Reticular Chemistry and Metal-Organic Frameworks for Clean ...yaghi.berkeley.edu/pdfPublications/09cleanEnergy.pdf · be organic molecules, inorganic clusters, dendrimers, peptides,

682 MRS BULLETIN • VOLUME 34 • SEPTEMBER 2009 • www.mrs.org/bulletin

AbstractReticular chemistry concerns the linking of molecular building blocks into

predetermined structures using strong bonds. We have been working on creating anddeveloping the conceptual and practical basis of this new area of research. As a result,new classes of crystalline porous materials have been designed and synthesized:metal-organic frameworks, zeolitic imidazolate frameworks, and covalent organicframeworks. Crystals of this type have exceptional surface areas (2,000–6,000 m2/g)and take up voluminous amounts of hydrogen (7.5 wt% at 77 K and 3–4 × 106 Pa),methane (50 wt% at 298 K and 2.5 × 106 Pa), and carbon dioxide (140 wt% at 298 K and 3 × 106 Pa). We have driven the basic science all the way to applicationswithout losing sight of our quest for understanding the underlying molecular aspects ofthis chemistry. The presentation was focused on the design concepts, synthesis, andstructure of these materials, with emphasis on their applications to onboard energystorage.

predetermined structures in which suchunits are repeated and are held togetherby strong bonds. The more brief definitionis that reticular chemistry is “stitchingmolecules together by strong bonds.” Thereason I have emphasized these two termsis because they are the new aspects of thischemistry. For the first time outsidemolecular chemistry, we are able to con-ceive of a structure and go to the labora-tory and make it. In collaboration withMichael O’Keeffe, we have developed theblueprint for extended structures that gobeyond molecules and 1D structures.11

This is a new region of structure space inchemistry, and we want the buildingblocks to be linked by strong bonds so thatwe can make robust materials that are use-ful as catalysts or as storage materials. Theability to do this is the reason for the pop-ularity of this field.

The basic thinking behind reticularchemistry is that all extended structuresare composed entirely of joints and links,which correspond to vertices and edges ofa net. In each of such structures, there arebranching points and links that join them.This way of thinking helps in understand-ing how one can assemble building blocksinto predetermined structures, as I willdemonstrate later. This approach coversthe entire gamut of compounds in chem-istry. Figure 1 shows a number of struc-tures: 0D, nanoparticles; 1D, chains; 2D,sheets; and 3D, networks. Notice that allof these structures are composed of inter-sections that are square in shape. The onlydifference between these structures is theangle between the squares. And so, ifthere is a way first to make molecularsquares and second to place them at therequired angles to make these shapes,then the synthesis should produce com-pounds that are based on these blueprints.

This point is illustrated in Figure 2using a basic structure in inorganic chem-istry, copper acetate, in which four car-boxylates (–COO−) are chelating thecopper centers (shown in yellow). Ofcourse, the copper acetate structure is ter-minated with methyl groups to make adiscrete molecule. Our strategy was toreplace methyl with a ditopic link (a linkwith two connecting points) and thus linkthese together into larger and extendedstructures. If completed, then this struc-ture would act as a square, and, depend-ing on what the link is, squares could beassembled into various shapes. This cop-per acetate unit is a secondary buildingunit, and the geometry of the unit isdefined by the points of extension (such as the carboxylate C atoms in most car boxylate MOFs). Having an assemblycomposed of metal–oxygen bonds,

IntroductionI am delighted to be here and I am

pleased and honored to receive thisaward. In this lecture, I will show how onecan take a simple idea from basic scienceand planning through to application. The field of metal-organic frameworks(MOFs)1 is rapidly expanding; more than1,000 new MOF structures have beenreported each year for the last few years.2This is an unparalleled development—it isthe fastest growing field in materialschemistry, and it is also the first time thatone has such a diverse class of new mate-rials to investigate.

I call the chemistry I will be discussingtoday “reticular chemistry.” I will describethe basic thinking behind reticular chem-istry and the design strategy and showthat this chemistry leads to a cornucopia

of new materials. The focus today will beon polyhedral and periodic MOFs, but I will also mention covalent-organicframeworks (COFs)3 and more recentwork on zeolitic imidazolate frameworks(ZIFs).4 I will then briefly describe the appli-cations of MOFs to clean energy, hydro-gen5–7 and methane storage,8 and carbondioxide capture.9 Finally, I will describe themass production of these materials byBASF in collaboration with my group.10

Reticular ChemistryThe meaning of the word reticular is

“net-like.” I define reticular chemistry asthe chemistry concerned with the linkingof molecular building blocks—these canbe organic molecules, inorganic clusters,dendrimers, peptides, and proteins—into

Reticular Chemistryand Metal-OrganicFrameworks forClean Energy

Omar M. Yaghi and Qiaowei LiThis article is an edited transcript of the MRS Medal presentation given by Omar Yaghi(University of California, Los Angeles) on November 28, 2007, at the 2007 MaterialsResearch Society Fall Meeting in Boston. Yaghi was awarded the Medal for “his pioneeringwork on the synthesis, structure, and theory of metal-organic frameworks.” The MRS Medalrecognizes a specific outstanding recent discovery or advancement that is expected to havea major impact on the progress of any materials-related field.

