membrane reactor in which the membrane is perm-selective to oxygen. Another possible route for the partial oxidation of methane is the use of catalytic membrane reactors in which the membrane acts as both separation layer and reaction media. In this chapter new membranes to be used in syngas production and in hydrogen production will be discussed.

Both zeolite and MOF (metal organic framework) membranes should be able to separate a fluid mixture due to the size and shape of its components and can be called, therefore, “molecular sieve membrane”. During the last years, novel tools have been developed for the synthesis of more powerful zeolite membranes. By rigorously adopting these tools like seeding, use of macroporous ceramic and metal supports, microwave heating, playing with zeta potentials etc., in a relatively short time scale the first MOF membranes with molecular sieve properties could be developed which have almost the same state of development like advanced zeolite membranes.

We present a review of both experiments and atomistically-detailed simulations of both liquid and gas transport through carbon nanotube membranes. Carbon nanotubes have exceptional transport properties due to the remarkable smoothness of the potential energy surface inside carbon nanotubes. Membranes composed of carbon nanotubes have the potential to provide unparalleled performance as gas and liquid separation membranes. Recent novel fabrication techniques that permit the assembly of vertically aligned carbon nanotubes in membrane platforms are enabling the testing of that transport performance. Potential applications of carbon nanotube membranes in desalination are especially promising, as computer simulations indicate that membranes having nanotubes of the correct diameter will be highly efficient at rejecting ions while allowing transport of water. Furthermore, recent experimental studies suggest that ion exclusion can be controlled by electrostatic interactions between the ions and fixed charges on the carbon nanotubes.

Following the discovery by researchers at UOP on Mixed Matrix Membranes (MMMs), the latest emerging membrane materials comprise molecular sieve entities embedded in a polymer matrix. This type of materials can potentially surpass the “upper bound” limit of the permeability-selectivity relationship by means of combining the easy processability of polymers with the superior gas separation properties of rigid molecular sieve materials. It is well-known that the success of MMMs in gas separation mainly lies on the proper selection of inorganic and polymeric materials and the approaches to obtain an intimate polymer-zeolite interface as well as a practical membrane configuration. Since then, an intensive research has been devoted to improve the interface quality by developing some modification techniques, such as the silane modification on the zeolite surface, the introduction of compatibilizers between polymer and particle phases, and high membrane processing temperature. On the other hand, in view of MMM configuration, to meet a high productivity requirement for the industrial application, the asymmetric hollow fiber membrane is a preferred configuration due to its desirable characteristics like large surface-to-volume ratio and high flux. Hence, this chapter will briefly outline the concept, materials selection, and challenges of MMMs containing molecular sieves in the polymer matrix. Subsequently, it will focus on the key of mixed matrix hollow fiber membrane fabrication and introduce attractive avenues to overcome its challenges for gas separation applications.

Mixed-Matrix Membranes (MMM) could provide a near term solution to the permeability/selectivity trade-off with polymeric gas separation membranes and bridge the gap with inorganic membranes. While zeolite and carbon additives have shown promise, the corresponding mixed-matrix membranes are limited by low loadings and interfacial defects such that the true potential of MMMs has not been realized. Metal-Organic Frameworks (MOF), on the other hand, provide exceptionally high surface areas and organic linkers that could enhance selective gas transport and improve polymer/particle interactions. In this review, recent theoretical and experimental results for MOF-based mixed-matrix membranes will be described.

This paper summarized part of the material development for fuel cell conducted in my group in the last 10 years, based on new functionalized polymers with phosphonic, sulfonic as well as oxadiazole and triazole sites.

Perfluoropolymers are characterized by outstanding chemical, thermal and solvent resistance, therefore they represent natural candidates for demanding environments. In membrane separations and electrochemical processes perfluoropolymers enable substantial cost reductions, energy savings and environmental benefits. This contribution is aimed at giving an overview of the materials chemistry of perfluoropolymer membranes currently used in gas and vapour separations, in the chlor-alkali industry and in fuel cells, and on the perspectives for new challenging applications.

