Microporous and Mesoporous Materials 157 (2012) 137–145

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Microporous and Mesoporous Materials

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Large scale fluorine-free synthesis of hierarchically porous iron(III) trimesateMIL-100(Fe) with a zeolite MTN topology

You-Kyong Seo a,b, Ji Woong Yoon a, Ji Sun Lee a,b, U-Hwang Lee a, Young Kyu Hwang a, Chul-Ho Jun b,Patricia Horcajada c, Christian Serre c, Jong-San Chang a,⇑a Research Group for Nanocatalyst, Biorefinery Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yusung, Daejeon 305-600, Republic of Koreab Department of Chemistry, Center for Bioactive Molecular Hybrid, Yonsei University, Seodaemoonku, Seoul 120-749, Republic of Koreac Institut Lavoisier, UMR CNRS 8180, Université de Versailles Saint Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France

a r t i c l e i n f o

Article history:Received 1 August 2011Received in revised form 25 January 2012Accepted 1 February 2012Available online 3 March 2012

Dedicated to Professor Gérard Férey on theoccasion of his 71th birthday.

Keywords:Large scale synthesisPorous iron(III) trimesateMetal–organic frameworkHydrophilicityHydrophobicity

1387-1811/$ - see front matter � 2012 Elsevier Inc. Adoi:10.1016/j.micromeso.2012.02.027

⇑ Corresponding author.E-mail address: jschang@krict.re.kr (J.-S. Chang).

a b s t r a c t

The hierarchically mesoporous iron(III) trimesate MIL-100(Fe) with a zeolite-MTN topology is known asan advanced functional material that is biocompatible. In this work, the large scale synthesis of MIL-100(Fe) has been achieved by hydrothermal reactions using suitable conditions without HF for the largescale synthesis. Although such conditions are narrow, the concurrent change of iron precursor andincrease in the concentration of reaction mixture give rise to a synergetic effect leading to an increasein the crystallinity of F-free MIL-100(Fe). This method, combined with two purification steps (solventextraction and chemical treatment with NH4F) leads to a highly porous F-free material obtained through-out a very high space–time-yield (>1700 kg/m3 day). Possible formation mechanisms of MIL-100(Fe)under hydrothermal conditions have been proposed in terms of four steps such as hydrolysis, deprotona-tion, self-assembly, and polycondensation. The resulting material exhibits similar physicochemical prop-erties to those of the one prepared in the presence of HF, except for a slight difference in sorptioncapacities of gases and liquid vapors corresponding to the difference of pore volume. Regardless of theuse of HF, the purified MIL-100(Fe) possesses very high uptakes for both non-polar toluene and polar eth-anol probe molecules due to the respective interactions with hydrophilic and hydrophobic sites in theframework. Finally, hydrophobicity measurements confirm that the dehydrated MIL-100(Fe) is morehydrophobic than conventional zeolite beta (SiO2/Al2O3 = 25) and commercial iron trimesate (BasoliteF300) from BASF SE.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Porous metal–organic frameworks (MOFs) are currently animportant class of advanced functional materials due to their novelcoordination structures, diverse topologies, and potential applica-tions [1–3]. Moreover, unique features of MOFs such as exception-ally high porosities, with regular pores and world-record surfaceareas, well-defined crystalline structures, and the lack of non-accessible bulk volume have attracted the attention of both acade-mia and industry [1–4].

Some of us have recently discovered the zeotype cubic chro-mium(III) carboxylates with giant cages labeled MIL-100(Cr) [5]and MIL-101(Cr) [6] (MIL stands for Materials of Institut Lavoisier)using chromium and the cheap and simplest aromatic carboxylatessuch as terephthalate (1,4-benzenedicarboxylate or BDC) andtrimesate (1,3,5-benzenetricarboxylate or BTC), respectively. This

ll rights reserved.

approach was then extended to the cheaper and environmentallyfriendly iron metal, leading to the isostructural iron(III) trimesateMIL-100(Fe) [7]. In the usual classification of porous solids, MIL-100 and MIL-101 structures represented the first examples of per-fectly crystallized mesoporous solids, which are the two largestnon-proteinic structures ever evidenced [3]. These solids possessseveral unprecedented features such as hierarchical pore struc-tures including a zeotype architecture, mesoporous cages accessi-ble through microporous windows, an exceptionally high cellvolume and surface area, numerous unsaturated metal Lewis acidsites, and high hydrothermal and chemical stability. Because ofthese unique characteristics, these topical MOFs have been re-cently subjected to many investigations concerning surface func-tionalization [8], catalysis [9], selective gas sorption [10,11], drugcarrier [12], energy-efficient dehumidification [13] and adsorptionheat transformation [14].

The crystalline iron(III) trimesate MIL-100(Fe) is built up fromhydrid super-tetrahedral units, made from oxo-centered trimersof iron(III) octahedra and trimesate ligands, creating a three-dimensional cubic structure with two types of mesocages (25–29 Å)

138 Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145

accessible through microporous windows (5–9 Å) and a large BETsurface area (>2000 m2/ g). Each iron trimer (three iron(III) octahe-dra sharing a common vertex of l3-O) is coordinated to six carbox-ylates, two coordinated water molecules and one anion, whichdepending on the synthesis conditions, is either F� or OH� anions.Moreover, we have recently demonstrated a progressive partialand controlled reducibility of FeIII ion sites of MIL-100(Fe) into FeII

coordination sites upon heating under vacuum, leading to an inter-esting stronger interaction with unsaturated molecules through aback-donation effect [10,11]. Besides these unique features, MIL-100(Fe) is also known as a cheap and biocompatible material, whichhave proved a lack of in vivo toxicity after the intravenous adminis-tration of very high doses for its use in drug delivery [12].

