Article pubs.acs.org/cm
Host−Guest Stabilization of a Zeolite Strained Framework: In Situ Transformation of Zeolite MTW into the Less Dense and More Strained ITW Alex Rojas,† María Luisa San-Roman,‡ Claudio M. Zicovich-Wilson,§ and Miguel A. Camblor†,* †
Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, Sor Juana Inés de la Cruz, 3, 28039 Madrid, Spain Centro de Investigaciones Químicas and §Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209 Cuernavaca (Morelos), Mexico
‡
S Supporting Information *
ABSTRACT: The new organic structure-directing agent 1ethyl-2,3-dimethylimidazolium, in conjuction with fluoride anions, shows selectivity toward pure silica zeolite ITW. At low water contents this zeolite crystallizes directly, while at higher water contents, the denser and more stable in the absence of occluded species MTW crystallizes first and then transforms in situ into ITW. A detailed physicochemical and structural characterization and a periodic density functional theory analysis are provided, and we show crystallographic and DFT evidence for a significant distortion of the SiO4 tetrahedra, attributable to a polarization of the Si−O bond that helps relax the otherwise strained silica framework. A comparative analysis of five closely related imidazolium cations suggests the importance of both hydrophilicity and conformational flexibility in determining their selectivity as structure-directing agents. KEYWORDS: zeolite synthesis, structure direction, ITW, in situ transformation, fluoride, imidazolium
■
INTRODUCTION Structure direction,1,2 the factors determining the zeolite that crystallizes under a set of given crystallization conditions, continues to be a central issue in zeolite science. Very recently, DFT calculations allowed the conclusion that, in zeolites with double four-membered rings (D4R), which are highly strained for pure silica compositions, host−guest interactions bring about a global polarization of the Si−O bond that enhances the flexibility of the silica framework, making it accessible for crystallization.3,4 This represented the first sound theoretical rationale for the long claimed structure-directing effect of fluoride anions toward zeolites with D4R units.5,6 The polarization and stabilization effect has been shown to be strong enough to revert the order of stability of two zeolites when fluoride was combined with the small and rigid 1,3,4trimethylimidazolium (134TMI):4 while this cation can directly produce zeolite ITW, under specific conditions the denser zeolite TON crystallizes first and then transforms in situ into the less dense zeolite ITW. The latter, when devoid of guests, is less stable than TON not only because of its lower density but also because it contains structurally strained D4R units. Later, the smaller and also rigid 1,3-dimethylimidazolium (13DMI) was shown to behave similarly, while 1,2,3-trimethylimidazolium (an isomer of 134TMI) showed a very large specificity for ITW.7 The observation of in situ phase transformations between zeolite phases under crystallization conditions is interesting because it directly provides an experimental order © 2013 American Chemical Society
of stability of the as-made zeolites (under those conditions and as far as the transformation is not driven by a significant change of conditions by, for instance, degradation of the organic cation). Here we report that 1-ethyl-2,3-dimethylimidazolium (1E23DMI), which is larger and more flexible than the mentioned imidazolium cations, is also highly selective to ITW but, under given conditions, it may first crystallize the denser zeolite MTW, which then transforms in situ into ITW. This contrasts with the behavior of 1-ethyl-3-methylimidazolium (1E3MI, an isomer of 134TMI and 123TMI), which behaves as a very unselective structure-directing agent (SDA) or, rather, as a pore filler of default structures.8 We discuss the different selectivity as SDA of the five cations on the basis of size, shape, conformational flexibility, and hydrophilicity.
