Al pairing in 8-membered rings drives superior methanol amination on CHA zeolites

Abstract

In protonic acid-catalyzed methanol amination, achieving high selectivity toward methylamine (MMA) and dimethylamine (DMA) while suppressing dimethyl ether (DME) formation remains challenging. The catalytic performance of protonic zeolites in this reaction depends on Brønsted acidity, which is controlled by both the Si/Al ratio and the spatial distribution of framework Al. CHA zeolite is particularly attractive because of its unique pore architecture and tunable acidity. Here, we synthesized CHA zeolites with Si/Al ratios from 3.4 to 4.3 and systematically varied their Al distributions, without using organic structure-directing agents (OSDAs). The synthesis exploited the synergistic effect of alkali metal cations (Na + , Cs + ) and CHA seeds with different Al contents (SAPO-34, SSZ-13, and pure silica CHA). The Al content of the seed determined the Si/Al ratio of the final product, while the seed type controlled the Al distribution. At a fixed Si/Al ratio of 3.8, the sample synthesized with a SAPO-34 seed (CHA-S34-3.8) showed the best performance. At 350 °C and 0.813 h -1 , and at 400 °C and 4.3 h -1 , MMA + DMA yield reached 86.6% and 87.6%, respectively, with no detectable DME after stabilization. These values represent the highest reported performance for methanol amination. Characterization revealed that enrichment of Al pairs in the eight-membered rings, combined with optimal acid density and strength, accounts for the superior activity. This study provides mechanistic insight into the role of Al distribution and offers a strategy to design high-performance zeolite catalysts for methanol amination.