338 Chem. Mater. 2010, 22, 338–346 DOI:10.1021/cm902528y
Novel Large-Pore Aluminophosphate Molecular Sieve STA-15 Prepared Using the Tetrapropylammonium Cation As a Structure Directing Agent Zhongxia Han,† A. Lorena Picone,† Alexandra M.Z. Slawin,† Valerie R. Seymour,† Sharon E. Ashbrook,† Wuzong Zhou,† Stephen P. Thompson,‡ Julia E. Parker,‡ and Paul A. Wright*,† †
School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St. Andrews, Fife, KY16 9ST, United Kingdom and ‡Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom Received August 17, 2009. Revised Manuscript Received October 12, 2009
The novel aluminophosphate STA-15 (St Andrews microporous solid-15) is prepared by hydrothermal synthesis in the presence of tetrapropylammonium hydroxide (TPAOH), which acts as a structure directing agent. The crystallization is accelerated by the addition of low concentrations of tetraphenylphosphonium or other bulky organic cations, and the purity is improved by the addition of silica to the gel, but these additives are not included in the final crystalline product. The structure of STA-15 was solved by a combination of synchrotron X-ray powder diffraction and modeling. The asprepared form of STA-15 (Iba2, a = 14.7953(1) A˚, b = 27.3634(3) A˚, c = 8.34464(6) A˚ at 100 K) has unit cell composition Al32P32O128(OH)1.8TPA1.8 3 3H2O. It has a system of one dimensional channels, limited by strongly elliptical 12-membered rings (12 tetrahedral cations and 12 oxygen atoms, 8.7 5.7 A˚ ) in which the TPAþ cations reside. The charge-balancing hydroxide ions are coordinated to framework Al, as shown by 27Al and 31P MAS NMR. STA-15 is stable to removal of the organic and hydroxyl species upon calcination in oxygen, leaving a microporous solid with a pore volume of 0.11 cm3 g-1 and showing uptakes of n-hexane and toluene (at 297 K, p/po = 0.10) of 0.48 and 0.69 mmol g-1, respectively. Introduction Microporous inorganic solids with zeolitic properties are of great importance as sorbents and catalysts and have high potential in emerging technologies.1,2 Among these solids, aluminosilicates, silicas and aluminophosphates with tetrahedrally-connected frameworks are of most general interest, because of their high thermal stability and properties as catalysts. For aluminophosphate-based molecular sieves,3-7 the inclusion of Mg for Al, or Si for P, introduces acidity for solid acid catalysts, 1,5,8,9 and the inclusion of redox metals, such as Mn, Co, or *To whom correspondence should be addressed. E-mail: paw2@ st-andrews.ac.uk. Tel: (0)1334 463793. Fax: (0)1334 463808.
(1) Wright, P. A. Microporous Framework Solids; RSC Publishing: Cambridge, U.K., 2007 (2) Davis, M. E. Nature 2002, 417, 813. (3) Wilson, S.T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (4) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U.S. Patent 4,310,440, 1982 (5) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1984, 106, 6092. (6) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. U.S. Patent 4,440,871, 1984 (7) Patarin, J.; Paillaud, J. L.; Kessler, H. In Handbook of Porous Solids; Schuth, F., Sing, K. S. W., Weitkamp, J., Eds.; Wiley-VCH: New York, 2002; p 815. (8) Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. Acc. Chem. Res. 2003, 36, 317. (9) Wright, P. A.; Sayag, C.; Rey, F.; Lewis, D. W.; Gale, J. D.; Natarajan, S.; Thomas, J. M. J. Chem. Soc. Faraday Trans. 1995, 91, 3537.
pubs.acs.org/cm
Fe, for Al produces redox active solids suitable for selective oxidation catalysis.1,10-12 Although the mechanism of the crystallization of zeolites and aluminophosphates remains an open question of current interest,13,14 the role of inorganic and organic species as structure directing agents, or templates, is of fundamental importance. Indeed, for the aluminophosphate solids, first discovered by Wilson, Lok, and Flanigen et al.,3-6 all of the most porous frameworks are prepared in the presence of organic amines or alkylammonium cations that are occluded in the solid after crystallization, and must be removed by calcination in oxygen to render the solids porous. As a result, there is particular interest in those materials that can be prepared using readily available organic species, and initial synthetic efforts concentrated on the use of tetraalkylammonium cations (NR4þ, where R = CH3, C2H5, C3H7, and C4H9). The tetrapropylammonium cation (TPAþ) has been reported to act as a structure directing agents for the aluminophosphates AlPO4-5 and AlPO4-40 (the latter favored by the presence of low (10) Thomas, J. M.; Raja, R.; Sankar, G.; Bell, R. G. Acc. Chem. Res. 2001, 34, 191. (11) Thomas, J. M.; Raja, R. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 13732. (12) Raja, R.; Thomas, J. M.; Xu, M. C.; Harris, K. D. M.; GreenhillCooper, M.; Quill, K. Chem. Commun. 2006, 448. (13) O’Brien, M. G.; Beale, A. M.; Catlow, C. R. A.; Weckhuysen, B. M. J. Am. Chem. Soc. 2006, 128, 11744. (14) Cubillas, P.; Castro, M.; Jelfs, K. E.; Lobo, A. J. W.; Slater, B.; Lewis, D. W.; Wright, P. A.; Stevens, S. M.; Anderson, M. W. Cryst. Growth Des. 2009, 9, 4041.
