J . Phys. Chem. 1986, 90, 5183-5190
5183
Aluminum Siting in Mordenite and Dealumination Mechanism Philippe Bodart, Janos B. Nagy,* Guy Debras,+Zelimir Cabelica, and Peter A. Jacobsf Laboratoire de Catalyse, Facult& Universitaires de Namur, B-5000 Namur, Belgium (Received: July 19, 1985)
The dealumination of mordenite by acidification (HCI, "0,) and by SiCI4 and steaming treatments is followed by high-resolution magic-angle-spinning solid-state 29Siand 27AlNMR spectroscopy. The combined use of these techniques leads to the determination of the silicon-aluminum ordering. It is demonstrated that aluminum atoms preferentially occupy tetrahedral positions in the four-membered rings of the mordenite structure. In addition, a mechanism of dealumination can be inferred, consisting in removing the aluminum atoms two by two from the four-membered rings. It is also possible to compute the number of SiOH groups left in the unit cell after the progressive removal of aluminum atoms: four groups per extracted AI atom are generated in the beginning of dealumination and this number gradually decreases to two, suggesting that a structural reorganization must necessarily occur for substantial dealumination. These silanol groups are also identified by cross-polarization MAS 29SiNMR spectroscopy.
Introduction
Mordenite is a natural and synthetic zeolite with an idealized composition Na8A18Si40096.nH20.Its structure is orthorhombic (Cmcm), with unit cell parameters a = 1.81, b = 2.05, and c = 0.75 nm.' The framework can be reconstructed either from a combination of 5-1 secondary building units (consisting of one single 5-ring with an attached tetrahedron) or from an assembly of single 6-rings sheets linked through single 4 - r i n g ~ . ~The aluminosilicate skeleton so generated exhibits a pore system consisting of parallel linear channels with 12- or 8-ring apertures. Four nonequivalent crystallographic types of tetrahedra can be distinguished: TI and T2 in the 6-rings sheets and T, and T4 located in the 4-rings (Figure l).I One unit cell contains 16 T I , 16 T2, 8 T3, and 8 T4sites. Data obtained from single crystal X-ray diffraction refinements on natural ptilolite samples3v4and from theoretical calculationsS suggest the preferential siting of aluminum in the 4-rings (sites T3and T4) of the structure. A recent solid-state N M R study also supports these results.6 In order to increase its thermal stability and the strength of its acid sites, mordenite was submitted to various dealumination treatment^.^-^ The ultrastabilized samples were characterized by using various physicochemical techniques such as X-ray diffraction,Iwi2infrared spectroscopy,i1*i2thermal analysis and adsorption measurements,I2 and solid-state magic-angle-spinning (MAS) N M R s p e c t r o s ~ o p y . ~With ~ ~ ~ the ~ ~ help - ~ ~ of this latter technique, Debras et aL6 showed the presence of extralattice octrahedral A1 atoms and suggested the subsequent formation of defects in the structure. They also proposed the preferential siting of A1 atoms on sites T, and T4. Using 27A1and 29Si N M R spectroscopy, Klinowski et al.&and Ripmeester et aLi3studied the dealumination of mordenite with SiC14 and aqueous HCl, respectively. More recently, Hays et aLi6 compared different ultrastabilization processes by using 29Si NMR. The aim of the present work is to combine different physicochemical techniques, namely EDX, XRD, IR, and multinuclear (29Siand 27Al)solid-state N M R spectroscopy, to study the aluminum distribution and the mechanisms of aluminum removal from the mordenite framework when the latter is submitted to acid leaching, steaming, or SiC14 treatment. Physicochemical Characterization of the Samples
Chemical Analysis. The structure and crystallinity of the samples were checked by X-ray diffraction (XRD). The Si and A1 contents were determined by atomic absorption (AA), energy-dispersive X-ray analysis (EDX), and/or proton-induced y-ray *To whom all correspondence should be addressed. Present address: Labofina S.A., Zoning Industriel, B-6520 Feluy, Belgium. Laboratorium voor Oppervlakte Chemie, Katholieke Universiteit Leuven, B-3030 Leuven, Belgium.
*
0022-3654/86/2090-5183$01.50/0
emission (PIGE) as described previously.6q20*21The Si/Al ratios were additionally inferred from solid-state 27A1N M R intensities by using a calibration curve.22 Standards used include 100% crystalline zeolites of various compositions (Si/Al from 2.4 to >150).6*22 ZR Measurements. The infrared (IR) spectra of the hydroxyl region of the mordenite samples were measured on a P E 580B dispersive spectrometer, equipped with data station. Self-supporting wafers ( 5 f 1 mg of zeolite per cm2 of wafer) were suspended in a cell mounted in the spectrometer. This cell was continuously purged with dry helium during heating of the sample. Dehydration of the samples occurred at 673 K, while the spectra were recorded at 423 K. To reduce background noise, spectra were averaged 25 times in the range of 3800-3000 cm-'. Solid-state N M R Measurements. The high-resolution magic-angle-spinning (HRMAS) 29SiN M R spectra were obtained (1) Meier, W. M. Z . Kristallogr. 