Magnetic
resonance
in
reaction
engineering:
beyond
spectroscopy
Lynn
F
Gladden
Magneticresonance(MR),intheformofnuclearmagnetic resonance(NMR)spectroscopy,iswellestablishedin characterisingcatalystsandstudyingrelativelysmallcatalytic samplesinsitu.However,MR-basedmeasurementsof molecularadsorptionanddiffusion,alongwithMR micro-imagingandflowmappingofferanadditionaltoolkitof methodstostudyboththecatalystanditsworking environment.Focussingonrecentadvancesinthe implementationofMRmethodsinreactionengineeringwe considerMRmeasurementsyieldinginformationattwoquite differentlength-scales:first,theabilityofMRrelaxometryand diffusometrytostudymolecularadsorptionanddiffusion processeswithintheporespaceofheterogeneouscatalysts; second,theuseofmicro-imagingandMRflowimagingto criticallyevaluatetheclosurerelationshipsandboundary conditionsusedinnumericalsimulationsofcatalyticreactor operation.
Addresses
UniversityofCambridge,DepartmentofChemicalEngineeringand Biotechnology,PembrokeStreet,CambridgeCB23RA,UnitedKingdom
Correspondingauthor:Gladden,LynnF(lfg1@cam.ac.uk)
CurrentOpinioninChemicalEngineering2013,2:331–337 ThisreviewcomesfromathemedissueonReactionengineeringand catalysis
EditedbyMarc-OlivierCoppensandTheodoreTTsotsis ForacompleteoverviewseetheIssueandtheEditorial Availableonline22ndJune2013
2211-3398#2012TheAuthor.Publishedby ElsevierLtd.
http://dx.doi.org/10.1016/j.coche.2013.05.005
Introduction
Whenconsideringtheroleofmagneticresonance(MR)in catalysis, we naturally thinkof nuclearmagnetic reson-ance(NMR)spectroscopyasusedforthecharacterisation of solid state structure and speciation of the catalyst surface[1–6,7].Alongsidetheseadvances,anincreasing diversityofMRtechniquesforstudyingcatalyticfunction
insituhasbeenpioneeredbyanumberofgroups.Insitu
NMR methods can be broadly divided into two main
areas: those methods that operate under ‘batch’ con-ditions in sealed vessels [8–10] and those that operate under ‘flow’conditions[11,12,13].The motivationfor the vast majority of these studies has been to access chemicalinformation—forexample,thecharacterisation of acidic and basic sites at the catalyst surface, the oxidation state of surface active species, identification ofparticularmechanisticstepsinthecatalyticconversion. In modelling a catalytic process, clearly the chemical engineernot only needsthis information but alsogives consideration to the interplay between the chemical processes occurring and the influence of the associated rates ofadsorption, desorption andmasstransfer within the catalyst, as well as the local boundary conditions imposedonthecatalystbythehydrodynamic character-isticsofthegivenreactorenvironment.
DespitethelimitationsofMRinnotbeingabletostudy samplesandsampleenvironmentscontainingsignificant amounts offerromagnetic material,MR remainsone of thefewmeasurementmethodswhichcanprovide chemi-calinformationviaspectroscopicmeasurement,whilein the same sample environment being able to measure molecular transport without need for use of tracers or invasiveprobes;allthesemeasurementscanbemadein opticallyopaquemedia[14,15].Thisarticleattemptsto illustratehowMRmethods,whichcomplement spectro-scopicdata,arenowsufficientlyadvancedandrobustthat they can begin to yield valuable information at the microscale where we can probe molecular adsorption and diffusion phenomena, while at the macroscale we cancriticallyevaluatetheassumptionsusedintheoretical analysis and numerical simulationof reactor behaviour. The toolkit of MR techniques now available to the chemical engineer can help us understand the relative importance of differentchemicaland physical phenom-enaindeterminingcatalyticperformanceandhencegain insight into improving catalyst and reactor design, and process operation.
The
microscale:
what
is
happening
inside
the
pore?
