ULM-18, a Fluorinated Gallium Phosphate with ... - ACS Publications

8588

J. Phys. Chem. B 1998, 102, 8588-8598

ULM-18, a Fluorinated Gallium Phosphate with Perforated Layers: XRD and NMR, Structure Determination, and HF Localization in a D4R Francis Taulelle,*,† Ago Samoson,‡ Thierry Loiseau,§ and Ge´ rard Fe´ rey§ RMN et Chimie du Solide, UMR 7510 ULP-Bruker-CNRS, UniVersite´ Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France, Institute of Chemical Physics and Biophysics, AKAD TEE 23, Tallinn 0026, Estonia, and Institut LaVoisier, IREM UMR 173, UniVersite´ de Versailles-Saint Quentin, 45, AVenue des Etats-Unis, 78035 Versailles Cedex, France ReceiVed: April 14, 1998; In Final Form: July 3, 1998

Ga4(PO4)5HF-1.5 N2C6H18-H2O (or ULM-18) is an oxyfluorinated gallium phosphate obtained by hydrothermal synthesis using N,N,N′,N′-tetramethylethylenediamine as a template. The structure is described in the triclinic space group P-1 (n°2) with a ) 8.5264(7) Å, b ) 9.2512(7) Å, c ) 17.870(2) Å, R ) 101.742(7)°, β ) 99.137(7)°, γ ) 87.020(6)°, V ) 1362.3(2) Å3, and Z ) 2. The two-dimensional framework is built up from sheets containing double four-ring units (D4R) connected by PO4 tetrahedra. Along [010] and [011], 8-ring channels makes the layers hollow. The D4R units consist of four PO4 tetrahedra, three GaO4F trigonal bipyramids, and one GaO5 trigonal bipyramid. The D4R structure contains a templating fluorine. 19F MAS at high speed and 71Ga DOR lead to elucidation of their environment. 31P NMR MAS, RFDR, and double-quanta analyses are used to assign phosphorus. 31P-{1H} CPMAS shows that an hydrogen is inside the D4R unit. Those features together make this building unit a quite unusual D4R conformation. For sake of the rationalization of the framework construction, this D4R is compared to known MPO D4R (M ) Al, Ga, Fe, V, Co), and the results of this comparison suggest a general scheme for proton insertion in the structure.

Introduction The use of fluorides in hydrothermal synthesis is of great interest for producing microporous materials. Since 1986, Guth and Kessler1 have developed this route for the study of the crystallization of zeolites at different pH values. The fluoride method has also been extended to the preparation of phosphatebased compounds.2 Some of these phases have the same structures as those of aluminosilicate zeolites (for example, GaPO4-LTA). However, new three-dimensional frameworks have exclusively been obtained by using fluorine. The best example is cloverite,3 an oxyfluorinated gallium phosphate with three-dimensional 20-ring channels system. Since 1992, we focused our attention on the synthesis of new fluorinated phosphates (noted ULM-n) in the systems M2O3P2O5-HF-amine-H2O (M ) Al, Ga, Fe).4-7 In these phases, the fluorine is directly involved in the coordination sphere of the metal atom M and also interacts strongly with the terminal amino group of the organic template. In our recent studies, we undertook different syntheses which implies the use of mixture of amines molecules during the reaction process. In this context, a fluorinated gallophosphate noted ULM-16 has been obtained in the presence of the cyclohexylamine and the tripropylamine in the starting mixture. Only the cyclohexylamine molecules are trapped in the 16-ring channels of the ULM-16 framework. To see the influence of the tripropylamine in this mixture, attempts were made by using other amine templates as the N,N,N′,N′-tetramethylethylenediamine. In this paper, we describe the synthesis and structural characterization of a new * To whom correspondence should be addressed. E-mail: taulelle@ chimie.u-strasbg.fr. † RMN et Chimie du Solide. ‡ Institute of Chemical Physics and Biophysics. § Instiut Lavoisier.

oxyfluorinated gallophosphate or ULM-18, with intercalated diprotonated N,N,N′,N′-tetramethylethylenediamine. Experimental Section Synthesis. The title compound was prepared by hydrothermal synthesis under autogenous pressure in the presence of N,N,N′,N′tetramethylethylenediamine and cyclohexylamine. The reactants were gallium oxyhydroxide (GaO(OH) was prepared by direct reaction of gallium metal with water at 220 °C under autogenous pressure for 3 days), phosphoric acid (85% H3PO4, Prolabo RP Normapur), fluorhydric acid (40% HF, Prolabo RP Normapur), N,N,N′,N′-tetramethylethylenediamine (99%+, Aldrich, noted TMEDA), and cyclohexylamine (99%+, Aldrich, noted CHA). The molar ratio was 1 Ga:1 P:1 F:0.5 TMEDA:0.65 CHA:40 H2O. The chemical composition proposed by the formula obtained by diffraction is for fluorine 1.96% (in mass). Elemental analysis of fluorine made gives exactly the same results 1.96%. Attempts have then been carried out for obtaining ULM-18 as a pure phase by using tripropylamine (98%, Aldrich, noted TPA) instead of cyclohexylamine. The molar ratio was 1 Ga:1 P:1 F:0.5 TMEDA:0.6 TPA:40 H2O. The reaction has been performed in a similar way to that using the mixture containing CHA. For both preparations, the pH of the reaction is 5. For the formation of the ULM-18 phase, the TPA or CHA molecules are used as a pH buffer. In the absence of these molecules, another phase is obtained and its structure determination is in progress.8 The mixture was placed, without stirring, in a PTFE-lined stainless steel autoclave, heated at 180 °C for 24 h, and then cooled to room temperature over 24 h. The product was filtered off, washed with distilled water, and dried at room temperature. Examination under an optical microscope revealed two white

10.1021/jp9818481 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/29/1998

Fluorinated Gallium Phosphate

J. Phys. Chem. B, Vol. 102, No. 43, 1998 8589

TABLE 1: NMR Acquisition Conditions nucleus resonating frequency reference probehead radiofrequency field

