A study of fast tracer desorption in molecular sieve crystals

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Abstract
A modification of the customary NMR (Nuclear Magnetic Resonance) pulsed field gradient technique is shown to allow the observation of tracer desorption phenomena in a time interval of 4 … 1 000 ms. The processes observed are only limited by molecular transport in the individual crystallites. Other influences, such as intercrystalline transport resistances and the finite rate of adsorbate supply or adsorption heat dissipation, as known from traditional sorption experiments, can be excluded.The method is applied to several zeolitic adsorbate‐adsorbent systems: methane…hexane/NaX; ethane/NaCaA. The tracer desorption curves show the expected dependences on paraffin chain length, sorbate concentration and crystallite size. Comparison with the coefficients of intracrystalline diffusion shows that desorption is limited by intracrystalline transport, excluding the existence of structural surface resistance for these systems. Application of the technique to small zeolite crystallites allows the observation of molecular transport phenomena considerably slower than accessible until now by customary NMR techniques.

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