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 Al-27 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 Al-27 MAS NMR analysis combined with 1H 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.