Conversion of methane to acetonitrile over GaN catalysts derived from gallium nitrate hydrate co-pyrolyzed with melamine, melem, or g-C3N4: the influence of nitrogen precursors

Abstract

Co-pyrolyzing gallium nitrate hydrate and melamine, melem, or g-C₃N₄ generates gallium nitride (GaN) for the conversion of methane to acetonitrile (AcCN). The solid-state-pyrolysis-made GaN catalysts exhibited better activity than commercial GaN. Among the as-prepared catalysts, GaN made by using g-C₃N₄ with a N/Ga ratio of 2 (i.e., GaN-(C₃N₄)-(2)) was the most attractive: a high initial methane conversion (28.2%), a high initial AcCN productivity (151 μmol g_cat⁻¹ min⁻¹), and a 6 h accumulated AcCN yield (5816 μmol g_cat⁻¹) were obtained at 700 °C with a space time of 3000 mL_CH4 g_cat⁻¹ h⁻¹. GaN-(C₃N₄)-(2) had finely dispersed GaN crystals and enriched amorphous CN species (e.g., sp² N and CN groups), and both are important in promoting the methane conversion rate. GaN agglomeration, coke deposition, and depleted CN species contributed to the deactivation of the catalyst, and a nitridation–activation process could rejuvenate the activity partially. The analysis of the structure–activity correlation revealed that the accumulated AcCN yield had an inverse trend with respect to the crystallite size of GaN and the sp³/sp² ratio of the N environment.