PFG NMR Measurement of Molecular Diffusion in Cation-Free Zeolites of Type LTA

Fig. 1 Self-diffusion coefficients of (+) methane, (x)ethane, (Δ)propane [7], and of (○)methanol [8] in NaCaA in comparison with the first results obtained with two different specimens of cation-free Si-LTA, viz. of (■)ethane and (▲)propane in specimen I and of (□)ethane, (⃟)ethene and (●)methanol in specimen II. 1. Introduction In elucidating the origin of the discrepancies in the message of different experimental techniques on intracrystalline diffusion in zeolites, theoretical approaches correlating the rate of translational migration with the potential fields exerted by the zeolite framework on the diffusing molecules have attained particular relevance. Due to some residual ambiguity in the theoretical approaches for the cation-sorbate interaction, in this way especially weighty evidence is provided in the case of cation-free zeolitic adsorbate-adsorbent systems. The comparison of experimental diffusion studies with the results of molecular simulations for n-alkanes in silicalite-1 is a prominent example of such investigations [1]. Though to a considerable extent MD simulations have been devoted to molelcular diffusion in cation-free LTA [2, 3], only with the recent advent of this material with crystal sizes of the order of 10 to 20 μm [4] genuine experimental studies with this important type of material has become possible. We are going to present the first results of PFG (pulsed field gradient) NMR diffusion measurements with cation-free, purely siliceous LTA.