Most syntheses of MOFs are generally performed by solvother-mal or hydrothermal process via a cooperative ionic interaction be-tween organic linkers containing poly-carboxylate anions andinorganic cations constructing the skeleton of crystalline frame-works together with solvent as a template [15]. In particular,MIL-100(Fe) has been generally synthesized by hydrothermalmethods using HF as a mineralizing agent to improve its crystallin-ity and promote the crystal growth of the final product [16,17].While HF is technically a weak acid in water, it is corrosive and dif-ficult to handle meaning that the use of HF hinders mass produc-tion of MOFs at elevated temperature and pressure. A Germancompany, BASF SE has recently succeeded in the large scale pro-duction of several selected MOFs by solvothermal and electro-chemical methods [4]. Taking into account its physicochemicalproperties and non-toxicity, MIL-100(Fe) can be a good candidateas a new adsorbent and catalyst. For the scale-up production, itis essential to establish a synthetic pathway of the material with-out the use of HF. In this work, we suggest possible conditions tocrystallize MIL-100(Fe) in the absence of fluoride using hydrother-mal conditions on the large scale as well an adapted purificationmethod for obtaining the highly porous solid. A possible multi-stepformation mechanism of MIL-100(Fe) is also discussed. Finally,some insights on the physicochemical properties, including elec-trokinetic and sorption properties, of F-free MIL-100(Fe) have beendisclosed.

2. Experimental

2.1. Hydrothermal synthesis of MIL-100(Fe)

F-containing MIL-100(Fe) (MIL-100(Fe)_F) was prepared fromhydrothermal reaction of trimesic acid with metallic iron, HF, nitricacid and H2O at 160 �C for 8 h as reported elsewhere [10]. The com-position of reaction mixture was 1.0Fe0:0.67BTC:2.0HF:0.6H-NO3:277H2O. The as-synthesized MIL-100(Fe)_F was sufficientlypurified by solvent extraction treatments using hot water and

Table 1Synthesis conditions and physicochemical properties of MIL-100(Fe).

SampleNo.a

Iron precursor Presenceof HF

Compositio(Fe:BTC:HF

1 Fe metal Yes 1:0.66:2:22 Fe metalb No 1:0.66:0:23 Fe metalb No 1:0.66:0:54 FeCl3�6H2O No 1:0.66:0:65 Fe(NO3)3�9H2O No 1:0.66:0:26 Fe(NO3)3�9H2O No 1:0.66:0:5

a Samples (MIL-100(Fe)_NF or MIL-100(Fe)_F) synthesized under hydrothermal conditiwater and hot ethanol.

b This metallic iron precursor was dissolved in 1.0 M aqueous nitric acid solution (moc BET surface area.d Total pore volume.e BET surface area of samples further purified with aq. 38 mM NH4F.

ethanol without chemical treatment with NH4F (Table 1, SampleNo. 1). The chemical formula of MIL-100(Fe)_F has been assignedto [Fe3O(H2O)2F0.81(OH)0.19){C6H3(CO2)3}2�nH2O (n � 14.5)], basedon elemental analysis [10].

To develop a new recipe for the F-free synthesis, metallic ironmetal was dissolved in 1.0 M aqueous nitric acid solution (molarratio of Fe/HNO3 = 1.67) instead of HF. Then, trimesic acid wasadded to the solution. The final composition was1Fe0:0.67BTC:0.6HNO3:xH2O (x = 55–280). After stirring for 1 h,the mixture was heated up to 160 �C and kept at that temperaturefor 12 h. However, the F-free synthesis of MIL-100(Fe) using metal-lic iron was not successful because the resulting product has beenobtained with a low yield and poor porosity (Table 1, Sample Nos. 2and 3). As another trials, the hydrothermal synthesis of F-free MIL-100(Fe) (MIL-100(Fe)_NF) has been carried out by using differentiron precursors (iron chloride or iron nitrate) at high concentra-tions of precursors in the absence of HF. The use of iron chloride(Table 1, Sample No. 4) is shown to provide improved porosity ascompared with the use of metallic iron but the product yieldshould be still improved. On the other hand, the use of iron nitrate(Table 1, Sample Nos. 5 and 6) is successful to increase both theporosity as well as the product yield.

For a typical synthesis using the nitrate precursor,Fe(NO3)3�9H2O was completely dissolved in water. Then, trimesicacid was added into previous solution, stirring at room temperaturefor 1 h. The final composition was 1Fe(NO3)3�9H2O:0.67BTC:xH2O(x = 55–280). The reactant mixture was loaded in a Teflon-linedpressure vessel. The reactor vessel was heated up to 160 �C andkept at the same temperature for 12 h. The pH remains acidicthroughout the synthesis. The reactor volume used ranges from 1to 10 l. Considering time required for heating and cooling the 10 lreactor, total synthesis time for the large scale synthesis was about1 day.

After the hydrothermal reaction, the light orange solid wasrecovered by filtration and washed with deionized water. The as-synthesized MIL-100(Fe)_NF was further purified by two-step pro-cesses using double solvent extraction with hot water and ethanoland then, chemical treatment with an aqueous NH4F solution. Forabout 15 g hydrated solid, the solvent extraction has been per-formed using 700 ml water at 70 �C for 3 h and then 700 ml hotethanol at 65 �C for 3 h until no detection of colored impuritiesin the mother solution. For further purification, chemical treatmentwith an aqueous of 38 mM NH4F solution (700 ml) at 70 �C for 3 hwas used to give the purified MIL-100(Fe). The solid was finallydried overnight at less than 100 �C in air.

