Revealing and Controlling Atomic-Scale Stacking Disorder in SAPO-34 for Enhanced MTO Performance

Area of Science:

• Materials Science
• Catalysis
• Nanotechnology

Background:

• SAPO-34 is a key catalyst for methanol-to-olefins (MTO) but suffers from limited operational stability.
• Catalyst deactivation is a major challenge in industrial MTO processes, necessitating improved catalyst design.
• Stacking disorder, a subtle structural defect, has been identified as a previously unrecognized cause of SAPO-34 deactivation.

Purpose of the Study:

• To identify the atomic-level origins of SAPO-34 deactivation.
• To investigate the role of stacking disorder in SAPO-34 catalyst performance during MTO reactions.
• To develop strategies for synthesizing stacking-disorder-free SAPO-34 with enhanced catalytic stability.

Main Methods:

• Advanced electron microscopy techniques, including 3D electron diffraction (3D ED) and integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM).
• Electron ptychography for high-resolution structural analysis of SAPO-34 crystals.
• Synthesis and characterization of SAPO-34 using various organic structure directing agents (OSDAs).
• Methanol-to-olefins (MTO) process testing to evaluate catalyst performance and lifetime.

Main Results:

• Atomic-level stacking disorders were uncovered in conventionally synthesized SAPO-34, linked to smaller *aei*-cages and accelerated deactivation.
• A larger OSDA, N,N,N-trimethyladamantan-1-aminium (TMAda+), was found to suppress *aei*-cage formation, yielding stacking-disorder-free SAPO-34 with extended MTO lifetime.
• Cost-effective amine OSDAs were developed, successfully producing stacking-disorder-free SAPO-34 with MTO performance comparable to TMAda+-directed catalysts.
• SAPO-34 synthesized with N,N-dimethylcyclohexylamine demonstrated superior catalytic durability.

Conclusions:

• Stacking disorder is a critical factor limiting SAPO-34 catalyst lifetime in MTO processes.
• Structural design using appropriate OSDAs is crucial for eliminating stacking disorder and enhancing SAPO-34 stability.
• The development of cost-effective OSDAs offers a viable pathway for producing highly stable SAPO-34 catalysts for industrial MTO applications.