Multinuclear MAS NMR Studies of Sodalitic Framework Materials

1454

J. Phys. Chem. B 2000, 104, 1454-1463

Multinuclear MAS NMR Studies of Sodalitic Framework Materials Geoffrey M. Johnson,† Philip J. Mead,† Sandra E. Dann,‡ and Mark T. Weller*,† Department of Chemistry, UniVersity of Southampton, Highfield, Southampton, SO17 1BJ, U.K., and Department of Chemistry, Loughborough UniVersity, Loughborough, Leicestershire, LE11 3TU, U.K. ReceiVed: August 20, 1999; In Final Form: October 27, 1999

A wide range of sodalite framework materials, M8[TT′O4]6X2 where T ) Al, Ga, Si, T′ ) Be, Al, Si, Ge, have been characterized using 27Al, 29Si, and 71Ga magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Structural parameters, such as functions of the framework T-O-T′ angle, correlate linearly with the determined chemical shift values and provide relationships, as a function of T′, which will facilitate characterization of more complex zeolitic compounds containing such species. The effects of changing a particular neighboring framework cation on the resonance position is controlled by variations in both framework bond angles/distances and electrostatic effects; these contributions are resolved.

Introduction Aluminosilicate sodalites of general formula M8[AlSiO4]6‚ X2, where M ) Na, Li, Ag..., and X ) Cl, Br, (ClO4)..., are composed of alternating SiO4 and AlO4 corner-sharing tetrahedraforming four and six rings that make up the basic β-cage unit common to many zeolites.1-10 The structure of the sodalite framework is illustrated in Figure 1. Substitution of the aluminum and silicon in zeolites by other tetrahedra-forming ions diversifies the compound range considerably for each framework type. The consequent structural modifications may influence potential applications such as ion exchange and redox activity. Framework components T and T′ may be taken from a long list of tetrahedron-forming cations including Si, Al, Ga, Ge, As, P, B, Be.11 Examples of substituted structures include AlGe-RHO, GaSi-ANA, GaGe-NAT, and BeP-FAU.12-15 The determination of the structural parameters of framework materials is frequently carried out using powder diffraction methods. Neutron diffraction in particular is important for such studies due to the need to pinpoint the framework oxygen position. The use of powder methods also allows an extended range of materials to be investigated, as in this work, since growth of good quality single crystals is often impossible for all compositions. This is particularly so for ion-exchanged derivatives of a specific framework structure. MAS NMR spectroscopy has been widely applied to zeolitic systems, yielding information concerning framework ordering, framework composition, acidity, mechanisms for dealumination, and, for some simple materials, correlations between chemical shift and framework structure. The nonframework species can also be probed to yield data regarding coordination and mobility of M+ exchangeable cations in addition to coordination, geometry, and mobility of guest molecules or anions.16-23 One of the most useful parameters available from MAS NMR spectra for the characterization of framework structures is the chemical shift, which may be directly related to a number of local structural features. The most important factors include the nature of the neighboring tetrahedral cations (T′) and the T-O-T′ * E-mail: [email protected]. Telephone/fax: +44 1703 593592. † University of Southampton. ‡ Loughborough University.

Figure 1. Sodalite structure showing a single β-cage for Na8[AlSiO4]6‚ Cl2.

bond angle, although additional parameters, for example the distribution of nonframework cations in the vicinity of the NMR active nucleus, may also be influential. To parameterize the various structural effects on the chemical shift of a specific nucleus it is necessary to study an extensive range of materials. In terms of framework structures this is best achieved in the sodalite system where it is possible to enclathrate a wide variety of anions and cations within the intracrystalline voids. The sodalite structure is also stable for a broad variety of framework compositions. This allows the comparison of chemical shift data for a wide range of cell parameters or T-OT′ angles, and thus permits a more accurate elucidation of the effects of neighboring tetrahedral framework cation type and other factors on the resonance position. Deconvolution of the effects of various structural parameters on the chemical shift then allows useful structural information to be derived from NMR data alone. For more complex zeolitic phases, including those with partial framework substitutions, ion exchanged derivatives, and less crystalline materials, such information can be very useful in allowing structural features to be predicted solely from their NMR spectra.