19

31

F

470 MHz CFCl3 MAS 2.5 mm 100 kHz

volume Z density (calculated) absorption coefficient F(000) crystal size crystal faces absorption corrections transmission factors theta range for data collection index ranges reflections collected independent reflections refinement method data/restraints/parameters goodness-of-fit on F∧2 final R indices [I > 2sigma(I)] R indices (all data) largest diff. peak and hole

P{1H}

202 MHz 85% H3PO4 MAS 4 mm 62.5 kHz

TABLE 2: Crystal Data and Structure Refinement for ULM-18 identification code empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions

31

P

ULM-18 C9 H28.50 F Ga4 N3 O21 P5 967.58 293(2) K 0.71069 Å triclinic P-1 a ) 8.5264(7) Å b ) 9.2512(7) Å c ) 17.870(2) Å R ) 101.724(7)° β ) 990.137(7)° γ ) 87°.020(6)° 1362.3(2) Å3 2 2.359 Mg/m3 4.311 mm-1 959 0.361 × 0.152 × 0.551 mm {010}, {001}, {100} Gaussian method Tmin ) 0.168, Tmax ) 0.472 2.25-29.98° -11 e h e 11, -12 e k e 12, 0 e l e 25 7225 7225 [R(int) ) 0.0000] full-matrix least-squares on F2 7225/0/395 0.598 R1 ) 0.0279, wR2 ) 0.0962 R1 ) 0.0279, wR2 ) 0.0962 1.279 and -1.278 e.Å-3

crystalline phases: transparent small plateletshaped crystals corresponding to ULM-18 and an unidentified powdered component. Due to their morphology difference, these two phases can be easily separated. Thermal Analysis. Upon heating, the thermogravimetric analysis indicates that the gallophosphate begins to decompose at 350 °C. It is assumed that the compound continuously loses H2O, HF, and TMEDA molecules. At 600 °C, powder X-ray diffraction pattern of the residue shows broad peaks corresponding to a mixture of GaPO4 (crystobalite form) and Ga2O3. Nuclear Magnetic Resonance. NMR has been run on Bruker MSL 300 and DSX 500. 31P has been acquired in MAS high power decoupling and CPMAS at 7.05 T. 31P RFDR and double quanta experiments as well as 71Ga DOR and 19F; 1H MAS have been acquired at 11.7 Ts. The Bruker probeheads used were MAS 4 mm double bearing with decoupling doubletuned circuitry for the MSL and the DSX and 2.5 mm MAS spinning up to 35 kHz for fluorine experiments on the DSX. A probe for DOR experiment modified since its first design9 allows for a coupling to a special pneumatic unit with control and automatic stabilization of inner and outer rotors speeds. The outer rotor has been controlled between 1400 and 1800 Hz with a inner/outer ratio of ca. 4.1. The pulse programs used were single-pulse experiment (SPE) for MAS acquisition, and high power decoupling (MAS-HPD) or cross-polarization (CPMAS), RFDR and double quanta, the

71

Ga

121 MHz NH4H2PO4 MAS 4 mm 62.5 kHz

98 MHz Ga(NO3)3/H2O MAS 2.5 mm/DOR 250 kHz/50 kHz

TABLE 3: Atomic Coordinates (×104) Equivalent Isotropic Displacement Parameters (Å2 × 103) for ULM-18. U(eq) Is Defined as One-Third of the Trace of the Orthogonalized Uij Tensor atom type

x

y

z

U(eq)

Ga(1) Ga(2) Ga(3) Ga(4) P(1) P(2) P(3) P(4) P(5) F(1) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15) O(16) O(17) O(18) O(19) O(20) OW N(1) N(2) N(3) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9)

3954(1) 1619(1) 5051(1) 4986(1) 2277(1) 3736(1) 2677(1) 3362(1) 5085(1) 4168(2) 535(2) 2473(2) 2975(2) 3043(2) 5203(2) 3359(2) 3952(2) 2216(2) 1856(2) 1438(2) 3924(2) 3505(2) 2785(2) 5816(2) 4444(2) 1826(2) 3324(2) 5593(2) 5769(2) 5973(3) 7478(3) 2912(3) -323(3) 662(3) 4678(3) 2331(4) 2272(4) 877(3) -598(4) -199(5) -83(4) -1369(5) -585(5)

2693(1) 6823(1) 4539(1) 8813(1) 3258(1) 4560(1) 6558(1) 9478(1) 1210(1) 6844(2) 3360(2) 2864(2) 4772(2) 2065(2) 5400(2) 3339(2) 3800(2) 5541(2) 6446(2) 6431(2) 5278(2) 8068(2) 10881(2) 1586(2) 9859(2) 8770(2) 11197(2) 10425(2) 2776(2) 10331(2) 11149(3) 4230(2) 11824(3) 11456(3) 4222(3) 4438(4) 2846(3) 11699(4) 11389(4) 12708(4) 10027(4) 10934(4) 13418(4)

227(1) 8182(1) 7433(1) 7696(1) 1758(1) -1101(1) 6593(1) 9151(1) 6652(1) 8074(1) 1817(1) 906(1) 2159(1) 2198(1) -706(1) -704(1) -1940(1) -1165(1) 5768(1) 7121(1) 6621(1) 6853(1) 9649(1) 354(1) 8609(1) 8654(1) 6377(1) 7330(1) 6847(1) 5983(1) 4908(1) 4770(1) 9000(1) 6962(1) 4958(2) 3972(2) 4896(2) 7823(2) 8133(2) 6664(2) 6570(2) 9320(2) 9294(3)

