Cation pair formation in copper and palladium exchanged MFI zeolite frameworks - a theoretical study

Area of Science:

• Materials Science
• Computational Chemistry
• Catalysis

Background:

• MFI frameworks are crucial materials in catalysis.
• Understanding cation coordination within these frameworks is key to optimizing catalytic activity.
• Copper and palladium cations are important in various catalytic reactions.

Purpose of the Study:

• To investigate the coordination behavior of copper (Cu) and palladium (Pd) cations within MFI zeolite channels.
• To determine optimal positions for cation pair formation and dimerization.
• To evaluate the catalytic potential of these cation configurations for reactions like NO dissociation and methane partial oxidation.

Main Methods:

• Utilized the Quantum Mechanics/Molecular Mechanics (QM/MM) approach within the ONIOM method.
• Employed Density Functional Theory (DFT) for the high layer of QM/MM models.
• Examined various ring structures and associations within the MFI framework.

Main Results:

• Divalent cations prefer six-member ring coordination in MFI channels.
• Monovalent Cu+ and Pd+ cations form dimers ([Pd-Pd]2+ and [Cu-Cu]2+).
• Pd-Pd dimers are stable regardless of Si, Al ordering, while Cu-Cu dimers require specific charge distributions.
• Oxygen-bridged Pd cations (Pd-OH+-Pd) facilitate proton transfer for NO protonation and NO dissociation.
• Narrow-angle [Cu-O-Cu] sites enhance methanol desorption in methane partial oxidation, while [Pd-O-Pd] sites show superior performance.

Conclusions:

• Cation dimers in MFI channels, particularly Pd-Pd, offer energetically favorable pathways for NO dissociation.
• [Pd-O-Pd] active sites are more effective than [Cu-O-Cu] for methane activation and methanol production.
• The specific coordination and dimerization of Cu and Pd cations significantly influence their catalytic properties in MFI frameworks.