Porous Materials

Edited by

Duncan W. BruceUniversity of York, UK

Dermot O’HareUniversity of Oxford, UK

Richard I. WaltonUniversity of Warwick, UK

A John Wiley and Sons, Ltd, Publication

Porous Materials

Inorganic Materials Series

Editors:

Professor Duncan W. BruceDepartment of Chemistry, University of York, UK

Professor Dermot O’HareChemistry Research Laboratory, University of Oxford, UK

Professor Richard I. WaltonDepartment of Chemistry, University of Warwick, UK

Series Titles

Functional OxidesMolecular MaterialsLow-Dimensional SolidsPorous MaterialsEnergy Materials

Porous Materials

Edited by

Duncan W. BruceUniversity of York, UK

Dermot O’HareUniversity of Oxford, UK

Richard I. WaltonUniversity of Warwick, UK

A John Wiley and Sons, Ltd, Publication

This edition first published 2011� 2011 John Wiley & Sons, Ltd

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Library of Congress Cataloging-in-Publication Data

Porous materials / edited by Duncan W. Bruce, Dermot O’Hare, Richard I. Walton.p. cm. — (Inorganic materials series)

Includes bibliographical references and index.ISBN 978-0-470-99749-9 (cloth)1. Porous materials. I. Bruce, Duncan W. II. O’Hare, Dermot. III. Walton, RichardI. TA418.9.P6P667 2010620.1016—dc22

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Contents

Inorganic Materials Series Preface ix

Preface xi

List of Contributors xiii

1 Metal-Organic Framework Materials 1

Cameron J. Kepert

1.1 Introduction 11.2 Porosity 3

1.2.1 Framework Structures and Properties 31.2.2 Storage and Release 181.2.3 Selective Guest Adsorption and Separation 211.2.4 Heterogeneous Catalysis 27

1.3 Incorporation of Other Properties 311.3.1 Magnetic Ordering 321.3.2 Electronic and Optical Properties 411.3.3 Structural and Mechanical Properties 51

1.4 Concluding Remarks 54Acknowledgements 56References 56

2 Mesoporous Silicates 69

Karen J. Edler

2.1 Introduction 692.2 Nomenclature 702.3 Methods of Preparation 712.4 Surfactant Aggregation 722.5 Silica Source 752.6 Template Removal 792.7 Synthetic Routes and Formation Mechanisms 83

2.7.1 True Liquid Crystal Templating 832.7.2 Cooperative Self-Assembly 872.7.3 Evaporation-Induced Self-Assembly 99

2.8 Properties and Characterisation 108

2.9 Macroscopic Structures 1172.10 Applications 124References 128

3 Ordered Porous Crystalline Transition Metal Oxides 147

Masahiro Sadakane and Wataru Ueda

3.1 Introduction 1473.2 Scope and Limitations of this Review 1483.3 Microporous Transition Metal Oxide Materials 1493.4 Mesoporous Transition Metal Oxide Materials 153

3.4.1 Soft Template Method 1543.4.2 Hard Template Method 1553.4.3 Mesoporous Oxides of Group 4 Elements (Ti, Zr) 1573.4.4 MesoporousOxidesofGroup5Elements(Nb,Ta) 1703.4.5 Mesoporous Oxides of Group 6 Elements

(Cr, Mo, W) 1723.4.6 Mesoporous Oxides of Group 7 Elements (Mn) 1723.4.7 Mesoporous Oxides of Elements of Groups

8–11 (Fe, Co, Ni, Cu) 1733.4.8 Mesoporous Oxides of Lanthanide Elements (Ce) 174

3.5 Macroporous Materials 1743.5.1 Macroporous Monometal Oxides 1773.5.2 MacroporousOxidesofGroup4Elements (Ti,Zr) 1913.5.3 MacroporousOxidesofGroup5Elements(V,Nb) 1913.5.4 MacroporousOxidesofGroup6Elements(Cr,W) 1923.5.5 Macroporous Oxides of Elements of Groups

7–11 (Mn, Fe, Co, Ni, Cu) 1933.5.6 Macroporous Oxides of Lanthanide Elements

(La, Ce, Nd, Sm, Eu) 1943.5.7 Macroporous Multi-Component Metal Oxides 1943.5.8 Two-Step Templating Method 2073.5.9 Applications 207

3.6 Conclusion 209References 209

4 Templated Porous Carbon Materials: Recent Developments 217

Yongde Xia, Zhuxian Yang and Robert Mokaya

4.1 Introduction 2174.2 Microporous Carbon Materials 221

vi CONTENTS

4.2.1 Zeolites as Hard Template 2214.2.2 Clays as Hard Template 2294.2.3 Other Microporous Materials as Hard

Template 2314.3 Mesoporous Carbon Materials 231

4.3.1 Conventional Hard Template SynthesisStrategy 232

4.3.2 Cost-Effective Strategies for the Synthesis ofMesoporous Carbons 240

4.3.3 Soft-Template Synthesis Strategy for OrderedMesoporous Carbons 241

4.3.4 Ordered Mesoporous Carbons with GraphiticPore Wall 244

4.3.5 Mesopore Size Control 2464.3.6 Morphology Control 247

4.4 Macroporous Carbon Materials 2524.4.1 Silica Colloidal Crystals as Hard Template 2524.4.2 Polymer Microspheres as Template 2544.4.3 Dual Template Method 255

