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27Al&{1H} cross-polarization and ultrahigh-speed 27Al MAS NMR spectroscopy in the characterization of USY zeolites
Volume 182, number 2
CHEMICAL PHYSICS LETTERS
26 July 1991
27A1(‘H) cross-polarization and ultrahigh-speed 27A1MAS NMR spectroscopy in the characterization of USY zeolites Lars Kellberg a, Magnus Linsten b and Hans J. Jakobsen a a Department ofChemistry, Univenity ofAarhus, DK-8000Aarhus C, Denmark b Eka NobelALl, S-445 01 Bohus, Sweden Received 8 March 199 1;in final form 9 May I99 1
?41 MAS NMR spectroscopy with ‘H cross-polarization is shown to be an efficient technique fordistinguishing non-framework from framework Al in USY zeolites and has revealed the presence of non-framework tetrahedral Al. Field-dependent measurements and simulations show that the non-framework 30 ppm 27A1resonance observed for USY zeolites must be considered an individual signal and not part of a second-order quadrupole lineshape as recently suggested. Ultrahigh-speed spinning (I!,= 1820 kHz) is demonstrated to be a prerequisite for obtaining quantitative results for these systems by 2’Al MAS NMR. Finally, experimental evidence for the similarity between the non-framework Al phases in USY zeolites and steamed silica-alumina is presented.
1. Introduction Since the discovery of ultrastable Y (USY) zeolites [ I], the relationships between structure and properties of these important catalytic materials have been extensively studied [2,3]. The structural changes accompanying hydrothermal dealumination have been followed especially by 29Si and *‘Al MAS NMR spectroscopy [ 4-61. 29Si MAS NMR has been used for determination of the framework Si/Al ratio, F(Si/Al) [2,3], a quantity strongly related to the degree of stabilization and catalytic properties. For USY zeolites, F(Si/Al) increases as a function of steaming temperature and time, whereas the total bulk Si/Al ratio, as measured by XRF, PIXE, etc., remains constant even for highly dealuminated samples [ 41. Thus, relatively high amounts of nonframework aluminum (NFA) must be present in these zeolites. However, the presence of Si-OH groups, as evidenced by 29Si{‘H} cross-polarization (CP) MAS NMR [5,7], may throw doubt on the accuracy of F(Si/Al) determined by 29Si MAS NMR. The reason is that signals from, e.g., (SiO),SiOH groups overlap with the signal from Si( 1Al) units 120
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resulting in a too-low calculated F( Si/Al) for highly dealuminated zeolites. Although the detailed structure of USY zeolites and the nature of the NFA species could vary according to the exact steaming conditions, their 27A1MAS NMR spectra generally consist of signals at approximately 60 ppm, assigned to framework Al (FA), and at about 30 and 0 ppm attributed to NFA [ 8-101. These observations suggest “Al MAS NMR as being a simple method for determination of the NF,4%, but the values obtained are consistently too low [ 2-41. The concept of “NMR-invisible Al” introduced in such cases must be associated with NFA sites having large quadrupole interactions. Recently 2D quadrupole nutation NMR [ 8,111 have contributed to the understanding of the nature and quantitative aspects for the NFA, i.e. the AINFA/Altota’%.However, according to one of these studies [ 8 1, we note that the signal at 30 ppm, assigned to a pentacoordinated Al species [ 91, should not be considered an individual resonance but as part of a tetrahedral second-order quadrupolar-broadened 27A1lineshape. This Letter shows that the recently introduced method of 27Al{‘H} CP/MAS NMR [ 12,131 contributes to understanding the nature of NFA in USY 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
Volume 182.number 2
CHEMICALPHYSICSLETTERS
zeolites and, thereby, the quantitative aspects of 27A1 MAS NMR for these materials. Furthermore, we show that ultrahigh-speed spinning (v,~ 18 kHz),
compared to the spinnings speeds employed sofar, significantly changes the intensities of the resonances. This technique significantly improves the quantitative analysis of USY zeolites based on 27A1 MAS NMR and was inspired by the fact that 27A1 quadrupolar couplings as large as 15 MHz may be determined and studied quantitatively using v, z 20 kHz [14].
