Structure-Directing Effect of ( S )-(−)- N -Benzylpyrrolidine-2-methanol and Benzylpyrrolidine in...

pubs.acs.org/cmPublished on Web 06/29/2009r 2009 American Chemical Society

Chem. Mater. 2009, 21, 3447–3457 3447DOI: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-Hortig::uela,*,†,‡,§ Ana B. Pinar,§ Joaquın P�erez-Pariente,§ and Furio Cor�a†,‡

†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,Gower Street, WC1E 6BT London, United Kingdom, and §Instituto de Cat�alisis y Petroleoquımica-CSIC,

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 thattakes into account the stability of the guest species in solution as well as the possibility of occludingsolvent 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 thenanoporous STA-1 material (SAO framework type), and compare results with experimentalcharacterization. Despite the fact that up to 8 BP or BPM molecules can be loaded within thenanoporous framework, the relative stability of the SDAmolecules in solution and the simultaneousincorporation of water limit the amount of organic content. The most stable arrangementscorrespond to the occlusion of 6 BPMor 5 BPmolecules per SAO unit cell, accompanied respectivelyby 9 and 22 water molecules in order to complete space filling, leading to a cooperative structure-directing action between organic SDA and water molecules during crystallization. The organicmolecules act as primary structure-directing agents, and the water molecules as secondary space-filling species whose role is to completely fill the void space of the pore system and provide furtherstabilization to the nanoporous framework. We demonstrate the necessity of including in thecomputational models the occlusion of water molecules as well as to account for the stability ofSDA and water molecules in solution for realistically studying the structure-directing effect oforganic molecules in the synthesis of hydrophilic aluminophosphate nanoporous frameworks. Wetherefore provide a useful computational tool for studying the occlusion of guest species in host-guest systems, an issue which is essential for controlling the potential applications of these materials.

Introduction

Crystalline nanoporous materials attract industrialinterest for processes such as catalysis, molecular sieving,gas separation, and ion exchange,1-3 which exploit themolecular dimensions and the crystalline nature of thenanoporous structure to discriminate between moleculeswith even subtle steric differences. Many different atomscan be incorporated within the oxide network of thesecrystalline nanoporous solids, giving place to a range ofmaterials with different compositions, several of whichhave useful catalytic properties. Since their discovery byWilson et al. in 1982,4 the synthesis of nanoporousaluminophosphates (AlPO4) has been widely studied,yielding a diversity of structural types comparable to thatof the previously known aluminosilicate-based zeolites.5

Known nanoporous AlPO4 structures include poly-morphs that are common to both SiO2 and AlPO4

compositions, but also structures that have no zeoliticcounterpart. In AlPO4 materials, there is a strict alterna-tion of Al3þ and P5þ ions; both can be isomorphicallyreplaced by heteroatoms, giving rise to acid, redox andeven bifunctional properties.Nanoporous oxides are synthesized through hydro-

thermal methods, where the source of the inorganic ions,water, and, generally, an organic species, are heated in anautoclave for a time ranging from a few hours to weeks.The inclusion of the organic molecules is usually requiredto direct the crystallization toward a certain nanoporousstructure, and so they are called structure directing agents(SDAs). The role of these organic molecules has beentraditionally described as a “template effect”6 to indicatethat the organic molecules organize the inorganic tetra-hedral units into a particular topology around themselvesduring the nucleation process, providing the initial build-ing blocks from which crystallization of the nanoporousstructures will take place. Nevertheless, new theories

*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. (6) Gies, H.; Marler, B. Zeolites 1992, 12, 42.

