Solid-State NMR Spectroscopy of Inorganic Materials - ACS Publications

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Downloaded by STANFORD UNIV GREEN LIBR on August 1, 2012 | http://pubs.acs.org Publication Date: March 18, 1999 | doi: 10.1021/bk-1999-0717.ch010

Investigation of the Host-Guest Structures of Molecular Sieves by One- and Two-Dimensional Solid-State NMR Techniques C. A. Fyfe, A. C. Diaz, A. R. Lewis, J. M. Chézeau, H. Grondey, and G. T. Kokotailo Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada

One of the most important characteristics of molecular sieve systems is the size and shape selectivity they display towards organic molecules. Unfortunately, it is very difficult to obtain detailed, reliable structures for these host/guest systems to properly understand the details of the interactions because of the lack of large enough single crystals for single crystal diffraction experiments. In this lecture we will outline the development of a protocol to use one and two dimensional NMR experiments to determine these structures which should be of general applicability.

A most important characteristic of molecular sieve systems which is common to their applications as catalysts, sorbents and in gas separations is the size and shape selectivity toward adsorbed organic molecules conferred by the molecular dimensions of their channel and cage systems (1-4). Because of their small crystallite dimensions, powder rather than single-crystal XRD techniques must be used for structure determinations. While it is possible to define framework topologies and structures with powder X-ray diffraction, particularly if Rietveld analysis and synchrotron radiation are used (5,6), it is very difficult to reliably determine the structures of organic sorbate/framework complexes which would yield important information on the detailed nature of the interactions. Important exceptions in this regard are the single crystal XRD studies of van Koningsveld and co-workers who determined detailed high-quality structures of the high-loaded forms of p-xylene and p-dichlorobenzene in zeolite ZSM-5 (7,8). These are the most reliable zeolite/sorbate structures to date. In recent years high resolution solid state NMR has emerged as an important technique complementary to XRD in the investigation of zeolite structures, being particularly sensitive to short to medium range geometries and orderings (9). For some time we have worked to develop new approaches to the investigation of zeolite ©1998 American Chemical Society In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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284 structures by combining solid-state NMR techniques and XRD studies with the aim of ultimately being able to determine the 3-D structures of their complexes with sorbed organic molecules by NMR. In this paper, we outline the development of these techniques and their current standing.


Results and Discussion In high-resolution solid state NMR, the widths of the signals from dilute spin-1/2 nuclei are determined by the degree of crystallinity and the perfection of the local ordering. This can be achieved in the case of zeolites by investigating high-quality, completely siliceous systems where there is only the Si(4Si) local environment present. As illustrated in Figure 1 for (a) ZSM-12 (70) and (b) ZSM-5 (77), sharp resonances are now observed whose numbers and relative intensities reflect the number and occupancy of the crystallographically inequivalent T-sites in the unit cell. In the case of A1P0 materials, there is exact alternation of Al and Ρ giving completely and perfectly ordered frameworks in the as-synthesized materials. This effect is quite general for perfectly crystalline and ordered solids. These spectra may be used to monitor various structural transformations, for example, those induced by temperature as in the case of ZSM-5 (72,75), or in the case of AIPO4 materials by the hydration/dehydration of octahedral Al sites. Of particular importance, they yield information on the interaction of organic sorbates with the molecular sieve framework. For example, Figure 2c shows the Si spectrum of ZSM-5 with p-xylene present at a loading of two molecules per unit cell (u.c.). Comparison with Figure lb shows that the number of T-sites has decreased from 24 to 12 indicating a change in symmetry from monoclinic to orthorhombic induced by the absorption of the organic molecules. Further, the similarities between the spectra of ZSM-5 in the presence of p-xylene and p-dichlorobenzene and p-chlorotoluene (Figure 2) indicate that the interactions, at least in this case, are based on the size and shape of the organic molecule since the CH3 and CI substituents have the same steric factors but the molecules differ in most other aspects (14). The difficulty in using these spectra further is that the assignment of the resonances to the different T-sites is generally not known, although there may be some information from the intensities if the site occupancies are different. In the case of ZSM-12 and ZSM-5, all of the site occupancies are the same and no assignments are possible. This problem can be solved by using two dimensional homonuclear correla tion experiments, such as COSY and INADEQUATE, to establish the three-dimen sional (Si-O-Si) connectivity pattern within theframeworkwhen the topology is known (75). Figure 3 shows such an experiment on ZSM-12 (70). These experiments yield the assignments of the resonances shown in Figure 3. The above experiments are based on the scalar Si/ Si J-coupling which operates through the bonding network. Knowledge of the assignments may now be used to gain additional information on the details of the structures and the various changes which they can undergo. The last step in the extension of these solid state NMR techniques is to apply them to the investigation of the three dimensional structures of zeolite-sorbate 4

