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Structure Directing Effect of (1S,2S)-2-Hydroxymethyl-1-benzyl-1-methylpyrrolidinium in the Synthesis of AlPO-5 Luis Go´mez-Hortigu¨ela,*,†,‡,§ Raquel Garcı´a,† Fernando Lo´pez-Arbeloa,| Furio Cora`,‡,§ and Joaquı´n Pe´rez-Pariente† Instituto de Cata´lisis y Petroleoquı´mica-CSIC, C/Marie Curie 2, 28049 Cantoblanco, Madrid, Spain, Department of Chemistry, Kathleen Lonsdale Building, Third Floor, UniVersity College London, Gower Street, WC1E 6BT, London, United Kingdom, Thomas Young Centre at UniVersity College London, Gower Street, WC1E 6BT, London, United Kingdom, and Departamento de Quı´mica Fı´sica, UniVersidad del Paı´s Vasco-EHU, Apartado 644, 48080, Bilbao, Spain ReceiVed: February 26, 2010; ReVised Manuscript ReceiVed: March 22, 2010
A pure diastereoisomer of a new chiral organic molecule with two asymmetric atoms, (1S, 2S)-2-hydroxymethyl1-benzyl-1-methylpyrrolidinium, has been prepared and used as structure directing agent (SDA) for the crystallization of the one-dimensional AlPO-5 microporous aluminophosphate. Our main objective is to induce chiral supramolecular arrangements of the SDA molecules within the microporous structure. The SDA molecules can be arranged as monomers or dimers within the nanochannels of AlPO-5, the aggregation being driven by π-π type interactions between the aromatic rings. It has been found that increasing the crystallization time leads to a higher organic content (and lower water incorporation) together with a higher formation of dimers, which indicates that the incorporation of the organic molecules as dimers is thermodynamically favored. We also observe a partially reversible photoinduced rearrangement of the SDAs; the aggregation is enhanced by exciting the occluded monomers with the appropriate wavelength. A recently developed computational model has been applied to determine the stable loading of SDAs and water molecules, which has been found to be of 1.0 SDA and 7 water molecules per unit cell, with water forming stable 6-ring clusters between consecutive dimers, in good agreement with the experimental observations. Introduction Microporous materials have been widely employed in industrial applications because of their molecular sieving and ionexchanging capacities,1-3 which exploit the molecular dimensions and the crystalline nature of the microporous structure to discriminate between molecules with even subtle steric differences. Many different dopant atoms can be incorporated within the framework of these crystalline microporous materials, giving place to a range of doped materials with different compositions, many of them with useful catalytic properties. In this context, chiral microporous materials are of particular interest because they can combine shape selectivity and enantioselectivity, which are desirable for asymmetric catalysis and separation.4 Several approaches have been attempted to introduce chirality in microporous materials, such as the immobilization of homogeneous chiral catalysts inside the inorganic solid matrix5 or the anchoring of chiral modifiers to the microporous materials in the vicinity of the active sites.6 In these cases, the microporous host may give additional selectivity by molecular sieving effects.7 However, in both cases, the chiral information is contained in the molecular component attached to an otherwise achiral solid. A more interesting approach comes from the synthesis of an intrinsically chiral framework, in which chirality is imprinted in the inorganic solid. Among the known zeolite structures, only few are known to contain chiral channels,4 though in a recent paper chiral networks have been identified * Corresponding author. E-mail:
[email protected]. † Instituto de Cata´lisis y Petroleoquı´mica-CSIC. ‡ University College London. § Thomas Young Centre at University College London. | Universidad del Paı´s Vasco-EHU.
in 20 more zeolite structures;8 indeed, in that work, the authors demonstrated that chiral zeolites are actually able to perform enantioselective operations. Very recently, a new chiral zeolite topology has been discovered.9 The synthesis of microporous materials is based on hydrothermal methods and generally involves the addition of organic molecules10-12 to the synthesis gels. These molecules, which are usually called structure directing agents (SDAs), are encapsulated within the void space of the nascent frameworks during crystallization, remaining occluded at the end of the process. Hence, the size and shape of the SDA plays an important role in determining the outcome of the microporous materials syntheses; indeed, a correlation between the shape of the molecule and that of the pores crystallized in its presence13 has often been recognized. In this context, the main strategy traditionally followed to obtain a chiral microporous framework is the use of an asymmetric organic molecule as SDA to impart chirality into the inorganic framework. For a transfer of the chirality from the SDA molecule to the inorganic framework to occur, a close structural relationship between the host and the guest species should exist. However, the way in which such chiral nature is transferred from the molecular SDA component to the long-range order structure is still an unknown phenomenon, and the study of the structure directing mode of action of this type of chiral molecules is essential to understand a possible chiral imprinting into the solid. A priori, not only an asymmetric SDA molecular nature, but also an asymmetric long-range order of the SDAs is desirable to achieve such a chirality transfer. In recent work, we have studied the use of aromatic molecules as SDAs in the synthesis of microporous aluminophosphates (AlPOs);14 in these molecules, the presence of the aromatic rings
10.1021/jp101756d 2010 American Chemical Society Published on Web 04/13/2010
