Microporous Materials for Separation Membranes E-Book

Xiaoqin Zou

• Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 910

Veröffentlichungsjahr: 2019

Beschreibung

A guide to membrane separation based on a variety of porous materials with promising separation applications

Microporous Materials for Separation Membranes offers an in-depth guide that explores microporous materials? potential for membrane applications. The authors?two experts on the topic?examine a wide range of porous materials that have application potential including: microporous silica, porous carbons, zeolites, metal-organic frameworks (MOFs), and porous organic frameworks (POFs).

Comprehensive in scope, the book covers a broad range of topics on membrane separations such as: hydrogen recovery, carbon dioxide capture, air purification, hydrocarbon separation, pervaporation, and water treatment. In addition, this up-to-date resource explores the most recent materials for preparing microporous membranes and explores the most promising applications for industrial use. This important book:

-Examines the use of microporous materials as membranes to perform with different gases and liquids
-Offers an overview of the basic knowledge of membrane separation and an intense examination of separations
-Describes the state-of-the-art of membrane separation with porous materials
-Highlights the most promising applications of industrial interest

Written for scientists working in the fields of membranes, gas and liquid, Microporous Materials for Separation Membranes offers a valuable guide to the potential of microporous materials for membrane applications.

Leseprobe

Table of Contents

Cover

Preface

1 Introduction of Microporous Membranes

1.1 Introduction

1.2 Historical Development of Membranes

1.3 Microporous Materials

1.4 Fundamentals of Membrane Separation

1.5 Membrane Configurations

1.6 Features of Microporous Membranes

1.7 Conclusions

References

2 Microporous Silica Membranes

2.1 Introduction

2.2 Membrane Synthesis

2.3 Intermediate Layers

2.4 Support

2.5 Modification of Silica Membranes

2.6 Microporous Silica Membranes for Hydrogen Separation

2.7 Microporous Silica Membranes for Carbon Dioxide Separation

2.8 Microporous Silica‐Based Membranes for Pervaporation Process

2.9 Microporous Silica‐Based Membranes for Desalination

2.10 Conclusions and Future Trends

References

3 Carbon‐Based Membranes

3.1 Introduction

3.2 Carbon Membrane Preparations

3.3 Selective Surface Flow Membranes

3.4 Advantages and Disadvantages of Carbon Membranes

3.5 Carbon Nanotubes

3.6 Porous Graphene

3.7 Carbon‐Based Mixed Matrix Membranes (MMMs)

3.8 New Advances and Challenging Aspects

3.9 Conclusions

References

4 Microporous Membranes for Water Purification

4.1 Introduction

4.2 Types and State‐of‐the‐Art Microporous Membranes in Water Purification

4.3 Removal of Water Contaminants (Inorganics, Organics, Biological)

4.4 Membrane Desalination

4.5 Membrane Surface Engineering

4.6 Conclusions

References

5 Mixed Matrix Membranes

5.1 Introduction

5.2 Principles of Mixed Matrix Membranes

5.3 MMMs Made of Zeolites

5.4 MOF‐Based MMMs

5.5 POF‐Derived MMMs

5.6 MMMs Containing Other Porous Fillers

5.7 Conclusions

References

6 Zeolite Membranes

6.1 Introduction

6.2 Synthesis Techniques for Zeolite Membranes

6.3 Crystal Growth in Zeolite Layers

6.4 Microstructures of Zeolite Films

6.5 Membrane Characterizations

6.6 Conclusions and Outlook

References

7 Gas Separations with Zeolite Membranes

7.1 Introduction

7.2 H

2

Recovery

7.3 Air Separation

7.4 CO

2

Capture

7.5 N

2

/CH

4

Separation

7.6 H

2

S Capture

7.7 Kr/Xe Separation

7.8 Post‐modification of Zeolite Membranes

7.9 Conclusions and Outlook

References

8 Pervaporation with Zeolite Membranes

8.1 Introduction

8.2 Pervaporation Process

8.3 Alcohol Dehydration

8.4 Organic–Organic Separation

8.5 Acid Separation

8.6 Membrane Reactors for Various Separations

8.7 Conclusions and Outlook

References

9 MOF Membranes and Their H

2

Separation Properties

9.1 Introduction

9.2 Fabrication of MOF Membranes

9.3 Controlling and Characterizing MOF Membranes

9.4 Pore Chemistry for H

2

Separation

9.5 Conclusions

References

10 CO

2

Capture with MOF Membranes

10.1 Introduction

10.2 CO

2

Capture and Separation Strategies

10.3 Chemistry of MOFs for CO

2

Recognition

10.4 Membrane Design for CO

2

Separation

10.5 Conclusions

References

11 MOF Membranes for Vapor or Liquid Separation

11.1 Introduction

11.2 Selective Separation of Chemicals Via Pervaporation

11.3 Organic Solvent Nanofiltration

11.4 Chiral Resolution

11.5 Stability of MOF Membranes

11.6 Conclusions and Outlook

References

12 Microporous Organic Framework Materials for Membrane Separations

12.1 Introduction

12.2 Porous Structures and Free Volumes

12.3 Hydrogen Recovery

12.4 Carbon Dioxide Capture

12.5 Air Separation

12.6 Other Gas Separations

12.7 Emerging Liquid Separations

12.8 Conclusions

References

Index

End User License Agreement

List of Tables

Chapter 1

Table 1.1 Microporous materials.

Table 1.2 Industrial membrane technologies.

Chapter 2

Table 2.1 Summary of microporous silica membranes for hydrogen separations.

Table 2.2 Microporous silica membranes for CO

2

separation.

Table 2.3 Membrane pervaporation performances for organic–water and organic–orga...

Table 2.4 Performance comparison of silica‐based membranes for desalination.

Chapter 3

Table 3.1 Features of vertically aligned (VA)‐CNT membranes and mixed (composite...

Table 3.2 Advantages and limitations of different perforation methods.

Chapter 4

Table 4.1 Membrane technologies and major contaminants in source waters that can...

Table 4.2 Membrane types and processes and essential and additional desired prop...

Table 4.3 Microporous membranes used for removing targeted organic contaminants,...

Table 4.4 Relevant findings of vertically aligned (VA) and mixed matrix (MM) CNT...

Table 4.5 Efficiency of various porous membranes in water desalination with resp...

Table 4.6 Recent studies on the surface modification for membrane antifouling.

Chapter 5

Table 5.1 Glass transition temperatures (

T

g

) of common polymers.

Table 5.2 A summary of different solvents used for fabricating MMMs.

Table 5.3 Different microporous materials and their properties.

Table 5.4 MMMs made of zeolites and polymers for gas separation.

