Synthesis of MIL 88(Sc)

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Synthesis, characterisation and adsorption properties of microporous scandium

carboxylates with rigid and flexible frameworks

John P.S. Mowat a, Stuart R. Miller a,b,Alexandra M.Z. Slawin a, Valerie R. Seymour a,Sharon E. Ashbrook a, Paul A. Wright a,

a School of Chemistry, University of St. Andrews, Purdie Building, North Haugh, St. Andrews, Fife KY16 9ST, United Kingdomb Institut Lavoisier, 45, Avenue des Etats-Unis, Universit de Versailles St-Quentin en Yvelines 78035, Versailles, France

a r t i c l e i n f o

Article history:

Received 14 October 2010Received in revised form 10 December 2010Accepted 11 December 2010Available online 17 December 2010

Keywords:

Scandium carboxylatesMicroporous MOFsMIL-53CO2 adsorption45Sc MAS NMR

a b s t r a c t

Conditions for the synthesis of each of the scandium terephthalate frameworks Sc2L3, MIL-53(Sc)(Sc(OH)L) and MIL-88(Sc) (Sc3O(H2O)2(OH)L3) (L= 1.4-benzenedicarboxylate (BDC)) have been estab-lished. In addition, the MOFs MIL-100(Sc) (Sc3O(H2O)2(OH)L0 2, L0 = 1,3,5-benzenetricarboxylate (BTC))andsocMOF (Sc3O(H2O)3,(NO3

)L00 1.5, L00 = 3,30,5,50-azobenzenetetracarboxylate (ABTC)), have been syn-thesised for the first time with Sc. These materials have been characterised by powder and single crystalX-ray diffraction,1H,13C and 45Sc MAS NMR, and gas adsorption. MIL-53(Sc) is a highly flexible breathingframework that adopts many different forms, depending on the amount and type of adsorbate included.The structures of the as-prepared solvent-containing form, MIL-53(Sc)-DMF (DMF = dimethylformamide)and of the desolvated, hydrated form, MIL-53(Sc)-H2O, have been determined. The former structure pos-sesses one kind of partially open channel, whereas the latter includes two kinds of channel, closed andslightly open. The hydrated structures configuration has not previously been observed for MIL-53 mate-rials. 45 Sc MAS NMR is a sensitive probe for the Sc environment: distinctive lineshapes are observed forisolated ScO6 octahedra, corner-sharing chains of ScO4(OH)2 octahedra and Sc3O(O2C-)6(OH, H2O)3 tri-mers of octahedra. N2 adsorption at 77 K indicates that the flexible frameworks MIL-88(Sc) and MIL-

53(Sc) show no porosity in their desolvated, closed forms, whilst the rigid frameworks Sc2BDC3(0.26 cm3 g1), MIL-100(Sc) (0.72 cm3 g1) and Sc-ABTC (0.57 cm3 g1) have so far been prepared withappreciable permanent porosity. For CO2 adsorption on desolvated solids, remarkable behaviour is shownby the flexible MIL-53(Sc), which opens in two stages to a maximum capacity of >13 mmol g1, while therigid frameworks show uptakes at 196 K (atp/po= 0.40) of 5.5 mmol g

1 (Sc2BDC3), 21.3 mmol g1 (MIL-

100(Sc)) and 13.1 mmol g1 (Sc-ABTC).2010 Elsevier Inc. All rights reserved.

1. Introduction

The trivalent metal terephthalate metal organic frameworksMIL-53, MIL-68, MIL-88 and MIL-101 (terephthalate = 1,4-ben-zenedicarboxylate = BDC = L) are some of the most important

microporous types of MOF in terms of their porosity, their flexibil-ity and their thermal stability [1]. For example, MIL-101(Cr), Cr3O(-H2O)2(X)L3, (X = OH, F) exhibits a pore volume of 2.15 cm

3 g1,[15] MIL-53(Cr), Cr(OH)Land MIL-88(Fe), Fe3O(H2O)2(X)L3, show50%and 125% unit cell expansions, respectively[3]upon adsorbate up-take and MIL-53(Al) is thermally stable up to temperatures of773K inair[4]. This combination of properties has led to an exam-ination of the performance of these solids in hydrogen storage,adsorption and separation, catalysis and drug delivery.

For these structure types, their physical properties and adsorp-tion behaviour canbe strongly dependent on the metal cation type,particularly for those flexible solids able to breathe with uptakeand release of adsorbed molecules. For the Al and Cr forms ofMIL-53, for example,[415]the removal by heating of free tere-

phthalic acid occluded in the as-prepared form leads to a largeopen-pore form which upon cooling and exposure to air readilytakes up water to form a smaller closed-channel phase. By contrast,thermal removal of molecules from the pores of MIL-53(Fe) resultsin the closure of its channels in a two stage process forming first anintermediate phase (MIL-53(Fe)-INT) with triclinic symmetry andcontaining two distinct channel types, closed and partially open,and then after heating at higher temperature a second phaseMIL-53(Fe)-HT, with a single type of closed-channel[10,11]. Theclosed form opens again upon adsorbate uptake.

The syntheses of terephthalate MOFs with the trivalent metalcations Al3+, V3+, Cr3+, Fe3+, Ga3+ and In3+ have been reported inconsiderable detail [14,911,1517]. Further exploration of the

1387-1811/$ - see front matter 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.micromeso.2010.12.016

Corresponding author. Tel.: +44 (0)1334 463793; fax: +44 (0)1334 463808.

E-mail address: [email protected](P.A. Wright).

Microporous and Mesoporous Materials 142 (2011) 322333

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials

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choice of trivalent cation used in the syntheses with terephthalicacid has extended to Sc3+, the ionic radius of which (0.745 ) issimilar to that of In3+ (0.80 ) and so intermediate between thoseof the trivalent first row transition metals and Ga3+ (0.620.64 )and those of the trivalent cations of the smaller lanthanides suchas Yb3+ (0.87 ) and the related Y3+ (0.90 ) which form solidswithout permanent porosity such as Ln2L3(H2O)4 [18]. Initialhydrothermal syntheses with Sc3+ gave the unprecedented smallpore structure type, Sc2L3 [19](designated Sc2BDC3 here) whichhas been shown to possess promising properties for the adsorptionof small molecules and also catalysis via Lewis acid sites present atthe surface[20]. Solvothermal preparation of Sc2BDC3 gave singlecrystals that were used in crystallographic studies of the adsorp-tion of the small molecules H2, CO2, CH4 and C2H6 [20]. Further-more, solvothermal reaction of Sc3+ with 2,5-dihydroxy-1,4-benzene dicarboxylic acid indicated it was possible to prepare ananalogue of the MIL-88 structure, although this decomposed uponremoval of solvent.[21]. Very recently Ibarra et al. have reportedthe synthesis of MIL-88(Sc) using scandium chloride as the metalsource[22].