1961, 115, 439. (2) Meier, W. M. In Natural Zolites, Occurrence, Properties, Uses; Sand, L. B., Murnpton, F. A., Eds.; Pergammon: Oxford, 1976, p 99. (3) Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1968, 13, 169 and references therein. (4) Meier, W. M.; Meier, R.; Gramlich, V. Z . Kristallogr. 1978, 147, 329. (5) Derouane, E. G.; Fripiat, J. G. In Proceedings of the 6 f h International Zeolite Conference, Reno, 1983; Olson, D., Bisio, A,, Eds.; Butterworths, Guildford, U.K., 1984; p 717. (6) Debras, G.; B. Nagy, J.; Gabelica, Z.; Bodart, P.; Jacobs, P. A. Chem. Lett. 1983, 199. (7) Chen, N. Y.; Smith, F. A. Inorg. Chem. 1976.15, 295 and references therein. (8) Klinowski, J.; Thomas, J. M.; Anderson, M. W.; Fyfe, C. A,; Gobbi, G. C. Zeolites 1983, 3, 5. (9) Hidalgo, C. V.; Kato, M.; Hattori, T.; Niwa, M.; Murakami, Y. Zeolites 1984, 4, 175. (10) Olson, R. W.; Rollmann, L. D. Inorg. Chem. 1977, 16, 651. (1 1) Ha, B. H.; Guidot, J.; Barthomeuf, D. J. Chem. SOC.,Faraday Tram. 1 1979, 75, 1245. (12) Beyer, H. K.; Belenykaja, I. M.; Mishin, I. W.; Boberly, Y. Stud. Surf. Sci. Catal. 1984, 18, 133. (13) Ripmeester, J. A.; Majid, A.; Hawkins, R. E.; J . Inclusion Phenom. 1983, 1 , 193. (14) B. Nagy, J.; Gabelica, Z.; Bodart, P.; Debras, G.; Derouane, E. G.; Jacobs, P. A. J . Mol. Catal. 1983, 20, 328. (15) Gabelica, Z.; B. Nagy, J.; Badart, P.; Debras, G.; Derouane, E. G.; Jacobs, P. A. In Zeolites: Science and Technology:Ribeiro, F. R. et al., Eds.; Martinus Nijhoff: The Hague, 1984; p 193. (16) Hays, G. R.; van Erp, W. A.; Alma, N . C. M.; Couperus, P. A,; Huis, R.; Wilson, A. E. Zeolites 1984, 4, 377. (17) Bodart, P.; Gabelica, Z.: B. Nagy, J.; Debras, G. In Zeolites: Science and Technology; Ribeiro, F. R. et al., Eds.; Martinus Nijhoff The Hague, 1984; p 21 1. (18) Fejes, P.; Hannes, I.; Kiricsi, I.; Pfeifer, H.; Freude, D.; Oehme, W. Zeolites 1985, 5, 45. (19) Sand, L. B. US.Patent 3436174, 1969. (20) Gabelica, Z.; Blom, N.; Derouane, E. G. Appl. Catal. 1983, 5, 227. (21) Debras, G.; Derouane, E. G.: Gilson, J.-P.; Gabelica, Z.; Demortier, G . Zeolites 1983, 3, 37. (22) B. Nagy, J.; Gabelica, Z.; Debras, G.; Derouane, E. G.; Gilson, J.-P.; Jacobs, P. A. Zeolites 1984, 4, 133.
0 1986 American Chemical Society
5184
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Bodart et al.
TABLE I: Dealumination of Mordenite by Acid Leaching sample
Si/AI'
parent zeolite
exptl conditions
AA
EDX
2 7 ~ NMR 1
av
5.5 6.1 5.8 20.5 20.6 31.2
5.6 6.6 7.1 19.2 22.3 28.0
5.4 6.3 7.1 21.6 27.6 30.7
5.5 6.3 6.7 20.4 23.5 30.0
Na-Zb H-Z- 1 H-2-2 H-Z-3 H-Z-4 H-2-5
Na-2 Na-2 Na-2 Na-Z Na-Z
4 M "03, 293 K, 8 h 4 M HNO,, 323 K, 24 h 4 M H N 0 3 . 363 K, 24 h 6 M H N 0 3 , 363 K, 24 h 14 M HNO,, 363 K, 24 h
Na-M' H-M- 1 H-M-2 H-M-3 H-M-4 H-M-5 H-M-6 H-M-7
Na-M Na-M H-M- 1 H-M- 1 H-M- 1 H-M-2 H-M-2
2 M HCI, 298 K, 16 h 6 M HCI, 298 K, 16 h 2 M HCI, 353 K, 8 h 6 M HCI, 353 K, 8 h 15 M HCI, 353 K, 8 h 6 M HCI, 353 K, 8 h 15 M HCI, 353 K. 8 h
5.P 6.1 6.8 6.7 6.9 7.1 7.0 1.6
a AA atomic absorption; EDX energy-dispersive X-ray analysis; 27AlN M R results obtained from a calibration curve.6 Commercial Na-Zeolon. CSynthetic Na-MOR prepared according to ref 19. dPIGE (proton-induced y-ray emission) results in the whole series H-M.
TABLE 11: Dealurnination of Mordenite by Steaming and SiCl, Treatments parent zeolites samples
Si/AI"
AI/Na"
Na-Mb H-M- 1' H-M-3b H-M-6b "4-Z
5.7 6.7 6.1 7.0 5.5c
0.9 9.4 53 146
final samples
dealumina tion process
samples
Si/AI"
AI/Na"
HZO/N2/873 K H20/N2/873 K H20/N,/873 K H,O/N,/873, K N2/873 K; SlCI,/N2/873 K
S-M S-M- 1 S-M-3 S-M-6 SiCI,-Z
5.7
1
8.0 7.7d
31d
"Measured by PIGE. bSee Table I. 'SilAl of parent Na-2, commercial specifications. dEDX value.
on a Bruker CXP-200 spectrometer operating in the Fourier transform mode. A radio frequency (rf) field of 49.3 Oe was used for the a / 4 pulses of 29Si (39.7 MHz). Time intervals between pulse sequences were typically 3 s, and at least 2000 free induction decays were accumulated per sample. Chemical shifts (6) were measured from external tetramethyl~ilane.~,'~ For the cross-polarization (CP) measurements, the additional ' H (200.0 MHz) rf field was 9.8 Oe. The CP-MAS spectra were recorded by using a single contact sequence, the contact and the recycle times being 10.0 ms and 6 s, r e ~ p e c t i v e l y . ~ ~ 27Al N M R spectra were recorded at 52.1 MHz. Chemical shifts, in ppm, were measured with respect to Al(H20)63' as an external reference. An rf field of 10.0 Oe was used for the a/12 pulses of 27Al. Actually, at this small angle, both tetrahedral and octahedral species can be determined q ~ a n t i t a t i v e l y . ~This ~.~~ is further confirmed by comparing the chemical composition (Si/Al) obtained by AA or EDX and by 27Al N M R in initial (Na-Z) or dealuminated samples (H-Z): the results are in good agreement independent of the relative amounts of tetrahedral or octahedral A1 (Table I). Time intervals between pulse sequences were 0.1 s, and at least 5000 free induction decays were accumulated per sample. For the MAS experiments, the conical Delrin rotors were spun at 3 kHz.
