Effects of hydrogen pretreatment on the acidic and catalytic properties of gallium-supported H-ZSM-5 in n-hexane aromatization

Abstract

The interactions between gallium and H-ZSM-5 on gallium-supported H-ZSM-5 catalysts were investigated. Gallium was introduced into H-ZSM-5 by impregnation [Ga/Z5(im)] and physical mixing [Ga/Z5(mix)]. Temperature-programmed desorption studies of ammonia show that the number of strong acid sites (HTP) of H-ZSM-5 decreased after hydrogen pretreatment, while the number of weak acid sites (LTP) increased at the same time. The fall of the HTP is compensated for by the rise of the LTP. If the pretreatment time was long enough, the HTP would disappear completely. The new acid center (Ga +-Z −) was also an active site for acid catalyzed reactions (cracking, oligomerization and cyclization) as H +-Z −. The main factor affecting the catalytic activity was the dispersion of gallium rather than the oxidation state. The yield of aromatics on the reduced and reoxidized catalysts were similar. On hydrogen pretreatment, the residual gallium species (probably in the form of Ga 2O) sinter and block the aperture and/or the internal channel of zeolite. Because Ga/Z5(im) sintered more seriously than Ga/Z5(mix), the activity and aromatic yield of Ga/Z5(mix) were superior to those of Ga/Z5(im), especially at higher space velocity.