Glassy amorphous perfluoropolymers with a large spectrum of permeability and selectivity - Teflon AF, Hyflon AD, Cytop - are applied today in gas and vapor separation and in membrane contactors. Their use in petrochemistry instead of energy intensive unit operations and for process intensification is hindered by the polymer permeability - selectivity trade-off. Mixed matrix or hybrid polymer - filler membranes are in principle able to overcome this limitation. Examples are given of the opportunities offered by such hybrid membranes.

Polyperfluorosulfonic acids (Nafion, Hyflon Ion, 3M and the like) are today the materials of choice for low temperature fuel cells, but modest durability, low performances and high costs limit their use in niche applications. At medium temperatures - 90-160°C - Nafion loses water, proton conductivity and mechanical resistance, and degrades irreversibly from 110°C on. Several research strategies, such as better polymers, compounding with fillers, alternative proton conducting systems, scaffolds and reinforcements, have improved this picture in the last years, but have not found a viable solution yet for the huge automotive market. A combination of the incremental progress achieved by each of these strategies holds promise to find a solution.

Polymer Electrolyte Membrane Fuel Cells (PEMFC) are widely based on Nafion due to its high proton conductivity. Crucial role for the transport in hydrated Nafion is played by its nanophasesegregated structure in which hydrophilic phase is embedded in a hydrophobic matrix. Molecular Dynamics (MD) simulations of Nafion 117 using two extreme monomeric sequences may be an useful tool to study the transport: one very blocky and other very dispersed. Both produce a nanophasesegregated structure with hydrophilic and hydrophobic domains. The blocky Nafion leads to a characteristic dimension of phase-segregation that is ~ 60 % larger than for the dispersed system. This leads to a water diffusion coefficient for the dispersed case that is ~25% smaller than for the blocky case (0.46x10^-5 cm²/s vs. 0.59x10^-5 cm²/s at 300K). The experimental value (0.50x10^-5 cm²/s) is within the calculated range. In addition, the vehicular diffusion of hydronium is not affected significantly by the monomeric sequence: therefore, the hopping diffusion is the main mechanism for the proton transport.

In this work, we investigate a new molecular architecture in which water-soluble dendrimers are grafted onto a linear polymer for application to Polymer Electrolyte Membrane Fuel Cells (PEMFC). Using full-atomistic MD simulations, we examined the nanophase-segregation and transport properties in hydrated membranes with this new architecture. In order to determine how the nature of the linear polymer backbone might affect membrane properties, we considered three different types of linear polymers such as poly (epichlorohydrin) (PECH), poly (styrene) (PS) and poly (tetrafluoroethylene) (PTFE) each in combination with the second-generation sulfonic poly aryl ether dendrimer to form PECH-D2, PS-D2, and PTFE-D2. Our simulations show that the extent of nanophase-segregation in the membrane increases in order of PECH-D2 (~20 Å) < PS-D2 (~35 Å) < PTFE-D2 (~40 Å) at the same water content, which can be compared to 30~50 Å for Nafion and ~30 Å for Dendrion at the same water content. We find that the structure and dynamics of the water molecules and transport of protons are strongly affected by the extent of nanophase segregation and water content of the membrane. As the nanophase-segregation scale increases, the structure in water phase, the water dynamics and the proton transport approach those to those in bulk water. Based on the predicted proton and water transport rates, we expect that the PTFE-D2 may have a performance comparable with Nafion and Dendrion.

Nanotechnology with its “bottom-up” approach gives many opportunities to the membrane science and technology community for the developing advanced membranes. In this chapter, the fabrication of separation membranes from block copolymers is described. In addition, a strategy to manipulate the self-assembly of block copolymers is proposed highlighting the recent advances. As reported, the block copolymers offer exquisite nanostructures that lead to fabricate membranes with controlled pore size (nanopores), as well as dense films with tailored properties for CO₂ capture.

