Abstract:

The distributions of framework aluminum (Al) in zeolites critically govern the location and speciation of active copper (Cu) centers, thereby influencing their performance in ammonia selective catalytic reduction (NH₃-SCR) of nitrogen oxides (NOₓ). Conventional Cu-SSZ-39 (Cu-SSZ-39-T) exhibits excellent hydrothermal stability but limited low-temperature activity (150-225 °C) due to a low concentration of Al in 8-membered rings (8MRs) that inhibits the formation of active [Cu(OH)]⁺-Z species. Herein, an SSZ-39 zeolite synthesized with potassium ions (SSZ-39-K) achieved a significantly higher 8MR Al fraction (37.6%). Density functional theory calculations and H₂-temperature-programmed reduction analyses confirmed that the increased 8MR Al population facilitated the formation of [Cu(OH)]⁺-Z species. Aged Cu-SSZ-39-K exhibited nearly twice the NO_x conversion of aged Cu-SSZ-39-T in the 150-225 °C range while maintaining comparable high-temperature activity (250-550 °C) under a gas hourly space velocity of 250,000 h^-1. Enhanced low-temperature performance is particularly beneficial for mitigating NO_x emissions during cold-start phase. Moreover, SSZ-39-K was synthesized with a high crystallization yield (~65%), nearly double the highest yield (33%) reported for direct synthesis routes. This work establishes a robust strategy for tailoring Al distributions in SSZ-39 zeolites, offering an effective pathway to improve low-temperature NH₃-SCR performance and promote practical implementation.

Key words: SSZ-39 zeolite, Al regulation, Low-temperature activity, Ammonia selective catalytic reduction, Enriching framework Al sites