223
J . Phys. Chem. 1993, 97, 223-229
Transformations of the All3 Polyoxycation Intercalated in the Layered Lattice of Moo3 Erwin Lalik,' Waclaw Kolodziejski, Anton Led,$and Jacek Klinowski' Department of Chemistry, University of Cambridge, Lensfeld Road, Cambridge CB2 1 EW, U.K. Received: March 16, 1992; In Final Form: October 14, 1992
Moo3 partially and fully intercalated using a solution of aluminum chlorohydrate and AlC13 hydrolyzed with NaOH, both containing [A104Al,2(OH)~4(H~O)~2]7+ polyoxycations, has been studied using 27Almagic-anglespinning (MAS) NMR, 1H-27Alcross-polarization and 27Alquadrupole nutation with MAS. There are three main lines from 6-coordinated Al: a line at 14 ppm and at 0 ppm and a very broad line with a maximum close to 3u,r/2~in the quadrupole nutation spectra. The line at 0 ppm is assigned to Al(H20)63+ ions, and the likely origin of the other two lines is discussed. The chemical shift and the quadrupole coupling constant of the line from 4-coordinated A1 at 62-65 ppm in the partially intercalated sample are different from those in the completely intercalated one. We propose that hydrolytic abstraction of the A13+cation from the bulk of the guest species leads to the formation of an A1124+species. Introduction
The [A104AlI~(OH)24(H~O)l~]7+ polyoxycation is the species most often intercalated into layered host lattices. The central aluminum atom in the tridecamer is coordinated to four oxygens and surrounded by twelve 6-coordinated A1 atoms (Figure 1). This structure is correct for the species in solution and is also found in certain aluminum compounds.' Although it is agreed that the polyoxycation can be introduced between the layers of host compounds from solutions of hydrolyzed aluminum salts, the exact structure of the cation may not always be preserved after intercalation, even before the sample is calcined. The fact that the interlayer distance of an intercalate is larger than that of the host by the diameter of the All3 polyoxycation indicates that the host lattice takes up the Al13unit. 27Almagic-anglespinning (MAS) NMR spectra of All 3-intercalated materials contain a line from 4-coordinated A1 at ca. 64 ppm and a broad line at ca. 0 ppm corresponding to 6-coordinated Al.*-' The position of the resonance from 4-coordinated A1 is similar to that observed for Al13 in solution,s where the line from the 6-coordinated A1 is not observed. However, 6-coordinated A1 in solid Al13can be observed by MAS NMR. The intensity ratio of the lines from 6- and 4-coordinated A1 is often different from the expected value of 12, which may be due to the radio-frequency pulse-width effect or the relative populations of the site^.^,^ The resonance at 0 ppm is a composite containing a peak from the monomeric Al(H20)63+cation and a line from at least one other 6-coordinated species.235 Elemental analysis and measurements of the cation-exchange capacity have shown that the effective charge of the guest species in clays is lower than the nominal value of 7+, and it has been proposed that the actual charge is 4+ ,4.9. IO The host lattice of Al13-intercalatedMoo3 is aluminum-free. Intercalation is easy and, unlike in clays, the XRD pattern of the product shows a number of well-resolved reflection^.^,^ Also, irrespective of whether aluminum chlorohydrate or AlC13 hydrolyzed with NaOH is used for intercalation, the same increase in the interlayer spacing is observed, from 1 1.5 8, in the starting molybdenum bronze to 18.1 A.5 The XRD pattern indicates that the Moo3 layers in the intercalate are shifted with respect to one another, so that the terminal oxygen atoms of the Moo6 octahedra of adjacent layers point toward each other (Figure l), while in thestarting material they point toward the spacebetween the Moo6 octahedra of the neighboring layers.5 In view of the Present address: Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 1, 30-239 Krak6w, Poland. 1 On leave from the Walther-Meissner-Institute, 8046 Garching, Germany. +
bL
18.1
H
Figure 1. Schematic representationoftheAl13polyoxycation intercalated into MOO3.
highcrystallinity of thesamples, theorientationof theAl13species between the layers (see Figure 1) could be determined using onedimensional Fourier analysis." Crystallinity of the intercalate decreases upon heat treatment, with the formation of aluminum molybdate starting at ca. 400 OC (XRD), and we have reported preliminary NMR work for as-synthesized and heat-treated sample^.^ Here we discuss the results of conventional27AlMAS NMR, IH-2'Al cross-polarization (CP) with MAS, andz7A1MAS quadrupole nutation experiments, carried out with A1 polyoxycations introduced into a layered host lattice of Moo3 from different solutions and for partial and complete intercalation. Experimental Section
The sodium form of the molybdenum bronze was prepared by soaking Moo3(recrystallized at 600 OC in the flow of oxygen for 24 h) in an aqueous solution of Na2Mo04and Na2S2O4according to Thomas and McCarron.I2 The X-ray diffraction (XRD) pattern of the as-prepared material is in agreement with that given in the original report, giving the interlayer distance of 1 1.5 8, for the fully hydrated material. Two intercalating solutions
0022-3654/58/2091-0223%04.00/0 0 1993 American Chemical Society
224 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993
Lalik et al.
