Synthesis, Structure, and Crystallization Study of a Layered Lithium

Jan 19, 2012 - Synopsis. A novel lithium metal−organic framework material constructed from the ligand thiophene-2,5-dicarboxylate forms under solvot...
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Synthesis, Structure, and Crystallization Study of a Layered Lithium Thiophene-Dicarboxylate Racha El Osta,† Michel Frigoli,† Jérôme Marrot,† Manuela E. Medina,‡ Richard I. Walton,*,§ and Franck Millange*,† †

Institut Lavoisier, Université de Versailles St-Quentin en Yvelines (UMR CNRS 8180), 45 Avenue des Etats-Unis, 78035, Versailles Cedex, France ‡ Departamento de Nuevas Arquitecturas en Química de Materiales, Instituto de Ciencia de Materiales de Madrid, C/Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049 Madrid, Spain § Department of Chemistry, The University of Warwick, Coventry, CV4 7AL, U.K. S Supporting Information *

ABSTRACT: A layered lithium carboxylate, Li4[C4H2S(CO2)2]2[C3H7NO]2, crystallizes under solvothermal conditions (160−180 °C) from lithium nitrate and the ligand 2,5-thiophenedicarboxylate in N,N-dimethylformamide as solvent. Single-crystal X-ray diffraction (Pbca a = 10.0216(18) Å, b = 18.327(4) Å, c = 24.871(5) Å) shows that the material is constructed from lithium-centered tetrahedral units linked by edge- and corner-sharing to give tetrameric clusters. These inorganic building units are linked in two dimensions by the bidentate ligand to give a layered structure in which additionally coordinated dimethylformamide projects into the interlayer region. A study of the crystallization of the material using time-resolved in situ energy-dispersive X-ray diffraction reveals that the material crystallizes directly from solution at the reaction temperature following an induction period. Analysis of the crystallization curves suggests that nucleation does not extend far into the crystallization period and that crystal growth is one-dimensional. These findings are corroborated by the observation of relatively large, anisotropic crystals by scanning electron microscopy.



INTRODUCTION Metal−organic framework (MOF) materials of the s-block metals have attracted some attention recently and have been the subject of an extensive review by Banerjee and Parise.1 As with other families of MOFs, the interest in the materials stems from the ability to construct extended network structures by combining a multidentate organic linker with metals of either known coordination preference or with some specific functionality to prepare materials with some degree of predictability in their structures.2−6 One particular goal in this area is the rational synthesis of highly porous structures that may have applications in gas storage, molecular separation, or shape-selective catalysis.7 In the case of s-block metals, the coordination geometry of the metal is less well-defined than for transition-metal centers, since bonding is more ionic in nature. Thus the geometry and connectivity of the organic linkers play a more important role in defining the structure of MOFs of these metals.1 Nevertheless, strong metal-carboxylate bonds are expected which should lead to robust framework materials, and indeed Banerjee et al. prepared a three-dimensional (3D) lithium carboxylate Li2(2,6-NDC) (NDC = naphthalene dicarboxylate) that was thermally stable to over 600 °C.8 Furthermore, the low relative weight of metals such as lithium and magnesium potentially may provide materials with favorable gravimetric properties for gas sorption, permitting a high weight percent of adsorbent.9 © 2012 American Chemical Society

In the case of lithium carboxylate materials, a number of frameworks have now been reported. When 1,4-benzene dicarboxylate10 and 2,6-naphthalene dicarboxylate8 are used as linkers, the structures formed are constructed from pairs of edge-shared lithium-centered tetrahedra, linked by corners to give antifluorite inorganic sheets, which are pillared by the organic linker to give a 3D structure. Using pyridine dicarboxylates, greater structural diversity is possible, with chain, layered, or 3D structrues formed, depending on the orientation of the carboxylate functionalities in various isomers of the linker.9,11 In the lithium materials reported so far the metal center is always tetrahedrally coordinated, but these tetrahedra may be linked with each other in a number of ways to give various inorganic structural building units. In terms of properties, lithium carboxylates have been termed “ultralight MOFs” to indicate their favorable gravimetric properties toward gas sorption. 9 Abrahams et al. used the bifunctional isonicotinate linker to prepare the first porous lithium metal− organic framework material that showed reversible sorption of a number of gases.12 Recently, Zhao et al. described 3D lithiumbased framework structures prepared using mixed ligands (one neutral and one negatively charged) that show hydrogen Received: December 1, 2011 Revised: January 5, 2012 Published: January 19, 2012 1531

