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2009Metal–organicframeworksissue

Reviewingthelatestdevelopmentsacrosstheinterdisciplinaryareaof

metal–organicframeworksfromanacademicandindustrialperspective

GuestEditorsJeffreyLongandOmarYaghi

Pleasetakealookattheissue5tableofcontentstoaccess

theotherreviews.

In2005,thedailyglobalconsumptionofpetroleumexceeded/超过/

83millionbarrelsresultingintherelease/释放/ofalmost11billion

metrictonnes/公吨/ofcarbondioxideintotheatmosphere.1

Consumptionisexpectedtoincreasesteadily/稳定/overthenext

50years,driveninpartbyhigherdemandsindeveloping

nations.Amid/其中/concernsthattheatmosphericlevel

ofcarbondioxidewillirreparablydamagetheglobalecosystem,

researchintocarbon-neutralreplacementsforfossil

fuelsisexpanding.Analternativefuelforautomotive

transportationisofparticularinterestandwouldhavea

substantial/大量的实质的/impact/影响/oncarbonemissions,withmorethan

132millionvehicles/工具/intheUnitedStatesalone.2

Batteryandfuel-celltechnologiesarestrongcandidates/候选者/to

replacegasoline/汽油/anddiesel/柴油/engines.Inparticular,hydrogen

isanattractiveenergycarrierbecauseitiscarbon-free,

abundantly/丰富的/availablefromwater,andhasanexceptionalmass

energydensity.3Unfortunately,hydrogenisanextremely

volatile/挥发性的/gasunderambient/周围的/conditions,resultingina

volumetricenergydensitythatismuchtoolowforpractica

applications.Foron-boarduse,hydrogenmustbecompressed

toveryhighpressuresorstoredcryogenically/低温/,bothofwhich

costenergyandsubstantial/大量的实质的/lyincreasevehicleweight.Thegoal

thereforeistodesignlow-cost,light-weightmaterialsthatcan

reversibly/可逆的/andrapidlystorehydrogennearambient/周围的/conditions

atadensityequaltoorgreaterthanliquidhydrogen.TheUS

DepartmentofEnergy2010targetsforahydrogenstorage

systemare:

acapacityof45gH2perL,arefuelling/补充燃料/timeof

10minorless,alifetimeof1000refuelling/补充燃料/cycles,andanability

tooperatewithinthetemperaturerange_30to501C.4,5Itis

importanttonotethatthesetargetsarefortheentirestorage

system,suchthattheperformanceofastoragematerialmust

beevenhigherinordertoaccountforthestoragecontainer

and,ifnecessary,temperatureregulating/校正/apparatus/设备/.

Hydrogenbindstosurfacesbyweakdispersive/分散的/interactions

（physisorption）orthroughstrongerchemicalassociations

（（chemisorption）.Physisorptioncorrelateswithsurfacearea,

withgreatergasuptakefavoredbyhighersurfacearea.Thus,

materialswithlargesurfaceareasandlowdensities,suchas

metal–organicframeworksandcertainactivatedcarbons,are

attractiveforhydrogenstorageapplications.Inthetemperature

regimedesiredforautomotiveapplications,however,

dispersive/分散的/forcescannotfacilitatesubstantial/大量的实质的/hydrogen

uptake.Themodularnatureofmetal–organicframeworks

allowsforthefacile,orderedincorporationofnewfunctionalities

toenhancethehydrogenstorageproperties.Here,

wereviewthecurrentstateofhydrogenstorageinmetal–

organicframeworks,focusingonstrategiesforimproving

thestoragecapacityofthesecompounds.Thedesignof

newframeworksdependsonadetailedchemicalunderstanding

oftheinteractionofhydrogenatsiteswithinthestructure.

Wethereforealsodiscussbrieflysomespectroscopictools

thatareavailabletointerrogatehydrogenbindinginthese

systems.

H2adsorptioninmetal–organicframeworks

Excessversustotaluptake

Mostarticlesdealingwithhydrogenstorageinmetal–organic

frameworksreporttheH2uptakecapacityatapressureof

ca.1bar,whereexcessandtotaladsorptionvaluesarenearly

identical.However,sincepressuresofupto100barare

deemedsafeforautomotiveapplications,measurementsat

higherpressures,wherethesetwoquantitiescandiffer

considerably,havebecomecommon.Excessadsorptionrefers

totheamountofH2takenupbeyondwhatwouldbe

contained,underidenticalconditions,withinafreevolume

equivalenttothetotalporevolumeofthesample.Thus,

thisquantityapproximatestheamountofH2adsorbed

onthesurfaceswithinthematerial.Sincetheefficiencyof

packingandcompressinggasmoleculeswithintheconfinesof

theporesofamicroporoussolidislessthanthatachievedina

freevolume,theexcessadsorptionwillreachamaximumat

somepressure（typically20–40bar）andthendecrease.Despite

thedecrease,measurementsatpressuresabovethemaximum

inexcessadsorptionareofvalueforassessingthecompressibility

ofH2withinthematerialandevaluatingthetotal

1295

oneshouldbecarefultoemployanaccurateintermolecularH2

potentialenergyfunction18andtoensurethatthecomparison

dataareforanauthenticsample.19Thesestudiesindicatethe

presenceofjustvanderWaals-typeinteractionsbetweenH2

andmostframeworks,consistentwiththeapproximate

correlationofH2uptakeat77Kwithsurfaceareaandthe

verylowstoragecapacitiesobservedat298K.Indeed,with

justtwoelectrons,H2formsextremelyweakvanderWaals

bonds,resultinginisostericheatsofadsorptionthatare

typicallyintherange4–7kJmol_1.