*'AI
2 7 ~ MAS 1 NMR
CPIMAS
0.0
64.6
1
14.4
sample 3
sample 3
62.3
I
- 0.5
14.3
sample 1
- 0.2
sample 1
80
60
40
20
0
-20
-40
ppm from AI(H20)e3+ Figure 3. 27A1CP/MAS NMR spectra of samples 1 and 3. 100
50
0
- 50
- 100
ppm from AI(H20)63+
Figure2. Z7AI MAS NMRspectraof theAll3polyoxycation intercalated into MoO3 from ACH (sample l), partially intercalated from NaOH/ AICI, (sample 2), and completely intercalated from NaOH/AICII (sample 3).
were used. Solution I (0.1 mol/dm3 in Al) was prepared by dissolving powder aluminum chlorohydrate, ACH (Reheis; 47% of AlzOl) in water, and aged at 80 OC for 2 h. Solution I1 with OH/Al = 2.0 and 0.02 mol/dm3 in Al, was prepared by slow hydrolysis of AIC13.6H20 (Aldrich; 99.8%) with dropwise added dilute NaOH. A low A1 concentration was used to ensure that All3 species, rather than lower oligomers, are generated during hydrolysis.I3 For complete intercalation, two samples of solid bronze were treated for 24 h at room temperature with an excess ofsolution I (sample 1) andsolution I1 (sample 3) without stirring.
Partially intercalated sample 2 was made using a smaller volume of solution 11. Each preparation step was monitored by XRD, which showed the presence of unconverted molybdenum bronze in sample 2. 27AlNMR Bloch decays were recorded a t 104.26 MHz with very short, 0.7-ws (less than lo0), radio-frequency pulses and 0.5s recycle delays. Zirconia MAS rotors 4 mm in diameter were driven by nitrogen gas at 16 kHz. The magic angle was set precisely by observing the 79Br resonance of KBr.14 lH-27Al CP/MAS spectral5were recorded with a singlecontact, a contact time of 500 ws, a IH 90° pulse of 3.4PS, a recycle delay of 5 s, and a spinning rate of 7 kHz. The Hartmann-Hahn condition was established in one scan on a sample of pure and highlycrystalline kaolinite using similar acquisition parameters. Because only the central (J/2 +1/2) transition is observed, excitation
-
The Journal of Physical Chemistry, Vol. 97, No. I , 1993 225
Al13Polyoxycation Intercalated in Moo3
-
F2 cross sections
27A1 liquid
63.1
- 0.2
11.5
342 kHz
- 0.2
105 kHz
ppm from AI(H,0)c3+ F,
cross - sections
j
15.0 ppm
x5
50
100
b
- 50
ppm from AI(H20),3+ Figure4. 27AlNMR spectra of (a) ACH solution and (b) NaOH/AICIJ solution. *'AI
quadrupole nutation sample 1
9.5 ppm
-02
h 4.5 ppm
x5
2
- 0.2 ppm
x l
400 ppm kom w4$)63'
Figure 5. 27Alquadrupole nutation spectrum of sample 1 intercalated from the ACH solution.
300
200
100
kHz Figure 6. Cross-sectionsof the Z7Alquadrupole nutation spectrumshown in Figure 5: (above) along the F2 axis; (below) along the FIaxis.
226 The Journal of Physical Chemistry, Vol. 97,No. I , I993
Lalik et al. -05
*'AI
A
quadrupole nutation sample 2 140
62 3
72 kHz
152*!ikHz
80
60
40
20
0
-20
-40
F2 ppm from A1(H20)63*
Figure 7. 27Alquadrupole nutation spectrum of sample 2 partially intercalated from NaOH/AICI3 solution.
is selective and therefore the Hartmann-Hahn condition is where YAI and YH denote the gyromagnetic ratios of 27Aland IH and B is the radio-frequency field strength. Quadrupole nutation spectra16 were measured with a radiofrequency field o,r/27r > 100 kHz (the exact values are specified with the spectra) and with MAS at 12 kHz. A total of 48 data points were collected in the t l dimension in increments of 1 ps. A sine-bell-squared apodization and zero filling were used in both dimensions, and the FIDs were doubly Fourier transformed in the magnitude mode.