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Figure 1. Tetrameric building units seen in the lithium-organic frameworks (a) Li4[C4H2S(CO2)2]2[C3H7NO]2 (b) Li2(4,4′-SDB) (SDB = sulfonyldibenzoate)36 and (c) Li2(2,5-PDC)(DMF) (2,5-PDC = 2,5-pyridine dicarboxylate).11

storage properties at 77 K and ambient pressure13 and also prepared an unsual framework using 4-pyridinol as the linker, which gave a structure with cubane-like Li4 building units and zeolitic gas sorption properties.14 Another interesting feature of metal−organic framework materials is their structural flexibility, and dynamic structural behavior has been reported in lithiumbased MOF, where reversible removal and uptake of coordinated solvent gives a structural transformation involving bond breaking and formation around the lithium centers.11 In this paper we report the synthesis and structure of a lithium metal−organic framework constructed from the ligand thiophene-2,5-dicarboxylate. This linker has been used previously to prepare a number of metal organic framework materials, including networks constructed from zinc,15 indium,16 cadmium,16 cobalt,17 manganese,17 and various lanthanides18,19 as metal centers. We have also made a study of the crystallization of the new material using time-resolved, in situ X-ray diffraction: this allows noninvasive measurement within the solvothermal reaction vessel of the extent of crystallization with time.20 Despite the interest in the rational design of new materials, there are presently rather few reported in situ studies of MOF crystallization.21−29



direct methods, developed by successive difference Fourier syntheses, and refined by full-matrix least-squares on all F2 data using SHELXTL V6.14.32,33 All non-H atoms were refined anisotropically. The H atoms were placed in calculated positions and refined by using a riding mode. data: C18H18Li4N2O10S2, Mw = 514.22, orthorhombic space group Pbca; cell dimensions: a = 10.0216(18) Å, b = 18.327(4) Å, c = 24.871(5) Å, V = 4567.9(16)Å3; Z = 8; μ = 0.29 mm−1; 232890 reflections measured at RT; independent reflections: 6662 [5171 Fo > 4σ (Fo)]; data were collected up to a 2θmax value of 60.06° (99.8% coverage). Number of variables: 329; R1 = 0.0425, wR2 = 0.1193, S = 1.057; highest residual electron density 0.497 e.Å−3 (all data R1 = 0.0559, wR2 = 0.1333). CCDC 852579. This number contains the supplementary crystallographic data, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Temperature-Dependent X-ray Powder Diffraction. Patterns were recorded using a Siemens D5000 diffractometer (θ−θ mode, Co Kα radiation, λ = 1.7903 Å) equipped with an Anton Paar HTK16 high temperature device and an M Braun linear position-sensitive detector (PSD) under static air. Patterns were scanned with a resolution of 0.0147° and a divergence slit of 0.1° over the angular range 2θ = 6− 24° to observe the most intense low-angle Bragg reflections. Temperature steps of 10 K from 293 to 773 K were typically used. Time-Resolved Diffraction. In situ energy-dispersive X-ray diffraction (EDXRD) spectra were recorded using Beamline F3 of the HASYLAB facility, Germany. This beamline receives white-beam radiation with energy of 13.5−65 keV and the incident X-ray beam is typcially collimated to 20 × 20 μm2. Reactions were performed within 12 mm diameter DURAN tubes fitted with PBT screw caps and PTFE-coated gaskets. Reagents, using the molar ratios given above, based on 0.52 g of lithium nitrate, were stirred for a few minutes before introducing 2 mL of the resulting suspension into the tube which was placed in a circulating oil heater equipped with a magnetic stirring device. EDXRD patterns were recorded in 120 s intervals using a fixed angle solid-state Ge detector whose angle (2θ) was calibrated using the characteristic Bragg peaks of a premade solid sample of the material. In the EDXRD experiment, a Bragg peak is characterized by an energy, E/keV, related to its d-spacing, d/Å, according to

