PFG NMR technique

The pulsed field gradient nuclear magnetic resonance (PFG NMR) technique is experimentally distinct from the radiotracer technique but the principle is very similar. A fraction of the nuclei of each constituent is labelled by flipping their spins and monitoring the diffusion of these species. Both techniques are influenced by the presence of neutral associates unlike the Hittorf/Tubandt method. 

Hills and Snaar [15] used the PFG-NMR technique to study cellular tissue and related multicompartment systems. By fitting their data they showed how to obtain dynamic information about membrane permeability, and they intended to use the given strategy to study plant tissue and food preparations. 

The coefficients of intraciystalline self-diffusion of the n-alkanes from propane to n-hexane adsorbed in zeolite ZSM-5 are studied by means of the PFG NMR technique over a temperature range from -20 to 380 °C (96). The diffusivities are found to decrease monotonically with increasing chain lengths. Over the considered temperature range, the diffusivities in ZSM-5 are found to be intermediate between those for Na X and NaCa A zeolite, and the diffusivities of n-alkanes are independent of the Si/Al ratio of the zeolite lattice (96). 

On the basis of the known results one may conclude that this novel PFG NMR technique can be a routine method for characterization of diffusivities in zeolitic materials. 

Fig. 26.3. H-difTusion coefficient measured on TSA.28H2O by the PFG-NMR technique , (a) Echo attenuation as a function of applied magnetic field gradient showing separation of the contributions from the mother liquor and the solid, (b) The resultant diffusion coefficient as a function of the reciprocal temperature the diffusion coefficient calculated from the proton conductivity measured by a.c.-impedance spectroscopy is given for comparison (see Chapters 29-31).

Kortunov et al [55] have used the PFG NMR technique to measure the diffusion of linear alkanes within the crystals and within the macropores of HY and REY based cracking catalysts. At 600°C Dmacro/Dmicro 10 but, since the crystal size is about 1 xm while the particle size is about 100 pm the ratio of the diffusional... 

The PFG NMR technique was first described by Stejskal and Tanner,and it offers the advantage of allowing the measurement of diffusion coefficients over a wider range of values and more precise definition of the time period over which diffusion is being measured. Assuming the diffusion to be time-independent, the echo attenuation for a single... 

MMMs made of poly-(l,4-phenylene ether-ether-sulfone) (PPEES) and ZIF-8 with filler loadings of 10, 20, and 30wt% were studied for CO2 diffusion using pulsed field gradient (PFG) NMR techniques.The self-diffusion coefficient increased from 2.1 x 10 cm s for the pristine PPEES membrane to 9.3x 10 cm s for the 30wt% ZIF-8/PPEES-MMM. ZIF-8 provides Langmuir adsorption sites for CO2 molecules, thus, the gas adsorption in the MMM increases with the filler content. ... 

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