3448 Chem. Mater., Vol. 21, No. 14, 2009 G�omez-Hortig::uela et al.

developed by means of computational simulations byCaratzoultas et al.,7-10 based on previous experimentalevidence for the formation of cagelike silica clustersassisted by the presence of tetramethylammonium col-lected by Kinrade et al.,11,12 are currently emerging; thesesuggest that the role that these SDA molecules playduring crystallization of nanoporous materials is morecomplex than the one described in the initial templatetheory. The exact role of the SDAs is still far from beingproperly understood, and structure direction is a majoropen issue in molecular sieve science. Controlling thisfeature would enable the synthesis of new topologies, aswell as allow researchers to gain control over crystal sizesand morphologies and the location of heteroatoms, ifpresent.The organic SDA molecules are encapsulated within

the nascent nanoporous structure during its crystalliza-tion, developing strong nonbonded interactions with theframework and thus contributing to the final stability ofthe system. It is well-known that nanoporous frameworksare metastable, and their intrisic stability increases withthe framework density, i.e., denser materials are morestable than open frameworks. The organic moleculesoccluded within the void spaces of open frameworksinteract (through nonbonded interactions) with the sur-rounding oxide network, providing the required thermo-dynamic stabilization tomake viable the crystallization ofsuch structures. Therefore, the SDA molecules direct thecrystallization of the nanoporous frameworks not only byproviding a volume from which the inorganic ions areexcluded but also by altering the relative thermodynamicstability of different framework structures via the devel-opment of different SDA-framework interactions.Despite the large number of studies about the role of

these organic molecules in the crystallization of nanopor-ous materials, the water molecules are another importantcomponent of the synthesis gel that has attracted lessattention. Water is present during the synthesis of bothpure-silica and aluminophosphate nanoporous frame-works, and it indeed influences the crystallization mecha-nism.7-10 Silica-based nanoporous materials do notusually occlude water molecules because of the hydro-phobic nature of the SiO2 network; however, the higherhydrophilicity of AlPO4-based networks, coming fromtheir molecular-ionic nature (the network can be seenas Al3þ and PO4

3- units ionically bonded,13 in contrast toSiO2 networks that are mainly covalent), and the strictalternation of Al3þ and PO4

3- ions in the network, allows

them to interact more strongly with water molecules andthus incorporate themwithin the nascent structure duringcrystallization. Recently we have demonstrated the im-portant role that water molecules play, in addition to theorganic SDA molecules, in the energy balance and in thestructure directing effect of the AFI structure.14 In addi-tion, the energetics involved in the H-bond networkformation of water clusters embeddedwithin nanoporousaluminophosphates, leading to a clear structural relation-ship between the framework and the water cluster, and itsrole on the material formation have been recently ad-dressed.15

Previous works carried out in our laboratories showedthe 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 todirect the crystallization of the STA-1 nanoporous ma-terial (SAO framework type) at higher SDA concentra-tions and in the presence of Zn.19 Results showed that at acrystallization temperature of 150 �C, both BP and BPMled to the crystallization of pure STA-1, whereas at thehigher temperature of 190 �C, only BPM directed thecrystallization of pure STA-1; instead, BP led to amixtureof STA-1 and ZnAPO-5 (AFI framework type). Detailsof the synthesis and characterization of these materialswill be given elsewhere.19 The SAO framework structurehas rarely appeared in the literature; to the best of ourknowledge, the only work related to this structure corre-sponds 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 usedas SDAs. The SAO structure is composed by a three-dimensional channel system, with two 12 MR channelsystems perpendicular to each other, one along the [100]direction with a diameter of 6.5 � 7.2 A2 and the otheralong the [001] direction with a diameter of 7.0� 7.0 A2

(Figure 1), resulting in a framework density of 13.4 T/1000 A3, one of the lowest among synthesized AlPOsstructures. The two channel systems are interconnected,giving place to channel intersections with a large fractionof void space. The stabilization of such a large voidstructure requires a very efficient structure directionduring the synthesis; moreover, and because the size ofthe empty channels exceeds the normal molecular sizes, itis likely that structure direction of SAO requires someform of supramolecular aggregation or cooperativitybetween SDAs or SDA and water molecules.

(7) Caratzoulas, S.; Vlachos, D.; Tsapatsis, M. J. Phys. Chem. B 2005,109, 10429.