Published on Web 12/18/2009
r 2009 American Chemical Society
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Table 1. Gel Compositions (Molar Ratios) and Resultant Products for Hydrothermal Syntheses of STA-15a Al(OH)3
H3PO4
1 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.85
1 1 1 1 1 1 1 1 1
SiO2
Co(Ac)2
TPAþ
0.05
0.64 0.64 0.64 0.64 0.64 0.64 0.60 0.64 0.64
0.2 0.2 0.2 0.2 0
TPPþ
0.108 0.108
K222
0.008
H2O 70 70 70 70 70 70 70 36 70
product (PXRD label, Figure 1) STA-15 þ AlPO4-5 (a) STA-15 þ impurity (b) poorly crystalline STA-15 (c) STA-15 (d) STA-15 þ minor impurity (e) STA-15 (f ) STA-15 (g) AlPO4-5 (h) STA-15 þ minor impurityb
a All preparations based on 6.24 mmol H3PO4. TPAOH added to give an initial pH of 6.5-7.0. This typically required TPAþ/P molar ratio of 0.60-0.64. All syntheses in the table were performed at 190 °C for 168 h. a-d in new PTFE liners, others in used liners washed with hot dilute nitric acid. b Preparation seeded with 0.04 mmol of as-prepared AlPO4-STA-15.
levels of added tetramethylammonium TMAþ),3,15 and the silicoaluminophosphates SAPO-5,16 SAPO-40,5,6,16 and SAPO-34,17 where the product phase depends on the reaction conditions and presence of additives that are not included in the crystals. In addition, SAPO-37 is templated by a mixture of TPAþ and TMAþ cations, where both are included in the final product.5,18 The ability of an organic molecule to be able to direct to more than one structure is well known in zeolites and related materials, the commercially available hexamethonium cation has recently been shown to give the novel aluminophosphate-based EMM-319 and had previously been shown to template the silicoaluminophosphate SAPO-17.20 In this paper, we report the synthesis of a novel large pore aluminophosphate, STA-15 (St. Andrews microporous material-15) using the tetrapropylammonium cation (TPAþ) as a structure directing agent. That a new material was obtained using reactants and conditions that had previously been investigated is remarkable, and underlines the critical dependence of phase formation on kinetic factors deriving from details of gel chemistry. The new solid was prepared during an ongoing program of research in which combinations of organic bases are examined for their co-templating abilities:21 in the synthesis of STA-15, the addition of low concentrations of large charged molecules tetraphenylphosphonium (TPPþ) or the azaoxacryptand 4,7,13,16,21,24-hexaoxa-1, 10-diazabicyclo[8.8.8]hexacosane (K222) along with TPAþ results in the acceleration of crystallization, without their incorporation. Experimental Section Synthesis. Hydrothermal syntheses were performed by combining sources of potential framework species Al, P (and in some cases Si) with distilled water and organic additives. All reagents (15) Lourenco, J.P.; Ribeiro, M. F.; Ramoa Ribeiro, F.; Rocha, J.; Onida, B.; Garrone, E.; Gabelica, Z. Zeolites 1997, 18, 398. (16) Dumont, N.; Gabelica, Z.; Derouane, E. G.; Di Renzo, F. Microporous Mater. 1994, 3, 71. (17) Felix, D. L.; Strauss, M.; Ducati, L. C.; Pastore, H. O. Microporous Mesoporous Mater. 2009, 120, 187. (18) Maistriau, L.; Dumont, N.; Nagy, J. B.; Gabelica, Z.; Derouane, E.G. Zeolites 1990, 10, 243. (19) Afeworki, M.; Dorset, D. L.; Kennedy, G. J.; Strohmaier, K. G. Chem. Mater. 2006, 18, 1705. (20) Valyocsik, E. W.; von Ballmoos, R. U. S. Patent 4,778,780, 1988. (21) Castro, M.; Garcia, R.; Warrender, S. J.; Wright, P. A.; Cox, P. A.; Fecant, A.; Mellot-Draznieks, C.; Bats, N. Chem. Commun. 2007, 3470.
