Monitoring the Activation Process of the Giant Pore MIL-100(Al) by Solid State NMR

Published: August 10, 2011

r 2011 American Chemical Society 17934 dx.doi.org/10.1021/jp206513v | J. Phys. Chem. C 2011, 115, 17934–17944

ARTICLE

pubs.acs.org/JPCC

Monitoring the Activation Process of the Giant Pore MIL-100(Al) bySolid State NMRMohamed Haouas,*,§ Christophe Volkringer,†,‡ Thierry Loiseau,†,‡ G�erard F�erey,† and Francis Taulelle§

Institut Lavoisier de Versailles (UMRCNRS 8180), §Tectospin Group and †Porous Solids Group, Universit�e de Versailles St Quentin enYvelines, 45, avenue des Etats-Unis, 78035 Versailles, France

bS Supporting Information

’ INTRODUCTION

Porous hybrid organic�inorganic crystalline materials providea rich and various multidimensional architectures with tunablechemical and physical properties suitable for many applications.1�4

In particular, their interest in the field of green chemistry ispromising regarding their potential use as a solid catalyst or catalystsupport similar to zeolites and mesoporous materials.5�10 Theirability in insertion and trapping of large wasted organic mol-ecules is unique, due to their adsorption property directly relatedto the specific structural design of their pore and channel systems.Some of them exhibit very high apparent surface area values with thepresence of extra-large pore highly crystalline frameworks.11�15

Besides the adsorption property, acidity is also a significantfeature in catalysis. The presence of Lewis acidic sites in thehighly porous chromium(III) trimesate MIL-100 has been ob-served in a CO sorption IR study.16,17 The authors showed thatthe departure of terminal water molecules attached to the μ3-oxocentered trimeric units Cr3O is observed upon heating. Initially,chromium cations are 6-fold coordinated and upon dehydrationthe formation of some 5-fold coordinated chromium atomsoccurs without destruction of the structure. Such coordinativelyunsaturated sites (cus) are at the origin of the Lewis acidityproperties.18,19 Their amount was quantified using CO as a

probe molecule. Among the three chromium atoms constitut-ing the μ3-O trimer, two of them were each bound to oneterminal water molecule, and the third was bonded to a fluorineatom (coming from the HF present in hydrothermal reac-tion medium) or a hydroxide group (Figure 1). Therefore, theglobal framework chemical formula of MIL-100(Cr) was de-duced to be CrIII3OFx(OH)1�x(H2O)2 3 {btc}2 (btc stands for1,3,5-benzenetricarboxylate). It should be mentioned that thecrystalline structure of MIL-100 was initially solved by means ofthe AASBU method.20 These materials showed a remarkablestability toward water adsorption.21 The catalytic performance inLewis acid reactions on someMOFs has been demonstrated.22,23

Aluminum-based MOF presents many advantages with respectto their use as heterogeneous catalyst, among them their low cost,remarkable chemical and thermal stability, and high Lewisacidity. Even though trivalent metal-based MOF are rather rarecompared to those with divalent metal, the number of Al-containing MOFs content is continuously increasing.24�35

Received: July 10, 2011Revised: August 9, 2011

ABSTRACT: The structure of mesoporous MIL-100 consists of Al3 aluminumoctahedra trinuclear units sharing a common oxo vertex μ3-O, interconnected through1,3,5-benzenetricaboxylate (btc) ligands. Each Al is bonded to four frameworkcarboxylate functions, and the corresponding octahedra exhibit therefore a terminalposition occupied either by a water molecule or by a hydroxide group. The ability of areversible removal of coordinated water upon dehydration/rehydration process hasbeen investigated using solid state NMR techniques. Double resonance techniquessuch as 1H{27Al} TRAPDOR (transfer population in double resonance) and 27Al-{1H} HETCOR (heteronucleus correlation) were used to probe the proximitybetween species containing protons and the inorganic framework. On the otherhand, 1H�1H correlation experiments using the DQ-BABA and RFDR sequences were employed to investigate the interactionbetween the different kinds of protonic species, including those of the organic framework. The compound shows a remarkablethermal stability up to 370 �C, with small structural alterations leading to a lowering of its crystallographic symmetry. Only one watermolecule per Al3 trimer was found to leave the trimer, producing only one coordinatively unsaturated site (cus) at 350 �C.Compared to the case for chromium isotype MIL-100(Cr), the difference of generated number of cus site per trimer explains at thesame time the different origin of the stability of the aluminum trimer versus chromium, and the different chemistry that it will inducebetween both MIL-100(Al) and MIL-100(Cr). The as-synthesized compound contains an important amount of extra-framework(EF) trimesic acid encapsulated into the large pores, and most of it could be removed upon DMF/water activation. However,0.3�0.5 molecules of (H3btc)EF per Al3 trimer were found strongly interacting with the framework and their complete removal wasdifficult to achieve.

17935 dx.doi.org/10.1021/jp206513v |J. Phys. Chem. C 2011, 115, 17934–17944

The Journal of Physical Chemistry C ARTICLE

The aluminum analogue of MIL-100 was successfully pre-pared recently without using the fluoride route.34 The three-dimensional open framework revealed the arrangement of super-tetrahedral (ST) building blocks based on the MTN topologyrelated to the zeolite ZSM-39. Each STmotif is built up from fourμ3-oxo-centered trinuclear unit (Al3O) located at each cornerand linked to the four btc ligands located on each face of thetetrahedron. In contrast to the chromium and iron analoguecompounds, no terminal fluoride ions can be found partiallysubstituting some hydroxide groups. The primary objective ofthis contribution is the investigation by solid state NMR of thereactivity of these trimeric units μ3-oxo centered for the alumi-num-based compound MIL-100(Al). By NMR spectroscopy ithas been possible to quantify cus Al sites and assess unambigu-ously the coordination state of the Al nucleus.36,37

’EXPERIMENTAL SECTION

Materials. MIL-100(Al) was hydrothermally synthesized ac-cording to a previously published procedure.34 The as-synthe-sized product was activated following a two-step process. First,the solid was treated in anhydrous dimethylformamide (DMF) at150 �C (5 h) under solvothermal conditions. Typically, 1 g ofsolid per 100 mL of DMF was used. The aim of this step was todissolve into DMF the extra-framework organic species (usuallyhardly soluble in water) entrapped within the pores of the solid.The second step consisted of separating the resulting solid andtreating it again under reflux overnight in a suspension of water.The amounts used were 1 g of solid per 1 L of distilled water. Thisstep allows exchange of entrapped molecules between DMF andwater. The efficiency of the activation procedure was checkedusing various techniques including, elemental analysis, IR, TGA(thermogravimetric analysis), and solid state NMR.In situ thermodiffraction experiments were carried out on a

