research article investigation by raman spectroscopy of...

Research ArticleInvestigation by Raman Spectroscopy of the DecompositionProcess of HKUST-1 upon Exposure to Air

Michela Todaro12 Antonino Alessi13 Luisa Sciortino1 Simonpietro Agnello1

Marco Cannas1 Franco Mario Gelardi1 and Gianpiero Buscarino1

1Dipartimento di Fisica e Chimica Universita di Palermo 90123 Palermo Italy2Dipartimento di Fisica e Astronomia Universita di Catania 95123 Catania Italy3Laboratoire H Curien UMR CNRS 5516 Universite de Lyon 42000 Saint-Etienne France

Correspondence should be addressed to Gianpiero Buscarino gianpierobuscarinounipait

Received 29 August 2016 Revised 28 October 2016 Accepted 14 November 2016

Academic Editor Artem E Masunov

Copyright copy 2016 Michela Todaro et alThis is an open access article distributed under theCreative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

We report an experimental investigation by Raman spectroscopy of the decomposition process of Metal-Organic Framework(MOF) HKUST-1 upon exposure to air moisture (119879 = 300K 70 relative humidity)The data collected here are compared with theindications obtained from a model of the process of decomposition of this material proposed in literature In agreement with thatmodel the reported Raman measurements indicate that for exposure times longer than 20 days relevant irreversible processes takeplace which are related to the occurrence of the hydrolysis of Cu-O bonds These processes induce small but detectable variationsof the peak positions and intensities of the main Raman bands of the material which can be related to Cu-Cu Cu-O and O-C-Ostretching modes The critical analyses of these changes have permitted us to obtain a more detailed description of the processof decomposition taking place in HKUST-1 upon interaction with moisture Furthermore the reported Raman data give furtherstrong support to the recently proposed model of decomposition of HKUST-1 contributing significantly to the development of acomplete picture of the properties of this considerable deleterious effect

1 Introduction

Metal-Organic Frameworks (MOFs) are a new class of hybridcompounds constituted by combination of metallic clustersand organic linkers [1ndash4] The formation of a crystallineporous matrix is achieved by reacting these two basic ele-ments in appropriate synthesis conditions [1ndash4] The possi-bility to adapt crystalline structures to a specific request interms of appropriate choice ofmetallic centers linkers lengthpore dimensions and high surface area makes MOFs veryinteresting in awide range of application fields as catalysis andbiomedical field as drug delivery systems chemical sensorsgas storage and so on [1ndash4]

One of the most promising MOFs from the point ofview of fundamental and applied research is HKUST-1synthesized for the first time in 1999 [5] It is a referencematerial for adsorption of small molecules and gases andselective capture and transporting of a large variety of gasesIn particular several recent studies have been focused on the

hydrogen methane and carbon dioxide storage in order touse alternative clean energy sources and with the intent toeliminate greenhouse gases from the atmosphere [6ndash9]

The outstanding adsorption properties of HKUST-1 arestrictly related to its structural characteristics Indeed thecubic crystalline structure of this material whose chemicalcomposition is Cu3(BTC)2(H2O)3 is composed of two Cu2+ions linked together by four carboxylate groups to form apaddle-wheel unit Each carboxylate bridge is part of 135-benzene tricarboxylate (BTC) linker molecule A picture ofthis structure is reported in Figure 1

The crystal structure involves large cavities with diameterof about 9 A and small pockets with diameter of about 6 AThe principal adsorption sites of polar molecules as NH3 andH2Oare those located on axial binding site of Cu2+ ion calledopen metal sites [5]

The good affinity of HKUST-1 with respect to watermolecules has been extensively investigated in the past In-deed it is well known that the adsorption of water molecules

Hindawi Publishing CorporationJournal of SpectroscopyVolume 2016 Article ID 8074297 7 pageshttpdxdoiorg10115520168074297

2 Journal of Spectroscopy

CuO

CH

Figure 1 Schematic representation of the paddle-wheel unit ofas synthesized HKUST-1 Each Cu2+ ion is coordinated with fourcarboxylate groups belonging to four 135-benzene tricarboxylate(BTC) linker molecules and one water molecule

on openmetal sites actually takes place within fewminutes ofexposure tomoisture [10]This feature has been already takeninto account in industrial applications because for instancethe absorption of some gases is assisted by the presence ofwater molecules in matrix [11 12]

