Probing Ge Distribution in Zeolite Frameworks by Post-Synthesis

Jul 10, 2012 - Probing Ge Distribution in Zeolite Frameworks by Post-Synthesis ... germanium-containing zeolites prepared under pure alkaline media...
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Probing Ge Distribution in Zeolite Frameworks by Post-Synthesis Introduction of Fluoride in As-Made Materials Xiaolong Liu,† Ugo Ravon,‡ Françoise Bosselet,† Gérard Bergeret,† and Alain Tuel*,† †

IRCELYON, UMR 5256 CNRS-Université Lyon 1, 2 Avenue A, Einstein 69626 Villeurbanne Cedex, France Kaust Catalysis Center (KCC), 4700 King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia

‡

S Supporting Information *

ABSTRACT: A new method has been developed to introduce fluoride in the structure of as-made germanium-containing zeolites prepared under pure alkaline media. Incorporation of fluoride species occurs without modification of the framework composition (Si/Ge ratio) and of the crystallinity, as evidenced by Xray diffraction and electron microscopy. After incorporation, 19F solid-state NMR has been used to probe the location and distribution of Ge atoms in the framework. In the case of ITQ-13 and ITQ-17, which can be prepared from both hydroxide and fluoride routes, incorporated F anions are located in the same structural units as those occupied when zeolites are prepared in the presence of fluoride. In the case of ITQ22 and ITQ-24, fluoride goes mainly in D4R units, which appear to be in the most energetically favorable positions for these zeolites. All experiments clearly show that zeolites prepared in the absence of fluoride in the synthesis medium are enriched in germanium, compared to the same materials obtained from F-containing gels. Moreover, Ge plays a strong structure-directing role by replacing Si atoms preferentially in D4R, leading to zeolites with mainly [4Si, 4Ge] units in the framework. In the particular case of ITQ-22, a new line observed around −2 ppm in 19F NMR spectra has been tentatively assigned to [3Si, 5Ge] D4R units, which corroborates the structural data obtained via X-ray diffraction. KEYWORDS: germanium, zeolite, fluoride, nuclear magnetic resonance, post-synthesis modifications

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surrounding framework. In all-silica zeolites, 19F NMR chemical shifts are generally ranging from −38 ppm to −75 ppm, depending on the size and morphology of the cage in which F ions are occluded.13 As an example, F ions occluded in the small [46] (D4R) cage of an octadecasil are observed at −38 ppm, whereas those occluded in the large [415462] cage of a nonasil silicate give a 19F NMR signal at −76 ppm. Moreover, for a given geometry of the cavity, the chemical shift strongly depends on the composition and gives precious information on the extent of framework substitution and the distribution of heteroelements. In the particular case of D4R units, the unique signal at −38 ppm observed for all-silica systems is split into several NMR lines in Ge-containing zeolites, each line corresponding to a specific Ge distribution in the D4R cages.2,14 This property has been used to confirm the preferential location of Ge atoms in D4R units and to follow Ge incorporation in various zeolite frameworks.4−9,15,16 However, when zeolites are prepared in the absence of fluoride under alkaline pH conditions, the only way to evaluate Ge distribution in the framework is to refine the structure and estimate the percentage of Ge at each T-site.17,18 We have recently reported that F ions could be easily removed from zeolite frameworks, even in the presence of organic molecules inside the pores.19 The extent of fluoride removal depended on

INTRODUCTION The introduction of germanium in the synthesis of all-silica zeolites led to the discovery of many new zeolite structures over the last two decades.1 By increasing the average T−O bond distance and decreasing the T−O−T angle, Ge stabilizes structural units in which angles are below 140°. This is particularly true for small units like D4R, in which the stressed T−O−T bonds are relaxed by substituting some of the Si atoms by Ge.2,3 Corma’s group took advantage of the structurestabilizing effect of Ge and of the fact that F− anions can also stabilize the formation of D4R units to prepare a series of germanosilicates from concentrated fluoride-containing gels.4−9 Because of the presence of small D4R units, the corresponding structures generally possess low framework densities (FD) with large or even extra large pores. Some of these structures can also accommodate a small fraction of Al atoms, introduced either by direct synthesis or by post-synthesis treatments.10,11 As for many heteroelements, the substitution of framework Si by Ge atoms modifies the 29Si NMR spectrum of the zeolite. In particular, new lines are observed around −100 ppm/TMS, assigned to Si atoms with a least one Ge atom in the first coordination sphere.2,12 Nevertheless, the technique is not very informative and it does not give clear indication on the distribution of Ge atoms in the framework. More-precise information on Ge incorporation can be indirectly obtained by 19 F NMR. 19F is not only an abundant nucleus (100% natural abundance); its spin quantum value is I = 1/2 and its chemical shift is very sensitive to the physical and chemical nature of the © 2012 American Chemical Society