What informationmightwe requireto aidourabilityto design acatalyst,andtounderstandandpredictcatalyst behaviour?Ofparticularinterestarethefollowing ques-tions:
Whichspeciesdominateattheporesurface?
Howdoesmolecularmobilityvarywithinthepore?Is thereastronginfluencefromthesurfacewhichrestricts
moleculartransportclosetothesurface?Ifso,canwe quantifyit?
Can we decouple the influences of adsorption and diffusioninsidethepore?
Can we predict the way multi-component mixtures behaveinside acatalystpore?
Notonlyshouldanswersto thesequestionsadvanceour ability to design and select catalysts with preferred activity and selectivity characteristics, but also the measurementsobtainedwillprovideinputdataand vali-dation of input datato kinetic schemes used in under-standingcatalyst functionand aidingcatalystdesign.In thisregard,itisusefultothinkofthesemeasurementsas anothertooltoaddtothosealreadyusedinimplementing microkinetic analyses. Using a microkinetic approach onlythekineticallysignificantstepsareincludedinthe kinetic scheme and then using theoretical principles, numericalsimulation (e.g. applicationof Density Func-tional Theory) or experimental measurement, the kineticsofeachof thesestepsarequantified—thereby producingaquantitativedescriptionoftheoverallprocess [16].Excellent examples of theapproach in use have beenreported[17–19].Oneofthepowerfulattributesof MR is that the individual methods are readily imple-mented under reaction conditions. Indeed, it will be usefulto use MR techniquesto characterise adsorption anddiffusionprocessesasafunctionoftemperatureand pressuretoexplorehowaccuratelywecanpredict multi-component transport processes and phase behaviour underrealisticoperatingconditions.
Which species dominate at the pore surface? The recent
implementationofthismeasurementcamefromparallel workundertakenincharacterisingrock-corewettability, asrequiredfordevelopingoilrecoveryprocesses[20].In thisfieldtwo-dimensionalMRrelaxometryexperiments areusedtocharacterisetherelativestrengthofinteraction of hydrocarbon and aqueous species with the interior surface of the rock—the parallel between hydro-carbon/brine/rockandamulti-componentmixturewithin asolidcatalystisobvious.Theseexperimentsarebased onsimple,robust measurementsof nuclearspin relaxa-tiontimes;relaxationtimescharacterisethereturnofthe nuclearspinsystemtoequilibriumfollowingexcitationof thenucleusofinterestusingradio-frequencyexcitation. In particular, the spin–lattice (T1) and spin–spin (T2) relaxation timesaremeasured. The higherthevalue of theT1/T2ratioofagivenchemicalspecies,thegreaterthe strength of interaction between that species and the surface.Motivatedby astudyof 2-butanone hydrogen-ationin mixed 2-propanol/water solvents, this method-ologywas usedto demonstratetherelativestrengths of interactionofwater,analcohol(2-propanol)andaketone (2-butanone)withPd/Al2O3andRu/Al2O3catalysts[21].
A very recent example of using this method to guide solventselectionforthehydrogenationof phenylbutan-2-oneisshowninFigure1,whichshowstheT1/T2dataand reactionratedataacquiredforphenylbutan-2-oneinthe presence of fivedifferentsolvents. In thisexample, we seethattherelative strengthofinteractionbetweenthe individual speciesis significantlydifferent. A clear cor-relationbetweenreactionrateandtherelativestrengthof surface interaction of thereactant species compared to thesolventspecieswasobserved;thelesscompetitivethe solventisforthecatalystsurface,thefasterthereaction rate.Nocorrelationwas observed betweenpolarity and hydrogen solubility characteristics of the solvents. Althoughtheseexperimentsare,inprinciple, straightfor-ward,caremustbetakenin performingthe two-dimen-sionalinversion of the datafrom which the final T1–T2 spectrum is obtained [22]; and there remains further research to be done to optimise theimplementation of thisapproachto studycomplexmulti-componentinside catalyst pores. The method becomesincreasingly chal-lenging if 13C observation is employed due to the far lowerMR sensitivityand naturalabundance of the13C nucleus.However,theadvantageofusing13Cisthatdue tothefargreaterspectralrangeof13Ccomparedto1H,it is significantly easier to discriminate between the multiplespectralresonancespresentinatypicalreaction mixture.
