| Popis: |
Elemental analysis, temperature-programmed desorption of ammonia (TPDA), and 27 Al magic angle sample spinning (MAS) NMR were combined to quantitate the amount of framework tetrahedral aluminum in ZSM-5 zeolites, either pretreated in controlled conditions to avoid alteration of structural aluminum or modified by exposure to boron trichloride to intentionally vary the aluminum content. The cracking of n-hexane at 811 K (alpha test) was used to evaluate the relative acid catalytic activity of these materials. In both cases, activity is governed by the framework tetrahedral aluminum content. Some of the critical variables for 27 AI MAS-NMR quantitation are delineated. 11 B MAS-NMR was used as a complementary technique to confirm the substitution of aluminum by boron in the zeolite framework. The ion-exchange capacity stays nearly constant as a result of such an atom interchange. A mechanism is proposed. |
