Alexander Hoffman metal-organic frameworks Experimental and computational vibrational ...

126 Pages·2017·17.67 MB·English

Experimental and computational vibrational spectroscopy of
metal-organic frameworks
Alexander Hoffman
Supervisors: Prof. dr. ir. Veronique Van Speybroeck, Prof. dr. ir. Henk Vrielinck
Counsellors: Irena Nevjestic, Ir. Louis Vanduyfhuys, Ir. Sven Rogge, Ir. Jelle Wieme, Hannes
Depauw
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in Engineering Physics
Department of Applied Physics
Chair: Prof. dr. ir. Christophe Leys
Vakgroep Vastestofwetenschappen
Chair: Prof. dr. Freddy Callens
Faculty of Engineering and Architecture
Academic year 2016-2017
This work has been performed at the Center for Molecular Modeling and the Department of
Solid State Sciences.
Preface
This thesis forms the final chapter of my studies in Engineering Physics. It was the hardest, but
definitely the most interesting part of the journey, which would not have been possible without
the support of some wonderful people.
Mydeepestthanksgoesouttomysupervisors,professorVanSpeybroeckandprofessorVrielinck,
for providing me such an interesting research subject. Thank you professor Van Speybroeck for
the guidance throughout the year, especially during the monthly meetings, which kept me on
track. Besides,I’mverygratefulforyoursupportwithregardtomyinternshipatSCK.Thatwas
a wonderful experience, which helped me to bring this thesis to a successful conclusion. Most of
all, I’m glad you provide me with the opportunity to continue my research in the coming years.
Of course, I also wish to thank professor Vrielinck for his wholehearted support. Whenever I
needed help, you were there to give advice or assist me. You created an environment where I
could do all the necessary experiments.
Furthermore, I want to thank my counsellors who helped bringing this work to a higher level.
Thanks Irena for the assistance with the experiments and making me familiar with the equip-
ment. ThankyouLouisforyourideasandyourhelpwithallthecomputationalproblems. Thank
you Sven for your help throughout the year, especially for your numerous corrections that made
this work much more pleasant to read. Also your frequent random visits to the students office
offered a welcome distraction. Next, I want to thank Jelle for his continuous support. Your idea
to start up DFT simulations formed the basis for the major computational results in this work.
Finally, I want to thank Hannes for all his help. Thank you for the synthesis of the samples
and for making pellets in the glovebox. Special thanks goes out to Els De Canck for helping me
with the Raman measurements.
Writing this thesis required a lot of hard work, but the company of my fellow thesis students
made it a very pleasant undertaking. Therefore, I want to thank Aran, Maarten, and Sander
for sharing an office with me and creating the best possible working atmosphere. Thank you
Aran for all the little Python and LATEXrelated help, Sander for sharing your graphic skills, and
Maarten for your insight whenever I got stuck on something. You all had your contribution to
this work. I also want to thank the other CMM students for the relaxing chats during lunch
break. Moreover, I wish to thank all the other members of the CMM and the Department of
Solid State Sciences for considering me as one of their team.
Last but not least, I wish to thank my parents, my sisters, and my brother for their support.
Although they were not familiar with the subject I was working on, they kept on motivating me
to make the best out of it. They also provided me with a comfortable home so I only had to
focus on my studies. Therefore, you have my eternal gratitude.
Alexander Hoffman
June 2, 2017
i
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Experimental and computational vibrational
spectroscopy of metal-organic frameworks
by Alexander Hoffman
Supervisors: prof. dr. ir. Veronique Van Speybroeck, prof. dr. ir. Henk Vrielinck
Counsellors: Irena Nevjestic´, dr. ir. Louis Vanduyfhuys, ir. Sven Rogge, ir. Jelle Wieme, Hannes Depauw
Master’s dissertation submitted in order to obtain the academic degree of
Master of Science in Engineering Physics
Department of Applied Physics
Chair: prof. dr. ir. Christophe Leys
Vakgroep Vastestofwetenschappen
Chair: prof. dr. Freddy Callens
Faculty of Engineering and Architecture
Academic year 2016-2017
Overview
This work combines experimental and computational techniques to investigate the vibrational
spectrum of the three different phases (narrow pore, large pore, and hydrated) of MIL-53(Al), a
prototypical example of a flexible metal-organic framework (MOF). To reliably assess the vibra-
tional spectra of these three MIL-53(Al) phases with infrared (IR) and Raman measurements,
various experimental techniques are compared. The interpretation of these experimental spec-
tra is supported by means of theoretical computations. The origin of the prominent absorption
bands in the mid- and far-IR regions of the experimental spectra of the narrow and large pore
phasesareidentifiedwithstaticdensityfunctionaltheory(DFT)simulations. Apartfromascal-
ing factor, a very good correspondence between the experimental and computational IR spectra
in the complete mid- and far-IR is found. Next, to incorporate anharmonic and temperature
effects, ab initio molecular dynamics (AIMD) simulations are performed at room temperature.
