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Design of cost-efficient and photocatalytically activeZn-basedMOFsdecoratedwithCu2OnanoparticlesforCO2methanation

MaríaCabrero-Antonino,a,†SoniaRemiro-Buenamañana,b,†ManuelSouto,cAntonioA.

García-Valdivia,dDuaneChoquesillo-Lazarte,eSergioNavalón,aAntonioRodriguez-

Diéguez,d*GuillermoMínguezEspallargas,c*andHemenegildoGarcíaa,b,*

Supporting Information

1.Generalmethodsandmaterials2.StructuralfeaturesofMOF(Zn)3.Catalytictests4.Determinationofvalencebandenergiesandopticalgaps5.References

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2019

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1. Generalmethodsandmaterials

Materials and reagents. All the reagents and solvents employed in this study were of analyticalreagentorHPLCgradeandweresuppliedbySigma-Aldrich.

SynthesisofMOF(Zn)-1.3H-benzotriazole-5-carboxylate(10mg,0.06mmol)wasdissolvedin0.5mLofDMFbeforeadding0.5mLofdistilledwater.Separately16.51mg(0.09mmol)ofZn(CH3COO)2wasdissolvedin0.5mLofdistilledwaterbeforeadding0.5mLofDMF.Bothsolutionsweremixedandtheresultingsolutionwasplacedinaclosedglassvesselandintroducedinanovenat95°Cfor12h,afterwhichyellow/brownsinglecrystalswere isolated,whichwerewashedwithwater/methanol.Yield:55%basedonmetal.Heatingatdifferent temperaturesproducedifferentdegreesofbreathingasrevealedbysingle-crystaldiffraction(seebelow).

SynthesisofMOF(Zn)-2.MOF(Zn)-2isobtaineduponheatingat400°Cfor2hcrystalsofMOF(Zn)-1.

Synthesis of MOF-74(Zn). Synthesis of MOF-74(Zn) was adapted from a previously reportedprocedure.S1Amixtureof2,5-hydroxy-1,4-benzenedicarboxylicacid(H2DHBDC)(2.4g,12.1mmol)andzincnitratetetrahydrate,Zn(NO3)2·4(H20)(179mg,60.5mmol)weredissolvedinDMF(240mL)andwater(120mL),dissolvedandheatedto105°Cfor20hoursandthencooledtoroomtemperature.Yellowneedlecrystalswereobtainedinyield51%(basedonH2DHBDC)thatwerewashedwithDMFandmethanol,anddriedinair.

Synthesis of MIL-125(Ti)-NH2. MIL-125(Ti)-NH2 was prepared following previously reportedprocedures.S22-Aminoterephtalicacid(1.43g,7.9mmol)wasdissolvedin20mLofanhydrousN,N-dimethylformamide(DMF).Anhydrousmethanol(5mL)wasthenaddedtothereactionmixtureandthesystemsonicatedfor20min.ThereactionmixturewastransferredtoaTeflon-linedautoclave(50mL)andtitaniumisopropoxide(1.36g,4.8mmol)wassubsequentlyadded.Theautoclavewassealedandheatedat110°Cfor72h.AftercoolingtheTeflontoroomtemperature,theresultingprecipitatewasfiltered,washedwithDMFatroomtemperaturefor12hand,then,washedwithDMFat120°Cfor 12 h. This washing procedure was repeated usingmethanol as solvent. Finally, the solid wasrecoveredbyfiltrationanddriedintheovenat100°C.

SynthesisofCuO2@MOFsusingthephotodepositionmethod(PD).CopperNPsweredepositedinthepreviouslysynthesizedMOFs(MOF(Zn)-1,MOF(Zn)-2,MOF-74(Zn)andMIL-125(Ti)-NH2)usingthePDmethod.S3EachMOFwasdispersedbysonicationinamixtureofwater(13mL)andmethanol(5mL).Then,Cu(NO3)2·3H2Owasdissolvedinwater(2mL)anditwasaddedtotheMOFsolutionatinitial1wt%.Thesolutionwaspurgedwithargonfor20min.Finally,themixturewasirradiatedusinganUV-VisiblelightXelamp(300W)for4h.Theresultingsolidwasfiltered,washedwithwateranddriedintheovenat100°Cfor24h.

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2. StructuralfeaturesofMOF(Zn)

Upon heating the as-synthetised material (MOF(Zn)), the solvent molecules (DMF and H2O) areremoved from the channels causing a reversible continuous shrinkage of the structure.S4,S5 ThreedifferentMOFswithadifferentdegreeofbreathinghavebeenisolatedafter2hheatingat100°C,200 °C, and 350 °C, denoted MOF(Zn)-100, MOF(Zn)-200 (corresponding to MOF(Zn)-1), andMOF(Zn)-350.Interestingly,uponfurtherheatingat400˚C,thecrystalstructureexpandsandamuchmoreopenstructureisobtained(MOF(Zn)-2).

