J . Phys. Chem. 1994,98, 1302-1305
1302
ESCA Studies of Framework Silicates with the Sodalite Structure. 1. Comparison of Purely Siliceous Sodalite and Aluminosilicate Sodalite Bruno Herreros, Heyong He, Tery L. Barr,*’+and Jacek Klinowski’ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CBl2 IEW, U.K. Received: June 29, 1993; In Final Form: October 29, 1993’
The first detailed X-ray photoelectron spectroscopy (ESCA) study of materials with the sodalite structure shows how the purely siliceous sodalite prepared in a nonaqueous medium differs from the conventional aluminosilicate sodalite and from various zeolites and clay minerals. The purely siliceous sodalite gives a unique set of binding energies and valence band patterns similar to those for silica, while the ESCA pattern of an aluminosilicate sodalite is similar to that of zeolite Na-A. The small differences in binding energies for aluminosilicate sodalite are due to the presence of some K+ and Mg2+ and the fact that the framework Si/Al ratio is slightly higher than unity. While the binding energies for zeolite Na-A and aluminosilicate sodalite form a common set, their ESCA patterns differ substantially from that for the layered aluminosilicate kaolinite. ESCA is thus shown to be sensitive to structure as well as composition.
Introduction In 1887 Sir William Thomson (later Lord Kelvin) demonstratedl that the truncated octahedron (more correctly a tetrakaidodecahedron), the simplest space-filling polyhedron apart from the cube, divides space with minimum partitional area. The tetrakaidodecahedron is thus the three-dimensional equivalent of a regular hexagon, the most economical divisor of a plane. The frameworkof sodalite is a periodic array of such [4686]polyhedra which are known in mineralogy as sodalite or @ cages. The cages are in turn built from corner-sharingSiO4”- and A 1 0 P tetrahedra. The negative charge of the framework brought about by the presence of the aluminate tetrahedra is neutralized by chargebalancing cations, typically Na+. In addition, sodalite cages may accomodate a variety of guest species, such as inorganic salts, water, or organic molecules. Many natural and synthetic materials with the sodalite structure and a variety of enclathrated species have been described.2 Sodalite may have a range of compositions2 but generally has almost equal amounts of tetrahedral Si and A1 in its framework. A typical unit cell formula is Na6A16Si6024*2NaCl. Because all tetrahedral sites in sodalite are crystallographically equivalent and only one kind of cages is present, sodalite is an archetypal molecular sieve. Sodalite is normally synthesized from strongly basic media under mild hydro thermal condition^.^ However, by using ethylene glycol as solvent, Bibby and Dale4 prepared a purely siliceous sodalite. The unit cell composition of the product is Si]2024-2C2H4(OH)2, and the glycol is encapsulated in the sodalite cage^.^^^ We report an X-ray photoelectron spectroscopy (ESCA) study of sodalites including its purely siliceous analogue. Our aim was (i) to carry out the first detailed ESCA study of sodalite, (ii) to see how the purely siliceous sodalitediffers from its aluminosilicate analogue, and (iii) to compare the results with those for various zeolites and clays.6 This work is part of a concerted effort to extend the range of application of the technique to cover all types of silicates. Experimental Section Synthesis of Purely Siliceous Sodalite. The starting materials were ethyleneglycol, Cab-0-Si1 fused silica, and sodium hydroxide in the molar ratio 4 SiOz:NaOH:40 CzHd(OH)z, slightly different from that originally de~cribed.~ NaOH was first added to the Permanent address: Department of Materials and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53201. Abstract published in Adoance ACS Abstracts, January 1, 1994. @