In this way a complete picture of the low-frequency vibrational modes that can be associated
with the breathing mechanism typical for MIL-53(Al) is obtained. Finally, a unique vibrational
fingerprint is identified for each of the phases, which allows to unequivocally determine which
phase is present in a given experimental sample.
Keywords
Metal-organicframeworks, vibrationalspectroscopy, infrared, Raman, densityfunctionaltheory,
molecular dynamics
Experimental and computational vibrational
spectroscopy of metal-organic frameworks
AlexanderHoffman
Supervisors: Prof. dr. ir. V.Speybroeck,Prof. dr. ir. H.Vrielinck
Counsellors: I.Nevjestic´,dr. ir. L.Vanduyfhuys,ir. S.Rogge,ir. J.Wieme,H.Depauw
Abstract—Thisworkcombinesexperimentalandcomputational
techniquestoinvestigatethevibrationalspectrumofthethreedif-
ferentphases(dehydratednarrowpore, largepore, andhydrated
narrow pore) of MIL-53(Al), a prototypical example of a flexible
metal-organicframework(MOF).Thesevibrationalspectraserve
as fingerprints for the material and can be used to describe the
phasetransition. Toreliablyassessthevibrationalspectraofthese
threeMIL-53(Al)phaseswithinfrared(IR)andRamanmeasure-
ments,variousexperimentaltechniquesarecompared. Theinter-
pretationoftheseexperimentalspectraissupportedbymeansof
theoreticalcomputations. Theoriginoftheprominentabsorption
Fig.1. ThetwodehydratedphasesofMIL-53(Al): thelpphase(left)
bandsinthemid-andfar-IRregionsoftheexperimentalspectraof
andthedehydratednpphase(right).
thenarrowandlargeporephasesareidentifiedwithstaticdensity
functionaltheory(DFT)simulations.Apartfromascalingfactor,a
verygoodcorrespondencebetweentheexperimentalandcomputa-
tionalIRspectrainthecompletemid-andfar-IRisfound.Next,to the np phase and contains adsorbed water molecules in
incorporateanharmonicandtemperatureeffects,abinitiomolecu-
thepores.
lardynamics(AIMD)simulationsareperformedatroomtempera-
ture.Inthiswayacompletepictureofthelow-frequencyvibrational Breathing MOFs, such as MIL-53(Al), are often the
modesthatcanbeassociatedwiththebreathingmechanismtypical subjectofinvestigation,becauseoftheirinterestingprop-
forMIL-53(Al)isobtained.Finally,auniquevibrationalfingerprint
erties, which can lead to applications as shock absorber
isidentifiedforeachofthephases,whichallowstounequivocally
determinewhichphaseispresentinagivenexperimentalsample. [6], in drug delivery [8], in gas adsorption [9], etc. The
Keywords—metal-organicframeworks,vibrationalspectroscopy, properties of breathing MOFs can be investigated by vi-
infrared,Raman,densityfunctionaltheory,moleculardynamics brationalspectroscopy. Spectroscopictechniquesareof-
ten applied on MOFs in order to characterize the mate-
I. INTRODUCTION rial [10] as they provide a vibrational fingerprint, which
isuniqueforthestructure. Moreover,itcanbeemployed
Metal-organic frameworks (MOFs) are a recently de-
toexaminetheinfluenceofguestmolecules,forexample
veloped class of micro- and/or mesoporous crystalline
H O[11]andCO [12],onthestructure. Alsomechani-
2 2
materials consisting of metal-oxide clusters connected
calproperties,suchaselasticity,canbestudiedbymeans
through organic linkers. The almost infinite number of
ofvibrationalspectroscopy,becauselow-frequencyvibra-
possible MOFs and their easy synthesis [1] are two fac-
tionalmodesinfluencethelatticedynamics[13].
tors that make them very promising materials for future
In this work, the different phases of the flexible MIL-
applications,suchascatalysis[2]andgasseparation[3].