Preparation of the Samples Dealumination by Acid Leaching. A commercial Na-Zeolon (Na-Z) from Norton was progressively dealuminated by using aqueous nitric acid. Another synthetic Na-mordenite (Na-M) was prepared according to conventional procedures19 and dealuminated by aqueous hydrochloric acid. Table I lists the different treatments and the chemical compositions of the resulting products. Dealumination by Steaming or SiC14 Treatment. The samples Na-M, H-M-1, H-M-3, and H-M-6 (Table I) obtained respectively by direct hydrothermal synthesis and by HCI leaching were heated at 473 K under nitrogen flow and then submitted (23) B. Nagy, J.; Gabelica, 2.;Derouane, E. G. Chern. Lett. 1982, 1105. (24) Samoson, A.; Lippmaa, E. Phys. Rev. B Condens. Mufter 1983, 28, 6567. ( 2 5 ) Fenzke, D.; Freude, D.; Frohlich, T.; Haase, J. Chern. Phys. Lett. 1984, 111, 171.
I
8c
-b-
a) viewed along t h e c-axis
oa
-b-
c) 6-rings sheet, viewed along t h e a-axis Figure 1. Mordenite framework (a) viewed along the
c axis. 6-Rings
sheet (b) of tetrahedra viewed along the u a x k 2
to steam between 473 and 873 K (heating rate, 85 K/h; water flow, 75 mL/h). The samples were left at 873 K for 10 h under the same atmosphere and then cooled to room temperature under nitrogen. The treatment and composition of the initial and final phases are listed in Table 11. The final sample S-M lost 20%
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5185
Aluminum Siting in Mordenite
TABLE III: Determination of the SVAI Ratios of the Zeolite Framework from the Relative Amounts of Tetrahedral (T) and Octahedral (0)AI and the Overall Chemical Composition of the Naand Acid-Treated Mordenite Samples
T
-
Si/AI=5.5
samples
Na-Z H-Z- 1 H-Z-2 H-Z-3 H-Z-4 H-Z-5
H-M-1 H-M-2 H-M-3 H-M-4 H-M-5 H-M-6 H-M-7
Si/AI=23.5
2 7 ~ NMR 1
AIT, %
100 81 75 89 93 93
5.7 6.7 6.8 6.7 6.9 7.1 7.0 7.6
Na-M
H-Z-4
chem compn Si/Al" 5.5 6.3 6.7 20.4 23.5 30.0
lattice compn Si/Alb AIT/ucC
Alo, % 0
19 25 11
7 7
100 77
0 23 29 28 28
71
72 72 69 63 67
5.5 7.7 8.9 22.9 25.3 32.2
7.4 5.5 4.8 2.0
5.7
7.2 4.9 4.5 4.7 4.5 4.2 4.0 3.9
31
8.7 9.6 9.3 9.6 10.3
37 33
11.3
1.8
1.4
11.1
+
I
I
200
I
I
100
cb
I
I
0
I
I
-100
I
I
From Table I. (Si/Al)lattice = [(A10 A~T)/A~T](S~/A~),,,,,. CAIT/uc = number of tetrahedral A1 atoms per unit cell; AIT/uc = 48/ [1 + (Si/Al)~atticel.
-200
I
I
I
(ppm vs AI (H,0)3,')
1
1
Si (1Al) SiOH Si(0AI) -n
Figure 2. High-power solid-state 27AlNMR spectra of Na-Zeolon and its dealuminated forms H-Z-2 and H-Z-4.
M
Si/Al=5.5 Na-Z
-
b (ppm / A I (H20)63+)Figure 3. Solid-state 27AlNMR spectra with magic-angle-spinning of the mordenite samples dealuminated by acid leaching (H-M-6) and by acid leaching followed by steaming (S-M-6). of its initial crystallinity (as measured by XRD), while S-M-1, S-M-3, and S-M-6 retained their crystallinity. NH4-Zeolon (NH4-Z) was obtained through ion exchange of the commercial Na-Zeolon (Norton) with ammonium nitrate. This sample was calcined at 873 K under N, and contacted at the same temperature with a SiCl,-saturated stream of N 2 (Table II).8 The sample was finally washed with cold water and dried. The product so obtained (SiCl,-Z) retained its crystallinity.
Results and Discussion Dealumination of Mordenite by Various Chemical Treatments. Solid-state MAS ,'AI and 29SiN M R Study. The MAS ,'A1 N M R spectra of a parent Na mordenite and of its dealuminated analogues are presented in Figures 2 and 3. The Na-Z and Na-M spectra (the latter is not shown) are characterized by a unique resonance line a t 6 N 50 ppm vs. Al(H20)63+,corresponding to the tetrahedrally coordinated AI of the lattice (Air). An additional line at 6 = 0, present for the acid-treated samples (H-Z-2, H-Z-4, and H-M-6), belongs to octahedral A1 species extracted from the lattice (A10).698913The narrowness of this line (800 Hz) suggests that the octahedral Al(II1) ions are fully hydrated and occupy probably cationic positions. From these spectra, the relative amounts of tetrahedral and octahedral A1 atoms can be determined. These data together with the Si/AI atomic ratios obtained by the chemical analyses allow one to compute the real composition of the zeolitic framework if it is assumed that all
H-2-4
I \
d) H-2-5
I \
C)
I \
Si/AI=30.0
I
-I
I
I
I
I
-120
-x)O
cb
(ppm vs TMS)
Figure 4. Variation of HRMAS 29SiNMR spectrum during mordenite dealumination by acid leaching.
tetrahedral A1 atoms belong to the lattice (Table 111). The H R M A S 29Si N M R spectra obtained for the parent compound Na-Z and its counterparts dealuminated by acid
5186 The Journal of Physical Chemistry, Vol. 90, No. 21. 1986
Bodart et al.
