Synthesis and Structure Determination of a Novel LayeredAluminophosphate Material Templated with 1-Phenylethylamine:

[AlPO4(OH)](NH 3C2H4C6H5)

Robert W. Dorner,* Malek Deifallah, David S. Coombes, C. R. A. Catlow, and Furio Cora`*

DaVy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albemarle Street,London W1S 4BS, United Kingdom

ReceiVed January 11, 2007. ReVised Manuscript ReceiVed March 7, 2007

The synthesis and crystal structure of a novel layered aluminophosphate is described. The structurewas solved from single-crystal X-ray diffraction data and confirmed by high-resolution powder diffractionand computational studies. The as-synthesized layered material, with composition [AlPO4(OH)]-(NH3C2H4C6H5), crystallizes in the monoclinic space groupC2/c with a ) 39.06(10) Å,b ) 5.31(13) Å,c ) 9.67(2) Å,R ) 90°, â ) 94.6(4)°, andγ ) 90°. In the aluminophosphate layers, the six-coordinatedaluminum polyhedra form infinite chains that are cross-linked by phosphate groups. These inorganiclayers are stabilized via strong hydrogen bonding to the protonated organic templates, involving theterminal oxygen atoms of the phosphate groups. Using quantum mechanical (QM) and interatomic potential(IP) techniques, we established the location of the protons in the layer and the structure’s stability.

Introduction

Since the work of Wilson et al. in the early 1980s, therehas been a large increase in the number of known frameworktopologies of aluminophosphates1,2 many with interestingcatalytic and adsorption properties.3 Structure direction inthe hydrothermal synthesis of these materials occurs generallythrough derivatives of organic amines, with the organicgroups usually being alkanes. It is still not clearly understoodhow certain organic molecules act as templates or structure-directing agents (SDAs). Some frameworks, e.g., AlPO-18,can be synthesized using several different organic molecules,examples beingN,N-diisopropylethylamine4 or tetraethylam-monium hydroxide,5 and some templates can form severaldifferent frameworks, e.g., triethylamine can form the CHA6

and the AFI topology.7

In this paper, we describe the synthesis and crystalstructure of a two-dimensional aluminophosphate, [AlPO4-(OH)](NH3C2H4C6H5), obtained using 1-phenylethylamineas the organic SDA. Similar layered structures have beenreported previously8,9 and can be found in nature as the

mineral tancoite with octahedral aluminophosphate chains.10

However, the arrangement of the linking phosphate groupspresent in our new structure and the phenyl-based anchorshave not been seen before.

Layered aluminophosphate materials are usually synthe-sized with diaminoalkanes as SDAs, with the diaminegenerally doubly protonated and bridging adjacent alumi-nophosphate layers. The focus on the choice of SDAs hasbeen on alkane-based amines, because aromatic amines (e.g.,aniline) do not tend to mix easily with the gel duringsynthesis. To the best of our knowledge, no new topologywithin the aluminophosphate materials has been synthesizedusing a phenyl- or benzyl-based template.

Phenyl- and benzyl-based compounds have so far receivedlittle attention in the synthesis of aluminophosphates becauseof difficulties mixing the template with the gel composition.Aniline, the most readily available aromatic amine, formsAlPO4-C, although this material normally does not needan organic SDA to be synthesized. Aniline tends not to mixwith the gel normally; it instead forms a layer on top of thegel, with the pH of the solution remaining at+2. Aniline isa relatively weak base (pKb) 9.7)11 and is only weaklyhydrated.

The hydrophilicity of the compound, and hence itssolubility in the synthesis gel, can be modified via theintroduction of inductive groups (e.g., alkane groups) to thephenyl ring11 or by replacing aniline by benzylamine.Compounds such as 1-phenylethylamine (pKb) 4.22),12

N-methylaniline (pKb) 4.42), benzylamine (pKb) 3.66),

* Corresponding author. E-mail: furio@ri.ac.uk (F.C.); robd@ri.ac.uk(R.W.D.). Fax: 44 (0)20 7670 2958. Tel: 44 (0)20 7409 2992.(1) Wilson, S. T.; Lok, B. M.; Flaningen, E. M. U.S. Patent 4310440,

1982.(2) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannon, T. R.; Flaningen,

E. M. J. Am. Chem. Soc.1982, 104, 1146.(3) Szostack, R.Molecular SieVes; Blackie Academic and Professional:

London, 1992.(4) Concepcion, P.; Blasco, T.; Nieto, J. M. L.; Vidal-Moya, A.; Martinez-

Arias, A. Microporous Mesoporous Mater.2004, 67, 215.(5) Wendelbo, R.; Akporiaye, D.; Andersen, A.; Dahl, I. M.; Mostad, H.

B. Appl. Catal., A1996, 142, L197.(6) Denavarro, C. U.; Machado, F.; Lopez, M.; Maspero, D.; Perezpariente,

J. Zeolites1995, 15, 157.(7) Liu, Y.; Withers, R. L.; Noren, L.Solid State Sci.2003, 5, 427.(8) Massa, W.; Yakubovich, O. V.; Karimova, O. V.; Dem’yanets, L. N.