0022-365419412098-1302$04.50/0
glycol, and the mixture was stirred for 6 h until the base dissolved. Fused silica was then added under stirring, producing a thick gel which was transferred into a Teflon-lined stainless steel autoclave and heated under autogeneous pressure at 180 OC for 3 days. The crystallineproduct was finally filtered, washed withdistilled water, and dried at room temperature. It crystallizes in the cubic space group Im3m, with a cell parameter a = 8.83 A (as compared with 8.94 A for a conventional aluminosilicate sodalite). A synthetic aluminosilicate sodalite, made using standard procedure^,^ was also examined in the as-prepared state. Other results referred to in this study were described in earlier publications.”l I Solid-state NMR. Magic-angle-spinning (MAS) NMR spectra were acquired at 9.4 T using a Chemagnetics CMX-400 spectrometer with rotors 4 mm in diameter spun in nitrogen gas at 8-10 kHz. 29Sispectra were measured at 79.5 MHz with 45O radiofrequency pulses and 30-s recycle and A1 spectra at 104.3 MHz with very short, 0.6-ps (less than loo), radiofrequency pulses and 0.3-s recycle delays. IH-W cross-polarization (CP) MAS NMR spectra were measured at 100.63 MHz with 4-ms contact times. The length of the IH and I3C u/2 pulses was 3 ps, recycle delay 10 s, and MAS rate 8 kHz. IH29SiCP/MAS spectra were recorded at 7.05 T using a Bruker MSL-300 spectrometerwith a single contact pulse sequence, 8-ms contact time, 4.8-ps IH 90° pulse, and 10-s recycle delay. The Hartmann-Hahn condition for ‘H-27A1 and lHJ9Si CP/MAS was established using kaolinite12 and for IH-I3C CP/MAS using hexamethylbenzene. Chemical shifts of 27Alare given in parts per million from external Al(H2O)p and the shifts for 29Siand I3C from external tetramethylsilane (TMS). ESCA Measurements. ESCA measurements were carried out using a Vacuum Generators ESCALAB system at the University of Wisconsin-Milwaukee in the conventional mode with A1 Ka X-rays. Powdered samples were either sprinkled onto doublesided adhesive Scotch tape and the excess shaken off or pressed as a wafer onto a thin indium foil. The tape or the foil was then mounted onto a stainless steel specimen stub. No problems with differential charging were encountered (see below). In order to remove charging shifts and deal with Fermi edge coupling problems, binding energies were scaled against the C( 1s) peak (set to 284.6 eV) of the alkane part of the ethylene glycol retained in the sodalite cage and also against any adventitious carbon.6,10.11,I 3 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1303
Framework Silicates with Sodalite Structure
TABLE 1: Core-Level Binding Energies and L i n e Widths (in Parentheses) in eV, Based on C(ls) at 284.6 eV. Also Included are Select Values for the Width at Half Maximum of Various Valence Bands material Si(2p) O(1s) valence band A1 (2p)
’H - 29Si CP 1 MAS
purely siliceous sodalite
Si02 (silica) purely siliceous sodalite zeolite E M - 5 zeolite Na-Y zeolite Na-X -60
-40
-80
-100
-120
-140
zeolite Na-A
-160
montmorillonite kaolinite aluminosilicate sodalite
103.5 (1.8) 103.2 (2.6) 103.1 102.55 (1.7) 102.0 (1.8) 101.1 (1.7) 102.75 (2.05) 102.45 (2.3) 101.5
532.9 (1.9) 532.45 (2.2) 532.45 532.0 (2.05) 531.1 (1.65) 530.5 (1.75) 532.0 (2.4) 531.5 (2.45) 530.9
10.4 10.4 10.4 10.2
14.5 14.4 (1.55) 13.95 (1.6) 13.5 (1.6) 74.8 (2.0) 14.3 (2.2) 13.6
9.8 9.1 10.2 10.2 a
Line width obscured by interference from In(4d). (b) spinning
-40
(b) aluminosilicate sodalite
sample
-60
-60
-100
-120
-140
-160
ppm from TMS
Figure 1. 1H-29Si C P spectra of purely siliceous sodalite: (a) static sample, (b) with MAS. 350
300
250
200
150
100
50
0
1H-13CCPIMAS purely siliceous sodalite
(a) purely siliceous sodalite
64 19 ppm
65
80
75
70
65
60
55
50
45
1
40
ppm from TMS
Figure 2. lH-l3C CP/MAS spectrum of purely siliceous sodalite.