References 258

5 Synthetic Silicate Zeolites: Diverse Materials Accessible

Through Geoinspiration 265

Miguel A. Camblor and Suk Bong Hong

5.1 Introduction 2655.2 Zeolites: Some Definitions 2675.3 Zeolite Structures 2695.4 Chemical Composition of Silicate Zeolites 270

5.4.1 Naming Zeolites 2725.4.2 Loewenstein’s Rule 273

5.5 Zeolite Properties 2745.6 Zeolite Applications 2755.7 Zeolite Synthesis 279

5.7.1 The Synthetic Zeolites as Geoinspired Materials 2795.7.2 Thermochemistry of Zeolite Synthesis 2815.7.3 Organic Structure-Directing Agents 2845.7.4 Structure-Direction by Flexible, Hydrophilic

OSDAs 2895.7.5 Double OSDA Strategies 2955.7.6 Structure-Direction by T-Atoms 297

CONTENTS vii

5.7.7 Zeolite Synthesis from Nonaqueous Solvents 3075.7.8 The Fluoride Route to Zeolites 3085.7.9 Structure-Direction Issues in the Fluoride

Route to Pure-Silica Zeolites 3125.7.10 Topotactic Condensation of Layered Silicates 315

5.8 Concluding Remarks 316Acknowledgements 316References 317

Index 327

viii CONTENTS

Inorganic Materials Series

Preface

Back in 1992, two of us (DWB and DO’H) edited the first edition ofInorganic Materials in response to the growing emphasis and interestin materials chemistry. The second edition, which contained updatedchapters, appeared in 1996 and was reprinted in paperback. The aimhad always been to provide the reader with chapters that while notnecessarily comprehensive, nonetheless gave a first-rate and well-referenced introduction to the subject for the first-time reader. Assuch, the target audience was from first-year postgraduate studentupwards. Authors were carefully selected who were experts in theirfield and actively researching their topic, so were able to provide anup-to-date review of key aspects of a particular subject, whilst pro-viding some historical perspective. In these two editions, we believeour authors achieved this admirably.

In the intervening years, materials chemistry has grown hugely andnow finds itself central to many of the major challenges that face globalsociety. We felt, therefore, that there was a need for more extensivecoverage of the area and so Richard Walton joined the team and, withWiley, we set about a new and larger project. The Inorganic MaterialsSeries is the result and our aim is to provide chapters with a similarpedagogical flavour but now with much wider subject coverage. Assuch, the work will be contained in several themed volumes. Many ofthe early volumes concentrate on materials derived from continuousinorganic solids, but later volumes will also emphasise molecular andsoft matter systems as we aim for a much more comprehensive coverageof the area than was possible with Inorganic Materials.

We approached a completely new set of authors for the newproject with the same philosophy in choosing actively researchingexperts, but also with the aim of providing an international perspec-tive, so to reflect the diversity and interdisciplinarity of the now verybroad area of inorganic materials chemistry. We are delighted withthe calibre of authors who have agreed to write for us and we thank

them all for their efforts and cooperation. We believe they have donea splendid job and that their work will make these volumes a valu-able reference and teaching resource.

DWB, YorkDO’H, OxfordRIW, Warwick

July 2010

x INORGANIC MATERIALS SERIES PREFACE

Preface

Porosity in the solid-state is a topic of long-standing attention in materialsscience and the case of the zeolites exemplifies the importance of porousmaterials across many disciplines of science. Here, the study of naturallyoccurring silicate minerals led to the discovery of synthetic analogues inthe laboratory that now have huge commercial value, ranging from large-scale industrial petroleum cracking catalysts to household applications inwater-softening additives in detergents. This is an important example ofhow curiosity-driven, fundamental research in complex inorganic struc-tures, and how they might be assembled in a controlled way, ultimatelycan lead to novel materials with societal benefit.

The field of porous materials has, however, undergone dramatic devel-opment in the past few decades, particularly with the increasingly routineuse of advanced structural probes for studying the structure and dynamicsof the solid state. Porosity in inorganic materials now extends from thenanoscale up to the macroscale and is a highly researched area, particularlysince the idea of design in synthesis is being realised by control of solutionchemistry in the crystallisation of complex extended structures. Propertiesare increasingly the goal in this field: novel solid hosts for confinement ofmatter on the nanoscale, highly specific shape selective catalysts for energy-efficient organic transformations, new media for pollutant removal, andgas storage materials for energy applications. The role of the syntheticchemist remains key to the discovery and classification of porous materials;the fact that novel porous materials are still reported at an increasing rate inthe chemical literature demonstrates the vitality of the field.

