Low‐silica Cu‐CHA Zeolite Enriched with Al Pairs Transcribed from Silicoaluminophosphate Seed: Synthesis and Ammonia Selective Catalytic Reduction Performance

Abstract

AbstractCu‐exchanged low‐silica CHA zeolites (Si/Al≤4) synthesized without organic templates are promising candidate catalysts for ammonia selective catalytic reduction of nitrogen oxides (NH3‐SCR), but their practical application is restricted due to the low hydrothermal stability. Here, inspired by the transcription from duplex DNA to RNA, we synthesized Al pairs enriched low‐silica CHA zeolite (CHA‐SPAEI, Si/Al=3.7) by using silicoaluminophosphate (SAPO) featured by strict alternation of −Al−O−P(Si)−O−Al−O− tetrahedra as seed. The proportion of Al pairs in CHA−SPAEI is 78 %, which is much higher than that in the conventional low‐silica CHA (CHA‐LS, 52 %). After hydrothermal ageing at 800 °C for 6 h, Cu‐exchanged CHA‐SPAEI shows NO conversion above 90 % within 225–500 °C under a gas hourly space velocity of 200,000 h−1, which is much better than that of Cu‐exchanged CHA‐LS. The spatial close proximity of Al pairs in CHA‐SPAEI is confirmed by the 27Al double‐quantum single‐quantum two‐dimensional NMR analyses. The strict −P(Si)−O−Al−O−P(Si)−O‐ sequence in the fragments from the dissolution of SAPO seed promotes the Al pairs with the −Al−O−Si−O−Al−O− sequence via a transcription process. The utilization of aluminophosphate‐based zeolites as seeds opens up a new avenue for the regulation of the Al distribution in zeolites.