VARIOUS TYPES OF GA IN MFI METALLOSILICATES - CHARACTERIZATION AND CATALYTIC ACTIVITY

Ga was introduced into MFI structures in three ways: direct synthesis, postsynthetic modification of boralites, and 'solid state exchange' w ith HZSM-5. The state of Ga species was characterized by Ga-71 MAS NMR , XRD and TPR; additional information was provided from acidity measur ements, which were carried out by FTIR spectroscopy of adsorbed aceton itrile-d(3) and by TPD of ammonia. The catalytic activity of samples p repared 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 relatively slight decrease i n acidity compared to H[Al]ZSM-5. The reduction of Ga starts above 600 degrees 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), Tw o types of acid centers appear and Ga species are much more readily re ducible, but with increasing Ga content its reduced fraction substanti ally decreases. Ga introduction by 'solid state exchange' does not cha nge the acidity strength of HZSM-5, the extent of reduction is relativ ely low but a part of Ga is reduced at similar to 350 degrees C. Catal ytic activity of the catalysts except the latter sample is low which i mplies that the presence of strong acid sites together with easily red ucible Ga species enhances the oxidative dehydrogenation as well as ar omatization of ethane, In some cases, the CO oxidation by O-18(2) was examined and it was found that the oxygen of the zeolite lattice is in corporated into CO2. (C) 1998 Elsevier Science B.V.