Molecular Modeling, Multinuclear NMR, and Diffraction Studies in the

Jul 6, 2010 - School of Chemistry, University of St. Andrews, Purdie Building, North Haugh, St. ... A Multinuclear NMR Study of Six Forms of AlPO-34: ...
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Molecular Modeling, Multinuclear NMR, and Diffraction Studies in the Templated Synthesis and Characterization of the Aluminophosphate Molecular Sieve STA-2 Maria Castro,† Valerie R. Seymour,† Diego Carnevale,† John M. Griffin,† Sharon E. Ashbrook,*,† Paul A. Wright,*,† David C. Apperley,‡ Julia E. Parker,§ Stephen P. Thompson,§ Antoine Fecant,| and Nicolas Bats| School of Chemistry, UniVersity of St. Andrews, Purdie Building, North Haugh, St. Andrews, KY16 9ST, United Kingdom, Department of Chemistry, Durham UniVersity, South Road, Durham, DH1 3LE, United Kingdom, Diamond Light Source, Harwell Science and InnoVation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom, and IFP-Lyon, BP. 3, 69390, Solaize, France ReceiVed: May 6, 2010; ReVised Manuscript ReceiVed: June 15, 2010

Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO4 form of STA-2, bis-diazabicyclooctane-butane (BDAB). This is incorporated as a divalent cation within the large cages of STA-2, as determined via a combination of solid-state 13C and 15N MAS NMR, supported by 14N and 1H-15N HMQC solution NMR and density functional calculations. Asprepared AlPO4 STA-2 containing cationic SDA molecules achieves neutrality by the inclusion of hydroxide ions bridging between 5-fold coordinated framework Al atoms. Synchrotron X-ray powder diffraction data of the dehydrated as-prepared form indicates triclinic symmetry (Al12P12O48(OH)2 · BDAB, P1, a ) 12.3821(2) Å, b ) 12.3795(2) Å, c ) 12.3797(3) Å, R ) 63.3585(8)°, β ) 63.4830(7)°, γ ) 63.4218(7)°) with the distortion from rhombohedral R3j symmetry resulting from the partial order of hydroxide ions in bridging Al-OH-Al sites within cancrinite cages. Upon calcination in oxygen, the organic SDA is removed, leaving AlPO4 STA-2 with a pore volume of 0.22 cm3 g-1 (R3j, Al36P36O144, a ) 12.9270(2) Å, c ) 30.7976(4) Å). Dehydrated calcined AlPO4 STA-2 has two crystallographically distinct P and Al sites: 31P MAS NMR resolves the two distinct P sites, and although 27Al MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D 27Al f 31P MQ-J-HETCOR correlation spectroscopy confirms that each framework Al is linked to the two different P sites via Al-O-P connections in a 3:1 ratio (and vice versa for P linked to different Al). The 27Al and 31P resonances are assigned to the crystallographic Al and P sites by calculation of the NMR parameters using the CASTEP DFT program for an energy-minimized AlPO4(SAT) framework. Introduction Aluminophosphate molecular sieves (AlPOs), first reported by Wilson et al. in 1982,1 exhibit a wide range of porous structures that possess important adsorbent and, when suitably chemically doped, catalytic properties.2–7 Almost all of these solids are prepared via the use of organic structure directing agents (SDAs). These may be amines (which may be protonated under the reaction conditions) or alkylammonium cations, and where the structure directing agent for AlPO4-based solids bears a permanent positive charge, a mechanism must exist to permit charge balance because the tetrahedral AlPO4 framework is neutral. The partial substitution of divalent metal cations for Al or of Si for P can achieve this.8–11 Another mechanism of charge balance without framework substitution is for anions from the synthesis solution to remain coordinated to aluminum cations in the as-prepared form of the material. This is wellknown for AlPO4 prepared from fluoride-containing solutions, such as UT-6, the aluminophosphate-fluoride version of AlPO434, and the framework topology type CHA12 in which Al * To whom correspondence should be addressed. E-mail: paw2@ st-andrews.ac.uk (P.A.W.); [email protected] (S.E.A.). † University of St. Andrews. ‡ Durham University. § Diamond Light Source, Harwell Science and Innovation Campus. | IFP-Lyon.

exhibits both tetrahedral AlO4 and octahedral AlO4F2 coordination. In the absence of F- in the gel, hydroxide ions can fulfill the same charge-balancing role, and AlPO4-1713 and AlPO41814 have both been prepared with OH groups coordinated to framework Al. In AlPO4-17, the OH group bridges across two Al cations in the top and bottom six-membered rings (6MRs) of a cancrinite cage, whereas in AlPO4-18, a hydroxyl group bridges across two Al cations in a 4MR of the host framework. Upon calcination of these aluminophosphate-fluorides or hydroxides, the fluoride or hydroxide ions are removed together with the organic template to leave porous solids.12–14 The aluminophosphate-based solid STA-2 was first reported in 1997.10 The framework structure (topology type SAT) belongs to a family of zeolitic solids that are built up by stacking six-membered rings (6MRs) at different positions in the xy plane of a hexagonal unit cell (A, (0,0); B, (2/3,1/3); C, (1/3,2/3)) and then connecting them via 4MRs. The stacking sequence of 6MRs for STA-2 is ABBCBCCACAAB, with a single column of the structure consisting of double six-membered rings (D6Rs), cancrinite cages, and elongated cages that are interconnected via elliptical 8MRs (Figure 1). The structure directing agent used for this solid was the dicationic 1,4bis-N-quinuclidiniumbutane (BQNB), Scheme 1, structure 1, and the solid was synthesized as part of an experimental program investigating the use of diquinuclidinium cations

10.1021/jp104120y  2010 American Chemical Society Published on Web 07/06/2010

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J. Phys. Chem. C, Vol. 114, No. 29, 2010 12699 tron powder X-ray diffraction and multinuclear NMR, and the mechanism of charge balance via the coordination of hydroxyl ions to framework Al has been investigated. Additionally, AlPO4 STA-2 has been calcined to give a porous solid, and NMR studies on this, together with DFT calculations of 27Al and 31P chemical shifts, enable the full assignment of 27Al and 31P MAS NMR spectra. Experimental Section

Figure 1. Part of the structure of STA-2, determined for MgAlPO STA-2,10 shown as a sequence of cancrinite cages, double six rings, and elongated cages stacked along the c axis. (left) Framework only and (right) with bis-quinuclidiniumbutane SDA included.