53(Al) are investigated by means of experimental and
AsubclassconsistsoftheflexibleMOFs,someofwhich
computational IR and Raman spectroscopy. To reliably
can undergo a large change in pore volume under exter-
assessthelow-frequencyvibrationalspectrum,whichcan
nal stimuli. This behavior is called breathing when the
be used to describe the phase transition in MIL-53(Al),
changeinporevolumeinducesaphasetransition.
special techniques will be introduced. From computa-
A prototype example of a breathing MOF is MIL- tionalpointofview,bothstaticanddynamicdensityfunc-
53(Al) [4], which consists of infinite chains of corner tionaltheory(DFT)simulationsareperformed.
sharing AlO4(OH)2 octahedra connected through 1,4- Theremainderofthisabstractisorganizedasfollows.
benzenedicarboxylate (BDC) linkers. In anhydrous cir- In section II, the theoretical framework is given for the
cumstances, this structure can appear in the dehydrated calculation of IR spectra from static simulations, and IR
narrow pore (np) and the large pore (lp) phase (see Fig- and Raman spectra from dynamic simulations. After-
ure1). Transitionsbetweenthesetwophasesoccurunder wards, the experimental and computational details are
the influence of temperature [5], pressure [6], and guest provided (section III). The results are presented in sec-
molecules[7]. Inthepresenceofwater, athirdstructure tionIV,endingwithaconclusioninsectionV.
is possible, which has a slightly larger cell volume than
II. THEORETICALBACKGROUND
A.HoffmaniswiththeCenterforMolecularModelingandtheDe-
There are mainly two different simulation techniques
partmentofSolidStateSciences,GhentUniversity(UGent),Gent,Bel-
gium.E-mail:[email protected]. bywhichvibrationalspectracanbepredicted: staticand
dynamic simulations. In static simulations, the potential III. METHODOLOGY
energysurface(PES)ofthemolecularsystemisapproxi-
A. Experimentaldetails
matedbya3N dimensionalparabolicsurface,withN the
numberofatoms.Thisallowsforafullquantummechan- For the measurement of the IR spectra correspond-
icaltreatment, becausethenuclearSchro¨dingerequation ing to the dehydrated np, lp, and hydrated np phase
canbesolvedanalytically. Indynamicsimulations,New- several experimental setups were used. For the attenu-
ton’s equations of motion are solved numerically, taking ated total reflection (ATR) measurements, a Bruker Ver-
into account the whole PES. These dynamic simulation tex80vFouriertransforminfrared(FTIR)interferometer
willhenceincludeallpotentialanharmonicitiespresentin was used equipped with a DLaTGS detector and a Glo-
thePES.However,thenucleiaretreatedasclassicalpar- bar source. The spectrometer allowed to measure spec-
ticles.Fromstaticanddynamicsimulationtechniques,IR tra in both the mid- (600-4000 cm 1) and far-IR (100-
−
andRamanspectracanbegenerated. 600 cm 1) range. For mid-IR and far-IR measurements
−
a KBr and Mylar multilayer beamsplitter were used, re-
A. IRintensitiesfromstaticsimulations
spectively.Thespectrawererecordedwitharesolutionof
For the calculation of the IR intensities of the vibra- 0.5cm−1 and10to40scanswereaccumulated,depend-
tional modes, the structure needs to be equilibrated by ing on the quality of the spectrum. The ATR measure-
means of a geometry optimization. Afterwards, a nor- ments were performed using a single-bounce diamond
mal mode analysis is performed on the optimized struc- ATRcrystalonwhichMIL-53(Al)waspressedbyasmall
ture, whichyieldstheeigenfrequenciesandeigenvectors lever.Duringmeasurementsthesamplecompartmentwas
of the dynamical matrix evaluated at the gamma point. keptundervacuumconditions,exceptinthedehydration
The dynamical matrix is defined as the discrete Fourier experiment.