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(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-Hortig::uela, L.; P�erez-Pariente, J.; Cor�a, F.Chem.;Eur. J.

2009, 15, 1478.(15) Henry, M.; Taulelle, F.; Loiseau, T.; Beitone, L.; F�erey, G.

Chem.;Eur. J. 2004, 10, 1366.(16) G�omez-Hortig

::uela, L.; L�opez-Arbeloa, F.; Cor�a, F.; P�erez-Pariente,

J. J. Am. Chem. Soc. 2008, 130, 13274.(17) G�omez-Hortig

::uela, L.; P�erez-Pariente, J.; Blasco, T. Microporous

Mesoporous Mater. 2007, 100, 55.(18) G�omez-Hortig

::uela, L.; P�erez-Pariente, J.; Blasco, T. Microporous

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K. J.; Kvick, A.; Graafsma, H.Angew. Chem., Int. Ed. 1997, 36, 81.

Article Chem. Mater., Vol. 21, No. 14, 2009 3449

We have recently developed a computational metho-dology that permits to study the energetic effect of waterin the synthesis of nanoporous structures; both coopera-tive and competitive water/SDA effects can be describedin themodel, which has been successfully applied to studystructure direction to the AFI topology.14 However, theAFI structure is a relatively simple structure in terms oftopological features, since it contains only one-dimen-sional not-interconnected 12MR channels; we now applyour computational model to the more complex SAOframework to test its validity and predictive ability. Inthis work, we employ the same computational methodol-ogy described in ref 14 to study the occlusion of the twoSDAmolecules (BP and BPM) and water within the SAOframework, and compare the computational results withexperimental data. We shall also compare our newlydeveloped protocol with the computational models tra-ditionally used for studying the occlusion of SDA mole-cules in nanoporous frameworks, which are limited toconsider the maximum loading of SDAs.

Computational Details

The goal of the computational study is to understand and

compare the energetics involved in the occlusion of the organic

(BP andBPM)molecules andwaterwithin the SAOnanoporous

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 calcula-

tions. Because of the usually very acidic pH of the synthesis gels

of AlPO4 frameworks, the amine group of the SDAmolecules 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 mole-

cular net charge ofþ1) were calculated by the charge-equilibra-

tion 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 frame-

work, 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.14,27,28Only the aluminum chargewasmodi-

fied 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

watermolecules were-0.82 andþ0.41 for oxygen and hydrogen

atoms, respectively; this charge distribution has been shown to

describe well the properties of water containing systems.29

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.

(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) DeVita, A.; Gillan,M. J.; Lin, J. S.; Payne,M. C.; Stich, I.; Clarke,

L. J. Phys. Rev. B 1992, 46, 12964.(27) G�omez-Hortig

::uela, L.; Cor�a, F.; Catlow, C. R. A.; P�erez-Pariente,

J. Phys. Chem. Chem. Phys. 2006, 8, 486.(28) G�omez-Hortig

::uela,L.;M�arquez-�Alvarez,C.;Cor�a,F.;L�opez-Arbeloa,

F.; P�erez-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.


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 SDAandwatermolecules. 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 SDAmolecules

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 simu-

lation steps:

(1) First, maximum loading of the organic SDA is deter-

mined.We shall gradually remove SDAmolecules 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 12MR 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 SDAmolecules 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 mole-

cules were generatedmanually for the lower SDAdensities (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

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 config-

uration was taken for subsequent calculations with water.