TheX-rayintensitydatawerecollectedonaBrukerD8Venturediffractometer(MOF(Zn),MOF(Zn)-RT, MOF(Zn)-100, MOF(Zn)-350 and MOF(Zn)-1) and on a Rigaku Oxford Diffraction Supernovadiffractometer(MOF(Zn)-1andMOF(Zn)-2),usingMo-Kαradiation.ThedataforMOF(Zn),MOF(Zn)-RT, MOF(Zn)-100, and MOF(Zn)-350were integrated with SAINTS6 and corrected for absorptioneffectswithSADABSS7.ThedataforMOF(Zn)-1andMOF(Zn)-2wereprocessedwithCrysAlisProS8,andabsorptioncorrectionsbasedonmultiple-scannedreflectionswerecarriedoutwithSCALE3ABSPACKinCrysAlisPro.AllcrystalstructuresweresolvedwithSHELXTS9andrefinedwithSHELXLS9.TheOLEX2softwarewas used as a graphical interface S10. Anisotropic atomic displacement parameterswereintroduced for all non-hydrogen atoms. Hydrogen atomswere placed at geometrically calculatedpositionsandrefinedwiththeappropriateridingmodel,withUiso(H) = 1.2Ueq(C,O)(1.5formethylgroups).

Eventhoughcrystaldatawascollectedupto0.77Angstromsresolution,crystalsstilldiffractedquiteweaklyathighangleduetotheirratherlowquality(withexceptiontoMOF(Zn)-RTandMOF(Zn)-1)and data were cut off according to intensity statistics, using restraints and constraints duringrefinement.

InMOF(Zn),itwasnotpossibletoclearlyseeelectron-densitypeaksindifferencemapswhichwouldcorrespondwithacceptablelocationsfortheHatomsbondedtoO1W.Therefore,therefinementwascompletedwithnoallowance for theseHatoms in themodel. The contributionsof thesemissingatoms(formulae,formulaweight,etc.)havebeenincludedintheCIF.

In MOF(Zn)-RT, a DMF solvent molecule was found disordered over two alternative positions(0.52:0.48 ratio), and was refined with rigid groups, partially equal anisotropic displacementparametersandrestraintsongeometryanddisplacementparameters.Inthisheavy-atomstructureitwasnotpossibletoclearlyseeelectron-densitypeaks indifferencemapswhichwouldcorrespondwithacceptablelocationsfortheHatomsofthedisorderedDMFmolecule.Therefore,therefinementwascompletedwithnoallowancefortheseHatomsinthemodel.Thecontributionsofthesemissingatoms(formulae,formulaweight,etc.)havebeenincludedintheCIF.

During the refinement ofMOF(Zn)-100, a number of ISOR restraints had to be used to obtainreasonable displacement parameters for all non-hydrogen atoms. Their Uij values were thereforerestrained to be approximately spherical. Severely disordered DMF inMOF(Zn)-100 could not bemodelledreasonably,andwerethereforeremovedfromthediffractiondata(usingtheOLEX2Masktool)butconsideredforcalculationofempiricalformula,formulaweight,density,linearabsorptioncoefficientandF(000).Asolventmaskwascalculatedand322.0electronswerefoundinavolumeof

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642.0Å3inonevoid.Thisisconsistentwiththepresenceof2DMFmoleculesperformulaunitwhichaccountfor320.0electrons.

AnumberofISORrestraintshadtobeusedtoobtainreasonabledisplacementparametersforallnon-hydrogen atoms inMOF(Zn)-350. Their Uij values were therefore restrained to be approximatelyspherical. In thisheavy-atomstructure itwasnotpossible to clearly seeelectron-densitypeaks indifferencemapswhichwouldcorrespondwithacceptablelocationsfortheHatomsbondedtoO1W,O2WandO3.Therefore,therefinementwascompletedwithnoallowancefortheseHatomsinthemodel.Thecontributionsofthesemissingatoms(formulae,formulaweight,etc.)havebeenincludedintheCIF.

