1. by Dieter Freude, Christian Chmelik, Jörg Kärger, Jürgen Haase Universität Leipzig, Institut für Experimentelle Physik, Linnéstraße 5, 04103 Leipzig, Germany MAS PFG NMR Diffusometry and MAS NMR Spectroscopy of Paraffin-Olefin Mixtures Adsorbed in MOF ZIF-8 Magic-Angle Spinning Pulsed Field Gradient Nuclear Magnetic Resonance as an Established Tool for Diffusometry of Interface Materials gradient coils for pulsed field gradients, maximum 1 T / m rotor with sample in the rf coil zrzr rot  10 kHz θ B 0 = 9  21 T

2. Introduction to pulsed field gradient (PFG) NMR r.f. pulse t  /2  gradient pulse t g max = 25 T / m magnetization   t  free induction Hahn echo  Spin recovery by Hahn echo without diffusion of nuclei:

3. PFG NMR diffusion measurements base on radio frequency (rf) pulse sequences. They generate a spin echo, like the Hahn echo (two pulses) or the stimulated spin echo (three pulses). At right, a sequence for alternating sine shaped gradient pulses and longitudinal eddy current delay (LED) consisting of 7 rf pulses, 4 magnetic field gradient pulses of duration , intensity g, observation time , and 2 eddy current quench pulses is presented. PFG NMR, signal decay by diffusion of the nuclei The self-diffusion coefficient D of molecules is obtained from the decay of the amplitude S of the FID in dependence on the field gradient intensity g by the equation FID, amplitude S     rf pulses gradient pulses     g    ecd

4. Fast rotation (1  60 kHz) of the sample about an axis oriented at the angle 54.7° (magic-angle) with respect to the static magnetic field removes all broadening effects with an angular dependency of Chemical shift anisotropy, internuclear dipolar interactions, first-order quadrupole interactions, and inhomogeneities of the magnetic susceptibility are averaged out. It results an enhancement in spectral resolution by line narrowing for solids and for soft matter. The transverse relaxation time is prolonged. High-resolution solid-state MAS NMR rot zrzr θ B0B0

5. MAS PFG NMR  diffusometry with spectral resolution Spectral resolution is necessary for studies of mixture diffusion ω r = 0 kHz ω r = 10 kHz 0.5 1.01.5 2.0 δ = 0.02 ppm  ppm -2024  ppm Example: n-butane + isobutane in zeolite Na-X Example: ethene + ethane in MOF ZIF-8 δ = 0.5 ppm

6. Metal-Organic Frameworks (MOFs) Potential applications in storage, separations, and catalysis caused a remarkable progress of research activities on metal-organic frameworks (MOFs) [1,2]. The mass transfer of molecular mixtures inside the nanopores and through the outer surface is essential for the applicability of the particular system which is zeolitic imidazolate framework 8 (ZIF-8) [3,4] in the present study. Direct access to the transfer through the outer surface and the mobility in the framework was obtained by our previous IR and interference microscopic investigations [5]. It could be shown that the self-diffusivity exceeds the transport diffusivity if molecular clustering dominates the molecular mobility. For the understanding of the molecular transport detailed information about the self-diffusion of the adsorbed molecules are needed. One of the major differences of MOFs compared to classical nanoporous materials, such as zeolites, is the flexibility of the host lattice. Also for the new MOF subclass of ZIFs (zeolitic imidazolate frameworks) such effects were reported. Gücüyner et al. [6] found a gate- opening effect upon adsorption of an ethene/ethane mixture on ZIF 7. Although a similar effect was not observed in recent permeation measurements through a ZIF 8 membrane for this mixture, the existence of a structural change upon adsorption cannot be ruled out in general. [1]U. Mueller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt, J. Pastre, J. Mater. Chem. 16 (2006) 626-636. [2]G. Feréy and C. Serre, Chem. Soc. Rev., 38 (2009) 1380-1399. [3]H. Bux, F.Y. Liang, Y.S. Li, J. Cravillon, M. Wiebcke, J. Caro, J. Am. Chem. Soc. 131 (2009) 16000-16001. [4]X.C. Huang, Y.Y. Lin, J.P. Zhang, X.M. Chen, Angew. Chem. Int. Ed., 45 (2006) 1557-1559. [5]C. Chmelik, H. Bux, J. Caro, L. Heinke, F. Hibbe, T. Titze, J. Kärger, Phys. Rev. Lett. 104 (2010) 085902. [6]C. Gücüyener, J. van den Bergh, J. Gascon, F. Kapteijn, J. Am. Chem. Soc. 132 (2010) 17704-17706.

7. synthesis: H. Bux, J. Caro, Hannover crystal size: 10 nm … 400 µm window size: ca. 3.4 Å cavity size: ca. 12 Å unit cell: a = b = c  17 Å Cage-cut: potential landscape mIM bridge ZIF = zeolitic imidazolate framework, ZIF-8 ↔ SOD

8. Solid-state NMR spectroscopy and diffusometry Magic-angle spinning NMR spectroscopy on 1 H and 13 C nuclei in the ZIF-8 framework and in the adsorbed molecules was performed in the field of 17.6 Tesla. Diffusometry on 1 H nuclei of the adsorbed molecules and the molecules in the gas phase was done in the temperature range 283  363 K.