Reticular Chemistry and Metal-Organic Frameworks for Clean Energy

MRS BULLETIN • VOLUME 34 • SEPTEMBER 2009 • www.mrs.org/bulletin 683

oxygen–carbon bonds, and then carbon–carbon bonds, as links, results in robustmaterials. The fact that the building unit isa chelated entity leads to a rigid unit thatcan only link as a square and has strongbonds with covalent characters (bondenergy is approximately 360 kJ/mol), as strong as carbon–carbon bonds (carbon–carbon single bond energy in alkane isapproximately 348 kJ/mol). At the heartof the development of MOFs is that wehave found a way to make rigid molecularunits of well-defined molecular shapesthat can be linked to produce robustextended structures.

Terephthalate or benzenedicarboxylate,illustrated in Figure 3, can be thought ofas an entity with three angles to control.The carboxylate groups can make a copla-nar bend in the middle of the link (θ) byusing the meta-carboxylate. It can alsotwist the carboxylate planes around thelinker axis relative to one another (ϕ) orbend the planes of the carboxylatestoward each other (ψ). If we can controlthese three angles, then that link providesmetric information or angular informa-

tion that will direct linkage of thosesquares into the desired structure. As anillustrative example, if we take the meta-benzenedicarboxylate with an θ angle of120° and carry out the chemistry that produces that square, the copper acetateunit, we obtain the truncated cube octahedron, cuboctahedron,12 shown inFigure 4. A very simple reaction in N,N-dimethylformamide and ethanol at80°C for 24 hours produces this structure.The planes of linked squares in this struc-ture are at 120°, and the bent links areexactly what were required to bring thisto fruition. We can make a polyhedronthat is about 3.4 nanometers in externaldiameter, with 1.5 nanometers in internaldiameter.12 The yellow ball in the pictureis there to make the structure clear, butalso to indicate the largest molecule thatcould fit inside the pore without contact-ing the van der Waals radii of the atomsmaking up the walls. This is a nanoparti-cle, in which the positions of all the atoms,and the way they are linked, are known.We know exactly what the bond lengthsare, the atoms on the outside, the atoms

0D 1D 2D 3D

Figure 1. Design of nanoparticles (0D), chains (1D), sheets (2D), and networks (3D). Allthese structures are composed of intersections that are square in shape (red), which areconnected by ditopic linkers (black). The dimensionality of linked square units is controlledby use of precise linker geometry.

4-coordinatedCu2(CO2)4

Secondary building unit

a b

Figure 2. Example of a secondary building unit (SBU). (a) In copper acetate SBU, twocopper atoms (yellow) are chelated by four carboxylates (O, red; C, white). The whole SBUis very rigid, because they are constructed by strong bonds. (b) The geometry of SBU isprecisely controlled because of the fixed coordination geometry of metals. Cu2(CO2)4 canbe considered as a square (red), which can be linked to four linkers.

Figure 3. Control angles for metal-organic frameworks constructed frombenzenedicarboxylic acid(C6H4(COOH)2). θ, bending in themiddle of the link with carboxylategroups (COO−) coplanar; ψ, bending ofthe planes of the carboxylates towardeach other; ϕ, twisting of thecarboxylate planes about the linker axisrelative to one another. C, black; O, red.

a

b

Figure 4. Truncated cuboctahedron.(a)Twelve paddle-wheel units (Cu, blue;O, red; C, black) were linked by 1,3-benzenedicarboxylate (m-BDC) to forma large truncated cuboctahedron of 15angstroms diameter void. The yellowball in the picture indicates the largestmolecule that could fit inside the porewithout contacting the van der Waalsradii of the atoms making up the walls.Hydrogen atoms have been omitted forclarity. (b) The same structure wasillustrated by connecting red squares(paddle-wheel secondary building units)with black rods (organic linkers).

on the inside, the metrics of everything inthe structure, and its geometry. Using theconditions that produce this intersection,we can functionalize the phenylene ring(in black) shown in Figure 4, and as longas we do not change the angle of 120°, wecan functionalize the surface of thissphere without changing its underlyingstructure. And thus we are able to createthe amino-, -OH, -bromo, -allyl, t-butyl-and -OC12H25 functionalized spheres,13,14

all based on the same truncated cubocta-



hedron blueprint. This is very importantfor various applications, such as design ofdevices, and if their exact structures areknown, they can be patterned on surfacessuch as graphite.13