InMOF(Zn)-1,SADI,FLAT,RIGUandEADPcommandswereusedtoobtainreasonablegeometryandanisotropic displacement parameters for the DMF solvent molecule. This molecule is disorderedacross a twofold rotation axis; bonds to symmetry equivalents were suppressed using PART -1command. Inthisheavy-atomstructure itwasnotpossibletoclearlyseeelectron-densitypeaks indifferencemapswhichwouldcorrespondwithacceptablelocationsfortheHatomsofthedisorderedDMFmolecule.Therefore,therefinementwascompletedwithnoallowancefortheseHatomsinthemodel.Thecontributionsofthesemissingatoms(formulae,formulaweight,etc.)havebeenincludedintheCIF.

WholemoleculedisorderofthebtcaligandwasmodelledinMOF(Zn)-2viaAFIX,DFIX,FLAT,EADP,andRIGUcommands.Theratiobetweenthetwocomponentswasrefinedfreelyandconvergedat0.48:0.52.ADFIXcommandwasusedtorestrainthegeometryoftheµ2-bridgingformatoligandinordertoexhibitaO-CH-OmoietybetweensymmetryrelatedZn2atoms(symmetryoperation:-x,+y,5/2-z).Figure1.d.showsa labelledORTEPplotshowingtheμ2-bridging formato ligand linkingtwosymmetry-relatedhalf-occupancyZn2atoms.CrystaldataandrefinementdetailsarelistedinTableS1.

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TableS1.Selectedcrystallographicdataforallcompounds.

Compound MOF(Zn)-1 MOF(Zn)-2

Activationtemperature Assynthesised 24hatRT 2hat100°C 2hat200°C 2hat350°C 2hat400°C

Spacegroup C2/c C2/c C2/c C2/c C2/c C2/c

Chemicalformula C20H24N8O9Zn3 C20H22N8O8Zn3 C20H22N8O8Zn3 C17H15N7O7Zn3 C14H16N6O10Zn3 C15H7N6O7Zn4

FormulaMass(g/mol) 716.58 698.56 698.56 625.47 624.44 644.75

a(Å) 17.7182(14) 18.037(2) 18.94(3) 19.2623(14) 19.544(2) 16.643(2)

b(Å) 13.0967(12) 12.5871(13) 10.098(16) 9.9839(6) 9.0665(12) 14.593(2)

c(Å) 11.0610(10) 11.0828(13) 10.957(17) 11.0852(7) 11.0933(13) 10.964(2)

β(°) 94.014(4) 93.046(5) 90.71(4) 90.246(8) 90.176(4) 90.296(16)

V(Å3) 2560.4(4) 2512.6(5) 2095(6) 2131.8(2) 1965.7(4) 2662.8(7)

Z 4 4 4 4 4 4

T 150K 120K 120K 120K 120K 120K

ρcalc(g·cm-3) 1.859 1.847 2.215 1.949 2.110 1.608

2qrange(°) 2.305–22.960 2.658–25.197 2.286–19.167 3.676–25.052 2.476–25.051 3.354–17.216

Data/Restraints/Parameters 1765/0/195 2238/144/226 848/90/132 1877/27/173 1588/90/150 805/263/156

Nºreflections 15414 7245 7160 16741 5727 6607

Independentreflections[I>2σ(I)]

1765 2238 848 1877 1588 805

GoF 1.106 1.072 1.664 1.126 1.060 1.165

RFactor[I>2σ(I)] R1=0.0402,wR2=0.0828

R1=0.0494,wR2=0.0831

R1=0.1798,wR2=0.4035

R1=0.0296,wR2=0.0666

R1=0.0625,wR2=0.1336

R1=0.1035,wR2=0.2575

RFactor(alldata) R1=0.0587,wR2=0.0891

R1=0.0860,wR2=0.0918

R1=0.2122,wR2=0.4232

R1=0.0349,wR2=0.0689

R1=0.1151,wR2=0.1578

R1=0.1239,wR2=0.2778

CCDCCode 1916085 1916086 1916087 1916083 1916082 1916084

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FigureS1. (a)Representationof thedifferent crystal structuresobtainedduring thebreathingprocessuponincreasingthetemperature.(b)DifferentviewsofthestructureofMOF(Zn)-1.(c)DifferentviewsofthestructureofMOF(Zn)-2.(d)AlabelledORTEPplotat50%probabilityleveloftheasymmetricunitofMOF(Zn)-2showingtheμ2-bridgingformatoligandlinkingtwosymmetry-relatedhalf-occupancyZn2atomsandthedisorderedbtcaligand(symmetryoperations:$1:-x,+y,5/2-z;$2:-x,-y,2-z;$3:-1/2-x,1/2+y,3/2-z;$4:1/2+x,1/2+y,+z).

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FigureS2.ExperimentalandsimulatedPXRDspectraoftheMOFactivatedat200˚C(MOF(Zn)-1)andat400˚C(MOF(Zn)-2).