Chapter 6

Table 6.1 Chemical structures and abbreviations of possible ISS types [89].

Table 6.2 Increase of the permselectivities (PS) derived for different gas mixtu...

Chapter 7

Table 7.1 Separation pattern of the AIPO

4

‐5 membrane tested at 91 °C and at a pr...

Table 7.2 Membranes for single gas permeation of Kr and Xe.

Table 7.3 Performance comparison of various membranes in Kr/Xe separations.

Chapter 9

Table 9.1 Summary of MOF membranes for H

2

separation.

Chapter 10

Table 10.1 MOF membranes for CO

2

separations.

Chapter 11

Table 11.1 Polycrystalline MOF membranes for PV separation process.

Table 11.2 MMMs based on MOFs for PV separation process.

Table 11.3 MMMs based on MOFs for organic solvent nanofiltration process.

Table 11.4 MOF membranes or films for chiral separation process.

List of Illustrations

Chapter 1

Figure 1.1 Historical development of membrane separation technology fro...

Figure 1.2 Representative classes of carbonaceous materials.

Figure 1.3 Sol–gel process for the formation of silica polymers.

Figure 1.4 The formation process of some representative zeolites.

Figure 1.5 A scheme for the self‐assembly of metal–organic frameworks b...

Figure 1.6 Molecular models of representative porous polymers.

Figure 1.7 Chemical structures of different catalogues of porous organi...

Figure 1.8 Schematic representation of a supported membrane.

Figure 1.9 Gas transport mechanisms in membranes.

Figure 1.10 Change of flux with time for UF membrane showing asymptotic...

Figure 1.11 Schematic of concentration and potential gradients in a wel...

Figure 1.12 Schematic of concentration gradients adjacent to a single c...

Figure 1.13 Schematic diagrams of the commonly used membranes.

Figure 1.14 Pressure‐driven membrane separation processes.

Figure 1.15 An early plate‐and‐frame design for the separation of heliu...

Figure 1.16 A tubular UF membrane system in which tubes are connected i...

Figure 1.17 An unfolded view of a spiral wound module.

Figure 1.18 Two types of hollow fiber modules: (a) shell‐side feed and ...

Figure 1.19 Four important parameters – pore size, interconnected pores...

Chapter 2

Figure 2.1 Schematic diagram of a microporous silica membrane.

Figure 2.2 Sol–gel process of microporous silica by polymeric and collo...

Figure 2.3 Schematic of the template approach for the synthesis of micr...

Figure 2.4 Illustration of CVD setup for depositing microporous silica ...

Figure 2.5 Various shapes of supports made of different materials.

Figure 2.6 Proposed mechanism for the instability of the pore structure...

Figure 2.7 The changes of gas permeances in function of time for Ni‐dop...

Figure 2.8 Permeance vs. kinetic diameter for several gas species teste...

Figure 2.9 SEM images of the cross section of an NS membrane: (a) asymm...

Figure 2.10 A schematic diagram for the pervaporation process.

Figure 2.11 Time course of the pervaporation performance on a BTESE‐der...

Figure 2.12 Schematic of water and hydrated ion diffusion through the p...

Figure 2.13 Schematic of the structure of the microporous organosilica ...

Chapter 3

Figure 3.1 The basic structures of (a) carbon molecular sieve, (b) grap...

Figure 3.2 (a) The production of carbon molecular sieve membranes throu...

Figure 3.3 SEM pictures of a hollow fiber manufactured by Carbon Membra...

Figure 3.4 Conceptual illustration for the structures of CNT membranes:...

Figure 3.5 Process flow for the fabrication of VA‐CNT membranes using t...

Figure 3.6 Functionalization fashions of CNT membranes.

Figure 3.7 Schematic pictures of the two types of graphene‐based membra...

Figure 3.8 A nitrogen‐functionalized pore with dangling carbon replaced...

Figure 3.9 (a) Final structure of the GO monolayer generated by the Mon...

Figure 3.10 (a) Scanning electron microscopy (SEM) and (b) atomic force...

Figure 3.11 Schematic diagrams of (a) the penetration processes of diff...

Figure 3.12 Five strategic directions of focus to orientate future rese...

Chapter 4

Figure 4.1 Representative types of membranes on the basis of solute siz...

Figure 4.2 Graphene pores of (a) hydrogenated and (b) hydroxylated func...

Figure 4.3 SEM images of the CNF membrane: (a) upper view with an inset...

Figure 4.4 Contact angle measurements of Ag/PAA–CNTs membrane under (a–...

Figure 4.5 Representative photographs of contact angle measurements: (a...

Figure 4.6 (a) Photographs of ultrathin graphene nanofiltration membran...

Figure 4.7 Morphology of the GO membrane: (a) digital photo, (b) SEM cr...

Figure 4.8 SEM images for the double‐layer nanoporous membrane fabricat...

Figure 4.9 The hierarchical layer of TiO

2

nanowires UF membrane: (a and...

Figure 4.10 Trapping of salts and movement of water molecules from sali...

Figure 4.11 (a) Schematic representation of the penetration processes o...

Figure 4.12 Structure and morphology of a polyamide (PA) thin‐film comp...

Figure 4.13 (a) Cross‐sectional view of a zeolite membrane grown direct...

Figure 4.14 Schematic representation of membrane modification using nan...

Figure 4.15 Variation of hydrophilicity with a change in concentration ...

Chapter 5

Figure 5.1 Historical growth of academic publications related with the ...

Figure 5.2 Systematic diagram of a mixed matrix membrane consisting of ...

Figure 5.3 Three types of phase inversion processes.

Figure 5.4 Schematic diagram of three nonideal morphologies in MMMs: in...

Figure 5.5 Schematic of the silane coupling process between zeolite and...

Figure 5.6 Schematic representation of major functionalities in MOF‐bas...

Figure 5.7 Structures of some typical MOFs as fillers in MMMs.

Figure 5.8 Scheme demonstrating favorable interactions between fragment...

Figure 5.9 Schematic illustration of the cavity‐occupying concept for t...

Figure 5.10 Profile of metal–organic framework fillers in mixed matrix ...

Figure 5.11 PAF/polymer MMM by intermixing glassy polymer and PAF‐1: (a...

Figure 5.12 (a) The synthesis protocol for preparing PAF‐56P and (b) PA...

Figure 5.13 (a) Schematic representations of the synthesis of COFs and ...

Figure 5.14 Schematic representation showing the postulated role that M...

Figure 5.15 (a) Porous imine cage CC3 synthesized from 1,3,5‐triformylb...