Results and Discussion Purely Siceous Sodalite. The 29Si CP/MAS spectrum of purely siliceous sodalite consists of a single very sharp single peak at -1 17.3 ppm from TMS (see Figure lb). Even the spectrum of a static sample (Figure l a ) is relatively narrow and symmetric, reflecting the strictly cubic environment of the silicon atoms. The IH-l3C CP/MAS spectrum (Figure 2) consists of a single sharp peak at 64.19 ppm corresponding to the encapsulated ethylene glycol, indicating that all glycol molecules are located on equivalent sites in the structure. The ESCA results are given in Table 1 and Figures 3-7. Also given in Table 1 is a representative set of results for several
200
160
120
80
40
0
Binding energy (eV)
Figure 3. (a) ESCA survey scan of purely siliceous sodalite; (b) ESCA survey scan of aluminosilicate sodalite confirming a significant presence of C, K, and Mg. reference materials. A “survey” ESCA scan for the purely siliceous sodalite is shown in Figure 3a, and Figure 4 documents the C( 1s) region, demonstrating that the principal elemental peaks detected are due to Si and 0 and that substantial amounts of C and moderate amounts of Na are also present. The carbon signal comes from the glycol molecules and the adventitious carbon species which are always present on the outer surface of airexposed m a t e r i a l ~ . ~The J ~ intense peak at 288.5 eV is the C(1s) signal primarily from the hydrocarbon parts of these species (with a charging shift of 3.9 eV). The size and shape of the shoulders in this C( 1s) spectrum indicate the presence of various types of carbon-xygen units, most of them C-OH groups of the ethylene glycol. Thevarious sodiumpeaks indicate that someof the sodium
Herreros et al.
1304 The Journal of Physical Chemistry, VoI. 98. No. 4, 1994 (b) aluminosilicate sodalite
c
.0 E
-2
(1s)
(a) purely siliceous sodalite
r
18
536
534
532
530
528
526
524
522
520
Binding energy (eV)
Figure 6. Key O(1s) peak binding energies adjusted to C(1s) = 284.6
eV . 295
290
285
280
275
Binding energy (eV)
Figure 4. C(1s) region of a high-resolution ESCA spectrum of purely siliceous sodalite.
I\
(b) aluminosilicate sodalite
20
108
106
104
102
100
98
96
94
92
90
Binding energy (eV)
Figure 5. Key Si(2p) peak binding energies adjusted to C(1s) = 284.6 eV.
(which is abundant in the aluminosilicate sodalite) is retained in the purely siliceous material. The sizes of these peaks indicate that the amount of sodium is small. On the basis of the binding energies, we attribute the majority of this sodium to residual NaOH. This is supported by the absence of any other anions.I4 The Si(2p) and O(1s) spectra in Figures 5 and 6 are the ESCA “signatures” of the purely siliceous sodalite. This conclusion is based on (i) the relative narrowness and the Lorentzian-Gaussian shape of the lines, (ii) their relative intensities, (iii) the binding
15
io
b
0
Binding energy (eV)
Figure 7. Valence band ESCA spectra: (a) Purely siliceous sodalite; (b) a-silica; (c) aluminosilicate sodalite; (d) zeolite Na-A. The dotted line removes the In(4d) satellite peak, providing a more accurate rendition of the true valence band spectrum.
energies (see below), and (iv) the nature of the corresponding valence band features (see Figure 7). The widths and shapes of the peaks are those of a singular species. The one exception is in the O(1s) spectra because of the presence of a small amount of NaOH. However, NaOH is not detected in the O(1s) ESCA spectrum because its O( 1s) binding energy is apparently almost identical to those for the purely siliceous sodalite, resulting in a O( 1s) line which is narrow (2.2 eV) and symmetric.