10.1021/jp9929521 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/01/2000

Multinuclear MAS NMR Studies of Sodalitic Materials This paper reports the MAS NMR chemical shift data for a large (approximately 75) number of materials adopting the sodalite framework, M8[TT′O4]6‚X2, where T ) Al, Si, Ga and T′ ) Si, Al, Ga, Be, Ge, recently synthesized by us and other workers, and the correlation of these data with selected structural parameters. We are able, for the first time, to resolve the differing effects of particular framework factors, e.g., geometric and electrostatic, on the chemical shift data for 29Si, 27Al, and 71Ga. Experimental Section The synthesis of sodalites, M8[TT′O4]6‚X2, with frameworks containing Al, Ga, Be, Si, and Ge has been accomplished for a wide variety of anions (X) and cations (M), and full preparative methods have been published elsewhere.24-31 MAS NMR spectra were recorded on a Varian 300 MHz at Durham or a Bruker AM300 spectrometer at Southampton with typical spinning rates between 2.5 and 4.5 kHz. Reference samples used were TMS for 29Si, saturated Al(acac)3 in benzene for 27Al (δ=0 ppm as for 1 M Al(H2O)63+), and 1 M Ga(NO3)3 for 71Ga. Structural information obtained from powder neutron or X-ray diffraction data and crystallographic parameters for individual materials, as well as an overview of structural trends within the sodalite family, are reported elsewhere.24-32 Powder X-ray diffraction data were collected on a Siemens D5000 diffractometer over a period of 15 h over the 2θ range 20-120° with a step size of 0.02°. Powder neutron diffraction data were acquired on the POLARIS or LAD diffractometers on the time-of-flight source at ISIS, Rutherford Appleton Laboratory, or on the constant wavelength D2B instrument at the Institut Laue Langevin, Grenoble, with run times between 4 and 8 h. Structure refinement was performed using the GSAS package,33 providing information such as framework bond angles and cell parameters. Sodalites for which framework species T * T′ adopt the space group P4h3n, while those for which T ) T′ crystallize in the I4h3m space group. In both framework arrangements, a single fixed site exists for the tetrahedral cation and the framework geometry is controlled solely by the oxygen position. Results MAS NMR chemical shift data, 29Si, 27Al and 71Ga, from the various sodalites are summarized in Table 1(a-c); refined cell parameters and T-O-T′ bond angles are also given. In Table 1, sodalites of composition M8[ABO4]6‚X2 are denoted M[AB]X; in certain cases, partial substitution of extraframework sodium by lithium and potassium yields materials of composition MyNa8-y[ABO4]6‚X2, denoted My[AB]X. Estimated errors (esd) for the values are not given in these tables, but are typically 0.04° for T-O-T angles, and 0.0004 Å for cell parameters. The data for the pure silicate represents that of a silica sodalite containing glycol, with three resonances due to three different silicon sites within the structure.34 Figures 2, 3, and 5 plot the variation of chemical shift for the different NMR active nuclei as a function of structural parameters involving framework T-O-T bond angle and T-T′ separation distance. 29Si MAS NMR Data. The low natural abundance of 29Si (ca. 4.7%) gives rise to a dilute spin system which, coupled with the absence of a quadrupolar moment, means that this is an ideal nucleus for MAS NMR studies of zeolitic moieties. Aluminosilicates with a silicon-to-aluminum ratio of unity, when combined with the requirements of Loewenstein’s rule,35 give a single Q4 (Si-(OAl)4) band due to perfect ordering of the Al and Si framework atoms. This is the case for all of the aluminosilicate sodalites studied here, with analogous cases for

J. Phys. Chem. B, Vol. 104, No. 7, 2000 1455 the other framework types synthesized, i.e., perfect ordering of beryllium and gallium in combination with silicon. Therefore, sodalites of the type M8[TSiO4]6‚X2 are ideal for 29Si MAS NMR studies since they are highly symmetrical, possess a single silicon environment, and are easily characterized. For aluminosilicates, the 29Si chemical shift, δ, has previously been related to the average T-O-T bond angle, θ, for zeolites36,37 and for aluminosilicate sodalites by Weller and Wong38, producing the relationship

δ/ppm ) 1.89 - 0.631 θ /°

(1)

More general expressions for aluminosilicate structures have been proposed, such as that of Ramdas and Klinowski39 who examined the effect of aluminum content and average intraframework atomic distance on δ for a range of zeolitic compounds including tetramethylammonium (TMA) silica sodalite and proposed the following relationship

δ/ppm ) 143.03 + 7.95n - 20.34 ΣdTT /Å

(2)

where n is the number of aluminums surrounding each silicon, ΣdTT is the sum of the four average Si-T distances around each Si(OAl)n(OSi)4-n unit assuming Si-O(1) and Al-O(1) bond lengths of 1.62 and 1.75 Å, respectively, and hence defined as

ΣdTT /Å ) [3.37n + 3.24(4 - n)]sin(θ/2)

(3)

The introduction of various levels of aluminum for silicon alters the paramagnetic contribution to δ (the 7.95n term in eq 2), but does not appear highly significant for aluminosilicate sodalites compared with the deshielding of the silicon nucleus through electron withdrawal from the shared oxygen as the aluminum content is augmented. The effective electronegativity of the four oxygens in the T-O framework bonds has been related to the 29Si chemical shift by Radeglia and Engelhardt.40,41 They used their previous observation that the electronegativity of the oxygen depends on the s hybridization of the oxygen orbitals in the T-O bonds,42 in conjunction with the relation between the degree of s hybridization and T-O-T bond angle proposed by Klessinger 43, to form the correlation

δ/ppm ) δ 0/ppm + an + bcos θ/(cos θ - 1)

