Selective multi-component diffusion measurement in zeolites by pulsed field gradient NMR

doi:10.1016/j.micromeso.2005.08.031

Microporous and Mesoporous Materials

Volume 90, Issues 1–3, 20 March 2006, Pages 271-277

Dedicated to the late Denise Barthomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science

https://doi.org/10.1016/j.micromeso.2005.08.031 Get rights and content

Abstract

The potential of pulsed field gradient (PFG) NMR for selective diffusion measurement in multi-component liquids is far from being fulfilled in multi-component diffusion studies with zeolites. We present two recent developments in PFG NMR instrumentation, which will significantly improve the measuring conditions for multi-component diffusion in zeolites and other nanoporous materials. They include options for an enhancement of the sensitivity with respect to smaller displacements by a novel principle of field gradient pulse matching and with respect to selectivity between different components by combining PFG NMR with magic angle spinning (MAS) NMR with a microimaging gradient system. The potentials and limitations of the two options are demonstrated by the first results of selective PFG NMR self-diffusion measurements with zeolitic adsorbate–adsorbent systems containing as much as four different species of guest molecules.

Introduction

In essentially all technical applications, notably for gas separation and matter conversion [1], [2], [3], zeolites get into contact with multi-component systems rather than with one component only. The great potentials of PFG NMR for selective diffusion measurements in multi-component systems have been impressively demonstrated by several research groups [4], [5], [6], culminating in the simultaneous, separate measurement of the diffusivities of as much as eight components [4].

This type of measurement is based on the fact that after Fourier transform of the NMR signal (the spin echo) from time into frequency domain, depending on the respective chemical shifts, the signal of each individual component (more correctly: of even each individual group with identical electronic surroundings) may be separately observed [4], [5], [6], [7], [8], [9]. From the attenuation of this signal with increasing intensity of the magnetic field gradients one directly determines the respective diffusivity. In fact, in signal decoding by the so-called DOSY (Diffusion-Ordered SpectroscopY) technique [10], [11] one takes the other way round by using the determined diffusivities for attributing the lines within the NMR spectrum to the corresponding species.

With multi-component molecular systems adsorbed in zeolites, this measuring principle is impaired by two, simultaneously operating mechanisms: First, as a consequence of both the mobility reduction in the zeolite pore network and of the heterogeneity of the magnetic field immanent to samples with beds of zeolite crystallites, the line widths of the NMR signal of adsorbed molecules are much broader than in the neat liquid. A distinction between different lines and, hence, selective diffusion measurements thus become much more complicated [12], [13].

Second, the reduced molecular mobility and the finite size of the zeolite crystallites which for measuring intracrystalline diffusion have to host the diffusing molecules during the total observation time, require the observation of diffusion paths much smaller than generally necessary for neat liquids. High sensitivity with respect to small displacements, however, require the application of field gradient pulses with great intensity, which—in turn—imply the risk of measuring artefacts, since they increases with the absolute (rather than with the relative) magnitude of the difference between the gradient pulse intensities [14], [15], [16]. Detection and elimination of this mismatch is based on the application of a constant field gradient in addition to the pulsed ones. Such a constant gradient, however, would additionally increase the signal line widths and would thus exclude selective diffusion measurements.

In the present paper we are going to demonstrate how either of these limitations may, at least partially, be overcome. First examples of the benefit attainable in this way are given. One of these options, viz. the reduction of the disturbing influence of sample heterogeneity by combining PFG NMR with MAS NMR, is based on previous, systematic studies of the interrelation between crystal morphology and the signal of the adsorbed molecules [17], [18]. In these measurements, the availability of large crystals of zeolite NaX [19], kindly provided by Professor Sergey Petrovich Zhdanov and his group, and the thus provided option of comparison with smaller (commercially available) crystals, was of particular relevance. The diffusion measurements which we are going to present in this study have been performed with the same material.

Section snippets

PFG NMR with ultra-high-intensity magnetic field gradient pulses

Fig. 1 displays the scheme of the pulse sequence applied for the selective diffusion measurements of zeolitic adsorbate–adsorbent systems by PFG NMR with ultra-high-intensity magnetic field gradient pulses [20]. The sequences of the r.f. pulses (top) and of the field gradient pulses (middle) are those of Cotts 13-interval sequence [21]. It has been introduced as an efficient means to overcome artefacts in the diffusion measurements by internal field gradients. They are an immediate consequence

Results and discussion

Fig. 2(a) shows the ¹H NMR spectra of the adsorbate mixtures considered in our experiments. They have been obtained by Fourier transform of the spin echoes produced by the pulse sequences presented in Sections 2.1 PFG NMR with ultra-high-intensity magnetic field gradient pulses, 2.2 MAS PFG NMR with a microimaging gradient system, respectively, from time domain into frequency domain. The dramatic gain in resolution due to MAS is immediately visible.

Analysis of the spin echo decay ψ due to the

Conclusion

Novel options for applying PFG NMR to selective multi-component diffusion studies in zeolitic adsorbate–adsorbent systems have been presented. They include the possibility of operating with ultra-high-intensity magnetic field gradient pulses also in the cases of selective, i.e. Fourier-transform PFG NMR measurements. Since the application of field gradients of such large intensities are inevitable for diffusion measurement with molecular displacements over small distances (sub-micrometer

Acknowledgements

We dedicate this paper to the memory of Professor Sergey Petrovich Zhdanov. The zeolite material synthesized in his famous laboratory in St. Petersburg has played an important role in the development of PFG NMR to a versatile technique for diffusion measurement in nanoporous material. This statement holds true with even the most recent developments in the field as presented in this contribution.

We thank Bruker Biospin for giving us the option of MAS PFG NMR measurement of zeolitic

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Selective multi-component diffusion measurement in zeolites by pulsed field gradient NMR

Abstract

Introduction

Section snippets

PFG NMR with ultra-high-intensity magnetic field gradient pulses

Results and discussion

Conclusion

Acknowledgements

Pressure Swing Adsorption

Catalysis and Zeolites

Handbook of Heterogeneous Catalysis

Prog. Nucl. Magn. Reson. Spectrosc.

Aust. J. Chem.

J. Magn. Reson. Ser. A

Micropor. Mesopor. Mater.

Micropor. Mesopor. Mater.

Micropor. Mesopor. Mater.

Prog. NMR Spectrosc.

J. Phys. Chem.

J. Am. Chem. Soc.

J. Magn. Reson.

J. Magn. Reson.

Adv. Magn. Reson.

Colloids Surf. A

Magn. Res. Chem.

Synthetic Zeolites

J. Magn. Reson.