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FigureS3.Thermogravimetricanalysis(TGA)profileofMOF(Zn)ataheatingrateof5ºC/minunderaconstantstreamofN2.

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3. CatalytictestsCatalystcharacterization

Highresolutiontransmissionelectronmicroscopyimages(HR-TEM)andscanningTEMimagesindarkfieldmode(DF-STEM)wererecordedonaJEOLJEM2100Finstrumentoperatingat200kW.AnOxforddetectorcoupledtotheSTEMinstrumentwasemployedtomeasurethecompositionofselectedMOFareas.Themetalparticlesizedistributionandstandarddeviationwereestimatedbycountingabout300nanoparticles.ThemorphologyandcompositionoftheMOFsampleswerefurthercharacterizedusingascanningelectronmicroscope(SEM,Zeissinstrument,AURIGACompact)coupledwithaEDXdetector. Isothermal nitrogen adsorption experiments were carried out using an ASAP 2010Micromeriticsdevice.X-rayphotoelectron(XP)spectrawerecollectedonaSPECSspectrometerwithaMCD-9detectorusingamonochromaticAl(Ka=1486.6eV)X-raysource.Spectradeconvolutionwasperformedwith the CASA software using the C 1s peak at 284.4 eV as binding energy reference.Inductively coupledplasmaatomic emission spectroscopy (ICP-AES) hasbeenused to confirm themetal contentof the catalystpreviouslydigested in concentratednitric acid. The contentofCu2OmetalloadedwasdeterminedbyIPC-AESbefore(0.92%wtCu)andafter(0.89%wtCu)use.

Photocatalyticmethanationtests

Forthephotoassistedmethanation,aquartzphotoreactorequippedwithaheatingmantletocontrolthedesiredtemperatureduringtheexperimentwasutilized.Thephotocatalyst(15mg)aspowderwasplacedasabedinsidethereactor,thesystemwaspurgedandchargedwithH2,andthenCO2wasadded until a ratio 4 to 1 was achieved. The photoreactor was heated up to 215 °C, once thetemperaturewasstable,thephotocatalystwasirradiatedunderUV-VislightusingaXelamp(300W)aslightsource.Thereactionwasmonitoredbytakingperiodicallygassamplesfromthereactorthatwere injected into anAgilent 490MicroGCequippedwith two channels and thermal conductivitydetectors.OnechannelallowsanalyzingH2,O2,N2andCOwithaMolSieve5Åcolumnwhiletheotherchannel shows CO2, CH4 and short chain hydrocarbons using a Pore Plot Q column. Gases werequantifiedcalibratingtheinstrumentanalyzingofgasmixtures.Thelimitofdetectionofthemicro-GCofthereactionthatisestimatedtobebelow0.1μg·cat–1·h–1.

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FigureS4. (a)RepresentativeDF-STEM imageofCu2O@MOF(Zn)-1. (b)EDSanalysisof twocoppernanoparticles(spectrumAandB).

FigureS5.(a,b)DF-STEMimagesofCu2O@MOF(Zn)-2.(c)Copperparticlesizedistribution

Spectrum B

Spectrum A

25nm

SpectrumA

SpectrumB

100 nm

0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-1111-1212-1313-1414-1515-1616-170

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uenc

y (%

)

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50 nm200 nm

a)b)

c)

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FigureS6.DF-STEM imagesof (a)Cu2O@MIL-125(Ti)-NH2 and (b,c)Cu2O@MOF-74(Zn). The insetsshowthecopperparticlesizedistribution, theaveragecopperparticlesizeandstandarddeviationobtainedaftercountingmorethan300particles.

Figure S7. SEM images of (a,b,c) Cu2O@MOF(Zn)-1 and (d, e, f) Cu2O@MOF(Zn)-2, revealing themorphologyofthecrystals.

0-1 1-2 2-3 3-4 4-50

10

20

30

40

50

Freq

uenc

y (%

)

Copper particle size (nm)

4.29± 0.95

2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-130

4

8

12

16

20

24

28

Freq

uenc

y (%

)

Copper particle Size (nm)

6.07± 1.52

a)b)

20µm 20µm 20µm

20µm 20µm 20µm

a)b)c)

d)e)f)

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FigureS8.SEMimagesof(a)Cu2O@MOF(Zn)-1and(b)Cu2O@MOF(Zn)-2.(c)ElementalanalysismappingofaselectedareaofCu2O@MOF(Zn)-1.