Synthesis of AlPO-5 allows self-assembly through π-π type interactions forming supramolecular aggregates that are the actual structure directing agents.15-20 In particular, we observed that (S)-N-benzyl-2pyrrolidinemethanol (BPM) directs efficiently the synthesis of AFI-type microporous AlPOs.17,21 The use of this molecule as SDA provides a rich supramolecular chemistry that can enhance its molecular chiral nature and, thus, favor the eventual transfer of the chirality to the microporous framework. BPM was rationally designed by means of a computational study to achieve a chiral supramolecular arrangement of the molecules within the one-dimensional channels of the AFI structure.17 The computational study suggested that the structure directing effect of BPM occurred through the aggregation of two molecules to form dimers, and that the most stable location of these molecules within the AFI framework involved a helicoidal, and hence chiral, arrangement of the BPM dimers, where consecutive dimers were always rotated by the same angle (-90°). This preferred rotation came from the asymmetric nature of the BPM molecule, associated with the hydroxymethyl substituent. The AFI structure itself is achiral, but under our synthesis conditions the chirality of the SDA self-assembly may be replicated in the framework by the ordered inclusion of dopant ions. We have observed, for instance, that MgAPO-5 synthesized with BPM is more active than materials of the same composition synthesized with other (and nonchiral) SDAs,22 although the origin of this increased activity at the atomic level is not yet completely understood. Following that work, we are currently designing new SDA molecules derived from BPM, where we attempt to enhance the chirality transfer from the molecular SDA component to the framework structure. To achieve this goal, several assumptions can be made. On one hand, it would be desirable to increase the SDA molecular rigidity, because it is known that more rigid molecules lead to more specific structure-directing effects, a required condition for the transfer of chirality to occur. The asymmetric nature of chiral molecules is enhanced by rigidity, because in very flexible molecules (and so with many different conformers), the molecular asymmetry may be averaged out. On the other hand, it may be desirable to assess what effect is introduced by the use of SDAs with two or more asymmetric atoms. When several diastereoisomers with different absolute configurations are available, it is essential to use pure diastereoisomers with a particular absolute configuration to investigate the effect on the overall chirality in the solid phase. Based upon these grounds, we have synthesized a new SDA molecule, (1S,2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium (BMPM), which is derived from BPM by attaching a methyl group to the tertiary N atom, leading to a new asymmetric center, the N atom (Figure 1-right). Such molecular transformation increases the rigidity of the SDA and indeed modifies the conformation in vacuo of the pyrrolidine ring, as can be clearly seen in Figure 1 where the geometry-optimized structures of BPM and BMPM are compared. The new substituent also enhances the asymmetry of the SDA. By controlling the order of addition of the substituents (benzyl and methyl) to the N atom during the synthesis of this molecule, we are able to obtain the pure diastereoisomer (1S,2S)-2-hydroxymethyl-1benzyl-1-methylpyrrolidinium, with the two asymmetric atoms in an “S” absolute configuration,23 leading to a very well-defined spatial configuration. The present work describes the results obtained by employing a combination of experimental and computational techniques to study the structure-directing effect of the S,S-diastereoisomer of BMPM in the synthesis of microporous AlPO materials.
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Figure 1. Two perpendicular views (top and bottom) of (S)-N-benzyl2-pyrrolidinemethanol (BPM; left) and (1S,2S)-2-hydroxymethyl-1benzyl-1-methylpyrrolidinium (BMPM; right) molecules in the most stable conformations in vacuo. Structures have been obtained with full geometry optimizations at the Hartree-Fock level and with a 6-31G* basis set.
Fluorescence spectroscopy is applied to study the aggregation behavior of BMPM within the microporous structure; moreover, a new computational model we have recently proposed for studying structure direction of organic molecules in hydrophilic microporous materials24,25 is used to predict the most stable organic content and the location of the guest species within the framework. The combined application of complementary experimental and computational techniques is essential to gain atomic level detail on the structure direction of these chiral molecules. Such fundamental understanding is required to underpin future research efforts aimed at the design of new SDAs that can promote the long sought imprinting of chiral functionality into inorganic structures. Experimental Details (a) BMPM Synthesis. Pure (1S,2S)-2-hydroxymethyl-1benzyl-1-methylpyrrolidinium was prepared by methylation of the starting amine (S)-1-benzyl-2-pyrrolidinemethanol (SigmaAldrich) with methyl iodide in ethanol, leading to the quaternary ammonium cation, (1S,2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium (BMPM) iodide. This procedure yields the pure (S,S)-diastereoisomer due to steric reasons, because the attack of the methyl group to the molecular N atom always takes place through the opposite side to the location of the bulky benzyl substituent. The final product was characterized by 13C and 1H NMR (Figure SI-1, Supporting Information) and chemical CHN analysis (calcd for C13H20NOI: C, 46.8; H, 6; N, 4.2. Found: C, 47.1; H, 5.9; N, 4.3). The chiral purity of the diastereoisomer obtained was determined from the integral measurements of 1H NMR spectrum of BMPM iodide salts (see Figure SI-1, Supporting Information), confirming that only the (S,S)-diastereoisomer was obtained. The hydroxide form of the (S,S)-BMPM quaternary ammonium salt was obtained by ion exchange with an anionic resin (Amberlite IRN-78, exchange capacity: 4 mequiv/g, Supelco). The hydroxide concentration of the BMPM solution was determined by titration with a 0.05 M HCl solution (Panreac) using phenolphthalein (Aldrich) as indicator. (b) AlPO-5 Synthesis. AlPO-5 was prepared from a gel having the following molar composition: 1 BMPM/1 P2O5/1 Al2O3/40 H2O. This gel was prepared by adding the aluminum source, pseudoboehmite (Catapal Pural SB, 75.3 wt % Al2O3), to a solution of phosphoric acid (Riedel-de Haen, 85 wt %) and water, and stirred for 1 h in a closed recipient. The titrated