Reference adsorbents used for comparison in sorption experi-ments were the commercial iron trimesate (Sigma–Aldrich, Baso-lite F300) and the NH4-form zeolite beta (Zeolyst CP814E, SiO2/Al2O3 = 25, SBET = 680 m2/g). The zeolite beta was converted into

n:H2O)

Productyield (%)

SBET

(m2/g)cVtotal

(cm3/g)d

78 82 2050 0.9078 35 1090 0.656 32 1190 0.69

45 1490 (1830)e 0.9478 70 1520 0.936 80 1800 (1970)e 1.15

ons in a 1 l reactor at 160 �C for 12 h and then purified by solvent extraction with hot

lar ratio of Fe/HNO3 = 1.67) instead of HF.

(d)

nsity

(a.u

.)

Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145 139

the H-form zeolite by calcination in air at 550 �C for 24 h beforeuse.

2.2. Characterization of MIL-100(Fe)

The crystallinity and purity of iron trimesate samples werechecked by X-ray powder diffraction analysis (XRPD; Rigaku D/Max 2200 PC, Cu Ka radiation). Thermogravimetric (TG) analysiswas carried out in a thermogravimetric analyzer (TA Instruments,Universal V4.5A). Analysis was performed in dry nitrogen flow of100 ml/min. The temperature was increased from 32 to 700 �Capplying a heating rate of 10 �C/min. Before TG measurement,the sample was hydrated in a chamber with 70% relative humidityat 30 �C for 2 days. The particle morphology and crystal size wereanalyzed using a scanning electron microscope (SEM, Philips,XL30S FEG). The surface atomic ratio (C/Fe) was analyzed by X-ray photoelectron spectroscopy (XPS, Axis Nova). The nitrogen con-tent was analyzed by Elemental Analyzer (Flash EA 1112 series/CEInstruments).

The zeta potential of each sample was measured in water sol-vent (Otsuka electronic ELS-Z). The dried sample (10 mg) wasadded in 10 ml of water, and then the solution was well dispersedby sonication at room temperature for 1 h. After then, the solutionwas adjusted to each pH by using 0.05 M NaOH and HCl to measurethe zeta potential.

The nitrogen sorption experiments were performed at �196 �Con a volumetric sorption analyzer (Micromeritics TriStar 3020)after dehydration of the sample (ca. 0.2 g) at 150 �C for 12 h underhigh vacuum (1 � 10�5 torr). Sorption experiments of propylenewere performed in another volumetric sorption analyzer (Microm-eritics Tristar 3000) as described in the literature [8]. The vaporphase adsorption experiments of toluene and ethanol were carriedout at 30 �C using an intelligent gravimetric analyzer (IGA, HidenAnalytical Ltd.). The samples (ca. 40 mg) were dehydrated underhigh vacuum (1 � 10�5 torr) before adsorption, and the adsorbate(toluene and ethanol) was purified by the method of freeze–pump–thaw. The Weitkemp hydrophobicity indices (HI) todetermine the hydrophobicity of porous materials have beendetermined by competitive breakthrough sorption experimentsof toluene and water as probe molecules according to a method re-ported in the literature [18]. The breakthrough experiments ofdehydrated samples (particle size ranging from 0.1 to 0.3 mm)were carried out in a 1/4 in. stainless-steel column with 100 mmlength at 35 �C using a vapor mixture of water (p = 5600 Pa) andtoluene (p = 6200 Pa) in helium (15 ml/min) as a carrier gas. Thevapor compositions passing through the adsorbent column weremonitored every 1 min on a gas chromatograph (Donam DS6200GC) equipped with a thermal conductivity detector and a capillarycolumn. The hydrophobic index was calculated with the followingWeitkamp equations using the breakthrough profiles:

HI ¼ XðtolueneÞ=XðwaterÞ

where X(toluene) is toluene adsorption capacity (g/g-dry solid) andX(water) is water adsorption capacity (g/g-dry solid).

5 10 15 20 25 30

(c)

(b)

Inte

2 θ (degree)

(a)

Fig. 1. XRPD patterns of F-free MIL-100(Fe) samples synthesized using a differentiron precursor at 160 �C for 12 h: (a) Fe0 (Sample No. 3, MIL-100(Fe)_NF3), (b) Fe0

(Sample No. 1, MIL-100(Fe)_F), (c) FeCl3�6H2O (Sample No. 4, MIL-100(Fe)_NF4),and (d) Fe(NO3)3�9H2O (Sample No. 6, MIL-100(Fe)_NF6).

3. Results and discussion

3.1. F-free hydrothermal synthesis and purification

During the hydrothermal synthesis of metal(III) trimesates MIL-100, the most influent chemical parameters are pH (acidic), con-centration and temperature (higher temperature for MIL-100(Cr)than MIL-100(Fe)) [5,7]. Besides MIL-101(Cr), crystalline metal(III)trimesates such as MIL-100(Fe) and MIL-100(Cr) are known to re-quire the presence of fluorine as a mineralizing agent; otherwise

poor crystalline phases are generally formed. The importance offluorine effect is clearly shown in the synthesis of MIL-100(Cr).While MIL-100(Cr) (SBET = 1980 m2/g) is hydrothermally obtainedat 220 �C for 8 h under acidic conditions in presence of fluoride an-ions, it is not the case in absence of fluorine atoms, whatever thesynthesis conditions explored so far (i.e. metal precursors, pH, tem-perature and concentration of reactants). This obviously indicates apivotal role of fluoride anion in nucleation as well as crystallizationof MIL-100(Cr).