9(1) 10(1) 12(1) 13(1) 10(1) 10(1) 11(1) 9(1) 14(1) 19(1) 20(1) 17(1) 16(1) 19(1) 14(1) 15(1) 16(1) 17(1) 16(1) 18(1) 17(1) 18(1) 12(1) 14(1) 17(1) 14(1) 22(1) 21(1) 18(1) 27(1) 48(1) 18(1) 23(1) 23(1) 20(1) 34(1) 32(1) 29(1) 30(1) 38(1) 39(1) 40(1) 42(1)

latest being a C7 double-quanta variant10 experiment. Other NMR acquisition conditions are reported, for all nuclei, in Table 1. Structure Determination. A suitable single crystal of ULM18 was isolated for the X-ray diffraction experiment. Its quality was tested by optical observation and Laue photographs. Intensity data were collected on a Siemens AED2 four-circle diffractometer. The conditions of the data measurements are summarized in Table 2. Intensities were corrected for Lorentz and polarization effects, and an absorption correction based on the crystal morphology was applied. Atomic scattering factors and anomalous dispersion coefficients were taken from reference.11 The structure was solved by direct method using the program SHELXS-86 (option TREF 20).12 Gallium and phosphorus atoms were first located and all the remaining atoms except hydrogen were revealed from the Fourier difference maps

8590 J. Phys. Chem. B, Vol. 102, No. 43, 1998

Taulelle et al.

TABLE 4: Selected Bond Legths [Angstroms] and Angles [Degrees] for ULM-18 Ga(1)*O(14) Ga(1)*O(2) Ga(1)*O(6) Ga(1)*O(5)#1 Ga(1)*O(13)#2 Ga(2)*O(8)#3 Ga(2)*O(16) Ga(2)*O(10) Ga(2)*O(1)#4 Ga(2)*F(1) P(1)*O(1) P(1)*O(2) P(1)*O(3) P(1)*O(4) OW‚‚‚H(6C) OW‚‚‚O(20) OW‚‚‚O(17) OW‚‚‚O(9) O(14)-Ga(1)-O(2) O(14)-Ga(1)-O(6) O(2)-Ga(1)-O(6) O(14)-Ga(1)-O(5)#1 O(2)-Ga(1)-O(5)#1 O(6)-Ga(1)-O(5)#1 O(14)-Ga(1)-O(13)#2 O(2)-Ga(1)-O(13)#2 O(6)-Ga(1)-O(13)#2 O(5)#1-Ga(1)-O(13)#2 O(8)#3-Ga(2)-O(16) O(8)#3-Ga(2)-O(10) O(16)-Ga(2)-O(10) O(8)#3-Ga(2)-O(1)#4 O(16)-Ga(2)-O(1)#4 O(10)-Ga(2)-O(1)#4 O(8)#3-Ga(2)-F(1) O(16)-Ga(2)-F(1) O(10)-Ga(2)-F(1)

1.853(2) 1.865(2) 1.868(2) 1.919(2) 2.004(2) 1.827(2) 1.832(2) 1.839(2) 1.854(2) 2.213(2) 1.503(2) 1.525(2) 1.540(2) 1.542(2) 2.70 2.72 2.84 2.86 121.94(8) 118.53(8) 118.41(8) 98.21(7) 92.10(8) 90.12(7) 90.40(7) 85.82(7) 83.10(7) 170.89(7) 113.78(9) 127.60(9) 115.83(9) 94.10(9) 98.91(8) 94.03(8) 85.95(7) 86.60(7) 81.19(7)

syntheses. The geometry of the cyclohexylamine molecule was not found in the structure. The structure was refined by fullmatrix least-squares analysis using the SHELX-93 program.13 The hydrogen atoms on the template molecules were placed geometrically (option HFIX of the program SHELX-93). Anisotropic thermal parameters were applied to all atoms except to hydrogen refined isotropically. The refinement of the data leads to final residuals R and R2w were 0.028 and 0.096, respectively for 7225 independent reflections (I > 2((I)). The structure is described in the triclinic space group P-1 (n°2) with a ) 8.5264(7) Å, b ) 9.2512(7) Å, c ) 17.870(2) Å, R ) 101.742(7)°, β ) 99.137(7)°, γ ) 87.020(6)°, V ) 1362.3(2) Å3, Z ) 2. Atomic coordinates, selected bond distances, and angles are respectively listed in Tables 3 and 4. The chemical formula for ULM-18 deduced from the single-crystal X-ray diffraction analysis is Ga4(PO4)5HF-1.5 N2C6H18-H2O. Results Description of the Structure. The structure of ULM-18 consists of macroanionic layers normal to [001] of formula [Ga4(PO4)5HF]3- intercalated with [(CH3)2NHCH2CH2NH(CH3)2]2+ cations and H2O. One of the two basic building units of the inorganic sheet is closely related to the double four-ring cage (D4R) encountered in different zeolite-like materials as Linde A,14 cloverite,3 octadecasil15 ULM-5,4 Mu-1,16 and Mu3.17 It is composed of four PO4 tetrahedra, each sharing three vertexes with three GaO4F trigonal bipyramid and one GaO5 trigonal bipyramid (Figure 1). This unit can be represented as a cube the corners of which are occupied by the gallium and phosphorus atoms alternatively. The four types of phosphorus

Figure 1. Polyhedral representation of the asymmetric unit. Medium gray circle indicate the encapsulated fluorine.