SCHEME 1

[C7H13N-(CH2)n-NC7H13]2+ with linking polymethylene groups of different lengths (n ) 3-10) as potential structure directing agents. Crystallization in the magnesioaluminophosphate (MgAPO) system with SDAs of different lengths was studied in detail. Using SDAs with n ) 3 gave MgAPO17(ERI) and with n ) 4 and 5 gave STA-2 (with impurity MgAPO-56 (AFX)15 under some conditions10,16). Longer templates gave MgAPO-5 or STA-1.9 The framework structure of STA-2 was solved from single crystals of the MgAPO form, and subsequent solid-state NMR of MgAPO STA-2 with Mg/P ) 0.1516 showed all Al was in tetrahedral coordination so that all charge balancing of the BQNB template arose from substitution of Mg2+ for Al3+. The SAPO form was also prepared with SDAs with n ) 4 and 5, although with cocrystallization of a SAPO-56 impurity. STA-2 is stable to template removal, and its ability to crystallize with different chemical compositions suggested further study, for example, of the mechanism by which charge balance of the cationic template is achieved in the AlPO4 form. However, one of the starting chemicals for the synthesis of the BQNB SDA for STA-2 (quinuclidine) is expensive. To facilitate more extensive studies and to improve the possibilities of the future applicability of STA-2, alternative, cheaper templates based on diazabicyclooctane (DABCO) were proposed, modeled, synthesized, and used in the preparation of solids. As a result, AlPO4 STA-2 has been prepared using the DABCO-based SDA bis-DABCO-butane, BDAB, (Scheme 1, structure 2) and compared with the version prepared with quinuclidine, BQNB. The bis-DABCO molecules have both quaternary ammonium and tertiary amine N atoms so that their charge can vary from 2+ to 4+: we have determined the charge of BDAB in as-prepared STA-2 as divalent. The AlPO4 STA-2 material prepared by this new route has been characterized by synchro-

Modeling Methods. Modeling was used to simulate the potential action of DABCO units linked by polymethylene chains as SDAs for STA-2 in place of similar cations prepared using diquinuclidine. DABCO- and quinuclidine-based SDAs were investigated in parallel, and because MgAPO-56 (structure type AFX)15 had been observed as an impurity in previous STA-2 preparations, the two series of molecules were also modeled as potential templates in the AlPO4-56 structure. Energy minimization of “templates” in the framework structures was performed using MS Modeling 4.1 software (Materials Studio), within the program Discover.17 The DABCO templates were modeled with the apical N atoms unprotonated (as was later confirmed to be the case by experiment). The Universal forcefield18 was used, and the charges were kept as current. The criteria for convergence tolerance were energy ) 0.001 kcal/ mol and force ) 0.5 kcal/mol/Å. During simulations, atoms’ positions were relaxed but the cell volume was kept constant. Once the simulation was complete, the torsion energy of the SDA was extracted. This is the energy difference between the configuration of the SDA in its minimum energy state outside the solid and its adopted configuration within the cages of the AlPO4 structures, when only the intramolecular interactions are considered. Synthesis of SDAs. The SDAs BQNB and bis-DABCObutane (BDAB) were prepared in the bromide form by the Menschutkin reaction mechanism, as described previously.9,10 For bis-DABCO-pentane dibromide, the general procedure of Abbiss and Mann for the reaction of DABCO with dibromoalkanes was used.19 Details are given in the Supporting Information. Synthesis of Aluminophosphates. Before use in the aluminophosphate syntheses, each bromide salt was converted to a solution of the hydroxide form by reaction of an aqueous solution of the salt with excess silver (I) oxide, followed by filtration to remove the solid residue and rotary evaporation to reduce the volume. A basic form of the template is required to increase the initial pH of the gel to 7, a suitable pH for the synthesis of open framework aluminophosphates. The alkylammonium hydroxide solution was added to give a starting pH of 7, and the gels were heated at 190 °C for 48 h. Two potential DABCO-based SDAs suggested by the modeling (based on C4 and C5 methylene chains) were used in crystallizations from AlPO4, MgAPO, and SAPO gel compositions: the previously investigated BQNB was used under the same conditions for comparison (Table 1). Characterization. Laboratory X-ray powder diffraction of the solid products of these syntheses was performed on a Stoe Stadi P diffractometer, using monochromated Cu KR1 radiation, over a period of 1 h. Samples of as-prepared AlPO4 STA-2 were also analyzed in Debye-Scherrer mode on beamline ID-31 at the ESRF at 100 K (λ ) 0.7999 Å).20 In addition, a sample of as-prepared AlPO4 STA-2 was loaded into a 0.5 mm quartz glass capillary attached to a vacuum line, where it was dehydrated at 250 °C for 3 h to remove water and sealed under a vacuum of 10-4 mbar. Another sample was calcined at 550 °C for 12 h in

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TABLE 1: Hydrothermal Synthesis Conditions and Product Phases Using BQNB, BDAB, and BDAP as Structure Directing Agents SDA BQNB BDAB BDAP

inorganic compositiona

T/°C

t/h

product phase

AlPO4 MgAPO SAPO AlPO4 MgAPO SAPO AlPO4 MgAPO

190 190 190 190 190 190 190 190

48 48 168 48 48 168 48 48

STA-2 MgAPO-56 STA-2, SAPO-56 STA-2 STA-2, MgAPO-56 STA-2, SAPO-56 unidentified phase STA-2, MAPO-5

