Detailed interpretation of the 29Si and 27AI high-field MAS n.m.r, spectra of zeolites offretite and omega

C. A. Fyfe,* G. C. G o b b i , G . J . K e n n e d y a n d the la te J . D. Graham1" Guelph Waterloo Centre for Graduate Work in Chemistry, Guelph Campus, Department of Chemistry &Biochemistry, University of Guelph, Guelph, Ontario N1G 2 W1, Canada

and R. S. O z u b k o a n d W. J . M u r p h y * Esso Petroleum Canada, Research and Engineering Department, Sarnia, Ontario, Canada N7T 7M1

and A. Bothner-By, J. Dadok and A. S. Chesnick Department of Chemistry, Carnegie-Mellon University, Pittsburgh, PA, US.A. 15213, USA (Received 5 October 1984)

The 29Si MAS n.m.r, spectra of highly dealuminated zeolites are shown to exhibi t separate Si(OAI) resonances due to crystal lographical ly inequivalent latt ice sites. The chemical shift dispersion due to crystal lographic inequivalence is of the same order of magni tude as that from the presence of aluminium atoms in the first coordination sphere and can result in the overlapping of resonances. Thus, the interpretat ion of the 29Si MAS n.m.r, spectra of zeolites of low Si/AI ratios can be ambiguous. The correct peak assignments are presented for the zeolites offret i te and omega and the problems involved in obtaining quant i tat ively reliable Si/AI ratios from the spectra discussed. In the case of offret i te it is not possible to completely describe the distr ibut ion of sil icon and aluminium over its two non-equivalent crystal lographic sites, al though the Si/AI ratios for a number of samples are shown to agree wi th a random distr ibution. For zeolite omega it is possible to complete ly describe the non random Si distr ibution and to predict the aluminium distr ibution, which is in excel lent agreement wi th that obtained independent ly from the high field (14.1T) 27AI MAS n.m.r, spectrum. All data indicate that Si and AI distr ibut ions in omega are not

random in nature.

Keywords: Spectroscopy; 29Si n.m.r.; 27AI n.m.r.; offretite; omega; dealumination

I N T R O D U C T I O N

Considerable insight has been gained in recent years into the structures of zeolites from the study of their 29Si and 27A1 MAS n.m.r, spectra I . It was first shown by L ippmaa and co-workers 2, that the '-"~Si MAS n.m.r. spectra were sensitive to the nature of the atoms (Si o, AI) attached at the vertices of the SiO4 tetrahedra. Five peaks were observed at characteristic shift values and originally attr ibuted to the five possible local silicon environments, Si(nAl), i.e. Si(4A1), Si(3AI), Si(2AI), Si(1AI) and Si(0A1). It is now recognised that the chemical shift ranges must be extended somewhat I and that caution must be exercised in interpretation of spectra showing single resonances. Despite these indica- tions that the spectra may be more complex than origin- ally thought, the discrimination due to these first co- ordination sphere effects is still the basic starting point in the interpretation of zeolite 29Si spectra. Particularly important is that Si/A1 ratios may be obtained from the spectra assuming Lowenstein's Rule s and using equa- tion (1) where the relevant peak intensities /Si(nAI) a r e obtained from the five peaks in the 298i MAS n.m.r. spectrum. Since the AI content is measured indirectly

* Authors to whom enquiries should be addressed 1" Formerly Department of Chemistry, University of Northern Illinois, De Kalb, Illinois, USA.

by its effect on the silicon spectra, only lattice alu- minium is included in the ratio, in contrast to chemical methods of analysis which measure total silicon and a lumium contents, independent of source.

. . . . . . .