atoms are in nearly regular tetrahedral surrounding with P-O distances within 1.503-1.556 Å. A fluorine atom is occluded in the D4R cage and affects drastically the surrounding of the gallium atoms. The polyhedron around of one of the four gallium atoms (Ga(1)) is a distorted trigonal bipyramid (Ga-O ) 1.853-1.868 Å (trigonal plane), Ga-O ) 1.919-2.004 Å (apex)). At first glance, the three other gallium atoms (Ga(2), Ga(3), and Ga(4)) could be considered as being tetrahedrally coordinated with Ga-O distances in the ranges 1.818-1.880 Å. But the fluorine atom inside the octameric unit strongly modifies the O-Ga-O angles of these three gallium atoms. The fluorine is off center within the cube and is preferentially linked to three gallium atoms: Ga(2) (Ga-F ) 2.213 Å), Ga(4) (Ga-F ) 2.246 Å), and Ga(3) (Ga-F ) 2.353 Å). It results in significant distortion for the polyhedron around these gallium atoms, (O-Ga-O ) 94.03-127.60°) from the ideal tetrahedron. In the vicinity of the gallium atoms, the fluorine tends to increase their coordination number to five leading to a trigonal bipyramid which better represents the coordination state of these gallium. The distance to the fourth gallium (Ga(1)) is much longer (Ga-F ) 3.157 Å) and thus prevents significant Ga(1)-F interactions. To finish the description of the structure, one has to mention an additional PO4 tetrahedron sharing only two of its vertexes with two D4R units (dP(5)-O(18) ) 1.529 Å; dP(5)-O(19) ) 1.543 Å). The other two remaining oxygen atoms are terminal with two different P-O distances: one (O(17)) is quite short (dP(5)-O(17) ) 1.504 Å) and is typical of a phosphoryl PdO group;18 the other (O(20)) must correspond to a hydroxyl group



Figure 4. Representation of the hydrogen bond interactions between the asymmetric unit and the N,N,N′,N′-tetramethylethylenediamine and water molecules.

Figure 2. Projection of the structure of ULM-18 near [100]. Open circles indicate water molecules. Hydrogen atoms of the organic template are omitted for clarity.

Figure 5. 19F MAS spectrum, referenced at 0 ppm/CFCl3, MAS spinning speed 30 kHz.

Figure 3. Projection of the structure of ULM-18 near [010] showing the 8-ring channels. Open circles indicate water molecules. Hydrogen atoms of the organic template are omitted for clarity.

due to the lengthening of the P-O distance (dP(5)-O(20) ) 1.580 Å). Bond-valence calculations19,20 for the O(20) oxygen indicates that it has unsatisfied valence (i.e., 1.10 valence units instead of 2 for O2-) and this is in good agreement with the presence of a neighboring hydrogen shared with the water molecule nearby (O(w)). The D4R units linked together with the isolated PO4 tetrahedra, generate sheets corresponding to the stacking of two octameric units along the [001] direction (Figure 2). This gives rise to the formation of 8-ring channels parallel to the layer plane along [010] (Figure 3) and crossing with 8-ring channels along [011]. It makes the layers hollow and this is quite specific to this structure. The sheets are separated by the N,N,N′,N′tetramethylethylenediamine and the water molecules. To achieve charge balance, the organic molecules must be protonated twice. The cohesion of the structure, Figure 4, is ensured through preferential strong hydrogen bonds between terminal oxygen

O(9) of tetrahedron P(3)O4 and hydrogen of ammonium group of the intercalated TMEDA cation (dO(9)‚‚‚H(1)-N(1) ) 1.74 Å). The water molecule is hydrogen bonded to terminal O group of the P(5)O4 tetrahedron (dP(5)-O(20)-H‚‚‚Ow ) 2.72 Å). A second N,N,N′,N′-tetramethylethylendiamine molecule is trapped in the 8-ring channels along [011]. The two distinct nitrogen atoms form hydrogen bonds to terminal oxygen O(17) of P(5)O4 subunit (dO(17)‚‚‚H(3)-N(3) ) 1.73 Å) and to bridging oxygen O(13) of P(4)-O(13)-Ga(1) bond (dO(13)‚‚‚H(2)-N(2) ) 1.99 Å). The proximity of the ammonium group near this bridging oxygen contributes to increase the Ga-O distance up to 2.004 Å (apex) of the trigonal bipyramid (the opposite apical Ga-O distance is 1.919 Å). At last, if the inorganic framework has the formula [Ga4(PO4)5HF]3- and the diamines are protonated in ammonium groups, the localization of the one hydrogen left in the inorganic structure is to be elucidated. This is the reason of the NMR study. NMR Results MAS 31P, 19F, 1H, and 71Ga. The MAS spectrum of 31P exhibits four peaks at 0.9, -5.1, -10.9, and -18.7 ppm. This MAS spectrum has been used instead of projections in the RFDR experiment, Figure 9. They are in a 1:2:1:1 ratio, reflecting a total of five different atoms of phosphorus. This agrees with the five inequivalent phosphorus atoms proposed by the diffraction analysis. Two of them are accidentally coincident at -5.1 ppm in NMR. A tentative assignment can be done by using the relationship relating 31P chemical shifts to their mean


Figure 6. 71Ga DOR experiment, external rotor spinning speed ) 1600 Hz and (internal/external) rotors spinning speed ratio of 4.1.

Figure 7. 1H MAS spectrum.

d(P-O) distance. The relation is a linear dependence between mean distance and chemical shift:

δ iso ) 780 d(P-O) - 1219 with d(P-O) in angstroms and δ iso in ppm relative to 85% H3PO4. From this first estimate, a ranking of chemical shifts from the mean distance obtained by the diffraction analysis is derived. The ranking is monotonic, and even if the parameters of the linear relation are not accurate to describe the actual chemical shift, they allow a first assignment of the sites as follows: P3 at 0.9 ppm, (P5,P4) at -5.1 ppm, P2 at -10.9 ppm, and P1 at -18.7 ppm. The 19F NMR exhibits a signal at about -60 ppm. D4R with fluorine inside is found in octadecasil,15,21 cloverite,3 and AlPO4-16.22 The signal observed is in agreement with values obtained for this type of environment. However, it has, Figure 5, a composite line shape that can be described by a sum of three Lorentzians. This is at contrast to single lines observed in cloverite or AlPO-16 or octadecasil. The factor which may differentiate the fluorine and which is not detected by Xdiffraction might very well be hydrogen localized inside the D4R. The 71Ga MAS spectrum, obtained at a spinning speed of 30 kHz, shows a large massive unstructured signal in the range of 0 ppm. The DOR spectrum of the compound, Figure 6, at contrast, shows two set of lines belonging to two different type of sites at -4.3 and at 29 ppm. A decomposition of the

