Nano Single-Crystal-Assembled MEL Zeolites with Specific Al Pairs for Efficient Low-Temperature Selective Catalytic Reduction of NO x by Methanol

Developing efficient zeolite-based catalysts for the methanol-selective catalytic reduction of NOx (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% NOx 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 (Alp) 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 Alp 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.