AL-27 AND SI-29 SOLID-STATE NMR-STUDIES OF DEALUMINATED MORDENITE

Al-27 and Si-29 MAS and CP MAS NMR have been used to monitor structura l modifications of mordenite subjected to hydrothermal dealumination, acid leaching and calcination. Six, four- and possibly five-coordinate extraframework aluminium species are formed upon expulsion of alumini um from the framework. H-1-Si-29 CP MAS NMR monitors the creation of S i-OH defect sites. Calcination of dealuminated mordenite greatly reduc es the number of such defects, indicating that they are substantially 'healed' by silicon atoms from other parts of the crystal, and also vi a dehydration of hydroxy groups to form bridging oxygen sites. The occ upancies of crystallographically inequivalent Si(OAI) sites calculated from Si-29 MAS NMR spectra of dealuminated mordenite show that alumin ium atoms occupy some of the T2 tetrahedral sites in the parent materi al.