CHARACTERIZATION OF THE MOLECULAR-EXCHANGE PROCESS OBSERVED FOR DIMETHYL METHYLPHOSPHONATE ADSORBED ON A SORPTIVE REACTIVE RESIN MIXTURE BY P-31 MAGNETIZATION-TRANSFER AND 2-D EXCHANGE MAS NMR

The molecular exchange of dimethyl methylphosphonate (DMMP) between the reactive and sorptive components in a mixture of three macroreticular resins (a high surface area carbonaceous adsorbent and strong-base and strong-acid ion-exchange resins) is characterized by P-31 MAS NMR. Data from P-31 MAS magnetization-transfer NMR experiments performed on four samples possessing 2, 6, 9 and 14% DMMP are analyzed using a modified two-site exchange model which takes into account that only a fraction of DMMP molecules adsorbed on the carbonaceous adsorbent (site B) are able to exchange with those residing on the ion-exchange resins (site A) during the time scale of the experiment. The exchange lifetime determined in this manner for DMMP adsorbed on the carbonaceous adsorbent (k(B)-1) remains fairly constant at ca. 50 ms for the four samples. The exchange lifetime for DMMP residing on the ion-exchange resins (k(A)-1) is observed to increase for lower weight-percent loadings: 250 ms (14%), 330 ms (9%), 500 ms (6%) and 670 ms (2%). P-31 MAS 2-D exchange NMR experiments indicate an optimum mixing time (tau(m)) of 0.2-0.5 s for the 14% sample, which is consistent with the measured exchange lifetimes.