SSZ-39 Zeolite with Tunable Acid Sites: A Catalyst for Selective Methylamine Synthesis

The selective synthesis of methylamines─monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA)─via methanol ammoniation is an important industrial process. Although small-pore zeolites like CHA and RHO offer improved selectivity, their combined MMA and DMA yield (yield_MMA+DMA) remains limited to 79.3% under industrial conditions (400 °C, methanol weight hourly space velocity of 4.3 h^-1). Here, we report the performance of the SSZ-39 zeolite in this reaction. H-SSZ-39 with a Si/Al ratio of 5 achieved a yield_MMA+DMA of 85.9% under industrial conditions. Partial Na⁺ exchange further improved the performance by selectively neutralizing strong Bro̷nsted acid sites responsible for TMA formation, increasing yield_MMA+DMA to 90.6%. Solid-state ²³Na multiple-quantum magic-angle spinning (MQMAS) NMR and Rietveld refinement of powder X-ray diffraction (PXRD) showed that Na⁺ ions first occupy eight-membered rings (8MRs) and migrate to double six-membered rings at higher exchange levels. Acid site analysis and density functional theory (DFT) calculations revealed that Al-OH groups in 8MRs possess the strongest Bro̷nsted acidity and high Na⁺ affinity. These results indicate that Na⁺ exchange in SSZ-39 proceeds in a specific order, offering valuable insights into the rational design of zeolite-based catalytic systems.