Typical gel compositions: AlPO4 ) Al(OH)3:H3PO4:0.4 R(OH)2:40 H2O; MgAPO ) 0.2 Mg(OAc)2:0.8 Al(OH)3:H3PO4:0.5 R(OH)2:40 H2O; SAPO ) Al(OH)3 · H2O:0.8 H3PO4:0.2 SiO2:0.4 R(OH)2:40 H2O (R ) SDA). In each case, the alkylammonium hydroxide was added to give a starting gel pH of 7. a

flowing oxygen and then loaded into a quartz glass capillary and dehydrated and sealed as above. The samples were analyzed in Debye-Scherrer mode at the X-ray synchrotron, station I-11, at the Diamond Light Source at 100 K (λ ) 0.82602 Å).21 Scanning electron microscopy on a JEOL JSM-5600 SEM was used to determine the morphology and particle size. Elemental analysis (carbon, hydrogen, nitrogen) was performed on a Carlo Erba instruments EA 1110 CHNS analyzer. Thermogravimetric analysis was performed at 5 °C min-1 in flowing air. The micropore volume of a calcined sample of AlPO4 STA-2 was measured by N2 adsorption at 77 K using a Hiden IGA gravimetric analyzer, converting the mass of N2 adsorbed in the micropores at P/P0 ) 0.1 into a volume by assuming adsorbed nitrogen has the density of liquid nitrogen at 77 K. Solid-State NMR. NMR spectra were acquired using a Bruker Avance III 600 spectrometer, equipped with a widebore 14.1 T superconducting magnet. Powdered samples were packed into conventional 4 mm rotors and rotated at MAS rates between 7 and 14 kHz (27Al, 31P) or 60 kHz (1H). For calcined STA-2, the open rotor was heated overnight at 100 °C, prior to the collection of data to ensure the sample was dehydrated. Chemical shifts are recorded in parts per million relative to 85% H3PO4 for 31P, 1 M Al(NO3)3(aq) for 27Al, TMS for 13C, and nitromethane for 15N. For 13C and 15N, spectra were acquired using cross-polarization (CP), with a contact pulse (ramped for 1 H) of 1 ms (13C) and 5 ms (15N), respectively, and 1H decoupling (SPINAL32 or TPPM with ω1/2π of 100 kHz) applied throughout acquisition. High-resolution 27Al triplequantum MAS NMR spectra were acquired using a phasemodulated split-t1 shifted-echo pulse sequence,22 with the third pulse chosen to be selective for the central transition. Twodimensional heteronuclear correlation experiments were performed using an MQ-J-HETCOR experiment, with transfer via an INEPT sequence from 27Al (using central-transition selective pulses) to 31P.23 For MQMAS spectra, the isotropic dimension is scaled according to the convention in ref 24. Solution-State NMR. Conventional 14N and 1H-15N HMQC NMR spectra were recorded using Bruker AVANCE 400 and 500 spectrometers, respectively, for two samples of the bromide form of BDAB dissolved in D2O at neutral, acidic, and basic pH. Full experimental details are given in the Supporting Information. NH3 was chosen as a reference for the spectra: on this scale, the conventional standard of nitromethane is at 380.23 ppm. Sample preparation in acidic solution was performed by dissolving 200 mg of the bromide salt of BDAB in 0.45 mL of D2O and adding 80 µL of HCl (36% in weight), corresponding

Castro et al. to two equivalents of H+. Sample preparation in basic solution was performed by dissolving 100 mg of the bromide salt of BDAB in 0.45 mL of D2O and adding 11.2 mg of NaOH, corresponding to one equivalent of OH-. DFT Calculations. DFT calculations (using the B3LYP hybrid functional and 6-31+G(d,p) level of theory) were performed to evaluate the influence of the three possible nitrogen environments (NR4+, NR3, and NR3H+) for the BDAB template on the chemical shift and quadrupolar constant (CQ).25 The calculation of the isotropic magnetic shieldings and quadrupolar couplings were obtained with the CSGT (continuous set of gauge transformations) method and were referenced to NH3. For calcined STA-2, DFT calculations were performed using CASTEP, a plane-wave pseudopotential code,26 which utilizes the GIPAW formalism23,27–31 to reconstruct the allelectron wave function in a magnetic field. The generalized gradient approximation (GGA) PBE functional and the core-valence interactions described by ultrasoft pseudopotentials32 were used. Integrals over the Brillouin zone were performed using a Monkhurst-Pack grid with a k-point spacing of 0.04 Å-1. Wave functions were expanded in planewaves with a kinetic energy smaller than the cutoff energy, typically ∼680 eV. Structural parameters for the asprepared solid (the unit cell and atomic positions of the framework) were obtained from the single-crystal data of MgAPO STA-2,10 and a starting model for the framework was reproduced from these parameters in the primitive cell using periodic boundary conditions and assuming all tetrahedral cations to be Al and P. Geometry optimization was also performed within CASTEP (using the Broyden-FletcherGoldfarb-Shannon (BFGS) method) on the empty AlPO4 framework, corresponding to the calcined sample, by varying both lattice parameters and atomic coordinates, without symmetry constraints. To calculate NMR parameters from the optimized structure, reference shieldings, σref, of 555.23 and 284.0 ppm were used for 27Al and 31P, respectively, obtained from previous work.23,28 To calculate the quadrupolar coupling, a quadrupole moment, eQ, of 140.3 mB was used for 27Al. Results and Discussion Modeling. A series of hypothetical SDAs based on DABCO and quinuclidine connected by polymethylene chains of different lengths (from -(CH2)3- up to -(CH2)6-) were modeled in the cages of both STA-2 and AlPO4-56 structures, and their nonbonding interaction energies were calculated. The bisDABCO SDAs were modeled with the apical nitrogen atoms unprotonated (as observed experimentally, below), and the longrange electrostatic interactions, due to charges on the framework and the templates, were not taken into account, as they are not expected to affect the relative order of the interactions. The results are summarized in Figure 2. Total enthalpy values cannot be directly compared between the AlPO4 SAT and AFX structures because each one has a large structure-dependent value. The modeling results for the bis-quinuclidinium and bisDABCO series are very similar in both structures. Therefore, it should be possible to replace the bis-N-quinuclidinium SDAs by their bis-DABCO analogues. The results for the bis-Nquinuclidinium (bis-quin-Cn) series for STA-2 are in agreement with previous experimental results observed for MgAPO crystallization.15 SDAs with C4 and C5 linking chains form STA-2 with MAPO-56 or a layered phase as impurities; whereas bisquin-C3 leads to MgAPO-17(ERI), the longer bis-quin-C6 gives

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Figure 2. (top) Histograms showing the nonbonding energies of interaction with (top) AlPO4(STA) and (bottom) AlPO4(AFX), of potential SDAs in which two quinuclidine groups or two diazabicyclooctane groups are linked by polymethylene groups (-(CH2)n- of different lengths, n ) 3-6). These SDAs are labeled QuinC4 and DABCO-Cn, respectively. (botton) Torsion energies of potential SDAs modeled within (left) AlPO4(STA) and (right) AlPO4(AFX) structure types.