4 /Si(,AI)

n =()

4 0.25n Isi(,Al)

n = 0

(1)

More recently, we have investigated the effects of de- alumination on the 29Si MAS n.m.r, spectra 4.5. The results indicate that the mechanism of the residual line- broadening in the 29Si spectra of low Si/A1 ratio mate- rials is the distribution of environments caused by the Si and AI distributions in the second and further co- ordination spheres. In highly siliceous zeolites narrow lines are observed at the high-field extreme of the Si[4Si] resonances 4. Further, for zeolites with more than one crystallographically inequivalent site, multiple resonances are observed whose number and intensities may be directly related to the framework structure de- termined by X-ray diffraction techniques 5. In general, the chemical shift dispersion due to site inequivalence is of the same order of magni tude as that from the pres- sence of an a luminium atom in the first coordination

0 1 4 4 - 2 4 4 9 / 8 5 / 0 3 0 1 7 9 - 0 5 $03.00 © 1985 Butterworth & Co. (Publishers) Ltd. ZEOLITES, 1985, Vol 5, May 179

MAS n.m.r, spectra of offretite and omega: C. A. Fyfe et al.

Heater E lement,L/..~. I 0 (vertical tube "0

furnace)

Zeol i te 0

Quartz D i s c - - - - ~ -

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: ) O O O O O O O O O O O O O O

: ) O O O O O O O O O O O O O O

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- - - Quartz Tube

0 0 0

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0 0

• " --- Glass Wool

~ 1 2 0 Saturated Air

Figure 1 Schematic representation of the apparatus used for hydrothermal dealuminations

sphere. Thus, peak overlap may result, and the assign- ment of peaks is more complex than simply being due to the five possible silicon environments as discussed above. From a study of the highly siliceous analogues we have presented revised assignments for the zeolites offretite, erionite and omega 5. These dealumination experiments have been repeated by Thomas and co- workers who obtained similar spectra for highly sili- ceous offretite and omega 6. In addition, Ja rman et al. have independently arrived at revised peak assignments for zeolite omega (synthetic mazzite) from a study of the effect of gallium on the 29Si chemical shifts of the gallium analogue 7.

In this paper, we extend our original findings in this area and examine in more detail the effects of site in- equivalence on the 29Si spectra for the two zeolites offretite and omega (synthetic mazzite).

EXPERIMENTAL

29Si MAS n.m.r, spectra were obtained at 9.4T (proton frequency of 400 MHz) using previously described equipment 8. 27A1 MAS n.m.r, spectra were obtained at 9.4T and 14.1T (400 and 600 M H z for IH), the latter using a probe system to be described. The spectra were deconvoluted in terms of Gaussian curves. Si/AI ratios were determined by direct injection using inductively coupled plasma emission spectroscopy 9.

Offretite was synthesized according to the litera- turO °. The sample of omega (synthetic mazzite) was a commercial sample (Union Carbide Carp; E L Z - 9 - 5 ) with a Si/AI ratio of 4.2. Dealuminations were carried out hydrothermally with the apparatus shown in Figure 1. The zeolites were cation exchanged with 2 M ammonium chloride, calcined and then reacted in the quartz tube with water vapour at atmospheric pressure for 12 h at 700°C. The samples were thoroughly washed, re-exchanged and subjected to further treat- ments if necessary. We have found that this procedure

may be used to successfully dealuminate a wide range of zeolite frameworks 4.SJ'J2.

RESULTS AND DISCUSSION

Figures 2 and 3 show the 29Si MAS n.m.r, spectra of zeolites offretite and omega and their completely sili- ceous analogues prepared as described in the experi- mental section. As indicated in the introduction, if the distribution of Si and A1 in the first coordination sphere is the only important feature, the five peaks may be assigned to Si(4A1), Si(3A1) etc. and the Si/AI ratio cal- culated from equation (1). In the case of offretite, a

Si(1AI) S i (2AI )

O f f r e t i t e

S i (2AI )

Si(OAI)

S i (OAI ) Si (1AI)

Si(OAD

Si(OAI)

I I I I

- 9 0 - 1 0 0 - 1 1 0 - 1 2 0

ppm f r o m T M S Figure 2 29Si MAS n.m.r, spectra of offretite at 79.5 MHz (9.4T). Upper spectrum: low Si/A[ ratio material showing peak assignments. Lower spectrum: highly siliceous analogue showing two signals in the ratio of 2:1 from the T-site inequivalence