Bruker AXS D5005 powder diffractometer using a diffracted-beam-graphite monochromator (Cu KR) and equipped with anAnton Paar HTK1200 oven camera. X-ray powder diffractiondata were collected in air between 20 and 600 �Cover the angularrange 2�18� (2θ) with a counting time of 2 s step�1 and a steplength of 0.04� (2θ).The TG experiments were carried out on a TA Instruments

type 2050 thermoanalyzer TA under oxygen gas flow with aheating rate of 1 �C min�1.Solid State NMR. The 27Al, 13C, and 1H magic angle spinning

(MAS) NMR spectra have been obtained at a resonancefrequency of 130.3, 125.8, and 500.1 MHz, respectively, using aBruker Avance 500 MHz spectrometer. Chemical shifts arereferenced to tetramethylsilane (TMS) for 1H and 13C, and 1 M

aqueous Al(NO3)3 for27Al. All NMR experiments have been

performed using a 2.5mmMAS probe in a zirconium oxide rotor.The 1HMAS NMR spectra have been acquired with a spectral

width of 30 kHz, a 45� flip angle, a recycle delay time of 3 s and asample spinning rate of 30 kHz. A total of 80 transients wereaveraged for each spectrum. The 1H T1 have been measured withthe a saturation-recovery method and found to be less than 3 s.1H 2DDQMAS experiments were performed with four cycles ofthe back-to-back (BABA) recoupling sequence.27 The t1 incre-ments were set equal to one rotor period. The RFDR experi-ments were acquired with NS = 16, t1 increment = 66.6 μs, andmixing times τm ranging from 0.266 and 5.33 ms, leading to atotal acquisition time of a 2D spectrum of ca. 5 h. One 180� pulseper rotor period was introduced during the mixing time.38 In theTRAPDOR (transfer population in double resonance) NMRexperiments,39,40 a spin�echo pulse was applied to the 1H channelwith simultaneous irradiation of 27Al almost on-resonance, duringthe first period of evolution time (τ). In theory, the continuousirradiation reintroduces the effects of the heteronuclear dipolarcoupling that are normally removed by magic-angle samplerotation. Two experiments, i.e., without and with 27Al irradiation,are performed for one rotor cycle, and the difference betweenthese two spectra reveals the signals of the protons in closeproximity to aluminum atoms. An 27Al radio frequency (rf) fieldamplitude of 131 kHz was used.The 27Al MAS NMR spectra have been acquired with a

spectral width of 30 kHz, and a pulse width of 0.4 μs (about15� tip angle for selective central transition), a recycle delay timeof 0.1 s, and a spinning rate of 30 kHz. A total of 4k scans wereaveraged for each. The 2D 27Al{1H} HETCOR (heteronucleuscorrelation) (CPMAS, cross polarization MAS) NMR spectrahave been acquired with a range of contact times between 0.25and 4 ms to gain information about the spatial relations betweendifferent proton groups and Al. The experiments have beenperformed using a high spinning rate ωr of 30 kHz and a low rffield strengths ω(27Al) and ω(1H) for the contact pulses, i.e.,5 and 42 kHz for 27Al and 1H, respectively, which correspond toHartman�Hahn conditions: ω(1H) ≈ ω(27Al)Sel + ωr orω(1H) ≈ (2I + 1)ω(27Al) + ωr. A recycle delay of ca. 1 s anda spectral width of 30 kHz have been used. A total of 512 scanswere averaged for each of 77�32t1 incrementss (144�64 FID),with the latter set to one rotor period (33.3 μs). The spectralwidth for the F2 dimension was set to 30 kHz. For the 2D NMRexperiments, the Sates�Haberkorn�Ruben (hypercomplex)method has been used to achieve quadrature detection in t1.

27AlMQMAS (multi-quantum MAS) NMR experiments were per-formed with the standard z-filter scheme, a single continuous-wave pulse for excitation of triple quantum (3Q) coherences, andthe FAM II sequence41 for 3Qf 0Q conversion. The excitationand refocusing pulses were obtained using a rf field of 118 kHz,whereas the selective π/2 pulse in the z-filter used a rf field of5 kHz. The dwell time in t1 (isotropic) dimension of MQMASmeasurements was synchronized with the spinning rate of therotor. Data acquisitions were performed using 2016 scans pert1 step and a 0.1 s delay between scans. 27Al spectra were fittedwith the DMFIT software package42 to decompose into indivi-dual contributions.In 13C{1H} CPMAS experiments, values of νRF (

13C) and νRF(1H) during CP and νRF (

1H) during XiX decoupling were 68.4,55.9, and 64 kHz, respectively. The number of scans (NS) was 4096.Several samples of activated materials with different dehydra-

tion levels have been studied by thermal treatment at different

Figure 1. Hydration/dehydration process on coordinatively unsatu-rated sites (cus) in Cr3(μ3-O) trimer of MIL-100 from ref 16.



temperatures ranging from 130 to 350 �C. The starting solid wasfirst calcined in static air at a given temperature for 4 h. Thesample was then quickly (within 5 min) packed in the NMRrotor, and the filled rotor was placed again in the oven withoutthe caps at the same temperature for an additional 1 h. The rotorwas finally sealed immediately (within 1�2 min) with tightTorlon caps. This procedure was found to be efficient enoughwith respect to water reuptake from ambient moisture. Nosignificant rehydration occurred during the NMR experiments.According to 1H NMR less than 1% of rehydration has observedafter 4 days of NMR measurements under MAS. To check thedehydration/rehydration cycle, the calcined sample was exposedto a moisture atmosphere in a desiccator over saturated NaClsolution and analyzed again by NMR.

’RESULTS AND DISCUSSION

Activation of MIL-100. Activation is a key process to prepareuseful solid porous materials as catalysts, adsorbents, and mo-lecular sieves. The procedure for MOFs has been rarely inves-tigated43 except for their implication in the alteration of thesurface chemistry.44 Without prior activation, the presence ofoccluded species within MOF compounds could seriously blockaccess to their pore system, reducing considerably their potentialsurface area. For instance, anions such as NO3

�, Cl�, SO42�, etc.

present initially in the reagents of the metal source could betrapped within the cages and channels of the solid. Also, neutralmolecules (mainly the solvent used) could be present but theirdeparture is usually easy and can occur simply by mild thermaltreatment when their saturated vapor pressure is low enough.The most difficult species to remove are the extra-frameworkligands, especially when they still interact with the inorganicnetwork. Such interactions will be particularly strong if the ligandis directly coordinated to the metal of the structure. For the samestructural topology involving different metals, the strength ofthese interactions will be a function of the Lewis acidity of themetallic site. For instance, one can expect a general increase ofsuch affinity changing the metal from chromium to iron oraluminum. This is indeed the case in the series ofMIL-100 phaseswhere no activation was found necessary forMIL-100(Cr),45 anddifferent activations are needed for MIL-100(Fe)5 and MIL-100(Al). Only one activation step was necessary for the former,5

and two steps were needed for the later. A detailed study of theactivation of MIL-100(Al) is presented below using varioustechniques to characterize the materials at different stages.The elemental analyses of the as-synthesized and the final

activated samples are compared to the theoretical values forMIL-100(Al) in Table 1. The effect of activation on the chemicalcomposition is clear, showing a significant decrease of C and Ncontents due to removal of most entrapped species in the pores.

