PFG NMR diffusivity measurements

A large variety of zeolites (e.g., ZSM-12, -22, -23, -48, AIPO4-5, -8, -11, and VPI-5) contain systems of parallel channels with diameters on the order of the molecular dimensions. Molecular propagation in this type of adsorbent represents a special case of diffusion anisotropy, since the main elements of the diffusion tensor referring to the plane perpendicular to the direction of the channel system are equal to zero. In the first PFG NMR diffusion measurements of methane in ZSM-12 and AIPO4-5, only a lower limit on the order of I0 m"s could be determined for diffusivity in the channel direction (160). This value is two orders of magnitude below the diffusivity of methane in the straight channels of zeolite ZSM-5 (see Fig. 14). Since the channel diameters of ZSM-12... 

The variation of the ionicity ratio with temperature is much harder to use in reaching any conclusions because convective flow will cause an overestimation of the PFG-NMR diffusion measurements the further away the measurement is from the ambient temperature. While... 

Solid polymer electrolytes for lithium batteries applications are commonly prepared by dissolving a lithium salt in poly(ethylene oxide) (PEO)-based materials. Chiappone et al. investigated these systems by a Li and NMR study yielding local dynamics and mass transport by temperature-dependent Ti and PFG-NMR diffusion measurements. 

In principle, any of the techniques described for studying self-diffusion may be applied to both single- and multicomponent systems. So far, however, most selfdiffusion measurements of multicomponent systems have been carried out by PFG NMR. Such measurements are possible by applying a set of samples with deuterated compounds with only one species (namely that to be measured) in the protonated form [98], by simultaneously considering different nuclei (e.g. H and 19F [79]), and by Fourier transform PFG NMR [91,92]. 

Table 2 Comparison of PFG NMR Diffusivities with the Results of Quasielastic Neutron Scattering and MD Simulations and of Macroscopic Nonequilibrium ( Sorption ) Measurements ...

Abstract In this chapter the main macroscopic experimental methods for measuring diffusion in microporous solids are reviewed and the advantages and disadvantages of the various techniques are discussed. For several systems experimental measurements have been made by more than one technique, and in Part 3 the results of such comparative studies are reviewed. While in some cases the results show satisfactory consistency, there are also many systems for which the apparent intracrystaUine diffusivities derived from macroscopic measurements are substantially smaUer than the values from microscopic measurements such as PFG NMR. Recent measurements of the transient intracrystalline concentration profiles show that sirnface resistance and intracrystalline barriers are both... 

Adsorption dynamics. IH NMR was employed beyond spectral studies by performing diffusion measurements Applying IH PFG-NMR diffusion experiments to surfactants adsorbed to latex particles in dilute dispersions, a method was developed for the investigation of surfactant adsorption dynamics [23, 26]. Since surfactant molecules were occurring in two sites, i.e. in solution and as adsorbed surfactant, each site exhibited a different diffusion coefficient and was distinguished in a PFG experiment. This offered a convenient way to vary the relevant experimental time scale, which is determined by the spacing of the gradient pulses A... 

As a typical example, in the long time limit, t —> oo, of diffusion confined to spheres of radius R, the effective diffusivity of PFG NMR measurements is found to be [4, 8, 10]... 

Fig. 3.1.5 Temperature dependence of the coefficient of long-range self-diffusion of ethane measured by PFG NMR in a bed of crystallites of zeolite NaX (points) and comparison with the theoretical estimate (line). The theoretical estimate is based on the sketched models of the prevailing Knudsen diffusion...

Fig. 3.1.6 Temperature dependence of the intraparticle diffusivity of n-octane in an FCC catalyst and the intracrystalline diffusivity of n-octane in large crystals of USY zeolite measured by PFG NMR. The concentration of n-octane in the samples was in all cases 0.62 mmol g 1. Lines show the results of the extrapolation of the intracrystalline diffusivity and of the intraparticle diffusivity of n-octane to higher temperatures, including in particular a temperature of 800 K, typical of FCC catalysis.

Comparison between xf a as determined on the basis of Eq. (3.1.15) from the microscopically determined crystallite radius and the intracrystalline diffusivity measured by PFG NMR for sufficiently short observation times t (top left of Figure 3.1.1), with the actual exchange time xintra resulting from the NMR tracer desorption technique, provides a simple means for quantifying possible surface barriers. In the case of coinciding values, any substantial influence of the surface barriers can be excluded. Any enhancement of xintra in comparison with x a, on the other side, may be considered as a quantitative measure of the surface barriers. 

Pulsed field gradient NMR has become a standard method for measurement of diffusion rates. Stilbs [272] and others have exploited in particular the FT version for the study of mixtures. An added advantage of PFG-NMR is that it can be employed to simplify complex NMR spectra. This simplification is achieved by attenuation of resonances based on the differential diffusion properties of components present in the mixture. 

Pulsed field gradient NMR (PFG-NMR) spectroscopy has been successfully used for probing interactions in several research fields.44-53 The method was developed by Stejskal and Tanner more than 40 years ago54 and allows the measurement of self-diffusion coefficient, D, which is defined as the diffusion coefficient in absence of chemical potential gradient. 

Every et al. determined diffusion coefficients of Nafion 117 and BPSH 40 using a modified PFG NMR method. Nafion 117 and BPSH 40 were immersed in a MeOH solution of known concentration, and diffusion coefficients were measured as a function of MeOH concentration. For a Nafion... 

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