MgAPO-36(ATS) or MgAPO-5(AFI). In the case of AlPO456, modeling shows bis-quin-C3 as the most favorable template, followed by bis-quin-C4 and bis-quin-C5. Experimentally, only bis-quin-C4 forms MgAPO-56 because bis-quin-C3 has a tendency to form MgAPO-17, which has a shorter cage than MgAPO-56. These molecules are long and quite flexible, so the torsion energy is another parameter that could be important in cases when competitive crystallization is a possibility. The torsion energy estimates the adaptability of the organic molecules within the structure. In this case, results can be directly compared from one structure to the other (Figure 2). The torsion energies of Quin-C4 (BQNB) and DABCO-C4 (BDAB) are low in the STA-2 structure, indicating that the molecules do not have to twist significantly to fit into the cage. For the AlPO4-56 structure, these SDAs show higher torsion energy values, making them less favorable. The C5 SDAs possess higher values in both structures than the C4 analogues. Therefore, if the torsion energy is an important parameter, the C4 analogues BQNB and BDAB are expected to be the favored SDAs for STA-2. Figure 3 shows the minimized configurations of the DABCO-based SDAs within the STA-2 and AlPO4-56 cages. As observed from the calculated energies, when the SDAs are too long, the torsion energy makes them less favorable, and the templates connected by six methylene groups need to twist to fit into the cage. From this

modeling, the most promising novel SDAs to replace their more expensive quinuclidine-based analogues are BDAB and BDAP, particularly BDAB. Synthesis. Laboratory X-ray powder diffraction of the products indicates that AlPO4 STA-2 is synthesized using BQNB and BDAB as SDAs but not using BDAP (Table 1). Instead, the use of BDAP under these conditions resulted in an unidentified phase that lost crystallinity upon calcination. Synthesis from MgAPO gels gave MgAPO-56 (with BQNB) or mixtures of STA-2 with MgAPO-56 (with BDAB) or with MgAPO-5 (with BDAP). For the synthesis from SAPO gels, the use of BQNB gave STA-2 with minor SAPO-56 impurity, whereas BDAB gave a poorly crystalline mixture of STA-2 and SAPO-56. The two AlPO4 STA-2 materials crystallize as micrometersized particles. Refinement of the unit cells of the two as-prepared samples (using the Rietveld refinement option in the GSAS suite of programs33 with the synchrotron PXRD collected at the ESRF) indicated that the samples were phase pure: whereas AlPO4 STA-2(BQNB) is indexed as rhombohedral R3j, the high-resolution diffraction gave peak splittings for AlPO4 STA-2(BDAB) that were better indexed as triclinic (Table 2). Chemical Composition of As-Prepared AlPO4 STA-2. The chemical composition of the as-prepared STA-2 samples was

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Figure 3. Energy-minimized configurations of bis-DABCO-based SDAs with different lengths of linking methylene chains (C6N2H12)-(CH2)n-(C6N2H12), n ) 3-6, in (top) AlPO4(STA) and (bottom) AlPO4(AFX).

TABLE 2: Unit Cell Parameters of AlPO4 STA-2 Materials sample SDA diffractometer wavelength/Å temperature/K space group a/Å b/Å c/Å R (°) β (°) γ (°) V/Å3

as-prepared AlPO4(STA-2)

as-prepared AlPO4(STA-2)

as-prepared, dehydrated AlPO4(STA-2)

calcined, dehydrated AlPO4(STA-2)

BQNB ID-31 0.79990 100 R3j 13.0323(3) 13.0323(3) 29.6010(9) 90 90 120 4353.9(3)

BDAB ID-31 0.79990 100 P1 12.3905(2) 12.3920(2) 12.3894(3) 63.401(1) 63.498(1) 63.452(1) 1447.80(3)

BDAB I-11 0.826019 100 P1 12.3821(2) 12.3795(2) 12.3797(3) 63.3585(8) 63.4830(7) 63.4218(7) 1443.39(3)

BDAB I-11 0.826019 100 R3j 12.9270(2) 12.9270(2) 30.7977(5) 90 90 120 4457.0(1)

established by a combination of NMR, TGA, and elemental analysis. The solid-state 13C CP MAS NMR spectrum (Figure 4a) of as-prepared AlPO4 STA-2(BQNB) indicates that the organic SDA is incorporated intact with no resonances other than those expected for the template molecule. The observed shifts (at δ ) 19.4, 19.8, 23.9, 56.6, and 64.1 ppm corresponding to carbons 5, 1, 4, 3, and 2, respectively) are in broad agreement with both the solution-state NMR results and the DFT calculations (both given in full in the Supporting Information), with any small differences arising from the presence of the surrounding aluminophosphate framework. For AlPO4 STA-2(BDAB) (Figure 4b), all the most intense resonances are those from the

intact SDA (at δ ) 19.7, 45.7, 54.5, and 64.6 ppm, corresponding to carbons 1, 4, 3, and 2, respectively). Again, this is in broad agreement with the solution-state NMR results and the DFT calculations (see the Supporting Information). There are also some minor signals corresponding to breakdown products that are incorporated (marked with *), but these only make up a few percent of the total C content. 15 N MAS NMR was also performed to observe any differences upon protonation between BQNB and BDAB, as shown in Figure 4. The charge on BQNB is unequivocally 2+, whereas BDAB can, in principle, be 2+, 3+, or 4+, depending upon the protonation state of the apical nitrogen atoms. As expected, the