Received: May 16, 2012 Revised: July 6, 2012 Published: July 10, 2012 3016

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the Ge content in the zeolite, particularly in D4R units.20 Preliminary experiments on all-silica ZSM-5 had shown that the process was reversible, and that F could also be reincorporated in zeolites prepared under alkaline pH conditions without changes in crystallinity and crystal habit.19 In the present work, we demonstrate that fluoride can be introduced in the structure of as-made Ge-containing zeolites prepared using the hydroxide route. The incorporation of fluoride occurs without any modification of the composition (Si/Ge ratio) and crystallinity of the zeolite. After incorporation, the corresponding 19F NMR spectra give unprecedented data on Ge distribution in these solids, which corroborates and completes XRD results of the literature.

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Thermal analysis data were obtained on a Netzsch Model STA 449F1 apparatus. Samples were heated in air from 25 °C to 1000 °C at a heating arte of 2 °C/min. The crystallinity of the zeolites was determined via X-ray diffraction (XRD) on a Bruker (Siemens) Model D5000 diffractometer using Cu Kα radiation. Diffractogrammes were collected between 3° and 70° (2Θ) with steps of 0.02° and 1 s per step.

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RESULTS AND DISCUSSION Previous experiments on silicalite-1 had suggested that fluoride could be reversibly removed and reincorporated in the as-made zeolite, without significant modifications of the crystallinity and crystal morphology.19 Later, we have reported the facile removal of F species from Ge-containing zeolites, particularly from the small D4R units, provided that the latter contain a high amount of Ge atoms.20 However, results were only qualitative and the method had not been quantified, particularly concerning fluoride reincorporation. Therefore, the method was first validated and quantified on an ITQ-17 zeolite prepared following the fluoride route with Si/Ge = 10 in the gel (Si/Ge = 9 in the final zeolite, see Table 1). This zeolite was

EXPERIMENTAL SECTION

Syntheses. All zeolites were synthesized using tetraethyl orthosilicate (TEOS, Aldrich 98%), germanium oxide (GeO2, 99.99%, Aldrich), and homemade hexamethonium hydroxide solutions. Hexamethonium bromide (HMBr2), prepared by reaction of dibromohexane with trimethylamine in ethanol, was converted into the hydroxide form by reaction with Ag2O. For ITQ-13, the preparation followed the recipe reported by Corma et al.5 Germanium oxide was first dissolved in an aqueous solution of hexamethonium hydroxide, followed by the addition of the silica source. The mixture was stirred under complete evaporation of the alcohol, formed upon hydrolysis of TEOS. The gel with the composition SiO2−0.1GeO2− 0.25HM2+−5H2O was placed in a stainless-steel Teflon-lined autoclave and heated at 175 °C for 2 weeks under slow rotation (60 rpm). ITQ17 was prepared following the same procedure, except that the gel was partially dehydrated at room temperature for 48 h before crystallization. During the dehydration period, the H2O/SiO2 ratio decreased to ca. 2. ITQ-22 was synthesized with the same SDA molecule (HM2+ cation) using the procedure reported by Sastre et al.21 The synthesis method was similar to that used to prepare ITQ-13 and ITQ-17, but the gel composition was different: SiO2−0.2GeO2−0.3HM2+−6H2O. ITQ-24 was synthesized from a gel with the following molar composition: SiO2−0.2GeO2−0.03Al2O3−0.3HM2+−6H2O, obtained by adding aluminum isopropoxide to the gel used for ITQ-22.18 For comparison, ITQ-13 and ITQ-17 were also prepared following the standard procedure in the presence of HF.5 In both cases, HF was added to the previous gels (HF/SiO2 molar ratio = 0.5) and the solid gel was mechanically mixed prior to autoclaving. After crystallization, all solids were recovered by filtration, washed with distilled water, and dried overnight at 110 °C. Incorporation of Fluoride in Zeolite Structures. Fluoride was incorporated in the structure of as-made zeolites following a recipe previously reported in the literature.19,20 Zeolites were mixed with NH4F (0.2 g NH4F/g of zeolite), the solid mixture was ground and finally heated at 150 °C for 24 h. After cooling temperature, the solid was washed with water and dried at 110 °C overnight. Physicochemical Techniques. Elemental analysis data were obtained from the IRCELYON-CNRS Analytical core facilities; metal contents were determined by inductively coupled plasma− optical emission spectroscopy (ICP-OES) (Horiba Jobin−Yvon Activa). Scanning electron microscopy (SEM) pictures were obtained on a Hitachi Model S800 microscope. Solid-state nuclear magnetic resonance (NMR) spectra were recorded on a Bruker DSX 400 spectrometer. 29Si MAS experiments were carried out at 79.4 MHz in 4 mm rotors spun at 10 kHz. Data were collected using a standard one-pulse sequence with 5 μs (π/3) pulses and 60 s delay. 19F NMR spectra were obtained at 376.3 MHz using a 2.5 mm probe head and a spinning speed of 18 kHz. The pulse length and the delay between two consecutive pulses were 3 μs (π/4) and 5 s, respectively. 29Si and 19F chemical shifts were referred to tetramethylsilane (TMS) and CFCl3, respectively.