transform of the mass-weighted Hessian. Subsequently, TheBrukerVertex80vwasnotprovidedwithanin-situ
theIRintensitiescanbedetermined: heatingdevice. Forthatpurpose,weusedaNicolet6700
diffuse reflectance infrared Fourier transform (DRIFT)
2
α(ω)∼Xi (cid:12)(cid:12)Xα Z¯¯α∗√e¯Mα,iα(cid:12)(cid:12) δ(ωi−ω) (1) slsipoqeuucrictdreo.nmOiternotelgyrem(nTichdoe-IorRmleomdFeMiasshCueTrre-ASmceidneenttsteiwcfitceo)rreeapqneudrifpoapnremdEeTwdCiwthiItRha
(cid:12) (cid:12)
Here, e¯α,i represe(cid:12)nts the ith no(cid:12)rmalized eigenvector the DRIFT spectrometer using a KBr beam splitter. The
(cid:12) (cid:12)
of the dynamical matrix, ωi is the eigenfrequency corre- spectral resolution was 2 cm−1 and spectra are obtained
sponding to the ith eigenmode, Mα stands for the mass from1000accumulatedscans.Thepowderedsamplewas
ofatomα, andZ¯¯α∗ istheBorneffectivechargetensorof prepared by diluting it in KBr and placing it in a small
atomα. Thistensorisdefinedasthederivativeofthepo- cup inside the sample compartment, which was kept un-
larization P¯ with respect to the atomic displacement u¯α dervacuumconditionsduringmeasurements.
atzeroelectricfield,multipliedbytheunitcellvolume: The Raman spectra were recorded with a NXR
FT-Raman spectrometer from ThermoFisher Scientific,
∂P
Zα∗,ij =Vcell ∂u j (2) which was coupled to the DRIFT spectrometer so the
α,i(cid:12)E=0 sameinterferometercouldbeused. Itwasequippedwith
(cid:12)
B. IRandRamanspectrumfromdyn(cid:12)amicsimulations a1064nmNd:YVO4laserandanInGaAsdetector.Spec-
(cid:12) trawererecordedinthefrequencyrange150-4000cm 1
−
In molecular dynamics (MD) simulations, IR and Ra- witharesolutionof2cm 1 and500accumulatedscans.
−
manspectracanbecalculatedfromtheFouriertransform The powdered sample was kept in a flask under vacuum
oftheautocorrelationfunctionofthedipolemomentand conditions, which allowed for heating in between mea-
thecomponentsofthepolarizabilitytensor, respectively. surements.
This is a result of the fluctuation-dissipation theorem,
whichstatesthatfluctuationsinamaterialareequivalent B. Computationaldetails
tothechangesduetoasmallperturbation. Inparticular,
theIRspectruminanMDsimulationiscalculatedbythe Both static and dynamic DFT simulations were per-
followingexpression: formed on the dehydrated np and lp phase only. Sim-
ulations on the hydrated np phase were not conducted,
α(ω) ω2 ∞ dte−iωt M¯(0) M¯(t) (3) becausetheadsorptionsitesandmechanismsarenotyet
∼ · completelyrevealed[11],[14].
Z−∞ (cid:10) (cid:11)
with M¯ the dipole moment. In an MD simulation the
B.1 StaticDFT
dipole moment is determined at discrete time steps and
forafinitetimespan,whichturnsequation(3)intoadis- The IR spectra resulting from static simulations were
creteFourier transform. The RamanspectrumfromMD obtainedusingVASP[15]. TodeterminetheBorneffec-
simulations consists of a sum of expressions, analogous tive charges (Eq. (2)) a density functional perturbation
to equation (3), where the dipole moment is replaced by theory (DFPT) approach was applied. The PBE func-
thecomponentsofthepolarizabilitytensor. tional was adopted together with a projector-augmented
Fig.2. IRspectrumofMIL-53(Al)inthedehydratednpphaseintherange100-4000cm−1accordingtoexperiment(ATR)andDFTsimulations
(bothstaticanddynamic).