(4)

where δ0, a, and b are constants. Equations 1 to 4 can be used to test whether the relationships derived for aluminosilicates remain valid upon isomorphous substitution of framework constituents by plotting the chemical shift parameter, δ, against θ, sin(θ/2), and cos θ/(cos θ - 1), where θ represents the framework T-O-T angle. In addition, correlations involving cell parameter10 and secant θ44 were also investigated. A graph illustrating one of these relationships is given in Figure 2a, which shows δ(29Si) plotted against sin(θ/ 2) for silicon in the four different environments Si(OT)4, where T ) Si, Al, Ga, and Be. For each plot, linear regression analysis has been performed to give an equation of the form δ ) mx + c relating the dependent and independent variables. The results are presented in Table 2, along with the linear correlation constant illustrating the goodness of fit. All of the functions investigated produced excellent fits, indicating that the proposed expressions relating δ to framework angle or other parameters are relatively insensitive in terms of the specific function. Indeed these functions themselves are all closely linked through near to linear relationships over the range of θ examined. It should be noted that these graphs, which depend on functions of θ alone, are designed to allow only for the contribution of the

1456 J. Phys. Chem. B, Vol. 104, No. 7, 2000

Johnson et al.

TABLE 1: MAS NMR Chemical Shifts (a) Halide Sodalites chemical shift/ppm sodalite

a /Åa

θ /°b

27Al

29Si

Na[AlSi]Cl Li[AlSi]Cl Li[GaSi]Cl Na[GaSi]Cl K4[GaSi]Cl Li[AlGe]Cl Na[AlGe]Cl K5[AlGe]Cl Na[GaGe]Cl Na[AlSi]Br Li[AlSi]Br Li[GaSi]Br Na[GaSi]Br

8.88 8.47 8.54 8.96 9.10 8.68 9.04 9.27 9.12 8.93 8.51 8.58 9.00

138.1 125.6 122.5 133.7 138.7 123.0 133.1 140.6 129.0 140.4 127.2 122.6 135.9

63.8 72.7

-85.3 -76.4 -68.5 -78.8 -82.4

chemical shift/ppm

71Ga

201.7 182.2 175.5

76.2 69.7 63.7 -86.7 -77.8 -69.6 -80.2

62.0 71.7

198.1 202.2 179.6

sodalite

a /Åa

θ /°b

K4[GaSi]Br Li[AlGe]Br Na[AlGe]Br K4[AlGe]Br Na[GaGe]Br Na[AlSi]I Li[GaSi]I Na[GaSi]I K4[GaSi]I Li[AlGe]I Na[AlGe]I K4[AlGe]I Na[GaGe]I

9.19 8.68 9.09 9.33 9.17 9.03 8.65 9.09 9.28 8.75 9.18 9.37 9.27

141.5 122.9 134.9 142.2 130.5 144.5 124.8 138.8 144.2 124.3 137.5 143.7 133.2

27Al

29Si

71Ga

-84.7

170.9

76.1 68.2 62.5 -89.1 -71.4 -82.3 -87.2

60.0

194.7 199.5 178.1 165.8

74.2 66.1 61.6 189.4

(b) Sodalites Containing Oxo-Anions chemical shift/ppm sodalite

a /Åa

θ /°b

Li7[AlSi]MnO4 Na[AlSi]MnO4 K5[AlSi]MnO4 Na[AlGe]MnO4 Li7[AlSi]ClO4 Na[AlSi]ClO4 K[AlSi]ClO4 Na[GaSi]ClO4 Na[AlGe]ClO4 Li3[AlSi]ClO3

8.72 9.11 9.27 9.25 8.74 9.09 9.34 9.15 9.23 8.85

129.9 149.2 152.2 142.0 132.4 147.5 156.6 140.3 141.5 137.3

27

29

Al

Si

67.0 57.9 53.8 63.4 66.5 58.5 52.1 64.4 64.4

chemical shift/ppm

71

-80.7 -90.6 -94.5 -81.9 -90.2 -97.3 -84.1

sodalite

a /Åa

θ /°b

Na[AlSi]ClO3 K4[AlSi]ClO3 Na[GaSi]ClO3 Na[AlGe]ClO3 Na[AlSi]BrO3 K5[AlSi]BrO3 Li5[AlSi]BrO3 Na[AlGe]BrO3 Na[AlSi]ClO2

9.02 9.23 9.06 9.16 9.04 9.25 8.79 9.15 8.99

144.2 154.8 137.5 137.0 145.3 156.9 135.2 136.2 143.6

60.6 53.9

sodalite

a /Åa

θ /°b

27

Zn[BeSi]S Zn[BeSi]Se Zn[BeSi]Te [SiSi]Glycol [SiSi]Glycol [SiSi]Glycol Cd[AlAl]S Cd[AlAl]Te Ca[AlAl]S Ca[AlAl]Se Cd[AlAl]SO4 Ca[AlAl]Te Sr[AlAl]S Sr[AlAl]Se Sr[AlAl]CrO4

8.13 8.19 8.27 8.83 8.83 8.83 8.82 8.94 9.04 9.07 9.10 9.15 9.27 9.30 9.43

123.6 125.0 126.6 158 153 147 126.1 129.7 131.6 132.5 134.9 135.2 139.5 141.4 149.9