In spite of this one of the crucial aspects to consider is thestability of HKUST-1 with respect to a prolonged exposureto moisture which can affect the crystalline matrix [13 14]Indeed the range of applications of a MOF is significantlyreduced if the integrity of the crystalline matrix is compro-mised as a consequence of such treatments In fact somestudies have pointed out that HKUST-1 is not much stablewith respect to moisture In particular it has been shownthat when the material is exposed to moisture it adsorbsa lot of water molecules until they reach the liquid stateinto the cavities When this stage is obtained water becomesable to hydrolyze the Cu-O bonds breaking the crystallinenetwork of thematerial Recently the effects of the interactionof HKUST-1 with air moisture (119879 = 300K 70 relativehumidity) for a long period of time have been investigatedin detail by electron paramagnetic resonance (EPR) X-raydiffraction (XRD) scanning electron microscopy (SEM)and surface analysis techniques It has been shown thatthe decomposition process of HKUST-1 upon exposure tomoisture takes place through three different stages The firststage with duration of about 20 days is characterized bythe adsorption of a large amount of water molecules by thematerial Water molecules gradually fill the cavities of thematerial pushing towards the Cu2+ ions The authors [15]pointed out by XRD that during this first stage the materialundergoes a swelling but no irreversible structural changestake place in the material The decomposition process of thecrystalline matrix actually starts for times longer than 20days when the effects of the hydrolysis of the Cu-O on thestructure of the material become more and more evident onincreasing the duration of exposure to air [15] In particularduring the second stage of the process of decompositionwhich takes place for exposure times from 20 days to 50

days about 35 of the paddle-wheels become hydrolyzed andconsequently one of the four carboxylate bridges involved inthese paddle-wheels is detached Finally in the third stage ofthe process of decomposition which takes place for exposuretimes longer than 50 days a second detaching of a carboxylatebridge takes place in the same paddle-wheels already affectedin the previous stage of the process

In order to shed new light on the fundamental issueconcerning the structural effects induced by hydrolysis oncarboxylate MOFs here we present an experimental inves-tigation performed by Raman spectroscopy focused on theprocess of decomposition induced in HKUST-1 upon expo-sure to air moisture (119879 = 300K 70 relative humidity)and showing the potentialities of the technique to deepenthe understanding of the underneath process in terms ofmolecular groups changes

2 Materials and Methods

Commercial HKUST-1 in powder form was purchased fromSigma-Aldrich as Basolite C300 A sample of about 10mgof HKUST-1 was exposed to air at 119879 = 300K and relativehumidity (RH)of 70 for different timesWehavemonitoredthe vibrational properties of the sample of HKUST-1 withRaman spectroscopy from about 3 hours up to 165 days ofexposure to air Furthermore we have performed a measure-ment on the activated sample of HKUST-1

The Raman measurements were acquired in the regionfrom 150 to 1700 cmminus1 with a Bruker Senterra 120583-Raman spec-trometer equipped with a diode laser working at 120582 = 532nmWe have performed measurements both at high resolution(3ndash5 cmminus1) and at low resolution (9ndash15 cmminus1) The advantageof the low resolution mode is that it gives spectra withsignificantly higher signal-to-noise ratio with respect to thoseobtained in high resolution mode The nominal laser powerwas 02mW to avoid sample modifications For statisticalreasons each sample was measured in three different pointsand the resulting spectra were averaged

The spectrum of the activated sample of HKUST-1 wasacquired with a Horiba LabRam HR-Evolution 120583-Ramanspectrometer with a 532 nm laser and 1mW nominal laserpower at high resolution (4 cmminus1) No spectral distortionswere induced by laser power

The sample was put inside a high pressure LinkamTHMS600PS cell at 150∘C (heating ramp 100 Cmin) in atmo-spheric pressure and monitored starting from 10 minutes upto 1 hour recording Raman spectra

All the recorded spectra were subtracted by a baselineand subsequently normalized with respect to the intensity ofthe band at sim1000 cmminus1 We have chosen this peak for thenormalization because it is well known that it is a very stablesignal as it is related to the symmetric stretching of the C=Cbonds in the rigid benzene rings

3 Results

The high resolution Raman spectrum of the sample ofactivated HKUST-1 and that acquired for the sample after

Journal of Spectroscopy 3Ra

man

inte

nsity

(arb

uni

ts)

1650 1500 1350 1200 1050 900 750 600 450 300 150

Raman shift (cmminus1)