Thereareanumberofwaysinwhichtheseexperiments canbeusedandit becomesimportant to design exper-imentsthatanswerthespecificquestionof interest.For example,in theexample shownhere,theapproach has been used to identify a solvent which will maximise Figure1 1 10 100 1000 0 0.2 0.4 0.6 0.8 1 1.2
Reaction rate / % min-1
T1
/
T2
Current Opinion in Chemical Engineering
PlotofT1/T2valuescharacterisingthestrengthofsurfaceinteraction betweenfivedifferentsolventsandaPt/TiO2catalyst.TheT1/T2ratiofor thereactantphenylbutan-2-oneis49.Itisclearlyseenthatasthe strengthofthesolventinteractionwiththesurfaceincreases,therateof conversiondecreases.InorderofdecreasingT1/T2ratiothesolvents usedaretetrahydrofuran(197),1,2-dichloroethane(36), 4-tert-butyltoluene(20),cyclohexane(14)andn-hexane(9).
reactionrate—aslong ascompetitiveadsorption ofthe reactantandsolventistherate-determiningstep.
How doesmolecular mobilityvary inside a pore?If relaxo-metry techniques show promise in characterising com-petitive adsorption processes under in situ or operando
conditions,towhatextentcanwecharacterisemolecular mobilitywithapore?TheuseofPulsedFieldGradient (PFG)MRtechniques[23,24]formeasuringmolecular diffusion iswellestablishedandhasbeen demonstrated in application to studyingcatalyticmaterialsin both its spatiallyunresolvedandresolved modalities[25,26].By measuringthemolecularself-diffusioncoefficientinthe bulk liquid and inside the pore space a value of the tortuosityofthecatalystpelletisobtained.Thisisuseful measurementbutisitthewholestory?Themeasurement is providingan averagemeasure ofthemolecular diffu-sionfrom theentirepore spaceinsidethecatalyst. Itis generally accepted that aliquid-saturatedporecontains two distinct regionswith differing moleculardynamics; themajorityoftheporeisfilledwithliquidthatbehaves asifitweresimilartoabulkliquid.However,atthepore surface, the molecular dynamics are altered by inter-actions(adsorption) atthesolidsurfaceandlimitedfree diffusion paths. This surface behaviour is considered typicallytoextendtoliquidmoleculeswithonetothree monolayersfromtheporesurface[27,28].By implement-ingamorecomplexversionofthebasicPFGexperiment knownasthealternatingpulsedgradientstimulatedecho (APGSTE)sequence[29],ithasbeenpossibletoacquire data at more extreme signal attenuations and in the presence ofthelarge‘background’magneticfield gradi-ents that characterise inorganic porous media, thereby enabling measurement of the much slower diffusion processes in the ‘surface-influenced’ layer as well as the diffusion occurring in the ‘bulk’ of the pore space [30]. Considering thecase of diffusion of 1-octene in a 1wt%Pd/Al2O3catalyst,thisapproachhasbeenusedto determine two molecular diffusion coefficients of 1.310 9 and 1.710 11m2s 1 for ‘bulk’ pore and surface diffusion, respectively. Confirmation that this lower diffusion coefficient is indeed associated with a surfaceinteraction,asopposedtoasecondarypopulation of smaller pores, was obtained by treating the catalyst with a silanesurfacecoating, which removed the inter-actionbetweenthe1-octeneandthehydroxylsoriginally present on thealumina surface;under these conditions theslowerdiffusioncoefficientwasnolongerobserved. Howshouldweusethistypeofmeasurement?Thedirect measurementofmoleculardiffusivitywillhave immedi-atevalueasinputtomodelsofcatalyticreactorprocesses. Thesemeasurementsalsoinformusastohowchangesto thecatalystformulationorsurfacetreatmentmight influ-ence catalyst performance. It is also interesting to examinehowsuchameasurementrelatestothe predic-tionsofnumericalsimulationsofthesameprocess.This
has been done in a comparison of PFG MR determi-nationsofbulkporeandsurfacediffusionofisopropanol in g-alumina, with values predicted by molecular dynamics simulation [31]. Excellent agreement was observed between the MR values and the numerically predictedvalues;thesimulatedvaluessuggestedthatthe surface-influencedlayerasprobedbyMRcomprisedthe firsttwomolecular layersinteractingwiththesurface.