180 ZEOLITES, 1985, Vol 5, May

number of different assignments have been made to bring the observed Si/A1 ratios into agreement with those calculated from equation ( 1 ) 2 3 3 3 4 . In the past, peaks 1-5 in Figure 2 were assigned to Si(3A1), Si(2AI), Si(1A1), Si(0A1), and Si(0AI) following the assignment of the peaks in clinoptilolite 2. Klinowski and co- workers 13 subsequently suggested a revision in the peak assignments to Si(3A1), Si(2AI), Si(1A1), Si(1A1), and Si(0A1). More recently Nagy and co-workers 14 have assigned the peaks to Si(3A1), Si(1A1), Si(1AI), Si(0A1), and Si(0A1).

The spectrum ofdealuminated offretite clearly shows that separate signals are observed (in the ratio of 2:1 as expected from the crystal structure l~) for the silicons in the two crystallographically inequivalent sites. The effect of the site inequivalence ( = 5.3 ppm) is almost exactly the same as the effect of aluminium in the first coordination sphere ( = 5 . 5 ppm). Thus an artificial degeneracy exists in the low Si/AI spectrum, as indi- cated in Figure 2, due to peak overlap, assuming that the shift difference between the two sites is independent of the aluminium coordination. It is also apparent that any simple application of equation (1) will yield an erroneous Si/A1 ratio.

For a quantitative interpretation of the spectrum, a more complex formula must be used which is analogous to equation (1), but which explicitly takes into account the observed effect of crystallographic inequivalence on the chemical shifts. For the case of two sites A and B, the appropriate formula is given in equation (2) and a more general formula can be written for multiple sites.

4

(/ASi(,AI) + IBSiI,AI I) n = 0

4 (0.25n(IASi(,Al) + IBSi(,A])))

n=O

(2)

In this equation, the denominator is the aluminium content of the lattice as before, but the individual sum- mations cannot be equated with the aluminium distri- butions over the two sites A1A and AI B.

In offretite's 298i spectrum there are four major peaks (labelled 2 - 5 in Figure 2) which have relative areas from a deconvolution in terms of Gaussian curves of: 0.0959 ( -96 .9 ppm), 0.3407 (-102.0 ppm), 0.4085 (-107.2 ppm), and 0.1546 (-112.3 ppm). In this case, since there is exact overlap of peaks from different local silicon environments, the number of observables is re- duced and an exact description of the silicon distribu- tions between sites A and B (i.e. SiA(XAI) and Si,(xA1)) is not possible. Thus, while the two peaks at 6 =-112.3 ppm and 6 - 9 6 . 9 ppm may be assigned to SiB(0AI) and SiA(2AI ) respectively, the two inner peaks each consist of contributions from two silicon coordin- ations. Even if the Si/A1 ratio is known experimentally, the silicon distributions over the two sites cannot be de- duced from the spectrum if no assumptions at all con- cerning the nature of the distribution are made. If the Si/A1 ratio of the material were higher so that only three peaks were observed, an exact solution would be possible.

In related work, using a completely different approach, Melchior and co-workers 7 have reported a general formula from which the lattice Si/A1 ratio may be estimated from the spectrum, once the A1

MAS n.m.r, spectra of offretite and omega: C. A. Fyfe et al.

Si(1AI)

Omega

Si(2AI) / J

Si(2AI) Si(OAI)

, /Si (1AI)

Si(OAI)

Si(OAI)

i/ f

Si(OAI)

I I I I

- 9 0 - 1 0 0 - 1 1 0 - 1 2 0

ppm from TMS Figure 3 29Si MAS n.m.r, spectra of omega at 79.5 MHz (9.4T). Upper spectrum: low Si/AI ratio material showing peak assignments. Lower spectrum: highly siliceous analogue showing two signals in the ratio of 2:1 from the T-site inequivalence

distribution between crystallographic sites is known or assumed. However, the detailed distribution over the sites is still not determined.