In contrast, suitable conditions to crystallize MIL-100(Fe) in theabsence of fluoride have been found as listed in Table 1. Notably,synthesis conditions in the presence of HF are relatively wide,while those in the absence of HF are narrow. For instance, usingmetallic iron as a precursor under acidic conditions, MIL-100(Fe)_F (Sample No. 1, SBET = 2050 m2/g) was formed in presenceof HF, but only the poorly crystallized product (Sample No. 3,SBET = 1190 m2/g) was obtained with the use of nitric acid onlyfor the dissolution of metallic iron even after purification (Fig. 1a,Table 1). The change of synthesis temperature was not so effectiveto form highly crystalline MIL-100(Fe) at this condition. However,the concurrent change of iron precursor and concentration of reac-tion mixture gives rise to a synergetic effect leading to an increasein the crystallinity of MIL-100(Fe) in the absence of the fluoride an-ion. Best results in terms of metal precursor were obtained usingiron(III) nitrate instead of Fe0 or FeCl3�6H2O [19] (Fig. 1, Table 1).In particular, the concentration dependence on the formation ofMIL-100(Fe)_NF is clearly different from that of MIL-100(Cr). Foridentical synthesis parameters and molar ratio of the startingcompounds, an increase of reactant mixture using iron nitrate asa precursor is shown to be effective to increase the crystallinityof MIL-100(Fe)_NF. Whereas under diluted synthesis conditions aless crystalline product (Sample No. 5, SBET = 1520 m2/g) wasobtained (Table 1), the five times higher concentration of reactionmixture leads to the better crystalline solid (Sample No. 6, MIL-100(Fe)_NF6; SBET = 1800 m2/g), which is close to that obtained inpresence of HF.

As shown in Fig. 2, thermal analysis of MIL-100(Fe)_NF6 hasshown apparently three weight losses between 30 and 600 �C.The weight loss at first step, attributed to the departure of thewater inside the pore, is about 34.7%. The second one between84 and 200 �C (2.6%) comes from the water coordinated to the irontrimers. The final weight loss, between 200 and 550 �C (36.3%), isrelated to the decomposition of the trimesic acid. These lattertwo losses are in total agreement with the theoretical values (calc.:3.2% and 36.9%). In addition, elemental analysis of a purified

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght l

oss

(%)

Temperature (oC)

Fig. 2. TG profile of MIL-100(Fe)_NF6. Conditions: flow rate of N2, 100 ml/min;ramping rate, 10 �C/min.

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

700

(d)

(c)

(b)

(a)

Am

ount

ads

orbe

d (m

l-STP

/g)

P/P0

Fig. 3. Nitrogen physisorption isotherms of MIL-100(Fe)_NF6 at �196 �Cdependingon the purification stage: (a) filtration and washing with water, (b) solventextraction with hot water at 70 �C for 3 h, (c) solvent extraction with hot ethanol at65 �C for 3 h, and (d) chemical treatment with aq. 38 mM NH4F at 70 �C for 3 h.

3500 3000 2500 2000 1500 1000

(e)

(d)

(c)

(b)

(a)Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)

Fig. 4. FT-IR spectra of MIL-100(Fe)_NF6 depending on the purification stage:(a) trimesic acid as a reference compound, (b) filtration and washing with water,(c) solvent extraction with hot water at 70 �C for 3 h, (d) solvent extraction with hotethanol at 65 �C for 3 h, and (e) chemical treatment with aq. 38 mM NH4F at 70 �Cfor 3 h.

140 Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145

MIL-100(Fe)_NF indicates the presence of only a trace amount ofnitrogen (less than the detection limit) probably as a nitrate anionin the solid. Based on the TGA and elemental analysis data,the chemical formula of MIL-100(Fe)_NF is assigned to be[Fe3O(H2O)2(OH){C6H3(CO2)3}2�nH2O (n � 19.5)].

It should be noted that BET surface areas and pore volumes ofMOFs hydrothermally prepared under acidic conditions dependon the degree of purification as well as their crystallinity [20].It is mainly ascribed to the presence of residual impurities suchas non-reacted carboxylic acids in the pores and interparticlegrains, coordinated carboxylates and/or inorganic cations or an-ions from starting metal salts, besides the content of amorphousphases or other crystalline impurities. In fact, this often leads tothe discrepancy of textural properties of products even from thesame batch of synthesis. Therefore, the development of properpurification methods to yield purified MOFs is necessary becausehigh temperature calcination of MOFs in air is not generally pos-sible, except for few highly thermally stable MOFs such as theMIL-53 phase [17]. These purification methods include the disso-lution of organic acids with organic solvents [21], the dissolutionof water-soluble ionic species with hot water [20], the removal ofresidual organic acids as well as ionic species with reagents suchas aqueous alkaline fluoride solutions [20], and/or supercriticalpurification methods [22]. Here, we have developed a combinedmethod of solvent extraction (hot water and ethanol at 60–70 �C) followed by chemical treatment with fluorine-containingsalts (aqueous NH4F solution at 70 �C) for the purification ofas-synthesized MIL-101(Cr) [20]. Fig. 3 shows N2 physisorptionisotherms of MIL-100(Fe)_NF6 at �196 �C according to the purifi-cation stage. These isotherms clearly demonstrate the effect ofpurification treatments for the removal of impurities inside oroutside the pores. Additionally, IR spectra of MIL-100(Fe)_NF6indicate out that the purification method used here is particularlyeffective to remove residual trimesic acid (Fig. 4), as evidenced bythe decrease of the C=O stretching vibration at 1710–1720 cm�1