Framework Silicates with Sodalite Structure Some of the other features of these ESCA results become apparent from the analysis of the binding energies of the core lines and of the features of the valence band.8 Thus, we see from Table 1 and Figures 5 and 6 that the binding energies of Si(2p) and O(1s) lines from the purely siliceous sodalite are almost the same as from pure silica.6 The slightly lower values for sodalite may be due to the different structures of the two materials. It has been our contention that there are readily detectible shifts in the binding energy positions for different and that these shifts are primarily due to the nature of the immediate chemical bonding environment adjacent to the Si-0 bonds.9 Thus, for example, as more A1-0 units are inserted into a silica-type framework, the binding energy of the Si(2p) peak progressively decreases to reflect the enhanced covalency induced into the Si-0 bonds by the more ionic A1-0 u n i t ~ . ~As J ~a result, the Si(2p) and O(1s) binding energies in the purely siliceous sodalite and silica are higher than those in ZSM-5.6,8 We note from Table 1 and Figure 7a that the structure and character of the valence band for the purely siliceous sodalite are very similar to those for silica (see Figure 7b). Aluminosilicate Sodalite. Studies of many zeolites and clays allow us to suggest that the binding energies and valence band patterns for aluminosilicate sodalite should be very similar to those for zeolite Na-A (see Figure 7 and Table 1). This is based on the shift of ESCA binding energies and valence bands for similar s y s t e m ~ . ~These J ~ shifts are found to reflect the localized bonding chemistry in each structural type. Thus the ESCA features of aluminosilicate sodalite and zeolite Na-A, two frameworksilicates with Si/Al = 1 which obey the “Loewenstein rule” (which forbids AI-0-A1 linkages), should be very similar, while kaolinite, a clay silicate also with Si/A1 = 1 but built of octahedral aluminate and tetrahedral silicate units, should give substantially different binding energies (see Table l).6,7,9The structural difference between zeolite Na-A and sodalite may not be resolved by ESCA, except perhaps as subtle shifts at very high resolution. At the same time, we note that if all the aluminate units in framework silicates are located in tetrahedral environments, essentially substituted for some of the silicate units (and obeying the Loewenstein rule), then both the A1 and Si binding energies will progressively increase as the Si/Al ratio is i n ~ r e a s e d . ~Structural -~ variations have been found to influence these binding energies, but far less than the change from silicate to aluminate bonding chemistry within the same structural u n k 6 The binding energies for aluminosilicatesodalite should therefore be similar to those of zeolite Na-A, differing only slightly due to the structural differences, just as is the apparent result in comparing the ESCA results for the purely siliceous sodalite with silica. The sample of aluminosilicate sodalite contained OH- instead of the C1- which is normally present in the sodalitecages.2 ESCA detected no evidence of the presence of chlorine. The type of anion has no effect on the sodalite framework2 and thus does not influence the critical ESCA shifts. We note that the relative quantification of the Si(2p) to Al(2p) and the Na(2s) to the Al(2p) in aluminosilicate sodalite also agrees with the result of chemical analysis (Si/Al = 1.05) and suggests that the Na/A1 ratio is slightly lower than unity. The latter feature is explained by the presence, detected by ESCA, of K+ and Mg2+cations (see Figure 3b). The core-level binding energies andvalence band patterns from aluminosilicate sodalite (see Figures 5-7c and Table 1) shift dramatically with respect to the corresponding features for the purely siliceous structural analogue. Substantial shifts of the core lines of the aluminosilicate sodalite also occur with respect to the silica, whereas the key binding energies for the former are almost equal to those for zeolite Na-A (see Table 1 and Figures 5 and 6).6 In addition, thevalence band pattern for aluminosilicate sodalite is also almost identical to that for zeolite Na-A (see Figure 7c and d).8 The fact that there are small differences between the binding energy values for zeolite Na-A and