Here, we report the hydrothermal and/or solvothermal synthe-ses ofSc2BDC3, MIL-53(Sc) and MIL-88(Sc) in the terephthalate sys-tem. We have also prepared mixtures including MIL-101(Sc)[2]. Inaddition, we have prepared the scandium analogues of the micro-porous carboxylates MIL-100 (M3O(H2O)2(OH)L

0

2, M@Cr, Fe,L0 = 1,3,5-benzenetricarboxylate) and M3O(H2O)3,(NO3

)L00 1.5,M@In3+, L00 = 3,30,5,50-azobenzenetetracarboxylate, (thesocMOF ofEddaoudi et al.[23,24]referred to in this report as M-ABTC, M@In,Sc). Whereas the former has previously been obtained with Cr3+,Al3+ and Fe3+ the only reported trivalent metal form of the latteris with indium.

The phases obtained pure have been characterised by X-ray dif-fraction and solid-state NMR. Powder diffraction and where possi-ble single crystal diffraction has been performed on samples beforeand after removal of solvent or adsorption of water to identifyproduct phases and follow structural changes. This has included

the determination of novel structural variants of the MIL-53 struc-ture type.45Sc MAS NMR has been shown to be a sensitive probe ofthe local environment of scandium in solids, including frameworkstructures[25]. 45Sc MAS NMR spectra of microporous carboxyl-ates are found here to be strongly dependent on the Sc3+ coordina-tion environment, giving characteristic spectra for isolatedoctahedra, corner-sharing chains and trimers.

The thermal stability and adsorption properties of the micropo-rous scandium carboxylates have also been measured and are com-pared with reported values for the same structures prepared withother metals. Nitrogen adsorption measurements at 77 K and CO2adsorption measurements at 196 K, up to pressures of 0.9 bar, havebeen used to differentiate between rigid materials with permanentporosity and breathing structures with little porosity in the closed

form that may adapt to take up CO2. As expected, Sc2BDC3, Sc-ABTCand MIL-100(Sc) all show significant permanent porosity for bothgases, whereas dehydrated MIL-53(Sc) takes up CO2 in two stepsas the structure opens up at higher pressures. MIL-88(Sc) showslimited CO2adsorption.

2. Experimental

A first set of syntheses of scandium terephthalates was per-formed under hydrothermal conditions, varying the relative quan-tities of the scandium source (Sc2O3, 99.999%, Stanford MaterialsCorporation, or Sc(NO3)33H2O, 99.9%, Metal Rare Earth Limited),terephthalic acid and water, at temperatures in the range 373

493 K. After reaction, crystalline products were filtered and driedat 333 K. Solvothermal crystallizations were performed over the

same temperature range using dimethylformamide, DMF (Aldrich,>99.8%) or diethylformamide, DEF (Alfa Aesar, 98%) as solvents,and with scandium nitrate as the scandium source. Scandiumoxide is not reactive in these solvents. Reactions were performedwith terephthalic acid/Sc ratios between 1.0 and 2.0, and the ef-fects of adding pyridine (reported by Anokhina et al. to favourthe formation of MIL-53(Fe) in the Fe system[15]) were also exam-ined. Syntheses were performed in PTFE-line autoclaves, typicallywith volumes of 23 or 40 mL. After reaction, products were filteredand washed with ethanol to remove strongly smelling DMF, DEFand/or pyridine, before drying at 333 K. Illustrative conditionsand products are given inTable 1. The synthesis of scandium ana-logues of the other known microporous tri- and tetracarboxylatephases MIL-100 and In-ABTC was also investigated under hydro-thermal and solvothermal conditions, starting with literature pro-cedures and varying metal source, reactant ratios, solvents andreaction conditions (Table 2).

Identification of the product phases was achieved by powder X-ray diffraction. Diffraction patterns were obtained in DebyeScher-rer geometry within sealed 0.7 mm quartz glass capillaries on aStoe STADI/P diffractometer operating with monochromated CuKa1 X-rays (k= 1.54056 ). For structures with rigid frameworks,phase identification was possible by direct comparison with pub-lished powder diffraction patterns, but was complicated for thosephases that exhibit strong breathing effects (MIL-53 and MIL-88).For these, the XRD patterns display considerable variation becausethe patterns depend on the amount and type of occluded speciespresent in the pores. Indeed, confirmation of the synthesis ofMIL-53(Sc) was typically achieved by removal of occluded mole-cules from the as-prepared materials and then hydration to a rec-ognizable crystalline form.

Crystals of as-prepared MIL-53(Sc) and desolvated, then hy-drated MIL-53(Sc) were examined by single crystal diffraction(see below). Although the data from the heated, hydrated materialwere not of high quality they were sufficient to permit structuresolution to give a starting structural model that could be refined

against the powder X-ray data. The Rietveld refinement is de-scribed below. Rietveld structure refinement was also performedon an as-prepared MIL-88(Sc) containing DMF and water.

TGA was performed at 5 K min1 up to 1173 k in flowing air.Elemental analysis (carbon, hydrogen, nitrogen) was performedon pure samples using a Carlo Erba instruments EA 1110 CHNSanalyser.

Solid-state NMR spectra were obtained using a Bruker AvanceIII 600 MHz spectrometer, equipped with a wide-bore 14.1 Tsuperconducting magnet, resulting in Larmor frequencies of600.13 MHz for 1H, 150.9 MHz for 13C and 145.8 MHz for 45Sc.Powdered samples were packed into 4, 2.5 or 1.3 mm ZrO2 rotorsand rotated at MAS rates between 10 and 60 kHz. Chemical shiftsare given relative to TMS for 13C and 1H and 0.2 M ScCl3 (aq) for45

Sc. For 13

C, spectra were acquired using cross-polarisation, witha contact pulse (ramped for 1H) of 1 ms duration and 1H decoupling(SPINAL32 withx1/2p= 100 kHz) applied throughout acquisition.

Adsorption isotherms of N2 were measured at 77 K using aMicromeritics Tristar II 3020. Adsorption isotherms of CO2 weremeasured up to 0.9 bar at 196 K (the measured temperature of adry ice/ethanol mixture) using a Hiden IGA automatic gravimetricporosimeter. Additional gravimetric CO2adsorption measurementswere made on the flexible solids MIL-53(Sc) and MIL-88(Sc) up to0.9 bar inthe range263273 K witha GrantsOptimaGR150thermo-static recirculating water bath to control sample temperature andalso at 196, 230 and 300 K on a Hiden IGA capable of operating atelevated pressures (up to 20 bar). In each case the samples werepre-heated under flowingnitrogento removesolventandunreacted

ligand (where appropriate) and then degassedunder vacuumin theporosimeter at a temperature that would remove adsorbed water.

