Structure-Directing Effect of - American Chemical Society

Chem. Mater. 2009, 21, 3447–3457 3447 DOI:10.1021/cm901149a

Structure-Directing Effect of (S)-(-)-N-Benzylpyrrolidine-2-methanol and Benzylpyrrolidine in the Synthesis of STA-1: A New Computational Model for Structure Direction of Nanoporous Systems :: Luis G omez-Hortiguela,*,†,‡,§ Ana B. Pinar,§ Joaquı´ n Perez-Pariente,§ and Furio Cora†,‡ †

Department of Chemistry, Third Floor, Kathleen Lonsdale Building, University College London, Gower Street, WC1E 6BT London, United Kingdom, ‡Thomas Young Centre at University College London, alisis y Petroleoquı´mica-CSIC, Gower Street, WC1E 6BT London, United Kingdom, and §Instituto de Cat C/Marie Curie 2, 28049 Cantoblanco, Madrid, Spain Received April 25, 2009. Revised Manuscript Received June 11, 2009

We propose a new computational model to study structure direction in nanoporous materials that takes into account the stability of the guest species in solution as well as the possibility of occluding solvent molecules in the nanopores. The model is applied to study the structure directing effect of (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) and benzylpyrrolidine (BP) in the synthesis of the nanoporous STA-1 material (SAO framework type), and compare results with experimental characterization. Despite the fact that up to 8 BP or BPM molecules can be loaded within the nanoporous framework, the relative stability of the SDA molecules in solution and the simultaneous incorporation of water limit the amount of organic content. The most stable arrangements correspond to the occlusion of 6 BPM or 5 BP molecules per SAO unit cell, accompanied respectively by 9 and 22 water molecules in order to complete space filling, leading to a cooperative structuredirecting action between organic SDA and water molecules during crystallization. The organic molecules act as primary structure-directing agents, and the water molecules as secondary spacefilling species whose role is to completely fill the void space of the pore system and provide further stabilization to the nanoporous framework. We demonstrate the necessity of including in the computational models the occlusion of water molecules as well as to account for the stability of SDA and water molecules in solution for realistically studying the structure-directing effect of organic molecules in the synthesis of hydrophilic aluminophosphate nanoporous frameworks. We therefore provide a useful computational tool for studying the occlusion of guest species in hostguest systems, an issue which is essential for controlling the potential applications of these materials. Introduction Crystalline nanoporous materials attract industrial interest for processes such as catalysis, molecular sieving, gas separation, and ion exchange,1-3 which exploit the molecular dimensions and the crystalline nature of the nanoporous structure to discriminate between molecules with even subtle steric differences. Many different atoms can be incorporated within the oxide network of these crystalline nanoporous solids, giving place to a range of materials with different compositions, several of which have useful catalytic properties. Since their discovery by Wilson et al. in 1982,4 the synthesis of nanoporous aluminophosphates (AlPO4) has been widely studied, yielding a diversity of structural types comparable to that of the previously known aluminosilicate-based zeolites.5 *Corresponding author. E-mail: [email protected].

(1) Davis, M. E. Acc. Chem. Res. 1993, 26, 111. (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) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U.S. Patent 4 310 440, 1982. (5) http://www.iza-structure.org/databases. r 2009 American Chemical Society

Known nanoporous AlPO4 structures include polymorphs that are common to both SiO2 and AlPO4 compositions, but also structures that have no zeolitic counterpart. In AlPO4 materials, there is a strict alternation of Al3þ and P5þ ions; both can be isomorphically replaced by heteroatoms, giving rise to acid, redox and even bifunctional properties. Nanoporous oxides are synthesized through hydrothermal methods, where the source of the inorganic ions, water, and, generally, an organic species, are heated in an autoclave for a time ranging from a few hours to weeks. The inclusion of the organic molecules is usually required to direct the crystallization toward a certain nanoporous structure, and so they are called structure directing agents (SDAs). The role of these organic molecules has been traditionally described as a “template effect”6 to indicate that the organic molecules organize the inorganic tetrahedral units into a particular topology around themselves during the nucleation process, providing the initial building blocks from which crystallization of the nanoporous structures will take place. Nevertheless, new theories (6) Gies, H.; Marler, B. Zeolites 1992, 12, 42.

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Chem. Mater., Vol. 21, No. 14, 2009

:: G omez-Hortiguela et al.

developed by means of computational simulations by Caratzoultas et al.,7-10 based on previous experimental evidence for the formation of cagelike silica clusters assisted by the presence of tetramethylammonium collected by Kinrade et al.,11,12 are currently emerging; these suggest that the role that these SDA molecules play during crystallization of nanoporous materials is more complex than the one described in the initial template theory. The exact role of the SDAs is still far from being properly understood, and structure direction is a major open issue in molecular sieve science. Controlling this feature would enable the synthesis of new topologies, as well as allow researchers to gain control over crystal sizes and morphologies and the location of heteroatoms, if present. The organic SDA molecules are encapsulated within the nascent nanoporous structure during its crystallization, developing strong nonbonded interactions with the framework and thus contributing to the final stability of the system. It is well-known that nanoporous frameworks are metastable, and their intrisic stability increases with the framework density, i.e., denser materials are more stable than open frameworks. The organic molecules occluded within the void spaces of open frameworks interact (through nonbonded interactions) with the surrounding oxide network, providing the required thermodynamic stabilization to make viable the crystallization of such structures. Therefore, the SDA molecules direct the crystallization of the nanoporous frameworks not only by providing a volume from which the inorganic ions are excluded but also by altering the relative thermodynamic stability of different framework structures via the development of different SDA-framework interactions. Despite the large number of studies about the role of these organic molecules in the crystallization of nanoporous materials, the water molecules are another important component of the synthesis gel that has attracted less attention. Water is present during the synthesis of both pure-silica and aluminophosphate nanoporous frameworks, and it indeed influences the crystallization mechanism.7-10 Silica-based nanoporous materials do not usually occlude water molecules because of the hydrophobic nature of the SiO2 network; however, the higher hydrophilicity of AlPO4-based networks, coming from their molecular-ionic nature (the network can be seen as Al3þ and PO43- units ionically bonded,13 in contrast to SiO2 networks that are mainly covalent), and the strict alternation of Al3þ and PO43- ions in the network, allows