■
EXPERIMENTAL SECTION
Synthesis of 1-Ethyl-2,3-dimethylimidazolium Hydroxide. The 1-ethyl-2,3-dimethylimidazolim iodide salt was obtained by ethylation of 1,2-dimethylimidazole (0.1 mol, Aldrich, 98%) with ethyl iodide (0.15 mol, Aldrich, 99%) in chloroform (100 mL) at 50 °C with magnetic stirring for two days. The solid was recovered by rotary evaporation under vacuum at 80 °C (yield 99%). (Caution! Toxic vapors evolve. Use a trap and work under a f ume hood.) Received: November 16, 2012 Revised: January 23, 2013 Published: February 1, 2013 729
dx.doi.org/10.1021/cm303709e | Chem. Mater. 2013, 25, 729−738
Chemistry of Materials
Article
in the code manual.14 A shrinking factor 2 was also considered for sampling within the Brillouin Zone. The exchange-correlation functional was numerically integrated by adopting standard tolerances and a (75,974)p grid (see keyword XLGRID in the code manual).14 These conditions are satisfactory for an accurate enough evaluation of the geometries, vibrational properties, and relative energies for this system.3,7 The Born dynamical charge for each atom was computed as the average diagonal value of the tensor containing the derivatives of the total dipole moment with respect to the corresponding atomic coordinates at each vibration.14
As a way to characterize the hydrophobicity of the imidazolium, the phase transfer behavior of its iodide salt from water to chloroform was measured following an experimental procedure previously reported by Kubota et al.9 For the zeolite synthesis we have used either the synthesized iodide salt or the commercial chloride (Aldrich 97%). In both cases the halide was converted to the hydroxide form by anion exchange using Dowex monosphere 550A (OH) anion exchange resin. The hydroxide solutions were concentrated by rotary evaporation to around 1−1.5 mol/1000g. The final hydroxide concentration was determined by titration using phenolphthalein as indicator. Zeolite Synthesis. For the zeolite synthesis, tetraethylorthosilicate was hydrolyzed under magnetic stirring at room temperature in an aqueous solution of the hydroxide form of the organic cation. All the ethanol produced in the hydrolysis, and some water, were allowed to evaporate, and stirring was stopped when the desired composition was achieved. The amount of water evaporated was monitored by weight. Finally, HF (ca. 48% Aldrich, recently titrated) was added while stirring with a spatula for 15 min. The crystallization was carried out at 150 °C in Teflon vessels inside stainless steel autoclaves that were tumbled at about 60 rpm. At different intervals of time, the autoclaves were taken out from the oven, and the solid product was recovered, washed with ample amounts of deionized water, and dried at 100 °C. The final composition of the reaction mixtures was 1:0.5:0.5:x SiO2:1E23DMIOH:HF:H2O, and the water to silica ratio, x, was varied between 7.4 and 15.7. Additionally, we also carried out experiments with the same composition but in which all the silica was added in the form of either as-made 1E23DMI-MTW or as-made 1E23DMI-ITW. In those cases, the as-made zeolite was added to an equimolar mixture of 1E23DMIOH and HF, at the desired water dilution. Characterization. The recovered solids were identified by power X-ray diffraction (XRD) recorded with a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å), in the 2θ range from 5° to 45°. Multinuclear magic angle spinning nuclear magnetic resonance (19F, 13C, 29Si MAS NMR) spectra were recorded on a Bruker AV 400WB, and details are given elsewhere.7 Thermogravimetric analyses were performed under oxygen flow (100 mL/min) in an SDT Q600 TA Instruments equipment up to 1000 °C (heating rate 10 °C/min). CHN analyses were carried out in a LECO CHNS-932 instrument. For the structure solution, PXRD were obtained at the 8C2 beamline at Pohang Acceleration Laboratory (Pohang, Korea) using monochromatic synchrotron radiation (λ = 1.5500 Å), and the experimental details are given elsewhere.7 Field emission scanning electron microscopy (FE-SEM) images were obtained with a FEI NOVA NANOSEM 230 on uncoated as-made