J. Phys. Chem. 1995, 99, 3235-3239
3235
Solid-state NMR and ESCA Studies of the Framework Aluminosilicate Analcime and Its Gallosilicate Analogue Heyong He,? Chi-Feng Cheng,? Sudipta Seal: Tery L. Barr,**$and Jacek Klinowski*J Department of Chemistry, University of Cambridge, Lensfleld Road, Cambridge CB2 IEW, U.K., and Department of Materials and Laboratory for Suface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201 Received: June 22, 1994; In Final Form: October 4, 1994@
A detailed ESCA and solid-state NMR study has been carried out of the aluminosilicate analcime (SUA1 = 1.82) and its gallosilicate analogue (SUGa = 2.68). The core level binding energies in (Si,Al)-analcime are consistent with that for framework materials of similar elemental composition. The shift of binding energies in (Si,Ga)-analcime suggests an increased ionicity of the Ga-0 bond compared to the A1-0 bond. The ionicity of the Ga-0 bond is substantially lower than in cloverite, a GaPO4 molecular sieve, as a result of the greater covalency of the P-0 bond compared to the Si-0 bond.
Introduction
This work is part of our effort to extend our X-ray photoelectron spectroscopy (ESCA) and N M R studies of zeolites to other in order to test the scope of both techniques and to assess their value to geochemistry and other fields. Analcime is a framework aluminosilicate with the ideal unit cell composition Na16[A116Si32096].16H20. It is cubic with a = 13.7 A and space group Ia3d.4-7 Closely related to the mineral pollucite, analcime is considered to be intermediate between zeolites and the feldspathoids.8 Like all known hydrothermally prepared aluminosilicates, analcime obeys the Loewenstein rule9 which forbids the presence of A1-0-A1 linkages. Naturally occurring analcimes have a SilA1 ratio of 1.0-3.0, the most usual value being ca. 2.8 As far as we are aware, this work is the first ESCA study of analcime and the first detailed ESCA investigation of any feldspathoid. We have used NMR to probe its bulk chemistry and ESCA to examine the surfacelnear-surface chemistry of analcime and its gallosilicate analogue. Just as for A1045- in framework silicates, the Ga0d5- tetrahedron carries a negative charge. Our objective is to compare the ionicity of Si-0-A1 and Si-0-Ga bonds and the behavior of A1 and Ga in closely related structures. Experimental Section Synthesis of Aluminosilicate Analcime. The synthesis was carried out according to Barrer and Whitelo except that aluminum hydroxide hydrate (Aldrich, 54.5 wt %) and fumed silica (BDH, Cab-0-Si1 M-5) were used instead of amalgamated aluminum and silicic acid, respectively. The other starting materials were sodium hydroxide (Fisons, 98 wt %) and water. The oxide molar ratio of the synthesis gel was Na20:A1203: 5Si02:200H20. NaOH (1.60 g) was dissolved in 30 g of deionized water, mixed with 3.74 g of aluminum hydroxide hydrate, and stirred for 2 h until a milky solution was formed. In a separate container, 6.01 g of fumed silica was added to 40 g of deionized water. The two mixtures were combined with stirring and aged at room temperature, without stirring, for 14 h. Synthesis proceeded in an unstirred 10-mL Teflon-lined t University of Cambridge.
*
University of Wisconsin-Milwaukee. 'Abstract published in Advance ACS Abstracts, February 15, 1995.
