EFFECT OF GA/PROTON BALANCE IN GA/HZSM-5 CATALYSTS ON C-3 CONVERSION TO AROMATICS

Ga/HZSM-5 catalysts prepared by chemical vapor deposition (CVD) of GaCl3 into the pores of HZSM-5 have been characterized by temperature programmed techniques, such as reduction (TPR), oxidation (TPO), and desorption of NH3 (NH3-TPD), X-ray powder diffraction (XRD), and catalytic tests including propane and propene conversions and toluene disproportionation (TD). TPR shows formation of (GaO)+ ions and Ga2O3 particles. With increasing Ga loading, the formation of (GaO)+ ions is favored. Unlike methods which introduce Ga into HZSM-5 from Ga salts in aqueous solution, the CVD technique acts as a true ion exchange, replacing protons by (GaO)+ ions. As more protons are replaced by (GaO)+ ions, the conversion of propene decreases, but that of propane passes through a maximum and so does the selectivity to aromatics, both for propane and for propene. This is consistent with a bifunctional mechanism in which Brønsted acid sites catalyze oligomerization and ring closure, but Ga, in concert with protons, acts as a dehydrogenating site. Optimum yields are obtained with catalysts having Ga/(Ga + H+) ratios between 0.4 and 0.5. © 1994 Academic Press, Inc.