The five chapters in this volume cover some of the key families ofinorganic solids that are currently being studied for their porosity. Thearea of zeolites is still researched heavily since there remain long-standingquestions in understanding crystallisation and the extent to which novelmaterials, with structure and chemical properties are tuned for particularapplications, can be produced. The chapter on zeolite chemistry illus-trates this and takes a novel angle, describing how the synthesis of porousmaterials can be inspired by nature. Various other important families arecovered representing the scales of porosity from nanoporous throughmesoporous, and also showing how various chemical classes of materialcan be rendered porous by elegant synthetic approaches.

We are very pleased that well-respected authors who are active inresearch in this important area agreed to prepared chapters for thisvolume and thank them for their excellent results. We hope that thiscollection will provide a useful and up-to-date introduction to an area ofabiding interest in materials chemistry.

DWB, YorkDO’H, OxfordRIW, Warwick

July 2010

xii PREFACE

List of Contributors

Miguel Camblor Instituto de Ciencia de Materiales de Madrid (CSIC),Madrid, Spain

Karen J. Edler Department of Chemistry, University of Bath, UK

Suk Bong Hong School of Environmental Science and Engineering,Pohang University of Science and Technology (POSTECH), Pohang,Korea

Cameron J. Kepert School of Chemistry, University of Sydney, NSW,Australia

Robert Mokaya School of Chemistry, University Park, University ofNottingham, Nottingham, UK

Masahiro Sadakane Graduate School of Engineering, HiroshimaUniversity, Higashi-Hiroshima, Japan

Wataru Ueda Catalysis Research Center, Hokkaido University,Sapporo, Japan

Yongde Xia School of Chemistry, University Park, University ofNottingham, Nottingham, UK

Zhuxian Yang School of Chemistry, University Park, University ofNottingham, Nottingham, UK

1Metal-Organic Framework

Materials

Cameron J. KepertSchool of Chemistry, The University of Sydney, Sydney NSW, Australia

1.1 INTRODUCTION

In recent years there has been a rapid growth in the appreciation ofmolecular materials not just as arrangements of discrete molecular enti-ties, but as infinite lattices capable of interesting cooperative effects. Thisdevelopment has arisen on many fronts and has seen the emergence ofchemical and physical properties more commonly associated with non-molecular solids such as porosity, magnetism, and electrical conductivity.This chapter focuses on an area of molecular materials chemistry that hasseen an extraordinarily rapid recent advance, namely, that of metal-organic frameworks (MOFs).† These materials consist of the linkage ofmetal ions or metal ion clusters through coordinative bridges to form

Porous Materials Edited by Duncan W. Bruce, Dermot O’Hare and Richard I. Walton

� 2011 John Wiley & Sons, Ltd.

† Whilst certain qualifications on the use of the term ‘metal-organic framework’ have been put

forward (e.g., relating to formal bond valence and energy, ligand type, etc.),[3] the common usageof this term has spread well beyond these to become largely interchangeable with a number of

more general terms such as ‘coordination polymer’, ‘coordination framework’, ‘metallosupra-molecular network’ and ‘hybrid material’. As such, this term is used here, with some reluctance,

in its broadest general sense to encompass a very diverse range of material types in which metal

atoms are linked by molecular or ionic ligands.

frameworks that may be one-dimensional (1D), two-dimensional (2D) orthree-dimensional (3D) in their connectivity.[1–14]

In the broadest sense, the use of coordination chemistry to produceframework materials has been with us since the discovery of Prussian Bluemore than 300 years ago, with developments throughout the last centuryproviding an arrayof framework lattices spanning a range of different ligandtypes.[15, 16] The rapid expansion of this early work into more structurallysophisticated families of materials can be traced to two developments. First,the exploitation of the strong directionality of coordination bonding hasallowed a degree of materials design (so-called ‘crystal engineering’) inthe synthesis of framework phases. Here, the use of molecular chemistryhas allowed both the rational assembly of certain framework topologies –many not otherwise accessible in the solid state – and the control overframework composition through the incorporation of specific buildingunits in synthesis or through post-synthetic modification. Secondly, thecapability to construct materials in a largely predictive fashion has led tothe emergence of a range of new properties for these materials. This mostnotably includes porosity, as seen in the ability to support extensive voidmicropore volume, to display high degrees of selectivity and reversibilityin adsorption/desorption and guest-exchange, and to possess heteroge-neous catalytic activity. A range of other interesting functionalities havealso emerged, many in combination with reversible host–guest capabil-ities. A particularly attractive feature of the metal-organic approach toframework formation is the versatility of the molecular ‘tool-box’, whichallows intricate control over both structure and function through theengineering of building units prior to and following their assembly. Theadoption of this approach has been inspired in part by Nature’s sophis-ticated use of molecular architectures to achieve specific function, spanninghost–guest (e.g. ion pumping, enzyme catalysis, oxygen transport),mechanical (e.g. muscle action), and electronic (e.g. photochemical, elec-tron transport) processes. Following rapid recent developments the immen-sely rich potential of MOFs as functional solids is now well recognised.