wave basis set with cutoff energy of 500 eV. For the de- trumwasreproduced. TheresultsarevisualizedinFigure
hydratednpphaseak-meshwaschosenof2 6 6k- 2. Hereby,aLorentzianlinebroadeningwithafull-width
× ×
points,whilesimulationsforthelpphasewereperformed athalfmaximum(FWHM)of28cm 1 hasbeenapplied
−
withak-meshof2 6 2k-points. for all vibrational modes resulting from the static simu-
× ×
lation. Also the AIMD simulation is slightly broadened
B.2 Abinitiomoleculardynamics totakeintoaccounttheinstrumentallineshapepresentin
Ab initio molecular dynamics (AIMD) simulations theexperimentalspectrum.
wereperformedwiththeCP2Ksoftware[16], [17]. The Weobserveaverygoodagreementwithexperimentfor
simulationswereexecutedintheNVT ensembleatatem- boththestaticanddynamicsimulation. Forthehighfre-
perature of 300 K using a Nose´-Hoover chain thermo- quency modes we notice a blue-shift in the simulations
stat with 3 beads and a relaxation time of 100 fs. Again compared to the experimental result, whereas the lower
thePBEfunctionalwasused, thistimetogetherwiththe frequencymodesarered-shifted. Thisisacommonfea-
Gaussian plane wave method. Within this method, we tureinDFTcalculations[18].
usedGaussianbasissetsoftheTZV2Ptypeforallatoms
Forthediscussionofthespectra,wewillfocusonthree
exceptthealuminumatomforwhichtheDZVPbasisset
frequency regions. The first region, indicated in yellow,
was used. They were taken from the MOLOPT basis
contains most of the stretch modes. The two peaks with
set and were applied together with the Goedecker-Teter-
the highest absorbance in all spectra correspond to the
Hutterpseudopotential. Thecutoffoftheauxiliaryplane
CO stretching modes and their intensities are very well
2
waves were set at 400 and 600 Rydberg for the simula-
reproduced in the simulations. In between those CO -
2
tions on the dehydrated np phase and lp phase, respec-
stretchpeaks,weobserveanothersharppeakintheexper-
tively. Totalsimulationtimesof10psand20pswereob-
imental spectrum that can be assigned to the CC-stretch
tainedforsimulationsonthedehydratednpandlpphase,
betweenthephenylringandtheCO group. Inboththe
2
respectively. Inadvance,anequilibrationof3pswasper-
staticDFTandAIMDsimulation, thecalculatedoscilla-
formedonbothstructures.
torstrengthofthismodeistooweak.
IV. RESULTSANDDISCUSSION The second frequency region, in magenta, represents
most of the bending modes. The peak with the highest
A. IRspectrumofdehydratednpphase
intensity is a bending mode of the hydroxo-group in the
ThediamondATRmodewasselectedformeasurement metal-oxide backbone. The intensity of this mode is too
ofthemid-andfar-IRspectraofMIL-53(Al)inthedehy- high in the dynamic simulation and too low in the static
dratednpphase,becauseityieldedthebestqualityspectra simulation. TheotherIRactivemodesinthisregionhave
andmeasurementsinthetworangescouldbeperformed aloweroscillatorstrength,whichiswellpredictedbythe
inexactlythesameconditions.Bymeansofstaticanddy- dynamic simulation, but is underestimated in the static
namicDFTsimulationsonthesamestructuretheIRspec- simulation.
The last frequency region which we will discuss, is wavenumbershigherthan700cm 1. Therearesomein-
−
foundinthefar-IRregionandisindicatedincyan. Itcon- tensity differences due to the higher symmetry in the lp
sists of vibrational modes involving the aluminum atom phase and some peaks are slightly shifted. Most often
or linker deformations. We recognize in the experimen- thevibrationalmodeassociatedwithaCH-bendat1017
talandcomputationalspectrathreemainregionswithab- cm 1inthedehydratednpphaseandat1026cm 1inthe
− −
sorptionbands. Ingeneral,thestaticDFTsimulationre- lpphaseisusedtodifferentiatebetweenbothphases[10].
producestherelativeabsorbancebetterthanthedynamic However,weobserveamuchmorepronounceddifference
simulations. Between 500 and 700 cm 1, we find vi- around670cm 1,whereinthedehydratednpphaseasin-
− −
brational modes which are mainly related to stretching glepeakisvisible,whilethelpphaseischaracterizedby
modes of AlO , where O represents an oxygen atom adoublepeakaroundthatfrequency.