Ga

172.5

-84.0

27

Al

66.8 59.2 53.9 64.8 66.8 60.4

29

Si

-88.4 -94.5 -81.6

71

Ga

175.6

-88.7 -94.5 -82.5 -87.9

(c) Sodalites chemical shift/ppm sodalite

a /Åa

θ /°b

Na[AlSi]SCN Na[AlGe]SCN Na[AlSi]HCO2 Li6[AlSi]HCO2 K5[AlSi]HCO2 Na[GaSi]HCO2 Na[AlGe]HCO2 Na[AlSi]NO2 Na[GaSi]NO2 Li[GaSi]NO2 K2[GaSi]NO2 Na[AlGe]NO2 Li7[AlGe]NO2 K4[AlGe]NO2 Cd[BeSi]S Cd[BeSi]Se Cd[BeSi]Te

9.07 9.23 8.97 8.65 9.16 9.01 9.07 8.92 8.99 8.60 9.09 9.07 8.74 9.19 8.45 8.49 8.56

143.1 138.7 143.5 128.6 145.2 136.2 134.5 140.2 134.4 124.3 138.5 134.6 124.6 137.1 135.2 137.8 139.6

a

27

Al

58.5 64.5 61.5 69.2 56.0 68.3 62.4

68.0 74.2 64.9

29

Si

chemical shift/ppm

71

Ga

-90.0 -86.8 -79.0 -91.9 -80.5 -86.6 -79.8 -72.9 -83.1

179.8 179.7 199.7 176.5

-72.7 -73.6 -74.7

Al

29

Si

71

Ga

-67.8 -68.9 -70.2 -117 -114 -110 80.4 79.8 79.1 78.7 79.1 77.7 76.9 76.6 75.5

a ) unit cell parameter. b θ ) framework T-O-T′ angle.

neighboring oxygen atom to the chemical shift. Additional factors such as the T-T′ separation and the electrostatic influence of T′ are not allowed for or only partly allowed for in such correlations. Our results are in excellent agreement with the more restricted range reported by Newsam,37 who investigated a variety of zeolite materials including three sodium and lithium aluminosilicate sodalite samples. Comparison of the data from the different series of compounds, i.e., those with different T′, sheds light on the other factors, beyond the influence of the neighboring oxygen, which can affect the chemical shift. For the isomorphous series with Si(OAl)4 and Si(OGa)4, the results of our study show that the resonance position of 29Si is shifted upfield by 3.2 ppm from Si(OAl)4 to Si(OGa)4 at a fixed T-O-T angle. For T′ ) Be

and Si the chemical shift data lie to distinctly more positive and more negative values, respectively. Additional factors that control the influence of the next nearest neighbor on the chemical shift of the 29Si nucleus are the separation of these nuclei and the electron withdrawing ability of T′. Equation 2 can be used to predict the effect of the T-T′ separation that obviously alters depending on the size of the T′ cation. Figure 2b plots the 29Si chemical shift against the ΣdTT, the sum of the four average Si-T distances around each Si(OT)4 unit. This is a rather simpler version of eq 3 since n ) 4, and is thus defined as ΣdTT ) [4(Si-O + T-O)sin(θ/2)]. These data fit in with the general observation45 that as the silicon nearest neighbor is changed from Si to Al to Ga to Be, δ is shifted downfield due to its neighbor’s increasing electron

Multinuclear MAS NMR Studies of Sodalitic Materials

Figure 2. Si.

29Si

J. Phys. Chem. B, Vol. 104, No. 7, 2000 1457

MAS NMR shift vs (a) sin(θ/2), θ and (b) ΣdTT. Best line fits are shown to the four groups of material Si-(OT′) T′ ) Be, Al, Ga,

TABLE 2: Linear Correlations of 29Si Resonance Frequency with Structural Parameters framework AlSi GaSi BeSi

mc cc Rd m c R m c R

a /Åa

θb

cos θ/(cos θ-1)

sin(θ/2)

secθ

ΣdTT

-24.34 130.86 0.993 -25.02 145.00 0.999 15.69 59.70 0.999

-0.63 1.94 0.993 -0.79 26.47 0.998 -0.39 20.44 0.988

-166.34 -14.57 0.984 -159.81 -13.75 0.998 -81.38 -39.21 0.990

-210.58 111.40 0.986 -215.71 119.48 0.998 -108.23 27.13 0.990

-30.41 126.65 0.981 -24.06 114.17 0.996 -12.59 90.83 0.991

-15.64 111.59 0.987 -16.19 126.33 0.997 -8.33 27.13 0.990

a a ) unit cell parameter. b θ ) framework T-O-T′ angle. c Equations take the form: chemical shift, δ ) mx + c. d R is the linear correlation coefficient.