Table 1. Chemical Composition (F, Ge, and Al Contents) for the Various Samples) Initial Composition (wt %) sample ITQ-17 ITQ-13 ITQ-17 ITQ-22 ITQ-24

b

Final Composition (wt %)

F

Gea

F

Gea

1.51

9.8 16.2 22.9 21.2 17.2 (2.5)c

1.42 1.25 1.45 1.11 1.06

10.2 15.7 21.8 20.8 17 (2.4)

Ge content was determined on zeolites calcined at 550 °C in air. Sample prepared with HF, defluorinated with NH4OH, and treated with NH4F for reincorporation. cAluminum content in parentheses for ITQ-24. a b

chosen on the basis of preliminary experiments that showed that fluoride could be easily and almost totally removed from the structure, particularly at high Ge contents.20 The 19F NMR spectrum of the zeolite shows two main signals at −8 and −19 ppm along with a minor signal at −38 ppm (Figure 1a). Previous reports on the evolution of 19F NMR spectra with the relative occupation of the different T-sites in ITQ-17 suggested that NMR lines at −8 and −19 ppm correspond to [4Si, 4Ge] and [7Si, 1Ge] D4R units, respectively.6 Indeed, a unique signal was observed at −7 ppm when Ge occupation on T1-sites was ca. 50%, i.e. when D4R units contained 4 Ge atoms. The third line at −38 ppm had been previously assigned to all-silica D4R units.2,5 The zeolite was first defluorinated using 1 M NH4OH solutions, as described in one of our previous publications.19 This procedure removed ∼95% of fluoride species originally present in the zeolite and was particularly efficient for Gecontaining D4R units, as evidenced by 19F NMR (Figure 1b). However, the signal at −38 ppm remained unchanged, thus supporting our previous observations on all-silica ITQ-13.19 Fluoride was then reincorporated by mixing the dried defluorinated zeolite with NH4F, following the procedure described in the Experimental Section. The final zeolite was then compared with the initial material. Both zeolites possess very similar F and Ge contents (Table 1), indicating that the successive treatments did not affect the framework composition (the Si/Ge molar ratio remained constant) and that 3017