Results and Discussion 27AlMAS NMR spectra of intercalated samples, shown in Figure 2, contain resonances from 4-coordinated A1 at 62-65 ppm and sharp lines at between +15 and -15 ppm from 6-coordinatedAl. The latter overlap with a very broad line from 6-coordinated Al, ranging from ca. +10 to -100 ppm and displaying a typical quadrupolar line shape. The sharp peak due toA12(Mo04)3at-14.7 ppm5hasnot beendetected. Theintensity ratio of the lines from 6- and 4-coordinated A1 is 11.4,8.2, and 10.5 for samples 1-3, respectively. The intensity in the 6-coordinated region is probably underestimated because of the extensive wings of the broad line, which are difficult to integrate. The resonances at 62-65 ppm have been previously assigned to 4-coordinated A1 in the aluminum polyoxycation.2 Among the sharp peaks resolved between + 15 and -1 5 ppm, only those at ca. 14 and 0 ppm are observed for all the samples (Figure 2). On the basis of its position, we assign the peak at 0 ppm to monomeric Al(H20)63+.Being very mobile, the cation is not expected to cross-polarize, and this is confirmed by the iH-27A1CP/MAS spectra of samples 1 and 3 (Figure 3). We shall concern ourselves with the assignment of the sharp peak at 14 ppm and the broad feature in the background, both appearing in the 6-coordinated spectral region. CP works best for A1 sites of low mobility, located close to protons, such as 6-coordinated A1 of Ali3, formally linked to two
OH groups and one H2O molecule. We note that in sample 3 CP makes the sharp 6-coordinated A1 line at 14.4 ppm markedly stronger than the broad line at ca. + 10 to -50 ppm and also than that of 4-coordinated A1 at 64.3 ppm (see spectra without and with CP in Figures 2 and 3). The latter two are missing in the CP spectrum of sample 1, where two sharp lines of 6-coordinated A1 appear at 14.3 and 10.6 ppm. CP is inefficient for 4-coordinated A1 and therefore not responsible for the line at 64.3 ppm (Figure 3), since it is located far away from protons in the center of the polyoxycation and unconnected to OH groups or H20. We conclude that the sharp peak at 14 ppm cross-polarizes much better than the broad line in its background. The CP/MAS peak at 10.6 ppm (Figure 3) and the MAS peaks at +10 and -1 3 ppm (Figure 2) either correspond to minor speciescointercalated with All3 or to some species formed during intercalation. The former is fairly certain to occur in sample 1, since such species are known to exist in the ACH solution and we have detected them by Z7Al NMR. Thus the spectrum of diluted AlC13/NaOH solution (Figure 4a) contains only two sharp lines: at 63 ppm .and at 0 ppm, assigned to tridecamer and Al( H20)63+,respectively.* However, in the spectrum of ACH solution (Figure 4b), the same two are present together with two very broad features at 7 1.4 ppm and at 11.5 ppm. The latter is probably a composite, as indicated by the shoulder peak at 7.9 ppm. The two broad lines (Figure 4b) correspond to different species, both other than Al13.8 In the solid, the 7 1.4 ppm oligomer is apparently absent, but the other species may be cointercalated with Al13, and will therefore be responsible for the 10.6 ppm line in the ACH sample. We also note that two extra peaks at 9.1 and 4.5 ppm are present in the t7Alquadrupole nutation spectrum of sample 1 but absent for samples 2 and 3 (Figures 5-10), thus confirming that the intercalation from the ACH solution is not a "neat" process. It is probable that the lines at 9.8 and 9.6 ppm (Figure 2), at 10.6 ppm (Figure 3), and at 9.1 ppm (Figures 5 and 6), all belong to the same species, since the small variation of thespectral position can be explained by theexternal referencing and by a different overlap with the neighbor resonances. Thequadrupolenutation spectra (Figures 5-10) arevery helpful
