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

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 Bronsted acid site (BAS) at Ga/BAS ratio up to 0.7, however, only up to similar 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 GaHx bands on reduced Ga/H-ZSM-5 at 550 degrees C increases almost linearly at low Ga loadings (Ga/Al < 0.13), and then levels off. In contrast, no detectable GaHx 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 GaHx 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 degrees C to form GaOOH species, while the latter is inactive under the same conditions. 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. (C) 2020 Elsevier Inc. All rights reserved.

Automated summary

This study systematically investigated the speciation of gallium in Ga/H-ZSM-5 using FTIR spectroscopy, revealing the existence of two different types of Ga species: isolated Ga+ and Ga+-H+ pair sites. These different species have distinct properties and may exhibit different catalytic activities in alkane dehydrogenation reactions.