samples. Conformational Analysis. A conformational analysis of 1E23DMI and of the smaller 1E3MI was performed using the Conformers module of the Materials Studio suite of programs.10 The only rotation that bears a significant change in molecular shape is along the N(1)−C(6) bond of the ethyl group, so a systematic grid scan at 1° fixed steps of the C(2)−N(1)−C(6)−C(7) torsion angle was performed, followed by minimization using the cvff forcefield.11 The energies of the conformers are given in kcal mol−1 relative to the energy of the most stable conformer. The molecular volumes, taken as the volume enclosed by the Connolly surface,12 of several (stable or unstable) conformers of 1E23DMI and 1E3MI, as well as those of 123TMI, 134TMI, and 13DMI, were calculated using Materials Studio, with a Connolly radius of 1.4 Å, a van der Waals scale factor of 1, and an ultrafine grid resolution (0.15 Å intervals). Periodic DFT Calculations. All DFT calculations were performed adopting the periodic approximation at the B3LYP13 (hybrid density functional) level of theory as implemented in the CRYSTAL09 code.14 Geometry optimizations were carried out without symmetry constraints employing analytical gradients for both atomic positions and cell parameters. The basis set considered was a VDZ with polarization on Si, O, and F, the same as employed in previous works on related materials.4,7 The tolerances for the computation of the infinite series involved in energy calculations were those recommended
■
RESULTS Synthesis. Table 1 shows the synthesis results using 1E23DMI as SDA at different water/silica ratios and Table 1. Summary of Results for the Synthesis of SiO2 Zeolites Using 1E23DMI silica source TEOS
H2O/SiO2
time (days)
product
7.4
5 7 14 15 10 12 28 30 4 6 8 5 12 19 6 10 14 19 15 6 9 23
ITW ITW ITW ITW ITW ITW ITW ITW amorphous (+ITW) amorphous + ITW ITW + amorphous MTW + amorphous MTW MTW + ITW MTW + amorphous MTW MTW MTW ITW amorphous + MTW MTW + amorphous MTW + ITW + densec
1E23DMI-MTWa TEOS
9 12.8
TEOS
13
TEOS
13.5
TEOS
14.0
1E23DMI-ITWb TEOS
15 15.7
a
The silica source was as-made 1E23DMI-MTW. bThe silica source was as-made 1E23DMI-ITW. cTridymite-like dense phase.
crystallization times. Two zeolite phases were obtained, with the phase with a lower framework density (ITW, FD = 18.1 Si/ nm3) being favored at lower water silica/ratios, in agreement with Villaescusa’s rule.15 Furthermore, when the higher density phase (MTW, FD = 19.4 Si/nm3) is initially formed it then transforms in situ into ITW when the crystallization is prolonged. At the highest water/silica ratios tested the transformation also goes to a dense tridymite-like phase. By contrast at intermediate or low water/silica ratios ITW presents a large stability under the synthesis conditions. The boundary for which the first phase obtained is ITW or MTW appears to be quite well-defined for water/silica ratios somewhere between 13 and 13.5. To further check the relative stabilities of both phases two additional experiments were done with the as-made 1E23DMIMTW or 1E23DMI-ITW zeolites as silica source in conditions that favor the alternative phase. As listed in Table 1 (entries 2 and 7, respectively, see also Supporting Information Figure S1), MTW transformed into ITW at relatively concentrated conditions (water/silica = 9), while ITW remained untrans730
dx.doi.org/10.1021/cm303709e | Chem. Mater. 2013, 25, 729−738
Chemistry of Materials
Article
Table 2. Density and Enthalpy Data for Calcined SiO2 MTW and ITW Zeolites framework density (Si nm−3) Vmol (cm3 (mol SiO2)−1) ΔH298trans (kJ (mol SiO2)−1)
SiO2−MTW
SiO2−ITW
19.39 31.05 8.7 ± 0.8 6.98 ± 3.76
18.09 33.28
method crystallographya crystallographya calorimetryb correlationc VTZP-B3LYP+Dd
8.21 ± 3.90 15.43
From the reported structures of SiO2−MTW17 and SiO2−ITW.18 bBy high temperature solution calorimetry.19 cCalculated from an empirical correlation between molar volume (Vmol) and enthalpy.16 dBy periodic DFT calculations.4
a
Table 3. Unit Cell Composition of 1E23DMI-ITW %C
%H
%N
C/Na
H/Na
TGb
u.c. compositionc
9.22
1.64
3.29
3.3 (3.5)
6.9 (6.5)
82.8 (82.6)
|C7H13N2F|2 [SiO2]24:0.8H2O