Partialcharges,eitherpositiveornegative,onthemetal–

organicframeworksurfacecanprovideameansofstrengthening

thebindingofH2throughdipole–induceddipole

interactions.6,20Onlyafewcomputationalstudieshavedealt

withframeworksexhibitingsuchheterogeneoussurface

potentials.Thesehavefocusedmainlyonthechiefexperimental

strategyadopted,thatofutilizingframeworkswithexposed

metalcationsitesonthesurface.Anaddedcomplication

inperformingcalculationsonframeworksbearingopen

transitionmetalcoordinationsitesstemsfromthefactthat

thesemetalssometimeshaveopen-shellelectronconfigurations,

forwhichassignmentofthespinstatecanbedifficult.

Forinstance,therelativelystrongmetal–H2interactionswithin

Mn3[（Mn4Cl）3（BTT）8]2（H3BTT=benzene-1,3,5-tris（1Htetrazole））,

whichexhibitsanisostericheatofadsorptionof

10.1kJmol_1atzerocoverage,21havebeenattributed

variouslytoaspin-statechangeuponbinding22ortoa

classicalCoulombicattraction.23Understandingmetal–H2

interactionsofthistypeisinstrumentaltothedesignof

improvedstoragematerials,andthedevelopmentofcomputational

approachesthatcanreliablyhandleinteractions

withopen-shellmetalionswouldpresentanimportantstep

forward.

Clearly,increasingtheH2bindingenergywithinmetal–

organicframeworksisthemostimportantchallengefor

creatinghydrogenadsorbentsthatoperateat298K.Recent

workhasaddressedthisissueandpredictedoptimal

parametersforhydrogenstorageinmicroporousmaterials.

First,Langmuirisothermswereemployedtoderiveequations

thatallowthecalculationofanoptimaladsorptionenthalpy,

DHopt,foragivenadsorptiontemperature.24Accordingtothis

model,whichcanbereducedtotheempiricalequation

_DHopt/RT=6.1,amicroporousadsorbentoperating

between1.5and100barat298Kwouldideallyhavean

adsorptionenthalpyof13.6kJmol_1overtheentireH2

uptakecurve.Similarly,themodelallowsonetocalculate

theoptimaloperatingtemperatureforanadsorbentwitha

givenenthalpyofadsorption.Forinstance,itpredictsthata

typicalmetal–organicframeworkwithanaverageadsorption

enthalpyof6kJmol_1wouldfunctionoptimallyata

temperatureof131K.

Theaforementionedmodelhasrecentlybeenadjusted

throughintroductionofanentropy–enthalpycorrelation

term.25WhereasDSadshadpreviouslybeenassumedtobe

constantandequalto_8R,thenewmodelarguesthat

Langmuiradsorptionisinfactgovernedbyapositive

correlationbetweenentropyandenthalpy.Takingthis

empiricalcorrelationintoaccountsuggeststhatamaterial

operatingbetween1.5and30barat298KrequiresaDHoptof

22–25kJmol_1,whichissignificantlyhigherthanthat

obtainedwiththepreviousmodel.Thus,forpressuresranging

upto100bar,onewouldliketocreatenewmetal–organic

frameworksfeaturingsurfaceswithaDHoptofca.

20kJmol_1,representinganenhancementbyafactorof

3or4oversimplephysisorption.

Asexpected,inamicroporousmaterialwherephysisorption

andweakvanderWaalsforcesdominatetheadsorption

picture,thestoragedensityisalsogreatlydependentonthe

sizeofthepore.Calculationsonidealizedhomogeneous

materials,suchasgraphiticcarbonsandcarbonnanotubes,

predictthatmicroporousmaterialswith7A˚-wideporeswill

exhibitmaximalH2uptakeatroomtemperature.Ineffect,

a7A˚-wideslit-shapedporemaximizesthevanderWaals

potentialbyallowingexactlyonelayerofH2moleculesto

adsorbonopposingsurfaces,withnospaceleftinbetween.

Notably,at77Kalayersandwichedinbetweenthesetwo

opposingsurfacemonolayersbecomesfavorable,andtheideal

poresizeformaximumvolumetricH2uptakeat100baris

predictedtobe10A˚,regardlessofwhetheraslitshapeor

cylindricalporeshapeisconsidered.26

Finally,anidealhydrogenstoragematerialwouldbestable

toanypotentialimpuritiesthatmightcommonlybepresentin

H2gas（e.g.,H2S,carbon–sulfurcompounds,CO,CO2,N2,

H2O,andhydrocarbons）,andtoaccidentalexposuretothe

atmosphere.Indeed,metal–organicframeworksexhibiting

someofthebestperformancecharacteristics,suchas

Zn4O（BDC）3andMn3[（Mn4Cl）3（BTT）8]2,areknownto

decomposeinair,19,21,27,28whichwouldneedtobeaccounted

forinthedesignofastoragesystem.However,byproducing

frameworksfeaturingstrongm