Chapter 6

Figure 6.1 Comparative synthsis models of zeolite membrane preparation ...

Figure 6.2 Zeta potentials of suspended MFI crystals with different Si/...

Figure 6.3 Improvement of the LTA and FAU membrane qualities by using a...

Figure 6.4 Supported silicalite‐1 membrane in the separation of

p

/

o

‐xyl...

Figure 6.5 FESEM images of

a

‐ and

b

‐oriented silicalite‐1 films obtaine...

Figure 6.6 Schematic representation of a MFI zeolite membrane with defe...

Figure 6.7 Schematic presentation of the pore filling process (a redraw...

Chapter 7

Figure 7.1 Examples of the state‐of‐the‐art microporous materials as bu...

Figure 7.2 Mixture separation factor α for H

2

/

i

‐butane (a feed composit...

Figure 7.3 The O

2

/N

2

separation factor and the oxygen permeance of the ...

Figure 7.4 Comparison of the CO

2

/CH

4

separation selectivity versus the ...

Figure 7.5 Component fluxes of the binary (50 : 50) mixture of CH

4

[7]...

Figure 7.6 Mixture permeation selectivity for CO

2

: (a) as a function of...

Figure 7.7 CO

2

/N

2

separation factor versus gas permeation test duration...

Figure 7.8 CO

2

permeance and CO

2

/CH

4

selectivity at 298 K and 400 kPa p...

Figure 7.9 Top (A) and cross‐sectional view SEM (B) images of SAPO‐34 m...

Figure 7.10 (a) Kr permeance as a function of membrane thickness and (b...

Figure 7.11 Kr/Xe separation selectivity vs. Kr permeance over AlPO‐18,...

Figure 7.12 Kr permeance for SAPO‐34, ZIF‐8, and AlPO‐18 membranes as a...

Figure 7.13 Percent changes in Kr/Xe permeabilities and their selectivi...

Figure 7.14 Temperature dependence of binary Kr and Xe permeances and K...

Figure 7.15 Crystallographic structures of (a) ZIF‐8, (b) IRMOF‐1, (c) ...

Figure 7.16 Temperature‐dependent separation performance of

b

‐oriented ...

Figure 7.17 SEM cross section of a PDMS‐coated silica membrane supporte...

Figure 7.18 Schematic illustrations of H

2

and CO

2

permeation through th...

Chapter 8

Figure 8.1 Schematic of the pervaporation process.

Figure 8.2 Cross‐sectional sketches of modules for disk‐shaped (a) and ...

Figure 8.3 Plant for the dehydration of bioethanol by steam permeation ...

Figure 8.4 Pervaporation test for water/ethanol at 40 °C [74].

Figure 8.5 SEM images of (a) the seeded support, (b) optical picture, a...

Figure 8.6 Flux Q (□) and separation factor

α

(○) as a function of...

Figure 8.7 Zeolite nanosheet membrane: (a) TEM images of

b

‐oriented MFI...

Figure 8.8 Schematic of a membrane‐supported water–gas shift reaction: ...

Figure 8.9 Conversion enhancement by water removal via a hydrophilic ZS...

Figure 8.10 Schematic illustration of a WGS membrane reactor with modif...

Figure 8.11 ZSM‐5 membrane performance in xylene isomer separation (fee...

Chapter 9

Figure 9.1 Fabrication methods for MOF films or membranes.

Figure 9.2 Schematic representation of

in situ

crystallization of MOFs ...

Figure 9.3 Scheme for preparing ZIF‐90 membranes using 3‐aminopropyltri...

Figure 9.4 (a) Diffusion cell for ZIF‐8 membrane fabrication and (b) sc...

Figure 9.5 Representation of (a) spin‐coating setup [48] and (b) electr...

Figure 9.6 Schematic illustration of heteroepitaxially growing IRMOF‐3 ...

Figure 9.7 Schematic depiction of the electrochemical fabrication of MO...

Figure 9.8 (a) Chemical vapor deposition approach for the ZIF‐8 membran...

Figure 9.9 Liquid–liquid interfacial synthesis: (a) interfacial prepara...

Figure 9.10 (a) Schematic illustration of the fabrication of NAFS‐1 mem...

Figure 9.11 Powder X‐ray diffraction patterns of platelike MOF crystals...

Figure 9.12 Experimental N

2

adsorption–desorption isotherms of micro‐ZI...

Figure 9.13 TGA profile recorded in nitrogen and air environments for t...

Figure 9.14 (a) SEM and (b) elemental mapping images of a ZIF‐8 membran...

Figure 9.15 NMR spectrum of NU‐1000 (a) before and (b) after removal of...

Figure 9.16 Zr

6

node of NU‐1000 before (a) and after (b) removal of exc...

Figure 9.17 Single‐component gas permeation results of an MOF‐5 membran...

Figure 9.18 Representations of separation factor and H

2

permeance of (a...

Figure 9.19 Gas permeance for ZIF‐8 membrane vs. gas kinetic diameter: ...

Figure 9.20 Mixed gas permeances and H

2

/CO

2

selectivity of the ZIF‐95 m...

Figure 9.21 (a) Plot of H

2

/CO

2

permeances and separation factors for th...

Chapter 10

Figure 10.1 Fundamental classifications of CO

2

capture and separation s...

Figure 10.2 Block diagram representation of CO

2

capture strategies.

Figure 10.3 Flow diagram of a typical chemical absorption process for C...

Figure 10.4 Representation of physical absorption process to capture CO

Figure 10.5 Schematic representation of separation mechanisms: (a) Knud...

Figure 10.6 Schematic of chemical‐looping combustion system.

Figure 10.7 Schematic representation of the design and construction of ...

Figure 10.8 (a) CO

2

and N

2

sorption isotherms of 1 and the PCN‐6X serie...

Figure 10.9 CO

2

sorption isotherms and separation curves of UiO‐67, BUT...

Figure 10.10 Pore size tuning from supramolecular isomerism in diamondo...

Figure 10.11 Carbon dioxide adsorption isotherms at 25 °C (blue), 40 °C...

Figure 10.12 SEM images of the ZIF‐8 membrane fabricated on PDA‐modifie...

Figure 10.13 SEM images of (a) Ni foam, (b) Ni

3

S

2

nanoarrays on Ni foam...

Figure 10.14 SEM images of the CAU‐1 membrane: (a) upper view and (b) l...

Figure 10.15 (a) CO

2

/CH

4

permeances and separation factor (SF) for the ...

Chapter 11

Figure 11.1 MOF membranes for vapor or liquid separation.

Figure 11.2 SEM images (a–c and e, cross section; d, top view) and (f) ...