When the results of activated sample are compared with thetheoretical values expected for the same aluminum content,corresponding to a general formula of Al3O(OH)(btc)2(H2O)29,there is still 3% extra C within the sample, meaning that a fractionof extra-framework H3btc could not be removed by the proce-dure used. Traces of N are also present that could be due to eitherresidual DMF used in the first step of the activation or someNOx

species from aluminum nitrate used as reagent for the synthesis.It is interesting to notice that removal of the extra-frameworkH3btc is accompanied by an increase of adsorbed water pre-sumably filling the pore volume. This replacement occurs with afixed ratio of 15H2O/1H3btc, which leads to the two observedsituations: Al3O(OH)(btc)2(H2O)16(H3btc)0.9(HNO3)1.3 andAl3O(OH)(btc)2(H2O)22�25(H3btc)0.3(HNO3)0.2 for as-synthe-sized and activated materials, respectively (see Table S1 in Support-ing Information for details). This fits nicely with the calculated for-mula with only water as guest species: Al3O(OH)(btc)2(H2O)29.The IR spectrum of as-synthesizedMIL-100(Al) (Figure S1 in

Supporting Information) is consistent with the presence of extra-framework H3btc. Indeed, bands in the region 1800�1300 cm�1

assigned to C�O bonds of Al coordinated trimesate ligands areobserved at 1682, 1670, and 1404 cm�1, and others assigned toCdO (1720 cm�1) and C�O (1349 and 1273 cm�1) functionsof free trimesic acid entrapped within the pores are also present.The spectrum of the intermediate DMF-activated sample (FigureS2 in Supporting Information) shows a decrease of the lattergroup of bands with the appearance of new bands (1660 and1250 cm�1) due to DMF molecules. The spectrum of the finalactivated sample (Figure S3 in Supporting Information) pre-sented bands of framework trimesates due to carboxylate C�Ovibrations at 1683, 1668, 1422, and 1403 cm�1. The strong bandassigned to OH vibrations of water molecules (3428 cm�1)appears now much more intense indicating the presence of alarge amount of water molecules after removal of bulky organicspecies.Further information was obtained from thermogravimetric

data. TGA curves of as-synthesized, one-step DMF activated, andtwo-step DMF�water activated MIL-100(Al) (Figure 2)showed globally two-step weight losses. The first step up to400 �C corresponds to the departure of occluded species, andthe second occurring from 400 to 500 �C is due to the collapse ofthe organic framework. An easier departure of water in the20�200 �C range was observed for the final activated samplethan the intermediate activated sample and than the as-synthesized

Table 1. Chemical Composition in As-Synthesized and Ac-tivated MIL-100(Al) from Elemental Analyses Compared toTheoretical Values for a General Formula ofAl3O(OH)(btc)2(H2O)29

as-synthesized activated theoretical

Al 7.40 7.78 7.78

C 28.76 23.75 20.74

N 1.60 0.23 0.00

H 3.97 5.11 6.15

Figure 2. TGA curves (under O2, 1 �C/min) of (a) as-synthesized,(b) one-step DMF activated, and (c) two-step DMF-water activatedsamples of MIL-100(Al).



sample, consistent with the presence of bulky species partiallyblocking the pores in the as-synthesized and to a lesser extent inthe one-step activated materials. Additionally, the three samplespresent appreciatively the same amount of extra-frameworkspecies because they showed the same final weight loss (ca. 85wt %). A similar observation has been reported for MIL-110,44

indicating that in all cases the pore volumes are fully occupied byguest species, where water replaces other extra-framework spe-cies (see Table S2 in Supporting Information for details). Asexpected upon activation, the majority of extra-framework spe-cies are washed out, and the chemical composition changed fromAl3O(OH)(btc)2(H2O)16(H3btc)0.9(HNO3)0.2 to Al3O(OH)-(btc)2(H2O)24(H3btc)0.1 at the end of activation process.Activation of MIL-100(Al) was also monitored by 1H solid

state NMR (Figure 3). The spectrum of the as-synthesizedsample (Figure 3a) displays three main signals at 4.6, 8.0, and9.1 ppm assigned to water, aromatic protons of extra-frameworkH3btc, and framework btc, respectively.34 A similar spectrum hasbeen reported recently for MIL-100(Sc), with resonances corre-sponding to aromatic protons and the hydroxyl/water protons inthe trimer unit as well as an additional number of peaks withlower intensity attributed to residual synthesis solvent (DMF).46

The TRAPDOR experiment (Figure 3b) shows that only thesignal of water was significantly affected as a consequence ofproximity of some protons to the 27Al nuclei within ca. 2�3 Å,i.e., those corresponding to water directly connected to trimericAl units (Al�OH2). Qualitatively, this is a clear indication thatthe signal of water bound to Al overlaps with the large signal ofunbounded adsorbed water molecules, although under certaincircumstances, some semiquantitative data can be extracted withREDOR-type experiments.47 The spectrum of the activatedsample (Figure 3c) shows an important decrease of intensity ofthe 8 ppm signal as a result of the removal of a major part of theextra-framework H3btc. The resolution was also enhanced,allowing distinction of a couple of signals of aromatic protonsin different local environments of extra-framework H3btc. Thebroadening of signals due to extra-framework H3btc could resultfrom chemical shift distribution caused by random positions ofsuch species filling the pores. The weak narrow peaks ranging

from 0.3 to 1.6 ppm, which are partially overlapped with thestrong signal of water, are tentatively assigned to nonidentifiedDMF decomposition products, as will be clearly demonstratedlater from correlation experiments (1H�1H and 1H�27Al) thatthey are isolated species and do not interact with the framework.On the basis of quantitative analysis, the chemical formula of thesolid can be established as follows: Al3O(OH)(btc)2(H2O)18-(H3btc)1.0 before activation, and Al3O(OH)(btc)2(H2O)24-(H3btc)0.3 after activation, in agreement with results of othertechniques including IR, TGA, and elemental analysis. Decom-positions of the corresponding spectra are shown in the Support-ing Information (Figure S4).To gain more information on interactions between the