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Figure 4. (a, b) 13C and (c, d) 15N (14.1 T) CP MAS NMR spectra of AlPO4 STA-2 with (a, c) BQNB and (b, d) BDAB templates. Spectra are acquired using a cross-polarization sequence with a contact pulse duration of 1 ms (13C) and 5 ms (15N) and 1H decoupling in acquisition. In (a) and (b), 800 and 1200 transients, respectively, were averaged with a recycle interval of 5 s, whereas in (c, d), 32 768 transients were averaged with a recycle interval of 5 s. The MAS rates were (a) 10, (b) 12.5, and (c, d) 7 kHz. Asterisked peaks in (b) and (d) are attributed to 13C and 15N resonances from breakdown products of BDAB that become occluded in crystallizing STA-2.

spectrum for BQNB exhibits a single signal at -331.2 ppm, corresponding to the quaternary nitrogen atoms (NR4+). The BDAB spectrum exhibits two strong signals, one at -331.8 ppm for the quaternary nitrogen atoms and another (∼42.8 ppm upfield) at -374.6 ppm from the apical nitrogen atoms, which could be either nonprotonated (NR3) or protonated (NR3H+). A third, smaller, peak in the 15N MAS NMR spectrum is asterisked: this is attributed to the small quantity of breakdown products of BDAB observed in the 13C MAS NMR spectrum of the same material. The isotropic magnetic shielding of the nitrogen environments, calculated using DFT for molecular BDAB, predict NR3 as the most shielded species (i.e., with the lowest ppm), followed by NR3H+ deshielded by 30 ppm and NR4+ a further 20 ppm deshielded (see the Supporting Information for more details). Although not necessarily a good representation of the exact shift differences expected in the solid (as a result of crystal packing effects and the presence of the framework) or even in solution (where the solvent will also influence the shifts observed), the calculation does confirm that the chemical shifts are expected to follow the general trend NR3 < NR3H+ < NR4+. Additional 15N NMR spectra, acquired using 1 H-15N HMQC, for BDAB in solution, and for solutions with

additional acid or base, were able to confirm this conclusion, with a shift difference of ∼35 ppm observed between the quaternary nitrogen and apical NR3, but only an ∼17 ppm difference between the quaternary nitrogen and apical NR3H+ signal. This suggests that the signal observed at -374.6 ppm in the solid state results from unprotonated N, demonstrating the incorporation of a divalent BDAB molecule (further details in the Supporting Information). This conclusion is also supported by 14N solution-state NMR, which suggests that, at pH 7 (as used in the STA-2 synthesis), the BDAB cation is divalent. As 14N is a quadrupolar nucleus (with spin quantum number I ) 1), resonances in solution-state NMR spectra are often broadened as a result of fast relaxation if the magnitude of the quadrupolar coupling, CQ, is large. The 14 N quadrupolar coupling constants predicted by DFT calculations for a single BDAB molecule are very different for the three environments, with the pyramidal NR3 having a large CQ (5.8 MHz at the highest level of theory), in contrast to the much smaller values (∼0.5 MHz) found for the two tetrahedral species (see the Supporting Information). Experimentally, at pH 7, only a single sharp resonance is observed in the 14N spectrum (corresponding to the quaternary NR4+), as the signal from the

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Figure 5. (a, b) 27Al and (c, d) 31P (14.1 T) MAS NMR spectra of AlPO4 STA-2 with (a, c) BQNB and (b, d) BDAB templates. Spectra are the result of averaging (a) 80, (b) 64, (c) 24, and (d) 80 transients with recycle intervals of (a, b) 0.5, (c) 30, and (d) 10 s. The MAS rate was (a, c) 10 and (b, d) 14 kHz.

apical NR3 is too broad to observe. Upon the addition of acid, a second signal is then observed at a lower shift, resulting from the NR3H+ species (albeit exchange broadened). Further details of the solution-state NMR results can be found in the Supporting Information. The NMR data, therefore, strongly suggest that BDAB is included mainly as the divalent form. As a result, the charge on each SDA is likely to be balanced by two hydroxyl ions. Thermogravimetric analyses of the STA-2 materials indicate a gradual weight loss of ca. 8% up to around 300 °C, attributed to the loss of water, followed by a weight loss of ca. 17% centered at 375 °C due to the removal of the organic SDA plus associated water. If this information is combined with the elemental analysis and the charge on the SDAs, the following compositions can be calculated (expressed as the triclinic unit cell of as-prepared AlPO4 STA-2): AlPO4(BQNB), Al12P12O48(OH)2BQNB0.97 (H2O)9, Calcd: C, 11.11 wt %; N, 1.44%. Obsvd: C, 11.06%; N, 1.43%. AlPO4(BDAB), Al12P12O48(OH)2BDAB0.97 (H2O)9, Calcd: C, 9.89%; N, 2.88%. Obsd: C, 9.89%; N, 2.87%. Solid-State NMR of the As-Prepared STA-2 Framework. For the as-prepared STA-2 framework, 27Al and 31P MAS NMR spectra were measured to provide more information on the local environment of the framework cations and to elucidate how the charge-balancing hydroxyl anions are incorporated into the structure. Figure 5 shows that the 27Al MAS NMR spectra of AlPO4 STA-2 prepared with BQNB and BDAB as SDAs are very similar. Two overlapping resonances at ∼40 ppm are characteristic of tetrahedrally coordinated Al(PO)4 units (typically with δ of 45-30 ppm), whereas the resonances at lower ppm show that higher-coordinated Al species are also present.34