0022-365419512099-3235 $09.0010
autoclave at 260 "C for 5 days. The product was washed repeatedly with distilled water, filtered, and dried in an air oven at 100 "C. Synthesis of Gallosilicate Analcime. The synthesis of (Si,Ga)-analcime was described by Ponomareva et al." and Yelon et a1.12 The oxide molar ratio of the synthesis gel was NazO:Ga203:5SiO2:90H20. The required amount of NaOH was dissolved in deionized water, gallium oxide added, and the mixture heated at 90 "C for 2 h with stirring. After cooling to ambient temperature, the mixture was blended with the silica component (fumed silica) and aged at room temperature, without stirring, for 14 h. Synthesis proceeded in an unstirred Teflonlined autoclave at 200 "C for 5 days. The product was washed repeatedly with distilled water, filtered, and dried in an air oven at 100 "C. ESCA Measurements. Most of the ESCA spectra were recorded on the Vacuum Generators ESCALAB system at the Surface Analysis Facility of the University of WisconsinMilwaukee. A conventional A1 K a anode was used with resolution of AU(4f?/2) = 83.95 eV at a line width of ca. 1.0 eV. The background pressure during analysis was ca. 1 x Torr. Spectra recorded on an HP5950 spectrometer duplicated those obtained with the VG instrument. The materials studied are insulators and therefore give rise to charging shifts. These were removed, and the binding energy scale was established by fixing the C( 1s) binding energy of the hydrocarbon part of the adventitious carbonaceous species at 284.6 eV.2,13-15 Adventitious carbon is a surface impurity of all air-exposed solids, and this scale permits the realization of the so-called chemical and group shifts which often reveal the details of the chemistry of the material.14J5 NMR Measurements. Magic-angle-spinning (MAS) N M R spectra were recorded at 9.4 T on a Chemagnetics CMX-400 spectrometer. 29Sispectra were acquired at 79.5 MHz with 2-ps (30") pulses and 90-s recycle delays. 27Al spectra were measured at 104.3 MHz with very short, 0.6-ps (less than lo"), radio-frequency pulses and 0.3-s recycle delays. The rotors were spun in air at 4.5 kHz in a double-bearing probehead. 29Siand 27Al chemical shifts are quoted in ppm from external tetramethylsilane (TMS)and Al(H20)63+,respectively.
Results and Discussion NMR Measurements. The 29Si MAS NMR spectrum of (Si,Al)-analcime shown in Figure l a consists of five lines 0 1995 American Chemical Society
Framework Analcime and Its Gallosilicate Analogue
J. Phys. Chem., Vol. 99, No. 10, 1995 3231
TABLE 1: Comparison of the Chemical Shifts, 6, for (Si,Al)-Analcime (Framework Composition Nal,.ozA11,.o*SiM.90~) and (Si,Ga)-Analcime (Nal3.o4Gal3.o4Si~4.%0%~ 6. mm from TMS
sample (Si,Al)-analcime
(Sfl)NMR
(Si,Ga)-analcime
2.68
Si(4T) -84.79 (8.88)
1.82
A
Si(3T) -89.80 (28.76) -86.39 (9.86) 3.41
Si(2T) -94.88 (39.54) -93.29 (38.96) 1.59
Si(1T) -99.83 (18.92) -100.17 (41.89) -0.34
Si(0T) -104.84 (3.90) -107.01 (9.29) -2.17
T = Al or Ga. Relative peak areas (normalized to 100%)are given in parentheses under each value of the chemical shift. A denotes the difference (d)si,oa- (d)si.~~ for lines with the same value of T.
TABLE 2: Binding Energies Referenced to C(1s) = 284.6 eV and, in Parentheses, Corresponding Line Widths (f0.05 eV) for Analcime and Related Material* material SiAIIAb SiQp) A@P) Ga(3d) O( 1s) Na( 1s) aluminosilicate analcime 1.82 102.2 73.95 531.45 1071.85 (2.3) ( 1.95) (2.4) gallosilicate analcime 2.68 102.6 20.35 531.7 1071.5 (2.7) (2.5) (2.7) (weak) 1.o 101.1 73.5 530.5 1071.45 zeolite Na-A zeolite Na-X 1.25 101.95 73.9 53 1.05 1072.25 531.75 1072.5 zeolite Na-Y 2.5 102.55 74.2 cloveritec 21.6 531.9 (2.7) (2.5) Valence bandwidths for zeolites Na-A, Na-X, analcime, and Na-Y are 9.1, 9.7, 9.9, and 10.3 eV, respectively. From 29SiMAS NMR. [Ga?6~P76~02976(OH),92].192QF (QF denotes quinuclidine fluoride). Aluminosilicate analcime
Sulvey scan (bl Galloskate analcime Ga 1393 )
SI 12 PI
SI 12 SI
-e
z
J
C
,
,
,
.