EXPERIMENTAL SECTION

Synthesis. Lithium nitrate (0.138 g, 2 mmol, Aldrich) and 2,5thiophenedicarboxylic acid (0.172 g, 1 mmol) were dissolved in 5 mL of N,N-dimethylformamide (DMF). The mixture was placed in a Teflon container, sealed in a stainless-steel autoclave, and heated at 175 °C for 12 h under autogenous pressure. The resulting solid phase is homogeneous and constituted of colorless needles. Exposure to air does modify the appearance of the crystals: they become cloudy and therefore have to be kept in the mother liquor. Thermogravimetry. Thermogravimetric analysis was carried out on a Perkin Elmer STA 6000 thermogravimetric analyzer under a flow of oxygen gas at a heating rate of 3 K·min−1 from 293 to 873 K. Infrared Spectroscopy. Infrared spectroscopy was performed with a Thermo Scientific Nicolet iS10 FT-IR spectrometer. Single Crystal Diffraction. A broken part of a needle-like colorless crystal (0.3 mm × 0.25 mm × 0.25 mm) was selected under a polarizing optical microscope and mounted onto a cryoloop with the viscous oil-drop method for a single-crystal X-ray diffraction experiment. X-ray intensity data were collected on a Bruker X8APEX2 CCD area-detector diffractometer using Mo-Kα radiation (λ = 0.71073 Å) at room temperature. Nine sets of narrow data frames (10 s per frame) were collected at different values of θ for two initial values of φ and ω, respectively, using 0.5° increments of φ or ω. Data reduction was accomplished using SAINT V7.03.30 The redundancy (35.9) in data allowed a semiempirical absorption correction (SADABS V2.10)31 to be applied on the basis of multiple measurements of equivalent reflections. The structure was solved by

d=

6.1992 E sin θ

(1)

Data were normalized to the incident beam intensity by using the logged synchrotron beam current. The program PowDLL34 was used to process the data into a format suitable for peak fitting using the program XFIT35 using Pseudo-Voigt functions. The identification of the final solid product was confirmed after the synchrotron experiment using ex situ powder XRD (see Supporting Information).



RESULTS AND DISCUSSION The new lithium metal−organic framework material, Li4[C4H2S(CO2)2]2[C3H7NO]2, is constructed from tetrahedral lithium centers: three of the oxygen atoms forming the 1532

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Figure 2. Views of the crystal structure of Li4[C4H2S(CO2)2]2[C3H7NO]2 (a) view along a showing connectivity of the building units (b) view along c showing the layered motif and (c) view showing two stacked layers.

pyridine dicarboxylate),11 Figure 1c, in which one of the vertices is formed from coordinated nitrogen of the linker, and the tetrahedra are shared by just one common edge as well as vertices. The tetrameric units shown in Figure 1a are cross-linked by the bidentate dicarboxylate to give a layered motif, Figure 2a,b. Each carboxylate moeity connects an edge of one unit with a vertex on a neighboring unit. Half of the terminal oxygen vertices of the tetrameric unit arise from coordinated dimethylformamide molecules. Thus ribbons of linked tetramers extend along a in an undulating manner. The combination of the steric requirements of the coordinated linker and the DMF gives rise to layers in the ab plane. In this plane (Figure 2b), adjacent ribbons are oriented in opposite directions, cross-linked by pairs of the linkers, between which there is evidence for π−π interactions (∼3.75 Å between the centers of the thiophene rings). The hybrid layers are stacked parallel to the c-axis, Figure 2c, where the monodentate dimethylformamide molecules project into the interlayer region and are interleaved between adjacent sheets, with only van der Waals interactions apparently responsible for the layer stacking. The layered network is connected solely by the organic linkers; thus, according to the nomenclature of Cheetham et al. for hybrid materials, the structure may be denoted I0O2, falling in the category of a layered coordination polymer.37