Activated3 hours

Figure 2 High resolution Raman spectra of the HKUST-1 sampleactivated (black line) and after about 3 hours of exposure to airmoisture at 119879 = 300K (red line) Spectra are vertically shifted forviewing purposes after the normalization described in the text

about 3 hours of exposure to air (119879 = 300K RH 70) arereported in Figure 2

The two spectra show the same spectroscopic character-istics with respect to those reported in literature for activatedand hydratedHKUST-1 [16 17] In particular the intense peakat about 230 cmminus1 evident in the spectrumof activated sampleis replaced by two bands in that of the sample exposed to airfor 3 hours in line with results reported by other authors [16]

Regarding the spectrum of the sample exposed to air for3 hours between 150 and 600 cmminus1 the peaks of vibrationalmodes involving Cu2+ ions are evident More precisely thedoublet at 173ndash191 cmminus1 and the peak at 276 cmminus1 are assignedin literature to a stretching mode involving Cu-Cu dimer andCu-Ow where Ow indicates the oxygen of the water moleculeadsorbed on Cu2+ ion respectively [16ndash19] The doublet at449ndash502 cmminus1 whose first component is barely detectable isrelated to Cu-O stretching modes involving oxygen atoms ofcarboxylate bridges [16 17]

In the central region from 700 to 1100 cmminus1 it is possibleto recognize the peaks related to the vibrational modes ofbenzene rings In particular at 745 cmminus1 and at 828 cmminus1we observe the C-H out-of-plane bending modes of ringswhereas at 1006 cmminus1 the symmetric stretching mode of C=Cis well evident in the Raman spectrum [16ndash19] In the regionfrom 1400 to 1700 cmminus1 two vibrational modes of carboxylatebridges are presentThe first feature at 1460 cmminus1 correspondsto the symmetric stretching of O-C-O whereas the peak at1544 cmminus1 corresponds to the asymmetric stretching of O-C-O [16] At 1616 cmminus1 the C=C symmetric stretching ofbenzene ring is recognized [16 17]

In Figure 3 a comparison of the Raman spectra obtainedfor the HKUST-1 exposed to air moisture (119879 = 300K RH70) for different times is reported

1650 1500 1350 1200 1050 900 750 600 450 300 150


Ram

an in

tens

ity (a

rb u

nits)

3 hours20 days

40 days165 days

C=C

sym

stre

tch

C-O

-O as

ym st

retc

hC-

O-O

sym

stre

tch

C=C

sym

stre

tch

C-H

ben

d

C-H

ben

d

Cu-O

stre

tch

Cu-O

wstr

etch

Cu-C

u str

etch

Figure 3 High resolution Raman spectra of HKUST-1 acquiredafter about 3 hours and 20 40 and 165 days of exposure to air at119879 = 300K Spectra are vertically shifted for viewing purposes afterthe normalization described in the text

As shown in Figure 3 the Raman spectrum obtainedfor the sample exposed to air for about 3 hours containsessentially the same peaks as the spectra obtained after longerexposure to air up to 165 days This indicates that no newvibrations are induced by the decomposition process of thematerial in the investigated wavenumbers range In order toobtain a more detailed description of the variations inducedin the Raman spectra of HKUST-1 during the exposure ofthe material to air we have estimated the intensity and theposition of all the relevant peaks present in the Ramanspectra For this analysis we have taken advantages of thehigh signal-to-noise ratio of the spectra obtained in the lowresolution mode

In Figure 4 the variations induced by exposure ofHKUST-1 to air in the intensity and in the position of thepeak at about 174 cmminus1

attributed to a mode involving the

Cu-Cu stretching are presented A comparison among someindicative spectra in the region of this peak is also reportedin Figure 4(b) As shown we observe that both the amplitudeand the peak position of this band remain almost constantup to about 20 days of exposure to moisture At varianceFigure 4(a) suggests that for longer exposures the peakposition undergoes a blue shift and correspondingly the peakamplitude decreases However by inspection of the spectra ofFigure 4(b) it is easy to recognize that actually the situationis more complex In fact it emerges that in correspondencebetween the longest exposure times the peak exhibits at leasttwo well distinguishable components both peaked at largerwavenumbers with respect to the position observed after 3hours of exposure of the material to air moisture

Similarly in Figure 5 the changes induced in the peak atabout 502 cmminus1 related to Cu-O stretching are reported Asshown in this case we have found that this feature remainsstable up to about 20 days of exposure to air thereafter both


01 1 10 100

Time of exposure to air (days)

170

172

174

176

178

180

Peak

pos

ition

(cm

minus1 )

00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

Peak positionPeak intensity

(a)

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)

Raman shift (cmminus1)250 225 200 175 150

3 hours20 days

165 days

(b)