These copper acetate squares can nowbe linked in other ways by varying theangles of the linkers. If terephthalate isused, which predisposes the linkedsquares to be coplanar, a 2D square gridcan be made called MOF-2.15 If we place -bromo in the ortho position of –COO− onthe benzene ring, it turns one carboxylate90° to the other. If we are careful not toheat this reaction mixture, so as not toovercome the rotation barrier to turn twocarboxylates coplanar, the squares will beat 90°, thus leading to formation of a 3Dextended structure16 based on a NbO typeof net as illustrated in Figure 5.

I have demonstrated how to make 0D,1D, 2D, and 3D structures using simplechemistry, by taking a link and a metal ionand combining them under mild condi-tions to produce well-defined materials.1Most MOF syntheses are performed atambient pressure, and the reaction tem-peratures range from 25 to 200°C. Squaresare not the only possible joints. In organicchemistry is replete with metal- carboxylate clusters that could be used assecondary building blocks in variousshapes. Organic chemistry provides links,which, in themselves, could also have dif-ferent geometries. These links lead toquestions about other structures thatcould be made from a square and a trian-gle, a square and a tetrahedron, and so on.The possible structures from such combi-nations of shapes are almost infinite; onlya few are known. Our strategy has notbeen to enumerate all of the possibilities,but to realize that there are only a limitednumber of high symmetry structures thatwill be the most favored structures forsynthesis, whether they are based on tri-angles, squares, tetrahedra, or any othershape.17 Generally, only a handful of struc-tures need to be considered; in particular,those with just one kind of link are over-whelmingly the ones that are most commonly observed. Most, if not all,structures of this type are known.17

The structure based on triangles joinedby one kind of link is called the SrSi2 net-work (the Si substructure in that com-pound has this topology, and the net isgiven the symbol srs); for squares joined atright angles by one kind of link, they havethe nbo topology named for NbO; simi-larly, when tetrahedra are linked together,a structure with the diamond topology(dia) is obtained; octahedra linked togetherform a structure based on the primitivecubic lattice (pcu). These basic structures

are called default structures (Table I); theyare in a way “the platonic networks” forextended structures,18 and they play a roleanalogous to that of platonic solids inmolecular chemistry. We have identifiedthe most symmetric possibilities for link-ing trigonal prisms, hexagons, etc., into 3Dstructures.18,19 We also have identified thekey ways of linking different shapes suchas cubes and tetrahedra, squares and trian-gles, tetrahedra and triangles, tetrahedraand squares, squares and hexagons, andoctahedra and triangles.20 We have ana-lyzed the thousands of MOFs in the litera-ture, and their topologies are generally thesymmetric structures I have described.17

Some Examples of MOFsThe next few examples illustrate the

way this chemistry works. Figure 6 showsa structure based on the linking of trigonalprisms, which is made from a famous clus-ter in inorganic chemistry, composed ofthree octahedral metals linked to two setsof three carboxylates, arranged in a trigo-nal prismatic geometry.21 This is namedthe acs net after Andrea C. Sudik, the stu-dent who discovered this compound hav-ing this underlying net topology.

Figure 7 shows a structure linking thepaddle-wheel copper acetate structurewith adamantane tetracarboxylate, whichmakes the pts type of net, so named afterPtS.22 The same thing can be done byreplacing adamantane with carbon andfour benzene rings, and the pts net also isobtained.23

The result of linking octahedra togetherwould be a primitive cubic structure.

a b

Figure 5. A 3D extended structure based on a NbO type of net. (a) The crystal structure ofCu2[o-Br-C6H3(CO2)2]2 was deliberately designed to have an NbO framework by usingreaction conditions that produce the paddle-wheel unit, a square secondary building unit(SBU). Cu, blue square pyramid; O, red spheres; C, black spheres; Br, green spheres.Hydrogen atoms have been omitted for clarity. (b) Schematic illustration of NbO net fromred squares (paddle-wheel SBUs) with black rods (organic linkers).

