Structure-activity relationship of the gallium nitride for the direct non-oxidative methane coupling to ethylene

Canada is one of the top chemicals and plastics manufacturing countries. Abundantly available methane in natural and shale gases is a promising feed. Geopolitical uncertainty of crude oil makes CH4 a reliable feed for petrochemicals. Direct non-oxidative conversion of CH4 to petrochemical feedstocks like ethylene and aromatics, is not only greenhouse gas emission-free but also produces valuable H2 gas. High C−H bond strength and coking of catalysts are the major challenges. Most work is focused on molybdenum-containing zeolites. This research project focuses on an emerging class of nitride catalysts, specifically gallium nitride (GaN). The main objective of this work was to understand the structure-activity relationship of GaN for the CH4 coupling to C2H4. A proof of concept with commercial GaN in a fixed bed reactor was established, followed by the GaN catalyst development, elucidating underlying reaction mechanisms, and catalyst regeneration study. Direct CH4 activation was investigated for the first time in a fixed bed reactor under continuous operation over commercial GaN powder. The C2 species (C2H4) was the main product, and besides C2, benzene and toluene were the minor components. The catalyst gets deactivated by coke. Unlike the batch reactor where the residence time was in the order of h, benzene was not the main product in a continuous reactor. Using commercial GaN is not economical even though it has CH4 activation capability. Because of commercial viability, the focus was directed towards the nitride catalyst development. A new stable supported GaN/SBA15 catalyst was reported for the direct CH4 coupling to C2H4. Additionally, unsupported GaN catalysts were also synthesized. An important step in the catalyst synthesis was the conversion of Ga2O3 with ammonia to GaN. In detail, the effect of nitridation temperatures, as well as ammonia exposure time on the morphology and activity of the catalyst, have been investigated. The optimum nitridation temperatures were 700 °C and 750 °C for the GaN/SBA15 and the unsupported GaN catalyst, respectively. Supported catalysts were more stable and had 5−10 times higher product (C2H4) formation rates per g of Ga than the unsupported ones due to the higher surface area and Ga-dispersion inside the pores. Unlike oxides, the nitrides exhibited a higher C conversion efficiency in CH4 leading to higher C2H4 selectivity (71% for nitride, <58% for oxide) and lower coke selectivity (27% for nitride, 40% for oxide). Unsupported nitride catalysts deactivated within 3 h and had much higher coke selectivity (70%). CH4 activation over Ga2O3 resulted in the formation of H2O, CO2, and CO as the CH4 reacted with the lattice oxygen from Ga2O3, which did not occur over GaN catalyst. The supported catalyst was not just economical but had higher and stable C2H4 selectivity, and lower coke selectivity than the unsupported one. Comprehensive DFT modeling was conducted to investigate the reaction mechanism of CH4 coupling to C2H4 over GaN as well as Ga2O3. The calculation indicated that the C−H bond cleavage of the adsorbed CH3* species had the highest activation barrier for both catalysts. DFT modeling also confirmed the formation of H2O and COx through the reaction with lattice O, creating an O vacancy. Diffuse reflectance infrared Fourier transform spectroscopy, 13C solid-state NMR, and isotope labeling experiments validated the DFT results and showed the existence of Ga−CH3 species as well as confirmed that the rate-determining step is the formation of CH2* from CH3*. Other surface species like C=C, >CH2, CH3, N−H, O−H were detected. The regeneration capability and reusability of GaN catalysts was studied. Among multiple regenerating agents tested, the air was found to be the optimum one. The supported catalyst can not only be reused but also had long-term stability in terms of CH4 conversion and C2H4 yield

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