Figure 4 (a) Position (black squares) and intensity (red circles) of the Raman peak at about 174 cmminus1 for HKUST-1 as a function of time ofexposure to air moisture (b) Raman spectra in the region of the peak at about 174 cmminus1 acquired for HKUST-1 after 3 hours 20 days and 165days of exposure to air moisture


00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

01 1 10 100


498

500

502

504

506

508

Peak

pos

ition

(cm

minus1 )

(a)

3 hours20 days

165 days

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)

Raman shift (cmminus1)600 575 550 525 500 475 450 425 400

(b)

Figure 5 (a) Position (black squares) and intensity (red circles) of the Raman peak at about 502 cmminus1 for HKUST-1 as a function of time ofexposure to air moisture (b) Raman spectra in the region of the peak at about 502 cmminus1 acquired for the HKUST-1 after 3 hours 20 days and165 days of exposure to air moisture

the amplitude and the position of the peak gradually changeThe former reduces from 088 to 064 for exposure to airup to 165 days whereas the latter undergoes a red shift ofabout 2 cmminus1 This shift is small but well reliable as it fallsabove the experimental uncertainty as it is evident fromFigure 5(a) The amplitude difference of the peak at about502 cmminus1 between spectra at 20 and 165 days of exposure tomoisture is well recognizable in Figure 5(b)

Finally in Figure 6 the data concerning the peak atabout 1615 cmminus1 are summarized In this case we observethat the peak position remains essentially unchanged duringthe first 20 days of exposure of the material to air whereasit undergoes a red shift for longer exposure times Themaximum shift observed for this band is of about 2 cmminus1 Atvariance no relevant changes of the peak intensity take place

during 165 days of exposure of the material to air moistureFeatures similar to those here described were also found forthe peaks at 1460 cmminus1 and at 1544 cmminus1 due to the symmetricand asymmetric stretching of O-C-O groups All the otherpeaks not mentioned above were found to be essentially notaffected by the exposure of the material to air moisture up to165 days and will not be considered further

4 Discussion

The main purpose of the present investigation is to betterunderstand the processes of decomposition taking place inthe carboxylate MOF HKUST-1 upon exposure to air mois-ture Since in a recent work [15] a model has been proposedfor this process here we expect to find further support to

Journal of Spectroscopy 5

01 1 10 100


00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

1610

1612

1614

1616

1618

1620

Peak

pos

ition

(cm

minus1 )


(a)

3 hours20 days

165 days


00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)

1675 1650 1625 1600 1575

(b)

Figure 6 (a) Position (black squares) and intensity (red circles) of the Raman peak at about 1615 cmminus1 for HKUST-1 as a function of time ofexposure to air moisture (b) Raman spectra in the region of the peak at about 1615 cmminus1 acquired for HKUST-1 after 3 hours 20 days and165 days of exposure to air moisture

that model and to obtain a more detailed knowledge of theprocess This goal is actually achieved by the overall datapresented in the previous section and discussed in detail inthe following

The proposed model indicates that during the first 20days of exposure of HKUST-1 to air moisture no irreversibleprocesses take place In fact during this first stage largequantities of water molecules are absorbed by the materialjust filling the volume of the cavities In agreement with theexpectation that no irreversible changes take place duringthe first 20 days of exposure the Raman data reportedhere indicate no relevant changes affecting the peaks fallingwithin the range of investigated wavenumbers Model [15]also suggests that during this first stage of the process theelectronic energy levels of the Cu2+ ions significantly changeand that the interaction between the two Cu2+ ions withinthe paddle-wheels becomes stronger as a consequence of theinteraction of these ions with the large quantities of waterfilling the cavities and pushing in all directions This effectis actually not caught by the Raman spectroscopy in thepresent investigation as no change in the properties of theCu-Cu vibration is recognized The reason for this failureis probably due to the fact that the two Cu2+ are very faraway with respect to each other and consequently their bondstrength is expected to be very weak In fact the estimateddistance between them is of about 264 A [16] Since thevariations of the properties of this bond induced by exposureof the material to air moisture are expected to be smallperturbations of the pristine small value they presumably fallbelow the detection limit of our Raman equipment

On the basis of previous reports [15 18 19] it is knownthat upon exposure of HKUST-1 to air moisture for timesbetween 20 and 40 days the irreversible process of hydrolysisof the Cu-O bonds is active As a consequence of thisprocess one of the four carboxylate bridges is detached inabout 35 of the paddle-wheels of the material generatinga paramagnetic defect named E1015840