Acta Crystallogr., Sect. C1995, 51, 1246.(9) Simon, N.; Guillou, N.; Loiseau, T.; Taulelle, F.; Ferey, G.J. Solid

State Chem.1999, 147, 92.

(10) Ramik, R. A.; Sturman, B. D.; Dunn, P. J.; Poverennykh, A. S.Can.Mineral. 1980, 18, 185.

(11) Elliott, J. J.; Mason, S. F.J. Chem. Soc.1959, 2352.(12) Tuckerman, M. M.; Mayer, J. R.; Nachod, F. C.J. Am. Chem. Soc.

1959, 81, 92.

2261Chem. Mater.2007,19, 2261-2268

10.1021/cm070106u CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 04/07/2007

and phenylpropylamine (pKb) 3.80)13 with lower pKbvalues than aniline form a homogeneous mixture afteraddition to the gel, as the nitrogen is more hydrophilic.

Our synthetic strategy in the present work has been to alterthe features of the aromatic amine in the SDA molecules bythe addition of inductive groups to aniline and to employ abenzyl-based SDA, which we find leads to new materials.Our experimental study is supported by a computationalinvestigation of the resulting layered material, which helpsin deriving detailed structural information, particularly onthe proton distributions.

Methodology and Characterization

Synthesis.The synthesis of the layered material was carried outhydrothermally in a Teflon-lined autoclave under high pressure.Aluminum hydroxide hydrate (Al(OH)3xH2O, Aldrich), phosphoricacid (H3PO4, Aldrich, 85%), 1-phenylethylamine (H2N(C2H4)C6H5,Aldrich, 99%), and distilled water were introduced while beingstirred continuously in a Teflon container, with the molar ratio of1:1:1.3:30. A liner containing the gel was then placed in anautoclave and heated at 150°C for 24 h. The pH of the solution(+8) was the same at the beginning and end of the reaction. Thematerial was also synthesized in the presence of 10% zinc as wellas cobalt acetate tetrahydrate (Aldrich, 99%).

Other amines used in the synthesis gels wereN-methylaniline(Synchemica), benzylamine (Aldrich), and phenylpropylamine(Aldrich), which did not yield single crystals of sufficient size norcrystallinity to be analyzed using single-crystal X-ray diffractiontechniques. Our subsequent discussion concentrates therefore onthe material synthesized using the 1-phenylethylamine template.

X-ray Structure Collection and Structure Determination.Crystals of sufficient size and crystallinity could be collected torecord single-crystal data at a wavelength of 0.6869 Å (120 K) onStation 9.8 at the SRS in Daresbury.14 The crystal was mountedon a glass fiber using Fomblin polyether. Station 9.8 is a high-flux, tuneable, monochromatic, single-crystal diffraction stationemploying a D8 diffractometer and utilizing a Bruker-NoniusAPEXII CCD area detector. The monochromator employed was aSilicon (111) crystal with an asymmetric cut of 2.01°, cooled by aGaInSn alloy and mounted on a Huber rotatory table, which canbe positioned to a precision of 0.001°.

The crystal selected was synthesized from a gel mixture with a1.0:0.9:0.1:1.3:30 P:Al:Co:1-phenylethylamine:H2O molar ratio andhad a size of 0.05× 0.05 × 0.03 mm3. The space group wasassigned on the basis of systematic absences and intensity statistics,leading to a satisfactory refinement. The structure was solved usingthe single-crystal software Crystals.15 SHELXS was used to locatethe heavy atoms of Al and P, with the remaining atoms locatedthrough the difference electron density map. The phenyl H atomswere also found via a difference Fourier synthesis, and theirpositions were refined. The final refinement of the structure wasperformed on the data havingI > 2σ(I) and included anisotropicthermal parameters for all non-hydrogen atoms. A summary of thesingle-crystal X-ray experiment is given in Table 1. Final positionaland equivalent isotropic thermal parameters are listed in Table 2.The asymmetric unit and 3D crystal structure are shown in Figures1 and 2, clearly showing a 2D layered structure.

In addition, powder XRD measurements were performed. Thedata collected from Daresbury on Station 2.3 using a wavelengthof 1.306 Å were analyzed by Rietveld methodology using the GSASEXPGUI software.16,17 The data were collected with a stepincrement of 0.01° and the time for each step was 2 s. Two sets ofpatterns between 2 and 25° and five sets of patterns between 25and 55° were recorded and the data summed.

Scanning Electron Microscopy.The morphology of the crystalswas characterized by scanning electron microscopy employing aJeol JSM-630 IF scanning microscope. The material was obtainedin a plate-like layered structure (Figure 3).

Thermogravimetric Analysis. Thermogravimetric analysis (TGA)and differential thermal analysis (DTA) were performed on a PU(13) Hall, H. J.J. Am. Chem. Soc.1957, 79, 5697.

(14) Cernik, R. J.; Clegg, W.; Catlow, C. R. A.; Bushnell-Wye, G.; Flaherty,J. V.; Greaves, G. N.; Burrows, I.; Taylor, D. J.; Teat, S. J.; Hamichi,M. J. Synchrotron Radiat.1997, 4, 279.

(15) Watkin, D. J.; Prout, C. K.; Lilley, P. M.Crystals; ChemicalCrystallography Laboratory, University of Oxford: Oxford, U.K.