1600
1600
1400
1200
. 1000
(a) Aluminosilicate analcime
I\
. 600
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Figure 2. ESCA survey scan of (Si,Al)-analcime. +
(see Figures 3 and 5 and Table 2) and also the shape and width of the valence band (see Table 2 and Figure 6 ) . Two measurements gave (Si/Al)~sc* of 1.45 and 1.60. The core level binding energies are consistent with a framework aluminosilicate with ( S i / A l ) m ~= 1.82. Thus, Table 2 shows that (Si,Al)analcime produces binding energies between those for zeolites Na-X (SUA1 = 1.25) and Na-Y (SUA1 = 2.5) and closer to the values of Na-X. Thus, the material follows the pattern typical of all pure framework aluminoslicates in which the core level binding energies grow with the SUA1 ratio. Similar conclusions may be reached from the valence band results, where the three subband features and the 9.9-eV bandwidth for (Si,Al)-analcime are consistent with a framework aluminosilicate of SUA1 = 1.82. It is clear that while ESCA can distinguish between framework aluminosilicates with different SUA1 ratios, it cannot differentiate between materials with different structures, but it can with the same SUA1, in this case between (Si,Al)-analcime and zeolite X with SUA1 = 1.5. This predominant dependence of the core level and valence band patterns on the SUA1 ratio was also borne out in an examination of sodalites3I and
ultra marine^.^^ As we indicate below, changes in the shifting patterns unrelated to the SUA1 ratio are found when different group IIIA elements (A1 or Ga) are present in the framework. The pattern
1120
1080
1040
1000
96 0
Binding energy (eV)
Figure 3. High-resolution ESCA spectra, adjusted to C(1s) = 284.6 eV, of (a) Si(2p) in (Si,Al)-analcime and (b) Ga(3p) and Si(2p) in (Si,Ga)-analcime. is also broken when the comparison is between silicates of different fundamental structure, as in the case of comparisons of the ESCA results for zeolites and clay The Si(2p)/A1(2p) ratio suggests a surface SUA1 ratio for (Si,Al)analcime of = 1.5. The binding energies in zeolites with SUA1 1.5 (see Table 2) are consistent with this ratio. The values are thus indicative of a zeolite with a composition intermediate between zeolites Na-X and Na-Y. The line widths suggest that the aluminosilicate units are relatively singular with somewhat greater variety of the silicate types than the aluminate. The cations revealed are almost exclusively Na+ (with a small amount of Ca2+). The binding energies are also typical of a Na+ form of a framework aluminosilicate of this c o m p o ~ i t i o n . ~ ~ ~ ~ Gallosilicate Analcime. The Ga ESCA spectra of (Si,Ga)analcime are more complex than those of the other elements and, therefore, require careful consideration. We have detected two sets of gallium core lines, the near-valent Ga(3d) peak at 20.4 eV shown in Figure 5 and the Ga(3p) set (both G a ( 3 p ~ ) and Ga(3pllz) in Figure 3. Both of these spectra partly overlap
He et al.
3238 J. Phys. Chem., Vol. 99, No. IO, 1995 Aluminosilicate analcime
A
-
I \
(b) Gallosilicate analcime
e
Valence band
a
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12
10
8
6
4
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0
Binding energy (eV)
542 0
538 0
534 0
530 0
.
Si60
’
Binding energy (eV)
Figure 4. Narrow scans of O(1s) ESCA peaks of analcime. The binding energy scales have been adjusted to C(1s) = 284.6 eV. (a) (Si,Al)-analcimeand (b) (Si,Ga)-analcime. la1 Aiumincsiilcate anaicime
66.0
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7i.O
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Binding energy (eV) ( b ) Gallosilicate analcime
320
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160
160
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Binding energy (eV)
Figure 5. Narrow scan ESCA spectra of (a) Al(2p) peaks for (Si,Al)-analcime and (b) Ga(3d) peaks for (Si,Ga)-analcime. The binding energy scales have been adjusted to C(1s) = 284.6 eV. core lines from other elements. The Ga(3d) line is adjacent to the In(4d) line from the indium foil sample holder used as a relatively soft housing to hold the gallosilicate powder. Note in Table 2 that this Ga(3d) is singular enough to determine the binding energy as 20.35 eV and its line width as 2.5 eV. The latter value is consistent with those for the Si(2p) and O( 1s) of this sample, suggesting a relatively singular gallium species. The Ga(3p) spectrum overlaps the Si(2p) line. The Ga(3p) peaks are characteristically weaker and less well resolved for any gallium system than the Ga(3d) peaks. We have inserted the probable appearance of the Ga(3p3/~)and Ga(3pllz) lines based on the position and relative size of the Ga(3d) line. In this case, peaks of -3-eV width, 3 eV apart with the Ga(3p3/2) at -105.5 eV, are appropriate. Although the manifold of the Ga(3p) spectrum may suggest a mixture of several types of