2. Experimental A NaY zeolite of Si/A1=2.46 (sample 1) from Eka Nobel AB constituted the starting material for the preparation of all hydrothermal dealuminated (USY) zeolites in this work. Sample 1 was ammonium-ion exchanged at 80°C and then calcined at 600°C under 1 atm of steam for 2 h in a 2 cm deep bed (sample 2). Sample 2 was subjected to a second ammonium-ion exchange at 80°C followed by an aluminum-ion exchange at room temperature in a 77 mM aluminum sulphate solution [ 151 and finally calcined at 650°C under 1 atm of steam for 2 h in a 2 cm deep bed (sample 3). The sample of silicaalumina (sample 4) was obtained from Grace (grade 135) and was calcined (sample 5) using the same procedure as for sample 3. The treatment of the sam-
26 July 1991
ples with acetylacetone (acac) followed the procedure described by Grobet et al. [ 161, i.e. 1 g of hydrated zeolite was soaked in 2 ml of a solution of 38% V/V acac in ethanol at room temperature for 1 h. The excess solution was evaporated and the samples dried in a fume cupboard at room temperature. The results of the XRF analysis, e.g. total Si/Al ratios, for the samples l-3 are shown in table 1. “Al MAS NMR experiments were performed on Varian VXR400 S (9.4 T) and XL-300 (7.1 T) spectrometers at 104.21 and 78.16 MHz, respectively. Homebuilt MAS probes for 7 and 4 mm Si3N, rotors with sample volumes of 225 and 75 ~1 and maximum spinning speeds of 10 and 20 kHz, respectively, were employed. Spectra were acquired using short rf pulses. For the 4 mm probe ( tP(x/2) = 7.0 us for 1.O M AlC13), used to obtain the quantitative results in table 1, spectra obtained using pulse widths of 0.8 us (n/18 pulse), 1.5 ps (rc/9 pulse), 2.0 ps (x/7 pulse), and 2.8 ps (n/5 pulse) all gave identical relative intensities for the observed resonances indicating that their quadrupole interactions are of similar order of magnitude. The quantitative data (table 1) were determined from spectra acquired using a 1.O ps pulse width, vr= 18 kHz and a repetition delay of 1 s. All 27Al{‘H} CP/MAS experiments were performed at 9.4 T using a 7 mm CP/ MAS probe and employing moderate spinning speeds (4-7 kHz) because of the dependence of the optimum Hartmann-Hahn (H-H) conditions on spin-
Table 1 X-ray and MASNMR data for the USY zeolites studied in this work Sample
Unit-cell a,axis”) (A)
Crystalhmty b, (%)
Si/AI (XRF)
G/Al ” (NMR)
NF.4 d’ (%)
Partial NFA ei (%)
AI (96)
1 2 3
24.69 24.51 24.30
100 77 65
2.34 2.35 2.21
2.46 5.1 16
0 54 86
0 52 12
0 3 51
NFA ‘)
6(27A1)*) (ppm) 60.7 59.3, 29.8, 0.8 59.3,29.8, 0.6
a) Determined according to ASTMD3906-85. b, Bulk Si/AI ratio determined according to ASTMD3942-80. ‘) Framework Si/AI ratio determined by deconvolution of 29SiMASNMR spectra. d, Non-frameworkaluminum contents obtained by combining the XRF and 29SiMASNMR data. ‘) Partial non-framework aluminum contents determined from ultrahigh-speed spinning (v,= 18.2kHz) *7AlMAS NMR spectra (9.4 T) by deconvolution, however, neglectingthe presence of tetrahedral NFA in the resonance at 60 ppm. Q AI NFA is the percentage tetrahedral NFA constituting the “AI resonance at 60 ppm; these values are calculated from the NFA (column 6) and partial NFA (column 7) data (see text). 8) Center of gravity ppm values ( t 0.7 ppm) at 9.4 T and u,= 18.2kHz for the resolved signals obtained from the deconvoluted spectra, i.e. *‘AIchemical shifts uncorrected for the second-order quadrupole shifts at 9.4 T.
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ning speed [ 121. For each sample, the H-H condition was determined from an experiment of arrayed ‘H tf field strengths (o,~) at a constant “Al rf field strength (w,*,), and corresponds to WlH%31~0,~~ for the spinning speeds used. Optimized values for the contact time of 300 ps and the relaxation delay of 4 s were used for all CP experiments. Simulations and computations were performed on a Digital VAX62 10 computer.
3. Results and discussion Different stages of the ultrastabilization process for the Y zeolite are illustrated by the 27AlMAS NMR spectra of samples 1. 2 and 3 in fig. 1. For the unsteamed sample (1). only the 60 ppm signal from tetrahedral FA is observed (fig. la). During steaming, different NFA and/or dislodged framework aluminum (DFA) species are formed. This is illustrated in the spectra of samples 2 and 3 (figs. 1b and lc) which both show a resonance at about 0 ppm resulting from the formation of octahedral NFA. Both spectra also show the appearance of a resonance at about 30 ppm in addition to a very broad hump (extending from z-180 to 2230 ppm at 7.1 T and from E - 120 to z 160 ppm at 9.4 T) below the three more distinct resonances. These features are especially pronounced in the spectrum for the more strongly dealuminated sample (3) which also shows that the intensity of the tetracoordinated Al resonance at ~60 ppm has markedly decreased at the expense of an increase for all other resonances indicated above. The “,41 MAS NMR spectra for the dealuminated samples 2 and 3 are very similar to published spectra of USY zeolites [ 2-4,8- 10,171. Since little is known about the NFA and/or DFA phases in USY zeolites, it is not surprising that their 27A1MAS NMR spectra are qualitatively and quantitatively not very well understood. The most widely proposed candidates, forming at least a partial Al phase of NFA in USY zeolites, are [AI(O (3-n)+, AI(H20)2+, aluminas, boehmite, pseudoboehmites and silica-aluminas [ 2-4,9,17 1. To gain insight into the nature of the NFA, an experiment that is able to distinguish between F.4 and NFA/DFA would be welcome. We note that almost all suggested candidates for NFA 122
.
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_r ..(__ .,,,_ .,.._ __, c--J _-. ..-..
200
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0
-loo
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200
100
0
100 porn
Fig. 1. 27A1MAS NMR spectra of USY zeolites recorded at 9.4 T (left column) and 7.1 T (right column) using spinning speeds ~~~7-8 kHz and otherwise identical conditions. The spectra in (a) sample 1. (b) sample 2, and (c) sample 3 are plotted with ratios for the vertical expansion of (top to bottom) I :4: 8 and I : 8: 16 for the 9.4 T and the 7.1 T spectra. respectively, showing a larger second-order linebroadening at 7. I T.