Figure 11.3 Flow chart and PV performance of the UiO‐66 membrane for se...

Figure 11.4 (a) TGA curves of UiO‐66(Zr)‐(OH)

2

crystals before and afte...

Figure 11.5 (a) Experimental and simulated adsorption and desorption is...

Figure 11.6 Formation of the ZIF‐8/PDMS nanohybrid composite membrane b...

Figure 11.7 Comparison of the pervaporation performance: (a) flux norma...

Figure 11.8 FESEM micrographs of cross section and top surfaces for TFC...

Figure 11.9 Schematic diagram of MOF@GO membranes prepared by PASA filt...

Figure 11.10 (a) SEM image of TFC membrane surface after DMF dipping, (...

Figure 11.11 (a) Images of ZIF‐8 synthesis using the interfacial method...

Figure 11.12 Comparison of the microstructures of the TFC/TFN membrane ...

Figure 11.13 Top: two principal growth directions and of [Zn

2

(cam)

2

(dab...

Figure 11.14 (a) Leica picture of the surface of the Ni

2

(

L

‐asp)

2

(bipy) ...

Figure 11.15 Comparison of ZIF‐8 membranes in surface micrographs, (a) ...

Chapter 12

Figure 12.1 (a) Cross section of a porous solid illustrating open and c...

Figure 12.2 (a) Synthesis and molecular structures of PIM‐EA‐TB and PIM...

Figure 12.3 (a) Effect of pore size on the diffusivity and (b) pore siz...

Figure 12.4 (a) H

2

diffusion in tortuous and straight channels of micro...

Figure 12.5 H

2

permeability and selectivity over N

2

(a) and CH

4

(b) in ...

Figure 12.6 Two‐dimensional representations of the contorted PIM‐1 memb...

Figure 12.7 (a) Reaction scheme for the conversion of PIM‐1 to TZPIM (t...

Figure 12.8 (a) Schematic representation of chemical structure and mole...

Figure 12.9 CO

2

permeability and selectivity over N

2

(a) and CH

4

(b) in...

Figure 12.10 Macromolecular structures and fragment models of PIM‐1 and...

Figure 12.11 Chemical structure and molecular fragment of PIM‐BTrip‐TB ...

Figure 12.12 Schematic illustration of three bridged building blocks fo...

Figure 12.13 O

2

permeability and O

2

/N

2

selectivity in selected micropor...

Figure 12.14 (a) Diagram for the synthesis of tubular COF‐LZU1 membrane...

Figure 12.15 Surface‐initiated polymerization of the CMP membrane on a ...

Figure 12.16 (a) Schematic representation of COF membrane fabrication a...

Guide

Cover

Table of Contents

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Microporous Materials for Separation Membranes

Xiaoqin ZouGuangshan Zhu

Copyright

Authors

Xiaoqin Zou

Northeast Normal University

Faculty of Chemistry

No. 5268 Renmin Street

130024 Changchun

China

Guangshan Zhu

Northeast Normal University

Faculty of Chemistry

No. 5268 Renmin Street

130024 Changchun

China

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applied for

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Print ISBN: 978‐3‐527‐34397‐3

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Preface

Separation represents a very important industrial activity because a lot of labors and investments are devoted to separate the components of large quantities of mixtures into pure or purer forms. These processes involved account for 10–15% of the world's energy consumption. Membrane technology is widely accepted as a more efficient and less energy‐consuming separation and purification process compared with conventional ones (e.g. distillation, sublimation, crystallization, etc.). Its efficiency reflects not only on the large quantity processed per day but also on its unique powerfulness in some specific systems such as azeotropic liquids or solutes. Membrane‐based separation would use 90% less energy than distillation. Thus, membrane separation has attracted much attention from both academic and engineering fields, and this interest will definitely keep on increasing. Membrane separation works on selective transport of a particular species through a semipermeable membrane layer under a certain driving force. Pressure driven is the simplest in terms of the ability to separate species in gas or liquid feed streams with high flux. According to the membrane definition, materials play a central role because they govern the mass transport behaviors; thus consequently the selectivity and the permeability are two main parameters for membrane separation. Although some other books are available for membrane separation, this book is solely focused on the development of advanced microporous materials for membrane application from materials perspective.

Microporous materials possess one unique and important feature of pores with diameters at the molecular level (<2.0 nm). The porosity should have a specific impact on the functions of materials, and microporosity is of vital importance for molecular separation when these materials are processed into membranes. Typical examples are microporous silica, carbon molecular sieves, zeolites, organic–inorganic porous hybrids, and porous organic polymers. Microporous materials remain one of the driving forces for the development in the field of membrane separation. These materials comprise wonderfully complex structures and compositions. The fascinating architectures such as the pore size, the accessible void space, the dimensionality of the channel system, and the pore microenvironment endow them with new membrane chemistry. We are trying to comprehensively understand the synthesis, properties, and new functions in membrane separations, and we feel that the contents of this book will offer value to both the novice and the expert.

This book contains 12 chapters in total and possibly will be expanded intentionally when interesting materials are introduced in membranes from future researches. Chapter 1 gives us a brief introduction of microporous materials and membrane separation from chemistry and chemical engineering points of view. The aspects include the chemistries of microporous materials and membrane separation and the basics of membrane fabrications and module configurations. Subsequent chapters follow the road map of membrane separation with microporous materials, which are discussed in Chapter 1. It is very first discovered that silica can be processed into microporous membranes (Chapter 2). Microporous silica usually contains very small pores of 3–5 Å, suitable for membrane separation of small molecules such gas light gases, water, and small organics (Chapters 2 and 4). Porous carbons are another interesting class of microporous materials; therefore Chapter 3 covers broad topics in porous carbon synthesis, membrane modification, and separation engineering. The regular‐pore‐structured membranes of carbon nanotubes and graphene receive a particular attention not only in their interesting structures but also their usefulness in separation application, for instance, water treatment (Chapter 4). Zeolites and molecular sieves as a traditional and vigorous crystalline porous material with identified pore structures are another focus. Chapters 6, 7, and 8 are devoted to fully discuss zeolite membranes. Chapter 6 describes the chemistry of zeolite membranes encompassing crystal growth, microstructures of film layers, and membrane characterization. Chapter 7 deals with zeolite membranes for gas separations. Pervaporation is another branch of separation or purification with zeolite membranes since it is an important process in removing impurities or recovering useful components (Chapter 8). Metal–organic frameworks are an extensive class of crystalline materials with structural diversity and tunable porosity. The unparalleled growth in researches motivates us to put more emphasis on metal–organic framework‐based membranes. Membrane synthesis runs before its application; thus a separate chapter (Chapter 9) is included to overview synthesis routes and possible techniques to control the crystallization during membrane synthesis. Separation of gases of industrial interest such as hydrogen and carbon dioxide are given as examples to show separation capability of metal–organic framework membranes (Chapter 10). Initial attempts are being done in vapor or liquid separations; thus Chapter 11 gives us the landscape of various liquids such as organics purification, desalination, and chiral resolution. Porous organic frameworks are emerging as a special type of polymers because of enormous internal surface areas and are standing in the list of microporous materials. Chapter 12 briefly introduces the recent development of porous organic frameworks in the membrane application with potentiality in gas separations and water treatment. For the engineering aspect of our discussion, we have a chapter (Chapter 5) as another key component of this book to introduce the science and application of mixed matrix membranes.