them to interact more strongly with water molecules and thus incorporate them within the nascent structure during crystallization. Recently we have demonstrated the important role that water molecules play, in addition to the organic SDA molecules, in the energy balance and in the structure directing effect of the AFI structure.14 In addition, the energetics involved in the H-bond network formation of water clusters embedded within nanoporous aluminophosphates, leading to a clear structural relationship between the framework and the water cluster, and its role on the material formation have been recently addressed.15 Previous works carried out in our laboratories showed the high structure directing activity of benzylpyrrolidine (BP) and (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) in the synthesis of the AFI structure.16-18 In recent work, we have observed that these molecules are also able to direct the crystallization of the STA-1 nanoporous material (SAO framework type) at higher SDA concentrations and in the presence of Zn.19 Results showed that at a crystallization temperature of 150 °C, both BP and BPM led to the crystallization of pure STA-1, whereas at the higher temperature of 190 °C, only BPM directed the crystallization of pure STA-1; instead, BP led to a mixture of STA-1 and ZnAPO-5 (AFI framework type). Details of the synthesis and characterization of these materials will be given elsewhere.19 The SAO framework structure has rarely appeared in the literature; to the best of our knowledge, the only work related to this structure corresponds to its original discovery and structure solution,20 where diquinuclidinium ions of the form (C7H13N)(CH2)n-(C7H13N), where n varied from 7 to 9, were used as SDAs. The SAO structure is composed by a threedimensional channel system, with two 12 MR channel systems perpendicular to each other, one along the [100] direction with a diameter of 6.5 7.2 A˚2 and the other along the [001] direction with a diameter of 7.0 7.0 A˚2 (Figure 1), resulting in a framework density of 13.4 T/ 1000 A˚3, one of the lowest among synthesized AlPOs structures. The two channel systems are interconnected, giving place to channel intersections with a large fraction of void space. The stabilization of such a large void structure requires a very efficient structure direction during the synthesis; moreover, and because the size of the empty channels exceeds the normal molecular sizes, it is likely that structure direction of SAO requires some form of supramolecular aggregation or cooperativity between SDAs or SDA and water molecules.

(7) Caratzoulas, S.; Vlachos, D.; Tsapatsis, M. J. Phys. Chem. B 2005, 109, 10429. (8) Caratzoulas, S.; Vlachos, D.; Tsapatsis, M. J. Am. Chem. Soc. 2006, 128, 596. (9) Caratzoulas, S.; Vlachos, D.; Tsapatsis, M. J. Am. Chem. Soc. 2006, 128, 16138. (10) Caratzoulas, S.; Vlachos, D. J. Phys. Chem. B 2008, 112, 7. (11) Kinrade, S. D.; Knight, C. T. G.; Ple, D. L.; Syvitski, R. Inorg. Chem. 1998, 27, 4272. (12) Kinrade, S. D.; Knight, C. T. G.; Ple, D. L.; Syvitski, R. Inorg. Chem. 1998, 27, 4278. (13) Cor a, F.; Alfredsson, M.; Barker, C. M.; Bell, R. G.; Foster, M. D.; Saadoune, I.; Simperler, A.; Catlow, C. R. A. J. Solid State Chem. 2003, 176, 496.

:: (14) G omez-Hortiguela, L.; Perez-Pariente, J.; Cora, F. Chem.;Eur. J. 2009, 15, 1478. (15) Henry, M.; Taulelle, F.; Loiseau, T.; Beitone, L.; Ferey, G. Chem.;Eur. J.:: 2004, 10, 1366. (16) G omez-Hortiguela, L.; L opez-Arbeloa, F.; Cora, F.; Perez-Pariente, J. J. Am. Chem.::Soc. 2008, 130, 13274. (17) G omez-Hortiguela, L.; Perez-Pariente, J.; Blasco, T. Microporous Mesoporous Mater. 2007, 100, 55. :: (18) G omez-Hortiguela, L.; Perez-Pariente, J.; Blasco, T. Microporous Mesoporous Mater. 2005, 78, :: 189. (19) Pinar, A. B.; G omez-Hortiguela, L.; Perez-Pariente, J. 2009, manuscript in preparation. (20) Noble, G. W.; Wright, P. A.; Lightfoot, P.; Morris, R. E.; Hudson, K. J.; Kvick, A.; Graafsma, H. Angew. Chem., Int. Ed. 1997, 36, 81.

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Figure 1. SAO framework structure and topology of the pore system (blue-white) viewed in the yz plane; the main structural features are highlighted. The [100] channel is perpendicular to the viewed (yz) plane.