1(Cu) center which involves

a Cu ion in +1 oxidation state with reduced coordinationand a Cu2+ ion with the pristine coordination but modifiedenvironment [15] Finally the last stage of the process ofdecomposition of HKUST-1 starts for exposure times longerthan 50 days and it involves a further detaching of a secondcarboxylate bridge in the samepaddle-wheels already affectedin the previous stage of the process [15] Such final structurehas been attributed to a paramagnetic defect named E1015840

2(Cu)

centerTheoccurrence of relevant irreversiblemodification ofthematerial for exposure times longer than 20 days is actuallyconfirmed by the reported Raman data As shown in Figures4 5 and 6 some of the Raman bands suffer detectable peakshifts for such exposure times More in detail the blue shiftof the peak related to the Cu-Cu bonds shown in Figure 4indicates that the structure of the dimer significantly changesafter prolonged exposure to air moisture In addition the factthat this peak exhibits two distinguishable components after165 days of exposure is very interesting as it indicates that thepaddle-wheels of the material are of two different types withdifferent structures and characterized bywell distinguishableCu-Cu vibrations The presence of two structures presumablycorresponds to the hydrolyzed and nonhydrolyzed paddle-wheels within the pristine structure On the basis of themodel [15] the relative weight of these two components isabout 35 and 65 respectivelyThese quantities are roughlyconsistent with the relative amplitude of the two componentsshown in Figure 4 in the range from 150 cmminus1 to 200 cmminus1By careful inspection of the peak at about 174 cmminus1 for thespectra obtained for HKUST-1 exposed to air moisture forabout 3 hours and 20 days shown in Figure 4(b) a verysmall shoulder is recognized at about 178 cmminus1 This featureis presumably due to a small fraction of preexisting defectivepaddle-wheels essentially generated during the synthesis ofthe material Concerning the properties of the Cu-O bondstogether with a notable red shift it also suffers a considerablereduction of the peak intensity for exposure times longerthan 20 days This interesting result agrees with the model


indicating that after hydrolysis many Cu+1 ions suffer areduction of their coordination number with respect to thepristine structure

Finally we note that since the hydrolysis process involvesthe oxygen of the carboxylate bridge one expects also toobserve a change of the spectroscopic properties of the peaksat 1460 cmminus1 and at 1544 cmminus1 corresponding to the symmet-ric and asymmetric stretching of O-C-O groups Also thisexpectation has been fulfilled in our present investigationAs a matter of fact we have observed a gradual red shift ofthese bands for long exposure times of the material to airIt is worth noticing that in agreement with the model ofdecomposition proposed previously [15] we have observedno change of the intensities of these two bands This isdue to the decomposition process of the material that doesnot change the number of carboxylate bridges but just theproperties of their pristine coordination with the Cu2+ ionsWe have found similar changes in the peak at 1615 cmminus1 dueto the stretching mode of C=C bonds of the benzene ringThis is not surprising because such vibrations actually takeplace in the aromatic ring of the linker and consequentlythe modifications occurring in the carboxylate bridges areexpected to affect indirectly also the C=C vibration prop-erties Also in this case no change in the number of theC=C bonds is expected Accordingly as shown in Figure 6there is no change of the intensity of the peak at 1615 cmminus1 isrecognized

5 Conclusions

We report an experimental investigation by Raman spec-troscopy of the decomposition process induced by exposureof the MOF HKUST-1 to air moisture Our data indicate theoccurrence of relevant structural processes taking place in thematerial for exposure times longer than 20 days In agreementwith previous reports [15] these processes are related tohydrolysis which affects a significant fraction of the Cu-Obonds of the crystal irreversibly reducing the crystallineorder of HKUST-1 The coexistence of two types of paddle-wheelswith different structures corresponding to hydrolyzedand nonhydrolyzed paddle-wheels predicted by a previousstudy [15] has been confirmed by detecting a splitting ofthe Raman peak attributed to the Cu-Cu vibration in twowell distinguishable components The predicted change of thecoordination number of a fraction of Cu ions in the materialduring the decomposition process [15] is also confirmed bythe observation of a reduction of the intensity of the Ramanpeak at 502 cmminus1 attributed to Cu-O stretching Summa-rizing the overall data reported here support and extendthe model of the process of decomposition of HKUST-1upon exposure to air recently proposed contributing to reacha complete and reliable description of this technologicallyrelevant process

Competing Interests

The authors declare that there are no competing interestsregarding the publication of this paper