C C
of the BDC linker. Between 400 and 500 cm−1, the IR Whentheexperimentalspectraarecomparedwiththe
active modes can be related to the bending of the metal- spectra from the corresponding static DFT simulations,
oxide backbone. Finally, the modes from 300 cm−1 to we observe that the characteristic difference around 670
400cm−1 canbeassignedtodeformationsofthemetal- cm−1 is very well reproduced in our simulation. From
oxidebackbone. the static simulations, it is even possible to identify the
origin of the feature. In the IR spectrum of the lp phase
B. DifferenceinIRspectrumbetweenthedehydratednp the double peaked feature is a result of the large fre-
andlpphases quency difference between the symmetric and asymmet-
ric stretch of the AlOAl-chain in the metal-oxide back-
TheexperimentalIRspectrumcorrespondingtothelp
bone, which makes them both resolved. For the dehy-
phase had to be obtained by heating, since it is the only
drated np phase, the absorption bands corresponding to
feasible way which gives rise to the np-to-lp transition.
thesespecificmodeslieclosertogether,yieldingasingle
ThesemeasurementswerecarriedoutwithaDRIFTspec-
peak.
trometer equipped with a heating device. The measured
The very good match between simulation and experi-
frequency range was restricted to the mid-IR. In Figure
mentforboththedehydratednpandlpphaseandtheabil-
3theDRIFTspectraforthedehydratednpandlpphases
ity to reproduce the characteristic difference around 670
are compared together with their prediction from static
cm 1 providesuswithareliablemodeltoinvestigatethe
DFT simulations again with Lorentzian broadening with −
low-frequencymodesthatarerelatedwithbreathing.Fur-
a FWHM of 28 cm 1. It is clear that also the IR spec-
−
thermore,ourmodelallowstopredictthefar-IRspectrum
trumfromastaticDFTsimulationofthelpphaseshows
ofthelpphase,whichwewerenotyetabletomeasure.
remarkableresemblancewithexperiment.
C. Ramanspectrum
To obtain a more extended view on the vibrational
modes in MIL-53(Al), we performed Raman measure-
ments and simulations. This gives complementary in-
formation on the IR spectra thanks to different selection
rules. ExperimentalRamanspectraofthedehydratednp
andlpphasesofMIL-53(Al)arerepresentedinFigure4
alongwiththeRamanspectraobtainedfromAIMDsimu-
lations. Thereportedfrequencyrangeislimitedfrom150
to2100cm 1.
−
The experimental spectrum for the lp phase is in ex-
cellent agreement with earlier observations which report
the Raman spectrum for wavenumbers higher than 600
cm 1 [6], [19]. The spectrum for the dehydrated np
−
phaseresemblesthespectrumforthelpphaseapartfrom
some minor shifts. However, one characteristic differ-
enceisfoundaround1450cm 1,whichconsistsofvibra-
−
tionalmodesthatcanbeassignedtotheCO symmetric
2
stretch. In our measurements, we observe two peaks in
the lp phase and three peaks in the dehydrated np phase
Fig.3. IRspectraofMIL-53(Al)inthedehydratednpandlpphase aroundthatfrequency. Thisresultdeviatesslightlyfrom
intherange600-1800cm−1accordingtoexperiment(DRIFT)and theobservationsofHamonetal.[19]. Theyreportedalso
staticDFTsimulations.
two peaks for the dehydrated np phase, an intense peak
at higher wavenumbers and a less intense one at lower
Wefirstdiscussthedifferencesbetweentheexperimen- wavenumbers, of which the one at lower wavenumbers
talspectraofthedehydratednpandlpphase. Ingeneral, is red-shifted compared to the lp phase. As our Raman
there are only subtle differences between the spectra for spectrum of the dehydrated np phase shows a third peak
Figure5).