coordination sphere are from coordinated thiophene-2,5dicarboxylate, while the fourth is from coordinated solvent, N,N-dimethylformamide. As with the other lithium carboxylates already described in the literature, the metal center is tetrahedrally coordinated by four donor atoms, all oxygen in this case. The average Li−O bond distance in our material is 1.972 Å, which matches well the average Li−O distance of 1.976 Å seen in lithium carboxylates recorded in the Cambridge Crystallography Database.1 In some of the previously reported lithium carboxylates there is also coordinated dimethylformamide,1 for example, in Li2(2,5-PDC)·(DMF) (PDC = pyridine dicarboxylate). The tetrahedra in the new material share corners and edges to form tetrameric inorganic building units consising of a fourring of the primary building units (to use zeolite nomenclature), Figure 1a. In the previously reported phase Li2(4,4′-SDB) (SDB = sulfonyldibenzoate), the same four-ring of corner- and edge-shared tetrahedra are seen as in our material but the vertices consist of oxygens from only the linking ligand and not coordinated solvent, Figure 1b.36 The tetrameric unit in the previously reported compound is more regular: the tetrameric unit is distorted in the new material in such a way as to bring together the vertex shared oxygens (5.113 Å vs 5.255 Å) with a corresponding lengthening of the distances between the shared edges (3.508 Å vs 3.164 Å). A distinctly different tetrameric unit was reported in Li2(2,5-PDC)(DMF) (2,5-PDC = 2,51533

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Figure 3. Thermodiffraction recorded during the heating of Li4[C4H2S(CO2)2]2[C3H7NO]2 with TGA inset.

Infrared spectroscopy (Supporting Information) indicates the presence of vibrational bands characteristic of the organic moieties, with the -(O−C−O)-bands at 1530 (asymmetric stretch) and 1385 cm−1 (symmetric stretch), confirming the presence of carboxylate moieties within the solid. According to the crystal structure the strong absorption of 2,5-thiophenedicarboxylic acid at 1657 cm−1 should not be observed in the IR spectrum, since the carboxylic acid is deprotonated; however, a band in this region (at 1661 cm−1) is seen, due to the presence of coordinated DMF. IR spectroscopy also confirms the absence of any coordinated water or hydroxide. Thermogravimetric analysis (TGA) and thermodiffractometry, Figure 3, show the onset of loss of coordinated DMF at 400−450 K (total mass loss to 700 K = 29.0%, and expected mass loss = 28.4%), which is accompanied by a structural transition to give a more dense phase with loss of the large dspacing (001) peak. This occurs in two stages, as indicated on the thermodiffraction trace, but it was not possible to isolate the transient crystalline phases under ambient conditions. On further heating an abrupt mass loss at 730 K occurs which must correspond to the loss of the dicarboxylate linker, and this leads to an amorphous product, although the mass loss would suggest that the product phase is Li2CO3 (observed total mass loss = 69.0%, expected mass loss = 71.3%). Figure 4 shows a contour representation of EDXRD patterns measured during the crystallization of the lithium carboxylate at 160 °C. Starting from a clear solution, the most intense (001) Bragg peak is observed to steadily increase in intensity with time following an induction period where no crystalline material is observed. The observation of an induction time for crystallization is well-known in the hydrothermal crystallization of zeolites from gels and is related to the process of nucleation.38 In the case of MOFs, for the copper carboxylates HKUST-126 and MOF-14,25 crystallization was observed to be instantaneous by in situ EDXRD with Bragg peaks of product seen immediately on mixing the chemicals, whereas for the iron(II) carboxylate MIL-53, crystallization of the intended product was preceded by the formation of another framework,

Figure 4. 3D contour map of EDXRD data recorded during the crystallization of Li4[C4H2S(CO2)2]2[C3H7NO]2.