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units, F occupied only the smallest D4R units, as it does for zeolites prepared in the presence of HF. Moreover, the relative intensity of signals at −8 and −19 ppm clearly indicated that the composition of the cage (Si/Ge ratio) did not influence the extent of reincorporation: all [4Si, 4Ge] and [7Si, 1Ge] units were occupied and contained approximately 1F−/D4R (the total amount of fluoride was 1.95 F/u.c., where u.c. denotes unit cell). Reincorporation of fluoride in ITQ-17 was further supported by 29Si NMR (see the Electronic Supporting Information (ESI)). For ITQ-13, we previously reported that 29 Si NMR chemical shifts were very sensitive to the presence of F species in the zeolite and that the overall shape of the spectrum was modified when fluoride was removed from the structure.5 In the present case, NMR spectra are modified both upon removal and reincorporation of fluoride, and the similarity between spectra of the as-made zeolite and final zeolites (spectra a and c) strongly supports 19F NMR data. The possibility to remove and reincorporate fluoride in the structure of as-made zeolites implies that F− species diffuse through the lattice in the presence of the template. For pure silica zeolites, we had previously assumed that diffusion could occur through 5- or 6-ring windows, simultaneously to the propagation of SiO−···HOSi framework connectivity defects.19 Recent data on silicalite-1, an all-silica zeolite with the MFI framework type, even showed that F− and SiO− were located in the same cavity of the structure, thus supporting the concept of “exchange” between fluoride anions and framework defects.22 However, the way fluoride enters and leaves small D4R units in Ge-rich zeolites, as well as the role of germanium in the process, are still unclear and deserve further investigation. Fluoride was then reincorporated in a series of Ge-containing zeolites obtained under alkaline conditions, in the absence of F− ions. For these zeolites, the negative charges necessary to balance the positive charges of organic templates most likely result from Si−O− and/or Ge−O− framework defects, generated by interaction between hydroxyl groups and Si− O−Si or Si−O−Ge bonds, respectively.23 Highly coordinated Ge atoms with additional OH− groups attached to GeO4 tetrahedra could also generate negative charges in the framework but their existence has never been demonstrated.24 The first solid was an ITQ-13 zeolite synthesized with a Si/Ge molar ratio of 10 in the gel. Although the synthesis of Ge-rich ITQ-13 materials from OH− media has been reported in the literature, information on the Ge content and the distribution of Ge atoms in the framework are not available.25 It is interesting to note that the Ge content in the zeolite is quite high, suggesting that some of the silica species did not react and remained in solution (see Table 1). After fluoride incorporation, neither the Ge content (Si/Ge = 6.5) nor the crystallinity and morphology changed, as evidenced by chemical analysis, XRD, and SEM, respectively (see Table 1 and Figures 3 and 4). Chemical analysis of the treated zeolite confirmed the presence of fluoride (see Table 1) at a concentration of ca. 1.72 F/u.c. The 19F NMR spectrum of the treated zeolite is composed of a major peak at −8 ppm along with two other resonances at −19 and −55 ppm (Figure 5a). Signals at −8 and −19 ppm have already been observed on ITQ-17 and can be assigned to [4Si, 4Ge] and [7Si, 1Ge] D4R units, respectively. The other signal at −55 ppm corresponds to fluoride anions in Gecontaining [415262] cages.5 The NMR spectrum is significantly different from that of a zeolite prepared with the same gel composition, but in the presence of HF. Indeed, in the presence

Figure 1. 19F NMR spectra of ITQ-17 prepared in the presence of HF with Si/Ge = 10 in the gel: (a) as-made, (b) defluorinated with NH4OH, and (c) treated with NH4F for reincorporation.

reincorporation of fluoride was complete, and not only limited to the outer surface of the crystals. The quality of the XRD pattern confirmed that the structure was not damaged and that the zeolite remained highly crystalline during a removal/ incorporation cycle (Figure 2).

Figure 2. Powder XRD patterns of (a) as-made, (b) defluorinated, and (c) refluorinated ITQ-17.

Moreover, neither the ammonium treatment nor the reincorporation of fluoride changed the organic content in the structure, as clearly demonstrated by TGA measurements (the percentage of carbon was 9.49 and 10.18 in the initial and final zeolites, respectively). Reincorporation of fluoride in the structure of ITQ-17 was definitely supported by the presence of 19 F NMR signals with chemical shifts and intensities similar to those observed on the initial zeolite (Figure 1c). Although the BEC framework contains different types of structural building 3018

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Figure 5. 19F NMR spectra of (a) ITQ-13 and (b) ITQ-17 prepared with Si/Ge = 10 under alkaline conditions and treated with NH4F. Spectrum (c) corresponds to an ITQ-13 prepared with the same Si/ Ge ratio but in the presence of HF.

