Membrane Gas Separation - Hardcover

Hardcover

ISBN 10: 0470746211 ISBN 13: 9780470746219

Publisher: Wiley, 2010

Synopsis

Gas separation membranes offer a number of benefits over other separation technologies, and they play an increasingly important role in reducing the environmental impacts and costs of many industrial processes.

This book describes recent and emerging results in membrane gas separation, including highlights of nanoscience and technology, novel polymeric and inorganic membrane materials, new membrane approaches to solve environmental problems e.g. greenhouse gases, aspects of membrane engineering, and recent achievements in industrial gas separation. It includes:

• Hyperbranched polyimides, amorphous glassy polymers and perfluorinated copolymers
• Nanocomposite (mixed matrix) membranes
• Polymeric magnetic membranes
• Sequestration of CO2 to reduce global warming
• Industrial applications of gas separation

Developed from sessions of the most recent International Congress on Membranes and Membrane Processes, Membrane Gas Separation gives a snapshot of the current situation, and presents both fundamental results and applied achievements.

About this title

About the Author

Benny D.Freeman - Kenneth A. Kobe and Paul D. and Betty Robertson Meek & American Petrofina Foundation Centennial Professor of Chemical Engineering University of Texas at Austin, Center for Energy and Environmental Resources. He researches in polymer science and engineering and, more specifically, in mass transport in polymers. His work in this field started in North Carolina State University, where he worked as assistant, associate professor and full professor during the period 1989-2002. In 2002 he moved to Department of Chemical Engineering of the University of Texas at Austin and currently directs 18 Ph.D. students and one postdoctoral fellow performing fundamental research in mass transport in polymers. I have taught a variety of short courses in the membranes area for societies such the ACS, the North American Membrane Society, and the International Congress on Membranes. I have also taught a course on this topic at the graduate level at the university. I have co-edited 4 books. Professor Freeman will be the 2009 ACS Awardee in Polymer Science in 2009.

Yuri Yampolskii, Professor, Head of Laboratory of membrane gas separation, A.V.Topchiev Institute of Petrochemical Synthesis, Moscow, Russia. Since the middle of 70s has been engaged in the studies of membrane separation, polymer physical chemistry and related subjects. At present he is Head of the laboratory dealing with the problems of membrane separation and pervaporation, gas permeation properties of various polymeric materials, vapor separation processes, sorption thermodynamics, free volume in polymers. He has published about 200 papers in peer reviewed journals and authored or co-authored several books.

This book describes recent and emerging results in membrane gas separation, including highlights of nanoscience and technology, novel polymeric and inorganic membrane materials, new membrane approaches to solve environmental problems e.g. greenhouse gases, aspects of membrane engineering, and recent achievements in industrial gas separation. It includes:

• Hyperbranched polyimides, amorphous glassy polymers and perfluorinated copolymers
• Nanocomposite (mixed matrix) membranes
• Polymeric magnetic membranes
• Sequestration of CO2 to reduce global warming
• Industrial applications of gas separation

Developed from sessions of the most recent International Congress on Membranes and Membrane Processes, Membrane Gas Separation gives a snapshot of the current situation, and presents both fundamental results and applied achievements.

This book describes recent and emerging results in membrane gas separation, including highlights of nanoscience and technology, novel polymeric and inorganic membrane materials, new membrane approaches to solve environmental problems e.g. greenhouse gases, aspects of membrane engineering, and recent achievements in industrial gas separation. It includes:

• Hyperbranched polyimides, amorphous glassy polymers and perfluorinated copolymers
• Nanocomposite (mixed matrix) membranes
• Polymeric magnetic membranes
• Sequestration of CO2 to reduce global warming
• Industrial applications of gas separation

Developed from sessions of the most recent International Congress on Membranes and Membrane Processes, Membrane Gas Separation gives a snapshot of the current situation, and presents both fundamental results and applied achievements.

Excerpt. © Reprinted by permission. All rights reserved.

Membrane Gas Separation

John Wiley & Sons

Copyright © 2010 John Wiley & Sons, Ltd
All right reserved.
ISBN: 978-0-470-74621-9

Chapter One

Synthesis and Gas Permeability of Hyperbranched and Cross-linked Polyimide Membranes

Shinji Kanehashi, Shuichi Sato and Kazukiyo Nagai Department of Applied Chemistry, Meiji University, Tama-ku, Kawasaki, Japan

1.1 Introduction

Recently, the polymer science field has focused on the role of polymers as membrane materials with precise, well-ordered structures through the development of defined synthesis and analysis of polymers. Among these well-ordered polymers are the hyperbranched polymers (e.g. hyperbranched polyimides). Part of the interest in such polymers is due to the expectation that they could have different properties as compared to common linear polymers. Also, cross-linked polyimides have attracted much attention from researchers, as can be judged by a high number of publications.