TABLE I V Variation of the Relative HRMAS 2%i NMR Line Intensities of Mordenite at Various Degrees of Dealurnination by Acid Leaching relative line intensitiesb (%) at samples
Si/AI"
6 = -99
6 = -105
6 = -110
Si/Alc
Na-2 H-Z-I H-2-2 H-2-3 H-Z-4 H-Z-5
5.5 7.7 8.9 22.9 25.3 32.2
13 12 10 5 4 2
45 45 43 23 21 16
42 43 47 72 75 82
5.6 5.8 6.3 12 14 20
Na-M H-M- 1 H-M-2 H-M-3 H-M-4 H-M-5 H-M-6 H-M-7
5.7 8.7 9.6 9.3 9.6 10.3 11.1 11.3
12
45 48 45 43 42 41 46 41
43 42 45 46 47 53 48 52
5.8 5.9 6.1 6.2 6.3 7.5 6.8 7.3
10
10 11 11 6 6 7
I
I
0
1
I
- 110
c a(ppm vs TMS) Figure 6. CPHRMAS 29SiN M R spectrum of mordenite treated by
aqueous HCI (H-M-2).
I
16
12,
9)
"Lattice composition (Table 111). *Relative to the total N M R intensity. 'Recalculated by using the relation Si/AI = 1,/(xf,,0.25 n Z S ~ ( ~ ~assuming ~)) that SiOH defect groups do not contribute to the -105 ppm line intensity.
z -5
-4
-
-3
-
5 E0
+E 0
L E
e o
ft m u
I 2
0 0 - 2 iij
2* X
-1
0
1
2
3800- 3600 3800- 3boo
cm-1 Figure 5. Infrared spectrum of mordenite dealuminated by acid leaching
3 4 Alaxtract.d /
5
-
6
unit cell
a
lo
7
Figure 7. Evolution of the total number of SiOH groups and the amount generated per extracted AI atom with the amount of AI extracted per unit
in the 3600-cm-' region.
cell.
leaching (H-Z-2, H-Z-4, and H-Z-5) are presented in Figure 4. The initial spectrum (Na-Z) mainly consists of three resonance lines at -1 10, -105, and -99 ppm, corresponding to Si atoms with 0, 1, and 2 A1 atoms in their second coordination sphere, respectively. Moreover, silanol groups at defect lattice spots can contribute to the intensity of the N M R line at -105 ppm.6,23 The relative intensities of the 29SiN M R lines are recorded in Table IV for the different samples. As dealumination by acid leaching proceeds, the lines at 6 = -99 and -105 decrease while the relative line intensity of the line at 6 = -1 10 increases. This behavior unambiguously shows the extraction of A1 from the lattice leading to a decrease of the number of Si(lA1) and Si(2AI) configurations in the structure and to an increase of Si(OA1) configurations. The Si/Al ratio can be recalculated from the 29Si N M R line intensities by using the relation26
cm-l, which has to be. attributed to silanol type defects (Figure 5). It has been previously shown that cross-polarization in the 29SiN M R spectrum enhances the signal of SiOH defect groups.23 The 29SiN M R spectrum of H-M-2 enhanced by CP also shows a very large increase of the line at 6 = -105, while the two other lines are less affected (Figure 6). Consequently, the line at 6 = -105 contains the contributions of both Si( 1Al) configurations and defect SiOH species vibrating in IR at 3760 cm-'. The is therefore equal to6J6 intensity of this line
Si
1,
=
I-105
(26) Engelhardt, G.; Lohse, U.; Lippmaa, E.: Tarmak, M.; Mlgi, M. 2. Anorg. Allg. Chem. 1981, 482, 49.
+ ISiOH
(2)
On the other hand, the intensity of the line at -99 ppm (Z+) is only determined by Si(2Al) configurations: (-99
=
(3)
ISi(2AI)
Finally, the total Si(OA1) contribution to the spectrum stems from both Si(OA1) groups at 6 = -1 10 (LIl0)and SiOH groups at -105 PPm: ISi(OA1)
I, is the total intensity of the spectrum, and ZSi(nAl) is the contribution of Si atoms with nAl neighbors in their second coordination sphere. However, except for the parent compounds Na-Z and Na-M, this ratio is always smaller than the one given by the lattice composition (obtained by combining the 27AlN M R data and the bulk chemical analysis (Table IV)). The origin of this discrepancy can be explained by assuming a substantial contribution of silanol groups to the -105 ppm line. Direct evidence for the existence of SiOH groups is obtained by both 29Si'H CP measurements and infrared spectroscopy. As dealumination proceeds, a new infrared band is detected at 3760
ISi(IA1)
(s)
Z S ~ ( is ~ ~thus ~ )
framework
=
I-110
+ ISOH
(4)
computed from the relation
4
= 2
0.25
ISi(2AI)
+1
0.25
ISi(IA1)
(5)
in which ZSic2+, is directly measured on the 29SiN M R spectra (q 3) and the Si/Al ratio of the framework is already obtained from the 27AlN M R measurements and the bulk chemical analysis. It stands for the total area under the 29Si resonance envelope. The relative intensities of the different configurations so obtained are listed in Table V. The number of SiOH groups at defect lattice spots per extracted A1 atom is plotted as a function of the number of extracted A1 atoms per unit cell (Figure 7). In the beginning of the dealumination, up to four SiOH defects per
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5187
Aluminum Siting in Mordenite
TABLE V Evolution of the Different Si Configurations and of the Number of SiOH Defect Groups Generated per Extracted AI during Mordenite Dealumination by Acid LeachingSi configurations (% of total Si) lattice compn samples Si/A1 Al/uc SiOH Si(2A1) Si( 1Al) Si(OAl)* SiOH/Alextd0 0 13 45 42 Na-Z 5.5 7.4 17 11 28 61 3.7 H-Z- 1 7.7 5.5 3.0 18 10 24 66 H-Z- 2 8.9 4.8 15 5 8 1.3 87 H-Z-3 22.9 2.0 1.2 14 4 7 89 H-Z-4 25.9 1.8 0.6 8 2 8 90 H-Z-5 32.2 1.4
Na-M H-M- 1 H-M-2 H-M-3 H-M-4 H-M - 5 H-M-6 H-M-7
5.7 8.7 9.6
9.3 9.6 10.3 11.1 11.3
12 9 9
45 26
22 22
10
14 22
6 6
20 19 26
20
7
0 21
7.2
4.9 4.5 4.7 4.5 4.2 4.0 3.9
24
10
43 65 70 70
23
71
4.0 3.8 3.8 3.4 2.0 3.0
20
73
2.7
21
71
68
'Number of SiOH generated per AI extracted. bSi(OA1)t,t,I= Si(OA1) + SOH.