Figure 3. Powder XRD patterns of zeolites prepared under pure alkaline conditions ((a) ITQ-13 and (b) ITQ-17) after reincorporation of fluoride in the structure.

like those observed at −38 ppm in the spectrum of the solid prepared in the presence of HF cannot be obtained in pure OH− media. Moreover, previous data had shown that F− anions could be easily removed from [4Si, 4Ge] units in ITQ-13 but not from [7Si, 1Ge], suggesting that [4Si, 4Ge] D4R were the most stable units in the absence of fluoride. Therefore, in the absence of HF, the zeolite can only crystallize from Ge-rich D4R units, principally [4Si, 4Ge] units, which explains the high Ge content in the framework and the intense 19F NMR signal at −8 ppm. The second example was an ITQ-17 zeolite synthesized in OH− medium with Si/Ge = 10 in the gel. As previously, the germanium content in the framework was high, and corresponded to a Si/Ge molar ratio of 3 (see Table 1). The characteristics of the treated zeolite confirmed that fluoride reincorporation occurred without significant changes in crystallinity and framework composition (Table 1 and Figure 3). The 19F NMR spectrum of the zeolite showed a main signal at −8 ppm, indicating that F was located in D4R units, as is the case for zeolites directly prepared in the presence of HF (Figure 5b). The fluoride content was 1.45 wt %, corresponding to ∼0.96 F ions per D4R, a value close to that obtained on zeolites prepared with fluoride in the gel. Once more, the intense resonance at −8 ppm confirmed that D4R contained a high proportion of Ge atoms. As for ITQ-13, the distribution of Ge atoms in the framework is different from that obtained in the presence of HF (compare Figures 1a and 5b), which supports the strong stabilizing effect of Ge in D4R units. In the absence of fluoride anions, both ITQ-13 and ITQ-17 crystallize from gels in which Ge-rich D4R units are dominant. This leads to zeolites with high Ge contents (generally higher than the expected value), in which Ge occupies preferentially [4Si, 4Ge] units. In theory, if zeolites contained only [4Si, 4Ge] D4R and if Ge was exclusively located in these units, the theoretical Si/ Ge ratios would be 6 and 3 for ITQ-13 and ITQ-17, respectively. These values are in excellent agreement with ratios observed experimentally (6.5 and 3 for ITQ-13 and ITQ-

Figure 4. MEB pictures of selected zeolites before (left) and after (right) fluoride reincorporation in the structure.

of fluoride, the Si/Ge ratio in the zeolite (Si/Ge = 9.2) is close to the expected theoretical value and the corresponding 19F NMR spectrum shows four signals, in agreement with spectra previously reported in the literature (Figure 5c).5 A significant difference is the existence of a signal at −38 ppm when the zeolite is prepared in the presence of HF. All-silica D4R units 3019

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17, respectively), thus supporting the essential role of [4Si, 4Ge] D4R in the crystallization of germanosilicates from fluoride-free media. The method was further applied to ITQ-24, which is a germanosilicate with a three-dimensional pore system containing intersecting 10-MR and 12-MR channels.18 This zeolite has been obtained using different structure-directing agents within a quite wide Si/Ge range of compositions.23,26 With hexamethonium cations, ITQ-24 can only be obtained from Ge-rich gels in the absence of fluoride.21 However, the crystallization necessitates the presence of 4-coordinate aluminum species, which generates negative charges and partially ensures charge matching between the zeolite framework and organic HM2+ cations. Chemical analysis of the calcined zeolite gave 17 wt % Ge and 2.5 wt % Al, corresponding to a framework composition Si45.3Ge7.85Al2.85O112 (see Table 1). Moreover, TGA analysis showed that the as-made solid contained ∼2.8 HM2+ cations, i.e., 5.6 positive charges per unit cell. All aluminum species are tetrahedrally coordinated in the as-made zeolite, as evidenced by a unique signal at 55 ppm in the 27Al NMR spectrum (see the ESI). However, the amount of framework aluminum species is not enough to achieve the full compensation of the 2.8 HM2+ cations and ∼50% of the positive charges are balanced by framework defects. According to our previous results, these defects make possible the incorporation of fluoride anions in the as-made zeolite.19,20 Incorporation of fluoride in the zeolite did not alter the crystallinity of the zeolite, as evidenced by very intense and highly resolved XRD patterns for the as-made and treated solids (see the ESI). Moreover, the framework composition did not change, with the sSi/Ge and Si/Al ratios very close to those of the original zeolite (Table 1). In addition, the presence of a unique 27Al NMR signal at 55 ppm in both zeolites supports the absence of dealumination during the process (see the ESI). After incorporation, the 19F NMR spectrum is essentially composed of a unique signal at −9 ppm, corresponding to F− anions in [4Si, 4Ge] D4R units (see Figure 6a). Despite an excellent crystallinity of the zeolite, the signal is broad, compared to those observed on ITQ-13 and ITQ-17 (full width at half-maximum (fwhm) values of 8.5 ppm for ITQ-24 and 6 ppm for ITQ-17). This difference, which has not yet been elucidated, could result from the presence of aluminum species in the framework. A weak contribution is also present at ca. −20 ppm, suggesting the presence of [7Si, 1Ge] units, but at a low level. The fluoride content (see Table 1), which corresponds to ∼0.95 F−/D4R, suggests that fluoride incorporation was almost complete in D4R units. The refinement of the powder XRD pattern of ITQ-24 has shown that Ge atoms occupy essentially T1 sites, which correspond to the sites forming D4R units in the zeolite structure, and that all other sites could be considered as almost purely siliceous.18 Assuming that all Ge atoms are in D4R units, 19F NMR indicates that the unit cell contains ∼8 Ge atoms (half the number of T1 sites), in excellent agreement with chemical analysis. The 19F NMR spectrum of the zeolite also shows weak signals between −40 and −70 ppm, which intensity represents 4, in particular [2Si, 6Ge] units. They also concluded that Ge could be substituted in the silicate framework in a wide compositional range, but in an ordered fashion. More recently, Dodin et al. reported the synthesis of a new microporous germanosilicate IM-20 with D4R units containing 5Ge atoms (on average).16 The correspond-