Table I: Default Structures from EachBuilding Block Geometries and One

Kind of Linker.17

Building Block Default StructureGeometry

Trigonal srs

Tetrahedral dia

Square nbo

Trigonal bipyramidal bnn

Octahedral pcu

Figure 8 shows an octahedral cluster,which is a basic zinc acetate structure, withan oxide and four zinc centers that aretetrahedral, bridged by six carboxylates.This is a robust and rigid intersection.When it is linked with terephthalate, itforms the cubic structure based on the pcunet called MOF-5.24 If we have the reactionconditions that produce this intersection,then we can add any desired link tochange the functionality of the structureand the metrics. The thermal stability ofthese materials is quite high. Once the mol-ecules filling the pores are removed byheat, the structure has a large stabilityrange, up to about 500°C. In this case, thesolvent molecules in the pores of the as-synthesized material can be exchangedwith chloroform to remove unreactedlinks, metal salts, and original solvent mol-ecules in the pores. Chloroform is thenevacuated, and the resulting framework



a

b

c

Figure 6. Structures linking triangular prisms composed of three octahedral metals and two sets of three carboxylates arranged in a trigonal prismatic geometry. (a) Fe3O clustersand terephthalate (1,4-BDC) are reticulated into decorated and expanded versions of theacs net, a hexagonal ABA array of trigonal prisms. Fe, blue; O, red; C, black. (b) Topologicalview of this acs net, based on trigonal prismatic secondary building units (red) and ditopiclinks (gray). (c) Single-crystal x-ray structure of [Fe3O(1,4-BDC)3][FeCl4] (MOF-235)composed of oxo-centered iron trimers and benzenedicarboxylate links (views down z-axis). Fe, blue octahedra; O, red spheres; C, black spheres; [FeCl4]−, orange tetrahedra.Axial coordinating ligands and hydrogen atoms have been omitted for clarity.

a

c

b

Figure 7. MOF-12, Cu2(adamantane tetracarboxylate), an example of the PtS structure(pts-a). pts-a is the augmented net of pts, in which the vertices of the original net arereplaced by a group of vertices with the shape of the original coordination figure of the vertex. (a) Squares (red) and tetrahedra (green) assemble to adopt the pts net. (b) Cu2(OCO)4 cluster and adamantine cluster were chosen because of their square andtetrahedral geometry, respectively. Cu, blue; O, red; C, black. (c) Vertices in pts net wereoccupied by the clusters to form a decorated pts framework. Cu, blue squares; O, redspheres; C, black spheres.

does not lose any weight even whenheated up to 400–500°C.

Porosity of MOFsThe porosity of MOFs is greater than that

of any other porous material, double therecord for porous carbon. The surface areaof MOF-5 was initially reported at 2,900m2/g,24 but now MOF-5 can be activated toobtain 3,800 m2/g.25 In such a material, 60%is open space, into which gases and organicmolecules can be introduced.

To demonstrate the possibilities of find-ing materials with higher surface areasthan MOF-5, it is useful to examine thecalculated surface area of a hypotheticallayer of graphite (Figure 9a). The calcu-lated surface area on both sides of thissheet approaches 3,000 m2/g. However, ifwe strip this sheet into different shapes(Figure 9b–9d), we see a dramatic increasein the surface area. The reason for this isthat, in addition to the faces beingexposed to incoming gas molecules, theedges are also exposed. The edge of a six-member ring is about 3.7 angstroms, ontowhich gas molecules can bind. We knowthis from x-ray diffraction studies on gasmolecules in the pores, and we knowexactly where they bind in these structures.26

To make those high surface area materi-als, we added a tritopic triangular car-boxylate linker to combine with theoctahedral secondary building unit,which is the MOF-5 cluster, to make MOF-177 (Figure 10).27 MOF-177 is replete withexposed faces and exposed edges, and itshould have the high surface area that wewould have predicted from those calcula-tions. Unlike other porous materials,MOFs have pores without walls; they aremade entirely of struts and intersections.They are open scaffolds, where the strutsor the intersections are sites for gas mole-cules to enter. That is the reason they havevery high surface area, and this is the opti-mal way to create high surface area mate-rials. MOF-177 had a surface area of 4,500m2/g when we first reported it,27 and nowwe can obtain 5,500 m2/g5 by evacuatingthe pores completely. These are previouslyunheard of surface areas. Seven years ago,we had to be content with zeolite surfaceareas of 500 m2/g, mesoporous materialsof 1,000 m2/g, and porous carbon, whichis amorphous with 1,500 m2/g surfacearea (2,000 m2/g with some expensiveprocessing). Now, using simple chemistry,we can obtain 5,500 m2/g.

In addition to discrete metal oxide unitsand intersections, it is also possible to userod-like metal oxides as infinite clustersand then link those rods with organics(Figure 11).28 This unit is nothing more



Figure 8. Isoreticular metal-organic frameworks (IRMOFs). (a) MOF-5 was synthesizedfrom terephthalic acid (H2BDC) and zinc nitrate in N,N-diethylformamide (DEF). One of thecavities in the MOF-5 framework is shown. Eight clusters constitute a unit cell and enclosea large cavity. Each octahedral cluster, which is a basic zinc acetate unit, contains an oxideand four zinc centers that are tetrahedral, and these clusters are bridged by sixcarboxylates. (b) Structures of IRMOF-n (n = 1 through 7, 8, 10, 12, 14, and 16), labeledrespectively. ZnO4, blue polyhedra; O, red spheres; C, black spheres. The large yellowspheres represent the largest van der Waals spheres that would fit in the cavities withouttouching the frameworks. All hydrogen atoms have been omitted, and only one orientationof disordered atoms is shown for clarity.