Molar ratios, with ideal values for 1E23DMI in parentheses. bPercent solid residue after thermal analysis up to 1000 °C. The value in parentheses corresponds to the composition given in the last column. cSDA calculated from the N content, assuming that the SDA is intact, its charge is compensated for by fluoride anions, and the residue in the thermogram (TG%) is SiO2. The water content was calculated from the H exceeding the SDA content. a
As in a previous report,4 the lower density zeolite is favored at low water/silica ratios, despite being thermodynamically more stable, in its as-made form, than the denser phase. This detracts from the hypothesis that a low water/silica ratio may favor a higher supersaturation and hence more metastable phases, proposed as a possible explanation of Villaescusa’s rule.15 An interesting observation is that, as the degree of dilution of the synthesis mixture increases, the crystallization apparently slows down (as expected) in the water/silica range (up to about 13) in which ITW is the first phase that crystallizes. A further increase in the water/silica ratio, however, promotes a small acceleration of the MTW’s crystallization (compare results at water silica ratios of 13 and 13.5, for instance, in Table 1). This might perhaps suggest that, at the boundary of the crystallization fields in which each phase crystallizes first, there may be a competition for nucleation of both phases resulting in a slower crystallization. Characterization. Table 3 shows the chemical composition of the ITW zeolite obtained in this work. With regard to MTW, we note that the routine washing of the obtained product with ample deionized water yielded a very large excess of organics amounting to over six organic cations per unit cell of 56 SiO2 (Supporting Information Table S1). This is, of course, unrealistic, since that unit cell contains just two 12MR pores 5 Å long. As shown in the Supporting Information, the excess organics likely exists as hexafluorosilicates that are not detected by XRD and can be washed out with hot water (70 °C) at the expense of a significant loss of crystallinity (Supporting Information Figures S2−S4). This is likely due to reaction of dissolved fluoride with the zeolite. In turn, calcination of the routinely washed material produces MTW with good crystallinity plus a dense phase likely coming from decomposition of the hexafluorosilicate impurity (Supporting Information Figure S4). With regard to ITW, it contains two organic cations per unit cell of 24 SiO2, implying full occupancy of the large [44546484] cavities of the ITW zeolite. The corresponding thermogram (Supporting Information Figure S5) reveals a small weight loss of less than 1% between 200 and 400 °C. This may correspond to the combustion of organics residing in the outer shell of the crystallites, as the temperature is too high for water desorption in a hydrophobic zeolite and an analysis of the evolved gases by
formed in conditions of relatively high dilution (water/silica = 15). This is the second example in which a higher density phase crystallizes at an early stage and then transforms in situ into a more porous zeolite (MTW → ITW) after the reports of a TON → ITW transformation.4,7 The transformation occurs in spite of ITW being likely less stable, in the absence of occluded species, than MTW. We draw this conclusion not only because of the higher density of MTW but also because of the more strained nature of ITW, which contains a high density of double-4-rings (D4R) in its structure. Table 2 lists the framework density and enthalpy of transformation from quartz determined either experimentally or from calculations for both SiO2 materials. While ITW is less dense than MTW, their difference in enthalpy, determined from an experimental correlation between molar volume and enthalpy of transformation from quartz, is within experimental error.16 However, a periodic density functional theory (DFT) calculation showed ITW to be significantly more unstable than it would correspond to its molar volume.4 This was explained by its high concentration of D4R units, which