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 221
Al13Polyoxycation Intercalated in MOO,
F, cross-sections
F, cross-sections
- 0.5
c_ij
62.3 ppm
14.0 ppm
~
410 kHz
~
io
60
4-0
io
0
-io - i o
- 0.5 ppm
ppm from AI(HzO),~+
. 400
300
200
7
100
kHz Figure 8. Cross-sections of the 27Alquadrupole nutation spectrum shown in Figure 7: (a, left) along the
for further line assignment. The technique permits 27Alenvironments with different quadrupole coupling constants to be resolved along the FI axis and allows the estimation of the asymmetry of the relevant environments considering that stronger quadrupolar interaction (less symmetric chemical environment) corresponds to a higher line intensity at 3wrr/2* within the same F I cross-section. The broad line overlapping with the 14 ppm peak and with the 0 ppm peak of Al(HzO)6,+ is now separated along the FIaxis and shows the maximum at 2.9-, 2.7-, and 2.8wrr/ 27r for samples 1-3, respectively. The other resonances are located mainly at wrr/27r. In the F2 dimension the maximum of the broad line of sample 1 appears at ca. 5 ppm, while for samples 2 and 3 the maximum is difficult to find. We suggest two possible assignments of the 6-coordinated spectral region. According to the first, the broad line at 3wrr/2ir and the resonance at 14 ppm come from two different 6-coordinated A1 sites in the same intercalated polyoxycation. This is in accordance with the observation that both lines are present in the spectra of all the samples studied. The differences in the cross-polarization and in the quadrupole coupling constants could be explained as follows. The polyoxycation (Figure 1) contains four parallel planes in which the 6-coordinated A1 atoms are arranged. After intercalation, the outermost planes are nearer
F2 axis;
(b, right) along the F, axis.
to the layers of Moo3 than are the middle planes, so that some of the 6-coordinated A1 atoms are in the neighborhood of Mo atoms, while others (also 6-coordinated) are not." Thus the former are connected to Mo and to other A1 atoms, thereby experiencing a higher electric field gradient (less symmetric environment) than those in the central plane which are located in a purely A1 neighborhood. This would rationalize a difference in chemical shift and quadrupole coupling constant of the *'A1 lines. If bound to the Mo layer, the A1 polyoxycation will probably lose a water molecule, so that the sites located close to the Mo layer will cross-polarize worse than those in the middle plane. It follows that the broad asymmetric line may be associated with the A1 sites located next to the Moo3 layer, while the sharp and more symmetric line a t 14 ppm may correspond to A1 in the middle planes. We note that the value of 14 ppm is fairly high as compared to the reported positions for the line of 6-coordinated A1 in Al13 (below 7.1 ppm'v4). Furthermore, the magnitude mode of the quadrupole nutation spectra tends to display the sharp lines better than the broad one. One must also consider the low intensity of the 14 ppm line in the conventional MAS spectrum (Figure 2). The second possible assignment implies therefore that the narrow line a t 14 ppm comes from a different species than the broad line.
Lalik et al.
228 The Journal of Physical Chemistry, Vol. 97, No. I , 1993 27Al quadrupole nutation sample 3 0
A
64.6
70 kHz
I 2 n = 135 5 kHZ
.
80
60
40
20
0
.
-20
.
.
-40
F2 ppm from Al(H20)~~’
Figure 9. 27Alquadrupole nutation spectrum of sample 3 completely intercalated from NaOH/AICIj solution.
F2 cross sections
F1 cross sections
x l
64.6 ppm
w
x2
h
14.0 ppm
375 kHz
1
80
40
0
- 40
400
1
1
300
200
100
kHz
ppm from AI(H20)63+
Figure 10. Cross-sections of the 27Al quadrupole nutation spectrum shown in Figure 9.
