Membranes for Gas Separations

The following sections are included:

• Preface
• Contents

Mixed matrix membranes (MMMs) have gained lot of interest in the research community over the last few years in order to overcome the performance trade-off shown by the polymeric membranes. An MMM utilizes the benefits of both polymeric and inorganic materials and has the potential to show very high performance without much increase in the cost of fabrication. But the fabrication of an MMM without conceding the mechanical properties is very challenging. In this chapter, the materials selection and fabrication techniques of MMM in both flat sheet and hollow fiber configurations are discussed. Formation of defects at the interface is one of the biggest challenges in fabricating the MMM. Different techniques used in the literature to compatibilize the polymer and particle were summarized. In addition, the recent progress of MMM for gas separation applications and performance prediction models were provided in detail.

Organic–inorganic composite membranes have received increasing attention for molecular separation. In this chapter, recent progresses on ceramic-supported organic composite membranes for gas separation are briefly reviewed. The organic separation layer of the composite membranes includes traditional polymeric membrane materials such as polydimethylsiloxane (PDMS), poly(ethylene glycol) (PEG), and poly(amide-b-ethylene oxide) (PEBA), as well as emerging twodimensional graphene-based membrane materials. Fabrication method, physicochemical property, and gas separation performance of the membranes are discussed. Compared with the state-of-the-art, the developed ceramic-supported organic composite membranes exhibited excellent gas separation performance for carbon dioxide and hydrogen separation, suggesting great potential for practical application…

Metal organic frameworks (MOFs) have received significant attention as membrane materials in the last decade. MOFs typically have high porosities, tunable pore sizes, large surface areas, reasonable thermal and mechanical stabilities. These properties make them ideal membrane materials for gas separations. Considering the very large number of available MOF structures, it is not practically possible to test membranebased gas-separation performance of MOFs using purely experimental means. Molecular modeling methods are highly useful to complement experimental studies by providing insights into the gas-separation mechanisms of MOFs. Molecular simulation studies have been recently used to screen large numbers of MOFs for gas separation applications to identify the most promising MOF membranes. In this way, experimental efforts, time, and resources can be directed to the materials with the highest separation performance. The aim of this chapter is to review the recent advances in molecular modeling of MOF membranes for gas-separations and address the current opportunities and challenges in using molecular simulations for MOF membranes.

An increasing demand for clean energy has led to a search for alternative energies with less environmental impact, of which natural gas is an important option. The large consumption of natural gas requires an effective technology to improve the purity of natural gas, in which inert gases such as nitrogen play a large part. However, current technologies cannot provide economic solutions to remove nitrogen from natural gas, which leads to a decrease in the heat value of the natural gas. Membrane separation by zeolites represents a viable energy-saving method that can potentially offer an effective way to remove nitrogen from natural gas. In particular, SAPO-34 zeolite membranes have been extensively studied for carbon dioxide separation because of their exceptional molecular-sieving effects, higher thermal and chemical resistances. These membranes can also be effective for N₂–CH₄ separation due to their molecular-sieving effects and adsorption properties. In this mini review, we discuss the different technologies employed for nitrogen rejection from natural gas, emphasizing zeolite membranes as an effective and promising technology to economically separate N₂ from CH₄.

Although polymer blends have been utilized in several industrial processes, they have not yet found wide acceptance as materials for gas separations due to the demanding conditions of industrial separations. Blending different polymers into a single membrane is attractive because of the synergistic combination of properties. Miscible polymer blends display properties that are intermediate of their comprising individual polymers, while immiscible blends can lead to unique properties depending on what morphology is presented. The gas-separation performance of immiscible blends is largely dependent on the internal morphology resulting from poor mixing. In this chapter, morphology and control of morphology is discussed, along with recent gas-separation examples that illustrate each concept. Reviewing the state of polymer blend membranes for gas separation reveals one major factor: the control of morphology is imperative to developing systems with optimized performance.

Cross-linking has been widely utilized to modify polyimide nanostructures for membrane gas separations, such as increasing size-sieving ability and diffusivity selectivity for H₂–CO₂ and CO₂–CH₄ separation and improving resistance to plasticization derived from CO₂ and heavy hydrocarbons for CO₂–CH₄ and C₃H₆–C₃H₈ separations. However, there is a lack of fundamental understanding of the relationship between crosslinked structure and membrane gas-separation properties. This chapter critically reviews the effect of cross-linking on polymer physical properties (such as glass transition temperature, T_g) and current strategies adopted to cross-link polyimides for membrane gas separation. The information is synthesized to elucidate the effect of cross-linking on T_g and cross-linking density in polyimides, which is then used to interpret the changes of gas permeability and selectivity. The benefits of crosslinking in improving gas-separation properties are also illustrated in Robeson’s upper bound plots for H₂–CO₂, CO₂–CH₄, and C₃H₆–C₃H₈ separations.

Hydrogen production has become an increasingly important field due to the rise of fuel cells and the search for a renewable source of energy. Yet, simply producing the hydrogen is not enough. It must also be separated from the various other effluent gases, whether the hydrogen comes from natural gas, a petroleum source, or is split from water. Various membrane technologies include porous silica membranes that rely on molecular sieving and dense technologies wherein the hydrogen is transported through a metal or ceramic lattice. Because of their perfect selectivity toward hydrogen, dense membranes using either certain metals or proton-conducting ceramics have remained a topic of great interest for decades and will be the subject of this chapter…