are rich in lattice and/or surface OH groups. Furthermore, lattice defects resulting from the hydrolytic breakdown of the zeolite during steaming could be present as Al (OH), groups (DFA sites). Therefore, 27Al{‘H} CP/MAS NMR [ 12,131 could be an important method in elucidating the nature of NFA/ DFA. Via use of optimized CP parameters, fig. 2 shows a comparison of the *‘Al(‘H} CP/MAS NMR spectrum (fig. 2a) with the ordinary 27A1MAS spectrum (fig. 2b) for sample 3. It is observed that with CP, the intensities of the 60 and 30 ppm resonances become similar whereas the relative intensity of the 0 ppm signal is much larger. We should note that, contrary to the absolute intensities, the relative intensities of the three resonances depend only slightly on
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CHEMICAL PHYSICS LETTERS
nk_J
‘ i : /\ : 1 \
e
L-
60
20 O-20 -60
wm
“doublet” (30 ppm at 9.4 T) should shift its position to - 5.0 ppm at 7.1 T (fig. 2d). However, in the experimental 7.1 T spectrum (fig. 2e), the position of this resonance peak is nearly unchanged (Z 30 ppm). Thus, we conclude that the 60 and 30 ppm signals arise from separate *‘Al resonances. Furthermore, it is concluded that the 60 ppm resonance observed without CP (fig. 2a) consists of overlap of “Al resonances from zeolite FA and NFA/DFA (at least partly as AI( species), a conclusion also reached by Samoson et al. [ 81. We should note that in a CP experiment of sample 1, no resonance is observed from the zeolite FA. Although *‘Al MAS NMR is considered an important tool for achieving information about different Al species in zeolites, its quantitative use for USY zeolites has been very disappointing. Too-low values for the NFA% are consistently obtained by deconvolution of “Al MA.5 NMR spectra, an observation that has been attributed to “NMR-invisible Al”. However, the presence of tetrahedral NFA (observed form the present CP/MAS experiment) overlapping with the FA resonance would account for part of the missing Al. The existence of tetrahedral NFA is consistent with the observation [ 6, lo] that the intensity of the 60 ppm resonance converges towards a limiting value as a function of steaming time, and that it never vanishes as expected if it arises only from zeolite FA. Unfortunately, quantitative estimates of the amount of tetrahedral NFA based on *‘Al{‘Hj CP/MAS and 2D quadrupole nutation NMR [ 8,111 are associated with great uncertainty. To investigate the influence of ultrahigh-speed spinning and its possibility for improving the quantitative results obtained from 27A1MAS NMR, fig. 3 shows a comparison of three spectra for sample 3 recorded using ~~~7.3, 12.5 and 18.2 kHz and otherwise identical conditions. It is seen that the broad hump observed below the three resonances at moderate spinning speeds (fig. 3a) decreases with increasing V, and that it is completely eliminated only at 18 kHz. In addition, the absolute intensities for the three narrow (0, 30 and 60 ppm) resonances simultaneously increase by different percentages. This shows that the hump forms part of all NFA resonances and that their quadrupole interactions require ultrahigh spinning speeds for averaging the central transitions. Thus, quantification (deconvo-
h.__ C
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26 July I99 1
100
60
20 O-20 -60
w
Fig. 2. 17A1MAS NMR spectra (9.4 T) of sample 3 obtained (a) without and (b) with ‘H cross-polarization. Simulations of second-order quadrupole lineshapes for the “doublet” at -60 and 30 ppm m (b) using k72.4 ppm, Co=7.3 MHz, q=O.O are shown for magnetic fields of(c) 9.4 and (d) 7.1 T. For comparison, the 7.1 T “Al MAS NMR spectrum is shown in (e).
the optimum CP conditions, i.e. H-H match and contact time. The nearly equal intensity observed for the 60 and 30 ppm “doublet” is of special interest in relation to the 2D quadrupolar nutation experiments by Samoson et al. [S]. They concluded that the 30 ppm signal should be considered part (the low-frequency component) of a second-order quadrupolarbroadened lineshape (Co>, 6 MHz, LEO [ 8]), i.e., similar to the “doublet” observed in the “Al{‘H} CP/MAS spectrum (fig. 2b). Computer simulation of a second-order quadrupolar lineshape for the “doublet” observed in fig. 2b (i.e., assuming a single *‘Al resonance with q= 0) gives 6( 27Al) = 72.4 ppm and C,Z 7.3 MHz (fig. 2~). More importantly, simulation of the field dependence of this lineshape shows that the low-frequency component of the
123
\y b I! CHEMICAL PHYSICS LETTERS
Volume 182, number 2
a * ;;-_I_ 200
26July 1991
-i;:-::“:--
100
0
-100
200
100
pm
0
-100
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200 100
0 -100
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Fig. 3. *‘Al MAS NMR spectra (9.4 T) of sample 3 recorded at different spinning speeds (4 mm rotor), a pulse width of 1.0 ps and otherwise identical conditions. (a) 7.3 kHz, (b) 12.5 kHz, and (c) 18.2 kHz. All spectra are plotted using the same vertical expansion. Note that a residual hump is still observed at u,= 12.5kHz (b) and that only two low-intensity spinning sidebands from the 0 ppm resonance are left at Ye= 18.2kHz (c). The small peak marked with an asterisk is an Al impurity (Al-N) from the Si,N, rotor material.