We have recently developed a computational methodology that permits to study the energetic effect of water in the synthesis of nanoporous structures; both cooperative and competitive water/SDA effects can be described in the model, which has been successfully applied to study structure direction to the AFI topology.14 However, the AFI structure is a relatively simple structure in terms of topological features, since it contains only one-dimensional not-interconnected 12 MR channels; we now apply our computational model to the more complex SAO framework to test its validity and predictive ability. In this work, we employ the same computational methodology described in ref 14 to study the occlusion of the two SDA molecules (BP and BPM) and water within the SAO framework, and compare the computational results with experimental data. We shall also compare our newly developed protocol with the computational models traditionally used for studying the occlusion of SDA molecules in nanoporous frameworks, which are limited to consider the maximum loading of SDAs.

The goal of the computational study is to understand and compare the energetics involved in the occlusion of the organic (BP and BPM) molecules and water within the SAO nanoporous structure during crystallization. Molecular structures and the interaction energies of the SDAs and water with the AlPO4 network were described with the cvff forcefield.21 This force field was originally developed for small organic molecules but has been extended for materials science applications including the simulation of zeolite and related structures, for which it has been successfully applied recently.22-24

The framework atoms were kept fixed during all the calculations. Because of the usually very acidic pH of the synthesis gels of AlPO4 frameworks, the amine group of the SDA molecules is expected to be protonated in the gels and also when occluded within the forming nanoporous AlPO4 structures. Protonated BP and BPM ammonium cations have been studied; the net atomic charges for these organic ammonium ions (total molecular net charge of þ1) were calculated by the charge-equilibration method.25 To provide charge neutrality, the net molecular charge of þ1 of the SDAs had to be compensated by the inorganic framework. In the experimental work, charge balance is provided by the Zn dopants, which leads to Zn2þ/Al3þ replacements (and thus to a negative charge) in the framework. Because of the lack of evidence from the experiment about the location and distribution of the Zn ions, which are difficult to determine through the diffraction techniques employed for structure determination, and the extremely large number of different Zn distributions possible in the nanoporous framework, the net molecular charge of the SDAs was compensated by making use of a modified version of the “uniform charge background” model,26 in which the charge of each aluminum framework ion is reduced uniformly until charge neutrality is achieved. This model has been successfully applied to study related systems, mimicking in a realistic way the effect of the framework charges.14,27,28 Only the aluminum charge was modified because Zn2þ ions are known to substitute Al3þ (and not P5þ) ions in the AlPO4 lattices. Thereby, the atomic charges of framework oxygen and phosphrous ions were fixed to -1.2 and þ3.4, respectively, whereas the aluminum charge was reduced from the initial value of þ1.4 to ensure charge neutrality (depending on the SDA content). The atomic charges for the water molecules were -0.82 and þ0.41 for oxygen and hydrogen atoms, respectively; this charge distribution has been shown to describe well the properties of water containing systems.29

(21) Dauger-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler, A. T. Proteins: Struct., Funct., Genet. 1988, 4, 31. (22) Moloy, E. C.; Cygan, R. T.; Bonhomme, F.; Teter, D. M.; Navrotsky, A. Chem. Mater. 2004, 16, 2121. (23) Williams, J. J.; Smith, C. W.; Evans, K. E.; Lethbridge, Z. A. D.; Walton, R. I. Chem. Mater. 2007, 19, 2423. (24) Garcia, R.; Philp, E. F.; Slawin, A. M. Z.; Wright, P. A.; Cox, P. A. J. Mater. Chem. 2001, 11, 1421.

(25) Rappe, A. K.; Goddard, W. A.III. J. Phys. Chem. 1991, 95, 3358. (26) De Vita, A.; Gillan, M. J.; Lin, J. S.; Payne, M. C.; Stich, I.; Clarke, L. J. Phys. Rev.:: B 1992, 46, 12964. (27) G omez-Hortiguela, L.; Cora, F.; Catlow, C. R. A.; Perez-Pariente, J. Phys. Chem.:: Chem. Phys. 2006, 8, 486. (28) G omez-Hortiguela, L.; Marquez-Alvarez, C.; Cora, F.; L opez-Arbeloa, F.; Perez-Pariente, J. Chem. Mater. 2008, 20, 987. (29) Williams, J. J.; Smith, C. W.; Evans, K. E.; Lethbridge, Z. A. D.; Walton, R. I. Chem. Mater. 2007, 19, 2423.

Computational Details

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Because of the large dimensions of the SAO unit cell (56 T sites), a simple crystallographic cell was large enough to model the occlusion of SDA and water molecules. All the calculations have been performed with the OFF and Sorption Modules available in the Cerius2 software package.30