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3. X-ray crystallography

3.1. Single crystal X-ray diffraction

Crystals of MIL-53(Sc) of suitable quality for single crystal dif-fraction were prepared by crystallisation of a reaction mixtureSc(NO3)33H2O:terephthalic acid:pyridine:DMF of 1:1:15:300 at463 K for 72 h. These were observed to lose solvent rapidly uponfiltration and being allowed to stand under ambient conditions,leading to loss of crystal quality. This could be seen under the opti-cal microscope as the crystals jumped on the microscope stage asthey were viewed. To prevent this, a batch of the crystals was cov-ered immediately after removal from the autoclave by a poly-fluo-rinated oil (Paratone) to slow down solvent loss, and a single

crystal was transferred to a glass fibre and cooled to 173 K in aN2flow from a Rigaku X-streamTM cryostream. Data were collected

on this sample (labelled MIL-53(Sc)-DMF) at 173 K using a RigakuMM007 high brilliance RA generator (Cu Karadiation, confocal op-tics) and Saturn92 CCD system. At least a full hemisphere of datawas collected using x scans. Intensities were corrected for Lor-entz-polarisation and for absorption. The structures were solvedby direct methods. Hydrogen atoms bound to carbon were fixedin idealised positions. Structural refinements were performed withfull-matrix least-squares based onF2 by using the program SHEL-XTL[26]. Electron density observed in the difference Fourier syn-thesis close to the ScOSc bridging oxygen atom was refined asa proton, with the distance to O constrained to 2.09 . Crystallo-graphic details are given inTable 3.

In addition, single crystals from a sample of MIL-53(Sc), pre-pared in parallel using the same reaction conditions, were heated

for 12 h at 623 K in a flow of nitrogen to remove solvent, and thenallowed to cool in moist air. These adsorbed water to give the hy-drated form Sc(OH)LxH2O. Data were collected using the sameRigaku MM007 high brilliance RA generator (Cu Ka radiation, con-focal optics) and Saturn92 CCD system. Although the data were ofpoor quality, (Rint = 0.4669) they were sufficient to permit struc-ture solution via the SHELX suite of programs to give a startingstructural model (P1, a= 7.260(6) , b= 13.071(9) , c= 20.583(13) ,a= 72.54(4), b= 82.95(3), c= 86.57(4), RI = 0.4650) thatwas subsequently successfully refined against powder X-ray data.

3.2. Rietveld refinement against powder diffraction data

Laboratory X-ray powder diffraction of MIL-53(Sc)-H2O,

(Sc(OH)L0.7H2O) collected at room temperature from 5 to 602hover 14 h was used for structural analysis. Attempts to refine thestructure starting from models for the intermediate phase reportedfor MIL-53(Fe)[27]were unsuccessful, as the unit cell differed sig-nificantly from the experimental pattern. Indexing of the experi-mental data indicated a larger triclinic unit cell, which wasconsistent with the data obtained from our single crystal diffrac-tion and therefore the partial structural model obtained from thesingle crystal was used as a starting point for the Rietveld refine-ment. Because of the complexity of this triclinic MIL-53(Sc) struc-ture, bond restraints were applied (ScO, 2.09(2) ; OO, 2.97(2) ;OC, 1.27(2) ; aromatic CC, 1.37(2) ; aromatic-carboxylate CC1.45(2) ). Protons were omitted from the refinement, and the re-gions 9.710.54 2h, 11.312.2 2h and 14.615.2 2h excluded. The

excluded regions contained small peaks that could not be indexedon the observedtriclinic cell, and which were not present in similar

Table 1

Synthetic conditions and product phases in hydrothermal and solvothermal crystallisation of scandium terephthalates (terephthalic acid = H 2L).

Temp./K Solvent Scandium source Molar ratios in preparation Time/h Product

Sc H2L S olvent Pyridine

373 H2O Sc(NO3)33H2O 1.0 1.0 600 2472 Recryst. H2L373 DMF Sc(NO3)33H2O 1.0 1.2 200 2448 MIL-88(Sc)373 DMF Sc(NO3)33H2O 1.2 1.0 200 2448 MIL-88(Sc)373 DEF Sc(NO3)33H2O 1.0 1.2 300 2448 MIL-88(Sc)373 DEF Sc(NO3)33H2O 1.2 1.0 300 2448 MIL-88(Sc)413 H2O Sc(NO3)33H2O 1.0 1.0 600 2472 Recryst. H2L413 DMF Sc(NO3)33H2O 1.0 1.2 300 2448 MIL-88(Sc)413 DMF Sc(NO3)33H2O 1.2 1.0 300 2448 MIL-88(Sc)413 DEF Sc(NO3)33H2O 1.0 1.2 200 2448 MIL-88(Sc)

463 H2O Sc2O3 1.0 1.5 600 72 Sc2BDC3463 DMF Sc2O3 1.0 1.5 300 72 Recryst. H2L463 H2O Sc(NO3)33H2O 1.0 1.5 600 1672 Sc2BDC3[19]463 DEF Sc(NO3)33H2O 1.0 1.5 200 40 Sc2BDC3463 DEF Sc(NO3)33H2O 1.0 1.0 200 15 40 MIL-53(Sc)463 DMF Sc(NO3)33H2O 1.0 1.5 300 40 Sc2BDC3463 DMF Sc(NO3)33H2O 1.0 1.0 300 40 MIL-53(Sc) + Sc2BDC3463 DMF Sc(NO3)33H2O 1.0 1.5 300 15 40 MIL-53(Sc)463 DMF Sc(NO3)33H2O 1.0 1.0 300 15 40 MIL-53(Sc)

Table 2

Synthetic conditions and product phases in hydrothermal and solvothermal crystallisation of microporous scandium carboxylates with trimesic acid (H3L0) and

azobenzenetetracarboxylic acid (H4L00).

Temp./K Solvent Scandium source Molar ratios in preparation Time /h Product

Sc H3L0 Solvent

393423 DMF Sc(NO3)33H2O 1.0 0.5 300 2472 MIL-100(Sc)423 DEF Sc(NO3)3.3H2O 1.0 0.5 150 2472 MIL-100(Sc)

Sc H4L0 0 Solvent393 H2O Sc(NO3)33H2O 3.0 2.0 1800 48 H4L00

393 EtOH Sc(NO3)33H2O 3.0 2.0 900 48 H4L00

393493 DMF Sc(NO3)33H2O 3.0 2.0 450 48 Sc-ABTC393493 H2O Sc2O3 3.0 2.0 1800 72 H4L00 + Sc2O3

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(but less crystalline) samples treated in the same way. Oxygenatoms from water molecules known from TGA analysis to be pres-

ent in the solid were located in the channels. The final Rietveld plotis shown inFig. 1and the crystallographic data are summarized inTable 4and in theSupporting information. (Elemental analysis forMIL-53(Sc)-H2O, Sc(OH)L0.7H2O, from TGA: C expected, 40.2 wt.%;C measured, 40.06 wt.%, N measured 0.0 wt.%).