Acknowledgments

The authors thank the people of the LAMP group (httpwwwunipaitlamp) at the Department of Physics andChemistry of the University of Palermo for useful discus-sions The CHAB laboratories at ATeN center (Universityof Palermo httpwwwchabcenterhome Med-CHHABPONa3 00273) are acknowledged for the LabRam spectrom-eter and Linkam equipments use

References

[1] H Furukawa K E Cordova M OrsquoKeeffe and O M YaghildquoThe chemistry and applications of metal-organic frameworksrdquoScience vol 341 no 6149 Article ID 1230444 pp 1ndash12 2013

[2] J L C Rowsell and O M Yaghi ldquoMetal-organic frameworksa new class of porous materialsrdquo Microporous and MesoporousMaterials vol 73 no 1-2 pp 3ndash14 2004

[3] J Canivet A Fateeva Y Guo B Coasne and D FarrussengldquoWater adsorption in MOFs fundamentals and applicationsrdquoChemical Society Reviews vol 43 no 16 pp 5594ndash5617 2014

[4] L Sciortino A Alessi F Messina G Buscarino and F MGelardi ldquoStructure of the FeBTC metal-organic framework amodel based on the local environment studyrdquo The Journal ofPhysical Chemistry C vol 119 no 14 pp 7826ndash7830 2015

[5] S S-Y Chui S M-F Lo J P H Charmant A G Orpenand I D Williams ldquoA chemically functionalizable nanoporousmaterial [Cu3(TMA)2(H2O)3]119899rdquo Science vol 283 no 5405 pp1148ndash1150 1999

[6] J-R Li R J Kuppler and H-C Zhou ldquoSelective gas adsorptionand separation in metal-organic frameworksrdquo Chemical SocietyReviews vol 38 no 5 pp 1477ndash1504 2009

[7] K-S Lin A K Adhikari C-N Ku C-L Chiang and HKuo ldquoSynthesis and characterization of porous HKUST-1 metalorganic frameworks for hydrogen storagerdquo International Journalof Hydrogen Energy vol 37 no 18 pp 13865ndash13871 2012

[8] Y Liu C M Brown D A Neumann V K Peterson and C JKepert ldquoInelastic neutron scattering ofH2 adsorbed inHKUST-1rdquo Journal of Alloys and Compounds vol 446-447 pp 385ndash3882007

[9] J Getzschmann I Senkovska D Wallacher et al ldquoMethanestorage mechanism in the metal-organic framework Cu3(btc)2an in situ neutron diffraction studyrdquo Microporous and Meso-porous Materials vol 136 no 1ndash3 pp 50ndash58 2010

[10] F Gul-E-Noor D Michel H Krautscheid J Haase and MBertmer ldquoTime dependent water uptake in Cu3(btc)2 MOFidentification of different water adsorption states by 1H MASNMRrdquoMicroporous and Mesoporous Materials vol 180 pp 8ndash13 2013

[11] N C Burtch H Jasuja and K S Walton ldquoWater stability andadsorption in metal-organic frameworksrdquo Chemical Reviewsvol 114 no 20 pp 10575ndash10612 2014

[12] A Ozgur Yazaydin A I Benin S A Faheem et al ldquoEnhancedCO2 adsorption inmetal-organic frameworks via occupationofopen-metal sites by coordinated water moleculesrdquo Chemistry ofMaterials vol 21 no 8 pp 1425ndash1430 2009

[13] J B Decoste G W Peterson B J Schindler K L KillopsM A Browe and J J Mahle ldquoThe effect of water adsorptionon the structure of the carboxylate containing metal-organicframeworks Cu-BTC Mg-MOF-74 and UiO-66rdquo Journal ofMaterials Chemistry A vol 1 no 38 pp 11922ndash11932 2013


[14] Z Liang M Marshall and A L Chaffee ldquoCO2 adsorption-based separation by metal organic framework (Cu-BTC) versuszeolite (13X)rdquoEnergyampFuels vol 23 no 5 pp 2785ndash2789 2009

[15] M Todaro G Buscarino L Sciortino et al ldquoDecompositionprocess of carboxylate MOF HKUST-1 unveiled at the atomicscale levelrdquoThe Journal of Physical Chemistry C vol 120 no 23pp 12879ndash12889 2016

[16] C Prestipino L Regli J G Vitillo et al ldquoLocal structureof framework Cu(II) in HKUST-1 metallorganic frameworkspectroscopic characterization upon activation and interactionwith adsorbatesrdquoChemistry of Materials vol 18 no 5 pp 1337ndash1346 2006