Computational Protocol To understand the relative occlusion of SDA and water molecules in the SAO nanoporous framework, we apply the computational protocol first described in ref 14, which was successful in describing competition and cooperation effects of organic and water molecules in the AFI framework. The nanoporous interstices of the hydrophilic AlPO structure can be occupied not only by the organic molecules used as structure-directing agent (BP or BPM in our case) but also by water molecules, which are always present in hydrothermal syntheses. Under thermodynamic control, the relative amount of water and organic SDA molecules can be estimated by calculating the stabilization energy of the system at different SDA:water ratios in the nanopores. In our computational methodology, and for computational convenience, we identify a primary and a secondary space-filling molecule. In our case, the primary molecule is the organic SDA; this assumption, although not affecting the final results, does represent real experimental evidence because in the absence of SDA molecules the crystallization of nanoporous frameworks does not take place (under our experimental conditions, dense phases such as AlPO-C and trydimite are crystallized in the absence of efficient SDAs). The water molecules are a secondary space-filling agent, which, however, needs to be accounted for in the energy balance. They are inserted in the nanopores after the docking of the primary space-filling molecules (the SDAs). The computational protocol involves three consecutive simulation steps: (1) First, maximum loading of the organic SDA is determined. We shall gradually remove SDA molecules in subsequent steps. In the present case, the maximum SDA loading was observed to be 8 molecules per SAO unit cell for both BP and BPM, a set of 4 molecules along each 12 MR channel. Here and in the following, at each SDA loading, the sytems were relaxed via simulated annealing calculations, which consisted of heating of the system from 300 to 700 K with temperature increments of 10 K, and then cooling to 300 K again in the same way. Five-hundred MD steps of 1.0 fs (for a total of 0.5 ps) were run in every heating/cooling step. At the end of each cycle the system was geometry optimized. (2) From the maximum value obtained above, the number of SDA molecules is decreased in discrete steps. Packing values of 8, 7, 6, 5, 4, 2, and 0 SDA molecules per SAO unit cell have been studied. Although lower SDA densities were achieved with the molecules in monomeric form, packing values of 8 and 7 could only be reached by loading the SDAs as dimers along each channel. Because of the low mobility of the SDAs, specially in dimeric form, through the SAO channels in the simulated annealing step, different initial orientations of the SDA molecules were generated manually for the lower SDA densities (6, 5, 4, and 2 SDAs per SAO cell), thus ensuring the screening of all the possible intermolecular orientations. Four different initial configurations were tried for packing values of 6, 5, and 4 SDA molecules, with these located in the channels with benzyl rings in opposite sides (1) or facing each other (2), or located in the (30) Sorption and OFF Modules, version 4.6; Accelrys Inc.: San Diego, CA, 2001.

:: G omez-Hortiguela et al. intersections, with all benzyl rings pointing to the same (3) or to different (4) channels. Two different orientations were tried for the packing value of 2, with the SDA molecules in the channels (1) or in the intersections (2). For each loading and molecular orientation, the most stable location of the organic molecules (initially without water) was found by running 5 cycles of simulated annealing calculations, and the most stable configuration was taken for subsequent calculations with water. (3) Water molecules were then inserted in the organic-containing SAO systems through Grand Canonical (pVT ensemble) MonteCarlo (MC) simulations, where Coulomb interactions were explicitly included. A constant water partial pressure of 1000 kPa was used; this high pressure was selected in order to ensure the full loading of water and to accelerate the loading convergence. Such a high partial pressure is, however, not unrealistic for hydrothermal synthesis conditions. A total of 5 million configurations were sampled, ensuring that the loading of water molecules had reached equilibrium. Then, the location of both water and organic SDA molecules and the final internal energies (Ef) for each system were obtained by running 10 cycles of simulated annealing calculations. This computational 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 nanoporous framework. Because in the true synthesis experiments SDA and water molecules do not come from vacuo but from the synthesis gel, we have to take in account their stability in the synthesis gel in the final energetic balance. In our model, we represent the source of water and SDAs, respectively, as liquid water and an aqueous solution containing the SDA cations at the same concentration as in the synthesis gel employed experimentally (2 SDA:40 H2O). The energy cost of removing a water molecule and one SDA molecule from the aqueous solution are then given by the sol H2O (E sol H2O) and SDA (E SDA) solution energies. These energy values were estimated as described in our previous work,14 by running simulated annealing followed by geometry optimization calculations of the SDA solutions (2 SDA in 40 water molecules under periodic boundary conditions, PBC), making them consistent with the rest of our calculations. In this way, the SDA solution energies were calculated as -92.86 and -99.71 kcal/mol per SDA molecule for BP and BPM, respectively. As expected, this energy is higher for BPM because of the presence of the hydrophilic methanol (CH2OH) group that can form H-bonds with the water molecules. The H2O solution energy was calculated in the same way by running simulated annealing followed by geometry optimization calculations of a PBC system with 80 water molecules; this value was found to be -12.40 kcal/mol per water molecule. The simulated annealing calculations described here are efficient ways to sample a complex potential energy landscape, and have been used to calculate internal rather than free energies for water and SDA molecules in solution; these values do not represent therefore real estimations of the water vaporization and SDA solvation energies under true experimental conditions, but provide the stability of these species in water, consistent with the computational approach employed here to study the interaction energies with the nanoporous frameworks. The net stabilization provided by the occlusion of SDAs and water within the nanoporous frameworks will be given by the total energy of the system minus the energy of the water and SDA molecules in the water solution, as expressed in the following equation: vac sol vac sol þESDA Þ- nH2O ðEH2O þ EH2O Þ ð1Þ ΔEðnSDA Þ ¼ Ef - nSDA ðESDA

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Table 1. Elemental CHN Analysis, C/N Ratio, Organic Content (in SDA molecules per unit cell, calculated from the indicated experimental results), and Water Content (calculated from TGA data) SDA/u.c. calculated from SDA

C

H

N

C/N

BP 11.10 2.56 1.31 9.9 (11) BPM 14.85 2.69 1.63 10.6 (12)

Figure 2. TGA (solid line, left axis) and DTA (dashed line, right axis) of the STA-1 samples obtained at 150 °C with BPM (black) and BP (gray) as SDAs.