MOF-235, which contains different building units.26 Recently for the case of a copper hybrid framework containing a germanium metalloligand, an induction time for crystallization was seen, which interestingly was independent of temperature.39 Analysis of the in situ EDXRD data shown in Figure 4 was performed by integrating the Bragg peak area as a function of time. The normalized extent of crystallization, α, was determined by dividing the Bragg peak intensity by the maximum value reached at the end of the crystallization. To 1534

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account for decay in incident beam intensity over the course of the accumulation of data, the incident beam monitor counts were used to produce normalized crystallization curves. Kinetic analysis of solvothermal crystallizations has been approached using various mathematical models,20,40,41 and indeed for a variety of classes of material, ranging from microporous silicate zeolites,42−45 phosphate zeotypes,46−48 chalcogenides,49,50 and dense metal oxide materials.51−53 The most common approach has been to use the Avrami−Erofe’ev equation, eq 2.54−57 α = 1 − exp{ − [k(t − t0)]n }

(2)

Here, k is the rate constant of crystallization, t is the time coordinate, t0 is the induction time (if any) for the crystallization, and n is the Avrami constant which may contain information about the mechanism of crystallization. This expression is versatile enough to describe the form of many extent of crystallization curves and allows calculation of a rate constant of crystallization.40 However, the model was orginally developed in a study of solid-state reactions, such as the crystallization of alloys from melts, and the underlying assumptions used to derive the expression have little relevance for the chemistry involved in the formation and crystallization of solids from a solvothermal reaction, which is often a heterogeneous mixture of dissolved and solid reagents. In recent work on the crystallization of some Cu(II) MOFs,25 we showed that a different model, that proposed by Gualtieri for the hydrothermal crystallization of zeolites,58 allows more physical insight into crystallization; in particular, it separates nucleation and growth as two separate processes with different activation energies. Gualtieri showed that the extent of crystallization (α) vs time (t) may be expressed as 1

α=

{ − ( )}

1 + exp

t−a b

. {1 − exp[ − (k gt ) n]} (3)

where kg is the rate constant of crystal growth, while a and b are constants related to nucleation. In this case n describes the dimensionality of crystal growth (i.e. is not the same value of n as the Avrami exponent in eq 1). Importantly, if these values are known then a probability function for nucleation, PN vs time can be calculated. ⎧ (t − a)2 ⎫ ⎬ P N = exp⎨ − 2b2 ⎭ ⎩ ⎪







(4)

Figure 5 shows the crystallization curves at the three temperatures studies, with final fits obtained for the Gualtieri model (eq 3). Also shown is the simulated nucleation curve (eq 4) using the final fitted parameters (Table 1). The induction time for crystallization given in Table 1 is simply that determined by inspection of the data. In order to achieve a satisfactory fit with a physically meaningful set of rate constants for the three temperatures studied, the value of n in the Gualtieri model was fixed at 1 (other values gave inconsistent trends in rate constant with temperature). This would imply a one-dimensional (1D) crystal growth, which is borne out by the anisotropic crystals seen by SEM, Figure 6. This is in contrast to the 3D crystal growth seen for the copper(II) carboxylates HKUST-1 and MOF-14 when analyzed using the same kinetic model.25 The simulated nucleation curves shown in Figure 5 imply that for the lithium carboxylate, nucleation is almost

Figure 5. Fits to the crystallization curves of Li 4 [C 4 H 2 S(CO2)2]2[C3H7NO]2 using the Gualtieri at three temperatures (a) 160 °C, (b) 170 °C, (c) 180 °C. Points are experimental data, the red line is the simulated crystal growth, and the blue line the simulated probability of nucleation.

instantaneous: it occurs only at the earliest stages of crystallization, that is, it does not extend much into the period of crystal growth. This is also rather different behavior to HKUST-1 and MOF-14, where we found the formation of nucleation sites extended late into the crystallization period,25 an observation made independently by Zacher et al. for HKUST-1 using light scattering.24 The observation of relatively large crystals of the lithium framework would fit these observations, with the formation of a smaller number of nucleation sites at which crystal growth then takes place at. 1535

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Table 1. Kinetic Parameters for the Crystallization of Li4[C4H2S(CO2)2]2[C3H7NO]2 Fitted Using the Gualtieri Modela T/ °C

t0 / min

a/ min

b/ min

kg / min−1

kn / min−1

160 170 180

32 15 5

35.7(3) 16.87(22) 7.01(39)