2,965 m2/g 5,683 m2/g

7,745 m2/g6,200 m2/g

a

c

b

d

Figure 9. (a) Calculated surface area of a hypothetical layer of graphite. The calculatedsurface area on both sides of this sheet approaches 3,000 m2/g. When the sheet isstripped into different shapes (b)–(d), a dramatic increase is seen in the surface area.When the graphene sheet is fully decom posed into isolated six-member rings, the surfacearea reaches a maximum of 7,745 m2/g.

than zinc oxide, with zinc in blue and oxy-gen in red, forming a rod intersection.These rods can then be linked with organ-ics to make porous materials that are veryrobust. We have identified the most sym-metric blueprints that could result fromlinking rods (Figure 12).29 In the case ofbnn net, these rods are going into the pageto make a hexagonal pattern. The rods areat an angle to each other; the green rodsare at an angle to the red rods and arelinked by organics.

Structures based on assembly of rod-like units have been discussed little in theliterature, but we have recognized theirimportance, and we have identified themas the prime structures that could resultfrom linking such building blockstogether;29 the basic rods can be trigonalhelical or square helical, or they couldresemble ladders. We have made a num-ber of MOFs based on these blue-prints.28,29

So far, I have discussed a wide range ofmaterials and have covered a significantpart of the periodic table with the organiclinks. My goal is to make a MOF of everysingle element in the periodic table and toensure that it is porous and that its prop-erties are studied.

MOFs can be made from simple startingmaterials: terephthalic acid, which is usedfor MOF-5, is a main component of plas-tics; zinc oxide, which is a source of zincthat can be used in MOF synthesis, is usedin sun cream. The possibilities are endless,and this can be translated to real chem-istry, with nice structures, highly porousmaterials, and record-breaking surfaceareas.

BASF is one company dedicated to theproduction of MOFs, which is being scaledup to produce materials in ton scale. MOFscan be shaped for various applications,including catalysis and gas separation. Theprice of the raw materials for manganeseMOFs based on organic links is approxi-mately four dollars per kilo.

Clean Energy ApplicationsSo far this article has covered ways to

think about the chemistry, how the struc-tures are assembled, and the way they arecharacterized. Now I will describe a newprogram to address applications for cleanenergy. A new source of renewable energyhas not been found; instead, we are tryingto work on issues that can be related to theproduction of clean energy. Crystals ofMOF-5 are well-defined materials, withentire scaffolding; they have no walls,only struts and intersections. Our idea isto take gases that could occupy large vol-umes and compact them inside an MOFwithout requiring high pressure or low

Zn4O(O2C-C6H4-CO2)3: MOF-5

H2BDC + Zn(NO3)2·4H2O Zn4O(BDC)3·(DEF)7 a b



a b

c

Figure 10. MOF-177 is replete with exposed faces and edges and has a high surface area.(a) A 1,3,5-benzenetribenzoate (BTB) unit linked to three Zn4O units (H atoms are omitted).ZnO4 blue tetrahedra; O, red spheres; C, black spheres. (b) The structure projected down[001]. Only half of the c-axis repeat unit is shown. Colors as for (a). (c) A fragment of thestructure radiating from a central Zn4O: six-member rings are shown as grey hexagons, O atoms as red spheres, and Zn atoms as blue spheres.

Zn3O(CO2)4 MOF-69

a b

Figure 11. MOF-69A: An example of metal-organic frameworks from zinc oxide structureswith rod intersections. (a) Infinite Zn-O-C secondary building units are linked together by4,4'-biphenyldicarboxylate links. (b) A 3D network was formed with large channels runningalong the [001] direction. Zn, blue; O, red; C, grey.

temperature. This has now been realizedfor methane8 and carbon dioxide.9 It alsohas been realized for hydrogen at 77 K,5and we are investigating ways of com-pacting hydrogen inside MOFs at roomtemperature.25,30

