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

The molecular exchange of dimethyl methylphosphonate (DMMP) between th e reactive and sorptive components in a mixture of three macroreticula r resins (a high surface area carbonaceous adsorbent and strong-base a nd 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 mod ified two-site exchange model which takes into account that only a fra ction 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 lifetim e determined in this manner for DMMP adsorbed on the carbonaceous adso rbent (k(B)-1) remains fairly constant at ca. 50 ms for the four sampl es. The exchange lifetime for DMMP residing on the ion-exchange resins (k(A)-1) is observed to increase for lower weight-percent loadings: 2 50 ms (14%), 330 ms (9%), 500 ms (6%) and 670 ms (2%). P-31 MAS 2-D ex change 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 excha nge lifetimes.