where Ef refers to the energy (per unit cell) of the framework containing the SDA and water molecules, nSDA and nH2O are the number of SDA and water molecules per framework unit cell, vac respectively, Evac SDA and EH2O are the calculated energies of sol the molecules in vacuo, ESDA is the SDA solution energy and sol sol EH2O is the H2O solution energy. As discussed above, ESDA and sol EH2O account for the energetic cost of transferring water and SDA molecules from the aqueous solution to the nanoporous framework. Plotting the value of ΔE(nSDA) as a function of the number of SDA (primary space-filling) molecules, we can estimate the most stable loading ratio of SDA and water molecules within the nanoporous frameworks, in equilibrium with their aqueous solution. For future reference, we also define the interaction energy Eint as the change in energy between the molecules in the framework and in vacuo (i.e., neglecting the sol sol ESDA and EH2O terms) vac vac - nH2O EH Eint ¼ Ef - nSDA ESDA 2O

ð2Þ

Results

A. Experimental Results: SDA Content within the SAO Structure from TGA and EA Data. In this section, we discuss the experimental results collected to evaluate the content of organic and water molecules in the samples obtained at 150 °C (where STA-1 was the only phased observed); further details of the characterization of the samples will be given elsewhere.19 Thermogravimetric (TGA) (Figure 2) and elemental CHN (EA) (Table 1) analyses were performed in order to study the organic content occluded within the SAO structures. EA data indicate that the molecular integrity of BP and BPM is retained upon occlusion within the framework, because the C/N ratio of the SAO samples with BP (9.9) and BPM (10.6) are very similar to those of BP and BPM molecules in vacuum (11 and 12 for BP and BPM, respectively); in addition, 13C NMR results evidenced the resistance of these molecules to the hydrothermal treatments.17,18 TGA results show the presence of three strong desorption steps, one at temperatures below 200 °C that corresponds to the release of water,28 and the other two centered at around 400 and 650 °C that correspond to the desorption of the organic molecules

EA-C EA-N TGA waterTGA 4.1 5.2

4.5 5.9

5.2 6.1

22.3 13.6

occluded within the framework. A higher desorption rate at the water desorption step (∼100 °C) and a lower desorption rate at the second SDA desorption step (∼650 °C) are observed for the sample obtained with BP, suggesting in principle a lower organic and higher water content for the SAO structure obtained with this molecule. The organic and water contents can be calculated from either the TGA or EA data, in the latter case using C or N content values. TGA results, considering that the organic desorption corresponds to the weight loss at temperatures above 200 °C, give organic contents of 6.1 BPM and 5.2 BP molecules per unit cell, respectively. If the SDA content is calculated from the CHN EA results and using the C content, a lower SDA packing value of 5.2 BPM and 4.1 BP molecules per SAO unit cell, respectively, is found. However, if we use the N content in the EA data, we obtain higher values of 5.9 BPM and 4.5 BP molecules per SAO unit cell, in substantial agreement with TGA. This difference in the values obtained from the EA (C) results could be indicative of an incomplete combustion of the C content during the elemental analysis; complete combustion of the organic molecules during the CHN analysis of these materials is difficult to achieve (TGA data show that even at temperatures up to 800 °C the weight loss has not completely finished) because of the acid nature of the samples due to the presence of low-valence Zn2þ ions in the framework, which upon combustion of the organic molecules can be protonated and thus provide acid sites in the sample during the combustion process; this could lead to retaining a certain amount of C species (coke) in the sample and thus to a slight underestimation of the C content. Therefore, it is possible that the CHN analysis has not achieved a complete C combustion, what would lead to an underestimated calculated SDA content from the EA analysis results when using the C content. Given the good agreement of TGA and EA (N) data, we used them to estimate the experimental organic content to be of 6.0 ( 0.1 BPM and 4.9 ( 0.4 BP molecules per SAO unit cell. A common feature of all the characterization techniques is that the BPM content is higher than that of BP by approximately 1 molecule per unit cell, despite the similar overall molecular size of BP and BPM (BPM is even larger because of the additional CH2OH substituent). The water content can be estimated from TGA data only, in particular from the desorption peak centered at ∼100 °C. The TGA results show in this case a notably higher water content in the sample obtained with BP, with 13.6 and 22.3 water molecules per unit cell for the STA-1 structures obtained with BPM and BP, respectively.

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Table 2. Stabilization Energies [ΔE(nSDA)] of the Different Systems As a Function of the Organic Content (nSDA) (and initial orientation (orient.) as defined in the text) in the SAO Structurea BPM nSDA 8 7 6 6 6 6 5 5 5 5 4 4 4 4 2 2 0

orient.

1 2 3 4 1 2 3 4 1 2 3 4 1 2

BP

nH2O

ΔE(nSDA)

nSDA

1 7 9 5 9 6 16 15 13 16 24 25 23 26 52 50 78

-74.1 -176.4 -257.8 -228.9 -180.4 -177.4 -227.8 -244.1 -218.0 -226.3 -198.7 -208.5 -208.2 -186.1 -202.8 -214.5 -198.9

8 7 6 6 6 6 5 5 5 5 4 4 4 4 2 2 0

orient.