Figure 6. Representative valence band ESCA spectrum for (Si&)analcime. gallium species, the breadth and lack of definition of this manifold is primarily a result of the overlap with adjacent peaks. We therefore conclude that relative singularity of the gallium species indicated by the Ga(3d) spectrum is an accurate reflection of surface integrity of this material. We cannot preclude, however, the possibility of a modest (-10%) presence of a second phase (probably GazO3). The gallosilicate analogue of analcime had (Si/Ga)EscA = 3 based on comparisons of intensities of both the Ga(3d) and Ga(2p3/2) peaks on the one hand and the Si(2p) peak on the other. The differences of the relative depth of field of the Ga(3d) and Si(2p) must be considered along with the ratios of the photoelectron cross sections to determine the concentration ratios. It is apparent that, as expected, the binding energies of both Si(2p) and O(1s) are shifted in a positive direction compared to ( S i , A l ) - a n a l ~ i m e . ~ In~light , ~ ~ of our earlier ESCA studies of zeolites, the shifts appear to be more typical of a framework system with Si/IIIA of ca. Z 3 3 l 3 Our previous studies of related oxides suggest a reason for the absence of a “complete” positive binding energy shift.15,33 This is related to the substantial difference in the chemical bonding influence of Ga compared to Al. Thus, the insertion of Ga-0 bonds into a Si-0 framework in place of A1-0 bonds changes the balance of the covalencylionicityin the oxide bonds. As we have demonstrated e l s e ~ h e r e , ’ the ~ - Ga-0 ~ ~ ~ ~bond ~~~~ is much more ionic than the A1-0 bond. As a result, the Si-0 bond in materials with a significant concentration of Si-0Ga units will be more covalent than Si-0 in materials with the equivalent amount of Si-0-Al units. Thus, the lack of any significant positive binding energy shift of the Si(2p) peak in (Si,Ga)-analcime reflects both the increase in ionicity of the induced IIIA bond and the relative increase in the concentration of Si. The former reduces the positive shift to a value significantly below that for a framework aluminoslicate with SUA1 = 3, such as zeolite Na-L. We also find that the binding energy of Ga is not nearly as ionic (Ga(3d) in Table 2 is not as positive) as is the case for gallium in the cloverite molecular sieve.3 This is because the P-0 bond is significantly more covalent than the Si-0 bond.33 As we have argued in detai1,15s33in a system with M-0-M’ bonds, the more covalent the M’-0 bond, the more ionic the M - 0 bond. Thus, the extreme covalency (ca. 75-80%) of the P-0 bond in cloverite3 makes the Ga-0 bond much more ionic than when Ga-0 is substituted into a Si-0-bonded system (ca. 50% covalent). This manifests itself in ESCA as a reduction in Ga binding energies for (Si,Ga)-analcime compared to the corresponding values for cloverite (see Table 2).
Conclusions ESCA is able to distinguish different populations of A1-0Si and Ga-0-Si bonds in framework silicate analcime. The
J. Phys. Chem., Vol. 99, No. IO, I995 3239
Framework Analcime and Its Gallosilicate Analogue greater ionicity of the Ga-0 bond compared to the A1-0 bond is also reflected in the ESCA spectra. The binding energies of the key core level peaks all shift in value to register the resulting change in relative covalency/ionicity as the SUA1ratio is altered. The changes in binding energies are consistent with those for framework aluminosilicates with the same SUA1 ratio. Similar patterns of behavior are found for the valence bands, where both the size and subband structure of the valence band of the (Si,Al)-analcime are consistent with patterns previously established for zeolites.
Acknowledgment. We are grateful to the Fulbright Commission for a Professorial Fellowship for T.L.B. and to the Oppenheimer Fund for a Research Studentship for H.H. References and Notes (1) Barr, T. L. Zeolites 1990,IO, 760. Barr, T. L. Appl. Surj. Sci.