(16) Larson, A. C.; Von Dreele, R. B. Report LAUR 86-748; Los AlamosNational Laboratory: Los Alamos, NM, 2000.

(17) Toby, B. H.J. Appl. Crystallogr.2001, 34, 210.

Table 1. Crystallographic Data and Structure Refinement forLayered Material

identification layered materialempirical formula [AlPO4(OH)](NH3C2H4C6H5)fw 261 g mol-1

T 120 Kwavelength 0.68690 Åcryst syst, space group monoclinic,C2/cunit cell dimensions a ) 39.06(10) Å

b ) 5.31(13) Åc ) 9.67(2) ÅR ) 90°â ) 94.6(4)°γ ) 90°

V 2002 Å3

F(000) 594.0cryst size 0.05× 0.05× 0.03 mm3

no. of independent reflns (I > 0) 3337no. of obsd data [I > 2σ(I)] 2388final R indices R1 ) 0.0639

wR2 ) 0.073

Table 2. Atomic Coordinates and Equivalent Isotropic DisplacementParameters for Layered Material

atom x y z Ueq (Å2)

P1 0.04550(4) 0.2022(3) 0.26295(11) 0.01704Al1 0.00000 -0.3168(4) 0.25000 0.01837Al2 0.00000 0.00000 0.00000 0.01684O1 0.02704(10) -0.0177(7) 0.3379(3) 0.00086O2 0.03500(9) 0.1897(7) 0.1050(3) 0.01858O3 0.03282(10) 0.4544(7) 0.3178(3) 0.01982O4 0.08348(10) 0.1779(8) 0.2936(3) 0.02439O5 0.01925(10) -0.2802(7) 0.0835(3) 0.01953H12 0.023906 -0.431250 0.047812 0.04972N1 0.09246(13) -0.2652(10) 0.4398(4) 0.02869H1A 0.08906 -0.40020 0.38142 0.04303H1B 0.09458 -0.12358 0.38782 0.04303H1C 0.07401 -0.24902 0.49068 0.04303C1 0.12075(18) -0.5594(13) 0.6024(6) 0.03589H1D 0.09802 -0.57128 0.63831 0.05383H1E 0.13845 -0.57295 0.68122 0.05383H1F 0.12383 -0.69318 0.53673 0.05383C2 0.12429(15) -0.3042(12) 0.5321(5) 0.02611H2 0.12482 -0.17165 0.60652 0.03134C3 0.15654(15) -0.2745(12) 0.4566(5) 0.02720C4 0.18367(17) -0.4415(13) 0.4784(6) 0.03372H4 0.18183 -0.58155 0.53847 0.04047C5 0.21374(18) -0.4067(15) 0.4135(8) 0.04364H5 0.23240 -0.52023 0.43151 0.05236C6 0.21706(18) -0.2047(16) 0.3246(8) 0.0438H6 0.23735 -0.18446 0.27853 0.05266C7 0.19034(17) -0.0350(13) 0.3025(6) 0.03538H7 0.19239 0.10353 0.24010 0.04245C8 0.16031(16) -0.0685(12) 0.3678(6) 0.02873H8 0.14201 0.04921 0.35343 0.03448

2262 Chem. Mater., Vol. 19, No. 9, 2007 Dorner et al.

4K (Rigaku) equipped with a quadrupole mass spectrometer (MS,Anelva M-QA200TS), under a mixture of argon (90%) and oxygen(10%) atmospheres with a heating rate of 5 K/min between 22 and800 °C. The thermogram revealed a weight loss of 51.8% up to650°C (Figure 4). The residue collected after heating the material

to 800 °C was examined using X-ray diffraction employing aSiemens D4 diffractometer. The material has lost all long-rangeorder and is completely amorphous; the 51.8% weight loss can beexplained by the loss of the organic anchors (46.7%), which holdthe inorganic planes together as well as water molecules implyingdehydroxylation of the layers.

Decoupled Solid-State Magic Angle Spinning. Decoupledsolid-state magic angle spinning (MAS) NMR was acquired for27Al to gain further insight into the coordination chemistry of thealuminum.27Al MAS NMR spectra were recorded on a BrukerMSL-300 spectrometer at 78.2 MHz. All measurements were carriedout at room temperature, with Al(H2O)6+3 being used as the externalstandard.

Computational. The computational work was performed firstto determine the location of the protons in the layers, which isdifficult to characterize via XRD techniques, and second toinvestigate the structural stability of the layers. A two-step approachwas adopted using quantum mechanical (QM) and interatomicpotential (IP) techniques.

Computationally, the study of the AlPO structure itself is mostsuited to quantum mechanical techniques because of its unusualstructure incorporating 6-coordinated Al and 1-, 2-, and 3-coordi-nated O ions. To begin with, we exploited the layering of thestructure and cleaved the solid along the [100] direction, forminga single-layer model of composition [AlPO5H]-; the symmetry (C2/c) and cell parameters were taken from the experimentally derivedvalues (a ) 39.06 Å,b ) 5.31 Å, c ) 9.67 Å, R ) 90.0°, â )94.6°, γ ) 90°). The experimentally derived coordinates for Al, P,and O were used in this initial structure. Each primitive unit cellcontains five symmetry-unique O ions, labeled 1-5 (Figure 1); inprinciple, each can act as a protonation site. We therefore generatedfive protonated single-layer models, each corresponding to bindingof the proton to a different, symmetry unique oxygen. The latticeparameters of these two-dimensional computational models werenot varied during our QM calculations, but internal optimizationsof all the fractional coordinates were performed.

