Aromatization of ethane over gallium-promoted H-ZSM-5 catalysts

Ethane was partly converted to aromatics over gallium-promoted H-ZSM-5 catalysts having different Si/Al ratios (15,25,34,50,75 and 95) and gallium contents ranging from 1.5 to 6 wt.-%. Temperatures varied between 823 and 973 K; the ethane partial pressure was 80 kPa. A maximum aromatics selectivity of 57% was obtained for a gallium (3.1 wt.-% )/ZSM 5 (Si/Al = 15) catalyst at 873 K (ethane conversion: 28.2%;W/F= 19.2 g h mol⁻¹). The method of catalyst preparation (incipient wetness impregnation or ion exchange) had no influence on catalyst activity and selectivity. Kinetic measurements indicated that ethane is dehydrogenated by both Ga₂0₃ at the outside surface of the zeolite and by the acid centres of the H-ZSM-5 catalysts to ethylene; in a subsequent step the ethylene is oligomerized and cyclized in the channels of the zeolite. Aromatic hydrocarbons are formed after further dehydrogenation. Mainly benzene, toluene and xylenes were obtained.