withdrawal from the bridging oxygen. This deshields the silicon nucleus as is quantified by the 7.95n term in eq 2, though the data here indicate that replacing Si(OAl)4 by Si(OSi)4 in the sodalite structure for a fixed ΣdTT causes a decrease in chemical shift of around 20 ppm as opposed to just over 30 ppm predicted by eq 2. This 20 ppm shift for the replacement of four silicons by four aluminums corroborates that of 20 ppm derived by Radeglia and Engelhardt using δ(29Si) versus cos θ/(cos θ 1) for various silicates and aluminosilicates.41 Their relationship is described by eq 4, with n ) 4 and a ) 5, with cos θ/(cos θ

- 1) shown to be equally suitable to sin(θ/2) for describing the angular dependence of δ(29Si). Of greater novelty and interest in terms of substitution into zeolitic frameworks, are the effects on 29Si chemical shift caused by replacement of Al by Ga and Be. A previous study on gallosilicate sodalites reported that δ(29Si) was shifted by 6-8 ppm on replacement of aluminum by gallium,46 though determination of this value did not allow for the changing T-O-T angle which would be associated with the inclusion of gallium in the framework. With the data presented in this work it is


Johnson et al.

possible to predict the effect of gallium replacing aluminum both for a fixed T-O-T angle and for the relaxation of this angle to accommodate the larger gallium. For a fixed Si-O-T angle, where T ) Al or Ga, the separation of the best fit lines in Figure 2 is constant at 3.2 ppm. This represents the difference in the electrostatic effects of four nearest neighbor gallium atoms with respect to four aluminum sites; as with the Si(OAl)4-y(OSi)y series this effect is likely to be incremental, i.e., 0.8 ppm per gallium. For beryllium, the effect is around 2.2 ppm per Be substitution, though the range of data here is much more limited and this value is much less reliable. Equation 2 can therefore be rewritten to account for substitutional chemistry in zeolitic frameworks, beyond simple aluminosilicates, as

δ ) δo + ETn + Msin(θ/2)

(5)

where ET depends on the nature of T′ and is related to its electron withdrawing power and its proximity, M ) -210.58 and is the gradient for the AlSi materials, and δo ) 111.4 ppm. Empirically in this work ET is -5 ppm for Si, +0.8 for Ga, and +2.2 for Be (ET by definition is zero for Al), and the relative values are as would be expected on simple electrostatic grounds in terms of the electron withdrawing powers of Si4+, Al3+, Ga3+, and Be2+. One of the potential uses of the structural and chemical shift information presented in this paper is the prediction and interpretation of the NMR spectra of gallium- and berylliumsubstituted zeolites allowing identification of the site of substitution. For such an application the data need to be interpreted carefully, as the substitution of a specific ion will involve a local relaxation of the structure, i.e., the T-O-T bond angle will change as can the T-T separation, though this may be constrained by the lattice parameter. For example, replacing aluminum by gallium in a specific sodalite generally reduces the T-O-T bond angle by approximately 6 degrees and increases the Si-T distance by ca. 0.014 Å. The data presented here can be treated to allow for these effects. For example, the function ΣdTT (Figure 2b) has contributions from both the SiO-T bond angle and the Si-T separation and the vertical separation in δ can be used to represent the combined influences of the change in Si-O-T bond angle and the nature of T. This amounts to 7.8 or 1.9 ppm per gallium atom, and we would expect the 29Si resonance in any zeolitic material to be shifted to less negative chemical shift values by this amount on substitution of a neighboring aluminum by gallium. 27Al MAS NMR Data. Similar correlations to those between 29Si chemical shift and structural parameters have been presented for MAS NMR spectra of the 27Al nucleus. However, since it possesses a quadrupolar moment, spectral line broadening can often be a problem; this can be reduced by using high spinning rates or more complex techniques, such as double angle or dynamic angle spinning spectroscopy. The use of high spinning rates to reduce line broadening is preferred since it is more straightforward experimentally. In general, however, less research has been carried out on 27Al due to spectral complication. Jacobsen et al.47 adapted the relationships described above for 29Si to aluminosilicate sodalites and successfully correlated δ (27Al) with structural parameters such as T-O-T angle. Other correlations have been proposed, including the quadrupolar line broadening with tetrahedral distortion angle to measure asymmetry in AlO4 tetrahedra.48,49 In this work the range of materials has been extended considerably over these earlier studies. Figure 3(a and b) shows 27Al chemical shift plotted against sin(θ/2) and ΣdTT, with linear regression results for all six

correlations investigated summarized in Table 3. The aluminum spectra collected in this work for the Al(OSi)4 and Al(OGe)4 environments showed very little second-order quadrupolar broadening producing very narrow and symmetrical resonances of typical halfwidth 3-5 ppm. The aluminum environment in all of the materials studied was very close to tetrahedral. Several authors have reported that it is necessary to correct for quadrupolar coupling in order to extract precise chemical shift values from observed spectra. However, our untreated data are in excellent agreement with previously reported “corrected” values; for example, we obtained a δ(27Al) value of 63.8 ppm for NaCl sodalite compared with that of 64.1 ppm by Jacobsen et al.47 The maximum difference between our values and those previously reported is less than 0.7 ppm. This indicates that quadrupolar effects do not significantly interfere with data directly derived for 27Al MAS NMR values for these aluminosilicate sodalites with high symmetry aluminum sites. For the pure aluminate sodalites, M8[AlO2]12X2, quadrupolar effects, which are very significant, were corrected for as described previously.49 In a manner analogous to 29Si, δ(27Al) becomes more negative as the T-O-T angle, and hence s orbital contribution in the bond to oxygen, increases. In addition, for the more electropositive silicon the 27Al chemical shift is lower than that for germanium at the same T-O-T angle, with both of these being smaller than the pure aluminates.45,49 Linear regression analysis shows that the data from the aluminosilicates and pure aluminates are best fitted using the unit cell size, whereas aluminogermanates are most effectively correlated using the T-O-T as the independent variable. However, in all cases, the regression coefficients are excellent. Like the 29Si NMR data, the correlations for different Al(OT)4 environments, where T ) Si and Ge, can be fitted by a simple expression such as