In general, hyperbranched polymers have many orderly branching units whose structures are different compared to linear and randomly cross-linked polymers. According to the Commission on Macromolecular Nomenclature of the International Union of Pure and Applied Chemistry (IUPAC), a crosslink polymer is defined as a polymer having a small region in a macromolecule from which at least four chains emanate. It is formed by reactions involving sites or groups on existing macromolecules or by interactions between existing macromolecules. The word 'network' is also defined as a highly ramified macromolecule in which essentially each constitutional unit is connected to each other constitutional unit and to the macroscopic phase boundary by many permanent paths through the macromolecule, the number of such paths increasing with the average number of intervening bonds; the paths must on the average be coextensive with the macromolecule. In this chapter, we use the term crosslink polymer to describe a random cross-linked network between polymer segments.

Precisely branched polymers include hyperbranched polymers, dendrimers and dendrons. Dendrimers and dendrons are characterized by perfectly controlled structures in three dimensions such as tree branch architecture, and they have attractive features such as a well-ordered chemical structure, molecular mass, size and configuration of polymers. Although the precise order of shape of hyperbranched polymers is less than that of dendrimers and dendrons, hyperbranched polymers have unique properties such as low viscosity attributed to the lack of entanglement of polymer segments, and the possibility of chemical modification in terminal functional groups such as in dendrimers.

Synthesis of hyperbranched polymers is typically performed through the selfpoly-condensation reaction of AB₂-type monomers (Scheme 1.1). The theoretical study of the random AB_x polycondensation has already been reported by Flory in 1952. He pointed out that the synthesis of hyperbranched polymers from AB_x monomers should resemble linear polymers in their elusion of infinite network (i.e. gelation) formation, which cannot occur except through the intervention of other interlinking reactions. Since then, there have only been a few experimental data made available on hyperbranched polymers; some have even been overlooked due to the fact that the use of the term hyperbranched polymers began only in the late 1980s. However, in early 1990s, hyperbranched polyphenylene was synthesized from AB₂-type monomers. This marked the beginning of the reawakened hyperbranched polymer concept. A variety of hyperbranched polymers such as polyphenylene, polyimide, polyamide, polyester, polyetherketone and polycarbonate have been reported in recent years. It is important that hyperbranched polymers with feathers of closed dendrons can be synthesized through the self-polycondensation one-step reaction because dendrimers and dendrons are synthesized by multistep procedures (e.g. protection, coupling and deprotection cycles). Producing dendrimers and dendrons is also costly and requires complicated manufacturing processes for industrial applications.

On the one hand, linear aromatic polyimides have been generally used as electronic and aerospace materials because of their excellent mechanical strength, thermal, chemical and electronic/optic properties compared with other common amorphous polymers. Polyimides are also excellent membrane materials for gas separation due to their rigid chemical structures, allowing the production of larger functional free volume. Over the past decades, numerous polyimides have been synthesized and their gas transport properties have been investigated. Scheme 1.2 shows the chemical structures of acid anhydrides and diamines mentioned in this chapter.

On the other hand, hyperbranched polyimides not only have the features of other hyperbranched polymers (e.g. low viscosity, good solubility) but also possess high thermal and physical stability, which is attributed to their rigid imide ring. It is commonly known that the kinds of terminal functional groups affect their physical properties, such as glass transition temperature and solubility. Hyperbranched polyimides have weak polymer chain interactions (lack of entanglement) and this affects their density, dielectric constant, refractive index and other properties. It is expected that they could provide an alternative to conventional polymer materials as novel functional and high-value added materials. Furthermore, hyperbranched polyimides could have a well-ordered structure compared with linear polyimides, which have a random distribution of polymer segments. Therefore, hyperbranched polyimides are expected to have favourable gas separation performance since their controlled branched structure could be advantageous in separating small molecules. Since the early 2000s, research on hyperbranched polyimides as gas separation materials has been reported, and these studies are still in progress.

Plasticization behaviour induced by condensable gases and vapours (e.g. carbon dioxide, hydrocarbons and other organic vapours) in polymer membranes is still a painful problem in polymeric membrane-based gas separation applications. Recently, novel hyperbranched polyimides were prepared from telechelic polyimides and an attempt was made to improve its gas separation performance and physical stability by obtaining plasticization-resistant materials (see e.g. Chapters 4, 6 and 7 of this book).