The structure of as-prepared MIL-88(Sc)-DMF was determinedby Rietveld refinement on laboratory X-ray powder diffraction col-lected at room temperature from 570 2h over 16 h. Restraintswere applied to the starting model (prepared from the literatureexample MIL-88B(Cr) [17]), ScO, 2.075(3) ; OO, 2.95(5) ; OC, 1.27(2) ; aromatic CC, 1.37(1) ; aromatic-carboxylate CC1.45(2) . Residual electron density in the MIL-88(Sc) was fittedusing the disordered solvent model from the literature MIL-88B[17]and additional scattering positions were found by differenceFourier mapping. The Rietveld plot is shown inFig. 2and the crys-tallographic data are summarized in Table 4 and inthe Supplemen-tary information. (Elemental analysis on a MIL-88(Sc)-DMF sampleheated at 623 K for 12 h to remove all guest molecules (verified by

TGA, see Supplementary information) gave for [Sc3O(OH)(OH2)2L3]: C expected, 41.40 wt.%; C measured, 41.13 wt.%, Nmeasured 0.0 wt.%).

4. Results and discussion

4.1. Synthesis of scandium terephthalates

Synthetic studies in the scandium terephthalic acid systemhave yielded four microporous phases, three of which are ana-logues of structures prepared previously with other trivalent metalcations and are described here for the first time (Table 1). Confir-mation of products as MIL-53(Sc) was achieved by first removingall guest species by calcination under flowing N2 at 623 K for

Fig. 1. Rietveld profile fit to laboratory X-ray powder data for MIL-53(Sc)-H2O, Sc(OH)L0.7H2O (desolvated and hydrated). (Experimental data, red markers; fitted profile,green; difference plot, purple). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 3

Single crystal diffraction details for MIL-53(Sc)-DMF.

MIL-53(Sc)-DMF

Formula unit Sc(OH)LDMFFormula weight 299.18Calculated Density/g cm3 1.533Temperature/K 173Space group P n a 21

X-ray source Cu KaradiationWavelength/ 1.54178Unit cell/ a= 19.344(2)

b= 9.1802(14)c= 7.3013(11)a=b=c=90

Volume/3 1296.6(3)Z 4RF(all data) 0.0804 (2012)RF(I2rI) 0.0556 (1556)Max and min residual e density (e/3) 0.425, 0.444

Table 4

Rietveld refinement details for MIL-53(Sc)-H2O and MIL-88(Sc)-DMF.

MIL-53(Sc)-H2O MIL-88(Sc)-DMF

Formula unit Sc(OH)L0.7H2O Sc3OL3xDMFyH2OFormula weight 229.04 851.43Calculated density/g cm3 1.594 1.339Temperature/K 295 295Space group P1 P62cX-ray source Cu Karadiation Cu KaradiationDiffractometer STOE STADI P STOE STADI PWavelength() 1.54056 1.54056Unit cell ()a/ 7.267(2) 11.2190(9)b/ 13.452(4) 11.2190(9)c/ 20.677(7) 19.373(3)a/ 71.570(6) 90b/ 84.212(11) 90c/ 87.729(6) 120Volume/3 1908.77(22) 2112.27(24)No. Reflections 1088 211No. Atoms (Non-H) 58 12.46No. Restraints 169 38R 0.0516 0.0450

wR 0.0701 0.0670Max. and min. residual

e density (e/3)0.406, 0.379 0.485, 0.440

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10 h, suggested by TGA (see below) to be suitable conditions to re-move solvent and any free terephthalic acid from the pores. Thecalcined solid was allowed to take up moisture upon standing inair. MIL-53(Sc) materials treated in this way gave the triclinicstructural variant determined by single crystal and powder diffrac-tion as described above.

Under hydrothermal conditions, and using Sc2O3or Sc(NO3)3asthe scandium source, high temperatures (463 K) are required forreaction. The predominant phase to form is Sc2BDC3. This is thefirst reported use of the less expensive scandium oxide as the me-

tal source, rather than a scandium salt. Under solvothermal condi-tions no reaction was obtained using Sc2O3as a reagent, due to itslow solubility, but Sc(NO3)33H2O gave a series of products. Reac-tion at lower temperatures (373413 K) in DMF or DEF givesMIL-88(Sc) although synthesis in DMF at 373 K and in a large100 ml Parr autoclave gave a mixture with MIL-101(Sc) as the ma-jor phase, together with MIL-88(Sc). Using DMF as a solvent, hightemperature reactions (463 and 493 K) give MIL-53(Sc) or a mix-ture of MIL-53(Sc) and Sc2BDC3, depending on the metal:ligand ra-tio. If pyridine is included in the reaction, pure MIL-53(Sc) isobtained over a wider compositional range. Using DEF as a solvent,high temperature reactions with a 1:1.5 Sc:ligand ratio give largecrystals of Sc2BDC3 (4050 lm)

3 but the addition of pyridine fa-vours the crystallization of MIL-53(Sc).

4.2. Structural flexibility of MIL-53(Sc)

As-prepared, MIL-53(Sc) adopts slightly different structuralmodifications, depending on the solvents and even the volume ofthe autoclave (manifested as strongly different powder diffractionpatterns). This could be a result of the different particle sizes of theproducts, which might then lose the solvents involved in prepara-tion or washing to different extents upon drying. If no special stepsare taken, this solvent loss results in loss of single crystal quality,preventing structural analysis. It was possible, however, to deter-mine the single crystal structure of MIL-53(Sc) prepared in DMFby immersing the as-prepared crystal in oil immediately after re-moval from the autoclave and rapidly cooling and collecting data.

The structure is shown in Fig. 3, with crystallographic details giveninTable 3. Under these circumstances the structure adopts space

group P n a 21, (for the first time in MIL-53) with a = 19.344(2) ,b= 9.1802(14) ,c= 7.3013(11) . The ScO distance in this struc-ture is 2.10(2) : CO, 1.25(5) : CC(ring), 1.39(4) and CC(ring-carboxylate), 1.49(3) . It was possible to locate solvent DMFmolecules within the pores, which are fully ordered within thechannels, the opening of which (at its widest point across the chan-nels) is 4.3 (taking into account the van der Waals radii of Oatoms). The location of the bridging hydroxyl proton suggestedby the difference Fourier calculations and subsequent refinementindicates an interaction of the proton with an adjacent oxygen

atom bound to scandium (ScO(10)-HO(9)-Sc = 2.32 A

0

). A similarposition for the proton in empty MIL-53(Fe) was given by Devicet al. [29]on the basis of computer simulations of the structure.The orientation of the DMF molecules appears to be determinedby DMFhost and DMFDMF interactions.