different surface protonic sites, 2D SQ-DQ BABA and SQ-SQRFDRNMR experiments were conducted on the activated MIL-100(Al) sample (Figure 4). The 1H�1H DQ (Figure 4a) and1H�1H RFDR (Figure 4b) experiments provide complementaryinformation, where the 2D DQ-SQ spectrum provides short-range interaction within a few angstroms, whereas the 2D SQ-SQRFDR spectrum provides much longer range interaction typi-cally up to 12 Å with long mixing times (dozens of milliseconds).The former maps zero-quantum to single-quantum correlations,and the latter maps double-quantum to single-quantum correla-tions. Moreover, the DQ experiment enables autocorrelationsbetween identical crystallographic sites. The DQ spectrumshowed just one signal, the autocorrelation involving the rigidframework btc signal. The other signals did not show any DQcoherence, indicating their involvement in very weak dipolar1H�1H interactions, mainly due to the mobility of the extra-framework species. The RFDR experiment enabling long mixingtimes showed, however, all the signals and even with a higherresolution than the 1D spectrum. Mainly two cross peak correla-tions are visible, both involving the signal of framework btc. Theyindicate the spatial proximity between the framework btc andoccluded species into the cages, i.e., the extra-framework H3btcand adsorbed water. Magnetization transfer between protons ofbtc framework and extra-framework species could also be due tosome spin diffusion. This is clear evidence of involvement of boththe extra-frameworkH3btc and water in some specific interactionwith the network. Such a strong interaction with the frameworkexplains their difficulty to be removed by activation.The heteronuclear 1H�27Al correlation experiment shown in

Figure 5 provides information on the local interaction of protonicspecies with the inorganic part of the framework occurring on theactivatedMIL-100(Al) sample. The 27Al NMR spectrum ofMIL-100(Al) consisted of three signals corresponding to the sevencrystallographical independent octahedral Al sites of the struc-ture.34 The 27Al{1H}HETCOR spectra exhibit all 27Al signals inthe direct dimension (see Figure S5 in Supporting Informationfor spectral decomposition of the F2 projection) and

1H signals offramework btc and water in the indirect dimension. This meansthat only protons of both species are involved in CP transfer.Signals of extra-framework H3btc were not observed, presumablydue to their mobility. The signal of water is stronger with shortercontact time experiment, whereas those of the aromatic frame-work btc increase at longer contact time. This is due to tworeasons. First, the protons of water are closer to framework Althan those of aromatic btc when they are bonded to Al. Second,the water molecules are more mobile than the rigid frameworkbtc, confirmed when the corresponding proton T1F values arecompared, which are 0.9 and 2.9 ms for water and framework btcsignals, respectively.

Figure 3. 1H NMR (a) MAS and (b) 1H{27Al} TRAPDOR spectra ofas-synthesized MIL-100(Al) and (c) MAS spectrum of activated MIL-100(Al).



Thermal Behavior. The thermodiffractogram of MIL-100-(Al) (Figure S6 in Supporting Information) reveals that thecrystallinity was preserved upon heating to 370 �C. This isconsistent with the TGA results, which indicate a structural collapseoccurring at 370�400 �C (Figure 2). However, upon heating, therewas a progressive increase of the intensity of some small angle Braggdiffraction peaks that could reflect a minor structural evolution dueto the dehydration. The resolution of the thermodiffractograms islimited and does not allow us to follow the space group changeexpected by the change in coordination for aluminum. It isinteresting to mention that the aluminum-based compound MIL-100(Al) showed higher thermal stability than their chromium andiron isotypes, which loose their crystallinity for the same measure-ment conditions at 325 �C16 and 270 �C,5 respectively.

Figure 6 shows 27Al and 1H MAS NMR spectra of activatedMIL-100 at successive dehydrated forms, from fully hydrated(Figure 6a) to fully dehydrated (Figure 6e�g) and after sub-sequent rehydration (Figure 6h). Spectral decompositions areshown in the Supporting Information (Figure S7). The amountof water molecules within the structure can be monitoredquantitatively from the 1H NMR spectra. We can see clearlythat increasing the temperature of treatment reduces progres-sively the signal of water with a slight shift from 4.6 to 4.9 ppmprobably related to a change in H-bonding structure.48 After 4 hat 130 �C all physisorbed water molecules are evacuated and onlystructure water molecules are still present, characterized by ahigher field chemical shift. The remaining 8% of the initial total

Figure 5. 2D 27Al{1H} HETCOR spectra of activated MIL-100(Al)with (a) 0.25, (b) 1, and (c) 4 ms contact time.

Figure 4. 2D 1H�1H (a) back-to-back and (b) RFDR spectra ofactivated MIL-100(Al). Signals (*) are due to DMF decompositionproducts. The mixing time was 0.27 and 2.7 ms, respectively.



intensity signal is consistent with almost the amount of water inthe MIL-100 structure coordinating the Al sites in the trimericunits, which is the origin of the octahedral environment of thesesites. At this temperature treatment, a small signal appeared inthe 27Al spectrum around isotropic chemical shifts of 37 ppmconsistent with pentacoordinated Al species. Under the effect ofincreasing the temperature, the intensity of this signal growscontinuously until reaching a limit value corresponding to ca.30% of total Al at 250 �C. Higher temperatures up to 350 �C didnot change the spectrum, confirming the removal of totalbounded water molecules within the structure. The 1H NMRconfirms that almost all water had been evacuated in conjunctionwith the appearance of broad small signals from ca. 0 to 3 ppm,assigned to terminal O�H groups in the trinuclear units, whichresonate at 0.8�1.5 ppm for nonacidic terminal Al�OH groupsin zeolites and related materials.49�51 This would indicate that, inhydrated form, hydroxyl protons are involved in fast chemicalexchange process with adsorbed water. In such a circumstance,the chemical composition was Al3O(OH)(btc)2(H2O)0.1�0.2-(H3btc)0.1�0.5 in the 250�350 �C temperature range. Theseresults indicate that only one pentacoordinated Al per Al3 trimeris generated upon complete dehydration. This differs from twosuch coordinatively unsaturated sites per M3 trimer reported inthe chromium16 and iron5 MIL-100 analogues (Figure 1). Thiswould indicate that in the case of aluminum-based compoundonly one water molecule bonds to the Al3 trimer. On the otherhand, important structural changes occurred after dehydrationbecause additional octahedral Al sites were observed in 27AlNMR, as confirmed by 3QMAS NMR experiments (see nextsection) as well as splitting of framework trimesate 1H NMRsignal into a couple of resonances. Although the local structure of