Finally, this book showcases a wide array of topics that highlight the diversity of microporous materials chemistry and the synthetic principles and structural characteristics of microporous materials, as well as their applications in different separation systems. The objectives of this book are to summarize the most pertinent knowledges of microporous membranes and the most up‐to‐date advances of materials‐driven membrane separations. We hope that readers will find it beneficial to their future research and teaching endeavors. Any comments or suggestions about this book are warmly welcome since this book screens only the tip of microporous membranes. Toward this end, we would like to thank Huazhen Rong, Baisong Liu, Muhammad Faheem, Dr. Sheng Wu, Jialu Li, Hao Zhang, Dr. Kok‐Giap Haw, Guangli Yu, Yanqin Li, Ziyang Wang, and Dr. Zixi Kang for their outstanding contributions to several chapters, as well as the strenuous efforts from the editorial staff members.

24 December 2018

Xiaoqin Zou

Guangshan Zhu

1Introduction of Microporous Membranes

1.1 Introduction

Membrane technology is widely considered as an important process and is used in a broad range of applications, such as chemical and biological processes. Membrane separation represents one of the most important applications of membrane science and technology. Membrane separation is practiced on feed streams ranging from small gases to large colloids. The key property that is exploited for gas separation is the ability of a membrane to allow a particular gas species in the gas mixture to permeate freely through the membrane while hindering the permeation of other species. In liquid separation, the goal is to control the transport rate of a colloidal particle while retaining other particles in a reservoir.

1.2 Historical Development of Membranes

Membrane science and technology has a long history, and the studies can be traced back to the eighteenth century [1]. The development of membrane technology can be divided into two periods of early membranes and modern membranes. The concept of membrane permeation was proposed in 1740s. The word osmosis was coined by Abbé Nolet to describe the permeation of water through a diaphragm in 1748. In the nineteenth century, theories were started in laboratory to describe the physical/chemical behaviors of membranes. For example, in 1887, the limit law was developed by van't Hoff through the measurements of solution osmotic pressure with membranes made by Traube and Pfeffer. The law was intended to explain the behavior of ideal dilute solutions, the work of which also led directly to the van't Hoff equation. Almost at the same time, the concept of a perfectly selective semipermeable membrane was used by Maxwell and others in developing the kinetic theory of gases.

Early membranes were experimented with different types of bio‐diaphragm, such as animal bladders or fish casings. Later, membranes made of nitrocellulose were investigated because of reproducibility. In 1907, Bechhold devised a technique to prepare nitrocellulose membranes of graded pore size [2]. With extensive studies from other workers such as Elford [3], Zsigmondy and Bachmann [4], and Ferry, Bechhold's technique [5] was greatly improved to prepare high‐quality nitrocellulose‐based membranes. As a result of their efforts, microporous membranes were commercialized by the early 1930s. In the next 20 years, microporous membranes were applied to the microfiltration (MF) technology. For instance, MF membranes found their first significant application in the testing of drinking water in most Europe, particularly for Germany during the end of World War II. The research effort to develop these membranes as filters, sponsored by the US Army, was continued by the Millipore Corporation. In addition to the technique renewal, membrane materials were expanded to other polymers, such as cellulose acetate.

Modern membranes appeared during 1960s, but they are only used for laboratory purpose or small‐scale industrial application. Membranes suffered from four drawbacks of unreliability, slow transport, low selectivity, and large expense, which limited their widespread use as a separation process. Solutions to each issue have been resolved, which paved way to the wide application in separation processes. The significant event was witnessed by the transformation from a laboratory to an industrial process in membrane separation. The single most important work could be counted for the Loeb–Sourirajan process for making defect‐free, high‐flux, anisotropic reverse osmosis (RO) membranes [6]. These membranes consist of an ultrathin, selective surface film of cellulose acetate supported on a much thicker but much more permeable porous substrate. The technical support from scientific researchers and engineers, as well as the timely infusion of financial funds for research and development from the US Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of RO membranes. In parallel with industrial applications in environmental processes, the potentiality of membranes was realized for medical separation processes. The success was made in the artificial kidney, for example, Kolf and Berk [7] demonstrated the first successful artificial kidney in the Netherlands in 1945. Since the 1960s, the use of membranes in artificial organs has become a major lifesaving procedure after a longtime technology refinement. The sales of these devices comfortably dominated the membrane market, which largely exceeded that of the total industrial membrane separations. Another representative application of membranes is as blood oxygenator and controlled drug delivery system. The membrane techniques developed by ALZA (a company founded by Alex Zaffaroni) and its peers were widely used in the pharmaceutical industry to improve the efficiency and safety of drug delivery. The membrane technology is maturing in the next decade (1970s), experienced by a number of technique innovations taken place during this period. Sponsored by OSW, other membrane fabrication techniques on the basis of Loeb–Sourirajan one, including interfacial polymerization and multilayer composite casting and coating, were introduced for producing high‐performance membranes. Engineering the membranes with very thin selective layers down to 0.1 μm or even less has been accomplished by several companies. Methods of packaging membranes with large areas and different configurations such as spiral wound, hollow fine fiber, capillary, and plate‐and‐frame modules were also developed. Improvements and advances have been made in enhancing the membrane stability. The fruits of the OSW program came out in commercial membrane units in the late 1970s, and the modern membrane technology began in the early 1980s.

The development of modern membranes can be divided into the following phases (Figure 1.1).

Figure 1.1 Historical development of membrane separation technology from 1960 to 2010.