1 2 3 4 1 2 3 4 2 1 3 4 1 2

nH2O

ΔE(nSDA)

2 5 13 7 9 11 22 20 18 20 33 31 32 34 55 55 78

--190.3 -220.5 -235.7 -223.0 -224.6 -224.1 -242.6 -228.5 -233.4 -221.9 -211.7 -218.8 -221.3 -222.8 -193.5 -189.5 -198.9

a The stabilization energies are given in kcal/mol per unit cell. The SDA (nSDA) and water (nH2O) contents are given in molecules per unit cell. The relative orientations are those explained in the computational protocol section. The best arrangement for each SDA content is highlighted in bold.

B. Computational Results: Occlusion of SDA and Water Molecules within the SAO Structure. The water content and stabilization energies [ΔE(nSDA) from eq 1] as a function of the SDA content and molecular orientation are given in Table 2, whereas the stabilization energies for the most stable orientation at each SDA content are plotted in Figure 3. The curve of ΔE(nSDA) as a function of the organic content has a clear minimum at n= 6 for the BPM molecule, accompanied by the simultaneous occlusion of 9 water molecules per unit cell. Instead, the most stable packing value for BP is of 5 SDA and 22 water molecules per unit cell. Our model predicts therefore a higher SDA content in the SAO structure for BPM, in accordance to the experimental results. The packing values of 6 BPM and 5 BP molecules per unit cell are in quantitative agreement with those estimated from the experimental data. A good match between the predictions of our computational model and the experimental findings is also observed concerning the water content. It is interesting to note that despite up to 8 SDA molecules can be occluded in the SAO structure, the most stable occlusion is achieved at lower loadings, both for BP and BPM. As discussed earlier, loadings in excess of 6 SDA molecules require the presence of SDA dimers, whereas lower values can be achieved without the presence of such supramolecular arrangements. BP, and especially BPM, dimers were found to be stable in the AFI structure, which does not seem to be the case in SAO, probably because of a steric hindrance between dimers located in close neighborhood in perpendicular channels (Figure 4-top). The very efficient space filling achieved by BP or BPM dimers in SAO is confirmed by the very low water occlusion (only 1 and 2 water molecules per unit cell can be accommodated in spaces left empty by BPM and

Figure 3. Stabilization energy [ΔE(nSDA)] as a function of the SDA (BPM in solid black line and BP in solid gray line) content (nSDA) in the SAO structure. The experimental values with the error margins are shown by the dashed vertical lines (BPM in dashed black line and BP in dashed gray line).

BP dimers, respectively, at loading of 8); however, such a high space-filling efficiency is achieved partly at the expense of molecular distortions in the constrained environment. Because of the larger size of BPM, the dimeric arrangement with loading of 8 is particularly unstable for the BPM molecules. A decrease of the loading to 7 molecules per unit cell, still 4 of them forming dimers along each channel system, leads to a stabilization, although this is still not the best compromise between space filling and stability. The most stable packing value for BPM is found to be of 6 molecules per unit cell, with 9 additional water molecules to completely fill the void channels and provide additional stability; these water molecules form small water clusters stabilized by H-bonds between themselves and with the framework oxygens and with the OH groups of the BPM molecules. These contents are in good agreement with the experimental values of 6.0 (( 0.1) BPM and 13.6 water molecules per unit cell; the small excess of water observed experimentally might derive from water adsorbed on the external surface of the material. By analyzing the computational data, we observe that 4 SDA molecules are located along each channel, with the main molecular axis parallel to the channel direction (highlighted by yellow circles in Figure 4, middle-left); the other two are located in the channel intersections (green circles), with the aromatic ring in one channel and the pyrrolidine ring in the other. A more detailed picture of the location of the molecules in the channels or intersections under this packing value is given in Figure 5. The presence of SDA molecules in the channel intersections, in addition to the molecules that are located along the individual channels, still provides a very efficient space filling, but with a lower steric hindrance than at the packing value of 8, which is responsible for the stability of this SDA arrangement. A decrease in the organic content to 5 BPM molecules per unit cell is instead

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Figure 4. SDA and water location for BPM (left) and BP (right) at different organic contents: 8 (top), 6 (middle), and 5 (bottom) molecules per SAO unit cell. Yellow and green dashed circles highlight molecules located along the channel and in the intersections, respectively.

associated with a destabilization of the SAO structure obtained with BPM. The arrangement of BP molecules at loading of 6 (with 13 water molecules) is also stable, although the most stable arrangement for BP is found for an SDA loading of 5 molecules per unit cell. In this case, 4 molecules are

located in the channel intersections, and the remaining one along one of the channels. The lower SDA loading enables a higher water occlusion, of 22 water molecules per unit cell, which are arranged as H-bonded clusters in form of chains surrounding the organic molecules. Again, these contents are in quantitative agreement with the

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Figure 5. Location of BPM molecules within the SAO structure (6 BPM per SAO unit cell), highlighting the different locations in the nanoporous architecture: the molecule displayed in blue is located in the channel intersections, with one ring at each channel system, whereas molecules displayed in yellow and green are located aligned with the two channel systems.