(3) Water molecules were then inserted in the organic-con-

taining 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 SDAmolecules 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 ourmodel, 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

H2O (E H2Osol ) and SDA (E SDA

sol ) solution energies. These energy

values were estimated as described in our previous work,14 by

running simulated annealing followed by geometry optimiza-

tion 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, respec-

tively. 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 com-

plex 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 estima-

tions 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 ap-

proach 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:

ΔEðnSDAÞ¼Ef-nSDAðEvacSDAþEsol

SDAÞ-nH2OðEvacH2OþEsol

H2OÞ ð1Þ(30) Sorption and OFF Modules, version 4.6; Accelrys Inc.: San Diego,CA, 2001.


where Ef refers to the energy (per unit cell) of the framework

containing the SDA andwatermolecules, nSDA and nH2O are the

number of SDA and water molecules per framework unit cell,

respectively, ESDAvac and EH2O

vac are the calculated energies of

the molecules in vacuo, ESDAsol is the SDA solution energy and

EH2Osol is the H2O solution energy. As discussed above, ESDA

sol and

EH2Osol 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

ESDAsol and EH2O

sol terms)

Eint ¼ Ef - nSDAEvacSDA- nH2OE

vacH2O

ð2Þ

Results

A. Experimental Results: SDAContent within the SAO

Structure from TGA and EA Data. In this section, wediscuss the experimental results collected to evaluate thecontent of organic and water molecules in the samplesobtained at 150 �C (where STA-1 was the only phasedobserved); further details of the characterization of thesamples will be given elsewhere.19

Thermogravimetric (TGA) (Figure 2) and elementalCHN (EA) (Table 1) analyses were performed in order tostudy the organic content occluded within the SAOstructures. EA data indicate that the molecular integrityof BP and BPM is retained upon occlusion within theframework, because the C/N ratio of the SAO sampleswith BP (9.9) and BPM (10.6) are very similar to thoseof BP and BPM molecules in vacuum (11 and 12 for BPand BPM, respectively); in addition, 13C NMR resultsevidenced the resistance of these molecules to the hydro-thermal treatments.17,18 TGA results show the presenceof three strong desorption steps, one at temperaturesbelow 200 �C that corresponds to the release of water,28

and the other two centered at around 400 and 650 �C thatcorrespond to the desorption of the organic molecules

occludedwithin the framework.Ahigherdesorption rate atthewater desorption step (∼100 �C) anda lowerdesorptionrate at the second SDA desorption step (∼650 �C) areobserved for the sample obtained with BP, suggesting inprinciple a lower organic and higher water content for theSAO 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 Ncontent values. TGA results, considering that the organicdesorption corresponds to theweight loss at temperaturesabove 200 �C, give organic contents of 6.1 BPM and5.2 BP molecules per unit cell, respectively. If the SDAcontent is calculated from the CHN EA results and usingtheC content, a lower SDApacking value of 5.2BPMand4.1 BPmolecules per SAO unit cell, respectively, is found.However, if we use the N content in the EA data, weobtain higher values of 5.9 BPMand 4.5 BPmolecules perSAO unit cell, in substantial agreement with TGA. Thisdifference in the values obtained from the EA (C) resultscould be indicative of an incomplete combustion of theC content during the elemental analysis; complete com-bustion of the organicmolecules during theCHNanalysisof these materials is difficult to achieve (TGA data showthat even at temperatures up to 800 �C the weight loss hasnot completely finished) because of the acid nature of thesamples due to the presence of low-valence Zn2þ ions inthe framework, which upon combustion of the organicmolecules can be protonated and thus provide acid sites inthe sample during the combustion process; this couldlead to retaining a certain amount of C species (coke) inthe sample and thus to a slight underestimation of theC content. Therefore, it is possible that the CHN analysishas not achieved a complete C combustion, what wouldlead to an underestimated calculated SDA content fromthe EA analysis results when using the C content. Giventhe good agreement of TGA and EA (N) data, we usedthem to estimate the experimental organic content to beof 6.0 ( 0.1 BPM and 4.9 ( 0.4 BP molecules per SAOunit cell. A common feature of all the characterizationtechniques is that the BPM content is higher than thatof BP by approximately 1 molecule per unit cell, despitethe similar overall molecular size of BP and BPM (BPMis even larger because of the additional CH2OHsubstituent). The water content can be estimated fromTGA data only, in particular from the desorption peakcentered at ∼100 �C. The TGA results show in this casea notably higher water content in the sample obtainedwith BP, with 13.6 and 22.3 water molecules per unit cellfor the STA-1 structures obtained with BPM and BP,respectively.