At the time of writing this field is experiencing an unprecedented rateof both activity and expansion, with several papers published per day anda doubling in activity occurring every ca 5 years. Faced with this enormousbreadth of research, much of which is in its very early stages, the aim ofthis chapter is not to provide an exhaustive account of any one aspect ofthe chemistry of MOFs, rather, to provide a perspective of recent devel-opments through the description of specific representative examples,including from areas yet to achieve maturity. Following a broad overviewof the host–guest chemistry of these materials in Section 1.2, particular

2 METAL-ORGANIC FRAMEWORK MATERIALS

focus is given to the incorporation of magnetic, electronic, optical, andmechanical properties in Section 1.3.

1.2 POROSITY

1.2.1 Framework Structures and Properties

1.2.1.1 Design Principles

1.2.1.1.1 BackgroundThe investigation of host–guest chemistry in molecular lattices has a longhistory. Following early demonstrations of guest inclusion in variousclasses of molecular solids (e.g. the discovery of gas hydrates by Davyin 1810), major advances came in the mid twentieth century with the firststructural rationalisations of host–guest properties against detailed crys-tallographic knowledge. Among early classes of molecular inclusioncompounds to be investigated for their reversible guest-exchange proper-ties were discrete systems such as the Werner clathrates and variousorganic clathrates (e.g. hydroquinone, urea, Dianin’s compound, etc.),in which the host lattices are held together by intermolecular interactionssuch as hydrogen bonds, and a number of framework systems (e.g.Hofmann clathrates and the Prussian Blue family), in which the hostlattices are constructed using coordination bonding.[15, 16] A notableoutcome from this early work was that the host–guest chemistry ofdiscrete systems is often highly variable due to the guest-induced rearran-gement of host structure, and that the coordinatively linked systems – inparticular those with higher framework dimensionalities – generally dis-play superior host–guest properties with comparatively higher chemicaland thermal stabilities on account of their higher lattice binding energies.

Whilst the excellent host–guest capabilities of coordinatively bondedframeworks have been appreciated for many decades, the extension ofthis strategy to a broad range of metals, metalloligands and organic ligandshas been a relatively recent development. Concerted efforts in this areacommenced in the 1990s following the delineation of broad design princi-ples[1] and the demonstration of selective guest adsorption;[17] notably,these developments arose in parallel with the use of coordination bonds toform discrete metallosupramolecular host–guest systems.[18] A number ofdifferent families of coordinatively bridged material have since been devel-oped, each exploiting the many attractive features conferred by the

POROSITY 3

coordination bond approach. A consequence of this rapid expansion is thatmany inconsistencies have arisen in the terminology used to distinguishthese various families. In this chapter, the broadest and arguably mostfundamental distinction, i.e. the exploitation of coordination bonding toform frameworks consisting of metal ions and molecular or ionic ligands, isused to define this diverse class of materials.

1.2.1.1.2 MOF SynthesisIn comparing MOFs with other classes of porous solids many interestingsimilarities and points of distinction emerge. A comparison has alreadybeen made above with discrete inclusion compounds, for which it wasnoted that coordinative rather than intermolecular linkage confers a highdegree of control over materials’ structure and properties, whilst retain-ing the benefits associated with the versatility of molecular building units.At the other end of the spectrum, an equally useful comparison can bemade with other porous framework materials, which notably includezeolites and their analogues (e.g. AlPOs). Here, some close parallelsexist between the structural behaviours of the host lattices, but manyimportant differences exist relating to synthesis, structure and properties.One principal point of distinction is that the building units of MOFs arecommonly pre-synthesised to a high degree. This allows the achievementof specific chemical and physical properties through a highly strategicmulti-step synthesis in which the comparatively complex structure andfunction of the molecular units are retained in the framework solid. Thisability to retain the structural complexity of the covalent precursors is adirect result of the low temperature synthesis of MOFs (i.e. typically

the entrapment of solvent in channels and pores is less pronounced thanfor higher temperature synthetic routes. Secondly, and conversely, theenthalpic favourability of regular bond formation is a dominant drivingforce for framework formation. Through exploitation of the highly direc-tional nature of coordination bonding, a reasonable degree of controlover the structure of MOF lattices can thus be achieved. Extensive effortsin the use of well defined coordination geometries and suitably regularligands have led to the development of relatively sophisticated ‘crystalengineering’ principles, albeit with absolute control over polymorphismin many cases being subject to the whims of crystal nucleation and subtlesensitivities to temperature, solvent, etc.

Among a range of useful design principles for MOFs are the ‘node andspacer’[19, 20] and reticular ‘secondary building unit’ (SBU)[21, 22] appro-aches. Common to each of these is the concept of using multitopic ligandsof specific geometry to link metal ions or metal ion clusters with specificcoordination preferences. Using these approaches it is possible to distillframework formation to the generation of networks of varyingtopology‡[23–28] with the geometry of these being determined in largepart by the geometry of the molecular building units (see Figure 1.1). Inmany cases the geometry of the building units defines a single possiblenetwork topology if fully bonded; for example, the use of octahedral nodesand equal-length linear linkers generates the cubic a-Po network [seeFigure 1.1(i)]. In many cases, however, only the dimensionality of theresulting framework can be predicted with any reasonable degree ofcertainty, with very low energy differences arising due to torsional effects,intraframework interactions or subtle geometric distortions; for example,the use of tetrahedral nodes and linear linkers can generate a range of 3D4-connected nets that include cristobalite [diamondoid; Figure 1.1(f)],tridymite (lonsdaleite), and quartz. In many further cases still, even theprediction of network dimensionality is not straightforward; for example,square nodes and linear linkers can produce a 2D square grid and a 3DNbO-type net [Figure 1.1(e) and (h)], and triangular nodes and linear linkerscan produce a wide range of nets that vary only in their torsional anglesthrough the linear linkers, e.g., 0� torsion produces the hexagonal (6,3)