[17] N R Dhumal M P Singh J A Anderson J Kiefer and HJ Kim ldquoMolecular interactions of a Cu-based metal-organicframework with a confined imidazolium-based ionic liquid acombined density functional theory and experimental vibra-tional spectroscopy studyrdquo Journal of Physical Chemistry C vol120 no 6 pp 3295ndash3304 2016

[18] K Tan N Nijem P Canepa et al ldquoStability and hydrolyzationof metal organic frameworks with paddle-wheel SBUs uponhydrationrdquo Chemistry of Materials vol 24 no 16 pp 3153ndash31672012

[19] K Tan N Nijem Y Gao et al ldquoWater interactions in metalorganic frameworksrdquoCrystEngComm vol 17 no 2 pp 247ndash2602015

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CatalystsJournal of


CuO

CH

Figure 1 Schematic representation of the paddle-wheel unit ofas synthesized HKUST-1 Each Cu2+ ion is coordinated with fourcarboxylate groups belonging to four 135-benzene tricarboxylate(BTC) linker molecules and one water molecule

on openmetal sites actually takes place within fewminutes ofexposure tomoisture [10]This feature has been already takeninto account in industrial applications because for instancethe absorption of some gases is assisted by the presence ofwater molecules in matrix [11 12]

In spite of this one of the crucial aspects to consider is thestability of HKUST-1 with respect to a prolonged exposureto moisture which can affect the crystalline matrix [13 14]Indeed the range of applications of a MOF is significantlyreduced if the integrity of the crystalline matrix is compro-mised as a consequence of such treatments In fact somestudies have pointed out that HKUST-1 is not much stablewith respect to moisture In particular it has been shownthat when the material is exposed to moisture it adsorbsa lot of water molecules until they reach the liquid stateinto the cavities When this stage is obtained water becomesable to hydrolyze the Cu-O bonds breaking the crystallinenetwork of thematerial Recently the effects of the interactionof HKUST-1 with air moisture (119879 = 300K 70 relativehumidity) for a long period of time have been investigatedin detail by electron paramagnetic resonance (EPR) X-raydiffraction (XRD) scanning electron microscopy (SEM)and surface analysis techniques It has been shown thatthe decomposition process of HKUST-1 upon exposure tomoisture takes place through three different stages The firststage with duration of about 20 days is characterized bythe adsorption of a large amount of water molecules by thematerial Water molecules gradually fill the cavities of thematerial pushing towards the Cu2+ ions The authors [15]pointed out by XRD that during this first stage the materialundergoes a swelling but no irreversible structural changestake place in the material The decomposition process of thecrystalline matrix actually starts for times longer than 20days when the effects of the hydrolysis of the Cu-O on thestructure of the material become more and more evident onincreasing the duration of exposure to air [15] In particularduring the second stage of the process of decompositionwhich takes place for exposure times from 20 days to 50

days about 35 of the paddle-wheels become hydrolyzed andconsequently one of the four carboxylate bridges involved inthese paddle-wheels is detached Finally in the third stage ofthe process of decomposition which takes place for exposuretimes longer than 50 days a second detaching of a carboxylatebridge takes place in the same paddle-wheels already affectedin the previous stage of the process

In order to shed new light on the fundamental issueconcerning the structural effects induced by hydrolysis oncarboxylate MOFs here we present an experimental inves-tigation performed by Raman spectroscopy focused on theprocess of decomposition induced in HKUST-1 upon expo-sure to air moisture (119879 = 300K 70 relative humidity)and showing the potentialities of the technique to deepenthe understanding of the underneath process in terms ofmolecular groups changes

2 Materials and Methods

Commercial HKUST-1 in powder form was purchased fromSigma-Aldrich as Basolite C300 A sample of about 10mgof HKUST-1 was exposed to air at 119879 = 300K and relativehumidity (RH)of 70 for different timesWehavemonitoredthe vibrational properties of the sample of HKUST-1 withRaman spectroscopy from about 3 hours up to 165 days ofexposure to air Furthermore we have performed a measure-ment on the activated sample of HKUST-1

The Raman measurements were acquired in the regionfrom 150 to 1700 cmminus1 with a Bruker Senterra 120583-Raman spec-trometer equipped with a diode laser working at 120582 = 532nmWe have performed measurements both at high resolution(3ndash5 cmminus1) and at low resolution (9ndash15 cmminus1) The advantageof the low resolution mode is that it gives spectra withsignificantly higher signal-to-noise ratio with respect to thoseobtained in high resolution mode The nominal laser powerwas 02mW to avoid sample modifications For statisticalreasons each sample was measured in three different pointsand the resulting spectra were averaged