Figure 2. TGA (solid line, left axis) and DTA (dashed line, right axis) ofthe STA-1 samples obtained at 150 �Cwith BPM (black) and BP (gray) asSDAs.

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 EA-C EA-N TGA waterTGA

BP 11.10 2.56 1.31 9.9 (11) 4.1 4.5 5.2 22.3BPM 14.85 2.69 1.63 10.6 (12) 5.2 5.9 6.1 13.6


B. Computational Results: Occlusion of SDA and

Water Molecules within the SAO Structure. The watercontent and stabilization energies [ΔE(nSDA) from eq 1] asa function of the SDA content and molecular orientationare given in Table 2, whereas the stabilization energies forthe most stable orientation at each SDA content areplotted in Figure 3. The curve of ΔE(nSDA) as a functionof the organic content has a clear minimum at n= 6 forthe BPM molecule, accompanied by the simultaneousocclusion of 9 water molecules per unit cell. Instead, themost stable packing value for BP is of 5 SDAand 22watermolecules per unit cell. Our model predicts therefore ahigher SDA content in the SAO structure for BPM, inaccordance to the experimental results. The packingvalues of 6 BPM and 5 BP molecules per unit cell arein quantitative agreement with those estimated fromthe experimental data. A good match between the pre-dictions of our computational model and the experi-mental findings is also observed concerning the watercontent.It is interesting to note that despite up to 8 SDA

molecules can be occluded in the SAO structure, the moststable occlusion is achieved at lower loadings, both for BPand BPM. As discussed earlier, loadings in excess of6 SDA molecules require the presence of SDA dimers,whereas lower values can be achieved without the pre-sence of such supramolecular arrangements. BP, andespecially BPM, dimers were found to be stable in theAFI structure, which does not seem to be the case in SAO,probably because of a steric hindrance between dimerslocated in close neighborhood in perpendicular channels(Figure 4-top). The very efficient space filling achieved byBP or BPM dimers in SAO is confirmed by the very lowwater occlusion (only 1 and 2watermolecules per unit cellcan be accommodated in spaces left empty by BPM and

BP dimers, respectively, at loading of 8); however, such ahigh space-filling efficiency is achieved partly at theexpense of molecular distortions in the constrained en-vironment. Because of the larger size of BPM, the dimericarrangement with loading of 8 is particularly unstablefor the BPM molecules. A decrease of the loading to7 molecules per unit cell, still 4 of them forming dimersalong each channel system, leads to a stabilization,although this is still not the best compromise betweenspace filling and stability.The most stable packing value for BPM is found to be

of 6 molecules per unit cell, with 9 additional watermolecules to completely fill the void channels and provideadditional stability; these water molecules form smallwater clusters stabilized by H-bonds between themselvesand with the framework oxygens and with the OH groupsof the BPMmolecules. These contents are in good agree-ment with the experimental values of 6.0 (( 0.1) BPMand13.6 water molecules per unit cell; the small excess ofwater observed experimentally might derive from wateradsorbed on the external surface of the material. Byanalyzing the computational data, we observe that 4SDA molecules are located along each channel, with themain 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 andthe pyrrolidine ring in the other. A more detailed pictureof the location of the molecules in the channels or inter-sections under this packing value is given in Figure 5. Thepresence of SDA molecules in the channel intersections,in addition to the molecules that are located along theindividual channels, still provides a very efficient spacefilling, but with a lower steric hindrance than at thepacking value of 8, which is responsible for the stabilityof this SDA arrangement. A decrease in the organiccontent to 5 BPM molecules per unit cell is instead

Figure 3. Stabilizationenergy [ΔE(nSDA)] as a functionof the SDA(BPMin solid black line and BP in solid gray line) content (nSDA) in the SAOstructure. The experimental values with the error margins are shown bythe dashed vertical lines (BPM in dashed black line and BP in dashed grayline).