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(1S,2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium hydroxide aqueous solution was then added, and the stirring continued for 2 h. The gels were introduced into 60 mL Teflonlined stainless steel autoclaves and heated statically at 150 °C for 3 or 6 days. The resulting solids were separated by filtration, washed with ethanol and water, and dried at 60 °C overnight. (c) AlPO-5 Characterization. The crystallization of the AFI structure as a pure phase was assessed by X-ray diffraction (Seifert XRD 3000P diffractometer, Cu KR radiation); main diffractions between 4 and 40 2θ were used for the unit cell parameters determination. The organic content of the samples was studied by chemical CHN analysis (Perkin-Elmer 2400 CHN analyzer) and thermogravimetric analysis (TGA) (PerkinElmer TGA7 instrument). 1H and 13C NMR spectra of the organic salt were recorded on a Bruker200. Chemical shifts are quoted relative to TMS as an internal reference. The chemical nature of the SDA molecules within the solids was studied by solid state magic angle spinning nuclear magnetic resonance (MAS NMR). These MAS NMR spectra were recorded with a Bruker AV 400 WB spectrometer, using a BL7 probe. 1H to 13 C cross-polarization (CP) spectra were recorded using π/2 rad pulses of 4.5 µs for 1H, a contact time of 5 ms and a recycle delay of 3 s. For the acquisition of the spectra, the samples were span at the magic angle (MAS) at a rate of 5-5.5 kHz. Solid state UV-visible fluorescence spectra were recorded in a SPEX fluorimeter model Fluorolog 3-22 equipped with a double monochromator in both the excitation and the emission channels and a red-sensitive photomultiplier detector (Hammamatsu R928 photomultiplier) operating with a Peltier cooling system. The fluorescence spectra were registered in the frontface configuration in which the emission signal was detected at 22.5° with respect to the excitation beam. To collect the spectra, supported thin films of the crystalline AlPO materials on glass slide were ellaborated by solvent evaporation of AlPO suspensions in dichloromethane. Computational Details A computational study was performed to understand the molecular features governing the incorporation of these molecules within the microporous solids. The molecular structure of BMPM and water molecules and their interaction with the microporous AlPO framework were described with the cvff forcefield,26 which has been successfully applied recently for the simulation of zeolite host-guest systems27-29 and water containing systems.30 The AlPO framework atoms were kept fixed during all the calculations. The atomic charges for BMPM cations (with a net molecular charge of +1) were calculated by the charge-equilibration method.31 Due to the lack of evidence from experiments about the location and distribution of negatively charged structural defects that compensate for the positive charge of the SDA molecules, the BMPM charge was compensated by uniformly reducing the charge of all the T (Al + P) ions until charge neutrality. The atomic charge for O was fixed to -1.2, while charges for Al and P were gradually reduced from +1.4 and +3.4, respectively, until charge neutrality. The atomic charges in the water molecules were -0.82 and +0.41 for O and H, respectively. We have recently proposed a new computational model to study structure direction in the synthesis of hydrophilic aluminophosphate frameworks.24,25 Previous results demonstrated that both the inclusion of water in the model as well as an energy term to account for the transfer of water and SDAs from the gel to the microporous frameworks are required for reliably modeling the main aspects of structure direction.25 Our com-
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Figure 2. XRD patterns of samples obtained with BMPM after 3 (solid) or 6 (dashed) days of crystallization. The inset shows a magnification of the central diffractions, highlighting the shift of the diffraction peaks.
putational protocol allows us to estimate the total internal energy of the system as a function of the ratio between SDA and water molecules inside the microporous framework. The net stabilization can be plotted as a function of the SDA content using the following equation (see refs 24 and 25 for details): vac sol ∆Estab-SDA(nSDA) ) Ef - nSDA · (ESDA + ESDA )-
nH2O · (EHvac2O + EHsol2O)
(1)
where Ef refers to the internal energy of the AlPO framework containing the SDA and water molecules, nSDA and nH2O are vac the number of SDA and water molecules, respectively, E SDA and E Hvac2O are the calculated energies of the molecules in vacuo, sol and E Hsol2O are the energies of the SDA and water in and E SDA aqueous solution, which represent consistent estimations of the stability of the SDA and water molecules in the gel. Plotting the value of ∆Estab-SDA as a function of the number of SDA molecules (nSDA) enables us to estimate the most stable loading ratio of SDA and water molecules within the microporous frameworks, in equilibrium with their aqueous solution. The sol BMPM and water solution energies (E sol SDA and E H2O), calculated as detailed in previous works, are -88.1 and -12.4 kcal/mol, respectively. The most stable location of the organic/water molecules for the different organic contents was obtained by means of simulated annealing followed by geometry optimization techniques. All the calculations have been performed with the Cerius2 software provided by Accelrys Inc. All the energies are given in kcal/mol per unit cell. Results (A) Synthesis and Experimental Characterization of AlPO5-BMPM Solids. Hereafter, samples will be referred to as AFIBMPM-3d (3 days of crystallization) and AFI-BMPM-6d (6 days of crystallization). The XRD patterns of the aluminophosphate based materials synthesized in this work (Figure 2) indicate that, under our synthesis conditions, (1S,2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium directs the synthesis toward the crystallization of AlPO-5 (AFI-type structure). The AFI type structure is composed of one-dimensional not interconnected 12-membered-ring (MR) channels with a diameter of 7.3 Å. By comparing the two diffractograms, we can appreciate a slight shift to lower 2θ angles of the diffraction
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TABLE 1: CHN Chemical Analyses of the AlPO-5 Samples
a
sample
%C
%H
%N
C/Na
%BMPM
BMPMb (molec/u.c.)
