The power of metal–organic frameworks

BY NANCY MILLS

Big onthe

i n s i d e

Talk about room to spare! Withwalls a single atom thick, andinternal surface areas up to10 000 square metres per

gram, a metal–organic framework(MOF) can be up to 80–90% emptyspace. The rest of this porous materialconsists of metal atoms or smallinorganic clusters positioned at thecorners of a two- or three-dimensionalframework and linked by rigid organicmolecules.

MOFs, also known as porouscoordination polymers, are a majorgrowth area for research chemists andmaterials engineers. Their periodicsub-nanoscale porosity (0.3–5nanometres) means big potentialapplications in the automotive andenergy sectors, including storinghydrogen and methane to powervehicles and capturing carbon dioxidefrom coal-fired power stations. UsingMOFs for on-board methane storagein natural gas-powered vehicles, forexample, could deliver a driving rangecomparable to petrol-poweredalternatives – with dramaticallyreduced greenhouse emissions andrunning costs.

The importance of MOFs washighlighted in January 2012 whenChemical Reviews dedicated a specialissue to their properties andapplications. The journal’s editorialnoted that MOFs ‘epitomise the beautyof chemical structures and the powerof combining organic and inorganicchemistry’, and drew attention to the‘endless possibilities’ afforded bymanipulation of their inorganic andorganic components.

Closer to home, Victoria’s six 2011Young Tall Poppies included two early-career researchers who collaborate onMOF research at the AustralianSynchrotron: Matthew Hill from CSIROMaterials Science and Engineeringand David Turner from the MonashUniversity School of Chemistry.Matthew went on to become the state’sYoung Tall Poppy Scientist of the Year.

Record resultsMatthew‘s MOFs currently hold threeworld records for gas storage: roomtemperature storage of both hydrogenand methane, and zero-degreestorage of carbon dioxide.

‘Gases take up a lot of room, evenwhen they’re compressed,’ Matthewsays. ‘It’s one of the limiting factors inhydrogen- or methane-fuelled cars.However, if you bond a gas to asurface, the gas molecules pack inmuch more densely than if they weresimply squashed together. MOFs havelarge internal surface areas becauseevery atom is a new surface, and theycan therefore store more gas in asmaller space.’

There’s an old joke about fitting fivegiraffes into a Mini: two in the front, twoin the back and one in the glovebox.How do you fit five elephants into aMini? You can’t – it’s full of giraffes. Inthe case of MOFs, however, Matthewpredicted in 2009 on the basis ofearlier work with carbon and coppernanotubes that encouraging metal-substituted fullerenes to infiltrate theopen framework structure wouldincrease the attraction betweenmethane and MOF and boost their

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MOFs

With their spacious interiors, metal–organicframeworks offer many possibilities for research

chemists and materials engineers.

This beryllium-based MOF holds the recordfor hydrogen storage at room temperature. Matthew Hill, CSIRO

methane storage capacity. His teamrecently validated this experimentally,smashing the storage capacity recordand surpassing the USA Department ofEnergy target by 63%.

Similarly, in 2012, Matthew foundthat one gram of an MOF loaded withlithium atoms could capture as muchas 170 mL of carbon dioxide nearatmospheric pressure. That’sequivalent to compressing the carbondioxide to one-eighty-fifth of itsoriginal volume.

The record for hydrogen storagecapacity per gram of material at roomtemperature was achieved in 2009,after Matthew and his CSIROcolleagues developed the first-everMOFs made from extremelylightweight atoms.

Synthetic solutionsSynthetic methods for MOFs typicallyinvolve precipitating MOF crystalsfrom dilute solutions of suitableprecursors such as organometallic

compounds with hydrocarbon ligands,under the right reaction conditions.However, the synthesis is still not wellunderstood.

Over the past few years, CSIROresearchers have tackled severalaspects of MOF production andcharacterisation, including thedevelopment of a high-throughputplatform for preparing and screeningmetal–organic framework materials.The key is to use high-throughputsynchrotron X-ray powder diffractionto identify crystalline materials withthe large unit cells associated with aMOF’s ordered porosity. Synchrotronpowder diffraction offers shorteracquisition times and higherthroughput, improved signal-to-noiseratios and better resolution thanlaboratory methods. This meansresearchers can assess a largernumber of test reactions to determinethe best conditions for producingMOFs, and map their formation in realtime.

Another section of the CSIRO teamhas expertise in material engineering.Their aim is to grow MOFs in preciselocations and on substrates withparticular geometries, and prepareMOFs with embedded functionalnanoparticles; both aspects areimportant for the fabrication of porous-based miniaturised devices such asthose required for microfluidics andmicro-sensing applications.