are strained for a pure silica composition due to the acute angles involved and the relatively high covalent character of the Si−O bond. Taking this into account, i.e., comparing the experimental enthalpy of MTW with the enthalpy of ITW calculated by DFT, it is clear that ITW is, in the absence of occluded species, significantly less stable than MTW. Thus, the transformation of MTW into ITW indicates a significant stabilization of ITW by interaction with the occluded F− and 1E23DMI+ ions. We will show below that, similarly to the mentioned TON → ITW case previously reported,4,7 this interaction brings about a polarization of the SiO bond that decreases its covalent character and enhances the flexibility of the framework, thus relaxing the strain inherent to the ITW framework. As explained above, this is the first coherent rationalization of the structure-directing effect of fluoride toward D4R silica zeolites, and it was also proposed to explain the crystallization of the new chiral silica polymorph STW,20 which was previously considered to be unfeasible for pure silica compositions due to its lack of flexibility window21 or its too strained nature.22 731
dx.doi.org/10.1021/cm303709e | Chem. Mater. 2013, 25, 729−738
Chemistry of Materials
Article
mass spectrometry (not shown) revealed fragments with m/e = 17, 18, and 44. This step is followed by a large weight loss of around 16.7% between 500 and 750 °C (attributed to the remaining of the two 1E23DMI plus two fluoride ions per unit cell). Although the C/N ratio found by chemical analysis is close to that in the pristine SDA, the 13C MAS NMR spectrum of 1E23DMI-ITW casts some doubts about the integrity of the occluded cation (Figure 1, bottom). In the spectrum, besides
solution, but this C has a very low intensity in the spectrum of the iodide of 1E23DMI (Figure 1, top) and here is probably mingled in the background noise (carbons not directly bonded to H typically show a decreased intensity in proton-decoupled 13 C NMR due to a reduced nuclear Overhauser enhancement). The 1H NMR spectrum after dissolution of the zeolite shows all the resonances of 1E23DMI with the expected multiplicities and intensities (Figure 2). In order to avoid extra resonances that could tangle the spectra, no internal standard has been used for chemical shift reference. The spectra were instead referenced to agree with the spectra of the iodide, and this resulted in the residual HDO resonance shifted by 0.13 ppm from its “normal” 4.8 value. Such a shift could be caused by a 10 K decrease in the temperature of measurement or, more likely, to an enhanced hydrogen bond due to the presence of HF.23 We have observed a similar shift in experiments with other dissolved zeolites (this time with methanol as an internal reference).24 The 29Si MAS NMR spectrum of 1E23DMI-ITW, Figure 3, shows only two resonances at −108 and −116 ppm with an
Figure 1. Solid state 13C MAS NMR of 1E23DMI-ITW (bottom) and 13 C NMR in D2O of the same zeolite after dissolution using HF (middle) and of pristine 1E23DMI iodide (top).
three aromatic carbons in the 115−145 ppm region plus two signals around 35 and 45 ppm, corresponding to methyl and methylene carbons bonded to N, respectively, there is only one rather symmetrical signal at a higher field (10.6 ppm), where two well separated resonances would be expected (both methyl groups attached to C atoms, as seen in Figure 1, top, for the pristine cation in solution). 1E23DMI is considerably larger than other cations previously used to synthesize ITW,7 so in order to conclusively check the integrity of the cation, a small amount of zeolite was dissolved in HF, the solution was diluted in D2O, and the 13C and 1H NMR spectra were recorded. The resulting spectra confirm the integrity of the cation, as seen in Figures 1, middle, and 2. In the 13C NMR of the dissolved zeolite the high field signals of CH3−C are now clearly resolved (Figure 1, middle). The imidazolium C at position C2 of the ring is not observed in
Figure 3. 29Si MAS NMR spectra of 1E23DMI-ITW (from top to bottom: experimental, simulation, and deconvoluted components).