H 2 Oc (molec/u.c.)
AFI-BMPM-3d AF-PMPM-6d
8.12 9.10
1.74 1.80
0.91 0.98
10.4 10.8
10.94 12.21
0.92 1.04
6.4 5.6
The C/N ratio of the isolated molecule is 13. b Calculated from C + N chemical analysis data. c Calculated from TGA data.
peaks upon increasing the crystallization time, especially for diffraction peaks between 20 and 40°. The unit cell parameters of these two samples were found to be 13.68 and 8.46 Å for AFI-BMPM-3d, and 13.70 and 8.48 Å for AFI-BMPM-6d, with corresponding cell volumes of 1370.2 and 1377.6 Å3, respectively, evidencing a slight increase in the unit cell volume with longer crystallization times. The chemical integrity of the organic molecules inside the framework, and their resistance to the hydrothermal treatment, was verified by 13C CP-MAS NMR (Figure 2, Supporting Information). The same bands were observed in the 13C spectra of the two solid AFI samples and in that of the initial ammonium iodide, verifying the integral incorporation of the molecules within the solids. The organic content of the solids was also studied by chemical analysis (Table 1). The C/N ratios are close to those of the isolated molecules, again confirming the intact incorporation of BMPM within the solids. The total organic content was calculated from the C chemical analysis data and was found to be slightly higher for the sample prepared with a longer crystallization time (0.92 and 1.04 BMPM molecules per unit cell for AFI-BMPM-3d and AFI-BMPM-6d, respectively), evidencing a slight increase in the organic content with the crystallization time. The thermogravimetrical analyses (after the hydration treatment) are shown in Figure 3. A first intense weight loss centered at temperatures below 100 °C is observed, corresponding to the desorption of water molecules occluded within the structure; this weight loss is slightly more intense in the sample obtained after 3 days of crystallization. Assuming that the weight loss at temperatures below 150 °C is only due to the desorption of occluded water molecules, the water content is of 6.4 and 5.6 H2O molecules per unit cell for samples AFI-BMPM-3d and AFI-BMPM-6d, respectively. The desorption/combustion of the organic BMPM molecules occurs in the temperature range between 200 and 800 °C. In line with previous results, here we observe a slightly higher desorption rate for the AFI-BMPM6d sample, especially in the 200-300 °C temperature range, confirming a higher organic content in the latter sample, in qualitative agreement with the results from CHN chemical
Figure 3. TGA (left axis) and DTA (right axis) of AFI samples.
analysis. Therefore, these results evidence an increase of the organic content upon increasing the crystallization time (increase of ca. 12% from 3 to 6 days), with a consequent and corresponding decrease of the water content (decrease of ca. 14%). The presence of an aromatic phenyl ring in BMPM enabled us to study the aggregation state by fluorescence spectroscopy.32-37 The fluorescence spectra of the solid samples, after excitation at 260 nm, are shown in Figure 4. Let us first focus on the lines displayed in black, which correspond to the initial emission spectra. Two bands can be clearly observed; one, with a vibronic structure, is centered at 288 nm, and a second broader band in the 380-450 nm spectral range. These two bands can be assigned to the emission of the BMPM monomers and aggregates, respectively, in analogy with previous results obtained for BPM,18 in which the intensity of the second emission band showed a concentration-dependence. The aggregation of the BMPM molecules in the micropores of AlPO-5 is restricted to the formation of dimers due to the confined space of the 12 MR channels. The broad emission band in the 380-450 nm range seems to contain two different maxima at approximately 400 and 440 nm, more clearly distinguishable in sample AFIBMPM-3d, which might correspond to dimers with different π-π overlap efficiency. Indeed, the excitation spectrum of sample AFI-BMPM-6d (monitored at dimer emission, 435 nm; Figure SI-3, right, Supporting Information) shows two different maxima for the dimer excitation at wavelengths of 310 and 365 nm apart from that of the monomer, confirming the existence of two types of dimers. By comparing the initial emission spectra (black lines) of AFI-BMPM-3d (left) and AFI-BMPM-6d (right) in Figure 4, we can clearly observe an increase of the dimer concentration with the crystallization time. This result is in good agreement with the higher BMPM and lower water contents found for sample AFI-BMPM-6d (Table 1) with CHN and TG analyses. Interestingly, the emission band at 440 nm that corresponds to a more efficient π-π overlap of the aromatic rings is more intense than that centered at 400 nm in the AFIBMPM-6d sample, with the one at 400 nm being barely appreciable, while in the AFI-BMPM-3d sample, the 400 nm band is more intense. These observations suggest a predominance of strongly overlapping dimers in the sample obtained with a longer crystallization time, and so a reorganization of weakly overlapping (400 nm) to strongly overlapping (440 nm) dimers on increasing the crystallization time. While measuring the different spectra, we observed that the collection of the emission spectrum once the sample had been already irradiated with a λexc of 260 nm (i.e., after collecting an initial emission spectrum) led to a notable decrease of the monomer band intensity and a slight increase of the dimer band intensity. This seems to indicate that irradiation of the monomers can provoke a photoinduced rearrangement, with aggregation of monomers into dimers. To the best of our knowledge, this observation has not been reported previously and has prompted our interest in studying the phenomenon in detail. We collected several emission spectra, one after the other, which allowed us to measure the fluorescent emission of the occluded molecules
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Figure 4. Height-normalized solid state fluorescence spectra of samples AFI-BMPM-3d (left) and AFI-BMPM-6d (right), with a λexc ) 260 nm. Consecutive emission spectra were recorded one after the other (Em1-6). Spectrum Em6+10 min was recorded after waiting 10 min after collecting spectrum Em6.
