Freude, Dieter

Krause, Bärbel C.

Krishna, Rajamani

Kärger, Jörg

Romanova, Ekaterina

Stepanov, Alexander

van Baten, Jasper M.

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 1H NMR Signal Broadening in Spectra of MFI Type Zeolites Ekaterina Romanovaa, Bärbel C. Krausea, Alexander Stepanova, Jasper M. van Batenb,  R. Krishnab, Jörg Kärgera, Dieter Freudeaa Abteilung Grenzflächenphysik, Universität Leipzig, Linnéstr. 5,  04103 Leipzig, Germany, E-Mail: romanova@uni-leipzig.de b Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam,  Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands 1. Introduction Isotropic mobility of molecules adsorbed in zeolites should completely average out spin interactions which cause line broadening in nuclear magnetic resonance (NMR) spectroscopy. But it is well-known that additional application of magic-angle spinning (MAS), Hahn-Echo (HE) or Carr-Purcell (CPMG) pulse sequences reduces the effective line broadening in such systems. This is essential for pulsed field gradient (PFG) NMR experiments, since the cutback of line broadening corresponds to a prolongation of the effective transverse relaxation time T2, which increases the sensitivity of NMR diffuso-metry. Short relaxation times limit the duration of the gradient pulses in PFG NMR. 2. Results of 1H NMR For zeolite types like faujasite, which belong to a cubic symmetry group, MAS, CPMG or HE effect a drastic prolongation of T2, whereas under the influence of a non-cubic symmetry like MFI significantly shorter relaxation times are obtained, see Tab. 1.   T2FID/ms T2CPMG/ms T2HE/ms T2MAS/ms silicalite-1/methane 0.242 12.5 1.02 8.8 silicalite-1/n-butane 0.137 8.0 0.91 6.5 silicalite-1/n-hexane 0.142 10.4 1.24 5.5 Na-X/n-butane 0.518 62.0 1.65 14.4  Table 1: Relaxation times T2 of adsorbed molecules. CPMG measurements were performed with a π-pulse-distance of 2τ = 200 µs. The MAS frequency was 10 kHz. Samples of silicalite-1 were loaded with one molecule per channel intersection, the faujasite sample with two molecules per supercage.  0 2 4 6 8 10 0.010.11time / ms IntensityFIDτ = 500 µsτ = 250 µsτ = 100 µsτ = 50 µsτ = 10 µs Fig. 1: Plot of the echo decay of the1H NMR signal of n-butane applyingCPMG pulse sequences with different π-pulse distances of 2τ from 20 µs to 1000 µs.Silicalite-1 was loaded with one n-butanemolecule per channel intersection.Observation time intervals range from 0 to 10 ms. Extrapolating the curves to 0 ms, weobtain relative intensities of 99% for τ = 10 µs, 90% for 50 µs, 59% for 100 µs, 36% fo r250 µs, and 17% for τ = 500 µs. 1The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application© 2007, E. RomanovaDiffusion Fundamentals 6 (2007) 68.1 - 68.2 3. Results of static H NMR  Concluding the line spacing 17 2.25 kHz  in the 2H NMR spectrum of CD2 groups in der parameter of 17,25 / 60 ≈ 0.29 or −0.29. 4. Snapshots of molecular simulation  mozeolite faujasite which has a cubic symmetrzero. But for a loading of 4 butane moleculesparameter of −0.40 is obtained at a temperatu−0.29 obtained by 2H NMR. 5. Conclusions two, whereas the linewidth should be reduced to zero in the tropic rotation. Anisotropy of the mobility should not be expected for zeolite and also not for linear n-alkanes in zeolites of a cubic symmetry. For Fig. 2: Static 2H NMR spectra of selectively observed CD2 and CD3 groups of fully deuterated n-butane adsorbed in MFI type zeolites. Line fits yield quadrupole coupling constants of 11 kHz and 23 kHz and asymmetry parameters η = 0.4 and η = 0.24 for CD2 and CD3 groups, respectively. 300 −30 δν / kHz CD2 group  −40 0 40CD3 groupδν / kHz  MFI type zeolite, we got an orlecular simulation. For n-butane in the y we obtain the expected order parameter  per unit cell in zeolite silicalite-1, an order re of 300 K. This is not far from the value  The order parameter can be obtained byThe present studies allow the conclusion that the observed slower decaying weak component is part of the signal of molecules with a uniform mobility and can be explained by NMR relaxation effects. For linear molecules like n-alkanes the mobility in a confined environment can be anisotropic. For example, very fast rotation of linear n-alkanes around one rigid axis reduces the NMR linewidth compared to the immobile molecule by a factor of only case of fast isomethane in any  the latter, the molecule jumps between different adsorption sites in such a way that the molecular axis takes all orientations with respect to the external magnetic field with the same probability. This averages out all intramolecular dipolar interactions. However, for n-alkanes in zeolite silicalite-1 an adsorption in the channels is expected, and the channel system does not provide a cubic symmetry. With other words, in one crystallite the molecular orientation is not isotropically distributed. The static linewidth of alkane is some 10 kHz, and also a slight deviation from the cubic symmetry of orientation possibilities can cause a residual dipolar linewidth of about 1 kHz. Now we are only left with the explanation, why methane, with tetrahedral symmetry, also shows an anisotropic behavior in slicalite-1. This can be explained by the additional intermolecular dipolar interaction between the adsorbed molecules. It depends on the symmetry of adsorption sites, which is subjected to a cubic and non-cubic symmetry in zeolites faujasite and silicalite-1, respectively.  2

1 H NMR signal broadening in spectra of MFI type zeolites

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1 H NMR signal broadening in spectra of MFI type zeolites

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