‡A large number of different chemical classification systems exist for network topologies. Thesenotably include those based on simple chemical compounds (e.g. diamondoid/cristabolite-type),an (n,p) system used by Wells related to that of Schläfli that classifies according to the number oflinks in a loop (n) and the node connectivity (p) [e.g. (6,4)],[23] a three-letter system derived fromthat used for zeolites (e.g. dia,dia-a,dia-b,etc.),[24] and a 2D hyperbolic approach (e.g. sqc6).[25]

As an example, the chiral (10,3)-a network is known also as the SrSi2 net, srs, Laves net, Y*,3/10/c1, K4 crystal, and labyrinth graph of the G surface.

POROSITY 5

net, 109.5� torsion produces the chiral (10,3)-a net [see Figure 1.1(a–c)],etc. A further point of considerable complication from a design perspec-tive is the interpenetration of networks,[27] which has a profound influ-ence over the pore structure and therefore host–guest properties.

Si net of SrSi2

Pt3O4

(a)

(d)

NbO

(e) (f)

(g) (h) (i)

(b) (c)

Diamond (C)

Cooperite (PtS) 44 Square lattice Primitive cubic

Si net of ThSi2 63 Honeycomb

Figure 1.1 A selection of common network topologies for MOFs: (a) the3-connected SrSi2 [also (10,3)-a] net, shown distorted away from its highestsymmetry; (b) the 3-connected ThSi2 net; (c) the 2D hexagonal grid; (d) the Pt3O4net, which contains square planar and trigonal nodes; (e) the NbO net, whichcontains square planar nodes; (f) the diamondoid net; (g) the PtS net, whichcontains tetrahedral and square planar nodes; (h) the 2D square grid; and (i) thea-Po net. Reprinted with permission from M. Eddaoudi, D.B. Moler, H.L. Li, B.L.Chen, T.M. Reineke, M. O‘Keeffe and O.M. Yaghi, Acc. Chem. Res., 34, 319.Copyright (2001) American Chemical Society

6 METAL-ORGANIC FRAMEWORK MATERIALS

An important consequence of both the versatility of the molecular build-ing units and the accessibility of novel framework topologies is that MOFscan readily be synthesised that are both chiral and porous. Efforts in thisarea have seen the emergence of the first homochiral crystalline porousmaterials through two primary routes (see also Sections 1.2.3.2 and1.2.4.2): (1) the use of chiral ligands to bridge metal ions within networktopologies that would otherwise be achiral,[29–39] as first seen in the use of apyridine-functionalised tartrate-based ligand to form the porous homochiral2D layered framework POST-1, which consists of honeycomb-type ZnII-based layers;[29] and (2) the use of chiral co-ligands to direct the assembly ofachiral building units into chiral framework topologies,[40–43] as first seen inthe homochiral synthesis of an interpenetrated (10,3)-a network phase.[42]

Inexploiting the favourable thermodynamicsandkineticsofMOFcrystalgrowth, very large pores of uniform dimension and surface chemistry arecommonly achieved that would be inaccessible by other chemical routes.[44,45] For example, whereas the synthesis of mesoporous silicates (i.e. thosewith pore dimensions in the range 20–500 Å) generally requires surfactanttemplation and calcination to leave behind amorphous hosts with regularmesopores,[46] crystallineMOFs withpores up to47Å indimension[47] havebeen synthesised by the assembly of molecular building units from solution.In addition to favouring the formation of complex mesoscale architectures,the strength and directionality of the coordination bond also imparts arelatively high degree of stability to these. This is seen, for example, intheir reasonably high thermal (up to�500 �C in some cases) and chemicalstabilities (albeit with susceptibility to strongly coordinating guests such aswater being common), extremely low solubilities, and robustness to guestdesorption (see Section 1.2.1.2). Achievement of the latter feature, which ismost common in higher dimensionality (i.e. 2D and 3D) framework sys-tems, has led to this field providing the most porous crystalline compoundsknown, with void volumes occupying as much as �90 % of the crystalvolume.Theachievementof such lowvolumetricatomdensities through theuse of moderately light elements means that the gravimetric measures ofporosity and surface area are also extremely high. Among a number ofnotable families of highly porous MOFs are members of the MOF/IRMOF family (see Figure 1.2),[22, 48–51] MIL-nnn (in particular nnn ¼100, 101),[52, 53] ZIF-nnn (in particular nnn ¼ 95, 100)[54, 55] and NOTT-nnn series (in particular nnn¼ 100–109),[56, 57] which provide some of themost extreme measures of porosity and surface area yet achieved:e.g. among these ZIF-100 (see Section 1.2.3.1.1 and Figure 1.12) andMIL-101 have the largest pores, of dimension 35.6 and 34 Å, respectively;and MOF-177 and MIL-101 have Langmuir surface areas of 5640[58] and

POROSITY 7

5500 m2 g�1,[53] each more than double that of porous carbon, and gravi-metric pore volumes of 1.69[58] and 1.9 cm3 g�1,[53] respectively.