trimeric units should not be altered a lot, except departure of onewater molecule, the crystallographic structure of the overallnetwork may undergo some lowering of the global symmetry(see below in next section). Interestingly, these structural changesappeared perfectly reversible after rehydration as the full recoveryof the original 1H and 27Al NMR spectra demonstrated. Depend-ing on the nature of the metal, these results highlight theflexibility of theMIL-100 structure with respect to the hydration/dehydration cycle and their adsorption property.NMRCharacterization of the Dehydrated Form. It is obvious

from the thermal behavior study that the structure underwentsignificant crystallographic changes. Further NMR investigationshave been conducted on some dehydrated samples using 2D1H�1H DQ, RFDR, 2D 27Al MQMAS, 2D 27Al{1H} HETCOR,and 1D 13C{1H} CPMAS techniques. The 1H DQ and RFDRspectra (Figure 7) showed that the four aromatic btc signals, 8.9,9.3, 9.9, and 10.2 ppm, are all involved in mutual dipolar interac-tion, confirming their location in the same framework. In parti-cular, the DQ experiment reveals some specific correlations:between (i) 10.2 and 9.9 ppm, (ii) 9.9 and 9.4 ppm, and (iii)9.4 and 8.9 ppm, and the (iv) 9.9 ppm and (v) 8.9 ppmautocorrelations.Besides the appearance of new 27Al signals of pentacoordi-

nated sites upon dehydration, some correlated noticeable changesoccur in the octahedral chemical shift range. In Figure 8, the27Al MQMAS spectra of activated MIL-100(Al) treated at 150and 250 �C showed different octahedral aluminum sites comparedto those initially observed in the fully hydrated sample (Figure S8 inSupporting Information for spectral decomposition of F2 projec-tions). Furthermore, the results are also different compared toeach other, indicating that the local structure is also temperaturedependent. Detailed quadrupolar parameters of all signals in thedifferent samples are compiled in Table 2. Because aluminumatoms in MOF materials are mostly in an octahedral environ-ment as isolated aluminum clusters44 or infinite corner52 oredge53 sharing aluminum chains connected to each other throughcarboxylates, the flexibility of the structure around the aluminumcation appears much more rigid in such compounds in compar-ison to zeolites and zeotypes constructed with tetrahedral units.Consequently,27Al NMR spectra often experience line widthbroadening due to the strong quadrupolar interaction in thesecompounds with quadrupolar coupling constants up to ∼13MHz.28,29,33,52,53 In this respect, the observed quadrupolarcoupling constants recorded in the current study appeared tobe somehow moderate. After a treatment at 150 �C, significantchanges of quadrupolar line shape parameters were observed forthe originally 6-fold coordinated sites, with the appearance of anadditional pentacoordinated site. Further evolution of theseparameters occurred after 250 �C treatment, with further appear-ance of a second pentacoordinated site. Such changes of NMRparameters reflect the sensitivity of the27Al quadrupolar nucleustoward the crystallographic symmetry changes locally and also atlong distance order. The decrease of coordination number from 6to 5 upon dehydration not only affects the first coordination shellenvironment but also has a much longer range effect. It shouldalso be mentioned that seven distinct crystallographic Al atomshave been solved by XRD for the fully hydrated sample, whereasNMR has shown only three Al sites.34 These differences wereexplained in terms of lack of resolution in NMR parameters ofsome Al sites due to the resemblance of their respectiveenvironments, giving rise to almost identical overlapped signals,and quantitatively the seven crystallographic Al sites could be

Figure 6. 27Al and 1H MAS NMR of activated MIL-100 (a), andsubsequent thermal treatment at 130 �C (b), 150 �C (c), 200 �C(d), 250 �C (e), 300 �C (f), and 350 �C (g) and then rehydration of(g) overnight over saturated NaCl solution in closed desiccator (h). Theduration of thermal treatment was in the range 4�6 h.



regrouped into three distinct Al types. In the present work, de-hydration might cause degeneration of those NMR resonances,leading progressively to distinction of these seven Al crystal-lographic sites with transformation of some of them from 6-foldto 5-fold coordinated-Al.The 1H�27Al CP HETCOR spectra of sample heated at

150 �C, shown in Figure 9, confirm that no significant hetero-nuclear dipolar interaction exist involving the pentacoordinatedAl and protons from the OH and H2O groups of the structure.This is fully consistent with the departure of one terminal co-ordinated water molecule initially present in the trimeric unitresulting in the pentacoordinated Al. The only correlation observedfor the pentacoordinated Al signal involves the signal of frame-work trimesate. This result also suggests that some protons slow/intermediate exchange occurs between the water and OH boundto the trimer, shortening the T1F. Though trimeric units withμ3-hydroxo group exist with divalent cations such Co54 andNi,55,56 the absence of cross-correlation between the hydroxideproton signal and the pentacoordinated Al site is consistent withthe trimeric aluminum unit sharing the μ3-oxo ligand. Thehexacoordinated Al still gives strong cross-peaks at short contacttimewith all types of protons (OH,H2O, and trimesate), confirming

the proximity of all these ligands of Al(VI). At long contact time,only correlation with the framework trimesate signal remainspresent due to a long 1H T1F for this signal (ca. 7 ms) comparedto those of extra-framework trimesic acid (2 ms), water (0.3 ms),and hydroxide groups (ca. 2 ms). The terminal groups such asOH andH2O as well as extra-framework species should thereforepresent a certain mobility compared to the rigid frameworkorganic species, and consequently no significant dipolar interac-tions between those two kinds of protons take place in the solid,leading to somehow two separate protonic baths, i.e., rigidframework protons and mobile species. This is further supportedby the absence of any correlation in the 1H�1H DQ experiment(Figure 4a). The extra-framework trimesic acid proton resonanceclearly exhibits correlation exclusively with the hexacoordinatedAl sites, and not with the pentacoordinated Al sites, suggestingthat these organic species may interact electrostatically directlywith aluminum.The 13C{1H} CPMAS spectra of the activated MIL-100(Al)

before and after thermal treatment at 150 �C are shown inFigure 10. Spectra were recorded with a contact time of 5 and 3ms, respectively, that could explain the difference in signal-to-noise ratio. Both spectra consisted of groups of signals located in

Figure 7. 2D 1H�1H (a) back-to-back and (b) RFDR spectra of activated MIL-100(Al) treated at 130 �C. Expansions of the correlations are shown in(c) and (d), respectively. The mixing time was 0.27 and 5.3 ms, respectively.



the C aromatic chemical shifts ranging from 132 to 140 ppm, andothers in the carbonyl chemical shifts around 171�172 ppm (seeFigure S9 in Supporting Information for spectral decomposition).After dehydration, the signals underwent some line broadeningpresumably due to chemical shift distributions as a result ofchanges of local structural environment. The removal of terminalbounded water to the trimer must cause a lowering of symmetry.Location of Extra-Framework H3btc. Although the chro-

mium and aluminum based MIL-100 present the same tridimen-sional structure built from the metallic trimeric blocks and trimesate

ligands, the terminal groups of the trimer seem to behavedifferently. If considering a neutral network and empty poresystem, the global chemical formula expected would be M3O-(OH)(H2O)2(btc)2. The metallic trimer would then possess intheir terminal positions one hydroxo and two aquo ligands. In thecase of chromium based compound, it has been demonstrated bymeans of in situ IR that two water molecules indeed were foundbounded to the trimer Cr3.

