Various types of Ga in MFI metallosilicates: characterization and catalytic activity Original Research Article

Ga was introduced into MFI structures in three ways: direct synthesis, postsynthetic modification of boralites, and `solid state exchangeʹ with HZSM-5. The state of Ga species was characterized by 71Ga MAS NMR, XRD and TPR; additional information was provided from acidity measurements, which were carried out by FTIR spectroscopy of adsorbed acetonitrile-d3 and by TPD of ammonia. The catalytic activity of samples prepared was checked by the oxidative dehydrogenation and aromatization of ethane. The methods used showed that the direct synthesis leads to the substitution of T atoms by Ga and to a relatively slight decrease in acidity compared to H[Al]ZSM-5. The reduction of Ga starts above 600°C. The postsynthetic modification of boralites yields Ga both in framework and extraframework positions; with increased Ga content an extraphase Ga is formed, which most probably belongs to GaO(OH). Two types of acid centers appear and Ga species are much more readily reducible, but with increasing Ga content its reduced fraction substantially decreases. Ga introduction by `solid state exchangeʹ does not change the acidity strength of HZSM-5, the extent of reduction is relatively low but a part of Ga is reduced at ∼350°C. Catalytic activity of the catalysts except the latter sample is low which implies that the presence of strong acid sites together with easily reducible Ga species enhances the oxidative dehydrogenation as well as aromatization of ethane. In some cases, the CO oxidation by 18O2 was examined and it was found that the oxygen of the zeolite lattice is incorporated into CO2.