Canweseparatetheinfluencesofadsorption anddiffusionon catalystreactivityandconversion?Howaremolecule-molecule interactionsanddynamicsinfluencedbyconfinementwithinthe poresofcatalyticmaterials?Anaturalextensionoftheideas described previouslyisto use MRrelaxometry and dif-fusometrytoinvestigateifwecandiscriminatebetween therelativeimportanceofthestrengthoftheadsorption interactionandthemolecularmobilitywithintheporein determiningcatalystperformance;ofcourse,thesewillbe stronglycorrelatedclosetothecatalystsurface.Recently, thetwoapproacheshavebeenusedtoprobetheeffectof water on the catalytic oxidation of 1,4-butanediol in methanol over a Au/TiO2 catalyst [32]. The presence of water causes a significant reduction in conversion. Using MR this reduction in conversion was shown to beaccompaniedbybothreducedreactant-surface inter-action and reactant diffusivity. More detailed studies suggest that it is possible to identify the adsorption interactionasthegreaterinfluenceoncatalytic perform-ance. Relaxometry and diffusometry also allow us to probe the influence of pore confinement on molecular mobility and the creation or breakdown of molecular networks; theseideas are explored in arecent work by D’Agostinoetal.[33].Inparticular,thisworkstudiedthe relaxation and diffusion characteristics of a range of organicmoleculesincludingalkanes,alcoholsand carbo-nylscontainedwithinporousmaterials—TiO2,g-Al2O3 andSiO2.BothMRprobesweresensitiveto changesin molecule–moleculeinteractionsandmoleculardynamics introducedbyconfinementwithinthepore space.Most notably, strong evidence was obtained that polyols demonstrate enhanced mobility when inside the pore spacerelativetotheirbehaviourinthebulkliquidstate; this wasinterpreted as poreconfinementdisruptingthe extensive intermolecular hydrogen bonding network characteristic ofthesemolecules.
While MR is not the only technique that can probe surfaceinteractionsandmolecularmobility—vibrational spectroscopy and neutron scattering are obvious examples—it does have a numberof attributeswhich make it a particularly useful method; the ability to identifyprocessesoccurringattheporesurfaceasdistinct frommeasurementsaveragedwithintheporeis particu-larlyinteresting. Itis alsoworthrememberingthat MR measurements are readily implemented directly on sample environmentsoperating atany temperatureand pressure.