1
SI lOAll
711 '1-#
bl
S*ll
I I
A
I
S-M- 1
S-M-6
Figure 8. HRMAS 29SiNMR spectra of mordenite dealuminated by acid leaching and subsequent steaming and by ammonium exchange and subsequent SiCI4 treatment with and without cross-polarization. -90
extracted Al atom are generated in the structure. This corresponds to the original picture presented by K e d 7
-110
b (ppm vs TMS)
Figure 9. HRMAS 29SiNMR spectra of mordenite dealuminated by
steaming (S-M) and by increasing acid treatments and subsequent steaming (S-M-I, S-M-3, and S-M-6). I! Si
I
? Si
in
I
0 I
ii
Ill
+AI(H,O):+
However, the decrease of this number to lower values as dealumination proceeds can be the result of a partial healing of the structure defects. Similar conclusions have been arrived at by Beyer et aI.'* and Fejes et aI.'* Figure 8 presents the H R M A S 29Si N M R spectra with and without enhancement due to CP for samples dealuminated by acid leaching and consecutive steaming (S-M-6) and by ammonium exchange and subsequent SiC1, treatment (SiC14-Z). The spectrum of the highly dealuminated sample (S-M-6) shows three lines corresponding to Si(OA1) configurations. These lines are well resolved due to the very small amount of Al remaining in the lattice and possibly correspond to the four inequivalent crystallographic (27) Kerr, G.T. J . Catal. 1969, 1.5, 200.
sites, since it has been shown that the 29SiN M R chemical shift is proportional to the average T U T angle of a given site.28 On the basis of published T U T angles,I4 the lines at -1 12.2, -1 13.1, and -1 15.0 ppm can be attributed to TI, T4, and T2 + T3 sites, respectively. The corresponding line intensity ratios are respectively 2, 1, and 3 and are in agreement with the relative amounts of the crystallographic sites.34 The contribution of the -105 ppm line to the spectrum is very small (Figure 8). However, the reorganization of the structure is not complete as evidenced by the CPMAS experiment. Indeed, a line at -105 ppm corresponding to SiOH defect groups is detected by cross-polarization. An additional line appears at -90 ppm and could be attributed to Si(20H) or Si(lA1)OH defect groups, as has been recently proposed by Engelhardt et al.29 This latter line was not detected for the acid-leached samples. Dealumination by acid leaching and subsequent steaming of mordenite (S-M-6) results in a 21A1N M R spectrum characterized by two lines at 51.6 and at -6 ppm (Figure 3). They are quite broad and are attributed respectively to tetrahedral (1400 Hz) and octahedral (2100 Hz) A1 belonging to an extralattice aluminous phase formed during A1 extraction from the zeolitic lattice (28) Jarman, R. H. J . Chem. Soc., Chem. Commun. 1983, 512. (29) Engelhardt, G.; Lohse, U.; Magi, M; Lippmaa, E. Stud. Surf. Sci. Catal. 1984, 19, 23.
