Characterization of Ga Speciation in Ga/H-ZSM-5 By in-Situ Transmission FTIR Spectroscopy

Ga/H-ZSM-5 has been widely investigated for non-oxidative propane dehydrogenation (PDH). Despite numerous studies, the dependence of Ga speciation on the Si/Al ratio and the Ga/Al ratio at the reaction condition remain a topic of considerable discussion. We employ in-situ transmission Fourier Transformed infrared (FTIR) spectroscopy to identify the gallium species in Ga/H-ZSM-5 with different Ga/Al ratios (0 – 1.7) and different Si/Al ratios (15 – 39) at temperature and atmosphere similar to PDH. After reducing Ga/H-ZSM-5 at 550 °C, Brønsted acid OH band gradually decreases and disappears at high Ga/Al ratio, confirming the replacement of Brønsted acid site (BAS) by gallium after reduction. Pyridine adsorption at 150 °C on reduced Ga/H-ZSM-5 has been applied to quantitatively determine the relationship between consumption of BAS and Ga/Al ratio, which reveals a gallium ion to proton ratio of unity at Ga/Al <1. The gallium hydride has been detected in FTIR (2035 cm^-1 and 2053 cm^-1). The amount of formed gallium hydride at 550 °C for Si/Al = 15 linearly increases with the Ga/Al ratio in the range of 0 - 0.1, and keeps almost constant in the range of 0.1 to 0.7. However, less amount of gallium hydride is detected for Si/Al = 28 and no observable gallium hydride is detected for Si/Al = 40. It is postulated that Ga⁺－H⁺ pair and the isolated Ga⁺ could be formed after replacing BAS, and Ga⁺－H⁺ pair can transform to gallium hydride with the treatment of H₂ while isolated Ga⁺ cannot. In addition, water treatment at 150 °C on reduced Ga/H-ZSM-5 has been first proposed to distinguish different gallium species. Isolated Ga⁺ could react with H₂O to form GaOOH, which is detectable in FTIR (2030-2070 cm^-1), while Ga⁺－H⁺ pair cannot. Our results pave the way for investigating the mechanism of PDH.