Table 1 Observed and calculated Si/AI ratios for zeolite offretite

Sample Si/AI (n.m.r.) Si/AI (ICPES)

1 2.5 2.88 2 2.6 2.88 3 4.6 4.61 4 " 4.4 4.70 5 ° 8.2 7.60 6" 7.3 8.65 7" 8.2 7.20

• Sample prepared by dealumination

ZEOLITES, 1985, Vol 5, May 181

MAS n.m.r, spectra o f offretite and omega: C. A. Fyfe et al.

Equation (2) may still be applied providing an as- sumption is made regarding the overall Si/AI distribu- tion between non-equivalent sites. Assuming that the distribution over the two sites is completely random (which need not be true, as will be discussed subse- quently), the 2:1 ratio of the two peaks will be carried throughout the spectrum. Analysis of the spectra of the starting material and a series ofdea luminated materials yields the Si/AI ratios presented in Table 1. There is an approximate agreement between the two sets of data, indicating, perhaps that deviations from a random dis- tribution in this system are small. This evidence cannot be considered unambiguous given the number of pos- sible variables ifi the system.

Inspection of Figure3 indicates that a similar situation involving site inequivalence applies in the zeolite omega system. An additional complication exists in that the shift difference due to site inequivalence is larger (for Si(0AI)) than the effect of AI in the first coordination sphere. (The assignments given in the Figure differ from those of ref 5 where a drafting error was made.) For this zeolite, however, there is a clear and unam- biguous indication that the Si/AI distribution is not random as one can directly observe from the '-'TAI high field MAS n.m.r, spectra.

Figure 4 shows the '-'TA] MAS spectra at 9.4T and 14.1T without enhancement of any kind. In both cases, two peaks are observed, the resolution increasing with in- creasing field strength as expected. The ratio of these two peaks is 1.6/1 (+0.2) from both spectra. This ratio is different from that reported by Klinowski and Thomas 16 for the same sample. The reason for this dis- agreement is not entirely clear, although there are indications of resolution enhancement techniques being applied in their spectra (unsymmetrical peaks result from residual quadrupolar interactions and enhance- ment techniques should be used with caution in these cases). However, the main point is that the a luminium distribution is definitely not random in this sample as a peak ratio of exactly 2:1 would be observed. Further, although the larger peak might intuitively be assigned to the more populous T site, even this cannot be as- sumed, since the Si/A1 ratio is large enough to accom- modate the larger number of a luminium atoms being in the less common site.

Table 2 Spectral assignments and observed and calculated peak areas for the 29Si MAS n.m.r, spectrum of zeolite omega.* (Si/AI = 4.2)

Observed Calculated

Shift (6) Rel. area Ass ignment Shift (6) Rel. area Ass ignment

-- 93.1 0 .1126 IA(2AI) -- 93.1 0 .1126 IA(2AI) -- 98.6 0 .4154 IA(1AI) -- 98.6 0 . 4 1 5 4 IA(1AI) - - 103 .4 0 .2008 IA(061) - - 1 0 3 . 4 0 .1415 IA(0AI)

+ IB(2AI) - - 1 0 3 . 4 0 .0593 IB(2AI) - -107.1 0.1851 IB(1AI) - -107.1 0 .1851 1s(161) - -113.1 0 .0776 IB(0AI) - -113.1 0 . 0 7 7 6 IB(0AI)

• Deconvolution in terms of Gaussian curves

IA(OAI) IA(1AI) IA(2AI) - - =1.823 - - =2.244 - - =1.899 IB(0AI) IB(1AI) Is(2AI)

s , Si~ =2.079 \ ~ ,,] CALC =1.7 \ ~ ,]ExP =1.6

1 5 6 z

m

l = i i

80 70 60 50 40

ppm from AI(1120)63°

Figure 4 2761 MAS n.m.r, spectra at 104.2 MHz (9.4T) and 156.3 MHz (14.1T) as indicated showing two signals due to tetrahedral aluminium from the T-site inequivalence