experimental values of 4.9 (( 0.4) BP and 22.3 water molecules per unit cell. Lower packing values of 4 and 2 SDA molecules per unit cell lead to lower stabilization energies in both BP and BPM cases, and finally, the system where no organic but only water molecules are loaded (78 molecules per unit cell) has an even lower interaction energy. It is interesting to note the higher energy range of the calculated stabilization energies as a function of the organic loading observed for the BPM (from -70 to -258 kcal/mol) than for the BP molecules (from -190 to -243 kcal/mol); this might be due to the larger molecular size and stronger interaction of BPM with the framework owing to the methanol group. As a result of the stronger interaction, the energy profile as a function of loading has a sharp minimum at nSDA = 6 for BPM, whereas for BP, the profile is smoother and the packing values between 4 and 7 have relatively small energy differences. Finally, the stabilization energy developed by BPM is higher (-257.8 kcal/mol) than that for BP (-242.6 kcal/mol), suggesting a better structure-directing efficiency of BPM in the crystallization of the SAO structure. In fact, this is experimentally confirmed by the fact that although at 150 °C both molecules are able to direct the crystallization of pure STA-1, at 190 °C only BPM leads to pure STA-1, whereas BP leads to a mixture of SAO and AFI nanoporous structures. Discussion and Approximated Models of Structure Direction The most stable packing values predicted by our computational model are in quantitative agreement with the values found from the experimental characterization. The experimental results clearly indicate a lower packing value for the sample obtained with BP, a result that is also predicted by the computational study. The quantitative

agreement of our model with the experimental results provides further confidence on the reliability of our computational methodology in predicting the most stable organic/water contents, which applies not only to the AFI framework, for which it was originally developed,14 but also to more complex structures like the SAO framework considered here. To date, the structure-directing effect of organic molecules in the synthesis of nanoporous materials has been investigated computationally by identifying the location and interaction energy of the organic molecules alone within the pore systems, without taking into consideration the occlusion of secondary species such as water. Furthermore, the reference energy of the SDA molecules was taken as being that of the isolated molecules in vacuum rather than the molecules in solution, leading to the use of interaction (eq 2) rather than stabilization (eq 1) energies. In an attempt to estimate which effect the occlusion of water has over the computational results, in Figure 6-top, we report the interaction energies (eq 2) of the BP and BPM SDA molecules within the SAO structure as a function of loading without including water molecules. Not surprisingly, in these models, the highest interaction energy is reached for the highest number of SDA molecules able to be arranged within the framework without overlapping, i.e., 8 for both BP and BPM in our case. The traditional computational models of structure direction tend therefore to overestimate the organic content. In those cases where no secondary species can compete for the occlusion within the structure, as is the case for high-silica zeolites, whose hydrophobicity prevents the occlusion of water molecules, a simplified study of the interaction energy of the SDA molecules alone may eventually yield realistic results. However, in hydrophilic systems such as AlPO4 type frameworks, this clearly is not the case. As a second step to investigate the effect of approximations, we considered if the inclusion of water in the model

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Figure 6. Approximated models for the energy (in kcal/mol per u.c.) of hydrophilic nanoporous systems. Top, interaction energies (Eint, eq 2) predicted by models where only SDAs are considered; middle, interaction energies (Eint, eq 2) for models where both SDAs and water molecules are considered; bottom, stabilization energy [ΔE(nSDA), eq 1] predicted by models where only SDAs are considered.


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but employing interaction energies instead of stabilization energies, is enough to estimate the packing of the organic molecules in hydrophilic nanoporous materials. Figure 6-middle shows the calculated interaction energy (eq 2) for BP and BPM molecules in SAO as a function of loading, this time considering the occlusion of water (for each organic content, the number of water molecules is the same as discussed earlier) but neglecting the enersol ) and water getic cost of transferring the organic (ESDA sol (EH2O) molecules from the aqueous solution to the nanoporous framework. In this case, the energy profile is more structured but is biased toward the massive occlusion of water, perhaps not surprisingly as one SAO unit cell can accommodate up to 73 water molecules, each coming at zero energetic cost when considering Eint given by eq 2. It is well-known that hydrothermal zeotype synthesis in absence of organic SDAs does not yield nanoporous structures (with few exceptions); this is certainly true for SAO and so the Eint results of Figure 5-middle and the related computational models should be disregarded as not corresponding to experimental evidence. The BPM curve displays a local minimum at a packing value of 6, which is indicative of the optimal interaction, but the energy profile for BP is monotonically decreasing and no indication of stable SDA/water arrangements is present. As a third and final computational model, we considered the stabilization energies (eq 1), i.e., those including the energetic cost of transferring the SDA molecules from the gel but neglecting the incorporation of water (Figure 6-bottom). The curve shows a clear minimum for both molecules at loading of 6 SDAs per unit cell, which bears resemblance to experiment, although it is still unable to reproduce the difference between BP and BPM observed experimentally. At most, the results of Figure 5-bottom may be used only in a qualitative way, to identify the range of SDA loadings around which a more accurate quantitative study that involves water is performed. The results discussed above show that for the templated hydrothermal synthesis of hydrophilic nanoporous solids, it is necessary to include in the model not only the presence of water molecules accompanying the organic SDAs, but also a correct estimation of their stability in an aqueous solution which is the phase from which crystallization takes place. Our recently developed computational methodology accomplishes this extension, and therefore provides a necessary improvement over previously employed models. Despite up to 8 molecules can be accommodated within the SAO framework, only 6 BPM and 5 BP are loaded (accompanied by the occlusion of 9 and 22 water molecules, respectively), evidencing that the crystallization pathways do not always require the maximum filling of the pore system by the organic molecules. Moreover, despite the larger size of BPM, a higher amount of these molecules is occluded within the SAO structure compared to BP. This can be explained by the higher interactions of