As typical shift ranges for Al(OP)5 and Al(OP)6 units are 5-20 ppm and -5 to -20 ppm, respectively, the Al species in AlPO4 STA-2 are most likely five-coordinated. The position of these peaks are also similar to those found for 5-fold coordinated Al observed in ordered AlPO4-17 prepared with the protonated SDA hexanediamine.13 In that material, charge-balancing hydroxyl groups bridge between Al cations within cancrinite cages. The 27Al MAS NMR spectrum of as-prepared STA-2 does not change upon dehydration at 250 °C, indicating that any additional coordination of Al above 4 is due to coordination of hydroxyl groups rather than of water, which the TGA shows is mainly lost below this temperature. The OH proton resonance does not appear to be resolved in fast-spinning 1H MAS NMR but may be responsible for the residual intensity at around 5 ppm in such spectra of the dehydrated material. See the Supporting Information for more details. As 27Al is a quadrupolar nucleus (with a spin quantum number I ) 5/2), MAS spectra are broadened by the quadrupolar interaction, limiting resolution. This broadening can be removed, improving the resolution, using two-dimensional multiplequantum (MQ) MAS experiments. The 27Al triple-quantum MAS NMR spectra of the two materials are shown in Figure 6. They are very similar, with broadened resonances observed despite the resolution enhancement, owing to a distribution in NMR parameters, which probably results from some disorder in the OH- position. From the position of the resonances in the two-dimensional spectrum, it is possible to extract the average values of the isotropic chemical shift, , and the quadrupolar interaction (through the quadrupolar product PQ ) CQ(1 + ηQ2/ 3)1/2), which are given in Table 3. (Note that, for a quadrupolar nucleus, the observed shift of a resonance in a MAS spectrum

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J. Phys. Chem. C, Vol. 114, No. 29, 2010 12705 TABLE 3: Experimental 27Al (14.1 T) MAS NMR Parameters (Average Isotropic Chemical Shift, 〈δiso〉, and Average Quadrupolar Product, 〈PQ〉) of Templated STA-2, Extracted from the MQMAS NMR Spectra in Figure 6, for AlPO4 STA-2(BQNB) and STA-2(BDAB) species STA-2(BQNB) AlIV AlIV AlV STA-2(BDAB) AlIV AlIV AlV

Figure 6. 27Al (14.1 T) two-dimensional triple-quantum MAS NMR spectra of AlPO4 STA-2 with (a) BQNB and (b) BDAB templates. Spectra were recorded using a phase-modulated split-t1 shifted-echo pulse sequence22 and are the result of averaging (a) 96 and (b) 192 transients with a recycle interval of 0.5 s for each of (a) 197 and (b) 200 t1 increments of 50 µs. The MAS rate was (a) 10 and (b) 14 kHz.

depends not only upon the chemical shift but also on the magnitude of the quadrupolar interaction, preventing information on the chemical shift being extracted by simple visual inspection.) As seen in Figure 6, both MQMAS spectra exhibit two clearly resolved resonances corresponding to tetrahedrally coordinated Al (with δiso of 35-50 ppm), presumably deriving from the two types of 4-fold Al species in the STA-2 framework. In both materials, the average quadrupolar couplings for these species, between 2 and 3 MHz, are relatively small. However, in each case, there is an indication of a shoulder on the most deshielded

δiso (ppm)

〈PQ〉/MHz

49(2) 45(2) 17(2)

2.6(3) 3.0(3) 2.4(3)

49(2) 46(2) 17(2)

2.5(3) 3.0(3) 2.5(3)

resonance, indicating additional 4-fold Al with a slightly different local environment. A single resonance, corresponding to 5-fold coordinated Al, is observed in both spectra, although, in both cases, this signal is broad and appears to contain more than one component. When BDAB is used as the SDA, two low-intensity resonances are also observed, resulting from very small amounts of a 6-fold coordinated Al impurity, seen in both MAS and MQMAS spectra. The 5-fold coordinated Al results from the presence of hydroxyl groups in the samples, with the ratio of 5-fold to tetrahedral Al characteristic of the concentration of these extraframework species. Figure 5 shows that this ratio is similar for AlPO4 STA-2 prepared with either BDAB or BQNB (which can only bear a 2+ charge), providing additional evidence that the BDAB is also divalent in the solid. The ratio also gives information on the mode of coordination of the hydroxyl groups. If these were terminal ligands, the ratio of tetrahedral to 5-fold Al for the composition established above (2 OH- per triclinic unit cell) would be 10:2 (5:1), whereas if the hydroxyls bridge between Al cations (as seen for AlPO(ERI)), then the ratio would be 8:4 (2:1). Approximate deconvolution of the spectra (bearing in mind that the resonances are broadened by the quadrupolar interaction and by disorder) indicates that the observed value (1.5:1) is much closer to that expected for bridging hydroxyl groups. Furthermore, the MQMAS spectra show that the approximate (integrated) intensity of the remaining resonances for the two major tetrahedrally coordinated Al species is approximately 1:1, suggesting that there are similar numbers of bridging hydroxyl groups attached to Al on both crystallographic sites. Removal of physisorbed water does not change these spectra (see the Supporting Information), indicating that the coordinating species are hydroxyl groups rather than water molecules. The 31P MAS NMR spectra of AlPO4 STA-2 prepared with BQNB and BDAB as SDAs, shown in Figure 5, are also very similar, with a series of more intense resonances between -20 and -40 ppm and somewhat broader resonances with a weaker intensity at higher ppm in both cases. Deconvolution of the spectrum with BDAB as the template indicates that at least four distinct resonances are present. There is no relative enhancement of any of the signals by 1H f 31P cross-polarization CP (not shown), indicating that all are related to fully coordinated PO4 tetrahedra and do not result from directly bonded hydroxyl (i.e., POH) groups. There is also no close correlation of the peaks in the as-prepared samples with those of the calcined material (see below), indicating that the presence of proximate 5-fold Al has a strong downfield effect on the chemical shift, whereas the presence of disorder broadens the resonances. The spectral resonances, therefore, appear to arise from P species from the two topological positions in the STA-2 structure and with

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Figure 7. Bridging OH positions found to be occupied in the cancrinite cages of AlPO4 STA-2(BDAB). Al-OH bonds are depicted in blue.