Table 2. Stabilization Energies [ΔE(nSDA)] of the Different Systems As a

Function of theOrganic Content (nSDA) (and initial orientation (orient.) as

defined in the text) in the SAO Structurea

BPM BP

nSDA orient. nH2O ΔE(nSDA) nSDA orient. nH2O ΔE(nSDA)

8 1 -74.1 8 2 --190.3

7 7 -176.4 7 5 -220.5

6 1 9 -257.8 6 1 13 -235.7

6 2 5 -228.9 6 2 7 -223.06 3 9 -180.4 6 3 9 -224.66 4 6 -177.4 6 4 11 -224.15 1 16 -227.8 5 1 22 -242.6

5 2 15 -244.1 5 2 20 -228.55 3 13 -218.0 5 3 18 -233.45 4 16 -226.3 5 4 20 -221.94 1 24 -198.7 4 2 33 -211.74 2 25 -208.5 4 1 31 -218.84 3 23 -208.2 4 3 32 -221.34 4 26 -186.1 4 4 34 -222.8

2 1 52 -202.8 2 1 55 -193.5

2 2 50 -214.5 2 2 55 -189.50 78 -198.9 0 78 -198.9

aThe stabilization energies are given in kcal/mol per unit cell. TheSDA (nSDA) and water (nH2O) contents are given in molecules per unitcell. The relative orientations are those explained in the computationalprotocol section. The best arrangement for each SDA content is high-lighted in bold.


associated with a destabilization of the SAO structureobtained with BPM.The arrangement of BP molecules at loading of 6 (with

13 water molecules) is also stable, although the moststable arrangement for BP is found for an SDA loadingof 5 molecules per unit cell. In this case, 4 molecules are

located in the channel intersections, and the remainingone along one of the channels. The lower SDA loadingenables a higher water occlusion, of 22 water moleculesper unit cell, which are arranged as H-bonded clusters inform of chains surrounding the organicmolecules. Again,these contents are in quantitative agreement with the

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 unitcell. Yellow and green dashed circles highlight molecules located along the channel and in the intersections, respectively.


experimental values of 4.9 (( 0.4) BP and 22.3 watermolecules 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 organicbut 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 -190to -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 ofthe 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 packingvalues between 4 and 7 have relatively small energy

differences.Finally, the stabilization energy developed by BPM is

higher (-257.8kcal/mol) than that forBP (-242.6kcal/mol),

suggesting a better structure-directing efficiency of BPM

in the crystallization of the SAO structure. In fact, thisis experimentally confirmed by the fact that although at

150 �Cbothmolecules are able to direct the crystallizationof pure STA-1, at 190 �C only BPM leads to pure STA-1,

whereas BP leads to a mixture of SAO and AFI nanopor-

ous structures.

Discussion and Approximated Models of Structure

Direction

The most stable packing values predicted by our com-putational model are in quantitative agreement withthe values found from the experimental characterization.The experimental results clearly indicate a lower packingvalue for the sample obtainedwith BP, a result that is alsopredicted by the computational study. The quantitative

agreement of our model with the experimental resultsprovides further confidence on the reliability of ourcomputationalmethodology in predicting themost stableorganic/water contents, which applies not only to theAFIframework, for which it was originally developed,14 butalso to more complex structures like the SAO frameworkconsidered here.To date, the structure-directing effect of organic mole-