Taulelle et al. anisotropic pattern by a sum of Lorentzian leads to a area ratio of 3:1 for the -4.3 ppm/29 ppm sites. This agrees well with the two groups of sites with different environments GaP4F and GaP5. The coordination number of gallium is therefore the same for all of them, and there is no reason to infirm the structural proposition of three gallium in pentacoordinated state with a Ga-F bond with the fluorine inside the D4R unit, and a fourth pentacoordinated gallium with five phosphates groups. Chemical shifts of mixed ligands sphere have not yet been reported in gallium, but the values observed are in quite good agreement with a transposition of known equivalent values in aluminum. A1H NMR spectrum is shown on Figure 7. It exhibits broad features indicating that the organic molecules are quite rigidly bound to the inorganic network. Unfortunately, if this characteristic allows for a quite reasonable proposition for the positioning of hydrogen by analysis of the diffraction on the single crystal, it does not give any hint of the localization of the hydrogen present inside the inorganic framework, represented as “HF” in the chemical formula. RFDR and Double-Quanta Analysis of 31P. To obtain more details of the hydrogen-phosphorus localization, a CPMAS experiment is necessary. However, the tentative assignment proposed for 31P is not demonstrative enough to draw too firm conclusions. To get a proper attribution, we decided to compare the homoatomic radial distribution that we can extract from the structural proposition (Figure 8) with the topology that can arise from a dipolar network analysis. Several NMR experiments for elucidating coupling networks have been recently proposed. They make usage of dipolar couplings, taking advantage of the high resolution of MAS, with a necessary reintroduction of the dipolar interaction suppressed by MAS. This is done by a radio frequency driven recoupling23-25 or a double-quanta experiment.10,26,27 Both approaches are exploring the homonuclear dipolar couplings network. A simple magnetization transfer (RFDR) experiment has been performed at different mixing times. Figure 9 illustrates in two of these experiments that the closest neighbors of the atoms considered can give rise to some magnetization transfer. Considering the structure proposed by diffraction, the homoatomic topology network is established by naming the inequivalent phosphorus by their crystal number. The first tentative NMR assignment of phosphorus is therefore used. If it reveals correct, at short mixing time the closest neighbors should have nonnegligible off-diagonal intensities. The homoatomic distribution is given as first neighbors analysis in the Table 5. The off-diagonal components between 3-(4,5), and (4,5)-2, and (4,5)-1 is consistent with the shortest distances and the number of atoms within this distance. As the unresolved sites (4,5) participates to all the off-diagonal peaks of the RFDR experiment at short mixing time, some ambiguity is still present. The main advantage of a double-quantum experiment compared to RFDR is that it suppresses all single quantum contribution and leaves exclusively double quantum contributions. Such a feature has been proposed to double-quantum filter magnetization transfer experiments.27 We have collected double-quanta 2D experiments with pairs of C7 blocks. Figure 10 presents the pulse sequence as used. With the scheme proposed, the number of unambiguous neighbor in the analysis increases, since Pi-Pi neighbors are included. P2-P2 neighboring is thus revealed, confirming the assignment. But to get a better diagnosis of the assignment, one may consider what line should be on display in the 2D DQ experiment if the assignment is correct. Lines observed and



Figure 8. Homoatomic radial distribution of phosphorus from the X-ray diffraction results. Number of neighboring phosphorus as a function of the radius in angstroms.

TABLE 5: First-Neighbor Distances between Atoms of Phosphorus, within a 5 Å Sphere P3 P3 P5 P4

4.8338 4.7733

P2

4.7862

P1

4.8013

P5

P4

P2

P1

4.8338

4.7733

4.7862 4.8005 4.4746 4.8391 4.1174

4.8013

3.7846 4.8005

4.4746 4.8391 4.7032

4.7032 4.4641

4.4641

lines expected have been plotted on Figure 11. To plot such expected lines one has to know which Pi is close to which Pj in order to get a significant dipolar coupling constant. Then a pair of line is expected on the x-axis of the plot located at each δPi and δPj frequency, and their common position on the y-axis is at (δPi + δPj)/2. If the y ) x line is drawn on the plot, the Pi neighbor of Pi will have a peak on that line and the Pi-Pj neighbors will exhibit a off line pair. The double quanta may develop through the whole sample depending on the dipolar network. This property has been used to elucidate the presence of chains in fluoroapatites.28 In our sample the homoatomic radial distribution, plotted in Figure 8, indicates that there is for almost all the phosphorus a gap at 5 Å. One may therefore expect that, by increasing the number of evolution and refocusing periods, we can get all the pairs predicted by diffraction.

Figure 11 reveals the DQ pattern. It has some analogy with a diffraction experiment in a way. Some lines connecting PiPj must be missing (extinction conditions, but not bound here to the space group but to the topology), here P1-P1, P3-P3, and P5-P5 must miss. The latter extinction cannot be evidenced because of P4-P4 signal at the same place. As in a diffraction experiment, some other signals are present. However, not all are necessary to fix the definitive topology. Of the 10 lines that should be observed only one, P1-P2, is not observed. Of the five lines that should be absent, two are absent, P3-P3 and P1-P1, and three are ambiguous because of the superposition of P4 and P5. If another assignment was correct it would be obtained by a permutation of chemical shift and sites. No other matching permutation could be found, indicating now a correct assignment between crystallographic and NMR sites. CPMAS and MAS Decoupled 31P {1H}. Results of 31P CPMAS are plotted on Figure 12. At short contact time, there is a fast rise of magnetization followed by a slow one and then a decrease due to 1H T1F. At the border between the fast rising part and the slow rising one, there is a slight decrease, like a hump. The latter indication points to two different 1H T1F and TH-P. Cross-polarization curves of similar shape have been obtained on cloverite.29 A good simulation has been obtained thus with two independent variable contact time curves. The


Taulelle et al. TABLE 6:

31P

phosphorus site fast/slow rising amplitudes no. of P deduced from CPa TH-P short T1F short TH-P long T1F long

{1H} CPMAS P3 P4,5 P2 P1 (0.9 ppm) (-5.1 ppm) (-10.9 ppm) (-18.7 ppm) 0.54 0.52 0.45 0.50 1.4

1.8

1

0.7

0.25 ms 3.3 ms 6.5 ms 78 ms

0.24 ms 3.3 ms 7.9 ms 78 ms

0.39 ms 3.3 ms 8.5 ms 97 ms

0.36 ms 3.3 ms 8.3 ms 93 ms

i i a P deduced from CP is defined as Pi ) (ai fast + aslow)/∑(afast + aislow).