lution) of the relative intensities for the three resonances is easier and more reliable for spectra obtained at such rotor speeds. We should note that although the relative intensities for the three resonances as observed in fig. 3 at 12.5 and 18.2 kHz for sample 3 appear quite similar, this need not be the case for other USY zeolites. The spectra in fig. 3 clearly indicate why several published attempts of a quantitative analysis ( AINFA/Al’“‘a’%) using 27Al MAS NMR with moderate spinning speeds are doomed to fail. A comparison between the NFA% determined by deconvolution of the 27Al MAS spectrum in fig. 3c (assuming no NFA at 60 ppm) and the conventional determination by XRF/29Si MAS NMR is shown in table 1. It is noted that the agreement is much better than for the equivalent low-speed spinning results published in the literature [ 16,171. However, to obtain 100% agreement with the XRF/ 29Si MAS NMR results , ~50% of the 60 ppm resonance in fig. 3c must be assigned to tetrahedral NFA/DFA (table 1) in accord with the 27Al CP/ MAS spectrum (lig. 2b). Furthermore, the data in table 1 show that tetrahedral NFA is formed in appreciable amounts only during strong dealumination. In conclusion, ultrahigh-speed 27AlMAS NMR (eventually combined with the highest possible magnetic field strength) appears a prerequisite and the 124
most direct method for obtaining quantitative results of USY zeolites by 27A1NMR. Regarding the nature of the NFA, it is of interest to consider the characteristic changes of the 27AlMAS NMR spectra observed by controlled acac-treatment (i.e., without further dealumination) of USY zeolites [ 161. Fig. 4a shows the 27AlMAS NMR spectra of sample 3 before (left) and after (right) acactreatment. The intensity increase of the 0 ppm signal has been ascribed to complexation of acac with NFA [ 161. However, for the two spectra in fig. 4a, the intensity increase of the resonance at 0 ppm is approximately 12% and no significant intensity reduction of the broad hump is observed. Thus, from the results in fig. 3: the acac method cannot be considered a quantitative 27A1MAS NMR technique for USY zeolites. Fig. 4b shows that amorphous silicaalumina (sample 4), a proposed candidate for the NFA, exhibits the same behaviour upon acac-treatment as sample 3. No such behaviour was observed for y-A&O3 (fig. 4c) which indicates a silica-alumina rather than an alumina-type structure for at least part of the NFA phase (we note that neither yA1203nor sample 4 shows any appearance of a signal at 30 ppm). Further evidence for a silica-alumina phase in USY zeolites is obtained by comparing the spectra of sample 3 (fig. Sa) and a steamed sample 5 (fig. 5b) of the silica-alumina. Both spectra ex-
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CHEMICAL PHYSICS LETTERS
Fig. 4. *‘Al MAS NMR spectra (7.1 T) of (a) a USY zeolite (sample 3), (b) a silica-alumina (sample 4), and (c) y-A&O, before (left column) and after (right column) treatment with acetylacetone (acac; see section 2).
hibit the 30 and 0 ppm signals and a broad background hump. Thus, a high degree of similarity between the Al phases constituting NFA in USY zeolites and Al species in steamed silica-alumina is observed. We note that evidence for non-framework silica in steamed Y zeolites has been obtained from 29Si{IH} CP/MAS NMR [ 51. In conclusion, the importance of *‘Al{‘H} CP/ MAS NMR for distinguishing NFA from FA in USY zeolites combined with the ability to obtain quantitative information on the NFA% using ultrahigh spinning speeds has been demonstrated. It is shown that the 30 ppm signal in *‘Al MAS NMR spectra of USY zeolites should be considered an individual resonance and not part of a second-order quadrupolar lineshape. Finally, experimental evidence for a high degree of resemblance between the Al phases constituting the NFA in USYs and the Al species in steamed silica-alumina has been observed.
i m
26 July 1991
L
I"",,"'l"“l,,"lTml'"'l-,""~ -100 ppm 200 100 0
Fig. 5. Comparison of “Al MAS NMR spectra (7.1 T) of (a) a USY zeolite (sample 3), (b) a sample of steamed silica-alumina (sample 5, see section 2). and (c) the silica-alumina before steaming (sample 4). The spectra are shown with a ratio for the vertical expansionof (top to bottom) 10: 15: 1.
Acknowledgement The use of the solid-state Varian VXR-400 S NMR spectrometer, sponsored by Teknologistyrelsen, at the University of Aarhus NMR-Laboratory is gratefully acknowledged. We thank the Danish Research Councils (SNF and STVF), Carlsbergfondet, Direktclr Ib Henriksen Fond and Aarhus University Research Foundation for equipment grants.
References [ 11 C.V. McDaniel and P.K. Maher, Sot. Chem. Ind. ( 1968) 186. [2] G. Engelhardt and D. Mitchel,High-resolutionsolid-state NMR ofsilicates and zeolites (Wiley, New York, 1987).
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[3] J.M. Thomas and J. Klinowski, Advan. Catal. 33 (1985) 199. [ 41 J. Klinowski, C.A. Fyfe and G.C. Gobbi, J. Chem. Sot. Faraday Trans. 181 (1985) 3003. [ 51 G. Engelhardt, U. Lohse, A. Samoson, M. Magi, M. Tarmak and E. Lippmaa, Zeolites 2 (1982) 59. [ 61G.J. Ray. B.L. Meyersand C.L. Marshall, Zeolites 7 ( 1987) 307. [ 71 L. Kellberg, M. Linsten and H.J. Jakobsen, to be published. [ 81A. Samoson. E. Ltppmaa, G. Engelhardt, U. Lohse and H.G. Jerschkewitz, Chem. Phys. Letters 134 ( 1987) 589. [ 91 J. Gilson, G.C. Edwards, A.W. Peters, K. Rajagopalan, R.F. Wormsbecher, T.G. Roberie and M.P. Shatlock, J. Chem. Soc.Chem.Commun. (1987) 91. [ lo] A. Corma, V. Fort&, A. Martinez and J. Sanz, ACS Symp. Ser. 375 (1988) 17.
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[ 111 P.P. Man, J. Klinowski, A. Trokiner, H. Zanni and P. Papon, Chem.Phys.Letters
151 (1988) 143.
[ 121L. Kellberg, H. Bildsoe and H.J. Jakobsen, presented at the “10th
European
Experimental
NMR
Conference”,
Veldhoven, Holland (May, 1990).
[ 131 H.D. Morris and P.D. Ellis, J. Am. Chem. Sot. I I I (1989) 6045.
[ 1411. Skibsted, H. Bildsoe and H.J. Jakobsen, J. Magn. Reson., in press.
[ 151 M. Linsten and A. Frestad, PCT Int. Appl. WO 89,03, 367 (1989).
