CONTROL OF INTRAZEOLITIC GALLIUM CATION CONTENT AND ITS EFFECTS ON C-2 DEHYDROGENATION IN GA-MFI CATALYSTS

Ga-doped MFI zeolites are selective C-2 and higher alkane dehydrogenation catalysts. The balance between zeolite proton sites and exchanged Ga-cations, which is known to critically affect the formation of aromatics, is itself greatly affected by the extent of Ga reduction, steaming, and treatment with oxidizers. Also, although the reaction can be inhibited by H-2, and CH4, at short times on stream the presence of CH4 can in some cases enhance the formation of aromatics. The reaction of ethane at low alkane pressures has been used to separate these effects from site deactivation by coking. The kinetics studies were supplemented with site characterization experiments, including thermal analysis of adsorbed 1-propanamine, TPO of the carbonaceous residues, XRD, and FTIR of the catalyst and of adsorbed 1-propanamine and pyridine. These studies show that at reaction conditions small quantities of H2O and/or O-2 can transform intrazeolite Ga-cation sites back to protons and gallium oxide. This transformation inhibits the formation of aromatics except in the case of low levels of protons, where the yield to aromatics can actually increase. The results are interpreted in terms of the dual-function mechanism for alkane dehydrogenation. (C) 1995 Academic Press, Inc.