Investigation of Aluminum Site Changes of Dehydrated Zeolite H-Beta during a Rehydration Process by High-Field Solid-State NMR

Aluminum site changes of dehydrated H-Beta zeolite during a rehydration process are systematically investigated by 27 Al MAS and MQ MAS NMR at a high magnetic field up to 19.9 T. Benefiting from the high magnetic field, more detailed information is obtained from the considerably broadened and overlapped spectra of dehydrated H-Beta zeolite. Dynamic changes of aluminum sites are demonstrated during the rehydration process. In completely dehydrated H-Beta, invisible aluminums can reach 29%. The strength of quadrupole interactions for framework aluminum sites decreases gradually during the progressive water adsorption process. Extra-framework aluminum (EFAL) species, that is, penta- (34 ppm) and octahedral- (4 ppm) coordinated aluminum atoms, rise initially with increasing amount of water adsorption, and finally change into either tetra-coordinated framework or extra-framework aluminum in water saturated samples, with the remaining octahedrally coordinated aluminum lying at 0 and −4 ppm, respectively. Quantitative 27 Al MAS NMR analysis combined with 1 H MAS NMR indicates that some active EFAL species formed during calcination can reinsert into the framework during this hydration process. The assignment of aluminum at 0 ppm to EFAL cation and −4 ppm to framework aluminum is clarified for H-Beta zeolite.