Upon loss of solvent from as-prepared MIL-53(Sc), which beginsto occur upon drying, the diffraction pattern changes.Fig. 4showsdiffraction patterns of MIL-53(Sc) prepared in DMF in 4 forms: (i)as-prepared, washed with DMF and dried at 333 K; (ii) heated un-der a vacuum of 104 mbar at 573 K to remove as much solvent aspossible; (iii) the heated material subsequently exposed to moistair and (iv) an as-prepared sample, washed with ethanol and dried.

The pattern of the DMF-washed sample is indexed as mono-clinic, a= 11.92(1) , b= 18.07(2) , c= 7.33(1) , beta = 90.69.

This suggests a larger cross channel axis repeat (a= 11.92(1) )than observed for the structure solved by single crystal diffraction(b= 9.180(1) ), which corresponds to a more open channel struc-ture. This is thought to result from a higher DMF content in thissample than in the single crystal.

It has not yet been possible to index unambiguously the X-raydiffraction pattern of MIL-53(Sc) (Fig. 4(ii)) desolvated by heatingunder vacuum (MIL-53(Sc)-CAL (CAL = calcined)) and structuralstudies structure are ongoing. The diffraction pattern of the cal-cined and then hydrated sample has been fitted by refining thepartial structure obtained by single crystal diffraction, as describedabove. Final refined bond lengths are: ScO, 2.09(3) 2.11(4) : CO, 1.29(3) : CC(ring), 1.39(1) and CC(ring-carboxylate),1.49(2) , similar to those observed for MIL-53(Sc)-DMF. The finalrefined structure (Fig. 5) possesses two kinds of channel, withopenings that are only 1.0 or 2.8 across (i.e. parallel tob) in their

Fig. 2. Rietveld profile fit for as-prepared MIL-88(Sc)-DMF.



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free dimension. There is considerable tilting of the phenyl groupsof alternate rows of terephthalate units along z away from thechannel axis compared to the as-prepared MIL-53(Sc)-DMF, andthe average ScOHSc angle in the chains of octahedra decreasesfrom 122.9 to 117.8. This is among the lowest values observedfor MIL-53 and is a consequence of the long ScO bond. (See

Supporting information for a comparison of the known crystalstructures and structural details of MIL-53 type materials.).

The distorted structure is similar to that observed for partiallydehydrated MIL-53(Fe), where it is referred to as the intermediatephase, because it possesses both closed and partially open chan-nels. The hydrated MIL-53(Sc) structure differs fromthe intermedi-ate structure of MIL-53(Fe), however, because the mode ofoctahedral tilting leads to doubling along its caxis. As a result ithas a larger unit cell (P1 a= 7.2674(23) , b= 13.452(4) ,c= 20.677(7) , a= 71.57(1), b= 84.21(1), c= 87.73(1)) com-pared to that of MIL-53(Fe)-INT (P1 a= 6.887 , b= 10.558 ,c= 13.466 ,a= 109.9,b= 88.06,c= 104.0). (See Supplementaryinformationfor figure).

The ethanol-washed and subsequently dried MIL-53(Sc) sample

has a similar diffraction pattern to the heated/hydrated material,indicating that ethanol washing has removed the DMF, and

subsequent loss of ethanol and uptake of moisture from the atmo-

sphere has given a hydrated intermediate MIL-53(Sc) material.

4.3. Structural flexibility of MIL-88(Sc)

PXRD patterns for MIL-88(Sc) are given inFig. 6. That of the as-prepared form can be indexed in the hexagonal cell typical of MIL-88, P6 2 c, with a= 11.18 , c= 19.47 . Comparison with publishedMIL-88 materials indicates a partially open framework containingDMF and the structure obtained from Rietveld refinement is giveninFig. 7. Desolvation gives a material with lower apparent crystal-linity and diffraction peaks which could not be indexed with a sin-gle cell, but rather suggest multiple states of closure of theframework structure. Immersion in methanol results in the struc-ture opening to a large degree (hexagonal, P6 2 c: a= 15.62 ,

c= 15.96 ), in a similar way to that previously observed for MIL-88B(Cr).

4.4. Synthesis of microporous MIL-100(Sc) and Sc-ABTC

Synthetic conditions for MIL-100(Sc) and Sc-ABTC are given inTable 2. MIL-100(Sc) is prepared using the 1,3,5-benzenetricarb-oxylic acid, at 293 K in DMF, and the same conditions with H4ABTCyielded Sc-ABTC. Although sufficient to identify the products aspure phases, the powder patterns are not of high enough qualityto permit structure refinement (seeSupplementary information).It was possible using the Le Bail method [30]to refine the cubicunit cell parameters as F d 3 m, a= 75.436(8) for MIL-100(Sc),and P4 3 n, a= 22.4567(11) , for Sc-ABTC. The structure types

for the solids MIL-100(Sc) and Sc-ABTC, are illustrated inFig. 8to-gether with that of MIL-101(Sc) to aid further discussions.

4.5. Thermal properties of scandium carboxylates

Some details of the thermogravimetric analyses are given inTable 5. The TGA of Sc2BDC3is similar to that reported previously[19]. Sc2BDC3 is hydrophobic and does not contain water or organicsolvents after synthesis. TGA of MIL-53(Sc) and MIL-88(Sc) (i)as-prepared and dried and (ii) after calcination in N2 at 623 Kand subsequently exposed to the laboratory atmosphere are givenas Supplementary information. In each case the structure begins todecompose at around 723 K, with most of the weight loss above773 K. In the as-prepared, heated materials there is some weight

loss up to 723 K (10.5% for MIL-53(Sc), 19.5% for MIL-88(Sc), whichis attributed to solvent and adsorbed water. After allowing

Fig. 3. Structure of MIL-53(Sc)-DMF, viewed down the channel axis. The framework is in a partially openstate, withdimethylformamide molecules ordered within the pores.All channelsare identical. (ScO6 octahedrain purple, C atomsblackspheres, O red, N blue andH, green). (For interpretation of the references to colour in this figurelegend, thereader is referred to the web version of this article.)

Fig. 4. X-ray powder diffraction patterns of MIL-53(Sc) in the following forms: (i)as-prepared in DMF, (ii) desolvated by heating, (iii) desolvated and allowed torehydrate in moist air, (iv) ethanol-washed and exposed to moist air.

