Characterization of the molecular exchange process observed for dimethyl methylphosphonate adsorbed on a sorptive/reactive resin mixture by 31P magnetization-transfer and 2-D exchange MAS NMR

Abstract

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 31P MAS NMR. Data from 31P 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 −1 B) remains fairly constant at ca. 50 ms for the four samples. The exchange lifetime for DMMP residing on the ion-exchange resins ( k −1 A) is observed to increase for lower weight-percent loadings: 250 ms (14%), 330 ms (9%), 500 ms (6%) and 670 ms (2%). 31P MAS 2-D exchange NMR experiments indicate an optimum mixing time (τ m) of 0.2–0.5 s for the 14% sample, which is consistent with the measured exchange lifetimes.