different numbers and locations of 5-fold Al cations in their first tetrahedral cation coordination spheres. Structural Analysis of As-Prepared AlPO4 STA-2(BDAB). The high-resolution powder diffraction pattern of dehydrated as-prepared AlPO4 STA-2(BDAB) measured at 100 K at I-11, Diamond Light Source, was indexed as triclinic, with larger peak splittings than in the as-prepared, hydrated state. Because the 27Al MAS NMR spectrum is not changed by dehydration (and so the sample should still have the coordinated hydroxyl groups), it was thought that the diffraction data of the dehydrated phase could give a more accurate picture of these chargebalancing hydroxyls than those of the hydrated phase, where additional extraframework scattering from water would be present. AlPO4 STA-2(BDAB) was, therefore, examined further using Rietveld refinement via GSAS.33 Starting with a model for the framework derived from the single-crystal structure of MgAPO STA-2, the positions of all atoms in a primitive triclinic (P1) cell were calculated. The location of the BDAB template in its energy-minimized configuration as determined from molecular modeling was also input. The fit to the data was improved by allowing instrumental parameters, lattice parameters, and the location of framework atoms (Al, P, O) to vary, with restraints applied to Al-O and P-O bond distances. In addition, O-O bond distances in the PO4 tetrahedra were restrained, but those in the AlO4 tetrahedra were not. The position of the template was also refined, keeping tight constraints on C-C, C-N, and N-N distances and CNC and CCC angles. As shown previously, solid-state NMR indicates that there is 5-fold aluminum present. Difference Fourier maps show a variety of peaks related to scattering from the template that is incompletely described and also scattering within one of the two crystallographically distinct cancrinite cages. The two largest difference Fourier peaks in the cancrinite cage were close to the positions expected for a bridging hydroxyl oxygen, such as observed in AlPO4-17.13 Therefore, six potential positions for bridging hydroxyl groups were calculated (three in each cage) and input as oxygen atoms. Refining alternately the positions and occupancies of these oxygen atoms improves the fit, but only the two sites observed from the difference Fourier map were observed to have significant occupancy (Figure 7), with a total occupancy of 1 per unit cell, around one-half of

Castro et al. the expected quantity. The more highly occupied site in the bridging position, O92, (fractional occupancy 0.75) is at an Al-O distance of 2.3(1) Å from Al(16) and 2.2(1) Å from Al(25). Although these are too long for Al-O bonds, it should be remembered that, because of the disorder in the OH position, the refined Al positions are weighted averages of positions where the hydroxyl group is there or is not so that the true Al-O distances will be less. Furthermore, whereas the Al-O distances of the other hydroxyl O atom, O(91), (Al(14)-O, 2.4(3) Å and Al(23)-O, 1.6(2) Å) are alternately short and long, it is likely that the framework positions are dominated by their positions when the hydroxyl group is not there because the fractional occupancy is only 0.25. The final refinement (see Figure 8 for final fit and the Supporting Information for details) has mean Al-O and P-O bond lengths of 1.75(5) and 1.51(2) Å, respectively, and suggests that at least 50% of the chargebalancing hydroxyl groups are located in one of the cancrinite cages, bridging between Al cations. The other hydroxyl group could not be located. Taken together, these modeling, XRD, and NMR studies of the aluminophosphate form of STA-2 underline the strength of a combined approach to the synthesis and characterization of the as-prepared phase. Modeling is a useful tool to investigate the potential application of SDAs in the targeted synthesis of zeotypes with well-defined cages. BDAB was proposed as the favored DABCO-based template for STA-2 on the basis of overall nonbonded energy and torsion energies and was the most successful in the synthesis of the AlPO4 form of STA-2. A combination of NMR methods suggests that, like the diquinuclidine analogue, BDAB is included as a dication and that charge balancing occurs, at least in part, by the inclusion of hydroxyl groups that bridge between Al(1) and Al(2) cations. Although MQMAS NMR only clearly resolves two main tetrahedral resonances, there is some evidence for Al in different sites within these two broad peaks. At least one-half of the charge-balancing hydroxyl groups are disordered over two bridging sites in one of the cancrinite cages, and as a result of this partial ordering, the structure distorts from rhombohedral to triclinic. It is also noteworthy that, whereas the AFX structure type crystallizes competitively from MgAPO gel compositions with the new SDAs, it does not appear in the pure AlPO4 composition. A similar effect has been observed previously for AFX, where the ERI structure (which, like STA-2, has cancrinite cages) has been observed to crystallize preferentially in the AlPO4 gels, whereas SAPO-56 is formed from SAPO gels under similar conditions.16 We speculate that the charge-balancing mechanism by which hydroxyl groups are included in cancrinite cages favors the crystallization of AlPO4 materials that possess these cages over structures (such as those with the AFX topology) that do not. Structure of Calcined STA-2. Calcination of STA-2 in flowing oxygen removes the template, leaving a porous solid with the SAT topology type. Nitrogen adsorption at 77 K shows a Type I isotherm (see the Supporting Information) and indicates a pore volume of 0.22 cm3 g-1. The 27Al and 31P MAS NMR spectra of the calcined material, shown in Figure 9, indicate that all Al and P are tetrahedrally coordinated. For 27Al, two distinct resonances, each with a characteristic quadrupolar line shape, are partially resolved, while two distinct 31P resonances are also observed, in agreement with the rhombohedral structure refined previously for calcined MgAPO STA-2.10 The two 27Al resonances can be clearly distinguished by MQMAS, as shown in Figure 9c, with two sharp resonances in the isotropic spectrum. Chemical shift and

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Figure 8. Rietveld plot for the refinement of the structure of dehydrated, as-prepared AlPO4 STA-2(BDAB) against a synchrotron X-ray diffraction pattern from I-11 at the Diamond Light Source (Rwp ) 0.069, Rp ) 0.055).

quadrupolar parameters extracted from this spectrum (from the position of the resonances and from cross sections extracted parallel to δ2, as shown in Figure 9) are given in Table 4. The X-ray powder diffraction data measured for calcined AlPO4 STA-2 were used to confirm the framework structure and to refine the tetrahedral AlPO4 framework positions. Using the calcined MgAPO STA-2 framework as a starting model, and restraining the Al-O and P-O bond distances in the structural refinement of the AlPO4 framework, a good fit to the data was obtained, confirming the framework structure (R3j, a

) 12.9270(2) Å, c ) 30.7976(4) Å, Rwp ) 0.072, Rp ) 0.057; for details, see the Supporting Information). To assign the 27Al and 31P MAS NMR signals, a combined approach using 2D NMR and DFT calculations was adopted. The different tetrahedral sites in the structure, Al(1), Al(2), P(1), and P(2), can be considered by a description of the STA-2 framework as being built up of layers of cancrinite cages in which these cancrinite cages share 4MR faces (Figure 10). Each of these layers is attached to a similar layer of cancrinite cages above and below through D6Rs, and the adjacent layers are