A further distinguishing feature of MOFs over other classes of porousmaterials is the extremediversity of their surface chemistry, whichcan rangefrom aromatic to highly ionic depending on the chemical nature of thebuilding units used. This notably includes the achievement of multiplepore environments within individual materials.[31] An important conse-quence of this versatility is that the surface chemistry can be tuned for highlyspecific molecular recognition and catalytic processes (see Sections 1.2.2,1.2.3 and 1.2.4).

1.2.1.1.3 Post-Synthetic Modification of MOFsIn addition to the high degree of control over framework structure that canbe achieved prior to and during MOF synthesis, considerable controlcan be exercised following framework assembly by exploiting the porosityof MOFs.[1] Developments here have seen the emergence of a range of

Figure 1.2 A selection of MOFs based on tetranuclear Zn4O(CO2)6, dinuclearCu2(CO2)4 and 1D Zn2O2(CO2)2 secondary building units (left) and a range ofmultitopic carboxylate ligands (top). Reprinted with permission from D. Britt,D. Tranchemontagne and O.M. Yaghi, Proc. Natl. Acad. Sci. U.S.A, 105, 11623.Copyright (2008) National Academy of Sciences

8 METAL-ORGANIC FRAMEWORK MATERIALS

post-synthetic approaches in which framework structure and pore chemis-try are modified via low energy chemical pathways that involve the internalmigration of guest species. These processes occur topotactically, i.e. withsome retention of the parent structure, to generate metastable phases thatare commonly inaccessible through ‘one-pot’ syntheses.[59]

The simplest andmost common form of post-synthetic modification is thedesorption of guest molecules. This process, which in some cases is achievedmost optimally at low temperature in multiple low-energy steps (e.g.through activation by volatile solvents[60] or supercriticial CO2

[61]), com-monly leads to apohost phases that are structurally stable despite havingvery high surface energies. This is particularly so in cases where guestdesorption leaves behind bare metal sites (see Sections 1.2.2 and 1.2.4), anexample being the desorption of bound water molecules from theCu2(CO2)4(H2O)2 ‘paddlewheel’ nodes within [Cu3(btc)2(H2O)3](HKUST-1,[62] also MOF-199; btc ¼ 1,3,5-benzenetricarboxylate) (seeFigure 1.3). Guest desorption influences the host–guest properties of theframework in two ways. First, in generating a large unbound surface itallows the subsequent adsorption and surface interaction of guest moleculesthat would not otherwise have displaced those present at the surface follow-ing MOF synthesis (e.g. gases, aromatics into polar frameworks). Secondly,the modification of pore contents can have a pronounced influence onframework and pore geometry, thereby greatly modifying the adsorptionproperties of the host (see Sections 1.2.1.2 and 1.2.3.1.2).

The exchange of guest species can also dramatically influence hostframework properties. This is particularly the case for the exchange ofions within charged frameworks – a process that can change both the

–H2O

(a) (b)

+H2O

Figure 1.3 Reversible desorption of bound water molecules from theCu2(CO2)4(H2O)2 nodes within [Cu3(btc)2(H2O)3] (a) to produce [Cu3(btc)2] (b).This process occurs following the desorption of unbound guests (not shown). Cuatoms are drawn as spheres and a transparent van der Waals surface is shown

POROSITY 9

relative polarity of the framework surface and the framework geometry. Incontrast to zeolites, which in consisting of anionic frameworks generallyonly display cation exchange, MOFs can undergo both cation[63–65] andanion[1, 66, 67] exchange depending on their framework charges. Whilstsuch processes commonly involve the exchange of labile ions within thepores, the former notably also includes the reversible exchange of metalnodes from within the framework itself, as has been seen with the replace-ment of CdII within Cd1.5(H3O)3[(Cd4O)3(hett)8] (where hett is an ethyl-substituted truxene tricarboxylate) by PbII (see Figure 1.4);[68] in contrastto the analogous dealuminisation process in zeolites, which requires multi-ple steps under extreme thermal and chemical conditions, this exchangeprocess occurs at ambient temperature. Notably, the development of ion-exchange capabilities in MOFs has numerous other points of significance,for example in the development of proton conducting frameworks.[69, 70]

The incorporation of metal sites and other charged species into thepores of MOFs is in many cases driven by the energetics of complexationat the framework surface. Such a process may occur either throughcation/anion exchange or salt inclusion. The former has been achieved,for example, with the exchange of protons with titanium(IV) di-isoprop-oxide at chiral BINOL units (BINOL ¼ 1,10-di-2-naphthol) to generatematerials that display enantioselective catalytic activity.[35, 71] The lattermay involve either the complexation of metal ions at binding sites on theframework surface with concomitant inclusion of charge-balancinganions, or cation/anion complexation at bare surface metal sites withconcomitant inclusion of metal complex anions/cations into the pores.[72]