were used as supplied (Al(OH)3 3 0.2 H2O, Aldrich, 98%; H3PO4, Analar, 85% aq.; fumed SiO2, Fluka, 97%; TPAOH, Aldrich 1M (aq); TPPBr, Aldrich, 97%; K222, ABCR, 97%; Cobalt acetate, Co(Ac)2 3 4H2O, Aldrich). Details are given in Table 1. All gels were stirred at room temperature until homogeneous, then loaded in PTFE-lined stainless steel autoclaves and heated at 190 °C for 72 or 168 h. The initial experiments were performed in PTFE liners (23 mL capacity) that had not previously been used in the synthesis of aluminophosphates: subsequent reactions were performed in liners that were cleaned between syntheses by prolonged washing in hot dilute nitric acid. In the syntheses, after removing autoclaves from the oven and allowing them to cool, the reaction mixtures were suspended in water, and if necessary, the suspensions were sonicated to force separation of crystals from a suspension of amorphous material, which was decanted. In the case of the cobalt-containing preparation, larger crystals of an impurity phase were removed by resonication and exclusion of the coarsest fraction. The products were filtered, washed, and air dried at 60 °C. Characterization and Structure Determination. Laboratory X-ray powder diffraction of the crystalline fraction of these samples was performed on a Stoe STADI P diffractometer, using monochromated Cu KR1 X-radiation, over a period of 1 h. For structural studies, two pure samples of as-prepared STA-15, prepared in different ways (samples d and f of Table 1), were analyzed in Debye-Scherrer mode on beamline I11 at the Diamond Light Source at 100 K.22 In addition, samples of the same two STA-15 materials were calcined at 550 °C for 12 h in flowing oxygen and then loaded into 0.5 mm quartz glass capillaries attached to a vacuum line, where they were dehydrated at 200 °C for 3 h to remove water and sealed under vacuum. One calcined and dehydrated sample (d) was analyzed at 290 K in Debye-Scherrer mode on a Stoe STADI P diffractometer, using monochromated Cu KR1 X-radiation, over a period of 16 h. The other (f ) was analysed in Debye-Scherrer mode at the Diamond Light Source at 100 K under the same conditions as its corresponding as-prepared material. Rietveld analysis of the data was performed using the GSAS suite of programs.23 A single crystal of AlPO4 STA-15 (prepared after 3 days heating of a gel of type d in Table 1 ) was analyzed on a Rigaku diffractometer fitted with a rotating copper anode (Cu KR, λ=1.54178 A˚) and a CCD detector (Supporting Information).
(22) Thompson, S. P.; Parker, J. E.; Potter, J.; Hill, T. P.; Birt, A.; Cobb, T. M.; Yuan, F.; Tang, C. C. Rev. Sci. Inst. 2009, 80, 057107. (23) Larson, A. C.; von Dreele, R. B. Generalised Crystal Structure Analysis System; Los Alamos National Laboratory: Los Alamos, NM, 1998.
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The structure was solved and refined using the SHELXS program and refined using the SHELXL-97 code.24 The morphology of the products was studied by SEM and selected area inorganic chemical analysis was performed by EDX on a JEOL JSM-5600 SEM with a tungsten filament and an Oxford INCA Energy 200 analyser. Electron diffraction was performed on a calcined sample of STA-15 using a JEOL 2010 electron microscope operating at an accelerating voltage of 200 kV. The sample was ground and deposited from a suspension in acetone onto a holey carbon grid. ED patterns were taken directly onto film. Chemical analysis for carbon, hydrogen and nitrogen was performed on as-prepared samples using a CE Instruments EA 1110 CHNS analyser. Solid-state NMR experiments were performed on as-prepared and dehydrated, calcined STA-15 samples using a Bruker Avance III 600 spectrometer, equipped with a widebore 14.1 T magnet, yielding Larmor frequencies of 243.0 MHz for 31P, 156.4 MHz for 27Al and 150.9 MHz for 13C. Samples were packed in conventional 4 mm ZrO2 rotors and rotated at a rate of 10 kHz. For calcined samples, the open rotor was heated overnight at 100 °C prior to collection of the spectra. Chemical shifts are recorded in ppm relative to 85 % H3PO4 for 31P, 1 M Al(NO3)3 (aq) for 27Al and TMS for 13C. For 13C spectra were acquired using cross-polarization, with a contact pulse (ramped for 1H) of 1 ms and 1H decoupling (SPINAL32 with ω1/2π = 100 kHz) applied throughout acquisition. Relative spectral intensities were analysed using the DMFIT program.25 Density functional theory (DFT) calculations were carried out on the AlPO4 framework structure of STA-15 using the experimentally measured structure of calcined STA-15 at 290 K as a starting point. The structure was optimized and NMR parameters were