For this particular case of zeolite omega, exact des- cription of the distribution of all of the silicon co- ordinations and also of the a luminium atoms over the two crystallographic sites may be calculated. The spectrum is deconvoluted in terms of five Gaussian peaks as indicated in Figure 5, with the relative areas given in Table 2. In the case, four of the peaks may be assigned unambiguously as there is not exact peak overlap. Only one peak, 6 =--103.4 ppm, is due to con- tributions from two resonances, SiA(0A1) and SiB(2AI). The i r relative areas may be determined from the ex- perimental value of the total area and equation (2) using the experimental value of the Si/AI ratio (which is equal to 4.2 from the elemental analysis). The complete set of silicon distributions from this analysis is given in Table 2. There are small but significant deviations of the silicon occupancies over the two sites from the ideal (statistically random) distribution of 2:1, yielding an overall distribution of 2.08:1.

A check of the correctness of this general t reatment may be obtained by calculating the distribution of a luminium over the two sites which, as described above, is experimentally determined to be 1.6/1. The total dis- tr ibution over the two T sites is known to be 2:1 from the X R D structure, equation (3), and the total [All is given by equation (4). (Again it must be noted that only the total AI content is given by (4), the individual sum- mations cannot be equated by AI A and AIB. )

SiA + A1A 2 (3) Si B +AI~ 1

182 ZEOLITES, 1985, Vol 5, May

4 A1A + A18 = ~ " + IgSi(.AI))) (4) . .o(0"25n(IASl(,^O

The calculated value of A1A/A1B from this treatment is 1.7/1, in excellent agreement with the experimental value. This gives general support to the treatment, and also assigns the low-field peak in the 27A1 spectrum to aluminium in site A1A and the high-field peak to aluminium in site B (AIB).

In general, the analysis presented above will only be possible when the number of peaks in the 29Si spectrum is reduced by the Si/AI ratio being quite large. In these cases, a small change in the silicon distribution from random will therefore be reflected in a much larger dif- ference in the aluminium distribution. In many ways, the case of zeolite omega described above is a particu- larly favourable one, both from the accuracy and direct- ness with which the 29Si spectrum may be interpreted and also in the direct observation of the two aluminium sites in the ZTAI spectrum. It cannot, therefore, be ex- pected that such a detailed description of the T atom distributions will be possible for most zeolites with multiple sites. However, for the case of omega where the A1 distribution can be observed directly, it is non- random, and this is probably a general occurrence in zeolites.

ACKNOWLEDGEMENTS

The authors acknowledge the financial assistance of the Natural Sciences and Engineering Research Council of Canada in the form of an Operating Grant and Strategic Grant (Energy) (CAF) and Graduate Scholar- ships (GCG and GJK). The support of an Imperial Oil University Research Grant is also acknowledged. The 9.4T (400 MHz) n.m.r, spectra were obtained at the South Western Ontario High Field N M R Centre and the 14.1T (600 MHz) spectra were obtained at Car- negie-Mellon University. The authors acknowledge helpful discussion with Dr. R. Jarman.

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MAS n.m.r, spectra of offretite and omega: C. A. Fyfe et al.

I I I i I I

- 8 0 -1OO - 1 2 0

ppm f rom T M S Figure 5 Deconvolut ion of the 29Si MAS n.m.r, spectrum of zeolite omega in terms of Gaussian curves. Top: experimental spectrum at 79.5 MHz (9.4T). Bottom: deconvolut in in terms of five Gaussian curves. Middle: theoretical spectrum from the summation of the bot tom curves

8 Fyfe, C. A., Gobbi, G. C., Hartman, J. S., Lenkinski, R. E., O'Brien, J. H., Beange, E. R. and Smith, M. A. R. J. Magn. Res. 1982, 47, 168

9 Mackey, J. R. and Murphy, W. J. Chem. Lett. 1984, 8, 1275 10 Aiello, R. and Barrer, R. M. J. Chem. Soc. A. 1970, 1470 11 (a) Fyfe, C. A., Gobbi, G. C. and Kennedy, G. J. Chem. Lett.

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16 Klinowski, J., Anderson, M. W. and Thomas, J. M. J. Chem. Soc., Chem. Commun. 1983, 525

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