The Department of Energy (DOE)requirement for hydrogen to be used inautomobile fueling is that 6% by weight ofthe fuel tank should be hydrogen, or 45grams of hydrogen for each liter. MOFshave great advantages over other poten-tial materials since they have well-defined, known, and tunable structures.They need to have an adsorption energyof about 15–20 kJ/mol; that is essentiallybinding enthalpy of hydrogen moleculesinto the pores of the material.7 MOFs alsohave high surface area and facile diffusionin and out of the pores. Some MOFs havepore sizes of 7 to 8 angstroms; we can varythe composition and structure; and we canfine tune the electronic character of theadsorption sites. After about six years ofstudy on hydrogen, we have concludedthat it is possible to store hydrogen inMOF-177 at 7.5% by weight at 77 K, andthe total amount of hydrogen depositedwould be 12% by weight.31 We have there-fore achieved the first goal of the DOE, butat 77 K. This is important, because itdemonstrates that we can store sufficienthydrogen in MOFs for them to be usefulfor storage. Now we need to work on the adsorption energy at the surface.However, hydrogen can go into a pore, orleave the pore, with great facility. If MOF-177 were in the fuel tank of a vehicle, thefueling time would be about two and ahalf minutes, because hydrogen is heldthrough polarizing or weak forces into theframework.

Our best result is 62 grams of hydrogenper liter at 77 K, which is more than two-thirds of that obtained from liquid hydro-gen at 20 K.31 This is very exciting, becauseit means that a container filled with MOF-177, or another MOF not discussed here,can hold 50 percent more hydrogen thanone without MOF. That is very interestingfor people who want to store hydrogencryogenically for stationary applications.Our goal is to obtain the same hydrogenuptake numbers at room temperature.Current hydrogen adsorption record forMOF at room temperature is Mn3[(Mn4Cl)3(BTT)8]2 (H3BTT = benzene-1,3,5-tris(1H-tetrazole)), from Jeffery Long’sgroup, which achieved 12.1 g/L hydrogenuptake at 9 × 106 Pa and 298 K.6 In order toreach the goal, we are collaborating withBill Goddard at Caltech. He has calculatedthe effect of doping MOFs containing aro-matic linkers with lithium so that each six-member ring would have a lithium atom.



Figure 12. Metal-organic framework (MOF) examples of blueprints based on rod buildingblocks. The rods are at an angle to each other; the green rods are at an angle to the redrods and are linked by organics. A three-letter symbol system was developed to describethese invariant rod packings. pcu, primitive cube lattice; bnn, boron nitride net; hex,hexagonal lattice; ths-z, ThSi2. MOFs illustration: metal, blue polyhedra; oxygen, redspheres; carbon, black spheres.

Lithium associated with the aromaticrings provides strong stabilization ofmolecular H2. Using this concept, he pre-dicted that MOFs could achieve five per-cent hydrogen uptake at close to roomtemperature.30 This is an important devel-opment, and the community is now mov-ing toward making MOFs impregnatedwith lithium or other metals.

Obviously companies such as BASF arenot going to make these materials in largequantities due to the sophisticatedorganic link synthesis and expense, butwe can consider structures that would be

equivalent to this. We are currently exam-ining covalent organic frameworks(COFs). Structures that are entirely com-posed of six-member rings could easily bemade by taking tetrahedral units thathave boronates, as illustrated in Figure 13.These structures can be either self- condensed to make extended structuresthat are entirely composed of carbon–carbon bonds, carbon–boron bonds, andboron–oxygen bonds.3,32 These are allstrong bonds based on small atoms, sothey are robust. We can either self- condense or co-condense or co-polymerize

to make extended structures, and weknow what those structures would be.32

We have characterized them using x-raydiffraction and found it is possible tomake COFs that represent the least densematerials known to date in crystallineform.32 Figure 14 shows COF-108,32 with adensity of 0.17 grams per cubic centime-ter; the surface area is 4,700 m2/g, andGoddard’s calculations predict that COF-108 is going to have an uptake of hydro-gen at about 18.9 wt% at 77 K.33

Goddard’s calculations have independ-ently matched every experimental valuethat we have achieved in MOFs,34 so I am optimistic about obtaining room-temperature storage.

Methane is a cleaner fuel than petrol orcoal, and it represents two-thirds of ourglobal natural energy resources; however,half of the available methane is a strandedgas that is difficult to transport. Methane iscurrently being transported at −50°C; if wecould store methane at room temperature,it would be a great development. COF-102takes up about 250 milligrams of methanefor each gram of material at 298 K, at avery reasonable pressure of 6–8 × 106 Pa.35

That is an exciting prospect, since a tankfilled with MOF or COF under these con-ditions will hold twice the methane as onewithout those materials. This would dou-ble the range for automobiles that run onnatural gas. A coalition of about 20 compa-

Figure 13. Representative condensation routes to 3D covalent-organic frameworks.Boronic acids (blue) can undergo self-condensation to form B3O3 rings or form C2O2B rings upon condensation with hexahydroxytriphenylene (red), a planar triangular unit.