intensity ratio close to 2:1, which we assign to the silicon sites in ITW that belong and do not belong, respectively, to D4R units (see below). The spectrum is much alike that of 134TMIITW and much dissimilar to those of 13DMI-ITW and 123TMI-ITW, which show a higher resolution of Q 4 resonances for the reasons discussed in a previous report (mainly, a larger dispersion of Si−O−Si angles for the different Si crystallographic sites).7 The 19F MAS NMR spectrum displays a unique resonance at −39.7 ppm, i.e., in the typical region for F anions occluded in fully siliceous D4R units, with no signs of hexafluorosilicates or other impurities (Supporting Information Figure S6). Figure 4 shows the morphology of the starting zeolite and after hydrothermal treatment with a 1E23DMIOH + HF solution, for the experiments in which as-made zeolites were used as the silica source. The starting MTW zeolite is composed of long needles and transforms into large ITW crystals, while the morphology and crystal size of the starting ITW remains unaltered upon treatment. It is noteworthy that the ITW crystals in Figure 4b−d tend to align into elongated oriented aggregates. This effect has been observed in other ITW samples during this work and seems to be more apparent as the concentration of the starting mixture decreases (Supporting Information Figure S7): at the higher concentrations tried, the ITW crystals are small ( 13DMI > 134TMI) as their hydrophilicity, evaluated as the percentage of transfer from a water solution to chloroform, following the procedure established by Kubota et al.9 When 1E23DMI is included in the picture it is found to be the second more hydrophilic (Table 6) and the second more selective to ITW of all the cations that are able to produce this zeolite (Figure 7). However, the fifth imidazolium cation in Chart 1 (1E3MI), which is an isomer of 134TMI and 123TMI, has shown no ability to produce zeolite ITW and rather behaves as a very unselective SDA.8 It mainly yields the default TON structure and only at very low water/silica ratios (3.5) it is also able to produce zeolite MFI, a material that is not very demanding of specificity in structure direction. Furthermore, prolonged heating produces the transformation MFI → TON (in this case a less dense to more dense phase transformation).8 This occurs despite the fact that 1E3MI (1) is the most hydrophilic of the cations (see percentage of transfer from water to chloroform, Table 6), (2) has an intermediate size among the
Figure 8. Excess potential energy of 1E23DMI (circles) and 1E3MI (crosses) as a function of the torsion angle defined in Chart 1. The conformation found by Rietveld refinement in the zeolite, 175.7°, is marked with a big filled circle. 736
dx.doi.org/10.1021/cm303709e | Chem. Mater. 2013, 25, 729−738
Chemistry of Materials
Article
only slightly over 2 kcal mol−1 and, hence, are small barriers at the crystallization temperature of 423 K (RT = 0.84 kcal mol−1). By contrast, rotation of the ethyl group in 1E23DMI shows a considerably larger barrier around 0° because of steric strain when the ethyl group faces the methyl substituent at C(2) (around 0°). Thus, occlusion of 1E3MI in the slit shaped cavities of ITW with the ethyl group roughly aligned with the imidazolium ring shall bear a significant entropic cost, while this may be much reduced in the case of 1E23DMI. For this one, the final conformation found in the zeolite (see Figure 6) is close to the second maximum around ±180°. Additionally, 1E3MI is considerably more hydrophilic than the rest of cations in the series (Table 6). According to Kubota et al.,9 cations with an intermediate hydrophobicity behave better in the synthesis of high silica zeolites in hydroxide media, which can be understood as a result of the need of a high solubility in water and of a strong interaction with the hydrophobic silica surface. In the fluoride synthesis route, especially those leading to D4R zeolites, we think the silica surface is not as much hydrophobic as in the hydroxide route, because of a significant global polarization of the Si−O bond.3 In fact, it may be argued that a more hydrophilic cation may enhance this polarization effect, thus improving the flexibility of the framework and contributing to relaxing the strain associated with D4R units. Nonetheless, the very high hydrophilicity of 1E3MI may contribute, together with its high flexibility, to the observed lack of ability to direct the crystallization to anything other than default structures.