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MIL-53(Sc) that had been pre-treated in N2 at623 K for 12 h topickup atmospheric moisture, TGA shows a gradual weight loss ofaround 5% up to 723 K, attributed to water uptake to give a mate-rial with stoichiometry Sc(OH)L.0.7H2O. Very little weight loss oc-curs below 723 K from similarly-treated MIL-88(Sc), indicatingthat the sample does not readily re-adsorb water.

Upon heating, as-prepared Sc-ABTC loses weight in five steps,and is complete by 723 K (Supplementary information). A weightloss of 10% occurs below 473 K, corresponding to solvent loss.

Above 523 K weight loss is associated with structural breakdownas the ligand decomposes, because this temperature is lower than

seen in other structures (such as MIL-88(Sc) or MIL-100(Sc) withsimilar trimeric scandium units. The TGA of as-prepared MIL-100(Sc) (Supplementary information) shows a weight loss of 15%up to 573 K attributed to solvent loss followed by structural break-

down between 623 and 723 K, MIL-101(Sc) displays similar behav-ior (Supplementary information) with a solvent loss of 10% up to573 K followed by structural breakdown between 673 and 773 K.

4.6. Solid-state NMR of scandium carboxylates

Microporous scandium carboxylates are amenable for analysisby NMR, as investigation using 1H, 13C and 45Sc NMR is possible.Scandium is quadrupolar (with spin quantum number I= 7/2)and readily studied by NMR (unlike paramagnetic Cr3+ and Fe3+,for example) and has been included in a wider range of structuresthan Al3+, Ga3+ or In3+. Representative solid-state 1H, 13C and 45ScMAS NMR of Sc2BDC3, MIL-53(Sc), MIL-88(Sc), MIL-100(Sc) and

Sc-ABTC from which solvent has been removed are shown inFigs. 911, respectively.

The1H MAS NMR spectrum of Sc2BDC3(Fig. 9a) shows a singleresonance at a chemical shift typical of aromatic protons, althoughcrystallographically-distinct protons cannot be resolved. In con-trast, the spectrum of calcined then hydrated MIL-53(Sc) inFig. 9b shows three main groups of signals. By comparison withthe spectrum of MIL-53(Al) in the literature,[4]two of these canbe tentatively assigned as corresponding to aromatic protons(7.2 ppm) and bridging hydroxyl groups (2 ppm). The third res-onance (at 5.9 ppm) decreases significantly when the material isdehydrated, suggesting it is related to incorporated water. For MIL-88(Sc) two resonances are observed, corresponding to aromaticprotons and bridging hydroxyl groups (and/or water molecules)

bound to the Sc3+

cations in the trimers. A very similar spectrumis obtained for MIL-100(Sc), with resonances corresponding to

Fig. 5. Comparison of the framework structures of desolvated thenhydratedMIL-53(Sc)-H2O (right) withas-prepared MIL-53(Sc)-DMF (left, above and below). In MIL-53(Sc)-H2O the structure distorts to give rows of closed and partially open channels as a result of associated tilts in the ScO 6octahedra and the connecting terephthalate units.

Fig. 6. X-ray powder diffraction patterns of MIL-88(Sc): (bottom) as-prepared,containing DMF, (middle) fully solvent exchanged with methanol and (top) heatedto remove solvent.

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aromatic protons and the hydroxyl/water protons in the trimerunit. In addition, a number of peaks with lower intensity are ob-served, attributed to residual solvent (DMF), which can also be ob-served in the13C MAS NMR spectra. Finally, the 1H spectrum of Sc-

ABTC (Fig. 9e) is dominated by an intense resonance at 8.6 ppm,associated with the aromatic protons, suggesting that charge bal-ance in this material is not maintained by hydroxyl groups but,as suggested in the literature, [30]by nitrate groups. The peakswith weaker intensity can once again be attributed to residualDMF solvent molecules.

The 13C MAS NMR spectra of the four terephthalates shown inFig. 10contain two main groups of peaks; aromatic carbons be-tween 120 and 140 ppm and carboxylate carbons between 170and 180 ppm. Within the aromatic region there are two groupsof peaks; those resulting from protonated carbons at lower chem-ical shift, and carbons attached to carboxylate groups typically athigher shift. Within each group of resonances further fine structurecan be observed, attributed to crystallographically-distinctterephthalate groups. Most notable, perhaps, is the 1:2 doublet-like structure of the resonances in the Sc2BDC3, reflecting the two

different terephthalate linkers (and their populations) in the struc-ture. It is also noticeable that the linewidths in this spectrum aremuch narrower than those found for MIL-53(Sc), MIL-88(Sc) andMIL-100(Sc), suggesting greater disorder in the latter cases. For

dehydrated MIL-53(Sc), a number of carboxyl resonances (at leastfour) are observed, suggesting many different environments for thelinker molecules. In a number of cases additional resonances asso-ciated with the residual solvent are also observed (typically thecarboxyl carbon in DMF). This is perhaps more clearly seen in the13C MAS NMR spectra shown in theSupporting informationwherethe spectral range has been expanded to include the alkyl region.For Sc-ABTC (Fig. 10e) there is a significant shift of the aromaticcarbon now attached to nitrogen (to 151.9 ppm).

The45Sc MAS NMR spectra inFig. 11show considerable varia-tion as a result of the different structural environments that Scadopts. For Sc2BDC3 a relatively narrow lineshape is observed,suggesting a small quadrupolar coupling, consistent with the pres-ence of isolated ScO6 octahedra. A quadrupolar broadened line-shape is present, however, (see inset) consistent with aquadrupolar coupling (CQ) of 4.5 MHz, an asymmetry (gQ) of 0.2

Fig. 7. Views of the framework of as-prepared MIL-88(Sc)-DMF, down (left) the b axis and (right) thecaxis. Atomic positions refined as disordered solvent molecules areomitted for clarity.

Fig. 8. Scheme showing the structures of the rigid scandium carboxylate MOFs that contain Sc3O(O2C-)6(H2O, OH)3trimers, MIL-100, MIL-101 and Sc-ABTC.