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this molecule with the framework because of the presence of the strongly interacting methanol (CH2OH) group, suggesting a more efficient structure directing ability for this molecule; in fact, the stabilization provided by the SDA-framework interactions accounts for ca. 75% of the total stabilization energy in the BPM system, whereas this contribution is only 50% in the BP case. In both cases, a large amount of water is occluded accompanying the SDA molecules and is higher in the sample obtained with BP because of the lower organic occlusion. In this case, the stabilization provided by the occlusion of water molecules is as important as that of the SDA molecules; in fact, a cooperative effect seems to exist between the BP molecules and water molecules when directing the crystallization of the nanoporous structure, which was also found in the crystallization of the AFI structure.14 Let us consider now the requirements of the SDA molecules for directing the crystallization of the SAO nanoporous structure. The only SDA molecules known to direct the synthesis of the SAO structure prior to our study, i.e, the linear diquinuclidinium ions of the form (C7H13N)-(CH2)n-(C7H13N), where n varied from 7 to 9,20 had a molecular structure comprising two bulky quinuclidine rings linked by an aliphatic chain, which provided a high structural flexibility that enabled the molecule to locate one ring in each SAO channel.20 Our molecules are much smaller in size, but share the feature of having two organic rings linked by an aliphatic chain. As shown in Figures 4 and 5, the molecules are arranged in a very efficient way in terms of space-filling, where 4 molecules are located along each channel, and the other 2 are in the channel intersections, with the aromatic ring in one channel and the pyrrolidine ring in the other. In this respect, the presence of the methylene moiety bridging the two rings in the SDA molecule provides the structural flexibility required for the molecules to be accommodated in the channel intersections and with one ring in each channel, thus making these molecules very efficient in the synthesis of the SAO nanoporous structure. The presence of flexible molecules with two rings able to bridge adjacent channels therefore appears to be a requirement for the synthesis of SAO type materials. Our computational results confirm the importance of the structure directing role of the organic SDA molecules. It has been shown that the stabilization energy developed by the system where only water molecules, or a small amount of organic SDAs, are present (packing values of 0 and 2) is very low, demonstrating that the presence of the organic molecules is necessary in order to hold the large pore architecture of the nanoporous structure, thus making viable the crystallization pathway of the SAO structure. In addition, the presence of water molecules surrounding the organic molecules and interacting with the framework walls provides space-filling and additional stabilization to the nanoporous structure. In this case water molecules do not seem to exert any structuredirecting role since no structural relationship between water and the framework topology is observed, in

contrast to other cases where a clear relationship was found.15 To optimize the interaction with the framework, water molecules tend to locate close to the channel walls, thus maximizing the stabilization of the system, whereas the inner parts of the channels are filled by the organic SDA molecules. This observation about the location of water could be extrapolated to other functional groups like amino or hydroxi substituents, whose ability of forming H-bonds with the framework oxygen atoms would drive them to locate close to the channel walls, if the molecular geometry allows it. Therefore, the findings observed in our work could be useful for proposing new efficient types of SDA molecules for the synthesis of nanoporous aluminophosphates, in which a bulky organic core would be used to provide a large molecular size for supporting the open-framework architecture of the nanoporous structure, and a shell carrying strongly interacting groups like hydroxi or amino would provide the required strong interactions in order to compensate for the low intrinsic stability of the open frameworks. Conclusions In this work, we have applied a recently developed computational methodology in order to rationalize the co-occlusion of organic benzylpyrrolidine (BP) or (S)(-)-N-benzylpyrrolidine-2-methanol (BPM) SDA molecules and water during the crystallization of STA-1 nanoporous materials. The computational results are compared with experimental characterization. A good quantitative agreement has been found, showing the reliable performance of the computational methodology in predicting the SDA/water occlusion within even complex nanoporous aluminophosphate structures, such as SAO. Despite the channel intersections enabling the occlusion of up to 8 SDA molecules, we found that the most stable packing value is of 6 BPM and 5 BP molecules per SAO unit cell, accompanied, respectively, by 9 and 22 water molecules. At this organic loading, the interaction energy of the SDA molecules is optimal. Despite the larger size, a higher number of BPM compared to BP molecules is found in the most stable arrangement, in agreement with results previously observed for the AFI structure. This result arises from the higher interaction energy of the BPM molecule with the framework because of the strongly interacting methanol group. Our computational results confirm the fundamental role that the organic SDA molecules bear during crystallization of nanoporous frameworks. The stabilization energy obtained when only water molecules are occluded is much lower than that achieved by the occlusion of the organic molecules, indicating that the presence of the SDA molecules is necessary to stabilize the nanoporous architecture of the framework during crystallization. Finally, we stress the importance of including secondary species (water in this case) present in the synthesis gels that can be co-occluded with the organic molecules in the nanoporous framework during crystallization. The

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relative stability of the guest species in the gel solution in equilibrium with the solid gives an important contribution to the energy balance that must be accounted for. We have shown that neglecting these features would lead to wrong predictions from the computational models in terms of the most stable organic contents occluded within nanoporous materials, especially when studying hydrophilic systems that can occlude water as secondary guest species. Our new computational model provides therefore a useful tool for studying host-guest organo-inorganic


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systems whose applications largely depend on the occlusion of the guest species. Acknowledgment. L.G.H. and A.B.P. are grateful to the Spanish Ministry of Science and Innovation for postdoctoral and predoctoral grants, respectively. Financial support of the Spanish Ministry of Education and Science (Project CTQ2006-06282) is acknowledged. FC is supported by an RCUK Fellowship. The authors also thank Accelrys for providing their software, and Centro Tecnico de Inform atica for running some of the calculations.

Structure-Directing Effect of - American Chemical Society

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