20µm 10µm 10µm

5µm 5µm 5µm

5µm 5µm

C O

Zn Cu

a)

b)

c)

20µm 10µm 10µm

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FigureS9.(a)DF-STEMand(b)BF-STEMimagesoffreshCu2O@MOF(Zn)-1photocatalyst;theinsetcorrespondstothecopperparticlesizedistributionobtainedbycountingabout300particles.(c)SEMimage of fresh SEM image and (d) XPS Cu2p spectrum of Cu2O@MOF(Zn)-1 showing the bestdeconvolutiontoindividualcomponents.

20 nm

0-1 1-2 2-30

10203040506070

Freq

uenc

y (%

)

Copper particle size (nm)

1.61± 0.46a)b)

930 940 950 960 970

36000

36500

37000

37500

38000

38500

39000

39500

CPS

Binding Energy (eV)

Cu0/Cu+

2p3/2

Cu2+

2p1/2

Cu2+

Cu2+satellite

Cu0/Cu+

10µm

c)d)

25 nm

1.6nm

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FigureS10.XPSspectraofCu2O@MOF(Zn)-1.(a)C1sXPS,(b)O1sXPS,(c)N1sXPS,(d)Cu2pXPS,(e)Zn2pXPS.

FigureS11.XPSspectraofCu2O@MOF(Zn)-2.(a)C1sXPS,(b)O1sXPS,(c)N1sXPS,(d)Cu2pXPS,(e)Zn2pXPS.

280 285 2904000

6000

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CPS

Binding Energy (eV)

930 940 950 960 970

36000

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CPS

Binding Energy (eV)

395 400 405

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CPS

Binding Energy (eV)

525 530 535 54012000

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CPS

Binding Energy (eV)

1020 1030 1040 1050

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CPS

Binding Energy (eV)

a)b)c)

d)e)

C=C

O=C-O O=C-OM-O-

Cu0/Cu+

2p3/2

Cu2+

2p1/2

Cu2+

Cu2+satellite

Cu0/Cu+

Zn2p3/2

Zn2p1/2

525 530 53512000

14000

16000

18000

20000

22000

24000

CPS

Binding Energy (eV)

396 398 400 402 404 4069000

10000

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CPS

Binding Energy (eV)

930 940 950 96025500

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CPS

Binding Energy (eV)

280 285 2904000

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CPS

Binding Energy (eV)

1020 1030 1040 1050

32000

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CPS

Binding Energy (eV)

a)b)c )

d)e)Cu0/Cu+

2p3/2

Cu2+

2p1/2

Cu2+

Cu2+satellite

Cu0/Cu+

Zn2p3/2

Zn2p1/2

C=C

O=C-O

O=C-O

M-O-

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FigureS12.PowderX-raydiffractionpatternsofthedifferentcopper-supportedMOF-basedcatalysts(MOF(Zn)-1,MOF-74(Zn)andMIL-125(Ti)-NH2)beforeandafterthephotocatalytictests.

Figure S13.DF-SEM images andEDX spectrumof selected areaof Cu2O@MOF(Zn)-1 sample after

photocatalyticmethanationofCO2.

20 nm

0-1 1-2 2-3 3-4 4-50

10

20

30

40

50

60

Freq

uenc

y (%

)

Copper particle size (nm)

1.73± 0.91

50nm 25nm

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-5 0 5 10 15 20 2505

101520253035404550

Light

CH

4 (µ

mol

·gca

t-1)

Time (h)

Dark

Figure S14. Reusability of Cu2O@MOF(Zn)-1 for the CH4 production at 215 °C upon 2236Wm−2

irradiationusinga300WXelamp.PH2=1.05bar,PCO2=0.25bar.LegendFirstuseCu2O@MOF(Zn)-

1(●),SeconduseCu2O@MOF(Zn)-1(♦)andthirduseofCu2O@MOF(Zn)-1(■).

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Figure S15. Mass spectra of the reaction products after the photocatalytic reaction using

Cu2O@MOF(Zn)-1for22hoursat215°Cusing13C18O2.

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4. Determinationofvalencebandenergiesandopticalgaps

FigureS16.Solid-stateCyclicVoltammetry(CV)andDifferentialPulseVoltammetry(DPV)ofMOF(Zn)-1andMOF-74(Zn)usingTBAPF60.1MinCH3CNaselectrolyteand0.05V/sscanrate.Platinumwirewasusedascounterelectrodeandsilverwireaspseudoreferenceelectrode.Ferrocenewasaddedasinternalstandard.AllpotentialsarereportedversusAg/AgCl.

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FigureS17.(a)UV-Visdiffusereflectanceand(b)TaucplotforMOF(Zn)-1(redline)andMOF-74(Zn)(blackline).

200 300 400 500 600 700 800

F(R)

l (nm)

1 2 3 4 5 6 70

50

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ahn

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a)

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