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and an isotropic chemical shift of 3.5 ppm. In contrast, the 45ScMAS NMR spectrum of MIL-53(Sc), in which the Sc3+ cations arepart of corner-sharing (ScO4(OH)2) chains shows a much broader

lineshape, characteristic of a larger quadrupolar interaction.Although the lineshape displays features characteristic of quadru-polar broadening, a number of distinct Sc species are expected tobe present. These cannot be resolved by high-resolution experi-ments (not shown) as they have very similar NMR parameters,but are evident as fine structure in the spectrum. Although a de-tailed analysis of NMR parameters is prevented by the presenceof multiple overlapped resonances, the width of the spectral reso-nance is consistent with a quadrupolar coupling of 16 MHz (con-siderably larger than for Sc2BDC3) and an asymmetry of 0.3.There is also a significant change to the isotropic chemical shift(note this is different to the resonance position for a quadrupolarnucleus) to 59 ppm. The spectrum of MIL-88, in which the build-ing units are trimers shows an asymmetric lineshape, typical of adisordered material (resulting in a distribution of quadrupolarand chemical shift parameters). This arises as each Sc3+ is coordi-nated to one l3O, four carboxylate oxygens and either an OH groupor a water molecule (in the ratio 1:2). The spectra for MIL-100(Sc)and Sc-ABTC are both similar to that of MIL-88(Sc), particularly themuch higher chemical shift, also reflecting the presence of scan-dium trimers in these materials.

4.7. Gas adsorption on scandium carboxylates

Adsorption on the scandium carboxylates prepared in this studywas performed using N2and CO2(Table 5). Sc2BDC3and the struc-ture types MIL-100, MIL-101 and M-ABTC with other metal cationsare known to be rigid, highly porous materials, whereas MIL-53

and MIL-88 materials are known to be able to breathe with certainadsorbates.

4.7.1. Rigid frameworks

Sc2BDC3has previously been shown to have a pore volume forN2 of 0.26 cm

3 g1 (assuming adsorbed nitrogen has the densityof liquid nitrogen at the same temperature) [19,20]. Nitrogenadsorption on MIL-100(Sc) stirred in MeOH for 72 h, filtered andevacuated at 453 K shows an isotherm typical of this structure,the porosity of which extends into the mesoporous regime(Fig. 12). The measured pore volume is 0.72 cm3 g1 which whilsthigh, is considerably lower than that observed for MIL-100(Cr)which achieves 1.10 cm3 g1 [1]. Sc-ABTC, by contrast, has a porevolume for N2of 0.57 cm

3 g1, which is slightly larger than that re-

ported for In-ABTC (0.54 cm3

g1

). Adsorption of N2on the mixturecontaining MIL-101 does demonstrate some microporosity after

activation by heating at 293 K for 3 h (0.16 cm3 g1), although thisis much less than that observed for MIL-101(Cr), which approaches2.0 cm3 g1 after activation[2].Attempts to improve the porosity

of this material are ongoing.The adsorption of CO2 on these rigid frameworks was per-formed up to 1 bar at 196 K, close to its normal sublimation tem-perature, for the adsorbents prepared in pure form (Fig. 13).Sc2BDC3has a maximum uptake of 4.2 mmol g

1, Sc-ABTC a max-imum uptake in excess of 13 mmol g1, and MIL-100(Sc) uptake inexcess of 21 mmol g1 close to that observed for MIL-100(Cr).

4.7.2. Flexible frameworks

The adsorption isotherm of N2at 77 K on pre-heated MIL-53(Sc)evacuated at 573 K at 104 mbar prior to analysis does not demon-strate microporosity, indicating that MIL-53(Sc) is in a closed formunder these conditions. Similarly, the 77 K isotherm of N2on des-olvated MIL-88(Sc) shows very little uptake, indicating the struc-

ture is in its closed form. Notably, very recently publishedstudies show that the synthesis of MIL-88(Sc), followed by solventexchange with acetone and desolvation under vacuum [22]gives asolid with permanent porosity to N2of 0.25 cm

3 g1, indicating theimportance of the activation procedure.

CO2adsorption isotherms measured on MIL-88 at 196 K dependstrongly on the sample and its pretreatment. In general, low up-takes are observed, with values at p/po= 0.40 ranging from 0 to3.3 mmol g1. One explanation for measured uptakes is that undersome pre-treatments the lattice has not completely closed, as indi-cated by the multiphase XRD, leaving parts of the sample slightlyopen to CO2 adsorption. The uptake is much lower than that ex-pected if the structure were to open fully, as it is observed to doupon uptake of methanol. The higher methanol uptake results from

strong methanol-MIL-88(Sc) interactions.For MIL-53(Sc), appreciable adsorption was achieved for CO2at

196 K, although equilibration times were in some cases more than4 h per point on the isotherm(Fig. 14). Although there is almost nouptake below 30 mbar, a first step is observed beginning at50 mbar, with adsorption reaching 2.2 mmol g1 at 600 mbar. Asecond, much larger step (to 13.2 mmol g1) was observed to beginatca. 750 mbar, which showed a large hysteresis upon desorption.Shorter equilibration times were observed during measurement ofa second isotherm, at 268 K. Again an initial region of very low up-take is followed by a step to 2 mmol g1. This step occurs at800 mbar at this temperature.

In a separate set of experiments that were continued to higherpressures, isotherms were collected at 196, 233 and 310 K (Fig. 15).

Similar behavior is observed at 233 K as at 196 K, with an initialstep of 2 mmol g1 uptake being followed by a second, larger step

Table 5

Details of thermogravimetric analysis and adsorption performance of microporous scandium carboxylates, compared with related solids.

Structure(metal)

Solventloss/%

Dec.temp/K

N2uptake at 77 K (p/po= 0.9)/mmol g1

CO2 uptakeat 196 K (p/po= 0.40)/mmol g1

Activation conditions for samples inthis work

MIL-53(Sc) 7.30 703 1.35 13.15 (195 K) 623 K in air for 12 h, outgas at 573 Kin vacuo

MIL-53(Al) 36.00 773 15.62 10.5 (303 K 3 MPa)[4,28] MIL-88(Sc) 19.27 723 1.36 13 (195 K) 623 K in air for 12 h, outgas at 573 K

in vacuoMIL-88(Cr) 18.00 673 N.A N.A MIL-100(Sc) 12.70 623 27.0 21.34 (195 K) Stir in MeOH for 72 h, outgas at

453 K in vacuoMIL-100(Cr) 28.00 523 31.25 19.5 (303 K 3 MPa)[1] Sc-ABTC 17.88 513 16.46 13.05 (195 K) 473 K in air for 12 h, outgas at 393 K

in vacuoIn-ABTC N.A N.A 15.62 9.5 (273 K 3 Mpa)[23,24] Sc2BDC3 0 773 6.97 5.5[20] 623 K in air for 12 h, outgas at 573 K

in vacuo



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at higher pressure (>4 bar) that achieves over 13 mmol g1. At310 K, no sharp second step is observed, but rather a gradual in-crease of uptake with increasing pressure.