This chapter presents a review of numerous publications devoted to the concept and synthesis of hyperbranched and cross-linked polyimides. Also, gas permeation properties of these polymers are considered in detail.

1.2 Molecular Designs for Membranes

There exist different architectures of polymer macromolecules, as is shown in Figure 1.1.

Type I represents common linear polymers such as polysulfone, polycarbonate or polystyrene, for example. In glassy polymers, the movement of segments is frozen, though small-scale mobility of side groups is possible. In general they have good solubility in various organic solvents; however, their gas permeation properties in the presence of organic vapours are affected by plasticization phenomena.

Type II. In randomly cross-linked polymers the solubility in organic solvents gradually decreases with the increasing degree of crosslink density. Too frequent crosslinks result in the gelation of the polymer and a decline in gas permeability while simultaneously permselectivity can increase.

Type III. Hyperbranched polymers have numerous branch units. They have low viscosity, good solubility and are capable of being chemically modified in terminal functional groups. Hyperbranched polymers have a potential to be good gas separation materials because their molecular-sized spaces between branched polymers can be controlled.

Type IV. Dendrimers and dendrons have perfectly and orderly branched tree-like structures. Their molecular mass increases with the growth of the number of generation. Dendrimers and dendrons, like common organic molecules, are perfectly controlled in terms of chemical structure, molecular mass, configuration and distribution of polymers. Dendrons are well-ordered hyperbranched polymers and dendrimers are assembled from dendrons. It is expected that molecular-sized spaces between branched as well as hyperbranched polymers of dendrimers can be controlled and, therefore, could have high potential as gas separation membranes. An obvious disadvantage of dendrimers as membrane materials is their poor film-forming properties.

One of the key problems for polymeric gas separation membranes is gas and vapour-induced plasticization. The plasticization of polymers produces an enhancement of polymer chain mobility. It is a recognized fact that almost all polymeric membranes undergo swelling and plasticization under high pressure (concentration) of CO₂ and organic vapours, resulting in a significant loss in gas separation performance. One of the effective techniques against plasticization of polymers is the crosslink approach. There is a trade-off relationship between polymer crosslink density and gas permeability.

The mobility of polymer chains is larger for their polymer terminal chain ends as compared to that for the sections of macromolecules inside main chains. Therefore, plasticization may occur more easily around the polymer chain ends than in the polymer main chains. Moreover, if the number of polymer chain ends were minimized in a membrane, plasticization would be prevented. It is the hyperbranch structure that can create such behaviour in the case of rigid polymer chains.

Thus, we can state that the use of hyperbranched polyimides can enhance the resistance to plasticization of polymer membranes.

1.3 Synthesis of Hyperbranched Polyimides

Cross-linked (Type II) and hyperbranched (Type III) polyimides can be prepared for the use as gas separation membranes. There are no dendrimers and dendrons known, which would form free-standing membranes. Therefore, we focus on the synthesis of crosslinked (Type II) and hyperbranched (Type III) polyimides.

1.3.1 Amorphous Cross-linked Polyimides (Type II)

Generally, the aim of the study on crosslink polyimides is an attempt to enhance their gas selectivity and physical stability for gas-induced swelling and plasticization. Several crosslink techniques such as monoesterification and transesterification reactions of carboxylic acid, imide ring-opening reactions, grafted with epoxy reactions, UV-induced cross-linking and Diels–Alder-type cyclization reactions have been reported.

The monoesterification and transesterification reactions of carboxylic acids were performed using the following steps. The carboxylic acid-containing polyimide was monoesterificated under acid catalyst and thermal treatment, and the transesterification reaction was induced through further thermal treatment under vacuum. Many carboxylic acid-containing copolyimides have been synthesized and the crosslink reaction of the varieties of diol agents has been investigated. The structure of cross-linked membranes could be strongly affected by structures of the diol agent and polyimide compositions and annealing temperature after membrane formation. In the case of 6FDA-TMPD/DABA (3:2) cross-linked polyimides, 1,3-propanediol can be considered as an efficient crosslink agent.

The decarboxylation-induced cross-linking reaction of carboxylic acid is preceded by the reaction of the phenyl radical and the elimination of the carboxylic group by high temperature annealing. This decarboxylation-induced reaction is more sensitive to the reactivity of phenyl radicals rather than the effects of charge transfer complexing, oligomer and dianhydride formation. It was reported that the sites within the diamines section could be the TMPD methyl, biphenyl (between the carboxylic acid group) and at the site of cleaved CF₃ groups in 6FDA.