1E23DMI-MTW, TG/DTA, 19F MAS NMR, additional FESEM images, difference Fourier analysis figure, Rietveld plot, interatomic distances and angles, calculated and experimental 29 Si chemical shifts, and coordinates of the DFT optimized structure for 1E23DMI-ITW in pdf format and cif file for 1E23DMI-ITW. This material is available free of charge via the Internet at http://pubs.acs.org.
■
Corresponding Author
*Corresponding author. Email:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Financial support by the Spanish CICYT (Project Nos. MAT2009-09960 and MAT2012-31759) is gratefully acknowledged. We warmly thank Prof. S.B. Hong (POSTECH) for sharing his synchrotron beam time and Mr. J. Shin for collecting the powder XRD data at the Pohang Accelerator Laboratory (Pohang, Korea). We thank A. Valera for technical expertise (FE-SEM). A.R. acknowledges a JAE fellowship from CSIC and Fondo Social Europeo from EU.
■
REFERENCES
(1) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 757. (2) Gies, H.; Marler, B. Zeolites 1992, 12, 42. (3) Zicovich-Wilson, C. M.; San-Román, M. L.; Camblor, M. A.; Pascale, F.; Durand-Niconoff, J. S. J. Am. Chem. Soc. 2007, 129, 11512. (4) Zicovich-Wilson, C. M.; Gándara, F.; Monge, A.; Camblor, M. A. J. Am. Chem. Soc. 2010, 132, 3461. (5) Caullet, P.; Guth, J. L.; Hazm, J.; Lamblin, J. M.; Gies, H. Eur. J. Solid State Inorg. Chem. 1991, 28, 345. (6) Guth, J. L.; Kessler, H.; Caullet, P.; Hazm, J.; Merrouche, A.; Patarin, J. In Proceedings of the 9th International Zeolite Conference; von Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds.; ButterworthHeinemann: 1993; p 215. (7) Rojas, A.; Martínez-Morales, E.; Zicovich-Wilson, C. M.; Camblor, M. A. J. Am. Chem. Soc. 2012, 134, 2255. (8) Rojas, A.; Gómez-Hortigüela, L.; Camblor, M. A. J. Am. Chem. Soc. 2012, 134, 3845. (9) Kubota, Y.; Helmkamp, M. M.; Zones, S. I.; Davis, M. E. Microporous Mater. 1996, 6, 213. (10) Materials Studio 6.0; Accelrys Inc.: San Diego, CA, 2011. (11) Dauger-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler, A. T. Proteins: Struct., Funct., Genet. 1988, 4, 31. (12) Connolly, M. L. J. Appl. Crystallogr. 1983, 16, 548. (13) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (14) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL09 Users Manual; University of Turin: Turin, 2009. Available online; see http://www. crystal.unito.it. (15) Camblor, M. A.; Villaescusa, L. A.; Días-Cabañas, M. J Top. Catal. 1999, 9, 59. (16) Piccione, P. M.; Laberty, C.; Yang, S.; Camblor, M. A.; Navrotsky, A.; Davis, M. E. J. Phys. Chem. B 2000, 104, 10001. (17) Fyfe, C. A.; Gies, H.; Kokotailo, G. T.; Marler, B.; Cox, D. E. J. Phys. Chem. 1990, 94, 3718. (18) Yang, X.; Camblor, M. A.; Lee, Y.; Liu, H.; Olson, D. H. J. Am. Chem. Soc. 2004, 126, 10403. (19) Petrovic, I.; Navrotsky, A.; Davis, M. E.; Zones, S. I. Chem. Mater. 1993, 5, 1805. (20) Rojas, A.; Camblor, M. A. Angew. Chem., Int. Ed. 2012, 51, 3854.