λ)

Figure 9. RFDR experiments: (a) with a mixing time of 5.3 ms, (b) with a mixing time of 20 ms.

general equation used for a single site has the following form:30

MP(t) )

γH H 1 M (1 - e-(1-λ)t/THP)e-t/T1F γP P0 1 - λ

with

Figure 10. RFDR and DQ, C7 pulse sequences.

THP H T1F

≈0

One constraint has been imposed on fitting the curves. The value of T1F of the first, fast rising component is fixed to the same value for all the site simulations. It has been estimated in examining the possible values giving rise to the small bump between the fast rising part of the magnetization curve and the slow one. Only very few values around 3.3 ms ((0.1 ms) fulfill this condition. With such constraints and fit, the results obtained, Table 6, are the following: P5, P3, and P2 have very close hydrogen atoms that can give rise to a fast rising transfer of magnetization due to a large dipolar interaction. P4 has also probably a fast rising component because the amount of magnetization for the site at -5.1 ppm, common to P4 and P5, is about twice P3 or P2 components, for the fast as well as the low rising component. But for the P1 site, existence of an obvious close hydrogen is not indicated by the structure determination. However, a fast rising component is present as well as the slow component due to the distant organic molecules. But the fast rising component can only be explained by the presence a very close hydrogen. The only



Figure 12. 31P variable contact CP MAS 1H f 31P experiments at a field of 7.05 Teslas. [ 18.7 ppm site, 1 -10.9 ppm site, 9 -5.1 ppm, 2 0.9 ppm site.

into three components, indicating some disordered behavior of the hydrogen inside of the cavity. Furthermore, one can expect two situations for this D4R insider H+. It may stay where it is, or move around, jumping between possible sites. In the latter case, dipolar interaction between H and P would be averaged to a small value, which is not the case. This is also confirmed by multiplicity of fluorine signals. Discussion

Figure 11. DQ, C7 experiment, (a) expected Pi-Pj pairs in open circle and observed reported on black dots; (b) 2D double-quantum experiment with an evolution and reconversion period of 81.5 µs (10 C7 cycles).

sites able to accept hydrogen are located between the central fluorine and the surrounding oxygen inside the D4R. There are several internuclear F-O distances between 2.6 and 2.8 Å, characteristic of a strong hydrogen bond. The fluorine is bound to three gallium atoms, Ga(2,3,4). No positions having hydrogen around F close to a tetrahedral bond topology of three gallium and one hydrogen is possible. Is the central fluorine associated to a hydrogen to form a HF molecule localized at the center of the D4R? The charge compensating H+ must be localized inside the cavity, but it is not clear if this is enough to state that HF or a H+-F- ion pair is present in the D4R unit.21 The hydrogen may be in one single place for every D4R or its location may change due to different possible sites available. MAS fluorine NMR answers that question nicely. If D4R inner H+ would be in a single place, one would get, like in previous studies of D4R units with fluorine inside, a single line. As can be seen in Figure 5, the fluorine signal can be broken down

D4R units have been observed on alumino- or gallophosphate microporous compounds as well on silicates, vanadates, iron, or cobalt compounds.3,15,17,22,31-41 AlPO4-1631 and CoAPO-50 (AFY)40 are reported to be able to be synthesized without fluorides. They are described as being without an ion inside the D4R unit of the structure. In the case of AlPO4-16, the refinement of the structure may has overpass the necessary OH- and its associated H+. Both may appear as well as H2O. Catlow21 has suggested, supported by a theoretical calculation, that in the case of octadecasil, if OH or F is the central anion, OH might be more exchangeable than F. For AlPO4-16, a trace of such a behavior would lead in the solid to a more dispersed electronic density for OH than for F. OH, therefore, might not be detected in the structure. When fluorine is used instead, this exchange is slowed and the fluoride anion well localized in the D4R unit. AlPO4-16 has been characterized by Bennett and Kirchner31 without definite localization of the anion, whereas Schott-Darie et al.22,32 have synthesized the same compound in fluorinated conditions with F- inside the D4R. We suggest that OH- is probably inside the D4R unit in the case of Kirchner’s AlPO4-16. In the case of CoAPO, the charge balance need not to get an ion as a charge compensator inside D4R, and this may be the case of other structures with a charge lower than the equivalent AlPO or GaPO. Several D4R with a fluoride inside the D4R are reported and their plot is reported on Figure 13. The status of the Al-F or Ga-F or in general M-F bond is neither clear nor properly described. The case of cloverite is typical. Fluoride is described as being in the center of the D4R and both D4R units are mostly quoted as not differentiated. Further examination of the structure leads to a clear distinction of both D4R with one with fluoride bonded to four gallium of the D4R and the other one with the fluorine bonded to three gallium. Octadecasil has a fluoride in the center with no bonding with the outer atoms which are silicon atoms for which a five coordination number is unlikely. A recent structure from Eckert42 indicates that even in the case of silicon Si-F bonding may lead to five coordinated states. GaPO-LTA34 has four Ga-F bonds in the D4R. ULM-535 presents two Ga-F in its


Taulelle et al.