after increasing irradiation times of the samples (every scan involved irradiation of the sample for 1 min at λexc of 260 nm). The series of emission spectra at increasing irradiation exposure (labeled as Em1 to Em6) is shown in Figure 4. We observe that, in both samples, progressive irradiation of the sample leads to a gradual increase of the intensity of the dimer band at the expense of a gradual decrease of the monomer band, leading to an isoemissive point at around 375 nm. The increased intensity of the dimer band is more evident in the AFI-BMPM-3d sample. A similar result was found by measuring the excitation spectra, monitoring the emission at 295 (monomer emission) and 435 nm (dimer emission), where the monomer excitation intensity decreased by irradiating the sample, and the opposite for the dimer (Figure SI-3, Supporting Information). Therefore, these results suggest that irradiation of the monomer with λexc ) 260 nm, that is, excitation of the monomer, leads to a photoinduced molecular reorganization to form dimers. We performed a final experiment to determine if this photoinduced aggregation is reversible. After measuring the last emission spectrum (Em6), the sample was kept without irradiation for 10 min, and then a new emission spectrum was collected (spectrum Em6+10 min in Figure 4, dashed line). We observed that the intensity of the monomer band in this spectrum increased with respect to the previous spectrum, while that of the dimer band decreased, although without reaching the initial levels before irradiation, evidencing that the photoinduced aggregation is at least partially reversible and is reduced when the irradiation is removed. (B) Computational Study of the Occlusion of BMPM and Water in AlPO-5. The loading of BMPM molecules within the AlPO-5 structure was investigated computationally. BMPM loadings of 0.00, 0.66, 0.80, 1.00, 1.2, 1.26, and 1.33 SDA molecules per unit cell (SDA/u.c.) have been studied, each with two different molecular orientations, with benzyl rings facing each other (“fac”, in a conformation suitable to form dimers) and in opposite sides (“opp”, in a conformation that prevents dimer formation); no higher organic contents can be achieved for this molecule. For packing values of 1.20, 1.26, and 1.33, only the fac-orientation has been studied, because these loadings can only be achieved via SDA dimers. The stabilization energy as a function of the BMPM loading is plotted in Figure 5, and the relative BMPM and water loading as well as the stabilization energies are given in Table 2. Results on the stabilization energy, calculated as defined in eq 1, reveal that the most stable BMPM loading in the AlPO-5 structure, in equilibrium with the synthesis gel, corresponds to
1.0 BMPM and 7 water molecules per unit cell, with the BMPM molecules facing each other. This result is in very good agreement with the experimental values of 0.94 and 1.06 BMPM (and 6.4 and 5.6 water) molecules for samples AFI-BMPM-3d and AFI-BMPM-6d found by CNH and thermogravimetric analysis (Table 1). A packing value of 0.8 SDA/u.c. led to the inclusion of 8.4 and 9.0 water molecules per unit cell in the structure in the fac- and opp-orientations, respectively, of which four were located in the 6 MR channels. The stabilization energy of the two configurations is very similar (-53.7 and -52.0 kcal/mol per unit cell, respectively; Table 2). The most stable location of the SDA and water molecules is shown in Figure 6 (top); the water molecules located in the 12 MR channel form either chain clusters surrounding BMPM molecules or stable sixmembered rings between adjacent molecules. In both molecular orientations, the BMPM molecules do not form dimers, but rather they are encapsulated as monomers surrounded by water molecules. An increase of the BMPM packing value to 1.0 SDA/ u.c. stabilizes the fac-orientation (around 6 kcal/mol per unit cell more stable than the opp-orientation). While up to 0.8 SDA/ u.c. each SDA molecule is surrounded by water, the higher organic content of 1.0 or more SDA/u.c. is only achieved through the dimer configuration, with a direct overlap of planar aromatic rings, stabilized by π-π type interactions. The increase in the BMPM loading from 0.8 to 1.0 (fac) SDA/u.c. is accompanied by a decrease of the water content from 8.4 to 7 molecules per unit cell, 3 of which are located in the 12 MR channel, and 4 in the 6 MR channels. The most stable location of the BMPM/water molecules in this system (1.0 fac) is shown in Figure 6 (middle). We can observe that the BMPM molecules form dimers, with the water molecules located in between adjacent dimers. It is worth noting that water clusters formed between the dimers are invariably formed by rings of six water molecules stabilized by H-bonds between the water molecules and with the framework O atoms (Figure 7). Indeed, the geometric configuration of this water cluster provides a close match with the AFI channel topology. The stability of both the SDAs in the dimer configuration and the water molecules in the six-membered ring cluster could explain the stability of this SDA loading, where the best balance between space-filling efficiency and strong interaction with the framework is achieved, leading to the highest stabilization energy (-58.3 kcal/mol per unit cell). A further increase of the BMPM content to 1.2 SDA/ u.c. prevents inclusion of water within the 12 MR channels
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Figure 5. Stabilization energy (as defined in eq 1) of BMPM occluded within the AlPO-5 structure as a function of the SDA loading.