Polymers are the favorite materials for membrane applications because they have tunable properties, both chemical and physical ones, which allow the design of selective barriers, and also because they are prompt to give rise to active layers of low thicknesses under various forms, i.e. either as flat sheets, rolled sheets for spiral modules or even as hollow fibers; in addition, most of the time, polymers are also cheap materials that is always a good point for ulterior industrial developments. But in any case, polymer membranes remain sensitive materials and of course fragile ones, that constitutes their ‘Achilles’ heel. The particular key feature which mainly contributes to the strong potential of polymers is the large variety of chemical functions which can be found and modified when going from one polymer family to another one. Mastering both the structure and the functionality of polymers are certainly the clue to the design and to the preparation of advanced membranes such as mixed matrix or nanoporous membranes. Indeed, one of the fundamental aspects of the pervaporation transport is the strength of the physico-chemical interactions between the membrane and a given penetrant which can lead to the selectivity of the transport. Indeed, whereas in gas permeation the transfer is dominated by the diffusion step, the reverse situation is usually occurring with pervaporation where the sorption step is the decisive one. Unfortunately there is no universal membrane able to handle with success various separation problems, as far as molecular separations are targeted. Indeed the choice of a membrane is always closely related to the nature of the mixtures to be fractionized, i.e. water or organic removal, and even to the precise mixture composition; in the worst case, the use of an inappropriate membrane can even lead to its damage. Inorganic membranes are actually rarely used in pervaporation compared to organic ones; but it is certain that some specific needs will push the development of inorganic membranes in the near future thanks to the big improvements achieved in the last ten years in the controlled synthesis of inorganic structures having very narrow pores able for instance to promote a very efficient dehydration of alcohol mixtures. This paper intends to review the main routes which have been investigated for membrane design since the early time of pervaporation, and to analyze the current trends of polymeric membrane design and preparation published in the open literature. Taking into account the up-to-date knowledge in polymer science, some promising syntheses to new permselective pervaporation membranes are discussed as innovative routes.

Membrane technology is receiving more and more opportunities to become a true implemented technology in many industrial applications. It is perfectly suited to answer some of the demands that are put forward in the new trends towards sustainable production and process intensification. Since many of these applications are in solvent environments, suitable membranes need new requirements including solvent compatibility and nanoporosity. In this chapter, a general overview is given on the progress that is made in the development of nanoporous, solvent-stable membranes. Emphasis is put on the synthesis of these membranes as well as on there stability. Both polymeric and ceramic membranes are described. However, this chapter is mainly focused on ceramic membranes and more specific hybrid organic-inorganic membranes made by post-synthesis modification or in situ methods. Synthesis methods such as organosilane and phosphonic acid grafting as well as a more recently developed method are discussed. The general aspects, advantages and drawbacks of each method are discussed in detail.

This chapter describes the membrane materials currently used in applications of aqueous nanofiltration. The focus will be on the following materials: polyamide, polysulfone, and ceramics. Methods for the synthesis of such membranes will be described. Although these groups of materials comprise most of the (commercially) available nanofiltration membranes, other (experimental) materials for manufacturing of nanofiltration membranes will be discussed as well. Furthermore, new developments in fine-tuning membranes or enhancing membrane performances (flux increase, fouling resistance, catalytic activities) will be described, including chemical modifications of membrane structures, and the addition of nanoparticles to polymeric and ceramic membranes.

Membrane reactors are generally applied in high temperature reactions (>400 °C). In the field of fine chemical synthesis, however, much milder conditions are generally applicable and polymeric membranes were applied without their damage.

The successful use of membranes in membrane reactors is primary the result of two developments concerning: (i) membrane materials and (ii) membrane structures. The selection of a suited material and preparation technique depends on the application the membrane is to be used in. In this chapter a review of up to date literature about polymers and configuration catalyst/ membranes used in some recent polymeric membrane reactors is given.

The new emerging concept of polymeric microcapsules as catalytic microreactors has been proposed.