0.51(26) 0.31(28) 0.89(34)

0.0152(3) 0.0248(4) 0.062(2)

0.028(1) 0.059(1) 0.143(8)

Ea(growth)/ kJ mol−1

A(growth)/ min−1

Ea(nucleation)/ kJ mol−1

A(nucleation)/ min−1

114.7(±22)

9.72 × 1011

132.7(±7.8)

2.76 × 1014

a The rate of nucleation, kn, is given by 1/a, and A is the pre-exponential factor from Arrhenius analysis. The estimated uncertainties on kinetic parameters are those from the least squares fit of the model to the data, and the errors on the parameters from Arrhenius analysis are standard linear regression errors.

materials have a subtle dependence on the particular choice of metal, linker, solvent, and reaction conditions.



CONCLUSIONS Crystallization of a new layered lithium dicarboxylate material adds to the recent reports of lithium-organic hybrid materials that have been studied for their possible application as “lightweight” porous materials. The in situ diffraction study of crystallization provides new reference data for understanding the formation mechanism of MOFs, needed for the future predictability in the synthesis of novel complex materials.



ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S2: IR spectra and indexed powder XRD and EDXRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. SEM showing 1D morphology of the crystals.



Figure 7 shows Arrhenius plots of the effect of temperature on the rate constants, from which activation energies for both

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.M.); r.i.walton@warwick. ac.uk (R.I.W.).



ACKNOWLEDGMENTS We thank DESY for award of beamtime at HASYLAB and we are grateful to Professor Wolfgang Bensch (Kiel) for the loan of his heating device for the in situ experiments and to Mark Feyand (Kiel) for his assistance with use of Beamline F3. We thank Chrystelle Thouvenot (Versailles) for the SEM measurement.



REFERENCES

(1) Banerjee, D.; Parise, J. B. Cryst. Growth Des. 2011, 11, 4704. (2) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (3) Férey, G. Chem. Soc. Rev. 2008, 37, 191. (4) Long, J. R.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1213. (5) O’Keeffe, M. Chem. Soc. Rev. 2009, 38, 1215. (6) Kepert, C. J. In Porous Materials; Bruce, D. W., O’Hare, D., Walton, R. I., Eds.; John Wiley and Sons: Chichester, 2010. (7) Czaja, A. U.; Trukhan, N.; Müller, U. Chem. Soc. Rev. 2009, 38, 1284. (8) Banerjee, D.; Kim, S. J.; Parise, J. B. Cryst. Growth Des. 2009, 9, 2500. (9) Banerjee, D.; Kim, S. J.; Borkowski, L. A.; Xu, W. Q.; Parise, J. B. Cryst. Growth Des. 2010, 10, 709. (10) Kaduk, J. A. Acta Crystallogr. Sect. B-Struct. Sci. 2000, 56, 474. (11) Banerjee, D.; Kim, S. J.; Li, W.; Wu, H. H.; Li, J.; Borkowski, L. A.; Philips, B. L.; Parise, J. B. Cryst. Growth Des. 2010, 10, 2801. (12) Abrahams, B. F.; Grannas, M. J.; Hudson, T. A.; Robson, R. Angew. Chem. - Int. Ed. 2010, 49, 1087. (13) Zhao, X.; Wu, T.; Bu, X. H.; Feng, P. Y. Dalton Trans. 2011, 40, 8072.

Figure 7. Arrhenius plots for nucleation and growth of Li4[C4H2S(CO2)2]2[C3H7NO]2.

crystal growth and nucleation may be determined, Table 1. These values may be compared to those obtained for HKUST1 and MOF-14, where the same kinetic model was applied: it is noteworthy that we see considerably higher values of activation energy for both nucleation and growth for the lithium MOF (>100 kJ mol−1). For the two Cu(II) materials (also crystallized under solvothermal conditions at only slightly temperatures), the values are somewhat lower. Although it is difficult to draw any physical significance from these values, especially without an extensive set of results for a variety of materials, it is clear that the pathways involved in the crystallization of MOF 1536

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dx.doi.org/10.1021/cg201587u | Cryst. Growth Des. 2012, 12, 1531−1537