The Journal of Physical Chemistry, Vol. 98,No. 4, 1994 1305 aluminosilicate sodalite is not unexpected since the %/A1 ratio of the latter is higher. As expected, the shift is to higher Si(2p) and Al(2p) binding energies for the sodalite (see Table 1 and Figures 5 and 6 ) . Theother notable feature is that the core-level binding energies for aluminosilicate sodalite are not at all similar to those for kaolinite (see Table 1). In this case, ESCA is apparently sensitive to thestructuraldifferences (frameworkversus sheet) ofthe species as well as to the Si/Al ratio. Conclusions ESCA core-level shifts and valence band patterns of sodalitetype framework silicates are summarized in Table 1 and Figures 5-7. These show the following: (1) The purely siliceous sodalite gives a unique set of binding energies and valence band patterns which resemble those for silica. The small variations may be indicative of the detection by the ESCA of the structural differences. (2) The ESCA core-level binding energies and the valence band pattern for the purely siliceous sodalite and the aluminosilicate sodalite are very different. (3) The ESCA patterns of aluminosilicate sodalite and zeolite Na-A are similar. The small difference in binding energies may be due to the presence of K+ and Mg2+and the higher Si/Al ratio of the aluminosilicate sodalite. (4) In some cases ESCA is as sensitive to structure as it is to composition. Thus, while zeolite Na-A and aluminosilicate sodalite form a common binding energy set (with Si/Al- 1 and all tetrahedral Si-0 and A 1 4 alternating according to the Loewenstein rule), their ESCA patterns differ substantially from that of kaolinite which also has Si/Al= 1 but in which all A1-0 units are in octahedrally-bonded subsheets linked via apical oxygens to tetrahedral Si-0 subsheets.13 This study of the surface/near-surface chemistry of sodalites leaves many aspects unexamined. Although we have demonstrated that ESCA may differentiate between substantially different members of this group, there are other members of the sodalite-ultramarine family the differences between which are much more subtlee2 An ESCA study of these materials will be described in a future paper. Acknowledgment. We are grateful to Dr. P. J. Barrie, University College, London, for acquiring an N M R spectrum, the Fulbright Commission for a Professorial Fellowship for T.L.B., the Oppenheimer Fund for a Research Studentship for H.H., and Unilever Research, Port Sunlight, for a Research Studentship for B.H. References and Notes (1) Thomson, Sir William. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 1881, 24 (5th series), 503. (2) Deer, W. A.; Howie, R. A.; Zussman, J. An Introduction to the Rock-Forming Minerals, 2nd ed.; Longman: London, 1992; pp 496-502. (3) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (4) Bibby, D. M.; Dale, P. Nature 1985, 317, 157. (5) Richardson, J. W.; Pluth, J. J.; Smith, J. V.; Dytrych, W. J.; Bibby, D. M. J. Phys. Chem. 1988, 92, 243. (6) Barr, T. L. In Practical Surface Analysis, 2nd ed., Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, U.K., 1990; Chapter 8. (7) Barr, T. L. Appl. Surface Sci. 1983, 15, 1. (8) 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. (9) Barr, T. L. Zeolites 1990, 10, 760.
(10) Barr, T. L.; Klinowski, J.; He, H.; Alberti, K.; MOller, G.; Lercher, J. A. Nature 1993, 365, 429. (11) He, H.; Alberti, K.; Barr, T. L.; Klinowski, J. J. Phys. Chem., in press. (12) Rocha, J.; Klinowski, J. J . Magn. Reson. 1990, 90, 567. (13) Barr, T. L. Modern ESCA: The Principles and Practice of X-ray
Photoelectron Spectroscopy; CRC Press: Boca Raton, FL, in press. (14) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis, 2nd ed.; John Wiley: Chichester, U.K., 1990. (15) Barr, T. L. J. Vac. Sci. Technol. 1991, A9, 1793.