Quantum mechanical calculations have been performed withinthe density functional theory (DFT). The QM code CRYSTAL18

was employed for these calculations, using the hybrid DFTfunctional B3LYP. The CRYSTAL code has been successfully usedto model AlPOs in previous work.19-22 The Gaussian basis sets

(18) Saunders, V. R.; Dovesi, R.; Roetti, C.; Orlando, R.; Zicovich-Wilson,C. M.; Harrison, N. M.; Doll, K.; Civalleri, B.; Bush, I. J.; D’Arco,P.; Llunell, M.CRYSTAL 2003 User’s Manual; University of Torino:Torino, Italy, 2004.

(19) Cora, 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.

(20) Cora, F.; Catlow, C. R. A.J. Phys. Chem. B2001, 105, 10278.(21) Cora, F.; Saadoune, I.; Catlow, C. R. A.Angew. Chem., Int. Ed.2002,

41, 4677.

Figure 1. Asymmetric unit of the layered aluminophosphate.

Figure 2. Structure of the layered aluminophosphate, showing two templatemolecules hydrogen bonded to layers of the material.

Figure 3. SEM image of the layered material. The layering of the structureis also clearly visible in the crystal morphology.

Figure 4. Thermogravimetric analysis of the layered material under a 90%argon and 10% oxygen atmosphere (heating rate 5°C/min).

[AlPO4(OH)](NH3C2H4C6H5) Chem. Mater., Vol. 19, No. 9, 20072263

used to describe the electrons in the system were the same as thosedescribed previously19-22 and can be obtained from the onlinelibrary of the code.23

Ammonium ions (NH4+) were used in our single-layer model to

represent the protonated 1-phenylethylamine ions. Our rationale forusing protonated SDA ions arises from the observed change in pHfrom 2 to 8 of the homogeneous gel mixture, indicating that protonsare removed from solution upon addition of the SDA. Eachammonium ion used here mimics the ammonium end of one organictemplate, which is the only part of the SDA close enough to thelayer to have any significant chemical interaction with it. Theremaining part of the organic template is likely to be involved intemplate-template interactions and/or is necessary for space filling.To examine the AlPO layer when no SDAs are present, wegenerated a neutral single-layer system in which the ammoniumion and the OH- ion in the layer are removed. A full optimizationincluding the lattice parameters was conducted on this neutral layer.

The stable protonated layer identified in the QM work wasemployed in the second stage of our computational study, performedusing IP techniques. Energy minimization and Molecular Dynamics(MD) calculations were carried out on the system with theexperimentally derived composition, [AlPO4(OH)](NH3C2H4C6H5).The goal of this approach was to study the organic templatesresiding in the interlayer region and in particular to examine theforces responsible for holding adjacent layers together. The cvffforcefield24 available in the Open Forcefield (OFF) suite of methodsunder Cerius2 was selected for this function because it yields goodresults for such organic-organic intermolecular interactions.25 Theprocess adopted for this part of the computational study initiallyinvolved replacing the NH4+ ions present in the QM model by anequal number of protonated 1-phenylethylamine ions. The structureof the AlPO layers present in this system was fixed to that obtainedin the QM work. The charges assigned to atoms in the layers are+1.4 to Al, +3.4 to P, and-1.2 to O ions, as customary in AlPOcalculations performed with cvff.26 The FF type and charge of theSDA atoms were obtained using the direct atom typing and thecharge equilibration features available in Cerius2. The Ewaldmethod was selected to describe the nonbonded dispersive andCoulombic interactions. The smart minimizer function and an N,V, T ensemble set at 423 K with a time step of 0.001 ps for a totalsimulation time of 100 ps were used for the energy minimizationand MD calculations. To examine the forces responsible for holdingadjacent layers together, we calculated the energy of the systemcorresponding to different values of thea lattice parameter in therange of 35 Åe a e 150 Å. The values selected allowed us toexamine the system at small interlayer separation (i.e., compressionof the bulk system) and large interlayer separation (i.e., expansionof the bulk system). For the experimentally derived value of thealattice parameter, we performed a sequence of energy minimization,molecular dynamics, energy minimization steps. No disorder orreconstruction was observed after the MD calculations and in factthe internal energy calculated by energy minimization of thestructure before and after the MD step differed by only 1 meV.Therefore, for all other values of thea lattice parameter, only energyminimizations of the organic molecules were performed, to calculate

the system’s energy,Ea, as a function of the interlayer separation.The interaction energy between the layers,Elay, was then evaluatedaccording to

where∆Elay is the interaction energy per unit cell between twolayers (which are separated by 4 SDA ions per unit cell);Ea is thetotal energy at lattice spacinga; E∞ is the total energy at infinitelayer separation (measured ata ) 150 Å).

