Intracrystalline Transport Resistances in Nanoporous Zeolite X

Abstract

By applying pulsed-field gradient nuclear magnetic resonance (PFG NMR) in comparison to quasi-elastic neutron scattering (QENS), we sense by measurement of the diffusion of n-octane on different length scales, transport resistances in faujasite type X (which is isostructural with type Y and differs by the lower Si/Al ratio only) with mutual distances of less than 300 nm. Direct observation of the real structure of zeolite X by transmission electron microscopy identifies them as stacking faults of mirror-twin type on (111)-type planes of the cubic framework. Thus, direct experimental proof is given that, in general, nanoporous host systems such as zeolite crystals cannot be considered as a mere arrangement of cavities. It is rather the presence of structural defects that dominates their properties as soon as transport phenomena with practical relevance, including chemical conversion by heterogeneous catalysis and chemical separation by molecular sieving and selective adsorption, become relevant.

Hybrid Reverse Monte Carlo simulation of amorphous carbon: Distinguishing between competing structures obtained using different modeling protocols

“…Finally, we note that from our modeling studies it appears that the HRMC technique is capable of capturing the long range internal energy barriers of amorphous carbon materials with very good level of accuracy, at least for some gases. In general, macroscopic diffusivities are often several orders of magnitude smaller than microscopic diffusivities calculated by molecular dynamics simulation in crystalline or other ordered systems such as zeolites, largely because of the presence of defects and related energy barriers [58,60]; however our studies demonstrate that accurately constructed HRMC models are able to decrease this gap to within a small factor.…”

A new view of diffusion in nanoporous materials

“…Possible explanations of these differences included the existence of transport resistances within the zeolite bulk phase (internal barriers), acting in addition to the drag on molecular propagation by the genuine pore space. The existence of such resistances has recently been confirmed by both diffusion measurements over space scales of the order of the distances between these resistances [9 -12] and by direct observation by high-resolution transmission electron microscopy [13].…”

Imaging of transient guest profiles in nanoporous host materials: a new experimental technique to study intra-crystalline diffusion

“…As the classical example (Kärger and Ruthven 1981, 1989, 1992, PFG NMR followed typically the paths of molecular self-diffusion over distances much smaller than the crystal extensions while, during the classical uptake and release experiments, transport diffusion was followed into or out of the whole crystals. Today, we dispose of persuasive evidence (Vasenkov and Kärger 2002;Feldhoff et al 2009) that the resistances governing molecular transport over such different space scale may notably Fig. 7 Methanol, Ethane and Ethanol in ZIF-8 at 298 K. (a) The adsorption isotherm of ethane (upper ordinate scale) shows a usual ("type I") behavior, whereas the isotherms of methanol and ethanol (lower ordinate scale) exhibit a pronounced S-shape ("type III").…”