16 Furthermore, fluoride ions couldalternatively substitute partially the terminal hydroxide groups.Due to the low electronic density of the hydrogen atom,diffraction techniques could not distinguish between the hydro-xide and water ligands. Quantitative NMR, however, enables

Figure 8. 2D 27AlMQMAS spectra of activatedMIL-100(Al) treated at(a) 150 �C and (b) 250 �C.

Table 2. Line-Shape Parameters Corresponding to 27Al MASSignals of MIL-100(Al) Samples as a Function of TreatmentTemperature

treatment temp (�C) coordination no. δiso (ppm)a CQ (MHz)a ηQ

RTb 6 3.4 1.30 0.04

6 2.4 2.72 0.05

6 1.3 5.53 0.30

150 6 2.5 1.71 0.00

6 �3.5 2.96 0.00

6 1.4 5.44 0.04

5 37.4 3.31 0.00

250 6 2.5 2.50 0.00

6 1.5 3.50 0.00

6 �5.5 4.50 1.00

5 36.9 3.61 0.00

5 31.7 3.16 0.00aAccuracy: δiso ((0.8 ppm); CQ ((0.39 MHz). bNo thermal treat-ment; data from ref 34.

Figure 9. 2D 1H�27Al CP HETCOR NMR spectra of dehydratedMIL-100(Al) at 150 �C recorded with contact time (a) 0.25, (b) 1, and(c) 4 ms. 64 t1 increments with 1280 transients each were collected for(a) and (c), and 64 t1 increments with 512 transients each were collectedfor (b).



distinguishing between not only the two ligands but also theirquantifications. The quantity of the water bounded to the trimerin MIL-100(Al) was determined precisely while the temperaturewas varied. Figure 11 summarizes results of quantitative 27Al,using selective central transition excitations, and 1H NMR as afunction of temperature treatment. In Figure 11a, the fraction ofgenerated pentacoordinated Al reached a maximum value corre-sponding to the third of total Al. From 1H NMR, the amount ofwater per Al3 trimer (Figure 11b) can be deduced by taking intoaccount the signal area of the framework btc as internal quanti-tative reference. Combining both results, the plot of fraction ofpentacoordinated Al as a function of the amount of water moleculeper Al3 trimer (Figure 11c), would determine definitively theamount of water molecule bond to the Al3 trimer. The experi-mental points fit nicely with the theoretical linear curve corre-sponding to one water molecule per Al3 trimer (joining 33% AlV

to H2O/Al3 = 1) rather than two molecules per trimer (joining66%AlV toH2O/Al3 = 2). At low amount of pentacoordinated Al(4% at 130 �C) a deviation from the straight line is observed inFigure 11a, due to the fact that some bounded water moleculeswere removed before evacuating all nonbounded water. Thismayindicate that some of the physisorbed water molecules trappedwithin the pores were strongly interacting with the surface or theaccess to the pore systems was partially blocked. They are,however, totally evacuated at higher temperature, i.e., 150 �C.If there is only one water molecule bonded to the Al3 trimer,

this implies that the Al3 trimeric unit possesses, besides thiscoordinated water molecule, two hydroxide terminal groups orone hydroxide terminal group and a third coordinating ligand toexplain the exclusive octahedral environment of the three Al. Thefirst situation with two hydroxide groups can be ruled outbecause the ratios of 1H NMR signal intensities of OH groupsand aromatic protons of framework btc were always correspond-ing to 1OH/2btc, and consequently to 1OH/1Al3. The most

plausible explanation rationalizing all the experimental observa-tions would be rather a strong interaction of the extra-frameworkH3btc with the Al3 trimer through a CO�Al monodendatecoordinating mode. Figure 12 shows a model of such interactionwhere two of the carboxylic arms of the trimesic acid bridges twoAl3 trimers for a given super tetrahedral building block, which isthe typical assembly of Al3 trimers in the MIL-100 structure. Thesize and geometry of theH3btc molecule fit well with the distanceseparating two trimer units. According to this model, one H3btcmolecule interacts not only with two Al3 trimers but also possiblywith a third Al3 trimer. This would explain why upon successiveactivations there is always some extra-framework trimesic acidleft in the ratio 0.3H3btc/1Al3.

1H�1H 2D NMR as well as1H�27Al correlation experiments evidenced the proximity of theextra-framework H3btc to the framework and their particular

Figure 10. 13C{1H} CPMAS NMR spectra of (a) activated MIL-100(Al) and (b) dehydrated MIL-100(Al) at 150 �C.

Figure 11. (a) Fraction of pentacoordinated Al as a function of treat-ment temperature according to quantitative 27Al. (b) Amount of watermolecules per Al3 trimer as a function of treatment temperatureaccording to quantitative 1H. (c) Fraction of pentacoordinated Al as afunction of water molecules per Al3 trimer according to quantitative 27Aland 1H. The broken red lines are theoretical curves for one watermolecule bond to Al3 trimer. The theoretical curve in (b) is based onobserved pentacoordinated Al in (a).



mutual strong interaction. Such host�guest interactions wouldalso strengthen the thermal stability of the compound. Never-theless, the surface area and pore accessibility would be reducedby their presence.