Results and Discussion

Experimental Structure Determination and Charac-terization of the Layered Material. The crystal structureof the newly synthesized layered material revealed on thebasis of single-crystal X-ray diffraction data consists ofaluminophosphate layers, which are bound by electrostaticinteractions and hydrogen-bonding to protonated 1-phenyl-ethylamine molecules (Figure 2). Usually, only one organicmolecule can be found, connecting the sheets in the layeredaluminophosphates.9,27,28However, in this layered alumino-phosphate material, two 1-phenylethylamine molecules arelocated between the inorganic sheets, leading to a larged-spacing between the inorganic layers of approximately 14Å. Indeed, this is the largest known value to date; the largestpreviously determined spacing between layers in other 2Daluminophosphate materials is approximately 11 Å,29 arisingfrom the diaminooctane template’s relatively long alkanechain, rather than from two smaller template molecules asin this case. Although two organic templates are not knownto be found in-between aluminophosphate layers, this hasbeen observed in layered gallophosphates.30 Another interest-ing characteristic of our layered aluminophosphate is the factthat the two template molecules between the two sheetsappear to be mostly held together by van der Waals forces,a feature that will be discussed in greater detail in thecomputational study reported later. We note that the SEMimage (Figure 3) shows the material’s crystals to be of aplate-like nature.

The inorganic layer of the material is made up ofoctahedral aluminum atoms, which are connected to eachother through bridging oxygens, forming cis-chains. Alloctahedra are interconnected through edge-sharing, givingrise to a zigzag arrangement lying along the (010) axis. ThisAlPO material thus contains Al-O-Al linkages. However,the aluminum is in an octahedral arrangement and thus doesnot violate Lowenstein’s rule, which forbids Al-O-Albonds systems with tetrahedrally coordinated aluminum.Other layered materials are known with this type ofoctahedral Al linkage,9,27,29,31,32which is, however, an unusual

(22) Saadoune, I.; Catlow, C. R. A.; Doll, K.; Cora, F.Mol. Simul.2004,30, 607.

(23) Ramaswamy, V.; McCusker, L. B.; Baerlocher, C.MicroporousMesoporous Mater.1999, 31, 1.

(24) Dauber-Osguthorpe, P.; Roberts, V. A.; Dauber-Osguthorpe, J.; Wolff,J.; Genest, M.; Hagler, A. T.Proteins1988, 4, 31.

(25) Beale, A. M.; Sankar, G.; Catlow, C. R. A.; Anderson, P. A.; Green,T. L. Phys. Chem. Chem. Phys.2005, 7, 1856.

(26) Gomez-Hortiguela, L.; Cora, F.; Catlow, C. R. A.; Perez-Pariente, J.J. Am. Chem. Soc.2004, 126, 12097.

(27) Kongshaug, K. O.; Fjellvag, H.; Lillerud, K. P.Microporous Meso-porous Mater.2000, 38, 311.

(28) Tuel, A.; Lorentz, C.; Gramlich, V.; Baerlocher, C.J. Solid State Chem.2005, 178, 2322.

(29) Tuel, A.; Gramlich, V.; Baerlocher, C.Microporous MesoporousMater. 2001, 47, 217.

(30) Lakiss, L. S.-M. A. G. V. P. J.Solid State Sci.2005.(31) Huang, Q.; Hwu, S. J.Chem. Commun.1999, 2343.(32) Mali, G.; Meden, A.; Ristic, A.; Tusar, N. N.; Kaucic, V.J. Phys.

Chem. B2002, 106, 63.

∆Elay )Ea - E∞

2(1)

2264 Chem. Mater., Vol. 19, No. 9, 2007 Dorner et al.

feature for AlPO materials that generally have alternatingaluminum and phosphorus tetrahedra and avoid Al-O-Albridges.

Within the layers, the Al-O-Al chains are interconnectedto each other by tetrahedral phosphate groups above andbelow the (100) plane, which in turn are linked to threedifferent aluminum octahedra. The fourth oxygen on thephosphate tetrahedron points outward from the layer and ishydrogen bonded to the protonated amine group of the SDA.The phosphate groups running along thec-axis are inalternating arrangements, giving rise to a criss-cross pattern.These arrangements differ from those in the mineral tancoite,where the aluminum octahedra of the chains are alsosurrounded by four phosphate groups, which are not exactlyabove each other but staggered; here, the phosphate groupslie exactly in the same position above and below the aluminachains (i.e., along thea-axis), but adopt a staggered arrange-ment (Figure 5). Because of this arrangement of terminalphosphate groups, it may not have been possible to formthree-dimensional structures, unlike the case with MIL-12,which is an intermediate in the formation of ULM-4.33

In the aluminum octahedron, distances range from 1.835-(7) to 2.057(9) Å for Al(1) and from 1.826(3) to 1.958(2) Åfor Al(2). The mean values (1.914(9) and 1.902(6) Å forAl(1) and Al(2), respectively) are in good agreement withpreviously reported bond lengths for octahedral Al-Obonds.27 The P-O distances range from 1.502(0) to 1.580-(3) Å with an average distance of 1.540(7) Å, which is closeto the bond lengths reported in similar compounds.9 Thepresence of octahedral aluminum was confirmed by de-coupled27Al solid-state magic angle spinning NMR, givingrise to a peak at-7.93 ppm.

