Exploring Mass Transfer in Mesoporous Zeolites by NMR Diffusometry

Abstract

With the advent of mesoporous zeolites, the exploration of their transport properties has become a task of primary importance for the auspicious application of such materials in separation technology and heterogeneous catalysis. After reviewing the potential of the pulsed field gradient method of NMR (PFG NMR) for this purpose in general, in a case study using a specially prepared mesoporous zeolite NaCaA as a host system and propane as a guest molecule, examples of the attainable information are provided.

Diffusion Study by IR Micro-Imaging of Molecular Uptake and Release on Mesoporous Zeolites of Structure Type CHA and LTA

“…It is noteworthy that the PFG NMR diffusion studies with the same material reported in Reference [30] did also refer to the option of barrier-limited mass transfer in the purely microporous specimens of LTA. In Reference [32], PFG NMR studies with particles with notably larger diameters (up to 15 μm) allowed the unambiguous determination of intracrystalline diffusivities.…”

“…This technique allows the rate of the redistribution of the guest molecules within the system under study to be observed [30,31], which is generally ensured to be in equilibrium. Owing to the very specific potentials of PFG NMR and its versatility, it is even possible to record molecular diffusivities separately in each of the two pore systems and to check not only overall diffusion enhancement by the presence of the mesopores, but also the opposite effect due to mesopore blocking [32].…”

Probing Mass Transfer in Mesoporous Faujasite‐Type Zeolite Nanosheet Assemblies

“…Together with the fabrication of these materials, the exploration of various phenomena contributing to mass transfer in such materials and their quantitation has thus become a key topic of current research. [8][9][10][11][12][13] Experimental techniques for measuring mass transfer are commonly referred to as macroscopic if the information about mass transfer is deduced from the response of the whole system to well-defined changes in the surrounding atmosphere, such as a stepwise pressure increase (or decrease) in conventional uptake (or release) experiments. [14][15][16] During the course of the experiments, the molecular diffusion paths cover macroscopic dimensions, that is, distances much larger than the sizes of the crystals/particles of the sample, so that information about mass transfer within the individual crystals/particles must always be based on model assumptions.…”

“…Being sensitive to displacements from hundreds of nanometers to hundreds of micrometers, [15,[17][18][19] pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy has served as a particularly sensitive and universally applicable tool for the exploration of mass transfer in complex systems, [16,[20][21][22] including mesoporous zeolites. [9][10][11]13] This paper reports the first application of PFG NMR spectroscopy to diffusion studies in hierarchical pore spaces formed by assemblies of zeolite nanosheets. The potential of this type of porous materials has been impressively demonstrated in ref.…”

Mesostructured Zeolites

“…The nanometer-scale hierarchical pore network provides highly effi cient diffusional pathways between zeolitic micropores and mesopores in a confi ned space. Mehlhorn et al [ 83 ] reported the impact of the presence of mesopores in intracrystalline diffusion using a pulsed fi eld gradient NMR technique. In this context, many research groups have proposed a wide number of synthesis methods for such mesoporous or mesostructured zeolites.…”