The complexation of neutral metal species has also been used to modifypore chemistry, as seen with the reaction of MOF-5 with Cr(CO)6 to form[Zn4O((Z6-1,4-benzenedicarboxylate)Cr(CO)3)3], in which the aromatic

Figure 1.4 Reversible exchange of framework metal ions within Cd1.5(H3O)3[(Cd4O)3(hett)8] via a single-crystal-to-single-crystal process. Reprinted withpermission from S. Das, H. Kim and K. Kim, J. Am. Chem. Soc., 131, 3814.Copyright (2009) American Chemical Society

10 METAL-ORGANIC FRAMEWORK MATERIALS

linkers now take the form of the organometallic Cr(benzene)(CO)3piano-stool complex.[73]

A further strategy for framework modification involves electron transferbetween host and guest, a process that in principle provides amongst thestrongest of enthalpic driving forces for the inclusion (or removal) of cationsor anions and for the modification of framework properties. Redox activityat both the metal and ligand sites within the framework has been achieved.An example of the former is the oxidation of [NiII6(C26H52N10)3(btc)4]�n(-guest) (BOF-1; btc¼ 1,3,5-benzenetricarboxylate) by I2, in which oxidationof some of the NiII sites to NiIII results in the inclusion of triiodide ions intothe pores.[74] Examples of the latter include a number of dicarboxylateframework systems in which post-synthetic framework reduction leads tothe inclusion of alkali metal ions and to dramatic changes in hydrogen gasadsorption properties of the modified framework.[75, 76]

An equally powerful although less studied form of post-synthetic mod-ification treats MOF crystals as chemical substrates at which covalentgrafting can occur. The first use of this approach was the alkylation ofunbound pyridyl units within the homochiral framework POST-1(described in Section 1.2.1.1.2), a process that deactivates these sitescatalytically.[29] More recently, this approach has been used to confer arange of desirable host–guest properties to MOFs, with particular successseen with the grafting of a range of functional groups to the unboundamine group on the NH2-bdc (bdc ¼ 1,4-benzenedicarboxylate) linkerwithin IRMOF-3.[59] A notable consequence of this process is the mod-ification of chemical surface properties and the fine tuning of the dimen-sions of the pores and pore windows, with the systematic increase inorganic chain length leading to a corresponding decrease in surface areaof the framework due to pore occlusion.[77] Another notable example isthe two-step attachment of a catalytically active vanadium complexthrough ligand grafting (with �13 % conversion of the amine groups)followed by metal complexation to yield a material that exhibits hetero-geneous catalytic activity at the vanadium centres (see Figure 1.5).[78]

1.2.1.2 Structural Response to Guest Exchange

A common synthetic goal in MOF synthesis is the generation of frame-works that display zeolite-like rigidity to guest desorption andexchange[31, 50, 51, 79–90] (so-called ‘2nd generation materials’) ratherthan collapse irreversibly upon guest removal (‘1st generation materi-als’).[5] The host–guest chemistry of such systems is readily interpretable

POROSITY 11

using standard models, with rapid guest transport commonly occurringwithin the pores and Type I adsorption isotherms displayed. Importantly,these features lead to a high degree of predictability in the host–guestchemistry, with the framework structure able to be simulated as a rigidhost within which dynamic guest molecules migrate and bind,[91, 92] andwith guest selectivity depending principally on the size and shape of theguest molecules and the strength of the host–guest surface interactions.Such properties are highly desirable for a wide range of host–guestapplications.

In addition to the considerable interest in rigid frameworks, a veryinteresting feature of many MOFs is their high degree of frameworkflexibility. Materials of this type, which have been classified as ‘3rdgeneration materials’,[5] display flexing of their framework lattices inresponse to various stimuli; this most commonly involves response tothe desorption and exchange of guest molecules, but may also arise due tochanges in temperature, pressure, irradiation, etc. The adsorption iso-therms of materials that display guest-induced flexing typically exhibithysteretic behaviour due to the fact that the apohost phase has a differentpore structure from that of the adsorbed phase, with transformationbetween the two being an activated process. Structurally, the adsorptionproperties can range from intercalative behaviours in which stagedadsorption occurs through the gradual guest-induced opening of pores(cf. clays) to more cooperative behaviours in which guest adsorptioninfluences the structure of the entire MOF crystal (i.e. crystal and porehomogeneity are retained throughout the adsorption process). In materi-als of this general type the guest-selectivity is considerably more complexthan that of the zeolitic phases, with adsorption commonly depending on

NH2 N

OHO

HO

N

OO

VO

O

toluene –Hacac

+ V(O)acac2

Figure 1.5 Schematic for the functionalisation of IRMOF-3 (see Figure 1.2) withsalicylaldehyde and subsequent binding of a vanadyl complex (acac ¼ acetylacetonate).Reprinted with permission from M.J. Ingleson, J.P. Barrio, J.B. Guilbaud, Y.Z. Khimyakand M.J. Rosseinsky, Chem. Commun., 28, 2680–2682. Copyright (2008) Royal Societyof Chemistry