[ 161 P.J. Grobet, H. Geens, J.A. Martens and P.A. Jacobs, J. Chem. Sot. Chem. Commun. ( 1987) 1688. [17] P.J. Grobet, H. Geerts, M. Tielen, J.A. Martens and P.A. Jacobs, Stud. Surface Sci. Catal. 46 (1989) 721.
CHEMICAL PHYSICS LETTERS
26 July 1991
27A1(‘H) cross-polarization and ultrahigh-speed 27A1MAS NMR spectroscopy in the characterization of USY zeolites Lars Kellberg a, Magnus Linsten b and Hans J. Jakobsen a a Department ofChemistry, Univenity ofAarhus, DK-8000Aarhus C, Denmark b Eka NobelALl, S-445 01 Bohus, Sweden Received 8 March 199 1;in final form 9 May I99 1
?41 MAS NMR spectroscopy with ‘H cross-polarization is shown to be an efficient technique fordistinguishing non-framework from framework Al in USY zeolites and has revealed the presence of non-framework tetrahedral Al. Field-dependent measurements and simulations show that the non-framework 30 ppm 27A1resonance observed for USY zeolites must be considered an individual signal and not part of a second-order quadrupole lineshape as recently suggested. Ultrahigh-speed spinning (I!,= 1820 kHz) is demonstrated to be a prerequisite for obtaining quantitative results for these systems by 2’Al MAS NMR. Finally, experimental evidence for the similarity between the non-framework Al phases in USY zeolites and steamed silica-alumina is presented.
1. Introduction Since the discovery of ultrastable Y (USY) zeolites [ I], the relationships between structure and properties of these important catalytic materials have been extensively studied [2,3]. The structural changes accompanying hydrothermal dealumination have been followed especially by 29Si and *‘Al MAS NMR spectroscopy [ 4-61. 29Si MAS NMR has been used for determination of the framework Si/Al ratio, F(Si/Al) [2,3], a quantity strongly related to the degree of stabilization and catalytic properties. For USY zeolites, F(Si/Al) increases as a function of steaming temperature and time, whereas the total bulk Si/Al ratio, as measured by XRF, PIXE, etc., remains constant even for highly dealuminated samples [ 41. Thus, relatively high amounts of nonframework aluminum (NFA) must be present in these zeolites. However, the presence of Si-OH groups, as evidenced by 29Si{‘H} cross-polarization (CP) MAS NMR [5,7], may throw doubt on the accuracy of F(Si/Al) determined by 29Si MAS NMR. The reason is that signals from, e.g., (SiO),SiOH groups overlap with the signal from Si( 1Al) units 120
0009-2614/91/$
resulting in a too-low calculated F( Si/Al) for highly dealuminated zeolites. Although the detailed structure of USY zeolites and the nature of the NFA species could vary according to the exact steaming conditions, their 27A1MAS NMR spectra generally consist of signals at approximately 60 ppm, assigned to framework Al (FA), and at about 30 and 0 ppm attributed to NFA [ 8-101. These observations suggest “Al MAS NMR as being a simple method for determination of the NF,4%, but the values obtained are consistently too low [ 2-41. The concept of “NMR-invisible Al” introduced in such cases must be associated with NFA sites having large quadrupole interactions. Recently 2D quadrupole nutation NMR [ 8,111 have contributed to the understanding of the nature and quantitative aspects for the NFA, i.e. the AINFA/Altota’%.However, according to one of these studies [ 8 1, we note that the signal at 30 ppm, assigned to a pentacoordinated Al species [ 91, should not be considered an individual resonance but as part of a tetrahedral second-order quadrupolar-broadened 27A1lineshape. This Letter shows that the recently introduced method of 27Al{‘H} CP/MAS NMR [ 12,131 contributes to understanding the nature of NFA in USY 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
Volume 182.number 2
CHEMICALPHYSICSLETTERS
zeolites and, thereby, the quantitative aspects of 27A1 MAS NMR for these materials. Furthermore, we show that ultrahigh-speed spinning (v,~ 18 kHz),
compared to the spinnings speeds employed sofar, significantly changes the intensities of the resonances. This technique significantly improves the quantitative analysis of USY zeolites based on 27A1 MAS NMR and was inspired by the fact that 27A1 quadrupolar couplings as large as 15 MHz may be determined and studied quantitatively using v, z 20 kHz [14].