9. 1 H MAS NMR spectroscopy 1 H MAS NMR spectrum of a ZIF-8 sample loaded with two ethene and two ethane molecules per cavity. The spectrum was measured at L =750 MHz, rot = 10 kHz, T = 303. ethene ethane 1 H MAS NMR spectrum of the as-synthesized MOF ZIF- 8 measured at a Larmor frequency of L =750 MHz, a MAS frequency of rot = 17 kHz and a temperature of T = 322 K. Asterisks denote spinning side bands.

10. 13 C NMR spectroscopy 13 C CP { 1 H} MAS NMR 13 C CP MAS NMR spectrum of the non-loaded (dotted line) ZIF-8 sample and the sample loaded with four molecules ethene plus four molecules ethane per cavity (solid line), measured at L = 188 MHz, rot = 10 kHz and T = 303 K. Inlets increase the chemical shift scale by 10. 13 C MAS NMR 13 C MAS NMR proton decoupled spectrum of the MOF ZIF-8 loaded with four ethene and four ethane molecules p.c., measured at L = 188 MHz, rot = 10 kHz, T = 303. ethene ethane

11. Diffusometry of gas phase molecules Decay of MOF of the signals of gas phase molecules in ZIF-8 loaded with two ethene and two ethane molecules per cavity, measured at L =750 MHz, rot = 10 kHz, T = 303 K with a Hahn-echo pulse sequence with a pulse distance of 10 ms with two mono-polar gradient pulses (after the rf pulses) with a duration of 500 µs. The gradient intensity was varied between 0.0 and 0.1 T m . δ = 0.5 ppm Note the advantage of MAS PFG NMR diffusometry with respect to the PFG NMR diffusometry without spectral resolution: The latter would consider the sum of all unresolved signals for the determination of one averaged self-diffusion coefficient. We obtain D = 1.6 × 10  m 2 s  for both, the ethene and ethane, gas phase diffusivities.

12. Diffusometry of the adsorbed molecules 2D-presentation of the signal decay of MOF ZIF-8 loaded with two ethene and two ethane molecules per cavity, measured at T = 363 K with gradient pulse duration and observation time of 2 ms and 200 ms, respectively. The gradient intensity was varied between 0.05 and 0.5 T m .

13. Diffusometry of the adsorbed molecules Self-diffusion coefficients, D, of molecules in two mixtures of ethene and ethane molecules adsorbed in MOF ZIF-8 in dependence on T. D is given in units of 10 -10 m 2 s -1 and has a variance of ±10%.

14. Diffusometry comparisons Transport diffusion coefficients D T (triangles in the figure) which were derived in dependence on the concentration c of molecule mixtures ethene/ethane or single- component molecules from gas sorption uptake experiments by infra-red microscopy, IRM, on a large single crystal (300 µm size) at T = 298 K [1, 2]. Open and solid triangles denote the mixtures and single-components, respectively. Inverted and upright triangles denote ethene and ethane, respectively. The ethene/ethane ratios in the gas mixtures are 1/1.5 and 1.9/1. The latter ratio is denoted by upright bars in the open triangles. The concentration c corresponds to the sum of ethene plus ethane molecules per cage. Solid pentagons on the bottom denote the self-diffusion coefficient of ethane determined by tracer IR microscopy [1]. Solid asterisks (ethene) and solid spheres (ethane) at c = 4 and c = 8 mixture molecules per cage were taken from the MAS PFG NMR data in the table above for 283 and 303 K. [1] C. Chmelik, H. Bux, J. Caro, L. Heinke, F. Hibbe, T. Titze, J. Kärger, Phys. Rev. Lett. 104 (2010) 085902. [2] H. Bux, C. Chmelik, R. Krishna and J. Caro, J. Membr. Sci. 369 (2011) 284-289.

15. ConclusionsConclusions 1 H and 13 C MAS NMR spectroscopy show that there are no by-products or compounds with different short-range order in the synthesis products of ZIF-8. 13 C NMR spectroscopy gives a weak hint for a preferential adsorption of the molecules close to the methyl-groups of the imidazole-rings. However, no evidence for a gate-opening effect or another structural change upon adsorption of an ethene/ethane mixture is found. Four well-resolved signals were assigned to ethene and ethane molecules, which are adsorbed in the ZIF-8 crystals or non-adsorbed in the gas phase. The corresponding self-diffusion coefficients could be determined separately. The microscopic MAS PFG NMR diffusivities are in agreement with the mesoscopic diffusivities of IR microscopy. The diffusion selectivity is D ethene :D ethane = 5.5 at a loading of 4 molecules per cavity by both techniques. By accounting for the influence of the thermodynamic factor IRM transport diffusivities and NMR self-diffusivities could be directly transferred into each other. The latter is expected only for porous structures consisting of large cavities with narrow windows. The agreement between the results from both techniques is exceptionally good, if we consider the uncertainties in the determination of the absolute concentration and the diffusivities and the fact that crystals from different batches were investigated. The different diffusivities of ethene and ethane can be rationalized by the different size of molecules. This conclusion is supported by the higher activation energies of ethane diffusion compared to ethene. A possible difference in the guest-host interaction between the saturated and non-saturated molecule has no impact on the mobility of the molecules. MAS PFG NMR gives access to a multitude of different aspects of guest diffusion and adsorption. In particular if combined with non-equilibrium methods as IR microscopy a most detailed picture on molecular transport can be obtained which facilitates its understanding on a molecular level. Article in Press: Christian Chmelik, Dieter Freude, Helge Bux, Jürgen Haase, Micropor. Mesopor. Mater. (2011), doi:10.1016/j.micromeso.2011.06.009