’CONCLUSIONS

In summary, the activation and the dehydration�rehydrationprocess of a three-dimensional MOF-type aluminum trimesate(MIL-100) has been fully characterized by solid state 27Al, 13C,and 1H NMR as well as X-ray diffraction. 1H NMR MAS and1H{27Al} TRAPDOR experiments of as-synthesized and acti-vated MIL-100(Al) allowed us to identify framework and extra-framework components of the solid. 2D 1H�1H back-to-backand RFDR spectra of activated MIL-100(Al) are used to probetheir spatial proximities. Investigation of interaction within andwith the framework are performed using 2D 27Al{1H}HETCORexperiments at various contact times. 27Al and 1H MAS NMR ofactivated MIL-100 treated at various temperature up to 350 �Cevidenced a reversible dehydration�rehydration process accom-panied by removal of not only physisorbed water but also watermolecules of the framework. 2D 1H�1H back-to-back and RFDRspectra of partially dehydrated sample showed splitting of signalsinto several lines, which exhibit multiple cross-correlation due tostructural changes. Also, 2D 27AlMQMAS showed an increase inthe number of signals for both the octahedral and the generatedpentacoordinated Al sites as a consequence of the loweringcrystal symmetry. The 2D 1H�27Al CP HETCORNMR spectrarecorded at different contact times probed the dipolar interactionbetween protonated species and the two types of aluminum sites,i.e., Al(VI) and Al(V), within the dehydrated compound. Finally,13C{1H} CPMAS NMR of both activated and dehydrated MIL-100(Al) were performed to follow changes on the organic moietiesof the structure upon dehydration.

MIL-100(Al) generates coordinatively unsaturated sites, thoughdifferently from those occurring for its chromium analogue.Activation of MIL-100(Al) resulted in the removal of most (80wt %) extra-framework H3btc, according to quantitative 1HNMR, but the remaining organic species interact strongly withthe framework. From 1H�1H and 1H�27Al NMR correlations,the spatial proximity between the framework btc and occludedmobile H3btc into the cages was clearly demonstrated. Quanti-tative 1H and 27Al indicate only one water molecule per Al3trimer leaves at 250 �C, producing only one coordinately un-saturated in contrast to the chromium isotype. The 1H�27Al CPHETCOR spectra confirm that no significant heteronucleardipolar interaction involving the generated pentacoordinatedAl and protons from the OH and H2O groups of the structureas being far enough from these sites. Upon heating, the crystal-lographic structure underwent some alterations, mainly due to

the change of the symmetry of the Al3 trimer unit that is a fullyreversible process after rehydration. Indeed, besides the appear-ance of new 27Al signals of pentacoordinated sites upon dehy-dration, some correlated noticeable changes occur in both the27Al octahedral chemical shift range and the 1H aromatic btcsignals. 13C signals undergo, however, line broadening due tochemical shift distributions, resulting from the decrease of localstructural symmetry. The MIL-100(Al) network exhibits there-fore different structural flexibility, thermal stability, and innerchemistry, indicating a much stronger affinity to extra-frameworkbtc than the MOF MIL-100 isotype series. One can thereforeexpect a much different Bronsted acidity and a different Lewisacidity than its chromium or iron isotypes.

’ASSOCIATED CONTENT

bS Supporting Information. IR spectra of as-synthesizedand activated MIL-100(Al) samples; thermodiffractogram ofMIL-100(Al); tables of elemental and TGA analysis details; de-composition of 1H, 13C, and 27Al NMR of as-synthesized, activated,and dehydrated samples. This material is available free of chargevia the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Phone: (33) 1 39 254 254. Fax:(33) 1 39 254 476.

Present Addresses‡Unit�e de Catalyse et Chimie du Solide, UMR CNRS 8181,Universit�e de Lille Nord de France, 59652 Villeneuve d’Ascq,France.

’ACKNOWLEDGMENT

We thank Dr. Nathalie Audebrand (UMR CNRS 6226, Rennes,France) for her assistance for the XRD thermodiffractogram.

’REFERENCES

(1) F�erey, G. Chem. Soc. Rev. 2008, 37, 191.(2) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.;

Eddaoudi, M.; Kim, J. Nature 2003, 423, 705.(3) Alaerts, L.; Maes, M.; Giebeler, L.; Jacobs, P. A.; Martens, J. A.;

Denayer, J. F. M.; Kirschhock, C. E. A.; De Vos, D. E. J. Am. Chem. Soc.2008, 130, 14170.

(4) Colodrero, R.M.P.; Cabeza, A.;Olivera-Pastor, P.; Infantes-Molina,A.; Barouda, E.; Demadis, K. D.; Aranda, M. A. G. Chem.—Eur. J. 2009,15, 6612.

(5) Horcajada, P.; Surble, S.; Serre, C.; Hong, D. Y.; Seo, Y. K.;Chang, J. S.; Greneche, J. M.; Margiolaki, I.; F�erey, G. Chem. Commun.2007, 2820.

(6) Alvaro,M.; Das, D.; Cano,M.; Garcia, H. J. Catal. 2003, 219, 464.(7) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686.(8) Corma, A.; Garcia, H.; Leyva, A. J. Catal. 2004, 225, 350.(9) Huang, F. T.; Jao, H. J.; Hung, W. H.; Chen, K. M.; Wang, C. M.

J. Phys. Chem. B 2004, 108, 20458.(10) Liu, C. J.; Zou, J. J.; Yu, K. L.; Cheng, D. G.; Han, Y.; Zhan, J.;

Ratanatawanate, C.; Jang, B. W. L. Pure Appl. Chem. 2006, 78, 1227.(11) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.;

Matzger, A. J.; O0Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523.(12) F�erey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour,

J.; Surble, S.; Margiolaki, I. Science 2005, 309, 2040.

Figure 12. Possible interaction mode of extra-framework H3btc withthe trimers Al3 in MIL-100(Al).



(13) Hamon, L.; Serre, C.; Devic, T.; Loiseau, T.;Millange, F.; F�erey,G.; De Weireld, G. J. Am. Chem. Soc. 2009, 131, 8775.(14) Latroche, M.; Surble, S.; Serre, C.; Mellot-Draznieks, C.;

Llewellyn, P. L.; Lee, J. H.; Chang, J. S.; Jhung, S. H.; F�erey, G. Angew.Chem., Int. Ed. 2006, 45, 8227.(15) Llewellyn, P. L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.;

Hamon, L.; De Weireld, G.; Chang, J. S.; Hong, D. Y.; Hwang, Y. K.;Jhung, S. H.; F�erey, G. Langmuir 2008, 24, 7245.(16) Vimont, A.; Goupil, J. M.; Lavalley, J. C.; Daturi, M.; Surble, S.;

Serre, C.; Millange, F.; F�erey, G.; Audebrand, N. J. Am. Chem. Soc. 2006,128, 3218.(17) Vimont, A.; Leclerc, H.; Mauge, F.; Daturi, M.; Lavalley, J. C.;

Surble, S.; Serre, C.; F�erey, G. J. Phys. Chem. C 2007, 111, 383.(18) Bolis, V.; Busco, C.; Ugliengo, P. J. Phys. Chem. B 2006, 110,

14849.(19) Onfroy, T.; Li, W. C.; Schuth, F.; Knozinger, H. Phys. Chem.