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Figure 9. (a) 27Al and (b) 31P (14.1 T) MAS NMR spectra of calcined AlPO4 STA-2. Spectra are the result of averaging (a) 64 and (b) 24 transients with a recycle interval of (a) 0.5 and (b) 5 s. The MAS rate was 10 kHz. (c) 27Al (14.1 T) two-dimensional triple-quantum MAS NMR spectra of calcined AlPO4, recorded using a phase-modulated split-t1 shifted-echo pulse sequence.22 The spectrum is the result of averaging 192 transients with a recycle interval of 0.5 s for each of 256 t1 increments of 100 µs. The MAS rate was 10 kHz. (d) 27Al/31P (14.1 T) two-dimensional MQJ-HETCOR spectrum of calcined AlPO4, recorded using the pulse sequence (an 27Al phase-modulated split-t1 triple-quantum MQMAS experiment, followed by INEPT transfer to 31P) shown in ref 23. The spectrum is the result of averaging 192 transients with a recycle interval of 2.5 s for each of 128 t1 increments of 165 µs. The MAS rate was 10 kHz.

related by centers of symmetry in the middle of these D6Rs. This results in the formation of the large cavities that contain the SDAs in the as-prepared form. One type of each of Al and P is located in the 4MRs shared by cancrinite cages (Al(1) and P(2)), whereas the other set (Al(2) and P(1)) is located in the planar 6MRs of the cancrinite cage that are part of the D6Rs. As a result, each Al(1) is linked via O atoms to one P(1) and three P(2) and each Al(2) to one P(2) and three P(1). This 1:3 connectivity is clearly observed in the different intensities of the cross-peaks in the 27Al-31P MQ-J-HETCOR spectrum of

the calcined material shown in Figure 9d and in the slices through the two Al signals (at ca. 20 and 24 ppm) along the F2 dimension. Although this identifies possible Al(1)/P(2) and Al(2)/P(1) pairs of resonances, the spectrum cannot provide an unambiguous assignment. To assign the resonances, the 27Al and 31P NMR parameters were calculated using DFT within the CASTEP code and are given in Table 4. The structure used for the calculation was the one optimized within CASTEP using an experimental structure as a starting point. The parameters are in excellent agreement

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TABLE 4: Experimental and Calculated 27Al (14.1 T) MAS NMR Parameters (Isotropic Chemical Shift, δiso, Quadrupolar Product, PQ, Quadrupolar Coupling Constant, CQ, and Asymmetry, ηQ) for Calcined AlPO4 STA-2, Extracted from the MAS and MQMAS NMR Spectra in Figure 9 and Calculated Using CASTEP species

δiso (ppm)

PQ/MHz

Al1 Al2 P1 P2

experimental (14.1 T) 36.0 (5) 2.1(2) 42.0 (5) 3.9(2) -29.6 (2) -34.8 (2)

Al1 Al2 P1 P2

calculated (CASTEP) 2.33 3.86 -23.8 -30.6

38.2 45.9

CQ/MHz

ηQ

2.0(2) 3.5(1)

0.7(2) 0.85(5)

2.11 3.37

0.83 0.97

AlPO4 STA-2 can be rendered porous by calcination to give a rhombohedral structure with two crystallographically distinct Al and P sites, clearly observed by MAS and MQMAS NMR: 2D INEPT correlation spectroscopy readily resolves the Al and P sites and their connectivity. The four tetrahedral sites are assigned by calculation of the NMR spectra by DFT using the CASTEP program. Having unambiguously established the site assignment in this way, efforts are underway to determine quantitative details of substitution of Mg for Al and of Si for P in this material via solid-state NMR. Acknowledgment. Funding for this work was provided by the European Commission FP6 Marie Curie Research Training Network “INDENS” (MRTN-CT-2004-005503) (M.C. and P.A.W.), the EPSRC (V.R.S., J.M.G., and S.E.A.; EP/E041825), and the University of St. Andrews (D.C.). The EPSRC solidstate NMR facility at Durham University is thanked for collecting some of the spectra. Dr. Irene Margiolaki (ESRF) and Professor Chiu C. Tang (DLS) are thanked for help in collecting synchrotron X-ray powder diffraction data on asprepared and dehydrated and calcined materials, respectively. Supporting Information Available: The Supporting Information includes details of SDA synthesis, solution-state and solid-state NMR spectra and assignments, TGA, structural data for dehydrated, as-prepared and dehydrated, calcined AlPO4 STA-2, the adsorption isotherm for N2, and details of CASTEP structural optimization. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. Layers of cancrinite cages in the STA-2 structure (legend: green spheres, Al(1) atoms; red, Al(2); yellow, P(1); blue, P(2)).

with experiment and allow both Al and P species to be assigned. The assignment is also consistent with the 27Al-31P MQ-JHETCOR spectrum in Figure 9d. Conclusions Bis-diazabicyclooctane-butane (BDAB) dications have been found to template the crystallization of AlPO4 STA-2 in fluoridefree syntheses. BDAB is a novel structure directing agent, less expensive than the previously used bisquinuclidinium butane (BQNB). Modeling suggests that both SDAs are energetically favored in STA-2 compared with their longer or shorter analogues and also have lower torsion energies in STA-2 than in the potential competitive crystallization product AlPO4-56. A combination of chemical analysis and a range of NMR techniques confirms that the BDAB is included in the divalent form, and solid-state 27Al MAS NMR indicates that charge balance is achieved by the coordination of hydroxyl groups that bridge between two 5-fold coordinate Al cations. Highresolution powder X-ray diffraction of the as-prepared and the dehydrated, as-prepared AlPO4 STA-2(BDAB) shows that they display a triclinic distortion away from rhombohedral symmetry. Rietveld refinement suggests at least 50% of the bridging hydroxyl sites to be within the cancrinite cages, in positions seen previously in an as-prepared AlPO4-17 material that possesses the same type of cages. This suggests that the energetics of hydroxyl coordination to Al could play an important part in phase selectivity in the crystallization of otherwise neutral AlPO4 frameworks around cationic templates.

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