(6)

with δo ) 253.03, M ) -203.02 and refers to the aluminosilicate gradient, and ET ) 0.6 for Ge. For the pure aluminate sodalites, with neighboring aluminum sites, the data are not well represented by such an expression, which may indicate that additional parameters fully modeling the quadrupolar effects need to be incorporated into the relationships. In a manner analogous to δ(29Si), the function ΣdTT (Figure 3b) can be used to account for contributions from both the AlO-T bond angle and the Al-T separation, where T ) Si and Ge. In this case, the vertical separation in δ amounts to approximately 9.2 or 2.3 ppm per germanium atom. 71Ga MAS NMR Data. For the physical characterization of solids, application of 69Ga or 71Ga MAS NMR spectroscopy remains in a relatively early stage of development. Ga MAS NMR spectra in the literature are fairly uncommon, and those that have been cited generally display severe quadrupolar line broadening. It has, however, been successfully utilized by Zhong and Bray in their characterization of cesium gallate glasses with quite different Ga chemical shifts for tetrahedrally and octahedrally coordinated gallium,50 indicating that Ga MAS NMR could be an effective tool in the analysis of solids. Ione et al.51 have used 69Ga for a study on gallosilicate zeolites; other studies such as that of Timkin et al.,52 who employed both 69Ga and 71Ga, illustrated the importance of high MAS NMR spinning frequencies to reduce the chemical shift anisotropy effects on line widths. Of the two gallium isotopes, 71Ga is more attractive


Figure 3.

27Al

TABLE 3:

MAS NMR shift vs (a) sin(θ/2), θ and (b) ΣdTT. Best line fits are shown to the three groups of material Al-(OT′) T′ ) Al, Si, Ge.

27Al

MAS NMR Chemical Shift versus Structural Parameters

frameworkb AlSi AlGe AlAl


mc cc Rd m c R m c R

a /Åa

θb

cos θ/(cos θ-1)

sin(θ/2)

secθ

ΣdTT

-24.14 277.53 0.994 -20.77 256.24 0.984 -8.37 154.60 0.982

-0.62 149.69 0.989 -0.64 154.70 0.991 -0.22 107.99 0.963

-162.48 132.41 0.986 -133.92 123.01 0.990 -56.18 101.47 0.970

-203.02 253.03 0.990 -175.09 229.51 0.988 -70.69 143.55 0.970

-30.17 22.29 0.981 -20.26 38.73 0.986 -9.68 64.41 0.965

-15.31 256.15 0.991 -13.92 247.73 0.988 -5.05 143.55 0.970


because of its smaller quadrupolar moment, although it has a lower natural abundance: 39.6% compared with 60.4% for 69Ga. For the gallosilicates and gallogermanates of this work, the gallium environment is close to tetrahedral (e.g., lithium gallosilicate halides, O-Ga-O (×4) lies between 108.6 and 109.3°, and O-Ga-O (×4) lies between 109.8 and 111.2°) which means that the quadrupolar broadening and contribution to the chemical shift are small. Timken et al. reported that the

observed and corrected 71Ga chemical shifts differed by approximately 2 ppm for gallosilicate hydroxy sodalite, confirming that quadrupolar effects are relatively minor.52 The 71Ga spectra recorded here displayed broader peaks than those found for silicon or aluminum, but with typical symmetrical peak halfwidths of less than 20 ppm, the comparison with the few spectra reported in the literature is favorable. The spectrum shown in Figure 4, of gallogermanate iodide sodalite, typifies the spectra recorded in this work.


Johnson et al. zeolitic materials. However, studies reported by Jacobsen et al. also demonstrated that a 1:1 linear relationship exists between 29Si and 27Al resonances for sodalite materials.47 It was proposed that this stems from the parallel geometric changes which occur at the Si and Al sites as sodalite composition is altered. Such a relationship would be of importance given the large number of studies relating δ(29Si) to structural parameters such as T-O-T angles in zeolites: obtention of 27Al shift values would indirectly yield information concerning the local silicon environment. This is of practical use since typical acquisition times for 27Al spectra are considerably shorter than for 29Si, due to fast relaxation for Al which permits short pulse repetition to be applied. The study of Jacobsen et al. involved only five sodalites, and we have therefore reexamined this relationship using our extensive data set. A plot of δ(27Al) versus δ(29Si) is shown in Figure 6 (for 23 data points) and described by the equation

δ(27Al) ) 0.98 δ(29Si) + 146.63

Figure 4. 71Ga MAS NMR spectrum of Na8[GaGeO4]6‚I2. The narrow halfwidth indicates that the gallium environment is close to tetrahedral.