The arrangement of aluminum octahedra and tetrahedralphosphate groups gives rise to an unusual arrangement ofthree-, four-, and six-membered rings (MR) (Figure 5) inthe inorganic layer. Pseudo 2 MRs are made of twoaluminum octahedra linked together by two oxygen atoms,i.e., giving rise to the edge-sharing between the octahedra.Aluminum octahedra with a tetrahedral phosphate grouplocated just above the pseudo 2 MRs then give rise to the 3MRs. The 4 MRs are made up of 2 octahedral aluminumatoms in two different chains connected trans to tetrahedralphosphate groups, which bind the aluminum chains together.Four aluminum octahedra and two phosphate tetrahedra makeup the 6 MRs, which lie along thez-axis.

As noted earlier, the powder-XRD pattern of the samplecollected on Station 2.3 at the SRS, was analyzed usingGSAS Rietveld software to show that the crystal struc-ture observed is representative of the bulk material synthe-sized rather than an impurity. There was a very good matchbetween the single-crystal data and the powder data(Figure 6).

It is interesting to note that the other templates (N-methylaniline, benzylamine, and phenylpropylamine) did notyield crystals of sufficient size to collect single-crystal data(Figure 7). Moreover,N-methylaniline did not yield a phase-pure sample but produced large crystal impurities of theGismondine34 and the SBS structure35 that were analyzedusing single-crystal X-ray diffraction. The powder XRD

(33) Gerardin, C.; Loiseau, T.; Ferey, G.; Taulelle, F.; Navrotsky, A.Chem.Mater. 2002, 14, 3181.

Figure 5. Layers of the layered aluminophosphate material viewed (a) alongthe [100] axis showing octahedral aluminum chains interlinked by tetrahedralphosphate groups and (b) along the [010] axis showing the tetrahedralphosphate groups above and below the aluminum chains.

Figure 6. XRD pattern of layered material solved using the crystal dataobtained through single-crystal X-ray diffraction (wRp ) 9.7% andRp )7.7%).

Figure 7. XRD pattern of materials synthesized with (a)N-methylanilineand (b) phenylpropylamine.

[AlPO4(OH)](NH3C2H4C6H5) Chem. Mater., Vol. 19, No. 9, 20072265

pattern of the material synthesized with benzylamine showsthat it is very similar to the layered material formed by1-phenylethylamine (Figure 8).

Computational Analysis.The computational study aimedto establish the position of the proton within the layer andthe stability of the overall material. Out of the five possibleprotonation sites available in the layer, i.e., the five sym-metry-unique O ions, only two are found to be local minimain the potential energy surface (PES). These are the O ionslabeled as O1 and O5 in Figure 1 and are the two oxygenatoms bridging the Al ions in the Al-Al edge-sharing units.The oxygen ion labeled as O5 is coordinated to only twoaluminum ions; O1 however, is coordinated to 3 ions, twoAl, and one P ion in the layer. The oxygens labeled 2, 3,and 4, which are 1- and 2-coordinated, belong to Al-O-Punits and the terminal PdO group, and are not involved inthe Al-Al edge-sharing linkage. A proton initially locatedon one of the latter oxygen types moves to O5 upon geometryoptimization, indicating that there is no energy barrier forthese proton jumps. Examination of the relative energies ofthe protonated layers, after optimization, reveals that pro-tonation of sites O1 and O5 incurs at a noticeable energeticdifference, calculated as 1.52 eV per proton in favor of O5.Although O1 is a local minimum in the PES, protonation ofthis oxygen is unlikely to occur because of its relatively highenergy. O5, that is the oxygen bonded only to the twoaluminum ions, is therefore expected to be protonated onthe basis of the QM results.

The geometry-optimized structures (panels a and b ofFigure 9) of the AlPO layer obtained upon protonation ofO1 and O5 show major differences. It is important to firstnotice that the phosphorus atoms of the structure remainedtetrahedrally coordinated to four oxygens throughout ourcomputational study. This result is consistent with earlierwork25 indicating the molecular ionic nature of AlPOmaterials. All major differences observed in the optimizedstructures refer to the local environment around the Al ions.

The structure presented in Figure 10a corresponds to theoptimized layer when O1 is protonated. It bears no resem-blance to the experimentally determined one. In fact astructural reconstruction of the AlPO layer occurs, with half

of the Al ions being 4-coordinated in a tetrahedral geometry.Generally, this is the stable environment for Al in 3D AlPOframeworks, but not in the case of this layered material. Theother half of the Al ions in the system are 6-coordinated,forming AlO6 octahedra each with two long and four shortAl-O bonds.

The structure obtained when O5 is protonated yields a verygood match with experimental results in both coordinationnumbers and bond distances (strcutures b and c in Figure10). Upon closer examination, we find that protonation ofO5 gives uniquely octahedrally coordinated Al where pro-

(34) Baerlocher, C.; Meier, W. M.HelV. Chim. Acta1970, 53, 2080.(35) Bu, X. H.; Feng, P. Y.; Stucky, G. D.Science1997, 278, 2080.

