The Chemical State of Gallium in Working Alkane Dehydrocyclodimerization Catalysts. In situ Gallium K-Edge X-Ray Absorption Spectroscopy

Ga species in H-ZSM5 zeolites catalyze rate-limiting dehydrogenation steps during the conversion of alkanes to aromatics. Specifically, they promote the recombinative desorption of H-adatoms as H 2 and thus inhibit undesired cracking reactions. Here, we describe studies of the physical and chemical state of Ga using in-situ X-ray absorption at the Ga K-edge. Ga +3 species initially present in fresh catalysts reduce at temperatures below 770 K during hydrogen pretreatment or propane reactions. Reduced Ga is present in highly dispersed form without Ga nearest neighbors, probably as a monomeric hydride species coordinated to basic oxygens within zeolite channels. These hydride species are driven to release molecular hydrogen by the high surface hydrogen fugacities that develop during propane dehydrocyclodimerization on H-ZSM5. Reduced Ga species reoxidize to Ga +3 when samples are cooled to room temperature in flowing H 2, suggesting that active forms of Ga exist only at reaction conditions. This work illustrates the critical need for in-situ techniques to establish the chemical form of catalytic sites and the misleading conclusions that can arise from the exclusive use of pre- and postreaction characterization to suggest the nature of the catatlytic sites.