Figure 13. D4R of differents structures.

D4R. Mu-317 exhibits four Ga-F bonds and connects each D4R by one phosphorus and one gallium along its diagonal. It forms chains with only two interconnections between D4R, in contrast to other structures where all the eight corners of a D4R are

connected. ULM-17 39exhibits a fluorine inside both D4R units, and the vanadium polyhedra are in octahedral coordination with edge sharing dimers. The fluorine are connected to the four vanadium.

Fluorinated Gallium Phosphate ULM-18 has a 5:4 P:Ga ratio. It is a quite original case of D4R. It has a chemical formulation close to that of the gallium phosphate [(CH3)2NHCH2CH2NH(CH3)2]2+-[Ga4P5O20H]2-H2O reported by Chippindale et al.43 However, the latter structure does not contain a D4R unit. No structural comparison can be made on the basis of the chemical similarity. ULM-18 has three of the gallium polyhedra in a pentacoordinated state comparable to most of the D4R already mentioned. However, the last gallium polyhedron though pentacoordinated is linked to five phosphate groups, and not like the others to four phosphates and a fluorine. This one is linked to three phosphate groups belonging to the D4R and is connected by two other vertexes to the single phosphate linking D4R between them and to a phosphate group belonging to another D4R. There is no way in which this gallium might be linked to fluorine inside the D4R unit. We found only one structure with a comparable peculiarity, though topologically different. This is in an iron microporous compound, first approximately characterized by DeBord et al.37 and later by Lii.38 The fluorine is also connected to the four iron of the D4R. In cloverite, a CPMAS study has been run by Klinowski’s group29,44,45 and an NMR study of cloverite hydration made by Bradley.46 The limited resolution of 31P spectrum, due to too many different 31P sites, and an quite undefined state of the crystallographic structure when hydrated, unfortunately precludes any firm chemical conclusions. It appears that POH bond exist. In the quite detailed analysis of 1H-31P and the reverse 31P-1H transfer of magnetization29 the T P-H are in agreement with ours. The authors attribute this POH bond to an accessible one external to the D4R which is plausible, but no discussion has been done on hydrogen localization inside the D4R unit able to house a hydrogen in a manner comparable to the one observed in this study. The hydration state renders probably highly difficult such an analysis in cloverite. Two aspects are responsible for the number of F-M bonds inside a D4R unit, the topological constraints of the overall framework and the local presence or absence of an inside hydrogen. The first reason is due to the M/P ratio of the structure that imposes topological restrictions. To the second is due the pH of synthesis as it imposes some protonation state of the building blocks and of the templating species. This form of hydrogen localization may very well be quite general in microporous compounds. A detailed examination of the D4R units, especially related to the fluoride bonding to the metallic atoms of the unit as well as the cross polarization behavior may answer the localization question. The acidity of such a site might be quite high. The question of its accessibility is still opened. Actually, the mobility of the hydrogen inside the D4R seems low, at least in our case with fluorine inside. The case might be different if OH would have been inside; the lability of OH compared to F might be transferred to the charge compensator H+. The hydrolysis of Ga-O-P bonds, suggested by Klinowski, producing external accessible POH might be too high an energy process. Another possible way to promote accessibility of H+ would be to have water localized on the D4R, by a hydrogen bonding to a POGa bridging oxygen, allowing proton jumps from the inside of the D4R to the external H2O. Conclusions ULM-18 structure has been described by a combination of diffraction and NMR techniques. A single-crystal analysis has allowed to propose a well-defined starting structure. The 31P

J. Phys. Chem. B, Vol. 102, No. 43, 1998 8597 RFDR and double quantum have allowed us to assign NMR inequivalent and XRD inequivalent phosphorus. CPMAS 31P and 19F experiments have demonstrated that a hydrogen is localized inside the D4R unit of this structure. It must be very acidic, although its accessibility might be limited by its diffusion through the D4R limits. Examination of fluorine environments in a series of D4R units leads to the conclusion that occlusion of a fluoride inside the D4R proceeds through covalent bonds formation. These bonds may be the trace of the templating role of fluorine postulated by several authors. In the case of ULM18, the possibility that F- ion or HF molecule might be the templating agent remains open and will probably need to be the object of theoretical considerations. Without detailed information of the structure on the special places where the synthesis mechanisms have played a key role, it would be illusionary to get a proper retrosynthetic analysis. The aim of our approach is to combine synthesis understanding, in in-situ conditions,47 as well as getting traces of the last steps of the mechanism in the structure of the solids to try to access to the synthetic pathways, especially during nucleation, the most difficult step to resolve in formation of solids. In ULM-18 one may propose that the fluorine forms the reticulation node on which the D4R is formed, followed by an insertion of a H+ in the unit, decreasing the negative charge of the building block unit, to permit an infinite condensation of the solid. Acknowledgment. The authors thank Prof. M. Leblanc (University of Le Mans) for his help in the X-ray diffraction data collection. Prof. C. Ja¨ger from Jena is thanked for his help in getting the DQ 31P experiment working on our NMR spectrometer and Vincent Munch for acquiring some DQ spectra. Profs. Lii and Morris for providing preprint of their work concerning GaPO and FePO containing D4R. Dr. D. Palmer is thanked for his Crystalmaker version handling cloverite, allowing us to visualize the two different D4R. Elemental analysis has been done in “Service d’analyse de Vernaison, CNRS”. CNRS GdR 1164 funding has allowed to cover the expenses of this academic study that would have not been possible without it. Supporting Information Available: Tables of crystal data, atomic coordinates, bond lengths and bond angles, and anisotropic displacement parameters for ULM-18 (9 pages). Ordering information is given on any current masthead page. References and Notes (1) Guth, J. L.; Kessler, H.; Wey, R. Stud. Surf. Sci. Catal. 1986, 28, 121. (2) Kessler, H.; Patarin, J.; Schott-Darie, C. Stud. Surf. Sci. Catal. 1994, 85, 75. (3) Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merrouche, A.; Kessler, H. Nature 1991, 352, 320. (4) Ferey, G. J. Fluor. Chem. 1995, 72, 187. (5) Renaudin, J.; Ferey, G. J. Solid State Chem. 1995, 120, 197. (6) Loiseau, T.; Ferey, G. J. Mater. Chem. 1996, 6, 1073. (7) Cavellec, M.; Riou, D.; Ninclaus, C.; Greneche, J.-M.; Ferey, G. Zeolites 1996, 17, 250. (8) Loiseau, T.; Taulelle, F.; Fe´rey, G. Unpublished results. (9) Samoson, A.; Pines, A. ReV. Sci. Instrum. 1989, 60, 3239. (10) Lee, Y. K.; Kurur, N. D.; Helmle, M.; Johannessen, O. G.; Nielsen, N. C.; Levitt, M. H. Chem. Phys. Lett. 1995, 242, 304. (11) Hahn, T. International Tables of Crystallography; Kluwer: Dordrecht, 1987. (12) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (13) Sheldrick, G. M. SHELXL-93, a Program for the Refinement of Crystal Structure Determination; University of Go¨ttingen: Germany, 1993. (14) Simmen, A.; Patarin, J.; Baerlocher, C. 9th International Zeolite Conference 1993, 1, 433.