Figure 8. XRD patterns of layered compound synthesized with (a)1-phenylethylamine and (b) benzylamine.

Figure 9. Optimized structure shown as polyhedra with proton residingon the (a) O1 site and (b) O5 site.

Figure 10. Comparison of optimized structures with proton bonded tothe (top) O1 site, (middle) O5 site, and (bottom) experimentally obtainedpicture.

2266 Chem. Mater., Vol. 19, No. 9, 2007 Dorner et al.

tonation of the O ions in each of the AlO6 units occurs alongvertices situated trans with respect to each other. Theoptimized values of all bond lengths in the layer are within1% difference to the experimentally deduced values. As thesedifferences are extremely small, we can be confident in thereliability of the experimental structural refinement, thecalculated structure itself, and the suitability of the B3LYPfunctional to model AlPOs.

The combination of results discussed above clearlyindicates that only one protonation site is present in the layer,i.e., the oxygen ion labeled O5. In practice, because thisoxygen is always protonated, it should be described as partof a hydroxide ion. Anchoring of the hydroxide ion to thelayer is necessary to charge-balance the positive charge ofthe SDAs residing in the interlayer region and to yield theexperimentally observed octahedral coordination of Al. Thematerial can be considered to be an AlPO4 structure with ahydroxyl group that acts as a thermodynamic stabilizerincorporated into the framework. In the past, hydroxyl andfluoride groups have been shown to act as stabilizers in thesynthesis of zeolitic materials.36-38 In some cases (i.e., theNON and STT structures), the fluoride anions are directlybonded to one silicon T atom and are thus integrated intothe framework.39 Within the group of layered aluminophos-phates, some structures such as MIL-12 can only besynthesized in the presence of fluorine, which is oftenincorporated into the structure and stabilizes the material inthe process.9 We therefore consider that in the case of thislayered AlPO, the hydroxide group anchored to the layersacts as a thermodynamic stabilizer for the structure andtherefore plays a vital role in the construction of the layers.

The protonated SDA (1-phenylethylamine) acts not onlyas a space filler within the structure but also as a hydrogen-bonding donor to the framework oxygens. The inorganicsheets are being held together by H-bonding between oxygenatoms of the framework and hydrogen atoms attached to thenitrogen from the SDA, which stabilizes the structure. Thepresence of the hydroxyl group, O5H, is necessary to charge-balance the protonated SDA.

The second part of our computational work focused onthe role that the SDA molecules play in holding the AlPOlayers together. Our MD calculations, performed on theprotonated layers at full loading of SDAs and at theexperimental value of the interlayer separation, indicate thatthe templates are stable within the channels. No disorder and/or reconstruction of the SDA molecules is observed after100 ps of MD simulation at 423 K. This result indicates thatthe SDAs are tightly packed within the interlayer region ofthe AlPO. It also suggests that the removal of the SDAsduring calcination results in the creation of voids, leadingto the collapse of the AlPO framework.

Figure 11 shows the relative energy (calculated using eq1) as a function of the interlayer separation. Our resultsindicate that the optimal interaction energy between twolayers in the bulk material amounts to 26.1 kcal/mol per unitcell (comprised of 2 layers) and corresponds to the latticeparametera ) 38.3 Å, which is in reasonable agreementwith the experimentally derived value of 39.06 Å. Thecalculated interlayer binding corresponding to this value ofa, is -0.353 J/m2, a small, yet significant value. For all valuesof a, the SDAs remain close to the layers. Specifically, theammonium end of each 1-phenylethylamine SDA is stronglybound to the negatively charged inorganic layers, even atlarge layer separations (Figure 13). A combination ofnonbonded coulombic and dispersive interactions is thereforeresponsible for holding the SDAs and AlPO-layers together.

Finally, we attempted to model some of the effects ofcalcination on the structure using the same quantum me-chanical techniques described earlier. One of the effects ofcalcination on the AlPO-SDA adduct is the removal of theSDA and other nonframework ions, except those needed for

(36) Millini, R.; Perego, G.; Berti, D.; Parker, W. O.; Carati, A.; Bellussi,G. Microporous Mesoporous Mater.2000, 35-6, 387.

(37) Salehirad, F.; Aghabozorg, H. R.; Manoochehri, M.; Aghabozorg, H.Catal. Commun.2004, 5, 359.

(38) Liu, Z. Q.; Xu, W. G.; Yang, G. D.; Xu, R. R.MicroporousMesoporous Mater.1998, 22, 33.

(39) Camblor, M. A.; Villaescusa, L. A.; Diaz-Cabanas, M. J.Top. Catal.1999, 9, 59.

Figure 11. Interaction energy between adjacent layers as a function of interlayer separation.

Figure 12. (a) Optimized charge neutral AlPO layer when symmetry ispresent; (b) the optimized structure obtained removing all symmetryconstraints.

[AlPO4(OH)](NH3C2H4C6H5) Chem. Mater., Vol. 19, No. 9, 20072267

charge-balance. The QM calculations designed to examinethe effects of calcination focused on hypothetical systemsof one layer thickness; in our initial examination, the O5H-

group and all extraframework ions were removed, leaving aslab of composition AlPO4. Full geometry optimizations wereperformed on the slab (1) retaining all symmetry operatorsof the original AlPO5H-SDA system, and (2) with allsymmetry constraints removed. The structures obtained afterfull geometry optimization are shown in Figure 12.