TABLE 2: Computational Details of the Systems Studied system water 0.66-BMPM-fac 0.66-BMPM-opp 0.80-BMPM-fac 0.80-BMPM-opp 1.00-BMPM-fac 1.00-BMPM-opp 1.20-BMPM-fac 1.26-BMPM-fac 1.33-BMPM-fac
total BMPM/ stabilization supercell BMPM u.c. H2O/u.c. E 2 3 3 5 5 4 4 20 19 18
0 2 2 4 4 4 4 24 24 24
0.00 0.66 0.66 0.80 0.80 1.00 1.00 1.20 1.26 1.33
19.0 10.0 10.7 8.4 9.0 7.0 6.5 4.0 4.0 4.0
-27.5 -44.0 -44.7 -53.7 -52.0 -58.3 -52.5 -57.1 -52.4 -45.0
(Figure 6-bottom); only four water molecules per unit cell are observed, located in the 6 MR channels (Table 2). Such an increase in the organic content to 1.2 results in a slight decrease of the stabilization energy (-57.1 kcal/mol per unit cell). Further increases of BMPM content to 1.26 and 1.33 SDA/u.c. result in much lower stabilization energies (-52.4 and -45.0 kcal/ mol per unit cell, respectively) due to a steric repulsion between the tightly packed BMPM molecules. This loading is less stable than the optimal value of the similar SDA molecule BPM of 1.33 molecules per unit cell; the methyl group in BMPM stands out from the dimer structure and does not permit a good fitting between consecutive dimers (see Figure SI-4, Supporting Information). Because in the most stable arrangement (loading of 1.0 SDA/u.c.) adjacent BMPM dimers are separated by a ring of six water molecules, no long-range ordered structure of BMPM, similar to the BPM helix, can be observed.
Figure 6. Location of the SDA and water molecules inside the 12 MR channels of the AFI structure for different packing values: 0.8 (top), 1.0 (middle), and 1.2 (bottom). H-bonds are displayed as dashed blue lines. Methyl C atoms are displayed as balls. C atoms are displayed in gray, H in white, N in blue, and O in red.
Discussion We have employed successfully a new organic molecule, containing multiple-chiral centers, as SDA for the synthesis of a microporous material. The original goal of transferring the molecular chirality to the inorganic framework appears bound to failure as BMPM dimers are separated by water molecules and do not interact directly with one another, thus, lacking the long-range ordered self-assembly present in the related SDA molecule BPM.17 Our results demonstrate that the (S,S)-BMPM molecule directs efficiently, under our synthesis conditions, the crystallization of the AlPO-5 structure. Interestingly, we have observed that an increase of the crystallization time leads to a higher occlusion of BMPM molecules as dimers, resulting in a higher organic and corresponding lower water content, and to
Figure 7. Structure of the six-membered ring cluster of water molecules within the AFI channel.
a slight increase of the unit cell volume; this is probably a consequence of the higher incorporation of dimers, whose size is larger than that of the monomer-water adduct. Our observation provides an effective way of enhancing the incorporation of SDA dimers by increasing the crystallization time, at least in this system, although this appears as a postsynthetic optimization of the SDA content once the AlPO-5 framework is formed and, therefore, does not influence the optimal outcome of the synthesis. We have also discovered a photoinduced rearrangement of the SDAs, obtained by irradiating the samples with a
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Figure 8. Left: Cp-Cp (aromatic C atoms) RDF in dimers 1 (black line) and 2 (gray line). Right: Molecular structure of dimers 1 (top) and 2 (bottom).