The structure-chemical shift relationships derived for 29Si and 27Al were extended to the 71Ga nucleus; those obtained with θ/2 and ΣdTT are displayed in Figure 5(a and b) and linear regression values are summarized in Table 4. The data are in general agreement with the 184 ppm value for gallosilicate hydrosodalite reported by Timken and Oldfield.52 Again the sets of data may be related through an expression of the type


(7)

with δo ) -599.61, M ) -453.51 for GaSi materials, and ET ) 1.7 for Ge. Figure 5b shows δ(71Ga) plotted against ΣdTT for GaSi and GaGe sodalites. The vertical separation between these two almost parallel lines is 22.2 ppm, equating to approximately 5.6 ppm for the replacement of each gallium by germanium. However, very few data points were available for the gallogermanate materials, and studies of further materials of this type would be more instructive. Discussion It has been well documented that correlations between 29Si and 27Al MAS NMR chemical shifts and structural parameters can be used to provide information regarding local chemical and geometric environments of the tetrahedral components of

(8)

The linear correlation coefficient is 0.9919, confirming that a linear relation does exist between δ(27Al) and δ(29Si), which can effectively be described as a 1:1 relationship. (The equivalent correlation of Jacobsen et al. is δ(27Al) ) 1.03 δ(29Si) + 151.94.) We have extended the concept of correlating two types of chemical shift values recorded on the same material by plotting δ(71Ga) against δ(29Si) in Figure 7. The fact that this relationship is linear indicates that both Ga and Si undergo analogous changes in geometric environment (e.g., tetrahedral tilt angle and framework T-O-T bond angle32) with sodalite cell size/ composition. It is envisaged that the equation relating these chemical shift values may be used to predict δ(71Ga) from δ(29Si) spectral values

δ(71Ga) ) 2.04 δ(29Si) + 343.67

(9)

This may be of even more use than the Al/Si relationship, since Ga MAS NMR spectra are more difficult to obtain and can be subject to severe quadrupolar broadening which can make interpretation problematic. In addition, partially substituted gallium structures have been shown to be important catalytically (for example Ga doped pentasil is active in the aromatization of light alkanes53), but the location of Ga is often difficult to determine; developments which will facilitate characterization are therefore significant. Since 29Si does not possess a quadrupolar moment (I ) 1/2), unambiguous spectral information is usually relatively straightforward to extract and could be used in conjunction with our relationship to resolve the Ga environment. For a series of compounds M8[(Al/Ga)SiO4]6X2 with the same M and X, the 27Al chemical shift is plotted against the 71Ga chemical shift in Figure 8. After allowing for relaxation in the local structure as gallium dopes for aluminum, such a correlation may then be used to relate the chemical shifts of gallium and aluminum on the same site in a framework material. For

TABLE 4: Linear Correlation of 71Ga MAS NMR Results with Structural Parameters framework GaSi

mc

GaGe

cc Rd m c R

a /Åa

θb

cos θ/(cos θ-1)

sin(θ/2)

secθ

ΣdTT

-50.56 635.14 0.999 -57.29 720.33 0.998

-1.64 402.29 0.998 -2.06 463.52 0.999

-336.19 319.64 0.996 -435.53 366.24 0.999

-453.51 599.64 0.997 -571.06 713.45 0.999

-50.01 109.19 0.995 -67.89 90.20 0.999

-33.49 607.44 0.997 -36.93 675.23 0.998




Figure 5.

71Ga

MAS NMR shift vs (a) sin(θ/2), θ and (b) ΣdTT. Best line fits are shown to the two groups of material Ga-(OT′) T′ ) Si, Ge.

Figure 6.

27Al

MAS NMR shift vs 29Si MAS NMR shift for aluminosilicate sodalites.

example, changes in Al-O-T and Ga-O-T bond angles as gallium replaces aluminum may not be the same in all framework materials as found between a pair of sodalites and as assumed in this graph, but the correlations from Tables 3 and 4 could be used to correct for this. Using these relationships,

structural information can then be extracted for poorly crystalline or amorphous phases, doped zeolites, or solid solutions. Bradley and co-workers54 have investigated the relationship between27Al and 71Ga NMR chemical shifts for a range of structurally analogous aluminum and gallium compounds con-


Johnson et al.

Figure 7.

71Ga

MAS NMR shift vs 29Si MAS NMR shift for gallosilicate sodalites.

Figure 8.