ing 19F NMR signal appeared at −8.9 ppm (i.e., the position usually assigned to [4Si, 4Ge] D4R units). From the 19F NMR, a value higher than 4Ge/D4R can only be obtained, assuming that the line at −2 ppm corresponds to [3Si, 5Ge] D4R. Taking into account the relative intensities of the lines at −2 and −8 ppm (∼0.35:0.65), the number of Ge atoms calculated from NMR is thus 4.35 Ge/D4R, in very good agreement with the value obtained from refinement of the XRD pattern.

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CONCLUSIONS For Ge-containing zeolites prepared in the presence of F− anions, 19F NMR has shown to be a very powerful technique to get information on the location of fluoride species in the structure and on the composition of the cavities in which they are occluded. However, when zeolites crystallize from fluoridefree gels, the only way to get information on the framework composition is to refine the powder XRD pattern and estimate the Ge concentration on each T-site. The method developed in the present work, which consists in the post-synthesis incorporation of fluoride in as-made zeolites, represents a real alternative to X-ray diffraction (XRD) to estimate the Ge distribution in zeolite frameworks. We have demonstrated that F− anions could be easily introduced in a series of as-made zeolites without modification of the chemical composition and crystallinity. In most cases, fluoride is incorporated mainly in D4R units, which seem to be the most energetically favorable positions in the framework. After incorporation, fluoride has been used as a probe to reveal the framework composition using 19F solid-state NMR. As a general trend, we have observed that all Ge-containing zeolites prepared in the absence of fluoride contain a high proportion of germanium. The enrichment in germanium is particularly clear when comparing zeolites like ITQ-13 and ITQ-17, obtained in the presence and the absence of HF. In the absence of F− anions, frameworks essentially contain [4Si, 4Ge] D4R, which stabilize the structure and appear to be essential for the crystallization. Because of the sensitivity of 19F nuclei to the environment, NMR gives new and very precious information that are not necessarily easy to obtain by XRD. In particular, the existence of a 19F NMR line around −2 ppm allowed us to propose the existence of [3Si, 5Ge] D4R units in ITQ-22.

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ASSOCIATED CONTENT

* Supporting Information S

NMR spectra of ITQ-17 and ITQ-24 zeolites; XRD powder patterns of ITQ-22 and ITQ-24; XRD analysis of ITQ-22 with Ge occupancies on T-sites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication is based on the work supported by Award No. UK-C0017, made by King Abdullah University of Science and Technology (KAUST). The authors would like to thank Dr. Aude Demessence for SEM pictures. 3021

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dx.doi.org/10.1021/cm301511y | Chem. Mater. 2012, 24, 3016−3022