cules in the synthesis of nanoporous materials has beeninvestigated computationally by identifying the locationand interaction energy of the organic molecules alonewithin the pore systems, without taking into considera-tion the occlusion of secondary species such as water.Furthermore, the reference energy of the SDA moleculeswas taken as being that of the isolated molecules invacuum rather than the molecules in solution, leadingto the use of interaction (eq 2) rather than stabilization(eq 1) energies. In an attempt to estimate which effect theocclusion of water has over the computational results, inFigure 6-top, we report the interaction energies (eq 2) ofthe BP and BPM SDA molecules within the SAO struc-ture as a function of loading without including watermolecules. Not surprisingly, in these models, the highestinteraction energy is reached for the highest number ofSDAmolecules able to be arranged within the frameworkwithout overlapping, i.e., 8 for both BP and BPM inour case. The traditional computational models of struc-ture direction tend therefore to overestimate the organiccontent. In those cases where no secondary species cancompete for the occlusion within the structure, as is thecase for high-silica zeolites, whose hydrophobicity pre-vents the occlusion of water molecules, a simplified studyof the interaction energy of the SDAmolecules alonemayeventually yield realistic results. However, in hydrophilicsystems such asAlPO4 type frameworks, this clearly is notthe case.As a second step to investigate the effect of approxima-

tions, we considered if the inclusion of water in the model

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


but employing interaction energies instead of stabiliza-tion 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 ener-

getic cost of transferring the organic (ESDAsol ) and water

(EH2Osol ) molecules from the aqueous solution to the nano-

porous framework. In this case, the energy profile is more

structured but is biased toward the massive occlusion ofwater, perhaps not surprisingly as one SAO unit cell can

accommodate up to 73 water molecules, each comingat 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 nanoporousstructures (with few exceptions); this is certainly true for

SAO and so the Eint results of Figure 5-middle and therelated 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 noindication of stable SDA/water arrangements is present.As a third and final computational model, we consid-

ered 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-bottommay 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 per-

formed.The results discussed above show that for the templated

hydrothermal synthesis of hydrophilic nanoporous so-

lids, 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 crystal-

lization takes place. Our recently developed computa-

tional methodology accomplishes this extension, and

therefore provides a necessary improvement over pre-

viously employed models.Despite up to 8molecules can be accommodatedwithin

the SAO framework, only 6 BPM and 5 BP are loaded

(accompanied by the occlusion of 9 and 22 water mole-

cules, 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 occludedwithin the SAO structure compared

to BP. This can be explained by the higher interactions of

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


this molecule with the framework because of the presenceof the strongly interacting methanol (CH2OH) group,suggesting a more efficient structure directing ability forthis molecule; in fact, the stabilization provided by theSDA-framework interactions accounts for ca. 75% of thetotal stabilization energy in the BPM system, whereas thiscontribution is only 50% in the BP case. In both cases,a large amount of water is occluded accompanyingthe SDA molecules and is higher in the sample obtainedwith BP because of the lower organic occlusion. Inthis case, the stabilization provided by the occlusion ofwater molecules is as important as that of the SDAmolecules; in fact, a cooperative effect seems to existbetween the BP molecules and water molecules whendirecting the crystallization of the nanoporous structure,which was also found in the crystallization of the AFIstructure.14

Let us consider now the requirements of the SDAmolecules for directing the crystallization of the SAOnanoporous structure. The only SDA molecules knownto direct the synthesis of the SAO structure prior toour study, i.e, the linear diquinuclidinium ions of theform (C7H13N)-(CH2)n-(C7H13N), where n varied from7 to 9,20 had a molecular structure comprising two bulkyquinuclidine rings linked by an aliphatic chain, whichprovided a high structural flexibility that enabled themolecule to locate one ring in each SAO channel.20 Ourmolecules are much smaller in size, but share the featureof having two organic rings linked by an aliphatic chain.As shown in Figures 4 and 5, the molecules are arrangedin a very efficient way in terms of space-filling, where4 molecules are located along each channel, and the other2 are in the channel intersections, with the aromatic ringin one channel and the pyrrolidine ring in the other. In thisrespect, the presence of themethylenemoiety bridging thetwo rings in the SDA molecule provides the structuralflexibility required for themolecules to be accommodatedin the channel intersections and with one ring in eachchannel, thus making these molecules very efficient in thesynthesis of the SAO nanoporous structure. The presenceof flexible molecules with two rings able to bridge adja-cent channels therefore appears to be a requirement forthe synthesis of SAO type materials.Our computational results confirm the importance of