Chem. Phys. 2009, 11, 3671.(20) Mellot-Draznieks, C.; F�erey, G. Prog. Solid State Chem. 2005,

33, 187.(21) Kusgens, P.; Rose, M.; Senkovska, I.; Frode, H.; Henschel, A.;

Siegle, S.; Kaskel, S. Microporous Mesoporous Mater. 2009, 120, 325.(22) Alaerts, L.; Seguin, E.; Poelman, H.; Thibault-Starzyk, F.;

Jacobs, P. A.; De Vos, D. E. Chem.—Eur. J. 2006, 12, 7353.(23) Schlichte, K.; Kratzke, T.; Kaskel, S. Microporous Mesoporous

Mater. 2004, 73, 81.(24) Ahnfeldt, T.; Guillou, N.; Gunzelmann, D.; Margiolaki, I.;

Loiseau, T.; F�erey, G.; Senker, J.; Stock, N. Angew. Chem., Int. Ed.2009, 48, 5163.(25) Comotti, A.; Bracco, S.; Sozzani, P.; Horike, S.; Matsuda, R.;

Chen, J.; Takata, M.; Kubota, Y.; Kitagawa, S. J. Am. Chem. Soc. 2008,130, 13664.(26) Liu, L.; Wang, X.; Jacobson, A. J. Dalton Trans. 2010, 39, 1722.(27) Loiseau, T.; F�erey, G. J. Fluor. Chem. 2007, 128, 413.(28) Loiseau, T.; Lecroq, L.; Volkringer, C.; Marrot, J.; F�erey, G.;

Haouas, M.; Taulelle, F.; Bourrelly, S.; Llewellyn, P. L.; Latroche, M.J. Am. Chem. Soc. 2006, 128, 10223.(29) Loiseau, T.; Mellot-Draznieks, C.; Muguerra, H.; F�erey, G.;

Haouas, M.; Taulelle, F. C. R. Chim. 2005, 8, 765.(30) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.;

Henry, M.; Bataille, T.; F�erey, G. Chem.—Eur. J. 2004, 10, 1373.(31) Senkovska, I.; Hoffmann, F.; Froeba, M.; Getzschmann, J.;

Boehlmann,W.; Kaskel, S.MicroporousMesoporousMater. 2009, 122, 93.(32) Volkringer, C.; Loiseau, T.; Guillou, N.; F�erey, G.; Elkaim, E.

Solid State Sci. 2009, 11, 1507.(33) Volkringer, C.; Loiseau, T.; Guillou, N.; F�erey, G.; Haouas, M.;

Taulelle, F.; Audebrand, N.; Margiolaki, I.; Popov, D.; Burghammer, M.;Riekel, C. Cryst. Growth Des. 2009, 9, 2927.(34) Volkringer, C.; Popov, D.; Loiseau, T.; F�erey, G.; Burghammer,

M.; Riekel, C.; Haouas, M.; Taulelle, F. Chem. Mater. 2009, 21, 5695.(35) Volkringer, C.; Popov, D.; Loiseau, T.; Guillou, N.; F�erey, G.;

Haouas, M.; Taulelle, F.; Mellot-Draznieks, C.; Burghammer, M.;Riekel, C. Nat. Mater. 2007, 6, 760.(36) Simon, S. Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. Part B

2006, 47, 489.(37) Simon, S.; Vanmoorsel, G.; Kentgens, A. P.M.; Deboer, E. Solid

State Nucl. Magn. Reson. 1995, 5, 163.(38) Bennett, A. E.; Ok, J. H.; Griffin, R. G.; Vega, S. J. Chem. Phys.

1992, 96, 8624.(39) Deng, F.; Yue, Y.; Ye, C. H. Solid State Nucl. Magn. Reson. 1998,

10, 151.(40) Kao, H. M.; Grey, C. P. Chem. Phys. Lett. 1996, 259, 459.(41) Goldbourt, A.; Madhu, P. K.; Vega, S. Chem. Phys. Lett. 2000,

320, 448.(42) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.;

Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z. H.; Hoatson, G. Magn.Reson. Chem. 2002, 40, 70.(43) Nelson, A. P.; Farha, O. K.; Mulfort, K. L.; Hupp, J. T. J. Am.

Chem. Soc. 2009, 131, 458.

(44) Haouas, M.; Volkringer, C.; Loiseau, T.; F�erey, G.; Taulelle, F.Chem.—Eur. J. 2009, 15, 3139.

(45) F�erey, G.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; Surble,S.; Dutour, J.; Margiolaki, I. Angew. Chem., Int. Ed. 2004, 43, 6296.

(46) Mowat, J. P. S. M. J. P. S.; Miller, S. R.; Slawin, A. M. Z.;Seymour, V. R.; Ashbrook, S. E.; Wright, P. A. Microporous MesoporousMat. 2011, 142, 322.

(47) Chan, J. C. C.; Eckert, H. J. Chem. Phys. 2001, 115, 6095.(48) Koller, H.; Lobo, R. F.; Burkett, S. L.; Davis, M. E. J. Phys. Chem.

1995, 99, 12588.(49) Hunger, M.; Ernst, S.; Steuernagel, S.; Weitkamp, J. Micropor-

ous Mater. 1996, 6, 349.(50) Maeda, K.; Tuel, A.; Baerlocher, C. J. Chem. Soc., Dalton Trans.

2000, 2457.(51) Zahedi-Niaki, M. H.; Zaidi, S. M.; Kaliaguine, S. Microporous

Mesoporous Mater. 1999, 32, 251.(52) Volkringer, C.; Loiseau, T.; Guillou, N.; Ferey, G.; Haouas, M.;

Taulelle, F.; Elkaim, E.; Stock, N. Inorg. Chem. 2010, 49, 9852.(53) Volkringer, C.; Loiseau, T.; Haouas,M.; Taulelle, F.; Popov, D.;

Burghammer, M.; Riekel, C.; Zlotea, C.; Cuevas, F.; Latroche, M.;Phanon, D.; Knofelv, C.; Llewellyn, P. L.; Ferey, G. Chem. Mater. 2009,21, 5783.

(54) Uemura, S.; Spencer, A.; Wilkinso., G J. Chem. Soc., DaltonTrans. 1973, 2565.

(55) Ma, S. Q.; Wang, X. S.; Manis, E. S.; Collier, C. D.; Zhou, H. C.Inorg. Chem. 2007, 46, 3432.

(56) Reynolds, R. A.; Yu, W. O.; Dunham, W. R.; Coucouvanis, D.Inorg. Chem. 1996, 35, 2721.

Documents

Monitoring the Activation Process of the Giant Pore MIL-100(Al) by Solid State NMR