12 METAL-ORGANIC FRAMEWORK MATERIALS

the strength of the host–guest interaction (which needs to be sufficient todrive the framework deformation), as well as guest size and shape con-siderations. This is particularly the case for mixtures of guests, wherecooperative effects are commonly seen; e.g. the adsorption of one guestcan have a ‘gate-opening’ function to allow the inclusion of a secondguest that would not otherwise be adsorbed. Despite being generally lesspredictable than rigid frameworks, such materials have potential use in arange of applications that make use of their chemically selective adsorp-tion and/or hysteretic behaviour (e.g. for guest storage). A further point ofinterest here is that structural modification upon guest loading provides amechanism for molecular sensing.

At the present time it is not straightforward in all cases to predict inadvance whether MOFs will survive guest desorption, or the extent towhich their frameworks might distort upon desorption and subsequentadsorption. Some clear guiding principles exist, however. First, the rigid-ity of the building units has a clear influence on framework flexibility,with the strength of coordination bonding providing a useful initial guideas to the energetics of bond bending as well as thermal stability. Secondly,the extent of connectivity and topological underconstraint within theframework lattice has a key influence over whether low energy deforma-tions might occur; e.g. cf. rigidity of triangular network vs scissor actionof square grid. In considering whether host–guest interaction energies aresufficient to drive framework deformation or decomposition, a particu-larly important consideration is whether guests may bind at the metalnodes and thereby favour pronounced structural flexing, frameworkinterconversion or even dissolution; a relatively common limitation ofMOFs is their sensitivity to water vapour, with the metal nodes in somesystems being susceptible to water binding and ligand displacement.More subtle effects such as hydrogen bonding interactions, or evenweak intermolecular forces involving small gaseous guests, can fre-quently be sufficient to cause pronounced framework flexing.

1.2.1.2.1 Flexible FrameworksTwo different types of guest-induced flexibilities exist in MOF hostlattices. The first can be considered as essentially static in nature,involving bulk framework deformations that may be readily charac-terised using diffraction-based techniques and which are frequentlyobservable at the macroscale through changes in crystal dimensions.The second are dynamic and arise due to molecular vibrations or localguest-induced framework deformations away from the ‘parent’ struc-ture. The latter are not so readily detectable by diffraction methods and

POROSITY 13

are commonly inferred based on geometric considerations; for example,local distortions away from the bulk crystallographic structure havebeen shown to be necessary in certain cases to allow migration of gueststhrough narrow pore windows.[93] Given these complexities, considera-tions of the guest selectivity of flexible systems need necessarily extendbeyond simple ‘size and shape’ arguments towards the more complexconsideration of guest-driven host lattice modification.

A broad array of interesting flexing behaviours have been seen in MOFsystems, spanning intercalative-type behaviour in 2D layered systems tothe deformation of individual frameworks and the translation of inter-penetrated frameworks.[74, 79, 85, 94–99] The interdigitated 2D layer com-pound [Cu(dhbc)2(4,4

0-bpy)]�n(guest) (dhbc ¼ dihydroxybenzoate;4,40-bpy ¼ 4,40-bipyridine) displays pronounced interlayer contractionupon guest desorption, with a 30 % decrease in the c-axis length.[100] Thisprocess occurs without loss of polycrystallinity and involves the gliding ofaromatic units with respect to each other. Subsequent adsorption of guestmolecules leads to regeneration of the more open structure, with thecorresponding adsorption isotherms displaying activated, hystereticbehaviour in which a ‘gate-opening’ pressure is required beforeadsorption can occur.

The interdigitated bilayer phase [MII2(4,40-bpy)3(NO3)4]�n(guest)

(M ¼ Ni, Co, Zn)[79, 89, 93, 101] displays zeolite-like robustness upondesorption of ethanol guests from the parent phase[82] and two types offramework flexibility upon adsorption of other guests.[79] In situ singlecrystal diffraction characterisation during guest adsorption showed thatmolecular guests with dimensions too large for the pores of the apohostcan be adsorbed due to a progressive widening of the 1D pores withincreasing guest size associated with low energy scissor-type flexing of thebilayers. Even larger guests are adsorbed into this phase through adifferent pore expansion mechanism in which translation of the inter-penetrated bilayer nets with respect to each other leads to an increase inthe height of the 1D channels.

The MIL-53 family of 3D frameworks, with formula[MIII(OH,F)(bdc)]�n(guest) (M ¼ Al, Cr, Fe; bdc ¼ 1,4-benzenedicarbox-ylate), also display scissor-type flexing as a function of temperature andguest adsorption with considerable variation in the dimensions of the 1Dchannels.[102, 103] A comprehensive in situ powder X-ray diffraction exam-ination of guest adsorption into the Fe analogue, MIL-53(Fe), has demon-strated that the guest-induced breathing effect depends principally on thestrength of the interaction between host and guest rather than beingparticularly dependent on guest size (see Figure 1.6).[102] The principal

14 METAL-ORGANIC FRAMEWORK MATERIALS