Relation Between Structure and Diffusion in Nanostructured Porous Solids and in Lipid Membranes
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0899-N08-02.1
Relation between Structure and Diffusion in Nanostructured Porous Solids and in Lipid Membranes Sergey Vasenkov* and Jörg Kärger Physics department, Leipzig University Leipzig 04103, Germany *current address: Chemical Engineering Department University of Florida P.O. Box 116005 Gainesville, FL 32611-6005, USA
ABSTRACT Pulsed field gradient (PFG) NMR technique has been applied to study molecular transport in two different types of nanostructured materials, viz. in fluid catalytic cracking (FCC) catalysts and in lipid membranes. Diffusion studies have been performed for a broad range of molecular displacements covering displacements which are as small as a fraction of a micron. The effective diffusivities recorded on various length scales are used to evaluate the relevance of various transport modes in the particles of FCC catalysts for the rate of molecular exchange between catalyst particles and the surrounding atmosphere. This rate is shown to be primarily related to the diffusion in the meso- and macropores of the particles under the condition of fast molecular exchange between these pores and the zeolite crystals located in the particles. Studies of lipid membranes are focused on developing fundamental understanding of the influence of various types of domains on lateral mobility of lipids. A meaningful study of this influence requires an ability of monitoring lipid diffusion for different displacements that are smaller and larger than the domain size. First PFG NMR data along this direction are presented. INTRODUCTION Recent advances in PFG NMR technique, which allow monitoring molecular diffusion on nanometer and micrometer length scales, open a direct way to study relation between structure and transport in various nanostructured materials. In particular, direct PFG NMR measurements of molecular diffusion in fluid catalytic cracking (FCC) catalysts can be used to find new routes of optimization of these materials with respect to transport of reactant and product molecules [1]. Typically, each particle of a formulated FCC catalyst possesses a complex system of pores consisting of nanopores located in zeolite crystals and of macro- and mesopores located in the socalled “matrix”, which surrounds the crystals [2]. Here we report the results of the PFG NMR study of self-diffusion of guest molecules in FCC catalysts as well in USY zeolite. This zeolite represents the most important, catalytically active part of the FCC catalyst particles. The diffusion measurements have been performed at different temperatures and for the whole range of molecular displacements, which are essential for a detailed understanding of molecular transport in FCC catalysts. The results of these measurements have proved to be essential for a clarification of the role of various mechanisms of diffusion in transport limitations arising during catalytic reactions under typical FCC conditions.
0899-N08-02.2
Diffusion also plays a very important role in many functions of mammalian cell membranes. A number of recent studies have
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