The imide ring-opening reaction occurred between the polyimide and primary diamine agents. Many chemical cross-linking reactions between 6FDA-based polyimides and primary diamines have been investigated. They were carried out by immersing the polyimide membranes into the methanol solution of amine compounds. The structure of the cross-linked membranes could depend on the structures of the primary amine agents and the reaction conditions such as the reaction time and temperature. Furthermore, the gas permeation properties in 6FDA-TMPD modified by amine compounds were described. The cross-linking in 6FDA-TeMPD with dendrimers such as polyamidamine (PAMAM) and polypropyleneimide (DAB-AM) has also been reported. There was no doubt that they took place, as the measurements of gel fraction and FTIR data showed; in addition, the degree of crosslink density increased in the order of generations G1 > G2 > G3 at the same reaction. The dielectric constant increased with the reaction time owing to the decrease in the polymer chain's mobility and free volume.

The etherification reaction of polyimides is similar to the process of the imide ringopening reaction. It was demonstrated for the reaction of polyimides with primary diamine and epoxy agents (for example, for 6FDA-TeMPD polyimides and tetraglycidyldiaminodiphenylmethane (TGDDM), diethyltoluenediamine (DETDA), TMPDA, 1,3-phenylenediamine (PDA) and 4,4'-(9-fluorenylidene)dianiline (FDA)). The density of polymers and crosslink concentration increased with the increase in epoxy content.

It is known that the crosslink reaction proceeds with participation of photo reactive benzophenone and alkyl chains under UV irradiation. Many benzophenone-containing BTDA-based polyimides have been synthesized and their cross-linking investigated. The same effects as discussed earlier were observed due to increases in the UV irradiation time.

Synthesis and Gas Permeability of Hyperbranched and Cross-linked Polyimide Membranes 9 Acetylene-terminated or internal acetylene imide oligomers were investigated for aerospace and electronic applications, in particular because of their good thermal and environmental stability. In respect of membrane application the aim of these studies was an enhancement of the physical stability under high pressure CO₂, that is, resistance to plasticization. Recently, a co-polyimide was synthesized from 6FDA, TeMPD and 4,4-diaminodiphenylacetylene (p-intA) having internal acetylene structure. After thermal treatment at 400°C of such a membrane (an internal acetylene membrane), the cycloaddition of a Diels–Alder-type reaction occurred, according to the results of DSC and FT-Raman spectroscopy. The cross-linked membrane was insoluble; however, no densification of the membrane was observed.

1.3.2 Hyperbranched Polyimides (Type III)

Hyperbranched polyimides can result due to the self-polycondensation reactions of AB2-, A₂- and B₃-types. The preparation of hyperbranched polyimides involves chemical imidization of polyamic acid ester synthesized from AB₂-monomers, which are carboxylic dianhydrides containing an ether bond and a diamine. Polyamic acid in combination with a condensation agent is used because it is difficult to separate the synthesized polymer from AB₂-type monomers.

For example, it is possible to prepare hyperbranched polyimides from 3,5-dimethoxyphenol and 4-nitrophthalonitrile in the presence of diphenyl(2,3 - dihydro-2-thioxo-3-benzoxazolyl) phosphonate (DBOP) as a condensation agent at room temperature. Hyperbranched polyimide was obtained through thermal or chemical imidization of the precursor (polyamic acid) (Scheme 1.3). The obtained hyperbranched polyimide had a relatively great molecular mass (M_w) of about 190 000 g mol^-1 but low intrinsic viscosity of 0.30 dL g^-1. Therefore, it had a compact configuration and the lack of entanglement of polymer chains. The polymer obtained via chemical imidization was soluble in aprotic polar solvents such as tetrahydrofuran (THF), while the polymer from thermal imidization was insoluble in any solvents.

Other examples of self-polycondensation of an AB₂-type monomer containing an imide-ring via etherification reactions can be found in the literature. The self-polycondensation can be performed though nucleophilic etherification of silylated phenol and aryl fluoride in diphenylsulfone at 240°C under the presence of caesium fluoride (Scheme 1.4). This reaction involved a nucleophilic substitution of the halogen group interacting with the electron-attracting imide-ring. It gave an increase in number average molecular mass (Mn) value from 52 000 to 85 000 g mol^-1. The hyperbranched polyetherimide was soluble in common organic solvents and showed high thermal stability.

(Continues...)

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Edition details

• Publisher: Wiley
• Publication date: 2010
• Language: English
• ISBN 10: 0470746211
• ISBN 13: 9780470746219
• Binding: Hardcover
• Edition number: 1
• Number of pages: 392
• Editor: Freeman Benny, Yampolskii Yuri