5188 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 as no tetrahedral AI atoms in the lattice could be detected from the 29Si N M R spectra. Figure 9 emphasizes the necessity of using acidified samples for an effective dealurnination of the mordenite structure. Indeed, the lower the residual Na content of the samples, the more effective the dealumination, as shown by the higher resolution of the 29Si N M R spectra. In addition, the -105 ppm line due to configurations [Si( 1Al) + S O H ] decreases with decreasing Na content. The solid-state 29SiN M R spectrum of mordenite dealuminated by SiC14 is also presented in Figure 8. It consists of three lines at 6 = -99, -105, and -1 11 respectively corresponding to Si(2A1), Si(lAl), and Si(0AI) configurations. As the enhancement of the line at -105 ppm is relatively small in the CPMAS 29SiN M R spectrum, it is concluded that few defect SiOH groups have been generated in such a structure. This confirms the 27A1N M R data (Figure 3): the SEI4treatment only partly removes aluminum from the lattice, which is consequently deposited in the channels. This is caused by both reduced diffusivity of SiCI, in the channels of mordenite and by the presence of residual Na’ cations. The reduced diffisivity of SiC14 is responsible for the partial dealumination, while the residual Na+ cations can react with the aluminum chlorides to form NaA1Cl4 which is precipitated in the channels. Finally, these chlorides are hydrolyzed during washing to produce Al(H20)63+ Mechanism of Mordenite Dealumination and Distribution of Aluminum in the Lattice. The framework of mordenite is composed of four different crystallographic tetrahedral sites (T sites).] A unit cell contains 48 T atoms: 16 T I , 16 T2, 8 T3, and 8 T4. TI sites are located in the 6-rings sheets of the structure, shared between the 8- and 12-rings channels; T2 sites are also in the 6-rings sheets, in the 12-rings channels but not in the 8-rings; T, and T4 sites are located in the 4-rings, T, sites in the small channels and T4 sites in the 12-rings channels. Each TI or T2 atom has 3(T1,Tz)and l(T3,T4)neighboring sites and each T3 or T4 atom is surrounded by 2(Tl,T2) and 2(T3,T4) atoms. A priori, different aluminum distributions can be assumed for the mordenite framework, each obeying Loewenstein’s rule of aluminum avoidance:,O 1O statistical distribution on sites T I , T2, T3, and T4; 2’ specific siting on sites TI and T,; and 3’ specific siting on T3 and T4. For the statistical distribution among all sites, the different contributions of Si(nA1) configurations can be computed from the relative 29SiN M R line i n t e n ~ i t i e s ~ ’ * ~ ~
ZSi(nAl) is the line intensity of configuration Si(nAl), It is the total 29SiN M R line intensity, and
The Si/Al ratio is computed from formula 1. If the AI atoms are specifically located on TI and T2 sites (in 6-rings sheets), two types of distributions can be taken into account: (a) a purely statistical distribution or (b) a statistical distribution where a minimum charge density of the lattice is considered. For the first case of “purely” statistical distribution on sites T, and T2, the relative line intensities are given by
n varies between 0 and 3 in eq 7 and between 0 and 2 in eq 8
Bodart et al. is three and the number of T,,T2 neighbors for a T3 or T4 site is two. The total NMR line intensities of sites Tl,T2and T3,T4are given respectively by 32 - A ~ / u c IT,,T~ = 48 - Al/uc 4
(9)
and
The AI/Si and Al/(Al + Si) ratios are computed with respect to sites T,,T2. In eq 7, the distribution on sites T1,T2is described by a conditional probability, due to the aluminum avoidance principle. Equation 8, on the other hand, is based on a description by independent probabilities because no AI atoms may be found on sites T3,T4.,, In case b the distribution of A1 atoms follows a minimum charge density in the lattice. This distribution can be worked out on a model by locating one A1 atom in the lattice and placing the other AI atoms in such a way that the number of Si(nAl) groups (n # 0) is minimized. For Si/Al = 5, it is possible to locate AI in the lattice in such a way that there is no Si(ZA1) or higher Si(nA1) configurations (with n > 2) in the structure. For such a configuration, the Si/AI ratio becomes
Si -_ AI
It 0.25zSi(lAI)
and it is then possible to recalculate ZSi(lAI) for any Si/Al ratio greater than 5 , Le., for the composition range of our samples. If the A1 atoms are specifically located on sites T3and T4 (i.e., in the four-membered rings), the statistical distribution can be calculated as follows:
and
n varies between 0 and 2 in eq 1 1 and between 0 and 1 in eq 12 because the number of T3,T4neighbors for a T3or T4 site is two and the number of T3,T4 neighbors for a T I or T2 site is one. The total N M R line intensities of sites T3,T4and Tl,T2 are respectively given by
and
The Al/Si and AI/(Al + Si) ratios are computed with respect to sites T3,T4. Equations 11 and 12 are based on statistical considerations similar to those included in eq 7 and 8 (vide supra). At higher Si/AI ratios (dealuminated samples), two more possibilities can be included in the A1 distribution of sites T3and T4. One could exclude the configurations where the A1 atoms are diagonally sited in the 4-rings (a). This model leads to a minimum charge density. The second distribution excludes the configurations with 4-rings containing only one AI atom (b). This latter dis-
because the number of T,,T2neighboring sites for a TI or T, site (30) Loewenstein, W. Am. Mineral. 1954, 39, 92. (31) Engelhardt, G.; Ziegan, D.; Lippmaa, E.; Magi, M. Z . Anorg. Allg. Chem. 1980, 468, 35. ( 3 2 ) Mikowsky, R . J. Zeolites 1983, 3, 90.
(33) Bodart, P.; B. Nagy, J.; Gabelica, Z.; Derouane, E. G. Proceedings of the International Conference on Occurrence, Properties and Utilization of Natural Zeolites, Budapest, Aug 1985, in press. (34) Thomas, J. M.; Minowski, J.; Ramdas, S.; Hunter, B. K.; Tennakoon, D. T. B. Chem. Phys. Lett. 1983, 102, 158.
