27Al and 29Si solid-state NMR studies of dealuminated mordenite

Abstract

²⁷ Al and ²⁹Si MAS and CP MAS NMR have been used to monitor structural 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 aluminium from the framework. ¹H–²⁹Si CP MAS NMR monitors the creation of Si—OH defect sites. Calcination of dealuminated mordenite greatly reduces the number of such defects, indicating that they are substantially ‘healed’ by silicon atoms from other parts of the crystal, and also via dehydration of hydroxy groups to form bridging oxygen sites. The occupancies of crystallographically inequivalent Si(0AI) sites calculated from ²⁹Si MAS NMR spectra of dealuminated mordenite show that aluminium atoms occupy some of the T2 tetrahedral sites in the parent material.