The Impact of Mesopores on Mass Transfer in Nanoporous Materials: Evidence of Diffusion Measurement by NMR

Abstract

The purposeful incorporation of mesopores has become an important issue of the synthesis and post-synthesis modification of nanoporous solids for generating transport-optimized materials. The potentials of the pulsed field gradient (PFG) technique of NMR for assessing the transport properties of such materials are discusses and first case studies demonstrating the spectrum of information provided by the application of this technique to diffusion studies in hierarchical porous materials and the still existing limitations are presented.

Assemblies of Mesoporous FAU‐Type Zeolite Nanosheets

“…[3,4] Compared to the allover microporous channels in conventional zeolite crystals, such hierarchical pore systems, with wide transport pores intersecting the micropore network like motorways linking a narrow road system, are already proven to reduce diffusion limitations for molecules within zeolitic catalysts. [5][6][7][8][9][10] While for high-silica zeolites, such as ZSM-5, several methods for incorporating additional transport pores were developed during the past few years, [3,4,11] such a substantial collection of approaches is not available for low-silica zeolites. Herein, we present the synthesis and characterization results of Faujasite (FAU)-type zeolite X (Si/Al < 1.5) grown as house-of-cards-like nanosheet assemblies with intracrystalline mesoporosity.…”

Guest Diffusion in Binderless High‐Performance NaX Molecular Sieves

“…where the function P(z,t) is referred to as the mean propagator [7][8][9]. It denotes the probability (density) that, during time t, i.e., during the time interval between the two field gradient pulses, a molecule, arbitrarily selected within the sample, has been shifted along a distance z in the direction of the field gradient.…”

“…Diffusion measurement by PFG NMR is based on the observation of the PFG NMR signal and on recording its attenuation wcdg; t as a function of the width (d) and amplitude (g) of the field gradient pulses, which are applied during a socalled NMR spin echo experiment [7,8], separated from each other by the observation time t. The quantity c is a characteristic constant of the nucleus considered in the experiments (in general, like also in the present studies, the protons, the nucleus of hydrogen, with c proton ≈ 2.675 · 10 8 T -1 s -1 ), referred to as the gyromagnetic (or magnetogyric) ratio. The attenuation of the NMR signal (the spin echo) is correlated with the diffusion behavior within the sample by the relation…”

Numerical Simulation of Hindered Diffusion inγ-Alumina Catalyst Supports

“…PFG-NMR has been used to study the diffusion of probe molecules in various mesoporous samples: water in silica and alumina [12] or in mesoporous NaA zeolite [17], n-pentane and n-heptane in alumina and used hydroprocessing catalysts [18], n-octane and 1,3,5-triisopropylbenzene in mesoporous Y zeolithe [19] or n-hexane in micro-mesoporous activated carbon [20]. Hollewand and Gladden found a D NMR 0 =D NMR e ratio equal to 1.8 for water diffusing in a 182 m 2 /g specific surface area alumina support [12].…”