Effects of pretreatments on state of gallium and aromatization activity of gallium/ZSM-5 catalysts

In situ Ga K edge XANES study of the activation of Ga/ZSM-5 prepared by chemical vapor deposition of trimethylgallium

“…We surmise that the oxidation of monovalent Ga + results in the formation of [GaO] + , although one should also consider the possibility that small gallium oxide clusters form concomitant with the regeneration of acid protons. Dooley et al [15] have shown that oxidation of reduced Ga species is fast at 823 K but does not proceed to b-Ga 2 O 3 . The difference in reducibility between the dimethylgallium precursor and the oxidized Ga species is expressed by the minor extent of reduction of the latter species upon exposure to hydrogen at 773 K. Figure 6 shows that such treatment only leads to a weak shoulder on the low energy side indicating that the oxidized species cannot be reduced easily under these conditions.…”

“…The structure of the active intrazeolite Ga species and their reactivity in oxidative and reductive conditions has not been fully resolved yet, owing to the wide variety of possible structures including bulk Ga 2 O 3 aggregates on the external zeolite surface, small gallium oxide aggregates occluded in the zeolite micropore space, reduced Ga species such as Ga 2 O and cationic Ga species in oxidized or reduced form. Various methods to incorporate extra-framework gallium into the micropore space of HZSM-5 have been described [12] including impregnation [13,14] and ion-exchange [15] with Ga 3+ salts, physical mixing with Ga 2 O 3 [16] and chemical vapor deposition (CVD) of GaCl 3 [17,18]. Impregnation, ionexchange and physical mixing lead to poorly dispersed gallium oxide species, mostly present on the external zeolite surface.…”

A DFT Study of Hydrogen–deuterium Exchange over Oxidized and Reduced Gallium Species in Ga/HZSM-5 Zeolite

“…This is commercially applied in the Cyclar process [2,3]. It is generally believed that gallium acts as a promoter to recombinative hydrogen removal during alkane dehydrogenation [1,[4][5][6][7][8][9]. The nature of the active gallium species are still under debate and cationic species such as Ga + [8], GaH 2 + and GaO + replacing the Brønsted acid protons to compensate the framework charge [10] as well as small neutral gallium oxide clusters are considered.…”

New insights into hierarchical metal-containing zeolites; synthesis and kinetic modelling of mesoporous gallium-containing ZSM-5 for propane aromatization

“…Choudhary et al [7] and Al-Yassir et al [8] employed the in situ incorporation method (hydrothermal (one-pot)), for the proper dispersion of Ga species in the structure. It was also shown that steam [9] or chemical vapor deposition method [10] can promote the formation of extra framework Ga species.…”