2. Experimental A NaY zeolite of Si/A1=2.46 (sample 1) from Eka Nobel AB constituted the starting material for the preparation of all hydrothermal dealuminated (USY) zeolites in this work. Sample 1 was ammonium-ion exchanged at 80°C and then calcined at 600°C under 1 atm of steam for 2 h in a 2 cm deep bed (sample 2). Sample 2 was subjected to a second ammonium-ion exchange at 80°C followed by an aluminum-ion exchange at room temperature in a 77 mM aluminum sulphate solution [ 151 and finally calcined at 650°C under 1 atm of steam for 2 h in a 2 cm deep bed (sample 3). The sample of silicaalumina (sample 4) was obtained from Grace (grade 135) and was calcined (sample 5) using the same procedure as for sample 3. The treatment of the sam-
26 July 1991
ples with acetylacetone (acac) followed the procedure described by Grobet et al. [ 161, i.e. 1 g of hydrated zeolite was soaked in 2 ml of a solution of 38% V/V acac in ethanol at room temperature for 1 h. The excess solution was evaporated and the samples dried in a fume cupboard at room temperature. The results of the XRF analysis, e.g. total Si/Al ratios, for the samples l-3 are shown in table 1. “Al MAS NMR experiments were performed on Varian VXR400 S (9.4 T) and XL-300 (7.1 T) spectrometers at 104.21 and 78.16 MHz, respectively. Homebuilt MAS probes for 7 and 4 mm Si3N, rotors with sample volumes of 225 and 75 ~1 and maximum spinning speeds of 10 and 20 kHz, respectively, were employed. Spectra were acquired using short rf pulses. For the 4 mm probe ( tP(x/2) = 7.0 us for 1.O M AlC13), used to obtain the quantitative results in table 1, spectra obtained using pulse widths of 0.8 us (n/18 pulse), 1.5 ps (rc/9 pulse), 2.0 ps (x/7 pulse), and 2.8 ps (n/5 pulse) all gave identical relative intensities for the observed resonances indicating that their quadrupole interactions are of similar order of magnitude. The quantitative data (table 1) were determined from spectra acquired using a 1.O ps pulse width, vr= 18 kHz and a repetition delay of 1 s. All 27Al{‘H} CP/MAS experiments were performed at 9.4 T using a 7 mm CP/ MAS probe and employing moderate spinning speeds (4-7 kHz) because of the dependence of the optimum Hartmann-Hahn (H-H) conditions on spin-
Table 1 X-ray and MASNMR data for the USY zeolites studied in this work Sample
Unit-cell a,axis”) (A)
Crystalhmty b, (%)
Si/AI (XRF)
G/Al ” (NMR)
NF.4 d’ (%)
Partial NFA ei (%)
AI (96)
1 2 3
24.69 24.51 24.30
100 77 65
2.34 2.35 2.21
2.46 5.1 16
0 54 86
0 52 12
0 3 51
NFA ‘)
6(27A1)*) (ppm) 60.7 59.3, 29.8, 0.8 59.3,29.8, 0.6
a) Determined according to ASTMD3906-85. b, Bulk Si/AI ratio determined according to ASTMD3942-80. ‘) Framework Si/AI ratio determined by deconvolution of 29SiMASNMR spectra. d, Non-frameworkaluminum contents obtained by combining the XRF and 29SiMASNMR data. ‘) Partial non-framework aluminum contents determined from ultrahigh-speed spinning (v,= 18.2kHz) *7AlMAS NMR spectra (9.4 T) by deconvolution, however, neglectingthe presence of tetrahedral NFA in the resonance at 60 ppm. Q AI NFA is the percentage tetrahedral NFA constituting the “AI resonance at 60 ppm; these values are calculated from the NFA (column 6) and partial NFA (column 7) data (see text). 8) Center of gravity ppm values ( t 0.7 ppm) at 9.4 T and u,= 18.2kHz for the resolved signals obtained from the deconvoluted spectra, i.e. *‘AIchemical shifts uncorrected for the second-order quadrupole shifts at 9.4 T.
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ning speed [ 121. For each sample, the H-H condition was determined from an experiment of arrayed ‘H tf field strengths (o,~) at a constant “Al rf field strength (w,*,), and corresponds to WlH%31~0,~~ for the spinning speeds used. Optimized values for the contact time of 300 ps and the relaxation delay of 4 s were used for all CP experiments. Simulations and computations were performed on a Digital VAX62 10 computer.
3. Results and discussion Different stages of the ultrastabilization process for the Y zeolite are illustrated by the 27AlMAS NMR spectra of samples 1. 2 and 3 in fig. 1. For the unsteamed sample (1). only the 60 ppm signal from tetrahedral FA is observed (fig. la). During steaming, different NFA and/or dislodged framework aluminum (DFA) species are formed. This is illustrated in the spectra of samples 2 and 3 (figs. 1b and lc) which both show a resonance at about 0 ppm resulting from the formation of octahedral NFA. Both spectra also show the appearance of a resonance at about 30 ppm in addition to a very broad hump (extending from z-180 to 2230 ppm at 7.1 T and from E - 120 to z 160 ppm at 9.4 T) below the three more distinct resonances. These features are especially pronounced in the spectrum for the more strongly dealuminated sample (3) which also shows that the intensity of the tetracoordinated Al resonance at ~60 ppm has markedly decreased at the expense of an increase for all other resonances indicated above. The “,41 MAS NMR spectra for the dealuminated samples 2 and 3 are very similar to published spectra of USY zeolites [ 2-4,8- 10,171. Since little is known about the NFA and/or DFA phases in USY zeolites, it is not surprising that their 27A1MAS NMR spectra are qualitatively and quantitatively not very well understood. The most widely proposed candidates, forming at least a partial Al phase of NFA in USY zeolites, are [AI(O (3-n)+, AI(H20)2+, aluminas, boehmite, pseudoboehmites and silica-aluminas [ 2-4,9,17 1. To gain insight into the nature of the NFA, an experiment that is able to distinguish between F.4 and NFA/DFA would be welcome. We note that almost all suggested candidates for NFA 122
.
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200
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200
100
0
100 porn
Fig. 1. 27A1MAS NMR spectra of USY zeolites recorded at 9.4 T (left column) and 7.1 T (right column) using spinning speeds ~~~7-8 kHz and otherwise identical conditions. The spectra in (a) sample 1. (b) sample 2, and (c) sample 3 are plotted with ratios for the vertical expansion of (top to bottom) I :4: 8 and I : 8: 16 for the 9.4 T and the 7.1 T spectra. respectively, showing a larger second-order linebroadening at 7. I T.