The
macroscale:
validation
of
closure
laws
The application of MR to imaging hydrodynamics [34,35,36,37,38] and mapping chemical conversion [39–41] in reactor environments is well documented. Muchof this workhasfocused onfixedbeds.Itis now possible to measure directly the average concentration gradientsbetweentheintra-pelletandinter-pelletspace atdifferentpointsalongthedirectionofsuperficialflow along the bed. Can we push these limits of spatial resolutionfurthersothatwecandeterminethe concen-tration gradients at the external surface of individual pellets andsee how theserelate to thelocal hydrodyn-amicboundaryconditionsonthecatalystpelletandthe microstructureof thepellet? Such datawouldgiveusa first opportunity to test assumptions used in reactor design.When small % improvementsin rate and selec-tivitybecomeimportantin reactoroperation,theability toselectthecatalystanditsoperatingenvironmentonthe basisofdirectmeasurementmadeduringoperationunder realistic conditions becomes an attractive possibility. A fixed-bed reactordesigned to operate undercontinuous flow conditions at maximum operating conditions of 3508Cand30bar,andconstructedofnon-ferromagnetic material,hasrecentlybeencommissionedinour labora-tory.Obviouslythisreactoroffersustheexciting oppor-tunity of performing MR imaging and velocimetry experimentsatrelativelyhightemperatureandpressure, andcanbeusedtostudysomecatalystsatconditionsvery closetothosetheywouldexperienceinaworkingreactor. Justasimportantly,thereactorgivesustheopportunityto explorehowadsorptionanddiffusionprocessesvaryasa functionof temperatureand pressure—thereby identi-fyingwhenitisacceptabletouselow temperature/pres-sure measurements of these characteristics, and to evaluate the accuracy with which these properties can bepredictedatrelevant operatingconditions.The next generationofultra-fastMRtechniquescanbeexpected tooffernewopportunitiesinimagingunsteadyflowsand chemical conversion with subsecond time resolution. Already, forced pulsing, or periodic operation, of a trickle-bedreactorhasbeenreportedandintegratedwith modelling studies to explore how unsteady state oper-ationofafixedbedcaninfluencecatalystselectivityand activity—in this example, the hydrogenation of a -methylstyrene was considered [42]. Aswell as 1H and 13
C spatially resolved spectroscopy to map chemical compositionwithinreactors,theuse ofparahydrogen to probethemechanismofcatalytichydrogenationsisalso beingexplored[43,44];opportunitiesforexploiting para-hydrogen and other hyperpolarisation signal enhance-ment methods [45,46] will almost certainly find niche applicationsincatalysis.
Howelsemight MRcontributeatthemacroscale? Ima-gingmulti-phaseflowsisanareawhichhasseen signifi-cant advances over the past five years. With these advances come the opportunities to critically evaluate
closurerelationshipsandboundaryconditionswhichare applied in numerical simulations of multi-phase flows and, following incorporation of the appropriate kinetic scheme,fullreactor behaviour.Thisisanareain which thedesignoftheexperimentmustbegivenverycareful consideration.ExamplesofthisuseofMRarenowbeing reportedinapplicationtotricklebeds,gas–liquidbubble columnsandgas–solidfluidisedbeds;theapplicationsto gas–solidfluidisedbedsareperhapsthemostadvanced. In the context of fixed beds, Figure 2 illustrates the direction in which this work is developing. The figure showsthegasandliquidflowfieldthroughapackingof 5mm spheres (representing catalyst pellets) contained within a 4cm diameter column. The motivation for acquiring Figure 2 was indeed to explore the closure relationship between gas and liquid velocity [47], and toinvestigate ifthegas–liquid shearstressissignificant comparedtotheliquidsolid-shearstresswhenmodelling fixedbeds[48,49].Toinvestigatethesephenomenaitis necessarytoacquireimagesofthegasandliquidflowfield atthehighest possible spatialresolution andto acquire theimagesof bothflow fieldsatthesame spatial resol-ution,therebyminimisingmis-registrationeffects atthe gas–liquid interface. The technical challengehere is to overcomethefact thatbecausethegasdensityisabout threeordersofmagnitudesmallerthantheliquiddensity, so too will be the signal-to-noise ratio in the velocity imageofthegasphase.Theapproachusedtoacquiredata Figure2 27 mm 27 mm gas liquid 316 –158 199 –99 0 158 99 0 axial velocity (mm s-1)
Current Opinion in Chemical Engineering
GasandliquidvelocitymapofSF6(red/yellow)andwater(blue/green) duringtrickleflow.Thegasandliquidsuperficialvelocitieswere 15mms 1and2.3mms 1,respectively.Datawererecordedforagas– liquidflowina4cmdiametercolumnpackedwith5mmdiameter spheres.Thevelocityvectormeasuredisthevelocityinthedirectionof superficialflow(i.e.downthebed).19Fimagingisusedtoimagethegas flow.Thein-planespatialresolutionofboththegasandliquidflowfields is234mm234mm;thiswasachievedbyusingacompressedsensing acquisitionandimagereconstructionforthegasvelocityimage.