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In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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(a)

-toe

-109

-110 PPM

-112

-111

-113

(b)

ι

~

ι

1 0 8

ι

1

PPM

ι

1

FROM

I

I

I

I

I

TMS

~

29

Figure 1. (a) Si MAS NMR spectrum of ZSM-12. Reproduced with permission from ref. 10. Copyright 1990 American Chemical Society, (b) Si MAS NMR spectrum of ZSM-5. Reproduced with permission from ref. 11. Copyright 1988 American Chemical Society. 29


N


29

Figure 2. Si MAS NMR spectra of ZSM-5 loaded with 2 molecules per u.c. of (a) p-dichlorobenzene, (b) p-chlorotoluene, and (c) p-xylene. Adapted from ref. 15.



287

Τ

3

-10·.Ο

Τ4

τ

β

-109.0

-110.0 PPM

-111.0

-112.0

-113.0

29

Figure 3. Si INADEQUATE experiment on ZSM-12. Reproduced with permission from ref. 10. Copyright 1990 American Chemical Society.



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complexes. This can be done by using experiments such as cross-polarization (CP), REDOR and TEDOR, which are based on the through-space dipolar interaction. Because of the strong distance dependence, the distances between the T-sites in the framework (whose identities are now known) and nuclei on suitably isotopically substituted substrates may be determined, yielding the 3-D structure of the zeolitesorbate complex. To test the validity of this approach we have investigated a number of such experiments applied to the high-loaded form of zeolite ZSM-5 containing p-xylene where the answer is knownfromthe high-quality single crystal structure of van Koningsveld and co-workers (7). The simplest experiment is the CP technique with protons as the source nuclei. In order to localize the polarization source as much as possible, experiments were carried out with the two specifically deuterated p-xylenes (1) and (2).

CH

CD

3

(1)

3

(2)

Since the CP process is greatly dependent on molecular motions, these must be well understood for the system being studied. In the present work, these motions were investigated by wide line deuterium NMR of the sorbed organics. It was found that at 6-8 molecules/u.c, the methyl groups in the organic substrate rotate rapidly while the aromatic rings are essentially rigid but a proportion show some low frequency "ring-flips" around the 1,4-axis. However, care must be taken that no motions occur during the very long evolution times (100 ms) of these experiments (see on). It was established from C spin diffusion experiments (not shown) that no such motions occur when the experiments are carried out 0 °C. The effect of the distance dependence can be seen qualitatively from a comparison of the CP spectra with that from a simple one-pulse experiment as shown in Figure 4. The structure is orthorhombic with 24 T-sites of equal occupancy and the assignments of the resonances come as previously from 2-D INADEQUATE experiments (16-18). In the CP spectrum some signals are obviously enhanced compared to the others. The resonances due to the T-sites 1, 2, 10,12 and 16 are quite well resolved and these were used in the study. The spin-dynamics of the cross-polarization processfrom/ to S nuclei as a function of time is described by equation 1 (79). 13

'>=-(-y,,J"'W"/T„,J-«p(-Xj m

S(

s

,

P


289


(a)

-110 -111 -112 -113 -114 -115 -116 -117 -118 -119 -120

-121

ppm from TMS Figure 4. Si NMR spectra of ZSM-5 loaded with 8 molecules per u.c of pxylene (2). (a) CP/MAS with 5ms contact time and a recycle delay of 5s. (b) Quantitative Si MAS (recycle delay 350 s). The numbers indicate T-sites. 29


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Smax represents the theoretical maximum signal intensity obtainable from the polarization transfer, T^H) the proton T i value and TCP the cross-polarization time constant. Thus the S signal intensity as a function of time should consist of an exponential growth controlled by the cross polarization transfer and an exponential decay due the Tj process. Of particular interest, TCP can be related to the second moment of the IS dipolar interaction (Aa?is) as in equation 2 and is proportional to tis as in equation 3 (20). p

p

2

C (AR,n

Δ Φ

* · " > ™β

Solid-State NMR Spectroscopy of Inorganic Materials - ACS Publications

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