are rich in lattice and/or surface OH groups. Furthermore, lattice defects resulting from the hydrolytic breakdown of the zeolite during steaming could be present as Al (OH), groups (DFA sites). Therefore, 27Al{‘H} CP/MAS NMR [ 12,131 could be an important method in elucidating the nature of NFA/ DFA. Via use of optimized CP parameters, fig. 2 shows a comparison of the *‘Al(‘H} CP/MAS NMR spectrum (fig. 2a) with the ordinary 27A1MAS spectrum (fig. 2b) for sample 3. It is observed that with CP, the intensities of the 60 and 30 ppm resonances become similar whereas the relative intensity of the 0 ppm signal is much larger. We should note that, contrary to the absolute intensities, the relative intensities of the three resonances depend only slightly on
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nk_J
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e
L-
60
20 O-20 -60
wm
“doublet” (30 ppm at 9.4 T) should shift its position to - 5.0 ppm at 7.1 T (fig. 2d). However, in the experimental 7.1 T spectrum (fig. 2e), the position of this resonance peak is nearly unchanged (Z 30 ppm). Thus, we conclude that the 60 and 30 ppm signals arise from separate *‘Al resonances. Furthermore, it is concluded that the 60 ppm resonance observed without CP (fig. 2a) consists of overlap of “Al resonances from zeolite FA and NFA/DFA (at least partly as AI( species), a conclusion also reached by Samoson et al. [ 81. We should note that in a CP experiment of sample 1, no resonance is observed from the zeolite FA. Although *‘Al MAS NMR is considered an important tool for achieving information about different Al species in zeolites, its quantitative use for USY zeolites has been very disappointing. Too-low values for the NFA% are consistently obtained by deconvolution of “Al MA.5 NMR spectra, an observation that has been attributed to “NMR-invisible Al”. However, the presence of tetrahedral NFA (observed form the present CP/MAS experiment) overlapping with the FA resonance would account for part of the missing Al. The existence of tetrahedral NFA is consistent with the observation [ 6, lo] that the intensity of the 60 ppm resonance converges towards a limiting value as a function of steaming time, and that it never vanishes as expected if it arises only from zeolite FA. Unfortunately, quantitative estimates of the amount of tetrahedral NFA based on *‘Al{‘Hj CP/MAS and 2D quadrupole nutation NMR [ 8,111 are associated with great uncertainty. To investigate the influence of ultrahigh-speed spinning and its possibility for improving the quantitative results obtained from 27A1MAS NMR, fig. 3 shows a comparison of three spectra for sample 3 recorded using ~~~7.3, 12.5 and 18.2 kHz and otherwise identical conditions. It is seen that the broad hump observed below the three resonances at moderate spinning speeds (fig. 3a) decreases with increasing V, and that it is completely eliminated only at 18 kHz. In addition, the absolute intensities for the three narrow (0, 30 and 60 ppm) resonances simultaneously increase by different percentages. This shows that the hump forms part of all NFA resonances and that their quadrupole interactions require ultrahigh spinning speeds for averaging the central transitions. Thus, quantification (deconvo-
h.__ C
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26 July I99 1
100
60
20 O-20 -60
w
Fig. 2. 17A1MAS NMR spectra (9.4 T) of sample 3 obtained (a) without and (b) with ‘H cross-polarization. Simulations of second-order quadrupole lineshapes for the “doublet” at -60 and 30 ppm m (b) using k72.4 ppm, Co=7.3 MHz, q=O.O are shown for magnetic fields of(c) 9.4 and (d) 7.1 T. For comparison, the 7.1 T “Al MAS NMR spectrum is shown in (e).
the optimum CP conditions, i.e. H-H match and contact time. The nearly equal intensity observed for the 60 and 30 ppm “doublet” is of special interest in relation to the 2D quadrupolar nutation experiments by Samoson et al. [S]. They concluded that the 30 ppm signal should be considered part (the low-frequency component) of a second-order quadrupolarbroadened lineshape (Co>, 6 MHz, LEO [ 8]), i.e., similar to the “doublet” observed in the “Al{‘H} CP/MAS spectrum (fig. 2b). Computer simulation of a second-order quadrupolar lineshape for the “doublet” observed in fig. 2b (i.e., assuming a single *‘Al resonance with q= 0) gives 6( 27Al) = 72.4 ppm and C,Z 7.3 MHz (fig. 2~). More importantly, simulation of the field dependence of this lineshape shows that the low-frequency component of the
123
\y b I! CHEMICAL PHYSICS LETTERS
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a * ;;-_I_ 200
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100
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-100
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Fig. 3. *‘Al MAS NMR spectra (9.4 T) of sample 3 recorded at different spinning speeds (4 mm rotor), a pulse width of 1.0 ps and otherwise identical conditions. (a) 7.3 kHz, (b) 12.5 kHz, and (c) 18.2 kHz. All spectra are plotted using the same vertical expansion. Note that a residual hump is still observed at u,= 12.5kHz (b) and that only two low-intensity spinning sidebands from the 0 ppm resonance are left at Ye= 18.2kHz (c). The small peak marked with an asterisk is an Al impurity (Al-N) from the Si,N, rotor material.