the structure directing role of the organic SDAmolecules.It has been shown that the stabilization energy developedby the system where only water molecules, or a smallamount of organic SDAs, are present (packing values of0 and 2) is very low, demonstrating that the presence ofthe organic molecules is necessary in order to hold thelarge pore architecture of the nanoporous structure, thusmaking viable the crystallization pathway of the SAOstructure. In addition, the presence of water moleculessurrounding the organic molecules and interacting withthe frameworkwalls provides space-filling and additionalstabilization to the nanoporous structure. In this casewater molecules do not seem to exert any structure-directing role since no structural relationship betweenwater and the framework topology is observed, in

contrast to other cases where a clear relationship wasfound.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, whereasthe inner parts of the channels are filled by the organicSDA molecules. This observation about the location ofwater could be extrapolated to other functional groupslike amino or hydroxi substituents, whose ability offorming H-bonds with the framework oxygen atomswould drive them to locate close to the channel walls, ifthe molecular geometry allows it. Therefore, the findingsobserved in our work could be useful for proposing newefficient types of SDA molecules for the synthesisof nanoporous aluminophosphates, in which a bulkyorganic core would be used to provide a large molecularsize for supporting the open-framework architecture ofthe nanoporous structure, and a shell carrying stronglyinteracting groups like hydroxi or amino would providethe required strong interactions in order to compensatefor the low intrinsic stability of the open frameworks.

Conclusions

In this work, we have applied a recently developedcomputational methodology in order to rationalize the

co-occlusion of organic benzylpyrrolidine (BP) or (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) SDA mole-

cules and water during the crystallization of STA-1nanoporous materials. The computational results arecompared with experimental characterization. A good

quantitative agreement has been found, showing thereliable performance of the computational methodology

in predicting the SDA/water occlusion within even com-plex nanoporous aluminophosphate structures, such asSAO.Despite the channel intersections enabling the occlu-

sion of up to 8 SDA molecules, we found that the moststable packing value is of 6 BPM and 5 BP moleculesper SAO unit cell, accompanied, respectively, by 9 and

22 water molecules. At this organic loading, the interac-tion energy of the SDA molecules is optimal. Despite the

larger size, a higher number of BPM compared to BPmolecules is found in the most stable arrangement, inagreement with results previously observed for the AFI

structure. This result arises from the higher interactionenergy of the BPMmolecule with the framework because

of the strongly interacting methanol group.Our computational results confirm the fundamental

role that the organic SDA molecules bear during crystal-lization of nanoporous frameworks. The stabilization

energy obtained when only water molecules are occludedis much lower than that achieved by the occlusion of theorganic molecules, indicating that the presence of the

SDA molecules is necessary to stabilize the nanoporousarchitecture of the framework during crystallization.Finally, we stress the importance of including second-

ary species (water in this case) present in the synthesisgels that can be co-occludedwith the organic molecules inthe nanoporous framework during crystallization. The


relative stability of the guest species in the gel solution inequilibrium with the solid gives an important contribu-tion to the energy balance that must be accounted for.Wehave shown that neglecting these features would lead towrong predictions from the computational models interms of the most stable organic contents occluded withinnanoporous materials, especially when studying hydro-philic systems that can occlude water as secondary guestspecies. Our new computationalmodel provides thereforea useful tool for studying host-guest organo-inorganic

systems whose applications largely depend on the occlu-sion of the guest species.

Acknowledgment. L.G.H. and A.B.P. are grateful to theSpanishMinistry of Science and Innovation for postdoctoraland predoctoral grants, respectively. Financial support ofthe Spanish Ministry of Education and Science (ProjectCTQ2006-06282) is acknowledged. FC is supported by anRCUK Fellowship. The authors also thank Accelrys forproviding their software, andCentro T�ecnico de Inform�aticafor running some of the calculations.

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