8598 J. Phys. Chem. B, Vol. 102, No. 43, 1998 (15) Caullet, P.; Guth, J. L.; Hazm, J.; Lamblin, J. M.; Gies, H. Eur. J. Solid State Inorg. Chem. 1991, 28, 345. (16) Kallus, S.; Patarin, J.; Marler, B. Microporous Mater. 1996, 7, 89. (17) Reinert, P.; Patarin, J.; Loiseau, T.; Ferey, G.; Kessler, H. Microporous Mater. 1998. (18) Jones, R. H.; Chippindale, A. M.; Srinivasan, N.; Thmas, J. M. Chem. Commun. 1994, 565. (19) Brown, I. D. The bond valence method: an empirical approach to chemical structure and bonding. In Structure and Bonding in Crystal; A., O’Keefe, M., Alexandra,N., Bloch, A. N., Eds.; Academic Press: New York, 1981; Vol. 2, p 1. (20) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192. (21) George, A. R.; Catlow, C. R. A. Zeolites 1997, 18, 67. (22) Schott-Darie, C.; Patarin, J.; Le Goff, P. Y.; Kessler, H.; Benazzi, E. Microporous Mater. 1994, 3, 123. (23) Bennett, A. E.; Griffin, R. G.; Ok, J. H.; Vega, S. J. Chem. Phys. 1992, 96, 8624. (24) Bennett, A. E.; Rienstra, C. M.; Lansbury, P. T., Jr.; Griffin, R. G. J. Chem. Phys. 1996, 105, 10289. (25) Sun, B.-Q.; Costa, P. R.; Kocisko, D.; Lansbury, P. T., Jr.; Griffin, R. G. J. Chem. Phys. 1995, 102, 702. (26) Baldus, M.; Tomaselli, M.; Meier, B. H.; Ernst, R. R. Chem. Phys. Lett. 1994, 230, 329. (27) Geen, H.; Johannes, G.; Graf, R.; Shnell, I.; Spiess, H. W.; Titman, J. J. J. Magn. Reson. 1997, 125, 224. (28) Gyunggoo, C.; Yesinowski, J. P. J. Phys. Chem. 1996, 100, 15716. (29) Kolodziejski, W.; Klinowski, J. J. Phys. Chem. B 1997, 101, 3937. (30) Mehring, M. Principles of High-Resolution NMR in Solids, 2nd ed.; Springer-Verlag: Berlin, 1983.

Taulelle et al. (31) Bennett, J. M.; Kirchner, R. M. Zeolites 1991, 11, 502. (32) Schreyeck, L.; Caullet, P.; Mougenel, J. C.; Patarin, J.; Paillaud, J. L. Microporous Mater. 1997, 11, 161. (33) Feng, P. Y.; Bu, X. H.; Stucky, G. D. Nature 1997, 388, 735. (34) Reinert, P.; Schottdarie, C.; Patarin, J. Microporous Mater. 1997, 9, 107. (35) Loiseau, T.; Ferey, G. J. Solid State Chem. 1994, 111, 407. (36) Wragg, D. S.; Hix, G. B.; Morris, R. E. 1998. Personnal communication, and manuscript in preparation. (37) DeBord, J. R. D.; Reiff, W. M.; Warren, C. J.; Haushalter, R. C.; Zubieta, J. Chem. Mater. 1994, 9, 1194. (38) Lii, K.-H. J. Porous Mater. 1998. (39) Riou, D.; Taulelle, F.; Ferey, G. Inorg. Chem. 1996, 35, 6392. (40) Marcus, J. M. B. a. B. K. ACP1 CoAPO-50 (AFY). In InnoVation in Zeolite Materials Science, 1988. (41) Reinert, P.; Patarin, J.; Kessler, H. 1998. To be published. (42) Koller, H.; Wolker, A.; Eckert, H.; Panz, C.; Behrens, P. Angew. Chem. 1998, 36, 2823. (43) Chippindale, A. M.; Walton, R. I.; Turner, C. Chem. Commun. 1995, 1261. (44) Kolodziejski, W.; Klinowski, J. Chem. Phys. Lett. 1997, 266, 597. (45) Barr, T. L.; Klinowski, J.; He, H. Y.; Alberti, K.; Muller, G.; Lercher, J. A. Nature 1993, 365, 429. (46) Bradley, S. M.; Howe, R. F.; Hanna, J. V. Solid State NMR 1993, 2, 37. (47) Haouas, M.; Gerardin, C.; Taulelle, F.; Estournes, C.; Loiseau, T.; Ferey, G. J. Chim. Phys. 1998, 95, 302.

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