Materials engineer Paolo Falcaroand his colleagues are employingsynchrotron powder diffraction to scanhundreds of samples of MOF crystalsnucleated using silicon dioxidemesoporous monoliths withfunctionalised surfaces andmesoporous nanoparticles in solution.The group is considered one of theworld’s top four in the area of MOFfunctionalisation and patterning.

They have also used Fourier-transform IR reflectance andtransmission microspectroscopy at theAustralian Synchrotron to examine a

Chemistry in Australia22 | May 2012

David Turner (Monash University) and Rachel Williamson (Australian Synchrotron) on the microcrystallography beamline.

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new type of MOF grown onthe surface of seededsubstrates and patternedsurfaces created using deepX-ray lithography at theElettra synchrotron in Italy.Synchrotron Fourier-transformIR enabled detailedcharacterisation of seedcomposition as an importantstep towards understandinghow to use these to promoteMOF formation or supplycompounds such as drugs forcontrolled release through theporous MOF structure.

Paolo and his colleagues havediscovered that nano-structuredpolyhydrate zinc phosphate (alpha-hopeite) microparticles can act asMOF nucleation agents both in solutionand on complex two- and three-dimensional surfaces. Thealpha-hopeite microparticles can beused to accelerate MOF production,provide spatial control of MOF growth,or insert functional (e.g. metallic,polymeric or semiconducting)nanoparticles into MOFs for molecularsize-selective applications. Theimportance of their findings and thebeauty of their MOF nanoparticleshave put them on the front covers oftwo major journals in the last 12months.

‘The Australia Synchrotronrepresents a unique facility for thecontrol of MOF quality’, Paolo says.‘Our modifications induce distortion inthe MOF frameworks, and thesynchrotron is essential for identifyingthe conditions that yield the bestproperties.’

Crystal chemistry and engineeringAt Monash University, David Turner isinterested in what he describes insimple terms as ‘how best to trap ordetect molecules, and how moleculespack together to form materials’. Bothgoals involve ‘exploringintermolecular interactions that areweaker than standard chemical

bonds’. His research focuses onporous materials for gas storage (suchas MOFs) and more-fundamentalstudies of how molecules interact inthe solid state. He says that thesynthesis of porous systems such asMOFs is ‘very challenging because thenatural tendency is for materials not tocontain any hollow space’.

Because MOFs have relatively fewatoms per unit volume, they areunsuitable for crystallographic studiesusing a laboratory source. Theproblem is exacerbated by the factthat MOF crystals are typically around10-30 microns.

“For crystallography, the key pointis that the intensity of synchrotronradiation lets us structurallycharacterise materials that we wouldnever have a hope of characterising onin-house instrumentation,” David says.“This has blown the field wide open.Interesting compounds that might havebeen discarded years ago as ‘poorlydiffracting, microscopic rubbish’ cannow be fully explored.”

David says other areas of chemistryresearch benefit from similar methods.For example, his group is alsointerested in crystals composed ofdiscrete metal–organic cages. Crystalengineering is a key focus for anotherMonash group seeking to control thearrangement of molecules in the solidstate: ‘MOFs are really a subcategoryhere; we engineer them in the sameway, but with porosity as the main goal.’

In 2009, while working as part of a

Monash research teamled by Stuart Batten,David published thediscovery of a self-assemblingsupramolecular complexwith a complete catenaneand eight associatedtetrafluoroborate anions inthe asymmetric unit. Thisremarkable catenane

includes eight aromatic groups in acontinuous 2.5-nanometre array offace-to-face pi interactions along itslongest dimension. Not surprisingly,the researchers dubbed it an ‘octapi’complex. The structure was solved onthe Australian Synchrotron’s MX2microcrystallography beamline.

The quest for moreAs part of the quest for even greatersurface areas, Matthew and David andtheir colleagues made a series ofMOFs with metals such as magnesium,calcium, titanium, manganese, iron,cobalt, nickel, copper, zinc, tin andlead. They used the synchrotron’scrystallography beamlines todetermine the crystal structures oftheir MOFs as part of an investigationof how the size-to-charge ratios of themetal ions affect the gas adsorptionstrength of MOFs. As a result, theresearchers have discovered new andunexpected structures withapplications not only in gas storage,but also in photocatalysis, gasseparations and medical treatment.

With their diverse frameworkstructures, extraordinary porosity andpotential applications across manyareas of science and industry, it is clearthat MOFs will continue to fascinate.

Nancy Mills <nancy.mills@synchrotron.org.au> is theAustralian Synchrotron’s science writer. She says thepeople named in this article are just the tip of theresearch ‘iceberg’ and there are many otheroutstanding contributors to this work who could notbe named due to limited space.

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MOFs

May 2012

SEM image of a polyhydrate zincphosphate (alpha-hopeite) mi-croparticle seeding ultraporouscubic metal–organic crystals(MOF-5).

Paolo Falcaro, CSIRO