27Al

chemical shift plotted against 71Ga chemical shift for the compounds M8[(Si,Ge)TO4]6‚X2.

taining solely oxygen in the first metal coordination spheres. Their relationship was derived using data from a wide range of materials, including polyoxo complexes, oxyhydroxides, hydroxides, and selected zeolites such as X and Y (FAU), natrolite (NAT), and sodalite (SOD). However, the correlation resulting from our work relates solely to sodalites and hence would be expected to reflect more accurately the MAS NMR chemical shifts for zeolitic materials. The correlation resulting from our data analysis is described by the equation

δ(71Ga) ) 2.22 δ(27Al) + 42.03

(10)

with a linear regression coefficient, R ) 0.9924. (The equivalent correlation of Bradley et al. is (δ 71Ga) ) 2.83(δ 27Al) - 4.50). Such a relationship permits the prediction of 71Ga chemical shifts, if δ(27Al) is known, and a greater understanding of observed 71Ga chemical shifts. Although the relationships derived here cover only materials with a one-to-one ratio of framework atoms, they may be expanded to include zeolitic species in which partial isomorphous substitution on framework sites has been achieved. In

general there is a linear relationship between the chemical shift change and n for a series of compounds T(OT′1)4-n(OT′2)n. Hence, for example, the chemical shift changes seen in replacing four aluminum atoms by four gallium atoms may be divided by four to get the expected change for a single site substitution. Using our correlations, the 29Si chemical shift for the sodalite structure of the natural mineral tugtupite, Na8[Al2Be2Si8O24]Cl2, can be calculated. The predicted value of -94.4 ppm is in excellent agreement with the experimental value of -95.1 ppm.55 In addition, the correlations reported in this work may be used to determine the site of isomorphous replacement in complex framework structures. For example replacement of aluminum by gallium in a complex aluminosilicate zeolite structure can be studied by 29Si, 27Al, and 71Ga MAS NMR spectroscopy. At low levels of substitution, little change would be expected in the 29Si spectrum. However, comparison of the chemical shifts for 27Al and 71Ga in equivalent structures may allow determination of the site of substitution. Correlations such as those shown in Figure 8 may be used to this end. In addition to aiding the structural elucidation of partially substituted framework materials, amorphous or poorly crystalline samples

Multinuclear MAS NMR Studies of Sodalitic Materials could be analyzed using spectroscopy in conjunction with the relationships proposed here. Conclusions MAS NMR spectroscopy has been used in conjunction with powder diffraction to provide excellent correlations between chemical shift and a variety of structural parameters for many compositions within the sodalite family. These are in good agreement with those previously reported for aluminosilicates and have been extended to cover gallosilicate, aluminogermanate, gallogermanate, and beryllosilicate sodalites. These relationships may prove useful in the characterization of more complex zeolitic systems such as those involving partial framework substitution, and more particularly, for materials which are amorphous or poorly crystalline. Acknowledgment. We thank the EPSRC for use of Varian MAS NMR facilities at Durham, and D.C. Apperley and N.A. Davies for help with data collection. We gratefully acknowledge financial support for P.J.M. from Southampton University, and for G.M.J. from the EPSRC and Johnson Matthey. References and Notes (1) Taylor, D. Contrib. Mineral. Petrol. 1975, 51, 39. (2) Henderson, C. M. B.; Taylor, D. Spectrochim. Acta 1977, 33A, 283. (3) Godber, J.; Ozin, G. A. J. Phys. Chem. 1988, 92, 4980. (4) Hund, F. Z. Anorg. Allg. Chem. 1984, 511, 225. (5) Beagley, B.; Henderson, C. M. B.; Taylor, D. Mineral. Mag. 1982, 46, 459. (6) Weller, M. T.; Haworth, K. E. J. Chem. Soc., Chem. Commun. 1991, 10, 373. (7) Tomisaka, T.; Eu¨gster, H. P. Miner. J. 1968, 5 (4), 249. (8) Wong, G. Ph.D. Thesis, University of Southampton, 1990. (9) Hassan, I.; Grundy, H. D. Acta Crystallogr. 1984, B40, 6. (10) Weller, M. T.; Wong, G. Solid State Ionics 1989, 33, 430. (11) Newsam, J. M. Solid State Chemistry: Compounds; Oxford University Press: Oxford, 1992; Chapter 7. (12) Johnson, G. M.; Tripathi, A.; Parise, J. B. Microporous Mesoporous Mater. 1999, 28, 139. (13) Yelon, W. B.; Xie, D.; Newsam, J. M.; Dunn, J. Zeolites 1990, 10, 553. (14) Klaska, K. H.; Jarchow, O. Z. Kristallogr. 1985, 172, 167. (15) Gier, T. E.; Stucky, G. D. Zeolites 1992, 12, 770. (16) Kempa, P. B.; Engelhardt, G.; Buhl, J.-C.; Felsche, J.; Harvey, G.; Baerlocher, Ch. Zeolites 1991, 11, 558. (17) Nielsen, N. C.; Bildsøe, H.; Jakobsen, H. J.; Norby, P. Zeolites 1991, 11, 622. (18) Jelinek, R.; Chmelka, B. F.; Stein, A.; Ozin, G. A. J. Phys. Chem. 1992, 96, 6744.

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Multinuclear MAS NMR Studies of Sodalitic Framework Materials

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