1983,15, 1. (2) Barr, T. L.; Lishka, M. A. J . Am. Chem. SOC.1986,108, 3178. Barr, T. L.; Chen, L. M.; Mohsenian, M.; Lishka, M. A. J. Am. Chem. SOC. 1988,110, 7962. (3) Barr, T. L.; Klinowski, J.; He, H.; Alberti, K.; Muller, G.; Lercher, J. A. Nature 1993,365, 429. (4) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types; Butterworths: Sevenoaks, 1992. (5) Taylor, W. H. Z. Kristallogr. 1930,74, 1. (6) Knowles, C. R.; Rinaldi, F. F.; Smith, J. V. Indian Mineral. 1965, 6, 127. (7) Ferraris, G.; Jones, D. W.; Yerkess, Z. Z. Kristallogr. 1972,135, 240. (8) Deer, W. A,; Howie, R. A.; Zussman, J. An Introduction to the Rock-Forming Minerals, 2nd ed.; Longman Scientific: London, 1992; pp 496-502. (9) Loewenstein, W. Am. Mineralog. 1954,39, 92. (10) Barrer, R. M.; White, E. A. D. J . Chem. SOC.1952,1561. (11) Ponomareva, T. M.; Tomilov, N. P.; Berger, A. S. Zh. Neorg. Khim. 1979,24, 530. (12) Yelon, W. B.; Xie, D.; Newsam, J. M.; Dunn, J. Zeolites 1990,10, 553. (13) Barr, T. L. In Practical Surjace Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, 1990; Chapter 8.
(14) Briggs, D., Seah, M. P., Eds. Practical Surjace Analysis, 2nd ed.; Wiley: Chichester, 1990. (15) Barr, T. L. Modern ESCA: The Principles and Practise of X-ray Photoelectron Spectroscopy; CRC: Boca Raton, FL, 1994. (16) Engelhardt, G.; Lohse, U.; Lippmaa, E.; Tarmak, M.; M. Magi, M. Z. Anorg. A&. Chem. 1981,482, 49. (17) Klinowski, J.; Ramdas, S.; Thomas, J. M.; Fyfe, C. A.; Hartman, J. S. J . Chem. Soc., Farahy Trans. 2 1982,78, 1025. (18) McCusker, L. B.; Meier, W. M.; Suzuki, K.; Shin,S. Zeolites 1986, 6,388. (19) Newsam, J. M.; Jorgensen, J. D. Zeolites 1987,7,569. (20) Selbin, J.; Mason, R. B. J . Inorg. Nucl. Chem. 1961,20, 222. (21) Kiihl, G. H. J. Inorg. Nucl. Chem. 1971,33, 3261. (22) Vaughan, D. E. W.; Melchior, M. T.; Jacobson, A. J. In Intrazeolite Chemistry;Stucky, G. D., Dwyer, F. G., Eds.; ACS Symposium Series 218; American Chemistry Society: Washington, D.C., 1983; p 231. (23) Thomas, J. M.; Kliinowski, J.; Ramdas, S.; Anderson, M. W.; Fyfe, C. A,; Gobbi, G. C. In Intrazeolite Chemistry; Stucky, G. D., Dwyer, F. G., Eds.; ACS Symposium Series 218; American Chemical Society: Washington, D.C., 1983; p 159. (24) Newsam, J. M.; Jacobson, A. J.; Vaughan, D. E. W. J . Phys. Chem. 1986,90, 6858. (25) Barrer, R. M.; Baynham, J. W.; Bultitude, F. W.; Meier, W. M. J. Chem. SOC.1959,195. (26) Newsam, J. M. J . Phys. Chem. 1988,92, 445. (27) Yang, J.; Xie, D.; Yelon, W. B.; Newsam, J. M. J . Phys. Chem. 1988,92, 3586. (28) Xie, D.; Yang, J.; Yelon, W. B.; Newsam, J. M. In Proc. Symp. on Microstructure and Properties of Catalysts; Treacy, M. M. J., White, J. M., Thomas, J. M., Eds.; MRS Symp. Proc. vol. 111; Materials Research Society: Pittsburgh, PA, 1988; pp 147-154. (29) Sulikowski, B.; Klinowski, J. J . Chem. SOC.,Chem. Commun. 1989, 1289. (30) O’Keefe, M.; Hyde, B. G. In Structure and Bonding in Crystals; O’Keefe, M., Navrotsky, A., Eds.; Academic: New York, 1981; Vol. 1, p 227. (31) Herreros, B.; He, H.; Barr, T. L.; Klinowski, J. J. Phys. Chem. 1994,98, 1302. (32) He, H.; Barr, T. L.; Klinowski, J. J. Phys. Chem. 1994,98,8124. (33) Barr, T. L. J. Vac. Sci. Tech. 1991,A9, 1793. (34) Barr, T. L. Crit. Rev. Anal. Chem. 1991,22, 225. JP941569N