wavelength of 260 nm (corresponding to the monomer excitation). The reorganization of the excited molecules into dimers, which suggests instability of the excited monomers, is not surprising, as self-aggregation enables a more effective delocalization of the excited electrons and, thus, explains the enhanced stability of the dimers. This observation, if shown to be of a more general validity, could be useful for hybrid organoinorganic host-guest systems where maximum aggregation of the SDAs is required (for instance in optical applications). Indeed, photoirradiation of the synthesis gel could also be used to control the aggregation state of the SDAs and, thus, affect the final structure of the inorganic framework, although the use of hydrothermal synthesis methods at high pressures (in autoclaves) does not allow easy irradiation of the SDA molecules. Our computational model shows that the most stable BMPM content is of 1.0 molecule per unit cell, with benzyl rings facing each other and forming dimers stabilized by π-π type interactions; seven water molecules are also loaded, three of which in the 12 MR channels, which form six-membered ring clusters in between consecutive dimers; the remaining four water molecules are occluded in the 6 MR channels. The geometry of these six-membered ring water clusters yields a close match to the 12 MR AFI channels and strongly stabilizes the systems. Packing values of 0.8 and 1.2 SDA/u.c. represent the next most stable configurations with energy differences with respect to the most stable of less than 5 kcal/mol. The experimental results show that, in the sample obtained after 3 days of crystallization, 0.92 BMPM molecules per unit cell are loaded, while increasing the crystallization time to 6 days results in an increase of the BMPM content to 1.04 molecules per unit cell, with a higher fraction of dimers. Our combined experimental and computational study enables us to gain insight into the structure directing effect of these organic molecules, and shows that, in the first stages of the crystallization, the system incorporates a lower amount of organic molecules, around 0.8 molecules per unit cell, mostly in the form of monomers and accompanied by the incorporation of water molecules surrounding the organic SDAs. This observation suggests the monomer-water adducts to be the kinetically favored guest incorporation, which appears reasonable due to the smaller size and higher mobility of the
hydrated SDA monomer compared to the dimer. Increasing the crystallization time leads to a higher organic incorporation of 1.0 (or even 1.2) molecules per unit cell as dimers; these are the thermodynamically stable occluded species, according to our computational model. Higher SDA densities are less stable due to the orientation of the methyl groups, which stand out from the dimer configuration and face the adjacent dimer, thus preventing an effective interaction between consecutive dimers (Figure SI-3). It is finally worth noting the presence of two different overlapping fluorescence bands centered at 400 and 440 nm, assigned to two different BMPM dimers (Figure 4). Indeed a careful investigation of the simulations revealed that two types of dimeric configurations can be accommodated within the onedimensional 12 MR channels of the AlPO-5 structure, differing in the respective orientation of the BMPM units composing the dimers (Figure 8, right). In the first, which has been found to be the most stable (labeled as dimer1), the benzyl rings are oriented parallel to the channel direction. The second type of dimer has its benzyl rings bent with respect to the channel direction (labeled as dimer2); once occluded within the channel, the two orientations cannot be transformed into each other. Computational results show that the latter dimer conformation (2) is less stable than the former (1). However, at the beginning of the crystallization, the occlusion of dimers in conformation (2) may occur with a nonzero probability, although not being the most thermodynamically favored product, as we have previously observed for the monomer incorporation. This dimer conformation (2) involves a less-efficient overlap of the aromatic rings than that achieved in dimer1. To provide further evidence for this, we ran 200 ps of NVT MD simulations for the two types of dimers (at 300 K) and calculated the radial distribution function (RDF) of the aromatic C atoms (Cp) (Figure 8, left). Results evidenced that the overlap of the aromatic rings is more efficient in dimer1 than in dimer2 (the RDF of the aromatic C atoms shows a much more intense short-range order and at shorter distances for dimer1). The existence of these two types of dimers with different π-overlaps may explain the two distinct dimeric fluorescence bands, the one at 400 nm corresponding to dimer2 and the one at 440 nm to dimer1, because the emission at longer wavelengths (lower energies) implies a dimer with a
Synthesis of AlPO-5 stronger π-overlap. In fact, we have observed that increasing the crystallization time, which in turn drives the system toward the thermodynamically stable product, increases the band assigned to dimer1, which is the most stable dimer conformation. Conclusions We have found that the new chiral SDA molecule (1S,2S)2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium directs efficiently the crystallization of the undoped AlPO-5 structure. The incorporation of guest molecules, namely, organic and water molecules, depends upon the crystallization time: shorter crystallization times lead to a lower organic content (and, consequently, higher water incorporation), resulting in a lower formation of dimers, in what seems to be the kinetically favored structure-directing action. However, an increase of the crystallization time results in a slightly higher incorporation of organic molecules, with an enhancement of the SDA aggregation within the AlPO-5 channel, thus, being these dimers the thermodynamically favored structure-directing species. We have observed that the aggregation of the monomers into dimers within the AlPO-5 channels can be enhanced by photoexcitation. Two types of dimers can be formed within the AlPO-5 structure, with a different efficiency in the overlap of the aromatic rings, thus, resulting in different π-π-type interactions and different fluorescence bands. The most stable dimer corresponds to that with the highest aromatic overlap, emitting at longer wavelengths. Finally, we have also found that water molecules located between the organic molecules form stable six-membered ring clusters with a geometry complementary to that of the AlPO-5 channel, providing high stability to the system. Our recently developed computational model predicts the most stable organic content to correspond to 1.0 molecules per unit cell, in good agreement with the experimental results, demonstrating again its good performance for the study of structure-directing effects in hydrophilic microporous materials. Acknowledgment. Financial support of the Spanish Ministry of Education and Science (Project CTQ2006-06282) is acknowledged. L.G.-H. acknowledges the Spanish Ministry of Education and Science for a postdoctoral grant, and R.G. is grateful to CSIC for a JAE contract. F.C. is supported by an RCUK Fellowship. Accelrys is acknowledged for providing the software and Centro Tecnico de Informatica-CSIC for running some of the calculations. Supporting Information Available: Additional spectral data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Davis, M. E. Acc. Chem. Res. 1993, 26, 111.
