by Dieter Freude, Christian Chmelik, Jörg Kärger, Jürgen HaaseUniversität 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

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 forpulsed field gradients,

maximum 1 T / m

rotor with samplein the rf coil zr

rot 10 kHz

θ

B0 = 9 21 T

Introduction to pulsed field gradient (PFG) NMRIntroduction to pulsed field gradient (PFG) NMR

r.f. pulse t

/2

gradient pulse tgmax = 25 T / m

magnetization

t

free induction Hahn echo

B0

M x

y

z B0

x

y

z

5 4

1 2

3

B0

x

y

z

1 2

5 4

3

B0

M x

y

z

Spin recovery by Hahn echo without diffusion of nuclei:

PFG NMR diffusion measurements baseon radio frequency (rf) pulse sequences. They generate a spin echo, like the Hahn echo (two pulses) orthe stimulated spin echo (three pulses). At right, a sequence for alternatingsine shaped gradient pulses andlongitudinal 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 nucleiPFG NMR, signal decay by diffusion of the nuclei

kDSpg

DSS

exp

2

4exp 0

2

0

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

Fast rotation (160 kHz) of the sample about an axis oriented at the angle54.7° (magic-angle) with respect to the static magnetic field removes all broadening effects with an angular dependency of

o7.543

1cosarc

Chemical shift anisotropy,internuclear dipolar interactions,first-order quadrupole interactions, and inhomogeneities of the magnetic susceptibilityare 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 NMRHigh-resolution solid-state MAS NMR

.2

1cos3 2 rot

zr

θ

B0

MAS PFG NMR MAS PFG NMR diffusometry with spectral resolution diffusometry with spectral resolution

Spectral resolution is necessary for studies of mixture diffusion

ωr = 0 kHz

ωr = 10 kHz0.51.01.52.0

δ = 0.02 ppm

ppm

-2024ppm

Example: n-butane + isobutane in zeolite Na-X Example: ethene + ethane in MOF ZIF-8

δ = 0.5 ppm

Metal-Organic Frameworks (MOFs)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.

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 = zeolitic imidazolate framework, ZIF-8ZIF-8 ↔ SOD ↔ SOD

Solid-state NMR spectroscopy and diffusometrySolid-state NMR spectroscopy and diffusometry

Magic-angle spinning NMR spectroscopy on 1H and 13C nuclei in the ZIF-8 framework and in the adsorbed molecules was performed in the field of 17.6 Tesla.

Diffusometry on 1H nuclei of the adsorbed molecules and the molecules in the gas phase was done in the temperature range 283363 K.

11H MAS NMR spectroscopyH MAS NMR spectroscopy

1H 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

1H 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.

1313C NMR spectroscopyC NMR spectroscopy13C CP {1H} MAS NMR

13C 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.

13C MAS NMR

13C 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

Diffusometry of gas phase moleculesDiffusometry 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 m2s for both, the ethene and ethane, gas phase diffusivities.

Diffusometry of the adsorbed moleculesDiffusometry 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.

Diffusometry of the adsorbed moleculesDiffusometry 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 m2s-1 and has a variance of ±10%.

Loading per cavity \ Temperature 283 K 303 K 323 K 343 K 363 K

D (ethene) / 10 ms 2 ethene + 2 ethane 0.65 0.82 0.99 1.11 1.21

D (ethane) / 10 ms 2 ethene + 2 ethane 0.11 0.14 0.18 0.23 0.27

D (ethene) / 10 ms 4 ethene + 4 ethane 0.79 0.95 0.97 1.22 1.25

D (ethane) / 10 ms 4 ethene + 4 ethane 0.13 0.20 0.20 0.25 0.29

Diffusometry comparisonsDiffusometry comparisons

Transport diffusion coefficients DT (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].

0 1 2 3 4 5 6 7 8

10-11

10-10

10-9

ethane

DT o

r D

/ m

2 s-1

c / molecules per cage

ethene

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.

ConclusionsConclusions1H and 13C MAS NMR spectroscopy show that there are no by-products or compounds with different short-range order in the synthesis products of ZIF-8. 13C 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 Dethene:Dethane = 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