In the first phase, natural and synthetic polymers were employed for fabricating membranes in large scales. MF, ultrafiltration (UF), RO, and electrodialysis (ED) were well established in industries. The emergence of industrial membrane gas separation processes opened a new avenue in membrane technology. The first major product was the Monsanto Prism membrane for hydrogen separation [8]. The Dow Chemical Company started to produce membrane systems for separating nitrogen from air. Within a few years, Cynara and Separex tried to make polymeric membranes to separate carbon dioxide from natural gas. Gas separation technology is continuing to evolve and expand owing to the vast resources of polymers and matured membrane fabrication techniques. Further growth is experienced in the coming years with a milestone of pervaporation (PV) in the membrane road map. Gesellschaft für Trenntechnik (GFT), a small German engineering company, introduced the first commercial PV system for dehydration of alcohol and other solvents. Since then, a number of PV‐based plants have been installed for ethanol and isopropanol dehydration, particularly for bioethanol extraction from biomass. The third development phase, which began in the mid‐1990s, was the establishment of reliable and economical MF/UF membranes for the treatment of municipal water sources and for use in membrane bioreactors (MBR) in sewage treatment plants. Concurrent with the progress in polymer membranes, inorganic membranes evolved in the 1990s. Ceramic membranes represent a classic type of inorganic membranes. Artificial ceramic membranes were made from inorganic materials such as alumina, titania, zirconia oxides, silicon carbide, or some glassy materials. The first trial of ceramic membranes was employed for uranium enrichment in the nuclear industry in France in the late 1980s. After many of the nuclear plants were set up in France, other industrial application areas for the ceramic membranes were sought out. Meanwhile, academic research on ceramic membranes was conducted [9]. Most ceramic membranes manufacturers were based in France, and other companies outside France (e.g. Philips Ceramics Uden, Atech, Inopor, Jiangsu Jiuwu, LiqTech, and Mantec Technical Ceramics Ltd.) also participated in the family of manufacturing ceramic membranes. They are used in membrane operations for liquid filtration. In contrast to polymeric membranes, they can be used in separations where aggressive media (acids, strong solvents) are present. They also have excellent thermal stability that makes them usable in high temperature membrane operations. According to the timeline of membrane development, much effort in modern times has been done to improve membrane performance by creating new materials. Around 2000s, advanced membranes made of zeolites were synthesized [10]. The advent of zeolite membranes not only enriches the diversity of inorganic membranes but also brings benefits in enhancing separation performance in terms of selectivity and permeability. More recently, microporous polymeric materials are proposed for making molecular sieving membranes because of their ordered pores and well‐defined crystalline structures. Microporous materials can be catalogued into microporous silica, porous polymer, carbon, zeolites, metal–organic frameworks (MOFs), and porous organic frameworks (POFs) [11]. The scope of this book covers a wide range of topics of microporous membranes made of these materials.

1.3 Microporous Materials

In general, a solid skeleton comprising pores and/or voids is considered as a porous material. In practice, all solid materials can provide a porous medium; thus the chemical nature of porous solids is extremely rich, covering all important groups of materials: inorganic and organic crystals, carbons, polymers, glasses, ceramics, and metals. The International Union of Pure and Applied Chemistry (IUPAC) classifies porous materials according to their pore sizes: (i) microporous, with pores less than 2.0 nm; (ii) mesoporous, with pores from 2.0 to 50 nm; and (iii) macroporous, with pores between 50 and 1000 nm [12]. The pore size controls the accessibility to the pore volume, while the capacity is determined by the ratio between the skeleton and the empty space. A consequence of porous organization is the high specific surface area (SSA) of porous materials, which can vary from several hundred to several thousand square meters per gram of solid. Another important characteristic determining the properties of porous materials is their structural organization. Based on this last criterion, porous solids are divided into two major groups, that is, crystalline and amorphous. It is important to note that the properties of porous materials depend on their chemical nature. Thus, the combination of pore characteristics, structural organization, and chemical composition determines the overall property of a porous material and its possible area of application. Table 1.1 shows the different species of microporous materials according to their frameworks.

Table 1.1 Microporous materials.

Categories

Various materials

Properties

Carbonaceous materials

Activated carbons, ordered mesoporous carbon, carbide derivatives

Pure carbon component, easy availability, considerable porosity, high stability

Porous organic materials

CPNs, MOFs (MOF‐177, MIL‐101), COFs (COF‐1, COF‐10, COF‐108), ZIFs

Adjustable pore sizes, tunable pore surfaces, diversified skeletons

Zeolite molecular sieves

Ordered mesoporous silica, zeolite 13X, zeolite 5A, MCM‐41

Uniform pore structures, inorganic components, solid acidity

1.3.1 Carbonaceous Materials

Porous and nanostructured carbonaceous materials are very promising materials for numerous applications because of their unique pore structures, low cost, lightweight, and the abundance of natural raw materials used in their syntheses. Several classes of carbonaceous materials are discussed as follows (Figure 1.2).

Figure 1.2 Representative classes of carbonaceous materials.

1.3.1.1 Activated Carbon

Activated carbon, also called activated charcoal, is a form of carbon processed to have small, low‐volume pores that increase the surface area available for adsorption or chemical reactions. Activated carbon is carbon produced from carbonaceous source materials such as nutshells, coconut husk, peat, wood, coir, lignite, coal, and petroleum pitch. It can be produced by one of the following processes:

Physical activation: The source material is developed into activated carbons using hot gases. Air is then introduced to burn out the gases, creating a graded, screened, and de‐dusted form of activated carbon. This is generally done by using one or a combination of the following processes:

Carbonization: Material with carbon content is pyrolyzed at temperatures in the range of 600–900 °C, usually in inert atmosphere with gases like argon or nitrogen.

Activation/oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (oxygen or steam) at temperatures above 250 °C, usually in the temperature range of 600–1200 °C.

Chemical activation: Prior to carbonization, the raw material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, calcium chloride, and zinc chloride 25%)

[13]

. Then, the raw material is carbonized at lower temperatures (450–900 °C). It is believed that the carbonization/activation step proceeds simultaneously with the chemical activation. Chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating the material. Activated carbon may be the most widely used sorbent.

Its manufacture and use dates back to the nineteenth century. Its usefulness derives mainly from its large micropore and mesopore volumes and the resultant high surface area [14]. Activated carbon could be divided into activated carbon fibers (ACFs), activated carbon monoliths (ACMs), and powdered activated carbons (PACs), which were widely used for gas storage and separation [15], and they could be made from various precursors. Mostly, ACFs come from precursors such as polyacrylonitrile fiber, and PACs are obtained from different coal precursor anthracites or different kinds of ligneous materials such as coconut shells, grains, and bamboos. However, ACMs are made by the PACs and ACFs under high pressure and temperature or using some binders.