Figure 14. Space-filling illustration ofCOF-108, with a bor blueprint (thesame as boracite). COF-108 wasobtained from tetrahedral tetra(4-dihydroxyborylphenyl)methane andtriangular hexahydroxytriphenylene.Density = 0.17 g/cm3; surface area =4,700 m2/g. Hydrogen atoms areomitted for clarity. Carbon, boron, andoxygen atoms are represented asgreen, brown, and pink spheres,respectively.



nies have undertaken a project to convincethe world that a wide distribution of tankscontaining MOFs impregnated withmethane can be useful in transporta-tion.36,37 A prototype car, with an MOF fueltank, has been successfully driven throughfive continents.36 According to BASF, fueltanks of this type may be widely distrib-uted in the near future.

Carbon dioxide emission is an importantenvironmental issue. The world producesmore than four tons of carbon dioxide percapita every year. Carbon dioxide emittedfrom coal or natural gas power plants con-tains many other gases besides carbondioxide. At present, in order to meet theEPA levels of emission, the gas is passedthrough a membrane to separate carbondioxide from the exhaust gas mixture, andthen carbon dioxide is bubbled into anamine solution. This amine solution isheated and carbon dioxide is evolved, andthen it is piped into geologic formationsunder the ground for various purposes.This process costs each power plant a largepercentage of its power output.

If we had a material that stored carbondioxide, we could simply place it in thestacks and pipe carbon dioxide under theground directly after it is released fromthe stacks. About 33.5 millimols of carbondioxide can be stored in every one gram ofMOF-177 at room temperature at veryacceptable pressures.9 A tank filled withMOF can store nine times more carbondioxide than one without MOFs.

This is not the complete answer, how-ever, because we need materials that canselectively extract carbon dioxide from theexhaust flue gas. We have developedzeolitic imidazolate frameworks (ZIFs) forthis purpose.4,38,39

Zeolite structures arise from the charac-teristic Si-O-Si angle of about 145 degrees.In ZIFs, we have replaced silicon withtransition metals with tetrahedral geome-tries, such as Zn or Co, and oxygen isreplaced with imidazolate. The link stillhas two connecting points, such as oxy-gen, and the metals are tetrahedral; thelinking angle of metal-imidazolate-metalis the same as Si-O-Si in zeolite, so wehave a zeolite-like structure that is madeof transition metals and organics. One ofmy students, Kyosung Park succeeded inmaking the first ZIF, ZIF-8,4 which isbased on a sodalite (sodium aluminum sil-icate chloride) net, and it is composed ofzinc or cobalt tetrahedra and entirelymethylimidazolate links. This work hasessentially addressed two holy grails inzeolite chemistry: to develop a materialwith a high concentration of transitionmetals and a material with organics in itsbackbone. Pore environments in ZIFs can

a b

c d

ZIF-95

ZIF-100

FAU supercage

C60

Figure 15. Size comparison of (a) POZ B in ZIF-95, (b) MOZ in ZIF-100, (c) super cage infaujasite (FAU), and (d) C60. The MOZ cage has a 35.6 angstroms inner sphere diameter.All the cages are shown in ball and stick diagrams (Zn, red spheres in (a) and (b); Si/Al, redspheres in (c); C, red spheres in (d)). Different tilings are shown in different colors.

be tailored by different functionalities inorganic backbones, and a high concentra-tion of transition metals provides oppor-tunities in various applications, such ascatalysis.40

ZIFs are thermally very stable up to500°C, with absolutely no weight loss.Furthermore, unlike other porous materi-als and some MOFs, they are also chemi-cally stable. We have boiled ZIF-8 inbenzene, methanol, water, and sodiumhydroxide for periods up to a week, andthe material remains crystalline.4 X-raydiffraction experiments show there is nochange to the framework. It is easy toimagine the different applications in selec-tive gas capture and heterogeneous catal-ysis for materials with this robustchemical stability. Park delivered approxi-mately a dozen of these materials withinthree months, based on different zeolitestructures, and not just methylimidazolatecan be used but also imidazolate andother functionalized imidazolates. This isa beautiful result because these are delo-calized π-systems, whose electronic struc-ture could be changed and tailored by thefunctionality that is introduced on thoserings, such that they are highly selective tovarious gases.

Figure 15 illustrates two ZIFs, the“papa of ZIFs,” ZIF-95 (Figure 15a showsPOZ B cage in ZIF-95) and ZIF-100 with“MOZ” cage, the “mother of ZIFs”(Figure 15b). They are both built fromtetrahedra, and the MOZ cage is thelargest known in chemistry. These are

exceptional structures, in terms of theirsheer size. The POZ B cage in ZIF-95 is30.1 × 20.0 angstroms in inner diameter,and the size of the MOZ cage in ZIF-100 is35.6 angstroms in inner diameter.39 ZIF-100 has a Langmuir surface area of 780m2/g, but what makes it unique is thatthe MOZ cage has a very nice slit intowhich carbon dioxide can pass. Once ithas entered the pores, it interacts with theimidazolate rings and accumulates there,just as in a reservoir.

