Ga speciation in Ga/H-ZSM-5 by in-situ transmission FTIR spectroscopy

Journal Article · Thu Nov 26 00:00:00 UTC 2020 · Journal of Catalysis

DOI: https://doi.org/10.1016/j.jcat.2020.11.004 · OSTI ID:1861078

Yuan, Yong ^[1]; Brady, Casper ^[2]; Annamalai, Leelavathi ^[2]; Lobo, Raul F. ^[2]; Xu, Bingjun ^[3]

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1. Univ. of Delaware, Newark, DE (United States); University of Delaware
2. Univ. of Delaware, Newark, DE (United States)
3. Univ. of Delaware, Newark, DE (United States); Peking Univ., Beijing (China)

H-ZSM-5 supported Ga (Ga/H-ZSM-5) has long been recognized as a promising catalyst for nonoxidative dehydrogenation and dehydroaromatization of alkanes. However, Ga speciation under reaction conditions remains controversial. In this work, in-situ transmission Fourier Transform infrared (FTIR) spectroscopy is employed to systematically investigate Ga speciation in Ga/H-ZSM-5 with three Si/Al ratios (15, 28 and 39) and a wide range of Ga/Al ratios (0–1.7). Quantitative FTIR spectroscopy with pyridine reveals that one Ga atom roughly replaces one Brønsted acid site (BAS) at Ga/BAS ratio up to 0.7, however, only up to ~80% of the BAS on the H-ZSM-5 can be exchanged even with excess amounts of Ga. At a low Si/Al ratio of 15, the intensity of GaH_x bands on reduced Ga/H-ZSM-5 at 550 °C increases almost linearly at low Ga loadings (Ga/Al < 0.13), and then levels off. In contrast, no detectable GaH_x bands are observed on Ga/H-ZSM-5 with a high Si/Al ratio of 39, with Ga/Al ratios up to 1.3. The dependence of GaH_x bands on both the Si/Al ratios and the Ga/Al ratios shows that Ga speciation varies with BAS density in the zeolite. We hypothesize that paired BAS sites are preferentially exchanged with Ga⁺, leading to the formation of Ga⁺–H⁺ pair sites, while the exchange of isolated BAS form isolated Ga+ species. Using water as a probe molecule, we show that isolated Ga⁺ and Ga⁺–H⁺ pair sites have distinct properties, i.e., the former can be easily oxidized by water at 150 °C to form GaOOH species, while the latter is inactive under the same conditions. Furthermore, these results provide direct experimental evidence for the existence of two types of Ga species on reduced Ga/H-ZSM-5, highlighting the possibility that they have different catalytic activities in alkane dehydrogenation reactions.

• Research Organization: Univ. of Delaware, Newark, DE (United States)
• Sponsoring Organization: USDOE; USDOE Office of Energy Efficiency and Renewable Energy (EERE), Energy Efficiency Office. Advanced Manufacturing Office
• Grant/Contract Number: EE0007888
• OSTI ID: 1861078
• Journal Information: Journal of Catalysis, Journal Name: Journal of Catalysis Vol. 393; ISSN 0021-9517
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English