The spectrum of the activated sample of HKUST-1 wasacquired with a Horiba LabRam HR-Evolution 120583-Ramanspectrometer with a 532 nm laser and 1mW nominal laserpower at high resolution (4 cmminus1) No spectral distortionswere induced by laser power

The sample was put inside a high pressure LinkamTHMS600PS cell at 150∘C (heating ramp 100 Cmin) in atmo-spheric pressure and monitored starting from 10 minutes upto 1 hour recording Raman spectra

All the recorded spectra were subtracted by a baselineand subsequently normalized with respect to the intensity ofthe band at sim1000 cmminus1 We have chosen this peak for thenormalization because it is well known that it is a very stablesignal as it is related to the symmetric stretching of the C=Cbonds in the rigid benzene rings

3 Results

The high resolution Raman spectrum of the sample ofactivated HKUST-1 and that acquired for the sample after


man

inte

nsity

(arb

uni

ts)

1650 1500 1350 1200 1050 900 750 600 450 300 150


Activated3 hours







1650 1500 1350 1200 1050 900 750 600 450 300 150


Ram

an in

tens

ity (a

rb u

nits)

3 hours20 days

40 days165 days

C=C

sym

stre

tch

C-O

-O as

ym st

retc

hC-

O-O

sym

stre

tch

C=C

sym

stre

tch

C-H

ben

d

C-H

ben

d

Cu-O

stre

tch

Cu-O

wstr

etch

Cu-C

u str

etch








01 1 10 100


170

172

174

176

178

180

Peak

pos

ition

(cm

minus1 )

00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)


(a)

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)


3 hours20 days

165 days

(b)



00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

01 1 10 100


498

500

502

504

506

508

Peak

pos

ition

(cm

minus1 )

(a)

3 hours20 days

165 days

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)


(b)





4 Discussion



01 1 10 100


00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

1610

1612

1614

1616

1618

1620

Peak

pos

ition

(cm

minus1 )


(a)

3 hours20 days

165 days


00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)

1675 1650 1625 1600 1575

(b)







2(Cu)





5 Conclusions


Competing Interests


Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


man

inte

nsity

(arb

uni

ts)

1650 1500 1350 1200 1050 900 750 600 450 300 150


Activated3 hours







1650 1500 1350 1200 1050 900 750 600 450 300 150


Ram

an in

tens

ity (a

rb u

nits)

3 hours20 days

40 days165 days

C=C

sym

stre

tch

C-O

-O as

ym st

retc

hC-

O-O

sym

stre

tch

C=C

sym

stre

tch

C-H

ben

d

C-H

ben

d

Cu-O

stre

tch

Cu-O

wstr

etch

Cu-C

u str

etch








01 1 10 100


170

172

174

176

178

180

Peak

pos

ition

(cm

minus1 )

00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)


(a)

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)


3 hours20 days

165 days

(b)



00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

01 1 10 100


498

500

502

504

506

508

Peak

pos

ition

(cm

minus1 )

(a)

3 hours20 days

165 days

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)


(b)





4 Discussion



01 1 10 100


00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

1610

1612

1614

1616

1618

1620

Peak

pos

ition

(cm

minus1 )


(a)

3 hours20 days

165 days


00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)

1675 1650 1625 1600 1575

(b)







2(Cu)





5 Conclusions


Competing Interests


Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


01 1 10 100


170

172

174

176

178

180

Peak

pos

ition

(cm

minus1 )

00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)


(a)

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)


3 hours20 days

165 days

(b)



00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

01 1 10 100


498

500

502

504

506

508

Peak

pos

ition

(cm

minus1 )

(a)

3 hours20 days

165 days

00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)


(b)





4 Discussion



01 1 10 100


00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

1610

1612

1614

1616

1618

1620

Peak

pos

ition

(cm

minus1 )


(a)

3 hours20 days

165 days


00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)

1675 1650 1625 1600 1575

(b)







2(Cu)





5 Conclusions


Competing Interests


Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


01 1 10 100


00

02

04

06

08

10

Peak

inte

nsity

(arb

uni

ts)

1610

1612

1614

1616

1618

1620

Peak

pos

ition

(cm

minus1 )


(a)

3 hours20 days

165 days


00

02

04

06

08

10

Ram

an in

tens

ity (a

rb u

nits)

1675 1650 1625 1600 1575

(b)







2(Cu)





5 Conclusions


Competing Interests


Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




5 Conclusions


Competing Interests


Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

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