Nano Single-Crystal-Assembled MEL Zeolites with Specific Al Pairs for Efficient Low-Temperature Selective Catalytic...

Developing efficient zeolite-based catalysts for the methanol-selective catalytic reduction of NO_x (CH₃OH-SCR) is imperative for addressing the challenges associated with exhaust gas emissions. Herein, MEL zeolites assembled from nano single crystals were synthesized via a seed-assisted method and evaluated in CH₃OH-SCR reactions. Compared with MEL-D catalysts prepared via the direct synthesis method, the optimal MEL-SS catalysts achieved more than 80% NO_x conversion and 75% N₂ yield over 280-420 °C. Additionally, they also exhibit excellent resistance to SO₂ poisoning and superior long-term stability. The introduction of seeds induces the transformation of the gel into single-crystal nanorods, and their subsequent assembly yields uniform MEL-SS spheres. This special morphology guarantees MEL-SS-30 catalysts more framework paired Al (Al_p) sites than MEL-D zeolites with a similar SiO₂/Al₂O₃ ratio and Brønsted and Lewis acid sites. In situ DRIFTS and density functional theory (DFT) calculations demonstrate that the key intermediate formamide (CH₃NO) can be readily formed and adsorbed on the Al_p sites of the MEL-SS-30 catalyst. Specifically, it originates from the cascade reaction between methoxy (-CH₃O) and HONO species, where the former is the product of CH₃OH activation, and the latter is derived from the multistep reaction of CH₃OH and NO. Finally, a plausible transformation pathway of the key intermediate as well as the CH₃OH-SCR mechanism over MEL catalysts is proposed.