Fourier-transform pulsed-field-gradient1H nuclear magnetic resonance investigation of the diffusion of light n-alkanes in zeolite ZSM-5

Abstract

Fourier-transform pulsed-field-gradient NMR measurements have been used to analyse the diffusion of some n-alkanes (methane, n-butane and n-pentane) in zeolite ZSM-5. The intention of the NMR study was to compare results obtained by molecular dynamics calculations and by uptake measurements, using the same systems. Methane clearly exhibits a bi-exponential spin-echo attenuation. This indicates two types of diffusion: intracrystalline diffusion and long-range diffusion, which is a combination of inter- and intra-crystalline diffusion. In the case of small crystals the diffusion of methane into the macropores between the crystals dominates the decay. However, as expected, for larger crystals, the contribution of intracrystalline (or micropore) diffusion increases significantly. From the curves, the coefficient of intracrystalline diffusion of methane in ZSM-5 has been determined (3.8 × 10^–9 m² s^–1 at 25 °C). The NMR methane diffusion data are in good agreement with values obtained by molecular dynamics calculations. Subsequent NMR measurements of n-butane and n-pentane diffusion in ZSM-5 indicate that the diffusion decreases sharply with increasing chain length of the hydrocarbons (11 × 10^–11 and 4.4 × 10^–11 m² s^–1, respectively, at 25 °C and 20 kPa loading). To allow for a comparison with the diffusivities obtained independently by other techniques, the concentration dependence of the NMR self-diffusion coefficient of n-pentane in ZSM-5 was determined and was found to decrease with increasing sorbate concentration. In addition, from the temperature dependence of the diffusion rates the activation energies for the n-butane and n-pentane diffusion in ZSM-5 have been determined (8.4 and 12.6 kJ mol^–1 at 20 kPa, respectively). The PFG NMR and MD results for the diffusion of light n-alkanes in ZSM-5 have also been compared with relevant diffusion data from the literature (obtained using other techniques, i.e. uptake methods, ZLC, MT). The microscopic self-diffusivity (from PFG NMR and MD) differs systematically by ca. two orders of magnitude from the much slower, macroscopic diffusion observed by uptake, ZLC and MT methods. On the other hand, there is satisfactory agreement between the self-diffusivity of n-butane obtained with PFG NMR and the transport diffusivity of the same system measured using the frequency response method.