In the resulting calculated AlPO structures, we see thatthe one-dimensional chains of AlO6 octahedra present in theas-synthesized material are no longer present and are replacedby a chess-board-like arrangement of the Al ions in whichthe coordination polyhedra around Al are connected bycorner-sharing only. The PO4 tetrahedra are located aboveand below the plane but cover only half of the empty squaresof the chess-board pattern. In practice, this geometry definesdense one-dimensional chains in the structure, of composition[AlP2O8]3-, bonded via AlOx units (x ) 4 or 6) in theinterchain region. Al-O2-P edge sharing, which is anunstable feature in AlPOs is found for calculations both withand without symmetry constraints. For the case, in whichthe geometry optimization was performed with full symmetryconstraints, shown in Figure 12a, the one-dimensional[AlP2O4]3- chains consist of 6-coordinated AlO6 octahedraand 4-coordinated PO4 tetrahedra. These chains are heldtogether by 4-coordinated square planar AlO4 units in theinterchain region. The composition ratio of Al in 4- and6-coordination is 1. The presence of AlO4 units in squareplanar coordination suggests that this structure is unstable,and that Al in this geometry can be easily solvated andpossibly removed from the interchain region; therefore, wedo not expect this phase to be stable under harsh experimentalconditions, such as those present during calcination.

Performing a full geometry optimization on the layer withall symmetry constraints removed has instead predicted amore stable structure, which is shown in Figure 12b, in whichthe [AlP2O8]3- one-dimensional chains consist solely of AlO4

and PO4 tetrahedra and are held together by AlO6 octahedrain the interchain region. Akin to the previous calculation,the ratio of 4- and 6-coordinated Al ions is found to be 1,but now the Al ions linking the AlP2O8 chains are in a stable6-coordinated octahedral environment. The differences in thegeometry of the Al ions in the structures examined explainsthe energetic difference, calculated as 0.13 eV/unit cell infavor of the structure obtained in the absence of symmetry.This result confirms the instability of the square planar AlO4

units in comparison to the tetrahedral AlO4 geometry. Themore energy favorable conformation of the charge-neutral

AlPO layer, predicted in the absence of any symmetryconstraints, shows a major change in the bonding betweenions of the framework. Specifically, we find that the Al-O-Al zigzag linkage reconstructs and forms a linear chainof corner sharing AlO4 and PO4 tetrahedra that are held byAlO6 octahedra in the interchain space. The latter AlPO4

structure is very unstable; its energy difference with respectto the stable AlPO4 polymorph, i.e., Berlinite, is calculatedas 2.07 eV per AlPO4 formula unit. This value should becompared with the relative energy with respect to Berliniteof other stable microporous modifications, which at theB3LYP level is calculated in a range of 0.1-0.2 eV performula unit.20 The energetic instability of the calcinedlayered structure, as well as the reconstruction of the atomsin the layers discussed above, explain why the layered AlPOcollapses upon calcination; the one-dimensional chainsformed are unable to maintain the integrity of the layers.

These results indicate that the presence of charged species,the O5H- ions in the layer and the ammonium templates(i.e., the protonated 1-phenylethylammonium ions duringsynthesis) in the interlayer region are essential to the integrityand structural stability of the new layered AlPO describedhere.

Summary and Conclusion

The utilization of phenyl- and benzyl-based templates hasled to the formation of a layered aluminophosphate. Thestructure consists of octahedral aluminum chains, which areinterlinked by phosphorus tetrahedra into a 2D layeredstructure. Two template molecules separate subsequentinorganic layers, leading to the largest knownd-spacingobserved between layers in lamellar aluminophosphatematerials. The two template molecules are held togethermainly by van der Waals interactions, resulting in a stablematerial. Our calculations reveal that the synthesis of thisAlPO is only possible because of the presence of thehydroxyl (O5H)- ions. These ions are primarily needed firstto charge-balance the positive SDAs in the interlayer regionand second to coordinate the otherwise 4-coordinated Alforming octahedral Al, thus acting as a thermodynamicstabilizer to the formation of the layered structure. Uponmodeling the effects of calcination, our calculations predictthat removal of charged extraframework species, namely theO5H- group and the SDA, leads to structural reconstructionsof the AlPO layer, in which the zigzag AlO6 chains transforminto a 1-dimensional [AlP2O8]3- conformation, which isthermodynamically unstable.

Acknowledgment. We acknowledge Saudi Aramco andEPSRC for financial support. The authors thank ProfessorGopinathan Sankar, Dr. Simon Teat, Dr. Scott Woodley, andDr. John Warren for useful contributions and discussions.

Supporting Information Available: Crystallographic informa-tion in CIF format; table of bond lengths and angles in PDF format.This material is available free of charge via the Internet athttp://pubs.acs.org.

CM070106U

Figure 13. Structure of the layers at large separation, showing 1-phenyl-ethylamine’s affinity to remain bonded to the inorganic layers.

2268 Chem. Mater., Vol. 19, No. 9, 2007 Dorner et al.