A gram of activated carbon can have a surface area in excess of 500 m2 with 1500 m2 being readily achievable. Carbon aerogels, although more expensive, have even higher surface areas and are used in special applications. Under an electron microscope, the high‐surface‐area structures of activated carbon are revealed. Individual particles are intensely convoluted and display various kinds of porosity; there may be many areas where flat surfaces of graphite‐like materials run parallel to each other, separated by only a few nanometers or so. These micropores provide superb conditions for adsorption to occur, since the adsorbing material can interact with many surfaces simultaneously. Tests of adsorption behavior are usually done with nitrogen gas at 77 K under high vacuum, but in everyday terms activated carbon is perfectly capable of producing the equivalent, by adsorption from its environment, liquid water from steam at 100 °C and a pressure of 1 atm. Physically, activated carbon binds materials by van der Waals force or London dispersion force. Activated carbon does not bind well to certain chemicals, including alcohols, diols, strong acids and bases, metals, and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid. Activated carbons can be further modified by different chemical approaches. Acid–base, oxidation–reduction, and specific adsorption characteristics are strongly dependent on the composition of the surface functional groups [16]. The surface of conventional activated carbon is reactive, capable of oxidation by atmospheric oxygen and oxygen plasma [17], steam [18], carbon dioxide [19], and ozone [20]. Oxidation in the liquid phase is caused by a wide range of reagents (HNO3, H2O2, KMnO4) [21]. The formation of a large number of basic and acidic groups on the surface of oxidized carbon to sorption and other properties can differ significantly from the unmodified forms [16]. Activated carbon can be nitrogenated by natural products or polymers [22] or processing of carbon with nitrogenating reagents [23]. Activated carbon can interact with chlorine [24], bromine [25], and fluorine [26]. Sulfonic acid functional groups can be attached to activated carbon to give “starbons,” which can be used to selectively catalyze the esterification of fatty acids [27]. Formation of such activated carbons from halogenated precursors gives a more effective catalyst, which is thought to be a result of remaining halogens improving stability [28]. Some of the chemical properties of activated carbon have been attributed to the presence of the surface‐active carbon double bond [29].

Due to high surface areas, activated carbon is used in gas purification, decaffeination, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and respirators, filters in compressed air, and many other applications. One major industrial application involves the use of activated carbon in the metal finishing field. It is very widely employed for purification of electroplating solutions. For example, it is a main purification technique for removing organic impurities from bright nickel plating solutions. A variety of organic chemicals are added to plating solutions for improving their deposit qualities and for enhancing properties like brightness, smoothness, ductility, etc. Due to passage of direct current and electrolytic reactions of anodic oxidation and cathodic reduction, organic additives generate unwanted breakdown products in solution. Their excessive buildup can adversely affect the plating quality and physical properties of deposited metal. Activated carbon treatment removes such impurities and restores plating performance to the desired level. Activated carbon is also used to treat poisonings and overdoses following oral ingestion. Tablets or capsules of activated carbon are used in many countries as an over‐the‐counter drug to treat diarrhea, indigestion, and flatulence. Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as spill cleanup, groundwater remediation, drinking water filtration, air purification, and volatile organic compound capture from painting, dry cleaning, gasoline dispensing operations, and other processes. During early implementation of the 1974 Safe Drinking Water Act in the United States, EPA officials developed a rule that proposed requiring drinking water treatment systems to use granular activated carbon (GAC). Because of its high costs, the so‐called GAC rule encountered such strong opposition all across the country from the water supply industry, including the largest water utilities in California; thus the agency set aside the rule [30]. Research is being done testing the ability of various activated carbons to store natural gas and hydrogen gas. The porous material acts like a sponge for different types of gases. The gas is attracted to the carbon material via van der Waals forces. Some carbons have been able to achieve bonding energies of 5–10 kJ mol−1. The gas may then be desorbed when subjected to higher temperatures and either combusted to do work or in the case of hydrogen gas extracted for use in a hydrogen fuel cell. Gas storage in activated carbons is an appealing gas storage method because the gas can be stored in a low‐pressure, low‐mass, low‐volume environment that would be much more feasible than bulky onboard compression tanks in vehicles. The US Department of Energy has specified certain goals to be achieved in the area of research and development of nanoporous carbon materials, such as to satisfy the goal of the ALL‐CRAFT program. Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapors, odor, and other hydrocarbons from the air. The most common designs use a one‐stage or two‐stage filtration principle in which activated carbon is embedded inside the filter media. Activated carbon is also used in spacesuit primary life support systems. Activated carbon filters are used to retain radioactive gases within the air vacuumed from a nuclear boiling water reactor turbine condenser. The large charcoal beds adsorb these gases and retain them while they rapidly decay to nonradioactive solid species. The solids are trapped in the charcoal particles, while the filtered air passes through.

1.3.1.2 Carbon Nanotubes

Carbon nanotubes (CNTs) are allotropes of carbon with cylindrical pore structures [31]. Nanotubes are members of the fullerene structural family. Their name is derived from their long, hollow structure with the walls formed by one‐atom‐thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete angles, and the combination of the rolling angle and radius decides the nanotube properties, for example, metallic or semiconducting properties of the individual nanotube shell. Nanotubes are categorized as single‐walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). Individual nanotubes naturally align themselves into ropes held together by van der Waals forces, more specifically π‐stacking. The chemical bonding in nanotubes is composed entirely of sp2 bonds, similar to those of graphite. These bonds, which are stronger than the sp3 bonds found in alkanes and diamond, endow nanotubes with their unique strengths. SWNTs have outer diameters in the range of 1.0–3.0 nm with inner diameters of 0.4–2.4 nm. MWNTs can have outer diameters ranging from approximately 2.0 nm (double‐walled nanotubes) up to approximately 100 nm with tens of walls. Recently, three‐dimensional (3D) CNT architectures based on CNTs have been also highlighted for macroscopic all‐carbon devices. For instance, Lalwani et al. have reported a novel radical‐initiated thermal cross‐linking method to fabricate macroscopic, freestanding, porous, all‐carbon scaffolds using single‐walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes as building blocks [32]. These scaffolds possess macro‐, micro‐, and nanostructured pores, and the porosity can be tailored for specific applications. These 3D all‐carbon scaffolds may be used for the fabrication of the next generation of energy storage, supercapacitors, field emission transistors, high‐performance catalysis, photovoltaics, and biomedical devices and implants [33].