We have performed competitive studieson gases containing carbon dioxide,methane, and other power plant fluegases.38,39 First, we started with a materialthat has nothing in the pores and thenexposed it to a mixture of nitrogen andcarbon dioxide. The carbon dioxide wasentirely retained in the pores, while nitro-gen passed right through it. ZIFs are thusselective materials for carbon dioxide, andthey are twice as efficient as BPL carbon (a commercially available carbon productof Calgon), which is the “state of the art”material.

SummaryIn summary, I have described a number

of new materials, metal-organic frame-works (MOFs), covalent organic frame-works (COFs), and zeolitic imidazolateframeworks (ZIFs), with controlled com-position and structure. Within each ofthese classes, there are subclasses, andeach subclass has hundreds of structures.The ways in which we can take molecules



and link them together into structures, forany desired application, are seeminglylimitless. The applications are mainlybeing exploited for catalysis at present,but also for gas storage, carbon dioxideseparation, and other areas.

Starting a new field is exciting andrewarding. It has been stated thatShakespeare “only” strung words togetherto make beautiful sentences. I hope I demonstrated how, in the field of reticu-lar chemistry, we have “only” strung mol-ecules together to make crystals that arenot just of wondrous beauty but of unpar-alleled diversity and utility.

AcknowledgmentsI would like to acknowledge the help of

my colleagues and group members. I amfortunate to have had many students andpostdoctoral fellows in my group. I alsohad the pleasure of working with a number of dedicated collaborators who cared about reticular chemistry, espe-cially Michael O’Keeffe (Arizona StateUniversity), Ulrich Mueller (BASF,Ludwigshafen), and Ronald Allain, aretired VP of R&D at NALCO ChemicalCompany. I also had the pleasure of work-ing with many other colleagues around theworld who now stand as leaders in thefield of metal-organic frameworks andreticular chemistry. I am also grateful forthe support of NSF for a lot of the basicwork of reticular chemistry over the past14 years; the DOE for funding the studiesof gas separations and hydrogen storage;BASF for financial support and independ-ence; DOD for funding, and the DefenseThreat Reduction Agency for variousapplications related to personal protection.

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Omar M. Yaghi receivedhis PhD degree from theUniversity of Illinois-Urbana in 1990 withPro fessor Walter G.Klemperer. He was anNSF postdoctoral fellowat Harvard University(1990–1992) with Pro -

fessor Richard H. Holm. Yaghi also has beenon the faculties of Arizona State University(1992–1998) and the University of Michigan(1999–2006). His current position is theIrving and Jean Stone Chair in PhysicalSciences at UCLA. His early accomplishmentsin the design and synthesis of new materialshave been honored by the Solid StateChemistry Award of the American ChemicalSociety and Exxon Co. (1998) and the SacconiMedal of the Italian Chemical Society (2004).His work on hydrogen storage was recognizedby Popular Science Magazine, which listedhim among the “Brilliant 10” scientists andengineers in the United States (2006). Hereceived the U.S. Department of EnergyHydrogen Program Award for outstandingcontributions to hydrogen storage (2007) andwas the sole recipient of the MaterialsResearch Society Medal (2007) for pioneeringwork in the theory, design, synthesis, andapplications of metal-organic frameworks. Healso won the AAAS Newcomb ClevelandPrize for the best paper published in Science(2007), and he is the recipient of the AmericanChemical Society Chemistry of MaterialsAward (2009). His work encompasses thesynthesis, structure and properties of inorganic compounds and the design and construction of new crystalline materials. Hehas published more than 130 papers, whichhave received more than 180 citations perpaper. He is listed among the top 10 mosthighly cited chemists worldwide (1998–2008).

Yaghi can be reached by phone at 310-206-0398 or e-mail at [email protected].

Qiaowei Li received his BS(2004) in applied chem-istry from the Universityof Science and Technologyof China and his MS in chemistry from theUniversity of Michigan(2006). He is currently a PhD student at the

University of California-Los Angeles, work-ing with Professor Omar M. Yaghi. His cur-rent research interests include designing and constructing extended solid structures withcomplexities and the use of new porous crystalline materials for chemical appli -cations. Li can be reached by e-mail [email protected].


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Reticular Chemistry and Metal-Organic Frameworks for Clean ...yaghi.berkeley.edu/pdfPublications/09cleanEnergy.pdf · be organic molecules, inorganic clusters, dendrimers, peptides,