Our explanation for the MIL-53(Sc) CO2adsorption data is thatafter degassing the structure is in a fully closed form, as observedfor MIL-53(Fe)[11]. As the partial pressure of CO2 is increased, avalue is reached where an intermediate is formed, with one setof closed channels and one set of partially open channels that

can take up CO2molecules. This is likely to be similar to the struc-ture seen for the hydrated MIL-53(Sc) determined in this structure.

Further increase in the pressure of CO2results in full opening of thestructure to a large pore form, observed for MIL-53(Al and Cr). Thisopening occurs in a step at pCO2 ofca. 800 mbar at 196 K and at4 bar at 233 K, but only gradually at 310 K.

5. Summary

The scandium terephthalate MOFs Sc2BDC3, MIL-53(Sc) andMIL-88(Sc), the scandiumtrimesate MIL-100(Sc) and the scandium3,30,5,50-azobenzenetetracarboxylate Sc-ABTC (socMOF) have beenprepared pure by careful optimization of reaction conditions, and

the scandium terephthalate MIL-101(Sc) has also been preparedwithin mixtures. The synthesis of these structures in Sc-form is

Fig. 9. 1H (14.1 T)MASNMRspectra of(a) Sc2BDC3, (b) MIL-53(Sc) bothdehydratedand after subsequent rehydration, (c) MIL-88(Sc), (d) MIL-100(Sc) and (e) Sc-ABTC.Spectra were recorded at MAS rates of (a, c, d and e) 30 kHz and (b) 60 kHz.

Fig. 10. 13C (14.1 T)CP MAS NMR spectra of(a) Sc2BDC3, (b) MIL-53(Sc) dehydrated,

(c)MIL-88(Sc), (d)MIL-100(Sc) and(e) Sc-ABTC. The MAS rate was 10 kHz. Spinningsidebands are each marked with a .



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facilitated by the similarity of the ionic radius of Sc3+ to the radii ofthe trivalent cations that have previously been observed to formthem.

At moderate temperatures (373413 K), MIL-88(Sc) is the pre-ferred terephthalate to form, and MIL-101(Sc) is also observed.These structures contain the trimeric Sc3l3O unit, as previouslyobserved by Dietzel et al. and Ibarra et al.[21,22]whereas crystal-lisation of scandium terephthalates at higher temperatures giveseither MIL-53(Sc) (which contain chains of ScO6 octahedra), or

Sc2BDC3, which contains isolated ScO6 octahedra. The trimericSc3l3O unit is also observedin Sc-ABTC however, whichforms over

a wide temperature range.45Sc MAS NMR spectra are strongly char-acteristic of the environment of ScO6octahedra in these materials.Whereas isolated octahedra and edge-sharing octahedra give well-defined quadrupolar lineshapes, trimers give disordered line-shapes, resulting from the disordered distribution of two water

molecules and one hydroxyl group over the three Sc3+

cation sites.Among the structures, there are both rigid and flexible frame-

works, and thermal stabilities are at the top of the range observedfor those solids also prepared with other metals. The rigid frame-works MIL-100(Sc) and Sc-ABTC demonstrate high porosities(0.72 and 0.57 cm3 g1, respectively) similar to those observedfor their Cr and In analogues. MIL-88(Sc) behaves similarly to otherMIL-88 materials, showing major expansion upon exposure to po-lar molecules such as methanol, but we observed lowuptakes of N2and CO2using our activation methods.

MIL-53(Sc) is, like other MIL-53 materials, highly flexible. It hasbeen possible to determine the structure of MIL-53(Sc) containingsolvent DMF molecules, in which the framework structure is par-tially open, leaving space for the packing of the DMF molecules

in the channel. However, MIL-53(Sc) behaves differently fromother MIL-53 materials upon desolvation and in its adsorption

Fig. 12.Adsorption isotherms of N2 at 77 K onthe rigid scandiumcarboxylate MOFsSc2BDC3(d), Sc-ABTC (N) and MIL-100(Sc) (j).

Fig. 13. Adsorption and desorption branches of isotherms of CO2 at 196 K on therigid scandium carboxylate MOFs Sc2BDC3 (d), Sc-ABTC (N,) and MIL-100(Sc) (j)(open symbols for desorption branches).

Fig. 11. 45Sc (14.1 T) MAS NMR spectra of (a) Sc2BDC3, (b) MIL-53(Sc) dehydrated,(c)MIL-88(Sc), (d)MIL-100(Sc)and (e)Sc-ABTC. Spectra were recorded at MAS ratesof (a, c, d and e) 30 kHz and (b) 60 kHz.



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properties. Complete removal of solvent gives a closed phase (MIL-53(Sc)-CAL) that does not adsorb N2. Further work is ongoing todetermine this structure, but the formation of a closed structureupon desolvation/dehydration is much more similar to the behav-ior of MIL-53(Fe) than to that of MIL-53(Cr or Al) which open up.The uptake of water molecules from the atmosphere by MIL-53(Sc)-CAL gives rise to a novel triclinic structure with both closedand slightly open channels, in which there is tilting of alternate

rows of terephthalate groups associated with the incorporationof larger Sc3+ cations into the octahedral positions.The adsorption behaviour of MIL-53(Sc)-CAL for N2and CO2can

be discussed in the light of this observed structural behaviour. Thefully dehydrated material is unable to take up N2 or CO2 becausethe pores are too small. As the chemical potential of gaseous CO2isincreased, the structure opens in two steps. In the first step, around2 mmolg1 ofCO2 is admitted (possiblyas the framework adapts tobe similar to that determined for the hydrated sample) and in thesecond step the structure opens fully to take up >13 mmol g1.

In summary, a series of microporous scandium carboxylates hasbeen prepared, members of which exhibit variously high thermalstability and both rigid and flexible frameworks capable of highadsorption uptake and novel adsorption behaviour. They provide

a promising family of solids suitable for ongoing studies into appli-cation in adsorption, separation and catalysis, bearing in mind thatthe terephthalate groups can readily be included functionalisedand that loss of water molecules from scandium based trimericbuilding blocks gives coordinatively unsaturated Sc3+ cations,known to be active in Lewis acid catalysis.

Acknowledgements

We gratefully acknowledge the EPSRC for funding (VRS, PAW,SEA, EP/EO41825/1; JPSM).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2010.12.016.

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Fig. 14. Adsorption isotherms (up to 1 bar) of CO2 on dehydrated MIL-53(Sc) at196 K (closed squares, adsorption; open squares, desorption) and 268 K.

Fig. 15. Adsorption isotherms (up to 20 bar) of CO2on dehydrated MIL-53(Sc).

http://dx.doi.org/10.1016/j.micromeso.2010.12.016http://dx.doi.org/10.1016/j.micromeso.2010.12.016

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Synthesis of MIL 88(Sc)