in F-free MIL-100(Fe), assigned to residual trimesic acid, after fi-nal treatment with NH4F. During this latter purification step,there is possibly an exchange of coordinated carboxylate anionswith the fluoride anion and removal of residual organic acid fromthe pores through the formation of water soluble ammoniumtrimesate [20]. BET surface areas of the resulting F-free MIL-100(Fe) samples range from 1800 to 2000 m2/g after completework-up of the as-synthesized solids obtained from identical syn-thesis conditions.

Besides, BASF SE has recently reported the commercial produc-tion of a porous iron trimesate (Basolite F300, Fe-EMOF) which isassumed to be obtained from an electrochemical route [4,23]. Thespace–time-yield (STY, kg of MOF per m3 of reaction mixture perday) of Basolite F300 in the large scale production is known to be20 kg/m3 day. The successful synthesis of F-free MIL-100(Fe) atthe high concentration of reactant mixture would be beneficialfor the large scale production due to high productivity. Hereby,we have been able to use the high concentrated reactant mixturein a 10 l scale synthesis reactor (using a volume of 6 l reaction mix-ture) to produce 0.75 kg of MIL-100(Fe) (MIL-100(Fe)_NF7) with amuch higher STY (120–130 kg/m3 day) than that reported for Baso-lite F300. Note that MIL-100(Fe)_NF7 obtained from the large scalesynthesis and purification processes keeps the same physicochem-ical properties as those of MIL-100(Fe)_NF6. Very recently, we havesucceeded in the larger scale production of F-free MIL-100(Fe) byoptimizing synthesis conditions in a 200 l metal alloy reactor vessel(Hastalloy C-276). Surprisingly, the final weight of the dried prod-uct (MIL-100(Fe)_NF8) in this production was 15.6 kg per a batch(Fig. 5). The space–time yield of this production is estimated toabout 450 kg/m3 day only in terms of the volume of the reaction

Fig. 5. A photograph of a transparent poly(methyl methacrylate) box (70 l)containing the dried MIL-100(Fe)_NF8 powder obtained from a large-scale batch(200 l).

Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145 141

mixture (around 70 l) and synthesis time (12 h). With the help ofpurification units including a large scale filter press, the BET surfacearea of the purified MIL-100(Fe)_NF8 reached up to 2280 m2/g (seeSupporting information Figs. S1 and S2). Moreover, it is further con-firmed that the synthesis time can be greatly reduced to 3 h withouta notable change in the quality of the product (MIL-100(Fe)_NF9,SBET = 2050 m2/g; see Supporting information Figs. S1 and S2). Thisindicates that the space-time yield for the production of MIL-100(Fe) could be dramatically increased up to more than 1700 kg/m3 day. A further systematic investigation on large scale productionof MIL-100(Fe) will be carried out later.

3.2. Proposition of synthesis pathways under hydrothermal conditions

To date, the use of isolated trimeric iron(III) acetate did not al-low to form iron(III) trimesate MIL-100(Fe), which is different fromtrivalent metal carboxylates with a MIL-88 topology [24]. How-ever, one has not determined yet if the l3-oxo-trimeric iron(III)octahedral units, constitutive unit of MIL-100(Fe), are formed: (i)in situ, i.e. through a controlled SBU (secondary building unit) syn-thetic approach where the trimeric inorganic unit keeps the integ-rity of the inorganic precursors during the formation of thecrystalline phase within the liquid phase, or (ii) only during the fi-nal condensation step leading to the desired solid. CrystallineMOFs are generally crystallized by four important steps [25]:hydrolysis, deprotonation, self-assembly, and (poly)condensation.In the very early reaction stage, it is assumed that no direct linkageof the carboxylic acids occurs with the trinuclear metal clusters be-

cause deprotonation of aromatic carboxylic acids is generallyslower than hydrolysis of trivalent metal ions [26]. It has been pre-viously shown that during the hydrolysis of Fe(III) salts, the Fe(III)ion readily undergoes hydrolytic oligomerization which producestrinuclear clusters hydrated through H-bonding [27]. Mechanismssuggested for the formation and aging of the hydrolytic Fe(III)polymer are known to involve the condensation of monomeric[Fe(H2O)6�n(OH)n](3�n)+ to form polymers including an octahedraltrimer in which Fe(III) ions are bridged by OH� groups or O2� ions[27,28]. Therefore, one possible mechanism for the crystallizationof MIL-100(Fe) involves the initial formation of trimeric inorganicunits of Fe(III) ions with a triangular arrangement of Fe(III) atomstriply bridged by a l3-O2� ion under aqueous acidic conditionswhereas exchange of O2� or OH� anions are likely to occur withcarboxylic groups from trimesic acid. It is noted that these trimericunits are equilibrated with the monomeric unit and other oligo-mers. Therefore, the crystallinity and purity of MIL-100(Fe) maybe dependent on how to keep the concentration of the trimericunit, being either purely inorganic or slightly hybrid, as well asits coordination geometry during the synthesis.