Techniques have been developed to produce nanotubes in sizable quantities, including arc discharge, laser ablation, high‐pressure carbon monoxide disproportionation, and chemical vapor deposition (CVD). Most of these processes take place in a vacuum or with process gases. The CVD growth method is popular, as it yields high purity and has a high degree of control over diameter, length, and morphology. Using particulate catalysts, large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth are making CNTs more commercially viable [34]. Vertically aligned CNT arrays are also grown by thermal CVD. A substrate (quartz, silicon, stainless steel, etc.) is coated with a catalytic metal (Fe, Co, Ni) layer. Typically that layer is iron and is deposited via sputtering to a thickness of 1.0–5.0 nm. A 10–50 nm underlayer of alumina is often also put down on the substrate first. This imparts controllable wetting and good interfacial properties. When the substrate is heated to the growth temperature (∼700 °C), the continuous iron film breaks up into small islands, and each island then nucleates a CNT. The sputtered thickness controls the island size, and this in turn determines the nanotube diameter. Thinner iron layers drive down the diameter of the islands, and they drive down the diameter of the nanotubes grown. The amount of time that the metal island can sit at the growth temperature is limited, as they are mobile and can merge into larger (but fewer) islands. Annealing at the growth temperature reduces the site density (CNT number mm−2) while increasing the catalyst diameter. CNTs can be functionalized to attain desired properties that can be used in a wide variety of applications. The two main methods of CNT functionalization are covalent and non‐covalent modifications. Because of their hydrophobic nature, CNTs tend to agglomerate, hindering their dispersion in solvents or viscous polymer melts. The resulting nanotube bundles or aggregates reduce the mechanical performance of the final composite. The surface of the CNTs can be modified to reduce the hydrophobicity and improve interfacial adhesion to a bulk polymer through chemical attachment [35].

CNTs are the strongest and stiffest materials discovered in terms of tensile strength and elastic modulus, respectively. This strength results from the covalent sp2 bonds formed between the individual carbon atoms. In 2000, a multiwalled carbon nanotube was tested to have a tensile strength of 63 GPa [36]. Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to 100 GPa, which is in agreement with quantum/atomistic models [37]. Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5.0% and can increase the maximum strain the tubes undergo before fracture by releasing strain energy. Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multiwalled carbon nanotubes and CNT bundles down to only several GPa [38]. This limitation has been recently addressed by applying high‐energy electron irradiation, which cross‐links inner shells and tubes and effectively increases the strength of these materials to approximately 60 GPa for multiwalled carbon nanotubes [37] and approximately 17 GPa for double‐walled CNT bundles [38]. The surface wettability of CNT is of importance for its applications in various settings. Although the intrinsic contact angle of graphite is around 90°, the contact angles of most as‐synthesized CNT arrays are over 160°, exhibiting a superhydrophobic property. By applying a voltage as low as 1.3 V, the extreme water‐repellent surface can be switched to a superhydrophilic one [39]. Another characteristic of CNTs is their unique electrical properties. Unlike two‐dimensional (2D) graphene, CNTs are either metallic or semiconducting along the tubular axis. For a given (n, m) nanotube, if n = m, the nanotube is metallic; if (n − m) is a multiple of 3, then the nanotube is semiconducting with a very small band gap. Otherwise the nanotube is a moderate semiconductor. Thus, all armchair (n = m) nanotubes are metallic, and nanotubes (6, 4), (9, 1), etc. are semiconducting [40]. CNTs are not semimetallic because the degenerate point (the point at zero energy where bonding π‐band meets antibonding π*‐band) is slightly shifted away from the K point in the Brillouin zone due to the curvature of the tube surface, causing hybridization between the σ* and π* antibonding bands and then modifying the band dispersion. Some exceptions also exist regarding metallic versus semiconductor behavior, because curvature effects in small diameter tubes can strongly influence electrical properties. For instance, a (5, 0) SWCNT that should be semiconducting in fact is metallic according to the calculations. Likewise, zigzag and chiral SWCNTs with small diameters that should be metallic have a finite band gap. In theory, metallic nanotubes can carry an electric current density of 4 × 109 A cm−2, which is more than 1000 times greater than those of metals such as copper, because copper interconnect current densities are limited by electromigration [41]. CNTs are being explored as conductivity‐enhancing components in composite materials, and many groups are attempting to commercialize highly conducting electrical wire assembled from individual CNTs. There are still some challenges to be overcome, such as reducing resistive nanotube‐to‐nanotube junctions and impurities, because they lower the electrical conductivity of the macroscopic nanotube wires by orders of magnitude compared with the conductivity of the individual nanotubes. Because of its nanoscale cross section, electrons propagate only along the tube's axis. As a result, CNTs are frequently referred to as one‐dimensional (1D) conductors. The maximum electrical conductance of an SWCNT is 2G0 (G0 = 2e2 h−1, a conductance of a single ballistic quantum channel) [42]. Due to the π‐electron system in determining the electronic properties of graphene, doping in CNTs differs from that of the same group of crystalline semiconductors (e.g. silicon). Graphitic substitution of carbon atoms in the nanotube wall by boron or nitrogen dopants leads to p‐type and n‐type behavior, respectively. However, some non‐substitutional (intercalated or adsorbed) dopants introduced into a CNT, such as alkali metals as well as electron‐rich metallocenes, result in n‐type conduction because they donate electrons to the π‐electron system of the nanotube. In contrast, π‐electron acceptors such as FeCl3 or electron‐deficient metallocenes function as p‐type dopants since they draw π‐electrons away from the top of the valence band.

Current use and application of nanotubes has mostly been limited to the use of bulk nanotubes, which is a mass of rather unorganized fragments of nanotubes. Bulk nanotube materials may never achieve a tensile strength similar to that of individual tubes, but such composites may, nevertheless, yield strengths sufficient for many applications. Bulk CNTs have already been used as composite fibers in polymers to improve the mechanical, thermal, and electrical properties of the bulk product. Easton‐Bell Sports Inc. has been in partnership with Zyvex Performance Materials, using CNT technology in a number of their bicycle components including flat and riser handlebars, cranks, forks, seat posts, stems, and aero bars. Zyvex Technologies has also built a 54′ maritime vessel (Piranha Unmanned Surface Vessel), as a technology demonstrator for CNT technology. CNTs help improve the structural performance of this vessel, resulting in a lightweight 8000 lb boat that can carry a payload of 15 000 lb over 2500 miles. Amroy Europe Oy manufactures Hybtonite carbon nanoepoxy resins, where CNTs have been chemically activated to bond to epoxy, resulting in a composite material that is 20–30% stronger than other composite materials. It has been used for wind turbines, marine paints, and a variety of sports gear such as skis, ice hockey sticks, baseball bats, hunting arrows, and surfboards. Other current applications include tips for atomic force microscope probes in tissue engineering. CNTs can act as scaffolding for bone growth [43]. There is also ongoing research in using CNTs as a scaffold for diverse microfabrication techniques [44]