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8327 (2) Naber, J. E.; de Jong, K. P.; Stork, W. H. J.; Kuipers, H. P. C. E.; Post, M. F. M. Stud. Surf. Sci. Catal. 1994, 84C, 2197. (3) Venuto, P. B. Microporous Mesoporous Mater. 1994, 2, 297. (4) Yu, J.; Xu, R. J. Mater. Chem. 2008, 18, 4021. (5) Mc Morn, P.; Hutchings, G. J. Chem. Soc. ReV. 2004, 33, 108, and references therein. (6) Davis, M. E. Microporous Mesoporous Mater. 1998, 21, 173, and references therein. (7) Bedioui, F. Coord. Chem. ReV. 1995, 144, 39. (8) Dryzun, C.; Mastai, Y.; Shvalb, A.; Avnir, D. J. Mater. Chem. 2009, 19, 2062. (9) Sun, J.; Bonneau, C.; Cantı´n, A.; Corma, A.; Dı´az-Caban˜as, M. J.; Moliner, M.; Zhang, D.; Li, M.; Zhou, X. Nature 2009, 458, 1154. (10) Lok, B. M.; Cannan, T. R.; Messina, C. A. Zeolites 1983, 3, 282. (11) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756. (12) Zones, S. I.; Nakagawa, Y.; Lee, G. S.; Chen, C. Y.; Yuen, L. T. Microporous Mesoporous Mater. 1998, 21, 199. (13) Gies, H.; Marler, B. Zeolites 1992, 12, 42. (14) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U.S. Patent 4310440, 1982. (15) Go´mez-Hortigu¨ela, L.; Cora`, F.; Catlow, C. R. A.; Pe´rez-Pariente, J. J. Am. Chem. Soc. 2004, 126, 12097. (16) Go´mez-Hortigu¨ela, L.; Pe´rez-Pariente, J.; Cora`, F.; Catlow, C. R. A.; Blasco, T. J. Phys. Chem. B 2005, 109, 21539. (17) Go´mez-Hortigu¨ela, L.; Cora`, F.; Catlow, C. R. A.; Pe´rez-Pariente, J. Phys. Chem. Chem. Phys. 2006, 8, 486. (18) Go´mez-Hortigu¨ela, L.; Lo´pez-Arbeloa, F.; Cora`, F.; Pe´rez-Pariente, J. J. Am. Chem. Soc. 2008, 130, 13274. (19) Go´mez-Hortigu¨ela, L.; Pe´rez-Pariente, J.; Lo´pez-Arbeloa, F. Microporous Mesoporous Mater. 2009, 119, 299. (20) Go´mez-Hortigu¨ela, L.; Hamad, S.; Lo´pez-Arbeloa, F.; Pinar, A. B.; Pe´rez-Pariente, J.; Cora`, F. J. Am. Chem. Soc. 2009, 131, 16509. (21) Go´mez-Hortigu¨ela, L.; Pe´rez-Pariente, J.; Blasco, T. Microporous Mesoporous Mater. 2007, 100, 55. (22) Gomez-Hortiguela, L.; Marquez-Alvarez, C.; Sastre, E.; Cora`, F.; Perez-Pariente, J. Catal. Today 2006, 114, 174. (23) Dehmlow, E. V.; Klauck, R.; Du¨ttmann, S.; Neumann, B.; Stammler, H. Tetrahedron: Asymmetry 1998, 9, 2235. (24) Go´mez-Hortigu¨ela, L.; Pe´rez-Pariente, J.; Cora`, F. Chem.sEur. J. 2009, 15, 1478. (25) Go´mez-Hortigu¨ela, L.; Pinar, A. B.; Pe´rez-Pariente, J.; Cora`, F. Chem. Mater. 2009, 21, 3447. (26) Dauger-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler, A. T. Proteins: Struct., Funct., Genet. 1988, 4, 21. (27) Moloy, E. C.; Cygan, R. T.; Bonhomme, F.; Teter, D. M.; Navrotsky, A. Chem. Mater. 2004, 16, 2121. (28) Williams, J. J.; Smith, C. W.; Evans, K. E.; Lethbridge, Z. A. D.; Walton, R. I. Chem. Mater. 2007, 19, 2423. (29) Garcia, R.; Philp, E. F.; Slawin, A. M. Z.; Wright, P. A.; Cox, P. A. J. Mater. Chem. 2001, 11, 1421. (30) Hill, J. R.; Minihan, A. R.; Wimmer, E.; Adams, C. J. Phys. Chem. Chem. Phys. 2000, 2, 4255. (31) Rappe, A. K.; Goddard III, W. A. J. Phys. Chem. 1995, 95, 3358. (32) Corma, A.; Rey, F.; Rius, J.; Sabater, M. J.; Valencia, S. Nature 2004, 431, 287. (33) Suib, S. L.; Kostapapas, A. J. Am. Chem. Soc. 1984, 106, 7705. (34) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Am. Chem. Soc. 1993, 115, 10438. (35) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Phys. Chem. 1993, 97, 13380. (36) Hashimoto, S.; Ikuta, S.; Asahi, T.; Masuhara, H. Langmuir 1998, 14, 4284. (37) Thomas, K. J.; Sunoj, R. B.; Chandrasekhar, J.; Ramamurthy, V. Langmuir 2000, 16, 4912.
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