Although these trimeric units are assumed to be first stabilizedin aqueous acidic solutions, the complicated chemistry of thesetrinuclear clusters depends on the chemical nature of the cationsand their acido-basic properties. As explained above for the hydro-thermal synthesis of MIL-101(Cr), the fluoride anion appears tostabilize the formation of trimeric inorganic building units of MIL-100(Fe). This is probably related to the high electronegativity offluorine atoms, which could then stabilize the iron trimer, leadingto the effective crystallization and, therefore, to the formation oflarger crystals (Fig. 6). Actually, fluoride was found to be involvedin the terminal bond of an iron(III) cation in the trimeric iron(III)species of MIL-100(Fe) [10]. The use of the fluoride anion expandsthe synthetic conditions to form the crystalline MIL-100 phase, butthe conditions are limited in the absence of fluoride as revealed inMIL-100(Fe). In the absence of fluoride, a hydroxide anion is as-sumed to compensate the charge of iron(III) cation, as previouslyobserved for the MIL-100(Fe)_NF synthesized from FeCl3 [19]. Inthe case of iron nitrate precursor, it is also likely that nitrate anionsare in competition with hydroxides for such a role.

Concerning the stability of inorganic building units, anotherimportant factor for the synthesis is the deprotonation rate of car-boxylic acids. It is directly related to the acid strength of carboxylicacids, which can be expressed by pKa values, e.g. 3.12 (pKa1), 3.89(pKa2) and 4.70 (pKa3) for trimesic acid [29]. If the carboxylic acidsare not fully deprotonated in the synthetic conditions, the resultingMOF solids could not be highly crystalline due to the formation ofdefects or due to the inhibition of polycondensation, as illustratedin the crystallization of the zirconium terephthalate UiO-66 [30].The l3-oxo-centered trimeric Fe(III) octahedra are less stable thanthose of the Cr(III) analogue at hydrothermal conditions, meaningthat the synthesis temperature of MIL-100(Fe) is limited to lessthan 200 �C.

In the synthesis of MIL-100(Fe), the self-assembly of the inor-ganic trimers and carboxylic acids can be readily recognized interms of supramolecular interactions [24]. The trimers may be sub-sequently exchanged with a trimesate anion to form PBUs (primarybuilding units) likely with several trimers and trimesate that areinvolved, that might lead under hydrothermal conditions to furthercondensation leading to the final solid built up from hybrid super-tetrahedra ½FeIII

3 ðl3-OÞXðC6H4ðCO2Þ3Þ2ðH2OÞ2� (X = F� or OH�). Priorto this process, it is necessary to break hydration layers betweenthem connecting with H-bonding [28]. Crystallization by polycon-densation of trimeric PBUs is another self-assembly process thatinvolves molecular recognition [24,25]. Once the self-assemblystarts, the polycondensation will occur to continuously releasewater molecules in the units, forming the final phase. Controlled

Fig. 6. SEM images of (a) MIL-100(Fe)_F and (b) MIL-100(Fe)_NF7.

142 Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145

synthesis conditions are able to promote the occurrence of thesespecific building-units, which serve to propagate the infinite crys-tal structure of MIL-100(Fe).

3.3. Physicochemical and sorption properties of MIL-100(Fe)

MIL-100(Cr) and MIL-101(Cr) are known to be stable underhydrothermal conditions above 100 �C [20]. Likewise, robust MIL-100(Fe), thermally stable in air up to 320 �C [19], is also stable overmonths under air atmosphere as well as when treated with variousorganic solvents (alcohols, chloroform, acetonitrile, etc.) at room orhigher temperature. Moreover, it is obviously stable even afterexposure to boiling water for a week as illustrated in Fig. 7. In con-trast, even though highly cited MOFs such as MOF-177 and MOF-5have relatively high thermal stabilities, these materials are actuallyunstable and easily decomposed in the presence of moisture[31,32]. Therefore, the high hydrothermal stability of MIL-100(Fe), together with its high sorption capacity and unique sorp-tion characteristics, make it an attractive candidate of functionalmaterials for many potential applications [10,11].

Electrokinetic properties of solid particles in an aqueous solu-tion, including the isoelectric point, are helpful to characterizethe surface charge of particles at the solid/solution interface [33].In particular, information about the surface charges of MOF parti-cles suspended in water as a function of pH is valuable for design-ing material processing such as fabrication, film coating andsolution suspension [34]. The relation between surface chargeand pH in MIL-100(Fe)_F and MIL-100(Fe)_NF7 in aqueous solu-tions is plotted in Fig. 8. The isoelectric points (IEPs) of these sam-ples are almost same and observed at pH 4.4–4.5, close to the pKa

5 10 15 20 25 30

(c)

(b)

(a)

Inte

nsity

(a.u

.)

2 θ (degree)

Fig. 7. XRPD patterns of MIL-100(Fe)_NF7 (a) before and after exposure to boilingwater at 100 �C for (b) 3 days and (c) 7 days.

of trimesic acid. Below the IEP, the electrokinetic potentials (zetapotentials) of both crystals are positively charged due probablyto the presence of the positively charged species, Fe-OH2

+ andFe-FH+ on crystal surfaces of MIL-100(Fe). Above the IEP, the neg-atively charged species on the crystal surfaces are dominant spe-cies, probably deprotonated trimesate moieties. Note that thecharge is however not higher than ±30 mV, value which is relatedwith the interparticulate electric repulsion enough to ensure agood dispersion (without aggregation) in solution.