atthisrelativelyhigh spatialresolutionin thegasphase imagehasbeentoundersampletheacquireddatasothat forafixedacquisitiontimeitisnowpossibletoachievefar greatersignalaveraging.Theparticularapproachusedhas been that of Compressed Sensing—a well-established signalprocessingmethodology[50]but onewhichhas beenlittleusedinMRandnothithertoreportedinMR flow imaging[51].
Similar undersampling strategies have been used to undersampledataacquisitionsinapplicationtostudying two-phase gas–liquid bubbly flows (Figure 3). Here, signal is being acquired from the liquid phase and so the reductioninnumberof datapoints sampledcanbe exploiteddirectlyinincreasingthetemporalresolutionof the images acquired. For this system, two-dimensional velocitymapsofasinglevelocitycomponentoftheliquid flowfieldaroundairbubblesrisingthroughstagnantwater havebeenacquiredatarateof188framess 1[52]; two-dimensionalmapsofthevx,vyandvzvectorflowfieldsare acquired at a rate of 63framess 1. This imaging capa-bility has the ability to capture bubble and flow field behaviour in real time during bubble coalescence and break-up—these measurements will provide data on which to test our ability to describe coalescence and break-upeventsinawaywhichhashithertobeen imposs-ible,particularlyinhigh gasvoidagesystems.
In the field of granular flows, MR has been used in combination with Discrete Element Modelling simu-lations to discriminate between closure laws on the
gas–solid interaction or drag force, and the particle– particle interaction. In bothcases MRvelocimetry data arerecordedfor thesolidsflow. Ofcourse, thisrequires thesolidparticlesto haveanMRsignalassociatedwith them and for this reasonthe solid particles are usually mustard or poppyseeds(which naturallycontainoil— and hence 1H), or liquid-filledporous particles[53–55]. Theimageshavethenbeenusedtotesttheaccuracyof proposed closure models [56–58]. Among the many results reportedin thesepapers,thework demonstrates thatthesimulationpredictionsemployingagivenclosure canagreetodifferingextentswithexperimentdepending onwhetheratwo-dimensionalorthree-dimensional geo-metryisconsidered.OneoftheadvantagesofMRisthat thereactorgeometrystudiedcantakeany sizeorshape and therefore it is possible to image flows in a three-dimensionalcolumnjustaseasilyasa pseudo-two-dimen-sionalflat-walledcell.Variousquestionsarisefromthese observations—andtheiranaloguesexistinexperiments performed on gas–liquid and gas–liquid–solid systems. To whatextentdoclosuresandmulti-phaseflow corre-lations derived from experimental geometries, greatly simplified compared to the real three-dimensional pro-blem, accurately predict three-dimensional behaviour? MR offersthe opportunityto beginto explore some of theseimportant challenges.
Conclusions
The ability of MRtechniquesto image hydrodynamics and chemicalconversionis now wellknownto reaction engineers. Theaim ofthis articlehasthereforebeen to introduce some of the emergingareas of application of MR in the field of catalysis and reaction engineering. Recent developments in physical reactor environments that can be operated in a MR magnet, combined with signalprocessingtechniquesonlyrecentlyappliedinMR meanthatitisnowpossibleto explorephenomenathat werepreviously inaccessibleto studybyMR—and,in mostcases,byanyother technique.
Acknowledgements
TheauthorwishestothankEPSRCforfundingmuchofthework described,primarilythroughgrantsEP/G011397/1andEP/F047991/1,and alsoJohnsonMattheyplcandExxonMobilfortheircontinuedsupportof thiswork.ShealsowishestothankQueen’sUniversityBelfastforproviding thereactordataandDrsM.LuteckiandJ.MitchellforacquiringtheMR datashowninFigure1.
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