The Journal of Physical Chemistry, Vol. 90, No. 21, I986 5189
Aluminum Siting in Mordenite 100,
60
‘ I
I
a-
‘experimental
40-
100,
exptl calcd
I
Si / AI * Figure 10. Comparison between the evolution of the experimental relative populations of the different Si(nAl) configurations obtained by 27A1 and 29SiNMR spectroscopies and those recalculated from various AI distributions in the zeolitic lattice: (a) statistical among all sites; (b) statistical on TI and T2sites; (c) on T,and T2 sites with a minimum charge density; (d) statistical on T, and T,; (e) on TI,and T4sites with a minimum charge density; (f) on T3and T4sites with two or no AI atoms in the 4-rings. The dashed lines represent the experimental data; the solid
lines represent the calculated distributions. tribution is also suggested by recent theoretical c a l c ~ l a t i o n s . ~ Finally, if the A1 atoms are specifically distributed on either T I or T2 sites, statistical distributions identical with those calculated by eq 11-14 are obtained. In Figure 10 the experimental line intensities (Table V) are compared with the theoretical ones computed from the different types of AI distributions. It is quite clear that the statistical distribution of aluminum on all sites is not able to reproduce the experimental results (Figure loa). The same conclusion holds for a statistical distribution in the 6-ring sheets, Le., on sites TI and T2 (Figure lob). The introduction of the minimum charge density in the latter case does not improve the computed values: not only does a substantial discrepancy still exist between the experimental and theoretical curves but also the Si(2A1) contribution is absent from the theoretical distribution, in complete disagreement with experiments (Figure 1Oc). It can be easily seen from Figure 10d that the statistical distribution either on sites T, T4 or on TI only or on T2only does not hold for the samples which are differently dealuminated. In order to adequately explain the experimental results, it is of paramount importance to include the mechanism of dealumination in the correct computation of the aluminum distribution. Parts e and f of Figure 10 depict the two possible mechanisms of dealumination if A1 atoms are specifically located on sites T3,T4. The model allowing either one or two A1 atoms in the 4-ring and keeping the minimum charge density is not able to reproduce the experimental results. Finally, a very good agreement is found between the calculated and experimental values in the whole range of Si/A1 ratios for the model, where the AI atoms are specifically sited in the 4-rings and the configurations with only one A1 atom are excluded (Figure 1Of). We therefore propose an adequate
+
TABLE VI: Experimental and Computed Relative 29SiNMR Intensities and Si/AI Ratios of Mordenite Dealuminated by SKI4 Treatment Si configuration (% of total Si)
Si(2A1) 6.7 7.3
Si(1AI) 16.2 14.5
Si(0AI) 77.1 78.2
Si/AI 13.8 13.8
picture for the mechanism of mordenite dealumination by acid leaching. In the initial sample, nearly all 4-rings contain two A1 atoms. As dealumination starts, 4-rings with no A1 atoms appear in the structure and the remaining 4-rings thus contain either two or no A1 atoms. In addition, silanol defect groups appear, the number of which is close to four per extracted A1 atoms. The model implies that both A1 atoms are quasi simultaneously extracted from the 4-ring. As dealumination proceeds, the structure is reorganized even at 353 K by recombining the silanol defect groups. Indeed, the number of SiOH per extracted A1 decreases from 4 to 0.6 during dealumination (Table V and Figure 7 ) . One has to note that, formally, similar conclusions can be drawn from specifically siting A1 atoms on either T I or T2sites and by dealuminating the structure by extracting the AI atoms two by two. Nevertheless, despite the mathematical agreement, this possibility is rejected because there is no physical reason why the Al atoms should be taken out two by two from a chainlike structure (Al-0-Si-0-A1-0-Si-0), (TI or T, sites in the 6-ring sheets). In addition, as it was already mentioned above, both X-ray diffraction studies3s4and theoretical calculations5 suggest the preferential siting of A1 in the 4-rings of the structure. The proposed model is also able to explain the change in the 29SiN M R spectra or mordenite dealuminated by SiCI,. Table VI shows the Si/Al ratio of the framework recalculated in a similar manner from EDX and 27AlN M R results. A good agreement is found between this value and the one computed from 29SiN M R data. The relative N M R line intensities are also correctly reproduced if it is supposed that A1 atoms are specifically located on sites T3 and T4 and that they are removed two by two from the framework. Moreover, only a small amount (less than 1%) of SiOH group is produced, confirming the effective replacement of A1 by Si during the SiC14 treatment.
Conclusions Dealumination of mordenite was achieved by acid leaching, steaming, and SiCI4 treatment. By the use of solid-state 27Al N M R combined with EDX and/or PIGE measurements, it was possible to find the composition of the zeolitic lattice, excluding the contribution of extralattice Al. Acid leaching provides samples with various degrees of dealumination. By a combination of infrared spectroscopy and solid-state 29SiN M R (with and without cross-polarization), SiOH defect groups were detected and their amount quantified. In the beginning of the process, the extraction of A1 generates about four SiOH groups per AI atom extracted. Further dealumination leads to a reorganization of the structure, even at 363 K, as shown by the decreasing amount of defects. Comparison of the solid-state 29SiN M R line intensities with those calculated for six possible distributions of A1 in the zeolitic lattice leads to the conclusion that A1 atoms are located in the 4-rings of the structure, each ring containing either two or no A1 atoms. As a consequence, both A1 atoms of a ring are extracted quasi simultaneously during dealumination. It has to be emphasized that this quantitative analysis is based on the following assumptions: 1. Tetrahedral and octahedral components in the 27AlN M R spectra are quantitative. This is justified by using small angle pulses. 2. The small intensity at ca. -99 ppm in the 29SiN M R spectra is always and entirely due to Si(2A1). 3. The AI atoms occupy sites T3 and T4. 4. The deconvolution of the 29SiN M R spectra to three different lines, Le., Si(2Al), Si(lA1) + Si(OH), and Si(OAI), is carried out with a precision of about 5%.
5190
J . Phys. Chem. 1986, 90, 5190-5193
Dealumination by steaming only occurs with acidified or ammonium-exchanged samples. AI is then extracted and deposited on the channels or on the outer surface of the crystallites as an alumina phase, as shown by the typical 27AlN M R spectrum. The solid-state 29Si N M R spectrum is highly resolved due to the absence of Si(nA1) contribution ( n C 0). Three distinct N M R lines correspond to Si(OA1) configurations of T I , T,, and T, + T3 sites, respectively. Enhancement of the 29SiN M R spectrum by cross-polarization confirms the presence of SiOH groups.
Acknowledgment. We thank Prof. E. G. Derouane for valuable discussions. We are indebted to G. Daelen for taking the N M R spectra and to F. Vallette for technical assistance. G. Debras and P. Bodart thank IRSIA-IWONL for financial support. P. A. Jacobs acknowledges NFWO-FNRS (Belgium) for a research position as "Onderzoeksleider". Registry No. AI, 7429-90-5; HCI, 7647-01-0; HN03, 7697-37-2; SiCI,, 10026-04-7.