The existence of inorganic and organic moieties in the structureallows hydrophilic and hydrophobic parts to coexist within theporous solid, leading to an amphiphilic environment suitable forthe adsorption of a large number of molecules [10,11,35]. In gen-eral, the size, shape, and polarity of the sorbate molecules andthe available pore volume govern the equilibrium sorption uptakesof porous materials [36,37]. Water and ethanol molecules, whichare small and polar sorbate probes, can be used to evaluate thehydrophilic character of porous materials. In contrast, non-polararomatic molecules such as benzene or toluene can be used to esti-mate the hydrophobilic character of the materials. In order to con-firm the surface hydrophobicity and hydrophilicity of MIL-100(Fe),dynamic breakthrough sorption experiments using the binary mix-ture of toluene and water have been carried out together with sta-tic measurements of sorption isotherms on ethanol and toluene asprobe molecules.

According to the hydrophobicity measurements by break-through sorption experiments, the Weitkamp hydrophobicity in-dex (HI) values of MIL-100(Fe)_F and MIL-100(Fe)_NF7previously dehydrated at 150 �C are 2.87 and 2.75, respectively(Fig. 9). These values are not significantly different, within the

3 4 5 6

-30

-20

-10

0

10

20

30

40

Zeta

pot

entia

l (m

V)

pH

Fig. 8. Isoelectric points (IEPs) of MIL-100(Fe)_F (dash line) and MIL-100(Fe)_NF7(solid line).

0 5 10 15 20 25 30 35 40 450.0

0.2

0.4

0.6

0.8

1.0

1.2

C/C

0

Time (min)

MIL-100Fe_F(water) MIL-100Fe_F(Toluene) MIL-100Fe_NF3(water) MIL-100Fe_NF3(Toluene)

Fig. 9. Competitive breakthrough sorption curves of water and toluene for MIL-100(Fe)_F and MIL-100(Fe)_NF7 at 35 �C using a vapor mixture of water(p = 5600 Pa) and toluene (p = 6200 Pa) in helium (15 ml/min) as a carrier gas.

Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145 143

range of experimental errors. For comparison, the evaluated HI ofzeolite beta (SiO2/Al2O3 = 25) was also determined, being 1.88, inagreement with the literature [38]. These results point out thatMIL-100(Fe) is more hydrophobic than zeolite beta. Fig. 10 showssingle component adsorption isotherms of toluene and ethanol va-

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100

Ethanol Toluene

Amou

nt a

dsor

bed

(wt %

)

P/P0

(a)

Fig. 10. Vapor phase adsorption isotherms of toluene and ethanol at 30 �C: (a) MIL-100(Fat 150 �C for 12 h under high vacuum.

Table 2Physicochemical and sorption properties of several iron trimesates.

Sample Surface atomicratio of C/Fea

SBET

(m2/g)bVtotal

(cm3/g)b

MIL-100(Fe)_F 7.8 2050 0.90MIL-100(Fe) _NF7i 6.9 1970 1.04 (0.97)c

Basolite F300j 2.5 950 0.60 (0.44)c

a Surface atomic ratio (C/Fe) analyzed by X-ray photoelectron spectroscopy (XPS).b BET surface area; total pore volume.c Micropore volume calculated by the Dubinin–Radushkerich plot.d The Weitkamp hydrophobicity index.e Equilibrium sorption uptake of ethanol at 30 �C and p/p0 = 0.9.f Equilibrium sorption uptake of toluene at 30 �C and p/p0 = 0.9.g Equilibrium sorption uptake of propylene at 30 �C and 101 kPa.h Isoelectric point.i MIL-100(Fe) obtained from the F-free synthesis on the large scale (10 l) followed by

NH4F solution.j A commercial iron trimesate purchased from Sigma–Aldrich in a powder form.

pors at 30 �C on two MIL-100(Fe) samples. Interestingly, it can becommonly seen on both samples that toluene gives rise to two-step adsorption isotherms at a very low p/p0 (p0 is the saturationpressure of the adsorbate at 30 �C) while ethanol yields three-stepisotherms. Interestingly, the similar three-step isotherm was alsoobserved for the adsorption of polar solvents (water and ethanol)in MIL-101(Cr) [39]. MIL-100(Fe)_F adsorbs up to 0.63 g/g(6.84 mmol/g) of toluene at p/p0 > 0.3, while MIL-100(Fe)_NF7has a reasonably lower sorption capacity (0.60 g/g or 6.51 mmol/g), which is consistent with the difference of surface areas betweenthem. In adsorption isotherms of ethanol at 30 �C, the ethanol up-take of MIL-100(Fe)_F reaches 0.66 g/g (14.3 mmol/g) at p/p0 > 0.4while MIL-100(Fe)_NF7 has a lower sorption capacity (0.61 g/g or13.2 mmol/g) as similar to the toluene uptake. The occurrence ofthree-step isotherm in the ethanol adsorption might be due tothe presence of coordination on Lewis metal sites as well as doublemicropore windows and mesoporous cages within the framework.High uptakes of non-polar toluene and polar ethanyol from the sin-gle-component adsorption isotherms can be attributed to the coex-istence of two-types of adsorption sites showing hydrophobicity orhydrophilicity in the framework.

