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

• Published in: Journal of catalysis Vol. 140; no. 1; pp. 209 - 225
• Main Authors: Meitzner, G.D., Iglesia, E., Baumgartner, J.E., Huang, E.S.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.03.1993; Elsevier
• Subjects: 400201 - Chemical & Physicochemical Properties; ABSORPTION SPECTROSCOPY; ALKANES; AROMATICS; Catalysis; CATALYST SUPPORTS; CATALYSTS; Catalysts: preparations and properties; CHEMICAL REACTIONS; CHEMICAL STATE; Chemistry; CRACKING; CYCLIZATION; DECOMPOSITION; DEHYDROGENATION; DESORPTION; DIMERIZATION; ELEMENTS; Exact sciences and technology; GALLIUM COMPOUNDS; General and physical chemistry; HYDRIDES; HYDROCARBONS; HYDROGEN; HYDROGEN COMPOUNDS; INORGANIC ION EXCHANGERS; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ION EXCHANGE MATERIALS; MATERIALS; MINERALS; NONMETALS; ORGANIC COMPOUNDS; POLYMERIZATION; PROPANE; PYROLYSIS; SILICATE MINERALS; SORPTION; SPECTROSCOPY; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; THERMOCHEMICAL PROCESSES; VALENCE; X-RAY SPECTROSCOPY; ZEOLITES; Gallium; Catalytic reaction; Hydrocarbon; Aliphatic compound; Saturated compound; Zeolite; Experimental study; EXAFS spectrometry; Molecular sieve; Characterization; Heterogeneous catalysis; Impregnation; Dehydrogenation; Preparation; Alkane; Catalyst; Propane; Modified material
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1006/jcat.1993.1079

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.