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

Area of Science:

• Catalysis
• Materials Science
• Chemical Engineering

Background:

• Methanol ammoniation is crucial for industrial methylamine production (monomethylamine, dimethylamine, trimethylamine).
• Small-pore zeolites like CHA and RHO show promise but have limited MMA and DMA yields (79.3%) under industrial conditions.
• Optimizing zeolite catalysts is key to improving selectivity and yield in methylamine synthesis.

Purpose of the Study:

• To evaluate the performance of SSZ-39 zeolite for selective methylamine synthesis via methanol ammoniation.
• To investigate the effect of sodium ion (Na+) exchange on SSZ-39's catalytic properties and product selectivity.
• To understand the mechanism of Na+ ion exchange and its impact on acid site neutralization and TMA formation.

Main Methods:

• Testing H-SSZ-39 zeolite (Si/Al ratio of 5) under industrial conditions (400 °C, WHSV 4.3 h⁻¹).
• Performing partial Na+ exchange on SSZ-39 and evaluating its effect on yield.
• Utilizing solid-state 23Na MQMAS NMR and PXRD with Rietveld refinement to study Na+ ion locations.
• Conducting acid site analysis and Density Functional Theory (DFT) calculations to probe acidity and ion affinity.

Main Results:

• H-SSZ-39 achieved a combined MMA and DMA yield (yieldMMA+DMA) of 85.9% under industrial conditions.
• Partial Na+ exchange increased yieldMMA+DMA to 90.6% by neutralizing strong acid sites that favor TMA formation.
• NMR and PXRD revealed that Na+ ions preferentially occupy eight-membered rings (8MRs) before migrating to double six-membered rings.
• DFT calculations identified Al-OH groups in 8MRs as having the strongest Brønsted acidity and highest Na+ affinity.

Conclusions:

• SSZ-39 zeolite significantly enhances selective methylamine synthesis compared to conventional zeolites.
• Strategic Na+ exchange in SSZ-39 effectively controls acidity, boosting MMA and DMA yields.
• The ordered Na+ ion exchange mechanism in SSZ-39 provides a pathway for rational catalyst design in zeolite-based systems.