calculated for 27Al and 31P. The CASTEP26,27 code was used, which employs the gauge including projector augmented wave (GIPAW)26 algorithm, to reconstruct the allelectron wave function in a magnetic field. The generalized gradient approximation (GGA) PBE functional was used and the core-valence interactions were described by ultrasoft pseudopotentials. Integrals over the Brillouin zone were performed using a Monkhorst-Pack grid with a k-point spacing of 0.04 A˚-1. Wavefunctions were expanded in planewaves with a kinetic energy smaller than the cut-off energy of 60 Ry. Calculations were performed using the EaStCHEM Research Computing Facility, which consists of 136 AMD Opteron processing cores partly connected by Infinipath high speed interconnects. Typical NMR calculation times were 78 hours using 28 processors. The isotropic chemical shift, δiso, is given by -(σiso - σref), where σiso, is the isotropic shielding. Reference shieldings, σref, of 553.2 ppm, and 280.4 ppm were used for 27Al and 31P, respectively, obtained from previous work.28 The structure was geometry optimized within the CASTEP program (with all atomic positions and the unit cell dimensions allowed to vary). No symmetry restrictions were applied. TGA was performed at 10 °C min-1 in air using a TA Instruments SDT 2960 thermogravimetric analyser. The N2 adsorption isotherm at 77 K was measured for STA-15 (24) Sheldrick, G. M. SHELXL97. Acta Cryst 2008, A64, 112. (25) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70. (26) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys. Cond. Matter 2002, 14, 2717. (27) Pickard, C. J.; Mauri, F. Phys. Rev. B 2001, 63, 245101. (28) C. Ashbrook, S. E.; Cutajar, M.; Pickard, C. J.; Walton, R. I.; Wimperis, S. Phys. Chem. Chem. Phys. 2008, 10, 5754.
Han et al.
Figure 1. Powder X-ray diffraction patterns of the products of reactions a-h of Table 1. Samples d, f and g are pure STA-15.
previously calcined in O2 at 550 °C for 12 h, using a fully automated Hiden IGA gravimetric apparatus. STA-15 was dehydrated at temperatures of 150 °C or 200 °C under vacuum (10-4 Torr) in the IGA instrument prior to adsorption, but this did not affect the uptake. The adsorption isotherms of toluene and n-hexane were measured by following pressure changes on a glass vacuum line of calibrated volumes fitted with PTFE taps. The solvents were dried over molecular sieves and adsorbed gases removed by freeze-thaw cycles and evacuation. The calcined STA-15 was dehydrated under a vacuum of 10-4 Torr and 200 °C for 3 hours prior to measuring the isotherms.
Results and Discussion Synthesis and Characterization of As-Prepared STA-15. Details of some of the hydrothermal syntheses (190 °C for 168 h) are given in Table 1, and powder diffraction patterns are shown in Figure 1. Using only TPAOH as a potential structure directing agent and with a gel composition Al(OH)3 3 0.2 H2O/H3PO4/TPAOH/H2O = 1:1:0.64:70 gave a mixture of the known phase AlPO4-5 and a second phase, subsequently identified as a novel large pore solid and named STA-15. Reducing the Al/P ratio to 0.9 gave STA-15 with a small amount of an unidentified impurity and adding fumed silica to the reactant mixture gave STA-15 of low crystallinity. The addition of small amounts of TPPþ (molar ratio TPPþ/P = 0.108:1) or K222 (K222/P = 0.008:1) gave highly crystalline STA-15 from silica-containing gels (yields of 4050 % on P). (Adding K222 at higher concentrations resulted in the crystallization of AlPO4-42, for which it is a known structure directing agent.29,30) Subsequent reactions (e, f, g) in PTFE liners previously used in the above STA-15 syntheses and subsequently washed in hot dilute nitric acid (29) Schreyeck, L.; D’Agosto, F.; Stumbe, J.; Caullet, P.; Mougenel, J. C. Chem. Commun. 1997, 1241. (30) Maple, M. J.; Philp, E. F.; Slawin, A. M. Z.; Lightfoot, P.; Cox, P. A.; Wright, P. A. J. Mater. Chem. 2001, 11, 98.
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Figure 3. Solid-state 27Al MAS NMR of (below) as-prepared and (above) calcined STA-15.
Figure 2. Representative scanning electron micrographs of STA-15 prepared in the presence of the additive TPPþ after (below) 3 days and (above) 7 days at 190 °C.
gave pure or nearly pure STA-15 in the presence or absence of silica or organic additives, probably as a result of seeding effects because of residual STA-15 left after washing. When preparation b, with Al/P = 0.9 and H2O/P = 70, was repeated with half the original water content, AlPO4-5 crystallized instead. After 72 h of heating the corresponding gels without organic additives, preparations b, c, and g of Table 1 gave very low yields of STA-15 (yield