The high intensity of the latter suggests that it could be assigned to all 6-coordinated A1 sites of the intercalated polyoxycations. We speculate that the peak at 14 ppm could correspond to some
oligomer species formed from Al(H20)b3+ions during intercalation, since this peak is absent from the spectrum of the AlClj/ NaOH solution (Figure 4) but is present in the MAS (Figure 2)
Al13Polyoxycation Intercalated in Moo3
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 229 means that the latter corresponds to a more symmetric environment. We conclude that the symmetry of the central A1 atom in the intercalate decreases with the progress of intercalation and that intercalated All3 polyoxycations undergo some chemical transformation. We note that during the aging of AlI3in solution, resulting in a formation of a “defective”A112polyoxycation,17the 4-coordinated A1 line shifts from 62.9 ppm for Al13to 64.5 ppm for A112in the aged solution, which is in good agreement with the change on going from a partially to a fully intercalated Moo3 in our MAS spectra (Figure 11). With one of the A1 octahedra missing, the symmetry of the central tetrahedron decreases,since it is now surrounded by 11 octahedra instead of 12, and this may explain our quadrupole nutation results. We therefore suggest that the All3 guest species loses one of its A1 octahedra as intercalation progresses, possibly by a hydrolytic abstraction of a1u min u m
2 7 ~ 1MAS
64.4
[A104A1,2(OH)24(H20),2]7+ + 5H20=
I“;
partial intercalation (sample 2)
[A104A1,1(0H)24(H20), 114’ + A1(H20)63+ A13+cations formed between the layers are mobile and may be exchanged by fresh All3 cations from the solution, which in turn undergo A1 abstraction. In this way, the host lattice will be enriched in A1124+rather than in A113’+. Because of its lower charge, a larger number of Al12~+ cations is necessary to neutralize the layer charge. The effective charge of the intercalated species of 4+ instead of 7+ is in agreement with the work made on pillared clays.9J0 We have postulated a similar hydrolytic decomposition of the [Bi(OH)12]6+guest species in M003.~
Acknowledgment. We are grateful to Shell Research, Amsterdam, and Unilever Research, Port Sunlight, for support. References and Notes
75
io
65
60
55
So
ppm from AI(H20)63+ Figure 11. Expansion of the *’A1 MAS N M R lines from 4-coordinated AI sites in samples 2 and 3.
and in quadrupole nutation spectra (Figures 7-10) of samples 2 and 3. The Al(H20)63+ions necessary for that to happen could come from the intercalation solution itself or from the decomposition of the All3 polyoxycations (vide infra). Expansion of the MAS spectrum of samples 2 and 3, prepared from AlCls/NaOH (Figure 11) reveals that the line of 4-coordinated A1 consistsof two resonances, at 62.3 and 64.4 ppm, with the former dominant in the partially intercalated sample and the latter in the fully intercalated material. The relevant quadrupole nutation spectra (Figures 7-10) detect one line of 4-coordinated Al. In the FIdimension the 64.6 ppm line of sample 3 is spread toward 2wrr/2u more than the 62.3 ppm line of sample 2. This
(1) Kunwar, A. C.; Thompson, A. R.;Gutowsky, H. S.;Oldfield, E. J . Magn. Reson. 1984, 60, 467. (2) Fripiat, J. J. Catal. Today 1988, 2, 281. (3) Anderson, M. W.;Klinowski, J. Znorg. Chem. 1990, 29, 3261. (4) Plee, D.; Borg, F.; Gatineau, L.; Fripiat, J. J. J . Am. Chem. Sot. 1985, 107, 2362. (5) Lerf, A.; Lalik, E.; Kolodziejski, W.;Klinowski, J. J . Phys. Chem. 1992, 96, 7389. (6) Pinnavaia, T. J.; Landau, S . D.; Tzou, M.-S.; Johnson, I. D. J . Am. Chem. SOC.1985, 107, 7222. (7) Deng, Z.; Lambert, J.-F. H.; Fripiat, J. J. Chem. Mater. 1989,1,640. (8) (a) Akitt, J. W.;Farthing, A. J . Magn. Reson. 1978, 32, 345. (b) Akitt, J. W.;Mann, B. E. J . Magn. Reson. 1981, 44, 584. (9) Pinnavaia, T. J.; Tzou, M.-S.; Landau, S.D.; Rayathatha, R.H. J . Mol. Cafal. 1984, 27, 195. (10) Figueraz, F. Catal. Reu.-Sci. Eng. 1988, 30 (3), 457. (11) Nazar, L. F.; Liblong, S . W.;Yin, X.T. J. Am. Chem. Soc. 1991, 113, 5890. (12) Thomas, D. M.; McCarron, E. M., 111. Mater. Res. Bull. 1986, 21, 945. Elders, J. M. J . Chem. Soc., Dalton Tram. 1988, 1347. (13) Akitt, J. W.; (14) Frye, J. S.;Maciel, G. E. J . Magn. Reson. 1982, 125, 48. (15) (a) Blackwell, C. S.;Patton, R. L. J . Phys. Chem. 1984,88, 6135. (b) Morris, D. H.; Ellis, P. J . Am. Chem. Sot. 1989, 1I I , 6045. (c) Rocha, J.; Liu, X.;Klinowski, J. Chem. Phys. Left. 1991, 182, 531. (16) Samoson, A.; Lippmaa, E. J . Magn. Reson. 1988, 79, 255. (17) Fu, G.; Nazar, L. F.; Bain, A. D. Chem. Mater. 1991, 3, 602.