Temperature Dependence of the Luminescence Emission of Ruthenium( I I ) Cornpiexes Containing the Ligands 2,2'-Bipyrldine and Dipyrido[ 3,2-c :2',3'-e Ipyridazine Francesco Barigelletti,lnAlberto Juris,ln*b Vincenzo Balzani,* and Alex von Zelewsky"
Peter Belser,IC
Istituto FRAE-CNR and Istituto Chimico "G. Ciamician" dell'Universitri, Bologna, Italy, and Institute for Inorganic Chemistry, University of Fribourg, Fribourg, Switzerland (Received: September 27, 1985; In Final Form: April 15, 1986)
The luminescence behavior (emission spectra and emission lifetimes) of R~(taphen),~+, Ru(bpy)(taphen)?+, and Ru(b~y)~(taphen)*+ (taphen = dipyrido[3,2-~:2',3/-e]pyridazine, bpy = 2,2'-bipyridine) has been studied in propionitrile-butyronitrile ( 4 5 v/v) solutions in the temperature range 85-350 K. The results obtained are illustrated and discussed in comparison with those previously obtained for Ru(bpy),'+. The Ru(taphen),*+ complex exhibits a very peculiar behavior. Its emission lifetime (i) decreases strongly in the temperature range of solvent melting (100-150 K), (ii) increases with increasing temperature from 150 to 230 K, and (iii) decreases again at higher temperature (230-330 K). The maximum of the emission band moves to the red and the luminescence intensity decreases in the melting region, but at higher temperatures (150-230 K) the emission maximum undergoes a blue shift and the emission intensity increases. Above 230 K the emission maximum continues to move to the blue, while the intensity decreases. Such a complicated temperature dependence of the photophysical behavior is discussed on the basis of a sequence of excited states which includes metal-to-ligand charge-transfer levels of (C, antisymmetric) $ and (Czosymmetric) x ligand orbital origin. The mixed ligand complexes exhibit a behavior qualitatively similar to that of Ru(taphen),*+, except that the emission lifetime remains approximately constant in the temperature range 150-280 K.
Introduction
A better knowledge of the parameters which govern the photophysical properties of transition-metal complexes2-6 is fundamental for the progress of photochemistry and essential for the design of new photo sensitizer^.^-^^ Temperature dependence studies of the luminescence behavior can yield important pieces of information concerning the energy ordering of the various excited states and the dynamics of interstate c o n v e r ~ i o n . ~ - ~ ~ 9 the ' ~ complexes of the RuContinuing our i n ~ e s t i g a t i o n s ~ Jon (11)-polypyridine family, we have recently studiedI5 the spectroscopic, electrochemical, and luminescence properties of the Ru(bpy)>"(taphen);+ complexes ( n = 0-3, bpy = 2,2'-bipyridine, taphen = dipyrid0[3,2-~:2',3'-e]pyridazine; Figure 1). We report (1) (a) Istituto FRAE-CNR. (b) Istituto Chimico 'G. Ciamician". (c) University of Fribourg. (2) For reviews, see ref 3-6. (3) Crosby, G. A. Acc. Chem. Res. 1975, 8, 231. (4) Kemp, T. J. Prog. React. Kinet. 1980, 10, 301. (5) DeArmond, M. K.;Carlin, C. M. Coord. Chem. Reu. 1981, 36, 325. (6) Demas, J. N. J. Chem. Educ. 1983, 60, 803. (7) Graetzel, M., Ed. Energy Resources through Photochemistry and Catalysis; Academic: New York, 1983. (8) Meyer, T. J. Prog. Inorg. Chem. 1983, ZZ, 94. (9) Balzani, V.; Juris, A.; Barigelletti, F.; Belser, P.; von Zelewsky, A. Riken Q. 1984, 78, 78. (10) Whitten, D. G. Acc. Chem. Res. 1980, Z3, 83. (1 1) Balzani, V.; Bolletta, F.; Ciano, M.; Maestri, M. J . Chem. Educ. 1983, 60, 447.
(12) Sutin, N. Pure Appl. Chem. 1980, 52, 2717. (13) Juris, A.; Barigelletti, F.;Balzani, V.; Belser, P.; von Zelewsky, A. Inorr. Chem. 1985. 24. 202. (y4) Barigelletti, F.f Belser, P.; von Zelewsky, A,; Juris, A.; Balzani, V. J . Phys. Chem. 1985, 89, 3680. (15) Juris, A.; Belser, P.; Barigelletti, F.; von Zelewsky, A,; Balzani, V. Inorg. Chem. 1986, 25, 256.
0022-3654/86/2090-5 190$01 .50/0
now a detailed study on the temperature dependence of the luminescence emission (energy, intensity, and lifetime) of the same complexes in the temperature range 84-350 K. Experimental Section
PF6- salts of the R~(bpy),_,(taphen),~+ complexes were prepared and purified as described el~ewhere.'~,'~ Two independent batches gave compounds having identical absorption, excitation, and emission spectra. All the complexes were stable in propionitrile-butyronitrile solutions for days, but for longer time periods or upon heating above 350 K they underwent a decomposition reaction. The most reactive appeared to be R~(bpy)~(taphen),+, which could not be studied above 320 K. All the experiments were carried out in a mixture of freshly distilled propionitrile and butyronitrile ( 4 5 v/v). A diluted solution (10-5-10-4 M) of each complex was sealed under vacuum in an 1-cm quartz cell after repeated freeze-pump-thaw cycles. The cell was then placed inside a Thor C 600 nitrogen flow cryostat, equipped with a Thor 3030 temperature controller and indicator. The absolute error on the temperature is estimated to be 1 2 K. The uncorrected emission spectra were obtained with a PerkinElmer MPF-44B spectrofluorimeter equipped with a Hamamatsu R 928 phototube. Emission spectra were recorded by exciting in the lowest energy absorption maximum and were independent of the excitation wavelength. Excitation spectra were performed with a Perkin-Elmer LS-5 spectrofluorimeter. Emission lifetimes were measured by a modified Applied Photophysics single-photon-counting apparatus equipped with a thyratron gated N2 lamp (337 nm) or by a JK system 2000 Nd3+:YAG DPLY4 laser. The decay was monitored at the maximum of the emission band. Data (16) Belser, P.; von Zelewsky, A . Helc. Chim. Acta 1980, 63, 1675.
0 1986 American Chemical Society