lution) of the relative intensities for the three resonances is easier and more reliable for spectra obtained at such rotor speeds. We should note that although the relative intensities for the three resonances as observed in fig. 3 at 12.5 and 18.2 kHz for sample 3 appear quite similar, this need not be the case for other USY zeolites. The spectra in fig. 3 clearly indicate why several published attempts of a quantitative analysis ( AINFA/Al’“‘a’%) using 27Al MAS NMR with moderate spinning speeds are doomed to fail. A comparison between the NFA% determined by deconvolution of the 27Al MAS spectrum in fig. 3c (assuming no NFA at 60 ppm) and the conventional determination by XRF/29Si MAS NMR is shown in table 1. It is noted that the agreement is much better than for the equivalent low-speed spinning results published in the literature [ 16,171. However, to obtain 100% agreement with the XRF/ 29Si MAS NMR results , ~50% of the 60 ppm resonance in fig. 3c must be assigned to tetrahedral NFA/DFA (table 1) in accord with the 27Al CP/ MAS spectrum (lig. 2b). Furthermore, the data in table 1 show that tetrahedral NFA is formed in appreciable amounts only during strong dealumination. In conclusion, ultrahigh-speed 27AlMAS NMR (eventually combined with the highest possible magnetic field strength) appears a prerequisite and the 124
most direct method for obtaining quantitative results of USY zeolites by 27A1NMR. Regarding the nature of the NFA, it is of interest to consider the characteristic changes of the 27AlMAS NMR spectra observed by controlled acac-treatment (i.e., without further dealumination) of USY zeolites [ 161. Fig. 4a shows the 27AlMAS NMR spectra of sample 3 before (left) and after (right) acactreatment. The intensity increase of the 0 ppm signal has been ascribed to complexation of acac with NFA [ 161. However, for the two spectra in fig. 4a, the intensity increase of the resonance at 0 ppm is approximately 12% and no significant intensity reduction of the broad hump is observed. Thus, from the results in fig. 3: the acac method cannot be considered a quantitative 27A1MAS NMR technique for USY zeolites. Fig. 4b shows that amorphous silicaalumina (sample 4), a proposed candidate for the NFA, exhibits the same behaviour upon acac-treatment as sample 3. No such behaviour was observed for y-A&O3 (fig. 4c) which indicates a silica-alumina rather than an alumina-type structure for at least part of the NFA phase (we note that neither yA1203nor sample 4 shows any appearance of a signal at 30 ppm). Further evidence for a silica-alumina phase in USY zeolites is obtained by comparing the spectra of sample 3 (fig. Sa) and a steamed sample 5 (fig. 5b) of the silica-alumina. Both spectra ex-
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Fig. 4. *‘Al MAS NMR spectra (7.1 T) of (a) a USY zeolite (sample 3), (b) a silica-alumina (sample 4), and (c) y-A&O, before (left column) and after (right column) treatment with acetylacetone (acac; see section 2).
hibit the 30 and 0 ppm signals and a broad background hump. Thus, a high degree of similarity between the Al phases constituting NFA in USY zeolites and Al species in steamed silica-alumina is observed. We note that evidence for non-framework silica in steamed Y zeolites has been obtained from 29Si{IH} CP/MAS NMR [ 51. In conclusion, the importance of *‘Al{‘H} CP/ MAS NMR for distinguishing NFA from FA in USY zeolites combined with the ability to obtain quantitative information on the NFA% using ultrahigh spinning speeds has been demonstrated. It is shown that the 30 ppm signal in *‘Al MAS NMR spectra of USY zeolites should be considered an individual resonance and not part of a second-order quadrupolar lineshape. Finally, experimental evidence for a high degree of resemblance between the Al phases constituting the NFA in USYs and the Al species in steamed silica-alumina has been observed.
i m
26 July 1991
L
I"",,"'l"“l,,"lTml'"'l-,""~ -100 ppm 200 100 0
Fig. 5. Comparison of “Al MAS NMR spectra (7.1 T) of (a) a USY zeolite (sample 3), (b) a sample of steamed silica-alumina (sample 5, see section 2). and (c) the silica-alumina before steaming (sample 4). The spectra are shown with a ratio for the vertical expansionof (top to bottom) 10: 15: 1.
Acknowledgement The use of the solid-state Varian VXR-400 S NMR spectrometer, sponsored by Teknologistyrelsen, at the University of Aarhus NMR-Laboratory is gratefully acknowledged. We thank the Danish Research Councils (SNF and STVF), Carlsbergfondet, Direktclr Ib Henriksen Fond and Aarhus University Research Foundation for equipment grants.
References [ 11 C.V. McDaniel and P.K. Maher, Sot. Chem. Ind. ( 1968) 186. [2] G. Engelhardt and D. Mitchel,High-resolutionsolid-state NMR ofsilicates and zeolites (Wiley, New York, 1987).
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[3] J.M. Thomas and J. Klinowski, Advan. Catal. 33 (1985) 199. [ 41 J. Klinowski, C.A. Fyfe and G.C. Gobbi, J. Chem. Sot. Faraday Trans. 181 (1985) 3003. [ 51 G. Engelhardt, U. Lohse, A. Samoson, M. Magi, M. Tarmak and E. Lippmaa, Zeolites 2 (1982) 59. [ 61G.J. Ray. B.L. Meyersand C.L. Marshall, Zeolites 7 ( 1987) 307. [ 71 L. Kellberg, M. Linsten and H.J. Jakobsen, to be published. [ 81A. Samoson. E. Ltppmaa, G. Engelhardt, U. Lohse and H.G. Jerschkewitz, Chem. Phys. Letters 134 ( 1987) 589. [ 91 J. Gilson, G.C. Edwards, A.W. Peters, K. Rajagopalan, R.F. Wormsbecher, T.G. Roberie and M.P. Shatlock, J. Chem. Soc.Chem.Commun. (1987) 91. [ lo] A. Corma, V. Fort&, A. Martinez and J. Sanz, ACS Symp. Ser. 375 (1988) 17.
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[ 111 P.P. Man, J. Klinowski, A. Trokiner, H. Zanni and P. Papon, Chem.Phys.Letters
151 (1988) 143.
[ 121L. Kellberg, H. Bildsoe and H.J. Jakobsen, presented at the “10th
European
Experimental
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Conference”,
Veldhoven, Holland (May, 1990).
[ 131 H.D. Morris and P.D. Ellis, J. Am. Chem. Sot. I I I (1989) 6045.
[ 1411. Skibsted, H. Bildsoe and H.J. Jakobsen, J. Magn. Reson., in press.
[ 151 M. Linsten and A. Frestad, PCT Int. Appl. WO 89,03, 367 (1989).
[ 161 P.J. Grobet, H. Geens, J.A. Martens and P.A. Jacobs, J. Chem. Sot. Chem. Commun. ( 1987) 1688. [17] P.J. Grobet, H. Geerts, M. Tielen, J.A. Martens and P.A. Jacobs, Stud. Surface Sci. Catal. 46 (1989) 721.