■
CONCLUSIONS 1E23DMI is a new SDA for the synthesis of zeolite ITW by the fluoride route. The system follows Villaescusa’s rule, as the phases that crystallizes first at low and high water/silica ratios are the one with lower framework density (ITW) and the one with higher density (MTW), respectively. However, and despite the likely higher stability of MTW in the absence of occluded species, this phase transforms into ITW under prolonged heating of the crystallizing mixture, suggesting largely stabilizing host−guest interactions in 1E23DMI-ITW. Both the experimental and DFT optimized structures of this zeolite show a significant deviation from regularity of the SiO4 tetrahedra, in agreement with the polarization of the Si−O bond determined by DFT. This has been previously proposed as an essential stabilizing factor for silica D4R zeolites prepared by the fluoride route. When the structure-directing effect of five closely related imidazolium cations are compared, 1E23DMI is found as the second most selective for ITW, after 123TMI. In fact, the deviation from tetrahedral regularity and the polarization of the Si−O bond is similar for the ITW zeolites synthesized with 1E23DMI and with 123TMI, which is the best SDA for this zeolite. We think this is likely related to their comparatively large hydrophilicity. By contrast, 1E3MI, which is even more hydrophilic and an isomer of 123TMI and 134TMI, behaves as a very unspecific SDA. We conclude this is likely due to its exceeding hydrophilicity and to a significantly larger conformational flexibility compared to 1E23DMI.
■
AUTHOR INFORMATION
ASSOCIATED CONTENT
S Supporting Information *
XRD patterns for zeolites before and after hydrothermal treatment, chemical analysis, multinuclear NMR and XRD for 737
dx.doi.org/10.1021/cm303709e | Chem. Mater. 2013, 25, 729−738
Chemistry of Materials
Article
(21) Kapko, V.; Dawson, C.; Treacy, M. M. J.; Thorpe, M. F. Phys. Chem. Chem. Phys. 2010, 12, 8531. (22) Sastre, G.; Corma, A. J. Phys. Chem. C 2010, 114, 1667. (23) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512. (24) Rojas, A.; Camblor, M. A. To be published. (25) Shayib, R. M.; George, N. C.; Seshadri, R.; Burton, A. W.; Zones, S. I.; Chmelka, B. F. J. Am. Chem. Soc. 2011, 133, 18728. (26) Altomare, A.; Caliandro, R.; Camalli, M.; Cuocci, C.; Giacovazzo, C.; Moliterni, A. G. G.; Rizzi, R. J. Appl. Crystallogr. 2004, 37, 1025. (27) Visser, J. W. J. Appl. Crystallogr. 1969, 2, 89. (28) Toby, B. H. J. Appl. Crystallogr. 2005, 38, 1040. (29) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86748; Los Alamos National Laboratory: Los Alamos, NM, 1994. (30) Toby, B. H. J. Appl. Crystallogr. 2001, 34, 210. (31) Thompson, P.; Cox, D. E.; Hastings, J. B. J. Appl. Crystallogr. 1987, 20, 79. (32) Finger, L. W.; Cox, D. E.; Jephcoat, A. P. J. Appl. Crystallogr. 1994, 27, 892. (33) Stephens, P. W. J. Appl. Crystallogr. 1999, 32, 281. (34) Thomas, J. M.; Klinowski, J.; Ramdas, S.; Hunter, B. K.; Tennakoon, D. T. B. Chem. Phys. Lett. 1983, 102, 158. (35) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2011, 44, 1272. (36) (a) Zefirov, Yu. V. Russ. J. Inorg. Chem. 2001, 46, 568. (b) Batsanov, S. S. Inorg. Mater. 2001, 37, 871. (37) Papiz, Z.; Andrews, S. J.; Damas, A. M.; Harding, M. M.; Highcock, R. M. Acta Crystallogr., Sect. C 1990, 46, 172.
738
dx.doi.org/10.1021/cm303709e | Chem. Mater. 2013, 25, 729−738