Initially, MIL-53(Al) is observed in the hydrated np
phase. Thisphaseischaracterizedbytheshiftinthebend
modeofthehydroxogroupfrom985cm 1 inthedehy-
−
dratedphaseto1125cm 1 inthehydratedphase. More-
−
over, the IR spectrum of the hydrated np phase contains
bending, rocking, and wagging modes of adsorbed wa-
ter molecules around 650 cm 1, which broadens the vi-
−
brationalbands. Becausewearemeasuringunderatmo-
sphericconditions,wealsoobserverotational-vibrational
bandsduetowaterandCO moleculesinairthataresu-
2
perposedonthespectrumofthehydratednpphase. The
absorptionbandsfromthewatermoleculesarefoundbe-
tween1400and2000cm 1,whilearound670cm 1 we
− −
observethedegeneratebendingmodesofCO molecules.
2
Every15minutesaspectrumisrecordedandafterafew
hours all the water has been removed from the pores.
ThisisclearlyvisiblebytheshiftoftheOH-bendmode.
Thatthisisonlyaresultofthedryingprocedurebecomes
clearfromtheunchangedintensityintheCO absorption
2
Fig.4. RamanspectraofMIL-53(Al)inthedehydratednpandlpphase
bands.
intherange150-2100cm−1 accordingtoexperimentandAIMD
simulations.
inbetweenthesetwopeaks, itissuspectedtoarisefrom
amixtureofthelpanddehydratednpphase. Thiscanbe
duetoheatingofthepowderbythelaser,whichturnsthe
samplepartiallyinthelpphase.
LookingattheAIMDresults,weobserveagainagood
correspondencebetweensimulationsandexperimentsfor
bothphaseswhenwefocusonthepresenceoftheRaman
active peaks, although there is a deviation in intensities
forsomepeaks. Moreover,aswealreadyencounteredin
thesimulatedIRspectra,alsothesimulatedRamanspec-
traarered-shiftedcomparedtotheexperiment. Moreim-
portantly,weseethatthesimulationforthelpphasepre-
dicts two peaks with high intensity around 1450 cm 1,
−
while the simulation for the dehydrated np phase results
Fig.5. DehydrationprocessinMIL-53(Al)byadaptingthehumidity
in a Raman spectrum where only one major peak is vis- ofthesurroundingairvisualizedbyATRIRspectroscopy.Initially,
ible around that frequency together with a low intensity MIL-53(Al)isfoundinthehydratednpphase(purple),butitgrad-
uallyundergoesatransitiontothedehydratednpphase(red). The
peakatsomewhatlowerfrequency. Thisisincorrespon-
absorptionbandsofatmosphericwaterandCO2aresuperposedon
dencewithliterature[19]andconfirmsthereasoningthat thespectrumofMIL-53(Al)
ourmeasurementofthedehydratednpphaseisactuallya
mixtureofthedehydratednpandlpphases.
D. DehydrationofMIL-53(Al) V. CONCLUSIONS
WhilenosimulationsonthehydratedMIL-53(Al)were InthisworkthreedifferentphasesofMIL-53(Al)were
performed, some interesting experimental observations investigated by means of IR and Raman spectroscopy.
concerning the dehydration process of MIL-53(Al) were The spectra were obtained both by experiment and DFT
detected. Most often, we dehydrated MIL-53(Al) by calculations. Besides the frequently studied mid-IR
bringing the structure under vacuum conditions. How- range,weextendedourresearchtothelow-frequencyvi-
ever, we observed that the dehydration of MIL-53(Al) brational modes of MIL-53(Al), which have never been
merely depends on the humidity of the surrounding air. examined before. For the dehydrated np and lp phase
WeplacedaMIL-53(Al)powder,startinginthehydrated weobtainedaremarkablecorrespondencebetweenexper-
np phase, together with silica desiccator material in the imentandsimulationsinboththemid-andfar-IRrange.
closed sample compartment under ambient conditions. With IR spectroscopy we managed to identify a new
By meansof ATR IR measurements the evolution ofthe characteristicfeatureforthelp-to-nptransition. Usually,
hydration in the pores of MIL-53(Al) was followed (see thesmallshiftinCH-bendaround1020cm 1 isusedto
−