3.4. MIL-100(Fe) and commercial iron trimesate

The comparison of the physicochemical properties of both,the commercially available iron trimesate (Basolite F300) and

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100 Ethanol Toluene

Amou

nt a

dsor

bed

(wt %

)

(b)

P/P0

e)_F and (b) MIL-100(Fe)_NF7. Before measurements, the samples were dehydrated

HId Q(EtOH)(mmol/g)e

Q(Tol)(mmol/g)f

Q(C3H6)(mmol/g)g

IEPh

2.75 14.3 6.84 5.37 4.42.87 13.2 6.51 5.17 4.51.26 6.77 3.06 2.42 3.9

purification with sequential treatments with hot water, hot ethanol and aqueous

144 Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145

MIL-100(Fe)_NF7 produced from large scale synthesis, could be veryuseful for the design of commercial applications (see Table 2). Thestructure of Basolite F300 has been so far unknown because it is con-sidered an amorphous-like phase as illustrated in the XRD pattern(Fig. 11). Physicochemical properties of iron trimesates are listed

5 10 15 20 25 30

(a)

(a)

Inte

nsity

(a.u

.)

2 θ (degree)

Fig. 11. XRPD patterns of (a) MIL-100(Fe)_NF7 and (b) Basolite F300.

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600(a)

(b)

Am

ount

ads

orbe

d (m

l/g)

P/P0

Fig. 12. Nitrogen physisorption isotherms at�196 �C: (a) purified MIL-100(Fe)_NF7and (b) Basolite F300. Before measurements, the samples were dehydrated at150 �C for 12 h under high vacuum.

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50 Ethanol Toluene

Amou

nt a

dsor

bed

(wt %

)

P/P0

Fig. 13. Vapor phase adsorption isotherms of toluene and ethanol on Basolite F300at 30 �C. Before measurements, the samples were dehydrated at 150 �C for 12 hunder high vacuum.

in Table 2. BET surface area and micropore volume of Basolite F300are 950 m2/g and 0.44 cm3/g, respectively, which are half of thoseof the MIL-100(Fe)_NF7 (Table 2, Fig. 12). Considering different N2

physisorption isotherm shapes of MIL-100(Fe)_NF7 and BasoliteF300, the pore structure of Basolite F300 appears to be different tosome extent from that of MIL-100(Fe).

The surface atomic ratio of C/Fe in the Basolite F300 measuredby XPS was only 2.5, which is much lower than those in the MIL-100(Fe)_NF7 (6.9) and the MIL-100(Fe)_F (7.5). The much lowersurface ratio in the Basolite F300 is assumed to be attributed tothe presence of dense inorganic impurities and/or non porousamorphous phases since the theoretical molar ratio of C/Fe in theformula of MIL-100(Fe) is 6. According to hydrophobicity measure-ments, the Basolite F300 (HI = 1.26) is more hydrophilic than theMIL-100(Fe)_NF3 (HI = 2.87), in agreement with the presence ofinorganic impurities. From zeta potential measurements, an IEPof the Basolite F300 is observed at lower pH (3.9) than that ofthe MIL-100(Fe)_NF7 (4.5). Sorption uptakes of several gases andliquid vapors for the Basolite F300 are only about a half of thoseobserved for the MIL-100(Fe)_NF7, which is consistent with thedifference of micropore volume between them. In contrast toMIL-100(Fe), the Basolite F300 does not exhibit the three-stepisotherm in ethanol adsorption (Fig. 13). The sorption and IEP datasuggest that the surface properties of the Basolite F300 are signif-icantly different from those of MIL-100(Fe). Further characteriza-tion of MIL-100(Fe) and Basolite F300 on unsaturated iron sitesand the framework reducibility is under progress.

4. Conclusion

We have demonstrated that the hierarchically mesoporous iro-n(III) trimesate MIL-100(Fe) is well prepared from hydrothermalreactions in the absence of HF by using high concentration of reac-tant mixture, leading to a very high space–time-yield (>1700 kg/m3 day) on the large scale. It was confirmed that the F-free MIL-100(Fe) obtained from the large scale synthesis has similar physi-cochemical properties than MIL-100(Fe) prepared in presence ofHF. MIL-100(Fe) has shown very high uptakes for gases and liquidvapors as compared with those of commercially available irontrimesate from BASF. The high sorption ability for both non-polartoluene and polar ethanol probe molecules of MIL-100(Fe) hasbeen attributed to the respective interactions with hydrophilicand hydrophobic sites in the framework. However, hydrophobicityof MIL-100(Fe) is higher than those of conventional zeolite betaand commercial iron trimesate. Considering the unique sorptionproperties and hydrothermal stability, it is concluded thatMIL-100(Fe) is an excellent functional porous material for manypotential applications. The synthesis pathways of the crystallineMIL-100(Fe) phase under hydrothermal reactions could be explainedby a series of four steps including hydrolysis, deprotonation,self-assembly, and polycondensation. The success in the largescale synthesis of MIL-100(Fe) may encourage extensive studieson potential applications as well as many attempts to commercialproduction of MOFs.

Acknowledgements

This work was supported by the Korea CCS 2020 R&D Program(KCRC, KN-1134), the Institutional Collaboration Research Program(ISTK, SK-1110) and the International Collaboration Program(KICOS-MEST, KN-1127). French authors also acknowledge CNRSfor international scientific collaboration program (PICS No. 5443(2010–2012)). All authors thank Prof. Gérard Férey for his scientificencouragement to us. We would like also to thank Mr. In Tae Jangand Dr. D.Y. Hong for their synthesis and helpful discussion.

Y.-K. Seo et al. / Microporous and Mesoporous Materials 157 (2012) 137–145 145

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2012.02.027.

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